group-III Nitrides Research Articles

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.

Control of the 3-Fold Symmetric Shape of Group III-Nitride Quantum Dots: Suppression of Fine-Structure Splitting.

Controlling the in-plane symmetry of wide-bandgap semiconductor quantum dots (QDs) is essential for room temperature quantum photonic applications using polarization entangled photon pairs. Herein, we report the formation of 3-fold symmetric group III-nitride QDs at the apex of a triangular pyramid via a self-limited growth mechanism. We employed the in-plane rotational symmetry of the c-plane of a Wurtzite crystal and the large built-in piezoelectric field to reduce fine-structure splitting. The QDs exhibit emission that is distinguishable from that of sidewall quantum wells, and the biexciton-exciton cascade possesses a single-photon nature. We observed the relatively low optical polarization anisotropy and small fine structure splitting under the measurement limit (270 μeV) with the 3-fold symmetric QD. In contrast with current strategies that consider group III-nitride QDs as strongly polarized single-photon emitters, our approach for controlling the QD symmetry provides a new perspective on such QDs, as polarization-entangled photon pairs.

Giant excitonic absorption and emission in two-dimensional group-III nitrides

Absorption and emission of pristine-like semiconducting monolayers of BN, AlN, GaN, and InN are systematically studied by ab-initio methods. We calculate the absorption spectra for in-plane and out-of-plane light polarization including quasiparticle and excitonic effects. Chemical trends with the cation of the absorption edge and the exciton binding are discussed in terms of the band structures. Exciton binding energies and localization radii are explained within the Rytova-Keldysh model for excitons in two dimensions. The strong excitonic effects are due to the interplay of low dimensionality, confinement effects, and reduced screening. We find exciton radiative lifetimes ranging from tenths of picoseconds (BN) to tenths of nanoseconds (InN) at room temperature, thus making 2D nitrides, especially InN, promising materials for light-emitting diodes and high-performance solar cells.

Characterization of epitaxial titanium nitride mediated single-crystal nickel oxide grown on MgO-(100) and Si-(100)

Single-crystal nickel oxide (NiO) was grown, using epitaxial titanium nitride (TiN) as a preorienting interlayer, on both the lattice-matching substrate of magnesium oxide in the (100) surface orientation, MgO-(100), and a lattice-mismatched silicon (100) substrate, Si-(100), by high-temperature pulsed-laser deposition. To the best of the authors’ knowledge, this is the first report of its kind in the literature. The high-temperature sputter-deposited TiN interlayer is crucial for forming a bottom contact for the implementation of a device, and as a lattice-matching layer for NiO and MgO. The structural, surface-related, and elemental properties of the as-grown NiO/TiN/MgO(100) and NiO/TiN/Si(100) samples were determined by high-resolution transmission electron microscopy (HRTEM), x-ray diffraction (XRD), thin-film x-ray diffraction, atomic force microscopy, and scanning transmission electron microscopy in conjunction with energy-dispersed x-ray spectroscopy. XRD rocking curve data confirmed that the NiO layers were single crystalline on both template substrates, and the structural quality of NiO/TiN on the lattice-matching MgO substrate surpassed that on the Si substrate. XRD φ-scan data confirmed the cube-on-cube stacking of NiO and TiN. An analysis of HRTEM fast Fourier transform (FFT) images further confirmed the single crystallinity of the NiO and TiN layers, while lattice mismatches at the NiO/TiN, TiN/MgO, and TiN/Si interfaces were examined using the FFT line profile measurements of HRTEM. The resulting thin film of single-crystalline NiO can be used as a transparent conducting electrode in group-III oxide and group-III nitride semiconductor devices, and in such electrochemical processes as solar hydrogen generation and nitrogen reduction reactions.

Nano-spectroscopic and nanoscopic imaging of single GaN nanowires in the sub-diffraction limit

The optical spatial resolution limit prohibits a precise spectroscopic characterization of objects at the nanoscale. However, plasmonic assisted microscopy at the nanoscale can overcome the diffraction limit owing to the confinement of the localized electric field. We report the spectroscopic imaging of a single GaN nanowire (NW) of diameter ∼200 nm with the help of Au nanoparticle assisted plasmonic tip-enhanced Raman spectroscopy (TERS), well below the diffraction limit of ∼750 nm. The challenge in the achievement was that the insignificant value of change of polarizability in the partially ionic III–V nitrides made the Raman scattering intensity too low for spectroscopic studies. Consequently, the enhancement factors for different observed Raman modes for the wurtzite GaN NW were also found to be significantly low. Spectroscopic imaging of such low scattering efficiency group III-nitride single GaN NW is achieved in the sub-diffraction limit by careful selection of the TERS substrate. Similarly, the nanoscopic optical imaging of a single GaN NW of diameter ∼10 nm using the confined electromagnetic wave is also achieved with the help of the plasmonic assisted near-field scanning optical microscopic technique in the sub-diffraction limit.

