Strain-balanced Quantum Research Articles

Growth of InGaAs/InAlAs superlattices for strain balanced quantum cascade lasers by molecular beam epitaxy

This study investigated the molecular beam epitaxy (MBE) growth conditions of strain-balanced (SB) In0.669GaAs/In0.362AlAs superlattices (SLs) for SB quantum cascade lasers (QCLs). The growth modes and properties of SB SLs are strongly affected by the growth conditions. The properties of the SB SLs were evaluated using atomic force microscopy (AFM) and high-resolution X-ray diffraction (HRXRD) analysis. Following the establishment of optimized conditions for SB SLs growth, SB QCL were grown. The HRXRD analysis and transmission electron microscopy (TEM) measurements showed that the grown samples exhibited abrupt interfaces and good structural quality. An epitaxial wafer was processed into a Fabry-Perot cavity with a ridge structure. The device operated in pulsed mode emitting ∼4.7 µm at room temperature with a peak power of 650 mW and a slope efficiency of 870 mW/A.

Triple-junction solar cells with 39.5% terrestrial and 34.2% space efficiency enabled by thick quantum well superlattices

<h2>Summary</h2> Multijunction solar cell design is guided by both the theoretical optimal bandgap combination as well as the realistic limitations to materials with these bandgaps. For instance, triple-junction III-V multijunction solar cells commonly use GaAs as a middle cell because of its near-perfect material quality, despite its bandgap being higher than optimal for the global spectrum. Here, we modify the middle cell bandgap using thick GaInAs/GaAsP strain-balanced quantum well (QW) solar cells with excellent voltage and absorption. These high-performance QWs are incorporated into a triple-junction inverted metamorphic multijunction device consisting of a GaInP top cell, GaInAs/GaAsP QW middle cell, and lattice-mismatched GaInAs bottom cell, each of which has been highly optimized. We demonstrate triple-junction efficiencies of 39.5% and 34.2% under the AM1.5 global and AM0 space spectra, respectively, and the global efficiency is higher than previous record six-junction devices.

GaAs-based strain-balanced GaNxAs1-x-yBiy type-I and -II multi-quantum wells for near-infrared photodetection range

Electronic band structure of t2-GaNx2As1-x2-y2Biy2/t1-GaNx1As1-x1-y1Biy1 strain-balanced quantum wells (QWs) elaborated on GaAs substrate was theoretically studied. Our study is based on the band anti-crossing model combined with the envelope function formalism. Using the zero stress balance model, the strain compensation criteria (N and Bi compositions, and thickness ratio (t1/t2) for the strain-balanced QWs is investigated. The critical thickness for each strained GaNAsBi layer is also discussed. Through the results, we found that the control of the compositions (x1,y1) and (x2,y2) for the tensile and compressive strained layers respectively, could modulate the band alignment of the strain-balanced QWs. The use of strain-balanced GaNx2As1-x2-y2Biy2/GaNx1As1-x1-y1Biy1type-II ”W” QWs is quite promising candidate for GaAs-based near-infrared photodetection range as compared to strain-balanced type-I QWs counterparts.

Photoelectrochemical water splitting using strain-balanced multiple quantum well photovoltaic cells

Strain-balanced GaInAs/GaAsP quantum wells were incorporated into the classical GaInP/GaAs tandem photoelectrochemical water splitting device to increase the range of photon absorption and achieve higher solar-to-hydrogen efficiencies.

Thin-film multiple-quantum-well solar cells fabricated by epitaxial lift-off process

Thin-film solar cells fabricated using the epitaxial lift-off (ELO) process enable considerable cost reduction as well as light-trapping. A polyimide film was used as a support substrate for thin film devices to address the poor temperature tolerance of poly(ethylene terephthalate) films. Samples were stuck to polyimide films by an Au–Au bonding process. It was found that the insertion of a wetting layer enhanced the adhesion between a polyimide film and an Au layer. The achieved bonding was strong enough to carry out the ELO process, and no degradation was observed in cell performance. Thin-film solar cells with strain-balanced multiple quantum wells were also fabricated using the ELO process to benefit from the light-trapping effect. Quantum efficiency enhancement was observed in the long-wavelength range of 850–980 nm compared to the non-ELO-processed devices. The photo-absorption enhancement was due to the Fabry–Perot resonance between the surface and the rear Au electrodes.

