C. M. Yang

The role of quantum‐confined excitons vs defects in the visible luminescence of SiO2 films containing Ge nanocrystals

Synthesis of Ge nanocrystals in SiO2 films is carried out by precipitation from a supersaturated solid solution of Ge in SiO2 made by Ge ion implantation. The films exhibit strong room-temperature visible photoluminescence. The measured photoluminescence peak energy and lifetimes show poor correlations with nanocrystal size compared to calculations involving radiative recombination of quantum-confined excitons in Ge quantum dots. In addition, the photoluminescence spectra and lifetime measurements show only a weak temperature dependence. These observations strongly suggest that the observed visible luminescence in our samples is not due to the radiative recombination of quantum-confined excitons in Ge nanocrystals. Instead, observations of similar luminescence in Xe+ -implanted samples and reversible PL quenching by hydrogen or deuterium suggest that radiative defect centers in the SiO2 matrix are responsible for the observed luminescence.

Electroluminescence and photoluminescence of Ge‐implanted Si/SiO2/Si structures

Electroluminescent devices were fabricated in SiO_2 films containing Ge nanocrystals formed by ion implantation and precipitation during annealing at 900 °C, and the visible room‐temperature electroluminescence and photoluminescence spectra were found to be broadly similar. The electroluminescent devices have an onset for emission in reverse bias of approximately −10 V, suggesting that the mechanism for carrier excitation may be an avalanche breakdown caused by injection of hot carriers into the oxide. The electroluminescent emission was stable for periods exceeding 6 h.

Island growth and coarsening in thin films---conservative and nonconservative systems

A model is developed for time‐dependent growth and coarsening of islands in conservative and nonconservative thin‐film systems with substrate coverages in the range 0 0 but constant during coarsening, a dynamically scaling distribution and a steady state are also approached asymptotically. The form of the initial distribution has a significant effect on the coarsening kinetics in the tra...

Selective solid phase crystallization for control of grain size and location in Ge thin films on silicon dioxide

Selective solid phase crystallization for control of grain size and location in polycrystalline thin Ge films on amorphous silicon dioxide substrates is described. The approach consists of solid phase crystal nucleation at predefined nucleation sites, which consist of metal islands deposited on top of the amorphous Ge film, followed by lateral solid phase epitaxial growth. Grain sizes as large as 30 μm have been achieved in 50‐nm‐thick Ge films at temperatures less than 475 °C.

Correlation of size and photoluminescence for Ge nanocrystals in SiO2 matrices

Abstract Synthesis and size-dependent photoluminescence has been performed for Ge nanocrystals in SiO2 matrices with average diameters between 2 and 9 nm, formed by room-temperature ion implantation into SiO2 followed by precipitation during vacuum thermal anneals. Nanocrystal size distributions obtained from electron microscopy data were used in conjunction with a quantum-confined exciton recombination model [T. Takagahara and K. Takeda, Phys. Rev. B 46 (1992) 15578] to generate calculated photoluminescence spectra, which were compared with experimental spectra. Qualitative agreement between calculated and observed spectra could be obtained when: (i) the non-radiative and radiative carrier recombination rates were approximately equal for nanocrystal sizes corresponding to visible emission, (ii) the spatial extent of the nanocrystal confinement potential is assumed to be slightly smaller than the measured size of the nanocrystal, and (iii) coalescence of individual nanocrystals produces electronic states corresponding to single nanocrystals with larger volume.

Group III–V and Group IV Quantum Dot Synthesis

Semiconductor structures that exhibit quantum confinement effects in three dimensions have attracted considerable attention owing to their potential as tools for exploration of conceptually simple mesoscopic systems, and also because of their potential for new optoelectronic devices. In order to observe unique quantum dot transport and optical properties at room temperature, the characteristic dimensions of the carrier confining potentials and structures should be less than 10–20 nm. Although the electronic structure issues are quite different for group III–V semiconductors (prototypically GaAs/AlGaAs) than for group IV semiconductors (prototypically Si and Ge), growth of dense arrays of small (≤ 10 nm), uniformly-sized structures are important goals for both materials systems. In particular, there is a compelling need for development of synthesis techniques capable of making denselypacked,uniformly-sized structures which are less than 10–15 nm in size, over large areas.