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Showing papers on "Quantum well published in 2016"


Journal ArticleDOI
TL;DR: Perovskite quantum wells yield highly efficient LEDs spanning the visible and near-infrared as discussed by the authors. But their performance is not as good as those of traditional LEDs, and their lifetime is shorter.
Abstract: Perovskite quantum wells yield highly efficient LEDs spanning the visible and near-infrared.

1,419 citations


Journal ArticleDOI
TL;DR: In this paper, the authors demonstrate continuous-wave InAs/GaAs quantum dot lasers directly grown on silicon substrates with a low threshold current density of 62.5 cm−2, a room-temperature output power exceeding 105mW and operation up to 120°C.
Abstract: Reliable, efficient electrically pumped silicon-based lasers would enable full integration of photonic and electronic circuits, but have previously only been realized by wafer bonding. Here, we demonstrate continuous-wave InAs/GaAs quantum dot lasers directly grown on silicon substrates with a low threshold current density of 62.5 A cm–2, a room-temperature output power exceeding 105 mW and operation up to 120 °C. Over 3,100 h of continuous-wave operating data have been collected, giving an extrapolated mean time to failure of over 100,158 h. The realization of high-performance quantum dot lasers on silicon is due to the achievement of a low density of threading dislocations on the order of 105 cm−2 in the III–V epilayers by combining a nucleation layer and dislocation filter layers with in situ thermal annealing. These results are a major advance towards reliable and cost-effective silicon-based photonic–electronic integration.

682 citations


Journal ArticleDOI
TL;DR: In this article, the authors show how a new quantum spin Hall (QSH) paradigm based on substrate-supported atomic monolayers of a high-Z element can be achieved by making use of a new QSH paradigm.
Abstract: Quantum spin Hall (QSH) materials promise revolutionary device applications based on dissipationless propagation of spin currents. They are two-dimensional (2D) representatives of the family of topological insulators, which exhibit conduction channels at their edges inherently protected against scattering. Initially predicted for graphene, and eventually realized in HgTe quantum wells, in the QSH systems realized so far, the decisive bottleneck preventing applications is the small bulk energy gap of less than 30 meV, requiring cryogenic operation temperatures in order to suppress detrimental bulk contributions to the edge conductance. Room-temperature functionalities, however, require much larger gaps. Here we show how this can be achieved by making use of a new QSH paradigm based on substrate-supported atomic monolayers of a high-Z element. Experimentally, the material is synthesized as honeycomb lattice of bismuth atoms, forming "bismuthene", on top of the wide-gap substrate SiC(0001). Consistent with the theoretical expectations, the spectroscopic signatures in experiment display a huge gap of ~0.8 eV in bismuthene, as well as conductive edge states. The analysis of the layer-substrate orbitals arrives at a QSH phase, whose topological gap - as a hallmark mechanism - is driven directly by the atomic spin-orbit coupling (SOC). Our results demonstrate how strained artificial lattices of heavy atoms, in contact with an insulating substrate, can be utilized to evoke a novel topological wide-gap scenario, where the chemical potential is located well within the global system gap, ensuring pure edge state conductance. We anticipate future experiments on topological signatures, such as transport measurements that probe the QSH effect via quantized universal conductance, notably at room temperature.

461 citations


Journal ArticleDOI
03 Oct 2016-ACS Nano
TL;DR: Efficient blue LEDs based on the colloidal, quantum-confined 2D perovskites are demonstrated, with precisely controlled stacking down to one-unit-cell thickness, with record-high external quantum efficiencies in the green-to-blue wavelength region.
Abstract: Solution-processed hybrid organic–inorganic lead halide perovskites are emerging as one of the most promising candidates for low-cost light-emitting diodes (LEDs). However, due to a small exciton binding energy, it is not yet possible to achieve an efficient electroluminescence within the blue wavelength region at room temperature, as is necessary for full-spectrum light sources. Here, we demonstrate efficient blue LEDs based on the colloidal, quantum-confined 2D perovskites, with precisely controlled stacking down to one-unit-cell thickness (n = 1). A variety of low-k organic host compounds are used to disperse the 2D perovskites, effectively creating a matrix of the dielectric quantum wells, which significantly boosts the exciton binding energy by the dielectric confinement effect. Through the Forster resonance energy transfer, the excitons down-convert and recombine radiatively in the 2D perovskites. We report room-temperature pure green (n = 7–10), sky blue (n = 5), pure blue (n = 3), and deep blue (n...

