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Surface Science 601 (2007) 1635–1641

Transient desorption of HD and D2 moleculesfrom the D/Si(100) surfaces exposed to a modulated H-beam

A.R. Khan, A. Takeo, S. Ueno, S. Inanaga, T. Yamauchi, Y. Narita, H. Tsurumaki,A. Namiki *

Department of Electrical Engineering, Kyushu Institute of Technology, Kitakyushu 804-8550, Japan

Received 13 December 2006; accepted for publication 25 January 2007Available online 3 February 2007

Abstract

We have studied the direct and indirect abstraction of D adatoms by H on the Si(100) surfaces by employing a pulsed H-beam.Desorptions of HD molecules is found to occur promptly as a result of direct abstraction at the beam on-cycles. In contrast, we findthat D2 desorption induced by adsorption of H atoms, i.e., the so-called adsorption-induced desorption (AID), occurs even at the beamoff-cycles. The D2 rate curves measured with the pulsed-H beam are decomposed into four components characterized with the reactionlifetimes of 60.005, 0.06 ± 0.01, 0.8 ± 0.1, and 30 ± 5 s. We propose that the fastest and the second fastest AID channels are related tothe thermodynamical instability of (1 · 1) dihydride domains locally formed on the (3 · 1) monodeuteride/dideuteride domains. The 0.8 sAID channel is attributed to the desorption occurring at the stage when (3 · 1) monodeuteride/dideuteride domains are built up upon Hadsorption onto the (2 · 1) monohydride surface. The 30 s AID path is attributed to the thermal desorption accompanied by the shrink-age of the (3 · 1) domains which were excessively formed during the beam on-cycles on the (2 · 1) monohydride surface. Atomistic mech-anisms are proposed for these three AID pathways.� 2007 Elsevier B.V. All rights reserved.

Keywords: Hydrogen; Si surface; Desorption; Abstraction; Adsorption

1. Introduction

The dangling bonds on Si surfaces can be terminated byH atoms [1–3]. Sticking probability of H atoms onto Si sur-faces is high while the surface coverage is low. After suffi-cient H dose the Si surfaces get saturated with H [4–7].This saturation does not necessarily mean that H atomsno longer react with the surface but it rather means thatthey are still reactive toward adsorption even on the termi-nated surfaces [8]. The adsorption of H atoms onto themonohydride Si(1 00) surface gives rise to the breakageof Si dimer bonds as well as their backbonds to form higherhydrides [9,10]. Moreover, H atoms abstract H adatoms to

0039-6028/$ - see front matter � 2007 Elsevier B.V. All rights reserved.

doi:10.1016/j.susc.2007.01.042

* Corresponding author.E-mail address: namiki@ele.kyutech.ac.jp (A. Namiki).

generate molecule desorption from H(D)-terminated sur-face [11–15]. At such a stage of the surface saturation underH exposure the adsorption and adatom abstraction by Hare supposed to be balanced [7,16,17].

So far, two types of adatom abstraction by H via a di-rect and an indirect pathway have been discriminated onthe Si surfaces by means of an isotopic labelling of adatomswith D [12–14]: The reaction of D abstraction by H,H + D/Si(100)! HD, has been called abstraction orABS [18–21]. As a general belief, it has been consideredthat the HD desorption along the ABS path is prompt[22,23]. A kinematic collision mechanism or an Eley–Ri-deal mechanism including a hot atom mechanism, by whichincident H atoms directly abstract D adatoms, was as-sumed to explain the ABS [11,12,15,16,22,23]. Such a sim-ple kinematic collision model predicts that the ABS obeys afirst-order rate law in D coverage (hD). However, the HD

1636 A.R. Khan et al. / Surface Science 601 (2007) 1635–1641

desorption generated along the ABS was found to be gov-erned by a second-order rate law in hD [14,18,24], suggest-ing the incident H atoms interact with two adatoms toabstract one of them. In addition to the ABS, an indirectpath to induce recombinative desorption of adatoms,H + D/Si(10 0)! D2, was also observed [12–14]. Des-orbed molecules along this pathway include no primaryH atoms. This type of homonuclear recombination fol-lowed by desorption has been called an adsorption-induceddesorption (AID) [19–21]. A fourth-order rate law in hD

has been found in AID [14,18,20,21,24], suggesting thatH atoms interact with four adatoms to generate AID.The experiments on the AID rates showed up a featureof strong Ts dependence peaking at around 600 K, tendingto zero rate at both temperature sides around 300 K and750 K [18,20].

