Self aligned silicon quantum wires on ag(1 1 0)

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

Self-aligned silicon quantum wires on Ag(1 1 0)

C Leandri a, G Le Lay a, B Aufray a,*, C Girardeaux b, J Avila c,d, M.E Da´vila c, M.C Asensio c,d, C Ottaviani e, A Cricenti e

a

CRMCN-CNRS, Campus de Luminy, Case 913, 13288 Marseille Cedex 9, France

b

L2MP, Campus de Saint Je´roˆme, 13397 Marseille Cedex 20, France

c

Instituto de Ciencia de Materiales de Madrid (CSIC), 28049 Cantoblanco, Madrid, Spain

d

LURE, Baˆt 209 D, Universite´ Paris-Sud, BP 34, 91898 Orsay, France

e Instituto di Struttura della Materia, CNR, Via Fosso del Cavaliere, 00133 Rome, Italy

Received 9 September 2004; accepted for publication 21 October 2004

Available online 13 December 2004

Abstract

Upon deposition of silicon onto the (1 1 0) surface of a silver crystal we have grown massively parallel one-dimen-sional Si nanowires They are imaged in scanning tunnelling microscopy as straight, high aspect ratio, nanostructures, all with the same characteristic width of 16 A˚ , perfectly aligned along the atomic troughs of the bare surface Low energy electron diffraction confirms the massively parallel assembly of these self-organized nanowires Photoemission reveals striking quantized states dispersing only along the length of the nanowires, and extremely sharp, two-compo-nents, Si 2p core levels This demonstrates that in the large ensemble each individual nanowire is a well-defined quan-tum object comprising only two distinct silicon atomic environments We suggest that this self-assembled array of highly perfect Si nanowires provides a simple, atomically precise, novel template that may impact a wide range of applications

2004 Elsevier B.V All rights reserved

Keywords: Silver; Silicon; Self-assembly; Nanowires; Scanning tunneling microscropy; Photoelectron spectroscopy

In the quest for electronics on the nanoscale,

one-dimensional (1D) quantum structures are

ex-pected to play a key role[1,2] Systems that might

act as nanowires (NWs) are of major importance,

but are rather difficult to prepare experimentally

[3] Such NWs bear great potential to exhibit exo-tic and attractive physical phenomena [4] In re-cent years, several self-organized quantum wire arrays have been grown upon depositing metals

on semiconductor [3–9] or on metallic surfaces exhibiting regularly spaced steps[10,11] Self-orga-nized formation of quasi-one-dimensional surface

0039-6028/$ - see front matter
2004 Elsevier B.V All rights reserved.

doi:10.1016/j.susc.2004.10.052

* Corresponding author Fax: +33 0 4 91 82 91 97.

E-mail address: aufray@crmcn.univ-mrs.fr (B Aufray).

oxide domains on Cu(1 1 0) leading to 1D

confine-ment of a Shockley surface state has been also

ob-served by Bertel and Lehmann[12] On wide band

gap b-SiC(100) substrates, the spontaneous

forma-tion of stable atomic lines, i.e., carbon lines with C

atoms in sp3 configuration on C-terminated

sur-faces as well as silicon lines at the phase transition

between Si-rich and Si-terminated surfaces has

also been observed [13,14] Given the central role

of silicon in microelectronics and the potential

occurrence of quantum size effects in silicon-based

devices [15], silicon NWs have attracted

consider-able interest[16,17] However, with respect to

pro-cedures used, producing Si NWs with controlled

sizes is far from being trivial and aligning them

in a well-ordered fashion, a crucial issue, is

another problem

We have succeeded in growing a massively

paral-lel assembly of straight silicon NWs on a clean,

nominally flat (misorientation  0.1) (1 1 0) silver

surface All NWs have the same orientation and

characteristic narrow width of 1.6 nm and are

two-atom thick; they reach eventually hundreds of

nanometers in length Strikingly, this ensemble

displays quantized electronic states with a 1D

dis-persion in valence band photoemission, while

high-resolution core level spectroscopy

demon-strates that all individual NWs within the assembly

have an identical and highly perfect atomic structure

which comprises two and only two distinct silicon

environments Hence, this nanowire array provides

a novel, simple and atomically precise macroscopic

template that may impact, not only future

electron-ics, but and also a wide range of fields[18]

