Durrani, Z. A. K.

Khalafalla, M.

Mizuta, Hiroshi

Nakazato, K.

Uno, S.

English

Southampton (e-Prints Soton)

New Insights In Electron Emission From Porous Silicon Diodes  Shigeyasu Uno a) c), Z. A. K. Durrani b) c), M. Khalafalla b), Kazuo Nakazato a) c), and Hiroshi Mizuta a) c)  a) Hitachi Cambridge Laboratory, Hitachi Europe Ltd., Cavendish Laboratory, Madingley Road, Cambridge, CB3 0HE, United Kingdom Phone:  +44-1223-467944   Fax:  +44-1223-467942   E-mail:  drsuno@phy.cam.ac.uk  b) Microelectronics Research Centre, University of Cambridge Cavendish Laboratory, Madingley Road, Cambridge, CB3 0HE, United Kingdom  c) CREST JST (Japan Science and Technology) Shibuya TK Bldg., 3-13-11 Shibuya, Shibuya-ku, Tokyo 150-0002, Japan  Introduction Porous silicon (PS) diode exhibits high-energy electron emission as illustrated in Fig. 1 (a). This device is a potential candidate for cold cathodes in future flat panel displays due to low turn-on voltage and high current stability [1-3]. As the PS layer has tree-like network of many nanometer size silicon dots [2, 3], conduction mechanism in the PS layer is also of great interest. The emission energy distribution exhibits a peak, which shifts toward higher energy with increasing positive bias applied to the Au electrode [2, 3]. This behavior is depicted in Fig.1 (b). In this work, we first quantitatively analyze the dependence of the peak energy value on the voltage applied to the Au electrode. We then study the shape of the distribution and its dependence on the applied voltage. The existing models are discussed based on the analysis.  Analysis Figure 2 demonstrates that the peak energy (Epeak), as a function of the applied voltage (Vps) shows linear dependence with unity slope, which we call the ‘linear law’ hereafter. As illustrated in Fig. 3, Epeak can be written as, Epeak = eVps – WAu – Eloss,   (1) where WAu and Eloss denote the work function of Au and the energy difference between the Fermi level of the silicon substrate and the peak energy level, respectively. In general, Eloss is a function of Vps and depends on nature of electron transport in the PS layer. The linear law, however, indicates that Eloss is nearly independent of Vps, and is a virtually constant parameter of each individual device as shown in Fig. 4. Figure 5 compares the energy distribution of different electron emitters. Note that the energy dispersion of the PS diode (PSD) is much larger than those of the carbon nanotube emitter (CNT) and the conventional metal tip field emitter (TIP). This indicates that the PS diodes cannot be modeled as the simple conventional field emitter arrays. The emission energy distribution of MOS emitter (MOS) has similar shape as that of PS diode. However, the PS diode and MOS emitters have different dependence on Vps. Figure 6 shows the impact of Vps on the energy distribution of a PS diode. Note that the increasing Vps dramatically changes the distribution in the range of the smaller energy than Epeak (negative ∆ E), while it has minimal impact in the range of the larger energy (positive ∆ E). This behavior is quite different from that observed in the MOS emitters, as shown in the inset. This indicates that the simple MOS model also fails to explain the electron emission from PS diode.  Discussion  One of the existing models to address the electron emission from PS diode is the quasiballistic emission model [2, 3, 10]. This model assumes that the electron-phonon scattering is strongly suppressed in the silicon nanocrystallites in the PS layer. However, theoretical background of this assumption is still unclear, and any straightforward reason of the nearly constant Eloss cannot be found in this model. Another model regards the PS layer surface as the conventional field emission arrays [6]. This model, however, fails to explain the results shown in Fig. 5. In order to explain the electron emission from the PS diodes, new model has to be developed, which will be proposed in the presentation.  