We report STM-induced desorption of H from Si(100)-H(2$\times1$) at negative
sample bias. The desorption rate exhibits a power-law dependence on current and
a maximum desorption rate at -7 V. The desorption is explained by vibrational
heating of H due to inelastic scattering of tunneling holes with the Si-H
5$\sigma$ hole resonance. The dependence of desorption rate on current and bias
is analyzed using a novel approach for calculating inelastic scattering, which
includes the effect of the electric field between tip and sample. We show that
the maximum desorption rate at -7 V is due to a maximum fraction of
inelastically scattered electrons at the onset of the field emission regime.Comment: 4 pages, 4 figures. To appear in Phys. Rev. Let

B. C. Stipe

B. N. J. Persson

Ben Yu-Kuang Hu

C. Thirstrup

D. P. Adams

D. Vanderbilt

F. Grey

F. Perez-Murano

G. P. Salam

J. A. Stroscio

J. G. Simmons

J. P. Perdew

J. Tersoff

J. W. Lyding

K. Stokbro

M. Sakurai

P. Avouris

P. Guyot-Sionnest

P. Hohenberg

R. E. Walkup

R. H. Fowler

S. Gao

T.-C. Shen

U. Quaade

W. Kohn

English

arXiv

We report STM-induced desorption of H from Si(100)-H(2×1) at negative sample bias. The desorption rate exhibits a power-law dependence on current and a maximum desorption rate at −7V. The desorption is explained by vibrational heating of H due to inelastic scattering of tunneling holes with the Si-H 5σ hole resonance. The dependence of desorption rate on current and bias is analyzed using a novel approach for calculating inelastic scattering, which includes the effect of the electric field between tip and sample. We show that the maximum desorption rate at −7V is due to a maximum fraction of inelastically scattered electrons at the onset of the field emission regime

Stokbro, K.

Thirstrup, C.

Sakurai, M.

Quaade, U.

Yu-Kuang Hu, Ben

Pérez Murano, Francesc

Grey, F.

