A density-driven phase transition between semiconducting and metallic polyamorphs of silicon

Amorphous and crystalline forms of silicon are well-known, tetrahedrally coordinated semiconductors. High-pressure studies have revealed extensive polymorphism among various metallic crystal structures containing atoms in six-, eight- and 12-fold coordination. Melting silicon at ambient or high pressure results in a conducting liquid, in which the average coordination is greater than four (ref. 3). This liquid cannot normally be quenched to a glass, because of rapid crystallization to the diamond-structured semiconductor. Solid amorphous silicon is obtained by synthesis routes such as chemical or physical vapour deposition that result in a tetrahedrally bonded semiconducting state. It has long been speculated that the amorphous solid and the liquid could represent two polymorphic forms of the amorphous state that are linked by density- or entropy-driven transformations. Such polyamorphic transitions are recognized to occur among several different types of liquid and glassy systems. Here we present experimental evidence for the occurrence of a density-driven polyamorphic transition between semiconducting and metallic forms of solid amorphous silicon. The experiments are combined with molecular dynamics simulations that map the behaviour of the amorphous solid on to that of the liquid state.     

Liquid-liquid phase transition in supercooled silicon

Silicon in its liquid and amorphous forms occupies a unique position among amorphous materials. Obviously important in its own right, the amorphous form is structurally close to the group of 4-4, 3-5 and 2-6 amorphous semiconductors that have been found to have interesting pressure-induced semiconductor-to-metal phase transitions. On the other hand, its liquid form has much in common, thermodynamically, with water and other 'tetrahedral network' liquids that show density maxima. Proper study of the 'liquid-amorphous transition', documented for non-crystalline silicon by both experimental and computer simulation studies, may therefore also shed light on phase behaviour in these related materials. Here, we provide detailed and unambiguous simulation evidence that the transition in supercooled liquid silicon, in the Stillinger-Weber potential, is thermodynamically of first order and indeed occurs between two liquid states, as originally predicted by Aptekar. In addition we present evidence to support the relevance of spinodal divergences near such a transition, and the prediction that the transition marks a change in the liquid dynamic character from that of a fragile liquid to that of a strong liquid.     

Quantitative determination of the volume fraction of intergranular amorphous phase in sintered silicon nitride

In this paper, a commercial liquid phase sintered silicon nitride with Y₂O₃ and Al₂O₃ additives is investigated. The material is characterised by a large, and stable, internal friction peak near 1150 °C. The peak is linked with the glass transition in intergranular amorphous volumes, whose presence is confirmed with transmission electron microscopy. An estimate of the volume fraction of the amorphous phase is calculated from the difference in stiffness below and above the glass transition temperature. The procedure relies on accurate Young’s modulus data, which were obtained with the impulse excitation technique (IET). The amount of amorphous pocket phase was estimated at 12.4 vol.%. For the first time, microstructural evidence supporting this estimate is obtained, using analysis of scanning electron microscopy (SEM) images. The rather large amount of amorphous matter explains the limited high temperature potential of the material, which was primarily and successfully developed for wear applications.     

Effect of implanted phosphorus ions on the crystallization of amorphous silicon films under the action of pulsed excimer laser radiation

We have studied the effect of implanted phosphorus ions on the crystallization of thin amorphous silicon films under the action of nanosecond radiation pulses of a XeCl excimer laser. The amorphous silicon films with a thickness of 90 nm, obtained by plasmachemical deposition on glass substrates, were implanted with phosphorus ions at a dose of 3 × 10¹⁴ and 3 × 10¹⁵ cm⁻². The subsequent laser treatments were performed using energies both above and below a threshold corresponding to the fusion of amorphous silicon. The structure of the silicon films was studied using Raman scattering spectroscopy. An analysis of the experimental data shows that implanted phosphorus stimulates nucleation, especially in the case of liquid phase crystallization. The results are of interest for the development of the technology of thin-film transistors on nonrefractory substrates.     

Thermodynamic properties of amorphous silicon via tight binding simulations

An atomic-scale structure of amorphous silicon, generated by reverse Monte Carlo, has been used as a starting configuration for finite temperature molecular dynamics simulations performed by an orthogonal tight binding Hamiltonian. Structural, dynamic, elastic and electronic structure properties have been investigated in the range of temperatures up to and above the melting transition. The amorphous silicon structure undergoes a melting transition at a temperature sensibly smaller than that of the crystalline structure. Above this temperature, the structure has the same properties of an under-cooled liquid and it has a metallic behavior.     

