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Formation of Si(111)3 × 3 -B and Si epitaxy on Si(111)3 × 3 -B: LEED-AES study
466
Surface Science 195 (1988)466-474 North-Holland, Amsterdam
F O R M A T I O N OF Si(lll)Cr3 x v/3"-B A N D Si EPITAXY O N Si(lll)V~" × v/3-B: L E E D - A E S S T U D Y V.V. KOROBTSOV, V.G. LIFSHITS a n d A.V. ZOTOV Institute of Automation and Control Processes, Academy of Sciences of the USSR, Vladivostok 6900:12, USSR
Received 6 May 1987; accepted for publication 5 October 1987
Accumulationof boron on the surface of hishly B-doped Si(lll) samples undergoing thermal evaporation in ultrahigh vacuum at 1250 to 1350o C and Si epitaxy on B-enriched Si surfaces have been studied by LEED and AES techniques. It has been found that boron accumulation is accompanied by structural reordering of the surface leading to the formation of an ordered Si(111)¢r3× V/3-Bsurface structure. Epitaxial growth of Si over Si(lll)v/3"X V~"was conducted in two regimes:(a) by solid phase epitaxy, i.e. via epitaxial crystallization of deposited amorphous silicon at 700-950*C and Co) by conventional molecular beam epitaxy in which silicon was deposited onto the substrate heated to 600-1000 o C. It has been found that boron redistribution during epitaxial growth is determined by bulk diffusion for solid phase epitaxy and by surface segregation for conventional molecularbeam epitaxy.
1. Introduction The annealing of silicon samples at elevated temperatures is known to result in considerable redistribution of impurities within the sample. In the case of boron-doped silicon, the high-temperature annealing leads to accumulation of boron in a surface layer in which the boron concentration is several orders of magnitude higher than that in the bulk. The k n o w n iiterature data [1,2] concern the annealing of silicon with boron concentration below 1018 c m -3 doping level. The thermal behavior of highly B-doped silicon is of interest since accumulation of boron in considerable amounts at the sample surface may result in noticeable surface reconstructions. The results of the present paper show that accumulation of boron at the Si(111) surface during annealing of silicon which contains b o r o n at a concentration of 2 × 1019 c m - 3 results in formation of a Si(lll)v~" × vt3--B surface structure. The second object of this paper was the investigation of Si epitaxy on the Si(lll)~/3" x ¢~'-B surface. The main attention was focused on the study of the evolution of surface structure and boron redistribution during epitaxial growth. 0039-6028/88/$03.50 © Elsevicz Science Publishers B.V. (North-Holland Physics Publishing Division)
V. V. Korobtsov et ai. / L E E D - A E S study of Si( l l l )vt3 × ~I3-B
467
2. Experimental Annealing of the samples and experiments on epitaxial growth were carried out in LAS-600 and DEL-300 Riber ultrahigh-vacuum systems with a base pressure of 2 × 10-10 Ton'. The samples used were rectangular wafers (15 × 4 × 0.4 mm3) of 0.005 f~ cm boron-doped Si(lll). The samples were heated by passing electrical current through them. The sample temperature was measured by an optical pyrometer for temperatures above 800* C. The sample temperatures below 800"C were determined by a thermoeouple attached to the back side of the sample. The error in the measurements associated with removal of heat through the thermocouple was taken into account. The value of this error as a function of sample temperature was determined in calibrating experiments according to the compensation technique described in ref. [3]. Silicon films were deposited from a sublimation source (rectangular silicon wafer of 40 f~ cm B-doped Si resistively heated at 1350" C). The deposition rate was 180 A/rain, the pressure in the vacuum chamber during deposition was below 8 × 10 -1° Torr. Low-energy electron diffraction (LEED) and Auger electron spectroscopy (AES) were used to characterize the structure and composition of the sample surfaces during annealing and epitaxial growth. The boron surface concentration was estimated from AES measurements corrected for elemental sensitivities [41.
