DTIC ADA482500: The Structure of Si(112):Ga-(N x 1) Reconstructions

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by  Defense Technical Information Center

SurfaceScience423(1999)L265–L270
SurfaceScienceLetters
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The structure of Si(112):Ga-(N 1) reconstructions
A.A. Baski a, S.C. Erwin b, L.J. Whitman b,*
aDepartmentofPhysics,VirginiaCommonwealthUniversity,Richmond,VA23284,USA
bNavalResearchLaboratory,Washington,DC20375,USA
Received1December1998;acceptedforpublication21December1998
Abstract
We have studied the structure of Si(112):Ga-(N×1) reconstructions using atomic-resolution scanning tunneling
microscopy and first-principles calculations. The nanofaceted clean Si(112) surface becomes planar following the
adsorption ofGa, which forms long chains on the surface interrupted by isolated quasi-periodic defects. The defects
create a mixture of (N×1) structures (N#4–7) with 5×1 and 6×1 unit cells most common. We demonstrate that
thisstructureconsistsofachainofGaatomsadsorbedatthe(111)-likestepedgewithinthe(112)unitcell,andthat
the defects are Ga vacancies where the exposed step edge Si atoms form a dimer bond. Calculations performed as a
functionofvacancyperiodconfirmthatthesurfaceenergyisminimizedatN=5–6,whencompressivestrainassociated
withthe Si–Gabonds iseVectively minimized.© 1999ElsevierScienceB.V. All rightsreserved.
Keywords: Density functional calculations; Gallium; High index single crystal surfaces; Scanning tunneling microscopy; Silicon;
Surfaceenergy;Surfacestructure,morphology,roughness,andtopography
A variety of systems have been explored as step edge ‘atomic-wire’ system [3–7]. However, it
candidates for the growth of self-assembled wires was recently shown that clean Si(112) is faceted
on the nanometer scale. For example, V-shaped [8],andonlybecomesplanarfollowingtheadsorp-
grooves have been used as templates for growing tion of Ga [9]. This large-scale surface restructur-
quantumwirearrays[1],andvicinalsurfaceshave ing indicates that Ga interacts strongly with the
been used to nucleate atomically thin wires at the substrate, and therefore the self-assembly in this
step edges [2]. In these systems it is generally case is a fundamentally diVerent phenomenon.
assumed that the substrate is structurally inert – Thebulk-terminatedSi(112)surfaceconsistsof
that is, the known morphology of the surface narrow (111) terraces [two (1×1)-unit cells wide]
determines the geometry of the heterostructure, separated by single-height steps, and it has long
with the self-assembly governed by interactions beenassertedthatGa‘wires’adsorbalongthestep
within the overlayer. Based on a variety of experi- edges [4–7]. As shown by scanning tunneling
ments over the past two decades, the adsorption microscopy(STM),thecleanSi(112)surfacecon-
of Ga on Si(112) has been described as such a sists of quasi-periodic nanofacets (~10nm width)
with(111)and~(337)orientations[8].Following
*Correspondingauthor.Fax:[phone]; Gaadsorptionandannealing,thesurfacemorphol-
e-mail:[email protected]. ogy becomes planar [i.e. all terraces have a (112)
0039-6028/99/$–seefrontmatter©1999ElsevierScienceB.V.Allrightsreserved.
PII: S0039-6028(99)00005-9
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The structure of Si(112):Ga-(N×1) reconstructions
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14. ABSTRACT
We have studied the structure of Si(112):Ga-(N×1) reconstructions using atomic-resolution scanning
tunneling microscopy and first-principles calculations. The nanofaceted clean Si(112) surface becomes
planar following the adsorption of Ga, which forms long chains on the surface interrupted by isolated
quasi-periodic defects. The defects create a mixture of (N×1) structures (N#4-7) with 5×1 and 6×1 unit cells
most common. We demonstrate that this structure consists of a chain of Ga atoms adsorbed at the
(111)-like step edge within the (112) unit cell, and that the defects are Ga vacancies where the exposed step
edge Si atoms form a dimer bond. Calculations performed as a function of vacancy period confirm that the
surface energy is minimized at N=5-6, when compressive strain associated with the Si-Ga bonds is
effectively minimized.
