Threshold Current Decreases Research Articles

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.

Lasing Threshold Reduction of AlGaN‐Based Ultraviolet‐C Laser Diodes Using Strain Relaxed Lower Cladding Layer

The dependence of the performance of ultraviolet‐C (UV‐C) laser diodes on the strain relaxation of lower cladding layer (LCL) is numerically investigated. As the strain relaxation increases from 0% to 100%, the threshold current density decreases from 25 to 12.7 kA cm−2, and the slope efficiency increases from 0.039 to 0.087 W A−1, thanks to an increase in internal quantum efficiency (IQE). The improvement of IQE is mainly due to the enhanced hole injection resulting from the increased hole concentration in the distributed polarization doping layers and the reduced hole blocking barrier height of the upper waveguide layer with increasing the strain relaxation. Additionally, the effective screening of the polarization field within the quantum well ameliorates the quantum‐confined Stark effect, further improving the IQE. Our study reveals that strain relaxed LCL is imperative to achieve low‐threshold UV‐C laser diodes.

Performance enhancement of an AlGaN-based deep-ultraviolet laser diode using a two-stepped doped lower waveguide

Subject of study. The performance of a deep-ultraviolet laser diode is improved using a two-stepped Si-doped lower waveguide. Method. The impact of the variations on AlGaN-based deep-ultraviolet laser diodes has been evaluated based on the simulated results after theoretical calculations. The two AlGaN-based ultraviolet laser diodes as a traditional device and the proposed device have been analyzed comparatively based on their performance within a nomination wavelength region range of 269–280 nm. Main results. Using a two-stepped Si-doped lower waveguide, the lasing threshold current decreases in the proposed device D2 in comparison to the traditional device D1. The operating threshold voltage of D1 is 13.8 V, and that of D2 is 4.24 V, respectively, with lasing threshold currents of 0.4 A and 0.002 A, respectively. Practical significance. The optimization of the design of AlGaN-based ultraviolet laser diodes following the perfect bulk aluminium nitride substrate in the traditional device is the crucial factor of the practical significance of the scientific field utilized in the proposed device. The performance of deep-ultraviolet laser diodes is improved when a suitably constructed two-stepped Si-doped lower waveguide substitutes the traditional lower waveguide. The noted improvements are the reduction in total optical loss with the improved optical confinement factor. These are primarily due to an increase in the hole injection current and a decrease in the electron leakage current.

Size-dependent mobility of skyrmions beyond pinning in ferrimagnetic GdCo thin films

Magnetic skyrmions are swirling magnetic textures that can be efficiently driven with spin-orbit torques with a deflected trajectory. However, pinning slows skyrmions down and alters their trajectory, which prevents a quantitative comparison to analytical models. Here, we study skyrmions driven by spin-orbit torques at room temperature in ferrimagnetic GdCo thin films, an amorphous material with low pinning. Above a sharp current depinning threshold, we observe a clearly linear velocity increase with current that extrapolates to zero and a constant deflection angle, reaching high velocities up to 200 m/s. The mobility increases and the depinning threshold current decreases with the skyrmion diameter, which we vary using an external magnetic field. An analytical model based on the Thiele equation quantitatively reproduces these findings with a single fitting parameter. This validates the linear flow regime description and shows, in particular, the important role of skyrmion size in its dynamics.

The effect of humidity on the degradation mechanisms of GaN-based green laser diodes

The degradation behaviors of InAlGaN-based green laser diodes in humid ambients were studied. Nonhermetic laser diodes were aged at three different relative humidity conditions at constant current aging mode. For all nonhermetic devices failure occurred as a result of a large increase in the threshold current and sharp decrease of light output power. The reverse leakage current for the failures did not increase when the threshold current increased and slope efficiency decreased, indicating that the change in the threshold current was a result of a change in internal and/or mirror loss. Besides the threshold current increase, the photoluminescence intensity of the ridge region decreased more than 80% after aging compared with the non-aged companions. The moisture oxidized the p-Ohmic contact region of the semiconductor and caused the increase of the serial resistance. The mirror facet degradation was observed in scanning electron microscope. This led to change in reflectivity and light scattering and eventually to a large shift in the threshold current and slope efficiency of the device.

