17 Growth of Semiconductor and Oxide Nanowires

Dr. Anchal Srivastava

Contents of this Unit

1.  Introduction

2.  Growth of Nanowires

2.1 Liquid Phase Technique

2.1.1. Template Based Methods

2.1.1.1. Template Preparation

2.1.1.2. Deposition Methods

2.2. Vapor Phase Techniques

2.2.1. Pulse Laser Deposition Techniques

2.2.2. Thermal Evaporation Techniques

3. Summary

Learning Outcomes

• After studying this module, you shall be able to
• Know about one dimensional (1D) nanowires.
• Learn various synthesis techniques for growth of semiconducting and oxide nanowires.

1. Introduction

One-dimensional (1D) structures have been called in different ways: nanowires, nanorod, fibers of fibrils, whiskers, etc. The common characteristic of these structures is that all they have a nanometer size in one of the dimensions, which produces quantum confinement in the material and changes its properties. Figure 1 shows different examples of nanorods and nanowires made in different materials.

Fig. 1. Figure showing various 1D nanostructures.

Nanowires are structures with at least one of their dimensions in the 1–100 nm range; typically they are several microns in length with diameter under 100 nm. Nanoscale materials exhibit a very high surface-to-volume ratio. A 30 nm iron particle has about 5% atoms on the surface, and a 10 nm particle has 20% of atoms on the surface. In contrast, a 3 nm particle has 50% of atoms on the surface. This makes the surface or interface phenomena to dominate over bulk effects and strongly influence adsorption, solubility, reactivity, catalysis, etc. where surface effects are important. In addition, as mentioned earlier, the size-dependent properties make nanoscale materials attractive in various applications. In any application, the material selection is made based on one or more desirable properties of a material over its competition. When physical, chemical, optical, electrical, magnetic, and other properties change at the nanoscale, it is easy to understand the widespread impact of nanotechnology across all industrial sectors. Fundamental to this revolution is the development of nanostructured materials, and inorganic nanowires (INWs) form a class of nanomaterials that is expected to play a prominent role. Attempts have been made in recent years to grow the above materials in the form of one-dimensional nanowires. The nanowires are single crystals with well-defined surface structural properties. When their diameter is less than the Bohr radius, the resulting quantum confinement is of great interest in studying the excitonic behavior of low-dimensional solids for its effect on electron transport, band structure, and optical properties. In electronics, their one dimensionality offers the lowest dimension transport channel for the best field effect transistor (FET) scalability. The interesting properties of various nanowires make them better candidates for a wide range of applications as seen in Table 1.

It is also possible to conceive of systems wherein various nanowires can be employed for different functions as shown in Fig. 2: silicon or germanium nanowires for processors, phase change nanowires for memory, oxide nanowires for sensing, thermoelectric nanowires for power generation, III–V nanowires for optoelectronic components, and some examples to build integrated systems.

Fig. 2. Figure showing future of inorganic nanowires.

2. Growth of Nanowires:

Nanowires are structures with at least one of their dimensions in the 1–100 nm range; typically they are several microns in length with diameter under 100 nm. Growth processes must be able to preferentially support growth in one dimension. The key requirements for growing nanowires are that there must be a reversible pathway or condition near equilibrium between a fluid phase such as solution, melt, or vapor and a solid phase, and also the atoms in solid phase should have high surface or bulk mobility. Nanowires have been synthesized using a variety of techniques. The nanowire synthesis methods can be divided into two broad categories: liquid- or solution-based techniques and vapor-phase techniques. The vapour phase techniques include methods such as chemical vapor deposition (CVD) using catalyst metals, chemical vapor transport, reactive vapor transport, laser ablation, carbothermal reduction, chemical beam epitaxy (CBE), thermal evaporation and thermal decomposition, and plasma- and current-induced methods. Liquid- or solution-based techniques include approaches such as sol-gel synthesis, hydrothermal processes, and electrodeposition. Many liquid-phase techniques utilize templates for producing one-dimensional (1-D) materials though; techniques without the use of templates are being developed rapidly. In this module we will learn about growth apparatus for these techniques along with source material, growth conditions, substrate preparation, catalyst preparation and post growth processing.

