VOLUME 85, NUMBER 9 PHYSICAL REVIEW LETTERS 28 AUGUST 2000 Metal-Semiconductor Nanocontacts: Silicon Nanowires Uzi Landman, 1 Robert N. Barnett, 1 Andrew G. Scherbakov, 1 and Phaedon Avouris 2 1 School of Physics, Georgia Institute of Technology, Atlanta, Georgia 30332-0430 2 IBM Research Division, T.J. Watson Research Center, Yorktown Heights, New York 10598 (Received 16 November 1999) Silicon nanowires assembled from clusters or etched from the bulk, connected to aluminum electrodes and passivated, are studied with large-scale local-density-functional simulations. Short 0.6 nm wires are fully metallized by metal-induced gap states resulting in finite conductance e 2 h. For longer wires 2.5 nm nanoscale Schottky barriers develop with heights larger than the corresponding bulk value by 40% to 90%. Electric transport requires doping dependent gate voltages with the conductance spectra exhibiting interference resonances due to scattering of ballistic channels by the contacts. PACS numbers: 73.40.Sx, 73.40.Cg, 73.20.Dx As the relentless miniaturization of electronic devices is reaching the nanometer scale, current device concepts may have to be radically modified due to the nonscalable nature of materials in this size range, with an emphasis on quan- tum mechanical effects [1]. However, while of significant fundamental interest and of high technological relevance, answers to such issues, in the form of reliable estimates of properties calculated and/or measured for nanoscale mate- rials structures, pertaining to characteristics of individual nanoscale device components and their interconnections, are largely unavailable. We report on large-scale ab initio simulations [2], pro- viding first insight into the structures, electronic spectra, and transport properties of surface-passivated silicon nanowires (SiNWs) [3], etched from bulk Si or prepared via a novel cluster assembly, and their contacts to alu- minum electrodes. For a very short 0.6 nm SiNW bridging the Al electrodes we find full metallization by metal-induced-gap states (MIGS) resulting in a finite electronic conductance e 2 h, while for longer wires 2.5 nm highly localized interfacial dipoles form which together with pinning of E F at the neutrality level result in nanoscale Schottky barriers with heights larger than the barrier at the corresponding bulk interface by 40% to 90%, depending on the type of SiNW and the interfacial atomic structure. Transport through the longer wires requires doping dependent gate voltages and the conduc- tance spectra exhibit size-dependent oscillations due to interference resonances, originating from scattering of the ballistic conductance eigenchannels from the contacts. In constructing the SiNWs we considered two strate- gies: (1) assembly of wires from silicon clusters [4], i.e., formation of cluster-derived (CD) SiNWs; see Fig. 1 where shown in (ii) is a Si 24 cluster bridging the two Al electrodes, referred to as Si 24 NW, and displayed in (iii) is a wire made of five base-sharing Si 24 clusters attached to the Al electrodes, referred to as Si 96 NW, and (2) etching of wires out of bulk Si; see Fig. 1(i) where shown is an electrode-attached diamond-structured (DS) wire with its long axis along the [211] direction and exposing 1 11 and 01 1 surfaces [3a,3d], referred to as Si 94 NW [5]. In all cases each of the electrodes is comprised of 116 Al atoms in a face-centered cubic structure exposing (111) facets, and all the Si dangling bonds not involved in bonding with the electrodes are passivated by hydrogen. The electronic spectrum of each of the fully hydrogen- passivated free (unattached) longer wires exhibits a fundamental energy gap between the valence and con- duction states [6]. The local density of states (LDOS) for the short cluster bridge [Si 24 NW, Fig. 2(e)] and for the bridge with the cluster doped endohedrally by an Al atom [Si 24 AlNW, Fig. 2(f)] reveals a finite DOS in the gap both in the region of the wire bonded directly to the Al electrode (I) and the one in the middle of the cluster bridge (II). This correlates with a calculated finite electronic conductance [7] through the undoped bridge (G  0.45g 0 , where g 0  2e 2 h) with a significant enhancement upon doping G  1.33g 0 . For the longer wires, the LDOS calculated in different regions of the wires [defined in Fig. 2(g)] are shown in Figs. 2(a)–2(d). For all wires the LDOS at the interfacial regions closest to the metal electrode differ from those in the middle section of the wire, showing for the former a finite DOS in the gap indicating a metallization of these re- gions (see, in particular, region I), with a gradual “empty- ing” of the gap as a function of distance from the electrode. Convergence to the corresponding “bulk” limit (i.e., to that calculated at the middle segment of the wire) is rather fast, occurring over a range of 5 Å [compare regions III and IV with regions I and II in Figs. 2(a) and 2(d)]. While doping is likely to be unadvisable in nanoscale systems because of expected large device-to-device statis- tical variation in dopant concentrations [8], the CD SiNWs offer a unique doping strategy through insertion of (inter- stitial) dopants into the cluster cages which are stabilized by endohedral doping. Modifications of the electronic properties of the CD Si 96 NW wire via doping with Al atoms are illustrated in Figs. 2(b) and 2(c), respectively, for partial (Si 96 Al 2 NW, where only the second and fourth cages are doped) and full [Si 96 Al 5 NW; see Fig. 1(b)] dop- ing. The LDOS is enhanced in the gap for regions [I and II in Figs. 2(b) and 2(c)] closest to the metal electrodes, and 1958 0031-9007 00  85(9)  1958(4)$15.00 © 2000 The American Physical Society