Properties of silicon dioxide layers with embedded metal nanocrystals produced by oxidation of Si:Me mixture
© Novikau et al; licensee Springer. 2011
Received: 21 September 2010
Accepted: 16 February 2011
Published: 16 February 2011
A two-dimensional layers of metal (Me) nanocrystals embedded in SiO2 were produced by pulsed laser deposition of uniformly mixed Si:Me film followed by its furnace oxidation and rapid thermal annealing. The kinetics of the film oxidation and the structural properties of the prepared samples were investigated by Rutherford backscattering spectrometry, and transmission electron microscopy, respectively. The electrical properties of the selected SiO2:Me nanocomposite films were evaluated by measuring C-V and I-V characteristics on a metal-oxide-semiconductor stack. It is found that Me segregation induced by Si:Me mixture oxidation results in the formation of a high density of Me and silicide nanocrystals in thin film SiO2 matrix. Strong evidence of oxidation temperature as well as impurity type effect on the charge storage in crystalline Me-nanodot layer is demonstrated by the hysteresis behavior of the high-frequency C-V curves.
During the last decade, much attention has been focused on the investigation of semiconductor and metallic nanocrystals (NCs) or nanoclusters embedded in dielectric matrices. The interest is motivated by possible applications of such nanocomposite structures. Particularly, semiconductor or metal NCs embedded in SiO2 dielectric layer of a metal-oxide-semiconductor field-effect transistor may replace SiN x floating gate in conventional Flash memory devices, allowing for thinner injection oxides, and subsequently, smaller operating voltages, longer retention time, and faster write/erase speeds [1–3]. The performance of such memory structure strongly depends on the characteristics of the NCs arrays, such as their size, shape, spatial distribution, electronic band alignment.
Several approaches have been recently tested for the formation of NCs in dielectric layers. Among those, self-assembling of NCs in dielectric layers fabricated by the low-energy ion implantation and different deposition techniques has been studied by several groups [4–7]. A strong memory effect in MOS devices using oxides with Si or Ge NCs was reported in [4, 6]. However, the implantation of Ge at the silicon-tunnel oxide interface creates trap sites and results in the degradation of the device performance . The growth technique using MBE deposition of 0.7-1 nm thick Ge layer followed by rapid thermal processing was implemented in [8, 9]. An alternative method for Ge NCs production  consists of the following steps: low pressure chemical vapor deposition of thin Si-Ge layer, thermal wet or dry oxidation, and thermal treatment in an inert ambient (reduction). Recently, a method to form an ultrathin nanocomposite SiO2:NC-Me layers at room temperature by combining the deposition of Si:Me mixed layer on the pre-oxidized Si substrate and its further oxidation in the glow discharge oxygen plasma was proposed .
In this article, a similar approach was used to produce thin SiO2 layers with an embedded layer of metal NCs. Au and Pt were chosen as metal components in Si:Me mixtures since both metals are believed to catalyze Si oxidation thus reducing the processing temperature, while neither Au nor Pt form stable oxides. Both Pt and Au embedded as NCs in dielectric matrix are attractive materials in plasmonics . In addition, both metals have much higher electron work functions compared to semiconductors, particularly, Ge, and it is interesting to investigate the effect of the NC work function on the electrical properties of the MOS stack with embedded NCs. As the first step, a thin Si:Me layer with the precisely pre-defined composition was grown by pulsed laser deposition (PLD) technique. The oxidation of Si:Me mixture was expected to result in the segregation of the noble metal in NCs distributed in the SiO2 matrix. By means of analyzing the Si(O x ):Me elemental depth distributions as a function of the annealing temperature and/or time, we attempted to investigate the kinetics of the composite structure formation. This information was supplemented by microstructural transmission electron microscopy (TEM) analysis and further--by electrical measurements on metal/SiO2:Me-NC/Si capacitors.
The composition of and the metal depth distribution in the samples were measured using Rutherford backscattering spectrometry (RBS) with a He+ beam at E = 1.5 MeV. The spectra were taken simultaneously at two different scattering angles, θ = 10° and θ = 75°, with the former geometry being used to calculate the integral metal concentration in Si:Me, while the latter one to observe possible changes in the metal distribution upon oxidation. The experimental spectra were analyzed using the RUMP software . The structural quality and the phase composition were analyzed using the TEM in both plain-view and cross-sectional geometries using a Philips CM20 instrument operating at U = 200 kV. MOS capacitors with In electrodes were fabricated, and the high-frequency C-V measurements were carried out using a serial HP4156B instrument.
Results and discussion
The self-assembling phenomenon of the formation of metal and silicide NCs in SiO2 can be explained using two mechanisms. A solubility of impurities in SiO2 is quite low, and therefore the structures obtained after metal segregation and piling up between two SiO2 layers (tunnel oxide and SiO2 capping layer) were transformed into the supersaturated solution. It is well known that under the thermal treatment the decomposition of supersaturated solution takes place eventually resulting in the phase separation and the formation of the metal NCs in a dielectric (oxide) matrix. On the next stage, the Ostwald ripening of the formed NCs occurs. This implies the diffusion of metal atoms from the valley regions of the islands toward their respective centers forming spherical nanocrystals to achieve greater volume-to-surface ratio. In our model, the initial NCs are formed during the oxidation of the Si:Me layer. After the oxidation is completed, the sample is still kept at elevated temperature facilitating the coalescence of Me NCs.