GaAsP/Si tandem solar cells: Realistic prediction of efficiency gain by applying strain-balanced multiple quantum wells

The combination of tunable direct bandgap III-V absorbers with active Si substrates promises high-efficiency tandem solar cells. To yield the ideal III-V bandgaps between 1.6 and 1.8eV which is considered to achieve current matching with the Si bottom-cell, however, either a big lattice-mismatch or dilute nitrides had to be considered so far. Here, we propose a new structure for a two-terminal III-V-on-Si solar cell which bases on strain-balanced III-V multi-quantum wells (MQWs) embedded in a metamorphic GaAsP top-cell matrix. Strain-balanced MQWs extend the absorption edge to a longer wavelength and enable a reduction of the As content in the GaAsP metamorphic top-cell matrix. And accumulation of carriers in MQWs favors radiative recombination, which is beneficial for high efficiency while deep quantum wells lower the charge carrier collection efficiency. Here, we predict solar energy conversion efficiencies over 42.6% with an entire MQW as thin as 500nm. The applied model takes into account the drawbacks of MQWs, such as limited light absorption and the bottleneck of charge carrier collection from the confinement.

Current gain and external quantum efficiency modeling of GeSn based direct bandgap multiple quantum well heterojunction phototransistor

This paper aims to provide the performance characteristics of proposed, strain balanced direct band gap multiple quantum wells (MQWs) hetero phototransistor (HPT) made of SiGeSn/GeSn alloys grown on Si substrate which is compatible with recent CMOS fabrication technology. This also presents a comprehensive comparison of this proposed structure with the existing HPT structure made of indirect gap Ge/SiGe MQWs. Alloys of Ge and Sn grown on Si platform shows about tenfold increase in absorption over Ge at C and L-bands due to direct nature of band gap in GeSn. Initial work begins the solution of continuity equation to solve the different terminal current densities and optical gain of the multiple quantum well structure. Main analysis was concentrated on finding the external quantum efficiency depending on the doping variations of emitter and base, base width etc. Finally the photocurrent density variations are estimated for the structure and compared with existing indirect band gap HPT. The calculated values for direct band gap GeSn HPT device are found to be comparable with those for indirect band gap SiGe device to flourish as a potential candidate of photo detectors for the present day telecommunication network.

Room-temperature mid-infrared quantum well lasers on multi-functional metamorphic buffers

The modern commercial optoelectronic infrastructure rests on a foundation of only a few, select semiconductor materials, capable of serving as viable substrates for devices. Any new active device, to have any hope of moving past the laboratory setting, must demonstrate compatibility with these substrate materials. Across much of the electromagnetic spectrum, this simple fact has guided the development of lasers, photodetectors, and other optoelectronic devices. In this work, we propose and demonstrate the concept of a multi-functional metamorphic buffer (MFMB) layer that not only allows for growth of highly lattice-mismatched active regions on InP substrates but also serves as a bottom cladding layer for optical confinement in a laser waveguide. Using the MFMB concept in conjunction with a strain-balanced multiple quantum well active region, we demonstrate laser diodes operating at room temperature in the technologically vital, and currently underserved, 2.5–3.0 μm wavelength range.

Wide bandgap, strain-balanced quantum well tunnel junctions on InP substrates

In this work, the electrical performance of strain-balanced quantum well tunnel junctions with varying designs is presented. Strain-balanced quantum well tunnel junctions comprising compressively strained InAlAs wells and tensile-strained InAlAs barriers were grown on InP substrates using solid-source molecular beam epitaxy. The use of InAlAs enables InP-based tunnel junction devices to be produced using wide bandgap layers, enabling high electrical performance with low absorption. The impact of well and barrier thickness on the electrical performance was investigated, in addition to the impact of Si and Be doping concentration. Finally, the impact of an InGaAs quantum well at the junction interface is presented, enabling a peak tunnel current density of 47.6 A/cm2 to be realized.

Room temperature continuous wave, monolithic tunable THz sources based on highly efficient mid-infrared quantum cascade lasers.

A compact, high power, room temperature continuous wave terahertz source emitting in a wide frequency range (ν ~ 1–5 THz) is of great importance to terahertz system development for applications in spectroscopy, communication, sensing, and imaging. Here, we present a strong-coupled strain-balanced quantum cascade laser design for efficient THz generation based on intracavity difference frequency generation. Room temperature continuous wave emission at 3.41 THz with a side-mode suppression ratio of 30 dB and output power up to 14 μW is achieved with a wall-plug efficiency about one order of magnitude higher than previous demonstrations. With this highly efficient design, continuous wave, single mode THz emissions with a wide frequency tuning range of 2.06–4.35 THz and an output power up to 4.2 μW are demonstrated at room temperature from two monolithic three-section sampled grating distributed feedback-distributed Bragg reflector lasers.

