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Substitution of surface-adsorbed As atoms to P atoms in atomic layer epitaxy
applied
surface science Applied Surface Science 82/83 (1994) 257-262
ELSIWIER
Substitution of surface-adsorbed As atoms to P atoms in atomic layer epitaxy Hitoshi Ikeda *,l, Yoshiki Miura, Naoyuki Takahashi, Akinori Koukitu, Hisashi Seki Deparhent
ofApplied
Chemistry, Faculty of Technology, Tokyo University of Agriculture and Technology, Koganei, Tokyo 184, Japan
Received 9 May 1994; accepted for publication 5 July 1994
Abstract A systematic study on the substitution reaction of surface-adsorbed As atoms to P atoms was performed using atomic layer epitaxy (ALE). The reaction’s dependence on P4 exposure time, on P4 partial pressure and on growth temperature was investigated quantitatively. A mechanism consisting of two kinds of substitution is proposed. Investigation showed that the substitution reaction rate is proportional to the fraction of superficial As atoms and P4 partial pressure. The activation energy of the substitution reaction is 81.8 kJ/mol.
1. Introduction Current epitaxial growth techniques are required to fabricate heterostructures, such as quantum wells and superlattices, with atomically controlled interfaces. To put the structures to work in practical devices and to improve their performance, it is essential to understand the crystal growth mechanism on an atomic scale. Recently, in atomic layer epitaxy (ALE), we have found that P atoms substitute for surface-adsorbed As atoms, when GaAs grown surfaces are exposed to P4. An understanding of this phenomenon will contribute greatly to our knowledge of the epitaxial growth mechanism, the atomically abrupt interface required for superlattices and quantum well fabrication.
* Corresponding author. ’ Present address: OJU Electric Industry Co., Ltd., Research and Development Group, 550-5 Higashiasakawa-cho, Hachioji-shi, Tokyo 193, Japan. 0169-4332/94/$07.00
Basically, the growth of heterostructures consisting of different kinds of Group V-atoms, such as InGaAs/InP and InAs/InP, has problems with atom substitution occurring on the epitaxial surface. On the grown surface which becomes the heterointerface after the growth, arsenic atoms terminating on the surface are substituted by the phosphorus component of the next layer and vice versa. Although optimum growth conditions yielding an abrupt interface have been reported [1,21, the mechanism of substitution has not yet been clarified. One reason is the difficulty to estimate quantitatively the substitution within one atomic layer. This paper reports As to P substitution on the grown surface (As + P substitution) investigated using an atomic layer epitaxy (ALE) technique. ALE of III-V compounds is done by either metalorganic vapor-phase epitaxy (MOVPE) or halogen-transport VPE. However, the latter is better suited to growth mechanism investigation, because it easily provides monolayer growth over a wide range of growth
0 1994 Elscvier Science B.V. All rights reserved
SSDI 0169-4332(94)00227-4
258
H. lkeda
et al. /Applied
Surface
Science
82 / ;Y3 (I 994) 257-262
2. Experimental
The apparatus used in this study is shown schematically in Fig. 1. Halogen-transport ALE growth and As + P substitution were performed at atmospheric pressure in a vertical reactor. The reactor consisted of purge, exposure and source zones, the temperature of each controlled independently using resistive furnaces. As the figure shows, the reactor tube is divided into three chambers for exposure to each of the source species. Hydrogen gas is introduced from the top of the reactor and divided among the three exposure chambers. Each chamber has branch tubes connecting to each source zone for GaCl, As, and P4. GaCl was formed by chemical reaction between hydrogen chloride and gallium metal at 780°C. The Group V sources were solid arsenic and phosphorus. The partial pressures were controlled by the temperatures at the source zones. The GaAs substrate, exactly (100) oriented, was tipped on the rod as shown in Fig. 1. In upward and downward rod movement and rotation, the substrate was dipped into each source species and pulled up into the hydrogen atmosphere. In this way, it was possible to perform ALE exposure and purge sequences. Before the experiments below were con-
a
Exhaust Fig.
I. Schematic view of the apparatus
conditions, especially so in the gallium system. As a preliminary investigation, we chose the GaAs/GaP system and halogen-transport ALE to study the As --f P substitution phenomenon systematically.
