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In-situ observation of Ga adsorption during TMGa exposure on GaAs(001) surfaces with various As coverages
applied
surface science ELSEVIER
Applied Surface Science 82/83 (1994) 263-268
In-situ observation of Ga adsorption during TMGa exposure on GaAs(OO1) surfaces with various As coverages Shigeyuki Otake *, Akira Sakamoto, Masateru Yamamoto, Izumi Iwasa Foundation Research Laboratory, Fuji Xerox Co., Ltd., 2274 Hong0 Ebina-shi, Kanagawa 243-04, Japan
Received 28 May 1994; accepted for publication 4 July 1994
Abstract We measured transients of surface photoabsorption signals during TMGa exposure on c(4 X 4)-like, 2 X 4-like and Ga-terminated GaAs(OO1) surfaces in the MOCVD reactor. Ga-containing species were continuously deposited during TMGa exposure on Ga-terminated surfaces. On the other hand, self-limiting deposition was observed on c(4 X 4)-like and 2 x 4-like surfaces. The growth thickness in one cycle of ALE process on the 2 X 4-like surface was 0.7-0.8 compared to that on the c(4 X 4)-like surface. The stable duration of the SPA signal under TMGa exposure was longer than the residency time of methyl groups on the surface which was obtained from the transients during purge after TMGa exposure. Hence the methyl groups are continuously supplied to the surface by decomposition of TMGa during the exposure period under self-limiting condition.
1. Introduction Atomic layer epitaxy (ALE) is an attractive growth method with its potentially high controllability and uniformity. Nishizawa et al. [l] applied ALE on GaAs(001) surface in the MOCVD reactor using trimethylgallium (TMGa) and arsine (AsH,). Details of the surface chemistry including adsorption, desorption, and reaction processes have been studied intensively. An extensive summary of these results has been given by Creighton and Banse [2]. But there are still contradictions in the experimental results. The residency time of methyl groups, for example, is reported to be less than a second [2-41 and longer than 10 s [5,6] at around 450°C.
* Corresponding author. Tel: +81-462-38-3111; Fax: +81462-37-1371.
Recent progress in optical monitoring methods [7,8] makes it possible to observe directly the state of growing surfaces in the MOCVD reactor. Reconstructions of the GaAs(001) surface under atmospheric pressure were observed by reflectance-difference spectroscopy (RDS) [9] and X-ray diffraction [lo-121. We have to take these reconstructions into consideration to explain the ALE mechanism. Kobayashi and Horikoshi [13] and Aspnes et al. [3,14,15] have observed surfaces under TMGa exposure using surface photoabsorption (SPA) and RDS, respectively. The surface reconstructions of Asterminated surfaces used in these works were c(4 X 4)-like and excessively As-adsorbed surfaces. The 2 X 4-like surface was not examined yet. In the previous paper we observed two kinds of As desorption processes on the GaAs(001) surface using the SPA method [16] which we assigned as the desorption process of excess As and the desorption process
0169-4332/94/$07.00 0 1994 Elsevier Science B.V. All rights reserved SSDI 0169-4332(94)00228-2
S. Otake et al. /Applied
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from c(4 X 4)-like to 2 X 4-like surface [17]. This assignment agrees with the result obtained by Kamiya et al. [9] using the RDS method. In this paper, we report directly observed results of adsorption and desorption of species related to TMGa on the c(4 X 4)-like, the 2 X 4-like and the Ga-terminated GaAs(001) surfaces by the SPA method. The layer thickness during the ALE process on these surfaces was observed simultaneously.
2. Experimental
details
In this SPA study, we measured the reflected intensity of a p-polarized Ar-ion laser (488 nm) beam from GaAs(OO1) wafers placed in the MOCVD reactor. The experimental set-up was reported previously [16]. The angle of incidence was 73” and the azimuth of incidence was parallel to [ilO] on the (001) surface. The H, flow rate in the MOCVD reactor was 1.5 X 10-j mol/s at a total pressure of 12 kPa. ASH, and TMGa or TMAl were introduced alternatingly. The TMGa and TMAl flow rates were 0.03 and 0.036 /I.(micro)mol/s, respectively. ASH, partial pressures of 0.06 and 6.0 Pa were used.
