Isotype Heterojunction Research Articles

Tubular g‐C3N4 Isotype Heterojunction: Enhanced Visible‐Light Photocatalytic Activity through Cooperative Manipulation of Oriented Electron and Hole Transfer

A tubular g-C3 N4 isotype heterojunction (TCNH) photocatalyst was designed for cooperative manipulation of the oriented transfer of photogenerated electrons and holes to pursue high catalytic performance. The adduct of cyanuric acid and melamine (CA·M) is first hydrothermally treated to assemble into hexagonal prism crystals; then the hybrid precursors of urea and CA·M crystals are calcined to form tubular g-C3 N4 isotype heterojunctions. Upon visible-light irradiation, the photogenerated electrons transfer from g-C3 N4 (CA·M) to g-C3 N4 (urea) driven by the conduction band offset of 0.05 eV, while the photogenerated holes transfer from g-C3 N4 (urea) to g-C3 N4 (CA·M) driven by the valence band offset of 0.18 eV, which renders oriented transfer of the charge carriers across the heterojunction interface. Meanwhile, the tubular structure of TCNH is favorable for oriented electron transfer along the longitudinal dimension, which greatly decreases the chance of charge carrier recombination. Consequently, TCNH exhibits a high hydrogen evolution rate of 63 μmol h(-1) (0.04 g, λ > 420 nm), which is nearly five times of the pristine g-C3 N4 and higher than most of the existing g-C3 N4 photocatalysts. This study demonstrates that isotype heterojunction structure and tubular structure can jointly manipulate the oriented transfer of electrons and holes, thus facilitating the visible-light photocatalysis.

Investigation of Shot Noise in Avalanche Photodiodes

PbWO4 crystal-Hamamatsu S8148 avalanche photodiode (APD) assembly has been used in the barrel section of the CMS electromagnetic calorimeter. The shot noise of the photodiode is one of the important parameters for the energy resolution of the crystal-APD system. The major source of this noise is the statistical variations in the rate at which primary charge carriers are generated and recombine. Thus, the shot noise varies with position of the primary charge carriers generated in photodiode. In this work, the shot noise properties of the Hamamatsu S8148 APD structure and zinc sulfide-silicon (ZnS-Si) isotype heterojunction APD structure have been compared for the PbWO4 photons. Calculations were made with a Single Particle Monte Carlo simulation technique.

Facile fabrication of acidified g-C3N4/g-C3N4 hybrids with enhanced photocatalysis performance under visible light irradiation

A highly efficient visible-light-driven acidified g-C3N4 (ACNS)/g-C3N4 isotype heterojunction photocatalysts were synthesized by ultrasonic dispersion assisted electrostatic self-assembly strategy for the first time. The photocatalytic oxidation ability of the novel photocatalysts were evaluated using methyl orange (MO) as a target pollutant. The obtained ACNS/g-C3N4 photocatalysts were characterized by X-ray powder diffraction (XRD), Fourier transform infrared spectrometry (FTIR), UV–vis diffuse reflection spectroscopy (DRS), Scanning electron microscopy (SEM), Transmission electron microscopy (TEM) and X-ray photoelectron spectroscopy (XPS) methods. The photocatalysts exhibited a significantly enhanced photocatalytic performance in degrading MO under visible light illumination (λ>420nm) compared with the pristine ACNS and g-C3N4 solely. The optimal ACNS content for the photocatalytic activity of the heterostructured composites was determined. The 30wt.% ACNS/g-C3N4 exhibited the highest photocatalytic activity, which showed a reaction rate constant as high as 0.0216min−1, 4.3 times higher than that of bare g-C3N4. The mechanism of the photocatalysts was investigated by determination of reactive species in the photocatalytic reactions and photoluminescence technique. The quenching effects of different scavengers displayed that the reactive h+ and O2− played major role in the reaction systems. The synergic effect between the ACNS and g-C3N4 was found to lead to an improved photo-generated carrier separation and hence the photocatalytic activities of the composite photocatalysts were increased significantly.

MBE-grown GaAs:Si/GaAs:Be tunnel diodes for multijunction solar cells

We present the results of optimization of the design and molecular beam epitaxy (MBE) growth technology of N-AlGaAs:Si/n+-GaAs:Si/p+-GaAs:Be/P-AlGaAs:Be heterostructures for tunnel diodes (TDs). The achieved maximum peak current density level (Jp = 513 A/cm2) allows these TDs to be used for cascade connections both in multijunction solar cells and in structures of tunnel-coupled laser diodes. The initial region of the J–U curve of TDs exhibits nonlinearity that is explained by a residual potential barrier left in the p+–P–p+ isotype heterojunction (confining the active region of TD) as a result of non-optimum doping of Al0.4Ga0.6As alloy.