3. Results and discussion
3.1. Formation of Si(111)Vc3 × ~ - B surface structure The evaporation rate of boron atoms from a silicon surface is known to be considerably lower than that of silicon atoms. The ratio of rates is about 1:100 according to the data of ref. [5]. This leads to the accumulation of boron at the surface of sublimated B-doped silicon. In the present work, the boron accumulation at the surface of 0.005 ~ cm boron-doped Si(1!1) during annealing at 1250 to 1350 °C was directly monitored by the growth of the B KLL peak (179 eV) in the Auger spectra. Simultaneously the evolution of the surface structure was controlled by LEED observations. Combined LEED-AES data (fig. I) revealed that the. sample ~,urfac~ un~,so~s suu~tuxtu transfo~ations during surface accumulation of boron. The sample cleaned by short annealing ( T = 1200 o C, t = 3 rnin) exhibited a sharp Si(lll)7 × 7 LEED pattern typical for a clean (111) silicon surface and AES indicated no detectable contaminations (including boron) at the surface. However after the first minutes of annealing at temperatures above 1250~C the B KLL peak exceeded the noise level. The boron accumulation at the
,168
It'. V. Korobtsov et at / L E E D - A E S study
of m ( l n ) ~ x dY-B ((3
{
..e
~(
t xl )
0.04 0.02 0
1oo
200 300 onnealincj time, rain.
Fig. 1. Evolution of surface structure and composition of 0.005 [~ cm B-doped ~i(111) sample during annealing at 1275 ° C according to LEED-AES data: (a) intensities o f . ,,o~:tion spots corresponding to different LEED patterns versus annealing time; (b) boro~ conce,,.ation at the surface versus annealing time.
surface was accompanied by the decrease of intensities of extra-spots in the (7 x 7) LEED pattern and, at a boron concentration (Ca) of about 1.3 at%, a (1 × 1) LEED pattern was observed. The further increase of B concentration resulted in (1 × 1) to (¢~-× Vc3) structural transformation of the sample surface. The faint diffuse extra-spots of the (¢~ x ¢~-) structure could be seen in the LEED pattern at C B --1.5 at% and their intensities grew with the growth of boron surface concentration. The increase of 1/V~'-extra-spot intensities was observed up to the greatest boron concentrations at the surface achieved in these experiments (C B = 15 at%), the intensities of extra-spots becoming comparable with that of the main spots. Fig. 2 shows typical LEED patterns observed during annealing. It should be pointed out that, at all stages of annealing, there was no indication of dements other than Si and B on the surface at the highest AES sensitivity. Therefore, vve consider that surface reordering was caused by boron accumulation at the smface. AES analysis combined with sputtering of the sample by 5 k,~v ar~on ions revealed that boron is accumulated not only at the surface but in a rather thick (several hundred A,) near-surface region. Fig. 3 shows schematically a possible structure formed by boron atoms with lattice period ~ times that of the two-dimensional lattice of the silicon (111) plane. One can see that the structure shown in fig. 3 has an elemental composition of CB : Csi = 1 : 6, i.e., B concentration CB = 0.14. The obtained
V.V. Korobtsov et aL / LEED-AES study of Si( l l l )~[3 X ~/3-B
469
Fig. 2. LEED patterns observed during annealing: (a) Si(lll)7 x7; (b) Si(lll)l x 1; (c) Si(111)¢r3 x
Ountce11un c ell ® Si a t o m s
o B atoms
Fig. 3. Hypothetic Si(lll)vr3 x vf3-B structure formed by B atoms on (111)Si surface. Note that ~11 Si atoms of Si(lll) double layer are shown.
470
KV. Korobtsov et aL / LEED-AES study of Si(ll l)¢r3 x ¢r3-B
value agrees with our AES data. As for the Si(111)1 × 1 structure seen at the initial period of annealing, it is unlikely to be formed by boron atoms since the observed boron concentration corresponding to S i ( l l l ) l × 1 is obviously insufficient to form a structure with such period. Thus, the S i ( l l l ) l × 1 LEED pattern is believed to indicate the loss of the long-range order at boron concentrations exceeding the equilib~um solubility limit.
