Bulk GaN Research Articles

Comprehensive device modeling for AlGaN/GaN based Schottky barrier diodes with beveled p-GaN termination to control electric field

In this article, we have systematically investigated the impact of different structural parameters on the electrical characteristics for AlGaN/GaN based Schottky barrier diodes (SBDs) with beveled p-GaN termination by using TCAD simulation tools. The p-GaN termination can decrease the electric field at the Schottky contact region, thereby suppressing the electrical field magnitude in the Schottky contact region. Then, the breakdown voltage can be enhanced without remarkably sacrificing the forward conduction characteristics. However, in spite of the reduced electric field magnitude in the Schottky contact region, the electric field will possess the peak value at the edge of p-GaN termination. Therefore, the premature breakdown can occur when the electric field therein reaches critical value. Hence, we have comprehensively manipulated the electric field distributions by designing different p-GaN terminations and detailed optimization strategy for the AlGaN/GaN based Schottky barrier diodes has been conducted. Moreover, we find that the strong electric field at the p-GaN termination edge can be reduced by using beveled p-GaN termination, which can extend the electric potential into the bulk GaN region. With the developed defected-related physical models, we also find that the p-GaN termination suppress the capture and release process for the carriers by defects, which improves the dynamic characteristics for the proposed devices.

Band order reversal and valley engineering driving to dual high electron and hole mobility in strained monolayer BS

Strain engineering is an effective method to tune the physical properties of materials. In this work, we found that applying 5% biaxial compression to monolayer boron sulfur can simultaneously induce a band order reversal of valence bands and valley engineering of conduction bands. This change significantly enhances the room temperature intrinsic mobility, boosting it from 2 to 260 cm2 V−1 s−1 for holes and 63 to 543 cm2 V−1 s−1 for electrons. The high hole mobility surpasses that of bulk GaN and silicon. We further demonstrate that the valence band order reversal not only lifts the px/y state above the pz states, giving to a more dispersive highest valence band, and thus smaller electron scattering channels and hole effective mass, but also shifts the stronger σz bonding to weaker π-bonding characteristics, resulting in smaller electron–phonon coupling strength. Those combined effects results in a substantial increase in hole mobility.

Density functional theory study on the electronic structures and piezoelectric properties of hydrogenated monolayer II-VI semiconductor XYH2 (X = Zn, Cd; Y = S, Se, Te)

Piezoelectric materials have been widely used in various fields such as sensing and energy catalysis. This paper primarily investigates the structural, electronic and piezoelectric properties of hydrogenated II-VI monolayer semiconductor XYH2 (X = Zn, Cd; Y = S, Se, Te) by density functional theory. Results reveal that XYH2 monolayers exhibit stable hexagonal structures and tunable bandgaps ranging from 3.27 eV to 4.30 eV. Density functional perturbation theory is employed to investigate the out-of-plane piezoelectric coefficients of XYH2 monolayers. CdSH2 exhibits a value of −1.8 pm/V of out-of-plane piezoelectric coefficients which surpasses typical piezoelectric materials such as bulk ZnS (0.076 pm/V), ZnO (0.21 pm/V), CdS (0.143 pm/V), CdSe (0.104 pm/V), BN (0.33 pm/V) and GaN (0.96 pm/V). Our results indicate that hydrogenated II-VI monolayers can be a new type of novel piezoelectric semiconductors and hold a potential application for the piezoelectric devices.

High-performance negative differential resistance characteristics in homoepitaxial AlN/GaN double-barrier resonant tunneling diodes

In this work, high-performance negative differential resistance (NDR) characteristics are demonstrated in homoepitaxial AlN/GaN double-barrier resonant tunneling diodes (RTDs). The devices are grown by plasma-assisted MBE on bulk GaN substrates and exhibit robust and repeatable NDR at RT. A high peak current density of 183 kA cm−2 is simultaneously demonstrated with a large peak-to-valley current ratio of 2.07, mainly benefiting from the significantly reduced dislocation density and improved hyper-abrupt heterointerfaces in the active region, which boosts the electron quantum transport in the resonant tunneling cavity. The achievement shows the promising potential to enhance the oscillation frequency and output power of GaN-based RTD oscillators, imperative for next-generation high-power solid-state compact terahertz oscillators application.

