GERMANIUM-TIN: A MATERIAL AND TECHNOLOGY FOR

GROUP-IV PHOTONICS INTEGRATION ON SILICON

A DISSERTATION

SUBMITTED TO THE DEPARTMENT OF ELECTRICAL ENGINEERING

AND THE COMMITTEE ON GRADUATE STUDIES

OF STANFORD UNIVERSITY

IN PARTIAL FULFILLMENT OF THE REQUIREMENTS

FOR THE DEGREE OF

DOCTOR OF PHILOSOPHY

Robert Chen

May 2014

http://creativecommons.org/licenses/by-nc/3.0/us/

This dissertation is online at: http://purl.stanford.edu/pg608zy8724

2014 by Robert Chen. All Rights Reserved.

Re-distributed by Stanford University under license with the author.

This work is licensed under a Creative Commons Attribution-Noncommercial 3.0 United States License.

ii

http://creativecommons.org/licenses/by-nc/3.0/us/http://creativecommons.org/licenses/by-nc/3.0/us/http://purl.stanford.edu/pg608zy8724

I certify that I have read this dissertation and that, in my opinion, it is fully adequatein scope and quality as a dissertation for the degree of Doctor of Philosophy.

James Harris, Primary Adviser

I certify that I have read this dissertation and that, in my opinion, it is fully adequatein scope and quality as a dissertation for the degree of Doctor of Philosophy.

Krishna Saraswat

I certify that I have read this dissertation and that, in my opinion, it is fully adequatein scope and quality as a dissertation for the degree of Doctor of Philosophy.

Jelena Vuckovic

Approved for the Stanford University Committee on Graduate Studies.

Patricia J. Gumport, Vice Provost for Graduate Education

This signature page was generated electronically upon submission of this dissertation in electronic format. An original signed hard copy of the signature page is on file inUniversity Archives.

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Abstract

The germanium-tin (GeSn) alloy system is a highly engineerable, Group-IV

material system that has the potential to yield a useful direct bandgap, making it a desirable

material for developing light emitters and other photonic devices. Furthermore, its Group-

IV nature makes it electronically compatible with silicon and is important for ubiquitous

integration into current silicon-based chips. In this dissertation, we explore several

properties, features, design, and integration of GeSn alloys on silicon for photonics.

First, we show how Sn-alloying enhances the efficiency of Ge-based light emission

and demonstrate its potential as a light source for Group-IV photonics. Furthermore, we

discuss how the pseudomorphic (fully-strained) GeSn/Ge heterostructure system has many

features in device design, including improved quantum efficiency and quantum

confinement. We motivate, develop, and integrate pseudomorphic GeSn heterostructure

devices into a pseudomorphic GeSn quantum-well light-emitting diode and a

pseudomorphic GeSn quantum-well microdisk resonator. Additionally, the latter device

leverages a special etch-stop property of GeSn and fabrication method to form high-quality,

suspended materials and structures on lattice-mismatched Ge/Si heteroepitaxy. This

technology and the demonstration of a GeSn-based microdisk resonator on silicon

represent a significant advance towards developing a GeSn-based laser on silicon.

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vii

Acknowledgements

The GeSn project was and is one of those incredibly risky and challenging, yet

incredibly rewarding, adventures that academics (for better or worse) decide to partake. As

Ivan Sutherland discusses in his famous speech titled Technology and Courage1, academics

constantly face risk and overwhelming discouragement in such adventures that can lead to

an atrophy of motivation and possibly even failure. Courage, as Sutherland describes, is

the key trait needed to overcome the overwhelming odds of a risky and challenging

situation, where courage is defined as stepping on to thin ice when you know the ice is thin.

As an entering graduate student, I cant say that I had courage because I didnt know how

thin the ice was; however, those leading and pushing the project in 2008 were courageous.

This leads me to my first set of acknowledgements of Professor James Coach Harris, Jr.

and Dr. Yijie Huo. Coach has been a wonderful advisor and does so in a style that allows

students to develop courage in the quickest possible manner, which is with the freedom to

explore ideas, fail many times, and learn from past experiences. A failure is not punished,

a crazy idea is not discouraged, and not once has Coach advised me to not do something

he knew I wanted to try, both technically and professionally. From that, I can say that I

have built courage throughout graduate school.

Yijie has also been a wonderful guide and mentor throughout my graduate studies,

offering guidance, support, interest, and generosity to an extent that is unparalleled. As the

leading graduate student on the project when I first joined, he brought me up to speed with

1 The full, transcribed document can be found at: http://cseweb.ucsd.edu/~wgg/smli_ps-1.pdf

viii

the project and got me involved in growth, characterization, and fabrication. His style of

allowing students to become hands on quickly (and trusting them) is a powerful model that

Ive tried to integrate into my own mentoring style. The rest of the group would

undoubtedly agree that if or when Yijie decides to leave Stanford, it will be difficult to fill

his shoes.

The past and future academics that have been involved with GeSn at Stanford have

been instrumental in driving and developing GeSn technology. In addition to Yijie, Dr. Hai

Lin, who co-developed with Yijie the InGaAs platform for strain control of pseudomorphic

Ge(Sn) films, was extremely productive, helpful, and courageous during her work with me

on GeSn materials. She investigated numerous basic material properties of GeSn and

SiGeSn, and her materials growth and characterization experience has been invaluable to

me. Colleen Shang, who is the future of GeSn at Stanford, has been a pleasure to work with

and mentor. Shes helped remind me that the research environment should be fun, has

pushed me to expand my thinking on the applications of GeSn technology, and has forced

me to keep up-to-date on making sure I can explain technical topics. Professors Ted I.

Kamins, Jelena Vuckovic, and Krishna C. Saraswat have also been extremely helpful in

technical and professional discussions. Ive had the pleasure of working with many

students in Jelenas and Prof. Saraswats groups in developing Ge-based technology.

Dr. Suyog Gupta, a fellow GeSn investigator, has been a fantastic collaborator and

friend. Suyogs intelligence, smarts, mindset, and passion were essential to the success that

GeSn has had at Stanford. His discovery of GeSn as an etch-stop material and connection

with Applied Materials undoubtedly enabled much of the latter work, and having someone

ix

to bounce ideas off of in technology, fabrication/processing, and life (usually over a dark

beer) was a blessing.

Along with Yijie and Hai, Dr. Angie Lin and Dr. Tomas Sarmiento composed the

family of core MBE growers during the MBE portion of this work. I cant imagine how

any of the MBE work would have been completed without the support and knowledge of

Angie and Tomas. Angie, Tomas, and I spent many long nights, weekends, and holidays

babysitting the MBE chambers, repairing equipment, and fixing random plumbing and

pump issues. Despite all the tough times and hours put into the tools, I dont have a single

devastating memory of working on the chambers because when you work with Angie and

Tomas, it doesnt feel like work. Angie has been extremely valuable as a co-worker,

collaborator, and friend. Tomas has also been extremely valuable as a co-worker and

friend, as well as a useful resource of discussions on the optical properties of materials.

The rest of the Harris Group, both past, present, and future, have been invaluable

to my development and sanity in graduate school. My first encounter with the Harris Group

was with an alum, Professor Seth R. Bank at the University of Texas at Austin. As a friend

and mentor, he helped develop my interest in GeSn and provided me with an environment

to develop my interests as an undergraduate. Throughout graduate school, hes been an

excellent resource for technical and professional development. Drs. Meredith M. Lee and

Thomas D. OSullivan have been wonderful friends and role-models that have helped me

determine what I wanted to get out of graduate school. Meredith and Tom were both

heavily involved with the Stanford Optical Society and strongly valued both the technical

and non-technical aspects of being a successful contributor to science. Their leadership and

courage are inspirational. Dr. Sonny Vo and (soon-to-be Drs.) Sara Harrison and Ed Fei

x

have also been a pleasure to interact with and have brought a large amount of energy and

joy to our group over the years. Sara has been an awesome adjacent-cube buddy for the

past couple of years and has made my former cube spot a much cleaner place to work. I

thank the rest of the Harris Group, especially those living in the MBE office, for being

great people to interact with during the day. Having such a great family away from home

makes work seem less like work. Not to forget: our wonderful, growing family of fish

(Fermi, Dirac, Higgs, Boson, and Einstein).

Our industry partners have also been critical for development of GeSn technology.

The APIC and Applied Materials teams have been essential. APIC has supported the GeSn

work at Stanford throughout my tenure here, both financially and technically. Id like thank

Dr. Yi-Chiau Huang at Applied Materials, who grew all the CVD samples and has been a

wonderful collaborator. Furthermore, Id like to thank Dr. Andrew Kellock (IBM), Chuck

Hitzman (Stanford SNL/SNC), Dr. Kin Man Yu (LBNL), Richard Geiger (Sigg Group at

PSI), Prof. Jerome Faist (ETH Zurich), Dr. Charlie Rudy (Byer Group), Dr. Gary Shambat

and Jan Petykiewicz (Vuckovic Group), Dr. Robert Kudraweic (Wroclaw Poland), and

Prof. Seongjae Cho for their collaboration in developing GeSn technology, either through

direct or indirect ways.

