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Physica E 40 (2008) 2508–2512 A new mechanism for modulation of Schottky barrier heights on silicon nanowires J. Piscator à , O. Engstro ¨ m Department of Microtechnology and Nanoscience, MC2, Chalmers University of Technology, SE-412 96 Go ¨ teborg, Sweden Available online 10 August 2007 Abstract For nanowires with Schottky barriers on the end surfaces, charges on the walls of the wire are close enough to the metal–semiconductor interface to influence the Schottky barrier. This is similar to an effect in planar structures, where impurities with energy levels below the Fermi level in the bulk of the substrate material will change charge state in the depletion region of a metal–semiconductor structure if the Schottky barrier is high enough to bring the impurity energy level above the Fermi level. The mechanism for barrier modulation is the same in both cases and occurs in nanowires as a result of the wire geometry. r 2007 Elsevier B.V. All rights reserved. PACS: 81.07.Lk, 61.72.Ày, 73.30.+y Keywords: Silicon nanowires; Doping; Schottky contact; Oxide charge 1. Introduction In the historical discussion of Schottky barriers, most of the focus has been on planar structures and the occurrence of dipole potentials at the metal–semiconductor (MS) interface for modulating the barrier heights between the extreme cases set by early predictions by Sch ottky and Mott on one hand and of Bardeen on the other [1–3]. These treatments explain the lowering of the Schottky barrier as a phenomenon taking place in intimate contact with the interface between the metal and the semiconductor [4] or as a result of electron wave function penetration from the metal into the semiconductor [5]. A second possibility to influence an effective barrier height is to introduce a high doping in the semiconductor, thus thinning the barrier and allowing for tunneling [6]. In silicon technology, the MS structure has received an increased interest for coming transistor generations. In order to reduce source/drain resistance and overcome the ‘‘short channel effect’’ for gate lengths in the 20 nm range and below, MS structures are considered as replacements of traditional p–n junctions as source and drain contacts [7]. In the search for methods to lower effe ctive Schottky barrier heights, ideas of segregat- ing dopants like As and B, with shallow energy levels close to the metal have been demonstrated [8]. This is the same method as used for creating ohmic contacts by n + and p + doping, differing only by the depth of the latter. In the present paper, we demonstrate two alternative possibilities for introducing charge in the vicinity of the metal. For planar structures a similar possibility exists as for the ohmic contact solution described above by doping with deep impurities which contributes their charge only close to the metal. For wires with dimensions in the nanometer range, surface charges can be used to influence barrier properties due to their specific geometry. In the first case, we demonstrate, by using experimental data from literature, how a deep double donor impurity can serve as a barrier modulator, in the second case our own experi- mental data on silicon nanowires point out the effect [9]. 2. Planar structures modified by bulk doping with deep impurities Recently, Schottky barrier modulation was demon- strated for NiSi contacts on planar silicon surfaces, where the semiconductor was doped with sulphur [10]. This ARTICLE IN P RESS www.elsevier.com/locate/physe 1386-9477/$ - see front matter r 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.physe.2007.07.018 à Corresponding author. Tel.: +46 31 7721862; fax: +46 31 7723622. E-mail address: johan.piscator@chalmers.se (J. Piscator). element creates a double donor in silicon with a ground state for the first capture d electron at an energy of about 0.55 eV from the conduction band edge. Subsequently, the second electron is captured into an energy level at about 0.30 eV [11]. Considering a shallow phosphorus doping of about 10 15 cm À3 as used in the experiment of Ref. [10], this corresponds to a Fermi level position at 0.28 eV from the conduction band. Therefore, in the bulk of an n-type semiconductor, the sulphur levels are only slightly ionized at room temperature and the resistivity of the sample is mainly unchanged. However, the NiSi metal contact has a barrier height of about 0.65 eV on n-type silicon, which means that the upper sulphur level passes the Fermi level in the depletion region, emits the captured electron and becomes positively charged as demo nstrated by the band diagram in Fig. 1. Close to the MS interface a small contribution from the doubly charged energy level at 0.55 eV may occur. Depending on the profile of the sulphur concentration close to the MS interface, the charge may give rise to a very thin energy barrier, thus enabling tunneling at the barrier tip and a lowering of the effective Schottky barrier height. Assuming a sulphur dopant distribution with two exponential tails as obtained from ion implantation and segregation close to the NiSi/Si interface as shown in Ref. [10], the charge occurring in the depletion region can be expressed as Q ¼ qN 01 exp À x x 01 þ qN 02 exp À x x 02 þ qN D x, (1) where q is the electron charge, x is the length coordinate into the semiconductor volume perpendicular to and with origin at the MS interface, N 01 and N 02 are the surface concentrations of the two sulphur profiles, x 01 and x 02 are their decay lengths and N D is the shallow doping concentration in the semiconductor bulk. Using Eq. (1) in Poisson’s equation, and the materials parameters from Ref. [10] as mentioned in Fig. 2, the shape of the conduction band is demonstrated with and without sulphur doping. One notices a considerable decrease of the barrier width, facilitating tunneling of electrons and a decreasing effective Schottky barrier height. Fig. 3 demonstrates the barrier height at 1 nm from the MS interface as a function of the sulphur surface concentration taking this depth as a reasonable value for substantial tunneling. This gives an energy lowering similar to the experimental data as given in Ref. [10]. 