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The improved resistively-heated furnace with two heaters established a vertical thermal gradient to control nucleation during AlN single crystals Physical Vapor Transport (PVT) growth on polycrystal tungsten substrates. During the high temperature (> 1850 °C) heating process, the reverse temperature field (i.e., the temperature difference between the sublimation zone and the crystalline zone ΔT < 0) was obtained to reduce the number of nuclei on the tungsten substrate. During growth, the proper positive values of ΔT T were chosen to content the supersaturation values (0.25 < S < 0.3). The reverse temperature condition during high temperature (> 1850 °C) cooling was fulfilled to avoid recrystallization on grown AlN crystal. AlN single crystals made through the method were characterized by X-ray diffractions (XRD) and Raman spectroscopy. [ABSTRACT FROM AUTHOR]

This paper focuses on the detailed investigation of the structural and electronic properties of wurtzite AlN crystals doped by Li with and without oxygen with the first principles calculation. All the calculations have exhibited significant structural distortions. Compared with the monoclinic doping, the oxygen codoping has improved the structure deformation and lowered the formation energy of Li dopants. The calculated electronic density of states (DOS) reveals that all doping configurations still preserve semiconductor characteristics. The states around the valence band maximum cross the Fermi level, which implies p-type doping. The induced extra levels are extremely localized and flat in Li-doped AlN while much more delocalized in oxygen codoped models. The mono-doping of Li is in general energetically unfavorable while the codoping improves the formation and makes the intercalation of Li more stable in AlN. According to the results, the codoping configuration of Li with O in AlN has provided a useful way of modifying the corresponding properties. [ABSTRACT FROM AUTHOR]