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Isothermal desorption process of Cl-covered Si(1 1 1) studied by surface differential reflectivity spectroscopy
Surface Science 527 (2003) 21–29 www.elsevier.com/locate/susc
Isothermal desorption process of Cl-covered Si(1 1 1) studied by surface differential reflectivity spectroscopy Masatoshi Tanaka *, Shigeru Minami 1, Kenichi Shudo, Eriko Yamakawa
2
Department of Physics, Faculty of Engineering, Yokohama National University, 79-5 Tokiwadai, Hodogaya-ku, Yokohama 240-8501, Japan Received 15 May 2002; accepted for publication 21 January 2003
Abstract The isothermal desorption of silicon chlorides from a chlorine-saturated Si(1 1 1) rest atom surface at about 900 K has been investigated by means of in situ real-time surface differential reflectivity spectroscopy. The spectral features at 1.8 eV (A) and 2.5 eV (B) arise mainly from missing adatom dangling bonds and missing adatom back bonds of the dimer–adatom–stacking-fault (DAS) structure, respectively. The feature A decays twice as fast as B does, which indicates that A also originates from the dangling bonds on the rest atom surface. The decay of A, which is considered to represent the desorption of silicon dichlorides, follows first-order kinetics, and the activation energy for this process is 2.3 eV. The decay of B, which is considered to represent the formation of the DAS structure, also follows first-order kinetics and the activation energy is 2.8 eV. The mechanisms of the desorption of silicon dichlorides and the formation of the DAS structure are discussed on the basis of the experimental results. Ó 2003 Published by Elsevier Science B.V. Keywords: Reflection spectroscopy; Thermal desorption; Silicon; Chlorine; Etching
1. Introduction Layer-by-layer etching of semiconductor surfaces is a fundamental technique to fabricate nanometer-scale structures. Etching with halogen gases is widely used in the semiconductor industry and is also a promising candidate method for layerby-layer etching. Halogen etching consists of sev-
*
Corresponding author. Tel./fax: +81-45-339-4201. E-mail address: [email protected] (M. Tanaka). 1 Present address: National Instruments Japan Corporation, 2-4-1 Shibakoen, Minato-ku, Tokyo 105-0011, Japan. 2 Present address: Technology Operations Systems, Merrill Lynch Japan Securities Co., Ltd., 1-1-3 Otemachi, Chiyoda-ku, Tokyo 100-8180, Japan.
eral stages: adsorption and migration of halogen molecules on the surface, desorption of silicon halides, and reconstruction of the clean surface. The atomic-scale mechanisms of these processes should be elucidated before an actual application of layerby-layer etching. Etching of the Si(0 0 1) surface is studied in connection with industrial applications, though etching of the Si(1 1 1) surface is also of interest because the dimer–adatom–stacking-fault (DAS) structure has a variety of sites with different chemical reactivities. Chloride desorption from Si(1 1 1) is a model process for the layer-by-layer etching studies and the static properties at each stage have been revealed mainly by scanning tunneling microscope (STM) studies. Most studies have been concerned
0039-6028/03/$ - see front matter Ó 2003 Published by Elsevier Science B.V. doi:10.1016/S0039-6028(03)00071-2
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M. Tanaka et al. / Surface Science 527 (2003) 21–29
with the desorption from a Cl-saturated surface having polychloride species. After desorption due to annealing at about 700 K or ultraviolet laser irradiation, Si adatoms are removed and the socalled rest atom surface covered with Cl atoms appears [1–5]. X-Ray photoelectron spectroscopy [6,7] and surface-enhanced X-ray absorption fine structure [8] studies confirmed that only monochloride species remain after annealing above 673 K. In accordance with the above observations, a temperature-programmed desorption (TPD) study revealed that polychlorides species are desorbed as a peak at 690 K, and a laser-induced thermal desorption (LITD) study showed that the SiCl3 species almost disappears above 630 K [9]. When the Cl-covered rest atom surface is heated above 900 K, chlorine atoms are desorbed mainly as SiCl2 , as shown by a TPD study [7,9]. In this way, one Si layer is taken off, and a clean 7 7 DAS structure is subsequently restored. In spite of these studies, the dynamic processes of the desorption of silicon chlorides and the reconstruction to form the 7 7 DAS structure are still poorly understood. Real-time observation is essential to investigate the kinetics. Optical methods are superior to others, because they are noninvasive, nondestructive, and capable of very rapid response. Surface differential reflectivity spectroscopy (SDR) was proved to be a powerful tool for the real-time study of hydrogen adsorption on Si(1 1 1) 7 7 [10]. Differential reflectivity is defined as DR=R ¼ ðRa Rc Þ=Rc , where Ra and Rc are the reflectivities of the H-covered and clean surfaces, respectively. Spectral signatures of adsorption on adatom dangling bonds and breaking of adatom back bonds were identified from the calculation of the DR=R spectrum for the hydrogenated 7 7 surface [11,12]. These spectral features arise from the surface states of the clean surface, so that the DR=R spectrum does not depend on the adsorbate. The adsorption process of chlorine on Si(1 1 1) 7 7 has been elucidated by the same method [13,14]. These features develop with time during adsorption processes, whereas in the desorption process, they decay with time as the clean surface structure is restored. The variations can provide information about the desorption process.
This paper is the first report on the real-time observation of the desorption process by means of SDR. The process of isothermal desorption of silicon chlorides from a Cl-saturated Si(1 1 1) rest atom surface and the restoration of the DAS structure were investigated in the temperature range from 873 to 933 K. The time courses of the dangling bond recovery and the back bond recovery were determined from the magnitude of the corresponding spectral features. The rate constant, the order of reaction and the activation energy were obtained from the temperature dependence of the time courses. The kinetics of the desorption processes and the restoration process are discussed on the basis of the results obtained.
2. Experimental Measurements were performed in an ultrahigh vacuum chamber at a base pressure of 2 108 Pa. A specimen of 5 18 0.38 mm3 was cut from a Bdoped p-type Si(1 1 1) wafer (10–15 X cm). The specimen was resistively heated to 1420 K for several seconds and then slowly cooled to obtain the 7 7 DAS structure. The 7 7 structure was confirmed by low-energy electron diffraction. Chlorine gas was generated with an AgCl electrochemical cell [15]. The Si(1 1 1) surface was saturated with chlorine atoms (50 L; 1 L ¼ 1.33 104 Pa s) at room temperature [13]. The Cl-covered 7 7 rest atom surface obtained by annealing at 743 K is not stable, but partially reconstructs a Cl-terminated 1 1 bulk-like surface [3]. Other STM studies [16,17] have shown that additional Si atoms required for the reconstruction are supplied from steps and that the movement of the step enlarges the 1 1 domain. Accordingly, in our study, the surface was annealed at 743 K for 2 min in order to enlarge the 1 1 domain. Otherwise, the reconstruction and the desorption of both polychloride and monochloride species will proceed at the same time during isothermal desorption at higher temperatures. After the annealing at 743 K, isothermal desorption of silicon chlorides from the Ô1 1Õ rest atom surface was observed by SDR at between 873 and 933 K. The temperature was monitored by an electronic pyrometer.
M. Tanaka et al. / Surface Science 527 (2003) 21–29
The experimental setup for SDR has been described in detail elsewhere [14]. Light from a halogen tungsten lamp or a deuterium lamp was polarized and introduced into the chamber. The p-polarized light was incident on the surface at an angle of 70° from the surface normal. The specularly reflected light and the reference light were introduced via optical fibers to a spectrograph. The spectra of both beams were detected by a dual photodiode array, and the intensity of the reflected spectrum was normalized with respect to the reference spectrum. The wavelength ranged from 250 to 850 nm. Although one spectrum can be output every 20 ms, photoinduced electrons were accumulated at each pixel for 10 s to improve the signal-to-noise ratio. At each temperature, measurements on a clean surface and a Cl-covered surface were performed alternately so as to minimize uncertainties due to instability in the experimental setup.
