Si–Ge–Sn alloys: From growth to applications

In this review article, we address key material parameters as well as the fabrication and application of crystalline GeSn binary and SiGeSn ternary alloys. Here, the transition from an indirect to a fundamental direct bandgap material will be discussed. The main emphasis, however, is put on the Si–Ge–Sn epitaxy. The low solid solubility of α-Sn in Ge and Si of below 1 at.% along with the large lattice mismatch between α-Sn (6.489 Å) and Ge (5.646 Å) or Si (5.431 Å) of about 15% and 20%, respectively, requires non-equilibrium growth processes. The most commonly used approaches, i.e. molecular beam epitaxy (MBE) and chemical vapor deposition (CVD), will be reviewed in terms of crucial process parameters, structural as well as optical quality and employed precursor combinations including Germanium hydrides, Silicon hydrides and a variety of Sn compounds like SnD₄, SnCl₄ or C₆H₅SnD₃. Special attention is devoted to the growth temperature window and growth rates being the most important growth parameters concerning the substitutional incorporation of Sn atoms into the Ge diamond lattice. Furthermore, the mainly CVD-driven epitaxy of high quality SiGeSn ternary alloys, allowing the decoupling of band engineering and lattice constant, is presented. Since achieving fundamental direct bandgap Sn-based materials strongly depends on the applied strain within the epilayers, ways to control and modify the strain are shown, especially the plastic strain relaxation of (Si)GeSn layers grown on Ge.Based on recently achieved improvements of the crystalline quality, novel low power and high mobility GeSn electronic and photonic devices have been developed and are reviewed in this paper. The use of GeSn as optically active gain or channel material with its lower and potentially direct bandgap compared to fundamentally indirect Ge (0.66 eV) and Si (1.12 eV) provides a viable solution to overcome the obstacles in both fields photonics and electronics. Moreover, the epitaxial growth of Sn-based semiconductors using CMOS compatible substrates on the road toward a monolithically integrated and efficient group IV light emitter is presented.     

Fabrication and ionic conductivity of amorphous Li–Al–Ti–P–O thin film

Li⁺ ion conducting Li–Al–Ti–P–O thin films were fabricated on ITO-glass substrates at various temperatures from 25 to 400 °C by RF magnetron sputtering method. When the substrate temperature is higher than 300 °C, severe destruction of ITO films were confirmed by XRD (X-ray diffraction) and the abrupt transformation of one semi-circle into two semi-circles on the impedance spectra. These as-deposited Li–Al–Ti–P–O solid state electrolyte thin films have an amorphous structure confirmed by XRD and a single semicircle on the impedance spectra. Good transmission higher than 80% in the visible light range of these electrolyte thin films can fulfill the demand of electro-chromic devices. Field emission scanning electron microscopy and atomic force microscopy showed the denser, smoother and more uniform film structure with the enhanced substrate temperature. Measurements of impedance spectra indicate that the gradual increased conductivity of these Li–Al–Ti–P–O thin films with the elevation of substrate temperature from room temperature to 300 °C is originated from the increase of the pre-exponential factor (σ₀). The largest Li-ion conductivity can come to 2.46 × 10^? 5 S cm^? 1. This inorganic solid lithium ion conductor film will have a potential application as an electrolyte layer in the field such as lithium batteries or all-solid-state EC devices.     

Investigation of directional solidified Al–Ti alloy

Al–1 wt% Ti alloy was directionally solidified upwards under argon atmosphere under the two conditions; with different temperature gradients (G = 2.20–5.82 K/mm) at a constant growth rate (V = 8.30 μm/s) and with different growth rates (V = 8.30–498.60 μm/s) at a constant temperature gradient (G = 5.82 K/mm) in a Bridgman furnace. The dependence of characteristic microstructure parameters such as primary dendrite arm spacing (λ₁), secondary dendrite arm spacing (λ₂), dendrite tip radius (R) and mushy zone depth (d) on the velocity of crystal growth and the temperature gradient were determined by using a linear regression analysis. A detailed analysis of microstructure development with models of dendritic solidification and with previous similar experimental works on dendritic growth for binary alloys were also made.     

Effects of post-annealing on electrical properties of amorphous Ga-doped Zn–Sn–O semiconductor films

We investigated the effect of post-annealing on the electrical properties of amorphous gallium-zinc-tin oxide (a-GZTO) films with different Ga contents. The films were deposited at room temperature by sputtering and annealed in air for 1 h. It was found that the doping with Ga, which acts as the carrier suppressor, contributes to the thermal stability of characteristic properties of a-GZTO thin films. The film with a small amount of Ga showed significant variations in carrier concentration according to the annealing temperature. Increases in carrier concentration and mobility can be ascribed to the reduction of subgap density of states by annealing. After annealing at 400 °C, however, the enrichment of Zn cations in surface region resulted in considerable changes in chemical bonding states and consequently, the carrier concentration decreased by two orders of magnitude for the low Ga-doped ZTO film.     

