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

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 National Renewable Energy Laboratory 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 College of Engineering University of Illinois

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. College of Engineering University of Illinois

Basic crystallography and thermodynamics College of Engineering University of Illinois

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

Chalcopyrite Cu(In,Ga)(S,Se) 2 Metal Chalcogen Polar Non-polar Polar Metal Chalcogen College of Engineering University of Illinois

Ternary Phase Diagram …and right away we know we are in trouble. What is observed: β : ~CuIn 3 Se 5 γ : CuIn 5 Se 8 In 2 Se 3 Cu 2 Se CuInSe 3 Pseudobinary …is valence compensating College of Engineering University of Illinois

Pseudobinary Phase Diagram Note: extended solubility in α phase. Low-temperature chalcopyrite- sphalerite transition suggests low cation ordering energy. College of Engineering University of Illinois

Deposition Methods College of Engineering University of Illinois

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 College of Engineering University of Illinois

Selenization • Deposit metals separately • React with a Se source (Se vapor or H 2 Se). • 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 +Se College of Engineering University of Illinois

MOCVD Epitaxy by MOCVD Hydrogen selenide at Hahn Meitner Institute Trimethyl Ga or trimethyl In Complex Cu compound: use bubbler source. Low rate. Flow Controllers & Vents Computer Control Source Gases Buffer Volume Best growth: Cu-rich films Load Lock 500-550°C Reactor Valve Pumps Scrubber Safety Enclosure Toxic Gas Alarm System College of Engineering University of Illinois

Hybrid Method Growth Process: • Similar to MBE • Evaporate Se • Sputter metals • Optional rf coil for ionization Growth rate ~ 1 µ m/hr. Substrate temperature: 550-725°C Epitaxy on GaAs: Cu-Ga target for Cu(In,Ga)Se 2 Typical hole mobility Cu target for pure CuInSe 2 ~280 cm 2 /V-sec Cu and Ga targets for CuGaSe 2 Typ. Carrier Lifetime: ~ 0.4 nsec College of Engineering University of Illinois

Band Structure College of Engineering University of Illinois

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 of Se-p : Cu-d repulsion. College of Engineering University of Illinois

Experimental UPS • Se capped surface (112) Se cleaned thermally shows E f - E V ~ 0.47 eV. • Agrees well with data from other labs Cu 3d College of Engineering University of Illinois

Optoelectronic Properties College of Engineering University of Illinois

Photoluminescence Data Cu-rich films show sharp emissions: Donor-acceptor emissions Weak band-band emission Group-III rich films show broad emissions. Polycrystals same as epilayers Data: S. Siebentritt et.al. Hahn Meitner Institute College of Engineering University of Illinois

Photoluminescence Lifetime NREL Lifetimes 0.2-4 nsec 100000 SSI GSE EPV Consistent with direct- 10000 ISET UI261B gap band structure UI261A 1000 UI291 UI286 Microsecond decay 100 (not shown shows deep state) 10 0.E+00 1.E-09 2.E-09 3.E-09 4.E-09 5.E-09 6.E-09 Epilayers have short lifetimes. Suggests minority carrier traps. College of Engineering University of Illinois

Electroluminescence Defect-to-defect, not band-to-band At least 4 fixed peaks 10 4 recombination. Emission Intensity (Arb. Units) Red: 283 K Yellow: 238 K Photon energy Blue: 171 K 1000 Purple: 110 K does not track the Increasing gap change with 100 Current Ga. 10 Similar to CIS PL data. 1 900 1000 1100 1200 1300 1400 1500 Wavelength (Å) College of Engineering University of Illinois

Experimental Data for deep levels Thermophotocapacitance Cathodoluminescence IEC Device UIUC Epilayer Device Defect 1 Defect 2 Defect 1 Defect 2 Both Layers 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 1E+18 Fit based on data from Temperature 1E+17 Data from Y. Photocapacitance Strzhemechny and L. 1E+16 (J. Heath and D. Cohen, Brillson, Ohio State U. U. Orgegon) 1E+15 1E+14 1E+13 1E+12 0.5 0.8 1.1 1.4 Energy Above Valence Band (eV) Quantitative estimate of Subgap states produce defect state densities. clear CL emission spectra College of Engineering University of Illinois

