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Ideal hydrogen termination of the Si(111) surfaceG. S. Higashi, Y. J. Chabal, G. W. Trucks, and Krishnan RaghavachariCitation:Appl. Phys. Lett. 56, 656 (1990); doi: 10.1063/1.102728View online: http://dx.doi.org/10.1063/1.102728View Table of Contents: http://apl.aip.org/resource/1/APPLAB/v56/i7Published by theAmerican Institute of Physics.Related ArticlesSpecific channels for electron energy dissipation in the adsorbed systemJ. Chem. Phys. 138, 104201 (2013)Conduction band offset at GeO2/Ge interface determined by internal photoemission and charge-corrected x-rayphotoelectron spectroscopies

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Appl. Phys. Lett. 102, 103101 (2013)The role of dimensionality in the decay of surface effectsJ. Chem. Phys. 138, 084707 (2013)Additional information on Appl. Phys. Lett.Journal Homepage: http://apl.aip.org/Journal Information: http://apl.aip.org/about/about_the_journalTop downloads: http://apl.aip.org/features/most_downloadedInformation for Authors: http://apl.aip.org/authors

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Ideal hydrogen termination of the 51 ( i i i ) surfaceG. S. Higashi, Y. J. Chabal, G. W. Trucks, and Krishnan RaghavachariAT&T Bell Laboratories, Murray Hill, New Jersey 07974(Received 29 June 1989; accepted for publication 4 December 1989)Aqueous HF etching of silicon surfaces result s in the removal of the surface oxide and leavesbehind silicon surfaces terminated by atomic hydrogen. The effect of varying the solution pHon the surface structure is studied by measuring the SiH stretch vibrations with infraredabsorption spectroscopy. Basic solutions (pH = 9-10) produce ideally terminated Si( 111)surfaces with silicon monohydride C-S i -H) oriented normal to the surface. The surface isfound to be very homogeneous with low defect density ( < 0.5%) and narrow vibrationallinewidth (0.95 cm I).

HF acid etching is a key step in producing silicon surfaces which are contamination-free and chemically stable forsubsequent processing in the semiconductor industry. 13 Indeed, chemical oxidation and subsequent HF treatment of Sisurfaces are used prior to the growth of gate oxide, wheresurface contamination an d interface control are crucial todevice performance.4 Th e chemical stability of the HF-treated surface was long thought to be explained by fluorine termination. Recently, however, it has become understood thatfluorine is a minority species on the surface and that theremarkable surface passivation achieved is explained by Htermination ofsilicon dangling bonds, 3,5,6 protecting the surface from chemical attack. Th e structure of the H-terminated surface also plays a role in the initial stages of gate oxideformation and possibly affects the resulting SiiSiOz interfacial properties. Recently, it was shown7 ,8 that aqueous HFsolutions (H F in H 20 of various concentrations) induce microscopic roughness on both Si ( 111) as well as Si ( 100). Inthis study, we find that varying the pH of the HF solutionsdrastically alters the microscopic roughness and the natureof the associated H termination of the surface. In particular,basic solutions produce ideally terminated SiC 111) surfacesthat are microscopically smooth.

Ou r approach is to measure the infrared absorption inthe SiH stretch region ( ~ 2 1 0 0 cm I) using a multiple internal reflection geometry which gives nearly equal sensitivity to components ofvibrations parallel an d perpendicular tothe surface.7-1o From these spectra the relative concentrations ofdifferent hydride species and their orientation can bedetermined. In particular, for well-ordered surfaces, the defect density can be quantified.

