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Epitaxial growth of CdTe(0 0 1) studied by scanning tunnelling microscopy
ELSEVIER
Journal of Crystal Growth 184/185 (1998) 203-207
Epitaxial growth of CdTe(0 0 1) studied by scanning tunnelling microscopy D. Martrou, J. Eymery, P. Gentile, N. Magnea* CEA Grenoble DRFMCJSPZM, I7 Au. des Martyrs, 38054 Grenoble, France
Abstract Scanning tunnelling microscopy is used to study the growth mechanisms of CdTe( 1 0 0) in molecular beam epitaxy. The 2D growth proceeds by the formation of square-shaped islands with (10 0) edges. This effect leads to the self-organisation of terraces for vicinal surfaces with (1 0 0) steps. 0 1998 Elsevier Science B.V. All rights reserved. PAC.9
68.55; 68.35.B~; 61.16
Kevwords:
Surfaces; Vicinal surfaces; Scanning
tunnelling
The recent development of quantum nanostructures has raised a strong interest in the search for self-organised growth mechanisms which could provide a perfectly ordered array of quantum dots or wires. Among the most studied methods is the Stranski-Krastanov process which leads by strain relaxation, to the formation of defect-free 3D islands [ 11. But the drawback is a large dispersion of the island size and of the distance between the islands. A second method has been proposed which uses low-index stepped surfaces where t&e step edges act as preferential incorporation sites for impinging atoms [a]. The success of the last method in the fabrication of nanostuctures implies a
*Corresponding [email protected].
author. Fax: + 33 4 76 88 31 33; e-mail:
microscopy
well-ordered array of steps with a minimal density of kinks. For materials such, as (0 0 1) the small fluctuations of terraces size and shape indicate that this step pattern could be used to grow a wellordered array of quantum wires. In silicon or gallium arsenide this type of control is made difficult by the strong anisotropy of the growth along the (1 1 0) directions. This anisotropy has been explained as a consequence of the (2 x 1) reconstruction of the (0 0 1) surface which makes the diffusion and the sticking coefficient at the step edges very different for A and B steps [3]. Scanning tunnelling microscopy (STM) of CdTe(0 0 1) grown by molecular beam epitaxy (MBE) shows that, in this compound, such an anisotropy does not exist. Indeed, we have observed that the equilibrium shape of 2D islands consists of squares with equivalent edges parallel to the (1 0 0)
0022-0248/98/$19.00 cs 1998 Elsevier Science B.V. All rights reserved. PII SOO22-0248(97)00753-7
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D. Martrou et al. /Journal
of Cystal
directions. We have developed a model based on the symmetry of the C(2 x 2) reconstruction which explains that under Cd-rich conditions the 2D growth proceeds by the flow of the (1 0 0) rather than the (1 1 0) steps. For these conditions, parallel and equally spaced terraces are then obtained on vicinal surfaces with (1 0 0) steps. We first discuss the atomic structure of the (10 0)CdTe surface. The STM images are obtained with a VT Omicron microscope coupled to a small MBE chamber equipped with a Cd and a Te cell. After deposition at 300°C the samples are cooled down to 240°C in order to freeze the equilibrium configuration, and then quickly transferred under UHV to keep the surface free of any contamination. Fig. 1 shows the morphology of a CdTe(1 0 0) surface obtained after the deposition of 50 nm of CdTe, grown at 300°C with a Cd/Te ratio = 2 on a surface initially smoothed at 360°C under a Cd overpressure. This surface is relatively smooth and exhibits wide terraces of several tens of nanometers separated by 0.324 nm high monomolecular steps resulting from a misorientation of f 0.1’ of the Cd0.96Zn0.04Te substrate. Such a morphology is indicative of a 2D growth with a surface diffusion length of Te atoms comparable to or greater than
Growth 184/185 (1998) 203-207
