SYNTHESIS OF THIN FILMS

FILMS - FORM?

• Supported - substrate type - effect of interface – single crystal (oriented) or glassy

• Free standing - synthetic strategy - air/liquid or liquid/liquid interface or substrate lift-off

• Epitaxial - lattice matching and tolerance with underlying substrate

• Superlattice - artificial multilayer – periodic or aperiodic

• Patterned - chemical or physical lithography – bottom-up synthesis or top-down physical methods

FILMS - WHEN IS A FILM THICK OR THIN?

• Monolayer - atomic, molecular thickness

• Multilayer - compositional superlattice - scale - periodicity

• Bulk properties - scale - thickness greater than e,h)

• Quantum size effect - 2D spatial confinement – quantum confined along z, free electron behavior along x,y – called quantum wells – enable range of quantum devices

THIN FILMS VITAL IN MODERN TECHNOLOGY

• Protective coatings

• Optical coatings - antireflection coatings

• Electrochromic windows – photochromic spectacles

• Dielectric film – low k circuit packaging – high k transistor gate insulation

• Optical filters

• Microelectronic devices and circuits

• Optoelectronic devices and circuits

• Photonic devices and circuits

THIN FILMS VITAL IN MODERN TECHNOLOGY

• Electrode surfaces – solar cells, fuel cells, lithum solid state batteries

• Xerography, photography

• Electrophoretic, electrochromic, electrodewettability displays

• Catalytic and photocatalytic surfaces

• Nanoporous membranes – gas separation

• Information storage - magnetic, magneto-resistant, magneto-optical, optical, flash

FILM PROPERTIES - ELECTRICAL, OPTICAL, MAGNETIC, MECHANICAL, ADSORPTION,

PERMEABILTY, CHEMICAL

• Thickness and Surface : Volume ratio

• Surface vs bulk structure - surface reconstruction, dangling bonds – unsatisfied valencies, surface roughness

• Hydrophobicity - hydrophilicy - wettability

• Composition – surface vs bulk

• Texture - single crystal, microcrystalline, orientation, glassy

• Form - supported or unsupported (free-standing) - nature of substrate - patterned or un-patterned

METHODS OF SYNTHESIZING THIN FILMS

• ELECTROCHEMICAL, PHYSICAL, CHEMICAL

• Cathodic or anodic

• Electroless deposition

• Laser ablation

• Cathode sputtering, vacuum evaporation, e-gun

• Thermal oxidation, nitridation, sulfidation

METHODS OF SYNTHESIZING THIN FILMS

• ELECTROCHEMICAL, PHYSICAL, CHEMICAL

• Liquid/melt phase epitaxy

• Self-assembly - surface molecule anchoring, organization, close-packing, monolayers or multilayers

• Discharge (plasma) techniques - RF, microwave

• Chemical vapor deposition CVD, metal organic chemical vapor deposition MOCVD

• Molecular beam epitaxy, supersonic cluster beams, aerosol deposition

ANODIC OXIDATIVE DEPOSITION OF FILMS

• Deposition of metal oxide filmsmetal oxide films, such as alumina, titania by oxidation of metal electrode in aqueous salts or acids

• Deposition of conducting polymer filmsconducting polymer films by oxidative polymerization of monomer, such as thiophene, pyrrole, aniline, acetylene

ANODIC OXIDATION OF Al IN OXALIC OR PHOSPHORIC ACID TO FORM ALUMINUM OXIDE

• Pt|H3PO4, H2O|Al ECCell

• Al Al3+ + 3e- Anode

• PO43- +2e- PO3

3- + O2- Cathode

• 2Al3+ + 3O2- -Al2O3 (annealing) -Al2O3

• Voltage control of oxide thickness

• Al3+/O2- diffuse through growing layer of Al2O3

ANODIC OXIDATION OF PATTERNED Al DISC TO MAKE PERIODIC NANOPOROUS Al2O3 MEMBRANE

2Al + 3PO43- Al2O3 + 3PO3

3-

2Al + 3C2O42- Al2O3 + 6CO + 3O2-

How to remove residual Al and Al2O3 barrier layer???

