High resolution transmission electron microscopy and aberration-corrected scanning transmission electron microscopy (STEM) reveal a new void defect in GaN, Si-doped GaN, and InGaN. The voids are pyramid shaped with symmetric hexagonal {0001} base facets and {10-11} side facets. The pyramid void has a closed or open core dislocation at the peak of the pyramid, which continues up along the [0001] growth direction. The closed dislocations have a 1/3 11-20 edge dislocation Burgers vector component, consistent with known threading dislocations. The open core dislocations are hexagonal shaped with pure screw character, {10-10} side facets, varying lateral widths, and varying degrees of hexagonal symmetry. STEM electron energy loss spectroscopy spectrum imaging revealed a larger C concentration inside the void and below the void than above the void. We propose that carbon deposition during metal organic chemical vapor deposition growth acts as a mask, stopping the GaN deposition locally. Subsequent layers of GaN deposited around the C covered region create the overhanging {10-11} facets, and the meeting of the six {10-11} facets at the pyramid's peak is not perfect, resulting in a dislocation.
InGaN light emitting diodes (LEDs), which have become key components of the lighting technology owing to their improved power conversion efficiencies and brightness, still suffer from efficiency degradation at high injection levels. Experiments showing sizeable impact of the barrier height provided by an electron blocking layer (EBL) or the electron cooling layer prior to electron injection into the active region strongly suggest that the electron overflow resulting from ballistic and quasi-ballistic transport is the major cause of efficiency loss with increasing injection. Our previous report using a first order simple overflow model based on hot electrons and constant LO phonon scattering rates describes well the experimental observations of electron spillover and the associated efficiency degradation in both nonpolar m-plane and polar c-plane LEDs with different barrier height EBLs and electron injection layers. LEDs without EBLs show three to five times lower efficiencies than those with Al0.15Ga0.85N EBLs due to significant electron overflow to the p-type region in the former. For effective means of thermalization in the active region within their residence time and possibly longitudinal optical phonon lifetime, the electrons were cooled prior to their injection via a staircase electron injector, i.e. an InGaN staircase structure with step-wise increased In composition. The investigated m-plane and c-plane LEDs with incorporation of staircase electron injector show comparable electroluminescence performance regardless of the status of EBL. This paper discusses hot electron effects on efficiency loss, means to cool the electrons prior to injection.
The relative roles of radiative and nonradiative processes and the polarization field on the light emission from blue (∼425 nm) InGaN light emitting diodes (LEDs) have been studied. Single and multiple double heterostructure (DH) designs have been investigated with multiple DH structures showing improved efficiencies. Experimental results supported by numerical simulations of injection dependent electron and hole wavefunction overlap and the corresponding radiative recombination coefficients suggest that increasing the effective active region thickness by employing multiple InGaN DH structures separated by thin and low barriers is promising for LEDs with high efficiency retention at high injection. The use of thin and low barriers is crucial to enhance carrier transport across the active region. Although increasing the single DH thickness from 3 to 6 nm improves the peak external quantum efficiency (EQE) by nearly 3.6 times due to increased density of states and increased emitting volume, the internal quantum efficiency (IQE) suffers a loss of nearly 30%. A further increase in the DH thickness to 9 and 11 nm results in a significantly slower rate of increase of EQE with current injection and lower peak EQE values presumably due to degradation of the InGaN material quality and reduced electron-hole spatial overlap. Increasing the number of 3 nm DH active regions separated by thin (3 nm) In0.06Ga0.94N barriers improves EQE, while maintaining high IQE (above 95% at a carrier concentration of 1018 cm−3) and showing negligible EQE degradation up to 550 A/cm2 in 400 × 400 μm2 devices due to increased emitting volume and high radiative recombination coefficients and high IQE. Time-resolved photoluminescence measurements revealed higher radiative recombination rates with increasing excitation due to screening of the internal field and enhanced electron and hole overlap at higher injection levels. To shed light on the experimental observations, the effect of free-carrier screening on the polarization field at different injection levels and the resulting impact on the quantum efficiency were investigated by numerical simulations.
AlGaN/GaN heterojunction field effect transistors (HFETs) with 2 μm gate length were subjected to on-state-high-field (high drain bias and drain current) and reverse-gate-bias (no drain current and reverse gate bias) stress at room and elevated temperatures for up to 10 h. The resulting degradation of the HFETs was studied by direct current and uniquely phase noise before and after stress. A series of drain and gate voltages was applied during the on-state-high-field and reverse-gate-bias stress conditions, respectively, to examine the effect of electric field on degradation of the HFET devices passivated with SiNx. The degradation behaviors under these two types of stress conditions were analyzed and compared. In order to isolate the effect of self-heating/temperature on device degradation, stress experiments were conducted at base plate temperatures up to 150 °C. It was found that the electric field induced by reverse-gate-bias mainly generated trap(s), most likely in the AlGaN barrier, which initially were manifested as generation-recombination (G-R) peak(s) in the phase noise spectra near 103 Hz. Meanwhile electric field induced by on-state-high-field stress mainly generated hot-electron and hot-phonon effects, which result in a nearly frequency independent increase of noise spectra. The external base plate temperatures promote trap generation as evidenced by increased G-R peak intensities.
