Effect of electric bias on trapping and release of excitons in GaN/(Al,Ga)N quantum wells

Abstract

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.

Electrostatic modulation of excitonic fluid in GaN/AlGaN quantum wells by deposition of few-layer graphene and nickel/gold films

“…This means that both FLG and NiAu modify the potential landscape of IXs in the plane of the QW, although FLGinduced potential barrier is almost six times smaller. The result for NiAu is consistent with our previous evaluations [33,34]. Because IXs screen out the built-in potential according to Eqs.…”

“…( 1) depend on the surface potential. Therefore, to identify precisely these two contributions and reliably determine E 0 in three different areas of the excitonic landscape, we rely on the fact that, under a given surface potential (or, equivalently, corresponding value of the built-in electric field, F z ) not only the emission energy, but also the integrated intensity I IX is density dependent [6,27,33,34,45,50]:…”

“…This make it possible to avoid spurious currents induced by an applied electric bias that are known to be detrimental to the IX coherence [30][31][32]. As a different approach, as we have shown previously, the IX potential landscape can be patterned in these QWs by simple deposition of nickel/gold (NiAu) layers [33,34]. This is illustrated in Fig.…”

“…This ensures better photon collection efficiency. While an external bias can still be applied to the electrodes in order to manipulate the electrostatic environment [34], it is appealing to find zero-bias approaches to enrich the available potential landscapes for IXs, e.g., by changing the composition of the electrodes.…”