Nondestructive, Room Temperature Determination of the Nature of the Band-Bending (Carrier Type) in Group III Nitrides Us
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FRED H. POLLAK ", Z.C. FENG ***,M. SCHURMAN ***and
R.A. STALL *** * Physics Department and New York State Center for Advanced Technology in Ultrafast Photonic Materials and Applications, Brooklyn College of the City University of New York Brooklyn, NY 11210 ** E-mail: [email protected] "*** EMCORE Corporation, 394 Elizabeth Avenue, Somerset, NJ 08873 ABSTRACT Using contactless electroreflectance and surface photovoltage spectroscopy at room temperature we have nondestructively evaluated the band bending (carrier type) at the surface of epitaxial n- and p-type GaN/sapphire samples as well as at both the InGaN surface and the GaN at the InGaN/GaN interface of samples of epitaxial InGaN grown on top of thick GaN epilayers/sapphire, having average n- and p-type character. INTRODUCTION GaN and InGaN semiconductor compounds are the subject of intensive investigations because of their importance to both science and technology for applications towards hightemperature electronics, light emitters, and detectors operating in the blue and ultraviolet wavelength range [1,21. Currently, GaN and InGaN materials, to a large extent, are obtained by the epitaxial growth on sapphire or SiC or other substrates, by metalorganic chemical vapor deposition (MOCVD), molecular beam epitaxy (MBE), and other technologies [1,2]. Because of the large differences in the lattice constants and thermal expansion coefficients between GaN (and InGaN) and sapphire, a high density of dislocations and N-vacancies exist in the epitaxial GaN (and InGaN) layers. The N-vacancies in GaN produce n-type doping centers, leading to the natural high level of n-type doping background in nominally undoped epitaxial GaN films grown on sapphire or other foreign substrates [3,4]. This has caused difficulty in p-type doping GaN (and InGaN). A few years ago, Amano et al. [3] succeeded in obtaining p-type GaN films using Mg doping, a low-energy electron-beam irradiation treatment and an AIN buffer layer technique. S. Nakamura et al. [4] succeeded in obtaining p-type GaN films, using a GaN buffer layer and post-growth thermal annealing of the Mg doped GaN layers at temperatures beyond 7001C in the N2-ambient. The re-activated acceptors due to these thermal treatments compensated the high density of the N-vacancy-related n-type background. It is, therefore, very important to develop quick, non-destructive and reliable techniques to determine the n-or p-type nature of GaN and InGaN layers, and
their related band bendings in the surface and interfaces for device applications. The nondestructive techniques of contactless electroreflectance (CER) and surface photovoltage spectroscopy (SPS) have proven to be valuable experimental methods to investigate the properties of semiconductors (bulk/thin film) [5-7], semiconductor 573 Mat. Res. Soc. Symp. Proc. Vol. 482 ©1998 Materials Research Society
microstructures [5-11] (quantum wells, quantum dots, heterojunctions), surfaces/interfaces [5,6,8] and actual device configurations [5,12,13] (heterojunction bipolar transistors,
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