Journal of Alloys and Compounds 362 (2004) 261–264 Structure of GaAs:Be crystals studied by X-ray quasi-forbidden reflections I. Frymark a,∗ , G. Kowalski a , M. Kami ´ nska a , A. Krotkus b a Institute of Experimental Physics, University of Warsaw, Ho˙ za 69,00-681 Warsaw, Poland b Semiconductors Physics Institute, 2600 Vilnius, Lithuania Received 17 October 2002; received in revised form 16 November 2002; accepted 16 January 2003 Abstract The X-ray quasi-forbidden reflection method (XQFR) employs for example (002) and (006) reflections in the GaAs structure. They are called quasi-forbidden because of very small differences in scattering factors between gallium and arsenic. These reflections are very sensitive to the chemical composition and hardly sensitive to large crystal lattice defects. The method is based on the comparison of quasi-forbidden reflection intensities for the investigated crystal with the intensities obtained for a reference, quasi-ideal crystal. One can build theoretical models of lattice point defects distributions and local distortions and calculate appropriate structure factors. The kinematical theory of X-ray diffraction then allows comparison with the experimental data obtained with the XQFR method. In our preliminary study we employed only the 006 reflection to verify positions of the beryllium atoms and how they are incorporated into gallium arsenide lattice and to study the local microscopic structures around them. © 2003 Elsevier B.V. All rights reserved. Keywords: X-ray diffraction; Gallium arsenide; Beryllium doping 1. Introduction Low temperature (LT) GaAs grown by molecular beam epitaxy (MBE) has attracted extensive attention due to ex- ceptional physical and electrical properties. High resistivity, short carrier-trapping times and high carrier mobility make it a unique semiconductor that has found many applications in electronic and optoelectronic devices. The most important defects in LT GaAs are As antisites (As Ga ) and Ga vacan- cies (V Ga ) [1]. The most intensively studied is As Ga , called EL2 defect, in semi-insulating (SI) GaAs. The influence of the EL2 defect on crystal lattice properties and its metasta- bility were previously investigated by other X-ray methods [2,3]. In order to increase thermal stability of LT GaAs and to shorten the lifetimes of charge carriers a p-type dopant like beryllium is used [4,5]. The only data available which deal with the question of the Be position in the lattice come from Raman spectra measurements [6]. Their conclusion is that Be atoms probably take substitutional positions. ∗ Corresponding author. Tel.: +48-22-553-2325; fax: +48-22-622-6154. E-mail address: ifrymark@fuw.edu.pl (I. Frymark). 2. Theoretical basis Weak (quasi-forbidden) reflections can be described by the kinematic theory of X-ray diffraction. Their integrated intensity is proportional to |F hkl | 2 , where F hkl is the structure factor of the relevant lattice [7]. For gallium arsenide with a Ga atom taken as the origin of the coordinates system F hkl = [f As e i(h+k+l)/2 + f Ga ] × [1 + e i(h+k) + e i(h+l) + e i(k+l) ]. (1) If h + k + l = 4n + 2(n = 0, 1, 2, ... ), then F hkl = 4(f Ga - f As ). (2) f As and f Ga as usually mean atomic form factors of the re- spective atoms, hkl—Miller indices of the reflecting planes. Due to small differences in scattering factors between gallium and arsenic these hkl reflections have very low intensity and consequently they are called quasi-forbidden reflections. If Ga and As sites are fully occupied by the respective atoms, one can treat Eq. (1) as the ideal-crystal formula. 0925-8388/$ – see front matter © 2003 Elsevier B.V. All rights reserved. doi:10.1016/S0925-8388(03)00595-4