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Ar+-laser-assisted subatomic-layer epitaxy of Si
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CRYSTAL GROWTH
Journal of Crystal Growth 169 (1996) 672-680
Ar+-laser-assisted subatomic-layer epitaxy of Si Yoshiyuki Suda *, Masahiro Ishida, Mitsutomi Yamashita Faculo' of Technology, Tolg'o Unicersityof Agriculture and Technology, 2-24-16Naka-cho, Koganei, Tok)'o 184. Japan
Received 2 January 1996; accepted 29 May 1996
Abstract
Si submonolayer-by-submonolayer epitaxy or subatomic-layer epitaxy (SALE) from Si2H 6 on Si(001) has been carried out by repeating Si2H 6 exposure and surface excitation induced by the combination of substrate resistive heating and Ar + laser irradiation. As the average substrate temperature or the laser irradiation power increases, the surface morphology of a grown film changes from a convex shape to a concave shapeothrough a trapezoid shape. The roughness of a fiat area of the trapezoid film is within _+2 ,~ per growth thickness of 100 A, and a substrate temperature window of ~ 15°C and a laser power window of ~ 0.25 W, where such a flat growth surface and a constant growth rate are obtained, has been observed. The ranges of these windows have been estimated to correspond to the same variation of the surface temperature in the laser irradiation area during the laser irradiation. This result together with the result of the analyses on growth thickness distribution profiles suggests that the laser irradiation works as a thermal effect. Thus, in the Ar+-laser-assisted SALE method, the growth surface morphology then the growth mode is controlled by the surface temperature during the laser irradiation. An Ar + laser is a useful tool to control the surface temperature.
1. I n t r o d u c t i o n
Atomic-layer epitaxy (ALE) is an epitaxial film growth technique which, in its ideal form, produces self-limited one-monolayer (ML) film growth per growth cycle. Much attention has recently been paid to the A L E of group IV semiconductors with the development of new S i / G e optoelectronic materials and devices that consist of artificial atomic-scale layer structures. Generally, for A L E of group IV semiconductors consisting of one element, an appropriate source gas, which exhibits just one-monolayer saturation coverage, has to be found. If the saturation coverage is more than or less than one, a growing
* Corresponding author. Fax: +81 423 88 7129.
surface is expected to become rough and exhibit a three-dimensional structure. Nearly 1 M L growth rate per growth or adsorption cycle (GR) were first reported for Si A L E using a S i H 2 C l J S i system with H 2 as a reducer gas [1] and for Ge A L E using a ( C 2 H s ) z G e H z / G e system [2]. W e first focused on hydride molecules as a group-IV A L E source gas, which is inherently free from contaminants such as C and C1, and reported saturated adsorption reaction using a S i z H 6 / S i ( 0 0 1 ) system [3-5]. Since then, several source g a s / s u b s t r a t e systems, which exhibit a nearly 1 M L growth rate, have been reported to date including S i H ~ C l J S i with H as a reducer [6], G e H a / S i [7,8], cracked S i H 2 C I 2 / S i [9], and ( C H 3 ) 2 G e H 2 / G e [10]. However, in the case of S i z H 6 / S i [11-13] and S i 3 H 8 / S i [14] systems, GR
0022-0248/96/$15.00 Copyright © 1996 Elsevier Science B.V. All rights reserved PII S0022-0248(96)00454-X
E Suda et al./Journal of Co'stal Growth 169 (1996) 672-680
is less than 1, and for a SiH4/Si system, the temperature window exhibiting GR = 1 has been very limited [7]. Thus, some consideration is needed to apply hydride molecules to ALE more widely for its potential advantage of being contaminant free. In our previous works [3-5,13], we have reported that SizH 6 adsorbs dissociatively at random sites on Si(001) at room temperature and the adsorbed species are interpreted to be -H, - S i = H 2 and - S i - H 3. This H termination limits the adsorption coverage of Si to less than 1 ML. However, from our recent results of time-dependent RHEED observations during annealing a SieH6-exposed Si(001) surface with self-resistive heating, it has been concluded that Si adatoms migrate to produce wide terraces accompanied by or following hydrogen desorption during the surface annealing, and an atomically flattened surface is obtained after the surface annealing [13,15]. The terrace growth time exhibits the Arrhenius relation to the annealing temperature and the activation energy of the terrace growth is found to be 1.6 _+ 0.4 eV. From these results, we have proposed a subatomiclayer epitaxy (SALE) technique with which Si is grown by submonolayer-by-submonolayer by alternately repeating Si 2H 6 exposure and surface thermal excitation [13,15]. Using the RHEED technique and a single-domain Si(001) 2 X 1 surface, surface structural change in the Si SALE growth process has also been investigated by repeating a SALE growth cycle, where one growth cycle consists of Si2H 6 exposure and substrate resistive heating [13]. The result indicates that nearly single 2 × 1 and 1 × 2 domains appear alternately every almost two growth cycles, which supports the SALE growth principle. Thus, even if the Si2H 6 saturation coverage is less than one, Si is epitaxially grown by submonolayer-bysubmonolayer due to the surface-flattening process induced by surface thermal excitation, we have also demonstrated Si submonolayer-by-submonolayer digital growth from Si2H 6 using Ar + ion laser irradiation and resistive substrate heating as surface excitation sources [13,15]. However, the role of the surface excitation by Ar + ion laser irradiation has not been fully understood. In this study, we have carried out Si SALE growth from Si2H 6 on Si(001) using Ar + ion laser irradiation and resistive substrate heating as surface excitation sources and investigated the growth conditions
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and the growth mechanisms of the Ar+-laser-assisted SALE method. Either the laser power or the substrate temperature was varied. From the results of the SALE experiments, we have first demonstrated a substrate temperature window and a laser power window where an atomically flat growth surface is obtained. The effect of Ar ÷ laser irradiation is well understood as a thermal effect. The results have also indicated that the flattening process then the surface morphology is controlled by the surface temperature during the surface excitation process of the growth cycle, and that its surface temperature is controlled by the combination of the laser irradiation and thermal annealing.
2. Experimental procedure SALE growth experiments were carried out using a load-locked ultrahigh vacuum chamber with a base pressure of < 5 × 10 -l° TorT. Si(001) substrates used in these experiments were 5 × 25 mm plates cleaved from p-type 5 - 7 f~. cm wafers (sample A) and p-type 3-5 F t . c m wafers (sample B). These substrates were degreased before insertion into the vacuum chamber. Clean double-domain Si(001)2 × 1 surfaces were obtained by annealing the substrates at ~ 600°C for 1 h and heating them to ~ 1100°C for > 3 s. 99.98% Si2H 6 was introduced into the chamber through a gas doser using a computer-controlled o n - o f f air valve. The substrates were resistively heated to regulate the surface temperature, which is monitored using a pyrometer and by measuring the substrate electrical resistance. SALE growth from S i z H 6 o n Si(001) was carried out using an Ar + ion laser. One growth cycle of the SALE experiments consists of (1) Si2H 6 exposure, (2) evacuation, (3) laser irradiation, and (4) cooling. The laser was operated with a multiline mode and the wavelength was in the visible region of 458-515 nm. The laser beam was focused through a lens and its beam diameter at half-maximum intensity was 150 + 25 /xm. The timing chart of the sequential SALE process is illustrated in Fig. 1. t 1, t 2, t 3 and t 4 c o r r e s p o n d to the periods of four processes in one cycle of the SALE growth, respectively. The Si2H 6 exposure pressure was set to be ~ 3 X 10 - 4 Torr. At the instant when the Si2H 6 gas valve was turned on, the
674
E Suda et al. / Journal of Co'stal Growth 169 (1996) 672-680 i [(
it
1 cycle
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power of 5.4 W. In all S A L E growth experiments, the periods of t 1, t2, t 3 and t 4 were set to be 10, 10, 5 and 5 s, respectively, and the number of growth cycles were 360. The thickness profiles of the growth films were measured by a surface profilometer.
