Search

We investigated the electrical property of reflective non-alloyed Rh contacts to p-Al0.45Ga0.55N contact layers and the performance of 278 nm flip-chip (FC) LEDs with the Rh and Ni/Au contacts as a function of Mg concentration. The three contact layers contained Mg concentration of 5.7 × 1019 cm−3 (sample A), 1.0 × 1020 cm−3 (sample B), and 1.4 × 1020 cm−3 (sample C). The Rh contacts to the sample A showed non-ohmic behaviour, while the samples B and C were ohmic with contact resistivity of 3.2 × 10−1 and 3.4 × 10−2 Ωcm2, respectively. The FC-LEDs with the sample A, B, and C with the Rh contacts yielded forward voltages of 7.26, 6.24, and 6.30 V, and output power of 10.46, 10.85, and 10.44 mW at 50 A cm−2, respectively, whereas that with the sample C (Ni/Au contact) gave 6.51 V and 6.26 mW. The FC-LEDs with p-Al0.55Ga0.45N (sample D) and p-Al0.61Ga0.39N (sample E) contact layers (with Mg dopant of 8.0 × 1019 cm−3) displayed forward voltage of 7.41 and 8.72 V at 50 A cm−2 and output power of 11.88 and 15.25 mW, respectively. The Wall-plug efficiency of the sample C, D, and E was estimated to be 3.31, 3.21, and 3.50%, respectively. These results show FC-LEDs fabricated with highly Mg-doped p-AlGaN and Rh contact demonstrate reliable performance upon operating at 100 A cm−2.

The protective activity of prickly pear cactus (Opuntia ficus indica var. saboten) fruit juice and its main constituent, betanin, were evaluated against stress-induced acute gastric lesions in rats. After 6 h of water immersion restraint stress (WIRS), gastric mucosal lesions with bleeding were induced in Sprague–Dawley rats. Pretreatment of a lyophilized powder containing O. ficus indica var. saboten fruit juice and maltodextrin (OFSM) and betanin significantly reduced stress lesions (800–1600 mg/kg). Both OFSM and betanin effectively prevented the decrease in gastric mucus content as detected by alcian blue staining. In addition, OFSM significantly suppressed WIRS-induced increases in the level of gastric mucosal tumor necrosis factor-α and myeloperoxidase (MPO). Betanin alone was only effective in decreasing MPO. These results revealed the protective activity of OFSM against stress-induced acute gastric lesions and that betanin may contribute to OFSM's gastric protective activity, at least in part. When OFSM and betanin were taken together, OFSM exerted gastroprotective activity against stress-induced gastric lesions by maintaining gastric mucus, which might be related to the attenuation of MPO-mediated damage and proinflammatory cytokine production.

To suppress bipolar population and hence electron–hole recombination outside quantum dots (QDs), tunneling-injection of electrons and holes into QDs from two separate quantum wells was proposed earlier. Close-to-ideal operating characteristics were predicted for such a double tunneling-injection (DTI) laser. In the Stranski–Krastanow growth mode, a two-dimensional wetting layer (WL) is initially grown followed by the formation of QDs. Due to thermal escape of carriers from QDs, there will be bipolar population and hence electron–hole recombination in the WL, even in a DTI structure. In this work, the light–current characteristic (LCC) of a DTI QD laser is studied in the presence of the WL. Since the opposite sides of a DTI structure are only connected by the current paths through QDs and the WL is located in the n-side of the structure, the only source of holes for the WL is provided by QDs. It is shown that, due to the zero-dimensional nature of QDs, the rate of the hole supply to the WL remains limited with increasing injection current. For this reason, as in the other parts of the structure outside QDs (quantum wells and optical confinement layer), the parasitic electron–hole recombination remains restricted in the WL. As a result, even in the presence of the WL, the LCC of a DTI QD laser becomes increasingly linear at high injection currents, which is a further demonstration of the potential of such a laser for high-power operation.

A laser structure is studied, which exploits tunneling-injection of electrons and holes into quantum dots (QDs) from two separate quantum wells (QWs). An extended theoretical model is developed allowing for out-tunneling leakage of carriers from QDs into the opposite-to-injection-side QWs (electrons into the p-side QW and holes into the n-side QW). Due to out-tunneling leakage, parasitic recombination of electron-hole pairs occurs outside QDs – in the QWs and optical confinement layer. The threshold current density j th and the characteristic temperature T 0 are shown to be mainly controlled by the recombination in the QWs. Even in the presence of out-tunneling from QDs and recombination outside QDs, a tunneling-injection laser shows potential for significant improvement of temperature stability of j th – the characteristic temperature T 0 remains very high (above 300 K at room temperature) and not significantly affected by the QD size fluctuations.