3. Schottky barriers at nanowire end surfaces modified by surface charge For nanowire widths in the 10 nm range, surface charges may exist close enough to the MS interface to influence the shape of the Schottky barrier. The result of a theoretical calculation for the barrier lowering of a wire with 10  10 nm 2 cross section is shown in Fig. 4. Positive ARTICLE IN P RESS Fig. 1. Band diagram of a NiSi–Si contact at zero bias showing the two energy levels of a sulphur double donor and the Fermi level for a doping concentration of 8  10 14 cm À3 . 0 1×10 −6 2×10 −6 3×10 −6 4×10 −6 5×10 −6 6×10 −6 x[cm] 0.1 0.2 0.3 0.4 0.5 0.6 E C [eV] b a Fig. 2. Conduction band edge as a function of depth at thermal equilibrium (a) with and (b) without sulphur doping. Parameter values for the sulphur profile were taken from Ref. [10] as: N 01 ¼ 5  10 18 cm À3 , N 02 ¼ 5  10 17 cm À3 , x 01 ¼ 10nm, and x 02 ¼ 30nm (see Eq. (1)). The shallow donor doping of the semiconductor is N D ¼ 8  10 14 cm À3 , the Fermi level is at zero energy and the Schottky barrier height is 0.65 eV. 0 1×10 19 2×10 19 3×10 19 4×10 19 5×10 19 N 01 [cm −3 ] 0 0.1 0.2 0.3 0.4 0.5 0.6 E C [eV] Fig. 3. Conduction band edge at x ¼ 1 nm versus increasing sulphur concentration, N 01 . N 02 is simultaneously taken as N 02 ¼ 0.1  N 01 . J. Piscator, O. Engstro ¨ m / Physica E 40 (2008) 2508–2512 2509 elementary point charges are placed on the surface of the wire to a concentration of 4  10 12 cm À2 and mirrored in the metal. The barrier lowering for this concentration at a distance of 1 nm from the MS interface, enough for appreciable carrier tunneling, is seen to be about 0.2 eV. For our experiments, samples with Pd 2 Si/Si Schottky contacts on the end surfa ces of silicon wires were prepared on SOI material by electron beam lithography. By using the substrate as a back-gate, the potential distribution along the wire could be chosen to separate electron and hole injection. The sample configuration, contact geometry and charge distribution are demonstrated in Fig. 5. The preparation started from an SOI wafer with a silicon film thickness of 55 nm and a buried oxide layer (BOX) of 145 nm. After thinning the silicon film to 30 nm and performing patterning by e-beam lithography and plasma etching, wires with a 30  30 nm 2 cross section were created. By oxidation of the wires in dry atmosphere at 800 1C for 90 min, the wires were embedded in a SiO 2 shell of about 10 nm, with a silicon core of 15–20 nm width. Following evaporation and patterning of Pd, MS struc- tures of Pd 2 Si solely in contact with the end surfaces of the wires were obtained by annealing at 250 1C for 20 min. Using the Pd 2 Si contacts as source and drain and the silicon substrate as a back-gate, a transistor configuration can be defined as shown in Fig. 5(a). Transfer characteristics at different temperatures for a constant drain voltage of À5 V are shown in Fig. 6. Three different regimes can be observed among the graphs. In regime A, the negative gate voltage pushes the energy bands of the wire to higher energies, thus thwarting the injection of electrons from drain. At the source contact, the electric field is increased for negative gate voltages, which allows for hole injection. In regime B, the energy bands are lowered such that hole injection is thwarted and the electric field at drain is increased. This situation favours injection of electrons from drain. Finally, in regime C, the injected electron current has reached values such that the Schottky diode at source needs to be markedly forward biased in order to bring about enough carrier transport. This lifts the ARTICLE IN P RESS Fig. 4. (a) Schematic 10  10 nm 2 nanowire geometry with a metal contact at the end surface. (b) Conduction band lowering 1 nm from the interface due to an introduced positive point charge distribution surrounding the wire. The point charges correspond to a surface state density of 4  10 12 cm À2 . J. Piscator, O. Engstro ¨ m / Physica E 40 (2008) 2508–25122510 energy bands and allows for ambipolar transport limited by the two MS structures in combination. Therefore, from Arrhenius plots of the injection currents in the three regimes A, B and C, activation energies corresponding to the electron and hole barriers of the Pd 2 Si Schottky contacts at source and drain can be determined. Results from such a treatment of the data in Fig. 6 are shown by the filled squares in the plot of Fig. 7. The values in regime A are found in the range of 0.15 eV, while regime B has a sharp maximum at 0.55 eV and regime C again decreases the activation energy to about 