M. I. N. da Silva

Synthetic melanin thin films: Structural and electrical properties

Scanning probe microscopy was used to investigate the structural and electrical organization at the nanoscopic level of hydrated melanin thin films synthesized by oxidizing L-3-(3,4-dihydroxyphenyl)-alanine (L-dopa) in dimethyl sulfoxide. Atomic force microscopy (AFM) provided the morphologies of the L-dopa melanin films. Electrostatic force microscopy and conductive-AFM were used to spatially resolve the electrical properties of the material. Using a simple parallel plate capacitor model a method to measure the charge distribution on the sample was developed. The correlations between topography, electric charge, and current images of the sample demonstrated that the hydration process produces a restructuring of melanin observed not only through topographic variations, but also through the creation of areas with different electrical properties.

On three dimensional self-organization and optical properties of InAs quantum-dot multilayers

We report on experiments aimed at producing three-dimensional self-organization in InAs quantum-dot multilayers embedded in GaAs. These InAs/GaAs quantum-dot multilayers have been grown by molecular beam epitaxy. Employing atomic force microscopy, we have analyzed the island density in samples with different number of periods of InAs/GaAs bilayers The results reveals a decrease and a tendency to saturation of the island density with an increase in the number of periods, as a three-dimensional self-organization characteristic of these samples. Optical properties of the samples are examined via photoluminescence spectroscopy. The evolution of the quantum-dot photoluminescence peak position indicates an increment in the mean size of the buried islands and a relative homogenization in size of the quantum dots, as the number of periods increases. The results of the optical measurements agree with the morphological data, and characterize a spatial process of self-organization, related to the increment of the nu...

Improvements in focused ion beam micromachining of interconnect materials

Focused ion beam micromachining (FIBM) of integrated circuits continues to be an important tool for design debug, editing, and verification; for metrology; and for process control. FIBM of copper interconnects has presented challenges not faced when micromachining aluminum interconnects and the introduction of low-k dielectrics present additional challenges. A new approach to chemically assisted FIBM of thin film Cu, SiO2, and SiLK low-k material using polar precursor molecules has been investigated. Polar alcohols were used to reduce the sputter rate of SiO2 and SiLK while having a minimal effect on the Cu sputter rate. A new FIBM process based on the reduction of the FIB Ga+ energy from the typical 25 to 15 keV is also introduced. The new low energy FIBM process was shown to increase the sputter rate of polycrystalline Cu with strong (111) crystallographic texture by a factor of 2.5. This increase in the sputter rate of Cu combined with a slight reduction of the sputter rate of SiO2 and SiLK results in ...

Identification and quantification of iron silicide phases in thin films

Iron silicide samples were grown on Si (111) substrates by solid phase epitaxy and reactive deposition epitaxy. The different iron silicide phases and their correlations with the growth parameters were analyzed by x-ray photoelectron spectroscopy, conversion electron Mossbauer spectroscopy, x-ray diffraction, atomic force microscopy, and magnetic force microscopy. The authors investigated the potential of each technique for identifying and quantifying of the phases. In particular, the authors used a semiquantitative analysis of magnetic force microscopy images to spatially resolve the semiconductor β-FeSi2 phase.

Electrical transport properties of silicon delta-doped Al0.30Ga0.70As samples showing suppression of the DX center features

Photoconductivity and photo-Hall density measurements using an infrared light emitting diode as the light source were carried out on single silicon delta-doped Al0.30Ga0.70As samples as a function of temperature. The samples were grown by molecular beam epitaxy at 530 °C and 600 °C. We have studied the effect of etching the cap layer on the electrical transport properties. An observed persistent photoconductivity effect is explained using a model of parallel conduction in two nearby spatially separated channels. We will present evidence that the DX center is not active for nearly ideal delta-doped samples. We have proposed that the DX-center level related to the conduction-band minimum is strongly dependent on the silicon delta-doping density and on the growth conditions.

The EL2-like metastable defect and the n- to p-type transition in silicon planar-doped GaAs

Through photo-Hall measurements at temperatures below about 120 K, we have observed the presence of a deep donor defect, with characteristics similar to those of the EL2 center, in planar-doped GaAs samples grown by molecular beam epitaxy at 300 °C. We have shown that this EL2-like center can account for the remarkable photoreleasing of electrons and holes into the conduction and valence bands, respectively. The two different kinds of carriers accumulate in two spatially separated channels, which can secondarily account for the n- to p-type transition we have observed.

Cross-sectional Scanning Probe Microscopy of GaN-based p–n heterostructures

Abstract In this work, the structural and electrical properties of a GaN-based p–n heterostructure are studied using cross-sectional Atomic Force Microscopy, Friction Force Microscopy, Electrical Force Gradient Microscopy, and Surface Potential Microscopy. Using Atomic Force Microscopy and Friction Force Microscopy, we were able to identify and measure the thickness of the layers present in the heterostructures. The electrical conductivity type of the different layers as well as the p–n junction, and piezoelectric fields were identified and studied using Electric Force Gradient Microscopy and Surface Potential Microscopy.

Magneto-optical properties of stacked self-assembled InAs quantum dots

In this article, we report magneto-photoluminescence measurements on stacked self-assembled InAs quantum dots. By applying a magnetic field parallel to the growth direction, we determined the exciton reduced mass and exciton radius from the photoluminescence (PL) peak energy. We observed an asymmetric increase of the full width at half maximum of the quantum dots PL peak to the high-energy side that we associate to the size selectivity of the oscillator strength of the ground state transitions. The observed increase of the integrated intensity of the quantum dots line is explained in terms of the reabsorption of the photons emitted by the GaAs substrate and the InAs wetting layer. These effects are related to the multilayer structure of the sample.

Electrical characterization of InGaN quantum well p -n heterostructures

Abstract In this work, two methods for electrical characterization of InGaN quantum well p–n heterostructures at the nanometer level are presented. Cross-sectional Electrical Force Microscopy and High Resolution Electron Beam Induced Current (HR-EBIC) are used to study and identify regions of the cross-sectional surface of InGaN heterostructures with different types of electrical conductivity, the location of the InGaN quantum well, the location of the p–n junction, and the depletion layer. HR-EBIC was implemented in a Scanning Transmission Electron Microscope to take advantage of the high resolution chemical imaging capabilities of this microscope, such as Z-Contrast and Energy Dispersive X-ray Spectroscopy, and the small spread of the high energy electron beam in the electron transparent thin sample that allows electron beam induced current imaging with nanometer resolution.