Regulating absorption loss and carrier injection efficiency in ultraviolet laser diodes by changing waveguide layer structure

Performance of UV laser diodes (LDs) with different structure of waveguide (WG) layers is investigated by simulation and experimental methods. It is found that the threshold current firstly decreases and then increases when the GaN WG thickness increases from 80 to 400 nm, and it also increases when a part of GaN WG layer is changed to be AlGaN. The variation of slope efficiency is accompanied with a decrease of threshold current. It is found that the optical absorption loss decreases, the carrier loss in upper WG layer increases and hole injection efficiency decreases. Finally, a high continuous-wave (CW) output power of 3.8 W at λ = 386.5 nm is achieved by using 120 nm-thick GaN WG layers. The threshold current density is 1.78 kA/cm2, the slope efficiency is about 1.5 W/A below the injection current of 2 A, and the highest wall plug efficiency (WPE) is about 20.2% at an injection current of 2.5 A.

Experimental investigation of stripe cavity length effect on threshold current density for InP/AlGaInP QD laser diode

InP/AlGaInP QD laser diodes with various cavity lengths grown on GaAs substrate were synthesized by (MOVPE) technique. L-I characteristics at various temperatures (190–390K) were measured for InP/AlGaInP QD laser diodes to investigate the effect of the various stripe cavity length on threshold current density and characteristic temperature. It is found that the threshold current increases with temperature, reaches a peak, then drops as temperature increases, before it increases again at temperatures over 300K due to carrier redistribution in the quantum dot states with temperature. It is found also that the threshold current density decreases with increasing cavity length due to the increasing threshold gain with the cavity length. However, as the cavity length increases beyond certain length, the rate of the decrease of the threshold current with cavity length decreases. One important finding is that the characteristic temperature of threshold current decreases significantly with the cavity length at the operating temperature range from 190K to 360K.

Improved performance of GaN-based blue laser diodes using asymmetric multiple quantum wells without the first quantum barrier layer.

An asymmetric multiple quantum well (MQW) without the first quantum barrier layer is designed, and its effect on the device performance of GaN-based blue LDs has been studied experimentally and theoretically. It is found that compared with LD using symmetrical multiple quantum well, device performance is improved significantly by using asymmetric MQW, i.e. having a smaller threshold current density, a higher output optical power and a larger slope efficiency. The threshold current density decreases from 1.28 kA/cm2 to 0.86 kA/cm2, meanwhile, the optical power increases from 1.77 W to 2.52 W, and the slope efficiency increases from 1.15 W/A to 1.49 W/A. The electroluminescence characteristics below the threshold current demonstrate that asymmetric MQW is more homogeneous due to the suppressed strain and piezoelectric field. Furthermore, theoretical calculation demonstrates that the enhancement of electron injection ratio and reduction in optical loss are another reason for the improvement of device performance, which is attributed to a smaller electron potential barrier and a more concentrated optical field distribution in the asymmetric structure, respectively. The new structure design with asymmetric MQW is concise for epitaxial growth, and it would also be a good possible choice for GaN-based LDs with other lasing wavelengths.

Design of diode laser using slab coupled optical waveguide with graded refractive index structure

Thickness and refractive index of slab layer have major effects in the electro-optical characteristics of slab coupled optical waveguide diode lasers. In this research, ordinary step refractive index profile was replaced by a broadened waveguide with linear graded refractive index profile and optimized doping. The results show such design can significantly improve the power conversion efficiency (PCE). According to these results, by using the asymmetric barrier in the sides of active region, the threshold current decreases and slop efficiency (SE) increases. The PCE and SE in the proposed structure are respectively 12% and 0.21 W/A higher than those in conventional step profile.