The Stanford Optical Society (OSA) has also been crucial for my professional

development and has been a wonderful way to make friends with scientists who value both

the technical and non-technical aspects of science and research. Ive made numerous close

friends through OSA that I hope will follow me throughout my life. Id also like to thank

the support from all my friends at Stanford that Ive made through OSA, the Lyman CA

program, the Harris Group, and other various encounters.

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Lastly, Id like to thank my closest friends outside of Stanford and my family for

their guidance and support through graduate school. When people arent sure what theyre

supporting you through but do so unconditionally, thats love. Benny has been a great

friend throughout the years and has always been there for support when needed. Victoria

has been an amazing friend and guide throughout the majority of graduate school, always

providing useful advice and support. My Mom has always been supportive and encourages

me to enjoy life to the fullest in the happiest way possible. My Dad has been a major

inspiration for always shooting high and understanding that the road to success is never

easy, requires hard work, guts, a lot of determination, and the support of others. Leo and

Frances have been huge support centers in having a family on the west coast. The

adventures of the West Coast Family will live on.

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Table of Contents

Abstract ............................................................................................................................... v

Acknowledgements ........................................................................................................... vii

Table of Contents ............................................................................................................. xiii

List of Tables .................................................................................................................. xvii

List of Figures .................................................................................................................. xix

Chapter 1 Introduction ................................................................................................... 1

1.1 Integration of Photonic Devices on Silicon.......................................................... 3

1.2 Enhancing Germaniums Light-Emitting Potential ............................................. 7

1.3 A Historical Review of GeSn Technology Development .................................. 12

1.4 Challenges in Developing GeSn Materials ........................................................ 19

1.5 Goals and Organization of This Dissertation ..................................................... 25

Chapter 2 Growth and Characterization Tools ............................................................ 27

2.1 Tools for Materials Growth of GeSn Alloys ...................................................... 28

2.1.1 Molecular beam epitaxy .............................................................................. 30

2.1.2 Chemical vapor deposition ......................................................................... 33

2.2 Chemical and Structural Characterization of GeSn Alloys ................................ 36

2.2.1 X-ray photoemission spectroscopy ............................................................. 36

2.2.2 Secondary ion mass spectrometry ............................................................... 39

2.2.3 Auger electron spectroscopy ....................................................................... 40

2.2.4 Rutherford backscattering spectrometry ..................................................... 41

2.2.5 Atomic force microscopy ............................................................................ 44

2.2.6 X-ray diffraction ......................................................................................... 45

2.2.7 Transmission and scanning electron microscopy for imaging .................... 49

2.3 Optical Characterization of GeSn Alloys ........................................................... 50

2.3.1 Absorption/Transmission and Photoreflectance Spectroscopy ................... 50

2.3.2 Photoluminescence and Electroluminescence Spectroscopy ...................... 54

Chapter 3 Strain-Reduced GeSn Alloys Grown on InGaAs/GaAs (001) by MBE ..... 67

xiv

3.1 Solving the Strain Issue with Lattice-Matched InGaAs Buffers ........................ 69

3.2 MBE Growth of Strain-Reduced GeSn on InGaAs/GaAs (001) ........................ 72

3.3 Compositional and Structural Characterization of GeSn Films ......................... 75

3.4 Optical Properties of Strain-Reduced GeSn Films ............................................. 82

3.4.1 Photoluminescence of Strain-Reduced GeSn Alloys .................................. 83

3.4.2 Time-dependent Absorption of Strain-Reduced GeSn Alloys.................... 93

3.4.3 GeSn Resonators ......................................................................................... 95

3.5 Summary, Outlook, and Acknowledgements ................................................... 102

Chapter 4 Pseudomorphic GeSn Grown on Ge-Buffered Si (001) by CVD ............. 105

4.1 Understanding the Effects of Compressive Strain on the GeSn Bandstructure 107

4.2 Theoretical Investigations of Pseudomorphic GeSn/Ge Heterostructures ....... 109

4.2.1 Band Edge Calculations for Pseudomorphic GeSn/Ge Heterostructures . 110

4.2.2 Gain Calculations for Pseudomorphic GeSn/Ge Quantum Wells ............ 116

4.3 CVD Growth of Pseudormorphic GeSn on Ge-Buffered Si ............................ 120

4.3.1 Developing a High-Quality, Relaxed Ge Buffer on Si ............................. 121

4.3.2 Critical Thickness Considerations for Pseudomorphic GeSn/Ge ............. 122

4.3.3 CVD Growth of GeSn Films..................................................................... 123

4.4 Structural Characterization of Pseudomorphic GeSn....................................... 124

4.5 Thermal Stability of Pseudomorphic GeSn Films with High Sn Content........ 127

4.5.1 Sample Growth and Experimental Procedures ......................................... 128

4.5.2 Sn Presence on the Surface and in the Film .............................................. 129

4.5.3 Confirmation of Photoluminescence from the GeSn Film ....................... 134

4.5.4 Effect of Annealing on Photoluminescence from GeSn Films ................. 136

4.5.5 Implications of Thermal-Stability Issues for Device Development ......... 138

4.6 Quantum Confinement in Pseudomorphic GeSn/Ge Quantum-Wells ............. 139

4.7 Benefits of Ge Cap Passivation for GeSn Emission ........................................ 140

4.8 Summary and Acknowledgements ................................................................... 142

Chapter 5 Pseudomorphic GeSn/Ge Quantum-Well Light-Emitting Diode ............. 145

5.1 Material Stack and Fabrication ........................................................................ 146

5.2 Luminescence Studies ...................................................................................... 148

xv

5.2.1 Luminescence Comparisons with Photoluminescence and

Electroluminescence ............................................................................................... 149

5.2.2 Light-Current Characteristics in Electroluminescence for 50-m and 100-

m Devices ............................................................................................................. 151

5.2.3 Discussion of Linear, Super-Linear, and Sub-Linear Trends ................... 154

5.3 Summary and Acknowledgements ................................................................... 155

Chapter 6 GeSn-Based Microdisk Resonators on Si for Lasers ................................ 157

6.1 Challenges of Resonator Development for Ge-based Lasers on Si ................. 158

6.1.1 Challenges with High-Quality, High-Index-Contrast Resonators ............ 160

6.1.2 Previous Strategies for Ge(Sn)-based Microdisk Resonators on Si .......... 162

6.1.3 An Etch Stop: The Elegant Solution for Precise Definition of Suspended 3D

Structures ................................................................................................................ 164

6.2 Developing Ge(Sn)-based Microdisk Resonators Using a GeSn Etch-Stop Layer

165

6.2.1 Fabrication of High-Quality, Suspended Ge(Sn)-based Structures on Si . 167

6.2.2 Material Stack and Fabrication of Pseudomorphic GeSn Quantum-Well

Microdisk Resonators ............................................................................................. 169

6.3 Photoluminescence Measurements of GeSn Quantum-Well Microdisk

Resonators ................................................................................................................... 171

6.3.1 Measurement Setup for Microphotoluminescence ................................... 172

6.3.2 Strong Whispering-Gallery-Mode Resonances from

Microphotoluminescence Measurements ................................................................ 174

6.3.3 The Lasing Potential of Single 20-nm GeSn Quantum-Well Microdisk

Resonators ............................................................................................................... 177

6.3.4 Strategies for Reaching Net Modal Gain in GeSn Microdisk Resonators 180

6.4 Summary, Outlook, and Acknowledgements ................................................... 181

Chapter 7 Conclusions and Suggested Future Work ................................................. 185

List of Abbreviations ...................................................................................................... 189

Appendix A: Electron Inelastic Mean Free Path ............................................................ 193

Appendix B: Determination and Usage of Relative Sensitivity Factors......................... 195

Appendix C: Parts List for the Microluminescence Setup.............................................. 205

Appendix D: Processing of Luminescence Data ............................................................ 209

Appendix E: Continuous-Wave vs. Pulsed Signal in Lock-In Scheme .......................... 217

xvi

Appendix F: Material Properties for Theoretical Calculations ....................................... 221

Appendix G: Process Flow for Microdisks with CF4 Selective Etch ............................. 223

References ....................................................................................................................... 235

xvii

List of Tables

Table 1-1. Summary of Theoretical Predictions and Models of GeSn. ............................ 14

Table 1-2. Summary of Approaches to GeSn Growth. References shown here may refer

to only the first publication of a particular effort. Work done on amorphous materials or

work with dilute Ge were omitted. s.p. = single phase and p.c. = poly crystalline when

noted. ................................................................................................................................. 15

Table 1-3. Table of Common Cubic Substrates and the Sn Contents of Matched GeSn

Films. ................................................................................................................................ 24

Table 3-1. Summary of Samples Grown for Photoluminescence Experiments................ 74

Table 4-1. List of Recipes Used in the Rapid Thermal Annealing Study of High Sn-

Content Films and Their Results. ................................................................................... 129

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List of Figures

Figure 1-1. Limitation of Electrical Interconnects. As the length of an electrical

interconnect increases, its bandwidth limit decreases. A 1 x 1 m2 line reaches an RC-

limited bandwidth of 3.5 GHz at less than 10 mm length. Reproduced from Ref. [8]. ...... 6

Figure 1-2. Bandsturcture E- Diagrams of Ge and -Sn. a) Bandstructure of Ge

because the lowest valley in the CB for Ge is at , Ge represents an indirect bandgap

material. b) Bandstructure of -Sn on the other hand, -Sn is semi-metallic with a CB

minimum lower in energy than the VB maximum, both at zone-center (). Reproduced

from Ref. [24]. .................................................................................................................... 8

Figure 1-3. Illustration of Improved Carrier Occupation in by Reducing in Ge.

