M. Steinmair

Anomalous piezoresistance effect in ultrastrained silicon nanowires.

In this paper we demonstrate that under ultrahigh strain conditions p-type single crystal silicon nanowires possess an anomalous piezoresistance effect. The measurements were performed on vapor-liquid-solid (VLS) grown Si nanowires, monolithically integrated in a microelectro-mechanical loading module. The special setup enables the application of pure uniaxial tensile strain along the <111> growth direction of individual, 100 nm thick Si nanowires while simultaneously measuring the resistance of the nanowires. For low strain levels (nanowire elongation less than 0.8%), our measurements revealed the expected positive piezoresistance effect, whereas for ultrahigh strain levels a transition to anomalous negative piezoresistance was observed. For the maximum tensile strain of 3.5%, the resistance of the Si nanowires decreased by a factor of 10. Even at these high strain amplitudes, no fatigue failures are observed for several hundred loading cycles. The ability to fabricate single-crystal nanowires that are widely free of structural defects will it make possible to apply high strain without fracturing to other materials as well, therefore in any application where crystallinity and strain are important, the idea of making nanowires should be of a high value.

Pressure-induced orientation control of the growth of epitaxial silicon nanowires.

Single crystal silicon nanowires (SiNWs) were synthesized with silane reactant using Au nanocluster-catalyzed one-dimensional growth. We have shown that under our experimental conditions, SiNWs grown epitaxially on Si(111) via the vapor-liquid-solid growth mechanism change their growth direction as a function of the total pressure. Structural characterization of a large number of samples shows that SiNWs synthesized at a total pressure of 3 mbar grow preferentially in the 111 direction, while the one at 15 mbar favors the 112 direction. Specifically by dynamically changing the system pressure during the growth process morphological changes of the NW growth directions along their length have been demonstrated.

Ga∕Au alloy catalyst for single crystal silicon-nanowire epitaxy

Epitaxial growth of single crystalline silicon nanowires along the ⟨111⟩ directions was obtained on Si (100) and Si (111) substrates by gold-gallium-nanoparticle-catalyzed chemical vapor deposition with a SiH4 precursor. In comparison to the pure gold catalyst, the proportion of the nanowires growing perpendicular to the substrate is much higher and the wires show almost no kinks. The average diameter is smaller, and the diameter and length distributions are narrowly dispersed. By making a particular choice of growth conditions, it is possible to realize either rodlike or tapered silicon nanowires, which may be desirable for applications as field emitters.

Orientation specific synthesis of kinked silicon nanowires grown by the vapour-liquid-solid mechanism.

Kinked silicon nanowires (Si-NWs) were synthesized in a well reproducible manner using gold nanocluster-catalyzed quasi-one-dimensional growth on Si(111) substrates with silane (SiH(4)) as the precursor gas. The kinking is considered to be due to the change in the growth direction induced by the sudden change of the pressure during Si-NW synthesis. Structural high resolution transmission electron microscopy (HRTEM) characterization of the sample shows that epitaxial Si-NWs synthesized on Si(111) substrates at a total pressure of 3 mbar grow along the {111} direction, while the ones at 15 mbar favour the {112} direction. By dynamically changing the system pressure during the growth process morphological changes of the NW growth directions along their length have been shown, resulting in kinked nanowires. The crystallographic orientation relation of the kinking between the 3 and 15 mbar ranges has been analysed by TEM. It is shown that no defects or grain boundaries in the intersection between the two sections of the Si-NWs are necessary to form such kinks between different wire directions.

Growth of branched single-crystalline GaAs whiskers on Si nanowire trunks

In this paper we present the hetero-epitaxial growth of single-crystalline GaAs whiskers on Si(111)-nanowire trunks forming hierarchical star-like structures with a six-fold symmetry. These hierarchical nanostructures have been successfully formed utilizing both vapor?liquid?solid (VLS) growth by low-pressure chemical vapor deposition (LPCVD) and molecular-beam epitaxy (MBE) techniques. High-resolution transmission electron microscopy (HRTEM) studies revealed the [111] growth direction of the core Si nanowires (Si-NWs) with six {112} facet planes. The sequentially grown branches are single-crystalline hexagonal GaAs nanowhiskers which grow preferably in the [0001] direction and are perpendicular to the {112} facets of the Si-NW backbone. Photoluminescence (PL) measurements confirm the good crystalline quality of the GaAs nanowhiskers and a blueshift of about 30?meV compared to bulk zinc blende-type GaAs. The ability to prepare rotationally branched NW structures should open new opportunities for both fundamental research and applications including monolithic three-dimensional nanoelectronics and nanophotonics.