Trap modulated photoresponse of InGaN/Si isotype heterojunction at zero-bias

n-n isotype heterojunction of InGaN and bare Si (111) was formed by plasma assisted molecular beam epitaxy without nitridation steps or buffer layers. High resolution X-ray diffraction studies were carried out to confirm the formation of epilayers on Si (111). X-ray rocking curves revealed the presence of large number of edge threading dislocations at the interface. Room temperature photoluminescence studies were carried out to confirm the bandgap and the presence of defects. Temperature dependent I-V measurements of Al/InGaN/Si (111)/Al taken in dark confirm the rectifying nature of the device. I-V characteristics under UV illumination, showed modest rectification and was operated at zero bias making it a self-powered device. A band diagram of the heterojunction is proposed to understand the transport mechanism for self-powered functioning of the device.

P–P heterojunction sensor of self-assembled polyaniline nano-thin film/microstructure silicon array for NH3 detection

A novel P–P isotype heterojunction sensor was developed by modifying microstructure silicon array (MSSA) with self-assembled polyaniline (PANI) nano-thin film for ammonia (NH3) detection at room temperature, and the comparative study of response performance was focused on sensors with varied etching time of MSSA. The results indicated that PANI nanorods network was successfully deposited on MSSA surface, and PANI/MSSA sensor exhibited superior NH3-sensing properties than PANI/plane silicon sensor, which should be ascribed to three-dimensional structure and p-p heterojunction enhancement effect. Meanwhile, the optimized sensor with 4min etching time exhibited high response, good reversibility, repeatability and selectivity when exposed to NH3.

Construction of g-C3N4/S-g-C3N4 metal-free isotype heterojunctions with an enhanced charge driving force and their photocatalytic performance under anoxic conditions

g-C3N4/S-g-C3N4 metal-free isotype heterojunction catalysts with an enhanced charge driving force were prepared by a two step calcination method. The results indicate that this method can lead to a more efficient charge-carrier migration.

One-chip multicolor electroluminescence from an isotype heterojunction light-emitting diode

Both direct current (DC) and alternating current (AC) driving electroluminescence were obtained from isotype heterojunction (n-i-n: n-ZnO/i-HfO2/n-GaN) light-emitting diodes (LEDs) fabricated by a pulsed laser deposition system. The n-ZnO film maintained the same growth orientation as the n-GaN film and was of high crystalline quality even on a polycrystalline high-k HfO2 thin film. The as-produced n-i-n LEDs can emit strong visible light or dominant ultraviolet light at ∼392 nm, depending on the polarity of the applied DC voltages. The individual spectrum under either forward or reverse bias can be integrated to one spectrum by applying 50 Hz AC driving voltages (sinusoidal signals). More importantly, near white-light can be obtained by tuning the symmetric driving AC sinusoidal signals to the asymmetric ones. This simple and facile method only by applying AC asymmetric signals to achieve white light emission on one single chip may provide an easy route for the white-light solid-state lighting industry.

NiO/Si heterostructures formed by UV oxidation of nickel deposited on Si substrates

Nickel oxide (NiO) is one of few metal-oxide semiconductors showing a p-type conductivity and can be combined with an n-type semiconductor to fabricate a pn diode. In this study, isotype and anisotype heterojunction diodes were fabricated by ultraviolet (UV) oxidation at 350 °C of metallic Ni deposited on p- and n-Si substrates. The I-V characteristics of the NiO/p-Si diode do not show rectification, while those of the NiO/n-Si diode show rectification with a difference of two orders of magnitude between the currents at 2 and −2 V. The linear C−2 -V characteristics of the NiO/n-Si and secondary ion mass spectroscopy profile confirm that the UV oxidation temperature is low enough so as not to affect deep inside the Si. The NiO layers formed by the oxidation of Ni under UV illumination are found to be more conductive and more suitable for the diode fabrication than those by the thermal oxidation of Ni without UV illumination at the same temperature. Because the oxidation temperature is relatively low and can be further reduced, the UV oxidation can become an important process technology to form various metal-oxide semiconductors from printed metals for future printed flexible electronics.

