Cu(In,Ga)(S,Se) 2 Crystal Growth, Structure, and Properties Angus - - PowerPoint PPT Presentation

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Cu(In,Ga)(S,Se) 2 Crystal Growth, Structure, and Properties Angus - - PowerPoint PPT Presentation

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Cu(In,Ga)(S,Se) 2 Crystal Growth, Structure, and Properties Angus Rockett Department of Materials Science The University of Illinois With support from: 1101 W. Springfield Avenue, Urbana, IL 61801, USA 217-333-0417 arockett@uiuc.edu The

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College of Engineering University of Illinois

Cu(In,Ga)(S,Se)2 Crystal Growth, Structure, and Properties

Angus Rockett

Department of Materials Science The University of Illinois

1101 W. Springfield Avenue, Urbana, IL 61801, USA 217-333-0417 arockett@uiuc.edu

With support from:

The National Renewable Energy Laboratory
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College of Engineering University of Illinois

CIGS

  • Chalcopyrites have been proposed as

spin-polarized electron emitters.

  • They are also interesting for other

applications including thin film transistors.

  • Primary application -- solar cells
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Useful Properties

  • Engineerable energy gap in useful range

(< 1eV to >2 eV).

  • Very high optical absorption coefficient.
  • Usable as polycrystals.
  • Native defects “harmless” (all shallow).
  • Few observable problems with impurities.
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College of Engineering University of Illinois

Basic crystallography and thermodynamics

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Disordering energy is low so there are many point defects A polar compound so charged surfaces could be a problem c a Cu In or Ga S or Se c/2a ratio varies from >1 (high In) to <1 (high Ga)

Chalcopyrite Cu(In,Ga)(S,Se)2

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College of Engineering University of Illinois

Chalcopyrite Cu(In,Ga)(S,Se)2

Polar Metal Metal Chalcogen Chalcogen Polar Non-polar

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Ternary Phase Diagram

…and right away we know we are in trouble.

Cu2Se In2Se3 CuInSe3

Pseudobinary

…is valence compensating

What is observed:

γ: CuIn5Se8 β: ~CuIn3Se5

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College of Engineering University of Illinois

Pseudobinary Phase Diagram

Note: extended solubility in α phase. Low-temperature chalcopyrite- sphalerite transition suggests low cation

  • rdering energy.
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Deposition Methods

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Evaporation

  • Multisource Evaporation
  • High rate

High rate

  • Easy control

Easy control

  • Difficult to scale

Difficult to scale

  • High temperature

High temperature process process

Most epitaxy by MBE Most solar cells this way

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College of Engineering University of Illinois

+Se

Selenization

  • Deposit metals separately
  • React with a Se source (Se vapor or H2Se).
  • Similar processes with sulfides
  • Sequential, easy to control
  • High film stress
  • Reaction of metal layers & phases formed are critical,

processes complex.

Heating lamps

Metal sources

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College of Engineering University of Illinois

MOCVD

Scrubber Pumps Valve Load Lock Reactor Buffer Volume Computer Control Flow Controllers & Vents Source Gases Safety Enclosure Toxic Gas Alarm System

Hydrogen selenide Trimethyl Ga or trimethyl In Complex Cu compound: use bubbler source. Low rate. Best growth: Cu-rich films 500-550°C

Epitaxy by MOCVD at Hahn Meitner Institute

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Growth rate ~ 1µm/hr. Substrate temperature: 550-725°C Cu-Ga target for Cu(In,Ga)Se2 Cu target for pure CuInSe2 Cu and Ga targets for CuGaSe2

Typical hole mobility ~280 cm2/V-sec

  • Typ. Carrier Lifetime:

~ 0.4 nsec

Hybrid Method

Growth Process:

  • Similar to MBE
  • Evaporate Se
  • Sputter metals
  • Optional rf coil for

ionization Epitaxy on GaAs:

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Band Structure

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Band Structure - Theory

Se-S alloys mostly affect valence band edge. In-Ga alloys mostly affect conduction band. Cu-Ag alloys have minor effects on both bands. Gaps are low because

  • f Se-p : Cu-d

repulsion.

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Experimental UPS

  • Se capped surface (112)Se cleaned

thermally shows Ef - EV ~ 0.47 eV.

  • Agrees

well with data from

  • ther labs

Cu 3d

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Optoelectronic Properties

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Photoluminescence Data

Data: S. Siebentritt et.al. Hahn Meitner Institute

Cu-rich films show sharp emissions: Donor-acceptor emissions Weak band-band emission Group-III rich films show broad emissions. Polycrystals same as epilayers

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Photoluminescence Lifetime

10 100 1000 10000 100000 0.E+00 1.E-09 2.E-09 3.E-09 4.E-09 5.E-09 6.E-09 NREL SSI GSE EPV ISET UI261B UI261A UI291 UI286

Lifetimes 0.2-4 nsec Consistent with direct- gap band structure Microsecond decay (not shown shows deep state) Epilayers have short lifetimes. Suggests minority carrier traps.

