Growth Of Silicon Nanowires

Published Date: 02 Nov 2017

Student Name: Febin Paul

Student ID : P12212927

Submitted in partial fulfilment of the module : Nanomaterials and Nanoelectronics

Submitted to : Dr. Shashi Paul

LIST OF CONTENTS

CONTENTS PAGE NO.

Abstract

Aim and Objectives:

Introduction

Experimental Procedure

An overview

4.1.1 Silicon Nanowire Synthesis

4.1.2 High Temperature Chemical Vapour Deposition (PECVD)

4.1.3 Low Temperature Chemical Vapour Deposition (PECVD)

4.1.4 Plasma Enhanced Chemical Vapour Deposition (PECVD)

4.1.5 Catalyst Deposition by Thermal Evaporation

4.2 Experimental Procedures

Analysis, Result and Discussion

Catalyst Material Analysis: An Introduction

5.1.1 Gold- the fallen angel

5.1.2 A Quest for Alternate Catalyst Materials

5.1.2.1 A general overview

5.2 Alternate Catalyst Materials: Result and Analysis

5.2.1 Nickel Oxide

5.2.2 Copper

5.2.3 Gallium

5.2.4 Silver

5.2.5 Aluminium

Conclusion

1. Abstract

Silicon Nanowires have been the hot topic of research round the globe for over a decade now. These low dimensional nanostructures behave very differently from their bulk material predecessors. Many quantum phenomenon become prominent at such small scale, and because of these many peculiar effects can be observed in Nanowires. Among all the techniques by which Silicon Nanowires can be made, the Plasma Enhanced Chemical Vapor Deposition (PECVD) technique is the most popular. This review is intended to discuss the growth and properties of Silicon Nanowires (SiNWs) by using different catalyst in the PECVD technique. Later the experimental results are analysed to explain the differences in each of the cases and finally trying to point out an optimum catalyst for better control and growth of the SiNWs.

2. Aim and Objectives:

The aim of the report is to get an overall idea about all the processes involved in the synthesis of the Silicon Nanowires. This includes getting familiar with the devices and the operations, and the concepts upon which the operation is based on. The objectives include:

Depositing different metal catalyst on the substrate by the method of thermal evaporation.

Synthesis of the Silicon Nanowires (SiNWs) by PECVD method.

Critically analysing the samples for growth of the SiNWs by SEM.

Investigating the electronic properties by measuring the Optical Bandgap by UV Spectroscopy.

3. Introduction

With the rapid development in the semiconductor and the electronic industry, we have witnessed the development of some incredibly fast and high performance device. With the advent of the sleek electronic devices, it has become very important for the designers to make the fundamental devices much smaller. But due to scaling constraints, the bulk Silicon cannot be miniaturised further. So, the quest for creating devices using lower dimensional materials like Carbon Nanotubes (CNT) and SiNWs have been researched and investigated a lot for the last few years. Though nanotubes have been a tough contender, but it could not go beyond the territories of the laboratory due to the constraints in reproducibility and placement. This has given the Nanowires an advantage over the other. The researches for the optimized growth for the Nanowires have been advancing so that the growth and the properties of the nanowires can be controlled. Due to these, Silicon Nanowires are expected to play a very important role in the future devices and optoelectronics. The growth of SiNWs using different catalyst materials have been investigated in this assignment. The bandgap of the nanowires were also investigated and tried to be explained in this assignment.

4. Experimental Procedure

4.1 An overview

4.1.1 Silicon Nanowire Synthesis

There are various techniques of growing Silicon nanowires, the Vapour- Liquid- Solid (VLS) and Vapour Solid Solid (VSS) technique being just two of them. The technique by which they are synthesized depends on the application for which they need to be employed. The Chemical Vapour Deposition (CVD) is a very important method which employs the VLS mechanism for the growth of the nanowires. The VLS method is known so, because of the state of the phase of the participating materials. The substrate, on which the catalyst is deposited, is in Solid state. The catalyst itself melts at the eutectic temperature, and therefore is in the Liquid state and thirdly, the Silicon being supplied is in Vapour state. The VLS method involves the interaction of all these material, even if all of them are in different phase. On the other had VSS technique deals with only two phases- Solid and Liquid. In this method the catalyst is still not in molten state, so it is in Solid phase as is the substrate also, whereas the interacting Silicon is in Vapour phase. Many metals like aluminium and zinc are known to form SiNW by the VSS technique. In this report, the CVD method will be investigated in detail. Later the CVD will be compared with the growth by PECVD method, and analysed why PECVD is better in many ways than CVD. Later the nanowires grown by this technique will be analysed to investigate its electronic properties.

