Investigation of au and in as solvents for the growth of silicon nanowires on si(1 1 1)

Đây là một bài báo khoa học về dây nano silic trong lĩnh vực nghiên cứu công nghệ nano dành cho những người nghiên cứu sâu về vật lý và khoa học vật liệu.Tài liệu có thể dùng tham khảo cho sinh viên các nghành vật lý và công nghệ có đam mê về khoa học

Physica E 40 (2008) 2462–2467

Investigation of Au and In as solvents for the growth

of silicon nanowires on Si(1 1 1) Andrea Kramer  , Torsten Boeck, Peter Schramm, Roberto Fornari

Institute for Crystal Growth, Berlin 12489, Germany Available online 14 February 2008

Abstract

This paper reports on the bahavior of Au and In as solvents for the growth of silicon nanowires on a Si(1 1 1) substrate via vapor–liquid–solid (VLS) mechanism Gold is the mostly used solvent for growing silicon nanowires but in the present work indium was also applied, as it may bring some advantages for later electronic application of the wires

The main focus of this work is the behavior of gold and indium on a silicon substrate but also the different morphologies and distributions of the grown wires are compared

Individual metal droplets have been located in pre-structured nanopores to serve as starting points for wire growth The method used

to exactly position the metal droplets and thus obtain a regular arrangement of nanowires is also presented

r2008 Elsevier B.V All rights reserved

PACS: 62.23.Hj; 68.03.Cd; 68.08.Bc; 81.16.Rf

Keywords: Nanostructures; Silicon; Physical vapor deposition; Vapor–liquid–solid mechanism; Gold; Indium; Surface tension; Surface energy; Solubility; Focused ion beam structuring

1 Introduction

Nanowire-based devices are of great interest in diverse

areas ranging from electronics, optoelectronics and sensor

components to biotechnology [1–3] Among different

fabrication methods for nanowires, chemical vapor

deposi-tion (CVD) and physical vapor deposideposi-tion (PVD) are the

most widely applied The experimental conditions depend

not only on the growth method but also on the chosen

nanowire material[4–6] Common aim of all approaches is

a perfect control of wire growth by experimental

para-meters and a possibility to position the nanowires which is

essential for most of the applications

In this work, the investigation of Au and In as solvents

for the growth of silicon nanowires on Si(1 1 1) via PVD by

means of the well-known vapor–liquid–solid (VLS)

me-chanism will be presented Silicon nanowires are mostly

grown from gold droplets It is still a controversial issue

how gold is incorporated into the wire and thus how

it influences the electronic properties of the wire Gold

is a deep-level defect in bulk silicon and if this is also true for nanowires grown from gold droplets, an alternative metal for the growth would be necessary For this reason, apart from gold we also tried indium as solvent for the growth

2 Experimental

In all our experiments, Si(1 1 1) substrates were initially cleaned by an RCA standard process[7]in order to remove organic contaminations The substrate was dipped into an

HF (40%, w/v):H2O solution at a ratio of 1:5 to remove the native oxide from the silicon surface before inserting it into the ultra-high vacuum (UHV) chamber where the growth process took place The nanowire growth procedure consisted of three steps: the first one was the desorption

of residual oxide at a substrate temperature of 850 1C, the second one was metal evaporation from an effusion cell at

a substrate temperature of 550 1C in order to form droplets

on the substrate, and the last step was the evaporation of silicon at the same substrate temperature and at a rate of

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1386-9477/$ - see front matter r 2008 Elsevier B.V All rights reserved.

doi: 10.1016/j.physe.2008.01.011

Corresponding author Tel.: +49 30 6392 3050.

E-mail address: kramer@ikz-berlin.de (A Kramer).

silicon evaporation appeared to be very inhomogeneous

when a desorption step had been carried out (Fig 1) Large

droplets with diameters of about 10–20 mm and with

density of about 600 mm 2, as well as many tiny indium

deposits of sizes below 100 nm, located in the free space

between the larger droplets, were observed When the

desorption step was omitted, the distribution was seen to

be much more homogeneous with droplet diameters of

about 200 nm and density of about 7.7  106mm 2(Fig 2)

The gold distribution after a desorption step was

examined by scanning electron microscopy (SEM) and

transmission electron microscopy (TEM) measurements

(Fig 3) Again, different sizes of droplets were detected

The larger ones had diameters of about 100 nm and a

density of about 1.5  106mm 2, which can be seen in the

SEM image

But in contrast to the indium experiments, the

distribu-tion of gold droplets did not change when skipping the

desorption step

Zakharov et al [8] also found an inhomogeneous

distribution of gold droplets in the range between 10 and

300 nm under comparable experimental conditions

To explain differences in droplet formation between

indium and gold, we will consider in the following the

effects of different diffusion coefficients of gold and indium

on silicon, the solubility of substrate atoms in the two

metals, the surface tension of gold and indium and the

surface energy of silicon and silicon oxide

The diffusion coefficients at temperatures around 550 1C for indium and gold on a clean Si(1 1 1) surface are 0.30 and 0.12 m2/s, respectively [9,10] They are of the same order of magnitude and thus cannot account for our very different experimental results

