Gas Heterostructures Research Articles

Tunable Electronic, Optoelectronic, and Photocatalytic Properties of MoS2 and GaS Monolayers in the MoS2/GaS Heterostructure

AbstractThe electronic structure calculations show that the GaS and MoS2 monolayer and MoS2/GaS hetero‐bilayer systems are semiconducting materials. It is also justified by the density of states. In the hetero‐bilayer system, the band gap is reduced. It causes an increase in the absorption, compared to their monolayers, of incident light in the visible region. This may be because of the charge transfer and the interlayer interaction between these monolayers in the heterostructure form. The calculated higher extinction coefficient causes the fast absorption of light, hence, the increased absorption coefficient. The heterostructure system makes the refractive index and reflection coefficient of GaS and MoS2 monolayers tuneable. The static reflection coefficient is in the following order GaS < MoS2/GaS < MoS2. It means that the MoS2/GaS heterostructure system can increase the static refractive index of GaS and decrease that of MoS2 monolayers simultaneously. The calculated band edge positions show that the MoS2 and GaS monolayers have excellent water oxidation and reduction capability, respectively. These predictions show that the proposed heterostructure may be a potential candidate for nanoelectronics and optoelectronics.

Technology of production and photoelectric characteristics of AlB10 heterojunctions based on silicon

Currently, the usage of electronic devices with diverse applications is prevalent in numerous commercial, industrial, electrical, and military sectors. The development of new semiconductors is essential for the advancement of highly sensitive, fast-responding, multifunctional, and high-precision devices and installations. Thin films based on semiconductor monocrystalline substrates are being obtained by scientists in leading research centers worldwide. Production technologies are being improved, optimal conditions are being determined, and the structural and unique physical properties of the obtained thin films are being studied. Additionally, research is being conducted to broaden the light absorption spectra of gas, temperature, and pressure-sensitive heterostructures. In this work, the results of a low-temperature technology development for obtaining films of refractory materials based on the presence of the eutectic state in boron-metal oxide systems are presented. The electro-physical properties of the obtained AlB10 films are investigated over a wide temperature range. The discussion revolves around the development of the technology for manufacturing metal-dielectric-metal (MDM) structures based on AlB10. The results of investigating the volt-ampere characteristics (VAC) of Ni-AlB10-Ni and Al-AlB10-n-Si structures are presented. The method employed for obtaining AlB10 films involves thermal evaporation of a B+Al2O3 mixture in a vacuum at a temperature of 1350-1400°C.

First-principles study of two-dimensional HfS2/GaS van der Waals heterostructure for photocatalytic action

Exploring excellent performance of photocatalysts for hydrogen/oxygen evolution reactions is an effective way for solar energy applications. In this work, a HfS 2 /GaS van der Waals heterostructure (vdwH) has been designed to achieve efficient and spontaneous water decomposition. Through first-principles calculation, HfS 2 /GaS vdwH has been proved to be a stable and promising photocatalyst with the following obvious advantages: both water reduction and oxidation potentials are located in the band gap of HfS 2 /GaS vdwH (2.04 eV), ensuring that water splitting can occur. The type-Ⅱ band structure and built-in electric field ensure the effective separation of photoelectron-hole, and the high mobility of electrons and holes can also predict high photocatalytic activity . By calculating the free energy of the oxygen evolution reaction on the surface of GaS and the hydrogen evolution reaction on the surface of HfS 2 of the heterostructure , it's found that the water decomposition reaction can occur spontaneously under light. Furthermore, after 1–6% in-plane biaxial tensile strain was applied, the redox reaction of photocatalytic decomposition water can still occur on the HfS 2 /GaS vdwH, and the application of strain can effectively regulate the optical and electronic properties. Therefore, HfS 2 /GaS vdwH could be a promising candidate material for the photocatalytic decomposition of water. • The spatial separation of photogenerated carriers is realized in type-II HfS 2 /GaS heterostructure. • The electron mobility can be as high as 10 3 cm 2 V −1 s −2 . • Free energy calculation shows that the water decomposition reaction can occur spontaneously under light. • HfS 2 /GaS heterostructure reach high light absorption coefficients of 3.6 × 10 5 cm −1 .

