Preparation and characterization of close-spaced vapour transport thin films of ZnSe for heterojunction solar cells

Thm Solid Fdms, 127 (1985) 9-27

9

ELECTRONICS AND OPTICS

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 CLOSE-SPACED V A P O U R T R A N S P O R T T H I N FILMS O F ZnSe F O R H E T E R O J U N C T I O N SOLAR CELLS J. M. PAWLIKOWSKI* Institute of Energy Converston, Unwerstty of Delaware, Newark, DE 19711 (U S A ) (Recewed June 27, 1983, accepted September 4, 1984)

Thin films of ZnSe were deposited onto several types of substrates, including glass coated with indium tin oxide and semitransparent metal layers, using the closespaced vapour transport (CSVT) method. The ZnSe film parameters such as crystal orientation, resistivity, photoluminescence and absorption spectra, and photogeneration efficiency were investigated, and the effects of the deposition conditions, doping and post-deposition heat treatment on these parameters were determined. The ZnSe films were found to be polycrystalline with (111) preferred orientation independent of the substrate used. The dark resistivity of the films was reduced by about three orders of magnitude to approximately 1000 D cm by post-deposition annealing. It is suggested that zinc vacancies and copper residual impurities acting as acceptors are the main recombination centres and are responsible for compensation effects in the films. The fundamental absorption edge was found to be at about 2.67 eV and the values of the absorption coefficient above the absorption edge were established. Doping had a detectable effect on the absorption curves and the photoluminescence spectra. Spectral photoresponse curves computed for the ZnSe films using the absorption coefficient values found here seem to be in a qualitative agreement with the experimental data. Finally, application of the ZnSe thin films to ZnSe-Zn3P2 n - p heterojunction solar cells is discussed.

1. INTRODUCTION Increasing attention has recently been paid to the development and application of various photoelectric conversion devices. Zinc selenide (ZnSe), which is a I I - V I compound with a crystal structure of the cubic zinc blende type, a direct band gap of 2.67 eV, an electron affinity of 4.09 eV and an electron mobility of about 530 cm 2 V - 1 s- 1 at room temperature 1, has been found to be a very promising material for optoelectronic devices such as light-emitting diodes 2. It can also be used in heterojunction solar cells as a "window" material z. Zinc phosphide (Zn3P2), which has a direct energy gap of about 1.51 eV at 3 0 0 K 3 and an electron diffusion length o! about 5-101,tm a, has also been * Present address Inshtute of Physics, Technical Umverslty of Wroc|aw, Wroclaw, Poland 0040-6090/85/$3 30

© Elsevier Sequoia/Printed in The Netherlands

l0

J M. PAWLIKOWSKI

investigated as a promising solar cell material However, the best parameters obtained to date in Mg/Zn3P 2 configurations s are below expectatmns. Therefore an approprmte heterojunction partner for ZnsP 2 must be sought. The parameters of these semiconductors, particularly the electron affinity values, suggest that ZnSe is a promising n-type partner for Zn 3Pz; initial results 6 confirmed this expectation. In this paper we review previous approaches to the deposition of n-type ZnSe films (Section 2.1) and describe in detail our method of dealing with this problem (Secnon 2.2) Our search for appropriate light-transparent conductive substrates is discussed in Section 3 together with the properties of the thin films obtained. The effects of the deposition conditions, doping and post-deposition heat treatment on the parameters of the ZnSe films were investigated by means of X-ray diffraction examination and electrical, optical and photoelectric measurements. Finally, comments on the application of ZnSe thin films to the ZnSe/Zn3P2 heterojunctlon structure are presented in Section 4. 2.

EXPERIMENTAL DETAILS

2.1. Review of deposttton methods for ZnSe thm films Many approaches to the growth of ZnSe thin films have been used and apparent difficulty in obtaining the n-type high conductivity film has often been cited. Among the deposition methods for ZnSe thin films, the following are the most frequently used' close-spaced vapour transport (CSVT), liquid phase epitaxy (LPE), molecular beam epltaxy (MBE), organometallic chemical vapour deposition (OMCVD), physical vapour deposition (PVD), 1.e. thermal evaporation in vacuum, and the gas transport (GT) method. They are reviewed briefly below on the basis of some representative papers 7 33. Single-crystal GaAs substrates have most frequently been used 7's'l°'la in the CSVT method v 11 and G a P 9 GaAs ° 6p ° 4 7, germanium v and silicon 7 substrates have also been employed. Substrate temperatures were typically maintained in the range 998-1058 K with the source at 1133-1273 K. Zinc was applied I°'I1 from a second source during the deposition and low resistivity films (of the order of 1 f2 cm) ° 1~ were obtained, presumably as a result of gallium doping from the substrate. In the L PE method 12 16 ZnSe 1s ZnSe: AI 16 and ( 111 ), (110) and (100) surfaces of ZnSx Sel x (0 <~ x ~< 1) single crystals doped with galhuml 2,13 have been used as substrates. Substrate temperatures were in the range 1123 1323 K and layers of low resistivity (several ohm centlmetres) were obtained with/~e = 5-100 c m 2 V I s I and n = 1016_ 1017 c m - 3 ; again, gallium was successfully used as a dopant 12. L PE was also attempted using ZnTe(110) single crystals 14 as the substrate, but the results of optical and electrical measurements showed that a high resistivity solid solution layer was formed at the interface In the MBE method 17 22 only GaAs substrates with (100) 17 21, (110)22 and (111) 19.20 surfaces have been used. Gallium from an additional source was used as an intentional d o p a n 0 9 21 and resistlvities as low as 0.07 f~ cm were obtained 19'z°. In this method substrate temperatures can be as low as 573-673 K. Single-crystal layers about 3 gm thick were obtained at 633 K and operation of the gallium source at 748 K resulted in a resistivity of 0.05 fl cm 21

