480†nm InGaN-based cyan laser diode grown on Si by interface engineering of active region.

InGaN-based long wavelength laser diodes (LDs) grown on Si are highly desirable for expanding the applications in laser display and lighting. Proper interface engineering of high In-content InGaN multi-quantum wells (MQWs) is urgently required for the epitaxial growth of InGaN-based long wavelength LD on Si, because the deteriorated interfaces and crystalline quality of InGaN MQWs can severely increase the photon scattering and further exacerbate the internal absorption loss of LDs, which prevents the lasing wavelength of InGaN-based LDs from extending. In this work, a significantly improved morphology and sharp interface of the InGaN active region are obtained by using a graded-compositional InGaN lower waveguide (LWG) capped with a 10-nm-thick Al0.1Ga0.9N layer. The V-pits density of the InGaN LWG was one order of magnitude reduction from 4.8 × 108 to 3.6 × 107 cm-2 along with the root-mean-square surface roughness decreasing from 0.3 to 0.1 nm. Therefore, a room-temperature electrically injected 480 nm InGaN-based cyan LD grown on Si under pulsed current operation was successfully achieved with a threshold current density of 18.3 kA/cm2.

Enhanced carrier confinement and radiative recombination in GaN-based lasers by tailoring first-barrier doping.

Very limited 1-3 pairs of quantum-wells (QWs) are preferred for GaN-based laser diodes (LDs), which require more careful engineering of the carrier transport than LEDs. In this work, the first-barrier doping level of QWs is found to significantly affect the carrier confinement and distribution for GaN-based LDs. The first-barrier doping exceeding 2×1018 cm-3 will make the bottom QW return to the parasitic state, yielding unexpected photons absorption and even Auger recombination. The underlying physical mechanism is discussed in terms of the calculated energy-band diagram, carrier confinement, and distribution. And all the experimental findings are consistent with the physical model.

Suppression of substrate mode in GaN-based green laser diodes.

Parasitic substrate mode readily appears in GaN-based laser diodes (LDs) because of insufficient optical confinement, especially for green LDs. Substrate modes affect the behavior of a LD severely, including the laser beam quality, the optical output power, the longitudinal mode stability, and the maximum modulation speed. In this article, systematic studies on the n-cladding layer (CL) design to suppress the substrate mode of GaN-based green LDs were carried out. We established a contour map to describe the relationship between the optical confinement (determined by the thickness and the refractive index) of n-CL and the substrate mode intensity by simulating the near-field pattern and the far-field pattern. We found that it was difficult to obtain the Gaussian-shape far-field pattern using AlGaN as a cladding layer due to the appearance of cracks induced by tensile strain. However, this can be realized by introducing quaternary AlInGaN as a cladding layer since refractive index and strain can be tuned separately for quaternary alloy.

Continuous-wave electrically injected GaN-on-Si microdisk laser diodes.

Silicon photonics has been calling for an electrically pumped on-chip light source at room temperature for decades. A GaN-based microdisk laser diode with whispering gallery modes grown on Si is a promising candidate for compact on-chip light source. By suppressing the unintentional incorporation of carbon impurity in the p-type AlGaN cladding layer of the laser, we have significantly reduced the operation voltage and threshold current of the GaN-on-Si microdisk laser. Meanwhile the radius of the microdisk laser was shrunk to 8 µm to lower the thermal power. The overall junction temperature of the microdisk laser was effectively reduced. As a result, the first continuous-wave electrically pumped InGaN-based microdisk laser grown on Si was achieved at room temperature.

Performance improvement of InGaN-based laser grown on Si by suppressing point defects.

High performance InGaN-based laser diodes (LDs) monolithically grown on Si is fundamentally interesting and highly desirable for photonics integration on Si platform. Suppression of point defects is of crucial importance to improve the device performance of InGaN-based LDs grown on Si. This work presents a detailed study on the impact of point defects, such as carbon (C) impurities and gallium vacancies (VGa), on the device characteristics of InGaN-based LDs grown on Si. By suppressing the VGa-related defect within the waveguide layers, reducing the thermal degradation of InGaN-based quantum wells, and controlling the C impurity concentrations within the thick p-type cladding layers, the as-fabricated InGaN-based LDs grown on Si exhibited a significantly reduced threshold current density of 2.25 kA/cm2 and an operation voltage of 4.7 V.

