Magnetic field detection with extremely high spatial resolution is crucial to applications in magnetic storage, biosensing, and magnetic imaging. Here, we present the concept of using a spin torque oscillator (STO) to detect magnetic fields by measuring the frequency of the oscillator. This sensor's performance relies predominantly on STO properties such as spectral linewidth and frequency dispersion with magnetic field, rather than signal amplitude as in conventional magnetoresistive sensors, and is shown in measured devices to achieve large signal to noise ratios. Using macrospin simulations, we describe oscillator designs for maximizing performance, making spin torque oscillators an attractive candidate to replace more commonly used sensors in nanoscale magnetic field sensing and future magnetic recording applications.

Raman spectroscopy has been used to determine built-up deformation in GexSi1−x/Si strained-layer superlattice grown by molecular beam epitaxy. By comparing peak positions in commensurate superlattices and single layers with those from incommensurate thick layers of the same composition we can obtain a quantitative determination of strain. Linewidths are affected by the presence of inhomogeneous strain, dislocations, and disorder. Lines are always narrower in superlattice samples, indicating better crystalline quality. In particular, the Raman line from the Si layers of the strained-layer superlattices is indistinguishable from that from single-crystalline Si in both linewidth and frequency. This is consistent with the expectation that the entire lattice mismatch is accommodated as a homogeneous tetragonal strain in the alloy layers only.

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The strain-induced splitting of the heavy-hole (hh) and light-hole (lh) valence bands for 4-pmthick GaAs/Si is examined on a microscopic scale using linear polarized-cathodoluminescence imaging and spectroscopy. The energies and intensities of the hh-and lh-exciton luminescence are quantitatively analyzed to determine spatial variations in the stress tensor. The results indicate that regions near and far from the microcracks are primarily subject to uniaxial and biaxial tensile stresses, respectively. The transition region where biaxial stress gradually converts to uniaxial stress is analyzed, and reveals a mixing of lh and hh characters in the strain-split bands.In the past decade, improvements in epitaxial-growth techniques have made feasible the hybridization of GaAs and Si technologies for applications of high-speed electronic and optoelectronic devices, despite the 4. 1% lattice mismatch.The ratio of the GaAs and Si thermalexpansion coeScients is about 2.5 at typical growth temperatures in excess of 700 C, and the subsequent cooling to room temperatures results in a large tetragonal distortion which can cause wafer bowing and the formation of microcracks to relieve partially the thermal stress. The effects of stress on the splitting of the j = 2 heavy-hole (hh; m~= + -' , ) and light-hole (lh; m1 =+ -, ' ) bands at k=0 have been previously studied by several authors using photoluminescence (PL), ' photoluminescence excitation (PLE), ' ' cathodoluminescence (CL), ' and photoreflectance (PR);"' optical transitions involving these states exhibit polarization selection rules which depend on the strain. 'In this paper, we present the results of a novel approach in which linearly polarized CL scanning electron microscopy (SEM) is used to analyze spatial variations in the stress tensor for GaAs/Si. Yacobi and co-workers, ' using CL, have demonstrated the presence of local variations in the luminescence near microcracks caused by variations in stress. With the enhancement of a linear-polarizationdetection scheme, we demonstrate that a quantification of the exciton luminescence intensities and energies in the CL spectra can be performed; this leads to a definitive evaluation of the micrometer-scale spatial variations of the hh and lh characters in the strain-split valence bands. The present approach, in conjunction with polarization selection rules which depend on the form of the stress tensor, enables a determination of the spatial distribution of the stress. Scanning monochromatic CL images are presented and reveal a polarization anisotropy within the sample. From the results, we show that the stress is predominantly uniaxial along the microcracks and predominantly biaxial in the regions between the cracks.The samples examined in this study were grown by atmospheric-pressure metal-organic chemical-vapor deposition (MOCVD) at the Spire Corporation. ' A threestep method was used to grow the sample and is described here: the Si substrate [oriented 2' oA' (001) in the (110) direction] was heated in hydrogen to over 100...

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