Copper-Stabilized Si/Au Nanowhiskers for Advanced Nanoelectronic Applications

Abstract

We report here the growth and functional properties of silicon-based nanowhisker (NW) diodes produced by the vapor–liquid–solid process using a pulsed laser deposition technique. For the first time, we demonstrate that this method could be employed to control the size and shape of silicon NWs by using a two-component catalyst material (Au/Cu ≈ 60:1). During the NW growth, copper is distributed on the outer surface of the NW, whereas gold sticks as a droplet to its top. The length of NWs is defined by the total amount of copper in the catalyst alloy droplet. The measurements of the electrical transport properties revealed that in contact with the substrate, individual NWs demonstrate typical I–V diode characteristics. Our approach can become an important new tool in the design of novel electronic components.

Introduction

Quasi-one-dimensional and two-dimensional objects are the basis for most of the modern microelectronic and nanoelectronic devices.¹⁻³ As such, silicon nanowires or nanowhiskers (NWs) are unique one-dimensional nanostructures with a high aspect ratio, which makes them crucially important for various applications including semiconductor logics, memory, solar energy cells, sensors, and nanoelectromechanic devices.⁴⁻⁹ NW production requires a reliable control of multiple parameters, such as the diameter, height, crystal orientation, concentration of doping impurities, and so forth.

One of the established methods for the NW growth is the vapor–liquid–solid (VLS) technique. A metal catalyst deposited on a single crystal silicon substrate forms a eutectic liquid alloy droplet, which creates an area with minimal energy on the surface. Placed in a NW-forming environment,¹⁰ these droplets absorb both the material from the substance-reach atmosphere and the drifting atoms from the surface. Once the metal–silicon alloy gets to the saturated state, the silicon atoms begin to diffuse through and crystallize underneath the droplets, forming the NWs.¹¹ The NW body is formed by pure silicon with traces of the catalyst material. The VLS technique also allows a precise doping of NWs during the growth (including catalyst doping^11,12). Gold is one of the most common catalyst materials applied in this process. It can also be used as a basis for a double-component alloy.¹³ In particular, the Au/Cu alloy was applied to fabricate germanium¹⁴ or IV–V semiconductor¹⁵ NWs. The unique features of the VLS process enable us to control the NW structure and functionality by changing such parameters as the substrate temperature,¹⁶ the amount of growth substance in the reaction chamber atmosphere,¹⁷ and so forth. This approach employs the growth of self-organized structures, where each NW might act as a separate electronic component. This results in a dramatic decrease of the size of the produced devices and significant simplification of the fabrication process. Silicon NWs can be produced via the VLS technique by chemical vapor deposition,^18,19 electron-beam evaporation,²⁰ molecular beam epitaxy,²¹ magnetron sputtering,²² and pulsed laser deposition^23,24 (PLD). Among these technologies, PLD is the most flexible one in terms of plasma energy range (10–100 eV) and growth speeds. Also, PLD provides a unique opportunity of precise multielement codeposition with low dopant atomic concentrations (<1%).

A successful application of semiconducting NWs as a universal electronic component requires a precise control of the NW doping type and structure.²⁵ As an effective tool for manipulating functional properties, doping could have various side effects on the NW surface geometry, leading to an unstable growth, nonsymmetric side expansion, formation of a jagged surface,^26,27 and so forth. The key feature of our work is the use of the two-component Au/Cu alloy catalyst for PLD growth of silicon NWs, with gold as the main catalytic metal and copper as the doping element, where Cu is used to control the geometrical form without contaminating the NW structure. This opens a new way to the controlled growth of the NWs, giving new insights into precise doping of Si NWs and showing that the alloy catalyst has a great potential in controlling the shape of PLD-produced NWs.

Results and Discussion

In this study, the growth of NWs was performed by PLD in two steps according to the VLS technique: the formation of metal catalyst clusters on the substrate (n-type monocrystalline silicon (111)) was followed by the catalyst-assisted growth of the NWs at p-type silicon target sputtering. The catalyst droplets were seeded by simultaneous codeposition of gold and copper, at the 60:1 ratio (see the Supporting Information).

The diameter of NWs is proportional to the size of the initial Au/Cu catalyst cluster. The latter depends directly on the substrate temperature, which is normally set in the range of 500–600 °C. Atomic force microscopy images of metal clusters formed at different temperatures are shown in Figure 1a,b,d. Single cluster profiles (Figure 1c) clearly demonstrate the way to control the cluster size: higher temperatures lead to the formation of bigger volume droplets separated by larger gaps between them.