281 citations


Proceedings Article
02 Dec 2016
TL;DR: This work demonstrates 1.3 µm quantum dot lasers grown directly on silicon substrates without offcut or germanium layers, with thresholds down to 30 mA and lasing up to 90°C, with 20 dB higher tolerance to reflections compared to quantum well lasers on silicon.
Abstract: We demonstrate 1.3 µm quantum dot lasers grown directly on (001) silicon substrates without offcut or germanium layers, with thresholds down to 30 mA and lasing up to 90°C. Measurements of relative intensity noise versus feedback show 20 dB higher tolerance to reflections compared to quantum well lasers on silicon.

105 citations


Journal ArticleDOI
TL;DR: A new series of hybrid perovskite emitters with a general formula of (C4H9 NH3)2(CH3NH3)n−1PbnI3n+1 (where n = 1, 2, 3), which possesses a multiple quantum well structure, which results in a continuously tunable bandgap and emission spectra.
Abstract: Organic-inorganic hybrid perovskites have the potential to be used as a new class of emitters with tunable emission, high color purity and good ease of fabrication. Recent studies have so far been focused on three-dimensional (3D) perovskites, such as CH3NH3PbBr3 and CH3NH3PbI3 for green and infrared emission. Here, we explore a new series of hybrid perovskite emitters with a general formula of (C4H9NH3)2(CH3NH3)n−1PbnI3n+1 (where n = 1, 2, 3), which possesses a multiple quantum well structure. The quantum well thickness of these materials is adjustable through simple molecular engineering which results in a continuously tunable bandgap and emission spectra. Deep saturated red emission was obtained with a peak external quantum efficiency of 2.29% and a maximum luminance of 214 cd/m2. Green and blue LEDs were also demonstrated through halogen substitutions in these hybrid perovskites. We expect these results to open up the way towards high performance perovskite LEDs through molecular-structure engineering of these perovskite emitters.

101 citations



Journal ArticleDOI
TL;DR: The quantum size effect results in a giant optical Kerr susceptibility, which is four orders of magnitude higher than the intrinsic value of bulk gold and several orders larger than traditional nonlinear media, making quantum metallic films a promising candidate for integrated nonlinear optical applications.
Abstract: With the size of plasmonic devices entering into the nanoscale region, the impact of quantum physics needs to be considered. In the past, the quantum size effect on linear material properties has been studied extensively. However, the nonlinear aspects have not been explored much so far. On the other hand, much effort has been put into the field of integrated nonlinear optics and a medium with large nonlinearity is desirable. Here we study the optical nonlinear properties of a nanometre scale gold quantum well by using the z-scan method and nonlinear spectrum broadening technique. The quantum size effect results in a giant optical Kerr susceptibility, which is four orders of magnitude higher than the intrinsic value of bulk gold and several orders larger than traditional nonlinear media. Such high nonlinearity enables efficient nonlinear interaction within a microscopic footprint, making quantum metallic films a promising candidate for integrated nonlinear optical applications.

88 citations


Journal ArticleDOI
TL;DR: This implementation combines a two-dimensional electron gas formed in a semiconductor quantum well grown on the surface of an s-wave superconductor with a nearby array of magnetic tunnel junctions (MTJs).
Abstract: We propose a versatile platform to investigate the existence of Majorana bound states (MBSs) and their non-Abelian statistics through braiding. This implementation combines a two-dimensional electron gas formed in a semiconductor quantum well grown on the surface of an $s$-wave superconductor with a nearby array of magnetic tunnel junctions (MTJs). The underlying magnetic textures produced by MTJs provide highly controllable topological phase transitions to confine and transport MBSs in two dimensions, overcoming the requirement for a network of wires. Obtained scaling relations confirm that various semiconductor quantum well materials are suitable for this proposal.

83 citations


Journal ArticleDOI
TL;DR: In this article, the theoretical and experimental results for the electronic and optical properties of atomically thin (1 and 2 monolayers) GaN quantum wells with AlN barriers are presented.
Abstract: We present the theoretical and experimental results for the electronic and optical properties of atomically thin (1 and 2 monolayers) GaN quantum wells with AlN barriers. Strong quantum confinement increases the gap of GaN to as high as 5.44 eV and enables light emission in the deep-UV range. Luminescence occurs from the heavy and light hole bands of GaN yielding E ⊥ c polarized light emission. Strong confinement also increases the exciton binding energy up to 230 meV, preventing a thermal dissociation of excitons at room temperature. However, we did not observe excitons experimentally due to high excited free-carrier concentrations. Monolayer-thick GaN wells also exhibit a large electron-hole wave function overlap and negligible Stark shift, which is expected to enhance the radiative recombination efficiency. Our results indicate that atomically thin GaN/AlN heterostructures are promising for efficient deep-UV optoelectronic devices.

