Development of new characterization techniques for III-V nanowire devices

Abstract: This dissertation presents the new methods and techniques developed to investigate the properties of nanowires (NWs) and NW devices and the results obtained using these methods. The growth and characterization of NWs have become a large research field because NWs have been shown to improve the properties of many semiconductor applications such as transistors, solar cells, and light emitting diodes. The structural composition, optical properties, and electric characteristics of NWs and NW devices are affected by effects at the atomic level. The surface of NWs plays a crucial role when it comes to these characteristics because of the large surface to volume ration of the NW structures. This makes the characterization of these structures, at the atomic level, a key factor, for understanding the underlying mechanisms, and for the development of even more suitable structures. Here, the composition of III-V semiconductor materials and the electronic properties of III-V semiconductor NWs are investigated using scanning tunneling microscopy (STM) and X-ray photoelectron spectroscopy (XPS). A new method for studying contacted NWs on insulating substrates with STM is described, and the results from investigations of InAs-GaSb Esaki diode NWs are presented. The ability to study the NW side facets with STM while at the same time being able to apply a potential along the NW makes it possible to connect the device performance with the NW characteristics found with the STM. The conductivity of up-standing, as grown NWs is also measured with an STM using the novel technique called top contact mode. The method is used to evaluate, the Schottky barrier height of the Au GaAs interface in GaAs NWs, and the conductivity of InP and InAs NWs. This method makes it possible to measure the electric conductivity of the NWs without any additional device fabrication, making it more reliable due to the good ohmic electrical contacts established to the NW. It also, in contrast to conventional methods, enables well-controlled surface treatment of the NW side facets which is used to show how surface oxides influence NW conductance.XPS and the more penetrating hard X-ray photoelectron spectroscopy (HAXPES) are used to evaluate the homogeneity and the growth of HfO2 films on InAs as well as the mechanism behind the self-cleaning effect of the InAs native oxide. This information is especially important for the continued work on semiconductor transistors where the HfO2 is one of the best candidates to be used as the gate oxide and where the interface between the oxide and semiconductor is crucial for device performance.