ZnSe

THIN FILMS FOR HETEROJUNCTION SOLAR CELLS

11

In the O M C V D method 23-26 spinel, sapphire and BeO at 1023 K were initially used as substrates23; GaAs(100) single crystals were employed in subsequent investigations 24"26. ZnSe layers of n type with a claimed room temperature resistivity of 0.05 f~ cm were grown on GaAs at 613-623 K using triethylalumlnium as the dopant. An electron mobility of up to 400cm 2 V -1 s 1 and n ~ l017 cm -3 were obtained. However, a high degree of donor compensation was observed. In the PVD method 27-29 Ge(100) with an oxygen monolayer on the surface 27, p-type GaAs and CdTe 28 and (001), (110) and (11 l) surfaces of NaC1 29 have been used as substrates. Post-deposition treatment of layers for 24 h at 773 K in zinc vapour (the layers were already doped with gallium, indium or zinc) resulted in low resistlvlties of the order of several ohm centimetres 28. In the G T method 3° 33 GaAs(111) 3o, sapphire 31 (100) surfaces of n-type GaAs and GaAso 6Po 4 32, and GaAs(100) 33 were used as substrates. Hydrogen was used as both the transport and reactive gas 3°'31. Cathodoluminescence and photoluminescence studies indicated a deviation from stolchiometry and/or the presence of copper impurity centres 3°, and the room temperature resistivity was about 10 f~ cm 29 o r 10 4 ~ cm 31 The contribution from trace impurities to the electronic properties was established 33. Very high growth rate was achieved by focusing the vapour flux at the exit 32, and a resistivity of about 10 f~ cm was obtained at substrate temperatures of 873-973 K. It should be mentioned that the ion-plating 34 and photodeposltion 3s methods have also been used to deposit ZnSe films. It can be concluded that good results (particularly with respect to the properties and high conductivity of ZnSe thin films) were obtained when the CSVT technique was used. However, it is essential to use single-crystal gallium compounds as substrates. Moreover, the substrate orientation was found to have an effect on the resistivity Presumably gallium diffusing from the GaAs substrate and/or the vapour phase plays an important role as a dopant. Aluminium and indium have also been used as donor-type dopants. It should be noted that this type of substrate requires a higher substrate temperature during deposition which usually helps to improve the crystalhnity of the layer. The presence of an additional zinc vapour source during deposition was found to be an important factor in reducing the layer resistivity, since the zinc vacancy In ZnSe is a double acceptor with a singly ionized level near the valence band and a doubly ionized level higher in the band gap 36"37. Recently, it has also become clear that the properties of ZnSe are strongly influenced by trace impurities in the form of both shallow donors and shallow acceptors T M .

2.2. Deposition of ZnSe thm films The CSVT method was chosen for this work on the basis of the literature survey (see Section 2.1) as well as because of the main aim, i.e. the preparation of a ZnSe-Zn3 P2 heterojunction, since Zn3 P Edeposition by means of the CSVT method is already established 39. Low temperature (LT) and high temperature (HT) variants of the growth technique were employed. The arrangement used is shown schematically in Fig. 1. In the LT method (Fig. l(a)) the substrate temperature can be as low as 623 K for a source temperature of 1123 K and a source-to-substrate distance of 10 ram. The set-

12

J. M. PAWLIKOWSKI

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Fig 1 The quartz system used to deposit the ZnSe thin films by means of the CSVT method in (a) the LT, (b) the HT and (c) the high temperature double-deposmon modes a,a', quartz rods, b,b', thermocouples, c, c', graphite blocks, d, ZnSe source, d', ZnsP 2 source, e, e', tantalum screens; f, f', mica screens, g, tantalum multlhole screen, h, substrate u p s h o w n in Fig. l(a) was specifically d e s i g n e d for Z n S e d e p o s i t i o n o n t o Z n a P 2 wafers w h i c h s h o u l d be k e p t at a r e a s o n a b l y low t e m p e r a t u r e . I n the H T m e t h o d (Figs. l(b) a n d l(c)) b o t h the s o u r c e a n d the s u b s t r a t e t e m p e r a t u r e c a n be m a i n t a i n e d in a wide r a n g e of v a l u e s u p to a b o u t 1273 K. T h e a r r a n g e m e n t s h o w n i n Fig. l(c) was a p p l i e d to d e p o s i t b o t h Z n S e a n d Z n a P 2 films m the s a m e c h a m b e r w i t h o u t b r e a k i n g the v a c u u m a n d w i t h as s h o r t a t i m e as p o s s i b l e b e t w e e n the d e p o s i t i o n s . A n a d d i t i o n a l zinc s o u r c e was i n c l u d e d in the r e a c t i o n c h a m b e r to p r o v i d e the

