Metal Oxide Semiconductor Diodes Research Articles

Reduction in Gap State Density near Valence Band Edge at Al2O3/p‐type GaN Interface by Photoelectrochemical Etching and Subsequent SiO2 Cap Annealing

The process‐dependent properties of Al2O3/p‐type GaN (p‐GaN) interfaces formed by atomic layer deposition at 300 °C after photoelectrochemical (PEC) etching are reported. For investigating the gap states at the Al2O3/p‐GaN interface, metal‐oxide‐semiconductor (MOS) diodes are fabricated and examined by sub‐bandgap‐light‐assisted and temperature‐dependent capacitance–voltage (C–V) measurements. PEC etching prior to Al2O3/p‐GaN interface formation is conducted with the etching depth varied in the range between 12.5 and 32.1 nm. The C–V characteristics of the MOS diodes without PEC etching indicate Fermi‐level pinning due to the near‐surface defect level in p‐GaN at 0.7 eV above the valence band edge EV and a high density of gap states around the midgap. However, all samples with PEC etching exhibit C–V characteristics, indicating a reduction in the density of the defect states at EV + 0.7 eV and midgap states. Still, PEC etching after capless annealing at 800 °C for the activation of Mg acceptors cannot reduce the density of gap states near the valence band edge. On the other hand, annealing of a sample with a SiO2 cap layer at 800 °C after PEC etching can reduce the gap state density near the valence band edge.

Interface state density distribution near conduction band edge at Al2O3/Mg-ion-implanted GaN interface formed after activation annealing using AlN cap layer

An interface state density (Dit) distribution near the conduction band edge (EC) at the Al2O3/Mg-ion-implanted GaN interface was measured after ion implantation, annealing with an AlN protective cap, and cap layer removal. Mg ions were implanted into n-GaN with a Si concentration of 6 × 1017 cm−3 at a maximum Mg concentration of 2 × 1017 cm−3, resulting in the maintenance of the n-type conduction in GaN even after the activation of Mg dopants. Activation annealing was carried out at 1250 °C for 1 min using an AlN cap layer. The complete removal of the AlN cap layer was accomplished by wet etching, which was confirmed by x-ray photoelectron spectroscopy. The photoluminescence spectrum showed donor–acceptor-pair emission after annealing, indicating the activation of Mg acceptors. By applying the capacitance–voltage method to a completed metal–oxide–semiconductor diode, we derived a continuous distribution of relatively low Dit below 5 × 1012 cm−2 eV−1, which increased monotonically toward EC in the range from EC − 0.15 to EC − 0.45 eV. Compared with the Dit distribution of the as-implanted sample, the density of the discrete level at EC − 0.25 eV generated by divacancies markedly decreased upon 1250 °C annealing.

Bowtie Nanoantenna Coupled Metal-Oxide-Silicon (p-Doped) Diode for 28.3 THz IR Rectification.

Low-temperature waste heat in the infrared (IR) wavelength region offers an opportunity to harvest power from waste energy and requires further investigation in order to find efficient conversion techniques. Although grating-coupled metal-oxide-semiconductor (MOS) diode devices offer efficient conversion from low and moderate-temperature thermal sources, the integration of such diodes with a nanoantenna structure has yet to be explored. We propose a bowtie nanoantenna coupled with a p-doped MOS diode for IR to direct current (DC) conversion without any bias voltage at 28.3 THz. The nanoantenna was designed and optimized to provide maximum field enhancement in a 4 nm-thick oxide layer at the resonant frequency. The device was fabricated following the complementary MOS (CMOS) fabrication process and measured in a custom DC and optical characterization setup using a 10.6 μm wavelength CO2 laser. The results reveal two different types of devices with linear and nonlinear I-V curves having kΩ and MΩ zero-bias resistance, respectively. The linear device generates a micron-level open-circuit voltage (Voc) with clear polarization dependence from the laser input, but the nonlinear case suffers from a weak noise-like signal. Finally, we analyze two types of devices using thermoelectric and tunneling effects and discuss the future direction of nanoantenna-integrated MOS devices for efficient IR harvesters.