SiC patterned master harder than Al to make nanoimprint replica

ANODIC OXIDATION OF PATTERNED Al DISC TO MAKE PERIODIC NANOPOROUS Al2O3 MEMBRANE

Aqueous HgCl2 dissolves Al to give Hg and Al(H2O)63+

and H3PO4 dissolves Al2O3 barrier layer to give Al(H2O)6

3+ - yields open channel membrane

ANODIC OXIDATION OF LITHOGRAPHIC PATTERNED Al TO PERIODIC NANOPOROUS Al2O3

Not bad for chemistry!!!

Hexagonal close packed nanochannel membraneHexagonal close packed nanochannel membrane

40V

60V

80V

ANODIC OXIDATION OF LITHOGRAPHIC PATTERNED Al TO PERIODIC NANOPOROUS Al2O3

Voltage control of channel diameter

50-500 nm accessible

PROPOSED MECHANISM OF ALUMINA PORE FORMATION IN ANODICALLY OXIDIZED ALUMINUM

SELF ORGANIZED SELF LIMITING GROWTH OF PORES

Electric and strain fields guide and organize hcp channel growth

Templated synthesis of metal barcoded nanorods

Collection of multi-metal nanorods imaged in an optical microscope by the different reflectivity’s of different metal segments, Science 2001, 294, 137

• Optical (A) and field emission scanning electron microscopy FE-SEM (B) images of an Au-Ag multi-stripe nanorods

• 550-nm Au stripes and Ag stripes of 240, 170, 110, and 60 nm -top to bottom

60 nm

240 nm

170 nm

110 nm

550 nm

Orthogonal assembly on nanorods. Butyl isonitrile is bound non-selectively to Pt and Au surfaces. Aminoethanethiol displaces isonitriles selectively on gold but not on platinum. Rhodamine isocyanate is reacted with terminal amino groups to fluorescently label gold segments.

-NH-CS-NH-

thiourea linkage of rhodamine fluorescent

dye to Au segment

DNA sandwich hybridization assay on metal barcode nanorods

- Science 2001, 294, 137

SYNTHESIS OF CHEMICALLY POWERED NANOROD MOTORS

?Ozin et al Chem Comm, AdvMater 2006, Mallouk et al JACS 2005

cork-screw.mov

ANODIC OXIDATION OF Si TO FORM POROUS Si: THROWING SOME LIGHT ON SILICON

• Typical electrochemical cell to prepare PS by anodic oxidation of heavily doped p+-type Si

• PS comprised of interconnected nc-Si with H/O/F surface passivation

• nc-Si right size for QSEs and red light emission observed during anodic oxidation – electroluminescence

ELECTRONIC BAND STRUCTURE OF DIAMOND SILICON LATTICE

• band structure of Si computed using density functional theory with local density and pseudo-potential approximation

• diamond lattice, sp3 bonded Si sites

• VB maximum at k = 0, the point in the Brillouin zone, CB minimum at distinct k value

• indirect band gap character, very weakly emissive behavior

• absorption-emission phonon assisted

• photon-electron-phonon three particle collision very low probability, thus band gap emission efficiency low, 10-5%

SEMICONDUCTOR BAND STRUCTURE: CHALLENGE, EVOKING LIGHT EMISSION FROM Si

• Effective Mass Approximation Rexciton ~ 0.529/mo where = dielectric constant, reduced mass of exciton mo = memh/(me + mh)

• Note exciton size within the bulk material defines the size regime below which significant QSEs on band structure are expected to occur, clearly < 5 nm to make Si work

REGULAR OR RANDOM NANNSCALE CHANNELS IN ANODICALLY OXIDIZED SILICON WAFERS

• Anodized forms of p+-type Si wafer

• Showing formation of random (left) and regular (right) patterns of pores

• Lithographic pre-texturing directs periodic pore formation

PORE FORMING PROCESS IN ANODICALLY OXIDIZED SILICON WAFERS

• Basics of electrochemical cell - p+-Si wafer anode in contact with aqueous HF electrolyte – simplified electrochemistry:

