Strained Quantum Wells Research Articles

Semiconductor Optical Amplifiers with Wide Gain Bandwidth and Enhanced Polarization Insensitivity Based on Tensile-Strained Quantum Wells.

The paper presents a wide-bandwidth, low-polarization semiconductor optical amplifier (SOA) based on strained quantum wells. By enhancing the material gain of quantum wells for TM modes, we have extended the gain bandwidth of the SOA while reducing its polarization sensitivity. Through a combination of tilted waveguide design and cavity surface optical thin film design, we have effectively reduced the cavity surface reflectance of the SOA, thus decreasing device transmission losses and noise figure. At a wavelength of 1550 nm and a drive current of 1.4 A, the output power can reach 188 mW, with a small signal gain of 36.4 dB and a 3 dB gain bandwidth of 128 nm. The linewidth broadening is only 1.032 times. The polarization-dependent gain of the SOA is below 1.4 dB, and the noise figure is below 5.5 dB. The device employs only I-line lithography technology, offering simple fabrication processes and low costs yet delivering outstanding and stable performance. The designed SOA achieves wide gain bandwidth, high gain, low polarization sensitivity, low linewidth broadening, and low noise, promising significant applications in the wide-bandwidth optical communication field across the S + C + L bands.

Low disorder and high valley splitting in silicon

The electrical characterisation of classical and quantum devices is a critical step in the development cycle of heterogeneous material stacks for semiconductor spin qubits. In the case of silicon, properties such as disorder and energy separation of conduction band valleys are commonly investigated individually upon modifications in selected parameters of the material stack. However, this reductionist approach fails to consider the interdependence between different structural and electronic properties at the danger of optimising one metric at the expense of the others. Here, we achieve a significant improvement in both disorder and valley splitting by taking a co-design approach to the material stack. We demonstrate isotopically purified, strained quantum wells with high mobility of 3.14(8) × 105 cm2 V−1 s−1 and low percolation density of 6.9(1) × 1010 cm−2. These low disorder quantum wells support quantum dots with low charge noise of 0.9(3) μeV Hz−1/2 and large mean valley splitting energy of 0.24(7) meV, measured in qubit devices. By striking the delicate balance between disorder, charge noise, and valley splitting, these findings provide a benchmark for silicon as a host semiconductor for quantum dot qubits. We foresee the application of these heterostructures in larger, high-performance quantum processors.

Strain control and its effect on the optical properties of InGaN/GaN multiple quantum wells

The strain modulation mechanism of green InGaN quantum wells (QWs) was investigated by designing and growing additional InGaN/GaN strain modulation layers with different Indium (In) content and structures. Electroluminescence (EL) measurements were employed to analyze the strain in InGaN QWs by checking the change of EL peak energy with injection current. The experimental results suggest that proper In content in the InGaN strain modulation layer is required to reduce the compression strain in multiple quantum wells (MQWs). Meanwhile, the material quality and flatness of the pre-grown modulation layer may significantly affect the homogeneity and the strain state of MQWs. Temperature-dependent photoluminescence (TDPL) and micro-luminescence were measured to characterize the optical properties of MQWs prepared with strain modulation layers. By carefully modulating the strain in QWs, the InGaN/GaN green multiple quantum wells (MQWs) exhibit higher In incorporation and uniform luminescence.

Germanium wafers for strained quantum wells with low disorder

We grow strained Ge/SiGe heterostructures by reduced-pressure chemical vapor deposition on 100 mm Ge wafers. The use of Ge wafers as substrates for epitaxy enables high-quality Ge-rich SiGe strain-relaxed buffers with a threading dislocation density of (6±1)×105 cm−2, nearly an order of magnitude improvement compared to control strain-relaxed buffers on Si wafers. The associated reduction in short-range scattering allows for a drastic improvement of the disorder properties of the two-dimensional hole gas, measured in several Ge/SiGe heterostructure field-effect transistors. We measure an average low percolation density of (1.22±0.03)×1010 cm−2 and an average maximum mobility of (3.4±0.1)×106 cm2/Vs and quantum mobility of (8.4±0.5)×104 cm2/Vs when the hole density in the quantum well is saturated to (1.65±0.02)×1011 cm−2. We anticipate immediate application of these heterostructures for next-generation, higher-performance Ge spin-qubits, and their integration into larger quantum processors.

