Electrical and Surface Characterization for InAs/GaAs Site-Controlled Quantum Dots in Schottky Diodes

Abstract

In this thesis, a research study is held as a part of the Photonic quantum cellular automata (PhotonicQCA) project at the Tampere University of Technology. The study includes both surface and electrical characterization for the site-controlled quantum dots (QDs). The surface characterization includes the statistics of the pits diameter. The statistics shows that for 30 and 40 nm pits, the occupational probability with QDs is low. The best case is obtained with 50 nm. Additionally, the problem with 60 nm pits was due to filling the pit with multiple QDs. Thus, 50 nm diameter pits were selected for the project. The research work in the thesis continues to study the double pits with different spacing and orientation. The aim of the study is to find out which orientation and spacing shall fulfill the needs of the project. The study includes three orientation; [011] double pits, [0-11] double pits, and [010] cross double pits. For each orientation, 10 different spacing’s between the pits are studied from 45 to 100 nm. The [011] orientation affords the minimum spacing between the pits. Additionally, the spacing between the two QDs (after the pits are filled) increases steadily with the increase of the distance be-tween the pits.

The electrical characterization of the QDs is performed in Schottky diode structure where the capacitance is measured as a function of reverse applied voltage at different temperatures. There are 9 samples which are involved in the study. The samples are prepared in a way to observe the effect of each fabrication step on the overall capacitance. Due to the chemical cleaning step which is needed before the sample is loaded to the Molecular beam epitaxy reactor, a remarkable defect layer is created which trap the electrons and screen the depletion region. Thus, the capacitance does not change normally with the applied voltage and the site-controlled QDs layer does not contribute to the overall capacitance which is kept constant until the depletion layer pass over this parasitic defect layer.