Sn Compositions Research Articles

Synthesis of group-IV ternary and binary semiconductors using epitaxy of GeH3Cl and SnH4

Ge1−x−ySixSny alloys were grown on Ge buffers via reactions of SnH4 and GeH3Cl. The latter is a new CVD source designed for epitaxial development of group-IV semiconductors under low thermal budgets and CMOS-compatible conditions. The Ge1−x−ySixSny films were produced at very low temperatures between 160 and 200 °C with 3%–5% Si and ∼5%–11% Sn. The films were characterized using an array of structural probes that include Rutherford backscattering, x-ray photoelectron spectroscopy, high-resolution x-ray diffraction, scanning transmission electron microscopy, and atomic force microscopy. These studies indicate that the films are strained to Ge and exhibit defect-free microstructures, flat surfaces, homogeneous compositions, and sharp interfaces. Raman was used to determine the compositional dependence of the vibrational modes indicating atomic distributions indistinguishable from those obtained when using high-order Ge hydrides. For a better understanding of the growth mechanisms, a parallel study was conducted to investigate the GeH3Cl applicability for synthesis of binary Ge1−ySny films. These grew strained to Ge, but with reduced Sn compositions and lower thicknesses relative to Ge1−x−ySixSny. Bypassing the Ge buffers led to Ge1−ySny-on-Si films with compositions and thicknesses comparable to Ge1−ySny-on-Ge; but their strains were mostly relaxed. Efforts to increase the concentration and thickness of Ge1−ySny-on-Si resulted in multiphase materials containing large amounts of interstitial Sn. These outcomes suggest that the incorporation of even small Si amounts in Ge1−x−ySixSny might compensate for the large Ge–Sn mismatch by lowering bond strains. Such an effect reduces strain energy, enhances stability, promotes higher Sn incorporation, and increases critical thickness.

Design of tensile-strained GeSn/SiGeSn structure for low threshold mid-infrared laser application

The plasticity of GeSn alloy energy band has promoted the development of silicon-based photoelectric integration and optical interconnection. A tensile-strained GeSn/SiGeSn double heterostructure laser wrapped with Si3N4 stress liner is designed and characterized theoretically. The triaxial tensile strain is introduced into the GeSn/SiGeSn heterostructure laser by the Si3N4 linear stressor. The lower threshold current density and higher optical gain of the GeSn/SiGeSn laser can be achieved by tuning the band structure and carrier distribution in the active region with tensile strain and Sn compositions. Compared with the relaxed device, the value of ne ,Γ/ne ,total for the Ge0.90Sn0.10/Si0.315Ge0.499Sn0.186 heterostructure laser wrapped with 300 nm Si3N4 linear stressor is increased to 30.6% at n e,total of 1018 cm−3, the laser λ can be extended to 3.44 μm, and the J th is reduced from 206 to 59 A/cm2.

Challenges for room temperature operation of electrically pumped GeSn lasers

Recent demonstrations of room-temperature lasing in optically pumped GeSn show promise for future CMOS compatible lasers for Si-photonics applications. However, challenges remain for electrically pumped devices. Investigation of the processes that limit device performance is therefore vital in aiding the production of future commercial devices. In this work, a combined experimental and modelling approach is utilised to explore the dominant loss processes in current devices. By manipulating the band structure of functioning devices using high hydrostatic pressure techniques at low temperature, the dominant carrier recombination pathways are identified. This reveals that 93±\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$~\\pm ~$$\\end{document}5% of the threshold current is attributable to defect-related recombination at a temperature, T = 85 K. Furthermore, carrier occupation of L-valley states (carrier leakage) is responsible for 1.1±\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$~\\pm ~$$\\end{document}0.3% of the threshold current, but this sharply increases to 50% with a decrease of just 30 meV in the L-Γ\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$\\Gamma$$\\end{document} separation energy. This indicates that thermal broadening of a similar order may reproduce these adverse effects, limiting device performance at higher temperatures. Temperature dependent calculations show that carrier occupation of indirect valley L-states strongly affects the transparency carrier density and is therefore very sensitive to the Sn composition, leading to an effective operational temperature range for given Sn compositions and strain values. Recommendations for future device designs are proposed based on band structure and growth optimisations.