In contrast to the HD desorption characterized with thequick response to a modulated H-beam, the AID reactionresponds to it either quickly or slowly [20,25]. It was no-ticed that the observed AID rate curves (F(t)) as a functionof exposure time (t) consists of several components havingspecific reaction lifetimes [25]. In order to evaluate reactionlifetimes, Inanaga et al. [25] defined the desorption proba-bility function, n(t),

nðtÞ ¼X

i

cie�t=si : ð1Þ

Here ci and si are the intensity and the reaction lifetime ofthe ith component, respectively. Hence, F(t) could be ex-pressed as a convolution of n(t) under the application ofH-impulse function g(t),

F ðtÞ ¼Z t

0

nðt � t0Þgðt0Þdt0: ð2Þ

As a result of the best curve fitting, three components char-acterized with the reaction lifetimes of about 0.05, 0.8 and30 s were obtained. It was argued that all these AID path-ways are related to the (3 · 1) monohydride/dihydrdie do-mains present on the (2 · 1) monohydride surface. On sucha (3 · 1) monohydride/dihydrdie phase, rows of monohy-dride Si dimers (HSi–SiH) and dihydrides (HSiH) are alter-nately arranged [26]. This phase can develop under Hexposure even at temperatures as high as 600 K wherethe so called b2 thermal desorption (TD) flashes out allthe dihydrides if no further H atoms are supplied [20].The in situ, real time observation of the surface coverageunder H exposure indeed exhibited the overpopulation ofH adatoms beyond the saturation coverage attained afterturning off the H exposure [27,28]. The rate equation anal-ysis of the H uptake on the Si(1 00) surface also predictspresence of the excess dihydrides [21].

The resolution of reaction lifetimes evaluated with thebest curve fitting of Eq. (2) to experimental rate curvesmay be limited by the rise- and fall-time of a H-impulseused. Indeed, the lifetime of 0.05 s evaluated for the ratecurve measured previously with the modulated H-beamhaving ’0.1 s rise- and fall-times of the H-impulse was

judged to be an upper limit of the reaction lifetime, sincefor a sharper H-impulse function a shorter reaction timewas preliminarily obtained [25]. In this work, we reinvesti-gate the ABS and AID using a sharper H-impulse with arise- and fall-time of ’0.02 s. In addition to the threeAID components obtained previously [25], the fastest com-ponent with a lifetime of 60.005 s is obtained. The temper-ature dependence of the four AID components aremeasured. Possible atomistic mechanisms for the AIDpathways are proposed on the Si(1 00) surface.

2. Experimental

The H or D beam was generated by an induction-cou-pled plasma of Ar-mixed H2 or D2 gases, respectively, fora radio frequency (13.56 MHz) power of 200 W. Their fluxis �0.01 ML s�1 at the sample surface. The beam passesthrough three differentially pumped chambers connectedto each other through 3 mm diameter apertures by whichthe beam is well collimated. In the second chamber a beamchopper driven by a vacuum tight pulse motor wasequipped, generating a modulated H-beam with a duty ra-tio of 5%. The modulated H-beam has a 0.1 s width with a’0.02 s rise- and fall-time at the rotation of 0.5 Hz. It wasadmitted onto D-terminated Si(1 00) surfaces to induceHD and D2 desorptions. Gas densities of desorbed HDand D2 molecules were detected simultaneously with aquadrupole mass spectrometer (QMS) with an angle inte-grated mode. Signal pulses from the QMS were fed into amulti-channel scaler (MCS, 2048 channels, dwell time of0.9 ms) triggered with the rotating chopper. First 30 or60 pulses in the train of the pulsed desorption signals wereaccumulated for HD or D2, respectively, corresponding toN(t) in Eq. (3). Nascent desorption rate spectrum F(t) canbe obtained from the rate equation

