- Email: [email protected]
MBE and conventional MBE
Journal of Crystal Growth 198/199 (1999) 1130—1135
A detailed comparison of the degree of selectivity, morphology and growth mechanisms between PSE/MBE and conventional MBE G. Bacchin*, T. Nishinaga Department of Electronic Engineering, The Graduate School of Engineering, The University of Tokyo, 7-3-1 Hongo, Bunkyo-ku, Tokyo 113, Japan
Abstract In this paper we compare the degree of selectivity, morphology and growth mechanisms of selective area growth (SAG) performed by periodic supply molecular beam epitaxy (PSE/MBE) and by conventional MBE. In particular we focus on the SAG of GaAs on GaAs(0 0 1) and GaAs(1 1 1)B substrates patterned with a SiO mask. We have observed that the degree of selectivity is considerably higher by PSE/MBE than by conventional MBE when comparing either the samples grown under the same growth conditions or grown under the same effective growth rate. The latter observation confirms that the effectiveness of the PSE technique relies on the enhancement of the decomposition, which occurs during the interruption period, of the polycrystalline islands formed on the mask. Atomic force microscope (AFM) observations proved that the PSE technique does not damage the morphology of the GaAs epilayers. Atomically smooth GaAs epilayers have been grown under completely selective conditions both on GaAs (0 0 1) and GaAs (1 1 1)B substrates. Besides, we have discussed the mechanisms of PSE/MBE and we have evaluated the relevance of the migration and the re-evaporation of the Ga adatoms. 1999 Elsevier Science B.V. All rights reserved. PACS: 81.15.Hi; 81.05.Ea; 81.65.Cf; 61.16.Ch Keywords: GaAs; SiO ; Selective area growth; Periodic supply epitaxy; Molecular beam epitaxy
1. Introduction Selective area growth (SAG) is a very important tool for the fabrication of new electronic, optoelectronic and photonic devices. In contrast to metalor* Corresponding author. Tel.: #81 3 3812 2111; fax: #81 3 5803 3975; e-mail: [email protected].
ganic chemical vapor deposition (MOCVD) and metalorganic molecular beam epitaxy (MOMBE), selective growth is very difficult for solid source molecular beam epitaxy (MBE) under the more usual growth conditions. Indeed, the suppression of growth on the mask needed to achieve true SAG is easier for metalorganic compounds than for elemental group III materials. However, MBE has,
0022-0248/99/$ — see front matter 1999 Elsevier Science B.V. All rights reserved. PII: S 0 0 2 2 - 0 2 4 8 ( 9 8 ) 0 0 9 9 1 - 9
G. Bacchin, T. Nishinaga / Journal of Crystal Growth 198/199 (1999) 1130–1135
when compared to other techniques, some advantages such as the ultrahigh vacuum environment, which allows very clean growth conditions and in situ observations, monolayer control of the epilayer thickness, negligible carbon contamination and the use of sources that are neither toxic nor dangerous. Different attempts have been carried out to grow selectively by MBE employing high growth temperatures [1,2], very low growth rates [3] or by MBErelated techniques [4—9]. But, only recently has the PSE technique been developed to give us a very powerful tool to achieve selective growth by MBE at a substrate temperature low enough to allow the growth of atomically smooth epilayers [10]. SAG has been demonstrated by PSE/MBE for GaAs [10—12], ZnSe [13], AlGaAs [14,15] and even AlAs [15]. In this paper, we conduct atomic force microscope (AFM) observations proving the effectiveness of the PSE/MBE technique in the SAG of GaAs on GaAs(1 0 0) and GaAs(1 1 1)B substrates. Besides, we discuss the morphology of the epilayers and the growth mechanisms involved.
2. Experimental details All the growth experiments were carried out using an ULVAC MBC-300 MBE machine. The GaAs (0 0 1) and (1 1 1)B substrates were of the epiready type grown by the Bridgman technique. The SiO film, used as the mask, was deposited by spinning on an organic solution (OCD, Tokyo Ohka) followed by baking at 500°C. The thickness of the SiO film was estimated to be 65 nm by AFM. The substrates were patterned by conventional photolithography, by using a mask with different line-shaped openings (windows). Their widths were 1.5 and 3 lm with a separation of 10 and 30 lm. The PSE technique consists of a periodic repetition of cycles in which every deposition period is followed by an interruption period with only the As flux on and no Ga flux [10,15]. The continuous supply of As flux is necessary to maintain the flatness and the smoothness of the growing layer. During the interruption period there can be enough time for GaAs polycrystalline islands, eventually
1131
accumulated on the mask during the deposition period, to be decomposed and for the decomposed adatoms to re-evaporate or to migrate toward the window regions. The PSE parameters are the deposition time (q ), the interruption time (q ) and the ratio of the interruption time to the deposition time (R). Typical growth conditions employed in the experiments described in this paper were a substrate temperature, an As flux and a growth rate of 640°C, 1.4;10\ Torr and 0.125—0.250 lm/h, respectively.