Reentrant glass transition leading to ultrastable metallic glass

Polyamorphs are often observed in amorphous matters, and a representative example is the reentrant glass transition in colloid systems. For metallic amorphous alloys, however, the cases reported so far are limited to metallic glasses (MGs) that undergo electronic transitions under gigapascal applied pressure, or the presence of two liquids at the same composition. Here we report the first observation of a reentrant glass transition in MGs. This unusual reentrant glass transition transforms an MG from its as-quenched state (Glass I) to an ultrastable state (Glass II), mediated by the supercooled liquid of Glass I. Specifically, upon heating to above its glass transition temperature under ambient pressure, Glass I first transitions into its supercooled liquid, which then transforms into a new Glass II, accompanied by an exothermic peak in calorimetric scan, together with a precipitous drop in volume, electrical resistance and specific heat, as well as clear evidence of local structural ordering on the short-to-medium-range scale revealed via in-situ synchrotron X-ray scattering. Atomistic simulations indicate enhanced ordering of locally favored motifs to establish correlations in the medium range that resemble those in equilibrium crystalline compounds. The resulting lower-energy Glass II has its own glass transition temperature higher than that of Glass I by as much as 50 degrees. This route thus delivers a thermodynamically and kinetically ultrastable MG that can be easily retained to ambient conditions.     

添加AlN对机械合金化Zr65Al7.5CU17.5Ni10非晶态合金热稳定性的影响

以成分为Zr65Al7.5Cu17.5Ni10的元素的粉末混合物及AIN颗粒为起始材料,经机械合金化形成非晶态合金为基体的复合材料,AIN添加量为5％－30％（体积分数,下同）,利用X射线衍射（XRD）,透射电子 显微镜（TEM）和差示扫描量热计（DSC）分析了含AIN复合材料的结构特性,玻璃转变与晶化行为,TEM观察表明,AIN第二相粒子弥散分布在晶Zr基合金基体上,粒子尺寸为20－200nm,仍为初始的晶体结构,与未添加AIN的Zr基非晶态合金相比,含5％－10％AIN的复合材料仍表现出较宽的过冷液态温度区域,玻璃转变温度（Tg)和晶化激活能（Ex)没有显著变化,但晶化起始温度（Tx)向高温移动大约10K,导致过冷液态温度区域的扩宽,AIN含量增至30％,明显的玻璃转变消失,Tx升高的20K。     

铝诱导晶化法低温制备多晶硅薄膜

为了满足在普通玻璃衬底上制备多晶硅薄膜晶体管有源矩阵液晶显示器,低温（＜６００℃）制备高质量多晶硅薄膜已成为研究热点．本文研究了一种低温制备多晶硅薄膜的新工艺：金属诱导非晶硅薄膜低温晶化法．在非晶硅薄膜上蒸镀金属铝薄膜,并光刻形成铝膜图形,而后于氮气保护中退火．利用光学显微镜和拉曼光谱等测试方法,研究了Ａｌ诱导下非晶硅薄膜的晶化过程,结果表明;在５６０℃退火６ｈ后;铝膜下的非晶硅已完全晶化,确定了所制备的是多晶硅薄膜．初步探讨了非晶硅薄膜金属诱导横向晶化机理．     

N-type crystalline silicon films free of amorphous silicon deposited on glass by HCl addition using hot wire chemical vapour deposition

Since n-type crystalline silicon films have the electric property much better than those of hydrogenated amorphous and microcrystalline silicon films, they can enhance the performance of advanced electronic devices such as solar cells and thin film transistors (TFTs). Since the formation of amorphous silicon is unavoidable in the low temperature deposition of microcrystalline silicon on a glass substrate at temperatures less than 550 degrees C in the plasma-enhanced chemical vapour deposition and hot wire chemical vapour deposition (HWCVD), crystalline silicon films have not been deposited directly on a glass substrate but fabricated by the post treatment of amorphous silicon films. In this work, by adding the HCl gas, amorphous silicon-free n-type crystalline silicon films could be deposited directly on a glass substrate by HWCVD. The resistivity of the n-type crystalline silicon film for the flow rate ratio of [HCl]/[SiH4] = 7.5 and [PH3]/[SiH4] = 0.042 was 5.31 x 10(-4) ohms cm, which is comparable to the resistivity 1.23 x 10(-3) ohms cm of films prepared by thermal annealing of amorphous silicon films. The absence of amorphous silicon in the film could be confirmed by high resolution transmission electron microscopy.     

低温保护剂溶液固化性质的DSC研究

利用差示扫描量热仪,研究了乙二醇、丙三醇、1,3丙二醇、1,3丁二醇和2,3丁二醇水溶液的水合性质、玻璃化转变温度和反玻璃化转变温度,得出了这些性质与溶液浓度的关系.研究发现未冻水含量在低浓度时没有明显的规律性,而在中高浓度则随着溶液浓度的增大而增大.对于玻璃化转变,中低浓度区溶液形成的是部分晶体的玻璃态,紧接着在高浓度区溶液形成完全的玻璃态.部分玻璃化的温度不随浓度变化,而玻璃化的温度随浓度增大而升高.形成反玻璃化的浓度范围比较小,反玻璃化的温度随浓度增大而升高.保护剂在玻璃化转变温度和反玻璃化转变温度的明显差异,则反映它们在玻璃化能力方面的强与弱.     

