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Structural change of selenium-treated GaAs(001) surface observed by STM
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applied surface science ELSEVIER
Applied Surface Science 107 (1996) 58-62
Structural change of selenium-treated GaAs(001) surface observed by STM Y. Haga *, S. Miwa, E. Morita Sony Corporation Research Center, Yokohama 240, Japan Received 12 October 1995; accepted 24 December 1995
Abstract We have controlled the surface structures of selenium (Se) treated GaAs(001) using a Se molecular beam in a molecular-beam epitaxy (MBE) system and obtained a variety of surface reconstructions. The detailed surface structures were observed in-situ using a high-performance scanning tunneling microscope (STM) system. After producing Ga-stabilized GaAs(001) (4 X 6) reconstructions by thermal cleaning, Se was evaporated slowly onto the surfaces. As the Se deposition time was increased, the surface structure could be changed from (4 × 6) to (4 X 3) to (2 X 3) and finally to the (2 X 1) reconstruction. All surfaces were well ordered and STM images were obtained successfully for all of these reconstructions. The (4 × 3) structure was observed to be similar to (2 X 3), however, dimers might be buckled in alternate directions together with the nearest neighbors in the [1~0] direction. The (2 X 3) structure consisted of elliptical protrusions in the dimer rows forming an additional periodicity along the dimer row direction. For the (2 × 1) surface, simple dimers row structures were observed. These structural changes were thought to be caused by irregularities in the Se-Se or Se-Ga dimers and Ga vacancies under the surface. We found that Se coverage played an important role as well as heat treatment in this phase change. 1. Introduction Passivation of compound semiconductor surfaces by chalcogen atoms has attracted considerable attention in recent years because of its beneficial effect on the electronic properties of surfaces and interfaces, such as enhanced photoluminescence [1], increased sensitivity of Schottky-barrier height to the metal work function [2], and reduced band-gap surface states [3]. In order to understand the passivafion mechanism and the initial stage of growth for ZnSebased I I - V I semiconductors on I I I - V semiconductors, many experimental and theoretical studies have
* Corresponding author.
been carried out [4-13] but with limited success. Takatani et al. have investigated Se covered GaAs(001) surfaces using reflection high-energy electron diffraction (RHEED) [14]. They claimed that there are at least three reconstructions for the Se-adsorbed surface and reversible phase transitions were observed between the (4 × 3), (2 × 3) and (2 × 1) phases during heat treatment. Li et al. have reported from the observation of the Z n S e / G a A s interface by transmission electron microscopy (TEM) that a GazSe3-1ike structure (defect induced zincblend) was generated at the interface [15]. This means that Se-induced surface structures form Ga vacancies under the surface in order to satisfy the electron counting. For the analysis of further structural and electrical properties, it is essential to con-
0169-4332/96/$15.00 Copyright © 1996 Elsevier Science B.V. All rights reserved. PI1 S 0 1 6 9 - 4 3 3 2 ( 9 6 ) 0 0 5 0 4 - 1
59
Y. Haga et aL /Applied Surface Science 107 (1996) 58-62
trol the surface atomically to generate such reconstructions even at room temperature. In this work, we have controlled the Se-treatment of GaAs(001) surfaces by heat treatment and a Se molecular beam in a molecular-beam epitaxy (MBE) system and obtained a variety of reconstructions even after the samples were cooled down to room temperature. The details of the surface structures were analyzed in-situ using a high-performance ultra-high vacuum (UHV) STM system.
2. Experimental The experiments were performed in an UHV STM system connected to an MBE chamber. Details concerning this system have been reported elsewhere [16,17]. Prior to Se exposure, thermally cleaned GaAs(001) Ga-stabilized surfaces were prepared so that we would have to consider only on the reaction between Ga and Se. A Si-doped n-type ( ~ 1 × 1018 cm -3) GaAs(001) wafer was used as the substrate. The substrate was chemically etched in a KOH : H 2 0 : H202 solution and then thermally cleaned in the MBE chamber without an As flux in order to start with Ga-stabilized surfaces. During the cleaning, the evolution of the surface structures was monitored by RHEED. After (4 × 2), (4 × 3), or (4 × 6) patterns were observed at 510°C where Gastabilized surfaces were formed, Se was evaporated slowly at the same temperature onto the surfaces using a Knudsen cell with a pressure of 4 × 10 .6 Pa. When a desired reconstruction was observed by RHEED, Se exposure was stopped immediately to keep the structure. After the Se treatment, the sample was cooled and transferred from the MBE chamber to the STM chamber through vacuum. STM imaging was performed at room temperature in the constantcurrent mode mainly under conditions of a tunneling current of 20 pA and sample bias of - 3 to - 4 V.
