Comparison of properties of thin films of CuInSe2 and its alloys produced by evaporation, RF-sputtering and chemical spray pyrolysis

Applications of Surface Science 22/23 (1985) 645-655 North-Holland, Amsterdam

645

C O M P A R I S O N OF PROPERTIES OF THIN FILMS OF CulnSe 2 AND ITS ALLOYS P R O D U C E D BY EVAPORATION, RF-SPUTTERING AND C H E M I C A L SPRAY PYROLYSIS J.J. L O F E R S K I , C. C A S E , M. K W I E T N I A K L. C A S T A N E R *** a n d R. B E A U L I E U

*, P.M. S A R R O

**,

Division of Engineering, Brown University, Providence, Rhode Island 02912, USA

Received 27 August 1984; accepted for publication 6 November 1984

This paper summarizes a six-year research program aimed at preparation and characterization of thin films of the chalcopyrite semiconductor CulnSe2 suitable for photovoltaic solar cells and other semiconductors devices. The thin film deposition methods used were evaporation, flash evaporation, RF-sputtering and chemical spray pyrolysis. Problems with producing films of reproducible properties by these methods are discussed. Gaps in our knowledge of the dependence of the opto-electronic properties of CulnSe2 crystals and films on structural defects and other imperfections are pointed out.

I. Introduction This p a p e r s u m m a r i z e s research at B r o w n U n i v e r s i t y on the p r e p a r a t i o n a n d c h a r a c t e r i z a t i o n of thin films of C u I n S e 2 i n t e n d e d for use as the p h o t o v o l t a i c a l l y active s e m i c o n d u c t o r ( P V A S ) in solar cells. A s is well k n o w n , solar cells m a d e from this m a t e r i a l have exhibited efficiencies up to 12% [1,2]. T h e films to be discussed in this p a p e r were p r o d u c e d by v a c u u m e v a p o r a t i o n of the previously synthesized c o m p o u n d from a resistance h e a t e d crucible, flash e v a p o r a t i o n , R F - s p u t t e r i n g a n d chemical spray pyrolysis. Typically these films had a thickness of several microns, a thickness sufficient to a b s o r b most of the p h o t o n s having an e n e r g y equal to or g r e a t e r than the b a n d gap of C u I n S % (about 1.0 eV). This p a p e r i n t e n d s to identify p r o b l e m s which i m p e d e progress toward higher p e r f o r m a n c e C u I n S e 2 solar cells a n d o t h e r s e m i c o n d u c t o r devices m a d e from this material a n d from similar chalopyrite s e m i c o n d u c t o r s . * Present address: Telecom Australia Research Laboratories, Melbourne, Australia. ** Present address: Department of Electrical Engineering, National Research Laboratories, Delft, The Netherlands. *** Present address: Escuela Tecnica Superior de Ingenieros de Telecommunicacion Barcelona, Spain. 0378-5963/85/$03.30 © Elsevier Science Publishers B.V. ( N o r t h - H o l l a n d Physics Publishing Division)

¢~4~,

.I..I.l.oferski et al.

I~'operties of thin lihn.s ol ( 'ulnSe:

2. How the films were characterized 2. 1. ( ? , n t ~ o s i t i o n

The local composition of the ( ' u l n S c , lilms was d e t e r m i n e d b~ energ5 dispersive analysis of X-rays ( F D A X ) a n d / o r by wave dispersive analysis of X-rays (WDX). These m e t h o d s result in compositions accurate onl,, to about I % , an a c c u r a c y far helow lhat required to predict the values of e~cn such basic p a r a m c t c r s as resistivity and mobilit,,. However, K I ) A X and W D X permit one It) identify fihns whose composition deviates grossl), from its stochiometric valuc. In the present state of their d c v c l o p n l c t m the elcclr(mic properties of C u l n S c , and chalcopyrilc s e m i c o n d u c t o r s similar t() il (e.g, ( ' u l n S 2, CuGaSc_+, clc.) arc controlled by controlling stochiomctry. Noufi ci al. 131 showcd that if the Sc/metal (Cu + In) ratio is unit~, or close to it, lilms in which the Cu/ln ratio is g r c a t c r than unity (say 1.1) arc p-type. If the ( ' u / I n ratio is less than unity (say {).9), their resistivity incrcascs and the~ b e c o m e n-type. W h e n the ( ' u / I n ratio is nnitv or very close to unity, the film type and resistivity b c c o m c very sensitive to the So/rectal ratio. Films with excess Se arc p-type: f l m s deficient in Sc arc n-typc. Of coursc - c x c c s s and deficient" mean concentration differences on a s e m i c o n d u c t o r scale, i.e. between several parts per t h o u s a n d and several parts per million. These experiments indicate the importance of controlling stoichiometrv in the C u l n S e e. W h e n we synthesize CulnSc+ p o w d e r for evaporation or for usc in preparing targets for RF-sputtering, the starting materials (Cu, In, Sc) arc 99.9999% pure. T h e y are weighed to an accuracy of one part in 11)". A small a m o u n t (0.1%) of excess Se is a d d e d to the mixture. The mixture is sealed in an evacuated silica a m p o u l e and heated slowly to the melting t e m p e r a t u r c (1050°C). The t e m p e r a t u r e of thc material is reduced rapidly by withdrawing the a m p o u l e from the hot zone of the furnace. The resulting ingot is g r o u n d into powder, which is again sealed in an evacuated silica a m p o u l e and annealed close to the melting t e m p e r a t u r e for several days. The X-ray diffraction pattern of the p o w d e r p r o d u c e d from the ingot is recorded. The process is r e p e a t e d as m a n y times as may be necessary to p r o d u c e the desired X-ray diffraction pattern as described in the next section. F o r conducting E D A X analysis, we d e v e l o p e d a c o m p u t e r p r o g r a m which applies a Z A F correction to the E D A X spectrum ( Z = atomic number, A = X-ray absorption coefficients, F fluorescence coefficients). However, the raw data fed into the c o m p u t e r consists of some o v c r l a p p m g spectral lines which complicate interpretation and limit the accuracy of compositional analysis. 2.2.

CO,stal structure

Fig.

1 compares the lattice of the typical chalcopyrite semiconductor

./.3.. Loferski et al. / Properties of thin films of CulnSe2

647

A

b

O.

• Zn

oCn

oS

,In

oS

Fig. 1. Structures of zincblende (a) and chalcopyrite (b).

CulnS 2 (CulnSe 2 has the same structure) with the lattice ZnS (the sphalerite structure). Fig. 2 is the X-ray diffraction pattern of the chalcopyrite structure. CulnS 2 contains two metal sublattices instead of the single metal sublattice of ZnS. This additional order in the chalcopyrite materials results in the presence of weak ("superlattice") lines in the chalcopyrite X-ray diffraction pattern (like the lines labelled 101, 103, 211 in fig. 2) superposed over the intense sphalerite lines (e.g. 112, 220, 312 of fig. 2). To our knowledge, there does not exist any study of the electrical property differences between films which exhibits the sphalerite diffraction pattern

i

Cu In Se 2

100

112

80

a = 5.782~, c = 11.62 ~,

220 204

>..-

i-60 H-z

312 116

40 taJ o7;

20

~74 ,

90 °

80°

332 316

400

[

0i8 ,

70°

,

60 °

,

50°

105,

,

40 °

10[

30°

,

20°

]

10°

2e Fig. 2. Chalcopyrite structure in the (110) projection and its X-ray diffraction pattern.

0°

J.J. Loferski et al. / Properties of thin films of CulnSee

648

and films which exhibit the chalcopyrite diffraction pattern. It is known that the highest efficiency CulnSe~ solar cells were made from films of the chalcopyrite structure. In general, it has been the experience of the Brown PV group that films of the chalcopyrite structure are less frequently obtained than films of the salphalerite structure. For example, fig. 3 compares the ]

F

I

i

i

0 0 OJ OJ

N

0 C-J

Z

rO

w

t-Z

7-

co

t~3 . ~)

< -J UJ

0 0

L

L

J

I

L

72

68

64

60

56

T

r

-

' 52

4L8

4'4

4~0

T

~ 36

~ 32

t 28

24

20

1

~

r

r

r

16

b c~ 0 N

o

f,l

r,3°

~_'Z J

r~

GO

_

o°,.,

i

"el

I

72

68

64

60

56

52

4 8 4 4 40 2 8 (deg)

36

32

28

24

20

16

Fig. 3. X - r a y difiraction pattern of ( ' u l n S c 3 lilms clcpositcd o n ( ' r A u ;vith spra,, r a t e o f (a)

7

ml/min a n d

(b) I 1 m l / m i n .