3. Results and discussion 3.1. Selective growth by conventional MBE and by PSE/MBE Fig. 1 shows the AFM image of a GaAs epilayer typically grown by conventional MBE on patterned substrates. As it is clearly visible, there is a relevant accumulation of GaAs polycrystalline islands on the SiO mask. There are two ways to avoid the accumulation of these islands. One way is to increase the substrate temperature and the other is to decrease the growth rate. However, the increase of the substrate temperature leads to a macroscopically rough epi-surface. Thus, to
Fig. 1. AFM image of a GaAs epilayer (central part) grown by conventional MBE on a patterned GaAs(0 0 1) substrate with a growth rate of 0.250 lm/h at 640°C.
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achieve completely selective growth with no polycrystalline islands on the mask, the growth rate should be decreased significantly [3]. Fig. 2 and Fig. 3 show two sets of AFM images of GaAs epilayers grown by conventional MBE and by PSE/MBE on GaAs(0 0 1) and GaAs(1 1 1)B substrates, respectively. Fig. 2a and Fig. 3a, illustrate the effect of a reduced growth rate in conventional MBE. At a growth rate of 0.125 lm/h the number of polycrystalline islands is strongly reduced. However, completely selective growth is still not achieved despite the relatively high growth
temperature and low growth rate that were employed. These difficulties can be overcome by the PSE technique, which provides a new way to increase the degree of selectivity based on the decomposition of the polycrystalline islands. Therefore, complete selectivity can be achieved using conditions not selective for conventional MBE. In particular, SAG becomes possible at substrate temperatures for which the trade-off between surface morphology and selectivity can be solved.
Fig. 2. AFM images of GaAs epilayers grown on patterned GaAs(0 0 1) substrates at 640°C: (a) by conventional MBE with a growth rate of 0.125 lm/h; (b) by PSE/MBE with q "30 s, q "30 s (R"1) and a growth rate during the deposition period of 0.250 lm/h (effective growth rate of 0.125 lm/h); (c) by PSE/MBE with q "30 s, q "75 s (R"2.5) and a growth rate during the deposition period of 0.250 lm/h.
G. Bacchin, T. Nishinaga / Journal of Crystal Growth 198/199 (1999) 1130–1135
1133
Fig. 3. AFM images of GaAs epilayers grown on patterned GaAs(1 1 1)B substrates at 640°C: (a) by conventional MBE with a growth rate of 0.125 lm/h; (b) by PSE/MBE with q "30 s, q "30 s (R"1) and a growth rate during the deposition period of 0.250 lm/h (effective growth rate of 0.125 lm/h); (c) by PSE/MBE with q "30 s, q "75 s (R"2.5) and a growth rate during the deposition period of 0.250 lm/h.