the step separation. The main points of interest are the orientation of the step edges which are parallel to the (1 0 0) directions and the square or rectangular shape of the isolated islands. This is very different from what is observed in Si or GaAs where the islands are highly anisotropic with their long edges along the (1 1 0) axis [3,4]. We interpret this major difference as a result of the symmetry of the surface reconstruction. For Si and GaAs the surface reconstruction during MBE is n(2 x 1) because of the dimerisation of Si or As surface atoms along (1 - 1 0). But for CdTe stabilised under Cd-rich conditions the surface reconstruction is C(2 x 2) [S]. This is shown in Fig. 2a, obtained with atomic resolution, where we identify the rows of Cd atoms along the (1 0 0) directions, each Cd atom being situated at the corner of a square of 0.648 nm side. As shown in Fig. 2a, this symmetry is reproduced identically on a larger scale to produce the shape of the monomolecular islands. From the atomic structure of the Cd C(2 x 2) reconstruction, we have built a model explaining the morphology of the CdTe(1 0 0) surface. The main assumptions are: (i) Cd C(2 x 2) has a surface coverage of $. The Cd atoms are aligned along the (1 0 0) axis, they are not dimerised and they are in
Fig. 1. Large-scale (500 nm x 350 nm) STM image of a Cd-rich CdTe(1 0 0) surface obtained by MBE(@Cd/@Te = 2). The (1 1 0) and (1 - 1 0) directions are parallel to the vertical and horizontal axis, respectively. Steps run along the (1 0 0) direction. The tunnelling conditions are: V, = - 2.2 V, It = 0.2 nA.
D. Martrou
et al. ! Journal
sf Qvstal
Growth 1841185 (1998) 2033207
205
Fig 2. (a) Atomic scale STM image showing the C(2 x 2) reconstruction and monomolecular islands. (1 1 0) stripes starting from each intersection of (1 0 0) steps have a (2 x 1) symmetry and separate the C(2 x 2) domains. (b) Atomic model of a monomolecular island of CdTe on the Cd-rich C(2 x 2) reconstruction. The 0.5Cd coverage imposes the dimerisation of Te bonds and, consequently, (1 0 0) oriented edges. Stripes of (2 x 1) reconstruction start from each corner and define C(2 x 2) domains. The low levels of the (1 0 0) steps incorporate three Cd atoms which will act as preferential sticking nucleii for Te, molecules so that the step flow perpendicular to the four (1 0 0) directions is isotropic
the plane of the Te underlayer as a result of molecular bonding [S]. Ab initio calculations confirm this description [6]. (ii) The Te atoms have C molecular bond as a result of their incorporation in the dimer form Tez. An atomic representation of this model is shown in Fig. 2b. On a Cd-rich surface the monomolecular island takes a square shape with (1 0 0) edges. Starting from each corner, we also see a stripe made of two rows of Cd which locally adopt the (2 x 1) reconstruction between the C(2 x 2) domains. This is clearly observed in the experimental data of Fig. 2a where the (2 x 1) structure appears at each intersection of (1 0 0) steps. This model also shows that the (1 0 0) edges are made of a succession of Cd atoms in the lower level, forming a staircase-like structure. These sites, made of three Cd atoms, will be very reactive for the incorporation of the Tez molecules diffusing on the C(2 x 2) surface. Then
the lateral growth of the island will proceed mainly perpendicular to the (1 0 0) edges with the same velocity for the four equivalent (1 0 0) directions, as experimentally observed. As we will show below this peculiar growth mode will strongly affect the morphology of vicinal surfaces. From our model one can expect two distinct behaviours depending on whether the steps are (1 1 0) or (1 0 0) oriented: smooth and straight borders for (1 0 0) steps but rough ledges for the (1 1 0) steps because the step orientation defined by the miscut angle is at 45” from the preferential (1 0 0) growth. In Fig. 3 we compare the STM pictures for the two kinds of steps. Fig. 3a is obtained after CdTe deposition and smoothing under Cd excess on a CdTe substrate misoriented 2” toward (1 0 0). As expected, the steps remain parallel to the (1 0 0) direction and are straight over severals tens of nanometres. In areas free of substrate defects, the
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D. Martrou et al. /Journal of Crystal Growth 1841185 (1998) 203-207
Fig. 3. (a) STM images (160 nm x 140 nm) after MBE of a CdTe vicinal surface misoriented 2” toward (1 0 0) showing straight and smooth step edges running parallel to the (1 0 0) direction. (b) Same image (300 nm x 250 nm) as 3(a) is obtained on a CdTe vicinal surfaces with (1 - 1 0) A steps. The terraces initially aligned along the (1 - 1 0) axis are split into triangular domains with (1 0 0) edges.
