Structure and growth rate of ZnTe films grown by isothermal closed space sublimation

doi:10.1016/j.jcrysgro.2007.06.030

Journal of Crystal Growth

Volume 307, Issue 2, 15 September 2007, Pages 253-258

https://doi.org/10.1016/j.jcrysgro.2007.06.030 Get rights and content

Abstract

Results on isothermal closed space sublimation (ICSS) of ZnTe using different experimental conditions are presented. Wide ranges of exposure and purge times were used to study the properties of the adsorption and desorption processes in this material. Moreover, the atmosphere and the type of substrates were also varied. It was found that epitaxial ZnTe layers are always obtained in either [1 1 1] or [1 0 0] oriented GaAs substrates regardless the used gas atmosphere. In contrast, samples grown on [1 0 0] oriented Si substrates showed powder-like X-ray diffraction features with a strong preferential orientation in the [1 1 1] direction. In both kinds of substrates, there were obtained growth rates considerably larger than one monolayer per cycle for samples grown with large exposure times to the sources. The influence of different process like gas absorption in the substrate holder, gas transport in the substrate compartment and multilayer adsorption on these large growth rates is discussed.

Introduction

Atomic layer epitaxy (ALE) is one of the methods commonly used for growing very thin epitaxial films and/or other small-dimension structures [1], [2], [3]. In ALE the growing surface is exposed cyclically to the vapours of reactants (sources) in order to get a saturated surface reaction. However, between successive exposures to the sources there is a purge or “dead” step in which incoming vapours are removed from the growing surface environment and the surface might evolve to a stable state through desorption. Therefore, the self-regulation feature of ALE might be attained by means of the saturation of chemisorption of the reactants (during the exposure to the sources) or by desorption (during the purge). Anyhow, ALE growth is not controlled or governed by the dose of reactant but rather by the surface (stable coverage) itself. For this reason, very thin films can be obtained with accurate control of composition and thickness. Typically, molecular beam epitaxy (MBE) [3] and chemical vapour deposition (CVD) [2] have been used to obtain self-regulated (SR) growth regimes based in a self-terminated reaction during the exposure step of the growth cycle.

In previous papers, the SR growth of ZnTe using an isothermal closed space sublimation (ICSS) approach has been well documented by the authors [4]. The more complicated case of CdTe growth by ICSS has been also analysed [5], [6] in more recent works. In this last case, the role of the purge time in obtaining a stabilized surface was demonstrated. Both exposure and purge time intervals were varied in the 3–20 s range. In fact, it was observed that during the exposure time to the Cd source, in spite of the isothermal condition used, several monolayers of this element were deposited, and large purge time intervals were needed to desorb the non-stable outermost part of the layers. For this reason, the combination of short exposure times to the sources and large purge times, led to SR growth of this binary compound. In this sense the regulation mechanism in CdTe growth by ICSS was found to be different to that observed in other growth techniques like MBE or CVD. The reaction was not self-terminated in the exposure step (or at least not at a few monolayers). We explained this fact as due to multilayer adsorption. However we still obtained a SR growth when the purge time was large enough for allowing re-evaporation of the non-stable physisorbed outermost part of the multilayer films. In this way, while there was not a self-terminated reaction during the exposure step, it was observed a self-terminated desorption in the purge step. We found that under this condition a SR growth rate of 2 ML/cycle was obtained for CdTe. SR growth at more than 1 ML/cycle has been already observed for CdTe by other authors [7].

In previous papers, combinations of exposure and purge times used in ZnTe growth were not adequate to check the occurrence of multilayer adsorption of this material. In the present work, we extend the study on ZnTe ICSS by using wide ranges of exposure and purge times in order to explore the occurrence of multilayer adsorption in this material. The effect in the growth rate behaviour of ZnTe layers of other processes different to multilayer adsorption are considered and evaluated. Moreover, the influence of the gas atmosphere as well as the crystallographic orientation and type of the substrate are also analysed.

