Multiple Quantum Well Samples Research Articles

Porous pseudo-substrates for InGaN quantum well growth: Morphology, structure, and strain relaxation

Strain-related piezoelectric polarization is detrimental to the radiative recombination efficiency for InGaN-based long wavelength micro-LEDs. In this paper, partial strain relaxation of InGaN multiple quantum wells (MQWs) on the wafer scale has been demonstrated by adopting a partially relaxed InGaN superlattice (SL) as the pseudo-substrate. Such a pseudo-substrate was obtained through an electro-chemical etching method, in which a sub-surface InGaN/InGaN superlattice was etched via threading dislocations acting as etching channels. The degree of strain relaxation in MQWs was studied by x-ray reciprocal space mapping, which shows an increase of the in-plane lattice constant with the increase of etching voltage used in fabricating the pseudo-substrate. The reduced strain in the InGaN SL pseudo-substrate was demonstrated to be transferable to InGaN MQWs grown on top of it, and the engineering of the degree of strain relaxation via porosification was achieved. The highest relaxation degree of 44.7% was achieved in the sample with the porous InGaN SL template etched under the highest etching voltage. Morphological and structural properties of partially relaxed InGaN MQWs samples were investigated with the combination of atomic force and transmission electron microscopy. The increased porosity of the InGaN SL template and the newly formed small V-pits during QW growth are suggested as possible origins for the increased strain relaxation of InGaN MQWs.

Atomic-scale insights of indium segregation and its suppression by GaAs insertion layer in InGaAs/AlGaAs multiple quantum wells

The In segregation and its suppression in InGaAs/AlGaAs quantum well are investigated by using high-resolution x-ray diffraction (XRD) and photoluminescence (PL), combined with the state-of-the-art aberration corrected scanning transmission electron microscopy (Cs-STEM) techniques. To facility our study, we grow two multiple quantum wells (MQWs) samples, which are almost identical except that in sample B a thin GaAs layer is inserted in each of the InGaAs well and AlGaAs barrier layer comparing to pristine InGaAs/AlGaAs MQWs (sample A). Our study indeed shows the direct evidences that In segregation occurs in the InGaAs/AlGaAs interface, and the effect of the GaAs insertion layer on suppressing the segregation of In atoms is also demonstrated on the atomic-scale. Therefore, the atomic-scale insights are provided to understand the segregation behavior of In atoms and to unravel the underlying mechanism of the effect of GaAs insertion layer on the improvement of crystallinity, interface roughness, and further an enhanced optical performance of InGaAs/AlGaAs QWs.

The investigation of photoluminescence properties in InxGa1-xN/GaN multiple quantum wells structures with varying well number

The photoluminescence (PL) properties of InGaN/GaN multiple quantum wells (MQWs) samples with a different quantum well number (two, three, and four) grown by molecular beam epitaxy have been studied in the temperature range between 3 and 300 K. The redshift-blueshift-redshift in the PL peak energy of InGaN well, which is known that as S-shaped behavior, is observed with increasing temperature in the investigated samples. S-shaped behavior is associated with carrier localization states and inhomogeneity in InGaN/GaN MQWs samples. This behavior is explained with the theoretical band tail model. The parameters showing the effect of carrier localization degree are determined and discussed in detail. Furthermore, it is presented the activation energy of charge carriers by using InGaN related-normalized PL peak intensity as a function of the reverse of temperature and Arrhenius function for all samples. The highest PL intensity is obtained in sample B with 3-QWs. Determining carrier localization effect and activation energy plays a vital role in luminescence efficiency and quality of InGaN/GaN MQWs devices.

Understanding the Luminescence Characteristics of Ultraviolet InGaN/AlGaN Multiple Quantum Wells with Different In Gradients

The electroluminescence (EL) properties of InGaN/AlGaN ultraviolet light-emitting multiple quantum wells (MQWs) with identical average In content but different In gradients (In content increases linearly, along the growth direction) are investigated numerically. It is found that the luminescence efficiency is improved, and the EL spectral peak wavelength becomes longer for the MQW sample with a larger In gradient. Since the influence of In gradient is different for the conduction and valence bands in InGaN layers, the distribution of electrons and holes in QWs may be changed, leading to a redshift of EL spectra. In particular, when the In gradient increases, the overlap integral of electron-hole wavefunction in InGaN QWs increases, resulting in a higher radiative recombination rate and an enhanced EL intensity.

Anomalous Temperature Dependence of Photoluminescence Caused by Non-Equilibrium Distributed Carriers in InGaN/(In)GaN Multiple Quantum Wells.

