P-type Gallium Research Articles

Charge Transfer Kinetics and Thermodynamics Control the Energy Conversion Efficiency of a Gallium Phosphide Solar Hydrogen Photocathode.

P-type gallium phosphide (GaP) photocathodes for hydrogen evolution from water have a theoretical energy conversion efficiency of 12% based on the 2.4 eV optical band gap of the material. The performance of actual GaP photocathodes is much lower, for reasons not entirely clear. Here we use vibrating Kelvin probe surface photovoltage (VKP-SPV), open circuit potential (OCP) measurements, and photoelectrochemical (PEC) experiments to evaluate the kinetic and thermodynamic factors that control energy conversion with GaP photocathodes for the hydrogen evolution reaction (HER). We find that the open circuit photovoltage of the bare GaP-H2O junction is limited by recombination at surface states and that an CdS overlayer increases both photovoltage and photocurrent due to formation of a n-p-junction. An optimized GaP/CdS/Pt photocathode drives hydrogen evolution with a quantum efficiency of 62% at 400 nm and 0.0 V RHE and an open circuit photovoltage of 0.43 V at 250 mW cm-2. The Pt cocatalyst increases the photocurrent due to improve HER kinetics but reduces the photovoltage by promoting recombination. Added hydrogen or oxygen gas raise or lower the photovoltage by modifying the electrostatic barrier (band bending) in GaP. This shows that the GaP/CdS junction is not "buried" but behaves like a Schottky junction whose charge separating properties are controlled by the electrochemical potential of the electrolyte. The dynamic junction properties need to be considered in the design of optimized hydrogen evolution photoelectrodes and photocatalysts. Additionally, the work reveals that PEC or OCP measurements tend to underestimate the photovoltage because they do not account for changes in the electrochemical potential at the electrode-liquid contact. In contrast, the VKP-SPV method provides the open circuit photovoltage value directly. By combining the photovoltage data with OCP data, the minority carrier electrochemical potential at the electrode-liquid contact can be measured in a contactless way. This provides an improved understanding of illuminated photoelectrodes for the production of solar fuels.

Cathodoluminescence studies of electron injection effects in p-type gallium oxide

Cathodoluminescence studies of electron injection effects in p-type gallium oxide

Synthesis and characterizations of zinc oxide nanorods by cost effective aqueous hydrothermal technique for solid state lighting applications

Zinc oxide (ZnO) is recognized as the most promising and often utilized semiconducting material due to its exceptional physical, chemical, optical, and relatively low-cost production features, as well as its ecologically beneficial nature. ZnO is a prominent candidate owing to its wide band gap (3.37 eV at room temperature) as well as its high exciton binding energy (60 meV) for the next generation of ultraviolet and blue optoelectronic devices. This would be achieved through the development of hybrid heterostructured LED devices with low manufacturing costs by blending gallium nitride (GaN) with zinc oxide (ZnO), which serves as the most efficient opto-electronic device. Due to its high electron mobility, high transparency in the visible wavelength spectrum, and low work function, ZnO is also a highly investigated transparent conducting oxide thin film for several applications. Besides, their incredible optical capabilities, ZnO-based optical devices, such as light-emitting diodes (LEDs) and laser diodes (LDs), have drawn a great deal of interest. This reported research work demonstrates the growth of high-density, high-aspect-ratio ZnO nanorod (NR) arrays on p-type gallium nitride substrates employing low-cost facial aqueous hydrothermal syntheses with optimised growing conditions. Additionally, a top-down chemical etching technique is adapted for fabricating zinc oxide nanotubes. High-quality ZnO nanorods with high intensity that were a few microns long were found across the GaN substrate with FESEM analysis. The chemical composition of the grown nanorods is confirmed with EDAX analysis. The wurtzite crystalline structure and c-axis orientation of the grown ZnO NR-arrays were identified in X-ray diffraction studies. A Schimadzu UV–Vis–NIR spectrophotometer was used to record the optical curve, and the energy gap was determined to be 3.24 eV by plotting the Tauc curve. The good optical quality of the harvested nanorods is ensured by the narrow near-band-edge (NBE) emission at 384 nm and defect emission peaks detected through photoluminescence analyses. Grown specimens are also subjected to Raman spectrum analyses, and the obtained results are presented.

