Characterization of the defect levels in copper indium diselenide

Solar Cells, 30 (1991) 151-160

151

Characterization of the defect levels in copper indium diselenide F. A. A b o u - E l f o t o u h , H. M o u t i n h o , A. Bakry, T. J. C o u t t s a n d L. L. K a z m e r s k i Solar Energy Research Institute, 1617 Cole Boulevard, Golden, CO 80401 (U.S.A.)

(Received October 25, 1990)

Abstract High-resolution photoluminescence (PL) measurements were carried out at 10 K to identify the energy levels associated with the various defect states dominating the semiconductor CuInSe2 (CIS). PL measurements were taken on the bare surfaces of both thin film and single-crystal (polished and cleaved) samples and through a (Cd, Zn)S window layer deposited by thermal co-evaporation onto the CIS absorber surface. A complete energy band diagram is proposed which identifies the origin of the 12 intrinsic defect states expected in this material. The effects of surface and heat treatments, used in device fabrication processing, on the existence and generation of defect states (deep and shallow) are identified and correlated with the device performance. The inferior single-crystal device performance is correlated with presence of a high density of process-generated radiative surface recombination states and trap levels.

1. I n t r o d u c t i o n C o p p e r indium diselenide (CIS) has b e e n established as a viable thin film s e m i c o n d u c t o r for solar cells. B e c a u s e of substantial r e c e n t p r o g r e s s (cells a n d m o d u l e s e x c e e d i n g 11% c o n v e r s i o n efficiencies with d e m o n s t r a t e d stabilities), the c o s t benefits of this cell m a k e it c o m p e t i t i v e with o t h e r p h o t o v o l t a i c candidates. The cell p e r f o r m a n c e has evolved despite the uncertainties r e g a r d i n g the relationships a m o n g the e l e c t r o - o p t i c a l p r o p e r t i e s o f CuInSe2, the t y p e s of defects a n d their electrical role in the material. Therefore, substantial efforts are still required to clarify the defect c h e m i s t r y that d e t e r m i n e the efficiency of this t y p e o f cell. As an e x t e n s i o n of p r e v i o u s efforts [ 1 - 3 ] , the objective o f the w o r k r e p o r t e d in this p a p e r is the f u r t h e r c h a r a c t e r i z a t i o n of the defects in singlecrystal CIS a n d polycrystalline material, since t h e s e limit the efficiency of b o t h [4] solar cells. The p u r p o s e o f the single-crystal studies is to establish a f o u n d a t i o n for the u n d e r s t a n d i n g of the m o r e c o m p l e x case o f polycrystalline thin film CIS material, the material u s e d in c o m m e r c i a l solar cells. In particular, it was i n t e n d e d to identify the origins o f the v a r i o u s intrinsic d e f e c t states o f the CIS s e m i c o n d u c t o r , including t h o s e resulting f r o m v a r i o u s s u r f a c e a n d heat t r e a t m e n t s , and to establish a c o m p a r i s o n b e t w e e n thin films a n d single crystals. It was also i n t e n d e d to clarify the relationship b e t w e e n the

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152 type and origin of the dominant defect states, the material chemical composition, and the junction behavior.

2. E x p e r i m e n t a l m e t h o d s

This study was per f or m ed on p-type CIS single crystals and thin films. Samples cut from crack-free crystals (obtained from R. Thomlinson, Salford University, U.K.) were mechanically polished and etched in m e t h a n o l - b r o m i n e solutions of different concentrations. Thin film materials were also used in this study (supplied by IEC) and these were prepared by the three-source co-evaporation method. The chemical compositions of the samples under investigation were measured using wavelength dispersive X-ray spect rom et ry with a CAMECA electron microprobe (Model MBX) and a Physical Electronics (Model 600) scanning Auger microprobe. Spectroscopic scanning tunneling microscopy (SSTM) was used to identify several major defect types in the CIS single-crystal surfaces. High-resolution photoluminescence (PL) measurements were carried out at 10 K to identify the energy levels associated with the various defect states. Excitation was provided using the 647 /~ argon laser line at different powers ( 5 - 1 0 0 mW unfocused). The PL measurements were taken on the bare surfaces of both the thin film and single-crystal (polished and cleaved) samples and through a (Cd, Zn)S window layer deposited by thermal co-evaporation onto the CIS surface. Schottky devices were fabricated by depositing aluminum dots (100 nm thick) onto the cleaved surface of the p-type CIS with a gold back contact. Au/nCuInSe2 Schottky devices were also fabricated by thermal evaporation of gold dots (100 nm thick) onto the front surface and a 1.0 /zm gold layer on the back. Current-density-voltage (J-V), capacitance-voltage (C-V), and deep level transient s pe c t r os c opy (DLTS) measurements were perform ed on both the metal/CIS Schottky devices and the (Cd, Zn)S/CIS heterostructures, to investigate the presence of surface states and the deep levels associated with the defect states in the CIS.

