Nitride Semiconductors Research Articles

Direct Observation of Photoinduced Charge Separation at Transition-Metal Nitride-Semiconductor Interfaces.

Optically excited hot carriers from metallic nanostructures forming metal-semiconductor heterostructures are advantageous for enhancing photoelectric conversion in the sub-band gap photon energy regime. Plasmonic gold has been widely used for hot carrier excitation, but recent works have demonstrated that plasmonic transition-metal nitrides have higher efficiencies in injecting hot electrons to adjacent n-type semiconductors and are more cost-effective. To collect direct evidence of hot carrier excitation from nanostructures, imaging of hot carriers is essential. In this work, photoexcited Kelvin probe force microscopy (KPFM) is used to image hot carriers excited in transition-metal nitride nanostructures forming heterostructures with semiconductors. Among available transition-metal nitrides, we select zirconium nitride (ZrN) for this study. Additionally, both p-type and n-type titanium dioxides (TiO2) are selected to study the transport of hot holes and hot electrons. The KPFM results indicate that for ZrN and p-type TiO2 heterostructures, hot holes are injected into the p-type TiO2 across the Schottky contact. In the case of ZrN and n-type TiO2 heterostructures, hot electrons are injected into the n-type TiO2 across the ohmic contact. Because transition-metal nitrides are known to be more effective than gold at injecting hot carriers into adjacent semiconductors, unambiguously determining the mechanisms of hot carrier transportation of transition-metal nitrides using photoexcited KPFM will facilitate additional studies on hot carrier applications with transition-metal nitrides.

Structural, electronic, and optical properties of the gallium nitride semiconductor by means of the FP-LAPW method.

In the present paper, the structural, electronic, and linear optical properties of different phases of the gallium nitride (GaN) have been investigated. The zinc blende and wurtzite phases of the GaN have been studied using the full-potential linearized augmented plane wave method (FP-LAPW). In our study, many approximations have been used, such as the local density approximation (LDA), the generalized gradient approximation (GGA), the Engel and Vosko generalized gradient approximation (EV-GGA), and the modified Becke-Johnson (mBJ) potential exchange. As a result, we found a very good agreement with literature experimental results for the energy band gap using the mBJ approximation with a scaling factor of 98% and 80% for the zinc blende and wurtzite phases, respectively.

(Invited) Photo-Electrochemical Etching and Porosification of III-Nitride Semiconductors

Gallium nitride (GaN), which is a III-nitride semiconductor, has wide direct bandgap (3.4 eV), high breakdown electric field (3.3x106 V/cm) and high saturation electron velocity (2.7 x107 cm/sec). On the basis of excellent optical and electrical properties, significant progress has been achieved in the GaN and III-nitride based devices, such as ultraviolet laser diodes, white light-emitting diodes, and high-frequency power transistors. In addition, GaN and its alloys are getting much attention as building block materials for electrochemical (EC) energy conversion systems such as chemical sensors, water splitting, and artificial photosynthesis due to their superior chemical stability, direct transition, and widely tunable bandgap by alloying. For the fabrication of nitride semiconductor devices, the plasma-assisted dry etching process has commonly been used. However, various types of damage and defects induced by the bombardment of reactive ions and radicals leads to severe operational stability problems in optical and electrical devices.Photo-electrochemical (PEC) etching probably solves these problems on nitride semiconductors. PEC etching possesses some big advantages such as low-damage process conducted at room temperature. In this presentation, we introduce our recent work on the PEC etching and porosification of III-nitride semiconductors. The etching features such as smooth and flat etching and porosification etching have been controlled by changing the PEC conditions [1,2]. The porous structure formed by the present method has attractive features such as a high-density array of pores exhibits high specific surface area, low reflectance, and high absorptance properties. In addition, this technique does not require any complicated process such as lithography, indicating higher productivity than other nanostructure fabrication techniques such as reactive ion etching and selective-area growth.One example is the formation of straight pores along with the electric field induced in the vertical to the GaN substrate. This kind of straight pores was formed in the dark condition under the high-electric filed applied. Here, we found that the pore diameter was enlarged by the irradiation of UV light to the GaN surface during the porosification. In the case of the irradiation of UV light with a photon energy (hv) above the bandgap energy (E g), the pore diameter was enlarged only in the top region with a depth less than 1µm, where the photo-absorption occurred only near the surface. On the other hand, under the illumination with hvbelow E g, the pore diameter was enlarged overall pore from the top to the bottom. This kind of photo-assisted etching can be explained by the sub-bandgap absorption during the porosification. Sub-bandgap light with a photon energy below the bandgap energy was transmitted through bulk GaN, but a certain type of photo-absorption additively occurred at the pore tip where high electric filed was concentrated. To confirm such specific absorption process, we systematically investigated the correlation between the photocurrents and photon energy by changing the light intensity. The photocurrents observed when the high-electric field was applied to n-GaN were well reproduced by calculation taking into account the Frantz-Keldysh effect [3]. The results suggest that the independent control of pore diameter and depth is possible by controlling the PEC conditions.Acknowledgement: This work was supported by JSPS KAKENHI-JP16H06421, 20H02175.[1] Y. Kumazaki et al., J. Appl. Phys., 121, 184501 (2017).[2] Y. Kumazaki et al., J. Electrochem. Soc., 164, H477 (2017).[3] M. Toguchi et al., J. Electrochem. Soc., 166, H510 (2019). Figure 1

