Experimental and theoretical results of Mg-doped superlattices consisting of uniformly doped AlxGa1−xN, and GaN layers are presented. Acceptor activation energies of 70 and 58 meV are obtained for superlattice structures with an Al mole fraction of x=0.10 and 0.20 in the barrier layers, respectively. These energies are significantly lower than the activation energy measured for Mg-doped bulk GaN. At room temperature, the doped superlattices have free-hole concentrations of 2×1018 cm−3 and 4×1018 cm−3 for x=0.10 and 0.20, respectively. The increase in hole concentration with Al content of the superlattice is consistent with theory. The room temperature conductivity measured for the superlattice structures is 0.27 S/cm and 0.64 S/cm for an Al mole fraction of x=0.10 and 0.20, respectively. X-ray rocking curve data indicate excellent structural properties of the superlattices. We discuss the origin of the enhanced doping, including the role of the superlattice and piezoelectric effects. The transport properties of the superlattice normal and parallel to the superlattice planes are analyzed. In particular, the transition from a nonuniform to a uniform current distribution (current crowding) occurring in the vicinity of contacts is presented. This analysis provides a transition length of a few microns required to obtain a uniform current distribution within the superlattice structure.
A new nitride semiconductor, single crystalline ZnGeN 2 has been successfully grown by MOCVD for the first time. The epitaxial ZnGeN 2 is found to be of hexagonal wurtzite lattice without ordering of the zinc and germanium atoms in the pseudomorphic Group III sublattice. Lattice constants of the ZnGeN 2 are a = 3.186 ± 0.007 Α, c = 5.174 ± 0.012 A, which gives c/a = 1.624.
Pyroelectric properties of high-quality (0001)AlN films grown on (111) Si were studied. The pyroelectric coefficient p was measured using the dynamic method. The value was in the range of 6–8 μC/(m2 K), yielding a p/ε figure-of-merit of 0.8–0.95. The pyroelectric coefficient was independent of temperature and applied bias. Leakage current as low as 1–2×10−9 A/cm2 was measured at 5 V on large area devices. The obtained results indicate that AlN films can be used in pyroelectric thin-film devices.
Thin films of yttrium-rich YBa2Cu3O7−x with c-axis orientation prepared by plasma-enhanced metalorganic chemical vapor deposition have been examined by high-resolution transmission electron microscopy. Yttria precipitates smaller than 50 (Å) in size have been identified in the matrix. They are uniformly distributed, have a high density as large as 1024 per cubic meter and are oriented with respect to the matrix. The magnetic field dependence of the critical current density of the thin films indicates that the yttria precipitates are effective flux pinning centers.
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