Nanoscale Electrodeposition of Metals and Semiconductors From Ionic

7/30/2019 Nanoscale Electrodeposition of Metals and Semiconductors From Ionic

1/9

Nanoscale electrodeposition of metals and semiconductors from ionicliquids

W. Freyland *, C.A. Zell, S.Zein El Abedin, F. Endres

Institute of Physical Chemistry, Physical Chemistry of Condensed Matter, University of Karlsruhe, Kaiserstrasse 12, Postfach 6980, D-76128 Karlsruhe,

Germany

Received 15 October 2002; received in revised form 25 March 2003; accepted 3 April 2003

Abstract

The electrocrystallization of Ni, Co and their respective alloys with Al and the electrodeposition of Ge on Au(1 1 1) and

Si(1 1 1):H have been studied in the underpotential (UPD) and o verpotential (OPD) range. To this end, in situ electrochemical STM

and STS measurements have been performed in ionic electrolytes, the room temperature molten salts or ionic liquids AlCl3/

[BMIm]'Cl( and [BMIm]'PF6(. The larger electrochemical windows of these ionic electrolytes in comparison to aqueous media

enables the investigation of electrodeposition of these elements and compounds on a nanometer scale. We present and compare

recent results of 2D and 3D phase formation of Ni and Co electrodeposition. Clear differences are obser ved in the 2D phase

formation*/Ni monolayer vs. Co island formation*/which are discussed in the light of the distinct values of the interfacial free

energies of these two metals. In the overpotential (OPD) range, Ni deposition proceeds by a columnar growth of 3D Ni clusters

along step edges, whereas Co clusters grow homogeneously at potentials below (/0.17 V vs. Co/Co(II). Electrodeposition of

NixAl1(x and CoxAl1(x is found to be very similar. In both cases, codeposition starts at a potential clearly positi ve of the Al/

Al(III)-Nernst potential and with increasing Al content smaller grains are observed. The composition of the respective alloy clusters

has been probed in situ by STS spectra and it is found that the effecti ve tunneling barriers at different potentials E scale with thecluster composition determined from independent conventional electrochemical and spectroscopic measurements. Finally, we report

first investigations of electrodeposition of ultrathin Ge films with varying thickness on Au(1 1 1) and Si(1 1 1):H. Probing the

electronic structure of these films by in situ STS spectra a metal/semiconductor transition is indicated with increasing film thickness

above 2/3 nm. This is discussed in comparison with ultrathin Ge films grown by expitaxial vapour deposition.

# 2003 Elsevier Science Ltd. All rights reserved.

Keywords: Electrocrystallization; Electrochemical STM and STS; Ni, Co and Ge electrodepositions; Ionic electrolytes

1. Introduction

Although electrochemical scanning tunneling micro-

scopy (EC-STM) has been developed only during thelast decade, it has already entered modern textbooks in

electrochemistry [1]. It is, at present, the only method

that can probe the structure at the electrode/electrolyte

interface and its change during electrochemical pro-

cesses in situ with atomic or nanometer resolution.

Measuring the differential tunneling conductance (I vs.

U spectra) an additional in situ analytical information

on the nanometer scale can be obtained by scanning

tunneling spectroscopy (EC-STS) [2].

So far, these scanning probe techniques have beenapplied in a variety of electrochemical studies, but

exclusively with aqueous electrolytes, see e.g. [3/8]. In

this way, the electrodeposition of elements and com-

pounds is limited by the decomposition potential of

water of /1.2 V. Taking this into account we have

started with STM experiments in ionic electrolytes,

molten salts at elevated temperatures [9] and room

temperature ionic liquids [10,11]. They possess clearly

larger electrochemical windows*/e.g. up to 7 V for

[BMIm]'PF6( at a tungsten electrode [12]*/and thus

enable electrodeposition of a wider range of metals,

alloys and also semiconductors.

* Corresponding author. Tel.: '/49-721-608-2100; fax: '/49-721-

608-6662.

E-mail address: [email protected] (W.

Freyland).

Electrochimica Acta 48 (2003) 3053/3061

www.elsevier.com/locate/electacta

0013-4686/03/$ - see front matter# 2003 Elsevier Science Ltd. All rights reserved.

doi:10.1016/S0013-4686(03)00378-5
mailto:[email protected]:[email protected]


2/9

This aspect is the main objective of the present paper.