We believe that a thin oxide layer forms during in-sertion of the sample into the UHV chamber in spite

of the preceding HF-dip There are hints in the literature

[11] that deposition of gold onto a thin layer of SiO2on Si(1 1 1) favors the decomposition of SiO2, i.e that gold contributes to cleaning the surface This could explain why the gold distribution is the same with or without desorption step

Unfortunately, no literature data about the enhancement

of decomposition of a silicon oxide layer by indium were found From the phase diagrams In–Si and Au–Si (Fig 4),

it can be seen that the solubilities of silicon in gold and indium at our growth temperatures are 420 and o1 mol%, respectively It could be argued that also the solubilities of SiOx in gold and indium are significantly different and thus that indium does not enhance the decomposition of an oxide layer If this is actually the case, the indium distribution will then depend on whether a desorption step has been applied or not

Let us consider now the role of surface tension and surface energy of the different components of our experiment From the phase diagram Au–Si, we expect to have a liquid Au–Si alloy at our growth temperatures with

a silicon concentration of about 25 mol% For this concentration, Naidich et al [12]found a surface tension

of about 1.0 J/m2at 1500 1C No data could be found in the literature for the surface tension of indium–silicon alloys However, as the solubility of silicon in indium at our growth temperature is less than 1 mol%, we take the surface tension of pure indium as an approximation which

is 0.6 J/m2at its melting point (157 1C)[13] As the surface tension of most liquids decreases in a nearly linear fashion with increasing temperature[14], there is a wide difference between the surface tension of the Au–Si alloy and the In–Si alloy at our growth temperatures

Fig 1 SEM image of the indium droplet distribution after a desorption

step had been carried out.

Fig 2 SEM image of the indium droplet distribution when the desorption step had been skipped.

Since the state of the substrate surface after desorption

and the vacuum conditions are the same during

evapora-tion of gold and indium, the different liquid–solid–vapor

interface dynamics can be ascribed to the surface tension of

the solvent Liquids with high surface tension tend to form

droplets with a small contact area with the underlying

substrate whereas liquids with lower surface tension tend to

wet the substrate This could explain the formation of

smaller droplets in the case of gold than in the case of

indium on a bare silicon surface, i.e after desorption step for indium

Without desorption step, indium forms smaller droplets which can be explained by the different surface energies of silicon and silicon oxide The surface energy of silicon at its melting temperature (1410 1C) is 0.9 J/m2 and it decreases

in a nearly linear way with increasing temperature[13], i.e

it is higher than 0.9 J/m2at our growth temperatures As we

do not know the exact composition of the surface after inserting the sample into our growth chamber, we take data

of similar surfaces from the literature as an approximation Asay and Kim [15] expect the surface energy of a not exactly specified silicon oxide surface to be higher than 0.1 J/m2 at room temperature Janczuk and Zdziennicka

[16] determined the surface energy of quartz in the temperature range from 200 to 1000 1C and found out that it changed only slightly from 0.19 to 0.18 J/m2 This indicates that the silicon oxide surface energy is always smaller than the silicon surface energy which is not surprising if one thinks of a crystalline silicon surface and

an amorphous oxide surface This explains why the bare silicon tends to minimize the free surface by maximizing the contact area between indium and silicon This leads to larger droplets compared to those on the silicon oxide surface

The size and distribution of gold and indium droplets on the silicon surface that we observed by SEM and TEM after cooling down (Figs 1–3) may be therefore reasonably explained considering the influence of solubilities and surface energies on the mechanism of formation of droplets with or without desorption step

Silicon nanowires were obtained after silicon evapora-tion on substrates with indium and on those with gold Without desorption step, however, no wire growth from indium could be realized Furthermore, the results with indium and gold differed in direction and distribution of the wires

The sample where indium was used as solvent showed no wire growth from the large droplets, while in the space between, a not completely closed silicon layer was found

In the cavities of this layer, silicon nanowires appeared sporadically (Fig 5)

Fig 3 TEM and SEM images of the gold distribution on a sample The arrows indicate different sizes of droplets.

1500

1200

900

600

300

0

mole Si/(Si+In) 1500

1200

900

600

300

0

mole Si/(Si+Au)

Fig 4 Binary phase diagrams; top: In–Si and bottom: Au–Si.

