DLTS and CV Analysis of Doped and N-Implanted GaN
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ABSTRACT We studied by deep level transient spectroscopy (DLTS) and capacitance-voltage (CV) measurements the effects of doping (Zn, S), nitrogen implantation and annealing of n-type GaN grown on sapphire by MOVPE. The DLTS spectra of the as grown samples show two defect levels which are assumed to be identical with recently reported levels [10, 11]. In N-implanted GaN a third level is introduced which is not detectable in our as grown samples. This levels concentration follows the increasing N-implantation density. The depth profiles of its concentration correlate with the distribution of implantation defects expected from Monte-Carlo simulation. After annealing at 900'C for 60s the additional defect level vanishes. The DLTS spectrum then resembles those of annealed as grown samples. The n-type carrier concentration (CV measurements) increases in samples with low N-implantation dose. This implantation effect can be removed also with the RTA step. The increasing carrier concentration provides evidence that the N vacancy is a donor in GaN. For Zn and S doped GaN deep defect levels has been found, which are reported here.
INTRODUCTION Gallium nitride has recovered great interest in research and development of optoelectronic devices and materials since Akasaki et al. [1] showed that p-type doping is achievable by low- energy-electron-beam irradiation. The direct bandgap energy of EG = 3.4 eV at room temperature allows the realization of blue-light-emitting devices. Nakamura et al.
have realized LED and laser diode technology [3, 4]. To optimize devices based on GaN it is necessary to understand the doping mechanisms of this material. In recent studies Binari et al. [7] and Pearton et al. [6] have shown that implantation damage, which produces vacancies in large concentration, leads to a highly resistive material. On the other hand the nitrogen vacancy (VN) has been discussed for a long time to be responsible for the intrinsic n-type doping of GaN [8]. To achieve high n-type carrier concentration intentionally doping is necessary. As a group-V element sulfur is a candidate for this purpose. For this reason we investigated the effect of S doping. The doping with Zn is of interest due to the blue luminescence center [2] and the acceptor states which are assigned to Zn [5].
EXPERIMENT The analysis of shallow donor concentration was done by using the conventional capacitance voltage (CV) technique which uses the fundamental relationship between the dynamic capacity of the space-charge region of a Schottky contact and the net carrier concentration. The measurement of the capacitance was done with a BOONTON 72B capacitance meter. The test signal frequency was 1MHz and held at a level of 1OOmV. Deep level transient spectroscopy (DLTS) was used to investigate ionization energies and concentrations of deep levels between Ec - 100meV and EC - 900meV. DLTS was performed 531 Mat. Res. Soc. Symp. Proc. Vol. 423 01996 Materials Research Society
with a commercial POLARON DL 4600 equipment. All DLTS measurements shown in this pap
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