3. Results and discussion Prior to Se deposition, a Ga-stabilized GaAs(001) (4 × 6) surface was observed by STM. Many STM studies on As-stabilized structures, such as the (2 × 4) or c(2 × 8) have been undertaken [18-21], however,
(a)
[1]-01
Fig. 1. (a) STM image of the GaAs(001) Ga-stabilized (4×6) surface (36 × 36 nm2). (b) STM image of the GaAs(001) Ga-stablized (4×6) surface (18 × 18 nma).
there are few reports on Ga-stabilized surfaces [18,21-23]. This is presumably because the Gastabilized surface is obtained only by careful thermal cleaning in UHV without an As atmosphere. For cleaning at 510°C, the surface showed (4 × 2), (4 × 3) or (4 × 6) RHEED patterns. This variation is likely to come from surface irregularities caused by chemical etching or heat treatment. Due to the KOH treatment, the surface oxide should be relatively thin and easy to remove, however, the real temperature might be a little higher than 510°C measured by IR pyrometer for the (4 × 6) structure. Fig. l(a) and (b) show respectively the large and small-area STM images of the Ga-stabilized (4 × 6) surface. The surface looks very rough and many
60
Y. Haga et al. /Applied Surface Science 107 (1996) 58-62
contaminants, perhaps oxide or carbon, are observed. A stripe-like structure, however, can be observed along the [170] direction in the flat areas. The distance between the stripes (bright and dark) is about 2.4 nm and close to the X6 periodicity in the [110] direction. This image is very similar to the reported Ga-stabilized structures [19,23,24] which are designated as (2 X 6) or (2 X 6 / 3 X 6). Se was deposited on the Ga-stabilized GaAs(001) surface described above at 510°C. As the Se deposition time was increased, the surface structure changed from (4 × 6) to (4 × 3) to (2 × 3) and finally to the (2 × 1) reconstruction. Any of these structures could be maintained by stopping Se exposure and were stable even after the sample was cooled to room temperature for STM observation. We could successfully obtain STM images for all of these reconstructions. When the Se deposition was started, the RHEED patterns at first changed to the (4 X 3) reconstruction. Fig. 2(a) shows an STM image Of GaAs(001) with a (4 × 3) reconstruction, and a cross-section is drawn along A - B in Fig. 2(b). The structure is observed to be similar to (2 × 3) (shown later), however, dimers might be buckled in alternate directions together with the nearest neighbors and form pairs to make the 4 X periodicity in the [110] direction. This suggests the existence of some charge transfer in the dimer layers. One possibility of such buckling is that dimers at this surface might be partially formed by Se and Ga atoms in so-called hetero-dimers. In this case, Se atoms were diffused into the sub-layer and been replaced by Ga. The (4 X 3) reconstruction quickly changed to the (2 X 3) under the Se exposure. Fig. 3 shows an STM image of a GaAs(001) surface with (2 X 3) RHEED patterns. This structure consists of elliptical protrusions in the dimer rows to make an additional periodicity along the dimer row direction. The distance between protrusions is about 1.2 nm, which is close to a 3 x periodicity in the [110] direction. For this structure, one dimer out of three is likely to be missing in the dimer rows. If the structure is formed by simple Se dimer rows, the (2 X 1) reconstruction leaves one excess electron for each Se dimer. In the (2 X 3) surface, since each Se-dimer desorption leaves more excess electrons (3 electrons for a (2 X 3) unit), filled state dangling bonds or G a - G a bonds
(a)
[110]
[11-0]
(b)
A
B
Fig. 2. (a) STM image of the Se-treatedGaAs(001) (4X 3) surface (32 x 32 nmZ). (b) Cross-sectionalong A-B in (a). might form dimers in the second layer with the Se dimer missing to compensate the excess electrons. Continuing Se treatment, the RHEED patterns
7L [1Y0] Fig. 3. STM image of the Se-treated GaAs(001) (2X3) surface (36 X 36 nm2).