Si H 26 Exposure
off
on A r * Ion Laser Irradiation
off
Fig. 1. The timing chart of the sequential processes in one growth cycle of the Ar+-laser-assisted subatomic-layer epitaxy (SALE) method.
pressure rose up to ~ 2 X 10 -3 Torr, and then, the pressure settled down to the set value of ~ 3 X 10 -4 Torr during t I. After evacuation of the S i e H 6, the pressure reached to the order of 10-8 Torr during t 3. The laser irradiation was switched on and off using a computer-controlled light shutter. Substrate temperature during the S A L E growth was regulated by controlling the DC current passing through the substrate so that the average surface temperature monitored by the pyrometer during Si2H 6 exposure (t]) and evacuation (t e) was kept constant. This average surface temperature is hereafter addressed as substrate temperature Tsub. The value of iC,ub is set to be between 235 and 320°C. In the period of laser irradiation, the laser-irradiated area was thermally excited by the combination of the laser irradiation and the resistive heating itself. A temperature profile on the surface during the laser irradiation exhibits a Gaussian-like shape [13,15]. The surface temperature in the laserirradiated area during the laser irradiation is hereafter denoted by To . To clarify the effect of the laser excitation, two series of the S A L E experiments were carried out. In one series of S A L E experiments, the incident laser powers were set at values ranging from 4.8 to 5.4 W under the same substrate temperature of 235°C. In the other series of S A L E experiments, the substrate temperatures were set at values ranging from 250 to 320°C under the same incident laser
3. Results and discussion 3.1. Substrate temperature effects on thickness profile o f S A L E growth f i h n
Fig. 2 shows typical thickness distribution profiles of films grown by the S A L E method at substrate temperatures of 260-300°C with an incident laser power of 5.4 W. In these experiments, sample A wafers were used as substrates. As the substrate temperature increases, the thickness distribution profile changes from a convex shape to a concave shape through a trapezoid shape. The relation between the growth thickness per growth cycle (GR) and the substrate temperature T,ub in the S A L E growth is shown in Fig. 3. As the value of GR for a film with a dent in the center, an average value between the maximum thickness in the rim of the dent and the minimum thickness at the bottom of the dent is used. The substrate temperature was varied from 250 to 320°C. In the middle substrate temperature range AT~uw of about 270 to 280°C, a flat surface area is observed in the center of the growth film. The
2
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Fig. 2. Thickness distribution profiles of films (sample A) grown by the Ar+-laser-assisted subatomic-layer epitaxy (SALE) method with an incident laser power of 5.4 W at substrate temperatures of 260 to 320°C. The periods of t l, tz, t3 and t4 in one SALE growth cycle were 10, 10, 5 and 5 s, respectively.
E Suda et al./Joumal of Co'stal Growth 169 (1996) 672-680 2
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surface roughness in the flat area in the most flat case is measured to be about _+0.015 M L / c y c l e the value of which corresponds to about +_2 A for the film of 100 A thickness. Thus, the atomically flat surface is obtained. In the lower substrate temperature range of less than ~ 270°C, the growth thickness distribution profile becomes a convex shape and the GR value is smaller than that of a film grown under the middle substrate temperatures. In the higher substrate temperature range of more than ~ 280°C, the growth thickness distribution profile becomes a concave shape. For the conventional ALE growth technique, a temperature window is referred to as the temperature range where GR is 1 ML/cycle. As the SALE growth technique is originally a submonolayer-by-submonolayer growth method, the similar temperature window is defined as the temperature range where constant GR and an atomically flat surface is obtained. This temperature window is observed in Fig. 3 and its substrate temperature window AT~u w and the GR value is about 15°C and about 0.63 ML/cycle, respectively. The surface temperature distribution during the laser irradiation has been calculated using the Green's function method [16]. In the calculation, the laser beam is treated as a Gaussian and penetrating source. Full width at half maximum (FWHM) of the laser beam is set to be 146 /zm. From this calculation, the
675
minimum temperature within the flat area of a growth film, which corresponds to the temperature at the rim of the flat area, is estimated to be 600 + 40°C for films grown with an incident laser power of 5.4 W. In our previous investigation on surface thermal excitation of a Si2H6-exposed surface using a RHEED technique [15], more than 575°C is necessary to produce Si adatom migration or terrace growth enough to obtain an atomically flat surface when the excitation time is 5 s, which corresponds to a period of 13 in this experiment. Therefore, the minimum temperature within the flat area corresponds to the threshold temperature which produces sufficient surface flattening induced by Si adatom migration. This threshold temperature is denoted by T~. In other words, agreement between the RHEED results and the result of the surface temperature calculation suggests that Si migration should occur in the flat area during the laser irradiation. Measured atomic-scale flatness in the flat area probably evidences the Si migration. In our previous RHEED study on the adsorption mechanism of Si2H 6 on Si(001) [13], it was found that the adsorption coverage of Si2H 6 at surface temperatures less than ~ 300°C was less than 1 over the wide Si2H 6 dosages of 18-1.8 × 10 ~5 langmuir and hydrogen termination on the surface tends to prevent further Si e H6 adsorption. On the other hand, in our time-dependent RHEED experiments [15], terrace growth of a Si2H6-exposed surface completes within ~ 10 s when the annealing temperature is 575°C. An electron energy-loss spectroscopy (EELS) study [4] has also indicated that hydrogen desorbs from a Si~H6-exposed surface almost completely with a 15 s anneal at 570°C when the surface is annealed at progressively higher temperatures. Thus, the hydrogen desorption almost completes during Si adatom migration, and surface temperature, at which Si adatom migration completes, is close to the value of TL for a given annealing period. Therefore, when the surface temperature during the laser irradiation is less than T~, hydrogen desorption is not complete and the residual hydrogen in the next Si2H 6 exposure cycle prevents further reaction of Si2H 6. Thus, the thickness of a film grown at a surface temperature of < TL is smaller than that of the film with a flat area. Surface temperature TC during the laser irradia-
676
Y. Suda et al./ Journal of C~stal Growth 169 (1996) 672-680
