The Interplay between Strain,Sn Content,and Temperature on Spatially Dependent Bandgap in Ge1−x Sn x Microdisks

Germanium–tin (GeSn) microdisks are promising structures for complementary metal–oxide–semiconductor-compatible lasing. Their emission properties depend on Sn concentration, strain, and operating temperature. Critically, the band structure of the alloy varies along the disk due to different lattice deformations associated with mechanical constraints. An experimental and numerical study of Ge_1−x Sn _x microdisk with Sn concentration between 8.5 and 14 at% is reported. Combining finite element method calculations, micro-Raman and X-ray diffraction spectroscopy enables a comprehensive understanding of mechanical deformation, where computational predictions are experimentally validated, leading to a robust model and insight into the strain landscape. Through micro-photoluminescence experiments, the temperature dependence of the bandgap of Ge_1−x Sn _x is parametrized using the Varshni formula with respect to strain and Sn content. These results are the input for spatially dependent band structure calculations based on deformation potential theory. It is observed that Sn content and temperature have comparable effects on the bandgap, yielding a decrease of more than 20 meV for an increase of 1 at% or 100 K, respectively. The impact of the strain gradient is also analyzed. These findings correlate structural properties to emission wavelength and spectral width of microdisk lasers, thus demonstrating the importance of material-related consideration on the design of optoelectronic microstructures.     

First-principles study on the electronic and optical properties of SnxGe1 − x

The electronic and optical properties of the direct band gap alloys Sn_xGe_1 − x (x = 0.000, 0.042, 0.083, 0.125, 0.167, and 0.208) have been studied by using the generalized gradient approximation in the framework of the density functional theory. The calculated lattice constants obey Vergard's law. The band structures show that the alloys have direct band gap and the band gaps can be tunable by Sn contents. The optical properties of the Sn_xGe_1 − x alloys with the physical quantities such as the complex dielectric function, the energy-loss function and the static dielectric constant, respectively, are shown to support the potential application of infrared devices in the future.     

Ge(001)衬底上分子束外延生长高质量的Ge1-xSnx合金

Ge_1-xSn_x是一种新型IV族合金材料, 在光子学和微电子学器件研制中具有重要应用前景. 本文使用低温分子束外延(MBE)法, 在Ge(001)衬底上生长高质量的Ge_1-xSn_x合金, 组分x分别为1.5%, 2.4%, 2.8%, 5.3%和14%, 采用高分辨X射线衍射(HR-XRD)、卢瑟福背散射谱(RBS) 和透射电子显微镜(TEM)等方法表征Ge_1-xSn_x合金的材料质量. 对于低Sn组分(x≤ 5.3%)的样品, Ge_1-xSn_x合金的晶体质量非常好, RBS的沟道/随机产额比(χ_min)只有5.0%, HR-XRD曲线中Ge_1-xSn_x衍射峰的半高全宽(FWHM)仅100' 左右. 对于x=14%的样品, Ge_1-xSn_x合金的晶体质量相对差一些, FWHM=264.6'. 关键词： 锗锡合金 锗 分子束外延     

Tunable band structure in diamond-cubic tin-germanium alloys grown on silicon substrates

Novel chemical methods based on deuterium-stabilized Sn hydrides and ultra-high-vacuum chemical vapor deposition were used to grow Sn_xGe_1−x alloys directly on silicon. Device-quality, strain-free films with a Sn-fraction as high as x=0.2 were obtained. The optical properties provide evidence for a well-defined Ge-like band structure. In particular, the direct band gap E₀ is reduced to a value as low as 0.41 eV for Sn_0.14Ge_0.86. The growth of these high-optical quality infrared materials creates entirely new opportunities for band gap engineering on Si.     

Studies on electrical switching behavior and optical band gap of amorphous Ge–Te–Sn thin films

Amorphous thin film Ge₁₅Te_85−x Sn _x (1≤x≤5) and Ge₁₇Te_83−x Sn _x (1≤x≤4) switching devices have been deposited in sandwich geometry using a flash evaporation technique, with aluminum as the top and bottom electrodes. Electrical switching studies indicate that these films exhibit memory type electrical switching behavior. The switching fields for both the series of samples have been found to decrease with increase in Sn concentration, which confirms that the metallicity effect on switching fields/voltages, commonly seen in bulk glassy chalcogenides, is valid in amorphous chalcogenide thin films also. In addition, there is no manifestation of rigidity percolation in the composition dependence of switching fields of Ge₁₅Te_85−x Sn _x and Ge₁₇Te_83−x Sn _x amorphous thin film samples. The observed composition dependence of switching fields of amorphous Ge₁₅Te_85−x Sn _x and Ge₁₇Te_83−x Sn _x thin films has been understood on the basis of Chemically Ordered Network model. The optical band gap for these samples, calculated from the absorption spectra, has been found to exhibit a decreasing trend with increasing Sn concentration, which is consistent with the composition dependence of switching fields.     

