Quality Of Quantum Wells Research Articles

Optical Properties and HR-XRD Study of /InGaAs/Al Ga As/GaAs Structures with InAs Quantum Dots and Different Capping Layers

The Al0.30Ga0.70As/InGaAs/AlGaInAs/Al0.30Ga0.70As structures with embedded InAs quantum dots (QDs) covered by strain reduced AlGaInAs capping layer have been investigated in as grown state by means of a photoluminescence (PL) and high resolution X ray diffraction (HR-XRD) methods. Two types of QD structures with different quantum well (QW) capping layers: Al0.10In0.15Ga0.75As (#1) and Al0.40In0 .15Ga0.45As (#2) are compared. It is revealed that the QD emission in the structure with Al0.10In0.15Ga0.75As capping is characterized by the highest PL intensity of the ground state (GS) band and smaller the full with at half maximum (FWHM), compared to #2 structures. The variation of the GS emission peak versus temperature has been monitored within the range of 10-400K for the as grown film states and compared with shrinkage of the energy bandgaps in the InAs and GaAs bulk crystals. The results show that the efficiency of Ga/Al/In intermixing in #2 is less than in #1. HR-XRD study has shown that the both QD structures are characterized by best quality of QW interfaces with the high numbers of Pendellösung peaks. The HR-XRD scans were numerically modelled for the analysis of QW layer compositions. The peculiarities of PL spectra of the studied QD structures are analyzed and the advances of studied QD structures have been discussed.

Efficient emission of InGaN-based light-emitting diodes: toward orange and red

Indium gallium nitride (InGaN)-based light-emitting diodes (LEDs) are considered a promising candidate for red-green-blue (RGB) micro displays. Currently, the blue and green LEDs are efficient, while the red ones are inefficient for such applications. This paper reports our work of creating efficient InGaN-based orange and red LEDs on silicon(111) substrates at low current density. Based on the structure of InGaN yellow LEDs, by simply reducing the growth temperature of all the yellow quantum wells (QWs), we obtained 599 nm orange LEDs with peak wall-plug efficiency (WPE) of 18.1% at 2 A / cm 2 . An optimized QW structure was proposed that changed two of the nine yellow QWs to orange ones. Compared with the sample containing nine orange QWs, the sample with two orange QWs and seven yellow QWs showed similar emission spectra but a much higher peak WPE up to 24.0% at 0.8 A / cm 2 with a wavelength of 608 nm. The improvement of peak WPE can be attributed to the improved QW quality and the reduced active recombination volume. Subsequently, a series of efficient InGaN-based orange and red LEDs was demonstrated. With further development, the InGaN-based red LEDs are believed to be attainable and can be used in micro LED displays.

Effect of Barrier Temperature on Photoelectric Properties of GaN-Based Yellow LEDs**Supported by the National Key Research and Development Program of China (Grant No. 2016YFB0400103), the National Natural Science Foundation of China (Grant No. 61704069), and the Major Special Science and Technology Project of Jiangxi Province (Grant No. 20182ABC28003)

The effect of growth temperature of barriers on photoelectric properties of GaN-based yellow light emitting diodes (LEDs) is investigated. It is found that as the barrier temperature increases, the crystal quality of multi-quantum wells (MQWs) and the quality of well/barrier interface are improved, and the quantum well is thermally annealed, so that the indium atoms in the quantum well migrate to the equilibrium position, reducing the phase separation of the quantum well and improving the crystal quality of quantum wells (QWs). However, the external quantum efficiency (EQE) of the samples begins to decrease when raising the barrier temperature even further. One explanation may be that the higher barrier temperature destroys the local state in the quantum well and reduces the well/barrier interface quality. Therefore, a suitable barrier temperature is proposed, contributing to the improvement of the luminous efficiency of the yellow LEDs.

Comparison between InP-based quantum dot lasers with and without tunnel injection quantum well and the impact of rapid thermal annealing

An InP based tunnel injection quantum dot (QD) laser and a reference quantum dot laser designed to emit at 1.55 μm were grown by molecular beam epitaxy. The lattice matched tunnel injection (TI) structures consist of an InGaAs quantum well (QW), an InAlGaAs barrier and multiple InAs quantum dot layers. Quantum well and quantum dot test structures as well as the actual laser structures were treated with rapid thermal annealing (RTA) at varying temperatures (680–780 °C). Photoluminescence measurements were performed at 10 K on all samples and a strong effect of this annealing process on the emission properties is observed. The laser structures were processed into broad area lasers. Power-current characteristics were measured and evaluated. A strong improvement in device performance is observed after annealing for QD lasers while for TI-QD lasers no significant improvement was obtained, which might be caused by detuning of band energy alignment and different impact of the RTA process on the material quality of QW and QD layers.

