Schottky barrier formation for passivated semiconductor surfaces

surface science ELSEVIER

Applied Surface Science 92 (1996) 362-366

Schottky barrier formation for passivated semiconductor surfaces R. Saiz-Pardo *, R. Rinc6n, F. Flores Departamento de Fisica de la Materia Condensada C-XII, Facultad de Ciencias, Universidad Aut6noma de Madrid, E-28049 Madrid, Spain

Received 14 December1994; acceptedfor publication4 March 1995

Abstract

The effect on the Schottky barrier height of H-monolayer passivation of the Si(lll) surface is explored. In our calculations we have analyzed K/Si(111) interfaces (O = 1/3 ML) with and without a H interlayer. Our results show that the effect of passivation is to reduce the Schottky barrier height, ~bb,, by 0.23 eV. Comparison is made with previous results on passivated GaAs(110) surfaces.

I. Introduction

Schottky barriers (SB) are at the heart of many technological devices [1]. Although our knowledge of those interfaces has improved a lot in the last few years, we still need a procedure to tune their barrier heights to the value required by the chosen application [2]. SB heights for covalent and I I I - V semiconductors are mainly controlled by the interaction between the metal adatoms and the semiconductor dangling bonds [2]. Defects may play some role in the barrier formation if their density is above 1013 cm -2 [1]. For lower densities, intrinsic dangling bonds play the major role. Moreover, for ideal interfaces, the intrinsic charge neutrality level (CNL)-defined by the midpoint of the semiconductor optical g a p - p r a c t i -

* Corresponding author. Fax: + 34 1 3974950.

cally coincides with the interface Fermi level [1]. Real metal lattices and, in particular, the geometry and the chemistry of the last metal layer, define an extrinsic CNL, which has to be calculated for each particular case [3]. The extrinsic CNL fixes the Fermi level and the SB height. Notice that the average of the extrinsic CNLs associated with all the interface geometries define the intrinsic CNL [2]. The detailed understanding of the mechanism controlling the SB height [1,2] achieved for the defect free metal-semiconductor interfaces allows us to suggest methods to control the SB formation. In this paper, we explore the possibility of modifying the surface structure of dangling bonds, by depositing an appropriate interface between the metal and the semiconductor. In particular, we analyze theoretically Si(111) passivated surfaces, with semiconductor dangling bonds saturated with appropriate atoms [4]. In Section 2, we present a summary of the method used to calculate the electronic properties of an

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R. Saiz-Pardo et al. /Applied Surface Science 92 (1996) 362-366

interface. In Section 3 we present our main results. In particular, we discuss the specific case of a Si(111)-H saturated surface, and analyze how the SB height changes upon the deposition of K. This has been studied by considering the deposition of K onto both clean Si(111) and Si(lll)-H surfaces. In Section 4 we present our conclusions and a summary of our results.

2. Method of calculation

In our theoretical approach we have followed a selfconsistent LCAO method supplemented with a LD prescription to calculate many-body effects. Ref. [5] gives a full description of our method; here, we only present a very brief summary of the main ideas. The starting point is the following Hamiltonian:

363

spectively, and introduce the following Kohn-Sham Hamiltonian:

jq= oe +

XC ^

(5)

i,tr

This Hamiltonian defines the electronic properties of our system. In our calculations, we obtain the orbital occupancies, (fli~), in a selfconsistent way since the exchange-correlation potential and the Hartree contribution Ui(ai~) + E~(J,.j(~j~) + Jij( njcr )) depend on ( ~ ). We also mention that the effective one-electron Hamiltonian is solved using Green's function techniques [5]. In particular, the semiinfinite crystal is projected onto the last twelve layers, and the resulting film is joined to the adsorbed species. Green's function techniques allow us to obtain the one-electron density of states associated with each orbital and their occupancies.

(1) where

3. Results

~¢ " "t ~o.e.= E E T . i t r . ~ E T~r[ "ij~Cio'Cjo'dt'C)o'Oio ") i,o" tr ,( i,j)

(2)

defines the one-electron contribution, and I~ m .b.= E Ui(°)ni "f ~li i

+½ ~., (Jti°)~i~,~j~+J'i~°)~i~ja)

(3)

i,j~ i,o"

the electron-electron interaction. In Eq. (2), E and T describe the different orbital levels and their hopping interactions; their specific values are obtained by using the wavefunctions, ~bi, of the independent atoms forming the system. In Eq. (3), U and J define the different intrasite and intersite Coulomb interactions associated with the atomic wavefunction, Hamiltonian (1) can be reduced to an effective one-electron Hamiltonian by introducing the Hartree and the exchange-correlation potentials associated with each i orbital [5]. In particular, we define ~EH(/lit r )

8ni~"

~ E Xc ( nio" )

, Vixc

8ni~

(4)

where Ert(ni,) and EXC(n/~) are the Hartree and the exchange-correlation energies of the system, re-

The previous method has been applied, firstly, to calculate the interaction between a H monolayer and an unreconstructed Si(111) surface [6]. (Experimental evidence has shown that a H monolayer removes the reconstruction on this surface.) The semiconductor electronic properties are described using Vogl et al.'s parameters [7]. In a second step, we calculate the effect of depositing one third of a K monolayer onto both clean unreconstructed S i ( l l l ) and H-passivated S i ( l l l ) surfaces. Comparing these two cases we shall deduce the effect of passivation on the SB formation.

