Effect of Impurities on Raman and Photoluminescence Spectra of AlN Bulk Crystals
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Effect of Impurities on Raman and Photoluminescence Spectra of AlN Bulk Crystals A. Sarua 1 , S. Rajasingam1 , M. Kuball1 , N. Garro2 , O. Sancho 2 , A. Cros2 , A. Cantarero2 , D. Olguin3 , B. Liu4 , D. Zhuang4 , J. H. Edgar4 1 H. H. Wills Physics Laboratory, University of Bristol, Bristol BS8 1TL, United Kingdom 2 Institut de Ciencia dels Materials, Universitat de Valencia, 46980 Paterna, Valencia, Spain 3 Depto. de Fisica, Centro de Investigacion y de Estudios Avanzados del Instituto Politecnico Nacional, AP. 14 740, Mexico, D.F., 07300 Mexico 4 Department of Chemical Engineering, Kansas State University, Manhattan, Kansas 66506 ABSTRACT Raman scattering and photoluminescence (PL) spectroscopy with sub-bandgap excitation has been applied to explore tracing of common impurities (in particular of oxygen) in AlN. Bulk AlN crystals grown by the high temperature sublimation method were studied. PL bands have been observed at around 375 nm and at 560-660 nm and have been attributed to oxygen and to nitrogen vacancy/aluminium excess defects, respectively. The 375 nm UV PL band was found to shift with oxygen concentration. Micro-Raman spectra of the bulk AlN samples were measured in different polarisations. Besides normal Raman modes of AlN the presence of additional vibrational modes was detected. The modes were discussed and tentatively attributed to oxygen and silicon local vibrational modes (LVMs) in AlN.
INTRODUCTION There is considerable interest in the growth and study of bulk III-V nitride materials, as they are attractive as substrates for high-power, high-bandwidth electronic and opto-electronic devices. Bulk AlN substrates up to 1cm2 have been employed for HFET fabrication [1]. Apart from current size limitations, residual impurities in the bulk crystals are an important issue, since they adversely affect the properties of AlN. For example AlN has shown incorporation levels of oxygen exceeding 4 at% [2], which decreases its thermal conductivity. Optical methods like Raman spectroscopy or photoluminescence (PL) offer non-destructive way to probe and study impurities and defects in semiconductor materials. A foreign atom in a crystal lattice often results in an energy level situated in the bandgap, whose energy correlates with the defect configuration. Foreign atoms occupying lattice sites can also result in a local vibrational mode (LVM), which can be Raman or/and infrared active. The possibility of tracing impurities via LVMs in IR and Raman spectra has been successfully demonstrated in the past in the case of Si and GaAs [3], however, there is relatively little information available on LVMs in AlN. Works on impurities in bulk AlN reported so far include defect related luminescence and optical absorption spectroscopy [2,4,5]. Some results on LVMs of hydrogen decorated impurity complexes in hexagonal GaN were reported [6,7]. In this work we investigate the effect of impurities in bulk AlN samples on first-order Raman and PL spectra.
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EXPERIMENTAL DETAILS Bulk 2H AlN samples were grown using a
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