High-pressure optical and vibrational properties of InN and InGaN

Abstract

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.

The optical and vibrational properties of the InGaN alloy as a function of composition have also been investigated. The composition dependence of the fundamental bandgap of InGaN has been studied with optical absorption, and high-pressure photoluminescence measurements have been carried out to determine the pressure coefficients of the optical emission. In turn, the optical and acoustic phonons of InGaN have been investigated as a function of alloy composition at ambient conditions. From Raman measurements on InGaN thin films, we have found that strain importantly affects the dependence on composition of the optical modes. An analysis to correct for the strain-induced shifts as well as to assess the effect of compositional inhomogeneities on the optical phonon frequencies of InGaN is provided.

For the study of the acoustic modes of InGaN at room pressure, we have performed high-resolution Brillouin spectroscopy measurements, which have allowed us to determine the velocity of the surface acoustic waves of InGaN as a function of composition. With the aid of theoretical simulations based on the Green’s function formalism, these velocities have been used to evaluate the compositional dependence of the elastic constants of the alloy. Additional Raman-scattering measurements on InGaN/GaN superlattices have allowed us to detect the folded acoustic modes and observe the linear dispersion of the LA modes in InGaN, as predicted by elastic continuum theory.

Finally, the behavior of the optical modes of InGaN under pressure has been investigated by Raman spectroscopy on epilayers grown on GaN/Al2O3 and Si(111) substrates. This study, which

was initially aimed at determining the Grüneisen parameters of the optical phonons, has revealed that the experimental results strongly depend on the compressibility of the substrate material. We conclude that the pressure coefficients of free-standing InGaN should follow a linear dependence between those of GaN and InN.