- Email: [email protected]
MBE growth of calcium and cadmium fluoride nanostructures on silicon
Applied Surface Science 175±176 (2001) 619±628
MBE growth of calcium and cadmium ¯uoride nanostructures on silicon N.S. Sokolov*, S.M. Suturin Solid State Optics Department, Ioffe Physico-Technical Institute, Russian Academy of Sciences, 26 Polytechnicheskaya, 194021 St. Petersburg, Russia Accepted 8 October 2000
Abstract Molecular beam epitaxy (MBE) growth of CdF2 and CaF2 layers and nanoscale heterostructures on Si(1 1 1) and Si(0 0 1) substrates has been investigated. The surface morphology of CaF2 and CdF2 layers on Si(1 1 1), studied by atomic force microscopy (AFM) was found to depend strongly on the growth temperature. The high temperature 2D growth mode would produce atomically ¯at surface. In contrast to that, well-pronounced pyramids indicating the 3D growth mode were observed on the layers grown at low temperature. Short-period CdF2±CaF2 superlattices (SLs) were also studied. Analysis of re¯ection high energy electron diffraction (RHEED) intensity oscillations showed asymmetry of the ``CaF2 on CdF2'' and ``CdF2 on CaF2'' interfaces resulting in SL surface smoothening. The possible mechanisms relevant to this phenomenon have been discussed. Atomic force microscopy (AFM), and RHEED have been used to study formation of CaF2 epitaxial nanostructures on Si(0 0 1). Different types of nanostructures have been grown including ultra-thin 2D wetting layer, quasi 1D wires at high growth temperature and well-organized dots at lower temperatures. An important role of the wetting layer in the transformation of the surface morphology at the initial stages of CaF2 growth on Si(0 0 1) at high temperatures has been demonstrated. # 2001 Elsevier Science B.V. All rights reserved. Keywords: MBE; AFM; Fluoride; Silicon; Superlattice; Facetting; Nanostructures
1. Introduction Epitaxial growth of CaF2 on Si(1 1 1) substrates by molecular beam epitaxy (MBE) has been investigated by a number of research groups (see, e.g. [1±5]). Considerably less efforts have been undertaken to explore the MBE growth and properties of CdF2 heterostructures. However, it was demonstrated that *
Corresponding author. Tel.: 7-812-247-6411; fax: 7-812-247-1017. E-mail address: [email protected] (N.S. Sokolov).
high crystal quality CdF2 epitaxial layers [6] and CdF2±CaF2 superlattices (SLs) [7] can be grown. The studies revealed a number of speci®c features related to the growth of such heterostructures which have been recently discussed in [8]. The photoluminescence (PL) studies of CaF2±CdF2 SLs with CaF2 layers doped with Eu revealed an interesting feature of tunneling-assisted photoionization [9] of Eu2 leading to a considerable drop of the Eu2 PL intensity monitored by a continuous excitation of the 4f 7 ! 4f 6 5d transition. This effect can be attractive for designing ¯uoride-based non-volatile memory.
0169-4332/01/$ ± see front matter # 2001 Elsevier Science B.V. All rights reserved. PII: S 0 1 6 9 - 4 3 3 2 ( 0 1 ) 0 0 0 7 5 - 7
620
N.S. Sokolov, S.M. Suturin / Applied Surface Science 175±176 (2001) 619±628
It was found that CaF2 epitaxial growth on Si(0 0 1) results in very different from CaF2/Si(1 1 1) surface morphology and epitaxial relations strongly depending on growth temperature [10,11]. At certain growth conditions, formation of CaF2 quasi 1D islands was observed [12]. These can be very attractive as a lithographic mask for fabrication of new micro-, optoand magneto-electronic devices with the required characteristic size smaller than 10 nm. In this work, we applied re¯ection high energy electron diffraction (RHEED) and atomic force microscopy (AFM) to study comprehensively the formation of single CaF2 and CdF2 layers and SLs on Si(1 1 1), as well as to have an insight in the variety of selfassembled CaF2 nanostructures produced on Si(0 0 1) in a wide range of growth conditions. 2. Experimental The growth of samples was carried out in an ultrahigh vacuum chamber, with the base pressure inside maintained below 10 8 Pa. The substrate temperature could be varied from room temperature to 13008C. The growth was carried out at a rate of several nanometer per minute from a molecular beam formed by evaporating small pieces of required material in a carbon or CaF2 crucible (the latter was used to hold CdF2 material). The growth chamber was equipped with a RHEED diffractometer and a TV system for taking RHEED patterns. Silicon substrates with a miscut of several arc minutes were cleaned following the Shiraki technique and annealed inside the chamber at 12508C for 1.5 min. The oxide layer was evaporated during the annealing to reveal a clean Si surface con®rmed by a 7 7 superstructure on Si(1 1 1) and a 2 1 superstructure on Si(0 0 1), both observed by RHEED. The surface morphology of the grown structures was investigated by AFM. The AFM instrument manufactured by NT-MDT (Zelenograd, Russia) allowed surface topography measurements of an area up to 8 mm 8 mm in the contact and resonant modes. An angstrom resolution in the normal direction could be easily achieved, the lateral resolution being estimated as 10 nm, depending on the cantilever tip sharpness.
