Oxygen plasma etching of carbon nano-structures containing nitrogen

Journal of Non-Crystalline Solids 352 (2006) 1314–1318 www.elsevier.com/locate/jnoncrysol

Oxygen plasma etching of carbon nano-structures containing nitrogen J.J.S. Acun˜a a, C.A. Figueroa a,*, M.E.H. Maia da Costa a, P. Paredez a, C.T.M. Ribeiro b, F. Alvarez a a

Instituto de Fı´sica ‘Gleb Wataghin’, Universidade Estadual de Campinas, P.O. Box 6165, Campinas, SP 13083-970, Brazil b Instituto de Fı´sica de Sa˜o Carlos, Universidade de Sa˜o Paulo, P.O. Box 369, Sa˜o Carlos, SP 13560-250, Brazil Available online 20 March 2006

Abstract In this paper we report a study of the oxygen plasma etching effect on CNx nano-structures grown on tiny nickel islands (1–5 nm) previously deposited onto oxidized silicon wafers. In order to eliminate the ill-formed structures, broad oxygen ion beam plasma was used to irradiate the nano-structured CNx material (3.4 at.% N). The structures were prepared by ion beam assisted deposition (IBAD) and etched in situ by an oxygen ion beam at room temperature. In situ characterization by XPS and ex situ Raman, FEG-SEM, AFM, and field emission measurements were employed to study the evolution of the nano-structures. Raman spectra show two narrow and well defined D and G bands (disorder and graphitic bands) in the formed nano-structure.  2006 Elsevier B.V. All rights reserved. PACS: 73.61.Wp; 52.77.Dq; 81.05.Tp Keywords: Carbon nano-structures; Oxygen etching; Field emission

1. Introduction Since carbon nano-tubes were discovered, many studies dealing with synthesis, purification, and characterization have been reported [1,2]. The peculiarities of these compounds make carbon nano-structures excellent candidates for field emitting devices [3,4]. Doping the structures during synthesis to improve their field emission properties is a challenge. Having five valence electrons, nitrogen inclusion could change the electronic properties of the material by acting as a donor impurity when incorporated into graphitic-like structure [5]. To achieve this goal, the structural control during fabrication is mandatory to optimize the field emission properties of the material. However, amorphous carbon and defects introduced by nitrogen are unde-

*

Corresponding author. Tel.: +55 19 3788 5350; fax: +55 19 3788 5376. E-mail address: cafiguer@ifi.unicamp.br (C.A. Figueroa).

0022-3093/$ - see front matter  2006 Elsevier B.V. All rights reserved. doi:10.1016/j.jnoncrysol.2005.10.027

sirable sub-products that degrade the electronic behavior of the structure [6]. In order to refine the final compound, some purification techniques are generally used. Fullerene structures are purified by oxidation, a procedure that eliminates the amorphous carbon phase present in the raw material [7–9]. Physically, the organized structures such as fullerenes, nano-domes, -tubes, -onions, and -horns containing graphitic planes are more stable than amorphous carbon and defective structures [10]. Consequently, a post oxidation treatment after carbon nano-structure formation will, in principle, eliminate the amorphous and defective parts of the material [11]. Moreover, the mechanism of oxygen ion beam etching in carbon nano-structures and its correlations with field emission properties are not fully understood [12]. In this paper we report a study of the oxygen ion beam etching effect on CNx nano-structures grown by ion beam assisted deposition (IBAD). The effect of oxygen etching was studied in situ by XPS. Afterward, ex situ Raman, FEG-SEM, AFM, and field emission measurements were also employed for further characterization. Finally, field

J.J.S. Acun˜a et al. / Journal of Non-Crystalline Solids 352 (2006) 1314–1318

emission properties were correlated to electronic and structural behaviors of the treated material. 2. Experimental CNx nano-structures were prepared by IBAD from a high purity graphite (99.9995%) target sputtered by argon ions. Simultaneously, a nitrogen beam assisted the growth. In situ purification of CNx nano-structures by oxygen ion beam etching was performed at different oxidation times (in the range 0–90 s). After oxidation, an annealing process by heating until 400 C during 10 min guarantees the elimination of the volatile compounds. The series of samples were grown at constant substrate temperature (650 C). The nitrogen content was obtained assisting the growing film with a Nþ 2 beam (8 eV) as reported before by the prior works of the group [18]. The Si h1 0 0i substrates were p-doped (15 X/cm2, 10 mm · 10 mm), mirror polished and a 600 nm SiO2 layer grown by wet oxidation at 1050 C on the Si wafers. Before the CNx deposition, nickel islands were grown on the substrates (1–5 nm size) by in situ sputtering of a Ni target and subsequent annealing at 650 C during 3 min in vacuum (