Title: Localization landscape theory of disorder in semiconductors II: Urbach tails of disordered quantum well layers,
Abstract: Urbach tails in semiconductors are often associated to effects of
compositional disorder. The Urbach tail observed in InGaN alloy quantum wells
of solar cells and LEDs by biased photocurrent spectroscopy is shown to be
characteristic of the ternary alloy disorder. The broadening of the absorption
edge observed for quantum wells emitting from violet to green (indium content
ranging from 0 to 28\%) corresponds to a typical Urbach energy of 20~meV. A 3D
absorption model is developed based on a recent theory of disorder-induced
localization which provides the effective potential seen by the localized
carriers without having to resort to the solution of the Schrödinger equation
in a disordered potential. This model incorporating compositional disorder
accounts well for the experimental broadening of the Urbach tail of the
absorption edge. For energies below the Urbach tail of the InGaN quantum wells,
type-II well-to-barrier transitions are observed and modeled. This contribution
to the below bandgap absorption is particularly efficient in near-UV emitting
quantum wells. When reverse biasing the device, the well-to-barrier below
bandgap absorption exhibits a red shift, while the Urbach tail corresponding to
the absorption within the quantum wells is blue shifted, due to the partial
compensation of the internal piezoelectric fields by the external bias. The
good agreement between the measured Urbach tail and its modeling by the new
localization theory demonstrates the applicability of the latter to
compositional disorder effects in nitride semiconductors.