Lightweight Ti–Zr–Be–Al bulk metallic glasses with improved glass-forming ability and compressive plasticity

A series of lightweight Ti–Zr–Be–Al bulk metallic glasses (BMGs) have been developed through the addition of Al to Ti–Zr–Be ternary glassy alloy. By replacing Be with Al, the critical size of the glassy rod has been increased from 5 mm for Ti₄₁Zr₂₅Be₃₄ alloy to 7 mm for Ti₄₁Zr₂₅Be₂₉Al₅ alloy, while the yield strength of Ti₄₁Zr₂₅Be_34 ? xAl_x (x = 2–10) has been greatly enhanced, resulting in a significant increase of the specific strength which is defined as yield strength/density. Among these newly developed Ti–Zr–Be–Al BMGs, Ti₄₁Zr₂₅Be₂₆Al₈ glassy alloy exhibits a high specific strength of 4.33 × 10⁵ Nm/kg and a very large compressive plastic strain of 47.0%, which are much larger than those (3.69 × 10⁵ Nm/kg and 2.9%, respectively) for Ti₄₁Zr₂₅Be₃₄ glassy alloy. The present results show that Al is an effective alloying element for improving the glass-forming ability (GFA) and mechanical properties of Ti-Zr-Be glassy alloy.     

Electron Microscopy of the Microstructure of Nb–Ti Tapes

Crystallography Reports - The microstructure of a cold-rolled tape made of Nb–50 wt % Ti superconducting alloy before and after heat treatment has been investigated by transmission electron...     

Characterization and hydrogenation of methyl oleate over Ru/TiO2, Ru–Sn/TiO2 catalysts

The hydrogenation of methyl oleate over 2%Ru/TiO₂, 4.7%Sn/TiO₂ and 2%Ru–4.7%Sn–TiO₂ catalysts was studied. The catalysts were prepared by the impregnation method. The characterization techniques used were: (i) specific surface area (B.E.T. method); (ii) pore volume and mean pore diameter; (iii) thermogravimetric analysis (TGA); (iv) temperature programed reduction (TPR); (v) X-ray diffraction (DRX); and (vi) scanning electron microscopy (SEM). The results indicate that the presence of tin increases the difficulty of the ruthenium reduction, evidencing a possible metal–metal interaction. However, tin addition to the TiO₂ and the catalysts calcinations did not significantly influence the values obtained for the surface area and mean pore diameter. The DRX analyses showed that the catalysts have a crystalline structure, mostly in the anatase form. The metal-support interaction occurred probably due to the formation of RuTi of Ru/TiO₂ catalyst. In Ru–Sn/TiO₂ catalyst, the metal–metal interaction occurs between ruthenium and tin, resulting in Ru₃Sn₇. The catalysts reaction revealed that tin addition in ruthenium affects the activity of catalyst considerably in the hydrogenation of methyl oleate reaction. However, the activity decrease is accompanied by a considerable increase of alcohols yield, in particular of unsaturated alcohols.     

Bulk glass formability for Cu–Hf–Zr–Ag and Cu–Zr–Ag–Si alloys

The effects of the gradual substitution of Zr by Hf on glass formability and thermal stability in the Cu₄₅Zr_45?xHf_xAg₁₀ alloys and the effects of small additions of Si on glass formability in the Cu₄₅Zr₄₅Ag₁₀ alloy are reported and discussed. The samples were prepared as ribbons of thickness in the range 25–200 μm by melt spinning and as conical bulk shapes, with a length of 50 mm and cone base diameters in the range 2–10 mm, by suction die casting. The alloy Cu₄₅Zr₄₅Ag₁₀ had a critical cylindrical rod diameter for glass formation, d_c, of 3.5 mm but substitution of 1.5 and 3.5 at.% Zr by Hf resulted in substantial increases to 5.5 and 4.5 mm, respectively. However, for x in the range 5–40 at.%, d_c was reduced to <1 mm_. The small addition of Si proved to be beneficial to the glass forming ability (GFA), increasing d_c up to 5.5 mm for 0.5 at.% Si. Thus the chemical and atomic size similarities of Hf and Zr do not guarantee that bulk glass formation will be maintained on substituting large proportions of Zr by Hf though small substitutions of Hf and of minor additions of Si were beneficial to the GFA. These effects are discussed in terms of the possible influence of the alloying additions on the liquid structure and on the number density of heterogeneous nucleants.     