Summary The peak energy plotted as a function of applied voltage obeys the linear law, indicating that Eloss is nearly constant of each device. Increasing Vps dramatically changes the distribution in the range of smaller energy than Epeak, while it has minimal impact in larger energy range. Any existing model cannot explain these findings.  Acknowledgements The authors are indebted to Dr. A. Kojima and Prof. N. Koshida for their helpful discussions.  References [1] Koshida et al., Jpn. J. Appl. Phys. 34, p. L705 (1995) [2] Koshida et al., Appl. Surf. Sci., 146, p. 371 (1999) [3] Komoda et al., J.Vac. Sci. Technol. B, 17 p. 1076 (1999) [4] Koshida et al., Electro-chemical Society 201st Meeting, Philadelphia, United States, #463 (2002) [5] Nakajima et al., Appl. Phys. Lett. 81 p. 2472 (2002) [6] Mimura et al., Tech. Dig. 15th Int. Vacuum Microelectronics Conf. Lyon, France, PM18 (2002). [7] Brorson et al., J. Appl. Phys. 58 p. 1302 (1985) [8] Purcell et al., Appl. Phys. Lett. 67 p. 436 (1995) [9] Bonard et al., Appl. Phys. A69 p. 245 (1999)   [10] Kojima et al., Tech. Dig. SSDM 2002, “A Monte Carlo Simulation of Ballistic Transport in Nanocrystalline Silicon Diode”, (2002)  Fig. 1     (a) Schematic Illustration of the electron emission from porous silicon (PS) diode. Positive bias applied to the Au electrode (Vps) injects electrons into PS layer. Electrons are accelerated in the PS layer, and some high energy electrons overcome the work function of Au, being ejected into vacuum. (b) Emission energy distribution measured from vacuum level. Peak energy Epeak moves toward higher energy with increasing Vps.Fig. 3     Band Diagram under positive bias Vps. Oxide layers in PS are not shown for simplicity [2]. The peak energy level Epeak can be written as Epeak = eVps –W Au –E loss, where WAu and Eloss denote the work function of Au and the energy difference between the Fermi level of the n-type silicon substrate and the peak energy level, respectively .Fig. 2     Evolution of the peak energy with increasing applied voltage. Note that Epeak varies linearly with unity slope, which we call the ‘linear law’.Fig. 4     Values of Eloss extracted from data in Fig. 2 using the relation shown in Fig. 3. Note that Eloss is nearly constant for each device.Energy from vacuum levelN(E)VpsVpsn-SiAu(a) (b)PSEmissionEpeak0 5 10 15 20 25 30 35 40 05101520Epeak [eV]Slope 1.0 Koshida (Ref. 2) Komoda (Ref. 3) Koshida (Ref. 4) Nakajima (Ref. 5) Mimura (Ref. 6)Vps [V]0 5 10 15 20 25 30 35 40 05101520Vps [V]Eloss [eV] Koshida (Ref. 2) Komoda (Ref. 3) Koshida (Ref. 4) Nakajima (Ref. 5) Mimura (Ref. 6)n-Si Porous Silicon Au VacuumeVpsWAuEpeakElossEfEcEcEfVacuum LevelFig. 5     Comparison of energy distributions among various electron emitters. The horizontal axis is the energy measured from the peak energy (∆ E), and the vertical axis is normalized so that the number of electrons at the peak equals unity.Fig. 6     Impact of increasing Vps on the emission energy distribution of  PS diode. The inset is the same plot of MOS emitters. -6 -4 -2 0 2 4 600.20.40.60.81CNTTIPMOSPSD∆ E [eV]Normalized Number of Electrons [a. u.]PSD : PS diode (Ref. 2)MOS : MOS-type emitter (Ref. 7)TIP : Metal tip field emitter (Ref. 8)CNT: Carbon nanotube emitter (Ref. 9)-6 -4 -2 0 2 4 600.20.40.60.8111V 12V 15VVps=18VPSD (Ref. 3)∆ E [eV]Normalized Number of Electrons [a. u.]-2 0 2 4 600.51MOS (Ref. 7)9.9MV/cm6.54.0∆ E [eV]

New insights into electron emission from silicon nanocrystallites

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