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On the mechanism of growth-rate enhancement by photocatalysis in metalorganic vapor phase epitaxy of ZnSe
Journal of Crystal Growth 117 (1992) 107—110 North-Holland
Jo~~o,
CRYSTAL GROWT H
On the mechanism of growth-rate enhancement by photocatalysis in metalorganic vapor phase epitaxy of ZnSe Akihiko Yoshikawa and Tamotsu Okamoto Department of Electrical and Electronics Engineering, Faculty of Engineering, Chiba University, 1-33 Yayoi-cho, Chiba-shi 260, Japan
The growth and/or reaction mechanism of photoassisted MOVPE of ZnSe using DMZn and DMSe has been studied. Some evidence is shown which indicates that the dissociation of Se sources, i.e., DMSe or H 2Se, may be initiated by the effect of photoinduced excess holes at the ZnSe surface, and this is the rate-limiting reaction for the growth rate enhancement under photo-irradiation. Furthermore, a brief discussion is provided concerning how the excess holes contribute to the dissociation of these Se sources at the surface. Some plausible reaction models will be proposed.
1. Introduction Photo-assisted metalorganic vapor phase epitaxy (MOVPE) of ZnSe-based wide-gap H—VI compounds has attracted considerable attention as a new MOVPE-based low-temperature epitaxy method [1—3].It has already been shown that the temperature for growth of ZnSe using dimethyl zinc (DMZn) and dimethyl selenide (DMSe) as reactants can be lowered to about 300°Cby irradiating with photons; this is attributed to the effect of photo-induced excess carriers in ZnSe. On the basis of the experimental results reported to date [1—7],we have proposed a growth model in which photo-induced excess holes can contribute to the growth rate enhancement. It has been shown that the proposed model can interpret how growth rate enhancement is affected by growth parameters such as irradiating-photon energy, deposited layer thickness, and the quality of the layer [3—71. However, the actual mechanism by which excess holes can contribute to growth rate enhancement is still unclear, at the In this surface, paper,i.e., we the propose surface a new reaction reaction of DMZn model and DMSe including H 2 by the effect of photocatalysis. First, some evidence will be shown which indicates that the decomposition of DMSe may 0022-0248/92/$05.00 © 1992
—
be initiated by the effect of excess holes and that this is the rate-limiting reaction for growth rate enhancement. Next, surface reaction models between DMZn and DMSe to form ZnSe will be proposed. It is also shown that the proposed model can be applied for the growth of ZnSe using H2Se as a Se source instead of DMSe.
2. Experimental procedure ZnSe layers were grown on (100) GaAs using a low-pressure photo-MOVPE apparatus reported previously [3—7].DMZn and DMSe were used as reactants. The reactor pressure during growth was kept at 50 Torr by supplying excess hydrogen gas [3,4]. H2Se was also used as a Se source to compare the effects of photo-irradiation in both cases when using DMSe and H2Se. In order to avoid a reaction between DMZn and H2Se before reaching the substrate, H 2Se was introduced into the reactor by using a separate nozzle, and the reactor pressure was reduced to less than2)1 was Torr.used A 488 as the nm irradiation Ar laser line source. (<50AmW/cm 457.9 nm line was also used when the growth temperatures were below 300°C,because the energy bandgap of ZnSe at temperatures below 250°C becomes
Elsevier Science Publishers B.V. All rights reserved
A. Yoshikawa, T Okamoto
108
/ Mechanism ofgrowth-rate enhancement byphotocatalvsi.s
larger than the energy of the 488 nm line, resulting in insufficient photo-absorption by the ZnSe layer [3,4].