GeSn合金的晶格常数对Vegard定律的偏离

在Si (001)衬底上, 以高质量的弛豫Ge薄膜作为缓冲层, 先后生长Sn组分x分别为2.5%, 5.2%和7.8%的完全应变的三层Ge_1-xSn_x合金薄膜. 在Si (001)衬底上直接生长了x分别为0.005, 0.016, 0.044, 0.070和0.155的五个弛豫Ge_1-xSn_x样品. 通过卢瑟福背散射谱、高分辨X射线衍射和X射线倒易空间图等方法测量了Ge_1-xSn_x合金的组分 与晶格常数. 实验得到的晶格常数相对Vegard定律具有较大的正偏离, 弯曲系数b=0.211 Å.     

Self-assembling of C and Sn in Ge

Self-assembling of isoelectronic C and Sn impurities in Ge is predicted. The formation of the 1C4Sn tetrahedral cells is thermodynamically profitable in Ge-rich C_xSn_yGe_1−x−y (4x<y) alloys in the ultra dilute C impurity limit with 1×10^-8x1×10^-3. The concentrations of Sn atoms when all C atoms are surrounded only by Sn atoms are estimated for the lower molecular beam epitaxy, intermediate annealing and higher bulk crystallization temperatures. The origin of this phenomenon is a considerable decrease of the strain energy after self-assembling. The same self-assembling in Si is thermodynamically non-profitable due to the large cohesive energy of Si–C chemical bonds.     

A TIN-119 Mössbauer and electrical conductivity study of the system SnxGe1-xSe (0 ⩽ x ⩽ 1)

¹¹⁹Sn Mössbauer parameters and electrical conductivity data for phases in the system Sn_xGe_1-xSe (0 ? x ? 1) are reported. The Mössbauer data for the material Sn_0.8Ge_0.2Se show that the electron distribution around the Sn at this composition is anomalous. There is, however, little indication of the presence of such an anomaly in the electrical conductivity and Hall Effect data. The implications of these observations are discussed.     

Strained and strain-relaxed epitaxial Ge1-xSnx alloys on Si（100） substrates

Epitaxial Ge1-xSnx alloys are grown separately on a Ge-buffer/Si(100) substrate and directly on a Si(100) substrate by molecular beam epitaxy (MBE) at low temperature. In the case of the Ge buffer/Si(100) substrate, a high crystalline quality strained Ge0.97Sn0.03 alloy is grown, with a χmin value of 6.7% measured by channeling and random Rutherford backscattering spectrometry (RBS), and a surface root-mean-square (RMS) roughness of 1.568 nm obtained by atomic force microscopy (AFM). In the case of the Si(100) substrate, strain-relaxed Ge0.97Sn0.03 alloys are epitaxially grown at 150°C-300°C, with the degree of strain relaxation being more than 96%. The X-ray diffraction (XRD) and AFM measurements demonstrate that the alloys each have a good crystalline quality and a relatively flat surface. The predominant defects accommodating the large misfit are Lomer edge dislocations at the interface, which are parallel to the interface plane and should not degrade electrical properties and device performance.     

Potential of Si1−xGex alloys for Auger generation in highly efficient solar cells

Recent investigation on Si solar cells demonstrated the utility of Auger generation for the creation of more than merely one electron/hole pair per absorbed photon. The semiconductor Si requires a minimum photon energy of about 3.4 eV for this internal carrier multiplication. The current of a Si cell is therefore not significantly increased by Auger generation when the cell is illuminated by an air mass 1.5 spectrum, which contains only few photons with energies above 3.4 eV. Use of Si_1–x Ge _x alloys promises a lower onset energy. Unfortunately, incomplete data on band structures ofrandom Si_1–x Ge _x alloys preclude a detailed quantitative discussion of the full potential for these materials. Nevertheless, (i) analogies to our own quantum efficiency data from pure Si, (ii) the calculated band structure of the hypothetical,ordered zincblende type Si_0.5Ge_0.5 crystal, and (iii) optical data for Si_1–x Ge _x alloys indicate an optimum Ge content ofx=0.6 tox=0.7.     