Over All Water Splitting By Anti-Photocorrosive Core-Shell Composite Sulfide Photocatalyst Synthesized Via Acid Dissolution Process

Some of the most significant challenges of the 21st century are related to energy exhaustion and global warming. In order to overcome these issues, it is necessary to produce sustainable energy without carbon dioxide emissions. Since the discovery of the Honda-Fujishima effect (H2O + 2h ν → 1/2 O2 + H2) in 1972, photocatalytic water splitting has attracted considerable attention as a solution to energy exhaustion. Since hydrogen is a clean source of energy that does not generate carbon dioxide when employed, water splitting has been considered an effective measure against global warming. The most commonly studied photocatalyst is TiO2. However, hydrogen production from water using TiO2 requires ultraviolet light, which deems it inefficient. In order to achieve a high conversion efficiency, which is considered suitable for practical applications, it is necessary to achieve a minimal absorption of visible light. To satisfy this requirement, a visible-light-responsive photocatalyst should be employed in the water splitting process. However, many of the photocatalysts with high visible light responsiveness, such as sulfide semiconductors and nitride semiconductors, have photocorrosive issues. This is an issue where hole carriers generated by photo-excitation cause the oxidative decomposition of the photocatalyst itself. To date, two approaches have been reported to suppress photocorrosion. The first is to transfer photocorrosive holes to different photocatalysts. The transfer of holes to the valence band of another photocatalyst has been reported. (H. Nagakawa et al., ACS Omega, 2018, 3, 10, 12770-12777.) In the previous study, the hydrogen generation efficiency was increased by transporting the excited electrons and holes from CdS to SiC. This approach successfully suppressed the photocorrosion of CdS. The second photocorrosion suppression method is to protect the photocorrosion-susceptible photocatalyst with different stable photocatalysts, thereby reducing exposure to the solution and preventing elution. The current authors reported that this method promoted the water splitting reaction in a CdS composite photocatalyst. (H. Nagakawa et al., ACS Applied Energy Materials, 2018, 1, 12, 6730-6735.) In the study, we achieved an overall water splitting under visible light using a home-synthesized tri-composite, in which CdS was covered with a photocatalyst on both the oxidation and reduction sides. Previously, the use of CdS/SiC/TiO2 tri-composite photocatalysts was found to promote the consumption of holes on CdS and reduce the exposure of CdS to the solution, thereby suppressing photocorrosion. However, since oxidation of water cannot be achieved with the oxidizing power of SiC, in this study, we employed WO3 instead of SiC and realized stronger oxidizing power by constructing a Z-scheme. In addition, we employed CdWO4 as a photocatalyst on the reduction side in order to enable the transport of electrons excited on CdS to the conduction band of CdWO4. A highly controlled CdS/CdWO4 core–shell composite was synthesized by a simple method via the dissolution of CdS under acidic conditions. The eluted Cd2+ reacted with the dissolved WO4 2- in solution in situ, allowing selective deposition on the CdS surface for uniform shell synthesis. (H. Nagakawa et al., RSC Advances, 2020, 10, 1, 105-111.) As a result, recombination was suppressed and the hydrogen production ability was improved compared to that of the dispersion system of CdS and WO3. Furthermore, water splitting was achieved by reducing the CdS exposure and suppressing photocorrosion. Hydrogen evolution experiments were conducted in vial bottles under Ar flow. First, 1 wt % Pt was deposited on the prepared photocatalyst using H2PtCl6·6H2O through photo-deposition under visible light (λ > 420 nm) in 30 vol% methanol aqueous solution. The synthesized CdS/WO3/CdWO4-Pt composite photocatalyst (0.03 g) and pure water (15 mL) were prepared in the reaction cell. A Xe lamp with a UV cutoff filter (λ > 420 nm) was used as a visible-light source. A 0.1 mL aliquot of the evolved gas was collected using a gastight syringe, and the sampled gas was injected into the gas chromatograph to determine the amounts of evolved hydrogen and oxygen. As a result, hydrogen and oxygen were constantly produced at a ratio of 2 : 1 while suppressing photocorrosion. This is the first report of water splitting by a powder-based photocatalyst using CdS under visible light. Figure 1

