A novel deposition technique for compound semiconductors on highly porous substrates: ILGAR

Thin Solid Films 361±362 (2000) 113±117 www.elsevier.com/locate/tsf

A novel deposition technique for compound semiconductors on highly porous substrates: ILGAR J. MoÈller, Ch.-H. Fischer*, H.-J. Muf¯er, R. KoÈnenkamp, I. Kaiser, C. Kelch, M.C. Lux-Steiner Dept. Heterogenous Material Systems, Hahn±Meitner Institute Berlin, Glienicker Strab e 100, D-14109 Berlin, Germany

Abstract ILGAR (ion layer gas reaction), a novel low-cost technology for the preparation of sul®dic thin layers is described, which can be analogously applied for other chalcogenides. The process consists of three steps: (1) application of a precursor solution on a substrate by dipping or spraying, (2) drying in an inert gas stream, (3) sulfurization of the solid precursor (e.g. a metal halide) by hydrogen sul®de gas. This cycle is repeated until the desired layer thickness is obtained. Not only on smooth, but also on structured and porous substrates the method allows the deposition of homogenous thin ®lms following the microscopic structure, where other methods often have problems with shading. Once the ®lm is closed, the growth per dip cycle is constant and reproducible during the process. The binary compounds CdS, Cu2S, In2S3 and also the ternary CuInS2 have been prepared by ILGAR on glass and on porous TiO2 or SiO2. The layers were characterised by XRD, SEM and EDX. q 2000 Elsevier Science S.A. All rights reserved. Keywords: Chemical deposition; Thin ®lms; Chalcogenides; Porous titanium dioxide; Porous silica

1. Introduction Besides expensive physical techniques, such as evaporation or sputtering, various chemical methods exist for the preparation of inorganic thin ®lms. Chemical vapour deposition (CVD) and metal organic chemical vapour deposition (MOCVD) are technically complex and costly. Compared to these the chemical bath deposition (CBD) works with a rather simple equipment. For the generation of sul®dic thin layers single step and sequential multi step processes are known. In the ®rst case a bath contains a solution of the desired metal cation (often together with a complex former) and a sul®de precursor, e.g. thiourea or thioacetamide generating hydrogen sul®de or sul®de ions upon heating [1±3]. The growth takes place layer by layer and/or via cluster deposition. Only a small window for the process parameters (concentration, pH, temperature, reaction time) allows good results in many cases. The choice of the reagents needs special care [4] and it is very dif®cult to vary the layer thickness, because the material properties change with reaction time. Sequential methods consist of two steps at least, one for the deposition of the starting compound and another, where the reagent and/or energy is added. Sol-gel processes (typically for oxides) belong to this * Corresponding author. Tel.: 149-30-8062-2017; fax: 149-30-80623199. E-mail address: ®[email protected] (C.H. Fischer)

category as well as the SILAR technique (successive ion layer adsorption and reaction) [5]. In the latter case sul®de ®lms are produced by successive dipping of a substrate in a metal salt solution and then in a bath of alkali sul®de. Between these dips rinsing steps might be included. These cycles are repeated several times and the insoluble sul®de is formed layer by layer on the substrate surface. Within the framework of the eta-solar cell project [6] the thin ®lm deposition of a sul®dic semiconductor absorber on a several mm thick porous titanium dioxide layer is required with the demand that the ®lm follows the inner surface of the highly structured substrate and does not just close the pores covering the outer surface. The present work describes the development of a low-cost process for this purpose.

2. Experimental 2.1. Materials The TiO2 substrates were prepared by spray pyrolysis or screen printing (the latter were obtained from Institut fuÈr angewandte Photovoltaik, Gelsenkirchen, Germany). Float glass substrates were cleaned with isopropanol in an ultrasonic bath prior to the deposition. The chemicals CdCl2 (Alfa, Germany), CuCl (Alfa, Germany), CuI (Merck, Germany), InCl3 (Alfa, Germany), HCl (Merck, Germany), acetic acid (Merck, Germany), acetonitril (Merck,

0040-6090/00/$ - see front matter q 2000 Elsevier Science S.A. All rights reserved. PII: S 0040-609 0(99)00797-X