Typical
growth
details
conditions
Temp. = 450 - 480 “C = 2.5x10A atm PAs4
PGSKI= 1.0~ 10e4atm = 7.5x10m4- 1.O~lO~*atrn
PP4
7
GaCl Step l(8 s)
Time Fig. 2. Typical growth conditions
+
and time sequence of one operational
cycle
H. Ikeda et al. /Applied Surface Science 82 / 83 (1994)
ducted, GaAs and GaP ALE growth was conformed with this system. Fig. 2 shows typical time sequences and growth conditions for this study. First, to fabricate an Asterminated surface, one GaAs monolayer was grown on the substrate surface by halogen-transport ALE. Next, the As-terminated surface was exposed to the phosphorus species. In this stage, the surface As atoms were substituted by the P atoms. Next, over this, another As-terminated surface was formed by one monolayer ALE growth of GaAs. Thus, one operational cycle consisted of six steps: exposures to GaCl (step 11, As, (step 3) and P4 (step 5) and the purges of these species by hydrogen (steps 2, 4 and 6). This cycle was repeated 600 or 1800 times per run. The fraction of the As -+ P substitution reaction was estimated quantitatively by measuring the composition of the grown layer, that is, the composition of GaAs, P, _-x. The composition of the layers was measured by electron spectroscopy for chemical analysis (ESCA). Double-crystal X-ray diffraction (DC-XRD) was used to measure lattice parameters perpendicular to the surface plane and the peak diffraction pattern width. Grown layer thickness was measured by observing the cleaved surface with a Nomarski microscope.
3. Results and discussion Fig. 3 shows the growth rate per cycle as a function of exposure time to P4 species of Asterminated surfaces after GaAs ALE growth at 450 and 480°C. Growth rates were calculated from the thickness of grown layers and the lattice parameter perpendicular to the surface plane measured by DCXRD, that is, by dividing the layer thickness by the product of half of the lattice parameter and the number of cycles run. In this study, after ALE growth of one GaAs molecular layer, the Asterminated surface was exposed to the P4 species (step 5 in the experimental sequence). As the figure shows, growth rates are independent of the P,-species exposure time, indicating that one operational cycle consisting of six steps provides one GaAs,P,_, molecular layer. Furthermore, the composition of the grown GaAs,P, _-x layer corresponds to the fraction in which P atoms substitute for superficial As atoms.
257-262
259
PWI = l.OxlO~atm
PAS = 2SrlO~atm PP~ = 2.5x10Aatm
’
1.5’
I
I
I
Temp. [“Cl 0 :450 0 :480
0.0’ I
o
Time Sequence [s] Exposure - Purge GaCl 8-9 As4 4-9 _ P4 t-9 _ I
I
10
20
P4 Exposure
I 30
Time [s]
Fig. 3. Growth rate per cycle as a function of P4 exposure time.
Fig. 4 shows the double-crystal X-ray rocking curves of three samples with different P,-species exposure times. The diffraction peak intensity from the substrates is standardized. As the exposure time increases, the distances between peaks from grown layers and substrate peaks widen, indicating that the fraction of As + P substitution increases with P4 exposure time. The peak width of grown layers measures a few hundred arc-seconds. Considering the grown layer thinness and lattice mismatch, we found that high-quality GaAs,P, _-x layers were obtained and that reproducible substitutions occur cycle by cycle. The relationship between grown layer composition and P4 exposure time is shown in Fig. 5. Solid circles represent the data from ESCA analysis. Moreover, in the figure, there are an open square and a circle at an exposure time of 24 s. These two data are from the lattice parameter perpendicular to the surface plane by DC-XRD measurement and Vegard’s law for different cycle sample, 600 and 1800 cycles respectively, under the same growth conditions. They differ, because of strains in the grown layers, but ESCA analysis shows that the composition of these two samples is almost the same as in the figure. Thus, reproducible substitutions occur despite strain. ESCA is more useful in measuring the composition
260
H. Ikeda et al. /Applied
Surface Science 82 /83
(1994) 257-262
Double Crystal X-Ray Chart
Temp. PAsi
= 450°C
Pcacl= l.OxlO+atm Ph =2SxlO’atm
=2.5xlO”atm
Time Sequence [s]
Time Sequence [s]
. :ESCA
,” ]DC_X_R~~ Gatepost 1‘gUrge l-l,
t= L_
10
0 [arc-set] X-ray rocking
20
-9 -9
30
40
P4 Exposure Time [s] Fig. 5. Composition time.
