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硼在Si(100)表面引起的原子结构特征

https://www.wendangku.net/doc/0a13393611.html, Atomic structures of boron-induced protrusion features on

Si(100)surfaces

Zhenghui Liu,Zhaohui Zhang,?and Xing Zhu

State Key Laboratory for Arti?cial Microstructures and Mesoscopic Physics, School of Physics,Peking University,Beijing100871,China

(Dated:August19,2007)

Abstract

It is known that ultrahigh doping can be realized for boron on Si(100)substrates while boron-induced features on a heavily boron doped Si(100)surface cannot form any periodic structure.Here we demonstrate that boron-induced features actually result from the adsorption of boron-silicon addimers,owing to the underneath substitutional boron atoms at the second layer.Furthermore, more closely arranged boron atoms at the second layer make the energy of the(2×1)surface lower, and the whole second layer can be completely occupied by boron atoms while the surface is still (2×1)reconstructed.

Boron (B)is a widely used p-type dopant in silicon-based semiconductor technology and its particular properties in the silicon crystal have been interesting for up-to-date silicon devices [1–3].Ultrahigh doping can be realized for B on Si(100)substrates with a volume concentration of up to 25%[4,5].The understanding of the related atomic process involves the B e?ects on silicon surfaces,since the B doping in silicon is inevitably related to the B segregation and B-induced atomic structures on silicon surfaces [6].It has been found from scanning tunneling microscope (STM)observations that the segregated B atoms from silicon can induce a (√3×√3)structure on the (111)surface [7].Such a B-Si alloy surface becomes so stable that a silicon epitaxy can be changed,giving rise to stacking faults in a grown ?lm [8].On a Si(100)surface,however,all the published STM observations have demonstrated a common fact that the segregated B atoms from a heavily B doped sample cannot accumulate enough so as to change the original (2×1)reconstruction,regardless of how high the annealing temperature and how long the annealing time [9–12].This exper-imental fact doubts a conclusion from some Auger measurements and STM investigations that a Si(100)surface can reconstruct into c-(4×4)with B atoms of 0.5monolayer [13].It is already well known in the literatures that the segregated B atoms on a Si(100)surface just induce randomly distributed protrusions,most of which are paired,some single and some tripled or several assembled,but all never aggregate a patch of periodic structure [9–12].Such behaviors of protrusions have been confusing in understanding the Si(100):B surfaces for more than ten years,and so far there is no acceptable model for them.Because Si(100)is usually used as substrates for device manufacture,it is undoubtedly important to ?nd out the B doping capacity of a Si(100)surface and the related surface structures.In this letter we will reveal the atomic structures of the B-induced protrusion features and will further discuss the B doping capacity on Si(100)surfaces.

We ?rst check Si(100):B surfaces that present the B-induced protrusions in experiment.Our samples were prepared by annealing heavily B doped Si(100)samples (ρ=0.01?·cm)in an ultrahigh vacuum,as had ever done by others [9],and they were observed by using an STM equipped in the vacuum system.Fig.1shows one of our high-resolution STM images,where arrow I points to a single protrusion,arrow II to paired protrusions,and arrow III to tripled protrusions.In a block of tripled protrusions the middle one is clearly bigger and higher than the other two on its both sides.In addition,a few percent of protrusions may also appear in a di?erent size or brightness in STM images (not shown here).All 2

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III

FIG.1:An empty state STM image presents common features of B-induced protrusions and striking bending of atom rows beside all the protrusions.The image was acquired at a sample voltage of2.0V and a tunneling current of0.2nA.arrow I points to a single protrusion,Arrow II to paired protrusions,and arrow III to tripled protrusions.The bending of atom rows next to the protrusions is marked by connected dots.The two connected circles indicate the position of a dimer.

the protrusions are located between two dimer rows,which can be recognized in the image with the aid of two connected circles that represent a dimer[14].Compared with dimers as marked,we can see that each protrusion makes four adjacent atoms invisible,which belong to four adjacent dimers,and the whole appearance is mirror symmetric with respect to the dimer orientation.It is interesting to note a striking phenomenon in our STM observations that the atom rows beside all the protrusions become bent as if they were attracted by the protrusions,which is indicated in the?gure by connected dots.The dotted atoms next to a protrusion can deviate from their original positions by as large as0.8?A.This surface strain phenomenon provides a new evidence about B-Si interaction on a Si(100)surface and will be particularly considered in our modeling below.

