Picosecond cathodoluminescence

We have developed an original time resolved cathodoluminescence (TRCL) set-up with temporal performances similar to those of conventional time resolved optical techniques, keeping the capability to get structural information through the secondary electron mode of an electron microscope (SEM). This system allows for performing ultrafast spectroscopy on nanostructures leading insight into phenomena like transport, carrier capture and carrier recombination. A traditional TRCL is based on the use of a SEM column equipped with an electrostatic beam blanking unit and a photon counting system for the detection. Electron pulses of rise and decay times ~ 200 ps, varying width from 1ns to 1µs and a repetition rate up to 1 MHz are used for the excitation. Due to the long duration of the pulse compared to characteristic relaxation times, the system under investigation is set in a quasi-equilibrium state before the study of the luminescence decay. This leads to a non-straightforward interpretation of the temporal luminescence profile; moreover resolution is limited to 250 ps. In order to overcome such limitations, we have replaced the thermionic electron gun of a SEM with a home-made ultrafast electron gun. Femtosecond mode-locked laser pulses are focused on a metal photocathode to create electron bunches. An extraction electrode and an anode accelerate photoelectrons up to 30 kV. The electron lens systems of the microscope column focus the photoelectron beam on the sample. Luminescence emitted as a consequence of the probe beam excitation is spectrally analyzed with a monochromator and it is then collected with a streak camera for temporal analysis. This set up (picoCL) has achieved unprecedented combined spatial and temporal resolutions. The test for the spatial resolution is carried out on gold particles grown on a carbon substrate: a sample currently used to test commercial SEMs resolution. We prove that the ultrafast electron gun brightness is high enough to focus electrons on a probe diameter of 50 nm still having enough current to obtain a secondary electron image of the sample. The temporal width Δt of the electron pulses is measured by an indirect method. We compare the time resolved photoluminescence response (200 fs laser pulses excitation) of a semiconductor sample with that obtained with the picoCL. A Δt (FWHM) = 12 ± 1 ps is found. As a first study with the picoCL we investigate the time resolved luminescence emission from quantum structures located in InGaAs/AlGaAs tetrahedral pyramids. An In0.10Ga0.90As quantum dot (QD) formed just below the top of the pyramid is connected to several types of low-dimensional barriers: InGaAs quantum wires (QWRs) on the edges of the pyramid, InGaAs quantum wells (QWs) on the (111)A facets and segregated AlGaAs vertical quantum wire (VQWR) and AlGaAs vertical quantum wells (VQWs) formed at the centre and at the pyramid edges. PicoCL is successful in identifying the spectral features of the different nanostructures. Indeed the rise and decay times of their luminescence emissions vary strongly with the location of the excitation point on the pyramid. The intricate and complex carrier transport among the different quantum structures is enlightened: our results suggest the scenario that after excitation on the facet or on the edge of the pyramid, carriers diffuse towards the central structures (QD and VQWR) via the QWR. According to these findings we model the carrier diffusion along the QWR and fit our experimental data. A carrier mobility of 1300 cm2/Vs is found.

Ganière, Jean-Daniel
Lausanne, EPFL
Other identifiers:
urn: urn:nbn:ch:bel-epfl-thesis3206-8

 Record created 2005-03-16, last modified 2018-01-27

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