Homogeneous Plastic Flow of Fully Amorphous and Partially Crystallized Zr41.2Ti13.8Cu12.5Nil0Be22.5 Bulk Metallic Glass

The homogeneous plastic flow of fully amorphous and partially crystallized Zr41.2Ti13.8Cu12.5Ni10Be22.5 bulk metallic glass (Vitl) has been investigated by compression tests at high temperatures in supercooled liquid region. Experimental results show that at sufficiently low strain rates, the supercooled liquid of the fully amorphous alloy reveals Newtonian flow with a linear relationship between the flow stress and strain rate. As the strain rate is increased, a transition from linear Newtonian to nonlinear flow is detected, which can be explained by the transition state theory.Over the entire strain rate interval investigated, however, only nonlinear flow is present in the partially crystallized alloy, and the flow stress for each strain rate is much higher. It is found that the strain rate-stress relationship for the partially crystallized alloy at the given temperature of 646 K also obeys the sinh law derived from the transition state theory, similar to that of the initial homogeneous amorphous alloy. Thus, it is proposed that the flow behavior of the nanocrystalline/amorphous composite at 646 K is mainly controlled by the viscous flow of the remaining supercooled liquid.     

Influence of microstructure and hydrogen concentration on amorphous silicon crystallization

Hydrogenated amorphous silicon samples were deposited on glass substrates at different temperatures by high frequency plasma-enhanced chemical vapor deposition. In this way, samples with different hydrogen concentrations and structures were obtained. The transition from an amorphous to a crystalline material, induced by a four-step thermal annealing sequence, has been followed. Effusion of hydrogen from the films plays an important role in the nucleation and growth mechanisms of crystalline silicon grains. Measurements of hydrogen concentrations, Raman scattering, X-ray diffraction and UV reflectance showed that an enhanced crystallization was obtained on samples deposited at lower substrate temperatures. A correlation between these measurements allows to analyze the evolution of structural properties of the samples. The presence of voids in the material, related to disorder in the amorphous matrix, results in a better quality of the resulting nanocrystalline silicon thin films.     

Fast Melting of Amorphous Silicon Carbide Induced by Nanosecond Laser Pulse

We report the first experimental detailed study of laser induced surface melting on the nanoscale time scale of amorphous silicon carbide layers produced by ion implantation. Time-resolved reflectivity has been used to observe the fast liquid–solid–liquid transition features, and transmission electron microscopy (TEM) was used in order to study the structure resulting after the fast solidification following the laser induced melting. By means of the evaluation of the laser fluences required to induce melting of amorphous layers of different thickness on top of a crystalline substrate, we evaluated the thermal diffusion coefficient and the melting point of the amorphous material which occurred much lower than for crystalline material. Moreover, we give evidence of amorphous-to-crystal transitions occurring in the solid phase on the nanosecond time scale, for laser irradiation at fluences below the melting threshold. A quite different crystalline structure is observed for crystallization from the liquid phase than from the solid phase.     

Polycrystalline silicon films on glass grown by amorphous-liquid-crystalline transition at temperatures below 330 °C

An approach to deposit polycrystalline silicon layers on amorphous substrates is presented. It is shown that metastable amorphous silicon can be transformed into its more stable microcrystalline structure at a temperature below 330 °C via an intermediate liquid solution stage. In particular, the interaction of liquid indium nanodroplets in contact with amorphous silicon is shown to lead to the formation of circular polycrystalline domains. Crystallinity of these domains is analyzed by micro-Raman spectroscopy. The droplet size necessary for the onset of crystallization is related to the temperature of the film. Full coverage of the substrate with microcrystalline silicon has been obtained at 320 °C within less than one hour. These films might act as seeding layers for further enlargement by steady-state liquid phase epitaxy.     

