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Showing papers on "Gallium nitride published in 2016"


Journal ArticleDOI
TL;DR: The synthesis of 2D gallium nitride (GaN) via a migration-enhanced encapsulated growth (MEEG) technique utilizing epitaxial graphene is demonstrated and it is established that graphene plays a critical role in stabilizing the direct-bandgap, 2D buckled structure.
Abstract: A method to synthesize 2D layers of gallium nitride on SiC is reported. Epitaxial graphene preliminarily grown on SiC allows intercalation of gallium atoms on the SiC substrate and stabilizes the 2D gallium nitride islands formed by ammonolysis. The spectrum of two-dimensional (2D) and layered materials ‘beyond graphene’ offers a remarkable platform to study new phenomena in condensed matter physics. Among these materials, layered hexagonal boron nitride (hBN), with its wide bandgap energy (∼5.0–6.0 eV), has clearly established that 2D nitrides are key to advancing 2D devices1. A gap, however, remains between the theoretical prediction of 2D nitrides ‘beyond hBN’2,3 and experimental realization of such structures. Here we demonstrate the synthesis of 2D gallium nitride (GaN) via a migration-enhanced encapsulated growth (MEEG) technique utilizing epitaxial graphene. We theoretically predict and experimentally validate that the atomic structure of 2D GaN grown via MEEG is notably different from reported theory2,3,4. Moreover, we establish that graphene plays a critical role in stabilizing the direct-bandgap (nearly 5.0 eV), 2D buckled structure. Our results provide a foundation for discovery and stabilization of 2D nitrides that are difficult to prepare via traditional synthesis.

594 citations


Journal ArticleDOI
TL;DR: In this article, a review of the fundamental material properties of gallium nitride (GaN) as they relate to silicon carbide (SiC) and SiC is presented.
Abstract: Power semiconductor devices based on silicon (Si) are quickly approaching their limits, set by fundamental material properties. In order to address these limitations, new materials for use in devices must be investigated. Wide bandgap materials, such as silicon carbide (SiC) and gallium nitride (GaN) have suitable properties for power electronic applications; however, fabrication of practical devices from these materials may be challenging. SiC technology has matured to point of commercialized devices, whereas GaN requires further research to realize full material potential. This review covers fundamental material properties of GaN as they relate to Si and SiC. This is followed by a discussion of the contemporary issues involved with bulk GaN substrates and their fabrication and a brief overview of how devices are fabricated, both on native GaN substrate material and non-native substrate material. An overview of current device structures, which are being analyzed for use in power switching applications, is then provided; both vertical and lateral device structures are considered. Finally, a brief discussion of prototypes currently employing GaN devices is given.

253 citations


Journal ArticleDOI
TL;DR: In this article, a comprehensive study of the mechanisms of Ohmic contact formation on GaN-based materials is presented, discussing the role of single metals composing the stack and the modification induced by the thermal annealing, either on the metal layers or at the interface with GaN.

194 citations


Journal ArticleDOI
TL;DR: In this paper, the performance of three HEMT power switch technologies, namely, Si IGBT, SiC MOSFET, and Gallium nitride (GaN) power switches at 600-V class is evaluated in single-phase T-type inverter.
Abstract: In this paper, benchmark of Si IGBT, SiC MOSFET, and Gallium nitride (GaN) HEMT power switches at 600-V class is conducted in single-phase T-type inverter. Gate driver requirements, switching performance, inverter efficiency performance, heat sink volume, output filter volume, and dead-time effect for each technology is evaluated. Gate driver study shows that GaN has the lowest gate driver losses above 100 kHz and below 100 kHz, SiC has lowest gate losses. GaN has the best switching performance among three technologies that allows high efficiency at high-frequency applications. GaN-based inverter operated at 160-kHz switching frequency with 97.3% efficiency at 2.5-kW output power. Performance of three device technologies at different temperature, switching frequency, and load conditions shows that heat sink volume of the converter can be reduced by 2.5 times by switching from Si to GaN solution at 60 $^{\circ }$ C case temperature, and for SiC and GaN, heat sink volume can be reduced by 2.36 and 4.92 times, respectively, by increasing heat sink temperature to 100 $^{\circ }$ C. Output filter volume can be reduced by 43% with 24, 26, and 61 W increase in device power loss for GaN-, SiC-, and Si-based converters, respectively. WBG devices allow reduction of harmonic distortion at output current from 3.5% to 1.5% at 100 kHz.

