Electrical Characterization of Magnesium-Doped Gallium Nitride Grown by Metalorganic Vapor Phase Epitaxy
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ABSTRACT We have applied frequency-dependent capacitance measurements and admittance spectroscopy on metalorganic vapor phase epitaxy GaN:Mg to study the electronic states associated with Mg doping. Samples with different Mg doping levels were grown and annealed in nitrogen. Lateral dot-and-ring Schottky diodes using Au/Ti were fabricated. After a 800 'C anneal, frequency-dependent measurements show that the capacitance is reduced at a higher frequency, most likely due to the inability of a deep center to maintain an equilibrium ionization state under a high frequency modulation. The net ionized acceptor concentrations was found to be greater at a higher Mg doping level. Admittance spectroscopy, in which the conductance is monitored as a function of temperature, verifies the existence of at least one impurity-related acceptor level with an activation energy of - 140 meV. A reduction in the annealing temperature was found to lead to a lower net ionized acceptor concentration, as well as a higher activation energy.
INTRODUCTION The successful p-type doping in metalorganic vapor phase epitaxy (MOVPE) GaN using Mg has allowed the realization of blue light emitting p-n junction diodes. Treatments, such as
electron beam irradiation [1] or thermal annealing [2], are usually required to activate the p-type conduction. Electrical characteristics of these Mg-related acceptors are, however, not well understood. The Mg acceptor binding energy has been reported to be 250 and - 160 meV from the donor-acceptor pair transition and from temperature-dependent photoluminescence (PL) experiments respectively [3]. The temperature-dependent Hall measurements revealed Mgrelated activation energies of 125 and 157 meV for two samples from the same growth run but annealed for 60 and 25 seconds respectively [4]. The 60-second annealed sample had more electrically active acceptors. In this paper, we report the electrical characterization of GaN:Mg, using frequency-dependent capacitance measurements and admittance spectroscopy, in order to study the electronic states associated with Mg doping. Both the effects of Mg doping levels and thermal annealing temperatures were investigated. It is well known that deep traps greatly affect the free carrier profiles, as determined by capacitance-voltage (C-V) measurements [5]. Given the aforementioned energy level depths associated with Mg, as well as the fact that Mg-doping is responsible for most or all of the observed p-type conductivity, the small signal capacitance of Mg-doped GaN is expected to be dependent on measurement frequency. We had also found that deep level transient spectroscopy (DLTS) could not be successfully used to characterize these Mg-related deep levels, principally due to the low diode capacitance level at low measurement temperatures. In contrast, admittance spectroscopy is better-suited for the study of these relatively shallow and/or fast levels [6]. 601 Mat. Res. Soc. Symp. Proc. Vol. 423 01996 Materials Research Society
EXPERIMENTS Mg-doped GaN samples were grown in a hor
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