P. M. Fauchet

Blue luminescence in films containing Ge and GeO2 nanocrystals: The role of defects

X-ray measurements show that GeO2 and Ge nanocrystals with a radius from 25 down to 1.5 nm are formed in an oxide matrix after annealing. Under ultraviolet excitation, both types of films luminesce around 3.1 eV, with identical photoluminescence (PL) line shapes and subnanosecond PL dynamics. The strongest PL intensity was found for the films containing GeO2 crystals and for the largest nanocrystals. These results are a clear indication that although the blue luminescence is without a doubt correlated with the formation of Ge (or GeO2) nanocrystals, it is not produced by the radiative recombination of excitons confined in the nanocrystals. Possible mechanisms for the luminescence include defects at the nanocrystal/matrix interface or in the matrix itself.

Thermal crystallization of amorphous Si/SiO2 superlattices

Annealing of amorphous Si/SiO2 superlattices produces Si nanocrystals. The crystallization has been studied by transmission electron microscopy and x-ray analysis. For a Si layer thinner than 7 nm, nearly perfect nanocrystals are found. For thicker layers, growth faults and dislocations exist. Decreasing the a-Si layer thickness increases the inhomogeneous strain by one order of magnitude. The origin of the strain in the crystallized structure is discussed. The crystallization temperature increases rapidly with decreasing a-Si layer thickness. An empirical model that takes into account the Si layer thickness, the Si/SiO2 interface range, and a material specific constant has been developed.

Temperature stability for silicon-based photonic band-gap structures

A method for minimizing thermal effects on silicon-based photonic band-gap (PBG) structures is demonstrated. The reflectance resonance positions of as-anodized one-dimensional porous silicon (PSi) PBG microcavities exhibit 3 nm redshifts when heated up to 100 °C, which significantly alters the light propagation properties of the structures. By carefully controlling the oxidation conditions of the PSi microcavities, it is possible to reduce the shift to less than 0.5 nm. High-resolution x-ray diffraction experiments directly link variations of the silicon strain during heating to shifts of the reflectance resonance. The pressure change induced by the proper oxidation level can compensate for the effect of the temperature change and, thus, stabilize the resonance position.

Electro‐optic sampling of 1.5‐ps photoresponse signal from YBa2Cu3O7−δ thin films

Photoresponse signals with widths as short as 1.5 ps are observed from epitaxial YBa2Cu3O7−δ  thin films using electro‐optic sampling techniques. Voltage transients less than 2 ps wide are seen in 100‐ and 200‐nm films exposed to 150‐fs laser pulses and cooled to 79 K. At low bias currents, the amplitude of the fast response varies linearly with the bias current, suggesting a kinetic inductive mechanism. A negative transient about 15‐ps long is also seen that may provide evidence for nonequilibrium recombination of excited quasiparticles into Cooper pairs. At high bias currents or large laser fluences, a fast tail with a decay time of about 10 ps appears in the response followed by a slow, resistive bolometric component due to sample heating. Nonequilibrium aspects of the photoresponse and the origin of the fast tail are discussed.

PHONON-ASSISTED TUNNELING AND INTERFACE QUALITY IN NANOCRYSTALLINE SI/AMORPHOUS SIO2 SUPERLATTICES

We report on the interface quality and phonon-assisted tunneling in nanocrystalline Si (nc-Si)/amorphous SiO2 (a-SiO2) superlattices (SLs) prepared by magnetron sputtering and thermal crystallization of nanometer-thick a-Si layers. Phonon-assisted tunneling is observed in a bipolar nc-Si based structure, which confirms that the nc-Si/a-SiO2 junction is not only abrupt but also nearly defect free. The conclusion is supported by capacitance–voltage measurements from which the estimated interface defect density is found to be ∼1011 cm−2 for an eight-period SL. Such high quality interfaces hold considerable promise for the development of nc-Si SL quantum devices.

Spatiotemporal shaping of half-cycle terahertz pulses by diffraction through conductive apertures of finite thickness

We demonstrate a simple quasi-optical technique for spatiotemporal shaping of half-cycle terahertz-radiation pulses. We show, both experimentally and theoretically, that properly polarized half-cycle pulses can be modulated temporally by diffraction through a conductive aperture of finite thickness. We use the finite-difference time-domain method to solve Maxwell’s equations for such a geometry and show that it can explain all the experimentally observed features. In the case of thick aperture, a planar waveguide model can also be used to describe the propagation of the pulse through the aperture, with excellent agreement with the experimental results.

Extraordinary crystallization of amorphous Si/SiO2 superlattices

Abstract Ordered Si crystals with nanometer sizes are prepared in a Si/SiO 2 superlattice structure using rf sputtering and plasma oxidation. Decreasing the a-Si layer thickness to 1.9 nm increases the inhomogeneous strain by one order of magnitude. An exponential increase of the crystallization temperature with decreasing thickness is found. A new model using the melting temperature and the interior amorphous crystallization temperature is reported. The extension of the model to Ge/SiO 2 and Si/SiO x superlattices is demonstrated.

Self-organization and ordering in nanocrystalline Si/SiO2 superlattices

Abstract The solid phase crystallization of nanometer-thick layers of disordered Si confined between layers of amorphous SiO 2 has been achieved using high temperature annealing. For ultrathin Si layers (∼1– 3 nm thick) crystallization was not possible even after extensive annealing at temperatures up to 1100°C, because of the high strain fields introduced by the SiO 2 layers. However, for thicker layers (∼4– 20 nm thick) a variety of Si nanocrystals ranging in shape from spheres to bricks could be spontaneously formed and, in suitable cases, oriented along the 〈1 1 1〉 crystallographic direction. This formation of organized nanocrystals is an important step towards the construction of Si/SiO 2 quantum devices.

Silicon Light Emitters: Preparation, Properties, Limitations, and Integration with Microelectronic Circuitry

Starting with Canham’s discovery in 1990 that porous silicon (PSi) can emit bright light in the visible range of the spectrum, there has been a strong interest in silicon light emitters. PSi and other light-emitting forms of silicon contain nanostructures or crystallites in the nanometer size range. Throughout most of the 1990’s, the intense visible luminescence from nanoscale silicon crystallites has been a source of numerous investigations and considerable debate. Today, most of the controversies have been put to rest. However, much less has been written about nanoscale Si light-emitting devices, in part because some of their characteristics are less than ideal and not well understood. This paper reviews the status of nanoscale silicon light emitters. It starts with a survey of the manufacturing methods used to produce nanoscale Si. Next, key physical, optical, electrical, and structural properties of nanoscale Si are examined. The fabrication of electroluminescent devices (LEDs) is then discussed. We focus on the stability, efficiency, speed, and spectral characteristics of nanoscale Si light emitters. Recent results obtained on microcavity PSi LEDs and 1.5μm LEDs produced by doping PSi with erbium are discussed. Finally, the integration of PSi LEDs with microelectronic circuitry is reported and the prospects for practical devices are briefly examined.