(Invited) Synthesis of Photoactive Hexagonal-Shape GaN Nanoplate Using Solid Nitrogen Source in Molten Salt

Gallium nitride (GaN), is a group III nitride semiconductor. Thin films of GaN have beenwidely adopted for both commercial and fundamental research, such as in light-emitting diodes (LEDs) and UV light water-splitting. Much interest has also been paid to the creation of nanostructures of GaN including the morphology of wires, rods, sheets, seeds and hollows. The nitridation reactions are often conducted using gas flow techniques for growing GaN crystals. In this process, a nitrogen-rich gas, such as ammonia, is usually used as a source of nitrogen. We have reported a new technique to synthesize low cost GaN powder under moderate condition. Li3N or LiNH2 were used as a high reactive solid nitrogen source to improve the reaction rate and lowering the reaction temperature. Crystalline wurtzite GaN powder with several hundred nanometer size was successfully synthesized. But the reaction yield was not so high because its solid phase reaction. Molten salts are often used as reaction media to increase the reaction rate and the crystallinity of ceramics. In this study, the synthesis of GaN nanoplate has been established by using LiCl as the molten salt at relatively low temperatures. Also, we anticipated that the formation of hexagonal-shape GaN nanoplates instead random-shaped crystals could enhance the charge transport in the photoelectrochemical applications. Here, we present the effect of the molten salt on the crystal growth of the GaN microstructure and the photoelectrochemical properties of the GaN electrodes.GaCl3 was reacted with LiNH2 at temperature of 400 - 650oC, LiCl was used as molten salts. LiCl has a melting point of 614°C. The graphite crucible with the starting materials (GaCl3, LiNH2) and molten salts (LiCl) was put into a sealed stainless reaction vessel. The reaction temperature was at 400 - 650°C. The products were washed with water and HCl solution after cooling.Figure 1 shows SEM photographs of GaN samples synthesized at (a) 550oC, (b) 600oC, (c) 650oC, (d) 700oC and (e) 800oC. The hexagonal-plate morphologies were observed at 600°C, 650°C, and 700°C, GaN formed with the particle size smaller than 500 nm. These GaN nanoplates are termed as 2D HS-GaN. Notably, by increasing temperature up to 700°C, particle sizes increased to be more than 300 nm and the crystal shape changed from granular to hexagonal. However, at 800°C, the crystal size decrease and the particles adopted a random plate-like shape. It is consider that thermal decomposition occurred at 800°C. The thickness of the 2D HS-GaN nanoplates varied from 50 to 80 nm by increasing the temperature from 600°C to 700°C. It is most likely that the LiCl assisted synthesis facilitated a more homogeneous reaction than that of the solid phase synthesis at 550°C. In the molten salt, the mobility of the reactants GaCl3 and LiNH2 is higher than that in the solid phase. The increased mobility causes the reactants to be uniformly diffused within a short period. Thus, at temperatures, more than 600°C, GaCl3 and LiNH2 in the molten salt allowed the reaction to proceed at an atomic level. The photoelectrochemical characterization revealed that the hexagonal structure gave a higher anodic photocurrent by a factor of up to 2 compared to the random-shaped GaN obtained at a higher temperature. To our belief, the photo-electrochemical properties of 2D HS-GaN electrode indicate that it has significant potential for photo-catalytic water splitting. Figure 1

