p interfaces as a function of deposition parameters

Surface and Coatings Technology 110 (1998) 68–72

Chemical changes of ITO/p and ZnO/p interfaces as a function of deposition parameters M.A. Martı´nez *, M.T. Gutie´rrez, C. Maffiotte Departamento de Energı´as Renovables (CIEMAT), Avda. Complutense 22, E-28040 Madrid, Spain Received 27 March 1998; accepted 22 July 1998

Abstract The influence of deposition parameters of p-type amorphous silicon on the chemical properties of different transparent conductive oxides ( TCO)/p a-Si:H interfaces (ITO/p and ZnO/p) has been investigated by X-ray photoelectron spectroscopy ( XPS). p samples were prepared by plasma-enhanced chemical vapour deposition (PECVD) from a SiH , B H and He mixture, 4 2 6 whereas TCO were made by r.f.-magnetron sputtering. In0 has been detected on all ITO/p surfaces as a consequence of the reduction of this TCO by hydrogen resulting from silane decomposition during p-layer formation. Oxygen is released from the indium oxide crystalline lattice, which can react with the silicon to form silicon oxide, SiO , 1≤x≤2. No presence of Zn0 has x been observed on any ZnO/p bilayer. Owing to this fact, ZnO/p interfaces are more abrupt than the ITO/p interfaces and, therefore, they are good to be applied in amorphous silicon-based solar cells. © 1998 Elsevier Science S.A. All rights reserved. Keywords: a-Si:H deposition parameters; ITO/p and ZnO/p interfaces

1. Introduction In order to obtain a further improvement in stabilized efficiency of amorphous silicon hydrogenated (a-Si:H ) solar cells, growing attention is being paid to the transparent conductive oxide ( TCO) material that forms the front electrode. In particular, the TCO/p contact properties and their influence on solar-cell parameters have been the subject of a number of investigations [1– 4], but the study of this interface is still a matter of debate. Transparent conductors for the front surface of a-Si:H solar cells have generally been made using either tin oxide, indium tin oxide (ITO) or zinc oxide. Each material has its advantages and disadvantages. Tin oxide and ITO yield a higher open circuit voltage because they present a more stable interface with amorphous silicon, and zinc oxide yields a higher short-circuit current because of its greater transparency in the bulk of the film. Boron-doped a-Si:H is prepared on TCO in a plasma CVD process by dissociation of silane and diborane, which implies generation of Si, B, SiH , B , H, H , and x x 2 * Corresponding author. Tel: +34 9134 66669; Fax: +34 9134 66037; e-mail: [email protected]

their radicals, impinging during deposition on the first monolayers of TCO. Beside atomic mixing induced by particles hitting the surface of growing film, excited particles interact with the TCO, leading to processes such as reduction of metal oxide, diffusion of metal into the growing a-Si:H and oxidation of silicon at the interface. Microcrystalline p layers have been known to produce greater short-circuit currents and voltages than amorphous p layers in silicon solar cells grown on metal substrates. The current is higher because the microcrystalline material is more transparent than amorphous material. The voltage is higher because doping is more effective in moving the Fermi level in the less defective microcrystalline material. However, a-Si:H solar cells made on glass superstrates can be grown neither on indium tin oxide nor on zinc oxide. The conditions (high plasma power and hydrogen dilution) that can originate microcrystalline p layers also destroy the TCO by reducing it to metallic layers. In order to avoid this problem, different approaches related to the introduction of intermediate layers and plasma treatments have been considered [5]. However, taking into account the paper of Matsuda [6 ], it is possible to conclude that dilution with hydrogen is not an essential condition for the formation of micro-

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M.A. Martı´nez et al. / Surface and Coatings Technology 110 (1998) 68–72

crystalline silicon. An approach involving the dilution of silane (SiH ) in He and applying high r.f.-power 4 densities has been tested [7–11], and the results obtained have demonstrated its suitability as an alternative to the conventional hydrogen dilution [12]. In recent studies related to optimization of p layers, it has been found that r.f.-power density is quite a critical parameter whose value may determine different properties with relatively small variations [13]. In this paper, we analyse, by XPS studies, the chemical changes that occur at the surface of indium tin oxide and aluminium-doped zinc oxide, in contact with a p-type a-Si:H layer prepared using silane diluted in helium at different r.f.-power densities.