Y. Haga et aL /Applied Surface Science 107 (1996) 58-62
(a)
T[11o] [1T0~ (b)
I [110] [1~o] Fig. 4. (a) STM image of the S'e-treated GaAs(001) (2 × 1) surface (40X40 nm2). (b) STM image of the Se-treated GaAs(001) (2 X 1) surface (20 x 20 nm2).
finally changed to the (2 × 1), and since this reconstruction did not change further, it appears to be the final phase of this transition. Fig. 4(a) and (b) show STM images of the GaAs(001) (2 × 1) surface on which Se has been deposited. In Fig. 4(a), the surface has many defects, such as holes, bright spots, and steps; however, the (2 × 1) surface appears to consist of simple dimer row structures at least in the top layer. In Fig. 4(b), each dimer can be well resolved in the dimer rows. The dimer rows are shifted relative to each other by a half dimer width in the [130] direction at the crack-like defect running between upper left and lower right. This is an example of an anti-phase domain boundary. It is not likely that these reconstructions are
61
formed only from the Se dimer structure in the top layer. The reasons are as follows. First, the heat treatment of the GaAs(001) without an As molecular beam should leave many Ga (and As) vacancies. Second, considering the Se pressure and treatment time, the Se coverage should be more than one monolayer even for the (4 X 3) structure that is formed by the shortest Se treatment. Finally, from the viewpoint of electron counting as described above, if the (2 × 3) or (2 × 1) reconstruction consists of only a simple Se dimer row structure, the dimerizafion should leave excess, electrically unstable electrons and Ga vacancies should exist under the surface. There is a structural model for the (2 × 1) surface that Pashley et al. have proposed, which is a complicated Ga2Se3-1ike structure with ordered Ga vacancies, which is formed by 4-layer reconstructed region [12]. As the Se deposition proceeds, the reconstructed region is expected to grow inward. Thus although the Se treatment is a process of making such a complicated reconstructed structure, unfortunately we can only observe by STM the top layer that is formed of Se-dimers and their arrangements such as dimer vacancies and buckling. The details of these structures should be considered from the viewpoint of electron transfer and stress in the lattice required to stabilize the surface energy. The technique required to produce and to observe any desired surface reconstruction will become increasingly important for controlling and investigating the electrical properties of Se-treated GaAs(001) surfaces.
4. Summary Structural changes of Se-terminated GaAs(001) surfaces have been controlled by a Se molecularbeam and observed in-situ by STM. As the Se treatment proceeded, the surface structure could be changed from (4 × 6) to (4 × 3) to (2 X 3) and finally to the (2 X 1) reconstruction. All surfaces were well ordered and STM imaging was carried out successfully for all of these reconstructions. The (4 X 3) structure was observed to be similar to (2 X 3), however, dimers might be buckled in alternate directions together with the nearest neighbors in the
62
E Haga et al./Applied Surface Science 107 (1996) 58-62
[110] direction. The (2 X 3) structure consisted of elliptical protrusions in the dimer rows forming an additional periodicity along the dimer row direction. For the (2 X 1) surface, simple dimer row structures were observed. These structural changes were thought to be caused by irregularities of the S e - S e or S e - G a dimers and Ga vacancies under the surface. We found that Se coverage played an important role as well as heat treatment in this phase change.
Acknowledgements The authors wish to thank Dr. J. Seto and Dr. S. Arakawa for their encouragement during the course of this work.
