Showing papers on "Quantum well published in 2022"

InGaN quantum well with gradually varying indium content for high-efficiency GaN-based green light-emitting diodes

Abstract: High-efficiency GaN-based green LEDs are of paramount importance to the development of the monolithic integration of multicolor emitters and full-color high-resolution displays. Here, the InGaN quantum well with gradually varying indium (In) content was proposed for improving the performance of GaN-based green LEDs. The InGaN quantum well with gradually varying In content not only alleviates the quantum-confined Stark effect (QCSE), but also yields a low Auger recombination rate. Consequently, the gradual In content green LEDs exhibited increased light output power (LOP) and reduced efficiency droop as compared to constant In content green LEDs. At 60 A/cm2, the LOPs of the constant In content green LEDs and the gradual In content green LEDs were 33.9 mW and 55.2 mW, respectively. At 150 A/cm2, the efficiency droops for the constant In content green LEDs and the gradual In content green LEDs were 61% and 37.6%, respectively. This work demonstrates the potential for the gradual In content InGaN to replace constant In content InGaN as quantum wells in LED devices in a technologically and commercially effective manner.

A 357.9 nm GaN/AlGaN multiple quantum well ultraviolet laser diode

Abstract: Ultraviolet (UV) and deep-UV light emitters are promising for various applications including bioagent detection, water and air purification, dermatology, high-density optical storage, and lithography. However, to achieve shorter UV laser diodes (LDs), especially for the LDs with lasing wavelength less than 360 nm, requires high AlN mole fraction AlGaN cladding layer (CL) and waveguide (WG) layers, which usually leads to generate cracks in AlGaN epilayer due to lack of lattice-matched substrates. Meanwhile, due to high resistivity of high AlN mole fraction Mg doped AlGaN layers, only few groups have reported GaN-based LDs with emission wavelength shorter than 360 nm[1−8], and up to now, there is no room temperature continuous-wave (CW) operation UV LDs with a lasing wavelength shorter than 360 nm ever reported. Previously, we have reported a UV LD with lasing wavelength of 366 nm[9]. In this paper, a higher AlN mole fraction of AlGaN WG layers is employed to shorten the LD emission wavelength to less than 360 nm. A lasing wavelength of 357.9 nm is achieved. Firstly, two GaN-based UV LD structures with different AlN mole fraction of AlGaN WG layers were grown on c-plane GaN substrate by metal organic chemical vapor deposition (MOCVD), TMGa/TEGa, TMAl, TMIn and NH3 were used as Ga, Al, In and N sources, respectively. The schematic diagram of the structure of GaN-based UV LDs is shown in Fig. 1(a). It consists of a 1-μm thick n-type GaN layer, a 500 nm n-type Al0.07Ga0.93N cladding layer (CL) with n-doping concentration of 3 × 1018 cm–3, a 20 nm n-type Al0.25Ga0.75N hole blocking layer (HBL), a AlGaN lower WG (LWG) layer, an unintentional doped GaN/Al0.07Ga0.93N multiple quantum well (MQW) active region, a AlGaN upper WG (UWG) layer, a 20-nm p-type Al0.3Ga0.7N electron blocking layer (EBL), a 500-nm p-type Al0.07Ga0.93N CL layer, and a 40 nm p-GaN contact layer. After the epitaxial growth, a 10 μm-wide ridge stripe along the <1-100> direction was formed by dry etching on the epitaxial layers. The cavity of the laser diode with a length of 600 μm was fabricated by cleaving the epitaxial film and substrate together along the {1-100} plane. A Ni/Au contact was evaporated onto the p-type GaN layer and a Ti/Al contact was evaporated onto the n-type GaN layer. Then the output power versus current (P–I) curves were recorded at room temperature using a calibrated Si detector. The RT output power of one fabricated UV LDs, LD1, which is with about 4.5% AlN mole fraction AlGaN WG layers, as a function of injected current is measured under pulsed operation condition and shown in Fig. 1(b). It is found that the output power of LD1 is nonlinear with the injection current and it has an abrupt increase at the injection current of 1.5 A, corresponding to 25 kA/cm2. This threshold current value of lasing is similar to the reported ones by other groups[3, 4, 10–12]. The output power is about 11 mW at an injection current of 1.7 A. The inset shows the photo of lasing LD1 and the far field pattern of laser beam in blue color formed on the white paper screen. The leakage modes in the laser beam are not observed because the UV light should be absorbed by thick GaN layers existing in both n side and p side. Fig. 1(c) shows the electroluminescence spectrum of UV LD under pulsed operation condition at an injection around the threshold current of 1550 mA. It can be seen that the peak wavelength is 357.9 nm, and the full width at half maximum (FWHM) is about 0.3 nm which is obtained by Gaussian fitting to the emission peak. Another prepared UV LD, LD2, has a lower AlN mole fraction of AlGaN WG layer, which is approximately 3.5% for both lower and upper WG layers. The front and rear cleaved facets of laser diode were uncoated. The P–I curve and stimulation emission spectrum of LD2 are presented in Fig. 1(d). Compared with LD1, the wavelength of stimulation emission increases about 5 to 362.6 nm. Such an increase is attributed to the increased absorption of high energy UV light in the AlGaN WG layers with lower AlN mole fraction. In addition, it is found that in comparison with LD1, the optical characteristics of LD2 are improved obviously. The threshold current of LD2 is 760 mA, corresponding to 12.7 kA/cm2. The output power is 258 mW at the injection current of 1.9 A and the slope efficiency is about 0.23 W/A. The higher threshold current density of LD1 compared with LD2 may be attributed to the weaker optical confinement factor, which is due to a smaller difference in the AlN mole fraction of AlGaN CL and WG layers for LD1. It leads to a large percentage of light penetrating into CL or even to GaN layers. Therefore, the absorption loss of LD1 is larger than that of LD2. However, it is noted that during the growth of Al Correspondence to: D G Zhao, dgzhao@red.semi.ac.cn Received 13 DECEMBER 2021. ©2022 Chinese Institute of Electronics SHORT COMMUNICATION Journal of Semiconductors (2022) 43, 010501 doi: 10.1088/1674-4926/43/1/010501