Analytical Expression for Thermionic Transport Through Isotype Heterojunction Interfaces of Arbitrary Doping Ratio

We present an analytical expression for the current density across an isotype heterojunction valid for arbitrary doping concentration ratios. This result generalizes the standard expression found in the literature, which is limited by the assumption that the doping concentration ratio is equal to one. This result relies on the solution of a transcendental equation associated with the heterojunction boundary conditions by means of the Lambert W function. As done in the derivation of the standard expression, the generalization only considers thermionic emission, but the method can equally be applied for other transport mechanisms. The general result mathematically contains the expression for the current density across a metal-semiconductor Schottky contact as a limiting case, unifying the treatment of these two heterointerfaces into a single general analytical description.

Design of polarization-dipole-induced isotype heterojunction diodes for use in III–N hot electron transistors

The design of III–nitride-based hot electron transistors (HETs) is investigated using different diode design methodologies. Barrier-limited forward bias current and low reverse leakage current are demonstrated for the emitter-base diode using a barrier formed by a high-Al% AlGaN layer as a polarization-dipole layer. Two different base-collector diode designs are compared, one using 30% AlGaN as the barrier and the other using 10% InGaN as a polarization-dipole barrier. The InGaN polarization-dipole approach is shown to exhibit much lower reverse leakage currents. The impact of threading dislocation density on diode characteristics is also discussed.

Properties of sol–gel synthesized n-ZnO/n-GaN (0001) isotype heterojunction

The electronic and optical properties of sol–gel synthesized n-ZnO/n-GaN (0001) isotype heterojunction were reported. By incorporating ZnO–GaN with the same wurtzite structure and the similar lattice constant, a heterojunction was fabricated. The junction properties were evaluated by measuring the electrical characteristics. The n-ZnO/n-GaN heterostructure exhibits a non-ideal I–V behavior with the ideality factor greater than unity that could be ascribed to the interfacial layer, interface states and series resistance. The forward turn on voltage is about 0.7 V and the reverse breakdown voltage is more than 2 V. The optical band gaps of the ZnO film and GaN using optical absorption method were found to be 3.272 eV and 3.309 eV, respectively. The fundamental absorption edge in the film is formed by the direct allowed transitions.

In Situ Construction of g-C3N4/g-C3N4 Metal-Free Heterojunction for Enhanced Visible-Light Photocatalysis

The photocatalytic performance of the star photocatalyst g-C3N4 was restricted by the low efficiency because of the fast charge recombination. The present work developed a facile in situ method to construct g-C3N4/g-C3N4 metal-free isotype heterojunction with molecular composite precursors with the aim to greatly promote the charge separation. Considering the fact that g-C3N4 samples prepared from urea and thiourea separately have different band structure, the molecular composite precursors of urea and thiourea were treated simultaneously under the same thermal conditions, in situ creating a novel layered g-C3N4/g-C3N4 metal-free heterojunction (g-g CN heterojunction). This synthesis method is facile, economic, and environmentally benign using easily available earth-abundant green precursors. The confirmation of isotype g-g CN heterojunction was based on XRD, HRTEM, valence band XPS, ns-level PL, photocurrent, and EIS measurement. Upon visible-light irradiation, the photogenerated electrons transfer from g-C3N4 (thiourea) to g-C3N4 (urea) driven by the conduction band offset of 0.10 eV, whereas the photogenerated holes transfer from g-C3N4 (urea) to g-C3N4 (thiourea) driven by the valence band offset of 0.40 eV. The potential difference between the two g-C3N4 components in the heterojunction is the main driving force for efficient charge separation and transfer. For the removal of NO in air, the g-g CN heterojunction exhibited significantly enhanced visible light photocatalytic activity over g-C3N4 alone and physical mixture of g-C3N4 samples. The enhanced photocatalytic performance of g-g CN isotype heterojunction can be directly ascribed to efficient charge separation and transfer across the heterojunction interface as well as prolonged lifetime of charge carriers. This work demonstrated that rational design and construction of isotype heterojunction could open up a new avenue for the development of new efficient visible-light photocatalysts.

