Growth of NLO Chalcopyrite Materials by Omvpe
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theories. Since then, applications using nonlinear effects have multiplied. Examination of the nonlinear optical (NLO) properties of chalcopyrite crystals began in 1972. A summary of early crystal growth results and material properties of both the I-Ifl-VI 2 and lI-IV-V 2, chalcopyrite analogs of il-VI and Ill-V materials, respectively, as well as a chapter on nonlinear effects can be found in Shay and Wernick [1]. Preparation of these crystals generally used bulk growth or liquid phase techniques. Equilibrium thermodynamics control both of these techniques. Non-equilibrium approaches, molecular beam epitaxy (MBE) or organometallic vapor phase epitaxy (OMVPE), for example, offer advantages for growing crystals that might otherwise be unstable or metastable and for growing thin-layered structures where composition and thickness control are essential for device performance. Crystals must satisfy four criteria to be useful in nonlinear optics[2]: 1. 2. 3. 4.
They must have adequate and uniform nonlinearity. They must be adequately transparent. They must have adequate, uniform birefringence for phasematching. Crystal faces must be relatively immune to damage by intense optical radiation.
Major shortcomings of chalcopyrites in NLO are high absorption losses, nonuniform crystallinity, and relatively small-sized crystals. We believe that OMVPE may address these shortcomings. One advantage of epitaxial growth by OMVPE is that material properties can be 507
Mat. Res. Soc. Symp. Proc. Vol. 484 01998 Materials Research Society
very uniform. We expect the uniformity of nonlinearity and birefringence to be excellent. Optical losses (reduction in transparency) result from impurities and defects found in crystals. Again, epitaxial growth techniques such as OMVPE are capable of producing material that contains fewer impurities than host substrates. Non-equilibrium control of the growth environment may provide a means to alter the densities of native point defects that are difficult to control in bulk-grown crystals. OMVPE allows rapid, flexible changes of gas-phase reactant concentrations and resulting solid compositions. We have grown several of the II-IV-V 2 compounds for applications other than nonlinear optics as well as ZnGeP 2, which is of keen interest for NLO. A summary of the compounds grown and their applications is given in Table I. ZnSiAs 2 provides an example that illustrates the advantages of OMVPE growth of these materials. In earlier work using traditional vapor phase epitaxy (open tube with elemental zinc source), Kx1 and KOC2 lines of copper x-rays failed to resolve for ZnSiAs 2 growth on Ge. They do resolve for OMVPE growth (using dimethylzinc) on Ge and GaAs substrates, suggesting improved crystalline quality[3]. Other workers using OMVPE to grow ZnSiAs 2 have subsequently reported similar resolution of the Koc radiation[4]. Bulk ZnGeP 2, grown at North Carolina State University, illustrates a second advantage of OMVPE; Figure 1 shows mass spectral analyses of the bulk-grown material and an epitaxial sampl
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