3. Nanostructures on Si(1 1 1) 3.1. CaF2 and CdF2 pyramidal growth It was recently demonstrated [8] that both CaF2 and CdF2 grow ¯at at a suf®ciently high temperature on the (1 1 1) surface of a Si or CaF2 substrate. When grown at a lower temperature, both materials tend to form pyramids with triangular basement sides parallel to h1 1 0i. The two materials differ in the upper temperature limit of the pyramid formation which is 3508C for CdF2 and 6008C for CaF2. Pyramids produced on CaF2 at 5008C (Fig. 1a) are 150 nm laterally and only 2±3 nm in height, which ensures that the terraces on the pyramid slopes can be easily identi®ed by AFM. In contrast, CdF2 pyramids obtained at 3008C show slopes about 10 times steeper (Fig. 1b), with an 108 inclination to the (1 1 1) substrate plane. The reason for the pyramid growth is the tendency for the next layer to nucleate on top of the growing one well before the latter is completed. The maximal size of a ¯at island, which has not yet turned to a nucleation site for the next layer, is a parameter that determines the inclination of the pyramid facets. The lower admolecular mobility and the higher energy barriers at step edges result in steeper pyramid facets. The pyramids tend to grow larger, both in width and height, with more material deposited, the smaller pyramids being swallowed up by the bigger ones. 3.2. CaF2±CdF2 superlattices (SLs) In spite of the fact that pyramids are characteristic of both ¯uorides, the surfaces of low temperature CaF2±CdF2 SL with the ¯uoride layers less than 30 ML thick prove to be remarkably ¯at. During the SL growth, CaF2 and CdF2 layers are deposited on the substrate in turn at the same temperature of 1008C. Fig. 2 shows the time behavior of the specular beam intensity I00, monitored during the growth of the ®rst three SL layers, each 20 ML thick. Distinct oscillations in I00 with a 1 ML period are clearly seen, their amplitude varying with the material being deposited. It is found that the specular beam is much brighter for CaF2 than for CdF2. This may be due to the difference in the electronic structure of Ca and Cd atoms. Noteworthy, the RHEED intensity increases
N.S. Sokolov, S.M. Suturin / Applied Surface Science 175±176 (2001) 619±628
621
Fig. 1. Pyramidal mounds (a) on CaF2 at 5008C and (b) on CdF2 at 3008C.
gradually during the CaF2-on-CdF2 growth, while the growth in the reverse order leads to an abrupt intensity drop. This indicates that a relatively thick CaF2 layer is to be deposited to recover the morphology typical for CaF2 surface. At the same time, just a couple of CdF2 monolayers are suf®cient to form a continuous coverage. This asymmetry can be explained in the following way. The growth of CdF2 at 1008C starts on a comparatively smooth CaF2 buffer (Fig. 3a) and results in the
surface facetting, as indicated in Fig. 3b. One can suggest that the diffusion length of CaF2 admolecules on the CdF2 surface is greater than on the CaF2 surface. The CaF2 admolecules diffuse readily down the CdF2 facets to ®rst occupy the positions in the pits of the facetted surface rather than to cover the entire surface uniformly. The CdF2 surface would be covered completely as soon as the amount of deposited CaF2 material is suf®cient to ®ll the pits. This makes up 10 ML of ¯uoride, according to Fig. 2. An increase in the
622
N.S. Sokolov, S.M. Suturin / Applied Surface Science 175±176 (2001) 619±628
Fig. 2. Specular beam intensity during the growth of ®rst SL layers.
oscillation amplitude during the CaF2 deposition corresponds to the overall surface ¯attening due to the facet overgrowth. The CaF2 epitaxial surface grown at 1008C also has facetted morphology. The facets, however, are much smaller than on CdF2 because of the shorter diffusion length of the CaF2 admolecules. Thus, the surface to be overgrown by the subsequent CdF2 layer has a smoother topography (Fig. 3c) and is readily covered by a few CdF2 monolayers. Besides, the diffusion length of CdF2 admolecules on the CaF2 surface is shorter than that on the CdF2 surface, which is also a prerequisite for a fast transformation to a uniform coverage. The oscillations fade out during the CdF2 growth, most likely due to the development of facetted morphology. In addition, a noticeable increase of the specular beam intensity during the growth interruption (see Fig. 2) between the layers of different materials indicates the surface ¯attening, which may be due to the non-equilibrium nature of the facetted surface. Thus, a combination of the two ¯uoride materials with different diffusion lengths allows thick heterostructures to be grown with relatively ¯at surfaces and interfaces. AFM measurements carried out for SLs with different periods show that a shorter SL period results in a ¯atter surface. Another advantage of the CaF2±CdF2 SLs is that the layers are alternatively compressed (CaF2) or stretched (CdF2) with respect to the Si substrate which allows the SL to grow up to hundreds of nanometer of total thickness without lattice relaxation. High resolution X-ray diffractometry has shown that short-period SLs have a high structural perfection [13]. The o-curve width for SL re¯ection is found to be as small as 13 arc s.
Fig. 3. Surface topography during the growth of ®rst SL layers: (a) 1st CaF2 layer; (b) 1st CdF2 layer; (c) 2nd CaF2 layer.
4. Nanostructures on Si(0 0 1) Unlike Si(1 1 1), the Si(0 0 1) surface is less intensively studied from the point of ¯uoride growth. The main difference is expected to arise here from the fact
N.S. Sokolov, S.M. Suturin / Applied Surface Science 175±176 (2001) 619±628
that the (0 0 1) plane in the ¯uorite lattice has a much higher energy than the (1 1 1) plane [10]. This may lead to structure growth in the [1 1 1] direction, which is not normal to the substrate plane. This is the main reason why CaF2 deposition on Si(0 0 1) does not produce a more or less uniform coverage, as is the case with Si(1 1 1), but rather leads to the formation of selfassembling nanostructures of different shapes and lattice orientations.
