Growth of ZnSe Epitaxial Layers and ZnSe/ZnS Superlattices By Pulsed Laser Deposition
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GROWTH OF ZnSe EPITAXIAL LAYERS AND ZnSe/ZnS SUPERLATTICES BY PULSED LASER DEPOSITION Y. RAJAKARUNANAYAKE, Y. LUO, B. T. ADKINS AND
A. COMPAAN
Department of Physics and Astronomy, Univ. of Toledo, Toledo, OH 43606 ABSTRACT
We report the successful growth of ZnSe and ZnSe/ZnS superlattices on GaAs by pulsed laser deposition. An XeCl excimer laser operated at 308 nm was used to ablate/evaporate Il-VI bulk targets in an ultra high vacuum enclosure. For typical growth temperatures in the range 350'-450*C we obtained epitaxial layers with excellent optical properties. The laser power and fluence were varied to produce growth rates in the range 0.1-1 A/pulse. The photoluminescence of the pulsed laser deposited ZnSe layers showed dominant bound and free exciton features. The superlattice samples showed large blue shifts (-400 meV) in the photoluminescence as the layer thicknesses were varied. These results are consistent with strong quantum confinement of the heavy holes in the ZnSe layers (valence band offset for ZnSe/ZnS - -850±100 meV), while the electrons are not confined to either layer because of very small conduction band offsets Strong exciton photoluminescence exhibited by our samples indicates that pulsed laser deposition is a promising growth technique for the fabrication of iI-VI epitaxial layers and strained layer superlattices for visible light emitter applications. INTRODUCTION
The materials ZnSe and ZnS have attracted much attention in the recent years because of their direct, wide band gaps that are ideal for short wavelength optoelectronics and blue light emitter fabrication[I,2]. The ability to grow high quality epitaxial layers and superlattices of these II-VI materials is a prerequisite for future devices based on these materials. Epitaxial growth of wide band gap II-VI heterostructures on GaAs by modern low temperature growth techniques such as molecular beam epitaxy (MBE)[2], and novel doping techniques[3,4] to achieve p-type conductivity in ZnSe have enabled the fabrication of electrically pumped ZnSe based blue lasers capable of room temperature operation.[4] We have applied pulsed laser deposition (PLD) as an alternate growth technique for producing ZnSe and ZnS epitaxial layers and ZnSe/ZnS strained layer superlattices on GaAs. Our results show that pulsed laser deposition offers the potential of producing 1I-VI epitaxial layers rivaling the quality of MBE-grown layers.[2] In this paper we describe the pulsed laser growth of ZnSe and ZnSe/ZnS superlattices by PLD, and their characterization by low temperature photoluminescence (PL). PLD has already established its reputation as an excellent growth technique for producing high quality, high-transition-temperature ceramic superconductors (e.g. YBCO).[5] One of the key advantages of PLD is its ability to evaporate nearly congruently multi-element targets and produce directional plumes with well defined kinetic energy distributions which can be easily controlled by varying the laser fluence, photon energy, etc.[6] The evaporated species contain
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