Two-wavelength Excited Photoluminescence Research Articles

Detection of Nonradiative Recombination Centers in GaPN by Combining Two‐Wavelength Excited Photoluminescence and Time‐Resolved Photoluminescence

Presence and influence of nonradiative recombination (NRR) centers in an intermediate band (IB)‐type material, GaP1–xNx (), are studied by two‐wavelength excited photoluminescence (TWEPL) method and time‐resolved photoluminescence (TRPL) measurement at 77 K. With the use of below‐gap excitation (BGE) light in addition to an above‐gap excitation (AGE), the PL peak intensity is found to increase which indicates the presence of NRR centers and a secondary excitation from the IB to conduction band (CB). Depending on the effect of different BGE energies, an energy diagram on the distribution of NRR centers and NRR process is interpreted. The saturation of PL increase is attributed to the trap‐filling effect in NRR centers, which allows us to modify the rate equation. The NRR parameters are evaluated by a qualitative simulation of the modified rate equations of one‐level model together with the lifetime determined by TRPL. In continuation of evaluating NRR parameters by rate equation analysis, the addition of TRPL measurement improves accuracy and approaches the determination of NRR parameters. A successful characterization of NRR centers leads to a proper optimization of IB‐type solar cells (IBSCs).

Spectroscopy of Nonradiative Recombination Levels by Two‐Wavelength Excited Photoluminescence

The intensity of photoluminescence (PL) changes when an additional below‐gap excitation (BGE) light modifies the electronic occupation of one of the crystalline defects acting as nonradiative recombination (NRR) levels and shifts the balance between radiative and NRR rates. The photon energy of above‐gap excitation (AGE) light selects the layer and determines the depth of inspection, whereas that of BGE corresponds to the energy distribution of the NRR levels. The essential advantage of two‐wavelength excited PL (TWEPL) lies on a systematic combination of BGE spectroscopy and AGE spectroscopy for a variety of materials. It provides the noncontacting and nondestructive detection of NRR levels, distributing in a whole wafer as well as in a microscopic volume. Density and the electron and hole capture rates of NRR level 1, Nt1, Cn1, and Cp1, and those of level 2, Nt2 and Cp2, are determined consistently by the TWEPL measurement with the aid of time‐resolved PL. A brief review is given on the principle of the TWEPL, basic model of analysis, and experimental results of detecting NRR levels in GaAs/AlGaAs, InGaAs/GaAs, InAs/GaAs, GaN, InGaN, AlGaN, and GaPN. The method gives us a guiding compass toward the efficiency improvement of electronic devices from which PL is observable.

Spectral Change of E− Band Emission in a GaAs:N δ-Doped Superlattice Due to Below-Gap Excitation and Its Discrimination from Thermal Activation

In this study, we examined the E− band luminescence of a GaAs:N δ-doped superlattice (SL) grown by metal organic vapor phase epitaxy with 0.15% nitrogen (N) using two-wavelength excited photoluminescence. It was observed that the photoluminescence (PL) intensity of the low-energy peak (P2) at 1.38 eV of the E− band was quenched larger compared to the high-energy peak (P1) at 1.41 eV. This was due to the superposition of below-gap excitation (BGE) light of energy 0.95 eV over the above-gap excitation light of energy 1.45 eV on the SL structure at a fixed temperature of 12 K. On the other hand, at higher temperatures, the PL intensity of the high-energy peak P1 was quenched higher compared to the low-energy peak P2 without any addition of the BGE light. We interpreted the experimental results by considering the carrier recombination (CR) model and concluded that the observed PL spectral and intensity change of the E− band emission due to BGE does not result from the thermal activation, but from the optical excitation among the E− band, conduction band, and CR levels in GaAs:N δ-doped structure. We concluded that to achieve high-efficiency intermediate band-type solar cells, it is essential to understand the CR mechanism through CR levels by determining their origin and eliminating them from the material.

Study of Nonradiative Recombination Centers in n-GaN Grown on LT-GaN and AlN Buffer Layer by Below-Gap Excitation

Nonradiative recombination (NRR) centers in n-type GaN samples grown by MOCVD technique on a LT-GaN buffer layer and aAlN buffer layer have been studied by two wavelength excited photoluminescence (TWEPL). The near band-edge photoluminescence (PL) intensity decreases due to the superposition of below-gap excitation (BGE) light of energies 0.93, 1.17 and 1.27 eV over above-gap excitation (AGE) light of energy 4.66 eV. The decrease in PL intensity due to the addition of the BGE has been explained by a two levels recombination model based on SRH statistics. It indicates the presence of a pair of NRR centers in both samples, which are activated by the BGE. The degree of quenching in PL intensity for the sample grown on LT-GaN buffer layer is stronger than the sample grown on AlN buffer layer for all BGE sources. This result implies that the use of the AlN buffer layer is more effective for reducing the NRR centers in n-GaN layers than the LT-GaN buffer layer. The dependence of PL quenching on the AGE density, the BGE density and temperature has been also investigated. The NRR parameters have been quantitatively determined by solving rate equations and fitting the simulated results with the experimental data.