One-Dimensional Nanomaterials in Resistive Gas Sensor: From Material Design to Application

With a series of widespread applications, resistive gas sensors are considered to be promising candidates for gas detection, benefiting from their small size, ease-of-fabrication, low power consumption and outstanding maintenance properties. One-dimensional (1-D) nanomaterials, which have large specific surface areas, abundant exposed active sites and high length-to-diameter ratios, enable fast charge transfers and gas-sensitive reactions. They can also significantly enhance the sensitivity and response speed of resistive gas sensors. The features and sensing mechanism of current resistive gas sensors and the potential advantages of 1-D nanomaterials in resistive gas sensors are firstly reviewed. This review systematically summarizes the design and optimization strategies of 1-D nanomaterials for high-performance resistive gas sensors, including doping, heterostructures and composites. Based on the monitoring requirements of various characteristic gases, the available applications of this type of gas sensors are also classified and reviewed in the three categories of environment, safety and health. The direction and priorities for the future development of resistive gas sensors are laid out.

A Review of THz Modulators with Dynamic Tunable Metasurfaces.

Terahertz (THz) radiation has received much attention during the past few decades for its potential applications in various fields, such as spectroscopy, imaging, and wireless communications. To use terahertz waves for data transmission in different application systems, the efficient and rapid modulation of terahertz waves is required and has become an in-depth research topic. Since the turn of the century, research on metasurfaces has rapidly developed, and the scope of novel functions and operating frequency ranges has been substantially expanded, especially in the terahertz range. The combination of metasurfaces and semiconductors has facilitated both new opportunities for the development of dynamic THz functional devices and significant achievements in THz modulators. This paper provides an overview of THz modulators based on different kinds of dynamic tunable metasurfaces combined with semiconductors, two-dimensional electron gas heterostructures, superconductors, phase-transition materials, graphene, and other 2D material. Based on the overview, a brief discussion with perspectives will be presented. We hope that this review will help more researchers learn about the recent developments and challenges of THz modulators and contribute to this field.

Design and material growth of AlGaN-channel two-dimensional-electron gas heterostructures employing strain-controlled quaternary AlGaInN barrier layers

In order to improve the thermal stability in AlGaN-channel two-dimensional electron gas (2DEG) heterostructures, we newly designed strain-controlled quaternary AlGaInN barrier layers, and then we grew them by metalorganic chemical vapor deposition. A 2DEG density as high as 2.5 × 1013 cm−2 and a good surface morphology were obtained for an (Al0.64Ga0.36)0.976In0.024N/Al0.19Ga0.81N heterostructure, in which in-plane tensile strain in the barrier layer was estimated to be 0.51%. It was also confirmed that the (Al0.64Ga0.36)0.976In0.024N/Al0.19Ga0.81N heterostructure showed an excellent thermal stability in its 2DEG properties even at high temperatures up to 900 °C. As a result of their thermal stability improvement, we were also able to confirm that the contact resistance was improved by applying high-temperature annealing.

Improved mobility in InAlN/AlGaN two-dimensional electron gas heterostructures with an atomically smooth heterointerface

We attempted to improve the mobility of InAlN/AlGaN two-dimensional electron gas (2DEG) heterostructures by achieving an atomically smooth heterointerface in metalorganic chemical vapor deposition processes. In the result, it was confirmed that the high-growth-rate AlGaN layer was very effective to improve the surface morphology. The atomically smooth surface morphology with a root-mean-square roughness of 0.26 nm was achieved for an Al0.15Ga0.85N layer under the growth rate of approximately 6 µm/h. Furthermore, nearly lattice-matched In0.17Al0.83N/Al0.15Ga0.85N 2DEG heterostructures with the atomically smooth heterointerface exhibited a 2DEG mobility of 242 cm2 V−1 s−1 with a 2DEG density of 2.6 × 1013/cm2, which was approximately 1.5 times larger than the mobility in a sample grown under original conditions.

High-k/GaN interface engineering toward AlGaN/GaN MIS-HEMT with improved Vth stability

Metal–insulator–semiconductor (MIS) capacitor structures were fabricated on AlGaN/GaN two-dimensional electron gas heterostructure material in order to investigate important aspects of the gate module of a corresponding MIS-high electron mobility transistor device. The process sequence started with an initial wet chemical surface treatment of the as-grown semiconductor material followed by an atomic layer deposition of Al2O3 (high-k first). The electrical analysis focused on the gate leakage current as well as on the shift of the threshold voltage (Vth) upon bias stress in the off- and the on-state regions. The high-k first samples showed much better Vth stability compared to lithographically processed samples, in which the high-k deposition was performed after ohmic contact formation and just before the gate electrode metallization. These results reflect a superior quality of the high-k/GaN interface for the processed structures according to the high-k first approach.

