Oscillatory Modulation of light emissions from excitons of electron-hole pairs

What is it about?

This study investigates a 0D-2D mixed-dimensional heterostructure comprising an array of InAs quantum dots embedded between AlAs potential barriers, all integrated within a GaAs p-i-n diode. We report a distinct oscillation in the excitonic photoluminescence (PL) as a function of applied bias, which occurs in out-of-phase with the observed DC-photocurrent oscillations. This work provides a detailed exploration of the physical mechanisms driving these periodic variations and examines the intrinsic relationship between these oscillations within the device. While previous studies have attempted to characterize photocurrent oscillations in similar devices, those models didn’t account for the new insights provided by our simultaneous PL measurements. In this mixed-dimensional heterostructure, photocurrent and PL serve as complementary probes: photocurrent tracks the transport of photoexcited carriers through the sample, whereas PL monitors the emission from these carriers upon recombination. Because these two techniques capture different carriers, both are essential to form a unified understanding of the underlying physics. The oscillations are due to periodic resonant tunnelling of electrons from a two-dimensional electron gas (2DEG) in to the quantum dots. As a result of momentum narrowing of these carriers upon tunnelling from 2D to 0D, the tunnelling becomes coherent leading to increased depletion of the carriers in the GaAs where 2DEG is present. This in turn decreases the observed PL of excitons in GaAs. Such a modulation of excitonic PL has been hardly studied before.

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Why is it important?

The oscillations we observe in our experiments are fundamentally distinct from the standard plateaus observed in the current-voltage characteristics of a Coulomb Blockade. Notably, the oscillatory features occur exclusively under illumination and vanish in dark conditions. Furthermore, while standard Quantum Point Contact experiments focus on single-dot behavior, the oscillations observed here emerge from a collective ensemble of quantum dots. Most importantly, the photo-generated AC conductance changes sign across the dark value; this clearly differentiates the mechanism from a Coulomb Blockade, where conductance does not typically oscillate between positive and negative values. We attribute these observed photocurrent and PL oscillations to a collective phenomenon: the periodic occurrence of resonant tunneling at the 0D-2D heterojunction. This sample was previously investigated using photocapacitance measurements, which revealed similar oscillations attributed to the periodic polarization of indirect excitons. Because these measurements were performed over a large area, such persistent oscillations suggest a collective spatial ordering of indirect excitons, hinting at the formation of a Bose-Einstein Condensate (BEC). Such a state requires momentum-space narrowing, which is facilitated by the resonant tunneling of carriers from 2DEG into the quantum dots. This implies that resonant tunneling becomes spatially coherent at specific bias voltages, leading to a periodic transition between coherent and incoherent carrier transport across the barriers. Consequently, the simultaneous analysis of photocapacitance, photocurrent, and photoluminescence provides a comprehensive framework for explaining the complex, collective behavior of carriers within the heterostructure.

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