Carrier Escape Research Articles

Structural, Optical and Electrical Properties of Self-Assembled InAs Quantum Dots Based p–i–n Devices Grown on GaAs Substrate by Molecular Beam Epitaxy for Telecommunication Applications

This work aims to investigate the structural, electrical, and optical properties of InAs quantum dots (QDs) grown by Molecular Beam Epitaxy on GaAs substrates. As-made samples were thoroughly characterized using different techniques, including Atomic Force Microscopy (AFM), X-ray diffraction (XRD), and highresolution X-ray diffraction (HRXRD). The patterns of HRXRD revealed an excellent crystallinity of the nanostructure with a maximum diameter of 25 nm as demonstrated by AFM images. The photoluminescence (PL) spectra showed two distinct bands centered at 835 and 1210 nm, and the intensity of these wavelengths increased with decreasing temperature. A redshift accompanied by a decrease in the FWHM as a function of temperature was observed as a consequence of the thermal escape of carriers. The Ideality factor (n), built-in potential energy, and series resistance at different temperatures were also determined from current-voltage characteristics curves.

A qualitative analysis of the impact of P composition in GaAs1−xPx capped InAs/GaAs quantum dot hetero-structure

In this paper, a virtual GaAs1−xPx is used as a capping layer to InAs/GaAs quantum dot system. The strain profile, photoluminescence peak and rate of carrier escape out of the quantum dot inside the proposed structure are thoroughly investigated. The fractional composition of P in GaAs1−xPxis varied from 0.1 to 0.6 and a corresponding shifting of photoluminescence peak from 1176 ​nm to 1137 ​nm is observed. The thermal emission rate equal to 1.599 x 1010 ​s−1 and 6.437 1 x 108 ​s−1 for electron and hole respectively are observed for the proposed structure. A two-dimensional strain matrix evaluated for the mentioned range of P composition exhibits smoother normalization of hydrostatic strain inside the capping layer as compared to GaAs0.86Sb0.14 capped device. Moreover, the effect of capping layer thickness on the strain profile, photoluminescence peak and carrier escape rate are investigated. The results of this analysis concludes that GaAs1−xPx for x = 0.1 is a useful candidate for the capping layer in quantum dot photodetector application.

Quaternary – alloyed capping for strain and band engineering in InAs sub – monolayer quantum dots

In the last few years, a considerable amount of attention has been given to the optimization of several capping layers (CLs) in InAs/GaAs sub – monolayer (SML) quantum dots (QDs) heterostructure for a sustainable room – temperature (RT) operation in 1.3–1.55 μm telecommunication range (O, C – bands). In particular, the quaternary – alloyed capping (Q – CL: Q – Sb: InxGa1−xAsySb1−y, Q – N: InxGa1−xAsyN1−y, and Q – SbN: GaAsxSbyN1−x−y) has been explored in this work to reach the above – stated photoluminescence (PL) emission wavelength and we compared it with the conventional InAs/InxGa1−xAs QDs using Nextnano software. This tool is based on 8 – band k.p model that solves the self – consistent Schrödinger – Poisson equations analytically to obtain the envelope functions of the eigen energy levels (both electrons and holes) and finally the absorption spectra. Although Q – CL help in preserving QD morphology by strain reduction, it also allowed us create deep carrier confinement potential near conduction and valence band offsets i.e. CBO (VBO) where there is a minimum carrier escape probability towards GaAs barriers (more thermal stability) thus fostering a prolonged PL emission wavelength at 300 K. By changing atomic constituent's composition in Q – CLs, the obtained ground – state (GS) band – to – band absorption energy are 1.31 (Q – Sb22%, 1.25 (Q – N1.8%), 1.37 μm (Q – SbN2.2%))). Such deep confinement aid in the dark current reduction and can promote for high temperature operation by improving the PL quantum yield by several orders of magnitude. The inherent material properties of CL composing the surfactant effect of Sb, larger bond strength of tensile – strained dil.N, and phase separation effects of Sb–N alloys are advantageous in this regard to corroborate further PL redshift.

Electronic and Optical Properties of InAs QDs Grown by MBE on InGaAs Metamorphic Buffer.

