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The 54th AVS International Symposium, October 14, 2007

Nanometer-scale Science and Technology

Observation of Si/Co/Cu/Co surface and interface processes for nanostructure formation by scanning High electron Energy diffraction

Haji Shirinzadeh

Materials and Energy Research Center, Karaj, Iran. PO box 31787/316 [email protected]

. Abstract. We observe the oxidation process on clean Si surfaces using high-resolution scanning reflection electron diffraction and form nanostructures on them, through focused electron-beam (EB) induced surface reactions. Si thermal oxidation occurs layer by layer, and the interface between the oxide film (<1.5 nm thickness) and Si substrate becomes atomically abrupt. When the sample is heated to 700-800 °C, resulting in the exposure of a clean Si substrate. The typical width of the clean Si `open windows' is about 10 nm. Using selective reactions during heating after the deposition of Si and Co films on the patterned samples, Si and Co nanoislands with 25 nm size are formed on Si surfaces.

Keywords: reflective high energy electron diffraction (RHEED), epitaxial growth, giant

Introduction A significant advantage of Scanning high energy electron diffraction (SHEED) is a simply-assembled device incorporates UHV compatible surface analytical tools directly into the substrate preparation and growth chamber. Such tools assist in monitoring and confirming epitaxial growth as well as optimizing procedures and possibility troubleshooting as well. UHV condition (pressure of less than 10

-11

Torr) are for the universal

acceptance due to ultra clean and reproducible conditions. . because Electron Beam (EB) is an inherently slow growth process it is possible to achieve extreme dimensional control over both major compositional variation of nanometric epitaxial layers of Co/Cu/Co were successfully grown on the (111) plane of a silicon single crystal wafer.

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The 54th AVS International Symposium, October 14, 2007

Nanometer-scale Science and Technology

Experiment. Pure elements (99.99%) of Co, Cu were used. and substrate temperature being was 600630oC conformation of substrate Si surface by SHEED. At the rate 1A/s to thickness of 250450 at ambient room temperature. The base was 3X10-10 torr and 5X10-9 torr during deposition to reduce the inter diffusion at the interface as non-equal baron deposition. We exam crystal structure of the film surface were in-situ and through of all the growth 30KeV reflection of high-energy electron diffraction (HREED). The magnetic properties of all samples were measured with applied field up to films of tree compositions were investigated; 250.Co/Cu, 400Co/Cu, 450. Respectively. The thin film are prepared in ­situ in a SHEED system equipped with two Electron Beam (EB) made in a deposition system are analyzed by single UHV set up techniques. Two important aspects of surface study of SHEED growth under clean ultrahigh vacuum conditions to observe substrate surface and quality of surface during layer deposition. Furthermore. It is hearting that the construction of phase diagram has been made possible from the knowledge gained on surface atomic structures using (SHEED) [6-10]. In SHEED an electron beam which have energy in the range 30 KeV is incident at a glancing angle of 1-2o. To the crystal surface which is set

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perpendicular to the electron beam, allowing SHEED to be used during layer deposition. The technique provides a very sensitive yet simple's diagnostic tool for observing change in structure of surface layer as function of growth parameters. It is possible to obtain in formation on surface structure. Microstructure and smoothness from SHEED pattern. To be deposited in an evaporator are directly related to the frequency of their reaction with the background gaseous species, systems with ever improving vacuum are necessity. The standard SHEED system is shown schematically in Fig.1.the same system was used for growth of Co-Cu-Co. the system is pumped combination of diffusion pump and Titanium sublimation pump, the latter being enclosed in a liquid-nitrogen shroud Sample preparation: all the thin film samples used in present study were prepared in SHEED model by vg scientific, thin film deposition system. The rough pumping of the chamber is done by diffusion pump and final base pressure of 10.10-11 torr is obtained by 120/s sonic oil diffusion pump. There are two electron guns of 10KeV each with a water cooled hearth of single 10cc crucibles. The thickness of the film is measured ex-situ process controller. Multiple pump control unit VG ionization gauge measures the pressure in the chamber. A technique that has been to be capable of

The 54th AVS International Symposium, October 14, 2007

Nanometer-scale Science and Technology

producing high-quality films of the cuprates [10] and the maggnaties [11]. Details of the

apparatus and procedures used can be found elsewhere [12].