tion is controlled by the combination of the incident laser power and the resistive substrate heating itself. When the substrate temperature is increased under constant laser power, the m a x i m u m temperature at the center of a laser irradiation area exceeds another threshold temperature (Tff), which causes a dent in the center of a growth film. Laser irradiation with the same laser power at the same substrate temperature as used in the S A L E experiment itself did not produce a dent in the laser irradiation area on a Si substrate surface. This result explains that the laser irradiation does not cause Si evaporation within the experimental conditions of this work. As is shown in Fig. 3, the thicknesses of films with a dent, where the thickness is defined as an average value between the m a x i m u m thickness in the rim of the dent and the minimum thickness at the bottom of the dent, are almost the same as those of films with a flat area. These facts imply that the dent is primarily caused by Si adatom migration to the circumference or lower temperature side and Si evaporation or etching and Si adsorbate desorption are considered to be minor effects. To confirm the quantitative levels of these effects, more precise experiments are needed. However, it is reasonable to expect that Si migration becomes more active before Si evaporation occurs. In the case of films with a flat area, the surface temperature in the flat area during laser irradiation is high enough to produce complete hydrogen desorption, and the amount of Si deposition in one cycle of the S A L E process is determined by the Si2H 6 exposure process for t~. As is explained in the next section, the deposition or the adsorption of Si2H 6 on Si(001) under the same Si2H 6 pressure used in the S A L E experiments of this work is reaction limited at surface temperatures of up to > 800°C. The surface temperature in the S i z H 6 exposure process during t~ is almost the same as the value of T~ub, which is < 320°C. Thus, Si deposition in the S i z H 6 exposure process is reaction limited and is not supplied-mass limited. However, the growth rate saturates rapidly with increasing substrate temperature as shown in Fig. 3. This result indicates that the activation energy in the S i z H 6 adsorption process is very small and the Si2H 6 adsorption is nearly self-limited up to at least ~ub = 290°C. The amount of the corresponding Si deposition was about 0.63 M L / c y c l e . Surface temperature distribution calculation, de-
scribed in this section, was further carried out for the case when the laser power is 5.4 W and the substrate temperature Tsu b is between 240 and 300°C. In this case, surface temperature T~ within 100 /zm from the center of the laser beam spot during the laser irradiation increases by 1.5 _+ 0.08°C when the substrate temperature is increased by I°C. Thus, the substrate temperature window AT~u w of ~ 15°C corresponds to the surface temperature window A Tw of ~ 22.5 _+ 1.2°C. This ATw also corresponds to Tff -T~.
3.2. Laser irradiation effects on thickness profile of SALE growth film Thickness distribution profiles of films grown by the S A L E method at a substrate temperature of 235°C with incident laser powers ranging from 4.8 to 5.4 W are shown in Fig. 4. The relation between the growth thickness per growth cycle (GR) and the laser power in the S A L E growth is shown in Fig. 5. As the value of G R for a film with a dent in the center, an average value between the m a x i m u m thickness in the rim of the dent and the minimum thickness at the bottom of the dent is used. In these experiments, sample B wafers were used as substrates. In the incident laser power range of about 4.9 to 5.1 W, the thickness distribution profiles exhibit a fiat area on the grown film surface. In the higher incident laser power range of > 5.1 W, a dent is
2 Ar ÷ Laser
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Sample B
......... ;5.4W ;5.0W
rr ~oo 0.5 (.9 0 -300
-150
0
150
300
Distance from the Center (pm)
Fig. 4. Thickness distribution profiles of films (sample B) grown by the Ar+-laser-assisted subatomic-layer epitaxy (SALE) method at a substrate temperature of 235°C with incident laser powers ranging from 4.8 to 5.4 W. The periods of t I. t 2, t 3 and t4 in one SALE growth cycle were 10. 10. 5 and 5 s, respectively.
E Suda et al./Journal of Co'stal Growth 169 (1996) 672-680
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5.6
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Fig. 6. Typical thickness distribution profiles of a film grown by the Ar+-laser-assisted subatomic-layer epitaxy (SALE) method, and a film grown by the Ar+-laser-inducedchemical vapor deposition (CVD) method.
Laser Power (W)
Fig. 5. The relation between the growth thickness per growth cycle (GR) and the laser power in the Ar+-laser-assisted SALE growth. formed at the center of the grown films. In the lower laser incident power range of < 4.9 W, the thickness distribution profiles exhibit a convex shape and the values of GR are smaller than those for films grown with laser incident powers of > 4.9 W. The values of GR for films exhibiting a flat surface area in these experiments were about 0.9 M L / c y c l e . The roughness of the most flat film, as is shown in Fig. 4, was +_0.017 M L / c y c l e . This value corresponds to the roughness of about _+2 A for the film of 100 ,~ thickness. Thus, a film grown by the SALE method using an Ar + ion laser whose power is set within the appropriate power range exhibits an atomically flat surface. Thus, these changes in the thickness distribution profiles of films grown using different laser powers well corresponds to the change in the thickness distribution profiles of films grown at different substrate temperatures. In Fig. 5, a laser power window, where a flat surface and constant GR value are obtained, is also observed. The range of the laser power window is about 0.25 W. The value of the temperature in the center of the laser-irradiated area during the laser irradiation has been estimated using a laser-induced chemical vapor deposition (laser-CVD) method. In this experiment, Si films were continuously grown by both continuous Ar + ion laser irradiation and continuous Si2H 6 exposure. This Si2H 6 exposure pressure is the same
as used in the SALE experiments. The laser power was varied between 4 to 5.4 W. Fig. 6 shows typical thickness distribution profiles of a film grown by the laser-CVD method, and a film grown by the laser-assisted SALE method for comparison. All films grown by the laser-CVD method show a Gaussian-like shape, which reflects the laser power density distribution profile. Fig. 7 shows the relation between the film growth rate and the laser power for films grown by the laser-CVD method. As is shown in Fig. 7, GR is nearly proportional to the inverse of the laser power, suggesting that Si is grown according to the Arrhenius relation and laser power is nearly proportional to the surface temperature during its growth. The relation between the growth temperature and the 103 Laser-CVD Si on Si(001) ~" 102
T ub = 235 °C
.< n¢-
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Fig. 7. The relation between the film growth rate and the laser power for films grown by the Ar+-laser-induced chemical vapor deposition (CVD) method.
