****NON-TECHNICAL ABSTRACT**** This project will address two semiconductor issues of great scientific and technological interest. The first one relates to the precise motion of impurities in semiconductors at high temperatures caused by a process called diffusion. Mixed crystal layers grown on standard silicon wafers generate mechanical stress, which in turn affects carrier mobility and dopant diffusion. These effects shall be studied and determined quantitatively so that the full potential of crystal alloying can be reached. The results of this study will benefit the formation of smaller and faster integrated circuits. The second project focuses on semiconductor nanocrystals. To make a semiconductor have particular conducting properties, atoms of other elements are often added. This process is known as doping the semiconductor, and the added atoms are called ?dopants.? Doping nanocrystals is challenging because dopant atoms move to the nearby nanocrystal surface where they no longer function as expected. Neutron Transmutation Doping (NTD) will be used to transform some of the semiconductor atoms into dopants via thermal neuron capture followed by radioactive decay. The NTD process can be performed at room temperature where the dopants are ?frozen? in their place. If successful, the NTD process applied to nanocrystals may lead to the ultimate miniaturization of transistors and diodes. Graduate students assisted by undergraduates will perform all the research described. The discovery-oriented research will lead to Masters and Ph.D. degrees, maintaining a highly trained workforce, which is crucial for US competitiveness.
The performance of Integrated Circuits can be improved by raising the electron and hole mobilities through the application of mechanical stress. Growing epitaxial layers of SiGe alloys can induce the necessary stress. This project will make use of isotopically controlled, stressed as well as relaxed Si(1-x)Ge(x) multilayer structures to determine the diffusivities and diffusion mechanisms of the major dopants; Boron, Arsenic and Phosphorus. Secondary Ion Mass Spectroscopy will be the main tool to determine the depth distribution of dopants and host crystal atoms. The resulting information will be useful in the design and fabrication of advanced semiconductor devices. A second focus of the project is the doping of Ge and Si nanocrystals, a formidable problem caused by segregation to the nearby surface. The Neutron Transmutation Doping (NTD) process of istopically enriched nanocrystals such as 70Ge or 74Ge makes possible ?cold? doping via thermal neutron capture followed by radioactive decay: 70Ge + n produces the acceptor 71Ga; while 74Ge + n produces the donor 75As. A rich range of doped structures can be envisioned assuming the formation of core-shell, bi-lobe and alloyed nanocrystals can be controlled. Graduate students working towards their MS and Ph.D. degrees will conduct the experimental research assisted by undergrads. This training at the cutting edge of semiconductor science and technology contributes to the maintenance of a domestic pool of experts, which are vital for the U.S. industrial competitiveness.