Post-annealing treatment in improving high dielectric constant MgO-based metal-oxide-semiconductor diodes

Metal-oxide-semiconductor (MOS) diodes with a high dielectric constant magnesium-oxide (MgO) insulating layer were fabricated using a magnetron radio frequency sputtering system. MgO has a high dielectric constant of approximately 11.2, which is three times higher than the dielectric constant (3.9) of silicon dioxide (SiO2), thereby ensuring a three times thicker gate oxide and reducing gate leakage current while maintaining the same capacitance density. Post-annealing treatment was employed on the MgO film to study how annealing treatment affects the electrical characteristics of MOS diodes. It was observed that the post-annealing treatment of MgO effectively diminished the gate leakage current by approximately one order, thereby increasing the rectification ratio from 8.5 × 103 to 6.8 × 104 for the MOS diodes with as-deposited and post-annealed MgO. In contrast to the MOS diodes with as-deposited MgO, the post-annealing treatment of MgO significantly decreased the flatband voltage shift from 7.8 to 1.3 V and reduced the fixed oxide charge density from 1.3 × 1012 to 2.3 × 1011 cm−2; also, the interface trap charge density was suppressed from 1.8 × 1013 to 3.2 × 1012 cm−2 eV−1. Large quantities of fixed oxide charge attracted more electrons accumulated at the Si surface, which decreased the barrier height from 0.85 to 0.81 eV for the MOS diodes with post-annealed and as-deposited MgO. Moreover, x-ray photoelectron spectroscopy showed that the oxide charges were caused by the defects inside MgO, particularly oxygen vacancies. The oxygen vacancies were compensated by the oxygen atoms introduced from the air during the post-annealing treatment.

Electroluminescence in metal-oxide-semiconductor tunnel diodes with a crystalline silicon/silicon dioxide quantum well

We investigated electroluminescence (EL) in metal-oxide-semiconductor (MOS) diodes with an indium tin oxide (ITO) film/ultrathin silicon dioxide (SiO2) film/ultrathin crystalline-silicon (Si) layer/buried SiO2 layer/Si substrate structure at a negative voltage applied to the ITO film. The EL peak shifted toward higher energy with decreasing Si-layer thickness. The dependence of the EL-peak energy on the Si-layer thickness shows that the EL is due to one-dimensional quantum confinement. Characterization of the MOS diodes suggested that electrons tunneling through the ultrathin SiO2 film from the conduction band (CB) of the ITO film are injected into higher energy states in the CB of the ultrathin Si layer with a negative voltage applied to the ITO film, and the tunneling electrons radiatively recombine with holes accumulated near the Si/SiO2 interface in the ultrathin Si layer.

Formation of thermally grown SiO2/GaN interface

An attempt was made to form a thermally grown SiO2/GaN interface. A Si layer deposited on the c-plane GaN surface was oxidized in an O2 atmosphere to form a SiO2 layer. The formation of SiO2 with a bandgap of 8.6 eV was confirmed by x-ray photoelectron spectroscopy. Metal–oxide–semiconductor diodes were fabricated and tested to characterize the interface by electrical measurements. The capacitance–voltage (C–V) characteristics measured at 1 MHz showed that a longer oxidation time resulted in a steeper slope. However, it was unavoidable that a bump in a C–V curve appeared after a long oxidation time. The electron trap distributions derived from C–V curves exhibited a discrete-level trap at 0.7 eV from the conduction band edge. This discrete-level trap was an acceptor-like trap that can be assigned to a Ga vacancy. An insufficient oxidation led to a high leakage current owing to the asperities of the residual polycrystalline Si layer. Although the leakage current was improved by extending the oxidation time, an excessively long oxidation time resulted in a slight increase in the leakage current. We cannot deny the possibility of the diffusion of Ga atoms into SiO2 during oxidation. Moreover, the cross-sectional transmission electron microscopy and energy-dispersive x-ray spectroscopy of a sample formed with an excessively long oxidation time indicated the formation of a Ga oxide interlayer without a severe disorder. Most possibly, the formation of the Ga oxide interlayer by excess oxidation improved the interface properties.

