We report on intersubband absorption and vertical transport phenomena in regular GaN-AlN-based superlattice structures grown by MBE. We found that for barrier thicknesses larger than about 25 A, the optical intersubband absorption peaks at a much smaller energy than the photovoltage. A simple model based on the oscillator strength of the corresponding transition and the Wentzel-Kramers-Brillouin-method can qualitatively explain the experimental facts. For barrier thicknesses on the order of 20 A, a much smaller shift between the measured intersubband absorption and photovoltage peaks is observed. This energy difference is due to the strong barrier height dependence of the tunneling probability, which favors vertical transport at the upper edge of the minband formed by the excited states.
For the sample with the smallest well thickness of 17 A, we were able to detect light at 1.55 μm up to room temperature. Due to a considerable surface roughness, p- and s-polarized detector signals were almost identical in this particular device and a Gaussian line broadening of the photovoltage spectrum was observed.
Furthermore, the transient behavior of the DC component of the photovoltaic signal under strong illumination with a halogen lamp was investigated at different temperatures. Besides the intersubband absorption/tunneling process contribution to the signal, a strong influence of optical quenching and persistent photoconductivity due to midgap defects in GaN occurred under illumination of the sample with energies above 1.1 eV.
Similar investigations on lattice-matched InAlN/GaN MOVPE grown superlattice structures were carried out. They revealed valuable information about the conduction band discontinuity of this novel material combination.

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