191 citations


Journal ArticleDOI
TL;DR: The screening of ternary zinc nitride semiconductors using first-principles calculations of electronic structure, stability and dopability identifies as-yet-unreported CaZn2N2 that has earth-abundant components, smaller carrier effective masses than gallium nitride and a tunable direct bandgap suited for light emission and harvesting.
Abstract: Nitride semiconductors are attractive because they can be environmentally benign, comprised of abundant elements and possess favourable electronic properties. However, those currently commercialized are mostly limited to gallium nitride and its alloys, despite the rich composition space of nitrides. Here we report the screening of ternary zinc nitride semiconductors using first-principles calculations of electronic structure, stability and dopability. This approach identifies as-yet-unreported CaZn2N2 that has earth-abundant components, smaller carrier effective masses than gallium nitride and a tunable direct bandgap suited for light emission and harvesting. High-pressure synthesis realizes this phase, verifying the predicted crystal structure and band-edge red photoluminescence. In total, we propose 21 promising systems, including Ca2ZnN2, Ba2ZnN2 and Zn2PN3, which have not been reported as semiconductors previously. Given the variety in bandgaps of the identified compounds, the present study expands the potential suitability of nitride semiconductors for a broader range of electronic, optoelectronic and photovoltaic applications.

190 citations


Journal ArticleDOI
19 Feb 2016-ACS Nano
TL;DR: Lattice-matched epitaxial growth of molybdenum disulfide (MoS2) directly on gallium nitride (GaN), resulting in high-quality, unstrained, single-layer MoS2 with strict registry to the GaN lattice presents a promising path toward the implementation of high-performance electronic devices based on 2D/3D vertical heterostructures.
Abstract: When designing semiconductor heterostructures, it is expected that epitaxial alignment will facilitate low-defect interfaces and efficient vertical transport. Here, we report lattice-matched epitaxial growth of molybdenum disulfide (MoS2) directly on gallium nitride (GaN), resulting in high-quality, unstrained, single-layer MoS2 with strict registry to the GaN lattice. These results present a promising path toward the implementation of high-performance electronic devices based on 2D/3D vertical heterostructures, where each of the 3D and 2D semiconductors is both a template for subsequent epitaxial growth and an active component of the device. The MoS2 monolayer triangles average 1 μm along each side, with monolayer blankets (merged triangles) exhibiting properties similar to that of single-crystal MoS2 sheets. Photoluminescence, Raman, atomic force microscopy, and X-ray photoelectron spectroscopy analyses identified monolayer MoS2 with a prominent 20-fold enhancement of photoluminescence in the center regions of larger triangles. The MoS2/GaN structures are shown to electrically conduct in the out-of-plane direction, confirming the potential of directly synthesized 2D/3D semiconductor heterostructures for vertical current flow. Finally, we estimate a MoS2/GaN contact resistivity to be less than 4 Ω·cm(2) and current spreading in the MoS2 monolayer of approximately 1 μm in diameter.

