Tin Based Absorbers for Infrared Detection, Part 1

Presented By: Justin Markunas

IR Detection Introduction

Applications:•Military: night vision, IR target detection•Space: weather forecasting, astronomy•Industrial: quality control, failure analysis

Atmospheric absorption breaks IR spectrum into several bands:•SWIR: 1.4-3m•MWIR: 3-5 m•LWIR: 8-12 m•VLWIR: >12 m

Current Technology

Epitaxially grown Hg(1-x)CdxTe on lattice matched Cd(1-y)ZnyTe

•x-value adjusts bandgap from 0 eV (x=0) to 1.56 eV (x=1)

Advantages:•High detectivity•Able to sense the entire IR spectrum•Fast detectors due to large carrier mobilities.

Two color photovoltaic pixel arrays are currently being produced

•Capable of 40m pitch•Backside illumination is common (Cd(1-y)ZnyTe bandgap > 1.56eV)

Disadvantages:•High cost•Difficult to process•Require cooling to operate well (especially LWIR)

Competing Technologies

Microbolometers•Use materials with high thermal coefficient of resistance that are heated by incident radiation•No cooling requirements•Slow

Quantum Well Infrared Photodetector (QWIP) Arrays•III-V superlattices absorb IR with intraband processes•Fabricated by standard growth and processing•Absorption strength maximized at 45° angle

Others (past and present)•Hg1-xCdxSi/CdTe/Si•PtSi/Si Schottky barrier diodes•Extrinsic Si and Ge photoconductors•Lead Salts (PbSnTe)•Quantum dot infrared photodetectors

Basic Properties of Tin

Two allotropes of Tin:

White Tin (-Phase) Gray Tin (-Phase)•Tetragonal structure•Metallic form of tin

•Cubic Structure•Semimetallic with 0 eV direct bandgap•Extremely brittle

Phase Transition Occurs around 13°C•Occurs spontaneously over time

Melting Point ~ 232° C

Lattice Constant (-Phase): 6.49Å

Key Issues

•Gray tin has a 0eV bandgap

•13°C Phase Transition

Bandgap Adjustment

Quantum size effect•Confinement of electrons and holes changes the electronic structure•Thin film can be roughly defined as 1-D quantum square well:

22

2

L

n

m

Results from quantitative model•Peak Bandgap: .43eV•Absorption edge > 2.9m•Drop in peak due to increased role of surface structure on electronic properties

Growth of Metastable -Sn

Delaying the phase transition•Pseudomorphic epitaxial growth raises transition temperature

Key requirement for pseudomorphic growth•Epilayer must be thinner than some critical thickness•Critical thickness is inversely proportional to substrate/epilayer mismatch

-Sn Grown on CdTe by MBE

CdTe lattice constant: 6.482 Å (mismatch < .1%)

Growth Parameters adjusted for optimal stability:•Substrate orientation•Substrate temperature•Growth rate•Total film thickness

Determination of Stability:•Sample placed on hotplate under a microscope•Phase change is readily observable•Reproducible to ±1° C

-Sn Grown on CdTe by MBE

Results:•Substrate orientation: both (100) and (110) provided best results•Substrate temperature: increased temperature improved stability (100-150 °C is optimal) •Growth rate: slower rate improves stability (.1-.5 m/s)•Total film thickness: thicker films decreased stability (750-1000 Å can be achieved)•High substrate quality is critical•Highest temperature achieved before transformation: 107 °C

Key Issue:

•Stability is important, but IR absorption is critical•need ~2-12 m of Sn for sufficient absorption•requires Sn/CdTe superlattices to maintain quantum size effects

-Sn/CdTe Superlattices

-Sn/CdTe superlattices were grown and their properties were monitored by RHEED•Growth occurred at 100 °C

CdTe Buffer ~250Å

CdTe Substrate (110)

-Sn 50ÅCdTe 50Å-Sn 50ÅCdTe 50Å

-Sn 50ÅCdTe 50Å

Results:•Stable superlattices were grown for several periods•After 10 periods, quality degraded substantially•Partly due to nonideal CdTe growth conditions

Conclusions

•Thickness required for good absorption not achieved

•Quality of CdTe substrates appears to be a problem

•Similar experiments performed with InSb (a = 6.48 Å) showed comparable results

References

A. Rogalski, “Infrared Detectors: Status and Trends,” Progress in Quantum Electronics, vol. 27, pp. 59-210, 2003.

S. Groves and W. Paul, “Band Structure of Gray Tin,” Physical Review Letters, vol. 11(5), pp. 194-196, Sep. 1963.

F. Vnuk, A. DeMonte, and R.W. Smith, “The effect of pressure on the semiconductor-to-metal transition temperature in tin and in dilute Sn-Ge alloys,” J. Appl. Phys., vol. 55(12), pp. 4171-4176, Jun. 1984.

B.I. Craig and B.J. Garrison, “Theoretical examination of the quantum-size effect in thin grey-tin films,” Physical Review B, vol. 33(12), pp. 8130-8135, Jun. 1986.

R.F.C. Farrow, “The stabilization of metastable phases by epitaxy,” J. Vac. Sci. Technol. B, vol. 1(2), pp. 222-228, Apr.-Jun. 1983.

J.L. Reno, “Effect of growth conditions on the stability of -Sn grown on CdTe by molecular beam epitaxy,” Appl. Phys. Lett., vol. 54(22), pp. 2207-2209, May 1989.

H. Höchst, D.W. Niles, and I.H. Calderon, “Interface and growth studies of -Sn/CdTe(110) superlattices,” J. Vac. Sci. Technol. B, vol. 6(4), pp. 1219-1223, Jul.-Aug. 1988.