Thin Solid Films 517 (2008) 297–299

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Formation of high-density Si nanodots by agglomeration of ultra-thinamorphous Si films

Hiroki Kondo a,⁎, Tomonori Ueyama a, Eiji Ikenaga b, Keisuke Kobayashi b, Akira Sakai a,Masaki Ogawa c, Shigeaki Zaima a

a Graduate School of Engineering, Nagoya University, Furo-cho, Chikusa-ku, Nagoya 464-8603, Japanb Japan Synchrotron Radiation Research Institute, 1-1-1, Kouto, Sayo-cho, Sayo-gun, Hyogo 679-5198, Japanc EcoTopia Science Institute, Nagoya University, Furo-cho, Chikusa-ku, Nagoya 464-8603, Japan

⁎ Corresponding author. Fax: +81 52 789 2760.E-mail address: hkondo@alice.xtal.nagoya-u.ac.jp (H

0040-6090/$ – see front matter © 2008 Elsevier B.V. Aldoi:10.1016/j.tsf.2008.08.079

a b s t r a c t

a r t i c l e i n f o

Available online 20 August 2008

High-density and similarly

Keywords:Si nanodotSi nanocrystalAmorphous siliconScanning electron microscopy (SEM)Angle-resolved X-ray photoelectronspectroscopy (ARXPS)

-sized Si nanodots were formed by annealing ultra-thin amorphous Si (a-Si)films deposited on SiO2/Si substrates in vacuum. Dependences of density and diameter of the Si nanodots onthe a-Si film thickness and, annealing temperature and time were investigated by scanning electronmicroscopy. It is found that drastic increase (decrease) in the density (diameter) occurred at an a-Si thicknessof 1 nm. By agglomeration of sub-nanometer thick a-Si films, a density larger than 1012 cm−2, an averagediameter smaller than 5 nm, and a dispersion of diameter less than 15% were achieved.

© 2008 Elsevier B.V. All rights reserved.

1. Introduction

Si nanodots are one of typical materials used in nanotechnologies,because of their unique and useful functions originating fromquantized electron energy state. In the practical applications, itis important to form high-density and similarly-sized Si nanodots.Although various formation techniques have been developed so far, itis generally difficult to achieve high-density and nanometer-size withlow dispersion of distribution simultaneously [1–3]. For example, in achemical vapor deposition method, which is a commonly used forma-tion technique of Si nanodots, chemical treatment or radical radia-tion is necessary to achieve a density larger than 1012 cm−2 [3–4]. Inthis study, we performed a formation technique of high-density andsimilarly-sized Si nanodots using ultra-thin amorphous Si (a-Si) filmsdeposited on SiO2/Si substrates.

2. Experimental

After chemical cleaning, a SiO2 layer was formed on a p-Si (100)wafer (1–10 Ω cm) by thermal oxidation at 1000 °C in dry oxygen. ASiO2/Si substrate was set in an ultrahigh-vacuum (UHV) chambersystem at a base pressure less than 2×10−9 Torr. An a-Si layer with athickness ranging from 0.1 to 3.0 nm was deposited on the SiO2/Sisubstrate by electron beam evaporation at room temperature. The

. Kondo).

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a-Si/SiO2/Si samples were successively annealed at temperatures from390 to 630 °C for 1 to 30min in the same chamberwithout air exposure.The number density and the size of Si nanodots were studied byscanning electron microscopy (SEM) and transmission electron micro-scopy (TEM) observations. Angle-resolved X-ray photoelectron spectro-scopy (ARXPS) was also performed to study chemical properties of thenanodots.

3. Results and discussions

Fig.1 shows SEM images of Si nanodotsmade from (a) 0.3-, (b) 0.5-,(c) 1.0- and (d) 3.0-nm-thick a-Si films annealed for 2 min at tempera-tures of 550 °C, 550 °C, 600 °C, and 630 °C, respectively. As the a-Sithickness decreases, the diameter of Si nanodots and the dispersion oftheir size distribution were found to drastically decrease and thedensity increase. Dependencies of the density and diameter of Sinanodots on the a-Si thickness are shown in Fig. 2. It should be notedthat drastic increase (decrease) in the density (diameter) occurs atan a-Si thickness of 1 nm. Additionally, while a dispersion of diameterwas about 30% in the case of the a-Si film thicker than 1 nm, thatobtained from the sub-nanometer-thick a-Si film was less than15%. Fig. 3 shows SEM images of Si nanodots made from (a) 0.3- and(b) 3.0-nm-thick a-Si films annealed for 1 min at temperatures of550 °C and 600 °C, respectively. It can be found that, in the case of the0.3-nm-thick a-Si film, high-density Si nanodots were formed evenafter the annealing for 1 min. On the other hand, though a few Sinanodots were formed, the a-Si film was remaining around the Sinanodots in the case of the 3.0-nm-thick a-Si film. This result indicatesdifferent nucleation rates depending on the a-Si thicknesses.

