Applied Surface Science 258 (2012) 2639–2642 Contents lists available at SciVerse ScienceDirect Applied Surface Science j our nal ho me p age: www.elsevier.com/loc ate/apsusc Site-specific electron-induced cross-linking of ortho-carborane to form semiconducting boron carbide Frank L. Pasquale, Jeffry A. Kelber ∗ Department of Chemistry, University of North Texas, Denton, TX 76203, USA a r t i c l e i n f o Article history: Received 26 May 2011 Received in revised form 17 October 2011 Accepted 20 October 2011 Available online 29 October 2011 Keywords: Boron carbide Semiconducting solid films Electron beam bombardment UPS and XPS a b s t r a c t Semiconducting boron carbide (B 10 C 2 H x ) films have been formed by bombardment of condensed ortho- carborane (closo-1,2-dicarbadodecaborane) multilayers on polycrystalline copper substrates by 200 eV electrons under ultra-high vacuum conditions. The film formation process was characterized by X-ray and ultraviolet photoelectron spectroscopies. Electron bombardment results in the cross-linking of the icosahedral units. The cross-linking is accompanied by a shift in the B(1s) binding energy indicating site- specific cross-linking between two boron sites on adjacent carborane icosahedra. An additional shift in valence band binding energies attributed to the surface photovoltage effect is indicative of the formation of a p-type semiconductor. This is the first report of B 10 C 2 H x formation by electron bombardment of condensed films, and the data indicate that this method is a viable route towards formation of ultra-thin films of tailored composition and cross-linkages for emerging nanoelectronics and sensor applications. © 2011 Elsevier B.V. All rights reserved. 1. Introduction X-ray and ultraviolet-excited photoemission spectra (XPS, UPS) demonstrate that condensed layers of ortho-carborane can be cross-linked by 200 eV electrons under ultra-high vacuum (UHV) conditions to form p-doped semiconducting boron car- bide (B 10 C 2 H x ) films. This is the first report of site-specific cross-linking in such films, with cross-linking occurring between B sites on adjacent icosahedra. The results also suggest that this method can be used to form selected-area ultra thin films for emerging nanoelectronics and radiation sensing applications. Broad interest in B 10 C 2 H x films arises due to potential applica- tions of these amorphous molecular solids in neutron detection [1–4], semiconductor devices [5–14], biomedical applications [15] and magnetoresistance-based devices [12,14,16,17]. Boron carbide films have been deposited by plasma-enhanced chemical vapor deposition (PECVD) [1–3,5–9,12,13,16–21], synchrotron radiation- assisted chemical vapor deposition (SRCVD) [6–8,10–11,22,23], and rf and dc magnetron sputtering [24–28]; all of which yield similar electronic and physical properties. We demonstrate here that exposure of condensed orthocarborane films to 200 eV elec- tron beam bombardment at 110 K results in site-specific bond scission and cross-linking of the icosahedral cage units. Shifts in XPS core level spectra indicate that cross-linking of ortho-carborane ∗ Corresponding author at: Mail Stop 305070, University of North Texas, Denton, TX 76203, USA. Tel.: +1 940 565 3265. E-mail address: kelber@unt.edu (J.A. Kelber). icosahedra occurs selectively by bonding through the boron sites between adjacent cage units, resulting in p-type B 10 C 2 H x films. UPS data indicate that the valence band electronic structure of these films is broadly similar to that of films formed by other methods [2,5,10–12,14,22,26,27,29]. 2. Experimental Experiments were undertaken in a turbomolecularly pumped UHV analysis system with a typical operating pressure of 3–8 × 10 -10 Torr. Photoemission spectra were acquired with a 140 mm mean radius hemispherical analyzer (Physical Electron- ics), equipped with a channel plate detector operated at constant pass energy mode of 23.5 eV and 0.585 eV, for XPS and UPS, respectively. These conditions yielded core level binding ener- gies reproducible to within 0.1 eV. The substrate, a 1 cm × 1 cm copper foil, was mounted on a sample holder which permitted regulation of sample temperature by a combination of liquid nitro- gen cooling and resistive heating, with a type-K thermocouple mounted close to the sample. The substrate was cleaned by 3000 eV Ar ion bombardment. XPS data were acquired using a commer- cial unmonochromated AlK X-ray source, operated at 15 kV and 300 W. UPS spectra were acquired using a commercial, differ- entially pumped He I discharge source. All photoelectrons were collected normal to the surface. The analyzer energy scale was calibrated with respect to the Fermi level of clean polycrystalline copper at 110 K. Cross-linking of the condensed film was done using an electron gun (Kimball Physics), operated at 200 eV and an emis- sion current of 4 A. Ortho-carborane (Aldrich) was admitted into 0169-4332/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.apsusc.2011.10.110