GaN-based Laser Diodes Research Articles

Degradation mechanism during catastrophic optical damage in 385 nm GaN-based ultraviolet laser diodes

The failure mechanism of 385 nm GaN-based ultraviolet (UV) laser diodes (LDs) has been investigated. The degradation factors were studied by analyzing scanning electron microscopy (SEM), cathodoluminescence, and transmission electron microscopy (TEM) after aging. In degraded UV LD, degradation is easily observed on the anterior cavity facet, with holes found in the epitaxial GaN material, primarily concentrated in the quantum wells and waveguide regions of the LDs. This shows that the degradation may be closely related to the higher photon energy of the UV laser beam emitted by UV LDs. In the aging process of samples, emission of UV lasers leads to deterioration at the interface and in the semiconductor. This degradation leads to continuous heat accumulation and may create a positive feedback loop. Ultimately, this results in the failure of the UV LD. This study presents a possible major factor for the shorter lifetime of UV LD compared to blue LD. Therefore, it is very significant to improve the interface quality for extending the lifetime of UV LDs.

Optical and electrical degradation behavior of GaN-based UV-A laser diodes

The degradation mechanism of GaN-based ultraviolet (UV-A) laser diodes (LDs) is analyzed by studying the changes of their electrical and optical properties after aging process with the operation current below the threshold current (Ith). After aging treatment, Ith increases, the slope efficiency decreases, and the leakage current increases. In particular, the emission spectra of aged LD show significant broadening, with additional peaks on the shorter wavelength side. Both cathodoluminescence and deep level transient spectrum results indicate defects can develop around the active region of the LD, even when the operation current is below Ith. This leads to an increase in trap-assisted tunneling current and non-radiative recombination. The observed defects may result from the diffusion of water molecule in the environment from the cavity facet into the active region and formation of ON–CN and VGaON complex defects. These defects not only reduce the effective carrier concentration injected into the quantum well but also increase non-radiative recombination. These results will contribute to our understanding of the degradation mechanism of UV LDs and the preparation of long lifetime UV LDs.

Effects of TMIn Flow Rate During Quantum Barrier Growth on Multi-quantum Well Material Properties and Device Performance in GaN-Based Laser Diodes

Abstract Multidimensional influence of indium composition in barrier layers on GaN-based blue laser diodes (LDs) are discussed through both material quality and device physics perspectives. LDs with higher indium content in the barriers demonstrate a notably lower threshold current and shorter lasing wavelength compared to those with lower indium content. Our experiments reveal that higher indium content in the barrier layers can partially reduce indium composition in the quantum wells, a novel discovery. Employing higher indium content barrier layers leads to improved luminescence properties of MQW region. Detailed analysis reveals that this improvement can be attributed to better homogeneity in the indium composition of the well layers along the epitaxy direction. InGaN barrier layers suppressed lattice mismatch between barrier and well layers, thus mitigating the indium content pulling effect in well layers. In supplement to experimental analysis, theoretical computations are performed, showing that InGaN barrier structures can effectively enhance carrier recombination efficiency and optical confinement of LD structure, thus improving output efficiency of GaN-based blue LDs. Combining these theoretical insights with our experimental data, we propose that higher indium content barriers effectively enhance carrier recombination efficiency and indium content homogeneity in quantum well layers, thereby improving the output performance of GaN-based blue LDs.

Improving photoelectric characteristics of GaN-based green laser diodes by inserting electron blocking layer with gradient Al composition

This paper proposes four new gradient composition electron blocking layer (EBL) structures to enhance the photoelectric performance of GaN-based green laser diodes (LDs). The optical and electrical properties of new structure LDs are theoretically analyzed by LASTIP. It is observed that the implementation of a gradient composition EBL structure increases the effective barrier height for electrons, thereby better inhibiting the electron current overflow from the active region. In addition, the optical field leakage is effectively suppressed, lower threshold current and higher output power are obtained. The four different new structures have obvious differences in the improvement of the hole current injection and slope efficiency of multiple quantum well (MQW) LDs, and the underlying reasons for these phenomena are explored. Simulation results indicate that adopting a gradually descending Al component EBL structure yields optimal improvements in the photoelectric performance of LDs.

