The Ga−Si (Gallium-Silicon) system

Template-assisted growth of Ga-based nanoparticle clusters on Si: effect of post-annealing process on the Ga ion beam exposed 2D arrays fabricated by focused ion beam nanolithography

We demonstrate template-assisted growth of gallium-based nanoparticle clusters on silicon substrate using a focused ion beam (FIB) nanolithography technique. The nanolithography counterpart of the technique steers a focussed 30 kV accelerated gallium ion beam on the surface of Si to create template patterns of two-dimensional dot arrays. Growth of the nanoparticles is governed by two vital steps namely implantation of gallium into the substrate via gallium beam exposure and formation of the stable nanoparticles on the surface of the substrate by subsequent annealing at elevated temperature in ammonia atmosphere. The growth primarily depends on the dose of implanted gallium which is in the order of 107atoms per spot and it is also critically influenced by the temperature and duration of the post-annealing treatment. By controlling the growth parameters, it is possible to obtain one particle per spot and particle densities as high as 109particles per square centimetre could be achieved in this case. The demonstrated growth process, utilizing the advantages of FIB nanolithography, is categorized under the guided organization approach as it combines both the classical top-down and bottom-up approaches. Patterned growth of the particles could be utilized as templates or nucleation sites for the growth of an organized array of nanostructures or quantum dot structures.

Low-temperature electronic transport of manganese silicide shell-protected single crystal nanowires for nanoelectronics applications

Recently, core-shell nanowires have been proposed as potential electrical connectors for nanoelectronics components. A promising candidate is Mn₅Si₃ nanowires encapsulated in an oxide shell, due to their low reactivity and large flexibility. In this work, we investigate the use of the one-step metallic flux nanonucleation method to easily grow manganese silicide single crystal oxide-protected nanowires by performing their structural and electrical characterization. We find that the fabrication method yields a room-temperature hexagonal crystalline structure with the c-axis along the nanowire. Moreover, the obtained nanowires are metallic at low temperature and low sensitive to a strong external magnetic field. Finally, we observe an unknown electron scattering mechanism for small diameters. In conclusion, the one-step metallic flux nanonucleation method yields intermetallic nanowires suitable for both integration in flexible nanoelectronics as well as low-dimensionality transport experiments.

Low growth rate synthesis of GaAs nanowires with uniform size

The growth of nanowires (NWs) with uniform sizes is crucial for future NW-based electronics. In this work, an efficient one-step process is introduced for the growth of uniform gallium arsenide NWs on the native oxide surface of Si, which could be even considered as an alternative for expensive and sophisticated patterning approaches. The proposed strategy considers a Ga pre-deposition step leading to the formation of droplets with homogeneous sizes. That is followed by controlled nucleation of gallium arsenide from those droplets only. Our key to controlling the nucleation of gallium arsenide is to perform the NW growth at temperatures above 580 ± 10 °C and low Ga fluxes. By this method, the statistical distribution of the length and diameter of the vertically grown NWs decreased to about 3%–6% of their averaged values. Moreover, 100% epitaxial growth was realized. Besides, the growth of undesired parasitic islands is addressed and accordingly suppressed. Our study focuses on NW low growth rates, which is so far not investigated in the literature and, could be of great interest e.g. for in situ growth studies.

Synergetic effect in rolling GaIn alloy droplets enables ultralow temperature growth of silicon nanowires at 70 °C on plastics

Ultralow temperature growth of silicon nanowires (SiNWs) directly upon cheap plastics is highly desirable for building high performance soft logics and sensors based on mature Si technology. In this work, a low temperature growth of SiNWs at only 70 °C has been demonstrated for the first time, upon polyethylene terephthalate plastics, by using gallium-indium (GaIn) alloy droplets that consume an amorphous Si (a-Si) layer as the precursor. The GaIn alloy droplets enable a beneficial synergetic effect that helps not only to reduce the melting temperature, but also to install a protective Gibbs adsorption layer of In atoms, which are critical to stabilize the rolling catalyst droplet, against otherwise rapid diffusion loss of Ga into the a-Si matrix. Ultra-long SiNWs can be batch-produced with a precise location and preferred elastic geometry, which paves the way for large scale integration. At <70 °C, a transition from rolling to sprawling dynamics is observed by in situ scanning electron microscopy, caused by reduced diffusion transport and rapid formation of discrete nuclei in the alloy droplet, which provides the basis for continuous growth of SiNWs. This unique capability and critical new understanding open the way for integrating high quality c-Si electronics directly over flexible, lightweight and extremely low cost plastics.

