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In this work, an enhancement-mode (E-mode) AlGaN/GaN-based high-electron-mobility transistor (HEMT) with a graded AlGaN cap layer (GACL) is proposed and numerically studied by Silvaco technology computer-aided design. The GACL is designed with a decreasingly graded Al composition x along [0001] direction and the initial x is smaller than the Al composition of the Al0.2Ga0.8N barrier layer (BL). This GACL scheme can simultaneously produce high-concentration polarization-induced holes and negative net polarization charges at the GACL/BL interface. This can facilitate the separation of the conduction band (E C) and Fermi level (E F) at the 2DEG channel and therefore benefit the normally-OFF operation of the device. The optimized graded-AlGaN-gated metal-semiconductor HEMT can achieve a large threshold voltage of 4 V. Furthermore, we demonstrated that shortening the gate length on the GACL and inserting an oxide layer between the gate and GACL can be both effective to suppress gate leakage current, enhance gate voltage swing, and improve on-state drain current of the device. These numerical investigations can provide insights into the physical mechanisms and structural innovations of the E-mode GaN-based HEMTs in the future.

In this work, the device characteristics of GaN-based high-electron-mobility transistors (HEMTs) were systematically investigated by the direct current (DC) and low-frequency noise (LFN) measurements within the temperature ranging from 300 K to 4.2 K. The temperature-dependent behavior of the on- and off-state electrical properties was statistically analyzed, highlighting an overall improved device performance under cryogenic temperatures. In addition, the LFN of the device exhibited an evident behavior of 1/f noise from 10 Hz to 10 kHz in the measured temperature range and can be well described by the carrier number fluctuations with correlated mobility fluctuations (CNF/CMF) model down to 4.2 K. Based on this model, we further extracted and discussed the defect-related behavior in the devices under low-temperature environments. These experimental results provide insights into the device characteristics of GaN-based HEMTs under cryogenic environments, motivating further studies into the GaN-based cryo-devices and systems.

In this work, we design and fabricate a deep ultraviolet (DUV) light-emitting array consisting of 10 × 10 micro-LEDs (μ-LEDs) with each device having 20 μm in diameter. Strikingly, the array demonstrates a significant enhancement of total light output power by nearly 52% at the injection current of 100 mA, in comparison to a conventional large LED chip whose emitting area is the same as the array. A much higher (~22%) peak external quantum efficiency, as well as a smaller efficiency droop for μ-LED array, was also achieved. The numerical calculation reveals that the performance boost can be attributed to the higher light extraction efficiency at the edge of each μ-LED. Additionally, the far-field pattern measurement shows that the μ-LED array possesses a better forward directionality of emission. These findings shed light on the enhancement of the DUV LEDs performance and provide new insights in controlling the light behavior of the μ-LEDs.

Self-assembled AlGaN nanowires (NWs) are regarded as promising structures in the pursuit of ultraviolet photodetectors (UV PDs). However, AlGaN NW-based PDs currently suffer from degraded performance partially due to the existence of outstanding surface-related defects/traps as a result of their large surface-to-volume-ratio. Here, we propose an effective passivation approach to suppress such surface states via tetramethyl ammonium hydroxide (TMAH) solution treatment. We successfully demonstrate the fabrication of UV PDs using TMAH-passivated AlGaN quantum-disk NWs and investigate their optical and electrical properties. In particular, the dark current can be significantly reduced by an order of magnitude after surface passivation, thus leading to the improvement of photoresponsivity and detectivity. The underlying mechanism for such a boost can be ascribed to the effective elimination of oxygen-related surface states on the NW surface. Consequently, an AlGaN NW UV PD with a low dark current of 6.22 × 10−9 A, a large responsivity of 0.95 A W−1, and a high detectivity of 6.4 × 1011 Jones has been achieved.

Gallium oxide (Ga2O3) is rapidly emerging as a material of choice for the development of solar blind photodetectors and power electronic devices which are particularly suitable in harsh environment applications, owing to its wide bandgap and extremely high Baliga figure of merit (BFOM). The Ga2O3 based devices show robustness against chemical, thermal and radiation environments. Unfortunately, the current Ga2O3 technology is still not mature for commercial usage., Thus, extensive research on the growth of various polymorph of Ga2O3 materials has been carried out. This article aims to provide an overview of the current understanding of epitaxial growth of different phases of Ga2O3 by various growth techniques including pulsed laser deposition (PLD), molecular beam epitaxy (MBE), metal-organic chemical vapour deposition (MOCVD), sputtering, mist chemical vapour deposition (Mist CVD) and atomic layer deposition (ALD).The review also investigates the factors such as the growth temperature, pressure, carrier gas, III/V ratio, substrate as well as doping which would influence the synthesis and the stability of meta stable phases of Ga2O3. In addition, a through discussion of growth window is also provided using phase diagrams for aforementioned epitaxial deposition methods.

Light detection in the deep-ultraviolet (DUV) solar-blind waveband has attracted interest due to its critical applications, especially in safety and space detection. A DUV photodetector based on wide-bandgap semiconductors provides a subversive scheme to simplify the currently mature DUV detection system. As an ultra-wide-bandgap (4.4–5.3 eV) semiconductor directly corresponding to the DUV solar-blind waveband, Ga2O3 has an important strategic position in the prospective layout of semiconductor technology owing to its intrinsic characteristics of high breakdown electric field, excellent tolerance of high/low temperature, high resistance to radiation, and rich material systems. As the only native substrate that can be fabricated from melt-grown bulk single crystals, β-Ga2O3 has attracted a lot of attention both in power-electronic and photo-electronic devices. In addition, other metastable phases (e.g. α, ϵ, γ) of Ga2O3 have attracted great interest due to their unique properties. In this work, we discuss the advances in achieving bulk and film Ga2O3 materials with different crystal phases. In addition, the latest achievements with polymorphous Ga2O3-based solar-blind photodetectors (SBPDs) and the methods to enhance their performance, including doping, annealing, and transparent electrodes, are also discussed. Furthermore, as the most desirable application, DUV imaging technologies based on Ga2O3 SBPDs are systematically summarized. Finally, conclusions regarding recent advances in Ga2O3 SBPDs, remaining challenges, and prospects are presented and discussed.