In the first part, we show representative results of EC-

STM measurements with nanometer resolution of Ni

and Co electrodeposition on Au(1 1 1) from AlCl3/

[BMIm]'Cl( in the underpotential (UPD) and OPD

range. The different behaviour of 2D and 3D phase

formation of these two metals will be discussed. We thencompare the alloy formation of NixAl1(x and

CoxAl1(x on a nanometer scale with emphasis on

recent EC-STS measurements of the compositional

changes of alloy clusters. Finally, first STM and STS

results of the electrodeposition of Ge films on Au(1 1 1)

and Si(1 1 1):H are presented focusing on the apparent

thickness induced metal/non-metal transition in these

ultra thin films.

2. Experimental

The room temperature molten salts employed in this

study, AlCl3/[BMIm]'Cl(and [BMIm]'PF6

(, have

the following advantages for EC-STM measurements:

they have low vapour pressures even at elevated

temperatures, have large electrochemical windows, pos-

sess a sufficiently high solubility of the respective metal

or semiconductor halides and, finally, the AlCl3 contain-

ing melts can be varied from Lewis-acidic to -basic

characteristics and are particularly suited for Al alloy

electrodeposition. Especially from the STM point of

view they have shortcomings: they are sensitive to

hydrolysis*/especially the AlCl3 containing liquids*/

and their specific electrical conductivity is relativelyhigh, of the order of 10(2 to 10(1 V(1 cm(1 [13]. This

requires a specially developed EC-STM set up similar to

the one first described in Ref. [9]. In brief, the electro-

chemical cell together with the STM scanner and the

micrometer screws for coarse positioning are mounted

inside a vacuum tight stainless steel housing filled with

high purity Ar gas (O2B/1 ppm, H2OB/2 ppm). The

electronics of the scanner is specially sealed so that

attack by hydrolysis products like HCl is strongly

reduced. Both W and Pt/Ir electrochemically etched

STM tips have been used which were electrically

insulated by epoxide paint.For the metal electrodeposition experiments reported

here the ionic electrolyte AlCl3/[BMIm]'Cl( has been

used. Solutions of Ni(II) and Co(II) of (59/0.1))/10(3

mol l(1 have been prepared by anodic dissolution of the

respective metals at potentiostatic control. Substrates of

flame annealed Au(1 1 1) on quartz (Berliner Glas KG)

have been employed. Wires of Ni and Co, respectively,

were used as counter and reference electrodes, which

proved to be very stable. Ge electrodeposition on

Au(1 1 1), mica (Molecular Imaging Corp.) and

Si(1 1 1):H was studied in liquid [BMIm]'PF6( satu-

rated with GeCl4 or GeBr4 at room temperature (see

also [14]). Preparations and fillings of the electrochemi-

cal cells and all assemblies of the EC-STM experiments

have been performed in an Ar glove box (O2B/1 ppm,

H2OB/2 ppm). The STM and STS measurements have

been made with a DI nanoscope E (Veeco Instruments,

Mannheim) or an MI controller (Molecular Imaging,

Witec GmbH, Ulm). For the electrochemical measure-

ments an MI picostat potentiostat has been used.

Further details of sample preparation, synthesis andpurification of the ionic liquids, and experimental

procedures are given in [15,16].

3. Results and discussion

3.1. Overview of UPD and OPD deposition of Ni and Co

on Au(1 1 1) from AlCl3/[BMIm]'Cl(

A first overview of the relevant redox processes of the

Ni and Co electrodeposition in the potential limits

between bulk Au oxidation A and bulk Al depositionF is given by the cyclic voltammograms in Fig. 1. In the

UPD range two redox couples are of interest. The

couple B/Bl is assigned to Au(1 1 1) step edge oxidation

which has been shown in a previous STM study with the

same electrolyte [11]. A second redox process C/Cl with

an oxidation peak at 0.35 V vs. Ni/Ni(II) is clearly seen

in the Ni cyclic voltammogram, but is only weakly

indicated in the UPD of Co. This UPD process will be

further elucidated and discussed below on the basis of

detailed STM images. It is not observed in Ni electro-

deposition from an aqueous Watts electrolyte [17].

Bubendorff et al. [18], however, found that UPD of Ni

Fig. 1. Cyclic voltammogram of Ni (upper panel) and Co (lower

panel) electrodeposition on Au(1 1 1) from AlCl3/[BMIm]'Cl(

(molar ratio 58:42); scan rate: 0.1 V s(1, electrode area0/0.38 cm2.