Effects of magnetic, electric, and intense laser fields on the optical properties of AlGaAs/GaAs quantum wells for terahertz photodetectors

Abstract: We investigate optical absorption coefficients and changes in the refractive index in single- and double-step GaAs/AlGaAs quantum wells under the influence of magnetic, electric, and intense laser fields. For single-step quantum wells, increasing the intensity of the external laser and electric fields induces a red-shift in the total optical absorption coefficient and change in the refractive index and increases their amplitudes; however, increasing the magnetic field generates a blueshift in the optical absorption and change in the refractive index. For double-step quantum wells, the transition energy between the ground and first excited state is strongly decreased. In addition, the total optical absorption coefficients and changes in the refractive index incur a blue shift when the electric and magnetic fields are increased and a red-shift with increasing laser fields. Our analysis of coupling matrix elements, probability densities, and energy levels show that intense laser, electric, and magnetic fields can be harnessed to adjust and tune total optical absorption coefficients and refractive indices for terahertz applications.

Lightly strained germanium quantum wells with hole mobility exceeding one million

Abstract: We demonstrate that a lightly strained germanium channel ([Formula: see text]) in an undoped Ge/Si 0.1 Ge 0.9 heterostructure field effect transistor supports a two-dimensional (2D) hole gas with mobility in excess of [Formula: see text] cm 2 /Vs and percolation density less than [Formula: see text] cm −2 . This low disorder 2D hole system shows tunable fractional quantum Hall effects at low densities and low magnetic fields. The low-disorder and small effective mass ([Formula: see text]) defines lightly strained germanium as a basis to tune the strength of the spin–orbit coupling for fast and coherent quantum hardware.