ZnSe

THIN FILMS FOR HETEROJUNCTION SOLAR CELLS

13

excess zinc and to modify the zinc partial pressure during the growth of the ZnSe layers. In some deposition series additional aluminium or gallium shot was placed in the ZnSe source to dope the layer being grown. Hydrogen at various flow velocities was used as the carrier gas for ZnSe deposition and argon was used for Zn3P 2 deposition in the system shown in Fig. l(c). In the H T method a flow baffle was employed to provide more uniform hydrogen or argon flow at higher pressures. The starting materials were as follows: ZnSe powder (purity, 99.999~o), alumlnium shot (purity, 99.999~o) and gallium chunks (purity, 99.999~o), all supplied by Alfa Division (Danvers, MA), and zinc shot (purity, 99.9999~o) obtained from the United Mineral and Chemical Corporation (Hudson, NY). ZnSe films were deposited onto the following substrates: (1) glass (Corning 7059); (2) glass covered with an evaporated or r.f.-sputtered alumlmum thin film; (3) glass covered with an r.f.-sputtered indium tin oxide (ITO) thin film followed by a sputtered aluminium thin film; (4) glass covered with a sputtered ITO thin film; (5) glass covered with a sputtered aluminlum thin film followed by an evaporated zinc thin film; (6) alumlnium foil (Clecon Metals, Cleveland, OH); (7) bulk ZnaP 2 wafer. Mica covered with ITO was also used as a substrate at high substrate temperatures (873-973 K) but the results were poor; the ZnSe layers frequently peeled off on exposure to air. All but three substrates ((1), (6) and (7)) were pretreated prior to ZnSe deposition: the glass substrates (1) were cleaned in a detergent and washed with delonized water and methanol, the alumlnlum substrates (6) were polished mechanically, etched with 5~o K O H and washed with deionized water, and the Zn3P 2 substrates (7) were polished mechanically, etched with 1~o bromine solution in methanol and washed with methanol. The glass substrates (1) were used in determinations of the crystal orientation and resistivity of the films as well as in measurements of absorption and photoluminescence spectra. The other substrates were used in the preparation of thin film ZnSe/Zn3P2 heterojunctlons. The main parameters of the deposition process were as follows: (a) the source temperature T~; (b) the substrate temperature Td; (c) the deposition time z; (d) the distance d between the source and the substrate; (e) the pressures Pr, PH and PA of the residual gases, the hydrogen or the argon respectively during a deposition These parameters fell within the following ranges: T~ = 1043-1223 K; Ta = 603-843 K, z = 8-310 min, d = 8-12 mm, pr = 35-60 mTorr, PH = 82-450 mTorr and PA ~ 760 Torr. The deposition parameters of some ZnSe films are given in Table I. The post-deposition heat treatment of the films was performed under equihbrium conditions at temperatures in the range of 673-773 K for times ranging from 20 min to several hours. The films together with some zinc shot were sealed in fused silica capsules under a vacuum of about 10 -6 Torr. The annealing temperature Ta was restricted by the substrate used and, in the case of ITO-coated glass, by the properties of ITO 3.

RESULTS AND DISCUSSION

Because of the main aim of this work, i.e. the development of a thin film technology for the ZnSe window of a thin film ZnSe/ZnaP 2 heterojunction solar cell, the search for an appropriate substrate onto which to deposit the ZnSe films was

14

J. M. PAWLIKOWSKI

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THIN FILMS FOR HETEROJUNCTION SOLAR CELLS

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hmited (except for Zn3P2) to highly conductive materials which were as transparent as possible to hght. Obviously this requirement excluded the single-crystal GaAs wafers employed previously (see Section 2.1). Glass or quartz plates covered with a layer o f l T O seemed to be a natural backcontact material for ZnSe/Zn3P2 devices. However, it was found that ITO only provided good ohmic contact with ZnSe when Td did not exceed about 773 K during deposition. At higher values of Td the resistivity of ITO increased and it seemed to produce a rectifying junction with ZnSe. It was also found that ITO tended to produce non-ohmic contacts with ZnSe (or even damaged the film) when the ZnSe/ ITO sandwich was annealed at Ta >~ 773 K after ZnSe deposition. Semitransparent alumlnium or any combination of semitransparent alumimum with ITO and semitransparent zinc layers was investigated because it also had the advantage of being an n-type dopant. The reaction which occurred between alummmm and ZnSe during the deposition process resulted in an increasing number of pinholes in the aluminium layer. Relatively good ohmic contact between alumlnium and ZnSe was found; however, it was accompanied by relatively high optical losses when devices were illuminated from the aluminium side. Zn3P 2 wafers are natural substrates for studies of the ZnSe/ZnaP 2 heterojunction interface. However, the highest substrate temperature that could be applied was strongly limited (Td < 673 K) owing to possible diffusion of phosphorus atoms and/or dopants from Zn3P 2 into the growing layers. Despite the disadvantages mentioned above, the ZnSe films with the best overall qualities for heterojunction apphcations were obtained on the A1/glass and ITO/glass substrates. Uniform flat pinhole-free layers of ZnSe with a relatively large-grain polycrystalline structure were deposited. Figure 2 shows such a film deposited onto a glass substrate. Some of the parameters of these films are discussed below.

