Films of Various Shot Numbers of Control Beam

To figure out the formation process of the laser-induced dots, we irradiated the control beam on germanium film with thickness of 9 Å (6.4 ML) with various laser shot numbers and the same average fluence of 60 mJ/cm²in a region.

The result is shown in Figure 3.8. In the sample of irradiation with 200 shots, we

can observe the formation of structures of height about 3 nm, which are connected to each other. In the image of 1000 shots, the connections between the structures disappear and islands of height about 8 nm form. When the shot number comes to 10000 shots, the dots become smaller and higher with height of 13 nm. And then some of the dots gathered to be a bigger dots with height of 15 nm on the sample with shot numbeer 20000 shots.

The mechanism of the process of the laser-induced formation is unclear and all of these were observed only in small region of several tens of micrometers. More mechanisms are needed to understand this phenomena.

Figure 3.8: 2D and 3D AFM images of irradiated germanium film with thickness of 9 Å (6.4 ML). The laser shot numbers are (a) 200 shots, (b) 1000 shots, (c) 10000 shots, and (d) 20000 shots, respectively.

Figure 3.9: (a) AFM images of irradiated germanium film with thickness of of 9 Å (6.4 ML). The laser shot numbers is 40000 shots. (b) Line profile of the ordered dots from lower left to upper right, and (c) from lower right to upper left.

When it comes to 40000 shots, we see from Figure 3.9 (a) that the dots become larger with base diameter of 200 nm and height of 30 nm. The dots ordered in two directions are surprisingly observed. Figure 3.9 (b) and (c) shows the beam profile of the ordered dots from lower left to upper right and lower right to upper right, respectively. We found that in the direction of lower right to upper right, the distances between the dots are near to our wavelength of control beam, which is 355 nm. It indicates that it is due to the interference between the incident beam and the surface scattered wave as shown in Section 1.4.3.

3.2.3 Films of Various Effective Thickness

With the same average fluence of 60 mJ/cm² in a region, we irradiate laser with 20000 shots on samples with various germanium film thickness. The result of 2D and 3D view of AFM images are shown in Figure 3.10 and the 3D view of AFM images of one dot and their line profiles are shown in Figure 3.11. The base diameter of these films with thickness of 2 Å (1.4 ML), 9 Å (6.4 ML), 14 Å (10.0 ML), and 18 Å (12.9 ML) are about 14 nm, 90 nm, 170 nm, and 360 nm, respectively. And the height is 1.3 nm, 15 nm,

Figure 3.10: 2D and 3D AFM images of irradiation with 20000 laser shots on germanium film with thickness of (a) 2 Å (1.4 ML), (b) 9 Å (6.4 ML), (c) 14 Å (10.0 ML), and (d) 18 Å (12.9 ML).

16 nm, and 27 nm, respectively. We can see that both base diameter and height increases with the larger thickness of film. However, increment of base diameter is much more than that of height. The reason of this phenomenon might be that the thicker germanium film provides more materials for the dot formation. And the base diameter increases much more because the strain is stronger along the direction of the silicon surface due to the 4.2% lattice mismatch between silicon substrate and germanium film.

Figure 3.11: 3D AFM images of one dot and their line profiles of irradiation with 20000 laser shots on germanium film with thickness of (a) 2 Å (1.4 ML), (b) 9 Å (6.4 ML), (c) 14 Å (10.0 ML), and (d) 18 Å (12.9 ML).

Figure 3.12: (a) The histogram of the base diameter and (b) the height of Figure 3.11 (a).

With the film thickness of 2 Å (1.4 ML), we got the highest density of dots, which is 1.6×10¹¹cm⁻². And we also saw the smallest dot of height of about 1.3 nm and base diameter of about 14 nm among our experiments. The histogram of base diameter and the height are shown in Figure 3.12 (a) and (b), respectively. Although the sheet density and

the mean dot size do not reach what we have seen in Section 1.3.1, we develop a new way to make quantum dots formation. Once we understand the mechanism of this phenomena, maybe we could control the density and size of the quantum dots. There are big potentials to make the smaller and more concentrated quantum dots.