In Chemical Vapour Deposition, the silicon needed for the nanowire growth is obtained from a precursor, which is cracked by increased temperature. At certain temperature these compounds crack and silicon gets isolated, which settle on the catalyst on the surface of the substrate. When the catalyst reaches the ‘eutectic temperature’ it melts and become a droplet, at which the silicon starts dissolving. After sometime, the droplet of catalyst reaches the saturation point. Anymore silicon dissolving in it, will result in a precipitation of the silicon out of the droplet. Consequently, the progressive precipitation of silicon out of the drop of catalyst results in the formation nanowires. The figure below shows the process of CVD.

Figure 1: The graphical representation of CVD.1

This technique of growing nanowires, was first proposed by Wagner and Elllis in their seminal article published back in March 1964.2 Different types of precursors are used depending upon the type of CVD growth is being employed. The CVD growth can be broadly classified into two classes depending upon the temperature being used: High Temperature CVD and Low Temperature CVD.3 In both the cases, the growth of the nanowires depend on different parameters and can thus be used to optimise different aspects of the dimensions of the nanowire like the diameter4, 5 or length6.

4.1.2 High Temperature Chemical Vapour Deposition (PECVD)

The high temperature CVD, as the name suggests employs high temperature for the growth of nanowires. The process temperature affect many other factors, like the choice of the catalyst to be used or the precursor to be used. The high temperature being referred to, is defined as any temperature higher than 700 degree Celsius3. The most commonly used precursors for CVD are Silane (SiH4), Disilane (Si2H6), Dichlorosilane (SiH2) and Tetrachlorosilane (SiCl4). Since the halogens are much electronegative than the Hydrogen, replacing a Hydrogen by Chlorine in the compound would mean greater stability. Meaning that it would require greater temperature to crack compared to its Hydrogen counterparts. For Tetrachlorosilane, the growth temperature range from 800 degree Celsius7-9 to beyond 1000 degree Celsius11. Due to this reason, the high temperature CVD uses Tetrachlorosilane and Dichlorosilane as precursor for the growth process. Another advantage of using the above precursors is that, if supplied with Hydrogen, it produces HCl during the process, which helps etch away unwanted oxide during the process.

Like every thermally activated process, the SiNW growth using Au catalyst also shows an exponential growth with increase in temperature. Due to this exponential dependence, the growth rates can be quite substantial at such high temperature. A growth rate of um/min7, 10, or even um/sec11 has been observed using high temperature CVD. But such high growth rate is attained only at the cost of the controllability of the process at sub- micrometre lengths of nanowires.

But a problem faced in high temperature CVD is something called the Ostwald Rippening12-16. It is usually observed that at high temperature the catalyst material also starts migrating on the substrate surface. The smaller droplets of catalysts thus combine together to form bigger drops. This is particularly undesirable because it disturbs the alignment and the size of the nanowires.7 It then becomes very difficult to align the catalyst in particular pattern.

4.1.3 Low Temperature Chemical Vapour Deposition (PECVD)

The name is derived from the process temperature used for the growth of nanowires. The temperature for low temperature is below 700 degree Celsius3. At lower temperature, the energy supplied to the system is also comparatively less. So the typical precursor for low temperature CVD is Silane (SiH4) and Disilane (Si2H6). They are less stable, and so need less temperature to crack. In contrast to Tetrachlorosilane, Silane decomposes at 350 degree Celsius.

At such low temperature, the dependence of growth rate on the temperature changes. In this process, the growth rate is highly dependent on the supply of the precursor gases. According to the data published by Lew et al,17 the growth of the nanowires for similar temperature were linearly dependent on the supply of the Silane. The nanowire growth with Silane as the precursor shows an exponential dependence on the inverse temperature. Another important phenomenon observed in the low temperature CVD is that the radial growth is less compared to the axial growth. In fact, the radial growth is about 2 orders of magnitude less than the axial growth. Due to this the wire gets tapered at it grows taller. This property has been employed effectively to grow nanocones.18 The illustration below shows the cracking of the precursor and the deposition of Silicon on the surface of the substrate.