Electrical Characterization of Gallium Nitride Thin Film Synthesized By Electrochemical Deposition Method

Introduction : Group III-nitride semiconductor materials are the basis of modern electronic allowing for the development of diodes, transistor, LEDs and photovoltaic. Newer generation of semiconductors rely heavily on nanotechnology and advance in nanomaterials. Group III-nitride semiconductor materials such as GaN, AlN and InN are promising materials since their properties are ideal for use in optoelectronic devices, high power and high temperature electronic devices. These materials have wide energy band gaps between 1.9 to 6.3 eV and, for electronic applications, these materials are usually synthesised by means of chemical vapour deposition or molecular beam Epitaxy and these techniques are high cost, and recently room temperature electrochemical growth.The advantages of electrochemical deposition are the controllability of the thickness and surface morphology by varying the deposition parameters, cost-effectiveness, the minimum waste generation, long bath lifetime and easy set-up, in this study room tempreature electrochemical deposition has been used to synthesis the GaN nitride on Si. Summary: the GaN thin films grown by electrochemical deposition on Si (111) substrate using mixture of Ga(NO3)3 and NH4NO3, then dissolved in deionized water at the room temperature with different current of deposition after investigated the grown by using XRD, SEM and AFM, thereafter fabricated the Schottky diode on the GaN thin films and measured the IV, CV and DLTS on it. Result: The XRD results of GaN thin films grown under the different conditions are shown in Figure 1. Sample a (1 mA during 30 mint) and b (3 mA during 30 mint) were prepared at room temperature at an electrodeposition current of 1 mA, 3 mA for 30 respectively. The XRD analysis showed the presence of hexagonal and cubic structure with crystallite sizes ranging from 130 to 140 nm. From AFM and SEM images, however, the results varied with growth conditions and substrate. Good quality Schottky diodes was shown on the GaN thin film and DLTS measurements were performed. Introduction : Group III-nitride semiconductor materials are the basis of modern electronic allowing for the development of diodes, transistor, LEDs and photovoltaic. Newer generation of semiconductors rely heavily on nanotechnology and advance in nanomaterials. Group III-nitride semiconductor materials such as GaN, AlN and InN are promising materials since their properties are ideal for use in optoelectronic devices, high power and high temperature electronic devices. These materials have wide energy band gaps between 1.9 to 6.3 eV and, for electronic applications, these materials are usually synthesised by means of chemical vapour deposition or molecular beam Epitaxy and these techniques are high cost, and recently room temperature electrochemical growth.The advantages of electrochemical deposition are the controllability of the thickness and surface morphology by varying the deposition parameters, cost-effectiveness, the minimum waste generation, long bath lifetime and easy set-up, in this study room tempreature electrochemical deposition has been used to synthesis the GaN nitride on Si. Summary: the GaN thin films grown by electrochemical deposition on Si (111) substrate using mixture of Ga(NO3)3 and NH4NO3, then dissolved in deionized water at the room temperature with different current of deposition after investigated the grown by using XRD, SEM and AFM, thereafter fabricated the Schottky diode on the GaN thin films and measured the IV, CV and DLTS on it. Result: The XRD results of GaN thin films grown under the different conditions are shown in Figure 1. Sample a (1 mA during 30 mint) and b (3 mA during 30 mint) were prepared at room temperature at an electrodeposition current of 1 mA, 3 mA for 30 respectively. The XRD analysis showed the presence of hexagonal and cubic structure with crystallite sizes ranging from 130 to 140 nm. From AFM and SEM images, however, the results varied with growth conditions and substrate. Good quality Schottky diodes was shown on the GaN thin film and DLTS measurements were performed. Figure 1

Modulation of the two-dimensional electron gas channel in flexible AlGaN/GaN high-electron-mobility transistors by mechanical bending