American Physical Society

Diposit Digital de Documents de la UAB

VOLUME 80, NUMBER 12 P HY S I CA L REV I EW LE T T ER S 23 MARCH 199826STM-Induced Hydrogen Desorption via a Hole ResonanceK. Stokbro,1 C. Thirstrup,2 M. Sakurai,2 U. Quaade,1 Ben Yu-Kuang Hu,1 F. Perez-Murano,1,3 and F. Grey11Mikroelektronik Centret, Danmarks Tekniske Universitet, Bygning 345ø, DK-2800 Lyngby, Denmark2Surface and Interface Laboratory, RIKEN, Saitama 351, Japan3Department of Electronic Engineering, Universitat Autonoma de Barcelona, 08193 Bellaterra, Spain(Received 10 October 1997)We report STM-induced desorption of H from Sis100d-Hs2 3 1d at negative sample bias. Thedesorption rate exhibits a power-law dependence on current and a maximum desorption rate at 27 V.The desorption is explained by vibrational heating of H due to inelastic scattering of tunneling holeswith the Si-H 5s hole resonance. The dependence of desorption rate on current and bias is analyzedusing a novel approach for calculating inelastic scattering, which includes the effect of the electricfield between tip and sample. We show that the maximum desorption rate at 27 V is due to amaximum fraction of inelastically scattered electrons at the onset of the field emission regime. [S0031-9007(98)05616-6]PACS numbers: 61.16.Ch, 68.10.Jy, 79.20.La, 81.65.CfSingle atom manipulation with a scanning tunneling mi-croscope (STM) has been reported for several systems anda variety of physical mechanisms has been proposed toaccount for such manipulation [1,2]. Among the worksrelevant for the present Letter we mention single hydro-gen atom desorption from the Sis100d-Hs2 3 1d surface[3], and dissociation of single O2 molecules on Pt(111)[4]. These manipulations were performed at positivesample bias, and the underlying microscopic mechanismhas been related to vibrational heating by inelastic scatter-ing of tunneling electrons with an electron resonance on thesample. There have been theoretical predictions that a re-lated mechanism may operate at negative sample bias [5],which involve inelastic scattering of a tunneling hole witha hole resonance on the sample. However, to our knowl-edge there has been no experimental confirmation of sucha mechanism, probably because high tunnel currents andsample biases are needed to obtain high inelastic scatter-ing rates with low-lying hole resonances (see Fig. 1).In this Letter, we present evidence for a desorptionmechanism involving a hole resonance, for STM-induced hydrogen desorption from the monohydrideSis100d-Hs2 3 1d surface in ultrahigh vacuum [3].Whereas in previous studies [3,6,7], hydrogen desorptionhas been studied at positive sample bias, here we reporthydrogen desorption at negative bias. The desorptionprocess is modeled by vibrational heating of hydrogencaused by inelastic scattering of tunneling holes with theSi-H 5s hole resonance. The inelastic scattering rates arecalculated using a novel method based on first principleselectronic structure theory, and desorption rates obtainedby solving the Pauli master equation for a truncatedharmonic potential well [8]. With this model we obtaindesorption rates as function of tunnel current and samplebias that are in agreement with the experimental data.We find a maximum desorption rate at sample bias 27 V,which coincides with the onset of the field emission orFowler-Nordheim regime [9,10]. We show that this18 0031-9007y98y80(12)y2618(4)$15.00results from a competition between polarization of thehole resonance, which increases the fraction of inelasticscattered electrons, and domination of Fermi-level contri-butions to the tunnel current in the field emission regime.The experiments were performed on n-type Si(100)(ND ­ 1 3 1018 cm23) samples using a JEOLJSTM-4000XV microscope at a base pressure of1 3 10210 torr. Atomic hydrogen was absorbed on theclean Sis100d-s2 3 1d reconstructed surface to obtain themonohydride s2 3 1d phase, in a manner identical to pre-vious reports [7], and W tips were used. The desorptionexperiments were carried out by scanning the STM tipat speed, s, sample bias, Vb , and tunnel current, I , andsubsequently imaging the affected area to determine thenumber of Si sites where desorption occurred. Figure 2shows a typical example of desorption at negative samplebias of hydrogen along a single dimer row. We firstFIG. 1. (a) Inelastic tunneling of a hole into an adsorbate in-duced hole resonance with density of states, r0. The higherbarrier for tunneling into the hole resonance compared to tun-neling into Fermi-level states, means that only a fraction ofthe total tunnel current will pass through the hole resonance.(b) Schematic illustration of relative energy dependent proba-bilities, rns«d, for inelastic hole tunneling with energy transfernh¯v0 to the adsorbate.