Investigation of native defects and impurities in X-N (X = Al, Ga, In)

Defects have a decisive influence on the performance of III-V devices. In this paper, we systematically investigated the native point defects and impurities in group III nitride semiconductors X-N (X = Al, Ga, In) using density functional theory calculations. We studied the geometric and mechanical properties of these nitrides first, the bulk moduli were calculated and analyzed. Then by considering both anion-rich and cation-rich conditions, formation energies and corresponding thermodynamic charge transition levels of these defects were presented. Specifically, our results indicate the elastic stiffness is reduced with the introduction of defects. The nitrogen antisite, the magnesium-nitrogen substitution, and oxygen-X (X = Al, Ga, In) substitution have the most significant decrease in the bulk moduli, which are consistent with their formation energies. For native defects, the nitrogen vacancies are the most stable ones, but still are energetically unfavorable for the experimentally detected n-type conductivity of these nitrides. Interstitials and antisite defects are uneasy to be formed in n-type situations, but they could act as acceptors in p-type nitrides. For magnesium and oxygen impurities, our results have shown that the magnesium usually acts as p-type dopants, and the incorporated oxygen impurities could be the reason for the experimentally detected n-type conductivity. By studying and understanding the properties of these defects and impurities, we address new possibilities for designing and performance improvement of the group III nitride devices.

Disorder effects in nitride semiconductors: impact on fundamental and device properties

AbstractSemiconductor structures used for fundamental or device applications most often incorporate alloy materials. In “usual” or “common” III–V alloys, based on the InGaAsP or InGaAlAs material systems, the effects of compositional disorder on the electronic properties can be treated in a perturbative approach. This is not the case in the more recent nitride-based GaInAlN alloys, where the potential changes associated with the various atoms induce strong localization effects, which cannot be described perturbatively. Since the early studies of these materials and devices, disorder effects have indeed been identified to play a major role in their properties. Although many studies have been performed on the structural characterization of materials, on intrinsic electronic localization properties, and on the impact of disorder on device operation, there are still many open questions on all these topics. Taking disorder into account also leads to unmanageable problems in simulations. As a prerequisite to address material and device simulations, a critical examination of experiments must be considered to ensure that one measures intrinsic parameters as these materials are difficult to grow with low defect densities. A specific property of nitride semiconductors that can obscure intrinsic properties is the strong spontaneous and piezoelectric fields. We outline in this review the remaining challenges faced when attempting to fully describe nitride-based material systems, taking the examples of LEDs. The objectives of a better understanding of disorder phenomena are to explain the hidden phenomena often forcing one to use ad hoc parameters, or additional poorly defined concepts, to make simulations agree with experiments. Finally, we describe a novel simulation tool based on a mathematical breakthrough to solve the Schrödinger equation in disordered potentials that facilitates 3D simulations that include alloy disorder.

Electron transport within bulk cubic boron nitride: A Monte Carlo simulation analysis

Through the use of a semi-classical three-valley Monte Carlo electron transport simulation analysis, we analyze the nature of the steady-state and transient electron transport processes that occur within the zinc-blende phase of bulk boron nitride. For the purposes of our steady-state analysis, the dependence of the electron drift velocity on the applied electric field strength is examined. For our transient electron transport analysis, however, we study how an ensemble of electrons, initially in thermal equilibrium, i.e., zero-field, responds to the sudden application of a constant and uniform applied electric field. The results obtained, corresponding to the specific case of bulk zinc-blende boron nitride, are then contrasted with those corresponding to a number of other compound semiconductors of interest within the III–V nitride semiconductor genome.

