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A valved thermal cracker was used as the Mg dopant source for growing p-type GaN, and benefits of this device over conventional effusion cells are outlined. The thermally energetic Mg cracker source, with an independent, valved-flux control, was used to study the behavior of Mg incorporation into GaN. A mathematical model of the cracker's operation was derived for enhanced doping control. This enhanced control was demonstrated with abrupt and linear profiles of Mg doping. To observe effects of the thermal energy of the Mg flux on Mg incorporation, two cracking tip temperatures (which thermally energized the Mg flux) 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 to 900 °C. This increased incorporation, along with other detailed SIMS observations, suggests a shift in the saturation point toward a lower flux as the thermal energetics of the beam are increased. The best sample grown with this new source had an average resistivity of 0.65 Ω cm, an average mobility of 8 cm 2 /Vs, and an average hole concentration of 1.2E18 cm −3 . |
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