400
in MOVPE of ZnSe TEMPERATURE ~C)
GROWTH
01
•
/
E
3. Results and discussion
irradiated
//_o~—~4~:nmsornw/&n2)
1
3.1. Thermal stability of DMZn and DMSe
DMZn-H 25e ~~—~.•(Ar’Laser
,.~
X
DMZn-H25e~ (non-irradiated)
Thermal stability of DMZn and DMSe in an atmosphere of 5 Torr hydrogen was examined using a quadrapole mass spectrometer (QMS). In order to avoid the effect of DMZn—DMSe adducts, the experiment for each source was performed separately. The source gases were introduced into a reactor by using an inlet-nozzle and the distance between the nozzle and the heater was about 5 cm. Furthermore, the gaseous species in the reactor was sampled and introduced into a QMS analyzer by using a thin stainless pipe (1/4 inch outer diameter). Since the source gases were also decomposed in the QMS analyzer itself, it was very difficult to determine which species were thermally decomposed in the reactor. We therefore measured the change in intensity with temperature for the source molecules themselves, i.e., DMZn~and DMSet The results are shown in fig. 1. It is shown that DMSe is stable, at least in the temperature range examined (<380°C), but the dissociation of DMZn begins at about 200°C. Therefore, it has been found that it is necessary for DMSe to be dissociated by the
to
TEMPERATURE(’C) .
400
300
____
200
100
Pressure Slorr Atmosphere H2
I
0.110
-.
2
10
16
1’~
20
22
24
26
ooy~r(K’) Fig. 1. Thermal stability of DMZn and DMSe.
28
3.0
D~1Zn-DM5e
o1
(ArLaser irradiated.4579r-ni~mW/cm2)
15
2.0
25
30
iooo/i (K~) Fig. 2. Effectsof photo-irradiation on the temperature dependence of the growth rate of ZnSe in the cases using DMSe and H2Se as Se sources.
direct or indirect (secondary) effect of photoirradiation because of its thermal stability. 3.2. Comparison of the results when using DMSe and H2Se as Se sources For the Se sources in MOVPE of ZnSe, both alkyls and hydrides, such as DMSe and H2Se, respectively, can be used. Therefore, in order to investigate the growth mechanism, effects of the photo-irradiation on the growth rate enhancement have been compared for the two different Se sources. The results are shown in fig. 2. When using H2Se, it is shown that the growth rate can be enhanced at temperatures from 100 to 250°C, even though the layer can be grown without photo-irradiation. When using DMSe, however, the growth rate without photo-irradiation is less than 300 A/h in the temperature range exammcd; consequently, results are shown in the figure only for the case of photo-irradtion. The important point which should be emphasized in fig. 2 is the temperature range where growth rate enhancement by photo-irradiation can be expected. When using DMSe, the temperature must be higher than 250°C;but when using H2Se, growth rate enhancement is remarkable even at such a low temperature as 100°C.It is reasonable to propose that the difference in the temperature
A. Yoshikawa, T. Okamoto Me~ N Mé—Zr~’-—Me
/
/ Mechanism of growth-rate enhancement by photocatalysis in MOVPE of ZnSe
Me’
5é
H’
H’ Se
Fig. 3. Structure and atomic charge in DMZn, DMSe and H.,Se. Details are given in table I.