GeSn-Based Multiple-Quantum-Well Photodetectors for Mid-Infrared Sensing Applications

Abstract: Silicon (Si)-based mid-infrared (MIR) photonics has promising potential for realizing next-generation ultra-compact spectroscopic systems for various applications such as label-free and damage-free gas sensing, medical diagnosis, and defense. The epitaxial growth of Ge1-xSnx alloy on Si substrate provides the promising technique to extend the cut-off wavelength of Si photonics to MIR range by Sn alloying. Here, we present the theory and simulation of heterojunction p-i-n MIR photodetectors (PDs) with Ge0.87Sn0.13/Ge0.92Sn0.08 quantum-wells with an additional Ge0.91Sn0.09 layer to elongate the photoabsorption path in the MIR spectrum. The incorporation of QW pairs (N) enables the light-matter interaction due to the carrier and optical confinement in the active region. As a result, the spectral response of the device is enhanced in the MIR range. Devices with varying N were compared in terms of various figure-of merits including dark-current, a photocurrent-to-dark current ratio, detectivity, spectral responsivity, and noise equivalent power (NEP). Additionally, parasitic capacitance-dependent RC and 3dB bandwidth were also studied using a small-signal equivalent circuit model. The proposed device exhibited the extended photodetection wavelength at ∼ 3370 nm and [Formula: see text] up to ∼ 7.3×103 with a dark current of ∼ 56.3 nA for N=8 at 300 K. At a bias of -3V, the proposed device achieved the spectral responsivity of 0.86 A/W at 2870 nm and 0.55 A/W at 3300 nm, detectivity more than 2.5×109 Jones and a NEP less than 2.1×10-13 WHz-0.5 for N=8 at 3250 nm. The calculated 3dB bandwidth of 47.8 GHz, the signal-to-noise ratio (SNR), and linear dynamic range (LDR) of 93 dB and 74 dB were achieved at 3300 nm for N=8 . Thus, these results indicate that the proposed GeSn-based QW p-i-n PDs pave the pathway towards the realization of new and high-performance detectors for sensing in the MIR regime.

Improvement of 650-nm red-emitting GaIn0.17N/GaIn0.38N multiple quantum wells on ScAlMgO4 (0001) substrate by suppressing impurity diffusion/penetration

Abstract: This study aims to improve the crystalline quality of 650-nm GaIn0.17N/GaIn0.38N red-emitting multiple quantum wells (MQWs) fabricated on a ScAlMgO4 (SCAM) substrate. When using the SCAM substrate, the diffusion and/or penetration of impurities, including Mg, Sc, O, and Al, from the SCAM substrate poses as a challenge. To address this issue, we introduced an Al0.74In0.26N layer between the SCAM substrate and MQWs, which was lattice-matched to the substrate. The Al0.74In0.26N layer effectively blocked the diffusion of impurities from the SCAM substrate into the adjacent layers during the metal-organic vapor epitaxy (MOVPE) growth. For further suppression, a thick AlN layer was deposited on the back of the SCAM substrate before the MOVPE growth, which effectively suppressed impurity penetration from the growth surface. The structure proposed in this study improved the crystallinity and the surface roughness of MQWs, resulting in the improvement of internal quantum efficiency by approximately three times compared to that of the conventional sample.

High Power Mid-Infrared Quantum Cascade Lasers Grown on GaAs

Abstract: The motivation behind this work is to show that InP-based intersubband lasers with high power can be realized on substrates with significant lattice mismatch. This is a primary concern for the integration of mid-infrared active optoelectronic devices on low-cost photonic platforms, such as Si. As evidence, an InP-based mid-infrared quantum cascade laser structure was grown on a GaAs substrate, which has a large (4%) lattice mismatch with respect to InP. Prior to laser core growth, a metamorphic buffer layer of InP was grown directly on a GaAs substrate to adjust the lattice constant. Wafer characterization data are given to establish general material characteristics. A simple fabrication procedure leads to lasers with high peak power (>14 W) at room temperature. These results are extremely promising for direct quantum cascade laser growth on Si substrates.