Role of tin clustering in band structure and thermodynamic stability of GeSn by atomistic modeling

Synthesis of device-quality GeSn materials with higher Sn compositions is hindered by various factors, such as Sn segregation, clustering, and short-range ordering effects. In the present work, the impact of the clustering of Sn atoms in a GeSn semiconductor alloy was studied by density functional theory using SG15 pseudopotentials in a Synopsys QuantumATK tool, where the thermodynamic stability, effective band structure, indirect and direct bandgaps, and density of states (DOS) were computed to highlight the difference between a cluster-free random GeSn alloy and a GeSn alloy with Sn–Sn clusters. A 54-atom bulk Ge1–xSnx (x = 3.71%–27.77%) supercell was constructed with cluster-free and a first nearest neighbor Sn–Sn clustered GeSn alloy at each composition for this work. Computation using the generalized gradient approximation exchange-correlation functional showed that the thermodynamic stability of GeSn was reduced due to the clustering of Sn, which increased the formation energy of the GeSn alloys by increasing the Hartree potential energy and exchange-correlation energy. Moreover, with the effective band structure of the GeSn material at a Sn composition of ∼22%, both direct (Eg,Γ) and indirect (Eg,L) bandgaps decreased by a large margin of 40.76 and 120.17 meV, respectively, due to Sn–Sn clustering. On the other hand, Eg,Γ and Eg,L decrease is limited to 0.5 and 12.8 meV, respectively, for Sn composition of ∼5.6%. Similar impacts were observed on DOS, in an independent computation without deducing from the electronic band structure, where the width of the forbidden band reduces due to the clustering of Sn atoms in GeSn. Moreover, using the energy bandgaps of GeSn computed with the assumption of it being a random alloy having well-dispersed Sn atoms needs revision by incorporating clustering to align with the experimentally determined bandgap. This necessitates incorporating the effect of Sn atoms clustered together at varying distributions based on experimental characterization techniques such as atom probe tomography or extended x-ray absorption fine structure to substantiate the energy bandgap of the GeSn alloy at a particular composition with precision. Hence, considering the effect of Sn clusters during material characterization, beginning with the accurate energy bandgap characterization of GeSn would help in mitigating the effect of process variations on the performance characteristics of GeSn-based group IV electronic and photonic devices such as varying leakage currents in transistors and photodiodes as well as the deviation from the targeted wavelength of operation in lasers and photodetectors.

Diffusion dominant thermal transport in mixed valent Ba4Sn4Se9

New material developments are of paramount importance from the perspective of developing a fundamental understanding of material properties as well as transitioning from materials to devices. Among the different classes of materials of interest, mixed valence Sn compositions have been known to display technologically important properties and are of interest for the design and synthesis of novel compositions for targeted applications. Herein, we report on electronic, optical and thermal properties of the previously unexplored mixed valence composition Ba4Sn4Se9, a material with a direct optical band gap of 2.36 eV. As revealed by single crystal synchrotron X-ray diffraction, Ba4Sn4Se9 crystallizes in the orthorhombic space group Pnma with a large unit cell comprised of 58 atoms. Heat capacity data reveals a low Debye temperature with many low frequency optic modes. Our low temperature thermal conductivity data and analyses show that thermal diffusion dominates thermal transport above 80 K. Furthermore, high lattice anharmonicity contributes to the very low thermal conductivity. Our results and analyses will motivate further investigations of this as well as other compositions in the Ba-Sn-Se system, as well as other mixed valence chalcogenides for technological applications.

Theoretical Analysis of GeSn Quantum Dots for Photodetection Applications.

