γ-In2Se3 thin films obtained by annealing sequentially evaporated In and Se layers in flowing argon

Vacuum/volume

Pergamon PII: s0042-207x(97)00094-8

48/number IO/pages 871 to 87811997 0 1997 Elsevier Science Ltd All rights reserved. Printed in Great Britain 0042-207x/97 $17.00+.00

y-ln,Se, thin films obtained by annealing sequentially evaporated In and Se layers in flowing argon M Emziane, S Marsillac, J Ouerfelli, J C Bern&de, R Le Ny, Equipe couches minces et materiaux G.P.S.E.-F.S. T. N., 2, rue de la Houssiniere, BP 92208, 44322, Nantes, cedex 3, France

nouveaux,

received 2 June 1997

Polycrystalline thin films of y-I&Se, were obtained by sequentially evaporating In and Se on glass substrates followed by an annealing in flowing argon. The obtained films were analyzed by X-ray diffraction, scanning electron microscopy, X-ray photoelectron spectroscopy and microprobe analysis and optical and photoconductive measurements were also made. The effects were studied of the deposition atomic ratio (R=[Se]/[lnll, the annealing temperature and duration on the film structure and properties. It was found that whatever the deposition conditions, the resultant films were homogeneous, stoichiometric and monophased. Textured films were obtained when the deposition atomic ratio R was 3/2 with annealing at 673 K for 30 min. 0 1997 Elsevier Science Ltd. All rights reserved

Introduction The layered semiconductors are of great interest because of photovoltaic properties arising from their particular structure.’ Among these semiconductors, indium selenide In,Se,, belonging to the III-VI compounds family, has been the subject of many recent studies.*-” In thin film form, the compound has valuable optical and electrical properties and is thus of interest for photovoltaic applications using low-cost solar cells.’ Several deposition and annealing techniques for producing thin films have been used by different research groups.4,8,‘0,‘2 All the foregoing techniques have advantages, and disadvantages such as the need for complex equipment,4 difficulties when controlling stoichiometry” and the coexistence of more than one compound and/or phase in the final product.4-h.‘3 In an earlier paper,2 we showed that single phase y-In,Se, films were obtained by post-annealing thin In/Se layers which had been sequentially deposited. The multi-layers were sealed in a Pyrex ampoule for annealing and it was found that film texture was very sensitive to the selenium pressure in the ampoule during the annealing. It was concluded that the problem existed to find the optimum Se- pressure to result in a suitable thickness of the outer Se layer. It also followed to optimize the process, both the temperature and the duration of the annealing. The aim of the present study is to describe a simple and more reproducible technique for obtaining 1;-In,Se, polycrystalline thin films by sequentially evaporating In and Se on glass substrates followed by an annealing in flowing Ar. Currently, annealing in flowing argon is widely used to obtain homogeneous and highly oriented thin films of different compounds’“” and it seems to give promising results, e.g. it has been

used in our laboratory to obtain MoS, photoconductive films,” although to prepare MoS, a high temperature (1073 K) was needed. In the present study, we show that textured photoconductive films of y-In,Se, are obtained at only T = 673 K. The purity of the argon used was 99.999 at%. Experimental Film preparation process Films were sequentially vacuum evaporated at a gas pressure of IO-+’ Pa, using two tungsten crucibles, and pure (99.999%) In and Se. Clean soda-lime glass substrates were used with film deposition at room tamperature. The glass substrates were cleaned chemically and by ultrasound. The number of layers varied from 6 to 12 for each constituent. The thickness of the films as well as the evaporation rate, 0.3 rim/s for In and 0.5 rim/s for Se, were controlled by a quartz crystal thickness monitor. The layer thicknesses were calculated to achieve the desired composition and varied from 20 to 70 nm. Each deposit was covered by a last layer of selenium only to prevent sample oxidation during transfer from the depositing apparatus to the annealing oven. Films were annealed for 15, 30 and 45min at different temperatures in range 543 to 823 K in flowing argon with the flow rate kept constant at 0.5dm3/min. After heat treatment, the samples were slowly cooled to room temperature (2-3 ‘/nun) in the Ar atmosphere. Indium selenide films corresponding to y-In2Se3 of thickness from 400 nm to 1100 nm were prepared by this process. Film characterization. The obtained films were analyzed ray diffraction (XRD) to investigate compound and/or

