Todd W. Simpson

InGaAs/InP quantum well intermixing studied by cross-sectional scanning tunneling microscopy

Cross-sectional scanning tunneling microscopy (STM) is used to study lattice matched InGaAs/InP quantum well (QW) intermixing induced by ion implantation and thermal annealing. Different strain development in QWs (determined by STM topography of elastic relaxation in cross sectionally cleaved samples) is found to be dependent upon the range of the implanted ions relative to the QWs. It is found that the quantum wells remain latticed matched to the barrier layers after intermixing when ions are implanted through the multiple quantum well (MQW) stack. A shallow implantation in which ions are implanted into the cap layer above the MQW stack leads to tensilely strained wells and compressively strained interfaces between wells and barriers. The strain development in the latter case is attributed to different degrees of interdiffusion on the group III and group V sublattices. Finite element elastic computations are used to extract the group V and group III interdiffusion length ratio, and results using differen...

Doping effects on the kinetics of solid‐phase epitaxial growth of amorphous alumina thin films on sapphire

The effects of doping on the kinetics of solid‐phase epitaxial growth of amorphous alumina have been studied. Amorphous alumina Al2O3) thin films, 200–265 mm thick, were deposited on to (0001) sapphire substrates by electron‐beam evaporation. Iron or chromium atoms were uniformly doped into the films during deposition to cation concentrations below 5 cationic %. The kinetics of the epitaxial growth were studied at 800–1050 °C in flowing oxygen gas by in situ time‐resolved reflectivity techniques as well as by ion backscattering and channeling techniques. A phase transformation sequence from amorphous through gamma to alpha alumina has been observed in all the undoped and doped films. The transformation from γ to α alumina is a thermally activated process with an activation energy of 5.0±0.2 eV, independent of the presence of dopants. However, the presence of dopants affects the overall transformation rate. Fe enhances while Cr slows the growth rate relative to the undoped case.

Formation of iron or chromium doped epitaxial sapphire thin films on sapphire substrates

This work summarizes results of a simple procedure to incorporate dopants into the near surface region of single‐crystal sapphire. We demonstrate the formation of iron‐doped and chromium‐doped sapphire thin films by solid‐phase epitaxial growth. Amorphous alumina films of about 200–350 nm thickness were deposited onto single‐crystal sapphire substrates. Fe or Cr ions were introduced into the films during deposition. A post‐deposition thermal process was performed in oxidizing ambients at 800–1400 °C to induce epitaxial growth and to incorporate dopants. The epitaxial relationship of the grown film with the substrate was confirmed by both ion channeling and cross‐sectional transmission electron microscopy. The growth kinetics were determined by time‐resolved reflectivity measurements for different dopant concentrations. Ion channeling angular scans revealed that the Fe or Cr ions are incorporated onto octahedral sites (Al3+ sites) in the corundum structure as expected in equilibrium. External optical trans...

Digital in-line soft x-ray holography with element contrast

Digital in-line soft x-ray holography (DIXH) was used to image immobilized polystyrene and iron oxide particles and to distinguish them based on their different x-ray absorption cross sections in the vicinity of the carbon K-absorption edge. The element-specific information from the resonant DIXH images was correlated with high-resolution scanning electron microscopy (SEM) pictures. We also present DIXH images of a cell nucleus and compare the contrast obtained for nuclear components with the appearance in optical microscopy.

HYDROGEN CATALYZED CRYSTALLIZATION OF STRONTIUM TITANATE

The crystallization rate of amorphous strontium titanate is enhanced by more than an order of magnitude during thermal annealing in water vapor as compared to a dry ambient. Time resolved optical reflectivity (TRR) has been combined with Rutherford backscattering spectrometry (RBS) and ion channelling to investigate this effect. Thin amorphous films (0.6 μm) were produced on single‐crystal substrates of (100) strontium titanate by bombardment with 1.9 or 2.0 MeV Pb ions. Specimens were annealed under controlled ambient conditions (H2O, D2O, vacuum, 265–430 °C) and the solid phase epitaxial crystallization monitored in situ by TRR (633 nm). The TRR data were calibrated ex situ by transmission electron microscopy and RBS measurements. Isotope substitution, nuclear reaction analysis, and secondary‐ion‐mass spectrometry were utilized to reveal the uptake of hydrogen and oxygen into the implanted layer. Hydrogen is identified as the only species which penetrates to the crystal/amorphous interface. It is shown ...

