Ion implantation, diffusion, and solubility of Nd and Er in LiNbO 3
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I. INTRODUCTION
LiNbO3 is one of the best materials for integrated optical applications.1"3 Its crystallographic structure and its high ferroelectric Curie temperature of approximately 1420 °C permit the diffusion of various elements without the need for ferroelectric repoling.4 This enables the formation of optical waveguides by Ti indiffusion or proton exchange techniques.5'6 The material also permits the application of ion implantation for optical waveguide formation.7 If a high-dose Ti implantation is performed, the amorphized LiNbO3 matrix can be restored by Solid Phase Epitaxial Regrowth (SPE). After SPE a steep Ti dopant profile and a restored matrix polarization produce excellent optical waveguides.8 Most LiNbO3 IOCs are modulators, demodulators, and switches; additionally, resonators, parametric oscillators, and more complex devices have been realized.1'2'3'9 The first fluorescence measurements on a Nd: LiNbO3 crystal, which had been doped in the melt and had an additional waveguide fabricated by proton exchange techniques, have been reported.10 For integrated applications, a local doping technique, which affects only some of the waveguides, would be more attractive, especially since large wafers of LiNbO3 are available in very good quality. The present study is motivated by the goal of future fabrication of integrated optical amplifiers and lasers by selectively introducing rare earth (RE) ions into the channels and resonators of waveguiding structures. First attempts of Nd indiffusion from an evaporated metallic film into the LiNbO3 substrate resulted in intolerable surface roughening and unsatisfactory diffusion results.11 Ion implantation has solved this problem to a large extent and first results on Er-implanted LiNbO3 waveguides have been published.12 II. EXPERIMENTAL
For all experiments optical grade wafers of x- and z-cut orientation were divided on a diamond saw into 134 http://journals.cambridge.org
J. Mater. Res., Vol. 6, No. 1, Jan 1991 Downloaded: 16 Mar 2015
samples of 10 x 5 X 1 mm.13 These samples were securely clamped against a Cu backplate within the implantation chamber. Its temperature was kept at 80 K, 300 K, or 620 K during implantation. The final results did not depend significantly on implantation temperature, and in the following we will discuss the 300 K data only. The ion beam was generated by inserting pure rare earth metal foil into a Freeman-type arc chamber running on HCl-gas.14 Due to the limited resolution of the separation magnet all Nd or Er isotopes were implanted according to their natural abundances. The ion energy was varied from 30 keV to 400 keV; most results communicated here were obtained at 200 keV. Scanned beam currents were typically 5-10 fiA/cm2; the dose was fixed to 1016/cm2. After a series of samples had been implanted under identical conditions, they were immediately annealed in an oxygen atmosphere for their individual soaking times, except for one which was retained for reference "as-implanted". For the RBS/ channeling evaluation of the LiNbO3 lattic
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