Multiple Quantum Wells Active Region Research Articles

Nanomaterials-Based Multiple Quantum Wells for High Photovoltaic Conversion Solar Cells

Using metalorganic chemical vapor deposition (MOCVD), molecular beam epitaxy (MBE) and pulse Laser deposition (PLD) techniques on GaN, Silicon, Silicon Carbide and sapphire substrates, high efficiency InGaN/GaN solar cells are reported with a particular emphasis on the work and achievements made with multi-junction tandem and Nanomaterials (Quantum well (QW), Multiple Quantum Wells (MQW), and Quantum Dots (QD)). An effective method for increasing photon absorption in ultrathin cells made for the best possible photovoltaic response is the InGaN/GaN QW system. To maximize light absorption, the quantum well and barrier thicknesses and number of wells in the MQW active region must be adjusted.

UV-A to UV-B electroluminescence of core-shell GaN/AlGaN wire heterostructures

Core-shell GaN/AlGaN multiple quantum wells (MQWs) embedded in a p–n junction are integrated on the upper part of GaN microwires grown by silane-assisted metal organic vapor phase epitaxy. Dispersed wires are then contacted by electron beam induced deposition for fabrication of single wire UV-LED devices. Rectifying diode-like behavior is first demonstrated for both GaN and GaN/AlGaN p-n junctions without a MQW active region. A weak leakage current in the GaN/AlGaN core-shell heterostructure is attributed to an additional conduction path along wire sidewalls. Electroluminescence at 340 nm in UV-A is demonstrated using a GaN (2.6 nm)/Al0.3Ga0.7N (3 nm) heterostructure embedded in a GaN/Al0.3Ga0.7N p–n junction. This value is even decreased to 310 nm by reducing the well thickness to 0.9 nm and increasing the Al-content of barriers (up to 60%) integrated in the GaN/Al0.3Ga0.7N p–n junction. This work demonstrates UV-B emission based on single wire core-shell UV-LEDs.

Improved Performance of InGaN/AlGaN Multiple‐Quantum‐Well Near‐UV Light‐Emitting Diodes with Convex Barriers and Staggered Wells

The physical mechanism of improving the photoelectric performance of InGaN/AlGaN‐based near UV light‐emitting diode (LED) with convex quantum barrier and staggered quantum well (QW) is studied by numerical simulation. The simulation results indicate that the voltage–current characteristics of the LED structure with convex quantum barrier and staggered QW are effectively improved compared with the traditional multiple quantum well (MQW) structure, and its electroluminescence (EL) intensity and light output power are significantly improved. The main physical mechanisms are: on the one hand, the convex quantum barrier with a lower average Al component can reduce the polarization electric field at the interface between the quantum barrier and QW as well as the effective potential barrier of holes, improve the spatial separation of electron and hole wave functions, promote the injection efficiency of carriers, and improve the uniformity of carrier distribution in the MQWs active region; On the other hand, staggered QWs can provide stronger carrier confinement effect and further increase the overlap of electron and hole wave functions, so as to improve the carrier radiative recombination efficiency; In a word, this work provides a valuable reference for obtaining high‐performance near‐ultraviolet LED.

Suppression of hole leakage by increasing thickness of the first AlGaN barrier layer for GaN/AlGaN ultraviolet light-emitting diode

The influence of thickness of the first AlGaN barrier layer, which is closest to the n-type GaN layer, on the luminescence characteristics of ultraviolet GaN/AlGaN multiple-quantum-well (MQW) light-emitting diodes (LEDs) is investigated numerically. It is found that the luminescence efficiency of LED is enhanced as the thickness of first AlGaN barrier increases. According to the energy band structures and carrier distributions in the MQW active region, it is found that the hole leakage can be suppressed by the thicker first barrier, which may be ascribed to the increased width and height of the triangular potential barrier induced by the polarization electric field in the first AlGaN barrier layer. Therefore, the concentration of holes in the whole MQW active region is increased, which improves the luminescence efficiency of the ultraviolet LEDs with thick AlGaN first barrier layer.