3.2. Si epitaxy on Si(l ll)vr3 X ~ - B surface In the present paper, the epitaxial growth was conducted in two regimes: by solid phase epitaxy (SPE regime) and by conventional molecular beam epitaxy (conventional MBE regime).
3.2.1. SPE regime After the formation of a Si(111)¢r3" × V~'-B surface structure with boron concentration of about 8 at%, the deposition of silicon was carried out maintaining the ultrahigh vacuum. During deposition the substrate temperature was about 150 ° C due to the heat radiated from the sublimation source. The films obtained by such deposition had an amorphous structure as revealed by LEED and transmission electron microscopy. The deposited amorphous films were crystallized in situ at 700 to 950 *C by passing electrical current through the sample. Immediately after the crystallization, the film surface exhibited a (7 × 7) LEED pattern with no indication of boron according to AES. If the annealing was continued the accumulation of boron at the surface accompanied by surface reordering from (7 x 7) through (1 × 1) to (¢r~ X ¢~) structure was detected by LEED-AES observations. Fig. 4 shows the annealing time in which the B surface concentration reaches the value of 1.3 at% and a (7 × 7) to (1 × 1) transition takes place versus the square of film thickness (d 2) at different annealing temperatures. The parabolic type of the dependences observed is believed to indicate the diffusion w_echanism of boron redistribution during annealing. In order to estimate the diffusion constants from these experimental data we use the solution of a well-known problem in classical diffusion, namely, the diffusion into semi-infinite space from the surface with constant impurity concentration:
C(x, t)= Co erfc(x/2v/-D~-),
(1)
where Co is the concentration at the surface and D is the diffusion coefficient. Inserting into eq. (1) Co = 0.08 and C(d, t) = 0.013 gives:
D=d2/3.6t.
(2)
V, V. Korobtsov et al. / LEED-AES study of Si(l 1 l)vf3 x V~-B
471
I 840°C
".24oo ,¢..
T 8800C 1200
05°C 9450C a-.--"r--
01
I
I
2345
I
i
t
67 d =, ,to-Iz cm2
Fig. 4. Formation time of (1 x 1) structure versus square of r;!m thickness for different annealing temperatures.
The diffusion coefficient estimated according to eq. (2) from the data of fig. 4 was found to be (fig. 5): D = 18 e x p ( - 3,,7 e V / k r )
(cm s-l).
(3)
The obtained constants of boron diffusion in silicon agree well with known data determined at elevated temperatures of 1100 to 1350°C (Do = 10.5-25
900 I
1(]14
T, °G 850 I
0
$ E (.3
c; 16ss
8.2
8.4
8.6 8.8 9.0 404"/T, K-I
Fig. 5. Temperature dependence of diffusion coefficient of boron in silicon determined from data of fig. 4.
472
V. V, Korobtsov et aL / L E E D - A E S
study of $i011)~[3 X ~f3-B
cm 2 s -t, E = 3.51-3.69 eV) [6]. It should be pointed out that this coincidence of data is observed in spite of the fact that the calculation model used in this work is rather rough and differs essentially from the real situation. Firstly, the model describes the diffusion into the semi-infinite space while, in the experiment, the diffusion was limited by the outer surface of the film where boron accumulation takes place. Secondly, in the model, boron concentration at the film/substrate interface is considered to be constant while it should obvim,sl~ diminish during diffusion. The first inexactitude of the model tends to overestimate the value of the diffusion coefficient and the second one to underestimate it. The inexactitudes seem to compensate each other (or both of them are rather small) resulting in reasonable values of diffusion constants.