W. Freyland et al. / Electrochimica Acta 48 (2003) 3053 /30613054


3/9

on Au(1 1 1) can occur, depending on the nature of

anions present in the electrolyte solution. A similar

observation has been made for Co electrodeposition on

Au(1 1 1) in various electrolytes [19]. In the OPD range,

three reduction waves can be distinguished which are

assigned as follows: bulk deposition of Ni or Co at D, Al

deposition approaching F. The interpretation of wave E

by Ni/Al alloy deposition is consistent with previous

electrochemical studies [20]. In the case of Co and Co/

Al deposition a more complex growth kinetics has to be

considered (see [21]). This is indicated by a detailed

analysis of the dissolution peak El in the CVs recorded

at different scan rates with different reverse potentials

[21] and will be further analyzed with the aid of STM

and STS results below.

3.2. 2D phase formation of Ni and Co on Au(1 1 1)

In the UPD range of the redox couple C/Cl (Fig. 1)

2D nucleation of Ni and Co on Au(1 1 1) exhibits a

clearly distinct behaviour. This is demonstrated by the

STM images in Fig. 2. In the case of Ni deposition a

complete monolayer is formed within several minutes if

the potential is switched from 0.5 to 0.1 V vs. Ni/Ni(II).

This is illustrated by the regular hexagonal superstruc-

ture (Moire pattern) with a lattice constant of 239/1 A

and with a modulation amplitude of 0.6 A; this is due to

the lattice misfit between the Au(1 1 1) surface and the

Ni monolayer (dNi0/2.49 A; dAu0/2.885 A). This

interpretation is supported by simultaneously measuredcurrent transients which yield an integrated value of the

charge of 5309/50 mC cm(2 corresponding to a 0.9 ml

of Ni [22]. It is interesting to note that in Ni electro-

deposition from an aqueous Watts electrolyte a com-

plete Ni monolayer is only observed in the OPD range at

a potential of(/0.1 V vs. Ni/Ni(II) [17]. In contrast to

Ni, visible 2D phase formation during Co electrodeposi-

tion from the ionic liquid starts only at slightly negative

potentials and is characterized by statistically distribu-

ted monoatomically high islands (apparent height 1.79/

0.1 A in comparison to a Au(1 1 1) step edge height of

2.39/0.1 A). Island formation at the first stages of Co

electrodeposition has also been reported for aqueous

electrolytes [19,23]. In the AlCl3/[BMIm]'Cl( ionic

liquid, however, this island growth prevails up to

potentials of(/0.15 V vs. Co/Co(II) (Fig. 2b). Details

of the kinetics of the island growth are discussed in Ref.

[21]. Here, we restrict to a qualitative explanation of the

different nucleation mechanisms of Ni and Co as

evidenced in Fig. 2.

In a simple thermodynamic approach of nucleation a

distinction between island and layer growth is deter-

mined by the change of the free enthalpy, DG2D, of the

formation of a 2D nucleus. For a circular monolayer

island of radius R , atomic volume V and latticeconstant a, DG2D is given by (see e.g. [24]):

G2D0R2aV(1'R2aV(1=3'2RE: (1)

Here, the first term describes the energy gain when the

film is formed by electrodeposition corresponding to a

change in the electrochemical potential Dh , the second

term contains the energy cost for the formation of the

new interfaces, and sE in the last term is the line tension.

In the following, we first consider the role ofDs on the

formation of a 2D island/film in equilibrium with the

vapour phase (Dh is replaced by Dm0/kT ln(p /p0), with

p00/saturation pressure). In this case, Ds is given by the

Fig. 2. STM images of 2D nucleation of Ni and Co on Au(1 1 1): (a)

Moire pattern of a Ni monolayer electrodeposited at E0/0.11 V vs. Ni/

Ni(II), Etip0/0.2 V, Itun0/3 nA; (b) Atomically high Co islands with

radii of 2 nm5/R5/6 nm grown on Au(1 1 1) terraces at a potential

E0/(/0.15 V vs. Co/Co(II), Etip0/0 V, Itun0/3 nA; the two larger

frizzy islands are Au islands.

