Intersubband absorption at λ∼1.55 μm in well- and modulation-doped GaN/AlGaN multiple quantum wells with superlattice barriers

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Abstract
Intersubband optical absorption around 1.55 μm has been measured in GaN/AlGaN multiple quantum wells (MQWs) grown by molecular-beam epitaxy. First, peak absorption wavelengths as short as 1.41 μm are reported for ultranarrow, ⩾11 Å wide, well-doped MQWs with high, 85%, AlN mole-fraction barriers. Second, in order to enable modulation doping as well as the use of lower AlN mole-fraction barriers, we designed and fabricated QWs embedded in barriers consisting of a short period superlattice of narrow GaN QWs and only 65% AlN mole-fraction barriers. The resulting electron Bragg confinement allows peak absorption wavelengths as short as 1.52 μm. Furthermore, the structures can now be modulation doped through doping of the narrow superlattice wells and subsequent charge transfer into the active well. We observe a reduction of the absorption linewidth, from ∼200 to ∼130 meV, for these structures.