623
4.1. Si(0 0 1) surface It is known that a clean Si(0 0 1) surface is dimerized to show 2 1 reconstruction. On a vicinal surface, the dimerization rotates by 908 on the adjacent terraces separated by a single atomic step. This results in alternating 2 1 and 1 2 reconstruction across the surface unless the steps group in pairs [14]. In our case, the RHEED patterns taken from the pregrowth treated
Fig. 4. AFM image (a) and RHEED pattern (b) of CaF2 1.5 ML deposited on Si(0 0 1) at 4508C.
624
N.S. Sokolov, S.M. Suturin / Applied Surface Science 175±176 (2001) 619±628
Si(0 0 1) substrates in the 1 1 0 and 1 0 0 azimuths show that both dimerization directions are present at the surface in comparable shares. AFM measurements show the vicinal Si surface with ¯at terraces separated by straight equidistant atomic steps, most of them multiples of 2 ML high. Thus, there is the step doubling, but areas of both dimerization orientations may arise, especially in the vicinity of step kinks [14]. 4.2. Low-temperature CaF2 growth Ð dot formation Grown on Si(0 0 1) below 5508C, CaF2 tends to form an array of uniformly distributed islands of a
nearly square shape and a side of 15±20 nm (Fig. 4a). The islands are 3±8 nm in height with the sides oriented along the 1 1 0 and 1 1 0 directions. The RHEED pattern of the structure in Fig. 4b shows transmission dots, which correspond to the CaF2 lattice oriented as that of Si. Besides, the Si(0 0 1) 2 1 superstructure can be distinguished in the pattern even after 2±3 ML of CaF2 are deposited. This leads us to the conclusion that the area between the dots is bare Si, implying that the Si±¯uoride bonding is not strong enough compared to that of ¯uoride± ¯uoride. The admolecules diffusing during the growth along the surface would climb up the already existing
Fig. 5. AFM image (a) and RHEED pattern (b) k to the wires; (c) ? to the wires of CaF2 1.5 ML deposited on Si(0 0 1) at 7008C.
N.S. Sokolov, S.M. Suturin / Applied Surface Science 175±176 (2001) 619±628
islands rather than nucleate new ones. The AFM measurements con®rm that the surface topography roughens fast with more material deposited. As the {1 1 1} planes have the lowest energy in the ¯uorite lattice, the islands are expected to be facetted with these planes to form square-based pyramids. The pyramid size is too small to measure the facet slope accurately using AFM. However, the facetting described above was clearly observed in the RHEED patterns.
625
4.3. High-temperature CaF2 growth Ð wire formation Deposition of several CaF2 MLs on Si(0 0 1) above 7008C produces quite a different sort of nanostructures. The material aggregates in wires of several microns long (Fig. 5a), running along either the 1 1 0 or 1 1 0 directions. In contrast to [12], we observed only one wire orientation on a particular substrate depending on which direction
Fig. 6. AFM images of CaF2 (a) 1/2 ML and (b) 1 ML deposited on Si(0 0 1) at 7508C.
626
N.S. Sokolov, S.M. Suturin / Applied Surface Science 175±176 (2001) 619±628
was closer to the surface steps on Si. At intermediate temperature, however, structures of both orientations were obtained. The wires were15±20 nm wide and 3±8 nm high, their ends cut along the h1 0 0i directions. The area between the wires was covered by ¯uoride. The RHEED pattern taken with an e-beam perpendicular to the wires (Fig. 5c) shows transmission dots originating from the CaF2 lattice orientation other than that of Si. The CaF21 1 0 axis proves to be normal to the substrate plane while the CaF21 1 0 axis coincides
with Si1 1 0. These epitaxial relations agree with those observed in [12]. The lattice matching between Si and CaF2 is achieved in that con®guration only along the CaF21 1 0 direction. RHEED measurements indicate that this is the direction along which the wires are aligned. A mismatch of 45% is expected in the CaF21 0 0 direction, which is the direction across the wire. The large mismatch is a possible reason for the wire to be restricted in transverse growth. The other reason of CaF2 anisotropic growth may be related to the fact that vertical (1 1 1) facets
Fig. 7. AFM image (a) and RHEED pattern ? to the wires (b) of CaF2 1.5 ML deposited on a wetting layer at 5508C.
N.S. Sokolov, S.M. Suturin / Applied Surface Science 175±176 (2001) 619±628
form the ends of the wires. The molecules, which attach the wire in the base of vertical facets form a larger number of bonds than the molecules at the base of the inclined facets. This would also favor the preferential growth along the wires. The RHEED pattern taken with an e-beam parallel to the wires (Fig. 5b) shows 3 1 reconstruction reported earlier [15] for CaF2 growth on a Si(1 1 0) surface. This fact is another con®rmation of the unusual (1 1 0) orientation of the CaF2 lattice. As in the previous case, the facetting was clearly observed by RHEED with the e-beam along the wires. The inclination of the streaks in the RHEED pattern corresponds to CaF2 {1 1 1} facets. The expected shape of the wires is an elongated hut with two {1 1 1} sidewalls. 4.4. High-temperature CaF2 growth Ð wetting layer On a closer examination of the RHEED pattern evolution during the growth has shown that the wires are not formed before a single CaF2 monolayer is deposited. AFM images taken from the 1/2 and 1 ML of CaF2 grown at 7508C are presented in Fig. 6. Unlike thicker structures, the ®rst CaF2 monolayer seems to grow ¯at and cover the substrate uniformly, acting as a wetting layer. The wetting layer nucleates in small islands elongated across silicon steps (Fig. 6a). Once completed, the wetting layer bears resemblance to the underlying vicinal Si surface except for the step edges being broken along the 1 1 0 or 1 1 0 directions. The formation of the wetting layer was earlier observed in STM studies of ultrathin CaF2 layers on Si(0 0 1) [16]. The RHEED measurements exhibit no features of a clean Si surface. Completely different patterns are obtained in the 1 1 0 and 1 1 0 azimuths: a 3 1 superstructure is observed in the ®rst and a faded 2 1 superstructure in the other azimuth. Thus, the growth of a single CaF2 monolayer converts the two-domain Si(0 0 1) surface to an anisotropic and apparently a single-domain surface of the wetting layer. To con®rm this, several CaF2 MLs deposited on the wetting layer at low temperature result in formation of wires rather than dots (Fig. 7a). The RHEED pattern taken in the azimuth across epi-wires indicates that the CaF2 lattice is oriented in the same way as in high-temperature wires, i.e. with the CaF21 1 0 axis normal to the substrate (Fig. 7b).