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Germany) and tetrahydrofuran (Merck, Germany) were of analytical grade. 2.2. Characterisation of the ®lms The following equipment was used for the characterisation of the layers: XRD: Bruker D8, SEM with EDX: Leo 430, Optical spectroscopy: Cary 500. 2.3. The ILGAR process After dipping in the metal halide precursor solution for a few seconds the substrate was dried in a nitrogen stream and transferred in a closed reaction chamber. Then hydrogen sul®de gas is introduced and the excess is pumped into a scrubber. The reaction time depends on the material: 10 s for CdS and Cu2S, two minutes for In2S3 and CuInS2. The cycle is repeated until the desired ®lm thickness is obtained. Alternatively the precursor solution can be applied by an ultrasonic sprayer. All actions were carried out by an x,y,zrobot (ISEL, Germany), which also controlled the electric valves for N2 and H2S. 2.4. Typical ILGAR conditions CdS:10 22 mol/l CdCl2 in tetrahydrofuran plus 5% water. Cu2S:10 22 mol/l CuCl in pure acetonitril or methanol plus 20% water, adjusted to pH 2.5 by hydrochloric acid. In2S3:10 22 mol/l InCl3 in acetonitril plus 5% (v/v) acetic acid. CuInS2:CuCl and InCl3, 10 22 mol/l each in acetonitril plus 5% (v/v) acetic acid, the primary layers were annealed at 5008C for 30 min in a 5% H2S/Ar atmosphere. 3. Results and discussion One main feature of the eta-solar cell is the widely enlarged surface of the substrate, e.g. porous TiO2, covered by a very thin absorber layer. Due to this arrangement the absorption is strongly enlarged. For this purpose it is crucial that the absorber ®lm follows the internal porous surface. Just a covering of the outer surface or completely ®lled pores would not meet the demands of the concept of the eta-solar cell. The expensive physical preparation methods seem neither adequate for a low-cost product nor suitable for a deposition in pores. Chemical bath processes are favoured. In a one-pot process the precipitation of clusters can take place increasing the risk of plugged substrate pores. Therefore a sequential process should be superior since the dissolved starting material reaches the complete surface, whereas the reaction to the insoluble product occurs mainly at the ®nal position.

sequentially dipped in cadmium chloride and in sodium sul®de solution with a rinsing step in between. Unfortunately, the ®lms showed crusts in the scanning electron microscope (SEM), their homogeneity was not satisfying. Two problems might arise when the chemical reaction is carried out in a liquid bath, i.e. at a solid±liquid interface: Adsorbed precursor ions are in equilibrium with dissolved ones. Therefore the risk of cluster formation and homogenous precipitation in solution must be considered. Second, diffusion in liquids is relatively slow as compared to the gas phase. Precipitation on the walls of the pores narrows their diameter. This could make the diffusion of the dissolved ions even more dif®cult in the case of small pore sizes. Moreover, in ionic solutions the effective pore diameter can even be considerably narrower depending on the ion strength due to the electric double layer at the interface. This effect has been extensively studied for porous silica gels in size exclusion chromatography [9]. Therefore, a homogenous deposition inside of pores by SILAR seems to be problematic. 3.2. The ILGAR concept The previous results led to the concept of ILGAR (ion layer gas reaction, patents pending [10,11]) shown schematically in Fig. 1. In general, the method is suitable for all insoluble chalcogenides. Here we describe the deposition for various sul®des. The process consists basically of three steps, one in a liquid and two in a gas phase. A solution of a precursor compound containing the metal ion of the desired sul®de is applied on a substrate by dipping or spraying. In the next step residual solvent is removed by an inert gas stream. Finally the solid precursor layer is sulfurized by hydrogen sul®de gas according to Eq. (1). The remaining protons form the acid of the precursor anion, e.g. HCl, when a chloride has been used. This volatile and well soluble compound is easily removed either by evaporation or by rinsing. The cycle is repeated until the ®nal layer thickness

3.1. CdS on porous TiO2 by SILAR With cadmium sul®de the SILAR-process was tested on porous TiO2 in analogy to [7,8], i.e. the substrate was

Fig. 1. Scheme of the three basic steps of the ILGAR process.

