Experimental and theoretical investigation of defects at (1 0 0) Si1−xGex/oxide interfaces

The identification of a nontrigonal Ge dangling bond at SiO₂/Si_1−xGe_x/SiO₂ heterostructures and its electrical activity are discussed, both from experimental and theoretical points of view. This dangling bond is observed from multifrequency electron-spin resonance experiments performed at 4.2 K, for typical Ge concentrations in the range 0.4 ≤ x ≤ 0.85. The electrical activity of this defect is revealed from capacitance-voltage characteristics measured at 300 and 77 K, and is found to behave like an acceptor defect. First-principles calculations of the electronic properties of this Ge dangling bond indicate that its energy level approaches the valence band edge of the Si_1−xGe_x layer as the Ge content increases, confirming its acceptor-like nature.     

Influence of barrier on partial discharge activity by a conducting particle in liquid nitrogen under AC voltages adopting UHF technique

The UHF signals are generated due to PD formed by particle movement in liquid nitrogen under AC voltages. The levitation voltage of a particle in liquid nitrogen measured through UHF technique and by conventional PD measurement technique is the same, confirming the sensitivity of UHF technique for identification of PD activity. The frequency content of UHF signal generated due to particle movement in liquid nitrogen, under AC voltages, lies in the range 0.5–1.5 GHz. The characteristics of UHF signal generated due to particle movement between the barrier and high voltage/ground electrode is much similar to the signal generated by particle movement in clean electrode gap. Pseudo resonance phenomena can occur in liquid nitrogen due to particle movement. It is also observed that the partial discharge magnitude, in general, be high when the particle moves between the barrier and high voltage electrode when compared to the barrier and the ground electrode. Percentage of clay in epoxy nanocomposites has not altered the levitation voltage of the particle in the electrode gap. Zero span analysis clearly indicates that pseudo resonance occurs when particle moves (in a short gap) between the barrier and high voltage/ground electrode.     

Synthesis of high-performance graphene nanosheets by thermal reduction of graphene oxide

High-performance graphene nanosheets have been synthesized by thermal reduction of graphene oxide (GO) under ethanol atmosphere. The reduced GO nanosheets were characterized by X-ray photoelectron spectroscopy, Raman spectroscopy and electrical transport measurements, respectively. The results indicated that the thermal reduction of GO under ethanol atmosphere can effectively remove the oxygen-containing functional groups and restore its graphic structure compared to the ones obtained using hydrazine or hydrogen. The electrical measurements indicated that the electrical mobility of single-layer graphene sheet reduced under ethanol atmosphere at 900 °C can reach 29.08 cm² V⁻¹ S⁻¹.     

Effect of oxygen flow rate and radio-frequency power on the photoconductivity of highly ultraviolet sensitive ZnO thin films grown by magnetron sputtering

We have investigated the effect of oxygen flow rate and radio-frequency (RF) power on the photoconductivity properties of ZnO thin films grown by magnetron sputtering and correlated the changes to the structural qualities. The electrical measurements show that the carrier concentration decreases with increase in the oxygen flow rate which is attributed to the probable increase in the oxygen vacancy (V_O)-related defects. The photocurrent spectra show that as the oxygen content increases, the films become lesser and more sensitive to the visible and ultraviolet (UV) lights respectively. As a result, the photo-to-dark current ratio (gain) increases to a value of 1.10 × 10⁶. As the RF power increases from 50 W to 150 W, the films become more conducting. The photoconductivity results show that as the RF power increases, the UV gain decreases slowly indicating that highly UV sensitive films can be grown at lower RF power.     

A quaternary lead based perovskite structured materials with diffuse phase transition behavior

