Yuantao Zhang

N-polar InGaN provides an attractive route to long-wavelength optoelectronic devices. However, the growth of N-polar InGaN quantum wells with high In composition is still limited by the trade-off between In incorporation and crystalline quality. Although underlayer strain engineering has been widely used to address this trade-off in metal-polar InGaN, its role in N-polar InGaN growth remains insufficiently understood. In this work, first-principles calculations are employed to elucidate the effects of biaxial strain in the underlying N-polar GaN on In-related adsorption and incorporation during InGaN growth. Our calculations show that tensile strain stabilizes InGaN adsorption relative to GaN adsorption at both step terminations without altering their distinct step selectivities, thereby promoting In incorporation through a GaN-dominated step-flow pathway without aggravating alternating-step attachment asymmetry. After lattice incorporation, tensile strain generally weakens In segregation, with this tendency also modulated by surface reconstruction. A quantitative strain-composition relationship is further established for N-polar InGaN under MOCVD conditions based on a cooperative multi-In incorporation pathway. Importantly, this relationship reproduces the experimentally observed strain dependence of In composition. This work provides atomic-scale insights into strain-modulated N-polar InGaN growth and offers guidance for high-quality InGaN quantum wells with high In composition.

Realizing the epitaxial growth of N-polar GaN/AlGaN heterostructures on GaN substrates is essential for advancing high-performance N-polar GaN HEMT devices. However, the growth of N-polar GaN films on GaN substrates by metal–organic chemical vapor deposition (MOCVD) remains challenging. Here, we show that N-polar GaN grown directly on untreated N-face free-standing GaN substrates exhibits three-dimensional islanded growth and poor coalescence. Surface analysis indicates that residual surface SiOx is a key interfacial factor limiting film coalescence. First-principles calculations further suggest that surface Si can be readily oxidized upon air exposure, leading to the formation of residual SiOx. Guided by these insights, we introduced an HF pretreatment to reduce the surface oxide prior to growth, enabling high-quality epitaxial growth of N-polar GaN films on GaN substrates and the subsequent fabrication of N-polar GaN/AlGaN heterostructures. The resulting heterostructure exhibited a room-temperature two-dimensional electron gas mobility of 1030 cm2/V·s and a sheet density of 8.2 × 1012 cm–2. To the best of our knowledge, this work represents the first demonstration of MOCVD-grown N-polar GaN/AlGaN heterostructures on GaN substrates. These results establish a practical MOCVD route for N-polar nitride heterostructures on GaN substrates and highlight the critical role of surface chemistry in N-polar GaN homoepitaxy.

Weak polarization electric field (PEF) III-nitride LEDs provide a promising pathway toward high-performance micro-LEDs for next-generation displays and wearable devices. To realize this promise, improving the efficiency of weak-PEF LEDs is crucial, as it remains the central requirement for next-generation optoelectronic applications. Here, we demonstrate record-high efficiency weak-PEF LEDs on polar c-plane with InGaN/AlGaN digital alloy (DA) superlattice barriers, enabled by optimized pulse-growth mode of metal-organic chemical vapor deposition (MOCVD). In addition, we reveal strong lateral carrier confinement in these LEDs. We improve the structural quality and optical performance of weak-PEF InGaN/DA multiple quantum wells (MQWs) by optimizing the pulse-growth conditions. Further, we realize weak-PEF DA-based blue LEDs with peak external quantum efficiency up to 15%, which is the highest value in current studies of its kind. Importantly, weak-PEF DA-based LEDs have strong lateral carrier confinement, leading to less efficiency degradation from sidewall effects than conventional GaN-based MQWs LEDs. This work represents a meaningful step toward high-performance weak-PEF III-nitride LEDs, particularly for micro-LED applications where both efficiency and carrier confinement are critical.

In this study, high-quality wurtzite Al0.9Sc0.1N thin films were grown on GaN templates via pulsed laser deposition (PLD). The influence of in situ thermal pretreatment of the substrate, growth temperature, and laser repetition rate on the microstructural evolution and electrical properties of the films was investigated. As revealed by in situ x-ray photoelectron spectroscopy, surface oxides are effectively removed through in situ thermal cleaning at 850 °C, and this step facilitates the nucleation of the (0002) orientation. Regarding growth kinetics, optimization results show that an ideal kinetic window for adatom migration is provided by a substrate temperature of 550 °C combined with a low laser repetition rate of 2 Hz. Under these optimized conditions, exceptional crystalline quality was achieved. A record-low full width at half maximum (FWHM) of approximately 180 arc sec was recorded for the (0002) plane in the x-ray rocking curve, while an FWHM as low as 175 arc sec was observed for the (101¯2) plane. This performance is comparable to state-of-the-art thin films fabricated via molecular beam epitaxy and sputtering. As revealed by temperature-dependent electrical characterization, the leakage current mechanism is dominated by Schottky emission at low electric fields, transitioning to Poole–Frenkel emission at high electric fields. Dielectric performance was significantly enhanced by the superior crystalline quality of the sample grown at 2 Hz, yielding a breakdown field of approximately 12.7 MV/cm (breakdown voltage approximately 57 V). Furthermore, distinct ferroelectric switching characteristics were exhibited by the optimized films. Collectively, these results demonstrate PLD as a competitive, cost-effective technique.