Room-temperature electrically pumped InGaN-based microdisk laser grown on Si.

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.

Picosecond tunable gain-switched blue pulses from GaN laser diodes with nanosecond current injections.

We investigated the gain-switching properties of GaN-based ridge-waveguide lasers on free-standing GaN substrates with low-cost nanosecond current injection. It was observed that the output pulses with intense injection consisted of an isolated short pulse with a duration of around 50 ps at the high-energy side and a long steady-state component at the lower energy side independent of the electric pulse duration. The energy separation between the short pulse and steady-state component can be over 30 meV, favoring short-pulse generation with the spectral filtering technique. The duration of the steady-state component can be tuned freely by controlling the duration and voltage of the electric pulse, which is very useful for generating pulse-width-tunable optical pulses for various applications.

Green laser diodes with low threshold current density via interface engineering of InGaN/GaN quantum well active region.

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 cm-2. The output power of the green LD is 58 mW at a current density of 6 kA cm-2 under continuous-wave operation at room temperature.

Effects of Miscut on Step Instabilities in Homo-Epitaxially Grown GaN.

The rough morphology at the growth surface results in the non-uniform distribution of indium composition, intentionally or unintentionally doped impurity, and thus impacts the performance of GaN-based optoelectronic and vertical power electronic devices. We observed the morphologies of unintentionally doped GaN homo-epitaxially grown via MOCVD and identified the relations between rough surfaces and the miscut angle and direction of the substrate. The growth kinetics under the effect of the Ehrlich-Schwoebel barrier were studied, and it was found that asymmetric step motions in samples with a large miscut angle or those grown at high temperature were the causes of step-bunching. Meandering steps were believed to be caused by surface free energy minimization for steps with wide terraces or deviating from the [11¯00] m-direction.

Growth Behaviors of GaN on Stripes of Patterned c-Plane GaN Substrate.

Growth behaviors of GaN on patterned GaN substrate were studied herein. Spiral and nucleation growth were observed after miscut-induced atomic steps disappeared. The morphology of nucleation growth at different temperature is explained by a multi-nucleation regime introducing critical supersaturation. Simulated results based on a step motion model successfully explain the growth behaviors on stripes. These findings can be applied to control the surface kinetics of devices such as laser diodes grown on patterned substrate.

Effect of Graded-Indium-Content Superlattice on the Optical and Structural Properties of Yellow-Emitting InGaN/GaN Quantum Wells.

We have improved the material quality of the high indium composition InGaN/GaN multiple quantum wells (MQWs) grown on free-standing GaN substrates using the graded-indium-content superlattice. We found that by adopting a graded-indium-content superlattice structure, the spectral FWHM of the yellow emitting InGaN/GaN MQW was reduced from 181 meV to 160 meV, and the non-radiative recombination lifetime increased from 13 ns to 44 ns. Besides, the graded-indium-content superlattice can mitigate strain relaxation in high indium composition MQWs as shown by the TEM diffraction patterns.

Narrow-Linewidth GaN-on-Si Laser Diode with Slot Gratings.

This letter reports room-temperature electrically pumped narrow-linewidth GaN-on-Si laser diodes. Unlike conventional distributed Bragg feedback laser diodes with hundreds of gratings, we employed only a few precisely defined slot gratings to narrow the linewidth and mitigate the negative effects of grating fabrication on the device performance. The slot gratings were incorporated into the ridge of conventional Fabry-Pérot cavity laser diodes. A subsequent wet etching in a tetramethyl ammonium hydroxide solution not only effectively removed the damages induced by the dry etching, but also converted the rough and tilted slot sidewalls into smooth and vertical ones. As a result, the threshold current was reduced by over 20%, and the reverse leakage current was decreased by over three orders of magnitude. Therefore, the room-temperature electrically pumped narrow-linewidth GaN-on-Si laser diode has been successfully demonstrated.

Room-temperature continuous-wave electrically pumped InGaN/GaN quantum well blue laser diode directly grown on Si.

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.

On-wafer fabrication of cavity mirrors for InGaN-based laser diode grown on Si.

