Tunnel injection and power efficiency of InGaN/GaN light-emitting diodes

The results of studying the influence of the finite tunneling transparency of injection barriers in light-emitting diodes with InGaN/GaN quantum wells on the dependences of the current, capacitance, and quantum efficiency on the p-n junction voltage and temperature are presented. It is shown that defectassisted hopping tunneling is the main transport mechanism through the space charge region (SCR) and makes it possible to lower the injection barrier. It is shown that, in the case of high hopping conductivity through the injection barrier, the tunnel-injection current into InGaN band-tail states is limited only by carrier diffusion from neutral regions and is characterized by a close-to-unity ideality factor, which provides the highest quantum and power efficiencies. An increase in the hopping conductivity through the space charge region with increasing frequency, forward bias, or temperature has a decisive effect on the capacitance-voltage characteristics and temperature dependences of the high-frequency capacitance and quantum efficiency. An increase in the density of InGaN/GaN band-tail states and in the hopping conductivity of injection barriers is necessary to provide the high-level tunnel injection and close-to-unity power efficiency of high-power light-emitting diodes.     

Statistical parameters of kinetics of heteroadagulation of microdrops of solar oil on carbon cloth

The statistical parameters of hydrodynamic heteroadagulation of microdrops of solar oil with sizes of 0.5–2 μm have been obtained for the first time. The problem is of a great practical significance because the agglomeration of microdrops leads to substantial acceleration of purification of aqueous phase from them according to hydrodynamic heteroadagulation (HHA). HHA has been studied in the broad range of the rotation frequencies of rotor discs from 200 to 2000 min^?1. Root-mean-square deviations S have been calculated in all points of kinetic curves and also their relative errors δ% have been obtained. The methods of analyzing the u-distributions and Student t-distributions have been used. The trends of random deviations of kinetic curves of heteroadagulation from mean values have been determined.     

Determination of the coefficient of light attenuation in thin layers of light-emitting diodes

A method for determining the coefficient of light attenuation in a thin GaN layer of light-emitting structures is suggested. The method is based on the measurements of electroluminescence intensity in parallel and perpendicular directions to the layer. The light-attenuation coefficient of 90 ± 15 cm^?1 was obtained for the GaN layer of an InGaN light-emitting structure.     

Effect of the joule heating on the quantum efficiency and choice of thermal conditions for high-power blue InGaN/GaN LEDs

The heat model of a light-emitting diode (LED) with an InGaN/GaN quantum well (QW) in the active region is considered. Effects of the temperature and drive current, as well as of the size and material of the heat sink on the light output and efficiency of blue LEDs are studied. It is shown that, for optimal heat removal, decreasing of the LED efficiency as current increases to 100 mA is related to the effect of electric field on the efficiency of carrier injection into the QW. As current further increases up to 400 mA, the decrease in efficiency is caused by Joule heating. It is shown that the working current of LEDs can be increased by a factor of 5–7 under optimal heat removal conditions. Recommendations are given on the cooling of LEDs in a manner dependent on their power.     

Sprayed CuInS2 thin films for solar cells: The effect of solution composition and post-deposition treatments

CuInS₂ thin films were prepared by spray pyrolysis from solutions with different compositions. Etching in KCN solution and thermal treatments in vacuum and hydrogen were applied to as-deposited films. KCN etching removes conductive copper sulfide from the surface of Cu-rich films but has no effect on matrix composition. Vacuum annealing at 500°C and hydrogen treatment at 400–500°C purifies the films, prepared from the solutions with the Cu/In=1, from secondary phases, reduces chlorine content and improves crystallinity. Vacuum annealing results in n-type films due to the formation of In₂O₃ phase. Treatment in hydrogen reduces oxygen-containing residues and results in p-type CuInS₂ films with resistivity close to 10 Ω cm.     

Nonuniformity of carrier injection and the degradation of blue LEDs

The distribution of electroluminescence (EL) intensity over the area and in the course of time before and after the optical degradation of blue InGaN/GaN LEDs is studied. Current-voltage characteristics have been recorded. It is found that the initially bright luminescence near the region of metallization of the p-contact turns weak after the degradation of an LED. The time delay of ～20–40 ns is observed in the distribution of EL intensity over the area of LEDs after their degradation. We suppose that a rise in the excess current after degradation is due to the density increasing of the InGaN/GaN interface states and the formation of an electrical dipole, which lowers the potential barriers in p-GaN and n-GaN layers. The corresponding increase of capacitance leads to a time delay in the spreading of the injection current and in the distribution of the emission brightness over the area. The lateral nonuniformity of the carrier injection into the quantum, well before and after optical degradation, is attributed to diffusion and electromigration of hydrogen, induced by mechanical stress. The metallization of the p-contact may be the source of mechanical stress.     

Efficient singlet oxygen generation upon two-photon excitation ofnew porphyrin with enhanced nonlinear absorption

We demonstrate efficient generation of singlet oxygen upon two-photon excitation with 150-fs 780-nm laser pulses of a new porphyrin photosensitizer molecule whose two-photon absorption cross section has been considerably enhanced by chemical design