X-ray Photoelectron Spectroscopy Evaluation on Surface Chemical States of GaN, InGaN and AlGaN Heteroepitaxial Thin Film
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ABSTRACT The surface chemical states of MOCVD grown GaN, AlGaN and InGaN, and the influence of different dopants have been studied with x-ray photoelectron spectroscopy (XPS). The results show that for most of the samples the N Is peak can be fitted with a dominant GaN peak at the binding energy of 397.2 ± 0.2 eV and a small N-H peak at the binding energy of 398.5 ± 0.2 eV, while Ga 3d can be deconvoluted into three peaks, i.e., elemental Ga at 18.5 ± 0.1 eV, GaN at 19.7 ± 0.1 eV, and Ga 20 3 at 20.4 ± 0.1 eV. Si-doping appears to have small influence on the surface chemical states of GaN. Compared with Sidoping, the influence of Mg-doping appears -to be larger. In addition to a change in the component intensities, Mg-doping also causes the N ls and Ga 3d peaks to broaden. The ternary AlxGal.xN (x - 0.025) sample shows aluminum surface segregation, while the undoped In.Gal.xN (x -0.12) shows indium surface deficiency. INTRODUCTION With the progress in crystal epitaxial growth techniques, rnI-V nitrides are of intense research interest in recent years. The I1-V nitrides GaN, AIN, InN and their ternary compounds are of significant technological importance for semiconductor device applications in the blue and UV wavelengths, such as light emitting diodes and semiconductor lasers,1-6 due to their large direct bandgaps. The nitrides of gallium, indium and aluminum form a continuous alloy system whose direct bandgaps cover the range from 1.9 eV for InN, to 3.4 eV for GaN, and 6.2 eV for AIN.' Moreover, these nitrides possess extremely high hardness,6 very large heterojuction offsets, high thermal conductivity and high melting temperature, which make them promising candidates for high temperature and high power devices. However, many technological and scientific barriers still exist in this materials system. The most dominant ones are stoichiometric nitrogen incorporation, p-type doping, and the availability of lattice- and thermally matched substrates. Nevertheless, much improvement 26 has been made and GaN based light emitting diodes are now commercially available. Currently, GaN and GaN-based rnI-V nitride semiconductor materials are being intensively investigated. The two most commonly used growth techniques are metalorganic chemical vapor deposition (MOCVD) and molecular beam epitaxy (MBE). 3'-3 Il-V nitrides 60 with increasingly better crystalline quality, as confirmed by XRD, have been reported. -' , 1418Scanning electron microscopy (SEM) and high resolution transmission electron microscopy (HRTEM) have been employed to study the surface morphology and crystal orientation relationship between the rn-V nitride films and the substrate respectively, 2' 7, 11-17 and the dopant depth distribution has been investigated with secondary ion mass spectrometry (SIMS). 11-2 The optical transmission property in the UV to visible range has also been investigated.17-20 The most important light emitting property of Ir-V nitride based LEDs and its relationship with growth parameters, such as substrate temperature and precursor flow
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