Carrier Localization Effect Research Articles

Observation of carrier localization in cubic crystalline Ge2Sb2Te5 by field effect measurement

The tunable disorder of vacancies upon annealing is an important character of crystalline phase-change material Ge2Sb2Te5 (GST). A variety of resistance states caused by different degrees of disorder can lead to the development of multilevel memory devices, which could bring a revolution to the memory industry by significantly increasing the storage density and inspiring the neuromorphic computing. This work focuses on the study of disorder-induced carrier localization which could result in multiple resistance levels of crystalline GST. To analyze the effect of carrier localization on multiple resistant levels, the intrinsic field effect (the change in surface conductance with an applied transverse electric field) of crystalline GST was measured, in which GST films were annealed at different temperatures. The field effect measurement is an important complement to conventional transport measurement techniques. The field effect mobility was acquired and showed temperature activation, a hallmark of carrier localization. Based on the relationship between field effect mobility and annealing temperature, we demonstrate that the annealing shifts the mobility edge towards the valence-band edge, delocalizing more carriers. The insight of carrier transport in multilevel crystalline states is of fundamental relevance for the development of multilevel phase change data storage.

Carrier dynamics of two distinct localized centers in 530 nm InGaN green light-emitting diodes

We investigate carrier localization effect and recombination dynamics of 530 nm InGaN green light-emitting diodes (LEDs) using temperature-dependent photoluminescence (TDPL) and time-resolved photoluminescence (TRPL) methods. Two distinct emission peaks located in low-energy and high-energy sides were observed in PL spectrum at 10 K by Gaussian fitting. Both temperature-dependent blue-shift and TRPL decay time decreasing with photon energy of the two peaks indicate the presence of two localized states. Formation of In rich clusters in the InGaN wells was clearly observed by atom probe tomography (APT). Finally, the electroluminescent spectrum points out that both peaks originate from radiative recombination of localized states.

Enhancement of carrier localization effect and internal quantum efficiency through In-rich InGaN quantum dots

InGaN-based light emitting diodes have a problem known as green gap. InGaN quantum dots have been proven to be a kind of promising structure to solve this problem due to its strong carrier localization effect. In this study we fabricated InGaN/GaN quantum dots and conventional InGaN/GaN multiple quantum wells by tuning growth mode and compared their quantum confined stark effect and internal quantum efficiency by means of intensity-dependent as well as temperature-dependent photoluminescence measurements. It was found that the surface morphology transited from two dimensional step flow to three dimensional quantum dots with increasing the well thickness. In addition the quantum confined stark effect was weakened as a result of releasing the compressive strain. Furthermore, the internal quantum efficiency of the quantum-dot structures was four times higher than that of the conventional multiple quantum wells due to the enhancement of carrier localization effect.

Electroluminescence properties of InGaN/GaN multiple quantum well-based LEDs with different indium contents and different well widths

Two InGaN/GaN multiple quantum well (MQW)-based blue light emitting diodes (LEDs) emitting photons at approximately the same wavelength, with different indium contents and well widths, are prepared, and the temperature-dependences of their electroluminescence (EL) spectra at different fixed injection currents are investigated. The results show that, compared with sample B with its lower indium content and larger well width, sample A with its higher indium content and smaller well width, has a stronger carrier localization effect and higher external quantum efficiency (EQE) at the lower fixed currents; however, upon increasing the injection current, both the localization effect and EQE for sample A decrease at a faster rate. The former is mainly attributed to the deeper potential levels due to the larger indium fluctuations originating from the higher indium content, and to the smaller well width-induced stronger carrier quantum-confine effect (QCE); the latter is mainly attributed to the more significant growing in the electron leakage and/or electron overflow originating from the smaller well width and larger lattice mismatch-induced stronger piezoelectric field, and to the more significant reduction in carrier localization effect originating from the smaller well width-induced smaller density of high-energy localized states.