Fig 2. ZnSe film BJ-217on glass observedusing reflectedhght 3.1. Deposition rate

The film thickness was measured using a Dektak surface profile measuring system (Sloan) with a glass substrate as the reference. The thicknesses were generally

16

J.M. PAWLIKOWSKI

in the range 0.1-5 Ixm. The deposition rate calculated on the basis of the total deposition time ranged from about 0.5 to 50 nm min-1, depending on the source temperature. The dependence of the deposition rate on other deposition conditions was also studied and the following conclusions were drawn. (i) The deposition rate was weakly dependent on the deposition time in the range 90-235 min, but it increased more rapidly at longer times. This was probably due to the fact that some time was needed to establish the deposition conditions, in particular the rate of evaporation from the source and the ZnSe vapour pressure. (ii) The deposition rate was weakly dependent on the substrate temperature Td: for Td In the range 623-723 K the rate decreased with increasing Td, whereas no relation could be established for Td in the range 723-813 K. (lil) No correlation was observed between the deposition rate and the hydrogen pressure m the range 84-450 m T o r r or between the deposition rate and the presence (or absence) of excess zinc vapour during deposition.

3 2 Crystalortentatton X-ray diffraction analysis was performed to establish the preferred crystal orientations of the as-grown ZnSe thin films. X-ray scans were made using Cu K s radiation (2(~1) = 1.540 51 ~ and 2(Ctz) = 1.544 39 ~). These peaks were resolved by making a high resolution scan (0.04 ° c m - 1)4o. The crystallographic order of the ZnSe thin films was found to be reasonable for all substrate types. The best order was observed for films deposited onto metal-covered substrates, particularly alumlnium-coated glass. The m a m diffraction peak corresponded to the (111) orientation and was consistently dominant in all the samples measured. Peaks corresponding to (311) and (220) orientations were also frequently observed. In addition, peaks corresponding to the (422), (331), (511) and (400) orientations were observed occasionally. The relative intensities of the three main diffraction peaks are listed m Table II for the three different substrate types. 3.3. Reslstwity The resistivity of the ZnSe films on glass substrates was measured using the T A B L E II RATIO OF THE INTENSITIESOF THE DIFFRACTION PEAKS FOR SOME ZnSe THIN FILMS

Sample

Type of substrate

Relattve mtenslty (° o) (111)

(220)

(311)

B J-27 B J-216 B J-217

Glass

100 100 100

11 2 0a

20 4 4

B J-71 B J-72

A1

100 100

0" 0a

0a 22

BJ-324 BJ-328 BJ-344

AI thin film on glass

100 100 100

1 3 5

9 5 11

a Below the hmxts of d e t e c t a b l h t y

ZnSe

THIN FILMS FOR HETEROJUNCTION SOLAR CELLS

17

four-point probe method and was also estimated from the current-voltage characteristics of Au/ZnSe Schottky-type devices and ZnSe/Zn3P2 heterojunctions. R.f.-sputtered aluminium was used as the ohmic contact with ZnSe in the four-point probe measurements. The resistivity of as-grown films deposited by the LT method was high (p > 105 f~ cm under room illumination) and was independent of both the type of substrate used and the deposition conditions. Deposition of the films from a ZnSe:A1 or ZnSe: Ga source in the presence of zinc vapour (provided by an extra source) by means of the H T method did not improve the film conductivity substantially (the resistivity values were scattered around 105 f~ cm). It has recently been found that the optical and electrical properties of ZnSe are not dominated by native Schottky-type defects, and evidence for the important role of both shallow donors and shallow acceptors from trace impurities has been provided 2. Hence, intentional doping to obtain low resistivity n-type ZnSe has to be applied together with special treatment to minimize the concentration of shallow and deep acceptors (e.g. zinc vacanoes), donor-vacancy complexes and distant donor-acceptor pairs. The high values of the resistivity of our films may indicate insufficient doping a n d / o r non-effective minimization of the defect and impurity contributions. However, it was found in subsequent work that even the presence of large amounts (more than 1 at.~) of a well-known donor (gallium) in the films does not reduce the resistivity substantially. This may indicate the dominant role of zinc vacancies and/or trace impurities m controlling the resistivity of the films. Hence we used post-deposition heat treatment (see Section 2.2) under equilibrium conditions to reduce the film resistivity by compensating for the zinc vacancies. The resistivity of the ZnSe films deposited by the LT method was reduced, but only by an insignificant amount. This result was independent of the type of substrate. However, annealing the films grown by the H T method (in the presence of excess zinc and doped with alummlum) reduced the resistivity by up to three orders of magnitude. In the best (although accidental) case a resistivity as low as about 800 ~ cm in the dark and about 20 f2 cm under simulated air mass 1 illumination (87.5 mW c m - 2) was observed in the aluminium-doped ZnSe films. The defect chemistry model and the self-compensation effect have been used to explain the changes produced in the net electron concentration and resistivity of ZnSe:AI and Z n S e : G a samples by heat treatment in zinc vapour under equilibrium conditions 41. On the basis of the data collected in ref. 41 we can expect the net electron concentration [ e - ] in the ZnSe:AI film to be much higher than that in the ZnSe: Ga film after equilibrium and cooling when both films are highly doped (more than 1019cm-3). At low doping (less than 101acm -3) the difference can be negligible. However, the appropriate annealing conditions are required in all cases and can be computed from the data of ref. 41. The calculation indicates that the postdeposition equilibration should last about 400 days or more when it is performed at 723-773 K (see Section 2.2). Therefore, either the annealing temperature should be higher than 773 K or the annealing time should be much longer than several hours to provide the appropriate conditions for the process. However, these conditions could not be apphed; glass substrates cannot be annealed at temperatures above about 793 K and annealing times of about 400 days are unacceptable. This seems to

