- Email: [email protected]
Antimony adsorption on silicon
Surface Science 137 (1984) 397-411 North-Holland, Amsterdam
ANTILOG
A~SOR~~ON
R.A. METZGER
397
ON SILICON *
and F.G. ALLEN
~[eetrical Engineering Department, U~i~ers~iyuj Cdifomia, Los Angeles, Cul~ornia 90024, U.S.4 Received
9 August
1983; accepted
for publication
26 October
1983
The kinetic behavior of antimony films adsorbed on clean silicon surfaces has been measured versus temperature and coverage using LEED, Auger and thermal desorption. Sb adsorbs in two distinct modes, seen in a high and a low temperature desorption peak. The high temperature peak fills up to one monolayer on the Si(ll1) surface with a desorption rate K, given by K, = K,o = with XI, = 1.5 x 109 s-l, E, = 2.46 eV, and IV,, = Sb coverage. Growth of eWE,/kTW,s, this phase is tw~dimensionai. Higher Sb coverage adsorbs in the low temperature peak as bulk Sb on Sb with E, = 1.49 eV and grows without population limit in a three-dimensiona mode. The residence time of Sb atoms on the surface, 7 = Kg’, varies from 10’ to 10 s over the temperature rang 600 to 950°C; this determines transient smearing during MBE growth of Si.
1. ~n~~uetion During molecular beam epitaxial (MBE) growth of silicon using evaporative antimony doping, we find that inco~oration of Sb proceeds in a two-step process: Sb atoms first adsorb in a surface layer with a finite residence time, and then either desorb again or incorporate into the growing crystal. We have described the inco~oration process for the growing crystal and the resultant doping effects elsewhere [I]. This paper describes our findings on the adsorption-desorption kinetics of these Sb films. We have made measurements in two regimes: (1) room temperature and above with no incident silicon flux, and (2) MBE growth temperatures above 600°C with an incident silicon flux of - 0.25 monolayer/s. In general, we find that: (1) the rate of desorption greatly exceeds that of incorporation during MBE growth, (2) the Sb adsorption-deso~tion process is unaffected by the presence of the Si flux except at high Sb flux and low substrate temperatures, and (3) Sb residence times are long enough to cause serious “smearing” of doped profiles below 750°C [l]. Some previous data on Sb-Si adsorption is available in the MBE studies reported by Bean [2], Koenig et al. [3,4], and Tabe and Kajiyama (5). Tabe and * Supported
in part by grant No. ECS-00227
from the National
0039-6028/84/$03.00 0 Elsevier Science Publishers (North-Holland Physics Publishing Division)
Science Foundation.
B.V.
398
R.A. Metager, FG. Allen / Antmony
ad.rorption on siiicon
Kajiyama found that Sb adsorbs on Si in two distinct phases: a high temperature phase which can accommodate approximately one monolayer and a lower temperature phase which forms only below 440°C after the high temperature phase is complete, and which can accommodate an unlimited population of Sb at room temperature. Our studies bear out Tabe and Kajiyama’s general findings and describe the kinetics of this behavior in quantitative terms. We find many similarities in this system to other metal-simiconductor systems that have been studied, particularly that of bismuth on silicon [6,7]. We report here techniques applicable to the Sb-Si system of particular usefulness for adsorption-desorption studies which are readily carried out in MBE systems.
2. Experimental apparatus Our Si MBE system consists of a modified stainless steel Varian station with vat-ion pumps and titanium gettering and is capable of sustaining a base pressure of 5 X lo- ” Torr. Surface analyses include low energy electron diffraction (LEED), Auger electron spectroscopy (AES) and quadrupole mass spectroscopy used for both detection in desorption studies and for residual gas analysis. Silicon is evaporated from a molten pool of pure Si surrounded by solid silicon in a molybdenum crucible by means of a 6 kW electron bombardment gun. The Sb beam is generated in a resistively heated pyrolitic boron nitride effusion cell. The electron gun and effusion cells are housed in a liquid nitrogen shroud. Typical operating pressure during Si growth is 3 X lo-’ Torr. Samples are inserted into the chamber via a load-lock with sample insertion time from atmospheric pressure to start of run on the order of one hour. The silicon substrate is held between molybdenum clips and is resistively heated. Atomically clean starting surfaces as determined by LEED patterns are typically produced by heating the Si substrate to 1200°C for 1 to 2 min. Auger analysis has disclosed no surface impurities other than some residual carbon, sometimes present at a small fraction of one monolayer. Sb doping during MBE growth is carried out at substrate temperatures over the range of 600 to 900°C.
3. Model for adaption-deso~tion 3. I. Differential
kinetics
equation for N,,
We now develop a kinetic model describing Sb behavior during Si MBE growth in which the incident Sb flux F is entrapped on the Si surface with a mean residence time of 7. The entrapped Sb atoms may then either desorb or
R.A. Metzger, F.G. AI/en / Antimony adsorption on silicon
399
incorporate into the growing film. There is accumulated on the surface a layer of adsorbed dopant atoms of density No, (atoms/cm*). This layer acts as a reservoir that produces doping in the growing crystal. We write a kinetic equation for this system as: dN,s/di
= F-
K,N;,
- K,N&.
(1)
s), I(, and K, are the desorption and F is the incident Sb flux (atoms/cm2. incorporation coefficients and p and q are the desorption and incorporation represents the incorporating Sb which produces orders. The term KIN& doping and the term K,N& represents the desorbing Sb. We now anticipate the result developed below that p = 1, and we use the result derived from our doped film studies that q = 1 to illustrate sticking and transient behaviour for Sb on Si. Using p = 1, q = 1, we proceed as follows: The sticking coefficient S is defined as the fraction of incident Sb incorporated into the growing film, S = K,N,s/F. Under
(2)
steady state conditions
F = K,N,,
dNo,/dt
= 0 and eq. (1) gives
+ K,N,,.
(3)
Substituting eq. (3) into eq. (2) results in an expression steady state conditions S=
K,
K, No, KEN,,
for S measured
+ K,N,,
= K, + K,
’
under
(4)
3.2. Transient behavior It was found 900°C S was
in the doping study [l] that over the temperature range 700 to < 1, so eq. (4) implies that K, +z K,. For K, GE K, and (an initial Sb surface concentration at the beginning of Si N,,( f = 0) = N,,, growth) eq. (1) can be solved for N,,(t):
No&) = Nosoe-K,r
x”,(
+ -
1 -
,-W)_
The resulting doping in the film, NDB(t), is equal to the incorporation KIN,,(t), divided by the silicon growth rate, u, in cm/s,
As c goes to infinity N oH,, = K,F/vK,
(5) rate,
and steady state is reached, = SF/v.