GaN high-electron mobility transistors (HEMTs) based on nitrogen-polar (N-polar) nitride films are promising for high-frequency millimeter-wave applications. However, realizing high-mobility N-polar GaN/AlGaN heterostructures by metal-organic chemical vapor deposition (MOCVD) remains challenging. In this work, N-polar GaN/AlGaN heterostructures are grown on SiC substrates by MOCVD. By lowering the growth temperature of the high-resistance GaN (HR-GaN) template layer, the two-dimensional electron gas (2DEG) mobility is significantly increased to 1947 cm2/V s, the highest value reported for N-polar GaN/AlGaN heterostructures on SiC. The mobility enhancement is primarily attributed to the effective reduction of oxygen impurity concentration in N-polar heterostructures at lower growth temperature of HR-GaN, leading to weakened ionized impurity scattering toward 2DEG. Importantly, reducing the growth temperature of HR-GaN also substantially increases its sheet resistance, which not only suppresses leakage current of N-polar HEMTs, but also contributes partially to the 2DEG mobility enhancement. This work presents an effective approach for enhancing the mobility of N-polar heterostructures, which is beneficial for the development of high-performance millimeter-wave N-polar GaN HEMT devices.

Light-emission devices based on InGaN/GaN quantum well (QW) bring about an ongoing revolution in general lighting. One of the highly deliberated discussions in this field is the steep efficiency drop with the increasing indium content of InGaN/GaN QW, posing a critical challenge to InGaN-based long-wavelength optoelectronic devices. Unfortunately, the factors that underlie the limitation remain unclear. Here, by using femtosecond transient absorption spectroscopy, we investigate the carrier dynamics of InGaN/GaN QW and find that the luminescence efficiency of InGaN/GaN QW is closely related to the localization states (LSs), i.e., dot-like In-rich InGaN clusters, in the InGaN layer. We demonstrate that the increase in the indium content can not only decrease the potential depth of LSs to weaken the localization binding effect and enhance the possibility of electrons being trapped by defects, but also enhance the density of LSs to increase the recombination channels and enlarge the full width at half maximum of the luminescence spectra. With these findings, we propose a model of carrier dynamics to deeply understand the emission mechanisms of InGaN/GaN QW, paving a way towards realizing high-performance InGaN-based optoelectronic devices.

The β-Ga2O3/p-GaN heterojunction avalanche photodetector (APD) with separated absorption and multiplication (SAM) structure was fabricated by metal–organic chemical vapor deposition. Through x-ray diffraction and scanning electron microscopy characterization, the Ga2O3 films exhibit relatively high crystalline quality. Through reasonable design of the electron concentration and thickness of the Ga2O3 layers, the device achieves an ultrahigh avalanche gain (Gain) of 1.75 × 107, an external quantum efficiency (EQE) of 4.53 × 107%, and a high responsivity (R) of 9.28 × 104 A/W under 254 nm solar-blind ultraviolet illumination (5 μW/cm2). The working mechanism of SAM-APD was further validated through TCAD simulations. Compared to conventional APDs, the SAM structure can significantly enhance avalanche gain and EQE, thereby improving the weak signal detection level of the APD effectively.

Red light-emitting diodes (LEDs) are at the forefront of current research in III-nitride semiconductors. The researches have shown that nitrogen-polar (N-polar) InGaN material facilitates the incorporation of indium, and N-polar InGaN-based LEDs are more conducive to carrier injection and confinement. This confers significant application potential for N-polar InGaN in red LEDs. In this work, we developed a novel metal-organic chemical vapor deposition (MOCVD) growth process for N-polar InGaN/GaN quantum wells (QWs) and successfully fabricated N-polar red LEDs. We achieved a significant red shift in the emission wavelength of the N-polar QWs by increasing the growth pressure of the well layer and reducing the V/III ratio, as well as modulating the stress of GaN template layer. Compared to metal-polar QWs, N-polar QWs fabricated using the new growth process exhibited a wavelength red shift exceeding 100 nm at the same growth temperature. Utilizing the new growth process, the efficiency of N-polar green LEDs increases by 1.5 times compared to previous results. Further, we fabricated N-polar red LEDs with an emission wavelength of 615 nm. This work demonstrates the feasibility and advantages of using N-polar InGaN for red LEDs fabrication, contributing to the advancement of nitride-based red LEDs.