A secret luminescence killer in deepest QWs of InGaN/GaN multiple quantum well structures

This work suggests new alternative explanation why a single InGaN quantum well (QW) or the deepest QWs in the multiple quantum well (MQW) structures suffer with a high non-radiative recombination rate. According to SIMS results, positron annihilation spectroscopy and photoluminescence measurements we suggest that vacancy of Ga in complex with hydrogen atoms can play a dominant role in non-radiative Shockley-Read-Hall recombination of the deepest QWs in InGaN/GaN MQW structures. Vacancy of gallium originate dominantly in GaN buffer layers grown at higher temperatures in H2 atmosphere and are transported to the InGaN/GaN MQW region by diffusion, where they are very effectively trapped in InGaN layers and form complex defects with hydrogen atoms during epitaxy of InGaN layers. Trapping of gallium vacancies is another suggested mechanism explaining why the widely used In containing prelayers help to increase the luminescence efficiency of the InGaN/GaN MQW active region grown above them. Understanding the mechanism why the luminescence efficiency is suppressed in deeper QWs may be very important for LED community and can help to develop new improved technologies for the growth of InGaN/GaN MQW active region.

Wave-shaped temperature dependence characteristics of the electroluminescence peak energy in a green InGaN-based LED grown on silicon substrate

This study aimed to investigate temperature dependencies at different injection currents (ICs) of the electroluminescence (EL) spectra from a green InGaN/GaN light-emitting diode (LED) based on multiple quantum wells (MQWs) grown on a Si substrate in a wide range of ICs (0.001–350 mA) and temperatures (6–350 K). The results show that the temperature-changing characteristic of the EL peak energy gradually evolves from an approximately V-shaped temperature dependence into a wave-shaped (three-step blueshift) dependence with increasing IC. Finally, it emerges as an approximately inverted V-shaped temperature dependence. The behavior reflects the fact that the emission related to InGaN is significantly influenced by the changing recombination dynamics of carriers with rising temperature or IC. This is attributed to the presence in the MQW active region of a stronger carrier localization effect across three zones with different average In contents. Moreover, with the decline of the temperature at lower ICs, the temperature behavior of the external quantum efficiency (EQE) value is dominated by the deactivated non-radiative centers. This phenomenon occurs not only in the higher temperature range but also at lower temperatures due to more In-content-induced structural defects, which are confirmed by measurements of the integrated EL intensity as well as the EQE dependence on IC.

Green laser diodes with constant temperature growth of InGaN/GaN multiple quantum well active region

Increasing trench defect density in green InGaN/GaN multiple quantum wells (MQWs) has been reported to be one cause for “green gap” and creates a challenge in fabricating high-performance green laser diodes (LDs). In this article, we report methods to suppress the density of trench defects. Suppressed defect density in InGaN/GaN MQWs results in greatly improved optical quality, as indicated by increasing photoluminescence (PL) intensity and nonradiative recombination lifetime, and decreasing PL full-width-half-maximums (FWHMs), which can be as narrow as 113 meV. This enabled constant temperature growth of the InGaN/GaN MQW active region of the green LD structure and greatly improved slope efficiency.

Influence of light absorption on the performance characteristics of UV LEDs with emission between 239 and 217 nm

The development of ultraviolet AlGaN multiple quantum well (MQW) light emitting diodes (LEDs) in the wavelength range between 239 and 217 nm is presented. The effects of aluminum composition in the MQW active region and of the underlying AlxGa1−xN:Si current spreading layer on the emission characteristics and operating voltages are investigated. A strong reduction in output power is observed with decreasing emission wavelength which is partly attributed to light absorption within the underlying AlxGa1−xN:Si. Additionally, a reduced carrier injection efficiency is identified as the root cause for the reduced emission power with decreasing emission wavelength. Emission powers at a dc current of 20 mA between 310 and 0.15 μW have been achieved for LEDs emitting between 239 and 217 nm. The maximum light output in pulsed mode operation of these LEDs ranged between 4.6 mW and 3.6 μW, respectively.

Factors Determining the Carrier Distribution in InGaN/GaN Multiple-Quantum-Well Light-Emitting Diodes

Factors determining the carrier distribution in InGaN/GaN multiple-quantum-well (MQW) light-emitting diodes (LEDs) are studied via photoluminescence and temperature-dependent electroluminescence spectra. Employing a dichromatic LED device, we demonstrate that the carrier recombination rate should be considered playing an important role in determining the carrier distribution in the MQW active region, not just the simple hole characteristics such as low mobility and large effective mass.

