Strong confinement-induced engineering of the g factor and lifetime of conduction electron spins in Ge quantum wells.

Control of electron spin coherence via external fields is fundamental in spintronics. Its implementation demands a host material that accommodates the desirable but contrasting requirements of spin robustness against relaxation mechanisms and sizeable coupling between spin and orbital motion of the carriers. Here, we focus on Ge, which is a prominent candidate for shuttling spin quantum bits into the mainstream Si electronics. So far, however, the intrinsic spin-dependent phenomena of free electrons in conventional Ge/Si heterojunctions have proved to be elusive because of epitaxy constraints and an unfavourable band alignment. We overcome these fundamental limitations by investigating a two-dimensional electron gas in quantum wells of pure Ge grown on Si. These epitaxial systems demonstrate exceptionally long spin lifetimes. In particular, by fine-tuning quantum confinement we demonstrate that the electron Landé g factor can be engineered in our CMOS-compatible architecture over a range previously inaccessible for Si spintronics.

Laser Level Scheme of Self-Interstitials in Epitaxial Ge Dots Encapsulated in Si.

Recently, it was shown that lasing from epitaxial Ge quantum dots (QDs) on Si substrates can be obtained if they are partially amorphized by Ge ion bombardment (GIB). Here, we present a model for the microscopic origin of the radiative transitions leading to enhanced photoluminescence (PL) from such GIB-QDs. We provide an energy level scheme for GIB-QDs in a crystalline Si matrix that is based on atomistic modeling with Monte Carlo (MC) analysis and density functional theory (DFT). The level scheme is consistent with a broad variety of PL experiments performed on as-grown and annealed GIB-QDs. Our results show that an extended point defect consisting of a split-[110] self-interstitial surrounded by a distorted crystal lattice of about 45 atoms leads to electronic states at the Γ-point of the Brillouin zone well below the conduction band minimum of crystalline Ge. Such defects in Ge QDs allow direct transitions of electrons localized at the split-interstitial with holes confined in the Ge QD. We identify the relevant growth and annealing parameters that will let GIB-QDs be employed as an efficient laser active medium.

Lasing from Glassy 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. 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 defect-free Si matrix show extraordinary optical properties if partially amorphized by Ge-ion bombardment (GIB). In contrast to conventional SiGe nanostructures, these QDs exhibit dramatically shortened carrier lifetimes and negligible thermal quenching of the photoluminescence (PL) up to room temperature. Microdisk resonators with embedded GIB-QDs exhibit threshold behavior as well as a superlinear increase of the integrated PL intensity with concomitant line width narrowing as the pump power increases. These findings demonstrate light amplification by stimulated emission in a fully SIT-compatible group IV nanosystem.

Integration, photostability and spontaneous emission rate enhancement of colloidal PbS nanocrystals for Si-based photonics at telecom wavelengths.

We experimentally investigate PbS nanocrystal (NC) photoluminescence (PL) coupled to all-integrated Si-based ring resonators and waveguides at telecom wavelengths. Dissolving the NCs into Novolak polymer significantly improves their stability in ambient atmosphere. Polymer-NC blends of various NC concentrations can be applied to and removed from the same device. For NC concentrations up to 4vol%, the spontaneous emission rate into ring-resonator modes is enhanced by a factor of ~13 with respect to that into a straight waveguide. The PL intensity shows a linear dependence on the excitation intensity up to 1.64kW/cm(2) and stable quality factors of ~2500.

Sb-mediated Ge quantum dots in Ti-oxide-Si diode: negative differential capacitance.

The negative differential capacitance (NDC) effect is observed on a titanium-oxide-silicon structure, formed on n-type silicon with embedded germanium quantum dots (QDs). The Ge QDs were grown by an Sb-mediated technique. The NDC effect was observed for temperatures below 200 K. We found that approximately six to eight electrons can be trapped in the valence band states of Ge QDs. We explain the NDC effect in terms of the emission of electrons from valence band states in the very narrow QD layer under reverse bias.

Hybrid nanoimprinted laser based on conjugated conductive polymer and nanocrystal quantum dots.

A composite of the conjugate conductive polymer poly(2-methoxy-5-(2-ethylhexyloxy)-1,4-phenylenevinylene) (MEH-PPV) and CdSe/ZnS quantum dots (QDs) is utilized for nanoimprinted lasers after drop-casting onto one-dimensional omnidirectional Bragg mirrors. Strong amplified spontaneous emission is observed at the resonances of both the QDs and the polymer. Clearly resolved, low-threshold lasing was found after nanoimprinting of distributed-feedback gratings on the composite surface. Energy transfer between the polymer molecules and the QDs is demonstrated by excitation spectroscopy.

Si rib waveguide photodetector with an ordered array of Ge islands for 1.5 microm.

This work demonstrates a rib waveguide photodetector based on a vertical Si p-i-n junction with Ge islands operating in the spectral region around lambda=1.55 microm at room temperature. A vertical stack of four layers of Ge islands is grown by molecular beam epitaxy on a silicon-on-insulator. Each layer is organized in a two-dimensional square grid with a period of 460 nm. The spectral response of the detector extends well beyond 1.6 mum at 300 K. The absorption length of approximately 135 microm (at 1/e decrease of intensity) at lambda=1.55 microm along the waveguide allows for relatively small-size devices for all-on-one-platform integration.

g-factor tuning and manipulation of spins by an electric current.

We investigate the Zeeman splitting of the two-dimensional electron gas in an asymmetric silicon quantum well, performing electron-spin-resonance (ESR) experiments. Applying a small dc current we observe a shift in the resonance field due to the additional current-induced Bychkov-Rashba type of spin-orbit field. We also show that a high frequency current may induce electric dipole spin resonance very efficiently. We identify different contributions to this type of ESR signal.