IMPROVED STOICHIOMETRY MEASUREMENTS USING 4HE ELASTIC BACKSCATTERING : EXPERIMENT AND SIMULATION

Abstract We have measured absolute spectra for MeV energy 4He ions backscattered from amorphous silicon, SiO2 and Si3N4 targets. The simulated backscattering spectra from a-Si are only brought into acceptable agreement with experiment when we adopt 4He stopping cross section values recently reported by Konac et al. [Nucl. Instr. and Meth. B 136–138 (1998) 159]. When these same values are used to simulate RBS spectra from SiO2 and Si3N4 targets, good agreement is found for ions incident at 2 MeV but there is ∼8% disagreement for 1 MeV ions. The implications for stoichiometry determinations based on RBS are discussed.

Ultrahigh-density, nonlithographic, sub-100 nm pattern transfer by ion implantation and selective chemical etching

A self-assembled array of nanometer-sized holes in alumina has been adapted as a mask for conventional, broad-area, ion implantation. The mask pattern, made up of nanoholes arranged in a two-dimensional triangular array with a 100 nm period and a 55 nm diameter pore size, has been successfully transferred onto single crystal (100) SrTiO3 substrates using 200 and 500 keV energy Pt ion bombardments, at fluences sufficient to amorphize the exposed areas. The amorphized material was removed by selective chemical etching resulting in a periodic array of holes about 55 nm in diameter and 115 nm deep. This parallel, nonlithographic approach is adaptable to submicron depth, variable array geometry and scale, and to any material where a selective etch can be found for the irradiated volume.

Deactivation and diffusion of boron in ion-implanted silicon studied by secondary electron imaging

Secondary electron (SE) imaging in a scanning electron microscope is used to map electrically active dopant distributions of B-doped superlattices in Si. By comparing SE contrast profiles with secondary ion mass spectroscopy data, it is shown that B is electrically deactivated when the damage caused during Si implantation falls onto a doped region. Following a 450 °C anneal, the effect of the implantation damage is severely reduced in the SE profiles and the B is partially reactivated. An 815 °C anneal results in transient enhanced diffusion of some of the B with the remainder trapped in an inactive immobile peak.

Secondary defect formation in self-ion irradiated silicon

Abstract A detailed experimental study has been made of the evolution of extended secondary decondary defects which form during rapid thermal anneals of 0.5 MeV energy self-ion irradiated silicon. The implant fluence (2 × 10 15 ions/cm 2 ), flux and substrate temperature (91°C) were chosen so that primary damage levels were well below saturation. Cross-sectional transmission electron microscopy (X-TEM), Rutherford backscattering-channeling spectroscopy (RBS-C) and variable-energy positron annihilation techniques (VEP) have been used to allow partial discrimination between vacancy- and interstitial-type defects. The growth and development of the defect band and of specific types of extended defects within the band has been followed up to anneal temperatures of 1000°C, where the majority are shown to have dissipated. X-TEM has revealed the formation of a previously unreported tubular defect which is found in a narrow temperature range of 700–765°C. The occurrence of this defect correlates with the positron annihilation analysis which shows that a small concentration of defects with vacancy character is present after annealing in the same temperature range. In addition, positron annihilation analysis has allowed an assessment of the role played by defects lying in regions appearing defect-free by the other techniques (RBS-C and TEM). The implications of these findings to existing models involving secondary defect production are discussed.

Suppression of dislocation formation in silicon by carbon implantation

We have examined the role of carbon co‐implantation in the formation of secondary defects in self‐ion‐irradiated Si(100). Implantation of Si ions (540 keV energy, 1015 ions/cm2 at 1.3×1011 ions/cm2/s, Ti=90 °C) followed by a 900 °C, 15 min anneal leads to the growth of an extended defect band at the end of range. Range matched‐carbon co‐implantation (300 keV energy, 1015 ions/cm2 plus 500 keV energy 1015 ions/cm2 of 1.5×1011 ions/cm2/s, Ti=90 °C) can be used to modify this defect development dramatically. While direct co‐implantation of carbon and silicon ions to similar concentrations has no apparent effect on the formation of extended defects, such formation is suppressed when the implanted C is incorporated substitutionally into the silicon lattice. These results are discussed in the context of recent reports on C suppression of the transient enhanced diffusion of boron.