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Understanding the interfacial phonon transport is essential for optimizing the thermal management of microelectronics, especially for high energy density devices. Some calculations have suggested that introducing interfacial defects or disorders will increase the interfacial thermal conductance, which helps heat dissipation, while some studies suggested otherwise. In this work, we introduced substitutional impurities in GaN by growing a ∼2-nm-thick AlxGa1−xN or InxGa1−xN alloyed interlayer at the Al/GaN interface and measured the interface thermal conductance by time-domain thermoreflectance at room temperature. Our results show that substituting Ga atoms near the interface with either lighter Al atoms or heavier In atoms at a nominal concentration of 20% or less will not necessarily change the thermal conductance of the Al/alloy interface but is detrimental to the thermal transport across the total Al/GaN interface, which provides an experimental guideline for the thermal design of GaN-based devices. [ABSTRACT FROM AUTHOR]

A ground-breaking roadmap of III-nitride solid-state deep-ultraviolet light emitters is demonstrated to realize the wafer-scale fabrication of devices in vertical injection configuration, from 2 to 4 inches. The epitaxial device structure is stacked on a GaN template instead of conventionally adopted AlN, where the primary concern of the tensile strain for Al-rich AlGaN on GaN is addressed via an innovative decoupling strategy, making the device structure decoupled from the underlying GaN template. Moreover, the strategy provides a protection cushion against the stress mutation during the removal of substrates. As such, large-sized wafers can be obtained without surface cracks, even after the removal of the sapphire substrates by laser lift-off. Wafer-scale fabrication of 280 nm vertical injection deep-ultraviolet light-emitting diodes is eventually demonstrated, where a light output power of 65.2 mW is achieved at a current of 200 mA, largely thanks to the significant improvement of light extraction. This work will definitely speed up the application of III-nitride solid-state deep-ultraviolet light emitters featuring high performance and scalability. The efficiency of deep-ultraviolet light sources is critical for applications like disinfection. Here, the authors employ a novel decoupling strategy using GaN templates to fabricate 280 nm vertical injection diodes, achieving a light output power of 65.2 mW at 200 mA. [ABSTRACT FROM AUTHOR]

An equilibrium carrier statistics approach with a partial ionization model is proposed to determine the energy level of CN deep donors in p-type GaN with heavy Mg doping. Unlike usual compensating centers that are assumed to be fully ionized under equilibrium, partial ionization of the CN donor was taken into consideration. The energy level of the CN donor is determined to be EV + (0.20 ± 0.01) eV at elevated temperatures (∼350 K) using such a partial ionization model. The donor level for an isolated C center at a low temperature limit is further calculated considering the doping and temperature effects, which is EV + (0.32 ± 0.01) eV. Furthermore, the ionization ratio of CN is found to be dependent on the C concentration and can then be estimated to be in the range of 0.3–0.8. Such a partial ionization characteristic of CN may capture/emit free carriers during device operation and should be taken into account when analyzing device reliability. [ABSTRACT FROM AUTHOR]

GaN Schottky diodes show great potential in high-power terahertz frequency multipliers. The thermal characteristics of GaN Schottky diodes with single and double-row anode arrangements are described in this paper. The temperature distribution inside the Schottky diode is discussed in detail under the coupling condition of Joule heat and solid heat transfer. The effects of different substrates and substrate geometric parameters on the thermal characteristics of the Schottky diode chips with single and double-row anode arrangements are systematically analyzed. Compared with that of the chip with single-row anode arrangement, the maximum temperature of the chip with double-row anode arrangement can be reduced by 40 K at the same conditions. For chips with different substrates, chips with diamond substrates can withstand greater power dissipation when reaching the same temperature. The simulation results are instructive for the design and optimization of Schottky diodes in the terahertz field. [ABSTRACT FROM AUTHOR]

Lattice‐asymmetry‐driven selective area sublimation (SAS) process of GaN is systemically investigated by exploring the in situ dynamic evolution of the decomposition pathway under ultra‐high vacuum. The rationale of the SAS is confirmed as a strong anisotropic decomposition driven by lattice‐asymmetry of wurtzite crystal: the sublimation preferably starts along the ‐c axis due to the relatively lower decomposition energy barrier. Finally, the fabrication of site‐ and size‐controlled GaN nanowires has been achieved by utilizing the SAS process, exhibiting good controllability on the sidewall of nanowires. These findings shed light on the thermodynamic mechanism of the lattice‐asymmetry‐driven sublimation process in III‐nitrides, providing an efficient alternative approach for the tailoring of semiconductor micro/nanostructures. [ABSTRACT FROM AUTHOR]

