Peter Mascher

Point defects and luminescence centres in zinc oxide and zinc oxide doped with manganese

Abstract The green and yellow luminescence centres in ZnO and Mn-doped ZnO are investigated. Water vapour enhances green luminescence of ZnO in the surface, and causes a two band luminescence phenomenon (green and yellow35in the bulk. Mn behaves as quencher for both the green and the yellow luminescence of ZnO. Positron lifetime measurements show the existence of at least two distinct vacancy-type defects in all materials. At the present time it seems that a VZn · V0 divacancy exists, and that luminescence is due to interstitial zinc and oxygen.

X-ray-diffraction study of crystalline Si nanocluster formation in annealed silicon-rich silicon oxides

The formation and subsequent growth of crystalline silicon nanoclusters (Si-ncs) in annealed silicon-rich silicon oxides (SRSOs) were studied by glancing angle x-ray diffraction. SRSO samples with Si concentrations (y) of 0.40, 0.42, and 0.45 were grown by inductively coupled plasma-enhanced chemical-vapor deposition (PECVD). Samples with y=0.42 grown by electron-cyclotron-resonance PECVD were also studied. Annealing treatments were performed at temperatures (T) of 900, 1000, and 1100 °C for times (t) between 0.5 and 3 h in flowing Ar. As-grown SRSO films did not present signs of Si clusters (amorphous or crystalline); however, (111), (220), and (311) Bragg peaks corresponding to c‐Si were clearly seen after annealing at 900 °C for the y=0.45 sample, but only barely seen for the y=0.42 and undetected for the y=0.40 samples. For T=1000°C, all studied SRSO samples clearly showed the c‐Si diffraction peaks, which became narrower with increasing t and T. From the width of the Si (111) peaks, the mean size of ...

Vertically aligned wurtzite CdTe nanowires derived from a catalytically driven growth mode

Substrate-based catalytic growth modes have been widely used to fabricate vertically aligned nanowires for most technologically relevant semiconducting systems, with CdTe being a notable exception. The catalysts that promote a one-dimensional nanoscale growth mode in other systems seem to fail, creating the need for an alternative approach. Here, we demonstrate how nanowire structures can be derived from a newly developed catalytically driven process. The vertically aligned nanowires produced are highly faceted and share an epitaxial relationship with the underlying substrate. The nanowire structures could also be described as nanorods; they show a high degree of size uniformity over large areas with heights of 300 nm imposed by growth dynamics. Two-dimensional x-ray diffraction techniques indicate that the CdTe exists in the wurtzite crystal structure instead of the zinc blende structure normally associated with the bulk material. The work presented here adds these substrate-based wurtzite CdTe nanowires to the growing list of potential building blocks for nano-based devices.

Electron cyclotron resonance chemical vapor deposition of silicon oxynitrides using tris(dimethylamino)silane

A new compound, tris(dimethylamino)silane was used as an organosilicon source for the deposition of silicon oxynitride thin films. The depositions were carried out at low substrate temperatures (<150 °C) in an electron cyclotron resonance plasma enhanced chemical vapor deposition reactor. Films with compositions varying from Si3N4 to SiO2 were deposited on silicon substrates by varying the N2/O2 flow ratio to the plasma chamber. In situ ellipsometry measurements of the film optical index were well correlated with film composition. Auger electron spectroscopy showed that only low levels of carbon (<3 at. %) were present, while Fourier transform infrared spectroscopy showed low levels of bonded hydrogen. The deposition rate of high quality Si3N4 was as high as 220 A/min.A new compound, tris(dimethylamino)silane was used as an organosilicon source for the deposition of silicon oxynitride thin films. The depositions were carried out at low substrate temperatures (<150 °C) in an electron cyclotron resonance plasma enhanced chemical vapor deposition reactor. Films with compositions varying from Si3N4 to SiO2 were deposited on silicon substrates by varying the N2/O2 flow ratio to the plasma chamber. In situ ellipsometry measurements of the film optical index were well correlated with film composition. Auger electron spectroscopy showed that only low levels of carbon (<3 at. %) were present, while Fourier transform infrared spectroscopy showed low levels of bonded hydrogen. The deposition rate of high quality Si3N4 was as high as 220 A/min.

