Photocurrent spectroscopy of bound-to-bound intraband transitions in GaN/AlN quantum dots

Using photocurrent infrared spectroscopy, we characterized bound-to-bound transitions in the conduction band of samples with high density of GaN/AlN quantum dots (QDs). TE (TM) polarized peaks at 0.15-0.23 eV (0.7-0.9 eV) depending on the dot size, are associated with S- Px,y (S- Pz) transitions. The transition energies and the oscillator strengths were analyzed by numerical solution of the Schrödinger equation in three dimensions for truncated hexagonal pyramid dots. Assuming peak broadening due to dot size fluctuations, we derived the dot dimensions for which all observed photocurrent peak energies can be accounted for. It is shown that for these dimensions to agree with the quantum well limit both nonparabolicity and anisotropy of the conduction band must be taken into account. The appearance of photocurrent due to bound-to-bound intraband transitions within the QDs is attributed to lateral hopping conductivity. Analysis of the photoresponse magnitude due to optical excitation of electrons to different states in the QD yields a two-dimensional mobility edge at ∼1 eV above the GaN conduction-band edge. The hopping model is further supported by the temperature dependence of the dark conductivity and its sensitivity to the size and the density of the QDs.