69

parameters determine the chemical changes produced in the interface. Film thicknesses were obtained by using a Dektak 3030 surface profile measurement unit. Compositional studies were carried out using a Perkin Elmer Phi 5400 XPS system. Samples were excited with a MgKa X-ray source operated at 300 W input, and superficial analyses were carried out at a base pressure of 10−9 Torr and with a spot size of 1 mm2. Depth profile data were obtained by 3-kV Ar+ sputtering at 10−7 Torr. ˚ min−1 from Sputtering rates were estimated to be 80 A Ta O data. The exit angle was maintained at 45° in 2 5 all cases.

3. Results and discussion 2. Experimental ITO and Al-doped ZnO were prepared using a commercial r.f.-magnetron sputtering system (Leybold Heraeus Z-400). For this purpose, two different oxidized targets with a diameter of 75 mm were obtained from Cerac Inc.: In O :SnO 95/5 wt%, 99.99% purity for 2 3 2 ITO films and ZnO:Al O 98/2 wt%, 99.99% purity for 2 3 Al-doped ZnO films. Glass substrates with an area of 20×11 mm2 were placed parallel to the target surface, at a substrate–target distance of 35 mm. They were then ultrasonically cleaned with organic solvents before loading into the sputtering chamber. Argon and oxygen mass-flow rates, sputtering time, r.f.-power density and substrate temperature, were optimized in order to achieve the best electrooptical and structural properties; the most relevant results can be found elsewhere [14–21]. It can be concluded that by precisely controlling the O mass-flow rate, it is possible 2 to obtain yield a low resistivity and high transmittance of visible light for ITO and ZnO layers prepared at a high substrate temperature (380 °C ) and without intentionally annealing the substrate [19,21]. p-type a-Si:H layers were prepared in a r.f. (13.56 MHz) glow-discharge, capacitively coupled, commercial plasma reactor (Plasma Technology DP-800) with two 38-cm-diameter electrodes 1.5 cm apart. The gas flow rates were controlled by means of Tylan mass controllers. Suitable gas cabinets and a gas detection system were used to ensure safe gas handling. Samples were produced on TCO substrates from gas mixtures of SiH , He and B H . The influence of the deposition 4 2 6 parameters on the optoelectronic properties of p-type layers has been reported elsewhere [13]. In the present study, substrate temperature, T, process pressure, P, and He, W and gas flows of SiH , W , B H , W He 4 SiH4 2 6 B2H6 have been maintained constant. The effects of r.f.-power density (RFP) and corresponding deposition time on TCO/p interface properties have been studied in an attempt to understand how these p-layer preparation

For device to perform properly, each layer has to be optimized according to the desired optoelectronic properties, the thickness and the eventual grading of composition. In addition, the functioning of the device depends also on the transitions between the different layers, which we might call the interface regions. Even though the electrooptical properties of the individual layer materials are homogeneous over the layer thickness and the genuine interface regions on an atomistic scale do not exhibit indigenous regions states, interface-related phenomena are operative and affect the device performance. These effects have been caused by the preparation conditions and the resulting growth process. Some of them, due to adjacent and different layers, represents the interface between the TCO and the p-doped a-Si:H layer. The reduction of TCO in hydrogen plasma can lead to optical losses, the resulting metallic particles give rise to additional absorption, thereby limiting the short current density of the solar cell. In order to verify these types of interfacial phenomena, a set of p-type a-Si:H samples has been prepared on ITO substrates [19] with the following fixed parameters: P=1000 mTorr, T=250 °C, W =6 sccm, W = SiH B2H6 1 sccm, W =193 sccm and the r.f. 4P varied from 88 to He 154 mW cm−2. Taking into account the evolution of growth rate with the r.f.-power density, the deposition time has been varied from 1160 to 2185 s to obtain the ˚. desired thicknesses, 250–300 A XPS analyses of ITO/p surface have demonstrated that the superficial composition is not dependent on the amorphous silicon deposition parameters studied. The X-ray photoelectron spectrum plotted in Fig. 1 indicates that besides typical silicon, boron and oxygen signals corresponding with a p-doped a-Si:H layer, indiumrelated peaks appear on surface. We think that these, In MNN, In 3p and In 3d, are associated with the diffusion of indium from ITO substrate. Considering the In 3d XPS peak, two different binding energies 5/2 can be deduced: one related to indium oxide

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Fig. 1. XPS superficial spectrum corresponding with a p-type a-Si:H layer prepared on an ITO substrate by PECVD at 154 mW cm−2.