dNðtÞdt¼ F ðtÞ � aNðtÞ; ð3Þ

where a is the pumping speed of the evacuation system.From the exponential decrease of D2 gas density injectedinto the reaction chamber through the same chopper ofthe H-beam, we obtained a = 14.8 s�1 for the uncalibratedgas density measured by the QMS. The validity of the eval-uated a was further confirmed by checking that the modu-lated D2 beam profile could be indeed reproduced by Eq.(3). A commercially available Si(1 00) wafer (B doped,�10 X cm) was cut into a 13 · 22 · 0.5 mm3 specimen. Itwas first sputtered with a 2 keV Ar+ beam with 60� inci-dence with respect to the surface normal at 773 K. Surfacecleanliness was checked with an Auger electron spectrome-ter and confirmed that C(KLL) and O(KLL) intensitieswere both less than 1% of the Si(LVV) intensity. The sur-face was further flashed out at 1223 K for 30 s and then an-nealed at 1083 K for 10 min. It was then slowly cooleddown to the desired temperatures with a cooling rate of0.5 K/s to ensure well-ordered surface reconstruction.

A.R. Khan et al. / Surface Science 601 (2007) 1635–1641 1637

Other details of the experiments have been described else-where [13].

3. Results and discussion

3.1. ABS

We measured rate curves of HD molecules desorbedalong the ABS path from the D/Si(100) surfaces, i.e.,H + D/Si(1 00)! HD. The initial D coverages (h0

D) werecontrolled so that the abstraction experiments were carriedout under the condition of presence or absence of (3 · 1)domains prior to H exposure. The D-saturated Si(1 00) sur-faces were prepared with sufficient D dose at given surfacetemperatures below 600 K. The amount of dideuterides onthe saturated surfaces decreases with Ts due to the b2 TD.On the other hand, the 1.0 ML D/Si(100) surface, whereno dideuterides were present before H exposure, was pre-pared by sufficient D dose at 600 K. As we found previ-ously [25], the desorption yield of HD molecules isapproximately proportional to h0

D. It is not seriously af-fected by dideuterides but slightly decreased with Ts below600 K. These facts suggest that the ABS is mostly direct innature and hence the reaction must be prompt. This expec-tation was confirmed as follows. The nascent desorptionrate spectra F(t) were obtained from Eq. (3) after calculat-ing dN(t)/dt for the measured raw rate curves N(t). Fig. 1shows an example of F(t) obtained on the 1.0 ML D/Si(1 00) surface at 573 K. It is obvious that the ABS ratecurve is characterized with a steep rise and a steep fallpromptly chasing the applied H-impulse. Indeed, the mea-sured ABS rate could be well fit with a single componenthaving the reaction lifetime of 0.005 s as shown in Fig. 1.However, one should note that the value of 0.005 s is anupper limit because the rise time (�0.02 s) of the used H-impulse is longer than the evaluated value.

So far, the ABS reaction has been interpreted with thedirect Eley–Rideal (ER) mechanism [7,11,14], the hot atom

Fig. 1. Nascent HD desorption rate curve (open circles) measured at573 K for h0

D ¼ 1:0 ML on the Si(100) surface. The width of the Himpulse is about 0.1 s. The solid line is the best fit curve by Eq. (2) in thetext with the parameter of s = 0.005 s.

(HA) mechanism [12,15,22,23], and the hot complex (HC)mechanism [18,19]. The HA mechanism claims that inci-dent H atoms first get trapped in the excited states ofchemisorption. They are energetically rich keeping theadsorption energy, being referred to H*. H*s are consideredto be able to freely migrate over the surface. They may col-lide with adatoms to generate ABS, H* + D/Si(100)!HD. Thus, the HA mechanism is an extended version ofthe ER mechanism. Since both mechanisms do not explainthe observed second-order rate law in hD, they are ruledout from possible ABS mechanisms.

In contrast to the HA mechanism, the HC mechanism isbased on such an idea that incident H atoms are firsttrapped in vibrationally excited states of H chemisorptioninto a DSi–SiD cell, being denoted as (H + DSi–SiD)*

