Electrostatic potential and charge distribution at interfaces and dislocations in group III nitrides

Significant advancements in the performance of gallium nitride (GaN) based optoelectronics and high-power electronic devices have been achieved in the past decade. However, some basic issues in this material system are not well understood, such as potential profiles and the charge distribution across indium gallium nitride (InGaN) quantum wells and heterostructures, and the electronic charge states of threading dislocations. A clear picture of the potential profiles across these structures can certainly help to sharpen the understanding of the light emission mechanisms of quantum wells and electron transport properties in devices.; The subject of the present work is the experimental measurement by electron holography of the electrostatic potential profiles across a variety of structures. Phase and amplitude information of the electron wave passing through the specimens are recorded in terms of interference patterns between the sampling electron beam passing through the specimen and a reference beam traveling through a vacuum. The electrostatic potential profile in the specimen is derived from the recorded information, and the charge density is determined using Poisson's equation.; The potential variations at interfaces and the electric fields in InGaN quantum wells have been measured. The accuracy of this method and the detrimental effect of the high-energy electron beam on samples have also been analyzed. Potential profiles across threading dislocations in samples with different doping conditions have confirmed that their charge states actually depend on the Fermi-level and growth stoichiometry of materials. The correlation between the potential profiles at dislocations and the intensity minima in cathodoluminescence images shows that the drop in luminescence at the dislocations is closely related to depletion of free carriers due to electrostatic charges at the dislocations.; This work represents an initial exploration in the use of electron holography to solve critical issues in the field of nitride semiconductors. It opens up new windows, questions, and excitement, and also generates a need for continued research.