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The characteristic behavior of hot-wall deposited BaGa2Se4 layers was investigated as a function of temperature. The structural quality of the layers was found to be affected by tensile strain due to the coincidence-lattice mismatch. The carrier mobility showed an aspect of two scattering mechanisms. As the temperature was increased to 100 K, the mobility increased as a function of T1 due to impurity scattering. Also, at temperatures higher than 100 K, its behavior was reduced to two different-temperature slopes caused by lattice scattering. One was a function of T−3/2 at T > 200 K, and the other was a function of T−1/2 for 100 200 K. From the behavior of log n(T) versus 1/T, three donor-trap levels due to native defects were observed. By tracking the spectral photocurrent (PC) behavior with decreasing temperature, we found that the three PCpeak positions shifted toward shorter wavelengths and their intensities were dramatically decreased. These spectral PC peaks were due to the band-to-band transition. These band-gap variation were well matched by Eg(T) = Eg(0)−2.06×10 −3T2/(T +230.7). By the selection rule, the crystal-field and the spin-orbit splitting were found to be 0.2031 and −0.2259 eV, respectively. In the log Jph versus 1/T plot, we found that the spectral PC-intensity behavior was related to two donor-trap levels by comparing with the carrier concentration results. In conclusion, the dramatic decrease in the spectral PC intensity was attributed to trapping centers.

The dc-photoconductive characteristic on the hot-wall grown CaAl2Se4 (CAS) layers was explored as a function of temperature. From the photocurrent (PC) measurement, three PC peaks A, B, and C corresponded to the intrinsic transitions, which represent the band-to-band transitions from the valence-band states of Γ2(A), Γ3 + Γ4(B), and Γ3 + Γ4(C) to the conduction-band state of Γ1, respectively. Based on these PC results, the optical band-gap energy was well matched by E g (T) = E g (0) − 4.94 × 10−3 T 2/(T + 552), where E g (0) is found to be 3.8239, 3.8716, and 3.8801 eV for three peaks A, B, and C, respectively. Thus, the effect of the crystal field and spin-orbit splitting (These values were extracted out to be 47.7 and 8.5 meV, respectively.) was observed and calculated by means of the PC spectroscopy. However, PC intensity gradually decreased with decreasing temperature unlike an ordinary behavior. In the log J ph vs 1/T plot, two dominant traplevels were observed to be 20.81 meV at temperatures of 300 - 70 K and 1.18 meV at temperatures below 70 K. Consequently, we extract out that these trapping centers caused by native defects in CAS confine the PC intensity as temperature decreases.

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 BaAl 2 S 4 layers, which were identified to be a cubic structure in space group Pa3, were grown using the hot wall epitaxy method attached with the reservoir tail. Also, the coincidence lattice mismatch of the grown layers was shown to be −2.52% due to the influence of the compressive strain. The Hall effect measurement showed the different temperature-dependent decrease of mobility at a temperature above 100 K. One was T −1/2 at the temperature range of 100 T T −3/2 at the temperature of T>180 K. The mobility decreased in proportion to T 1 at a low temperature range of T E g ( T )= E g (0)−7.556×10 −4 T 2 /( T +523), where E g (0) is estimated to be 4.0596, 4.1053, and 4.1094 eV.

The bias-dependent behavior of the photocurrent (PC) and photoluminescence (PL) of sputtered ZnO layers has been investigated. Based on PC spectroscopy results, the PC intensity of the observed free exciton increased strongly up to electric field of 60 V/cm, after which its rate of increase slightly reduced due to disturbance of field-assisted dissociation of radical ion pairs, which leads to photocarrier generation. Thus, the energy of excitonic PC peaks showed a tendency to red-shift with increasing electric field, being attributed to the induced Stark effect. Therefore, it is concluded that the strong interaction between free excitons and photogenerated PC carriers leads to displacement or widening of the spectrum. In the PL measurements, near-band-edge (NBE) and violet emissions were observed. With increasing electric field, two PL emissions were progressively quenched. The combined PL/PC results reveal that the PL ions associated with the NBE and violet emissions readily interact with the PC carriers of photogenerated electrons and holes. This behavior reduces the recombination ratio and the lifetime of PL ions. So, the PL intensity quenching originates from a decrease in the number of carriers participating in recombination. Consequently, we find that the quenching mechanism of the NBE and violet emissions is strongly related to low external electric field.