[18]. Because of energy corrugation, HCs are considerednot to be able to migrate over the surface. In energy relax-ation, they may disintegrate either into a HD molecule anda singly occupied Si dimer �Si–SiD or into HSiD and �SiDvia breaking the Si dimer bond. The former path is theABS, while the latter one contributes to AID as will be dis-cussed later. This HC-mediated ABS mechanism seems toreasonably explain the second-order rate law if we takepossible isotope effects on the ABS into consideration: Itwas observed that isotope effects on the ABS becomesprominent as D adatoms are substituted with H atomsand thereby the surface is coadsorbed by H and D[24,29], i.e., abstraction of lighter adatoms is facile thanheavier one. The observed isotope effect in ABS could bedue to possible quantum effects on H reaction such as en-ergy barriers affected by zero point energy, tunneling effi-ciencies through barriers, and attempting frequencyfactors in transition. During H exposure, HCs tend to beformed in a DSi–SiH cell where one of the two D atomsin the DSi–SiD has been substituted by H. For (H +HSi–SiD)*, H abstraction by H is preferred to D abstrac-tion by H. This means that in order to generate HDdesorption along the ABS pathway, terminated Si dimersincluded in HCs should be exclusively DSi–SiD. The prob-ability to find two D atoms in the same Si dimer is propor-tional to h2

D. This may rationalize the second-order ratelaw observed in the ABS. Furthermore, the lifetime ofthe vibrationally excited Si–H is known to be less than afew nanoseconds [30]. If the energy dissipation of HCs isrelated to deexcitation of the Si–H(D) vibration, the reac-tion lifetime of the ABS may be the order of the energyrelaxation time of the Si–H vibrations.

3.2. AID

The time response of D2 molecules desorbed along theAID pathway, i.e., H + D/Si(100)! D2, was also mea-sured as a function of Ts as well as h0

D. An example ofD2 rate curve measured for h0

D ¼ 1:2 ML at 503 K is plot-ted in Fig. 2. Contrasted to the HD rate curves, the nascentD2 rate curves are found to be characterized with a slow in-crease in the on-cycles of the modulated H-beam and a

Fig. 2. Nascent D2 rate curve F(t) measured at 503 K for h0D ¼ 1:2 ML on

the Si(100) surface. The rate curve is decomposed into four componentscharacterized with the lifetimes of 0.005, 0.06, 0.8 and 30 s.

Fig. 4. Plots of integrated D2 yields of different lifetime componentsversus Ts for the 1 ML D/Si(100) surfaces. The yield of the componentwith the longest lifetime of 30 s increases with Ts.

1638 A.R. Khan et al. / Surface Science 601 (2007) 1635–1641

long tail in the off-cycles. This feature observed in time re-sponse suggests that the AID process includes delayeddesorptions. Using Eqs. (1) and (2) the nascent D2 ratecurve F(t) was decomposed into four components, A, B,C, and D, by means of a best curve fitting method. Resultswere plotted in Fig. 2. The lifetimes of the components A,B, C, and D were determined to be 0.005 ± 0.001,0.06 ± 0.01, 0.8 ± 0.1, and 30 ± 5 s, respectively. The reac-tion lifetime of the component A is as short as that of theHD rate, and therefore it should be reminded that the mea-sured lifetime is an upper limit. The rate curves measuredin the different conditions for Ts and h0

D were also decom-posed into four components along the same method. As aresult, all the rate curves measured at various temperatureswere found to be fit with the same lifetimes as obtainedabove for the curve plotted in Fig. 2. However, the intensi-ties of the four components varied sensitively depending onboth Ts and h0

D as plotted in Figs. 3 and 4. Here Fig. 3shows the case measured for the initial D coverages satu-rated at given Ts, thus containing dideuterides, and Fig. 4

Fig. 3. Plots of integrated D2 yield versus Ts for the D-saturated Si(100)surfaces rich with monohydrides and dihydrides. The yield of the longestlifetime (30 s) component increases with Ts. On the other hand, the yield ofshorter lifetime components decreases.

shows the case for h0D ¼ 1:0 ML where no dideuterides

were present before H exposure. Comparing Fig. 3 withFig. 4 one can notice that the yield of the four componentsbecomes larger on the D-saturated surfaces than on the1.0 ML D-covered surface for Ts 6 500 K, suggestingdideuteride (dihydrides) play a role in AID. The yield ofthe component A, B, and C decreases with Ts on the D-sat-urated surfaces, whereas they tend to show a slight increaseon the 1 ML D-covered surface. On the other hand, thecomponent D characterized with the longest reaction life-time of 30 s remarkably increases in its intensity with Ts.