Enhanced luminescence efficiency in GaN-based blue laser diodes by H plasma technology

To the best of our knowledge, this paper is the first to report the application of H plasma treatment technology to the treatment of laser diode ridge. Through the H plasma passivation on the ridge of the laser diode, a neutral complexes layer (i.e., Mg-H) is formed on the ridge, which effectively reduces ridge leakage current, thus reducing the threshold current of the laser diode and significantly improving the slope efficiency. The ridge were treated with H plasma using the Oxford Plasmalab System 100 ICP 180. The lasers' leakage current, optical power, emission wavelength, and other parameters were measured using a Cascade150 + B1505A probe station system, along with matched optical power meters and fiber optic spectrometers. Specifically, this study successfully fabricates a GaN-based blue laser diode characterized by a threshold current as low as 0.42 A and a slope efficiency as high as 1.96 W/A. Compared with the traditional silicon oxide-mediated ridge treated laser, the threshold current of the laser passivated by H plasma is reduced by 0.13 A, and the slope efficiency is increased by 0.56 W/A. This research not only enhances the performance of laser diodes but also has the potential to expand their application in multiple fields.

Optimum Design of InGaN Blue Laser Diodes with Indium-Tin-Oxide and Dielectric Cladding Layers.

The efficiency of current GaN-based blue laser diodes (LDs) is limited by the high resistance of a thick p-AlGaN cladding layer. To reduce the operation voltage of InGaN blue LDs, we investigated optimum LD structures with an indium tin oxide (ITO) partial cladding layer using numerical simulations of LD device characteristics such as laser power, forward voltage, and wall-plug efficiency (WPE). The wall-plug efficiency of the optimized structure with the ITO layer was found to increase by more than 20% relative to the WPE of conventional LD structures. In the optimum design, the thickness of the p-AlGaN layer decreased from 700 to 150 nm, resulting in a significantly reduced operation voltage and, hence, increased WPE. In addition, we have proposed a new type of GaN-based blue LD structure with a dielectric partial cladding layer to further reduce the optical absorption of a lasing mode. The p-cladding layer of the proposed structure consisted of SiO2, ITO, and p-AlGaN layers. In the optimized structure, the total thickness of the ITO and p-AlGaN layers was less than 100 nm, leading to significantly improved slope efficiency and operation voltage. The WPE of the optimized structure was increased relatively by 25% compared to the WPE of conventional GaN-based LD structures with a p-AlGaN cladding layer. The investigated LD structures employing the ITO and SiO2 cladding layers are expected to significantly enhance the WPE of high-power GaN-based blue LDs.

Investigation of the mechanism of carrier recombination in GaN-based blue laser diodes before lasing

Abstract The carrier recombination behavior of GaN-based blue laser diodes (LDs) is studied and analyzed by experiments and simulation calculations before lasing, with a particular focus on the role of Auger recombination. It is found that Auger recombination plays a crucial role in the decrease in differential efficiency and threshold current of GaN-based blue LDs. The theoretical calculation results show that a large Auger recombination rate may lead to a dominant recombination channel before lasing, which could exceed the radiation recombination and result in an obvious decrease in the differential efficiency. Such a high Auger recombination will dissipate a large number of carriers in the quantum well, resulting in deterioration of device performance, a higher threshold current and a lower efficiency. This work presents a method to evaluate Auger recombination through differential efficiency and also provides evidence that suppressing the Auger recombination rate is beneficial to improve the performance of blue LDs.

Atomic-level quantum well degradation of GaN-based laser diodes investigated by atom probe tomography

Gallium nitride (GaN)- based lasers are extensively employed in display, lighting, and communication applications due to their visible laser emission. Despite notable advancements in their performance and reliability, sustained device functionality over extended periods remains a challenge. Among the diverse mechanisms contributing to degradation, the deterioration of quantum wells poses a persistent obstacle. In this study, we investigated the atomic-level degradation of quantum wells within GaN-based laser diodes utilizing atom probe microscopy. Our analysis revealed a substantial increase in indium fluctuation, accompanied by the formation of indium protrusions at the quantum well interfaces, which provides a credible explanation for the observed increase in FWHM (full width at half maximum) of the spontaneous spectra of lasers following prolonged operation. Additionally, magnesium analysis yielded no evidence of diffusion into the quantum well region. Combined with prior studies, we attribute the degradation of quantum wells primarily to the formation of indium-related non-radiative recombination centers.

