Growth characteristics of silicon nanowires synthesized by vapor–liquid–solid growth in nanoporous alumina templates
Introduction
In the last few years, nanomaterials, which are of fundamental and technological interest, have attracted increasing attention since carbon nanotubes were first discovered in the 1990s [1]. The main driving force is the shrinking feature size of silicon (Si) CMOS devices, in which the critical dimension is anticipated to be below 0.1 μm by 2007. This small length scale presents immense technical challenges for manufacturing processes, such as photolithography and interconnects. As a result, there is increasing interest in the synthesis and assembly of nanomaterials, such as nanotubes and nanowires, which may serve as the building blocks for the next generation of electronic devices.
Semiconducting nanowires have recently attracted increasing interest due to the unique fundamental properties and potential applications of these structures. Boron-doped Si nanowires, for example, have been used for sensor applications such as pH and protein binding [2]. Recently, Gudiksen et al. [3] demonstrated the fabrication of gallium arsenide (GaAs)/gallium phosphide (GaP) nanowire superlattices and Si nanowire p–n junctions which may find applications in nanoscale electronics and photonics.
A common process for semiconductor nanowire synthesis is vapor–liquid–solid (VLS) growth. The well-known VLS mechanism for Si nanowire growth was initially proposed and demonstrated by Wagner and Ellis [4], [5]. In this process, gold (Au) is used to catalyze the decomposition of a Si-containing source gas, such as silane (SiH4) or silicon tetrachloride (SiCl4). Au and Si then form a liquid phase alloy at a eutectic temperature of 363°C. Finally, Si nanowires crystallize and grow from the supersaturated alloy. In this process, the diameter of the nanowire is determined by the original size of the Au catalyst particle as well as growth conditions. Synthesis of Si nanowires with different diameters has been demonstrated using techniques in which the Au particles were formed by metal deposition and photolithography (<20 μm) [6], [7], laser ablation (6–20 nm) [8], evaporation (∼15 nm) [9] or nanocluster formation (6–31 nm) [10].
The majority of studies of VLS growth carried out thus far have focused on the effect of metal catalyst size and growth conditions on the structural properties of the nanowires. The effect of processing parameters on the growth rate of the nanowires has not been examined in detail. The impact of growth temperature and SiH4 partial pressure on the structural properties of Si nanowires was reported by Westwater et al. [11], [12] The optimum conditions for growing straight, single crystal Si nanowires on the surface of a Si wafer were at moderate temperature (∼600°C) and at low total pressure (∼0.1 Torr).
In this study, nanoporous membranes were used as templates to control the diameter of VLS-grown Si nanowires. This technique combines the advantages of both template-directed synthesis and VLS growth to produce straight, single crystal Si nanowires with well-controlled diameters ranging from 100 to 340 nm [13]. Si nanowires grown out of the membranes were found to be either single crystal with growth orientation 〈1 0 0〉 [13] or bicrystalline containing a single (1 1 1) twin boundary along the growth axis [14]. Template-directed synthesis provides a convenient platform to study the effect of process parameters on nanowire growth since the nanowire diameter is determined by the pore size of the membrane rather than the growth conditions. In this study, the effect of growth temperature and SiH4 partial pressure on the growth rate of the nanowires was investigated. The experimental results are compared with those previously reported for low-pressure chemical vapor deposition of Si from a SiH4 source.
Section snippets
Experimental procedure
Commercially available anodic alumina membranes (Whatman Scientific) with a nominal pore diameter of 200 nm and thickness of 60 μm were used as templates in this study. Alumina membranes with pore diameters ranging from 4 nm to greater than 200 nm can be produced through the anodization of aluminum in various acids as described by Routkevitch et al. [15]. A schematic of the nanowire fabrication process is as shown in Fig. 1. The Au catalyst for VLS growth was electrodeposited into the membranes
Model of SiH4 diffusion and reaction in a nanopore
VLS growth in nanoporous templates requires careful consideration and selection of reaction conditions. In conventional VLS growth, the metal catalyst particle is either supported on a surface or produced in the gas phase and is readily accessible to the vapor phase growth species. In template-directed VLS growth, the metal catalyst particle is buried deep within the pore. Consequently, a careful choice of reaction conditions is required to ensure that the vapor phase species diffuses far into
Effect of temperature
The effect of temperature on the growth rate of the Si nanowires was initially investigated. Au plugs with an average thickness of 0.24 μm were electrodeposited into the nanoporous membranes at a distance of 25 μm from the membrane surface. Growth experiments were then performed at times ranging from 2 to 40 min, growth temperatures between 400°C and 500°C, a total pressure of 13 Torr and a SiH4 partial pressure of 0.65 Torr. Fig. 4 shows a cross-section of the membrane after 8 min of growth time at
Discussion
The VLS growth process can be summarized in the following four steps [22]: (1) mass transport of SiH4 from the gas phase to the Au surface; (2) reaction of SiH4 on the Au surface; (3) diffusion of Si through the Au–Si eutectic liquid phase; (4) crystallization of Si from the supersaturated Au–Si eutectic liquid. In this study, the experimental conditions (low temperature, low pressure) were chosen such that the mass transport of SiH4 from the bulk gas phase to the Au surface within the pore
Conclusions
The effect of reaction conditions on the growth rate of Si nanowires via template-directed VLS growth from a SiH4 source was investigated. The Si nanowire growth rate follows an Arrhenius relation over the temperature range of 400–500°C with an activation energy of 22 kcal/mol. Decomposition of SiH4 through deposition of Si on the membrane surface and pore walls was observed when the temperature was greater than 500°C. The activation energy is considered to be associated with the heterogeneous
Acknowledgements
This work was supported by the National Science Foundation under grant number DMR-0103068 and The Pennsylvania State University Materials Research Science and Engineering Center (MRSEC) on Collective Phenomena in Restricted Geometries.
References (26)
- et al.
J. Crystal Growth
(1994) - et al.
J. Crystal Growth
(1996) - et al.
J. Crystal Growth
(1982) - et al.
J. Crystal Growth
(1971) J. Crystal Growth
(1975)Nature
(1991)- et al.
Science
(2001) - et al.
Nature
(2002) - et al.
Appl. Phys. Lett.
(1964) - et al.
J. Appl. Phys.
(1964)
Science
Appl. Phys. Lett.
Appl. Phys. Lett.
Cited by (170)
Nanofabrication through molding
2022, Progress in Materials ScienceAdvanced VLS growth of gold encrusted silicon nanowires Mediated by porous Aluminium Oxide template
2021, VacuumCitation Excerpt :Further, they had overcome this limitation with electroless deposition of Au-catalyst into the AAO pores in their later work [37]. Though, this template-directed synthesis has good control over diameter, density and uniformity of the SiNWs formed, requires long reaction time (gaseous precursors are made to react with the metal catalyst buried deep within the pores) with lower yield and requires a careful selection of reaction conditions [38,39]. From the literature, AAO templates have been utilised for the fabrication of SiNWs.
Preparation of high-purity straight silicon nanowires by molten salt electrolysis
2020, Journal of Energy ChemistrySynthesis and Characterization of Silicon Based Anode Materials
2017, Materials Today: Proceedings