Direct Observation of Bulk and Interface States in GaN on Sapphire grown by Hydride Vapor Phase Epitaxy
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Direct Observation of Bulk and Interface States in GaN on Sapphire grown by Hydride Vapor Phase Epitaxy S. H. Goss1, A. P. Young2, L. J. Brillson1,2, D. C. Look3, R. J. Molnar4 Center for Materials Research, Ohio State University, Columbus OH1 Department of Electrical Engineering, Ohio State University, Columbus OH2 Phone (614)-292-8015, Fax (614)-688-4688 Wright State University, Dayton OH3 Massachusetts Institute of Technology Lincoln Labs, Lexington MA4 Abstract We have used an ultrahigh vacuum scanning electron microscope to carry out cross sectional secondary electron imaging, cathodoluminescence spectroscopy, and cathodoluminescence imaging on GaN grown on sapphire by hydride vapor phase epitaxy. These measurements provide evidence for deep level defects highly localized at the GaN/sapphire interface as well as defects extending into both the semiconductor film and the substrate. The different spatial distributions of these radiative defects provide information on the physical origin of these electrically active features. Introduction Gallium nitride is proving to have superior electronic and optical properties compared to other semiconductors for high power, high frequency microelectronics. However, GaN must be grown epitaxially as thin films on ordered substrates in order to produce useful devices. The reason for thin film growth of GaN material on alternate substrates is due to the lack of GaN material as a substrate. For economic reasons, sapphire is the most common substrate and the 14% GaN/sapphire lattice mismatch leads to dislocations and chemical interactions at the growth interface. In turn, these imperfections can lead to electrically active defect sites that degrade the properties of the thin film device. In particular, increased conductivity has been reported for such GaN/sapphire interfaces that can lead to back – channel conduction and degraded high electron mobility transistor (HEMT) performance. 1,2 A promising method of growing GaN is hydride vapor phase epitaxy (HVPE). The growth rates are sufficiently high such that GaN can be grown quickly and the relatively low equipment costs for this technique make it economically feasible. We have carried out secondary electron imaging in cross section, cathodoluminescence spectroscopy (CLS), and CL imaging of GaN grown on sapphire by HVPE. Cross sectional images of the GaN/sapphire structure clearly identify the boundaries of GaN film, GaN/sapphire interface and bulk sapphire. CL images of this structure reveal clear changes in the quality of the GaN epilayer near the GaN/sapphire interface. CL spectra exhibit features characteristic of the GaN near –band-edge (NBE) emission, defects within the GaN and the sapphire, as well as at the interface. A series of CL spectra taken of regions within this structure provide a profile of these spectral features from the sapphire substrate through the interface to the free GaN surface. These defect profiles reveal deep level defects highly localized at the GaN/sapphire interface as well as other deep level states
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