Blue GaN-based DFB laser diode with sub-MHz linewidth

Distributed feedback laser diodes (DFBs) serve as simple, compact, narrow-band light sources supporting a wide range of photonic applications. Typical linewidths are on the order of sub-MHz for free-running III-V DFBs at infrared wavelengths, but linewidths of short-wavelength GaN-based DFBs are considerably worse or unreported. Here, we present a free-running InGaN DFB operating at 443 nm with an intrinsic linewidth of 685 kHz at a continuous wave output power of 40 mW. This performance is achieved using a first-order embedded hydrogen silsesquioxane (HSQ) surface grating. The frequency noise is measured using a cross-correlated self-heterodyne frequency discriminator, and two estimations of integrated linewidth are evaluated using 1/π integration and β-separation line integration methods.

High-speed GaN-based laser diode with modulation bandwidth exceeding 5 GHz for 20 Gbps visible light communication

Visible light communication (VLC) based on laser diodes demonstrates great potential for high data rate maritime, terrestrial, and aerial wireless data links. Here, we design and fabricate high-speed blue laser diodes (LDs) grown on c-plane gallium nitride (GaN) substrate. This was achieved through active region design and miniaturization toward a narrow ridge waveguide, short cavity length, and single longitudinal mode Fabry–Perot laser diode. The fabricated mini-LD has a low threshold current of 31 mA and slope efficiency of 1.02 W/A. A record modulation bandwidth of 5.9 GHz (−3 dB) was measured from the mini-LD. Using the developed mini-LD as a transmitter, the VLC link exhibits a high data transmission rate of 20.06 Gbps adopting the bit and power loading discrete multitone (DMT) modulation technique. The corresponding bit error rate is 0.003, satisfying the forward error correction standard. The demonstrated GaN-based mini-LD has significantly enhanced data transmission rates, paving the path for energy-efficient VLC systems and integrated photonics in the visible regime.

Investigation on spontaneous recombination mechanisms in GaN based laser diodes under low injection current

This study works on the spontaneous recombination mechanisms of GaN-based laser diodes (LDs) under low injection current by examining their power–current (P–I) curves and electroluminescence spectra. Our investigation focuses on the behavior of differential efficiency in LDs under low injection current, revealing that a competition between impurity-related yellow emissions and band-edge blue emissions leads to a change in total luminescence efficiency. Using both experimental and simulating methods, the yellow emission peak is primarily attributed to carrier recombination in deep-level defects located on the LD's p-side. A detailed explanation to the differential efficiency changing mechanism is beneficial to improve the GaN-based LD performance in future fabrication.

Effect of Asymmetric InAlGaN/GaN Superlattice Barrier Structure on the Optoelectronic Performance of GaN-Based Green Laser Diode

An asymmetric InAlGaN/GaN superlattice barrier structure without the first quantum barrier layer (FQB) is designed, and its effect on the optoelectronic performance of GaN-based green laser diode (LD) has been investigated based on simulation experiment and analytical results. It is found that, compared with conventional GaN barrier LD, device performance is significantly improved by using FQB-free asymmetric InAlGaN/GaN superlattice barrier structure, including low threshold current, high output power, and high photoelectric conversion efficiency. The threshold current of LD with novel structure is 16.19 mA, which is 22.46% less than GaN barrier LD. Meanwhile, the output power is 110.69 mW at an injection current of 120 mA, which is 16.20% higher compared to conventional LD, and the wall-plug efficiency has an enhancement of 9.5%, reaching 20.27%. FQB-free asymmetric InAlGaN/GaN superlattice barrier layer can reduce optical loss, suppress the polarization effect, and improve the carrier injection efficiency, which is beneficial to improve output power and photoelectric conversion efficiency. The novel epitaxial structure provides theoretical guidance and data support for improving the optoelectronic performance of GaN-based green LD.