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Y. Suda et al. / Jounzal of C~stal Growth 169 (1996) 672-680
growth rate for thermally grown Si epitaxy from Si2H 6 has been investigated by many researchers [17] and the activation energy in the Arrhenius relation for the Si growth rate is constant over the wide growth temperature range. Using this relation together with the growth rate of films grown by the laser-CVD method, the surface temperature of the laser-irradiated area during the laser irradiation has been estimated. When the laser power is 5.4 W and the value of Tsub is 235°C, the maximum surface temperature for sample B is estimated to be ~ 820°C. When the laser power is within 4 to 5.4 W, a change of 0.1 W in the laser power corresponds to a change of ~ 9°C in the surface temperature Ta. These results indicate that the laser power window of 0.25 W corresponds to the surface temperature window ATw of 0.25 X ( 9 / 0 . 1 ) = 22.5°C. This value is in good agreement with the value of ATw estimated from the substrate temperature window in the previous section. The fact of this agreement, together with the result that the surface morphology variation with increasing laser power is the same as observed when the substrate temperature is increased, suggests that the laser irradiation functions as a thermal effect and does not cause apparent photochemical desorption. Such a photochemical effect for a Si2H6/Si(001) system has been reported by other researchers [18] using a synchrotron radiation light with a peak photon energy of 100 eV. However, maximum photon energy of the laser light used in this work is 2.7 eV and the value is smaller than a S i - H bonding energy of 3.8 eV [19]. Due probably to this small photon energy, photo-excited hydrogen desorption is not apparently observed in this work. From the analyses of the results of the SALE experiments, Si migration is also expected to occur during growth by the laser-CVD method. However, a concave or a trapezoid surface was not observed, as is already described with Fig. 6. This fact suggests that the Si adatom migration velocity is smaller than the deposition rate. Thus, surface temperature evaluated using the growth rate for the laser-CVD method is considered to be somewhat underestimated, and, for the case of the value of the Si2H 6 pressure used in this work, Si deposition from SizH 6 is reaction limited and is not supplied-mass limited at least up to ~ 820°C. The GR value for the films grown with different
laser powers using sample B ( ~ 0.9) is higher than that for the films grown at different substrate temperatures using sample A ( ~ 0.63). Our primary RHEED observations through the process of alternating Si 2 H 6 exposure at room temperature and resistive heating using a single domain 4°-off Si(001) 2 X 1 surface indicate that the number of the alternate cycle to produce a 1 × 2 ( 2 X 1 ) to 2 × 1 ( 1 x 2 ) surface change is a little bit smaller than 2. This implies that the corresponding GR value is a little bit larger than 0.5. The GR value for sample A is close to this value. One explanation for the higher GR value for sample B is that the rate of decrease in the surface temperature for sample B is slower than the case for sample A due to the resistivity difference between sample A and sample B and that more cooling time is needed for sample B. However, more precise experiments are needed to clarify the reason for this GR value difference. 3.3. Growth modes and growth conditions in SALE method Fig. 8 summarizes the relation between the thickness distribution profile and the surface temperature P~
°134 o Distance from the Center
T sLib2
o Distance from the Center
Fig. 8. The relation between the thickness distribution profile and the surface temperature Tc during the laser irradiation in the SALE method.
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Y. Suda et al. / Journal of Crystal Growth 169 (1996) 672-680
during laser irradiation in the SALE method. When the laser power (PL) or the substrate temperature (~ub) is increased, three growth modes corresponding to three types of surface morphologies of a convex, a trapezoid, and a concave shape are observed. In the convex surface growth mode, the value of GR is determined by the hydrogen-desorption-limited reaction. In the flat surface growth mode, the value of GR is determined by the reaction-limited reaction or self-limited reaction during the Si2H 6 exposure process. Both ranges of the laser power window and the substrate temperature window, where the flat surface growth mode is observed, correspond to the same value of the range of the surface temperature window (ATw = T~ - T~). When r~ < T~, the surface morphology becomes convex, and, when TC > T~, the surface morphology becomes concave. When the laser power is increased while the substrate temperature is set constant, the surface temperature increases only in the laser-irradiated area. When the substrate temperature is increased while the laser power is set constant, all of the surface temperature distribution profile moves parallel towards higher temperatures. FWHM of the surface temperature distribution is broader in the case that the substrate temperature is increased than in the case that the laser power is increased as illustrated in Fig. 8. Therefore, FWHM of the thickness distribution profile of a film grown by increasing the substrate temperature is expected to be broader than that of a film grown by increasing the laser power. This FWHM difference in the thickness distribution profiles is apparently observed in Figs. 2 and 4, and the value of FWHM of the thickness distribution profile of a film grown by increasing Tsub is ~__270 nm and that of a film grown by increasing the laser power is ~ 230 nm. Thus, the growth mode in the laser-assisted SALE method is controlled by the surface temperature during the laser irradiation and the surface temperature is controlled by the combination of Ar + laser irradiation and substrate heating.
4. Conclusions Si submonolayer-by-submonolayer epitaxy from SiaH 6 on Si(001) has been carried out by repeating Si2H 6 exposure and surface excitation induced by
the combination of substrate resistive heating and Ar ÷ ion laser irradiation. This is the method of Ar+-laser-assisted subatomic-layer epitaxy. As the average substrate temperature or the laser irradiation power increases, the surface morphology of a grown film changes from a convex shape to a concave shape through a trapezoid shape. The results of this work together with our previous RHEED results suggest that the growth rate of a convex-shaped film is determined by hydrogen-desorption-limited reaction during laser irradiation, and that the growth rate of a trapezoid-shaped film is determined by selflimited reaction during Si2H 6 exposure. Surface temperature calculation together with our previous RHEED results suggest that Si adatom migration should occur in the flat area of a trapezoid-shaped film during laser irradiation. The roughness of the flat area is within _ 2 A per growth thickness of 100 ]~, and a substrate temperature window of ~ 15°C and a laser power window of ~ 0.25 W, where such a flat growth surface and a constant growth rate are obtained, has been observed. The ranges of these windows have been estimated to correspond to the same variation of the surface temperature in the laser irradiation area during the laser irradiation. The corresponding surface temperature window is ~ 23°C. This result together with the results of the analyses on growth thickness distribution profiles suggest that the laser irradiation works as a thermal effect. In the Ar+-laser-assisted SALE method, the growth surface morphology then the growth mode is controlled by the surface temperature during the laser irradiation. Ar + ion laser is a useful tool to control the surface temperature. o
Acknowledgements This work was supported in part by a Grant-in-Aid from the Ministry of Education, Science, Sports and Culture, the Izumi Science and Technology Foundation, and the Casio Science Promotion Foundation.
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[3] Y. Suda. D. Lubben, T. Motooka and J.E. Greene, Microphysics of Surfaces, Beams and Adsorbates, 1989 Technical Digest Series, Vol. 8 (Optical Society of America, Washington, DC, 1989) p. 23. [4] Y. Suda, D. Lubben, T. Motooka and J.E. Greene, J. Vac. Sci. Technol. B 7 (1989) 1171. [5] Y. Suda, D. Lubben, T. Motooka and J.E. Greene, J. Vac. Sci. Technol. A 8 (1990) 61. [6] S. Imai, T. Iizuka, O. Sugiura and M. Matsumura, Thin Solid Films 225 (1993) 168. [7] J. Murota, M. Sakuraba and S. Ono, Appl. Phys. Lett. 62 (1993) 2353. [8] M. Sakuraba, J. Murota, T. Watanabe, Y. Sawada and S. Ono, Appl. Surf. Sci. 82/83 (1994) 354. [9] C. Sasaoka and A. Usui, Appl. Surf. Sci. 82/83 (1994) 348. [10] S. Sugahara, T. Kitamura. S. Imai and M. Matsumura, Appl. Surf. Sci. 82/83 (1994) 380. [11] D. Lubben, R. Tsu. T.R. Bramblett and J.E. Greene, J. Vac. Sci. Technol. A 9 (1991) 3003.