High performance continuous-wave InP-based 2.1 μm superluminescent diode with InGaAsSb quantum well and cavity structure suppression

Broadband spectrum electrically pumped InP-based superluminescent diodes with high output power in continuous-wave (CW) mode are reported. The optimized active region and high quality of quantum wells are guaranteed for controlling the wavelength and acquiring large mode gain. Three kinds of cavity structures were designed to provide strong suppression of reflections allowing strong amplified spontaneous emission without any sign of lasing modes. All device spectrums are centered around 2.1 μm, and at low injection current, the full width at half maximum (FWHM) of the widest spectrum is 99 nm. After the optimization of the waveguide structure for higher output power, the maximum CW output power is lifted up to 30 mW with the two lobe far field pattern in a single output facet at 300 K and still the FWHM of 41 nm. This work is of great importance to pave the way for demonstrating compact and efficient light sources based on InP material systems. Considering the InP material compatibility with the mature device fabrication process, especially the buried heterostructure, device characteristics would be better improved and be more promising for applications.

Mobility of carrier in the single-side and double-side doped square quantum wells

Abstract We present a theoretical study of the effect from doping of quantum wells (QWs) on enhancement of the mobility in one-side (1S) and two-side (2S) doped square infinite quantum well. Within the variational approach, we introduce the enhancement factor defined by the ratio of the overall mobility in the 2S doped square quantum wells to that in the 1S doped counterpart with the same sheet carrier density and interface profiles. The enhancement is fixed by the sample parameters such as well width and sheet carrier density. We propose two-side doping as an efficient way to upgrade the quality of QWs. Our theory is able to well reproduce the recent experimental data about low-temperature transport of electrons and holes in one-side and two-side doped square QWs.

A Comparison of Zinc and Carbon Doped on the Atomic Interdiffusion of InGaAs/AlGaAs Quantum Wells Laser Structures After Annealing

We have compared a zinc and a carbon doped on the atomic interdiffusion of InGaAs/AlGaAs after annealing using Electrochemical capacitance voltage (EC-V), X-ray diffraction and photoluminescence (PL) measurements. Electrochemical capacitance voltage measurements revealed that the carrier concentration in the Zn-doped p++GaAs contact layers decreased after annealing at 900oC for 60 sec, indicating that some of the Zn acceptors were passivated or outdiffused from the surface. In contrast to the C-doped samples, an increase of carrier concentration was observed after annealing. X-ray rocking curve confirmed this result where the amount of lattice contraction increase after annealing which is attributed to the presence of the substitutional carbon CAs. Photoluminescence results showed that a large energy shift was observed in the Zn-doped samples compare with C- doped samples. Photoluminescence measurements after etching to various depth showed similar luminescence defects in both Zn- and C-doped samples. Photocurrent measurements showed the quality of quantum well was improve after annealing in C-doped samples due to activation of carbon doped.

Structural and Optical Characteristics of GeSn Quantum Wells for Silicon-Based Mid-Infrared Optoelectronic Applications

This paper reports the study of Ge0.95Sn0.05/Ge0.91Sn0.09/Ge0.95Sn0.05 single quantum well (SQW) and double quantum wells (DQWs). The quantum well (QW) structures were grown on Ge buffered Si substrates using an industrial standard reduced-pressure chemical vapor deposition system. Pseudomorphically grown structures were observed using x-ray diffraction measurements. Defect-free interfaces between each layer were revealed using cross-sectional transmission electron microscopy. Atomic-scale high-resolution transmission electron microscopy and Fourier transform patterns exhibited the high crystalline quality of QWs. Temperature-dependent photoluminescence (PL) was performed, and the emission peaks attributed to the QW region were identified. The dominant optical transition changed from direct bandgap transition at 300 K to indirect bandgap transition at 10 K. Theoretical calculation showed the type-I band alignment for the QWs. Moreover, the Γ and L valley electron distributions and non-radiative lifetimes were evaluated, which further explained the PL characteristics of the QW samples.

EBIC investigations on polar and semipolar InGaN LED structures

AbstractPolar and semipolar LED structures with InGaN quantum wells (QWs) were investigated in a scanning electron microscope (SEM) using simultaneously electron beam induced current (EBIC) and cathodoluminescence (CL). EBIC yields important information about depletion region, leakages, and the overall functionality of the pn‐junction and allows to determine the diffusion length of the generated minority carriers on both sides of the pn‐junction. Spectrally and spatially resolved CL measurements yield both, information about the QW quality and about centers of non‐radiative recombination. EBIC measurements were carried out in top‐view and in cross‐section between and room temperature. The diffusion lengths measured for polar and semipolar structures prove the better crystal quality of the polar structures in terms of the higher carrier diffusion length and its increase for lower temperatures, as expected. On the semipolar structures, top‐view EBIC measurements detect specific areas, where the pn‐junction seems to be disturbed. The comparison of EBIC and CL measurements shows interesting correlations of the EBIC signal and the emission wavelengths of the quantum wells.

