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This letter reports a successful fabrication of room-temperature electrically injected AlGaN-based near-ultraviolet laser diode grown on Si. An Al-composition step down-graded AlN/AlGaN multilayer buffer was carefully engineered to not only tackle the huge difference in the coefficient of thermal expansion between AlGaN template and Si substrate, but also reduce the threading dislocation density caused by the large lattice mismatch. On top of the crack-free n-AlGaN template, high quality InGaN/AlGaN quantum wells were grown, sandwiched by waveguide and optical cladding layers, for the fabrication of edge-emitting laser diode. A dramatic narrowing of the electroluminescence spectral line-width, an elongated far-field pattern, and a clear discontinuity in the slope of light output power plotted as a function of the injection current provide an unambiguous evidence of lasing.

It is becoming a challenging issue for users to choose a satisfied service to fit their need due to\ud the rapid growing number of cloud services and the vast amount of service type varieties. This paper proposes\ud an effective cloud service recommendation approach, named personalized cloud service recommendation\ud (P-CSREC), based on the characterization of heterogeneous information network, the use of association rule\ud mining, and the modeling and clustering of user interests. First, a similarity measure is defined to improve the\ud average similarity (AvgSim) measure by the inclusion of the subjective evaluation of users’ interests. Based\ud on the improved AvgSim, a new model for measuring the user interest is established. Second, the traditional\ud K-Harmonic Means (KHM) clustering algorithm is improved by means of involving multi meta-paths to\ud avoid the convergence of local optimum. Then, a frequent pattern growth (FP-Growth) association rules\ud algorithm is proposed to address the issue and the limitation of traditional association rule algorithms to offer\ud personalization in recommendation. A new method to define a support value of nodes is developed using\ud the weight of user’s score. In addition, a multi-level FP-Tree is defined based on the multi-level association\ud rules theory to extract the relationship in higher level. Finally, a combined user interest with the improved\ud KHM clustering algorithm and the improved FP-Growth algorithm is provided to improve accuracy of cloud\ud services recommendation to target users. The experimental results demonstrated the effectiveness of the\ud proposed approach in improving the computational efficiency and recommendation accuracy

The fabrication of a blue–violet semiconductor laser on a silicon substrate enhances the integration of optoelectronics with electronics.

Current laser-based display and lighting applications are invariably using blue laser diodes (LDs) grown on free-standing GaN substrates, which are costly and smaller in size compared with other substrate materials.1–3 Utilizing less expensive and large-diameter Si substrates for hetero-epitaxial growth of indium gallium nitride/gallium nitride (InGaN/GaN) multiple quantum well (MQW) structure can substantially reduce the cost of blue LDs and boost their applications. To obtain a high crystalline quality crack-free GaN thin film on Si for the subsequent growth of a blue laser structure, a hand-shaking structure was formed by inserting Al-composition step down-graded AlN/AlxGa1−xN buffer layers between GaN and Si substrate. Thermal degradation in InGaN/GaN blue MQWs was successfully suppressed with indium-rich clusters eliminated by introducing hydrogen during the growth of GaN quantum barriers (QBs) and lowering the growth temperature for the p-type AlGaN/GaN superlattice optical cladding layer. A continuous-wave (CW) electrically pumped InGaN/GaN quantum well (QW) blue (450 nm) LD grown on Si was successfully demonstrated at room temperature (RT) with a threshold current density of 7.8 kA/cm2.

A thermal resistance measurement method of high accuracy for GaN/InGaN laser diodes (LDs) is presented based on the forward-voltage method. Three items are optimized in order to improve the accuracy of the measurement. (i) Measurement time delay is shortened to 300 ns by using the single trigger function of a MDO4104-3 Mixed Domain Oscilloscope. (ii) The measured results are revised based on the simulation result. (iii) The accuracy of the measurement is further improved by the real-time monitoring of the sensor current. Thermal resistance of the LD operating under different injection current is measured by using this method. The thermal resistance of TO56 packaged GaN/InGaN LD is 30.2 ± 0.3 K/W.

