Multiple Quantum Well Structures Research Articles

Investigation of optical polarization characteristics of ultraviolet-C AlGaN multiple quantum wells by angle-resolved cathodoluminescence

AlGaN-based ultraviolet-C (UV-C) light-emitting diodes (LEDs) face challenges related to their extremely low external quantum efficiency, which is predominantly attributed to the remarkably inadequate transverse magnetic (TM) light extraction efficiency (LEE). In this study, we employ angle-resolved cathodoluminescence (ARCL) spectroscopy to assess the optical polarization of (0001)-oriented AlGaN multiple quantum well (MQW) structures in UV-C LEDs, in conjunction with a focused ion beam and scanning electron microscopy (FIB/SEM) system to etch samples with various inclination angles (θ) of sidewall. This technique effectively distinguishes the spatial distribution of TM- and transverse electric (TE)-polarized photons contributing to the luminescence of the MQW structure. CL spectroscopy confirms that UV-C LEDs with a θ of 35° exhibit the highest CL signal compared to samples with other θ. Furthermore, we establish a model using finite difference time domain (FDTD) simulation to validate the mechanism of the outcomes. The complementary contribution of TM and TE photons at different specific angles are distinguished by ARCL and confirmed by simulation. At angles near the sidewall, the CL is dominated by the TM photons, which mainly contribute to the increased LEE and the decreased degree of polarization (DOP) to make the spatial distribution of CL more uniform. Additionally, this method allows us to analyze the polarization of light without the need for polarizers, enabling the differentiation of TE and TM modes. This distinction provides flexibility for selecting different emission mode based on various application requirements. The presented approach not only opens up new opportunities for enhanced UV-C light extraction but also provides valuable insights for future endeavors in device fabrication and epitaxial film growth.

Optical proximity sensors using multiple quantum well didoes

InGaN/GaN multiple quantum well (MQW) diodes perform multiple functions, such as optical emission, modulation and reception. In particular, the partially overlapping spectral region between the electroluminescence (EL) and responsivity spectra of each diode results in each diode being able to sense light from another diode of the same MQW structure. Here, we present a noncontact, optical proximity sensing system by integrating an MQW-based light transmitter and detector into a tiny GaN-on-sapphire chip. Changes in the external environment modulate the light emitted from the transmitter. Reflected light is received by the on-chip MQW detector, wherein the carried external modulation information is converted into electrical signals that can be extracted. The maximum detection proximity is approximately 17 mm, and the displacement detection accuracy is within 1 mm. Based on the detection of distance, we extend the application of the sensor to vibration and pressure detection. This monolithic integration design can replace external discrete light transmitter and detector systems to miniaturize reflective sensor architectures, enabling the development of novel optical sensors.

Effect of InAs insertion layer on the structural and optical property improvement of InGaAs/InAlAs multiple quantum wells

A 100 periods In0.53Ga0.47As/In0.52Al0.48As multiple quantum wells (MQWs) with and without 2 ML InAs insertion layers (ISLs) were grown on semi-insulating GaAs (001) substrates by molecular beam epitaxy (MBE). The stress-strain was weakened by inserting two monolayers of InAs between the In0.53Ga0.47As well layer and the In0.52Al0.48As barrier layer. High-resolution transmission electron microscopy (HRTEM) was investigated to visually characterize the crystal quality of MQW structures with and without InAs ISLs. The effect of structural and optical properties of the In0.53Ga0.47As/In0.52Al0.48As MQWs by InAs-inserting were characterized by X-ray diffraction (XRD) and photoluminescence (PL) spectra. The research shows that the introduction of the 2 ML InAs ISLs can eliminate the strain and reduce the average dislocation density from 2.4×108 cm−2 to 1.4×108 cm−2 in the InGaAs/InAlAs MQWs structure, then control the interface between InGaAs and InAlAs, and finally realize the improvement of the quantum well structural and optical properties with the PL intensity increased by one order of magnitude. Our studies have great significance to the fabrication of GaAs-based multiple quantum well optoelectronic devices, which promote the further development of high-performance versatile optoelectronic applications.