To illustrate the effects of the PSE technique, we have shown in Fig. 2b and Fig. 3b, the AFM images of epilayers grown under the same conditions as the one shown in Fig. 1. The PSE parameters were q "30 s and R"1 with a growth rate of 0.250 lm/h during the deposition time, which corresponds to an effective growth rate of 0.125 lm/h. Despite the fact that the substrate temperature and the growth rate are the same in both cases, one can see that in the former case most of the polycrystalline islands are not formed on the mask. It is possible to argue that the effect of the PSE technique relies on the reduction of the effective
growth rate and its benefits may not be clear. However, the decomposition process of the polycrystalline islands is basically a non-linear process since bigger islands are difficult to decompose, the decomposition time being more than simply proportional to the amount of material. It has been shown theoretically [16] that the degree of selectivity is larger in PSE/MBE above a critical value of R (which is about 1.0 for the employed growth conditions) and increases rapidly for further increases in R. Indeed, when comparing the density of polycrystalline islands in Fig. 2b and Fig. 3b with the corresponding Fig. 2a and Fig. 3a, it is clear
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that the increase of the degree of selectivity by PSE/MBE is higher than by conventional MBE even at the same effective growth rate. This implies that the efficacy of the PSE technique is due to the decomposition of the polycrystalline islands rather than a decrease of the effective growth rate. The benefits of the PSE/MBE technique are further demonstrated in Fig. 2c and Fig. 3c with the results of experiments carried out with R"2.5 and a growth rate of 0.250 lm/h during the deposition time. It can be clearly observed that atomically smooth GaAs epilayers were grown both on GaAs(0 0 1) and (1 1 1)B substrates, achieving at the same time completely selective growth across the mask regions for both samples. 3.2. Morphology of the epilayers The problem of the trade-off between morphology and selectivity is a main concern especially for the growth of GaAs on GaAs(0 0 1) substrates. For the rather critical growth conditions employed, we can observe from Fig. 2 that an atomically smooth epilayer is formed even on this substrate with the formation of a ridge with (n11)A sidewalls. On the other hand, atomically smooth and flat top (1 1 1)B surfaces are observed in Fig. 3. It is important to point out that the morphology of the epilayer is not damaged by employing the PSE technique. This is a consequence of the constant presence of the As flux even during the inter ruption period. Moreover, while polycrystalline islands are decomposed on the mask, the Ga adatoms in the epilayer have more time for diffusion producing an even smoother epilayer because of the annealing effect of the interruption period. 3.3. Growth mechanisms for selective area growth The main difference between SAG by PSE/MBE and by conventional MBE (or other techniques) is that the former does not require the suppression of nucleation of polycrystalline islands on the mask, while it is a necessary requirement in the latter case. Indeed, the interruption period provides the time for the decomposition of the islands. The Ga adatoms coming from the decomposed islands can either re-evaporate from the mask or
migrate from the mask toward the surrounding window regions. To evaluate the relevance of Ga adatom migration to the growth in the window, cross sections of epilayers have been analyzed and are shown in Fig. 2c. The cross sections of the epilayers that have been grown on line-shaped open windows with either the same widths and different separation or the same separation but different widths have been compared. The cross sections of epilayers grown on windows of different widths showed a slight increase in their average thickness when the window is narrower due to the re-distribution of the same amount of material that migrated from the mask onto a narrower region. From this data we have estimated the migration coefficient to be 2% while the re-evaporation from the mask accounts for 98%. The re-evaporation from the epilayers, due to the relatively high growth temperature, was estimated to be 35% of the directly deposited material. On the other hand, the cross sections of the epilayers grown on windows with different separation, surprisingly showed a comparative increase of the average thickness for smaller separation. This cannot be explained by a simple migration effect since one would have expected the opposite effect. To understand this behavior, we suppose that thermal stress, generated between the SiO mask and GaAs substrate, is not negligible, especially when the mask to window ratio is high (3.5 and 10.7 for the small and large separation, respectively). We have carried out a first-order approximation of the stress effect and we have found that even a small stress effect can justify the observed behavior. After the correction due to stress, it was found that the migration effect accounts for at least 5%, confirming its very low significance compared to the re-evaporation of the Ga adatoms from the mask. Besides, the combination of thermal re-evaporation and stress effect account for a 40—50% loss of the material directly deposited on the epilayers. We can conclude that the decomposition of the polycrystalline islands and the re-evaporation of the Ga adatoms from the mask are the most relevant phenomena in the SAG by PSE/MBE, while the migration effect plays an almost negligible role.
G. Bacchin, T. Nishinaga / Journal of Crystal Growth 198/199 (1999) 1130–1135
1135
4. Conclusion
References
In this paper SAG carried out by PSE/MBE and by conventional MBE has been compared. It has been shown that the degree of selectivity is higher if the PSE technique is employed, both under the same growth conditions and under the same effective growth rate. We were able to grow atomically smooth GaAs epilayers, while at the same time achieving complete selectivity, not only on GaAs(1 1 1)B substrates but also on less stable GaAs(0 0 1) substrates. The effectiveness of the PSE technique relies on a different mechanism for SAG, that is the decomposition of initially formed polycrystalline islands on the mask rather than the suppression of their nucleation on the mask.