Structural transformation and phase change properties of Se substituted GeTe

GeTe1−xSex (0 ≤ x ≤ 1.0) alloys have been prepared both in bulk and thin film forms to study the effect of selenium (Se) substitution for tellurium (Te) on the phase change properties. It is observed that with increasing Se substitution in GeTe, the structure transforms from rhombohdral structure to orthorhombic structure. Rietveld Refinement analysis support the phase transformation and show that the short and long bond lengths in crystalline GeTe decrease with increasing Se substitution but the rate of reduction of shorter bond length is more than the longer bond length. The GeTe1−xSex thin films undergo amorphous to crystalline phase change when annealed at high temperatures. The transition temperature shows an increasing trend with the Se substitution. The contrast in electrical resistivity between the amorphous and crystalline states is 104 for GeTe, and with the Se substitution, the contrast increases considerably to 106 for GeTe0.5Se0.5. Devices fabricated with thin films show that the threshold current decreases with the Se substitution indicating a reduction in the power required for WRITE operation. The present study shows that the crystalline structure, resistance, bandgap, transition temperature and threshold voltage of GeTe can be effectively controlled and tuned by the substitution of Te by Se, which is conducive for phase change memory applications.

Experimental Investigation on the Current Carried by a Cathode Spot of Vacuum Arc in Axial Magnetic Fields

The splitting and threshold currents of vacuum arc cathode spots in axial magnetic fields (AMFs) up to 220 mT are investigated experimentally. Measurements are made on electrodes of Cu and CuCr25 with electrode separations of 2 and 6 mm. The electrode diameter is 30 mm. All the experiments are conducted in a demountable vacuum chamber evacuated continuously by a turbo-molecular pump to a pressure of the order of 10−3 Pa. The spots are photographed using a high-speed digital camera with an exposure time of $2~\mu \text{s}$ . The AMF is applied by means of two permanent magnets placed symmetrically inside the chamber. The experimental results indicate that the splitting and threshold current decrease in the presence of AMF and tend to certain constant values when the field gets stronger. The splitting and threshold currents are considerably larger than Cu electrodes than those with CuCr25 electrodes. With increasing electrode separation, the splitting current decreases while the threshold current increases. Possible explanations are proposed to account for the experimental observations.

Suppression the formation of V-pits in InGaN/ GaN multi-quantum well growth and its effect on the performance of GaN based laser diodes

The mechanism for the formation of V-pits in InGaN/GaN multi-quantum well (MQW) growth and its effect on the performance of GaN based laser diodes (LDs) are investigated in detail. It is observed that the V-pits in InGaN/GaN MQWs begin at the GaN barrier layer rather than InGaN well layer due to the low atomic migration ability of Ga atoms at low growth temperature. In addition, it is found that the formation of V-pits can be suppressed remarkably when a small amount of In atoms is introduced during GaN barrier layer growth. This may be attributed to the decrease of the potential barrier for atom migration around the dislocation region. In this way, the thermal stability of InGaN/(In)GaN MQWs is enhanced. Therefore, the threshold current decreases abruptly when using InGaN/InGaN MQWs instead of InGaN/GaN MQWs.

Effect of Mg doping concentration of electron blocking layer on the performance of GaN-based laser diodes

Performance of InGaN-based laser diodes (LDs) with different Mg concentrations of electron blocking layer (EBL) is investigated by simulation and experimental methods. It is found from the simulation results that the threshold current decreases and slope efficiency increases, when the Mg concentration of EBL increases from 2 × 1018 to 6 × 1019 cm−3; it is attributed to the suppression of the leakage of electrons and the enhancement of the injection of holes due to the variation of potential barrier for them as the increase of Mg concentration of EBL. These simulation results agree well with the experimental ones, when the Mg concentration of EBL is lower than 7.5 × 1018 cm−3. However, it deteriorates when the Mg concentration increases to 1.2 × 1019 cm−3. It may be due to the increase of the absorption loss of LDs.

Numerical study of metallic semiconductor nanolasers with double-concave cavity structures

