Nanotechnology towards the 21st Century

Thin Solid Films 341 (1999) 120±125

Nanotechnology towards the 21st Century K. Tanaka* Joint Research Center for Atom Technology (JRCAT), National Institute for Advanced Interdisciplinary Research (NAIR), 1-1-4 Higashi, Tsukuba, Ibaraki 305, Japan

Abstract A project entitled `Atom Technology' was initiated in ®scal 1992 for a planned period of 10 years under the MITI sponsored National R&D Program. The `Atom Technology' project aims at systematically establishing technology for the handling of individual atoms and molecules on a solid surface or in a three-dimensional space, as a generic technology for various ®elds of industry. This project, closely adjacent to science, emphasizes the following three key focuses; atom manipulation, nanoscale self-organization, and critical-state phase control, with two basic approaches of in situ dynamical observations (experimental) as well as abinitio calculations (theoretical). In this paper, I pick up several topics from our recent activities in JRCAT and describe some technical details with respect to the above three key focuses. Topics include (1) the removal of an atomic layer from a Si(111) 7 £ 7 surface by scanning tunneling microscope (STM), (2) sizeselective growth of SiH cluster ions in a specially designed ion trap, (3) nanoscale wire construction on a Ga-covered Si(111) surface in a self-organizing manner, (4) nanometer-scale selective epitaxial growth of Si using a 0.3 nm thick oxide ®lm window, and (5) colossal magneto resistance in perovskite-type manganese oxides. q 1999 Elsevier Science S.A. All rights reserved. Keywords: Nanotechnology; Atom manipulation; Silicon; Thin oxide ®lm

1. Introduction In a few years, we shall have the 21st Century. It is a ®nde-siecle just now. In this century, we had a number of major changes in the ®elds of science and technology. Since the invention of transistors half a century ago, electronics has been intimately involved in our daily life, and now has grown to one of key industries. However, its high growth rate up to now will not necessarily be guaranteed in the coming century. For instance, in case of LSI memory chips, where the memory capacitance has increased with a rate of four times in 3 years, the pace is approaching to physical and technological limits, and the extrapolation of the current technology may suggest the presence of a nonsurmountable wall at 0.01 mm resolution 30 years later in the next century The same applies to the materials science. An arti®cial superlattice ®lm fabricated by depositing a few atoms-thick layers of different elements one over another is an assembly of interfaces, as it were. Hence, the main arena is the world of non-equilibrium, where no text book is available, and neither phase diagrams nor almanacs have authoritative power. Some revolutionary concept or approach is urgently needed.

* Tel.: 1 81-298-54-2701; fax: 1 81-295-54-2785; e-mail: [email protected].

An ultimate technology emerges out of such backgrounds as a great need, namely, static and dynamic observation and the manipulation of materials and material formation at atomic and/or molecular levels, or the handling of individual atoms and molecules. This radical technology closely adjacent to science may be called `atom technology', which corresponds to `nanotechnology' in a more widely accepted terminology. Not only in electronics, but also in the chemical industry and biotechnology, in reference to the development of new catalysts, and decoding and manipulation of genes, the manipulation technology of atoms and molecules is urgently needed. This is a typical generic technology of inter-industrial and interdisciplinary nature. A project entitled `Atom Technology' was thus organized in ®scal 1992 for meeting those needs, under the MITI sponsored national R&D Program for a planned period of 10 years with an expected total budget of about US$ 250 million.

2. The atom technology project and its main focuses The atom technology project aims at systematically establishing technology for handling individual atoms and molecules on a solid surface or in a three-dimensional space, as a future generic technology for various ®elds of industry. Naturally, the project is characterized by a long-

0040-6090/99/$ - see front matter q 1999 Elsevier Science S.A. All rights reserved. PII: S00 40-6090(98)0150 8-9

K. Tanaka / Thin Solid Films 341 (1999) 120±125

121

Fig. 1. Extraction of silicon atoms and the control of surface atomic structures on Si(111)-7 £ 7 surfaces by scanning tunneling microscopy (Komeda et al. [1]).

