Electronic Bandgap and Refractive Index Dispersion of Single Crystalline Epitaxial ZnGeN 2
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ABSTRACT The electronic band gap of single crystalline ZnGeN 2 epitaxial layer grown on sapphire substrate by metal organic chemical vapor deposition has been measured by optical transmission and room temperature photoluminescence. The band gap energy is 2.99eV at room temperature, and the band gap is a direct transition type. The interference oscillations of the transmission spectra together with rutile prism coupling measurements have been used to determine the refractive index and the dispersion characteristics of the single crystal ZnGeN 2 below the band gap energy. The rutile prism coupling measurement displays the wave guide modes of the film at 632.8nm wavelength of the He-Ne laser, enabling determination of the film thickness and refractive index precisely at the wavelength. The refractive index of ZnGeN 2 crystal is 2.35 at 6328A wavelength. The measured refractive index dispersion curve can be fitted with the firstorder Sellmeier equation n 2(X) = A + X2/(X2-B), using fitting parameters A=4.3 1, B=0.076. INTRODUCTION Recently Zhu et al reported growth of single crystalline layers of ZnGeN 2 by metal organic chemical vapor deposition technology on sapphire substrates and GaN/sapphire wafers [1]. A theoretical investigation of the electronic structure of ZnGeN 2 was also reported [2]. The goal of this research is to explore the potential of a new family of wide band gap nitride semiconductors for short wavelength optoelectronic and high temperature electronic
applications. The advance in crystal growth technology is making this historically third attempt possible. ZnGeN 2 is one material in the II-IV-N 2 compound family, which belongs to the "periodic compounds" classified by Hall [3]. These compounds are characterized as the nearest crystallochemical and electronic analogues of the group IV element crystals, where the number of average valence electrons per atom is four. The tetrahedral coordination of the atoms in this crystal, predominantly covalently bonded, results in a wurtzite-type lattice as the stable structure, like its III-V compound analogues, the group III-nitrides. The first research on this material system was initiated in the early 1970s [4,5], using a high temperature, high pressure powder synthesis method. The resulting poly-crystalline bulk powders identified the existence of some of the compounds. The second attempt involved chloride-hydride vapor phase epitaxy [6], which resulted in polycrystalline films. In the previous paper the authors described the growth resulting in the first single crystalline ZnGeN 2 layers, and characterized the lattice structure and crystalline quality in terms of X-ray diffraction data. The lattice parameters are in fairly good agreement with the data obtained by Lang et al on polycrystalline powder samples [4,5]. The epitaxial single crystal however is found to be disordered, instead of Zn/Ge ordering in the sublattice in the freely nucleated poly-grains [5]. In this article, we report on electronic band gap and refractive indices of ZnGeN 2 determined from
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