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The photoconductive characterization of hot-wall-deposited MnAl2Se4 layers was investigated as a function of temperature. By measuring the photocurrent (PC) spectroscopy, we identified PC peaks corresponding to an intrinsic transition, i.e., a band-to-band transition. According to the selection rule, the crystal field and the spin-orbit splitting in the valence band were found to be 150.3 and 9.2 meV, respectively. With decreasing temperature, the position of the PC peak shifted toward the short wavelength region, and its variation was matched well by using E g (T) = E g (0) − 3.19 × 10−3 T2/(T + 488). E g (0) was estimated to be 3.5616, 3.7119, and 3.7211 eV corresponding to E g (A), E g (B), and E g (C) in the valence band in order of increasing energy, respectively. The PC intensity also decreased dramatically with decreasing temperature. From a semi-logarithmic plot of the PC density vs. the reciprocal temperature, the dominant trap level was extracted as 19.7 meV in the high-temperature region. Consequently, we found that trapping centers due to native defects in the MnAl2Se4 layers were responsible for the decrease in the PC intensity with decreasing temperature.

The epitaxial growth of photoconductive BaIn 2 S 4 layers was first achieved through the hot-wall-epitaxy method. In spite of an existing large lattice mismatch between the substrate and layer, BaIn 2 S 4 layers were epitaxially grown along the (440) direction onto a GaAs (100) substrate. Thus, the lattice mismatch was well interpreted through a coincidence site lattice model. From the relationship between the reciprocal temperature and the carrier concentration, the three donor levels were found to be 1.3, 20.2, and 78.3 meV below the conduction band. These donor levels are caused by the native defects originating from slight stoichiometric deviations. From the temperature dependence of the Hall mobility, two specific scatterings were observed. One, at high temperatures ranging over 180 K, is mainly due to the acoustic phonon mode of lattice vibrations through a deformation potential. The other, at low temperatures ranging below 100 K, is ascribed to the dislocation scattering. From the photocurrent (PC) measurement, three PC peaks due to band-to-band transitions were observed. Also, based on the analysis of optical absorption and PC spectra, the optical band gap has been compared and matched well with E g ( T )= E g (0)−3.95×10 −3 T 2 /( T +499), where E g (0) is estimated to be 3.0597, 3.2301, and 3.2606 eV for the transitions corresponding to the valence band states of peaks A, B and C, respectively.

The epitaxial growth of photoconductive BaAl2Se4 layers, which have a tetragonal structure, was first achieved through the hot wall epitaxy method. From the Hall effect result, in the high temperature range of T>230 K, the mobility decreased as a function of T−3/2 and its scattering was mainly due to the acoustic phonon mode of lattice vibrations through a deformation potential. Thus, at the intermediate temperature range (100

N2O sensors made with ZnO-based ZnCdO films were grown on Pyrex substrates by using the RF co-sputtering method. The structure of the N2O sensor was electrode/sensor/glass/illuminant. The mechanism of the photo-assisted oxidation and reduction process on the surface of the N2O sensors was investigated using light from a UV lamp and violet light emitting diode (LED). For photon exposure wavelengths of 365 and 405 nm, the sensitivity of the ZnO-based ZnCdO sensors was measured. From these measurements, the values of the sensitivity of the sensors with x = 0, 0.01, and 0.05 were found to be S = 1.44, 1.39, and 1.33 under LED light with a wavelength of 405 nm, respectively. These sensitivities were compared to those of SnO2 and WO3 materials measured at operating temperatures of 300–600 °C. Also, under exposure with UV light, the response times were observed to be 130 to 270 sec. These response times were slightly slower than that for the traditional method of thermal heating. However, they indicate that the described photon exposure method for N2O detection can replace the conventional heating mode. Consequently, we demonstrated that portable N2O sensors for room-temperature operation could be fabricated without thermal heating.