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J. M. PAWLIKOWSKI

be one of the major reasons why annealing is only partially successful in decreasing the film resistivity. The polycrystalline structure of the films may be another reason for the high resistivity.

3.4. Photolummescence spectra M a n y papers on the luminescence of ZnSe have been published (e.g. refs. 7, 13-15, 17, 18, 21, 24, 30, 33, 38, 42-50) since its light-emitting properties were first discovered. Luminescence is a very useful tool for investigating the doping processes in ZnSe since m a n y features of the ZnSe emission spectra have been attributed to impurities and native defects acting as donor and/or acceptor centres. Photolumlnescence (PL) measurements were performed at 7 7 K using a P e r k i n - E l m e r M P F 44 fluorescence spectrophotometer with the entrance set at )~ = 380 nm and a xenon lamp as the exciting light source. A photomultiplier equipped with a P e r k i n - E l m e r 561 recorder was used to record the data. The PL spectra of some samples were also investigated using the 512 nm line of a high power argon ion laser with a silicon detector and a conventional lock-in system to detect the response 51. In both cases a set of filters was used to check the correctness and repeatability of the recorded plots. The experimental results of the PL measurements of films deposited using the CSVT method are shown in Fig. 3 (curves a, b, and c). The PL spectrum of a physically vapour-deposlted ZnSe film 5z is also shown (curve d). The photoluminescence of the ZnSe powder used as the source material and sublimed ZnSe powder doped with CuC1 (curves e and f respectively) was also measured 51 for comparison with the spectra of the films deposited by CSVT. Also shown in Fig. 3 is the spectrum of a ZnSe single crystal (Eagle-Picher) (curve g) (in fact, each slab consists of a basic grain and several peripheral grains). The main broad band in the region 500-750 nm was observed in all the samples. However, its detailed position was found to depend on the origin of the samples. An emission at 555-560 nm (about 2.22 eV) was found in the spectra of source material sublimed and doped with CuC1 and was also a component of the broad emission of films deposited by CSVT in the presence of excess zinc. Since it is dominant in CuCl-doped material and is similar to the so-called copper green emission, it is suggested that its origin can be ascribed to donor-acceptor transition luminescence involving copper related states. The dominant donors involved could be gallium, chlorine and bromine (all of which are commonly present in ZnSe) and the aluminium dopant. The energy positions of both the D z , donors (aluminium and gallium) and the Dse donors (bromine and chlorine) lie 2 0 - 3 0 m e V below the conduction band. Cuzn and the recently proposed53 neutral hole-binding complexes CUzn-Clse + or C u z . - C u , + could act as acceptor states. The emission from the source material and from aluminlum-doped films deposited by CSVT in the presence of excess zinc has a peak at about 610 nm (about 2.03 eV), while that from films deposited by CSVT which have not been intentionally doped and from physically vapour-deposited films has a peak at 620-630 nm (about 2.0 eV). The first emission is similar to the well-known self-activated luminescence (SAL), and the second is similar to copper red luminescence. D o n o r - a c c e p t o r emission revolving (1) a "self-activated" centre related to a zinc vacancy (probably a

ZnSe THIN FILMS FOR HETEROJUNCTION SOLAR CELLS

25

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600 700 800 WAVELENGTH ( n m ) Fig 3. PL spectra of CSVT-deposlted ZnSe film BJ-21 (curve a), post-deposiuon-annealed CSVTdepomted ZnSe film BJ-22a (curve b), CSVT-deposited ZnSe film BJ-217 (curve c), phymcaily vapourdeposited ZnSe' Ga film 600081 (curve d), ZnSe source material (curve e), ZnSe source material subhmed and doped with CuCl (curve f) and a ZnSe single crystal (Eagle-Picher) (curve g). Curves e and g are raised to slmphfy the figure. The reset shows the low energy PL band

G a z n - V z , or VZn2 - - D ÷ acceptor complex) in the first case and (ii) a Cu+-related level or compensated substitutional-copper atom associated with a selenium vacancy in the second case. The Cu-Vse association is favoured by the possibility that a number of dissociated selenium atoms are reduced by hydrogen during the deposition. It should be noted that the separation between the peaks at 610 nm and at 620-630 nm is not very much less than the total error of the wavelength scale; therefore this separation (although unambiguous) cannot be established firmly. Moreover, the differences between the growth conditions usually produce different defect densities and therefore different numbers of impurity imperfections and differences in the formation of active complexes. Hence it is possible that the two emission mechanisms discussed above are superimposed. Nevertheless, the luminescence properties of CSVT-grown ZnSe films at wavelengths in the range 600-640 nm are mainly due to the copper residual impurity and "self-activated" defect centres. The low energy PL band seen in the CSVT-grown films around 910 nm (see inset in Fig. 3) possesses a lower intensity and was not observed in the films grown in

