Photocurrent spectroscopy investigations of Mg-related defects levels in p-type GaN
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Semiconductor Physics Research Center and Department of Semiconductor Science and Technology, Chonbuk National University, Chonju 561-756, KOREA. *Author to whom correspondence should be addressed [email protected]
ABSTRACT The defect levels associated with Mg impurity in p-type GaN films were systematically investigated in terms of doping concentration by photocurrent spectroscopy. Mg-doped GaN samples were grown on sapphire substrate by metal organic chemical vapor deposition and annealed in nitrogen atmosphere at 850 for 10 minutes. At room temperature, PC spectra showed two peaks at 3.31 and 3.15 eV associated with acceptor levels formed at 300 and 142 meV above valence band in as grown samples. But, after the thermal annealing, PC spectra exhibited various additional peaks depending on the Mg concentration. In the GaN samples with Mg concentration around 6 7 1017 cm-3, we have observed PC peaks related to Mg at 3.31 as well as 3.02 eV and carbon acceptor at 3.17 eV. For moderately Mg doped GaN samples, i.e., the hole concentration p=3 4 1017 cm-3, additional peak was observed at around 0.9 eV which can be attributed to defects related to Ga vacancy. For relatively low Mg doped samples whose hole concentrations are 1 2 1017 cm-3, additional broad peak was observed at around 1.3 eV. This peak may be related to the yellow band luminescence. As the Mg concentration is increased, the concentration of Ga vacancies can be reduced because Mg occupies the substitutional site of Ga in GaN lattice. When the hole concentration is above 6 7 1017 cm-3, the yellow luminescence and Ga vacancy related peaks disappeared completely. INTRODUCTION Gallium nitride (GaN) has been one of the most promising materials for blueultraviolet (UV) lasers, short wavelength radiation detectors and high-temperature electronic devices. This is evident from several impressive device achievement in the last few years, including the injection light-emitting diodes(LED)[1], laser diodes[2], solarblind UV detectors[3], and high-temperature transistors[4]. Since deep levels in GaN affect significantly the photo-electric properties of material and devices, they were investigated using a number of techniques in samples grown by different methods and post treatment conditions [5-18]. For n-type and undoped GaN samples, a shallow donor(SD) at about 30 meV below the conduction band were usually observed by temperature dependent Hall and photoluminescence(PL) measurements[6,7], and a double donor(DD) at about 0.7 eV below the conduction band edge(Ec) as well as a deep level at Ec - 2.0 eV were recognized by optically detected magnetic resonance (ODMR) and persistent photoconductivity (PPC) techniques [6,13]. On the other hand, a deep level at 1.4 eV above the top of the valence band (Ev) were detected using photocurrent (PC) and photoemission capacitance transient spectroscopy methods [9,14,17]. However, one discordance seems to appear for the Mg-related deep level; the Hall measurement
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gives an activation energy of 0.16 eV while the
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