Semiconductors play an important role in our today’s lives. Our societies could not operate normally without semiconductors. They are used in devices that form new infrastructures in our societies. Decades ago, a power outage was considered catastrophic while it had mainly local consequences. Today we are living in a world where a problem in a server could instantly affect the lives of people living on another continent. This is the motivation behind numerous research on semiconductors in academia and the industry. Due to the strategic role of the semiconductor industry, governments started to pour funding into this section and to pass legislation related to it such as European Chips Act. Many different areas are involved in the semiconductor industry, from mining and recycling raw materials to designing chips and circuits, building equipment used in production lines for making semiconductor device components, etc.

After the very frst transistor made of Ge was built in 1947, the size of electronic and photovoltaic components made of semiconductors are constantly getting smaller following Moore’s law, resulting in faster and more effcient devices, which are smaller, light weighted, and widely accessible and affordable. On the other hand, size reduction makes us facing what is called the "devil’s invention" by Wolfgang Pauli: surfaces. By reducing the size of components, the role of the surface becomes more effective and important on the performance and properties of the devices. The surface part of a crystalline semiconductor compared to the bulk material beneath it naturally contains more defects, which cause electrical and optical losses in devices made of semiconductors.

This PhD thesis was dedicated to make possible improvements on the defectrich surface of Si and GaAs semiconductors by using ultra-high vacuum (UHV) and chemical treatments. One target has been that the treatments studied are scalable and therefore could be utilized on an industrial scale. Further investigation was done by testing the effect of the above treatments on selected devices. The presented experimental results suggest that by applying UHV pre-and post-treatments on Si, defect density could be reduced and the minority carrier life-time could be increased. Also, reduction in leakage current in the devices made of Si after these surface treatments are reported. A simple wet chemical method for growing gallium oxide nanocrystals on the surface of GaAs was developed which could reduce surface losses in optoelectronics devices made of GaAs.