We investigate the effect of strain on the two-dimensional electron gas (2DEG) channel in a flexible Al0.25Ga0.75N/GaN high-electron-mobility transistor (HEMT) by mechanical bending to prove the concept of active polarization engineering to create multifunctional electronic and photonic devices made of flexible group III-nitride thin films. The flexible HEMTs are fabricated by a layer-transfer process and integrated with a 150-μm-thick Cu film. The strain values are estimated from high-resolution x-ray diffraction and Raman spectroscopy in 4-cm bend-down and −4-cm bend-up test conditions. The strain-induced piezoelectric polarization can alter the charge density of the 2DEG in the channel at the AlGaN/GaN interface and thus modify the output characteristics of the flexible HEMTs. Accordingly, output characteristics show an increase in output current by 3.4% in the bend-down condition and a decrease by 4.3% in the bend-up condition. Transfer characteristics show a shift of threshold voltage, which also supports the 2DEG channel modulation during bending. Computational simulation based on the same structure confirms the same current modulation effect and threshold voltage shift. Furthermore, the electrical characteristics of the flexible HEMTs show a repeatable dependence on the strain effect, which offers potential for electro-mechanical device applications.

Vertical alignment of InN- and GaN-based nanopillar crystals grown on a multicrystalline Si substrate

In this study, alignments of nanopillar crystals consisting of indium nitride (InN) and gallium nitride (GaN) based semiconductors were successfully controlled by inserting “steering” nanocrystals between a multicrystalline Si substrate and nanopillar crystals. The directions of the axes of the nanopillar crystals were vertically aligned owing to the effect of the steering nanocrystals even when the nanopillar crystals were grown in conditions in which the directions of the crystal axes tended to be randomly oriented. This technique will enable the realization of novel optoelectronic devices that use nanopillar-shaped crystals consisting of group-III nitride semiconductors fabricated on various non-single crystalline substrates.

GaN-based LEDs using Homo-Epitaxial technology and the Progress and Challenges of HVPE method

GaN and other group III-nitrides are promising materials for optoelectronic devices applications. Currently, sapphire is the most widely used substrate material for GaN LEDs due largely to economic considerations. During the past decade or so, free-standing GaN substrates fabricated via HVPE have gained technological importance and circumventing the shortcomings of sapphire substrates in terms of thermoconductivity and lattice mismatch. This technology, however, does have its own set of challenges. This review summarizes the current state of freestanding GaN substrate technology, the challenges it is facing, and outlook.

Calibration of Polarization Fields and Electro-Optical Response of Group-III Nitride Based c-Plane Quantum-Well Heterostructures by Application of Electro-Modulation Techniques

Calibration of Polarization Fields and Electro-Optical Response of Group-III Nitride Based c-Plane Quantum-Well Heterostructures by Application of Electro-Modulation Techniques

Calibration of Polarization Fields and Electro-Optical Response of Group-III Nitride Based c-Plane Quantum-Well Heterostructures by Application of Electro-Modulation Techniques

The polarization fields and electro-optical response of PIN-diodes based on nearly lattice-matched InGaN/GaN and InAlN/GaN double heterostructure quantum wells grown on (0001) sapphire substrates by metalorganic vapor phase epitaxy were experimentally quantified. Dependent on the indium content and the applied voltage, an intense near ultra-violet emission was observed from GaN (with fundamental energy gap Eg = 3.4 eV) in the electroluminescence (EL) spectra of the InGaN/GaN and InAlN/GaN PIN-diodes. In addition, in the electroreflectance (ER) spectra of the GaN barrier structure of InAlN/GaN diodes, the three valence-split bands, Γ9, Γ7+, and Γ7−, could selectively be excited by varying the applied AC voltage, which opens new possibilities for the fine adjustment of UV emission components in deep well/shallow barrier DHS. The internal polarization field Epol = 5.4 ± 1.6 MV/cm extracted from the ER spectra of the In0.21Al0.79N/GaN DHS is in excellent agreement with the literature value of capacitance-voltage measurements (CVM) Epol = 5.1 ± 0.8 MV/cm. The strength and direction of the polarization field Epol = −2.3 ± 0.3 MV/cm of the (0001) In0.055Ga0.945N/GaN DHS determined, under flat-barrier conditions, from the Franz-Keldysh oscillations (FKOs) of the electro-optically modulated field are also in agreement with the CVM results Epol = −1.2 ± 0.4 MV/cm. The (absolute) field strength is accordingly significantly higher than the Epol strength quantified in published literature by FKOs on a semipolar ( 11 2 ¯ 2 ) oriented In0.12Ga0.88N quantum well.

