A novel way of measuring lifetime at the nanometer scale using specific fast electron‐matter interactions
Authors: Meuret Sophie , Tizei Luiz , Auzelle Thomas , Cazimajou Thibault , Bourrellier Romain , Songmuang Rudee , Chang Huan‐Cheng , Treussart François , Daudin Bruno , Gayral Bruno , Kociak Mathieu 
Published: December 2016

Charge carrier lifetime is a key parameter for understanding the physics of electronic or optical excitations. For example the excited state can unveil details of environmental influence, specifically the role of non‐radiative transitions. From a practical point of view, lifetimes can largely determine the performances of devices, such as Light Emitting Devices (LEDs) or photovoltaic cells. These usually rely on nanometer scale structures for which small details, such as the presence of single point defects, have to be known with atomic precision. Despite the success of super resolution optical microscopies, they fail as general tools for lifetime measurement at the nanometer scale. In this presentation we will show how we can take advantage of the nanometer probe size formed in a Transmission Electron Microscope and a phenomenon that we recently discovered (referred hereafter as the bunching effect [1]), to study lifetimes of emitter at the nanometer scale without using a pulsed electron gun.

The effect takes its name from the fact that the autocorrelation function g
(2)
(τ) of the CL signal coming from quantum emitters (points defects or more generally single photon emitters –SPE‐, quantum confined structures…) may exhibit a peak at zero delay – which is a fundamental difference with PL. To measure this effect, we use an intensity interferometry experiment that measures the CL g
(2)
(τ). Figure 1 shows that, at low incoming electron currents (I < 100 pA), the g
(2)
(τ) of the CL signal intensity I(t) displays a large nanosecond‐range peak at zero delay (g
(2)
(0) > 35) (bunching), the amplitude of which depends on the incoming electron current. This behavior strongly departs from the PL g
(2)
(τ) function which is flat when multiple independent SPE are excited. In this presentation we will show that it occurs because an emitter, like a quantum well, will be excited multiple times by a single electron and will emit a bunch of photons on a time window close to its radiative lifetime. As it will be proved, by simply fitting the experimental curve of the g
(2)
(τ) function by an exponential we can retrieve the lifetime of the emitter.

Using this effect we were therefore able to measure very efficiently lifetimes of Gallium Nitride quantum wells (QWs) separated by less than 15 nm, together with their emission energy and atomic structure (Figure 2). Experiments on well separated individual quantum structures shows an excellent agreement with combined time‐resolved μ‐photoluminescence. We also demonstrate the possibility to measure the lifetimes of emitters of different kinds (defects, QWs, bulk) within a distance of a tenth of nanometers even for spectrally overlapping emissions. This technique is readily applicable to large ensembles of single photon sources and various emitters such as QWs, quantum dots, point defects and extended defects, such as stacking faults (SF).