[12] H. Akazawa, Y. Utsumi, T. Urisu and M. Nagase, Phys. Rev. B 47 (1993) 15946. [13] Y. Suda, M. Shirahama and M. Ishida, Advanced Materials '93, VI/A, Eds. H. Sakai, S. Kawai and H. Koinuma, Trans. Mater. Res. Soc. Jpn., Vol. 19A (Elsevier. Amsterdam, 1994) p. 149. [14] S. Imai, S. Takagi, O. Sugiura and M. Matsumura, Jpn. J. Appl. Phys. 30 (1991) 3646. [15] Y. Suda, M. Ishida, M. Yamashita and H. Ikeda, Appl. Surf. Sci. 82/83 (1994) 332. [16] M. von Allmen, Laser-Beam Interactions with Materials (Springer, Berlin, 1987) p. 49. [17] T. Aoyama, K. Miyanaga and T. Tatsumi, Ext. Abstr. '93 Inter. Conf. on Solid State Devices and Materials (Makuhari, 1993) p. 383. [18] H. Akazawa, Y. Utsumi. T. Urisu and M. Nagase, Phys. Rev. B 47 (1993) 15946. [19] P.M. Agrawal, D.L. Thompson and L.M. Raft, J. Chem. Phys. 92 (1990) 1069.
ELSEVIER
,. . . . . . . .
CRYSTAL GROWTH
Journal of Crystal Growth 169 (1996) 672-680
Ar+-laser-assisted subatomic-layer epitaxy of Si Yoshiyuki Suda *, Masahiro Ishida, Mitsutomi Yamashita Faculo' of Technology, Tolg'o Unicersityof Agriculture and Technology, 2-24-16Naka-cho, Koganei, Tok)'o 184. Japan
Received 2 January 1996; accepted 29 May 1996
Abstract
Si submonolayer-by-submonolayer epitaxy or subatomic-layer epitaxy (SALE) from Si2H 6 on Si(001) has been carried out by repeating Si2H 6 exposure and surface excitation induced by the combination of substrate resistive heating and Ar + laser irradiation. As the average substrate temperature or the laser irradiation power increases, the surface morphology of a grown film changes from a convex shape to a concave shapeothrough a trapezoid shape. The roughness of a fiat area of the trapezoid film is within _+2 ,~ per growth thickness of 100 A, and a substrate temperature window of ~ 15°C and a laser power window of ~ 0.25 W, where such a flat growth surface and a constant growth rate are obtained, has been observed. The ranges of these windows have been estimated to correspond to the same variation of the surface temperature in the laser irradiation area during the laser irradiation. This result together with the result of the analyses on growth thickness distribution profiles suggests that the laser irradiation works as a thermal effect. Thus, in the Ar+-laser-assisted SALE method, the growth surface morphology then the growth mode is controlled by the surface temperature during the laser irradiation. An Ar + laser is a useful tool to control the surface temperature.
1. I n t r o d u c t i o n
Atomic-layer epitaxy (ALE) is an epitaxial film growth technique which, in its ideal form, produces self-limited one-monolayer (ML) film growth per growth cycle. Much attention has recently been paid to the A L E of group IV semiconductors with the development of new S i / G e optoelectronic materials and devices that consist of artificial atomic-scale layer structures. Generally, for A L E of group IV semiconductors consisting of one element, an appropriate source gas, which exhibits just one-monolayer saturation coverage, has to be found. If the saturation coverage is more than or less than one, a growing
* Corresponding author. Fax: +81 423 88 7129.
surface is expected to become rough and exhibit a three-dimensional structure. Nearly 1 M L growth rate per growth or adsorption cycle (GR) were first reported for Si A L E using a S i H 2 C l J S i system with H 2 as a reducer gas [1] and for Ge A L E using a ( C 2 H s ) z G e H z / G e system [2]. W e first focused on hydride molecules as a group-IV A L E source gas, which is inherently free from contaminants such as C and C1, and reported saturated adsorption reaction using a S i z H 6 / S i ( 0 0 1 ) system [3-5]. Since then, several source g a s / s u b s t r a t e systems, which exhibit a nearly 1 M L growth rate, have been reported to date including S i H ~ C l J S i with H as a reducer [6], G e H a / S i [7,8], cracked S i H 2 C I 2 / S i [9], and ( C H 3 ) 2 G e H 2 / G e [10]. However, in the case of S i z H 6 / S i [11-13] and S i 3 H 8 / S i [14] systems, GR
0022-0248/96/$15.00 Copyright © 1996 Elsevier Science B.V. All rights reserved PII S0022-0248(96)00454-X
E Suda et al./Journal of Co'stal Growth 169 (1996) 672-680
is less than 1, and for a SiH4/Si system, the temperature window exhibiting GR = 1 has been very limited [7]. Thus, some consideration is needed to apply hydride molecules to ALE more widely for its potential advantage of being contaminant free. In our previous works [3-5,13], we have reported that SizH 6 adsorbs dissociatively at random sites on Si(001) at room temperature and the adsorbed species are interpreted to be -H, - S i = H 2 and - S i - H 3. This H termination limits the adsorption coverage of Si to less than 1 ML. However, from our recent results of time-dependent RHEED observations during annealing a SieH6-exposed Si(001) surface with self-resistive heating, it has been concluded that Si adatoms migrate to produce wide terraces accompanied by or following hydrogen desorption during the surface annealing, and an atomically flattened surface is obtained after the surface annealing [13,15]. The terrace growth time exhibits the Arrhenius relation to the annealing temperature and the activation energy of the terrace growth is found to be 1.6 _+ 0.4 eV. From these results, we have proposed a subatomiclayer epitaxy (SALE) technique with which Si is grown by submonolayer-by-submonolayer by alternately repeating Si 2H 6 exposure and surface thermal excitation [13,15]. Using the RHEED technique and a single-domain Si(001) 2 X 1 surface, surface structural change in the Si SALE growth process has also been investigated by repeating a SALE growth cycle, where one growth cycle consists of Si2H 6 exposure and substrate resistive heating [13]. The result indicates that nearly single 2 × 1 and 1 × 2 domains appear alternately every almost two growth cycles, which supports the SALE growth principle. Thus, even if the Si2H 6 saturation coverage is less than one, Si is epitaxially grown by submonolayer-bysubmonolayer due to the surface-flattening process induced by surface thermal excitation, we have also demonstrated Si submonolayer-by-submonolayer digital growth from Si2H 6 using Ar + ion laser irradiation and resistive substrate heating as surface excitation sources [13,15]. However, the role of the surface excitation by Ar + ion laser irradiation has not been fully understood. In this study, we have carried out Si SALE growth from Si2H 6 on Si(001) using Ar + ion laser irradiation and resistive substrate heating as surface excitation sources and investigated the growth conditions
673
and the growth mechanisms of the Ar+-laser-assisted SALE method. Either the laser power or the substrate temperature was varied. From the results of the SALE experiments, we have first demonstrated a substrate temperature window and a laser power window where an atomically flat growth surface is obtained. The effect of Ar ÷ laser irradiation is well understood as a thermal effect. The results have also indicated that the flattening process then the surface morphology is controlled by the surface temperature during the surface excitation process of the growth cycle, and that its surface temperature is controlled by the combination of the laser irradiation and thermal annealing.
