Multiple Quantum Well Structures Research Articles

General way to obtain multiple defect modes in multiple photonic quantum-well structures.

We introduce a method to obtain and describe the multiple defect modes in the multiple photonic quantum-well structure [(AB)mC]n(AB)m. It is found that the relationship between structural parameters and defect modes can be obtained by analytically solving the dispersion relationship for the photonic crystal with its periodic unit [(AB)mC], and a laconic and unifying expression can be used to describe the energy levels of both the interface modes and defect modes. The internal mechanism to determine the related parameters is revealed. We believe that these findings can be used to provide direct guidance for practical application.

GaN/AlGaN multiple quantum wells grown on transparent and conductive (-201)-oriented β-Ga2O3 substrate for UV vertical light emitting devices

GaN/AlGaN multiple quantum wells (MQWs) are grown on a 2¯01-oriented β-Ga2O3 substrate. The optical and structural characteristics of the MQW structure are compared with those of a similar structure grown on sapphire. Scanning transmission electron microscopy and atomic force microscopy images show that the MQW structure exhibits higher crystalline quality of well-defined quantum wells when compared to a similar structure grown on sapphire. X-ray diffraction rocking curve and photoluminescence excitation analyses confirm the lower density of dislocation defects in the sample grown on a β-Ga2O3 substrate. A detailed analysis of time-integrated and time-resolved photoluminescence measurements shows that the MQWs grown on a β-Ga2O3 substrate are of higher optical quality. Our work indicates that the 2¯01-oriented β-Ga2O3 substrate can be a potential candidate for UV vertical emitting devices.

Effect of strain modification on crystallinity and luminescence of InGaN/GaN multiple quantum wells grown by MOCVD

The effects of compressive strain modification on green-emitting InGaN/GaN multiple quantum wells (MQWs) are comprehensively investigated by inserting a bulk InGaN prestrain underlying layer with different indium doping contents. Systematic measurements of the structural, morphological and optical properties are conducted. Better crystallinities are obtained for both modified samples, in which the density of V-pits is reduced from 4.0 × 108 /cm2 in the original sample to 1.9 × 108 /cm2 and 3.3 × 108 /cm2. Compared with the conventional InGaN/GaN MQWs, the photoluminescence (PL) emission wavelength of the strain-modified MQW structure with an InGaN (In = 0.03) prestrain layer is blueshifted by ~ 6 nm as a result of the reduced built-in piezoelectric field, while the other MQW structure with an InGaN (In = 0.1) layer has an ~ 15 nm redshift, which is attributed to not only the ~ 1% increase in the In content in the QWs and the ~ 0.6 nm increase in the well width due to the composition pulling effect, but also the ~ 0.19 degree of strain relaxation in the InGaN QWs. The low-temperature PL (LTPL) intensities of both samples are approximately twice as large as that of the conventional one, whereas the internal quantum efficiency is three times larger at the maximum. These tremendous improvements are mainly due to the weakened quantum confined stark effect and stronger carrier localization states, as confirmed by LTPL and temperature-dependent PL (TDPL) measurements.

Optimized Ge1−<italic>x</italic>Sn<italic>x</italic>/Ge Multiple-Quantum-Well Heterojunction Phototransistors for High-Performance SWIR Photodetection

We present the design and analysis of three-terminal Ge1− x Sn x /Ge multiple-quantum-well (MQW) heterojunction phototransistors (HPTs) for high-performance short-wave infrared (SWIR) photodetection. The active layer incorporates Ge1− x Sn x /Ge MQW structures between base–collector junctions pseudomorphically grown on Ge-on-Si substrates, thereby providing compatibility with CMOS platforms for low cost and scalable manufacturing technology. Introducing Sn into the MQW structures can effectively reduce the direct bandgap, thereby considerably extending the photodetection range in the SWIR spectral range with improved absorption efficiencies. In addition, the use of MQW structures as the active layer can enhance the absorption coefficient via quantum-confinement effects, thus further enhancing the photoresponses. We develop theoretical models to calculate the strained electronic band structures and optical absorption coefficients for the Ge1− x Sn x /Ge MQW structures, and the current gain is evaluated as a function of the barrier/well thicknesses, number of periods of the MQW structures, and doping concentrations of the layers for the HPT structures. The calculation results show that the absorption coefficient can be significantly enhanced by increasing the Sn content in the Ge1− x Sn x /Ge MQW active layer, and the current gain can be considerably enhanced by optimizing the HPT structures. With the enhanced absorption coefficient and current gain, high-responsivity photodetection can be achieved in the SWIR spectral range, thus making this device very promising for a wide range of SWIR photodetection applications.