Influence of Indium Content on the Unintentional Background Doping and Device Performance of InGaN/GaN Multiple-Quantum-Well Solar Cells

We experimentally investigate the influence of indium composition on the photovoltaic performance of InGaN/GaN multiple-quantum-well (MQW) solar cells. Three MQW structures with different In content are grown via metal-organic chemical vapor deposition, and then fabricated into solar cell devices. The solar cells with lower In content show stronger photovoltaic response, which may be ascribed to both smaller piezoelectric fields in InGaN well layers and better material quality of MQW active region. In addition, based on the capacitance–voltage measurements by which the profiles of apparent carrier concentration can be determined, we find that in the higher-In-content solar cells the background electron concentration in unintentionally doped InGaN QWs is higher. Consequently, the effective volume of active region, or the width of depletion region, may become smaller at zero bias and correspondingly the light absorption by InGaN active layers is reduced. This may be attributed to the increased nitrogen vacancy-related defects which are easier induced into In-rich InGaN alloys as a source of shallow donors.

Carrier transport and emission efficiency in InGaN quantum-dot based light-emitting diodes

We present a study of blue III-nitride light-emitting diodes (LEDs) with multiple quantum well (MQW) and quantum dot (QD) active regions (ARs), comparing experimental and theoretical results. The LED samples were grown by metalorganic vapor phase epitaxy, utilizing growth interruption in the hydrogen/nitrogen atmosphere and variable reactor pressure to control the AR microstructure. Realistic configuration of the QD AR implied in simulations was directly extracted from HRTEM characterization of the grown QD-based structures. Multi-scale 2D simulations of the carrier transport inside the multiple QD AR have revealed a non-trivial pathway for carrier injection into the dots. Electrons and holes are found to penetrate deep into the multi-layer AR through the gaps between individual QDs and get into the dots via their side edges rather than via top and bottom interfaces. This enables a more homogeneous carrier distribution among the dots situated in different layers than among the laterally uniform quantum well (QWs) in the MQW AR. As a result, a lower turn-on voltage is predicted for QD-based LEDs, as compared to MQW ones. Simulations did not show any remarkable difference in the efficiencies of the MQW and QD-based LEDs, if the same recombination coefficients are utilized, i.e. a similar crystal quality of both types of LED structures is assumed. Measurements of the current–voltage characteristics of LEDs with both kinds of the AR have shown their close similarity, in contrast to theoretical predictions. This implies the conventional assumption of laterally uniform QWs not to be likely an adequate approximation for the carrier transport in MQW LED structures. Optical characterization of MQW and QD-based LEDs has demonstrated that the later ones exhibit a higher efficiency, which could be attributed to better crystal quality of the grown QD-based structures. The difference in the crystal quality explains the recently observed correlation between the growth pressure of LED structures and their efficiency and should be taken into account while further comparing performances of MQW and QD-based LEDs. In contrast to experimental results, our simulations did not reveal any advantages of using QD-based ARs over the MQW ones, if the same recombination constants are assumed for both cases. This fact demonstrates importance of accounting for growth-dependent factors, like crystal quality, which may limit the device performance. Nevertheless, a more uniform carrier injection into multi-layer QD ARs predicted by modeling may serve as the basis for further improvement of LED efficiency by lowering carrier density in individual QDs and, hence, suppressing the Auger recombination losses.

Strongly transverse-electric-polarized emission from deep ultraviolet AlGaN quantum well light emitting diodes

The optical polarization of emission from ultraviolet (UV) light emitting diodes (LEDs) based on (0001)-oriented AlxGa1−xN multiple quantum wells (MQWs) has been studied by simulations and electroluminescence measurements. With increasing aluminum mole fraction in the quantum well x, the in-plane intensity of transverse-electric (TE) polarized light decreases relative to that of the transverse-magnetic polarized light, attributed to a reordering of the valence bands in AlxGa1−xN. Using k ⋅ p theoretical model calculations, the AlGaN MQW active region design has been optimized, yielding increased TE polarization and thus higher extraction efficiency for bottom-emitting LEDs in the deep UV spectral range. Using (i) narrow quantum wells, (ii) barriers with high aluminum mole fractions, and (iii) compressive growth on patterned aluminum nitride sapphire templates, strongly TE-polarized emission was observed at wavelengths as short as 239 nm.