Summary and Future Prospective

We found out a new way to control the formation of germanium on silicon self-assembled QDs. By laser irradiation, we can see the laser-induced formation of dots under the critical thickness of spontaneous S-K mode growth. What's more, the larger the thick-ness is, the larger the base diameter and height of the dots are. That is, we can control the size of the quantum dots by varying thickness of the germanium film. In this way, the smallest average base diameter of about 17 nm and the density of dots of 1.6×10¹¹cm²are reached. Although both of them do not exceed the achievement from other works, there are great potentials to make them smaller and denser after we understand the mechanisms of this new method of quantum-dots formation. However, the restriction of the beam pro-file of control beam make it hard to form a sufficient large region with such small and dense quantum dots.

Since we believe that the laser-induced formation of the quantum dots is as result of the gradient of the fluence, we are planning to make the control beam split into two beams and interfere each other on the sample as shown in Figure 4.1 (a). On the interference patterns, there are full of gradient of fluence. By slightly moving the position of mirror of the split beam, the large fluence part of the interference fringe will irradiate averagely

on the whole region (Figure 4.1 (b)). Therefore, we can make the whole irradiated region full of uniform quantum dots.

Figure 4.1: (a) Schematic setup of the interference of split control beams on the sample.

(b) Interference fringe moveing back and forth to make whole plan full of quantum dots.

As seen in Figure 2.18, the optical absorption of germanium is about 40 times higher than that of silicon at wavelength of 532 nm. Thus in our future work, we're also planning to switch the wavelength of control beam from 355 nm to 532 nm. The beam will be absorbed by the germanium film only, providing the direct heating on germanium film and not on the silicon substrate [19]. Maybe it will help us to find out the mechanism of this interesting process of laser-induced formation of germanium quantum dots (Figure 4.2).

Figure 4.2: Schematic mechanism of the laser-induced formation of the quantum dots. In this graph the absorption of silicon is very small relatively to the absorption of germanium for light with wavelength of 355 nm.

[1] Mitsuru Sugawara. Self-Assembled InGaAs/GaAs Quantum Dots. Academic Press, 1999.

[2] Rudolf Bratschitsch and Alfred Leitenstorfer. Quantum dots: Artificial atoms for quantum optics. Nature Materials, 5:855--856, 2006.

[3] M. S. Hegazy and H. E. Elsayed-Ali. Growth of Ge quantum dots on Si(100)(2×1) by pulsed laser deposition. Journal of Applied Physics, 99:054308, 2006.

[4] I. Kamiya, Ichiro Tanaka, and H. Sakaki. Control of size and density of self-assembled InAs dots on (0 0 1)GaAs and the dot size dependent capping process.

Journal of Crystal Growth, 201-202:1146--1149, 1999.

[5] Y. R. Li, Z. Liang, Y. Zhang, J. Zhu, S. W. Jiang, and X. H. Wei. Growth modes transition induced by strain relaxation in epitaxial MgO thin films on SrTiO3 (001) substrates. Thin Solid Films, 489:245--250, 2005.

[6] D. J. Eaglesham and M. Cerullo. Dislocation-free Sranski-Krastanow growth of Ge on Si(100). Physical Review Letters, 64:1943--1946, 1990.

[7] J. L. Liu, S. Tong, and K. L. Wang. Handbook of Semiconductor Nanostructures and Nanodevices. American Scientific, 2005.

[8] J. L. Huang. Feasibility study of the control of the size distribution of Ge/Si quantum dots grown with pulsed laser deposition by laser pre-processing of the substrate.

Master's thesis, National Central University, Taiwan, 2013.

[9] Kang L. Wang, Fellow IEEE, Dongho Cha, Jinlin Liu, and Christopher Chen. Ge/Si self-assembled quantum dots and their optoelectronic device applications. Proceed-ings of IEEE, 95:1866--1883, 2007.

