Au-induced lateral crystallization of a-Si1−xGex (x: 0–1) at low temperature

Thin Solid Films 508 (2006) 44 – 47 www.elsevier.com/locate/tsf

Au-induced lateral crystallization of a-Si1 xGex (x: 0–1) at low temperature Tomohisa Aoki, Hiroshi Kanno, Atsushi Kenjo, Taizoh Sadoh, Masanobu Miyao * Department of Electronics, Kyushu University, 6-10-1 Hakozaki, Fukuoka 812-8581, Japan Available online 10 November 2005

Abstract Au-induced lateral crystallization of amorphous Si1 x Gex (x: 0 – 1) on SiO2 at a low temperature (400 -C) has been investigated. Although the growth velocity decreased with increasing Ge fraction, growth velocity exceeding 20 Am/h was obtained for all Ge fractions. As a result, polySi1 x Gex with large areas (> 20 Am) was obtained at a low temperature (400 -C). This is a great advantage of Au-induced lateral crystallization compared with Ni. However, the concentrations in the surface regions (depth: 0 – 20 nm) of the lateral growth regions were high (10 – 30%), though those in the deeper regions (depth: 20 – 50 nm) were as small as 1 – 2%. Removing of the surface regions with the high Au concentrations and gettering of Au atoms in the deeper regions are necessary to apply the grown layers to the device fabrication. D 2005 Elsevier B.V. All rights reserved. Keywords: Crystallization; Poly-SiGe; Metal-induced lateral crystallization; Eutectic reaction

1. Introduction Low temperature formation of polycrystalline silicongermanium (poly-SiGe) films on insulating substrates has been expected to realize the advanced system-in-displays and threedimensional ultralarge-scale integrated circuits (ULSI). To prevent the diffusion of dopant atoms and the softening of glass substrates, the formation temperature should be lower than 500 -C. In line with this, the recrystallization processes of amorphous SiGe (a-SiGe) on SiO2 have been extensively investigated. Melt-growth processes such as laser annealing enabled the formation of poly-SiGe with large grains (¨ 5 Am); however, Ge atoms were not distributed uniformly in the films, and surface ripples (¨ 15 nm in height) were observed [1]. The solid-phase crystallization (SPC) of SiGe realized uniform distribution of Ge atoms and the flat surface; however, high temperature annealing (> 550 -C) was required [2,3]. Recently, the low-temperature solid-phase crystallization of a-Si was realized using the catalytic effect of the silicidation species such as Ni [4 –6], where NiSi2 formed at a low temperature acted as a seed for lateral solid-phase crystallization. We have applied this technique to crystallize a-Si1 x Gex (x: 0 –1) at a low temperature (500 -C), and obtained polySiGe with large grains (¨ 10 Am) [7– 9]. However, the lateral * Corresponding author. Tel.: +81 92 642 3951; fax: +81 92 642 3974. E-mail address: [email protected] (M. Miyao). 0040-6090/$ - see front matter D 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.tsf.2005.07.317

growth velocity (1 Am/h at 550 -C) was not sufficiently high, and uniform crystallization was obtained only for samples with low Ge fractions (< 30%) [7,8]. One possible solution for these problems is utilization of other types of reactants such as Ag, Al, In, and Au. This is because these metals cause eutectic reactions with Si and Ge [10 – 13], which are expected to induce liquid-phase growth of SiGe with all Ge fractions at a low temperature. Among them, we selected Au, because the eutectic temperatures for Au – Si (363 -C) and Au –Ge systems (361 -C) are very close [14,15]. In the present paper, we report our new findings concerning the important role of Au on the metal-induced lateral crystallization of a-Si1 x Gex (x: 0– 1). 2. Experimental procedure In the experiment, p-type Si substrates with the (100) orientation were used. They were covered with SiO2 films (160 nm thick) by dry oxidation, and then a-Si1 x Gex (x: 0– 1) layers (50 nm thick) were deposited on the SiO2 films by using a molecular beam epitaxy system (base pressure: 5  10 11 Torr), where Si and Ge were evaporated (rate: ¨0.1 nm/s) using the electron-beam gun and the Knudsen cell, respectively, with keeping the substrates at room temperature. The composition ratio (x) in a-Si1 x Gex was controlled by monitoring Si and Ge fluxes, and calibrated by Auger electron spectroscopy (AES). Subsequently, Au films (¨30 nm thick)

