Jianping Liu

AlGaN-based ultraviolet (UV) laser diodes (LDs), with emission wavelength in the 280–365 nm range, are promising for applications in medical diagnostics and biological sensing, making them a prominent research focus in both academia and industry in recent years. A key challenge in their development is the large stress induced during the epitaxial growth of LD structures, which arises from the lack of lattice-matched substrates, and severely degrades the quantum efficiency and overall LD performance. This study presents an in-depth investigation into the growth mode and stress evolution of thick Al _0.2 Ga _0.8 N template. Firstly, we used the compressive stress between the Al _0.2 Ga _0.8 N layer and AlN/Sapphire substrate to form spontaneously three-dimensional growth to annihilate dislocations. Secondly, based on the Nakajima's theory of elasticity, we refined the conventional theoretical models for AlGaN strain relaxation of the S–K growth mode and critical thickness by considering the crucial role of threading dislocations (TDs) in releasing compressive stress. The experimentally measured critical thickness for three-dimensional growth was consistent with the calculated results. Furthermore, a crack-free high-quality 5 μ m-thick Al _0.2 Ga _0.8 N template was successfully grown on an AlN/sapphire substrate.

Photonic‐crystal surface‐emitting lasers (PCSELs) have emerged as promising light sources for high‐power, high‐brightness applications such as LiDAR, free‐space communication, and material processing, owing to their unique capabilities for single‐mode operation and narrow beam divergence. However, extending PCSELs into the blue wavelength regime remains challenging, primarily due to the low refractive index and short wavelength of GaN‐based materials, which weakens optical feedback and increases the fabrication difficulty and optical scattering loss, resulting in very high lasing thresholds. In this work, we demonstrate regrowth‐free, watt‐class GaN‐based PCSELs operating at 430.5 nm, fabricated on GaN substrates using an optimized epitaxial structure and photonic crystal design. The device achieved a threshold current density of 4 kA/cm ² , and a peak output power of 3.2 W at room temperature, which, to our knowledge, is the highest output power reported. This work provides a viable pathway toward high‐power visible PCSELs with simplified fabrication processes, offering significant potential for high‐brightness lighting and display applications in the future.

We fabricated optically pumped GaN‐based VCSELs and systematically analyzed and compared the quality factor ( Q ), angle‐resolved characteristics, lasing threshold, emission spectra, and near‐field patterns between the planar and plano‐concave cavities. The plano‐concave cavity achieved Q factor >20 000 due to strong 3D optical confinement, whereas the planar cavity yielded Q values of only a few hundred. The planar cavity exhibits a single longitudinal mode lasing without high‐order modes and a single bright spot of the near‐field pattern image. On the other hand, the plano‐concave cavity enables both single/multiple longitudinal mode(s) lasing depending on cavity length ( L ), consistently accompanied by multiple high‐order lasing modes. Its near‐field pattern shows a circular Gaussian spot. Angle‐resolved results demonstrate the characteristics of 3D optical confinement in a plano‐concave cavity and 1D confinement in the planar cavity. The threshold of a plano‐concave cavity depended weakly on cavity length, increasing the threshold from 2.5 mJ cm ^{− 2} at L = 1 µm to 9.7 mJ cm ^{− 2} at L = 31 µm. By comparison, the planar cavity shows a dramatic threshold increase due to sharply rising internal losses, from 4.4 mJ cm ^{− 2} at L = 1 µm to more than 25 mJ cm ^{− 2} at L = 4 µm. These results provide valuable guidance for the future research of VCSELs.

Visible light communication (VLC) is attracting growing interest for energy-efficient, high-bandwidth links for next-generation data centers. GaN-based blue edge-emitting micro-laser diodes (micro-LDs) with cavity lengths down to 40 μm are demonstrated based on optical resonance between a cleaved facet and an etched facet with laterally deposited distributed Bragg reflector. The threshold current as low as 9.6 mA and slope efficiency of 0.32 W/A are achieved for an 80 μm-cavity-length device. The VLC characteristics of 140 μm micro-LD show a broader −3 dB modulation bandwidth of 429 MHz and a higher data rate of 5.3 Gbps, compared with those of standard 500 μm LD. These results demonstrate the potential application of GaN-based blue edge-emitting micro-LDs as a compact and low-power laser platform for augmented/virtual reality displays and short-distance optical communication for inter-chip optical interconnections.