(445.1–445.7 eV ) and the other to metallic indium (443.4–443.7 eV ) (see Table 1). Metallic indium has appeared as a consequence of the reduction of this TCO by hydrogen resulting from silane decomposition during p-layer formation. Banerjee et al. [22] reported a similar reduction of ITO substrates in both hydrogen and inert gas plasma. The darkening of the samples observed by visual inspection corroborates the existence of a certain amount of metallic particles. The depth profiles of different analysed elements have been represented in Fig. 2. Indium is present on the surface (depth=0 nm), and its atomic concentrations increase as the p amorphous silicon layer is sputtered, Fig. 2(a). A low atomic concentration of tin is detected because it acts as a dopant in indium oxide. Fig. 2(b) shows the evolution of the O 1s, Sn 3d, In 3d and Si 2p peaks as a function of the cycles tested. Once more, the presence of indium on the surface is observed. From Fig. 3, it can be seen that there is a shift in the In 3d 5/2 peak towards higher binding energies as the r.f.-power density decreases. Pure ITO substrate is considered as the reference. We believe that such a displacement could be related to the variation in the oxidation state from 3+ (In O ) to 0 (In0 or In–Sn). In order to prove this 2 3 hypothesis, the deconvolution of these peaks has been performed, and the most relevant results can be seen in Table 1 for different r.f.-power densities and corresponding deposition times. The ratio of metallic particles to oxidized ones has been observed to be more dependent

Fig. 2. Depth profiles evolution related to oxygen, tin, indium and silicon peaks of an ITO/p-a-Si:H sample made at 115 mW cm−2.

on deposition time since the highest time, 2185 s, is associated with the highest metallic percentage, 33%. However, when deposition times are reasonably similar, 1165 and 1460 s, the final oxidation state does not change appreciably (In0=10–11%, In3+=90–89%). If the a-Si:H layer grows slowly, it fails to protect the ITO from the hydrogen impact caused by SiH decomposi4 tion, whereas rapid growth can limit the formation of the accumulation layer, thus improving the contact properties. Considering these data in which the diffusion and presence of indium on a p-type a-Si:H surface have been evidenced, we have studied other distinct TCO as substrates with the objective of avoiding this sort of phenomenon. Thereby, another series of films has been

Table 1 Binding energy and atomic concentration data associated with the In 3d XPS peak of several p-type a-Si:H samples made at different r.f.-power 5/2 densities and corresponding deposition times on ITO substrates Power density (mW cm−2)

t

0 88

— 2185

115

1165

154

1460

PECVD

(s)

Binding energy (eV )

Atomic concentration (%)

444.2 443.7 445.7 443.4 445.4 443.5 445.1

100 33 67 11 89 10 90

(In3+) (In0) (In3+) (In0) (In3+) (In0) (In3+)

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Fig. 3. Effect of the r.f.-power density used in the p-type a-Si:H fabrication process on the In 3d XPS signal. 5/2

made, having Al-doped zinc oxide as the transparent conductive oxide [21]. The amorphous-silicon deposition conditions are the same as those corresponding with the ITO/p samples in order to compare the chemical changes that took place in their interfaces. ZnO/p superficial XPS measurements have demonstrated that only typical silicon, boron and oxygen signals related to p a-Si:H films appear on the film surface, and no zinc peaks have been detected under any of the deposition conditions studied. This behaviour, illustrated in Fig. 4 for an amorphous silicon layer made at 154 mW cm−2, differs from that observed for layers deposited on an ITO substrate (see Fig. 1). This indicates that no diffusion of zinc atoms from substrate towards growing amorphous silicon has taken place. A depth profile analysis of the different elements presented, silicon, zinc and oxygen, corroborates that the presence of metallic atoms is detected only after several cycles checked, thereby showing a more abrupt ZnO/p interface. Therefore, this TCO can be considered to be of a better quality than ITO (Fig. 5). An analysis of the Zn 2p peak has indicated that 3/2 its associated binding energy does not change either

Fig. 4. XPS superficial spectrum corresponding with a p-type a-Si:H layer prepared on a ZnO substrate by PECVD at 154 mW cm−2.