24 [s]
Fig. 4. Double-crystal sure times.
4 t
As4
of P4 exposure
which may occur in step 5 in the experiment quence, is as follows:
* curves for various
of grown layer as a function
P4 expo-
of grown layers. Composition is presented logarithmically in the figure, and ESCA data in the figure shows a linear relationship between the logarithm of composition x in GaAs.P,_. and P4 exposure time. Fig. 6 shows the relationship, indicated by ESCA data, between grown layer-composition and P4 partial pressure. P4 exposure was fixed at 16 s. As expected, the higher the partial pressure of P4, the more P-rich the composition of the grown layer. However, even at the highest possible P4 partial pressure our apparatus could provide (1.0 X 10e2 atm), a layer with composition x = 0 could not be grown. Regarding the substitution phenomenon, at the substitution stage, phosphorus species were supplied in the vapor phase, and substituted arsenic species were removed in the vapor phase. The substitution reaction of surface-adsorbed As atoms to P atoms,
As( surface)
+ f P, --+ P( surface)
Temp. = 450°C PGP,CI= 1.0x10M4atm 1.0
+ i As,,
se-
(1)
PAS =2.5x10Aatm
I
I
l :Experimental
0.8
0.6
Exposure - Purge
P4 Partial Pressure Fig. 6. Composition pressure.
of grown
[atm]
layer as a function
of P4 partial
H. Ikeda et al. /Applied Surface Science 82 / 83 (1994)
257-262
261
where A&surface) and P&&ace) denote surface-adsorbed As and P atoms. The rate equation for this reaction is
substitution occurred in step 5 (P4 exposure) in the experiment sequence, P + As substitution apparently occurred in step 3 (As, exposure). We propose a substitution mechanism that pro-d[eAs(s”rface~]/dt=kl[BA~~s”rface~]m[P41n. (2) ceeds on the surface during these experiments. As shown schematically in Figs. 7a and 7b, we consider Here, k, is the rate constant, [6)As(surface)]the two kinds of substitution: (a) the substitution of surface-adsorbed fraction of As atoms to P atoms surface-adsorbed As atoms to P atoms (As + P subafter step 5 in the experiment sequence, [P,] the stitution) in step 5 and (b) the substitution of P atoms partial pressure of P4, t the exposure time to Pq, and in the second Group V layer under the Ga surface m and n are the orders of each component. During layer to As atoms (P + As substitution) in step 3. the run, [P,] is independent of time, that is, constant. Type (a) substitution corresponds to the model de] is 1. From Fig. 5, we find When t is 0, [6)As(suIface) scribed by Eq. (2). The type (b) model has P atoms that m equals unity. Eq. (3) solves integral Eq. (2): in the second Group V layer, that is, in the Group V ln[ &+“rface)] = -mJt. (3) layer just under the Ga surface layer, substituted by As atoms, while one monolayer As is grown on the This equation well explains the relationship between Ga layer, in step 3. In halogen-transport ALE, after the grown layer composition and P4 exposure time in exposure to GaCl in step 1, the grown surface is Fig. 5. However, the behavior of the data in Fig. 6, terminated with Ga atoms [3,4]. On the Ga-terminated especially at higher P4 partial pressure, cannot be (001) surface in a zinc blende structure, Group V explained by Eq. (3). According to Eq. (31, at a high atoms are exposed at the surface. We could therefore P4 partial pressure, As + P substitution should be easily find it possible for Group V atoms in the complete and the grown layer whose composition x second Group V layer to substitute for other Group is 0, that is, GaP should be obtained after step 5, as V atoms under the Group III surface layer. Such a shown in Fig. 7. This suggests that not only As -+ P substitution as shown in Fig. 7b was reported by Y. but also P + As substitution occurs. While As + P Kobayashi and N. Kobayashi [5] using the surface photo-absorption method. STEP 5 (P4 exposure) STEP 6 (purge) For P -+ As substitution, we have a rate equation As+P substitution similar to Eq. (2). In step 3, growth conditions such as As, partial pressure and As, exposure time are (a> fixed through these experiments. Therefore, the P + As substitution rate is proportional to the P-atom fraction in the second Group V layer. The final composition of the grown layer is then: STEP 3 (As4 exposure) P-As
STEP 4 (purge)
substitution
1 - x = k, %(Z”d layer)
=
k,(l
-
?4s(s”rface))~
(4)
Here, k, is a constant and OrcZhd,ayer) is the P-atom fraction in the second Group V layer. Since the Group V surface layer on which As + P substitution has occurred in step 5 becomes the second Group V layer in step 3 in the next cycle, OrcZnd,ayer) is equal to 1 - %s(surface)*From Eqs. (3) and (4),
(b)
0
:Ga
0
:As
x=k,exp(-k,t[P,]“)+l-k,. k,,
Fig. 7. Proposed substitution mechanism: (a) substitution of surface-adsorbed As atoms to P atoms; (b) substitution of P atoms in the second Group V layer under the Ga surface layer to As atoms.