term and precompetitive nature, and not directly targeting device technology. It explores new materials, new processing and new phenomena with a special emphasis on a nanometer range. In order to implement the project, the Joint Research Center for Atom Technology (JRCAT) was founded under the agreement between the National Institute for Advanced Interdisciplinary Research (NAIR) and the Angstrom Technology Partnership (ATP), which is a concentrated research center of scientists from national laboratories, universities, private sectors and overseas research organizations. JRCAT promotes the equal partnership of those scientists participating from different organizations and, thereby, creates a multidisciplinary effort in the ®eld of nanotechnology. This project emphasizes the following three key focuses; (1) atom manipulation, (2) nanoscale self-organization, (3) critical-state phase control. In order to look into those three key focuses we need at least two basic tools; `in situ dynamic measurement and control' as an experimental tool and a `abinitio calculation' as a theoretical tool. 2.1. Atom manipulation Scanning probe microscopy (SPM) has opened up the new world of nanotechnology for observing and manipulating individual atoms and molecules on solid surfaces.

However, its technological level is still far from maturity. Beam-probe techniques as well as mechanical-probe techniques and particle trapping techniques such as ion trap or others for manipulating atoms in a three-dimensional free space will also be needed for future atom manipulation with wider controllability. 2.2. Nanoscale self-organization `Self-organization' might be considered as a concept which is complementary to that of `Atom Manipulation' because the former can be characterized as a group behavior of a whole system consisting of a lot of atoms or molecules. When one wants to fabricate materials or devices only by the atom manipulation technique, one has to spend too long a time to ®nish up. Obviously, actual fabrication of materials and devices should not be pursued by atom-by-atom processes but by some sort of self-organization. We need a self-organization process including self-ordering, self-assembly, self-limiting phenomena through which a huge number of nanostructures can be fabricated in parallel processing, with atomic accuracies and within a practically acceptable time. 2.3. Critical-state phase control It is a unique concept of our project that, when a system is set up at a critical state adjacent to the phase transition, one

122

K. Tanaka / Thin Solid Films 341 (1999) 120±125

atoms from the Si(111) 7 £ 7 surface using a biased tungsten tip of a scanning tunneling microscope (STM) [1]. In their experiment p-type (B-doped, 0.01 V cm) Si substrates and an electrochemically etched W tip were used and atom removal was made at room temperature. As shown in Fig. 1, the double layers of the Si atoms are removed from the surface by tip motions, up and down at a tip bias of 2 V, p p and new structures with 2 £ 2, 3 £ 3 and c(2 £ 4) reconstructions are created in the hole, 0.3 nm in depth, depending on its size. This is the ®rst observation of layer-bylayer atomic manipulation on the Si surface using an STM tip without giving any damage to subsurface layers. They also observed a characteristic behavior in the tip-

Fig. 2. Nanoscale wire construction on Ga-covered Si(111) surface formed by using a STM probe in a self-organizing manner (Fujita et al. [2]).

can possibly control the macroscopic phase of the system from one to the other by a subtle perturbation. The phase transition that is in consideration here includes various phenomena; structural transformation from amorphous to crystalline, electronic transition from metal to insulator and so on. This concept gives a sort of guiding principle for new materials, new processings and new phenomena, and also may produce a new approach to atom manipulation as well as nanoscale self-organization. With regard to the three key research focuses, JRCAT consists of eight experimental groups and two theoretical groups for a total 100 researchers, each of which is conducting research of broad spectrum under the guidance of their respective group leader. In experimental groups, the object materials include semiconductors (Si, II±VI, III±V, amorphous), organic substances, magnetic thin ®lms and transition metal oxides, while the underlying technologies and phenomena range from atom manipulation, not only on solid surfaces but also in three-dimensional space, to phase transition in strongly-correlated electron systems. Several topics are selected from our recent activities and some details are described below.

3. Selected topics and results 3.1. Layer-by-layer atomic manipulation on Si(111) 7 £ 7 surfaces Komeda et al. achieved layer-by-layer removal of Si

Fig. 3. STM images of Si(001) surface passivated with a 0.3 nm-thick oxide ®lm having several windows (dark areas) (a) and after the selective epitaxial growth of Si (bright areas) in window areas (b) (Fujita et al. [3]).

K. Tanaka / Thin Solid Films 341 (1999) 120±125

123

Fig. 4. Schematic representation of size-selective growth of SiH cluster ions in a quadrupole ion trap (Kanayama [5]).

substrate current during the tip excursion, which may be related with the nature of the Si nanoscale wire [1]. 3.2. Self-organization of nanoscale wire on Ga-covered Si(111) surfaces

Fig. 5. Fragmentation pattern of Si6Hx1 clusters (top) (Murakami and Kanayama [6]) and theoretically predicted stable structures (bottom) (Miyazaki et al. [7]).