81 citations



Journal ArticleDOI
TL;DR: In this article, the first integrated waveguide modulator-laser diode (IWM-LD) at 448 nm was presented, offering the advantages of small footprint, high speed, and low power consumption.
Abstract: To date, solid-state lighting (SSL), visible light communication (VLC), and optical clock generation functionalities in the blue-green color regime have been demonstrated based on discrete devices, including light-emitting diodes, laser diodes, and transverse-transmission modulators. This work presents the first integrated waveguide modulator–laser diode (IWM-LD) at 448 nm, offering the advantages of small footprint, high speed, and low power consumption. A high modulation efficiency of 2.68 dB/V, deriving from a large extinction ratio of 9.4 dB and a low operating voltage range of 3.5 V, was measured. The electroabsorption characteristics revealed that the modulation effect, as observed from the red-shifting of the absorption edge, resulted from the external-field-induced quantum-confined Stark effect. A comparative analysis of the photocurrent versus wavelength spectra in semipolar- and polar-plane InGaN/GaN quantum wells (QWs) confirmed that the IWM–LD based on semipolar (2021) QWs was able to operat...

Journal ArticleDOI
TL;DR: In this paper, the authors demonstrate MQW solar cells with effective bandgaps ranging from 1.31 eV to as low as 1.15ÕeV, which implies the merit of high photovoltage as compared with bulk cells with the same bandgap.
Abstract: Bandgap engineering of strain-balanced InGaAs/GaAsP multiple quantum wells (MQWs) allows high-quality materials with an absorption edge beyond GaAs to be epitaxially grown in Ge/GaAs-based multijunction solar cells. We demonstrate MQW solar cells with effective bandgaps ranging from 1.31 eV to as low as 1.15 eV. The bandgap-voltage-offset of MQWs is found to be independent of effective bandgaps and superior to a bulk reference by approximately 0.1 V. This implies the merit of high photovoltage as compared with bulk cells with the same bandgap in addition to their widely bandgap-tunable property. Towards the realization of fully lattice-matched quad-junction devices, we demonstrate a 70-period, 1.15-eV bandgap MQW cell as a promising material in 0.66/1.15/1.51/1.99-eV quad-junction cells, whose practical efficiency has a potential to achieve over 50%. With such a large period number of MQWs, the reverse-biased external quantum efficiency reaches an average of over 60% in the spectral region corresponding to a 1.15-eV subcell; this is achieved with only a-few-percent drop at short-circuit condition. The device presented here reaches the target open-circuit voltage and over 75% of the current density required for realizing a 1.15-eV subcell in a 50%-efficient quad-junction solar cell. We believe that future devices which exploit light-trapping structures and enhanced carrier extraction will be able to reach the desired target. Copyright © 2015 John Wiley & Sons, Ltd.

Journal ArticleDOI
TL;DR: In this paper, the weak van der Waals bonds between h-BN atomic layers break easily, allowing the MQW structure to be mechanically lifted off from the sapphire substrate using a commercial adhesive tape.
Abstract: Recent advances in epitaxial growth have led to the growth of III-nitride devices on 2D layered h-BN. This advance has the potential for wafer-scale transfer to arbitrary substrates, which could improve the thermal management and would allow III-N devices to be used more flexibly in a broader range of applications. We report wafer scale exfoliation of a metal organic vapor phase epitaxy grown InGaN/GaN Multi Quantum Well (MQW) structure from a 5 nm thick h-BN layer that was grown on a 2-inch sapphire substrate. The weak van der Waals bonds between h-BN atomic layers break easily, allowing the MQW structure to be mechanically lifted off from the sapphire substrate using a commercial adhesive tape. This results in the surface roughness of only 1.14 nm on the separated surface. Structural characterizations performed before and after the lift-off confirm the conservation of structural properties after lift-off. Cathodoluminescence at 454 nm was present before lift-off and 458 nm was present after. Electrolumi...