Decomposition of ammonia on ZrB2(0001)

Zirconium diboride has been recently identified as a promising substrate for the growth of Group-III nitride semiconductors using reactive vapors that include ammonia as the nitrogen source. Adsorption energies and dissociation pathways of NH3 on the (0001) surface of ZrB2 were investigated using density functional theory calculations. Our results indicate that NH3 readily adsorbs onto the ZrB2 surface terminated with Zr and decomposes to atomic N and H with relatively small activation barriers. The resulting atomic species are found to be mobile with the computed diffusion barriers between 0.11 and 0.78 eV.

Review—Visible LEDs: More than Efficient Light

The lighting sector has seen a tremendous technology revolution in the recent few decades. This process has been fueled by light emitting diodes (LEDs), when group-III nitride materials became available for semiconductor devices by groundbreaking technological discovery of p-type conductivity in gallium nitride (GaN). Ever since this Nobel prize winning discovery, the race of improving the efficiency and the efficacy of visible LEDs has been the dominant driving force, enabled by novel designs and processes to exploit novel materials, to improve material quality, electrical and thermal resistance, light extraction and beam shaping. Technology trends like miniaturization and integration of electronics and photonics have led LED markets to expand to a variety of novel application fields, such as illumination, sensing, healthcare and visualization, that are far beyond the traditional general lighting market. Integration of LEDs and other electronic devices like CMOS transistor or photodiodes adds novel functionalities in the smallest dimensions.

Thermodynamic model for metalorganic vapor-phase epitaxy of N-polar group-III nitrides in step-flow growth mode: Hydrogen, competitive adsorption, and configuration entropy

A thermodynamic model for metalorganic vapor-phase epitaxy (MOVPE) of the N-polar $(000\overline{1})$ binary group-III nitrides (AlN, GaN, and InN) in the step-flow growth mode is proposed based on the Burton, Cabrera, and Frank (BCF) theory. The coverages of the group-III adatoms are thermodynamically evaluated under competitive adsorption with hydrogen, which is used as a carrier gas or dissociated from the ${\mathrm{NH}}_{3}$ source gas during MOVPE. The chemical potentials of the group-III and H adatoms on N-polar group-III nitride surfaces are modeled using the respective bond energies with the surface N atoms of the nitride and the vibrational frequencies of the adatoms. The coverages of the coadsorbed group-III and H adatoms are calculated using the Langmuir adsorption isotherm with these chemical potentials. The configuration entropy of the group-III adatoms bridges the gap between the thermodynamic model and the BCF theory. The coverage of the group-III adatoms plays a role like partial pressure of the group-III gas in the thermodynamics. The equilibrium coverage of the group-III adatoms and the equilibrium pressure of the ${\mathrm{NH}}_{3}$ gas are evaluated from the conditions of Gibbs energy balance between the sources (group-III adatom and ${\mathrm{NH}}_{3}$ gas molecule) and products (group-III nitride and $3/2\phantom{\rule{0.28em}{0ex}}{\mathrm{H}}_{2}$ gas molecules) and of speed balance between group-III and N incorporation into step kinks. Fair agreements with the experimentally optimized growth conditions for MOVPE of N-polar GaN and InN are obtained by this method. Among the examined binary group-III nitrides, AlN growth is hardly affected by ${\mathrm{H}}_{2}$ gas pressure, GaN growth is controlled well by ${\mathrm{H}}_{2}$ gas pressure, and InN growth is strongly inhibited by ${\mathrm{H}}_{2}$ gas. A criterion for selecting the ${\mathrm{NH}}_{3}$/group-III flow ratio for maximum products/cost and minimum waste of the materials is demonstrated using the growth model and the estimated growth parameters. The offcut angle dependence of the growth rate on the vicinal substrates is also investigated.