by Xphase 871

M Emziane et al: l;-ln,Se, thin films formation and to determine grain size and preferential orientation; a Siemens X-ray goniometer with CuKa line was used in the study. The surface film and depth profiling composition were obtained from X-ray photoelectron spectroscopy (XI’S) measurements. The quantitative XPS studies were based on the determination of the Se 3d and In 3d,z peak areas with 0.57 and 3.8 respectively as sensitivity factors given by the manufacturer (Leybold). The experimental conditions have been described previously.’ The XPS analysis was made at Nantes: University of Nantes-CNRS. Electron microprobe analysis (PGT-IMIX PTS model) was used to determine the chemical composition of the films. For the observation of their surface and cross-section morphology, a JEOL-type JSM 6400F scanning electron microscope (SEM) was employed. Additional film properties were found using optical and photoconductive measurements. The optical measurements were done at room temperature using a spectrophotometer CARY and the optical density measured in the wavelength interval 4002000 nm. Raman spectra were measured at room temperature using a Bruker model RFS 100. The samples were excited by a (Nd:YAG) laser with a wavelength of 1064nm. The laser power and the scale number necessary to obtain good signal to noise ratio were 100 mW and 500 respectively. The photoconductivity measurements were made at the CPMOH in Bordeaux I with a lock-in amplifier and at room temperature. Experimental results Initially, we have tried to optimize the deposition and annealing conditions by studying film structure using XRD and SEM. Then having checked the composition of some typical samples, the optical and photoconductive properties of the best samples were measured. Structural and morphological studies. After sequential deposition of the In/Se...In/Se layers, the XRD analysis showed that no In,Se, compound existed before annealing and the samples were opaque to the naked eye. After annealing in flowing argon

(543 K < T< 823 K; 15 min < t < 45 min), the films crystallized (Figure 1) and became brown/orange-coloured and transparent. Whatever were the composition of the initial sequence (R = 3’2 or R = 5/2), the annealing duration (1545 min) and temperature (543-823 K). all the diffraction peaks visible on the XRD spectra could be assigned to the y-ln,Se, hexagonal phase according to the JCPDS data.2” However the texture of the films and the grain size depend strongly on the experimental conditions as shown in Table 1. The degrees of preferential orientation F(O,O,I) and F(h,k,O): i.e. crystallites with the c axis perpendicular and parallel to the substrate plane respectively, were estimated from the formula given by Klug and Alexander.” The size of the crystallites cannot be deduced accurately from the full width at half maximum (FWHM) method. because this value for the diffraction peaks of the films is about the same as that obtained with the reference powder, thus we conclude that the film grains are large.” Therefore we report in Table I the FWHM of the peak (006) which allows the relative estimation of the grain size. The c and u lattice constants, deduced from the XRD spectra and reported in Table 1, were determined using the expression for hexagonal crystals.2 For photovoltaic applications, it is known that thin films should be textured with large grain size.” In the case of hexagonal structure, dangling bonds are mainly present on the face parallel to the c axis, therefore it is necessary to obtain a texture with F(0.0,1) = 1 to have a minimum trapping effect of the photo-generated carriers. Table 1 shows that the films obtained from samples with large initial Se excess (R = 5/2) are badly textured (F(h,k,O) > F(0,O.n. the grains are relatively small, and there is some discrepancy between the calculated lattice constants and those of reference.“’ Better results have been obtained when stoichiometric (R = 3/2) starting samples are used. Not only F(0,0,1) is the highest obtained, but also the FWHM is the smallest (i.e. the grain size the largest) and the lattice constants are nearly those expected. which demonstrates the better quality of these films. Some attempts have been done about the annealing duration (I 5 min < t-c45 min). It appears that while the texturation of the sample is similar, the diffraction peaks are more intense and the lattice constants are optimum after half an hour of annealing.

Table 1. XRD analysis Initial composition R = [Se]/[In] 5;‘2

JCPDS 312

872

Annealing Thickness

(nm)

Temperature

conditions (K) Duration

(min)

F(O,O,I)

F(0.0)

c(nm,

19.278 19.290 19.308 19.341 19.323 19.382 19.272 19.188 19.296 19.190 19.296 19.320 19.380 19.296 19.260

425 425 425 425 425 425

513 623 673 123 113 823

30

30 30 30 30 30

Amorphous 0.18 0.19 0.18 0.12 0.13

0.34 0.26 0.23 0.25 0.21

500 500 500 500 900 100 1100 1000 900

573 623

30 15 30 45 30 30 30 30 30

0.05 0.10 0.60 0.10 0.60 0.70 0.98 0.04 0.09

0.19 0.32 0.17 0.23 0.17 0.17 0.01 0.47 0.18

data

673

723 823

x10)