A lead based quaternary compound composed of 0.25(PbZr_0.52Ti_0.48O₃) + 0.25(PbFe_0.5Ta_0.5O₃) + 0.25 (PbF_0.67W_0.33O₃) + 0.25(PbFe_0.5Nb_0.5O₃) - (PZT-PFT-PFW-PFN) was synthesized by conventional solid-state reaction techniques. It showed moderate high dielectric constant, low dielectric loss, and two diffuse phase transitions, one below the room temperature ∼261 K and other above ∼410 K. X-ray diffraction (XRD) patterns revealed a tetragonal crystal structure at room temperature where as scanning electron micrograph (SEM) indicates inhomogeneous surface with an average grain size of 500 nm-3 μm. Well saturated ferroelectric hysteresis loops with good saturation polarization (spontaneous polarization, P_s ∼ 30.68 μC/cm²) were observed. Temperature-dependent ac conductivity displayed low conductivity with kink in spectra near the phase transition. In continuing search for developing new ferroelectric materials, in the present study we report stoichiometric compositions of complex perovskite ceramic materials: (PZT-PFT-PFW-PFN) with diffuse phase transition behavior. The crystal structure, dielectric properties, and ferroelectric properties were characterized by XRD, SEM, dielectric spectroscopy, and polarization. 1/? versus (T) plots revealed diffuse relaxor phase transition (DPT) behavior. The compositional variation on the phase transition temperature, dielectric constant, and ferroelectric to paraelectric phase transitions are discussed.     

Fast response ultraviolet Ga-doped ZnO based photoconductive detector

A metal-semiconductor-metal photoconductive detector was fabricated using high quality Ga-doped ZnO film epitaxially grown onto alumina substrate by spray pyrolysis. The photocurrent increases linearly with incident power density for more than two orders of magnitude. Reflectance and photocurrent measurements were carried out to study optoelectronic properties of Ga-doped ZnO thin film. Both spectra are consistent with each other showing good response in UV than visible region. Peak responsivity of about 1187 A/W at 5 V bias for 365 nm light was obtained in UV region.     

Design and fabrication of linear pixel arrays for organic photovoltaic modules to achieve scalable power output

We describe a design and fabrication method to enable simpler manufacturing of more efficient organic solar cell modules using a modified flat panel deposition technique. Many mini-cell pixels are individually connected to each other in parallel forming a macro-scale solar cell array. The pixel size of each array is optimized through experimentation to maximize the efficiency of the whole array. We demonstrate that integrated organic solar cell modules with a scalable current output can be fabricated in this fashion and can also be connected in series to generate a scalable voltage output.     

Synthesis of optical transparent and electrical conductive polymer/nanocarbon composite films by infiltration method

Optical transparent and electrical conductive (PMMA)/nanocarbon composite films can be prepared through the infiltration of the polymer between the carbon aggregates of a preformed percolation cluster. This cluster is prepared by Langmuir-Blodgett technique as a thick film (2-20 μm in thickness). The performed direct current electrical measurements have shown that PMMA/Multiwall Nanotubes present a lower electrical surface resistivity and a higher optical transmittance than PMMA/carbon nanoparticles composite films. Much simpler and cheaper than other methods, the infiltration method allowed us to prepare composite films with a 75% in transparency and 1 MΩ/square in surface electrical resistivity. The composite films prepared by infiltration method have a good adhesion to the glass substrates but in some specific conditions can be integrally removed as free-standing films.     

Highly conductive, transparent flexible films based on open rings of multi-walled carbon nanotubes

Open rings of multi-walled carbon nanotubes were stacked to form porous networks on a poly(ethylene terephthalate) substrate to form a flexible conducting film (MWCNT-PET) with good electrical conductivity and transparency by a combination of ultrasonic atomization and spin-coating technique. To enhance the electric flexibility, we spin-coated a cast film of poly(vinyl alcohol) onto the MWCNT-PET substrate, which then underwent a thermo-compression process. Field-emission scanning electron microscopy of the cross-sectional morphology illustrates that the film has a robust network with a thickness of ~ 175 nm, and it remarkably exhibits a sheet resistance of approximately 370 Ω/sq with ~ 77% transmittance at 550 nm even after 500 bending cycles. This electrical conductivity is much superior to that of other MWCNT-based transparent flexible films.     

Nanocrystallized tetragonal metastable ZrO2 thin films deposited by metal-organic chemical vapor deposition for 3D capacitors

ZrO₂ is a potential candidate for the realization of 3D capacitors on silicon for future Systems-on-Chip. This paper reports on the deposition of ZrO₂ thin films by metal-organic chemical vapor deposition on planar and 3D structures. Physico-chemical as well as electrical properties of the films are investigated. It is shown that the change of phase and microstructure of the film due to annealing at 900 °C under O₂ impacts directly on the electrical performance of the capacitors. Capacitance densities are 2 nF/mm² for planar capacitors and reach 8 nF/mm² for capacitors with pores etched in silicon with a 4:1 aspect ratio.