The effects of temperature dependent recombination rates on performance of InGaN/GaN blue superluminescent light emitting diodes

The effects of temperature dependent radiative and nonradiative recombination (Shockley–Read–Hall, spontaneous radiative, and Auger coefficients) on the spectral and power characteristics of a blue multiple quantum well (MQW) superluminescent light emitting diode (SLD or SLED) have been studied. The study is based on the rate equations model, where three rate equations corresponding to MQW active region, separate confinement heterostructure (SCH) layer, and spectral density of optical power are solved self-consistently with no k-selection energy dependent gain and quasi-Fermi level functions at steady state. We have taken into account the temperature effects on Shockley–Read–Hall (SRH), spontaneous radiative, and Auger recombination in the rate equations and have investigated the effects of temperature rising from 300K to 375K at a fixed current density. We examine this procedure for a moderate current density and interpret the spectral radiation power and light output power diagrams. The investigation reveals that the main loss due to temperature is related to Auger coefficient.

A novel theoretical model for broadband blue InGaN/GaN superluminescent light emitting diodes

A broadband superluminescent light emitting diode with In0.2Ga0.8N/GaN multiple quantum wells (MQWs) active region is investigated. The investigation is based on a theoretical model which includes the calculation of electronic states of the structure, rate equations, and the spectral radiation power. Two rate equations corresponding to MQW active region and separate confinement heterostructures layer are solved self-consistently with no-k selection wavelength dependent gain and quasi-Fermi level functions. Our results show that the superluminescence started in a current of ∼120 mA (∼7.5 kA/Cm2) at 300 K. The range of peak emission wavelengths for different currents is 423–426 nm and the emission bandwidth is ∼5 nm in the superluminescence regime. A maximum light output power of 7.59 mW is obtained at 600 mA and the peak modal gain as a function of current indicates logarithmic behavior. Also, the comparison of our calculated results with published experimental data is shown to be in good agreement.

Strain-balanced InGaN/GaN multiple quantum wells

InGaN/GaN multiple quantum well (MQW) structures suffer from a high amount of compressive strain in the InGaN wells and the accompanied piezoelectric field resulting in both a blue shift in emission and a reduction of emission intensity. We report the growth of InxGa1−xN/GaN “strain-balanced” multiple quantum wells (SBMQWs) grown on thick InyGa1−yN templates for x > y by metal organic chemical vapor deposition. SBMQWs consist of alternating layers of InxGa1−xN wells and GaN barriers under compressive and tensile stress, respectively, which have been lattice matched to a thick InyGa1−yN template. Growth of the InyGa1−yN template is also detailed in order to achieve thick, relaxed InyGa1−yN grown on GaN without the presence of V-grooves. When compared to conventional InxGa1−xN/GaN MQWs grown on GaN, the SBMQW structures exhibit longer wavelength emission and higher emission intensity for the same InN mole fraction due to a reduction in the well strain and piezoelectric field. By matching the average lattice constant of the MQW active region to the lattice constant of the InyGa1−yN template, essentially an infinite number of periods can be grown using the SBMQW growth method without relaxation-related effects. SBMQWs can be utilized to achieve longer wavelength emission in light emitting diodes without the use of excess indium and can be advantageous in addressing the “green gap.”

InGaN/GaN quantum wells for polariton laser diodes: Role of inhomogeneous broadening

Contrary to the case of III-nitride based visible light-emitting diodes for which the inhomogeneous linewidth broadening characteristic of InGaN-based multiple quantum well (MQW) heterostructures does not appear as a detrimental parameter, such a broadening issue can prevent a microcavity (MC) system entering into the strong light-matter coupling regime (SCR). The impact of excitonic disorder in low indium content (x ∼ 0.1) InxGa1–xN/GaN MQW active regions is therefore investigated for the subsequent realization of polariton laser diodes by considering both simulations and optical characterizations. It allows deriving the requirements for such MQWs in terms of absorption, emission linewidth, and Stokes shift. Systematic absorption-like and photoluminescence (PL) spectroscopy experiments are performed on single and multiple In0.1Ga0.9N/GaN quantum wells (QWs). Micro-PL mappings reveal a low temperature PL linewidth of ∼30 meV, compatible with SCR requirements, for single QWs for which the microscopic origin responsible for this broadening is qualitatively discussed. When stacking several InGaN/GaN QWs, a departure from such a narrow linewidth value and an increase in the Stokes shift are observed. Various possible reasons for this degradation such as inhomogeneous built-in field distribution among the QWs are then identified. An alternative solution for the MC design to achieve the SCR with the InGaN alloy is briefly discussed.

