490 Applied Surface Science 50 (1991) 490-495 North-Holland Identification of the double and single acceptor state of isolated Nioa in GaAs N. Hizem, G. Bremond, L. Mayet, M. Gavand, J. Gregoire, G. Guillot Laboratoire de Physique de la MatiOre (URA CNRS 358), INSA de Lyon, Batiment 502, 69621 Villeurbanne Cddex, France and W. Ulrici Akademie der Wissenschaften, Zentralinstitut fiir Elektronenphysik, Hausvogteiplatz 5-7, Berlin, Germany Received 27 November 1990; accepted for publication 23 January 1991 The question of the exact energy positions of isolated Nioa as well as their optical and electrical properties have not yet been fully clarified in GaAs. We present a systematic study by deep-level transient spectroscopy, deep-level optical spectroscopy and optical absorption performed on several Ni-doped GaAs materials: n- and p-type LEC (liquid-encapsulated Czochralski) grown and p-type layers grown by liquid-phase epitaxy (LPE). All the electrical and optical results are up to now relatively coherent with the following identifications: (i) the double-acceptor charge state (Ni+/Ni 2+) is at E c - 0.4 eV, (ii) the single-acceptor charge state (NiE+/Ni 3+) is at E v + 0.2 eV. However, when Ni is introduced during LPE in p-type materials we do not detect the NiE+/Ni 3+ level which suggest a very low solubility of Ni in the LPE growth conditions. 1. Introduction It has been shown that some 3d transition metals are present as inevitable contaminants in GaAs crystals. In general, they act as deep accep- tors and can have an adverse effect on the proper- ties and performances of devices based on GaAs. Therefore, it is important to understand their elec- tronic behaviour. Recent studies have revealed that these transition metals occupy the unper- turbed substitutional gallium site, exist in differ- ent charge states with respect to the position of the Fermi level, and easily form complexes with other impurities or native defects [1]. One of these transitions metals which concerns our work is Ni because in GaAs it is commonly used in the technology of ohmic contacts. It has been shown that the fabrication of such contacts gives rise to a deep diffusion of nickel in GaAs with Ga vacancies and so it may enter the crystal during contact formation. This is significant be- cause Ni can have a negative effect on the minor- ity carrier hole lifetime in n-type GaAs and can be responsible for the high-resistivity layer observed below ohmic contacts. Several studies of the energy states which ap- pear in the forbidden gap of GaAs after nickel diffusion have reported that Ni like most of the 3d transition metals forms an acceptor level in GaAs [1,2]. But it is the only isolated impurity reported up to now which introduces a stable double accep- tor level in the gap of GaAs [3]. Nevertheless, the question of the exact energy positions of the dou- ble (Ni+/Ni 2+) and the single (Ni2+/Ni 3+) acceptor levels of isolated NiGa as well as their optical and electrical properties have not been fully clarified. Partin et al. [4], and Kumar and Ledebo [5], have suggested after DLTS (deep-level transient spectroscopy) measurements the level at about E c - 0 . 4 eV as being the double-acceptor 0169-4332/91/$03.50 © 1991 - Elsevier Science Publishers B.V. (North-Holland)