Photoluminescence Study of Li-Implantation into ZnSe Epitaxial Layers Grown by Molecular Beam Epitaxy
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PHOTOLUMINESCENCE STUDY OF Li-IMPLANTATION INTO ZnSe EPITAXIAL LAYERS GROWN BY MOLECULAR BEAM EPITAXY T. Yasuda*, M. K. Jin*, J. Gaines** and J. L. Merz* *Department of Electrical and Computer Engineering, University of California, Santa Barbara, CA 93106 **Philips Laboratories, 345 Scarborough Road Briarcliff Manor, New York 10516
ABSTRACT Lithium ion-implantation was performed on ZnSe layers which were grown on GaAs substrates by MBE. Strong photoluminescence emission due to acceptor-bound excitons was observed from samples implanted with lithium doses of up to 1015 cm-2 and annealed under optimized conditions for rapid thermal annealing. Lithium implantation results in low damage to the ZnSe as well as the formation of a shallow acceptor level. Thermal degradation of both implanted and unimplanted epilayers caused by the rapid thermal annealing was inviestigated at various annealing temperatures. We find that, under certain annealing conditions, unimplanted samples show greater thermal degradation than lithium implanted samples. The photoluminescence spectrum of an unimplanted sample annealed at 800'C shows considerable degradation; however, a lithium-implanted sample annealed at the same temperature still shows strong luminescence in the excitonic region. INTRODUCTION For the fabrication of practical blue light emitting devices from ZnSe, control of p-type material is essential. Recently, significant improvement of crystal growth has been achieved as a result of the use of the modem growth techniques of molecular beam epitaxy (MBE) and metalorganic chemical vapor deposition (MOCVD). Several groups have reported p-type ZnSe by doping with lithium, nitrogen and oxygen [1-10]. However, it is still very difficult to control the doping of these impurities during growth due to the difficulty of identifying appropriate dopant sources as well as doping techniques. Ion implantation offers the advantage that specific impurities can be introduced under highly controllable and reproducible conditions. Pioneering work has been done to achieve p-type conversion of ZnSe bulk crystals by ion implantation using various impurities, such as lithium, nitrogen and phosphorus [11-13]. However the conductivity of these samples was not high enough to allow fabrication Recent improvement in crystal growth offers high purity, uniform and large-area ZnSe epilayers, enabling further systematic investigations of ion implantation into ZnSe [4, 14-16]. Ion implantation must be followed by an annealing process, because implantation creates considerable crystalline damage, which must be removed. However, the annealing process also causes thermal damage, resulting from evaporation of Zn and Se, thermal strain between ZnSe and GaAs, and perhaps other processes, such as diffusion of Ga from the substrate. As ZnSe appears to be much more susceptible to damage during heat treatment, compared with Si and GaAs [16], it is necessary to reduce the implantation damage and to keep the annealing temperature as low as possible. In this paper, we describe
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