3. Results The thick lines in Fig. 1 show the relative variation of p-polarized reflectance spectra during the isothermal desorption at 903 K. The vertical axis is DR=R ¼ ðRa Rc Þ=Rc , where Ra and Rc are reflectances from the Cl-covered surface and the clean surface at 903 K, respectively. The spectrum at 20 s has a negative peak B at around 2.4 eV, and a weakly negative shoulder A at around 1.7 eV. There are positive peaks at around 3.4 and 4.3 eV. The feature A is depressed faster than B, and almost disappears at 400 s, whereas B remains even at 800 s. These spectral features are quite similar to those observed for Cl adsorption at room temperature, except for the relative magnitude of the feature A [13,14]. The feature A is smaller than B in the desorption process at 903 K, whereas the opposite is the case in the adsorption process. The feature A is ascribed to transitions between surface states involving adatom dangling bond states below and above EF , whereas B is ascribed to transitions from adatom back bond states to bulk states [11]. The peak energies of the features in the higher energy range coincide with those of the E1 and E2 edges in the reflectance spectrum, and these
0.01
23
903 K
0.00 SA
20 s
SB A B 0.00
100 s
0.00 200 s 0.00 400 s 0.00 800 s
-0.01 1
2
3
4
5
Fig. 1. Variation of p-polarized reflectance spectra during the isothermal desorption at 903 K (thickest lines). It can be seen that two features, A from the adatom dangling bond and B from the adatom back bond, decay with time. Thick lines are best-fit curves (aSA þ bSB ) for the experimental plots, whereas thin lines represent component spectra aSA and bSB .
features are ascribed to bulk transitions. The transitions relevant to the features A and B are related to missing surface states of the clean 7 7 DAS structure. The decay of these features therefore corresponds to the restoration of the structure of the clean surface. Each DR=R spectrum in Fig. 1 is reproduced by a linear combination (aSA þ bSB : shown by thick lines) of two component spectra, SA and SB , representing the features A and B, respectively. Thin lines represent the decomposition of each spectrum into two component spectra, aSA and bSB . The spectral feature of SB was obtained from the spectrum when the contribution of A was negligible, namely the DR=R spectrum at around 600 s. SA was obtained by subtracting SB from the DR=R spectrum at 20 s. The magnitude of SB was determined so that SA crosses the zero line at around 2.7 eV, because the component spectrum originating from
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M. Tanaka et al. / Surface Science 527 (2003) 21–29 1.0
0.01 0.8
0.00 0.6
-0.01 0.4
desorption (903 K) adsorption (313 K)
-0.02 SA SB 1
2
873 K
903 K
0.2
-0.03
933 K
3
4
0.0
5
0
200
400
600
800
(a) Fig. 2. Component spectra corresponding to the adatom dangling bond (SA ) and adatom back bond (SB ). Thick lines are the spectra for the desorption process at 903 K, whereas thin lines are for the adsorption process at 313 K.
1.0
0.8
0.6
the missing adatom dangling bond does so in the case of adsorption at room temperature [14]. Fig. 2 compares the component spectra SA and SB for the desorption process at 903 K (thick lines) and the adsorption process at 313 K (thin lines). The spectra for the adsorption have been reduced with the same factor so as to make the peak-to-peak value of the negative peak of SA at 1.7 eV equal to that for the desorption. Overall spectral line shapes for the two processes resemble each other except for the peak energy and the relative magnitude of the negative peak of SB at around 2.5 eV. The energy shifts are 0.11 eV for SA and 0.20 eV for SB , and they are comparable to the energy shift of the E1 and E2 structures in the reflectance spectrum, )2.2 104 eV/K [18]. On the other hand, the relative magnitude of the negative peak of SB at around 2.5 eV for the desorption is about 1.5 times that for the adsorption. This is reasonably explained by the fact that two back bonds of each adatom are broken in the adsorption process, while three back bonds are missing on the rest atom surface on which the isothermal desorption takes place. The coefficients a and b at other times can be determined by fitting the observed spectra by using SA and SB . The a and b thus obtained are plotted in Fig. 3(a) and (b), to represent the time changes of the features A and B. They are normalized so that the values fitted by Eq. (3)
873 K 0.4 903 K 0.2 933 K 0.0
0
200
400
600
800
(b) Fig. 3. Time courses of the coefficients (a) a and (b) b at 873 K (d), 903 K (N) and 933 K (j). Solid lines are best-fit curves obtained by using first-order kinetics.
described below are 1 at 0 s. The decay of a represents the recovery of the adatom dangling bond, whereas the decay of b represents the recovery of the adatom back bond. The results between 873 and 933 K are shown. The decay rates increase with temperature, suggesting that the relevant processes are thermally activated. The decay of a and b was analyzed as follows. The decay rate Rd is expressed in terms of the rate constant jðmÞ and the order of the process m as Rd ¼
dxðtÞ m ¼ jðmÞ xðtÞ ; dt
ð1Þ
where x stands for a or b. In the case of a thermally activated process, the temperature dependence of jðmÞ can be expressed in terms of an activation energy Ed ;
M. Tanaka et al. / Surface Science 527 (2003) 21–29
ðmÞ
jðmÞ ¼ j0 exp
Ed kB T
;
25
0.1
ð2Þ
where kB and T are the BoltzmannÕs constant and the temperature, respectively. For a first-order process (m ¼ 1), the solution of Eq. (1) is written as ð1Þ
xðtÞ ¼ x0 ej t ;
ka 0.01
ð3Þ
where x0 is the initial value of x. The solid lines in Fig. 3 are best-fit curves obtained by using Eq. (3). These curves reproduce well the overall features of the decay of a and b. The data were best fit by assuming the first-order kinetics. The decay rates ja and jb were obtained from these fits at each temperature. ja corresponds to the recovery rate of the dangling bond, whereas jb corresponds to the recovery rate of the back bond. The temperature dependences of ja and jb are shown in Fig. 4 with solid and open circles, respectively. The solid lines are the best linear fits to the plots. The activation energies in the recovery processes of the dangling bond and the back bond were determined from the slopes of the solid lines as Ea ¼ 2:3 eV and Eb ¼ 2:8 eV. The errors in these Ea and Eb values are 0.3 and 0.5 eV, respectively.
4. Discussion The results of the SDR experiments are summarized in Table 1 and compared with the results of other isothermal desorption studies on
kb 0.001
1.06
1.08
1.10
1.12
1.14
1.16
Fig. 4. Logarithmic plots of the decay rates ja (d) and jb ( ). Solid lines are best linear fits to the plots, giving the activation energies Ea ¼ 2:3 0:3 eV and Eb ¼ 2:8 0:5 eV.
Cl-saturated Si(1 1 1). The order of the process, the rate constant at 903 K and the activation energy are listed. For the recovery of the dangling bond, the rate constant determined from SDR (feature A) is compared with those from ultraviolet photoelectron spectroscopy (UPS) [19], Auger electron spectroscopy (AES) [20] and the fast component of second harmonic generation (SHG) [21]. UPS and AES detect the density of the chlorine atom remaining on the surface, whereas SDR (feature A) and SHG detect the density of the dangling bond. For the recovery of the back bond, the result of SDR (feature B) is compared with that of the slow component of SHG.
