SHIMAWAKI Hidetaka

嶋脇 秀隆

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SHIMAWAKI, Hidetaka

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嶋脇, 秀隆

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シマワキ, ヒデタカ

Hachinohe Institute of Technology Repository / 八戸工業大学学術リポジトリ

Nanocrystalline Silicon basedMetal-oxide-semiconductor Cathodes著者 SHIMAWAKI Hidetaka著者別名 嶋脇 秀隆journal orpublication titleThe bulletin of Reseach Institute forIn erdisciplinary Science, Hachinohe Instituteof Technologyvolume 7page range 1-4URL http://id.nii.ac.jp/1078/00002326/Creative Commons : 表示 - 非営利 - 改変禁止http://creativecommons.org/licenses/by-nc-nd/3.0/deed.jaNanocrystaline Silicon based Metal-oxide-semiconductor Cathodes Hidetaka SHIMAWAKI?Abstract Metal-oxide-semiconductor(MOS)cathodes with an array structure have been fabricated based on nanocrystal-line silicon covered with a thin oxide film prepared by a pulsed laser ablation technique and their emission properties have been investigated.The electron emission occurred at the gate voltage higher than the work function of the Au gate,and the emission efficiency reached about 2% at the gate voltage of 25 V.Although the current decreased slightly,it was relatively stable.We also observed the emission image on the phosphor screen and the emission divergence was estimated to be less than 3°.Key words:nanocrystaline silicon,electron emission,MOS,tunneling cathode 1. Introduction A metal-oxide-semiconductor(MOS)cathode is a promising candidate as a fine electron source for applications of vacuum nanoelectronic devices,because the cathode operates at low extraction volt-age,and produces uniform and highly directional emission.Furthermore,the cathode is insensitive to environment.However,the device is limited in emis-sion current by its low efficiency of less than 1% due to the strong phonon-electron scattering in both the oxide layer and the gate electrode during traveling through the conduction band of the oxide and the gate electrode.In fact,a considerable increase in emis-sion current has been measured and a large number of electrons have been detected at the energies lower than the original work function of the gate electrode by cesiation［1］.Lowering the work function of the gate electrode is a practical way for improving the emission current,i.e., the emission efficiency.Recently,the modified MOS cathodes have reported high emission efficiencies over 10%,where the oxide layer is replaced with semi-insulating layers such as non-doped Si,SiOx films,or nanocrystaline Si consis-ting of oxidized porous Si and poly-Si［2-5］.Although the mechanism is not stil understood,the use of films containing nanocrystaline structures taking the place of the oxide layer is a promising way to improve the performance on the MOS cathode.In a previous paper［6］,we have reported the emis-sion characteristics of MOS cathodes based on nano-crystaline Si(nc-Si)prepared by a pulsed laser ablation(PLA)technique.This technique has the advantage of fabricating nanocrystaline Si covered with a thin oxide layer in contrast to other fabrication methods.It was demonstrated electron emission at the gate voltage as low as the work function of the gate metal and the high emission efficiency of 4% by reducing the thickness of the Au gate metal.How-ever,the energy of emitted electrons was widely distributed and showed strong dependence on the applied gate voltage.A lot of nanoholes in the thin Au film were observed,suggesting emission includes electrons tunneled through thin metal and directly emitted from nanotips aligned under nanoholes obser-ved in metal.In this work,we investigate the emission character-istics of an nc-Si MOS cathode array for emission with a narrow spread of electron energy.2. Experiments Figures 1(a)and(b)show a schematic structure of an nc-Si planar cathode and its fabrication process,respectively.The device is a thin film diode struc-ture,which consists of nanocrystaline silicon(nc-Si)particles covered with an oxide film on an n-type silicon substrate as an electron source and a thin top metal electrode as an extraction gate.The emission area of the device is set an array pattern of 50μm-diametric circles in an area of 500μm in diameter.The nc-Si layer was deposited by a pulsed laser ablation technique using a Si disc target,which was irradiated with the fourth harmonic(λ＝266 nm)of Nd:YAG laser(Spectron,SL856)light with an energy of about 80 mJ/cm?,a pulse width of 13 ns and a repetition rate of 10 Hz.To create the tunneling― 1―平成21年1月7日受理?