Characterization and ohmic contact properties of indium tin-oxide films prepared on p-type GaN using electron-cyclotron-resonance plasma-sputter deposition

The optical and electrical properties of indium tin oxide (ITO) films deposited using electron cyclotron resonance (ECR) plasma-sputter deposition and the ohmic contact properties between the ITO films and p-type gallium nitride (p-GaN) were evaluated. The electrical properties of the ITO films were evaluated in terms of the dependence on the tin (IV) oxide (SnO2) content of the ITO target and radio frequency (RF) power. The ohmic contact properties between the ITO films and p-GaN were also characterized by varying the top-surface Mg concentration of the p-GaN substrate. An ohmic contact was achieved without post-annealing treatment by depositing the film on a p-GaN high-dope substrate with a top-surface Mg concentration of 2.0 × 1020 cm−3 under low RF-power deposition using an ITO sputter target with 10 wt.% SnO2. The interface between the ITO films and p-GaN that achieved an ohmic contact was observed using scanning transmission electron microscopy (STEM) and was very smooth with no crystal disorder. This indicates that the crystallinity of the ITO films and p-GaN surface is a factor affecting ohmic-contact properties. The inter-diffusion between the ITO films and p-GaN surface was also evaluated using transmission electron microscopy energy dispersive X-ray spectroscopy (TEM-EDS), and no inter-diffusion was observed.

Observation of 2D-magnesium-intercalated gallium nitride superlattices

Since the demonstration of p-type gallium nitride (GaN) through doping with substitutional magnesium (Mg) atoms1,2, rapid and comprehensive developments, such as blue light-emitting diodes, have considerably shaped our modern lives and contributed to a more carbon-neutral society3–5. However, the details of the interplay between GaN and Mg have remained largely unknown6–11. Here we observe that Mg-intercalated GaN superlattices can form spontaneously by annealing a metallic Mg film on GaN at atmospheric pressure. To our knowledge, this marks the first instance of a two-dimensional metal intercalated into a bulk semiconductor, with each Mg monolayer being intricately inserted between several monolayers of hexagonal GaN. Characterized as an interstitial intercalation, this process induces substantial uniaxial compressive strain perpendicular to the interstitial layers. Consequently, the GaN layers in the Mg-intercalated GaN superlattices exhibit an exceptional elastic strain exceeding −10% (equivalent to a stress of more than 20 GPa), among the highest recorded for thin-film materials12. The strain alters the electronic band structure and greatly enhances hole transport along the compression direction. Furthermore, the Mg sheets induce a unique periodic transition in GaN polarity, generating polarization-field-induced net charges. These characteristics offer fresh insights into semiconductor doping and conductivity enhancement, as well as into elastic strain engineering of nanomaterials and metal–semiconductor superlattices13.

III-Nitride Materials: Properties, Growth, and Applications

Since the activation of magnesium (Mg) in p-type gallium nitride (GaN) [...]

Reversible Thermoelectric Regulation of Electromagnetic and Chemical Enhancement for Rapid SERS Detection.

Actively controlling surface-enhanced Raman scattering (SERS) performance plays a vital role in highly sensitive detection or in situ monitoring. Nevertheless, it is still challenging to achieve further modulation of electromagnetic enhancement and chemical enhancement simultaneously in SERS detection. In this study, a silver nanocavity structure with graphene as a spacer layer is coupled with thermoelectric semiconductor P-type gallium nitride (GaN) to form an electric-field-induced SERS (E-SERS) for dual enhancement. After applying the electric field, the intensity of SERS signals is further enhanced by over 10 times. The thermoelectric field enables fast and reproducible doping of graphene, thereby modulating its Fermi level over a wide range. The thermoelectric field also regulates the position of the plasmon resonance peak of the silver nanocavity structure, rendering synchronous dual electromagnetic and chemical regulation. Additionally, the method enables the trace detection of severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2). A detailed theoretical analysis is performed based on the experimental results and finite-element calculations, paving the way for the fabrication of high-efficient E-SERS substrates.

Influence of Energetic Particles and Electron Injection on Minority Carrier Transport Properties in Gallium Oxide

The influence of various energetic particles and electron injection on the transport of minority carriers and non-equilibrium carrier recombination in Ga2O3 is summarized in this review. In Ga2O3 semiconductors, if robust p-type material and bipolar structures become available, the diffusion lengths of minority carriers will be of critical significance. The diffusion length of minority carriers dictates the functionality of electronic devices such as diodes, transistors, and detectors. One of the problems in ultrawide-bandgap materials technology is the short carrier diffusion length caused by the scattering on extended defects. Electron injection in n- and p-type gallium oxide results in a significant increase in the diffusion length, even after its deterioration, due to exposure to alpha and proton irradiation. Furthermore, post electron injection, the diffusion length of an irradiated material exceeds that of Ga2O3 prior to irradiation and injection. The root cause of the electron injection-induced effect is attributed to the increase in the minority carrier lifetime in the material due to the trapping of non-equilibrium electrons on native point defects. It is therefore concluded that electron injection is capable of “healing” the adverse impact of radiation in Ga2O3 and can be used for the control of minority carrier transport and, therefore, device performance.