Enhanced optical property in quaternary GaInAsSb/AlGaAsSb quantum wells

High quality GaInAsSb/AlGaAsSb quantum wells (QWs) have been grown by molecular beam epitaxy using proper interface treatments. By controlling the group-V elements at interfaces, we obtained excellent optical quality QWs, which were free from undesired localized trap states, which may otherwise severely affect the exciton recombination. Strong and highly efficient exciton emissions up to room temperature with a wavelength of 2.2 μm were observed. A comprehensive investigation on the QW quality was carried out using temperature dependent and power dependent photoluminescence (PL) measurements. The PL emission intensity remains nearly constant at low temperatures and is free from the PL quenching from the defect induced localized states. The temperature dependent emission energy had a bulk-like behavior, indicating high quality well/barrier interfaces. Because of the uniformity of the QWs and smooth interfaces, the low temperature limit of inhomogeneous line width broadening is as small as 5 meV.

Low-temperature illumination and annealing of ultrahigh quality quantum wells

The effects of low temperature illumination and annealing on fractional quantum Hall (FQH) characteristics of a GaAs/AlGaAs quantum well are investigated. Illumination alone, below 1 K, decreases the density of the 2DEG electrons by more than an order of magnitude and resets the sample to a repeatable initial state. Subsequent thermal annealing at a few Kelvin restores the original density and dramatically improves FQH characteristics. A reliable illumination and annealing recipe is developed that yields an energy gap of 600 mK for the 5/2 state.

Spin Transition in theν=8/3Fractional Quantum Hall Effect

We present here the results from a density dependent study of the activation energy gaps of the fractional quantum Hall effect states at Landau level fillings ν=8/3 and 7/3 in a series of high quality quantum wells. In the density range from 0.5×10(11) to 3×10(11) cm(-2), the 7/3 energy gap increases monotonically with increasing density, supporting its ground state being spin polarized. For the 8/3 state, however, its energy gap first decreases with increasing density, almost vanishes at n~0.8×10(11) cm(-2), and then turns around and increases with increasing density, clearly demonstrating a spin transition.

Effect from Doping of Quantum Wells on Enhancement of The Mobility Limited by One-Interface Roughness Scattering

We present a theoretical study of the effect from doping of quantum wells (QWs) on enhancement of the mobility limited by one-interface roughness scattering. Within the variational approach, we introduce the enhancement factor defined by the ratio of the overall mobility in symmetric two-side doped square QWs to that in the asymmetric one-side counterpart under the same doping and interface profiles. The enhancement is fixed by the sample parameters such as well width, sheet carrier density, and correlation length. So, we propose two-side doping as an efficient way to upgrade the quality of QWs. The two-interface roughness scattering is also incorporated to make comparison.

Utilizing the interface adsorption of nitrogen for the growth of high-quality GaInAsN/GaAs quantum wells by metal organic chemical vapor deposition for near infrared applications

We have investigated the composition and optical properties of GaInAsN/GaAs single quantum wells grown using metal organic chemical vapor epitaxy at 500 °C. Using time-of-flight secondary ion mass spectrometry and photoluminescence spectroscopy, we have shown the presence of a 1–2 nm thick nitrogen-rich interfacial layer at the first interface grown. The inhomogeneous asymmetric distribution of nitrogen atoms along the growth direction is attributed to the dominance of surface kinetics, nonlinear dependence of N incorporation on In content, and the strain gradient effect on the effective diffusion of N. We have utilized this finding to grow high quality quantum wells.

Analysis of Improved Efficiency of InGaN Light‐Emitting Diode With Bottom Photonic Crystal Fabricated by Anodized Aluminum Oxidxe

AbstractThe improved performance of a bottom photonic crystal (PC) light‐emitting diode (LED) is analyzed based on internal quantum efficiency (ηint) and light‐extraction efficiency (ηex). The bottom PC is fabricated by anodized aluminum oxide nanopatterns and InGaN quantum wells (QWs) are grown over it. Transmission electron microscopy images reveal that threading dislocations are blocked at the nanometer‐sized air holes, resulting in improved optical emission efficiency of the QWs. From temperature‐dependent photoluminescence measurements, the enhancement of ηint is estimated to be 12%. Moreover, the enhancement of ηex is simulated to be 7% by the finite‐difference time‐domain method. The fabricated bottom PC LED shows a 23% higher optical power than a reference, which is close to the summation of enhancements in ηint and ηex. Therefore, the bottom PC improves LED performance through higher optical quality of QWs as well as increased light extraction.

