Effect of tunneling current on the growth of silicon islands on si(111) surfaces with a scanning tunneling microscope

Đây là một bài báo khoa học về dây nano silic trong lĩnh vực nghiên cứu công nghệ nano dành cho những người nghiên cứu sâu về vật lý và khoa học vật liệu.Tài liệu có thể dùng tham khảo cho sinh viên các nghành vật lý và công nghệ có đam mê về khoa học

Si(111) surfaces with a scanning tunneling microscope Alexander A Shklyaev 1, Motoshi Shibata, Masakazu Ichikawa *

Joint Research Center for Atom Technology (JRCAT), Angstrom Technology Partnership (ATP), 1-1-4 Higashi, Tsukuba,

Ibaraki 305-0046, Japan

Received 5 July 1999; accepted for publication 15 November 1999

Abstract

The early-stage growth rate of a silicon island on a Si(111) surface at the center of the interaction between a sample and the tip of a scanning tunneling microscope (STM ) was defined as a function of the tunneling current The tunneling current dependence of the rate has a maximum at 0.3 nA, and the decrease of the rate at tunneling currents above 0.3 nA was related to the effect of electron flow on atom transfer by field-induced directional diffusion The early-stage growth rate was about three times higher than the late-stage growth rate, which was almost independent of the tunneling current The results suggest that atom transfer by field-induced diffusion on the sample surface was gradually substituted by atom transfer from the STM tip by field-induced silicon atom re-evaporation as the island grew from 0 to about 12 nm in height © 2000 Elsevier Science B.V All rights reserved

Keywords: Diffusion and migration; Field effect; Growth; Scanning tunneling microscopy; Semiconducting surfaces; Silicon; Surface structure, morphology, roughness, and topography

of atoms involved in the transfer cannot be con-trolled well by the voltage of the pulses or by the

A surface structure can be modified by

transfer-pulse duration [5,6 ] We have recently demon-ring individual atoms and molecules with a

scan-strated that growth of a three-dimensional (3D) ning tunneling microscope (STM ) [1,2] Such

silicon island on a Si(111) surface occurs at the modifications are usually achieved when micro- or

center of the tip–sample interaction after applying millisecond voltage pulses are applied between the

elevated negative bias voltages (6–10 V ) to the sample and the STM tip [3,4] The creation of

STM tip at a constant tunneling current [7] The nanostructures, which involves the transfer of

hun-island was created by transferring atoms towards dreds and thousands of atoms, can be performed

the center from the area around the island

by increasing the voltage of the pulses [5,6 ]

In this work, in order to outline the conditions However, in ultrahigh vacuum ( UHV ) conditions,

for reproducible island formation, the kinetics of there is only a probability of producing a

nan-island growth — that is, nan-island height as a function

of the duration of tip–sample interaction — was

* Corresponding author Fax: +81-298-54-2577.

obtained for various tunneling currents We

deter-E-mail address: ichikawa@jrcat.or.jp (M Ichikawa)

mined the early- and late-stage growth rates from

1 On leave from the Institute of Semiconductor Physics,

Novosibirsk 630090, Russia. these kinetic data The tunneling current

depen-0039-6028/00/$ - see front matter © 2000 Elsevier Science B.V All rights reserved.

PII: S 0 0 39 - 6 0 28 ( 99 ) 0 11 6 5 -6

sample interaction very likely occurs in the growth The reproducible formation of the 3D silicon

islands on the Si(111) surface with the STM has

of islands over 4 nm in height Island growth was

not stable at tunneling currents above 1.0 nA At been observed at small tunneling currents of less

than 0.5 nA At a given duration of the voltage large tunneling currents, a pit or a pit near an

island could appear on the surface The corre- pulse, the scattering of the island height for islands

grown in one experimental run was within 10%, sponding tip–sample interaction process was

accompanied by fluctuations of the tunneling cur- and the height of islands increased gradually when

the duration increased [7] Fig 1 shows STM data rent and mechanical vibrations of the STM tip in

the direction normal to the surface for the islands grown at a larger tunneling current

of 1.2 nA for various durations of the tip–sample interaction At this large tunneling current, the height of the islands increases with increasing

2 Experimental

voltage pulse duration, but the increase is not gradual A big scattering of the height was

