P. J. Simpson

Infrared spectroscopy of hydrogen in ZnO

Zinc oxide (ZnO) is a wide-band gap semiconductor that has attracted tremendous interest for optical, electronic, and mechanical applications. First-principles calculations by [C. G. Van de Walle, Phys. Rev. Lett. 85, 1012 (2000)] have predicted that hydrogen impurities in ZnO are shallow donors. In order to determine the microscopic structure of hydrogen donors, we have used IR spectroscopy to measure local vibrational modes in ZnO annealed in hydrogen gas. An oxygen–hydrogen stretch mode is observed at 3326.3 cm−1 at a temperature of 8 K, in good agreement with the theoretical predictions for hydrogen in an antibonding configuration. The results of this study suggest that hydrogen annealing may be a practical method for controlled n-type doping of ZnO.

Quantum confinement in Si and Ge nanostructures

We apply perturbative effective mass theory as a broadly applicable theoretical model for quantum confinement (QC) in all Si and Genanostructures including quantum wells(QWs), wires (Q-wires), and dots(QDs). Within the limits of strong, medium, and weak QC, valence and conduction band edge energy levels (VBM and CBM) were calculated as a function of QD diameters, QW thicknesses, and Q-wire diameters. Crystalline and amorphous quantum systems were considered separately. Calculated band edge levels with strong, medium, and weak QC models were compared with experimental VBM and CBM reported from X-ray photoemission spectroscopy (XPS), X-ray absorption spectroscopy (XAS), or photoluminescence(PL). Experimentally, the dimensions of the nanostructures were determined directly, by transmission electron microscopy(TEM), or indirectly, by x-ray diffraction (XRD) or by XPS. We found that crystalline materials are best described by a medium confinement model, while amorphous materials exhibit strong confinement regardless of the dimensionality of the system. Our results indicate that spatial delocalization of the hole in amorphous versus crystalline nanostructures is the important parameter determining the magnitude of the band gap expansion, or the strength of the quantum confinement. In addition, the effective masses of the electron and hole are discussed as a function of crystallinity and spatial confinement.

Impurity gettering to secondary defects created by MeV ion implantation in silicon

Impurities in MeV-implanted and annealed silicon may be trapped at interstitial defects near the projected ion range, Rp, and also at vacancy-related defects at approximately Rp/2. We have investigated the temperature dependence of impurity trapping at these secondary defects, which were preformed by annealing at 900 °C. The binding energies of Fe, Ni, and Cu are greater at the vacancy-related defects than at extrinsic dislocation loops. During subsequent processing at temperatures up to 900 °C, the amount of these impurities trapped at Rp/2 increases with decreasing temperature while the amount trapped at Rp decreases, with most of the trapped metals located at Rp/2 in samples processed at temperatures ≲ 700 °C. However, intrinsic oxygen is trapped at both types of defects; this appears to have little effect on the trapping of metallic impurities at extrinsic dislocations, but may inhibit or completely suppress the trapping at vacancy-related defects.

Quantum confinement in Si and Ge nanostructures: Theory and experiment

The role of quantum confinement (QC) in Si and Ge nanostructures (NSs) including quantum dots, quantum wires, and quantum wells is assessed under a wide variety of fabrication methods in terms of both their structural and optical properties. Structural properties include interface states, defect states in a matrix material, and stress, all of which alter the electronic states and hence the measured optical properties. We demonstrate how variations in the fabrication method lead to differences in the NS properties, where the most relevant parameters for each type of fabrication method are highlighted. Si embedded in, or layered between, SiO2, and the role of the sub-oxide interface states embodies much of the discussion. Other matrix materials include Si3N4 and Al2O3. Si NSs exhibit a complicated optical spectrum, because the coupling between the interface states and the confined carriers manifests with varying magnitude depending on the dimension of confinement. Ge NSs do not produce well-defined luminesc...

Detection of current‐induced vacancies in thin aluminum–copper lines using positrons

In situ depth‐resolved positron annihilation spectroscopy (PAS) is used to show dynamic formation of vacancies in 1 μm×1 μm Al‐0.5 wt % Cu lines under current flow. We show that the number of vacancies in these lines increases when a dc current (8×104 A/cm2) is applied. This increase in vacancy concentration is substantially greater than that due to thermal vacancy generation alone (4×1018 cm−3 versus 3×1017 cm−3). Isothermal measurements (with no current flow) yield a vacancy formation energy of 0.60±0.02 eV. These results show that PAS can be used to examine the initial stages of interconnect damage due to electromigration.

Vacancy generation resulting from electrical deactivation of arsenic

Electrical deactivation of arsenic in highly doped silicon has been studied using the positron‐beam technique. Direct experimental evidence linking the formation of arsenic‐vacancy complexes (i.e., Asn‐v) to the deactivation process is reported. The average number of arsenic atoms per complex, n≳2, was determined by comparing the observed complex concentrations with those of the deactivated arsenic inferred from Hall‐effect measurements.

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...

Chemical information in positron annihilation spectra

Positron annihilation spectra of arsenic‐ and gold‐implanted silicon are compared with spectra from bulk samples of arsenic and gold. Spectra with strongly reduced background intensities were recorded using a two detector coincidence system with a variable‐energy positron beam. It is shown that features in the high‐momentum region of the spectra (∼514–520 keV) can be identified with particular elements and that this identification is independent of structure, i.e., whether the element forms the bulk or is an implanted impurity. Proportionality between the intensity of characteristic spectral features and the fraction of annihilating positrons is also demonstrated, using the native oxide on a silicon wafer as a test case.

Observation of fluorine-vacancy complexes in silicon

We show direct evidence, obtained by positron annihilation spectroscopy, for the complexing of fluorine with vacancies in silicon. Both float zone and Czochralski silicon wafers were implanted with ...

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.