Selective Area Epitaxy of Highly Strained InGaAs Quantum Wells (980-990 nm) in Ultrawide Windows Using Metalorganic Chemical Vapor Deposition.

We employed the selective-area-epitaxy technique using metalorganic chemical vapor deposition to fabricate and study samples of semiconductor heterostructures that incorporate highly strained InGaAs quantum wells (980-990 nm emission wavelength). Selective area epitaxy of InGaAs quantum wells was performed on templates that had a patterned periodic structure consisting of a window (where epitaxial growth occurred) and a passive mask (where epitaxial growth was suppressed), each with a width of 100 µm for every element. Additionally, a selectively grown potential barrier layer was included, which was characterized by an almost parabolic curvature profile of the surface. We conducted a study on the influence of the curvature profile of the growth surface on the optical properties of InGaAs quantum wells and the spatial distribution of composition in an ultrawide window. Our results showed that, under fixed selective-area-epitaxy conditions, the composition of the InxGa1-xAs and the wavelength of the quantum-well emission changed across the width of the window. Our study demonstrates that increasing the curvature profile of the growth surface of highly strained quantum wells leads to a transition in the photoluminescence wavelength distribution profile across the window, from quasi-parabolic to inverted parabolic.

Surface Morphology and Photoluminescence Spectra of Pseudomorphic {InGaAs/GaAs} Superlattices on GaAs (100), (110), and (111)A Substrates

The production of superlattices with pseudomorphically strained quantum wells (QWs) {InxGa1–xAs/GaAs} grown by molecular beam epitaxy on GaAs substrates with (100), (110), and (111)A crys-tallographic surface orientations is reported. The quality of the crystal structure of epitaxial samples is assessed using the atomic force microscopy of their surface. The manifestation of a piezoelectric field in the photoluminescence spectra is reported.

Low temperature micro-photoluminescence spectroscopy of microstructures with InAsP/InP strained quantum wells

Ridge microstructures were prepared by reactive ion etching (RIE) of a series of stacked InAs x P quantum wells (QWs) with step graded compositions grown on InP by molecular beam epitaxy. These microstructures were characterized by low temperature micro-photoluminescence. The photoluminescence (PL) emission associated with each of the QWs was clearly identified and a model for their line shape was implemented. PL line-scans were measured across etched ridge stripes of various widths in an optical cryostat, with a spatial resolution of 1 µm. The model for the PL spectra allowed accurate extraction of the local PL integrated intensities, spectral positions and line widths. Two different RIE processes, using CH4/H2 and CH4/Cl2, were investigated. The PL line-scans showed strong variations of the integrated PL intensities across the etched stripes. The PL intensities for all QWs increased gradually from the edge to the center of the ridge microstructures, over a length scale of 10–20 µm. On the other hand, the spectral peak position of the PL lines remained constant (within an accuracy of 0.2–0.4 meV, depending on which QW was considered) across the microstructures. These observations are discussed in terms of the mechanical stress induced by the RIE processes, the relaxation of the biaxial built-in compressive stress in the InAsP QWs (induced by the free surfaces at the vertical etched sidewalls), and also by the non-radiative recombination at these sidewalls. Altogether, this study illustrates the contribution that specially designed test structures, coupled with advanced spectroscopic characterization, can provide to the development of semiconductor photonic devices (e.g. lasers or waveguides) involving RIE processing.