We used a UHV STM combined with a scanning

reflection electron microscope (SREM ) The con- observed for relatively small durations (between 1

and 17 s) as shown in Fig 1a and c The island dition of the tungsten tip of the STM and the

manipulation of the tip on the silicon surface could grew to about 12 nm in height when the voltage

pulse duration was extended to 103 s

be monitored continuously with the SREM

Details of the apparatus have been described else- From the STM images, we obtained the height

of the islands as a function of the voltage pulse where [8] Clean Si(111) surfaces were prepared

by flash direct-current heating at 1200°C After duration as shown in Fig 2 To get a description

of these kinetic data, island growth was charac-setting a constant tunneling current between 0.09

and 3 nA and a negative tip bias of−8 or −10 V terized by the early- and late-stage growth rates

It has been measured that the islands, grown with for a duration between 1 and 103 s, the STM tip

was positioned over the surface to create an island the STM, have a high aspect ratio (height divided

by base length) of about 0.3 [7] We shall assume The STM images were obtained with a tip bias

voltage of either 2.0 or −2.0 V and a constant that all islands are cone-shaped with an aspect

ratio of 0.3 The amount of silicon in such an current between 0.3 and 0.6 nA We repeatedly

used the same STM tip to create islands To island is approximately 3H3, where H is the height

of the island grown for the voltage pulse duration sharpen the tip when the tip apex was crushed, we

applied a voltage pulse between the tip and the t The growth rate R of the islands can be defined

as R =3d(H3)/dt This definition corresponds to

sample The heating of the tip by the electric

Fig 1 (a, b) STM images of silicon islands grown at a constant current of 1.2 nA and a tip bias voltage of −10 V In order to create the islands (from left to right), the bias voltage was applied (a) for 1, 2, 4, 6, 9, 12 and 17 s and (b) for 8, 17, 33, 60, 120, 240, 480 and 960 s (c, d ) Height profiles along the island rows that are indicated by arrows in (a) and (b), respectively.

stage growth rate for H larger than 4 nm The

results of the fit are shown in Fig 2 The good approximation to the data for island heights larger than 4 nm by a constant rate indicates that the late-stage growth rate was almost independent of the island height The early-stage growth rate was significantly higher than the late-stage growth rate and, as a function of the tunneling current, had a maximum at 0.3 nA as shown in Fig 3

The scattering of the island height in one experi-mental run increased with increasing tunneling current The increase of the scattering coincided with the appearance of fluctuations of the tunnel-ing current Durtunnel-ing the growth of islands at a bias

Fig 2 Island height as a function of the duration of bias

volt-voltage of −10 V and a tunneling current of

age Islands were grown at a negative tip bias voltage of −10 V

and at a constant tunneling current of 1.2 nA The solid lines 1.2 nA or larger, the fluctuations could enhance

represent the approximation of the data by a function by themselves and could eventually reach about

H =[(a+Rt)/3]1/3 (see text) for the late growth stage and the

500 nA in amplitude As a result of the

fluctua-insert represents the early growth stage The scattering of the

tions, structures appeared on the silicon surface

data exceeded the accuracy of measurements of the island

height, which was within 10% (as shown for a point at 240 s). that were like a pit or a pit near an island (Fig 4)

The large fluctuations could flatten the apex of the STM tip, as could be seen in SREM images of the

the equation H=[(a+Rt)/3]1/3, where a and R

tip and in the quality of STM images obtained were used as the fitting parameters in order to

after the fluctuations In the constant tunneling

obtain the growth rate R from the experimental

current mode of the STM operation, the fluctua-data We thus obtained the early-stage growth rate

tions were accompanied by mechanical vibrations

by using the range of voltage pulse durations at

which H was between 0 and 4 nm, and the late- of the STM tip in the direction normal to the

the electric field, p is the static dipole moment, a

is the polarizability of the atoms on the surface

(aE

ris the induced dipole moment), and the electric

field E

r at the sample surface decreases with

increasing radial distance r from the center of the

Fig 3 (a) Early- and late-stage growth rates, and (b)

early-tip–sample interaction When the mobile silicon

stage growth rate as a function of the tunneling current for

atoms on the surface have a positive electronic

tip bias voltages of (a) −10 V and (b) −8 V The accuracy of

measurements of the early-stage growth rate at −10 V was charge, the interaction between the electric field,

about 15–20% The lines with arrows indicate the regions of created by applying a negative tip bias voltage, tunneling currents at which island growth was unstable.

and both the static and induced dipole moments directs diffusion toward the center of the tip– sample interaction, providing the island growth surface The vibration was observed by the SREM