Defect structure of carbon rich a-SiC:H films and the influence of gas and heat treatments

A comprehensive study of carbon rich a-SiC:H films using optical absorption measurements, Fourier transform infrared spectroscopy, thermal desorption measurements, atomic force microscopy, and positron lifetime and Doppler-broadening techniques suggests that open volumes are formed in the films, due to incomplete breaking of the source molecule during film deposition. These open volumes are interconnected and can effectively trap gases from the ambient, during the film growth or after deposition. With increasing temperature the gases are desorbed from the internal surfaces of these open volumes and are released from the sample. This increases the areal density of the defects and is observable in positronium formation and annihilations of positrons with surface electrons. The growth of a nanocrystalline structure is observable upon annealing. At sufficiently high temperatures thermal breaking of Si–H and C–H bonds occurs and results in irreversible structural changes and film densification due to new C–C b...

Plasmon Field Effects on the Nonradiative Relaxation of Hot Electrons in an Electronically Quantized System: CdTe-Au Core-Shell Nanowires

The intense electromagnetic fields of plasmonic nanoparticles, resulting from the excitation of their localized surface plasmon oscillations, are known to enhance radiative processes. Their effect on the nonradiative electronic processes, however, is not as well-documented. Here, we report on the enhancement of the nonradiative electronic relaxation rates in CdTe nanowires upon the addition of a thin gold nanoshell, especially at excitation energies overlapping with those of the surface plasmon oscillations. Some possible mechanisms by which localized surface plasmon fields can enhance nonradiative relaxation processes of any quantized electronic excitations are proposed.

Optical attenuation in defect-engineered silicon rib waveguides

The excess optical attenuation at wavelengths around 1550nm induced by subamorphous dose ion implantation of silicon-on-insulator rib waveguides has been quantified. Optical attenuation is related to the introduction of lattice defects such as the silicon divacancy. After 2.8MeV Si+ implantation at a dose of 2.5×1014cm−2, the attenuation is greater than 1000dBcm−1. Using positron annihilation spectroscopy to determine the vacancy-type defect concentration, it is demonstrated that the absorption component of the excess attenuation can be predicted using a simple analytical expression. Additional losses are suggested to result from a defect induced change in the real part of the refractive index of the silicon waveguide. A processing strategy for ensuring that the absorption component dominates the excess attenuation is described, and it is shown that selective implantation of a relatively low dose of inert ions is an efficient method for the reduction of optical cross talk in silicon photonic circuits.

The formation of light emitting cerium silicates in cerium-doped silicon oxides

Cerium-doped silicon oxides with cerium concentrations of up to 0.9 at. % were deposited by electron cyclotron resonance plasma enhanced chemical vapor deposition. Bright cerium related photoluminescence, easily seen even under room lighting conditions, was observed from the films and found to be sensitive to film composition and annealing temperature. The film containing 0.9 at. % Ce subjected to anneal in N2 at 1200 °C for 3 h showed the most intense cerium-related emission, easily visible under bright room lighting conditions. This is attributed to the formation of cerium silicate [Ce2Si2O7 or Ce4.667 (SiO4)3O], the presence of which was confirmed by high resolution transmission electron microscopy.

Formation and oxidation of Si nanoclusters in Er-doped Si-rich SiOx

The mechanisms for the formation and oxidation of Si nanoclusters (Si-ncls) are elucidated by means of the study of their effects on the photoluminescence of Er in Er-doped Si-rich SiOx (x<2) films. We find that the light emission of Er is the most intense in films with a Si concentration of ∼40% after annealing at 875°C in an argon ambient, which yields an optimum Si-ncl size. The nucleation rate of Si-ncls increases with temperature, however, they stabilize around a critical size which increases with annealing temperature. We determine that the activation energy for the formation of Si-ncls is 1.4±0.5eV. During annealing in an oxygen ambient Si-ncls are oxidized. The resultant oxide reduces the efficiency of energy transfer from them to Er ions and thus the light emission of Er. The activation energy for the oxidation is 1.06±0.03eV.

Influence of the annealing temperature and silicon concentration on the absorption and emission properties of Si nanocrystals

Silicon nanocrystals embedded in a silicon-rich silicon-oxide matrix have been fabricated at different silicon contents (38%, 40%, and 49%) using plasma-enhanced chemical vapor deposition and annealing at different temperatures in the range from 900 °C to 1100 °C. Their optical properties have been investigated by photoluminescence and transmittance measurements. Strong, room-temperature emission bands at ∼1.6 eV have been observed for all samples, with intensities dependent on the annealing temperature and Si content of the samples. From transmittance measurements, a redshift of the absorption edge has been detected when increasing the annealing temperature or Si content.