An increase of integrated photoluminescence (PL) intensity has been observed in a GaN-based multiple quantum wells (MQWs) sample. The integrated intensity of TDPL spectra forms an anomalous variation: it decreases from 30 to 100 K, then increases abnormally from 100 to 140 K and decreases again when temperature is beyond 140 K. The increased intensity is attributed to the electrons and holes whose distribution are spatial non-equilibrium distributed participated in the radiative recombination process and the quantum barrier layers are demonstrated to be the source of non-equilibrium distributed carriers. The temperature dependence of this kind of spatial non-equilibrium carriers’ dynamics is very different from that of equilibrium carriers, resulting in the increased emission efficiency which only occurs from 100 to 140 K. Moreover, the luminescence efficiency of MQWs with non-equilibrium carriers is much higher than that without non-equilibrium carriers, indicating the high luminescence efficiency of GaN-based LEDs may be caused by the non-equilibrium distributed carriers. Furthermore, a comparison analysis of MQWs sample with and without hydrogen treatment further demonstrates that the better quantum well is one of the key factors of this anomalous phenomenon.

Patterned sapphire substrates cause a wavelength shift of green InGaN light-emitting diodes

InGaN/GaN multiple quantum wells (MQWs) were fabricated under the same growth conditions on the planar and patterned sapphire substrates (PSS) with 10 nm and 20 nm sputtering AlN layers, respectively. Photoluminescence and electroluminescence results both showed that MQWs samples have significant differences in emission wavelengths. The wavelength of the samples on planar substrate is about 20 nm longer than that on the PSS. For samples with the same substrate, but different AlN layer thickness, also exhibit a small wavelength shift. High-resolution X-ray diffraction also revealed that the periodic thickness of MQWs on a planar substrate is thicker than that on PSS. Thermodynamic simulation was carried out to verify the effect of PSS on the heat conduction of GaN film. The PSS embedded in GaN film will affect its heat dissipation ability, and thus influence the wavelength of the MQWs samples by affecting the growth temperature.

Low-Loss Buried-Heterostructure Optical Waveguide Based on Impurity-Free-Vacancy-Diffusion Quantum Well Intermixing

A new method for fabricating a high-quality buried-heterostructure optical waveguide using quantum well intermixing (QWI) has been demonstrated. By patterning a SiO2 thin film on top of a multiple quantum well (MQW) heterostructure, rapid thermal annealing (RTA) could induce laterally local QWI, resulting in a bandgap blueshift and a simultaneous decrease in the refractive index. Both lateral bandgap and index engineering could be attained along the MQW plane, which could be used for a buried-heterostructure optical waveguide. Two SiO2 strips with 3, 5 and 7 μm windows were fabricated for waveguide on a 1540 nm InGaAsP MQW sample. A 120 nm blueshift under the SiO2 area was observed, leading to the index contrast of 0.07. Far-field optical diffraction measurements were also performed to yield angles of 13.9°, 12.8° and 10.6°. A narrower window resulted in a narrower optical waveguide width and exhibited a larger diffraction angle, suggesting that QWI defined the buried optical waveguide. In addition, an electroabsorption modulator was also made by buried waveguide. A −10 dB low optical insertion loss and a 15 dB high extinction ratio in a 500 μm long waveguide were obtained, indicating that a buried heterostructure could be used for photonic devices and integration applications.

Evaluation of polarization field in InGaN/GaN multiple quantum well structures by using electroluminescence spectra shift**Project supported by the National Key Research and Development Program of China (Grant Nos. 2016YFB0400803 and 2016YFB0401801) and the National Natural Science Foundation of China (Grant Nos. 61674138, 61674139, 61604145, 61574135, and 61574134).

In order to investigate the inherent polarization intensity in InGaN/GaN multiple quantum well (MQW) structures, the electroluminescence (EL) spectra of three samples with different GaN barrier thicknesses of 21.3 nm, 11.4 nm, and 6.5 nm are experimentally studied. All of the EL spectra present a similar blue-shift under the low-level current injection, and then turns to a red-shift tendency when the current increases to a specific value, which is defined as the turning point. The value of this turning point differs from one another for the three InGaN/GaN MQW samples. Sample A, which has the GaN barrier thickness of 21.3 nm, shows the highest current injection level at the turning point as well as the largest value of blue-shift. It indicates that sample A has the maximum intensity of the polarization field. The red-shift of the EL spectra results from the vertical electron leakage in InGaN/GaN MQWs and the corresponding self-heating effect under the high-level current injection. As a result, it is an effective approach to evaluate the polarization field in the InGaN/GaN MQW structures by using the injection current level at the turning point and the blue-shift of the EL spectra profiles.