Electrical properties of isotype heterojunctions n-TiN/n-Si

Titanium nitride and silicon are promising materials for use in various photoelectric devices due to their physical properties, so the research of the isotype heterojunctions n-TiN/n-Si is of considerable interest. The article studies the electrical properties of the heterojunctions formed by deposition of thin-film ТiN of conductivity of n-type on single crystal supports of Si of conductivity of n-type by the method of reactive magnetron sputtering of pure titanium target in mixtures of argon and nitrogen at continuous voltage. The partial pressures of argon and nitrogen are 0.35 Pa and 0.7 Pa, respectively, at constant magnetron power 120 watts. The volt - ampere characteristics of the heterojunction TiN / Si were measured by the measuring complex SOLARTRON SI 1286, SI 1255. There is an analysis of temperature dependence of height of potential barrier and successive resistance of the heterojunction. The concentration of surface states (mismatch dislocations) N ss at the heterojunctions interface constitutes 2.67 × 10 13 sm -2 . It was determined that the dominant mechanism of current flow through the heterojunction n-TiN/n-Si at direct bias is well described in the tunnel model

Electrical characterization and modelling of n–n Ge-Si heterojunctions with relatively low interface state densities

We investigated the electrical behavior of n–n Ge-Si isotype heterojunction diodes prepared by surfactant-mediated epitaxy of relaxed n-Ge layers on (100) n-Si substrates. Current-voltage characteristics were measured at different temperatures between 10 °C and 90 °C. The experimental results were interpreted with a new heterojunction model based on Shockley-Read-Hall kinetics for electron and hole capture/emission at the interface traps, which describes the bias dependent interface and semiconductor charges, the trap-mediated currents, and the thermionic electron transmission current. The modeled thermionic electron emission current was in excellent agreement with the experimental current-voltage characteristics in the whole temperature range for negative (≥−0.5 V) and positive (≤0.1 V) Ge biases. Trap-mediated currents were much smaller for reasonable trap capture cross sections σ ≤ 10−14cm2. From the experimental data, we extracted an electron barrier height of 0.59 eV at room temperature and an effective density of interface traps of only 5·1012cm−2eV−1 near the Si midgap. The charge carrier exchange between these traps with the Ge side was found to be much more efficient than with the Si side. The presence of a hole inversion layer at the interface proved to be essential for the interpretation of the heterojunction characteristics.

Sno2-N-Si Isotype Heterojunction Solar Cell with Efficiency of 7.1%

This paper presents the results of a study of the photovoltaic effect of SnO2/n-Si solar cell formed by chemically sprayed SnO2 on signal crystal silicon the main of using SnO2 chemically sprayed thin films include low cost and the risibility of production in large scale, the photovoltaic characteristics included short circuit current (Jsc), open circuit voltage (Voc), where the maximum (Jsc) and (Voc)obtained at AM1 were 14.3 (mA cm-2) and 630(mV), respectively. The fill factor (FF) was (0.68). Photo-induced open carrier lifetime of these cells is about (29 μs).

Study on the luminescence properties of n-ZnO/i-NiO/n-GaN isotype heterojunction diodes

The n-ZnO/i-NiO/n-GaN isotype heterojunction diode was fabricated by metal organic chemical vapor deposition (MOCVD) on the n-GaN/sapphire substrate. The current–voltage (I–V) measurement showed typical rectification characteristics with a turn-on voltage of 2.5V. An ultraviolet emission peak around 363nm was observed from the EL spectra at room temperature. High ultraviolet light extraction efficiency was proved by a superlinear curve of the intensity ratio of ultraviolet to visible emission. The mechanism of electroluminescence has been tentatively elucidated in terms of band diagrams of the heterojunction.

A Facile Band Alignment of Polymeric Carbon Nitride Semiconductors to Construct Isotype Heterojunctions

A Facile Band Alignment of Polymeric Carbon Nitride Semiconductors to Construct Isotype Heterojunctions

A Facile Band Alignment of Polymeric Carbon Nitride Semiconductors to Construct Isotype Heterojunctions

A Facile Band Alignment of Polymeric Carbon Nitride Semiconductors to Construct Isotype Heterojunctions

Rectifying properties of sol–gel synthesized Al:ZnO/Si (N–n) thin film heterojunctions

We report on the rectifying behavior of sol–gel synthesized Al (1 and 3at%):ZnO/Si (N–n) thin film isotype heterojunctions. The films were dense and uniform over the substrate and show polycrystalline morphology with defined grain boundaries. The current–voltage (I–V) characteristics of the junctions at room temperature and high temperature in air ambient were found to be asymmetric with an increase in rectification ratio (If/Ir) from 1.29 to 3.70 for 1at% and from 0.60 to 2.54 for 3at% of Al (at a bias voltage of 5V) for increase in temperature upto 150°C. The I–V characteristics of the junctions were explained on the basis of high temperature carrier injection and single carrier dynamics.