X-ray photoelectron spectroscopy of electrodeposited cadmium mercury telluride thin films and their native surface oxide
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ontero Instituto de Ciencia de Materiales, CSIC, Campus de la Universidad Auto´noma de Madrid, 28049 Madrid, Spain
Y. Laaziz L.P.S.C.M. Faculte´ des Sciences Semlalia, De´pt. Physique B.P.S./15, 40000 Marrakech, Morocco (Received 6 April 2001; accepted 9 April 2001)
Electrodeposited thin films of CdTe and CdxHg1−xTe with 1 − x ⳱ 0 and approximately 0.1 were characterized by x-ray photoelectron spectroscopy. The composition of the bulk and the thickness and composition of the native surface oxide before and after Ar+ ion sputtering were determined. A surface oxide layer 20-Å thick constituted mainly of alloyed TeO2 and CdO was found over a nearly stochiometric bulk. Chemical diffusion of Hg, Cd, and Te to the surface was observed. Hg appears to inhibit oxidation of the telluride, mainly of Te. Ar sputtering removed undesirable oxides and impurities off the films.
I. INTRODUCTION
CdxHg1−xTe (CMT) is currently one of the most studied semiconductors because of its applications in infrared technology and photovoltaic devices. Mercury is weakly bonded, and when a metal is deposited on CMT, it is easily lost from the surface. This is important when making contact with metals in optoelectronic devices and solar cells.1–3 Another surface problem, the composition of the native oxide on CMT films, has been the source of some controversy.4 On the other hand, anodic oxides on CMT films have an important function in photoconductive detector structures. It is, therefore, important to study the physical and chemical properties of the anodic oxide/ CMT interface to establish and understand the mechanisms involved in the electrical and thermal properties of metal–insulator–semiconductor (MIS) devices.4,5 Moreover, surface oxidation could be used to prevent carbon contamination of the CdTe surface, which is not easily removed. There is also great interest in the effect of oxidation on the CdTe surface.6 Before CMT, CdTe was considered as a semiconductor with an optimum energy band gap (Eg ≈ 1.45 eV) for use in solar cells with CdS.7–9 Even nowadays, CdTe is still used as a photovoltaic material.10,11 The use of Hg as a dopant for CdTe opened up a new way to control the band gap, which depends on the amount of Hg, leading to the production of CdxHg1−xTe thin films. This 1942
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J. Mater. Res., Vol. 16, No. 7, Jul 2001 Downloaded: 14 Mar 2015
semiconductor has been used in infrared detectors with low values of x, typically x < 0.7, and can be used as photovoltaic material for x > 0.8. For this last purpose, the possibility of controlling the gap between 1.1 and 1.5 eV (x ⳱ 0.8 and x ⳱ 1, respectively12) with Cd-rich CMT is very useful for making high-efficiency multigap structures to optimize the use of the solar spectrum in the photovoltaic effect. There are many techniques to grow CMT films. Among the most interesting ones because of its low cost, ease, and use in industry is electrodeposition.13–15 The concentration of Hg2+ in the electrolytic bath and the deposition potential are the fundamental vari
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