The fabrication of high‐speed electronic and communication devices has rapidly grown the demand for high mobility semiconductors. However, their high cost and complex fabrication process make them less attractive for the consumer market and industrial applications. Indium nitride (InN) can be a potential candidate to fulfill industrial requirements due to simple and low‐cost fabrication process as well as unique electronic properties such as narrow direct bandgap and high electron mobility. In this work, 3 µm thick InN epilayer is grown on (0001) gallium nitride (GaN)/Sapphire template under In‐rich conditions with different In/N flux ratios by molecular beam epitaxy. The sharp InN/GaN interface monolayers with the In‐polar growth are observed, which assure the precise control of the growth parameters. The directly probed electron mobility of 3610 cm2 V‐1 s‐1 is measured with an unintentionally doped electron density of 2.24 × 1017 cm‐3. The screw dislocation and edge dislocation densities are calculated to be 2.56 × 108 and 0.92 × 1010 cm‐2, respectively. The step‐flow growth with the average surface roughness of 0.23 nm for 1 × 1 µm2 is confirmed. The high quality and high mobility InN film make it a potential candidate for high‐speed electronic/optoelectronic devices. [ABSTRACT FROM AUTHOR]

The control of carbon (C) impurity plays a crucial role in achieving high mobility GaN drift layers for GaN vertical power devices. We investigate the effects of NH3 partial pressure on the C incorporation in GaN and find out that the NH3 partial pressure is the key parameter controlling the C concentration. It is found that the C concentration is inversely proportional to the square of NH3 partial pressure under a fixed growth rate. Further physical analysis indicates that higher NH3 partial pressure results in a lower surface N vacancy (VN) concentration and, thus, the lower C incorporation efficiency. Under such a guidance, the C concentration in GaN can be reduced to as low as ∼1.7 × 1015/cm3 at a growth rate of 1 μm/h. As a consequence, a record high electron mobility of 1227 cm2/V·s at the room temperature is achieved for a GaN drift layer grown on Si substrates. Our strategy is effective in C control and paves the way for realizing high performance kV-class GaN vertical power devices on Si substrates. [ABSTRACT FROM AUTHOR]

Vertical GaN‐on‐Si devices are promising for the next‐generation high‐voltage power electronics with low cost and high efficiency. However, their applications are impeded by the limited thickness of crack‐free GaN layers and high threading dislocation density (TDD) in the layer. Buffer layers are crucial for stress control while they usually behave with poor surface morphology, which causes early stress relaxation in GaN and limited thickness. Hereby, a terrace engineering approach for the buffer layers is proposed. Through tuning the supersaturation ratio, the terrace width can be manipulated and an atomically smooth AlGaN buffer layer can be realized, which effectively reduces the compressive stress relaxation and provides a firm foundation for thick GaN growth. As a result, a 7.5 µm thick GaN drift layer with TDD as low as 8.6 × 107 cm−2 is achieved on Si substrates. The room temperature electron mobility of the GaN drift layer can be raised up to 1210 cm2 V−1 s−1. The fabricated PiN diode shows a high breakdown voltage of 1058 V as well as a high on/off ratio of 1012. This work thus truly demonstrates the potential of high‐performance and low‐cost GaN‐based electronic as well as optoelectronic devices on Si platforms. [ABSTRACT FROM AUTHOR]

Heavy carbon (C) doping is of great significance for semi-insulating GaN in power electronics. However, the doping behaviors, especially the atomic configurations and related self-compensation mechanisms, are still under debate. Here, with the formation energy as the input parameter, the concentrations of C defects with different atomic configurations are calculated by taking the configurational entropy into account. The result shows that the concentrations of tri-carbon complexes (CNCiCN, where Ci refers to interstitial carbon) and dicarbon complexes (CNCGa) cannot be neglected under heavy doping conditions. The concentration of CNCiCN can even exceed that of CN at sufficiently high doping levels. Especially, we suggest that it is the tri-carbon complex CNCiCN, instead of the commonly expected CGa, that acts as the self-compensation centers in semi-insulating GaN under heavy C doping conditions. The results provide a fresh look on the long-standing problem about the self-compensation mechanisms in C doped GaN. [ABSTRACT FROM AUTHOR]

We report planar AlN/GaN resonant tunneling diodes (RTDs) fabricated using a nitrogen ion implantation isolation process on silicon substrates. The active area of AlN/GaN RTDs is defined by nitrogen ion implantation. A planar RTD consists of two different-sized RTDs connected in series, where the larger one acts as a resistor and the performance of the planar RTD is determined by the smaller one. Repeatable and hysteresis-free negative differential resistances without degradation are observed through 500 continuous bidirectional voltage sweeps. The peak current density is 15.5 kA/cm2. The RTD exhibits stable negative differential resistance (NDR), with the peak-to-valley current ratio varying from 1.39 to 1.28 as the temperature increases from 77 to 295 K. This practicable and reproducible ion implantation process is compatible with silicon fabrication technology. It, thus, provides a feasible method for device design of GaN-based RTDs and facilitates the implementation of complex monolithic microwave integrated circuits based on planar III-nitride RTDs on large-size silicon wafers. [ABSTRACT FROM AUTHOR]

