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Báo cáo hóa học: " Memory properties and charge effect study in Si nanocrystals by scanning capacitance microscopy and spectroscopy"

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  1. Lin et al. Nanoscale Research Letters 2011, 6:163 http://www.nanoscalereslett.com/content/6/1/163 NANO EXPRESS Open Access Memory properties and charge effect study in Si nanocrystals by scanning capacitance microscopy and spectroscopy Zhen Lin1*, Georges Bremond1†, Franck Bassani2† Abstract In this letter, isolated Si nanocrystal has been formed by dewetting process with a thin silicon dioxide layer on top. Scanning capacitance microscopy and spectroscopy were used to study the memory properties and charge effect in the Si nanocrystal in ambient temperature. The retention time of trapped charges injected by different direct current (DC) bias were evaluated and compared. By ramp process, strong hysteresis window was observed. The DC spectra curve shift direction and distance was observed differently for quantitative measurements. Holes or electrons can be separately injected into these Si-ncs and the capacitance changes caused by these trapped charges can be easily detected by scanning capacitance microscopy/spectroscopy at the nanometer scale. This study is very useful for nanocrystal charge trap memory application. IBM, 1982 (Nobel Prize awards in 1986), it has become Recently, the self-assembled silicon nanocrystals (Si-ncs) a powerful high-spatial-resolution tool for nanoscale that are formed within ultrathin SiO2 layer are consid- semiconductor analysis or characterization comparing to ered to be a promising replacement of this conventional several conventional methods for such as x-ray, nuclear, floating gate [1,2]. These isolated Si-ncs embedded in electron and ion beam, optical and infrared and chemi- between a tunnel and a top dielectric layer serve as the cal technique. It can provide simultaneous topography charge storage nodes and exhibit many physical proper- and various physical feature images with some addi- ties even at room temperature such as Coulomb block- tional electrical applications such as scanning capaci- ade [3], single-electron transfer [4] and quantization tance microscopy (SCM) [6,7], electrostatic force charges effect [5] which differ from bulk crystals. It can microscopy (EFM) [8], scanning resistance microscopy reduce the problem of charge loss encountered in con- [9] and Kelvin probe force microscopy [10]. In amount ventional memories, cause thinner injection oxides and of these techniques, SCM became one of the most use- hence smaller operating voltages, better endurance and ful methods for the capacitance characterization of faster write/erase speeds. So, the characterisation and semiconductor as its non-destructive detection of varies understanding of its charging mechanism in such nanos- electrical properties with high resolution such as dopant tructure is of prime importance. profiling variation [11], silicon p-n junction [12] and Although the conventional I-V and C-V characteriza- carrier injection [13], etc. tion methods for memory application provide a vast In this letter, scanning capacitance microscopy and amount of macro information, these methods lack the spectroscopy (SCS) were used to study the memory ability of discriminating structural and material proper- properties and charge effect of the Si-ncs materials in ties on a nanometer scale. Since atomic force micro- ambient temperature. scopy (AFM) was invented by Binning and Rohrer in Figure 1 shows the formation of these isolated Si-ncs. First, a 4-nm-thick thermal oxide was grown as the tun- * Correspondence: zhen.lin@insa-lyon.fr nelling oxide on an amorphous Si substrate. Subse- † Contributed equally 1 quently, Si layer was deposited by molecular beam Institut des Nanotechnologies de Lyon, UMR 5270, Institut National des Sciences Appliquées de Lyon, Université de Lyon, Bât. Blaise Pascal, 20, epitaxy over a very thin SiO2 layer, 5 nm in thickness, at avenue Albert Einstein - 69621 Villeurbanne Cedex, France ambient temperature and was thermally annealed at Full list of author information is available at the end of the article © 2011 Lin 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.