627
The wetting layer is believed to arise from the chemical reaction at the ¯uoride±silicon interface resulting in ¯uorine depletion and the formation of strong Ca±Si bonds. A similar reaction is known to take place at elevated temperature during CaF2 deposition on a Si(1 1 1) [2], Si(0 0 1) [16] or GaAs(0 0 1) [17] surface. It is supposed that the way in which the bonds are formed favor further growth of the CaF2 lattice with the [1 1 0] axis normal to the substrate, regardless of the temperature at which the further growth is carried out. 5. Conclusion The growth of ¯uoride-on-silicon nanostructures was studied on Si(1 1 1) and on Si(0 0 1) substrates. The surface of single ¯uoride ®lms on Si(1 1 1) was found to have pyramids of the size and shape varying with the growth temperature and particular ¯uoride used. Interestingly, a combination of CaF2 and CdF2 in the superlattice favors the growth of structures with a very ¯at surface. This phenomenon is attributed to the pyramid formation going on different scales for the two ¯uorides. The difference in the diffusion rates depending on what admolecules diffuse on which surface results in the effect of mutual smoothing observed for the SLs. Calcium ¯uoride grown on Si(0 0 1) was found to form self-assembling nanostructures with the shape and lattice orientation varying with the growth conditions. Below 5508C, CaF2 aggregates in dots with the lattice orientation identical to that of the Si substrate. Above 7008C, wires grow with the CaF2 1 1 0 axis normal to the substrate and the 1 1 0 axis parallel to Si1 1 0 and along the wire. All the wires are aligned equally along either Si1 1 0 or Si1 1 0, depending on which of the axes is closer to the substrate step direction. The wires are preceded by a 1 ML wetting layer, which is supposed to be due to a chemical reaction at the CaF2±Si interface at elevated temperature. Acknowledgements The authors appreciate the stimulating discussions of the epitaxial growth mechanisms with V.P. Ulin
628
N.S. Sokolov, S.M. Suturin / Applied Surface Science 175±176 (2001) 619±628
and N.L. Yakovlev. This work was partially funded by the Russian Foundation for Basic Research, the Russian Ministry of Science and INTAS (Grant 97-10528).
References [1] L.J. Schowalter, R.W. Fathauer, et al., J. Appl. Phys. 58 (1) (1985) 302. [2] M.A. Olmstead, R.I.G. Uhrberg, R.D. Bringans, R.Z. Bachrach, Phys. Rev. B 35 (1987) 7526. [3] C.A. Lucas, D. Loretto, G.C.L. Wong, Phys. Rev. B 50 (1994) 14340. [4] N.S. Sokolov, J.C. Alvarez, N.L. Yakovlev, Appl. Surf. Sci. 60/61 (1992) 421. [5] J. Viernow, D.Y. Petrovykh, F.K. Men, A. Kirakosian, J.-L. Lin, F.J. Himpsel, Appl. Phys. Lett. 74 (15) (1999) 2125. [6] A.K. Kaveev, R.N. Kyutt, M.M. Moisseeva, L.J. Schowalter, Y.V. Shusterman, N.S. Sokolov, S.M. Suturin, N.L. Yakovlev, J. Cryst. Growth 201/202 (1999) 1105.
[7] N.S. Sokolov, S.V. Gastev, A.Y. Khilko, R.N. Kyutt, S.M. Suturin, M.V. Zamoryanskaya, J. Cryst. Growth 201/202 (1999) 1053. [8] N.S. Sokolov, S.M. Suturin, Thin Solid Films 367 (1/2) (2000) 112. [9] S.M. Suturin, S.A. Basun, S.V. Gastev, J.M. Langer, R.S. Meltzer, N.S. Sokolov, Appl. Surf. Sci. 162/163 (2000) 472. [10] L.J. Schowalter, R.W. Fathauer, Crit. Rev. Solid State Mater. Sci. 15 (1989) 367. [11] S.V. Novikov, N.S. Sokolov, N.L. Yakovlev, Abstacts of National Conference on Molecular Beam Epitaxy, Novosibirsk, 1990. [12] D. Loretto, F.M. Ross, C.A. Lucas, Appl. Phys. Lett. 68 (1996) 2363. [13] R.N. Kyutt, A.Y. Khilko, N.S. Sokolov, Appl. Phys. Lett. 70 (12) (1997) 1563. [14] J.E. Grif®n, G.P. Kochanski, J.A. Kubby, P.E. Wierenga, J. Vac. Sci. Technol. A 7 (3) (1989) 1914. [15] W.K. Liu, X.M. Fang, P.J. McCann, Appl. Phys. Lett. 67 (12) (1995) 1695. [16] T. Sumiya, T. Miura, H. Fujinuma, S. Tanaka, Surf. Sci. 376 (1997) 192. [17] Y.Y. Maruo, M. Oshima, T. Waho, T. Kawamura, Jpn. J. Appl. Phys. 28 (1989) L299.