Sn自溶剂含量对Al掺杂Ⅷ型Sn基单晶笼合物电传输特性的影响

通过Sn自熔剂法制备了Al掺杂Ⅷ型Sn基单晶笼合物Ba8Ga10Al6Snx(x=40,50,60;Sn40,Sn50,Sn60),并研究Ba8Ga10Al6Snx单晶笼合物的结构和电传输特性对自熔剂Sn初始含量的依赖性.结果表明,Al的实际含量随Sn自熔剂含量的增加而基本保持不变,说明Sn的起始含量对Al在该笼合物中固溶度的影响较小;室温下Sn60样品的载流子浓度较高,这可能是因Al在笼合物Ga8 Ga16Sn30中的占位不同而导致费米能级附近能带色散关系发生变化所引起;另一方面,在300～600 K的温度范围内,获得较高功率因子的是Sn初始含量为50的样品,在488 K处获得最大值1.82×10-3 W·m-1·K-2;获得较低功率因子的是Sn初始含量为40的样品,而功率因子较低主要是由于该样品电导率较低.     

Sn1-xNxO2材料光电性质的研究

基于密度泛函理论的第一性原理,采用广义梯度近似(GGA)下的线性缀加平 面波(FP-LAPW)方法,应用WIEN2K软件计算了超晶胞结构Sn1-xNxO2材料的总态密度、能带结构和光学折射率及介电函数虚部.计算结果表明掺杂后费米能级向低能方向移动,随着掺杂量的增加,Sn1-xNxO2材料的价带和导带的分裂程度增强,禁带宽度逐渐减小,并且在1.35～ 2.50 eV的能量范围上形成了杂质带,其主要来源是N的2p态上的电子.分析Sn1-xNxO2材料的能带结构可知掺杂前后均是直接跃迁半导体,掺杂后其介电函数谱和折射率也与带隙相对应地发生红移,介电谱的跃迁峰与电子从价带到导带的跃迁有关,从理论上指出光学性质与电子结构之间的内在关系.     

Thermodynamic prediction of bulk metallic glass forming alloys in ternary Zr–Cu–X (X = Ag,Al, Ti,Ga) systems

Prediction of bulk metallic glass (BMG) forming compositions has always been a challenge due to thermodynamic and kinetic constraints. In the present investigation, a parameter based on the enthalpy of chemical mixing (?H_chem) and the mismatch entropy (?S_σ/k_B) has been used to correlate with glass forming ability in some Zr based BMGs. The new thermodynamic parameter, P_HS = ?H_chem × ?S_σ/k_B, is found to have strong correlation with glass forming ability in the configurational entropy (?S_config/R) range of 0.9–1.0. P_HS has been calculated for compositions in Zr–Cu–Ag, Zr–Cu–Al, Zr–Cu–Ti and Zr–Cu–Ga ternary systems. It is observed that in all the systems studied, the best BMG composition (highest critical diameter (Z_c) of glass formation) is the one that corresponds to the highest negative P_HS value. Present approach using P_HS could be road map to design new BMG forming compositions.     

Microstructure and properties of spark plasma sintered Fe–Cr–Mo–Y–B–C bulk metallic glass

Fabrication of Fe-based amorphous alloy using spark plasma sintering (SPS) process has been reported. Fully amorphous compacts with ～95% relative density were successfully sintered at temperature about 100 °C lower than glass transition temperature (T_g: 575 °C). Formation of crystalline Fe₂₃(C, B)₆ phases within near-fully dense (～99%) amorphous matrix is observed at sintering temperatures (>550 °C) close to glass transition temperature. Microstructure evolution in sintered compacts indicated that density, degree of crystallinity, and mechanical properties can be effectively controlled by optimizing SPS parameters.     