In the illustration here, goes from a positive value to a negative value,

indicating a change from an indirect to direct bandgap semiconductor. This is a simplified

diagram in reality, the two valleys are in a continuous band when moving along .

............................................................................................................................................. 9

Figure 1-4. Calculated Carrier Occupation for Ge as a Function of . a) For the

range of electron concentrations studied here, the enhancement when compared to bulk

Ge can be improved more than two orders of magnitude under certain conditions. b)

Furthermore, the bottom graph shows the fraction of carriers occupying as a function of

both and carrier concentration. The advantages of a reduced are

greater when the carrier concentrations are low. .............................................................. 10

Figure 1-5. Bandedges and Reduction of for Tensile-strained Ge. a) The

application of 1.7% biaxial tensile strain leads to a direct-bandgap Ge material. b) The

energy difference, , monotonically reduces towards achieving direct bandgap. 11

Figure 1-6. Predicted Bandgap Energy of GeSn as a Function of Sn Alloying. This

calculation uses the linear interpolation model. The simple model suggests that ~22% Sn

incorporation will result in direct-bandgap GeSn. Reproduced from Ref. [40]. .............. 13

Figure 1-7. Summary of Early Work on Bandgap Extraction of GeSn Films. Black colors

refer to the direct gap and red colors refer to the indirect gap. The dashed lines represent

the predictions from linear interpolation. Results from He et al.[2], Bauer et al.[62], and

de Guevara et al.[65] are shown. The green line is a representation of the direct gap

energy with a bowing parameter of 2.4 eV. ...................................................................... 17

Figure 1-8. Phase Diagram of GeSn in the Low-Sn Regime. The left portion of the

diagram marked (Ge) represents the diamond-cubic () phase. Reproduced from Ref.

[87]. ................................................................................................................................... 20

xx

Figure 1-9. Predicted Critical Thickness for GeSn Grown on Ge. Two theoretical

methods from Matthews-Blakeslee and People & Bean are shown. ................................ 22

Figure 1-10. Transmission Electron Micrograph of Relaxed Ge on Si. These images

represent 3-cycle LT/HT-HR/anneal Ge growth results (a-c) with thickness of 1.44 m. A

dense network of dislocations is confined to the Ge/Si interface using MHAH. d) are

cross-section images of a 4-cycle sample. Reproduced from Ref. [95]. ........................... 23

Figure 2-1. Reaction Curve Illustrating Kinetics vs Thermodynamics. A local minimum

occurs at state A, but the system can be kinetically driven to state B. ............................. 29

Figure 2-2. A Cartoon Schematic of a Molecular Beam Epitaxy Chamber. The cartoon

highlights many standard components and analysis capabilities of most MBE systems. 31

Figure 2-3. The Chamber and Processes of Chemical Vapor Deposition Growth. a) A

basic demonstration of precursor gas flow in CVD. b) Precursor gas processes that can

occur in a CVD chamber. c) Adatom diffusion processes for 2D film growth in CVD.

Reproduced from Ref. [99]. .............................................................................................. 34

Figure 2-4. Sample X-ray Photoemission Spectroscopy Surface Scan of GeSn. The

survey scan can be used to quickly and easily determine the compilation of elements on

the sample surface. For GeSn, Ge, Sn, O, and C peaks are usually seen. ........................ 37

Figure 2-5. High-Resolution X-ray Photoemission Spectroscopy Peak Scans for

Prominent Ge and Sn Binding Energies with their Oxygenated Shifts. a) The Ge3d peak.

b) The Sn3d doublet peak. Sputtering the top surface removes Ge and Sn oxides to reveal

elemental binding energies at 29.5 eV and 484.9/493.2 eV, respectively. ....................... 38

Figure 2-6. An Example of a Secondary Ions Mass Spectrometry Depth Profile of GeSn.

Two isotopes of Ge and Sn were tracked during this depth scan. .................................... 40

Figure 2-7. Auger Electron Spectroscopy Scanning Electron Micrograph and Sn Surface

Map of Annealed GeSn Films. The technique shows excellent overlay of surface features

with strong Sn content for useful mapping of features with chemical composition. ........ 41

Figure 2-8. An Example Rutherford Backscattering Spectrometry Scan of GeSn. a) is for

10% Sn while b) is for 6.5% Sn. The scans show peaks from Sn (far right plateau), Ge

(middle plateau), and Si (left plateau). Reproduced from Ref. [80]. ................................ 43

Figure 2-9. An Example Atomic Force Microscope Surface Scan of Buffered-Oxide-

Etched Silicon Nitride. The beautiful patterns are likely a result of poor surface cleaning

or partially removed silicon nitride. .................................................................................. 45

Figure 2-10. A Schematic of the X-ray Diffraction Geometry. Reproduced from Ref.

[105]. ................................................................................................................................. 46

xxi

Figure 2-11. Example of a Symmetric - X-ray Diffraction Scan in (004) Reflection.

In (004), this is equivalent to a symmetric - with the x-axis angle being twice a -

scan. Scans taken by Dr. Hai Lin. ............................................................................... 48

Figure 2-12. An Example X-ray Diffraction Reciprocal Space Map of 0.16% Strained

GeSn on InGaAs. The GeSn and InGaAs peaks are aligned in Qx, as indicated by the

pseudomorphic line. The GaAs and InGaAs peaks are almost aligned along the relaxed

line. Scans taken by Dr. Hai Lin. ...................................................................................... 49

Figure 2-13. An Example of Absorption of GeSn Films with Increasing Sn Contents (x).

The general shape of the curves shift towards lower energies with increasing Sn content.

Reproduced from Ref. [2]. ................................................................................................ 51

Figure 2-14. The Refractive Index (n) and Decay/Loss Parameter (k) for a Model

Material with 3 Absorption Lines, as Determined by the Drude-Lorentz Model.

Reproduced from Ref. [107]. ............................................................................................ 53

Figure 2-15. An Example of Photoreflectance from GeSn Films with Varying Sn Content.

The transition point can be seen to move towards lower energies with increasing Sn

content for mostly-unstrained GeSn films. Scans taken by Dr. Hai Lin and reproduced

from Ref. [90]. .................................................................................................................. 54

Figure 2-16. Schematic of Photoluminescence and an Example Spectra from Ge. a)

Emission can occur from either the direct or indirect valley, giving off light with different

photon energies. b) Collected spectra provide evidence of bandgap transitions as shown

by the peak contributions. When the energy scale in b) is switched to the y-axis as shown

in a), we see how the peaks align well with band-edge transitions. ................................. 56

Figure 2-17. Schematic of the Harris Group Microphotoluminescence Setup. Component

details are described in Appendix C. ................................................................................ 60

Figure 2-18. The Inside of a Spectrometer Showing a Czerny-Turner Configuration. .... 62

Figure 2-19. Demonstration of Peak Aliasing Present in Photoluminescence Experiments

with a Diffraction Grating. First-order peaks can replicate to HOD peaks without proper

filtering. With proper filtering (FEL1400 Longpass), we see that these HOD peaks

disappear. .......................................................................................................................... 64

Figure 3-1. Lattice Number Line for GeSn and InGaAs Alloys. A relaxed InGaAs buffer

can be used to lattice match to up to 50% Sn, more than needed for interesting

investigations of semiconducting GeSn. ........................................................................... 70

Figure 3-2. An Example of Sn Precipitation as Seen in Transmission Electron

Microscopy. a) shows an as-grown film, b) shows a PDA-treated 8% Sn film, and c)

shows a high-resolution image of precipitates in b). Reproduced from Ref. [111]. ......... 71

xxii

Figure 3-3. Graph Used for Lattice Matching GeSn with InGaAs. As the Sn content

increases, an increased amount of In must be used in the buffer in order to lattice match.

The graph here assumes 100% relaxation in the InGaAs buffer. ..................................... 73

Figure 3-4. Rutherford Backscattering Spectrometry on 6% Sn Grown on InGaAs in

Random and Channeling Configurations. a) Spectra can be fit well to simulation of a

known stack, assisting in issues with peak overalp. b) Random and channeled spectra

showing a = 9%. The sample is 2906. Scans taken by Dr. Kin Man Yu at LBNL.