Journal ArticleDOI
TL;DR: In this paper, the experimental data and the theoretical predictions of the low temperature optical properties of polar and non-polar InGaN/GaN quantum well structures were compared and compared.
Abstract: In this paper, we compare and contrast the experimental data and the theoretical predictions of the low temperature optical properties of polar and nonpolar InGaN/GaN quantum well structures. In both types of structure, the optical properties at low temperatures are governed by the effects of carrier localisation. In polar structures, the effect of the in-built electric field leads to electrons being mainly localised at well width fluctuations, whereas holes are localised at regions within the quantum wells, where the random In distribution leads to local minima in potential energy. This leads to a system of independently localised electrons and holes. In nonpolar quantum wells, the nature of the hole localisation is essentially the same as the polar case but the electrons are now coulombically bound to the holes forming localised excitons. These localisation mechanisms are compatible with the large photoluminescence linewidths of the polar and nonpolar quantum wells as well as the different time scales and form of the radiative recombination decay curves.

Journal ArticleDOI
01 Jul 2016
TL;DR: Calculations showed excellent agreement with the experiment, key for future laser design.
Abstract: Optical gain, absorption and spontaneous emission spectra for GaAs 0.978 Bi 0.022 /GaAs laser diodes are measured experimentally and compared with theory. Internal optical losses of 10–15 cm−1 and peak modal gain of 24 cm−1 are measured at threshold. The results of calculations showed excellent agreement with the experiment, key for future laser design.

Journal ArticleDOI
TL;DR: The ability of the active layer to be released from the silicon substrate and to be grown on silicon-on-insulator substrates opens the way to future developments of nitride nanophotonic platforms on silicon.
Abstract: Deep ultra-violet semiconductor lasers have numerous applications for optical storage and biochemistry. Many strategies based on nitride heterostructures and adapted substrates have been investigated to develop efficient active layers in this spectral range, starting with AlGaN quantum wells on AlN substrates and more recently sapphire and SiC substrates. Here we report an efficient and simple solution relying on binary GaN/AlN quantum wells grown on a thin AlN buffer layer on a silicon substrate. This active region is embedded in microdisk photonic resonators of high quality factors and allows the demonstration of a deep ultra-violet microlaser operating at 275 nm at room temperature under optical pumping, with a spontaneous emission coupling factor β = (4 ± 2) 10−4. The ability of the active layer to be released from the silicon substrate and to be grown on silicon-on-insulator substrates opens the way to future developments of nitride nanophotonic platforms on silicon.

Journal ArticleDOI
TL;DR: In this paper, a new quantum well geometry was proposed to characterize the nontrivial topology in bismuth through high-resolution ARPES measurements aided by a new well geometry.
Abstract: Characterization of the nontrivial topology in bismuth through high-resolution ARPES measurements is aided by a new quantum well geometry.

Journal ArticleDOI
TL;DR: In this article, the authors proposed a method for accurately determining the valley splitting in Si/SiGe double quantum dots embedded in a superconducting microwave resonator. And they showed that low lying valley states in the double quantum dot energy level spectrum lead to readily observable features in the cavity transmission.
Abstract: The band structure of bulk silicon has a sixfold valley degeneracy. Strain in the Si/SiGe quantum well system partially lifts the valley degeneracy, but the materials factors that set the splitting of the two lowest lying valleys are still under intense investigation. Using cavity input-output theory, we propose a method for accurately determining the valley splitting in Si/SiGe double quantum dots embedded in a superconducting microwave resonator. We show that low lying valley states in the double quantum dot energy level spectrum lead to readily observable features in the cavity transmission. These features generate a ``fingerprint'' of the microscopic energy level structure of a semiconductor double quantum dot, providing useful information on valley splittings and intervalley coupling rates.

Book ChapterDOI
03 Jul 2016
TL;DR: A silicon optical interposer with the bandwidth-density of 15Tbps/cm2 at 125 C was demonstrated using flip-chip bonding method in this paper, where quantum dot lasers on silicon by wafer-bonding technique are also discussed.
Abstract: We discuss recent progresses in silicon photonics integrating quantum dot lasers. High temperature stability and high feedback-noise tolerance of the quantum dot lasers are advantageous features for application to silicon photonics. A silicon optical interposer with the bandwidth-density of 15Tbps/cm2 at 125 C was demonstrated using flip-chip bonding method. Quantum dot lasers on silicon by wafer-bonding technique are also discussed.