2.1 Liquid-Phase Techniques:

There have been a number of approaches for nanowire synthesis using liquid phase. These can be divided broadly into two categories: template-based and template-free approaches.

2.1.1. Template-Based Methods:

Templates provide 1-D channels for guiding the growth or deposition of materials in 1-D form. There are three primary types of templates: positive template, negative template, and surface step templates. The negative template method is the most commonly used among the three. There are also three main deposition methods using these templates: electrodeposition, sol-gel, and CVD. Other less popular techniques involving templates include chemical precipitation or polymerization reactions and electroless deposition.

2.1.1.1. Template Preparation:

Positive template approach:

In a positive template approach, 1-D nanostructures such as carbon nanotubes (CNTs), DNA, and polymer are used as templates to deposit the material for nanowire production. This is schematically shown in Fig. 3(a). CNTs provide an ideal confined platform for chemical reactions. In principle, it is possible to obtain nanowires of any material by simply depositing it (using sputter coating, electrodeposition, or thermal treatment) onto CNT. However, this is not possible for all types of materials because of weak metal–carbon interaction. Physical vapour deposition results in discontinuous structures on the CNT after deposition. However, titanium has strong interactions with the CNT and hence it can be used as a buffer layer to increase the adhesion for obtaining nanowires of other metals such as Au, Pd, Fe, Al, Pb, etc.

Fig. 3. Schematic showing nanowire electrodeposition techniques based on (a) positive and (b) negative tempates.

Negative template approach:

In a negative template approach, a prefabricated hollow cylindrical structure is used to deposit the material of interest inside the hollow pores in the 1-D form. This is shown schematically in Fig. 3.1b. Potential templates include nuclear track-etched membranes (TEMs), anodic or anodized alumina membranes (AAMs), diblock copolymer, nanopore arrays in glass, mesoporous silica, and other porous or hollow structures.

AAM is another commonly used negative template, produced by wellknown anodization process. Anodization involves placing an Al foil in a chemical acid bath (electrolytic solution) under a direct current which develops a self-assembled pore structure as shown in Fig. 4a. The Al sheet becomes the anode whereas acidic solution becomes the cathode. During anodization an oxide film forms at the anode,

while hydrogen evolves at the cathode (Pt)

The electrode potential at the anode is given by

Thus, the reaction at the anode is determined by the pH, which in turn depends on the electrolyte and the temperature. As the anodization progresses, the growing oxide layer is also dissolved by the acid (reverse reaction of Reaction 3.1), leading to formation of pores as shown in Fig. 4b. Thus the acid action is balanced by oxidation rate to form microscopic pores, 10–150 nm in diameter and lengths up to several microns. The bottom of each pore also consists of thin (10–100 nm thick) oxide barrier layer over the metallic Al surface. Also pores are formed only when Al anodization is done under acidic onditions (pH < 5) using acids such as H₂SO₄, H₂C₂O4, and H₃PO₄.

Fig. 4. Simplifed schematics illustrating (a) an anodization process and (b) a nanoporous alumina channel.

2.1.1.2. Deposition Methods:

There are various methods for deposition of materials inside, outside, or on the templates for creating materials of choice in 1-D form. These methods include electrodeposition, sol-gel, CVD, chemical precipitation, or polymerization reactions, electroless deposition, etc. Each deposition method works better with a type of template for a certain type of material system.

Electrochemical methods:

In electrochemical template synthesis approaches, the working electrode (cathode, where deposition is done) consists of a prefabricated template or an electroactive support for growing nanowires and the electrodeposition is carried out in a standard three-electrode electrochemical cell with a counter electrode and a reference electrode. The cathode or working electrode is connected to the negative terminal of an external direct current power supply and anode to the positive terminal. Both these electrodes are immersed in the electrolyte that contains the metal ions to be deposited on the cathode during the experiment. The deposition starts from the base of the electrode resulting in well-defined structures through the templates as nanowires.