References [1] B.J. Skromme, C.J. Sandroff, E. Yablonovitch and T. Gmitter, Appl. Phys. Lett. 51 (1987) 2022. [2] M.S. Carpenter, M.R. Melloch and T.E. Dungan, Appl. Phys. Lett. 53 (1988) 66. [3] J.-F. Fan, H. Oigawa and Y. Nannichi, Jpn. J. Appl. Phys. 27 (1988) L1331. [4] C.J. Sandroff, M.S. Hegde, L.A. Farrow, R. Bhat, J.P. Harbison and C.C. Chang, J. Appl. Phys. 67 (1990) 586. [5] H. Oigawa, J. Fan, Y. Nannichi, H. Sugahara and M. Oshima, Jpn. J. Appl. Phys. 30 (1991) L322. [6] S.A. Chambers and V.S. Sundaram, J. Vac. Sci. Technol. B 9 (1991) 2256.
[7] H. Hirayama, Y. Matsumoto, H. Oigawa and Y. Nannichi, Appl. Phys. Lett. 54 (1989) 2565. [8] S.M. Mokler and P.R. Watson, J. Vac. Sci. Technol. A 9 (1991) 1374. [9] R.D. Bringans, D.K. Biegelsen, J.E. Northrup and L.-E. Swartz, Jpn. J. Appl. Phys. 32 (1993) 1484. [10] D. Li and M.D. Pashley, Phys. Rev. B 49 (1994) 13643. [11] D.K. Biegelsen, R.D. Bringans, J.E. Northrup and L.-E. Swartz, Phys. Rev. B 49 (1994) 5424. [12] M.D. Pashley and D. Li, J. Vac. Sci. Technol. A 12 (1994) 1848. [13] H. Shigekawa, H. Oigawa, K. Miyake, Y. Also, Y. Nannichi, T. Hashizume and T. Sakurai, Appl. Phys. Lett. 65 (1994) 607. [14] S. Takatani, T. Kikawa and M. Nakazawa, Phys. Rev. B 45 (1992) 8498. [15] D. Li, Y. Nakamura, N. Otsuka, J. Qiu, M. Kobayashi and R.L. Gunshor, J. Vac. Sci. Technol. B 9 (1991) 2167. [16] S. Miwa, Y. Haga, E. Morita, S. Arakawa, T. Hashizume and T. Sakurai, Jpn. J. Appl. Phys. 32 (1993)1508. [17] Y. Haga, S. Miwa and E. Morita, J. Vac. Sci. Technol. B 12 (1994) 2107. [18] M.D. Pashley, K.W. Haberern, W. Friday, J.M. WoodaU and P.D. Kirchner, Phys. Rev. Lett. 60 (1988) 2176. [19] D.K. Biegelsen, R.D. Bringans, J.E. Northrup and L.-E. Swartz, Phys. Rev. B 41 (1990) 5701. [20] E.J. Heller and M.G. Lagally, Appl. Phys. Lett. 60 (1992) 2675. [21] T. Hashizume, Q.K. Xue, J. Zhoe, A. Ichimiya and T. Sakrurai, Phys. Rev. Lett. 73 (1994) 2208. [22] S.L. Skala, J.S. Hubacek, J.R. Tucker, J.W. Lyding, S.T. Chou and K.-Y. Cheng, Phys. Rev. B 48 (1993) 9138. [23] C.W. Snyder, J. Sudijono, C.-H. Lam, M.D. Johnson and B.G. Orr, Phys. Rev. B 50 (1994) 18194. [24] Q. Xue, T. Hashizume, J.M. Zhou, T. Sakata, T. Ohno and T. Sakurai, Phys. Rev. Lett. 74 (1995) 3177.