Section snippets

Experimental details

General details of the growth process and the growth system can be found elsewhere [3], [4], [5]. A sketch of the growth system is shown in Fig. 1. The quartz reactor (a) contains a graphite crucible which is placed in a plate-shaped temperature profile. The crucible has two parts: the upper part (b) is movable and holds the substrate (c), while the lower one has two wells (d) for the elemental sources and an aperture (e) that allows the upper part communicate with the controlled pure (He, Ar

Structural characterization

Fig. 2 shows a reciprocal lattice map around the symmetric (1 1 1) Bragg reflection region on GaAs and ZnTe for a typical ZnTe/GaAs (1 1 1) sample. The $(\bar{2} 2 4)$ non-symmetric peak was also measured. From the analysis of the peak positions of the GaAs and ZnTe Bragg reflections, lattice parameters of a_∥=6.102 Å and a_⊥=6.1002 Å were calculated for ZnTe in the directions parallel to the interface (in $[\bar{1} \bar{1} 2]$ azimuth) and in growth direction ([1 1 1] direction). Therefore, within the precision of the

Conclusions

ZnTe films were grown onto [1 1 1] oriented GaAs using different atmospheres and [1 0 0] oriented Si substrates in Ar atmosphere. On GaAs, the samples grown epitaxially regardless the gas atmosphere used. On Si, however, a powder-like structure was observed from X-rays measurements. In this case, ZnTe polycrystalline films were found to have strong preferential [1 1 1] orientation. By using a range of exposure and purge times wider than in previous papers [4], [5], it was possible to verify the

Acknowledgements

OdeM and JMD acknowledge the SAB2005-0023 project supported by the Secretary of State for Universities and Research of Spain and the Visiting Scholar Programme VS-404 from the International Centre for Theoretical Physics. EML is grateful for the research fellowship from the Alexander von Humboldt Foundation.

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Citation Excerpt :
In this paper we demonstrate the use of the Isothermal Close Space Sublimation (ICSS), a cost-effective technique, to prepare CdxZn1−xTe alloys with GC. This physical vapor deposition technique, in which the substrate is exposed sequentially to the elemental sources, has been previously employed for growing epitaxial as well as polycrystalline ZnTe, CdTe and CdxZn1−xTe with constant composition [17,18]. This technique revealed a regulated growth rate determined by the multilayer adsorption of the vapor elements in every exposure, the growth rate being controlled by both the exposure and the purge time.
Graded composition Cd_xZn_1−xTe films were prepared by growing several alternate layers of CdTe and ZnTe by the Isothermal Close Space Sublimation technique. The thickness of both kinds of layers was modified along the structure to produce an increase of the average concentration of CdTe towards the surface of the films. Due to Zn/Cd inter-diffusion during the growth process the sequence of layers was converted into a relatively smooth graded composition film. According to X-ray diffraction characterization the layers grew coherently with the (100) oriented GaAs substrates although they showed a relatively high mosaicity. θ−2θ plots show very wide diffraction peaks as expected from variable composition samples. The band gap grading effect in light absorption was also verified through transmission measurements, using transparent substrates. Graded composition profiles of the thin films were confirmed by x-ray photoelectron and secondary ion mass spectroscopies. Moreover, quantitative Cd, Zn and Te profiles were obtained by the analysis of Rutherford backscattering spectra of the samples. This analysis showed a CdTe molar fraction profile ranging from approximately x=0.8 at the surface of the sample and x=0.35 at the interface with the substrate. The possibility of growing graded composition layers using a simple and cost-effective procedure was demonstrated. This could be interesting in technological applications like Cd_xZn_1−xTe layers of variable composition in CdS/CdTe solar cells.

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¹: Also at: Fachbereich Physik, Universität Dortmund, Otto-Hahn-Strasse 4, D-44221 Dortmund, Germany.

²: Also at: Instituto de Ciencia de Materiales de Madrid, ICMM-CSIC, Cantoblanco 28049 Madrid, Spain.

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