© 1998 The American Physical SocietyVOLUME 80, NUMBER 12 P HY S I CA L REV I EW LE T T ER S 23 MARCH 1998FIG. 2. (a) A single line of desorbed H from theSis100d-Hs2 3 1d surface as a result of a line scan at 27 Vand 3.0 nA. The picture is obtained at 22 V and 0.2 nA,and bright regions are due to increased density of states ofH-free Si atoms. (b) The experimental corrugation (solid line)along the line in (a) compared with the theoretical corrugationof a missing H defect including electric field effects (dashedline) and without electric field effects (dotted line). Results inthe range 27 Å , x , 9 Å are calculated using a cs4 3 4dcell with one missing H atom and outside this range using a(2 3 1) cell.measured the dependence of the desorption rate, R, on thetunnel current for sample biases of Vb ­ 27 and 25 V.The results are shown in Fig. 3 (inset). For both biasesthere is a power-law dependence of the desorption rateupon current, R ­ R0sIyIdesda , with exponent a ø 6.In this equation Ides is the tunnel current that givesrise to a fixed desorption rate R0. In order to find thevoltage dependence of the desorption rate, we measureIdessVbd with Vb in the range 210 to 24 V, as shownin Fig. 3. The measurements were obtained by scanning30 nm along a single dimer row at s ­ 2 nmys, andIdes is defined as the current that gives rise to desorptionof 50% of the hydrogen along such a scan line, whichcorresponds to R0 ­ 4 s21.The main feature of Fig. 3 is that significantly higherbias and tunnel currents are required at negative bias,compared to the positive bias case [3]. Whereas IdessVbddecreases monotonically at positive Vb [3], it displaysa minimum at negative Vb , increasing for Vb , 27 V.Experimentally, the lithographic resolution at negativebias is comparable to that at positive bias. At the highestFIG. 3. Current, Ides , as a function of sample bias, Vb , forconstant desorption rate R ­ 4 s21. Circles show experimentalresults, and the solid line shows the theoretical result. Thedashed line shows the current as a function of sample bias forconstant electric field E ­ 0.8 VyÅ in the tunnel gap. Insetshows RsId for Vb ­ 27 V (triangles) and 25 V (crosses),and from least-squares fits of R ~ Ia to the data we obtaina ­ 5.7 6 0.7 (27 V) and 6.3 6 1.3 (25 V). Lines showtheoretical calculations and have exponents a ­ 6.4 (27 V)and 7.1 (25 V).current levels there is a tendency for the tip resolutionto be affected by the lithography process. This limitsthe voltage range over which IdessVbd can be readilymeasured, and may explain why no detailed investigationof desorption at negative sample bias has been madepreviously. We note that the behavior of IdessVbd atnegative sample bias has been confirmed independentlyon a separate STM system [11].At negative sample bias, electrons accelerated acrossthe tunnel gap impinge on the tip, so the possibilityof direct excitation by collision can be ruled out as adesorption mechanism. In contrast, at positive samplebias, direct excitation of the Si-H bond is believed to playthe dominant role for Vb . 4 V [3,6]. Shen et al. [3]have proposed vibrational heating of hydrogen [12] bytunneling electrons scattering inelastically with the Si-H6sp electron resonance as a desorption mechanism forpositive sample bias and Vb , 4 V. A similar mechanismcan function at negative sample bias, since an electrontunneling from the sample to the tip may excite the Si-H5s hole resonance, which upon deexcitation can transferenergy to the hydrogen atom [5]. This process can beviewed as inelastic scattering of a tunneling hole withthe Si-H 5s hole resonance, as illustrated in Fig. 1. Acharacteristic feature of vibrational heating by inelasticscattering is a power-law dependence of the desorptionrate on current [13] in agreement with the experimentalobservations.To make a quantitative analysis of the dependence ofthe desorption rate on tunnel current and sample bias,we develop a method for obtaining inelastic scatteringrates from first principles electronic structure calculations.Since the inelastic scattering rate depends on the tip-sample2619VOLUME 80, NUMBER 12 P HY S I CA L REV I EW LE T T ER S 23 MARCH 1998distance, d, while the measured quantities are I and Vb ,we first calculate the relation between I , Vb , and d.For this purpose we use a high voltage extension ofthe work by Tersoff and Hamann [14], that includesthe effect of the electric field between tip and sample[15]. The tunnel current is obtained from the local den-sity of states (DOS) of the Sis100d-Hs2 3 1d surface,rsr, «, Ed ­Pm jcmsr, Edj2ds« 2 «md, at tip position r,and wave functions cm for electrons with energy «m arecalculated in an external field E. The electric field is mod-eled by a planar electric field outside the surface and itsmagnitude determined from Vb and d. The tunnel currentis given by [15]I ­ CWZ eVb0je2Rw ks«djrsd 1 Rw, «, Ed d« , (1)where distances are in bohrs, current in amperes, and