Fabrication of electronic structures on diamond using UV laser etching

Transistors are semiconductor devices with the main functions of amplifying or switching electrical signals. Conventional semiconductors such as Si and GaAs have inherent restraints that limit the range of power and frequency they can deliver. To satisfy the requirements for the emerging technologies such as high-speed wireless communications and electric vehicles, progress in power and frequency range of transistors is necessary. Devices based on diamond are seen as an important next-generation technology because its thermal and electrical properties provide a range of potential features that are superior to other semiconductor materials for transistor applications. However, a major problem in realizing a practical diamond technology is the high resistance of the extrinsic diamond, which has hindered development of stable low on-resistance transistors. This problem has also afflicted the most promising form of conductivity formed on hydrogen-terminated diamond surfaces and the field effect transistors (FETs) based upon on it. Several parameters affect the surface resistance including crystal orientation, surface roughness, geometry and capping material.

In this thesis, a novel UV laser-based technique is used to address the high resistance of hydrogenated diamond surfaces and to improve the current density of the FETs. The technique uses short UV laser pulses to etch the surface, and to remove carbon atoms ranging from depths corresponding to a fraction of a carbon monolayer to hundreds of nms. Upon etching with high UV doses (removal of approximately 600 carbon monolayers), a surface with (100) orientation is transformed into a ridged surface with {111} facets. Here, the capability of UV etching in surface roughening and transforming the surface orientation is used as a tool for manipulating the surface resistance. It is found that surfaces etched with high UV doses exhibit five times improved conductance compared to unexposed surfaces and provide a record low resistivity measurement of 0.9 kΩ sq^-1. The enhancement is larger than expected on the basis of geometric arguments and surface transformation. Further investigations reveal that even low UV doses (removal of less than one carbon monolayer) improve the resistance. The UVtreated surfaces lead to a likely permanent enhancement - observed to be stable over 16 days after hydrogenation.

In order to investigate the underlying reasons for the observed enhancement, surface sensitive studies including Hall effect, X-ray photoelectron spectroscopy and photoluminescence measurements are carried out. These measurements show that even a sub-monolayer etch reduces the resistance by bringing about a large increase in the carrier concentration on hydrogenated surfaces, with a slight decrease in the carrier mobility. The sub-monolayer removal is found to cause a shift in the maximum of the valence band allowing more electrons to be transferred to the adsorbates, leading to an increase in the carrier concentration. Furthermore, an increase in the C=O concentration on the oxygen-terminated surface and nitrogen vacancy generation in the subsurface is detected which point towards generation of defects on the surface upon UV laser etching. A likely relationship between etch-induced defects and steps on the surface and carrier concentration is proposed and discussed.

The UV etching is combined with a conventional FET fabrication technique for fast prototyping of FETs with improved performance. Laser etching was used to reduce the on-resistance, and also used post hydrogenation to make insulating regions and reduce the number of lithography steps. The fabricated FETs show up to six times enhancement in drain current compared to the FETs on unprocessed surfaces. In comparison to the state-of-the-art FETs with NO2 activation, UV laser etching provided a similar drain current but with excellent prospects for long term stability.

Future optimization of the UV laser etching, combined with submicron-sized devices and advanced capping oxides, point towards the development of a practical diamond FET technology with outstanding power and frequency performance.