W. Freyland et al. / Electrochimica Acta 48 (2003) 3053 /3061 3055


4/9

equilibrium interfacial energies sik according to the

Young equation for complete wetting:

D0F;V'S;F(S;V; (2)

where F,V0/film/vapour; S,F0/substrate/film; and

S,V0/substrate/vapour interface. If Ds0/0, the sub-

strate is completely wet by the film and layer growth isfavoured. On the other hand, ifDs!/0 the substrate is

partially wet by the deposit corresponding to island

growth. In order to calculate Ds for the interesting case

of Ni and Co deposition we have to estimate sAu,Ni and

sAu,Co since no experimental or theoretical data are

available. In general, the sik scale with the energies of

transformation of the two neighbouring phases i and k.

Therefore, we may assume that sS,F&/sS,V, sF,V holds.

With this assumption and the literature data of the

metals of interest here*/sCo,V0/2709 mJ m(2 [25],

sNi,V0/1850 mJ m(2 [26], sAu,V0/1500 mJ m

(2

[27]*/we find for Co a clearly positive value for Ds ,i.e. island formation is favoured. Taking into account

the relatively large uncertainties of the experimental sikdata of metals, no clear conclusion can be drawn in the

case of Ni deposition. Finally, we have to comment

critically, that these considerations are based on the

thermodynamic values of the interfacial free energies

and do not contain a correction for the finite micro-

scopic thickness of the deposited films. This is crucial for

a quantitative understanding of the wetting behaviour of

thin films [28].

An important question remains, in how far the above

results for the vapour deposition are relevant for the

electrodeposition and nucleation of Ni and Co onAu(1 1 1)? In this case, we have to consider that the

coordination chemistry of the metal cations in the ionic

liquid possibly influences the deposition and growth

behaviour. Previous studies on complex formation of

transition metals in Lewis-acidic chloroaluminate room

temperature molten salts indicate that both Co and Ni

form very weakly coordinated [M(AlCl4)3]( complexes

[13,29]. This high chemical similarity of Co and Ni

allows to exclude coordination chemistry as an explana-

tion for their different growth behaviour during electro-

deposition. For the same reason, we do not think that

the first term in Eq. (1) can account for this difference.However, the crystal structures and surface energies of

the two metals are clearly different and so the determin-

ing contribution in DG2D is that in Ds . For the

following reason, we can assume that Ds for the

metal/vapour and for the metal/electrolyte interface are

comparable in magnitude. Although the absolute values

of the sik are strongly reduced going from the vapour to

the electrolyte interphase, their difference should vary

only slightly. This is indicated by our STS measurements

of the effective tunneling barriers of the metal/electrolyte

interface (M0/Ni, Co, Au(1 1 1)), which yield a con-

stant reduction by /4 eV for all three metals relative to

the vacuum level. With the assumption that the inter-

facial free energies of the metals are reduced by acorresponding constant amount we suggest a Ds!/0 for

the Co electrodeposition from the ionic electrolyte, i.e.

island formation and growth. This is consistent with our

STM observations.

3.3. 3D cluster growth of Ni and Co

Passing the Nernst potential a transition from 2D to

3D nucleation occurs for Ni electrodeposition. At

slightly negative potentials 3D Ni-clusters start to

grow preferentially at the step edges of Au(1 1 1)

Fig. 3. STM images of 3D cluster formation of Ni and Co on

Au(1 1 1): (a) Decoration of Au step edges by columnar Ni clusters at

E0/(/0.03 V, Etip0/0.02 V, Itun0/5 nA; (b) Formation of a rough and

dense Co cluster film at E0/(/0.2 V, Etip0/(/0.25 V, Itun0/3 nA.



5/9

terraces and at defects of the Ni-monolayer*/see the

example of Fig. 3a. With increasing OPD these clusters

grow in size and number. At a constant potential of(/

0.2 V vs. Ni/Ni(II) they achieve a height of/30 A and a

diameter of/100 A over a time period of 12 min. When

the potential is decreased, 3D cluster growth sets in all

over the Au(1 1 1) surface [22]. The nucleation kinetics is

similar to that reported for Ni cluster growth on an

Ag(1 1 1) substrate in aqueous solution [30]. It can be

accounted for by a simple model whereby the verticalcluster growth is determined by the strain energy of the

structural defects at the substrate, ad metal step edges

(see also [15]).