range for the growth rate enhancement is attributed to the difference in the chemical properties of DMSe and H2Se. That is, the photo-induced excess holes probably contribute to the dissociation of Se sources, because the temperature ranges for the growth rate enhancement should be almost the same if the dissociation of DMZn could be initiated by excess carriers. Further evidence to support this idea has been obtained from the photoluminescence properties of the ZnSe layer grown under photoirradiation ~ 3.3. Structure and atomic charge distribution in DMZn, DMSe and I~I7Se
109
88. The results are shown in fig. 3 and table 1. It has been found that the DMZn has a straight structure and no dipole moment, and that Zn is positively charged. On the other hand, Se in DMSe and H,Se is negatively charged and these molecules have a finite dipole moment. Thus, it has been found that the polarity of the charges is negative for Se and positive for Zn, as expected. Therefore, we can expect that Se sources may be dissociated by the charge transfer of positive holes to them. Furthermore, since the DMSe and H2Se have a finite dipole moment and the Se in these molecules has negative charges, they will be effectively adsorbed on the positively charged surface~facing the Se atoms on the surface. This will be desirable for Se sources being effectively dissociated at the surface. 3.4. Surface reaction model
The possibility for the dissociation of Se sources by positive holes is discussed in this section. First, we have qualitatively considered the possibility of dissociation of source molecules by charge transfer at the surface. The oxidation number of Se in DMSe and H2Se is —2 and that of Zn in DMZn is + 2 [8]. Of course, the oxidation number of elemental Se and elemental Zn is 0. Therefore, we can expect that DMSe and H ,Se may be dissociated by positive holes and that DMZn may be dissociated by negative electrons, because the transfer of positive holes means the oxidation of Se and that of negative electrons means the reduction of Zn. In order to confirm this idea, we calculated the structure of the source molecules and the atomic charge distribution in them by using GAUSSIAN
In this section, we briefly discuss how the excess holes contribute to the growth rate enhancement. In previous sections, some evidence has been presented which indicates the possibility of the Se source dissociation by the photo-induced positive holes. When considering a surface reaction model, we recall that the presence of hydrogen is necessary for the reaction between DMZn and DMSe to form ZnSe [3—8]. The question is how the hydrogen gases contribute to the photocatalytic reactions between DMZn and DMSe to form ZnSe. We have two plausible reaction models. Fig. 4 shows one of our models for the surface reaction of DMSe and hydrogen to form H2Se at the Zn-terminated (100) surface of ZnSe under photo-irradiation.
Table 1 Structure and atomic charge distribution in DMZn, DMSe and H2Se Molecule
DMZn DMSe H,Se
Bonding angle (deg)
C-X--C
H-C-H
X-C-H
1813 100.6
106.9 108.7
111.9 110.2
—
—
—
H-X-H — —
92.6
Bonding distance (A)
Atomic charge (esu)
C-X
C-I-I
X
1.9440 1.9041
t.0080 1.0864
—
—
X-H — —
1.4323
0.874 —0.164 —0.249
C
H
Dipole moment (debye)
1.064 —0.164
0.209 0.082 0.124
0 1.80 1.34
—
—
X denotes Zn or Se in DMZn or in Se compounds, respectively; C and H denote carbon and hydrogen atoms, respectively.
A. Yoshikawa, T. Okamoto
110
(a)
H
2
which model is more plausible for the photocatalytic reaction of interest, study of the surface reaction using optical probing technique is now under progress. Details for the reaction model
H2
en
en
~
I
C’, I
/
~
Se
Surtae
/ Mechanism of growth-rate enhancement by photocatalysis in MOVPE of ZnSe
~
\
‘Zn
~
Zrr
‘s~‘~e~’~ (b)
will be discussed elsewhere. CHi,
H
H
H
H”~
Surt~g /
\
/
“
--
~‘
4. Summary
Zn Zn Zn ‘~e~’ ‘~e~~
Fig. 4. Proposed reaction model between DMSe and H, to form CH4 and H2Se at the ZnSe surface with the aid of photoinduced excess holes,
The reaction can be expressed by the following formulae: DMSe
+
2H2
—*
H2Se
+
2 CH4
(s),
(1)
H2Se + DMZn ZnSe + 2 CH4 (s), (2) where (s) denotes the surface reaction. First, DMSe and hydrogen react to form CH4 and H2Se on the surface with the aid of excess holes. Then the H2Se react with DMZn to form ZnSe at low temperatures. One of the key points of this model is that the model can be applied to both cases using DMSe and H2Se as Se sources. The .other plausible model is expressed by the following formulae: —~
DMSe—s2CH3+Se
(s),
CH5+H2-*CH4+H Se
+
2 H
H2Se
+
—~
H2Se
DMZn
—*
(3)
(g),
(4)
(s), ZnSe
(5) +
2 CH4
(s),
(6)
The growth and/or reaction mechanism of photo-assisted MOVPE of ZnSe using DMZn and DMSe has been studied. Some evidence has been presented which supports the idea that Se sources may be decomposed by the effect of photo-induced excess holes at the ZnSe surface and this is the rate-limiting reaction for the growth rate enhancement by photo-irradiation. Furthermore, we have discussed how the excess holes contribute to the dissociation of these Se sources at the surface, and some plausible reaction models have been proposed.