Reliability limitations from crystal defects in thick GaN epitaxial layers

The vertical power switching devices made from GaN and related nitride semiconductors contain a high density of crystal defects, especially in the thick epitaxial drift layers. Most of these defects are present initially in starting wafers and some are generated during device processing. There is little conclusive evidence so far on the exact role that the crystal defects play on device performance, manufacturing yield, and more importantly, long-term field-reliability especially when devices are operating under extreme stressful high voltage environments. This paper provides the progress of characterization of thick GaN power semiconductor material epitaxial layers and the potential impact crystal defects may have on high-density power switching electronics. The investigation also presents simulated results of the electric field distribution through thick epitaxial (greater than 10 mm) region.

Single-step synthesis of efficient nanometric boron carbon nitride semiconductor for photocatalysis

Carbon-based materials have demonstrated great advantages over other metal-based photocatalyst materials. With the recent development in textile industries, residual commercial dyes discharged out as wastewater is a serious environmental concern. Herein, we have developed Boron Carbon Nitride nanosheets via thermal condensation (TBCN) and solvothermal route (SBCN). X-ray diffraction, X-ray photoelectron spectroscopy and Ultraviolet-Diffuse Reflectance Spectroscopy used to describe the surface optical properties of as-prepared samples. The results were compared to identify a facile single-step route for the synthesis of pure BCN photocatalyst materials thereby to avoid additional treatments for purification of the synthesised material. The optical bandgap of TBCN was estimated to 2.68 eV, resembling a typical semiconductor. XPS study revealed the presence of residual starting material as impurities in the as synthesised SBCN sample. Photodegradation of Malachite green, Acid green and Reactive blue dyes were carried out using the synthesised samples and the photoactivity of TBCN & SBCN were studied. The BCN photocatalyst after photodegradation was further characterized to understand the structural change of the material. Toxicological studies were conducted using the treated and untreated water by Mung bean (Vigna radiata L.). Based on the results, this study provides a facile single-step approach for the synthesis of BCN with great durability and stability which are promising for removal of textile dyes from wastewater.

Development and Application of Resistance Strain Force Sensors.

Resistance strain force sensors have been applied to monitor the strains in various parts and structures for industrial use. Here, we review the working principles, structural forms, and fabrication processes for resistance strain gauges. In particular, we focus on recent developments in resistance stress transfer for resistance strain force sensors and the creep effect due to sustained loads and/or temperature variations. Various error compensation methods to reduce the creep effect are analyzed to develop a metrology standard for resistance strain force sensors. Additionally, the current status of carbon nanotubes (CNTs), silicon carbide (SiC), gallium nitride (GaN), and other wide band gap semiconductors for a wide range of strain sensors are reviewed. The technical requirements and key issues of resistance strain force sensors for future applications are presented.

Nanoscale structural and electrical properties of graphene grown on AlGaN by catalyst-free chemical vapor deposition

The integration of graphene (Gr) with nitride semiconductors is highly interesting for applications in high-power/high-frequency electronics and optoelectronics. In this work, we demonstrated the direct growth of Gr on Al0.5Ga0.5N/sapphire templates by propane (C3H8) chemical vapor deposition at a temperature of 1350 °C. After optimization of the C3H8 flow rate, a uniform and conformal Gr coverage was achieved, which proved beneficial to prevent degradation of AlGaN morphology. X-ray photoemission spectroscopy revealed Ga loss and partial oxidation of Al in the near-surface AlGaN region. Such chemical modification of a ∼2 nm thick AlGaN surface region was confirmed by cross-sectional scanning transmission electron microscopy combined with electron energy loss spectroscopy, which also showed the presence of a bilayer of Gr with partial sp2/sp3 hybridization. Raman spectra indicated that the deposited Gr is nanocrystalline (with domain size ∼7 nm) and compressively strained. A Gr sheet resistance of ∼15.8 kΩ sq−1 was evaluated by four-point-probe measurements, consistently with the nanocrystalline nature of these films. Furthermore, nanoscale resolution current mapping by conductive atomic force microscopy indicated local variations of the Gr carrier density at a mesoscopic scale, which can be ascribed to changes in the charge transfer from the substrate due to local oxidation of AlGaN or to the presence of Gr wrinkles.