20

J. M. PAWLIKOWSKI

the presence of excess zinc. This band was found in the Eagle-Picher samples but was not detectable in the PL spectra of either the ZnSe source powder or the ZnSe:CuC1 powder samples sx. Post-deposition anneahng of CSVT-grown films in zinc vapour resulted in suppression of the 910 nm band relatwe to the main peak at 620-630 nm (the ratio of the intensity before annealing to that after annealing changed noticeably). A more pronounced effect was observed in the Eagle-Picher samples m which the 910 nm band was not found after heat treatment in molten zinc. Any assignment of this emission to known impurity centres in ZnSe could only be highly speculative, but it should be noted that the presence of zinc during both the deposition and post-deposmon anneahng was essential for this emission to be suppressed The results of the effect of post-deposition treatment on the main peak of the PL spectra were inconclusive.

3.5 Absorptton coeffictent and optical band gap The absorption coefficient of the ZnSe thin films was investigated at 300 K primarily to establish the transmittivity of the films for device applications and to detect any possible dependence of this property on the method of preparation. Measurements of the optical absorptivity A, where - A = log T and T is the transmittiwty of the medium, were made at 300 K for wavelengths in the range 0.3-0.85 ~tm using a Cary 14 spectrophotometer. In some cases the measurements were also performed using a Perkln-Elmer 330 spectrophotometer equipped with an integrating sphere. The absorption coefficient ~ was calculated using the equations for light passing normally through (1) a thin plane-parallel film including multiple reflection and interference effects in the region of interference fringes 54, (2) a thick absorbing film including multiple reflections and neglecting interference effects 54, (3) a thick strongly absorbing film neglecting multiple reflections 54 and (4) a thin absorbing film including multiple reflections and light-scattering and absorption effects other than those caused by band-to-band transitions 5s. A detailed discussion of the determination of the absorption coefficient of semiconducting thin films is given elsewhere 55. The absorptlwties of 27 CSVT-grown ZnSe films with thicknesses ranging from 0.17 to 4.8 ~tm were measured. The results.calculated with multiple reflections included are shown in Fig. 4. Four ZnSe films with thicknesses covering the entire range investigated were chosen as examples. The shapes of the ~(hog) plots for the other films are similar to those shown m Fig. 4 but the values ofct depend on the film thicknesses. High values of the absorption coefficient below the fundamental absorption edge were found for all the films investigated. This effect is frequently observed in thin semiconductor films and is mainly attributed to scattering losses on the surface. Differences between the values of ct computed using the various methods listed above for the same film were found to be higher than the experimental error when calculated below the absorption edge and approximately equal to the error when calculated above the absorption edge (the total error in 0t was due mainly to the thickness measurements and did not exceed 20% over the whole spectrum). The ct(h~o) plots including light scattering and other losses were found to have the lowest

ZnSe

21

THIN FILMS FOR HETEROJUNCTION SOLAR CELLS

values and to show the least dependence on the film thickness; they are believed to be close to the true ~(hco) plots of the ZnSe films investigated. Examples of ~(hco) spectra of ZnSe films which have been corrected for optical losses are shown in Fig. 5. They were used as the basis for estimating the energy band gap Eg and the absorption coefficient ~g (Section 3.6). Since ZnSe is a direct band gap semiconductor, the fundamental absorption edge has been fitted using the wellknown formula 54 for direct interband transitions (in the simple parabolic two-band model) in the absence of electron-hole interactions:

e2(2mr)3/2

inc,v(0) 12(hco_ Eg)Z/2

(1)

cxhco = 6nmonh2 ceo

He,v(0 ) is the optical matrix element at k = 0 and is given in the Kane energy band model by

Ino v(0)l 2 = m_~2zp2 '

(2)

h

where P is an element of the momentum matrix, n is the refractive index, m o is the free-electron mass and m,- 1 = me- t + mh- 1 (me and mh are the effective masses of the densities of states in the conduction and valence bands respectively). The other WAVELENGTH (#m) 045 O4

05 I

T

I

I

WAVELENGTH (,urn) 045

04

i

ooooooOOO~OOo°~

E u ¢1

~ - ~ - ~ieeoeelee~oe° 105

/~

105

eoeoeI~°

i

uE o

104

oel 104

oo

o i

24

26

28

30 32 ~oJ ( e V )

34 -

103

5

I 27

2t9

:3I I ~ u (eV)

313

5

Fig. 4. A b s o r p t i o n coefficients of the Z n S e films BJ-12 (curve a), BJ-21 (curve b), BJ-22 (curve c) and BJ-217 (curve d) at 300 K. Fig. 5. A b s o r p U o n coefficients of the Z n S e films BJ-12 (O) and BJ-24 and BJ-219 (@) at 300 K after c o r r e c t i o n for optical l o s s e s - - , , theoretical plots (see text for detads)