Coaxial InxGa1–xN/GaN Multiple Quantum Well Nanowire Arrays on Si(111) Substrate for High-Performance Light-Emitting Diodes

We report the growth of high-quality nonpolar (m-plane) and semipolar (r-plane) multiple quantum well (MQW) nanowires (NWs) for high internal quantum efficiency light emitting diodes (LEDs) without polarization. Highly aligned and uniform In(x)Ga(1-x)N/GaN MQW layers are grown coaxially on the {1-100} sidewalls of hexagonal c-axis n-GaN NWs on Si(111) substrates by a pulsed flow metal-organic chemical vapor deposition (MOCVD) technique. The photoluminescence (PL) measurements reveal that the wavelength and intensity of an MQW structure with various pairs (2-20) are very stable and possess composition-dependent emission ranging from 369 to 600 nm. The cathodoluminescence (CL) spectrum of individual In(x)Ga(1-x)N/GaN MQW NW is dominated by band-edge emission at 369 and 440 nm with a relatively homogeneous profile of parallel alignment. High-resolution transmission electron microscopy (HR-TEM) studies of coaxial InxGa1-xN/GaN MQW NWs measured along the [0001] and [2-1-10] zone axes reveal that the grown NWs are uniform with six nonpolar m-plane facets without any dislocations and stacking faults. The p-GaN/In(x)Ga(1-x)N/GaN MQW/n-GaN NW coaxial LEDs show a current rectification with a sharp onset voltage at 2.65 V in the forward bias. The linear enhancement of power output could be attributed to the elimination of piezoelectric fields in the In(x)Ga(1-x)N/GaN MQW active region. The superior performance of coaxial NW LEDs is observed in comparison with that of thin film LEDs. Overall, the feasibility of obtaining low defect and strain free m-plane coaxial NWs using pulsed MOCVD can be utilized for the realization of high-power LEDs without an efficiency droop. These kinds of coaxial NWs are viable high surface area MQW structures which can be used to enhance the efficiency of LEDs.

Optical investigation of GaInP-AlGaInP quantum-well layers for high-power red laser diodes

We investigated the effect of Si diffusion on the optical properties of GaInP-AlGaInP multiple quantum wells (MQWs) grown at different growth temperatures. Photoluminescence (PL) measurements showed that the emission wavelength was shortened and the PL intensity decreased with increasing growth temperature. Secondary-ion mass spectroscopy (SIMS) measurements showed that the Si concentrations in the MQW active region were similar regardless of the growth temperature. Different growth temperatures of the MQW layers might give rise to different compositions in the MQW layers, resulting in different emission wavelengths. We also investigated the temper-aturedependent optical properties of MQW layers grown with different well thicknesses (6, 8, and 10 nm). For the sample with a well thickness of 10 nm, better crystalline quality, better GaInP-AlGaInP interfaces and higher activation energy were observed compared to other samples. However, the activation energies for all three samples were found to be smaller than the carrier confinement energy. The carrier loss in the GaInP-AlGaInP QW might be related to defects and impurities in the AlGaInP layer.

Spectral competition of chirped dual-wavelength emission in monolithic InGaN multiple-quantum well light-emitting diodes

In this work, spectral competition of chirped dual-wavelength emission in monolithic InGaN multiple-quantum well (MQW) light-emitting diodes (LEDs) is studied numerically. In addition to the crystalline quality that is generally desired for good LED performance, the simulation results show that effective suppression of piezoelectric polarization is another key issue toward the realization of efficient monolithic dual-wavelength emission in InGaN MQW LEDs. With appropriately tailored configuration of MQW active region, the detrimental effects of hetero-interfaces and polarization charges are diminished and thereby balanced dual-wavelength emission can be achieved.

Non-equilibrium quantum well populations and active region inhomogeneity in polar and nonpolar III-nitride light emitters

Strong disparity of electron and hole transport in III-nitride materials is commonly accepted as a main reason for inhomogeneous carrier injection in multiple-quantum well (MQW) active regions of light emitters operating in visible spectral range. In this work, we show that two more factors, specifically (i) excessive depth of III-nitride QWs and (ii) strongly non-equilibrium character of electron and hole populations in optically active QW, are responsible for the active region inhomogeneity in GaN-based light emitters. Modeling shows that electron and hole populations of deep III-nitride QWs are highly imbalanced and substantially deviate from thermodynamic equilibrium with corresponding mobile carrier subsystems in the device active region. In turn, large residual QW charges provide strong impact on the active region electrical uniformity and QW injection conditions. We demonstrate that, as a result of non-equilibrium effects in QW population, even nonpolar III-nitride light emitters with deep QWs suffer from inhomogeneous carrier injection, large QW residual charges, and overall electrical non-uniformity of MQW active regions.