J.J. Loferski et al. / Properties of thin films of CulnSe:

649

X-ray diffraction patterns of two CuInSe 2 films prepared by chemical spray pyrolysis at two different rates. The upper pattern produced at the lower rate contains the chalcopyrite lines (101, 103, 211) while the lower one contains only the sphalerite lines. Note also the presence of indium oxide lines in the upper pattern; such lines can occur in both sputtered and sprayed films. Clearly, their presence indicates that the atmosphere of the chamber in which the films are being deposited contains oxygen. Thus X-ray diffraction spectra can also provide information about other materials and phases which are present in the films. However, sensitivity is limited to about 5% so that the absence of lines of In203 in the lower spectrum of fig. 3 does not mean that indium oxide is entirely absent from that film.

2.3. Conductivity and Hall effect Four point probe m e a s u r e m e n t s were routinely made to estimate the resistivity of the films. A hot probe was used to determine conductivity type. Occasional Van der Pauw method Hall effect measurements were conducted to determine carrier concentration and mobility at room temperature. The dependence of these parameters on temperature was not measured for our films and, in fact, the literature does not contain any significant body of information about the temperature dependence of these properties in CulnSe 2 films or crystals. Thorough studies of the temperature dependence of these fundamental electrical parameters on films having controlled different compositions and structures are sorely needed before we can claim a proper understanding of CulnS%. Data on single crystals must be obtained so that m e a s u r e m e n t s of the mobility and carrier concentration of films will indicate how closely the films replicate the properties of crystalline CulnSe z.

2. 4. Optical properties In our laboratories, films of CuInSe 2 are simultaneously deposited on a transparent substrate (pyrex, silica, sapphire) and on a conducting substrate (molybdenum or gold/molybenum covered sintered alumina). The optical transmission (and also electrical properties) are measured on the films deposited on the insulating, transparent substrates. It is assumed that the properties of films deposited on the conducting substrates are the same as those of films deposited on the transparent substrates. Unfortunately, we do not know whether this is really so. We do know that the morphology of films simuiteneously deposited on these two kinds of substrates heated to the same substrate temperature are quite different as shown by SEM photographs and therefore it would not be surprising if their electrical properties, which should be very sensitive to grain size and orientation, differ, even significantly.

65~

J.J. Loferski et al. / Properties of thin films of CulnSe:

2.5. Thickness measurements, morphology observation

A Sloan Dektak surface profiler is used to measure thickness of the fihns: its accuracy is about 5%. In some cases, the thickness has also been measured with the help of a metallurgical microscope. An A M R 1000 scanning electron microscope is used to exanaine the grain structure of the films. Grain sizes in the range of 1 ,am can be obtained in films produced by evaporation, flash evaporation and RE-sputtering [3], but grain sizes less than 1 ,am are the rule in films produced by chemical spray pyrolysis (CSP) [4,5]. Presumably this is related to the lower temperature of the substrate (<200°C) used to date from CSP deposition: temperatures in the 450--500°C range are found to be necessary to achieve micron grams in RF-sputtered films [6].

