Observation of electromigration effect upon Si-MBE growth on Si(001) surface

Vacuum/volume 41/numbers 4-6/pages 933 to 937/1990

0042-207X/90$3.00 + .00 © 1990 Pergamon Press plc

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Observation of electromigration effect upon S i - M B E growth on Si(O01) surface M

I c h i k a w a and T Doi, Central Research Laboratory, HitachL Ltd, Kokubunji, Tokyo 185, Japan

Si(O01) surface topography changes during annealing, and Si molecular beam epitaxial (MBE) growth is observed by reflection electron microscopy using microprobe reflection high-energy electron diffraction. When the sample is heated above 600°C, one of the monoatomic steps to which the 2 x I dimer axis is normal or parallel becomes less stable than the other, depending on the direction of the sample electric current used for heating. Si atoms are detached from the unstable steps and captured by the stable steps. This causes selective growth c'f one of the two 2 x 1 domains on the surface and biatomic step growth during Si MBE at high substrate temperature (,,~900°C).

1. Introduction

Recently, Enta et al L reported that the major and minor 2 x 1 domains on a nearly single domain surface can be converted by changing the direction of the direct electric current used for heating the sample. Using uhv reflection electron microscopy, Kahata and Yagi 2 observed that the conversion process between the major and minor domains was caused by the currentinduced movement of monoatomic steps on the surface. In our previous study 3"4, we found that biatomic step growth occurred during Si-MBE growth at a high substrate temperature (~900~C). The cause of biatomic step growth, however, was not clarified. Very recently we observed that the biatomic step growth has strong dependence on the sample current direction 5. It is, therefore, important to investigate the currentinduced reactions at atomic steps in order to understand the MBE growth mechanism. In this study, we observed the detailed surface topography changes of Si(001) surfaces during annealing and Si-MBE growth, using microprobe reflection high-energy electron diffraction. 2. Experimental method

The experiments were performed using microprobe reflection high-energy electron diffraction (RHEED) 6'7. Scanning electron microscope images produced using one diffraction spot intensity (reflection electron microscope (REM) images) can be obtained. In situ observation of surface topography is performed by taking REM images. The base pressure of the sample chamber is about 8 × 10 - ~ torr, increasing to about 5 x 10 -1° torr during Si-MBE. The Si sample was cut from a precisely oriented ( < 2 ' ) Si(001) wafer. Surface cleaning by several flashings at around 1200°C for 3s produced a mixed surface structure of 2 × 1 and 1 x 2 domains. However, annealing at 900 C for 20 rain produced a nearly single (2 x l)-domain structure on the surface. The experiments were performed using the nearly single-domian structured surface. 3. Results and discussion

3.1. Conversion between 2 x 1 and 1 x 2 domains. Figure 1 shows the conversion process from a nearly single 2 x 1 domain

surface to a nearly single 1 x 2 domain surface during anneal ing of the sample at 800°C (current density is about 100 A cm-2). The bright areas show 2 x 1 domains in which the dimer axis is in the horizontal direction in the image and the dark areas show 1 x 2 domains in which the dimer axis is in the vertical direction. The surface in the image becomes higher from right to left. Monoatomic steps exist between the two domains. At first, the sample current passed in the step-up direction, as indicated by the arrow in Figure l(a). When the direction was changed to step-down, the minor 1 x 2 domain grew selectively on the surface [(b),(c)] and nearly single (1 x 2)-domain surface was obtained after 20min annealing (d). During this process, the S~-type monoatomic steps (the notation used in Chadi's paper s) to which the dimer axis is normal, moved to the upper-terrace side. The Sb-type monoatomic steps moved to the lower-terrace side as shown in the inserted illustration. The distances of the movement are nearly equal in the initial stage. This is confirmed when dark contrast contamination idicated by the arrow is used as a scaling point. When the current direction was reversed again (step-up direction), the minor 2 x 1 domains grew on the surface as shown in Figure 2(b and c) and nearly a single (2 x 1)-domain surface was obtained again (d). During this process, the Sb-type steps moved to the upper-terrace side and the S~-type steps moved to the lower-terrace side, again the movement distances of S, and Sb steps are nearly equal. We measured the temperature dependence of spreading velocity v of the minor domain terrace in the range of 600-900'C. It was found that the velocity can be represented by v = vo exp( - E m / k T ) ,

(1)

where Vo is a constant of proportionality and Em is the activation energy of the terrace spreading velocity, which is 1.3 + 0.1 eV determined by the Arrhenius plot for both of the 2 x 1--*1 x 2 and the 1 x 2 ~ 2 x 1 domain conversions, as shown in Figure 3. The flux density of Si atoms evaporating for the surface is known to be about l x 109 ( a t o m s . c m - 2 . s -I) at 800°C 9, which is quite small compared with the atomic density on the Si(001) surface ( ~ 7 x 1014 atoms, cm-2). This means that the 933