k,
(5)
and n obtained from experimental data are 38.24 atm-’ . s-l, 0.784 and 1, respectively. The calculated curves using Eq. (5) are shown in Figs. 5 and 6 as solid lines. The calculated curves and
H. Ikeda et al. /Applied Surface Science 82 /83
262
Temperature [“Cl 470
480 -2
450
460
0.3
” 2s
0.2
(1994) 257-262
substitution, As + composition range substitution is not Figs. 5 and 6. The of the substitution
P and P + As. However, in the of the As-rich one, the P + As thought to be so dominant as in slope gives the activation energy reaction, 81.8 kJ/mol.
Ea = 8 1.8 kJ/mol 4. Conclusion
El = x 2 g ‘5 ‘Z $ 0
0.1 0.09 0.08 0.07
U
0.06
Time Sequence [s] Exposure - Purge GaCl 89 Ass 4 - 9 P4 16 - 9
1.32
1.34
Pti~c~ = 1.0x 10-4atn PAW = 2.5xl04atn PP.8 = 2.5x10-4atn
1.36
1.38
103/ T [K-l] Fig. 8. Arrhenius
experimental the following
plots of substitution
data agree excellently. for Eq. (2):
The substitution reaction of surface-adsorbed As atoms to P atoms was investigated quantitatively using halogen-transport ALE. We have shown the existence of two kinds of substitution, (a) surface-adsorbed As atoms to P atoms, and (b) P atoms in the second Group V layer to As atoms through the Ga surface layer. The activation energy of substitution is 81.8 kJ/mol. The substitution of As atoms to P atoms is well explained by the rate equation -d[
&,s(5urracej]/dt
= 3=4[
‘&surface)] [P, 1.
We then have Acknowledgement
-d[ %s(s”rface)]/dt= 38.24[ %s(surface)] P417 (6) This equation is the reaction rate corresponding only to As -+ P substitution occurring in step 5. Calculations from Eq. (6) are shown as dashed lines in Figs. 5 and 6. The dashed line in Fig. 6 shows that As + P substitution is complete and GaP is obtained at a high P4 partial pressure. The final composition shown as solid lines in Figs. 5 and 6 is more As-rich than that of dashed lines due to the P + As substitution occurring in step 3 in the next cycle. Fig. 8 shows the Arrhenius plot of As --) P substitution. The composition 1 - x increases with growth temperature. Almost all data lie on one straight line. As described before, these data involve two kinds of
This work was supported in part by Grant-in-Aid No. 05650006 from the Ministry of Education, Science and Culture of Japan.