(6b)
400
R.A. Meirger, F.G. Allen / Antimory
Therefore, where
doping
profiles
will show transients
adsorprron on srlrcon
with a characteristic
7 = l/K,.
lifetime
7.
(7)
The present work is undertaken to determine coefficient K, through surface studies. 3.3. Determination
the properties
of p and K,
Under the condition that K, < K,, which was found this work, eq. (I) becomes, for p not yet determined, = F-
dN,,/dt
of the desorption
K,N&.
to hold throughout (8)
We now present a method to solve eq. (8) in order to determine both p and K,. The method used requires the measurement of N&T, F). The Sb flux F is calibrated by direct measurement of Sb fifms grown on Si room temperature substrates with the resultant thickness measured with an in-situ thickness monitor calibrated later by an interferometer measurement of the film. With a clean Si substrate at room temperature a known amount of Sb is deposited, N,,
= Ft*,
(9)
where t* is the deposition time and F the calibrated flux. No, is measured by placing the Si substrate in direct line of site with the mass spectrometer (distance 10 cm) and flashing the Si substrate from room temperature to 1200°C in approximately 10 s. This desorbs all of the Sb. The resultant Sb signal from the mass spectrometer is I(t). We define the experimentally measured area under the flash-off curve as N;,
=
I
I(r)
The actual determined
dt. surface coverage factor cy given by
N IX =cwN*DS =Ft*.
N,,
is related
to N&
by an experimentally
(11)
Over the full range of ND, values covered, cy was constant to within lo%, the deviation being due to a slight nonlinearity in the mass spectrometer response to a sudden burst of gas. Some small fraction of the Sb may have evaporated as multi-atomic molecules, though we failed to detect these in the mass spectrometer. Variations in this fraction would affect ty. This method has been utilized to resolve Sb layers of as little as 10M4 monolayer. This measurement method permits the determination of K, and p as follows: From eq. (8) under steady state conditions F=
KDN&,
K, = K,,
(12) ,-hi~~-,
(13)
R.A. Metrger,
F.G. Allen / Antimoy
401
adsorption on silicon
K,, corresponds to a vibrational frequency and E, is the desorption energy. We will make the assumption that K,, and p are independent of temperature. The desorption order p is determined by keeping the substrate temperature (and K,) constant, so that eqs. (12) and (13) give where
K,
= F/N&
= constant.
(14)
We then vary Fat constant temperature, measure No, by the flash/integration method, and determine p from the resulting relation of F to No,. Eq. (14) implies that N,, has reached its steady state value. This steady condition is achieved by leaving the heated substrate in the Sb beam for lengthening intervals of time while measuring No, periodically after each interval. Steady state has been reached when No, does not increase for lengthening times in the Sb beam. K, can then be determined under steady state conditions by varying the substrate temperature T under a constant Sb flux F such that K,
= K,,
ePEDjkT = F/N&.
(15)
The flux F is calibrated and No, is measured by the flash/integration Taking the natural logarithm of both sides of eq. (15) we have In K, = ln( F/N&) Plotting ln( K,,)
= In K,,
method.
- E,/kT.
ln( K,) versus l/T should then give a straight line with an intercept and a slope of -E&k, if the model is valid.
(16)
of
4. Experimental results Sb kinetics have been studied on all three low index silicon surface orientations: (ill), (loo), and (110). Since the behaviour of Sb on all three orientations is similar, a detailed analysis on the (111) orientation will be presented followed by summaries of (100) and (110) behavior. Before detailed analysis of K, and p can be attempted, it is necessary to find the number of Sb binding sites on the silicon surface and to determine which sites are involved during adsorption. This is done by a combination of thermal desorption spectroscopy, Auger plots, and LEED patterns. In all three methods, all Si surfaces receive a chemical cleaning before insertion into the station and a thermal cleaning in vacuum (1 to 2 min at 1200°C) in order to achieve a sharp characteristic LEED pattern and Auger spectrum, before surfaces are studied. 4. I. Thermal desorption spectra The desorption spectra for Sb on Si(ll1) are shown in fig. 1, taken with a substrate temperature of the form T = TO+ /?t, where TO is the initial temper-
402
R.A. Metrger,
I(T)
0.1
I(T)
0.50
NT)
Monolayers
L---A-
F.G. Allen / Antimony
Monolayer8
L---ALT
T I(T)
Monolayers
0.25
adsorption on silicon
1.0 Monolsyers
T
T
Fig. 1. Thermal desorption spectra I(T) for Sb on the Si(llf) surface as a function of substrate temperature T. Two distinct peaks are observed: a high temperature peak saturates at 1.0 monolayer followed by a low temperature peak which has no population limit at room temperature.
fi the heating rate and t time. This figure shows Sb coverage varying from 0.1 to 2.5 monolayers. For coverages from 0.1 to 1.0 monolayer * a single peak is observed, and for a constant substrate heating rate, j3, the desorption peak maximum occurs at the same temperature For all coverages. (See also fig. 6.) As an example, for j? = 3.33 K/s, the peak temperature T, = 950°C. As the 1.0 monolayer point is exceeded all subsequent Sb enters a second low temperature peak where no upper population limit is observed. The (100) orientation exhibits identical behavior, except that the high temperature peak saturates at 0.5 monolayers. The (110) orientation is more difficult to interpret and shows three or four peaks. These consist of two or three closely spaced peaks which saturate at a cumulative total of 1.0 monolayer, followed by the lower temperature peak which contains all subsequent Sb. In all three orientations, the low temperature desorption peak maximum occurs at appro~mately 450°C but this peak maximum shifts to higher temperatures with higher coverages. We believe that the high temperature peak represents Si-Sb bonding, while the low temperature peak represents bulk Sb evaporation from Sb.
ature,
* I monolayer surface
on the (Ill) = 7.85 X 10 I4 sites/cm’ and 9.6 X 1014 sites on the f 110) surface.
surface.
6.8X 1014 sites/cm’
on the (100)
R.A. Metrger, E G. Allen / Antimony adsorption on silicon
403
4.2. LEED patterns Fig. 2 shows the LEED pattern on the (111) orientation as a function of Sb coverage for a room temperature substrate. The clean surface produces the standard (7 x 7) pattern. As coverage increases from 0 to 1.0 monolayer, the pattern becomes blurred and then sharpens at a coverage of 1.0 monolayer to exhibit a (1 x 1) pattern. As coverage increases, the (1 X 1) pattern slowly decreases in intensity until it vanishes at a coverage of - 30 equivalent monolayers. The (100) behavior is similar, in that it starts with a (2 X 1) surface which blurs as coverage increases to 0.5 monolayers, at which time it exhibits a (1 x 1) pattern. This remains with decreasing distinctness until a coverage of - 30 equivalent monolayers. The (110) pattern was difficult to interpret, but the LEED pattern slowly decreased in intensity until it vanished at - 10 equivalent monolayers. Changes in (110) reconstructed patterns could not be determined.