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is obtained. ÿ
mMen1 mn=xX x2 solid 1mn=2H2 Sgas ! Mem Sn=2

solid

1 mnHXgas

or ads

…1†

Me ˆ metal; X ˆ halide or other anion; n ˆ metal valency It should be stressed that the ILGAR process is an ion exchange reaction, chemically completely different from the sulfurization of metal layers by H2S or sulfur according to Eq. (2) or (3), respectively, which are redox reactions and need typically higher temperatures. mMe 1 n=2H2 Sgas ! Mem Sn=2 1 nH2

…2†

mMe 1 n=2S ! Mem Sn=2

…3†

The solid has to be suf®ciently ionic to enable a gas phase sulfurization at a low temperature, preferentially room temperature. The great advantage in comparison to SILAR is that the process takes place at a solid±gas interface. A liquid phase does not exist, where clusters could be formed. The homogeneity of the solid precursor ®lm determines the quality of the ®nal layer, because the primary crystallites are only chemically converted into those of the product. Upon solvent evaporation the solute forms smooth layers supposed the solvent wets the substrate. Easily evaporating organic solvents and a laminar drying stream favour a uniform product. Moreover, a reagent gas penetrates narrow pores better than ionic species in a liquid. This improves the deposition in deep regions. 3.3. CdS by ILGAR The excellent homogeneity of ILGAR layers on porous spray pyrolysis TiO2 is demonstrated by a deep uniformly yellow CdS ®lm prepared in 15 dips from a CdCl2/tetrahyfuran solution with subsequent H2S sulfurization according to Eq. (1). The SEM micrograph (Fig. 2, left) of this sample is almost identical with that of the original substrate (Fig. 2, right) indicating that the ILGAR ®lm is homogeneous and follows exactly the substrate morphology.

115

For growth rate studies CdS was deposited on ¯oat glass (CdCl2 concentration ˆ 4 £ 1023 mol/l). The layer thickness was determined after different numbers of dip cycles. This was achieved by evaluation of the spectroscopic transmission and re¯ection data. It turned out that after a few dips, as soon as the substrate is completely covered, the average growth rate per cycle is constant (Fig. 3). This allows an easy and reproducible adjustment of the ®lm thickness. Of course, the bath concentration has a great in¯uence on the deposition rate as it determines the precursor amount deposited from a single dip [12]. 3.4. CuInS2 by ILGAR For the absorber of the eta-solar cell a semiconductor compound with low solubility is needed in order to allow chemical deposition. The copper indium disul®de (CIS) is a promising absorber for chalcopyrite thin ®lm solar cells typically prepared by evaporation of the two metals followed by high temperature reaction either with sulfur or hydrogen sul®de [13]. By ILGAR such a ternary compound can be prepared by two principal ways. Route 1: The binary sul®des Cu2S and In2S3 are deposited sequentially according to Eqs. (4) and (5), the less soluble one (Cu2S) ®rst, otherwise the better soluble (In2S3) would redissolve by ion exchange (Eq. (6)). It should be mentioned that InCl3 is sulfurized in a slightly acidic solution necessary for complete dissolution (e.g. addition of 5% acetic acid or dilute hydrochloric acid). Strong acids on the other hand, would prevent any indium sul®de deposition. The reaction is slower with InCl3 than with CuCl (120 s as compared to 10 s). With short sulfurization times most of the precursor would be washed out during the next dip resulting in a insuf®cient growth rate. 2CuClsolid 1 H2 Sgas ! Cu2 Ssolid 1 2HClgas 2InCl3 In2 S3

solid 13H2 Sgas

1 solid 16Cusolv

! In2 S3

solid 16HClgas

! 3Cu2 Ssolid 1 2In31 solv

…4† …5† …6†

Fig. 2. SEM micrographs of spray pyrolysis TiO2 with (left) and without (right) an ILGAR CdS ®lm. From the almost identical appearance of the deeply yellow product and the colourless substrate it is concluded that the CdS layer follows precisely the microscopic morphology.