Impact of gamma-ray irradiation on capacitance–voltage characteristics of Al2O3/GaN MOS diodes with and without post-metallization annealing

Atomic layer deposited Al2O3/GaN metal-oxide-semiconductor (MOS) diodes with and without post-metallization annealing (PMA) were irradiated with gamma-rays. Capacitance–voltage measurements were made before and after irradiation to investigate trap formation in Al2O3 films and interface states between Al2O3 and GaN. Negative flat-band voltage shifts were observed. The flat-band voltage shift depends on the Al2O3 thickness, showing different distributions of gamma-ray-induced positive charges for samples with and without PMA. The interface state density of the PMA sample slightly increased after irradiation, but was lower than that of the sample without PMA before irradiation.

New Room Temperature Ammonia Gas Sensor Synthesized by a Tantalum Pentoxide (Ta2O5) Dielectric and Catalytic Platinum (Pt) Metals

A new room temperature (25 °C) ammonia gas sensor based on a metal-oxide-semiconductor (MOS) diode is reported. The device structure is synthesized by a sputtered tantalum pentoxide (Ta2O5) dielectric, evaporated platinum nanoparticles (Pt NPs), and a Pt thin film on a GaN/AlGaN heterostructure. Pt NPs can effectively increase the specific surface area and related catalytic reactivity of Pt metal. In experiment, the studied Pt NP/Pt/Ta2O5/GaN/AlGaN MOS device shows good ammonia sensing properties including a high sensing response of 74.4 under 1000 ppm NH3/air gas and a sub-ppm (100 ppb) detecting level at room temperature. The studied MOS diode has the advantages of low power, low cost, and a widespread concentration range (0.1–1000 ppm NH3/air) for ammonia sensing operation. The ammonia sensing mechanism and a thermodynamic analysis to study the related interface coverage are included in this article. The studied MOS diode also exhibits advantages of good selectivity toward ammonia gas and a simple device structure.

Efficient methodology for estimation of metal effective work function, interface trap, and fixed oxide charges in metal-oxide-semiconductor capacitors with dual layer high-κ/SiO2 dielectric

A capacitance-voltage (C–V) based efficient methodology is demonstrated for precise estimation of process-induced various fixed charge distributions in the dielectric layer, interface trapped charges at the silicon/oxide interface, and the effective work function (EWF) of the metal gate on double layer high-κ/silicon dioxide (SiO2) stack in metal-oxide-semiconductor (MOS) capacitors. The present technique takes care of the variation of the work function of the silicon substrate either due to wafer-to-wafer variation or nonuniformity of the doping level throughout the entire wafer. The analysis is verified with experimentally obtained high-frequency C–V results by varying only the physical thickness tHfO2 of the hafnium oxide (HfO2) dielectric layer on an interfacial SiO2 film of a fixed thickness tox in tantalum nitride (TaN)/HfO2/SiO2/p-Si MOS diodes. A value of 4.5 eV was obtained for EWF of physical vapor deposited TaN on HfO2. Furthermore, the calculations indicate the presence of a significant amount of positive bulk charges in the high-κ layer, in addition to interface trapped positive charges Qit at the Si/SiO2 interface and negative effective charges at both the high-κ/SiO2 and Si/SiO2 interfaces.

Electrical and Structural Properties of All-Sputtered Al/SiO2/p-GaN MOS Schottky Diode

