Deep Level Formation in Undoped and Oxygen-Doped GaN
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Deep Level Formation in Undoped and Oxygen-Doped GaN J. M. Gregie, R. Y. Korotkov, and B. W. Wessels Materials Research Center and Department of Materials Science and Engineering, Northwestern University, Evanston, IL 60208 ABSTRACT Deep level defects in oxygen doped GaN grown by metal-organic vapor phase epitaxy were investigated. Using steady-state photocapacitance (SSPC) spectroscopy, three deep levels with optical ionization energies of 1.0, 1.4, and 3.25 eV were observed in both nominally undoped and oxygen-doped samples. The total deep level defect concentrations ranged from 6x1015 cm-3 in undoped films to 3x1016 cm-3 in oxygen-doped films. The concentration of the 3.25 eV level defect increased upon oxygen doping, while the concentration of the 1.0 and 1.4 eV levels were essentially dopant independent. From the measured concentrations the formation energies of the defects were calculated and compared to energies calculated using density functional theory.
INTRODUCTION Deep level defects play an important role in the performance of GaN based electronic devices. Trapping centers can limit the switching frequency of transistors or cause current collapse in FETs [1]. In optical devices, deep levels can lead to non-radiative recombination. A number of deep level defects have been previously observed in n-type GaN, using steady state photocapacitance (SSPC) and deep level optical spectroscopy (DLOS) techniques [2]. Various investigations have measured defects in nominally undoped GaN, as well as Mg [3], and Si [4] doped GaN. The effects of nitrogen annealing [3] and hydrogenation [5] have also been reported. Deep levels have been identified throughout the band gap from 1.0 eV to 3.2 eV below the conduction band. Reported concentrations range from 1014 cm-3 to nearly 1017 cm-3. The stability and resultant concentration of defects, including deep levels are determined by the position of the Fermi level [6]. Thus, the deep level defect concentration depends on the concentration of the shallow dopant. In this study, the concentration and stability of deep level defects in undoped and deliberately oxygen-doped [7] n-type GaN were determined using steady-state photocapacitance. From the concentration, the formation energies of the defects were calculated and compared to theory.
EXPERIMENT All of the GaN thin films in this investigation were grown using metal-organic vapor phase epitaxy (MOVPE) in a horizontal-flow quartz reactor. Both undoped and doped samples were n-type. Trimethylgallium and ammonia were the reactant gases and hydrogen was used as the carrier gas. To dope the films with oxygen, a 500 ppm oxygen in nitrogen mixture was added to the reactant gas. Films were grown at 1060 oC to a total thickness of ~2 µm. To
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fabricate Schottky diodes, Au dots with a diameter of 475 µm were deposited through a shadow mask, using either e-beam or thermal evaporation. Indium was used as the ohmic contact. To determine the energy level and concentration of point defects in the films, steady-state photocapacitanc
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