Showing papers by "Paulo V. Santos published in 2002"

Superhigh-frequency surface-acoustic-wave transducers using AlN layers grown on SiC substrates

Abstract: We demonstrate the operation of surface-acoustic-wave (SAW) transducers fabricated on AlN/SiC structures at frequencies as high as 19 GHz. The high SAW velocity of the AlN film is enhanced by the even higher sound velocity of the SiC substrate, enabling us to achieve these frequencies with a SAW wavelength of 400 nm.

Guided propagation of surface acoustic waves in AlN and GaN films grown on 4H-SiC(0001) substrates

Abstract: The propagation modes of surface acoustic waves ~SAW! in AlN and GaN layers are identified for the case of the acoustic wave being confined in the epitaxial layers by the fast-velocity SiC substrate. Higher-order Rayleigh modes emerge when the SAW wavelength is reduced to be below the layer thickness. We also report on the observation of an extremely-high-velocity SAW mode, which exists when the SAW wavelength is about an order of magnitude larger than the layer thickness. These guided modes enable us to utilize SAW velocities which can be even larger than the Rayleigh velocity in the substrate while maintaining the large electromechanical coupling coefficient of the overlayer.

Surface-acoustic-wave-induced carrier transport in quantum wires

Abstract: The ambipolar transport of photogenerated electron-hole pairs by surface acoustic waves (SAW's) in coupled GaAs quantum wells (QW's) and quantum wires (QWR's) is investigated by spatially and time-resolved photoluminescence. Experimental configurations for SAW propagation direction parallel or perpendicular to the QWR's have been studied. In the first configuration, the QWR confinement potential inhibits lateral carrier diffusion. The carriers are then efficiently transported along the wire as well-defined charge packages with a repetition rate corresponding to the SAW frequency. In the second configuration, we demonstrate that the SAW can be used to transfer electron-hole pairs generated in the QW into the QWR. We also provide clear evidence for the extraction of carriers from the QWR into the QW, when the SAW piezoelectric field is sufficiently strong to overcome the QWR confinement potential.

Propagation of surface acoustic waves in a GaAs/AlAs/GaAs heterostructure and micro-beams

Abstract: The dispersion of the Rayleigh wave in a strain-unrelaxed GaAs/AlAs/GaAs heterostructure is evaluated. We determine the wavelengths at which the velocity of surface acoustic waves (SAWs) is maximum or comparable to that in GaAs. The information is utilized to design interdigital transducers to investigate the SAW transmission properties when GaAs micro-beams are realized in the SAW propagation path by selectively etching the buried AlAs layer. A frequency shift as well as an attenuation of the SAW transmission peak is observed for certain SAW wavelengths within the range of the enhanced SAW velocity in the unpatterned heterostructure.

Elastic and magnetic properties of epitaxial MnAs layers on GaAs

Abstract: We have investigated the elasto- and magneto-optical properties of MnAs layers epitaxially grown on (001) GaAs for temperatures around the structural (hexagonal/orthorhombic) and magnetic (ferromagnetic/paramagnetic) phase transition of MnAs at ${T}_{c}\ensuremath{\sim}40\ifmmode^\circ\else\textdegree\fi{}\mathrm{C}.$ The phase transition is accompanied by a large variation of the MnAs lattice parameter a of $\ensuremath{\sim}1%,$ which induces a strong and anisotropic strain field in the MnAs/GaAs heterostructures. The latter was measured by detecting the optical anisotropy induced on the GaAs substrate by means of polarization-sensitive light transmission measurements. The experimental results show clear evidence for the quasi-uniaxial strain induced on the GaAs substrate during the phase transition, which extends over a temperature range of $\ensuremath{\sim}30\ifmmode^\circ\else\textdegree\fi{}\mathrm{C}$ in the MnAs/GaAs heterostructures. The strain levels are well reproduced by an elastic model for the heterostructures which assumes that the strain is transferred across the MnAs/GaAs interface without relaxation. The elastic properties during the phase transition were compared to the average magnetization probed using a SQUID magnetometer and to the magnetization near the front and the back surfaces of the MnAs films detected using the magneto-optical Kerr effect. The smaller temperature range of the phase transition observed in the magneto-optical Kerr effect measurements indicates a lower stability of the ferromagnetic phase near the surface of the MnAs layers.

