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.