Comparative Study of trap levels observed in undoped and Si-doped GaN
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Comparative Study of trap levels observed in undoped and Si-doped GaN C. B. Soha, D. Z. Chib,*, H. F. Lima and S. J. Chuaa,b# a) Centre of Optoelectronics, Department of Electrical and Computer Engineering, National University of Singapore, Singapore 117576 b) Institute of Materials Research and Engineering, 3 Research Link, Singapore 117602 ABSTRACT In this paper, deep level defects in undoped and Si–doped GaN have been studied using digital deep level transient spectroscopy. Common trap levels at Ec -ET ~ 0.15-0.20 eV and 0.590.62 eV were detected for both undoped and Si-doped samples. For the doped samples, three additional defect levels at Ec-Et ~ 0.11, 0.28, and 0.45 eV were detected. The concentration of the 0.15-0.20 eV was found to be much higher in undoped GaN that also shows higher dislocation density. Based on this correlation and the logarithmic capture behavior observed for this level, indicative of extended defect nature, we attribute the 0.15-0.20 eV level to dislocation related defects. On the other hand, the 0.28 and 0.45eV trap levels are tentatively attributed to Sirelated defects simply due to the fact that these two levels were observed only in Si-doped GaN. The 0.11eV trap level, which exhibits an exponential capture kinetic, is believed to be related to nitrogen vacancies. INTRODUCTION Gallium nitride (GaN) possesses some unique features such as a large bandgap (3.39 eV), high saturation velocity (2x107 cm/s), and high thermal stability, with a large breakdown field of 2 MV/cm. These enable GaN to emerge as a unique material for multiple applications such as light emitting diodes, blue lasers, light power electronic devices and solar-blind ultraviolet detectors [1,2]. However, the performance of devices can be limited by the deep levels present within the GaN band gap, which act as traps and/or recombination-generation centers. These deep levels cause non-radiative transitions and control the compensation mechanism in weakly doped GaN layer [3]. The deep levels in GaN, in general, are either related to structural defects like dislocations, native point defects (and/or their complex) such as nitrogen vacancies [4], or impurity-related defects. Doping of GaN with dopants may introduce deep levels, in addition to shallow donor or acceptor levels. For example, it has been found that Mg doping introduces a deep level at Ec – Et ~ 0.58-0.62 eV [5]. For Si-doped GaN, though there are several reports on the observation of various deep levels [4,5,6,7], the nature and origin of these levels, i.e., whether they are extended or point-like defects and which defect levels are associated with Sirelated defects, have not been well understood. In this paper, we compare the deep level spectra and properties in undoped and Si-doped GaN using Schottky diode structure. Both temperature scan and isothermal deep level transient spectroscopy (DLTS) measurements were carried out to obtain detailed information of the electronic and structural properties of the deep levels. Possible physical sources of the deep levels
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