MnAl2Se4 layers were grown using the hot-wall deposition technique with an attached reservoir tail. Precise control of the vapor pressure in the reservoir was thought to play an important role in the grown of a stoichiometric layer. From the relation between the reciprocal temperature and the carrier concentration, we extracted the dominant trap level as 96.1 meV in the high-temperature region and 13.9 meV in the middle-temperature region. Thus, from a log-log plot between the mobility and the temperature, the mobility showed the different temperature-dependent decreases of the mobility at temperatures above 100 K: T−1/2 in the temperature range of 100 225 K. The mobility decreased in proportion to T1 in the low-temperature range of T < 100 K. By analyzing the optical absorption results, the bandgap variation matched Eg(T) = Eg(0) − 3.19 × 10−3T2/(T + 488) well, where Eg(0) is estimated to be 3.5616 eV. Consequently, low-temperature growth of MnAl2Se4 layers was achieved by using the hot-wall deposition technique.

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 photocurrent (PC) of hot-wall-epitaxy-grown BaIn2S4 layers was studied at different temperatures and for different photoresponse intensities. With increasing temperature, the position of the PC spectra tended to shift toward longer wavelength. These PC peaks corresponded to band-to-band transitions caused by intrinsic transitions from the valence band states to the conduction band states. Also, the bandgap variations were well matched by the equation E g(T) = E g(0) − 3.79 × 10−3 T 2/(T + 499), where E g(0) was estimated to be 3.0597 eV, 3.2301 eV, and 3.2606 eV for transitions corresponding to the valence band states Γ 4(z), Γ 5(x), and Γ 5(y), respectively. By use of the selection rule and results from the PC spectroscopy, the crystal field and the spin–orbit splitting were found to be 0.1703 and 0.0306 eV, respectively. Thus, the PC intensity gradually decreased with decreasing temperature. The decrease of PC intensity was caused by the presence of trapping centers associated with native defects in the BaIn2S4 layers. The trap level was found to be a shallow donor-level type of 20.4 meV, 1.6 meV below the conduction band. Consequently, these trap levels, which are related to native defects in BaIn2S4 layers, are believed to limit PC intensity with decreasing temperature.

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

The epitaxial growth of the photoconductive MnAl 2 S 4 with a layered-type rhombohedral structure was first achieved by means of the hot-wall epitaxy method. From the Hall effect measurement, two dominant scattering mechanisms on the mobility were extracted. One was an acoustic phonon scattering caused by lattice vibrations at the high-temperature range of T >100 K and the other one was an impurity ion scattering at the low-temperature range of T PC ) measurement, the A , B , and C peaks were observed at whole temperature range. The electronic origin of these three peaks were caused by the band-to-band transitions from the valence band states of Γ 4 (z), Γ 5 (x), and Γ 5 (y) to the conduction band state of Γ 1 (s), respectively. Thus, the parameters of the crystal-field and the spin–orbit splitting were directly extracted by means of PC spectroscopy, and those values were 3.5 and 39.9 meV, respectively. The band gap energies estimated from PC and absorption measurements were well expressed by E g ( T )= E g ( 0 )−2.80×10 −4 T 2 /( T +287), where E g ( 0 ) were found to be 3.7920, 3.7955, and 3.8354 eV at the valence band states of Γ 4 (z), Γ 5 (x), and Γ 5 (y), respectively.