3. R e s u l t s a n d d i s c u s s i o n

An energy band diagram has previously been p r o p o s e d which explained most of the defect chemistry of CIS within a wide range of composition [5, 6]. These diagrams, however, did not account for all 12 intrinsic defect states e x p e c t e d in this material. Extension of this effort has resulted in identification of additional levels associated with intrinsic defects which dominate the CIS. The results are summarized in the energy band diagram shown in Fig. 1. This diagram was developed utilizing the material composition (as a guide to the likely defect levels), the calculated formation energies of the possible intrinsic defect states [4], and the published ionization energies [7]. The PL emission from the cleaved surfaces of three p-type CIS single

153

Fig. 1. Energy band diagram of CuInSe2. _

i

i

i

i I Inse - Vou

i

i

P

I

0.918(eV) incu- VCu -0.955(eV) CB- Secu (eV) ~. r- 0.978

Inse. VSe

0.093(eV) /[0"9~T~

"E ,~O Q.

0.750

-b

Cu" 3u i

'

Cu~ In Se 2 a 22.79 27.22 49.99 b 22,97 25.62 51.41 c 23.77 2516 5108

0.9:'75 Photon

energy

1.200 (eV)

Fig. 2. PL en~ssion from CIS single crystals of different compositions. crystal samples with different compositions is shown in Fig. 2. The sample with the lowest copper content ([Cu]/[In] ratio Am is 0 . 8 3 7 ) is dominated by a PL transition due to Inse-Vcu at 0 . 9 1 8 eV (curve a in Fig. 2). The s e c o n d major PL emission at 0 . 9 5 5 eV is attributed to the Incu-Vcu transition followed by the 0 . 8 9 3 eV peak resulting from Inse-Vs~. This sample had almost no stoichiometry deviation, i.e. there is almost exactly 50 at.% Se. In contrast, the sample with an excess of selenium and a slightly higher [Cu]/[In] ratio (curve b in Fig. 2, A m = 0.896), is dominated by the conduction band (CB)-Secu recombination responsible for the PL peak at 0 . 9 7 8 eV. This overlaps the PL emission at 0 . 9 5 5 eV, which is due to the Incu-Vcu recombination, in addition to the signal at 0 . 9 2 6 eV, due to Inc~-Se~, transitions.

154

Curve c of Fig. 2 is the spectrum for a sample with an excess of selenium (51.08 at.%). This exhibits the largest [Cu]/[In] ratio ( A m = 0 . 9 4 4 ) and it is dominated by the same emission as sample b, plus a signal at 0.94 eV resulting from the Secu-Vcu transition present at relatively equal concentrations. Direct evidence of the Vcu and Incu defects responsible for the majority carrier type in sample a has been provided by SSTM imaging of the 112 surface (which contains only indium and copper) and the E9 grain boundary of the 112 plane as shown in Figs. 3(a) and 3Co) respectively.

(a)

(b) Fig. 3. S S T M images from (a) the 112 cleavage plane and (b) the E9 grain boundary on the (112) plane of the p-type sample a.

