Self-ion-induced swelling of germanium

Ge substrates of (1 0 0) orientation were irradiated with 1.0 MeV Ge ions at temperatures in the range from -180 degreesC to 500 degreesC. Pronounced swelling of the irradiated material up to 4000 Angstrom, associated with the formation of a porous surface layer, was evident only for temperatures between -50 degreesC and 200 degreesC. The extent of swelling was found to be insensitive to temperature in this range and exhibited an approximately linear dose dependence for doses up to 1 x 10(17) Ge cm(-2). For temperatures outside this range, only sputtering effects were observed. The structure of the porous surface layer was examined by transmission electron microscopy. For samples irradiated at 22 degreesC, it was shown to have a density similar to 30% that of bulk Ge. These layers were further shown to be stable during subsequent annealing to 500 degreesC, (C) 2001 Elsevier Science B.V. All rights reserved

Linear optical properties of Ge nanocrystals in silica

The absorption and extinction spectra of Ge nanocrystals in silica formed by ion implantation are studied using photothermal deflection and transmission spectroscopies. It is found that scattering makes a significant contribution to the extinction spectrum, damping the spectral features and resulting in a Rayleigh scattering-like ω4 dependence. In contrast, the spectra measured by photothermal deflection clearly show features such as the E1/E1 + Δ1 transitions. The Tauc gap is extracted to be ∼0.7 ± 0.1 eV. © 2001 American Institute of Physics. [DOI: 10.1063/1.1409591]

Rapid, substrate-independent thickness determination of large area graphene layers

Phase-shifting interferometric imaging is shown to be a powerful analytical tool for studying graphene films, providing quantitative analysis of large area samples with an optical thickness resolution of ≥ 0.05 nm. The technique is readily able to identify single sheets of graphene and to quantitatively distinguish between layers composed of multiple graphene sheets. The thickness resolution of the technique is shown to result from the phase shift produced by a graphene film as incident and reflected light pass through it, rather than from path-length differences produced by surface height variations. This is enhanced by the high refractive index of graphene, estimated in this work to be n G 2.99 ± 0.18. © 2011 American Institute of Physics

Nonlinear optical properties of semiconducting nanocrystals in fused silica

Degenerate four-wave mixing (DFWM) is used to examine the nonlinear optical response of Ge nanocrystals in a silica matrix. The nanocrystals are formed by implanting 1.0 MeV Ge ions into silica to a dose of 3.0×1017 Ge cm-2 and annealing at 1100°C. The particle size is well described by a log normal distribution with a mean particle diameter of 3.0 nm and a dimensionless geometric standard deviation of 0.25. The nonlinear optical response, measured at 800 nm with pulse lengths in the range 100-1000 fs, is found to exhibit two patterns of behaviour. For short (180 fs) pulses the DFWM signal is shown to exhibit a third-order dependence on pulse energy (Kerr nonlinearity) and to have a relaxation time of ∼≤ 1 ps, independent of energy. However, for long pulse lengths (600 fs) the signal exhibits a higher order (∼4th order) dependence on pulse energy and has a relaxation time (∼10 ps) which increases with increasing pulse energy. Extreme pulse energies are shown to cause irreversible changes in the sample. © 1999 Elsevier Science B.V. All rights reserved

Nonlinear optical response of Ge nanocrystals in a silica matrix

Time-resolved degenerate-four-wave-mixing measurements were used to study the nonlinear optical response (intensity-dependent refractive index) of Ge nanocrystallites embedded in a silica matrix. Nanocrystals were fabricated by ion-implanting silica with 1.0 MeV Ge ions to fluences in the range from 0.6 to 3 × 1017 Ge cm-2, followed by annealing at 1100°C for 60 min. For the highest fluence, this resulted in nanocrystals with a log-normal size distribution, having a geometric mean diameter of 3.0 nm and a dimensionless geometric standard deviation of 0.25. The intensity-dependent refractive index |n2| was measured at a wavelength of 800 nm and found to increase linearly with increasing Ge fluence. For the highest fluence, |n2| was determined to be in the range 2.7-6.9 × 10-13 cm-2 W-1, depending on the duration of the excitation pulse; values were consistently smaller for shorter pulse lengths. Relaxation of the nonlinear response was found to have two characteristic time constants, one <100 fs and the other ∼1 ps. © 1999 American Institute of Physics

Ion irradiation of GeSi Si strained-layer heterostructures

The strain in GeSi/Si strained layer heterostructures is studied as a function of ion-irradiation and thermal annealing conditions and correlated with the defect microstructure in the GeSi alloy layer. For room temperature irradiation, compressive strain within the alloy layer increases with increasing ion fluence for both low (projected range of ions within the alloy layer) and high energy (projected range of the ions greater than alloy thickness) irradiation. In contrast, elevated temperature irradiation results in an increase in strain for low-energy irradiation, but a decrease for high-energy irradiation. For example, strain relaxation is observed in layers irradiated with 1 MeV Si-28(+) at 253 degrees C. During subsequent annealing to 750 degrees C, the strain is partially recovered but relaxes again at temperatures &gt; 750 degrees C. This behavior is shown to be consistent with the evolution of intrinsic (vacancy-type) defects within the alloy layer

Ion irradiation of GeSi Si strained-layer heterostructures

The strain in GeSi/Si strained layer heterostructures is studied as a function of ion-irradiation and thermal annealing conditions and correlated with the defect microstructure in the GeSi alloy layer. For room temperature irradiation, compressive strain within the alloy layer increases with increasing ion fluence for both low (projected range of ions within the alloy layer) and high energy (projected range of the ions greater than alloy thickness) irradiation. In contrast, elevated temperature irradiation results in an increase in strain for low-energy irradiation, but a decrease for high-energy irradiation. For example, strain relaxation is observed in layers irradiated with 1 MeV Si-28(+) at 253 degrees C. During subsequent annealing to 750 degrees C, the strain is partially recovered but relaxes again at temperatures &gt; 750 degrees C. This behavior is shown to be consistent with the evolution of intrinsic (vacancy-type) defects within the alloy layer

The effect of irradiation temperature on post-irradiation strain levels in GexSi1-x/Si strained layer heterostructures

Double crystal x-ray diffraction (DCXRD), transmission electron microscopy (TEM), and Rutherford backscattering spectroscopy and channelling (RBS-C) were used to study the effect of irradiation temperature and ion-energy on the perpendicular strain (epsilon perpendicular to in GexSi1-x/Si strained layer heterostructures. Room temperature irradiation was shown to increase epsilon perpendicular to. This effect was adequately modelled by assuming that irradiation caused additional strain due to excess interstitial distribution. The effects of irradiating at 253 degrees C were more complex, resulting in: a) an increase in epsilon perpendicular to when the radiation damage profile was confined to the alloy layer; or b) a decrease in epsilon perpendicular to when the damage profile extended through the alloy/substrate interface. Strain relaxation at elevated temperature is believed to be effected by loop-like defects which nucleate at or near the alloy/substrate interface during high energy irradiation