APPLIED PHYSICS LETTERSVOLUME 83, NUMBER 12

22 SEPTEMBER 2003

Site-selective silicon adatom desorption using femtosecond laser pulsepairs and scanning tunneling microscopy
D. N. Futaba,a) R. Morita, and M. Yamashitab)
Department of Applied Physics, CREST, Japan Science and Technology Corporation (JST),
Hokkaido University, Sapporo 060-8628, Japan

S. Tomiyama
Department of Applied Physics, Hokkaido University, Sapporo 060-8628, Japan

H. Shigekawa
Institute of Applied Physics, 21st COE, CREST, Japan Science and Technology Corporation (JST),
University of Tsukuba, Tsukuba 305-8573, Japan

共Received 28 April 2003; accepted 28 July 2003兲
We performed an experimental study of silicon adatom desorption from the Si共111兲-7⫻7 surface
using femtosecond laser pulse pair excitation with 80 fs pulse duration, 800 nm center wavelength,
300 mW average power, and a 100 MHz repetition rate. Using scanning tunneling microscopy, we
directly recorded the desorption characteristics at each delay setting for each of the four adatom
binding sites. The study revealed a preferential dependence between the delay time and the adatom
sites within a 66.6 –1000 fs delay range. © 2003 American Institute of Physics.
关DOI: 10.1063/1.1613361兴

Bonding at surfaces holds fundamental importance inour understanding and manipulation of chemistry and materials. Among them, femtosecond 共fs兲 laser-induced breaking
or forming chemical bonds can provide crucial insight into
these surface-related processes. A considerable amount of research has already been done on the study of the interaction
between light and solid-state surfaces,1,2 in particular, on
semiconducting surfaces, for both the fundamental understanding and the technological importance to solid state devices. Structural changes have been observed to occur as aresult of this interaction using lasers, and electronic processes have been shown to play important roles when thelaser intensity was below the melting threshold.3–5 Chargetransfer and rearrangement are also thought to be key inmaking and breaking surface chemical bonds. In addition,
the coupling of photoexcited electrons to the nuclear motionof the surface species and the time scale involved in bonddissociation and formation are two important issues. However, such optically induced dynamic surface phenomena
have not been well studied at the atomic scale.6
The Si共111兲-7⫻7 surface, whose structure consists ofdistortions from the ideal bonding tetrahedral s p 3 bonding
geometry, relies heavily on electron-lattice interaction forstructural stability.7 This reconstructed silicon surface presents four distinct adatom sites, which differ both electronically and dynamically8,9 and is thought they could serve assites to excite localized vibrations. The purpose of this letteris to experimentally demonstrate that the fs delay-time difference in optical pulse pair excitation plays a role in thesite-selective desorption of Si adatoms under below-melting
power irradiation. Using a scanning tunneling microscope
共STM兲 to directly observe the numbers of vacancies, we ina兲Electronic mail: dfutaba@eng.hokudai.ac.jp
Author to whom correspondence should be addressed; electronic mail:mikio@eng.hokudai.ac.jpb兲