• Si Si(4+) + 4e

• Si(4+) + 6F(-) [SiF6]2-

• 4H(+) + 4e 2H2

• Mechanism of natural self-limiting process for regular pore formation based on wider band gap of PS compared to bulk Si and respective redox potentials for anodic oxidation

KEY ISSUES: ORIGIN OF PHOTO- AND ELECTROLUMINESCENE OF POROUS SILICON

• Origin of luminescence key point- as bulk Si is indirect band gap semiconductor with very weak light emission

• Models for light emission include quantum-spatial confinement, siloxenes, and SiOH

• Luminescent nc-Si structure requires SiO, SiH surface bonds - caps dangling bonds -removes killer traps in band gap

• Size dependence of k, selection rules, scaling laws determine light emission properties

• Mechanical, photochemical, chemical stability are key factors for devices – safety too - care with humidity control and toxic silane evolved Si[H]surface + H2O SiH4

• Efficient e-h charge-injection required for practical LED

MAKING NANOCRYSTALLINE SILICON LUMINESCENT: CAPPING

CB

VB VB

CB

Sin HxSin

(SiH)

*(SiH)

capping Si cluster dangling bond with H, F, O forms bonding-antibonding SiH -orbitals, moves killer trap states out of the gap facilitates radiative over non-radiative relaxation

LIGHT WORK BY THE SILICON SAMURAIWHERE IT ALL BEGAN AND WHERE IT IS ALL GOING???

FROM CANHAM’S 1990 DISCOVERY OF PL AND EL ANODICALLY OXIDIZED p+-DOPED Si WAFERS, TO NEW LIGHT EMITTING SILICON NANOSTRUCTURES, AND DREAM OF SILICON OPTOELECTRONICS AND PHOTONIC COMPUTING – ACTUALLY BIOSENSORS EVENTUALLY

CHEMICAL VAPOUR DEPOSITION

• Pyrolysis, photolysis, chemical reaction, discharges - RF, microwave facilitated deposition processes

• Epitaxial films, correct matching to substrate lattice

• CH4 + H2 (RadioF, MicroW) C, diamond films (perfect non-stick frying pan – inert, hard, transparent, non-stick, high thermal conductivity)

• Et4Si (thermal, air) SiO2

• SiCl4 or SiH4 (thermal T, H2) a-H:Si or nc-H:Si

• SiH4 + PH3 (RF) n-Si (ppm P)

CHEMICAL VAPOUR DEPOSITION

• Si2H6 + B2H6 (RF) p-Si (ppm B)

• Single source precursor SiH3SiH2SiH2PH2 (RF) n-Si

• Me3Ga (laser photolysis, heating) Ga

• Me3Ga + AsH3 + H2 (T,P) GaAs + CH4

• Si (laser evaporation, molecular beam, high to low P supersonic jet, ionization) Sin

+ (size selected MS - cluster deposition) Si

H HH

H H

H

H

H HH H

H

H

H

HH

H

H

H

H

H

HH

H

H

H

H

HH

HHH

H

H

H

Amorphous hydrogenated silicon a-H:Si, easy to form thin film by CVD

Hydrogen capping of dangling surface sp3 bonds

Reduces surface electron killer traps

Enhances electrical conductivity compared to a-Si but less than bulk c-Si

Poly-domain texture

Useful for pn and pin junction large area solar cell devices

REMOVING DANGLING BONDS BY Si-H CAPPING

CB

VB VB

CB

Sin HxSin

(SiH)

*(SiH)

capping Si cluster dangling bond with H, F, O forms bonding-antibonding SiH -orbitals, moves killer trap states out of the gap facilitates charge transport and radiative relaxation

