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The recent progress in III-nitride tunnel junction (TJ) contacts for optoelectronic devices is summarized in this review paper. Due to the rapid developments of various III-nitride based optoelectronic devices for different emerging applications, TJs are of interest for these devices to achieve more efficient device performance. This review paper first briefly describes the working principles of TJs and the advantages of employing TJs in optoelectronic devices. The significant advancements in III-nitride TJs realized by metalorganic chemical vapor deposition (MOCVD) and molecular beam epitaxy (MBE) are addressed, as well as their TJ device performances. Lastly, the critical challenges for employing III-nitride TJs in wide bandgap ultraviolet (UV) light emitters are discussed for future investigations. [ABSTRACT FROM AUTHOR]

The electrical performances of III-nitride blue micro-light-emitting diodes (µLEDs) with different tunnel junction (TJ) epitaxial architectures grown by metalorganic chemical vapor deposition are investigated. A new TJ structure that employs AlGaN is introduced. The current density–voltage characteristic is improved by incorporating AlGaN layer above the n-side of the TJ layer, and the effects of the AlGaN/GaN superlattices is examined. Based upon the data from band diagram simulation, net positive polarization charge is formed at the AlGaN/GaN interface, which results in a reduction in tunneling distance and increase in tunneling probability. Moreover, similar electrical improvement is observed in various device dimensions and is independent of operating current density, suggesting that AlGaN/GaN biaxial tensile strain or current spreading is not the main contribution for the improvement. Finally, the effects on the efficiency performances are determined. While the maximum external quantum efficiency of the TJ devices remains identical, the wall-plug efficiency of µLEDs is enhanced significantly by the proposed AlGaN-enhanced TJ design. This work reveals the possibility of energy efficient TJ contact with high transparency in the visible wavelength range. [ABSTRACT FROM AUTHOR]

AlGaN germicidal ultraviolet (GUV) light emitting diodes (LEDs) are one of the most promising disinfection technologies in fighting the COVID-19 pandemic; however, GUV LEDs are still lacking in efficiency due to low p-type doping efficiency in p-AlGaN. The most successful approach for producing conductive p-type AlGaN is the implementation of a polarization-enhanced short period Al $_{\mathbf {x}}$ Ga $_{\mathbf {1-}\mathbf {x}}$ N/Al $_{\mathbf {y}}$ Ga $_{\mathbf {1-}\mathbf {y}}$ N superlattice (SL) structure, which enhances hole injection and reduces device operating voltage. In this report, we investigated different aspects of the superlattice including the Al $_{\mathbf {x}}$ Ga $_{\mathbf {1-}\mathbf {x}}$ N and Al $_{\mathbf {y}}$ Ga $_{\mathbf {1-}\mathbf {y}}$ N alloy constituent compositions, ${x}$ and ${y}$ , period thickness, total thickness, and Mg dopant concentration in terms of LED performance as well as electrical, optical, and morphological characteristics. The polarization-enhanced p-type doping in the AlGaN superlattice was also investigated computationally, giving excellent agreement with experimental results. Highly efficient UVC LEDs (279 nm) with EQE of 2% at 5 A/cm2 were demonstrated. A maximum output power of 5.5 mW (56 mW/mm2) was achieved at 100 mA. [ABSTRACT FROM AUTHOR]

The impact of AlGaN growth conditions on AlGaN:Si resistivity and surface morphology has been investigated using metalorganic chemical vapor deposition. Growth parameters including growth temperature, growth rate, and trimethylindium (TMI) flow have been systematically studied to minimize the resistivity of AlGaN:Si. We observed a strong anticorrelation between AlGaN:Si conductivity and growth temperature, suggesting increased silicon donor compensation at elevated temperatures. Secondary ion mass spectrometry and positron annihilation spectroscopy ruled out compensation by common impurities or group-III monovacancies as a reason for the observed phenomenon, in contrast to theoretical predictions. The underlying reason for AlGaN:Si resistivity dependence on growth temperature is discussed based on the possibility of silicon acting as a DX center in Al0.65Ga0.35N at high growth temperatures. We also show remarkable enhancement of AlGaN:Si conductivity by introducing TMI flow during growth. A minimum resistivity of 7.5 mΩ cm was obtained for n-type Al0.65Ga0.35N, which is among the lowest reported resistivity for this composition. [ABSTRACT FROM AUTHOR]