M Ichikawa and T Doi: Electromigration and Si-MBE growth on Si(O01 )

(a)

2 x 1 (-q--

(C)

5 rain

i

)

(b)

3 min

( i ---)

5 I~m 15m

(d) l x 2 (20 rain) i .~ll,.-

.-. 7 ) 2x 1 •

,,,';,,ix2x

,-,

S

Figure 1. REM images during the conversion process from 2 x 1 domain surface to 1 x 2 domain surface at 800°C. The images were taken using a 2 x 1 reconstruction spot intensity. The current direction is shown by the arrow. Illustration of atomic step movement during the conversion process is also shown. • • shows the dimer axis direction of 2 x 1 structure.

evaporation during the experiment was negligible. The conversion was caused by atom migration from the S,-steps to the Sb-steps in the case of 2 × 1 --* 1 × 2 domain conversion and by migration from the Sb-steps to the S,-steps in the case of 1 × 2 ~ 2 × 1 domain conversion. This problem can be treated by a simple model where the unstable and the stable steps supply and capture adsorbed atoms, respectively. A one-dimensional steady-state diffusion equation gives the adsorbed atom density; n ( x ) n ( x ) = (n~ - n u ) x / l + no,

(2)

where l is the terrace width, and nu and ns are adsorbed atom densities at the unstable and stable steps, respectively, In the above discussion, the effect of the electric field supplied to the sample was neglected since the voltage between the sample edges ( ~ 2 cm) was several volts.

934

As the atomic current j is - D ity of the terrace is given by

• dn/dx,

v = 2a2j = 2aZO(nu - ns)/l,

the spreading veloc(3)

where D is surface diffusion coefficient and a is surface lattice length. When n u ,> n s is satisfied, the temperature dependence of velocity is represented by v = 2a2Dono/l • e x p { - ( E , + E k ) / k T },

(4)

where D = D o e x p ( - E , / k T ) , E , is the activation energy of surface diffusion, n , , = n o e x p ( - E u / k T ) , E u is the activation energy required to move atoms from step sites to adsorbed sites, and n o is atom site density. Equations (1) and (4) indicate that Er, = E, + E k = 1.3 eV. Si-MBE experiments at low substrate temperature (450°C) 3'4 showed that movement of the monoatomic steps did not occur

M Ichikawa and T Doi. Electromigration and Si-MBE growth on Si(O01 )

(a) 1-x2

(c) 5 rain

(i--~)

(b) 3 min

is

(-,-i)

lX2sJb.J~,F

(--i)

(d) 2 x l (20 rain)

5p.rn :1~m

, 56

'Sa--,~

Figure 2. REM images during the conversion process from 1 × 2 domain surface to 2 × 1 domain surface at 800°C.

within an accuracy of 0.1 #m during the growth. Following the discussion of Lewis and Campbell ~° and K u r o d a " , atoms which impinge on the surface at distances less than Xs from the step can interact with the step without being captured by two-dimensional nuclei on the terraces, xs is represented by xs = ( D / J ) 1 / 4 ,

(5)

where J is the flux density of Si atoms deposited on the surface. No occurrence of atomic step movement indicates xs < 0.1 jim. This gives E s > 0.7 eV when the thermal vibration frequency is supposed to be 10~2s -~. Then the activation energy Ek required to migrate atoms from step or kink sites to adsorbed sites, is less than 0.6 eV. It is known that the activation energy E k is about half the evaporation energy TM ~3. Since the evaporation energy is about 4 eV for Si 14, this shows that the step became very unstable because of the effect of sample electric current.

The above discussion indicates that the 2 x 1 domain conversion was caused by the instability of one of the atomic steps induced by the sample electric current, depending on the stepup or step-down direction of the current. 3.2. MBE growth. The effect of sample electric current upon Si-MBE growth was examined by changing the substrate temperature at a constant Si growth rate. The growth rate is about 0.1 monolayer (ML) per minute. The growth mode was nearly independent of the current direction. It was found that the growths were dominated by two-dimensional nucleation growth at low substrate temperature (~450°C), monoatomic step propagation at higher temperature (~750°C) and by biatomic step propagation at even higher temperature (~900°C), as already observed in our previous study 3'4. The sample electric current effect on the growth was not obvious at substrate temperatures lower than 750°C. Figure 4 935

M Ichikawa and T Doi: Electromigration and Si-MBE growth on Si(O01 )

10 e - - 2xl - - l x 2 x--1x2--2x1 r-

E ~L

";1 15

>~1o-'

I

0.5

I

I

I

I

I

1.5

1.0

lO00/'r (K-1)

Figure 3. Substrate temperature (T) dependence of spreading velocity of minor domain terrace. O; 2 x l --*1 x 2 conversion, × ; l × 2--*2 x 1 conversion. The data were taken at terrace width of about 2/~m.

