Efficient radiative recombination

The efficient formation of singlet excitons from the positive and negative charge carriers, which are injected via the metallic contacts and transported as positive and negative polarons (P+ and P ) in the layer, and the efficient radiative recombination of these singlet excitons formed are crucial processes for the function of efficient electroluminescence devices. 

Efficient radiative recombination in tetrahedrally-bonded amorphous films depends critically on the presence of hydrogen which removes non-radiative recombination centers from the middle of the gap. Recent PL experiments on amorphous silicon-germanium alloys show that the intensity of the PL at low energies is independent of germanium content. This observation may require a reinterpretation of the commonly accepted explanation for the dominant PL process in these alloys. 

In LEDs, electrons are generally injected into a type active layer. The internal quantum efficiency, ie, the photons per injected electrons, is given by the radiative recombination rate divided by the total recombination rate ... 

Thushigh internal quantum efficiency requires short radiative and long nonradiative lifetimes. Nonradiative lifetimes are generally a function of the semiconductor material quaUty and are typically on the order of microseconds to tens of nanoseconds for high quahty material. The radiative recombination rate, n/r, is given by equation 4 ... 

For a simplified case, one can obtain the rate of CL emission, =ft GI /e, where /is a function containing correction parameters of the CL detection system and that takes into account the fact that not all photons generated in the material are emitted due to optical absorption and internal reflection losses q is the radiative recombination efficiency (or internal quantum efficiency) /(, is the electron-beam current and is the electronic charge. This equation indicates that the rate of CL emission is proportional to q, and from the definition of the latter we conclude that in the observed CL intensity one cannot distii pish between radiative and nonradiative processes in a quantitative manner. One should also note that q depends on various factors, such as temperature, the presence of defects, and the... 

The overall efficiency of LED emission depends on three factors which vary between the different types of LEDs. These are the efficiency of electron-hole production, the radiative efficiency of recombination, and the efficiency of extraction of the optical signal from the junction. 

As already said in the introduction, simultaneous doping with n- and p-type impurities represents a way to overcome the low radiative recombination efficiency in our systems, so, starting from the already described hydrogenated Si-NCs, and following the work of Fujii et al. [32-34], we have... 

Carrier and exciton dynamics in InGaN/GaN MQWs have also been studied at a high optical pumping power [34], At 7 K, a radiative decay lifetime of 250 ps was observed for the dominant transition at a generated carrier density of 1012/cm2. The time-resolved measurement showed that the decay of PL has a bimolecular recombination characteristic. At room temperature, the carrier recombination was found to be dominated by non-radiative processes with a measured lifetime of 130 ps. Well width dependence of carrier and exciton dynamics in InGaN/GaN MQWs has also been measured [35]. The dominant radiative recombination at room temperature was attributed to the band-to-band transition. Combined with an absolute internal quantum efficiency measurement, a lower limit of 4 x 10 9 cm3/s on the bimolecular radiative recombination coefficient B was obtained. At low temperatures, the carrier... 

Deep levels interact with free carriers as either recombination centres or traps. Consequently, deep levels can significantly influence the photoelectric or electronic properties of a semiconductor. For example, in the active region of light emitting diodes deep levels can act as efficient non-radiative recombination centres and significantly limit the internal quantum efficiency. Other applications may utilise deep levels for the benefit of device performance. 

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