155 It is therefore clear that p-type CIS can be grown as c o p p e r - p o o r material (22.79 at.%) with near zero molecularity deviation. The p-type crystals with a selenium excess, however (curves b and c, Fig. 2), can be grown either as c o p p e r p o o r or slightly c o p p e r rich [2], but the defect chemistry and corresponding electrical behavior are considerably different. The copperdeficient crystals are dominated by Vcu defects. These exhibit high carrier concentration (Na> 5 × 1016 cm -3) and, if combined with Secu anti-site defects, the carrier mobility is increased ( ~ > 50 cm 2 V -1 s - l ) . The presence of a high density of the Incu defect, however, reduces ~ substantially. The effect of surface treatments of the CIS single crystal is also dependent on the initial type and relative concentration of the dominant defect states. Some samples are severely affected by mechanical polishing, while others are affected very slightly. Figure 4 (curves a and c) shows a comparison of the PL emission from sample a of Fig. 2 after cleaving and then after mechanical polishing. It is clear from this that polishing created several additional defects in the range 0 . 9 5 4 - 0 . 9 8 4 eV. Curve a of Fig. 4 was taken at an incident power of 7 mW whereas curve b was obtained using a power of 14 mW. Hence, increasing the power also increases the transition energy of the additional states. At lower energy, reduced excitation power causes them to overlap the Incu-Vcu emission. The effect of mechanical polishing on sample a (which had a indium excess) is more p r o n o u n c e d t h a n for sample b (indium content close to 25 at.%). This is shown in Fig. 5. For this, the PL signal at 0.926 eV emitted by the Inc~-Sein transition dominated after polishing and the 0.952 eV emission due to the Secu-Vc~ transition be c a m e well defined. The radiative surface recombination state at 0.978 eV, which is superimposed on the CB-Secu transition, is shifted to 0.990 eV by increasing the excitation power from 7 to 14 mW but it does not dominate the PL emission of this sample as it did for sample a.

I

I

I

I Inse _VCu - ~ 0.917 (eV)

I

0.921 ( e v ) ~ Incu - VCu \ ~.954 (eV) \~

[ c~ I

"~ O~

~ 1%

Inse'Vse _1 . 0.8921 ( e V i l ~

; \0.984 I \(eV)

I Cu 22.79

i I In Se2 27.22 49.09

a - Polished surface (7 mW) b - Polished surlace (14 mW~

c - c l e a v ~ suMace (7 mw) A.B-Energyrangeofthe radiative surface

--0.977

-5

0.'/50

0.975

1.200

Photonenergy(eV) Fig. 4. Effect of mechanical polishing on radium-rich CIS single crystal.

156

CB - Secu 0.977 [eV) . ~

Cu 2297

a ~0~ "~

InCu Seqn

~

/

in 25.92

Se~ 51.41

a - Cleaved surface b - Polished surface (14 mW] c Polished surface {7 roW)

._c

Q_

0.750

0.975 Photon energy (eV)

1.200

Fig. 5. Effect of m e c h a n i c a l polishing on selenium-rich CIS single crystal.

Inse CUln 0.880 (eV) 0.892 (e~)

"

0.920 (eV) -

Inou - Vcu ~-0.940 (eV) cu

J

lr

0936 (eV)

2l/1

o

In

se2

, c,oa,o0so,aoe C

-

o .o,ac

Thin film

o9ol (eV)

o /~ 0.750

_-a

0.975

1.200

Photon e n e r g y (eV)

Fig. 6. PL e m i s s i o n from CIS single crystal ( c l e a v e d and p o l i s h e d surfaces) and thin film with the s a m e c o m p o s i t i o n .

In contrast, the PL spectrum of sample c (Fig. 6) did not show any additional radiative surface recombination states after polishing. However, the reduction in the integrated intensity of the PL spectrum of the polished surface indicates the formation of non-radiative surface states that quench the radiative recombination transitions. The PL emission of this sample is compared with that from a thin film sample of similar composition (supplied by Boeing). It is clear that the defect configuration of the thin film material is dominated by a strong peak at 0.880 eV (attributed to Inse-Cu~) in addition to the main emission from the single-crystal sample, including the 0.901 eV signal attributed to Inse-Vcu recombination. Hall measurements have shown that the carrier mobility of the thin film material is twice that of the singlecrystal material. The carrier concentration, however, is higher in the single