The all-sputtered Al/SiO2/p-GaN metal-oxide-semiconductor (MOS) Schottky diode was fabricated by the cost-effective radio-frequency sputtering technique with a cermet target at 400 °C. Using scanning electron microscope (SEM), the thicknesses of the electrodes, insulator SiO2 layer, and p-GaN were found to be ~250 nm, 70 nm, and 1 µm, respectively. By Hall measurement of a p-Mg-GaN film on an SiO2/Si (100) substrate at room temperature, the hole’s concentration (Np) and carrier mobility (μ) were found to be Np = 4.32 × 1016 cm−3 and μ = 7.52 cm2·V−1·s−1, respectively. The atomic force microscope (AFM) results showed that the surface topography of the p-GaN film had smoother, smaller grains with a root-mean-square (rms) roughness of 3.27 nm. By I–V measurements at room temperature (RT), the electrical properties of the diode had a leakage current of ~4.49 × 10−8 A at −1 V, a breakdown voltage of −6 V, a turn-on voltage of ~2.1 V, and a Schottky barrier height (SBH) of 0.67 eV. By C–V measurement at RT, with a frequency range of 100–1000 KHz, the concentration of the diode’s hole increased from 3.92 × 1016 cm−3 at 100 kHz to 5.36 × 1016 cm−3 at 1 MHz, while the Fermi level decreased slightly from 0.109 to 0.099 eV. The SBH of the diode at RT in the C–V test was higher than in the I–V test because of the induced charges by dielectric layer. In addition, the ideality factor (n) and series resistance (Rs) determined by Cheung’s and Norde’s methods, other parameters for MOS diodes were also calculated by C–V measurement at different frequencies.

Effects of Dosage Increase on Electrical Properties of Metal‐Oxide‐Semiconductor Diodes with Mg‐Ion‐Implanted GaN Before Activation Annealing

The effect of increasing the dosage on the electrical properties of Mg‐ion‐implanted GaN before activation annealing is investigated to obtain knowledge on the defect levels generated by ion implantation. To probe the near‐surface region, GaN metal‐oxide‐semiconductor (MOS) structures with Al2O3 are used. Two kinds of MOS diodes with Mg‐ion dosages of 1.5 × 1011 and 1.5 × 1012 cm−2 implanted at 50 keV are prepared. Although anomalous capacitance–voltage (C–V) characteristics are observed for the low‐dosage sample, they are improved by annealing at 600 °C for 3 h. However, for the high‐dosage sample, more severe and persistent frequency dispersion is observed in the C–V characteristics, which is not improved by the same annealing. On the basis of the detailed analysis of capacitance–frequency (C–f) characteristics, it is concluded that the discrete interface trap at 0.2–0.3 eV below the conduction band is responsible for the frequency dispersion observed for the high‐dosage sample. Combined with the results of deep‐level transient spectroscopy, it is highly likely that the bulk deep levels affect the C–V and C–f characteristics. The possibility that the dominant deep levels are changed by the increase in Mg‐ion dosage is discussed.

Hydrogen Sensing Characteristics of a Metal–Oxide–Semiconductor Diode With Bimetallic Catalysts and a GaO x Dielectric

A new metal–oxide–semiconductor (MOS) diode with bimetallic catalysts and a GaOx dielectric is employed herein to fabricate a hydrogen sensor. Bimetallic catalysts, including Pt nanoparticles (NPs) and a Pd thin film, are formed by the proper vacuum thermal evaporation (VTE) approach, and a GaOx dielectric is produced by H2O2 treatment on the GaN surface. The presence of this bimetallic structure can effectively increase the surface area-to-volume ratio and provide a “spill-over” effect. This can substantially enhance the dissociation and adsorption of hydrogen molecules and atoms. The use of a GaOx dielectric effectively suppresses the surface leakage current and increases the adsorption sites for hydrogen atoms. Experimentally, excellent hydrogen sensing properties, including a very high sensing response of $1.1 \times 10^{7}$ under 1% H2/air gas at 300 K, an extremely low detection level (≤100 ppb H2/air), a widespread hydrogen concentration sensing range, and a relatively fast sensing speed, were obtained. From a thermodynamic analysis, it is clear that the hydrogen adsorption of the studied device is an exothermic reaction. Therefore, based on the above-mentioned advantages, the studied Pt NP/Pd thin film/GaOx/GaN-based MOS diode shows promise for high-performance hydrogen sensing applications.