Green syngas production from Earth-abundant and renewable resources is a viable means to achieve carbon neutrality. However, this critical path to realize net-zero carbon dioxide emissions has remained extremely challenging. In this work, by coupling GaN nanowires (NWs) with dual gold–indium nanoparticles (NPs) onto wafer-scale silicon, a novel and well-developed photocatalytic architecture of AuIn NPs/GaN NWs/Si has been assembled for the first time to produce syngas with bio-derived glycerol, water, and concentrated light as the only inputs. By correlative experimental and theoretical investigations, the AuIn/GaN interface is found to be synergistically favorable for dehydrogenating the edge functional group of –CH2OH of the glycerol skeleton, followed by the cleavage of the inert C(sp3)–C(sp3) bond toward an ethylene glycol-based intermediate with a greatly reduced reaction energy barrier in glycerol solution. Owing to the exceptional optoelectronic and catalytic properties, syngas yields a benchmarking activity of 149.3 mmol g−1 h−1 under mild conditions without an extra energy input other than light. The H2/CO ratio can be broadly tailored from 19.3 to 0.9, thus favoring various downstream products. Water plays a vital role in the highly efficient formation of syngas by providing active species e.g., hydroxyl radicals through photocatalysis. Owing to the widespread distribution of biomass, water, and sunlight, and the industrially available semiconductor platform, this work promises to efficiently and selectively produce next-generation distributed syngas. [ABSTRACT FROM AUTHOR]

Identifying atomic configurations of impurities in semiconductors is of fundamental interest and practical importance in designing electronic and optoelectronic devices. C impurity acting as one of the most common impurities in GaN, it is believed for a long time that it substitutes at Ga site forming CGa with +1 charge-state in p-type GaN, while it substitutes at N site forming CN with -1 charge-state in n-type GaN. However, by combining x-ray absorption spectroscopy and first-principles simulations, we observed that C is mainly occupying the N site rather than the Ga one in p-GaN. We further reveal that this is due to an H-induced EF-tuning effect. During growth, the existing H can passivate Mg dopants and upshifts the EF to the upper region of bandgap, leading to the CN formation. After the p-type activation by annealing out H, although the EF is pushed back close to the valence band maximum, whereas the extremely large kinetic barrier can prevent the migration of C from the metastable CN site to ground-state CGa site, hence stabilizing the CN configuration. Additionally, the CN with neutral charge-state ( C N 0 ) in the p-GaN is further observed. Therefore, the real C-related hole-killer in p-type GaN could be CN rather than the commonly expected CGa. Our work not only offers the unambiguous evidence for the C defect formation in p-GaN but also contributes significantly to an in-depth understanding of the C-related hole-killers and their critical role on electrical and optoelectrical properties of p-GaN and even p-AlGaN. [ABSTRACT FROM AUTHOR]

Room temperature (RT) operation is one of the crucial requirements for the practical applications of single photon emitters. Here, RT triggered single photon emission from self‐assembled GaN quantum dots (QDs) embedded in AlN nanowires grown by molecular beam epitaxy on Si (111) substrate is demonstrated. Photoluminescence of GaN/AlN QDs show strong emission in the range of 3.6–4.1 eV at 4.7 K, with average full width at half maximum of ≈2.7 meV, the sharpest one reaches as narrow as 1.6 meV. The QD exhibits a fast‐radiative lifetime of ≈0.98 ns at 4.7 K due to strong quantum confinement. A clear antibunching effect is observable up to RT, with a second‐order correlation function at zero time delay g(2)(0) = 0.35 ± 0.05, confirming single photon emission at RT, manifesting strong potential for practical quantum information applications. [ABSTRACT FROM AUTHOR]

We report repeatable AlN/GaN resonant tunneling diodes (RTDs) grown on a silicon substrate by plasma-assisted molecular-beam epitaxy. The RTDs exhibit stable negative differential resistance without hysteresis at room temperature, where no degradation is observed even after 500 continuous bidirectional sweeps. The peak-to-valley current ratio is 1.36, and the peak current density is 24.38 kA/cm2. When the temperature is changed from 77 to 475 K, the peak current remains almost unchanged and the valley current increases gradually, resulting in a reduced peak-to-valley current ratio from 1.59 to 1.07. Our work softens the material quality constraints on realizing the room-temperature repeatable negative differential resistance and paves the way to low-cost III-nitride-based monolithic and hybrid microwave integrated circuits on large-size silicon wafers. [ABSTRACT FROM AUTHOR]