T. Aoki et al. / Thin Solid Films 508 (2006) 44 – 47

Au pattern

Au 100

Concentration (%)

a-Si1-xGex SiO2

annealing

45

lateral growth area

80

a-Si area

Si

60 40

(a) Au

20 0

Au

0

10

20

30

Distance (µm) lateral growth area

poly-Si1-xGex 100

Concentration (%)

SiO2 Fig. 1. Schematic experimental procedure for Au-induced lateral crystallization of a-Si1 x Gex on SiO2.

were evaporated on the a-Si1 x Gex films and then patterned by lift-off process with photolithography. Finally, these samples were annealed at 400 – 550 -C in a dry nitrogen ambient. The cross sectional views during crystallization are schematically shown in Fig. 1. The crystal structure and quality of grown Si1 x Gex layers were characterized by Nomarski optical microscopy and microprobe Raman spectroscopy (spot diameter: ¨ 1 Am). The composition of the grown layers was evaluated by energydispersive X-ray spectroscopy (EDX) and AES. All measurements were carried out at room temperature.

Ge 60 40

100µm

400oC, 4 h

(b) Au

20 0

10

20

30

Distance (µm) Au concentration (%)

60

40

(c)

20

0

Fig. 2(a) – (d) show Nomarski optical micrographs of Si1 x Gex (x: 0– 1) samples obtained by the Au-induced lateral crystallization at 400 -C. The annealing time for each sample is shown in the figure. The Au patterns, which act as the seeding

a-Ge area

80

0

3. Results and discussion

400oC, 4 h

Au-pattern

0

20

40

60

80

100

Ge fraction (%) Fig. 3. Line scan profiles (Au, Si, and Ge) in Au-induced lateral growth area (400 -C, 2 h) obtained by EDX for a-Si/SiO2 (a) and a-Ge/SiO2 (b), and Au concentration in the lateral growth area as a function of Ge fraction (c).

50µm

(e) 400°C, 2h

Au

(a) Si 400oC, 2 h

Au

(b) Si0.6Ge0.4 50µm

400oC, 2 h

50µm

Intensity(a.u.)

Si Sub Ge–Ge Si–Ge

Ge:0%

Si–Si

Ge:40%

Ge:70% Ge:100%

Au

(c) Si0.3Ge0.7

Au

(d) Ge

200

300

400

500

600

Raman Shift(cm-1)

Fig. 2. Nomarski optical micrographs (a) – (d) and Raman spectra (e) for samples with different Ge fractions obtained by Au-induced lateral crystallization. The annealing times for samples are shown in the figures.

T. Aoki et al. / Thin Solid Films 508 (2006) 44 – 47

poly-Si0.6Ge0.4 50µm

Au

Au 550oC,2h (a) 7µm

50µm

50µm

Au

550oC,2h (b) 23µm

550oC,2h (c) 50µm

Lateral growth length(µm)

46

60

(d)

Ge

Au (550 0 0°C, 2h) 40

Si 0.3Ge 0.7

20

Si Si 0.6Ge 0.4

0

0

10

20

40

30

50

Au–pattern line width(µm) Fig. 4. Nomarski optical micrographs of samples (Ge fraction: 0.4) with different Au-pattern line widths of 7 (a), 23 (b), and 50 Am (c); and lateral growth length of samples (Ge fraction: 0 – 1) as a function of Au-pattern line width (d). The annealing was performed at 550 -C for 2 h.