Fig. 1. Plan-view SEM images of Si nanodots formed by annealing (a) 0.3-, (b) 0.5-,(c) 1.0- and (d) 3.0-nm-thick a-Si films at a temperature ranging from 550 to 630 °C for2 min.

Fig. 2. a-Si thickness dependence of density and diameter of Si nanodots.

Fig. 4. Si 1s photoelectron spectra obtained from Si nanodots formed by annealing(a) 0.3 and (d) 3.0 nm-thick a-Si films.

298 H. Kondo et al. / Thin Solid Films 517 (2008) 297–299

According to these observation results, it is suggested that, hemi-spherical grain (HSG) growth occurred at the surface of the a-Si filmsthicker than 1 nm after annealing [1–2]. It was also confirmed by TEMobservation that crystalline grains with sizes of several tens ofnanometers were formed in those samples. Contrary, no crystalline

Fig. 3. Plan-view SEM images of Si nanodots formed by annealing (a) 0.3- and(b) 3.0-nm-thick a-Si films for 1 min at temperatures 550 and 600 °C, respectively.

grains were found in TEM in the case of a-Si thickness thinner than1 nm.

Fig. 4 shows Si 1s photoelectron spectra for Si nanodots made from(a) 0.3- and (d) 3.0-nm-thick a-Si films on the SiO2(35 nm)/Si substratesby annealing. These spectra were measured using high-resolution hardX-ray radiation at the hard X-ray undulator beam line (BL47XU) of theSuper Photon ring, 8 GeV (SPring-8). The energy of the incident beamwas 7925.5 eV in this experiment. Deconvoluted spectra are also indi-cated in the figure. Si4+ peaks found at 1844 eV are attributed to the SiO2

films. On the other hand, Si0+ peaks at around 1839 eVoriginate fromtheSi nanodots on the sample surfaces rather than the Si substrates, sincetake-off-angle of the photoelectron was set at 8°. Other three peakscome from suboxide at the SiO2/Si interfaces. The Si0+ peak from the Sinanocrystalsmade from the 3.0-nm-thick a-Sifilmdetected at a bindingenergyof 1839 eVhada full-width athalf-maximum(FWHM)of 0.81eV,which corresponds to that reported for Si substrates [5]. On the otherhand, the Si0+ peak has a larger FWHM (1.16 eV) and slightly shifts to ahigher binding energy side in the case of the 0.3-nm-thick a-Si film. InFig. 5, FWHMvalues of the Si0+ peaks for the Si nanodotsmade from the0.3- and 3.0-nm-thick a-Si films are plotted against the take-off-angle.FWHM values of the Si4+ peaks from the underneath SiO2 are alsoplotted. Due to the high-energy X-ray radiation, Si 1s photoelectronsexcited in the Si substrate was detected at the high take-off-anglemeasurements and thus the FWHMvalues of the Si0+ peaks are shown tobe small. Note that the values of Si0+ increase as the take-off-angledecreases in the case of the 0.3-nm-thick a-Si film. Large FWHM valuesat low take-off-angles possibly indicate that observed Si nanodots formsan amorphous phase, in which there are variation of bond-length andbond-angle of Si atoms.

Fig. 5. FWHM of Si0+ and Si4+ peaks observed in Si 1s photoelectron spectrum obtainedfrom Si nanodots formed by annealing 0.3-(●○) and 3.0-(■,□)nm-thick a-Si films.

299H. Kondo et al. / Thin Solid Films 517 (2008) 297–299

In summary, we have performed the formation of Si nanodotsby annealing thin a-Si films with various thicknesses. Density anddiameter of the Si nanodots drastically changed at an a-Si thickness of1 nm. Si 1s photoelectron spectra excited from the Si nanodots werealso different between the Si nanodotsmade from the a-Sifilms thickerand thinner than 1 nm. High-density and nanometer-sized Si nanodotscan successfully be formed byagglomerating sub-nanometer thick a-Sifilms as are amorphous.

References

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