Transmission electron microscopy study of diamond nucleation and growth on smooth silicon surfaces coated with a thin amorphous carbon film

Abstract

Amorphous carbon (a-C) films with high contents of tetrahedral carbon bonding (sp³) were synthesized on smooth Si(100) surfaces by cathodic arc deposition. Before diamond growth, the a-C films were pretreated with a low-temperature methane-rich hydrogen plasma in a microwave plasma-enhanced chemical vapor deposition system. The evolution of the morphology and microstructure of the a-C films during the pretreatment and subsequent diamond nucleation and initial growth stages was investigated by high-resolution transmission electron microscopy (TEM). Carbon-rich clusters with a density of ∼10¹⁰ cm⁻² were found on pretreated a-C film surfaces. The clusters comprised an a-C phase rich in sp³ carbon bonds with a high density of randomly oriented nanocrystallites and exhibited a high etching resistance to hydrogen plasma. Selected area diffraction patterns and associated dark-field TEM images of the residual clusters revealed diamond fingerprints in the nanocrystallites, which played the role of diamond nucleation sites. The presence of non-diamond fingerprints indicated the formation of Si–C-rich species at C/Si interfaces. The predominantly spherulitic growth of the clusters without apparent changes in density yielded numerous high surface free energy diamond nucleation sites. The rapid evolution of crystallographic facets in the clusters observed under diamond growth conditions suggested that the enhancement of diamond nucleation and growth resulted from the existing nanocrystallites and the crystallization of the a-C phase caused by the stabilization of sp³ carbon bonds by atomic hydrogen. The significant increase of the diamond nucleation density and growth is interpreted in terms of a simple three-step process which is in accord with the experimental observations.

Introduction

The development of smooth, thin diamond films using plasma-assisted chemical vapor deposition (CVD) techniques depends greatly on the surface condition. Surface pretreatment methods to enhance diamond nucleation and growth include seeding or activating the substrate surface by mechanical abrasion and chemomechanical polishing techniques which use diamond powders (or pastes) and hard abrasive particles [1], [2], [3], [4]. These surface treatments result in the deposition of residual diamond grits or carbonaceous precursors and/or the generation of high surface free energy nucleation sites through the removal of surface contaminants and the formation of surface steps, kinks and imperfections. However, such surface preparation methods are not appropriate for applications requiring undamaged surfaces such as optical windows, lenses and smooth wear-resistant coatings. Pretreatment methods yielding high diamond nucleation densities without altering the original surface topography are therefore highly desirable.

The most common approaches for achieving high nucleation densities without inducing permanent surface damage involve the deposition of different carbonaceous precursors such as carbon fibers [5], fullerene clusters [6], [7], and thin films of graphitic carbon [8], diamond-like carbon [9] and hard amorphous carbon (a-C) [10], [11]. The formation of an amorphous silicon carbide interfacial layer in bias-enhanced microwave plasma CVD has been reported to yield high diamond nucleation densities on unscratched and polished silicon and refractory metal surfaces, depending on the time of exposure to a methane-rich hydrogen plasma and the magnitude of the negative bias voltage applied to the substrate during the pretreatment [12], [13], [14], [15]. It was argued that the biasing process actually produces diamond nuclei, as opposed to just creating diamond nucleation sites, in addition to removing and/or suppressing the formation of a surface oxide [12].

Diamond nucleation densities in the range of 10⁴–10¹¹ cm⁻² have been observed, depending on the thickness and microstructure of the predeposited carbon film [7], [11] and methane concentration, chamber pressure, and exposure time to a low temperature, methane-rich hydrogen plasma [16]. The highest diamond nucleation densities (∼10¹⁰ cm⁻²) and better quality diamond films were obtained with hard a-C films synthesized on smooth silicon surfaces by a vacuum arc plasma deposition technique (also known as cathodic arc) using a pulsed bias voltage of −100 to −200 V [17], [18]. Scanning electron microscopy and Raman spectroscopy studies revealed that diamond nucleation occurred from submicron residual clusters generated during the pretreatment [10]. The high diamond nucleation densities obtained were attributed to the large number of nanoparticles produced during the pretreatment which provided numerous high surface free energy nucleation sites, and the high resistance of the nanoparticles to etching by atomic hydrogen resulting from the significant percentage of tetrahedral (sp³) atomic carbon configurations [7], [10], [16], [17].

Although the previous studies have demonstrated that enhancement of the diamond nucleation density on smooth silicon surfaces subjected to a methane-rich hydrogen plasma is due to the formation of high density, etch-resistant residual nanoparticles (or clusters), detailed information about the structure of these residual clusters and the associated mechanism of diamond nucleation was not obtained, mainly due to the resolution limits of the microscopy and Raman spectroscopy techniques used in these investigations. The main objective of the present work, therefore, was to further investigate the nucleation and initial growth of diamond on pretreated smooth silicon substrates coated with a hard, ultrathin a-C film produced by cathodic arc deposition. High-resolution TEM was used to characterize the submicron residual clusters generated during the pretreatment and to observe their structural evolution during the nucleation and initial growth stages of the diamond films.