[1] A. Okamoto, K. Ohata, Appl. Phys. Lett. 51 (1987) 1512. [2] F. Allegretti, M. Inoue, T. Nishinaga, J. Crystal Growth 146 (1995) 354. [3] A. Okamoto, K. Ohata, J. Appl. Phys. 66 (1989) 3413. [4] N. Yamamoto, N. Kondo, Y. Nanishi, J. Crystal Growth 108 (1991) 433. [5] J.H. Thompson, G.A.C. Jones, D.A. Ritchie, E.H. Linfield, M. Houlton, G.W. Smith, C.R. Whitehouse, J. Crystal Growth 127 (1993) 732. [6] K. Pak, I. Saitoh, N. Ohshima, H. Yonezu, J. Crystal Growth 140 (1994) 244. [7] T. Sugaya, Y. Okada, M. Kawabe, Jpn. J. Appl. Phys. 31 (1992) L713. [8] M. Kawabe, J. Crystal Growth 150 (1995) 370. [9] S. Yokoyama, J. Oogi, D. Yui, M. Kawabe, J. Crystal Growth 95 (1989) 32. [10] F.E. Allegretti, T. Nishinaga, J. Crystal Growth 156 (1995) 1. [11] G. Bacchin, K. Tsunoda , T. Nishinaga, Surf. Rev. Lett. 5 (1998) 731. [12] F.E. Allegretti, C. Roberts, J.H. Neave, J. Vac. Sci. Technol. A 15 (1997) 954. [13] A. Ueta, I. Suemune, K. Uesugi, M. Arita, A. Avramescu, T. Numai, H. Machida, N. Shinomoyama, Jpn. J. Appl. Phys. 36 (1997) 5044. [14] G. Bacchin, K. Tsunoda, T. Nishinaga, J. Crystal Growth 174 (1997) 616. [15] G. Bacchin , T. Nishinaga, J. Crystal Growth 191 (1998) 599 [16] G. Bacchin, Ph. D. Dissertation, The University of Tokyo, 1998.
Acknowledgements This work was supported by JSPS Research for the Future Program in the Area of Atomic-Scale Surface and Interface Dynamics under the project of “Self-assembling of Nanostructures and its control”. G. Bacchin would like to express his thanks for the Monbusho Scholarship.
A detailed comparison of the degree of selectivity, morphology and growth mechanisms between PSE/MBE and conventional MBE G. Bacchin*, T. Nishinaga Department of Electronic Engineering, The Graduate School of Engineering, The University of Tokyo, 7-3-1 Hongo, Bunkyo-ku, Tokyo 113, Japan
Abstract In this paper we compare the degree of selectivity, morphology and growth mechanisms of selective area growth (SAG) performed by periodic supply molecular beam epitaxy (PSE/MBE) and by conventional MBE. In particular we focus on the SAG of GaAs on GaAs(0 0 1) and GaAs(1 1 1)B substrates patterned with a SiO mask. We have observed that the degree of selectivity is considerably higher by PSE/MBE than by conventional MBE when comparing either the samples grown under the same growth conditions or grown under the same effective growth rate. The latter observation confirms that the effectiveness of the PSE technique relies on the enhancement of the decomposition, which occurs during the interruption period, of the polycrystalline islands formed on the mask. Atomic force microscope (AFM) observations proved that the PSE technique does not damage the morphology of the GaAs epilayers. Atomically smooth GaAs epilayers have been grown under completely selective conditions both on GaAs (0 0 1) and GaAs (1 1 1)B substrates. Besides, we have discussed the mechanisms of PSE/MBE and we have evaluated the relevance of the migration and the re-evaporation of the Ga adatoms. 1999 Elsevier Science B.V. All rights reserved. PACS: 81.15.Hi; 81.05.Ea; 81.65.Cf; 61.16.Ch Keywords: GaAs; SiO ; Selective area growth; Periodic supply epitaxy; Molecular beam epitaxy
1. Introduction Selective area growth (SAG) is a very important tool for the fabrication of new electronic, optoelectronic and photonic devices. In contrast to metalor* Corresponding author. Tel.: #81 3 3812 2111; fax: #81 3 5803 3975; e-mail: [email protected].