1D metal chains or stripes on silicon surfaces

have attracted considerable interest because of

en-hanced many-body interactions leading possibly to

an exotic state described by the Luttinger liquid

framework, or, typically, to metal-insulator

transi-tions [7,19–21] However, conversely, only very

few studies concern the reverse silicon-on-metal

systems Two investigations concern gold and

cop-per noble metal substrates[22,23] In the last case,

short atomic silicon chains, albeit presenting many

defects, and displaying no localized electronic

states, could be grown on top of an initial 2D

sur-face alloy by depositing silicon onto clean Cu(1 1 0)

surfaces

We have deposited silicon in situ under ultra-high vacuum (UHV) (typical silicon coverage

0.25 monolayer (ML) in silver (1 1 0) surface atom density) from a direct-current heated piece

of silicon wafer (flashed at1250 C), controlling the evaporation flux with a quartz monitor and the deposition at room temperature (RT) by Auger electron spectroscopy To limit any possible inter-mixing we have chosen a silver (1 1 0) substrate, since numerous works have demonstrated the atomic abruptness of the silver-silicon interface compared to the diffusiveness of the gold-silicon one and the reactivity of the Cu–Si one, which forms silicides [24] The clean, nominally flat (1 1 0) surface (misorientation 0.1) was prepared

by standard, repeated cycles of Ar+ bombard-ments and annealing

As imaged in scanning tunnelling microscopy (STM) at RT in Fig 1(a), thin silicon NWs, reaching up to about 30 nm in length, are formed

at the early stages of the deposition at RT, appar-ently from the self-assembly of nanodots, which appear as their swiftly diffusing building blocks The density of the nanowires is typically 1.4 ·

1012cm 2at RT; as will be seen later it can be re-duced upon mild annealing All these NWs are perfectly aligned along the [ 1 1 0] direction of the Ag(1 1 0) surface, showing rounded protru-sions (Fig 1(b)), equally spaced every second sil-ver atomic distance (2a2= 0.577 nm); some of them appear too large to represent single atoms (the atomic diameters of Si and Ag in the bulk crystals are 0.288 and 0.235 nm respectively) The 2a2 periodicity indicates that the NWs are not simply composed of Si [ 1 1 0] rows with a

‘‘bulk-like’’ inter-atomic distance; in such a case

a 4a2 periodicity would be expected, given the excellent match between four Ag atomic distances and three silicon ones along [ 1 1 0] Indeed, the negligible misfit permits the perfect epitaxial growth of silver (1 1 1) crystallites on the Si(1 1 1) surface with common [1 1 0] directions [25] The NWs have the same definite width of 1.6 nm, which corresponds to four silver atomic distances (4a1) along the Ag[0 0 1] direction, and a maxi-mum apparent height of0.2 nm (Fig 1(c)); their mutual separations vary between 1.5 and 15 nm The NWs are markedly asymmetric along their

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widths, as shown by the height profile, which

eventually indicates that their atomic structure

may not be trivial, although we can not exclude

tip convolution effects Upon mild annealing at

230C for about 10 min they further markedly

elongate, keeping the same narrow width, well

be-yond 100 nm, as shown inFig 2; in this case their

density is reduced typically by a factor 7 Just

from the STM images we can not give a reliable

atomic model of the NWs However, we surmise

that their very narrow width is due to a strong

epitaxial strain, consequently the actual geometry

might resemble one of the metallic bulk silicon

phases obtained at high pressures, e.g., the b-tin

like phase or rather the simple hexagonal phase

[26] If true, this would point to a possible

super-conductivity of the NWs

As seen inFig 3(a), LEED patterns display, in

addition to the integer order spots of the

unrecon-structed Ag(1 1 0) surface, thin streaks elongated

along the [1 0 0]* reciprocal direction, either

con-necting these spots or situated in half-order

posi-tion along the orthogonal [ 1 1 0]* direcposi-tion In

excellent agreement with the STM images, these

patterns corroborate, at the macroscopic scale,

the order within the NWs with a 2a2periodicity

along their lengths, the narrow width of the silicon

NWs, and a lack of periodicity in the perpendicu-lar direction, reflecting their variable separations Since the NWs differ only in length, these un-equal separations are no obstacle to probe the macroscopic electronic response using advanced synchrotron radiation photoemission (PES) meth-ods We have performed high-resolution (HR) angle-integrated (AI) measurements at RT of the valence bands (VBs) and of the Si 2p core-levels (CLs) A typical Si 2p spectrum is shown in Fig