........................................................................................................................................... 76

Figure 3-5. Depth-Profile Scans Using X-ray Photoemission Spectrometry and Secondary

Ion Mass Spectrometry. Data has been processed using a RBS-calibrated standard sample

for both tools. The extracted Sn contents match very well between the two techniques. 77

Figure 3-6. Atomic Force Microscopy Surface Scans on GeSn as a Function of Growth

Temperature. The RMS surface roughness improves slightly with increased temperature.

We note that the highest growth temperature is a (10 x 10 m2 scan). ............................ 79

Figure 3-7. Atomic Force Microscopy Surface Scans of GeSn as a Function of Increasing

Sn Content. 4.5% Sn is on 10% InGaAs (2564), 7% Sn is on 25% InGaAs (2625), and

8.8% Sn is on 25% InGaAs and also capped with 25% InGaAs (2634). ......................... 80

Figure 3-8. Atomic Force Microscopy Surface Scans of Thick GeSn on InGaAs. Samples

shown are 2906 (left), 2962 (middle), and 2910 (right) with RMS surface roughness of

0.83, 0.80, and 0.61 (in flat regions) nm, respectively. .................................................... 81

Figure 3-9. Transmission Electron Micrograph of Strain-Reduced GeSn on InGaAs.

Smaple has around 50 nm of 7% Sn on a 10% InGaAs buffer (2625). The high-resolution

scan shows excellent ordering and absence of precipitation or dislocations. Scans taken

by Dr. Yijie Huo. .............................................................................................................. 82

Figure 3-10. Sanity Check of GeSn Photoluminescence by Comparison to an InGaAs

Buffer-Only Sample. The InGaAs buffer-only sample shows no emission around 2200

nm, whereas the GeSn sample does. ................................................................................. 83

Figure 3-11. Photoluminescence of Unannealed Films in Table 3-1. The x-axis has been

switched to a photon energy scale since this unit is more useful in the context of bandgap.

This same data is also shown in Ref. [3]. ......................................................................... 85

Figure 3-12. Photoluminescence of Annealed Films in Table 3-1. PL intensities increase

substantially when annealing for low Sn-content samples, but do not for higher Sn

contents. This same data is also shown in Ref. [3]. .......................................................... 86

Figure 3-13. Bandgap Bowing Parameter Fit to Data from Strain-Adjusted Peaks

Extracted from Photoluminescence Experiments. A best-fit bandgap bowing parameter of

2.1 is extracted, suggesting a crossover to direct-bandgap GeSn with 7.1% Sn. This same

data is also shown in Ref. [3]. ........................................................................................... 88

xxiii

Figure 3-14. Temperature-Dependent Photoluminescence of Pure Ge on GaAs and 8.6%

Sn on InGaAs. a) Sample A in Table 3-1 showing dominant indirect-gap PL at LT with a

direct peak appearing at elevated temperatures. b) Sample E in Table 3-1 showing

presumably dominant direct-gap PL at all temperatures with an apparent s-curve at low

temperatures. ..................................................................................................................... 90

Figure 3-15. Photoluminescence Peak Position for 8.6% Sn (Sample E) as a Function of

Temperature with Varshni Fit. The fit works very well at higher temperatures but

deviates from the expected trend at low temperatures. ..................................................... 91

Figure 3-16. Pump-Probe Carrier Lifetime Measurements with Best Fits to Exponential

Decay. The solid lines are best fits to the measured data. Scans and analysis done by

Richard Geiger at PSI. ...................................................................................................... 94

Figure 3-17. Scanning Electron Micrograph of a GeSn Waveguide Resonator on InGaAs

(left) and a TE-Mode Profile Simulation (right). .............................................................. 95

Figure 3-18. Scanning Electron Micrograph of a 4% Sn Microdisk Resonator on InGaAs

and Optical Image of a Top-View Fiber-Tape Probe. a) SEM image of a fabricated GeSn

microdisk resonator on InGaAs taken by Dr. Seongjae Cho. b) Optical microscope image

of the tapering process. The same images are presented in Ref. [123]. ............................ 96

Figure 3-19. Microdisk Transmission Spectra from Fiber-Taper Coupled Microdisks. a)

GeSn microdisks with 1% Sn show WGM resonances above 1550 nm. b) GeSn

microdisks with 4% Sn, however, do not. The loss of resonances with 4% Sn signifies a

smaller bandgap. The same images are presented in Ref. [123]. ...................................... 98

Figure 3-20. Setup Schematic for Q-Switched Pumping of Waveguides. The schematic

shows many basic components for pump and imaging; however, the spectrometer is not

used due to low collection efficiency.............................................................................. 101

Figure 4-1. Schematic of GeSns Bandstructure with Biaxial Strain. Sn-alloying can

make the material direct-bandgap, while compressive strain can reverse that situation to

yield an indirect-bandgap material. This same schematic is shown in Ref. [131]. ......... 108

Figure 4-2. Deformation Potential Theory for Tensile-Strained Ge. The calculations

predicted a direct-gap crossover with 1.7% biaxial tensile strain, which matches

published predictions. This was used to demonstrate the validity of the code. .............. 112

Figure 4-3. Band edge Calculations of Pseudomorphic GeSn Alloys on Ge. a) As Sn is

alloyed, is predicted to shrink and type-I heterostructures are present. The

dashed lines represent the bandedge energy of surrounding Ge. b) A reduction of

is predicted to drop from 136 to 74 meV with around 8% Sn. These results are also

shown in Ref. [131]......................................................................................................... 114

Figure 4-4. Results from GeSn Gain Calculations. a) Gain calculations show a reduction

of the threshold carrier concentration as a function of increasing Sn content. b) The

xxiv

calculated net gain for the TE mode can be quite high at moderate carrier densities. These

results are also shown in Ref. [131]. ............................................................................... 119

Figure 4-5. Calculated Bound-state Energies for and L in a Pseudomorphic GeSn/Ge-

QW as a Function of QW Width. The bound-state energy transition is increased quite

rapidly for due to the small effective mass of carriers in . ........................................ 120

Figure 4-6. Demonstration of Improvements Using the Multiple Hydrogen Annealing

Heteroepitaxy Method. Systematic improvements in the surface roughness and threading

dislocation density are seen with repeated cycles. Figure adapted from Ref. [95]. ........ 122

Figure 4-7. Atomic Force Microscope Surface Scans of Pseudomorphic GeSn on Ge

Films. a) Ultra-smooth, featureless surfaces are seen from a 30-nm 10% Sn film (B4S13).

b) A triple QW structure with 20-nm thick 8% Sn QWs (B12) shows a similar surface

morphology. The right AFM was taken by Colleen Shang. ........................................... 125

Figure 4-8. X-ray Diffraction Reciprocal Space Maps of Pseudomorphic GeSn Showing

Alignment Along Qx. These are the same samples shown in Figure 4-7, respectively,

where a) is a 30-nm 10% Sn layer and b) is a triple QW structure. Scans taken by Colleen

Shang and Dr. Suyog Gupta............................................................................................ 126

Figure 4-9. An Example Cross-Section Transmission Electron Micrograph of GeSn

Grown by Chemical Vapor Deposition. Figure reproduced from Ref. [140], TEM scan

courtesy of AMAT. ......................................................................................................... 127

Figure 4-10. Atomic Force Microscope Surface Scans of Annealed Samples. The film

appears stable up to annealing recipes of 400 oC for 500 s; however, films become

unstable after annealing at 450 oC. This same data is shown in Ref. [78]. ..................... 130

Figure 4-11. Large-Area Atomic Force Microscope Surface Scans of 500C-60 and 500C-

60 HCl. Annealed samples produces features aligned along directions, as seen in

a). b) After an HCl etch, surface nanodot features are removed. c) Overlay profiles of

nanodots and their removed craters after HCl etching. This same data is shown in Ref.

[78]. ................................................................................................................................. 130

Figure 4-12. Auger Electron Spectroscopy of Surface Nanodots Demonstrating Their Sn-

Rich Surface Composition. a) SEM image of surface nanodots and a corresponding AES

Sn-peak scan. b) SEM image of a sample annealed and then subjected to an HCl etch,

removing surface Sn. This same data is shown in Ref. [78]. .......................................... 131

Figure 4-13. X-ray Diffraction Scans Comparing Annealed and Unannealed Samples. a)

Comparison of XRD-RSM (224) scans before and after annealing to form surface

nanodots suggests that the bulk-like structure is mostly maintained, and that the GeSn

film remains pseudomorphic. b) (004) - scans suggest a marginal loss of Sn in the

bulk of the film. This same data is shown in Ref. [78]. .............................................. 133

xxv

Figure 4-14. Photoluminescence Comparison from a GeSn Film and with the GeSn Film

Removed. Loss of the low-energy peak confirms after surface etching confirms that low-

energy photons originate from the top GeSn layer. This data is also shown in Ref. [78].

