Mg Doped GaN Using a Valved, Thermally Energetic Source: Enhanced Incorporation, Control and Quantitative Optimization
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Mg Doped GaN Using a Valved, Thermally Energetic Source: Enhanced Incorporation, Control and Quantitative Optimization Shawn D. Burnham, W. Alan Doolittle, Gon Namkoong, and Walter Henderson Electrical and Computer Engineering, Georgia Institute of Technology, Atlanta, GA 30332 ABSTRACT In this study, a thermally-energetic Mg source with an independent, valved-flux control was used to study the behavior of Mg incorporation into GaN. To observe effects of the thermal energy of the Mg flux on Mg incorporation, two Mg flux temperatures were investigated: one (900°C) well above the melting point of Mg and one (625°C) slightly below the melting point of Mg. Alternating Mg-doped and undoped GaN layers were grown at steps of increasing Mg flux, retaining a constant thermal energy, from below the saturation limit, to above the saturation limit. Results were analyzed and compared using secondary ion mass spectroscopy (SIMS). For a constant measured Mg flux, the incorporated Mg increased by more than an order of magnitude when the Mg thermal source temperature was raised from 625°C to 900°C. During SIMS analysis, the energy spectra of sputtered Ga atoms were fairly constant for a Mg flux above the saturation limit, and shifts for a Mg flux slightly below the critical flux for saturation, indicating a conductivity change, and possibly providing a quantitative means of optimizing p-type conduction. Furthermore, Mg incorporation into GaN strongly depends on the III-V flux ratio. During this study it was also observed that Mg incorporation into GaN was enhanced on a rough growth-layer surface under N-rich conditions, while a smoother growth-layer surface resulted in lower Mg incorporation, even under N-rich conditions 1. INTRODUCTION Acceptable levels and control of p-type doping of GaN are a necessity for the advancement of promising electronic and optoelectronic devices such as ultraviolet light emitting diodes, or high-speed and high-power heterojunction bipolar transistors (HBTs). Magnesium was the first p-type dopant of GaN successfully used 2, and is now the most widely used and studied of the possible Ga-substitutionals. However, due to a high vapor pressure 3 at low temperatures, low sticking coefficient, surface accumulation effects 4, relatively low solubility 5, deep ionization energy 6, and compensation at high dopant concentrations 5, acceptable levels and control of Mgdoping of GaN have been a challenge, limiting the development of this promising compound semiconductor. Control of Mg as a dopant in molecular beam epitaxy (MBE) can be especially difficult because of the high vacuum environment. The temperature needed for dopant-level vapor pressures of Mg is relatively small 3 and the vapor pressure curve has a large slope at small temperatures, making it highly sensitive to thermal fluctuations. Conventional effusion cells use thermal energy to control the flux of the material in the cell. They can demonstrate very sluggish response, and have limited flux control due to steep vapor pressure curves of Mg ver
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