Single junction solar cell employing strain compensated GaAs0.965Bi0.035/GaAs0.75P0.25 multiple quantum wells grown by metal organic vapor phase epitaxy

Single junction solar cells employing 30-period and 50-period GaAs0.965Bi0.035/GaAs0.75P0.25 (Eg ∼ 1.2 eV) multiple quantum wells (MQWs) as base regions were grown by metal organic vapor phase epitaxy. Room temperature photoluminescence measurements indicated a peak spectral emission at 1.18 eV, and the spectral dependence of the external quantum efficiency measured from the fabricated devices shows the extended absorption edge relative to that of GaAs. The fabricated devices with anti-reflection coating employing a 50-period MQW structure exhibit 23% improvement in the conversion efficiency, 4% in the open-circuit voltage, 9% in the short-circuit current density, and 9% in the fill factor, compared to those from the devices employing a 30-period MQW structure in the base region, under AM1.5 direct illumination.

Nonpolar and semipolar ultraviolet multiple quantum wells on GaN/sapphire

The surface morphologies, anisotropic quality and optical properties of nonpolar and semipolar ultraviolet multiple quantum wells (MQWs) structures on GaN/sapphire deposited by metal organic chemical vapor deposition are investigated. The crystal planes of the MQWs are identified as nonpolar (112¯0) plane and semipolar (112¯2) plane by the 2θ−ω scan of high-resolution X-ray diffraction (HRXRD). The results of the atomic force microscope and scanning electron microscope show that the surface morphologies of nonpolar and semipolar MQWs structures have obvious differences. The X-ray rocking curves of nonpolar and semipolar MQWs on-axis diffraction planes by HRXRD demonstrate that the semipolar MQWs structure has a better crystalline quality and a weaker anisotropy of crystal quality than nonpolar MQWs structure. The temperature dependence photoluminescence (PL) spectra reveal that the nonpolar MQWs structure have a stronger yellow band than semipolar MQWs structure. Besides that, the PL spectra of nonpolar MQWs structure also show a blue-shift as temperature increases within a certain range, and then, when the temperature increases continuously, it turns to a red-shift. However, for the semipolar MQWs structure, only the red-shift is observed in PL spectra.

On-chip multicomponent system made with an InGaN directional coupler.

An on-chip multicomponent system is implemented on a III-nitride-on-silicon platform by integrating a transmitter, InGaN waveguide, InGaN directional coupler, and receivers onto a single chip. The transmitter and the receiver share an identical InGaN/GaN multiple-quantum-well (MQW) diode structure and are produced by using the same wafer-level process flow. The receiver sensitively responds to the short-wavelength half of the emission spectrum of the transmitter, thus realizing the multicomponent system with the capability for inplane light communication. A SiO2 isolation layer is employed to decrease the p-n junction capacitance, thus improving the modulation rate without modifying the MQW structure. The wire-bonded monolithic multicomponent system experimentally demonstrates inplane data transmission at 80Mbps and spatial light communication at 100Mbps, paving the way for diverse applications from on-chip power monitoring to inplane light communication in the visible light spectrum.

Large-scale atomistic simulations demonstrate dominant alloy disorder effects in GaBixAs1−x/GaAs multiple quantum wells

Bismide semiconductor materials and heterostructures are considered a promising candidate for the design and implementation of photonic, thermoelectric, photovoltaic, and spintronic devices. This work presents a detailed theoretical study of the electronic and optical properties of strongly-coupled GaBi$_x$As$_{1-x}$/GaAs multiple quantum well (MQW) structures. Based on a systematic set of large-scale atomistic tight-binding calculations, our results reveal that the impact of atomic-scale fluctuations in alloy composition is stronger than the inter-well coupling effect, and plays an important role in the electronic and optical properties of MQW structures. Independent of QW geometry parameters, alloy disorder leads to a strong confinement of charge carriers, a large broadening of the hole energies, and a red shift in the ground-state transition wavelength. Polarisation-resolved optical transition strengths exhibit a striking effect of disorder, where the inhomogeneous broadening could exceed an order of magnitude for MQWs, in comparison to a factor of about three for single quantum wells. The strong influence of alloy disorder effects persists when small variations in the size and composition of MQWs typically expected in a realistic experimental environment are considered. The presented results highlight the limited scope of continuum methods and emphasise on the need for large-scale atomistic approaches to design devices with tailored functionalities based on the novel properties of bismide materials.