3.2.2. Conventional MBE regime Deposition of silicon was carried out onto the Si(lll)v~ × V~-B substrate held at temperatures ranging from 600 to 1000 * C. Experiments revealed that epitaxial growth in conventional MBE regime is accompanied by surface segregation of boron. Fig. 6 shows the dependence of boron concentration at the surface on the thickness of epitaxial film deposited onto the substrate held at 730 ° C. One can see that surface concentration decreases gradually with the growth of the film. As a result the surface structure transforms from (V~ x V~-) to (1 x 1) and then to (7 x 7) typical for the clean o~dered (111) silicon surface. A similar dependence of boron surtace concentration on MBE-grown film thickness was observed for all growth temperatures studied. The difference
to°
MBE REGIME
0.10
T= 730°C
0.05
o
5oo
1ooo
d,.~ ,~5oo
I " - ' - ( ~ x ~1~).--~,11111,4,,. ( I x t ) - ~ 7 / ) P - ( 7 " 7 ) . - ~
Fig. 6. Boron concentration at the surface versus thickness of MBE-grown film. The diagram below the plot shows the evolution of LEED pattern. Deposition temperature is 730°C, depo¢ition rate is 180 ~/rnin.
V. V. Korobtsoo et aL / L E E D - A E S study of Si( l l l )vf3 X ¢r~.S
rj
S
103 o~
473
10 2
J I
600
i
I
I
,
800
i
1000 T, °C
Fig. 7. Smearing of concentration profile versus deposition temperature.
was in the rate of decrease of boron concentration at the surface and thus in the value of smearing of concentration profile. Fig. 7 shows smearing of the boron concentration profile as a function of deposition temperature in the range from 600 to 1000°C. The value of smearing ( ~ d ) is presented by the thickness of the grown film when the boron concentration at the surface drops to 1.5 at~ and a (1 × 1) LEED pattern forms. This corresponds to a decrease by about a factor 7 of boron concentration compared with the initial concentration at the substrate surface. One can see in fig. 7 that the smearing width remains constant at temperatures ranging from 800 to 1000°C but diminishes by about two orders of magnitude with the decrease of temperature from 800 to 600 ° C. The decrease of smearing is believed to be associated with kinetical limitations for boron segregation at lower temperatures. The estimation yields that, at temperatures below 800 ° C, the boron diffusion length 2v~-t for the time of deposition of one silicon monolayer ( - 1 s) becomes less than the interatomic distance.
4. Conclusions
In this paper we have presented new data for the thermal annealing behavior of highly-doped Si:B samples and Si epitaxy at B-enriched Si surfaces. These data show that: (1) boron accumulation at the (111) surface of highly-doped Si:B during high-temperature annealing leads to the structural reordering of the surface resulting in formation of an ordered Si(lll)v~ x ¢~-B surface structure;
474
V. IX. Korobtsov et al. / LEED-AES study of Si(l 11)V~ × ~I3-B
(2) boron redistribution in solid phase epitaxial growth of a Si film on Si(111)¢r3 × v~-B is determined by bulk boron diffusion solely; (3) constants of boron diffusion in silicon (Do= 18 cm 2 s -1, E - 3 . 7 eV) estimated in SPE experiments in the 850 to 950°C range agree well with known literature data determined at elevated temperatures of 1100 to 1350 o C; (4) epitaxial growth of a Si film on Si(111)~/3 × v~-B in conventional MBE regime is accompanied by surface segregation of boron.
Acknowledgement
The authors gratefully acknowledge V.B. Akilov, B.K. Churusov and I.G. Kaverina for their assistance in the experiments.