N-polar AlN nucleation layers grown by hot-wall MOCVD on SiC: Effects of substrate orientation on the polarity, surface morphology and crystal quality

Hot-wall metalorganic vapor phase epitaxy enables a superior quality of group-III nitride epitaxial layers and high electron mobility transistor structures, but has not yet been explored for N-polar growth. In this work, we aim at achieving N-polar AlN nucleation layers (NLs) with optimized properties for subsequent growth of GaN device heterostructures. The effects of substrate orientation on the polarity, surface morphology and crystalline quality of AlN NLs on on-axis C-face SiC (0001̄), C-face SiC (0001̄) off-cut towards the [112̄0] by 4°, and Si-face SiC (0001) are investigated. The results are discussed in view of growth mode evolution with growth temperature and substrate orientation. It is demonstrated that N-polar AlN NLs with step-flow growth mode and 0002 rocking curve widths below 20 arcsec can be achieved on off-axis C-face SiC substrates.

Remote epitaxy using graphene enables growth of stress-free GaN

The properties of group III-Nitrides (III-N) such as a large direct bandgap, high melting point, and high breakdown voltage make them very attractive for optoelectronic applications. However, conventional epitaxy on SiC and sapphire substrates results in strained and defective films with consequently poor device performance. In this work, by studying the nucleation of GaN on graphene/SiC by MOVPE, we unambiguously demonstrate the possibility of remote van der Waals epitaxy. By choosing the appropriate growth conditions, GaN crystals can grow either in-plane misoriented or fully epitaxial to the substrate. The adhesion forces across the GaN and graphene interface are very weak and the micron-scale nuclei can be easily moved around. The combined use of x-ray diffraction and transmission electron microscopy demonstrate the growth of stress-free and dislocation-free crystals. The high quality of the crystals was further confirmed by photoluminescence measurements. First principles calculations additionally highlighted the importance of the polarity of the underlying substrate. This work lays the first brick towards the synthesis of high quality III-N thin films grown via van der Waals epitaxy.

Effects of nitrogen flow rate on the morphology and composition of AlGaN nanowires grown by plasma-assisted molecular beam epitaxy

Nitrogen flow rate is one of the important growth parameters for the growth of group-III nitride nanowires in plasma-assisted molecular beam epitaxy. However, nitrogen flow rate has received less attention compared to the group-III metal fluxes since its effects are not as prominent as that of the metal fluxes. In this study, we investigated the effects of nitrogen flow rate on the morphology and composition of AlGaN nanowires. Reducing the nitrogen flow rate improved the structural uniformity and increased the Al composition. We present a composition change model and show that excess nitrogen suppresses Ga desorption by recombining the Ga atoms, thereby causing a change in the composition of AlGaN. It was confirmed that the influence of the nitrogen flow rate on the Al composition varied with the growth temperature. These results provide insights into the role of nitrogen flow rate on the growth of AlGaN nanowires and suggest that more sophisticated growth control is possible by considering the nitrogen flow rate.

Output characteristics of thin-film flexible piezoelectric generators: A numerical and experimental investigation

Flexible piezoelectric generators are promising energy harvesters for future self-powered electronics by converting surrounding mechanical energy into electricity. Although many types of flexible piezoelectric generators have been demonstrated, their output behaviors are still not systematically reported due to the complexity since the behaviors depend on many ingredients including piezoelectric device parameters and external mechanical energy conditions. In this research, numerical simulations based on a buckle-bending model were carried out to systematically investigate the output characteristics of flexible piezoelectric generators. The reliability of the numerical results were verified through experimentation using group III-nitride thin-film flexible piezoelectric generators. The ideal open-circuit voltage and short-circuit current density are proportional to the strain and strain rate of the piezoelectric material, respectively. For a specific device at a certain mechanical condition, the piezoelectric output varies with load resistance and shows a maximum power density at a certain load resistance, i.e., optimum load resistance. The optimum load resistance increases linearly with piezoelectric material thickness, inverse of device area, and bending time, while does not change with flexible substrate thickness, and bending extent. The optimum power density increases linearly with piezoelectric material thickness, square of flexible substrate thickness, inverse of bending time, and bending extent, while does not change with device area. The detailed understanding of the output characteristics of flexible piezoelectric generators can help the optimization of device configuration for better piezoelectric energy harvesting.