MBE growth of calcium and cadmium ¯uoride nanostructures on silicon N.S. Sokolov*, S.M. Suturin Solid State Optics Department, Ioffe Physico-Technical Institute, Russian Academy of Sciences, 26 Polytechnicheskaya, 194021 St. Petersburg, Russia Accepted 8 October 2000
Abstract Molecular beam epitaxy (MBE) growth of CdF2 and CaF2 layers and nanoscale heterostructures on Si(1 1 1) and Si(0 0 1) substrates has been investigated. The surface morphology of CaF2 and CdF2 layers on Si(1 1 1), studied by atomic force microscopy (AFM) was found to depend strongly on the growth temperature. The high temperature 2D growth mode would produce atomically ¯at surface. In contrast to that, well-pronounced pyramids indicating the 3D growth mode were observed on the layers grown at low temperature. Short-period CdF2±CaF2 superlattices (SLs) were also studied. Analysis of re¯ection high energy electron diffraction (RHEED) intensity oscillations showed asymmetry of the ``CaF2 on CdF2'' and ``CdF2 on CaF2'' interfaces resulting in SL surface smoothening. The possible mechanisms relevant to this phenomenon have been discussed. Atomic force microscopy (AFM), and RHEED have been used to study formation of CaF2 epitaxial nanostructures on Si(0 0 1). Different types of nanostructures have been grown including ultra-thin 2D wetting layer, quasi 1D wires at high growth temperature and well-organized dots at lower temperatures. An important role of the wetting layer in the transformation of the surface morphology at the initial stages of CaF2 growth on Si(0 0 1) at high temperatures has been demonstrated. # 2001 Elsevier Science B.V. All rights reserved. Keywords: MBE; AFM; Fluoride; Silicon; Superlattice; Facetting; Nanostructures
1. Introduction Epitaxial growth of CaF2 on Si(1 1 1) substrates by molecular beam epitaxy (MBE) has been investigated by a number of research groups (see, e.g. [1±5]). Considerably less efforts have been undertaken to explore the MBE growth and properties of CdF2 heterostructures. However, it was demonstrated that *
Corresponding author. Tel.: 7-812-247-6411; fax: 7-812-247-1017. E-mail address: [email protected] (N.S. Sokolov).
high crystal quality CdF2 epitaxial layers [6] and CdF2±CaF2 superlattices (SLs) [7] can be grown. The studies revealed a number of speci®c features related to the growth of such heterostructures which have been recently discussed in [8]. The photoluminescence (PL) studies of CaF2±CdF2 SLs with CaF2 layers doped with Eu revealed an interesting feature of tunneling-assisted photoionization [9] of Eu2 leading to a considerable drop of the Eu2 PL intensity monitored by a continuous excitation of the 4f 7 ! 4f 6 5d transition. This effect can be attractive for designing ¯uoride-based non-volatile memory.
0169-4332/01/$ ± see front matter # 2001 Elsevier Science B.V. All rights reserved. PII: S 0 1 6 9 - 4 3 3 2 ( 0 1 ) 0 0 0 7 5 - 7
620
N.S. Sokolov, S.M. Suturin / Applied Surface Science 175±176 (2001) 619±628
It was found that CaF2 epitaxial growth on Si(0 0 1) results in very different from CaF2/Si(1 1 1) surface morphology and epitaxial relations strongly depending on growth temperature [10,11]. At certain growth conditions, formation of CaF2 quasi 1D islands was observed [12]. These can be very attractive as a lithographic mask for fabrication of new micro-, optoand magneto-electronic devices with the required characteristic size smaller than 10 nm. In this work, we applied re¯ection high energy electron diffraction (RHEED) and atomic force microscopy (AFM) to study comprehensively the formation of single CaF2 and CdF2 layers and SLs on Si(1 1 1), as well as to have an insight in the variety of selfassembled CaF2 nanostructures produced on Si(0 0 1) in a wide range of growth conditions. 2. Experimental The growth of samples was carried out in an ultrahigh vacuum chamber, with the base pressure inside maintained below 10 8 Pa. The substrate temperature could be varied from room temperature to 13008C. The growth was carried out at a rate of several nanometer per minute from a molecular beam formed by evaporating small pieces of required material in a carbon or CaF2 crucible (the latter was used to hold CdF2 material). The growth chamber was equipped with a RHEED diffractometer and a TV system for taking RHEED patterns. Silicon substrates with a miscut of several arc minutes were cleaned following the Shiraki technique and annealed inside the chamber at 12508C for 1.5 min. The oxide layer was evaporated during the annealing to reveal a clean Si surface con®rmed by a 7 7 superstructure on Si(1 1 1) and a 2 1 superstructure on Si(0 0 1), both observed by RHEED. The surface morphology of the grown structures was investigated by AFM. The AFM instrument manufactured by NT-MDT (Zelenograd, Russia) allowed surface topography measurements of an area up to 8 mm 8 mm in the contact and resonant modes. An angstrom resolution in the normal direction could be easily achieved, the lateral resolution being estimated as 10 nm, depending on the cantilever tip sharpness.