Synthesis of compositionally controllable Cu<Subscript>2</Subscript>(Sn<Subscript>1−x</Subscript>Ge<Subscript>x</Subscript>)S<Subscript>3</Subscript> nanocrystals with tunable band gaps

In this work, we show that compositionally controlled Cu₂(Sn_1–xGe_x)S₃ nanocrystals can be successfully synthesized by the hot-injection method through careful tuning the Ge/(Sn+Ge) precursor ratio. The band gaps of the resultant nanocrystals are demonstrated to be linearly tuned from 1.45 to 2.33 eV by adjusting the composition parameter x of the Ge/(Sn+Ge) ratio from 0.0 to 1.0. The crystalline structures of the resultant NCs have been studied by the X-ray diffraction (XRD), high-resolution transmission electron microscopy (HRTEM), select area electron diffraction (SAED), and Raman spectroscopy. A ligand exchange procedure is further performed to replace the native ligands on the surface of the NCs with sulfur ions. The photoresponsive behavior indicates the potential use of as-prepared Cu₂(Sn_1–xGe_x)S₃ nanocrystals in solar energy conversion systems. The synthesis of compositionally controlled Cu₂(Sn_1–xGe_x)S₃ nanocrystals reported herein provides a way for probing the effect of Ge inclusion in the Cu-Sn-S system thin films.     

Statistical model for the formation of the alloy

The electronic structures of most semiconductor alloys are smooth functions of their composition. Binary alloys of group IV semiconductors are usually easy to prepare at any concentration, but this is not the case for the Ge_1-xSn_x alloy. Homogeneous alloys as required for nano- and optoelectronics device applications have proved difficult to form for x above a temperature-dependent critical concentration, above which Sn exhibits the tendency to segregate in the metallic cubic β phase, spoiling the semiconducting properties. The underlying mechanism for this segregation and critical concentration was not known.Through previous accurate ab initio local defect calculations we estimated the scale of energies involved in the immediate environment around a large number of Sn defects in Ge, the relaxed configurations of the defects, and the pressure directly related to the elastic field caused by the defects. This detailed information allowed us to build a simple statistical model including the defects most relevant at low x, namely substitutional α-Sn and non-substitutional β-Sn (in which a single atom occupies the centre of a Ge divacancy). Our model enables us to determine at which concentration β defects, which exhibit a tendency to segregate, can be formed in thermal equilibrium. These results coincide remarkably well with experimental findings, concerning the critical concentration above which the homogeneous alloys cannot be formed at room temperature. Our model also predicts the observed fact that at lower temperature the critical concentration increases.     

Structural and optical properties of amorphous GexSn1-x alloys

The structural and optical properties of amorphous Ge_xSn_1-x alloys have been studied in the range x ? 0.5. The atomic arrangement is found to be tetrahedral with a random nearest neighbor environment. Both the average and the minimum optical gaps decrease with increasing Sn content, with the latter extrapolating to 0 at x ≈ 0.0. It is suggested that the variation of the optical and electronic properties with Sn concentration will provide a sensitive test of current theories of amorphous solids.     

FP-LMTO method to calculate the structural,thermodynamic and optoelectronic properties of SixGe1−xC alloys

The structural and electronic properties of the ternary Si_xGe_1?xC alloys have been calculated using the full-potential linear muffin-tin-orbital (FP-LMTO) method based on density functional theory within both local density approximation (LDA) and generalised gradient approximation (GGA). The calculated equilibrium lattice constants and bulk moduli are compared with previous results. The concentration dependence of the electronic band structure and the direct and indirect band gaps are investigated. Using the approach of Zunger and co-workers, the microscopic origins of the band gap bowing are investigated also. Moreover, the refractive index and the optical dielectric constant for Si_xGe_1?xC are studied. The thermodynamic stability of the alloys of interest is investigated by means of the miscibility. This is the first quantitative theoretical prediction to investigate the effective masses, optical and thermodynamic properties for Si_xGe_1?xC alloy, and still awaits experimental confirmations.     