InGaP-based quantum well solar cells: Growth, structural design, and photovoltaic properties

Raising the efficiency ceiling of multi-junction solar cells (MJSCs) through the use of more optimal band gap configurations of next-generation MJSC is crucial for concentrator and space systems. Towards this goal, we propose two strain balanced multiple quantum well (SBMQW) structures to tune the bandgap of InGaP-based solar cells. These structures are based on InxGa1−xAs1−zPz/InyGa1−yP (x &amp;gt; y) and InxGa1−xP/InyGa1−yP (x &amp;gt; y) well/barrier combinations, lattice matched to GaAs in a p-i-n solar cell device. The bandgap of InxGa1−xAs1−zPz/InyGa1−yP can be tuned from 1.82 to 1.65 eV by adjusting the well composition and thickness, which promotes its use as an efficient subcell for next generation five and six junction photovoltaic devices. The thicknesses of wells and barriers are adjusted using a zero net stress balance model to prevent the formation of defects. Thin layers of InGaAsP wells have been grown thermodynamically stable with compositions within the miscibility gap for the bulk alloy. The growth conditions of the two SBMQWs and the individual layers are reported. The structures are characterized and analyzed by optical microscopy, X-ray diffraction, photoluminescence, current-voltage characteristics, and spectral response (external quantum efficiency). The effect of the well number on the excitonic absorption of InGaAsP/InGaP SBMQWs is discussed and analyzed.

Can a Hot-Carrier Solar Cell also be an Efficient Up-converter?

The hot-carrier solar cell is a very ambitious device concept, which has a thermodynamic efficiency limit of around 84% when operated at the maximum power point. However, if the same device is instead operated at open circuit, then it becomes an efficient radiator of blackbody radiation at the temperature of the hot carriers. Such a configuration is similar to a thermal photovoltaic converter, but one in which the thermal gradient is maintained by a hot electron-hole gas rather than by a physically hot lattice. In this scheme (see Fig. 1), a low-bandgap hot-carrier material is placed behind a conventional solar cell and absorbs sub-bandgap photons, generating a hot-carrier distribution which re-radiates this energy, some of which can be collected by the solar cell located above. The additional photons have been thermally up-converted by a “hot-carrier radiator.” We will discuss the thermodynamic efficiency limit of a hot-carrier radiator placed behind a conventional single-junction solar cell, and present some experimental results toward developing a proof-of-concept device using strain-balanced quantum wells.

InP-Based Active and Passive Components for Communication Systems at 2 μm

Progress on advanced active and passive photonic components that are required for high-speed optical communications over hollow-core photonic bandgap fiber at wavelengths around 2 μm is described in this paper. Single-frequency lasers capable of operating at 10 Gb/s and covering a wide spectral range are realized. A comparison is made between waveguide and surface normal photodiodes with the latter showing good sensitivity up to 15 Gb/s. Passive waveguides, 90° optical hybrids, and arrayed waveguide grating with 100-GHz channel spacing are demonstrated on a large spot-size waveguide platform. Finally, a strong electro-optic effect using the quantum confined Stark effect in strain-balanced multiple quantum wells is demonstrated and used in a Mach-Zehnder modulator capable of operating at 10 Gb/s.

Anisotropic Emission in Strain-Balanced Quantum Well Solar Cells

Strain-balanced quantum well solar cells (SB-QWSC) extend the photon absorption edge beyond that of bulk GaAs by incorporation of quantum wells in the i-region of a pin device. The strain-balanced quantum well solar cell benefits from a fundamental efficiency enhancement due to anisotropic emission from the quantum wells. This anisotropy arises from a splitting of the valence band due to compressive strain in the quantum wells, suppressing a transition which contributes to emission from the edge of the quantum wells. We have studied both the emission light polarized in the plane perpendicular (TM) to the quantum well which couples exclusively to the light hole transition and the emission polarized in the plane of the quantum wells (TE) which couples mainly to the heavy hole transition. It was found that the spontaneous emission rates TM and TE increase when the quantum wells are deeper. We have also demonstrated that the photo-generated carriers can escape from the QWs with near unity efficiency, via a thermally-assisted tunneling process, because gain is several orders greater than radiative recombination.