183 citations


Journal ArticleDOI
TL;DR: It is found that the much lower thermal conductivity of ZnO originates from the smaller phonon group velocities, larger three-phonon scattering phase space and larger anharmonicity, which will provide in-depth understanding of phonon dynamics for the design of w-ZnO-based electronics.
Abstract: Wurtzite Zinc-Oxide (w-ZnO) is a wide bandgap semiconductor that holds promise in power electronics applications, where heat dissipation is of critical importance. However, large discrepancies exist in the literature on the thermal conductivity of w-ZnO. In this paper, we determine the thermal conductivity of w-ZnO using first-principles lattice dynamics and compare it to that of wurtzite Gallium-Nitride (w-GaN)--another important wide bandgap semiconductor with the same crystal structure and similar atomic masses as w-ZnO. However, the thermal conductivity values show large differences (400 W/mK of w-GaN vs. 50 W/mK of w-ZnO at room temperature). It is found that the much lower thermal conductivity of ZnO originates from the smaller phonon group velocities, larger three-phonon scattering phase space and larger anharmonicity. Compared to w-GaN, w-ZnO has a smaller frequency gap in phonon dispersion, which is responsible for the stronger anharmonic phonon scattering, and the weaker interatomic bonds in w-ZnO leads to smaller phonon group velocities. The thermal conductivity of w-ZnO also shows strong size effect with nano-sized grains or structures. The results from this work help identify the cause of large discrepancies in w-ZnO thermal conductivity and will provide in-depth understanding of phonon dynamics for the design of w-ZnO-based electronics.

134 citations


Journal ArticleDOI
TL;DR: In this paper, the gate degradation was found to be weakly dependent on temperature with an activation energy of 0.1 eV, and the maximum allowed gate operating voltage was estimated using the Weibull statistics.
Abstract: Gate reliability of normally-off p-type-GaN/AlGaN/GaN high-electron mobility transistors grown on Si substrate subjected to forward bias stress at different gate voltages and temperatures was analyzed. Stress-induced gate current degradation was found to be consistent with the percolation process. Obtained time-to-breakdown data were interpreted using the Weibull statistics, and the maximum allowed gate operating voltage was estimated. The gate degradation was found to be weakly dependent on temperature with an activation energy of 0.1 eV.

133 citations


Journal ArticleDOI
TL;DR: A type of single-crystal gallium nitride mesoporous membrane is fabricated and its supercapacitor properties are demonstrated for the first time, which may expand the range of crystals as high-performance electrode materials in the field of energy storage.
Abstract: A type of single-crystal gallium nitride mesoporous membrane is fabricated and its supercapacitor properties are demonstrated for the first time. The supercapacitors exhibit high-rate capability, stable cycling life at high rates, and ultrahigh power density. This study may expand the range of crystals as high-performance electrode materials in the field of energy storage.

122 citations


Journal ArticleDOI
TL;DR: In this paper, the authors demonstrate optical driving at a petahertz frequency in the wide-bandgap semiconductor gallium nitride, which corresponds to instantaneous light-induced switching from insulator to conductor.
Abstract: Experiments showing that electron dynamics can be controlled on attosecond timescales suggest that wide-bandgap semiconductors could be exploited for petahertz signal processing technologies. High-speed photonic and electronic devices at present rely on radiofrequency electric fields to control the physical properties of a semiconductor1, which limits their operating speed to terahertz frequencies (1012 Hz; ref. 2). Using the electric field from intense light pulses, however, could extend the operating frequency into the petahertz regime (1015 Hz; ref. 3). Here we demonstrate optical driving at a petahertz frequency in the wide-bandgap semiconductor gallium nitride. Few-cycle near-infrared pulses are shown to induce electric interband polarization though a multiphoton process. Dipole oscillations with a periodicity of 860 as are revealed in the gallium nitride electron and hole system by using the quantum interference between the two transitions from the valence and conduction band states, which are probed by an extremely short isolated attosecond pulse with a coherent broadband spectrum. In principle, this shows that the conductivity of the semiconductor can be manipulated on attosecond timescales, which corresponds to instantaneous light-induced switching from insulator to conductor. The resultant dipole frequency reaches 1.16 PHz, showing the potential for future high-speed signal processing technologies based on wide-bandgap semiconductors.