All optical control of comb-like coherent acoustic phonons in multiple quantum well structures through double-pump-pulse pump-probe experiments.

We present an advancement in applications of ultrafast optics in picosecond laser ultrasonics - laser-induced comb-like coherent acoustic phonons are optically controlled in a In0.27Ga0.73As/GaAs multiple quantum well (MQW) structure by a high-speed asynchronous optical sampling (ASOPS) system based on two GHz Yb:KYW lasers. Two successive pulses from the same pump laser are used to excite the MQW structure. The second pump light pulse has a tunable time delay with respect to the first one and can be also tuned in intensity, which enables the amplitude and phase modulation of acoustic phonons. This yields rich temporal acoustic patterns with suppressed or enhanced amplitudes, various wave-packet shapes, varied wave-packet widths, reduced wave-packet periods and varied phase shifts of single-period oscillations within a wave-packet. In the frequency domain, the amplitude and phase shift of the individual comb component present a second-pump-delay-dependent cosine-wave-like and sawtooth-wave-like variation, respectively, with a modulation frequency equal to the comb component frequency itself. The variations of the individual component amplitude and phase shift by tuning the second pump intensity exhibit an amplitude valley and an abrupt phase jump at the ratio around 1:1 of the two pump pulse intensities for certain time delays. A simplified model, where both generation and detection functions are assumed as a cosine stress wave enveloped by Gaussian or rectangular shapes in an infinite periodic MQW structure, is developed in order to interpret acoustic manipulation in the MQW sample. The modelling agrees well with the experiment in a wide range of time delays and intensity ratios. Moreover, by applying a heuristic-analytical approach and nonlinear corrections, the improved calculations reach an excellent agreement with experimental results and thus enable to predict and synthesize coherent acoustic wave patterns in MQW structures.

Single peak deep ultraviolet emission and high internal quantum efficiency in AlGaN quantum wells grown on large miscut sapphire substrates

Deep ultraviolet (DUV) AlxGa1-xN/AlyGa1-yN-based multiple quantum wells (MQWs) were grown on sapphire substrates with miscut angles of 0.2° and 4°. Significantly reduced edge type dislocation density and in-plane anisotropy of dislocation distributions were demonstrated for MQWs grown on high miscut sapphire substrate. Up to 10-fold enhanced MQW emission was observed for MQWs grown on 4° miscut sapphire compared to that grown on 0.2° miscut sapphire, and internal quantum efficiency (IQE) as large as 88% was achieved. Compositional inhomogeneity and thus formation of potential minimum were ambiguously demonstrated by localized cathodoluminescence spectra across the step bunched surface of the MQW sample. However, from the macroscale point of view, single peak PL luminescence was shown for MQW grown on high miscut sapphire without peak separation, suggesting a strongly carrier localization effect. A three-dimensional atom diffusion and desorption model describing the MQW growth was also proposed. This work provides a promising approach towards the realization of highly-efficient DUV emitters.

Structural, morphological, optical and electrical characterization of InGaN/GaN MQW structures for optoelectronic applications

InGaN/GaN multiple quantum well (MQW) structures were grown on c-plane sapphire substrate using metal organic chemical vapour deposition technique by varying the MQW periods. The indium composition and thickness were estimated using high-resolution X-ray diffraction. InGaN well, GaN barriers and Indium composition were estimated as 3 nm, 18 nm and 16–18% using epitaxy smooth fit software. Reciprocal space mapping revealed that InGaN/GaN MQW samples were coherently strained. High-resolution transmission electron microscopy images confirmed good interface between the InGaN/GaN MQW structures. Atomic force microscopy and scanning electron microscopy exhibit decrease in the surface roughness with increase in the number of InGaN/GaN MQW periods with respect to the number of defects comprising of threading dislocations and hexagonal V-pits. Self-organized In(Ga)N like nanostructures with spiral growth mechanism was also observed due to the low temperature growth of p-GaN layer. The photoluminescence spectra of the MQWs showed a red-shift when the number of QW periods was increased due to quantum confined stark effect. Hall Effect measurement displayed good semiconducting behavior in the InGaN/GaN MQW structures. The carrier concentration values also emphasized adequate variations when number of periods was increased.

