  A giant built-in electric field in the growth direction makes excitons in
wide GaN/(Al, Ga)N quantum wells spatially indirect even in the absence of any
external bias. Significant densities of indirect excitons can accumulate in
electrostatic traps imprinted in the quantum well plane by a thin metal layer
deposited on top of the heterostructure. By jointly measuring
spatially-resolved photoluminescence and photo-induced current, we demonstrate
that exciton density in the trap can be controlled via an external electric
bias, which is capable of altering the trap depth. Application of a negative
bias deepens the trapping potential, but does not lead to any additional
accumulation of excitons in the trap. This is due to exciton dissociation
instigated by the lateral electric field at the electrode edges. The resulting
carrier losses are detected as an increased photo-current and reduced
photoluminescence intensity. By contrast, application of a positive bias washes
out the electrode-induced trapping potential. Thus, excitons get released from
the trap and recover free propagation in the plane that we reveal by
spatially-resolved photoluminescence.