Some aspects of substrate pretreatment for epitaxial Si nanowire growth.

We report on the influence of the surface pretreatment for vapor-liquid-solid growth of epitaxial silicon nanowires with gold catalyst and silane precursor on Si(111) substrates. In this paper we make it obvious that a thin native oxide layer on the Si substrate-as is present under most technological conditions-or a thin layer of oxide formed on top of the catalytic gold particle restrain nucleation and nanowire growth. High resolution transmission electron microscopy, and electron energy loss spectroscopy were utilized to demonstrate Si diffusion from the substrate through the catalytic Au layer and further the formation of a thin oxide layer atop. Based on this observation we present a sample pretreatment practice, making the catalyst insensitive for further oxide formation, thereby preserving epitaxy for nanowire synthesis.

Scalable Approach for Vertical Device Integration of Epitaxial Nanowires

In this letter, we demonstrate the simultaneous vertical integration of self-contacting and highly oriented nanowires (NWs) into airbridge structures, which have been developed into surround gated metal oxide semiconductor field effect transistors (MOSFETs). With the use of conventional photolithography, reactive ion etching (RIE), and low pressure chemical vapor deposition, a suspended vertical NW architecture is formed on a silicon on insulator (SOI) substrate where the nanodevice will later be fabricated on. The vapor-liquid-solid (VLS) grown Si-NWs are contacted to prepatterned airbridges by a self-aligned process, and there is no need for postgrowth NW assembly or alignment. Such vertical NW architecture can be easily integrated into existing ICs processes opening the path to a new generation of nonconventional nano devices. To demonstrate the potential of this method, surround gated vertical MOSFETs have been fabricated with a highly simplified integration scheme combining top-down and bottom-up approaches, but in the same way, one can think about the realization of integrated nano sensors on the industrial scale.

Focused electron beam induced deposition of gold catalyst templates for Si-nanowire synthesis

A new approach using focused electron beam induced deposition (FEBID) to deposit catalyst particles is reported for the synthesis of single crystalline silicon nanowires (SiNWs) grown by low pressure chemical vapor deposition (LPCVD). The FEBID deposited gold dot arrays fabricated from an acac-Au(III)-Me(2) precursor were investigated by AFM and EDX. The depositions were found to form a sharp tip and a surrounding halo and consist of only 10 at.% Au. However, SiNWs could be synthesized on the deposited catalyst using the vapor-liquid-solid (VLS) method with a mixture of 2% SiH(4) in He at 520 °C. NW diameters from 30 nm up to 150 nm were fabricated and the dependency of the NW diameter on the FEBID deposition time was observed. TEM analysis of the SiNWs revealed a [110] growth direction independent of the NW diameter. This new method provides a maskless and resistless approach for generating catalyst templates for SiNW synthesis on arbitrary surfaces.

In place growth of vertical Si nanowires for surround gated MOSFETs with self aligned contact formation

We demonstrate the simultaneous vertical integration of self contacting and highly oriented nanowires (NWs) into airbridge structures. With the use of conventional semiconductor processing techniques and vapour-liquid-solid (VLS) growth a suspended vertical NW architecture is formed on a silicon on insulator (SOI) substrate where the nanodevice will later be fabricated on. The VLS grown Si-NWs are contacted to prepatterned airbridges by a self aligned process and there is no need for post-growth NW assembly or alignment. To demonstrate the potential of this method surround gated vertical MOSFETs have been fabricated with a highly simplified integration scheme combining top-down and bottom-up approaches.

MBE Growth of GaAs Whiskers on Si Nanowires

We present the growth of GaAs nanowhiskers by molecular beam epitaxy on Si (111) nanowires grown by low‐pressure chemical vapor deposition. The whiskers grow in the wurtzite phase, along the [0001] direction, on the {112} facets of the Si nanowire, forming a star‐like six‐fold radial symmetry. The photoluminescence shows a 30 meV blue shift with respect to bulk GaAs, additionally a GaAs/AlAs core‐shell heterostructure shows increased luminescence.