Hall Effect Carrier Concentration: Hole Mobility: Temperature, T [K] 300 100 50 30 20 10 20 -3 ) 10 19 CGS, x=1.00 Hole Mobility (cm 2 /Vs) 1000 Hole Concentration (cm CIGS x=.20 10 18 CIGS x=.56 10 17 CIGS x=.11 10 16 10 15 10 14 100 10 13 10 12 10 100 0 10 20 30 40 50 Temperature, T [K] -1 ] 1000/T [K College of Engineering University of Illinois

Hall Effect -- vs. composition No change in electrically-active defects with Cu/III composition. 10 19 10 19 -3 ) 10 18 10 18 State Concentration (cm 10 17 10 17 10 16 10 16 10 15 10 15 N A1 10 14 10 14 N A2 N D 10 13 10 13 10 12 10 12 0.9 0.95 1 1.1 1.1 1.1 1.2 0.7 0.75 0.8 0.85 0.9 0.95 1 Cu/In Ratio Cu/(In+Ga) Ratio College of Engineering University of Illinois

Hall-effect -- vs. composition …except for Ga, which increases p at high Ga contents. 10 22 Acceptor State Density [cm -3 ] 10 21 10 20 Deep acceptor (135 meV) 10 19 10 18 10 17 10 16 Open points: Ga gradient Shallow acceptor (40 meV) Closed points: uniform Ga 10 15 10 14 0 0.2 0.4 0.6 0.8 1 Ga/(In+Ga) College of Engineering University of Illinois

Growth Orientation Effects • (112) and (002) very similar, phonon scattering for (112) and (002) • p (220) 20x lower, mobility lower, defect limited Film Orientation (112) Hole Concentration, p (cm -3 ) (002) 10 17 1000 (220) Hall Mobility (cm 2 /V-sec) 10 16 10 15 100 10 14 10 13 10 3 3.5 4 4.5 5 5.5 6 6.5 7 100 150 200 250 300 Inverse Temperature, 1000/T [K -1 ] Temperature, T (K) College of Engineering University of Illinois

Implant Changes Low dose Se implant -- mostly reversible changes. as deposited as implanted annealed 1000 10 19 -3 ) Hole Concentration (cm Mobility, µ (cm 2 /Vs) 100 Implant 10 17 damage Implant damage 10 10 15 1 10 13 10 11 0.1 2 4 6 8 10 12 70 80 100 200 300 1000/T (K ) Temperature T (K) Cr implant similar College of Engineering University of Illinois

Point Defects Summary • Shallow acceptor: Cu vacancy Conduction Band • Deep acceptor: Cu In (divalent) In Cu +1 or V se +1 ? In Cu +2 or V se +2 ? • Defects 0.8 and 1.0 eV above valence band: probably Se 0.9 eV vacancy 0.7 eV Cu In -2 0.3 • Above do not shift with Cu In -1 0.13 Ga/(In+Ga) E f V Cu -1 0.04 • Additional deep states? Valence Band College of Engineering University of Illinois

Surface Energy & Growth Mechanisms College of Engineering University of Illinois

Surface Morphology Contrast enhanced Summary: (112) close-packed Best epitaxy on GaAs (111). Very low angle (<1°) pyramidal facets. Rough surfaces on GaAs (110). Epitaxial temperatures: T s = 540 ° C (220); 640 ° C (100); and 700 ° C (112) (220)/(204) surface Facets to (112) planes, one smooth, one rough. Pure CuInSe 2 : Ga diffuses from the substrate (100)/(002) surface Elongated ripples with Kirkendall voids form at the asymmetric rectangular interface in the GaAs substrate. pits sometimes present. College of Engineering University of Illinois

(112) Se Close Packed Surface Steps: 1-3 ML high • Growth by step nucleation and 5 µ µ m x 5 m x 5 µ µ m AFM image 5 m AFM image propagation • Very flat • 180° growth twins ~40 µ m apart Electron Channeling Patterns Very triangular facets , strong step orientation College of Engineering University of Illinois