In Situ Transmission Electron Microscopy Measurements of Ge Nanowire Synthesis with Liquid Metal Nanodroplets in Water

The growth of Ge nanowires in water inside a liquid transmission electron microscope (TEM) holder has been demonstrated at room temperature. Each nanowire growth event was stimulated by the incident electron beam on otherwise unsupported liquid Ga or liquid In nanodroplets. A variety of conditions were explored, including liquid metal nanodroplet surface condition, liquid metal nanodroplet size and density, formal concentration of dissolved GeO₂, and electron beam intensity. The cumulative observations from a series of videos recorded during growth events suggested the following points. First, the conditions necessary for initiating nanowire growth at uncontacted liquid metal nanodroplets in a liquid TEM cell indicate the process was governed by solvated electrons generated from secondary electrons scattered by the liquid metal nanodroplets. The attained current densities were comparable to those achieved in conventional electrochemical liquid-liquid-solid (ec-LLS) growths outside of a TEM. Second, the surface condition of the liquid metal nanodroplets was quite influential on whether nanowire growth occurred and surface diffusion of Ge adatoms contributed to the rate of crystallization. Third, the Ge nanowire growth rates were limited by the feed rate of Ge to the crystal growth front rather than the rate of crystallization at the liquid metal/solid Ge interface. Estimates of an electrochemical current for the reduction of dissolved GeO₂ were nominally in line with currents used for Ge nanowire growth by ec-LLS outside of the TEM. Fourth, the Ge nanowire growths in the liquid TEM cell occurred far from thermodynamic equilibrium, with supersaturation values of 10⁴ prior to nucleation. These collective points provide insight on how to further control and improve Ge nanowire morphology and crystallographic quality by the ec-LLS method.

Thermodynamic criteria of the end-of-life silicon wafers refining for closing the recycling loop of photovoltaic panels

The collected end-of-life (EoL) silicon wafers from the discharged photovoltaic (PV) panels are easily contaminated by impurities such as doping elements and attached materials. In this study, the thermodynamic criteria for EoL silicon wafers refining using three most typical metallurgical refining processes: oxidation refining, evaporation refining, and solvent refining were systemically and quantitatively evaluated. A total of 42 elements (Ag, Al, Au, B, Be, Bi, C, Ca, Ce, Co, Cr, Cu, Fe, Ga, Gd, Ge, Hf, In, La, Mg, Mn, Mo, Na, Nb, Ni, Os, P, Pb, Pd, Pt, Re, Ru, Sb, Sn, Ta, Ti, U, V, W, Y, Zn, Zr) that are likely to be contained in the collected EoL silicon-based PV panels were considered. The principal findings are that the removal of aluminum, beryllium, boron, calcium, gadolinium, hafnium, uranium, yttrium, and zirconium into the slag, and removal of antimony, bismuth, carbon, lead, magnesium, phosphorus, silver, sodium, and zinc into vapor phase is possible. Further, solvent refining process using aluminum, copper, and zinc as the solvent metals, among the considered 14 potential ones, was found to be efficient for the EoL silicon wafers refining. Particularly, purification of the phosphorus doped n-type PV panels using solvent metal zinc and purification of the boron doped p-type PV panels using solvent metal aluminum are preferable. The efficiency of metallurgical processes for separating most of the impurity elements was demonstrated, and to promote the recycling efficiency, a comprehensive management and recycling system considering the metallurgical criteria of EoL silicon wafers refining is critical.

