Undercooled Silicon Research Articles

Fast Reversible Phase Change Silicon for Visible Active Photonics

AbstractBoth amorphous and crystalline silicon are ubiquitous materials for electronics, photonics, and microelectromechanical systems. On‐demand control of Si crystallinity is crucial for device manufacturing and to overcome the limitations of current phase‐change materials (PCM) in active photonics. Fast reversible phase transformation in silicon, however, has never been accomplished due to the notorious challenge of amorphization. It is demonstrated that nanostructured Si can function as a PCM, since it can be reversibly crystallized and amorphized under nanosecond laser irradiation with different pulse energies. Reflection probing on a single nanodisk's phase transformations confirms the distinct mechanisms for crystallization and amorphization. The experimental results show that the relaxation time of undercooled silicon at 950 K is 10 ns. The phase change provides a 20% nonvolatile reflectivity modulation within 100 ns and can be repeated over 400 times. It is shown that such transformations are free of deformation upon solidification. Based on the switchable photonic properties in the visible spectrum, proof‐of‐concept experiments of dielectric color displays and dynamic wavefront control are shown. Therefore, nanostructured silicon is proposed as a chemically stable, deformation free, and complementary metal–oxide‐semiconductor compatible (CMOS) PCM for active photonics at visible wavelengths.

Nucleation properties of undercooled silicon at various substrates

When solidifying molten silicon in contact with a foreign substrate, the interfacial interaction between silicon and the substrate material can affect the nucleation behavior and therefore the final grain structure. This work illuminates the nucleation properties of different materials using differential scanning calorimetry to measure the undercooling below the melting temperature of molten silicon in contact with silicon oxides and nitrides. Both dry and wet thermal oxides show greater than 120 °C of undercooling before nucleation occurs, while silicon nitride consistently shows less than 17 °C. Variability in measured undercooling was minimized by improving coating uniformity but is not necessarily affected by overall sample deformation. A double-layer coating of silicon nitride over dry oxide reduced the dry oxide undercooling to <56 °C due to inter-layer thermal stresses, which can be minimized by reducing the outer layer of silicon nitride to <40 nm. The double-layer SiNx-SiO2 coating produced larger grains than the SiNx coating alone, when applied to a wafer recrystallization process.

An Investigation on the Local Structure of Silicon: Liquid to Undercooled Regime

We present the modelled local structure for undercooled silicon beginning from its liquid state, ~1730K to ~1550K. The modelling procedure was achieved by using reverse Monte Carlo (RMC) modelling technique fitting to x-ray static structure factors. The calculated radial distribution functions satisfied experimental observes either liquid or undercooled region. To make a detailed analysis on the modelled local environment we have focused on the distributions both average numbers of atoms within first coordination shell and bond angles whereas the uniqueness of model is discussible. In order to construct model that is more close to nature, the minimum and maximum bond lengths and the average coordination number constraints could have been used. The predicted results using RMC technique show that there is a possible structural transition and it slightly transforms into covalent-like bounded open network structure from its metallic structure, while decreasing temperature.

Dynamics of liquid and undercooled silicon: Anab initiomolecular dynamics study

First-principles molecular dynamics simulations of liquid and undercooled silicon have been performed in order to study the evolution of dynamic properties across the melting point. The calculated dynamic structure factors show collective excitations at small wave vectors above the melting point, in good agreement with recent experimental data, while the transverse current correlation functions do not display propagating shear modes. Upon undercooling, our results evidence the occurrence of well-defined shear modes that result from an enhancement of local tetrahedral arrangement associated to strong covalent bonding suggesting that propagating shear waves might be sustained by a concerted motion of tetrahedra.

Liquid-Liquid Phase Transition in Undercooled Silicon: Structural, Electronic and Dynamical Aspects

ABSTRACTWe have proposed a new mixed approach by combining efficiently classical and first-principles molecular dynamics to study undercooling of liquid silicon, regardless of a specific empirical interaction model. Our results show an enhancement of the local tetrahedral ordering in the deep undercooled region associated to the appearance of propagating shear waves and give a strong support to the existence of a transition between a high density liquid to a low density liquid near 1050 K. An analysis of the structural, dynamics and electronic properties is proposed to elucidate these features.