3.1. Si(111)-H passivated surface H passivates the Si(111) surface. Theoretical and experimental works [6] have shown that H leaves the Si surface unreconstructed, with H bonded to the Si dangling bonds, located on top of the Si surface atoms. In our calculations, we have assumed the H atoms bond to an ideal Si(111) surface as described above. We have allowed the H layer to relax in the direction perpendicular to the surface and we have looked for the minimum of the chemisorption energy.

364

R. Saiz-Pardo et al./ Applied Surface Science 92 (1996) 362-366 30

25-

20-

o 15¢1 10-

5-

0 -12

o

-10

-8

-6

-4

-2

0

2

gn, erg'll (elO

-1-

Fig. 2. Total density of states for the Si(l 11)-H system.

~.,m_2. t-3-

g

-4-

2

;,

A

;

;

;

;

,o

z (Q.u.)

Fig. 1. (a) Ideal S i ( l l l ) surface with different points where the adatoms can be located. Small circles correspond to the Si second layer. (b) Chemisorption energy for a H monolayer as a function of the H-Si distance.

Fig. la shows the ideal S i ( l l l ) surface with the different sites of adsorption considered in this paper. Fig. lb shows the chemisorption energy for the H monolayer adsorbed at an on-top position, as a function of the H - S i distance. The chemisorption energy per atom, at the most favourable position is 2.7 eV. Fig. 2 shows the total density of states for H and the twelve last layers of Si; these results clearly show that H passivates the Si(111) surface, since no density of states appears in the semiconductor gap.

3.2. Si(l l l)-H / K interface In the next step, we have considered how a K layer is adsorbed on the passivated Si(111)-H surface. In our calculations, we have minimized the energy for a (V~ × V~-) surface cell, with one K per three Si atoms. We have found the energy minimum for the three-fold adsorption site, H a , shown in Fig. l, and have calculated a chemisorption energy of

0.53 eV per atom. Notice that this low energy corresponds to a weak bond between the adsorbed alkali metal atom and the chemically saturated S i ( l l l ) surface. Fig. 3 shows the local density of states calculated for this adsorption geometry. The important point to notice is the following: due to the K adsorption, a new density of states starts to develop around the bottom of the semiconductor conduction band. This new density has Si character, and pins the Fermi energy at 0.75 eV above the semiconductor valence band top. The Fermi level calculated in this way should be close to the final position which would be

3O

25t 2O

~, 15-

10-

5-

0

I

I

I

I

I

I

-12

-10

-8

-6

-4

-2

I

I

;"

2

gnerg'U (elO

Fig. 3. Total density of states for the Si(lll)-H/KVr3"xyr3 system.

365

R. Saiz-Pardo et a l . / Applied Surface Science 92 (1996) 362-366

obtained for a thick metal layer [2]. Instead of investigating this limit, we have tried to find out how the Fermi level position at the interface depends on the H monolayer. To this end we have studied the Si(111)-K(vt-3" × ~/'3) interface and have calculated the position of the interface Fermi energy.

3.3. Si(I I1)-K(vl3 × ~f3 ) interface The adsorption of alkali metals on Si(ll 1) has been analyzed for different reconstructions. Cs on Si(lll)-2 × 1 [8] seems to eliminate the clean surface reconstruction and forms a (¢~ × f 3 ) R 3 0 ° structure, probably with the alkali atoms located on three-fold sites. The Si(111)-2 x 1 reconstruction is, however, stable upon K adsorption [9]. Furthermore results for K on Si(lll)-7 × 7, although somewhat ambiguous [10], suggest that the surface reconstruction is not substantially modified by the alkali metal deposition. Recently, Weitering et al. [11] have argued that in this surface K atoms bond directly to the Si dangling bonds leaving all the Si-Si bonds intact. In order to understand the effect of passivation on the SB we have neglected the S i ( l l l ) reconstructions and have analyzed K deposited on an ideal Si(111) surface, assuming that the alkali atoms form a (V~-× v~')R30 ° structure. This is the structure found for Cs on Si(ll 1)-2 × l, and it has the advantage of allowing us to make a direct comparison between the clean and passivated surfaces. We should also stress that the differences found between de SB heights for K and Cs are very small, suggesting that the crucial ingredient for explaining the SB formation is the interaction between the Si dangling bonds and the K atoms. Our results, for this structure ( S i ( l l l ) - K f 3 × V~-), show that K is adsorbed on to a three-fold geometry (H 3 site on Fig. 1), and that the chemisorption energy is 2.4 eV. (Similar results have been obtained by Northrup [12] and Moullet et al. [13] for Na on Si(111).) This energy is much larger than the one found for K adsorbed on the Si(ll I)-H passivated surface, reflecting the larger reactivity of the clean surface. Fig. 4 shows the local density of states associated with the Si(111)-KV~- × v~- interface. In this calculation, the Fermi level is pinned by the surface states associated with the Si dangling bonds: this is under-