3. Problems associated with various deposition techniques 3. 1. Evaporation

We have previously pointed out that vacuum evaporation of a charge of synthesized CulnSe 2 powder from a resistance heated crucible rarely results in films having stoichiometric composition [7]. The experiments comparing films produced by evaporation from an open boat, from a baffled boat and by flash evaporation were summarized in a report from our laboratories [8]. The results of compositional analyses of these films are shown in fig. 4. In this figure the In (vertical axis) and Se (horizontal axis) contents measured by E D A X are normalized to Cu content of the films. The point corresponding to perfectly stoichiometric films would lie at the exact center of the square. The shaded circle defines departures from stoichiometry equal to or less than 20%. The figure shows the copper deficiency line which is a diagonal line from the center to the upper right-hand corner. Compositions along this line would have the correct In : Se ratio (1 : 2) but would be Cu deficient. The excess Se line is a horizontal broken line from the center to the mid-point of the right edge of the square: along this line the Cu : In ratio is correct (1 : 1), but the sample contains excess selenium. The charge used for all of these evaporations contained excess Se and therefore it is not surprising that most of the films represented on this diagram contain excess selenium. For these experiments, the source temperatures were in the 1050-1350°C range for both flash evaporation and for evaporation from a baffled boat and between 900 and 1000°C for evaporation from an open boat. The substrate temperatures were in the 400-450°C range. Some of the specimens which were particularly far off stoichiometry were placed in an ampoule containing stoichiometric CulnSe 2 powder. The ampoule contain-

J.J. Loferski et al. / Properties of thin films of CulnSe2 A

FLASH EVAPORATION

0 STD. EVAPORATION OPEN BOAT

Cu=l ,

2.0 1.5

,0~2~

6

651

[] STD. EVAPORATION

__-- COPPER DEFFICIENCY LINE SELENIUM EXCESS LINE

1.o

O.S 0

I

0

(a)

I

I

2 3 SELENIUM EDAX ANALYSISAS DEPOSITED 2.0

1.5

1

|

i

!

Eu=

' ' 2}

,,o

:}

0.5 0

(b)

BEFORE H.T.

AFTER H.T.

5 I

I

1

2

I

3

/* SELENIUM EFFECT OF HEAT TREATMENTON STOICHIOMETRY

Fig. 4. Stoichiometry of the films produced by standard and flash evaporation and effect of heat treatment. ing the film and CuInSe2 powder was then evacuated, sealed and heated for 16 h at 500°C. It was expected that the films would exhibit improvements in stoichiometry. The results, shown in fig. 4b, were inconclusive. None of the points corresponding to films after heat treatment shifted into the circle representing less than 20% departure form stoichiometry. Perhaps other t i m e - t e m p e r a t u r e combinations would result in stoichiometric films but at least the particular heat treatment employed by us did not produce the desired result of transforming poor films into good ones. Similar treatments have been used on samples closer to stochiometry; their stoichiometry improved. From these experiments, we concluded that (1) films deficient in copper were always n-type even though they contained excess selenium; (2) the stoichiometry of flash evaporated films depends more on source temperature than substrate t e m p e r a t u r e (higher source temperatures are preferred); (3)

652

.LJ. Loferski et al. / Properties of thin films of CuInSe,

nearly stoichiometric films produced by flash evaporation were usually of the chalcopyrite structure: (4) in as-deposited films, the grain sizes were in the (t.l-I # m range; (5) heat treatment increased the grain size into I ,u range but did not improve stoichiometry much. 3.2. RF-sputtering

In our sputtering apparatus, ions of a heavy inert gas (we use argon) b o m b a r d a target, dislodging atoms and/or molecules with a kinetic energy large enough to allow them to reach the substrate which is about 5 cm away from the target. The argon pressure is in the 1-5 × 1(I e T o r r range during deposition. For metal targets, a DC bias is applied between the target and the anode with the target area serving as cathode. For targets ol ',~,ver electrical conductivity, like CulnSe~, an RF-voltage is used to accelerate the argon ions and to generate the plasma. In our system, typical RF-power is about 5 W/cm 2 of target and RF-voltage, 500-2000 V. Our targets consist of cold pressed, sintered p-type CulnSc~ powder: they arc about 5 c m in diameter. One expected advantage of the sputtering process is that the composition of the film will replicate the composition and, therefore, the electrical properties of the target. This will be true provided that the target remains cool enough to preclude evaporation and the target does not decompose during ion b o m b a r d m e n t . Sputtered ions arrive on the substrate with a kinetic energy in the 10-50eV range as compared to ().2ql.3eV for ions deposited by evaporation. Therefore, when one semiconductor is sputtered over another (e.g. CdS onto CulnS%) to form a heterojunction, the interface may contain undesirable interface states resulting from this b o m b a r d m e n t . This partly explains why our all sputtered CdS/CuIns% cells have always been poorer performers than cells in which the CdS films are deposited by evaporation [6]. The most serious problem we have encountered in our six years of studying RF-sputtered CulnSe 2 films has been the lack of reproducibility of films produced from the same target under what appear to be identical conditions. This suggests that at least one of the necessary conditions for replicating films is being violated. Martil et al. [9] have reported similar reproducibility problems in the course of sputtering CdS and CdTe. In fact, none of the best performing thin film solar cells reported to date have been based on sputtered photovoltaically active films. Clearly, more fundamental information about the RF-sputtering of c o m p o u n d semiconductors must be developed before we can m a k e an assessment of the role this technology will play in the fabrication of thin film solar cells and other semiconductor devices. For example, we need to know what species are present in the chamber during sputtering. Diagnostic equipment, like mass spectrum and