Influence of laser irradiation on optical properties of GaAsBi/GaAs quantum wells

This paper is focused on investigation of the impact of laser irradiation on the structural and optical properties of bismide-based multiple quantum wells (MQWs). The MQW structures, composed of 5 GaAsBi quantum wells, 7 nm thick, separated by 10 nm-thick GaAs barriers, were grown by molecular beam epitaxy on GaAs (100)-plane oriented semi-insulating substrates at 330°C temperature. The bismuth content in as-grown GaAsBi wells evaluated from the measurements of HR-XRD rocking curves was about 6%. HR-TEM and AFM studies of the MQWs evidenced sharp interfaces between the wells and barriers, and a smooth, droplet-free surface, respectively. HR-TEM images also evidenced a homogeneous bismuth distribution in the wells. The spatially-resolved photoluminescence study of GaAsBi/GaAs MQWs revealed the enhancement of PL emission efficiency of up to 80% with no shift of the spectral position after intense laser irradiation. The obtained results were explained by improvement of the GaAsBi crystal quality.

The charge confinement effect of quantum-well Alq[formula omitted]-based OLEDs by dual-pulsed transient electroluminescence

In this paper, the dual-pulsed transient electroluminescence (EL) characteristics in organic light-emitting diodes (OLEDs) with NPB/Alq3 multiple quantum-well (MQW) structures are investigated in detail. The threshold duty-ratio ( δth) is found in the NPB/Alq3 MQW devices, whereas the bilayer NPB/Alq3 device does not appear. Then the δth in single and double wells are 0.68 and 0.35, respectively. In the single QW device, the second pulsed intensity will be greater than the first pulsed strength after reaching the δth, while the double QW device is the opposite. This is due to the charge diffusion effect in the Alq3 well layer near the cathode of MQW devices. As the well numbers increase, it is different from the fall time remaining almost constant, the transient delay time will increase linearly, which is totally caused by the recombination position of excitons.

Terahertz response in the quantum-Hall-effect regime of a quantum-well-based charge-sensitive infrared phototransistor

The characteristics of a charge-sensitive infrared phototransistor (CSIP) based on a GaAs/AlGaAs multiple quantum-well (QW) structure are studied under a magnetic field. In the CSIP, the upper QWs serve as a floating gate that is charged by photoexcitation. The photoinduced charges are detected using the resistance of the lowest QW conducting channel. The conducting channel exhibits the integer quantum Hall effect (QHE) in a perpendicular high magnetic field, yielding the magnetic field dependence of the terahertz (THz) response ΔR. We found two different features of ΔR. One is that ΔR switches sign across the QHE plateau, which is explained simply by an increased electron density in the conducting channel. The other feature is observed as an enhanced positive ΔR when a potential barrier is formed in the conducting channel. The latter mechanism can be interpreted as the promotion of edge/bulk scattering due to photoinduced charges. These findings suggest ways to enhance the THz response by using magnetic fields and potential barriers.

Monolithic high performance InGaN/GaN white LEDs with a tunnel junction cascaded yellow and blue light-emitting structures

We propose a phosphor-free dual wavelength monolithic white LED comprising a tunnel junction that separates a yellow light-emitting InGaN/GaN multiple quantum well (MQW) structure without an electron blocking layer (EBL) from a blue light-emitting MQW structure. Using a well-calibrated APSYS simulation software we investigate its performance in terms of output power, wall plug efficiency, and CIE plot of the emitted light and analyze its operation on the basis of band diagram, carrier distribution, EL spectra and radiative recombination rate taking into account the carrier confinement and carrier reuse effects due to removal of an EBL and incorporation of a tunnel junction, respectively. At the injection current density of 250 A/cm2, our proposed white LED exhibits 232% improvement in output power as compared to a conventional blue LED and only 40% droop reduction of wall plug efficiency as opposed to 56% in a reported white LED. The Commission Internationale de l'Eclairage (CIE) coordinates of the emitted light from the proposed white LED structure are found to be (0.310, 0.309) at 100 A/cm2 which signify a good quality white light emission.