Lysosome fusion mediates acid sphingomyelinase translocation to and activation in plasma membrane to form membrane lipid raft (LR) signaling in coronary arterial endothelial cells (CAECs). The molecular mechanism mediating this lysosome fusion to cell plasma, however, remains poorly understood. The present study attempted to evaluate whether vesicles associated membrane proteins‐2 (VAMP‐2) mediates lysosome fusion and LR clustering, leading to endothelial dysfunction in CAECs. By immunohistochemistry, VAMP‐2 was found to be abundantly expressed in the endothelium of bovine coronary arteries. Fluorescent confocal microscopy showed that VAMP‐2 was significantly aggregated with lysosome marker lamp‐1 and LRs when the CAECs were stimulated by FasL, a well‐known LR clustering stimulator. Using FM1‐43 quenching and dequenching technique, lysosome fusion to plasma membrane in response to FasL was blocked by VAMP‐2 inhibitor, tetanus toxin. In addition, fluorescence resonance energy transfer (FRET) detection demonstrated that Fas L‐induced FRET between LRs marker and VAMP‐2 increased by 21.2 ± 3.3 during FasL stimulation. Functionally, inhibition of VAMP‐2 significantly restored FasL‐induced endothelial dysfunction in coronary arterial rings. In conclusion, VAMP‐2 on lysosome membrane in CAECs is critical to the lysosome fusion to LR areas of plasma membrane in CAECs and this VAMP‐2‐mediated lysosome fusion may contributes to FasL‐induced endothelial dysfunction (Supported by NIH grants HL057244, HL091464 and HL075316).
In15.7%Al84.3%N/AlN/GaN heterojunction field effect transistors have been electrically stressed under four different bias conditions: on-state-low-field stress, reverse-gate-bias stress, off-state-high-field stress, and on-state-high-field stress, in an effort to elaborate on hot electron/phonon and thermal effects. DC current and phase noise have been measured before and after the stress. The possible locations of the failures as well as their influence on the electrical properties have been identified. The reverse-gate-bias stress causes trap generation around the gate area near the surface which has indirect influence on the channel. The off-state-high-field stress and the on-state-high-field stress induce deterioration of the channel, reduce drain current and increase phase noise. The channel degradation is ascribed to the hot-electron and hot-phonon effects.
We apply a number of all-optical time-resolved techniques to study the dynamics of free carriers in InGaN quantum structures under high excitation regime. We demonstrate that carrier lifetime and diffusion coefficient both exhibit a substantial dependence on excitation energy fluence: with increasing carrier density, carrier lifetime drops and diffusivity increases; these effects become more apparent in the samples with higher indium content. We discuss these experimental facts within a model of diffusion-enhanced recombination, which is the result of strong carrier localization in InGaN. The latter model suggests that the rate of non-radiative recombination increases with excitation, which can explain the droop effect in InGaN. We use the ABC rate equation model to fit light induced transient grating (LITG) kinetics and show that that linear carrier lifetime drops with excitation (i.e. excess carrier density). We do not observe any influence of Auger recombination term, CN3, up to the maximum carrier density that is limited due to the onset of very fast stimulated recombination process. To support these conclusions, we present spectrally resolved differential transmission data revealing different recombination rates of carriers in localized and extended states.
Diffusion lengths of photo-excited carriers along the c-direction were determined from photoluminescence (PL) measurements in p- and n-type GaN epitaxial layers grown on c-plane sapphire by metal-organic chemical vapor deposition. The investigated samples incorporate a 6 nm thick In0.15Ga0.85N active layer capped with either 500 nm p- GaN or 1300 nm n-GaN. The top GaN layers were etched in steps and PL from the InGaN active region and the underlying layers was monitored as a function of the top GaN thickness upon photogeneration near the surface region by above bandgap excitation. Taking into consideration the absorption in the active and underlying layers, the diffusion lengths at 295 K and at 15 K were measured to be about 92 ± 7 nm and 68 ± 7 nm for Mg-doped p-type GaN and 432 ± 30 nm and 316 ± 30 nm for unintentionally doped n-type GaN, respectively. Cross-sectional cathodoluminescence line-scan measurement was performed on a separate sample and the diffusion length in n-type GaN was measured to be 280 nm.
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