We present the optical characterization of GaAs-based InAs quantum dots (QDs) grown by molecular beam epitaxy on a digitally alloyed InGaAs metamorphic buffer layer (MBL) with gradual composition ensuring a redshift of the QD emission up to the second telecom window. Based on the photoluminescence (PL) measurements and numerical calculations, we analyzed the factors influencing the energies of optical transitions in QDs, among which the QD height seems to be dominating. In addition, polarization anisotropy of the QD emission was observed, which is a fingerprint of significant valence states mixing enhanced by the QD confinement potential asymmetry, driven by the decreased strain with increasing In content in the MBL. The barrier-related transitions were probed by photoreflectance, which combined with photoluminescence data and the PL temperature dependence, allowed for the determination of the carrier activation energies and the main channels of carrier loss, identified as the carrier escape to the MBL barrier. Eventually, the zero-dimensional character of the emission was confirmed by detecting the photoluminescence from single QDs with identified features of the confined neutral exciton and biexciton complexes via the excitation power and polarization dependences.

Frequency response and carrier escape time of InGaAs quantum well-dots photodiode

p-i-n photodiodes comprising dense arrays of InGaAs quantum dots (referred to as quantum well-dots) were fabricated, and the basic physical processes affecting their high-speed performance were studied for the first time by measuring the frequency response under illumination with photons absorbed either in the quantum well-dots (905-nm illumination) or mainly in GaAs layers (860-nm illumination). A GaAs p-i-n photodiode of similar design was also measured for comparison. A maximum −3 dB bandwidth of 8.2 GHz was measured for the 905-nm light illumination, and maximum internal −3 dB bandwidth of 12.5 GHz was estimated taking into account the effect of RC-parasitic by the equivalent circuit model. It was found that the internal response is mainly controlled by the carrier drift time in the depletion region; this process can be characterized by a field-dependent effective velocity of charge carriers in the layered heterostructure, which is approximately half the saturation velocity in GaAs. The carrier escape from the InGaAs quantum well-dots was found to has less effect; the escape time was estimated to be 12–17 ps depending on the reverse-bias voltage applied.

Degradation mechanisms of 1.3 μm C-doped quantum dot lasers grown on native substrate

The aim of this paper is to analyse the degradation modes of 1.3 μm InAs quantum dot laser diodes (QD LDs) grown by using carbon as a p-type dopant, as an alternative to beryllium. The devices were stressed at high current densities, to favor the onset of excited-state (ES) emission, and to study the related degradation phenomena. The study of QD LDs is of fundamental importance for the photonic integrated circuits (PICs). The investigation is based on two types of stress tests: 1) a current step stress and 2) a constant current stress. With these experiments we demonstrate that a) the current is a determining factor for the onset of the ES during the operation of such devices. b) the onset of ES is responsible for a rapid quenching of the ground state (GS) emission. c) stress induces a significant increase in the threshold current of the devices, that is ascribed to the lowering of the injection efficiency, which in turn can be explained by the easier escape of carriers from quantum dots (QDs) from ES energy levels. d) the contribution of defects on the optical and electrical degradation is also discussed.

Correlation between photoluminescence and electroluminescence in GaN-related micro light emitting diodes: Effects of leakage current, applied bias, incident light absorption and carrier escape

Accurately predicting the peak wavelength and intensity of the electroluminescence spectrum through the photoluminescence method which can be measured without connecting electrodes is not only practically important for the development of micro-LEDs that cannot be fully inspected for device development, but there is also a fundamental importance in understanding the light emitting mechanism of the LEDs. In this study, the correlation and detailed mechanism between PL and EL are systematically analyzed by considering the effects of carrier escape, carrier accumulation, applied bias, leakage current, incident light wavelength and light absorption in order to provide a clear path for all-optical inspection of light emitting diodes and complete understanding of the luminescence mechanism in GaN-related quantum wells. • To correlate the photoluminescence to the electroluminescence in GaN-related micro LEDs, detailed mechanism of PL and EL are systematically analyzed. • Direct experimental results on the sequential depletion process and on the wavefunction overlap in an individual quantum well were obtained. • In order to correlate the PL with EL, matching the concentration of carriers participating in recombination in both measurements is important. • Effect of carrier leakage plays an important role in EL in micro-LEDs which operates just above the turn on voltages. • Simultaneous excitation of 325 nm and 405 nm lasers can provide most similar carrier distribution as in EL by injecting carriers both in QWs and p-GaN.

