Recombination dynamics in low‐dimensional nitride semiconductors

Abstract

Optical properties induced by two major effects, potential fluctuation and piezoelectric fields, have been assessed to interpret the emission mechanism in low-dimensional nitride semiconductors because the former leads to the exciton/carrier localization, and the latter to the quantum confined Stark effect (QCSE). Degenerated white-light pump-and-probe spectroscopy has been employed to assess which factor plays an important role in the series of In x Ga 1-x N multiple quantum well (MQW) structures whose well widths are 3 nm, 5 nm and 10 nm. Moreover, photoluminescence (PL) mapping with scanning near-field optical microscopy (SNOM) has revealed the dense distribution of island-like structures, the size of which ranges from 20 nm to 70 nm in a 3 nm-thick In x Ga 1-x N single quantum well (SQW) structure emitting at blue spectral region. The most devices are grown toward c-direction with hexagonal structures, so that large internal electric field is induced to the growth direction. This electric field is in the order of MV/cm, so the optical dipole, as well as transition energies is reduced due to the separation between electron and hole wavefunctions. Such effects are called as Franz-Keldysh effect and quantum confined Stark effect (QCSE). The second one is potential fluctuation induced by inhomogeneity of both In compositions and well widths. This limits the number of density of states due to the formation of localized tail states. Because of these effects, large Stokes shift between absorption and emission is observed, as well as significant blue shift of emission energy with increasing injected-carrier density. The problem is that it is very difficult to know with conventional optical measurement which factor limits the recombination mechanism because both two effects can contribute to observed phenomena. It should be noted here that the major factor changes with the sample, because the effect of internal electric field is enhanced with increasing well width, and is reduced with increasing background doping concentration and with carrier injection, and also the degree of potential fluctuation is changed with the difference in growth conditions. Therefore, the precise optical characterization is needed in each sample to determine the recombination mechanism.

Recent developments in the III‐nitride materials

“…There have been lively discussions in the literature concerning what process dominates the recombination in these QW systems, the localization effects or the internal field effects [48][49][50][51]. Clearly both are important.…”

In‐clustering effects in InAlN and InGaN revealed by high pressure studies

“…The presence of In‐content inhomogeneities is commonly used as an explanation of enhanced light emission from In‐containing alloys of group‐III nitrides 1–5. In‐content fluctuations may cause localization of excitons, thus reducing the rate of nonradiative recombination due to dislocations.…”

Microstructure and luminescence dynamics of ZnCdO films with high Cd content deposited on different substrates by DC magnetron sputtering method

“…The film with the largest cadmium content demonstrates the largest value of the internal quantum efficiency. It may be caused by several factors: (і) decrease in the concentration of the intrinsic point defects, which may play a role of the non-radiative recombination centers and (іі) strong exciton localization, which can be a reason for the enhancement of the probability of the radiative transitions [51]. Taking into account the first factor (i), we have found a principle relationship between the value of the deviation of the stoichiometric ratio ∆S, which may be expressed as: ∆S =n O /(n Zn +n Cd ) -1, and cadmium content in the film (Fig.…”