0.25 eV. All these values are lower than corresponding energy barriers found for electrons and holes for planar Pd 2 Si structures. It can also be seen that there are no sharp transitions from the ARTICLE IN P RESS Fig. 5. (a) Transistor device structure on SOI with Pd 2 Si source and drain contacts and a back-gate. (b) Cross section of the nanowire with charge close the Si SiO 2 interface. (c) SEM image of fabricated structure. Fig. 6. Transfer characteristics measured at different temperatures from 303 to 363 K at a fixed V DS of À5 V. Regions A, B and C correspond to hole injection, electron injection and a combination of the two, respectively. Fig. 7. Effective barrier heights extracted from the temperature depen- dence of the current in Fig. 6. Regions A, B and C can be identified. After introduction of positive charge the electron barrier is lower and the hole barrier is correspondingly higher. J. Piscator, O. Engstro ¨ m / Physica E 40 (2008) 2508–2512 2511 injection of one carrier type to another, instead the change is gradual as the gate voltage is swept and the potentials change correspondingly. In order to investigate whether oxide charge may influence barrier heights in accordance with the theoretical estimate abo ve, positive oxide charge was created in the SiO 2 shell surrounding the silicon wire. This was done by irradiating the structure with UV light and verifying the positive charging from the voltage shift in a capacitance versus voltage measurement on a M OS structure prepared on the same sample chip with the insulator being the buried oxide. The result is shown by the circles in Fig. 7. Now the activation energies in regime A, mainly influenced by the hole barrier has increased while the values in regime B influenced by the electron barrier has decreased. On the other hand, the data in regime C, influenced by both barriers are mainly on the same level as before UV irradiation. 4. Discussion The introduction of charge by a deep impurity close to the MS interface of a planar structure decreases the Schottky barrier in n-type silicon only if the impurity acts as a donor. Two mechanisms contribute to the effect: (i) the change of charge state occurring in the depletion region when the impurity energy level passes the Fermi level and, indirectly, and (ii) the segregation of impurities close to the MS interface. For barrier modulation on p-type silicon, a similar effect would be expected from a deep double acceptor like for instance Zn with an energy level for the most shallow captured hole at about 0.3 eV from the valence band edge [12]. For a planar MS structure, where the semiconductor doping is low enough not to influence the barrier height, the sum of the electron and hole barriers is equal to the semiconductor band gap. In the data of Fig. 7, this sum is below the 1.17 eV band gap value of silicon which would be expected from the sum of activation plots [13]. Also the values for the electron and the hole activation energies, respectively, are both lower than the values of Schottky barriers obtained on planar structures for Pd 2 Si/Si MS junctions [14]. This indicates that tunneling occurs already before the introduction of positive oxide charge due to the added electric field at the contacts created by the gate potential. In addition to this, combined injection of both electrons and holes can take place and what is measured is the resulting barrier height. However, it is important to note that the changes of activation energies go in opposite direction in the tw o regimes A and B, where hole injection dominates in first case and electron injection in the latter. This is expected from the influence of a positive oxide charge as demonstrated in Fig. 4. Moreover, in regime C, where both carrier types are injected, the activation energy is mainly the same because the introduced positive oxide charge changes the barriers in different directions. Acknowledgments This work was financed by the Swedish Foundation for Strategic Research through the NEMO project and by the European SiNANO Network of Excellence. References [1] W. Mo ¨ nch, Rep. Prog., Phys. 53 (1990) 221. [2] R.T. Tung, Mater. Sci. Eng. R 35 (2001) 1. [3] A. Cowley, S.M. Sze, J. Appl. Phys. 36 (1965) 3212. [4] R.T. Tung, Phys. Rev. B 64 (2001) 205310. [5] V. Heine, Phys. Rev. 138 (1965) 1696. [6] J. Knoch, M. Zhang, Q.T. Zhao, St. Lenk, S. Mantl, Appl. Phys. Lett. 87 (2005) 263505. [7] M. Ono, M. Koyama, A. Nishiyama, Solid State Electron. 51 (2007) 732. [8] J. Knoch, M. Zhang, J. Appenzeller, S. Mantl, Appl. Phys. A 87 (2007) 351. [9] J. Piscator, O. Engstro ¨ m, Appl. Phys. Lett. 90 (2007) 132107. [10] Q.T. Zhao, U. Breier, E. Rije, St. Lenk, S. Mantl, Appl. Phys. Lett. 86 (2005) 062108. [11] O. Engstro ¨ m, H.G. Grimmeiss, J. Appl. Phys. 47 (1976) 4090. [12] J.M. Herman III, C.T. Sah, Phys. Status Solidi A 14 (1972) 405. [13] H.D. Barber, Solid State Electron. 10 (1969) 1039. [14] O. Engstro ¨ m, H. Pettersson, B. Sernelius, Phys. Status Solidi A 95 (1986) 691. ARTICLE IN P RESS J. Piscator, O. Engstro ¨ m / Physica E 40 (2008) 2508–25122512 . Physica E 40 (2008) 2508–2512 A new mechanism for modulation of Schottky barrier heights on silicon nanowires J. Piscator à , O. Engstro ¨ m Department of. the barrier height at 1 nm from the MS interface as a function of the sulphur surface concentration taking this depth as a reasonable value for substantial