Optical and nano-characterization of gallium nitride and its alloys for ultraviolet photodetector applications

The doping of GaN and its alloys was investigated to determine the factors that limit their conductivity. To achieve high p-type conductivity in GaN, GaN:Mg was deliberately codoped with shallow donors to suppress the formation of compensating native donors, primarily nitrogen vacancy (VN) and its complexes. Codoping changes the Fermi energy, which has been predicted to control the defect stability. The effect of codoping on the defect stability was studied by photoluminescence (PL). Upon codoping the absolute intensity of the 2.8 eV PL band, a donor-acceptor pair transition involving VN, decreased by more than an order of magnitude. This indicated that the compensating native donor formation can be effectively suppressed by means of codoping to increase the Fermi energy, which is explained by an equilibrium thermodynamic model using local density approximation calculations.; In an effort to increase the p-type conductivity of GaN, the effect of limited Mg solubility on the doping behavior was investigated. Heavily Mg doping at or near the solubility limit results in strong compositional/potential fluctuations. Fluctuations related Urbach tail transition was observed in the optical absorption. The Urbach energy increases with increasing potential fluctuation magnitude. Nanoscale microstructure of the fluctuations was determined by near field scanning optical microscopy (NSOM) and scanning Kelvin probe microscopy (SKPM). A characteristic wavelength of 300--500 nm was observed for an average Mg doping concentration of 4 x 10 19 cm-3. From the SKPM image the local Mg concentrations were calculated. For GaN:Mg with a Mg concentration of 4 x 10 19 cm-3, a RMS value of 5.4 x 10 19 cm-3 was determined for the compositional fluctuations. The potential fluctuation magnitude between 0.07 and 0.14 eV was calculated from PL studies.; The optical properties of InGaN alloys were studied in an effort to determine the origin of their high background electron concentrations. The effect of limited solubility of In in GaN on the alloying behavior of InGaN was investigated. NSOM study indicated the presence of strong alloy fluctuations in InGaN. Both their characteristic wavelength and magnitude increased with increasing In concentration, which was analyzed in terms of a spinodal decomposition model.