The temperature-dependent photoresponse characteristics of MnAl2S4 layers have been investigated, for the first time, by use of photocurrent (PC) spectroscopy. Three peaks were observed at all temperatures. The electronic origin of these peaks was associated with band-to-band transitions from the valence-band states Γ4(z), Γ5(x), and Γ5(y) to the conduction-band state Γ1(s). On the basis of the relationship between PC-peak energy and temperature, the optical band gap could be well expressed by the expression E g(T) = E g(0) − 2.80 × 10−4 T 2/(287 + T), where E g(0) was estimated to be 3.7920 eV, 3.7955 eV, and 3.8354 eV for the valence-band states Γ4(z), Γ5(x), and Γ5(y), respectively. Results from PC spectroscopy revealed the crystal-field and spin–orbit splitting were 3.5 meV and 39.9 meV. The gradual decrease of PC intensity with decreasing temperature can be explained on the basis of trapping centers associated with native defects in the MnAl2S4 layers. Plots of log J ph, the PC current density, against 1/T, revealed a dominant trap level in the high-temperature region. By comparing PC and the Hall effect results, we confirmed that this trap level is a shallow donor 18.9 meV below the conduction band.

A P-doped ZnO surface layer on undoped ZnO wafers was prepared by phosphorus (P) ion implantation. Hall effect measurement revealed p-type conduction in such layers annealed at 800°C. This indicates that acceptor levels are present in P-doped ZnO, even though the ZnO is still n-type. Micro-Raman scattering in −z(xy)z geometry was conducted on P-implanted ZnO. The E 2 high mode shift observed toward the high-energy region was related to compressive stress as a result of P-ion implantation. This compressive stress led to the appearance of an A 1(LO) peak, which is an inactive mode. This A 1(LO) peak relaxed during thermal annealing in ambient oxygen at temperatures higher than 700°C. The P2p3/2 peak observed at 135.6 eV by x-ray photoelectron spectroscopy is associated with chemical bond formation leading to 2(P2O5) molecules. This indicates that implanted P ions substituted Zn sites in the ZnO layer. In photoluminescence spectroscopy, the P-related peaks observed at energies ranging between 3.1 and 3.5 eV originated from (A0, X) emission, because of PZn-2VZn complexes acting as shallow acceptors. The acceptor level was observed to be 126.9 meV above the valence band edge. Observation of this P-related emission indicates that ion implantation results in acceptor levels in the P-doped ZnO layer. This suggests that the P2O5 bonds are responsible for the p-type activity of P-implanted ZnO.

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 temperature-dependent photoresponse properties of As-doped p-ZnO crystals prepared by using the As-implantation method have been investigated by using photocurrent (PC) and photoluminescence (PL) spectroscopy. Two peaks, A and C, associated with band-to-band transitions were observed in the PC spectra. From the PL spectra, the neutral acceptor bound exciton emission, (A0, X), due to As-related acceptors was extracted. The temperature dependences of these peaks were well described by Eg(T) = Eg(0) — (3.10 × 10−4)T2/(143 + T). Also, the values of Eg(T) were estimated to be 3.412, 3.418, and 3.468 eV at the valence-band states of Γ9(A), Γ7(B), and Γ7(C), respectively. Thus, the parameter of the crystal-field splitting was directly extracted by means of PC spectroscopy, and its value was 0.0503 eV at 13 K. However, the behavior of the PC was different from that generally observed in other semiconductors. The PC intensities progressively decreased with falling temperatures in regions below 150 K. From plots of log JPCvs. 1/T, where JPC is the PC current-density, a dominant level was observed at mid-range temperatures. By comparing the PC and the PL results, we confirmed an activation energy of 21.5 meV, corresponding to a partial dissociation energy of the (A0, X) emission. Consequently, trapping centers, which are imperfections related to As ions in As-implanted p-ZnO crystals, are thought to limit the PC intensity with decreasing temperatures below 150 K.

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 photocurrent (PC) variation of photoconductive ZnAl2Se4 layers was investigated as a function of temperature. Three PC peaks corresponding to band-to-band transitions were observed. From the relation between the temperature and the PC-peak position, the temperature-dependent band-gap energy E g was well described by E g (T) = E g (0) − (1.94 × 10−3)T 2/(468 + T). Also, E g (0) was estimated to be 3.5268, 3.6148, and 3.7178 eV at the valence band states of Γ4(z), Γ5(x), and Γ5(y), respectively. A characteristic splitting of the valence band governed by the selection rule was first observed through the method of PC spectroscopy. We noted that the crystal-field splitting and spin-orbit splitting were 0.088 eV and 0.103 eV, respectively. With decreasing temperature, however, the PC intensities decreased, contrary to our expectation. From the log J ph vs. 1/T plot, a dominant level of 37.8 meV was observed in the high-temperature region. Thereby, we suggest that this trapping center due to native defects and impurities limits the PC response with decreasing temperature.

