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Characteristics of ZnO films grown on glass substrates with various MgO buffer layer thicknesses have been investigated. All the layers are deposited by plasma-assisted molecular beam epitaxy. Without the MgO buffer layer, surface cracks due to the difference in the thermal expansion coefficients between ZnO and the quartz substrate are easily observed. With the MgO buffer layer, the cracks drastically decrease. The surface morphology changes with different MgO buffer layer thicknesses due to the compressive-tensile stress balance. The ZnO films with a medium MgO buffer layer thickness show a dotlike surface. All samples except those with the dotlike surface exhibit c-axis oriented polycrystalline structures. The photoluminescence emission intensity is also improved by increasing the buffer layer thickness until the optimum MgO buffer layer thickness of approximately 150 nm is reached. Few cracks appear with the reduction of the photoluminescence (PL) intensity when the buffer layer thickness is more than the optimum value. All of the as-grown ZnO films without cracks exhibit n-type conductivity.

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Symmetrical emitter-collector planar Ga1-x Al x As/GaAs/Ga1-y Al y As double heterojunction bipolar transistors have been designed and fabricated. Crystal growth and doping are carried out using liquid phase epitaxy (LPE) technique. Five layers consisting of collector, base, emitter, emitter contact and mask layer, Ga0.6Al0.4As, have been grown consecutively by LPE on GaAs (n+) substrate. After etching the mask layer to open a region for base diffusion, the sample was carried back to the LPE furnace to grow the Zn-doped Ga0.6Al0.4As in order to diffuse Zn to the p-type base. After the process, both the diffusion layer and the mask layer were selectively etched. Ohmic contacts were then made. Finally, the DC characteristics of the transistors were measured to show their symmetrical behaviors.