Fermi-level pinning and interface states at PbSi(111) interface

0038-1098/92$5.00+.00 Pergamon Press Ltd

Solid State Communications, Vol. 82, No. 11, pp. 863-866, 1992. Printed in Great Britain.

FERMI-LEVEL

PINNING AND INTERFACE Pb-Si( 111) INTERFACE Stefano

STATES AT

Ossicini and F. Bernardini

Dipartimento di Fisica and INFM ,Universit& di Modena, Via Campi 213/A, 41100 Modena, Italy (Received 15 March 1992 by C. Calandra)

The first theoretical analysis of the electronic properties of the Pb-Si(ll1) interface at monolayer coverage is presented. We have used the self-consistent linear m&n-tin orbital method with the atomic-sphere-approximation in a “supercell” geometry. The results show the presence of a discrete true interface state which is responsible of the pinning of the Fermi level and of the large Schottky barrier heights found for this system.

Introduction The

system

(0.94 formed

by the deposition

of Pb on the Si( 111) surface

is receiving

a great

been proposed

as a model structure

(SBH)

on the

for Pb-Si(ll1)

interface structure, Since the discovery

contacts

interface silicon,

depends

strongly

is the only system the local electronic

NiSiz-Si(ll1)

interfaces

x

between

x

fi)R

30”.Pb

l3 this

cussion

and perhaps

case of the two epitaxial Si( 111) interface. the atomic

orientations

Considerable

structure,

is still

is far more complicate

exists

consists

of a 1:l mixture

in the about

structure

understanding

responsible of the link

and the electronic

proper-

we have demonstrated

systems, systems

owing the a.nd recon-

that the Linear

Muf-

(LMTO) method in the Atomic Spheres (ASA) within the local density approxi-

with atomic number equal to zero, allowing the investigation of surface and interface systems in the same version normally used for bulk problems. We present here the first theoretical study of the PbSi(lll)

of Pb and Si adatoms

11.12

Very recently

0.9 eV below

ASA (SLMTO-ASA) approach is to describe the vacuum region by a solid formed by empty sphere atoms, i. e.

and the number of dis-

Hibma et al. 7~10have shown using pho-

toelectron spectroscopy that at monolayer coverage the Pb-Si( 111) system gives even higher values for the SBH

one about

mation (LDA) is a powerful tool for the calculation of the electronic properties of clean surfaces and interfaces at low coverage 14-16. The basic idea of this surface LMTO-

tinct phases; thus it has been argued that the variations of the SBH arise from different numbers of Pb atoms which are in contact with the Si surface 6,8 or that the interface

two discrete

bulk band gap of

The first one was considered

the atomic

fin Tin Orbital Approximation

for the NiSi2( 11 I )-

controversy

the coverage

one close and the other

Recently

struc-

under disthan

in the projected

have been performed for the Pb-Si difficulty in treating such complicate structions.

ture. When a thick layer of Pb is grown on the two reconstructions two different n-type SBH are measured: 0.70 eV and 0.90 eV ‘. The reason of this large difference

photoemission

ties of this interface, one needs further experimental and theoretical studies. Until now no theoretical calculations

which emphasizes the importance of structure for Schottky barrier forma-

7 and a Si(lll)(&

levels located

respec-

for metal-silicon

they could identify

for the Fermi- level pinning. It is clear that, for a better

tion at ordered interfaces. In fact two different structures can be formed at the interface for a monolayer coverage; a Pb-Si(lll)7

are exceptional

using a.ngle-resolved

(ARUPS)

the Fermi level.

which can be modified by annealing. of the relation between atomic struc-

ture and SBH at epitaxial

heights

Moreover

spectroscopy

for t.he growth of met5 have height

These

contacts.

deal of

attention at the moment’-‘*. The reason is twofold: due to the unreactivity of the Pb-Si interface this system has als on semiconductors; moreover Heslinga. ef al. recently reported that the n-type Schottky barrier

eV and 1.04 eV, for the two orientations

tively).

of thin layers

interface

using the SLMTO-ASA

goal is to investigate

the presence

the relation

structural

between

and electronic

Calculations and results The detailed interface structure 863

method.

of interface

states

Our and

properties.

of the real Pb-Si( 111)

Pb-Si(111) I N T E R F A C E

864

system is still under discussion, thus we have started our theoretical study using a well ordered unreconstructed Pb-Si(111) l x l interface at monolayer coverage. Firstly this systems is very interesting in itself, secondly regions of l x l Pb termination have been observed and studied experimentally 1,s,11,12 finally the theoretical results for the ideal case will give new insight into the experimental data. In the calculation, the Pb-Si(111) 1 × 1 interface is simulated by a 12 layers Si(111) slab covered with a monolayer of Pb atoms and a vacuum region made of empty sphere layers. Our supercell crystal is formed by a threedimensional unit cell with one atom per (111) layer; the total number of atoms per unit cell is 4 2 : 1 2 Si + 12 ES for the Si side + 2 Pb atoms on both sides of the slab + 16 ES to simulate the vacuum. For the radii of the spheres we use the standard ASA packing requirement starting from non overlapping spheres around each atom. Details of the calculation will be published elsewhere 17 The calculations have been done at different geometries in the one monolayer regime. The Pb atoms were

(d) 1.0-

0.6-

0.01.6-

o %

(c)

