Molecular beam epitaxy of II-VI compounds

Abstract

The II-VI compounds are desirable for integrated optics due to their high electro-optic coefficients, wide transparency range from the visible to beyond 10 μm, and the continuously adjustable refractive index offered by their ternary alloys. Films of waveguide thickness (1/2–2 μm) were grown here by evaporation, under ultrahigh vacuum, of the constituent elements (Zn, Cd, Se, Te) from separately liquid-nitrogen-shrouded graphite Knudsen cells onto temperature-controlled chemomechanically polished single-crystal substrates. A quartz-crystal deposition monitor operating at the growth temperature was used in a novel way to determine the group-VI/II impingement rate ratio which would produce stoichiometric films, by measuring deposition rate as a function of the impingement rate ratio. Dissociative reevaporation of growing films sets an upper limit to growth temperature; 1 μm/h reevaporation rates were calculated to be reached at 400 °C for CdTe, 470 °C for ZnTe and CdSe, and 570 °C for ZnSe. At impingement rates corresponding to about 1 μm/h, ZnSe ceased to deposit at 125 °C below that point and the other three compounds at 50 °C below that point. Over a 50 °C range below the cessation of deposition, epitaxy was obtained for all II-VI compounds, as determined by in situ LEED. Film surface topography was examined by Nomarski differential-interference-contrast microscopy. Crystallographic quality depended slightly and surface smoothness depended very strongly on substrate orientation in the following decreasing order of quality: (1) GaAs(100); (2) GaAs(110); (3) CdS(0001), CdSe(0001). On GaAs, ZnTe and Zn(SeTe) films were much smoother than ZnSe films, 1-μm-thick films being smooth to tens of angstroms. The following system is recommended for integrated optics: ZnTe waveguide on Zn(SeTe) to optically isolate the waveguide from the high-index substrate, InAs(100) for 0.6% lattice constant mismatch.