ganic chemical vapor deposition (MOCVD) and metalorganic molecular beam epitaxy (MOMBE), selective growth is very difficult for solid source molecular beam epitaxy (MBE) under the more usual growth conditions. Indeed, the suppression of growth on the mask needed to achieve true SAG is easier for metalorganic compounds than for elemental group III materials. However, MBE has,
0022-0248/99/$ — see front matter 1999 Elsevier Science B.V. All rights reserved. PII: S 0 0 2 2 - 0 2 4 8 ( 9 8 ) 0 0 9 9 1 - 9
G. Bacchin, T. Nishinaga / Journal of Crystal Growth 198/199 (1999) 1130–1135
when compared to other techniques, some advantages such as the ultrahigh vacuum environment, which allows very clean growth conditions and in situ observations, monolayer control of the epilayer thickness, negligible carbon contamination and the use of sources that are neither toxic nor dangerous. Different attempts have been carried out to grow selectively by MBE employing high growth temperatures [1,2], very low growth rates [3] or by MBErelated techniques [4—9]. But, only recently has the PSE technique been developed to give us a very powerful tool to achieve selective growth by MBE at a substrate temperature low enough to allow the growth of atomically smooth epilayers [10]. SAG has been demonstrated by PSE/MBE for GaAs [10—12], ZnSe [13], AlGaAs [14,15] and even AlAs [15]. In this paper, we conduct atomic force microscope (AFM) observations proving the effectiveness of the PSE/MBE technique in the SAG of GaAs on GaAs(1 0 0) and GaAs(1 1 1)B substrates. Besides, we discuss the morphology of the epilayers and the growth mechanisms involved.
2. Experimental details All the growth experiments were carried out using an ULVAC MBC-300 MBE machine. The GaAs (0 0 1) and (1 1 1)B substrates were of the epiready type grown by the Bridgman technique. The SiO film, used as the mask, was deposited by spinning on an organic solution (OCD, Tokyo Ohka) followed by baking at 500°C. The thickness of the SiO film was estimated to be 65 nm by AFM. The substrates were patterned by conventional photolithography, by using a mask with different line-shaped openings (windows). Their widths were 1.5 and 3 lm with a separation of 10 and 30 lm. The PSE technique consists of a periodic repetition of cycles in which every deposition period is followed by an interruption period with only the As flux on and no Ga flux [10,15]. The continuous supply of As flux is necessary to maintain the flatness and the smoothness of the growing layer. During the interruption period there can be enough time for GaAs polycrystalline islands, eventually
1131
accumulated on the mask during the deposition period, to be decomposed and for the decomposed adatoms to re-evaporate or to migrate toward the window regions. The PSE parameters are the deposition time (q ), the interruption time (q ) and the ratio of the interruption time to the deposition time (R). Typical growth conditions employed in the experiments described in this paper were a substrate temperature, an As flux and a growth rate of 640°C, 1.4;10\ Torr and 0.125—0.250 lm/h, respectively.
3. Results and discussion 3.1. Selective growth by conventional MBE and by PSE/MBE Fig. 1 shows the AFM image of a GaAs epilayer typically grown by conventional MBE on patterned substrates. As it is clearly visible, there is a relevant accumulation of GaAs polycrystalline islands on the SiO mask. There are two ways to avoid the accumulation of these islands. One way is to increase the substrate temperature and the other is to decrease the growth rate. However, the increase of the substrate temperature leads to a macroscopically rough epi-surface. Thus, to
Fig. 1. AFM image of a GaAs epilayer (central part) grown by conventional MBE on a patterned GaAs(0 0 1) substrate with a growth rate of 0.250 lm/h at 640°C.
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achieve completely selective growth with no polycrystalline islands on the mask, the growth rate should be decreased significantly [3]. Fig. 2 and Fig. 3 show two sets of AFM images of GaAs epilayers grown by conventional MBE and by PSE/MBE on GaAs(0 0 1) and GaAs(1 1 1)B substrates, respectively. Fig. 2a and Fig. 3a, illustrate the effect of a reduced growth rate in conventional MBE. At a growth rate of 0.125 lm/h the number of polycrystalline islands is strongly reduced. However, completely selective growth is still not achieved despite the relatively high growth
temperature and low growth rate that were employed. These difficulties can be overcome by the PSE technique, which provides a new way to increase the degree of selectivity based on the decomposition of the polycrystalline islands. Therefore, complete selectivity can be achieved using conditions not selective for conventional MBE. In particular, SAG becomes possible at substrate temperatures for which the trade-off between surface morphology and selectivity can be solved.