Sn掺杂对ZnO晶体形貌和磁性的影响

采用水热法,在ZnO中添加SnCl2.2H2O作前驱物,3M KOH作矿化剂,温度430℃,填充度35%,反应24h,合成了掺杂Sn的ZnO晶体。当前驱物中添加SnCl2.2H2O可以明显影响部分晶体形态,使正极面c轴方向的生长速度受到抑制,较大面积显露正极面c{0001},同时也显露负极面-c{000 1}、正锥面p{10 10}、负锥面-p{101 1}和柱面m{10 10}。磁性测量结果显示Sn可微量掺入ZnO晶格中,且呈现顺磁性特征。X射线衍射和X光荧光能谱分析表明,SnCl2.2H2O的添加量较大时,还伴随生成金红相SnO2棒状晶体。     

Ti5 Al2 C3与Ti2 AlC、Ti3 AlC2结构、弹性和电子性质的第一性原理对比研究

采用第一性原理的方法,研究了Ti5 Al2 C3在高压下的结构、弹性和电子性质,并与Ti2 AlC和Ti3 AlC2进行了对比分析.结果表明:三者的晶格常数a值近似相等,而Ti5 Al2 C3的c值和V值则分别近似等于Ti2 AlC和Ti3 AlC2的对应值之和;通过晶体结构分析认为Ti2 AlC和Ti3 AlC2分别由Al-Ti-C-Ti链和Al-Ti-C-Ti-C-Ti链交替排列构成,而Ti5 Al2 C3的结构主链Al-Ti-C-Ti-Al-Ti-C-Ti-C-Ti刚好是Ti2 AlC和Ti3 AlC2的主链之和;综合分析认为Ti5 Al2 C3可由Ti2 AlC与Ti3 AlC2组合而成.三者的结构参数都随着压力增大而减小,Ti5 Al2 C3的弹性性质介于Ti2 AlC和Ti3 AlC2之间,满足Ti3 AlC2>Ti5 Al2 C3>Ti2 AlC;通过电子态密度分析认为三者均具有共价键和金属键的共同性质.     

Ti掺杂6H-SiC电学性质研究

使用物理气相传输方法(PVT)制备了2英寸Ti掺杂与非故意掺杂6H-SiC衬底,并对衬底进行热处理.使用拉曼光谱仪、低温光致发光谱(LTPL)和非接触电阻率测试对衬底晶型、掺杂元素和电阻率进行了表征.结果表明,Ti元素有效掺入6H-SiC中,Ti掺杂对PVT方法生长的6H-SiC衬底晶型稳定性无影响,Ti掺杂衬底与非故意掺杂衬底均为6H-SiC,热处理后Ti掺杂衬底电阻率达到1010～1011Ω·cm.初步认为Ti掺杂衬底热处理过程中产生的大浓度碳空位Vc是引起Ti掺杂样品电阻率上升的主要原因.     

Free volume and viscosity of Fe–Co–Cr–Mo–C–B–Y bulk metallic glasses and their correlation with glass-forming ability

In this work, the variation of the glass-forming ability (GFA) in Fe–Co–Cr–Mo–C–B–Y alloy system with Co addition has been investigated from the viewpoints of non-isothermal viscosity and free volume. The best GFA for the alloy containing 7% Co is found to closely relate with its highest viscosity, and the minimum sizes and lowest concentrations of the intermediate and largest open volumes. The results may provide more insights into the formation mechanism of metallic glasses.     

Hydrogen in Cu?Ti and Ni?Ti Metallic Glasses

The hydrogen solubility and its effect on the crystallization of Cu Ti and Ni Ti glasses were studied by differential scanning calorimetry, thermogravimetry, and X-ray diffraction. Dependence of the crystallization products of the hydrogenated Ti-based alloys on the hydrogen content was found. Whereas in Cu-Ti alloys hydrogenation leads to drastic decreasing in the thermal stability due to phase separation in the amorphous state and to formation of microcrystalline structure during crystallization, in Ni Ti system hydrogen produces hydrides with Ni as well with Ti, which after heat treatment decompose, and finally the same crystalline phases as in unhydrogenated alloy are formed. The isothermal crystallization kinetics of the maximum hydrogenated Cu₅₀Ti₅₀ amorphous alloy was also investigated to obtain additional information about this transformation leading to nano-crystalline material.     

Electronic conduction in (Bi,Pb)–Sr–Ca–Cu–O granular superconductors

Granular superconductors are very interesting materials owing to their untypical electrical properties caused by the presence of Coulomb effects, electron tunnelling, Josephson coupling between granules and various aspects of disorder. The electrical properties of (Bi,Pb)–Sr–Ca–Cu–O granular superconductors obtained by the solid state crystallization method have been studied in this paper. The materials may be considered as a system of granules of a high-temperature suprconductor embedded in the insulating matrix. The main parameter which determines the properties of granular materials is the tunnelling conductivity between the neighboring grains. Composed of small weakly coupled granules they are characterized by hopping conductivity exponential dependence. Samples containing relatively large granules of the 2212 phase (above 30 nm), with larger inter-grain conductivity, above the transition temperature, exhibit a logarithmic temperature dependence of resistivity.