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

The variation in the photocurrent (PC) in photoconductive MgGa2Se4 layers was investigated as a function of temperature. Three peaks corresponding to band-to-band transitions were observed in the PC spectra at low temperatures. Thus, the crystal field splitting and the spin orbit splitting were found to be 0.164 and 0.092 eV, respectively, using PC spectroscopy. From the relations between peak position and temperature, the temperature dependence of the bandgap energy Eg was well described by E g (T) = E g (0) − (8.87 × 10−4)T 2/(251 + T). Also, the values of E g (0) were estimated to be 2.3412, 2.5047, and 2.5967 eV for the Γ4(z), Γ5(x), and Γ5(y) valence band states, respectively. With decreasing temperature, however, the PC intensities decreased, contrary to our expectation. In the log J ph vs. 1/T plot, a dominant level of 64.9 meV was observed at high temperatures. Thus, we confirmed that the activation energy of 64.9 meV was related to the dissociation energy of shallow donors due to native defects below the conduction band. Thereby, we suggest that trapping centers due to native defects and impurities limit the PC signal at lower temperatures.

Using the hot wall epitaxy method, we grew CdS epilayers on GaAs substrates. From the photocurrent (PC) measurement, the peaks corresponding to excitons A and B were observed in the short-wavelength region. Also, a crystal field splitting of 0.026 eV was extracted through the PC spectroscopy method. The PC intensities decreased with decreasing temperature. In the log Jphvs. 1/T plot, the dominant level was observed in the high-temperature region, and its activation energy was 33.9 meV. From the photoluminescence (PL) experiment, the PL intensities of the free exciton were exponentially reduced with increasing temperature. From the relations of log IPLvs. 1/T, an activation energy of 29.7 meV was observed in the high-temperature region. By comparing the PC and the PL results, we found that these activation energies were the thermal dissociation energies of the free exciton at the ground state and corresponded to the binding energies of the free exciton. Thus, we suggest that this trapping center limits the PC signal with decreasing temperature. Furthermore, the band gap energy of CdS as a function of temperature was well described by Eg(T) = 2.581 − (6.1 × 10−4)T2/(248 + 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.

Cd1-x Zn x O alloy layers were grown by a radio-frequency magnetron co-sputtering system. By X-ray diffraction measurement, the Cd1-x Zn x O layers were observed to have a single-phase cubic structure at x ≤ 0.21. However, polycrystalline having mixed phases was observed at x = 0.29. It seems that the high miscibility of Zn flux in CdO leads to a large fluctuation in phase segregation caused by the hexagonal ZnO and cubic CdO structure within CdZnO. Therefore, the maximum Zn-composition ratio without phase segregation was found to be x = 0.21. The bandgap energies of the Cd1−x Zn x O layers between x = 0 and x = 0.21 were distributed from 2.574 eV to 2.892 eV, respectively, while the bandgap energy of x = 0.29 was a slightly smaller value of 2.834 eV. After rapid-thermal-annealing (RTA) treatment at 600°C, the bandgap energies of each x component decreased, especially the x = 0.29 layer, which converted into the double phase of a cubic-and-hexagonal structure existing in the cubic CdO (111) and hexagonal ZnO (0001) phases. Consequently, reliable formation and crystallinity improvement of the Cd1−x Zn x O layers were achieved through an x composition ratio below x = 0.29 and an RTA method of short-time thermal-annealing below 600°C.

