Papers by Keyword: Admittance Spectroscopy

Abstract: The ZnSe/Si heterojunction is of specific interest since this structure provides effective solar cell and enables the integration of wide bandgap device in silicon circuits. It is known that the quality of the diode and the current transport mechanisms across the heterojunction may be greatly influenced by the quality of the interface and depends on the crystallinity of the film layer. In this work, n-ZnSe/p-Si (100) heterojunction was fabricated by thermal evaporating ZnSe thin films on p-Si (100) substrates. The current-voltage characteristics of n-ZnSe/p-Si (100) heterojunction were investigated in temperature range 20-300 K. Some important parameters such as barrier height, ideality factor and series resistance values evaluated by using thermionic emission (TE) theory and Cheung’s method at room temperature are n = 2.910, φ_B0 = 0.832 eV and 8.5910³ Ω, respectively. The temperature dependence of the saturation current and ideality factor are well described by tunneling enhanced recombination at junction interface with activation energy and characteristic energy values about 1.293 eV and E₀₀ = 95 meV, respectively. The carrier concentration of ZnSe thin films about 3.16×10¹³ cm^-3 was deduced from the C-V measurements at room temperature. Admittance spectroscopy was employed for analysis of the defect energy levels situated in depletion region. The results showed that there was a single trap level whose position in the band gap was close to 0.04 eV above valence band. The results of this work may be useful for application such as heterojunction solar cells.

Abstract: In this work, we analyze compensating defects which are formed after implantation of aluminum (Al) into n-type 4H-SiC epitaxial layers and subsequent thermal annealing. These defects reduce the expected free charge carrier density by 84% for a low doped layer with [Al]_impl ≈ 9ž·10¹⁶ cm^-3 and by 27 % for a high doped layer with [Al]_impl ≈ 2·ž10¹⁹ cm^-3. Furthermore, an electrical activation ratio of implanted aluminum ions of 100 % is calculated. The ionization energy of implanted aluminum as measured by Hall effect and admittance spectroscopy ranges from 101 meV to 305 meV depending on the doping concentration.

Abstract: We have investigated the electrical properties of n-type 4H-SiC in-situ germanium-doped homoepitaxial layers grown by chemical vapor deposition. Germanium is an isoelectronic impurity and, therefore, not expected to contribute to the conductivity. However, Hall effect measurements taken on samples with and without germanium revealed an enhanced mobility by a factor of ≈2 at T ≈ 55 K in the germanium-doped sample despite equal free electron concentration and equal compensation. Deep level transient spectroscopy (DLTS) measurements taken on germanium-doped samples reveal negative peaks indicating the presence of charged extended defects.

Abstract: A single-carrier hole-only device of ITO/MoO₃/NPB/Al was used to measure the hole mobility of NPB layer, where NPB is a hole transport material. The hole mobility of NPB at different electric fields was obtained from the hole transit time (τ_dc) which was extracted from the negative differential susceptance (ΔB) as a function of frequency. To eliminate the impact of the contact resistance on the mobility measurement, an equivalent circuit model has been introduced to calculate the capacitance and the conductance of the NPB layer. By fitting the Nyquist plot of the impedance spectroscopy using the equivalent circuit, the contact resistance can be extracted from the impedance measurements. It is found that the mobility at low electric field is lower than what was reported in literature after the consideration of the contact resistance. It is also found that the calculated hole mobility of the NPB layer follows the Poole-Frenkels rule.

Abstract: In this work, the dielectric response of ZnO thin films has been studied over a temperature range of 200 K - 550 K. The dielectric response of polycrystalline ZnO thin films in the frequency domain was measured from 42 Hz - to 5 MHz with a small AC signal amplitude at different temperatures. Influence of the light on conductivity has been also investigated. A universal power law relation was brought into picture to explain the frequency dependence of AC conductivity. The temperature dependence of AC conductivity was analyzed in detail. The activation energy obtained from the temperature dependence of AC conductivity was attributed to the shallow trap-controlled space charge conduction in the bulk of the sample.