3. Results 3.1. SPA signal subsequent purge
during
TMGa
exposure
and
Fig. 1 shows the transients (A, B, C) of the SPA signal of various surfaces during TMGa exposure and subsequent H, purge at 460°C. Transient A was on the c(4 x 4)-like surface which was formed under ASH, partial pressure of 0.06 Pa at 460°C. Transient B was on the 2 X 4-like surface which was prepared by annealing the c(4 X 4)-like surface at 550°C in the H, flow for about 200 s and cooled down to 460°C. Transient C was on the Ga-terminated surface which was formed by annealing the TMGa exposed c(4 X 4)like surface in the H, flow at 460°C until all methyl groups were desorbed from the surface. The SPA signals from the 2 X 4-like and the Gaterminated surfaces were 0.8% and 1.25% higher than that of the c(4 X 4)-like surface, respectively.
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150
Fig. 1. Transients of the SPA signal from various surfaces during TMGa exposure and subsequent H, purge: (A) on the ~(4 X 4).like surface, (B) on the 2X4-like surface, (C) on the Ga-terminated surface.
In transient A, the SPA signal increased as soon as the surface was exposed to TMGa (t = O-5 s). After the increase, a gradual decrease (t = 5-30 s) was observed. During the purge period (t > 30 s) after the TMGa exposure, the signal increased about 1% and became constant at the final stage (t = 150 s). In transient B, the SPA signal decreased as soon as TMGa was exposed. Then a gradual decrease (t = 5-30 s) similar to transient A was observed. After TMGa exposure the signal increased and became constant at the same value as transient A. A decrease was observed just after TMGa exposure in transient C, and then the signal increased linearly until TMGa was turned off. During the purge period after the TMGa exposure, the signal increased and became constant. Next we measured the layer thickness by observing the SPA signal while GaAs was grown on the GaAs/AlAs/GaAs structure. At first AlAs was grown on a GaAs substrate using 30 cycles of AsH,(6 Pa)/purge/TMAl/purge = 8/2/2/2 s. Then GaAs was grown using cycles of AsH,(0.06 Pa)/purge/ TMGa/purge = 80/15/5/15 s. The baseline of the SPA signal varied sinusoidally as the thickness of the upper GaAs layer increased. Thus by choosing the GaAs thickness in such a way that the reflectivity changes almost linearly, we could monitor the thick-
S. Otake et al. /Applied
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ness of the layer as a linear function of reflectivity. Fig. 2 shows three SPA signal transients (#l, 2, 3) during a cycle of AsH,(0.06 Pa)/purge/TMGa/ purge/AsH,(0.06 Pa> on the GaAs/AlAs/GaAs structure at 470°C. Three cycles were executed successively and signals were normalized at the starting reflectivity of each cycle. These three traces were started from the c(4 X 4)-&e surfaces and ended as the c(4 X 4)-&e surfaces, but differing in the duration of purge after TMGa and in the duration of TMGa exposure. Transient #l was the SPA signal during a cycle of ASH,/ purge/ TMGa/ purge AsH, = 1.50/ 15/30/ lO/ 150 s. Transient #2 was measured with a long purge period (70 s) after the TMGa exposure. Transient #3 was measured with a short TMGa exposure period (5 s), i.e. TMGa was turned off just after the fast increase. The increment of the signal level under the AsH, pressure before (t < 0 s) and after the TMGa exposure (t > 150 s) corresponds to the thickness increase in a cycle. In these three cycles, the signal increments were the same. This result shows that the same amount of Ga was deposited on the surface during these three cycles. Ga-containing species were adsorbed within the fast signal increase (less than 5 s in this case), and Ga
(1994) 263-268
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Fig. 3. Transients of SPA signal on the 2X4-&e and on the c(4X4)_like surfaces during a cycle of ALE growth on the GaAs/AlAs/GaAs structure.
was not deposited during the gradual decrease (t = 5-30 s) in spite of the TMGa exposure. In other words, Ga deposition was inhibited during the gradual decrease. The duration of the purge period after the TMGa exposure did not influence the total amount of deposited Ga. Therefore, the increase in the signal during the purge period after the TMGa exposure was caused by desorption of methyl groups, not by Ga-containing species, such as monomethylgallium and dimethylgallium.