Nearly lattice-matched InAlN/AlGaN two-dimensional electron gas heterostructures grown by metalorganic chemical vapor deposition

Nearly lattice-matched InAlN/AlxGa1−xN (x = 0.1, 0.21, and 0.34) heterostructures with a 1-nm-thick AlN interfacial layer were grown on AlN/sapphire templates by metalorganic chemical vapor deposition. Capacitance–voltage and Hall effect measurements revealed that two-dimensional electron gases (2DEGs) with high densities exceeding 2 × 1013/cm2 were generated at the heterointerface for all samples. It was confirmed that the generation of high-density 2DEGs can be explained as being due to internal polarization effects. The sheet resistance increased from 1,267 to 1,919 Ω/sq with the increase in Al content in the AlGaN channel, owing to the decreases in 2DEG density and mobility.

Impact of AlN Spacer on Metal–Semiconductor–Metal Pt–InAlGaN/GaN Heterostructures for Ultraviolet Detection

We report on metal–semiconductor–metal (MSM) photodetectors (PDs) fabricated on InAlGaN/GaN two-dimensional electron gas (2DEG) heterostructures. Electrical and photodetection properties were compared in two structures with and without an AlN spacer between the barrier (InAlGaN) and the GaN. The presence of the spacer hugely reduces the leakage current, allowing biasing at higher voltages. In photodetection, gain is obtained in both structures at a high bias. The photocurrent transient behavior revealed a faster response for excitation energy close to the GaN band edge than for energy above the barrier band edge. The fabrication and improvement of this type of device can lead to integration with the already mature high-electron-mobility transistor (HEMT) technology.

High speed photodetectors based on a two-dimensional electron/hole gas heterostructure

We report on high-speed metal-semiconductor-metal (MSM) resonant cavity enhanced photodetectors based on Schottky-contacted (Al,In)GaAs heterostructures containing both electron and hole quantum wells. Interdigitated detectors were fabricated and characterized with and without an underlying Distributed Bragg Reflector (DBR). All detectors had very low dark currents and high linear responsivities. The fastest measured temporal response with a 16 ps full-width at half-maximum and a 29 ps fall time was demonstrated on a device with 1 μm gap between electrodes and an underlying DBR. Single quantum well detectors have previously demonstrated increased responsivity and speed but were limited by a slow decaying tail in the high speed photoresponse, attributed to the long collection path of minority carriers. The use of an electron and hole well, separated by a 110 nm absorption region as well as an underlying DBR, eliminates the slow tail by providing an enhanced collection path for both optically generated electrons and holes. Here, we present the fabricated device structure along with the DC and high speed photoresponse under varying incident powers. We briefly compare these results to those of the previous single well devices and attribute improvements in the time response tail to enhanced diffusion created by the presence of the separated dual well structure.

Sensing domain wall pinning in the longitudinal magnetoresistance of a two-dimensional electron gas

We investigate the sensing of domain wall pinning in thin Co wires positioned on top of a two-dimensional electron gas (2DEG) heterostructure by measuring the longitudinal resistance of the 2DEG as the magnetic field is swept, in an analogy to the Barkhausen effect. For comparison, we also measure the magnetoresistance of the ferromagnetic film in the same device in a subsequent sweep. Compared to the Hall measurements, the longitudinal measurement has the advantage of sensing magnetic activity over longer lengths, while compared to the measurement of the magnetoresistance in the ferromagnetic wire, it offers complementary information related to the pinning and unpinning of the domain wall, due to its sensitivity only to the out-of-plane magnetic field component.

Monte Carlo studies of the intrinsic time-domain response of nanoscale three-branch junctions

We present a Monte Carlo time-domain study of nanostructured ballistic three-branch junctions (TBJs) excited by both step-function and Gaussian picosecond transients. Our TBJs were based on InGaAs 2-dimensional electron gas heterostructures and their geometry followed exactly the earlier experimental studies. Time-resolved, picosecond transients of both the central branch potential and the between-the-arms current demonstrate that the bandwidth of the intrinsic TBJ response reaches the THz frequency range, being mainly limited by the large-signal, intervalley scattering, when the carrier transport regime changes from ballistic to diffusive.

Hybrid AlGaN/GaN-ZnO-Nanowire Gas Sensors

The potential of AIGaN/GaN heterostructures integrated with zinc oxide (ZnO) nanowires for gas sensing applications is demonstrated. Single crystal ZnO nanowires, serving as sensing probes, were selectively grown between two ohmic electrodes of AIGaN/GaN two dimensional electron gas heterostructures by thermal oxidation of sputtered zinc films in air. Electron diffraction and transmission electron microscopy showed the ZnO-nanowires to be crystalline structures oriented in the [001] direction. The fabricated structures were used to detect ethanol, acetone and methanol in a nitrogen background. The results indicate that the hybrid AIGaN/GaN-ZnO nanowires gas sensors are operable over a broad range of temperatures and could potentially be integrated with devices for wireless environmental monitoring.

