A convenient picture of the ZnGeN₂ lattice can be realized by replacement of the Ga atoms in the GaN lattice by alternating Zn and Ge atoms. This replacement results in a 2x2x1 orthorhombic lattice accompanied by a slight distortion of cell shape, and bond angles, and a bimodal distribution of bond lengths.¹ GaN and ZnGeN₂ have nearly identical lattice constants² of the underlying wurtzite lattice, and nearly identical band gaps.³ However, we find that the Raman spectra for the two materials are substantially different. The period doubling in two directions in the c-plane compared to the wurtzite lattice results in 78 phonon modes for ZnGeN₂, all of them Raman active,⁴ versus 6 Raman-active modes for GaN. Here, we present polarized Raman spectra on individual, oriented single crystal ZnGeN₂ for several scattering geometries. The crystallites, grown by a vapor-liquid-solid method using NH₃ and elemental Zn and Ge, are single crystal rods, as determined by electron diffraction, of lengths of the order of 100 μm and approximately hexagonal cross sections of up to 6 μm in width. A comparison of our results with a previously published unpolarized micro-Raman spectrum for polycrystalline material reveals major differences². These include our observation of many individually resolved Raman peaks, and the absence of spectral weight for frequencies above 850 cm^-1. The previously published spectrum shows a strong Raman signal in the entire region from 850-1300 cm^-1. This portion of the spectrum, which, again, is absent in our spectrum, has been attributed tentatively to second harmonic overtones of the single phonon spectrum.⁴ A comparison with theory, including a discussion of the relevant selection rules for different scattering geometries, will be presented.This work was supported partially by grants from the Department of Education ( APR P200A030186), the National Science Foundation (DMR-0420765) and the Air Force Office of Scientific Research (F49620-03-1-0010).¹ S. Limpijumnong, S.N. Rashkeev and W.R.L. Lambrecht, MRS Internet J. Nitride Semicond. Res. 4S1, G6.11 (1999).² R. Viennois, T, Taliercio, V. Potin, A. Errebbahi, B. Gil, S. Charar, A. Haidoux, and J.-C. Tédenac, Mat. Sci. Eng. B82, 45 (2001).³ K. Du, C. Bekele, C.C. Hayman, J.C. Angus, P. Pirouz, and K. Kash, “Synthesis and Characterization of ZnGeN₂ grown from Elemental Zn and Ge Sources”, submitted to J. Crystal Growth.⁴ Walter R.L. Lambrecht, Erik Alldredge and Kwiseon Kim, Phys. Rev. B72, 155202 (2005).

Bulk AlN and SiC crystals are currently grown using Physical Vapor Deposition (PVT) technique. This technique has some characteristic features as compared to various Chemical Vapor Deposition (CVD) methods, related to growth of the crystals in more or less tightly closed crucibles. In this case, total pressure and vapor composition in the crucible cannot be directly controlled but are spontaneously determined by the conditions of mass and species exchange between the crucible and the ambient. This effect essentially worsens controllability of the PVT technique and hampers its modeling analysis and optimization.We have developed original models of the AlN and SiC PVT growth that describe this effect and provide the correct prediction of the total pressure and vapor composition in both the hermetically closed and somewhat open crucibles. The models give also detailed description of the interplaying processes of heat exchange (conductive, convective, and radiant), gas flow dynamics, multi-component species diffusion, and surface chemical kinetics. It is essential that they allow modeling the process evolution during long growth of large crystals, including gradual movement of the evaporation/crystallization fronts and shift of the initial growth conditions. The models are validated using the available experimental data and embodied as the commercial computational codes Virtual Reactor AlN TM and Virtual Reactor SiC TM.The developed codes are applied to find the optimal technological conditions for growth of bulk AlN and SiC crystals. Computations with the codes revealed some unexpected effects, including possible high jumps of the species partial pressures at the Knudsen layers on the reactive surfaces, considerable increase of the total pressure in the crucible due to the diffusion of inert carrier gas from the ambient, complicated movement of the evaporation/crystallization fronts relative to the isotherms, disappearance of the species diffusion in the strictly stoichiometric vapor, and others. The results of computations were applied to study of particular machines for sublimation growth of bulk AlN and SiC crystals. Optimization of the processes allowed growing the crystals of the desired slightly convex shape that favors high quality of the crystals.

Solid electrolytes are an important class of materials used in fuel cells, sensors, or batteries. In the case of anion conductors solids with high oxygen mobility are most prominent. Zirconia doped with aliovalent oxides such as yttria or scandia is commonly used in Solid Oxide Fuel Cells or automotive oxygen sensors. These zirconia-based materials can be described as anion-deficient fluorite-type structures. This structure type tolerates a large amount of oxygen vacancies and an activation energy of around 1 eV was found for the jump process of the oxygen ions. Thinking about solids with mobile nitrogen ions, fluorite-type based nitride oxides are promising candida