In order to investigate the structural features of the protrusions we performed ab initio calculations using CASTEP computer program[15],which employs the plane-wave pseu-dopotential method based on the density functional theory(DFT)within the localized density approximation(LDA).For such calculations one has to periodically arrange the protrusion blocks on a Si(100)-(2×1)surface and the distance between them should be large enough to prevent their interaction.In practice we could construct a periodic block with a top area of6×3unit cells of the Si(100)surface and a thickness of ten atomic layers owing to

(a)(c)

+2V -2V -2V

+2V (b)

(d)FIG.2:Structures and STM simulations of two models that have ever been proposed.(a)and (b)Optimized structure (Upper:top view,lower:side view)and its STM images simulated from the denuded silicon atom model.(c)and (d)Optimized structure (Upper:top view,lower:side view)and its STM images simulated from the B-adsorption model.In side view of (c),an atom is supposed to be adsorbed as indicated by a dashed circle to form a closed ring.

our limited computation capacity.The positions of the atoms of the bottom four layers were ?xed during the structural optimization.The plane-wave kinetic energy cut-o?was 160eV and the k-points for the Brilliouin-zone sampling were 1×2×1using the Monkhorst-Pack scheme.The STM simulations were generated from the results of CASTEP calculations with Terso?-Hamann approach [16].

Before engaged in our modeling we would like to have a look at two models that have ever been proposed for the paired protrusions,in order to present some clues for further consideration.One is the denuded silicon atom model [9]and the other is the B adsorption model [10].Based on our ab initio calculations,the structures and the STM images of these two models were simulated.From the optimized atomic structures and their STM images shown in Fig.2,we can see that in the denuded silicon atom model a B atom located at the third layer can pull the surface atoms toward its side while in the B-adsorption model the adsorbed B clearly pushes the adjacent atoms away.But both of them cannot correctly reproduce the typical features of protrusions as STM images display.Considering the surface strain induced by B as indicated in Fig.1,it seems necessary to have an adsorbed dimer that connects two neighboring dimer rows so as to form a closed ring,as indicated in Fig.2c,and such a ring would contain several B atoms to shrink in its size,since a B-Si or B-B bond is shorter than a Si-Si one [7].4

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B (1

1C 2

C 1

D 12B 2FIG.3:The possible positions of B atoms in a ring.The surface energies of symmetrically paired rings are shown in Tab.I.

TABLE I:Surface energies[17]of symmetrically paired rings with B atoms at di?erent positions.The possible positions of B atoms in a ring is marked in Fig.3.Positions

Energy (eV)(A 1)

-0.56(A 1,C 1)

-3.31(A 1,C 2)

-3.06(A 1,D)

-2.87(A 1,C 1,C 2)

-4.35(A 1,C 1,D)

-3.41(A 1,C 2,D)

-3.32(A 1,C 1,C 2,D)-3.24

Based on the calculated surface energies and the simulated STM images of symmetrically paired rings with various Si-B con?gurations,we were able to determine the atomic structure that reproduces the features of paired protrusions.As shown in Fig.3,a ring mentioned above is related to an adsorbed dimer (A 1and A 2),four surface atoms (B 1,B 1,B 2,and B 2),two subsurface atoms (C 1and C 2)and one atom at the third layer (D 1).If an adsorbed dimer contains a B atom,this B atom will be invisible in the simulated STM images,because a B atom just has three covalent electrons and these electrons are all combined into covalent bonds at deep energy levels.Thus a protrusion may be reproduced with a B atom at the position marked by A 1and with a Si atom at the position A 2.Connected with an adsorbed dimer are four surface atoms.Because a block of B-induced protrusions have the mirror symmetry with respect to the orientation of the dimers,occupation of B atoms at these four sites must have the same symmetry.Therefore,these four sites might contain four,two or zero B atoms.However,in all the cases of B atoms at these four sites,the simulated STM 5

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(a)(d)

(b)

FIG.4:Atomic structure of paired protrusions.(a)Optimized structure of symmetrically paired Si-B rings.Upper:top view;lower:side view.(b)A simulated STM image at sample voltage of 2V.The connected dots marked indicate the displacements of the atoms toward the protrusions.(c)Appearance of protrusions changes with sample voltage.Upper row:experimental results;Lower row:simulated results.(d)Relative heights of protrusions with respect to the average height of the surrounding surface atoms.