Acknowledgment This work was partly supported by the Grantsin-Aid for Scientific Research and Scientific Research for Priority Areas, from the Ministry of Education, Science and Culture in Japan. References [1] Sg. Fujita, A. Tanabe, T. Sakamoto, M. Isemura and Sz. Fujita, Japan. J. AppI. Phys. 26 (1987) L2000. [2] T. Yasuda, Y. Koyama, J. Wakitani, J. Yoshino and H.
where (g) denotes the reaction in the gas phase. In this model, DMSe molecules are dissociated into CH3 radicals and Se at the surface due to the effect of photocatalysis. The presence of hydrogen in the atmosphere is necessary to prevent the reaction expressed by
Japan. AppI. Phys. (1989) L1628. [3] Kukimoto, A. Yoshikawa, T. J.Okamoto, T. 28 Fujimoto, H. Onoue, K. Haseyama, S. Yamaga and H. Kasai, in: Proc. SPIE Symp. on Laser/Optical Processing of Elecronic Materials, Santa
CH3
[51A.
Clara, CA, Oct. 1989 [SPIE Proc. 1190 (1989) 25]. [4] A. Yoshikawa, T. Okamoto, T. Fujimoto, K. Onoue, S. Yamaga and H. Kasai, Japan. J. AppI. Phys. 29 (1990) L225.
+
CH3
—~
C2H6
(g),
(7)
and also to promote the reaction shown in formula (4). Although the above two models are speculative, they explain why the hydrogen gases are necessary for the reaction. In order to confirm
Yoshikawa, T. Okamoto and T. Fujimoto, J. Crystal Growth 107 (1991) 653. [6] T. Okamoto and A. Yoshikawa, Japan. J. AppI. Phys. 30 (1991) L156. [7] A. Yoshikawa and T. Okamoto, J. Crystal Growth 115 (1991) 274. [81RB. 1-leslop and K. Jones. Inorganic Chemistry (Elsevier, Amsterdam, 1976) ch. 8.
Jo~~o,
CRYSTAL GROWT H
On the mechanism of growth-rate enhancement by photocatalysis in metalorganic vapor phase epitaxy of ZnSe Akihiko Yoshikawa and Tamotsu Okamoto Department of Electrical and Electronics Engineering, Faculty of Engineering, Chiba University, 1-33 Yayoi-cho, Chiba-shi 260, Japan
The growth and/or reaction mechanism of photoassisted MOVPE of ZnSe using DMZn and DMSe has been studied. Some evidence is shown which indicates that the dissociation of Se sources, i.e., DMSe or H 2Se, may be initiated by the effect of photoinduced excess holes at the ZnSe surface, and this is the rate-limiting reaction for the growth rate enhancement under photo-irradiation. Furthermore, a brief discussion is provided concerning how the excess holes contribute to the dissociation of these Se sources at the surface. Some plausible reaction models will be proposed.
1. Introduction Photo-assisted metalorganic vapor phase epitaxy (MOVPE) of ZnSe-based wide-gap H—VI compounds has attracted considerable attention as a new MOVPE-based low-temperature epitaxy method [1—3].It has already been shown that the temperature for growth of ZnSe using dimethyl zinc (DMZn) and dimethyl selenide (DMSe) as reactants can be lowered to about 300°Cby irradiating with photons; this is attributed to the effect of photo-induced excess carriers in ZnSe. On the basis of the experimental results reported to date [1—7],we have proposed a growth model in which photo-induced excess holes can contribute to the growth rate enhancement. It has been shown that the proposed model can interpret how growth rate enhancement is affected by growth parameters such as irradiating-photon energy, deposited layer thickness, and the quality of the layer [3—71. However, the actual mechanism by which excess holes can contribute to growth rate enhancement is still unclear, at the In this surface, paper,i.e., we the propose surface a new reaction reaction of DMZn model and DMSe including H 2 by the effect of photocatalysis. First, some evidence will be shown which indicates that the decomposition of DMSe may 0022-0248/92/$05.00 © 1992
—
be initiated by the effect of excess holes and that this is the rate-limiting reaction for growth rate enhancement. Next, surface reaction models between DMZn and DMSe to form ZnSe will be proposed. It is also shown that the proposed model can be applied for the growth of ZnSe using H2Se as a Se source instead of DMSe.
