Multiple Quantum Well Structures Research Articles

Demonstration of a three-active-section DFB laser with photon-photon resonance and detuned loading effects to obtain enhanced bandwidth and maintained high output power

Enhancement of the modulation bandwidth of the directly modulated semiconductor laser has attracted tremendous attention for applications in optical fiber communication and optical interconnects. However, modulation bandwidth is enhanced by some special technology such as integrating the active region or passive region regularly, which lead to low power and complex fabrication. Here we design and demonstrate a monolithic integrated three-active-section distributed feedback (TAS-DFB) laser with enhanced bandwidth theoretically and experimentally. The laser includes three sections: the DFB section, feedback section and active phase section. The three sections share the same multiple quantum-well structure. To enhance the modulation bandwidth beyond the intrinsic modulation bandwidth, both photon-photon resonance (PPR) and detuned loading (DL) effects are utilized to achieve over 55 GHz. The active phase section shows promise in achieving high bandwidth with PPR and relatively high output power of ∼ 10 mW at 25 °C under continuous-wave (CW) operation simultaneously. The fabrication process of the TAS-DFB laser is simplified due to the conventional and identical active layer structure.

Engineering of Impact Ionization Characteristics in GaAs/GaAsBi Multiple Quantum Well Avalanche Photodiodes.

The presence of large bismuth (Bi) atoms has been shown to increase the spin-orbit splitting energy in bulk GaAsBi, reducing the hole ionization coefficient (β) and thereby reducing the excess noise seen in avalanche photodiodes. In this study, we show that even very thin layers of GaAsBi introduced as quantum wells (QWs) in a GaAs matrix exhibit a significant reduction of β while leaving the electron ionization coefficient, α, largely unchanged. The optical and avalanche multiplication properties of a series of GaAsBi/GaAs multiple quantum well (MQW) p-i-n structures with nominally 5 nm thick, 4.4% Bi GaAsBi QWs, varying from 5 to 63 periods and corresponding barrier widths of 101 to 4 nm were investigated. From photoluminescence, ω-2θ X-ray diffraction, and cross section transmission electron microscopy measurements, the material was found to be of high quality despite the strain introduced by the Bi in all except the samples with 54 and 63 QW periods. Photomultiplication measurements undertaken with different wavelengths showed that α in these MQW structures did not change appreciably with the number of QWs; however, β decreased significantly, especially at lower values, the noise factor, F, is reduced by 58% to 3.5 at a multiplication of 10, compared to a similar thickness bulk GaAs structure without any Bi. This result suggests that Bi-containing QWs could be introduced into the avalanching regions of APDs as a way of reducing their excess noise.

Influence of low-temperature cap layer thickness on the structure and luminescence of InGaN/GaN multiple quantum wells

In order to effectively regulate the luminescence performance of MQW and enhance its optical quality, it is crucial to investigate the InGaN/GaN MQW's structure and luminescence properties. In this study the focus is how they are affected by low-temperature cap (LT-cap) layer's thickness which is grown above each InGaN well layer during growth process of InGaN/GaN multiple quantum well (MQW) samples in MOCVD system. This was achieved by analyzing high resolution X-ray diffraction (HRXRD) spectra, electroluminescence (EL) spectra, temperature-dependent photoluminescence (TDPL) spectra, and micro-area fluorescence imaging of these samples. The results show that changes in LT-cap layer's thickness even have no significant impact on some structural parameters of MQW, such as the thickness of the well layer, but have an influence on the In component of the well layer. Due to the existence of LT-cap layer, dissociation of InGaN can be effectually reduced. In addition, the augmentation of LT-cap layer's thickness will make polarization effect of the QW sample more remarkable, so that the blue shift of the EL peak with the augmentation of current injection increases. The change of LT-cap layer's thickness will also influence distribution of the tail states of the quantum wells, which leads to a different localization states for injected carriers. As LT-cap layer becomes getting thicker, the material's internal quantum efficiency (IQE) tends to decrease, which may result from an increase in non-radiative recombination centers.