images of Si-B addimers do not accord with the experiments and the surface energies are high.So the occupations of B atoms at these four sites are not favored.The last three sites at the second and the third layer are located along the symmetry axis and might be equally available for B atoms to occupy.Thus,the possible sites for B atoms would be those marked by A 1,C 1,C 2and D.The calculated surface energies for a block of paired rings with various Si-B con?gurations at these four sites are listed in Tab.I.Clearly,one B atom in an adsorbed dimer and two B atoms at the positions of the second layer (A 1,C 1,C 2),forming symmetrically paired Si-B rings,lead to the minimum of surface energy of -4.35eV.

Fig.4a shows the optimized atomic structure of paired rings with B atoms at positions A 1,C 1,and C 2.The observed surface strain is reproduced in our simulated STM images,as shown in Fig.4b,where small displacements of atoms are marked with connected dots.Meanwhile,it is determined that the protrusion features are actually contributed by the silicon atoms in the adsorbed dimers.Furthermore,the observed protrusions become weak 6

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TABLE II:Surface energies[17]of paired protrusions arranged in di?erent period

Period(4×2)(6×2)(6×3)

Energy(eV)-2.35-3.29-4.35

with decrease of absolute sample voltage value,which is more pronounced for STM obser-vations at positive sample voltages than at negative ones[9].We show such phenomenon in Fig.4c,where the images in the upper row are from our experiments,those in the lower row from our simulations.Fig.4d shows the relative heights of protrusions with respect to their surrounding surface atoms,which were measured and simulated.On the whole,the simulated results agree well with the experimental observations.

There is still an artifact in the simulated STM images that the atoms surrounding the paired protrusions appear in di?erent brightness.This problem could arise from the close arrangement of calculated blocks because the similar phenomena are often observed in exper-iments when two protrusion blocks are located close enough.We consider that there exists some interaction between the protrusion blocks,because the surface energy calculated for a block of paired protrusions increases when the block size is decreased,as shown in Tab II. This result suggests that the dense arrangement of protrusions is energetically unfavorable, which agrees with the experiments.Such interaction may result from the surface strain as shown in Fig.1.The calculation of a sparse enough arrangement of the protrusion blocks would help identify the artifact,if we could have enough computer capacity.On the other hand,the dynamic e?ects of the dimers may be also a reason.Our simulation is based on the calculations at0K,while the Si dimers are usually in?ip-?op motion induced by thermal activation at room temperature[18]or by the probe e?ect of STM[19],which would lead the atoms to appear in the same height during STM observations.

As demonstrated above,a block of paired protrusions results from two symmetrically paired rings,each of which contains one B atom in the addimer and two B atoms at the positions of the second layer.According to the simulated STM images shown in Fig.4and the calculated surface energies shown in Tab I,we can see that the Si-B addimers display the main STM features of the paired protrusions while the B atoms at the second layer are indispensable for stabilizing the structures.From the viewpoint of surface energy,the Si-B addimers could not survive if there were no B atoms underneath at the second layer,

(a)

(c)

FIG.5:Si(100)-(2×1)surfaces resulting from B atoms occupying the whole second layer.(a)Structure of the clean (2×1)surface;(b)Structure of the (2×1)surface with a complete second layer of B atoms.(c)A ?lled state STM image presenting the surface with B at the second layer,which was acquired at a sample voltage of -1.6V and a tunneling current of 0.3nA.As a comparison,the inset shows a ?lled state STM image of the surface with low B coverage,which was acquired at a sample voltage of -2.0V and a tunneling current of 0.2nA.A block of paired protrusions is marked by a frame on both ?gure and inset.

but the protrusion density were limited because of their strong interaction,although the B density at the second layer could be high.After removing the Si-B addimers in the structural optimization,we found that more closely arranged B atoms at the second layer make the energy of (2×1)surface lower and when the whole second layer is occupied by B atoms the

surface energy decreases by a value of 30.1meV/?A

2with respect to the clean Si(100)-(2×1)surface.Fig.5a and 5b show the side views of optimized Si(100)-(2×1)surfaces without any B atom and with pure B atoms at the second layer,respectively.Because of the short Si-B bonds the surface atoms are lowered by 0.8?A ,as indicated in Fig.5a and 5b.We prepared such surfaces by annealing samples at 1200?C,which had been implanted by a B dose of 1016atoms/cm 2at energy of 60keV.Fig.5c shows a ?lled state STM image of such surfaces,in which there are also protrusions randomly dispersed on the (2×1)-reconstructed surface.But compared to the Si(100)-(2×1)surfaces with low B coverage as shown in the inset of Fig.5c,the dimers of the (2x1)surface become much lower than the protrusions by about 1.0?A and single protrusions become more than paired ones.According to our simulations these protrusions have the same structures as demonstrated above,but the second layer 8

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underneath the(2×1)surface should be completely doped with B.