GeSn alloys have recently emerged as complementary metal-oxide-semiconductor (CMOS)-compatible materials for optoelectronic applications. Although various photonic devices based on GeSn thin films have been developed, low-dimensional GeSn quantum structures with improved efficiencies hold great promise for optoelectronic applications. This study theoretically analyses Ge-capped GeSn pyramid quantum dots (QDs) on Ge substrates to explore their potential for such applications. Theoretical models are presented to calculate the effects of the Sn content and the sizes of the GeSn QDs on the strain distributions caused by lattice mismatch, the band structures, transition energies, wavefunctions of confined electrons and holes, and transition probabilities. The bandgap energies of the GeSn QDs decrease with the increasing Sn content, leading to higher band offsets and improved carrier confinement, in addition to electron-hole wavefunction overlap. The GeSn QDs on the Ge substrate provide crucial type-I alignment, but with a limited band offset, thereby decreasing carrier confinement. However, the GeSn QDs on the Ge substrate show a direct bandgap at higher Sn compositions and exhibit a ground-state transition energy of ~0.8 eV, rendering this system suitable for applications in the telecommunication window (1550 nm). These results provide important insights into the practical feasibility of GeSn QD systems for optoelectronic applications.

Influence of samarium modification on the phase-change performance and phase structure of tin antimonide

This work presents the optimization of the crystallization behavior and reliability of Sn15Sb85 thin films by doping Sm element. The phase transition behaviors induced by thermal were investigated by in situ resistance measurement. With the addition of Sm element, Sn15Sb85 film exhibits the superior crystallization temperature (232 °C) and data conservation (172.32 °C for 10 years), larger activation energy of crystallization (4.91 eV) and crystalline resistance (∼103 Ω), which contributes to the increased thermal stability of the amorphous state and decrease in the programming energy. The Sm-doping can broaden the energy band gap from 0.55 to 1.07 eV. The amorphous Sm and Sn compositions could retard grain growth and refine grain size from 21.13 to 11.13 nm, combining with x-ray diffraction and x-ray photoelectron spectroscopy. The surface morphology of Sn15Sb85 film becomes smoother after Sm doping as determined by atomic force microscopy images, resulting in the improved interfacial reliability. Phase change memory devices based on Sm0.095(Sn15Sb85)0.905 films can successfully achieve the complete SET and RESET reversible operation process with high operating speed (200 ns) and low power consumption (1.6 × 10−10 J). The results suggest that doping the proper concentration of Sm element will be an effectual solution to adapt and optimize the crystallization properties of Sn15Sb85 phase change material.

Dark Current Analysis on GeSn p-i-n Photodetectors.

Group IV alloys of GeSn have been extensively investigated as a competing material alternative in shortwave-to-mid-infrared photodetectors (PDs). The relatively large defect densities present in GeSn alloys are the major challenge in developing practical devices, owing to the low-temperature growth and lattice mismatch with Si or Ge substrates. In this paper, we comprehensively analyze the impact of defects on the performance of GeSn p-i-n homojunction PDs. We first present our theoretical models to calculate various contributing components of the dark current, including minority carrier diffusion in p- and n-regions, carrier generation-recombination in the active intrinsic region, and the tunneling effect. We then analyze the effect of defect density in the GeSn active region on carrier mobilities, scattering times, and the dark current. A higher defect density increases the dark current, resulting in a reduction in the detectivity of GeSn p-i-n PDs. In addition, at low Sn concentrations, defect-related dark current density is dominant, while the generation dark current becomes dominant at a higher Sn content. These results point to the importance of minimizing defect densities in the GeSn material growth and device processing, particularly for higher Sn compositions necessary to expand the cutoff wavelength to mid- and long-wave infrared regime. Moreover, a comparative study indicates that further improvement of the material quality and optimization of device structure reduces the dark current and thereby increases the detectivity. This study provides more realistic expectations and guidelines for evaluating GeSn p-i-n PDs as a competitor to the III-V- and II-VI-based infrared PDs currently on the commercial market.