Figure 2: Fig (a) shows the nucleation the Silicon from the precursor and deposition on the catalyst in low temperature CVD. Fig (b) and (c) shows the tapering of the nanowires due to increase rate of axial growth compared to the radial growth in low temperature CVD. Due to this the nanostructures shape up like nanocones.18

4.1.4 Plasma Enhanced Chemical Vapour Deposition (PECVD)

From the above discussion we compared the low temperature CVD and the high temperature CVD. Though the low temperature CVD can be used to grow nanowires, the application is restricted to the use of precursors. This is because not every precursor would crack at such low temperature. It is due to this reason, that the PECVD has been developed. In this method, the precursor gas attained the energy from Radio Frequency power which is supplied to it. This electrical power energizes the precursor and breaks it into the respective elemental components and ions and radicals, which then settles on the substrate to form nanowires. The table below shows the difference in the temperature at which different precursors crack, for CVD and PECVD.

Film

Precursors

CVD

PECVD

Silicon nitride

SiH4 or SiH2Cl2 and NH3

750 C

200-500 C

Silicon dioxide

SiH4 and O2 [or often N2O]

350-550 C

200-400 C

TEOS (Tetraethyl orthosilicate, Si(OC2 H 5)4)and O2

700-900 C

300-500 C

Amorphous silicon

SiH4

550-650 C

200-400 C

Table 1: The table shows the difference in temperature of growth in CVD and PECVD.

Due to the alternate supply of energy, the growth of nanowires become possible at a little above room temperature.* But this doesn’t mean that the overall growth process takes place in a low temperature environment, the bombarding plasma itself raises the temperature of the substrate surface to a considerable level.* The PECVD chamber operates on the same principle as that of the CVD, in which a precursor is cracked and the cracked silicon deposits on the substrate. But, in PECVD, the substrate is kept between the electrodes across which the RF signal is applied. The plasma is produced above the sample and the precursor is supplied to the chambers from centre of the system. The figure below show the insides of a PECVD chamber.

Figure 3: The figure shows the cross sectional view of a PECVD chamber.

The Plasma produced gives the silicon enough energy for it to traverse on the surface of substrate, just like in the High Temperature CVD. This increases the conformality of the layer as compared to the Low Temperature CVD. The PECVD has the advantages of both the High Temperature and Low Temperature CVD. Not only is the deposited layer more conformal, but the quality of the layer is also very fine. The quality of the layer can be adjusted using the RF power adjustment. This is because the layer quality can be adjusted by the ion bombardment energy. When the ions bombard the surface at greater energy, the denser the deposited film becomes. But this is true only to certain extent. If the RF power is increased beyond this optimum energy, the ions acquire such high energy that is starts knocking off the particles from the surface of the substrate. The radicals starts knocking off the catalyst as in the case of sputtering.* This optimum energy depends on the layer of substance being deposited.

4.1.5 Catalyst Deposition by Thermal Evaporation

The catalyst material can be deposited using different techniques depending upon the nature of the catalyst being used. The most common method of deposition is by Evaporation method. The method involves heating the metal sample with the help of high current. When the catalyst material gets heated then it turns into vapour. This vapour then gets deposited on the substrate, which is kept at a distance, but in the field of vision of the catalyst material. The metal is either kept in a boat, which in turn is heated, or the metal in itself carries the current and gets heated. The entire apparatus is kept under high vacuum condition. This is because high vacuum condition increases the mean free path of the metal catalyst. With the increase in the mean free path, the probability of more number of catalyst molecule reaching the substrate substantially increases. But the deposition rate in this method depends upon the vapour pressure of the material being evaporated. This leads to a big limitation to this method. The evaporation method cannot be used to evaporate different compound molecules. This is because the vapour pressure of the elements in the compound may not be the same. If it differ too much then it would result in the deposition of a material with stoichiometric composition that is entirely different to that of the material that was to be evaporated. Due to this compound material are deposited using a technique called Sputtering. In sputtering, the material to be deposited is bombarded by heavy and inert molecule like Argon. When the Argon molecules bombard the sample at such high energy, the sample gets ripped off from the surface and then gets deposited on the substrate. In all the experiments conducted for the purpose of this report, evaporation was used to deposit the catalyst onto the substrate. Even in the case of Nickel Oxide (NiO), evaporation was used due to similar vapour pressure of Nickel and Oxygen.

