R.N. Gayen

Synthesis of DLC films with different sp2/sp3 ratios and their hydrophobic behaviour

Diamond-like carbon (DLC) films were deposited on glass substrate by sputtering of a vitreous carbon target in Ar + H2 plasma. The sp2/sp3 content in the films depended on the relative amount of hydrogen in the Ar + H2 plasma. The films were characterized by Fourier transformed infrared studies, Raman studies, scanning electron microscopy, atomic force microscopy and optical measurements. Hydrophobicity in these films was studied by measuring the contact angles of the water droplets and it was found that the films were extremely hydrophobic. The results are interpreted in terms of hybridization of carbon in these DLC films.

Enhanced UV detection by transparent graphene oxide/ZnO composite thin films

All solution processed transparent thin films of graphene oxide (GO) and zinc oxide (ZnO) in different compositions prepared by a simple two-step chemical synthesis method have been studied for their UV detection properties. The preparation of GO through oxidation of graphite flakes is followed by sol–gel spin coating deposition of the GO–ZnO composite films on glass substrates. The surface morphology, microstructure and composition of the samples have been studied to confirm the formation of composite thin films comprising wurtzite-ZnO nanocrystallites and GO flakes. Optical studies demonstrate that both the transparency and optical band gap of the samples as estimated from wavelength dependent transmittance curves decrease with the increase of GO content in the films, while the charge carrier concentration increases by 5 fold. The in-plane current–voltage (I–V) measurements with two silver electrodes on the GO–ZnO film show a significant enhancement of the photosensitivity in comparison to ZnO films when they are exposed to UV light of different intensities. The response time (t90-response) is nearly three times smaller for GO–ZnO composite films as compared to that of pure ZnO. This improvement is attributed to the defect state modulation and carrier density improvement of the thin films with incorporation of GO, which is encouraging to propel optical, electrical and hence optoelectronics applicability of ZnO composite based transparent devices.

Temperature dependent current transport of Pd/ZnO nanowire Schottky diodes

Zinc oxide (ZnO) nanowire based Schottky barrier diodes are fabricated by depositing Pd metal contact on top of vertically well-aligned ZnO nanowire arrays. A vertical array of ZnO nanowires on indium tin oxide (ITO) coated glass substrates is synthesized by hybrid wet chemical route. Scanning electron microscopy (SEM), x-ray diffraction (XRD) and x-ray photoelectron spectroscopy (XPS) measurement confirm the formation of stoichiometric well-aligned hexagonal (h-ZnO) nanowire arrays with wurtzite structure. Temperature dependent current?voltage (I?V) measurements on palladium-ZnO (Pd/ZnO) nanowire Schottky junctions in the temperature range 303?383 K exhibit excellent rectifying character. From these nonlinear I?V plots, different electrical parameters of diode-like reverse saturation current, barrier height and ideality factor are determined as a function of temperature assuming pure thermionic emission model. The ideality factor is found to decrease while the barrier height increases with the increase in temperature. The series resistance values calculated from Cheung?s functions also show temperature dependency. Such behavior can be attributed to the presence of defects that traps carriers, and barrier height inhomogeneity at the interface of the barrier junction. After barrier height inhomogeneity correction, considering a Gaussian distributed barrier height fluctuation across the Pd/ZnO interface, the estimated values of mean barrier height and modified Richardson constant are more closely matched to the theoretically predicted value for Pd/ZnO Schottky barrier diodes. The variation of density of interface states as a function of interface state energy is also calculated.

Optical properties of Si-doped GaN nanocrystals in SiO2/GaN/SiO2 thin film structure

SiO2/n-GaN/SiO2 composite films in the nanocrystalline form were deposited using a multi-target sputtering system onto fused silica substrates at a system pressure ~25 Pa. Different sets of nanocomposite films with different ratios of the sizes of the nanocrystallites (d) and intercrystallite distances (s) were deposited by changing the substrate temperature during sequential sputtering of the targets. The films were characterized by measuring the microstructural and optical properties. Photoluminescence measurements were carried out at 300 K. The experimental absorption spectra could be faithfully described by considering the combined effect of scattering by the ultra-small crystallites and broadening due to inhomogeneity in the sizes of the films.

Electrical characteristics and rectification performance of wet chemically synthesized vertically aligned n-ZnO nanowire/p-Si heterojunction

Vertically well-aligned n-ZnO nanowire (NW) thin films were deposited onto p-Si substrates by a two-step wet chemical technique to form a p–n heterojunction diode. The morphological and structural characteristics of the ZnO NW performed by scanning electron microscopy (SEM) and x-ray diffraction (XRD) revealed well-aligned h-ZnO NW with a wurtzite structure. A direct optical band gap of 3.30 eV was calculated from the transmittance trace obtained using a UV–VIS–NIR spectrophotometer. The electrical characteristics of the heterojunction diode were studied by capacitance–voltage (C–V) measurement at room temperature, and current–voltage–temperature (I–V–T) measurements performed in the 300–400 K range. The C–V measurements yield a carrier concentration of 1.3 × 1016 c.c.−1 for the ZnO NW thin film. The ideality factor (n) was found to decrease, while the barrier height ( b0) increased with the increase in temperature, when calculated using a thermionic emission model from the non-linear I–V–T plots. The series resistance (R s) calculated by the Cheung–Cheung method decreased with the increase in temperature. The mean barrier height (0.718 eV) and modified Richardson constant (28.4 A cm−2 K−2) calculated using a Gaussian distribution of barrier heights (considering barrier height inhomogeneity) were closer to the theoretical value than those calculated from the linear approximation of the ln(I s/T 2) versus 1000/T plot. The variation of the density of interface states with interface state energy was also studied. The n-ZnO NW/p-Si heterojunction diode performed very good half wave rectification in the frequency range 50 Hz–10 kHz, when a sinusoidal ac voltage of amplitude 4.5 V was applied across it.