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L266 A.A.Baskietal./SurfaceScience423(1999)L265–L270
orientation] with a mixture of (5×1) and (6×1) terminated unit cell in the [1:10] direction).
structures. It has been proposed that the (N×1) Previous LEED studies have found that
periodicity is caused by periodic vacancies within Ga/Si(112)exhibitsboth(5×1)and(6×1)recon-
the Ga chains [4–7]. In this Letter, we use both structions[4,5,7].FromourSTMresultsitisclear
STM and first-principles total-energy calculations thatthereisactuallyarangeof‘localperiodicities’
to show definitively that this vacancy model is fluctuating around an average value between 5a
indeed correct. and 6a.
TheSTMexperimentswereperformedinultra- Identifying the defects requires a structural
high vacuum using Si wafers oriented to within model for the Ga adsorption. A model was first
0.5° of (112). To obtain a clean surface each suggested by Jung et al. [5], based on LEED and
sample was radiatively heated to 1150°C for 60s AES, in which Ga adsorbs at the step edge within
(pressure≤2×10−9Torr).Galliumwasdeposited each bulk-terminated (112) unit cell to form Ga
from a heated tungsten basket onto samples at chains. The observed (6×1) reconstruction was
temperatures below 150°C. After depositing interpreted as one-sixthGa vacancies, which leave
>1ML of Ga, the surface was annealed to unbonded pairs of Si step edge atoms to rebond
500±50°C for 10min, resulting in a ‘6×1’ low asdimers,asshowninFig.2.Asubsequentangle-
energy electron diVraction (LEED) pattern resolved AES study provided structural data sup-
thought to correspond to 5/6 Ga atoms per bulk- porting this model [6]. This model has the attrac-
terminated(112)unitcell[5,7].TheAugerelectron tive feature that it immediately explains the
spectroscopy (AES) Ga (55eV)-to-Si (92eV) absence of adjacent defects in the STM images:
peak height ratio on this surface was 0.04±0.005, two adjacent vacancies would necessarily lead to
similar to that observed previously [4], indicating a Si dimer plus a Si step edge atom with two
that we produced a comparable surface structure. dangling bonds – an energetically unfavorable
STM images of the filled and empty electronic geometry. However, the atomic-resolution STM
states were acquired at room temperature with a images cannot easily be interpreted in terms of
constant current between 0.1 and 0.3nA and bias this structure given that ‘bumps’ are observed in
voltages between 1.0 and 2.5V. thefilledstateswhereavacancyissupposedtolie.
Fig.1 shows gray-scale STM images of the Inordertodeterminethestructuredefinitively,we
Si(112):Ga-(N×1)surfaceexhibitingwell-ordered have performed extensive first-principles total-
:
row structures oriented along the [110] direction. energy calculations.