Spatial-dispersion-induced acoustic anisotropy in semiconductor structures

Abstract: Brief Report, we show that the superposition of the two mechanisms induced by an acoustic mode in ~Al,Ga!As quantum well ~QW! structures also removes the invariance of the optical properties under a 90° rotation around the z 5@001# growth axis. Specifically, we demonstrate that the phase relationship between the acoustic ~u! and the piezoelectric ( F) fields composing the wave function of elastic modes differs by 180° for modes propagating along the x8 and y8 axes, thus leading to different optical properties. The phase difference, which is also expected in zinc blende crystals, arises from the distinct transformation properties of the two fields under a 90° rotation of the crystal. The investigations were performed by using photoluminescence ~PL! spectroscopy to map the acoustic and the piezoelectric fields induced by a surface acoustic wave of the Rayleigh type ~SAW! propagating on the QW structure. The samples consist of a 12-nm GaAs QW with Al0.3Ga0.7As barriers grown by molecular-beam epitaxy on GaAs ~001!. The QW is placed 100 nm below the surface. SAW’s propagating along the x8 and the y8 surface directions were generated by identical aluminum split-finger interdigital transducers ~IDT’s ! deposited on the sample surface, as illustrated in the inset of Fig. 1. The IDT’s were designed for operation at a wavelength l SAW of 5.6 mm, corresponding to a frequency v SAW /(2p) of 520 MHz at 12 K. The PL measurements were carried out at 12 K using a confocal microscope with illumination and detection areas with a diameter of about 2 mm. The continuous radiation from a Ti:sapphire laser ( l L5765 nm) was employed as excitation source. The setup also allows for interferometric measurements of the vertical ~i.e., parallel to z) surface displacement induced by the SAW, which was used to determine the absolute magnitude of the SAW acoustic field. The latter will be specified in terms of the ratio Pl between the acoustic power and the width of the SAW beam. Photoluminescence spectra of the GaAs QW recorded under SAW’s propagating along the two perpendicular directions x8 and y8 are shown in Figs. 1~a! and 1~b!, respectively, for different values of Pl . In the absence of a SAW,

Radiative recombination during ambipolar carrier transport by surface acoustic waves in GaAs quantum wells

Abstract: We report on the defect-assisted radiative recombination of photogenerated electrons and holes in GaAs quantum wells (QWs) during the transport by surface acoustic waves (SAWs). The studies were performed by detecting the spatial distribution of the photoluminescence (PL) with a resolution of a few micrometers. Under a SAW, a high PL intensity is observed on spatially localized spots along the SAW propagation path. This high PL intensity is attributed to recombination of the carriers, which are transported by the SAW, induced by charged defects located on or close to the QW plane.

Laser interference structuring of a-Ge films on GaAs

Abstract: We have investigated the laser interference crystallization (LIC) of amorphous germanium films on (100)-oriented GaAs substrates using nanosecond laser pulses. We demonstrate that LIC can produce periodic arrays of epitaxially crystallized Ge lines on GaAs with submicrometer widths. The gratings display a surface undulation with faceted surfaces, which depends on laser fluency. The undulation is attributed to the lateral solidification process induced by the temperature gradients created during the LIC process.

Pulsed laser crystallization of SiGe alloys on GaAs

Abstract: We have investigated the crystallization of amorphous SiGe films deposited on crystalline GaAs (001) substrates using ns laser pulses. Analysis of the film structure using Raman spectroscopy indicates the formation of heteroepitaxial Si xGe1/GaAs structures for Si compositions up to x = 25%. Higher compositions lead to polycrystalline films. This is attributed to the increased lattice mismatch between Si xGe1 and GaAs as the Si fraction in the alloy increases

Microscopic carrier dynamics in quantum wells modulated by high-frequency lateral fields

Abstract: We have investigated the dynamics of photogenerated carriers in GaAs quantum wells under the influence of high-frequency fields produced by metal gratings and by surface acoustic waves (SAW's) using spatially and time-resolved photoluminescence (PL). The frequency and phase of the PL oscillations induced by the high-frequency field yield information about the spatial distribution of the carriers and, in the case of SAW's, about the band-gap modulation induced by the SAW strain.

Pulsed laser crystallization and structuring of a-Ge on GaAs

Abstract: We have investigated the crystallization and structuring of amorphous Ge (a-Ge) films deposited on crystalline GaAs (1 0 0) substrates by nanosecond laser pulses. Epitaxial Ge films on GaAs are obtained for laser fluencies that completely melt the Ge film, but not the substrate. Higher fluencies lead to a partial melting of the substrate and to the formation of a (GaAs)1−xGe2x epitaxial alloy at the interface with the substrate. We demonstrate the fabrication of line gratings of crystalline Ge on GaAs produced by laser interference structuring. The gratings display an unusual surface undulation with faceted surfaces, which is ascribed to a lateral solidification process.