Cheng-Yen Wen

Formation of Compositionally Abrupt Axial Heterojunctions in Silicon-Germanium Nanowires

Sharp Nanowires The potential for using nanowires in devices can be limited by the ability to synthesize them from two or more materials while maintaining compositional purity at the interfaces. Instead of using liquid droplets at the eutectic point when the melting point is at a minimum, Wen et al. (p. 1247) show that generating the wires at solid alloy catalysts allows fabrication of silicon germanium wires with atomically sharp interfaces. The system works well because an AlAu alloy composition was chosen in which Si and Ge have a low solubility but which have a high enough eutectic temperature so that nanowire growth is not limited by the reactivity of the Si and Ge precursors. A solid alloy catalyst is used to synthesize atomically sharp interfaces in silicon-germanium nanowires. We have formed compositionally abrupt interfaces in silicon-germanium (Si-Ge) and Si-SiGe heterostructure nanowires by using solid aluminum-gold alloy catalyst particles rather than the conventional liquid semiconductor–metal eutectic droplets. We demonstrated single interfaces that are defect-free and close to atomically abrupt, as well as quantum dots (i.e., Ge layers tens of atomic planes thick) embedded within Si wires. Real-time imaging of growth kinetics reveals that a low solubility of Si and Ge in the solid particle accounts for the interfacial abruptness. Solid catalysts that can form functional group IV nanowire-based structures may yield an extended range of electronic applications.

Growth system, structure, and doping of aluminum-seeded epitaxial silicon nanowires.

We have examined the formation of silicon nanowires grown by self-assembly from Si substrates with thin aluminum films. Postgrowth and in situ investigations using various Al deposition and annealing conditions suggest that nanowire growth takes place with a supercooled liquid droplet (i.e., the vapor-liquid-solid system), even though the growth temperatures are below the bulk Al/Si eutectic temperature. Wire morphology as a function of processing conditions is also described. It is shown that when Al environmental exposure is prevented before wire growth a wide process window for wire formation can be achieved. Under optimum growth conditions, it is possible to produce excellent crystal quality nanowires with rapid growth rates, high surface densities, low diameter dispersion, and controlled tapering. Photoelectron spectroscopy measurements indicate that the use of Al leads to active doping levels that depend on the growth temperature in as-grown nanowires and increase when annealed. We suggest that these structural and electronic properties will be relevant to photovoltaic and other applications, where the more common use of Au is believed to be detrimental to performance.

Controlling the growth of Si/Ge nanowires and heterojunctions using silver-gold alloy catalysts.

We describe a new catalyst for group IV nanowire heterostructures, based on alloying Ag with Au, that combines the ability to control catalyst phase and nanowire structure with good environmental stability. Compared to other alloy catalysts, we show a higher oxidation resistance of AgAu and more consistent crystal shapes and catalyst/nanowire orientation relationships during growth. We show that AgAu catalysts are also stable against diffusion during growth, making them capable of forming long nanowires with uniform diameters. Furthermore, we demonstrate the growth of compositionally abrupt Si/Ge heterojunctions with good reproducibility and yield, switching individual nanowires between vapor-liquid-solid and vapor-solid-solid growth to optimize growth rates by control of the catalyst state. The stability and properties of AgAu catalysts potentially open up a promising and practical route toward control of group IV heterostructure nanowires.

Strategies To Control Morphology in Hybrid Group III-V/Group IV Heterostructure Nanowires.

By combining in situ and ex situ transmission electron microscopy measurements, we examine the factors that control the morphology of hybrid nanowires that include group III-V and group IV materials. We focus on one materials pair, GaP/Si, for which we use a wide range of growth parameters. We show through video imaging that nanowire morphology depends on growth conditions, but that a general pattern emerges where either single kinks or inclined defects form some distance after the heterointerface. We show that pure Si nanowires can be made to exhibit the same kinks and defects by changing their droplet volume. From this we derive a model where droplet geometry drives growth morphology and discuss optimization strategies. We finally discuss morphology control for material pairs where the second material kinks immediately at the heterointerface and show that an interlayer between segments can enable the growth of unkinked hybrid nanowires.