Anisotropic emission and photon-recycling in strain-balanced quantum well solar cells

Strain-balanced quantum well solar cells (SB-QWSCs) extend the photon absorption edge beyond that of bulk GaAs by incorporation of quantum wells in the i-region of a p–i–n device. Anisotropy arises from a splitting of the valence band due to compressive strain in the quantum wells, suppressing a transition which contributes to emission from the edge of the quantum wells. We have studied both the emission light polarized in the plane perpendicular (TM) to the quantum well which couples exclusively to the light hole transition and the emission polarized in the plane of the quantum wells (TE) which couples mainly to the heavy hole transition. It was found that the spontaneous emission rates TM and TE increase when the quantum wells are deeper. The addition of a distributed Bragg reflector can substantially increase the photocurrent while decreasing the radiative recombination current. We have examined the impact of the photon recycling effect on SB-QWSC performance. We have optimized SB-QWSC design to achieve single junction efficiencies above 30%.

Epitaxial lift-off of quantum dot enhanced GaAs single junction solar cells

InAs/GaAs strain-balanced quantum dot (QD) n-i-p solar cells were fabricated by epitaxial lift-off (ELO), creating thin and flexible devices that exhibit an enhanced sub-GaAs bandgap current collection extending into the near infrared. Materials and optical analysis indicates that QD quality after ELO processing is preserved, which is supported by transmission electron microscopy images of the QD superlattice post-ELO. Spectral responsivity measurements depict a broadband resonant cavity enhancement past the GaAs bandedge, which is due to the thinning of the device. Integrated external quantum efficiency shows a QD contribution to the short circuit current density of 0.23 mA/cm2.

Modelling of GaAsP/InGaAs/GaAs strain-balanced multiple-quantum well solar cells

A model of strain balanced quantum well solar cells is presented, together with a high efficiency design for a GaAsP/InGaAs/GaAs device. The effect of tensile and compressive strain on bandstructure is considered in order to compute the electron and hole dispersion relation E(k) in conduction and valence bands. The optical transitions in quantum well and barrier are evaluated and the quantum efficiency, dark current and the photocurrent calculated. Experimental data quantum efficiency and dark current are compared with theoretical calculations in the presence of strain, showing a good agreement. The resulting model is initially applied to a GaAsP/InGaAs/GaAs solar cell and the structure optimised to yield the greatest output power. The model is also applied to the problem of determining the highest efficiencies achievable for quantum well solar cells as a function of strain and confirms the high efficiency potential of strained quantum well solar cells.

Strain Balanced Epitaxial Stacks of Quantum Dots and Quantum Posts

AbstractThe self assembly of quantum dots by heteroepitaxy of lattice‐mismatched semiconductors is based on elastic energy relaxation, which spontaneously occurs at the growth front when the largest atoms in the crystal cluster together. Because a larger covalent radius is related to weaker bonds, and this is in turn fundamentally related to smaller bandgaps, the formation of quantum dots leads to a confinement potential for electrons and/or holes. This effect has applications ranging from ultralow threshold diode lasers to highly efficient solar cells and usually requires the stacking of multiple quantum dots. The number of layers is limited by the stress accumulated during growth due to the larger covalent radius of the atoms that constitute the quantum dots. This accumulated stress can be relieved by introducing in the epitaxial layers compensating atomic species with a smaller covalent radius, enabling a reduction of spacer layer thickness. In the limit of sub‐nanometer spacer thickness, the quantum dots, which have a tendency to line up vertically, fuse into a quantum post. The current efforts to optimize the properties of strain balanced quantum dot stacks and quantum posts are reviewed.

Strain balanced quantum posts

Quantum posts are assembled by epitaxial growth of closely spaced quantum dot layers, modulating the composition of a semiconductor alloy, typically InGaAs. In contrast with most self-assembled nanostructures, the height of quantum posts can be controlled with nanometer precision, up to a maximum value limited by the accumulated stress due to the lattice mismatch. Here, we present a strain compensation technique based on the controlled incorporation of phosphorous, which substantially increases the maximum attainable quantum post height. The luminescence from the resulting nanostructures presents giant linear polarization anisotropy.

Influence of the growth temperature on the performances of strain-balanced quantum cascade lasers

The effect of substrate temperature, during epitaxial growth, on the performances of strain-balanced quantum cascade lasers based on a three quantum well active region and operating at λ≈4.6 μm is presented. Based on a comparison with a density matrix model of these devices, the optimum performances obtained at a growth temperature of 515 °C, are interpreted as arising from a value of the interface roughness correlation length (Λ=85 Å) close to the optimum one computed by the model (Λ=100 Å).