Growth of AlGaN-based multiple quantum wells on SiC substrates

Al0.2Ga0.8N/Al0.45Ga0.55N multiple quantum wells (MQWs) were grown on SiC substrates by metal–organic chemical vapor deposition. We studied the influence of well width and barrier width on the structural and optical properties of AlGaN MQWs in details. The MQWs structures prepared in this work all exhibited good periodicity and abrupt interfaces. When the barrier width was fixed, the thinner well was benefit to increase the integrated photoluminescence (PL) intensity of the MQWs and obtain a shorter ultraviolet wavelength. And the thicker barrier was also contributed to improve the optical properties while the well width remained the same. Compared with the effect of the well width, the emission wavelength of MQWs was less dependent on the barrier width. A 318-nm room-temperature PL emission was achieved when the well width and barrier width were 2.5 and 11 nm, respectively.

Theoretical Investigation of Tensile-Strained GeSn/SiGeSn Multiple Quantum Well Laser Wrapped in Si 3N4 Liner Stressor

A tensile-strained GeSn/SiGeSn multiple quantum well (MQW) laser wrapped in Si3N4 liner stressor is designed and characterized theoretically. A biaxial tensile strain is introduced into the GeSn/SiGeSn MQW laser by the Si3N4 liner stressor. The boosting effects of tensile strain on the threshold current density J th and optical gain in GeSn/SiGeSn lasers are attributed to the modulation of energy band structure and carrier distribution in the GeSn wells. Tensile-strained Ge0.90Sn 0.10/Si0.161Ge0.695Sn0.144 MQW laser wrapped in 500 nm Si3N 4 liner stressor achieves a J th reduction from 476 to 168 A/cm2 and a significant enhancement of optical gain, as compared to the relaxed control device without Si3N4 . The design of GeSn/SiGeSn MQW structure wrapped in Si3N4 liner stressor provides a practical way to realize high performance GeSn based mid-infrared laser.

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.

A theoretical study of the influence of barrier thickness variations on optical properties of a semiconductor multiple quantum well slow light device

The influence of barrier thickness variations on the operation of GaAs/AlGaAs multiple quantum well (MQW) slow light devices based on coherence population oscillations (CPOs) is explained. The variations are shown to affect the slow down factor (SDF) and bandwidth of these devices. Bloch equations and the analytical model in fractional dimension are used to analyse and simulate the slow light device. It is shown that other physical parameters of MQW structures (QW width and barrier alloy concentration) affect significantly the optical properties of the device. The presented approaches make it possible to achieve suitable values of SDF and focal energy by adjusting the barrier thickness, QW width and aluminium content. The maximum range of the centre frequency tuning is estimated to be about 1 THz in our calculations, while the slow down factor can reach a high value of 8.5 × 104.

Enhanced light emission from AlGaN/GaN multiple quantum wells using the localized surface plasmon effect by aluminum nanoring patterns

We investigate the localized surface plasmon (LSP) effect by Al nanorings on the AlGaN/GaN multiple quantum well (MQW) structure emitting at 365 nm. For this experiment, first, the size of Al nanorings is optimized to maximize the energy transfer (or coupling) between the LSP and MQW using the silica nanospheres. Then, the Al nanorings with an outer diameter of 385 nm, which exhibit a strong absorption peak in the near-ultraviolet region, are applied to the top surface of the AlGaN/GaN MQW. The photoluminescence (PL) intensity of the MQW structure with Al nanorings increased by 227% at 365 nm compared to that without Al nanorings. This improvement is mainly attributed to an enhanced radiative recombination rate in the MQWs through the energy-matched LSPs by the temperature-dependent PL and time-resolved PL analyses. The radiative lifetime was about two times shorter than that of the structure without Al nanorings at room temperature. In addition, the measured PL efficiency at room temperature of the structure with Al nanorings was 33%, while that of the structure without Al nanorings was 19%, implying that LSP-QW coupling together with the nanoring array pattern itself played important roles in the enhancement.

Effects of 4° misoriented sapphire substrate on optical property of green InGaN/GaN multiple quantum wells

The effects of 4° misoriented sapphire substrate on optical property of green InGaN/GaN multiple quantum wells (MQWs) grown on planar c-plane sapphire and vicinal cut sapphire substrates were investigated. About an order of magnitude higher photoluminescent intensity of MQWs on vicinal substrate was observed compared to that of MQWs on planar substrate. Atomic force microscope and scanning electron microscope found there was a large number of V-shape pits on the structure grown on vicinal substrate. Result of Raman demonstrated that there was more stress in MQWs structure on planar substrate. It was identified by high-resolution X-ray diffraction and transmission electron microscope that the structure on vicinal substrate had higher crystalline quality. V-shape MQWs grown on semipolar sidewalls of V-shape pits on vicinal substrate played an important role in improving luminescent intensity.

