Structural transformation of implanted diamond layers during high temperature annealing

Abstract

In the recent years graphitization of ion-beam induced amorphous layers became the basic tool for device fabrication in diamond. The etchable graphitic layers can be removed to form free-standing membranes into which the desired structures can be sculpted using FIB milling. The optical properties of the devices fabricated using this method are assumed on the model of sharp diamond–air interface. The real quality of this interface could depend on degree of graphitization of the amorphous damage layers after annealing. In the present work the graphitization process was studied using conventional and analytical TEM. It was found that annealing at 550 °C results in a partial graphitization of the implanted volume with formation of the nano-crystalline graphitic phase sandwiched between layers of tetrahedral amorphous carbon. Annealing at 1400 °C resulted in complete graphitization of the amorphous layers. The average size of graphite nano-crystals did not exceed 5 nm with predominant orientation of c-planes normal to the sample surface.

Introduction

The progress in the fabrication of synthetic diamond has resulted in increasing number of its potential applications. Single crystal diamond has also attracted enormous interest as a solid state platform for quantum information processing. Nitrogen-vacancy (N-V) color centers in diamond show remarkable quantum properties such as long coherence times and single spin readout, and can be used as qubits in a quantum computer architecture [1], [2], [3]. In order to take advantage of these properties, it is highly desirable to fabricate photonic components in diamond at the micro and even nano-scale level. In a previous work we demonstrated the ability of fabricating three-dimensional structures in diamond at the micro-scale level using lift-out method [4]. MeV ion implantation was used to create a buried damage layer which transformed into a graphite-like layer upon high temperature thermal annealing. The graphitic layer can be selectively etched to form a free-standing membrane into which the desired structures can be sculpted using focused ion beam (FIB) milling. Ion implantation with tens of keV ion energy [5] or multiple energy implantation techniques [6] when combined with FIB milling allows device fabrication in diamond at the micro- and nano-scale. The modeling of the optical properties of the devices fabricated using this method [4], [5], [6], [7], [8] are based on the assumption of a sharp diamond–air interface. The real quality of this interface could depend on many factors, for example, on the degree of graphitization of the amorphous damage layers after annealing. Kalish et al. [9] reported complete graphitization of the implanted layer in diamond after a 20 min annealing at 600 °C. However, a recent study [10] using high-resolution electron microscopy (HREM) revealed the presence of a transition area with pockets of crystalline diamond in the graphite matrix near surface, after high temperature vacuum annealing of implanted layer in diamond. Thus, the real quality of the diamond surface after chemical removal of graphitic layer can be far from ideal. In the present work the processes of the ion-beam-induced amorphisation and graphitisation in diamond were studied using cross-sectional conventional and analytical TEM. The graphitisation of the amorphized layers was carried out at annealing temperatures of 550 °C and 1400 °C.