The epitaxial growth of photoconductive BaIn2Se4, which has orthorhombic structures, was first achieved through the hot wall epitaxy method. The electrical and optical characteristics of these epilayers were discussed. From the Hall effect measurement, in a high-temperature range of T>150 K, the mobility decreased as a function of T−1 and its scattering was mainly due to the acoustic phonon mode of lattice vibrations through a deformation potential. In contrast, the mobility decreased in proportional to T0.6 in a low-temperature range of T

ZnCdO oxide thin films have been deposited by using the radiofrequency cosputtering method to prepare ozone sensors for room-temperature operation. The sensors were fabricated in the order: electrode/sensor/glass/illuminant. The mechanisms of photo-assisted oxidation and reduction on the surface of the ZnCdO ozone sensors were investigated. Free electrons and holes were generated by exposure of the surface of the ZnCdO ozone sensor to ultraviolet light, tending to enhance surface absorption of dissociated O2 molecules and dissociation of adsorbed oxygen ions, respectively. Thereby, the sensitivity of the ZnCdO ozone sensor was increased. This photon exposure method can replace the conventional heating mode used in ozone sensors. Moreover, 405-nm light from a light-emitting diode, corresponding to photon energy of 3.061 eV, was found to assist the processes of oxidation and reduction due to the chemical reaction of the ozone gas. We present a possible route for fabrication of portable ZnCdO ozone sensors for room-temperature operation.

The epilayer growth of the MgGa2Se4 compounds was successfully achieved through the hot wall epitaxy method. The grown layer was accumulated along the direction onto the GaAs(100) substrate. From the Hall effect measurement, the mobility was determined to be 264 cm2/Vs at 293 K. At a high temperature range (T>150 K), it tended to decrease as a function of T−3/2 by increasing the temperature, and increase as a function of T3/2 at the low-temperature range (T

We grew modulated BeZnO layers by hybrid plasma-assisted molecular-beam epitaxy/electron-beam deposition. A wide band-gap investigation of the modulated BexZn1−xO by means of photocurrent (PC) spectroscopy was conducted. The band-gap energy was directly acquired from the wavelength of the PC peak, caused by the band-to-band transition. By increasing x, which was the rate of the Be elements, the optical band-gap energy was empirically fitted by EBeZnO(x) = 6.32(x–1)x + 7.305x + 3.295. We expect that this finding can open new possibilities for wide band-gap engineering of BeZnO layers, which can be utilized as barrier layers in active layers consisting of ZnO/BeZnO quantum well structures and solar-blind ultraviolet photodetectors.

ZnO based oxide system BexZn1−xO alloy of various compositions has been successfully grown by the RF co-sputtering method. The crystallinity of the BexZn1−xO alloys has been remarkably improved after the post-annealing at 600 °C compared with the BexZn1−xO alloys post-annealed at other temperatures. The x value of the BexZn1−xO layers has been increased from 0.022 to 0.17 by adjusting the RF-power of the Be target. Also, the optical bandgap energy has been modulated from 3.2218 to 3.7978 eV, respectively. Based on our results, a bandgap bowing parameter of BexZn1−xO alloy has been extracted out to be 4.5 eV. These findings could be useful to fabricate the ZnO/BexZn1−xO quantum structures and bandgap modulation for deep ultraviolet-light-emitting diodes.

The photoconductive cadmium gallium selenide (CdGa2Se4) layer was grown through the hot wall epitaxy method. From the photocurrent (PC) measurements, the three peaks in the PC spectra were associated with the band-to-band transitions. The PC intensities were observed to decrease with decreasing temperature. The valence-band splitting on CdGa2Se4 was also observed by means of the PC spectroscopy. The crystal field splitting and the spin orbit splitting turned out to be 0.1604 and 0.4179 eV at 10 K, respectively. The temperature dependence of the optical band gap on the CdGa2Se4 was estimated using the PC. The band-gap energy of CdGa2Se4 at room temperature was 2.5446 eV.

This is a report on the electrical characterization of gallium nitride (GaN) nanowire (NW) p–n junction diodes. These diodes were formed by assembling n-GaN NWs on p-Si (1 0 0) substrates using alternating current (AC) dielectrophoresis (DEP). The AC DEP was optimized with a bias voltage of 15 V p−p at a frequency of 1 kHz. The hetero-junction single GaN nanowire p-n diode (n-GaN NW/p-Si) showed well-defined current rectifying behavior with a forward voltage drop of 1.2–2.0 V at a current density of 10–60 A/cm 2 . The GaN nanowire p–n diodes had a high parasite resistance in the range of >470 kΩ. We observed that these high resistances were mostly the result of the metal contacts to the n-GaN NWs. We also found that these parasite resistances were reduced by the formation of an additional capping layer on the top of the n-GaN NW as well as high temperature annealing.

Epitaxial BeZnO layers, the candidates for active barrier layers composed of the ZnO/BexZn1−xO quantum well structure, were successfully grown using the hybrid beam deposition method. The Be rate of x in the grown BexZn1−xO layer was estimated to be 0.11. The optical properties caused by the thermal quenching phenomenon of BeZnO, an ultraviolet light emitting material, were studied using the temperature dependence of photoluminescence spectra. Emission of the neutral donor-bound exciton shows a tendency to quench the intensity and extend the spectral width with increasing temperatures. This tendency is related to the phonons generated by the lattice vibration of the host material in BeZnO. These findings led us to conclude that the phonons may be participating in the quenching process.

