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Urbach tails in semiconductors are often associated to effects ofcompositional disorder. The Urbach tail observed in InGaN alloy quantum wellsof solar cells and LEDs by biased photocurrent spectroscopy is shown to becharacteristic of the ternary alloy disorder. The broadening of the absorptionedge observed for quantum wells emitting from violet to green (indium contentranging from 0 to 28\%) corresponds to a typical Urbach energy of 20~meV. A 3Dabsorption model is developed based on a recent theory of disorder-inducedlocalization which provides the effective potential seen by the localizedcarriers without having to resort to the solution of the Schr\"odinger equationin a disordered potential. This model incorporating compositional disorderaccounts well for the experimental broadening of the Urbach tail of theabsorption edge. For energies below the Urbach tail of the InGaN quantum wells,type-II well-to-barrier transitions are observed and modeled. This contributionto the below bandgap absorption is particularly efficient in near-UV emittingquantum wells. When reverse biasing the device, the well-to-barrier belowbandgap absorption exhibits a red shift, while the Urbach tail corresponding tothe absorption within the quantum wells is blue shifted, due to the partialcompensation of the internal piezoelectric fields by the external bias. Thegood agreement between the measured Urbach tail and its modeling by the newlocalization theory demonstrates the applicability of the latter tocompositional disorder effects in nitride semiconductors.

Localization landscape theory of disorder in semiconductors II: Urbach tails of disordered quantum well layers