3D growth in Co electrodeposition on Au(1 1 1) from

AlCl3/[BMIm]'Cl( starts at potentials below (/0.17 V

vs. Co/Co(II). The Co 2D islands grow in size without

forming a coherent monolayer before new islands form

on top. As a result, a relatively rough film of several

monolayers thickness is observed by STM (Fig. 3b). The

nucleation kinetics of Co electrodeposition has been

studied at various OPDs and follows classical nucleation

theory with a critical number of 1/2 atoms in thepotential range (/0.05 V!/h!/(/0.2 V [21].

3.4. Ni/Al and Co/Al alloy formation

Codeposition of metal/aluminum alloys from chlor-

oaluminate melts has been studied by several groups

employing conventional electrochemical methods*/see

e.g. [20,31/33]. In agreement with our results in Fig. 1

they find that Al codeposition sets in at potentials which

lie clearly above the Al/Al(III) equilibrium potential, i.e.

Ni/Al codeposition starts at /0.3 V vs. Al/Al(III) and

that of Co/

Al at/

0.35 V vs. Al/Al(III)*/

the Al-Nernst potential lies at (/0.5 V vs. Ni/Ni(II) and at(/0.6

V vs. Co/Co(II). The compositions and structures of the

deposited alloy films have been analyzed by various

electrochemical and ex situ structural techniques. For a

NixAl1(x film a detailed analysis of the diffraction

peaks suggests a grain size of/10 nm [20]. The in situ

STM measurements of this study yield the following

trend for the variation of the alloy film morphology of

NixAl1(x and CoxAl1(x . At high x clusters of 1/2 nm

height and 10 nm diameter are observed. Reducing x ,

the number density of clusters increases fast and they

merge and form larger aggregates. Approaching the Al

deposition potential, smaller Al-rich clusters of 2/3 nmdiameter can be distinguished from the larger alloy

aggregates. A typical example is shown by the STM

image in Fig. 4 for a CoxAl1(x deposit. An in situ

analysis of the composition of the single clusters is

possible by measuring the tunneling spectra. For this

aim we have measured the I/U curves for different

clusters at various deposition potentials and have

determined the effective tunneling barrier f according

to [2]:

I0constgU

O

exp((A(f(eV)1=2d) dV; (3)

Fig. 4. CoxAl1(x alloy deposition on Au(1 1 1) from AlCl3/

[BMIm]'Cl( melts: (a) STM image of CoxAl1(x alloy clusters

deposited at (/0.7 V vs. Co/Co(II); (b) I/U curve of the cluster

marked by a cross in Fig. 4a; (c) Variation of the effective tunneling

barrier f (E) with deposition potential E in CoxAl1(x alloy deposi-

tion; error bars represent the scattering off (full symbols) determined

from various I/U curves; STS-spectra were taken with W and Pt-Ir

tips and at several clusters at otherwise constant conditions of tip

distance and potential E.



6/9

where A/1.025 eV(1/2 A(1 and d is the tip-cluster

distance. The tunneling barrier f is determined by the

average workfunctions of tip and substrate. Therefore, if

the tip remains the same in a series of measurements

with varying potential E, f (E) measures the variation

of the substrate workfunction and thus yields an

information of the composition x . A typical I/U curveof CoxAl1(x taken at a cluster marked by a cross in the

STM image is given in Fig. 4. The corresponding f(E)

dependence is shown in the lower panel of Fig. 4. The

interesting result is that within the deposition range of

CoxAl1(x the relative change of f(E) within experi-

mental errors quantitatively correlates with independent

electrochemical measurements of the cluster composi-

tion x [20,34]. A similar result we got for NixAl1(xdeposition [15]. Since recording of the tunneling spectra

takes only several ms, this is a valuable analytical probe

to study compositional changes during electrocrystalli-

zation with nanometer resolution.

3.5. Electrodeposition of ultra thin Ge films and thickness

induced metal/non-metal transition

Semiconducting nanostructures and ultrathin films

have experienced tremendous research interest in recent

years because of their potential to develop novel

electronic and optoelectronic devices. An example are

ultrathin films of Ge whose microscopic and electronic

structure has been characterized by a variety of methods

in the thickness range from submonolyers to several tens

of monolayers*/see e.g. [35/41]. A detailed review ofthe kinetics and thermodynamics of thin film growth