Monolithic III-nitride photonic circuit on a single chip

Inserting multiple quantum wells (MQWs) into a p–n junction, III-nitride MQW diodes can separately function as a light transmitter, modulator, and receiver under different bias conditions. Owing to the spectral overlap between the emission and responsivity spectra, the emitted light from the transmitter is able to be modulated and detected by the modulator and receiver, which have identical MQW structures. Here, we develop a compatible fabrication process to monolithically integrate an III-nitride light transmitter, waveguides, Y-splitter, modulators, Y-combiner, and receiver into a tiny chip. An on-chip 405 nm light communication system is established and exhibits a transmission rate of 260 Mbps in the non-return-to-zero on-off keying scheme. The results pave a feasible route to develop sophisticated monolithic photonic circuit on an III-nitride-on-silicon platform.

Ruddlesden–Popper and Dion–Jacobson Perovskites in Multiple Quantum Wells Light‐Emitting Diodes

AbstractLow dimensional perovskite forms multiple quantum wells (MQWs) structures that can be an efficient structure for light‐emitting diodes (LEDs). Here the use of different barrier molecules is demonstrated to create Ruddlesden–Popper and Dion–Jacobson 2D perovskite and implement them in LEDs. It is found that the aromatic barrier molecules are better performing in Perovskite LED (PeLED) compared to linear barrier molecules. The benzylammonium (BnzA), n = 5, shows the highest external quantum efficiency (EQE) of 16.65% compared to the di‐ammonium and the linear barrier molecules. The BnzA forms n = 1 inside the 3D perovskite, which provides a non‐radiative energy transfer from the wider bandgap, n = 1 perovskite, to the smaller bandgap, 3D perovskite, that presents radiative recombination. Optical characterizations support the MQW structure whereas physical characterizations show unique morphology of separate nanocrystals laying on top of thin perovskite film during the n = 5 BnzA crystallization. In addition, it is revealed that the barrier molecule type, in this case, the BnzA, is responsible for the formation of the MQW structure whereas other barrier molecules didn't show this structure. The formation of an additional low n‐value phase inside the 3D perovskite is beneficial for the enhancement of the radiative recombination which results in high EQE.

Insights into the Growth Orientation and Phase Stability of Chemical-Vapor-Deposited Two-Dimensional Hybrid Halide Perovskite Films.

Chemical vapor deposition (CVD) offers a large-area, scalable, and conformal growth of perovskite thin films without the use of solvents. Low-dimensional organic-inorganic halide perovskites, with alternating layers of organic spacer groups and inorganic perovskite layers, are promising for enhancing the stability of optoelectronic devices. Moreover, their multiple quantum-well structures provide a powerful platform for tuning excitonic physics. In this work, we show that the CVD process is conducive to the growth of 2D hybrid halide perovskite films. Using butylammonium (BA) and phenylethylammonium (PEA) cations, the growth parameters of BA2PbI4 and PEA2PbI4 and mixed halide perovskite films were first optimized. These films are characterized by well-defined grain boundaries and display characteristic absorption and emission features of the 2D quantum wells. X-ray diffraction (XRD) and a noninteger dimensionality model of the absorption spectrum provide insights into the orientation of the crystalline planes. Unlike BA2PbI4, temperature-dependent photoluminescence measurements from PEA2PbI4 show a single excitonic peak throughout the temperature range from 20 to 350 K, highlighting the lack of defect states. These results further corroborate the temperature-dependent synchrotron-based XRD results. Furthermore, the nonlinear optical properties of the CVD-grown perovskite films are investigated, and a high third harmonic generation efficiency is observed.

Experimental evidence of hole injection through V-defects in long wavelength GaN-based LEDs

Hole injection through V-defect sidewalls into all quantum wells (QWs) of long wavelength GaN light emitting diodes had previously been proposed as means to increase efficiency of these devices. In this work, we directly tested the viability of this injection mechanism by electroluminescence and time-resolved photoluminescence measurements on a device in which QW furthest away from the p-side of the structure was deeper, thus serving as an optical detector for presence of injected electron–hole pairs. Emission from the detector well confirmed that, indeed, the holes were injected into this QW, which could only take place through the 101¯1 V-defect sidewalls. Unlike direct interwell transport by thermionic emission, this transport mechanism allows populating all QWs of a multiple QW structure despite the high potential barriers in the long wavelength InGaN/GaN QWs.

