Etude des proprietes optoelectroniques de nanocristaux colloidaux a faible bande interdite : application a la detection infrarouge

Abstract

Colloidal semiconductor nanocrystals are nanomaterials synthesized in solution. Below a certain size, these nanocrystals acquire quantum confinement properties: their optoelectronic properties depend on the nanoparticle size. In the visible range, colloidal nanocrystals are quite mature. The next objective in this field is to get infrared colloidal nanocrystals. Mercury selenide (HgSe) and mercury telluride (HgTe) are potential candidates. The goal of this PhD work is to strengthen our knowledge on optical, optoelectronic and transport properties of these nanocrystals, in order to design an infrared detector. To do so, we studied the electronic structure of HgSe and HgTe for different sizes and surface chemistries. We can then determine the energies of the electronic levels and the Fermi energy, quantify doping level. We show that the nanocrystal size has an influence on doping level, which gets more and more n-type as the nanocrystal size gets larger. We even observe a semiconductor-metal transition in HgSe nanocrystals as the size is increased. The doping control with surface chemistry is then investigated. By using dipolar effects or oxidizing ligands, we show a doping control over several orders of magnitude. Thanks to these studies, we are able to propose a HgTe based device for detection at 2,5 um, which structure allows to convert effectively the absorbed photons into an electrical current and to get a high signal over noise ratio. We get a photoresponse of 20 mA/W and a detectivity of 3.10^9 Jones.