and, in particular, of the effect of strain on the structure

of ultrathin Ge films on Si is given by Lagally and

coworkers [35]. For films grown at high temperatures,

typically at 500/700 8C and annealed for several hours,

Si/Ge interlayer mixing seems to be suppressed by a

large kinetic barrier. At low coverage dimer vacancies

form and order in a (2n) reconstruction thus reducing

the surface strain energy. Pure Ge has a lower surface

energy than Si and thus completely wets the substrate up

to /3 ml. Above this thickness a transition from 2D to

3D growth (Stranski/Krastanov mode) occurs. It ischaracterized by the formation of hut-like clusters with

canted ends, sometimes of prismatic shape with perfect

facet planes [35]. They occur preferentially at lower

temperatures (B/500 8C) and are believed to be a

metastable intermediate phase. This onset of the transi-

tion from 2D to 3D phase formation is clearly identified

in STM investigations [35] and is supported by several

spectroscopic studies like interference enhanced Raman

spectroscopy [38] or UPS [41]. The critical thickness for

the transition varies in these different studies which

certainly is due to the different temperatures where the

films have been grown and studied.

In most of these studies, Ge films on various

substrates have been prepared by vapour deposition

techniques like MBE or CVD at elevated temperatures.

In this section, we report new measurements of Ge

electrodeposition from an ionic electrolyte on Au(1 1 1)

and Si(1 1 1):H at room temperature. In the discussionof the STM and STS results, we focus on the variation

of the electronic structure going from very thin to thick

deposited Ge films.

As an example, a cyclic voltammogram of the

[BMIm]'PF6( ionic melt saturated with GeCl4 and

measured with a H-terminated Si(1 1 1) electrode is

shown in Fig. 5, see also [42]. Measurements have

been performed with a Pt quasi reference, the potentials

are given vs. Ge bulk deposition. Similar to the

interpretation given for the Au(1 1 1) interface [42] we

assign the two broad reduction peaks at /0.8 V and (/

0.3 V vs. Ge/Ge(IV), respectively, to the reduction steps

Fig. 5. Cyclic voltammogram of Ge electrodeposition from

[BMIm]'PF6( saturated with GeCl4 on Si(1 1 1):H; electrode area:

0.4 cm2; scan rate: 1 mV s(1.

Fig. 6. STM image and I/U tunneling spectrum of a Si(1 1 1):H

substrate with 48 miscut recorded at the Si/ionic liquid interface at

open circuit potential (1.8 V vs. Ge deposit), Etip0/2.8 V, Itun0/1 nA,

height scale: 0/10 A; STS spectra are averaged from measurements at

different sites with W and Pt/Ir tips.



7/9

Fig. 7. (a) STM image and I/U tunneling spectrum of a /1 nm thick Ge film electrodeposited from a GeBr4 saturated (/5)/10(3 mol l(1)

[BMIm]'PF6( melt on Au(1 1 1) (see text), E0/0.3 V, Etip0/0.9 V, Itun0/1 nA, height scale: 0/10 A; thickness has been estimated from the z -piezo-

displacement after dissolution, same in Fig. 7b and c; all potentials E vs. Ge/Ge(IV). (b) STM image, I/U curve and cluster height profile of a Ge

film of 2/4 nm thickness deposited from a GeCl4 saturated [BMIm]'PF6

( melt on Au(1 1 1) (see text), E0/(/0.05 V, Etip0/1.05 V, Itun0/1 nA. (c)

STM image and I/U curve of a 4/6 nm thick Ge film deposited on a Si(1 1 1):H substrate, E0/(/0.1 V, Etip0/0.8 V, Itun0/1 nA.



8/9

Ge(IV)/Ge(II) and Ge(II)/Ge(0), respectively. Ob-

viously the CV exhibits irreversible behaviour which is

not understood in detail. For instance, the role of a

finite solubility of Ge in its respective halides is

unknown.

STM images on Au(1 1 1) and Si(1 1 1):H and in situ

I/U curves have been taken at different deposition

potentials at room temperature. In Fig. 6, the topogra-

phy of the Si substrate in contact with the ionic liquid is

shown at the open circuit potential of/1.8 V vs. Ge/Ge(IV). The corresponding I/U spectra measured at

different sites of the interface yield a gap energy of 1.19/

0.1 eV which is in good agreement with the literature

value of bulk silicon at room temperature [43]. The

variation of the microscopic and electronic structure of

Ge deposits of different thickness is illustrated in Fig. 7

by a few STM and STS results representing measure-

ments on Au(1 1 1) and Si(1 1 1):H. In the UPD range,

coherent rough Ge films are observed. This is demon-

strated in Fig. 7a, for the example, of a 5/1 nm thick

film deposited from a GeBr4 saturated [BMIm]'PF6

(

melt on a Au(1 1 1) substrate. The I/U curve measuredon this film exhibits clearly metallic characteristics (Fig.