Constructing of amorphous/PbSe/amorphous multiple quantum wells with record high thermoelectric properties

Quantum well (QW) is one of the proposals to improve the thermoelectric properties. However, previous studies have shown that the large electron and phonon tunneling across the layers in semiconductor-based QWs and oxide-based QWs, respectively, are the two main challenges that need to be addressed. In this work, PbSe/amorphous-SrTiO3 (α-STO) supperlattices with a multiple quantum-well (MQW) structure can overcome both limitations. We demonstrate that the PbSe/α-STO MQW shows a high power factor of ∼ 24.5 μW cm−1K−2 at room temperature due to the strong quantum confine effect. In addition, the amorphous-SrTiO3 layer can engender intrinsically low thermal conductivity (0.27± 0.05 W m−1 K−1). The maximum estimated zT value is 2.7 for PbSe/α-STO MQW at room temperature, which is more than 1800 % higher than that of pure PbSe films (zT = 0.15).

Effects of Thermal-Strain-Induced Atomic Intermixing on the Interfacial and Photoluminescence Properties of InGaAs/AlGaAs Multiple Quantum Wells.

Quantum-well intermixing (QWI) technology is commonly considered as an effective methodology to tune the post-growth bandgap energy of semiconductor composites for electronic applications in diode lasers and photonic integrated devices. However, the specific influencing mechanism of the interfacial strain introduced by the dielectric-layer-modulated multiple quantum well (MQW) structures on the photoluminescence (PL) property and interfacial quality still remains unclear. Therefore, in the present study, different thicknesses of SiO2-layer samples were coated and then annealed under high temperature to introduce interfacial strain and enhance atomic interdiffusion at the barrier-well interfaces. Based on the optical and microstructural experimental test results, it was found that the SiO2 capping thickness played a positive role in driving the blueshift of the PL peak, leading to a widely tunable PL emission for post-growth MQWs. After annealing, the blueshift in the InGaAs/AlGaAs MQW structures was found to increase with increased thickness of the SiO2 layer, and the largest blueshift of 30 eV was obtained in the sample covered with a 600 nm thick SiO2 layer that was annealed at 850 °C for 180 s. Additionally, significant well-width fluctuations were observed at the MQW interface after intermixing, due to the interfacial strain introduced by the thermal mismatch between SiO2 and GaAs, which enhanced the inhomogeneous diffusion rate of interfacial atoms. Thus, it can be demonstrated that the introduction of appropriate interfacial strain in the QWI process is of great significance for the regulation of MQW band structure as well as the control of interfacial quality.

Miniature GaN optoelectronic temperature sensor.

The combination of plastic optical fiber (POF) with monolithically integrated transmitter and receiver is becoming increasingly attractive for the development of miniature optoelectronic sensing systems. Here, we propose a temperature sensing system by integrating a GaN optoelectronic chip with a POF and aluminum (Al) reflector. Owing to the overlap between electroluminescence and responsivity spectra of multiple quantum well (MQW) diodes, both the transmitter and the receiver having identical MQW structures are monolithically integrated on a tiny GaN chip by using the same fabrication process flow. Environmental temperature change leads to thermal deformation in the Al reflector, which reflects the transmitted light back with a light pulse. The reflected light is coupled into the guided POF again and sensed by the on-chip receiver. Finally, the temperature information is read out as electrical signals. When the ambient temperature changes from 20.1°C to 100°C, the optically induced electrical signal decreases from -3.04 µA to -3.13 µA. The results suggest that the monolithically integrated GaN device offers a promising option for optoelectronic temperature sensing systems.

Extended emission wavelength beyond 2.2 μ m in strained multiple-quantum-well laser using InGaAsSb material grown on InP substrate

We report on the growth and lasing characteristics of 2.3%-compressive-strained InGaAsSb multiple-quantum-well (MQW) lasers on InP substrates with emission wavelengths near 2.2 μm. MQW structures with four wells were grown by metalorganic molecular beam epitaxy at 500 °C. X-ray diffraction and photoluminescence results showed that the InGaAsSb well layers were grown with the assistance of the Sb surfactant effect. The emission wavelengths of the MQW lasers with well thicknesses of 6.4 and 8.4 nm were 2.190 and 2.278 μm, respectively. For the MQW laser with the well thickness of 8.4 nm, the threshold current under continuous-wave operation was 22 mA at 15 °C, and the characteristic temperature was estimated to be 53 K in the temperature region from 15 to 35 °C and 42 K in the region from 35 to 55 °C. The laser with the 8.4-nm-thick well had an emission wavelength about 90 nm longer than that of the one with the 6.4-nm-thick well, but the lasing characteristics of the two were comparable.

