T. S. Jeong

Raman scattering and photoluminescence of As ion-implanted ZnO single crystal

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 800u200a°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...

Sensing Mechanism and Behavior of Sputtered ZnCdO Ozone Sensors Enhanced by Photons for Room-Temperature Operation

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 3.061xa0eV, 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.

Wide band-gap investigation of modulated BeZnO layers via photocurrent measurement

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)xa0=xa06.32(x–1)xxa0+xa07.305xxa0+xa03.295. 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.

Effect of Thermal Annealing on the Characteristics of Phosphorus-Implanted ZnO Crystals

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 E2high 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 A1(LO) peak, which is an inactive mode. This A1(LO) peak relaxed during thermal annealing in ambient oxygen at temperatures higher than 700°C. The P2p3/2 peak observed at 135.6xa0eV 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 3.5xa0eV originated from (A0, X) emission, because of PZn-2VZn complexes acting as shallow acceptors. The acceptor level was observed to be 126.9xa0meV 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.

Investigation of the Photocurrent in Hot-Wall-Epitaxy-Grown BaIn2S4 Layers

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 Eg(T)xa0=xa0Eg(0)xa0−xa03.79xa0×xa010−3T2/(Txa0+xa0499), where Eg(0) was estimated to be 3.0597xa0eV, 3.2301xa0eV, and 3.2606xa0eV for transitions corresponding to the valence band states Γ4(z), Γ5(x), and Γ5(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 0.0306xa0eV, 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 20.4xa0meV, 1.6xa0meV 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.

Spectral Behavior of Bias-Dependent Photocurrent and Photoluminescence in Sputtered ZnO Layers

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 60xa0V/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.

An analysis of temperature-dependent absorption and photocurrent spectra in BaAl2Se4 layers

The temperature-dependent photoresponse behavior of BaAl2Se4 layers has been investigated through the analysis of optical absorption and photocurrent (PC) spectra. Based on these results, the optical band gap was well expressed by Eg(T)u2009=u2009Eg(0)u2009−u20094.39u2009×u200910−4T2/(Tu2009+u2009250), where Eg(0) is estimated to be 3.4205, 3.6234, and 3.8388u2009eV for the transitions corresponding to the valence band states Γ3(A), Γ4(B), and Γ5(C), respectively. From the PC measurement, three peaks A, B, and C corresponded with the intrinsic transitions from the valence band states of Γ3(A), Γ4(B), and Γ5(C) to the conduction band state of Γ1, respectively. According to the selection rule, the crystal field and spin orbit splitting were found to be 0.2029 and 0.2154u2009eV, respectively, through the direct use of PC spectroscopy. However, the PC intensities decreased with lowering temperature. In the log Jph versus 1/T plot, the dominant trap level at the high-temperature region was observed and its value was 12.7u2009meV. This level corresponds ...

Photocurrent Spectroscopic Study of Temperature-Dependent Photoresponse and Valence-Band Splitting in MnAl2S4 Layers

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 Eg(T)xa0=xa0Eg(0)xa0−xa02.80xa0× 10−4T2/(287xa0+xa0T), where Eg(0) was estimated to be 3.7920xa0eV, 3.7955xa0eV, and 3.8354xa0eV 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 3.5xa0meV and 39.9xa0meV. 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 Jph, 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 18.9xa0meV below the conduction band.