Experiments are performed on 0.5 X 19 X 38 mm 3 n-doped (p-lOO-300 n em ) SiC I 11} and Si( 100) sampleswith 45 bevels on each of the short sides. The infrared radiation exiting the interferometer is focused at normal incidence onto the input bevel, is internal ly reflected ~ 75 times,and exits the output bevel to be collected and refocused ontoa liquid-N2 -cooled InSb photodetectof. Samples are prepared by conventional thermal oxidation ( 1000 Aoxide)to move the interface away from any residua! polishing damage. After HF acid removal of the thermal oxide, the Si samples are placed into the HF solution of interest. They are thenremoved, placed into a dry N 2 purged environment, andspectra are recorded. Background reference spectra are ob-

tained by measuring the transmission of chemically oxidized8i surfaces using HCl:H20 z or H 2 S04 :HzOz solutions.In the course of ou r work on HF etching, we have notedthat the concentration of aqueous HF alters the observedspectra. In contrast to the well-defined (narrow - 10 cm -1 )vibrational features when dilute HF (1-10% in H 20 ) isused,7.8 the vibrational lines are broad ( ~ 3 0 cm - 1) whenconcentrated HF (49% in H 20 ) is used, and the band ischaracterized by a higher density of defect modes (dihydride). Since the pH varies from 1.0 to 2.0 with water dilution (relative concentrations ofR + an d OH-) and appearsto affect the morphology of the H-passivated surfaces, astudy of he effects ofpH was undertaken. A common way toincrease thepH an d maintain it constant at pH ~ 5.0 is to useammonium fluoride (N H 4 F) as a buffering agent (7:1NH 4 F:HF). Thus, we used this buffered HF solution as astarting point and varied its pH by adding ammonium hydroxide (N H4 0H ) to raise the pH ( pH=5 to 12) orhydrochloric acid (HeI) to lower th epH (pH = 5-0).

Fo rpH greater than 4.0, we observe a dramatic changein the hydrogen termination, compared to solutions previously prepared with lowerpH. 7,8 A comparison of the SiHstretch spectra is shown in Fig. 1(a). Th e spectrum shown indashed lines corresponds to a Si ( 1 I 1) surface prepared indilute HF (100:1 H 20:HF) and is characterized by a largedensity of defect modes (coupled monohydride 1\1 and dihydride D). 7,8 Th e (111) terraces are terminated by both monohydride (M ) and trihydride (n . In contrast, the spectrumof the surface prepared in pH modified buffered HF(pH = 9-10) is dominated by one very sharp vibrationalline at 2083.7 cm - 1, perpendicular to the surface. The observation of a single, narrow mode, oriented perpendicular tothe surface, makes its assignment as the monohydride of anideally terminated SiC 111) surface unambiguous. Ou r earlier studies7,8 on surfaces with high defect densities mistakenly assigned this mode to the monohydride on adstructures because of its frequency position. 11

Th e defect densit y can be estimated from the area underthe s-polarized spectrum because only H modes at defectshave vibrational components parallel to the surface. Aftercorrection for misalignment (-2 ) resulting in an apparentcontributionat 2083.7 cm -1 in thes-polarized spectrum andfor screening of the perpendicular mode, 12 the defect densityis found to be -0.5% (every 700 A for one-dimensional

656 AppL Phys. Lett. 56 (7), 12 February 1990 0003-6951/90/070656-03$02,00 @ 1990 American Institute of PhYSics 65 6

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roughness or every 30 A for two-dimensional roughness).This number is comparable to what one would expect for themisorientation of the sample ( ~ O S , or 360 A betweensteps).

Another measure of surface homogeneity is thelinewidth of the SiH stretch vibration. The inset of Fig. 1shows an expanded view of this line, measured with higherresolution (0.25 em - I) to resolve its linewidth (0.95 cm-- 1 ) .Since the homogeneous linewidth of the SiH stretch is expected to be - 1 em - 1 at room temperature, due to anharmonic coupling to lower frequency modes, 13 the inhomogeneous contribution to the linewidth is likely to be small ( 0.1cm- 1 ) . It should be stressed that, since the frequency of theSiH stretch vibration depends on how the Si is bonded to itsneighbors,14 and since its weak dynamic dipole moment(e* - 0.1) 15 precludes line narrowing due to dipole interactions,I6 the SiR measured width is a sensitive measure ofinhomogeneities. I}