Cd1−xZnxO alloy layers were grown by using a radio-frequency magnetron co-sputtering system. From the X-ray diffraction measurement, the Cd1−xZnxO layers were observed to have a cubic structure until a single phase for x ≤ 0.21. However, a polycrystalline form having mixed phases was observed at x = 0.29. It seems that the high miscibility of Zn in CdO leads to a large fluctuation in phase segregation caused by the hexagonal ZnO and cubic CdO structures within CdZnO. The band gap energies of the Cd1−xZnxO layers for values of x between x = 0 and x = 0.21 were distributed from 2.574 to 2.892 eV, respectively, while the band gap energy for x = 0.29 was a value of 2.834 eV due to the polycrystallinity. From the ozone sensing measurement, the working temperature of the Cd1−xZnxO layers was found to be 200 ◦C. Thus, a sensor with x = 0.21 showed a sensitivity of S = 1.31 at 100 ppb. Furthermore, the response and the reduction times of the sensor were found to be 184 and 210 sec, respectively. From these results, we found that the Cd1−xZnxO layer has a capability for use as a new material for ozone sensing.

Zn1-xCdxO layers were deposited on the sapphire substrate using the radio-frequency magnetron co-sputtering system. The grown Zn1-xCdxO layers were carried out in the post-annealing treatment for 1 min at the 800 °C oxygen-ambient by the rapid thermal annealing (RTA) method. X-ray diffraction (XRD) experiment shows that the Zn1-xCdxO layers are changed from the single phase of the hexagonal structure at 0≤x ≤0.08 to the double phase of hexagonal-and-cubic structure at x =0.13. Thus, the maximum Cd-composition ratio with the hexagonal structure was found out to be x =0.08. Also, the crystallinity of Zn1-xCdxO layers at x =0.13 was remarkably improved by the RTA annealing treatment. This crystal quality improvement was thought to be associated with the relaxation of the compressive strain remaining in the Zn1-xCdxO layers. Therefore, the results of XRD and transmittance lead that the crystal quality of the Zn1-xCdxO layers forming the hexagonal ZnO phase is better than that forming the cubic CdO phase. Consequently, the reliable formation and the crystallinity of the Zn1-xCdxO layers were achieved by using the RTA method of short-time thermal-annealing at the high temperature. (© 2010 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Wide band-gap BeZnO layers were grown on Al 2 O 3 (0 0 0 1) substrate using radio-frequency magnetron co-sputtering. The rate of Be x Zn 1− x O crystallized as a hexagonal structure was x =0.2. From the X-ray photoelectron spectroscopy measurement, the O–Zn bonds relating the crystal structure and the Be–O bonds related to the deviation of the stoichiometry in the BeZnO layer were caught at 530.4 and 531.7 eV in the O 1s spectrum, respectively. Thus, the observance on the Be 1s peak of 113.2 eV associated with the bonding Be–O indicates that the sputtered Be atoms are substituted for the host-lattice site in ZnO. This Be–O bonding shows a relatively low intense and broadening spectrum caused by large fluctuation of Be content in the BeZnO layer. From the photoluminescence and transmittance measurement, the free exciton and the neutral donor-bound exciton (D 0 , X) emissions were observed at 3.7692 and 3.7313 eV, respectively, and an average transmittance rate over 95% was achieved in a wide ultraviolet (UV)–visible region. Also, the binding energy for the (D 0 , X) emission was extracted to be 37.9 meV. Through the wide band-gap material BeZnO, we may open some possibilities for fabricating a ZnO-based UV light-emitting diode to be utilized as a barrier layer comprised of the ZnO/BeZnO quantum well structure and/or an UV light emitting material itself.

We have studied the effect of electric fields on the near-band-edge (NBE) emissions in bulk zinc oxide (ZnO) by using photoluminescence and photocurrent (PC) spectroscopy simultaneously. The intensity-quenching and peak-shift effects of the free exciton spectra were observed with increasing electric field. From the PC result, we find out that the free excitons are disturbed by the PC carriers of the photo-created electrons and holes. This disturbance reduces the recombination ratio and the lifetime of free excitons. Therefore, the intensity-quenching effect was attributed to the decrease in the recombination of free excitons. Thus, the shift of the free exciton peaks was related to Stark effect induced by electric field. As a result, we have found that these phenomena are caused to the exciton–electron scattering due to a strong interaction between the excitons in the conduction band and the photo-generated electron carriers with increasing the applied electric field.

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.

