The effect of impurities and defects on the optical properties of AlN was investigated. High-quality AlN single crystals of more than 20mm2 size were examined. Different crucible materials and growth procedures were applied to the growth of bulk AlN by physical vapor transport method to vary the defect and the impurity concentrations. The crystalline orientation was investigated by Raman spectroscopy. Glow discharge mass spectrometry was used to determine the trace concentration of the incorporated impurities such as oxygen and carbon. The photoluminescence emission and absorption properties of the crystals revealed bands around 3.5 and 4.3eV at room temperature. Absorption edges ranging between 4.1 and 5.95eV were observed. Since no straight correlation of the oxygen concentration was obtained, a major contribution of oxygen or oxygen-related impurities was ruled out to generate the observed emission and absorption bands in the Ultraviolet spectral range. The carbon-related impurities and intrinsic defects might contribute to the observed optical properties. The absorption coefficient for AlN single crystals has been derived for the spectral range below the band edge.

Epitaxial layers of Ga 1−x Mn x N with concentrations of up to x = 0.015 have been grown on c-sapphire substrates by metalorganic chemical vapour deposition. No ferromagnetic second phases were detected via high-resolution x-ray diffraction. Crystalline quality and surface structure were measured by x-ray diffraction and atomic force microscopy, respectively. No significant deterioration in crystal quality and no increase in surface roughness with the incorporation of Mn were detected. Optical measurements show a broad emission band attributed to a Mn-related transition at 3.0 eV that is not seen in the underlying GaN virtual substrate layers. Room temperature ferromagnetic hysteresis has been observed in these samples, which may be due to either Mn-clustering on the atomic scale or the Ga 1−x Mn x N bulk alloy.

The suppression of the ferromagnetic behaviour of metal–organic chemical vapour deposition grown Ga1−xMnxN epilayers by silicon co-doping, and the influence of the Fermi level position on and its correlation with the magnetic and optical properties of Ga1−xMnxN are reported. Variation in the position of the Fermi level in the GaN bandgap is achieved by using different Mn concentrations and processing conditions as well as by co-doping with silicon to control the background donor concentration. The effect on Mn incorporation on the formation of defect states and impurity induced energy states within the bandgap of GaN was monitored by means of photoluminescence absorption and emission spectroscopy. A broad absorption detected around 1.5 eV is attributed to the presence of a subband introduced by Mn induced energy states due to temperature independent transition energies and linewidths. The intensity and the linewidth of the absorption band correlate with the Mn concentration. Similarly, the magnitude of the magnetization decreases as the Fermi level approaches the conduction band, as the Fermi energy is increased above the Mn (0/−) acceptor state. Silicon concentrations>1019 cm−3 caused the complete loss of ferromagnetic behaviour in the epilayer. The absorption band at 1.5 eV is also not observed upon silicon co-doping. The observed spectroscopic data favour a double-exchange-like mechanism rather than an itinerant free carrier mechanism for causing the ferromagnetism. This behaviour significantly differs from the properties reported for widely studied (Ga, In)MnAs.

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High quality AlN single crystals grown by physical vapour transport and by sublimation of AlN powder were investigated by Raman, photoluminescence (PL) and absorption spectroscopy. Absorption edges of the AlN single crystals varying from 4.1 eV to 5.9 eV as determined by transmission measurements. Near band edge absorption, PL and glow discharge mass spectroscopy identified impurities such as oxygen, silicon, carbon, and boron that contribute to the absorption and emission bands below the bandgap. The absorption coefficients were derived from UV (6 eV) to FIR (60 meV) spectral range. The exact crystal orientation of the samples, and their low carrier density were confirmed by Raman spectroscopy.