Upcycling of carbon dioxide towards fuels and value-added chemicals poses an opportunity to overcome challenges faced by depleting fossil fuels and climate change. Herein, combining highly controllable molecular beam epitaxy growth of gallium nitride (GaN) under a nitrogen-rich atmosphere with subsequent air annealing, a tunable platform of gallium oxynitride (GaN1-xOx) nanowires is built to anchor rhodium (Rh) nanoparticles for carbon dioxide hydrogenation. By correlatively employing various spectroscopic and microscopic characterizations, as well as density functional theory calculations, it is revealed that the engineered oxynitride surface of GaN works in synergy with Rh to achieve a dramatically reduced energy barrier. Meanwhile, the potential-determining step is switched from *COOH formation into *CO desorption. As a result, significantly improved CO activity of 127 mmol‧gcat−1‧h−1 is achieved with high selectivity of >94% at 290 °C under atmospheric pressure, which is three orders of magnitude higher than that of commercial Rh/Al2O3. Furthermore, capitalizing on the high dispersion of the Rh species, the architecture illustrates a decent turnover frequency of 270 mol CO per mol Rh per hour over 9 cycles of operation. This work presents a viable strategy for promoting CO2 refining via surface engineering of an advanced support, in collaboration with a suitable metal cocatalyst. Thermocatalytic hydrogenation holds great promise for commercial utilization of carbon dioxide, but the process is energy-intense with high temperature and pressure requirements. Here, the authors engineer a GaN1-xOx rhodium nanoparticle catalyst for CO2 to CO hydrogenation that functions at temperatures as low as 170 °C. [ABSTRACT FROM AUTHOR]

Thermal sublimation, a specific method to fabricate semiconductor nanowires, is an effective way to understand growth behavior as well. Utilizing a high‐resolution transmission electron microscope (TEM) with in situ heating capability, the lattice‐asymmetry‐driven anisotropic sublimation behavior is demonstrated of wurtzite GaN: sublimation preferentially occurs along the [0001¯$000\bar{1}$] and [0001] directions in both GaN thin films and nanowires. Hexagonal pyramidal nanostructures consisting of six semipolar {11¯01}$\{ {1\bar{1}01} \}$ planes and one (0001¯$\bar{1}$) plane with the apex pointing to the [0001] direction are generated as a sublimation‐induced equilibrium crystal structure, which is consistent with the lattice‐asymmetry‐driven growth behaviors in wurtzite GaN. These findings offer a new insight into the thermal stability of wurtzite GaN and provide essential background for tailoring the structure of III‐nitrides for atomic‐scale manufacturing. [ABSTRACT FROM AUTHOR]

Traditional GaAs-based frequency multipliers still exhibit great challenges to meet the demand for solid-state high-power THz sources due to low breakdown voltage and heat dissipation of the Schottky barrier diode (SBD). In this study, a GaN SBD chain was fabricated with n−/n+-GaN structure. As a consequence, the breakdown voltage of 54.9 V at 1 μA and cut-off frequency of 587.5 GHz at zero bias were obtained. A 120 GHz frequency-doubler module based on the GaN SBD chain was designed and fabricated. When driven with 500 mW input power in a continuous wave, the output power of the frequency-doubler module was 15.1 mW at 120 GHz. Moreover, the experiments show that the frequency-doubler module can endure an input power of 2 W. In addition, it is worth noting that the SBD chain works well at an anode temperature of 337.2 °C. [ABSTRACT FROM AUTHOR]

To promote the modulation bandwidth of GaN-based blue micro-light emitting diodes ($\mu $ -LEDs) for visible light communication, the multiple quantum well active region has been improved via approaches based on low-temperature (LT) GaN insertion layer in barriers. $\mu $ -LED arrays with 10 $\times $ 10 pixels have been fabricated. The device using LT-GaN interlayer has an output power of 2.14 mW and a modulation bandwidth of 220 MHz, with an enhancement factor of 2.5 and 1.7, respectively, as compared with the device without LT-GaN interlayer. It is believed that these improvements are a result of the suppression of strain-related quantum-confined Stark effect and defects/dislocations by LT-GaN interlayer based optimization. The LT-GaN interlayer not only facilitates a tensile strain in barriers, making it more lattice-compatible with InGaN well to reduce the strain there, but also has an effect on blocking the propagation of threading dislocation into upper layers. Our work provides an effective approach to enhance the modulation bandwidth of III-nitride $\mu $ -LEDs using LT interlayer. [ABSTRACT FROM AUTHOR]