Influence of quantum confined Stark effect and carrier localization effect on modulation bandwidth for GaN-based LEDs

We have fabricated GaN-based light-emitting diodes (LEDs) with different quantum well (QW) thicknesses to investigate the influence of the quantum confined Stark effect (QCSE) and carrier localization effect on the carrier recombination processes under both direct current (DC) and alternating current (AC) biases. At low current density, QCSE dominates the carrier recombination and decreases the radiative recombination rate. With increasing the current density, QCSE will be screened by injected carriers, and both optical power and modulation bandwidth can be increased. When the polarization field is completely compensated, the carrier localization effect starts to dominate. By reducing the influence of the QCSE and carrier localization effect, a high modulation bandwidth of ∼700 MHz was achieved at a low current density of 425 A/cm2 for the LED with 5 nm QW. Our findings will pave an alternative solution for co-optimization of the modulation bandwidth and efficiency for LEDs at a relatively low current density for visible light communications.

Impact of carrier localization on recombination in InGaN quantum wells and the efficiency of nitride light-emitting diodes: Insights from theory and numerical simulations

We examine the effect of carrier localization due to random alloy fluctuations on the radiative and Auger recombination rates in InGaN quantum wells as a function of alloy composition, crystal orientation, carrier density, and temperature. Our results show that alloy fluctuations reduce individual transition matrix elements by the separate localization of electrons and holes, but this effect is overcompensated by the additional transitions enabled by translational symmetry breaking and the resulting lack of momentum conservation. Hence, we find that localization increases both radiative and Auger recombination rates, but that Auger recombination rates increase by one order of magnitude more than radiative rates. Furthermore, we demonstrate that localization has an overall detrimental effect on the efficiency-droop and green-gap problems of InGaN light-emitting diodes.

Effect of Carrier Localization on Electrical Transport and Noise at Individual Grain Boundaries in Monolayer MoS2.

Despite its importance in the large-scale synthesis of transition metal dichalcogenides (TMDC) molecular layers, the generic quantum effects on electrical transport across individual grain boundaries (GBs) in TMDC monolayers remain unclear. Here we demonstrate that strong carrier localization due to the increased density of defects determines both temperature dependence of electrical transport and low-frequency noise at the GBs of chemical vapor deposition (CVD)-grown MoS2 layers. Using field effect devices designed to explore transport across individual GBs, we show that the localization length of electrons in the GB region is ∼30-70% lower than that within the grain, even though the room temperature conductance across the GB, oriented perpendicular to the overall flow of current, may be lower or higher than the intragrain region. Remarkably, we find that the stronger localization is accompanied by nearly 5 orders of magnitude enhancement in the low-frequency noise at the GB region, which increases exponentially when the temperature is reduced. The microscopic framework of electrical transport and noise developed in this paper may be readily extended to other strongly localized two-dimensional systems, including other members of the TMDC family.

Monte Carlo simulations of disorder in ZnSnN2 and the effects on the electronic structure

In multinary compound semiconductors, cation disorder can decisively alter the electronic properties and impact potential applications. $\mathrm{ZnSn}{\mathrm{N}}_{2}$ is a ternary nitride of interest for photovoltaics, which forms in a wurtzite-derived crystal structure. In the ground state, every N anion is coordinated by two Zn and two Sn cations, thereby observing the octet rule locally. Using a motif-based model Hamiltonian, we performed Monte Carlo simulations that provide atomistic representations of $\mathrm{ZnSn}{\mathrm{N}}_{2}$ with varying degrees of cation disorder. Subsequent electronic structure calculations describe the evolution of band gaps, optical properties, and carrier localization effects as a function of the disorder. We find that octet-rule conserving disorder is practically impossible to avoid but perfectly benign, with hardly any effects on the electronic structure. In contrast, a fully random cation distribution would be very detrimental, but fortunately it is energetically highly unfavorable. A degree of disorder that can realistically be expected for nonequilibrium thin-film deposition leads to a moderate band-gap reduction and to moderate carrier localization effects. Comparing the simulated structures with experimental samples grown by sputtering, we find evidence that these samples indeed incorporate a certain degree of octet-rule violating disorder, which is reflected in the x-ray diffraction and in the optical absorption spectra. This study demonstrates that the electronic properties of $\mathrm{ZnSn}{\mathrm{N}}_{2}$ are dominated by changes of the local coordination environments rather than long-range ordering effects.