References [l] T. Fukui, .I. Cryst. Growth 107 (1988) 301. [2] H. Kamei and H. Hayashi, J. Cryst. Growth 107 (1991) 567. [3] A. Koukitu, H. Ikeda, H. Suzuki and H. Seki, Jpn. J. Appl. Phys. 30 (1991) L1712. [4] A. Koukitu, N. Takahashi. Y. Miura and H. Seki, Jpn. J. Appl. Phys. 33 (1994) L613. [5] Y. Kobayashi and N. Kobayashi, Jpn. J. Appl. Phys. 31 (1992) 3988.
surface science Applied Surface Science 82/83 (1994) 257-262
ELSIWIER
Substitution of surface-adsorbed As atoms to P atoms in atomic layer epitaxy Hitoshi Ikeda *,l, Yoshiki Miura, Naoyuki Takahashi, Akinori Koukitu, Hisashi Seki Deparhent
ofApplied
Chemistry, Faculty of Technology, Tokyo University of Agriculture and Technology, Koganei, Tokyo 184, Japan
Received 9 May 1994; accepted for publication 5 July 1994
Abstract A systematic study on the substitution reaction of surface-adsorbed As atoms to P atoms was performed using atomic layer epitaxy (ALE). The reaction’s dependence on P4 exposure time, on P4 partial pressure and on growth temperature was investigated quantitatively. A mechanism consisting of two kinds of substitution is proposed. Investigation showed that the substitution reaction rate is proportional to the fraction of superficial As atoms and P4 partial pressure. The activation energy of the substitution reaction is 81.8 kJ/mol.
1. Introduction Current epitaxial growth techniques are required to fabricate heterostructures, such as quantum wells and superlattices, with atomically controlled interfaces. To put the structures to work in practical devices and to improve their performance, it is essential to understand the crystal growth mechanism on an atomic scale. Recently, in atomic layer epitaxy (ALE), we have found that P atoms substitute for surface-adsorbed As atoms, when GaAs grown surfaces are exposed to P4. An understanding of this phenomenon will contribute greatly to our knowledge of the epitaxial growth mechanism, the atomically abrupt interface required for superlattices and quantum well fabrication.
* Corresponding author. ’ Present address: OJU Electric Industry Co., Ltd., Research and Development Group, 550-5 Higashiasakawa-cho, Hachioji-shi, Tokyo 193, Japan. 0169-4332/94/$07.00
Basically, the growth of heterostructures consisting of different kinds of Group V-atoms, such as InGaAs/InP and InAs/InP, has problems with atom substitution occurring on the epitaxial surface. On the grown surface which becomes the heterointerface after the growth, arsenic atoms terminating on the surface are substituted by the phosphorus component of the next layer and vice versa. Although optimum growth conditions yielding an abrupt interface have been reported [1,21, the mechanism of substitution has not yet been clarified. One reason is the difficulty to estimate quantitatively the substitution within one atomic layer. This paper reports As to P substitution on the grown surface (As + P substitution) investigated using an atomic layer epitaxy (ALE) technique. ALE of III-V compounds is done by either metalorganic vapor-phase epitaxy (MOVPE) or halogen-transport VPE. However, the latter is better suited to growth mechanism investigation, because it easily provides monolayer growth over a wide range of growth
0 1994 Elscvier Science B.V. All rights reserved
SSDI 0169-4332(94)00227-4
258
H. lkeda
et al. /Applied
Surface
Science
82 / ;Y3 (I 994) 257-262
2. Experimental
The apparatus used in this study is shown schematically in Fig. 1. Halogen-transport ALE growth and As + P substitution were performed at atmospheric pressure in a vertical reactor. The reactor consisted of purge, exposure and source zones, the temperature of each controlled independently using resistive furnaces. As the figure shows, the reactor tube is divided into three chambers for exposure to each of the source species. Hydrogen gas is introduced from the top of the reactor and divided among the three exposure chambers. Each chamber has branch tubes connecting to each source zone for GaCl, As, and P4. GaCl was formed by chemical reaction between hydrogen chloride and gallium metal at 780°C. The Group V sources were solid arsenic and phosphorus. The partial pressures were controlled by the temperatures at the source zones. The GaAs substrate, exactly (100) oriented, was tipped on the rod as shown in Fig. 1. In upward and downward rod movement and rotation, the substrate was dipped into each source species and pulled up into the hydrogen atmosphere. In this way, it was possible to perform ALE exposure and purge sequences. Before the experiments below were con-
a
Exhaust Fig.
I. Schematic view of the apparatus
conditions, especially so in the gallium system. As a preliminary investigation, we chose the GaAs/GaP system and halogen-transport ALE to study the As --f P substitution phenomenon systematically.