Crystal structure of rare earth and group III nitride alloys by ab initio calculations

The ground state phases of ternary alloys of rare earth and group III nitride semiconductors have been investigated within the density functional theory. The most energetically favorable crystal phases among possible cubic and hexagonal structures, i.e., the rock salt, zinc blende, wurtzite, and hexagonal BN, were determined. The type of a unit cell and the lattice parameters of the materials are presented as a function of their composition. Furthermore, effects of strain on ground states of group III and rare earth nitride materials are discussed. The findings presented in this work discloses the wurtzite type materials as being stable with relatively low contents of rare earth elements. It is expected that the wurtzite phase will be very persistent only in the La-based systems. Nevertheless, the two-dimensional hexagonal atomic layers are revealed as being a metastable phase for all alloys studied. This finding supports the conclusion of previous experimental reports for Sc-doped GaN systems that the presence of rare earth ions in group III nitride materials leads to flattening of the wurtzite type layers.

All-optical transistor using deep-level defects in nitride semiconductors for room temperature optical computing

The essential device for optical computing is an all-optical transistor in which a weak “gate” light controls the strong “source” light. Particularly promising for application in logic operations are all-optical transistors using quasiparticles in a semiconductor because they can be easily integrated into circuits in a way similar to that of conventional electronic ones. However, the practical development of such devices has so far been limited due to extreme difficulties in achieving room temperature operation. In this work, we proposed and numerically verified a scheme of the high-temperature stable all-optical transistor, where light controls light by using deep-level defects in non-polar InGaN/GaN heterostructure and photo-exited holes as an intermediate medium. The developed optical switching concept fulfills all criteria for the useful all-optical transistor listed in Miller, Nat. Photonics 4, 3 (2010), in particular fan-out and cascadability, which are the most difficult to meet. For the design of our transistor, we applied an entirely new approach to III-nitride device physics: we turned usually undesirable deep-level defects into a key, active element of the transistor in which they realize on and off operations. Due to this, the developed device was able to obtain excellent operation stability in a wide temperature range up to 500 K as well as an extremely high on/off ratio (106) and gain (100). Finally, in order to show that the proposed transistor concept is feasible, we performed the gated-photoluminescence experiment for metal–oxide–semiconductor GaN structures.

A Simple Approach to Growth Mode of InN and InGaN Thin Films on GaN(0001) Substrate