......................................................................................................................................... 135

Figure 4-15. Comparison of Photoluminescence Spectra of GeSn Films After Various

Annealing Recipes. Systematic decrease in the PL intensity is seen with increasing

thermal annealing with a large drop apparent after nanodot formation (compare 400 and

450 oC). The periodic modulations originate from the vertical Fabry-Prot cavity, as

indicated in the inset, and the shift in the FSR is due to slight variations in the Ge buffer

thickness. This data is also shown in Ref. [78]. .............................................................. 137

Figure 4-16. Comparison of Photoluminescence Spectra of 11-nm and 31-nm GeSn

Quantum Well Samples. a) A mode hop in the peak PL emission is seen as the well

thickness changes, and the thinner well has a higher peak photon energy. b) The shift is

consistent with predicted ground-state energy differences due to quantum confinement, as

calculated. ....................................................................................................................... 140

Figure 4-17. Comparison of Photoluminescence Spectra from a Ge-capped Sample and

One with the Cap Etched Away. A large decrease in the PL intensity is seen after

removing the cap, suggesting a decrease in the quantum efficiency for light emission

likely related to increased surface recombination from carriers in the GeSn region. ..... 141

Figure 5-1. Material Stack and Structure for a GeSn Quantum-Well Light-Emitting

Diode. a) The Ge/GeSn/Ge QW LED is grown on a thick Ge buffer and contains a 34-

nm-thin, 7.5% Sn emitter. Ring contacts (10-m wide) are made to the bottom p-Ge

region by mesa etching to expose the p-Ge region. b) A plan view optical image of a 50-

m-diameter device. This figure has been submitted in a manuscript for publication. .. 147

Figure 5-2. X-ray Diffraction Reciprocal Space Map Along (224) for the Stack of Layers

Shown in Figure 5-1a. Diffraction peaks for the Ge and GeSn layers are clearly seen

along with interference fringes in between. The GeSn and Ge peaks are aligned in Qx,

signifying that GeSn is pseudomorphic to Ge. This figure has been submitted in a

manuscript for publication. ............................................................................................. 148

Figure 5-3. Representative Spectra Obtained from Photoluminescence and

Electroluminescence Measurements. The PL spectrum (blue) shows a high-energy peak

around 0.78 eV from direct-gap emission in Ge. The fringes in both PL and EL spectra

originate from the optical cavity formed by Ge-air and Ge-Si interfaces. The fringes fit

very well to the O-TMM calculations for a 4.385-m Ge cavity, as indicated by the

resonance spectra in black. This figure has been submitted in a manuscript for

publication....................................................................................................................... 150

Figure 5-4. Electroluminescence Spectra from a 50-m-Diameter-Mesa, GeSn Quantum-

Well Light-Emitting Diode at Various Injection Currents. Using an averaged injection

xxvi

current density given by the area of the junction, the injection currents studied represent

current densities of around 330 to 4560 A/cm2. As shown in the inset, the integrated EL

vs injection current curve has a very linear response (R2 = 0.9998) with a zero-EL

crossing at 1.4 mA, indicating good collection efficiency. The spectrum at 6.5 mA was

taken with a longer lock-in time constant to reduce noise. Only four spectra are shown for

clarity. This figure has been submitted in a manuscript for publication. ........................ 152

Figure 5-5. Electroluminescence Spectra from a 100-m-Diameter-Mesa, GeSn

Quantum-Well Light-Emitting Diode at Various Injection Currents. When compared to

the 50-m device in Figure 5-4, we notice slight sub-linear behavior with increased

injection current. ............................................................................................................. 153

Figure 6-1. The Three Core Components Needed to Make a Laser. These include a

pump/energy source, an optical gain medium, and a resonator (a DBR, DFB, PC, and

microdisk resonator are shown). ..................................................................................... 159

Figure 6-2. Illustration of Using a Ge Buffer to Improve the Material Quality of a Lattice-

Mismatched Epitaxial Film on Si. There is little or no contrast between the buffer and a

Ge-based epitaxial film. .................................................................................................. 161

Figure 6-3. An Example of a Ge-based Microdisk Resonator on Silicon. a) A completed

Ge microdisk resonator is formed by undercut etching the Si substrate, as seen in the

SEM image. b) Resonances in taper-collected PL measurements are seen from these

disks. Reproduced from Ref. [148]. ................................................................................ 164

Figure 6-4. Example of the High Selectivity of the CF4 Etch for Etching Ge Over GeSn.

a) Accelerated depiction of the fabrication process for demonstrating etch selectivity with

a 30-nm GeSn layer between two Ge layers. Circular mesas are formed, followed by CF4

selective etching. b) An SEM micrograph of 30-nm-thick GeSn potato chips. The

thickness of the GeSn disks matches the design thickness. The undercut due to the Ge

etch is around 2600 nm, whereas the thickness of GeSn is mostly unchanged. While

prolonged etches were not studied, this demonstrates that GeSn works as an etch stop

with CF4. The waviness in the GeSn potato chips is due to partial strain relaxation

when the straining Ge is removed. This data is also shown in Ref. [131]. ..................... 166

Figure 6-5. Fabrication Process for Forming Suspended 3D Structures. The process

illustrated (read left to right) is a method that provides full 3D protection for the desired

film to be suspended. The top and sides are protected with SiN, and the bottom is

protected with a GeSn etch-stop layer, indicated as the thin, orange layer. This data is

also shown in Ref. [131]. ................................................................................................ 167

Figure 6-6. Material Stack for Developing GeSn Quantum-Well Microdisk Resonators

(left) and its X-ray Diffraction Reciprocal Space Map (right). a) A GeSn QW is inserted

between two Ge layers, forming the QW active region. b) The XRD-RSM shows that this

stack is indeed pseudomorphic. This data is also shown in Ref. [131]. ......................... 170

xxvii

Figure 6-7. Scanning Electron Micrographs of Completed GeSn Quantum-Well

Microdisk Resonators. a) A completed microdisk resonator with a single 20-nm QW. b)

A completed microdisk resonator with a triple GeSn QW. a) is also shown in Ref. [131].

......................................................................................................................................... 171

Figure 6-8. Measured Q-Factor as a Function of Real Q-Factor for Due to Resolution

Limits in the Measurement Setup. Shown here are two curves from an instrument-limited

Qmax of 500 and 1000. ..................................................................................................... 173

Figure 6-9. Transmission Spectrum of the Thorlabs DMLP1180 Dichoric and a Camera

Image Showing Simultaneous Imaging of Sample and Pump Laser. a) The transmission

curve of the Thorlabs DMLP1180 shows a transmission window in the visible as well as

partial transmission for 980 nm. b) The optical microscope image shows the laser spot

and the sample surface simultaneously. .......................................................................... 174

Figure 6-10. Microphotoluminescence from a 2.7-m GeSn Quantum-Well Microdisk

Resonators. a) Large enhancement of the luminescence for the GeSn QW in a microdisk,

as evidenced by the comparison between the microdisk spectra and the bulk (as-grown)

spectra shown for the 1.4-mW excitation (the bulk spectra is almost entirely at the noise

floor). The 1.4-mW conditions are denoted by solid lines, and the 7.4-mW conditions are

denoted by dotted lines. Base represents recorded spectra when pumping the etched Ge

buffer region next to the microdisk. Strong WGM resonances are seen in the microdisk

spectra, which show great luminescence enhancement. The resonances marked by red

dots show relatively close energy spacings of ~0.026 eV. b) Increased pump power

increases the emission intensity for both the background luminescence and the WGM

luminescence. The inset shows extracted peak information for the strong peak near 0.6

eV (~2240 nm). The integrated peak luminescence increases only linearly, and the Q-

factor falls as the pump power is increased, indicating additional loss with increased

pump. This data is also shown in Ref. [131]................................................................... 175

Figure 6-11. Example of Q-Factors Extracted Using an Automatic Q Finder. Shown here

are four high-resolution scans and their full-widths and peak positions marked. .......... 176

Figure 6-12. Heating Effects in GeSn Quantum-Well Microdisk Resonators with

Increased Pump Intensity. The shifting peak position is a result of temperature-induced

index change, leading to changes in the FSR. This data is also shown in Ref. [131]..... 177

Figure 6-13. Estimated Pump Power Required as a Function of Required Carrier

Densities as Determined by Rate Equation Analysis. The highlighted region represents

the pump powers studied in this work. This data is also shown in Ref. [131]. .............. 178

Figure 6-14. Modeling of Free-Carrier Absorption in Ge for the Same Scenarios as

Shown in Figure 4-4 (left) and the Associated Net Modal Gain (right) for a 20-nm, 8% Sn

Quantum Well with 100-nm Ge Barriers. a) The calculated absorption loss for bulk Ge

with the carrier concentrations shown for GeSn. Carrier concentrations in Ge are

xxviii

determined using quasi-Fermi levels. b) Due to the low modal overlap with the GeSn

gain region, net modal gain is not predicted in this stack. This data is also shown in Ref.