Carrier localization in strong phase-separated InGaN/GaN multiple-quantum-well dual-wavelength LEDs

Dual-wavelength light-emitting diodes (LEDs) based on InGaN/GaN multiple quantum wells (MQWs) were grown by metal organic chemical vapor deposition. The strong phase separation in the InGaN layer led to high-density, indium-rich quasi-quantum dots (QDs) in the InGaN/GaN MQWs. The photoluminescence (PL) spectra consisted of green and blue emissions around 2.37 and 2.71 eV, which originated from the quasi-QDs and InGaN matrix, respectively. The effect of carrier localization on the two emissions was explored using a combination of energy-dependent time-resolved PL, temperature-dependent PL, and excitation-dependent PL measurements. Experimental results indicated the co-existence of shallow and deep localized states in the sample: the shallow localized states originated from the InGaN matrix, and the deep localized states originated from the phase-separated quasi-QDs. The green emission from the deep localized quasi-QDs showed a higher luminescence intensity and higher thermal stability than the blue emission from the shallow localized InGaN matrix. These results can be explained by our proposed model, which provides a considerable insight into the carrier localization and luminescence mechanism in strong phase-separated InGaN/GaN MQW dual-wavelength LEDs.

Optical Characterization of Quaternary AlInGaN Multiquantum Wells

Quaternary AlInGaN/AlInGaN multiquantum-well (MQW) structures were grown by metal-organic vapour-phase epitaxy (MOVPE). The influence of QW pairs and QW barrier width on the optical properties of the samples is investigated by means of temperature and incident-power dependent photoluminescence (PL) measurements. The PL emission peaks redshift and the PL peak intensities increase as the number of QW pairs increases. On the other side, a blueshift of the PL peak energy and reduction of the PL peak intensity are observed for the sample with thinner barrier width. These results are explained by carrier localization effect. The S-shaped temperature-dependence of the PL peak energies indicates the existence of localized states induced by the potential fluctuations. The estimated degree of localization effect is found to be enhanced for the sample with more QW pairs and thicker barrier layers. Observing the incident-power dependent PL spectra, the estimated amounts of localization effect of the samples are consistent with the incident-power induced blueshifts of PL peak energies and the slope exponents of power-law form of integrated PL intensity.

The influences of sputtered AlN buffer layer on AlInGaN based blue and near‐ultraviolet light emitting diodes

Instead of conventional GaN buffer layer, a sputtered AlN buffer layer is utilized on the patterned sapphire substrate in AlInGaN based 450‐nm blue and 380‐nm near‐ultraviolet light emitting diodes (LEDs). As a result, the full‐width at half maximum (FWHM) of GaN (002) and (102) X‐ray diffraction rocking curve decrease from 220 to 135 arcsec and from 245 to 172 arcsec, respectively. Consequently, peak external quantum efficiency (EQE) of 380‐nm LED is enhanced by 18–66%, while the EQE of the 450‐nm one shows inconspicuous improvement. This phenomenon implies that the radiative recombination efficiency is more sensitive to dislocation density in 380‐nm LED. Fitting results of temperature‐dependent photoluminescence spectra indicate that the carrier localization effect is weakened as indium composition decreases in the active region, which is regarded as one ordinary explanation. Moreover, carrier lifetimes in 450‐ and 380‐nm LEDs are also measured. The shorter carrier lifetime in 380‐nm LED is considered as another key factor.

Analysis of localization effect in blue-violet light emitting InGaN/GaN multiple quantum wells with different well widths**Project supported by the National Key Research and Development Program of China (Grant No. 2016YFB0401801), the National Natural Science Foundation of China (Grant Nos. 61674138, 61674139, 61604145, 61574135, 61574134, 61474142, 61474110, 61377020, and 61376089), Science Challenge Project, China (Grant No. JCKY2016212A503), and One Hundred Person

Four blue-violet light emitting InGaN/GaN multiple quantum well (MQW) structures with different well widths are grown by metal–organic chemical vapor deposition. The carrier localization effect in these samples is investigated mainly by temperature-dependent photoluminescence measurements. It is found that the localization effect is enhanced as the well width increases from 1.8 nm to 3.6 nm in our experiments. The temperature induced PL peak blueshift and linewidth variation increase with increasing well width, implying that a greater amplitude of potential fluctuation as well as more localization states exist in wider wells. In addition, it is noted that the broadening of the PL spectra always occurs mainly on the low-energy side of the PL spectra due to the temperature-induced band-gap shrinkage, while in the case of the widest well, a large extension of the spectral curve also occurs in the high energy sides due to the existence of more shallow localized centers.