Fig. 2. AFM images of GaAs epilayers grown on patterned GaAs(0 0 1) substrates at 640°C: (a) by conventional MBE with a growth rate of 0.125 lm/h; (b) by PSE/MBE with q "30 s, q "30 s (R"1) and a growth rate during the deposition period of 0.250 lm/h (effective growth rate of 0.125 lm/h); (c) by PSE/MBE with q "30 s, q "75 s (R"2.5) and a growth rate during the deposition period of 0.250 lm/h.
G. Bacchin, T. Nishinaga / Journal of Crystal Growth 198/199 (1999) 1130–1135
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Fig. 3. AFM images of GaAs epilayers grown on patterned GaAs(1 1 1)B substrates at 640°C: (a) by conventional MBE with a growth rate of 0.125 lm/h; (b) by PSE/MBE with q "30 s, q "30 s (R"1) and a growth rate during the deposition period of 0.250 lm/h (effective growth rate of 0.125 lm/h); (c) by PSE/MBE with q "30 s, q "75 s (R"2.5) and a growth rate during the deposition period of 0.250 lm/h.
To illustrate the effects of the PSE technique, we have shown in Fig. 2b and Fig. 3b, the AFM images of epilayers grown under the same conditions as the one shown in Fig. 1. The PSE parameters were q "30 s and R"1 with a growth rate of 0.250 lm/h during the deposition time, which corresponds to an effective growth rate of 0.125 lm/h. Despite the fact that the substrate temperature and the growth rate are the same in both cases, one can see that in the former case most of the polycrystalline islands are not formed on the mask. It is possible to argue that the effect of the PSE technique relies on the reduction of the effective
growth rate and its benefits may not be clear. However, the decomposition process of the polycrystalline islands is basically a non-linear process since bigger islands are difficult to decompose, the decomposition time being more than simply proportional to the amount of material. It has been shown theoretically [16] that the degree of selectivity is larger in PSE/MBE above a critical value of R (which is about 1.0 for the employed growth conditions) and increases rapidly for further increases in R. Indeed, when comparing the density of polycrystalline islands in Fig. 2b and Fig. 3b with the corresponding Fig. 2a and Fig. 3a, it is clear
1134
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that the increase of the degree of selectivity by PSE/MBE is higher than by conventional MBE even at the same effective growth rate. This implies that the efficacy of the PSE technique is due to the decomposition of the polycrystalline islands rather than a decrease of the effective growth rate. The benefits of the PSE/MBE technique are further demonstrated in Fig. 2c and Fig. 3c with the results of experiments carried out with R"2.5 and a growth rate of 0.250 lm/h during the deposition time. It can be clearly observed that atomically smooth GaAs epilayers were grown both on GaAs(0 0 1) and (1 1 1)B substrates, achieving at the same time completely selective growth across the mask regions for both samples. 3.2. Morphology of the epilayers The problem of the trade-off between morphology and selectivity is a main concern especially for the growth of GaAs on GaAs(0 0 1) substrates. For the rather critical growth conditions employed, we can observe from Fig. 2 that an atomically smooth epilayer is formed even on this substrate with the formation of a ridge with (n11)A sidewalls. On the other hand, atomically smooth and flat top (1 1 1)B surfaces are observed in Fig. 3. It is important to point out that the morphology of the epilayer is not damaged by employing the PSE technique. This is a consequence of the constant presence of the As flux even during the inter ruption period. Moreover, while polycrystalline islands are decomposed on the mask, the Ga adatoms in the epilayer have more time for diffusion producing an even smoother epilayer because of the annealing effect of the interruption period. 3.3. Growth mechanisms for selective area growth The main difference between SAG by PSE/MBE and by conventional MBE (or other techniques) is that the former does not require the suppression of nucleation of polycrystalline islands on the mask, while it is a necessary requirement in the latter case. Indeed, the interruption period provides the time for the decomposition of the islands. The Ga adatoms coming from the decomposed islands can either re-evaporate from the mask or