3. Nanostructures on Si(1 1 1) 3.1. CaF2 and CdF2 pyramidal growth It was recently demonstrated [8] that both CaF2 and CdF2 grow ¯at at a suf®ciently high temperature on the (1 1 1) surface of a Si or CaF2 substrate. When grown at a lower temperature, both materials tend to form pyramids with triangular basement sides parallel to h1 1 0i. The two materials differ in the upper temperature limit of the pyramid formation which is 3508C for CdF2 and 6008C for CaF2. Pyramids produced on CaF2 at 5008C (Fig. 1a) are 150 nm laterally and only 2±3 nm in height, which ensures that the terraces on the pyramid slopes can be easily identi®ed by AFM. In contrast, CdF2 pyramids obtained at 3008C show slopes about 10 times steeper (Fig. 1b), with an 108 inclination to the (1 1 1) substrate plane. The reason for the pyramid growth is the tendency for the next layer to nucleate on top of the growing one well before the latter is completed. The maximal size of a ¯at island, which has not yet turned to a nucleation site for the next layer, is a parameter that determines the inclination of the pyramid facets. The lower admolecular mobility and the higher energy barriers at step edges result in steeper pyramid facets. The pyramids tend to grow larger, both in width and height, with more material deposited, the smaller pyramids being swallowed up by the bigger ones. 3.2. CaF2±CdF2 superlattices (SLs) In spite of the fact that pyramids are characteristic of both ¯uorides, the surfaces of low temperature CaF2±CdF2 SL with the ¯uoride layers less than 30 ML thick prove to be remarkably ¯at. During the SL growth, CaF2 and CdF2 layers are deposited on the substrate in turn at the same temperature of 1008C. Fig. 2 shows the time behavior of the specular beam intensity I00, monitored during the growth of the ®rst three SL layers, each 20 ML thick. Distinct oscillations in I00 with a 1 ML period are clearly seen, their amplitude varying with the material being deposited. It is found that the specular beam is much brighter for CaF2 than for CdF2. This may be due to the difference in the electronic structure of Ca and Cd atoms. Noteworthy, the RHEED intensity increases
N.S. Sokolov, S.M. Suturin / Applied Surface Science 175±176 (2001) 619±628
621
Fig. 1. Pyramidal mounds (a) on CaF2 at 5008C and (b) on CdF2 at 3008C.
gradually during the CaF2-on-CdF2 growth, while the growth in the reverse order leads to an abrupt intensity drop. This indicates that a relatively thick CaF2 layer is to be deposited to recover the morphology typical for CaF2 surface. At the same time, just a couple of CdF2 monolayers are suf®cient to form a continuous coverage. This asymmetry can be explained in the following way. The growth of CdF2 at 1008C starts on a comparatively smooth CaF2 buffer (Fig. 3a) and results in the
surface facetting, as indicated in Fig. 3b. One can suggest that the diffusion length of CaF2 admolecules on the CdF2 surface is greater than on the CaF2 surface. The CaF2 admolecules diffuse readily down the CdF2 facets to ®rst occupy the positions in the pits of the facetted surface rather than to cover the entire surface uniformly. The CdF2 surface would be covered completely as soon as the amount of deposited CaF2 material is suf®cient to ®ll the pits. This makes up 10 ML of ¯uoride, according to Fig. 2. An increase in the
622
N.S. Sokolov, S.M. Suturin / Applied Surface Science 175±176 (2001) 619±628
Fig. 2. Specular beam intensity during the growth of ®rst SL layers.
oscillation amplitude during the CaF2 deposition corresponds to the overall surface ¯attening due to the facet overgrowth. The CaF2 epitaxial surface grown at 1008C also has facetted morphology. The facets, however, are much smaller than on CdF2 because of the shorter diffusion length of the CaF2 admolecules. Thus, the surface to be overgrown by the subsequent CdF2 layer has a smoother topography (Fig. 3c) and is readily covered by a few CdF2 monolayers. Besides, the diffusion length of CdF2 admolecules on the CaF2 surface is shorter than that on the CdF2 surface, which is also a prerequisite for a fast transformation to a uniform coverage. The oscillations fade out during the CdF2 growth, most likely due to the development of facetted morphology. In addition, a noticeable increase of the specular beam intensity during the growth interruption (see Fig. 2) between the layers of different materials indicates the surface ¯attening, which may be due to the non-equilibrium nature of the facetted surface. Thus, a combination of the two ¯uoride materials with different diffusion lengths allows thick heterostructures to be grown with relatively ¯at surfaces and interfaces. AFM measurements carried out for SLs with different periods show that a shorter SL period results in a ¯atter surface. Another advantage of the CaF2±CdF2 SLs is that the layers are alternatively compressed (CaF2) or stretched (CdF2) with respect to the Si substrate which allows the SL to grow up to hundreds of nanometer of total thickness without lattice relaxation. High resolution X-ray diffractometry has shown that short-period SLs have a high structural perfection [13]. The o-curve width for SL re¯ection is found to be as small as 13 arc s.
Fig. 3. Surface topography during the growth of ®rst SL layers: (a) 1st CaF2 layer; (b) 1st CdF2 layer; (c) 2nd CaF2 layer.
4. Nanostructures on Si(0 0 1) Unlike Si(1 1 1), the Si(0 0 1) surface is less intensively studied from the point of ¯uoride growth. The main difference is expected to arise here from the fact
N.S. Sokolov, S.M. Suturin / Applied Surface Science 175±176 (2001) 619±628
that the (0 0 1) plane in the ¯uorite lattice has a much higher energy than the (1 1 1) plane [10]. This may lead to structure growth in the [1 1 1] direction, which is not normal to the substrate plane. This is the main reason why CaF2 deposition on Si(0 0 1) does not produce a more or less uniform coverage, as is the case with Si(1 1 1), but rather leads to the formation of selfassembling nanostructures of different shapes and lattice orientations.
