Growth and Raman Spectroscopy of Single Crystal ZnGeN 2 Rods Grown from a Molten Zn/Ge Alloy
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Growth and Raman Spectroscopy of Single Crystal ZnGeN2 Rods Grown from a Molten Zn/Ge Alloy Timothy J. Peshek1, Shanling Wang2, John C. Angus3, and Kathleen Kash1 1 Department of Physics, Case Western Reserve University, 10900 Euclid Ave., Cleveland, OH, 44106 2 Department of Materials Science, Case Western Reserve University, 10900 Euclid Ave., Cleveland, OH, 44106 3 Department of Chemical Engineering, Case Western Reserve University, 10900 Euclid Ave., Cleveland, OH, 44106 ABSTRACT We present evidence for the growth of ZnGeN2 from a molten Zn/Ge alloy via the vaporliquid-solid mechanism. Hexagonally faceted, 3-4 µm wide by 20-40 µm long, single crystal rods of ZnGeN2 capped by a polycrystalline dome of ZnGeN2 were formed. A micro-Raman spectrum shows several individually resolved peaks and no spectral features above 825 cm-1, in contrast to a previously published spectrum for polycrystalline ZnGeN2, but in excellent agreement with recent theoretical predictions. INTRODUCTION The replacement of the Ga atoms of the III-V material GaN with its II-IV neighbors on the periodic table, Zn and Ge, gives rise to the II-IV-V2 analogue, ZnGeN2. It is remarkable how closely lattice matched ZnGeN2 is to GaN [1-9], and how similar are the band gaps, according to recent measurements [9]. The similarity in lattice constants makes ZnGeN2 a potentially attractive substrate for GaN devices. Furthermore, ZnGeN2, ZnSnN2 and ZnSiN2 together form an analogue to the GaN-InN-AlN family of semiconductors that is worthy of further examination. For example, it has been noted that the high vapor pressure of Zn at the ZnGeN2 growth temperature may tend to favor the formation of intrinsically p-type material via the incorporation of Zn vacancies [11], whereas GaN is intrinsically n-type. There has been relatively little work done on ZnGeN2 (and even less on ZnSnN2 and ZnSiN2). The growth of ZnGeN2 reported to date has been by a number of different techniques all using vapor phase reactions and yielding either polycrystalline material or films on highly mismatched substrates [1-9]. One previously reported study revealed ordering in the cation sublattice by neutron diffraction [11] This observation is consistent with the orthorhombic structure measured by x-ray diffraction, rather than the wurtzite structure expected if the cation sublattice were completely disordered. However, these and the x-ray diffraction results are also consistent with partial ordering of the cation sublattice. The phonon modes for orthorhombic ZnGeN2 (cation sublattice completely ordered) have been calculated; the Raman-active modes are very different from those of wurtzite GaN [12]. Therefore, measuring the phonon selection rules with Raman spectroscopy can provide new information on the ordering of the cation sublattice. This work reports growth of single crystal rods by a vapor-liquid-solid mechanism and has allowed the measurement of polarized Raman spectra.
GROWTH Zn and Ge were obtained from Alfa Aesar (each 99.999% pure, metals basis). The Zn was etch
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