Investigation on surface-plasmon-enhanced light emission of InGaN/GaN multiple quantum wells

We demonstrate surface-plasmon (SP) enhanced light emission from InGaN/GaN near ultraviolet (NUV) multiple quantum wells (MQWs) using Ag thin films and nano-particles (NPs). Two types of Ag NP arrays are fabricated on the NUV-MQWs, one is fabricated on p-GaN layer with three different sizes of about 120, 160 and 240 nm formed by self-assembled process, while the other is embedded close to the MQWs. In addition, the influence of the surface plasmon polariton (SPP) and localized surface plasmon (LSP) in NUV-MQWs has been investigated by photoluminescence (PL) measurement. Both PL measurements and theoretical simulation results show that the NUV light would be extracted more effectively under LSP mode than that of SPP mode. The highest enhancement of PL intensity is increased by 324% for the sample with NPs embedded in etched p-GaN near the MQWs as compared with the bare MQWs, also is about 1.24 times higher than the MQW sample covered with Ag NPs on the surface, indicating strong surface scattering and SP coupling between Ag NPs and NUV-MQWs.

InAs/InAsSb type-II strained-layer superlattices for mid-infrared LEDs

InAs/InAsSb type-II strained-layer superlattice (SLS) and multiple quantum well (MQW) structures have been studied for their suitability in the active region of mid-infrared LEDs operating at room temperature. A series of InAs/InAs1−xSbx superlattices with low antimony content (x = 3.8–13.5%) were grown by MBE on InAs substrates and characterised using x-ray diffraction and photoluminescence (PL). The 4 K PL spectra of these samples exhibit the expected peak shift to longer wavelength and a reduction in intensity as the Sb content is increased. Band structure simulations highlight the effects of changing the antimony content and the layer thicknesses, to tailor the overlap of the electron and hole wavefunctions and maximise the radiative recombination rate. Analysis of the temperature dependence of the PL emission spectra enabled the extraction of quenching energies that demonstrate some suppression of Auger recombination in both the MQW and SLS structures. The MQW samples exhibit a changeover in the dominant radiative recombination process above ~100 K associated with thermal emission of holes into the InAs barriers; this behaviour was not observed in the SLS samples. These SLS structures have the potential for use as the active region in room temperature mid-infrared LEDs.

85% internal quantum efficiency of 280-nm AlGaN multiple quantum wells by defect engineering

In this study, high internal-quantum-efficiency (IQE) AlGaN multiple quantum wells (MQWs) were successfully demonstrated on low-defect-density AlN templates with nano-patterned sapphire substrates. These templates consisted of AlN structures with 0∼30 periods superlattices (SLs) by alternating high (100) and low (25) V/III ratios under a low growth temperature (1130 °C). Compared to conventional high crystal-quality AlN epilayers achieved at temperatures ≥1300 °C, lower thermal budget can reduce the production cost and wafer warpage. Via optimization of the SL period, the AlN crystallinity was systematically improved. Strong dependence of SL period number on the X-ray full-width-at-half-maximum (FWHM) of the AlN epilayer was observed. The AlN template with 20-period SLs exhibited the lowest FWHM values for (0002) and (10ī2), namely 331 and 652 arcsec, respectively, as well as an ultra-low etching pit density of 1 × 105 cm−2. The relative IQE of 280 nm AlGaN MQWs exhibited a dramatically increase from 22.8% to 85% when the inserted SL increased from 0 to 20 periods. It has hardly ever been reported for the AlGaN MQW sample. The results indicate that the engineered AlN templates have high potential applications in deep ultraviolet light emitters.

Efficiency enhancement of InGaN amber MQWs using nanopillar structures

Abstract We have investigated the use of nanopillar structures on high indium content InGaN amber multiple quantum well (MQW) samples to enhance the emission efficiency. A significant emission enhancement was observed which can be attributed to the enhancement of internal quantum efficiency and light extraction efficiency. The size-dependent strain relaxation effect was characterized by photoluminescence, Raman spectroscopy and time-resolved photoluminescence measurements. In addition, the light extraction efficiency of different MQW samples was studied by finite-different time-domain simulations. Compared to the as-grown sample, the nanopillar amber MQW sample with a diameter of 300 nm has demonstrated an emission enhancement by a factor of 23.8.

Significant internal quantum efficiency enhancement of GaN/AlGaN multiple quantum wells emitting at ~350 nm via step quantum well structure design

Significant internal quantum efficiency (IQE) enhancement of GaN/AlGaN multiple quantum wells (MQWs) emitting at ~350 nm was achieved via a step quantum well (QW) structure design. The MQW structures were grown on AlGaN/AlN/sapphire templates by metal-organic chemical vapor deposition (MOCVD). High resolution x-ray diffraction (HR-XRD) and scanning transmission electron microscopy (STEM) were performed, showing sharp interface of the MQWs. Weak beam dark field imaging was conducted, indicating a similar dislocation density of the investigated MQWs samples. The IQE of GaN/AlGaN MQWs was estimated by temperature dependent photoluminescence (TDPL). An IQE enhancement of about two times was observed for the GaN/AlGaN step QW structure, compared with conventional QW structure. Based on the theoretical calculation, this IQE enhancement was attributed to the suppressed polarization-induced field, and thus the improved electron–hole wave-function overlap in the step QW.