3(b) together with its synthesis with two, spin–orbit splitted, components, as obvious on the raw data, using standard fitting procedures [27] These two components, separated by 0.24 eV, are remarkably narrow, with respectively 0.17 and 0.20 eV Full Widths at Half Maximum comparing favourably with the narrowest FWHMs of Si 2p bulk lines ob-tained for Sb covered Si(1 1 1) samples [27] This proves the perfect atomic order within the NWs and the existence of just two non-equivalent silicon environments Given the0.2 nm maximum height

of the NWs we can surmise that one of them may correspond to Si atoms (Si1) in direct contact with the Ag surface (hence, at the lowest BE because of the most effective metallic screening) and the other

to Si atoms (Si2) bonded to the (Si1) ones, although

we can not exclude the possibility that peculiar Ag

Fig 1 Topographic images of 0.25 monolayer of Si deposited on Ag(1 1 0) at room temperature: (a) 42 · 42 nm 2 overview with Si nanowires and nanodots, (b) 12.1 · 12.1 nm 2 zoom revealing the atomic rows of the bare substrate along the [ 1 1 0] direction and the profile of the nanowires, (c) height profile along the black line in (b) Imaging conditions: 1.7 V sample bias and 1.1 nA tunnel current

in (a) and (b) Note that the width of the NWs (1.6 nm) can serve as distance marker while their length direction points to the [ 1 1 0].

atom rows participate also in the structural

organi-zation of the NWs With photoelectron diffraction

experiments on each component, one could

determine precisely the two different local Si

envi-ronments, possibly solve the complete atomic

structure of the NWs, and, along with detailed

sim-ulations, interpret the protrusions seen in the STM

images Since the atomic structure of the thinnest

silicon wires is a matter of intense theoretical

re-search this structural determination might have a

decisive impact[17,28,29]

The best fits shown in Fig 3(b) were obtained

upon including an asymmetry parameter of 0.09,

higher than that reported for a pristine silver Ag

3d CL[30] This is direct evidence of the

metallic-ity of the Si NWs (all spectra taken at various

pho-ton energies, incidence and detection angles are

markedly asymmetric) This metallicity is

consis-tent with the fact that the density of states at the

Fermi energy increases compared to that of the ini-tial silver surface (Fig 3(c)), as well as with scan-ning tunnelling spectroscopy (STS) measurements (not shown here) performed on individual NWs: the I(V) spectra (tunnelling current versus sam-ple-to-tip voltage) do not significantly deviate from those performed on the pristine Ag surface This metallic character could be a proximity effect due metal-induced gap states, or be analogous to the 2D surface alloy initially formed by Si on Cu(1 1 0), or rather be the consequence of the stabilisation of a high-pressure silicon phase, as mentioned above[31,23,26]

The most striking result is the presence of new, discrete, electronic states, compared to the feature-less sp valence band of the pristine Ag(1 1 0) sur-face A maximum number of four new states were detected; they are clearly noticed in the mea-surement geometry of Fig 3(c) To precise their

Fig 2 Si nanowires (image size: 45 · 100 nm 2

) before (a), and after (b), annealing at 230C The diffusing companion nanodots disappear after complete incorporation for longer annealing times Imaging conditions: 1.7 V sample bias and 1.14 nA tunnel current

in (a) and 0.4 V and 0.7 nA in (b).

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nature, we further performed a detailed

angle-re-solved (AR) photoemission study These states,

which do not exist on pristine silver (no confusion

with the bare Ag(1 1 0) Y Schockley-type surface

state is possible [32]), do not disperse at normal

emission as a function of the photon energy This

proves that they are associated with the Si NWs In

the measurement geometry ofFig 4, that is, along

the direction of the NWs, the two deepest new

states, previously detected in AI-PES, are observed

at 2.4 and 3.1 eV BE at 51 off normal

emis-sion We emphasize that the photon energy, the

polarization direction, as well as the collection

angle, with respect to the wires strongly influence

the detection of the new states Besides a strongly

dispersive Ag bulk sp band, the analysis of

repre-sentative spectra taken along the NWs after anneal-ing230 C (Fig 4(a)), reveals that these two deep levels disperse markedly, by 0.4 eV The disper-sion relations of these two new states are plotted

in Fig 4(b) We stress that no dispersion at all was noticed in the direction orthogonal to the NWs; hence the dispersion is purely one-dimen-sional, as already shown in Ref.[11] Such behav-iour can be expected for quantum well levels due

to confinement within the NWs, i.e., the electronic wave is quantum mechanically confined in two directions: along the normal to the surface, as well

as perpendicular to the NWs, while the electronic movement is not restricted along the [ 1 1 0] direc-tion, leading to pronounced 1D dispersion along C–X in k-space