~i~i~ii~!~i?~?~i~:~.~i~i~i~i~i~!~.!i~i~ii~!..`.!ii: ~
applied surface science ELSEVIER
Applied Surface Science 107 (1996) 58-62
Structural change of selenium-treated GaAs(001) surface observed by STM Y. Haga *, S. Miwa, E. Morita Sony Corporation Research Center, Yokohama 240, Japan Received 12 October 1995; accepted 24 December 1995
Abstract We have controlled the surface structures of selenium (Se) treated GaAs(001) using a Se molecular beam in a molecular-beam epitaxy (MBE) system and obtained a variety of surface reconstructions. The detailed surface structures were observed in-situ using a high-performance scanning tunneling microscope (STM) system. After producing Ga-stabilized GaAs(001) (4 X 6) reconstructions by thermal cleaning, Se was evaporated slowly onto the surfaces. As the Se deposition time was increased, the surface structure could be changed from (4 × 6) to (4 X 3) to (2 X 3) and finally to the (2 X 1) reconstruction. All surfaces were well ordered and STM images were obtained successfully for all of these reconstructions. The (4 × 3) structure was observed to be similar to (2 X 3), however, dimers might be buckled in alternate directions together with the nearest neighbors in the [1~0] direction. The (2 X 3) structure consisted of elliptical protrusions in the dimer rows forming an additional periodicity along the dimer row direction. For the (2 × 1) surface, simple dimers row structures were observed. These structural changes were thought to be caused by irregularities in the Se-Se or Se-Ga dimers and Ga vacancies under the surface. We found that Se coverage played an important role as well as heat treatment in this phase change. 1. Introduction Passivation of compound semiconductor surfaces by chalcogen atoms has attracted considerable attention in recent years because of its beneficial effect on the electronic properties of surfaces and interfaces, such as enhanced photoluminescence [1], increased sensitivity of Schottky-barrier height to the metal work function [2], and reduced band-gap surface states [3]. In order to understand the passivafion mechanism and the initial stage of growth for ZnSebased I I - V I semiconductors on I I I - V semiconductors, many experimental and theoretical studies have
* Corresponding author.
been carried out [4-13] but with limited success. Takatani et al. have investigated Se covered GaAs(001) surfaces using reflection high-energy electron diffraction (RHEED) [14]. They claimed that there are at least three reconstructions for the Se-adsorbed surface and reversible phase transitions were observed between the (4 × 3), (2 × 3) and (2 × 1) phases during heat treatment. Li et al. have reported from the observation of the Z n S e / G a A s interface by transmission electron microscopy (TEM) that a GazSe3-1ike structure (defect induced zincblend) was generated at the interface [15]. This means that Se-induced surface structures form Ga vacancies under the surface in order to satisfy the electron counting. For the analysis of further structural and electrical properties, it is essential to con-
0169-4332/96/$15.00 Copyright © 1996 Elsevier Science B.V. All rights reserved. PI1 S 0 1 6 9 - 4 3 3 2 ( 9 6 ) 0 0 5 0 4 - 1
59
Y. Haga et aL /Applied Surface Science 107 (1996) 58-62
trol the surface atomically to generate such reconstructions even at room temperature. In this work, we have controlled the Se-treatment of GaAs(001) surfaces by heat treatment and a Se molecular beam in a molecular-beam epitaxy (MBE) system and obtained a variety of reconstructions even after the samples were cooled down to room temperature. The details of the surface structures were analyzed in-situ using a high-performance ultra-high vacuum (UHV) STM system.
2. Experimental The experiments were performed in an UHV STM system connected to an MBE chamber. Details concerning this system have been reported elsewhere [16,17]. Prior to Se exposure, thermally cleaned GaAs(001) Ga-stabilized surfaces were prepared so that we would have to consider only on the reaction between Ga and Se. A Si-doped n-type ( ~ 1 × 1018 cm -3) GaAs(001) wafer was used as the substrate. The substrate was chemically etched in a KOH : H 2 0 : H202 solution and then thermally cleaned in the MBE chamber without an As flux in order to start with Ga-stabilized surfaces. During the cleaning, the evolution of the surface structures was monitored by RHEED. After (4 × 2), (4 × 3), or (4 × 6) patterns were observed at 510°C where Gastabilized surfaces were formed, Se was evaporated slowly at the same temperature onto the surfaces using a Knudsen cell with a pressure of 4 × 10 .6 Pa. When a desired reconstruction was observed by RHEED, Se exposure was stopped immediately to keep the structure. After the Se treatment, the sample was cooled and transferred from the MBE chamber to the STM chamber through vacuum. STM imaging was performed at room temperature in the constantcurrent mode mainly under conditions of a tunneling current of 20 pA and sample bias of - 3 to - 4 V.