terraces have a quite regular size and are more or less equally separated by 8-9 nm, in agreement with the desorientation angle. Fig. 3b shows the surface morphology for a Cd,.g,Zn0,04Te substrate initially miscut 1” towards (1 1 0) to reveal A steps. While the initial step alignment along the (1 - 1 0) direction is more or less preserved on a macroscopic scale, each terrace is split along the (1 - 1 0) direction in quasi-equilateral triangles with two (1 0 0) edges. The amplitude of this roughness is strongly dependent on the growth mode (ALE, MBE) but the triangular shape of the terraces has always been observed on both type A or B vicinal surfaces. From this picture it is also clear that the triangular terraces tend to align perpendiculary to the step edges as a result of the correlation in the (1 0 0) directions. A possible origin of this kind of vertical correlation has to be searched for in the elastic deformation at the step edges and also in the electric dipolar interactions between steps. This vertical correlation has only been observed for Cdrich conditions with a C(2 x 2) reconstruction on the terraces and has never exceeded more than 10 steps. The absence of this growth anisotropy in spite of the (2 x 1) symmetry has also been observed for
growth on a nominal (1 0 0) surface with a Te flux twice as large as the Cd flux. This result indicates that the equilibrium shape of islands is not only determined by the geometrical orientation of the dimers as is the case for covalent materials (Si, Ge) [3]. In II-VI compounds whose bonds exhibit a strong ionic character, the electric dipolar interactions resulting from the transfer of electrons from Cd atoms to Te atoms could play a predominant role in the energy of steps and in their mutual interactions. In conclusion, scanning tunnelling microscopy imaging of CdTe(1 0 0) surfaces after molecular beam epitaxy has provided new insights into the growth process of this material. The symmetry of the C(2 x 2) surface reconstruction obtained for Cd-rich conditions determines the surface morphology. In the absence of any anisotropy for 2D growth, the monomolecular islands are squares aligned along the (1 0 0) directions. This can be modellised by considering the interactions between Cd and Te orbitals and the symmetry of the reconstruction. This original growth mode has important consequences for vicinal surfaces. A and B (1 1 0) steps cannot be regular and smooth for Cd-rich conditions whereas (1 0 0) steps can be. Thus, CdTe
D. Martrou et al. 1 Journal of Crystal Growth 1841185 (1998) 203-207
vicinal surfaces misoriented towards (1 0 0) are probably more suitable for fabricating artificial quantum wires or vertical superlattices. Nevertheless, (1 - 1 0) A steps are very attractive: we have recently observed that the terraces with (1 0 0) edges can self-organise on a macroscopic scale in a regular checkerboard array [7]. It will thus be possible to design a tile of nucleation sites on the surface which can be further used to fabricate an assembly of quantum dots with constant size and separation.
207
References Cl1 D.J. Eaglesham, M. Cerullo, Phys. Rev. Lett. 64(1990) 1943. PI P.M. Petroff, A.C. Gossard, W. Wiegmann, Appl. Phys. Lett. 45 (1984) 620.
c31 Y.W. MO, B.S. Swartzenbruner, f41 cs1 f61 f71
R. Kariotis, M.B. Webb, M.G. Lagally, Phys. Rev. Lett. 63 (1989) 2393. M.D. Pashley, K.W. Haberern, J.M. Gaines, Appl. Phys. Lett. 58 (1991) 406. M.B. Veron, V.H. Etgens, M. Sauvage-Simkin, S. Tatarenko, H.A. van der Vegt, Appl. Phys. Lett. 67 (1995) 3957. J. Eymery et al., to be published. D. Martrou et al., to be published.
