Deep Level Defects in Mg-Doped GaN
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ABSTRACT Deep level defects in Mg compensated GaN grown by metal-organic vapor phase epitaxy were investigated using photocapacitance spectroscopy measurements on Schottky barrier diodes. Addition of magnesium resulted in the formation of a series of deep centers with optical threshold energies of 1.0, 1.2, 1.8, and 3.1 eV. Upon annealing the epitaxial GCN in nitrogen at 850'C the ind-gap levels disappeared and only the trapping level at 3.1 eV remained. The mid-gap levels are ascribed to Mg dopant complexes which may in part be responsible for low doping efficiency of Mg in the as-grown, doped GaN. The deep level at 3.1 eV commonly observed from all Mgdoped GaN most likely involves the Mg acceptor. The photo-excited state of the 3.1 eV level had relaxation times of the order of 10' sec at 295 K. INTRODUCTION As-grown, epitaxial Mg-doped GaN is usually highly resistive [1,2]. However, after postdeposition nitrogen-ambient thermal annealing the p-type conductivity is improved [2]. The nature of the defect centers responsible for compensating the p-type material is not well understood [3-6]. Recently photoconductivity spectroscopy of Mg-doped GaN has been studied We have recently used photocapacitance and deep tail states have been observed [7]. spectroscopy to study deep level defects in Mg-doped GaN, a technique that is especially applicable for the study of deep levels in wide band-gap semiconductors [8,9]. In this study, both steady state and transient photocapacitance measurements were carried out on deliberately Mg doped GaN prepared by metalorganic vapor phase epitaxy to determine the deep level defects. The properties of the Mg related level at E.-3.1 eV were of special interest. EXPERIMENT The layers were prepared by atmospheric pressure metal-organic vapor phase epitaxy [ 10,11]. The films compensated by the addition of Mg during growth were all n-type as determined by Hall effect measurements. Schottky barrier diodes were used for photocapacitance measurements on both undoped and Mg-doped epitaxial GaN layers. They were fabricated by evaporating either gold or aluminum thin films on the GaN films through a shadow mask. The typical thickness 2 of the thin metal film is of the order of 500 A. The diode area was 0.6-1.0 mMm. Good Carrier rectifying current-voltage (I-V) characteristics were observed for the diodes. 3 1 7 3 7 concentrations were Ixl10 -2x×10' cm- in the autodoped, n-type GaN and 5xlO"-2x 10 cm in the Mg-doped GaN as determined by capacitance-voltage measurements. Steady state photocapacitance measurements were performed over the spectral range of 0.5-3.4 eV at room temperature. Photocapacitance spectra were measured by illuminating from low energy to high energy with monochromatic light from a Zeiss MM12 double prism monochromator equipped with a 250 W quartz-halogen lamp [12]. Transient photocapacitance was measured by illuminating with light at a fixed energy. The energy resolution was better than 525 Mat. Res. Soc. Symp. Proc. Vol. 423 0 1996 Materials Research Society
0.04 eV in all expe
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