Schmidt [17] studied, among other metals, indium as

solvent for silicon nanowire growth He applied an HF-dip

before inserting the samples into a UHV chamber but did

not apply any other cleaning steps He got a homogeneous

droplet distribution by annealing 4 nm of indium at a

growth temperature of 570 1C He did not get any

nanowires after flooding the chamber with diluted silane

and explained the absence of nanowires by considering the

surface tension of indium and the low solubility of silicon

in indium

Iacopi et al.[18]also studied indium as solvent for CVD

growth of silicon nanowires They treated the samples by

H2plasma after electrodeposition of indium nanoparticles

to reduce the surface oxidation of the metals as well as of

the substrate and they obtained in this way silicon

nanowires

Apart from CVD-based reports, no other works on the

growth of silicon nanowires from indium by means of PVD

were found

In our case, the growth of nanowires from indium seems

to be rather insensitive to change of parameters like

sub-strate temperature, rate of metal and silicon evaporation

On the other hand, very regular nanowires in [1 1 1]

direction were obtained on the samples where gold was

used as solvent (Fig 6) In this case, we found

unambig-uous correlations between experimental parameters and

grown wires: at higher substrate temperatures, larger

droplets of several 100 nm formed and led to thicker

nanowires, a higher gold evaporation rate led to smaller

distances between the wires and a higher silicon

evapora-tion rate led to a higher growth rate Wires also grew when

we did not perform the desorption step, which also

corroborates the theory that gold is able to solve a thin

surface oxide layer

There are also reports in the literature where nanowires

are grown by CVD with gold as solvent on a thick oxide

layer[19] We also performed experiments with gold on a

100 nm thick thermal oxide grown on silicon to find out

whether nanowires are growing or not However, we did

not get any nanowires Consequently, nanowire growth, at

least under our experimental conditions, is only possible on

a crystalline substrate, which again indicates that gold dissolves the thin native oxide on the ‘‘non-desorbed’’ substrate

To obtain a defined positioning of metal droplets, and thus a regular arrangement of nanowires, a reproducible process for the localization of single metal droplets in pre-structured nanopores was successfully developed in the course of this work FIB treatment was applied to silicon substrates before metal evaporation By adjusting metal evaporation rate and substrate temperature, individual indium or gold droplets formed preferentially within the pre-structured pores (Figs 7 and 8)

For indium, this was only possible when we skipped the desorption step After application of the desorption step, indium aggregates were found to be distributed randomly on the structured area, with no relation to the position of the nanopores By contrast, when we skipped the desorption step, tiny indium droplets formed in the nanopores This is in good agreement with the previous considerations about surface energies of silicon and silicon oxide The conclusion of our experiments is that it is not possible to stabilize small indium droplets for silicon nanowhisker growth by pre-structuring substrates when a desorption step is carried out

Fig 5 SEM images of a sample with indium as solvent after silicon deposition; left: overview of the sample and right: silicon nanowire grown in a cavity

of the not completely closed silicon layer.

Fig 6 SEM image of a sample with gold as solvent after silicon deposition.

On the other hand, gold droplets could be

well-distributed into nanopores after desorption

As a next step, we tried to grow silicon nanowires from

ordered gold droplets It was actually possible to grow

silicon nanowires in [1 1 1] direction from droplets which

had formed in the pre-structured nanopores (Fig 8) The

successful growth of nanowires from the droplets

em-bedded in the pores also means that the lattice planes which

had been damaged by FIB bombardment could be healed

during the growth process A recovery of the crystalline

structure around the pores is therefore possible even at the

relatively low temperatures used for the wire growth

4 Conclusions

The different behavior of gold and indium on Si(1 1 1)

has been described and analyzed An explanation based on

differences of solubilities of surface atoms in gold and

indium and on the different surface energies of the bare and

oxidized substrate as well as on the surface tension of the

liquid metal alloys has been presented

Silicon nanowires have been grown via VLS mechanism

with the use of gold and indium as solvent Indium would

be a favorable alternative for later electronic applications

Wires from indium could only be grown when an oxide

desorption step had been applied before indium and silicon

evaporation

The mechanism of wire growth from indium could not

be completely understood whereas wire growth from gold was well reproducible and could be perfectly governed by the parameters of the experiment

A method to obtain a defined arrangement of the wires was successfully developed It consisted in generating nanopores via FIB treatment on the substrate surface where metal droplets then preferentially formed Wire growth from an ordered array of gold droplets was successfully performed

Acknowledgement

The authors thank T Remmele for the TEM measure-ments

References

[1] Y Huang, C.M Lieber, Pure Appl Chem 76 (2004) 2051 [2] R Agarwal, C.M Lieber, Appl Phys A 85 (2006) 209.

Fig 7 Distribution of indium on a pre-structured substrate; top: after

desorption and bottom: without desorption.

Fig 8 SEM images of structured substrates; top: nanopores on a silicon substrate, middle: gold droplets in nanopores, and bottom: nanowires grown from gold droplets in nanopores.

[11] W Jun, C.E.J Mitchell, R.G Egdell, J.S Foord, Surf Sci 506 (2002)

66.

Solidi B 243 (2006) 3340.