The spacingbetween rowsis0.94nm,correspond- Thecalculationswereperformedina(112)slab
ing to the width of the bulk-terminated (112) unit geometry with six layers of Si and a vacuum layer
cell, confirming the local restoration of the basal equivalenttofivelayersofSi,withthesamesingle
orientation. Along these rows, isolated periodic (N×1) unit cell on both slab surfaces. Total ener-
defects are observed that appear as dark minima gies and forces were calculated within the local-
in empty-state images and slightly enhanced density approximation (LDA) with gradient cor-
maxima in filled-state images. The single defects rections [10], using Troullier–Martins pseudo-
are correlated across the rows and resemble potentials and a plane-wave basis with a kinetic
trenches perpendicular to the row direction in the energy cutoV of 8Ry, as implemented in the
empty states. At higher magnification [Fig.1c,d fhi96mdcode [11]. Because the surface Brillouin
and Fig.2], the filled-state ‘bumps’ range in zones of the diVerent supercells are not simply
appearance from symmetrical pairs of weak related, the individual total energies were required
maxima to more pronounced single maxima. tobecompletelyconvergedwithrespecttok-point
Similarly, in the empty states some defects appear sampling. Most importantly, full structural relax-
as single depressions along the row, and others as ation was performed on all but the innermost two
wider, asymmetrical depressions. Within a single layers untilthe surface energies wereconverged to
row the separation between these defects varies 0.1meVA˚−2. A variety of structural models were
from 4ato7a (a=0.34nm,the lengthofthe bulk- investigated, including Ga adsorbed on the (111)
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Fig.1.Gray-scaleSTMimagesofSi(112):Ga-(N×1)measuredforboththefilled(a,c)andempty(b,d)statesondiVerentareasof
the surface. The size of each image is indicated. The (N×1) reconstruction appears as rows oriented along the [1:10] direction
interruptedby quasi-periodicdefectsthat are correlatedalong [111:] (Nrangesfrom 4 to7,butis mostoften 5or6). Thedefects
appearasdeeptrenchesintheemptystatetopographyandasslightridgesinthefilledstates.Notethattheimagesin(a)and(b)
includemultiple(112)terraces.
terraces in T or atop sites, and along the step relative stabilityofthe diVerentvacancyperiodici-
4
edges with periodic vacancies or substitutional Si ties, we computed the equilibrium surface free
atoms within the chain. In the end, the step energy per unit area, c(h), as a functionof the Ga
edge/vacancymodel had thelowest surfaceenergy coverage, h=(N−1)/N:
and was the only structure consistent with the
c(h)=(2A)−1[E (h)−n E −n E ] (3)
STM images. t Ga Ga Si Si
Within the (N×1) step edge/vacancy model we where A is the surface area of the (N×1) unit
examined a wide range of vacancy periods, with cell, E(h) the total energy, n the number of
t Ga,Si
N ranging from 2 (every other Ga atom missing) atoms per cell, and E the chemical potentials.
Ga,Si
to8,aswellasa(1×1)structurewithnovacancies Notethatweareonlyinterestedinrelativesurface
(equivalent to N(cid:129)2). In order to compare the energies, which depend on the chemical potential
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Fig.2.Atomic-resolution dual-biasSTMimage ofthe (a)filled and(b) emptystates inone(5×1)area. (c)Simulated filled-state
imagecalculatedfora(5×1)stepedge/vacancymodelwheretheadjacentSiatomsatthevacancysiteformadimer.(d)Modelfor
the Si(112):Ga-(5×1) reconstruction after full structural relaxation. Ga atoms are dark gray and rebonded Si dimer atoms are
lightergray.
of Ga but not of Si in this case. For E we use and (6×1) vacancy periods, in remarkable
Ga
here the calculated energy per atom of bulk Ga, agreement with the average spacing observed
which is the thermodynamic upper limit for E ; experimentally. Because each calculation was lim-
Ga
the minimum of c(h) is unchanged for E within ited to a single (N×1) unit cell, we cannot com-
Ga
~1eV of this limit. As shown in Fig.3, where we ment on the energetics of the vacancy alignment
compare the relative energies with respect to that from row-to-row.
for no vacancies, a sharply defined minimum in One puzzling feature of this surface is the
the calculated surface energy occurs for (5×1) appearance of the vacancy defects as raised
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ment. We assume the variable appearance of the
defects in the room-temperature images is associ-
ated with higher-energy, asymmetrical variations
in the local vacancy structure.