[7] and appeared as a result of attempts to main- The kinetic data obtained in the present work

for island growth as a function of the tunneling tain the constant tunneling current by the feedback

circuit At large fluctuations, mechanical contact current for currents below 1 nA give us additional

insight into the mechanism of island formation between the STM tip and the sample could occur

The appearance of the large pit at the place of the with the STM At a constant tunneling current

mode of STM operation, a larger tunneling current tip–sample interaction as shown in Fig 4b might

be the result of the mechanical contacts Note that corresponds to a shorter tip–sample distance s

because I~V exp(−1.1sw1/2), where I is the

tunnel-atom transfer at mechanical tip–sample contacts

has been considered to explain the mound and pit ing current, V is the bias voltage, w is the effective

height of the tunnel barrier expressed in Volts, and formation on silicon surfaces in UHV when voltage

pulses were applied to a gold STM tip [5,6,9] s is in A ˚ [13,14] The decrease of s with increasing

I makes the electric field at the sample surface

At tunneling currents below 1 nA, the STM

operation was stable and the kinetic data of island stronger and therefore should increase the island

growth rate [7,10] However, the data in Fig 3 growth were reproducible The following

experi-mental data were obtained recently for the initial show that the early-stage growth rate increased

with increasing tunneling current only at small stage of the tip–sample interaction which results

in island growth [7,10] The application of elevated currents up to 0.3 nA, and the decrease of this rate

was observed at larger currents In our case of the bias voltages to the STM tip at a constant

tunnel-ing current created an area of disordered negative tip bias polarity, atom diffusion flows

towards the center of the tip–sample interactions; Si(111)-7×7 structure by field-induced

evapora-Fig 4 (a, b) STM images of Si(111) surfaces after tip–sample interactions at a tip bias voltage of −10 V and (a) at tunneling currents of 0.3, 0.7, 1.3 and 3.0 nA (from top to bottom) applied for 22 s at each point, and (b) for 25 s at tunneling currents marked

in the image (c, d ) Height profiles between the arrows marked in (a) and (b), respectively Structures like (a) a pit near an island and (b) a large pit are seen at the large tunneling current of 3.0 nA.

that is, in the direction opposite to the flow of growth As an island grows, the distance between

the area around the island and the STM tip electrons This is similar to the effect of the electron

wind force acting at electromigration [15,16 ] increases because, at a constant tunneling current,

the distance between the STM tip and the center Therefore, the effect of the electron flow on the

direction of atom movement is opposite to the of the island is fixed Therefore, the growth causes

the electric field to decrease at the area around the effect of the decrease of the potential energy

barri-ers for diffusion by the electric field, which is island and, hence, the growth should cause the

growth rate to decrease according to the mecha-responsible for island growth with the STM The

decrease of the early-stage growth rate as the nism of the early stage of growth described above

This corresponds to the fact that the late-stage tunneling current increased ( Fig 3) indicates that

the effect of the increasing electron flow on direc- growth rate is smaller than the early-stage growth

rate ( Fig 3a) However, at island heights between tional diffusion dominates the effect of the

increas-ing electric field when the tunnelincreas-ing current 4 and 12 nm, the growth rate remains significant

and is almost independent of the island height exceeds 0.3 nA

The kinetic data also show the difference Moreover, the early-stage growth rate has a strong

tunneling current dependence, whereas the late-between the early stage and the late stage of island

action The STM images have shown that, after the potential energy barrier for diffusion towards

the center of the tip–sample interaction In addi-the 6 s tip–sample interaction at a tip bias voltage

of−10 V, the diameter of disordered Si(111)-7×7 tion to this driving force, the interaction between

the electric field and the induced dipole moment structure around the island was about 60 nm