2. Experimental procedure SALE growth experiments were carried out using a load-locked ultrahigh vacuum chamber with a base pressure of < 5 × 10 -l° TorT. Si(001) substrates used in these experiments were 5 × 25 mm plates cleaved from p-type 5 - 7 f~. cm wafers (sample A) and p-type 3-5 F t . c m wafers (sample B). These substrates were degreased before insertion into the vacuum chamber. Clean double-domain Si(001)2 × 1 surfaces were obtained by annealing the substrates at ~ 600°C for 1 h and heating them to ~ 1100°C for > 3 s. 99.98% Si2H 6 was introduced into the chamber through a gas doser using a computer-controlled o n - o f f air valve. The substrates were resistively heated to regulate the surface temperature, which is monitored using a pyrometer and by measuring the substrate electrical resistance. SALE growth from S i z H 6 o n Si(001) was carried out using an Ar + ion laser. One growth cycle of the SALE experiments consists of (1) Si2H 6 exposure, (2) evacuation, (3) laser irradiation, and (4) cooling. The laser was operated with a multiline mode and the wavelength was in the visible region of 458-515 nm. The laser beam was focused through a lens and its beam diameter at half-maximum intensity was 150 + 25 /xm. The timing chart of the sequential SALE process is illustrated in Fig. 1. t 1, t 2, t 3 and t 4 c o r r e s p o n d to the periods of four processes in one cycle of the SALE growth, respectively. The Si2H 6 exposure pressure was set to be ~ 3 X 10 - 4 Torr. At the instant when the Si2H 6 gas valve was turned on, the
674
E Suda et al. / Journal of Co'stal Growth 169 (1996) 672-680 i [(
it
1 cycle
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;!
[
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power of 5.4 W. In all S A L E growth experiments, the periods of t 1, t2, t 3 and t 4 were set to be 10, 10, 5 and 5 s, respectively, and the number of growth cycles were 360. The thickness profiles of the growth films were measured by a surface profilometer.
Si H 26 Exposure
off
on A r * Ion Laser Irradiation
off
Fig. 1. The timing chart of the sequential processes in one growth cycle of the Ar+-laser-assisted subatomic-layer epitaxy (SALE) method.
pressure rose up to ~ 2 X 10 -3 Torr, and then, the pressure settled down to the set value of ~ 3 X 10 -4 Torr during t I. After evacuation of the S i e H 6, the pressure reached to the order of 10-8 Torr during t 3. The laser irradiation was switched on and off using a computer-controlled light shutter. Substrate temperature during the S A L E growth was regulated by controlling the DC current passing through the substrate so that the average surface temperature monitored by the pyrometer during Si2H 6 exposure (t]) and evacuation (t e) was kept constant. This average surface temperature is hereafter addressed as substrate temperature Tsub. The value of iC,ub is set to be between 235 and 320°C. In the period of laser irradiation, the laser-irradiated area was thermally excited by the combination of the laser irradiation and the resistive heating itself. A temperature profile on the surface during the laser irradiation exhibits a Gaussian-like shape [13,15]. The surface temperature in the laserirradiated area during the laser irradiation is hereafter denoted by To . To clarify the effect of the laser excitation, two series of the S A L E experiments were carried out. In one series of S A L E experiments, the incident laser powers were set at values ranging from 4.8 to 5.4 W under the same substrate temperature of 235°C. In the other series of S A L E experiments, the substrate temperatures were set at values ranging from 250 to 320°C under the same incident laser
3. Results and discussion 3.1. Substrate temperature effects on thickness profile o f S A L E growth f i h n
Fig. 2 shows typical thickness distribution profiles of films grown by the S A L E method at substrate temperatures of 260-300°C with an incident laser power of 5.4 W. In these experiments, sample A wafers were used as substrates. As the substrate temperature increases, the thickness distribution profile changes from a convex shape to a concave shape through a trapezoid shape. The relation between the growth thickness per growth cycle (GR) and the substrate temperature T,ub in the S A L E growth is shown in Fig. 3. As the value of GR for a film with a dent in the center, an average value between the maximum thickness in the rim of the dent and the minimum thickness at the bottom of the dent is used. The substrate temperature was varied from 250 to 320°C. In the middle substrate temperature range AT~uw of about 270 to 280°C, a flat surface area is observed in the center of the growth film. The
2
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Fig. 2. Thickness distribution profiles of films (sample A) grown by the Ar+-laser-assisted subatomic-layer epitaxy (SALE) method with an incident laser power of 5.4 W at substrate temperatures of 260 to 320°C. The periods of t l, tz, t3 and t4 in one SALE growth cycle were 10, 10, 5 and 5 s, respectively.
