Heteroepitaxial growth of nonlinear optical materials for frequency conversion in the mid and longwave IR

Abstract

We report recent results on heteroepitaxy of ZnSe on GaAs, GaSe on GaP, GaAs and GaN substrates, and GaAsxP1−x ternaries on GaAs, as well growths of OPGaP, OPGaAsP and OPZnSe on orientation-patterned (OP) GaAs templates. ZnSe and GaSe were chosen for their wide IR transparency and good NLO properties, taking advantages from the close lattice match of ZnSe to GaAs, α–Ga4Se6 (cubic) to GaP and GaAs, and β–GaSe (hexagonal) to GaN. For the same reason growth of OPGaP was attempted on OPGaAs templates. As for the GaAsxP1−x ternaries, the idea was to determine whether these ternaries can combine in one material the lower 2-photon absorption of GaP with the higher nonlinear susceptibility of GaAs since a GaAsxP1−x ternary always has a closer lattice match to either GaAs or GaP than the two parenting materials. Up to 500 µm thick layers grown by Hydride Vapor Phase Epitaxy (HVPE) on plain substrates and up to 300 µm thick OP structures grown on OP templates revealed smooth surface morphology (RMS < 2 nm from 1 μm2 AFM scan) and superior crystalline quality (FWHM < 100 arcsec of the XRD Omega scan). The growth of α–GaSe/GaAs can be considered as a low-dimensional Van-der-Waals heteroepitaxy. Further characterization of the heteroepitaxial layers by Nomarski optical imaging, linear transmission and nonlinear absorption measurements, TEM, SEM, AFM, and EDX revealed that in some of the cases a smooth transition between the substrate and the layer material can be achieved by the formation at the interface of an intermediate ternary layer with a gradually changing composition. It also indicated that during fast growth processes, such as HVPE, the strain built due to the lattice and the thermal mismatch between substrate and growing layer shall be relaxed in faster and more energy absorbing processes such as formation of voids or roughening surfaces (GaP/GaAs) or an immediate formation of stacking faults that originate still from the substrate (ZnSe/GaAs). All this may postpone or even replace the formation of the expected misfit dislocations frequently observed in the slower MOCVD and MBE growths.