The influence of well thickness on the photoluminescence properties of blue-violet light emitting InGaN/GaN multiple quantum wells

Photoluminescence (PL) properties of four blue-violet light emitting InGaN/GaN multiple quantum well (MQW) structures with varying well thickness were studied by means of room temperature PL (RTPL) spectra and temperature-dependent PL (TDPL). From the TDPL, two different kinds of dependencies of PL peak energy with increasing temperature are observed, i.e. an S-shape dependence and an inverted V-shape one. Since the In content in MQW structures is nearly identical for four samples, the difference in luminescence properties is mainly attributed to an increase in localization effects with increasing well thickness.

(Keynote) Lasing from SiGeSn/GeSn Multiple Quantum Well Structures

The amount of data collected, transferred and processed increased over the last decade with a substantial annual rate. With the internet of things in sight, this trend will be continued. Already today more than 10% of the electrical power produced in the western world is consumed by IT products, underlining the demand to drastically reduce the power consumption. In fact, energy consumption will be the main driver of development in information technology. Silicon photonics is considered to be an attractive pathway to drastically reduce power consumption in information technology based on classical Si microelectronics /1/. Moreover Si photonics are envisioned also as a promising pathway towards quantum computing/2/. Group IV alloys, namely Ge and SiGe on Si, are heavily investigated for the integration of modulators and detectors on Si. However, higher integration levels of optical data transfer on the chip will require the integration of low power laser. The latter is approached by a wide range of technologies to deposit III/V structures on Si as well as by bonding III/V lasers Si. Recently, a direct bandgap as well as optically pumped lasing was demonstrated for GeSn alloys deposited on Ge virtual substrates on Si (100) 8” wafers. Here, efforts are reported of this ongoing endeavor towards an electrically pumped group IV laser. The group IV alloy GeSn provide a direct band gap for Sn concentration above ~8%, which is far beyond the solid solubility limit of ~1%. Recently high quality GeSn alloys with Sn concentrations up to 14.5% could be grown at low temperatures and high growth rates by means reactive gas source epitaxy. These GeSn have a direct band gap, in the range of 0.48-0.63 eV dependening on the Sn concentration. Phosphorous and Boron doping has been realized to fabricate p-i-n junction in SiGeSn/GeSn structures. Thus these materials pave the road for the integration of optoelectronic circuitry on Si (100) substrates. Optically pumped laser in the Fabry Perot geometry as well as microdisc lasers have been fabricated from GeSn films, SiGeSn double heterostructures and SiGeSn/GeSn multiple quantum wells (MQW) on Ge virtual substrates. The threshold required to achieve lasing dropped from ~300 kW/cm2 for a thick GeSn film to about 30 kW/cm2 for a MQW structure. This can be attributed to reduced optical losses due to surface scattering as well as due to the reduced number of states in the multiple quantum wells, which allows carrier inversion at smaller pumping powers. However, due to the very small effective mass of electrons in the Г valley compared to large mass in the L valley, the quantum wells have to be designed carefully. For thin quantum wells the confinement shift of the subbands of the Г valley is larger than that for the L valley reducing the directness and may even turn it into an indirect material. Moreover, a (Si)GeSn buffer layer above critical thickness is required to relax the strain before the growth of the active region of the laser. The strain relaxation occurs predominantly via 90° step edge dislocations at the Ge/GeSn interface, within the hetero- and MQW structures no dislocations were detected by cross sectional TEM. However, besides defect engineering, care has to be taken in the design of the structures also to avoid losses due to absorption in GeSn and SiGeSn cladding layers, since the limited band offsets give rather stringent design rules. To achieve electrically pumped lasing double hetero- and MQW-structures have been grown using SiGeSn cladding and barrier layers. Parts of the SiGeSn cladding layers have been doped by P and B to achieve p-i-n junctions. First devices have been fabricated showing a superior electroluminescence efficiency of MQW structures compared to hetero- of homojunction devices. /1/ R. Soref, IEEE JOURNAL OF SELECTED TOPICS IN QUANTUM ELECTRONICS, 12, 1678 (2006) /2/ T. Rudolph APL PHOTONICS 2, 030901 (2017)