GaN-based InGaN/GaN multiple quantum wells (MQWs) structure having a high-quality epilayer and coherent periodicity was grown by metalorganic chemical vapor deposition. After thermal annealing of InGaN/GaN MQWs, the increase in temperature and annealing time caused the intermixing between the barrier and the wells, which in turn caused a decrease in periodicity on the high-resolution X-ray diffraction patterns. Thereby, we confirmed that the structural performance of InGaN MQWs is successively degrading with increasing thermal annealing temperature. Especially, InGaN MQWs of the sample annealed at 950 °C were profoundly damaged. The cathodoluminescence (CL) measurement indicated that MQWs emission intensity decreases with increasing thermal annealing temperature. Thus, the integrated CL intensity ratio of InGaN MQWs to GaN dramatically decreased while thermal annealing temperatures increased. This result caused the intermixing in MQWs to deteriorate the active layer performance. Furthermore, the peak position of MQWs showed a tendency of the red shift after high thermal annealing. It is suggested that the annealing-induced red shift in MQWs is attributed to the reduction of the inhomogeneity of the In content in the MQWs leading to the reduction of the quantized energies. Consequently, it indicates that the high temperature and the long-time thermal annealing would be inevitably followed by the structural destruction of InGaN MQWs.

GaN-based InGaN/GaN multi-quantum-well light emitting diode (MQW LED) structures were grown by metal organic chemical vapor deposition method. The optical properties of the LED structure have been investigated by using the photoluminescence and electroluminescence measurement. Both photoluminescence and electroluminescence results indicate that near pure InN clusters exist within the InGaN layers, which are responsible for the light emission in the LED. With increasing the Mg activation temperature of p-GaN layer, the optical properties of the LED structure tended to significantly degrade. This degradation was found to be deeply related to the variation of InN clusters in the active region. By the current–voltage measurement, a large forward voltage variation was observed. The voltage variation is caused to the conductivity variation of the p-GaN layer due to the different activation temperature. The turn-on voltage obtained from the best LED was 2.56 V and the forward voltage measured at 20 mA was 3.5 V. On the basis of these results, activation of the Mg-doped p-GaN layer must be carried out at the lowest possible value so as to obtain the better performance of LEDs.

The AgInS2 epilayers with a chalcopyrite structure grown using a hot-wall epitaxy method have been confirmed to be a high quality crystal. From the optical absorption measurement, the temperature dependence of the energy band gap on AgInS2/GaAs was found to be Eg(T)=2.1365 eV−(9.89×10−3 eV)T2/(2930+T). After the as-grown AgInS2/GaAs was annealed in AgInS2/GaAs has been investigated by using the photoluminescence (PL) at 10 K. The native defects of VAg, VS, Agint, and Sint obtained from PL measurements were classified as a donors or acceptors type AgInS2/GaAs to an optical p type. Also, we confirmed that In in AgInS2/GaAs did not form the native defects because In in AgInS2 did exist in the form of stable bonds.

We fabricated the AlGaN∕GaN metal-oxide-semiconductor heterostructure field-effect transistor (MOSHFET) using the oxidized Ni(NiO) as a gate oxide and compared electrical properties of this device with those of a conventional AlGaN∕GaN heterostructure field-effect transistor (HFET). NiO was prepared by oxidation of Ni metal of 100A at 600°C for 5min in air ambient. For HFET and MOSHFET with a gate length of 1.2μm, the maximum drain currents were about 800mA∕mm and the maximum transconductances were 136 and 105mS∕mm, respectively. As the oxidation temperature of Ni increased from 300 to 600°C the gate leakage current decreased dramatically due to the formation of insulating NiO. The gate leakage current for the MOSHFET with the oxidized NiO at 600°C was about four orders of magnitude smaller than that of the HFET. Based on the dc characteristics, NiO as a gate oxide is comparable with other gate oxides.

We have studied the influence of electric fields on the Mg-doped p-GaN by using Raman scattering and photocurrent (PC) measurement. It has been observed that the E2 (LO) mode was quenched upon increasing the electric field. To explain this effect, the combined results obtained from the electric-field-induced Raman scattering and the PC measurement were analyzed. As a result, we have found that the damping of E2 (LO) mode is caused to the phonon–hole scattering due to a strong interaction between the phonons and the photogenerated hole carriers with increasing the applied electric field.

