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Báo cáo hóa học: " Properties of silicon dioxide layers with embedded metal nanocrystals produced by oxidation of Si:Me mixture"

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Novikau et al. Nanoscale Research Letters 2011, 6:148 http://www.nanoscalereslett.com/content/6/1/148 NANO EXPRESS Open Access Properties of silicon dioxide layers with embedded metal nanocrystals produced by oxidation of Si:Me mixture Andrei Novikau1*, Peter Gaiduk1, Ksenia Maksimova2, Andrei Zenkevich2 Abstract 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. Introduction Si or Ge NCs was reported in [4,6]. However, the implantation of Ge at the silicon-tunnel oxide interface During the last decade, much attention has been focused creates trap sites and results in the degradation of the on the investigation of semiconductor and metallic device performance [4]. The growth technique using nanocrystals (NCs) or nanoclusters embedded in dielec- MBE deposition of 0.7-1 nm thick Ge layer followed by tric matrices. The interest is motivated by possible rapid thermal processing was implemented in [8,9]. An applications of such nanocomposite structures. Particu- alternative method for Ge NCs production [10] consists larly, semiconductor or metal NCs embedded in SiO2 of the following steps: low pressure chemical vapor dielectric layer of a metal-oxide-semiconductor field- deposition of thin Si-Ge layer, thermal wet or dry oxida- effect transistor may replace SiNx floating gate in con- tion, and thermal treatment in an inert ambient (reduc- ventional Flash memory devices, allowing for thinner tion). Recently, a method to form an ultrathin injection oxides, and subsequently, smaller operating nanocomposite SiO2:NC-Me layers at room temperature voltages, longer retention time, and faster write/erase speeds [1-3]. The performance of such memory struc- by combining the deposition of Si:Me mixed layer on ture strongly depends on the characteristics of the NCs the pre-oxidized Si substrate and its further oxidation in arrays, such as their size, shape, spatial distribution, the glow discharge oxygen plasma was proposed [11]. electronic band alignment. In this article, a similar approach was used to produce Several approaches have been recently tested for the thin SiO2 layers with an embedded layer of metal NCs. formation of NCs in dielectric layers. Among those, self- Au and Pt were chosen as metal components in Si:Me assembling of NCs in dielectric layers fabricated by the mixtures since both metals are believed to catalyze Si low-energy ion implantation and different deposition oxidation thus reducing the processing temperature, techniques has been studied by several groups [4-7]. A while neither Au nor Pt form stable oxides. Both Pt and strong memory effect in MOS devices using oxides with Au embedded as NCs in dielectric matrix are attractive materials in plasmonics [12]. In addition, both metals have much higher electron work functions compared to * Correspondence: andrei.novikau.by@gmail.com semiconductors, particularly, Ge, and it is interesting to 1 Belarusian State University, 4 prosp. Nezavisimosti, 220030, Minsk, Belarus investigate the effect of the NC work function on the Full list of author information is available at the end of the article © 2011 Novikau et al; licensee Springer. 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.
Novikau et al. Nanoscale Research Letters 2011, 6:148 Page 2 of 6 http://www.nanoscalereslett.com/content/6/1/148 electrical properties of the MOS stack with embedded oxidation. The experimental spectra were analyzed using NCs. As the first step, a thin Si:Me layer with the pre- the RUMP software [13]. The structural quality and the cisely pre-defined composition was grown by pulsed phase composition were analyzed using the TEM in laser deposition (PLD) technique. The oxidation of Si: both plain-view and cross-sectional geometries using a Philips CM20 instrument operating at U = 200 kV. Me mixture was expected to result in the segregation of the noble metal in NCs distributed in the SiO2 matrix. MOS capacitors with In electrodes were fabricated, and the high-frequency C-V measurements were carried out By means of analyzing the Si(Ox):Me elemental depth using a serial HP4156B instrument. distributions as a function of the annealing temperature and/or time, we attempted to investigate the kinetics of Results and discussion the composite structure formation. This information was supplemented by microstructural transmission elec- The typical RBS spectra from the as-grown and ther- tron microscopy (TEM) analysis and further–by electri- mally treated Si:Me/SiO2 /Si samples are presented in cal measurements on metal/SiO2:Me-NC/Si capacitors. Figure 2. The RBS spectra show that the thickness of as-deposited Si:Au layers is about 20 nm. The metal Experimental concentration in the deposited layers is in the range 2.5- N-type Si(001) wafers were used as substrates. The uni- 4.5%. The shift of both Au and Pt peaks to the lower energies upon thermal oxidation evidencing the pile up form SiO2 