Analysis of structural and electrical properties of Sn modified Ca0.6Sr0.4TiO3 ceramics

In this study, we synthesized Ca0.6Sr0.4Ti1-xSnxO3 ceramics using the solid-state reaction technique, varying the Sn compositions from x = 0.0 to 0.08. X-ray diffraction analysis revealed a slight modification in the crystallographic structure of calcium strontium titanate (CST) as the Sn content increased, while confirming the formation of single-phase ceramics. Parameters such as crystallite size, lattice strain, stacking fault, and dislocation density were estimated based on the XRD data. Crystallite size generally increased with higher tin concentrations, except for x = 0.08 where a decrease was observed. The FTIR spectra exhibited a broad band, and deconvolution using a Gaussian distribution was employed to identify structural units. Density measurements demonstrated densification with increasing Sn content. The activation energy values increased from 0.76 eV for undoped CST to 0.95 eV for x = 0.06, except for x = 0.08, which measured 0.89 eV, indicating the role of Sn in modifying the structure. Activation energy was determined through linear fitting of experimental data, revealing that the reciprocal temperature dependence of DC conductivity followed Arrhenius behaviour. These findings contribute novel insights into the synthesis and characterization of Ca0.6Sr0.4Ti1-xSnxO3 ceramics, highlighting the impact of Sn on the structural and electrical properties of CST.

FA/Cs-based mixed Pb–Sn perovskite solar cells: A review of recent advances in stability and efficiency

Perovskite solar cells (PSCs) have gained widespread attention due to their impressive optoelectronic properties, extraordinarily high efficiency, and low-cost manufacturing processes. PSCs are poised to usher in a new era of clean energy production. However, the use of methylammonium (MA) as the A-site cation in PSCs may result in poor thermal and atmospheric stability, limiting their potential for widespread commercialization. On the other hand, the most extensively studied perovskite systems with respect to B-site cations are pure Pb-based compositions due to their high device performance demonstrated to date. However, mixed Pb–Sn compositions can offer optimal bandgap energies for single-junction solar cells, suggesting the promise of future efficiency improvements. Thus, this review focuses on mixed Pb–Sn perovskite solar cells that use formamidinium (FA) or mixed cations of FA and cesium (Cs) without MA. These materials have bandgap energies suitable for use in either single-junction or multi-junction solar cells and show improved stability compared to their MA-based counterparts. We discuss the structural and optoelectronic properties of FA/Cs-based Pb–Sn perovskites and the efforts to improve their solar cell performance and stability through additive engineering, process optimization, and structural modifications. Our review highlights the potential for FA/Cs-based mixed Pb–Sn perovskites as a more stable and efficient alternative for future commercial applications of PSCs.

Gesn Alloys: From Optical to Electrical Pumped Lasers

Internet of Things applications, including medical diagnostics, imaging and industrial process controls, pollution monitoring or real-time motion control, push the extension of the Si photonics towards mid-infrared range.In recent years, group IV SiGeSn-based optoelectronic devices including light emitting, detectors, modulators, and CMOS devices have been developed. The main advantage of SiGeSn is the bandgap tunability in the range between 1.5 to 5 µm via Si and Sn compositions. Remarkably, the GeSn binary alloy becomes a direct bandgap semiconductor at a relative low Sn content of about 7-8 at.% opening the quest for the realization of monolithically integrated laser sources on Si. In this direction, from the first laser emission proof in 2015 remarkable progress has been done. Supported by epitaxial growth success, processing technology, and cavity design, the laser emission of optically pumped was continuously increased from 80K to 270K. The main role here was played by increasing the Sn content up a concentration of 17 at.%.The next major improvement came from layer transfer technology allowing defect engineering and heat management. Their combination made possible room temperature lasing. However, the real life application requires low laser threshold. Here the use of multiple quantum wells based on SiGeSn/GeSn heterostructures and strain engineering allow a remarkable threshold reduction to values of about 1 kA/cm2. Interestingly, modeling shows that the very epitaxial challenging of having high Sn contents can be reduced by band structure engineering via introducing tensile strain and, in addition, larger gains are possible compared with high-Sn strain free layers. The optical laser spectrum at 0 °C for a GeSn layer under 1 % biaxial tensile strain is shown in Fig 1a.The next step is the realization of an electrical pumped GeSn laser. The first data indicate that both waveguide and microdisk cavities can be electrically pumped leading to low temperature laser emission. The effect of the geometry and strain relaxation of under-etched cavities on the laser emission is under study. Figure 1b shows an exemplary spectrum and current-in light-out (LI) curve for a device with a SiGeSn/GeSn heterostructure taken at 50 K. Figure 1

Electrodeposition of Cu-Based Nanofoams and Sodiophilic Metals for Highly Stable Anode-Free Sodium Batteries

