Investigation of carbon profiles for enhanced boron confinement and improved carrier transport in strained silicon germanium nanolayers for heterojunction bipolar transistors.

摘要

This research covers a breadth of topics, in Chapters 1 through 7, ranging from the crystal lattice, to dopant diffusion in SiGe, to SiGe and SiGeC chemical vapor deposition, to the Si/SiGe and Si/SiGeC energy band structure, and NPN SiGeC HBT AC and DC characteristics. Chapters 8 and 9 contain the results of the research, which relates the film growth and carbon positioning to boron diffusion, sheet resistance, and device performance; specifically current gain, fmax, and noise figures of merit.; The first objective of the dissertation was to investigate carbon doping profiles in nano-layers (≤32 nm) of silicon germanium (Si1-xGe x), and provide an understanding of "remote carbon boron confinement" (RCBC), which is demonstrated to exploit the advantages of carbon to increase NPN HBT (heterojunction bipolar transistor) performance, reduce base resistance, and improve overall noise figures of merit. The second objective was to investigate the noise characteristics of this method compared to the standard method of placing carbon throughout the lattice, which is known as "uniform carbon boron confinement" (UCBC).; The current technological development towards smaller and faster devices has forced engineers and scientists to look into materials other than silicon, but which are highly compatible. A natural choice is the Si1-xGe x alloy, since Ge is also a Group IV. Si1-xGex has the same lattice structure as Si, but its lattice constant is 4.2% larger (aSi = 0.543nm, aGe = 0.567nm), and the bandgap is less than that of Si (Eg_Si = 1.11eV, Eg_Ge = 0.67eV). This opens the possibility of bandgap, strain, and dopant diffusion engineering, all of which affect the material and electronic properties of devices.; The primary benefit of carbon is to reduce the diffusion of boron in Si1-xGex thus keeping the base narrow for significantly reduced electron transit times and increased unity gain cutoff frequencies (fT). However the utilization of carbon reduces base conductivity and increases base recombination current, while reducing the built-in biaxial strain. This latter property may or may not be desirable, while the first two are not desirable having negative consequences for fmax and noise figures of merit such as NF min.; Chapter 8 discusses portions of this research, which identified a first-to-be-published method for carbon doping of silicon germanium films, which enhances the carbon confinement of boron, significantly reduces film resistance by approximately 23%, and increases the NPN HBT device performance. Chapter 9 discusses the improvements in noise figures of merit for an NPN HBT, which arise from improved film qualities presented in the Chapter 8 discussion. This research also illustrates that the device performance figures of merit such as base resistance, current gain, and subsequent noise figures are significantly determined during the epitaxial film growth.