Yellow luminescence band in undoped GaN revealed by two-wavelength excited photoluminescence

The below-gap emission components including yellow luminescence (YL) band of an MOCVD grown undoped GaN have been studied by the two-wavelength-excited photoluminescence (TWEPL). The nature of each emission line has been investigated by using an intermittent below-gap excitation (BGE) light of 1.17 eV on an above-gap excitation (AGE) light of 3.49 eV. The intensity of DAP and the YL decreased while it increased for IOX after irradiation of the BGE. The intensity change in PL after addition of the BGE implies the presence of defect levels in the energy position corresponding to the photon energy of the BGE. Possible recombination models are listed and examined. Only the recombination model in which the YL corresponds to the transition from a shallow donor to a deep state at about 1 eV above the valence band maximum satisfies our experimental result. The possible origin of this defect state is discussed.

Photoluminescence Characterization of Nonradiative Recombination Centers in Light Emitting Materials by Utilizing Below-Gap Excitation: A Mini Review of Two-Wavelength Excited Photoluminescence

We have developed an optical method of detecting and characterizing nonradiative recombination (NRR) centers without electrical contact. The method combines a below-gap excitation (BGE) light with a conventional above-gap excitation light in photoluminescence (PL) measurement, and discriminates the PL intensity change due to switching on and off the BGE. A quantitative analysis of the detected NRR centers became possible by utilizing the saturating tendency of the PL intensity change with increasing the BGE density due to trap filling effect. Some experimental results of AlGaAs, InGaN, and AlGaN quantum wells were shown to allocate the development and present status as well as to exemplify their interpretations.

A direct evidence of allocating yellow luminescence band in undoped GaN by two-wavelength excited photoluminescence

The behavior of below-gap luminescence of undoped GaN grown by MOCVD has been studied by the scheme of two-wavelength-excited photoluminescence. The emission intensity of shallow donor to valence band transition (IOX) increased while intensities of donor-acceptor pair transition and the Yellow Luminescence band (YLB) decreased after the irradiation of a below-gap excitation source of 1.17 eV. The conventional energy schemes and recombination models have been considered to explain our experimental result but only one model in which YLB is the transition of a shallow donor to a deep state placed at ∼1 eV above the valence band maximum satisfies our result. The defect related parameters that give a qualitative insight in the samples have been evaluated by systematically solving the rate equations and fitting the result with the experiment.

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.

Characterization of Cu(In,Ga)Se2 grown by MBE by two-wavelength excited photoluminescence spectroscopy

The properties of the defect level at 0.8eV above the valence band in Cu(In1−x,Gax)Se2 were studied by the two-wavelength excited photoluminescence (PL) method. The two-wavelength excited PL involves two different pumping light sources, that is, a 635nm diode laser used as an above-gap excitation and a 1550nm diode laser whose energy corresponds to the defect level resulting in having a role of charge saturation at the defect level. The intensity of the two-wavelength excited PL was stronger than that without 1550nm laser irradiation for all specimens at room temperature regardless of Ga concentration. The results suggest the 0.8eV defect level acts as recombination center at room temperature.

Nonradiative centers in InAs quantum dots revealed by two-wavelength excited photoluminescence

We studied nonradiative recombination centers in MBE-grown InAs/GaAs quantum dot (QD) structures with photoluminescence (PL) peak energies between 1.12 and 1.29 eV by the scheme of two-wavelength excitation. Temporally chopped below-gap excitation (BGE) light of 0.75 eV was superposed on a CW above-gap excitation light of 1.59 eV on the sample surface, and the resultant PL intensity change due to the BGE (BGE effect) was measured. Observed 25–35% decrease in PL intensity at 80–90 K implies a first discrimination of a pair of nonradiative centers activated at 0.75 eV inside the band gap of the QD region. Due to trap-filling, the BGE effect showed saturation with increasing BGE power. Its temperature dependence below 50 K was different from that of quantum wells, reflecting carrier dynamics peculiar to the QD.