Metastable (III–V)1−xIV2x alloys

Zincblende-to-diamond-lattice structural phase transitions should occur in metastable (III–V)_1?xIV_2x alloys at a transition composition x_c that is controllable by growth conditions. The effect of this transition should be visible both in the electronic as well as the vibrational properties of these alloys. For example, in the prototypical (III–V)_1?xIV_2x alloy, (GaAs)_1?xGe_2x, the observed anomalous V-shaped bowing of the direct gap is explained in terms of the phase transition, which occurs at the minimum of the “V.” Predictions are made for the band gaps of new metastable alloys, such as (GaSb)_1?xSn_2x. Consequences of this transition for (III–V)-IV superlattices are also discussed.     

Diamond cubic Sn-rich nanocrystals: synthesis, microstructure and optical properties

Diamond cubic Sn-rich nanocrystals were fabricated with radii less than 20 nm by post-growth annealing at T=750 °C of Sn_xGe_1-x alloys grown on Ge(001) by molecular beam epitaxy. The crystal phase of the Sn-rich nanocrystals was determined to be diamond cubic from Fourier transform analysis of high-resolution transmission electron microscopy images. Optical transmittance of these Sn_xGe_1-x/Ge (001) films demonstrated changes in optical absorption that can be attributed to absorption from the nanocrystals. The energy bandgap was measured to be 0.45 eV for nanocrystals arrays in Ge with a mean diameter of 32 nm. PACS 68.37.Lp; 78.67.Hc; 81.07.Ta; 81.16.Dn; 68.65.Hb     

Numerical analysis of SiGeSn/GeSn interband quantum well infrared photodetector

In this paper, detailed theoretical investigation on the frequency response and responsivity of a strain balanced SiGeSn/GeSn quantum well infrared photodetector (QWIP) is made. Rate equation and continuity equation in the well are solved simultaneously to obtain photo generated current. Quantum mechanical carrier transport like carrier capture in QW, escape of carrier from the well due to thermionic emission and tunneling are considered in this calculation. Impact of Sn composition in the GeSn well on the frequency response, bandwidth and responsivity are studied. Results show that Sn concentration in the GeSn active layer and applied bias have important role on the performance of the device. Significant bandwidth is obtained at low reverse bias voltage, e.g., 200?GHz is obtained at 0.28?V bias for a single Ge_0.83Sn_0.17 layer. Whereas, the maximum responsivity is of 8.6?mA/W at 0.5?V bias for the same structure. However, this can be enhanced by using MQW structure.     

Electronic structure of the Co4Sn6Te6 ternary skutterudite phase

The electronic structure of Co₄Sn_6–x Te_6+x (x = 0, 1) ternary skutterudite systems has been investigated using ab ‐initio band structure computation. The x = 0 system is a semiconductor like the binary Co₄Sb₁₂, but with a lower band gap. The best dopant concentration for Co₄Sn₆Te₆ is estimated to be lower than that of Co₄Sb₁₂, with the highest electronic fig‐ ure‐of‐merit Z_eT for the n‐doped system. Finally, the increased charge transfer between the 8c Co and 24g Sn and Te atoms in Co₄Sn₆Te₆ compared to that of Co₄Sb₁₂ could be one reason for the observed decrease of thermal conductivity in ternary skutterudite systems. (© 2007 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Zn掺杂p型Ba8Ga16ZnxGe30－x笼合物的合成及热电性能

用熔融法结合放电等离子烧结方法合成了Zn掺杂单相p型Ge基Ⅰ型笼合物Ba₈Ga₁₆Zn_xGe_30-x(x=3, 4, 5, 6)，探索Zn取代Ge对其热电性能的影响规律，结果表明：所制备的Ba₈Ga₁₆Zn_xGe_30-x化合物为p型传导，随Zn取代量x关键词： p型笼合物 合成 热电性能     

Ab‐initio thermodynamic calculations of Mg2BIV (BIV = Si,Ge, Sn) solid solutions

The thermodynamic properties of ternary Mg₂B^IV (B^IV = Si, Ge, Sn) solid solutions were first calculated by the ab‐initio density functional method. The results showed that there exist composition regions with d²G /dx² < 0 in Mg₂Si_1–x Sn_x and Mg₂Ge_1–x Sn_x systems, implying the possibility of spinodal decomposition of the pseudobinary solid solutions. It is suggested that the spinodal decomposition would be a potential way to obtain Mg₂B^IV based bulk in‐situ nanocomposites with reduced grain sizes and enhanced phonon scattering, and hence an improved thermoelectric figure of merit. (© 2008 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)