[131]. ............................................................................................................................... 180

Figure 6-15. Scanning Electron Micrographs Showing the Family of Simple 3D

Structures Made Using Our Fabrication Method and GeSn Etch Stop. These simple

structures include microdisks and membrane structures, and more complex structures are

possible. .......................................................................................................................... 183

CHAPTER 1: INTRODUCTION

1

Chapter 1

Introduction

hen I presented the outline of my defense talk on February 7, 2014, I referred

to the first experimental section as El Dorado2 for Group IV Photonics? for

a good reason. With the potential of yielding a useful direct bandgap, germanium-tin

(GeSn) is a material system with great promise for developing both electronic and photonic

devices. Such a direct-bandgap material could yield high-mobility electronic devices due

to absence of polar scattering in a low effective-mass material[1] and efficient photonic

devices with a direct-bandgap. Furthermore, its compatibility with a very famous Group-

IV material (silicon, Si) makes the alloy even more alluring, especially for monolithic

integration of photonic devices in todays processors and chips. The benefit of monolithic

integration is the ability to make thousands or millions of light sources on a chip which can

turn light-based devices paired with electronics into a commodity. This could enable high-

speed optical communication on-chip, especially with the development of a Si-compatible

2 El Dorado refers to the city of gold that was long sought-after by a handful of famous (and

unsuccessful) conquistadors beginning in the 16th century. The promise of great treasure lured several

explorers to search for the rumored city, but no city was ever to be found.

W

CHAPTER 1: INTRODUCTION

2

laser. Another impactful application is ubiquitous biosensing (lab-on-a-chip devices)

where photonics and electronics work together to detect a variety of analytes with optical

signatures (vibrational modes in a wavelength of interest). For many (including myself),

this was enough to turn a researcher into an investigador3.

However, as one well-versed on Spanish conquistador history might recall, El

Dorado was never found, and the city of gold remains a myth that led several explorers

astray. The analogy to El Dorado is pertinent, especially in the development of photonics,

because of the many steep challenges towards useful integration of GeSn on Si and

discrepancies related to predicted and experimental optical properties. For example, early

researchers spent ~15 years studying GeSn films before realizing that the bandgap changes

with Sn alloying did not follow a linear-interpolation model[2]. After that, it took another

~15 years before researchers were able to show that Sn alloying improved the quantum

efficiency for light emission in Ge[3], [4].

When the work composing this dissertation began in 20084, many questions loomed

around the realistic benefits that GeSn alloys could provide to the research and engineering

communities. Most of the work focused around overcoming the growth challenges of this

highly metastable material (which will be described in Section 1.4). As Ref. [3] is the work

of this author, GeSn had not been shown to improve the quantum efficiency for light

emission; in fact, there were no reports of light emission at all from GeSn epitaxial films.

Such a demonstration would be required before even considering GeSn for photonics.

Because experimental research in new materials for devices typically requires sufficient

3 Spanish for researcher, in the theme of El Dorado 4 The year 2008 will be referred to frequently in this chapter as a reference to the state of GeSn technology

prior to the contributions that will be presented in this dissertation.

CHAPTER 1: INTRODUCTION

3

motivation to begin investigations and progresses in a serial fashion (prediction to material

growth to optimization to devices to design iteration and engineering), new and unexpected

properties of GeSn and its heterostructures (with Ge, for example) did not have the

opportunity to be extensively explored prior to 2008. The goal of this dissertation is to

investigate the validity of GeSn as a useful material for photonics and to explore its obvious

and hidden properties towards device development.

Before diving into the work composing this dissertation, its important to understand

the details of why GeSn is an interesting material and why GeSn is a challenging material.

In the next three sections, Ill motivate Group-IV photonics, gain some historical

perspective on GeSn development, and highlight the main challenges involved with

growing or synthesizing GeSn films.

1.1 Integration of Photonic Devices on Silicon

The manipulation and applications of light represent some of the most impressive

and impactful technological developments that have taken place in the late 20th and early

21st centuries. Light is as pervasive as the air we breathe, and it carries vast amounts of

information and energy that can be leveraged in a multitude of ways to do a multitude of

things. Light in nature allows us to perceive the world around us, transmits energy from

the sun to power life on Earth, and allows for basic communication between creatures.

Light in the modern world finds its way into almost everything we use, from the cell phone

screens we look at every day and the cameras we use to stop time, to the remotes we use

CHAPTER 1: INTRODUCTION

4

to control our TiVo and the sensor that prevents our garage doors from crushing those

beneath.

Recently, light enabled a major and impactful application to our modern world:

optical communication. Optical communication, which uses various methods of light

modulation to communicate information, is well known for its usage in global high-speed

information transfer and will be responsible for enabling an expected zettabyte (1021 bytes)

of data transfer in 2015[5]. Because optical interconnects have many performance

advantages, optics is quickly replacing electrical interconnects in many scenarios,

including very short-scale communications (1-10 m) between cards and racks in data

centers5, as it becomes practical and cost-effective to do so[6]. A predicted future scenario

is the replacement of electrical interconnects with optical interconnects on Si chips. While

such a development would require investment in new research, this field known as Si or

Group-IV photonics to enable optical communications has received much support by large

companies such as Intel and IBM for the following reasons[7]:

Chip performance will soon be limited by a communications bottleneck. As

transistors continue to scale and attempts to follow Moores Law continue,

electrical interconnects will become the performance-limiting component.

Increasing the transistor density allows for increased computation, but the physical

processor size does not become smaller. Information still needs to be transmitted to

various components on-chip. The maximum RC-limited bitrate is proportional to

the wire cross-sectional area divided by the square of the length. Assuming that all

5 For example, Intel and Fujitsu released the Optical PCIe (OPCIe) Server which allows the processor and

storage components to be physically separated by 10-100 m, yet appear to be on the same mainboard.

CHAPTER 1: INTRODUCTION

5

dimensions scale (including the length of the interconnect) by a constant factor, the

maximum bitrate remains fixed. This maximum bitrate is limited to around 1016

2 bits/s, and an example calculation for a 1 x 1 m

2 interconnect is shown in

Figure 1-1[8]. Other practical concerns, such as cross-talk, power consumption,

impedance matching, and system complexity, make optical communications an

attractive alternative as it can serve the bandwidth needed in a more compact form.

Optical interconnects can reduce interconnect energy consumption. In a study by

workers at Intel in 2004, it was shown that 50% of the dynamic power consumption

(the largest component of power consumption on processors at the 130-nm node)

was due to interconnect power (line charging)[9]. Miller determined that optics can

provide a power savings over a 200 aF/m electrical line if a 10 fF optical detector

in the system was used for an interconnect length greater than 50 m[7]. Such a

feature can provide significant power savings in energy-hungry data centers that

constituted 1.1-1.5% of global electricity usage in 2010[10].

Clock and signal timing can be improved. To avoid bandwidth limitations

mentioned previously, electrical interconnect lines are kept short. The cost,

however, is the need for (several) repeaters which introduce issues with timing and

synchronization. Because optics on chips would not require repeaters, this issue

could be avoided or simplified significantly in a practical implementation[7].

CHAPTER 1: INTRODUCTION

6

Figure 1-1. Limitation of Electrical Interconnects. As the length of an electrical interconnect increases, its

bandwidth limit decreases. A 1 x 1 m2 line reaches an RC-limited bandwidth of 3.5 GHz at less than 10 mm

length. Reproduced from Ref. [8].

In order to become cost-effective to the point where optics can be considered as a

replacement for electrical interconnects on-chip, monolithic integration of the devices

required for optical communication on Si is necessary. Ideally, leveraging the processing

technology and techniques that are used for Si complementary metal-oxide-semiconductor

(CMOS) fabrication is ideal for low-cost integration. Additionally, the materials used to

develop these devices should be Si-compatible (in the Group-IV column of the periodic

table), meaning that the atomic elements composing devices do not drastically change the

electronic properties or performance of CMOS devices. This limits the choice of elements

to carbon (C), Si, Ge, and Sn as candidate materials to choose from when developing

photonic devices for optical, on-chip communication.

Already, several components needed for optical communication have been

developed with Si-compatible materials using CMOS-compatible processes, such as high-

CHAPTER 1: INTRODUCTION

7

speed Ge-on-Si photodetectors up to 40 GHz[11] (more devices reviewed in Ref. [12]),

high-speed Ge/SiGe quantum-confined Stark effect modulators[13][16], and low-loss

waveguides made from Si-on-insulator (SOI)[17]. An elusive device, however, has been

an efficient, Si-compatible light source owing to the fact that none of the elemental, Group-

IV elements display a useful direct bandgap. Recently, many advances have been made

towards Ge light sources due to their potential to emit light. Light-emitting diodes (LED)

on Si[18][20] and even a Ge laser pumped optically[21] and electrically[22] on Si have

been reported; however, the electrically-injected Ge laser displayed a threshold current

density of around 280 kA/cm2, which is not suitable for practical implementation. Intels

efforts towards light-source integration have thus led to a hybrid solution that incorporates

an InP-based III-V laser bonded to Si[23]. Ideally, a monolithic solution is desired. For this

effort, Ge-based solutions have the greatest chance for success if light-emitting

performance can be enhanced.