InGaN strained bulk layers were grown by low-pressure metalorganic chemical vapor deposition on c-plane GaN/sapphire templates. Two growth regimes, mass transport limited regime and indium desorption regime, were examined for InGaN growth. In the indium desorption regime, more indium source must be fed to keep a constant indium content. In the indium desorption regime, we found an abnormally enhanced GaN growth rate, which was proved to be related to the indium desorption and dependent on the growth temperature and the indium source flow. Due to the enhanced growth rate, the optical quality of In0.16Ga0.84N layers degraded significantly.

Carrier recombination behavior in c-plane GaN-based laser diodes (LDs) is numerically investigated by using the commercial software LASTIP. It is found that efficiency droop phenomenon does exist in GaN-based LDs before lasing, which is confirmed by experimental results. However, the current density corresponding to the peak efficiency of GaN-based LDs before lasing, Jmax, is nearly 40 A/cm2, which is much lower than that reported by other studies. The reported Jmax, measured from the cavity facet side is modulated by the absorption of quantum wells, which shifts the Jmax to a higher value. In addition, the currents due to various recombinations are calculated. It is found that Auger recombination affects the threshold current greatly, but it only plays a small role at high current injection levels.

True blue nitride laser diodes (LDs) are one of the key challenges for epitaxy of nitrides due to the variety of its potential applications. The growth of high temperature p-type layers may cause thermal degradation of the InGaN-based multiple quantum wells (MQWs) active region because of the annealing effect, since thick p-AlGaN layers were introduced as upper optical cladding layer in the LDs. The degradation was found in blue LDs grown on both Si and sapphire substrate. In the degraded LD wafer samples, “Dark” non-radiative MQWs regions were observed by microscopic photoluminescence. Formation of metallic indium precipitates and voids in these regions were confirmed by transmission electron microscope. The thermal degradation is attributed to the decomposition of indium-rich InGaN materials in the MQWs. The indium-rich InGaN materials were supposed to be accumulated at dislocation related V-shaped pits according to the surface morphology by atomic force microscope. To obtain high quality InGaN-based MQWs, one of the four methods can be introduced to eliminate the degradation. A lower thermal budget can suppress the decomposition of indium-rich InGaN clusters by a lower p-cladding layer growth temperature. The use of low threading dislocation density substrates results in low density indium-rich InGaN clusters. The introducing of H 2 carrier gas during the quantum barriers growth or a 2-step growth scheme with a higher quantum barrier growth temperature etches off the indium-rich InGaN clusters. The suppression of the thermal degradation in the MQWs makes it possible for lasing of blue laser diode directly grown on Si.

We have fabricated InGaN-based superluminescent diodes (SLDs) with one-sided oblique facet. The characteristics of the SLDs and laser diodes with the same cavity length (800 μm) were compared. The typical peak wavelength and the full width at half maximum of the spectrum in superluminescence regime are 445.3 and 7.7 nm for the SLDs with 800 μm cavity length. The characteristics of the SLDs with different cavity length were also demonstrated in a comparative way. It is found that the gain of the InGaN multi-quantum wells in blue spectral range is a linear function of the current density below gain saturation region. The lasing threshold current turns out to be higher for the shorter SLD (S-SLD) (400 μm), but the output light intensity of the longer SLD (800 μm) is higher than that of the S-SLD under the same current density. The gain saturation phenomenon was observed in S-SLD when it was biased at a current density larger than 27.5 kA/cm2. The increase of junction temperature was identified as the main reason for gain saturation through spectra analysis.