J.J. Loferski et al. / Properties of thin films of CulnSe2

653

optical spectrum analyzers, can provide the needed information. Sputtering conditions can then be reproduced more accurately and the desired degree of reproducibility attained. As previously reported [7], we can deposit by RF-sputtering p-type CulnSe 2 films of stoichiometric composition (within the accuracy of E D A X and WDX), having resistivities in the desired 1-10 ohm cm range and grain sizes in the 1 # m range. Some of these films produced n-CdS/p-CulnSe2 heterojunction solar cells having AM1 solar energy conversion efficiencies approaching 5% in devices having an area of about 0.5 c m 2. These devices [6] were characterized by high short circuit currents (35 mA/cm2), low open circuit voltages (0.31 V) and fill factors (-0.45).

3.3. Chemical spray pyrolysis Fig. 5 is a schematic representation of our apparatus for depositing CuInSe 2 films by chemical spray pyrolysis (CSP). The spray solution contains ions of the desired compound in a concentration ratio which is empirically demonstrated to produce a stoichiometric film on the substrate. One expects that this ratio should also be stoichiometric. However, in the case of CuInSe2, Bates et al. [10] found that substantial excess selenium must be present in the solution. In our most recent CSP experiments [4], we used Cu : In : Se ratios of 1 : 1 : 2.1, 1 : 1 : 2.2 and 1 : 1 : 2.3 by dissolving Cu2Ci 2,

HOT PLATE WITH MAGNETIC STIRRER

)LUTION

GUN

NITROGEN TANK

TEMPERATURE ~INDIEATOR

SAMPLE

I THERMOEOUPLE 110 VOLT '-~ VARIAC

~"-""- DRAIN FLASK Fig. 5. Schematic of spray pyrolysis system.