PtRh alloy nanoparticles were synthesized for use in enhancing the stability of methanol crossover and electrocatalytic activity for oxygen reduction in direct methanol fuel cells (DMFCs). From the electrochemical results, by comparing Pt in terms of methanol tolerance and oxygen reduction with Pt-based alloys, the PtRh(2:1) was an optimum catalyst for the elimination of CO poisoning during the oxidation of crossover methanol as well as oxygen reduction. A PtRh(2:1) alloy cathode showed a higher power density as well as better performance stability than Pt in DMFC unit cell test.

The maskless selective wet etching of p-GaN layer with KOH in ethylene glycol (KE) and H3PO4/H2SO4 (HH) acids was developed for the highly efficient light-emitting diodes (LEDs). The p-GaN surfaces textured by the selective wet etching process without using etch mask showed hexagonal and stripe shapes in the KE and HH solutions, respectively. The current-voltage (I-V) characteristics of the LED textured by KE and HH solutions showed improved electrical properties compared to the non-etched LED. This result could be attributed to a reduced contact resistance due to an increased contact area between the metal electrode and p-GaN layer. In addition, the light-output power of the LED textured by KE and HH solutions was improved by 29.4% and 36.8% relative to that of the non-etched LED. This result was attributed to the increase in probability of escaping photons from the LED and the reduction of surface defects by the maskless selective wet etching process. (© 2005 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

The selective wet etching of a p-GaN layer by using a solution of KOH in ethylene glycol (KE) was studied to enhance the optical and electrical performance of the GaN-based light-emitting diodes (LEDs). The surface of the p-GaN, which was selectively etched in the KE solution, showed hexagonal-shaped etch pits. The light-output power of etched LEDs was improved by 29.4% compared to that of the nonetched LED. This improvement was attributed to the increase in the probability of photons to escape due to the increased surface area of textured surface and the reduction in contact resistance of the ohmic layer resulting from the increased contact area and hole concentration on the textured p-GaN. The reverse leakage current of the LED was also greatly decreased due to the surface passivation and the removal of defective regions from the p-GaN.

A high light-extraction efficiency was demonstrated in the flip-chip light-emitting diode (FCLED) with a textured sapphire substrate. The bottom side of a sapphire substrate was patterned using a dry etching process to increase the light-extraction efficiency. Light output power measurements indicated that the scattering of photons emitted in the active layer was considerably enhanced at the textured sapphire substrate resulting in an increase in the probability of escaping from the FCLED. The light-output power of the FCLED was increased by 40.2% for a 0.4-mum deep FCLED with a periodic distance of 13-mum mesh-type texture on the bottom side of the sapphire substrate

Effect of the wetting layer (WL) on the output power of a double tunneling-injection (DTI) quantum dot (QD) laser is studied. Such a laser was proposed earlier to suppress bipolar population and hence electron-hole recombination outside QDs. In the Stranski-Krastanow growth mode, QDs are formed on an initially grown WL. The WL is directly connected to QDs by the processes of carrier capture and thermal escape. These processes are described in terms of the temporal cross-sections of electron and hole capture from the WL into QDs. The electron and hole densities and parasitic electron-hole recombination current density in the WL, and the output power of the device are calculated as functions of the temporal cross-sections. These calculations provide the basis for optimization of a DTI QD laser with the WL aimed at maximizing the output power. The larger the temporal cross-section of electron capture into QDs, the more efficient is the electron capture from the WL into QDs, and hence the higher is the output power. The smaller the temporal crosssection of hole capture into QDs, the less intensive is the hole thermal escape from QDs into the WL, the less intensive is the recombination in the WL, and hence the higher is the output power.

We develop a comprehensive theoretical model for a double tunneling-injection (DTI) quantum dot (QD) laser. Both electrons and holes are injected into QDs by tunneling from two separate quantum wells (QWs). Ideally, out-tunneling of each type of carriers from QDs into the opposite-to-injection-side QW should be completely blocked; as a result, the parasitic electron-hole recombination outside QDs will be suppressed and the light-current characteristic (LCC) of a laser will be strictly linear. To scrutinize the potential of a DTI QD laser for high-power operation and the robustness of an actual device, our model includes out-tunneling leakage of carriers from QDs. We complement our calculations by an analytical model and derive closed-form expressions for the LCC and carrier population across the layered structure. We show that, even in the presence of out-tunneling leakage, the flux of parasitic recombination outside QDs remains restricted with increasing injection current. As a consequence, the LCC exhibits a remarkable feature distinguishing the DTI QD laser from other types of injection lasers--it becomes increasingly linear and the slope efficiency grows closer to unity at high injection currents. The linearity is due to the fact that the current paths connecting the opposite sides of the structure lie entirely within the QDs--in view of the three-dimensional confinement in QDs, the out-tunneling fluxes of carriers from dots are limited.

