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Structural and electrical properties of Pd/p-GaN contacts for GaN-based laser diodes
摘要： In this paper, the properties of Pd-based p-contacts on GaN-based laser diodes are discussed. Pd is often the metal of choice for ohmic contacts on p-GaN. However, for Pd/p-GaN ohmic contacts, nanovoids observed at the metal/semiconductor interface can have a negative impact on reliability and also reproducibility. The authors present a thorough analysis of the microstructure of the Pd/p-GaN interface by x-ray photoelectron spectroscopy (XPS) and scanning transmission electron microscopy (STEM). STEM data show that the microvoids at the p-GaN/Pd interface form during rapid thermal annealing. A combination of the following effects is suggested to support the void formation: (1) the differences in thermal expansion coefficients of the materials; (2) excess matrix or impurity atoms in the semiconductor, at the interface, and in the metals, which are released as gases; and (3) the strong antisurfactant effect of Pd on Ga-rich p-GaN surfaces. A slow temperature ramp during contact annealing reduces the formation of voids likely by suppressing the accumulation of gases at the interface. XPS data show that the Ga/N ratio can be reduced by suitable cleaning of the p-GaN surface, which enhances Pd adhesion. As a result, the quality of the contact system is improved by the systematic optimization of the surface cleanliness as well as the annealing parameters, leading to void-free and clean Pd/p-GaN interfaces. The specific contact resistance, extracted from linear transmission line method measurements, is reduced by an order of magnitude to 2 × 10?3 Ω cm2 at 1 mA for the same epitaxial layer stack.
关键词： Pd/p-GaN contacts,STEM,nanovoids,ohmic contacts,rapid thermal annealing,XPS,GaN-based laser diodes
更新于2025-09-23 15:21:01
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Design of a Si-based Lattice-matched GeSn/SiGeSn Multi-quantum-well Laser
摘要： By calculating the heterojunction band alignment of GeSn/SiGeSn, a Ge0.9Sn0.1/Si0.14Ge0.71Sn0.15 multi-quantum-well laser was designed, where three Ge0.9Sn0.1 layers act as wells separated by three Si0.14Ge0.71Sn0.15 layers as barriers. The maximum TE gain reaches 7000 cm?1 at 0.5 eV, and the maximum TM gain reaches 5500 cm?1 at 0.52 eV. The modal gain of the Ge0.9Sn0.1/Si0.14Ge0.71Sn0.15 multi-quantum-well laser we proposed and designed can reach 100 cm?1 with a current density of 5 kA/cm2. The result indicates that it is possible to obtain a Si-based laser.
关键词： GeSn/SiGeSn laser,multi-quantum-well laser,Si-based laser
更新于2025-09-23 15:19:57
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Role of thermal fluid dynamics in alloying element distribution and weld porosity in powder feeding-based laser welding of Al alloy
摘要： This work develops a model incorporating the powder transport, laser heating, and molten pool dynamics to describe the mixing process of alloying element and reveal the mechanisms of bubble dynamics in powder feeding-based laser welding. Simulation results show that the different roles of recoil pressure: driving the molten metal flow at the rear part of keyhole in partial penetration and maintaining the stable keyhole profile in full penetration, are responsible for the difference between their melt flow behaviors. Driven by the melt flow, the alloying element transport path can be generalized as the shallow areas, the rear areas, the central and bottom areas in partial penetration. It permits a relatively uniform silicon distribution. However, two vortices rotating in the opposite direction in full penetration results in a higher concentration of Si in the upper part. Meanwhile, we conclude three steps for the bubble dynamics: bulge formation, bubble formation, bubble being captured by re-formed keyhole or being captured by the solid-liquid interface or escaping from the free surface. Such phenomena manifest that the contributions to the processing quality and metallurgy quality of welded joints come from the complex melt flow behaviors, which are confirmed by a series of experiments.
关键词： Thermal fluid dynamics,Alloying element distribution,Melt flow,Powder feeding-based laser welding,Weld porosity
更新于2025-09-23 15:19:57
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Underwater wire-feed laser deposition of the Ti–6Al–4V titanium alloy
摘要： Underwater wire-feed laser deposition (UWLD) of the Ti-6Al-4V alloy was firstly carried out utilizing a laser deposition nozzle, and the deposition appearance, geometry characteristics, microstructure and microhardness of deposited tracks were studied. At a lower gas flow rate, the absorption of residual water and scattering of aerosol particles on laser beam caused UWLD instability and decreased molten metal wettability. Upon increasing the gas flow rate, the aerosol particle density decreased, increasing the laser density and decreasing the cooling rate of molten metal; thus, the height and deposition angle of UWLD track decreased as the corresponding width and fusion depth increased. The microstructures of the deposited metal at a gas flow rate of 10 L/min were composed of the coarse lath α phase in upper region and the acicular martensite α′ with different size scales in middle and bottom regions. With increasing the gas flow rate to 20 L/min, the microstructure was primarily acicular martensite along with a small amount of α phase, and the martensite exhibited a larger grain size and stronger texture. At a gas flow rate of 20 L/min, the deposited process was stable and uniform deposition track without oxidation layer was obtained for UWLD Ti-6Al-4V alloy.
