Epitaxial ZnS, ZnSe and ZnS-ZnSe Superlattices Grown on (001)GaAs By Pulsed-Laser Ablation
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Epitaxial ZnS, ZnSe and ZnS-ZnSe Superlattices Grown on (001)GaAs By Pulsed-Laser Ablation J. W. McCamy,* Douglas H. Lowndes,t J. D. Budai,t G. E. Jellison, Jr.,t I. P. Hermant and S. Kimt * Department of Materials Science and Engineering, The University of Tennessee, Knox•vile, TN 37996-1200 t Solid State Division, Oak Ridge National Laboratory, Oak Ridge, TN 37831-6056 J:Department of Applied Physics, Columbia University, New York, NY 10027
Pulsed KrF (248nm) laser ablation of polycrystalline ZnS and ZnSe targets has been used to grow high quality, fully epitaxial ZnS and ZnSe thin films on (001) GaAs. Photoluminescence measurements of the ZnS thin films show strong edge emission, while ZnSe thin films show free excitonic as well as donor and acceptor peaks. By alternately ablating each target, strained layer superlattices of the form (ZnSe)m-(ZnS)n were grown with as many as 65 periods of compositional modulation. A ZnSxSel-x structure also was fabricated which simultaneously incorporated both continuously graded and abrupt compositional changes. INTRODUCTION Zinc sulfide (ZnS) and zinc selenide (ZnSe) are attractive thin-film optoelectronic materials because of their wide direct bandgaps (Eg = 3.7 eV and 2.7 eV, respectively). Potential applications for these materials in~lude optically bistable switching devices for information processing [1,2], optical waveguides [3,4], and blue LEDs or laser diodes [5,6]. The two techniques most often used to grow ZnS, ZnSe and ZnSxSel-x thin films and superlattices are metalorganic chemical vapor deposition (MOCVD) and molecular beam epitaxy (MBE). However, pulsedlaser ablation (PLA) has several practical advantages. These include stoichiometric transport of material from the ablation target to the substrate surface, when the laser beam conditions are correctly adjusted; growth of smooth films when the laser energy density, Eq, and the target surface morphology are controlled properly [13]; and, the ability to grade compositionally an epitaxial layer [7]. As with MBE, PLA films can be grown with submonolayer precision. Additionally, the composition of a PLA film is readily changed simply by exchanging solid ablation targets; consequently, strained layer superlattices of two or more materials are easily grown.by PLA [8]. We recently showed that PLA is an attractive method for growth of uniform, fully epitaxial (e.g., in-plane-aligned) ZnS and ZnSe thin films.and superlattices on (001) GaAs [9,10]. High resolution TEM (HRTEM), x-ray diffraction (XRD), and Rutherford backscattering spectrometry (RBS) showed that ZnS and ZnSe thin films grown by PLA are structurally comprable to those grown by MOCVD or MBE. However, it is not sufficient that PLA films simply be structurally 'good',. in order for PLA to be considered competitive with other growth techniques, for the applications noted above. Additional requirements are imposed on the optical and electrical properties of the films. Finally, the quality of PLA strained-layer superlattices must be investigated, and a capability to
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