Application of nano-patterned InGaN fabricated by self-assembled Ni nano-masks in green InGaN/GaN multiple quantum wells

The nano-patterned InGaN film was used in green InGaN/GaN multiple quantum wells (MQWs) structure, to relieve the unpleasantly existing mismatch between high indium content InGaN and GaN, as well as to enhance the light output. The different self-assembled nano-masks were formed on InGaN by annealing thin Ni layers of different thicknesses. Whereafter, the InGaN films were etched into nano-patterned films. Compared with the green MQWs structure grown on untreated InGaN film, which on nano-patterned InGaN had better luminous performance. Among them the MQWs performed best when 3 nm thick Ni film was used as mask, because that optimally balanced the effects of nano-patterned InGaN on the crystal quality and the light output.

Improving the short-wave infrared response of strained GeSn/Ge multiple quantum wells by rapid thermal annealing

In this work, the evolution of structural, optical and optoelectronic properties of coherently strained Ge0.883Sn0.117/Ge multiple quantum wells (MQWs) grown by molecular beam epitaxy under rapid thermal annealing (RTA) is systematically investigated. The MQW structure remains fully-strained state with RTA at 400 °C or below and disrupts at higher annealing temperatures due to Sn segregation and interdiffusion of Ge and Sn atoms. The GeSn well layers exhibit the strongest absorption in 2.0–2.4 μm after annealing at 400 °C and become transparent above 1.8 μm after RTA at 600 °C or beyond due to serve Sn segregation. Owing to improved crystal quality after RTA at 400 °C, the dark current of the fabricated metal-semiconductor-metal photodetector is effectively lowered by more than two times. Additionally, the responsivities at 1.55 and 2.0 μm are improved by 4.15 and 3.78 folds, respectively, compared to those of the as-grown sample. The results can be an insightful guidance for the development of high-performance short-wave infrared photonic devices based on Sn-containing group-IV low-dimensional structures.

Monolithically integrated photonic chips with asymmetric MQWs structure for suppressing Stokes shift

An asymmetric (ASY) multiple quantum wells (MQWs) structure consisting of emission and detection regions with different In components and thicknesses is presented for suppressing the Stokes shift in monolithically integrated photonic chips. Compared with conventional MQWs, the total luminescence intensity of ASY MQWs is improved due to the action of the potential field for transferring more holes to the quantum well near the n-GaN side. Meanwhile, resulted from about 25–30 nm redshift in response spectra, a 4.5-fold increase in the overlap of luminescence-detection spectra is realized. A photodetector exhibits a photo-to-dark current ratio of up to 107 at 0 V bias. Furthermore, the reported ASY MQWs diode shows a maximum bandwidth (−3 dB) of 173 MHz, suggesting that a monolithically integrated chip has tremendous potential on the application of the on-chip visible light communication.

Growth and characterization of GePb/Ge multiple quantum wells

GePb/Ge multiple quantum well (MQW) structures with a Pb content of up to 7.2% were successfully grown on Ge(100) substrates via sputtering epitaxy. Scanning electron microscopy revealed that both the Pb content in the GePb layer and Ge layer thickness affect the Pb surface segregation. High-resolution X-ray diffraction and cross-sectional transmission electron microscopy indicated that the GePb/Ge MQW structures had a high crystal quality. Moreover, the thermal stability of the GePb/Ge MQW structures was investigated, and the MQW structures were found to be stable at an annealing temperature of 500 °C. The band gaps of the GePb/Ge MQWs were measured, and the band gap regulation caused by the quantum confinement effect was verified. These results indicate that the MQW structure is a promising approach for realizing efficient GePb light sources.

Full-duplex visible light communication system using a single channel.

Multiple quantum well (MQW) III-nitride diodes can emit light and detect light at the same time. In particular, given the overlapping region between the emission spectrum and the detection spectrum, the III-nitride diode can absorb photons of shorter wavelengths generated from another III-nitride diode with the same MQW structure. In this study, a wireless visible light communication system was established using two pairs of identical III-nitride diodes with different wavelengths. In this system, two green light diode chips were used to transmit and receive green light signals on both sides. We have integrated two blue light chips with optical filtering in the middle of the optical link to carry out blue light communication, with one end transmitting and one end receiving. Simultaneously, green light was allowed to pass through two blue light chips for optical communication. Combined with a distributed Bragg reflection (DBR) coating, we proposed using four chips in one optical path to carry out optical communication between chips with the same wavelength and used the coating principle to gate the optical wavelength to filter the clutter of green light chips on both sides to make the channel purer and the symbols easier to demodulate. Based on this multifunctional equipment, advanced single-optical path, III-nitride, full-duplex optical communication links can be developed for the deployment of the Internet of Things.