vestigated the pulse pair delay-time dependence of Si adatom
desorption from the Si共111兲-7⫻7 surface. These findings canprovide a selective fabrication technique for nanometer-scale
devices as well as physical and chemical insight into ultrafast
dynamic processes at surfaces with atomic scale resolution.
We performed a room temperature desorption study of Si
adatoms from the Si共111兲-7⫻7 surface using a fs laser-STM
system 关Fig. 1共a兲兴. The STM was a commercially purchased
low-temperature ultrahigh vacuum instrument by UNISOKUCo., Ltd., Japan and was operated at a base pressure of⬃8.0⫻10⫺11 Torr. Si共111兲 samples were cut from an n-type,As-doped 共␳⫽0.001–0.005 ⍀/cm兲 Si共111兲 wafer and degassed under ultrahigh vacuum conditions at ⬃600° C for 10h. The samples were subsequently flash-heated at ⬃1150° C
for 10 s in order to form the 7⫻7 reconstruction. During
flash-heating, the pressure never exceeded 1.5⫻10⫺10 Torr.
Linearly polarized pulses from a mode-locked Ti:sapphire laser 共100 MHz repetition rate, 80 fs pulse width, 300
mW average power, and 800 nm central wavelength兲 weredirected into a Michelson-type interferometric delay andwere then focused to a 300 ␮m diameter spot on the sample
surface by a silver concave mirror 共focal length of 300 mm兲.
The p-polarized 共parallel to the reflection plane兲 laser pulses
were incident to the surface at approximately 45°. Duringlaser irradiation, the tip was withdrawn out of the path of thelaser to avoid desorbing any adsorbed species that may haveresided on the tip. The pulse pair delays ranged from 66.6 to1000 fs 共15.0 to 1.0 THz兲 with an irradiation time of 60 min.
The adatom removal yield was found roughly to be on theorder of 5 vacancies per 20 unit cells.
After verifying sample cleanliness by STM and characterizing the number of pre-existing adatom vacancies by examining several thousand unit cells, we irradiated the surface. After allowing for thermal drift to settle, the surface
was once again examined by STM. The desorption yield wasdetermined by counting the number of vacancies for images

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Appl. Phys. Lett., Vol. 83, No. 12, 22 September 2003

Futaba et al.

FIG. 1. 共a兲 Pulses from a mode-locked Ti:sapphire laser 共100 MHz repetition rate, 80 fs pulse width, 300 mW average power, and 800 nm central
wavelength兲 are directed into the STM vacuum chamber after being diverted
through Michelson-type interferometric delay. Flip mirrors 共F1–F4兲 allow
for pulse characterization during the experiment. 共b兲 A STM image of the7⫻7 surface before irradiation. I⫽0.2 nA; bias⫽1.5 V. Both polarities weretaken for each location to identify adsorbed gases. 共c兲 A STM image after 1
h of irradiation of two pulses. I⫽0.2 nA; bias⫽1.5 V. The circle indicates anadsorbed gas as identified from the negative bias image.

both before and after irradiation, and this procedure was repeated several times for each delay setting to confirm reproducibility. All STM observations were done using PtIr tips
and were taken in constant–current mode 共0.20 nA, 1.5 V兲.
STM images of sampled regions before and after laser
irradiation of 4 ␮ J/cm2 fluence reveal that the number ofvacancies increases after laser exposure 关Figs. 1共b兲–1共c兲兴.
STM images were taken in both polarities in order to rule out
the possibility that apparent vacancies were due to adsorbed
gases, which can appear as vacancies in one polarity and notin the other.10,11 An example of such a case is marked with acircle in the image. Accordingly, by using this as a rule, weare confident that the dark spots are adatom vacancies.
Upon incrementally decreasing the delay 共increasing thepulse pair frequency兲, we observed that the total desorption
yield showed an increasing trend with the exception at 12THz 关Fig. 2共a兲兴. The profile of the total desorption yield suggested that significant desorption occurred only below 333 fs
共above 3 THz兲.
We then examined the images further and compared thecenter-to-corner adatom desorption ratios for each delay setting. We found that the ratio ranged in value from as low as
0.876 to as high as 22.7 for each delay setting 关Fig. 2共b兲兴.
This revealed a desorption preference which varied with theexcitation delay time. In addition, it showed one delay setting 共8 THz兲 to have a particularly high selectivity for centersites. It seems that this result corresponds to the fact that thefaulted-half 共FH兲-center adatoms have the most prominent
peak at 8 THz for the surface phonon spectrum 共one of thevibrational frequencies of localized phonons at adatom sites兲,
while the FH-corner adatoms have no peaks at this fre-FIG. 2. 共a兲 Total net yield as a function of excitation frequency. The trend
indicates a general increase with higher frequencies 共shorter delays兲. 共b兲 The
center to corner adatom ratio for each of the same excitation frequencies
showing the preferential desorption between binding sites changes despite
showing differences in total yield.

quency.9 This discovery prompted us to further investigate
the adatom site dependence on the desorption yield.
As summarized in Fig. 3, a detailed investigation of thedesorption yields for each binding site revealed subtle trends
for each frequency setting and for each adatom site. This
breakdown of data is presented with the binding sites ar-FIG. 3. Desorption yield as a function of binding site and excitation frequency with a pulse pair 共1/delay time兲.
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Futaba et al.