119 citations


Proceedings ArticleDOI
01 Dec 2016
TL;DR: In this article, a normally off vertical GaN-based transistor on a bulk GaN substrate with low specific on-state resistance of 1.0 mΩ·cm2 and high off-state breakdown voltage of 0.7 kV is presented.
Abstract: A normally-off vertical GaN-based transistor on a bulk GaN substrate with low specific on-state resistance of 1.0 mΩ·cm2 and high off-state breakdown voltage of 1.7 kV is presented. P-GaN/AlGaN/GaN triple layers are epitaxially regrown over V-shaped grooves formed over the drift layer. The channel utilizes so-called semi-polar face with reduced sheet carrier concentration at the AlGaN/GaN interface, which enables high threshold voltages of 2.5 V and stable switching operations. Note that formation of carbon-doped insulating GaN layer formed on p-GaN well layer underneath the channel suppresses the punch-through current at off-state between the source and drain, which enables good off-state characteristics. The fabricated high-current vertical transistor achieves successful fast switching at 400V/15A. These results indicate that the demonstrated vertical GaN transistor is very promising for future high power switching applications.

Journal ArticleDOI
Rongming Chu1, Yu Cao1, Mary Chen1, Ray Li1, Daniel Zehnder1 
TL;DR: In this paper, the first demonstration of gallium nitride (GaN) complementary metal-oxide-semi-conductor (CMOS) field effect transistors (FETs) was reported.
Abstract: This letter reports the first demonstration of gallium nitride (GaN) complementary metal–oxide–semi-conductor (CMOS) field-effect-transistor technology. Selective area epitaxy was employed to have both GaN N-channel MOSFET (NMOS) and P-channel MOSFET (PMOS) structures on the same wafer. An AlN/SiN dielectric stack grown by metal–organic chemical vapor deposition served as the gate oxide for both NMOS and PMOS, yielding enhancement-mode N- and P-channel with the electron mobility of 300 cm2/V-s and hole mobility of 20 cm2/V-s, respectively. Using the GaN CMOS technology, a functional inverter integrated circuit was fabricated and characterized.

Journal ArticleDOI
TL;DR: Graphene layers transferred on amorphous carrier substrates is a promising alternative to bulk crystalline substrates for the epitaxial growth of high quality GaN nanostructures and a model for the nanowire density variation is proposed.
Abstract: Epitaxial growth of GaN nanowires on graphene is demonstrated using molecular beam epitaxy without any catalyst or intermediate layer. Growth is highly selective with respect to silica on which the graphene flakes, grown by chemical vapor deposition, are transferred. The nanowires grow vertically along their c-axis and we observe a unique epitaxial relationship with the ⟨2110⟩ directions of the wurtzite GaN lattice parallel to the directions of the carbon zigzag chains. Remarkably, the nanowire density and height decrease with increasing number of graphene layers underneath. We attribute this effect to strain and we propose a model for the nanowire density variation. The GaN nanowires are defect-free and they present good optical properties. This demonstrates that graphene layers transferred on amorphous carrier substrates is a promising alternative to bulk crystalline substrates for the epitaxial growth of high quality GaN nanostructures.

Journal ArticleDOI
TL;DR: In this paper, a synthesis of high structural quality and high-purity thick gallium nitride layers by crystallization from vapor phase (hydride vapor phase epitaxy (HVPE)) on 1, 1.5, and 2 inch substrates obtained by a solution (ammonothermal) growth method is presented.
Abstract: Homoepitaxial growth of high structural quality and high-purity thick gallium nitride layers by crystallization from vapor phase (hydride vapor phase epitaxy (HVPE)) on 1, 1.5, and 2 inch substrates obtained by a solution (ammonothermal) growth method is presented. Advantages and disadvantages of both growth technologies are described in detail. Structural, optical, electrical, and thermal properties of gallium nitride grown from the vapor phase are demonstrated and compared to properties of ammonothermally grown material. It is shown that a synergy of these two methods can create new opportunities for an efficient production of bulk gallium nitride crystals and then substrates. It is also shown that free-standing (products of slicing procedures) gallium nitride crystals obtained from growth by vapor phase on ammonothermal substrates can be successfully used as seeds for the next growth process by both discussed methods. Factors limiting HVPE and making it a 'wafer to wafer' technology are presented, clarified, and analyzed. Intentional introduction of silicon to growth of gallium nitride by HVPE and crystals with a high free carrier concentration and high structural quality are demonstrated. First electronic and optoelectronic devices fabricated on the free-standing gallium nitride substrates are shown.

