Concentration Of Nonradiative Recombination Centers Research Articles

Dislocation Filter Based on LT-GaAs Layers for Monolithic GaAs/Si Integration.

The use of low-temperature (LT) GaAs layers as dislocation filters in GaAs/Si heterostructures (HSs) was investigated in this study. The effects of intermediate LT-GaAs layers and of the post-growth and cyclic in situ annealing on the structural properties of GaAs/LT-GaAs/GaAs/Si(001) HSs were studied. It was found that the introduction of LT-GaAs layers, in combination with post-growth cyclic annealing, reduced the threading dislocation density down to 5 × 106 cm-2, the root-mean-square roughness of the GaAs surface down to 1.1 nm, and the concentration of non-radiative recombination centers in the near-surface GaAs/Si regions down to the homoepitaxial GaAs level. Possible reasons for the improvement in the quality of near-surface GaAs layers are discussed. On the one hand, the presence of elastic deformations in the GaAs/LT-GaAs system led to dislocation line bending. On the other hand, gallium vacancies, formed in the LT-GaAs layers, diffused into the overlying GaAs layers and led to an increase in the dislocation glide rate. It was demonstrated that the GaAs/Si HSs obtained with these techniques are suitable for growing high-quality light-emitting HSs with self-assembled quantum dots.

Room-temperature nonradiative recombination lifetimes in c-plane Al1−xInxN epilayers nearly and modestly lattice-matched to GaN (0.11 ≤ x ≤ 0.21)

Lattice-matched Al1−xInxN / GaN heterostructures with InN mole fraction (x) of 0.18 have attracted considerable interest for use in GaN-based optoelectronic devices. Because the light emission efficiency (ηemission) of Al1−xInxN alloys is far less than that of InxGa1−xN, understanding its causes is essential. For this purpose, room-temperature photoluminescence lifetime (τPLRT), which almost represents the nonradiative recombination lifetime that limits the internal quantum efficiency in low ηemission semiconductors, of c-plane Al1−xInxN epilayers nearly and modestly lattice-matched to GaN (0.11≤x≤0.21) was examined. For the epilayers grown on low threading dislocation density (TDD) GaN substrates (≪107cm−2), τPLRT principally decreased with increasing x, indicating a progressive increase in the concentration of nonradiative recombination centers (NRCs), NNRC. One of the probable causes is the growth temperature (Tg) reduction that is indispensable to incorporate more In, because in insufficient Tg regime higher Tg is preferred for enhancing the surface migration of adatoms to decrease the concentrations of vacancies that compose NRCs. The Al1−xInxN epilayers of the same x but grown on high TDD (>108cm−2) GaN-on-sapphire templates exhibited shorter τPLRT. Because the diffusion length of minority carriers was nearly zero in the Al1−xInxN epilayers, the shorter τPLRT indicates higher bulk NNRC in high TDD epilayers. The Al1−xInxN epilayers of considerably rough surface morphologies exhibited spatially inhomogeneous τPLRT, implying that excited carriers recombined everywhere at InN-rich to InN-poor portions, where NNRC were likely lower to higher, respectively, than the average due to the deviations in the surface stoichiometry at various non-c-plane surfaces at a given Tg.

Impact of a defect trapping layer on the reliability of 1.3 μm quantum dot laser diodes grown on silicon

In this paper, we studied the impact of a defect trapping layer on the degradation kinetics of 1.3 μm quantum dot laser diodes (QD LDs) for silicon photonics (SiPh). The purpose of the analysis is to prove how the inclusion of trapping layers (TLs) for misfit dislocations can improve the performance and reliability of QD LDs. To this aim we stressed two kinds of devices, that only differ from the inclusion of TLs. By submitting the samples to current step-stress, we tested the devices from mild to highly accelerated conditions. Our results demonstrate that a) the addition of TLs shifts the roll-off of the optical characteristics to higher current densities; b) in devices with TLs, excited state (ES) emission is reached at higher current densities, compared to standard devices; c) in terms of reliability, the addition of TLs results in a slower degradation of the threshold current, to a lower concentration of misfit dislocations in the active region; d) degradation is ascribed to the lowering of the injection efficiency, driven by the escape of carriers out of the active region, which causes an increase in the concentration of non-radiative recombination centers.

Free-standing large, ultrathin germanium selenide van der Waals ribbons by combined vapor-liquid-solid growth and edge attachment.