Nano-indentation study of dislocation evolution in GaN-based laser diodes

The slip systems and motion behavior of dislocations induced by nano-indentation technique in GaN-based LDs were investigated. Dislocations with burgers vector of b = 1/3 <112¯\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$\\overline{2}$$\\end{document}3> were introduced on either {112¯\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$\\overline{2}$$\\end{document}2} <112¯\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$\\overline{2}$$\\end{document}3>, or {11¯\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$\\overline{1}$$\\end{document}01} <112¯\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$\\overline{2}$$\\end{document}3> pyramidal slip systems in the upper p-GaN layer. Besides, {0001} <112¯\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$\\overline{2}$$\\end{document}0> basal slip system was also activated. The AlGaN/InGaN multi-layers in device can provide mismatch stresses to prevent dislocations from slipping through. It was observed that the density of dislocations induced by the indenter significantly decreased from the upper to the lower regions of the multi-layers. The a + c dislocations on pyramidal slip planes were mostly blocked by the strained layers.

Controlling GaN-Based Laser Diode Performance by Variation of the Al Content of an Inserted AlGaN Electron Blocking Layer.

The leakage of the electronic current of a laser diode (LD) has some significant influences on the performance of the LD. In this study, commercial simulation software LASTIP is used to numerically evaluate the performances of LDs by using different wavelengths and Al contents of the electron blocking layer (EBL). These LDs a adopt multilayer structure, which contains cladding layers, waveguide layers, multiple quantum well layers, contact layers and an AlxGa1-xN EBL. The influence mechanism of EBL is theoretically examined by analyzing the simulated performances. It is found that for short-wavelength violet LDs, the electrical and optical properties of the LD will reach the optimum state when the Al content (x) in the EBL is nearly 0.25. For long-wavelength green LDs, it will achieve optimum electrical and optical properties when the Al content in the EBL is as low as possible. We also compare the simulation results of LDs with emission wavelengths in the range of violet and green, including blue cyan, for a more general evaluation. According to the simulated results, it is verified that the influence of the EBL's Al content on LD performance enhances as the wavelength increases.

Improving temperature characteristics of GaN-based ultraviolet laser diodes by using InGaN/AlGaN quantum wells.

Temperature characteristics of GaN-based laser diodes are investigated. It is noted that the characteristic temperature of the threshold current (T0) decreases with decreasing lasing wavelength for GaN-based LDs. The performance deteriorates seriously for UV LDs at high temperature. It is ascribed to the increase of carriers escaping from quantum wells due to the lower potential barrier height. In this Letter, AlGaN is used as the barrier layer in UV LDs instead of GaN to improve the temperature characteristic of the threshold current and slope efficiency by increasing the potential barrier height of quantum wells. Based on this structure, a high output power of 4.6 W is obtained at the injection current of 3.8 A; its lasing wavelength is 386.8 nm.

Spontaneous Emission Studies for Blue and Green InGaN-Based Light-Emitting Diodes and Laser Diodes

We investigated the efficiency droop phenomenon in blue and green GaN-based light-emitting diodes (LEDs) and laser diodes (LDs), which poses a significant challenge in high-power LEDs and is characterized by a reduction in external quantum efficiency at higher injection currents. Utilizing identical epi-structures for blue and green LEDs and LDs, with variations only in indium composition, our experiments revealed a gradual blue shift in the emission wavelengths as the injection current increased. Notably, the blue LED demonstrated a smaller shift compared to the green LED. In addition, the full width at half maximum of emission spectra increased with increasing injection current density, indicative of efficiency droop. Significantly, LDs consistently exhibited lower junction temperatures despite operating at higher current densities. This is attributed to the enhanced heat dissipation capability of the ridge waveguide LD structure, which results in a narrower emission spectrum and reduced efficiency droop compared to mesa LED structures. These outcomes highlight the efficiency of the ridge waveguide LD structure in heat dissipation from the active layer, offering crucial insights for the advancement of high-power light-emitting devices.