異分野融合科学研究所/大学院工学研究科電子電気・情報工学専攻・准教授barrier in the interface between nc-Si particles,sur-faces of nc-Si particles were oxidized by an oxygen radical beam exposure during deposition,which was generated by radio frequency discharge with the RF power range of 250 to 350 W at the pressure of 5×10??Pa.The substrate temperature was kept at 450°C during deposition.Although we could not estimate the thickness of the thin oxide layer covering the surface of nc-Si,the oxide layer wil play as the tunneling barrier in the interface between nc-Si parti-cles.After rapid thermal oxidation at 700°C for 1 hour was adopted for forming a thin oxide layer near the top surface,a thin Au layer for the gate electrode was formed by sputter deposition.Finaly,Al elec-trodes were evaporated on a part of the Au film to reduce the resistance and the backside of the sub-strate to form an ohmic contact.We measured the emission characteristics and the energy distribution of emitted electrons by the experi-mental setup using a Faraday cup analyzer with double-meshes and a colector ilustrated in Fig.2.In the experiments,we defined the gate current measured at the gate electrode as a diode current,and the anode current measured at the first mesh as an emission current.We also observed the emission image on a phosphor screen.3. Results and discussion Figure 3 shows typical diode and emission currents of the cathode array,and emission efficiency,which are defined as the ratio of the emission current to the total current.Electron emission occurs at nearly 6 V,八戸工業大学異分野融合研究所紀要 第7巻― 2―Fig.1 Schematics of nc-Si MOS cathode array(a),and its fabrication process(b).Fig.2 Measurement setup of emission characteristics.which is as low as the work function of the Au gate electrode and the emission current rises rapidly with increasing gate voltage.Current saturation,which was observed previously［6］,does not occur at high fields.The emission efficiency reaches about 2% at the gate voltage of 25 V.The time dependence of the emission current for a constant gate voltage is shown in Fig.4 at the pressure of 5×10??Pa.Although the current decreases slightly,it is relatively stable.We have measured the energy distribution of emit-ted electrons from the cathode array using a Faraday cup analyzer with double-meshes and a colector.Figure 5 shows the energy spectra of emitted elec-trons from the cathode array at several gate voltages,which are normalized by each total emission current.The zero energy in the abscissa indicates the vacuum level of the Au gate electrode.FWHM of the spec-trum becomes broader and maximum energy moves to higher value with increasing gate voltage.How-ever,the change is not significant compared with that for the cathode without the arrayed structure report-ed previously［6］and the threshold energy of emitted electrons remains almost the same at near the work function of the gate metal,in contrast to the previous results［6］.This indicates that the energy distribution of the emitted electrons does not increase even with increasing gate voltage,because the field emission component through nanoholes in the metal gate decreases due to the arrayed structure.Figure 6 shows the emission image observed on the phosphor screen.The screen was located at 40 mm above the cathode array and biased at the accelera-tion voltage of 5 kV.The emission divergence is estimated to be about 3°.This value is smaler than that of the cathode without the arrayed structure,suggesting that uniformity of the field in the emission Nanocrystaline Silicon based Metal-oxide-semiconductor Cathodes― 3―Fig.3 Emission characteristics of an nc-Si MOS cathode array.Fig.4 Time dependence of the emission current of an nc-Si MOS cathode array.Fig.5 Energy spectra of emitted electrons from an nc-Si MOS cathode array at various gate voltage.Fig.6 Emission image from the nc-Si MOS cathode array.area is improved for the arrayed cathode.4. Conclusion We fabricated nanocrystaline silicon based MOS cathode arrays with thin gate metals,and examined their emission characteristics and energy distributions to improve the performance of the MOS cathode and understand the emission mechanism of nanocrystal-line based cathodes.The emission efficiency was improved by reducing the thickness of the gate metal.The use of films containing nanocrystaline structures taking the place of the oxide layer is a promising way to improve the performance on the MOS cathode.Acknowledgements A part of this work was carries out in the Labora-tory for Nanoelectronics and Spintronics of the Research Institute of Electrical Communication,To-hoku University.This work was supported in part by a Grand-in-Aid for Scientific Research from Japan Society for the Promotion of Science in Japan and the nationwide cooperative research projects organizing by RICE,Tohoku University.References［1］ H.Mimura,Y.Neo,H.Shimawaki,Y.Abe,K.Ta-hara,and K.Yokoo,Appl.Phys.Lett.88,123514(2006).［2］ K.Yokoo,G.Koshita,S.Hanzawa,Y.Abe,and Y.Neo,J.Vac.Sci.Technol.B14,2096(1996).［3］ X.Sheng,H.Koyama,and N.Koshida,J.Vac.Sci.Technol.B16,793(1998).［4］ N.Negishi,T.Chuman,S.Iwasaki,T.Yoshikawa,H.Ito,and K.Ogasawara,Jpn.J.Appl.Phys.,Part 2,36,L939(1997).［5］ A.Govyadinov, T.Novet, D.Pidwerbecki, S.Ramamoorthi,J.Smith,J.Chen,C.Otis,D.Neiman,and P.Benning,J.Vac.Sci.Technol.B23,853(2005).［6］ H.Shimawaki,Y.Neo,H.Mimura,K.Murakami,F.Wakaya,and M.Takai,J.Vac.Sci.Technol.B26,864(2008).［7］ H.Shimawaki,Y.Neo,and H.Mimura,J.Vac.Sci.Technol.B24,971(2006).― 4―八戸工業大学異分野融合研究所紀要 第7巻

Nanocrystalline Silicon based Metal-oxide-semiconductor Cathodes

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Nanocrystalline Silicon based Metal-oxide-semiconductor Cathodes

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