Among group IV monochalcogenides, layered GeSe is of interest for its anisotropic properties, 1.3 eV direct band gap, ferroelectricity, high mobility, and excellent environmental stability. Electronic, optoelectronic and photovoltaic applications depend on the development of synthesis approaches that yield large quantities of crystalline flakes with controllable size and thickness. Here, we demonstrate the growth of single-crystalline GeSe nanoribbons by a vapor-liquid-solid process over Au catalyst on different substrates at low thermal budget. The nanoribbons crystallize in a layered structure, with ribbon axis along the armchair direction of the van der Waals layers. The ribbon morphology is determined by catalyst driven fast longitudinal growth accompanied by lateral expansion via edge-specific incorporation into the basal planes. This combined growth mechanism enables temperature controlled realization of ribbons with typical widths of up to 30 μm and lengths exceeding 100 μm, while maintaining sub-50 nm thickness. Nanoscale cathodoluminescence spectroscopy on individual GeSe nanoribbons demonstrates intense temperature-dependent band-edge emission up to room temperature, with fundamental bandgap and temperature coefficient of Eg(0) = 1.29 eV and α = 3.0 × 10-4 eV K-1, respectively, confirming high quality GeSe with low concentration of non-radiative recombination centers promising for optoelectronic applications including light emitters, photodetectors, and solar cells.

Improved minority carrier lifetime in p-type GaN segments prepared by vacancy-guided redistribution of Mg

To accelerate the development of GaN power devices, reproducible fabrication of p-type GaN (p-GaN) segments by ion-implantation (I/I) that enables selective-area doping is preferred. In this Letter, the results of time-resolved photoluminescence (PL) and spatially resolved cathodoluminescence (CL) measurements on p-GaN fabricated by I/I are presented. The p-GaN samples of a 220-nm-deep box-shaped Mg concentration profile ([Mg] = 3×1018 cm−3) were fabricated by sequential I/I of Mg and N followed by atmospheric-pressure post-implantation annealing (PIA). During PIA, Mg redistribution was guided by N-I/I-induced vacancies. The intensities of PL peaks originating from Mg acceptors were increased by increasing the PIA temperature (Ta), indicating progressive activation of Mg and simultaneous decrease in the concentration of nonradiative recombination centers (NRCs). As a result, a record-long room-temperature PL lifetime for the near-band edge emission of Ga-polar Mg-implanted p-GaN (1.4 ps) was obtained when Ta = 1300 °C. Because the species of major vacancy-type defects in the present sample was determined as vacancy clusters comprised of few Ga-vacancies (VGa) and N-vacancies (VN) such as (VGaVN)2–3, the NRC concentration is roughly estimated at around 1017 cm−3 provided that (VGaVN)2–3 are major NRCs with the electron-capture-coefficient of 7×10−6 cm3 s−1. Cross-sectional CL linescan images revealed certain activation of Mg from the surface to the leading edge of the box-shaped [Mg] profile, while NRCs remained at the depths right below the box-shaped profile. The results support advantages of using the vacancy-guided Mg redistribution technique for controlling the activated [Mg] profile and realizing p-type Ga-polar GaN:Mg segments by I/I.

Reduced nonradiative recombination rates in c-plane Al0.83In0.17N films grown on a nearly lattice-matched GaN substrate by metalorganic vapor phase epitaxy

A record-long room-temperature photoluminescence (PL) lifetime (τPLRT) of approximately 70 ps was obtained for the sub-bandgap 3.7 eV emission band of a 300-nm-thick c-plane Al0.83In0.17N epilayer for the use in cladding layers of an edge laser structure, which were grown by metalorganic vapor phase epitaxy on a low threading dislocation density nearly lattice-matched GaN substrate. The recorded τPLRT value was twice as long as previously reported ones, indicating half concentration of nonradiative recombination centers. Room-temperature spatially resolved cathodoluminescence intensity images for the 3.7 eV band revealed nearly zero carrier diffusion length, which is consistent with the fact that τPLRT of 70 ps is 1/35 of the near-band-edge emission of the GaN substrate (2.4 ns). As the PL decay curves for the 3.7 eV band were sufficiently fitted by the stretched exponential function, the emission likely originates from extended states such as impurities, point defects, and their complexes, as well as localized states of uneven potential profile.

