High VI/II transport rate ratio for Al-doped ZnTe homoepitaxial layers grown by metalorganic vapor phase epitaxy leads to distinct shallow and deep donor-acceptor-pair (DAP) emissions in the photoluminescence spectrum together with donor-related bound excitonic emission (I d ), independent of the growth conditions. From the analysis of excitation power dependence of shallow DAP emission, donor and acceptor levels are estimated to be ~19.5 and ~53.5meV for Al-doped ZnTe layer, respectively. Thermal quenching effects of I d and shallow DAP were examined based on two step quenching processes, and the derived donor ionization energy is of ~ 19 meV and acceptor level is of ~52.8 meV, which are in good agreement with the result on its excitation power dependence for the latter case.Keywords: Photoluminescence, Al-doped ZnTe layer, metalorganic vapor phase epitaxy
1.INTRODUCTIONZnTe has a variety of potential applications such as pure green light emitting diodes (LEDs), modulators, and photovoltaic converters. Especially this compound is a promising material for pure green LEDs because of its direct transition band gap of 2.26 eV at room temperature. Also, the merit that it does not contain expensive metals such as In and Ga leads to the fabrication of LED with low cost. However, it is very difficult to prepare n-type ZnTe unfortunately, which greatly hindered the realization of most devices such as LEDs. So far, several efforts have been made to attain conductive n-type ZnTe using various dopant materials. Recently, Al has been considered to be a suitable n-type dopant in ZnTe, since n-type ZnTe is obtainable with Al by metalorganic vapor phase epitaxy (MOVPE) [1,2], molecular beam epitaxy [3], thermal diffusion [4,5] and so on. Very recently, we have succeeded the preparation of green ZnTe LED with pn junction by Al diffusion into a P-doped ZnTe [5], and a high efficiency comparable to commercial green GaP LED has been demonstrated in spite of homojunction structure [6]. However, it seems that the reproducibility or highly quality with respect to n-type ZnTe is still lacking at present. With respect to Al-doped ZnTe grown by MOVPE which is suitable for mass production for high-quality epitaxial layer, it is somehow very difficult to prepare even n-type ZnTe reproducibly. In order to improve the circumstance, it is important to acquire information related to compensation mechanism accompanied by Al doping. In this study, thus, we focused on estimation of acceptor level as well as donor level in Al-doped ZnTe layer grown by MOVPE based on the dependence of the photoluminescence (PL) spectra on excitation power or measurement temperature, since there is few information about activation energy of acceptors induced by Al doping.
EXPERIMENTALAl-doped ZnTe layers were prepared on (100) ZnTe substrates by MOVPE at atmospheric pressure. Dimethylzinc (DMZn) and diethyltelluride (DETe) were used as source materials, and trimethylaluminium (TMAl) as a dopant source. Hydrogen was employed as a carrier gas. After being degreased, t...