Effect of indium content and carrier distribution on the efficiency and reliability of InGaN/GaN-based multi quantum well light emitting diode

This paper investigates the mechanisms that limit the efficiency and the reliability of InGaN-based LEDs, by presenting an analysis carried out on devices having two quantum wells that emit at different wavelengths (405 nm and 495 nm). A color-coded test structure was designed in order to easily quantify the optical characteristics of both quantum wells, and to describe in detail the stress dynamics as a function of In-content and of the position of the quantum wells.The attention was focused on: (i) electrical degradation induced by constant current stress at high temperature, that was found to be related to a diffusion process; (ii) degradation of the optical parameters, which was dominantly ascribed to an increase in SRH recombination and to a decreased carrier injection efficiency; (iii) effect of indium content and carrier distribution on the efficiency and reliability of the devices. The results strongly suggest that the internal quantum efficiency is limited by a low injection efficiency and by the quantum confinement Stark effect. In addition, the results on optical degradation highlight the presence of different mechanisms that contribute to the degradation: the role of Shockley-Read-Hall recombination, carrier escape and Auger processes are discussed.

Photoluminescence of InGaN-based red multiple quantum wells.

Optical properties of InGaN-based red LED structure, with a blue pre-well, are reported. Two emission peaks located at 445.1 nm (PB) and 617.9 nm (PR) are observed in the PL spectrum, which are induced by a low-In-content blue InGaN single quantum well (SQW) and the red InGaN double quantum wells (DQWs), respectively. The peak shift of PB with increase of excitation energy is very small, which reflects the built-in electric field of PB-related InGaN single QW is remarkably decreased, being attributed to the significant reduction of residual stress in the LED structure. On the other hand, the PR peak showed a larger shift with increase of excitation energy, due to both the screening of built-in electric field and the band filling effect. The electric field in the red wells is caused by the large lattice mismatch between high-In-content red-emitting InGaN and surrounding GaN. In addition, the anomalous temperature dependences of the PR peak are well elucidated by assuming that the red emission comes from quasi-QD structures with deep localized states. The deep localization suppresses efficiently the escape of carriers and then enhances the emission in the red, leading to high internal quantum efficiency (IQE) of 24.03%.

Effects of thermal emission and re-trapping of photo-injected carriers on the optical transitions of InAs/GaAs quantum dots

Photoluminescence (PL) measurements are presented for self-assembled InAs/GaAs quantum dots (QDs). Under a very low excitation density, the 8K-PL spectrum of the QD sample shows two well-defined sub-bands. The PL sub-bands are clarified as emissions from the ground state (GS) and the first excited state (FES) of the dots. Different from that of the FES transition, a sigmoidal temperature-dependent variation is observed from the integrated PL intensity (IPLI) of the GS transition. This behavior is related to the carrier exchange between the excited states and the GS of the dots. A simple rate equation model, which takes into account the effects of the thermal escape and re-trapping of photo-injected carriers, is proposed to describe the sigmoidal variation of the IPLI. A good agreement between the model simulation and the experimental results is obtained and therefore supports the argument for the carrier exchange between the excited states and the ground states.

Defect effect on the performance of nonpolar GaN-based ultraviolet photodetectors

The anisotropy of GaN(11-20) makes it possible to fabricate polarized ultraviolet (UV) photodetectors (PDs) for applications in fields such as remote sensing and airborne astronomical navigation. The defect density has a significant effect on the performance of GaN(11-20)-based UV PDs. However, the mechanism through which different defects and their densities affect the performance of these devices is unclear. Therefore, in this work, we investigated the mechanisms of the screw or mixed dislocation, edge dislocation, and basal stacking fault (BSF) densities affecting the dark current, responsivity, and response time of GaN (11-20)-based PDs, respectively. We observed that the screw or mixed dislocation increased the dark current mainly through reducing the Schottky barrier height and forming leakage current, whereas the edge dislocation and BSF decreased the responsivity by reducing the electron mobility. Furthermore, all the three types of defects increased the response time through forming traps to recombine the holes with electrons and thus delaying the escape of carriers. These results are highly significant for developing nonpolar GaN-based UV PDs.

InAs/InGaAs quantum dots confined by InAlAs barriers for enhanced room temperature light emission: Photoelectric properties and deep levels