Fig. 5. Depth profiles variation related to zinc, oxygen, and silicon peaks of a ZnO/p a-Si:H sample made at 115 mW cm−2.

with the r.f.-power density used for the p-layer preparation or with deposition time (1022.5–1022.6 eV ). In addition, the Zn 2p line upon sputtering does not 3/2 show any shift during any of the cycles analysed (depth up to 115 nm). This kind of behaviour has not been observed for amorphous silicon samples produced using silane hydrogen dilution [23,24]. Thereby, Stiebig et al. [2] reported a certain evolution of the Zn 2p line 3/2 towards a higher binding energy, and they attributed this to an accumulation layer at the surface of the n-type ZnO caused by atomic hydrogen in the plasma. The electrical field is reduced, and consequently, the series resistance increases. Taking into account these results and the fact that the reduction of the metal oxides to metal liberates oxygen for the possible formation of silicon oxide, SiO with 1≤x≤2 [25], we have studied the deconvolux tion of the Si 2p line for producing Si0 and SiO atomic x concentrations as a function of the TCO used. Table 2 lists the atomic concentration data of the most severe p-type a-Si:H deposition conditions, 88 mW cm−2 and 2185 s, for ITO and ZnO substrates. From this, it can be proven that although, in both cases, Si 2p lines have been deconvoluted in several peaks associated with Si0 and SiO , respectively, the ITO/p bilayer contains a x higher amount of silicon oxide. We think that this effect has been produced because as a result of reduction of

M.A. Martı´nez et al. / Surface and Coatings Technology 110 (1998) 68–72

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Table 2 Si0 and SiO (1≤x≤2) atomic concentration of several p-type amorphous silicon layers deposited at different r.f.-power densities and associated x deposition times on ITO and ZnO substrates TCO

Power density (mW cm−2)

t (s) PECVD

Si0 atomic concentration (%)

SiO atomic concentration (%) x

— ITO ZnO

88 88 88

2185 2185 2185

60 41 61

40 59 39

ITO, oxygen is eliminated from the indium oxide crystalline lattice and could react with silicon to form silicon oxide, SiO . Thereby, whereas the percentage of Si0 is x about 61% when the ZnO substrate is utilized, it is only 41% when the substrate is ITO. Amorphous silicon deposited directly on glass, taken as the reference, has shown similar Si0 and SiO atomic concentrations to x those related to the ZnO/p layer. This fact corroborates once more that zinc oxide is not reduced under these p-layer deposition conditions. The results indicate that the ITO/p worsening can be solved by substituting the ITO by ZnO and using He instead of hydrogen for silane dilution.

4. Conclusions The dependence of chemical properties of ITO/p and ZnO/p interfaces as a function of p-type amorphous silicon preparation conditions, r.f.-power density and related deposition time has been studied by X-ray photoelectron spectroscopy. The investigations have revealed the existence of dynamic processes of atoms and molecules at the interface during deposition of p-type a-Si:H on ITO. From the results, migration and diffusion processes as well as a reduction of indium oxide to In0, as a consequence of hydrogen resulting from silane decomposition, can be deduced. No Zn0 has been detected on the ZnO/p surface. These results indicate that ITO/p worsening can be solved by substituting the ITO by ZnO and using He instead of hydrogen for silane dilution.

Acknowledgement This work has been supported by the Spanish Ministry of Industry and Energy through CIEMAT-DER Project: ‘‘Desarrollo de materiales y dispositivos fotovoltaicos’’ and the CYCIT Project TIC97-0409. The authors would like to thank ‘‘CIEMAT Project: Materiales Estructurales de Plantas Energe´ticas’’ for XPS measurements.

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