In this work, we demonstrated a thermal oxidation process for AlGaN nanowires and revealed the underlying mechanism of AlGaN to AlGaO transition for sub-250 nm deep ultraviolet photodetection. When the AlGaN nanowires were exposed to oxygen atmosphere below a temperature of 700 °C, only the Al–N bond was oxidized while the Ga–N bond remained intact, and thereafter, the AlGaN nanowires were slowly oxidized to AlGaON complex. However, as the oxidation temperature was elevated to above 700 °C, the AlGaN complex was transformed into AlGaO complex. Using this method, photodetectors based on AlGaON nanowires were fabricated and compared with its AlGaN counterpart. The AlGaON photodetector showed a response peak at deep ultraviolet of 242 nm with a high responsivity of 726 mA W−1, compared with the response peak at 276 nm in AlGaN nanowires. This work offers an alternative method towards the realization of high performance sub-250 nm deep ultraviolet photodetection.

Ⅲ-nitrides and beta-phase gallium oxide (β-Ga2O3) are currently two intensively investigated wide bandgap semiconductor materials for power electronics. Due to the relatively low lattice mismatch between the two material systems and the availability of bulk AlN, GaN and β-Ga2O3 substrates, epitaxial growth of Ⅲ-nitrides on β-Ga2O3 or vice versa has been realized. However, the design of power devices by integrating the two material systems is still lacking. Here we numerically investigate an AlN/β-Ga2O3 heterostructure by taking advantage of polarization-induced doping to realize high-performance enhancement-mode transistors. Induced by polarization effects at the AlN/β-Ga2O3 interface, a 2-dimensional electron gas (2DEG) concentration can reach up to 8.1×1019 cm−3 in the channel. On top of the channel, a p-GaN gate was introduced and eventually a normally-off AlN/β-Ga2O3 field-effect transistor with tunable positive threshold voltages was realized. Furthermore, we inserted an unintentionally doped (UID) GaN back barrier layer to suppress the drain leakage current. Eventually, the transfer and output characteristics of the proposed device with different structural parameters were further investigated and analyzed in the pursuit of high-performance Ⅲ-nitrides/Ga2O3-based power devices.

III-nitride deep ultraviolet (DUV) light-emitting diodes (LEDs) are identified as the promising candidate for energy-efficient, environment-friendly and robust UV lighting source in the application of water/air purification, sterilization, and bio-sensing. However, the state-of-art DUV LED performance is far from satisfaction for commercialization owing to its low internal quantum efficiency, large current leakage and efficiency droop at high current injection, etc. Extensive efforts have been devoted to properly designing the band structures of such luminescent devices to enhance their output power. In this review, we summarize the recent progress on various energy band designs and engineering of DUV LEDs, with particular of interest is paid on the various approaches in band engineering of the electron-blocking layer, quantum well, quantum barrier and the implementation of many novel structures such as tunnel junctions, ultrathin quantum heterostructures to enhance their efficiency. Those inspirational approaches pave the way towards the next generation of greener and efficient UV sources for practical applications.

The KAUST authors acknowledge the support of the GCC Research Program REP/1/3189-01-01, Baseline BAS/1/1664-01-01, and Equipment BAS/1/1664-01-07. The work at QU was supported by the GCC Research Program GCC-2017-007. The work at the Georgia Institute of Technology was supported in part by DARPA under Grant No. W911NF-15-1-0026 and NSF under Grant No. DMR-1410874. R.D.D. acknowledges the additional support of the Steve W. Chaddick Endowed Chair in Electro-Optics and Georgia Research Alliance.

Significant internal quantum efficiency (IQE) enhancement of GaN/AlGaN multiple quantum wells (MQWs) emitting at ~350 nm was achieved via a step quantum well (QW) structure design. The MQW structures were grown on AlGaN/AlN/sapphire templates by metal-organic chemical vapor deposition (MOCVD). High resolution x-ray diffraction (HR-XRD) and scanning transmission electron microscopy (STEM) were performed, showing sharp interface of the MQWs. Weak beam dark field imaging was conducted, indicating a similar dislocation density of the investigated MQWs samples. The IQE of GaN/AlGaN MQWs was estimated by temperature dependent photoluminescence (TDPL). An IQE enhancement of about two times was observed for the GaN/AlGaN step QW structure, compared with conventional QW structure. Based on the theoretical calculation, this IQE enhancement was attributed to the suppressed polarization-induced field, and thus the improved electron–hole wave-function overlap in the step QW.

AlxGa1−xN-based UV emitters in the form of a graded-index separate confinement heterostructure were grown on 6H-SiC. The devices consist of 100 nm AlxGa1−xN compositionally graded from x = 0.3 to 0 and x = 0 to 0.3 in two sides. Owing to polarization doping from naturally occurring or intentionally incorporated dopants, the two sides are of the doped p- and n-types, giving rise to a p–n junction. The doping level was calculated to be 1018 cm−3 for the p- and n-sides. It was found experimentally that this doping level requires the intentional incorporation of Mg and Si, and those devices show better rectification and stronger electroluminescence emission.