allelectron energies are relative to the sample Fermi energy,«F . In this equation ks«d ­p2msft 1 eVb 2 «dyh¯ isthe wave function inverse decay length, ft ­ 4.5 eVthe work function of the W tip [16], RW ­ 3 a.u. theatomic radius of W, and the normalization constant CW ­0.007R2W amperes 3 bohr is obtained from a calculationof a model W tip [15].We test the method by calculating the STM corruga-tion across a single missing H defect. The electronicstructure calculations are based on density-functionaltheory [17,18] within the generalized-gradient approxima-tion (GGA) [19] using 20 Ry plane-wave basis sets. TheSis100d-Hs2 3 1d surface is represented by a 12 layer(2 3 1) slab, and we use a cs4 3 4d slab to calculatethe corrugation of a single missing H defect. Ultrasoftpseudo-potentials [20] are used for both H and Si. Fig-ure 2(b) shows the result compared with the measuredcorrugation perpendicular to a desorption line. The theo-retical corrugation compares well with the experimentaldata when the electric field between tip and sample is in-cluded (dashed line) but not otherwise (dotted line).We next calculate the tunnel current of inelasticallyscattered electrons. Electrons may scatter inelasticallydue to long-range electrostatic interactions with a vibra-tional transition dipole moment, or due to local inelasticscattering with an electronic resonance [21]. Only a localinteraction can cause atomic scale desorption, so in thefollowing we restrict the analysis to resonance coupling.The resonance scattering event can be described by a localpolaron model with a linear electron-phonon coupling l,and we use a harmonic approximation for the Si-H bondpotential with frequency v0. To calculate the inelasticcurrent, In, which causes transitions from vibrational level0 to n, we combine previous models of inelastic-scattering[5,22] with the Tersoff-Hamann model of STM tunnelingbetween a surface and a W tip [14,15]. In the limit of abroad resonance, D À l, v0, we obtain [23]In ­ CW n!Z eVbn h¯v0je2Rw ks«djrnsd 1 Rw, «, Ed d« ,(2)2620where distances are in a.u. and In in amperes. We definea dimensionless parameter, K ­ pl2rsyD, where rs ­Pm ds«5s 2 «md is the average surface DOS around theresonance energy «5s , and D is the resonance width. Theweighted local DOS,rnsr, «, Ed ­ KnXmfmjk5sjmlj2njcmsr, Edj2ds« 2 «md ,is weighted with sKk5sjml2dn, where j5sl and jml arethe resonance state and sample eigenstates, respectively,and fm ­ jk5sjmlj2ysx 1 jk5sjmlj2d is the fraction ofelectrons which tunnel from the tip to state m via the5s resonance. The parameter x determines the frac-tion of electrons which does not tunnel via the reso-nance state, and we have estimated x ø 0.1 s­ 0.25 3maxm jk5sjmlj2d. We note that the calculations are quiteinsensitive to the value of x, since fm , 1 for «m ,«5s and fm , jk5sjmlj2yx for j«m 2 «5s j À D, thusx merely damps contributions from eigenstates not inresonance.We next calculate the parameters entering Eq. (2) forinelastic scattering with the Si-H 5s resonance. Froma frozen phonon calculation we obtain h¯v0 ­ 0.26 eV.Using the ground state energy of a free H atom we calcu-late Edes ­ 3.36 eV. To find k5sjml we project the elec-tronic eigenstates of the slab calculation onto the 5s wavefunction of a Si-H molecule, and the solid line in Fig. 4shows the partial DOS n5ss«d ­Pm jk5sjmlj2ds« 2«md. We find that the 5s resonance is centered at «5s ­25.3 eV relative to the Fermi level of an n-type samplewith an average width of D ­ 0.6 eV. Crosses in Fig. 4show rss«md ø ns«mdyjk5sjmlj2 from which we estimaters ø 1.2 eV21. The electron-phonon coupling is givenby l ­ «05sph¯y2Mv0, where M is the hydrogen massand «05s the derivative of «5s with respect to the Si-Hbond length z [5]. Our calculations show that «5svaries almost linearly with z in the range 1.25 Å , z ,2.25 Å with a slope «05s ­ 2.3 6 0.1 eVyÅ, and hencel ø 0.20 eV.From the inelastic currents we calculate desorptionrates by solving the Pauli master equation for the tran-sitions among the various levels of the oscillator [8]. Weinclude contributions from I1, I2, and I3 [24], and vibra-tional relaxations are described by a current independentrelaxation rate, g ­ 1 3 108 s21 [3,25]. We assume thatdesorption occurs when the energy of the H atom ex-ceeds the desorption energy Edes ­ 3.36 eV, correspond-ing to a truncated harmonic potential well with 13 levels.The solid lines in Fig. 3 (inset) show the calculated de-sorption rate as function of tunnel current for Vb ­ 25and 27 V. The agreement with the experimental data isexcellent. The desorption rate follows a powerlaw withexponent a ø 6.5. An analysis of the theoretical calcu-lation shows that the contribution from I2 dominates thedesorption rate, and therefore a ø Ny2, where N ­ 13is the number of levels in the truncated harmonic poten-tial well. This contrasts with