Effects of the prestrained growth of a low-In InGaN/GaN quantum well (QW) before the designated light-emitting high-In InGaN/GaN multi quantum wells (MQWs) in GaN-based laser diodes (LDs) are numerically investigated. Simulation results show that the inserted low-In InGaN/GaN QW does not remarkably change the absorption loss, and the ratio of recombination current of low-In InGaN/GaN QW over injection current is very small, especially after LD is lasing. However, it weakens the polarization fields, resulting in less tilted energy bands, and the overlap of electron-hole pairs is greatly increased, which reduces threshold carrier concentrations of MQWs, leakage current, and nonradiative recombination. The performance of LD is enhanced. Threshold current is reduced by 19%, and output power is increased by 59% at the injection current of 120 mA.

Hillocks on c-plane homoepitaxial GaN epilayers were investigated. They were observed on epilayers grown on [1 (1) over bar 00] direction miscut free-standing GaN substrates with miscut angle not larger than 0.2 degrees and were absent when substrate miscut angle increased to 0.4 degrees. Atomic force microscopy (AFM) and cathodoluminescence measurements reveal a close correlation between hillocks and dislocation clusters, while hillocks are absent on layers grown on GaN substrate free of dislocation clusters. We believe that the hillocks originate from spiral growth around dislocation clusters. Larger strain induced by dislocation accumulation may be responsible for the hillock formation around dislocation clusters. (C) 2013 Elsevier B.V. All rights reserved.

A method to calculate the reflectivity of the coated cavity facet was proposed, and the distribution of the optical power near the two coated cavity facets was calculated for GaN-based laser diodes. A new design method for reducing the optical power at the two cavity facets without changing the output power of laser diodes was discussed, which is helpful to optimize the cavity facet coating and raise the threshold current at which catastrophic optical damage occurs.

c -plane GaN-based blue laser diodes (LDs) were fabricated with Al-free cladding layers (CLs) and deepened etching depth of mesa structure, so the aspect ratio of the far-field pattern (FFP) of the laser beam can be reduced to as low as 1.7, which is nearly the same as conventional AlGaInP-based red LDs. By using GaN CLs, the radiation angle of the laser beam θ ⊥ is only 10.1° in the direction perpendicular to the junction plane. After forming a deeply etched mesa, the beam divergence angle parallel to the junction plane of FFP, θ // , increases from 4.9° to 5.8°. After using the modified structure, the operation voltage of LD is effectively reduced by 2 V at an injection current of 50 mA, but the threshold current value increases. The etching damage may be one of the main reasons responsible for the increase of the threshold current.

AlGaN-based near-ultraviolet light-emitting diodes (NUV-LEDs) emitting at 370 nm were grown on Si(111) substrates by metal-organic chemical vapor deposition. The effect of growth parameters of Si-doped n-type AlGaN thick layer on the material quality and optical performance was studied. Photoluminescence measurements showed that the near-band-edge emission of n-AlGaN was greatly increased and the yellow luminescence (YL) was substantially reduced, when the n-AlGaN layer was grown at a high temperature, a high chamber pressure, and a low growth rate. It was found that the reduced unintentional carbon incorporation in the n-AlGaN layer under those growth conditions was responsible for the improved optical property. The NUV-LED employing the optimized growth parameters of n-AlGaN showed an enhanced light output power and a suppressed YL emission, as well as a better color purity, as compared with the reference one. The results indicate that performance of NUV-LED can be significantly improved by suppressing unintentional carbon incorporation and the defects-related absorption/re-emission in the n-AlGaN.

This work reports the fabrication of via-thin-film light-emitting diode (via-TF-LED) to improve the light output power (LOP) of blue/white GaN-based LEDs grown on Si (111) substrates. The as-fabricated via-TF-LEDs were featured with a roughened n-GaN surface and the p-GaN surface bonded to a wafer carrier with a silver-based reflective electrode, together with an array of embedded n-type via pillar metal contact from the p-GaN surface etched through the multiple-quantum-wells (MQWs) into the n-GaN layer. When operated at 350 mA, the via-TF-LED gave an enhanced blue LOP by 7.8% and over 3.5 times as compared to the vertical thin-film LED (TF-LED) and the conventional lateral structure LED (LS-LED). After covering with yellow phosphor that converts some blue photons into yellow light, the via-TF-LED emitted an enhanced white luminous flux by 13.5% and over 5 times, as compared with the white TF-LED and the white LS-LED, respectively. The significant LOP improvement of the via-TF-LED was attributed to the elimination of light absorption by the Si (111) epitaxial substrate and the finger-like n-electrodes on the roughened emitting surface.