temperatures for Au –Si (363 -C) and Au – Ge (361 -C) systems [14,15]. In order to evaluate the crystal quality of the Auinduced lateral growth regions, microprobe Raman spectroscopy measurements were performed. The typical spectra of the annealed samples (400 -C, 2 h) are shown in Fig. 2(e). Three sharp peaks, originating from the vibration modes of Ge –Ge, Si– Ge, and Si– Si bonds, are clearly observed. The analysis of Raman shift showed that the grown layers were completely strain-free. In order to evaluate the concentration of Au in the lateral growth (400 -C, 2 h) area, EDX measurements were performed. The sample surface layers (10 nm) were sputtered before the measurement in order to remove surface contaminants. The line-scan profiles from the Au patterns to the amorphous regions for the samples with a-Si and a-Ge layers are summarized in Fig. 3(a) and (b), respectively. It is found that the Au atoms (10 – 30%) uniformly distribute in the growth areas. The residual concentration of Au is summarized as a function of Ge fraction in Fig. 3(c). This indicates that Au concentration increases with increasing Ge fraction. The Au concentration in Fig. 3(c) is very similar to those (19% and 28%) [14,15] necessary to form eutectics in Au –Si and Au –Ge systems. The in-depth concentration profiles of Au in the lateral growth regions were measured by AES. Although the concentrations in the surface regions (depth: 0 –20 nm) were high (10 –30%), those in the deeper regions (depth: 20 –50 nm) were as small as 1– 2%. The reason for the concentration

(a)

Lateral growth length(µm)

Lateral growth length(µm)

areas for crystallization, are located in the bottom right-hand areas of the photographs. It is clear that crystal growth propagated laterally from the Au patterns, and crystallized areas with the plane morphology were obtained for all Ge fractions (x: 0 – 1). In the case of pure Si, the Au-induced lateral growth length after annealing (400 -C, 2 h) was estimated to be 120 Am. Thus, a very high growth velocity (60 Am/h) was obtained at a low temperature (400 -C). This value is 15 times larger than that in Ni-induced lateral crystallization at 550 -C [7– 9]. In addition, it was found that the Au-induced lateral crystallization generated surface ripples (¨ 10 nm in height) in the regrown regions. This value is larger than those (¨ 2 nm in height) in the Ni-induced lateral crystallization, however acceptable for device fabrication. These phenomena can be explained on the basis of the eutectic reaction in Au –Si systems. When Au atoms diffuse laterally into a-Si, the melting temperature of a-Si is lowed from 1414 -C to the eutectic temperature of the Au – Si system (363 -C) [14]. Consequently, lateral-liquid-phase crystallization proceeds very fast from Au patterns. In addition, liquid-phase reaction generates the surface ripples larger than those for the Ni-induced solid-phase crystallization. Although zigzag-like growth was observed around the growth fronts, the Au-induced lateral crystallization realized uniform lateral growth (> 20 Am) even for the samples with high Ge fractions (> 40%). This is due to the closed eutectic

Si

100

50

Si0.6Ge0.4 Si0.3Ge0.7 Ge

0

0

1

2

Annealing time(h)

3

(b) Au-induced (400°C 2h)

100 Al-induced (500°C 2h)

50 Ni-induced (500°C 20h)

0

0

In-induced (400°C 2h)

50

100

Ge fraction(%)

Fig. 5. Lateral growth characteristics of Au-induced crystallization (400 -C) for samples with different Ge fractions (a), and Ge fraction dependence of lateral growth length obtained by Au-induced crystallization (400 -C, 2 h). The data [7,13] obtained by Ni-, Al- and In-induced lateral growth (550, 500, and 400 -C, respectively) are also shown.