The mechanism responsible for producing these dramatically different surface morphologies is elucidated by theobservation that one can go reversibly from a rough surface(Fig. 1, dashed curve) to a smooth, ideally terminated surface (Fig. 1, solid curve) by dipping the H-passivated surface in dilute aqueous HF or the pH modified HF solution,witho ut an intervening chemical oxidation. Since addition ofsilicon atoms to the surface from th e solution is unlikely, thesurface chemical changes must be occurring via etching ofthe H-terminated surface. In contrast, concentrated HF solutions do not alter the surface morphology (no change inconcentration of defect modes), implying that the HF aciditself does no t etch the surface.

A possible explana tion of the observed etching in dilutesolutions is the slow oxidation of the H-passivated surface,1718 followed by a fast removal of the surface oxide byHF. The second step involved in this etching process can beeliminated by removing the HF from the aqueous solution.

(0 ) ,1

ILlR/R '5 , , 0 -4PERREFLECTION8"45 'SI(HH

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FIG. 1. Internal icficction spectra ofHF-treated Si(1l1) surfaces: Ca) surface trea ted with pH modified buffered HF (p H = 9-10) (solid curve: resolution = 0.5 cm-- ') and with dilute HF (100:1 H 2 0:HF) (dashed curve:resolution = I em- ' ) ; (b) s polarization for surface treatment with pHmodified buffered rt F CpH = 9-10) (resolution = 2 cm - ;) . Inset: Highresolution spectrum of Si ( I l l) surface treated with pH modified bufferedHF( pH = 9-10).

657 Appl. Phys. Lett, Vol. 56, No.7, 12 February 1990

Confirming this idea, the spectra in Fig. 2 show the attack ofa RF prepared Si(111) surface by water. Surface preparation in buffered HF (pH = 5.0) results in a more defectivesurface, with coupled monohydride, dihydride, and trihydride clearly visible [Fig. 2 (a ) ]. Immersion in water for 5min results in the reduction in the amount of coupled monohydride and trihydride and complete removal of the dihydride, accompanied by a narrowing and strengthening of theline at 2083.7 cm '. The difference spectrum (Fig. 2, top)shows these effects more clearly. The observed stability ofthe ideally termi nated Si( 111) terrace (2083.7 cm- i mode)accounts for the preferential production of the 8i ( 111 ) surfaces.

Given that (111) planes are produced preferentially, itwould be expected that this phenomenon would lead toroughening of the Si(1oo) surface. While con centrated HFremoves silicon oxide leaving a rough H-passivated SiC 100)surface mostly covered with dihydride [Fig. 3(a)],8 treatment with buffered HF (pH = 5.0) still results in a roughsurface [Fig. 3(b)] but with an increase of monohydridemodes (both coupled and uncoupled), indicating the formation of small (111) facets.The mode at 2083.7 em -1 [arrow in Fig. 3(b ) ], characteristic of the ideally terminated (111) terraces, appearsboth ill s- and p-polarized spectra (dashed and solid curves,respectively) with similar intensities, showing that the SiH istilted from the (100) surface normal as expected for (111)facets. The other peaks in the range 2070-2090 cm-- I arecharacteristic of other monohydrides (coupledH--Si--Si-H or H attached to Si atoms with Si-Si bonding arrangements different than for bulk 8i) .

The startling result of this study is the formation of ahomogeneous rnonohydride phase on the ( 111) surface using these pH-modified buffered HF solutions. To our knowledge, this is the first t ime th e ideally terminated Si ( 111 )surface has been produced, whether in solution7R or byatomic H exposure of clean surfaces in ultrahigh vacuum. I9To understand how this termination is achieved, one mustunderstand why the surface is H termina ted in the first place.

SI HH )

2020 2060

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(0 )

(b )

2100 2140 2180FREQUENCY (0",-1)

FIG. 2. (a) Internal reflection spectrum of Si( I l l) treated with bufferedHF (pH = 5.0) and (b) subsequently rinsed in water. (b/a) Differencespectrum showing the decrease in the intensity of defect modes and the increase of the ideally terminated terrace mode.