Creation and Annihilation of Nonradiative Recombination Centers in Polycrystalline Metal Halide Perovskites by Alternating Electric Field and Light

AbstractMetal halide perovskites are promising optoelectronic materials. Their electronic properties however are rather unstable which is often assigned to ion migration. Ion migration can be readily influenced by an electric field (EF). Here, the response of photoluminescence (PL) of individual MAPbX3 (MA = CH3NH3, X = I, Br) sub‐micrometer‐sized polycrystals to EF is studied. Alternating EF with frequency higher than 10 Hz is found to reversibly quench PL. It is proposed that an alternating EF when applied together with light increases ion migration. This leads to a shift in the equilibrium between creation and annihilation of defects toward higher concentration of nonradiative recombination centers. The PL quenching is found to increase with increasing frequency of the field. This can be rationalized by the frequency dependence of the dielectric constant, leading to stronger internal fields for high modulation frequencies compared to, e.g., a constant EF with the same external amplitude. PL quenching and enhancement observed under constant EF are hypothesized to be due to a reconfiguration of already existing nonradiative recombination centers situated on grain boundaries. The control of perovskite PL by alternating EF reported here can find applications in optoelectronic devices.

Effects of selenisation temperature on photoluminescence and photoluminescence excitation spectra of ZnO/CdS/Cu2ZnSnSe4/Mo/glass

The effect of solar cell processing (including etching in KCN along with deposition of CdS and ZnO) on photoluminescence (PL) spectra and bandgap Eg (measured at 4.2 K by photoluminescence excitation) of Cu2ZnSnSe4 films, produced by selenising metallic precursors at 450 °C, 500 °C and 550 °C, was studied. Temperature and excitation intensity analysis of the P1 dominant band in the PL spectra of solar cells suggests that after processing this band still can be assigned to the free-to-bound recombination of free electrons with holes bound at deep acceptor levels influenced by valence band-tails. However processing increased the intensity of P1 and blue shifted it. The strongest effect was observed for the film selenised at 500 °C. For the film selenised at 450 °C the blue shift and increase in the intensity were smaller and only a slight intensity rise was found for the film selenised at 550 °C. The intensity increase we assign to a reduction in the concentration of non-radiative recombination centers on the surface because of the etching and changes in doping due to inter-diffusion of Cd, S, Se and Zn after the deposition of CdS. Such an inter-diffusion depends on the elemental composition of the films defining the chemistry of defects and influencing Eg which increased in the film selenised at 500 °C but decreased in the other films. Processing increased the P1 shift rate (j-shift) with excitation power change in all the films demonstrating a higher compensation degree in the solar cells which is consistent with the formation of an interface layer containing new donors CdCu.

Luminescence and Stimulated Emission of Polycrystalline Cu(In,Ga)Se2 Films Deposited by Magnetron-Assisted Sputtering

The stimulated emission of Cu(In,Ga)Se2 alloy thin films formed by magnetron-assisted sputtering onto a sodium-fluoride layer deposited onto a molybdenum layer on a glass substrate is observed. The structural and optical parameters of the films are determined by scanning electron microscopy, local X-ray spectral microanalysis, X-ray structural analysis, and low-temperature luminescence (T = 10 K) measurements in the range of excitation levels of 1.6–75 kW cm–2 provided by nanosecond nitrogen-laser pulses. The stimulated emission threshold corresponds to ~25 kW cm–2. Comparative analysis of the emission of Cu(In,Ga)Se2 thin films suggests that the introduction of sodium results in significant improvement of the structural quality, specifically, in a decrease in the density of energy states in the band tails and in a decrease in the concentration of nonradiative-recombination centers.

Influence of a Low-Temperature GaAs Dislocation Filter on the Perfection of GaAs/Si Layers

The influence of dislocation filters based on low-temperature layers (LT) of GaAs and postgrowth annealing on the perfection of GaAs/Si heterostructures is discussed. It is shown that LT-GaAs layers reduce the density of threading dislocations and surface roughness. Post-growth annealing at a temperature of 650 °C reduces the concentration of nonradiative recombination centers in GaAs/Si layers to a level close to the level in GaAs layers grown on a matched substrate.

Люминесценция и стимулированное излучение поликристаллических пленок Cu(In,Ga)Se-=SUB=-2-=/SUB=-, осажденных методом магнетотронного напыления

AbstractThe stimulated emission of Cu(In,Ga)Se_2 alloy thin films formed by magnetron-assisted sputtering onto a sodium-fluoride layer deposited onto a molybdenum layer on a glass substrate is observed. The structural and optical parameters of the films are determined by scanning electron microscopy, local X-ray spectral microanalysis, X-ray structural analysis, and low-temperature luminescence ( T = 10 K) measurements in the range of excitation levels of 1.6–75 kW cm^–2 provided by nanosecond nitrogen-laser pulses. The stimulated emission threshold corresponds to ~25 kW cm^–2. Comparative analysis of the emission of Cu(In,Ga)Se_2 thin films suggests that the introduction of sodium results in significant improvement of the structural quality, specifically, in a decrease in the density of energy states in the band tails and in a decrease in the concentration of nonradiative-recombination centers.