METAL ORGANIC CHEMICAL VAPOR DEPOSITION

MOCVDMOCVD

• Invented by Mansevit in 1968

• Recognized high volatility and chemical reactivity of metal organic compounds as sources for semiconductor thin film preparations

• Enabling chemistry: electronic, optical quantum devices

• Quantum wells and superlattices

• Occurs for 5-500 Angstrom layers

• Known as artificial superlattices

En = n22h2/2m*L2

Schematic energy band diagram of a quantum well structure showing confined electron and hole states produced by large Eg GaAlAs layers sandwiching small Eg GaAs depicting quantum size effects and some possible optical transitions

L

CB edge GaAlAs

VB edge GaAlAs

CB/VB edges GaAs

METAL ORGANIC CHEMICAL VAPOR DEPOSITION, MOCVD

• Quantum confined electrons and holes when thickness of quantum well L is comparable to the wavelength of an electron or hole at the Fermi level of the material, band diagram shows confined particle states and quantization effects for electrical and optical properties

• Discrete electronic energy states rather than continuous bands, given by solution to the simple particle in a box equation, assuming infinite barriers for the wells, m* is the effective mass of electrons and holes

• En = n22h2/2m*L2

• Tunable thickness, tailored composition materials, do it yourself quantum mechanics materials for the semiconductor industry

METAL ORGANIC CHEMICAL VAPOR DEPOSITION, MOCVD

• Quantum well structure synthesized by depositing a controlled thickness superlattice of a narrow band gap GaAs layer sandwiched by two wide band gap GaxAl1-xAs layers using MOCVD

• Ga(Al)Me3 + AsH3 (H2, T) Ga(Al)As + CH4

• Artificial superlattices, designer periodicity of layers, quantum confined lattices, thin layers, epitaxially grown, x determines electronic band gap

• Example: GaxAl1-xAs|GaAs|GaxAl1-xAs

MOCVD

• Example: GaxAl1-xAs|GaAs|GaxAl1-xAs

• n- and p-doping achievable by having excess As or Ga respectively in a GaAs layer

• Composition and carrier concentration controls refractive index (low RI cladding, TIR optical confinement) and electrical conductivity (p-n and p-n-p junction devices), in a semiconducting superlattice

• Enables electron (quantum) and photon (RI) confinement for electronic, optoelectronic and optical devices

• Multiple quantum well laser, quantum cascade laser, distributed feedback laser, resonant tunneling transistor, high electron mobility ballistic transistor (HEMT), laser diode

Resonant tunneling transistor

BAND GAP ENGINEERING OF SEMICONDUCTORS

• MOCVD, LPE, CVD, CVT, MBE all deposition techniques that provide angstrom precise control of film thickness

• Together with composition control one has a beautiful synthetic method for fine tuning the electronic band gap and hence most of the important properties of a semiconductor quantized film

BAND GAP ENGINEERING OF SEMICONDUCTORS

• Key is to achieve epitaxial lattice matching of film with underlying substrate

• Avoids interfacial lattice strain, elastic deformation, misfit dislocations, defects - all of these problems serve to increase carrier scattering, decrease charge-transport, increased quenching of e-h recombination luminescence (killer traps), thereby reducing the efficacy of the material for advanced device applications

MOCVD SINGLE SOURCE PRECURSORS

• Me3Ga, Me3Al, Et3In (synthesis GaCl3 + MeLi/R2Mg/RMgI)

• NH3, PH3, AsH3 (synthesis Mg3As2/HCl)

• H2S, H2Se

• Me2Te, Me2Hg, Me2Zn, Me4Pb, Et2Cd

• E.g. synthesize an IR detector based on p-n photodiode

• Me2Cd + Me2Hg + Me2Te (H2, 500oC) HgxCd1-xTe

• p-HgxCd1-xTe/n-HgxCd1-xTe• p- and n-doping requires precise control of Hg/Cd and Te stoichiometry• x determines the electronic bandgap – tuned to IR wavelength range for detector

• Toxic materials – safe handling and disposal of toxic waste!!!