22

J. M. PAWLIKOWSKI

symbols have their usual meanings. This simple a p p r o a c h has been widely used to estimate Eg to a first approximation. The best fit of eqn. (1) to the fundamental part of the absorption edge was obtained for Eg = 2.67 eV and P = 7 x 10- 8 eV cm (full curve in Fig. 5). A slightly better fit above the absorption edge was found for E g = 2.65eV and P = 6xl0-SeVcm (broken curve in Fig. 5); however, these parameters gave a p o o r e r fit to the fundamental edge. The following data were used to fit eqn. (1) to the experimental results: n = 2.7 (refs 56 and 57), m, = 0 . 1 7 m o (ref. 58) and mh = mhh = 1.44m o (ref. 59). A p o o r fit above the absorption edge is usually expected when the simple t w o - b a n d model is used in the absence of C o u l o m b attraction. The step in c~ hear ho9 = 3.1 eV seen on the experimental curves can be ascribed to transitions from the second valence level which is split by s p i n - o r b i t interactions. The value of Aso = 3.1 --2.67 eV = 0.43 eV is in g o o d agreement with Aso = 0.43 eV reported in ref. 60. Also, an optically estimated value of 2.56 eV for Eg at 300 K was p r o p o s e d some years ago 56 and has been reported recently 61. A visible change in the absorption spectrum as a result of the post-deposition heat treatment of the films in a zinc atmosphere was observed. This is shown in Fig. 6 for a film 0.41 lam thick annealed at 673 K for 1 h. The following changes were found in the ~(hco) plots after annealing: (1) a m u c h higher level of "as-measured" WAVELENGTH (,urn) 045 04

05

io5 _

I

I

t

t

~

•

oo~OOoOOooOO

I05 $

u a

i

I

/

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035

I

7

8 o

6

N

**o e°

10 4

I

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00

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2,5

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29 ~ (eV)

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Flg 6 AbsorpUon coeflicnent of the ZnSe film BJ-21 at 300 K before (O, before anneahng, O, after anneahng) and after Onset (--~, before anneahng, - - - , after anneahng)) correction for optncal losses

ZnSe

THIN FILMS FOR HETEROJUNCTION SOLAR CELLS

23

absorption below the absorption edge, (2) a slower rise of the fundamental part of the absorption edge and (3) a lower level of "as-measured" absorption above the absorption edge. However, these changes were minimized when the data were corrected for optical losses at the surface (see Fig. 6, reset). Therefore effect (1) is mainly due to surface changes induced by annealing. However, effects (2) and (3) are still visible after correction. They could be due to a higher doping level and/or the appearance of a second phase (e.g. ZnO which has occasionally been detected in annealed ZnSe films by means of Auger analysis). Although the magnitudes of the changes in the ~(h~o) spectra vary from one film to another, the trend shown in Fig. 6 seems to be typical for the CSVT-grown ZnSe films investigated to date.

3.6. Photogeneratlon efficiency A simple model was used to estimate the theoretical photoresponse of the ZnSe film. The following assumptions were made. (i) The rate of generation of free electron-hole pairs is given by

G(x,h~) = Io~g(hog)exp{-x~a(h~)}

(3)

where Io = I(1--R), R is the reflection coefficient of ZnSe, I is the intensity of the radiation illuminating the surface, and an and % are the absorption coefficients ascribed to total absorptivity (attenuation of hght) and effective absorptivity (generation of free current carriers) respectively and are obtained experimentally as the absorption coefficients before and after correction for optical losses respectively. (ii) The quantum efficiency of the internal photoprocesses is equal to unity. (iii) Photon losses in the metal (contact) layers are not included in the computations because they are almost independent of wavelength. (iv) The film thickness is assumed to be equal to the width of the built-in field (about 1 ~tm), and the collection efficiency of the built-in field region is equal to unity. The results of computations under air mass 1 conditions (87.5 mW c m - z ) are shown in Fig. 7 as the number of free electron-hole pairs per unit time per unit volume. The set of curves was obtained for various locations of the generation area inside the sample. The photoresponse decreases substantially as a function of the distance x from the illuminated surface, even in the vicinity ofx = 1 I.tm, owing to the rapid attenuation of the light. The photoresponse of the ZnSe thin films was measured as a part of that of thin film n-ZnSe/p-Zn3P2 heterojunctions TM. It was possible to separate the photoresponse of ZnSe from the whole spectrum of the heterojunction because of the relatively large difference between the energy band gaps of the two semiconductors and the low photoresponse of Zn3P 2 itself (particularly when illumination is from the ZnSe side). The theoretical plot is compared with the measured photoresponse spectrum in Fig. 8. The theoretical photoresponse was computed by integrating G(x, h~) over a layer 1 Ixm thick with an effective area of 0.8 mm 2 to fit the experimental data. When the simplicity of the model and the possible experimental errors are taken into account it can be seen that there is a good quahtative match between the theoretical and experimental curves m Fig. 8. The abrupt drop exhibited by both curves on the long wavelength side is due to the absence of carrier generation mechanisms below

24

J. M. P A W L I K O W S K I

ENEROYIeV)

iOiZ

35

25

30

ENERGY ( eV )

{Eg 16

z o

3S

30

25 i

14

//

/I

/ I

L/'/:

iOtO

12 i

109

5

I08

I0

8

6

/

/I

I I

111111 F /

0

/

//

I

//'/

z

o

4.