1.02 -
3.2. Ga deposition by ALE on c(4 x 4)-like and 2 X 4-like surfaces
$1 .Ol z 2 c( 1 .oo -
0.99’
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Fig. 2. SPA signal transients started from the c(4 X 4)like during a cycle of AsH,/purge/TMGa/purge/AsH, GaAs/AlAs/GaAs structure.
surfaces on the
We compared the layer thickness in one cycle of ALE growth on c(4 X 4)-like 2 X 4-like surfaces on the GaAs/AlAs/GaAs structure. Fig. 3 shows the SPA signal transients during a cycle of purge/ TMGa/purge/AsH,(0.06 Pa). The transients started from the c(4 X 4)-like and the 2 X 4-like surfaces and ended as the c(4 X 4)like surfaces. The difference in the initial signal level between the 2 X 4-like and the c(4 X 4)-like surfaces was 1.1% which was measured by exposing the 2 X 4-like surface to 0.06 Pa AsH,.
S. Otake et al. /Applied
Surface Science 82/X3
Temp. (“C)
1000/T
(K-‘)
Fig. 4. Arrhenius plot of methyl desorption rate estimated the transients during purge after TMGa exposure.
from
In Fig. 3, the increment between t < 0 and t = 160 s on the 2 X 4-like surface was 0.7 times as large as that on the c(4 X 4)-like surface. We measured the ratio of the layer thickness on 2 X 4-like and c(4 X 4)-like surfaces several times, and obtained values between 0.7 and 0.8.
3.3. Methyl desorption
rate
Fig. 4 shows an Arrhenius plot of the methyl desorption rate estimated from the increase in the SPA signal during the purge period after TMGa exposure. We obtained comparable time constants both on the c(4 X 4)-like and the 2 X 4-like surfaces, Data shown in this figure were obtained on the c(4 X 4)-like surface. The time constants of methyl desorption were 33 s at 460°C and 5 s at 500°C. The activation energy was 2.1 eV (48 kcal/mol).
4. Discussion The transients in Fig. 1 indicate that the adsorption of Ga is self-limited on the 2 X 4-like and the c(4 X 4)-like surfaces but not on the Ga-terminated
(1994) 263-268
surface. Continuous adsorption of Ga on the Gaterminated surface contradicts the selective adsorption (SA) mechanism [l&19] for ALE, which supposes that TMGa adheres to As but not to Ga. The self-limited Ga deposition on As-terminated 2 X 4like and c(4 X 4)-like surfaces supports the adsorbate inhibition (AI) mechanism [l] which supposes that adsorption of TMGa converts the As-terminated surface to a Ga-terminated surface covered with methyl groups and that the methyl groups inhibit further adsorption of TMGa. The AI mechanism also supposes that the selflimiting duration is shorter than the residency time of methyl groups on the surface. The time constant of methyl desorption, shown in Fig. 4, is about 20 s at 470°C. We obtained a stable duration of > 30 s during TMGa exposure at 460°C as shown in Fig. 1 and as long as > 120 s at 470°C. The amount of Ga on the surface does not change during the TMGa exposure period as shown in Fig. 2. From these results, we propose that methyl groups desorb from the surface with a time constant even during TMGa exposure but fresh methyl groups are supplied to the surface by decomposition of TMGa arrived at the desorbed sites. The coverage of methyl groups is determined by the balance of the desorption and the incorporation under the TMGa exposure. And the methyl groups continuously inhibit excess Ga adsorption for a duration longer than the residency time of the surface methyl groups. We call it dynamic adsorbate inhibition (DA11 model. Based on the DAI model, ALE at higher temperatures becomes possible if the arrival rate of TMGa on the surface is high. ALE was performed within the temperature range between 500 and 560°C by Ozeki et al. [19,20]. A growth rate of 1 ML/cycle was obtained under a TMGa exposure duration of 15 s. Their rate of TMGa exposure was more than 2000 ML/s (time constant = 4.5 X lop4 s). Even if the time constant of methyl desorption was less than 0.1 s, the GaAs surface was covered with methyl groups, from the viewpoint of the DA1 model. The residency times of methyl groups on the surface has been studied by various methods. The data obtained from SPA [5] and Auger electron spectroscopy (AES) [6j are close to our result. Shorter residency times were obtained by time-programmed desorption (TPD) [2], by time-resolved mass spec-
S. Otake et al. /Applied
Surface Science 82 /83