Fujita et al. have found a novel self-organization, by p p which the Si(111)- 3 £ 3 Ga surface are modi®ed on the nanometer scale with atomic accuracy [2]. A vicinal Si(111) surface tilting toward the [112] direction was cut from an n-type wafer and cleaved by heating up to 11008C in vacuum. Then, a nominally 1/3 monolayer of Ga was deposited onto the Si(111) surface at room temperature, followed by an annealing above 5008C, resulting in the p p uniform atomic layer of Ga with the 3 £ 3 structure fully covering the Si(111) surface, which was detected by p in situ STM at 5408C. Those Ga atoms on the Si(111)- 3 £ p 3 Ga surface can be locally removed if a high electric ®eld is applied between the STM tip and the surface, leaving a Si(111)7 £ 7 bare surface with a nanometer size [2]. In actual experiments, the STM tungsten tip was placed above a terrace of the Si(111)-3 £ 3 Ga surface and the sample bias of 7.0 V was applied during a period less than 0.3 s at 5608C. By this procedure Ga atoms were partially removed from the terrace and a Si(111)-7 £ 7 area appeared. The width of the 7 £ 7 area was just three times the width of a Si(111)-7 £ 7 unit cell, namely, 7.0 nm. This area started stretching along the [110] direction in a self-organizing manner and reached more than 140 nm in length after 9 min. STM images and brief explanations are given in Fig. 2.

124

K. Tanaka / Thin Solid Films 341 (1999) 120±125

3.3. Nanometer-scale Si selective epitaxial growth on Si(001) using an ultrathin oxide window Nanometer-scale Si crystals were produced by selective epitaxial growth on Si(001) surfaces passivated with 0.3 nm-thick oxide ®lms [3]. Samples used were cut from on-axis and miscut Si(001) wafers, and oxide ®lms were formed on clean Si(001) surfaces by the exposure to 2:7 £ 104 Pa of oxygen at 6408C for 10 min. According to careful analyses using the X-ray photoelectron spectroscopy the ®lm thickness was estimated to be 0.3 nm and more than one monolayer of Si was uniformly oxidized [3,4]. Window areas for the growth were produced by void formation during the thermal decomposition of oxide ®lms at 700±7308C. Using the void formation, Fujita et al. succeeded to grow nanoscale Si crystals selectively on the Si(001) surface passivated with a 0.3 nm-thick oxide ®lm [3]. Fig. 3 represents STM images of the surface that was observed (a) before and (b) 50 min after the Si growth started. As is clearly seen in the ®gure, voids of dark areas (0.50± 7 nm in depth) in Fig. 3a are covered by epitaxially grown Si crystals in Fig. 3b whose typical size is 20 nm in length and 0.8 nm in height. 3.4. Size-selective growth of SiH cluster ions in a novel ion trap Murakami and Kanayama succeeded in growing SinHx1 ions (n ˆ 210) size-selectively from silane gas in a newlydeveloped ion trap [5,6]. The ion trap consists of a set of two-dimensional quadrupoles surrounded by the ground (mesh) electrode, being operated by the external ®eld of an a.c. quadrupole superimposed on the static attraction [5]. In the region surrounding the quadrupole, ions are con®ned by the balance of the static attraction and the effective repulsion of the a.c. ®eld. This type of trap has a feature that it can simultaneously con®ne ions with a wide range of mass values [5]. The actual trap had an overall size of 17 cm in diameter and 20 cm in length. To the quadrupoles of 1 cm diameter, a.c. voltages with opposite phases of 200 V and 100 kHz were connected with a d.c. bias of 4 V. The whole system was installed in a vacuum chamber within ultimate pressure of 10 27 Pa. SiH4 gas of 10 4 Pa and a He buffer of 10 3 Pa were introduced and irradiated with electrons of 100 eV for 1 s for cluster growth [6] (see Fig. 4). From the detailed mass spectra measurements [6] it was concluded that (1) SinHx1 clusters were grown successfully in an ion trap from SiH4 gas, (2) several particularly stable structures do exist in 1 1 and Si10H12217 are inferred those ion clusters, and (3) Si6H12 to have the bulk fragment structures with hydrogen termi1 1 and Si10H024 should have the nation, while Si6H021 compact structures already known for the pure Si clusters [6]. Furthermore, quite recently, Miyazaki et al. have demonstrated through their ®rst-principles calculations on