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Table 1 Element composition of ILGAR CIS from EDX. Cu/In precursor ratio

1:1

Fig. 3. Average growth rate as a function of the number of dips for the ILGAR deposition of CdS from a 4 £ 1021 mol/l CdCl2/tetrahyfuran solution on glass.

Though CIS can be obtained from such a bilayer after annealing at 4808C in an Ar/H2S atmosphere (Eq. (7)), it is very dif®cult to establish the correct stoichiometry. Cu2 Ssolid 1In2 S3

solid

! 2CuInS2

solid

…7†

Route 2: Therefore an alternative method for the ILGAR± CIS preparation from a common solution of both copper and indium precursors was developed. Already ®rst experiments with CuI and InCl3 in acetonitril/water/HCl yielded CIS, when the resulting layer was sulfurized and annealed as before, but it was necessary to keep the layer slightly wet. The reason might be that CuI is not suf®ciently polar to enable sulfurization at the dry solid±gas interface at room temperature. So it is advantageous to replace CuI by CuCl and to work in completely water-free acetonitril with 5% acetic acid. After sulfurization brownish black layers of

Atom % Cu

In

S

28

20

51

excellent homogeneity are obtained. X-ray diffraction (XRD) in the grazing incidence mode shows already the CIS pattern before a thermal treatment indicating a reaction according Eq. (8). From the broad peaks the presence of nanocrystallites can be concluded. Annealing is carried out at 5008C for 30 min. An Ar/H2S atmosphere is used in order to complete conversion of minor amounts of incorporated and unconverted precursor. After this procedure the colour has turned black. The XRD signals of the CIS are narrower indicating a crystal growth. In the SEM ILGAR CIS ®lms are clearly visible (Fig. 4), but not as a plane cover. Still the original substrate structure can be recognised. CuClsolid 1InCl3solid 12H2 Sgas ! CuInS2solid 12HClgas

…8†

Deposition from an equimolar precursor solution on glass or screen printed TiO2 forms copper rich ®lms shown by EDX (Table 1) and by the presence of copper sul®de signals in the XRD. Obviously, a few percent of the InCl3 are not converted in each sulfurization step because of the lower reactivity. It is washed out during the following dip. For exact stoichiometry one could work at elevated temperature, with longer sulfurization times or with a slight excess of indium precursor. Interestingly, on porous silica an equimolar precursor generates ®lms showing only the CIS pattern (roquesite) in the grazing incidence XRD (Fig. 5). This means that also the properties of the substrate material have an in¯uence on the elemental composition in the case of ternary compounds.

Fig. 4. SEM micrograph of screen printed TiO2 with (right) and without (left) ILGAR CIS. The original structure can still be recognised in the product.

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obtained on glass depending on the precursor concentration. The preparation of intensively coloured CIS layers on porous TiO2 takes half an hour. Multilayers and doped ®lms could be accessible via ILGAR. The whole process can simply be automated by computer controlled x,y,z-robot and gas valves. First photovoltaic results with ILGAR-CIS were obtained and will be published elsewhere.

Acknowledgements

Fig. 5. X-ray diffractogram of ILGAR CIS on porous SiO2, measured in the grazing incidence mode in comparison with the JCPDS pattern of CuInS2.

The authors thank U. Gerloff and Wittmaack for the construction of the robot and the development of the related software and B. Mertesacker for the design of the substrate holder.

4. Conclusion

References

It has been shown that ILGAR is a powerful technique for the chemical thin ®lm deposition of chalcogenides at low temperatures. This is demonstrated for binary and even ternary sul®des: CdS, Cu2S, In2S3 and CuInS2. Not only smooth substrates can be used. The low-cost method has its special merits, when structured or porous substrates are to be covered with retained morphology. The reason for the homogeneity is the smooth solid precursor layer converted by a gaseous reactant (H2S). Typically, the process parameters are not critical, even for ternary compounds. Often room temperature is suf®cient. The precursor is completely consumed unlike in the thiourea CBD, where frequently more than 90% are wasted. The layer thickness is easily controlled via the number of dip cycles. The method is fast: In less than 10 min 20±50 nm thick CdS ®lms are

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