::°'-

o. '~

l ~7

,o, 105 03

I

d 04 05 WAVELENGTH (pro)

2

0

i 04

o5

06

WAVELENGTH (/,~m)

Fxg 7 Spectral plots of the photoresponse calculated for a Z n S e film at 300 K the generation space (1 m m 2 in area and 10 n m thick) is assumed to be located 100 nm (curve a), 300 n m (curve b), 500 n m (curve c), 1000 n m (curve d) a n d 3000 n m (curve e) below the dlummated surface, - -, as for curve a but with cq = ctg Fig. 8 Comparison between the theoretical ( - - ) and experimental (- - -) photoresponse of a ZnSe film at 300 K The experimental data are for the sample BJ-47 with an actlve area of 0 8 m m 2 and the configuration A u / Z n 3P 2 / Z n S e / I T O / g l a s s 62

the fundamental absorption edge (about 0.465 i,tm), and the gradual drop on the short wavelength side is due to the strong absorption of photons with hco > Eg in the vicinity of the surface. The high recombination rate of the free carriers generated close to the surface was not included in the model and seems to be the main cause of the difference between the theoretical and experimental plots. 4.

F I N A L COMMENTS

Thin films of ZnSe deposited by the CSVT method onto plain glass or glass coated with a conducting layer were found to be polycrystalhne with a strongly preferred (111) orientation. The films with the best qualities for heterojunction applications (i.e. suitable structure, good surface adhesion, absence of pinholes, smooth surface etc.) were obtained using the HT method (Fig. l(b)) with alumimumand ITO-coated glass substrates. It was found that the growth of good quahty films was favoured by relatively low deposition rates (0.5-50 nm m i n - 1). The value of the band gap and features of the PL spectra were found to be similar to those reported previously for bulk ZnSe.

ZnSe

THIN FILMS FOR HETEROJUNCTION SOLAR CELLS

25

The deposition of low resistivity films onto the substrates employed here by means of the CSVT method presented some problems. Undoped ZnSe films were expected to be highly resistive regardless of crystal perfection; therefore the films were intentionally doped with well-known donors such as aluminium and gallium. However, it has recently been found 2 that the properties of ZnSe are not dominated by native defects alone and evidence has been presented for the important role of shallow donors and acceptors from trace impurities. Therefore special treatment must be used with intentional n-type doping to minimize the concentration of these impurities. The existence of at least one residual impurity (copper) seems to have been established in the ZnSe films discussed here, and no method of removing copper from ZnSe films has yet been developed. Also the presence of zinc vacancies and/or complexes related to zinc vacancies has been revealed by the P L investigations. Post-deposition heat treatment in zinc vapour was used to mimmize the effect of zinc vacancies/complexes and some reduction in film resistivity was observed. However, the results of annealing were not repeatable which indicated the differences in the electronic properties of the "as-grown" films and suggested that the problem of trace impurities was still unresolved. In addition, the conditions for postdeposition annealing were not favourable (Section 3.3) and thus the role of zinc vacancies could not be minimized effectively. Therefore the lowest dark resistivity obtained for the films was about 1000 ~ cm. It should also be noted that annealing tended to increase the surface roughness of the films which resulted in higher optical losses. A typical configuration of a heterojunction solar cell employing a ZnSe film as the n-t'ype window and a p-type semiconductor such as Zn3P 2 as the base material is Au/Zn3P2/ZnSe/A1 (or ITO), with the order of deposition being either Zn3P 2 on ZnSe or ZnSe on Zn3P2. The second arrangement is less convenient because in practice ZnSe films cannot be annealed after deposition onto a Zn3P 2 substrate (the annealing temperature must not exceed about 673 K). In the Zn3P2/ZnSe configuration, the best parameters obtained to date are as follows62: open-circuit voltages Voc of up to 0.84 V in devices deposited onto aluminium/glass substrates and up to 0.68 V in devices deposited onto ITO/glass substrates; short-circuit currents Isc of up to 0.8 mA cm -2 in the first case given above and up to 3.8 mA c m - 2 in the second case. The main reason for the low value of Isc was the small photocurrent output from ZnaP 2 which was due to (1) the low net electron concentration in ZnSe which resulted in an extensive barrier or depletion region on the ZnSe side and (2) the presumed high recombination at the ZnaP2-ZnSe interface. Improvement of the deposition and doping technology of n-type ZnSe films seems to be of great importance. ACKNOWLEDGMENTS

The author is extremely grateful to Dr. M. Bhushan, Mr. B. E. McCandless, Dr. H. L. Hwang, Dr. J. D. Meakin and Mrs. S. Moore for technical assistance with various aspects of this work. Valuable discussions with Dr. M. Bhushan are gratefully acknowledged. The experimental part of this work was done under Solar Energy Research Institute contract XE-2-02048-1.

26

J M PAWLIKOWSKI

REFERENCES

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M Bhushan, Appl Phys Lett,40(1982) 51

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ZnSe THIN FILMS FOR HETEROJUNCTION SOLAR CELLS

27

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