Micro-PL analysis of high current density resonant tunneling diodes for THz applications

Low-temperature micro-photoluminescence (μPL) is used to evaluate wafer structural uniformity of current densities >5mA/μm2 InGaAs/AlAs/InP resonant tunneling diode (RTD) structures on different length scales. Thin, highly strained quantum wells (QWs) are subject to monolayer fluctuations, leading to a large statistical distribution in their electrical properties. This has an important impact on the RTD device performance and manufacturability. The PL spot size is reduced using a common photolithography mask to reach a typical high Jpeak for a given RTD mesa size (1 ∼ 100 μm2). We observe that for lower strain-budget samples, the PL line shape is essentially identical for all excitation/collection areas. For higher strain-budget samples, there is a variation in the PL line shape that is discussed in terms of a variation in long-range disorder brought about by strain relaxation processes. The RTD operating characteristics are discussed in light of these findings, and we conclude that strain model limits overestimate the strain budget that can be incorporated in these devices. We also highlight μPL as a powerful nondestructive characterization method for RTD structures.

Influence of the doping type on the temperature dependencies of the photoluminescence efficiency of InGaAlAs/InGaAs/InP heterostructures

The temperature dependencies of photoluminescence efficiency of InGaAlAs/InGaAs/InP heterostructures with 1550 nm range strained quantum wells have been studied. The heterostructures had nine In0.74Ga0.26As quantum wells which were separated by In0.53Al0.20Ga0.27As barriers. The barriers were δ-doped with n- and p-type dopants with different layer concentrations of charge carriers. The both p-type and n-type doping of barriers leads to slight increase in the efficiency when doping level not exceeding values of (1–5)·1012 cm−2. Moreover, doping with an n-type dopant leads to a significant increase in the width of the photoluminescence spectrum, while doping with a p-type dopant leads to a decrease in its width, compared to a heterostructure with undoped barrier layers.

Low Threshold 1550-nm Emitting QD Optically Pumped VCSEL

We present the first demonstration of a vertical cavity surface emitting laser (VCSEL) based on InAs quantum dots (QDs) on InP. A very high density of 10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">11</sup> cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-2</sup> per QD layer, and a thin spacing layer of 15 nm are used to enhance the QD modal gain within the VCSEL cavity. Continuous wave (CW) operation is demonstrated at room temperature on optically pumped devices emitting at telecommunication wavelength (1550 nm), being the ground state transition of QDs. A very low threshold of 1.4 mW is obtained, corresponding to a value 6 times smaller than the best threshold we obtained on similar VCSEL structures integrating 1550 nm strained quantum wells. Wavelength tuning experiments have been also conducted due to a wedge cavity design. Laser emission in CW operation has been achieved on a large spectral window: from 1530 nm up to 1573 nm. A stable output polarization state with a polarization ratio exceeding 20 dB is also measured.

Biaxial strain effects on the band structure and absorption coefficient of GaAs1-x-yNxBiy/GaAs MQWs calculated using k.p method

In this work, we derive the 16 band strain-dependent k·p Hamiltonian using finite difference method for dilute bismide-nitride compound GaNAsBi-based strained quantum wells (QWs). The effect of the biaxial strain (-0.57≤ε∥≤0.50%) on electronic band structure of GaNAsBi/GaAs double QWs (DQWs) operating at the near-infrared spectrum and telecommunications window (1.3 and 1.55 μm) are examined. We use the band anti-crossing model, envelop function formalism and Bir-Pikus theory in conjugation with k·p perturbation method to describe the optoelectronics properties (band offsets, absorption coefficient) behavior of the DQWs structures. Results show that the tensile strained GaNAsBi/GaAs DQWs can be used for the detection of 1, 0.95, and 0.80 eV energy range. This device seem be differentiated by higher carrier confinement and optical absorption compared to compressive strained one. The enhancement of spectral response is analyzed by increasing the QWs number and the coupling between them.

The strain, energy band and photoluminescence of GaAs0.92Sb0·08/Al0.3Ga0.7As multiple quantum wells grown on GaAs substrate

GaAsSb based materials have become the promising system for infrared semiconductor lasers and detectors. In this article, the strain, energy band structures and photoluminescence (PL) of GaAs0.92Sb0·08/Al0.3Ga0.7As strained quantum wells (QWs) grown with molecular beam epitaxy are systematically analyzed both theoretically and experimentally. The theoretical results are derived by Kane's model and k⋅p method, and the optical properties of a high-quality GaAs0.92Sb0·08/Al0.3Ga0.7As strained QWs sample are thoroughly investigated by excitation- and temperature-dependent PL measurements. The theoretical results show the strain has significant influence on the band structure of QWs. In experimental part, it is found that the light-hole exciton emission coexists with the heavy-hole exciton line in the temperature range of 50 K–300 K. However, the emission of localized excitons, which is caused by the nonuniformity of component in the GaAsSb well layer, takes over the light-hole exciton emission at lower temperatures (<50 K).