4.2 Experimental Procedure

This section consists of the actual procedures conducted during the experiment, based on the merits of the concepts explained above. The substrates are first prepared for the PECVD method. For this, every substrate is coated with a layer of catalyst. The deposition is done by Thermal Evaporation method. Using this technique we deposit Aluminium (Al), Gallium (Ga), Silver (Ag), Copper (Cu) and Nickel Oxide (NiO). A 20 nm thick layer is deposited using this technique. To begin with the evaporator chamber pressure is reduced to as low as 10-7 Torr. Then the metal catalyst is evaporated onto the substrate. The thickness of film being evaporated is measured in- situ using a piezo electric sensor, which detects the thickness of film depending on the phase difference in the resonant frequency at which it pulses.

Later this substrate is kept in the PECVD chamber, and silane, which is used as the precursor is passed into the chamber. The temperature is pre-set to 400 degree Celsius. Special care is taken to maintain proper vacuum. This is very important especially in case of highly reactive metals like Aluminium, which oxidises quickly when exposed to Oxygen. The chamber is first flushed using Hydrogen to prevent any lingering unwanted gases. This is done for 5 minutes. After this the precursor is slowly supplied to the substrate, and the RF power is set to 25 W. During this time the precursor split as per the chemical equation below to form Silicon, which settles on the substrate to form SiNWs.

SiH4(gas) Decomposition [SiHm] Si + m[H]

Special care must be taken to match the impedance of the PECVD chamber . If not done then there would be a loss in power due to the impedance mismatch, thereby the PECVD not receiving the entire 25W to produce plasma.

These samples are then checked using UV spectrometer for the absorption pattern of the sample. This absorption pattern is then used to calculate the optical band gap.

5. Analysis, Result and Discussion

5.1 Catalyst Material Analysis: an Introduction

5.1.1 Gold- the fallen angel

Gold (Au) has been studied as a catalyst material since the publication of Wagner and Ellis.2 It has been a hot topic for a long time and almost every aspect of its behaviour has been studied by many people during the course of time. Though the study of Gold as a catalyst is not the focus of the report, it is still important to know some of the key features of Gold. What makes Au such a favourite material for the study of nanowires? One of the main and trivial reasons is because of its chemical stability. The chemical stability means Au is stable against oxidation in air. This helps in the handling of the metal. Another advantage of using Gold is that the eutectic temperature of Gold is 363 degree Celsius(19 atom% Si).3 This temperature is about 700 degrees less than the melting point of Au. Such low eutectic temperature also means that Au is a good nomination for growing low temperature nanowires. It is also non- toxic, which helps in the handling of the material.

But all of the above advantages is weighed down by one serious problem of gold. Gold is known to contaminate the nanowires19-21. At high temperature, gold diffuses into the nanowires and creates defects in Silicon, which leads to strong and enhanced charge recombination. This means that the lifetime of a released charge in the SiNW decreases greatly due to the capturing of the released charges by the defects states generated due to the contamination of Gold. Due to this problem it becomes nearly impossible to use the Gold catalysed SiNW for a practical application in the semiconductor industries. This problem is intensified by the fact that it is chemically very stable, which makes the cleaning very difficult. All of these above mentioned problems have forced us to explore and analyse the performance of other material as catalyst for the nanowire growth.

5.1.2 A Quest for Alternate Catalyst Materials

5.1.2.1 A General Overview

The downfall of Gold as a catalyst has led to the exploration of different elements and their performance as a catalyst for the SiNW synthesis. Please note that the contamination problem of Gold does not render it useless. It merely means that it cannot be used for any application that relies on the energy level of the nanowires for its operation, like in MOSFET or solar cells. This report intends to investigate the property of a few of the metals as a catalyst by trying to grow nanowires. The metals are Aluminium (Al), Silver (Ag), Gallium (Ga), Copper (Cu) and Nickel Oxide (NiO).

The behaviour of a metal catalyst in the presence of silicon with respect to the change in temperature can be studied using a graph called Phase Diagram. A phase diagram in a narrow sense, shows the change in phases of the catalyst material and the solubility of Silicon in it at different temperatures and different percentages of Silicon. So it helps us understand many strange behaviour of the catalytic metals like eutectic temperature of the catalyst being lower than the melting point of both participating elements, which in our case is the catalyst metals and Silicon. It also explains the synthesis of Al-catalysed SiNW at 540 degree Celsius, which is 40 K below the eutectic temperature of the Al- Si system22. The graphs below shows the phase diagram of various metals with Silicon.

