Journal of Crystal Growth 246 (2002) 281–286 Vacancies as compensating centers in bulk GaN: doping effects K. Saarinen a, *, V. Ranki a , T. Suski b , M. Bockowski b , I. Grzegory b a Laboratory of Physics, Helsinki University of Technology, P.O. Box 1100, Espoo 02015 HUT, Finland b UNIPRESS, High Pressure Research Center, Polish Academy of Sciences, 01-142 Warsaw, Poland Abstract Gallium vacancy complexes have been identified in n-type bulk GaN by applying positron annihilation spectroscopy. Their formation is suppressed when the material becomes resistive by Mg doping, as expected from the behavior of the V Ga formation energy as a function of the Fermi level. In Be-doped GaN vacancies are observed even in resistive material. The positron lifetimes show that their open volume is larger than expected for the N vacancy. A possible identification is a V N 2Be Ga complex, where the atoms neighboring the N vacancy are strongly relaxed outwards, thus increasing the open volume. r 2002 Elsevier Science B.V. All rights reserved. PACS: 71.55.Eq; 61.72.Ji; 78.70.Bj Keywords: A1. Defects; A1. Doping; A1. Positron annihilation; B1. Gallium nitride 1. Introduction Bulk GaN crystals are ideal substrates for the epitaxy of GaN overlayers for optoelectronic components at the blue wavelength. Such material can be synthesized from liquid Ga in high N overpressure at elevated temperatures [1,2]. Nom- inally undoped GaN crystals usually show high n- type conductivity with the concentration of electrons exceeding 10 19 cm 3 . This is most likely due to the residual oxygen atoms acting as shallow donors [3,4]. When GaN is doped with Mg the electron concentration decreases. For sufficiently high amounts of Mg dopant the samples become semi-insulating. The same takes place with Be doping [5]. It is interesting to study how the movement of the Fermi level toward the midgap changes the formation of charged native defects such as the Ga vacancy. Another basic question is if there are differences in the doping effects induced by Mg and Be. Positrons in solids get trapped at neutral and negative vacancy defects, which can be experimen- tally detected by measuring the positron lifetime [6]. According to theory [7–9], vacancies and associated complexes are the dominant compen- sating centers formed at high doping levels. In this paper we first review our recent work [10–12] and show that a Ga vacancy acts as a native defect in n-type GaN crystals. Our data indicate further that the formation of Ga vacancies is suppressed by Mg doping. In this paper we pay special attention to Be doping. Interestingly, the positron data show the presence of vacancies in Be-doped *Corresponding author. Tel.: +358-9-451-3111; fax: +358- 9-451-3116. E-mail address: ksa@fyslab.hut.fi (K. Saarinen). 0022-0248/02/$-see front matter r 2002 Elsevier Science B.V. All rights reserved. PII:S0022-0248(02)01752-9