7a). This metallic behaviour is observed for Ge films on

Au(1 1 1) up to a thickness of about 1 nm. Increasing

the film thickness a band gap starts to open and the

morphology of the Ge films changes significantly. The

structure is determined by coherent 3D clusters of

trapezoidal shape and with sharp edges which have a

height of 2/4 nm. Fig. 7b gives, as an example, the STM

picture and the corresponding I/U spectrum of a Ge

deposit on Au(1 1 1). Similar characteristic cluster

shapes have been observed for the UPD of Ge on

Si(1 1 1):H. In the OPD range, thick Ge films can be

grown and stabilized at a given potential on Au(1 1 1)

and Si(1 1 1):H alike. An example is given in Fig. 7c.

Taking the band gap of 0.7 eV [43] as a measure, bulk

semiconducting properties occur above 10 nm filmthickness.

The overall change of the electronic structure as a

function of the film thickness is presented in Fig. 8.

Plotted are the apparent gap energies as determined

from the I/U curves of electrodeposited Ge films on

Au(1 1 1) and Si(1 1 1):H as a function of the a verage

film thickness d. This plot indicates that a metal/non-

metal transition occurs with increasing film thickness.

This is a remarkable observation, since metallic char-

acteristics of Ge so far are only known for the bulk

liquid state. The thin films studied here have been grown

at a relatively low temperature. At these conditions the

Fig. 7 (Continued)

Fig. 8. Variation of the apparent band gap energy with film thickness

d of electrodeposited Ge films:k, Au substrates; j, Si substrates (see

text).



9/9

growth is dominated by kinetics. So it may be that the

thin Ge films are far from equilibrium and the metallic

like behaviour reflects a metastable glassy state. Further

investigations are needed to clarify this hypothesis.

Acknowledgements

Financial support of this work by DFG through

project FR 299/17 and now through CFN, University of

Karlsruhe, and partially by the Fonds der Chemischen

Industrie is acknowledged.

References

[1] A.J. Bard, L.R. Faulkner, Electrochemical Methods, Fundamen-

tals and Applications, Wiley, New York, 2001.

[2] R.J. Hamers, in: D.A. Bonnell (Ed.), Scanning Tunneling Micro-

scopy and Spectroscopy: Theory, Techniques and Applications,VCH, Weinheim, 1993.

[3] O.M. Magnussen, J. Hotlos, R.J. Nichols, D.M. Kolb, R.J.

Behm, Phys. Rev. Lett. 64 (1990) 2929.

[4] R. Christoph, H. Siegenthaler, H. Rohrer, H. Wiese, Electrochim.

Acta 34 (1989) 1011.

[5] F.R.F. Fan, A.J. Bard, J. Electrochem. Soc. 136 (1989) 3216.

[6] A.A. Gewirth, H. Siegenthaler (Eds.), Nanoscale Probes of the

Solid/Liquid Interface, Nato ASI Series E 288, Kluwer Academic

Publishers, Dordrecht, 1995.

[7] W.J. Lorenz, W. Plieth, Electrochemical Nanotechnology,

Wiley/VCH, Weinheim, 1998.

[8] E. Budevski, G. Staikov, W.J. Lorenz, Electrochemical Phase

Formation and Growth, VCH, Weinheim, 1996.

[9] A. Shkurankov, F. Endres, W. Freyland, Rev. Sci. Instrum. 73

(2002) 102.[10] F. Endres, W. Freyland, B. Gilbert, Ber. Bunsen-Ges. Phys.

Chem. 101 (1997) 1075.

[11] C.A. Zell, F. Endres, W. Freyland, Phys. Chem. Chem. Phys. 1 (4)

(1999) 697/705.

[12] P.A.Z. Suarez, V.M. Selbach, J.E.L. Dullius, S. Einloft, C.M.S.

Piatnicki, D.S. Azambuja, R.F. de Souza, Electrochim. Acta 42

(1997) 2533.

[13] G. Mamantov, A.I. Popov (Eds.), Chemistry of Nonaqueous

Solutions, VCH, Weinheim, 1994.