A systematic study on the efficacy of low-temperature GaN regrown on p-GaN to suppress Mg out-diffusion

Embedding p-type gallium nitride (p-GaN) in AlxGa1-xN-based thin films has garnered significant interest as a versatile structure for bandgap engineering such as tunnel/super-junctions or current blocking/guiding functions in electronic devices. However, Mg, a p-GaN dopant, has an undesirable diffusive capacity into the nearby thin films at a high growth temperature (>1,000°C), resulting in structural challenges in device design. This study systematically investigated the low-temperature GaN (LT-GaN) layer regrown on p-GaN that suppresses Mg diffusion according to metal–organic chemical vapor deposition growth conditions. Prototype Al0.3Ga0.7N (40 nm)/GaN (140 nm) high-electron-mobility transistors (HEMTs) were regrown with LT-GaN on p-GaN (300 nm), and a high two-dimensional electron gas (2DEG) density of 3.13E12 cm−2 was achieved by inserting a 100-nm-thick LT-GaN layer grown at 750°C; in contrast, only 1.76E10 cm−2 2DEG density was obtained from Al0.3Ga0.7N/GaN HEMTs regrown directly on p-GaN (Mg: 4.0E19 cm−3). The fabricated Al0.3Ga0.7N/GaN HEMTs with 100-nm-thick LT-GaN demonstrated a high drain current density of 84.5 mA/mm with a low on-state resistance of 31 Ω·mm. The AlxGa1-xN/LT-GaN/p-GaN platform demonstrated here paves the way for various III-nitride-based structures with embedded p-GaN.

A novel stepped AlGaN hybrid buffer GaN HEMT for power electronics applications

In this paper, a novel p-type gallium nitride (p-GaN) high electron mobility transistor (HEMT) is proposed and investigated for high power electronic applications. In the proposed device, a stepped hybrid Aluminium GaN (AlGaN) buffer layer with low Al content has been introduced to improve the performance of the device. The low aluminium mole concentration decreases the surface defects which reduces the leakage current, thereby increasing the breakdown voltage of the device. To validate the simulation results, calibration of the device models has been performed with experimental results. Results present that the proposed device exhibits high breakdown voltage, high transconductance and low small signal capacitances. The proposed device achieves improvement in breakdown voltage, transconductance and small signal capacitances as compared to the conventional hybrid buffer high electron mobility transistor (HB-HEMT). Furthermore, the processing steps have been proposed for the possible fabrication of the proposed device. Also, the optimization is performed to achieve the optimal device performance with respect to breakdown voltage. Finally, the performance of the proposed device has been compared with the state-of-the-art devices.

Simulation And Design of a Vertical Gallium Nitride JFET with Dual Lateral Heterojunction Channels

A novel vertical gallium nitride (GaN) junction field-effect transistor (JFET) is reported in this article. Through the setup of dual lateral heterojunction channels in the body, the current density is high while the p-type gallium nitride growth area can be adjusted to easily achieve enhanced mode. Based on the structure parameter settings in this article, through the device simulation software TCAD, and under the condition of selecting the appropriate p-type GaN growth region, the threshold voltage of the device is 1.1 V, the source output current density can reach 7.2 kA/cm2, the breakdown voltage is about 750 V, and the on-resistance is 0.52 mΩ·cm2. Finally, the test circuit is built, and the switching characteristics of the device proposed in this paper are simulated under the consideration of parasitic inductance.

Hybrid biological/inorganic photocathode for H2 production based on a NiFeSe hydrogenase immobilized on electrodeposited CuGaS2

In order to mitigate global warming and pollution problems, new energy vectors need to be developed towards a fossil fuel-free society. Hydrogen is a great candidate for the role, since it can be cleanly obtained from water splitting. However, finding an efficient method that allows diminishing the high potentials required for the reaction is still a challenge to overcome. In order to tackle this task, a combination of a redox biocatalyst and an inorganic semiconductor that harnesses sunlight to enhance the H2 photoelectrochemical production has been studied. For this endeavor, we have developed a photocathode based on the p-type semiconductor copper gallium sulfide and the Desulfovibrio vulgaris Hildenborough NiFeSe-hydrogenase where the photoexcited semiconductor transfers excited electrons to the enzyme upon visible irradiation. The system is capable of producing photoelectrochemical biocatalytic proton reduction to H2 without any sacrificial agent while providing an approximately 400 mV decrease of the applied potential.