Journal ArticleDOI
TL;DR: Ma et al. as mentioned in this paper showed that two-dimensional materials made of tin and sulphur, selenium or tellurium can exist in an unusual state of matter known as a quantum spin Hall insulator.
Abstract: A family of materials with exotic quantum properties ideal for low-power electronics has been predicted by an international team. Yandong Ma from Jacobs University Bremen in Germany and his colleagues have theoretically shown that two-dimensional materials made of tin and sulphur, selenium or tellurium can exist in an unusual state of matter known as a quantum spin Hall insulator. Such systems conduct electrical current around their edges but not in their centres, making them useful in electronics because they are robust against impurities and require only low applied voltages to control electron flow. Ma and the team used density functional theory calculations to show that such a state exists at room temperature in SnS2, SnSe2 and SnTe2; materials that, unlike previous quantum spin Hall insulators, have a pentagonal atomic structure.

Journal ArticleDOI
TL;DR: In this paper, the growth and singlemode lasing operation of GaAs-AlGaAs core-multishell nanowires (NW) with radial single and multiple GaAs quantum wells (QWs) as active gain media were demonstrated.
Abstract: We demonstrate the growth and single-mode lasing operation of GaAs-AlGaAs core-multishell nanowires (NW) with radial single and multiple GaAs quantum wells (QWs) as active gain media. When subject to optical pumping lasing emission with distinct s-shaped input-output characteristics, linewidth narrowing and emission energies associated with the confined QWs are observed. Comparing the low temperature performance of QW NW laser structures having 7 coaxial QWs with a nominally identical structure having only a single QW shows that the threshold power density reduces several-fold, down to values as low as ∼2.4 kW/cm2 for the multiple QW NW laser. This confirms that the individual radial QWs are electronically weakly coupled and that epitaxial design can be used to optimize the gain characteristics of the devices. Temperature-dependent investigations show that lasing prevails up to 300 K, opening promising new avenues for efficient III–V semiconductor NW lasers with embedded low-dimensional gain media.

Journal ArticleDOI
TL;DR: It is demonstrated that a quantum well–embedded nanomembrane in a rolled-up geometry facilitates a 3D QW infrared photodetector (QWIP) device with enhanced responsivity and detectivity, offering a unique solution to high-performance focal plane array.
Abstract: Three-dimensional (3D) design and manufacturing enable flexible nanomembranes to deliver unique properties and applications in flexible electronics, photovoltaics, and photonics. We demonstrate that a quantum well (QW)–embedded nanomembrane in a rolled-up geometry facilitates a 3D QW infrared photodetector (QWIP) device with enhanced responsivity and detectivity. Circular geometry of nanomembrane rolls provides the light coupling route; thus, there are no external light coupling structures, which are normally necessary for QWIPs. This 3D QWIP device under tube-based light-trapping mode presents broadband enhancement of coupling efficiency and omnidirectional detection under a wide incident angle (±70°), offering a unique solution to high-performance focal plane array. The winding number of these rolled-up QWIPs provides well-tunable blackbody photocurrents and responsivity. 3D self-assembly of functional nanomembranes offers a new path for high conversion efficiency between light and electricity in photodetectors, solar cells, and light-emitting diodes.

Journal ArticleDOI
TL;DR: A pronounced conductance quantization of quantum point contact devices in InSb/InAlSb quantum wells is reported and crossings of subbands with opposite spins are investigated to extract the electron effective mass from magnetic depopulation of one-dimensional subbands.
Abstract: Because of a strong spin–orbit interaction and a large Lande g-factor, InSb plays an important role in research on Majorana fermions. To further explore novel properties of Majorana fermions, hybrid devices based on quantum wells are conceived as an alternative approach to nanowires. In this work, we report a pronounced conductance quantization of quantum point contact devices in InSb/InAlSb quantum wells. Using a rotating magnetic field, we observe a large in-plane (|g1| = 26) and out-of-plane (|g1| = 52) g-factor anisotropy. Additionally, we investigate crossings of subbands with opposite spins and extract the electron effective mass from magnetic depopulation of one-dimensional subbands.

Journal ArticleDOI
21 Apr 2016-ACS Nano
TL;DR: In this study, a self-consistent theoretical model is proposed to study the piezo-phototronic effect in the framework of perturbation theory in quantum mechanics and its validity and universality are well-proven.
Abstract: With enhancements in the performance of optoelectronic devices, the field of piezo-phototronics has attracted much attention, and several theoretical works have been reported based on semiclassical models. At present, the feature size of optoelectronic devices are rapidly shrinking toward several tens of nanometers, which results in the quantum confinement effect. Starting from the basic piezoelectricity equation, Schrodinger equation, Poisson equation, and Fermi’s golden rule, a self-consistent theoretical model is proposed to study the piezo-phototronic effect in the framework of perturbation theory in quantum mechanics. The validity and universality of this model are well-proven with photoluminescence measurements in a single GaN/InGaN quantum well and multiple GaN/InGaN quantum wells. This study provides important insight into the working principle of nanoscale piezo-phototronic devices as well as guidance for the future device design.