<sec>Metallic semiconductor nanolaser, as an ultra-small light source, has been increasingly attractive to researchers in last decade. It can have wide potential applications such as in photonic integrated circuits, on-chip interconnect, optical communications,etc. One obstacle to miniaturization of the laser size is that the loss increases rapidly with the cavity volume decreasing. In previous studies, a type of Fabry-Perot cavity with capsule-shaped structure was investigated and demonstrated both numerically and experimentally, showing that its cavity loss is reduced dramatically in contrast to the scenario of conventional rectangular cavities. However, when the cavity size is reduced down to nanoscale, capsule-shaped structure surfers high loss. To overcome this difficulty, in this paper, a novel type of double-concave cavity structure for metallic semiconductor nanolaser in a 1.55 μm wavelength range is proposed and numerically studied. The proposed structure consists of InGaAs/InP waveguide structure encapsulated by metallic clad, and has a cylindrical reflection end face and concave curved sidewalls. </sec><sec>The cylindrical reflection end face can push the resonant mode into the cavity center and reduce the optical field overlap with metallic sidewalls, which can reduce the metallic loss. The curved-sidewalls topologically reduce the electric field component perpendicular to the sidewalls, and thus reducing the plasmonic loss. By optimizing the waist width of the double-concave cavity structure, the radiation loss can be effectively reduced, resulting in the improvement of cavity quality factor and the decrease of threshold current. Finite-difference time-domain simulations are conducted to investigate the properties of the proposed cavity structures such as resonant mode distribution, cavity quality factor, confinement factor, threshold gain and threshold current in this paper. The numerical results show that the double-concave cavity laser with cavity volume as small as 0.258 <i>λ</i><sup>3</sup> increases 24.8% of cavity quality factor and reduces 67.5% of threshold current, compared with the conventional capsule-shaped one, demonstrating an effective improvement of metallic nanolaser. With those advantages, the proposed structure can be used for realizing the ultra-small metallic semiconductor nanolasers and relevant applications.</sec>

Enhancing the performance of GaN based LDs by using low In content InGaN instead of GaN as lower waveguide layer

Performance of InGaN based blue laser diodes (LDs) with different In content of (In)GaN lower waveguide (LWG) layers are investigated by simulation and experimental methods. It is found from the simulation results that the threshold current decreases and slope efficiency increases when the In content of InGaN LWG layer increases from 0% to 6%. However, when the In content is too high, the optical confinement factor of MQWs decreases, recombination rate in LWG layer increases and the surface morphology becomes roughness, all of these result in the deterioration of the performance of LDs with high In content InGaN LWG layer, the output power decreases from 97 to 83 mW at the injection current of 100 mA. It agrees well with the experiment results that the performance of LDs can be improved by using a low In content InGaN instead of GaN as lower waveguide layer.

Study of the characteristics of 976 nm Bragg grating external cavity semiconductor lasers

976 nm fiber Bragg grating (FBG) external cavity laser is the core component in erbium-doped fiber amplifiers (EDFA). In this paper, the theory model of FBG is modeled by transfer matrix method (TMM). The effects of Bragg grating length and modulation depth on the reflectivity, grating spectrum width and wavelength have been investigated. Then 976 nm laser modules with different grating reflectivity, i.e. 4%, 6%, 8% and 10% have been fabricated. By improving the reflectivity of FBG, the side-mode suppression ratio (SMSR) has been optimized to above 40 dB, and the threshold current decreases. The temperature cycling test indicates that the peak wavelength has a change rate [Formula: see text]0.0095 nm/[Formula: see text]C.

Enhancing the performance of InGaN/GaN multiple quantum well blue laser diodes by suppressing the overflow of holes

Transportation of carriers in c-plane InGaN-based blue laser diodes (LDs) has been investigated by both experimental and simulation methods. It is found that excepting the leakage of electrons, holes may overflow from InGaN/GaN multiple quantum wells (MQWs) and enter into InGaN lower waveguide (LWG) layer. It leads to the increase of recombination rate in LWG layer and results in the waste of carriers. Ultimately degrades the performance of LDs. In addition, we also found that reducing the thickness of GaN first barrier (FB) layer can suppress the leakage of holes due to the decrease of the effective depth of potential well in InGaN LWG layer, a lower effective depth of potential well in InGaN LWG layer is helpful the holes coming back to well layer and therefore the threshold current decreases to 80% when the FB layer thickness decreases from 20 nm to 1 nm.