troscopy [4] and by RDS [3]. The origin of the discrepancy awaits further study. Reconstruction of the As-terminated surface is another important factor to understand the ALE mechanism. In transient A and B of Fig. 1, the SPA signals reach the same reflectivity at the final stage of the purge period. Therefore, the Ga coverage on these surfaces should be the same in spite of the difference in the starting As coverages. Arsenic coverages of the c(4 X 4) surface and the 2 X 4 surface are reported to be 1.75 and 0.75, respectively [21]. Our ratio of growth rates on the 2 X 4-like surface and that on the c(4 X 4)-surface is 0.7-0.8. In our result, the growth rate is just 1 ML/cycle on the c(4 X 4)-like surface which was measured by optical interference using cycles of AsH ,(0.06 Pa>/ purge/ TMGa/purge = 80/15/5/15 s at 470°C. Therefore, the growth rate on the 2 X 4-like surface is 0.7-0.8 ML/cycle. The growth rate of just 1 ML/cycle was reported for ALE in the MOCVD reactor under AsH, partial pressures between several Pa and more than 100 Pa [3,15,19,20]. Under these ASH, pressures, surface reconstructions are the c(4 X 4)-like or the excessively As-adsorbed surfaces [2,9,17]. Although the As coverage is more than one monolayer on these surfaces, the growth rate is just one monolayer per cycle. Excessively adsorbed As must be desorbed during TMGa exposure. On the other hand, Goto et al. [6] observed 0.7 ML/cycle by exposing TMGa on the 2 X 4 reconstructed surfaces in the MOMBE chamber, in agreement with our growth rate on the 2 X 4-like surface. The amount of Ga-containing species adsorbed on the surface is equal to the coverage of the As when the 2 X 4-like surface was exposed to TMGa. Therefore, TMGa molecules decompose on the 2 X 4-like surface and supply the same number of Ga-containing species as the As atoms on that surface. It is remarkable that Ga adsorption limits itself on the 2 X 4-like surface at 0.75 ML. We have to consider the reason why the Ga atoms on the 2 X 4like surface do not adsorb Ga. The Ga adsorption rate on As-terminated surface is faster than that on Ga-terminated surface. Ga adsorb on to the As atoms on the As-terminated surface within a few seconds but adsorption of 1 ML Ga takes more than 30 s on Ga-terminated surface at 470°C. Hence As atoms on
(1994) 263-268
267
the 2 X 4-like surface are selectively covered with MMGa within a few seconds, and methyl groups hinder adsorption of Ga on Ga atoms.
5. Conclusion Ga adsorption on c(4 X 4)-like, 2 X 4-like and Ga-terminated surfaces during TMGa exposure was studied using the SPA method. We proposed a dynamic adsorbate inhibition (DAI) model for selflimiting supposing continuous methyl adsorption and desorption during TMGa exposure. Self-limited adsorption is observed not only on the c(4 X 4)-like surface but also on the 2 X 4-like surface. The layer thickness in an ALE cycle was 1 ML/cycle on the c(4 X 4)-like surface and 0.7-0.8 ML/cycle on the 2 X 4-like surface. The fact that Ga adsorption limits itself on the 2 X 4-like surface at 0.75 ML suggests that less than one monolayer adsorption of methyl groups can inhibit further Ga adsorption.
References [l] .I. Nishizawa, H. Abe, T. Kurabayashi and N. Sakurai, J. Vat. Sci. Technol. A 4 (1986) 706. [2] J.R. Creighton and B.A. Banse, Mater. Res. Sot. Symp. Proc. 222 (1991) 15. [3] D.E. Aspnes, E. Colas, A.A. Studna, R. Bhat, M.A. Koza and V.G. Keramidas, Phys. Rev. Lett. 61 (1988) 2782. 141 U. Memmert and M.L. Yu, Appl. Phys. L&t. 56 (1990) 1883. [5] N. Kobayashi and Y. Horikoshi, Jpn. J. Appl. Phys. 30 (1991) L319. [6] S. Goto, H. Ohno, Y. Nomura, Y. Morishita, A. Watanabe and Y. Katayama, J. Cryst. Growth 127 (1993) 1005. [7] D.E. Aspnes, J.P. Harbison, A.A. Studna and L.T. Florez, Phys. Rev. Lett. 59 (1987) 1687. [8] N. Kobayashi and Y. Horikoshi, Jpn. J. Appl. Phys. 28 (1989) L1880. [9] I. Kamiya, D.E. Aspnes, L.T. Florez and J.P. Harbison, Phys. Rev. B 46 (1992) 15894. [lo] H. Fuoss, D.W. Kisker, G. Renaud, K.L. Tokuda, S. Brennam and J.L. Kahn, Phys. Rev. Lett. 20 (1989) 2389. [ll] D.W. Kisker, H. Fuoss, K.L. Tokuda, G. Renaud, S. Brennam and J.L. Kahn, Appl. Phys. L&t. 56 (1990) 2025. [12] M. Sauvage-Sin&in, R. Pinchaux, J. Massies, P. Calverie, N. Jedrecy, J. Bonnet and I.K. Robinson, Phys. Rev. L&t. 62 (1989) 563. [13] N. Kobayashi and Y. Horikoshi, Jpn. J. Appl. Phys. 30 (1991) L319.