layer 6 nm in thickness (tunnel oxide) was of metal atoms at the SiO 2 /Si interface is clearly first grown in a dry oxygen ambiance. An amorphous Si: Me (Me = Au, Pt) layer 20 nm in thickness was then observed in RBS spectra. The observed evolution of Pt deposited by PLD at room temperature. The computer- and Au concentration profiles indicates the complete ized ultra-high vacuum (base pressure P = 10 -6 Pa) rejection of Me atoms from the oxide during thermal home-made PLD setup employing YAG:Nd laser ( l = oxidation of a-Si:Me layer. The detailed analysis of RBS 1,064 μm) and operating in the Q-switched regime (τ = data (Figure 2) reveals that Au and Pt segregation 15 ns) at the variable output energies E = 50-200 mJ depends on the oxidation conditions. In particular, and the repetition rates ν = 5-50 Hz was employed to neither evaporation nor diffusion of Au or Pt in SiO2 ablate from the elemental Si and Me (Me = Au, Pt) tar- layer takes place during thermal oxidation in dry O 2 . gets. The pre-calculated composition of the Si:Me mix- On the contrary, oxidation at higher temperatures ture necessary to form the desired nanocomposite results in a strong loss (about 30%) of Me from the structure was provided by choosing the exact ratio of Si SiO2 layer, apparently due to evaporation and partial dif- vs. Me deposition pulses in a deposition cycle during fusion into the Si substrate. the Si:Me layer growth. The sandwiched Si:Me/SiO2/Si The results of the plain-view TEM investigations (pub- samples were further thermally oxidized in dry oxygen lished elsewhere [14]) correlate well with the RBS data. ambient. To exclude the coalescence of the segregating Figure 3a clearly shows the well-separated clusters metal NCs, the thermal budget should be minimized. embedded in the SiO2 layer formed after thermal treat- Therefore, to determine the minimal temperatures to ment. The average size and the areal density of the oxidize Si:Me mixtures at our conditions, the prelimin- observed NCs were estimated to be from 10 to 20 nm and 2 × 1010 cm-2, respectively. To elucidate the struc- ary experiments were performed. It is worth noting that the presence of a noble metal in Si:Me mixture is found tural properties of metal NCs, the HRTEM analysis was to significantly reduce the oxidation temperatures as performed. The results for SiO 2 :NC-Pt are shown in compared to pure Si. Thus, the chosen oxidation condi- Figure 3b. The bright-field TEM micrograph of the Si: tions were T = 640-725°C for 60-540 min. Finally, the Pt-alloyed sample oxidized at T = 640°C for 5 h reveals thermally oxidized structures were subjected to rapid dark-gray clusters scattered on a light gray SiO2 back- thermal annealing in dry nitrogen ambient at T = 900°C ground. Careful examination of the clusters structure for 30 s. The sequential processing steps are shown in performed using the direct resolution of crystallographic Figure 1. A reference SiO2/Si sample with no metal NCs planes and selected area electron diffraction patterns was prepared for comparison. analysis (not shown) evidences the formation of plati- The composition of and the metal depth distribution num monosilicide (PtSi) crystalline phase in NCs. In in the samples were measured using Rutherford back- addition, unoxidized silicon islands were also identified. scattering spectrometry (RBS) with a He+ beam at E = Similar results were also obtained for Si:Au samples although no evidence of Au silicide formation was 1.5 MeV. The spectra were taken simultaneously at two different scattering angles, θ = 10° and θ = 75°, with the found (not shown). A previous study [11] describing detailed in situ investigation by X-ray photoelectron former geometry being used to calculate the integral spectroscopy of the Au chemical state evolution during metal concentration in Si:Me, while the latter one to the oxidation of the similarly produced Si:Au mixture observe possible changes in the metal distribution upon
Novikau et al. Nanoscale Research Letters 2011, 6:148 Page 3 of 6 http://www.nanoscalereslett.com/content/6/1/148 *** Si+Au .. . O* *** . •.. * * O* •. • ~30 nm NC-Me (Me:Au, Pt) ** * Si+Au SiO2 SiO2 SiO2 ~10 nm 3 Si 1 Si 2 Si Si 4 Figure 1 The proposed procedure of the MOS stack formation including SiO2 layers with the embedded metal NCs. Energy, KeV A 500 600 700 800 900 1000 1100 1200 1300 1400 O Si:Pt 1200 as grown 0 oxidation 60 min at 725 C 1000 Normalized Yield Si Pt 800 600 400 200 0 200 250 300 350 400 450 500 Channel Energy, KeV B 500 600 700 800 900 1000 1100 1200 1300 1400 1200 Si:Au as grown O 0 oxidation 60 min at 725 C 1000 0 Normalized Yield oxidation 60 min at 650 C 800 Si Au 600 400 200 0 200 250 300 350 400 450 500 Channel Figure 2 RBS spectra from as grown and thermally oxidized Si:Me/SiO2/Si samples: (a) RBS spectra (E = 1.5 MeV, θ = 75°) from Si:Pt/SiO2/Si samples thermally oxidized at T = 725°C for 60 min in O2 followed by thermal annealing in N2 at T = 900°C for 30 s. as compared with as-grown structure; (b) Au peak in RBS spectra evidences strong Au segregation during Si oxidation process at different temperatures.
