Yellow Band and Deep levels in Undoped MOVPE GaN
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Internet Journal o f
Nitride S emiconductor Research
Volume 1, Article 7
Yellow Band and Deep levels in Undoped MOVPE GaN. F. J. Sánchez, D. Basak, M. A. Sánchez-García, E. Calleja, E. Muñoz, I. Izpura, F. Calle, J. M. G. Tijero Dpt. Ingeniería Electrónica, E.T.S.I. Telecomunicación, Politécnica, Ciudad Universitaria B. Beaumont, P. Lorenzini, Pierre Gibart Centre de Recherche sur l'Hetero-Epitaxie et ses Applications, CRHEA-CNRS T. S. Cheng, C. T. Foxon Department of Physics, University of Nottingham J. W. Orton Department of Electrical and Electronic Engineering, University of Nottingham This article was received on June 4, 1996 and accepted on September 9, 1996.
Abstract Undoped layers of GaN grown by MOVPE on sapphire substrates have been characterized by photoluminescence, photocapacitance and photoinduced current transient spectroscopy (PICTS). Photocapacitance reveals in all samples two specific signatures at photon energies of 1 eV and 2.5 eV. The photocapacitance decrease observed at 1 eV seems to be due to an electron capture process from the valence band, whereas the capacitance increase at 2.5 eV is related to an electron emission process. The fact that the capacitance step at 1 eV is only seen after photoionization at energies above 2.5 eV, and the observed correlation between its amplitude and the photoluminescence intensity of the “yellow band», lead us to conclude that both transitions are linked to the same trap, which is also suggested to be responsible for the yellow band. The position of this trap, at 2.5 eV below the conduction band, is confirmed by PICTS measurements, that show a hole thermal emission activation energy of 0.9 eV at 350 K.
1. Introduction The origin and role of native defects and impurities in GaN is essential to understand their potential effects in device performance. Despite the fact that efficient devices based on GaN layers and its alloys (GaAlN, GaInN), showing a high density of dislocations (>1010 cm-2 ), have been achieved, the presence of extended and point defects can be detrimental on the device performance in the long term. It has been suggested that point defects like VN , GaI , and V Ga , among others, may be responsible for the high residual n-type character of undoped GaN, and for a reduced doping efficiency [1]. GaI are also suspected to generate a deep donor state in GaN that might be related to the ubiquitous “yellow band» in this material [2] [3] [4]. Other deep states related to impurities have been reported, like Fe, which is believed to generate an acceptor trap at EV + 2.5 eV [5]. Although Deep Level Transient Spectroscopy (DLTS) has been used by several authors to study the presence of deep defects in GaN, like Götz et al. in Si- doped GaN [6] and Hacke et al. in undoped and Mg-doped GaN [7] [8], there is no evidence about the origin of most of these traps. However, since the ability of this technique to detect deep traps in wide gap materials is limited by temperature, other techniques based on photoemission, have been used. Götz et al. [9] and Ba
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