The role of water (H2O) vapor in reducing background impurity concentration in epitaxial Ga2O3 thin films grown by metalorganic chemical vapor deposition (MOCVD) was studied. Adding H2O vapor to oxygen was found to increase the surface roughness and decrease the carrier concentration but have no effect on the structural quality and RT electron mobility of both the UID and doped films. The H2O vapor promotes the growth of the stable (110) and 1 ¯ 10 facets elongated along the [001] direction. Using [H2O] of 250 ppm, the β-Ga2O3 films showed a RMS roughness of >13 nm, which is much higher than that measured for similar films grown without H2O (RMS = 0.8 nm). The electron mobility of the UID Ga2O3 layers grown with and without water vapor was ∼150 cm2/V s, while the carrier concentration dropped by ∼2.5× after increasing the [H2O] to 6000 ppm. Similarly, for the lightly Si doped films, the RT carrier concentration dropped from 5.4 × 1015 cm−3 ([H2O] = 0 ppm) to 3.4 × 1015 cm−3 ([H2O] = 250 ppm), leaving the mobility unaffected. The results offer a new method of using water vapor assisted growth of Ga2O3 films with low RT carrier concentration that are required for the realization of high-performance electronic power devices with high breakdown voltages. [ABSTRACT FROM AUTHOR]

Abstract: Light-emitting diodes (LEDs) fabricated from gallium nitride (GaN) have led to the realization of high-efficiency white solid-state lighting. Currently, GaN white LEDs exhibit luminous efficacy greater than 150lmW−1, and external quantum efficiencies higher than 60%. This has enabled LEDs to compete with traditional lighting technologies, such as incandescent and compact fluorescent (CFL) lighting. Further improvements in materials quality and cost reduction are necessary for widespread adoption of LEDs for lighting. A review of the unique polarization anisotropy in GaN is included for the different crystal orientations. The emphasis on nonpolar and semipolar LEDs highlights high-power violet and blue emitters, and we consider the effects of indium incorporation and well width. Semipolar GaN materials have enabled the development of high-efficiency LEDs in the blue region and recent achievements of green laser diodes at 520nm. [Copyright &y& Elsevier]

An anomalous output conductance that resembled short-channel effects was observed in long-channel N-polar GaN-channel/AlGaN-back-barrier/GaN-buffer high electron mobility transistors. The phenomenon could not be reasonably explained by drain-induced barrier lowering, leakage currents, or impact ionization events. We propose that the output conductance was caused by the ionization of a donorlike hole trap state at the negatively polarized AlGaN-back-barrier/GaN-buffer interface that shifted the threshold voltage at the drain side of the gate, where a high-field depletion region developed beyond current saturation. No evidence of increased output conductance or related device performance degradation was apparent under small-signal high-frequency conditions. The output conductance was suppressed by introducing photogenerated holes that compensated the traps. The effect of several typical back-barrier designs on the dc output conductance was examined. [ABSTRACT FROM PUBLISHER]

Indium nitride (InN) is identified as a promising terahertz (THz) emitter based on the optical and electronic properties of high quality In- and N-face samples. Time domain THz spectroscopy has been employed to measure the pump wavelength and background carrier concentration dependence of THz emission from InN. There is no discernable difference between the In- and N-face InN samples, as expected for the improved crystalline quality and concomitant low background electron density and high mobility for both polarities. While there is only a weak dependence of THz signal on pump wavelength from 800 nm to 1500 nm, there is a strong dependence on background electron density. Modeling shows that the dominant mechanism for THz generation in bulk InN is the current associated with the diffusion of the photo-generated electrons at elevated electron temperature (photo-Dember effect) and the redistribution of the background electrons under drift, with larger screening from the higher mobility electrons as compared to holes. Compensation or p-type doping in conjunction with manipulation of the large internal electric fields in InN/InGaN nanostructures should lead to significant improvements in THz emitters. [ABSTRACT FROM AUTHOR]

We report on the development of AlGaN/GaN high-electron mobility transistors (HEMTs) grown on SiC using plasma-assisted molecular beam epitaxy (MBE). In this work, we show that performance comparable to state-of-the-art AlGaN/GaN HEMTs can he achieved using MBE-grown material. Buffer leakage was an important limiting factor for these devices. The use of either carbon-doped buffers, or low Al/N ratio in the nucleation layer growth were effective in reducing buffer leakage. Studies varying the thickness and concentration of the carbon doping were carried out to determine the effect of different carbon doping profiles on the insulating and dispersive properties of buffers. On devices without field plates, at 4 GHz an output power density of 12 W/mm was obtained with a power-added efficiency (PAE) of 46 % and gain of 14 dB. 15.6 W/mm with PAE of 56 % was obtained from these devices after field-plating. Two-tone linearity measurements of these devices were also earned out. At a C/13 level of 30 dBc, the devices measured had an output power of 1.9 W/mm with a PAE of 53 %. The effect of the Al/N ratio in the AlN nucleation layer on buffer leakage was studied. N-rich conditions yielded highly insulating GaN buffers without carbon doping. At 4 GHz, devices without field plates delivered 4.8 W/mm with a PAE of 62 %. At a higher drain bias (50 V), 8.1 W/mm with a PAE of 38 % was achieved. [ABSTRACT FROM AUTHOR]