Photoluminescence Study of GaAs Diffused with Li
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PHOTOLUMINESCENCE STUDY OF GaAs DIFFUSED WITH Li H. P. GISLASON, E. 0. SVEINBJORNSSON, B. MONEMAR* AND M. LINNARSSON* Science Institute, University of Iceland, Dunhaga 3, IS-107 Reykjavik, Iceland *Linkbping University, Department of Physics and Measurement Technology, S-581 83 Link6ping, Sweden ABSTRACT We present a detailed study of the photoluminescence (PL) properties of a wide range of GaAs material diffused with the group I element Li. The effects of the Li diffusion are investigated through its effects on existing photoluminescence bands in the as-grown material as well as the appearence of new such bands. Among new PL bands resulting from the Li doping of n-type and semi-insulating material the most pronounced ones are a strong deep band at 1.34 eV and shallower bands at 1.45 and 1.48 eV. The origin of these PL bands will be discussed. INTRODUCTION Passivation of shallow impurities in semiconductors by atomic hydrogen has been found to result from the formation of complexes including the shallow donor or acceptor species and atomic hydrogen. Recent example of this effect in GaAs is the Be-H complex, where the hydrogen is a donor passivating the Be acceptor [1, 2]. There have also been studies of donor-H complexes, where the hydrogen acts as an acceptor [3]. In view of the interest in the properties of hydrogen in semiconductors, we focus our attention on the behaviour of another group-I element in GaAs, namely Li. Li is a fast interstitial diffuser in GaAs with a high solubility at 800 oC both in undoped 3 3 GaAs, 1.6 x 1019 cm- , and in n- and p-type GaAs, 2.3 x 1019 cm- [4]. Nominally undoped GaAs samples which are diffused with Li at temperatures greater than 500 oC and cooled to room temperatures show a high resistivity. This has been explained by the action of Li÷ which rapidly diffuses through the crystal forming donor-acceptor complexes, thus compensating the crystal [5]. In this way Li compensates n-type Te-doped GaAs with concentrations as high as 5 x 1018 cm-3 [5]. Li is, however, also found to compensate strongly p-type material, in which case the interstitial Li donor is a natural candidate for the compensation mechanism. Studies of Li in GaAs were quite extensive in the early seventies, but little work has been done on this topic recently. The information from earlier work mainly comprises solubility and diffusion data and local mode energies from IR spectroscopy [6]. More recently, photoluminescence studies of Cu- and Li-codoped GaAs were reported [7]. There Li diffusion was found to reduce the intensity of the 1.36 eV PL band resulting from the primary Cu diffusion [8] at the same time as a new PL band at 1.41 eV emerged. The intensity of this new PL band was found to be correlated with the Li-diffusion procedure. In the same study a preliminary investigation of Lidoping alone was reported. A strong PL band at 1.34 eV as well as shallower bands in the photon energy range 1.46 - 1.49 eV were observed in two samples [7]. These optical spectra have been studied in detail in the present investiga
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