It has been established that the formation of point defects and their behaviors could be regulated by growth details such as growth techniques and growth conditions. In this work, we prove that C doping approaches have great influence on the charge state of C N , thus the interaction between H and C in GaN. For GaN with intrinsic C doping, which is realized by reducing the V/III ratio, C N mainly exists in the form of C N − charged from the higher concentration of V N and, thus, may attract H + by coulomb interaction. Whereas for the extrinsically C doped GaN with propane as the doping source, the concentration of V N is reduced, and C N mainly exists in neutral charge state and, thus, nearly does not attract H ions. Therefore, we demonstrate that the interplay between H and C atoms is weaker for the extrinsically C doped GaN compared to the intrinsically doped GaN, thus gives a clear picture about the different charge states of C N and the formation of C–H complexes in GaN with different C doping approaches. [ABSTRACT FROM AUTHOR]

The AlGaN/GaN Schottky barrier diodes (SBDs) working as high-power mixer and multiplier show great potential in millimeter wave (MMW) field owing to their high breakdown voltage. Nevertheless, its further application is severely limited by large reverse leakage current (Jr) since the two-dimensional electron gas (2DEG) channel is hard to be pinched off at low voltage. To address this limitation, a graded AlGaN/GaN heterostructure is introduced to extend the 2DEG channel into a quasi-three-dimensional electron slab. By comparing the fixed Al composition AlGaN/GaN SBD, Jr of the graded AlGaN/GaN SBD is significantly reduced due to the extension of channel carriers, confirming the effective Jr suppression effect of this structure. Furthermore, on this basis, a recessed anode structure is utilized to expect a smaller Jr. The results indicated that the graded AlGaN/GaN SBDs with air-bridge structure have achieved a pretty low Jr value (1.6 × 10−13 A at -15 V), and its cutoff frequency is as high as 60.6 GHz. It is expected that such SBDs with low Jr have significant advantages in future applications. [ABSTRACT FROM AUTHOR]

III‐nitride semiconductor films are usually achieved by epitaxial growth on single‐crystalline substrates (sapphire, silicon (Si), and silicon carbide). It is important to grow these films on non‐epitaxial substrates of interest such as polycrystalline substrates for exploring novel applications in electronics and optoelectronics. However, single‐crystalline III‐nitride films with uniform orientation on non‐epitaxial substrates have not yet been realized, due to the lack of crystallographic orientation of the substrates. Here, this work proposes a strategy of polarization‐driven‐orientation selective growth (OSG) and demonstrate that single‐crystalline gallium nitride (GaN) can in principle be achieved on any substrates. Taking polycrystalline diamond and amorphous‐silicon dioxide/Si substrates as typical examples, the OSG is demonstrated by utilizing a composed buffer layer consisting of graphene and polycrystalline physical vapor deposited (PVD) aluminium nitride (AlN). The polarization of the PVD AlN can effectively tune the strength of interfacial orbital coupling between AlN nuclei and graphene at different rotation angles, as confirmed by atom‐scale first‐principles calculations, and align the AlN nuclei to form a uniform orientation. This consequently leads to continuous single‐crystalline GaN films. The ability to grow single‐crystalline III‐nitrides onto any desired substrates would create unprecedented opportunities for developing novel electronic and optoelectronic devices. [ABSTRACT FROM AUTHOR]

In this Letter, an ultra-thin AlGaN/GaN heterostructure field effect transistor (HFET) with a total thickness of ∼200 nm was fabricated on sapphire substrates by combing physical vapor deposition and metal organic chemical vapor deposition growth methods. Thanks to the absence of a conventional semi-insulating thick GaN buffer by taking advantage of an ex situ sputtered AlN nucleation layer, we achieved a profound soft breakdown voltage of 1700 V accompanied by a 12.5 Ω·mm on-resistance and a low off-state leakage of 0.1 μA/mm in such ultra-thin HFET devices. Our work demonstrates an alternative strategy to fabricate GaN based power devices with high breakdown voltage and low cost. [ABSTRACT FROM AUTHOR]

We show the structural and optical properties of non-polar a-plane GaN epitaxial films modified by Si ion implantation. Upon gradually raising Si fluences from 5 × 1013 cm−2 to 5 × 1015 cm−2, the n-type dopant concentration gradually increases from 4.6 × 1018 cm−2 to 4.5 × 1020 cm−2, while the generated vacancy density accordingly raises from 3.7 × 1013 cm−2 to 3.8 × 1015 cm−2. Moreover, despite that the implantation enhances structural disorder, the epitaxial structure of the implanted region is still well preserved which is confirmed by Rutherford backscattering channeling spectrometry measurements. The monotonical uniaxial lattice expansion along the a direction (out-of-plane direction) is observed as a function of fluences till 1 × 1015 cm−2, which ceases at the overdose of 5 × 1015 cm−2 due to the partial amorphization in the surface region. Upon raising irradiation dose, a yellow emission in the as-grown sample is gradually quenched, probably due to the irradiation-induced generation of non-radiative recombination centers. [ABSTRACT FROM AUTHOR]