Silicon photonics has been longing for an efficient on-chip light source that is electrically driven at room temperature. Microdisk laser featured with low-loss whispering gallery modes can emit directional lasing beam through a closely coupled on-chip waveguide efficiently, and hence is particularly suitable for photonics integration. The realization of electrically pumped III-nitride microdisk laser grown on Si has been impeded by the conventional undercut structure, poor material quality, and a limited quality of GaN microdisk formed by dry etching. Here we report a successful fabrication of room-temperature electrically pumped InGaN-based microdisk lasers grown on Si. A dramatic narrowing of the electroluminescence spectral line-width and a clear discontinuity in the slope of light output power plotted as a function of the injection current provide an unambiguous evidence of lasing. This is the first observation of electrically pumped lasing in InGaN-based microdisk lasers grown on Si at room temperature.

High-power AlGaN-based 385 nm near-ultraviolet light-emitting diodes (UVA-LEDs) grown on Si(111) substrates are reported. The threading dislocation (TD) density of AlGaN was reduced by employing an Al-composition step-graded AlN/AlGaN multilayer buffer. V-shaped pits were intentionally incorporated into the active region to screen the carriers from the nonradiative recombination centers (NRCs) around the TDs and to facilitate hole injection. The light extraction efficiency was enhanced by the surface roughening of a thin-film (TF) vertical chip structure. The as-fabricated TF-UVA-LED exhibited a light output power of 960 mW at 500 mA, corresponding to an external quantum efficiency of 59.7%.

By observing the morphology evolution of green InGaN/GaN quantum well (QW) and studying the catholuminescence (CL) property, we investigate indium-segregation-related defects that are formed at green InGaN/GaN QW interfaces. Meanwhile, we also propose the approach and suggest the mechanism to remove them for green InGaN/GaN QW grown on both GaN templates and free-standing GaN substrates. By engineering the interface of green InGaN/GaN QWs, we have achieved green laser diode (LD) structure with low threshold current density of 1.85 kA cmsup-2/sup. The output power of the green LD is 58 mW at a current density of 6 kA cmsup-2/supunder continuous-wave operation at room temperature.

Recently, many groups have focused on the development of GaN-based green LDs to meet the demand for laser display. Great progresses have been achieved in the past few years even that many challenges exist. In this article, we analysis the challenges to develop GaN-based green LDs, and then the approaches to improve the green LD structure in the aspect of crystalline quality, electrical properties, and epitaxial layer structure are reviewed, especially the work we have done.

Semiconductor vertical-cavity surface-emitting lasers (VCSELs) with wavelengths from 491.8 to 565.7 nm, covering most of the ‘green gap’, are demonstrated. For these lasers, the same quantum dot (QD) active region was used, whereas the wavelength was controlled by adjusting the cavity length, which is difficult for edge-emitting lasers. Compared with reports in the literature for green VCSELs, our lasers have set a few world records for the lowest threshold, longest wavelength and continuous-wave (CW) lasing at room temperature. The nanoscale QDs contribute dominantly to the low threshold. The emitting wavelength depends on the electron–photon interaction or the coupling between the active layer and the optical field, which is modulated by the cavity length. The green VCSELs exhibit a low-thermal resistance of 915 kW−1, which benefits the CW lasing. Such VCSELs are important for small-size, low power consumption full-color displays and projectors. Researchers have used vertically oriented devices containing tiny quantum dots to improve the production of green laser light. While powerful red and blue lasers can be easily manufactured using multilayered semiconductors, finding material combinations that generate green wavelengths is trickier. Baoping Zhang from Xiamen University in China and co-workers cracked this problem by turning to vertical-cavity surface-emitting lasers (VCSELs) whose emission wavelength can be controlled by adjusting their cavity length. Fabricating a VCSEL with an active layer composed of indium–gallium–nitrogen quantum dots reduced the threshold currents needed to emit green light, thanks to the strong quantum confinement effects. The team demonstrated VCSELs with different wavelengths that covered the ‘green gap’ and used heat-dissipating materials to produce a continuous-wave device that operates at room temperature.