2. Experimental procedure ZnSe layers were grown on (100) GaAs using a low-pressure photo-MOVPE apparatus reported previously [3—7].DMZn and DMSe were used as reactants. The reactor pressure during growth was kept at 50 Torr by supplying excess hydrogen gas [3,4]. H2Se was also used as a Se source to compare the effects of photo-irradiation in both cases when using DMSe and H2Se. In order to avoid a reaction between DMZn and H2Se before reaching the substrate, H 2Se was introduced into the reactor by using a separate nozzle, and the reactor pressure was reduced to less than2)1 was Torr.used A 488 as the nm irradiation Ar laser line source. (<50AmW/cm 457.9 nm line was also used when the growth temperatures were below 300°C,because the energy bandgap of ZnSe at temperatures below 250°C becomes
Elsevier Science Publishers B.V. All rights reserved
A. Yoshikawa, T Okamoto
108
/ Mechanism ofgrowth-rate enhancement byphotocatalvsi.s
larger than the energy of the 488 nm line, resulting in insufficient photo-absorption by the ZnSe layer [3,4].
400
in MOVPE of ZnSe TEMPERATURE ~C)
GROWTH
01
•
/
E
3. Results and discussion
irradiated
//_o~—~4~:nmsornw/&n2)
1
3.1. Thermal stability of DMZn and DMSe
DMZn-H 25e ~~—~.•(Ar’Laser
,.~
X
DMZn-H25e~ (non-irradiated)
Thermal stability of DMZn and DMSe in an atmosphere of 5 Torr hydrogen was examined using a quadrapole mass spectrometer (QMS). In order to avoid the effect of DMZn—DMSe adducts, the experiment for each source was performed separately. The source gases were introduced into a reactor by using an inlet-nozzle and the distance between the nozzle and the heater was about 5 cm. Furthermore, the gaseous species in the reactor was sampled and introduced into a QMS analyzer by using a thin stainless pipe (1/4 inch outer diameter). Since the source gases were also decomposed in the QMS analyzer itself, it was very difficult to determine which species were thermally decomposed in the reactor. We therefore measured the change in intensity with temperature for the source molecules themselves, i.e., DMZn~and DMSet The results are shown in fig. 1. It is shown that DMSe is stable, at least in the temperature range examined (<380°C), but the dissociation of DMZn begins at about 200°C. Therefore, it has been found that it is necessary for DMSe to be dissociated by the
to
TEMPERATURE(’C) .
400
300
____
200
100
Pressure Slorr Atmosphere H2
I
0.110
-.
2
10
16
1’~
20
22
24
26
ooy~r(K’) Fig. 1. Thermal stability of DMZn and DMSe.
28
3.0
D~1Zn-DM5e
o1
(ArLaser irradiated.4579r-ni~mW/cm2)
15
2.0
25
30
iooo/i (K~) Fig. 2. Effectsof photo-irradiation on the temperature dependence of the growth rate of ZnSe in the cases using DMSe and H2Se as Se sources.
