Schottky Barrier Devices Research Articles

From GaN crystallinity to device performance: Nucleation mode vs Surface energy of single-crystalline AlN template

In this paper, the surface state of the single-crystalline AlN template's impact on the epitaxial growth of gallium nitride (GaN) was studied. Subsequently, Schottky barrier devices were fabricated and analyzed. The results indicate that surface states subjected to different treatments exhibit varying epitaxial growth modes at the initial nucleation stage, which have a correlated effect on surface topography, crystalline quality, strain and other material characteristics of the epitaxial GaN. Higher surface energy leads to a reduction in screw dislocation of GaN material, but has less influence on edge dislocation. Single-crystalline AlN templates with higher surface energy are more likely to exhibit Stranski-Krastanow and Frank-van der Merwe growth model and achieve high-quality epitaxial materials. Schottky devices prepared using GaN material with lower screw dislocation density exhibit lower reverse leakage current. First-principles simulation analysis revealed that the migration barrier of gallium and nitrogen atoms on the surface can be overcome with the larger surface energy of single-crystalline AlN template. This facilitates their migration on the surface, resulting in higher quality material epitaxy. The conclusions of this work provide insights for quality control of epitaxial GaN materials on single-crystalline AlN templates, as well as for studying device leakage in the backend of the device fabrication process, or similar homogeneous compound semiconductor heteroepitaxy studies.

New polymeric nanocomposite thin films for transparent optoelectronics: Impact of In(acac)3 on the structural, optical and photoelectrical characteristics of PVA films

Concerning the rapid progress in elastic and transparent optoelectronics, the need for cost-effective materials for transparent optoelectronic devices has become a mainstream research trend. Therefore, this report aimed to explore the modifications of the PVA characteristics due to In(acac)3 doping. New PVA:In(acac)3 nanocomposite thin films were fabricated using a cost-effective and facile spin-coating technique with varying the In(acac)3 loading concentration. The structural and molecular feature variations of PVA have been tracked using XRD and FTIR techniques, respectively. Furthermore, by varying In(acac)3′s concentration, the fabricated films' linear/nonlinear optical properties have been investigated symmetrically using the UV–visible-NIR spectrophotometric technique. The measured transmittance spectra of the prepared films showed the tunability of the energetic position of the fundamental edge while conserving the high transparency. The energy gap and Urbach energy of the prepared thin films have been calculated as a function of doping concentration. The nonlinear refractive index and optical susceptibilities were estimated for all the fabricated films. On the other hand, the photodetection response of the fabricated Schottky barrier devices was tested. The fabricated devices showed pronounced UV response, where the photocurrent density was significantly enhanced under UV illumination up to 0.1 mA/cm2 for the 20 wt.% In(acac)3 doped films. The photodetection performance of the manufactured nanocomposite thin films under repeatable On/Off switching cycles was tested. The optimum doping concertation of In(acac)3 was determined in terms of the analyzed optical and photodetection characteristics to be 20 wt.%. The obtained results support the careful reconsideration of the possibility of exploiting some cheap polymers as important components in advanced optoelectronic applications.

Non-zero crossing current–voltage characteristics of interface-type resistive switching devices

A number of memristive devices, mainly ReRAMs, have been reported to exhibit a unique non-zero crossing hysteresis attributed to the interplay of resistive and not yet fully understood “capacitive” and “inductive” effects. This work exploits a kinetic simulation model based on a stochastic cloud-in-a-cell method to capture these effects. The model, applied to Au/BiFeO3/Pt/Ti interface-type devices, incorporates vacancy transport and capacitive contributions. The resulting nonlinear response, characterized by hysteresis, is analyzed in detail, providing an in-depth physical understanding of the virtual effects. Capacitive effects are modeled across different layers, revealing their significant role in shaping the non-zero crossing hysteresis behavior. Results from kinetic simulations demonstrate the impact of frequency-dependent impedance on the non-zero crossing phenomenon. This model provides insight into the effects of various device material properties on the non-zero crossing point, such as Schottky barrier height, device area, and oxide layer.