Low threshold continuous-wave (CW) lasing of current injected InGaN quantum dot (QD) vertical-cavity surface-emitting lasers (VCSELs) was achieved at room temperature. The VCSEL was fabricated by metal bonding technique on a copper substrate to improve the heat dissipation ability of the device. For the first time, lasing was obtained at yellow-green wavelength of 560.4 nm with a low threshold of 0.61 mA, corresponding to a current density of 0.78 kA/cm2. A high degree of polarization of 94% were measured. Despite the operation in the range of "green gap" of GaN-based devices, single longitudinal mode laser emission was clearly achieved due to the high quality of active region based on InGaN QDs and the excellent thermal design of the VCSELs.

The dream of epitaxially integrating III-nitride semiconductors on large diameter silicon is being fulfilled through the joint R&D efforts of academia and industry, which is driven by the great potential of GaN-on-silicon technology in improving the efficiency yet at a much reduced manufacturing cost for solid state lighting and power electronics. It is very challenging to grow high quality GaN on Si substrates because of the huge mismatch in the coefficient of thermal expansion (CTE) and the large mismatch in lattice constant between GaN and silicon, often causing a micro-crack network and a high density of threading dislocations (TDs) in the GaN film. Al-composition graded AlGaN/AlN buffer layers have been utilized to not only build up a compressive strain during the high temperature growth for compensating the tensile stress generated during the cool down, but also filter out the TDs to achieve crack-free high-quality n-GaN film on Si substrates, with an X-ray rocking curve linewidth below 300 arcsec for both (0002) and (102) diffractions. Upon the GaN-on-Si templates, prior to the deposition of p-AlGaN and p-GaN layers, high quality InGaN/GaN multiple quantum wells (MQWs) are overgrown with well-engineered V-defects intentionally incorporated to shield the TDs as non-radiative recombination centers and to enhance the hole injection into the MQWs through the via-like structures. The as-grown GaN-on-Si LED wafers are processed into vertical structure thin film LED chips with a reflective p-electrode and the N-face surface roughened after the removal of the epitaxial Si(111) substrates, to enhance the light extraction efficiency. We have commercialized GaN-on-Si LEDs with an average efficacy of 150–160 lm/W for 1mm2 LED chips at an injection current of 350 mA, which have passed the 10000-h LM80 reliability test. The as-produced GaN-on-Si LEDs featured with a single-side uniform emission and a nearly Lambertian distribution can adopt the wafer-level phosphor coating procedure, and are suitable for directional lighting, camera flash, streetlighting, automotive headlamps, and otherlighting applications.

Growth conditions were explored to suppress the carbon impurity incorporation in AlGaN:Mg grown at low temperatures. Electrical properties of Al0.07Ga0.93N:Mg samples with various carbon concentrations were investigated by Hall measurements. A clear correlation between carbon concentration and electrical conductivity has been found. By reducing the carbon concentration from 2 x 10(18) to 5 x 10(16) cm(-3), the resistivity of p-Al0.07Ga0.93N decreases from 7.4 to 2.2 Omega.cm. From the results of the analysis of the charge neutrality equation, we found that the carbon concentration is close to the compensating donor concentration in the AlGaN:Mg samples, which suggests that carbon acts as the main compensating donor in AlGaN:Mg. (C) 2015 The Japan Society of Applied Physics