The spin injection into 2D electron gas (2DEG) in AlN/GaN heterostructures is studied by magneto‐transport measurements. An ultrathin AlN layer at the hetero‐interface acts as a barrier to form high‐quality 2DEG in the triangular quantum well and a tunneling barrier for the spin injection to overcome the conductance mismatch issue. In this study, Hanle signals and inversed Hanle signals are observed, proving that the spin injection is achieved in the 2DEG in the AlN/GaN heterostructure rather than in the interfacial states. The spin‐relaxation time in 2DEG at 8 K is found to be as long as 860 ps, which almost keeps constant with bias and decreases with increasing temperature. The spin‐relaxation process is illustrated as Rashba spin‐orbit coupling dominated D'yakonov Perel' mechanisms above 8 K. These results show the promising potential of 2DEG in AlN/GaN heterostructures for spin field‐effect transistor applications. [ABSTRACT FROM AUTHOR]

High quality GaN films on SiC with low thermal boundary resistance (TBR) are achieved by employing an ultrathin low Al content AlGaN buffer layer. Compared with the conventional thick AlN buffer layer, the ultrathin buffer layer can not only improve the crystal quality of the subsequent GaN layer but also reduce the TBR at the GaN/SiC interface simultaneously. The ultrathin AlGaN buffer layer is introduced by performing a pretreatment of the SiC substrate with trimethylaluminum followed by the growth of GaN with an enhanced lateral growth rate. The enhanced lateral growth rate contributes to the formation of basal plane stacking faults (BSFs) in the GaN layer, where the BSFs can significantly reduce the threading dislocation density. We reveal underling mechanisms of reducing TBR and dislocation density by the ultrathin buffer layer. We propose this work is of great importance toward the performance improvement and cost reduction of higher power GaN-on-SiC electronics. [ABSTRACT FROM AUTHOR]

In-polarity InN/In0.7Ga0.3N multiquantum wells (MQWs) were fabricated on a thick In0.7Ga0.3N interlayer/Ga-polarity GaN template by radio-frequency plasma-assisted molecular beam epitaxy. We then investigated how the lattice relaxation and piezoelectric field in InN wells affect their structural and photoluminescence (PL) properties, respectively. It was found that the critical thickness of InN well on In0.7Ga0.3N barrier was about 1 nm. A clear PL peak shift from 1.40 to 1.95 μm was observed depending on the InN well thickness from 0.7 to 2.0 nm. Correspondingly, PL-intensity reduction was also observed with increasing well thickness. No PL was observed for the sample with 4.1 nm thick InN wells. On the basis of theoretical estimation of transition energies in InN/In0.7Ga0.3N MQWs, it was confirmed that the quantum-confined Stark effect (QCSE) played an important role for both the observed PL peak shift and the decrease in intensity. The piezoelectric field in coherently grown InN wells was about 3 MV/cm but it was reduced to about 1–2 MV/cm for the samples with relaxed InN wells. It was confirmed that the InN wells must be thinner than the critical thickness (1 nm) in following two points: to reduce defects arising from lattice relaxation and to reduce QCSE leading to emission-peak redshift and a decrease in intensity. [ABSTRACT FROM AUTHOR]

We present a mechanism for the vertical leakage induced current degradation with identification of the properties of the relevant traps in AlGaN/GaN heterostructures on Si. The extent of the current degradation is determined by back-gating sweep measurements in double directions at different sweep rates and temperatures. It is found that the current degradation is only observed at relatively slow sweep rates and high temperatures. Time dependent back-gating measurements further suggest that the current degradation process is related to traps with long time constants. By comparing with the measurement results of samples on sapphire substrates, we confirm that the current degradation is caused by vertical leakage in heterostructures on Si. On the basis of the vertical leakage induced current degradation mechanism and in conjunction with the long-time degradation process, we measure both the trapping and detrapping processes of the relevant trap states to identify their properties. We find that there is a 0.6 eV capture barrier and a 0.67 eV emission barrier for the trap states, indicating that the trap states are of large lattice relaxation. [ABSTRACT FROM AUTHOR]

