Recently, pure green light emitting diode (LED) based on ZnTe has been realized [1,2]. By fabricating the hetero structure, the efficiency of the LED must be improved significantly. ZnMgTe ternary alloy is one of the candidates for the cladding layer since it has wider band-gap than ZnTe. Since ZnTe and MgTe have different crystal structure, the crystal structure of ZnMgTe is changed depending on the Mg content. The epitaxial growth under the non-equilibrium condition such as molecular beam epitaxy (MBE) often results in phase modification. In order to fabricate the LED on ZnTe substrate, it is important to know the crystal structure of ZnMgTe epitaxial layer on ZnTe. In this study, we have grown ZnMgTe epitaxial layers with a wide range of Mg content on ZnTe substrate by MBE.
The epitaxial growth was carried out using solid-source MBE system. The ZnTe (001) substrates were cleaned ultrasonically by organic solvents, and etched in a 1% Br-methanol solution and HF-distilled ionized water solution. In order to remove the native oxide and the contamination from the surface of ZnTe substrate, in-situ H radical cleaning was performed just before the growth. In the growth, a low-temperature ZnTe buffer layer was grown at 250 °C for 5 min, followed by the growth of ZnMgTe layer at 400 °C. In this study, the Mg/(Zn+Mg) flux ratio was changed between 0 and 0.26. During the growth, a reflection high energy electron diffraction (RHEED) pattern was observed.
The layers grown at the Mg/(Zn+Mg) flux ratio above 0.16 decomposed quickly by hydration just after exposing in air, indicating that the Mg content was very high. From the RHEED and the high-resolution x-ray diffraction patterns, all ZnMgTe layers on ZnTe were found to crystallize in a zincblende structure, suitable for fabricating LED. According to the Vegard's law, the Mg/(Zn+Mg) ratio in the ZnMgTe layer was calculated with the range between 0 and 0.85, which was about 8.5 times larger than the Mg/(Zn+Mg) flux ratio.
This study was partly supported by Industrial Technology Research Grant Program in 2005 from New Energy and Industrial Technology Development Organization of Japan.
[1] K.Sato et. al, Phys. Stat. Sol. (a), 180 (2000) 267.
[2] T.Tanaka et. al, Jpn.J.Appl.Phys. 42 (2003) L362.
|