The MBE growth and electrical characterisation of high resolution doped Si/GeSi structures
Exploring the properties and physical limits of nanometer scale structures and devices, emerged with the advent of epitaxial techniques such as Molecular Beam Epitaxy (MBE). This has correspondingly created challenges to the available characterisation techniques. Determination of carrier concentration profiles in semiconductor structures is of vital importance since the operation of devices depends on it. Of the commonly used techniques, conventional capacitance-voltage (CV) has a major drawback due to the breakdown voltage at high reverse bias particularly at highly doped structures. The most competitive techniques for carrier concentration profiling are the electrochemical CV (ECV) which does not suffer from this limitation, Spreading Resistance Profiling (SRP) and Hall combined with stripped measurement. This thesis reports experimental investigations of the capability and limitations of the ECV technique through comparisons with Secondary Ion Mass Spectroscopy (SIMS) and SRP on hitherto difficult profiling conditions in Si and, for the first time, carrier profiling in SilSiGe structures. The ECV technique is shown to be well capable of profiling Si structures doped with boron up to the solid solubility limits. It is also demonstrated for the first time that ECV is better suited to profiling ultra thin boron layers including deltalayers in Si than the SRP technique. The first attempts to profile boron doped Si/SiGe structures have revealed that this material system can be depthprofiled with the electrolytes used to profile Si under optimised conditions, providing that the Ge concentration is kept below 25%. The importance of the electrolytes, leakage current, and the models used are also discussed with specific samples. Also the changes in etch current density between Si and SiGe enabled Ge profiles to be obtained in Si/SiGe heterostructures. World record mobilities in strained SiGe channel MBE-grown normal structures are obtained through the use of very high substrate temperatures during growth whilst reducing the Ge concentration below 13% and limiting the thickness of the alloy layer. The theoretical calculations related to scattering mechanisms suggested that utilising high substrate temperatures results in reduction of both interface charge and interface roughness scattering, these being the dominant scattering mechanisms in the present material system.