Typical
growth
details
conditions
Temp. = 450 - 480 “C = 2.5x10A atm PAs4
PGSKI= 1.0~ 10e4atm = 7.5x10m4- 1.O~lO~*atrn
PP4
7
GaCl Step l(8 s)
Time Fig. 2. Typical growth conditions
+
and time sequence of one operational
cycle
H. Ikeda et al. /Applied Surface Science 82 / 83 (1994)
ducted, GaAs and GaP ALE growth was conformed with this system. Fig. 2 shows typical time sequences and growth conditions for this study. First, to fabricate an Asterminated surface, one GaAs monolayer was grown on the substrate surface by halogen-transport ALE. Next, the As-terminated surface was exposed to the phosphorus species. In this stage, the surface As atoms were substituted by the P atoms. Next, over this, another As-terminated surface was formed by one monolayer ALE growth of GaAs. Thus, one operational cycle consisted of six steps: exposures to GaCl (step 11, As, (step 3) and P4 (step 5) and the purges of these species by hydrogen (steps 2, 4 and 6). This cycle was repeated 600 or 1800 times per run. The fraction of the As -+ P substitution reaction was estimated quantitatively by measuring the composition of the grown layer, that is, the composition of GaAs, P, _-x. The composition of the layers was measured by electron spectroscopy for chemical analysis (ESCA). Double-crystal X-ray diffraction (DC-XRD) was used to measure lattice parameters perpendicular to the surface plane and the peak diffraction pattern width. Grown layer thickness was measured by observing the cleaved surface with a Nomarski microscope.
3. Results and discussion Fig. 3 shows the growth rate per cycle as a function of exposure time to P4 species of Asterminated surfaces after GaAs ALE growth at 450 and 480°C. Growth rates were calculated from the thickness of grown layers and the lattice parameter perpendicular to the surface plane measured by DCXRD, that is, by dividing the layer thickness by the product of half of the lattice parameter and the number of cycles run. In this study, after ALE growth of one GaAs molecular layer, the Asterminated surface was exposed to the P4 species (step 5 in the experimental sequence). As the figure shows, growth rates are independent of the P,-species exposure time, indicating that one operational cycle consisting of six steps provides one GaAs,P,_, molecular layer. Furthermore, the composition of the grown GaAs,P, _-x layer corresponds to the fraction in which P atoms substitute for superficial As atoms.
257-262
259
PWI = l.OxlO~atm
PAS = 2SrlO~atm PP~ = 2.5x10Aatm
’
1.5’
I
I
I
Temp. [“Cl 0 :450 0 :480
0.0’ I
o
Time Sequence [s] Exposure - Purge GaCl 8-9 As4 4-9 _ P4 t-9 _ I
I
10
20
P4 Exposure
I 30
Time [s]
Fig. 3. Growth rate per cycle as a function of P4 exposure time.
Fig. 4 shows the double-crystal X-ray rocking curves of three samples with different P,-species exposure times. The diffraction peak intensity from the substrates is standardized. As the exposure time increases, the distances between peaks from grown layers and substrate peaks widen, indicating that the fraction of As + P substitution increases with P4 exposure time. The peak width of grown layers measures a few hundred arc-seconds. Considering the grown layer thinness and lattice mismatch, we found that high-quality GaAs,P, _-x layers were obtained and that reproducible substitutions occur cycle by cycle. The relationship between grown layer composition and P4 exposure time is shown in Fig. 5. Solid circles represent the data from ESCA analysis. Moreover, in the figure, there are an open square and a circle at an exposure time of 24 s. These two data are from the lattice parameter perpendicular to the surface plane by DC-XRD measurement and Vegard’s law for different cycle sample, 600 and 1800 cycles respectively, under the same growth conditions. They differ, because of strains in the grown layers, but ESCA analysis shows that the composition of these two samples is almost the same as in the figure. Thus, reproducible substitutions occur despite strain. ESCA is more useful in measuring the composition
260
H. Ikeda et al. /Applied
Surface Science 82 /83
(1994) 257-262
Double Crystal X-Ray Chart
Temp. PAsi
= 450°C
Pcacl= l.OxlO+atm Ph =2SxlO’atm
=2.5xlO”atm
Time Sequence [s]
Time Sequence [s]
. :ESCA
,” ]DC_X_R~~ Gatepost 1‘gUrge l-l,
t= L_
10
0 [arc-set] X-ray rocking
20
-9 -9
30
40
P4 Exposure Time [s] Fig. 5. Composition time.