Table 1 The order of the process, the rate constant and the activation energy for the isothermal desorption determined by several methods Method
Order
Rate constant at 903 K (s1 )
Activation energy (eV)
(a) Recovery of the dangling bonds SDR (feature A) UPS (Cl 3p)a AES (Cl LMM)b SHG (fast)c
First First – First
8 103 7 103 7 103 2 102
2.3 0.3 2.2 – 2.1
(b) Recovery of the back bond SDR (feature B) SHG (slow)c
First First
4 103 2 104
2.8 0.5 2.4
a
Ref. [19]. Ref. [20]. c Ref. [21]. b
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4.1. Recovery of the dangling bond The result of SDR (feature A) agrees well with the results of other methods; the rate constants determined from SDR (feature A), UPS and AES are 7–8 103 s1 and the activation energies from SDR (feature A) and UPS are 2.2–2.3 eV. This agreement means that the same microscopic process is detected by the three different techniques. Moreover, the recovery rate of the dangling bond is about twice that of the back bond (SDR (feature B)). This is seemingly strange, because the dangling bond and the back bond should recover at the same time when the Si adatom is formed. However, this strangeness arises from the interpretation that the feature A in the DR=R spectrum originates only from the adatom dangling bond on the 7 7 DAS surface. UPS and AES directly respond to the desorption of chlorides, which produces the dangling bond on the rest atom surface. If the dangling bond on the rest atom surface is also reflected in the DR=R spectrum, the feature A can decay before the adatom is formed. In other words, the recovery of the dangling bond observed by SDR (feature A) can be faster than that of the back bond observed by SDR (feature B). Actually, the dangling bond states calculated for the relaxed 1 1 surface, which is identical to the rest atom surface, are located between EF 0:5 eV [22]. Compared with the dangling bond states calculated for the 7 7 DAS structure [12], the peak energy of the feature A for the rest atom surface is estimated to differ from that for the 7 7 DAS structure by less than 0.4 eV. On the other hand, the band width of the feature A is 1.2 eV [10]. Consequently, the peak energy difference affects the feature A only slightly, and the appearance of the dangling bond on the rest atom surface yields nearly the same decay of A as the appearance of the adatom dangling bond does. The recovery rate of the dangling bond determined from SHG is two or three times larger than that from other methods. The SHG intensity is dominated by not only the dangling bond density, but also the resonance factor. The resonance width for the SHG pumped at around 1064 nm is about 0.3 eV [23]. This value is about a quarter of the band width of SDR. SHG can therefore detect the
difference between the dangling bond state of the rest atom surface and that of the 7 7 DAS structure, so that the rate constant estimated from the SHG intensity is not necessarily the same as the recovery rate of the dangling bond density. TPD studies showed that the main desorption species from the Cl-saturated Si(1 1 1) surface above 873 K is SiCl2 [6]. In our isothermal desorption measurements, the main desorption species are also expected to be SiCl2 . As the simplest case, if randomly distributed monochloride species diffuse freely on the surface and two of them recombine to form the volatile SiCl2 species, the desorption rate is proportional to the square of the Cl density, which means that the process obeys second-order kinetics. However, the isothermal desorption measurements reveal first-order kinetics, irrespective of the methods used, as listed in Table 1. Moreover, breaking of three Si–Si back bonds, whose bonding energy is estimated as 4.23 eV in total, is required to extract the Si atom on the rest atom surface [24]. This disagrees with the experimentally obtained activation energy, 2.3 eV. Accordingly, the desorption of SiCl2 from an ideally Cl-covered rest atom surface is unlikely, and defects such as steps and craters should be taken into account. Desorption at the step has already been proposed by Pechman et al. in their STM study on the bromine etching of a Si(1 1 1) surface [25]. A step has one-dimensional nature, and a monochloride species bound on the step inevitably recombines with other monochloride species irrespective of the Cl density on the step. The desorption rate is therefore proportional to the density on the step, which means that it is a firstorder process. If the Cl density on the step is proportional to the Cl density on the rest atom surface, the desorption process observed by SDR (feature A) can be first-order. The proportionality is realized in the thermal equilibrium when the hopping rate from terrace site to step site is comparable to the rate of hopping back to terrace site from step site. In this case, the ratio of the Cl density on the step to that on the terrace is a constant having the Boltzmann factor (expðDE= ðkB T ÞÞ) determined by the energy difference, DE, between a Cl atom on the step and that on the terrace. The edge of craters or pits may also contribute to the
M. Tanaka et al. / Surface Science 527 (2003) 21–29
first-order desorption because of its one-dimensionality. The activation energy for the desorption process was found to be 2.3 0.3 eV. This value agrees well with the activation energies determined from UPS [19] and SHG [21]. According to the above desorption model, this activation energy represents the energy required to form a volatile SiCl2 molecule at defects such as steps, craters and pits, because the energy barrier for the Cl diffusion is estimated to be low, 0.9 eV [26]. The value of 2.3 eV is comparable to the calculated etching energy, 2.3–2.4 eV [27], and the calculated desorption energy, 2.38 eV [26], of the SiCl2 molecule. SDR, SHG and UPS detect the surface density of the dangling bond and the Cl atom, and they revealed first-order kinetics for the isothermal desorption. On the other hand, the methods that detect desorbed species, such as LITD [28], TPD [9], and monitoring the signal of the mass spectrometer in steady-state spontaneous etching [29], revealed second-order kinetics. This disagreement can be explained as follows. We will consider the results of the LITD [28] study on isothermal desorption first, and those of the TPD [9] and of the steady-state spontaneous etching [29] later in the next paragraph. In the LITD study, the Cl coverage was lower than that of the Cl-saturated surface obtained by Cl2 exposure, so that the result was not included in Table 1. The characteristic of second-order kinetics in isothermal desorption compared with first-order kinetics is long-lived desorption. Considerable amounts of Si clusters that come from adatoms are sustained on the rest atom surface after annealing at about 700 K of a halogen-saturated Si(1 1 1) surface, as observed by STM [3,30]. These islands or clusters must involve halogen atoms. These clusters have not so far been included in any model of the desorption process. Thus, we speculate that the SiCl2 species are desorbed from two sources, one being the step and the other, the SiClx cluster, and that the desorption from the cluster lasts longer than that from the step. SDR, SHG and UPS are highly sensitive to the surface and can not detect halogen atoms in clusters. The signal of these methods therefore disappears when Cl atoms on the terrace are almost exhausted, although the desorption from the