direct or indirect (secondary) effect of photoirradiation because of its thermal stability. 3.2. Comparison of the results when using DMSe and H2Se as Se sources For the Se sources in MOVPE of ZnSe, both alkyls and hydrides, such as DMSe and H2Se, respectively, can be used. Therefore, in order to investigate the growth mechanism, effects of the photo-irradiation on the growth rate enhancement have been compared for the two different Se sources. The results are shown in fig. 2. When using H2Se, it is shown that the growth rate can be enhanced at temperatures from 100 to 250°C, even though the layer can be grown without photo-irradiation. When using DMSe, however, the growth rate without photo-irradiation is less than 300 A/h in the temperature range exammcd; consequently, results are shown in the figure only for the case of photo-irradtion. The important point which should be emphasized in fig. 2 is the temperature range where growth rate enhancement by photo-irradiation can be expected. When using DMSe, the temperature must be higher than 250°C;but when using H2Se, growth rate enhancement is remarkable even at such a low temperature as 100°C.It is reasonable to propose that the difference in the temperature
A. Yoshikawa, T. Okamoto Me~ N Mé—Zr~’-—Me
/
/ Mechanism of growth-rate enhancement by photocatalysis in MOVPE of ZnSe
Me’
5é
H’
H’ Se
Fig. 3. Structure and atomic charge in DMZn, DMSe and H.,Se. Details are given in table I.
range for the growth rate enhancement is attributed to the difference in the chemical properties of DMSe and H2Se. That is, the photo-induced excess holes probably contribute to the dissociation of Se sources, because the temperature ranges for the growth rate enhancement should be almost the same if the dissociation of DMZn could be initiated by excess carriers. Further evidence to support this idea has been obtained from the photoluminescence properties of the ZnSe layer grown under photoirradiation ~ 3.3. Structure and atomic charge distribution in DMZn, DMSe and I~I7Se
109
88. The results are shown in fig. 3 and table 1. It has been found that the DMZn has a straight structure and no dipole moment, and that Zn is positively charged. On the other hand, Se in DMSe and H,Se is negatively charged and these molecules have a finite dipole moment. Thus, it has been found that the polarity of the charges is negative for Se and positive for Zn, as expected. Therefore, we can expect that Se sources may be dissociated by the charge transfer of positive holes to them. Furthermore, since the DMSe and H2Se have a finite dipole moment and the Se in these molecules has negative charges, they will be effectively adsorbed on the positively charged surface~facing the Se atoms on the surface. This will be desirable for Se sources being effectively dissociated at the surface. 3.4. Surface reaction model
The possibility for the dissociation of Se sources by positive holes is discussed in this section. First, we have qualitatively considered the possibility of dissociation of source molecules by charge transfer at the surface. The oxidation number of Se in DMSe and H2Se is —2 and that of Zn in DMZn is + 2 [8]. Of course, the oxidation number of elemental Se and elemental Zn is 0. Therefore, we can expect that DMSe and H ,Se may be dissociated by positive holes and that DMZn may be dissociated by negative electrons, because the transfer of positive holes means the oxidation of Se and that of negative electrons means the reduction of Zn. In order to confirm this idea, we calculated the structure of the source molecules and the atomic charge distribution in them by using GAUSSIAN
In this section, we briefly discuss how the excess holes contribute to the growth rate enhancement. In previous sections, some evidence has been presented which indicates the possibility of the Se source dissociation by the photo-induced positive holes. When considering a surface reaction model, we recall that the presence of hydrogen is necessary for the reaction between DMZn and DMSe to form ZnSe [3—8]. The question is how the hydrogen gases contribute to the photocatalytic reactions between DMZn and DMSe to form ZnSe. We have two plausible reaction models. Fig. 4 shows one of our models for the surface reaction of DMSe and hydrogen to form H2Se at the Zn-terminated (100) surface of ZnSe under photo-irradiation.
Table 1 Structure and atomic charge distribution in DMZn, DMSe and H2Se Molecule
DMZn DMSe H,Se
Bonding angle (deg)
C-X--C
H-C-H
X-C-H
1813 100.6
106.9 108.7
111.9 110.2
—
—
—
H-X-H — —
92.6
Bonding distance (A)
Atomic charge (esu)
C-X
C-I-I
X
1.9440 1.9041
t.0080 1.0864
—
—
X-H — —
1.4323
0.874 —0.164 —0.249
C
H
Dipole moment (debye)
1.064 —0.164
0.209 0.082 0.124
0 1.80 1.34
—
—
X denotes Zn or Se in DMZn or in Se compounds, respectively; C and H denote carbon and hydrogen atoms, respectively.