3.3. Atomistic AID mechanisms

The AID of which pathways can be categorized withreaction lifetimes should be understood from a thermody-namics point of view for the surface phases attained underH exposure. Northrup [31] calculates stability of surfacephases as a function of H chemical potential (lH), whichis an adjustable variable controlled by pressure of a H res-ervoir as well as Ts. The origin of lH is set so that SiH4

molecules are formed in the reaction system 4H +Si(100)! SiH4 without any energy cost at 0 K. Accordingto his results, if the surface system is in equilibrium with aH reservoir, it takes a unique phase for a certain range oflH. There are two critical levels of lH, l(2 · 1j3 · 1)(=�0.24 eV) and l(3 · 1j1 · 1) (=�0.09 eV), by which thepossible range of lH is discriminated for each surfacephase: A (2 · 1) monohydride phase is stable for lH 6

l(2 · 1j3 · 1), a (3 · 1) monohydride/dihydrdie phase isstable for l(2 · 1j3 · 1) 6 lH 6 l(3 · 1j1 · 1), and a(1 · 1) dihydride phase is stable for lH P l(3 · 1j1 · 1).Hence, the surface coverage is exclusively fixed to 0, 1.0,1.33 ML or 2.0 ML depending on lH. However, in thepresent case the surface system is not in equilibrium withthe H reservoir and the surface coverage takes a certainintermediate value in between the above fixed values. ForTs = 600 K, for example, during H exposure the presentH flux does not allow to cover the whole surface with the

A.R. Khan et al. / Surface Science 601 (2007) 1635–1641 1639

(3 · 1) monohydride/dihydrdie phase. The surface cover-age achieved under H exposure takes a value in the range1.0 ML < hH < 1.33 ML. For such an intermediate cover-age, the surface consists of (3 · 1) monohydride/dihydrdiedomains on the (2 · 1) monohydride surface. Since such a(3 · 1) monohydride/dihydride phase is unstable at around600 K, emission of molecules along, e.g. b2 TD channel,may decrease the monohydride/dihydrdie domains in theoff-cycles of the modulated H-beam. Nevertheless, the(3 · 1) domains may not be fully evacuated at the end ofthe off-cycles because sD is longer than the time durationof the off-cycles. One should remember that the presentexperiments elucidate various AID pathways when (3 · 1)domains dynamically develop and then shrink synchro-nously chasing the modulated H-beam. In the followingsubsections we will discuss possible atomistic AID path-ways on the basis of the measured reaction lifetimes.

3.3.1. Fast AID

Northrup assumes that equilibrium of H on the surfaceis faster than H desorption so that lH is pinned atl(2 · 1j3 · 1) throughout the surface. If otherwiselH > l(3 · 1j1 · 1), the surface coverage under H exposureexceeds 1.33 ML and thus (1 · 1) domains are present onthe (3 · 1) surface. In other words, if the surface phasesare in equilibrium with each other, three phases, (2 · 1),(3 · 1), and (1 · 1) phases, cannot coexist simultaneouslyon the surface. For example, (1 · 1) domains locallyformed on (3 · 1) domains on a (2 · 1) monohydride sur-face is thermodynamically unstable. Therefore, such(1 · 1) dihydride domains formed on the (3 · 1) domainsmust vanish by emitting molecules so that lH is pinnedat l(2 · 1j3 · 1). This implicates that pinning lH atl(2 · 1j3 · 1) is the driving force for the fast AID channel.Fig. 5 shows a ball and stick model which illustrates thismechanism. At step-1 a H atom is captured by a DSi–SiD cell to form a (H + DSi–SiD)*. As a possible relaxa-tion channel, the DSi–SiD dimer bonds will be broken byH in competition with the ABS. Then, four neighboring

Fig. 5. Top view of the ball and stick model to illustrate the mechanism ofthe fast AID generated on (3 · 1) domains at the on-cycles of the H-beam.Gray circles represent D adatoms. Big and small circles represent the firstand second layer Si atoms, respectively. A H atom breaks the Si dimerbond labelled by 2 and 3 in step-1, making four locally adjacent dihydrideslabelled as 1, 2, 3 and 4 in step-2. These four dihydrides are considered as a(1 · 1) dihydride domain locally formed on the (3 · 1) domain. Such a(1 · 1) local domain is thermodynamically unstable toward D2 emissionwhich leaves behind an occupied Si dimer and one dangling bond in step-3. Consequently, the original (3 · 1) domain is restored.

dihydrides are arranged as a local (1 · 1) domain in the(3 · 1) phase at step-2. Since this (1 · 1) domain is thermo-dynamically unstable a molecule is emitted and thereby theSi dimer is restored at step-3. The emission of a moleculefrom this configuration does not require any extra displace-ment of the created dihydrides. Hence this may be respon-sible for such a fast emission of molecules as observed inthe fastest AID with the reaction lifetime of 60.005 s. Fur-thermore, the backbonds of Si atoms labelled by 2 and 3are strained by the neighboring Si dimers at step-2. Thestress due to this strain and the Si dangling bonds createdby breaking the dimer bond further promote the moleculeemission to remake the Si dimer bond at step-3.