24 [s]
Fig. 4. Double-crystal sure times.
4 t
As4
of P4 exposure
which may occur in step 5 in the experiment quence, is as follows:
* curves for various
of grown layer as a function
P4 expo-
of grown layers. Composition is presented logarithmically in the figure, and ESCA data in the figure shows a linear relationship between the logarithm of composition x in GaAs.P,_. and P4 exposure time. Fig. 6 shows the relationship, indicated by ESCA data, between grown layer-composition and P4 partial pressure. P4 exposure was fixed at 16 s. As expected, the higher the partial pressure of P4, the more P-rich the composition of the grown layer. However, even at the highest possible P4 partial pressure our apparatus could provide (1.0 X 10e2 atm), a layer with composition x = 0 could not be grown. Regarding the substitution phenomenon, at the substitution stage, phosphorus species were supplied in the vapor phase, and substituted arsenic species were removed in the vapor phase. The substitution reaction of surface-adsorbed As atoms to P atoms,
As( surface)
+ f P, --+ P( surface)
Temp. = 450°C PGP,CI= 1.0x10M4atm 1.0
+ i As,,
se-
(1)
PAS =2.5x10Aatm
I
I
l :Experimental
0.8
0.6
Exposure - Purge
P4 Partial Pressure Fig. 6. Composition pressure.
of grown
[atm]
layer as a function
of P4 partial
H. Ikeda et al. /Applied Surface Science 82 / 83 (1994)
257-262
261
where A&surface) and P&&ace) denote surface-adsorbed As and P atoms. The rate equation for this reaction is
substitution occurred in step 5 (P4 exposure) in the experiment sequence, P + As substitution apparently occurred in step 3 (As, exposure). We propose a substitution mechanism that pro-d[eAs(s”rface~]/dt=kl[BA~~s”rface~]m[P41n. (2) ceeds on the surface during these experiments. As shown schematically in Figs. 7a and 7b, we consider Here, k, is the rate constant, [6)As(surface)]the two kinds of substitution: (a) the substitution of surface-adsorbed fraction of As atoms to P atoms surface-adsorbed As atoms to P atoms (As + P subafter step 5 in the experiment sequence, [P,] the stitution) in step 5 and (b) the substitution of P atoms partial pressure of P4, t the exposure time to Pq, and in the second Group V layer under the Ga surface m and n are the orders of each component. During layer to As atoms (P + As substitution) in step 3. the run, [P,] is independent of time, that is, constant. Type (a) substitution corresponds to the model de] is 1. From Fig. 5, we find When t is 0, [6)As(suIface) scribed by Eq. (2). The type (b) model has P atoms that m equals unity. Eq. (3) solves integral Eq. (2): in the second Group V layer, that is, in the Group V ln[ &+“rface)] = -mJt. (3) layer just under the Ga surface layer, substituted by As atoms, while one monolayer As is grown on the This equation well explains the relationship between Ga layer, in step 3. In halogen-transport ALE, after the grown layer composition and P4 exposure time in exposure to GaCl in step 1, the grown surface is Fig. 5. However, the behavior of the data in Fig. 6, terminated with Ga atoms [3,4]. On the Ga-terminated especially at higher P4 partial pressure, cannot be (001) surface in a zinc blende structure, Group V explained by Eq. (3). According to Eq. (31, at a high atoms are exposed at the surface. We could therefore P4 partial pressure, As + P substitution should be easily find it possible for Group V atoms in the complete and the grown layer whose composition x second Group V layer to substitute for other Group is 0, that is, GaP should be obtained after step 5, as V atoms under the Group III surface layer. Such a shown in Fig. 7. This suggests that not only As -+ P substitution as shown in Fig. 7b was reported by Y. but also P + As substitution occurs. While As + P Kobayashi and N. Kobayashi [5] using the surface photo-absorption method. STEP 5 (P4 exposure) STEP 6 (purge) For P -+ As substitution, we have a rate equation As+P substitution similar to Eq. (2). In step 3, growth conditions such as As, partial pressure and As, exposure time are (a> fixed through these experiments. Therefore, the P + As substitution rate is proportional to the P-atom fraction in the second Group V layer. The final composition of the grown layer is then: STEP 3 (As4 exposure) P-As
STEP 4 (purge)
substitution
1 - x = k, %(Z”d layer)
=
k,(l
-
?4s(s”rface))~
(4)
Here, k, is a constant and OrcZhd,ayer) is the P-atom fraction in the second Group V layer. Since the Group V surface layer on which As + P substitution has occurred in step 5 becomes the second Group V layer in step 3 in the next cycle, OrcZnd,ayer) is equal to 1 - %s(surface)*From Eqs. (3) and (4),
(b)
0
:Ga
0
:As
x=k,exp(-k,t[P,]“)+l-k,. k,,
Fig. 7. Proposed substitution mechanism: (a) substitution of surface-adsorbed As atoms to P atoms; (b) substitution of P atoms in the second Group V layer under the Ga surface layer to As atoms.