A. Yoshikawa, T. Okamoto
110
(a)
H
2
which model is more plausible for the photocatalytic reaction of interest, study of the surface reaction using optical probing technique is now under progress. Details for the reaction model
H2
en
en
~
I
C’, I
/
~
Se
Surtae
/ Mechanism of growth-rate enhancement by photocatalysis in MOVPE of ZnSe
~
\
‘Zn
~
Zrr
‘s~‘~e~’~ (b)
will be discussed elsewhere. CHi,
H
H
H
H”~
Surt~g /
\
/
“
--
~‘
4. Summary
Zn Zn Zn ‘~e~’ ‘~e~~
Fig. 4. Proposed reaction model between DMSe and H, to form CH4 and H2Se at the ZnSe surface with the aid of photoinduced excess holes,
The reaction can be expressed by the following formulae: DMSe
+
2H2
—*
H2Se
+
2 CH4
(s),
(1)
H2Se + DMZn ZnSe + 2 CH4 (s), (2) where (s) denotes the surface reaction. First, DMSe and hydrogen react to form CH4 and H2Se on the surface with the aid of excess holes. Then the H2Se react with DMZn to form ZnSe at low temperatures. One of the key points of this model is that the model can be applied to both cases using DMSe and H2Se as Se sources. The .other plausible model is expressed by the following formulae: —~
DMSe—s2CH3+Se
(s),
CH5+H2-*CH4+H Se
+
2 H
H2Se
+
—~
H2Se
DMZn
—*
(3)
(g),
(4)
(s), ZnSe
(5) +
2 CH4
(s),
(6)
The growth and/or reaction mechanism of photo-assisted MOVPE of ZnSe using DMZn and DMSe has been studied. Some evidence has been presented which supports the idea that Se sources may be decomposed by the effect of photo-induced excess holes at the ZnSe surface and this is the rate-limiting reaction for the growth rate enhancement by photo-irradiation. Furthermore, we have discussed how the excess holes contribute to the dissociation of these Se sources at the surface, and some plausible reaction models have been proposed.
Acknowledgment This work was partly supported by the Grantsin-Aid for Scientific Research and Scientific Research for Priority Areas, from the Ministry of Education, Science and Culture in Japan. References [1] Sg. Fujita, A. Tanabe, T. Sakamoto, M. Isemura and Sz. Fujita, Japan. J. AppI. Phys. 26 (1987) L2000. [2] T. Yasuda, Y. Koyama, J. Wakitani, J. Yoshino and H.
where (g) denotes the reaction in the gas phase. In this model, DMSe molecules are dissociated into CH3 radicals and Se at the surface due to the effect of photocatalysis. The presence of hydrogen in the atmosphere is necessary to prevent the reaction expressed by
Japan. AppI. Phys. (1989) L1628. [3] Kukimoto, A. Yoshikawa, T. J.Okamoto, T. 28 Fujimoto, H. Onoue, K. Haseyama, S. Yamaga and H. Kasai, in: Proc. SPIE Symp. on Laser/Optical Processing of Elecronic Materials, Santa
CH3
[51A.
Clara, CA, Oct. 1989 [SPIE Proc. 1190 (1989) 25]. [4] A. Yoshikawa, T. Okamoto, T. Fujimoto, K. Onoue, S. Yamaga and H. Kasai, Japan. J. AppI. Phys. 29 (1990) L225.
+
CH3
—~
C2H6
(g),
(7)
and also to promote the reaction shown in formula (4). Although the above two models are speculative, they explain why the hydrogen gases are necessary for the reaction. In order to confirm
Yoshikawa, T. Okamoto and T. Fujimoto, J. Crystal Growth 107 (1991) 653. [6] T. Okamoto and A. Yoshikawa, Japan. J. AppI. Phys. 30 (1991) L156. [7] A. Yoshikawa and T. Okamoto, J. Crystal Growth 115 (1991) 274. [81RB. 1-leslop and K. Jones. Inorganic Chemistry (Elsevier, Amsterdam, 1976) ch. 8.