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Defect-to-defect, not band-to-band recombination. Photon energy does not track the gap change with Ga. Similar to CIS PL data.

1 10 100 1000 104 900 1000 1100 1200 1300 1400 1500

Emission Intensity (Arb. Units) Wavelength (Å)

Red: 283 K Yellow: 238 K Blue: 171 K Purple: 110 K Increasing Current At least 4 fixed peaks

Electroluminescence

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Defect 1 Defect 2 Defect 1 Defect 2 Energy above VBE 0.8 0.9 0.8 1 E(gap) 1.22 Width 0.13 0.15 0.13 0.15 Band Tail width 0.022 Concentration 1.00E+14 3.50E+14 1.00E+14 1.10E+15 Band Tail DOS 1.00E+18 IEC Device UIUC Epilayer Device Both Layers

Experimental Data for deep levels

Thermophotocapacitance Cathodoluminescence

1E+12 1E+13 1E+14 1E+15 1E+16 1E+17 1E+18 0.5 0.8 1.1 1.4 Energy Above Valence Band (eV) Fit based on data from Temperature Photocapacitance

(J. Heath and D. Cohen,

  • U. Orgegon)

Quantitative estimate of defect state densities. Subgap states produce clear CL emission spectra

Data from Y. Strzhemechny and L. Brillson, Ohio State U.

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1012 1013 1014 1015 1016 1017 1018 1019 1020 10 20 30 40 50

Hole Concentration (cm

  • 3)

1000/T [K

  • 1]

300 100 50 30 20

Temperature, T [K]

CIGS x=.20 CIGS x=.56 CIGS x=.11 CGS, x=1.00

Carrier Concentration: Hole Mobility:

100 1000 10 100

Temperature, T [K] Hole Mobility (cm 2/Vs)

Hall Effect

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No change in electrically-active defects with Cu/III composition.

1012 1013 1014 1015 1016 1017 1018 1019 0.9 0.95 1 1.1 1.1 1.1 1.2

State Concentration (cm

  • 3)

Cu/In Ratio

0.7 0.75 0.8 0.85 0.9 0.95 1

Cu/(In+Ga) Ratio

NA1 NA2 ND 1012 1013 1014 1015 1016 1017 1018 1019

Hall Effect -- vs. composition

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1014 1015 1016 1017 1018 1019 1020 1021 1022 0.2 0.4 0.6 0.8 1

Acceptor State Density [cm -3] Ga/(In+Ga)

…except for Ga, which increases p at high Ga contents.

Open points: Ga gradient Closed points: uniform Ga

Hall-effect -- vs. composition

Deep acceptor (135 meV) Shallow acceptor (40 meV)

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  • (112) and (002) very similar, phonon scattering for (112) and (002)
  • p (220) 20x lower, mobility lower, defect limited

10 100 1000 100 150 200 250 300

Temperature, T (K) Hall Mobility (cm2/V-sec)

Film Orientation (112) (002) (220)

3 3.5 4 4.5 5 5.5 6 6.5 7

Inverse Temperature, 1000/T [K-1] Hole Concentration, p (cm -3)

1017 1016 1015 1014 1013

Growth Orientation Effects

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Low dose Se implant -- mostly reversible changes.

70 80 100 200 0.1 1 10 100 1000

Mobility, µ (cm 2/Vs) Temperature T (K)

300

as deposited as implanted annealed

1011 1013 1015 1017 1019 2 4 6 8 10 12

Hole Concentration (cm

  • 3)

1000/T (K )

Implant damage Implant damage

Implant Changes

Cr implant similar

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Point Defects Summary

Conduction Band Valence Band 0.9 eV 0.7 eV 0.3 0.13 0.04 Ef CuIn

  • 2

CuIn

  • 1

VCu

  • 1

InCu

+1 or Vse +1 ?

InCu

+2 or Vse +2 ?

  • Shallow acceptor: Cu vacancy
  • Deep acceptor: CuIn (divalent)
  • Defects 0.8 and 1.0 eV above

valence band: probably Se vacancy

  • Above do not shift with

Ga/(In+Ga)

  • Additional deep states?
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Surface Energy & Growth Mechanisms

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Surface Morphology

Summary:

Best epitaxy on GaAs (111). Rough surfaces on GaAs (110). Epitaxial temperatures: Ts = 540 ° C (220); 640 ° C (100); and 700 ° C (112) Pure CuInSe2: Ga diffuses from the substrate Kirkendall voids form at the interface in the GaAs substrate.

