Below-gap Excitation Light Research Articles

Temperature dependence of nonradiative recombination processes in UV-B AlGaN quantum well revealed by below-gap excitation light

Nonradiative recombination (NRR) processes through defect states and their temperature dependence in UV-B AlGaN MQW sample on sapphire substrate grown by MOCVD technique have been studied by photoluminescence (PL) spectroscopy. We detected NRR centers by adding a below-gap excitation light with photon energies from 0.93 eV to 1.46 eV on an above-gap excitation light of 4.66 eV. All the BGE energies decreased PL intensity at 25 K, and the most-distinct quenching is observed by 1.27 eV BGE at the same BGE photon number density. The temperature-dependent PL intensity for the BGE energy of 1.27 eV is interpreted by three NRR centers. The one-level model dominates over that of two-level model in the temperature range 58 K < T < 88 K. The two-level model prevails in other region of temperature. The combination of one-level and two-level models is consistent with the spectral peak-energy shift as a function of temperature.

Superlattice period dependence on nonradiative recombination centers in the n-AlGaN layer of UV-B region revealed by below-gap excitation light

Nonradiative recombination (NRR) centers in n-AlGaN layers of UV-B AlGaN samples with different numbers of superlattice (SL) periods (SLPs), grown on the c-plane sapphire substrate at 1150 °C by the metalorganic chemical vapor deposition technique, have been studied by using below-gap-excitation (BGE) light in photoluminescence (PL) spectroscopy at 30 K. The SLP affects the lattice relaxation of the SL and n-AlGaN layer. The PL intensity decreased by the superposition of BGE light of energies from 0.93 eV to 1.46 eV over the above-gap-excitation light of energy 4.66 eV, which has been explained by a two-level model based on the Shockley–Read–Hall statistics. The degree of PL quenching from n-AlGaN layers of the sample with SLP 100 is lower than those of other samples with SLP 50, 150, and 200. By a qualitative simulation with the dominant BGE energy of 1.27 eV, the density ratio of NRR centers in n-AlGaN layers of 50:100:150:200 SLP samples is obtained as 1.7:1.0:6.5:3.4. This result implies that the number of SLP changes lattice relaxation and determines the density of NRR centers in the n-AlGaN layer, which affects the performance of LEDs.

Nonradiative recombination centers in deep UV-wavelength AlGaN quantum wells detected by below-gap excitation light

Nonradiative recombination (NRR) centers in AlGaN multiple quantum well samples, grown on sapphire substrate at two different growth temperatures by the MOCVD technique, have been studied by using below-gap excitation (BGE) light in photoluminescence (PL) measurements at about 25 K. The PL intensity decreased by the superposition of BGE light of energies between 0.93 and 1.46 eV over above-gap excitation light of energy 4.66 eV. This is explained by a two-level model based on Shockley–Read–Hall statistics. The model indicates the presence of a pair of NRR centers in both samples, which are activated by the BGE. The degree of PL quenching for the sample grown at 1140 °C is higher than that of the sample grown at 1180 °C for BGE energies 0.93, 1.17, and 1.27 eV. The density ratio of 1.5, for the BGE energy of 1.27 eV, was obtained from a qualitative simulation. This result implies that a slight difference in growth conditions changes defect densities.

Characterization of nonradiative recombination centers in proton-irradiated InAs/GaAs quantum dots by two-wavelength-excited photoluminescence

Nonradiative recombination (NRR) centers in as-grown and proton-irradiated InAs/GaAs quantum dot (QD) structures have been studied by two-wavelength-excited photoluminescence (PL). The PL intensity quenching of GaAs and QD emissions due to the addition of a below-gap excitation light of 0.80 eV energy indicates the presence of defect levels acting as NRR centers. The method enables us to discuss the distribution of NRR centers in GaAs and/or InAs QD regions by selecting either conduction band excitation (2.33 eV) or intermediate band excitation (1.27 eV). We have found that the densities of NRR centers in GaAs layers and the effect of quenching on GaAs emissions increase monotonically with increasing proton irradiation fluence. The QD emission intensity, however, increases at a moderate fluence of 7 × 1011 protons/cm2 owing to the defect-assisted trapping of electrons into QDs. Further incorporation of NRR centers after 4 × 1012 protons/cm2 fluence quenches the QD-PL intensity below that of an unirradiated sample.

Trap and nonradiative centers in Ba₃Si₆O₁₂N₂:Eu²⁺ phosphors observed by thermoluminescence and two-wavelength excited photoluminescence methods.

We have studied trap centers and nonradiative (NRR) recombination centers in a Ba3Si6O12N2:Eu2+ (BSON), one of promising materials for efficient and stable phosphors in white LED lamp applications. The energy distribution of four trap centers was obtained by thermo-luminescence (TL) with the excitation energy of 5.59eV. By superposing a below-gap excitation light of 1.77eV and observing the intensity change of the 5d-4f emission of Eu2+ centered at 2.36eV in our two-wavelength excited photoluminescence (TWEPL) measurement, both transient and steady state enhancement were observed. Such peculiar behavior of photo-stimulation is attributed to the coexistence of trap centers and NRR centers: the photoexcitation of electrons from trap centers generates the transient component, while that from NRR centers maintains the steady state component. An optical detection of relatively faint contribution of defects became possible in order to improve further the reliability and efficiency of phosphor materials.

Improved quality of plasma assisted MBE‐grown GaN/AlGaN quantum wells revealed by two‐wavelength excited photoluminescence

Below-gap states in plasma assisted MBE-grown GaN/AlGaN quantum well (QW) structures have been studied by two-wavelength excited photoluminescence (PL). The decrease of the PL intensity with the addition of a below-gap excitation light source of 1.17 eV implies the presence of an energy-matched below-gap state. We have studied several QW structures grown on AlGaN or AlN buffer layers on sapphire substrates. We found improved internal quantum efficiency with increasing the number of QW's and with providing a sufficiently thick AlGaN or AlN buffer layer. (© 2003 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Below-gap recombination dynamics in GaN revealed by time-resolved and two-wavelength excited photoluminescence

In continuation to the steady state detection of below-gap states by combining a below-gap excitation (BGE) light with an above-gap excitation (AGE) light, we have studied the time-response of both donor-acceptor pair luminescence (DAP) and yellow luminescence (YL) in undoped GaN after switching off the BGE light. An extremely slow recovery of YL with a time constant of up to 28 s was observed, which is interpreted as a relaxation process of electrons from a below-gap state chosen by a BGE energy of 1.17 eV. Trap parameters were quantitatively determined by studying the intensity change and subsequent recovery time of DAP as a function of AGE and BGE power densities. The BGE energy dependence of the intensity change and subsequent recovery time of DAP and YL implies an energy distribution of traps in the band-gap of GaN. The combination of time-resolved PL measurement under two-wavelength excitation scheme was shown to be powerful for understanding the carrier recombination dynamics including YL among below-gap states.

Extremely Slow Relaxation Process of a Yellow-Luminescence-Related State in GaN Revealed by Two-Wavelength Excited Photoluminescence

In addition to detection of nonradiative recombination centers by a below-gap excitation (BGE) technique at steady state, we have studied the time-response of both donor-acceptor pair luminescence and yellow luminescence (YL) in GaN after switching off the BGE light. An extremely slow recovery of YL with a time constant of up to 28 s was observed, which is interpreted as a relaxation process of electrons from the state chosen by a BGE energy of 1.17 eV. The dependence of recovery time on the BGE energy and on the above-gap and below-gap excitation power densities became clear for the first time; it reflects the carrier recombination dynamics including YL among below-gap states.