Sol-gel synthesis:

The sol-gel template-based nanowire synthesis method relies on the capillary action (or electrophoresis) of the pores in the template to fill the pores with sol particles. In a simple process, as shown in Fig. 5, the template is dipped directly into the relevant sol (containing the precursor material) and heated for some time. Sols are the colloidal suspension of solid particles in a liquid dispersion. Sols are formed when the starting metal salt or precursor is processed through a series of reactions (e.g., hydrolysis) to form a colloidal suspension (the sol). Gels are the integrated networks (semisolid and liquid) comprising the sols along with the other liquid-phase solution ingredients formed by condensation reaction. Sol–gel process involves the transition of a system from a liquid sol to solid wet gel phase. The method is based on the hydrolysis and condensation reactions of precursors. The commonly used template is porous AAM. The pores of the template are filled slowly by compact stacking of the sol nanoparticles. If the deposition time is short, the particles deposit first on the walls, leaving voids in the centre eventually making nanotubes (Fig. 5a), whereas longer deposition time results in nanowires (Fig. 5b). The final product is obtained after a thermal treatment to remove the gel. Alternatively sol-gel electrophoresis can also be used where the template pores are filled by movement of sols under an applied electric field.

Fig. 5. Schematic illustrating various stages involved in nanowire/nanotube formation using sol–gel synthesis within templates: (a) deposition over shorter time results in tubular structures; and (b) deposition over longer duration results in nanowire structures.

Comparison of deposition methods based on the templates: In a positive template method, nanowire diameter can be controlled by controlling the amount of material deposited, but the length is not controlled. In negative template method, nanowire length can be controlled, but not its diameter, which is predetermined by the pore diameter.

2.2. Vapor-Phase Techniques:

Vapor-phase synthesis is the primary method for synthesizing nanowires in a number of material systems. Several vapor-phase methods have been developed to an advanced stage compared to liquid-phase methods. These methods can be broadly divided in to two categories: substrate-based and substrate-free direct methods.

Fig. 6. Schematics of two general methods used for vapor-phase synthesis of nanowires: (a) substratebased method using a horizontal chemical vapor deposition reactor; and (b) direct gas-phase nanowire synthesis method.

Substrate-based methods:

Fig. 6a represents a typical substrate-based approach where the substrate is placed downstream to the source. The reactor operating under vacuum is heated and the source vapors react to produce the responsible growth species, which then deposit onto the catalyst supported by the substrate. The substrate has been found to be essential for nanowire synthesis in the vapor-phase methods for the following reasons: (1) the substrate supports the catalyst clusters and allows control of cluster temperature and (2) induces preferential precipitation at substrate–catalyst cluster interface. So the use of a substrate is essential for maintaining 1-D growth in vapor-phase methods involving catalyst clusters.

Substrate-free methods:

There have been very few direct gas-phase (substrate free) methods developed to synthesize nanowires. Fig. 6b shows a simplified schematic of a vertical reactor where a microwave plasma discharge (reaction zone) generated by passing reactive gases is confined inside a vertical quartz tube. The source material is allowed to fall under gravity and it is collected in a cup. The key advantage of this approach over a substratebased method is bulk production. Since the use of a substrate limits the amount of nanowire which can be produced, this method can theoretically yield bulk quantities of nanowires.

2.2.1. Pulse Laser Deposition Techniques:

Pulse Laser deposition or laser ablation is another low-pressure, high-energetic, nonequilibrium, rapid condensation process for producing nanowires. The schematic of a typical laser ablation setup is shown in Fig. 7.

Fig. 7. Schematic of a typical reactor setup used for laser ablation.