Structural and optical properties of epitaxial ScxAl1−xN coherently grown on GaN bulk substrates by sputtering method

Three scandium aluminum nitride (ScAlN) thin films with different Sc compositions of 6%, 10%, and 14% were heteroepitaxially grown on n-type GaN bulk substrates by a low-temperature sputtering method. Atomically flat and smooth surfaces were observed by atomic force microscopy. The ScAlN films were coherently grown on GaN, and the c-axis lattice constants increased with increase in the Sc composition, confirmed by x-ray diffraction. The refractive index and the extinction coefficient of ScAlN were extracted by variable angle spectroscopic ellipsometry. The refractive index slightly increased and the extinction coefficient showed red shift with increase in the Sc composition. The optical bandgap of the ScAlN films was also extracted, which slightly shrunk with increase in the Sc composition.

Electronic structure, optical properties, and thermoelectric properties of Mg-doped GaN materials

This work was based on first principles calculations and investigated the electronic structure, optical properties, and thermoelectric properties of Mg doped GaN bulk materials. It analyzed the effect of Mg impurities defects on the properties of GaN bulk materials. Mg doping can transform GaN materials into p-type semiconductors, which is crucial for achieving p-n junctions or p-type layers in GaN based electronic devices. The calculation results showed that MgN-GaN and MgInterstitial-GaN(MgI-GaN) belong to n-type doping, while MgGa-GaN belongs to p-type doping. In addition, the optical properties and thermoelectric properties of different GaN doping models were calculated. It was found that the maximum ZT values of the MgN-GaN, MgGa-GaN, MgI-GaN doping models reached 4.46, 5.09 and 5.35, respectively. The Mg impurities can help improve the ZT value of semiconductors compared to intrinsic GaN material. This research result has significance for the application of GaN based semiconductor materials in thermoelectric fields.

Characteristics of Vertical Transistors on a GaN Substrate Fabricated via Na‐Flux Method and Enlargement of the Substrate Surpassing 6 Inches

The Na‐flux method is expected to be a key GaN growth technique for obtaining ideal bulk GaN crystals. Herein, the structural quality of the latest GaN crystals grown using the Na‐flux method and, for the first time, the characteristics of a vertical transistor fabricated on a GaN substrate grown using this method are discussed. Vertical transistors exhibit normally off operation with a gate voltage threshold exceeding 2 V and a maximum drain current of 3.3 A during the on‐state operation. Additionally, it demonstrates a breakdown voltage exceeding 600 V and a low leakage current during off‐state operation. It is also described that the variation in the on‐resistance can be minimized using GaN substrates with minimal off‐angle variations. This is crucial for achieving the large‐current chips required for future demonstration of actual devices. In addition, the reverse I–V characteristics of the parasitic p–n junction diode (PND) structures indicate a reduction in the number of devices with a significant leakage current compared to commercially available GaN substrates. Finally, a circular GaN substrate with a diameter of 161 mm, surpassing 6 inches, grown using the Na‐flux method is demonstrated, making it the largest GaN substrate aside from those produced through the tiling technique.

Efficient Second‐Harmonic Generation with Weak Polarization Sensitivity in Gallium Nitride Metasurfaces via Bound States in the Continuum

AbstractBound states in the continuum (BICs) are widely exploited in all‐dielectric metasurfaces to significantly enhance the Q‐factor and second harmonic generation (SHG). However, a higher SHG conversion efficiency is overshadowed by the narrow transparency window of all‐dielectric materials and strict requirement of polarization‐matching. Herein, an augmented SHG assisted is demonstrated by symmetry‐protected quasi‐BICs from z‐cut wurtzite GaN metasurfaces, whose wide transparent range suppresses self‐absorption of second harmonic. Strong resonances emerge under both X‐ and Y‐polarized excitations, where the multipole characters of each eigenmode are quantitatively identified via decomposition simulations. The local field enhancement enabled by the extreme energy confinement of quasi‐BICs contributes to an efficient SHG, recording a conversion efficiency up to 2 × 10−7 at moderately low input peak intensity (≈1 GW cm−2), orders of magnitude larger than bulk GaN. With proper design, the overlapping X‐ and Y‐polarized resonances can support SHG from excitation with arbitrary polarizations. The alleviation of polarization‐matching remarkably enhances the SHG power by 66.2% under unpolarized illumination condition. The results of intense SHG reveal GaN as a promising candidate for high‐performance nanoscale nonlinear photonic devices. The weak polarization sensitivity of the designed structure will chart a novel course for the advancement of next‐generation efficient all‐dielectric metasurfaces.