Journal of Crystal Growth 184/185 (1998) 203-207
Epitaxial growth of CdTe(0 0 1) studied by scanning tunnelling microscopy D. Martrou, J. Eymery, P. Gentile, N. Magnea* CEA Grenoble DRFMCJSPZM, I7 Au. des Martyrs, 38054 Grenoble, France
Abstract Scanning tunnelling microscopy is used to study the growth mechanisms of CdTe( 1 0 0) in molecular beam epitaxy. The 2D growth proceeds by the formation of square-shaped islands with (10 0) edges. This effect leads to the self-organisation of terraces for vicinal surfaces with (1 0 0) steps. 0 1998 Elsevier Science B.V. All rights reserved. PAC.9
68.55; 68.35.B~; 61.16
Kevwords:
Surfaces; Vicinal surfaces; Scanning
tunnelling
The recent development of quantum nanostructures has raised a strong interest in the search for self-organised growth mechanisms which could provide a perfectly ordered array of quantum dots or wires. Among the most studied methods is the Stranski-Krastanov process which leads by strain relaxation, to the formation of defect-free 3D islands [ 11. But the drawback is a large dispersion of the island size and of the distance between the islands. A second method has been proposed which uses low-index stepped surfaces where t&e step edges act as preferential incorporation sites for impinging atoms [a]. The success of the last method in the fabrication of nanostuctures implies a
*Corresponding [email protected].
author. Fax: + 33 4 76 88 31 33; e-mail:
microscopy
well-ordered array of steps with a minimal density of kinks. For materials such, as (0 0 1) the small fluctuations of terraces size and shape indicate that this step pattern could be used to grow a wellordered array of quantum wires. In silicon or gallium arsenide this type of control is made difficult by the strong anisotropy of the growth along the (1 1 0) directions. This anisotropy has been explained as a consequence of the (2 x 1) reconstruction of the (0 0 1) surface which makes the diffusion and the sticking coefficient at the step edges very different for A and B steps [3]. Scanning tunnelling microscopy (STM) of CdTe(0 0 1) grown by molecular beam epitaxy (MBE) shows that, in this compound, such an anisotropy does not exist. Indeed, we have observed that the equilibrium shape of 2D islands consists of squares with equivalent edges parallel to the (1 0 0)
0022-0248/98/$19.00 cs 1998 Elsevier Science B.V. All rights reserved. PII SOO22-0248(97)00753-7
204
D. Martrou et al. /Journal
of Cystal
directions. We have developed a model based on the symmetry of the C(2 x 2) reconstruction which explains that under Cd-rich conditions the 2D growth proceeds by the flow of the (1 0 0) rather than the (1 1 0) steps. For these conditions, parallel and equally spaced terraces are then obtained on vicinal surfaces with (1 0 0) steps. We first discuss the atomic structure of the (10 0)CdTe surface. The STM images are obtained with a VT Omicron microscope coupled to a small MBE chamber equipped with a Cd and a Te cell. After deposition at 300°C the samples are cooled down to 240°C in order to freeze the equilibrium configuration, and then quickly transferred under UHV to keep the surface free of any contamination. Fig. 1 shows the morphology of a CdTe(1 0 0) surface obtained after the deposition of 50 nm of CdTe, grown at 300°C with a Cd/Te ratio = 2 on a surface initially smoothed at 360°C under a Cd overpressure. This surface is relatively smooth and exhibits wide terraces of several tens of nanometers separated by 0.324 nm high monomolecular steps resulting from a misorientation of f 0.1’ of the Cd0.96Zn0.04Te substrate. Such a morphology is indicative of a 2D growth with a surface diffusion length of Te atoms comparable to or greater than
Growth 184/185 (1998) 203-207