Copper gallium selenide (CuGaSe2, CGS) layers were grown by the hot wall epitaxy method. The optimum temperatures of the substrate and source for growth turned out to be 450 and 610 °C, respectively. The CGS layers were epitaxially grown along the 〈1 1 0〉 direction and consisted of Ga-rich components indicating the slight stoichiometric deviations. Based on the absorption measurement, the band-gap variation of CGS was well interpreted by the Varshni's equation. The band-gap energies at low temperatures, however, had a higher value than those of other CGS. It suggests that the band-gap increase is influenced by the slightly Ga-rich composition. From the low-temperature photoluminescence experiment, sharp and intensive free- and bound-exciton peaks were observed. By analyzing these emissions, a band diagram of the observed optical transitions was obtained. From the solar cell measurement, an 11.17% efficiency on the n-CdS/p-CGS junction was achieved.

BeZnO layers, which are new materials for ultraviolet-light-emitting devices, were grown by hybrid beam method. The mobility and the carrier concentration on the BexZn1−xO layers of x = 0.28 were confirmed to be 2.83 cm2/V s and 4.16 × 1018 cm−3, respectively. Also, the optical properties attributed to the thermal quenching phenomenon of BeZnO were analyzed by photoluminescence as a function of temperature. With increasing temperature, the intensities and the spectral widths on the localized deep-level emissions of 3.6230 eV exponentially reduced and tended to broaden, respectively. Therefore, the temperature dependences of the full width at half maximum and the intensity were explained in terms of a configuration coordinate model. The broad emissions of 3.6230 eV without any fine structure were acted by a strong electron-phonon coupling due to the interaction between the radical beryllium and the ZnO host lattice. In addition, the Frank-Condon shift was found out to be 78.9 meV with the associated phonon energy of 15 meV. Thus, the activation energy of the nonradiative emission participating in the thermal quenching process was estimated to be 48.6 meV. Consequently, its value corresponds to the thermal dissociation energy requiring for the recombination of the conduction electron from the exciting state to the ground state.

Chemical bonds of As-implanted ZnO annealed in ozone molecular (O3) ambient were analyzed through the x-ray photoelectron spectroscopy as a function of the etching depth. With the etching depth increased to 25 nm from surface, the peaks of Zn2p and O1s core levels shifted toward the low-binding energy, and the bonding formation of As 3d core level gradually varied from As2O5 to As2O3. The new peaks were observed, which were posited to high-binding energy of 4.4 eV from Zn2p and O1s core levels. Finally, the chemical bonds of As-based oxides were found to consist of Zn(AsO3)2, As2O5, and As2O3. (© 2007 WILEY -VCH Verlag GmbH & Co. KGaA, Weinheim)

Be x Zn 1− x O (0⩽ x ⩽0.28) alloy films were epitaxially grown by hybrid electron-beam deposition system. The crystal structure of the Be x Zn 1− x O was investigated by X-ray diffraction pattern by varying the Be concentration. The crystal structure of the Be x Zn 1− x O films was hexagonal within the variance of the Be component. Also, the phase separation within the Be x Zn 1− x O alloy film was not observed. The photoluminescence emission peak of the Be x Zn 1− x O films revealed the same pattern till the Be mole fraction increase up to x =0.11. When the Be mole fraction is larger than x >0.11, the Ex(A) of free exciton at 3.624 eV was only observed in the Be x Zn 1− x O films. The band gap energy of the Be x Zn 1− x O can be tailored from 3.30 eV ( x =0) to 4.13 eV ( x =0.28) by alloying ZnO with BeO. Throughout this band gap tuning, we may open the possibilities for fabricating ZnO-based ultraviolet light emitting diode utilizing the active layer comprised of ZnO/Be x Zn 1− x O quantum well structure.

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.

An Mg-doped p-GaN layer was grown by the metalorganic chemical vapor deposition method. The dissociation extent of hydrogen-passivated Mg acceptors in the p-GaN layer through Mg activation annealing was estimated by using room-temperature cathodoluminescence (CL) spectroscopy. The CL measurement revealed that the CL spectra intensities tend to increase with increasing the activation annealing temperature. The sample annealed at 925 °C showed the most intense emission and the narrowest width among the emission peaks. Consequently, it was the most excellent dissociation extent of Mg–H complexes caused by the Mg activation annealing. The hole concentration under this optimum condition was 1.3×10 17 cm −3 at room temperature. The photoluminescence (PL) measurement showed a 2.8 eV band having characteristically a broad peak in heavily Mg-doped GaN at room temperature. By analyzing the PL results, we learned that this band was associated with the deep donor–acceptor pair (DAP) emission rather than with the emission caused by the transition from the conduction band to deep acceptor level. The four emission peaks in the resolved 2.8 eV band were emitted by transiting from deep donor levels of 0.14, 0.26, 0.40, and 0.62 eV below the conduction band to the shallow Mg acceptor level of 0.22 eV above the valence band.