Relative stability of local (1 · 1) dihydride domains maybe different depending on sites where they are formed. Ifthey are formed close to, e.g., the periphery of the (3 · 1)domains, surface vacancies, or already existing (1 · 1)dihydrdie domains, the four neighboring dihydrides tendto become stabler since the back bond strain is relaxed thereto some extent, resulting in the reaction lifetime longer.Thus it may be plausible that the lifetimes of the transient(1 · 1) domains are distributed depending on the local con-figurations of the sites where they were formed. The valueof 0.06 s may be understood as an upper bound for thepossible fast AID paths.

3.3.2. 0.8 s AID

The component C characterized with the lifetime of’0.8 s may be basically categorized to the fast AID. Thatis, the process occurs also through the instability of fourneighboring dihydrides when lH is pinned at l(2 · 1j3 ·1) for the surface consisting of a mixture of (3 · 1) and(2 · 1) phases. Taking the somewhat longer lifetime of0.8 s into consideration, four neighboring dihydrides maybe formed at a site less strained than those consideredabove for the fastest AID. As one of the adsorption sitesfavorable for them we invoke a boundary region in be-tween the (3 · 1) domains and the (2 · 1) monohydridephase. This idea was proposed for the first time by Qinand Norton [32] who suggested that molecules will be emit-ted as the area of the (3 · 1) domains build up under Hexposure. The conversion of a (2 · 1) phase to a (3 · 1)one along their model is illustrated with the ball and stickmodel in Fig. 6. Dihydrides at the boundaries could beformed by directly breaking the Si dimer bonds by H.Alternatively, they could be brought there from the regionof (2 · 1) monohydride phase as a result of surface diffu-sion. For Ts P 500 K, a pair of neighboring dihydridesare energetically unstable because of a repulsive interactionamong them due to steric avoidance between two terminat-ing H atoms [31]. Thence, the dihydrides diffuse across thesurface to pile up at the boundaries at step-2 in Fig. 6. Suchdihydrides piled up at the boundary region are energeti-cally unstable to emit a molecule, thereby enlarging(3 · 1) domains. One can easily notice that the rows ofHSi–SiH in the (3 · 1) domains is shifted by one row with

Fig. 6. Ball and stick model to illustrate the atomistic mechanism of the0.8 s AID generated at the phase boundary between the (3 · 1) domainand (2 · 1) surface. This model was given by Qin and Norton for the firsttime to explain the formation of anti-phase boundary present in betweenthe (3 · 1) and (2 · 1) phases [32]. H atoms break the Si dimers as labelledby 1 and 2, and 3 and 4 at step-1 and then form four neighboringdihydrides at step-2. These four adjacent dihydrides are thermodynami-cally unstable toward molecule emission. D2 desorption occurs from thislocal area shown as the dotted ellipse at step-2, leaving behind the Si dimeras labelled by 2 and 3. Consequently, the phase boundary line denoted byP.B. is shifted up by one lattice constant at step-3. Symbols are the same asthose defined in Fig. 5.

Fig. 7. Ball and stick model to illustrate the mechanism of the 30 s AID.At step-1, the site exchange reaction takes place via an isomerizationreaction between Si dimer labelled as 2 and 3 and a dideuteride labelled as4, making a pair of adjacent dihydrides at step-2. Instability of the twodihydrides gives rise to D2 molecule emission, leaving behind the doublyoccupied dimer at step-3. This decreases the area of the (3 · 1) domain,shifting P.B. by one lattice constant down. Symbols are the same as thosein Fig. 6.

1640 A.R. Khan et al. / Surface Science 601 (2007) 1635–1641

respect to those in the (2 · 1) monohydride domains, form-ing an anti-phase boundary [32].