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clusters, that can be detected by the LITD study, continues. Consequently, LITD can see a secondorder kinetics, even when SDR, SHG and UPS see first-order kinetics. In the TPD study [9], second-order kinetics was concluded to operate, since the peak temperature of the TPD spectrum at around 900 K shifted to lower temperature with increasing coverage. However, this peak shift can also be explained by desorption from the clusters, as follows. A cluster is formed by accumulating the adatom together with halogen atoms, and remaining halogen atoms terminate the dangling bonds on the rest atom surface. The number-ratio of the Cl atom in the cluster to that on the terrace is large at low coverage and small at high coverage. The energy barrier for desorption from the clusters in the temperature range must be higher than that for desorption from the step if the long-lived desorption observed by LITD arises from desorption from the clusters. The TPD spectrum at low coverage, dominated by the desorption from the cluster, may therefore peak at a temperature higher than the peak temperature at high coverage. Thus, two halogen sources for the desorption can account for the coverage-dependent peak shift in the TPD study, which leads to the second-order kinetics. Meanwhile, the steady-state spontaneous etching [29] also exhibited second-order kinetics, since the desorption rate was found to be proportional to the square of the coverage. However, this quadratic dependence can also be explained by desorption from clusters. The desorption at low coverage may be dominated by slow desorption from the clusters, whereas the desorption at high coverage reflects the fast desorption from the terrace. Consequently, the quadratic dependence of the desorption rate on the coverage does not necessarily mean second-order kinetics under a single desorption mechanism. In this way, the disagreement among the results from different methods can be reasonably interpreted by a model that includes the desorption from clusters. 4.2. Recovery of the adatom back bond The rate constant determined from SDR (feature B) is 4 103 s1 at 903 K. This is about 20
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M. Tanaka et al. / Surface Science 527 (2003) 21–29
times larger than the recovery rate of the slow component of SHG, as shown in Table 1. These results indicate that the slow component of SHG signal recovers very slowly after the adatom back bond recovers. The problem is, when does the feature B in the DR=R spectrum disappear? There are two possibilities: one is when the adatom sits on the T4 site, and the other is when the stackingfault structure is formed. Reflectance spectra were calculated with the semiempirical tight-binding method for two model structures [31]. One is 2 2 adatom–restatom model which includes the rest atom but none of dimers, stacking fault or corner hole, and the other is 3 3 DAS model which includes Si adatoms at the T4 site, dimers, the stacking fault and the corner hole, but no rest atom. The reflectance spectrum for the 3 3 structure exhibits a peak at around 2.5 eV due to the back bond contribution, which well reproduces the feature B for the 7 7 DAS structure. On the other hand, the reflectance spectrum for the 2 2 structure exhibits only a broad feature in that energy region. The feature B therefore does not disappear upon adatom formation only, but disappears when the DAS structure is formed. Consequently, it is reasonable to consider that the decay of B corresponds to the formation of a DAS structure, such as 5 5, 7 7, 9 9 structures, and the recovery of SHG intensity corresponds to the formation of the 7 7 DAS structure. As described in Section 4.1, the resonance width for SDR is expected to be about four times as large as the resonance width for SHG, so that SDR is less sensitive to the local structure than SHG. Consequently, SDR can not distinguish the 7 7 DAS structure from other n n DAS structures (n ¼ 3, 5, 9, 11 and so on). Various n n DAS structures have been found during reconstruction from the quenched 1 1 phase and the 7 7 DAS structure is formed through the size change process [32,33]. It takes a certain period of time to form the exact 7 7 DAS structure, so that the recovery of SHG intensity is much slower than that of decay of the feature B. Meanwhile, the recovery of the adatom back bond followed first-order kinetics, as shown in Table 1. This result suggests that the restoration of DAS structure is an independent event that takes place randomly on the surface, so that the
recovery rate of DAS structure is proportional to the area without DAS structure. Random nucleation and uniform growth of domains were observed by STM on extended terraces when the DAS structure is formed during Cl desorption [34]. The growth rate of an isolated domain will be proportional to the circumference of the domain. However, if the density of these domains is high and domain boundaries touch each other at early stage of the growth, the area of the DAS structure might approach the fixed value exponentially as a function of time, which appears to be first-order kinetics. The activation energy for the recovery of the back bond was found to be 2.8 0.5 eV. Since the energy needed for the diffusion of Si clusters is estimated to be lower than 1.5 eV [35,36], the activation energy corresponds to the energy required for the DAS structure formation. Although the DAS structure consists of several elements, such as dimers, a corner hole, and stacking faults, these elements are inherently inseparable [32]. Accordingly, the obtained activation energy should be compared with the formation energy for a faulted half unit cell, which is reported as 2.6 eV [35]. The agreement is good, which supports the validity of the interpretation that the feature B in the SDR spectrum disappears when the faulted half unit cell is formed.
5. Summary The process of isothermal desorption of silicon chlorides from the Cl-saturated Si(1 1 1) rest atom surface and the restoration of the DAS structure have been studied by means of SDR. The spectral features A and B arise mainly from missing adatom dangling bonds and missing adatom back bonds of the DAS structure, respectively. Time courses of the dangling bond recovery and the back bond recovery were determined from the magnitude of the corresponding spectral features. The rate constant, the order of reaction and the activation energy were determined from the temperature dependence of the time courses, and compared with the results obtained by other methods.
M. Tanaka et al. / Surface Science 527 (2003) 21–29
The feature A was found to decay when the dangling bond appears on the rest atom surface as a result of the desorption of silicon chlorides. The time course of the coefficient a reveals first-order kinetics for the desorption process, and the activation energy was found to be 2.3 0.3 eV. The associated desorption of SiCl2 at defects such as steps, craters and pits has been proposed as the mechanism for the first-order desorption process. The activation energy is ascribed to the energy required to form a volatile SiCl2 molecule at the defects. On the other hand, the feature B was found to decay when the DAS structure is formed. The time course of the coefficient b reveals firstorder kinetics for the reconstruction process, and the activation energy was found to be 2.8 0.5 eV. The activation energy is ascribed to the energy required to form a faulted half unit cell. Thus, mechanisms for the desorption processes and the restoration process at the atomic scale have been proposed on the basis of the experimental results, providing a deeper understanding of the fundamental processes involved in the halogen etching of a Si surface. Acknowledgements The authors thank Mr. T. Shirao for his assistance in the experiments. Discussion with Dr. K. Nakayama was most helpful. References [1] [2] [3] [4]
J.S. Villarrubia, J.J. Boland, Phys. Rev. Lett. 63 (1989) 306. J.J. Boland, J.S. Villarrubia, Science 248 (1990) 838. J.J. Boland, J.S. Villarrubia, Phys. Rev. B 41 (1990) 9865. M. Suguri, K. Shudo, F. Komori, Y. Murata, J. Phys. Condens. Matter 5 (1993) 6607. [5] K. Shudo, F. Komori, K. Hattori, Y. Murata, Surf. Sci. 320 (1994) 161. [6] R.D. Schnell, D. Rieger, A. Bogen, F.J. Himpsel, K. Wandelt, W. Steinmann, Phys. Rev. B 32 (1985) 8057. [7] L.J. Whitman, S.A. Joyce, J.A. Yarmoff, F.R. McFeely, L.J. Terminello, Surf. Sci. 232 (1990) 297.
