Electronic and Optical Properties of Deep Donors in Hydrogenated Al x Ga 1-x As:Si.
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ELECTRONIC AND OPTICAL PROPERTIES OF DEEP DONORS IN HYDROGENATED AlxGai-xAs:Si. R.MOSTEFAOUI*, R.LEGROS*, J.CHEVALLIER*, C.W.TU**, R.F.KOPF**; *Laboratoire de Physique des Solides, CNRS, 1 Place A.Briand, 92195 Meudon Cedex, FRANCE. **A.T.T. Bell Laboratories, Murray Hill, New Jersey 07974,USA. ABSTRACT: Transient conductivity and photoconductivity experiments have been performed on hydrogenated Alo.37Gao.63As:Si epilayers. The recapture kinetics of free electrons on the remaining deep donors is significantly affected under hydrogenation. The energy barrier height for capture on the deep donors drops from 210mev in the reference sample to 40mev in the hydrogenated sample. We also note a shift toward higher energies of the photoionization cross section versus photon energy curve. Different models are proposed for explaining the data. INTRODUCTION: Many published papers have shown that the electrical properties of n type AlxGai-xAs:Si (x > 0.2) are controlled by deep centers (called D-X centers) directly related to donor impurities. They are characterized by a repulsive barrier for both electron emission and capture leading to persistent photoconductivity. The exact structural description of these centers is still a subject of controversy (1)(2)(3). The presence of these defects has a detrimental influence on the properties of devices (4). So the elimination or the neutralization of these centers would have a large impact in microelectronics. The passivation of deep centers by hydrogen have been demonstrated in GaAs. NABITY et al have shown that the D-X centers in n-AlGaAs:Si are passivated after exposure of the crystals to a hydrogen plasma (5). In a more recent paper, we have reported large mobility increase after hydrogenation of AlxGai-xAs:Si. This effect has been related to the neutralization of shallow donors and D-X centers by hydrogen (6). For x = 0.37, the electrical properties after hydrogenation are governed by the ionization of remaining deep donors. In this work, we present the electronic and optical characteristics of these deep donors remaining after hydrogenation. EXPERIMENTAL: The AlGaAs samples were grown by molecular beam epitaxy on semi-insulating GaAs substrates. The sample structure consisted of a O.5pm undoped GaAs buffer layer, a 1000 A undoped AlxGai-xAs layer with graded x (0 < x < 0.37) in order to avoid modulation doping effects and a 1.5pm silicon doped AlxGal-xAs layer ([Si]=2xlO1 7cm- 3 , nH(300K)=5xlO01cm- 3 ). The Al fraction (x=0.37) was measured by electron microprobe and by photoluminescence at low temperature. Hydrogenation of the samples was realized by exposure to a R.F. hydrogen plasma. The exposure conditions were: T = 250°C, t 30 min., R.F. power = lOW. In this study, we have determined the Hall energy, the optical ionization energy Eo
Mat. Res. Soc. Symp. Proc. Vol. 163.
1990 Materials Research Society
794
and the capture energy barrier Ec of the deep donors present in AlGaAs:Si before and after hydrogenation. The Hall energy has been deduced from conventional Hall effe
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