Surface scattering-dependent electronic transport in ultrathin scandium nitride films

With the constant miniaturization of device technologies, it has become essential to understand and engineer the electronic properties of semiconductors in nanoscale dimensions. Scandium nitride (ScN), an emerging rock salt indirect bandgap semiconductor, has attracted significant interest for its interesting thermoelectric, plasmonic, neuromorphic computing, and Schottky barrier device applications. However, an in-depth understanding of the electronic transport, carrier scattering mechanism, and optical properties in ultrathin ScN films is still missing. Here, we show surface-scattering dominant electronic transport in epitaxial ScN films at nanoscale thicknesses. At the ultrathin dimensions, surface scattering increases significantly due to the large surface-to-volume ratio and growth-induced texturing. As a result, mobility decreases, and resistivity increases drastically with decreasing film thickness. Temperature-dependent electronic transport shows that the mobility of the ultrathin films decreases with increasing temperature due to the ionized-impurity and dislocation scattering. Electronic transport properties are further rationalized with x-ray diffraction and pole-figure analysis that shows that while the ultrathin films maintain their predominant 002 texture, their quality degrades with decreasing thickness. However, no significant changes are observed in the electronic structure of the films, as evidenced by x-ray photoelectron spectroscopy, photoemission measurements, and first-principles density functional theory calculations. Our results elucidate the impact of surface scattering on the ultrathin ScN films and would lead to miniaturized devices with improved efficiencies.

La/Ge stacked nanosheets designed as optical resonators, microwave oscillators and 5 G/6 G gigahertz receivers

Glass/Ge and La/Ge stacked layers, 100 nm thick, were prepared via thermal evaporation under a vacuum pressure of 10−5 mbar. Structural analysis confirmed amorphous growth of Ge nanosheets. Optically coating Ge onto La improved light absorption, energy band gap, and dielectric responses. In addition, dielectric spectra fitting using Lorentz approach showed increased free electron density, plasmon frequency, and drift mobility (up to 1153 cm2/Vs) in Ge due to lanthanum presence. Moreover, LGA Schottky barrier devices (La/Ge/Au) exhibited tunneling-type current conduction with 0.815 eV barrier height and 40 nm width. Resistance spectra of these Schottky barriers displayed negative resistance near 0.40 GHz, while capacitance spectra showed resonance-antiresonance behavior. La/Ge and LGA interface can be utilized for optical receivers and microwave resonators in 5 G/6 G technologies.

Ni/Ga2O3 based Schottky diode temperature sensor

Temperature sensors are the integrated part of electronics used in most of the applications. Present work demonstrates the temperature sensor based on ultra-wide bandgap semiconducting material, i.e.,Ga2O3. The material is known for its wider bandgap and radiation hardness properties. Ni/Ga2O3 based Schottky barrier device is simulated for sensing the temperature in the current range of 100 µA to 0.1 µA and in temperature range from 200 K to 600 K using SILVACO Atlas software. Relevant models such as bandgap narrowing, Schokely-Read-Hall, etc. were employed to ensure accuracy in the results. The simulated current–voltage (I-V) plots showed exponential variation and followed ideal thermionic emission model. The I-V plots were used to find the thermal sensitivity of device under considerations, which showed linearity in the measured current range. The maximum sensitivity is estimated as 3.22 mV/K at measuring current level of 0.1 µA.

Interface effects of Schottky devices built from MoS2 and high work function metals

Schottky junctions, formed by high work function metals and semiconductors, are important devices in electronics and optoelectronics. The metal deposition in traditional Schottky interfaces usually damages the semiconductor surface and causes defect states, which reduces the Schottky barrier height and device performance. This can be avoided in the atomically smooth interface formed by two-dimensional (2D) metals and semiconductors. For better interface tailoring engineering, it is particularly important to understand various interface effects in such 2D Schottky devices under critical or boundary conditions. Here we report the fabrication and testing of three types of MoS2 devices, i.e., using PtTe2, Cr and Au as contact materials. While the Cr/MoS2 contact is an ohmic contact, the other two are Schottky contacts. The van-der-Waals interface of PtTe2–MoS2 results in a well-defined OFF state and a significant rectification ratio of 104. This parameter, together with an ideality factor 2.1, outperforms the device based on evaporated Au. Moreover, a device in the intermediate condition is also presented. An abrupt increase in the reverse current is observed and understood based on the enhanced tunneling current. Our work manifests the essential role of doping concentration and provides another example for 2D Schottky interface design.

Temperature-dependent performance of Schottky-Barrier FET ultra-low-power diode

In this paper, for the first time, we apply the ultra-low-power (ULP) diode concept with Schottky Barrier (SB) transistors and analyze their performance in comparison to standard CMOS, using calibrated TCAD mixed-mode simulations. The negative impedance characteristics obtained in reverse mode with SB devices are shown to offer more stable current characteristics compared to CMOS, especially as a function of temperature. The origin of this behavior manifests itself in the fact that carriers tunneling through the barrier by field emission and carriers overcoming the barrier by thermionic emission both contribute to the total device current. This enables superior current performance over temperature. This enables ultra-low-power memory application over a larger temperature range, or with a denser cell area.