0.0 Monolayers
1.0 Monolayers
Sb
:. .a *. . . .*. . . .
.*.
l
. . . ..:..’ . . . . : 0. . ...*:.:*. . . . . .* .*.. . ** .. *o:.:* * 0:
. .
0:
‘. . .:.
.I:.:
.;*
.**:* *. . ‘,. . *
.*
.
.&.
. *
. .
.0
Sb
.
l:
:.
.
.
.
:*
0: .
l
‘0’
0
l
.
. l
. . 5.0 Monolayers
Sb
20.0
Monolayers
Sb
Fig. 2. LEED patterns (100 V) of Sb on the Si(ll1) surface as a function of Sb coverage. The pattern changes from (7 X 7) to (1 X 1) at approximately 1.0 monolayer of coverage. The (1 x 1) pattern does not vanish until 30 equivalent monolayers of Sb are deposited.
404
R.A. Metrger,
F.G. Allen / Antimony
adsorption on s~hcon
4.3. Auger spectra
Fig. 3 shows the Auger plot of both the Sb and the Si intensities as a function of Sb coverage on a room temperature clean (111) substrate. Two distinct regions are seen, with a breakpoint at approximately 1.0 monolayer. For coverage from 0 to 1.0 monolayer, the Sb signal increases linearly with coverage as the Si signal decreases linearly. As the 1.0 monolayer point is passed, the slopes of Si and Sb signal versus coverage decrease. The Si signal is not extinguished and the Sb signal does not saturate until - 20 equivalent monolayers. The (100) and (110) orientations are very similar, with their break points respectively seen at 0.5 and 1.0 monolayers. For all three surface orientations, all three of the above types of data show that Sb adsorbs on Si in two distinct binding states. The transition point between these two different states is seen simultaneously in the three different experimental results: (i) a distinct second desorption peak appears at low temperatures after the first monolayer is complete, and grows progressively thereafter; (ii) the LEED pattern changes its reconstructed pattern as the monolayer is passed and, (iii) there is an abrupt decrease in the slope of the Sb Auger signal versus coverage as the monolayer is passed. This behavior is
1.00 .90 .80 .70 .80 .50 .40 .30 .20 .lO 0
0
1
2
3
4
Sb COVERAGE
5
6
7
8
9
10
(Monolayers)
Fig. 3. Normalized Auger plot of Sb (0) and Si (A) for a Si(ll1) surface as a function of Sb coverage. A breakpoint is observed at a coverage of approximately 1.0 monolayers.
405
R.A. Merrger, F.G. Allen / Anirmony adsorption on s~hcon
consistent with a Stranski-Krastinov growth mode [8], which is two-dimensional in the first monolayer and becomes three-dimensional thereafter. For Si MBE growth we note that the low energy peak desorbs at 450°C. Since MBE growth occurs over the temperature range of 600 to 900°C only the single high energy site with a single desorption energy should characterize Sb behavior on Si during MBE growth. 4.4. Analysis
of desorption peaks
4.4.1. Low temperature peak We first analyze the low energy desorption peak. As was illustrated in the desorption spectra, the single low temperature peak appears to represent Sb on Sb. For this to be true, this desorption peak should be identical to bulk Sb evaporation. Bulk evaporation is described by: dN,,s/dt
= K,N&
The desorption
295
= K,
order
3$5
x constant,
is zero because
375
for
as one layer is evaporated,
415
TEMPERATURE
p = 0.
455
another
(17) is
495
F’C)
Fig. 4. Thermal desorption spectra I(T) of Sb on the Si(ll1) surface as a function of substrate temperature T and Sb coverage of the low temperature peak. The peak shifts to higher temperatures with increasing Sb coverage indicating zero order desorption.
R.A. Metrger, F.G. Allen / Antimony adsorptionon silicon
406
directly beneath it. In this manner evaporation is independent of N,, as illustrated in eq. (17) forp = 0. For a temperature ramp of the form T = To + j?t, for zero order desorption the temperature at which the desorption peak occurs will increase with coverage, simply because of the increased time needed to evaporate an increased amount of Sb. This is observed experimentally as illustrated in fig. 4, in which Sb coverage varies from 2.0 to 5.0 monolayers with p = 3.33 K/s. Under the conditions of an infinite amount of Sb, from eq. (18) where y is a constant, dN,,/dt
= Z(T)
Taking
the natural
In Z(T)o: By plotting
= yK,
= yK,,
logarithms
epE1~/hr.
(18)
of eq. (18)
-E,/kT.
(19)
In Z(T) versus l/T,
the resultant
TEMPERATURE 360
slope will be -ED/k
which will
(‘(2)
350
340
330
1.25
1000/T(K) Fig. 5. Plot of In I(T) versus l/T for Sb on the Si(ll1) determines a desorption energy E, = 1.49 eV.
surface for the low temperature peak which
R.A. Metzger,
F.G. Allen / Antimony
on silicon
adwption
407
determine the desorption energy of this low temperature peak. However, eq. (19) is only valid for an infinite amount of Sb. As Z(T) deviates from exp( - E,/kT) due to the finite amount of Sb, the slope of In Z(T) versus l/T will begin to decrease and go to zero at the peak maximum. Therefore, E, is determined from the initial portion of the desorption spectra where the slope is constant. For the 5.0 monolayer peak shown in fig. 4, In Z(T) versus l/T is shown in fig. 5. Fig. 5 shows these data over the temperature range of 330 to 360°C which represents the initial portion of the desorption spectra in fig. 4. This resulted in E, = 1.49 eV. If this lower temperature peak represents the bulk evaporation of Sb, the activation energy should be the same as that from the Sb effusion cell. Flux calibration of the effusion cell resulted in an activation energy of 1.41 eV. This is, within our experimental error, the same as the low temperature peak desorption energy. We therefore conclude that the low temperature peak represents bulk Sb evaporation from Sb.
33 30 27 24 21
18 15 12 9 8 3 0
I
I
0
5
lb
1’5
;o
2’5
3b
Q5
Sb FLUX, Fx lo’ ’ (atom/cm*-
4b
65
80
set)
Fig. 6. Sb surface coverage N os (atoms/cm’) versus Sb flux F for the high temperature peak (sub-monolayer coverage) where No, is proportional to F indicating first order desorption; p = 1.