The role of Si in GaN/AlN/Si(111) plasma assisted molecular beam epitaxy: polarity and inversion

The microstructure, polarity and Si distribution in AlN/GaN layers grown by plasma assisted molecular beam epitaxy (PAMBE) on Si(111) was assessed by scanning transmission electron microscopy (STEM). Samples grown under both metal- and nitrogen-rich conditions contained defects at the AlN/Si interface which suggest formation of an Al-Si eutectic. Correlated with this, interfacial segregation of Si was found in the samples. It is proposed that Si is dissolved in a eutectic layer floating on the AlN surface under metal-rich conditions. This Si is then incorporated into the film if the growth becomes nitrogen-rich, either intentionally or due to plasma source transients. These Si-rich layers appear to induce inversion of the nitride from nitrogen- to metal-polarity, and uncontrolled variations in the Si concentration cause occasional nonuniformity in the resulting inversion.

Eutectic Formation, V/III Ratio and Controlled Polarity Inversion in Nitrides on Silicon[1]

The crystallographic polarity of AlN grown on Si(111) by plasma assisted molecular beam epitaxy is intentionally inverted from N-polar to Al-polar at a planar boundary. The position of the inversion boundary is controlled by a two-step growth process that abruptly changes from Al-rich to N-rich growth conditions. The polarity inversion is induced by the presence of Si, which is incorporated from an Al-Si eutectic layer that forms during the initial stages of AlN growth and floats on the AlN surface under Al-rich growth conditions. When the growth conditions change to N-rich the Al and Si in the eutectic react with the additional N-flux and are incorporated into the solid AlN film. Relatively low levels of Al-Si eutectic formation combined with lateral variations in the Si incorporation lead to nonuniformity in the polarity inversion and formation of surprisingly narrow, vertical inversion domains. The results suggest that intentional incorporation of uniform layers of Si may provide a method for producing polarity engineered nitride structures.

Binary Phase Diagrams and Thermodynamic Properties of Silicon and Essential Doping Elements (Al, As, B, Bi, Ga, In, N, P, Sb and Tl)

Fabrication of solar and electronic silicon wafers involves direct contact between solid, liquid and gas phases at near equilibrium conditions. Understanding of the phase diagrams and thermochemical properties of the Si-dopant binary systems is essential for providing processing conditions and for understanding the phase formation and transformation. In this work, ten Si-based binary phase diagrams, including Si with group IIIA elements (Al, B, Ga, In and Tl) and with group VA elements (As, Bi, N, P and Sb), have been reviewed. Each of these systems has been critically discussed on both aspects of phase diagram and thermodynamic properties. The available experimental data and thermodynamic parameters in the literature have been summarized and assessed thoroughly to provide consistent understanding of each system. Some systems were re-calculated to obtain a combination of the best evaluated phase diagram and a set of optimized thermodynamic parameters. As doping levels of solar and electronic silicon are of high technological importance, diffusion data has been presented to serve as a useful reference on the properties, behavior and quantities of metal impurities in silicon. This paper is meant to bridge the theoretical understanding of phase diagrams with the research and development of solar-grade silicon production, relying on the available information in the literature and our own analysis.

Microscale Fracture Behavior of Single Crystal Silicon Beams at Elevated Temperatures

The micromechanical fracture behavior of Si [100] was investigated as a function of temperature in the scanning electron microscope with a nanoindenter. A gradual increase in K_C was observed with temperature, in contrast to sharp transitions reported earlier for macro-Si. A transition in cracking mechanism via crack branching occurs at ∼300 °C accompanied by multiple load drops. This reveals that onset of small-scale plasticity plays an important role in the brittle-to-ductile transition of miniaturized Si.

Thermally stable coexistence of liquid and solid phases in gallium nanoparticles

Gallium (Ga), a group III metal, is of fundamental interest due to its polymorphism and unusual phase transition behaviours. New solid phases have been observed when Ga is confined at the nanoscale. Herein, we demonstrate the stable coexistence, from 180 K to 800 K, of the unexpected solid γ-phase core and a liquid shell in substrate-supported Ga nanoparticles. We show that the support plays a fundamental role in determining Ga nanoparticle phases, with the driving forces for the nucleation of the γ-phase being the Laplace pressure in the nanoparticles and the epitaxial relationship of this phase to the substrate. We exploit the change in the amplitude of the evolving surface plasmon resonance of Ga nanoparticle ensembles during synthesis to reveal in real time the solid core formation in the liquid Ga nanoparticle. Finally, we provide a general framework for understanding how nanoscale confinement, interfacial and surface energies, and crystalline relationships to the substrate enable and stabilize the coexistence of unexpected phases.