Homogeneous Plastic Flow of Fully Amorphous and Partially Crystallized Zr_(41.2)Ti_(13.8)Cu_(12.5)Ni_(10)Be_(22.5) Bulk Metallic Glass

The homogeneous plastic flow of fully amorphous and partially crystallized Zr(41.2)Ti(13.8)Cu(12.5)Ni(10)Be(22.5) bulk metallic glass (Vitl) has been investigated by compression tests at high temperatures in supercooled liquid region. Experimental results show that at sufficiently low strain rates, the supercooled liquid of the fully amorphous alloy reveals Newtonian flow with a linear relationship between the flow stress and strain rate. As the strain rate is increased, a transition from linear Newtonian to nonlinear flow is detected, which can be explained by the transition state theory. Over the entire strain rate interval investigated, however, only nonlinear flow is present in the partially crystallized alloy, and the flow stress for each strain rate is much higher. It is found that the strain rate-stress relationship for the partially crystaltized alloy at the given temperature of 646 K also obeys the sinh law derived from the transition state theory, similar to that of the initial homogeneous amo     

Nanocrystalline silicon thin films on PEN substrates

We study the structural and electrical properties of intrinsic layer growth close to the transition between amorphous silicon (a-Si:H) and nanocrystalline silicon (nc-Si:H), deposited on glass and PEN without intentional heating. These samples showed different behaviour in Raman shift and XRD spectra when compared with that of samples deposited at 200 °C. Electrical properties of these films also reflect the transition between a-Si:H and nc-Si:H, and put in evidence some differences between the microstructure of the films grown on PEN and on glass.P- and n-doped layers were deposited onto glass substrate without intentional heating and at 100 °C with thicknesses ranging from 1000 nm to 35 nm. Conductivity measurements indicate the capability of doping this material, but, for very thin layers, substrate heating was found to be essential.     

Structural evolution during mechanical milling and subsequent annealing of Cu–Ni–Al–Co–Cr–Fe–Ti alloys

This study reports the structural evolution of high-entropy alloys from elemental materials to amorphous phases during mechanical alloying, and further, to equilibrium phases during subsequent thermal annealing. Four alloys from quaternary Cu_0.5NiAlCo to septenary Cu_0.5NiAlCoCrFeTi were analyzed. Microstructure examinations reveal that during mechanical alloying, Cu and Ni first formed a solid solution, and then other elements gradually dissolved into the solid solution which was finally transformed into amorphous structures after prolonged milling. During thermal annealing, recovery of the amorphous powders begins at 100 °C, crystallization occurs at 250–280 °C, and precipitation and grain growth of equilibrium phases occur at higher temperatures. The glass transition temperature usually observed in bulk amorphous alloys was not observed in the present amorphous phases. These structural evolution reveal three physical significances for high-entropy alloys: (1) the annealed state of amorphous powders produces simple equilibrium solid solution phases instead of complex phases, confirming the high-entropy effect; (2) amorphization caused by mechanical milling still meets the minimum criterion for amorphization based on topological instability proposed by Egami; and (3) the nonexistence of a glass transition temperature suggests that Inoue's rules for bulk amorphous alloys are still crucial for the existence of glass transition for a high-entropy amorphous alloy.     

Ti-based amorphous alloys with a wide supercooled liquid region

Ti-Cu-Ni-Co quaternary amorphous alloys produce by melt spinning were found to have a wide supercooled liquid region before crystallization, though no glass transition was observed in Ti-Cu binary amorphous alloys. The largest temperature interval of the supercooled liquid region (ΔT_x) is as large as 90 K for Ti₅₀Cu₂₅Ni₂₀Co₅.There is a tendency for ΔT_x to increase with an increase in storage modulus and with a decrease in loss modulus. It is therefore presumed that the increase in ΔT_x for the multicomponent amorphous alloy is due to the suppression of crystallization for the supercooled liquid resulting from the increase in viscosity.     

Preparation of gas hydrates by nonequilibrium condensation of molecular beams

Layers of amorphous ice saturated with carbon dioxide were prepared by the deposition of molecular beams of water and gas onto a substrate cooled with liquid nitrogen. Their heating is accompanied by glass transition (softening) and subsequent spontaneous crystallization. The glass transition and crystallization temperatures were determined from the change in dielectric properties during heating. The heat effects of the transformations were detected using differential thermal analysis. The crystallization of amorphous layers under conditions of deep metastability leads to the formation of crystalline hydrates. The avalanche nucleation of crystallization sites captures the gas molecules; therefore, they are not displaced by the movement of the crystallization front.     

Liquid-liquid transition without macroscopic phase separation in a water-glycerol mixture

The existence of more than two liquid states in a single-component substance and the ensuing liquid-liquid transitions (LLTs) has attracted considerable attention because of its counterintuitive nature and its importance in the fundamental understanding of the liquid state. Here we report direct experimental evidence for a genuine (isocompositional) LLT without macroscopic phase separation in an aqueous solution of glycerol. We show that liquid I transforms into liquid II by way of two types of kinetics: nucleation and growth, and spinodal decomposition. Although liquid II is metastable against crystallization, we could access both its static and dynamical properties experimentally. We find that liquids I and II differ in density, refractive index, structure, hydrogen bonding state, glass transition temperature and fragility, and that the transition between the two liquids is mainly driven by the local structuring of water rather than of glycerol, suggesting a link to a plausible LLT in pure water.