In summary,the STM observed protrusions and related surface strain result from the Si-B rings that may have B atoms in the addimers and at the subsurface layer.The protrusions cannot accumulate to form a patch of any periodic structure owing to the strong interaction between the structural modules.The subsurface layer can be completely doped with B, while the(2×1)reconstruction of the surface basically remains,which is very important for understanding ultrahigh doping of B on Si(100)surfaces.

This work is supported by the National Natural Science Foundation of China with Grant No.90406007,Grant No.10434010and Grant No.10574002,and by the Research Fund for the Doctoral Program of Higher Education with Grand No.20040001002,and by the Na-tional Basic Research Program of China(973Program)2007CB936800.The computation was made on the HP high-performance computer cluster provided by Center for Computa-tional Science and Engineering of Peking University.

?zhangzh@https://www.wendangku.net/doc/0a13393611.html,

[1] D.W.Donnelly,B.C.Covington,J.Grun,R.P.Fischer,M.Peckerar,and C.L.Felix,Appl.

Phys.Lett.78,2000(2001).

[2]L.S.Robertson,K.S.Jones,L.M.Rubin,and J.Jackson,J.Appl.Phys.87,2910(2000).

[3]J.F.Nielsen,J.P.Pelz,H.Hibino,C.W.Hu,I.S.T.Tsong,and J.Kouvetakis,Appl.Phys.

Lett.79,3857(2001).

[4] A.Vailionis,G.Glass,P.Desjardins,D.G.Cahill,and J.E.Greene,Phys.Rev.Lett.82,

4464(1999).

[5]G.Glass,H.Kim,P.Desjardins,N.Taylor,T.Spila,Q.Lu,and J.E.Greene,Phys.Rev.B

61,7628(2000).

[6]X.Luo,S.B.Zhang,and S.H.Wei,Phys.Rev.Lett.90,026103(2003).

[7]I.W.Lyo,E.Kaxiras,and P.Avouris,Phys.Rev.Lett.63,1261(1989).

[8]H.Hibino and T.Ogino,Mat.Sci.Eng.B87,214(2001).

[9]M.A.Kulakov,Z.Zhang,A.V.Zotov,B.Bullemer,and I.Eisele,Appl.Surf.Sci.103,443

(1996).

[10]T.Komeda and Y.Nishioka,Appl.Phys.Lett.71,2277(1997).

https://www.wendangku.net/doc/0a13393611.html, [11]Z.Zhang,M.A.Kulakov,B.Bullemer,I.Eisele,and A.V.Zotov,J.Vac.Sci.Technol.B14,

2684(1996).

[12]J.F.Nielsen,H.J.Im,J.P.Pelz,M.Krueger,B.Borovsky,and E.Ganz,J.Vac.Sci.Technol.

A17,1670(1999).

[13]Y.J.Wang,R.J.Hamers,and E.Kaxiras,Phys.Rev.Lett.74,403(1995).

[14]R.J.Hamers,P.Avouris,and F.Bozso,Phys.Rev.Lett.59,2071(1987).

[15]M.D.Segall,P.J.D.Lindan,M.J.Probert,C.J.Pickard,P.J.Hasnip,S.J.Clark,and

M.C.Payne,J.Phys.:Condens.Mat.14,2717(2002).

[16]J.Terso?and D.R.Hamann,Phys.Rev.Lett.50,1998(1983).

[17]The block energy is de?ned as surface energy of the paired protrusions minus that of the(2×1)

surface.

[18]M.Matsumoto,K.Fukutani,and T.Okano,Physical Review Letters90,106103(2003).

[19]M.Ono,A.Kamoshida,N.Matsuura,E.Ishikawa,T.Eguchi,and Y.Hasegawa,Phys.Rev.

B67,201306(2003).