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[14] E. Colas, D.E. Aspnes, R. Bhat, A.A. Studna, M.K. Koza and V.G. Keramidas, J. Cryst. Growth 94 (1989) 613. [15] D.E. Aspnes, I. Kamiya, H. Tanaka and R. Bhat, J. Vat. Sci. Technol. B 10 (1992) 1725. [16] A. Sakamoto, S. Otake and 1. Iwasa, Jpn. J. Appl. Phys. 32 (1993) L1318. [17] A. Sakamoto, S. Otake, M. Yamamoto and I. Iwasa, J. Cryst. Growth, 145 (1994) 22.
[18] G.B. Stringfellow, Organometallic Vapor-Phase Epitaxy (Academic Press, San Diego, CA, 1989) p. 363. [19] M. Ozeki, K. Mochizuki, N. Ohtuka and K. Kodama, Appl. Phys. Wt. 53 (1988) 1509. [20] K. Mochizuki, M. Ozeki, K. Kodama and N. Ohtuka, J. Cryst. Growth 93 (1988) 557. [21] D.K. Biegelsen, R.D. Bringans, J.E. Northrup and L.E. Swarm, Phys. Rev. B 41 (1990) 5701.
surface science ELSEVIER
Applied Surface Science 82/83 (1994) 263-268
In-situ observation of Ga adsorption during TMGa exposure on GaAs(OO1) surfaces with various As coverages Shigeyuki Otake *, Akira Sakamoto, Masateru Yamamoto, Izumi Iwasa Foundation Research Laboratory, Fuji Xerox Co., Ltd., 2274 Hong0 Ebina-shi, Kanagawa 243-04, Japan
Received 28 May 1994; accepted for publication 4 July 1994
Abstract We measured transients of surface photoabsorption signals during TMGa exposure on c(4 X 4)-like, 2 X 4-like and Ga-terminated GaAs(OO1) surfaces in the MOCVD reactor. Ga-containing species were continuously deposited during TMGa exposure on Ga-terminated surfaces. On the other hand, self-limiting deposition was observed on c(4 X 4)-like and 2 x 4-like surfaces. The growth thickness in one cycle of ALE process on the 2 X 4-like surface was 0.7-0.8 compared to that on the c(4 X 4)-like surface. The stable duration of the SPA signal under TMGa exposure was longer than the residency time of methyl groups on the surface which was obtained from the transients during purge after TMGa exposure. Hence the methyl groups are continuously supplied to the surface by decomposition of TMGa during the exposure period under self-limiting condition.
1. Introduction Atomic layer epitaxy (ALE) is an attractive growth method with its potentially high controllability and uniformity. Nishizawa et al. [l] applied ALE on GaAs(001) surface in the MOCVD reactor using trimethylgallium (TMGa) and arsine (AsH,). Details of the surface chemistry including adsorption, desorption, and reaction processes have been studied intensively. An extensive summary of these results has been given by Creighton and Banse [2]. But there are still contradictions in the experimental results. The residency time of methyl groups, for example, is reported to be less than a second [2-41 and longer than 10 s [5,6] at around 450°C.
* Corresponding author. Tel: +81-462-38-3111; Fax: +81462-37-1371.
Recent progress in optical monitoring methods [7,8] makes it possible to observe directly the state of growing surfaces in the MOCVD reactor. Reconstructions of the GaAs(001) surface under atmospheric pressure were observed by reflectance-difference spectroscopy (RDS) [9] and X-ray diffraction [lo-121. We have to take these reconstructions into consideration to explain the ALE mechanism. Kobayashi and Horikoshi [13] and Aspnes et al. [3,14,15] have observed surfaces under TMGa exposure using surface photoabsorption (SPA) and RDS, respectively. The surface reconstructions of Asterminated surfaces used in these works were c(4 X 4)-like and excessively As-adsorbed surfaces. The 2 X 4-like surface was not examined yet. In the previous paper we observed two kinds of As desorption processes on the GaAs(001) surface using the SPA method [16] which we assigned as the desorption process of excess As and the desorption process
0169-4332/94/$07.00 0 1994 Elsevier Science B.V. All rights reserved SSDI 0169-4332(94)00228-2
S. Otake et al. /Applied
264
Surface Science
82 /a.?
