Zinovchuk A. Effect of current crowding on the behavior of electron-hole plasma in many-layers light emitting structures

The work deals with the investigation of current crowding effect in visible (ІnGaN/GaN), near (AlGaAs/GaAs) and mid (ІnAsSbP/ІnAsSb) infrared many-layers light emitting diodes (LED). By means of local temperature measurements it is shown considerable overheating nonunifotmity caused by current crowding in ІnGaN/GaN multiple quantum well LEDs (L=460-470 nm). Combined influence of electrical and thermal effects results in "heat traps" - small high temperature zones (diameter 20 µm) inside active region. The temperature gradients in such local zones reach up to 10^(4) С/cm. It is shown both experimentally and theoretically that in planar mid infrared LEDs (L=3-5 m) based on ІnAsSbP/ІnAsSb double heterostructures current crowding effect depend on energy band gap of active layer (LEDs emitting wavelength). At identical bias current, more long-wave LEDs show more nonuniform electroluminescence distribution in comparison with short-wave once. Besides, the increasing of nonradiative recombination impact also caused the highest temperature overheating of long-wave LEDs. Despite of existing deficiencies of infrared LEDs it is shown that they can be successfully used in dynamic infrared scene simulation devices. Such advantages of LEDs as low time constant (modulation frequency >20 kHz), wide range of apparent temperatures, multispectral simulation (several sub-bands inside the mid-infrared range) and ability to simulate cold or low observable objects make it competitive with thermal emitters and lasers. The influence of current crowding on local overheating in LEDs (AlGaAs/GaAs flip-chip mesastructures, L=0.87-0.88 m) with 98 % internal quantum yield is investigated. It is shown that despite of a high internal quantum yield and favorable for current spreading two-sided mesastructure in the series with considerable overheating (T=50 C), AlGaAs/GaAs LEDs demonstrate nonuniformity of active region excess temperature distribution (temperature gradient >950 С/cm) which is responsible of devices catastrophic degradation. It is theoretically predicted two types of current crowding effect in mesastructures: contact current crowding at the contact metal surface and current crowding in active region.