High-pressure optical and vibrational properties of InN and InGaN

This thesis is devoted to the study of the optical and vibrational properties of indium nitride (InN) and indium gallium nitride (InGaN) at room and high-pressure conditions. For this purpose, we have employed spectroscopic tools such as absorption spectroscopy or Raman scattering in order to investigate a series of InN and InGaN thin films grown with different methods and on different substrates. For the high-pressure measurements, we have employed the diamond anvil cell technique. High-pressure optical absorption experiments on InN epilayers have allowed us to observe the direct-to-indirect bandgap transition at 15 GPa, where wurtzite InN (w-InN) transits to the rocksalt polymorph (rs-InN). Investigating w-InN samples with different levels of residual electron density, we have been able to estimate the bandgap pressure coefficient of intrinsic w-InN (32 meV/GPa). In addition, we have measured the indirect bandgap of rs-InN and its pressure dependence. We have also performed FTIR reflectivity measurements to determine the pressure dependence of the refractive index of w-InN and rs-InN. By fitting the experimental results with a model for the dielectric function, we have determined the pressure coefficient of the high-frequency dielectric constant of both phases. The pressure coefficient of the phonon frequencies of w-InN and their respective mode Grüneisen parameters have been measured by high-pressure Raman spectroscopy. After the wurtzite-to-rocksalt phase transition and the rocksalt-to-wurtzite backtransition upon decompression, the Raman features of both (amorphized) phases have been assigned in terms of first-principle lattice-dynamics calculations. Raman measurements on a heavily doped n-type sample have allowed us to detect a longitudinal-optical plasmon coupled mode, from which we have evaluated the pressure dependence of the electron effective mass of w-InN. With the aim of comparing the pressure behavior of the optical modes of rs-InN with that of a material exhibiting the rocksalt structure at ambient conditions, a high-pressure Raman-scattering study on rocksalt CdO is also presented.Peer Reviewe