Our first-principles results not only correctly
predict the ground-state equilibrium (5×1) and
(6×1)structures,butalsoprovideinsightintothe
physical mechanism that leads to their relative
stabilities. When there are no Ga vacancies, the
fully relaxed surface geometry shows that the step
edge Si–Ga bond is compressed by 7% relative to
a terrace Si–Ga bond length. Introducing widely
separated vacancies relieves this compression, but
only in the vicinity of the resulting Si dimer. With
more frequentvacancies, a larger fraction of these
compressedSi–Gabonds isrelieved;hence,Si–Ga
strain relief favors having the vacancies close
together. However, when the vacancies become
Fig.3.First-principlessurfaceenergiescalculatedasafunction two to three lattice constants apart, the average
ofvacancyperiod.Theenergy,displayedrelativetotheenergy strain in the Si–Ga chains changes from compres-
forcontinuous Gachains (novacancies), islowest for (5×1) sive to tensile, leading to an eVective short-range
and(6×1)periods,inagreementwithexperiment.
vacancyrepulsion.Theresolutionofthesecompet-
ing interactions at N=5–6 is a function of the
‘bumps’inthefilled-statetopography.Thisappar- eVective Si–Gaforce constantandthesubstrateSi
ent contradiction is neatly resolved by analyzing lattice constant, and can be described analytically
our numerically simulated constant-current STM by a one-dimensional Frenkel–Kontorova model
images of the ground-state structure – which also (aswillbereportedinaseparatepublication[12]).
show raised bumps at the defect sites (see Fig.2). In conclusion, we have studied the structure of
From the spatial relationship between the simu- Si(112):Ga-(N×1) reconstructions using atomic-
lated topography and the underlying atomic posi- resolution scanning tunneling microscopy (STM)
tions, it is clear that all of the maxima are and first-principles calculations. We demonstrate
associatedwith Siatoms;thatis, theGa atoms do thatthe(N×1)structureconsistsofachainofGa
not appear in the filled-state image. Thus, the atoms adsorbed at the (111)-like step edge within
verticalrowstructuresseeninFig.1a,candFig.2a the (112) unit cell, interrupted by periodic vacan-
arenotadsorbedGaatomsbutratherSistepedge cies where the exposed step edge Si atoms form a
atoms, and the raised features are associated with dimer bond, as previously proposed. Calculations
the dimer-bonded Si atoms at each vacancy site. performedasafunctionofvacancyperiodconfirm
Finally, although the dimers appear higher than the surface energy is minimized at N=5–6, when
the step edge atoms in the images, they are physi- the strain energy associated with the Si–Ga bonds
callyslightly lower. Their apparently raised height is eVectively minimized. Given the multilayer
is a purely electronic eVect: because the Si dimer restructuring of the Si(112) surface that occurs
bondishighlystrained,theassociatedsurfacestate uponadsorptionofGa,thissystemwouldbemore
is closer to the Fermi level, causing a relative accurately described as an adsorbate-induced sur-
increase in the local state density and a ‘bump’ in face reconstruction rather than a self-assembled
thefilled-statetopography.Theelectronicstructure array of ‘quantum wires’. It has recently been
of the empty state surface is relatively simpler, shown that the optical reflectivity of this surface
dominated by state density localized above each hasasignificantdirectionalanisotropy[7],andwe
Ga atom (not shown), in agreement with experi- expect thechain-like structureoftheGa overlayer
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L270 A.A.Baskietal./SurfaceScience423(1999)L265–L270
togive risetootherunusual electronicand optical Nakashima, M. Iwane, O. Matsuda, K. Murase, Jpn.
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Acknowledgements
A12(1994)1838.
[6]J.E. Yater, A. Shih, Y.U. Idzerda, Phys. Rev. B 51
This work was funded by the OYce of Naval (1995)7365.
Research, with computational work supported by [7]O.J. Glembocki, S.M. Prokes, Appl. Phys. Lett. 71
grants of HPC time from the DoD Shared (1997)2355.
[8]A.Baski,L.J.Whitman,Phys.Rev.Lett.74(1995)956.
Resource Centers MAUI and WPASC.
[9]A. Baski, L.J. Whitman, J. Vac. Sci. Technol. B 14
(1996)992.
[10]J.P. Perdew, J.A. Chevary, S.H. Vosko, K.A. Jackson,
M.R.Pederson,D.J.Singh,C.Fiolhais,Phys.Rev.B46
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