Optical Properties and Carrier Transport In

OPTICAL PROPERTIES AND CARRIER TRANSPORT IN Si/S1-xGex NANOSTRUCTURES by Eun Kyu Lee Defect-free epitaxial growth of Ge on a ~4 % lattice-mismatched single-crystal Si substrate is achieved using reduced dimension nanoscale heterostructures, where efficient structural relaxation might occur due to an enhanced adatom migrations and high surfaceto-volume ratio. For development of novel electronic or optical devices based on these novel structures, understanding of their electrical and optical properties is crucial. This study explores the optical properties and carrier transport in two different types of Si/Si1-xGex nanostructures: Ge nanowires (NWs) forming nanoscale heterojunctions with Si substrates and multilayers of SiGe clusters embedded into a Si matrix. Micron-long Ge nanowires grown on n+ (100) and p+ (111) Si substrates exhibit distinct current-voltage (I-V) characteristics, which are explained using a model of an abrupt and defect-free Ge NW/Si substrate heterointerfaces. From the measurements on Si/SiGe cluster superlattices (SLs) where SiGe clusters are vertically aligned due to strain-induced self-organization, a series of step-like structures are observed in the I-V characteristics obtained using mapping of electrical characteristics by a scanningtunneling-microscope (STM). The origin of this step-like I-V behavior and distinct peaks in differential conductivity is suggestively explained by a conduction energy band configuration that considers phonon-assisted carrier transfer at the interfaces between Si separating layers and Ge-rich SiGe clusters. Optical measurements show that the photoluminescence (PL) intensity in Si/SiGe cluster SL exhibits excitation dependent thermal quenching under continuous wave (CW) excitation by an Ar+ laser with the excitation intensities of 0.1-10 W/cm2 . A novel mechanism, where nonradiative carrier recombination is controlled by a competition between hole tunneling and hole thermionic emission over the valence band energy barriers at Si/SiGe heterointerfaces is suggested. In addition, the carrier transitions at a high photoexcitation using Q-switched pulsed Nd:YAG laser are found to be mediated by Auger processes with high energy Auger holes repopulating Si barriers in valence energy band. An efficient light emission due to radiative recombination of electron-hole condensates in Si separating layers of the Si/SiGe clusters SL is observed with a lifetime approaching 10 -8 s. These results, attributed to a mechanism similar to "Auger fountain" in quantum wells with type-II energy band alignment, indicate a new route toward light-emitters monolithically integrated into CMOS environment. OPTICAL PROPERTIES AND CARRIER TRANSPORT IN Si/ Si1-xGex NANOSTRUCTURES