Figure 4: Schematic phase diagrams of different metal catalyst with Si systems.(a) Au-Si, (b) Al-Si, (c) Ag-Si, (d) Zn-Si, (e) Ti-Si, (f) Pd-Si.3

Though there are many other metals which can be used for the synthesis of nanowires, many of them have the similar properties. The catalytic metals have been group into three different classes based on the phase diagram behaviour with Silicon. They are grouped as Type A, B and C catalysts.

Type A catalyst are similar to that of Gold. They have a simple phase diagram and have only a single eutectic temperature. They have the eutectic temperature at more than 10% silicon and they do not form metal- silicides. There are three metals in this class- Gold (Au), Silver (Ag) and Aluminium (Al).3

Type B catalysts, like Type A metals have a predominant single eutectic point. They also do not have the metal- silicide phase. But unlike Type A, these metals have the eutectic temperature at less than 1% silicon. There are three metals in this class- Indium (In), Gallium (Ga), or Zinc (Zn).3

Type C catalysts have a complex phase diagram with many eutectic points. They form metal- silicide and have more than one eutectic temperature. Also the least eutectic temperature is more than 800 degree Celsius. The metals in the Type C is Copper (Cu), Platinum (Pt), or Titanium (Ti).3

5.2 Alternate Catalyst Materials:

This section is intended to dig deep into the analysis of alternate catalyst materials based on the experiments conducted prior and result we got from each of the catalysts. We will analyse these catalyst based on the criteria mentioned and explained briefly in the previous section. Each of the catalysts have their own advantages and disadvantages, sometimes the later outweighing the former. The metals in particular that we will analyse further are Aluminium (Al), Silver (Ag), Gallium (Ga), Copper (Cu) and Nickel Oxide (NiO). We will also investigate the physical and electronic property of the nanowires that we obtain from each of these catalysts, by analysing the optical bandgap and the type of orientation of the nanowires (for example, crystalline or amorphous) by the absorption spectrum we obtain from the UV spectroscopy.

5.2.1 Aluminium: Aluminium is the other that has a strikingly similar phase diagram to that of Au. It has a eutectic temperature a little higher (577 degree Celsius) than that of Au, but even then it looks like that of the phase diagram of Au. There have been work reported of Al catalysed crystalline nanowire growth by CVD using temperature between 580-700 degree Celsius.23 But later some work done by Wang et al showed similar results but at a much lower temperature of around 540 degree C 24-26 and later also at 430-490 degree Celsius.22 This claim of growth by VLS method was later on questioned as this temperature is a lot lower than the eutectic temperature of aluminium by VLS method. So, it later that Wang et al 22 pointed out that the growth was not VLS, but VSS. The little pocket that exist in the lhs of the phase diagram shown in the figure shows that Silicon would dissolve in Aluminium at much lower temperature, provided proper Silicon pressure in maintained. In this region 1% Silicon can dissolve in Aluminium. Though the solubility of silicon is less, this is advantageous for some applications. It is particularly useful for the synthesis of axial Si-Ge heterostuctures. This is because for the synthesis of axial heterostructures, a sharp transition of precipitated material is expected. But when synthesized using Gold, this is not possible. One usually observes a gradual gradient instead of a sharp change, because of the solubility of silicon in Au.

Figure 5: Binary Phase diagram of Al- Si system

Another major advantage of aluminium is that, it does not contaminate the nanostructures. In fact when the level of defects were analysed, it was found that aluminium acts as a p- type dopant in the nanowires. The prospect of doping the nanowires even without having to use the vapour- phase dopant is particularly attractive.

However aluminium has one drawback because of its high chemical reactivity. Because of this it oxidises quickly. Care has to be taken at each point that the catalyst do not oxidise. In all our experiments, we tried to synthesize nanowires using PECVD method at 400 degree C, which can be within the VSS section of the phase diagram. Though we did not witness any growth of nanowires, we did observe presence of amorphous silicon from the UV spectroscopy. The absence of nanowires can be due to the lack of time, as the VSS method is slow and needs time for nanowire growth. The figure below shows the absorption spectrum of Al- catalysed substrate. As we can see that the optical bandgap calculated by the Tauc’s relation comes up to 1.4eV, which is near the range of amorphous Silicon.

Figure 6: The graph showing the UV spectrum of Aluminium.