3.3.3. Slow AID

Excess surface energy is stored in the enlarged (3 · 1)monohydride/dihydrdie domains as the H atoms areadmitted onto the surface in the on-cycles of the modulatedH-beam. Such excess surface energy must be relaxed in thesuccessive off-cycles of the H-beam by reducing the en-larged (3 · 1) domain area. The AID path with the 30 s life-time may be attributed to this path. Hence the AID couldbe considered as a sort of the b2 TD arising from the (3 · 1)phase. The kinetic experiments [33,34] so far done for theb2 TD revealed that the desorption activation energy is1.8–2.0 eV, and the molecular desorption obeys a second-order rate law in dihydride coverage. The theoretical calcu-lations could reproduce the measured desorption activationenergy [35,36]. The most reasonable model to describe theb2 TD may be 2HSiH! HSi–SiH + H2. The two neigh-boring dihydrides can be brought via an isomerizationreaction by which HSi–SiH and HSiH exchange their sitesin the (3 · 1) monohydride/dihydride domains. The recentSTM experiments [37] supports this b2 TD path. However,this picture may not be straightforwardly applied to theslow AID having the reaction lifetime of 30 s since it stilloccurs at temperatures even lower than the conventionalb2 TD. Furthermore, the reaction lifetime of 30 s is shorterthan that of the lifetime of b2 TD (P100 s) below 575 K.1

We may anticipate that the desorption proceeds basicallyalong the same pathway as for the conventional b2 TD

1 In order to estimate the desorption lifetime for the b2 TD path, wedescribe its rate constant in a quasi first-order Arrhenius form, i.e.,kðT sÞ ¼ me�Ea=kT s , where m is the pre-exponential factor and Ea is theactivation energy for desorption. Taking values that m = 3 · 1014/s andEa = 1.88 eV [21], we obtain k(575 K) = 10�2/s, i.e., the desorptionlifetime in b2 TD is evaluated to be approximately 100 s at 575 K.

channel, except that it occurs at the periphery of the(3 · 1) domains rather than their central area. Fig. 7 illus-trates a possible mechanism to describe the AID channelhaving the 30 s reaction lifetime occurring at an anti-phaseboundary. Since the tensile stress exerted to the Si latticethrough their backbonds at the phase boundary [32,38]may promote such an site-exchanging isomerization reac-tion between HSi–SiH (DSi–SiD) and HSiH (DSiD). Thenthe dihydride labelled with 4 at step-1 in Fig. 7 will move tothe position labelled with 2 to make a pair of dihydrides atstep-2. The two dihydrides will undergo the molecule emis-sion leaving behind a monohydride Si dimer of whichphase matches to that of the 2 · 1 monohydride phase. Inthis way, the phase boundary shifts by one lattice constantto the lower position, thereby resulting in the decrease ofthe (3 · 1) domain.

4. Summary

We investigated the transient desorption of HD and D2

molecules in the reaction system H + D/Si(100). Whilethe HD desorption induced by H atoms occurred promptlyas a modulated H-beam was admitted onto the surface, theD2 desorption occurred even after the H irradiation wasturned off in the off-cycles of the beam. The D2 desorptionhas been understood in terms of the adsorption-induceddesorption or AID. The D2 rate curves were decomposedinto four components characterized with the reaction life-times of 60.005, ’0.06, ’0.8, and ’30 s. These AID pathscould be related to the thermal instability of the Si–H(D)surface phases: The fastest and the second fastest AID pathswere considered to occur immediately when the (1 · 1) dihy-dride (dideuteride) domains are formed by the adsorptionof H atoms on the (3 · 1) domains. On the other hand,the 0.8 s AID path was related to the increase of the(3 · 1) domains when four neighboring dihydrides areformed at the boundaries between the (3 · 1) domains andthe (2 · 1) monohydride phase. The slow AID path charac-terized with the 30 s lifetime was attributed to the thermaldesorption from the (3 · 1) monodeuteride/dideuteride

A.R. Khan et al. / Surface Science 601 (2007) 1635–1641 1641

domains, which were excessively formed under H exposurebeyond the equilibrium coverage. Thus the slow AIDaccompanies a phase transition from the (3 · 1) to (2 · 1)phase. Ball and stick models were given to illustrate theseatomistic mechanisms for the three AID paths.

Acknowledgement

This work was financially supported by a Grant-in-Aidfrom the Ministry of Education, Science, Sport, and Cul-ture of Japan (No. 17002011).

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