Mobility limiting scattering mechanisms in nitride-based two-dimensional heterostructures with the InGaN channel

The scattering mechanisms limiting the carrier mobility in AlInN/AlN/InGaN/GaN two-dimensional electron gas (2DEG) heterostructures were investigated and compared with devices without InGaN channel. Although it is expected that InGaN will lead to relatively higher electron mobilities than GaN, Hall mobilities were measured to be much lower for samples with InGaN channels as compared to GaN. To investigate these observations the major scattering processes including acoustic and optical phonons, ionized impurity, interface roughness, dislocation and alloy disorder were applied to the temperature-dependent mobility data. It was found that scattering due mainly to interface roughness limits the electron mobility at low and intermediate temperatures for samples having InGaN channels. The room temperature electron mobilities which were determined by a combination of both optical phonon and interface roughness scattering were measured between 630 and 910 cm2 (V s)−1 with corresponding sheet carrier densities of 2.3–1.3 × 1013 cm−2. On the other hand, electron mobilities were mainly limited by intrinsic scattering processes such as acoustic and optical phonons over the whole temperature range for Al0.82In0.18N/AlN/GaN and Al0.3Ga0.7N/AlN/GaN heterostructures where the room temperature electron mobilities were found to be 1630 and 1573 cm2 (V s)−1 with corresponding sheet carrier densities of 1.3 and 1.1 × 1013 cm−2, respectively. By these analyses, it could be concluded that the interfaces of HEMT structures with the InGaN channel layer are not as good as that of a conventional GaN channel where either AlGaN or AlInN barriers are used. It could also be pointed out that as the In content in the AlInN barrier layer increases the interface becomes smoother resulted in higher electron mobility.

AlGaN/GaN two-dimensional electron gas Schottky barrier photodiodes with multiple MgxNy/GaN layers

In this study, AlGaN/GaN two-dimensional electron gas (2DEG) heterostructures were grown by metal organic chemical vapor deposition (MOCVD). It was found that we could reduce reverse leakage current and provide high-voltage operation by introducing multiple MgxNy/GaN layers into the conventional Schottky barrier photodiodes (SBPD). An atomic force microscopy (AFM) scan image showed that surface pits of TD terminations were hardly observed as the multiple MgxNy/GaN layers were grown before subsequently depositing a high-temperature (HT) AlGaN/GaN epitaxial layer. A larger Schottky barrier height (ΦB) and smaller ideality factor (n) extracted from the current–voltage (I–V) curve for SBPD with multiple MgxNy/GaN layers also suggested that better crystal quality and rectifying properties were achieved. The larger value of ΦB might be explained by the reduction of the TD density and interface state (IS) density.

Three dimensional potential distribution and quantized acoustoelectric current for a Al0.3Ga0.7As/GaAs two-dimensional electron gas heterostructure

We present the numerical results for the electrostatic potential distribution in AlGaAs/GaAs heterostructure with double split gates on the surface. The results are obtained from the self-consistent solution of the three dimensional Schrödinger–Poisson equation. The dependence of the potential on the applied gate voltages is discussed in detail. We pay special attention to the potential distribution along the electron transport direction in quasi-one-dimensional channel. The potential barrier heights calculated in the closed-channel-regime agree well with our experiment. The calculations show that the potential barrier height as a function of gate voltage differs strongly in the open-channel regime and the closed-channel regime. On the other hand, we calculate the quantized acoustic current by using the potential barrier obtained from self-consistent solution rather than using the simple analytical model. The results show that the quantized plateau accuracy is about 10−5–10−4 within the minimum slope of the current plateau.

The effect of AlN interlayer thicknesses on scattering processes in lattice-matched AlInN/GaN two-dimensional electron gas heterostructures