Investigation of degradation mechanism in GaN-based blue and ultraviolet laser diodes

We have studied the aging-induced degradation effect and the related mechanism of blue and ultraviolet (UV) laser diodes (LDs). First of all, the F parameter value, leakage current, and yellow luminescence intensity of LDs all increase after 24 h of the aging process, indicating that one of the reasons for the degradation of UV LDs may be the increase of the non-radiative recombination center in the material. Second, irreversible damage may be found on the front cavity surface of the UV and blue LDs. Due to the large UV photon energy, water molecules in the environment atmosphere are ionized to form OH− ions, which combine with dust in air to form SiO2 sediments and then attach to the front cavity surface. In addition, a large photon energy may cause damage to the anti-reflection film on the front cavity surface and lead to a too-high local temperature near the cavity surface, resulting in molten Ga droplets. Both sediment and the precipitation of molten GaN on the cavity surface will directly affect the function of the front cavity surface and the output power of the LD. In order to improve the reliability of the GaN-based UV LDs, it is necessary to reduce the density of material defects, select more stable coating materials on cavity facets, and improve the sealing property of the device package.

Study on carrier transport in InGaN upper waveguide layer of GaN-based blue laser diodes

The carrier transport capacity of the unintentionally doped InGaN upper waveguide (UWG) layer affects the hole injection efficiency of GaN-based blue laser diode. In this article, we studied the carrier transport property of UWG layer grown under various conditions by metal organic CVD. Hole diffusion length in these samples were obtained by photoluminescence. It is found that higher diffusion length can be obtained with growth temperature around 840 °C–870 °C and V/III ratio about 16 000. It is also found that reducing the threading dislocation density can enhance the carrier transport capacity of the UWG layer. Finally, blue laser diodes (LDs) were fabricated to confirm that increasing the effective diffusion length of carriers in UWG layers can help improve LDs performance.

Demonstrating the electron blocking effect of AlGaN/GaN superlattice cladding layers in GaN-based laser diodes

Electron leakage currents seriously limit the power conversion efficiencies (PCEs) of gallium nitride (GaN)-based laser diodes (LDs). To minimize the leakage currents, electron blocking layers are generally applied in the p-type region. However, few works have discussed the electron blocking effect of a p-cladding layer, which is found to be critical in suppressing the leakage currents of an LD. In this work, we compare the blocking performance of uniform AlGaN p-cladding layers and AlGaN/GaN superlattice (SL) p-cladding layers with the same average Al component respectively. Both light-emitting diodes (LEDs) and LDs with the same epitaxy structures are characterized by light–current (L–I) and current–voltage (I–V) measurements. The latest analytical model of leakage currents is applied to fit the L–I curves of LEDs, where smaller leakage coefficients are observed in the SL structures compared with the uniform-layer structures. Eighty LDs with varying ridge widths are studied by comparing the threshold current densities, slope efficiencies, and PCEs. The SL-based p-cladding layer shows statistically significant advantages over a uniform AlGaN layer. The blocking effects of both scattering- and bound-state electrons in SLs are investigated theoretically. Repetitive reflection and thermal relaxation are responsible for the blocking effect of scattering-state electrons. Simulation results indicate that the tunneling effect of bound-state electrons through a miniband mechanism is insignificant at a large injection level due to a negative differential conductivity by the Esaki–Tsu effect. We demonstrate a better electron blocking performance of p-cladding layers based on SLs than uniform AlGaN layers in GaN-based LDs.

Single-frequency violet and blue laser emission from AlGaInN photonic integrated circuit chips.

Chip-based, single-frequency and low phase-noise integrated photonic laser diodes emitting in the violet (412 nm) and blue (461 nm) regime are demonstrated. The GaN-based edge-emitting laser diodes were coupled to high-quality on-chip micro-resonators for optical feedback and mode selection resulting in laser self-injection locking with narrow emission linewidth. Multiple group III-nitride (III-N) based photonic integrated circuit chips with different waveguide designs including single-crystalline AlN, AlGaN, and GaN were developed and characterized. Single-frequency laser operation was demonstrated for all studied waveguide core materials. The best side-mode suppression ratio was determined to be ∼36 dB at 412 nm with a single-frequency laser emission linewidth of only 3.8 MHz at 461 nm. The performance metrics of this novel, to the best of our knowledge, type of laser suggest potential implementation in next-generation, portable quantum systems.