Abstract: Confocal Raman spectroscopy has been applied to investigate blend polycarbonate and ZnO thin layers with different thicknesses and different content of ZnO. The admittance spectroscopy have been applied to correlation of optical and electrical properties of these layers used in electroluminescence diodes and photovoltaic cells. The I-V (DC and AC) characteristics and thermally stimulated current (TSC) have been applied to the study of the deep levels in ZnO thin films grown by sol-gel method onto Si substrates. The surface spectroscopy morphology of the samples were investigated by scanning microscopy and X ray diffraction.

Abstract: The ZnO thin films have been produced on p-type Si and quartz substrates by the spin-coating method and after deposition were heated at different temperatures in the range from 650K to 850K. The photoluminescence (PL) and cathodoluminescence (CL) measurements were carried out at temperature range 12K-350K. I-V, C-V measurements were performed on the Al/ZnO/Si/Al structures at different temperatures. The structural properties of the ZnO thin films were carried out using x-ray and SEM method. The effects of the thickness variation and annealing temperature on the crystallinity parameters were observed. The electrical response of grains, grain boundaries, and contacts of the ZnO film was obtained.

Abstract: Admittance spectroscopy (AS) and deep level transient spectroscopy (DLTS) have been applied to B-doped thin polycrystalline diamond films deposited on p+-silicon by hot filament chemical vapour deposition. Films with two boron concentrations (1.5×10^19 cm-3 and 4×10^19 cm-3) were selected to study the effect of B concentration on the electronic states in CVD-diamond. We have investigated whether these deep states arise from point or extended defects. DLTS and AS find two hole traps, E1 (0.29±0.03 eV) and E2 (0.53±0.07 eV), in both films. A third level, E3 (0.36±0.02 eV) was also detected in the more highly doped film. The defect levels E1 and E2 exhibited behaviour typical of extended defects, which we suggest may be due to B segregated to the grain boundaries. In contrast, the defect level E3 exhibited behaviour characteristic of an isolated point defect, which we attribute to B-related centres in bulk diamond.

Abstract: Aluminum-doped 4H-SiC samples were either irradiated with high-energy electrons (170 keV or 1 MeV) or implanted with a box-shaped He+-profile. Admittance spectroscopy (AS) and deep level transient spectroscopy (DLTS) were employed to search for defect centers. AS spectra of as-grown as well as of electron-irradiated (170 keV or 1 MeV) 4H-SiC epilayers reveal the Al acceptor (ΔE(Al) = 200 meV) and an unknown defect (ΔE(SB) = 177 meV), while AS spectra of the He+-implanted and annealed sample show in addition to the Al-acceptor two energetically deeper acceptor-like defect centers (ΔE(RE3) = 255 meV and ΔE(KR3) = 375 meV). The KR3-center is not directly formed by the He+-implantation, it requires an annealing process. The DLTS spectra of the He+-implanted and annealed sample resolve a double-peak structure of the KR3-defect (ΔE(KR3A) = 380 meV and ΔE(KR3B) = 410 meV).

Abstract: ZnO varistors are multicomponent polycrystalline ceramics with highly nonlinear current-voltage characteristics and surge energy absorption capabilities. The voltage gradient of the ZnO varistor is inversely proportional to its average grain size. Recently, the rare-earth oxides have been reported as growth inhibitor of ZnO grains to obtain high voltage gradient. Although the dopant of rare-earth oxides can remarkably enhance the voltage gradients of varistor samples, their leakage currents and nonlinear coefficients deteriorate at the same time. In this paper, the sintering effects on electrical characteristics and admittance spectroscopies of ZnO varistor samples doped with yttrium oxides were investigated. Samples were fabricated under different sintering temperatures, including 1000 °C, 1100 °C, 1200 °C and 1300 °C. Then, the electrical characteristics and admittance spectroscopies of these varistor samples were measured. The measured current-voltage results behave special U-type curves related to sintering temperature. Furthermore, the admittance spectroscopy of these samples revealed the sintering effects on the intrinsic defects of ZnO varistors.