migrate from the mask toward the surrounding window regions. To evaluate the relevance of Ga adatom migration to the growth in the window, cross sections of epilayers have been analyzed and are shown in Fig. 2c. The cross sections of the epilayers that have been grown on line-shaped open windows with either the same widths and different separation or the same separation but different widths have been compared. The cross sections of epilayers grown on windows of different widths showed a slight increase in their average thickness when the window is narrower due to the re-distribution of the same amount of material that migrated from the mask onto a narrower region. From this data we have estimated the migration coefficient to be 2% while the re-evaporation from the mask accounts for 98%. The re-evaporation from the epilayers, due to the relatively high growth temperature, was estimated to be 35% of the directly deposited material. On the other hand, the cross sections of the epilayers grown on windows with different separation, surprisingly showed a comparative increase of the average thickness for smaller separation. This cannot be explained by a simple migration effect since one would have expected the opposite effect. To understand this behavior, we suppose that thermal stress, generated between the SiO mask and GaAs substrate, is not negligible, especially when the mask to window ratio is high (3.5 and 10.7 for the small and large separation, respectively). We have carried out a first-order approximation of the stress effect and we have found that even a small stress effect can justify the observed behavior. After the correction due to stress, it was found that the migration effect accounts for at least 5%, confirming its very low significance compared to the re-evaporation of the Ga adatoms from the mask. Besides, the combination of thermal re-evaporation and stress effect account for a 40—50% loss of the material directly deposited on the epilayers. We can conclude that the decomposition of the polycrystalline islands and the re-evaporation of the Ga adatoms from the mask are the most relevant phenomena in the SAG by PSE/MBE, while the migration effect plays an almost negligible role.
G. Bacchin, T. Nishinaga / Journal of Crystal Growth 198/199 (1999) 1130–1135
1135
4. Conclusion
References
In this paper SAG carried out by PSE/MBE and by conventional MBE has been compared. It has been shown that the degree of selectivity is higher if the PSE technique is employed, both under the same growth conditions and under the same effective growth rate. We were able to grow atomically smooth GaAs epilayers, while at the same time achieving complete selectivity, not only on GaAs(1 1 1)B substrates but also on less stable GaAs(0 0 1) substrates. The effectiveness of the PSE technique relies on a different mechanism for SAG, that is the decomposition of initially formed polycrystalline islands on the mask rather than the suppression of their nucleation on the mask.
[1] A. Okamoto, K. Ohata, Appl. Phys. Lett. 51 (1987) 1512. [2] F. Allegretti, M. Inoue, T. Nishinaga, J. Crystal Growth 146 (1995) 354. [3] A. Okamoto, K. Ohata, J. Appl. Phys. 66 (1989) 3413. [4] N. Yamamoto, N. Kondo, Y. Nanishi, J. Crystal Growth 108 (1991) 433. [5] J.H. Thompson, G.A.C. Jones, D.A. Ritchie, E.H. Linfield, M. Houlton, G.W. Smith, C.R. Whitehouse, J. Crystal Growth 127 (1993) 732. [6] K. Pak, I. Saitoh, N. Ohshima, H. Yonezu, J. Crystal Growth 140 (1994) 244. [7] T. Sugaya, Y. Okada, M. Kawabe, Jpn. J. Appl. Phys. 31 (1992) L713. [8] M. Kawabe, J. Crystal Growth 150 (1995) 370. [9] S. Yokoyama, J. Oogi, D. Yui, M. Kawabe, J. Crystal Growth 95 (1989) 32. [10] F.E. Allegretti, T. Nishinaga, J. Crystal Growth 156 (1995) 1. [11] G. Bacchin, K. Tsunoda , T. Nishinaga, Surf. Rev. Lett. 5 (1998) 731. [12] F.E. Allegretti, C. Roberts, J.H. Neave, J. Vac. Sci. Technol. A 15 (1997) 954. [13] A. Ueta, I. Suemune, K. Uesugi, M. Arita, A. Avramescu, T. Numai, H. Machida, N. Shinomoyama, Jpn. J. Appl. Phys. 36 (1997) 5044. [14] G. Bacchin, K. Tsunoda, T. Nishinaga, J. Crystal Growth 174 (1997) 616. [15] G. Bacchin , T. Nishinaga, J. Crystal Growth 191 (1998) 599 [16] G. Bacchin, Ph. D. Dissertation, The University of Tokyo, 1998.
Acknowledgements This work was supported by JSPS Research for the Future Program in the Area of Atomic-Scale Surface and Interface Dynamics under the project of “Self-assembling of Nanostructures and its control”. G. Bacchin would like to express his thanks for the Monbusho Scholarship.