Probing Electron Collisions in Nanostructures

Abstract: We live in a time of wireless technologies, robotics and small computers with very high computing power. This is mainly due to a large emphasis given towards the study of semiconductor materials which are the building blocks of today's electronic industry. One of the secrets behind creating faster computers is the ability to integrate more and more transistors on a small semiconductor chip. Gordon Moore, the co-founder of Intel predicted in 1965 that the number of transistors per integrated circuit will be doubled every two years. His prediction accurately worked for several decades. To fit in large number of transistors in a small computer chip, a reduction in the size of the transistors is a key. This is where the emergence of a relatively newer field of technology comes in to play, nano engineering. With nano engineering, transistors as small as few nm size can be fabricated. In 2016 Intel has reported a processor where 7.2 billions of transistors integrated only on a 456mm2 area chip. As the size of the transistors gets smaller and smaller, the laws of physics governing the motion of charge carriers through the devices has to be modified compared to larger transistors. One of the most common experimental setup is the measurement of current through the device which is connected to a potential difference (voltage). In macroscopic size electronic circuits, the dependence of the current on the applied voltage can be either linear or non-linear depending on the circuit elements. For a regular resistance, such as light bulb, the current is linearly proportional to the applied voltage (Ohm's law). However, if the circuit is composed of non-linear elements such as transistors or diodes, the dependence of the current on the applied voltage is non-linear, but typically the current increases with bias. This situation is different in nano scale electronic devices which are characterizedby discrete energy levels due to confinement. In this case the current displays discrete peaks which are dependent on the accessibility of energy levels for the applied bias. In the first part of this thesis, a study of electron transport properties of quantum dots has been performed. Due to confinement, the electron-electron (ee) interaction is enhanced in quantum dots compared to macroscopic size devices. As a result, understanding and careful description of the interaction types and their strength to the transport of charge carriers through the device is of great importance. One peculiar behavior in nanoscale devices is that the ee interaction becomes a factor that greatly determines the transport behavior across these devices. In the second part of the thesis, interaction of light with nanostructures is studied with the aim of simulating the microscopic physical processes in efficient quantum dot based solar cells. The photovoltaic effect in which a material generates an electric current as a result of exposure to light has been known for more than a century. Many countries are now giving priorities for utilization of renewable energy sources for electric power generation. Semiconductor based solar cells have already been used to convert solar energy to a usable form of electric current. However, they are not as popular yet as fossil fuel due to their limited efficiency. In 1961 a famous work by William Shockley and Hans Queisser puts a limit on the maximum theoretical conversion efficiency of a solar cell using a single p-n junctionto be not more than 33.7 %. Finding possibilities to circumvent the Shockley-Queisser limit is an active area of current research. In recent years, quantum-dot based solar cells demonstrated enhanced conversion efficiency. An understanding on a microscopic level how the charge carriers and the light field interacts in quantum dot based solar cells plays a key role in the design of the future solar cells. One mechanism which is described in this thesis is the multiple exciton generation (MEG) in which a generation of more than one electron-hole pairs (exciton) per absorbed photon enhances the conversion efficiency.