stable structures and energetics for Si6Hx clusters that clusters are categorized into at least three distinct families [7]. A fairly good coincidence between those experimental results and the theoretical prediction is seen in Fig. 5. 3.5. Colossal magnetoresistance (CMR) in perovskite-type manganese oxides The research group led by Tokura aims at exploring new electronic materials and related physics for the development of atom technology. Main materials they explored are perovskite-type manganese oxides as a typical stronglycorrelated electron system. First, Asamitsu et al. found out that a magnetic ®eld-induced structural phase transition in La12xSrxMnO3, from orthorhombic to rhombohedral, near the ferromagnetic ordering temperature (see Fig. 6) [8]. This phase transition accompanies a simultaneous transition from a paramagnetic insulator to a ferromagnetic conductor, namely, the giant magnetoresistance effect. Later, Kuwahara et al. found ®rst-order phase transitions between the charged-ordered antiferromagnetic insulator and the ferromagnetic metal in single crystals of ®lling and bandwidthcontrolled perovskite-type manganese oxides [9]. According to their tentative interpretation, the charge-ordered states or `electron crystals' can be melted by a magnetic ®eld, causing a change in resistivity by several orders of magnitude [9,10]. In particular, as shown in Fig. 7, the striction-coupled colossal magnetoresistance (CMR) was observed under a relatively low magnetic ®eld in a

Fig. 6. Magnetic ®eld-induced structural phase transition in La12xSrxMnO3 (Asamitsu et al. [8]).

K. Tanaka / Thin Solid Films 341 (1999) 120±125

125

we are now discussing on what subjects should be emphasized with more concrete targets. However, atom technology, or nanotechnology, always reminds me of Feynman's statement: `At any rate, it seems that the laws of physics present no barrier to reducing the size of computers until bits are the size of atoms, and quantum behavior holds dominant sway' (`Quantum Mechanical Computers' Optics News, Feb. 1985 pp. 1120). Perhaps this statement most strongly encourages people working in this area of nanotechnology. Acknowledgements This work is partly supported by the New Energy and Industrial Technology Development Organization and the Angstrom Technology Partnership.

Fig. 7. Colossal magnetoresistance for different temperatures in (Nd12ySmy)1/2MnO3 (Kuwahara et al. [10]). MR characteristics of other materials are also shown.

(Nd,Sm)1/2Sr1/2MnO3 single crystal [9,10]. The observed CMR effect in this system is shown in the ®gure for different temperatures, in comparison with other typical MR materials. 4. Concluding remark We have spent more than ®ve years in research activities of the Phase (1992±1997 FY) for the Atom Technology Project, and described here brie¯y some of our research output. Toward the Phase II (1998±2001 FY) of the project,

References [1] T. Komeda, R. Hasunuma, H. Mukaida, H. Tokumoto, Appl. Phys. Lett. 68 (1996) 3482. [2] K. Fujita, Y. Kusumi, M. Ichikawa, Appl. Phys. Lett. 68 (1996) 770. [3] K. Fujita, H. Watanabe, M. Ichikawa, Appl. Phys. Lett. 70 (1997) 2807. [4] H. Watanabe, K. Fujita, M. Ichikawa, Appl. Phys. Lett. 70 (1997) 1095. [5] T. Kanayama, Jpn. J. Appl. Phys. 33 (1994) L1792. [6] H. Murakami, T. Kanayama, Appl. Phys. Lett. 67 (1995) 2341. [7] T. Miyazaki, T. Uda, I. Stich, K. Terakura, Chem. Phys. Lett. 261 (1996) 346. [8] A. Asamitsu, Y. Morimoto, Y. Tomioka, T. Arima, Y. Tokura, Nature 373 (1995) 407. [9] H. Kuwahara, Y. Tomioka, A. Asamitsu, Y. Morimoto, Y. Tokura, Sci. 270 (1995) 961. [10] H. Kuwahara, Y. Tomioka, A. Asamitsu, Y. Morimoto, Y. Tokura, Sci. 272 (1996) 80.