Oxide semiconductors are essentially stable and environmental-friendly materials as well as possessing unique
multifunctional properties in conjunction with ultraviolet (UV) to deep UV (DUV) optical functions. Among them
ZnO and Ga<sub>2</sub>O<sub>3</sub>, having the bandgaps of about 3.3 and 4.9eV, respectively, are the promising candidates for exploring
their UV applications. This paper reports recent advances of ZnO and Ga<sub>2</sub>O<sub>3</sub> semiconductors focusing on their UV to
DUV optical functions and device applications. Since ZnO has reached to the actual application stage and future
development of the growth with chemical vapor deposition (CVD) is now strongly requested for mass production, here
we introduce a novel CVD growth technique, that is, ultrasonic spray assisted CVD, allowing safe and low-cost growth
of high quality ZnO-based films. Homoepitaxy on ZnO substrates resulted in step-flow growth, which has hardly been
achieved by metalorganic CVD. Ga<sub>2</sub>O<sub>3</sub> is expected for its DUV functions being supported by the availability of Ga<sub>2</sub>O<sub>3</sub>
bulk substrates. We show the potential applications of Ga<sub>2</sub>O<sub>3</sub> substrates for highly sensitive DUV photodetectors as
well as homoepitaxial step-flow growth of Ga<sub>2</sub>O<sub>3</sub> thin films by molecular beam epitaxy.
ZnCdSe/ZnSe multiple quantum well (MQW) transmission and reflection photomodulators operating at room temperature were fabricated employing quantum-confined Stark effect on the exciton absorption. Samples were grown on p-type GaAs substrates by MBE with an i-Zn<SUB>0.87</SUB>Cd<SUB>0.13</SUB>Se/ZnSe MQW heterostructure sandwiched by a ZnSe p-n junction. The transmission modulator was constructed with a Zn<SUB>0.87</SUB>Cd<SUB>0.13</SUB>Se/ZnSe MQW glued onto a piece of ITO film-covered glass with silver paste and epoxy. To avoid absorption in GaAs substrates, a window with a diameter of about 2 mm was opened using a selective etch. For the reflective use an Al mirror was deposited on the glass back surface, the device then operates in reflection with the light to be modulated making a double pass through the active quantum well region, thereby increasing the modulation amplitude. Measurement results are given in this paper for transmission, reflection, differential transmission, differential absorption, and differential reflection as a function of the incident photon wavelength and the applied field.
Photocurrent spectrum (PC) is reported both at room temperature and at 77 K on p-i-n Zn<SUB>1-x</SUB>Cd<SUB>x</SUB>Se/ZnSe multiple quantum well heterostructures (x approximately equals 0.12) grown by molecular beam epitaxy. Almost all the exciton transitions, allowed and forbidden, associated with the two lowest electron and three highest hole subbands are clearly observed. With the increase of well width from 22 angstrom to 54 angstrom, an evolution of excitonic structure in PC spectra is demonstrated. Theoretical calculations show a quite good agreement with the measurement results, and from the comparison between theory and experiment, an exiton binding energy for heavy hole of 28 meV, and for light hole of 20 meV are obtained for this type of quantum wells.
Gas-source molecular beam epitaxy (GSMBE) was applied for the growth of ZnMgSSe layers and quantum well (QW) structures. The source materials were elemental Zn and Se, as well as gas sources of bis- methylcyclopentadienyl-magnesium ((MeCp)<SUB>2</SUB>Mg) and H<SUB>2</SUB>S. Mg and S compositions were well controlled by the flow rate of (MeCp)<SUB>2</SUB>Mg and H<SUB>2</SUB>S, respectively. ZnSe/ZnMgSSe QWs with abrupt heterointerface have successfully been grown on -oriented GaAs substrates under in-situ monitoring of specular beam intensity oscillation in reflection high energy electron diffraction (RHEED). Photoluminescence (PL) at 4.2 K revealed sharp and intense emission from single QWs, which is attributed to n equals 1 heavy-hole free exciton. The photopumped lasing of a double heterostructure was achieved at room temperature with low threshold excitation intensity (110 kW/cm<SUP>2</SUP>), suggesting formation of well-defined heterostructures and promising potential of GSMBE for device applications.
We report the effect of the well width on room temperature operation of II-VI p-i-n quantum Stark effect modulators using ZnSe-ZnCdSe multiple quantum well structures within ZnSe p-n junctions. Results are given for the theoretical calculation of exciton binding energy and Stark shift, absorption and differential absorption as a function of the applied electric field. An optimum well width is estimated in current structures for the achievement of the largest (Delta) T/T by means of both theoretical and experimental approaches.