1.2 Enhancing Germaniums Light-Emitting Potential

The main motivation for engineering and developing Ge-based technology is related

to Ges electronic (leading to photonic) properties. Ge is an indirect-bandgap material with

a room-temperature (RT) band-to-band transition of 0.664 eV between the L-valley

in the conduction band (CB) and the light-hole (LH) and heavy-hole (HH) valleys in the

valence band (VB). The CB in Ge also has another nearby valley at the -point representing

a direct transition of 0.8 eV. Figure 1-2 shows these two critical points in the CB as

indicated by L6 and 7, respectively, in the calculated bandstructure diagram[24].

CHAPTER 1: INTRODUCTION

8

Figure 1-2. Bandsturcture E- Diagrams of Ge and -Sn. a) Bandstructure of Ge because the lowest valley in the CB for Ge is at , Ge represents an indirect bandgap material. b) Bandstructure of -Sn

on the other hand, -Sn is semi-metallic with a CB minimum lower in energy than the VB maximum, both

at zone-center (). Reproduced from Ref. [24].

The bandstructure for Ge represents a non-ideal situation for developing photonic

(and electronic6) devices because it yields an indirect bandgap where most electrons in the

CB will reside in the L-valley. Electrons in the L-valley require a two-step process

involving a phonon to participate in radiative recombination (denoted by vertical

transitions or small changes in the bandstructure picture), which is why indirect-bandgap

materials are much less efficient for light emission. For Ge, the indirect property is

exacerbated by the fact that the L-valley carries a four-fold degeneracy and has a much

larger density of states than the -valley (as a gauge, the effective mass is 0.217 m0 for L

and 0.038 m0 for in Ge [25]).

6 GeSn efforts are also focused on transport devices because the -valley has a much smaller effective mass

than the L-valley, which can result in improved drive currents in transistors.[42]

CHAPTER 1: INTRODUCTION

9

Figure 1-3. Illustration of Improved Carrier Occupation in by Reducing in Ge. In the illustration here, goes from a positive value to a negative value, indicating a change from an indirect to direct bandgap semiconductor. This is a simplified diagram in reality, the two valleys are in a continuous

band when moving along .

Reducing the energy difference between the and L valleys (referred to as ,

which is 136 meV in Ge at RT) can greatly enhance the quantum efficiency for light

emission and improve the fraction of carriers that can participate in direct-gap radiative

recombination. Under equilibrium statistics, a reduction in results in a greater

probability for carriers to occupy the -valley. This is illustrated schematically in Figure

1-3. The calculated results in Figure 1-4 show how reducing in Ge can have a huge

impact on improving the -valley occupation by more than two orders of magnitude in

going from = 136 meV to 0 meV for an electron concentration of = 8 x 1018 cm-3

in the CB.

CHAPTER 1: INTRODUCTION

10

Figure 1-4. Calculated Carrier Occupation for Ge as a Function of . a) For the range of electron concentrations studied here, the enhancement when compared to bulk Ge can be improved more than two

orders of magnitude under certain conditions. b) Furthermore, the bottom graph shows the fraction of carriers

occupying as a function of both and carrier concentration. The advantages of a reduced are greater when the carrier concentrations are low.

Understanding the importance of reducing , we search for ways to engineer Ge

to become more direct bandgap. Biaxial tensile strain is one technique explored

theoretically by Fischetti and Laux at IBM; their work, intended to illustrate how strain can

enhance electron and hole mobility, showed that with around 1.5-2.0% tensile-strain, Ge

can become direct bandgap[26]. A calculation using deformation potential theory is shown

in Figure 1-5 with a predicted crossover to direct bandgap of around 1.7% tensile strain.

Several methods have been used employ tensile strain, including the thermal coefficient of

expansion mismatch for Ge grown on Si[27], pseudomorphic (fully strained) growth on

CHAPTER 1: INTRODUCTION

11

templates/buffers with larger lattice constants[28][30], external stressors[19], [31][34],

and strain-enhanced suspended membranes[35], [36]. Our early work with major

contributions from Dr. Yijie Huo and Dr. Hai Lin explored tensile-strained Ge on indium-

gallium-arsenide (InGaAs) buffers, where the In composition was tuned to increase the

lattice constant of the buffer and induce tensile strain on pseudomorphic, epitaxial Ge

grown by molecular beam epitaxy (MBE)[29], [37]. The work illustrated strain-dependent

Raman shifts, photoluminescence (PL) emission peak shifts, and improved luminescence

efficiency at low-temperature (LT). Another way to make Ge direct bandgap is through Sn

alloying this is the focus of the next section.

Figure 1-5. Bandedges and Reduction of for Tensile-strained Ge. a) The application of 1.7% biaxial tensile strain leads to a direct-bandgap Ge material. b) The energy difference, , monotonically reduces towards achieving direct bandgap.

CHAPTER 1: INTRODUCTION

12

1.3 A Historical Review of GeSn Technology Development

Using Sn alloying to achieve direct-bandgap Ge was first suggested7 by Goodman in

1982[1] in an exploration of -Sn (the diamond cubic phase of Sn, stable below 13.5 oC),

with a review of prior bandstructure results from Groves-Paul[38] and Bloom-

Bergstresser[39]. Goodman noted that a direct-gap appeared to be possible by inspection

of bandstructure diagrams similar to those in Figure 1-2. The greater energy difference

between the -valleys in the two materials than the L-valleys suggested that the -valley

minimum would lower faster than the L-valley minimum with Sn alloying. While the

synthesis of such materials seemed ambitious due to the low solid solubility of Sn in Ge

(to be discussed further in the next section), Oguz et al. synthesized GeSn alloys using

radio-frequency (RF) sputtering and pulsed ultra-violet (UV) laser crystallization on a

variety of substrates one year later, demonstrating up to 22% Sn incorporation[40]. While

there were no useful results indicating the achievement of a direct-bandgap semiconductor,

embedded in the publication was a graph reproduced here in Figure 1-6 which illustrates

the predictions from the linear interpolation model of Ge and -Sn bandgaps.

7 Some of the original investigations of GeSn were done by Temkin, Connell, and Paul at Harvard

University in 1972 using sputter deposition of amorphous GeSn films, but there was no discussion in

producing a direct-bandgap material with their method[157].

CHAPTER 1: INTRODUCTION

13

Figure 1-6. Predicted Bandgap Energy of GeSn as a Function of Sn Alloying. This calculation uses the

linear interpolation model. The simple model suggests that ~22% Sn incorporation will result in direct-

bandgap GeSn. Reproduced from Ref. [40].

While these initial publications from Goodman and Oguz were far from promising,

they were able to motivate further work and investigation of GeSn alloys both theoretically

and experimentally. The linear interpolation model would lead the community for the next

15 years as it was supported with the results from more elaborate modeling techniques and

approximations for determining the bandgap of alloys, such as the virtual crystal

approximation (VCA). Subsequently, more advanced techniques with additional

corrections were implemented, including density functional theory (DFT), the empirical

pseudopotential method (EPM), local density approximation (LDA), linear muffin-tin

orbital (LMTO), and generalized gradient approximations (GGA). These methods will not

be discussed in detail here, but several recent implementation of these methods are

discussed in Dr. Suyog Guptas dissertation[41] and his publications[42], [43].

CHAPTER 1: INTRODUCTION

14

Table 1-1. Summary of Theoretical Predictions and Models of GeSn.

Reference

Year

Technique

Details

Crossover

Prediction

Oguz et al.[40] 1983 Linear interpolation 22% Sn

Jenkins & Dow[44] 1987 VCA 20% Sn

Mader &

Baldereschi[45]

1989 VCA Investigated zinc-

blend GeSn and alloys

26% Sn

Brudevoll et al.[46] 1993 DFT-LMTO Investigated zinc-

blend GeSn

//

Amrane et al.[47] 1995 EPM Investigated zinc-

blend GeSn

//

Zaoui et al.[48] 1996 EPM Investigated zinc-

blend GeSn

//

Chibane et al.[49] 2003 DFT-LDA Large bowing //

Moontragoon et

al.[50]

2007 DFT and VCA 17% Sn

Yin et al.[51] 2008 DFT-GGA 6.3% Sn

Chibane &

Ferhat[52]

2010 DFT-LDA ~10% Sn

Gupta et al.[42] 2011 DFT-GGA+U 8% Sn

Gupta et al.[43] 2013 VCA-EPM+P 7-8% Sn

A representative list of theoretical studies on the bandstructure of GeSn is listed in

Table 1-1. It is interesting to note how early VCA calculations corroborated results from

linear interpolation quite well, predicting a direct-gap crossover with greater than 20% Sn.

Most calculations typically display a non-linear term noted as a bowing parameter. This

bowing is represented in the following way:

,() = , + (1 ), (1 )

In the early models, this bowing parameter () was very weak and spanned values less

than 1.0 eV. Calculations with more advanced techniques, such as DFT and EPM, followed

in the early 1990s. As

CHAPTER 1: INTRODUCTION

15

Table 1-1 shows, ordered zinc-blend structures were investigated, which made it rather

difficult to extract any trends in the low-Sn-content regime of the GeSn alloy with only

three data points. As one can imagine, it may have been challenging and technologically

difficult to compute the bandstructure for disordered alloys with low Sn contents (increased

supercell size).