623
4.1. Si(0 0 1) surface It is known that a clean Si(0 0 1) surface is dimerized to show 2 1 reconstruction. On a vicinal surface, the dimerization rotates by 908 on the adjacent terraces separated by a single atomic step. This results in alternating 2 1 and 1 2 reconstruction across the surface unless the steps group in pairs [14]. In our case, the RHEED patterns taken from the pregrowth treated
Fig. 4. AFM image (a) and RHEED pattern (b) of CaF2 1.5 ML deposited on Si(0 0 1) at 4508C.
624
N.S. Sokolov, S.M. Suturin / Applied Surface Science 175±176 (2001) 619±628
Si(0 0 1) substrates in the 1 1 0 and 1 0 0 azimuths show that both dimerization directions are present at the surface in comparable shares. AFM measurements show the vicinal Si surface with ¯at terraces separated by straight equidistant atomic steps, most of them multiples of 2 ML high. Thus, there is the step doubling, but areas of both dimerization orientations may arise, especially in the vicinity of step kinks [14]. 4.2. Low-temperature CaF2 growth Ð dot formation Grown on Si(0 0 1) below 5508C, CaF2 tends to form an array of uniformly distributed islands of a
nearly square shape and a side of 15±20 nm (Fig. 4a). The islands are 3±8 nm in height with the sides oriented along the 1 1 0 and 1 1 0 directions. The RHEED pattern of the structure in Fig. 4b shows transmission dots, which correspond to the CaF2 lattice oriented as that of Si. Besides, the Si(0 0 1) 2 1 superstructure can be distinguished in the pattern even after 2±3 ML of CaF2 are deposited. This leads us to the conclusion that the area between the dots is bare Si, implying that the Si±¯uoride bonding is not strong enough compared to that of ¯uoride± ¯uoride. The admolecules diffusing during the growth along the surface would climb up the already existing
Fig. 5. AFM image (a) and RHEED pattern (b) k to the wires; (c) ? to the wires of CaF2 1.5 ML deposited on Si(0 0 1) at 7008C.
N.S. Sokolov, S.M. Suturin / Applied Surface Science 175±176 (2001) 619±628
islands rather than nucleate new ones. The AFM measurements con®rm that the surface topography roughens fast with more material deposited. As the {1 1 1} planes have the lowest energy in the ¯uorite lattice, the islands are expected to be facetted with these planes to form square-based pyramids. The pyramid size is too small to measure the facet slope accurately using AFM. However, the facetting described above was clearly observed in the RHEED patterns.
625
4.3. High-temperature CaF2 growth Ð wire formation Deposition of several CaF2 MLs on Si(0 0 1) above 7008C produces quite a different sort of nanostructures. The material aggregates in wires of several microns long (Fig. 5a), running along either the 1 1 0 or 1 1 0 directions. In contrast to [12], we observed only one wire orientation on a particular substrate depending on which direction
Fig. 6. AFM images of CaF2 (a) 1/2 ML and (b) 1 ML deposited on Si(0 0 1) at 7508C.
626
N.S. Sokolov, S.M. Suturin / Applied Surface Science 175±176 (2001) 619±628
was closer to the surface steps on Si. At intermediate temperature, however, structures of both orientations were obtained. The wires were15±20 nm wide and 3±8 nm high, their ends cut along the h1 0 0i directions. The area between the wires was covered by ¯uoride. The RHEED pattern taken with an e-beam perpendicular to the wires (Fig. 5c) shows transmission dots originating from the CaF2 lattice orientation other than that of Si. The CaF21 1 0 axis proves to be normal to the substrate plane while the CaF21 1 0 axis coincides
with Si1 1 0. These epitaxial relations agree with those observed in [12]. The lattice matching between Si and CaF2 is achieved in that con®guration only along the CaF21 1 0 direction. RHEED measurements indicate that this is the direction along which the wires are aligned. A mismatch of 45% is expected in the CaF21 0 0 direction, which is the direction across the wire. The large mismatch is a possible reason for the wire to be restricted in transverse growth. The other reason of CaF2 anisotropic growth may be related to the fact that vertical (1 1 1) facets
Fig. 7. AFM image (a) and RHEED pattern ? to the wires (b) of CaF2 1.5 ML deposited on a wetting layer at 5508C.
N.S. Sokolov, S.M. Suturin / Applied Surface Science 175±176 (2001) 619±628
form the ends of the wires. The molecules, which attach the wire in the base of vertical facets form a larger number of bonds than the molecules at the base of the inclined facets. This would also favor the preferential growth along the wires. The RHEED pattern taken with an e-beam parallel to the wires (Fig. 5b) shows 3 1 reconstruction reported earlier [15] for CaF2 growth on a Si(1 1 0) surface. This fact is another con®rmation of the unusual (1 1 0) orientation of the CaF2 lattice. As in the previous case, the facetting was clearly observed by RHEED with the e-beam along the wires. The inclination of the streaks in the RHEED pattern corresponds to CaF2 {1 1 1} facets. The expected shape of the wires is an elongated hut with two {1 1 1} sidewalls. 4.4. High-temperature CaF2 growth Ð wetting layer On a closer examination of the RHEED pattern evolution during the growth has shown that the wires are not formed before a single CaF2 monolayer is deposited. AFM images taken from the 1/2 and 1 ML of CaF2 grown at 7508C are presented in Fig. 6. Unlike thicker structures, the ®rst CaF2 monolayer seems to grow ¯at and cover the substrate uniformly, acting as a wetting layer. The wetting layer nucleates in small islands elongated across silicon steps (Fig. 6a). Once completed, the wetting layer bears resemblance to the underlying vicinal Si surface except for the step edges being broken along the 1 1 0 or 1 1 0 directions. The formation of the wetting layer was earlier observed in STM studies of ultrathin CaF2 layers on Si(0 0 1) [16]. The RHEED measurements exhibit no features of a clean Si surface. Completely different patterns are obtained in the 1 1 0 and 1 1 0 azimuths: a 3 1 superstructure is observed in the ®rst and a faded 2 1 superstructure in the other azimuth. Thus, the growth of a single CaF2 monolayer converts the two-domain Si(0 0 1) surface to an anisotropic and apparently a single-domain surface of the wetting layer. To con®rm this, several CaF2 MLs deposited on the wetting layer at low temperature result in formation of wires rather than dots (Fig. 7a). The RHEED pattern taken in the azimuth across epi-wires indicates that the CaF2 lattice is oriented in the same way as in high-temperature wires, i.e. with the CaF21 1 0 axis normal to the substrate (Fig. 7b).
