Light Sources in Semiconductor Photonic Materials

Abstract

In photonic materials the refractive index varies on a length scale comparable to the wavelength of light; as a consequence, propagation of light is strongly modified with respect to that in homogeneous dielectric materials. In this thesis experiments are reported with light souces in crystalline and random photonic materials. It had been predicted that photonic crystals could modify the radiative decay rate of embedded light sources. However, this prediction was never verified experimentally. We have been able to prove this prediction for the first time using CdSe nanocrystals as light sources in titania inverse opal photonic crystals. Spherical CdSe nanocrystals with a diameter between 3 nm and 6 nm were made by wet-chemical synthesis. These nanocrystals show so-called 'quantum-size-effects': the colour of the emission depends on the nanocrystals' size. Nanocrystals of 3 nm size emit blue light while nanocrystals of 6 nm emit red light. We measured the rate of emission, or the radiative lifetime, of the nanocrystals as a function of emission colour and observed that blue light is emitted about three times faster than red light. The rates were found to be in good agreement with quantum-mechanical calculations on small particles. Subsequently, we embedded the nanocrystals in our photonic crystals and again measured the radiative lifetime. We observed that the lifetime of the nanocrystals varied by a factor-of-three depending on the lattice parameter of the photonic crystal. With this we have confirmed the hypothesis that photonic crystals can indeed control the radiative lifetime of embedded light sources. Porous gallium phosphide (GaP) is a random photonic material that can be made by electrochemical etching of crystalline GaP. This process has been applied for many years. We observed that during etching light is generated at the pore fronts. We were able to reveal the mechanism of light generation and attributed it to a hot-carrier process. During etching a high electric field is required. As a consequence hot carriers are generated and these hot carriers give rise to emission. Besides exploration of the emission and its mechanism we were able to use the emission as an internal light source to measure the optical properties of porous GaP during electrochemical etching. This allows one to deduce the optical characteristics of porous GaP directly and over the complete visible range in a single experiment.