Figure 1.

Atomic force micrographs of Si(111) substrates (6.5 × 6.5 μm) after the deposition of catalyst clusters at T = 500 (a), 550 (b), and 600 °C (d); panel (c) shows the typical profiles of metal clusters for different deposition temperatures.

The catalyst cluster arrangement selected in our work for the next growth step corresponds to the substrate temperature of 600 °C. As shown on the scanning electron micrograph (SEM) in Figure 2a,b, NWs grow in different sizes and shapes depending on the type of the catalyst droplet. For the pure Au catalyst, the growth resulted in short conically shaped NWs (50–800 nm in height) oriented along the normal to the Si(111) substrate surface, with the same cone-base diameter as the initial droplet size. Meanwhile, with the Au/Cu catalyst, the NWs developed into elongated pillars with clearly shaped facets growing in {011} directions and topped with a spherical metal cap (Figure 2b). Part of the VLS mechanism connected with atom inflow from the substrate surface is demonstrated in Figure 2c–e. The image obtained by secondary electrons (Figure 2c,e) shows the geometrical outline of the surface with step-morphology terraces created by the Au/Cu catalyst droplet migration, similarly to the works.^28,29 The elemental contrast revealed by backscattered electrons in Figure 2d displays a sharp boundary between the Si NW body and the Au/Cu droplet. The row of silicon droplets on the side surface is left by the flow of the silicon adatoms from the substrate to the solid–liquid interface at the top (Figure 2e).

Figure 2.

SEM images of Si NW growth with Au (a) and Au/Cu (b) catalysts droplets on the Si(111) substrate surface, the Au/Cu droplets pseudocolored (in yellow). SEM of single NW growth in the ⟨111⟩ direction in secondary (c,e) and backscattered (d) electron modes. The row of silicon clusters on the facets (e) marks the Si atom upflow from the substrate.

The in-plane growth of NWs (formed under ultrahigh vacuum conditions and at the base pressure of about P ≈ 10^–8 Torr) could be explained by the influence of the hydrogen passivating layer produced after the hydrofluoric acid substrate treatment³⁰ (Figure 3a,b). The in-plane NWs could be used as building blocks for nanooptical and nanoelectronical devices,³¹ as well as for stretchable electronics.³² The planar growth mode could be easily transformed into vertical one by a proper choice of laser plasma energies.

Figure 3.

SEM micrographs: (a,b)—NW growth along the ⟨011⟩ direction and (c,d)—NW growth along the ⟨111⟩ direction. The droplets are pseudocolored (in yellow).

The energy of the silicon particles in the plasma was decreased by elevating the deposition chamber pressure to 5 mTorr with the introduction of pure argon (99.999% purity). The produced NWs were aligned vertically (Figure 3c,d) with the average distance of ∼1.5–2.0 μm and the NW diameter in the range of 100–280 nm. Transmission electron microscopy (TEM) data confirm that silicon NWs have a single crystalline structure with a very high degree of lattice perfection growing parallel to the ⟨111⟩ direction (Figure 4c–e). Inside the NWs, TEM also revealed defects common for cubic structures. For example, one observes coherent twin boundary, 60° around the ⟨111⟩ direction (Figure 4e) close to the Si/Au interface. Although the TEM data allow us to estimate the metal cluster (002) interplanar distance inside the NWs (Figure 4d): d₀₀₂ ≈ 2A, which might correspond either to AuCu (d₀₀₂ = 1.93A) or to Au (d₀₀₂ = 2.04A).

Figure 4.

(a) Overview SEM image of a single NW. (b) HAADF-STEM image of the Au/Cu cluster arrangement along the NW growth terraces. (c) Enlargement image from (b) showing Si dumbbells. (d) Au/Cu metal cluster inside the Si NW with the (002) interplanar distance ≈ 2A. (e) NW coherent twin boundary (with a few atom planes both sides marked with white dots), 60° around ⟨111⟩ near the interface Au/Si. (f–k) HAADF-STEM image of the NW and the corresponding maps of the elemental distribution for O, Al, Si, Cu, and Au. Note that Cu and Au are mostly concentrated in the top metal droplet. Part of the Cu atoms are evenly distributed over the side facets.