3. Results and discussion Prior to Se deposition, a Ga-stabilized GaAs(001) (4 × 6) surface was observed by STM. Many STM studies on As-stabilized structures, such as the (2 × 4) or c(2 × 8) have been undertaken [18-21], however,
(a)
[1]-01
Fig. 1. (a) STM image of the GaAs(001) Ga-stabilized (4×6) surface (36 × 36 nm2). (b) STM image of the GaAs(001) Ga-stablized (4×6) surface (18 × 18 nma).
there are few reports on Ga-stabilized surfaces [18,21-23]. This is presumably because the Gastabilized surface is obtained only by careful thermal cleaning in UHV without an As atmosphere. For cleaning at 510°C, the surface showed (4 × 2), (4 × 3) or (4 × 6) RHEED patterns. This variation is likely to come from surface irregularities caused by chemical etching or heat treatment. Due to the KOH treatment, the surface oxide should be relatively thin and easy to remove, however, the real temperature might be a little higher than 510°C measured by IR pyrometer for the (4 × 6) structure. Fig. l(a) and (b) show respectively the large and small-area STM images of the Ga-stabilized (4 × 6) surface. The surface looks very rough and many
60
Y. Haga et al. /Applied Surface Science 107 (1996) 58-62
contaminants, perhaps oxide or carbon, are observed. A stripe-like structure, however, can be observed along the [170] direction in the flat areas. The distance between the stripes (bright and dark) is about 2.4 nm and close to the X6 periodicity in the [110] direction. This image is very similar to the reported Ga-stabilized structures [19,23,24] which are designated as (2 X 6) or (2 X 6 / 3 X 6). Se was deposited on the Ga-stabilized GaAs(001) surface described above at 510°C. As the Se deposition time was increased, the surface structure changed from (4 × 6) to (4 × 3) to (2 × 3) and finally to the (2 × 1) reconstruction. Any of these structures could be maintained by stopping Se exposure and were stable even after the sample was cooled to room temperature for STM observation. We could successfully obtain STM images for all of these reconstructions. When the Se deposition was started, the RHEED patterns at first changed to the (4 X 3) reconstruction. Fig. 2(a) shows an STM image Of GaAs(001) with a (4 × 3) reconstruction, and a cross-section is drawn along A - B in Fig. 2(b). The structure is observed to be similar to (2 × 3) (shown later), however, dimers might be buckled in alternate directions together with the nearest neighbors and form pairs to make the 4 X periodicity in the [110] direction. This suggests the existence of some charge transfer in the dimer layers. One possibility of such buckling is that dimers at this surface might be partially formed by Se and Ga atoms in so-called hetero-dimers. In this case, Se atoms were diffused into the sub-layer and been replaced by Ga. The (4 X 3) reconstruction quickly changed to the (2 X 3) under the Se exposure. Fig. 3 shows an STM image of a GaAs(001) surface with (2 X 3) RHEED patterns. This structure consists of elliptical protrusions in the dimer rows to make an additional periodicity along the dimer row direction. The distance between protrusions is about 1.2 nm, which is close to a 3 x periodicity in the [110] direction. For this structure, one dimer out of three is likely to be missing in the dimer rows. If the structure is formed by simple Se dimer rows, the (2 X 1) reconstruction leaves one excess electron for each Se dimer. In the (2 X 3) surface, since each Se-dimer desorption leaves more excess electrons (3 electrons for a (2 X 3) unit), filled state dangling bonds or G a - G a bonds
(a)
[110]
[11-0]
(b)
A
B
Fig. 2. (a) STM image of the Se-treatedGaAs(001) (4X 3) surface (32 x 32 nmZ). (b) Cross-sectionalong A-B in (a). might form dimers in the second layer with the Se dimer missing to compensate the excess electrons. Continuing Se treatment, the RHEED patterns
7L [1Y0] Fig. 3. STM image of the Se-treated GaAs(001) (2X3) surface (36 X 36 nm2).