关键词： Ti-6Al-4V alloy,Laser propagation mechanism,Microstructure,Microhardness,Underwater wire-based laser deposition,Geometrical characteristics
更新于2025-09-19 17:13:59
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Redefinition the quasi-Fermi energy levels separation of electrons and holes inside and outside quantum wells of GaN based multi-quantum-well semiconductor laser diodes
摘要： Both accurate experiments and a self-consistent theoretical simulation using the near-ABC or ab initio method have uncovered the anomalous changes in the junction voltage (Vj ) of GaN-based lasers (Li et al 2013 Appl. Phys. Lett. 102 123501, Feng et al 2018 J. Phys. D: Appl. Phys. 51 095102 and Feng et al Appl. Phys. B 124 39). Here, further accurate emission spectral characteristics and band-gap simulation of a GaN based laser diodes (LDs) confirmed that previous researchers may have confused the junction voltage (Vj ) and the separation between the two Fermi levels of electrons and holes in the active region divided by the electronic charge (ΔEf /e). For the multiple-quantum wells (MQWs), there are obvious differences between them. For an ideal GaN-based LDs, despite the increase in junction voltage, physical quantities including the separation between the two Fermi levels of electrons and holes in the MQW region, and the optical net gain remain unchanged beyond the threshold region after an unpredictable sharp increase in the threshold region.
关键词： multi-quantum-well (MQW),steady state,junction voltage,quasi-Fermi energy levels separation,GaN based laser diodes (LD)
更新于2025-09-19 17:13:59
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Ultra-low-intensity-noise single-frequency fiber-based laser at 780 nm
摘要： With the rapid development of precision measurements relying on atom absorption such as atomic inertial sensors and optical magnetometry, advanced lasers are urgently required especially ultralow-noise lasers corresponding to the atom absorption line. For the D2 line of rubidium atoms, an ultralow-intensity-noise 780 nm single-frequency fiber-based laser with an output power of 340 mW is demonstrated. By eliminating polarization sensitivity in the noise suppression process based on the gain saturation of a semiconductor optical amplifier, a relative intensity noise of ?151 dB Hz?1 in the frequency range from 0.1 to 50 MHz is achieved, which approaches the shot noise limit.
关键词： ultralow-intensity-noise,shot noise limit,fiber-based laser,single-frequency,780 nm
更新于2025-09-16 10:30:52
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Control Technology of Ground-Based Laser Communication Servo Turntable via a Novel Digital Sliding Mode Controller
摘要： In this study, a sliding mode control (SMC) algorithm was proposed based on a novel reaching law to solve the nonlinear disturbance problem of a ground-based laser communication turntable. This algorithm is a chatter-free method, in which the coe?cient of sliding mode variable structure function is designed as an adaptive function, so the chattering of the sliding mode approaches zero. For any perturbed system, this algorithm can ensure a ?nite time for the system state to reach the sliding mode surface from any initial state. Additionally, the system will stabilize in the quasi-sliding mode domain (QSMD) with O(T3) width, where a narrower QSMD width corresponds to stronger robustness toward nonlinear disturbances. Both mathematical calculations and simulations veri?ed the sliding mode and stability of this control algorithm. Experimental results of the velocity closed-loop of pitch axis show that the proposed algorithm e?ectively improved the anti-nonlinear disturbance ability of the control system compared with the e?ects of the traditional digital PID and the existing chatter-reduced SMC algorithms, for smooth system operation.
关键词： quasi-sliding mode domain (QSMD),reaching law,sliding mode control (SMC),chatter-free,ground-based laser communication turntable
更新于2025-09-16 10:30:52
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Effects of deorbit evolution on space-based pulse laser irradiating centimeter-scale space debris in LEO
摘要： The aim of this work is to address the dynamic response rules of deorbit evolution for removing centimeter-scale dangerous space debris near the international space station (ISS) by space-based single laser station. According to the theory of orbital dynamics and laser irradiation technology, the deorbit model of space-based laser irradiating the centimeter-scale debris in low earth orbit (LEO) was established. The deorbit process of driving the centimeter-scale debris was simulated by analyzing the orbit and laser parameters in coplanar/non-coplanar conditions, and the distribution rules of irradiation distance with action time were also obtained. Furthermore, the dynamic characteristics of deorbit evolution were investigated by calculating the flight paths and orbital elements after the dangerous debris is irradiated, and the interaction mechanism between space-based nanosecond pulse laser and the dangerous debris was detailedly discussed by estimating collision risk between the dangerous debris and the ISS. The simulation results indicated that the collision risk between the dangerous debris and the ISS was successfully eliminated based on the designed removal scheme and given clearance windows. Therefore, it is highly important for exploring excellent clear method to master removal rules of centimeter-scale dangerous space debris in LEO by the space-based nanosecond pulse laser.
关键词： Low earth orbit,Deorbit model,Centimeter-scale debris,Dynamics behavior,Space-based laser
更新于2025-09-12 10:27:22