Gallium Arsenide Photonic Integrated Circuit Platform for Tunable Laser Applications

Abstract: An active-passive integration technique for operation near a wavelength of 1030 nm has been developed on a gallium arsenide (GaAs) photonic integrated circuit platform. The technique leverages quantum wells (QWs) that are slightly offset vertically from the center of the waveguide, and selectively removed prior to upper cladding regrowth to form active and passive regions. The active region consists of indium gallium arsenide (InGaAs) QWs, gallium arsenide phosphide (GaAsP) barriers, GaAs separate confinement heterostructure layers, and aluminum gallium arsenide (AlGaAs) cladding. Fabry Perot lasers with various widths were fabricated and characterized, exhibiting high injection efficiency of 98.8%, internal active loss of 3.44 cm−1, and internal passive loss of 4.05 cm−1 for 3 μm wide waveguides. The 3 μm, 4 μm, and 5 μm wide lasers demonstrated greater than 50 mW output power at 100 mA continuous wave (CW) current and threshold current as low as 9 mA. 20 μm wide broad area lasers demonstrated 240 mW output power, 35.2 mA threshold current under CW operation, and low threshold current density of 94 A/cm2 for 2 mm long lasers. Additionally, these devices exhibit transparency current density of 85 A/cm2 and good thermal characteristics with T 0 = 205 K, and Tη = 577K.

Bandgap engineering, monolithic growth, and operation parameters of GaSb-based SESAMs in the 2 – 2.4 µm range

Abstract: We present the detailed growth and characterization of novel GaSb-based semiconductor saturable absorber mirrors (SESAMs) operating in the 2–2.4 µm spectral range. These SESAMs at different wavelengths are bandgap engineered using ternary material compositions and without strain compensation. We observe that even when the thickness of quantum wells (QWs) exceeds the critical thickness we obtain strain relaxed SESAMs that do not substantially increase nonsaturable losses. SESAMs have been fabricated using molecular beam epitaxy with a AlAs 0.08 Sb 0.92 /GaSb distributed Bragg reflector (DBR) and strained type-I In x Ga 1-x Sb or type-II W-like AlSb/InAs/GaSb QWs in the absorber region. All the type-I SESAMs show excellent performance, which is suitable for modelocking of diode-pumped semiconductor, ion-doped solid-state, and thin-disk lasers. The recovery time of the type-II SESAM is too long which can be interesting for laser applications. The dependence of the SESAM design, based on its QW number, barrier material, and operation wavelength are investigated. A detailed characterization is conducted to draw conclusions from macroscopic nonlinear and transient absorption properties at different wavelengths in the 2–2.4 µm range for the corresponding devices.

Density-dependent two-dimensional optimal mobility in ultra-high-quality semiconductor quantum wells

Abstract: We calculate using the Boltzmann transport theory the density-dependent mobility of two-dimensional (2D) electrons in GaAs, SiGe, and AlAs quantum wells as well as of 2D holes in GaAs quantum wells. The goal is to precisely understand the recently reported breakthrough in achieving a record 2D mobility for electrons confined in a GaAs quantum well. Comparing our theory with the experimentally reported electron mobility in GaAs quantum wells, we conclude that the mobility is limited by unintentional background random charged impurities at an unprecedented low concentration of $\ensuremath{\sim}{10}^{13}\phantom{\rule{4pt}{0ex}}{\mathrm{cm}}^{\ensuremath{-}3}$. We find that this same low level of background disorder should lead to 2D GaAs hole and 2D AlAs electron mobilities of $\ensuremath{\sim}{10}^{7}$ and $\ensuremath{\sim}4\ifmmode\times\else\texttimes\fi{}{10}^{7}\phantom{\rule{4pt}{0ex}}{\mathrm{cm}}^{2}/\text{V}\phantom{\rule{0.16em}{0ex}}\text{s}$, respectively, which are much higher theoretical limits than the currently achieved experimental values in these systems. We therefore conclude that the current GaAs hole and AlAs electron systems are much dirtier than the state-of-the-art 2D GaAs electron systems. We present theoretical results for 2D mobility as a function of density, effective mass, quantum-well width, and valley degeneracy, comparing with experimental data.

