Three dopants (Be, Zn, Mg) were analysed in terms of diffusion through the crystal lattice of HVPE-GaN. Different crystallographic directions were investigated: [0001], [10-10] and [11-20]. Ion implantation was employed to create a thin layer of strongly doped GaN which acted as the diffusion source. Annealing in high nitrogen pressure was performed. Secondary ion mass spectrometry (SIMS) was used to measure the post-annealing depth profiles of implanted species. The measured profiles were used for calculation of diffusion coefficients and activation energies for all dopants and crystallographic directions. A strong dependence of diffusion on crystallographic orientation and impurities composition was observed.
Implantation of Ga ions into ammonothermal GaN crystals is proposed as a method of controlling the concentration of gallium vacancies. Ultra-high pressure annealing (UHPA) is expected to facilitate the diffusion of the implanted Ga ions into the entire volume of the crystal. Gallium vacancies are expected to be replaced by the Ga ions. Since Ga vacancies act as acceptors in GaN, reducing their content will result in a higher free electron concentration in the crystal.
Gallium ion implantation and UHPA processes will be presented in detail. Values of the main parameters of UHPA allowing for the complete replacement of the Ga vacancies with Ga ions in GaN crystals will be determined. The morphology as well as structural, optical, and electrical properties will be compared for samples without any treatment and those implanted and annealed. The ultimate goal is to obtain highly conductive GaN crystals with the lowest possible Ga vacancy concentration.
Diffusion of Be was investigated for the main crystallographic directions in HVPE-GaN: c [0001], m [10-10], and a [11-20]. Be was implanted into the samples at room temperature with a dose of 2.9e15 cm-2 with energy of 200 keV. Ultra-high pressure annealing (UHPA) was performed to repair the post-implantation damage and activate the dopant. The annealing was performed at different time (15 and 30 minutes) and temperature (1200 – 1400°C). Depth profiles of Be were measured by secondary ion mass spectrometry (SIMS). Diffusion coefficients were calculated using the complementary error function (erfc) or Boltzmann-Matano analysis. The determined diffusion coefficients were summarized in an Arrhenius plot: D(1/T). From this relation the pre-exponential factor D0 and the activation energy were calculated. In addition, it was possible to calculate the formation energy for interstitial Be as well as gallium vacancy defects.
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