[14] F. Endres, S. Zein El Abedin, Phys. Chem. Chem. Phys. 4 (2002)

1640.

[15] C.A. Zell, Ph.D. Thesis, Universitat Karlsruhe, 2002.

[16] F. Endres, Habilitation, Universitat Karlsruhe, 2002.

[17] F.A. Moller, J. Kintrup, A. Lachenwitzer, O.M. Magnussen, R.J.

Behm, Phys. Rev. B 56 (1997) 12506.

[18] J.L. Bubendorff, L. Cagnon, V. Costa-Kieling, J.P. Brucker, P.

Allongue, Surf. Sci. 384 (1997) L836.

[19] L. Cagnon, A. Gundel, T. Devolder, A. Morrone, C. Chappert,

J.E. Schmidt, P. Allongue, Appl. Surf. Sci. 164 (2000) 22.

[20] W.R. Pitner, C.L. Hussey, G.R. Stafford, J. Electrochem. Soc.143 (1996) 130.

[21] C.A. Zell, W. Freyland, Langmuir, 2003.

[22] C.A. Zell, W. Freyland, F. Endres, in: R.W. Berg, H.A. Hjuler

(Eds.), Molten Salt Chemistry, vol. 1, Elsevier, Paris, 2000, p. 597.

[23] M. Kleinert, H.-F. Waibel, G.E. Engelmann, H. Martin, D.M.

Kolb, Electrochim. Acta 46 (2001) 3129.

[24] H. Luth, Surfaces and Interfaces of Solid Materials, 3rd ed.,

Springer, Heidelberg, 1995.

[25] L.Z. Mezey, J. Giber, Jpn. J. Appl. Phys. 21 (1982) 1589.

[26] J.M. Blakeley, P.S. Maiya, in: J.J. Burke, et al. (Eds.), Surfaces

and Interfaces, Syracuse University Press, New York, 1967.

[27] J.C. Heraud, J.J. Metois, Acta Metall. 28 (1980) 1789.

[28] S. Dietrich, in: C. Domb, J. Lebowitz (Eds.), Phase Transitions

and Critical Phenomena, Academic Press, London, 1988.

[29] A.J. Dent, K.R. Seddon, T. Welton, J. Chem. Soc., Chem.

Commun. (1990) 315.

[30] S. Morin, A. Lachenwitzer, O.M. Magnussen, R.J. Behm, Phys.

Rev. Lett. 83 (1999) 5066.

[31] T.P. Moffat, J. Electrochem. Soc. 141 (1994) 3059.

[32] R.T. Carlin, P.C. Trulove, H.C. De Long, J. Electrochem. Soc.

143 (1996) 2747.

[33] Q. Zhu, C.L. Hussey, G.R. Stafford, J. Electrochem. Soc. 148

(2001) C88.

[34] R.T. Carlin, H.C. De Long, J. Fuller, P.C. Trulove, J. Electro-

chem. Soc. 145 (1998) 1598.

[35] F. Liu, F. Wu, M.G. Lagally, Chem. Rev. 97 (1997) 1045.

[36] R.Q. Yu, J. Fortner, S. Chaser, J.S. Lannin, Mater. Res. Soc.

Symp. Proc. 206 (1991) 403.

[37] S. Ogawa, J. Vac. Sci. Technol. 15 (1978) 363.[38] S. Kanakaraju, A.K. Sood, S. Mohan, J. Appl. Phys. 84 (1998)

5756.

[39] P. Castrucci, R. Gunnella, M. Crescenzi, M. Sacchi, G. Dufour,

F. Rochet, Phys. Rev. B 60 (1999) 5759.

[40] N. Marcovic, C. Christiansen, G. Martinez-Arizala, A.M. Gold-

mann, Phys Rev. B 65 (2001) 012501.

[41] L. Di Gaspare, G. Capellini, E. Cianci, F. Evangelisti, J. Vac. Sci.

Technol. 16 (1998) 1721.

[42] F. Endres, S. Zein El Abedin, Phys. Chem. Chem. Phys. 4 (2002)

1649.

[43] C. Kittel, Introduction to Solid State Physics, 2nd ed., Wiley, New

York, 1965.


Nanoscale Electrodeposition of Metals and Semiconductors From Ionic

Documents

Transcript of Nanoscale Electrodeposition of Metals and Semiconductors From Ionic