Group-III nitride semiconductors have attracted much attention for application in optoelectronic devices such as light emitting diodes and laser diodes. In particular, highly efficient light emitting devices using self-assembled quantum dots have been pained much attention. For InN and InGaN grown on GaN(0001) substrate, it has been reported that three-dimensional (3D) islands are formed after two-dimensional (2D) wetting layers, resulting in Stranski-Krastanow (SK) growth mode [1,2]. Moreover it has been reported that the growth modes of InN on GaN(0001) substrate during molecular beam epitaxy depend on growth conditions such as V/III ratio and substrate temperature[3]: N-rich condition favors 3D growth, whereas In-rich condition leads to 2D growth. However, it is still unclear what is the crucial factor for determining the growth modes of InN and InGaN depending on the growth condition. In this study, the growth mode of InN and InGaN thin films on GaN(0001) substrate are investigated using the macroscopic theory using free energy formula [4] with the aid of empirical potential calculations [5] as well as ab initio calculations [6]. Effects of pyramids island with {1-103} facets and misfit dislocation (MD) formation are taken into account for the evaluation of free energies of InN and InGaN thin films on GaN(0001) substrate.The calculations of cohesive energy difference ΔE as a function of InN film thickness for 2D coherent growth (2D-coherent) and 2D growth with MD (2D-MD) using empirical interatomic potentials reveals that 2D-MD is more stable than 2D-coherent for InN films beyond 0.7 monolayers (ML). On the basis of these results, the MD formation energy Ed and the effective elastic constant M are estimated to be 1.31 eV/Å and 5.60×1011 N/m2. The calculated result of Ed reasonably agrees with previous calculations for dislocation core energy in bulk InN (1.51 eV/Å) [7]. Moreover, the analysis of free energies using the value of Ed and M and surface energies obtained by ab initio calculations (~ 0.074 eV/ Å2) demonstrate that the critical thickness between growth 2D-coherent and 2D-MD under In-rich conditions is 0.45 ML. The calculated results are qualitatively consistent with the experimental results [3]. The calculated phase diagram for growth mode as functions of surface energy of (0001) and surface energy difference between (0001) and (1-103) planes suggest that growth mode boundary between 2D-MD and 3D coherent growth with hexagonal pyramid island formation (3D-coherent) strongly depends on the surface energy difference between (0001) and (1-103) planes. 2D-MD is favorable when the surface energy of InN(1-103) is larger than that of InN(0001), whereas 3D-coherent is favorable when the surface energies of InN(1-103) is smaller than InN(0001). These results imply that the relative stability between (0001) and (1-103) planes depending on the growth condition is decisive for the growth modes of InN on GaN(0001) substrate.[1] S. Ploch et al., Phys. Status. Solidi C 6, 574 (2009). [2] C. Adelmann et al., Appl. Phys. Lett. 76, 1570 (2000). [3] Y. F. Ng et al., Appl. Phys. Lett. 81, 3960 (2002). [4] K. Shiraishi et al., J. Cryst. Growth 237, 206 (2002). [5] K. E. Khor, and S. Das Sarma, Phys. Rev. B 38, 3318 (1988). [6] T. Akiyama et al., Phys. Status Solidi B 255, 1700329 (2018). [7] T. Ito et al., J. Cryst. Growth 298, 186 (2007).Figure: Phase diagrams of growth modes boundary between 2D-MD and 3D-coherent of InN on GaN(0001) substrate as functions of surface energy of (0001) and surface energy difference between (0001) and (1-103) planes. Figure 1

Engineering the Electronic, Thermoelectric, and Excitonic Properties of Two-Dimensional Group-III Nitrides through Alloying for Optoelectronic Devices (B1-xAlxN, Al1-xGaxN, and Ga1-xInxN).

Recently, two-dimensional (2D) group-III nitride semiconductors such as h-BN, h-AlN, h-GaN, and h-InN have attracted attention because of their exceptional electronic, optical, and thermoelectric properties. It has also been demonstrated, theoretically and experimentally, that properties of 2D materials can be controlled by alloying. In this study, we performed density functional theory (DFT) calculations to investigate 2D B1-xAlxN, Al1-xGaxN, and Ga1-xInxN alloyed structures. We also calculated the thermoelectric properties of these structures using Boltzmann transport theory based on DFT and the optical properties using the GW method and the Bethe-Salpeter equation. We find that by changing the alloying concentration, the band gap and exciton binding energies of each structure can be tuned accordingly, and for certain concentrations, a high thermoelectric performance is reported with strong dependence on the effective mass of the given alloyed monolayer. In addition, the contribution of each e-h pair is explained by investigating the e-h coupling strength projected on the electronic band structure, and we find that the exciton binding energy decreases with increase in sequential alloying concentration. With the ability to control such properties by alloying 2D group-III nitrides, we believe that this work will play a crucial role for experimentalists and manufacturers focusing on next-generation electronic, optoelectronic, and thermoelectric devices.

LaOCl‐Coupled Polymeric Carbon Nitride for Overall Water Splitting through a One‐Photon Excitation Pathway

AbstractIt is a challenge to explore photocatalytic materials for sunlight‐driven water splitting owing to the limited choice of a single semiconductor with suitable band energy levels but with a minimized band gap for light harvesting. Here, we report a one‐photon excitation pathway by coupling polymeric carbon nitride (PCN) semiconductor with LaOCl to achieve overall water splitting. This artificial photosynthesis composite catalyzes the decomposition of H2O into H2 and O2, with evolution rates of 22.3 and 10.7 μmol h−1, respectively. The high photocatalytic performance of PCN/LaOCl can be ascribed to the simultaneously accomplished reduction and oxidation of water on LaOCl and PCN domains, respectively, as well as the fast charge separation and migration induced by the interfacial electric field related to LaOCl modification. This study provides new insights on the development of composite photocatalysts for pure water splitting based on polymer‐based materials via charge modulation.