Vertical GaN Trench‐MOSFETs Fabricated on Ammonothermally Grown Bulk GaN Substrates

Herein, the fabrication and characterization of vertical GaN trench‐MOSFETs on ammonothermally grown bulk GaN substrates have been reported. A number of technological processes have been developed, including, among others, low‐resistance ohmic contacts to Ga‐face n‐GaN epitaxial layers, N‐face backside ohmic contact, vertical sidewall trench etching processes, surface preparation, and atomic layer deposition of gate dielectric layers and integrated with fabrication process flow of vertical power devices. The fabricated test structures are characterized by an output drain current of 288 ± 74 mA mm−1, threshold voltage of about 10 V, and field‐effect channel mobility 13.1 ± 5.0 cm2 (Vs)−1 at 10 V drain‐source voltage and up to 65 cm2 (Vs)−1 at 0.1 V drain‐source voltage. In addition, first, experiments toward high current multicell transistor fabrication are carried out. Multicell test devices with hexagonal topology with a total gate width of 11.1 mm and output current over 1 A are successfully fabricated and characterized.

Charge-Controlled Energy Optimization of the Reconstruction of Semiconductor Surfaces: sp3-sp2 Transformation of Stoichiometric GaN(0001) Surface to (4 × 4) Pattern.

It was demonstrated by ab initio calculations that energy optimization in the reconstruction of semiconductor surfaces is controlled by the global charge balance. The charge control was discovered during simulations of the influence of heavy doping in the GaN bulk, which changes sp3 to sp2 ratio in the reconstruction of stoichiometric GaN(0001), i.e., a Ga-polar surface. Thus, the reconstruction is not limited to the charge in the surface only; it can be affected by the charge in the bulk. The discovered new reconstruction of the GaN(0001) surface is (4 × 4), which is different from the previously reported (2 × 1) pattern. The undoped GaN reconstruction is surface charge controlled; accordingly, (3/8) top-layer Ga atoms remain in a standard position with sp3 hybridized bonding, while the remaining (5/8) top-layer Ga atoms are shifted into the plane of N atoms with sp2 hybridized bonding. The change in the charge balance caused by doping in the bulk leads to a change or disappearance of the reconstruction pattern.

Low field mobility in bulk GaN and its ternary AlGaN/GaN compounds (quantum kinetic approach)

Analytical expressions for the low-field mobility of charge carrier gases with three-(3D), two-(2D) and one-(1D) dimensionalities are obtained. Multi-ion ionized impurities scattering, acoustic and polar optic phonons are considered as scattering mechanisms. The calculated values of mobility are compared to known experimental data for bulk (3D) n-and p-type wurtzite, n-type zinc-blende GaN crystals and low dimensional (2D and 1D) ternary GaAlN compounds. The resulting analytical expressions give the dependences of mobility on dimensionality of charge carrier gas, its density, effective mass, temperature and confining dimensions. A comparison of the experimental and calculated temperature dependences of the mobility in bulk GaN crystals (3D) and in AlGaN/GaN nanowires (1D) shows that the mobility at 100\\,{\ ext{K}}$?> T>100K is determined by the scattering of charge carriers by polar optical phonons with an energy of 91.2 meV. The temperature dependences of mobility in Al0.25Ga0.75N/GaN heterostructures (2D) at 100\\,{\ ext{K}}$?> T>100K are in consistent with experiment for electron scattering by polar optical phonons with a noticeably higher energy of 160 meV. We associate this fact with the heterointerface, which according to well-known theoretical studies can change both the strength of electron polar optical phonons scattering and the energy of the phonons.

Impacts of vacancy complexes on the room-temperature photoluminescence lifetimes of state-of-the-art GaN substrates, epitaxial layers, and Mg-implanted layers