Photoelectrochemical Deposition of Silicon–Carbon Layer on P-Type Semiconductors and Aluminum–Copper Alloy in Ionic Liquid

Electrochemical deposition of silicon at room temperature is problematic due to the intrinsically low conductivity of the deposits. This study reports the photoelectrochemical (PEC) deposition of silicon (Si) and silicon–carbon (Si–C) layers from an ionic liquid at 40 °C using silicon tetrachloride (SiCl4) as a silicon precursor. Amorphous layers are deposited on p-type silicon (p-Si), p-type gallium arsenide (p-GaAs), and aluminum–copper alloy AA2024. The semiconductor substrates are activated by white LED illumination, which generates photoelectrons, thereby making the substrate conductive with respect to the cathodic reaction. The photoresponsiveness of the deposits is proven by the light-induced photocurrents on an optically inactive substrate made of the alloy AA 2024. The proposed method paves the way for the electrochemical modification of semiconductors and metals with Si and Si–C structures, which are applicable in various fields, such as batteries, anti-corrosion coatings, photovoltaics, or PEC electrodes for hydrogen production.

A novel p-GaN HEMT with AlInN/AlN/GaN double heterostructure and InAlGaN back-barrier

In this paper, a novel p-type gallium nitride (p-GaN) high electron mobility transistor (HEMT) is proposed and investigated for high power electronics applications. In the proposed device, an AlInN/AlN/GaN double heterostructure has been used. In addition, InAlGaN back-barrier has also been employed to improve the DC performance of the device. The double AlInN/GaN heterostructures offer low resistance, resulting to a high two-dimensional electron gas (2DEG) density. As a result the current drive efficiency of the device is significantly improved. On the other hand, an inserted InAlGaN back-barrier confines the electrons in the channel which further enhances the performance of the device. To validate the simulation results, calibration of the device models has been performed with experimental results. Results show that the proposed device exhibits low on-resistance (Ron), high drain current (Ids) and high transconductance (gm) as compared to the state-of-the-art devices. Also, the processing steps have been proposed for the possible fabrication of the proposed device. Finally, the optimization of the device design parameter is performed to achieve the optimal device performance with respect to Ron.

Conductometric gas sensor based on p-type GaN hexagonal pits /PANI for trace-level NH3 detection at room temperature

Herein, we demonstrate two ammonia (NH3) gas sensors based on p-type Gallium nitride (GaN) hexagonal pits (GaN-HP)/ Polyaniline (PANI) heterostructure prepared by wet etching process and in situ polymerization. The morphologies, structures of p-type GaN-HP/PANI sensors based on planer p-type GaN with etching time for 20 min and 30 min (GaN-HP/PANI-1 and GaN-HP/PANI-2) have been characterized. The responses of GaN-HP/PANI-1 and GaN-HP/PANI-2 are 3.2 and 6.3 times higher than that of pure PANI at room temperature, which is primarily due to the gas sensing improvement effect of p-p junction between p-type GaN-HP and PANI. Moveover, the GaN-HP/PANI-2 shows 1.94-fold enhancement in the gas sensing response toward 100 ppm NH3 and lower detection limits than that of GaN-HP/PANI-1, indicating that its performance is highly dependent on the etching time. Compared to GaN-HP/PANI-1, the excellent improvement of gas sensing for GaN-HP/PANI-2 can be attributed to the higher specific surface area, the stronger protonation degree of PANI and synergistic effects of the p-p junction effect. Furthermore, the selectivity, repeatability and the effect of relative humidity on the gas sensing performances of the two both sensors have also been explored. This work can provide a helpful reference for development of high-performance GaN-HP/PANI based gas sensors.