1.0-

0.5-

.,o 0.0.,o

1.0-

Vol. 82, No. 11

adsorbed on different sites on the Si surface: Pb atoms on the triangular filled sites (T4), with the metal atoms located directly above the subsurface Si atom (this is the site which is occupied by Pb atoms in the S i ( l l l ) ( v / 3 x x/3)R 30°-Pb structure), and Pb atoms on the on top site (T1), where the metal atoms are located directly above the Si surface atoms. In both cases the Pb-Si nearest neighbour distance is equal to 2.40 ~k; this is the outcome of the LEED investigation of Doust and Tear 9 for the T4 adsorption site at low coverage.In the two different atomic structures (T4 and T1) the number of adsorbed atoms equals the number of surface Si atom. First we will discuss the results for the interface with Pb at the T4 sites. The self-consistent local densities of states of the interface system projected on the silicon and lead sites (PDOS) are shown in Fig. 1. The PDOS of the central layer silicon atoms [panel (a)] is already similar to that of bulk silicon, showing that our supercell is large enough to simulate a realistic interface. Considering the silicon layer at the interface [panel (b)], we observe that the perturbation due to the presence of Pb atoms gives rise to states inside the gap. These are responsible of the metallic character of the Pb-Si system in agreement with the experiments 1,3,7,10. Another interface feature is clearly present at about 8 eV below the Fernfi level. These peaks are also present in the PDOS relative to the Pb adsorbed layer [panel (c)]. Both interface features (near the Fermi level and 8 eV below the Fermi level) were observed in the valence band photoemission spectra for the system formed by one monolayer of Pb adsorbed on S i ( l l l ) 3. Our results, that refer to the ideal lxl structure, compare favourably with the experimental data. The SBH is given by the energy difference between the bottom of the semiconductor conduction band and the Fermi level; it is well known that the minimum gap of semiconductor and thus the SBH are understimated by the LDA. This difficulty may be partially overcome considering that we correctly estimate the energy differ-

(b) 8

1.0-

6-

e~

0.6-

42-

0.01.5-

0

g

(a)

~-I

-4-

1.0-6 -0

0.6-

-100.0-18

-12

-8

-4

4

Energy E-E F (eV)

-12 -14 M

Fig.1 Site-projected density of states (PDOS) for the PbS i ( l l l ) system with the Pb atoms located at T, sites. The different curves show PDOS for: (a) central Si layer; (b) interface Si layer; (c) interface Pb layer; (d) first empty sphere layer on the vacuum site. Energies (in eV) are referred to the Fermi level.

K

M

Fig.2 Band structure along high symmetry directions of the two-dimensional Brillouin zone of the P b - S i ( l l l ) interface with the Pb atoms located at T4 sites. Dots and squares indicate interface states. Energies (in eV) are referred to the Fermi level.

Vol. 82, No. 11

Pb-Si(111) INTERFACE

ence between the Fermi level and the the valence-band maximum of Si, consequently by adding the experimental Si gap value (1.12 eV) we may compute the n-type SBH. In this way we obtain 0.60 eV. It is important to clarify the presence and the nature of the states which are responsible for the Fermi-level pinning at the interface and hence for the determination of the SBH. Figure 2 gives the band structure in the two-dimensional, hexagonal Brillouin zone. In the figure the interface states are marked by dots and squares, the tickness of the dots and squares is proportional to the wave functions composition of the interface states arising from the s and p orbitals located at the Pb adatoms. We see that the interface band located about 8 eV below the Fermi level, which shows a small dispersion, has mainly Pb s character. More important are the interface states located near the Fermi level in the band gap of bulk Si. Their energy location and dispersion are very similar to the experimental ones determined by ARUPS4; this confirms that our ideal lxl interface in the monolayer regime represents well the complex real systems. The upper discrete interface state crosses the Fermi level and is therefore responsible for the Fermi-level pinning; this state is mainly due to the interaction between the

(d)

865

p orbitals of the Pb adatoms and the dangling bonds of the surface Si atoms. Thus for the Pb-Si interface the Fermi-level pinning is due to a genuine interface states in agreement with the experimental result of Hibma et al. 7,10; the same mechanism has been considered previously only in the case of the NiSi~(111)-Si(111) epitaxial interface ls-19 As to the relation between the structural and electronic properties, Fig. 3 shows the calculated PDOS for the interface with Pb atoms located at T1 sites. Also in this case we note several interface features similar to the previous ones, however we see that the Fermi level is now shifted downwards in the gap and is located just at the top of the Si valence band. In this case the SBH is very high and is equal to the experimental Si gap (1.12 eV); the comparison with the results for the T4 case (0.60 eV) shows that there is a strong dependence of the electronic properties on the structural ones. Figure 4 shows the two-dimensional band structure for the 711 case; again we notice that the states pinning the Fermi level are true interface states located in the Si band gap. In summary we have performed the first theoretical investigation of the Pb-Si(111) interface; our calculations, performed for the ideal lxl interface in the monolayer regime at different geometries, show that for this system the interface barrier depends strongly on the bonding configuration, that the SBH can be very large and that interfacial states pinning the Fermi level are present in the band gap.

1.0.

Acknowledgements

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Financial support by Consiglio Nazionale delle Ricerche through Contracts No. 90.00658.PF69, No 89.00214.12 and 90.04139.CT12 and Ministero Universit£ e Ricerca Scientifica is acknowledged. The calculations have been performed at the Centro Interdipartimentale di Calcolo Automatico e Informatica Applicata (CICAIA), University of Modena and at the Centro di Calcolo Interuniversitario dell'Italia Nord Orientale (CINECA), Bologna.

(b)

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Fig.3 The same as in Fig. 1 for the P b - S i ( l l l ) system with the Pb atoms located at 7'1 sites,

-14 M

K

M

Fig.4 The same as in Fig. 2 for the P b - S i ( l l l ) system with the Pb atoms located at T1 sites.

866

Pb-Si(111) INTERFACE

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