[7,10] If on average every third silicon atom is of adatoms always pushes the diffusion flow

towards the center, where the electric field is removed from the surface bilayer in this area and

is transferred in the island, the height of the island stronger At a given temperature and dipole

moment, the adatoms should be weakly bonded will be 4.8 nm for cone-shaped islands with an

aspect ratio of 0.3 The estimated height is approxi- to the surface and able to diffuse under the electric

field However, not every surface can be modified mately two times larger than the height of

corre-sponding real islands, suggesting that part of the with the STM at room temperature, even if

ele-vated bias voltages are applied For example, the silicon removed can be accumulated on the STM

tip Note that the mechanism which includes STM-induced evaporation of SiO2 films has been

shown to occur only at temperatures higher than re-evaporation is consistent with the fact that the

probabilities for field-induced evaporation of nega- 450°C [20] The well-known ability of the STM to

manipulate atoms and molecules of adsorbates tive and positive silicon ions have been found to

be almost equal [3,17] Thus, the experimental [1,2] suggests that atoms which were adsorbed first

or deposited during the process can be involved in results obtained in this work indicate that two

mechanisms of atom transfer contribute to the continuous atom transfer with the STM As a

result, highly doped and compound nanostructures island growth: the continuous transfer of weakly

bonded silicon atoms along the sample surface is can be created As shown here, kinetic

measure-ments of the nanostructure growth provide data gradually substituted by silicon atom transfer from

the STM tip to the island as the island height which throw light on the mechanism of

nanostruc-ture formation

increases from 0 to about 12 nm

We have recently demonstrated that continuous

atom transfer can be used to create germanium

islands on germanium wetting layers on Si(111) 4 Conclusion

surfaces and that structures like nanowires can

also be formed when the STM tip is moved slowly The early- and late-stage rates of silicon island

growth on Si(111) surfaces with the STM were along the surface [18] It is important to define

the sort of conditions on the surface that are obtained as a function of tunneling current The

dependence of the early-stage growth rate has a required to create nanostructures by continuous

maximum at 0.3 nA, which was related to competi- References

tion between the electric-field-induced directional

diffusion and the effect of electron flow pushing [1] J.A Stroscio, D.M Eigler, Science 254 (1991 ) 1319.

[2] Ph Avouris, Acc Chem Res 28 (1995) 95.

the diffusion in the opposite direction At tunneling

[3] H Uchida, D Huang, F Grey, M Aono, Phys Rev Lett.

currents above 1 nA and at a tip bias voltage of

70 (1993) 2040.

−8 and −10 V, the growth of islands was unsta- [4] I.-W Lyo, Ph Avouris, Science 253 (1991) 173. ble The late-stage growth rate was about three [5] C.S Chang, W.B Su, T.T Tsong, Phys Rev Lett 72

(1994) 574.

times lower than the early-stage growth rate, and

[6 ] D Fujita, Q Jiang, H Nejoh, J Vac Sci Technol B 14

was almost independent of the tunneling current

(1996) 3413.

and height of the islands The difference between

[7] A.A Shklyaev, M Shibata, M Ichikawa, Appl Phys Lett.

the rates was associated with different mechanisms 74 (1999) 2140.

of island growth, in which atom transfer by field- [8] S Maruno, H Nakahara, S Fujita, H Watanabe, Y.

Kusumi, M Ichikawa, Rev Sci Instrum 68 (1997) 116.

induced diffusion on the sample surface in the

[9] T.T Tsong, Phys Rev B 44 (1991) 13703.

early growth stage is gradually substituted by the

[10] A.A Shklyaev, M Shibata, M Ichikawa, submitted for

transfer of silicon atoms, accumulated on the STM

publication.

tip during the initial stages of the tip–sample [11] T.T Tsong, G Kellogg, Phys Rev B 12 (1975) 1343. interaction, to the island by field-induced [12] L.J Whitman, J.A Stroscio, R.A Dragoset, R.J Celotta,

Science 251 (1991) 1206.

re-evaporation in the late growth stage

[13] G Binning, H Rohrer, Surf Sci 126 (1983) 236 [14] J.G Simmons, J Appl Phys 34 (1963) 1793.

[15] D Kandel, E Kaxiras, Phys Rev Lett 76 (1996) 1114 [16 ] E.S Fu, D.-J Liu, M.D Johnson, J.D Weeks, E.D

[17] A Kobayashi, F Grey, R.S Williams, M Aono, Science

259 (1993) 1724.

This work, partly supported by the New Energy

[18] A.A Shklyaev, M Shibata, M Ichikawa, in preparation.

and Industrial Technology Development

Organ-[19] K.D Brommer, M Galva´n, A.D Pino Jr., J.D

Joanno-ization (NEDO), was carried out at JRCAT under poulos, Surf Sci 314 (1994) 57.

an agreement between the National Institute for [20] M Shibata, Y Nitta, K Fujita, M Ichikawa, Appl Phys.

Lett 73 (1998) 2179.

Advanced Interdisciplinary Research and ATP