In Situ TEM Creation and Electrical Characterization of Nanowire Devices

We demonstrate the observation and measurement of simple nanoscale devices over their complete lifecycle from creation to failure within a transmission electron microscope. Devices were formed by growing Si nanowires, using the vapor-liquid-solid method, to form bridges between Si cantilevers. We characterize the formation of the contact between the nanowire and the cantilever, showing that the nature of the connection depends on the flow of heat and electrical current during and after the moment of contact. We measure the electrical properties and high current failure characteristics of the resulting bridge devices in situ and relate these to the structure. We also describe processes to modify the contact and the nanowire surface after device formation. The technique we describe allows the direct analysis of the processes taking place during device formation and use, correlating specific nanoscale structural and electrical parameters on an individual device basis.

Measurement of local Si-nanowire growth kinetics using in situ transmission electron microscopy of heated cantilevers.

A technique to study nanowire growth processes on locally heated microcantilevers in situ in a transmission electron microscope has been developed. The in situ observations allow the characterization of the nucleation process of silicon wires, as well as the measurement of growth rates of individual nanowires and the ability to observe the formation of nanowire bridges between separate cantilevers to form a complete nanowire device. How well the nanowires can be nucleated controllably on typical cantilever sidewalls is examined, and the measurements of nanowire growth rates are used to calibrate the cantilever-heater parameters used in finite-element models of cantilever heating profiles, useful for optimization of the design of devices requiring local growth.

Strain and stability of ultrathin Ge layers in Si/Ge/Si axial heterojunction nanowires.

The formation of abrupt Si/Ge heterointerfaces in nanowires presents useful possibilities for bandgap engineering. We grow Si nanowires containing thick Ge layers and sub-1 nm thick Ge quantum wells and measure the interfacial strain fields using geometric phase analysis. Narrow Ge layers show radial compressive strains of several percent, while stress at the Si/Ge interface causes lattice rotation. High strains can be achieved in these heterostructures, but we show that they are unstable to interdiffusion.

The growth and characterization of Si and Ge nanowires grown from reactive metal catalysts

We discuss the benefits of using metals other than Au to catalyze the growth of Si and Ge nanowires, emphasizing the opportunities that these non-conventional materials provide for tailoring electronic and structural nanowire properties. However, since these metals are more reactive than Au, their use creates constraints on wire growth conditions as well as difficulties in post-growth characterization. These issues are illustrated for Si and Si/Ge nanowires grown from Al, Cu and AuAl starting materials. The vacuum requirements for the deposition of the reactive metals are discussed as well as the effect of atmospheric exposure on the structure of wires observed post-growth with electron microscopy.

Producing Atomically Abrupt Axial Heterojunctions in Silicon–Germanium Nanowires by Thermal Oxidation

Compositional abruptness of the interfaces is one of the important factors to determine the performance of Group IV semiconductor heterojunction (Si/Ge or Si/SiGe) nanowire devices. However, forming abrupt interfaces in the nanowires using the common vapor-liquid-solid (VLS) method is restricted because large solubility of Si and Ge in the Au eutectic liquid catalyst makes gradual composition change at the heterojunction after switching the gas phase components. According to the VLS growth mechanism, another possible approach to form an abrupt interface is making a change of the semiconductor concentration in the eutectic liquid before precipitation of the second phase. Here we show that the composition in AuSiGe eutectic liquid on SiGe nanowires of low Ge concentration (≤6%)xa0can be altered by thermal oxidation at 700 °C.xa0During the oxidation process, only Si is oxidized on the surface of the eutectic liquid, and the Ge/Si ratio in the eutectic liquid is increased. The subsequently precipitated SiGe step at the liquid/solid interface has a higher Ge concentration (∼20%), and a compositionally abrupt interface is produced in the nanowires. The growth mechanism of the heterojunction includes diffusion of Si and Ge atoms on nanowire surface into the AuSiGe eutectic liquid and step nucleation at the liquid/nanowire interface.