Efficient room-temperature light-emitters based on partly amorphised Ge quantum dots in crystalline Si

Semiconductor light emitters compatible with standard Si integration technology (SIT) are of particular interest for overcoming limitations in the operating speed of microelectronic devices 1-3. Light sources based on group-IV elements would be SIT compatible but suffer from the poor optoelectronic properties of bulk Si and Ge. Here, we demonstrate that epitaxially grown Ge quantum dots (QDs) in a fully coherent Si matrix show extraordinary optical properties if partially amorphised by Ge-ion bombardment (GIB). The GIB-QDs exhibit a quasi-direct-band gap and show, in contrast to conventional SiGe nanostructures, almost no thermal quenching of the photoluminescence (PL) up to room-temperature (RT). Microdisk resonators with embedded GIB-QDs exhibit threshold-behaviour and super-linear increase of the integrated PL-intensity (IPL) with increasing excitation power Pexc which indicates light amplification by stimulated emission in a fully SIT-compatible group-IV nano-system

Optical properties of strain-engineered multilayer Si/SiGe nanostructures

The long carrier radiative lifetimes in indirect band gap semiconductors such as crystalline Si (c-Si) and Ge impede the development of efficient light-emitting devices and lasers. Multilayer Si/SiGe nanostructures are considered to be the strong candidates for efficient and high-speed optoelectronic devices integrated into CMOS platforms. Since c-Si and Ge have a considerable lattice mismatch of ~ 4.2%, Si/Si1-xGex(x \u3c 0.5) nanostructures in the form of nano-layers (NLs) or cluster multilayers (CMs) modify the band structure and create non-uniform strain distribution. Engineering of Si/Si1-xGexnanostructures with the predicted composition and interface abruptness, which controls spatial separation between electrons and holes and carrier radiative recombination rate, is critical in producing the desired fast and efficient photoluminescence (PL) peaked around 0.8-0.9 eV. This study investigates the structural, optical, and thermal properties of Si/Si1- xGexnanostructures with different layer thicknesses, Ge compositions, and SiGe heterointerface abruptness. A comprehensive experimental and theoretical analysis of Raman scattering in various Si/Si1-xGexmultilayered nanostructures with well-defined Ge composition (x) and layer thicknesses is presented. Using Raman and transmission electron microscopy data, Si/SiGe intermixing and strain are discussed and modeled. The studied samples exhibit significant dependence of the Raman scattering intensity on the excitation light penetration depth. Local temperature and thermal conductivity are calculated by analyzing the measured Stokes and anti -Stokes Raman spectra, and the developed model of heat dissipation in the samples under an intense laser illumination is in a good agreement with the experiment. A correlation is found between the SiGe/Si volume fraction ratio and thermal conductivity, which is explained and suggestions are made of applications of the developed model in the field of thermoelectric, electronic, and optoelectronic devices. In this thesis, PL measurements are focused on specifically designed Si/Si1-xGexnanostructures with a single 3-5 nm thick Si1-xGex layer with x ≈ 8% incorporated into Si/Si0.6Ge0.4 CMs. Under pulsed laser excitation, the PL decay associated with the Si0.92Ge0.08 N L is found to be nearly a 1000 times faster compared to that in Si/Si0.6Ge0.4 CMs, and the SiGe NL PL intensity does not saturate as a function of excitation energy density up to 50 mJ/cm2 . These dramatic differences in the observed PL properties are attributed to the difference in the structures of the Si/SiGe NL and CM heteroi nterfaces. A model considering Si/SiGe heterointerface composition and explaining the fast and slow time-dependent recombination rates is proposed and found to be in excellent agreement with the experimental data

Auger-mediated processes and photoluminescence in group iv semiconductor nanostructures