The scattering mechanisms governing the transport properties of high mobility AlInN/AlN/GaN two-dimensional electron gas (2DEG) heterostructures with various AIN spacer layer thicknesses from zero to 2 nm were presented. The major scattering processes including acoustic and optical phonons, ionized impurity, interface roughness, dislocation and alloy disorder were applied to the temperature-dependent mobility data. It was found that scattering due mainly to alloy disorder limits the electron mobility for samples having spacer layer thicknesses up to 0.3 nm. On the other hand, alloy scattering is greatly reduced as the AlN spacer layer thickness increases further, and hence the combination of acoustic, optical and interface roughness become operative with different degrees of effectiveness over different temperature ranges. The room-temperature electron mobility was observed to increase gradually as the AlN spacer layer increases. A peak electron mobility of 1630 cm2 V−1 s−1 was realized for the sample consisting of a 1 nm AlN spacer layer. Then, the electron mobility decreased for the sample with 2 nm AlN. Moreover, the measured 2DEG densities were also compared with the theoretical predictions, which include both piezoelectric and spontaneous polarization components existing at AlN/GaN interfaces. The experimental sheet carrier densities for all AlInN/AlN/GaN HEMT structures were found to be in excellent agreement with the theoretical predictions when the parasitic (unintentional) GaN layer deposited between AlN and AlInN was taken into account. From these analyses, 1 nm AlN spacer layer thickness is found to be the optimum thickness required for high electron mobility and hence low sheet resistance once the sheet carrier density is increased to the theoretically expected value for the sample without unintentional GaN layer.

Comparison of the transport properties of high quality AlGaN/AlN/GaN and AlInN/AlN/GaN two-dimensional electron gas heterostructures

The transport properties of high mobility AlGaN/AlN/GaN and high sheet electron density AlInN/AlN/GaN two-dimensional electron gas (2DEG) heterostructures were studied. The samples were grown by metal-organic chemical vapor deposition on c-plane sapphire substrates. The room temperature electron mobility was measured as 1700 cm2/V s along with 8.44×1012 cm−2 electron density, which resulted in a two-dimensional sheet resistance of 435 Ω/◻ for the Al0.2Ga0.8N/AlN/GaN heterostructure. The sample designed with an Al0.88In0.12N barrier exhibited very high sheet electron density of 4.23×1013 cm−2 with a corresponding electron mobility of 812 cm2/V s at room temperature. A record two-dimensional sheet resistance of 182 Ω/◻ was obtained in the respective sample. In order to understand the observed transport properties, various scattering mechanisms such as acoustic and optical phonons, interface roughness, and alloy disordering were included in the theoretical model that was applied to the temperature dependent mobility data. It was found that the interface roughness scattering in turn reduces the room temperature mobility of the Al0.88In0.12N/AlN/GaN heterostructure. The observed high 2DEG density was attributed to the larger polarization fields that exist in the sample with an Al0.88In0.12N barrier layer. From these analyses, it can be argued that the AlInN/AlN/GaN high electron mobility transistors (HEMTs), after further optimization of the growth and design parameters, could show better transistor performance compared to AlGaN/AlN/GaN based HEMTs.

Nuclear magnetic resonance probes for the Kondo scenario for the 0.7 feature insemiconductor quantum point contact devices

We discuss the expected features in nuclear relaxation and Knight shift measurements for the Kondo scenariofor the ‘0.7 feature’ of semiconductor quantum point contact (QPC) devices defined in two-dimensionalelectron gases (2DEGs). As the conductance is more sensitive to the nuclear polarization inthe centre of the QPC than that in the 2DEG leads, our analysis is focused on the regionnear to the centre of the QPC. We show that the exchange coupling of a bound electron inthe QPC with the nuclei would lead, in the region near to the centre of the QPC, to amuch higher rate of nuclear relaxation compared to that involving exchange of nuclear spinwith conduction electrons. Away from the centre of the QPC, we find that thedistance beyond which the latter (conduction electron) mechanism becomes equallyimportant is of the order of typical QPC lengths; thus, between these two electronicmechanisms, relaxation by coupling to the bound electron dominates within theQPC. Furthermore, we show that the temperature dependence of the nuclearrelaxation due to coupling to the bound electron is non-monotonic in contrast to thelinear-in-T relaxation from coupling with conduction electrons. Nuclear spin diffusion processes restrictthe range of validity of this analysis. We present a qualitative analysis of additionalrelaxation due to nuclear spin diffusion (NSD), and compare the nuclear relaxation timesassociated with NSD and the above electronic mechanisms. We discuss circumstances inwhich NSD will affect our results significantly, and suggest ways in which NSD may besuppressed in the QPC so that the Kondo physics may be unearthed. Nuclearrelaxation together with Knight shift measurements will help in verifying whether the‘0.7’ feature is indeed due to the presence of a bound electron in the QPC. Whilesome of the results have also been discussed in the context of paramagneticimpurities in bulk conductors, our analysis is intended for application to the0.7 effect in semiconductor systems. The qualitative and quantitative estimatesthat we make will allow experimental tests of the Kondo scenario for the0.7 feature of QPCs in two-dimensional electron gas heterostructures.