Systematic experimental and theoretical investigation of intersubband absorption inGaN∕AlNquantum wells

Abstract

We have studied the electronic confinement in hexagonal (0001) $\mathrm{Ga}\mathrm{N}∕\mathrm{Al}\mathrm{N}$ multiple quantum wells by means of structural (high-resolution x-ray diffraction and transmission electron microscopy) as well as optical characterizations, namely intersubband absorption and interband photoluminescence spectroscopies. Intense intersubband absorptions covering the $1.33--1.91\phantom{\rule{0.3em}{0ex}}\ensuremath{\mu}\mathrm{m}$ wavelength range have been measured on a series of samples with well thicknesses varying from $1\phantom{\rule{0.3em}{0ex}}\text{to}\phantom{\rule{0.3em}{0ex}}2.5\phantom{\rule{0.3em}{0ex}}\mathrm{nm}$. The absorption line shape exhibits either a pure Lorentzian shape or multiple peaks. In the first case the broadening is homogeneous with a state-of-the-art low value of $67\phantom{\rule{0.3em}{0ex}}\mathrm{meV}$. We deduce a dephasing time of the electrons in the excited subband ${T}_{2}$ of about $20\phantom{\rule{0.3em}{0ex}}\mathrm{fs}$. For structured spectra the absorption can be perfectly reproduced with a sum of several Lorentzian curves; the individual peaks originate from absorption in quantum well regions with thickness equal to an integer number of monolayers. We have also carried out simulations of the electronic structure which point out the relevance of the nonparabolicity and many-body corrections on the intersubband absorption energy. The intersubband absorption exhibits a blue shift with doping as a result of many-body effects dominated by the exchange interaction. An excellent agreement with the experimental data is demonstrated. The best fit is achieved using a conduction band offset at the $\mathrm{Ga}\mathrm{N}∕\mathrm{Al}\mathrm{N}$ heterointerfaces of $1.7\ifmmode\pm\else\textpm\fi{}0.05\phantom{\rule{0.3em}{0ex}}\mathrm{eV}$ and a polarization discontinuity $\ensuremath{\Delta}P∕({ϵ}_{0}{ϵ}_{r})$ of $10\ifmmode\pm\else\textpm\fi{}1\phantom{\rule{0.3em}{0ex}}\mathrm{M}\mathrm{V}∕\mathrm{cm}$.