Higashi et at 657

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ibJ

51 (100)

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25) of Si(111) surfaces leads immediately to theidea that anisotropic etching will occur when surfaces otherthan the (111) are used. The results obtained on the SiC 100)surfaces (Fig. 3) clearly show that the more basic the HFsolution, the more developed the ( 111) facets, although thesize of these facets remains limited, This finding stresses thatthe pH of the HF solutions used to clean Si(100) prior tooxidation must be chosen carefully to minimize roughness.

S. B. Christman, E. E. Chaban, and R. D. Yadvish aregratefully acknowledged for their technical support.

'w. Kern, Semicond. Int. April, 94 (1984).2F. J. Gnmthaner and P. J. Grunthaner, Mater. Sci. Rep. 1, 69 (1986).3M. Grundner and H. Jacob, App!. Phys. A 39,73 (1986)."Trace contamination at the 10 ppm level can reslIlt in threshold voltageshifts, device instability, lowered tra nsconducta nce, etc.'R . Ubara, T. Imura, and A. Hiraki, Solid State Commun. 50, 673 (1984)."E. Yab1onovitch, D. L. AHara, C. C. Chang, T. Gmitter, and T. B. Bright,Phys. Rev. Lett. 57, 249 (1986)."V. A. Burrows, Y. J. Chabal, G. S. Higashi, K. Raghavachari, and S. B.Christman. App!. Phys. Lett. 53, 998 (1988).

"Y. J. Chahal, G. S. Higashi, K. Raghavachari, and V. A. Burrows, J. Vac.Sci. Techno!. A 7, 2104 (1989)."N. J. Harrick, Imernal Reflection Spectroscopy (Wiley, New York, 1967),second printing (Harrick, Ossining, 1979).lOY. J. Chabal, Surf. Sci. Rep. 8, 211 (1988)."Origillally it was thought that the mode observed at 2077 em - I for H onSi( I l l ) 7X 7 might be characteristic of the ideal terrace monohydride. Itappears, however, that the strain around the terrace atoms may be responsible for shifting the frequency down by 6- 7 em - I.I C y ' J. Chabal and K. Raghavachari , Phys. Rev. Lett. 53, 282 (1984).13This is the smallest linewidth observed for any adsorbate on any substrat e

at room temperature. In particular, the monohydride on Si( 100) is characterized by a 3 em - I width, with 2 em - I due to dephasing and 1em - Idue to inhomogeneous broadening. See J. C. Tully, Y. J. Chabal, K. Raghavachari, J. M. Bowman, and R. R. Lucchese, Phys. Rev. B 31, 1184(1985).'"Yo J. Chabal and K. Raghavachari, Phys. Rev. Lett. 54, 1055 (1985)."Y. J. Chahal, in Chemistry and Physics a/Solid Surfaces VII, edited by R.Vanselow and R. F. Howe, Sprin ger Series in Surface Science. Vol. 10(Springer, Berlin, 1988) pp. 109-150.1('See, for example, Z. Schesinger, L. H. Greene, and A. J. Sievers, Phys.Rev. B 32, 2721 (1985), and references therein.17D. Graf, M. Grundner, and R. Shultz, J. Vac. Sci. Techno!. A 7, 808

(1989).IMM. Grundner and R. Schulz, AlP Conf. Proc. 167, 329 (1987).19H. Kobayashi, K. Edamoto, M. Onchi, and M. Nishijima, J. Chern. Phys.

78, ;429 (1983); Y. J. Chabal, G. S. Higashi, and S. B. Christman, Phys.Rev. B 28, 4472 (1983).

el lE. R. Weinberger, G. G. Peterson, T. C. Eschrich. and H. A. Krasinski, J.App!. Phys. 60, 3232 (1986)."G . W. Trucks, K. Raghavachari, G. S. Higashi, and Y. 1. Chahal (unpub

lished).Higashi et al. 658

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