In this study, the authors have demonstrated the growth of thin SiO2 layers with embedded metal and metal silicide NCs by the combination of Si:Me mixture by PLD at room temperature and its thermal oxidation. By means of this fabrication technique, it is possible to produce a sheet of crystalline metal nanocrystals at any desirable depth in the oxide. The metal segregation process during thermal oxidation results in the formation of a high areal density of crystalline Au and PtSi dots 10-20 nm in diameter which are distributed in the silicon dioxide at a distance of 5-6 nm from the crystalline Si substrate. The charge storage effect is evident from C-V characteristics on MOS capacitors, and the results indicate the injection of the electrons from the substrate. The flat-band voltage shift of about 1.2-1.8 V for V g sweeps of -5/+3 V is achieved. It is shown that the leakage current density depends mostly upon the oxidation conditions, and for both types of metal NCs (Au and PtSi), it was measured to be around 10-8 A/cm2. The reproducibility and the precision of the proposed fabrication technique (PLD and thermal treatment) to produce a 2 D array of well-separated nanocrystals in a SiO2 layer suggest that this method can be applied for the fabrication of functional MOS structures.
pulsed laser deposition
Rutherford backscattering spectrometry
transmission electron microscopy
We would like to acknowledge the help received from A. Orekhov (Institute of Crystallography, RAS) for high resolution TEM analysis.
This study is a part of the Belarusian Scientific Research Program "Electronics" and was funded also by the Belorussian and Russian Foundations for Fundamental Research (projects T08P-184/90023).
- Kwon YH, Park CJ, Lee WC, Fu DJ, Shon Y, Kang TW, Hong CY, Cho HY, Wang KL: Memory effects related to deep levels in metal-oxide-semiconductor structure with nanocristalline Si. Appl Phys Lett 2002, 80: 2502. 10.1063/1.1467617View Article
- Tiwari S, Rana F, Hanafi H, Hartstein A, Crabbe EF, Chan K: A silicon nanocrystals based memory. Appl Phys Lett 1996, 68: 1377. 10.1063/1.116085View Article
- Tiwari S, Rana F, Chan K, Shi L, Hanafi H: Single charge and confinement effect in nanocrystal memories. Appl Phys Lett 1996, 69: 1232. 10.1063/1.117421View Article
- Normand P, Kapetanakis E, Dimitrakis P, Tsoukalas D, Beltsios K, Cherkasin N, Bonafos C, Benassayag G, Coffin H, Claverie A, Soncini V, Agarwai A, Ameen A: Effect of annealing enviroment on the memory properties of thin oxides with embedded Si nanocrystals obtained by low-energy ion-beam synthesis. Appl Phys Lett 2003, 83: 168. 10.1063/1.1588378View Article
- Beyer V, von Borany J: Elemental redistribution and Ge loss during ion-beam synthesis of Ge nanocrystals in SiO2 films. Phys Rev B 2008, 77: 014107. 10.1103/PhysRevB.77.014107View Article
- Baron T, Pelissier B, Perniola L, Mazen F, Hartman JM, Rolland G: Chemical vapor deposition of Ge nanocrystals on SiO2. Appl Phys Lett 2003, 83: 1444. 10.1063/1.1604471View Article
- Choi WK, Chim WK, Heng CL, Teo LW, Ho V, Ng V, Antoniadis DA, Fitzgerald EA: Observation of memory effect in Germanium nanocrystals enbedded in an amorphous silicon oxide matrix of a metal-oxide-semiconductor structure. Appl Phys Lett 2002, 80: 2014. 10.1063/1.1459760View Article
- Kanjilal A, Hansen JL, Gaiduk P, Larsen AN, Cherkashin N, Claverie A, Normand P, Kapelanakis E, Skaratos D, Tsoukalas D: Structural and electrical properties of silicon dioxide layers with embedded Germanium nanocrystals grown by molecular beam epitaxy. Appl Phys Lett 2003, 82: 1212. 10.1063/1.1555709View Article
- Kanjilal A, Hansen JL, Gaiduk P, Larsen AN, Normand P, Dimitrakis P, Tsoukalas D, Cherkashin N, Claverie A: Size and aerial density distributions of Ge nanocrystals in a SiO2 layer produced by molecular beam epitaxy and rapid thermal processing. Appl Phys A 2005, 81: 363. 10.1007/s00339-004-2924-3View Article
- Novikau AG, Gaiduk PI, Pshenichnij EN, Nalivaijko OYu, Malishev VS, Plebanovich VI: Nanocrystal floating gate produced by CVD and thermal processing. Proceedings of the ICMNE, Moscow, Zvenigorod, Russia 2007, 0000: O3-O2.
- Zenkevich AV, Lebedinskii YuYu, Timofeyev AA, Isayev IA, Tronin VN: Formation of ultrathin nanocomposite SiO2:nc-Au structure by pulsed laser deposition. Appl Surf Sci 2009, 255: 5355. 10.1016/j.apsusc.2008.08.041View Article
- Atwater HA, Polman A: Plasmonics for improved photovoltaic devices. Nat Mater 2010, 9: 205. 10.1038/nmat2629View Article
- Computer Graphic Service[http://www.genplot.com]
- Maksimova K, Matveev Yu, Zenkevich A, Nevolin V, Novikov A, Gaiduk P, Orekhov A: Investigation of nanocomposite SiO2:Me structures, formed by metal segregation during thermal oxidation of Si:Me alloy layers. Perspektivnye Materialy 2010, 2: 33. (in Russian) (in Russian)
- Tan Z, Samanta SK, Yoo WJ, Lee S: Self-assembly of Ni nanocrystals on HfO2 and N-assisted Ni confinement for nonvolatile memory application. Appl Phys Lett 2005, 86: 013107. 10.1063/1.1846952View Article
This article is published under license to BioMed Central Ltd. This is an Open Access article distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/2.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.