Structural and optical studies of Co and Ti implanted sapphire
Introduction
Sapphire has a large range of technological applications due to its great chemical stability and high electrical resistivity. In the past, most of the work done in sapphire aimed at the improvement of its mechanical properties using ion implantation [1], [2], [3]. This nonequilibrium doping process often leads to compositions and structures impossible to obtain by conventional processes. However the doping by ion implantation introduces lattice defects, and the metastable states thus formed can be relaxed through thermal annealing in a variety of atmospheres. The new microstructures developed in the implanted region are strongly dependent on the annealing conditions [4], [5], [6]. In addition to the tribological changes a broad range of new optical and magnetic properties can be produced in this way. These new synthesized composite materials have great potential applications ranging from quantum dot lasers [7] to nonlinear optical devices [7], and have received a great deal of recent interest [8], [9]. Most of these properties are related with the precipitation of nanosized particles (metallic or semiconducting) dispersed in the sapphire matrix [10], [11]. To control the formation of such nanoparticles in α-Al2O3 it is necessary to study the interactions between the implanted species and implantation-induced defects, and the role of the annealing atmosphere. On the other hand, transition metal impurities are among the most important optically active ions in sapphire that give rise to luminescence bands from the infrared to the visible region [12], [13], [14].
In this work we studied the optical and structural modifications created in sapphire due to the implantation of Ti and Co ions, prior to and after annealing in oxidizing and reducing atmospheres.
Section snippets
Experimental details
Sapphire single crystals with (0 0 0 1), () and () orientations were implanted at room temperature (RT) with 100 keV titanium ions or 150 keV cobalt ions with fluences in the range of 1×1015 cm−2 to 5×1017 cm−2. Thermal annealings were carried out at 800 and 1000 °C for 1 and 3 h in reducing (p⩽10−7 mbar) and oxidizing atmospheres.
After implantation and after each stage of thermal annealing, RBS/channeling and TEM studies were performed to study the structural changes induced by the
Co implantation
Detailed angular scans along the main crystallographic directions show that for low concentrations (fluences of 1015 Co+/cm2) the Co ions are incorporated into Al lattice, assuming the trivalent charge state (Co3+) in order to maintain the charge neutrality [15]. Amorphization of the implanted region is only achieved for fluences higher than 5×1016 Co+/cm2 as can be concluded from the aligned RBS spectra shown in Fig. 1. The complete overlap between the random and aligned spectra through the
Conclusions
For low doses (up to 1×1015 cm−2) all the Ti and Co ions are incorporated into Al substitutional sites and remain stable up to a temperature of 1000 °C. A fluence of 5×1016 Ti+/cm2 creates a continuous amorphous layer through the implanted region while for Co, higher fluences are necessary (2×1017 Co+/cm2). Ambient atmosphere plays an important role in the annealing process of the implantation damage and final microstructure. For fluences of 2×1017 Co+/cm2, annealing in oxidizing atmosphere
Acknowledgements
We acknowledge our Technician Jorge Rocha for the implantations and FCT for its support through the PRAXIS XXI under the contract C/CTM/12067/1998.
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