A new mechanism for modulation of schottky barrier heights on silicon nanowires

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Physica E 40 (2008) 2508–2512

A new mechanism for modulation of Schottky barrier heights

on silicon nanowires

J Piscator  , O Engstro¨m Department of Microtechnology and Nanoscience, MC2, Chalmers University of Technology, SE-412 96 Go¨teborg, Sweden

Available online 10 August 2007

Abstract

For nanowires with Schottky barriers on the end surfaces, charges on the walls of the wire are close enough to the metal–semiconductor interface to influence the Schottky barrier This is similar to an effect in planar structures, where impurities with energy levels below the Fermi level in the bulk of the substrate material will change charge state in the depletion region of a metal–semiconductor structure if the Schottky barrier is high enough to bring the impurity energy level above the Fermi level The mechanism for barrier modulation is the same in both cases and occurs in nanowires as a result of the wire geometry

r2007 Elsevier B.V All rights reserved

PACS: 81.07.Lk, 61.72.y, 73.30.+y

Keywords: Silicon nanowires; Doping; Schottky contact; Oxide charge

1 Introduction

In the historical discussion of Schottky barriers, most of

the focus has been on planar structures and the occurrence

of dipole potentials at the metal–semiconductor (MS)

interface for modulating the barrier heights between the

extreme cases set by early predictions by Schottky and

Mott on one hand and of Bardeen on the other[1–3] These

treatments explain the lowering of the Schottky barrier as a

phenomenon taking place in intimate contact with the

interface between the metal and the semiconductor[4]or as

a result of electron wave function penetration from the

metal into the semiconductor [5] A second possibility to

influence an effective barrier height is to introduce a high

doping in the semiconductor, thus thinning the barrier and

allowing for tunneling [6] In silicon technology, the MS

structure has received an increased interest for coming

transistor generations In order to reduce source/drain

resistance and overcome the ‘‘short channel effect’’ for gate

lengths in the 20 nm range and below, MS structures are

considered as replacements of traditional p–n junctions as

source and drain contacts[7] In the search for methods to

lower effective Schottky barrier heights, ideas of segregat-ing dopants like As and B, with shallow energy levels close

to the metal have been demonstrated[8] This is the same method as used for creating ohmic contacts by n+and p+ doping, differing only by the depth of the latter

In the present paper, we demonstrate two alternative possibilities for introducing charge in the vicinity of the metal For planar structures a similar possibility exists as for the ohmic contact solution described above by doping with deep impurities which contributes their charge only close to the metal For wires with dimensions in the nanometer range, surface charges can be used to influence barrier properties due to their specific geometry In the first case, we demonstrate, by using experimental data from literature, how a deep double donor impurity can serve as a barrier modulator, in the second case our own experi-mental data on silicon nanowires point out the effect[9]

2 Planar structures modified by bulk doping with deep impurities

Recently, Schottky barrier modulation was demon-strated for NiSi contacts on planar silicon surfaces, where the semiconductor was doped with sulphur [10] This

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1386-9477/$ - see front matter r 2007 Elsevier B.V All rights reserved.

doi: 10.1016/j.physe.2007.07.018

Corresponding author Tel.: +46 31 7721862; fax: +46 31 7723622.

E-mail address: johan.piscator@chalmers.se (J Piscator).

element creates a double donor in silicon with a ground

state for the first captured electron at an energy of about

0.55 eV from the conduction band edge Subsequently, the

second electron is captured into an energy level at about

0.30 eV[11] Considering a shallow phosphorus doping of

about 1015cm3as used in the experiment of Ref.[10], this

corresponds to a Fermi level position at 0.28 eV from the

conduction band Therefore, in the bulk of an n-type

semiconductor, the sulphur levels are only slightly ionized

at room temperature and the resistivity of the sample is

mainly unchanged However, the NiSi metal contact has a

barrier height of about 0.65 eV on n-type silicon, which

means that the upper sulphur level passes the Fermi level in

the depletion region, emits the captured electron and

becomes positively charged as demonstrated by the band

diagram in Fig 1 Close to the MS interface a small

contribution from the doubly charged energy level at

0.55 eV may occur Depending on the profile of the sulphur

concentration close to the MS interface, the charge may

give rise to a very thin energy barrier, thus enabling

tunneling at the barrier tip and a lowering of the effective

Schottky barrier height

Assuming a sulphur dopant distribution with two

exponential tails as obtained from ion implantation and

segregation close to the NiSi/Si interface as shown in

Ref.[10], the charge occurring in the depletion region can

be expressed as

Q ¼ qN01 exp  x

x01

þqN02 exp  x

x02

þqNDx, (1)

