Growth conditions in molecular beam epitaxy for controlling CdSeTe epilayer composition

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Abstract

We studied the growth conditions for controlling CdSeTe epilayer composition. Te atoms are readily incorporated into CdSeTe epilayers. However, we could not control the Te composition by the beam intensity ratio of Te to Cd. Therefore, Te atoms are not preferentially chemisorbed on Cd atoms. Both the difference in the desorption rate of the physisorbed atoms and the difference in the formation energies of CdSe and CdTe affect the composition. The epilayers composition could be well controlled in the entire range by the beam intensity ratio of JSe/(JSe+JTe) when the epilayers were grown on the Cd-stabilized surface. A simple growth model is presented and the sticking probabilities of Se and Te were determined from the growth rate to be 0.1 and 1, respectively.

Introduction

Quantum dots of II–VI compounds in ZnSe matrix are attractive materials for visible-range optoelectronic devices. CdSe/ZnSe quantum-dots structures have the large conduction-band offset and green emissions were observed in the photoluminescence (PL) [1], [2], [3], [4], [5]. CdTe/ZnSe quantum-dots structures have the large valence-band offset and also show green emissions in the PL spectra [6]. CdSexTe1−x/ZnSe quantum-dots structures have the large conduction- and valence-band offsets at an appropriate composition and are expected to be a highly efficient light-emitting material in green region. However, to our knowledge, there have been very few reports on CdSeTe epilayers [7]. It is necessary to establish the growth conditions for controlling the composition of CdSeTe mixed crystals. The composition control of II–VIxVI1−x mixed crystals is difficult because the vapor pressures of group VI elements are high. Several researchers overcame this difficulty and succeeded in the composition control of ZnSexTe1−x films by molecular beam epitaxy (MBE) [8], [9], [10]. In this report, the growth conditions were studied for controlling CdSeTe epilayer composition referring to the successful ZnSexTe1−x studies.

Section snippets

Experimental procedure

The substrates used were GaAs(1 0 0). Elemental 6N–Cd, 6N–Se and 6N–Te were used as source materials. The growth temperature was 300°C. The thin ZnSe buffer layer was first grown on the GaAs substrate, and the CdSeTe epilayer was successively grown on it by MBE. The growth rates were about 0.5 μm/h and a typical growth time was 2 h.

Reflection high-energy electron diffraction (RHEED) patterns were observed during growth. X-ray diffraction measurement was performed and the composition of CdSexTe1−x

Results and discussion

The beam intensity ratios of Se (JSe/(JSe+JTe)) were varied to control the composition of CdSexTe1−x epilayers. Fig. 1 shows the JSe/(JSe+JTe) ratio dependence of the Se composition x in the epilayers. The JVI/JII ((JSe+JTe)/JCd) ratios are 0.75. The dashed line represents x=JSe/(JSe+JTe), which means Se and Te atoms are equally incorporated into CdSeTe epilayers. Our data are below this dashed line, which means that the Te atoms were readily incorporated into CdSeTe epilayers than the Se

Conclusions

The growth conditions were studied for controlling CdSeTe epilayer composition referring to the successful ZnSexTe1−x studies. The beam intensity ratios of Se (JSe/(JSe+JTe)) were varied to control the composition of CdSexTe1−x epilayers at a JVI/JII ratio of 0.75. The Te atoms were readily incorporated into CdSeTe epilayers compared to the Se atoms. The composition x increases steeply at JSe/(JSe+JTe) ratios around 0.7 and the composition control is difficult. We tried to control the Te

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