Schematic of cold wall MOCVD system

Water cooling

Thermocouple

Waste gases

Single crystal substrate on inductively heated or resistively heated susceptor – mass flow control of precursors

H2/AsH3/PH3

H2/InMe3/GaMe3

H2/PEt3

H2/n-dope H2S/p-dope ZnMe2

MOCVD deposited film

As Al As Al As Al As Al As Al As Al As Al As Al As Al As

GaMe

Me

MeGaMe

MeMe

AlMe

MeMe

MOCVD surface chemistry of precursors, nucleation and growth of product film on substrate

Precursor adsorption on single crystal oriented substrate - lattice matching epitaxy criteria - surface physisorption - chemisorption - surface diffusion - dissociative chemisorption - reaction - desorption

Different models for film nucleation and growth - depends whether surface diffusion involved - fixed vs mobile crystal nuclei

CH4

HH

AsH

H

MOCVD SINGLE SOURCE PRECURSORS

• Specially designed MOCVD reactors, hot and cold wall designs, controlled flow of precursors using digital mass flow meters directing precursors to heated single crystal substrate, induction or resistive heater, silicon carbide coated graphite susceptor for mounting substrate

• This chemistry creates problems for semiconductor manufacturers wrt safe handling and disposal of toxic waste

• Most reactions occur in range 400-1300oC, complications of diffusion at interfaces, disruption of atomically flat epitaxial surfaces/interfaces may occur during deposition

• Photolytic processes (photoepitaxy) help to bring the deposition temperatures to more reasonable temperatures

PHOTOEPITAXY

Making atomically perfect thin films under milder and more controlled conditions

Et2Te + Hg + H2 (h, 200oC) HgTe + 2C2H6

Exhaust gases H2/Et2Te

UV illumination

H2 gas windowBottom graphite, middle substrate, top HgTe film

Hg pool

PHOTOEPITAXYMaking atomically perfect thin films under milder and more controlled conditions

• Mullin and Tunnicliffe 1984

• Et2Te + Hg (pool) + H2 (hh, 200, 200ooCC) HgTe + 2C2H6

• Et2Te/Me2Cd + Hg (pool) + H2 (hh, 200, 200ooCC) HgxCd1-xTe + 2C2H6

• MOCVD preparation requires 500oC using Me2Te + Me2Hg/Me2Cd

• Advantages of photo-epitaxy

• Lower temperature operation, multi-layer formation, less damage of layers - ternaries HgxCd1-xTe, n- and p-doping, Te and Hg/Cd rich, p-n diodes, IR photodetectors, multi-layers, quantum size effect devices HgxCd1-xTe|HgTe|HgxCd1-xTe

• Lower interlayer diffusion, easy to fabricate multilayers

• Abrupt boundaries, less defects, strain and irregularities at interfaces

• Note that H2 gas window in apparatus prevents deposition of HgTe on observation port

• In this way CdTe can be deposited onto GaAs at 200-250oC even with a 14% lattice mismatch

• Key consideration - GaAs is susceptible to damage under MOCVD conditions 650-750oC

PHOTOEPITAXY

Making atomically perfect thin films under milder and more controlled conditions

REQUIREMENTS OF SUCCESSFUL MOCVD PRECURSOR

• RT stable

• No polymerization, decomposition

• Easy handling

• Simple storage

• Not too reactive

• Vaporization without decomposition

REQUIREMENTS OF MOCVD PRECURSORS

• Vaporization without decomposition

• Modest < 100oC temperatures

• Low rate of homogeneous pyrolysis, gas phase, wrt heterogeneous, surface, decomposition

• HOMO : HETERO rates ~ 1 : 1000

• Heterogeneous reaction preferred on substrate

• Greater than on other hot surfaces in reactor

REQUIREMENTS OF MOCVD PRECURSORS

• Not on supports or reaction chamber/vessel

• Ready chemisorption of precursor on substrate

• Detailed surface and gas phase studies of structure of adsorbed species, reactive intermediates, kinetics, vital for quantifying film nucleation and growth processes