where q is the electron charge, x is the length coordinate

into the semiconductor volume perpendicular to and with

origin at the MS interface, N01 and N02 are the surface

concentrations of the two sulphur profiles, x01 and x02are

their decay lengths and ND is the shallow doping

concentration in the semiconductor bulk Using Eq (1)

in Poisson’s equation, and the materials parameters from Ref [10] as mentioned in Fig 2, the shape of the conduction band is demonstrated with and without sulphur doping One notices a considerable decrease of the barrier width, facilitating tunneling of electrons and a decreasing effective Schottky barrier height Fig 3 demonstrates the barrier height at 1 nm from the MS interface as a function

of the sulphur surface concentration taking this depth as a reasonable value for substantial tunneling This gives an energy lowering similar to the experimental data as given in Ref.[10]

3 Schottky barriers at nanowire end surfaces modified by surface charge

For nanowire widths in the 10 nm range, surface charges may exist close enough to the MS interface to influence the shape of the Schottky barrier The result of a theoretical calculation for the barrier lowering of a wire with

10  10 nm2 cross section is shown in Fig 4 Positive

Fig 1 Band diagram of a NiSi–Si contact at zero bias showing the two

energy levels of a sulphur double donor and the Fermi level for a doping

concentration of 8  1014cm3.

x[cm]

0.1 0.2 0.3 0.4 0.5 0.6

EC

b a

Fig 2 Conduction band edge as a function of depth at thermal equilibrium (a) with and (b) without sulphur doping Parameter values for the sulphur profile were taken from Ref [10] as: N 01 ¼ 5  10 18 cm 3 ,

N 02 ¼ 5  10 17 cm 3 , x 01 ¼ 10 nm, and x 02 ¼ 30 nm (see Eq (1)) The shallow donor doping of the semiconductor is N D ¼ 8  10 14 cm 3 , the Fermi level is at zero energy and the Schottky barrier height is 0.65 eV.

0 0.1 0.2 0.3 0.4 0.5 0.6

EC

Fig 3 Conduction band edge at x ¼ 1 nm versus increasing sulphur concentration, N N is simultaneously taken as N ¼ 0.1  N

elementary point charges are placed on the surface of the

wire to a concentration of 4  1012cm2 and mirrored in

the metal The barrier lowering for this concentration at a

distance of 1 nm from the MS interface, enough for

appreciable carrier tunneling, is seen to be about 0.2 eV

For our experiments, samples with Pd2Si/Si Schottky

contacts on the end surfaces of silicon wires were prepared

on SOI material by electron beam lithography By using

the substrate as a back-gate, the potential distribution

along the wire could be chosen to separate electron and

hole injection The sample configuration, contact geometry

and charge distribution are demonstrated in Fig 5 The

preparation started from an SOI wafer with a silicon film

thickness of 55 nm and a buried oxide layer (BOX) of

145 nm After thinning the silicon film to 30 nm and

performing patterning by e-beam lithography and plasma

etching, wires with a 30  30 nm2 cross section were

created By oxidation of the wires in dry atmosphere at

800 1C for 90 min, the wires were embedded in a SiO2shell

of about 10 nm, with a silicon core of 15–20 nm width

Following evaporation and patterning of Pd, MS struc-tures of Pd2Si solely in contact with the end surfaces of the wires were obtained by annealing at 250 1C for 20 min Using the Pd2Si contacts as source and drain and the silicon substrate as a back-gate, a transistor configuration can be defined as shown inFig 5(a)

Transfer characteristics at different temperatures for a constant drain voltage of 5 V are shown inFig 6 Three different regimes can be observed among the graphs In regime A, the negative gate voltage pushes the energy bands of the wire to higher energies, thus thwarting the injection of electrons from drain At the source contact, the electric field is increased for negative gate voltages, which allows for hole injection In regime B, the energy bands are lowered such that hole injection is thwarted and the electric field at drain is increased This situation favours injection

of electrons from drain Finally, in regime C, the injected electron current has reached values such that the Schottky diode at source needs to be markedly forward biased in order to bring about enough carrier transport This lifts the