CI(nm. x 10)

FWHM(

7.094 7.095 7.098 7.101 7.096 7.128 7.101 7.087 7.101 7.080 7.101 7.098 7.118 7.087 7.084

0.2339 0.2318 0.2240 0.2100 0.2150 0.1934 0.2098 0.2329 0.2633 0.2059 0.2149 0.1866 0.2216 0.2110

)

M Emziane et al: y-/n2Se, thin films I

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20 Wg)

I

I

I

I

I

I

I

I

5

1

I

bl

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20(deg) Figure 1. XRD spectra of y-InzSe, films. (a) Sample with R = 5/2 annealed at 673 K for 30 min. (b) Sample with R = 3/2 annealed (c) Sample with R = 3/2 annealed at 673 K for 30min. (d) sample with R = 3!‘2 annealed at 723 K for 30min.

at 623 K for 30 min

M Emziane et al: ;,-ln,Se,

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All the results described above are in good agreement with the SEM study (Figures 2 and 3). The surface morphology of typical samples is presented in Figure 2 while scanning electron micrographs of the cross section are visualized in Figure 3. It can be seen (Figure 3) that there is a good correlation between the preferential orientation F(h,k,Z) and the grain orientation observed on the cross sections. For films obtained from samples with R = 51’2, the crystallites are small, quite disordered with a tendency to be parallel to the substrate plane [Figure 3(a)] which is in accord with the large value of the FWHM and the fact that F(h,k,O)>F(O,O,C). For samples with R = 3/2, the texturation shown in their cross section, in the SEM microgragraphs, in Figure 3(b), (c) and (d) follows that deduced from the XRD measurements. At low annealing temperature (573 K) the crystallites are small and disordered, and at high temperature (823 K) they are larger but some disorder is visible. The best films are those obtained by annealing at 673 K. The grains are perpendicular to the substrate plane and the height of the grains is equal to the film thickness. Small magnification allows to see that the films are homogeneous without any pinholes. Large magnification (Figure 2) corroborates the above discussion. There is a good adherence to the glass substrate. It should be noticed that the thicknesses of the films directly measured on the cross sections are in good agreement with the values obtained with a mechanical finger and those deduced from the vibrating quartz method. Quantitative analysis. In order to check the stoichiometry of the films, the samples have been studied by microprobe analysis and by XPS. The composition expected, In,Se,, is obtained with R = 3/2 and the variation from one point to another one never exceed 0.5% which is smaller than the accuracy of the measure. In the case of samples with R = 512, there is only a small bulk selenium excess (Table 2) which means that the initial Se excess leaves the sample during the annealing. However, even for these samples, it can be seen by XPS that there is a small Se deficiency at the film surface after annealing. This deficiency is limited to the first layers of the sample since after 1 min of etching the relative concentration of Se increases strongly. However we have only estimated the etching speed (about 5 nmlmin) and the relative sputtering efficiency of Se and In. Therefore it is difficult to estimate the thickness of the layer depleted in Se. This Se deficiency should be related to its high vapour pressure. The carbon and oxygen signals decreased as the effective sample depth analyzed increased and both have disappeared after l3 min of ion etching. This indicates that the carbon and oxygen impurities are originated from atmospheric contamination and are restricted to the near-surface region. It should be noted that never Na has been detected in the films, which means that, at least, if there is some Na diffusion from the substrate, its atomic concentration is too small to be detected by the techniques used in the present work. Optical and photoconductive characterizations. In order to confirm the occurence of the y-In,Se, phase, Raman scattering measurements have been performed on the best films. A typical Raman scattering spectrum is presented in Figure 4. The characteristic wavenumbers are reported in Table 3 where measurements of other authors are also presented. It can be seen that the results obtained in the present work are in good accordance with those of the reference.‘4 Moreover as discussed in a previous

Filgure 2. Surface morphology of y-In,Se, films. (a) Typical sample wilth R = 512 annealed at 673 K for 30 min. (b) Sample with R = 312 anneak :d at 573 K for 30 min. (c) Sample with R = 3:‘2 annealed at 673 K ft3r 30 min. (d) Sample with R = 3/2 annealed at 823 K for 30min.