The nanostructure and distribution of different chemical elements in an NW was controlled by high-angle annular dark-field scanning TEM (HAADF-STEM) with elemental mapping via energy-dispersive X-ray spectroscopy. It shows the presence of Au and Cu in the top droplet as well as on the side facets of Si NWs (Figure 4j,k). Remarkably, gold and copper do not mix and display a different surface distribution. A small fraction of the Au metal that escapes the cap forms gold nanoclusters sparsely distributed on the side walls of the NWs. A similar segregation of Au clusters on the sidewall of faceted Si nanowires was previously reported.²⁸ Copper also outflows the catalyst droplet, but in a much more intensive way, forming a cluster network on the facets (see Figure 4a,b for comparison). This could be connected to the well-known high mobility of Cu and to its ability to form epitaxial films on silicon {011} surfaces.³³ It is important to mention that the metal clusters on the surface are sparsely distributed but still lead to a weak leakage current.

Volt–ampere characteristics (I–V curves) of the fabricated NWs were measured by the two-probe method. The SEM image of an individual NW brought into the mechanical contact with the tungsten probe is shown in Figure 5c. The electron-beam-induced current (EBIC) image in Figure 5d was obtained at a negative external bias field, and higher brightness corresponds to a higher current and concentration of carriers. Thus, p-type silicon NWs grown on a n-type substrate without any additional impurity dopants exhibited an increased current at the NW-substrate boundary, indicating that the most applied voltage occurs at the interface. This fact evidences the existence of the p–n junction at the interface and of p-type doping NW.

Figure 5.

(a,b) Diode I–V curve of the p-type NW on the n-type substrate and the equivalent circuit. A weak leakage current flows through the metal clusters at the side facets, which is taken into account as resistor R1 in the equivalent circuit. (c) Microphotographs of the tungsten probe contact with the individual NW in secondary electron mode. (d) EBIC image of the investigation area shows the contrast meaning the electrical shorting of the circuit through the NW.

The curves obtained by the I–V measurements from a single NW are characteristic for p–n diodes (Figure 5a), demonstrating a weak rectifying character in the correspondence with the EBIC data. The curve is nearly linear at small values of applied voltage in both current directions, indicating a presence of an Ohmic shunt marked as R1 in the inset. The shunt defines the conductance at all reverse biases when the p–n junction current is limited by a high barrier, but it becomes less than the diode resistance at sufficiently high forward bias voltages.

For a further enhancement of electronic component reliability, NW oxidation and current leakage along the side surface can be prevented by applying a dielectric layer. For that, 8 nm of alumina was deposited by the atomic layer deposition (ALD) method³⁴ onto the side facets of NWs without breaking the vacuum.³⁵ Chemical elemental distribution maps (Figure 4g,h) revealed that aluminum oxide had grown conformally onto the silicon surface, whereas no continuous Al₂O₃ layer was formed on the gold droplet. Such area-selective deposition is typical for the chemical reactions used in ALD: a gold surface inhibits ALD growth as it lacks hydroxyl groups.

Conclusions

We experimentally demonstrated that silicon NW growth via a VLS process can be effectively controlled by doping of a gold catalyst with copper. The two metals play different roles in NW formation: Au–Si eutectic facilitates the transfer of silicon atoms at the growth interface, while Cu stabilizes the side facets and makes it possible to control the formation of the NWs. The height of the NWs depends on the silicon deposition rate and the number of NW nucleation clusters.

The as-grown NWs demonstrate an I–V characteristic specific for p–n junctions and can be directly used as individual components in electronic circuits. The layout of the NW configuration on a substrate can be directly controlled by lithographic processing, and isolating coatings, as demonstrated in the paper, can be fabricated by atomic layer deposition. Thus, the growth technique described in the paper can be effectively used for manufacturing modern nanoelectronic devices.

Glossary

Abbreviations

NW

nanowhisker

VLS

vapor–liquid–solid

PLD

pulsed laser deposition

EBIC

electron-beam-induced current

TEM

transmission electron microscopy

HAADF-STEM

high-angle annular dark-field scanning TEM

EDX

energy-dispersive X-ray spectroscopy

Supporting Information Available

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsomega.7b01640.

• Sample fabrication and characterization methods (PDF)

Author Contributions

The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript.

The authors declare no competing financial interest.