Theoretical and experimental analysis of the photoluminescence and photoluminescence excitation spectroscopy spectra of m-plane InGaN/GaN quantum wells

We present a combined theoretical and experimental analysis of the optical properties of m-plane InGaN/GaN quantum wells. The sample was studied by photoluminescence and photoluminescence excitation spectroscopy at low temperature. The spectra show a large Stokes shift between the lowest exciton peak in the excitation spectra and the peak of the photoluminescence spectrum. This behavior is indicative of strong carrier localization effects. These experimental results are complemented by tight-binding calculations, accounting for random alloy fluctuations and Coulomb effects. The theoretical data explain the main features of the experimental spectra. Moreover, by comparison with calculations based on a virtual crystal approximation, the importance of carrier localization effects due to random alloy fluctuations is explicitly shown.

Carrier localization effects in GaAs1−xSbx/GaAs heterostructures

We investigated the structural and optical properties of GaAs1−xSbx/GaAs heterostructures grown by molecular beam epitaxy on GaAs (001) substrates for Sb concentration up to 12% by means of high-resolution X-ray diffraction and photoluminescence. The correlation between our structural and optical analysis revealed that compositional fluctuations induced localized states which trap carriers at low temperature. Under low excitation power, the photoluminescence (PL) spectra are composed of two competing peaks in the temperature range of 30–80 K. The lower energy peak is associated with transitions from localized states in the band-tail of the density of states while the higher energy peak corresponds to transitions from free carriers. A model based on a redistribution process of localized excitons was used to reproduce the S-shape behavior of the temperature dependent PL. Reducing the growth temperature from 500 °C to 420 °C suppressed the S-shape behavior of the PL indicating a reduction in compositional variation.

Carrier Localization Effects in InGaN/GaN Multiple-Quantum-Wells LED Nanowires: Luminescence Quantum Efficiency Improvement and "Negative" Thermal Activation Energy.

Two-dimensional InGaN/GaN multiple-quantum-wells (MQW) LED structure was nanotextured into quasi-one-dimensional nanowires (NWs) with different average diameters with a combination approach of Ni nanoislands as mask + dry etching. Such nanotexturing bring out several appealing effects including deeper localization of carriers and significant improvement in quantum efficiency (e.g., from 4.76% of the planar MQW structure to 12.5% of the 160 nm MQW NWs) of light emission in the whole interested temperature range from 4 K to 300 K. With the aid of localized-state ensemble (LSE) luminescence model, the photoluminescence spectra of the samples are quantitatively interpreted in the entire temperature range. In terms of distinctive temperature dependence of photoluminescence from these samples, a concept of “negative” thermal activation energy is tentatively proposed for the MQW NWs samples. These findings could lead to a deeper insight into the physical nature of localization and luminescence mechanism of excitons in InGaN/GaN nanowires.

Hexamine (HMT) assisted wet chemically synthesized CuO nanostructures with controlled morphology and adjustable optical behavior

The present study reports a simple and cost effective chemical approach for conveniently synthesizing cupric oxide nanostructures (CuO-NSs). To promote confined and stable growth of CuO-NSs, hexamine (HMT) capping molecules were used in the reaction process. Addition of capping molecule could greatly influence the size confinement of CuO-NSs which in turn affects the optical properties. Significant variations in surface morphology were attributed to the presence of hydroxide anion in HMT during synthesis process. Transmission electron microscopy (TEM) results indicated that much larger CuO-NSs are formed without using HMT. XRD analysis revealed that the sample without HMT gives higher crystallite size with less strain. Energy dispersive X-ray analysis with elemental mapping confirms the stoichiometric proportion of CuO-NSs. Micro-Raman and Fourier transform infrared spectroscopy results confirmed the effect of HMT on monoclinic structure. The optical properties of the prepared CuO-NSs were investigated using UV–visible diffuse reflectance spectroscopy and photoluminescence (PL) spectroscopy. Optical band gap energy of CuO-NSs was increased with increasing HMT concentration from half to equimolar ratio. The PL spectra of CuO-NSs revealed the presence of emission bands in violet, blue-green and IR region due to band-gap expansion presence of oxygen vacancies and carrier localization effect in the defect levels. The PL results were in good agreement with TEM observations. Controllable morphology of CuO-NSs with adjustable optical properties has shown excellent applicability in many applications such as catalysts, gas sensors and solar cells.