Lattice-symmetry-driven epitaxy of hierarchical GaN nanotripods is demonstrated. The nanotripods emerge on the top of hexagonal GaN nanowires, which are selectively grown on pillar-patterned GaN templates using molecular beam epitaxy. High-resolution transmission electron microscopy confirms that two kinds of lattice-symmetry, wurtzite (wz) and zinc-blende (zb), coexist in the GaN nanotripods. Periodical transformation between wz and zb drives the epitaxy of the hierarchical nanotripods with N-polarity. The zb-GaN is formed by the poor diffusion of adatoms, and it can be suppressed by improving the ability of the Ga adatoms to migrate as the growth temperature increased. This controllable epitaxy of hierarchical GaN nanotripods allows quantum dots to be located at the phase junctions of the nanotripods and nanowires, suggesting a new recipe for multichannel quantum devices. [ABSTRACT FROM AUTHOR]

In order to solve the problems of GaN heteroepitaxy on sapphire substrate, some techniques were explored. Freestanding GaN substrates have been made by hydride vapor phase epitaxy (HVPE), laser lift-off (LLO), and chemical mechanical polishing techniques. Wafer bending and cracking in the HVPE growth were partly settled by pulsed flow modulation method. High-crystal quality was established for 1.2 mm thick GaN substrate by X-ray diffraction measurement, in which the full width of half maximum values were 72, 110 arcsec for (102), (002) peaks. A novel micro-size patterned sapphire substrate (PSS) and a nano PSS were also fabricated. High-power vertical structure light emitting diodes (VSLEDs) have been developed by Au-Sn eutectic wafer bonding, homemade micro-area LLO, and light extraction structure preparation. The high-injection-level active region with low temperature GaN sandwiched layers was used for low-efficiency droop. The light output power of VSLED was achieved as 400 mW driven at 350 mA, and the dominant wavelength is about 460 nm. The structures and properties of strain modulated superlattices (SLs) and quantum wells as well as advanced simulation of carriers transport across the electron blocking layer were investigated in laser diodes. The hole concentration was achieved as high as 1.6 × 10 cm in AlGaN/GaN SLs:Mg by inserting an AlN layer. High-quality AlGaN epilayers and structures were grown by MOCVD. Some device structures of UV LEDs and detectors were demonstrated. The emission wavelength of 262 nm UV LED has been successfully fabricated. At last, high-quality InN and InGaN materials for solar cell were grown by boundary-temperature-controlled epitaxy and growth-temperature-controlled epitaxy. Hall-effect measurement showed a recorded electron mobility of 3,280 cm/(V s) and a residual electron concentration of 1.47 × 10 cm at 300 K. [ABSTRACT FROM AUTHOR]

The spin splitting in GaN-based heterostructures has been investigated by means of circular photogalvanic effect experiments under uniaxial strain. The ratios of Rashba and Dresselhaus spin-orbit coupling coefficients (R/D ratios) have been measured in AlxGa1-xN/GaN heterostructures with various Al compositions. It is found that the R/D ratio increases from 4.1 to 19.8 with the Al composition of the AlxGa1-xN barrier varied from 15% to 36%. The Dresselhaus coefficient of bulk GaN is experimentally obtained to be 0.4 eV Å3. The results indicate that the spin splitting in GaN-based heterostructures can be modulated effectively by the polarization-induced electric fields. [ABSTRACT FROM AUTHOR]

GaN-on-Si(100): Epitaxy of Single-Crystalline GaN Film on CMOS-Compatible Si(100) Substrate Buffered by Graphene (Adv. Funct. In article number 1905056, Xuelin Yang, Kaihui Liu, Bo Shen, and co-workers achieve the epitaxy of single-crystalline gallium nitride (GaN) film on a complementary metal-oxide-semiconductor-compatible Si(100) substrate by applying a one-atom-thick single-crystalline graphene buffer layer. In-plane orientation, nucleation, Si(100), single-crystalline GaN films, single-crystalline graphene. [Extracted from the article]

Fabricating single‐crystalline gallium nitride (GaN)‐based devices on a Si(100) substrate, which is compatible with the mainstream complementary metal‐oxide‐semiconductor circuits, is a prerequisite for next‐generation high‐performance electronics and optoelectronics. However, the direct epitaxy of single‐crystalline GaN on a Si(100) substrate remains challenging due to the asymmetric surface domains of Si(100), which can lead to polycrystalline GaN with a two‐domain structure. Here, by utilizing single‐crystalline graphene as a buffer layer, the epitaxy of a single‐crystalline GaN film on a Si(100) substrate is demonstrated. The in situ treatment of graphene with NH3 can generate sp3 CN bonds, which then triggers the nucleation of nitrides. The one‐atom‐thick single‐crystalline graphene provides an in‐plane driving force to align all GaN domains to form a single crystal. The nucleation mechanisms and domain evolutions are further clarified by surface science exploration and first‐principle calculations. This work lays the foundation for the integration of GaN‐based devices into Si‐based integrated circuits and also broadens the choice for the epitaxy of nitrides on unconventional amorphous or flexible substrates. [ABSTRACT FROM AUTHOR]