  2. Lin et al. Nanoscale Research Letters 2011, 6:163 Page 2 of 5 http://www.nanoscalereslett.com/content/6/1/163 Figure 1 The formation of isolated Si-ncs. applied, which is the same as Ge nanocrystals. Too high 750°C for 20 min under ultrahigh vacuum. The dewet- DC bias amplitude, such as up to 2 V, will make the ting process leads to the formation of isolated Si nano- crystals having an average density of 4 × 1010 cm-2. SCM signal disappeared. Figure 3b illustrates this varia- tion in function to the DC bias. The higher the DC bias Veeco Digital Instruments 3100 Dimensions AFM amplitude, the stronger SCM signal intensity was. How- employing a Nanoscope V controller was used to con- ever, the lower the contrast between the Si-ncs and duct SCM and SCS measurements. The conductive tip dielectric layer was. Positive or negative modulation cor- that was selected was commercial Arrow-EFM PtIr coat- responds to different SCM phase. The best resolution ing tip. It has an average tip radius of less than 10 nm, and the best signal to noise ratio which correspond to cantilever spring constant: 2.8 N/m and resonance fre- the highest contrast between Si-ncs and dielectric layer quency: 75 kHz. SCM images were taken with a fixed in the image was obtained near -0.5 and 0.5 V with the bias frequency of 50 kHz, SCM lock-in phase of 90°and scan rate of 0.5 Hz. capacitance sensor frequency of 910 MHz. The ampli- AC bias was also investigated by fixing the DC bias to tude of direct current (DC)/direct voltage signal is 0.5 V which is one of the best DC bias as we mentioned strongly dependent on the modulation voltages and the above. The SCM line scan image with different AC bias magnitude of capacitance variation is generally a non- and its variation was shown in Figure 4. The contrast linear function of the carrier concentration. Figure 2 between the Si-ncs and dielectric layer changed with AC shows the topography and SCM image. The contrast bias. The higher the AC bias, the stronger the SCM sig- between Si-nc and the oxide layer was clear in SCM nal intensity was. However, too high AC voltage can image which indicates that the Si-nc has different capa- induce charge injection in the sample which will create citance from the oxide layer. parasitic capacitance and high noise. Here, 2 V AC bias In order to investigate the effects of DC bias and alter- was fixed during the scan. nating current (AC) bias to the SCM signal, the slows- Charge injection was done by separately applying (0.5, can was disabled and a typical line scan was performed. 1.0, 2.0, and 3.0 V) to the tip during the contact SCM In Figure 3a, the VAC bias was fixed to 2,500 mV. The scan. Then DC bias was set back to -0.5 V which was SCM image and signal variation with DC bias is shown the best as we chose for our signal. As the SCM signal in Figure 3a. Different DC bias during the scan can is dependent on the quantity of injected charges, it was cause different SCM signal. The best signal/noise ratio monitored for charge retention time study. The non- and highest contrast occurred when -1 or 0.5 V was Figure 2 Si nanocrystal images. (a) Topography (b) SCM data image.
  3. Lin et al. Nanoscale Research Letters 2011, 6:163 Page 3 of 5 http://www.nanoscalereslett.com/content/6/1/163 Figure 3 SCM image (a) and signal (b) versus different DC bias. Figure 5 Charge and discharge with different DC bias. linear function between the retention time and the DC bias is shown in Figure 5. The higher the DC bias (char- the negative charging with respect to the same DC char- ging voltage) was, the longer its discharge time was, ging intensity. When charge injection was done by more than 7 V, the charging process can’t be detected in sev- which means more carriers were injected into the Si-nc. Holes are much easier to be injected than electrons as eral minutes. This indicates that the charges were the retention time of positive charging was longer than trapped by the Si-nc which made the retention time much longer. Ramp processes between -2 and +2 V were done by SCS separately on and outside an isolated Si-nc without charge injection. Strong hysteresis window was observed on the isolated Si-nc. But outside the dot, this effect was too weak (see in Figure 6). Furthermore, SCS was used to quantitatively investigate trapped charge effect inside the isolated Si-nc. From the SCS signals, the curve shift direction and distance were observed differently by applying a DC bias of -10 or +10 V to the tip during charging (see in Figure 7). There is a shift of 0.91 V by +10-V charge while -0.74 V shift by -10-V charge. This relates to the fact that different type of carriers can be injected into these Si-ncs and the capacitance changes caused by these trapped charges can be easily detected by SCM at the nanometer scale. It also verified the pre- vious conclusion that holes are much easier to be injected and trapped than electrons. In this letter, Si-ncs were formed on top of a ther- mally grown silicon dioxide layer. SCM and SCS were used to study the memory properties and charge effect on the Si-ncs in ambient temperature. Applying DC bias to the conductive tip, charges were injected into the Figure 4 SCM image (a) and signal (b) versus different AC bias. Si-ncs which was recorded by the SCM images. The
  4. Lin et al. Nanoscale Research Letters 2011, 6:163 Page 4 of 5 http://www.nanoscalereslett.com/content/6/1/163 Figure 6 Ramp process for hysteresis window by SCS. Figure 7 SCS curve shift after charge injection by +10 and -10 V.