Direct bandgap III-V semiconductor lasers grown on silicon (Si) are highly desired for monolithic integration with Si photonics. Fabrication of semiconductor lasers with a Fabry-Pérot cavity usually includes facet cleavage, however, that is not compatible with on-chip photonic integration. Etching as an alternative approach holds a great advantage in preparing cavity mirrors with no need of breaking wafer into bars. However, gallium nitride (GaN) sidewalls prepared by dry etching often have a large roughness and etching damages, which would cause mirror loss due to optical scattering and carrier injection loss because of surface non-radiative recombination. A wet chemical polishing process of GaN sidewall facets formed by dry etching was studied in detail to remove the etching damages and smooth the vertical sidewalls. The wet chemical polishing technique combined with dry etching was successfully applied to the on-wafer fabrication of cavity mirrors, which enabled the realization of room temperature electrically injected InGaN-based laser diodes grown on Si.

Role of p53 in the inhibitory effects of interferon-alpha subtypes on proliferation of hepatocellular carcinoma cells.

While interferon-alpha (IFN-alpha) subtypes share a common specific receptor composed of two subunits, interferon-alpha receptor (IFNAR)-1 and IFNAR-2, their subtype activities are exhibited via several intracellular signaling pathways and thus subsequently show different biological effects. Anti-proliferative effects of single treatment with IFN-alpha subtypes or 5-fluorouracil (FU), and of combined treatment with each IFN-alpha subtype and 5-FU were examined on three hepatocellular carcinoma cell lines, HepG2, HLE and PLC/PRF/5. HepG2 and PLC/PRF/5 cells were susceptible to the combination treatment, but HLE cells were not. Proliferation of PLC/PRF/5 cells was also inhibited by the IFN-alpha subtypes singly. In addition, apoptosis was observed in HepG2 cells upon treatment with 5-FU alone and with the combination treatment, and in PLC/PRF/5 cells after single treatment with the IFN-alpha subtypes and after the combination treatment. IFN-alpha subtypes induced cell cycle arrest in the G2/M phase in HepG2 and PLC/PRF/5. Analyses by Western blotting and immunoprecipitation revealed increased p53 phosphorylation in HepG2 and PLC/PRF/5 cells but not in HLE cells after combined treatment. Single treatment with IFN-alpha subtypes promoted p53 activation only in PLC/PRF/5 cells. These results propose that IFN-alpha subtypes induce cells to undergo apoptosis through p53 activation directly and indirectly, in collaboration with 5-FU, further suggesting the presence of distinct signal pathways for IFN-alpha-induced apoptosis.

Reduction of Polarization Field Strength in Fully Strained c-Plane InGaN/(In)GaN Multiple Quantum Wells Grown by MOCVD.

The polarization fields in c-plane InGaN/(In)GaN multiple quantum well (MQW) structures grown on sapphire substrate by metal-organic chemical vapor deposition are investigated in this paper. The indium composition in the quantum wells varies from 14.8 to 26.5% for different samples. The photoluminescence wavelengths are calculated theoretically by fully considering the related effects and compared with the measured wavelengths. It is found that when the indium content is lower than 17.3%, the measured wavelengths agree well with the theoretical values. However, when the indium content is higher than 17.3%, the measured ones are much shorter than the calculation results. This discrepancy is attributed to the reduced polarization field in the MQWs. For the MQWs with lower indium content, 100% theoretical polarization can be maintained, while, when the indium content is higher, the polarization field decreases significantly. The polarization field can be weakened down to 23% of the theoretical value when the indium content is 26.5%. Strain relaxation is excluded as the origin of the polarization reduction because there is no sign of lattice relaxation in the structures, judging by the X-ray diffraction reciprocal space mapping. The possible causes of the polarization reduction are discussed.

Effects of interferon-alpha subtypes on the TH1/TH2 balance in peripheral blood mononuclear cells from patients with hepatitis virus infection-associated liver disorders.

