K. J. Hong

Effects of NiO addition in WO3-based gas sensors prepared by thick film process

Abstract NiO-doped WO3 thick films were prepared by the screen-printing technique. The microstructure and the electrical properties were investigated as functions of the amount of NiO, partial pressure of oxygen, concentration of NO2 and temperature. The grain growth is inhibited by the addition of NiO. The electrical conductance of undoped WO3 is high at low partial pressure of oxygen. The electrical conductance of NiO-doped WO3 in air increases with NiO content of up to 1.0 mol%, and then it decreases. The sensitivity for NO2 gas increases for samples with 1.0 mol% of NiO, and then it decreases for 10 mol% NiO samples.

Band gap energy and valence band splitting of p-CdIn2Te4 crystal by photocurrent spectroscopy

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.

Characteristic properties of Raman scattering and photoluminescence on ZnO crystals doped through phosphorous-ion implantation

P-doped ZnO was fabricated by means of the ion-implantation method. At the Raman measurement, the blue shift of the E2high mode and A1(LO) phonon of the inactive mode were observed after the P-ion implantation. It suggested to be caused by the compressive stress. Thus, Hall effect measurement indicates that the acceptor levels exists in P-doped ZnO while still maintaining n-type ZnO. From the X-ray photoelectron spectroscopy, the chemical bond formation of the P2p3/2 spectrum consisted of 2(P2O5) molecules. Therefore, the implanted P ions were substituted to the Zn site in ZnO. From the photoluminescence (PL) spectra, P-related PL peaks were observed in the energy ranges of 3.1 and 3.5 eV, and its origin was analyzed at PZn-2VZn complexes, acting as a shallow acceptor. With increasing temperatures, the neutral-acceptor bound-exciton emission, (A0, X), shows a tendency to quench the intensity and extend the emission linewidth. From the relations of the intensity and the linewidth as a function of temperatu...

Point-defect study from low-temperature photoluminescence of CdGa2Se4 layers through the postannealing in various ambient

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.

Temperature-dependent study of photocurrent signal in CdGa2Se4 layers

The photoconductive CdGa2Se4 layer has been investigated using photocurrent (PC) spectroscopy as a function of temperature. Three peaks corresponding to the band-to-band transitions were observed in the PC spectra for all temperature ranges. From the relations of peak position and temperature, the temperature dependence of the band-gap energy is precisely discussed. Also, contrary to our expectation, the PC intensities decreased with decreasing temperatures. From the relation of log Jph versus 1/T, where Jph is the PC density, two dominant levels by the exponential variation in the PC with varying temperature were observed, one at high temperatures and the other at low temperatures.

Temperature dependence and valence band splitting of the photocurrent response in undoped p-type CuInSe2 layers

In this study, the photocurrent (PC) spectroscopy of undoped p-type CIS layers has been investigated at temperatures ranging from 10to293K. Three peaks, A, B, and C, corresponded 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 at 0.0059 and 0.2301eV, respectively, and the temperature dependence of the optical band gap could be expressed by using the empirical equation Eg(T)=Eg(0)−(8.57×10−4)T2∕(T+129). But the behavior of the PC was different from that generally observed in other semiconductors: the PC intensities decreased with decreasing temperature. From the relation of logJph vs 1∕T, where Jph is the PC density, the dominant level was observed at the higher temperatures. We suggest that in undoped p-type CIS layers, the trapping center limits the PC signal due to native defects and impurities with decreasing temperature.

Nature of the AX center participating persistent photoconductivity effect in As-doped p-ZnO

The possible nature of metastable capture centers giving rise to persist photoconductivity (PPC) effect in As-doped p-ZnO was investigated using the photoluminescence result. Through the plot of log σph vs. 1/T and temperature-dependent PPC-decay process, the metastable trapping centers were extracted to be 15.1, 178.2, 180.6, and 291.9 meV. The shallow level of 15.1 meV was related to the binding energy of the neutral acceptor bound exciton. Also, the deep levels of 178.2 and 180.6 meV were caused by complex acceptor states of AsZn-2VZn located at 185 meV above the edge of the valence band. Furthermore, the trapping center of 291.9 meV was corresponded to the hole capture barrier of VZn located at 300 meV above the valence band. Therefore, these trapping centers were deeply related to the AX centers originating the native defects due to VZn or defect complexes of the As-implanted dopant in ZnO. Also, these defects, induced by the metastable AX centers, were concluded to be responsible for the PPC effect.

An analysis of temperature-dependent photocurrent-spectra in photoconductive CuGaSe2 layers

The photocurrent (PC) variation in photoconductive CuGaSe2 (CGS) layers had been investigated as a function of temperature. Three peaks A, B, and C of the PC spectra were associated with the band-to-band transitions. Thus, the parameters of the crystal-field splitting (Δcr) and spin-orbit splitting (Δso) were directly acquired through the PC measurement. The Δcr and Δso were 0.0903 eV and 0.2130 eV at 10 K, respectively. From the relations of peak position and temperature, the temperature dependence of the band-gap energy is well described by Eg(T)=Eg(0)−(8.63×10−4)T2/(336+T). Also, the Eg(0) is estimated to be 1.7952, 1.8855, and 2.0985 eV at the valence-band states of Γ7(A), Γ6(B), and Γ7(C), respectively. However, the behavior of the PC was different from that generally observed in other semiconductors, the PC intensities decreased with decreasing temperature. From the relation of log Jph versus 1/T, where Jph is the PC density, two dominant levels were observed, one at high temperature and the other a...

Hillock Formation of SnO2 Thin Films Prepared by Metal-Organic Chemical Vapor Deposition

SnO2 thin films were deposited at 375°C on an alumina substrate by metal-organic chemical vapor deposition (MOCVD). The number of hillocks on the thin-film surface increased with air annealing. The oxygen content and binding energy during air annealing at 500°C came close to those of stoichiometric SnO2. The cauliflower-like hillocks observed seem to be the result of the continuous migration of tiny grains to release the stress of an expanded grain.

High-Temperature Electrical Properties of NiO-Doped WO3 Thick Films

The electrical properties of NiO-doped WO3 thick films were investigated as a function of the amount of NiO, partial pressure of oxygen, temperature and NO2 concentration. NiO-doped WO3 thick films were prepared by the screen-printing technique. The grain growth is inhibited by the addition of NiO. The electrical conductance of undoped WO3 is high at a low partial pressure of oxygen. The electrical conductance of NiO-doped WO3 increases with NiO content of up to 1.0 mol%, and then decreases. The sensitivity for NO2 gas increased up to 1.0 mol% of NiO, and then decreased.