Four-wave mixing in 1.3 μm epitaxial quantum dot lasers directly grown on silicon

Abstract: This work compares the four-wave mixing (FWM) effect in epitaxial quantum dot (QD) lasers grown on silicon with quantum well (QW) lasers. A comparison of theory and experiment results shows that the measured FWM coefficient is in good agreement with theoretical predictions. The gain in signal power is higher for p-doped QD lasers than for undoped lasers, despite the same FWM coefficient. Owing to the near-zero linewidth enhancement factor, QD lasers exhibit FWM coefficients and conversion efficiency that are more than one order of magnitude higher than those of QW lasers. Thus, this leads to self-mode locking in QD lasers. These findings are useful for developing on-chip sources for photonic integrated circuits on silicon.

High-quality two-dimensional electron gas in undoped InSb quantum wells

Abstract: We report on transport experiments through high-mobility gate-tunable undoped InSb quantum wells (QWs). Due to the elimination of any Si modulation doping, the gate-defined two-dimensional electron gases in the QWs display a significantly increased mobility of $260\phantom{\rule{0.16em}{0ex}}000\phantom{\rule{0.16em}{0ex}}\mathrm{c}{\mathrm{m}}^{2}/\mathrm{Vs}$ at a rather low density of $2.4\phantom{\rule{4pt}{0ex}}\ifmmode\times\else\texttimes\fi{}{10}^{11}\phantom{\rule{0.16em}{0ex}}\mathrm{c}{\mathrm{m}}^{\text{--}2}$. Using magnetotransport experiments, we characterize spin-orbit interactions by measuring weak antilocalization. Furthermore, by measuring Shubnikov--de Haas oscillations in tilted magnetic fields, we find that the $g$ factor agrees with $\mathbf{k}\ifmmode\cdot\else\textperiodcentered\fi{}\mathbf{p}$ theory calculations at low magnetic fields but grows with spin polarization and carrier density at high magnetic fields. Additionally, signatures of Ising quantum Hall ferromagnetism are found at filling factor $\ensuremath{ u}=2$ for tilt angles where the Landau level energy equals the Zeeman energy. Despite the high mobility, the undoped InSb QWs exhibit no fractional quantum Hall effect up to magnetic fields of 25 T.

Investigation of Modulation Bandwidth of InGaN Green Micro-LEDs by Varying Quantum Barrier Thickness

Abstract: InGaN-based micro light-emitting diodes (micro-LEDs) with 5, 10, and 13 nm quantum barrier (QB) thickness were fabricated by metal-organic chemical vapor deposition (MOCVD) to investigate the influence of quantum-confined Stark effect (QCSE) on modulation bandwidth and luminous performance of devices. The room-temperature photoluminescence (PL), low-temperature time-resolved PL (TRPL), and electroluminescence (EL) results show that the decrease of QB thickness is beneficial to reduce QCSE of the device. The thinner QB thickness is good for improving the modulation bandwidth because the thinner QB is more beneficial to increase the total carrier recombination rate, but it is not conducive to the improvement of external quantum efficiency (EQE) due to degraded crystal quality. In addition, the modulation bandwidth of the device was calculated by using ABC model in combination with simulation. The calculation is in good agreement with the measured value. What is more, an optical link using an orthogonal-frequency division multiplexing (OFDM) modulation scheme was demonstrated. The transmission data rate increases with the increase of QB thickness from 1.309 to 1.773 Gbps, even though the modulation bandwidth decreases from 245 to 169 MHz. This work provides a way to balance the quantum efficiency and communication performance of green micro-LEDs.