627
The wetting layer is believed to arise from the chemical reaction at the ¯uoride±silicon interface resulting in ¯uorine depletion and the formation of strong Ca±Si bonds. A similar reaction is known to take place at elevated temperature during CaF2 deposition on a Si(1 1 1) [2], Si(0 0 1) [16] or GaAs(0 0 1) [17] surface. It is supposed that the way in which the bonds are formed favor further growth of the CaF2 lattice with the [1 1 0] axis normal to the substrate, regardless of the temperature at which the further growth is carried out. 5. Conclusion The growth of ¯uoride-on-silicon nanostructures was studied on Si(1 1 1) and on Si(0 0 1) substrates. The surface of single ¯uoride ®lms on Si(1 1 1) was found to have pyramids of the size and shape varying with the growth temperature and particular ¯uoride used. Interestingly, a combination of CaF2 and CdF2 in the superlattice favors the growth of structures with a very ¯at surface. This phenomenon is attributed to the pyramid formation going on different scales for the two ¯uorides. The difference in the diffusion rates depending on what admolecules diffuse on which surface results in the effect of mutual smoothing observed for the SLs. Calcium ¯uoride grown on Si(0 0 1) was found to form self-assembling nanostructures with the shape and lattice orientation varying with the growth conditions. Below 5508C, CaF2 aggregates in dots with the lattice orientation identical to that of the Si substrate. Above 7008C, wires grow with the CaF2 1 1 0 axis normal to the substrate and the 1 1 0 axis parallel to Si1 1 0 and along the wire. All the wires are aligned equally along either Si1 1 0 or Si1 1 0, depending on which of the axes is closer to the substrate step direction. The wires are preceded by a 1 ML wetting layer, which is supposed to be due to a chemical reaction at the CaF2±Si interface at elevated temperature. Acknowledgements The authors appreciate the stimulating discussions of the epitaxial growth mechanisms with V.P. Ulin
628
N.S. Sokolov, S.M. Suturin / Applied Surface Science 175±176 (2001) 619±628
and N.L. Yakovlev. This work was partially funded by the Russian Foundation for Basic Research, the Russian Ministry of Science and INTAS (Grant 97-10528).
References [1] L.J. Schowalter, R.W. Fathauer, et al., J. Appl. Phys. 58 (1) (1985) 302. [2] M.A. Olmstead, R.I.G. Uhrberg, R.D. Bringans, R.Z. Bachrach, Phys. Rev. B 35 (1987) 7526. [3] C.A. Lucas, D. Loretto, G.C.L. Wong, Phys. Rev. B 50 (1994) 14340. [4] N.S. Sokolov, J.C. Alvarez, N.L. Yakovlev, Appl. Surf. Sci. 60/61 (1992) 421. [5] J. Viernow, D.Y. Petrovykh, F.K. Men, A. Kirakosian, J.-L. Lin, F.J. Himpsel, Appl. Phys. Lett. 74 (15) (1999) 2125. [6] A.K. Kaveev, R.N. Kyutt, M.M. Moisseeva, L.J. Schowalter, Y.V. Shusterman, N.S. Sokolov, S.M. Suturin, N.L. Yakovlev, J. Cryst. Growth 201/202 (1999) 1105.
[7] N.S. Sokolov, S.V. Gastev, A.Y. Khilko, R.N. Kyutt, S.M. Suturin, M.V. Zamoryanskaya, J. Cryst. Growth 201/202 (1999) 1053. [8] N.S. Sokolov, S.M. Suturin, Thin Solid Films 367 (1/2) (2000) 112. [9] S.M. Suturin, S.A. Basun, S.V. Gastev, J.M. Langer, R.S. Meltzer, N.S. Sokolov, Appl. Surf. Sci. 162/163 (2000) 472. [10] L.J. Schowalter, R.W. Fathauer, Crit. Rev. Solid State Mater. Sci. 15 (1989) 367. [11] S.V. Novikov, N.S. Sokolov, N.L. Yakovlev, Abstacts of National Conference on Molecular Beam Epitaxy, Novosibirsk, 1990. [12] D. Loretto, F.M. Ross, C.A. Lucas, Appl. Phys. Lett. 68 (1996) 2363. [13] R.N. Kyutt, A.Y. Khilko, N.S. Sokolov, Appl. Phys. Lett. 70 (12) (1997) 1563. [14] J.E. Grif®n, G.P. Kochanski, J.A. Kubby, P.E. Wierenga, J. Vac. Sci. Technol. A 7 (3) (1989) 1914. [15] W.K. Liu, X.M. Fang, P.J. McCann, Appl. Phys. Lett. 67 (12) (1995) 1695. [16] T. Sumiya, T. Miura, H. Fujinuma, S. Tanaka, Surf. Sci. 376 (1997) 192. [17] Y.Y. Maruo, M. Oshima, T. Waho, T. Kawamura, Jpn. J. Appl. Phys. 28 (1989) L299.