In-situ mechanochemically tailorable 2D gallium oxyselenide for enhanced optoelectronic NO2 gas sensing at room temperature

The adverse effects of NO2 on the environment and human health promote the development of high-performance gas sensors to address the need for monitoring. Two-dimensional (2D) metal chalcogenides have been considered an emerging group of NO2-sensitive materials, while incomplete recovery and low long-term stability are the two major hurdles for their practical implementation. The transformation into oxychalcogenides is an effective strategy to alleviate these drawbacks, but usually requires multiple-step synthesis and lacks controllability. Here, we prepare tailorable 2D p-type gallium oxyselenide with the thicknesses of 3–4 nm, through a single-step mechanochemical synthesis that combines the in-situ exfoliation and oxidation of bulk crystals. The optoelectronic NO2 sensing performances of such 2D gallium oxyselenide with different oxygen contents are investigated at room temperature, in which 2D GaSe0.58O0.42 exhibits the largest response magnitude of 82.2% towards 10 ppm NO2 at the irradiation of UV, with full reversibility, excellent selectivity, and long term stability for at least one month. Such overall performances are significantly improved over those of reported oxygen-incorporated metal chalcogenide-based NO2 sensors. This work provides a feasible approach to prepare 2D metal oxychalcogenides in a single-step manner and demonstrates their great potential for room-temperature fully reversible gas sensing.

Photochemically-Activated p-Type CuGaO2 Thin Films for Highly-Stable Room-Temperature Gas Sensors

The photochemical activation process is a promising way to operate metal oxide gas sensors at room temperature. However, this technique is only used in n-type semiconductors. In this study, we report a highly stable p-type copper gallium oxide (CuGaO2) gas sensor fabricated through the facile sol-gel process. The sensor is capable of detecting O3 gas at room temperature, and its gas response can be further enhanced by ultraviolet (UV) activation. The highest gas response of 7.12 to 5 ppm O3 gas at a UV intensity of 10 mW cm−2 is achieved at room-temperature. In addition, the CuGaO2 sensor shows excellent long-term stability, with a degradation of approximately 3% over 90 days. These results strongly support the solution-processed CuGaO2 as a good candidate for room-temperature gas sensors.

Schottky barrier height engineering in vertical p-type GaN Schottky barrier diodes for high-temperature operation up to 800 K

We have fabricated vertical p-type gallium nitride (GaN) Schottky barrier diodes (SBDs) using various Schottky metals such as Pt, Pd, Ni, Mo, Ti, and Al. Current–voltage characteristics revealed that Schottky barrier heights determined using a thermionic emission (TE) model (ϕBTE) were ranged between 1.90 eV for Pt and 2.56 eV for Mo depending on the work function (ϕm) of the Schottky metals. Despite their low ϕm, Ti and Al gave unusually small ϕBTE probably due to the interfacial reaction between metal and p-type GaN. We also found that Mo/p-GaN SBDs exhibited a clear rectifying property even at 800 K, and the thermionic emission diffusion (TED) model explained well their high-temperature I–V characteristics. Furthermore, the temperature variation of Schottky barrier heights determined using a TED model (ϕBTED) almost agrees with half of the temperature variation of the bandgap energy. These findings will be helpful for the application of p-type GaN Schottky interfaces to high-power and high-temperature electronics.

Nanoporous GaN on p-type GaN: a Mg out-diffusion compensation layer for heavily Mg-doped p-type GaN

Embedding p-type gallium nitride (p-GaN) with controlled Mg out-diffusion in adjacent epitaxial layers is a key for designing various multi-junction structures with high precision and enabling more reliable bandgap engineering of III-nitride-based optoelectronics and electronics. Here, we report, for the first time, experimental evidence of how nanoporous GaN (NP GaN) can be introduced as a compensation layer for the Mg out-diffusion from p-GaN. NP GaN on p-GaN provides an ex-situ formed interface with oxygen and carbon impurities, compensating for Mg out-diffusion from p-GaN. To corroborate our findings, we used two-dimensional electron gas (2DEG) formed at the interface of AlGaN/GaN as the indicator to study the impact of the Mg out-diffusion from underlying layers. Electron concentration evaluated from the capacitance-voltage measurement shows that 9 × 1012 cm−2 of carriers accumulate in the AlGaN/GaN 2DEG structure grown on NP GaN, which is the almost same number of carriers as that grown with no p-GaN. In contrast, 2DEG on p-GaN without NP GaN presents 9 × 109 cm−2 of the electron concentration, implying the 2DEG structure is depleted by Mg out-diffusion. The results address the efficacy of NP GaN and its’ role in successfully embedding p-GaN in multi-junction structures for various state-of-the-art III-nitride-based devices.

Controlled electrochemical growth of micro-scaled As2O3 and Ga2O3 oxide structures on p-type gallium arsenide

Controlled electrochemical growth of micro-scaled As2O3 and Ga2O3 oxide structures on p-type gallium arsenide