Surfactant-controlled solid phase epitaxy of germanium on silicon

Promising modulation of self-assembled Ge-rich QDs by ultra-heavy phosphorus doping

Self-assembled Ge-rich quantum dots (QDs) can not only act as a prototype model for the fundamental studies of heteroepitaxial growth but also have great potential in optoelectronic devices for telecommunication and monolithic optical-electronic integrated circuits. Here, we report the unique features of Ge-rich QDs ultra-heavily doped with phosphorus (P) and embedded in a thin SiGe alloy film on Si (001) substrates. The ultra-heavy P doping considerably reduces the size of Ge-rich QDs and improves their uniformity. The inherent mechanism is associated with the reductions of both surface energy and the diffusion length of adatoms during QD growth promoted by the P dopants. Raman spectra indicate that the Ge content and strain in QDs are essentially not modified by the P doping. Particularly, the power- and temperature-dependent photoluminescence (PL) spectra demonstrate a type-I band alignment of Ge-rich QDs/SiGe alloy film due to the ultra-heavy P doping, which gives rise to additional low energy levels of electrons in QDs. Moreover, the PL of Ge-rich QDs is remarkably enhanced by ultra-heavy P doping at temperatures over 80 K. Over 3 times enhancement is obtained at 245 K. These results indicate that the overall quantum efficiency of Ge-rich QDs is substantially improved by the ultra-heavy P doping, which facilitates the applications of Ge-rich QDs in Si-based innovative optoelectronic devices.

Encapsulated solid phase epitaxy of a Ge quantum well embedded in an epitaxial rare earth oxide

An efficient method based on molecular beam epitaxy has been developed to integrate an epitaxial Ge quantum well buried into a single crystalline rare earth oxide. The monolithic heterostructure comprised of Gd2O3-Ge-Gd2O3 grown on an Si substrate exhibits excellent crystalline quality with atomically sharp interfaces. This heterostructure with unique structural quality could be used for novel nanoelectronic applications in quantum-effect devices such as nanoscale transistors with a high mobility channel, resonant tunneling diode/transistors, etc. A phenomenological model has been proposed to explain the epitaxial growth process of the Ge layer under oxide encapsulation using a solid source molecular beam epitaxy technique.

Aluminium-induced nanocrystalline Ge formation at low temperatures

The present work contributes to establishing the role of hydrogenation and of the substrates in the aluminium-induced crystallization process of amorphous germanium layers. For such a purpose, four series of a-Ge(Al) samples, deposited under identical nominal conditions, were studied: hydrogenated samples, H-free samples, and samples deposited on crystalline silicon and on glass substrates, respectively. On purpose, the impurity concentration was kept at a doping level (10⁻⁵<[Al/Ge]<2 × 10⁻³). Furthermore, the films were submitted to isochronal cumulative thermal annealing in the 200-550 °C range. Raman scattering spectroscopy was used to characterize the crystallization process. The role of Al impurity as a precursor seed for the crystallization of a-Ge:H has been clearly established, confirming that the metal-induced crystallization (MIC) phenomenon occurs at an atomic level. Moreover, it has been found that hydrogenation and the periodic nature of the substrate play a fundamental role in the appearance of crystal seeds at low temperatures. The evolution of crystallization with annealing temperature and the analysis of the distribution of crystallite sizes indicate that the formation of crystal seeds occurs at the amorphous film-substrate interface. The importance of fourfold-coordinated aluminium as the embryo of nanocrystal formation is discussed.