Results and discussion

During annealing. Cobalt and copper layers were applied to the silicon wafer using electron beam to evaporate laboratory grade targets that were constantly water-cooled during the process. The thickness of layers, which was in the nanometric range, was measured in-situ with a quartz piezoelectric thickness measurer. Surface crystallography of each layer was studied with a RHEED instrument of which output was digitalized by Lab View software. The layers were then annealed at 250°C for 45 minutes to gain mechanical stability resulting from interdiffusion. Copper contacts were glued to the only available side of the sample with silver paste and electrical conductivity of the sample was studied in a magnetic field according to four-point method The RHEED pattern of the silicon wafer after thermal etching is shown in figure 2a~2b. The unprocessed RHEED image of the first Co layer is shown in Figure 3(a) which depicts a high degree of crystallinity. The processed RHEED patterns of this surface is shown in Figure 3.(b) As it is anticipated from these images the first cobalt layer was perfectly crystalline while the quality of the crystallinity of the next layers reduced gradually. The RHEED patterns of the second Co layer are shown in Figure 4. Electrical resistance behavior of the composite layers was thoroughly investigated under applied magnetic field. A positive shift was observed in the electrical resistance of the layers as a magnetic field was applied to them. Data is shown in Figure 5. This can be attributed to the GMR properties of magnetic materials in magnetic fields

Acknowledgements

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The 54th AVS International Symposium, October 14, 2007

Nanometer-scale Science and Technology

I am very grateful to Dr. Dipten Bhattacharya for suitable discussions as well as for collaboration. The financial of this project has supported by Material and Energy research center with grant no............. under role of Ministry Sciences higher education Research Council of Iran.

References 1. V.S. Speriosu et al., Phys. Rev. B 44, 5358 (1991). 2. J. Bass, W.P. Pratt Jr., and P.A. Schroeder, Commenets Cond. Mat. Phys. 18, 223 (1998). 3. M.T. Johnson, P.J.H. Bloemen, F.J.A. den Broeder, and J.J. de Vriest, Rep. Pro.Phys. 59, 1409 (1996). 4. T. Valet and A. Fert, Phys, Rev. B 48, 7099 (1993). 5. S. Honda, T. Fujimoto, and M. Nawate, J. Appl. Phys. 80,5175 (1996). 6. Holzapfel, B. ; Huetten, A.; Eckert, D.; Mueller, K.-H.; Schultz, L.; Journal De Physique. IV : JP, v 7, n 1, Mar, 1997, p 637-638. 7. Wawro, A.; Baczewski, L.T.; Kalinowski, R.; Aleszkiewicz, M.; Rauluszkiewicz, J.; Thin Solid Films, v 306, n 2, Sep 11, 1997, p 326-330. 8. Emmerson, C.M.; Shen, T.-H.; Evans, S.D.; Journal of Magnetism and Magnetic Materials, v 165, n 1-3, Jan, 1997, p 301-303. 9. Albrecht, M.; Pohl, J.; Malang, E.U.; Kohler, J.; Wider, H.; Bucher, E.; Journal of Magnetism and Magnetic Materials, v 170, n 1-2, Jun 1, 1997, p 67-73. 10. Y. Enta; S. Suzuki; S. Kono; T. Sakamoto; Phys. Rev. B 39, 56 (1989) 11. D.N.Argyriou et al, Phys. Rev. Lett 76,1358 (1996) 12. 13 .A. Chakraborty et al. Phys. Rev. B 56, 8828 (1997)

Page 223, NS-ThP.14, October 14, 2007

The 54th AVS International Symposium, October 14, 2007

Nanometer-scale Science and Technology

Fig.1 schematic diagram of SHEED

Fig.2a, Thermal silicon wafer etching at 782o

Fig.2b, Silicon Wafer

Page 223, NS-ThP.14, October 14, 2007

The 54th AVS International Symposium, October 14, 2007

Nanometer-scale Science and Technology

600

500

Energy, eV

400

300

200

100

0 0 50 100 150 Time, pixel 200 250

Figure 1. Processed RHEED pattern of the thermally etched silicon wafer

Figure 3. (a) Unprocessed RHEED image and (b) hexagonal crystal structure [6] of (111) silicon.

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The 54th AVS International Symposium, October 14, 2007

Nanometer-scale Science and Technology

1.65 1.6 1.55 Intensity 1.5 1.45

(a)

11

8

12 9 6 1 4

10 7 5

13 3 2

1.4 1.35 1.3 0 50

100

150

200 Time, pixel

250

300

350

400

Figure 3. (a) Si/Co processed RHEED image and (b) Si/Co /Cu on (111) silicon.

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The 54th AVS International Symposium, October 14, 2007

Nanometer-scale Science and Technology

1200

(b)

1000

12 11 10 8

13

800 Energy, eV 600 400 1 200 0 0 50 100 6

9 7 5 3 2 4

150

200 Time, pixel

250

300

350

400

Figure 2. Processed RHEED patterns of the first cobalt layer.

3.65 3.6 3.55 3.5 3.45 3.4 3.35 3.3 0 1 1 2 2 3 3 4 4 5 Magnetic Field, kG

Resistance, Ohm

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