Date of Graduation
P-type doping in GaN remains a critical issue for GaN-based electronic and optoelectronic devices. Accurate temperature measurement of GaN during molecular beam epitaxy growth is one of the decisive factors to obtain high p-type conductivity due to background carrier concentrations, leading to the compensation effect on p-type doping, and to issues related to dopant incorporation and solubility. Thermocouples or optical pyrometers have been used to determine the substrate temperature of GaN in conventional MBE systems. Both require secondary calibration, leading to inaccurate reading and poor reproducibility of substrate temperature. In-situ cathodoluminescence (CL) occurring during reflection high energy electron diffraction (RHEED) is a strong candidate for a new technique allowing accurate measurement of the substrate temperature of GaN and has been used to determine substrate temperature of GaN for the growth of Mg doped GaN films. CL signal can be detected using either imaging CL technique or spectral measurement. In-situ CL measurements were performed in the MBE chamber with/without any growth flux. A relation between CL peak energy and substrate temperature is reasonably linear and inversely proportional and agreed well with the results by other group. CL measurements can be used to fix the temperature drift problems during the long growth of GaN films. Also, CL measurements can be transferred from one laboratory to another as well as one MBE chamber to another. Beryllium is potentially the shallowest p-type dopant in GaN but it is predicted that Be may incorporate on interstitial sites, acting as a double donor, instead of on substitutional sites. Theoretical calculations suggested using first principles calculations that co-doping with hydrogen and post-growth thermal annealing can improve the incorporation of Be on substitutional sites because the formation energy of Be-H complexes is smaller than that of Be interstitial donors and H can be removed easily by post-growth thermal annealing. Also, use of In as a surfactant or electron beam irradiation during growth was predicted to improve the incorporation of Be on substitutional sites. With these theoretical predictions, Be-doped GaN samples have been grown under atomic hydrogen, indium, and electron beam irradiation and selected samples were annealed after growth to activate Be acceptors. However, all Be-doped samples were n-type or semi-insulating. While as-grown N-polar GaN or Ga-polar GaN with high enough Be doping concentration to result in structural degradation have DAP PL at 3.38 eV, as-grown Ga-polar samples with [Be] â‰¤ 4x10 18 cm-3 have a new â€œDAPâ€ PL at 3.29 eV. Selected samples were annealed in a pure nitrogen gas or forming gas (12% H2 : 88% N2) and all annealed samples exhibit DAP PL at 3.38 eV, likely due to formation of a Be-Ga vacancy related complexes or some other extended complex. Current limits of hole concentration on Mg doping in c-plane GaN is about 1x1018 cm-3 due to saturation or reduction in p-type conductivity with increasing Mg concentration. Hole concentration in Ga-polar GaN is higher than that of N-polar GaN and Ga-polar GaN samples can be inverted to N-polar GaN by structures related to surface segregation during heavy doping with Mg in GaN. This induced polarity inversion is a primary limit on the hole concentration in c-plane GaN. To improve p-type conductivity in c-plane GaN films, increasing solubility and decreasing surface segregation of Mg are major issues. It was theoretically suggested that presence of a high density e-h pairs can increase solubility and decrease surface segregation of Mg and this effect was experimentally observed in Be-doped N-polar GaN sample. While most reports of Mg-doped GaN samples with high hole concentrations were grown at low growth temperature (â‰¤ 700Â°C), our best quality of undoped GaN films have been grown at high growth temperature of 750Â°C. We have grown Mg-doped GaN samples at high growth temperature of around 750Â°C with/without electron-beam irradiation during growth to investigate the effect of electron beam irradiation on p-type conductivity. The maximum hole concentration of âˆ¼6.5x1018 cm-3 was obtained by growing at Mg oven temperature of 413Â°C and substrate temperature of 840Â°C, corresponding to temperature of 750Â°C measured by CL measurements. Based on CL measurements at room temperature, activation energy of Mg is âˆ¼137 meV, likely due to heavy doping effects. Hole concentration of Mg-doped GaN samples depends on growth temperature and Mg oven temperature. Effect of electron beam irradiation during growth on p-type conductivity of Mg-doped GaN samples is not clear yet, based on Hall measurements and CL measurements.
Lee, Kyoungnae, "Issues for p-type doping of gallium nitride with beryllium and magnesium grown by rf-plasma assisted molecular beam epitaxy." (2007). Graduate Theses, Dissertations, and Problem Reports. 9263.