157 crystal ( s a m p l e c), w h i c h is d o m i n a t e d b y c o p p e r v a c a n c i e s active in the Incu-Vcu transition. The p r e s e n c e o f t r a p s in b o t h single-crystal and thin film j u n c t i o n s m a d e f r o m t h e s e s a m p l e s was also investigated using DLTS. Figure 7 shows the DLTS s p e c t r a l and Arrhenius plots o b t a i n e d f r o m single-crystal CIS devices o f the t y p e (Cd, Zn)S/p-CIS and AI/p-CIS respectively. Two m a j o r t r a p s w e r e f o u n d for the h e t e r o s t r u c t u r e device; t h e s e o c c u r r e d at 2 3 4 meV a n d 493 meV. Only o n e level at 282 meV was d e t e c t e d in the S c h o t t k y diode [8] m a d e o f the s a m e crystal. In the case of the thin film h e t e r o s t r u c t u r e device, a d e e p level at 530 meV was d e t e c t e d (Fig. 8). Although the d e e p trap o c c u r r e d at a p p r o x i m a t e l y the s a m e e n e r g y level in the single-crystal and thin film h e t e r o s t r u c t u r e devices, its density was a b o u t 2 - 3 o r d e r s o f m a g n i t u d e larger for the f o r m e r and this difference a c c o u n t s for the m u c h l o w e r o p e n circuit voltage c o m m o n l y s e e n for single-crystal cells.

IO00/T1

IO00/T2

88

310

Temperature (°K)

Fig. 7. DLTSspectrum and Arrhenius plots of (Cd, Zn)S/CIS single-crystal device (of composition 23.30, 25.47, 51.23). -3

I

I

I

I

I

2;0

3;0

O. -7" <~ ~.~ *9"

-4.5

-11"

-5.5 ~ -6.5 "J -7.5

~13-15 100

•

L

i

i

i

i

• •

-8.~!~

1'~0

'

313

' 31s 1000K

'

3!7

1;0

220

340

Temperature (°K)

Fig. 8. DLST spectrum and Arrhenius plot of a (CdS, Zn)CIS thin filmjunction (after annealing at 120 °C for 15 min).

158 Another trap level was obtained at 270 meV in the thin film A1/CIS junction as previously found using DLTS [8]. It is therefore clear that heating of the CIS during (Cd, Zn)S deposition, influences the trap location and density for both thin films and single crystals. However, the original location of the trap levels is dependent on the defect configuration of the starting material and this varies from sample to sample, even for similar compositions. The PL measurements performed on various samples of similar composition have shown that these can be dominated by different types and concentrations of intrinsic defects. This could be partially attributed to the preparation conditions which are somewhat uncontrollable because of the ternary nature of the material. In addition, the surface treatment during sample preparation also almost certainly modifies the dominant defect states. The equilibrium concentrations of the various defect states can also be changed by postpreparation annealing of the crystals [5], but it is preferable to modify heating during growth of the window layer since separate annealing will enhance the compensation ratio [9]. Therefore, a systematic study that relates the defect configuration of the material to the growth conditions of both thin films and single crystals is a necessary undertaking. The frequency-dependent capacitance characteristic was also used to study the interface behavior of both thin film and single-crystal junctions. The C - 2 - V plots obtained from the two heterostructure single-crystal devices numbered C2-8 and 6B (their respective conversion efficiencies are 1.2% and 4.1%) are shown in Figs. 9 and 10. It is clear that the characteristics for device C2-8 show not only a decrease in the slope with decreasing frequency, but also a non-linear dependence. Device 6B had an almost linear characteristic with a slight reduction in slope with frequency. This result confirms the presence of multiple surface states (at higher density in the C2-8 device) with variable distribution. The surface states include shallow acceptors and deep traps and the density of these decreases with increasing frequency. The C - 2 - V characteristics are obtained for devices of the types A1/p-CIS and (Cd, Zn)S/p-CIS prepared on polycrystalline thin films (supplied by IEC). Although a linear characteristic is obtained from the thin film Schottky 3 x 1019

2x102o

° lOOk.z 2 x 1019

t I

300 kHz 700 kHz

1 x 1019

~

J

r// z/ j

[] •

100 kHz 300 kHz

/~'

•

1 MHz

/. ,~/ /,)

-A/

/ A ," o -2.0

' = = i

i~ -1.0 Voltage (V)

o llillll O.0

-2.0

||||I

|iilll ====J 1.O

00

Voltage (V)

Fig. 9. Frequency-dependent C-2-V relation of (Cd, Zn)S/CIS single-crystal device (C2-8). Fig. 10. Frequency-dependeat C-2-V relation of (Cd, Zn)S/CIS single-crystal device (6B).