Tailoring strain and lattice relaxation characteristics in InGaAs/GaAsP multiple quantum wells structure with phosphorus doping engineering

Minimization of the internal strain in growing high quality strained multiple quantum wells (MQWs) has attracted enormous attention due to their high potential with optimized crystal quality for next-generation diode lasers technology applications. Among which the alternative multilayer structures with high performance of the strain quantum wells are prerequisite in the quantum cascade lasers and vertical cavity lasers. The periodic structure with an appropriate design strain-balanced MQWs are imperative for high performance devices. In present work, the strain compensated InGaAs/GaAsP multiple quantum well structures are developed. By engineering the composition of phosphorus in the barrier layer, i.e., 0.15 at.%, the best performance was achieved with high crystal quality via modulating the indium and phosphorus. Our systematic theoretical and experimental studies are performed upon the strain-balanced values investigations by complementary atomic force microscope and photoluminescence characterizations. The influence of variables on the measurement results and the internal correlated impact factors are discussed in detail. With tailored strain compensated structure, our developed InGaAs/GaAsP MQW opens new opportunities for innovative applications with optimized integration in next-generation diode lasers field.

Estimation of electron spin polarization from circularly polarized photoluminescence in strained quantum wells

A general method based on the valence band mixing is proposed to establish a correlation between electron spin polarization and the degree of circular polarization of luminescence in a strained quantum well (QW). Key issues that are critical for establishing such a correlation are highlighted by invoking 4 band k.p calculations in a strained QW system. The proposed method takes care of the complex valence band mixing in strained QWs by simultaneously analyzing the heavy hole (e-hh) and light hole (e-lh) related luminescence transitions along the quantization direction. It is shown that the ratio of the degree of circular polarizations measured for the e-hh and e-lh features is a sensitive probe to estimate the values of band mixing. The method is reasonably supported by carefully conducted experiments on a tensile strained GaAsP/AlGaAs QW sample in the temperature range of 10 to 150 K. The proposed method shall be helpful for understanding the spin injection/transfer across the interfaces of QW, especially for the cases where heavy hole-light hole coupling is expected to be strong, and both the features are experimentally observed. It might lead to new directions in the field of semiconductor spin-optoelectronics in QWs.

Analytical Investigation of Slow Light Systems with Strained Quantum Wells Structure under Applied Magnetic and Electric Fields Based on V-type EIT

In this paper, a slow light device, which is composed of GaAs/AlGaAs [110] strained quantum wells, has been studied to realize V-type electromagnetically induced transparency phenomenon. Influences of applying electric and magnetic fields are investigated theoretically on main properties of the slow light devices, such as slow down factor and refractive index, based on Bloch equations approach. This is the first study of applying external magnetic and electric fields impacts on proposed structure by using V-type electromagnetically induced transparency. By changing the external fields, we are able to modify central frequency of presented structure that provides appropriate conditions in order to be utilized in optical storages, optical buffers and all-optical switches. According to the simulation results, the peak value of slow down factor is estimated to near 5000 when power intensity of pump and signal are 10 kW/cm2 and 0.1 kW/cm2, respectively. With optimization the amount of external fields, the central frequency of system can reach positive shift as high as 1.1 THz and negative shift close to -1.1 THz.