Ohmic-to-Schottky Conversion in Monolayer Tellurene–Metal Interface via Graphene Insertion

Tellurene, a relatively new addition to the two-dimensional (2D) material family, has shown promising prospect in future nanoelectronics. Atomistic understanding of the electronic properties of 2D material-metal interfaces is crucial to promote optimal device performance. Monolayer tellurene exhibits an unusual Ohmic nature when interfaced with any metal surfaces, restricting its applications in vertical Schottky barrier devices. Using density functional theory (DFT), here, we describe a technique of adding a buffer layer to prevent the formation of Ohmic contact. Using six different metals (Ag, Au, Pt, Pd, Ru, and Ti), we show that the insertion of a graphene layer between tellurene and a metal surface can screen the metallization of tellurene and create Schottky barriers. The Schottky barrier heights (SBHs) can be now modulated using metal electrodes with different work functions (WFs) because the graphene layer partially depins the Fermi level. Our study provides quantum-chemical insights into the realization of vertical Schottky diodes using monolayer tellurene that could be a key component in future high-frequency nanoelectronic devices. © 2021 American Chemical Society.

Analysis of Electrical and Capacitance–Voltage of PVA/nSi

The present study examined polyvinyl alcohol, PVA, in its pure form without any additives. We investigated its application as a Schottky barrier device, which demonstrates the novelty of this research. We report the synthesis, current–voltage (I–V) and capacitance–voltage (C–V) characteristics, dielectric properties, x-ray diffraction of PVA/nSi. Several parameters, including the ideality factor (n), series resistance (Rs), barrier height (Φb), the saturation current (I0), and shunt resistance (Rsh) were estimated. It was found that the n values decrease with increasing temperature, setting the enhancement of the device features. (Φb) values tend to increase with temperature rise. Frequency, voltage and temperature dependence of electric modulus (( $$ M^{\prime}\& M^{\prime\prime} $$ ) was investigated, showing that $$ \left( {M^{\prime}} \right) $$ decreases with temperature (T) while the values of $$ (M^{'} $$ ) are increased with frequency (f). But at low frequencies $$ (M^{'} ) \,{\text{rises }}\,{\text{when }}\, T\, {\text{is }}\,{\text{raised}}. \,{\text{While }} $$ ( $$ M^{\prime\prime}) $$ tends to be reduced with increasing T at (f) = 10 and 103 Hz, with increases at (f) = 2 × 107 Hz. We observed semicircles in the Cole–Cole plots where the radii of curvature increased with temperature and voltage. I–V characteristic at different temperatures (298–423 K)

The effect of temperature and time on the properties of 2D Cs2ZnBr4 perovskite nanocrystals and their application in a Schottky barrier device

2D hybrid perovskites are promising materials for solar cell applications, in particular, cesium-based perovskite nanocrystals as they offer the stability that is absent in organic–inorganic perovskites.

A Current–Voltage Model for Double Schottky Barrier Devices

AbstractSchottky barriers (SBs) are often formed at the semiconductor/metal contacts and affect the electrical behavior of semiconductor devices. In particular, SBs are playing a major role in the investigation of the electrical properties of mono and 2D nanostructured materials, although their impact on the current–voltage characteristics is frequently neglected or misunderstood. In this work, a single equation is proposed to describe the current–voltage characteristics of two‐terminal semiconductor devices with Schottky contacts. The equation is applied to numerically simulate the electrical behavior for both ideal and nonideal SBs. The proposed model can be used to directly estimate the SB height and the ideality factor. It is applied to perfectly reproduce the experimental current–voltage characteristics of ultrathin molybdenum disulfide or tungsten diselenide nanosheets and tungsten disulfide nanotubes. The model constitutes a useful tool for the analysis and the extraction of relevant transport parameters in any two‐terminal device with Schottky contacts.

Advances in High-Resolution Radiation Detection Using 4H-SiC Epitaxial Layer Devices.