159 2x1017

~.5

§

m

700 kHz

•

1 Mhz

lx1017

0

-

-3

-2

- - ~

-1 Voltage (V)

0

1

Fig. 11. Frequency-dependent C-2-V relation of (Cd, Zn)S/CIS thin film device. diode [8], the slope decreases with decreasing frequency. However, a nonlinear characteristic is observed for the heterojunction device (Fig. 11), although the capacitance did not vary with the m e a s u r e m e n t frequency, thus confirming a very low density of deep traps.

4. Summary The principal observations in this investigation are the following: (1) The starting composition of single crystals of CIS provide a guide as to the likely defect levels as shown, for example, by the presence of transitions involving c o p p e r vacancies in samples which were c o p p e r poor. (2) However, energy levels and defect concentrations may be different for samples which have nominally identical compositions and they are evidently more influenced by the details of fabrication and subsequent thermal or mechanical processing steps. (3) Although a selenium excess always leads to crystals which are p type, even with almost exactly 50 at.% Se, it is possible to obtain p-type conductivity provided the crystals are c o p p e r poor. (4) The effect of mechanical polishing is governed by the initial defect configuration and cannot necessarily be predicted in advance. (5) Single-crystal heterostructure cells exhibit p o o r p e r f o r m a n c e because they have a m uch greater concentration of traps near the middle of the forbidden gap. It is suggested that these act as efficient recombination centers of excess p h o t o g e n e r a t e d carriers, thus increasing the reverse current and decreasing the open-circuit voltage. The high density of this trap is probably associated with the polishing step that is necessary for the fabrication of single-crystal cells.

Acknowledgement The authors wish to thank R. Birkmire and co-worker at (IEC) for the fabrication of the CIS devices and the supply of CIS thin films. This work

160

was performed by the Solar Energy Research Institute under contract DEAC 02-82 CH 10093 to the U.S. Department of Energy.

References 1 F. Abou-Elfotouh, L. L. Kazmerski, A. M. Bakry and A1-Douri, Proc. 21st IEEE Photovoltaic Specialists' Conf., Orlando, FL, May 21-25, 1990. 2 F. Abou-Elfotouh, S. Alkuhaimi, R. Moustafa, D. J. Dunlavy and L. L. Kazmerski, Proc. 19th IEEE Photovoltaic Specialists" Conf., New Orleans, LA, 1987, IEEE, New York, 1987, p. 77O. 3 F. A. Abou-Elfotouh, L. L. Kazmerski, T. J. Coutts, R. J. Matson, S. E. Asher, A. J. Nelson and A. B. Swartzlander, J. Vac. Sci. TechnoL A, 7 (3) (1989) 837. 4 D. Cahen, in S. K. Deb andA. Zunger (eds.), Ternary andMultinary Compounds, Materials Research Society, Pittsburgh, PA, 1987, p. 433. C. Rincon and S. M. Wasim, in S. K. Deb and A. Zunger (eds.), Ternary and Multinary Compounds, Materials Research Society, Pittsburgh, PA, 1987, p. 433. 5 F. A. Abou-Elfotouh, D. J. Dunlavy and T. J. Coutts, Sol. Cells, 27 (1989) 237. 6 F. A. Abou-Elfotouh, L. L. Kazmerski, T. J. Courts, D. J. Dunlavy, M. A. Almassary, S. Chaudhuri and R. W. Birkmirc, Proc. 20th IEEE Photovoltaic Specialists' Conf., Las Vegas, NV, 1988, IEEE, New York, 1989, p. 1520. 7 H. Neumann, Seminar, Centro de Estidias de Semiconductors, Universidad de las Andes, Merida, Venezuela, November 1984. H. Neumann, W. Horig, R. D. Tomlinson and N. Augerinos, Cryst. Res. Technol., 21 (6) (1986) 805. 8 F. A. Abou-Elfotouh, L. L. Kazmerski, H. R. Moutinho, A. J. Nelson and A. M. Bakry, Paper presented at the 37th American Vacuum Society Nat. Syrup., Toronto, Ontario, October 8-12, 1990. 9 F. Abou-Elfotouh, L. L. Kazmerski, R. J. Matson, D. J. Dunlavy and T. J. Coutts, J. Vac. Sci. Technol. A, 8 (4) (1990) 3251.