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[8] P.H. Citrin, J.E. Rowe, P. Einsenberger, Phys. Rev. B 28 (1983) 2299. [9] P. Gupta, P.A. Coon, B.G. Koehler, S.M. George, Surf. Sci. 249 (1991) 92. [10] C. Beitia, W. Preyss, R. Del Sole, Y. Borensztein, Phys. Rev. B 56 (1997) R4371. [11] C. Noguez, C. Beitia, W. Preyss, A.I. Shkrebtii, M. Roy, Y. Borensztein, R. Del Sole, Phys. Rev. Lett. 76 (1996) 4923. [12] C. Noguez, A.I. Shkrebtii, R. Del Sole, Surf. Sci. 331–333 (1995) 1349. [13] M. Tanaka, T. Shirao, D. Yamauchi, K. Ota, Appl. Surf. Sci. 169–170 (2001) 212. [14] M. Tanaka, T. Shirao, T. Sasaki, K. Shudo, H. Washio, N. Kaneko, J. Vac. Sci. Technol. A 20 (2002) 1358. [15] N.D. Spencer, P.J. Goddard, P.W. Dacies, M. Kitson, R.M. Lambert, J. Vac. Sci. Technol. A 1 (1983) 1554. [16] B.S. Itchkawitz, M.T. McEllistrem, J.J. Boland, Phys. Rev. Lett. 78 (1997) 98. [17] R.J. Pechman, X.-S. Wang, J.H. Weaver, Phys. Rev. B 52 (1995) 11412. [18] P.R.L. Zucca, Y.R. Shen, Phys. Rev. B 1 (1970) 2668. [19] H. Sakamoto, Y. Takakuwa, unpublished. [20] N. Aoto, E. Ikawa, T. Kikkawa, Y. Kurogi, Surf. Sci. 250 (1991) 253. [21] K. Shudo, T. Sasaki, M. Tanaka, T. Shirao, Solid State Commun., submitted for publication. [22] M. Schl€ uter, J.R. Chelikowsky, S.G. Louie, M.L. Cohen, Phys. Rev. B 12 (1975) 4200. [23] T. Suzuki, Phys. Rev. B 61 (2000) R5117. [24] S. Sakurai, T. Nakayama, Surf. Sci. 493 (2001) 143. [25] R.J. Pechman, T. Moriwaki, J.H. Weaver, G.S. Khoo, Surf. Sci. 341 (1995) L1085. [26] S. Sakurai, T. Nakayama, J. Cryst. Growth 237–239 (2002) 212. [27] P.V. Smith, P.-L. Cao, J. Phys.: Condens. Matter 7 (1995) 7125. [28] P. Gupta, P.A. Coon, B.G. Koehler, S.M. George, J. Chem. Phys. 93 (1989) 2827. [29] A. Szabo, P.D. Farrall, T. Engel, Surf. Sci. 312 (1994) 284. [30] H. Grub, J.J. Boland, Surf. Sci. 407 (1998) 152. [31] C. Noguez, A.I. Shkrevtii, R. Del Sole, Surf. Sci. 318 (1994) 342. [32] H. Tochihara, W. Shimada, H. Yamamoto, M. Taniguchi, A. Yamagishi, J. Phys. Soc. Jpn. 67 (1998) 1513. [33] T. Ishimaru, K. Shimada, T. Hoshino, T. Yamawaki, I. Ohdomari, Phys. Rev. B 60 (1999) 13592. [34] A. Feltz, U. Memmert, R.J. Behm, Surf. Sci. 307–309 (1994) 216. milauer, Phys. Rev. Lett. [35] B. Voighl€ander, M. K€astner, P. S 81 (1998) 858. [36] I.-S. Hwang, M.-S. Ho, T.-T. Tsong, J. Phys. Chem. Solids 62 (2001) 1655.
Isothermal desorption process of Cl-covered Si(1 1 1) studied by surface differential reflectivity spectroscopy Masatoshi Tanaka *, Shigeru Minami 1, Kenichi Shudo, Eriko Yamakawa
2
Department of Physics, Faculty of Engineering, Yokohama National University, 79-5 Tokiwadai, Hodogaya-ku, Yokohama 240-8501, Japan Received 15 May 2002; accepted for publication 21 January 2003
Abstract The isothermal desorption of silicon chlorides from a chlorine-saturated Si(1 1 1) rest atom surface at about 900 K has been investigated by means of in situ real-time surface differential reflectivity spectroscopy. The spectral features at 1.8 eV (A) and 2.5 eV (B) arise mainly from missing adatom dangling bonds and missing adatom back bonds of the dimer–adatom–stacking-fault (DAS) structure, respectively. The feature A decays twice as fast as B does, which indicates that A also originates from the dangling bonds on the rest atom surface. The decay of A, which is considered to represent the desorption of silicon dichlorides, follows first-order kinetics, and the activation energy for this process is 2.3 eV. The decay of B, which is considered to represent the formation of the DAS structure, also follows first-order kinetics and the activation energy is 2.8 eV. The mechanisms of the desorption of silicon dichlorides and the formation of the DAS structure are discussed on the basis of the experimental results. Ó 2003 Published by Elsevier Science B.V. Keywords: Reflection spectroscopy; Thermal desorption; Silicon; Chlorine; Etching
1. Introduction Layer-by-layer etching of semiconductor surfaces is a fundamental technique to fabricate nanometer-scale structures. Etching with halogen gases is widely used in the semiconductor industry and is also a promising candidate method for layerby-layer etching. Halogen etching consists of sev-
*
Corresponding author. Tel./fax: +81-45-339-4201. E-mail address: [email protected] (M. Tanaka). 1 Present address: National Instruments Japan Corporation, 2-4-1 Shibakoen, Minato-ku, Tokyo 105-0011, Japan. 2 Present address: Technology Operations Systems, Merrill Lynch Japan Securities Co., Ltd., 1-1-3 Otemachi, Chiyoda-ku, Tokyo 100-8180, Japan.
eral stages: adsorption and migration of halogen molecules on the surface, desorption of silicon halides, and reconstruction of the clean surface. The atomic-scale mechanisms of these processes should be elucidated before an actual application of layerby-layer etching. Etching of the Si(0 0 1) surface is studied in connection with industrial applications, though etching of the Si(1 1 1) surface is also of interest because the dimer–adatom–stacking-fault (DAS) structure has a variety of sites with different chemical reactivities. Chloride desorption from Si(1 1 1) is a model process for the layer-by-layer etching studies and the static properties at each stage have been revealed mainly by scanning tunneling microscope (STM) studies. Most studies have been concerned
0039-6028/03/$ - see front matter Ó 2003 Published by Elsevier Science B.V. doi:10.1016/S0039-6028(03)00071-2
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M. Tanaka et al. / Surface Science 527 (2003) 21–29
with the desorption from a Cl-saturated surface having polychloride species. After desorption due to annealing at about 700 K or ultraviolet laser irradiation, Si adatoms are removed and the socalled rest atom surface covered with Cl atoms appears [1–5]. X-Ray photoelectron spectroscopy [6,7] and surface-enhanced X-ray absorption fine structure [8] studies confirmed that only monochloride species remain after annealing above 673 K. In accordance with the above observations, a temperature-programmed desorption (TPD) study revealed that polychlorides species are desorbed as a peak at 690 K, and a laser-induced thermal desorption (LITD) study showed that the SiCl3 species almost disappears above 630 K [9]. When the Cl-covered rest atom surface is heated above 900 K, chlorine atoms are desorbed mainly as SiCl2 , as shown by a TPD study [7,9]. In this way, one Si layer is taken off, and a clean 7 7 DAS structure is subsequently restored. In spite of these studies, the dynamic processes of the desorption of silicon chlorides and the reconstruction to form the 7 7 DAS structure are still poorly understood. Real-time observation is essential to investigate the kinetics. Optical methods are superior to others, because they are noninvasive, nondestructive, and capable of very rapid response. Surface differential reflectivity spectroscopy (SDR) was proved to be a powerful tool for the real-time study of hydrogen adsorption on Si(1 1 1) 7 7 [10]. Differential reflectivity is defined as DR=R ¼ ðRa Rc Þ=Rc , where Ra and Rc are the reflectivities of the H-covered and clean surfaces, respectively. Spectral signatures of adsorption on adatom dangling bonds and breaking of adatom back bonds were identified from the calculation of the DR=R spectrum for the hydrogenated 7 7 surface [11,12]. These spectral features arise from the surface states of the clean surface, so that the DR=R spectrum does not depend on the adsorbate. The adsorption process of chlorine on Si(1 1 1) 7 7 has been elucidated by the same method [13,14]. These features develop with time during adsorption processes, whereas in the desorption process, they decay with time as the clean surface structure is restored. The variations can provide information about the desorption process.