Advances towards achieving the goal of miniature 4H-SiC based radiation detectors for harsh environment application have been studied extensively and reviewed in this article. The miniaturized devices were developed at the University of South Carolina (UofSC) on 8 × 8 mm 4H-SiC epitaxial layer wafers with an active area of ≈11 mm2. The thicknesses of the actual epitaxial layers were either 20 or 50 µm. The article reviews the investigation of defect levels in 4H-SiC epilayers and radiation detection properties of Schottky barrier devices (SBDs) fabricated in our laboratories at UofSC. Our studies led to the development of miniature SBDs with superior quality radiation detectors with highest reported energy resolution for alpha particles. The primary findings of this article shed light on defect identification in 4H-SiC epilayers and their correlation with the radiation detection properties.

Semi-transparent Schottky junction solar cell based on evaporated CdSe thin films: Influence of post-deposition air-annealing

FTO/CdSe/Al semi-transparent Schottky junction solar cells were fabricated by evaporating 300 nm CdSe thin films onto the FTO electrode and forming the Schottky junction by evaporation the aluminum onto the absorber layer. The performance of the samples was optimized by air-annealing the absorber layer prior to the evaporating of the top electrode. Also, the structural, optical and electrical properties of thermally evaporated CdSe thin films were investigated as a function of post-deposition air-annealing temperature. The surface morphology of the specimens was studied using field emission scanning electron microscope (FE-SEM) analysis. X-ray diffraction (XRD) analysis revealed the hexagonal structure of the samples. The crystallite size, micro-strain and dislocation density of CdSe thin films were evaluated using XRD patterns. The transmittance spectra in the wavelength range of 400−2500 nm were measured and then used to study other optical parameters. The optical energy band gap was decreased with air-annealing from 2.17 eV to 1.69 eV. It is observed that complex refractive index, relative density, complex dielectric constant and electrical polarizability of specimens were increased by air-annealing. Electrical conductivity and the photovoltaic efficiency of FTO/CdSe/Al Schottky barrier devices were improved by thermal annealing and annealed sample at 300 °C shows the best performance (η = 1.53 %). Obtained results show that CdSe thin films are a promising candidate to use as an absorber layer in semi-transparent solar cells and colorful photovoltaic windows.

Comparative study of GaN betavoltaic battery based on p-n junction and Schottky barrier diode

This paper presents a theoretical calculation model of GaN betavoltaic battery with the N63i radiation source. The output parameters of the GaN p-n junction battery and the GaN Schottky barrier battery are calculated based on Monte Carlo simulations. The calculation results show that when the thickness of N63i source is 2 μm, the GaN-N63i batteries nearly achieve optimized performance. For the GaN-based p-n junction battery, the maximum output power density, filling factor, energy conversion efficiency and total energy conversion efficiency are 0.359 μW/cm2, 94.6%, 23.2% and 6.96%, respectively. For the GaN-based Schottky barrier battery, the output performance is related to the selection of the Schottky metal. Among the metals of Ag, Ti, Au, Pd, Ni and Pt for the GaN Schottky barrier diodes, the Au-GaN Schottky barrier device is typically fabricated. The corresponding output parameters of the Au-GaN Schottky barrier battery are 0.223 μW/cm2, 92.2%, 14.4% and 4.33%, respectively.

Submicron Size Schottky Junctions on As-Grown Monolayer Epitaxial Graphene on Ge(100): A Low-Invasive Scanned-Probe-Based Study.

We report on the investigation of the Schottky barrier (SB) formed at the junction between a metal-free graphene monolayer and Ge semiconductor substratein the as-grown epitaxial graphene/Ge(100) system. In order to preserve the heterojunction properties, we defined submicron size graphene/Ge junctions using the scanning probe microscopy lithography in the local oxidation configuration, a low-invasive processing approach capable of inducing spatially controlled electrical separations among tiny graphene regions. Characteristic junction parameters were estimated from I-V curves obtained using conductive-atomic force microscopy. The current-voltage characteristics showed a p-type Schottky contact behavior, ascribed to the n-type to p-type conversion of the entire Ge substrate due to the formation of a large density of acceptor defects during the graphene growth process. We estimated, for the first time, the energy barrier height in the as-grown graphene/Ge Schottky junction (φB ≈ 0.45 eV) indicating an n-type doping of the graphene layer with a Fermi level ≈ 0.15 eV above the Dirac point. The SB devices showed ideality factor values around 1.5 pointing to the high quality of the heterojunctions.