For rooting the development of GaN-based optoelectronic devices, understanding the roles of midgap recombination centers (MGRCs), namely, nonradiative recombination centers and deep-state radiative recombination centers, on the carrier recombination dynamics is an essential task. By using the combination of time-resolved photoluminescence and positron annihilation spectroscopy (PAS) measurements, the origins of major MGRCs in the state-of-the-art GaN epilayers, bulk crystals, and Mg-implanted layers were identified, and their concentrations were quantified for deriving the capture coefficients of minority carriers. In this article, potential standardization of the room-temperature photoluminescence lifetime for the near-band-edge emission (τPLRT) as the concentration of major MGRCs well below the detection limit of PAS is proposed. For n-GaN substrates and epilayers grown from the vapor phase, τPLRT was limited by the concentration of carbon on N sites or divacancies comprising a Ga vacancy (VGa) and a N vacancy (VN), [VGaVN], when carbon concentration was higher or lower, respectively, than approximately 1016 cm−3. Here, carbon and VGaVN act as major deep-state radiative and nonradiative recombination centers, respectively, while major MGRCs in bulk GaN crystals were identified as VGa(VN)3 vacancy clusters in Na-flux GaN and VGa or VGaVN buried by a hydrogen and/or VGa decorated with oxygen on N sites, VGa(ON)3–4, in ammonothermal GaN. The values of τPLRT in n-GaN samples are compared with those of p-GaN, in which τPLRT was limited by the concentration of VGa(VN)2 in Mg-doped epilayers and by the concentrations of VGaVN and (VGaVN)3 in Mg-implanted GaN right after the implantation and after appropriate activation annealing, respectively.

Fabrication of free-standing GaN substrates using electrochemically formed porous separation layers

We have developed a pore-assisted separation (PAS) method for the fabrication of free-standing GaN substrates, where bulk GaN crystals were separated from seed GaN templates at electrochemically formed porous layers. The pore size was controlled by the electrochemical process conditions and must be greater than 100 nm to realize separation within whole wafers. A 2 inch free-standing GaN substrate having a low dislocation density of ∼2.7 × 106 cm−2 was realized by growth of an 800 μm thick GaN layer on the porous GaN template. A 3 inch free-standing GaN substrate was also fabricated by the PAS method, indicating its good scalability.

Exploration of the electronic structure and thermoelectric properties of the carbon-doped bulk GaN materials by first-principles calculations

As a common impurity, carbon was related to the physical properties of GaN-based materials closely. By using density functional theory and Boltzmann transport theory, the electronic structure and thermoelectric properties of carbon doped GaN were studied. The band structure, electronic density of states and thermoelectric properties were calculated. Comparing with the intrinsic bulk-GaN, (1) It was found that the Fermi level moves down, and the band gap becomes narrow for GaN with CN point defects. (2) Power factors become decrease, while thermal conductivity decreases more significantly. Therefore, electrical thermoelectric figure of merit (ZTe) increases, especially, the ZTe increase up to 1.150 at 500 K for GaN with CN point defects. The research could provide reference for experimental studies of GaN-based materials.

Evaluation of defect density in bulk gallium nitrides by photothermal deflection spectroscopy and steady-state photocapacitance methods

Bulk GaN samples were characterized by both photothermal deflection spectroscopy (PDS) and steady-state photocapacitance (SSPC) methods. The PDS signal intensity in the bandgap was found to correlate quantitatively with the defect density estimated by the SSPC method. The defect density of GaN bulks fabricated by hydride vapor phase epitaxy (HVPE) was decreased by controlling the incorporation of carbon and silicon impurities. Differences in the reciprocal of the slope near the valence band maximum and the signal intensity in the bandgap among HVPE GaN bulks could be detected by PDS, although they had the same crystalline quality. PDS can be used to evaluate the GaN bulks that have been improved with a highly insulative property caused by Fe- doping or low carbon incorporation.

Suppression of Mixing of Metastable Zincblende Phase in GaN Crystal Grown on ScAlMgO4 Substrates by Radio‐Frequency Plasma‐Assisted Molecular Beam Epitaxy

ScAlMgO4 (SAM) substrates have attracted significant attention as template substrates for fabricating bulk GaN crystals. Radio‐frequency plasma‐assisted molecular beam epitaxy (RF‐MBE) is preferred for the growth of the first GaN layer. However, the metastable zincblende (ZB) phase is mixed on the SAM substrate with a smaller terrace width, coupled with lower growth temperatures in MBE. Herein, the time evolution of GaN growth on SAM substrates is investigated to elucidate the mechanism underlying ZB mixing. ZB‐GaN is generated as stacking faults during the coalescence of the initially grown wurtzite‐ (WZ‐) GaN islands on adjacent SAM terraces, owing to the extraordinarily large step height close to three bilayers of WZ‐GaN. Therefore, high‐temperature growth on a SAM substrate with wider terraces suppresses the generation of the ZB phase in the initial growth stage. ZB‐GaN also decreases with an increase in the growth time. Based on these results, a 940 nm‐thick GaN layer is grown at 750 °C on the SAM substrate with a terrace width of 820 nm. A pure WZ‐GaN layer from the GaN/SAM interface with a flat surface with a root mean square value of below 1 nm and a dislocation density of 7.3 × 109 cm−2 is obtained.