T. Aoki et al. / Thin Solid Films 508 (2006) 44 – 47

reduction in the deeper regions is not clear. However, these results suggest that removing of the surface regions with the high Au concentrations and gettering of Au atoms in the deeper regions are necessary to apply the grown layers to the device fabrication. In the Au-induced lateral crystallization, we found out that the growth characteristics strongly depended on the Au-pattern sizes as shown in Fig. 4(a) – (c). Results are summarized in Fig. 4(d), which showed the lateral growth length of a-Si1 x Gex as a function of Au-pattern line width. It is found that the lateral growth length decreases with decreasing Au-pattern line width, especially in the narrow region (< 30 Am). We attribute such pattern-size dependent growth characteristics to the fact that a sufficient amount of Au atoms comparable to those of Si and Ge atoms are necessary to induce the eutectic reactions in the Si– Ge – Au system. The growth characteristics of Au-induced lateral crystallization (400 -C) for samples with sufficiently large Au-pattern size (> 500 Am  500 Am) are summarized as a function of annealing time and Ge fraction in Fig. 5(a) and (b), respectively. Our previously reported data [7,13] for the Ni-, Al- and In-induced growth (550, 500 and 400 -C, respectively) [13] are also shown for comparison. It is found that the Au-induced growth progresses rapidly and shows saturation in a short time (¨ 2 h). Although saturated growth length decreases with increasing Ge fraction, the length is much larger than that obtained by Ni- and In-induced crystallization. Especially, crystallization for all samples with a wide range of Ge fraction (x: 0– 1) was obtained only for Auinduced crystallization. In this way, the low-temperature (400 -C) crystallization of Si1 x Gex with all Ge fractions (x: 0– 1) has been achieved by employing eutectic reaction with Au for the first time. This is a great advantage for low temperature crystallization of SiGe. 4. Conclusions The Au-induced low-temperature (400 -C) lateral crystallization of a-Si1 x Gex (x: 0 –1) on SiO2 has been investigated. Eutectic reactions in the Au – Si– Ge system showed the Au-pattern size dependent lateral growth, however growth velocity was saturated for pattern sizes above 100 Am. As a

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result, a large growth area (> 20 Am) was obtained for aSi1 x Gex with all Ge fractions. This is a great advantage of Au-induced lateral crystallization compared with Ni. Although the concentrations in the surface regions (depth: 0– 20 nm) of the lateral growth regions were high (10 –30%), those in the deeper regions (depth: 20 – 50 nm) were as small as 1– 2%. Therefore, removing of the surface regions with the high Au concentrations and gettering of Au atoms in the deeper regions are necessary to apply the grown layers to the device fabrication. Acknowledgements A part of this work was supported by the Grant-in-Aid for Scientific Research of the Ministry of Education, Culture, Sports, Science and Technology of Japan. References [1] M. Miyao, T. Sadoh, S. Yamaguchi, S.K. Park, Tech. Rep. IEICE 101 (2001) 1. [2] S. Yamaguchi, N. Sugii, S.K. Park, K. Nakagawa, M. Miyao, J. Appl. Phys. 89 (2001) 2091. [3] J. Olivares, A. Rodriguez, J. Sangrador, T. Rodriguez, C. Ballesteos, Kling, Thin Solid Films 37 (1999) 51. [4] C. Hayzelden, J.L. Bastone, J. Appl. Phys. 73 (1993) 8279. [5] C. Hayzelden, J.L. Bastone, R.C. Cammarate, Appl. Phys. Lett. 60 (1992) 225. [6] M. Miyasaka, K. Makihara, T. Asano, E. Polychroniadis, J. Stoemenos, Appl. Phys. Lett. 80 (2002) 944. [7] H. Kanno, I. Tsunoda, A. Kenjo, T. Sadoh, M. Miyao, Appl. Phys. Lett. 82 (2003) 2148. [8] H. Kanno, I. Tsunoda, A. Kenjo, T. Sadoh, S. Yamaguchi, M. Miyao, Jpn. J. Appl. Phys. 42 (2003) 1933. [9] H. Kanno, A. Kenjo, T. Sadoh, M. Miyao, Appl. Phys. Lett. 85 (2004) 899. [10] B. Bian, J. Yie, B. Li, Z. Wu, J. Appl. Phys. 73 (1993) 7402. [11] J.H. Kim, J.Y. Lee, Jpn. J. Appl. Phys. 35 (1996) 2052. [12] Z. Tan, S.H. Heald, M. Rapposch, C.E. Bouldin, J.C. Woicik, Phys. Rev., B 46 (1992) 9505. [13] M. Miyao, H. Kanno, T. Sadoh, Proc. 1st Int. TFT Conf., Seoul, 2005, p. 32. [14] T.B. Massalski, in: J.L. Murray, L.H. Bennett, H. Baker (Eds.), Binary Alloy Phase Diagrams, vol. 1, American Society for Metals, Ohio, 1986, p. 312. [15] T.B. Massalski, in: J.L. Murray, L.H. Bennett, H. Baker (Eds.), Binary Alloy Phase Diagrams, vol. 1, American Society for Metals, Ohio, 1986, p. 263.