XPS study of the formation of ultrathin GaN film on GaAs(1 0 0) L. Bideux a, * , G. Monier a , V. Matolin b , C. Robert-Goumet a , B. Gruzza a a LASMEA, UMR 6602 CNRS, Blaise Pascal University, Campus Scientifique des Ce ´zeaux, 63177 Aubiere Cedex, France b Department of Electronics and Vacuum Physics, Charles University, V. Holesovickach 2, 18000 Prague 8, Czech Republic Received 18 September 2007; received in revised form 16 November 2007; accepted 27 December 2007 Available online 11 January 2008 Abstract The nitridation of GaAs(1 0 0) surfaces has been studied using XPS spectroscopy, one of the best surface sensitive techniques. A glow discharge cell was used to produce a continuous plasma with a majority of N atomic species. We used the Ga3d and As3d core levels to monitor the chemical state of the surface and the coverage of the species. A theoretical model based on stacked layers allows to determine the optimal temperature of nitridation. Moreover, this model permits the determination of the thickness of the GaN layer. Varying time of nitridation from 10 min to 1 h, it is possible to obtain GaN layers with a thickness between 0.5 nm and 3 nm. # 2008 Elsevier B.V. All rights reserved. Keywords: XPS; Gallium arsenide; Gallium nitride; Semiconductor–semiconductor thin film structure; Heterojunctions 1. Introduction Thin GaN films produced by nitridation, due to their high thermal and chemical stability, could be used for surface chemical passivation of GaAs [1]. Moreover, the high density of surface states of GaAs are known to induce surface Fermi level pinning [2]. GaN films are also expected to provide an effective electronic passivation because N atoms bonded with surface gallium [3] should not produce electronic states in the forbidden gap. GaN materials are important for a variety of device applications: light emitters and detectors operating in the visible to UV spectral range [4], high power microwaves devices [5] or high frequency field effect transistors [6]. Hexagonal GaN films are usually grown on Al 2 O 3 or SiC but GaAs(1 0 0) is a good candidate as substrate for the growth of cubic b-GaN. The best procedure to increase the quality of the c-GaN is achieved by a susbtrate nitridation before the deposition of a low temperature buffer layer [7]. In this case, the nitridation conditions are critical for this process. In this paper we report the nitridation of GaAs(1 0 0) surface with a glow discharge cell. The composition of the nitridated layer was quantitatively studied using XPS spectroscopy. Commercially available GaAs(1 0 0) wafers have been used in this work. Before introduction in the UHV chamber, samples were chemically cleaned using successive baths of deionised water, H 2 SO 4 and hot methanol. The experiments were carried out in a home-built UHV chamber equipped with XPS system (dual anode Al–Mg X-ray source and hemispherical electron energy analyser). XPS experiments were performed using a Mg Ka source (1253.6 eV) at an incident angle of 508 (normal detection, pass energy of the analyser equal to 20 eV). After introduction in the UHV chamber, GaAs substrates were cleaned using 1 keV Ar + ions with a current density of 3 mA cm 2 at 6  10 5 Torr. The nitridation process has been performed with a high voltage plasma discharge source (GDS). In this kind of nitrogen cell, continuous plasma was produced by a high voltage (about 2 kV) and a majority of N atomic species were created (nitrogen pressure: 1  10 4 Torr, ionic current: 1.0 mA cm 2 ). Before nitridation the GaAs(1 0 0) substrates were heated to 400 8C during 1 h, then the samples were kept under a nitrogen flow for a chosen time. 2. In situ cleaning of the substrates Ga3d and As3d peaks were recorded after ionic cleaning of GaAs. The background of spectra has been removed using the Shirley method. Cleaning and measurements have been made for eight substrates, and the results are reported in Fig. 1. One www.elsevier.com/locate/apsusc Available online at www.sciencedirect.com Applied Surface Science 254 (2008) 4150–4153 * Corresponding author. Fax: +33 4 73 40 73 40. E-mail address: bideux@lasmea.univ-bpclermont.fr (L. Bideux). 0169-4332/$ – see front matter # 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.apsusc.2007.12.058