(100)/(002) surface Elongated ripples with asymmetric rectangular pits sometimes present. (220)/(204) surface Facets to (112) planes,

  • ne smooth, one rough.

(112) close-packed Very low angle (<1°) pyramidal facets.

Contrast enhanced

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(112)Se Close Packed Surface

Very triangular facets, strong step

  • rientation

5 5 µ µm x 5 m x 5 µ µm AFM image m AFM image Steps: 1-3 ML high

  • Growth by step nucleation and

propagation

  • Very flat
  • 180° growth twins ~40 µm apart

Electron Channeling Patterns

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(112)Metal Close Packed Surface

Steps: 1-2 ML high

  • Growth by step nucleation and

propagation

  • Very flat
  • No growth twins over the entire

surface! Very triangular facets, strong step

  • rientation

5 5 µ µm x 5 m x 5 µ µm AFM image m AFM image

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(112) Surface Steps Compared

  • Both surfaces show triangular pyramids -- very similar

Metal-terminated Se-terminated

One type of step dominates More preference on Se-terminated surface

Inside corners! Layers nucleate near terrace edges!

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Implications for Steps

Island Perimeter 3 edges with one dangling bond per edge atom 3 edges with two dangling bonds per edge atom Same behavior for both metal and Se surfaces One step type dominates

1µm

Almost no single dangling bond steps observed! Step-edge reconstruction implied

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Layer Nucleation

Nucleation at step edges implies a barrier to atoms crossing downward over steps or binding to the upper step edge. Nuclei

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(220)/(204) Oriented CIGS

Layers facet spontaneously into polar (112) type planes. Smooth facets alternate with rough facets.

(220)/(204) epitaxial layer AFM image

Red: metal terminated Blue: Se terminated (112)A (112)B Indexing surface planes shows smooth planes are metal terminated

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(220)/(204) Oriented CIGS

  • Implication of the smooth/rough

surfaces:

(110) epitaxial layer AFM image

Diffusion transfers atoms to Se-terminated facets Nucleation and growth on Se- terminated surface.

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Composition Variations

  • EDS:
  • Probe size

~ 1nm

  • Noisy data
  • No obvious grain boundary change
  • Data generally follows the tie line
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CIS/GaAs Epitaxy

Kirkendal Voids w (112)Se facets GaAs CuInSe2

Angular Dark Field Image

Stacking faults & twins at interface

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Voids

  • From surface data

we know faceting is common.

  • Voids between

grains in polycrystals.

  • Voids within grains

at twin terminations and dislocations.

Nanovoids!

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Surface Chemistry

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CIGS Surface Chemistry

  • Use very smooth (112) surfaces to

study chemistry by angle-resolved photoelectron spectroscopy.

Angle θ controls probe depth θ

Photon in Photoelectron

  • ut

d d/cos(θ)

θ

X-ray/UV light source Photoelectron energy analyzer sample

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Angle-resolved Measurements

  • No chemical shift

between (112)metal and (112)Se.

0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1 442 444 446 448 450

Binding Energy (eV)

  • No chemical shifts with

photoelectron take-off angle Oxidized surface is (In,Ga) oxide

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Angle-resolved Measurements

  • Results show Cu-deficient Ga-rich

surface

Modelling suggests 1-2 monolayers with altered Cu-poor composition

Cu In Ga Se O 28 6 66 37 63 18 16 6 60 24 16 10 50 24 16 10 50

Best fit by monolayer, at.%

Air-exposed

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The CdS Heterojuction

  • The best CIGS solar cells have a heterojunction

made by dip-coating CdS onto the CIGS.

  • Plan: Use AR-PES to study the effect of the dip-

coating bath on the surface. CdS Dip HCl Etch

Control: HCl Etch w/o dip CdS-coated sample etched samples

CIGS

θ

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CdS Dip Coating Results

  • Results show ~0.7 cation monolayers of Cd atoms in

the first one (or two) atomic layers of the surface after etching.

  • Fermi-edge shifts but core levels similar to sputtered

& Se-capped samples.

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CdS Dip Coating Results

  • Comparison of

valence bands

  • Metal-terminated

has ~0.2 eV higher binding energy

  • Dip & etch samples

have component with ~1eV edge. He I

The oxidized surfaces have a minority component (1%) consistent with the sputtered edge

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Conclusions

  • Copper chalcogenides are fascinating and unique

materials.

  • Defects dominate the optoelectronic properties.
  • Growth mechanisms are unique and interesting.
  • Thanks again to everyone who helped with this

research, funded it, discussed results with me, and to you for listening!