408
R.A. Metrger,
FIG. Alien / Antimony
adsorption on silicon
4.4.2. High temperature peak-desorption order p As discussed above with eq. (14), the substrate temperature was held constant while the flux was varied and No, determined by the flash/integration method. The results are illustrated in fig. 6, where the substrate temperature was held at 850°C and N,, is plotted as a function of F. As shown, the plot of No, versus F results in a straight line, indicating that F/N,, = constant. This can only be satisfied in eq. (14) for p = 1, which shows that desorption is first order. This is further confirmed by the fact that, as shown in fig. 7, for coverages of 0.06 to 1.0 monolayers, when Sb was desorbed with a substrate heating rate of @ = 3.3 K/s, the temperature for the peak remained constant for this varying coverage. This can only result for a first order process, as seen from eq. (1) and (14), under the condition p = 1: dN,,/dt
= -K,,
=$(
eeEolkT K~,
No,.
,-b’k(T,,+B’)
N,,)
=
0.
,=t,,
NDS 7.8x10
14
4.3xld4 2.4x 10
14
1.4xld4 8.2xld3 4.9x1d3
I 4;o
8;lO
i3;0
TEMPERATURE
1000
1;oo
(OC)
Fig. 7. Thermal desorption spectra I(T) as a function of Sb surface coverage N,S (atoms/cm2) and substrate temperature T of the high temperature peak. No observed peak shifts as a function of Sb coverage indicate first order desorption; p = 1.
R.A. Metrger,
F.G. Allen / Antimony
ahorption
on silicon
409
This results in K,,
dND,+
e-E,/k(T,+br)
7
ND&K,,
e-Edk(To+Br)
)I
t=t,
=
0.
r= ;-,
Substituting
I
K,,
in the expression
e-Eo/kTp]2
NDs
_
for dN,,/dt
bEDKDo
from eq. (20)
e-Ed’kTp
ND,= 0.
KTp' I
This simplifies
to:
(22) This yields a single value of T,,independent of No,. Values of p other than 1 do not give such a result. Therefore, both these methods indicate that p = 1. TEMPERATURE
950 I
,^ ‘v
-2.0
I
850
800
1
750
700
I
I
-
z
9
900
(OC)
850
I
A
F = 3.35~10’~
0
F = 1.05~101~
I
\L Kg = K,oftXP(-ED’k~)
-3.0
E,,
I
-4.0
-
-5.0
-
-8.0
-
-7.0
=2.48eW
( .80
.86
.92
.98
1.04
1.10
1000/T(K) Fig. 8. Plot of In K, versus l/T for Sb on Si(ll1) T, is the predicted temperature at which K, temperature peak saturation of 1.0 monolayer.
for the high temperature reaches
its minimum
peak to determine value
due
K,.
to the high
410
R.A. Metzger, F.G. Allen / Antimony adsorption on silicon
4.4.3. Desorption energy of high temperature peak As described above, K, for the high temperature peak can be determined under steady state conditions by varying the substrate temperature T under a constant Sb flux. Fig. 8 shows ln( F/N,,) versus l/T for two different fluxes of 3.35 x lo’* and 1.05 X lo’* atoms/cm*. s. From eq. (15), K, can be determined from the high temperature regime as: K, = K,,
e-EI,/‘r,
(23)
where K,, = 1.5~10’s~‘andE,=2.46eV. As temperatures decrease, ln( K,) saturates to a minimum value. This is to be expected since ln( K,) = ln( F/N,,), and for a constant flux, as No, increases, it was found in desorption spectra that ND,__ saturates at 1.0 monolayer. Therefore, when the monolayer point is reached, K, will also saturate. This saturation point can be predicted, based on the flux value. The minimum value of K, defined as K, ,lU”occurring at a substrate temperature T,, is expressed as:
ED
T,, = k [In ho Based on the E,
Table 1 Desorption Orientation
(111) (100) (110)
Table 2 Residence
constant
(24)
- ln( ~~~~~~~~ I] and K,,
K,
values determined
of Sb from Si for different
in eq. (23)
orientations;
= K,,
K (so?)
(s-l)
(s- ‘)
2.46 3.05 2.08
1.5 x lo9 2.0 x lOI 2.2 x 10’
9.5 x 1om6 5.ox1o-6 2.1 x lo- 5
4.1x10~2 1.6x10-’ 2.6~ 10m2
surface
(600°C)
eeEDjkT
Eo (ev)
time, 7, of Sb atoms on the Si(lll)
K,
K,
T,, is shown in fig. 8
K,
versus temperature
TS”h
Kv
7 = l/K,
Pa
(S-I)
(s)
600
9.53XlOF”
650 700 750 800 850 900 950
5.60~10-~ 2.74x 10m4 1.15XlO~’ 4.22x10-s 1.38x10-* 4.o7xlo-2 0.110
1.05 x 1.79x 3.65 x 8.70 x 2.37 x 72.5 24.5 9.1
10s lo4 10’ lo2 10’
(9OO’C)
R.A. Metrger,
F.G. Allen / Antimony
411
adsorption on silicon
for the two different flux cases and the observed saturation of K, does occur at the predicted T,,point. This indicates that our method and results are reasonable. This same analysis was also carried out on the (100) and (110) orientations. Further analysis is required to interpret K, variations as a function of Si orientation. These results are summarized in table 1. As illustrated in eq. (7) the K, value determines the Sb residence time. The residence times r for Sb on Si(l11) are shown in table 2. A consequence of this residence time, 7, is the “smearing” of doping profiles during MBE growth. In this connection it was found that the K, values determined under conditions of no Si growth are also valid during Si growth, since the 7 values derived under no growth conditions are the same as those observed in transient doping profiles.
5. Conclusion With a combination of LEED, Auger and desorption spectroscopy, Sb adsorption behavior on Si has been quantitatively determined. It is characterized by two adsorptions sites reflected by two desorption peaks in thermal desorption spectra. A high temperature sub-monolayer peak whose kinetics determine doping transients during MBE growth is characterized by Sb-Si bonding and by two-dimensional growth with first order desorption. The second, low temperature peak, forming only after the first is complete, is characterized by bulk Sb evaporation and three-dimensional growth with zero order desorption.