GaN‐based spin light‐emitting diodes (spin‐LEDs) are attractive for realizing room‐temperature spin‐controlled light emission in efficient, color‐tunable devices. In this work, we demonstrate spin injection in a GaN spin‐LED fabricated on a Si substrate using wafer‐bonding technology. An inverted n‐i‐p LED structure with n ‐type GaN on top facilitates the injection of spin‐polarized electrons from a ferromagnetic Au/Co/MgO spin‐injector layer grown by molecular beam epitaxy. At room temperature, an electroluminescence circular polarization of approximately 8% is measured under an applied 1 T out‐of‐plane magnetic field in a surface‐emitting geometry. After considering contributions from the Zeeman effect and magnetic circular dichroism, about 6% circular polarization is attributed to the pure spin injection into GaN. This work is focused on the detailed structural and interface characterization of the spin‐LED and allows to understand its spin and optoelectronic properties. These results highlight the potential of GaN‐based spin–optoelectronic devices compatible with Si platforms.

Distributed polarization-doped (DPD) AlGaN cladding layer (CL) has been successfully used in AlGaN ultraviolet laser diodes (LDs), which overcomes the problem of low Mg doping efficiency and can reduce the internal loss of LDs. However, this graded AlGaN CL is rarely used in InGaN visible LDs. Our previous work reported green LDs with DPD AlGaN CL, which can reduce the threshold current density of LDs, but suffer from the problem of low slope efficiency, only 0.07 W/A. Therefore, LDs with different structures were analyzed by optical field and device simulation. It was found that the electron blocking layer (EBL) in the LD structure is necessary to hinder electron leakage, even if a graded AlGaN CL with a high initial Al composition is used. By adopting an EBL, the slope efficiency of LDs with DPD AlGaN CL increased to 0.54 W/A, while the threshold current density was only slightly increased and the operating voltage was significantly reduced, compared with our previous report. These green LDs with DPD AlGaN CL also show a better performance than traditional green LDs.

The p‐type of electrical activation via Mg implantation into GaN containing a trace of indium (1.54%) was demonstrated. Implantation was performed at room temperature, followed by post‐implantation annealing at 1230°C under atmospheric pressure. The improved activation efficiency at a lower annealing temperature of 1230°C (compared to 1400°C applied by multicycle rapid thermal annealing and ultra‐high pressure annealing was attributed to the lower bond formation energy of In–N (7.695 eV) compared to Ga–N (9.265 eV), suggesting Mg ions at interstitial sites preferentially replace In rather than Ga. Moreover, low‐rate dry etching and Tetramethylammonium Hydroxide (TMAH) treatment were applied to effectively remove the surface damages induced by ion implantation and high‐temperature annealing, resulting in a significant current switching ratio enhancement of ~ 10 ⁵ . Comparison of the electroluminescence spectra between Mg‐implanted and nonimplanted samples under identical conditions clearly confirmed effective electrical activation of Mg in In _1.54% Ga _98.46% N.

The carrier-injection efficiency and confinement within the active region play a crucial role in determining the threshold and slope efficiency of GaN-based vertical-cavity surface-emitting lasers (VCSELs). In this study, two types of composition-graded epitaxial structures were proposed: a composition-graded last quantum barrier (LQB) and a composition-graded electron blocking layer (EBL). GaN-based optically pumped VCSELs incorporating these structures exhibit significant improvements over conventional composition-uniform designs, with threshold reductions of 71.1% and 53.2%, respectively. Band structure analysis reveals that these performance improvements are attributed to enhanced electron injection efficiency and improved hole confinement within the active region as well. Furthermore, pulsed lasing operation in electrically injected VCSELs was demonstrated by employing the composition-graded LQB structure. This study systematically validated the effectiveness of the composition-graded LQB and EBL structures in improving GaN-based VCSEL performance from experimental perspectives. These findings provide valuable insights for the development of GaN-based VCSELs.

The high-temperature growth of p-type layers via metal-organic chemical vapor deposition (MOCVD) often leads to thermal degradation of the InGaN quantum wells (QWs) in green laser diodes. In this study, a hybrid MOCVD-MBE (molecular beam epitaxy) growth approach was employed to fabricate green laser diodes, achieving high device performance and excellent uniformity, with a lasing wavelength of 504.9 nm and a threshold current density of 2.8 kA/cm2. Comparative device performance investigation demonstrates that replacing the MOCVD-grown p-type layers with those grown by MBE effectively suppresses thermal degradation in green InGaN QWs.

Gallium nitride (GaN)-based lasers, spanning emission wavelengths from ultraviolet to green, are widely used as light sources in many application domains. Despite their widespread usage, the comprehensive analysis of atomic-level degradation remains challenging primarily due to limitations in metrology techniques. In this study, we investigated the aging-induced magnesium (Mg) clusters in GaN-based lasers utilizing atom probe tomography. Notably, we have identified Mg clusters of several nanometers in size within the p-type aluminum gallium nitride (p-AlGaN) cladding layer of aged lasers, marking the first observation of such clusters in this context. The presence of Mg clusters induces light scattering and absorption within the cladding layer, contributing to an increase in internal loss and a decrease in slope efficiency. Furthermore, our findings underscore the efficacy of atom probe tomography as a potent technique for conducting atomic-level device failure analysis of semiconductor devices.