5.2.2 Silver: Ag is another Type A catalyst which has similar phase diagram as that of Au. It has a rather high eutectic point (11 atom % Si at 836 degree C). There have been reports, where single crystalline Ag catalysed SiNWs have been produced at 950- 1050 degree Celsius using CVD method.27 Also it has been found that nanowires are found at as low as 650 degree C28, showing the presence of VSS growth. Later the phase diagram had to be revised to accommodate the pocket of VSS growth phase at the lhs, similar to that of aluminium29.

Thus the Ag- Si phase diagram seems very similar to that of the Al-Si phase diagram, except that the eutectic point of silver is at very high temperature, and also the VSS phase shows very low solubility of Silicon in Silver. Yet the results that have been obtained seem very promising in the VSS phase. This is again supported by the fact that the Ag induced defects are perfectly placed. It is neither very near to the centre, nor to the conduction or the valence band.

Figure7: Phase Diagram of Ag- Si binary system

In our experiment, we didn’t find any presence of nanowires. This is prominently because the temperature at which the growth was tried (400 degree C) was much lower than the eutectic temperature of Ag. Though it can be well within the pocket of VSS eutectic temperature of Ag (650 degree C).3 The absorption spectrum obtained from the UV spectroscopy shows the presence of amorphous silicon. So it can be suspected that there would have been a little growth of the nanowire by VSS method. We could also observe the agglomeration of the catalyst. This is seen in the image taken by TEM which is shown below.

Figure 8: The TEM image of Ag catalysed sample for the growth of SiNWs.

Figure 9: The graph showing the UV absorption spectrum of silver catalysed sample after putting it in PECVD chamber.

Figure 9 : The optical absorption spectrum of Ga catalysed sample

5.2.3 Gallium: Gallium is a Type 2 catalyst, which has very low solubility. But the most attractive part of Ga is that it has a low eutectic point (less than 0.01 atom %) and a eutectic temperature (30 degree C). This is the most exciting part of this metal, because this means that it would give good results at very low temperatures. This is supported by the fact that it has very low vapour pressure (10-7 mbar). It means that Ga would not just evaporate, but stick to the substrate and facilitate the formation of SiNWs. This fact has been confirmed when conical NWs were synthesized at very high temperature.30

Even in our experiments, we got really good results with Ga, where we witnessed high NW growth. The NWs were really long. Some of them grew as long as approximately 3um. This can be seen in the image. But as the image shows the NWs are not crystalline in nature. This suspicion is confirmed by the absorption spectrum which shows that the optical band gap of the Silicon is in the range of being amorphous.

Figure 10: The TEM showing the growth of nanowires in Ga catalysed sample at 400 degree Celsius.

Figure 11: The optical absorption spectrum of Ga catalysed sample.

5.2.4 Copper: Copper is a Type C catalyst with a very high eutectic temperature(802 degree C). It produces SiNWs of the grade similar to that of Au. It is also an important catalyst. Since Cu has been used in the interconnects of ICs already, it would be really compatible with the CMOS technology. But for the experiments we conducted, we got no results because the PECVD temperature was way below the eutectic temperature of Copper.

5.2.5 Nickel Oxide:The use of a metal oxide for the use of catalyst has not been known much. But Nickel has been known to produce NWs equivalent to the grade of Au. Nickel has a eutectic temperature of 993 degree C by VLS method. Also Nickel produces silicides, due to which it is grouped under Type C. These Ni silicides are very favourable because they are already used in the electronics for electrical contacts.

The absorption spectrum shows the presence of amorphous silicon due to the optical bandgap being more than 1.7 eV. But the TEM images show no growth of nanowires whatsoever.This might be due to the reason that the RF power must be so high enough to knock off the catayst material.

6. Conclusion: The growths of nanostructures have been discussed in detail in this report. As a final word it can be said that the growth of nanowires by PECVD can be optimised. It may be a Herculean task, but with proper study of the method and the phase diagram for different metal, the process can be conducting with more control. To summarize this section, the type-C catalysts work well, but only in the VLS growth mode, i.e. at high temperatures. At lower temperatures, where silicide-catalyzed VSS growth prevails, problems with the crystalline quality of the wires arise. The type-B catalysts such as In and Ga work, but only under rather harsh experimental conditions. Compared to In or Ga, growth using Zn seems to be easier, but there is no big advantage of Zn compared to Au, except for the contamination removal. Thus, in the end, for low-temperature

processes, everything boils down again to the use of the three type-A catalysts, Al, Au, and possibly Ag.