E Suda et al./Joumal of Co'stal Growth 169 (1996) 672-680 2
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surface roughness in the flat area in the most flat case is measured to be about _+0.015 M L / c y c l e the value of which corresponds to about +_2 A for the film of 100 A thickness. Thus, the atomically flat surface is obtained. In the lower substrate temperature range of less than ~ 270°C, the growth thickness distribution profile becomes a convex shape and the GR value is smaller than that of a film grown under the middle substrate temperatures. In the higher substrate temperature range of more than ~ 280°C, the growth thickness distribution profile becomes a concave shape. For the conventional ALE growth technique, a temperature window is referred to as the temperature range where GR is 1 ML/cycle. As the SALE growth technique is originally a submonolayer-by-submonolayer growth method, the similar temperature window is defined as the temperature range where constant GR and an atomically flat surface is obtained. This temperature window is observed in Fig. 3 and its substrate temperature window AT~u w and the GR value is about 15°C and about 0.63 ML/cycle, respectively. The surface temperature distribution during the laser irradiation has been calculated using the Green's function method [16]. In the calculation, the laser beam is treated as a Gaussian and penetrating source. Full width at half maximum (FWHM) of the laser beam is set to be 146 /zm. From this calculation, the
675
minimum temperature within the flat area of a growth film, which corresponds to the temperature at the rim of the flat area, is estimated to be 600 + 40°C for films grown with an incident laser power of 5.4 W. In our previous investigation on surface thermal excitation of a Si2H6-exposed surface using a RHEED technique [15], more than 575°C is necessary to produce Si adatom migration or terrace growth enough to obtain an atomically flat surface when the excitation time is 5 s, which corresponds to a period of 13 in this experiment. Therefore, the minimum temperature within the flat area corresponds to the threshold temperature which produces sufficient surface flattening induced by Si adatom migration. This threshold temperature is denoted by T~. In other words, agreement between the RHEED results and the result of the surface temperature calculation suggests that Si migration should occur in the flat area during the laser irradiation. Measured atomic-scale flatness in the flat area probably evidences the Si migration. In our previous RHEED study on the adsorption mechanism of Si2H 6 on Si(001) [13], it was found that the adsorption coverage of Si2H 6 at surface temperatures less than ~ 300°C was less than 1 over the wide Si2H 6 dosages of 18-1.8 × 10 ~5 langmuir and hydrogen termination on the surface tends to prevent further Si e H6 adsorption. On the other hand, in our time-dependent RHEED experiments [15], terrace growth of a Si2H6-exposed surface completes within ~ 10 s when the annealing temperature is 575°C. An electron energy-loss spectroscopy (EELS) study [4] has also indicated that hydrogen desorbs from a Si~H6-exposed surface almost completely with a 15 s anneal at 570°C when the surface is annealed at progressively higher temperatures. Thus, the hydrogen desorption almost completes during Si adatom migration, and surface temperature, at which Si adatom migration completes, is close to the value of TL for a given annealing period. Therefore, when the surface temperature during the laser irradiation is less than T~, hydrogen desorption is not complete and the residual hydrogen in the next Si2H 6 exposure cycle prevents further reaction of Si2H 6. Thus, the thickness of a film grown at a surface temperature of < TL is smaller than that of the film with a flat area. Surface temperature TC during the laser irradia-
676
Y. Suda et al./ Journal of C~stal Growth 169 (1996) 672-680
tion is controlled by the combination of the incident laser power and the resistive substrate heating itself. When the substrate temperature is increased under constant laser power, the m a x i m u m temperature at the center of a laser irradiation area exceeds another threshold temperature (Tff), which causes a dent in the center of a growth film. Laser irradiation with the same laser power at the same substrate temperature as used in the S A L E experiment itself did not produce a dent in the laser irradiation area on a Si substrate surface. This result explains that the laser irradiation does not cause Si evaporation within the experimental conditions of this work. As is shown in Fig. 3, the thicknesses of films with a dent, where the thickness is defined as an average value between the m a x i m u m thickness in the rim of the dent and the minimum thickness at the bottom of the dent, are almost the same as those of films with a flat area. These facts imply that the dent is primarily caused by Si adatom migration to the circumference or lower temperature side and Si evaporation or etching and Si adsorbate desorption are considered to be minor effects. To confirm the quantitative levels of these effects, more precise experiments are needed. However, it is reasonable to expect that Si migration becomes more active before Si evaporation occurs. In the case of films with a flat area, the surface temperature in the flat area during laser irradiation is high enough to produce complete hydrogen desorption, and the amount of Si deposition in one cycle of the S A L E process is determined by the Si2H 6 exposure process for t~. As is explained in the next section, the deposition or the adsorption of Si2H 6 on Si(001) under the same Si2H 6 pressure used in the S A L E experiments of this work is reaction limited at surface temperatures of up to > 800°C. The surface temperature in the S i z H 6 exposure process during t~ is almost the same as the value of T~ub, which is < 320°C. Thus, Si deposition in the S i z H 6 exposure process is reaction limited and is not supplied-mass limited. However, the growth rate saturates rapidly with increasing substrate temperature as shown in Fig. 3. This result indicates that the activation energy in the S i z H 6 adsorption process is very small and the Si2H 6 adsorption is nearly self-limited up to at least ~ub = 290°C. The amount of the corresponding Si deposition was about 0.63 M L / c y c l e . Surface temperature distribution calculation, de-
scribed in this section, was further carried out for the case when the laser power is 5.4 W and the substrate temperature Tsu b is between 240 and 300°C. In this case, surface temperature T~ within 100 /zm from the center of the laser beam spot during the laser irradiation increases by 1.5 _+ 0.08°C when the substrate temperature is increased by I°C. Thus, the substrate temperature window AT~u w of ~ 15°C corresponds to the surface temperature window A Tw of ~ 22.5 _+ 1.2°C. This ATw also corresponds to Tff -T~.
3.2. Laser irradiation effects on thickness profile of SALE growth film Thickness distribution profiles of films grown by the S A L E method at a substrate temperature of 235°C with incident laser powers ranging from 4.8 to 5.4 W are shown in Fig. 4. The relation between the growth thickness per growth cycle (GR) and the laser power in the S A L E growth is shown in Fig. 5. As the value of G R for a film with a dent in the center, an average value between the m a x i m u m thickness in the rim of the dent and the minimum thickness at the bottom of the dent is used. In these experiments, sample B wafers were used as substrates. In the incident laser power range of about 4.9 to 5.1 W, the thickness distribution profiles exhibit a fiat area on the grown film surface. In the higher incident laser power range of > 5.1 W, a dent is
2 Ar ÷ Laser
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Sample B
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0
150
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Fig. 4. Thickness distribution profiles of films (sample B) grown by the Ar+-laser-assisted subatomic-layer epitaxy (SALE) method at a substrate temperature of 235°C with incident laser powers ranging from 4.8 to 5.4 W. The periods of t I. t 2, t 3 and t4 in one SALE growth cycle were 10. 10. 5 and 5 s, respectively.
E Suda et al./Journal of Co'stal Growth 169 (1996) 672-680
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Fig. 6. Typical thickness distribution profiles of a film grown by the Ar+-laser-assisted subatomic-layer epitaxy (SALE) method, and a film grown by the Ar+-laser-inducedchemical vapor deposition (CVD) method.
Laser Power (W)
Fig. 5. The relation between the growth thickness per growth cycle (GR) and the laser power in the Ar+-laser-assisted SALE growth. formed at the center of the grown films. In the lower laser incident power range of < 4.9 W, the thickness distribution profiles exhibit a convex shape and the values of GR are smaller than those for films grown with laser incident powers of > 4.9 W. The values of GR for films exhibiting a flat surface area in these experiments were about 0.9 M L / c y c l e . The roughness of the most flat film, as is shown in Fig. 4, was +_0.017 M L / c y c l e . This value corresponds to the roughness of about _+2 A for the film of 100 ,~ thickness. Thus, a film grown by the SALE method using an Ar + ion laser whose power is set within the appropriate power range exhibits an atomically flat surface. Thus, these changes in the thickness distribution profiles of films grown using different laser powers well corresponds to the change in the thickness distribution profiles of films grown at different substrate temperatures. In Fig. 5, a laser power window, where a flat surface and constant GR value are obtained, is also observed. The range of the laser power window is about 0.25 W. The value of the temperature in the center of the laser-irradiated area during the laser irradiation has been estimated using a laser-induced chemical vapor deposition (laser-CVD) method. In this experiment, Si films were continuously grown by both continuous Ar + ion laser irradiation and continuous Si2H 6 exposure. This Si2H 6 exposure pressure is the same
as used in the SALE experiments. The laser power was varied between 4 to 5.4 W. Fig. 6 shows typical thickness distribution profiles of a film grown by the laser-CVD method, and a film grown by the laser-assisted SALE method for comparison. All films grown by the laser-CVD method show a Gaussian-like shape, which reflects the laser power density distribution profile. Fig. 7 shows the relation between the film growth rate and the laser power for films grown by the laser-CVD method. As is shown in Fig. 7, GR is nearly proportional to the inverse of the laser power, suggesting that Si is grown according to the Arrhenius relation and laser power is nearly proportional to the surface temperature during its growth. The relation between the growth temperature and the 103 Laser-CVD Si on Si(001) ~" 102
T ub = 235 °C
.< n¢-
101
o (.9
10° Sample B
lo,
o
o11
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I
03
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Fig. 7. The relation between the film growth rate and the laser power for films grown by the Ar+-laser-induced chemical vapor deposition (CVD) method.
