石墨薄膜的化学气相沉积

Abstract

Well-ordered graphite films with a thickness of a few graphene layers have been grown on Ni substrates by chemical vapor deposition(CYD) from a mixture of hydrogen and methane activated by a DC discharge. According to Auger, Raman and scanning tunnclingmicroscopy (STM) data the CVD graphite film thickness is about 1.5 - 0.5 nm. The graphene layers were perfectly adhered to the substrate surface excet for upthrusted ridges of a few tens of nanometers in height. STM has revealed an atomically smooth surface with theatomic arrangement typical of graphite between the ridges. A diference in the thermal expansion coeficients of nickel and graphite isconsidered as a reason for the ridge formation.

1. Introduction

Graphite is a widely used material with well-developedsynthesis methods and properties investigated in detail[1]. A peculiarity of graphite is its layered structure formedby parallel two-dimensional graphene sheets weakly cou-pled by van der Waals interaction. Each graphene sheetlooks like a hexagonal network of carbon atoms connectedby strong covalent “in-plane’ o-o bonds. The delocalizedelectrons appear only due to an additional t-t bonding ofelectronic orbitals oriented perpendicularly to the grapheneplane. They are responsible for the graphite electrical con-ductivity which is highly anisotropic. Recently, the uncon-ventional electric field and quantum Hall effects have beendemonstrated experimentally for a single sheet of graphitegraphene [2,3]. These experiments have triggered a greatinterest to the graphite films containing one or a very fewgraphene sheets [4-7], because the unusual electron behavior is considered as a result of two-dimensional confinement of charge carriers.

Graphene can be produced by mechanical exfoliation ofindividual layers from the surface of highly oriented pyrolytic graphite (HOPG) crystal [2-5]. Thin heteroepitaxialgraphite films consisting of a few graphene layers can beproduced by graphitization of silicon carbide surface (6,8]or dissociation of ethylene gas on Ni1 11) surface inultra-high vacuum (9]. However, such hand-made’graph-ene or heteroepitaxial thin films have relatively small lateralsizes and are suitable only for pure scientific studies. Fordeposition of extended films appropriate for practicalapplications a chemical vapor deposition (CVD) from acti-vated gaseous phase could be efficient. But up to now onlyrather thick graphite films with a high number of structuraldefects were deposited by CVD technique [1].In this paper, we describe a high yield technique ofdeposition of the large area graphitic films with thicknessof a few graphene layers. The results of structural characterization of such films are also reported.

2. Experimental

The graphite films were grown by chemical vapor deposition techniquefrom a hydrogen-methane gas mixture activated by DC discharge. Adetailed description of the deposition facility and the growth method has been done elsewhere [l0]. Our CVD system allows production of dif-ferent types of carbon films ranging from polycrystalline diamond to car-bon nanotubes. A type of the material deposited depends essentially on themethane concentration and on the substrate temperature. In this work, theprocess parameters corresponding to graphite phase deposition have beenchosen [l0l: the gas mixture composition H:CH. = 92:8; the total gaspressure P = 80 Torr; the substrate temperature of 950 °C. In these condi-tions the nano-graphite meso-porous layers were deposited after l-l.5 h[10]. The deposition procedure includes pumping out the reactor chamberby a rotary pump to the base pressure value of 10-3 Torr followed byintroduction of a pure hydrogen and ignition of DC discharge at pressureofabout 40 Torr. Then the hydrogen pressure was increased up to 80 Torrsimultaneously with increasing the discharge current. This procedure tookabout 10 min. It was followed by an introduction of the methane gas intothe reactor and an increase of its content up to 8 vol%. The discharge cur-rent was fixed at value about 0.5 A/cm’ being typical for the graphitephase formation.