L Journal of Alloys and Compounds 286 (1999) 271–275 Lattice parameters of GaN single crystals, homoepitaxial layers and heteroepitaxial layers on sapphire a, a a a b b * M. Leszczynski , P. Prystawko , T. Suski , B. Lucznik , J. Domagala , J. Bak-Misiuk , c d e e A. Stonert , A. Turos , R. Langer , A. Barski a High Pressure Research Center Polish Academy of Sciences, Sokolowska 29 /37, 01-142 Warsaw, Poland b ´ Institute of Physics Polish Academy of Sciences, Al. Lotnikow 32 /46, 02-668 Warsaw, Poland c Soltan’ s Institute of Nuclear Research, Hoza 69, Warsaw, Poland d Institute of Electronic Material Technology,, ul Wolczynska 133, Warsaw, Poland e ` ` ´ Commissariat a l’ Energie Atomique / Departement de Recherche Fondamentale sur la Matiere Condensee, SP2M, 38054 Grenoble Cedex, France Abstract Lattice parameters of gallium nitride were measured using diffraction of X-rays produced by the laboratory generators and the European Synchrotron Radiation Facility (ESRF). The following samples were examined: (i) bulk single crystals grown at high hydrostatic pressure, undoped and Mg-doped, (ii) homoepitaxial layers grown on those crystals, undoped, Mg-doped and Si-doped, (iii) heteroepitaxial layers grown on sapphire, (iv) heteroepitaxial layers on sapphire after high-pressure annealing treatment, (v) hetero- epitaxial layers on sapphire after implantation and thermal processes. The lattice parameters were measured versus free-electron concentration established by photoluminescence, far-infrared reflectivity and the Hall method. It is concluded that the main factor causing an increase of the lattice parameters of GaN are free electrons which act via the deformation potential of the conduction band. For heteroepitaxial layers, the strains caused by the mismatched substrates are discussed in terms of the mosaicity of these layers and thermal mismatch with respect to the substrates.  1999 Elsevier Science S.A. All rights reserved. Keywords: Semiconductors; X-ray diffraction; Gallium nitride; Lattice parameters 1. Introduction heteroepitaxial GaN layers by the rocking-curve analysis. For heteroepitaxial layers of GaN, the lattice parameters Gallium nitride and related compounds (Al,Ga,In)N are measured in order to estimate the strain induced by make the so called ‘‘blue revolution’’ by being used for different lattice parameters and thermal expansion between constructing new light emitters and detectors in the range the layers and the substrates [2,3]. For bulk crystals and from green to ultraviolet. The commercial blue and green homoepitaxial layers, we published introductory papers diodes are already on the market and first models of lasers showing the dependence of GaN lattice parameters on the operating to 10 000 h have been announced [1]. However, microstructure of the samples [4,5]. The present paper is the III-N compounds are not easy to be grown and an extension of those previously published and its main processed. The main problems faced by scientists and purpose is to show what information on crystallographic technologists are the following: (i) high melting points quality of GaN can be drawn from the precise measure- (about 2800 K for GaN), (ii) low decomposition tempera- ments of the lattice parameters. tures at low pressures (about 1300 K for GaN at 1 atmosphere), (iii) lack of easily available lattice matched substrates, (iv) low ionization rate of the main p-dopant, 2. Samples magnesium, (v) preparation of the ohmic electrical contact to the p-type material. X-ray diffraction is the main The following (00.1) oriented samples of gallium nitride experimental tool for evaluating the crystallographic qual- (wurtzite structure) were used: ity of the III-N samples. Most of the X-ray measurements (i) Bulk single crystals grown from the nitrogen solution are performed to examine only the mosaicity of the in pure gallium at high hydrostatic pressure (up to 15 kbar) and high temperature (about 1800 K) [6]. So far, these *Corresponding author. samples are the largest gallium nitride single crystals ever 0925-8388 / 99 / $ – see front matter  1999 Elsevier Science S.A. All rights reserved. PII: S0925-8388(98)01019-6