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 $1.33–1.91\mu \mathrm{m}$ wavelength range have been measured on a series of samples with well thicknesses varying from $1\text{to}2.5\mathrm{nm}$. 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 $67\mathrm{meV}$. We deduce a dephasing time of the electrons in the excited subband $T_2$ of about $20\mathrm{fs}$. 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 $1.7\pm 0.05\mathrm{eV}$ and a polarization discontinuity $\Delta P∕\left({ϵ}_0{ϵ}_r\right)$ of $10\pm 1\mathrm{M}\mathrm{V}∕\mathrm{cm}$.

We have studied the effect of growth and design parameters on the performance of Si-doped GaN/AlN multiquantum-well (MQW) structures for intersubband optoelectronics in the near infrared. The samples under study display infrared absorption in the $1.3–1.9\mu \text{m}$ wavelength range, originating from the photoexcitation of electrons from the first to the second electronic level in the QWs. A commonly observed feature is the presence of multiple peaks in both intersubband absorption and interband emission spectra, which are attributed to monolayer thickness fluctuations in the quantum wells. These thickness fluctuations are induced by dislocations and eventually by cracks or metal accumulation during growth. The best optical performance is attained in samples synthesized with a moderate Ga excess during the growth of both the GaN QWs and the AlN barriers without growth interruptions. The optical properties are degraded at high growth temperatures $\left(>720°\text{C}\right)$ due to the thermal activation of the AlN etching of GaN. From the point of view of strain, GaN/AlN MQWs evolve rapidly to an equilibrium average lattice parameter, which is independent of the substrate. As a result, we do not observe any significant effect of the underlayers on the optical performance of the MQW structure. The average lattice parameter is different from the expected value from elastic energy minimization, which points out the presence of periodic misfit dislocations in the structure. The structural quality of the samples is independent of Si doping up to ${10}^{20}{\text{cm}}^{-3}$ . By contrast, the intersubband absorption spectrum broadens and blueshifts with doping as a result of electron-electron interactions. This behavior is independent of the Si doping location in the structure, either in the QWs or in the barriers. It is found that the magnitude of the intersubband absorption is not directly determined by the Si concentration in the wells. Instead, depending on the Al mole fraction of the cap layer, the internal electric field due to piezoelectric and spontaneous polarization can deplete or induce charge accumulation in the QWs. In fact, this polarization-induced doping can result in a significant and even dominant contribution to the infrared absorption in GaN/AlN MQW structures.