Increasing the efficiency of graphene-based Schottky-barrier devices

Graphene’s high carrier mobility and ambipolar nature has the potential to improve electronic devices. The absence of a band-gap necessitates heterostructure devices. Schottky-barrier devices consisting of an interface between graphene and a semiconductor represent the simplest heterostructure. Despite its simplicity, graphene-based Schottky barrier devices are not well understood and exhibit low injection efficiencies. We here investigate the impact of graphene/metal interaction on the properties of the Schottky-barrier. Besides the commonly employed Au/graphene we use Pt/graphene contacts. We find that the injection efficiency for Pt is 5x higher than for Au and systematically study the origin of this behavior. We identify a large difference in Richardson’s constant due to changes in the density of surface states. The demonstrated ability to increase the injection current was applied to improve the efficiency of graphene-based Schottky solar cells by 13x.

Influence of Ta2O5 Interfacial Oxide Layer Thickness on Electronic Parameters of Al/Ta2O5/p-Si/Al Heterostructure

We describe the impact of Ta2O5 interfacial oxide layer thickness (ranging from 100-350 nm) on electrical and structural properties of Al/Ta2O5/p-Si/Al Metal-Insulator-Semiconductor (MIS) Schottky barrier diodes using RF magnetron sputtering. We studied the Schottky barrier device parameters such as ideality factor, barrier height and series resistance and are evaluated from current-voltage (I-V) measurements. The barrier height and ideality factor values are significantly varying with Ta2O5 oxide layer thickness and found to be 0.58 eV, 2.35, 0.71 eV, 2.10 and 0.78 eV, 1.87 for 20, 40 and 60 nm, respectively. It was noticed that the calculated barrier height and ideality values for this prepared Al/Ta2O5/p-Si/Al MIS Schottky barrier diode were greatly improved than those conventional metal-semiconductor (MS) Schottky diodes. The XRD studies revealed that the 100-nm thickness film exhibited poor crystallinity whereas 200 and 350 nm thickness films showed improved crystallinity with orthorhombic phase of β-Ta2O5. The presence of this orthorhombic phase of β-Ta2O5 is confirmed with FTIR studies. To explore the structural transformations in Ta2O5 films with varying thicknesses, Raman spectroscopy was utilized. In addition, the improvement in Schottky diode parameters was correlated with the enhanced crystallinity noticed in XRD studies.

Graphene–Perovskite Schottky Barrier Solar Cells

AbstractPerovskite solar cells have attained incredible power conversion efficiencies but it is still unclear whether photogenerated carriers are free or excitonic in nature. Originally, it is believed that they are exciton‐based devices, similar to organic or dye‐sensitized solar cells. However, the emergence of efficient planar devices as well as measurements of exciton binding energy in the range of 10–100 meV suggest that they may be free carrier‐based. In this work, the free carrier model is confirmed by building graphene/perovskite Schottky barrier solar cells, analogous to conventional metal/semiconductor Schottky barrier solar cells. To address the challenges of building such devices, solution‐processing techniques are extensively investigated for depositing perovskite films directly onto graphene in order to obtain an intimate contact between the graphene and perovskite. Interestingly, these graphene/perovskite Schottky barrier devices have reasonably good efficiency—up to 10.6%—and short circuit current densities only slightly lower than control devices. Furthermore, devices with neither a hole transport layer nor an electron transport layers have power conversion efficiencies of up to 6%. These results provide convincing evidence supporting the free carrier model for methylammonium lead iodide perovskites and offer insights on potential alternative designs for perovskite solar cells.

Analysis of loss mechanisms in Ag2ZnSnSe4 Schottky barrier photovoltaics

Recently, a new type of photovoltaic device based on an n-type Ag2ZnSnSe4 (AZTSe) absorber was demonstrated with an efficiency of over 5%. This work examines in detail several critical loss mechanisms in FTO/AZTSe/MoO3/ITO Schottky barrier devices. It is shown that the Schottky barrier height in the present devices under illumination is only ∼0.82 eV. With a barrier height of this magnitude, the device has an upper-bound of only 6.4% to the efficiency. Second, the AZTSe thin films used in the present devices exhibit exceptionally short minority carrier lifetimes (≤225 ps), as probed using two-photon spectroscopy. Third, hysteresis is observed in the J–V response of these devices. Pulsed electrical measurements reveal that reverse-bias voltage pulses create residual shunts in the device proportional to the pulse amplitude and duration. Reversible Ag-doping of the MoO3 buffer is proposed as an explanation of the observed shunting upon reverse bias. Therefore, improving the contact materials and the minority carrier lifetime is identified to be critically important for improving AZTSe-based photovoltaics. Finally, AZTSe is found to be photo-conductive with a Hall carrier density that increases by two orders of magnitude upon illumination. The origin of this effect is still unknown.