Self-Catalyzed Growth of Vertically Aligned InN Nanorods by Metal-Organic Vapor Phase Epitaxy

Vertically aligned hexagonal InN nanorods were grown mask-free by conventional metal-organic vapor phase epitaxy without any foreign catalyst. The In droplets on top of the nanorods indicate a self-catalytic vapor-liquid-solid growth mode. A systematic study on important growth parameters has been carried out for the optimization of nanorod morphology. The nanorod N-polarity, induced by high temperature nitridation of the sapphire substrate, is necessary to achieve vertical growth. Hydrogen, usually inapplicable during InN growth due to formation of metallic indium, and silane are needed to enhance the aspect ratio and to reduce parasitic deposition beside the nanorods on the sapphire surface. The results reveal many similarities between InN and GaN nanorod growth showing that the process despite the large difference in growth temperature is similar. Transmission electron microscopy, spatially resolved energy-dispersive X-ray spectroscopy, X-ray diffraction, X-ray photoelectron spectroscopy, and Raman spectroscopy have been performed to analyze the structural properties. Spatially resolved cathodoluminescence investigations are carried out to verify the optical activity of the InN nanorods. The InN nanorods are expected to be the material of choice for high-efficiency hot carrier solar cells.

Electrodeposition of crystalline silicon directly from silicon tetrachloride in ionic liquid at low temperature

Crystalline silicon was fabricated directly from silicon tetrachloride in ionic liquid at low temperature of 100 °C. SEM, TEM and SEAD revealed that as-deposited crystalline Si with diamond cubic crystal structure.

Selective etching of focused gallium ion beam implanted regions from silicon as a nanofabrication method

A focused ion beam (FIB) is otherwise an efficient tool for nanofabrication of silicon structures but it suffers from the poor thermal stability of the milled surfaces caused by segregation of implanted gallium leading to severe surface roughening upon already slight annealing. In this paper we show that selective etching with KOH:H2O2 solutions removes the surface layer with high gallium concentration while blocking etching of the surrounding silicon and silicon below the implanted region. This remedies many of the issues associated with gallium FIB nanofabrication of silicon. After the gallium removal sub-nm surface roughness is retained even during annealing. As the etching step is self-limited to a depth of 25-30 nm for 30 keV ions, it is well suited for defining nanoscale features. In what is essentially a reversal of gallium resistless lithography, local implanted areas can be prepared and then subsequently etched away. Nanopore arrays and sub-100 nm trenches can be prepared this way. When protective oxide masks such as Al2O3 grown with atomic layer deposition are used together with FIB milling and KOH:H2O2 etching, ion-induced amorphization can be confined to sidewalls of milled trenches.

Combining focused ion beam and atomic layer deposition in nanostructure fabrication

Combining the strengths of atomic layer deposition (ALD) with focused ion beam (FIB) milling provides new opportunities for making 3D nanostructures with flexible choice of materials. Such structures are of interest in prototyping microelectronic and MEMS devices which utilize ALD grown thin films. As-milled silicon structures suffer from segregation and roughening upon heating, however. ALD processes are typically performed at 200-500 °C, which makes thermal stability of the milled structures a critical issue. In this work Si substrates were milled with different gallium ion beam incident angles and then annealed at 250 °C. The amount of implanted gallium was found to rapidly decrease with increasing incident angle with respect of surface normal, which therefore improves the thermal stability of the milled features. 60° incident angle was found as the best compromise with respect to thermal stability and ease of milling. ALD Al2O3 growth at 250 °C on the gallium FIB milled silicon was possible in all cases, even when segregation was taking place. ALD Al2O3 could be used both for creating a chemically uniform surface and for controlled narrowing of FIB milled trenches.