The potential for high-power operation of a laser exploiting tunneling-injection of electrons and holes into quantum dots (QDs) from two separate quantum wells (QWs) is studied. An extended theoretical model is developed to account for out-tunneling leakage of carriers from QDs. Even in the presence of out-tunneling from QDs, the parasitic recombination flux outside QDs is shown to remain restricted with increasing injection current; correspondingly, the LCC becomes more and more linear and the slope efficiency closer to unity at high injection currents. The linearity is due to the fact that the current paths connecting the opposite sides of the structure lie entirely within QDs - in view of the threedimensional confinement in QDs, the out-tunneling fluxes of carriers from dots are limited.

High temperature stability of threshold current has been predicted for the semiconductor quantum dot (QD) laser. An ideal situation would be temperature-insensitive threshold current density j0 i.e. the characteristic temperature (a widely-accepted figure of merit of any diode laser from the viewpoint of temperature-stability of its y'th) defined as T0=(partln jth/partT)-1. In this paper the authors study the effect of out-tunneling leakage of carriers from QDs and hence of recombination outside QDs on the T-dependence of j0. The authors show that, even in the presence of such leakage, T0 remains very high. The authors analyze T0 versus the parameters of a GalnAsP/InP heterostructure lasing near 1.55 mum.

We investigate the impacts of quantum-well (QW) shapes on the performance of InGaN/GaN light-emitting diodes (LEDs) grown on c-plane sapphire substrates. Three different QW shapes are utilized, namely, rectangular, staircase, and trapezoidal QWs of thicknesses of 4, 5, and 6 nm. Various optoelectronic measurements are conducted on these samples to determine the correlation of the effect of piezoelectric field with device performances. It is found that the staircase QW consistently shows the reduced effect of piezoelectric field, which agrees well with the simulation results of the increased electron–hole overlap integral over the conventional rectangular QW. The nonconventional QW can reduce the effect of piezoelectric field and improve the LED performance accordingly.

The efficiency in non-polar m-plane InGaN/GaN light-emitting diodes (LEDs) with an electron injector (EI) layer was investigated using multiband effective mass theory. The electron overflow is shown to decrease with the inclusion of an electron-blocking layer. In particular, with the inclusion of the EI layer, the electron overflow is significantly reduced even for a large bias voltage. In the case of the single EI layer, the layer thicker than 13 nm would be needed to reduce electron overflows substantially. However, it will be difficult to grow epitaxially high-quality thick single EI layer. Instead, we show that the quantum-well structure with the superlattice-like EI layer is more effective in reducing the ballistic and quasiballistic electron transport across the active region, compared with a thick single EI layer.

Optical properties of blue AlInGaN/InGaN quantum well (QW) structures with a quaternary AlInGaN well layer were investigated by using the non-Markovian gain model with many-body effects. The band-gap expression of the AlInGaN materials was determined through a comparison with experimental results. We found that the emission peak can be enhanced by using quaternary AlInGaN well and is sensitive on In composition in the InGaN barrier. For example, the spontaneous emission coefficient for Al0.08In0.22Ga0.67 N/InxGa1−xN QW structures shows a maximum at In composition of 0.13 in the barrier and gradually decreases with increasing In composition. This is attributed to the fact that the quasi-Fermi-level separation linearly decreases with increasing In composition in the barrier due to the decrease in the conduction and valence band offsets. The AlInGaN/InGaN system with zero internal field is found to have smaller emission peak than the AlInGaN/InGaN system with nonzero internal field due to smaller band offsets.

We study the optical output power of a semiconductor laser, which exploits tunneling-injection of electrons and holes into quantum dots (QDs) from two separate quantum wells. Even if there is out-tunneling leakage of carriers from QDs, the intensity of parasitic recombination outside QDs remains restricted with increasing injection current. As a result, the light-current characteristic becomes increasingly linear, and the slope efficiency grows closer to unity at high injection currents-a fascinating feature favoring the use of tunneling-injection of both electrons and holes into QDs for high-power lasing. (C) 2008 American Institute of Physics. U.S. Army Research Office under Grant No. W911-NF-05-1-0308