Acknowledgements The assistance of S. Iyer and D. Streit instrumentation used in this work is gratefully
in developing acknowledged.
techniques
References [l] R.A. Metzger, PhD Thesis, UCLA (1983); also, R.A. Metzger and F.G. Allen, J. Appl. Phys., to be published. [2] J.C. Bean, Appl. Phys. Letters 33 (1978) 654. [3] V. Koenig, H. Kibbel and E. Kasper, J. Vacuum Sci. Technol. 26 (1979) 985. [4] V. Koenig, E. Kasper and H.J. Herzog, J. Crystal Growth 52 (1981) 151. [5] M. Tabe and K. Kajiyama, Japan. J. Appl. Phys. 22 (1983) 423. [6] Y. Saito, A. Kawazu and G. Tominaga, Surface Sci. 103 (1981) 563. [7] A. Kawazu, T. Otsuki and G. Tominaga, Japan. J. Appl. Phys. 20 (1981) 553. [8] G. Le Lay and R. Kern, J. Crystal Growth 44 (1978) 197.
and
ANTILOG
A~SOR~~ON
R.A. METZGER
397
ON SILICON *
and F.G. ALLEN
~[eetrical Engineering Department, U~i~ers~iyuj Cdifomia, Los Angeles, Cul~ornia 90024, U.S.4 Received
9 August
1983; accepted
for publication
26 October
1983
The kinetic behavior of antimony films adsorbed on clean silicon surfaces has been measured versus temperature and coverage using LEED, Auger and thermal desorption. Sb adsorbs in two distinct modes, seen in a high and a low temperature desorption peak. The high temperature peak fills up to one monolayer on the Si(ll1) surface with a desorption rate K, given by K, = K,o = with XI, = 1.5 x 109 s-l, E, = 2.46 eV, and IV,, = Sb coverage. Growth of eWE,/kTW,s, this phase is tw~dimensionai. Higher Sb coverage adsorbs in the low temperature peak as bulk Sb on Sb with E, = 1.49 eV and grows without population limit in a three-dimensiona mode. The residence time of Sb atoms on the surface, 7 = Kg’, varies from 10’ to 10 s over the temperature rang 600 to 950°C; this determines transient smearing during MBE growth of Si.
1. ~n~~uetion During molecular beam epitaxial (MBE) growth of silicon using evaporative antimony doping, we find that inco~oration of Sb proceeds in a two-step process: Sb atoms first adsorb in a surface layer with a finite residence time, and then either desorb again or incorporate into the growing crystal. We have described the inco~oration process for the growing crystal and the resultant doping effects elsewhere [I]. This paper describes our findings on the adsorption-desorption kinetics of these Sb films. We have made measurements in two regimes: (1) room temperature and above with no incident silicon flux, and (2) MBE growth temperatures above 600°C with an incident silicon flux of - 0.25 monolayer/s. In general, we find that: (1) the rate of desorption greatly exceeds that of incorporation during MBE growth, (2) the Sb adsorption-deso~tion process is unaffected by the presence of the Si flux except at high Sb flux and low substrate temperatures, and (3) Sb residence times are long enough to cause serious “smearing” of doped profiles below 750°C [l]. Some previous data on Sb-Si adsorption is available in the MBE studies reported by Bean [2], Koenig et al. [3,4], and Tabe and Kajiyama (5). Tabe and * Supported
in part by grant No. ECS-00227
from the National
0039-6028/84/$03.00 0 Elsevier Science Publishers (North-Holland Physics Publishing Division)
Science Foundation.
B.V.
398
R.A. Metager, FG. Allen / Antmony
ad.rorption on siiicon
Kajiyama found that Sb adsorbs on Si in two distinct phases: a high temperature phase which can accommodate approximately one monolayer and a lower temperature phase which forms only below 440°C after the high temperature phase is complete, and which can accommodate an unlimited population of Sb at room temperature. Our studies bear out Tabe and Kajiyama’s general findings and describe the kinetics of this behavior in quantitative terms. We find many similarities in this system to other metal-simiconductor systems that have been studied, particularly that of bismuth on silicon [6,7]. We report here techniques applicable to the Sb-Si system of particular usefulness for adsorption-desorption studies which are readily carried out in MBE systems.
2. Experimental apparatus Our Si MBE system consists of a modified stainless steel Varian station with vat-ion pumps and titanium gettering and is capable of sustaining a base pressure of 5 X lo- ” Torr. Surface analyses include low energy electron diffraction (LEED), Auger electron spectroscopy (AES) and quadrupole mass spectroscopy used for both detection in desorption studies and for residual gas analysis. Silicon is evaporated from a molten pool of pure Si surrounded by solid silicon in a molybdenum crucible by means of a 6 kW electron bombardment gun. The Sb beam is generated in a resistively heated pyrolitic boron nitride effusion cell. The electron gun and effusion cells are housed in a liquid nitrogen shroud. Typical operating pressure during Si growth is 3 X lo-’ Torr. Samples are inserted into the chamber via a load-lock with sample insertion time from atmospheric pressure to start of run on the order of one hour. The silicon substrate is held between molybdenum clips and is resistively heated. Atomically clean starting surfaces as determined by LEED patterns are typically produced by heating the Si substrate to 1200°C for 1 to 2 min. Auger analysis has disclosed no surface impurities other than some residual carbon, sometimes present at a small fraction of one monolayer. Sb doping during MBE growth is carried out at substrate temperatures over the range of 600 to 900°C.
3. Model for adaption-deso~tion 3. I. Differential
kinetics
equation for N,,
We now develop a kinetic model describing Sb behavior during Si MBE growth in which the incident Sb flux F is entrapped on the Si surface with a mean residence time of 7. The entrapped Sb atoms may then either desorb or
R.A. Metzger, F.G. AI/en / Antimony adsorption on silicon
399
incorporate into the growing film. There is accumulated on the surface a layer of adsorbed dopant atoms of density No, (atoms/cm*). This layer acts as a reservoir that produces doping in the growing crystal. We write a kinetic equation for this system as: dN,s/di
= F-
K,N;,
- K,N&.
(1)
s), I(, and K, are the desorption and F is the incident Sb flux (atoms/cm2. incorporation coefficients and p and q are the desorption and incorporation represents the incorporating Sb which produces orders. The term KIN& doping and the term K,N& represents the desorbing Sb. We now anticipate the result developed below that p = 1, and we use the result derived from our doped film studies that q = 1 to illustrate sticking and transient behaviour for Sb on Si. Using p = 1, q = 1, we proceed as follows: The sticking coefficient S is defined as the fraction of incident Sb incorporated into the growing film, S = K,N,s/F. Under
(2)
steady state conditions
F = K,N,,
dNo,/dt
= 0 and eq. (1) gives
+ K,N,,.
(3)
Substituting eq. (3) into eq. (2) results in an expression steady state conditions S=
K,
K, No, KEN,,
for S measured
+ K,N,,
= K, + K,
’
under
(4)
3.2. Transient behavior It was found 900°C S was
in the doping study [l] that over the temperature range 700 to < 1, so eq. (4) implies that K, +z K,. For K, GE K, and (an initial Sb surface concentration at the beginning of Si N,,( f = 0) = N,,, growth) eq. (1) can be solved for N,,(t):
No&) = Nosoe-K,r
x”,(
+ -
1 -
,-W)_
The resulting doping in the film, NDB(t), is equal to the incorporation KIN,,(t), divided by the silicon growth rate, u, in cm/s,
As c goes to infinity N oH,, = K,F/vK,
(5) rate,
and steady state is reached, = SF/v.
(6b)
400
R.A. Meirger, F.G. Allen / Antimory
Therefore, where
doping
profiles
will show transients
adsorprron on srlrcon
with a characteristic
7 = l/K,.
lifetime
7.