Journal ArticleDOI
TL;DR: In this article, a GaN vertical trench metal-oxide-semiconductor field effect transistor (MOSFET) with normally off operation was reported, with a threshold voltage of 4.8 V, blocking voltage of 600 V at gate bias of 0 V, and on-resistance of $1.7~\Omega $ (10 V).
Abstract: This letter reports a GaN vertical trench metal–oxide–semiconductor field-effect transistor (MOSFET) with normally-off operation. Selective area regrowth of n+-GaN source layer was performed to avoid plasma etch damage to the p-GaN body contact region. A metal-organic-chemical-vapor-deposition (MOCVD) grown AlN/SiN dielectric stack was employed as the gate “oxide”. This unique process yielded a 0.5-mm2-active-area transistor with threshold voltage of 4.8 V, blocking voltage of 600 V at gate bias of 0 V, and on-resistance of $1.7~\Omega $ at gate bias of 10 V.

Proceedings ArticleDOI
01 Dec 2016
TL;DR: By employing an interface protection technique to overcome the degradation of etched GaN surface in high-temperature process, highly reliable LPCVD-SiN x gate dielectric was successfully integrated with recessed-gate structure to achieve high performance enhancement mode (V th ∼ + 2.37 V @ I d = 100 μA/mm) GaN MIS-FETs with high stability and high reliability.
Abstract: By employing an interface protection technique to overcome the degradation of etched GaN surface in high-temperature process, highly reliable LPCVD-SiN x gate dielectric was successfully integrated with recessed-gate structure to achieve high-performance enhancement-mode (V th ∼ +2.37 V @ I d = 100 μA/mm) GaN MIS-FETs with high stability and high reliability. The LPCVD-SiN x /GaN MIS-FET delivers remarkable advantages in high Vth thermal stability, long time-dependent gate dielectric breakdown (TDDB) lifetime and low bias temperature instability (BTI).

Journal ArticleDOI
TL;DR: In this article, the impact of surface roughness after the recessed-anode formation on device characteristics is investigated, and an improved surface condition can reduce the leakage current and enhance the breakdown voltage simultaneously.
Abstract: In this letter, we demonstrate high-performance AlGaN/GaN Schottky barrier diodes (SBDs) on Si substrate with a recessed-anode structure for reduced turn-on voltage $V_{\mathrm {ON}}$ . The impact of the surface roughness after the recessed-anode formation on device characteristics is investigated. An improved surface condition can reduce the leakage current and enhance the breakdown voltage simultaneously. A low turn-on voltage of only 0.73 V can be obtained with a 50-nm recess depth. In addition, the different lengths of Schottky extension acting like a field plate are investigated. A high reverse breakdown voltage of 2070 V and a low specific ON-resistance of 3.8 $\text{m}\Omega \cdot \textrm {cm}^{2}$ yield an excellent Baliga’s figure of merit of 1127 MW/cm2, which can be attributed to the low surface roughness of only 0.6 nm and also a proper Schottky extension of 2 $\mu \text{m}$ to alleviate the peak electric field intensity in the SBDs.

Journal ArticleDOI
TL;DR: In this paper, the electronic and optical properties of titanium doped gallium nitride (GaN) nanowires were investigated using density functional theory (DFT) and showed that the donor states of near the Fermi level are mainly formed by Ti-3d orbital.

Journal ArticleDOI
TL;DR: In this article, a bilayer edge termination (ET) structure was proposed for GaN power diodes with a low threading dislocation defect density ( $10^{4}-10^{5}$ cm $^{-2}$ ) and a 15-μm-thick n-type drift layer with a free carrier concentration of $5\times 10−15$ cm.
Abstract: Vertical GaN power diodes with a bilayer edge termination (ET) are demonstrated. The GaN p-n junction is formed on a low threading dislocation defect density ( $10^{4}-10^{5}$ cm $^{-2}$ ) GaN substrate, and has a 15- $\mu \text{m}$ -thick n-type drift layer with a free carrier concentration of $5\times 10^{15}$ cm $^{-3}$ . The ET structure is formed by $N$ implantation into the p+-GaN epilayer just outside the p-type contact to create compensating defects. The implant defect profile may be approximated by a bilayer structure consisting of a fully compensated layer near the surface, followed by a 90% compensated (p) layer near the n-type drift region. These devices exhibit avalanche breakdown as high as 2.6 kV at room temperature. Simulations show that the ET created by implantation is an effective way to laterally distribute the electric field over a large area. This increases the voltage at which impact ionization occurs and leads to the observed higher breakdown voltages.