Table 1-2. Summary of Approaches to GeSn Growth. References shown here may refer to only the first

publication of a particular effort. Work done on amorphous materials or work with dilute Ge were omitted.

s.p. = single phase and p.c. = poly crystalline when noted.

Reference Year Technique Details Sn Content

Oguz et al.[40] 1983 RF sputtering Laser crystallization on various

substrates, 0.1 to 1 m thick

22%

Shah et al.[53] 1987 DC sputtering Ge and GaAs substrates, 1 m

thick

8% (s.p.)

15% (p.c.)

Pukite et al.[54]

Harwit et al.[55]

1989

1990

MBE Ge-buffered Si substrates, 200

nm thick[54], weak absorption

measurements in 700 nm thick

samples[55]

30% (s.p.)

32% (p.c.)

Gossmann[56] 1990 MBE Ge substrates 15% (s.p.)

Gurdal et al.[57] 1995 Temperature-

modulated MBE

Ge substrates, superlattice

structure

24% (s.p)

He & Atwater[2], [58] 1996

1997

Ar+ ion-assisted

MBE

Ge-buffered Si substrates,

Absorption measurements in

1997, Up to 300 nm thick

34% (s.p.)[58]

15% (s.p.)[2]

Lyman & Bedzyk[59] 1996 Surfactant-

mediated MBE

Bi surfactant on Ge substrates

Ragan & Atwater[60] 2000 MBE Ge Substrates, Absorption

measurements

11.5% (s.p.)

Bauer et al.[61], [62]

DCosta et al.[63]

2002

2003

2006

CVD with SnD4 Si substrates, ellipsometry

measurements

15% (s.p.)

18%

Perez Ladron de

Guevara et al.[64]

[66]

2003

2004

2007

RF magnetron

sputtering

Ge substrates, Absorption

measurements in 2004

14%

Takeuchi et al.[67],

[68]

Shimura et al.[69]

2007

2008

2009

MBE with arc-

plasma Sn

evaporation

Si, Ge, and Ge-buffered Si

substrates

Ge-buffered substrates

2.6%[67]

6.7%[68]

6.3%[69]

Lin et al.[70], [71]

Chen et al.[3]

2011 MBE InGaAs-buffered GaAs,

Photoluminescence

10.5% (s.p)

Yu et al.[72] 2011 MBE Ge substrates 14%

Vincent et al.[73]

Gencarelli et al.[74]

2011

2011

AP-CVD with

SnCl4

Ge-buffered Si substrates 8%

Werner, Oehme, et

al.[75][77]

2011

2013

MBE Ge-buffered Si substrates 0.5%[75]

4%[76]

12.5%[77]

Chen et al.[78] 2013 RP-CVD with

SnCl4

Ge-buffered Si substrates 10%

CHAPTER 1: INTRODUCTION

16

Tonkikh et al[79] 2013 MBE Ge substrates 6.3% with

enriched

clusters

Wirths et al.[80], [81] 2013 RP-CVD with

SnCl4

Si and Ge-buffered Si

substrates

6.5% (s.p.)

18% (a or p.c.)

Su et al.[82] 2013 MBE Si substrates, superlattice

structure

7%

Table 1-2 provides a summary and timeline of experimental results on GeSn

synthesis. Work from 1983 to 1996 mostly focused on the growth of GeSn alloys using

sputtering or MBE techniques on a variety of commercially available substrates. Driven by

predictions of a crossover near 20% Sn, most of the worked focused on incorporating large

amounts of Sn into the Ge lattice. However, published results focused on structural

characterization, and optical results on determining the bandgap as a function of Sn were

inconclusive (extremely weak absorption[55]) or irrelevant (transitions energies greater

than 1 eV[40]). Pukite et al. explicitly mentioned that determining bulk properties would

be challenging due to the need of thick, compositionally uniform GeSn films[54].

In 1997, He and Atwater at Caltech published the first results with clear indication

of optical transitions in the expected energy regime of 0.4-0.8 eV for GeSn alloys with up

to 15% Sn. Samples were grown using LT MBE (180 oC substrate temperature) on Ge-

buffered Si (001)[2]. Samples exhibited low residual strain and had thicknesses of up to

300 nm, which allowed for absorption measurements. The striking finding of this work was

that there was a strong deviation from linear interpolation and previously reported

VCA[44] and DFT[45] models as seen in Figure 1-7. Instead of < 1 eV, a value of 2.8

eV was extracted. Subsequent work by Bauer et al.[61], [62] and Perez Ladron de Guevara

et al.[65] corroborated this large deviation with optical ellipsometry and Fourier transform

infrared spectroscopy (FTIR), respectively.

CHAPTER 1: INTRODUCTION

17

Figure 1-7. Summary of Early Work on Bandgap Extraction of GeSn Films. Black colors refer to the

direct gap and red colors refer to the indirect gap. The dashed lines represent the predictions from linear

interpolation. Results from He et al.[2], Bauer et al.[62], and de Guevara et al.[65] are shown. The green line

is a representation of the direct gap energy with a bowing parameter of 2.4 eV.

The effect and implication of a large, positive bandgap bowing parameter is clear in

Figure 1-7. Instead of a direct bandgap crossover at 20-22%, a crossover at lower Sn

contents appeared to be likely. Assuming little or no bowing in the indirect gap, a crossover

around 5% Sn appeared to be possible. With experimental extraction of both gaps, the

crossover appears closer to 10% Sn. Several new theoretical investigations emerged

afterwards where corrections were made in order to better fit experimental data, as seen in

Table 1-1. Chibane et al. reported a DFT study in 2003 with a bowing parameter of 2.06

eV for 12.5% Sn[49]. They found the volume deformation (VD) and charge exchange (CE)

components in DFT were two significant contributions to the large bowing of the alloy.

Yin et al. suggested that with 25% Sn, VD, CE, and strain relaxation (SR) components

were fairly similar[51], but predicted a much larger bowing parameter when compared to

Ref. [49]. Even though theory was now producing numbers closer to experiment, empirical

CHAPTER 1: INTRODUCTION

18

evidence was guiding theory8. The main utility of such a technique is to extract other

parameters or trends. This was a goal of work by Gupta et al. where effective masses and

the trends of other CB valleys were extracted for design of and insight into GeSn-based

transport devices[43]. However even today, device modeling for GeSn-based laser

structures relies mostly on linear interpolation of material properties from Ge and -Sn

with bandgap energies as the only experimentally determined alloy parameter[25], [83]

[85].

Because of the difficulty in correctly predicting properties of GeSn alloys without

experimental correction, experimental results on GeSn alloys are incredibly valuable. For

light sources, perhaps the most important prediction that had not been demonstrated was

enhanced light emission. After all, improved carrier occupation is expected as Ge becomes

more direct gap (Figure 1-4), and PL from the direct gap of Ge was measured as early as

1955[86]. Despite the great body of experimental work that had been done on GeSn films,

no PL had been shown on even dilute Sn films as of 2008. Furthermore, much of the work

in the literature up to 2008 did not have precise strain control or consideration when

bandgap values were extracted. As mentioned before, strain can change the bandstructure

of materials quite substantially and should be considered for accurate determination of

basic material properties. A great impediment to achieving both PL and precise extraction

of basic material properties is the large set of practical challenges in developing high

quality, thick, and unstrained GeSn layers.

8 It is in my belief that this should be the other way around theory should help guide experiment. Theory

is most useful when it can predict new things (I believe this is a quote from a famous deceased scientist

whose name escapes me).

CHAPTER 1: INTRODUCTION

19

1.4 Challenges in Developing GeSn Materials

There are two9 key issues that make growing and integrating useful GeSn films

extremely challenging: low solid solubility of Sn in Ge and the large lattice constant of -

Sn. Figure 1-8 is the binary phase diagram for GeSn near the Ge-rich region[87]. The phase

diagram shows that the maximum solid solubility of Sn in Ge (for Ge-rich compositions)

is less than 1.2% at 350 oC and less than 0.5% at RT. Considering that a minimum of 5%

Sn would be required to achieve a direct bandgap10, incorporating large amounts of Sn

substitutionally would be difficult. However, there were subtle indications that this solid-

solubility limit could be exceeded, including early studies on the growth of -Sn at

temperatures greater than the transition temperature of 13.5 oC (where -Sn converts into

its tetragonal -Sn phase). Farrow et al. showed that -Sn could be grown on ordered InSb

(001) and CdTe (001) surfaces at 25 oC using MBE[88]. InSb and CdTe both have zinc-

blend (cubic) crystal structures with lattice constants of 6.479 and 6.48 , respectively,

which are closely matched the lattice constant of 6.489 of the diamond-cubic (DC) -Sn

phase. Tetragonal -Sn, however, has lattice parameters of a = b = 5.83 and c = 3.18 ,

meaning that -Sn could be preferentially and epitaxially stabilized with matched

templates. Furthermore, films grown with thicknesses of 500 nm remained in the -Sn

phase, even at elevated temperatures around 70 oC. These findings suggested that the

thermodynamic limitations of -