The setup is almost similar to that for thermal evaporation, except there is a port for introducing the laser beam and a target containing the source in the middle of the quartz tube. The laser, when it hits the target, generates a plume containing vapour fragments of the target. The target is made by pressing the source materials and the catalyst metal together into solid cylindrical pellets. The chamber temperature is maintained using the tube furnace at a level needed to sustain the catalyst-molten alloy or for the nanowire growth (typically between 800°C and 1200°C). The variables in this method include laser power, target composition, gas-phase composition, temperature, pressure, residence time of the vapor-phase species, and cold fnger conditions. Laser ablation in general is not a scalable process and therefore is suitable for laboratory investigation. A large range of semiconductor compounds such as binary III–V nanowires (GaAs, GaP, InP), binary II–VI nanowires (ZnS, ZnSe, CdSe), ternary III–V nanowires (GaAsP, InAsP), alloys (SiGe) have been synthesized by them. Similarly, metal oxides nanowires of Fe, Zn, Sn, Ga, etc., metal nanowires (e.g., B, Si, Ge etc.) have been obtained. Si nanowires have been produced using Si–Fe targets with a neodymium: yttriumaluminum-garnet (Nd:YAG) laser irradiation (2.0 W average power, 10 Hz pulse rate, λ 532 nm) in 50 sccm of Ar flow. The Si0.9Fe0.1 target was ablated to create the source vapors and the nanowires were deposited on the cold finger while the tube furnace was maintained at 1200°C. The resulting silicon nanowires were 10 nm thick and several microns long and consisted of Fe-rich clusters at their tips.

2.2.2. Thermal Evaporation Techniques:

A typical reactor setup used for thermal evaporation contains a horizontal tube furnace with provisions for gas flow, and pressure gauge with or without control as shown in a simplified schematic in Fig. 8. The furnace may have a single, two, or three heating zones. More than one zone allows a hotter source generation zone with a relatively cooler growth zone. Note that a vertical reactors with similar setup can also be used.

Fig. 8. Schematic of a typical reactor setup used for thermal evaporation.

The experiments are typically carried out at low pressures (achieved using a mechanical pump) so that the reactions over catalysts can be selective. Low-pressure operation can also help to reduce oxygen contamination whenever it is critical. The typical operating pressure is about 1–100 Torr. The desired source material to be heated is placed in an alumina crucible. The crucible material should have high melting point, good chemical stability, and hardness so that it can withstand high temperatures and chemical corrosion. The tube furnace heats up the source through conduction, gas convection, and radiation. Alternatively, the source material can also be heated by resistive heating of the crucible. It is important to bake and purge the reactor before each experiment to remove adsorbed gases from reactor walls and reduce contamination and oxygen partial pressure. The source gases diluted in an inert gas such as argon are used. The pure source vapors produced from thermal evaporation (e.g., ZnO vapor) or source vapor mixed with other feed gases (e.g., in vapor mixed with NH3 for InN growth) react on the substrate kept at a distance from the source boat. Note that the source materials do need to be at very high temperatures close to their melting, decomposition, or sublimation points to produce enough vapors. In downstream, there can be a provision for cold finger or a substrate where temperature is lower. This is the reason dual zone furnaces are preferred. Due to low pressures used, the nanowire growth occurs over a prolonged deposition time (few hours) if long nanowires are desired. Typically, the nanowire products are collected near the downstream end of the tube furnace. Several different types of growth can be conducted using this reactor, for example, catalyst-assisted and self-catalysed. Also, one can easily produce vertical arrays, epitaxial arrays on single crystal substrates, as well as randomly oriented nanowire films. In the case of catalyst-assisted growth, substrates coated with catalyst layers are placed downstream. In the case of self-catalysed schemes, the low-melting metal clusters are generated in situ on substrates during evaporation of solid oxide sources. A wide variety of nanowires such as metal oxides, metal sulphides, and semiconductor materials have been synthesized using thermal evaporation. This approach is most widely used to prepare metal oxides nanowires. Almost all kinds of metal oxides nanowires (both low-melting and high-melting metals) can be prepared by passing O2 over the metals in a horizontal tubular reactor. For example, SnO2 nanowires were produced from SnO powder using self-catalysis technique. SnO powder was heated at 680°C for 8 h in an alumina crucible, keeping the total pressure at 200 Torr and passing argon and oxygen gases. ZnO nanowires were produced by heating Zn powder at 850°C–900°C for 30 min and using Au-coated sapphire substrate.

3. Summary:

• In this module, you study
• Basic of 1D nanostructure: nanowire.
• Various synthesis approaches for the growth of nanowire. Different liquid and vapor phase techniques.