(7)
The present work is undertaken to determine coefficient K, through surface studies. 3.3. Determination
the properties
of p and K,
Under the condition that K, < K,, which was found this work, eq. (I) becomes, for p not yet determined, = F-
dN,,/dt
of the desorption
K,N&.
to hold throughout (8)
We now present a method to solve eq. (8) in order to determine both p and K,. The method used requires the measurement of N&T, F). The Sb flux F is calibrated by direct measurement of Sb fifms grown on Si room temperature substrates with the resultant thickness measured with an in-situ thickness monitor calibrated later by an interferometer measurement of the film. With a clean Si substrate at room temperature a known amount of Sb is deposited, N,,
= Ft*,
(9)
where t* is the deposition time and F the calibrated flux. No, is measured by placing the Si substrate in direct line of site with the mass spectrometer (distance 10 cm) and flashing the Si substrate from room temperature to 1200°C in approximately 10 s. This desorbs all of the Sb. The resultant Sb signal from the mass spectrometer is I(t). We define the experimentally measured area under the flash-off curve as N;,
=
I
I(r)
The actual determined
dt. surface coverage factor cy given by
N IX =cwN*DS =Ft*.
N,,
is related
to N&
by an experimentally
(11)
Over the full range of ND, values covered, cy was constant to within lo%, the deviation being due to a slight nonlinearity in the mass spectrometer response to a sudden burst of gas. Some small fraction of the Sb may have evaporated as multi-atomic molecules, though we failed to detect these in the mass spectrometer. Variations in this fraction would affect ty. This method has been utilized to resolve Sb layers of as little as 10M4 monolayer. This measurement method permits the determination of K, and p as follows: From eq. (8) under steady state conditions F=
KDN&,
K, = K,,
(12) ,-hi~~-,
(13)
R.A. Metrger,
F.G. Allen / Antimoy
401
adsorption on silicon
K,, corresponds to a vibrational frequency and E, is the desorption energy. We will make the assumption that K,, and p are independent of temperature. The desorption order p is determined by keeping the substrate temperature (and K,) constant, so that eqs. (12) and (13) give where
K,
= F/N&
= constant.
(14)
We then vary Fat constant temperature, measure No, by the flash/integration method, and determine p from the resulting relation of F to No,. Eq. (14) implies that N,, has reached its steady state value. This steady condition is achieved by leaving the heated substrate in the Sb beam for lengthening intervals of time while measuring No, periodically after each interval. Steady state has been reached when No, does not increase for lengthening times in the Sb beam. K, can then be determined under steady state conditions by varying the substrate temperature T under a constant Sb flux F such that K,
= K,,
ePEDjkT = F/N&.
(15)
The flux F is calibrated and No, is measured by the flash/integration Taking the natural logarithm of both sides of eq. (15) we have In K, = ln( F/N&) Plotting ln( K,,)
= In K,,
method.
- E,/kT.
ln( K,) versus l/T should then give a straight line with an intercept and a slope of -E&k, if the model is valid.
(16)
of
4. Experimental results Sb kinetics have been studied on all three low index silicon surface orientations: (ill), (loo), and (110). Since the behaviour of Sb on all three orientations is similar, a detailed analysis on the (111) orientation will be presented followed by summaries of (100) and (110) behavior. Before detailed analysis of K, and p can be attempted, it is necessary to find the number of Sb binding sites on the silicon surface and to determine which sites are involved during adsorption. This is done by a combination of thermal desorption spectroscopy, Auger plots, and LEED patterns. In all three methods, all Si surfaces receive a chemical cleaning before insertion into the station and a thermal cleaning in vacuum (1 to 2 min at 1200°C) in order to achieve a sharp characteristic LEED pattern and Auger spectrum, before surfaces are studied. 4. I. Thermal desorption spectra The desorption spectra for Sb on Si(ll1) are shown in fig. 1, taken with a substrate temperature of the form T = TO+ /?t, where TO is the initial temper-
402
R.A. Metrger,
I(T)
0.1
I(T)
0.50
NT)
Monolayers
L---A-
F.G. Allen / Antimony
Monolayer8
L---ALT
T I(T)
Monolayers
0.25
adsorption on silicon
1.0 Monolsyers
T
T
Fig. 1. Thermal desorption spectra I(T) for Sb on the Si(llf) surface as a function of substrate temperature T. Two distinct peaks are observed: a high temperature peak saturates at 1.0 monolayer followed by a low temperature peak which has no population limit at room temperature.
fi the heating rate and t time. This figure shows Sb coverage varying from 0.1 to 2.5 monolayers. For coverages from 0.1 to 1.0 monolayer * a single peak is observed, and for a constant substrate heating rate, j3, the desorption peak maximum occurs at the same temperature For all coverages. (See also fig. 6.) As an example, for j? = 3.33 K/s, the peak temperature T, = 950°C. As the 1.0 monolayer point is exceeded all subsequent Sb enters a second low temperature peak where no upper population limit is observed. The (100) orientation exhibits identical behavior, except that the high temperature peak saturates at 0.5 monolayers. The (110) orientation is more difficult to interpret and shows three or four peaks. These consist of two or three closely spaced peaks which saturate at a cumulative total of 1.0 monolayer, followed by the lower temperature peak which contains all subsequent Sb. In all three orientations, the low temperature desorption peak maximum occurs at appro~mately 450°C but this peak maximum shifts to higher temperatures with higher coverages. We believe that the high temperature peak represents Si-Sb bonding, while the low temperature peak represents bulk Sb evaporation from Sb.
ature,
* I monolayer surface
on the (Ill) = 7.85 X 10 I4 sites/cm’ and 9.6 X 1014 sites on the f 110) surface.
surface.
6.8X 1014 sites/cm’
on the (100)
R.A. Metrger, E G. Allen / Antimony adsorption on silicon
403
4.2. LEED patterns Fig. 2 shows the LEED pattern on the (111) orientation as a function of Sb coverage for a room temperature substrate. The clean surface produces the standard (7 x 7) pattern. As coverage increases from 0 to 1.0 monolayer, the pattern becomes blurred and then sharpens at a coverage of 1.0 monolayer to exhibit a (1 x 1) pattern. As coverage increases, the (1 X 1) pattern slowly decreases in intensity until it vanishes at a coverage of - 30 equivalent monolayers. The (100) behavior is similar, in that it starts with a (2 X 1) surface which blurs as coverage increases to 0.5 monolayers, at which time it exhibits a (1 x 1) pattern. This remains with decreasing distinctness until a coverage of - 30 equivalent monolayers. The (110) pattern was difficult to interpret, but the LEED pattern slowly decreased in intensity until it vanished at - 10 equivalent monolayers. Changes in (110) reconstructed patterns could not be determined.