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.

Since As doping in ZnO using ion implantation must always follow a damage caused by the energetic As ion penetration, there is a growing demand for crystallinity recovery from the surface damage through the post-implantation annealing. The results obtained from our experiments using double crystal X-ray diffraction and the atomic force microscopy indicate that the crystallinity recovery of the As-implanted sample presents the optimum condition when it is annealed at 800 °C for 1 h. From the Raman and photoluminescence measurements through the post-implantation annealing, the As-related optical properties were observed in As-implanted ZnO crystals. Thereby, we confirmed that the surface of the unimplanted ZnO was clearly converted into As-doped p-type ZnO layer by As ions.

After the CdIn 2 S 4 films grown by hot-wall epitaxy method were annealed in Cd-, S- and In-atmospheres, the point defects were investigated by using the low photoluminescence (PL) experiment. In the as-grown CdIn 2 S 4 films, the free excitons corresponding to the light hole and the heavy hole have been observed, and their splitting gap was 8.2 meV.The gap is associated with the strain caused by the lattice mismatch between the substrate and the film in the heterojunction growth. By means of thermal annealing in various atmospheres, the (D°,X) emission was observed to originate from the V S or Cd int . From this emission, the E D value of the donor impurity level is extracted to 0.1950 eV.Also, the emission between the free electrons and the acceptor holes (FA) became a dominant peak after S atmosphere treatment, and the E A value of the acceptor level is turned out to be 0.2371 eV.Thus, the origin of the FA emission is related to V Cd or S int . These PL results led us to confirm that CdIn 2 S 4 :S is converted into the optical p-type. In addition, the origin of the donor–acceptor pair emissions is caused by the recombination between donors such as V S or Cd int and acceptors such as V Cd or S int . Based on these PL results, we have schemed a new energy-level diagram of the recombination process in CdIn 2 S 4 .

We have converted the surface of undoped ZnO bulk into the As-doped p-ZnO layer by means of the As ion implantation method. After postimplantation annealing, the As-related properties were investigated by using Raman scattering and photoluminescence (PL) experiments. The Raman spectrum shows that the E2high peak obtained from the As-doped p-ZnO shifted toward the higher energy side of 0.55 cm−1 in comparison with that of the undoped ZnO bulk. This result is related to the stress increment of the sample surface due to the As implantation. After the As-implanted p-ZnO annealed at 800 °C, the PL spectroscopy reveals the neutral acceptor bound exciton emission (A∘,X) at 3.3589 eV. This emission shows a tendency to quench the intensity and extend the emission linewidth with increasing temperatures. Also, two As-related peaks associating with recombination emissions between free electrons and acceptor holes were observed at 3.3159 and 3.2364 eV. In addition, the I–V characteristic curves of the p–n homojunction...

Single crystals of p-CdIn2Te4 were grown by the Bridgman method without a seed crystal. From photocurrent measurements, it was found that three peaks, A, B, and C, correspond to the intrinsic transition from the valence band states of Γ7(A), Γ6(B), and Γ7(C) to the conduction band state of Γ6, respectively. The crystal field splitting and the spin orbit splitting were found to be 0.2360 and 0.1119 eV, respectively. The temperature dependence of the CdIn2Te4 band gap energy was given by Eg(T)=Eg(0)−(9.43×10−3)T2/(2676+T). The Eg(0) was calculated to be 1.4750, 1.7110, and 1.8229 eV at the valence band states of Γ7(A), Γ6(B), and Γ7(C), respectively. The band gap energy of p-CdIn2Te4 at room temperature was determined to be 1.2023 eV.