the step separation. The main points of interest are the orientation of the step edges which are parallel to the (1 0 0) directions and the square or rectangular shape of the isolated islands. This is very different from what is observed in Si or GaAs where the islands are highly anisotropic with their long edges along the (1 1 0) axis [3,4]. We interpret this major difference as a result of the symmetry of the surface reconstruction. For Si and GaAs the surface reconstruction during MBE is n(2 x 1) because of the dimerisation of Si or As surface atoms along (1 - 1 0). But for CdTe stabilised under Cd-rich conditions the surface reconstruction is C(2 x 2) [S]. This is shown in Fig. 2a, obtained with atomic resolution, where we identify the rows of Cd atoms along the (1 0 0) directions, each Cd atom being situated at the corner of a square of 0.648 nm side. As shown in Fig. 2a, this symmetry is reproduced identically on a larger scale to produce the shape of the monomolecular islands. From the atomic structure of the Cd C(2 x 2) reconstruction, we have built a model explaining the morphology of the CdTe(1 0 0) surface. The main assumptions are: (i) Cd C(2 x 2) has a surface coverage of $. The Cd atoms are aligned along the (1 0 0) axis, they are not dimerised and they are in
Fig. 1. Large-scale (500 nm x 350 nm) STM image of a Cd-rich CdTe(1 0 0) surface obtained by MBE(@Cd/@Te = 2). The (1 1 0) and (1 - 1 0) directions are parallel to the vertical and horizontal axis, respectively. Steps run along the (1 0 0) direction. The tunnelling conditions are: V, = - 2.2 V, It = 0.2 nA.
D. Martrou
et al. ! Journal
sf Qvstal
Growth 1841185 (1998) 2033207
205
Fig 2. (a) Atomic scale STM image showing the C(2 x 2) reconstruction and monomolecular islands. (1 1 0) stripes starting from each intersection of (1 0 0) steps have a (2 x 1) symmetry and separate the C(2 x 2) domains. (b) Atomic model of a monomolecular island of CdTe on the Cd-rich C(2 x 2) reconstruction. The 0.5Cd coverage imposes the dimerisation of Te bonds and, consequently, (1 0 0) oriented edges. Stripes of (2 x 1) reconstruction start from each corner and define C(2 x 2) domains. The low levels of the (1 0 0) steps incorporate three Cd atoms which will act as preferential sticking nucleii for Te, molecules so that the step flow perpendicular to the four (1 0 0) directions is isotropic
the plane of the Te underlayer as a result of molecular bonding [S]. Ab initio calculations confirm this description [6]. (ii) The Te atoms have C molecular bond as a result of their incorporation in the dimer form Tez. An atomic representation of this model is shown in Fig. 2b. On a Cd-rich surface the monomolecular island takes a square shape with (1 0 0) edges. Starting from each corner, we also see a stripe made of two rows of Cd which locally adopt the (2 x 1) reconstruction between the C(2 x 2) domains. This is clearly observed in the experimental data of Fig. 2a where the (2 x 1) structure appears at each intersection of (1 0 0) steps. This model also shows that the (1 0 0) edges are made of a succession of Cd atoms in the lower level, forming a staircase-like structure. These sites, made of three Cd atoms, will be very reactive for the incorporation of the Tez molecules diffusing on the C(2 x 2) surface. Then
the lateral growth of the island will proceed mainly perpendicular to the (1 0 0) edges with the same velocity for the four equivalent (1 0 0) directions, as experimentally observed. As we will show below this peculiar growth mode will strongly affect the morphology of vicinal surfaces. From our model one can expect two distinct behaviours depending on whether the steps are (1 1 0) or (1 0 0) oriented: smooth and straight borders for (1 0 0) steps but rough ledges for the (1 1 0) steps because the step orientation defined by the miscut angle is at 45” from the preferential (1 0 0) growth. In Fig. 3 we compare the STM pictures for the two kinds of steps. Fig. 3a is obtained after CdTe deposition and smoothing under Cd excess on a CdTe substrate misoriented 2” toward (1 0 0). As expected, the steps remain parallel to the (1 0 0) direction and are straight over severals tens of nanometres. In areas free of substrate defects, the
206
D. Martrou et al. /Journal of Crystal Growth 1841185 (1998) 203-207
Fig. 3. (a) STM images (160 nm x 140 nm) after MBE of a CdTe vicinal surface misoriented 2” toward (1 0 0) showing straight and smooth step edges running parallel to the (1 0 0) direction. (b) Same image (300 nm x 250 nm) as 3(a) is obtained on a CdTe vicinal surfaces with (1 - 1 0) A steps. The terraces initially aligned along the (1 - 1 0) axis are split into triangular domains with (1 0 0) edges.
