Investigation of the point defect structure of epitaxial gallium nitride

The defect structure of undoped and doped GaN was investigated. To determine the origin of the residual n-type donors as well as compensation mechanisms, epitaxial layers were prepared by atmospheric pressure metalorganic vapor phase epitaxy. GaN thin films with electron concentrations of 0.6–1 × 1017 cm−3 and electron mobilities of 450–520 cm2/Vs were obtained. Deliberately oxygen-doped layers were prepared to determine the importance of oxygen as a residual donor. The electron concentration of O-doped layers increased as the square root of the oxygen partial pressure. Variable temperature Hall effect and photoluminescence (PL) measurements indicated that oxygen was a shallow donor with a thermal ionization energy of 27 ± 2 meV. From the donor concentration measurements, a formation energy of 1.3 eV was calculated for oxygen indicating that it was highly soluble.; Compensation of unintentionally doped GaN films by native defects as well as impurities was observed by photoluminescence. Three donor-acceptor pair (DAP) transitions in undoped GaN were observed with maxima at 2.2, 2.5 and 2.9 eV. Quantitative analysis of the continuous wave PL and transient PL decay spectra indicated these bands were associated with donor acceptor pair (DAP) transitions involving a shallow donor and three acceptor states of different origins. A configuration coordinate model was utilized to explain broadening of these PL bands with temperature. The position of the zero phonon line of 3.098 eV for the 2.9 eV band was determined from the spectral fine structure at low temperature. Furthermore the acceptor involved in the 2.9 PL band was shown to couple with local and lattice phonons with energies of 36 and 91 meV, respectively.; A recombination model was developed to quantify the properties of acceptors involved in the DAP emissions. Acceptor concentration, formation energy, activation energy, radiative electron, non-radiative hole capture cross-sections were calculated using this model. Based on the comparison of acceptor formation energies and capture cross-sections, the identities of these acceptors were elucidated. The defect involved in the band at 2.2 eV was shown by varying the V/III ratio during the growth not to be a gallium vacancy, in contrast to the previous findings. The acceptor involved in 3.27 eV band was shown to be related to a family of defects involving the V_Ga.; P-type doping of GaN films was investigated to increase film conductivity. By co-doping with oxygen and magnesium the room temperature hole concentrations increased from 6 × 1016 to 2 × 1018 cm−3, and resistivity decreased from 8 to 0.2 Ωcm. Variable temperature Hall effect measurements indicated that the acceptor activation energy decreases from 170 ± 5 meV in Mg-doped films to 135 ± 5 meV upon oxygen doping. The increase in hole concentration results in, part from a decrease in the ionization energy of the acceptor. A decrease in the native deep donor defect concentration with oxygen co-doping was observed by PL spectroscopy.