Journal ArticleDOI
TL;DR: A much enhanced energy gap of 55 meV is demonstrated in heavily compressively strained QWs, which exceeds the highest possible gap on common II-VI substrates by a factor of 2-3, and extends the regime where the topological conductance prevails to much higher temperatures.
Abstract: The band gap of HgTe quantum wells can be controlled by adjusting their strain via a substrate lattice mismatch. \hskip-0.22emThe achieved band gap of 55 meV is well above room temperature thermal energy, which may allow for technical applications of these 2D topological insulators.

Journal ArticleDOI
TL;DR: The demonstration of active mode locking of an external-cavity quantum cascade laser that operates in the mode-locked regime at room temperature and over the full dynamic range of injection currents is reported.
Abstract: Compact, integrated mode-locked lasers can produce ultrashort pulses of light in the visible and near infrared, but are more difficult to achieve in the mid-infrared. Here, the authors demonstrate active mode locking of an external-cavity quantum cascade laser emitting at 5.2 micrometres at room temperature.

Journal ArticleDOI
TL;DR: The observation of a strikingly large linear magnetoresistance (LMR) up to 33 T with a magnitude of order 10^{5}% onto which quantum oscillations become superimposed in the quantum Hall regime at low temperature is observed.
Abstract: Charge density variations may be behind an unusual magnetic response seen in topological insulators and other novel materials.

Journal ArticleDOI
TL;DR: In this article, the complex optical conductivities for monolayer and few-layered MoS2 films were derived from their reflectance and transmittance responses, showing that the excitonic quantum confinement effect significantly modifies both the peak energy and magnitude of their optical conductivity, manifested by a gradual blueshift in energy and exponential attenuation in magnitude with decreasing layer number.
Abstract: Optical conductivity plays an important role in characterizing the optoelectronic properties of two-dimensional materials. Here we derive the complex optical conductivities for monolayer and few-layered MoS2 films from their reflectance and transmittance responses. We show that the excitonic quantum confinement effect significantly modifies both the peak energy and magnitude of their optical conductivity, manifested by a gradual blueshift in energy (consistent with two well-known models for quantum well systems) and exponential attenuation in magnitude with decreasing layer number. More importantly, the C excition induced optical conductivity peak exhibits the strongest dependence on the MoS2 layer number because of its largest Bohr radius among the A, B and C excitons. This unambiguously confirms the strong influence of quantum confinement effect in the optical conductivity of MoS2, shedding important insights into understanding its rich exciton-related optical properties and therefore facilitating potential applications in optoelectronic devices.

Journal ArticleDOI
TL;DR: It is shown that compressively strained Ge1−xSnx/Ge quantum wells (QWs) grown on a Ge substrate with 0.1 ≤ x’s width are a very promising gain medium for lasers integrated with an Si platform and reduction in the QW width shifts up the ground electron subband in theQW and increases occupation of the L valley in the barriers instead of the Γ valley inThe QW region.
Abstract: It is shown that compressively strained Ge1−xSnx/Ge quantum wells (QWs) grown on a Ge substrate with 0.1 ≤ x ≤ 0.2 and width of 8 nm ≤ d ≤ 14 nm are a very promising gain medium for lasers integrated with an Si platform. Such QWs are type-I QWs with a direct bandgap and positive transverse electric mode of material gain, i.e. the modal gain. The electronic band structure near the center of Brillouin zone has been calculated for various Ge1−xSnx/Ge QWs with use of the 8-band kp Hamiltonian. To calculate the material gain for these QWs, occupation of the L valley in Ge barriers has been taken into account. It is clearly shown that this occupation has a lot of influence on the material gain in the QWs with low Sn concentrations (Sn 15%). However, for QWs with Sn > 20% the critical thickness of a GeSn layer deposited on a Ge substrate starts to play an important role. Reduction in the QW width shifts up the ground electron subband in the QW and increases occupation of the L valley in the barriers instead of the Γ valley in the QW region.