Time-resolved terahertz spectroscopy of charge carrier dynamics in the chalcogenide glass As_30Se_30Te_40 [Invited

Broadband (1.6–18 THz) terahertz time-domain spectroscopy (THz-TDS) and time-resolved terahertz spectroscopy (TRTS) were performed on a 54 μm thick chalcogenide glass (As30Se30Te40) sample with a two-color laser-induced air plasma THz system in transmission and reflection modes, respectively. Two absorption bands at 2–3 and 5–8 THz were observed. TRTS reveals an ultrafast relaxation process of the photoinduced carrier response, well described by a rate equation model with a finite concentration of mid-bandgap trap states for self-trapped excitons. The photoinduced conductivity can be well described by the Drude–Smith conductivity model with a carrier scattering time of 12–17 fs, and we observe significant carrier localization effects. A fast refractive index change was observed 100 fs before the conductivity reached its maximum, with 2 orders of magnitude larger amplitude than expected for the optically induced THz Kerr effect, indicating that free carriers are responsible for the transient index change.

Temperature dependent carrier localization in AlGaInN near-ultraviolet light-emitting diodes.

We investigate the carrier localization effect in the low-indium ultraviolet AlGaInN light-emitting diodes with a 365 nm peak and a wide yellow luminous band over the visible range. Temperature-dependent electroluminescence spectra (EL) are measured under a wide range of temperature. We found that carrier localization effect relies on the carrier mobility and manifests itself by altering several macroscopic quantities, such as ELs and electrical resistance of the device. Under moderate injection densities, plots of EL peak energy vs. temperatures exhibits S-shapes. At low temperatures, line-width broadening in EL spectra and irregular humps in I-V curves were observed at similar level of injection densities. Both phenomena diminish as temperature increases and eventually disappear at room temperature. All the results stem from carrier localization and following delocalization effect. It suggests that the carrier mobility determine the degree of carrier localization effect - inactive carriers tend to be localized at low temperature but escape at high temperature from bindings of localization centers. As a result, carrier localization is intense only at low temperature for low-indium devices.

Distinctive mapping of strain and quantum size effects using depth-resolved photoluminescence in ZnO nanoneedles

In order to locate the spatially resolved influence of the strain, carrier localization, and quantum size effect (QSE) in tapered ZnO nanoneedles (NNs), the photoluminescence (PL) was measured as a function of the incident laser angle θ from 0∘ (normal to a surface) to 85∘. With increasing θ, the excitation point is spatially restricted along the axis of the NNs and varies from the ZnO buffer/sapphire interface to the tips of the NNs. In this way, we identified a strain-induced blue-shift of 25.3 meV at the ZnO buffer/sapphire interface, which corresponds to a tensile strain of 0.319%. The influence of strain and the concomitant indications of carrier localization decreased as the excitation point moved to a higher location along the NNs with increasing θ whereas the QSE revealed an abrupt blue-shift near the tips of the NNs. Furthermore, time-resolved PL measurement as a function of the excitation angle was used to distinguish the strain effect from the QSE. We observed two spatially competing tendencies: (1) the decay times are influenced by the increase in the interfacial strain and (2) the decay times are influenced by the decrease in the diameter-dependent QSE near the tips of the tapered ZnO NNs.

Charge carrier localization effects on the quantum efficiency and operating temperature range of InAsxP1−x/InP quantum well detectors

The effect of charge carrier localization resulting in “S-shaped” temperature dependence of the photoluminescence peak energy of InAsxP1−x/InP quantum wells (QWs) is distinctly revealed by the temperature dependent surface photo voltage (SPV) and photoconductivity (PC) processes. It is observed that the escape efficiency of carriers from QWs depends on the localization energy, where the carriers are unable to contribute in SPV/PC signal below a critical temperature. Below the critical temperature, carriers are strongly trapped in the localized states and are therefore unable to escape from the QW. Further, the critical temperature increases with the magnitude of localization energy of carriers. Carrier localization thus plays a pivotal role in defining the operating temperature range of InAsxP1−x/InP QW detectors.