654

J.J. Loferski et al. / Proper6es of thin films of CulnSee

lnCl 3 and N j N - d i m e t h y l s e l e n o u r e a in d e - o x y g e n a t e d , de-ionized water in sufficient a m o u n t s to p r o d u c e a 0.0035M solution of CulnSe2+ x (x -- 0. I, 0.2, 0.3). The substrate t e m p e r a t u r e s were d e t e r m i n e d by a t h e r m o c o u p l e fastened to a glass slide floating on a heated tin bath alongside the substrate o n t o which it was intended to deposit the C u l n S e : film; substrate temperatures ranged between 150 and 220 ° . In these experiments, we aimed at depositing C u l n S e 2 films over metallic films (aluminum and g o l d / c h r o m i u m ) e v a p o r a t e d over glass slides. In most previous reports of deposition of C u l n S e 2 by CSP, the films were deposited on glass slides. H o w e v e r , for solar cell application, it would be a d v a n t a g e o u s if the C u l n S e 2 were deposited on a g o o d metallic c o n d u c t o r which can serve as an ohmic contact. W e f o u n d that the most nearly stoichiometric films on metal films resulted from spraying a solution with Cu : In : Se ratio of I : 1 : 2.3. The X-ray diffraction patterns of films deposited at low spray rates (fig. 3) showed chalcopyritc lines but also included ln203 diffraction peaks, whereas the spectra of films deposited m o r e rapidly exhibited the sphalerite structure but contained less ln203. Grain size in these films as d e t e r m i n e d by observation in an S E M were of the o r d e r of several tenths of a micron. Photovoltaic devices were p r e p a r e d from these films by evaporating a layer of conducting (10-1-10 2 o h m c m ) CdS over the CSP C u l n S e 2 film. W h e n the best of these devices was illuminated by a simulated A M I source, the o b s e r v e d photovoltaic p a r a m e t e r s were V,~ = 0.130 V, I~,.= 2.3 m A / c m 2, fill factor = 0.27. An open circuit voltage of 0.33 V was observed on one of o u r devices, but I,~ was much lower than the value q u o t e d for the best cell. Chemical spray pyrolysis holds the promise of being an inexpensive m e t h o d for depositing thin films of C u l n S e 2 and related c o m p o u n d s like C u l n S 2 and their alloys like Culn~,Ga~ ySe2, etc. H o w e v e r , grain size in these films is too small for g o o d P V solar cells which a p p e a r to require grain sizes of at least 1 ,urn [2,6]. Grain size is controlled largely by substrate temperature. E x p e r i m e n t s with e v a p o r a t e d and sputtered films have shown that substrate t e m p e r a t u r e s between 450 and 550°C are n e e d e d to obtain grains of this dimension. As far as we know, no one has r e p o r t e d depositing CSP C u l n S e 2 films o n t o substrates heated into this range. T h e electrical properties of the CSP films have been studied in only a preliminary way. C o m p a r i s o n of the properties of CSP films with those of R F - s p u t t e r e d films deposited o n t o substrates maintained at the same temperature would permit an evaluation of the effect of residual impurities in the C S P films on their opto-electronic properties.

4. S u m m a r y and conclusions (1) C u l n S e 2 is a promising material for use in solar cells and in other

J.J. Loferski et al. / Properties of thin films of CulnSe2

655

semiconductor devices. Some thin films of this material have electrical properties in the range required for such application, but control of these properties and the reproducible preparation of films with the desired properties continues to be elusive. (2) Much more fundamental investigation of the properties of single crystal CulnSe 2 and of films of this material is needed before the full potential of this material and other similar chalcopyrite semiconductors can be determined.

Acknowledgements The preparation of this manuscript was partially supported by a grant from the Mobil Foundation. The work reported in this paper was supported by contracts from the US Department of Energy, the US Solar Energy Research Institute, the NSF Materials Research Laboratory at Brown and Standard Oil of Ohio.

References [1] J.L. Shay and S. Wagner, Appl. Phys. Letters 27 (1975) 89. [2] R.A. Mickelsen and W.S. Chen, in: Conf. Rec. 15th IEEE Photovohaic Specialists Conf., Orlando, FL, 1981 (IEEE, New York, 1981) p. 800. [3] R. Noufi, R. Axton, D. Cahen and S.K. Deb, in: Conf. Rec. 17th IEEE Photovoltaic Specialists Conf., Orlando, FL, 1984 (IEEE, New York, 1984). [4] P.M. Sarro, R.R. Arya, R. Beaulieu, T. Warminski and J.J. Loferski, in: Proc. 5th European Communities Photovoltaic Solar Energy Conf., 1983. [5] M. Gorska, R. Beaulieu, J.J. Loferski, B. Roessler and J. Beall, Solar Energy Mater. 2 (1980) 343. [6] J. Piekoszewski, J.J. Loferski, R. Beaulieu, J. Beall, B. Roessler and J. Shewchun, Solar Energy Mater. 2 (1980) 363. [7] J.J. Loferski, in: Proc. 1st Intern. Workshop on the Physics of Semiconductor Devices, 1981, New Delhi, p. 408. [8] L. Castaner, Second Quarterly Technical Progress Report, SERf Contract SERI-XI-98012-1. Period May 1979-August 1979. Report date October 1979. Brown University, Division of Engineering, Providence, RI. [9] I. Martil, G. Gonzalez Diaz and F. Sanchez-Quesada, Thin Solid Films 90 (1983) 253. [10] C.W. Bates and J.R. Mooney, Thin Solid Films 88 (1982) 279.