Elsevier

Applied Surface Science

Volume 418, Part A, 1 October 2017, Pages 388-392
Applied Surface Science

Full Length Article
Studies on interface between In2O3 and CuInTe2 thin films

https://doi.org/10.1016/j.apsusc.2016.12.124Get rights and content

Highlights

  • In2O3/CuInTe2 and CuInTe2/In2O3 structures grown over Si substrates.

  • Glancing angle X-Ray Diffraction (GXRD) shows the presence of CuInTe2 and In2O3 layers in Si/In2O3/CuInTe2.

  • GXRD suggest that the structure Si/CuInTe2/In2O3 has only In2O3 layer.

  • Micro Raman shows no characteristic peak of CuInTe2 in Si/CuInTe2/In2O3.

  • RBS shows Si/CuInTe2/In2O3 presence of additional layers and absence of CuInTe2 layer.

Abstract

Interface between dc sputtered In2O3 and stepwise flash evaporated CuInTe2 films were studied by probing Si/In2O3/CuInTe2 and Si/CuInTe2/In2O3 structures with the help of glancing angle X-ray diffraction, Rutherford backscattering spectrometry and micro-Raman spectroscopy. The results showed that in Si/In2O3/CuInTe2 structure, a ∼20 nm thick interface consisting of In, Cu and O had formed between In2O3 and CuInTe2 and was attributed to the diffusion of Cu from CuInTe2 into In2O3 film. On the other hand, in Si/CuInTe2/In2O3 structure, homogeneity of the underlying CuInTe2 film was found lost completely. An estimate of the masses of the constituent elements showed that the damage was caused by loss of Te from CuInTe2 film during the growth of In2O3 film on Si/CuInTe2.

Introduction

Thin film solar cells with I-III-VI2 film as absorber layer, Mo film deposited on glass substrate serves as back electrode, with the glass plate providing the necessary mechanical strength. An alternate approach [1], [2], [3] to the back metallic contact is to replace Mo film with a transparent conducting oxide (TCO) film. An important advantage [3] of using TCO electrodes is that, apart from providing electrical contact, they can transmit unabsorbed portions of the incident radiation, as TCO’s have an average transmittance of about 85% in the visible and near infrared regions. Solar cells with TCO back contact are sometimes termed as bifacial [3], as it is possible to illuminate from both sides and therefore forms the basic unit of multi-junction or tandem solar cells. The bifacial structures are commonly visualized in two different forms: a) the conventional form in which the cell is developed by first forming the back contact TCO on the glass plate and b) the “inverted” structure in which the back contact is deposited last. The latter structure is called “superstrate” type cell. Two major factors [3], [4] that affect the performance of the solar cells with TCO back contact are 1) changes in the structural and phase homogeneity of the absorber and the TCO films, 2) interface layers, if any, formed between TCO and absorber films due to chemical reaction or inter-diffusion across the interface. For instance, any change in the characteristics of the TCO increases the series resistance and reduce the transmitted light, which in turn will affect the other cells that are in tandem. Therefore, application of TCO as back contact necessitates a detailed prior study of interface between chalcopyrite and TCO thin films.

The present work deals with probing of the interface formed between stepwise flash evaporated CuInTe2 films and dc sputtered In2O3 films using Rutherford backscattering spectrometry (RBS), glancing angle X-ray diffraction (GXRD) and micro-Raman spectroscopy. CuInTe2 belongs to the chalcopyrite ternary compound semiconductor family of compounds and exhibits characteristics displayed by good solar radiation absorber materials. On the other hand, In2O3 is a wide band gap semiconductor (band gap ∼3.75 eV) with an average optical transmittance of ∼85% in the visible region [5]. Stoichiometric In2O3 shows semi-insulating characteristics [6], however, it becomes a degenerate n-type semiconductor with metallic characteristics in oxygen deficient form. It is apparent that the function of In2O3, when it is in contact with other materials variable from buffer to Ohmic contact by controlling the oxygen vacancies alone.

In order to probe the interfaces, two types of structures, viz., Si/In2O3/CuInTe2 and Si/CuInTe2/In2O3 have been formed. The interface formed between In2O3 and CuInTe2 in the former is analogous to the TCO-chalcopyrite interface in absorber based solar cells with TCO back contact [3] as CuInTe2 is deposited on In2O3 film. On the other hand, the interface between In2O3 and CuInTe2 thin films in the latter structure is similar to the chalcopyrite-TCO interface in “superstrate” type solar cells with TCO Ohmic contact [1], [3]. Since, the order in which In2O3 and CuInTe2 films are formed and the methods used for depositing the two are different, there is a possibility that interface characteristics differ.

Section snippets

Experiments

CuInTe2 and In2O3 films were grown by step wise flash evaporation method and dc reactive sputtering respectively. The details of growth of CuInTe2 and In2O3 thin films were presented elsewhere [7], [8]. Briefly, CuInTe2 films were grown onto substrates held at 573 K using step wise flash evaporation. In2O3 thin films were grown by direct current reactive sputtering technique using indium target of 5N purity in the presence of 2% of O2 and Ar mixture. Sputtering was done at a constant power of 100

GXRD studies

Typical GXRD patterns of Si/In2O3/CuInTe2 and Si/CuInTe2/In2O3 recorded with a glancing angle of 3° are displayed in Fig. 1a and b respectively. Analysis of the pattern shown in Fig. 1a revealed the reflections are from the different planes of CuInTe2 and In2O3 films indicating that both are homogeneous. Peaks corresponding to In2O3 are prefixed as INO. In Fig. 1a, reflections from CuInTe2 thin film dominate the pattern as it forms the top layer. On the other hand, the pattern shown in Fig. 1b

Conclusion

The interface between dc sputtered In2O3 and CuInTe2 thin films have been studied by analyzing Si/In2O3/CuInTe2 and Si/CuInTe2/In2O3 structures using GXRD, RBS and Raman spectroscopy. The results show In2O3 and CuInTe2 films have retained their homogeneity in the former structure but for the presence of a thin inhomogeneous interface region, whereas in the latter structure CuInTe2 film has been completely destroyed. The destruction of CuInTe2 film is attributed to the striking of the substrate

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