Infoscience

Thesis

Picosecond time resolved cathodoluminescence to study semiconductor materials and heterostructures

A large number of characterization tools for semiconductor based heterostructures are available nowadays. Most of these techniques deliver high temporal resolution (down to hundreds of femtoseconds) or good spatial resolution (down to sub nanometer resolution), but not both simultaneously. However, to get a complete picture of carrier recombination and diffusion processes in heterostructures, one needs a spectroscopic tool which simultaneously yields high temporal and spatial resolutions. The same kind of tool is also needed to obtain local excited carrier lifetimes in a disordered material. Due to the need of such a characterization tool, we have developed an original picosecond time resolved cathodoluminescence (pTRCL) setup using an ultrafast pulsed electron gun mounted on a scanning electron microscope (SEM). The basic idea of the construction was to replace the original thermionic electron gun by a pulsed electron gun. Electron pulses are produced in the gun by the photoelectric effect: ultraviolet (UV) light pulses illuminate a gold photocathode which leads to electron extraction. This setup reaches simultaneous spatial and temporal resolutions of 50 nm and 12 ps, respectively. Motivated by the exceptional luminescence efficiency of nitride based optoelectronic devices despite of the large defect densities, we first study local luminescence lifetime in the vicinity of threading dislocations (TDs) on GaN surface. We show that the effective luminescence lifetime decreases when approaching a TD. This variation is most probably due to the decrease of the non-radiative lifetime meaning that non-radiative recombination becomes predominant when approaching a dislocation. We measure cathodoluminescence (CL) on InGaN based double quantum well (QW) structures. The structures are optimized to improve spatial resolution for monochromatic CL measurements. The study of these structures reveals the particular surface morphology of InGaN QWs by atomic force microscopy (AFM) and its connection to optical properties: The InGaN QW exhibits deep valleys oriented in the < 1 – 100 > directions where the QW thickness almost decreases to zero. TDs populate these valleys. pTRCL measurements show that excited carriers diffuse away from the TDs in the valleys which act as energy barriers hindering the excited carriers from non radiative recombination on the TDs. This possibly explains the high luminescence efficiency of InGaN based heterostructures despite the high TD density. Results on bulk BN samples are presented in the last chapter on experimental results. BN presents a very high bandgap, estimated around 6 eV , which makes optical excitation difficult in addition to a very high luminescence efficiency. These two properties make BN an ideal candidate for future UV lasers and light sources. Another exciting fact about BN is the possibility to synthesize multi- and single wall nanotubes. We show that the near bandgap luminescence peaks have very short lifetimes (between 20 ps and 32 ps) despite the indirect bandgap structure (theoretical prediction) of BN. Nevertheless, we associate the observed luminescence peak to direct exciton recombination of the Frenkel type.

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