Growth evolution of surface nanowires and large anisotropy of conductivity on MgZnO/ZnO quantum wells based on M-nonpolar (10−10) ZnO

Self-organized surface nanowires were formed on M-nonpolar (10−10) ZnO homoepitaxial layer surfaces. High-resolution transmittance electron microscopy showed that the surface nanowires possessed a flat plateau at the top and a side facet with constant step spacing at the atomic scale. In and ex situ observations of layer growth revealed that anisotropic islands appeared after growing two-dimensional surfaces and that these gradually changed to nanowire structures with increasing layer thickness. Their growth origin, identified from theoretical parameter fittings based on a step-edge barrier model, was close to that of O-polar ZnO growth. A large difference in electron transport was clearly observed in Mg0.12Zn0.88O/ZnO quantum wells (QWs). Transport anisotropy correlated strongly with the surface morphology and with the structural quality of QWs, as confirmed by polarized photoluminescent spectroscopy. Finally, as a possible mechanism of anisotropic electron transport, we suggest that the electron motion perpendicular to the nanowire arrays was strongly restricted due to a potential barrier caused by an interface roughness of the surface nanowires.

The influence of antimony on the optical quality of highly strained GaInNAs/GaAs QWs investigated by contacless electroreflectance

AbstractContactless electroreflectance (CER) has been applied to study the influence of Sb atoms on the optical quality of highly strained GaInNAs/GaAs quantum wells (QWs). A set of QWs grown under various antimony fluxes has been analyzed in this work. The broadening of the CER resonance has been used as an indicator of the QW quality. It has been observed that broadening of the CER resonance related to the ground state transition decreases about three times (from ∼60 meV to 20 meV) after the incorporation of antimony into GaInNAs/GaAs QWs. Moreover, it has been observed that the broadening parameter does not decrease with the rise of antimony content from 0.5% to 2.0%. It means that only 0.5% Sb is enough to improve the optical quality of GaInNAs/GaAs QWs. (© 2007 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Optimization of GaAs (110) quantum well material growth technology by reflection high energy electron diffraction

Recently, there is an increasing interest in the molecular beam epitaxy growth of various high quality heterostructures on the nonpolar GaAs(110) surface for the unique properties which arise from this unconventional orientation. Considering that the Ga and As atoms are coplanar in GaAs(110) surface, the range of bestgrowth temperature is small. It is difficult to find the best growth condition by observing the change of reflection high energy electron diffraction (RHEED) pattern because this kind of (1×1) RHEED pattern is insensitive to growth temperature and V/III beam equivalent pressure ratio. In the process of the GaAs(110) quantum well growth, we observed the single and double period variation of oscillation of RHEED intensity. This implies that there are two growth modes (monolayer-by-monolayer and bilayer-by-bilayer) of GaAs quantum wells growing on the GaAs (110) substrate under different growth conditions. The measurements of transmission electron microscopy and photoluminescence at room temperature showed that the quantum wells have very bad optical property under the bilayer_by_bilayer growth mode, while the quantum wells grown under the monolayer-by-monolayer growthmode have much better optical property with rough interfaces. By means of RHEEDoscillations, high quality quantum wells have been grown on GaAs (110) substrate under optimized growth conditions.

Charge storage and screening of the internal field in GaN/AlGaN quantum wells

The optical spectra of a set of high quality quantum wells, with well characterized structural parameters, are presented here. We propose a quantitative calculation for the measured emission energies which takes into account the spontaneous and piezoelectric polarization fields and the field-screening effect due to electron-hole photogenerated pairs; such an effect is especially effective in wide QW samples which in fact behave like mesoscopic capacitors.

Mesoscopic-capacitor effect inGaN/AlxGa1−xNquantum wells: Effects on the electronic states

We show that wide GaN quantum wells behave like mesoscopic capacitors. The electron-hole pairs are separated by the spontaneous and piezoelectric polarization fields and accumulated in the well, resulting in a well-width dependent screening of the built-in field. The extent to which such screening is effective depends on the interplay between radiative and nonradiative recombination probabilities, which deplete the ground level of the quantum well, causing the recovery of the unscreened built-in field. The account of the mesoscopic capacitor effect provides a quantitative description of the optical spectra and of the time dynamics of a set of high quality quantum wells with well characterized structural parameters.