Novikau et al. Nanoscale Research Letters 2011, 6:148 Page 4 of 6 http://www.nanoscalereslett.com/content/6/1/148 Figure 3 Transmission electron microscopy analysis from a Si:Pt sample, oxidized at T = 640°C for 5 h in dry O2: bright-field plain- view (a) and high resolution (b) TEM images. Crystalline PtSi NCs exhibit a dark contrast on the gray background of the SiO2 layer. shift C-V curves in the direction of the stored negative indicated the formation of a metastable Au silicide dur- ing the room temperature deposition and its further charges, it is concluded that the charge trapping occurs decomposition to metallic Au upon oxidation. through the electron injection from the substrate into The self-assembling phenomenon of the formation of the oxide. No flat-band voltage shift was observed for metal and silicide NCs in SiO2 can be explained using the reference sample prepared with pure SiO2, oxidized at T = 850°C for 60 min in O2 ambient. It is therefore two mechanisms. A solubility of impurities in SiO2 is quite low, and therefore the structures obtained after concluded that the effect of charge storage is related to metal segregation and piling up between two SiO2 layers the NCs. (tunnel oxide and SiO2 capping layer) were transformed One of the major reasons for the loss of charge in the into the supersaturated solution. It is well known that floating gate structures is the leakage current. The mea- sured I-V curves (Figure 5) from Si:Au and Si:Pt samples under the thermal treatment the decomposition of supersaturated solution takes place eventually resulting oxidized in dry ambient reveal that the leakage current density can be reduced down to 10-8 A/cm 2. The low in the phase separation and the formation of the metal NCs in a dielectric (oxide) matrix. On the next stage, leakage currents achieved are explained by the high the Ostwald ripening of the formed NCs occurs. This quality of both tunneling and capping oxide formed by implies the diffusion of metal atoms from the valley dry thermal process compared with the deposited oxides regions of the islands toward their respective centers used in the alternative methods of MOS capacitor for- forming spherical nanocrystals to achieve greater mation [15]. It is found that the oxidation temperature volume-to-surface ratio. In our model, the initial NCs has also a strong effect on the leakage current, and are formed during the oxidation of the Si:Me layer. therefore the oxidation conditions should be optimized After the oxidation is completed, the sample is still kept for each type of embedded metal NCs. at elevated temperature facilitating the coalescence of Conclusion Me NCs. The effect of the oxidation temperature as well as the In this study, the authors have demonstrated the growth type of the embedded Me on the efficiency of the charge of thin SiO2 layers with embedded metal and metal sili- storage was studied by the high-frequency C-V measure- cide NCs by the combination of Si:Me mixture by PLD at ments. The hysteresis in C-V curves was found different room temperature and its thermal oxidation. By means for the structures containing Au and PtSi NCs (Figure 4). of this fabrication technique, it is possible to produce a The maximal value of the flat-band voltage shift U = 1.8 sheet of crystalline metal nanocrystals at any desirable V for the Vg sweep -5/+3 V was obtained for SiO2:NC- depth in the oxide. The metal segregation process during thermal oxidation results in the formation of a high areal Au based structures prepared by dry oxidation. On the density of crystalline Au and PtSi dots 10-20 nm in dia- contrary, in the case of SiO2:NC-PtSi, the maximal flat- band voltage shift was U = 1.2 V. By increasing V g meter which are distributed in the silicon dioxide at a distance of 5-6 nm from the crystalline Si substrate. The sweep up to 5 V, a gradual increase of the flat-band vol- charge storage effect is evident from C-V characteristics tage shift was achieved. Since high positive gate voltages
Novikau et al. Nanoscale Research Letters 2011, 6:148 Page 5 of 6 http://www.nanoscalereslett.com/content/6/1/148 SiPt SiAu 20 0 0 oxidation 9 h, 640 C oxidation 9 h, 640 30 0 0 oxidation 5 h, 640 C oxidation 5 h, 640 18 28 26 16 24 CSiO , pF 22 14 2 20 18 12 16 10 14 12 8 10 8 6 6 4 4 -5 -4 -3 -2 -1 0 1 2 3 -5 -4 -3 -2 -1 0 1 2 3 Gate voltage, V Figure 4 High-frequency C - V curves measured from Si:Au and Si:Pt samples, oxidized at T = 640°C for 5 and 9 h in dry O 2 , respectively. A gate voltage sweep from inversion to accumulation and from accumulation to inversion is shown on the figure by arrows. on MOS capacitors, and the results indicate the injection both types of metal NCs (Au and PtSi), it was measured to be around 10 -8 A/cm 2 . The reproducibility and the 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 precision of the proposed fabrication technique (PLD achieved. It is shown that the leakage current density and thermal treatment) to produce a 2 D array of well- depends mostly upon the oxidation conditions, and for separated nanocrystals in a SiO2 layer suggest that this 0 Oxidation at 640 C SiPt, 9 hours SiPt, 5 hours 2 Leakage current density, / m SiAu, 9 hours 1E-4 SiAu, 5 hours 1E-5 1E-6 1E-7 1E-8 Si/SiO2 structure with pure SiO2 1E-9 0 2 4 6 8 10 12 Gate voltage, V Figure 5 Leakage current vs. gate voltage characteristics obtained from the oxidized Si:Au and Si:Pt samples at T = 640°C. The I-V curve from the reference sample of pure SiO2 is shown for comparison.
