Thesis presented March 06, 2009

Abstract:
The aim of this work is to study the vertical electronic transport along the growth axis in AlN/GaN heterostructures. The comprehension of this phenomenon is of first importance in order to improve the design of unipolar devices such as resonant tunneling diodes, quantum well infrared photodetector or quantum cascade lasers.
This work begins with the study of GaN samples with a single AlN or AlGaN barrier, where the growth is performed by plasma-assisted molecular-beam epitaxy. I have analyzed the effect of the internal electric field generated by the barrier, and the role of the dislocations. To do so, I have developed some original experiments, such as photoluminescence spectroscopy under bias and conductive atomic force microscopy. Optical measurements provided information on the repartition of the internal electric field in the structure when we applied an external electric field, which confirmed the results obtained by simulations. Local conduction characterization revealed that pure screw dislocations are the main source of leakage current. The density of this kind of dislocations is fifty times smaller than the total dislocation density in the material, which raises the possibility to design components without conductive defects by scaling the patterns down to the micrometer scale.
In a second stage, I described the results of the study of samples with a double AlN barrier in a GaN matrix. Samples with a GaN quantum well width of 0.5 nm display a negative differential resistance around 1.5 V. This feature is observed only during the first measurement, and can be recovered only by applying a negative bias of about -4 V. In the samples with a wider well, we also observe two current states, the first one being attributed to leakage through dislocations and the second one to a non-resonant tunneling current assisted by defects.
Finally, I present two kinds of devices which use the vertical electronic transport in nitride heterostructures: charge transfer electro-optical modulators and quantum well infrared photodetectors. The good performance of the modulators confirms the feasibility of tunnel transport through an AlN barrier. For the quantum well photodetectors, it was demonstrated that they can operate in the photovoltaïc mode, but problems appear under bias. Further studies are required for the development of photoconductive infrared photodetectors.

Keywords:
Nitrides, vertical transport, conductive atomic force microscopy

On-line thesis.