Molecular beam epitaxy of GaN/AlGaN quantum wells on bulk GaN substrate in the step-flow or step meandering regime: Influence on indirect exciton diffusion

GaN/AlxGa1−xN quantum wells were grown by molecular beam epitaxy on high quality bulk (0001) GaN substrates. The quantum well thickness was set in the 6–8 nm range to favor the photoluminescence emission of indirect excitons. Indeed, such excitons are known to be spatially indirect due to the presence of the internal electric field which spatially separates the electron and hole wave functions. The growth conditions were optimized in view of minimizing the photoluminescence peak broadening. In particular, the impact of growth temperature (up to 900 °C) on the surface morphology, structural, and photoluminescence properties was studied. The diffusion of indirect excitons on the scale of tens of micrometers was measured with a micro-photoluminescence setup equipped with a spatially resolved detection. A dedicated model and its analysis allow us to extract from these measurements the exciton diffusion constant and to conclude on the optimum growth conditions for the GaN/AlxGa1−xN quantum well structures suited for studies of quantum collective effects in indirect exciton liquids.

Role of carbon in n-type bulk GaN crystals

In this work co-doping with silicon and carbon in gallium nitride grown by halide vapor phase epitaxy was performed. Native seeds of high structural quality with bowing radii of (0001) crystallographic planes higher than 15 m and threading dislocation density on the order of 5 × 104 cm−3 were used. The crystallized material was examined in terms of structural, optical, and electrical properties. For this purpose different characterization methods: X-ray diffraction, Raman spectroscopy, low-temperature photoluminescence, and Hall measurements, were applied. The physical properties of samples doped only with silicon and co-doped with carbon were analyzed and compared. However, during the crystallization process, due to high amount of Si and C, clusters of silicon carbide were unintentionally formed. The formation of these SiC clusters had an impact on the properties of the GaN crystals that were being studied. The research provides an initial insight into the complex studies of co-doping gallium nitride crystals. The results of this paper give us groundwork for more in-depth exploration of this topic.

Thermal behavior of irradiation-induced-deep level in bulk GaN used for fabricating blue light emitting diodes

Bulk GaN was irradiated by 2 MeV electron beam at a fluence of 5 × 1016 cm2 and studied by deep level transient spectroscopy (DLTS). After irradiation, a broad peak, including at least two traps, was detected. The trap D1 (EC – 0.16 eV) observed from a broad peak is induced during the annealing process below 550K and completely annealed out at 550K after ~ 10 hours. The annealing process at 550K also forms a new trap D2 (EC – 0.25 eV). From the isothermal study the activation energy of the trap D2 in the annihilation process is obtained and has a value of 1.3 ± 0.3 eV. The pre-factor of the annihilation process suggested this process to be related to the free-carrier capture by multi-phonon emission. From the thermal behavior, the trap D2 was suggested to be related to gallium vacancy.

Electron effective mass in GaN revisited: New insights from terahertz and mid-infrared optical Hall effect

Electron effective mass is a fundamental material parameter defining the free charge carrier transport properties, but it is very challenging to be experimentally determined at high temperatures relevant to device operation. In this work, we obtain the electron effective mass parameters in a Si-doped GaN bulk substrate and epitaxial layers from terahertz (THz) and mid-infrared (MIR) optical Hall effect (OHE) measurements in the temperature range of 38–340 K. The OHE data are analyzed using the well-accepted Drude model to account for the free charge carrier contributions. A strong temperature dependence of the electron effective mass parameter in both bulk and epitaxial GaN with values ranging from (0.18 ± 0.02) m0 to (0.34 ± 0.01) m0 at a low temperature (38 K) and room temperature, respectively, is obtained from the THz OHE analysis. The observed effective mass enhancement with temperature is evaluated and discussed in view of conduction band nonparabolicity, polaron effect, strain, and deviations from the classical Drude behavior. On the other hand, the electron effective mass parameter determined by MIR OHE is found to be temperature independent with a value of (0.200 ± 0.002) m0. A possible explanation for the different findings from THz OHE and MIR OHE is proposed.