This paper is the first report on the real-time observation of the desorption process by means of SDR. The process of isothermal desorption of silicon chlorides from a Cl-saturated Si(1 1 1) rest atom surface and the restoration of the DAS structure were investigated in the temperature range from 873 to 933 K. The time courses of the dangling bond recovery and the back bond recovery were determined from the magnitude of the corresponding spectral features. The rate constant, the order of reaction and the activation energy were obtained from the temperature dependence of the time courses. The kinetics of the desorption processes and the restoration process are discussed on the basis of the results obtained.
2. Experimental Measurements were performed in an ultrahigh vacuum chamber at a base pressure of 2 108 Pa. A specimen of 5 18 0.38 mm3 was cut from a Bdoped p-type Si(1 1 1) wafer (10–15 X cm). The specimen was resistively heated to 1420 K for several seconds and then slowly cooled to obtain the 7 7 DAS structure. The 7 7 structure was confirmed by low-energy electron diffraction. Chlorine gas was generated with an AgCl electrochemical cell [15]. The Si(1 1 1) surface was saturated with chlorine atoms (50 L; 1 L ¼ 1.33 104 Pa s) at room temperature [13]. The Cl-covered 7 7 rest atom surface obtained by annealing at 743 K is not stable, but partially reconstructs a Cl-terminated 1 1 bulk-like surface [3]. Other STM studies [16,17] have shown that additional Si atoms required for the reconstruction are supplied from steps and that the movement of the step enlarges the 1 1 domain. Accordingly, in our study, the surface was annealed at 743 K for 2 min in order to enlarge the 1 1 domain. Otherwise, the reconstruction and the desorption of both polychloride and monochloride species will proceed at the same time during isothermal desorption at higher temperatures. After the annealing at 743 K, isothermal desorption of silicon chlorides from the Ô1 1Õ rest atom surface was observed by SDR at between 873 and 933 K. The temperature was monitored by an electronic pyrometer.
M. Tanaka et al. / Surface Science 527 (2003) 21–29
The experimental setup for SDR has been described in detail elsewhere [14]. Light from a halogen tungsten lamp or a deuterium lamp was polarized and introduced into the chamber. The p-polarized light was incident on the surface at an angle of 70° from the surface normal. The specularly reflected light and the reference light were introduced via optical fibers to a spectrograph. The spectra of both beams were detected by a dual photodiode array, and the intensity of the reflected spectrum was normalized with respect to the reference spectrum. The wavelength ranged from 250 to 850 nm. Although one spectrum can be output every 20 ms, photoinduced electrons were accumulated at each pixel for 10 s to improve the signal-to-noise ratio. At each temperature, measurements on a clean surface and a Cl-covered surface were performed alternately so as to minimize uncertainties due to instability in the experimental setup.
3. Results The thick lines in Fig. 1 show the relative variation of p-polarized reflectance spectra during the isothermal desorption at 903 K. The vertical axis is DR=R ¼ ðRa Rc Þ=Rc , where Ra and Rc are reflectances from the Cl-covered surface and the clean surface at 903 K, respectively. The spectrum at 20 s has a negative peak B at around 2.4 eV, and a weakly negative shoulder A at around 1.7 eV. There are positive peaks at around 3.4 and 4.3 eV. The feature A is depressed faster than B, and almost disappears at 400 s, whereas B remains even at 800 s. These spectral features are quite similar to those observed for Cl adsorption at room temperature, except for the relative magnitude of the feature A [13,14]. The feature A is smaller than B in the desorption process at 903 K, whereas the opposite is the case in the adsorption process. The feature A is ascribed to transitions between surface states involving adatom dangling bond states below and above EF , whereas B is ascribed to transitions from adatom back bond states to bulk states [11]. The peak energies of the features in the higher energy range coincide with those of the E1 and E2 edges in the reflectance spectrum, and these
0.01
23
903 K
0.00 SA
20 s
SB A B 0.00
100 s
0.00 200 s 0.00 400 s 0.00 800 s
-0.01 1
2
3
4
5
Fig. 1. Variation of p-polarized reflectance spectra during the isothermal desorption at 903 K (thickest lines). It can be seen that two features, A from the adatom dangling bond and B from the adatom back bond, decay with time. Thick lines are best-fit curves (aSA þ bSB ) for the experimental plots, whereas thin lines represent component spectra aSA and bSB .
features are ascribed to bulk transitions. The transitions relevant to the features A and B are related to missing surface states of the clean 7 7 DAS structure. The decay of these features therefore corresponds to the restoration of the structure of the clean surface. Each DR=R spectrum in Fig. 1 is reproduced by a linear combination (aSA þ bSB : shown by thick lines) of two component spectra, SA and SB , representing the features A and B, respectively. Thin lines represent the decomposition of each spectrum into two component spectra, aSA and bSB . The spectral feature of SB was obtained from the spectrum when the contribution of A was negligible, namely the DR=R spectrum at around 600 s. SA was obtained by subtracting SB from the DR=R spectrum at 20 s. The magnitude of SB was determined so that SA crosses the zero line at around 2.7 eV, because the component spectrum originating from
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M. Tanaka et al. / Surface Science 527 (2003) 21–29 1.0
0.01 0.8
0.00 0.6
-0.01 0.4
desorption (903 K) adsorption (313 K)
-0.02 SA SB 1
2
873 K
903 K
0.2
-0.03
933 K
3
4
0.0
5
0
200
400
600
800
(a) Fig. 2. Component spectra corresponding to the adatom dangling bond (SA ) and adatom back bond (SB ). Thick lines are the spectra for the desorption process at 903 K, whereas thin lines are for the adsorption process at 313 K.
1.0
0.8
0.6
the missing adatom dangling bond does so in the case of adsorption at room temperature [14]. Fig. 2 compares the component spectra SA and SB for the desorption process at 903 K (thick lines) and the adsorption process at 313 K (thin lines). The spectra for the adsorption have been reduced with the same factor so as to make the peak-to-peak value of the negative peak of SA at 1.7 eV equal to that for the desorption. Overall spectral line shapes for the two processes resemble each other except for the peak energy and the relative magnitude of the negative peak of SB at around 2.5 eV. The energy shifts are 0.11 eV for SA and 0.20 eV for SB , and they are comparable to the energy shift of the E1 and E2 structures in the reflectance spectrum, )2.2 104 eV/K [18]. On the other hand, the relative magnitude of the negative peak of SB at around 2.5 eV for the desorption is about 1.5 times that for the adsorption. This is reasonably explained by the fact that two back bonds of each adatom are broken in the adsorption process, while three back bonds are missing on the rest atom surface on which the isothermal desorption takes place. The coefficients a and b at other times can be determined by fitting the observed spectra by using SA and SB . The a and b thus obtained are plotted in Fig. 3(a) and (b), to represent the time changes of the features A and B. They are normalized so that the values fitted by Eq. (3)
873 K 0.4 903 K 0.2 933 K 0.0
0
200
400
600
800
(b) Fig. 3. Time courses of the coefficients (a) a and (b) b at 873 K (d), 903 K (N) and 933 K (j). Solid lines are best-fit curves obtained by using first-order kinetics.