Effect of Nitrogen Flux on the Properties of N-Doped Zn<sub>1-x</sub>Mg<sub>x</sub>O Thin Films

Different Mg content in Zn1-xMgxO was also obtained by varying N2 flux. UV emission peak in photoluminescence (PL) spectra shift from 350 nm to 360 nm, and then to 352 nm, which is consistent with the result of absorption spectra of the Zn1-xMgxO:N films grown at different N2 flux. The emission intensity of the Zn1-xMgxO:N band edge peak was drastically reduced with increasing N2 flux, which was attributed to the concentration of nonradiative recombination centers increased with increasing N2 flux. The carrier concentration of N-doped Zn1-xMgxO increased with increasing N2 flux below 0.8 sccm, while those above 0.8 sccm decreased. The p-type Zn1-xMgxO:N with the maximum hole concentration 3.97×1017 cm-3 was obtained by optimizing growth conditions.

Defect-Resistant Radiative Performance of m-Plane Immiscible Al1-x Inx N Epitaxial Nanostructures for Deep-Ultraviolet and Visible Polarized Light Emitters.

Planar vacuum-fluorescent-display devices emitting polarized UV-C, blue, and green light are demonstrated using immiscible Al1-x Inx N nanostructures grown in nonpolar m-directions. Despite the presence of high concentration of nonradiative recombination centers, the Al1-x Inx N nanostructures emit polarized light with the luminescence lifetimes of 22-32 ps at 300 K. This defect-resistant radiative performance suggests supernormal localized characteristics of electron-hole pairs.

Reduction in the concentration of cation vacancies by proper Si-doping in the well layers of high AlN mole fraction AlxGa1–xN multiple quantum wells grown by metalorganic vapor phase epitaxy

Appropriate-amount Si-doping in the well layers significantly improved the luminescence efficiency of Al0.68Ga0.32N/Al0.77Ga0.23N multiple quantum wells. To understand the mechanisms, spatio-time-resolved cathodoluminescence measurements and self-consistent Schrödinger-Poisson calculations were carried out. The increase in the luminescence lifetime at room temperature, which reflects the decrease in the concentration of nonradiative recombination centers (NRCs), was correlated with increased terrace width of Si-doped wells. The results suggest the importance of H3SiNH2 doping-reactant formation that gives rise to enhanced decomposition of NH3 and provides wetting conditions by surface Si-N bonds, which reduce the total energy and concentration of NRCs composed of cation vacancies.

Low-resistivity m-plane freestanding GaN substrate with very low point-defect concentrations grown by hydride vapor phase epitaxy on a GaN seed crystal synthesized by the ammonothermal method

An m-plane freestanding GaN substrate satisfying both low resistivity (ρ = 8.5 × 10−3 Ω·cm) and a low point-defect concentration, being applicable to vertically conducting power-switching devices, was grown by hydride vapor phase epitaxy on a nearly bowing-free bulk GaN seed wafer synthesized by the ammonothermal method in supercritical ammonia using an acidic mineralizer. Its threading dislocation and basal-plane staking-fault densities were approximately 104 cm−2 and lower than 100 cm−1, respectively. A record-long fast-component photoluminescence lifetime of 2.07 ns at room temperature was obtained for the near-band-edge emission, reflecting a significantly low concentration of nonradiative recombination centers composed of Ga vacancies.

Temperature evolution of carrier dynamics in GaNxPyAs1−y−xalloys

The temperature dependence of carrier dynamics in GaNxAs1−yPy alloys has been investigated by time resolved photoluminescence. This investigation has shown that the decay time constant does not change significantly up to 100 K, and then starts to decrease rapidly above this temperature. Additionally, the decay times at the high-energy side of the spectrum decrease faster than those at the low-energy side. The effects have been explained by the interplay between carrier capture by radiative and nonradiative recombination centers. Detailed simulations show that the effect of carrier localization in the investigated materials is better described by double-scale potential fluctuations that are related to (i) distribution of localized states energy and (ii) bandgap fluctuations. In addition, it was observed that the increase in nitrogen concentration leads to a shorter decay time at room temperature, which is attributed to a larger concentration of non-radiative recombination centers. Furthermore, a post-growth annealing step leads to a longer decay time at room temperature, which is attributed to a reduction in non-radiative recombination centers. At low temperatures, the role of non-radiative centers is suppressed, and therefore the decay time does not differ significantly for samples with either different nitrogen concentrations or in both the as-grown and annealed samples.