Investigating carrier localization and transfer in InGaN/GaN quantum wells with V-pits using near-field scanning optical microscopy and correlation analysis

The V-pits and potential fluctuations in InGaN/GaN multiple quantum wells (MQWs) are key factors for understanding the performance of InGaN/GaN-based light-emitting diodes (LEDs). However, photoluminescence (PL) measurements using conventional optical microscopy only provide ensemble information due to the spatial resolution limit, known as the diffraction barrier, which hinders the analysis of dislocations and potential fluctuations. Here, in order to investigate the influence of the V-pits and potential fluctuations on local optical properties, we performed nanoscopic luminescence mapping for standard and V-pit InGaN/GaN MQWs samples with different sized V-pits using near-field scanning optical microscopy (NSOM) with illumination mode (I-mode) at various laser excitation powers. From the nanoscopic PL mapping data, we could clearly observe luminescence features associated with dislocations and potential fluctuations in the InGaN/GaN MQWs. We also employed correlation analysis to quantitatively analyze the nanoscopic PL mapping data for the different MQWs samples. Based on the results of NSOM PL with I-mode and correlation analysis, we could demonstrate that carrier transfer in the MQWs sample with large sized V-pits is suppressed by deeper potential fluctuations and higher energy barriers compared to the standard sample.

Effects of Nanoscale V-Shaped Pits on GaN-Based Light Emitting Diodes

This paper reviews the formation of nanoscale V-shaped pits on GaN-based light emitting diodes (LEDs) grown by the metal organic chemical vapor deposition (MOCVD) system and studies the effect of V-shaped pits on quantum efficiency. Since V-pits could provide potential barriers around threading dislocations to lessen non-radiative recombinations in such a high defect environment. In our study, multiple InGaN/GaN quantum well samples with different emission wavelengths of 380, 420, 460, and 500 nm were grown, each with different nanoscale V-shaped pits of three diameters for 150, 200, and 250 nm, respectively. It was found that the multiple quantum well (MQW) sample with larger V-pits had a lower pit density, but a relatively larger total V-pits defected area. The optimum diameter of V-pits showing the highest quantum efficiency from the MQW sample depended on the emission wavelength. MQW samples with wavelengths of 380 and 500 nm exhibited the best internal quantum efficiency (IQE) performance at the smallest V-pits area; however, the best performance for MQW samples with wavelength around 420 and 460 nm occurred when large V-pit areas were presented. Photoluminescence (PL) peak shifts and Raman shifts can provide a relationship between quantum-confined Stark effect (QCSE) and IQE, as well as a comparison between strain and IQE. The results obtained in this phenomenological study shall provide a useful guide line in making high-performance GaN-based LEDs with wide emission spectra.

The effects of indium aggregation in InGaN/GaN single and multiple quantum wells grown on nitrogen-polar GaN templates by a pulsed metalorganic chemical vapor deposition

In this study, the effects of indium aggregation in InGaN/GaN single and multiple quantum wells (MQW) grown on nitrogen-polar GaN templates by a pulsed metalorganic chemical vapor deposition are investigated. With the pulsed growth mode, InGaN decomposition and indium aggregation lead to InGaN mounds, which forms localized states for trapping carriers. In the MQW sample, a higher density and larger size of InGaN mounds imply that an enhanced indium aggregation can improve luminescence efficiency.

Enhanced resonant Raman scattering and optical emission of ZnO/ZnMgO multiple quantum wells

Multiple quantum wells (MQWs) with well-defined multilayer structure are at the foundation of fabricating optoelectronic devices with high quantum efficiency. In this study, we report the ZnO/ZnMgO MQWs fabricated under optimized conditions. At room temperature, the MQW samples, excited by a 325-nm He-Cd laser, exhibited intense ultraviolet photoluminescence (PL) tuned by the quantum confinement (QC) effect. At the same time, extremely enhanced multiple-phonon resonant Raman scattering (RRS) was observed in the first time in the PL spectra of ZnO-based MQW samples. The multiple-phonon RRS was ascribed to the ingoing and outgoing resonance with the barrier layers, and was found highly in accordance with that of ZnMgO epilayer deposited under the same conditions, thus strongly evidencing the MQW samples have well-defined multilayer structures. Using the PL spectra taken at ∼12 K, the PL shifts induced by QC effect were found in good agreement with the calculated results using Kronig-Penney model.