0.0 Monolayers
1.0 Monolayers
Sb
:. .a *. . . .*. . . .
.*.
l
. . . ..:..’ . . . . : 0. . ...*:.:*. . . . . .* .*.. . ** .. *o:.:* * 0:
. .
0:
‘. . .:.
.I:.:
.;*
.**:* *. . ‘,. . *
.*
.
.&.
. *
. .
.0
Sb
.
l:
:.
.
.
.
:*
0: .
l
‘0’
0
l
.
. l
. . 5.0 Monolayers
Sb
20.0
Monolayers
Sb
Fig. 2. LEED patterns (100 V) of Sb on the Si(ll1) surface as a function of Sb coverage. The pattern changes from (7 X 7) to (1 X 1) at approximately 1.0 monolayer of coverage. The (1 x 1) pattern does not vanish until 30 equivalent monolayers of Sb are deposited.
404
R.A. Metrger,
F.G. Allen / Antimony
adsorption on s~hcon
4.3. Auger spectra
Fig. 3 shows the Auger plot of both the Sb and the Si intensities as a function of Sb coverage on a room temperature clean (111) substrate. Two distinct regions are seen, with a breakpoint at approximately 1.0 monolayer. For coverage from 0 to 1.0 monolayer, the Sb signal increases linearly with coverage as the Si signal decreases linearly. As the 1.0 monolayer point is passed, the slopes of Si and Sb signal versus coverage decrease. The Si signal is not extinguished and the Sb signal does not saturate until - 20 equivalent monolayers. The (100) and (110) orientations are very similar, with their break points respectively seen at 0.5 and 1.0 monolayers. For all three surface orientations, all three of the above types of data show that Sb adsorbs on Si in two distinct binding states. The transition point between these two different states is seen simultaneously in the three different experimental results: (i) a distinct second desorption peak appears at low temperatures after the first monolayer is complete, and grows progressively thereafter; (ii) the LEED pattern changes its reconstructed pattern as the monolayer is passed and, (iii) there is an abrupt decrease in the slope of the Sb Auger signal versus coverage as the monolayer is passed. This behavior is
1.00 .90 .80 .70 .80 .50 .40 .30 .20 .lO 0
0
1
2
3
4
Sb COVERAGE
5
6
7
8
9
10
(Monolayers)
Fig. 3. Normalized Auger plot of Sb (0) and Si (A) for a Si(ll1) surface as a function of Sb coverage. A breakpoint is observed at a coverage of approximately 1.0 monolayers.
405
R.A. Merrger, F.G. Allen / Anirmony adsorption on s~hcon
consistent with a Stranski-Krastinov growth mode [8], which is two-dimensional in the first monolayer and becomes three-dimensional thereafter. For Si MBE growth we note that the low energy peak desorbs at 450°C. Since MBE growth occurs over the temperature range of 600 to 900°C only the single high energy site with a single desorption energy should characterize Sb behavior on Si during MBE growth. 4.4. Analysis
of desorption peaks
4.4.1. Low temperature peak We first analyze the low energy desorption peak. As was illustrated in the desorption spectra, the single low temperature peak appears to represent Sb on Sb. For this to be true, this desorption peak should be identical to bulk Sb evaporation. Bulk evaporation is described by: dN,,s/dt
= K,N&
The desorption
295
= K,
order
3$5
x constant,
is zero because
375
for
as one layer is evaporated,
415
TEMPERATURE
p = 0.
455
another
(17) is
495
F’C)
Fig. 4. Thermal desorption spectra I(T) of Sb on the Si(ll1) surface as a function of substrate temperature T and Sb coverage of the low temperature peak. The peak shifts to higher temperatures with increasing Sb coverage indicating zero order desorption.
R.A. Metrger, F.G. Allen / Antimony adsorptionon silicon
406
directly beneath it. In this manner evaporation is independent of N,, as illustrated in eq. (17) forp = 0. For a temperature ramp of the form T = To + j?t, for zero order desorption the temperature at which the desorption peak occurs will increase with coverage, simply because of the increased time needed to evaporate an increased amount of Sb. This is observed experimentally as illustrated in fig. 4, in which Sb coverage varies from 2.0 to 5.0 monolayers with p = 3.33 K/s. Under the conditions of an infinite amount of Sb, from eq. (18) where y is a constant, dN,,/dt
= Z(T)
Taking
the natural
In Z(T)o: By plotting
= yK,
= yK,,
logarithms
epE1~/hr.
(18)
of eq. (18)
-E,/kT.
(19)
In Z(T) versus l/T,
the resultant
TEMPERATURE 360
slope will be -ED/k
which will
(‘(2)
350
340
330
1.25
1000/T(K) Fig. 5. Plot of In I(T) versus l/T for Sb on the Si(ll1) determines a desorption energy E, = 1.49 eV.
surface for the low temperature peak which
R.A. Metzger,
F.G. Allen / Antimony
on silicon
adwption
407
determine the desorption energy of this low temperature peak. However, eq. (19) is only valid for an infinite amount of Sb. As Z(T) deviates from exp( - E,/kT) due to the finite amount of Sb, the slope of In Z(T) versus l/T will begin to decrease and go to zero at the peak maximum. Therefore, E, is determined from the initial portion of the desorption spectra where the slope is constant. For the 5.0 monolayer peak shown in fig. 4, In Z(T) versus l/T is shown in fig. 5. Fig. 5 shows these data over the temperature range of 330 to 360°C which represents the initial portion of the desorption spectra in fig. 4. This resulted in E, = 1.49 eV. If this lower temperature peak represents the bulk evaporation of Sb, the activation energy should be the same as that from the Sb effusion cell. Flux calibration of the effusion cell resulted in an activation energy of 1.41 eV. This is, within our experimental error, the same as the low temperature peak desorption energy. We therefore conclude that the low temperature peak represents bulk Sb evaporation from Sb.
33 30 27 24 21
18 15 12 9 8 3 0
I
I
0
5
lb
1’5
;o
2’5
3b
Q5
Sb FLUX, Fx lo’ ’ (atom/cm*-
4b
65
80
set)
Fig. 6. Sb surface coverage N os (atoms/cm’) versus Sb flux F for the high temperature peak (sub-monolayer coverage) where No, is proportional to F indicating first order desorption; p = 1.
408
R.A. Metrger,
FIG. Alien / Antimony
adsorption on silicon
4.4.2. High temperature peak-desorption order p As discussed above with eq. (14), the substrate temperature was held constant while the flux was varied and No, determined by the flash/integration method. The results are illustrated in fig. 6, where the substrate temperature was held at 850°C and N,, is plotted as a function of F. As shown, the plot of No, versus F results in a straight line, indicating that F/N,, = constant. This can only be satisfied in eq. (14) for p = 1, which shows that desorption is first order. This is further confirmed by the fact that, as shown in fig. 7, for coverages of 0.06 to 1.0 monolayers, when Sb was desorbed with a substrate heating rate of @ = 3.3 K/s, the temperature for the peak remained constant for this varying coverage. This can only result for a first order process, as seen from eq. (1) and (14), under the condition p = 1: dN,,/dt
= -K,,
=$(
eeEolkT K~,
No,.