MOCVD growth and thermal stability analysis of 1.2 µm InGaAs/GaAs multi quantum well structure

InGaAs/GaAs multiple quantum well (MQW) is desirable for many optoelectronic devices with the wavelength around 1.2 µm. However, the high defect density and instability caused by high strain have always been the problems. In this paper, an InGaAs/GaAs MQW structure was grown on GaAs substrate at low temperature by metal-organic chemical vapor deposition (MOCVD) technology. The InGaAs layers were grown with triethylgallium (TEGa) and trimethylgallium (TMGa), respectively. We studied the stability of InGaAs/GaAs MQW by evaluating the photoluminescence (PL) peak shift after annealing. It was found that the peaks of MQW grown by TEGa and TMGa were both blue-shifted after annealing. A smaller shift (<4 nm) in the MQW grown with TEGa exhibits better thermal stability compared to that of MQW grown with TMGa. This could be due to the weaker indium (In) and gallium (Ga) interdiffusion at the InGaAs/GaAs interface for the sample grown with TEGa. Furthermore, we fabricated a vertical external cavity surface emitting laser (VECSEL) based on the 6-period InGaAs/GaAs MQW grown with TEGa with an emission wavelength of ~1.2 µm.

Optical Properties of Perovskite-Organic Multiple Quantum Wells.

A comprehensive study of the optical properties of CsPbBr3 perovskite multiple quantum wells (MQW) with organic barrier layers is presented. Quantum confinement is observed by a blue‐shift in absorption and emission spectra with decreasing well width and agrees well with simulations of the confinement energies. A large increase of emission intensity with thinner layers is observed, with a photoluminescence quantum yield up to 32 times higher than that of bulk layers. Amplified spontaneous emission (ASE) measurements show very low thresholds down to 7.3 µJ cm−2 for a perovskite thickness of 8.7 nm, significantly lower than previously observed for CsPbBr3 thin‐films. With their increased photoluminescence efficiency and low ASE thresholds, MQW structures with CsPbBr3 are excellent candidates for high‐efficiency perovskite‐based LEDs and lasers.

The improvement properties of InGaAs/InGaAsP multiple quantum wells using the GaAs insertion layer

InGaAs/InGaAsP multiple quantum wells (MQWs) as semiconductor materials are commonly used in semiconductor lasers and solar cells. However, the problem of the increase in surface roughness due to the diffusion of P atoms in the barrier layer caused by the memory effect and the As/P exchange between the InGaAs well layer and the InGaAsP barrier layer has not been well resolved. In this work, the GaAs material was designed as the insertion layer (ISL) and the residual group-V source evacuation (RSE) methord was used in the structure of InGaAs/InGaAsP MQWs. It was found that when the GaAs ISL thickness was 6 nm and the RSE time was 20 s, the performance of InGaAs/InGaAsP MQWs was the best. Test results showed that the InGaAsP barrier ensured more radiative recombination in the MQWs structure. Meanwhile, the GaAs ISL reduced the diffusion of As and P atoms in the potential layer and improved the crystal quality. The InGaAs/InGaAsP MQWs with GaAs ISL create conditions for innovative applications of optimally integrated next-generation semiconductor lasers.

Optical properties of InGaN-based red multiple quantum wells

In this work, we present the characterization of red InGaN/GaN multiple-quantum-well (MQW) light-emitting diode structures. The optical properties of two MQW structures with different n-GaN underlayer thicknesses (4 and 8 μm) are studied and compared. The results of photoluminescence studies show that a thicker n-GaN layer is beneficial for obtaining higher In content for red MQWs. However, the sample with a thicker n-GaN layer has a poorer internal quantum efficiency, a larger full width at half maximum, and a shorter nonradiative recombination time, implying that there are stronger In-content fluctuations and more defects. Furthermore, red MQWs with higher In content are shown to exhibit more deep localized states. Our findings imply that in order to achieve high-efficiency InGaN MQWs for red emission, enhancing the uniformity of In-content distribution in the active region and decreasing nonradiative recombination centers are critical challenges.