Group IV semiconductors (Si, Ge) are inefficient light emitting materials due to their indirect bandgap structure. Nanostructures of Si, Ge, and SiGe however, have shown relatively high photoluminescence (PL) quantum efficiency (QE) at low carrier concentrations. At higher carrier concentrations, the PL QE of these nanostructures is drastically reduced due to the onset of a fast non-radiative process attributed to Auger recombination. Moreover, this onset occurs earlier in structures with reduced physical dimensions, than in bulk material. The study of Auger-mediated processes in group IV nanostructures is therefore critical to understanding the physics of carrier recombination and photonic device limitations. This work investigates recombination mechanisms in two such systems: the silicon/silicon germanium three-dimensional (3D) nanostructure system, and the silicon-on-insulator (SOI) system. Recombination mechanisms are studied by several experimental techniques. One approach explores the steady state PL spectroscopy and PL dynamics under pulsed excitations with varying concentrations of photo-generated charge carriers in the investigated systems. Another important technique uses selective, wavelength dependent photoexcitation to generate carriers up to varying depths in the nanostructures, enabling the understanding of local differences in PL properties through the thickness of structures. Several interesting observations are reported and underlying recombination mechanisms are discussed. For the Si/SiGe 3D nanostructure system, these include a reversible degradation of the PL after a few minutes of relative stability, an Auger Fountain mechanism that redistributes charge carriers within the nanostructure, and a severe reduction of the exciton diffusion length. For the SOI system, an apparently successful competition of the radiative recombination of carriers in a condensed excitonic phase with Auger processes is observed. The influence of the Si/SiO2 interface on the recombination mechanism in this system is emphasized. Results of the experiments show that the coexistence of a type II energy band alignment at Si/SiGe interfaces, the electron-hole-droplets in Si, and Auger-mediated processes results in several unusual photoluminescence properties in SiGe and Si nanostructures

Silicon-germanium nanowire heterojunctions: Optical and electrical properties

Semiconductor nanowires are quasi-one-dimensional objects with unique physical properties and strong potential in nanophotonics, nanoelectronics, biosensing, and solar cell devices. The next challenge in the development of nanowire functional structures is the nanowire axial heterojunctions, especially lattice mismatched heterojunctions. Si and Ge have a considerable lattice mismatch of ~ 4.2% as well as a mismatch in the coefficient of thermal expansion, and the formation of a Si1-xGex transition layer at the heterointerface creates a non-uniform strain and modifies the band structures of the adjacent Si and Ge nanowire segments. These nanostructures are produced by catalytic chemical vapor deposition employing vapor-liquid-solid mechanism on (111) oriented p-type Si substrate, and they exhibit unique structural properties including highly localized strain, and short-range interdiffusion/intermixing revealed by transmission electron microscopy, scanning electron microscopy and energy dispersive x-ray spectroscopy. Our studies of the structural properties of axial Si-Ge nanowire heterojunctions show that despite the 4.2% lattice mismatch between Si and Ge they can be grown without a significant density of structural defects. The lattice mismatch induced strain is partially relieved due to spontaneous SiGe intermixing at the heterointerface during growth and lateral expansion of the Ge segment of the nanowire, which is in part due to a higher solubility of Ge in metal precursors. The mismatch in Ge and Si coefficients of thermal expansion and low thermal conductivity of Si/Ge nanowire heterojunctions are proposed to be responsible for the thermally induced mechanical stress detected under intense laser radiation. The performed electrical measurements include current-voltage, conductance-voltage, transient electrical measurements under various applied voltages at temperatures ranging from 20 to 400K. We find that Si-Ge nanowire heterojunctions exhibit strong current instabilities associated with flicker noise and damped oscillations with frequencies close to 10-30 MHz. Flicker (or 1/f ) noise is characterized and analyzed on carrier number fluctuation model and mobility fluctuation model noise mechanism, respectively. The proposed explanation is based on a carrier transport mechanism involving electron transitions from Ge to Si segments of the NWs, which requires momentum scattering, causes electron deceleration at the Ge-Si heterointerface and disrupts current flow. Both Si/Ge heterojunctions and NW surface states are demonstrated to be the two dominant elements that strongly influence the electrical characteristics of nanowires