Hydrogen Sensor for High Temperature and Humid Environment Applications

There are great interest in hydrogen as an alternative energy source and a viable energy carrier. Compared to conventional fossil fuels, it is an emission-free fuel that can produce larger heat of enthalpy when reacted with oxygen. Hydrogen gas is extremely reactive with oxygen, and has very low ignition energy of 0.019 mJ. Especially, leaking gas from pressurized container may induce spontaneous flammable ignition due to its negative Joule-Thomson coefficient. Therefore, hydrogen gas sensing systems are essential in various hydrogen related applications. GaN based material system is well-suited to hydrogen sensing because of its wide bandgap for high temperature operation, and mechanical and chemical robustness for device reliability. Many types of devices based on the GaN, including Schottky diodes, metal oxide semiconductor diodes, GaN nanowires and AlGaN/GaN high electron mobility transistors have been developed for fast and sensitive detection of hydrogen. In this presentation, hydrogen sensing characteristics of highly sensitive GaN based hydrogen sensors with various crystal planes will be explained. Also, as a dramatic approach to improve the sensitivity of the device, active platinum nanostructure and photo-chemically etched GaN will be discussed. In addition, nano-porous membrane will be introduced for the reliable operation of hydrogen sensor in humid ambient.

Photovoltaic Energy Harvesting System Adapted for Different Environmental Operation Conditions: Analysis, Modeling, Simulation and Selection of Devices.

The present research work proposes a photovoltaic energy harvester and an appropriate direct current (DC)/DC converter for a harvesting system after the study of the devices and taking the operation conditions. Parameters such as power, efficiency and voltage are taken into account under different environment conditions of illumination and temperature in order to obtain the best possible response. For this reason, suitable metal-oxide semiconductor field-effect transistor (MOSFET), diode, coil, frequency, duty-cycle and load are selected and analyzed for a DC/DC converter with boost architecture.

Hydrogen sensing characteristics of a Pd/Nickel oxide/GaN-based Schottky diode

Hydrogen sensing characteristics of a novel metal-oxide-semiconductor (MOS) Schottky diode are thoroughly investigated. The MOS structure consists of a gallium nitride (GaN)-based semiconductor system, a nickel oxide (NiO) layer, and palladium (Pd) catalytic materials. A well-prepared Pd/NiO/GaN-based diode shows several advantages in relation to hydrogen sensing, including a simple structure, high sensing speed, wide flexibility for operation under both forward and reverse applied voltages, and a good sensing response of 8.1 × 103 under an applied forward voltage of 0.25 V, at 300 K in a 1% H2/air ambience. Furthermore, under an applied reverse voltage of −2 V and at a high temperature of 573 K, this MOS diode shows a response as high as 1.8 × 104 towards 1% H2/air mixture gas. The Schottky diode sensor with a novel Pd/NiO/GaN structure demonstrated in this study is a promising candidate for high-performance hydrogen sensing applications.

On the Ammonia Sensing Performance and Transmission Approach of a Platinum/Nickel Oxide/GaN-Based Metal-Oxide-Semiconductor Diode

New platinum (Pt)/nickel oxide (NiO)/GaN-based metal-oxide-semiconductor (MOS) diode-type ammonia sensor was fabricated and studied. In addition, a new grey polynomial differential recovery (GPDR) model was developed for the application of data transmission. The studied Pt/NiO/GaN-based MOS diode shows good ammonia sensing performance at relatively high temperatures ( $\ge 423$ K). A very high sensing response of 244.2 under 1000 ppm NH3/air gas and a low detecting level of 2 ppm NH3/air are obtained at 423 K. The studied device also shows operating flexibility in the applied forward and reverse voltages, and good reversibility in ammonia sensing. In order to expand the practical application of ammonia sensing, a GPDR model was developed to effectively reduce data redundancy by 64.22% and achieve a recovery rate of 99.79% compared with the original data. Therefore, the studied sensor device provides promise for ammonia sensing applications.