Photoluminescence characterization of InGaN/InGaN quantum wells grown by plasma‐assisted molecular beam epitaxy: Impact of nitrogen and galium fluxes

The impact of Ga and N fluxes on optical quality of InGaN multiple quantum wells grown by plasma assisted molecular beam epitaxy is studies by means of photoluminescence (PL) and time resolved photoluminescence (TRPL) technique. This investigation reveals that both PL spectrum intensity and broadening are affected by Ga and N fluxes but the optimal growth conditions for the smallest broadening and highest intensity can be different. These effects are explained by the changes in the concentration of nonradiative recombination centres. The experimental results are supported by simulation via hopping excitons model. The theoretical studies of impact of quantum wells inhomogeneity and concentration of nonradiative centres on photoluminescence intensity, decay times and quenching are presented and discussed. Both experimental and theoretical results show that nonradiative processes are present also at low temperature and the recombination efficiency can be lower than 100% even at low temperatures.

Structural and Luminescent Properties of (Y,Cu)-Codoped Zirconia Nanopowders

The effect of both CuO content (1 and 8 mol%) and calcination temperature (Tc = 500–1000°C) on light emitting and structural properties of ZrO2-Y2O3-CuO nanopowders was studied by photoluminescence (PL), X-ray diffraction (XRD) and Raman scattering methods. The samples, calcinated at Tc = 500–700°C, contain the tetragonal ZrO2 phase predominantly. The increase of Tc results in its transformation to monoclinic one, and the phase transformation happens at higher temperature at higher Cu content. It is shown that the increase of Tc from 500 to 800°C results in the Cu incorporation into nanocrystals from the surface entities. This manifests itself in the shift of XRD and Raman peaks of tetragonal phase. The PL spectra demonstrate the presence of two main PL components peaked at about 540 nm and 630 nm in the most samples. The increase of Tc from 500 to 1000°C results in the non-monotonic variation of total PL intensity as well as in minor changes in PL spectra shape. The evolution of PL intensity is explained by both the increase of a crystallinity degree and the variation of the concentration of non-radiative recombination centres. The transformation of PL spectra shape is ascribed to the appearance of radiative CuZr centres.

Influence of metal organic chemical vapour deposition growth conditions on vibrational and luminescent properties of ZnO nanorods

A detailed optical characterization by means of micro Raman and cathodoluminescence spectroscopy of catalyst-free ZnO nanorods grown by atmospheric-metal organic chemical vapour deposition has been carried out. This characterization has allowed correlating the growth conditions, in particular the precursors partial-pressures and growth time, with the optical properties of nanorods. It has been shown that a high Zn supersaturation can favor the incorporation of nonradiative recombination centers, which can tentatively be associated with ZnI-related defects. Characterization of individual nanorods has evidenced that ZnI-related defects have a tendency to accumulate in the tip part of the nanorods, which present dark cathodoluminescence contrast with respect to the nanorods bottom. The effect of a ZnO buffer layer on the properties of the nanorods has been also investigated, showing that the buffer layer improves the luminescence efficiency of the ZnO nanorods, revealing a significant reduction of the concentration of nonradiative recombination centers.

Some aspects of emission variation in InAs quantum dots coupled with symmetric quantum wells

The photoluminescence (PL), its temperature and power dependences, as well as PL inhomogeneity and x ray diffraction (XRD) has been studied in the symmetric In0.15Ga1−0.15As/GaAs quantum wells with embedded InAs quantum dots (QDs) (dot-in-a-well, DWELL) with different QD densities, obtained by the variation in QD growth temperatures. It is shown that four reasons are responsible for the difference in emission intensities, PL peak positions and PL inhomogeneity in studied QD structures: (i) the high concentration of nonradiative (NR) recombination centers in the capping In0.15Ga1−0.15As layer at low QD growth temperatures (470 °C), (ii) the QD density and size distributions for the structures with QD grown at 490–535 °C, (iii) the high concentration of NR recombination centers in the GaAs barrier at high QD growth temperatures (535 °C), and (iv) the variation nonmonotonous of elastic strain versus QD density. XRD study confirms that with decreasing density of QDs in DWELLs from 1.1×1011 cm−2 down to 1.3×1010 cm−2 at the rise of QD growth temperatures the level of compressive strain in DWELLs varies nonmonotonously. The reasons of compressive strain variation and the impact of this variation on emission parameters of DWELLs have been discussed as well.