Resonant tunneling diodes (RTDs) are candidates for high power terahertz oscillators, and form the basis for understanding the quantum confinement and vertical transport in quantum structures such as quantum cascade lasers and quantum cascade detectors. In this work, repeatable negative differential resistance (NDR) is achieved in AlN/GaN RTDs grown on sapphire substrate by plasma‐assisted molecular‐beam epitaxy. Two reproducible NDR regions sequentially following two preresonance replicas are demonstrated at room temperature. A current region exhibiting negative correlation with temperature and oscillation‐like features is first identified under reverse bias, which is interpreted as a combined contribution of weak resonant tunneling channels through different bound states in the well. The revealed peak‐to‐valley current ratio ranges from 1.1 to 1.8, and peak current density ranges from 5 to 164 kA cm−2. Using an analytic model, resonant tunneling transports in both bias directions are quantitatively characterized and show good agreements with experiment results, demonstrating the capability of accurate quantum transport control using III‐nitride grown on sapphire substrate. The findings will promote the implementation of low cost III‐nitride monolithic microwave circuits and resonant tunneling structures based on sapphire, SiC, and even silicon substrates. Repeatable room temperature negative differential resistance is demonstrated in polar AlN/GaN resonant tunneling diodes grown on sapphire. Two clear resonant peaks following two pre‐resonance replicas, plus one negative temperature correlated region are identified and show good agreement with theoretical calculations, demonstrating the capability of accurate quantum transport control using III‐nitrides. [ABSTRACT FROM AUTHOR]

We present a hot electron assisted vertical leakage/breakdown mechanism in AlGaN/GaN heterostructures on Si substrates by a combination of applying vertical and lateral bias conditions. Beyond a critical bias point, the vertical leakage current under the combined bias condition is larger than that under a pure vertical bias condition which results in a lower breakdown voltage. The critical bias has a positive temperature dependence. A model is proposed that highly energetic hot electrons can release trapped electrons from defects and even ionize them. The model is proved by investigating the detrapping and ionization mechanisms by changing hot electron energy. [ABSTRACT FROM AUTHOR]

By using in-situ NH 3 pulse flow cleaning method, we have achieved the repeated growth of high quality and uniformity GaN and AlGaN/GaN high electron mobility transistors (HEMTs) on 150 mm Si substrate. The two dimensional electron gas (2DEG) mobility is 2200 cm 2 /Vs with an electron density of 7.3 × 10 12 cm −2 . The sheet resistance is 305 ± 4 Ω/□ with ±1.3% variation. The achievement is attributed to the fact that this method can significantly remove the Al, Ga, etc. metal droplets coating on the post growth flow flange and reactor wall which are difficult to clean by normal bake process under H 2 ambient. [ABSTRACT FROM AUTHOR]

The challenge of p -type doping in Al-rich AlGaN is investigated based on first-principles calculations. We find that the p -type conductivity can be improved by replacing Al-rich AlGaN with Mg Ga δ -doping (AlN) m /(GaN) n ( m ≥ n ) superlattice (SL). The formation energy E f is the lowest and acceptor activation energy E A is the smallest for Mg substituting Ga in the SL. The E A increases if the doping position moves from GaN to AlN layer. Moreover, E A decreases with increasing the number of GaN monolayers. The E A can be reduced from 0.48 eV in AlN to 0.26 eV in (AlN) 5 /(GaN) 1 , 0.25 eV in (AlN) 1 /(GaN) 1 , 0.24 eV in (AlN) 4 /(GaN) 2 and 0.22 eV in (AlN) 3 /(GaN) 3 SLs. This will lead to a high hole concentration in the order of 10 18 cm −3 at room temperature, which is favorable for AlGaN-based deep ultraviolet optoelectronic devices. [ABSTRACT FROM AUTHOR]

The cutoff frequency of Schottky barrier diode (SBD) depends on its junction capacitance and series resistance. The two-dimensional electron gas (2DEG) at AlGaN/GaN interface has high carrier mobility and carrier concentration. However, AlGaN/GaN heterostructure SBD usually shows a high series resistance because of the thin 2DEG channel. In this work, multiple AlGaN/GaN heterojunctions are vertically stacked for forming multiple parallel 2DEG channels to reduce the series resistance. Multi-channel AlGaN/GaN-based air-bridge structure planar SBDs with a half through-hole are demonstrated. The series resistance of quintuple-channel SBD is only 39.5% of the single-channel SBD's. Moreover, a low capacitance is obtained by the Schottky electrode with a half through-hole structure. The low series resistance and the low capacitance contribute to a 16 GHz cutoff frequency in millimeter-wave band. [ABSTRACT FROM AUTHOR]