Fig 4 (a) Schematic 10  10 nm2nanowire geometry with a metal contact at the end surface (b) Conduction band lowering 1 nm from the interface due to

an introduced positive point charge distribution surrounding the wire The point charges correspond to a surface state density of 4  1012cm2.

energy bands and allows for ambipolar transport limited

by the two MS structures in combination

Therefore, from Arrhenius plots of the injection currents

in the three regimes A, B and C, activation energies

corresponding to the electron and hole barriers of the Pd2Si

Schottky contacts at source and drain can be determined

Results from such a treatment of the data in Fig 6 are

shown by the filled squares in the plot ofFig 7 The values

in regime A are found in the range of 0.15 eV, while regime

B has a sharp maximum at 0.55 eV and regime C again decreases the activation energy to about 0.25 eV All these values are lower than corresponding energy barriers found for electrons and holes for planar Pd2Si structures It can also be seen that there are no sharp transitions from the

Fig 5 (a) Transistor device structure on SOI with Pd 2 Si source and drain contacts and a back-gate (b) Cross section of the nanowire with charge close the Si SiO 2 interface (c) SEM image of fabricated structure.

Fig 6 Transfer characteristics measured at different temperatures from

303 to 363 K at a fixed V DS of 5 V Regions A, B and C correspond to

hole injection, electron injection and a combination of the two,

respectively.

Fig 7 Effective barrier heights extracted from the temperature depen-dence of the current in Fig 6 Regions A, B and C can be identified After introduction of positive charge the electron barrier is lower and the hole barrier is correspondingly higher.

injection of one carrier type to another, instead the change

is gradual as the gate voltage is swept and the potentials

change correspondingly

In order to investigate whether oxide charge may

influence barrier heights in accordance with the theoretical

estimate above, positive oxide charge was created in the

SiO2shell surrounding the silicon wire This was done by

irradiating the structure with UV light and verifying the

positive charging from the voltage shift in a capacitance

versus voltage measurement on a MOS structure prepared

on the same sample chip with the insulator being the buried

oxide The result is shown by the circles inFig 7 Now the

activation energies in regime A, mainly influenced by the

hole barrier has increased while the values in regime B

influenced by the electron barrier has decreased On the

other hand, the data in regime C, influenced by both

barriers are mainly on the same level as before UV

irradiation

4 Discussion

The introduction of charge by a deep impurity close to

the MS interface of a planar structure decreases the

Schottky barrier in n-type silicon only if the impurity acts

as a donor Two mechanisms contribute to the effect: (i) the

change of charge state occurring in the depletion region

when the impurity energy level passes the Fermi level and,

indirectly, and (ii) the segregation of impurities close to the

MS interface For barrier modulation on p-type silicon, a

similar effect would be expected from a deep double

acceptor like for instance Zn with an energy level for the

most shallow captured hole at about 0.3 eV from the

valence band edge[12]

For a planar MS structure, where the semiconductor

doping is low enough not to influence the barrier height,

the sum of the electron and hole barriers is equal to the

semiconductor band gap In the data ofFig 7, this sum is

below the 1.17 eV band gap value of silicon which would be

expected from the sum of activation plots [13] Also the

values for the electron and the hole activation energies,

respectively, are both lower than the values of Schottky

barriers obtained on planar structures for Pd2Si/Si MS junctions[14] This indicates that tunneling occurs already before the introduction of positive oxide charge due to the added electric field at the contacts created by the gate potential In addition to this, combined injection of both electrons and holes can take place and what is measured is the resulting barrier height However, it is important to note that the changes of activation energies go in opposite direction in the two regimes A and B, where hole injection dominates in first case and electron injection in the latter This is expected from the influence of a positive oxide charge as demonstrated inFig 4 Moreover, in regime C, where both carrier types are injected, the activation energy

is mainly the same because the introduced positive oxide charge changes the barriers in different directions

Acknowledgments This work was financed by the Swedish Foundation for Strategic Research through the NEMO project and by the European SiNANO Network of Excellence

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