Optical properties of single crystal diamond microfilms fabricated by ion implantation and lift-off processing

Abstract

We report the fabrication and optical characterisation of ~1 μm thick films of single crystal diamond prepared by an ion implantation and lift-off process. A comparison is made between type 1b material grown by high-temperature high-pressure and high purity type 2a material grown by chemical vapour deposition. Our results show that the fabrication process does not suppress strong emission from nitrogen-vacancy colour centres in the type 1b material. The 1332 cm^− 1 Raman line normally prominent in the CVD material is found to be strongly suppressed. A 20 nm thick damaged layer of sp³ carbon is identified on the underside of the films that indicates residual damage from the implantation process. These results are of significance for the fabrication of devices in diamond in which colour centres couple to discrete optical modes.

Introduction

Remarkable progress in the production of high-quality synthetic diamond, both by high-pressure-high-temperature (HPHT) and plasma-assisted chemical vapour deposition (CVD) techniques, has in the past few years opened up the possibility to exploit the unique properties of diamond for optical devices that span a wide range of applications. Reductions in the birefringence of synthetic material has already led to dramatic advances in the use of bulk diamond in the development of Raman lasers [1], [2], and of films of diamond ~100 μm thick for intra-cavity heat management in high power external cavity lasers [3].

The high transparency and low temperature dependence of the refractive index in diamond make the material attractive for stable single mode waveguiding and resonator structures that can find uses in optical communications systems. Such optical microstructures are also of significant interest to those engaged in the burgeoning fields of quantum photonics and spintronics, for whom diamond and its intrinsic and extrinsic defects have recently become one of the most attractive platforms [4], [5], [6], [7], [8], [9], [10].

The realisation of single mode optical structures raises additional challenges, however, such as controlled modulation of the refractive index and the fabrication of diamond surfaces of high optical quality. These devices need to be fabricated from structures of order 0.2 to 5μm in size. In particular the desire to construct single mode waveguides, two-dimensional photonic crystals, and diamond-containing microcavities motivates the fabrication of single crystal diamond films less than 1 μm thick [11], [12], [13], [14], [15]. Whilst in other semiconductor materials it is possible to perform a lift-off process through growth of a heterostructure that includes a sacrificial layer that can be etched to release a thin film, no sacrificial layer is known onto which homoepitaxial diamond growth is supported, and alternative routes need to be explored. Some structures have been fabricated using thin films of polycrystalline diamond [4], [16], [17] but these do not provide the requisite optical quality either in terms of surface finish or of low birefringence.

Previous studies have shown that free standing diamond structures can be fabricated by an ion implantation method [18], [19], [22], [23]. In the method described in [22], a collimated beam of monoenergetic helium ions is incident on the diamond surface, causing a damage profile that is highly localised in depth. Subsequent annealing of the diamond results in regions with damage in excess of ∼ 10²² vacancies/cm³ forming an sp² bonded graphite structure, while less damaged regions revert to the pristine diamond structure. The buried graphitised regions can then be etched using a combination of galvanic etching and boiling in oxidizing acids to leave only the single crystal diamond regions. The layer above the etched region can then be cut away and removed. For sub-1 μm layers, a dual energy beam can be used to produce a sandwich structure in which the film is defined between two damaged regions [23].

Here we report the optical properties of thin films of diamond prepared using the single ion implantation and lift-off method of reference [22]. We compare the properties of two films, one fabricated by the HPHT method, and one from high purity CVD material. These films are characterised using photoluminescence (PL) and reflectance spectroscopy, to identify the distributions of colour centres and impurities in them and the optical quality of the films for use as waveguides or intracavity media.