Journal ArticleDOI
TL;DR: In this article, an efficient approach to engineering the Al2O3/GaN positive interface fixed charges by post-dielectric annealing in nitrogen is demonstrated, which leads to a record high threshold voltage of 7.6 V.
Abstract: An efficient approach to engineering the Al2O3/GaN positive interface fixed charges by post-dielectric annealing in nitrogen is demonstrated. The remarkable reduction of interface fixed charges from $1.44 \times 10^{13}$ to $3 \times 10^{12}$ cm−2 was observed, which leads to a record high threshold voltage ( $V_{\mathrm{ TH}})$ of 7.6 V obtained in the Al2O3/GaN MOSFETs. The significantly reduced interface fixed charges and the corresponding remote scattering effect enable respectable improvement in the electron mobility that results in a high drain current density of 355 mA/mm in the device. These competitive results reveal that the method reported in this letter is promising in pushing $V_{\mathrm{ TH}}$ more positive and simultaneously achieving good device performance of normally-off GaN power devices.

Journal ArticleDOI
TL;DR: In this article, a GaN-on-Si epilayer-based fully vertical p-i-n rectifier with n-GaN facing up was shown to achieve 3.35 V at 1 A/cm2, a low differential on-resistance of 3.3 εm ε, and a breakdown voltage of 350 V. The results indicate that fully vertical rectifiers using GaN epilayers have great potential in achieving cost-effective GaN devices for high power and high voltage applications.
Abstract: Using GaN-on-Si epilayers, for the first time, fully vertical p-i-n diodes are demonstrated after Si substrate removal, transfer, and n-electrode formation at the top of the device. After SiO2 sidewall passivation, the vertical p-i-n diodes, with n-GaN facing up, exhibit $V_{{\mathrm{\scriptscriptstyle ON}}}$ of 3.35 V at 1 A/cm2, a low differential on-resistance of 3.3 $\text{m}\Omega $ cm2 at 300 A/cm2, and a breakdown voltage of 350 V. The corresponding Baliga's figure of merit is 37.0 MW/cm2, a very good value for GaN-based p-i-n rectifiers grown on Si substrates. The results indicate that fully vertical rectifiers using GaN-on-Si epilayers have great potential in achieving cost-effective GaN devices for high-power and high-voltage applications.


Journal ArticleDOI
TL;DR: In this article, a self-powered UV-photodetector device based on hybrid reduced-graphene-oxide (r-GO)/gallium nitride (GaN) was demonstrated.
Abstract: A simplistic design of a self-powered UV-photodetector device based on hybrid reduced-graphene-oxide (r-GO)/gallium nitride (GaN) is demonstrated. Under zero bias, the fabricated hybrid photodetector shows a photosensivity of ∼85% while the ohmic contact GaN photodetector with an identical device structure exhibits only ∼5.3% photosensivity at 350 nm illumination (18 μW/cm2). The responsivity and detectivity of the hybrid device were found to be 1.54 mA/W and 1.45 × 1010 Jones (cm Hz½ W−1), respectively, at zero bias with fast response (60 ms), recovery time (267 ms), and excellent repeatability. Power density-dependent responsivity and detectivity revealed ultrasensitive behaviour under low light conditions. The source of the observed self-powered effect in the hybrid photodetector is attributed to the depletion region formed at the r-GO and GaN quasi-ohmic interface.