Novikau et al. Nanoscale Research Letters 2011, 6:148 Page 6 of 6 http://www.nanoscalereslett.com/content/6/1/148 method can be applied for the fabrication of functional 10. Novikau AG, Gaiduk PI, Pshenichnij EN, Nalivaijko OYu, Malishev VS, Plebanovich VI: Nanocrystal floating gate produced by CVD and thermal MOS structures. processing. Proceedings of the ICMNE, Moscow, Zvenigorod, Russia 2007, 0000:O3-O2. 11. Zenkevich AV, Lebedinskii YuYu, Timofeyev AA, Isayev IA, Tronin VN: Abbreviations Formation of ultrathin nanocomposite SiO2:nc-Au structure by pulsed NCs: nanocrystals; PLD: pulsed laser deposition; RBS: Rutherford laser deposition. Appl Surf Sci 2009, 255:5355. backscattering spectrometry; TEM: transmission electron microscopy; MOS: 12. Atwater HA, Polman A: Plasmonics for improved photovoltaic devices. metal-oxide-semiconductor. Nat Mater 2010, 9:205. 13. Computer Graphic Service. [http://www.genplot.com]. Acknowledgements 14. Maksimova K, Matveev Yu, Zenkevich A, Nevolin V, Novikov A, Gaiduk P, We would like to acknowledge the help received from A. Orekhov (Institute Orekhov A: Investigation of nanocomposite SiO2:Me structures, formed of Crystallography, RAS) for high resolution TEM analysis. by metal segregation during thermal oxidation of Si:Me alloy layers. This study is a part of the Belarusian Scientific Research Program Perspektivnye Materialy 2010, 2:33, (in Russian). “Electronics” and was funded also by the Belorussian and Russian 15. Tan Z, Samanta SK, Yoo WJ, Lee S: Self-assembly of Ni nanocrystals on Foundations for Fundamental Research (projects T08P-184/90023). HfO2 and N-assisted Ni confinement for nonvolatile memory application. Appl Phys Lett 2005, 86:013107. Author details 1 Belarusian State University, 4 prosp. Nezavisimosti, 220030, Minsk, Belarus doi:10.1186/1556-276X-6-148 NRNU “Moscow Engineering Physics Institute”, 31 Kashirskoe shausse, 2 Cite this article as: Novikau et al.: Properties of silicon dioxide layers with embedded metal nanocrystals produced by oxidation of Si:Me 115409, Moscow, Russian Federation mixture. Nanoscale Research Letters 2011 6:148. Authors’ contributions AN participated in the RBS analysis and carried out the electrical characterization, participated in the design of the study and drafted the manuscript. KM carried out the pulsed laser deposition and experimental data analysis. PG conceived of the study, and participated in its design and coordination. AZ participated in the design of the study, coordinated TEM analysis and significantly contributed to the writing of manuscript. All authors read and approved the final manuscript. Competing interests The authors declare that they have no competing interests. Received: 21 September 2010 Accepted: 16 February 2011 Published: 16 February 2011 References 1. 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. 2. Tiwari S, Rana F, Hanafi H, Hartstein A, Crabbe EF, Chan K: A silicon nanocrystals based memory. Appl Phys Lett 1996, 68:1377. 3. Tiwari S, Rana F, Chan K, Shi L, Hanafi H: Single charge and confinement effect in nanocrystal memories. Appl Phys Lett 1996, 69:1232. 4. 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. 5. 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Appl Phys Lett 2003, 82:1212. 7 Open access: articles freely available online 9. Kanjilal A, Hansen JL, Gaiduk P, Larsen AN, Normand P, Dimitrakis P, 7 High visibility within the ﬁeld Tsoukalas D, Cherkashin N, Claverie A: Size and aerial density distributions 7 Retaining the copyright to your article of Ge nanocrystals in a SiO2 layer produced by molecular beam epitaxy and rapid thermal processing. Appl Phys A 2005, 81:363. Submit your next manuscript at 7 springeropen.com

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