  5. Lin et al. Nanoscale Research Letters 2011, 6:163 Page 5 of 5 http://www.nanoscalereslett.com/content/6/1/163 retention time of these trapped charges injected by dif- 12. Kopanski JJ, Marchiando JF, Lowney JR: Scanning capacitance microscopy measurements and modeling: Progress towards dopant profiling of ferent DC bias were evaluated and compared. By ramp silicon. J Vac Sci Technol B 1996, 14(l):242. process, strong hysteresis window was observed from 13. Hong JW, Shin SM, Kang CJ, Kuk Y, Khim ZG, Park , Sang-Il : Local charge the SCS signal. Furthermore, the SCS curve shift direc- trapping and detection of trapped charge by scanning capacitance microscope in the SiO2/Si system. Applied Physics Letters 1999, 75(12):1760. tion and distance were observed differently for quantita- tive measurements. This relates to the fact that holes or doi:10.1186/1556-276X-6-163 Cite this article as: Lin et al.: Memory properties and charge effect electrons can be separately injected into these Si-ncs study in Si nanocrystals by scanning capacitance microscopy and and the capacitance changes caused by these trapped spectroscopy. Nanoscale Research Letters 2011 6:163. charges could be easily detected by SCM/SCS at the nanometer scale. Acknowledgements Thanks X.Y. Ma for her helpful suggestions and Armel Descamps-Mandine from the CLYM platform facilities for his help and fruitful discussions on AFM measurements. Author details 1 Institut des Nanotechnologies de Lyon, UMR 5270, Institut National des Sciences Appliquées de Lyon, Université de Lyon, Bât. Blaise Pascal, 20, avenue Albert Einstein - 69621 Villeurbanne Cedex, France 2Institut Matériaux Microélectronique Nanosciences de Provence, UMR CNRS 6242, Avenue Escadrille Normandie-Niemen-Case 142, F-13397 Marseille Cedex 20, France Authors’ contributions ZL carried out the SCM and SCS experiment, studied these results and drafted the manuscript. GB participate the study of experiment results and manuscript writing. FB conducted the sample fabrication and the discussion. All authors read and approved the final manuscript. Competing interests The authors declare that they have no competing interests. Received: 19 September 2010 Accepted: 22 February 2011 Published: 22 February 2011 References 1. Blauwe JD: Nanocrystal nonvolatile memory devices. IEEE Transaction on Nanotechnology 2002, 1:72-77. 2. Tiwari S, Rana F, Chan K, Hanafi H, Wei C, Buchanan D: Volatile and non- volatile memories in silicon with nano-crystal storage. IEEE Int Electron Devices Meeting Tech Dig 1995, 521-524. 3. Gacem K, EI Hdiy A, Troyon M, Berbezier I, Szkutnik PD, Karmous A, Ronda A: Memory and Coulomb blockade effects in germanium nanocrystals embedded in amorphous silicon on silicon dioxide. J Appl Phys 2007, 102:093704. 4. Howell SW, Janes DB: Time evolution studies of the electrostatic surface potential of low-temperature-grown GaAs using electrostatic force microscopy. J Appl Phys 2005, 97:043703. 5. Thirstrup C, Sakurai M, Stokbro K, Aono M: Visible light emission from atomic scale patterns fabricated by the scanning tunneling microscope. Phys Rev Lett 1999, 82:1241. 6. Matey JR, Blanc J: Scanning capacitance microscopy. Journal of Applied Physics 1985, 57(5):1437-1444. 7. Barrett RC, Quate CF: Charge Storage in a Nitride-Oxide-Silicon Medium Submit your manuscript to a by Scanning Capacitance Microscopy. J Appl Phys 1991, 70:2725. journal and benefit from: 8. Lambert J, Guthmann C, Saint-Jean M: Relationship between charge distribution and its image by electrostatic force microscopy. J Appl Phys 7 Convenient online submission 2003, 93:5369. 7 Rigorous peer review 9. Shafai C, Thomson DJ, Simard-Normandin M, Mattiusi G, Scanlon PJ: Delineation of semiconductor doping by scanning resistance 7 Immediate publication on acceptance microscopy. Appl Phys Lelt 1994, 64. 7 Open access: articles freely available online 10. Henning AK, Hochwitz T, Slinkman J, Never J, Hoffman S, Kaszuba P, 7 High visibility within the field Daghlin C: Two-dimensional surface dopant profiling in silicon using 7 Retaining the copyright to your article scanning Kelvin probe microscopy. J Appl Phys 1995, 77:1888. 11. Huang Y, Williams CC, Slinkman J: Quantitative two-dimensional dopant profile measurement and inverse modeling by scanning capacitance Submit your next manuscript at 7 springeropen.com microscopy. Appl Phys Lett 1995, 66:344.
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