Fig 3 Low energy electron diffraction and angle integrated photoemission on a macroscopic surface area covered with h  0.25 monolayer of silicon at room temperature (a) LEED pattern taken at 43 eV primary energy (b) Si 2p core level spectrum (dots) and its synthesis (solid line overlapping the data points) with two asymmetric components (bottom curves) The spectrum was recorded at normal incidence at hm = 140 eV photon energy with the hemispherical photoelectron analyser axis (16 acceptance angle) aligned at 45 from the normal to the surface The fitting parameters are a spin–orbit splitting of 605 meV, a Lorentzian FWHM of 40 meV, Gaussian FWHMs of respectively 135 and 185 meV, an asymmetry parameter of 0.09 (c) Normal incidence valence band spectra (hm = 79 eV) limited to the sp region for the initial pristine Ag(1 1 0) surface (bottom curve) and for the same surface as in (a) and (b) (top curve) The detector axis is at 45 from the surface normal in the incidence plane, parallel to the direction of the nanowires The zero of energy is taken at the Fermi level and the relative intensities of the two spectra take into proper account all measurement conditions The total energy resolution is 40 meV.

To conclude, we stress that the quantized, sili-con NWs that we have grown and characterized under UHV might be stabilized by atomic hydro-gen termination, which could make them semicon-ducting [16] or oxidized, which could make them insulating Indeed, the growth can be further pur-sued We have done such tests, which show that extremely long, larger and much thicker crystalline nanowires, also perfectly aligned along the [ 1 1 0] direction can be produced [33] We plan to use these NWs as nucleation objects for further growth by Chemical Vapour Deposition We can also envisage to cover these Si NWs by overlayers and even the embedment of these nanostructures inside a silver matrix upon epitaxial regrowth of silver overlayers, which is very easy on Si surfaces Indeed, one can foresee the impact of such mas-sively parallel arrays of one-dimensional silicon metallic, semiconducting or insulating nanostruc-tures, from narrow, ultra-thin, nanowires to larger and thicker ones, in future electronics Another particularly exciting potentiality is for aligning large molecules, like C60, organic ones, nanotubes and polymers and interfacing with biological systems

Acknowledgment

The original LEED-AES and STM work started at the CRMC2-CNRS in Marseille as part

of the Thesis work of Christel Leandri; we espe-cially thank Dr H Oughaddou for help in the measurements and many discussions The expert technical assistance of A Ranguis, J.Y Hoarau and J.P Dussaulcy is greatly acknowledged We thank Dr P de Padova for stimulating discus-sions The angle-integrated photoemission experi-ments were carried out at the VUV beamline of the Italian synchrotron radiation facility ELET-TRA, in Trieste; we are grateful to the entire staff

of the beamline for help during the measurements The angle-resolved photoemission measurements were carried out at the Spanish–French SU8 beamline of the LURE, the French synchrotron radiation facility in Orsay; we warmly thank M.A Valbuena for help during the measurements

Fig 4 Angle-resolved photoemission valence band spectra and

dispersion relations of the deep lying quantum levels (QW a and

QW b ) from the silicon nanowires (a) a series of spectra at

different collection angles after annealing at 230 C (b)

Dispersion relations of the two quantum well states: the

binding energies of each state versus k || , the momentum of the

photoelectron parallel to the surface along the corresponding

C–X direction of the second and third (1 1 0) surface Brillouin

zones The colour code reflects the intensities of the different

features Experimental conditions: h  0.25 silicon ML; typical

resolutions of 1 and 50 meV; hm = 75 eV; binding energies are

referenced to the Fermi level; light was incident at 45 from the

surface normal, the plane of incidence is parallel to the [ 1 1 0]

direction of the wires the polar angles of detection in the

incidence plane are indicated; tick marks point to the positions

of the quantized states.

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Support from the ELETTRA and LURE staffs is

greatly acknowledged

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