References [1] L.N. Aleksandrov, R.N. Lovyagin and L.N. Safronov, Appl. Surface Sci. 11/12 (1982) 353. [2] L.N. Aleksandrov, Kinetics of Crystallization and Recrystallization of Semiconductor Films (Nauka, Novosibirsk, 1985) p. 64 (in Russian). [3] V. Pak and Yu.P. Krinsky, Prib. Sist. Upr. 7 (1978) 23. [4] L.E. Davis, N.C. MacDonald, C.W. Palmberg, H.E. Riach and R.E. Weber, Handbook of Auger Electron Spectroscopy (Physical Electronics, Eden Prairie, MN, 1976) p. 5. [5] V.A. Tolomasov, V.V. Vaskin, M.I. Ovsyannikov, R.G. Loginova and R.A. Rubtsova, Fiz. Tekh. Poluprovodn. 15 (1981) 104. [6] A.M. Smith, in: Fundamentals of Silicon Integrated Device Technology, Vol. 1, Eds. R.M. Burger and R.P. Donovan (Prentice-Hall, Englewood Cliffs, NJ, 1967) (Translation: Mir, Moscow, 1969, p. 214).
Surface Science 195 (1988)466-474 North-Holland, Amsterdam
F O R M A T I O N OF Si(lll)Cr3 x v/3"-B A N D Si EPITAXY O N Si(lll)V~" × v/3-B: L E E D - A E S S T U D Y V.V. KOROBTSOV, V.G. LIFSHITS a n d A.V. ZOTOV Institute of Automation and Control Processes, Academy of Sciences of the USSR, Vladivostok 6900:12, USSR
Received 6 May 1987; accepted for publication 5 October 1987
Accumulationof boron on the surface of hishly B-doped Si(lll) samples undergoing thermal evaporation in ultrahigh vacuum at 1250 to 1350o C and Si epitaxy on B-enriched Si surfaces have been studied by LEED and AES techniques. It has been found that boron accumulation is accompanied by structural reordering of the surface leading to the formation of an ordered Si(111)¢r3× V/3-Bsurface structure. Epitaxial growth of Si over Si(lll)v/3"X V~"was conducted in two regimes:(a) by solid phase epitaxy, i.e. via epitaxial crystallization of deposited amorphous silicon at 700-950*C and Co) by conventional molecular beam epitaxy in which silicon was deposited onto the substrate heated to 600-1000 o C. It has been found that boron redistribution during epitaxial growth is determined by bulk diffusion for solid phase epitaxy and by surface segregation for conventional molecularbeam epitaxy.
1. Introduction The annealing of silicon samples at elevated temperatures is known to result in considerable redistribution of impurities within the sample. In the case of boron-doped silicon, the high-temperature annealing leads to accumulation of boron in a surface layer in which the boron concentration is several orders of magnitude higher than that in the bulk. The k n o w n iiterature data [1,2] concern the annealing of silicon with boron concentration below 1018 c m -3 doping level. The thermal behavior of highly B-doped silicon is of interest since accumulation of boron in considerable amounts at the sample surface may result in noticeable surface reconstructions. The results of the present paper show that accumulation of boron at the Si(111) surface during annealing of silicon which contains b o r o n at a concentration of 2 × 1019 c m - 3 results in formation of a Si(lll)v~" × vt3--B surface structure. The second object of this paper was the investigation of Si epitaxy on the Si(lll)~/3" x ¢~'-B surface. The main attention was focused on the study of the evolution of surface structure and boron redistribution during epitaxial growth. 0039-6028/88/$03.50 © Elsevicz Science Publishers B.V. (North-Holland Physics Publishing Division)
V. V. Korobtsov et ai. / L E E D - A E S study of Si( l l l )vt3 × ~I3-B
467
2. Experimental Annealing of the samples and experiments on epitaxial growth were carried out in LAS-600 and DEL-300 Riber ultrahigh-vacuum systems with a base pressure of 2 × 10-10 Ton'. The samples used were rectangular wafers (15 × 4 × 0.4 mm3) of 0.005 f~ cm boron-doped Si(lll). The samples were heated by passing electrical current through them. The sample temperature was measured by an optical pyrometer for temperatures above 800* C. The sample temperatures below 800"C were determined by a thermoeouple attached to the back side of the sample. The error in the measurements associated with removal of heat through the thermocouple was taken into account. The value of this error as a function of sample temperature was determined in calibrating experiments according to the compensation technique described in ref. [3]. Silicon films were deposited from a sublimation source (rectangular silicon wafer of 40 f~ cm B-doped Si resistively heated at 1350" C). The deposition rate was 180 A/rain, the pressure in the vacuum chamber during deposition was below 8 × 10 -1° Torr. Low-energy electron diffraction (LEED) and Auger electron spectroscopy (AES) were used to characterize the structure and composition of the sample surfaces during annealing and epitaxial growth. The boron surface concentration was estimated from AES measurements corrected for elemental sensitivities [41.