Sodium-ion batteries (SIBs) represent a cost-effective and sustainable alternative to lithium-ion battery (LIBs).1 One of the main issues of SIB technology is the higher ionic radius of sodium, compared to lithium, which limits the cyclability due to unfavourable intercalation of Na+ ions into carbon-based anodes.2 Anode-free batteries represents a promising approach to improve the cyclability, changing the working principle at the anode from intercalation of Na+ ions to electrodeposition metallic sodium.3 Moreover, thanks to the anode-free configuration the energy density of the overall device can be drastically increased. To efficiently control the electrodeposition of the metallic layer of sodium, the current collector must be carefully designed. Copper nanofoams were produced by dynamic hydrogen-bubble template (DHBT) as high-surface area template for the sodium electrodeposition, as an innovative alternative to the traditionally flat electrodes. The electrodeposition of sodiophilic materials, i.e. tin and antimony, onto the nanofoam, was also investigated to control the nucleation of the metallic layer at the anode.4 Sn and Sb baths were optimized to guarantee an homogeneous distribution of the sodiophilic metal on the nanofoam. The morphology and the composition of the nanofoams was characterized and electrochemical characterization was performed in half-cell configuration, showing coulombic efficiency higher than 99% and good cyclability for 100 cycles, for both Sn and Sb compositions compared to the bare Cu. This demonstrates the feasibility of the usage of sodiophilic nanofoams as current collectors in anode-free sodium batteries. Vaalma, C., Buchholz, D., Weil, M. & Passerini, S. A cost and resource analysis of sodium-ion batteries. Nat. Rev. Mater. 2018 34 3, 1–11 (2018).Irisarri, E., Ponrouch, A. & Palacin, M. R. Review—Hard Carbon Negative Electrode Materials for Sodium-Ion Batteries. J. Electrochem. Soc. 162, A2476–A2482 (2015).Cohn, A. P. et al. Rethinking sodium-ion anodes as nucleation layers for anode-free batteries. J. Mater. Chem. A 6, 23875–23884 (2018).Seh, Z. W., Sun, J., Sun, Y. & Cui, Y. A highly reversible room-temperature sodium metal anode. ACS Cent. Sci. 1, 449–455 (2015).

Single‐Crystalline Ge1−xSnx/Si p–n Heterojunction Photodiodes with Sn Compositions up to 10%

AbstractGeSn/Si heterojunction photodiodes are attractive because they can extend light detection wavelength range. However, the development of such photodiodes via epitaxial growth faces great challenges due to unavoidable issues such as lattice and thermal mismatches between Si and GeSn. Here, print Si nanomembranes are transferred on GeSn/Ge/Si substrates to form the GeSn/Si heterojunction photodiodes. The p‐Ge0.977Sn0.023/n‐Si heterojunction photodiodes exhibit a good rectifying behavior with a low dark current density of 40 mA cm−2and responsivity of 0.41 A W−1at 1550 nm under a reverse bias of −2 V. In addition, the detection wavelength range of p‐Ge0.9Sn0.1/n‐Si is extended to 2100 nm because of the increased Sn composition. The bandgap calculation of as‐grown GeSn with various Sn compositions is carried out. It confirms that the enhanced responsivity and extended detection wavelength ranges are attributed to the reduced bandgap from 750 to 601 meV when the Sn composition is increased from 2.3% to 10%. The result shows that the transfer printing of a freestanding single‐crystalline Si nanomembrane to a bulk GeSn/Ge/Si substrate can provide an excellent alternative route for realizing GeSn/Si heterojunction photodiodes.