• Electrical, magnetic, optical films made in this way

• Semiconductors, metals, silicides, nitrides, oxides, mixed oxides (e.g., high Tc superconductors), sulfides, selenides

CRITICAL PARAMETERS IN MATERIALS PREPARATION FOR SYNTHESIS OF THIN FILMS

• Composition control - precise command over stoichiometry and adventitious carbonaceous deposits

• Variety of materials to be deposited

• Good film uniformity

• Large areas to be covered, > 100 cm2

• Precise reproducibility

CRITICAL PARAMETERS IN MATERIALS PREPARATION FOR SYNTHESIS OF THIN FILMS

• Growth rate, thickness control

• 2-2000 nm layer thickness

• Precise control of film thickness

• Accurate control of deposition, film growth rate

CRITICAL PARAMETERS IN MATERIALS PREPARATION FOR SYNTHESIS OF THIN FILMS

• Crystal quality, epitaxy

• High degree of film perfection

• Defects degrade device performance

• Reduces useable wafer yields

CRITICAL PARAMETERS IN MATERIALS PREPARATION FOR SYNTHESIS OF THIN FILMS

• Purity of precursors

• Usually less than 10-9 impurity levels

• Stringent demands on starting material purity

• Chemistry challenge, purifying, analyzing precursors at ppb level

• Demands exceptionally clean growth system otherwise defeats the object of controlled doping of films for device applications

CRITICAL PARAMETERS IN MATERIALS PREPARATION FOR SYNTHESIS OF THIN FILMS

• Interface widths

• Abrupt changes of composition, dopant concentration required, vital for quantum confined structures

• 30-40 sequential layers often needed

• Alternating composition and graded composition films

• 0.5-50 nm thickness required with atomic level precision

• All of the above has been more-or-less perfected in the electronics and optics industries – amazing achievement!!!

III-V BAND GAP ENGINEERING

• Designer semiconductors

• Single crystal substrate

• Single crystal layers

• Zinc blende lattice

• Lattice constant

• Composition

• Doping

• Thickness

• Multilayers

• Epitaxial lattice matching

• Control of Eg band gap and RI refractive index

• Operating wavelengths for optical telecommunication systems labeled in purple

1500nm

TECHNIQUES USED TO GROW SEMICONDUCTOR FILMS AND MULTILAYERED FILMS

• MOCVD• Liquid phase epitaxy

• Chemical vapor transport

• Molecular beam epitaxy

• Laser ablation

• Used for band gap engineering of semiconductor materials that function at 1.5 microns in near IR - integrating with glass fiber optics and waveguides

6InP/3GaAs/6InP EPITAXIALLY MATCHED SUPERLATTICE

TAILORED BAND GAPS - DESIGNER MOCVD GRADED COMPOSITIONGRADED COMPOSITION POTENTIAL WELLS

AlxGa1-xAs graded composition-gap superlattice

CB AlAs wide gap

CB GaAs narrow gap

VB GaAs narrow gap

VB AlAs wide gap

Designer quantum well architecture - band gap engineering - graded composition enables gradient potential – speeds mobility of electrons injected into channel - used to enhance performance in high electron mobility transistors HEMTs or build a quantum cascade laser

e

h

Tunable h

Federico Capasso co-inventor of the quantum cascade laser imagined small things when he used size and dimensionality of materials to

tailor their properties for electronic and optical devices

QUANTUM CASCADE LASER - A NICE EXAMPLE OF BAND GAP ENGINEERING BY MOCVD

White bands in the TEM are QWs made of narrow band gap GaInAs, which are sandwiched between barrier layers of wide band gap AlInAs ranging in thickness from atomic to 12 atomic layers

All wells are part of a QCLAll wells are part of a QCL

Voltage applied to device, electrons move down potential barrier from wide to narrow Eg QWs (tunnel from QW to QW) and emit a photon between two thickest QWs.

Electrons move on (tunnel) to the next stage to the right where the process repeats – hence cascade laser.

hh

h