(1994)
263-268
from c(4 X 4)-like to 2 X 4-like surface [17]. This assignment agrees with the result obtained by Kamiya et al. [9] using the RDS method. In this paper, we report directly observed results of adsorption and desorption of species related to TMGa on the c(4 X 4)-like, the 2 X 4-like and the Ga-terminated GaAs(001) surfaces by the SPA method. The layer thickness during the ALE process on these surfaces was observed simultaneously.
2. Experimental
details
In this SPA study, we measured the reflected intensity of a p-polarized Ar-ion laser (488 nm) beam from GaAs(OO1) wafers placed in the MOCVD reactor. The experimental set-up was reported previously [16]. The angle of incidence was 73” and the azimuth of incidence was parallel to [ilO] on the (001) surface. The H, flow rate in the MOCVD reactor was 1.5 X 10-j mol/s at a total pressure of 12 kPa. ASH, and TMGa or TMAl were introduced alternatingly. The TMGa and TMAl flow rates were 0.03 and 0.036 /I.(micro)mol/s, respectively. ASH, partial pressures of 0.06 and 6.0 Pa were used.
3. Results 3.1. SPA signal subsequent purge
during
TMGa
exposure
and
Fig. 1 shows the transients (A, B, C) of the SPA signal of various surfaces during TMGa exposure and subsequent H, purge at 460°C. Transient A was on the c(4 x 4)-like surface which was formed under ASH, partial pressure of 0.06 Pa at 460°C. Transient B was on the 2 X 4-like surface which was prepared by annealing the c(4 X 4)-like surface at 550°C in the H, flow for about 200 s and cooled down to 460°C. Transient C was on the Ga-terminated surface which was formed by annealing the TMGa exposed c(4 X 4)like surface in the H, flow at 460°C until all methyl groups were desorbed from the surface. The SPA signals from the 2 X 4-like and the Gaterminated surfaces were 0.8% and 1.25% higher than that of the c(4 X 4)-like surface, respectively.
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Time (s)
100
150
Fig. 1. Transients of the SPA signal from various surfaces during TMGa exposure and subsequent H, purge: (A) on the ~(4 X 4).like surface, (B) on the 2X4-like surface, (C) on the Ga-terminated surface.
In transient A, the SPA signal increased as soon as the surface was exposed to TMGa (t = O-5 s). After the increase, a gradual decrease (t = 5-30 s) was observed. During the purge period (t > 30 s) after the TMGa exposure, the signal increased about 1% and became constant at the final stage (t = 150 s). In transient B, the SPA signal decreased as soon as TMGa was exposed. Then a gradual decrease (t = 5-30 s) similar to transient A was observed. After TMGa exposure the signal increased and became constant at the same value as transient A. A decrease was observed just after TMGa exposure in transient C, and then the signal increased linearly until TMGa was turned off. During the purge period after the TMGa exposure, the signal increased and became constant. Next we measured the layer thickness by observing the SPA signal while GaAs was grown on the GaAs/AlAs/GaAs structure. At first AlAs was grown on a GaAs substrate using 30 cycles of AsH,(6 Pa)/purge/TMAl/purge = 8/2/2/2 s. Then GaAs was grown using cycles of AsH,(0.06 Pa)/purge/ TMGa/purge = 80/15/5/15 s. The baseline of the SPA signal varied sinusoidally as the thickness of the upper GaAs layer increased. Thus by choosing the GaAs thickness in such a way that the reflectivity changes almost linearly, we could monitor the thick-
S. Otake et al. /Applied
Surface Science 82 /83
ness of the layer as a linear function of reflectivity. Fig. 2 shows three SPA signal transients (#l, 2, 3) during a cycle of AsH,(0.06 Pa)/purge/TMGa/ purge/AsH,(0.06 Pa> on the GaAs/AlAs/GaAs structure at 470°C. Three cycles were executed successively and signals were normalized at the starting reflectivity of each cycle. These three traces were started from the c(4 X 4)-&e surfaces and ended as the c(4 X 4)-&e surfaces, but differing in the duration of purge after TMGa and in the duration of TMGa exposure. Transient #l was the SPA signal during a cycle of ASH,/ purge/ TMGa/ purge AsH, = 1.50/ 15/30/ lO/ 150 s. Transient #2 was measured with a long purge period (70 s) after the TMGa exposure. Transient #3 was measured with a short TMGa exposure period (5 s), i.e. TMGa was turned off just after the fast increase. The increment of the signal level under the AsH, pressure before (t < 0 s) and after the TMGa exposure (t > 150 s) corresponds to the thickness increase in a cycle. In these three cycles, the signal increments were the same. This result shows that the same amount of Ga was deposited on the surface during these three cycles. Ga-containing species were adsorbed within the fast signal increase (less than 5 s in this case), and Ga
(1994) 263-268
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thickness
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Fig. 3. Transients of SPA signal on the 2X4-&e and on the c(4X4)_like surfaces during a cycle of ALE growth on the GaAs/AlAs/GaAs structure.