875

M Emziane

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thin films

Figure 3. SEMs of the cross-section of y-In,Se, films. (a) Typical sample with R = 5,‘2 annealed at 673 K for 30min. (b) Sample with R = 3/2 annealed at 573 K for 30 min. (c) Sample with R = 3/2 annealed at 673 K for 30 min. (d) Sample with R = 3’2 annealed at 823 K for 30 min.

paper, some Raman frequencies are typical of the ;I-In$e, phase (22cm-‘, 83 cm-‘, 150cmand 230cm-‘).” In the same paper, it has been shown that y-In,Se, is a direct gap semiconductor. In the present work, the absorption coefficient s( has been calculated from the measured transmission (log(I!T) = OD(optica1 density)) of two samples with different thicknesses: ‘Z =

[ll(t*-fl)llog(~II~Z)

where t is the thickness of the film. The plot of (cc.hv)’ vs the photon energy is reported in Figure 5. The optical gap deduced from the high absorption domain is 1.80 eV. This value is corroborated by the photoconductivity results (Figure 6) where a strong increase of the photoconductivity is observed near 1.8 eV.

Discussion and conclusion We have shown that when )a-In,Se, films are obtained in an open reactor by annealing In/Se thin layers sequentially deposited, the 876

optimum annealing conditions are T = 673 K and t = 30 min when the starting sample composition is stoichiometric (R = 3/2). Moreover the films are homogeneous without pinholes. Electrodeposited films?’ post-annealed in the same conditions (T = 673 K. t = 30 min, Ar flow) are well crystallized but they do not exhibit the high degree of texture of the films obtained in the present work. The homogeneity is also far higher than that of the films described in ref. 14. The main difference between the technique described here and the best one proposed in our preceeding paper’ is the use of an open reactor during the annealing process in the present work while closed ampoule was used in the earlier process. If stoichiometric textured samples could be obtained with the preceeding process, the Se atmosphere during annealing was very difficult to control and the reproducibility of the results suffers of this difficulty. Moreover an annealing under dynamic vacuum was necessary at the end of the process to sublimate the selenium deposited at the sample surface during the annealing. In the present work this last annealing is not necessary. The vapour

M Emziane et al: y-ln,Se, thin films

Table 2. Quantitative

analysis

Initial composition R = [Se]/[In]

Annealing

5:2

513

30

613 773 823

30 30 30

573

30

613

30

123 823

30 30

Temperature

312

conditions

Composition

(K) Duration

Etching (min)

(min)

0 0

1 1 1 0 0 0 1 3 3 3 3 0 1

‘Microprobe

(at%)

time Se

In

46 61’ 52 61’ 62’ 46 60’ 31 52 52 60’ 60’ 60’ 21 48 60’

54 39’ 48 39’ 38’ 54 40’ 63 48 48 40’ 40’ 40’ 73 52 40’

analysis.

0.030

0.025

0.020 s ‘a 3 2

0.015

E 2 203

226

0.010

0.005

0.000 L

0

25

50

75

100

12.5

Wavenumber Figure 4. Raman

scattering

spectrum

of sample with R = 3/2 annealed

Table 3. Raman

scattering

22

175

200

225

250

cm-’

for half an hour at 673 K

measurements

Wave number Present work Refs 24,25,26

150

26

(cm-‘) 85 83

I51 150

203 205

226 224

a77

M Emziane

et al: ;+,Se,

thio films

voltaic properties of the thin films, those obtained suggested for low-cost solar cells applications.

above can be

Acknowledgements The authors wish to thank Mrs A. Barreau, P. Leray, L. Assmann and 0. Ligner for assistance in the experimental measurements. References

1.4

1.6

1.8

Figure 5. (~1.h)~vs /IVof samples

2 2.2 E cm with R = 3;Z obtamed

2.4

2.6

by annealing

at

673 K for half an hour.

I

i

6 t

1.5

I

6

I

4

2.5 E WI of samples with R = 3:‘2 obtained

Figure 6. Photoconductivity ing for half an hour at 673 K.

2

3 by anneal-

pressure of Se being very high we have shown that Se excess leaves the sample during the annealing. Moreover the annealing conditions (T = 673 K, t = 30 min, Ar flow rate = 0.5 dm3/min) are very easy to control, which explain the good reproducibility of the present results. The single phase films obtained are photoconductive, textured, homogeneous and stoichiometric. Because the reproducibility of these properties is high, which is important factor for photo-

878

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