,-b’k(T,,+B’)
N,,)
=
0.
,=t,,
NDS 7.8x10
14
4.3xld4 2.4x 10
14
1.4xld4 8.2xld3 4.9x1d3
I 4;o
8;lO
i3;0
TEMPERATURE
1000
1;oo
(OC)
Fig. 7. Thermal desorption spectra I(T) as a function of Sb surface coverage N,S (atoms/cm2) and substrate temperature T of the high temperature peak. No observed peak shifts as a function of Sb coverage indicate first order desorption; p = 1.
R.A. Metrger,
F.G. Allen / Antimony
ahorption
on silicon
409
This results in K,,
dND,+
e-E,/k(T,+br)
7
ND&K,,
e-Edk(To+Br)
)I
t=t,
=
0.
r= ;-,
Substituting
I
K,,
in the expression
e-Eo/kTp]2
NDs
_
for dN,,/dt
bEDKDo
from eq. (20)
e-Ed’kTp
ND,= 0.
KTp' I
This simplifies
to:
(22) This yields a single value of T,,independent of No,. Values of p other than 1 do not give such a result. Therefore, both these methods indicate that p = 1. TEMPERATURE
950 I
,^ ‘v
-2.0
I
850
800
1
750
700
I
I
-
z
9
900
(OC)
850
I
A
F = 3.35~10’~
0
F = 1.05~101~
I
\L Kg = K,oftXP(-ED’k~)
-3.0
E,,
I
-4.0
-
-5.0
-
-8.0
-
-7.0
=2.48eW
( .80
.86
.92
.98
1.04
1.10
1000/T(K) Fig. 8. Plot of In K, versus l/T for Sb on Si(ll1) T, is the predicted temperature at which K, temperature peak saturation of 1.0 monolayer.
for the high temperature reaches
its minimum
peak to determine value
due
K,.
to the high
410
R.A. Metzger, F.G. Allen / Antimony adsorption on silicon
4.4.3. Desorption energy of high temperature peak As described above, K, for the high temperature peak can be determined under steady state conditions by varying the substrate temperature T under a constant Sb flux. Fig. 8 shows ln( F/N,,) versus l/T for two different fluxes of 3.35 x lo’* and 1.05 X lo’* atoms/cm*. s. From eq. (15), K, can be determined from the high temperature regime as: K, = K,,
e-EI,/‘r,
(23)
where K,, = 1.5~10’s~‘andE,=2.46eV. As temperatures decrease, ln( K,) saturates to a minimum value. This is to be expected since ln( K,) = ln( F/N,,), and for a constant flux, as No, increases, it was found in desorption spectra that ND,__ saturates at 1.0 monolayer. Therefore, when the monolayer point is reached, K, will also saturate. This saturation point can be predicted, based on the flux value. The minimum value of K, defined as K, ,lU”occurring at a substrate temperature T,, is expressed as:
ED
T,, = k [In ho Based on the E,
Table 1 Desorption Orientation
(111) (100) (110)
Table 2 Residence
constant
(24)
- ln( ~~~~~~~~ I] and K,,
K,
values determined
of Sb from Si for different
in eq. (23)
orientations;
= K,,
K (so?)
(s-l)
(s- ‘)
2.46 3.05 2.08
1.5 x lo9 2.0 x lOI 2.2 x 10’
9.5 x 1om6 5.ox1o-6 2.1 x lo- 5
4.1x10~2 1.6x10-’ 2.6~ 10m2
surface
(600°C)
eeEDjkT
Eo (ev)
time, 7, of Sb atoms on the Si(lll)
K,
K,
T,, is shown in fig. 8
K,
versus temperature
TS”h
Kv
7 = l/K,
Pa
(S-I)
(s)
600
9.53XlOF”
650 700 750 800 850 900 950
5.60~10-~ 2.74x 10m4 1.15XlO~’ 4.22x10-s 1.38x10-* 4.o7xlo-2 0.110
1.05 x 1.79x 3.65 x 8.70 x 2.37 x 72.5 24.5 9.1
10s lo4 10’ lo2 10’
(9OO’C)
R.A. Metrger,
F.G. Allen / Antimony
411
adsorption on silicon
for the two different flux cases and the observed saturation of K, does occur at the predicted T,,point. This indicates that our method and results are reasonable. This same analysis was also carried out on the (100) and (110) orientations. Further analysis is required to interpret K, variations as a function of Si orientation. These results are summarized in table 1. As illustrated in eq. (7) the K, value determines the Sb residence time. The residence times r for Sb on Si(l11) are shown in table 2. A consequence of this residence time, 7, is the “smearing” of doping profiles during MBE growth. In this connection it was found that the K, values determined under conditions of no Si growth are also valid during Si growth, since the 7 values derived under no growth conditions are the same as those observed in transient doping profiles.
5. Conclusion With a combination of LEED, Auger and desorption spectroscopy, Sb adsorption behavior on Si has been quantitatively determined. It is characterized by two adsorptions sites reflected by two desorption peaks in thermal desorption spectra. A high temperature sub-monolayer peak whose kinetics determine doping transients during MBE growth is characterized by Sb-Si bonding and by two-dimensional growth with first order desorption. The second, low temperature peak, forming only after the first is complete, is characterized by bulk Sb evaporation and three-dimensional growth with zero order desorption.
Acknowledgements The assistance of S. Iyer and D. Streit instrumentation used in this work is gratefully
in developing acknowledged.
techniques
References [l] R.A. Metzger, PhD Thesis, UCLA (1983); also, R.A. Metzger and F.G. Allen, J. Appl. Phys., to be published. [2] J.C. Bean, Appl. Phys. Letters 33 (1978) 654. [3] V. Koenig, H. Kibbel and E. Kasper, J. Vacuum Sci. Technol. 26 (1979) 985. [4] V. Koenig, E. Kasper and H.J. Herzog, J. Crystal Growth 52 (1981) 151. [5] M. Tabe and K. Kajiyama, Japan. J. Appl. Phys. 22 (1983) 423. [6] Y. Saito, A. Kawazu and G. Tominaga, Surface Sci. 103 (1981) 563. [7] A. Kawazu, T. Otsuki and G. Tominaga, Japan. J. Appl. Phys. 20 (1981) 553. [8] G. Le Lay and R. Kern, J. Crystal Growth 44 (1978) 197.
and