Emission properties of self-assembled green-emitting InGaN quantum dots (QDs) grown on sapphire substrates by using metal organic chemical vapor deposition are studied by temperature-dependent photoluminescence (PL) measurements. As temperature increases (15–300 K), the PL peak energy shows an anomalous V-shaped (redshift—blueshift) variation instead of an S-shaped (redshift—blueshift—redshift) variation, as observed typically in green-emitting InGaN/GaN multi-quantum wells (MQWs). The PL full width at half maximum (FWHM) also shows a V-shaped (decrease—increase) variation. The temperature dependence of the PL peak energy and FWHM of QDs are well explained by a model similar to MQWs, in which carriers transferring in localized states play an important role, while the confinement energy of localized states in the QDs is significantly larger than that in MQWs. By analyzing the integrated PL intensity, the larger confinement energy of localized states in the QDs is estimated to be 105.9 meV, which is well explained by taking into account the band-gap shrinkage and carrier thermalization with temperature. It is also found that the nonradiative combination centers in QD samples are much less than those in QW samples with the same In content.

Two-step growth was employed to grow GaN quantum barriers (QBs) in InGaN green LD structures. A cap layer was grown at the same temperature as an InGaN quantum well (QW), and the temperature was then raised by around 130 °C to grow GaN QBs. The effects of low-temperature-grown cap (LT-cap) layers on the optical properties and microstructures of green LD structures were investigated. It was found that the LT-cap layer with an optimal thickness can improve the luminescence homogeneity and suppress the thermal decomposition of InGaN QWs. C-plane ridge waveguide laser diodes lasing above 500 nm were realized.

Local InGaN quantum well (QW) decomposition and resultant inhomogeneous luminescence in green laser diode (LD) epitaxial structures are investigated using micro-photoluminescence, Z-contrast scanning transmission electron microscopy, and high-resolution transmission electron microscopy. The local InGaN QW decomposition is found to happen during p-type layer growth due to too high thermal budget and may initiate at the InGaN/GaN QW upper interface probably due to the formation of In-rich InGaN clusters there. Reducing thermal budget and optimizing InGaN/GaN QW growth suppress the local InGaN QW decomposition, and green LD structures with homogeneous luminescence and bright electroluminescence (EL) intensity are obtained.

Effects of an inserted InGaN interlayer between active region and p-AlGaN electron blocking layer on electrical and optical characteristics of GaN-based blue laser diodes are numerically investigated. It is found that the inserted InGaN interlayer reduces the barrier height for hole injection into multiple quantum wells. Moreover, it is found that the background electron concentration of the undoped InGaN plays a critical role in the LD performance. A background electron concentration higher than 1 × 1017 cm-3 may induce undesired electron-hole recombination in this layer. In addition, we have calculated the dependences of optical confinement factor and internal absorption loss (IAL) on location, In composition, and thickness of the InGaN layer. A significant increase in OCF and a decrease in IAL are obtained by inserting the InGaN layer.

Using the finite-element method, the thermal resistances of GaN laser diode devices in a TO 56 package for both epi-up configuration and epi-down configuration are calculated. The effects of various parameters on the thermal characteristics are analysed, and the thicknesses of the AlN submount for both epi-up configuration and epi-down configuration are optimized. The obtained result provides a reference for the parameter selection of the package materials.

We report on the formation of Ohmic contacts with low resistance and high thermal stability to N-face n-GaN for vertical structure light emitters using a Ti(50nm)/Pt(50nm)/Au(50nm) metal scheme, where the Pt layer is introduced as a blocking layer to suppress the diffusion of Au onto the N-face n-GaN surface. It is shown that unlike the conventional Ti/Al/Ti/Au contacts, the Ti/Pt/Au contacts exhibit an Ohmic behavior with a relatively low specific contact resistivity of 1.1×10−4 Ω·cm2 even after annealing at 350°C. X-ray diffraction (XRD) measurements by synchrotron radiation and Auger electron spectroscopy (AES) examination are performed to understand the effects of heat treatment.