3. Results and discussion
3.1. Formation of Si(111)Vc3 × ~ - B surface structure The evaporation rate of boron atoms from a silicon surface is known to be considerably lower than that of silicon atoms. The ratio of rates is about 1:100 according to the data of ref. [5]. This leads to the accumulation of boron at the surface of sublimated B-doped silicon. In the present work, the boron accumulation at the surface of 0.005 ~ cm boron-doped Si(1!1) during annealing at 1250 to 1350 °C was directly monitored by the growth of the B KLL peak (179 eV) in the Auger spectra. Simultaneously the evolution of the surface structure was controlled by LEED observations. Combined LEED-AES data (fig. I) revealed that the. sample ~,urfac~ un~,so~s suu~tuxtu transfo~ations during surface accumulation of boron. The sample cleaned by short annealing ( T = 1200 o C, t = 3 rnin) exhibited a sharp Si(lll)7 × 7 LEED pattern typical for a clean (111) silicon surface and AES indicated no detectable contaminations (including boron) at the surface. However after the first minutes of annealing at temperatures above 1250~C the B KLL peak exceeded the noise level. The boron accumulation at the
,168
It'. V. Korobtsov et at / L E E D - A E S study
of m ( l n ) ~ x dY-B ((3
{
..e
~(
t xl )
0.04 0.02 0
1oo
200 300 onnealincj time, rain.
Fig. 1. Evolution of surface structure and composition of 0.005 [~ cm B-doped ~i(111) sample during annealing at 1275 ° C according to LEED-AES data: (a) intensities o f . ,,o~:tion spots corresponding to different LEED patterns versus annealing time; (b) boro~ conce,,.ation at the surface versus annealing time.
surface was accompanied by the decrease of intensities of extra-spots in the (7 x 7) LEED pattern and, at a boron concentration (Ca) of about 1.3 at%, a (1 × 1) LEED pattern was observed. The further increase of B concentration resulted in (1 × 1) to (¢~-× Vc3) structural transformation of the sample surface. The faint diffuse extra-spots of the (¢~ x ¢~-) structure could be seen in the LEED pattern at C B --1.5 at% and their intensities grew with the growth of boron surface concentration. The increase of 1/V~'-extra-spot intensities was observed up to the greatest boron concentrations at the surface achieved in these experiments (C B = 15 at%), the intensities of extra-spots becoming comparable with that of the main spots. Fig. 2 shows typical LEED patterns observed during annealing. It should be pointed out that, at all stages of annealing, there was no indication of dements other than Si and B on the surface at the highest AES sensitivity. Therefore, vve consider that surface reordering was caused by boron accumulation at the smface. AES analysis combined with sputtering of the sample by 5 k,~v ar~on ions revealed that boron is accumulated not only at the surface but in a rather thick (several hundred A,) near-surface region. Fig. 3 shows schematically a possible structure formed by boron atoms with lattice period ~ times that of the two-dimensional lattice of the silicon (111) plane. One can see that the structure shown in fig. 3 has an elemental composition of CB : Csi = 1 : 6, i.e., B concentration CB = 0.14. The obtained
V.V. Korobtsov et aL / LEED-AES study of Si( l l l )~[3 X ~/3-B
469
Fig. 2. LEED patterns observed during annealing: (a) Si(lll)7 x7; (b) Si(lll)l x 1; (c) Si(111)¢r3 x
Ountce11un c ell ® Si a t o m s
o B atoms
Fig. 3. Hypothetic Si(lll)vr3 x vf3-B structure formed by B atoms on (111)Si surface. Note that ~11 Si atoms of Si(lll) double layer are shown.