InAs/InGaAs heterostructures with quantum dots (QDs) have been studied for quite some time for light-emitting diodes operating from the near to the far infrared range. However, the room temperature QD emission is rather low, thus it is needed to look for improved structure designs allowing efficient enhancement of luminescence. In this work, we study the modification of photo- and thermo-electrical properties of InAs/In0.15Ga0.85As QD heterostructure, when introducing wide-bandgap In0.15Al0.85As confining barriers (CBs) at one or both sides of the QD layer. The structures demonstrate interband QD photoluminescence (PL) peaked at 1.2 μm measured from room temperature down to 10 K. The introduction of CBs allows to enhance the PL intensity by more than four orders of magnitude. The reason is a strong decrease of the thermal escape of both charge carriers confined in QDs and wetting layer, leading to a highly increased radiative recombination. The changes in optical transitions, involving quantum confinement states and defect-related levels, are studied by photocurrent (PC) measurements, also showing the expectable quenching of PC in the structures with CBs. The existing deep levels of defects are determined by thermally stimulated current spectroscopy. Having the same defect spectrum in all the studied structures, an increase in the defect density was detected near CBs at the QD layer. At low temperatures, defect traps in vicinity of the QDs layer caused the Coulomb screening of conductivity channel, that is studied by kinetics measurements in view of the CB introduction. The PC decrement under the constant illumination is theoretically explained by the screening. We confidently show that, despite of a slight increase in defects and PL blueshift in the QD structure with In0.15Al0.85As CBs, they can serve as improved active elements for energy-efficient QD lasers, single-photon emitters and optical amplifiers.

量子阱带间跃迁探测器基础研究（特邀）

Recently, the anomalous carrier transport in the quantum wells with the PN junction structures has been found experimentally, and the corresponding physical mechanism and the carrier transport model have been proposed. It is observed that the open circuit voltage or short-circuit current can be measured in the resonant excitation mode. Comparing the photoluminescence (PL) spectra of the two kinds of external circuits, it is found that the PL intensity decreased significantly under the short circuit condition. This suggests that the photogenerated carriers under the short circuit condition are not confined in the quantum well, but escaping from the junction region. However, this phenomenon of photocarriers escaping from the quantum wells is not found in the NN-type quantum well structure. Therefore, the effect of thermal excitation or tunneling is excluded to drive the carrier escaping from the quantum well. Based on this, the corresponding physical mechanism and carrier transport model are proposed. It is concluded that photogenerated carriers can escape from the quantum well directly under the built-in electric field of PN junction, and the radiative recombination luminescence occurs after the carrier escape process.

Excitation Intensity and Temperature-Dependent Performance of InGaN/GaN Multiple Quantum Wells Photodetectors

In this article, we investigate the behavior of InGaN–GaN Multiple Quantum Well (MQW) photodetectors under different excitation density (616 µW/cm2 to 7.02 W/cm2) and temperature conditions (from 25 °C to 65 °C), relating the experimental results to carrier recombination/escape dynamics. We analyzed the optical-to-electrical power conversion efficiency of the devices as a function of excitation intensity and temperature, demonstrating that: (a) at low excitation densities, there is a lowering in the optical-to-electrical conversion efficiency and in the short-circuit current with increasing temperature; (b) the same quantities increase with increasing temperature when using high excitation power. Moreover, (c) we observed an increase in the signal of photocurrent measurements at sub-bandgap excitation wavelengths with increasing temperature. The observed behavior is explained by considering the interplay between Shockley–Read–Hall (SRH) recombination and carrier escape. The first mechanism is relevant at low excitation densities and increases with temperature, thus lowering the efficiency; the latter is important at high excitation densities, when the effective barrier height is reduced. We developed a model for reproducing the variation of JSC with temperature; through this model, we calculated the effective barrier height for carrier escape, and demonstrated a lowering of this barrier with increasing temperature, that can explain the increase in short-circuit current at high excitation densities. In addition, we extracted the energy position of the defects responsible for SRH recombination, which are located 0.33 eV far from midgap.

Parasitic photon process versus productive photon process: a theoretical study of free-carrier absorption in conventional and hot-carrier solar cells

The effects of free-carrier absorption on conventional and hot-carriers solar cells are theoretically investigated in this work. The common view that free-carrier absorption in solar cells is ‘parasitic’ is re-examined, with the assistance of a theoretical framework and formulation developed and verified for calculating free-carrier absorption coefficients. In the case of spatial partitioning with photon absorption selectivity (e.g. solar cells with embedded quantum structures), free-carrier-absorption can facilitate and enhance carrier escape processes and increases photocurrents, especially in deep potential wells. Carrier heating resulting from free-carrier absorption is shown to be extremely beneficial to hot-carrier solar cells, especially for heavily-doped wide-band-gap optical absorbers. The energy conversion processes from carrier heating of free-carrier absorption could potentially make ideal hot-carrier solar cells function like solar thermal converters. As a result, their energy conversion efficiency is closer to the thermodynamic limit, regardless of optical absorbers’ band gap energy. It is illustrated that, as an optical process which is not limited by band gap energy, free-carrier absorption could benefit possible materials of hot-carrier solar cells regardless of their band gap energy. From this perspective, free-carrier absorption is far from a ‘parasitic’ process. Its usefulness depends on how we turn it into productive work.