described below are 1 at 0 s. The decay of a represents the recovery of the adatom dangling bond, whereas the decay of b represents the recovery of the adatom back bond. The results between 873 and 933 K are shown. The decay rates increase with temperature, suggesting that the relevant processes are thermally activated. The decay of a and b was analyzed as follows. The decay rate Rd is expressed in terms of the rate constant jðmÞ and the order of the process m as Rd ¼
dxðtÞ m ¼ jðmÞ xðtÞ ; dt
ð1Þ
where x stands for a or b. In the case of a thermally activated process, the temperature dependence of jðmÞ can be expressed in terms of an activation energy Ed ;
M. Tanaka et al. / Surface Science 527 (2003) 21–29
ðmÞ
jðmÞ ¼ j0 exp
Ed kB T
;
25
0.1
ð2Þ
where kB and T are the BoltzmannÕs constant and the temperature, respectively. For a first-order process (m ¼ 1), the solution of Eq. (1) is written as ð1Þ
xðtÞ ¼ x0 ej t ;
ka 0.01
ð3Þ
where x0 is the initial value of x. The solid lines in Fig. 3 are best-fit curves obtained by using Eq. (3). These curves reproduce well the overall features of the decay of a and b. The data were best fit by assuming the first-order kinetics. The decay rates ja and jb were obtained from these fits at each temperature. ja corresponds to the recovery rate of the dangling bond, whereas jb corresponds to the recovery rate of the back bond. The temperature dependences of ja and jb are shown in Fig. 4 with solid and open circles, respectively. The solid lines are the best linear fits to the plots. The activation energies in the recovery processes of the dangling bond and the back bond were determined from the slopes of the solid lines as Ea ¼ 2:3 eV and Eb ¼ 2:8 eV. The errors in these Ea and Eb values are 0.3 and 0.5 eV, respectively.
4. Discussion The results of the SDR experiments are summarized in Table 1 and compared with the results of other isothermal desorption studies on
kb 0.001
1.06
1.08
1.10
1.12
1.14
1.16
Fig. 4. Logarithmic plots of the decay rates ja (d) and jb ( ). Solid lines are best linear fits to the plots, giving the activation energies Ea ¼ 2:3 0:3 eV and Eb ¼ 2:8 0:5 eV.
Cl-saturated Si(1 1 1). The order of the process, the rate constant at 903 K and the activation energy are listed. For the recovery of the dangling bond, the rate constant determined from SDR (feature A) is compared with those from ultraviolet photoelectron spectroscopy (UPS) [19], Auger electron spectroscopy (AES) [20] and the fast component of second harmonic generation (SHG) [21]. UPS and AES detect the density of the chlorine atom remaining on the surface, whereas SDR (feature A) and SHG detect the density of the dangling bond. For the recovery of the back bond, the result of SDR (feature B) is compared with that of the slow component of SHG.
Table 1 The order of the process, the rate constant and the activation energy for the isothermal desorption determined by several methods Method
Order
Rate constant at 903 K (s1 )
Activation energy (eV)
(a) Recovery of the dangling bonds SDR (feature A) UPS (Cl 3p)a AES (Cl LMM)b SHG (fast)c
First First – First
8 103 7 103 7 103 2 102
2.3 0.3 2.2 – 2.1
(b) Recovery of the back bond SDR (feature B) SHG (slow)c
First First
4 103 2 104
2.8 0.5 2.4
a
Ref. [19]. Ref. [20]. c Ref. [21]. b
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M. Tanaka et al. / Surface Science 527 (2003) 21–29
4.1. Recovery of the dangling bond The result of SDR (feature A) agrees well with the results of other methods; the rate constants determined from SDR (feature A), UPS and AES are 7–8 103 s1 and the activation energies from SDR (feature A) and UPS are 2.2–2.3 eV. This agreement means that the same microscopic process is detected by the three different techniques. Moreover, the recovery rate of the dangling bond is about twice that of the back bond (SDR (feature B)). This is seemingly strange, because the dangling bond and the back bond should recover at the same time when the Si adatom is formed. However, this strangeness arises from the interpretation that the feature A in the DR=R spectrum originates only from the adatom dangling bond on the 7 7 DAS surface. UPS and AES directly respond to the desorption of chlorides, which produces the dangling bond on the rest atom surface. If the dangling bond on the rest atom surface is also reflected in the DR=R spectrum, the feature A can decay before the adatom is formed. In other words, the recovery of the dangling bond observed by SDR (feature A) can be faster than that of the back bond observed by SDR (feature B). Actually, the dangling bond states calculated for the relaxed 1 1 surface, which is identical to the rest atom surface, are located between EF 0:5 eV [22]. Compared with the dangling bond states calculated for the 7 7 DAS structure [12], the peak energy of the feature A for the rest atom surface is estimated to differ from that for the 7 7 DAS structure by less than 0.4 eV. On the other hand, the band width of the feature A is 1.2 eV [10]. Consequently, the peak energy difference affects the feature A only slightly, and the appearance of the dangling bond on the rest atom surface yields nearly the same decay of A as the appearance of the adatom dangling bond does. The recovery rate of the dangling bond determined from SHG is two or three times larger than that from other methods. The SHG intensity is dominated by not only the dangling bond density, but also the resonance factor. The resonance width for the SHG pumped at around 1064 nm is about 0.3 eV [23]. This value is about a quarter of the band width of SDR. SHG can therefore detect the
difference between the dangling bond state of the rest atom surface and that of the 7 7 DAS structure, so that the rate constant estimated from the SHG intensity is not necessarily the same as the recovery rate of the dangling bond density. TPD studies showed that the main desorption species from the Cl-saturated Si(1 1 1) surface above 873 K is SiCl2 [6]. In our isothermal desorption measurements, the main desorption species are also expected to be SiCl2 . As the simplest case, if randomly distributed monochloride species diffuse freely on the surface and two of them recombine to form the volatile SiCl2 species, the desorption rate is proportional to the square of the Cl density, which means that the process obeys second-order kinetics. However, the isothermal desorption measurements reveal first-order kinetics, irrespective of the methods used, as listed in Table 1. Moreover, breaking of three Si–Si back bonds, whose bonding energy is estimated as 4.23 eV in total, is required to extract the Si atom on the rest atom surface [24]. This disagrees with the experimentally obtained activation energy, 2.3 eV. Accordingly, the desorption of SiCl2 from an ideally Cl-covered rest atom surface is unlikely, and defects such as steps and craters should be taken into account. Desorption at the step has already been proposed by Pechman et al. in their STM study on the bromine etching of a Si(1 1 1) surface [25]. A step has one-dimensional nature, and a monochloride species bound on the step inevitably recombines with other monochloride species irrespective of the Cl density on the step. The desorption rate is therefore proportional to the density on the step, which means that it is a firstorder process. If the Cl density on the step is proportional to the Cl density on the rest atom surface, the desorption process observed by SDR (feature A) can be first-order. The proportionality is realized in the thermal equilibrium when the hopping rate from terrace site to step site is comparable to the rate of hopping back to terrace site from step site. In this case, the ratio of the Cl density on the step to that on the terrace is a constant having the Boltzmann factor (expðDE= ðkB T ÞÞ) determined by the energy difference, DE, between a Cl atom on the step and that on the terrace. The edge of craters or pits may also contribute to the
M. Tanaka et al. / Surface Science 527 (2003) 21–29