470
KV. Korobtsov et aL / LEED-AES study of Si(ll l)¢r3 x ¢r3-B
value agrees with our AES data. As for the Si(111)1 × 1 structure seen at the initial period of annealing, it is unlikely to be formed by boron atoms since the observed boron concentration corresponding to S i ( l l l ) l × 1 is obviously insufficient to form a structure with such period. Thus, the S i ( l l l ) l × 1 LEED pattern is believed to indicate the loss of the long-range order at boron concentrations exceeding the equilib~um solubility limit.
3.2. Si epitaxy on Si(l ll)vr3 X ~ - B surface In the present paper, the epitaxial growth was conducted in two regimes: by solid phase epitaxy (SPE regime) and by conventional molecular beam epitaxy (conventional MBE regime).
3.2.1. SPE regime After the formation of a Si(111)¢r3" × V~'-B surface structure with boron concentration of about 8 at%, the deposition of silicon was carried out maintaining the ultrahigh vacuum. During deposition the substrate temperature was about 150 ° C due to the heat radiated from the sublimation source. The films obtained by such deposition had an amorphous structure as revealed by LEED and transmission electron microscopy. The deposited amorphous films were crystallized in situ at 700 to 950 *C by passing electrical current through the sample. Immediately after the crystallization, the film surface exhibited a (7 × 7) LEED pattern with no indication of boron according to AES. If the annealing was continued the accumulation of boron at the surface accompanied by surface reordering from (7 x 7) through (1 × 1) to (¢r~ X ¢~) structure was detected by LEED-AES observations. Fig. 4 shows the annealing time in which the B surface concentration reaches the value of 1.3 at% and a (7 × 7) to (1 × 1) transition takes place versus the square of film thickness (d 2) at different annealing temperatures. The parabolic type of the dependences observed is believed to indicate the diffusion w_echanism of boron redistribution during annealing. In order to estimate the diffusion constants from these experimental data we use the solution of a well-known problem in classical diffusion, namely, the diffusion into semi-infinite space from the surface with constant impurity concentration:
C(x, t)= Co erfc(x/2v/-D~-),
(1)
where Co is the concentration at the surface and D is the diffusion coefficient. Inserting into eq. (1) Co = 0.08 and C(d, t) = 0.013 gives:
D=d2/3.6t.
(2)
V, V. Korobtsov et al. / LEED-AES study of Si(l 1 l)vf3 x V~-B
471
I 840°C
".24oo ,¢..
T 8800C 1200
05°C 9450C a-.--"r--
01
I
I
2345
I
i
t
67 d =, ,to-Iz cm2
Fig. 4. Formation time of (1 x 1) structure versus square of r;!m thickness for different annealing temperatures.
The diffusion coefficient estimated according to eq. (2) from the data of fig. 4 was found to be (fig. 5): D = 18 e x p ( - 3,,7 e V / k r )
(cm s-l).
(3)
The obtained constants of boron diffusion in silicon agree well with known data determined at elevated temperatures of 1100 to 1350°C (Do = 10.5-25
900 I
1(]14
T, °G 850 I
0
$ E (.3
c; 16ss
8.2
8.4
8.6 8.8 9.0 404"/T, K-I
Fig. 5. Temperature dependence of diffusion coefficient of boron in silicon determined from data of fig. 4.
472
V. V, Korobtsov et aL / L E E D - A E S
study of $i011)~[3 X ~f3-B
cm 2 s -t, E = 3.51-3.69 eV) [6]. It should be pointed out that this coincidence of data is observed in spite of the fact that the calculation model used in this work is rather rough and differs essentially from the real situation. Firstly, the model describes the diffusion into the semi-infinite space while, in the experiment, the diffusion was limited by the outer surface of the film where boron accumulation takes place. Secondly, in the model, boron concentration at the film/substrate interface is considered to be constant while it should obvim,sl~ diminish during diffusion. The first inexactitude of the model tends to overestimate the value of the diffusion coefficient and the second one to underestimate it. The inexactitudes seem to compensate each other (or both of them are rather small) resulting in reasonable values of diffusion constants.