Interferon-alpha (IFN-alpha) has recently been shown to modulate in vitro T helper (Th) 1-driven responses in the peripheral blood mononuclear cells (PBMC) of patients with hepatitis B virus or C virus infection. In this study, we examined the in vitro effects of IFN-alpha subtypes (IFN-alpha1, -alpha2, -alpha5, -alpha8, and -alpha10) on the Th1/Th2 balance in PBMC obtained from patients with hepatitis virus infection-associated liver disorders and chronic hepatitis (CH), in comparison with the effect on healthy control volunteer PBMC. The Th1-type cell percentages and Th1/Th2 ratios were significantly higher in the PBMC of patients when compared with controls both before and after cultivation in vitro, with the IFN-alpha subtypes. The IFNalpha-5 induced an increase in the Th2-type cell percentages in both control and patient PBMC, resulting in that IFN-alpha5 lowered the Th1/Th2 ratio in patients with CH. Furthermore, statistical analysis revealed that IFN-alpha8 significantly promoted an increase in the Th1/Th2 ratios of PBMC from patients with CH and liver cirrhosis (LC) but not that of PBMC from patients with LC-hepatocellular carcinoma (HCC) and HCC. These findings imply that hepatitis virus infection and its disease status modify the effects of IFN-alpha subtypes on Th1 and Th2 immune balance in patients. Our findings should help to elucidate the mechanisms underlying successful IFN therapy for hepatitis virus infection and prevention of hepatocellular carcinogenesis.

A novel phenomenon predicting the entry into a state of hibernation in Syrian hamsters (Mesocricetus auratus).

When Syrian hamsters (Mesocricetus auratus) are bred in a cold and short-day environment, most animals go into hibernation after a certain period of time. However, to date it has not been possible to predict which hamster will enter hibernation. In this study, we subcutaneously implanted thermo-loggers in hamsters bred in the cold environment, and recorded the subcutaneous temperature at short intervals until they went into hibernation. A time series analysis of temperature disclosed that a fall of 0.4 to 0.8 degrees C in subcutaneous temperature was seen 5 to 16 days before entering hibernation, and this phenomenon continued for three days or more. No hamster went into the hibernation without displaying this signal. Although the mechanism by which this phenomenon takes place is not clear, it is a sign from the body, which is useful for indicating if a hamster will enter hibernation shortly.

Inhibition of chlamydia trachomatis growth by human interferon-alpha: mechanisms and synergistic effect with interferon-gamma and tumor necrosis factor-alpha.

We have evaluated the effect of natural human interferon (IFN)-alpha on the growth of chlamydia trachomatis in human epithelial cells in vitro and revealed that IFN-alpha has reduced both growth and infectivity of C. trachomatis. The effect of IFN-alpha was reversed by the addition of exogenous L-tryptophan and iron to the culture medium, suggesting that antichlamydial effect of IFN-alpha was caused by depletion of intracellular tryptophan and iron, both of which are essential for chlamydial growth. When IFN-alpha was combined with another antichlamydial cytokines, IFN-gamma and tumor necrosis factor (TNF)-alpha, the effect was synergistically enhanced. Therefore, IFN-alpha would act coordinately with other cytokines such as IFN-gamma and TNF-alpha, and play an important role in host defense against infection and in the establishment of persistent chlamydial infection of host, in which the organism remains viable, but in a culture-negative state.

Royal Jelly prolongs the life span of C3H/HeJ mice: correlation with reduced DNA damage.

In this study, we investigate the effect of dietary Royal Jelly (RJ) on tissue DNA oxidative damage and on the life span of C3H/HeJ mice. In C3H/HeJ mice that were fed a dietary supplement of RJ for 16 weeks, the levels of 8-hydroxy-2-deoxyguanosine (8-OHdG), a marker of oxidative stress, were significantly reduced in kidney DNA and serum. Secondly, we determined the effect of dietary RJ on the life span in C3H/HeJ mice. The 50% mice survivals of intermediate- (about 6 mg/kg weight) and high-dose groups (about 60 mg/kg weight) were reached at significantly longer times than that of the control group according to the generalized Wilcoxon test (p<0.05). The average survival times were 88 weeks for the control group vs. 79 weeks for the low-dose group (about 0.6 mg/kg weight), 112 weeks for the intermediate-dose group and 110 weeks for the high-dose group, respectively, showing that RJ extended the average survival time by about 25% compared to the control group. However, RJ did not extend the total life span. These results indicated that dietary RJ increased the average life span of C3H/HeJ mice, possibly through the mechanism of reduced oxidative damage.