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Y. Suda et al. / Jounzal of C~stal Growth 169 (1996) 672-680
growth rate for thermally grown Si epitaxy from Si2H 6 has been investigated by many researchers [17] and the activation energy in the Arrhenius relation for the Si growth rate is constant over the wide growth temperature range. Using this relation together with the growth rate of films grown by the laser-CVD method, the surface temperature of the laser-irradiated area during the laser irradiation has been estimated. When the laser power is 5.4 W and the value of Tsub is 235°C, the maximum surface temperature for sample B is estimated to be ~ 820°C. When the laser power is within 4 to 5.4 W, a change of 0.1 W in the laser power corresponds to a change of ~ 9°C in the surface temperature Ta. These results indicate that the laser power window of 0.25 W corresponds to the surface temperature window ATw of 0.25 X ( 9 / 0 . 1 ) = 22.5°C. This value is in good agreement with the value of ATw estimated from the substrate temperature window in the previous section. The fact of this agreement, together with the result that the surface morphology variation with increasing laser power is the same as observed when the substrate temperature is increased, suggests that the laser irradiation functions as a thermal effect and does not cause apparent photochemical desorption. Such a photochemical effect for a Si2H6/Si(001) system has been reported by other researchers [18] using a synchrotron radiation light with a peak photon energy of 100 eV. However, maximum photon energy of the laser light used in this work is 2.7 eV and the value is smaller than a S i - H bonding energy of 3.8 eV [19]. Due probably to this small photon energy, photo-excited hydrogen desorption is not apparently observed in this work. From the analyses of the results of the SALE experiments, Si migration is also expected to occur during growth by the laser-CVD method. However, a concave or a trapezoid surface was not observed, as is already described with Fig. 6. This fact suggests that the Si adatom migration velocity is smaller than the deposition rate. Thus, surface temperature evaluated using the growth rate for the laser-CVD method is considered to be somewhat underestimated, and, for the case of the value of the Si2H 6 pressure used in this work, Si deposition from SizH 6 is reaction limited and is not supplied-mass limited at least up to ~ 820°C. The GR value for the films grown with different
laser powers using sample B ( ~ 0.9) is higher than that for the films grown at different substrate temperatures using sample A ( ~ 0.63). Our primary RHEED observations through the process of alternating Si 2 H 6 exposure at room temperature and resistive heating using a single domain 4°-off Si(001) 2 X 1 surface indicate that the number of the alternate cycle to produce a 1 × 2 ( 2 X 1 ) to 2 × 1 ( 1 x 2 ) surface change is a little bit smaller than 2. This implies that the corresponding GR value is a little bit larger than 0.5. The GR value for sample A is close to this value. One explanation for the higher GR value for sample B is that the rate of decrease in the surface temperature for sample B is slower than the case for sample A due to the resistivity difference between sample A and sample B and that more cooling time is needed for sample B. However, more precise experiments are needed to clarify the reason for this GR value difference. 3.3. Growth modes and growth conditions in SALE method Fig. 8 summarizes the relation between the thickness distribution profile and the surface temperature P~
°134 o Distance from the Center
T sLib2
o Distance from the Center
Fig. 8. The relation between the thickness distribution profile and the surface temperature Tc during the laser irradiation in the SALE method.
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during laser irradiation in the SALE method. When the laser power (PL) or the substrate temperature (~ub) is increased, three growth modes corresponding to three types of surface morphologies of a convex, a trapezoid, and a concave shape are observed. In the convex surface growth mode, the value of GR is determined by the hydrogen-desorption-limited reaction. In the flat surface growth mode, the value of GR is determined by the reaction-limited reaction or self-limited reaction during the Si2H 6 exposure process. Both ranges of the laser power window and the substrate temperature window, where the flat surface growth mode is observed, correspond to the same value of the range of the surface temperature window (ATw = T~ - T~). When r~ < T~, the surface morphology becomes convex, and, when TC > T~, the surface morphology becomes concave. When the laser power is increased while the substrate temperature is set constant, the surface temperature increases only in the laser-irradiated area. When the substrate temperature is increased while the laser power is set constant, all of the surface temperature distribution profile moves parallel towards higher temperatures. FWHM of the surface temperature distribution is broader in the case that the substrate temperature is increased than in the case that the laser power is increased as illustrated in Fig. 8. Therefore, FWHM of the thickness distribution profile of a film grown by increasing the substrate temperature is expected to be broader than that of a film grown by increasing the laser power. This FWHM difference in the thickness distribution profiles is apparently observed in Figs. 2 and 4, and the value of FWHM of the thickness distribution profile of a film grown by increasing Tsub is ~__270 nm and that of a film grown by increasing the laser power is ~ 230 nm. Thus, the growth mode in the laser-assisted SALE method is controlled by the surface temperature during the laser irradiation and the surface temperature is controlled by the combination of Ar + laser irradiation and substrate heating.
4. Conclusions Si submonolayer-by-submonolayer epitaxy from SiaH 6 on Si(001) has been carried out by repeating Si2H 6 exposure and surface excitation induced by
the combination of substrate resistive heating and Ar ÷ ion laser irradiation. This is the method of Ar+-laser-assisted subatomic-layer epitaxy. As the average substrate temperature or the laser irradiation power increases, the surface morphology of a grown film changes from a convex shape to a concave shape through a trapezoid shape. The results of this work together with our previous RHEED results suggest that the growth rate of a convex-shaped film is determined by hydrogen-desorption-limited reaction during laser irradiation, and that the growth rate of a trapezoid-shaped film is determined by selflimited reaction during Si2H 6 exposure. Surface temperature calculation together with our previous RHEED results suggest that Si adatom migration should occur in the flat area of a trapezoid-shaped film during laser irradiation. The roughness of the flat area is within _ 2 A per growth thickness of 100 ]~, and a substrate temperature window of ~ 15°C and a laser power window of ~ 0.25 W, where such a flat growth surface and a constant growth rate are obtained, has been observed. The ranges of these windows have been estimated to correspond to the same variation of the surface temperature in the laser irradiation area during the laser irradiation. The corresponding surface temperature window is ~ 23°C. This result together with the results of the analyses on growth thickness distribution profiles suggest that the laser irradiation works as a thermal effect. In the Ar+-laser-assisted SALE method, the growth surface morphology then the growth mode is controlled by the surface temperature during the laser irradiation. Ar + ion laser is a useful tool to control the surface temperature. o
Acknowledgements This work was supported in part by a Grant-in-Aid from the Ministry of Education, Science, Sports and Culture, the Izumi Science and Technology Foundation, and the Casio Science Promotion Foundation.
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