Effect of modulation-doping on luminescence properties of plasma assisted MBE-grown GaN/AlGaN quantum well

In order to improve the crystal quality of GaN-based light emitting devices, photoluminescence (PL) characterization of below-gap states in plasma assisted MBE-grown GaN/AlGaN quantum well (QW) structures has been done by utilizing a below-gap excitation (BGE) light in addition to an above-gap excitation light. The decrease of the band-edge PL intensity due to the addition of the BGE of 1.17 eV indicates the presence of an energy-matched below-gap state in the two-wavelength excited PL. In continuation to our previous efficiency improvement by applying modulation-doping to GaAs/AlGaAs QW's, we focused on several undoped and Si-doped GRN/AlGaN QW's. Experimental results showed that Si modulation-doping reduces the density of below-gap states in the QW region, hence it is promising for increasing internal quantum efficiency of GaN-based QW's.

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.

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

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

Study of MBE-Grown GaN/AlGaN Quantum Well Structures by Two Wavelength Excited Photoluminescence

We report experimental results of two wavelength excited photoluminescence (TWEPL) applied to GaN/Al0.2Ga0.8N multiple quantum well structures grown by plasma assisted molecular beam epitaxy. An increase or a decrease in photoluminescence (PL) indicate the presence of trap centers with dissimilar characteristics for each case. In these samples, a spatial inhomogeneity of trap distribution originating from the growth process was detected due to different change in PL for distinct regions. Also a time-dependent phenomenon in PL was observed: after a continuous mid-term irradiation with UV light (4.12 eV), the amount of change in PL, which is characteristic of TWEPL, became nil. We argue this phenomenon is a complex issue that can be due to migration of native defects or a kind of saturation process involving trap centers.

Absence of nonradiative recombination centers in modulation-doped quantum wells revealed by two-wavelength excited photoluminescence

Absence of nonradiative recombination (NRR) centers inside GaAs wells and at GaAs/Al0.2Ga0.8As hetero-interfaces in a Si modulation-doped GaAs/Al0.2Ga0.8As multiple quantum well (MQW) structure became clear for the first time by an improved two-wavelength excited photoluminescence (PL). The NRR parameters of modulation and uniform-doped MQWs were determined self-consistently by combining the analysis of the PL intensity change due to the below-gap excitation with the internal quantum efficiency and the recombination lifetime. These results showed the superiority of modulation-doping scheme over that of uniform-doping for light emitting devices.

Spectroscopy of non-radiative recombination centers in quantum wells by two-wavelength excited photoluminescence

Nonradiative recombination centers in Si-doped GaAs/Al x Ga 1− x As and GaN/In y Ga 1− y N quantum well (QW) structures were studied by the two-wavelength excited photoluminescence technique. For the former, a pair of trap centers was found in outer Al 0.4Ga 0.6As barrier layers: one of the pair is attributed to be induced by the diffusion of Si atoms from neighboring Al 0.2Ga 0.8As barrier layers. In case of GaN/In y Ga 1− y N, a distribution of traps was detected only inside the GaN layer, which is consistent with previous reports including the yellow luminescent band.

Distribution of below-gap states in undoped GaAs/AlGaAs quantum wells revealed by two-wavelength excited photoluminescence

The spatial distribution of below-gap states in undoped GaAs/AlGaAs quantum-well structures and their effect on the luminescence intensity has been determined by a spectroscopic measurement of two-wavelength excited photoluminescence. Tuning the photon energy of above-gap excitation revealed that these below-gap states were formed neither in the GaAs well layers nor at the GaAs/AlGaAs hetero-interface, but in the AlGaAs barrier layers. The properties of below-gap states in AlGaAs were found to be strongly dependent on growth temperature.

Below-Gap Spectroscopy of Undoped GaAs/AlGaAs Quantum Wells by Two-Wavelength Excited Photoluminescence

By using an improved method of two-wavelength excited photoluminescence, we observed a considerable decrease in the band-to-band photoluminescence intensity of an undoped GaAs/AlGaAs quantum well structure when a below-gap excitation was superposed on an above-gap excitation. This intensity quenching was attributed to the enhanced nonradiative recombination through below-gap states which were activated by the below-gap excitation. By changing the energy of above-gap excitation, it was shown that these below-gap states were formed neither inside GaAs well layers nor at the GaAs/AlGaAs heterointerface, but inside AlGaAs barrier layers. Tuning the photon energy of below-gap excitation revealed that the activation energy of the nonradiative recombination process via these below-gap states is around 1.2 eV. Lowering the above-gap excitation density to a single-photon-counting region enabled us to characterize below-gap states spectroscopically without the need of a high-power tunable laser source for the below-gap excitation.