[10] C. Teichert, M. G. Lagally, L. J. Peticolas, J. C. Bean, and J. Tersoff. Stress-induced self-organization of nanoscale structures in SiGe/Si multilayer films. Physical Re-view B, 53:16334--16337, 1996.

[11] A. I. Yakimov, A. V. Dvurechenskii, A. I. Nikiforov, S. V. Chaoekovskii, and S. A.

Tiis. Ge/Si photodiodes with embedded arrays of Ge quantum dots for the near infrared (1.3-1.5 µm) region. Semiconductors, 37:1383--1388, 2003.

[12] Zhi Liu, Tianwei Zhou, Leliang Li, Yuhua Zuo, Chao He, Chuanbo Li, Chunlai Xue, Buwen Cheng, and Qiming Wang. Ge/Si quantum dots thin film solar cells. Applied Physics Letters, 103:082101, 2013.

[13] J M MacLeod, C V Cojocaru, F Ratto, C Harnagea, A Bernardi, M I Alonso, and F Rosei. Modified Stranski-Krastanov growth in Ge/Si heterostructures via nanos-tenciled pulsed laser deposition. Nanotechnology, 23:065603, 2012.

[14] Mathieu Helfrich, Bernd Terhalle, Yasin Ekinci, and Daniel M. Schaadt. Controlling structural properties of positioned quantum dots. Journal of Crystal Growth, 371:39--44, 2013.

[15] M. Gherasimova, R. Hull, M. C. Reuter, and F. M. Ross. Pattern level assembly of ge quantum dots on si with focused ion beam templating. Applied Physics Letters, 93:023106, 2008.

[16] Amro Alkhatib and Ammar Nayfeh. A complete physical germanium-on silicon quantum dot self-assembly process. Scientific Reports, 3:2099, 2013.

[17] A. Pascale, I. Berbezier, A. Ronda, and P. C. Kelires. Self-assembly and ordering mechanisms of Ge islands on prepatterned Si(001). Physical Review B, 77:075311, 2008.

[18] S. Watanabe, Y. Yoshida, S. Kayashima, S. Yatsu, M. Kawai, and T. Kato. In situ observation of self-organizing nanodot formation under nanosecond-pulsed laser ir-radiation on Si surface. Journal of Applied Physics, 108:103510, 2010.

[19] A Pérez del Pino, E György, J Roqueta I C Marcus, and M I Alonso. Effects of pulsed laser radiation on epitaxial self-assembled Ge quantum dots grown on Si substrates.

Nanotechnology, 22:295304, 2011.

[20] Michale N. R. Ashfold, Frederik Claeyssens, Gareth M. Fuge, and Simon J. Henley.

Pulsed laser ablation and deposition of thin films. Chemical Society Reviews, 33:23--31, 2004.

[21] J. H. Chuei. Control of the crystallinity of carbon films grown with pulsed laser deposition by using another laser pulse for synchronous laser processing. Master's thesis, National Central University, Taiwan, 2013.

[22] S. T. Su. Using pulsed laser deposition for growing TiO₂ mesoporous films with structures optimized for increasing the efficiency of dye-sensitized solar cells. Mas-ter's thesis, National Chung cheng University, Taiwan, 2013.

[23] PV Education - Absorption Coefficient ( http:// pveducation.org/ pvcdrom/ pn-junction/absorption-coefficient ).

[24] Digital Instruments - Scanning Probe Microscopy Training Notebook.

[25] Nanosensors - Catalogue of PointProbe Plus Silicon -SPM-Probes.

[26] Museum of Science - Scanning Electron Microscope ( http:// legacy.mos.org/ sln/

SEM/ ).

[27] T. M. Burbaev, T. N. Zavaritskaya, N. N. Mel'nik V. A. Kurbatov, V. A. Tsvetkov, K. S. Zhuravlev, V. A. Markov, and A. I. Nikiforov. Optical properties of germanium monolayers on silicon. Semiconductors, 35:941--946, 2001.