Liquid-Liquid Phase Transformation in Silicon: Evidence from First-Principles Molecular Dynamics Simulations

We report results of first principles molecular dynamics simulations that confirm early speculations on the presence of liquid-liquid phase transition in undercooled silicon. However, we find that structural and electronic properties of both low-density liquid (LDL) and high-density liquid (HDL) phases are quite different from those obtained by empirical calculations, the difference being more pronounced for the HDL phase. The discrepancy between quantum and classical simulations is attributed to the inability of empirical potentials to describe changes in chemical bonds induced by density and temperature variations.

Critical undercoolings for the transition from the lateral to continuous growth in undercooled silicon and germanium

The effects of crystal–liquid interface energy and undercooling on a crystal–liquid interface structure and the growth mode of undercooled semiconductors have been investigated. A factor to predict the structure of the solid–liquid interface has been identified. Criterions expressed as the critical nucleation undercoolings (Δ T * 1 and Δ T * 2) and kinetic undercoolings (Δ T K1 * and Δ T K2 *) to judge the transition of growth mode in undercooled semiconductor have been developed. The growth mode of a semiconductor is a lateral one when nucleation undercooling is lower than Δ T * 1 or kinetic undercooling is lower than Δ T K1 *, a continuous one when nucleation undercooling is higher than Δ T * 2 or kinetic undercooling is higher than Δ T K2 *, and an intermediary one in the nucleation undercooling region from Δ T * 1 to Δ T * 2 or kinetic undercooling region from Δ T K1 * to Δ T K2 *. The critical undercoolings for the growth transition predicted from the present criterions for silicon and germanium are in very good agreement with the experimentally measured results.

Novel criterion for splitting of plate-like crystal growing in undercooled silicon melts

An undercooled drop of silicon was grown to crystals in containerless states with electromagnetic and electrostatic levitators. A high-speed video camera was used to monitor the growth rate and observe the crystal–melt interface as a function of undercooling. The morphology of the growing crystal changed from a mono-plate crystal to a multi-plate crystal, and then to faceted dendrite with increasing undercooling. The mono-plate and multi-plate crystals observed at undercooling of less than 100 K were shaped by a faceted planer interface and wavy-edge plane. The critical undercooling for the transition from mono-plate to multi-plate depends on the sample size; it was 50 and 80 K for samples of 5 and 1.7 mm in diameter, respectively. A novel criterion for the transition from mono-plate to multi-plate based on the instability of the wavy-edge plane is proposed.

Direct observation of the crystal-growth transition in undercooled silicon

Using an electromagnetic levitation facility with a laser heating unit, silicon droplets were highly undercooled in the containerless state. The crystal morphologies on the surface of the undercooled droplets during the solidification process and after solidification were recorded live by using a high-speed camera and were observed by scanning electron microscopy. The growth behavior of silicon was found to vary not only with the nucleation undercooling, but also with the time after nucleation. In the earlier stage of solidification, the silicon grew in lateral, intermediary, and continuous modes at low, medium, and high undercoolings, respectively. In the later stage of solidification, the growth of highly undercooled silicon can transform to the lateral mode from the nonlateral one. The transition time of the sample with 320 K of undercooling was about 535 ms after recalescence, which was much later than the time where recalescence was completed.

Estimation of maximum growth rates of undercooled silicon melt on (111) and (100) surfaces

Monte Carlo simulation data are used to study the behaviour of the melt growth from undercooled melt at different temperatures for both Si(111) and Si(100) surface. The dependence of the melt growth rate on the crystal orientation is demonstrated. In both cases, the authors find that the growth rate increases with the degree of undercooling until a diffusivity limited maximum is reached. The estimated maximum growth rates are 6.5 m s-1 and 14.5 m s-1 for stable single crystal at about 200 and 110 K of undercooling for Si(111) and Si(100) surfaces respectively. The results are in line with the experimental results measured by Cullis et al. (1984).

Solidification of Highly Undercooled Silicon: Bulk Nucleation and Amorphous Phase Formation

ABSTRACTRecent experimental and theoretical work suggests that bulk nucleation occurs in highly undercooled liquid silicon formed by the pulsed laser melting of amorphous layers. In this paper we discuss the dynamics of solidification of undercooled liquid silicon in terms of results from melting model calculations and from the theory of homogeneous bulk nucleation. The relationship between the occurrence of bulk nucleation and the ability to induce a liquid-to-amorphous phase transition by large undercoolings of the liquid is considered. It is concluded that the observed liquid-toamorphous transition in silicon cannot be explained by an equilibrium thermodynamic approach.