The relationship between the electrical, optical and material properties of transparent and conductive oxide films prepared by rf cosputtering indium–tin oxide (ITO) and zinc oxide (ZnO) targets has been investigated. The evolution from polycrystalline structure of an undoped ITO film to an amorphous-like ZnkIn2O3+k structure obtained from ZnO-doped ITO films is found to be responsible for the marked improvement in the electrical properties. A low surface roughness is also achieved from this amorphous structure. However, both electrical property and surface uniformity begin to degrade with increasing rf cosputtering power on the ZnO target that corresponds to a high atomic ratio of Zn/(Zn + In). The degradation mechanism is attributed to the appearance of a microcrystalline ZnO structure that is detrimental to the film resistivity. Furthermore, optical band gap calculated from the absorption edge of such cosputtered films also decreases with increasing ZnO impurities.
Crystalline structures, intrinsic stress, and surface roughness of piezoelectric ZnO films deposited on sapphire substrates by rf magnetron sputtering and sequential post-annealing treatments were investigated by X-ray diffraction (XRD) and atomic force microscopy (AFM) measurements. The rf power used and O2/(Ar+O2) gas ratio greatly affected the crystallinity and surface roughness of the ZnO film. A c-axis preferred orientation with superior surface roughness was achieved at a specific rf power and O2/(Ar+O2) gas flow ratio. In addition, the original intrinsic stress was also markedly released (stress=0.325 ×1010 dyn/cm2) and a uniform surface (Ra=2.42 nm) was produced by a post-annealing treatment at a temperature of 400 °C under oxygen ambient. An acoustic wave at the center frequency of 240 MHz (hZnO/λ=0.1) was obtained from the ZnO/sapphire-layered surface acoustic wave (SAW) device using this optimized ZnO film.
We report a coupled quantum dot (QD) structure for long wavelength laser applications. The structure comprises an InAs seed layer and a second InAs QD layer capped with an In0.33Ga0.67As capping layer. Cross-sectional transmission electron microscopy (TEM) images show a vertical alignment between the QD stacks, which causes the coupled QD sample to have a larger dot size and a lower dot density than the control sample. The laser with the coupled QD structure exhibits a markedly longer emission wavelength and a slightly higher threshold current density than lasers with a conventional QD structure, indicating that the coupled QD structure has potential for long wavelength applications.
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