Efficiency and Forward Voltage of Blue and Green Lateral LEDs with V-shaped Defects and Random Alloy Fluctuation in Quantum Wells

Abstract: For nitride-based blue and green light-emitting diodes (LEDs), the forward voltage $V_\text{for}$ is larger than expected, especially for green LEDs. This is mainly due to the large barriers to vertical carrier transport caused by the total polarization discontinuity at multiple quantum well and quantum barrier interfaces. The natural random alloy fluctuation in QWs has proven to be an important factor reducing $V_\text{for}$. However, this does not suffice in the case of green LEDs because of their larger polarization-induced barrier. V-defects have been proposed as another key factor in reducing $V_\text{for}$ to allow laterally injection into multiple quantum wells (MQWs), thus bypassing the multiple energy barriers incurred by vertical transport. In this paper, to model carrier transport in the whole LED, we consider both random-alloy and V-defect effects. A fully two-dimensional drift-diffusion charge-control solver is used to model both effects. The results indicate that the turn-on voltages for blue and green LEDs are both affected by random alloy fluctuations and V-defect density. For green LEDs, $V_\text{for}$ decreases more due to V-defects, where the smaller polarization barrier at the V-defect sidewall is the major path for lateral carrier injection. Finally, we discuss how V-defect density and size affects the results.

Role of critical thickness in SiGe/Si/SiGe heterostructure design for qubits

Abstract: We study the critical thickness for the plastic relaxation of the Si quantum well layer embedded in a SiGe/Si/SiGe heterostructure for qubits by plan-view transmission electron microscopy and electron channeling contrast imaging. Misfit dislocation segments form due to the glide of pre-existing threading dislocations at the interface of the Si quantum well layer beyond a critical thickness given by the Matthews–Blakeslee criterion. Misfit dislocations are mostly [Formula: see text] dislocations (b=a/2 <110>) that are split into Shockely partials (b=a/6 <112>) due to the tensile strain field of the Si quantum well layer. By reducing the quantum well thickness below critical thickness, misfit dislocations can be suppressed. A simple model is applied to simulate the misfit dislocation formation and the blocking process. We discuss consequences of our findings for the layer stack design of SiGe/Si/SiGe heterostructures for usage in quantum computing hardware.

InGaN micro-light-emitting diodes monolithically grown on Si: achieving ultra-stable operation through polarization and strain engineering

Abstract: Abstract Micro or submicron scale light-emitting diodes (µLEDs) have been extensively studied recently as the next-generation display technology. It is desired that µLEDs exhibit high stability and efficiency, submicron pixel size, and potential monolithic integration with Si-based complementary metal-oxide-semiconductor (CMOS) electronics. Achieving such µLEDs, however, has remained a daunting challenge. The polar nature of III-nitrides causes severe wavelength/color instability with varying carrier concentrations in the active region. The etching-induced surface damages and poor material quality of high indium composition InGaN quantum wells (QWs) severely deteriorate the performance of µLEDs, particularly those emitting in the green/red wavelength. Here we report, for the first time, µLEDs grown directly on Si with submicron lateral dimensions. The µLEDs feature ultra-stable, bright green emission with negligible quantum-confined Stark effect (QCSE). Detailed elemental mapping and numerical calculations show that the QCSE is screened by introducing polarization doping in the active region, which consists of InGaN/AlGaN QWs surrounded by an AlGaN/GaN shell with a negative Al composition gradient along the c -axis. In comparison with conventional GaN barriers, AlGaN barriers are shown to effectively compensate for the tensile strain within the active region, which significantly reduces the strain distribution and results in enhanced indium incorporation without compromising the material quality. This study provides new insights and a viable path for the design, fabrication, and integration of high-performance µLEDs on Si for a broad range of applications in on-chip optical communication and emerging augmented reality/mixed reality devices, and so on.

Identification of multi-color emission from coaxial GaInN/GaN multiple-quantum-shell nanowire LEDs