(a) Substrate

(c) 0.8 ML

(i---)

shows the MBE growth process at 750°C when the current direction is step-down. The 2 x I domain with bright contrast grew mainly on the down-sides of the steps of the narrow 2 x 1 domains. This indicates that the growth was dominated by monoatomic step propagation. It is found that the grown 2 x 1 domains are elongated in the vertical direction. This is confirmed by comparing the scale in the vertical direction with that in the horizontal direction. Chadi has reported that S,-type steps to which the dimer axis is normal have lowest step formation energy8. The Sa-type steps exist more in the elongated 2 x 1 domains than the Sb-type steps. This shows that the growth occurred by minimizing the step energy as observed previously3,4, indicating that the growth was not strongly affected by the sample electric current. At even higher temperature (,-,900°C), however, the sample electric current effect was clearly observed as shown in Figure 5. During the growth, the narrow 2 × 1 domains moved to the right side without changing size as indicated by the arrow, showing that the growth was dominated by 'biatomic step' propagation. The deposited Si atoms migrated on the surface and were captured by Sa and SFtype steps during the growth. However, since the Sa-type steps are unstable when the sample current

(b) 0.4 ML

(d) 1.9

ML

(i"--)

15 rnl m

Figure 4. REM images during Si-MBE growth at substrate temperature of 750°C. The current direction is step-down as indicated by the arrow. Deposited Si thicknesses are indicated in monolayer (ML) unit.

936

M Ichikawa and T Doi: Electromigration and Si-MBE growth on Si(O01 )

(a)

(b) 0.7 ML

Substrate

(c) 1.4 ML

(i--,-)

(d)3.3

ML

( i "~)

(i--*)

51aml'tm

Figure 5. REM images during Si-MBE growth at substrate temperature of 900°C. direction is step-down as shown in the previous section, Si atoms were detached from Sa-type steps and recaptured again by Sb-type steps. This suppressed the growth of the 2 × 1 domains on the surface. It is noted that there is an interaction between the two steps which prevents the Sa and Sb-type steps coalescing with each other. This preserves a finite 2 x 1 terrace width ( ~ 0.3 #m). This phenomenon is also observed in the 2 x 1 domain conversion process shown in Figures 1 and 2, which has been observed by Kahata and Yagi 2. The instability of Sa-type steps induced by the sample current in the step-down direction and the interaction between S, and Sb-type steps, caused biatomic step propagation during the growth. The instability of Sb-type steps induced by the sample current in the step-up direction also caused biatomic step propagation in which the narrow 1 × 2 domains moved to the right side without changing size, as shown in our previous study 3'4.

4. Conclusion We studied the sample electric current effect on the annealing process and Si-MBE growth. It was found that instability of the m o n o a t o m i c steps induced by the sample current caused selective growth of one of the two 2 x 1 domains on the surface and biatomic step growth during Si-MBE growth at high substrate temperature. Recently, Latyshev et al t5 observed sample current effect upon m o v e m e n t of atomic steps on Si(l 11) surfaces during

evaporation. Men et al L6 observed selective growth of one of the 2 x 1 domains on Si(001) surfaces caused by stress migration of Si atoms on the surface. The physics of the current and stress-induced phenomena has not been clarified yet. Studies of atomic step kinetics when external perturbations are applied, are interesting problems that should be further clarified.

References l y Enta, S Suzuki and S Kono, Phys Rev, B39, 5524 (1989). 2H Kahata and K Yagi, Japan J Appl Phys, 28, L858 (1989). 3 T Doi and M Ichikawa, J Crystal Growth, 95, 468 (1989). 4M lchikawa, Mater Sci Rep, 4, 147 (1989). 5 T Doi and M Ichikawa, Proc Ann Meet Phys Soc Japan, 2, 356 (1989). 6M Ichikawa and T Doi, Appl Phys Lett, 50, 1141 (1987). 7 M Ichikawa and T Doi, Reflection High-Energy Electron Diffraction and Reflection Electron Imaging of Surfaces, NATO ASI Series B, Vol. 188, p 343. Plenum Press, New York (1988). s D J Chadi, Phys Rev Lett, 59, 1691 (1987). 9 E Kasper, Appl Phys, A28, 129 (1982). to B Lewis and D S Campbell, J Vac Sci Technol, 4, 209 (1967). ~ T Kuroda, Private communication. t2 W K Burton, N Cabrera and F C Frank, Phil Trans R Soc, A243, 299 (1951). ~3j p Hirth and G M Pound, J Chem Phys, 26, 1216 (1956). 14j L Souchi6re and Vfi Thien Binh, Surface Sci, 168, 52 (1986). ~5A V Latyshev, A L Aseev, A B Krasilnikov and S I Stenin, Surface Sci, 213, 157 (1989). ~6 F K Men, W E Packard and M B Webb, Phys Rev Lett, 61, 2469 (1988). 937