Selective area grown photonic integrated chips for completely suppressing the Stokes shift

In this work, we report on the selective area growth (SAG) of InGaN multiple quantum well (MQW) structures to completely suppress the phenomenon of the Stokes shift in monolithically integrated photonic chips. The original green MQW region is designed as the integrated photodetector (PD), while the SAG blue MQW region acts as the integrated light-emitting diode (LED). The detection spectra of the PD can completely cover the emission spectra of the LED, greatly improving the on-chip optical connection by the complete suppression of the Stokes shift. Thus, the bottleneck of on-chip optical connection based on spectra-tail overlap in integrated photonic chips has been broken. Under the same operating current, the photocurrent of the SAG integrated PD reaches 11.8 μA, while the conventional chip achieves only 0.6 μA. By SAG method, the photo-to-dark current ratio of integrated PD exhibits about two orders of magnitude increase under 0 V bias. Undoubtedly, the SAG technology provides a strategy to further improve the on-chip optical signal transmission efficiency of the MQW structure integrated photonic chips.

InGaAs/AlGaAs MQWs grown by MBE: Optimizing GaAs insertion layer thickness to enhance interface quality and luminescent property

The refinement of interface structure in InGaAs/AlGaAs multiple quantum wells (MQWs) through molecular beam epitaxy (MBE) is crucial for enhancing their luminescence performance. The influencing mechanisms of the GaAs insertion layer (ISL) thicknesses on the interface structure and luminescence properties of InGaAs/AlGaAs MQWs were investigated by varying the GaAs ISL thickness variations and optimizing growth parameters. In our designed InGaAs/AlGaAs MQWs structures, GaAs ISL with thickness variables of 0 nm, 0.5 nm, 1 nm, 2 nm, and 4 nm were inserted between the interfaces of the InGaAs well layer and the AlGaAs barrier layer. Through experimental characterizations. The introduction of the GaAs ISL resulted in a gradual transition of the MQWs surface morphology from a three-dimensional (3D) island structure to a typical step-flow surface morphology, leading to the reduction in surface roughness and enhancement of hetero-interfaces quality. Moreover, the GaAs ISL with thickness exceeding 2 nm effectively inhibited the segregation of indium atoms, promoting the ordering of interface alloys and decreasing the polarization degrees from 3.76 % to 2.09 %. Temperature-dependent PL measurements indicated that the GaAs ISL could reduce interface defects and the density of non-radiative recombination centers, thereby enhancing the radiative recombination efficiency of MQWs and ultimately increasing the peak intensity of photoluminescence by 3.4 times. This work is significance for understanding the luminescent properties of InGaAs/AlGaAs MQWs, which provides reference highly efficient optoelectronic devices development.

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.

Effects of high-temperature annealing on vacancy complexes and luminescence properties in multilayer periodic structures with elastically strained GeSiSn layers

Effects of postgrowth high-temperature annealing on vacancy complexes and photoluminescence (PL) from GeSiSn/Si multiple quantum wells (MQWs) are studied. The series of PL peaks related to the vacancy-tin complexes was observed for as-grown samples including different structures, such as GeSiSn/Si MQWs, multilayer periodic structure with GeSiSn quantum dots (QDs), GeSn cross-structures upon GeSiSn/Si MQWs, and thick GeSiSn layers. The PL band intensity is significantly reduced after annealing at 700 °C corresponding to the reduction in vacancy density, as demonstrated by the positron annihilation spectroscopy (PAS) data. Such annealing also results in the appearance of the PL signal related to the interband optical transitions in GeSiSn/Si MQWs. However, the high temperature could negatively impact the sharpness of heterointerfaces due to Sn diffusion, thus limiting the PL efficiency. To improve the luminescence properties of GeSiSn/Si structures, we proposed a two-stage technique combining both the annealing and subsequent treatment of samples in a hydrogen plasma at 200 °C. The plasma treatment significantly reduces the PL band of vacancy-related defects, whereas annealing at a moderate temperature of ∼600 °C prevents the blurring of heterointerfaces. As a result, we demonstrate an increase in the relative efficiency of interband PL of type II GeSiSn/Si MQW structures emitting in the range of 1.5–2 μm.

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.