Abstract: Multi-color emission from coaxial GaInN/GaN multiple-quantum-shell (MQS) nanowire-based light-emitting diodes (LEDs) was identified. In this study, MQS nanowire samples for LED processes were selectively grown on patterned commercial GaN/sapphire substrates using metal–organic chemical vapor deposition. Three electroluminescence (EL) emission peaks (440, 540, and 630 nm) were observed, which were primarily attributed to the nonpolar m-planes, semipolar r-planes, and the polar c-plane tips of nanowire arrays. A modified epitaxial growth sequence with improved crystalline quality for MQSs was used to effectively narrow the EL emission peaks. Specifically, nanowire-based LEDs manifested a clear redshift from 430 nm to 520 nm upon insertion of AlGaN spacers after the growth of each GaInN quantum well. This demonstrates the feasibility of lengthening the EL emission wavelength since an AlGaN spacer can suppress In decomposition of the GaInN quantum wells during ramping up the growth temperature for GaN barriers. EL spectra showed stable emission peaks as a function of the injection current, verifying the critical feature of the non-polarization of GaN/GaInN MQSs on nanowires. In addition, by comparing EL and photoluminescence spectra, the yellow-red emission linked to the In-fluctuation and point defects in the c-plane MQS was verified by varying the activation annealing time and lowering the growth temperature of the GaInN quantum wells. Therefore, optimization of MQS nanowire growth with a high quality of c-planes is considered critical for improving the luminous efficiency of nanowire-based micro-LEDs/white LEDs.

Milliwatt-power far-UVC AlGaN LEDs on sapphire substrates

Abstract: AlGaN LEDs emitting < 230 nm UV light were fabricated on sapphire substrates. We employed a quantum well (QW) with an extremely thin barrier to enhance the quantum confinement of holes, wherein the calculation showed that the topmost valence subband became [Formula: see text]-like and increased the transverse-electric polarized emission. Additionally, we modified the Al composition of the spacer layer situated between the QW and an electron-blocking layer, which significantly improved the current-injection efficiency. The combination and optimization of these structures produced an LED emission of 228-nm UV light with an output power of 1.4 mW at 150 mA.

Enhanced interlayer neutral excitons and trions in MoSe2/MoS2/MoSe2 trilayer heterostructure

Abstract: Van der Waals heterostructures have recently emerged, in which two distinct transitional metal dichalcogenide (TMD) monolayers are stacked vertically to generate interlayer excitons (IXs), offing new opportunites for the design of optoelectronic devices. However, the bilayer heterostructure with type-II band alignment can only produce low quantum yield. Here, we present the observation of interlayer neutral excitons and trions in the MoSe2/MoS2/MoSe2 trilayer heterostructure (Tri-HS). In comparison to the 8 K bilayer heterostructure, the addition of a MoSe2 layer to the Tri-HS can significantly increase the quantum yield of IXs. It is believed the two symmetrical type-II band alignments formed in the Tri-HS could effectively promote the IX radiation recombination. By analyzing the photoluminescence (PL) spectrum of the IXs at cryogenic temperature and the power dependence, the existence of the interlayer trions was confirmed. Our results provide a promising platform for the development of more efficient optoelectronic devices and the investigation of new physical properties of TMDs.

Cubic GaN and InGaN/GaN quantum wells

Abstract: LEDs based on hexagonal InGaN/GaN quantum wells are dominant technology for many lighting applications. However, their luminous efficacy for green and amber emission and at high drive currents remains limited. Growing quantum wells instead in the cubic phase is a promising alternative because, compared to hexagonal GaN, it benefits from a reduced bandgap and is free of the strong polarization fields that can reduce the radiative recombination rate. Initial attempts to grow cubic GaN in the 1990s employed molecular beam epitaxy, but now, metal-organic chemical vapor deposition can also be used. Nonetheless, high phase purity requires careful attention to growth conditions and the quantification of any unwanted hexagonal phase. In contrast to hexagonal GaN, in which threading dislocations are key, at its current state of maturity, the most important extended structural defects in cubic GaN are stacking faults. These modify the optical properties of cubic GaN films and propagate into active layers. In quantum wells and electron blocking layers, segregation of alloying elements at stacking faults has been observed, leading to the formation of quantum wires and polarized emission. This observation forms part of a developing understanding of the optical properties of cubic InGaN quantum wells, which also offer shorter recombination lifetimes than their polar hexagonal counterparts. There is also growing expertise in p-doping, including dopant activation by annealing. Overall, cubic GaN has rapidly transitioned from an academic curiosity to a real prospect for application in devices, with the potential to offer specific performance advantages compared to polar hexagonal material.