Hydrogen in GaN
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obtained from experiments designed to isolate the dissociation kinetics from the total phenomenon of dopant activation which includes dissociation, recombination, and hydrogen migration. With the "depletion-layer" technique the dopant-hydrogen complexes are situated in the space-charge layer of a reverse-biased diode so that recombination is suppressed due to field-driven sweep-out of the dissociated charged hydrogen [10]. Such experiments have not yet been reported for Mg-doped GaN. Sources of Hydrogen The MOCVD growth technique is inherently rich in sources of hydrogen that can be incorporated into the growing film. The group III gaseous precursors for the AIGaInN system are metalorganic compounds (e.g., TMGa) that can produce hydrocarbon radicals (e.g., CH 3) and stable hydrocarbons (e.g., CH 4) as products of the growth reactions. The currently most widely used precursor for nitrogen is NH3. The precursors for dopant impurities are typically obtained from either metalorganics (e.g., Cp 2Mg and DEZn) or hydrides (e.g., SiH 4). Finally, the carrier gas is typically molecular hydrogen, although its stability renders H2 the least serious source of hydrogen incorporation. Hydrogen incorporation during MOCVD-growth of Mg-doped GaN has been demonstrated with secondary ion mass spectrometry (SIMS). In particular, it has been shown that the hydrogen concentration increases linearly with the Mg concentration [11]. It has also been reported with SIMS data that the hydrogen concentration decreases after furnace anneals which activate the Mg acceptors [11,12]. The furnace anneal is typically performed in the temperature range from 700 0C to 850 0 C for times of 5 to 60 min and in an ambient of flowing N2. High acceptor doping efficiencies have been achieved in Mg-doped GaN layers without post-growth treatment by MBE [13]. Chemically active nitrogen was obtained from N2 with an ECR plasma source, and none of the other precursors deliberately contained hydrogen. The high doping efficiency may reflect the absence of hydrogen in the growth process, although there are other fundamental differences between MOCVD and MBE (e.g., approximation to thermal equilibrium). EXPERIMENTAL RESULTS Hydrogen Diffusion Hydrogen diffusion into GaN depends strongly on the conductivity type of the material as well as on diffusion conditions (e.g., temperature and time). This is illustrated in Fig. 1 with depth profiles of deuterium in MOCVD-grown GaN [14]. The ptype conductivity was obtained from Mg-doped, furnace-activated GaN samples with a hole concentration at room temperature of 8x1017 cm- 3. The n-type GaN was Si doped to a concentration of 2x10 17 cm- 3. The deuterations were performed in a remote microwave plasma system [15], and the hydrogen isotope was detectable with high sensitivity by SIMS. For the p-type material, depth profiles are shown after deuteration at 400 0C and 600 0C. The amount of deuterium that has diffused into the p-type GaN at 600 0C is substantial and available to play a role in acceptor passivation [2,14,16], as furt
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