High-temperature annealing of colour centres in diamond.

Alan T Collins

Wheatstone Physics Laboratory, King’s College London, Strand, London WC2R 2LS, UK.

E-mail: Alan.Collins@kcl.ac.uk Telephone: +44 (0)20 7848 2584 Fax: +44 (0)20 7848 2420

Abstract

The announcement by General Electric that the colour of brown natural diamonds can be improved by annealing at high pressure and high temperature (HPHT) has stimulated renewed interest in the behaviour of point defects in natural diamond. Brown colour in these diamonds is associated with plastic deformation, but the precise nature of the centre producing the colour is not known. In diamonds containing negligible nitrogen (type IIa diamonds) the HPHT annealing results in a reduction of the brown colour, and it is assumed that this is due to a partial healing of the plastic deformation. In diamonds containing aggregated nitrogen (type Ia diamonds) the HPHT annealing produces yellow or yellow/green coloured specimens. These colours result from the production of nitrogen-vacancy-nitrogen (N-V-N) centres in either the neutral or negative charge states. The (N-V-N)⁰ gives rise to a vibronic absorption band with a zero-phonon line (ZPL) at 2.463 eV (503.2 nm) which is responsible for the yellow colour, and the (N-V-N)^- has a vibronic band with a ZPL at 1.257 eV (986.1 nm). Absorption from the high-energy tail of that band extends into the visible region and causes the green colour. HPHT annealing of type Ia diamond causes some of the nitrogen aggregates to dissociate, producing single, substitutional, nitrogen which is an electrical donor. A charge-transfer process between those (N-V-N)⁰ centres which are close to a nitrogen donor produces the (N-V-N)^- centres. Some N-V centres are also observed after HPHT annealing.

A luminescence band with a ZPL at 2.526 eV (490.7 nm) is produced at the slip traces associated with the plastic deformation. The reduction in intensity of this band, following HPHT annealing, provides strong evidence that changes are taking place at the slip traces during the annealing. The production of N-V-N and N-V centres suggests that vacancies are released from the slip traces, and captured by one of the forms of nitrogen.

In order to better understand these annealing phenomena, nitrogen-vacancy centres have been created by radiation damage, and annealing at 800 °C, in relatively dislocation-free type Ia diamonds and type Ib diamonds (those in which almost all the nitrogen is in single substitutional form). Some specimens have been progressively annealed at temperatures up to 1600 °C in an inert gas or in vacuo; other similar specimens have been annealed at the HPHT conditions used to enhance the colour of natural brown diamonds.

In the diamond containing (N-V-N)⁰ centres, a substantial concentration of (N-V-N)^- centres was formed after a 1 hour anneal at 1500 °C, showing that some single substitutional nitrogen is being formed by the dissociation of a radiation damage product. (1500 °C is too low a temperature for the dissociation of the nitrogen aggregates themselves.)

In those diamonds that were heated from 20 °C to 2300 °C over a period of 140 seconds, at a pressure of 5GPa, the (N-V-N)⁰ and (N-V)^- centres completely, or almost completely annealed out, and no (N-V-N)^- centres could be detected. In brown type Ia diamonds annealed in this way those optical centres are present in medium to high concentrations (producing the yellow or yellow/green colours). The present results therefore indicate that the production of these centres in HPHT-annealed type Ia diamonds is a dynamic process with the generation and annealing in competition.

Keywords: point defects, absorption and luminescence spectroscopy.