was not deposited during the gradual decrease (t = 5-30 s) in spite of the TMGa exposure. In other words, Ga deposition was inhibited during the gradual decrease. The duration of the purge period after the TMGa exposure did not influence the total amount of deposited Ga. Therefore, the increase in the signal during the purge period after the TMGa exposure was caused by desorption of methyl groups, not by Ga-containing species, such as monomethylgallium and dimethylgallium.
1.02 -
3.2. Ga deposition by ALE on c(4 x 4)-like and 2 X 4-like surfaces
$1 .Ol z 2 c( 1 .oo -
0.99’
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Fig. 2. SPA signal transients started from the c(4 X 4)like during a cycle of AsH,/purge/TMGa/purge/AsH, GaAs/AlAs/GaAs structure.
surfaces on the
We compared the layer thickness in one cycle of ALE growth on c(4 X 4)-like 2 X 4-like surfaces on the GaAs/AlAs/GaAs structure. Fig. 3 shows the SPA signal transients during a cycle of purge/ TMGa/purge/AsH,(0.06 Pa). The transients started from the c(4 X 4)-like and the 2 X 4-like surfaces and ended as the c(4 X 4)like surfaces. The difference in the initial signal level between the 2 X 4-like and the c(4 X 4)-like surfaces was 1.1% which was measured by exposing the 2 X 4-like surface to 0.06 Pa AsH,.
S. Otake et al. /Applied
Surface Science 82/X3
Temp. (“C)
1000/T
(K-‘)
Fig. 4. Arrhenius plot of methyl desorption rate estimated the transients during purge after TMGa exposure.
from
In Fig. 3, the increment between t < 0 and t = 160 s on the 2 X 4-like surface was 0.7 times as large as that on the c(4 X 4)-like surface. We measured the ratio of the layer thickness on 2 X 4-like and c(4 X 4)-like surfaces several times, and obtained values between 0.7 and 0.8.
3.3. Methyl desorption
rate
Fig. 4 shows an Arrhenius plot of the methyl desorption rate estimated from the increase in the SPA signal during the purge period after TMGa exposure. We obtained comparable time constants both on the c(4 X 4)-like and the 2 X 4-like surfaces, Data shown in this figure were obtained on the c(4 X 4)-like surface. The time constants of methyl desorption were 33 s at 460°C and 5 s at 500°C. The activation energy was 2.1 eV (48 kcal/mol).