Appl. Phys. Lett., Vol. 83, No. 12, 22 September 2003

ranged in increasing binding energy site 共i.e., FH-center,
unfaulted-half 共UH兲-center, FH-corner, and UH-corner兲12 to
make any related trends more easily identifiable. The mostobvious trends are the individual desorption profile for eachbinding site and the desorption profile at each excitation delay setting. Generally, FH-center sites showed the greatest
level of desorption. This seems to correlate with the fact that
the FH-center sites have the lowest binding energy among
the four adatom sites. Throughout the excitation frequency
spectrum, save the 1–3 THz 共to be discussed later兲 and the12 THz setting, the efficiency for the FH-center sites wasroughly unvarying. The UH-center sites, however, showed aslightly varying behavior with being strongly efficient at 8THz and considerably less efficient at 10, 12, and 15 THz.
Similarly distinct peaks existed at 10 and 15 THz for theFH-corner sites. The UH-corner sites exhibited maxima inthe desorption yield at 12 and 15 THz.
Examining the profiles for each excitation setting, weobserved that at lower frequencies, such as 1–3 THz, notonly was there an insignificant level of desorption, there appeared to be no prominent site-preferred vacancies 共Fig. 3兲.
With higher frequencies, 6 –12 THz, not only did the desorption efficiency increase, but also site-preferences emerged.
At 6 THz, the desorption levels at all sites increased with aprominent peak for the FH-center sites. At 8 THz, highly
selective profiles appeared with negligible desorption at twohigher binding-energy sites 共FH-corner and UH-corner兲 anddistinct peaks at the two lower ones 共FH-center and UHcenter兲. At 10 and 12 THz, in difference to prior trends where
we observed preferences between center and corner sites, weobserved a preference between subunits. At 10 THz, there
was a marked preference to the FH-center and FH-cornersites desorption, while at 12 THz this tendency was reversed.
Finally, at 15 THz, the desorption at all sites showed increased yields while showing no clear preferential behavior.
We believe that the process governing desorption is a
two-step excitation. The first pulse excites an electron in asurface bonding state to an intermediate state with a lifetime
␶, which is characteristic to the local electronic properties ofthe binding site. Additionally as deduced from Fig. 3, thislifetime is shorter than 333 fs. Using a single 1.55 eV photon, with the intensity below the melting threshold, the probability for removing an adatom is virtually zero. However, ifa second pulse interacts with the medium within the lifetimeof the excited surface bonding state, its energy can add to thefirst. In such a case, local phonons around the adatom, whose2335

electron contributes to the substrate bonding, are further excited, which gives rise to the possibility of bond breakage.
This two-photon process must therefore depend on the delaytime. Further, we believe that differences in the desorption
preferences stem from differences in the local dynamic properties surrounding each adatom site.9 To be complete, detailed energy-band structure calculations are needed to locate
the precise energy level involved.
More recent experimental results using s-polarized fs
pulses showed insignificant levels of desorption for a variety
of delay times. This result strongly suggests that surface
heating associated with fs-laser irradiation is not the dominant desorption mechanism particularly at short delay times.
Furthermore, it also suggests a dipole-like interaction between the adatom sites 共including the dangling bonds兲 andthe laser. Moreover, this gives rise to localized phonons
modes oriented along the surface normal direction.9
In summary, the desorption characteristics of silicon adatoms from the Si共111兲-7⫻7 surface were investigated using
fs laser pulse pair excitation and direct observation withSTM. A delay-time dependence on the overall desorption
yield and desorption character was observed. With shortened
delay time 共increased excitation frequency兲, the total desorption yield increased. In addition, preferences among adatom
binding sites were observed as a function of the delay time.
The authors gratefully acknowledge the helpful discussions with Professor C. Y. Fong.
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