The Metastability of the EL2 and DX Defects in GaAs and 3-5 Alloys
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THE METASTABILITY OF THE EL2 AND DX DEFECTS IN GaAs AND 3-5 ALLOYS H.J. von BARDELEBEN AND J.C. BOURGOIN Groupe de Physique des Solides de l'Ecole Normale Sup6rieure, Centre Natio2 place Jussieu, 75251 Paris Tour 23, nal de la Recherche Scientifique°, Cedex 05, France.
ABSTRACT GaAs and Gal_x AlxAs The metastability of the EL2 and DX defects in alloys is discussed in the context of recent hydrostatic pressure and photoEPR results. A unified model is presented based on the metastable trapping of carriers in excited effective-mass (EM) states derived from secondary minima. The metastability is attributed to small latconduction band (CB) tice relaxation effects.
INTRODUCTION The electronic properties of the native donor defect EL2 in GaAs and their relation with the atomic configuration have been extensively studied [1]. It has been shown that this defect can be transformed by low temperature optical excitation into a second configuration, which is separated from the ground state by a thermal barrier of - 0.3 eV [2]. Different models for this metastability have been proposed but the issue is still controversial [3-6]. Direct experimental verifications of the proposed metastable configurations are difficult in this case as no associated gap state has been detected. Recently, it has been shown that the single substitutional donors, obtained by doping with group IV at VI ions, are equally metastable defects in GaAs [7] and the GaAlAs alloy system [8-12]. The related donor states have been studied as a function of alloy composition x and hydrostatic pressure, two perturbations which both lead to a modification of the lowest conduction band (CB) structure. Three different ground states of these single donors have been evidenced : the effective-mass state derived from the r CB minimum, which is the ground state for alloy composition x < 0.2, the X CB related EM state, which is the ground state for x Z 0.8 as well as the -DX" level being the ground state for the intermediate composition 0.2 4 x < 0.8. The DX state follows the L CB minimum [13,14] and presents the characteristics of the L CB EM state [15]. By application of hydrostatic pressure or optical excitation at low temperature the DX center can be metastably transformed into an excited configuration, which contrary to the case of EL2 in GaAs is paramagnetic [11,12] and electrically [9] active. We have recently shown in the case of the Sn-DX center that the metastable state is the EM ground state derived from the secondary X CB minimum. We present in this paper new EPR results concerning the group VI related Te - DX center in Ga.6 7 Al. 33 As, which confirm the existence of this metastable state. We further analyze very recent results on EL2 under hydrostatic pressure as well as previously reported variations of the EL2 metastability in the GaAsP alloy system. We conclude that the metastability of the double donor EL2 is equally related to the population of an excited resonant EM state derived from the secondary L CB minimum. This leads us to the proposal of a unifie
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