Modeling power and linewidth of quantum dot superluminescent light emitting diode

A model for quantum dot (QD) superluminescent light emitting diode (SLED) is presented to study an optical power output and linewidth over a wide range of injection currents. The analysis is based on the photon and carrier rate equations including the effects of homogeneous and inhomogeneous broadening, carrier escape process, and high-current heating. The model is validated using experimental data available from the literature. The results show non-monotonic variations of the output optical power and linewidth of the SLED with the injection current density. It is seen that there exists an optimum injection current density for which the power–linewidth product becomes maximum for a given device length of the QD SLED.

Photovoltaic Response and Charge Redistribution Processes in GaAs/AlGaAs Multiple‐Quantum Wells Structure

The impact of radiative and non‐radiative processes on the photovoltaic response of GaAs/AlGaAs multiple‐quantum wells is investigated by temperature, excitation power, and chopping‐frequency‐dependent surface photovoltage (SPV) measurements. It is shown that a systematic study of SPV amplitude along with phase spectra can provide important information on thermal escape, carrier localization, and spatial redistribution of charge carriers. Characteristic features in SPV‐phase spectra related to quantum well (QW) transitions and their variation under an external perturbation are explained by the electron–electron interaction. It is found that a many‐body interaction inside the QW is responsible for the capture of charges at the heterointerfaces, which builds up an interface electric field and hinders the drift/diffusion of charges. Such an effect becomes dominant under higher excitation powers, causing a sub‐linear enhancement of photovoltage amplitude and an increase in phase delay as a function of photon flux. From the chopping‐frequency‐dependent SPV measurements, it is concluded that carriers during the drift in barrier layers are repeatedly captured by point defects and other QWs, which enhances the effective time response of the photovoltage signal. Results obtained in this study are beneficial in the engineering of quantum structures for high‐efficiency photovoltaics and non‐invasive characterization of electro‐optical processes.

Dependence of carrier escape lifetimes on quantum barrier thickness in InGaN/GaN multiple quantum well photodetectors.

We reported significant improvements in device speed by reducing the quantum barrier (QB) thicknesses in the InGaN/GaN multiple quantum well (MQW) photodetectors (PDs). A 3-dB bandwidth of 700 MHz was achieved with a reverse bias of -6 V. Carrier escape lifetimes due to carrier trapping in the quantum wells (QWs) were obtained from both simulation and experimental fitting, identifying carrier trapping as the major speed limiting factor in the InGaN/GaN MQW PDs.

Research on photo-generated carriers escape in PIN and NIN structures with quantum wells

Relevant experiments have shown that more than 90% of photo-generated carriers can escape from multiple quantum wells (MQWs) sandwiched between p-type and n-type layers (a PIN structure). The escape time of the photo-generated carriers is on the femtosecond scale, much less than their relaxation time. In contrast, photo-generated carriers cannot escape from MQWs sandwiched between two n-type layers (a NIN structure) with bias. We found that there is a high barrier for holes and that the electric field intensity in QWs is close to zero, resulting in holes accumulating and a low escape efficiency in a NIN structure with bias.

Enhancing carrier transport and carrier capture with a good current spreading characteristic via graphene transparent conductive electrodes in InGaN/GaN multiple-quantum-well light emitting diodes

In this work, InGaN/GaN multiple-quantum-wells light-emitting diodes with and without graphene transparent conductive electrodes are studied with current-voltage, electroluminescence, and time-resolved electroluminescence (TREL) measurements. The results demonstrate that the applications of graphene electrodes on LED devices will spread injection carriers more uniformly into the active region and therefore result in a larger current density, broader luminescence area, and stronger EL intensity. In addition, the TREL data will be further analyzed by employing a 2-N theoretical model of carrier transport, capture, and escape processes. The combined experimental and theoretical results clearly indicate that those LEDs with graphene transparent conductive electrodes at p-junctions will have a shorter hole transport time along the lateral direction and thus a more efficient current spreading and a larger luminescence area. In addition, a shorter hole transport time will also expedite hole capture processes and result in a shorter capture time and better light emitting efficiency. Furthermore, as more carrier injected into the active regions of LEDs, thanks to graphene transparent conductive electrodes, excessive carriers need more time to proceed carrier recombination processes in QWs and result in a longer carrier recombination time. In short, the LED samples, with the help of graphene electrodes, are shown to have a better carrier transport efficiency, better carrier capture efficiency, and more electron-hole recombination. These research results provide important information for the carrier transport, carrier capture, and recombination processes in InGaN/GaN MQW LEDs with graphene transparent conductive electrodes.