the low positive bias case,VOLUME 80, NUMBER 12 P HY S I CA L REV I EW LE T T ER S 23 MARCH 1998FIG. 4. The partial DOS, n5ss«d ­Pm jk5sjmlj2ds« 2 «md,of the 5s wave function of a Si-H molecule. The dotted lineshows the projection onto all Si-H molecular wave functions.Crosses show values of n5ss«mdyk5sjmlj2.Vb , 4 V, where I1 dominates and a ø 13 [3]. This dif-ference is due to the different lifetimes of the 5s and 6spresonances. Since the 6sp resonance is more short-lived(D ­ 1 eV), the energy transfer in each inelastic scatter-ing event is smaller and I1 dominates in this case [23].Furthermore, electrons with energy ,4 eV only samplethe low energy tail of the Si-H 6sp resonance, and r1 isrelatively broader than rn for n . 1; see Fig. 1.We next calculate IdessVbd and the results are shown inFig. 3, and we note that the minimum at 27 V is accuratelyreproduced by the theoretical calculation. This minimumdoes not coincide with the resonance energy «5s , but withthe onset of the field emission regime, as illustrated byI-V characteristics for this system at a constant field of0.8 VyÅ (dashed line). In field 0.8 VyÅ surface statesare band bent by ,1 eV [15] and therefore «5s ø 6 eV.Thus, in the range f26.5 V, 0 Vg the bias dependence ofIdes is mainly determined by the shape of the resonancewave function. Below 26.5 V the bias dependence isa result of competition between two different effects:polarization of the hole resonance and increasing Fermi-level conduction. The electric field polarizes the surface,displacing wave functions towards the tip. Since the 5sorbital is more polarized than bulk states, its overlap withthe tip state increases relative to other states, and thefraction of electrons causing double vibrational excitations,f2 ­ I2yI , increases with electric field, as observed in thevoltage range f27 V, 26.5 Vg. On the other hand, thegrowing tip-sample distance with increasing magnitude ofthe negative bias will increase the tunnel barrier of the5s state relative to states near the Fermi level, thus f2decreases with tip-sample distance. This effect becomesdominant at the onset of the field emission regime, wherethe conduction of states near the Fermi level is independentof the tip-sample distance, and f2 starts to decrease whenVb , 27 V.In conclusion, we have presented experimental mea-surements of the voltage and current dependent variationof the hydrogen desorption rate from the Sis100d-Hs2 31d surface at negative bias conditions. Based on a novelfirst principles theory of inelastic scattering, we haveshown that the desorption is caused by vibrational heat-ing of the H atom due to inelastic-scattering with the Si-H5s hole resonance.This work was supported by the Japanese Science andTechnology Agency and the Danish Ministries of Industryand Research in the framework of the Center for Nano-structures (CNAST). The use of national computer re-sources was supported by the Danish Research Councils.[1] J. A. Stroscio and D.M. Eigler, Science 254, 1319 (1991).[2] P. Avouris, Acc. Chem. Res. 28, 95 (1995).[3] T.-C. 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Grey, Report No. cond-mat/9710076 (1997).[16] CRC Handbook of Chemistry and Physics, 75th Edition,edited by D.R. Lide (CRC Press, New York, 1994).[17] P. Hohenberg and W. Kohn, Phys. Rev. 136, B864 (1964).[18] W. Kohn and L. J. Sham, Phys. Rev. 140, A1133 (1965).[19] J. P. Perdew et al., Phys. Rev. B 46, 6671 (1992).[20] D. Vanderbilt, Phys. Rev. B 41, 7892 (1990).[21] B. N. J. Persson, Phys. Scr. 38, 282 (1988).[22] S. Gao, Phys. Rev. B 55, 1876 (1997).[23] K. Stokbro et al., Report No. cond-mat/9802301 (1998).[24] In diverge for large n [5], and the divergence dependson lyD. In the present calculation we use nmax ­ 3, butobtain quite similar results using nmax ­ 2, 3 or 4.[25] P. Guyot-Sionnest, P.H. Lin, and E.M. Miller, J. Chem.Phys. 102, 4269 (1995).2621

STM-induced hydrogen desorption via a hole resonance

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Physical Review Letters

STM-Induced Hydrogen Desorption via a Hole Resonance

We report STM-induced desorption of H from Si(100)-H(2 X 1) at negative sample bias. The desorption rate exhibits a power-law dependence on current and a maximum desorption rate at -7 V. The desorption is explained by vibrational heating of H due to inelastic scattering of tunneling holes with the Si-H 5 sigma hole resonance. The dependence of desorption rate on current and bias is analyzed using a novel approach for calculating inelastic scattering, which includes the effect of the electric field between tip and sample. We show that the maximum desorption rate at -7 V is due to a maximum fraction of inelastically scattered electrons at the onset of the field emission regime

STM induced hydrogen desorption via a hole resonance

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