Journal ArticleDOI
TL;DR: In this article, the authors investigate the thermal boundary resistance and thermal conductivity of GaN layers grown on Si with 100 nm AlN transition layers using time domain thermoreflectance (TDTR).
Abstract: In this work, we investigate the thermal boundary resistance and thermal conductivity of GaN layers grown on Si with 100 nm AlN transition layers using time domain thermoreflectance (TDTR). The GaN layers ranged from 0.31 to 1.27 μm. Due to the challenges in determining the thermal boundary resistance of the buried interfaces found in this architecture, a new data reduction scheme for TDTR that utilizes a Monte Carlo fitting method is introduced and found to dramatically reduce the uncertainty in certain model parameters. The results show that the GaN thermal conductivity does not change significantly with layer thickness, whereas the resistance of the AlN layer decreases slightly with GaN thickness.

Journal ArticleDOI
TL;DR: In this paper, the polarity of the underlying nucleation layers of GaN nanowire arrays is determined by piezoresponse force microscopy (PFM), surface reconstructions, and polarity sensitive etching.
Abstract: We have demonstrated dramatic improvement in the quality of selective-area GaN nanowire growth by controlling the polarity of the underlying nucleation layers. In particular, we find that N-polarity is beneficial for the growth of large ordered nanowire arrays with arbitrary spacing. Herein, we present techniques for obtaining and characterizing polarity-controlled nucleation layers on Si (111) substrates. An initial AlN layer, which is demonstrated to adopt Al- (N-)polarity for N- (Al-)rich growth conditions, is utilized to configure the polarity of subsequently grown GaN layers as determined by piezoresponse force microscopy (PFM), polarity-dependent surface reconstructions, and polarity-sensitive etching. Polarity-dependent surface reconstructions observed in reflection high-energy electron diffraction (RHEED) patterns were found to be particularly useful for in situ verification of the nucleation layer polarity, prior to mask deposition, patterning, and selective-area regrowth of the GaN NW arrays. N-...

Journal ArticleDOI
TL;DR: In this paper, the role of carbon-related traps in GaN-based ungated high-electron mobility transistor structures has been investigated both experimentally and by means of numerical simulations.
Abstract: The role of carbon-related traps in GaN-based ungated high-electron mobility transistor structures has been investigated both experimentally and by means of numerical simulations. A clear quantitative correlation between the experimental data and numerical simulations has been obtained. The observed current decrease in the tested structure during backgating measurements has been explained simply by means of a thermally activated hole-emission process with $E_{A} = 0.9$ eV, corresponding to the distance of the acceptor-like hole-trap level from the GaN valence band. Moreover, it has been demonstrated by means of electrical measurements and numerical simulations that only a low percentage of the nominal carbon doping levels induces the observed current reduction when negative substrate bias is applied to the tested structure.

Journal ArticleDOI
TL;DR: In this article, the thermal conductivity of n-and p-type doped gallium nitride (GaN) epilayers having thicknesses of 3-4μm was investigated using time domain thermoreflectance.
Abstract: The thermal conductivity of n- and p-type doped gallium nitride (GaN) epilayers having thicknesses of 3–4 μm was investigated using time domain thermoreflectance. Despite possessing carrier concentrations ranging across 3 decades (1015–1018 cm–3), n-type layers exhibit a nearly constant thermal conductivity of 180 W/mK. The thermal conductivity of p-type epilayers, in contrast, reduces from 160 to 110 W/mK with increased doping. These trends—and their overall reduction relative to bulk—are explained leveraging established scattering models where it is shown that, while the decrease in p-type layers is partly due to the increased impurity levels evolving from its doping, size effects play a primary role in limiting the thermal conductivity of GaN layers tens of microns thick. Device layers, even of pristine quality, will therefore exhibit thermal conductivities less than the bulk value of 240 W/mK owing to their finite thickness.

