Electrical characteristics of Au/<Emphasis Type="Italic">n</Emphasis>-GaAs structures with thin and thick SiO<Subscript>2</Subscript> dielectric layer

The aim of this study, to explain effects of the SiO₂ insulator layer thickness on the electrical properties of Au/n-GaAs Shottky barrier diodes (SBDs). Thin (60 Å) and thick (250 Å) SiO₂ insulator layers were deposited on n-type GaAs substrates using the plasma enganced chemical vapour deposition technique. The current-voltage (I–V) and capacitance-voltage (C-V) characteristics have been carried out at room temperature. The main electrical parameters, such as ideality factor (n), zero-bias barrier height (? _Bo ), series resistance (R _s ), leakage current, and interface states (N _ss ) for Au/SiO₂/n-GaAs SBDs have been investigated. Surface morphologies of the SiO₂ dielectric layer was analyzed using atomic force microscopy. The results show that SiO₂ insulator layer thickness very affects the main electrical parameters. Au/n-GaAs SBDs with thick SiO₂ insulator layer have low leakage current level, small ideality factor, and low interface states. Thus, Au/n-GaAs SBDs with thick SiO₂ insulator layer shows better diode characteristics than other.     

Dielectric and Optical Properties of CdS–Polymer Nanocomposites Prepared by the Successive Ionic Layer Adsorption and Reaction (SILAR) Method

The successive ionic layer adsorption and reaction (SILAR) method has been used to grow epitaxial CdS–polymer nanostructures as thin films with different surface morphology and particle size. The main purpose of the study was to investigate the dielectric properties and a.c. electrical conductivity (σ _a.c.), by a.c. impedance spectroscopy between 1 kHz and 1 MHz, at room temperature, of CdS–polymer nanocomposites produced by use of 2, 6, and 10 cycles of SILAR. The surface morphology and optical absorption of the samples were characterized by scanning electron microscopy (SEM) and UV–visible spectroscopy, respectively. Determination of the energy gaps of CdS–polymer nanocomposites prepared by use of different numbers of cycles of SILAR reveals that the band gap decreases with increasing number of cycles (J. Cryst. Growth 305, 175–180, 2007). This behavior is because of the growth of nanoparticles in the matrix materials, and can be explained by changes in the amount of confinement as a consequence of particle size variation. SEM images also confirm that different numbers of cycles lead to different morphology. Frequency-dependent dielectric properties and a.c. electrical conductivity of the samples prepared by use of different numbers of cycles of SILAR were investigated, and comparative studies on some electrophysical properties of the samples are reported. Experimental results show that values of the dielectric constant (ε′), dielectric loss (ε″), dielectric loss tangent (tanδ), the real (M′) and imaginary (M″) parts of electric modulus, and σ _a.c. are highly dependent on the frequency and the number of cycles. It can be concluded that changing the frequency and the number of cycles substantially alters both the dielectric properties and a.c. electrical conductivity of the samples.     

A.c. and d.c. conduction processes in octakis[(4-tert-butylbenzylthio)-porphyrazinato]Cu(II) thin films with gold electrodes

The d.c. and a.c. electrical transport properties of Au/Pz/Au devices with various thickness of Pz(octakis[(4-tert-butylbenzylthio)-porphyrazinato]Cu(II)) layer have been investigated. Measurements revealed that, in contrast to previously investigated Au/Pc/Au structures, low voltage d.c. behaviour of the films can be described by the field-lowering mechanisms with a log(J) ∝ V ^1/2 current density-voltage characteristics under forward and reverse bias. For high reverse voltages, the observed ln(J/V ²) – 1/V characteristics indicated that the origin of conduction mechanism is Fowler–Nordheim tunnelling (FNT). On the other hand, the voltage dependence of current density at the higher forward-voltage region indicates that the mechanism of conduction in Au/Pz/Au devices is space charge limited conduction dominated by exponential trap distribution. A thickness independent barrier height was observed for tunnelling, while the total trap concentration show a general tendency to decrease with increasing film thickness. The a.c. conductivity showed two regions in the ln(σ _a.c.) – ln( f ) plots having different slopes, leading to the conclusion that for low frequency region, the dominant conduction mechanism is a small polaron tunnelling at all temperatures, whereas for high frequency region, correlated barrier hopping model is the dominant mechanism in the investigated devices.     

A comparative study on dielectric behaviours of Au/(Zn-doped PVA)/<Emphasis Type="Italic">n</Emphasis>-4H-SiC (MPS) structures with different interlayer thicknesses using impedance spectroscopy methods

Three different thicknesses (50, 150 and 500 nm) Zn-doped polyvinyl alcohol (PVA) was deposited on n-4H-SiC wafer as interlayer by electrospinning method and so, Au/(Zn-doped PVA)/n-4H-SiC metal–polymer–semiconductor structures were fabricated. The thickness effect of Zn-doped PVA on the dielectric constant (\(\varepsilon ^{\prime }\)), dielectric loss (\(\varepsilon ^{{\prime }{\prime }}\)), loss-tangent (tan \(\delta \)), real and imaginary parts of electric modulus (\(M^{\prime }\) and \(M^{{\prime }{\prime }})\) and ac electrical conductivity \((\sigma _{\mathrm{ac}})\) of them were analysed and compared using experimental capacitance (C) and conductance (\(G/\omega \)) data in the frequency range of 1–500 kHz at room temperature. According to these results, the values of \(\varepsilon ^{\prime }\) and \(\varepsilon ^{{\prime }{\prime }}\) decrease with increasing frequency almost exponentially, \(\sigma _{\mathrm{ac}}\) increases especially, at high frequencies. The \(M^{\prime }\) and \(M^{{\prime }{\prime }}\) values were obtained from the \(\varepsilon ^{\prime }\) and \(\varepsilon ^{{\prime }{\prime }}\) data and the \(M^{\prime }\) and \(M^{{\prime }{\prime }}\) vs. f plots were drawn for these structures. While the values of \(\varepsilon ^{\prime }\), \(\varepsilon ^{{\prime }{\prime }}\) and tan \(\delta \) increase with increasing interlayer thickness, the values of \(M^{\prime }\) and \(M^{{\prime }{\prime }}\) decrease with increasing interlayer thickness. The double logarithmic \(\sigma _{\mathrm{ac}}\) vs. f plots for each structure have two distinct linear regimes with different slopes, which correspond to low and high frequencies, respectively, and it is prominent that there exist two different conduction mechanisms. Obtained results were found as a strong function of frequency and interlayer thickness.