Deep Trap Characterization in GaN Using Thermal and Optical Admittance Spectroscopy
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of Experimental Physics, University of Magdeburg, PO Box 4120, 39016 Magdeburg
Institute of Solid State Physics, University of Bremen, PO Box 330440, 28334 Bremen **'1. Institute of Physics, University of Giessen, Heinrich Buff Ring 16, 35392 Giessen ABSTRACT Deep defect levels and the optical as well as thermal transitions of carriers from the levels into the corresponding bands were analyzed using Thermal and Optical Admittance Spectroscopy. High resistivity GaN-layers grown by MBE and heterostructures consisting of n-type GaN-layers grown with Low Pressure Chemical Vapor Deposition on 6H-SiC substrates are investigated. In the MBE-grown GaN layers we determine deep electron traps with thermal activation energies of EA=(0.45--0.04)eV and EA=(0.65_+0.03)eV. Furthermore, three different kinds of optical transitions were distinguished by Optical Admittance Spectroscopy: near band gap transitions including the transition between the valence band and a shallow donor 50meV below the conduction band, a peak at 2. 1eV associated with the yellow photoluminescence band and various deep level-band transitions in the infrared region. The high sensitivity of the TAS to interface defect states was used to investigate GaN/SiC heterostructures. We found an interface defect state at 70 ... 90meV. Furthermore, one level was obtained originating from the epitaxial GaN-layer having an activation energy of 63±3meV. A defect distribution was identified in the p-type SiC-substrate with activation energies between 160 meV and 180meV. INTRODUCTION Gallium nitride has evolved to a successful semiconductor material for electronic and optoelectronic devices. Of fundamental importance is a precise knowledge of all kinds of deep defects determining the quality of GaN-layers. Such deep defects cause non-radiative transitions and control the compensation mechanism in weakly doped or undoped GaN-layers. Therefore, these deep levels must be characterized. The most common measurement technique for detection of deep levels, the DLTS, is not applicable in highly compensated layers like, e.g. MBE grown GaN. Alternative methods, like Thermal and Optical Admittance Spectroscopy (TAS or OAS) show advantages compared to DLTS. Especially, the extension of the modulation frequencies for TAS and OAS down to extremely low ranges preserves the Schottky contacts even in high compensated materials where the space charge region is controlled by deep levels. Using thermal and optical admittance spectroscopy we characterise thermal activated deep defects and optical transitions between traps and the corresponding valence or conduction band in Schottky contacts on MBE grown GaN-layers.
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Mat. Res. Soc. Symp. Proc. Vol. 482 0 1998 Materials Research Society
A second important advantage of both methods of admittance spectroscopy is their sensitivity to defect states at interfaces. The energetic distribution of interface defects in SiC / GaN (grown by LPCVD) heterostructures is analysed by TAS. EXPERIMENT The samples were grown by MBE on sapphire substrates usi