30 2520,o 15105-

0 -12

-10

-8

-6

-k

-2

0 'Er

Ena*'gy (OV') Fig. 4. Total density of states for the Si(l 11)/Kv~ X vf3 system.

stood by realizing that the interaction of K with three Si dangling bonds (the atoms of the unit cell) yields a filled single level (with a K - S i character) and a doubly degenerate level (with a Si character) that is half filled with two electrons. Thus, we find two electrons filling the surface state of Fig. 4, that pins the Fermi energy. Notice also that the Fermi level in this calculation is located at 0.52 eV above the semiconductor valence band maximum.

4. Conclusions

Previous results show the evolution of the Fermi level with H~assivation for a Si(11 l) surface. For a S i ( l l l ) - K ( ~ / 3 × V~') interface the Fermi level is located 0.52 eV above the valence band top, while for the Si(111)-H/K()f3" × f 3 ) interface the Fermi level appears at 0.75 eV. This shows that H passivation tends to decrease the SB for electrons, 4)b~, by 0.23 eV. These results are in excellent agreement with recent data by Kampen and MSnch [14] who reported on barrier heights for Pb/Si(11 l) contacts without and with a H interlayer. These authors have observed a reduction of the SB of 0.22 eV due to the H interlayer. This trend is similar to the one found for GaAs(110), although smaller [4]. In this crystal, passivation (either H- or As-passivated surfaces) creates ohmic contacts due to an important shift, larger than 0.7 eV, of the Fermi level towards higher energies.

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R. Saiz-Pardo et aL /Applied Surface Science 92 (1996) 362-366

Although we find the same trend for Si, the effect is smaller for this semiconductor since the shift is reduced here to 0.23 eV. This can be rationalized considering the interaction of the semiconductor dangling bonds with the layer saturating the surface. In GaAs this interaction is rather asymmetric, with the Ga-like and the As-like dangling bonds presenting a different reactivity with the adsorbate, say H. In Si, the interaction is obviously symmetric with the Si dangling bond located in the middle of the semiconductor energy gap. This kind of symmetry is responsible of the different effects that passivation introduces in GaAs(110) and Si(111).

[2] [3]

[4] [5] [6] [7] [8]

Acknowledgements [9]

Support by the Spanish CICYT (contract No. PB92-0168-C), the EC (CHRX-CT93-0134) and Iberdrola is acknowledged.

References [1] E.H. Rhoderick and R.H. Williams, Metal-Semiconductor Contacts, 2nd ed. (Oxford Univ. Press, Oxford, 1988); W.

[10] [11] [12] [13] [14]

M~nch, Electronic Structure of Metal-Semiconductor Confacts (Kluwer, Dordrecht, 1990); F. FIores and C. Tejedor, J. Phys. C 20 (1987) 145; F. Bechstedt and M. Scheffier, Surf. Sci. Rep. 18 (1983) 145. F. F'lores and R. Miranda, Adv. Mater. 6 (1994) 540. M. Lannoo, C. Priester, G. Allan, I. Lefebvre and C. Delerue, in: Metallization and Metal-Semiconductor Interfaces, Ed. I.P. Batra (Plenum, New York, 1989) p. 259; A. Mufioz, J.C. Dufan and F. Flores, Appl. Surf. Sci. 41/42 (1989) 144. R. Saiz-Pardo, R. Rinc6n, R. P~rez and F. Flores, Surf. Sci. 307 (1994) 309. FJ. Garcfa-Vidal, A. Mart~n-Rodero, F. Flores, J. Ortega and R. P~rez, Phys. Rev. B 44 (1991) 11412. FJ. Garcla-Vidai, J. Merino, R. P~rez, R. Rinc6n, J. Ortega and F. Flores, Phys. Rev. B 50 (1994). P. Vogl, P. Hjalmarson and J.D. Dow, J. Phys. Chem. Solids 44 (1983) 365. K.O. Magnusson and B. Reihl, Phys. Rev. B 39 (1989) 10456. B. Reihi and K.O. Magnusson, Phys. Rev. B 42 (1990) 11839. K.O. Magnusson, S. Wicklund, R. Dudde and B. Reihl, Phys. Rev. B 44 (1991) 5657. H.H. Weltering, J. Chen, N.J. DiNardo and E.W. Plummet, Phys. Rev. B 48 (1993) 8119. J.E. Northrup, J. Vac. Sci. Technol. A 4(1986) 1404. 1. Moullet, W. Andreoni and M. Parrinello, Phys. Rev. B 46 (1992) 1842. T.U. Kampen and W. M~nch, Surf. Sci. 331-333 (1995) 490.