Y. Haga et aL /Applied Surface Science 107 (1996) 58-62
(a)
T[11o] [1T0~ (b)
I [110] [1~o] Fig. 4. (a) STM image of the S'e-treated GaAs(001) (2 × 1) surface (40X40 nm2). (b) STM image of the Se-treated GaAs(001) (2 X 1) surface (20 x 20 nm2).
finally changed to the (2 × 1), and since this reconstruction did not change further, it appears to be the final phase of this transition. Fig. 4(a) and (b) show STM images of the GaAs(001) (2 × 1) surface on which Se has been deposited. In Fig. 4(a), the surface has many defects, such as holes, bright spots, and steps; however, the (2 × 1) surface appears to consist of simple dimer row structures at least in the top layer. In Fig. 4(b), each dimer can be well resolved in the dimer rows. The dimer rows are shifted relative to each other by a half dimer width in the [130] direction at the crack-like defect running between upper left and lower right. This is an example of an anti-phase domain boundary. It is not likely that these reconstructions are
61
formed only from the Se dimer structure in the top layer. The reasons are as follows. First, the heat treatment of the GaAs(001) without an As molecular beam should leave many Ga (and As) vacancies. Second, considering the Se pressure and treatment time, the Se coverage should be more than one monolayer even for the (4 X 3) structure that is formed by the shortest Se treatment. Finally, from the viewpoint of electron counting as described above, if the (2 × 3) or (2 × 1) reconstruction consists of only a simple Se dimer row structure, the dimerizafion should leave excess, electrically unstable electrons and Ga vacancies should exist under the surface. There is a structural model for the (2 × 1) surface that Pashley et al. have proposed, which is a complicated Ga2Se3-1ike structure with ordered Ga vacancies, which is formed by 4-layer reconstructed region [12]. As the Se deposition proceeds, the reconstructed region is expected to grow inward. Thus although the Se treatment is a process of making such a complicated reconstructed structure, unfortunately we can only observe by STM the top layer that is formed of Se-dimers and their arrangements such as dimer vacancies and buckling. The details of these structures should be considered from the viewpoint of electron transfer and stress in the lattice required to stabilize the surface energy. The technique required to produce and to observe any desired surface reconstruction will become increasingly important for controlling and investigating the electrical properties of Se-treated GaAs(001) surfaces.
4. Summary Structural changes of Se-terminated GaAs(001) surfaces have been controlled by a Se molecularbeam and observed in-situ by STM. As the Se treatment proceeded, the surface structure could be changed from (4 × 6) to (4 × 3) to (2 X 3) and finally to the (2 X 1) reconstruction. All surfaces were well ordered and STM imaging was carried out successfully for all of these reconstructions. The (4 X 3) structure was observed to be similar to (2 X 3), however, dimers might be buckled in alternate directions together with the nearest neighbors in the
62
E Haga et al./Applied Surface Science 107 (1996) 58-62
[110] direction. The (2 X 3) structure consisted of elliptical protrusions in the dimer rows forming an additional periodicity along the dimer row direction. For the (2 X 1) surface, simple dimer row structures were observed. These structural changes were thought to be caused by irregularities of the S e - S e or S e - G a dimers and Ga vacancies under the surface. We found that Se coverage played an important role as well as heat treatment in this phase change.
Acknowledgements The authors wish to thank Dr. J. Seto and Dr. S. Arakawa for their encouragement during the course of this work.
References [1] B.J. Skromme, C.J. Sandroff, E. Yablonovitch and T. Gmitter, Appl. Phys. Lett. 51 (1987) 2022. [2] M.S. Carpenter, M.R. Melloch and T.E. Dungan, Appl. Phys. Lett. 53 (1988) 66. [3] J.-F. Fan, H. Oigawa and Y. Nannichi, Jpn. J. Appl. Phys. 27 (1988) L1331. [4] C.J. Sandroff, M.S. Hegde, L.A. Farrow, R. Bhat, J.P. Harbison and C.C. Chang, J. Appl. Phys. 67 (1990) 586. [5] H. Oigawa, J. Fan, Y. Nannichi, H. Sugahara and M. Oshima, Jpn. J. Appl. Phys. 30 (1991) L322. [6] S.A. Chambers and V.S. Sundaram, J. Vac. Sci. Technol. B 9 (1991) 2256.
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