3.2.2. Conventional MBE regime Deposition of silicon was carried out onto the Si(lll)v~ × V~-B substrate held at temperatures ranging from 600 to 1000 * C. Experiments revealed that epitaxial growth in conventional MBE regime is accompanied by surface segregation of boron. Fig. 6 shows the dependence of boron concentration at the surface on the thickness of epitaxial film deposited onto the substrate held at 730 ° C. One can see that surface concentration decreases gradually with the growth of the film. As a result the surface structure transforms from (V~ x V~-) to (1 x 1) and then to (7 x 7) typical for the clean o~dered (111) silicon surface. A similar dependence of boron surtace concentration on MBE-grown film thickness was observed for all growth temperatures studied. The difference
to°
MBE REGIME
0.10
T= 730°C
0.05
o
5oo
1ooo
d,.~ ,~5oo
I " - ' - ( ~ x ~1~).--~,11111,4,,. ( I x t ) - ~ 7 / ) P - ( 7 " 7 ) . - ~
Fig. 6. Boron concentration at the surface versus thickness of MBE-grown film. The diagram below the plot shows the evolution of LEED pattern. Deposition temperature is 730°C, depo¢ition rate is 180 ~/rnin.
V. V. Korobtsoo et aL / L E E D - A E S study of Si( l l l )vf3 X ¢r~.S
rj
S
103 o~
473
10 2
J I
600
i
I
I
,
800
i
1000 T, °C
Fig. 7. Smearing of concentration profile versus deposition temperature.
was in the rate of decrease of boron concentration at the surface and thus in the value of smearing of concentration profile. Fig. 7 shows smearing of the boron concentration profile as a function of deposition temperature in the range from 600 to 1000°C. The value of smearing ( ~ d ) is presented by the thickness of the grown film when the boron concentration at the surface drops to 1.5 at~ and a (1 × 1) LEED pattern forms. This corresponds to a decrease by about a factor 7 of boron concentration compared with the initial concentration at the substrate surface. One can see in fig. 7 that the smearing width remains constant at temperatures ranging from 800 to 1000°C but diminishes by about two orders of magnitude with the decrease of temperature from 800 to 600 ° C. The decrease of smearing is believed to be associated with kinetical limitations for boron segregation at lower temperatures. The estimation yields that, at temperatures below 800 ° C, the boron diffusion length 2v~-t for the time of deposition of one silicon monolayer ( - 1 s) becomes less than the interatomic distance.
4. Conclusions
In this paper we have presented new data for the thermal annealing behavior of highly-doped Si:B samples and Si epitaxy at B-enriched Si surfaces. These data show that: (1) boron accumulation at the (111) surface of highly-doped Si:B during high-temperature annealing leads to the structural reordering of the surface resulting in formation of an ordered Si(lll)v~ x ¢~-B surface structure;
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V. IX. Korobtsov et al. / LEED-AES study of Si(l 11)V~ × ~I3-B
(2) boron redistribution in solid phase epitaxial growth of a Si film on Si(111)¢r3 × v~-B is determined by bulk boron diffusion solely; (3) constants of boron diffusion in silicon (Do= 18 cm 2 s -1, E - 3 . 7 eV) estimated in SPE experiments in the 850 to 950°C range agree well with known literature data determined at elevated temperatures of 1100 to 1350 o C; (4) epitaxial growth of a Si film on Si(111)~/3 × v~-B in conventional MBE regime is accompanied by surface segregation of boron.
Acknowledgement
The authors gratefully acknowledge V.B. Akilov, B.K. Churusov and I.G. Kaverina for their assistance in the experiments.
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