Enhancement of Hydrogen Sensing Performance of a Pd Nanoparticle/Pd Film/GaO<italic>x</italic>/GaN-Based Metal–Oxide– Semiconductor Diode

A new Pd nanoparticle (NP)/Pd film/GaO x /GaN-based metal–oxide–semiconductor diode hydrogen sensor is fabricated and studied. In this paper, appropriate photochemical drop coating and an H2O2 surface treatment were used to form Pd NPs and a GaO x dielectric layer. The Pd NPs increased the surface area/volume ratio, and the presence of GaO x layer led to the effective dissociation of hydrogen molecules. The improved hydrogen sensing properties include a very high sensing response of $1.24 \times 10^{{7}}$ (in 1% H2/air gas at 300 K) and an extremely low detection level ( ${\le} 1$ ppm H2/air). Furthermore, based on a kinetic adsorption analysis, the activation energy was only 13.1 $\text {KJ}\cdot \text {mol}^{-{1}}$ that is beneficial for hydrogen sensing. The proposed device is therefore promising for high-performance hydrogen sensing applications.

Hydrogen sensing performance of a Pd/HfO2/GaOx/GaN based metal-oxide-semiconductor type Schottky diode

An interesting hydrogen sensor based on a Pd/HfO2/GaOx/GaN metal-oxide-semiconductor (MOS) structure is fabricated and demonstrated. The HfO2 and GaOx layers are prepared using a sputtering approach and hydrogen peroxide (H2O2) treatment. The hydrogen sensing characteristics of the studied device are comprehensively studied. Experimentally, good hydrogen sensing characteristics, including a high sensing response of 8.47 × 105, a low detection level of 5 ppm H2/air, and reversible, short response and recovery times upon exposure to different hydrogen concentrations and temperatures are obtained. The influence of humidity on hydrogen sensing performance is also studied. The exothermic action of the hydrogen adsorption process leads to a decreased hydrogen sensing response at higher temperatures. Consequently, the studied Pd/HfO2/GaOx/GaN MOS diode is promising for high-performance hydrogen sensing applications and integration with other GaN-based high-speed devices on a chip.

Giant Zero-Drift Electronic Behaviors in Methylammonium Lead Halide Perovskite Diodes by Doping Iodine Ions.

Methylammonium lead halide perovskites have attracted extensive attention for optoelectronic applications. Carrier transport in perovskites is obscured by vacancy-mediated ion migration, resulting in anomalous electronic behavior and deteriorated reliability of the devices. In this communication, we demonstrate that ion migration can be significantly enhanced by doping additional mobile I- ions into the perovskite bulk. Ionic confinement structures of vertical metal oxide semiconductor (MOS) and lateral metal semiconductor metal (MSM) diodes designed to decouple ion-migration/accumulation and electronic transport are fabricated and characterized. Measurement conditions (electric-field history, scan rate and sweep frequency) are shown to affect the electronic transport in perovskite films, through a mechanism involving ion migration and accumulation at the block interfaces. Prominent zero-point drifts of dark current-voltage curves in both vertical and lateral diode are presented, and further varied with the perovskite film containingthe different iodine-lead atomic ratio. The doped perovskite has a large ion current at grain boundaries, offering a large ion hysteresis loopand zero drift value. The results confirmthat the intrinsic behavior of perovskite film is responsible for the hysteresisof the optoelectronic devices, but also paves the way for potential applications in many types of devices including memristors and solid electrolyte batteries by doping the native species (I− ions) in perovskite film.

A study on electrical characteristics of Pentacene based MOS diodes and transistors

Recently, organic electronics has become an object of research in field of electronic devices because organic semiconductor materials have many advantages such as flexibility, low-cost production and low-temperature process. The purpose of this research is to fabricate metal-oxide-semiconductor (MOS) diodes and transistors using organic semiconductor materials called pentacene which is commonly used p-type organic semiconductor to fabricate organic devices such as MOS diodes and transistors because of its availability and stability. The lowest minimum capacitance in depletion region and the highest maximum capacitance in accumulation region obtained in this research were 0.1μF/cm2 at 5 V and 0.9μF/cm2 at -5 V, respectively. The highest magnitude of drain current measured in this research was around 1.4 μA at drain and gate voltage of -5 V.