Optical studies of diffusion, ion implantation and stimulated emission in CdTe epilayers and CdMnTe/CdTe quantum wells

Various experiments, involving P.L., P.L.E., D.C.X.R.D., reflectivity and magnetic field P.L.E. measurements were performed upon cleaved sections of M.B.E. fabricated CdTe epilayers and CdMnTe/CdTe superlattices. It has been shown that thermally annealing of superlattices at 250 deg. C yields measurable blueshifts in the optical spectra, attributable to interdiffusion of the Mn barrier ions. The shooting technique has been employed to calculate the diffusion coefficient of Mn barrier ions from the experimental results. This method yielded values of the diffusion coefficient, D, of 0.008 A2s-1 at 250 deg. C for Te overpressure annealing and 0.003 A2s-1 at 250 deg. C for Cd overpressure annealing for sample m550. CdTe/CdMnTe superlattices were also subjected to Pulsed Laser Annealing, (P.L.A.) with an XeCl laser. It has been shown that a laser induced heat pulse propagates through the superlattice stack, resulting in interdiffusion of the Mn barrier ions into the quantum wells. At an applied fluence of between 80 and 90 mJcm-2, melting is induced in the surface region of the structure, causing the destruction of the quantum wells in this region and yielding an alloy of CdMnTe. CdTe epliayers were implanted with nitrogen, copper and argon ions at doses of between 1011 and 1015 cm-2 at 100 keV and then laser annealed in order to restore the optical properties of the layers and yield activation of the impurity acceptors. The resultant P.L., P.L.E. and D.C.X.R.D. spectra provide evidence that post implantation P.L.A. results in the activation of all three impurity species. Cleaved cavities of a CdTe/CdMnTe superlattice were photopumped with a tuneable dye laser with amplifier stages, itself pumped via a frequency doubled, Q-switched Nd: YAG laser in order to achieve stimulated emission. Stimulated emission was observed at up to 60K for two cleaved cavities of widths of 1090 μm. and 470 μm. A threshold power density of 30.8 kWcm-2 at a temperature of 4K was measured for the wider laser cavity. The results obtained indicate that the gain mechanism is likely to be due to exciton localisation caused by inhomogeneous broadening of' the exciton resonance. (author)