Coexistence of two types of short-range order in Si–Ge–Sn medium-entropy alloys

Short-range chemical order (SRO) has been shown to play a decisive role in modulating a wide range of physical properties in medium-entropy alloys and high-entropy alloys. The enormous configurational space of these alloys implies multiple forms of SRO may exist, but such structural diversity has not been reported. Here, ab initio-based sampling reveals that Si–Ge–Sn medium-entropy alloys have two distinct forms of SRO, which carry different energies, distinct degrees of local ordering, and dissimilar electronic structures, and co-exist across a wide range of Si–Ge–Sn compositions. We rationalize the co-existence of two SROs through their virtual degeneracy of thermodynamic stability, due to a subtle balance in the change of enthalpy and configurational entropy upon transformation between the two SROs. Such co-existence of SROs suggests an inherent structural heterogeneity, a diffuse electronic structure, and a new route for band engineering in Si–Ge–Sn medium-entropy alloys. More generally, this study indicates possibility of the co-existence of multiple forms of SRO in medium- and high-entropy alloys.

Understanding the dopability of p-type Mg2(Si,Sn) by relating hybrid-density functional calculation results to experimental data

It is crucial to reach a sufficiently high carrier concentration in order to optimize the thermoelectric (TE) material in the development of Mg2 X (X= Si, Ge, and Sn)-based TE generators. While n-type Mg2(Si,Sn) has excellent TE properties, p-type shows suboptimal TE performance because of insufficient carrier concentration, in particular for Mg2Si and Si-rich Mg2(Si,Sn). A systematic investigation of Li-doped Mg2Si1-x Sn x has been performed as Li, in contrast to other typical dopants, has a high solubility in the material system and has been shown to yield the highest reported carrier concentrations. We observe that the carrier concentration increases with Li content, but the dopant efficiency decreases. With respect to the Si:Sn ratio, we find a clear increase in maximum achievable carrier concentration and dopant efficiency with increasing Sn content. The trends can be understood by employing defect formation energies obtained within the hybrid-density functional theory (DFT) for the binaries. Further, we use a linear interpolation of the hybrid-DFT results from the binaries to the ternary Mg2(Si,Sn) compositions and a simple single parabolic band model to predict the maximal achievable carrier concentration for the solid solutions, providing a simple guideline for experimental work. Finally, we show that the approach is transferable to other material classes. This work highlights that, besides dopant solubility, the interplay between intrinsic and extrinsic defects determines the achievable carrier concentration.

Role of deposition temperature and Sn content on structural, optical & electrical properties of In2O3 thin films

High quality indium tin oxide (ITO) thin films (In2−xSnxO3: x = 0, 0.1 and 0.2) have been grown by using pulsed laser deposition technique on quartz substrates. The structural, morphological, optical and electrical investigations of deposited films have been studied as a function of substrate deposition temperatures and the Sn compositions. X-ray diffraction (XRD) and high-resolution transmission electron microscopy (HRTEM) patterns affirm that each film is polycrystalline in nature with cubic bixbyite single phase structure which preferentially oriented along (222) Miller plane. The existence of chemical bonding and functional groups was investigated by FTIR spectroscopy. The TEM micrograph of films (@450°C) for x = 0.1 and x = 0.2 reveal spherical morphology with average particle size 63 nm and 51 nm, respectively. The SEM and AFM images show uniform flower like surface morphology and well-demonstrated nanosized spherical particles, respectively. The widening of the band gap of all the films were exclusively defined by Burstein-Moss shift. The Hall measurement reveals that each film is degenerate with n-type semiconducting nature along with high mobility. Low resistivity (2.024 × 10−4 Ω-cm) and high transparency (92.58%) along with high carrier concentration (8.915 × 1020cm−3) were optimized for x = 0.1 film at 450°C deposition temperature.