Boosting formic acid dehydrogenation via the design of a Z-scheme heterojunction photocatalyst: The case of graphitic carbon nitride/Ag/Ag3PO4-AgPd quaternary nanocomposites

The development of an efficient, eco-friendly, practical, and selective way to decompose formic acid (FA) into H2 and CO2 is crucial for the utilization of FA as a chemical hydrogen storage material in hydrogen economy. In this regard, photocatalytic FA dehydrogenation attracts great attention owing to its potential to meet the above-mentioned requirements. Interestingly, there is no example of heterojunction photocatalyst that tunes the hole potential of the semiconductor, resulted in a better photocatalytic activity. We report herein for the first time the design and fabrication of a novel Z-scheme heterojunction photocatalyst for FA dehydrogenation, denoted as g-CN/Ag/Ag3PO4-AgPd comprising graphitic carbon nitride (g-CN) and Ag3PO4 semiconductors, Ag and AgPd alloy nanoparticles (NPs). The designed g-CN/Ag/Ag3PO4-AgPd photocatalysts boosted the FA dehydrogenation by creating more positive hole potential and improving the charge separation efficiency of the two distinct semiconductors. The g-CN/Ag/Ag3PO4-AgPd photocatalysts provided a very high turnover frequency (TOF) of 2107 h−1 in the FA dehydrogenation under white-LED illumination at 50 °C. This TOF is 3.2 times and 44 times greater than those of g-CN/AgPd and g-CN/Pd binary non-Z-scheme heterojunction catalysts, respectively, under the same conditions and comparable to the best photocatalysts and heterogeneous catalysts reported in the FA dehydrogenation so far.

Calculated phonon modes, infrared, and Raman spectra in ZnGeGa2N4

Alloys between group III nitrides and the corresponding heterovalent II–IV-N2 compounds have recently been proposed as a further way of tuning the properties of nitride semiconductors. At 50% composition, a new ordered compound with composition ZnGeGa2N4 and space group Pmn21 was found, which has the lowest energy among wurtzite based structures. This structure obeys the local octet rule that every N is surrounded by two Ga, one Zn, and one Ge, ensuring local charge neutrality. Here, we investigate the vibrational properties of this new compound and provide predictions for its related infrared and Raman spectra, which may become useful for the characterization of this material. A group theoretical analysis, phonon frequencies and related Born effective charges, dielectric constants, infrared oscillator strengths, and Raman tensors are presented. Polarized infrared and Raman spectra for different scattering geometries are presented, as well as the phonon band structure and density of states.

Cathodoluminescence and x-ray photoelectron spectroscopy of ScN: Dopant, defects, and band structure

We have studied the optical band and defect transitions of ScN, a group IIIB transition metal nitride semiconductor with electronic and optoelectronic applications. Recent works have focused on the degenerate nature of ScN by substitutional impurities ON and FN, which shift the direct (X–X) gap transition to higher energies via the Burstein–Moss effect. We used cathodoluminescence spectroscopy (CLS) to observe optical signatures of both the midgap VN precursor to ON doping as well as above the direct (X–X) bandgap corresponding to band-to-band transitions from four separate conduction bands near the Γ point with the valence band minimum, in agreement with the calculated band structure diagrams. Thin film ScN grown by reactive magnetron sputtering displays mild degenerate doping by substitutional oxygen as indicated by elevated (X–X) transition energies and the presence of Sc–O bonding determined via x-ray photoelectron spectroscopy (XPS), while ScN grown by physical vapor transport exhibited the intrinsic, non-degenerate (X–X) bandgap predicted by theory. CLS reveals a sharp, sub-bandgap emission at 1.26 eV for sputter grown ScN on GaN, which we attribute to nitrogen vacancies (VN) based on surface sensitive CLS and XPS chemical trends. This finding is in strong agreement with theoretical calculations for VN predicting the formation of a defect energy level within the gap.