k,
(5)
and n obtained from experimental data are 38.24 atm-’ . s-l, 0.784 and 1, respectively. The calculated curves using Eq. (5) are shown in Figs. 5 and 6 as solid lines. The calculated curves and
H. Ikeda et al. /Applied Surface Science 82 /83
262
Temperature [“Cl 470
480 -2
450
460
0.3
” 2s
0.2
(1994) 257-262
substitution, As + composition range substitution is not Figs. 5 and 6. The of the substitution
P and P + As. However, in the of the As-rich one, the P + As thought to be so dominant as in slope gives the activation energy reaction, 81.8 kJ/mol.
Ea = 8 1.8 kJ/mol 4. Conclusion
El = x 2 g ‘5 ‘Z $ 0
0.1 0.09 0.08 0.07
U
0.06
Time Sequence [s] Exposure - Purge GaCl 89 Ass 4 - 9 P4 16 - 9
1.32
1.34
Pti~c~ = 1.0x 10-4atn PAW = 2.5xl04atn PP.8 = 2.5x10-4atn
1.36
1.38
103/ T [K-l] Fig. 8. Arrhenius
experimental the following
plots of substitution
data agree excellently. for Eq. (2):
The substitution reaction of surface-adsorbed As atoms to P atoms was investigated quantitatively using halogen-transport ALE. We have shown the existence of two kinds of substitution, (a) surface-adsorbed As atoms to P atoms, and (b) P atoms in the second Group V layer to As atoms through the Ga surface layer. The activation energy of substitution is 81.8 kJ/mol. The substitution of As atoms to P atoms is well explained by the rate equation -d[
&,s(5urracej]/dt
= 3=4[
‘&surface)] [P, 1.
We then have Acknowledgement
-d[ %s(s”rface)]/dt= 38.24[ %s(surface)] P417 (6) This equation is the reaction rate corresponding only to As -+ P substitution occurring in step 5. Calculations from Eq. (6) are shown as dashed lines in Figs. 5 and 6. The dashed line in Fig. 6 shows that As + P substitution is complete and GaP is obtained at a high P4 partial pressure. The final composition shown as solid lines in Figs. 5 and 6 is more As-rich than that of dashed lines due to the P + As substitution occurring in step 3 in the next cycle. Fig. 8 shows the Arrhenius plot of As --) P substitution. The composition 1 - x increases with growth temperature. Almost all data lie on one straight line. As described before, these data involve two kinds of
This work was supported in part by Grant-in-Aid No. 05650006 from the Ministry of Education, Science and Culture of Japan.
References [l] T. Fukui, .I. Cryst. Growth 107 (1988) 301. [2] H. Kamei and H. Hayashi, J. Cryst. Growth 107 (1991) 567. [3] A. Koukitu, H. Ikeda, H. Suzuki and H. Seki, Jpn. J. Appl. Phys. 30 (1991) L1712. [4] A. Koukitu, N. Takahashi. Y. Miura and H. Seki, Jpn. J. Appl. Phys. 33 (1994) L613. [5] Y. Kobayashi and N. Kobayashi, Jpn. J. Appl. Phys. 31 (1992) 3988.