Optical Gain in Type-II InAsN/GaSb Strained Quantum Wells Laser Diodes and Its Pressure Dependence

Theoretical calculations of the hydrostatic pressure influence on the electronic band structure and the optical gain of N-containing InAs0.98N0.02/GaSb quantum wells laser diodes are presented. A ${\mathbf {k}}\cdot {\mathbf {p}}$ model, taking into account, the p-like valence band, the s-like and p-like conduction band, and the nitrogen resonant level is introduced. We have used a unitary transformation that block diagonalizes the $16\times16$ Hamiltonian into two $8\times8$ blocks that are real symmetric in the finite difference formulation. Numerical results have been determined over a pressure range from 0 to 30 kbar. It has been shown that the hydrostatic pressure changes the electronic band structure and as a result the optical gain decreases with the hydrostatic pressure by about 80% and 50% for [111] and [001] growth directions, respectively for transverse electric modes. Hence, the emission spectrum is shifted to shorter wavelength (i.e., for [111] direction, from $2.65~\mu \text{m}$ to $1.16~\mu \text{m}$ ).

850-nm VCSELs With p-Type $\mathrm {{\delta }}$ -Doping in the Active Layers for Improved High-Speed and High-Temperature Performance

In this paper, we study the influence of p-type modulation doping on the dynamic/static performance of high-speed 850-nm VCSELs with highly strained multiple quantum wells. The studied device structure has a 3/ $2~\lambda $ asymmetric cavity design, which can let the internal transit time of injected carriers be as short as that of 1/ $2~\lambda $ cavity design and further improve its performance in terms of speed and output power for high single-mode operation. Our proposed VCSEL structure with p-type doping shows superior modulation speed with an output power comparable with that of the un-doped reference device under room temperature operation. Furthermore, when the operating temperature reaches 85 °C, there is a significant improvement in both the modulation speed and maximum power of the p-doped structures. According to our simulation, this can be attributed to the change in the quasi-Fermi levels of the injected carriers after the addition of p-doping in the active layers, which minimizes the electron leakage under high-temperature operation.

Calculation of strain compensation thickness for III–V semiconductor quantum dot superlattices

Models based on continuum elasticity theory are discussed to calculate the necessary thickness of a strain compensation (SC) layer for a superlattice (SL) of strained quantum wells (QW) or quantum dots (QD). These models are then expanded to cover material systems (substrates, QW or QD, and SC) composed of AlP, AlAs, AlSb, GaP, GaAs, GaSb, InP, InAs, or InSb, as well as the ternary, quaternary, and higher order material alloys possible in the Al/Ga/In/P/As/Sb systems. SC thickness calculation methods were compared against dynamical scattering simulations and experimental X-ray diffraction measurements of the InAs/GaP/GaAs QD/SC/Substrate superlattices of varying SC thickness. Based on the reduced (but not eliminated) strain present, a further modified strain compensation thickness is calculated to maximize the number of SL repeat units before the onset of misfit dislocations is also calculated. These models have been assembled into a free application on nanoHUB for use by the community.

Electromagnetically induced transparency in the strained quantum wells: Effects of structural parameters and geometrical size

In this work, the electromagnetically induced transparency in a strained GaN/InGaN/AlGaN quantum well system, is theoretically studied. Energy levels and wave functions are calculated using the transfer matrix method. Effects of In and Al compositions, the well and barrier widths on the real and imaginary parts of the optical susceptibility are investigated. Results show that the electromagnetically induced transparency can be controlled via the structural parameters and geometrical size.

Monolithic Broadband InGaN Light-Emitting Diode

A monolithic nonphosphor broadband-emission light-emitting diode is demonstrated, comprising a combination of high-density microstructured and nanostructured InGaN-GaN quantum wells fabricated using a top-down approach. Broadband emission is achieved by taking advantage of low-dimensional-induced strain-relaxation of highly strained quantum wells, combining light emitted from strain-relaxed nanotips at wavelengths shorter than the as-grown by as much as 80 nm with longer-wavelength light emitted from the larger nonrelaxed microdisks. The localized emission characteristics have been studied by spatially resolved near-field photoluminescence spectroscopy which enabled both the photoluminescence intensity and spectrum from individual nanotips to be distinguished from emission at the larger-dimensioned regions. Distinctive blue-green-yellow emission can be observed from the electroluminescent device, whose continuous broadband spectrum is characterized by CIE coordinates of (0.39, 0.47) and color rendering in...