Electronic and optical properties of SixGe1−x−ySny alloys lattice-matched to Ge

We present a combined experimental and theoretical analysis of the evolution of the near-band gap electronic and optical properties of Si$_{x}$Ge$_{1-x-y}$Sn$_{y}$ alloys lattice-matched to Ge and GaAs substrates. We perform photoreflectance (PR) and photoluminescence (PL) measurements on Si$_{x}$Ge$_{1-x-y}$Sn$_{y}$ epitaxial layers grown via chemical vapour deposition, for Si (Sn) compositions up to $x =$ 9.6% ($y =$ 2.5%). Our measurements indicate the presence of an indirect fundamental band gap, with PL observed $\approx$ 200-250 meV lower in energy than the direct $E_0$ transition identified by PR measurements. The measured PL is Ge-like, suggesting that the alloy conduction band (CB) edge is primarily derived from the Ge L-point CB minimum. Interpretation of the PR and PL measurements is supported by atomistic electronic structure calculations. Effective alloy band structures calculated via density functional theory confirm the presence of an indirect fundamental band gap, and reveal the origin of the observed inhomogeneous broadening of the measured optical spectra as being alloy-induced band hybridisation occurring close in energy to the CB edge. To analyze the evolution of the band gap, semi-empirical tight-binding (TB) calculations are employed to enable calculations for large supercell sizes. TB calculations reveal that the alloy CB edge is hybridized in nature, consisting at low Si and Sn compositions of an admixture of Ge L-, $\Gamma$- and X-point CB edge states, and confirm that the alloy CB edge retains primarily Ge L-point CB edge character. Our experimental measurements and theoretical calculations confirm a direct transition energy close to 1 eV in magnitude for Si and Sn compositions $x =$ 6.8 - 9.6% and $y =$ 1.6 - 2.2%.

High carrier lifetimes in epitaxial germanium–tin/Al(In)As heterostructures with variable tin compositions

Carrier lifetimes vs. Sn compositions of the Ge1−ySny/III–V heterostructures.

Theoretical impacts of single band gap grading of perovskite and valence band offset of perovskite/hole transport layer interface on its solar cell performances

Single bandgap (Eg)-grading in CH3NH3(SnxPb1−x)(I1−yBry)3 perovskites and valence band offset (VBO) of a perovskite/hole transport layer (HTL) interface of perovskite solar cells (PSCs) are theoretically scrutinized. Eg values at front and back is defined as Eg values of perovskite at light incident side and back side, respectively, which are changed from 1.10 eV to 1.80 eV based on Sn and Br compositions for different Eg-grading. It is disclosed that optimized single Eg-grading (1.47 / 1.33 eV at front/back) results in enhancement of cell performances because of effective utilization of broader solar spectrum and formation of electric field in direction of promoting charge separation. Furthermore, optimized carrier diffusion length of 2.5 μm and perovskite thickness of 0.75 μm yields the highest power conversion efficiency of 26.0%. CH3NH3(Sn0.07Pb0.93)I3 at front for front Eg of 1.47 eV and CH3NH3(Sn0.38Pb0.62)I3 at back for back Eg of 1.33 eV are thus suggested for optimized single Eg-grading. Moreover, optimized VBO is in a range from − 0.1 eV to + 0.2 eV regardless of Eg-grading to improve cell performances. The results indicated that PEDOT:PSS, Spiro-OMeTAD, and PTAA as HTL are ultimately suitable for CH3NH3 (SnxPb1−x)I3 based PSCs, whereas NiOx and CuOx as HTL is appropriate for CH3NH3 (SnxPb1−x)I3 and CH3NH3Pb(I1−yBry)3 based PSCs.

Green Synthesis of Ge1−xSnx Alloy Nanoparticles for Optoelectronic Applications

Compositionally controlled, light-emitting, group IV semiconductor nanomaterials have potential to enable on-chip data communications and infrared (IR) imaging devices compatible with the complementary metal−oxide−semiconductor (CMOS) technology. The recent demonstration of a direct band gap laser in Ge-Sn alloys opens avenues to the expansion of Si-photonics. Ge-Sn alloys showed improved effective carrier mobility as well as direct band gap behavior at Sn composition above 6–11%. In this work, Ge1−xSnx alloy nanoparticles with varying Sn compositions from x = 0.124 to 0.178 were prepared via colloidal synthesis using sodium borohydride (NaBH4), a mild and non-hazardous reducing reagent. Successful removal of the synthesized long-alkyl-chain ligands present on nanoparticles’ surfaces, along with the passivation of the Ge-Sn nanoparticle surface, was achieved using aqueous (NH4)2S. The highly reactive surface of the nanoparticles prior to ligand exchange often leads to the formation of germanium oxide (GeO2). This work demonstrates that the (NH4)2S further acts as an etching reagent to remove the oxide layer from the particles’ surfaces. The compositional control and long-term stability will enable the future use of these easily prepared Ge1−xSnx nanoalloys in optoelectronic devices.