ZnO based oxide system ZnxCd1−xO alloy of various compositions has been successfully grown by the RF co‐sputtering system. ZnxCd1−xO alloy films, which have x = 0, 0.01, 0.02, 0.05, 0.08, 0.13 and 1, were grown on Al2O3 substrate. From the XRD measurement, the ZnxCd1−xO films observed to be hexagonal structure at the range of 0≤x≤0.08. At x = 0.13, the mixed crystalline of the hexagonal‐cubic structure was measured. Also, the optical bandgap energy has been modulated from 3.3 eV to 2.4 eV. After post‐Annealing, the crystallinity of the ZnxCd1−xO films improved. Furthermore, this process suggests that it is very important to maintain the optical performance for LED and LD devices.

Hydrogenated amorphous silicon films were deposited on the unstrained and strained Si substrates by an electron cyclotron resonance plasma source. The photoluminescence spectra show that emission energies are different from each other. The redshift of photoluminescence induced by the biaxial tensile stress is increased with decreasing the temperature in the range of 0.11–0.17 eV. The stress effect also enhances the quantum efficiency significantly. The pseudomorphic growth of Si on a relaxed Si0.75Ge0.25 (100) surface provides the strain energy of about 0.17 eV. These comparable results indicate that the shift of emission energy is attributed to the stress effect perturbing the polysilane structure.

The CdGa2Se4 layers were grown by the hot-wall epitaxy method. From the absorption measurement, the band-gap variation in CdGa2Se4 was well interpreted using Varshni’s equation. After the postannealing in various ambient, the behavior of point defects in CdGa2Se4 was investigated by measuring photoluminescence (PL). Point defects originating from VCd, VSe, Cdint, and/or Seint were classified as donor or acceptor types. Thus, the Ga in CdGa2Se4 did not form native defects because the Ga existed in the form of stable bonds in CdGa2Se4. Based on these PL results, we schemed out a band diagram of the recombination process in CdGa2Se4.

The authors have fabricated ultraviolet (UV) laser diodes based on ZnO∕BeZnO films. The devices have p-n heterojunction structures with a multiple quantum well (MQW) active layer sandwiched between guide-confinement layers. The MQW active layer comprises undoped ZnO and BeZnO, while the two guide-confinement layers were As-doped p-type ZnO∕BeZnO and Ga-doped n-type BeZnO∕ZnO films, respectively. The exciton binding energy in the MQW region is exceptionally large (263meV). Exciton-related lasing was observed by optically pumping the MQWs. ZnO∕BeZnO-based diodes showed laser action by current injection at room temperature. The lasing mechanism is inelastic exciton-exciton collision.

A wide-band gap oxide alloy, BeZnO, is proposed and studied in this letter. The BeZnO films were deposited on sapphire substrates by our hybrid beam deposition growth method. The value of the energy band gap of BeZnO can be efficiently engineered to vary from the ZnO band gap (3.4 eV) to that of BeO (10.6 eV). BeZnO can be used for fabricating films and heterostructures of ZnO-based electronic and photonic devices and for other applications. Changes in the measured energy band gap and lattice constant values with Be content are described for BeZnO alloys.

The potential use of ZnO-based photonic and electronic devices has been demonstrated by the fabrication of prototype ultraviolet (UV) photodetector and field-effect transistor (FET) devices that contain films of p-type ZnO with arsenic as the p-type dopant. These p-type films have high crystalline quality and show long-term stability. The ZnO UV photodetectors are based on p-n junctions. The FETs are made with metal-semiconductor Schottky contacts on p-type ZnO and are normally off (enhancement) devices. The spectral and electrical characteristics of these devices are presented and explained.

The dc characteristics of the AlGaN∕GaN heterostructure field-effect transistors were examined at temperatures ranging from 25 to 260 °C under white light illumination. Drain current collapse measured was defined by the difference of drain current between light on and light off at Vgs=1V and Vds=5V. The surface-passivated device showed no drain current collapse, but the unpassivated device showed severe drain current collapse at 25 °C. Drain current and drain current collapse with an increase in temperature reduced, which resulted from the reduction of the electron mobility or saturation velocity and the thermal activation of the trapped electrons, respectively. Eventually, drain current collapse disappeared completely above 250 °C. The behavior of the temperature-dependent drain current collapse showed that the surface states for trapping electrons were continuously distributed with the temperature not having specific energy states.