4. Discussion The transients in Fig. 1 indicate that the adsorption of Ga is self-limited on the 2 X 4-like and the c(4 X 4)-like surfaces but not on the Ga-terminated
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surface. Continuous adsorption of Ga on the Gaterminated surface contradicts the selective adsorption (SA) mechanism [l&19] for ALE, which supposes that TMGa adheres to As but not to Ga. The self-limited Ga deposition on As-terminated 2 X 4like and c(4 X 4)-like surfaces supports the adsorbate inhibition (AI) mechanism [l] which supposes that adsorption of TMGa converts the As-terminated surface to a Ga-terminated surface covered with methyl groups and that the methyl groups inhibit further adsorption of TMGa. The AI mechanism also supposes that the selflimiting duration is shorter than the residency time of methyl groups on the surface. The time constant of methyl desorption, shown in Fig. 4, is about 20 s at 470°C. We obtained a stable duration of > 30 s during TMGa exposure at 460°C as shown in Fig. 1 and as long as > 120 s at 470°C. The amount of Ga on the surface does not change during the TMGa exposure period as shown in Fig. 2. From these results, we propose that methyl groups desorb from the surface with a time constant even during TMGa exposure but fresh methyl groups are supplied to the surface by decomposition of TMGa arrived at the desorbed sites. The coverage of methyl groups is determined by the balance of the desorption and the incorporation under the TMGa exposure. And the methyl groups continuously inhibit excess Ga adsorption for a duration longer than the residency time of the surface methyl groups. We call it dynamic adsorbate inhibition (DA11 model. Based on the DAI model, ALE at higher temperatures becomes possible if the arrival rate of TMGa on the surface is high. ALE was performed within the temperature range between 500 and 560°C by Ozeki et al. [19,20]. A growth rate of 1 ML/cycle was obtained under a TMGa exposure duration of 15 s. Their rate of TMGa exposure was more than 2000 ML/s (time constant = 4.5 X lop4 s). Even if the time constant of methyl desorption was less than 0.1 s, the GaAs surface was covered with methyl groups, from the viewpoint of the DA1 model. The residency times of methyl groups on the surface has been studied by various methods. The data obtained from SPA [5] and Auger electron spectroscopy (AES) [6j are close to our result. Shorter residency times were obtained by time-programmed desorption (TPD) [2], by time-resolved mass spec-
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troscopy [4] and by RDS [3]. The origin of the discrepancy awaits further study. Reconstruction of the As-terminated surface is another important factor to understand the ALE mechanism. In transient A and B of Fig. 1, the SPA signals reach the same reflectivity at the final stage of the purge period. Therefore, the Ga coverage on these surfaces should be the same in spite of the difference in the starting As coverages. Arsenic coverages of the c(4 X 4) surface and the 2 X 4 surface are reported to be 1.75 and 0.75, respectively [21]. Our ratio of growth rates on the 2 X 4-like surface and that on the c(4 X 4)-surface is 0.7-0.8. In our result, the growth rate is just 1 ML/cycle on the c(4 X 4)-like surface which was measured by optical interference using cycles of AsH ,(0.06 Pa>/ purge/ TMGa/purge = 80/15/5/15 s at 470°C. Therefore, the growth rate on the 2 X 4-like surface is 0.7-0.8 ML/cycle. The growth rate of just 1 ML/cycle was reported for ALE in the MOCVD reactor under AsH, partial pressures between several Pa and more than 100 Pa [3,15,19,20]. Under these ASH, pressures, surface reconstructions are the c(4 X 4)-like or the excessively As-adsorbed surfaces [2,9,17]. Although the As coverage is more than one monolayer on these surfaces, the growth rate is just one monolayer per cycle. Excessively adsorbed As must be desorbed during TMGa exposure. On the other hand, Goto et al. [6] observed 0.7 ML/cycle by exposing TMGa on the 2 X 4 reconstructed surfaces in the MOMBE chamber, in agreement with our growth rate on the 2 X 4-like surface. The amount of Ga-containing species adsorbed on the surface is equal to the coverage of the As when the 2 X 4-like surface was exposed to TMGa. Therefore, TMGa molecules decompose on the 2 X 4-like surface and supply the same number of Ga-containing species as the As atoms on that surface. It is remarkable that Ga adsorption limits itself on the 2 X 4-like surface at 0.75 ML. We have to consider the reason why the Ga atoms on the 2 X 4like surface do not adsorb Ga. The Ga adsorption rate on As-terminated surface is faster than that on Ga-terminated surface. Ga adsorb on to the As atoms on the As-terminated surface within a few seconds but adsorption of 1 ML Ga takes more than 30 s on Ga-terminated surface at 470°C. Hence As atoms on
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the 2 X 4-like surface are selectively covered with MMGa within a few seconds, and methyl groups hinder adsorption of Ga on Ga atoms.
5. Conclusion Ga adsorption on c(4 X 4)-like, 2 X 4-like and Ga-terminated surfaces during TMGa exposure was studied using the SPA method. We proposed a dynamic adsorbate inhibition (DAI) model for selflimiting supposing continuous methyl adsorption and desorption during TMGa exposure. Self-limited adsorption is observed not only on the c(4 X 4)-like surface but also on the 2 X 4-like surface. The layer thickness in an ALE cycle was 1 ML/cycle on the c(4 X 4)-like surface and 0.7-0.8 ML/cycle on the 2 X 4-like surface. The fact that Ga adsorption limits itself on the 2 X 4-like surface at 0.75 ML suggests that less than one monolayer adsorption of methyl groups can inhibit further Ga adsorption.
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