Journal ArticleDOI
TL;DR: Elect electrically driven, phosphor-free, white LEDs based on three-dimensional gallium nitride structures with double concentric truncated hexagonal pyramids are demonstrated and the electroluminescence spectra are stable with varying current.
Abstract: White light-emitting diodes (LEDs) are becoming an alternative general light source, with huge energy savings compared to conventional lighting. However, white LEDs using phosphor(s) suffer from unavoidable Stokes energy converting losses, higher manufacturing cost, and reduced thermal stability. Here, we demonstrate electrically driven, phosphor-free, white LEDs based on three-dimensional gallium nitride structures with double concentric truncated hexagonal pyramids. The electroluminescence spectra are stable with varying current. The origin of the emission wavelength is studied by cathodoluminescence and high-angle annular dark field scanning transmission electron microscopy experiments. Spatial variation of the carrier injection efficiency is also investigated by a comparative analysis between spatially resolved photoluminescence and electroluminescence. Phosphor-free white light-emitting diodes (LEDs) are realized that are based on gallium nitride (GaN) structures and exhibit stable color emission. White LEDs are fast replacing conventional lighting due to the very significant energy savings they offer, but phosphor-based white LEDs suffer from inherent conversion losses, high manufacturing cost, and poor thermal stability. Now, Yong-Hoon Cho and co-workers at Korea Advanced Institute of Science and Technology (KAIST) in South Korea have demonstrated electrically driven phosphor-free white LEDs with stable color emission. The LEDs contain gallium nitride structures with double concentric truncated hexagonal pyramids grown by metal-organic vapor-phase epitaxy. Each facet of a pyramid emits a different wavelength. The LEDs were found to consistently emit white light with relatively stable CIE color coordinates when the injection current was varied.

Journal ArticleDOI
TL;DR: In this paper, a continuous-wave (CW) operation of gallium nitride (GaN)-based vertical-cavity surface-emitting lasers (VCSELs) fabricated by epitaxial lateral overgrowth (ELO) using dielectric distributed Bragg reflectors (DBRs) as masks for selective growth was achieved.
Abstract: We have achieved continuous-wave (CW) operation of gallium nitride (GaN)-based vertical-cavity surface-emitting lasers (VCSELs) fabricated by epitaxial lateral overgrowth (ELO) using dielectric distributed Bragg reflectors (DBRs) as masks for selective growth. The GaN VCSELs exhibited CW operation at a wavelength of 453.9 nm, and the maximum output power was 1.1 mW, which is the highest value reported to date. GaN-based materials have presented challenges for obtaining DBRs with high reflectivity and a wide stopband, precise control of the cavity length and a lateral confinement structure to provide laser operation. The proposed VCSEL is immune to these concerns. Its two dielectric DBRs were obtained free from cracks. A high reflectance of more than 99.9% and a stopband with a width of 80–97 nm were obtained for both DBRs. The cavity length was controlled by epitaxial growth to as short as 4.5 µm. An ITO contact electrode on p-type GaN, which is required for a lateral confinement structure, showed electrical reliability under a high current density of 59.6 kA cm−2. The present data demonstrate that the fabrication process adopted here overcomes the shortcomings that have prevented the widespread use of GaN-based VCSELs.

Journal ArticleDOI
TL;DR: In this article, the geometric, electronic and optical properties of the graphene-like gallium nitride (GaN) monolayer paired with WS2 or WSe2 were studied systematically using the first-principles calculations.
Abstract: The geometric, electronic and optical properties of the graphene-like gallium nitride (GaN) monolayer paired with WS2 or WSe2 were studied systematically using the first-principles calculations. GaN interacts with WS2 or WSe2 via van der Waals interaction and all the most stable configurations of these two nanocomposites exhibit direct band gap characteristics. Meanwhile, the type-II heterojunctions are formed because the conduction band minimums and valence band maximums are respectively contributed by WS2 (or WSe2) and GaN. The imaginary parts of the dielectric function and the absorption spectra of the heterostructures were also calculated and the relatively improved optical properties were observed because of the new interband transitions. In addition, the band offsets as well as the intrinsic electric fields resulting from the interlayer charge transfer indicate that the electron-hole pairs recombination can be effectively inhibited, which is conducive for the photocatalysis process. Moreover, the band gaps of the heterostructures can be modulated by applying biaxial strains and even shift away the conduction band edge potential from the H+/H2 potential in a certain range, which further enhances the photocatalyst performance. The results indicate that GaN/WS2 or GaN/WSe2 nanocomposites are good candidate materials for photocatalyst or photoelectronic applications.