terraces have a quite regular size and are more or less equally separated by 8-9 nm, in agreement with the desorientation angle. Fig. 3b shows the surface morphology for a Cd,.g,Zn0,04Te substrate initially miscut 1” towards (1 1 0) to reveal A steps. While the initial step alignment along the (1 - 1 0) direction is more or less preserved on a macroscopic scale, each terrace is split along the (1 - 1 0) direction in quasi-equilateral triangles with two (1 0 0) edges. The amplitude of this roughness is strongly dependent on the growth mode (ALE, MBE) but the triangular shape of the terraces has always been observed on both type A or B vicinal surfaces. From this picture it is also clear that the triangular terraces tend to align perpendiculary to the step edges as a result of the correlation in the (1 0 0) directions. A possible origin of this kind of vertical correlation has to be searched for in the elastic deformation at the step edges and also in the electric dipolar interactions between steps. This vertical correlation has only been observed for Cdrich conditions with a C(2 x 2) reconstruction on the terraces and has never exceeded more than 10 steps. The absence of this growth anisotropy in spite of the (2 x 1) symmetry has also been observed for
growth on a nominal (1 0 0) surface with a Te flux twice as large as the Cd flux. This result indicates that the equilibrium shape of islands is not only determined by the geometrical orientation of the dimers as is the case for covalent materials (Si, Ge) [3]. In II-VI compounds whose bonds exhibit a strong ionic character, the electric dipolar interactions resulting from the transfer of electrons from Cd atoms to Te atoms could play a predominant role in the energy of steps and in their mutual interactions. In conclusion, scanning tunnelling microscopy imaging of CdTe(1 0 0) surfaces after molecular beam epitaxy has provided new insights into the growth process of this material. The symmetry of the C(2 x 2) surface reconstruction obtained for Cd-rich conditions determines the surface morphology. In the absence of any anisotropy for 2D growth, the monomolecular islands are squares aligned along the (1 0 0) directions. This can be modellised by considering the interactions between Cd and Te orbitals and the symmetry of the reconstruction. This original growth mode has important consequences for vicinal surfaces. A and B (1 1 0) steps cannot be regular and smooth for Cd-rich conditions whereas (1 0 0) steps can be. Thus, CdTe
D. Martrou et al. 1 Journal of Crystal Growth 1841185 (1998) 203-207
vicinal surfaces misoriented towards (1 0 0) are probably more suitable for fabricating artificial quantum wires or vertical superlattices. Nevertheless, (1 - 1 0) A steps are very attractive: we have recently observed that the terraces with (1 0 0) edges can self-organise on a macroscopic scale in a regular checkerboard array [7]. It will thus be possible to design a tile of nucleation sites on the surface which can be further used to fabricate an assembly of quantum dots with constant size and separation.
207
References Cl1 D.J. Eaglesham, M. Cerullo, Phys. Rev. Lett. 64(1990) 1943. PI P.M. Petroff, A.C. Gossard, W. Wiegmann, Appl. Phys. Lett. 45 (1984) 620.
c31 Y.W. MO, B.S. Swartzenbruner, f41 cs1 f61 f71
R. Kariotis, M.B. Webb, M.G. Lagally, Phys. Rev. Lett. 63 (1989) 2393. M.D. Pashley, K.W. Haberern, J.M. Gaines, Appl. Phys. Lett. 58 (1991) 406. M.B. Veron, V.H. Etgens, M. Sauvage-Simkin, S. Tatarenko, H.A. van der Vegt, Appl. Phys. Lett. 67 (1995) 3957. J. Eymery et al., to be published. D. Martrou et al., to be published.