first-order desorption because of its one-dimensionality. The activation energy for the desorption process was found to be 2.3 0.3 eV. This value agrees well with the activation energies determined from UPS [19] and SHG [21]. According to the above desorption model, this activation energy represents the energy required to form a volatile SiCl2 molecule at defects such as steps, craters and pits, because the energy barrier for the Cl diffusion is estimated to be low, 0.9 eV [26]. The value of 2.3 eV is comparable to the calculated etching energy, 2.3–2.4 eV [27], and the calculated desorption energy, 2.38 eV [26], of the SiCl2 molecule. SDR, SHG and UPS detect the surface density of the dangling bond and the Cl atom, and they revealed first-order kinetics for the isothermal desorption. On the other hand, the methods that detect desorbed species, such as LITD [28], TPD [9], and monitoring the signal of the mass spectrometer in steady-state spontaneous etching [29], revealed second-order kinetics. This disagreement can be explained as follows. We will consider the results of the LITD [28] study on isothermal desorption first, and those of the TPD [9] and of the steady-state spontaneous etching [29] later in the next paragraph. In the LITD study, the Cl coverage was lower than that of the Cl-saturated surface obtained by Cl2 exposure, so that the result was not included in Table 1. The characteristic of second-order kinetics in isothermal desorption compared with first-order kinetics is long-lived desorption. Considerable amounts of Si clusters that come from adatoms are sustained on the rest atom surface after annealing at about 700 K of a halogen-saturated Si(1 1 1) surface, as observed by STM [3,30]. These islands or clusters must involve halogen atoms. These clusters have not so far been included in any model of the desorption process. Thus, we speculate that the SiCl2 species are desorbed from two sources, one being the step and the other, the SiClx cluster, and that the desorption from the cluster lasts longer than that from the step. SDR, SHG and UPS are highly sensitive to the surface and can not detect halogen atoms in clusters. The signal of these methods therefore disappears when Cl atoms on the terrace are almost exhausted, although the desorption from the
27
clusters, that can be detected by the LITD study, continues. Consequently, LITD can see a secondorder kinetics, even when SDR, SHG and UPS see first-order kinetics. In the TPD study [9], second-order kinetics was concluded to operate, since the peak temperature of the TPD spectrum at around 900 K shifted to lower temperature with increasing coverage. However, this peak shift can also be explained by desorption from the clusters, as follows. A cluster is formed by accumulating the adatom together with halogen atoms, and remaining halogen atoms terminate the dangling bonds on the rest atom surface. The number-ratio of the Cl atom in the cluster to that on the terrace is large at low coverage and small at high coverage. The energy barrier for desorption from the clusters in the temperature range must be higher than that for desorption from the step if the long-lived desorption observed by LITD arises from desorption from the clusters. The TPD spectrum at low coverage, dominated by the desorption from the cluster, may therefore peak at a temperature higher than the peak temperature at high coverage. Thus, two halogen sources for the desorption can account for the coverage-dependent peak shift in the TPD study, which leads to the second-order kinetics. Meanwhile, the steady-state spontaneous etching [29] also exhibited second-order kinetics, since the desorption rate was found to be proportional to the square of the coverage. However, this quadratic dependence can also be explained by desorption from clusters. The desorption at low coverage may be dominated by slow desorption from the clusters, whereas the desorption at high coverage reflects the fast desorption from the terrace. Consequently, the quadratic dependence of the desorption rate on the coverage does not necessarily mean second-order kinetics under a single desorption mechanism. In this way, the disagreement among the results from different methods can be reasonably interpreted by a model that includes the desorption from clusters. 4.2. Recovery of the adatom back bond The rate constant determined from SDR (feature B) is 4 103 s1 at 903 K. This is about 20
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M. Tanaka et al. / Surface Science 527 (2003) 21–29
times larger than the recovery rate of the slow component of SHG, as shown in Table 1. These results indicate that the slow component of SHG signal recovers very slowly after the adatom back bond recovers. The problem is, when does the feature B in the DR=R spectrum disappear? There are two possibilities: one is when the adatom sits on the T4 site, and the other is when the stackingfault structure is formed. Reflectance spectra were calculated with the semiempirical tight-binding method for two model structures [31]. One is 2 2 adatom–restatom model which includes the rest atom but none of dimers, stacking fault or corner hole, and the other is 3 3 DAS model which includes Si adatoms at the T4 site, dimers, the stacking fault and the corner hole, but no rest atom. The reflectance spectrum for the 3 3 structure exhibits a peak at around 2.5 eV due to the back bond contribution, which well reproduces the feature B for the 7 7 DAS structure. On the other hand, the reflectance spectrum for the 2 2 structure exhibits only a broad feature in that energy region. The feature B therefore does not disappear upon adatom formation only, but disappears when the DAS structure is formed. Consequently, it is reasonable to consider that the decay of B corresponds to the formation of a DAS structure, such as 5 5, 7 7, 9 9 structures, and the recovery of SHG intensity corresponds to the formation of the 7 7 DAS structure. As described in Section 4.1, the resonance width for SDR is expected to be about four times as large as the resonance width for SHG, so that SDR is less sensitive to the local structure than SHG. Consequently, SDR can not distinguish the 7 7 DAS structure from other n n DAS structures (n ¼ 3, 5, 9, 11 and so on). Various n n DAS structures have been found during reconstruction from the quenched 1 1 phase and the 7 7 DAS structure is formed through the size change process [32,33]. It takes a certain period of time to form the exact 7 7 DAS structure, so that the recovery of SHG intensity is much slower than that of decay of the feature B. Meanwhile, the recovery of the adatom back bond followed first-order kinetics, as shown in Table 1. This result suggests that the restoration of DAS structure is an independent event that takes place randomly on the surface, so that the
recovery rate of DAS structure is proportional to the area without DAS structure. Random nucleation and uniform growth of domains were observed by STM on extended terraces when the DAS structure is formed during Cl desorption [34]. The growth rate of an isolated domain will be proportional to the circumference of the domain. However, if the density of these domains is high and domain boundaries touch each other at early stage of the growth, the area of the DAS structure might approach the fixed value exponentially as a function of time, which appears to be first-order kinetics. The activation energy for the recovery of the back bond was found to be 2.8 0.5 eV. Since the energy needed for the diffusion of Si clusters is estimated to be lower than 1.5 eV [35,36], the activation energy corresponds to the energy required for the DAS structure formation. Although the DAS structure consists of several elements, such as dimers, a corner hole, and stacking faults, these elements are inherently inseparable [32]. Accordingly, the obtained activation energy should be compared with the formation energy for a faulted half unit cell, which is reported as 2.6 eV [35]. The agreement is good, which supports the validity of the interpretation that the feature B in the SDR spectrum disappears when the faulted half unit cell is formed.
5. Summary The process of isothermal desorption of silicon chlorides from the Cl-saturated Si(1 1 1) rest atom surface and the restoration of the DAS structure have been studied by means of SDR. The spectral features A and B arise mainly from missing adatom dangling bonds and missing adatom back bonds of the DAS structure, respectively. Time courses of the dangling bond recovery and the back bond recovery were determined from the magnitude of the corresponding spectral features. The rate constant, the order of reaction and the activation energy were determined from the temperature dependence of the time courses, and compared with the results obtained by other methods.
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The feature A was found to decay when the dangling bond appears on the rest atom surface as a result of the desorption of silicon chlorides. The time course of the coefficient a reveals first-order kinetics for the desorption process, and the activation energy was found to be 2.3 0.3 eV. The associated desorption of SiCl2 at defects such as steps, craters and pits has been proposed as the mechanism for the first-order desorption process. The activation energy is ascribed to the energy required to form a volatile SiCl2 molecule at the defects. On the other hand, the feature B was found to decay when the DAS structure is formed. The time course of the coefficient b reveals firstorder kinetics for the reconstruction process, and the activation energy was found to be 2.8 0.5 eV. The activation energy is ascribed to the energy required to form a faulted half unit cell. Thus, mechanisms for the desorption processes and the restoration process at the atomic scale have been proposed on the basis of the experimental results, providing a deeper understanding of the fundamental processes involved in the halogen etching of a Si surface. Acknowledgements The authors thank Mr. T. Shirao for his assistance in the experiments. Discussion with Dr. K. Nakayama was most helpful. References [1] [2] [3] [4]
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