Electrical switching and aluminium speciation in Al-As-Te glasses

Bulk AlxAs30Te70-x glasses in the range 5≤x≤20 prepared by melt quenching method has been found to exhibit memory for x<10 and threshold switching for x≥10. Upon heating in a differential scanning calorimeter (DSC), glasses with x≤10 show crystallization reaction whereas glasses with x>10 do not show crystallization reaction. Glasses with x≤10 crystallized into Te and As2Te3 crystal structures when annealed at their crystallization temperature. Glasses with x≥15 crystallized to As2Te3 structure only. The glasses cooled from its melting temperature (Tm) in water at room temperature (28°C) solidified into crystalline structure for x=5, whereas x≥10 solidified into amorphous structure. Magic angle spinning nuclear magnetic resonance (MAS NMR) results show 27Al resides in [4]Al and [6]Al coordination. Raman spectra of the as-quenched, SET and RESET samples of the memory glass reveal that the SET state is crystalline and RESET state is amorphous. Raman spectra of threshold switching glass reveal no structural change even after 50 switching cycles. The progressive addition of Al promotes the transfer of lone-pair electrons of Te to Al atoms. We propose this lone-pair tailoring by Al may form a band above the valence band that decreases the density of charge carriers. As a result, a decrease in threshold current (Ith) and an increase in threshold voltage (Vth) is observed. The higher coordinated Al cross links the structural network heavily. This makes structural reorganization difficult resulting in the observed threshold switching in AlxAs30Te70-x for glasses with x>5.

Influences of material defects and temperature on current-driven domain wall mobility

Current-induced domain wall motion, which has potential application in the next-generation data storage and logic device, has attracted much interest in recent years. However, how the material defect and its joule heat influence current-driven domain wall motion in magnetic nanostripe is still unclear. This paper is to deal with these issues by using the Landau-Lifshitz-Gilbert spin dynamics. The results show that the material defect can pin domain wall motion and this pinning effect strongly depends on the defect concentration, location and shape. The pinning effect induced by the defect on domain wall motion results in the increase of threshold current, and the domain wall moves steadily and continuously. Specifically, the probability for domain wall motion induced by pinning effect is nonlinearly increasing with the increase of defect concentration. Namely, the increasing of the pinning ability with the increase of the defect concentration becomes fades away. Initially, when the defect is near to domain wall, the pinning ability is obvious. However, the pinning ability is not linearly increasing with the decrease of the initial distance between the defect and the domain wall. The results also show that the single defect is larger, the probability for domain wall motion induced by defect pining is bigger. Moreover, the material defect can suppress the domain wall trending toward breakdown and make domain wall move faster, but the suppressing ability is not obviously increasing with the increase of the defect concentration. On the other hand, the temperature field can remove the pinning phenomenon, which will result in the threshold current decrease. The decrease of the threshold current is of benefit to the working of the data storage and logic device. Also the temperature field can suppress the domain wall trending toward breakdown, but the suppressing ability is less than that of the defect. In addition, the Joule heat around defects can obviously eliminate the pinning effect of the defects, so the pinning effect for a few defects on current-induced domain wall motion can be ignored. Further analysis indicates that these effects are due to the change of the out-of-plane magnetization of the domain wall induced by the material defects and the temperature field, because the velocity of the domain wall motion induced by the applied current greatly depends on the out-of-plane magnetization of the domain wall.

Study on Two-Section DFB Lasers and Laser Arrays Based on the Reconstruction Equivalent Chirp Technique and Their Application in Radio-Over-Fiber Systems

Two-section distributed feedback (TS-DFB) semiconductor lasers based on the reconstruction equivalent chirp technique is proposed. A four-channel TS-DFB laser array (TS-DFB-LA) is theoretically studied and experimentally demonstrated. Compared with the conventional one-section DFB lasers, the proposed TS-DFB lasers have much higher output efficiency and higher side-mode suppression ratio (SMSR) when both the facets of the laser cavity are coated with antireflection coatings. About 39% increase in output efficiency, more than 10 mA decrease in threshold current, and about 5 dB increase in SMSR have been achieved experimentally. Furthermore, experimental results show that the wavelength spacing accuracy of the four-channel TS-DFB-LA can be improved by tuning the injection current of one of the two sections. Using the two-section structure, the dynamic characteristics including small-signal frequency response, relative intensity noise, and spurious-free dynamic range are also improved significantly. A radio-over-fiber link utilizing the TS-DFB laser as directly modulated light source was experimentally achieved. About 50 MSymbol/s 64-QAM signal with 10 GHz carrier was transmitted from the TS-DFB laser. After 40 km transmission in single-mode fiber, the average error vector magnitude of the whole link is 2.97%.