(Invited) Nanostructures and Crystal Defects in Thick GaN and SiC Epitaxial Layers for Power Electronic Switches

The state-of-the-art power switching devices made from SiC and GaN semiconductors contain a high density of crystal defects. Most of these defects are present 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 environments. This paper provides a review of the current state-of-the-art of SiC and GaN power semiconductor material technology, and the potential impact crystal defects may have on high-density power switching electronics. Group III-Nitride materials suffer from both extended and point defects, each of which will challenge the material’s application in both vertical and lateral power devices. The extended defects include vertical threading dislocations of both edge and screw type. The latter defects have been shown to be correlated to leakage in vertical two terminal device structures while the influence of the former is still undetermined and remains a critical research issue. Channel surfaces in vertical three terminal devices will also degrade due to vertical threading dislocations. These extended defects occur in all epitaxial layers grown on c-plane substrates (the predominant and largest area substrate type) and are the result of the lack of a high quality substrate bulk material as well as substrate surface. The present substrate technology after multiple efforts (either GaN, SiC or Si) still have a high defect density indicating that low-defect seeds may be non-existing. Point defects influence the background carrier concentration in low-doped layers and various recombination processes. The intrinsic nature of III-N materials makes nitrogen-vacancies a predominant point defect that automatically dopes the crystal n-type, but there are impurities (oxygen and carbon) from most growth environments that also contribute to conductivity. These defects are not well understood and not well controlled. Such defects must be minimized in order to realize thick epitaxial layers with drift regions that will support both high blocking voltages and low on-state resistances. A wide range of materials characterization methods have been applied to further understand these impurities. The semiconductor and dielectric materials used in the construction of power electronics switching devices employed for energy efficiency applications experience extreme electrical and thermal stresses during the on-state as well as when switching high voltages and high currents. For example, in a bipolar type power semiconductor switching device in order to achieve significant conductivity modulation and reduce the drift-region resistance, high-level injection of minority carriers is typical. If the semiconductor contains a high density of crystal defects, as is the case with WBG semiconductors, minority carrier recombination phenomenon can be adversely affected. For example, it has been shown more than a decade ago [3, 4] that basal plane dislocations (BPDs) present in SiC are excited by the energy released from minority carrier recombination process; the result is excess BPD generation and its glide within the semiconductor material that leads to an increase in the drift-region resistance and thermal run-away in a forward biased bipolar junction diode The reported experimental data for high-voltage SiC semiconductor strongly suggests that material defects have profound impact on its characteristics under extreme environments. Since GaN semiconductor has even higher density of material defects, this problem is further exacerbated especially in thick epitaxial GaN layers which are required for power switches beyond 5kV. The integration of the pillar junction scheme with the trench-gate GaN MOSFET processes developed at the University of Maryland will be reported. The thick pillar junction epitaxial layers have been grown by a commercial vendor, using a windowed GaN growth process and/or selective deep etching, followed by GaN regrowth to fabricate columns with opposite carrier polarities. The doping density of the n--pillars under the gate has been specified with background carrier concentration n ≤ 1x1016 cm-3, while the p-pillars will have a higher doping density (p ≥ 1x1018 cm-3) to maximize depletion in the n--pillar. The p-body and n+-source layers are grown by subsequent epitaxial growth across the whole structure. Our current research into optimized trench gate fabrication and gate dielectric processing has been used to fabricate gate trenches into the structure. These devices can be compared to traditional trench MOSFETs with extremely thick blocking layers of comparable thickness to the pillar junction layer in order to determine the role of defects in thick GaN epi layers in terms of limiting device performance and reliability.

Valence and conduction band offsets at beryllium oxide interfaces with silicon carbide and III-V nitrides

Beryllium oxide (BeO) is a large bandgap material with extreme properties that make it an ideal gate dielectric for pairing with other wide bandgap semiconductors such as silicon carbide (SiC) and gallium nitride (GaN). In this regard, the authors have utilized x-ray photoemission spectroscopy to determine the valence band offset (VBO) between atomic layer deposited (ALD) BeO and epilayers of the cubic form of silicon carbide (3C-SiC) grown on crystalline silicon (Si) substrates. The BeO VBO with 3C-SiC epilayers grown on both Si (111) and (001) substrates was determined to be 1.6 ± 0.1 and 1.5 ± 0.1 eV, respectively. Applying the band alignment rules of transitivity and commutativity, the authors additionally determine the VBO for BeO with GaN, aluminum nitride, and hexagonal boron nitride to be 0.9 ± 0.2, 0.7 ± 0.3, and 1.0 ± 0.2 eV, respectively. Utilizing the reported bandgap for ALD BeO (8.0 ± 0.1 eV) and literature values for SiC and the group III-nitrides (III-N), the authors find a type I band alignment with conduction band offsets >1 eV in all cases. These results indicate that BeO is a promising dielectric for wide bandgap SiC and III-N high-power, high-temperature, and high-frequency device applications.