F. Briones

Large magnetoresistance in Fe/MgO/FeCo(001) epitaxial tunnel junctions on GaAs(001)

We present tunneling experiments on Fe(001)/MgO(20 A)/FeCo(001) single-crystal epitaxial junctions of high quality grown by sputtering and laser ablation. Tunnel magnetoresistance measurements give 60% at 30 K, to be compared with 13% obtained recently on (001)-oriented Fe/amorphous-Al2O3/FeCo tunnel junctions. This difference demonstrates that the spin polarization of tunneling electrons is not directly related to the density of states of the free metal surface—Fe(001) in this case—but depends on the actual electronic structure of the entire electrode/barrier system.

Growth process of III–V compound semiconductors by migration-enhanced epitaxy

Abstract The growth mechanism of GaAs and AlGaAs in migration-enhanced epitaxy is investigated by RHEED observation and optical scattering measurements. The available Ga site density on a (2 × 4) reconstructed GaAs (001) surface is much less than the ideal density due to a missing As-dimer array structure. The layer-by-layer growth of GaAs by migration-enhanced epitaxy proceeds by repeated formation and annihilation of small Ga droplets. The growth process is also investigated by studying growth on singular and vicinal (001) GaAs planes. The observed step-flow growth is explained by considering the different chemical characteristics of the steps along the [110] and [ 1 10] directions. The results are compared with those of other growth methods such as molecular beam epitaxy and metalorganic vapor phase epitaxy.

Influence of buffer-layer surface morphology on the self-organized growth of InAs on InP(001) nanostructures

We have studied the influence of InP buffer-layer morphology in the formation of InAs nanostructures grown on InP~001! substrates by solid-source molecular-beam epitaxy. Our results demonstrate that when InP buffer layers are grown by atomic-layer molecular-beam epitaxy, InAs quantum dot-like structures are formed, whereas InP buffer layers grown by MBE produce quantum-wire-like structures. The optical properties of these corrugated structures make them potential candidates for their use in light-emitting devices at 1.55 mm.

Atomic layer molecular beam epitaxy (Almbe) of III–V compounds: Growth modes and applications

A new development of molecular beam epitaxy (MBE) for III–V compounds is described, based on cyclic perturbation of the growth front at atomic layer level by periodic pulsing, alternating or interrupting the molecular beams. The modification of the growth mechanism caused by this perturbations is discussed and related to periodic changes of surface stoichiometry which induce 2D growth mechanism by enhanced layer nucleation.Under appropriate modulation conditions, an atomic layer by layer growth mode can be achieved. A practical implementention of this mode, that we denote atomic layer MBE (ALMBE), is considered in which only group V beams are pulsed in a specially designed effusion cell. A number of growth applications of ALMBE are presented, including growth of highly mismatched heterostructures and short period superlattices containing two different group V elements such as arsenic and phosphorous.

Strain relaxation and segregation effects during self-assembled InAs quantum dots formation on GaAs(001)

In segregation effects during InAs growth on GaAs(001) and critical thickness for InAs self-assembled quantum dots are studied using a real time, in situ technique capable of measuring accumulated stress during growth. Due to a large (∼50%) surface In segregation of floating In, self-assembled dot formation takes place when less than one monolayer of InAs is pseudomorphically grown on GaAs. A picture of the growth process is discussed on the basis of the equilibrium between InAs and floating In dominated by the stress energy.

Large disparity between gallium and antimony self-diffusion in gallium antimonide

The most fundamental mass transport process in solids is self-diffusion. The motion of host-lattice (‘self-’) atoms in solids is mediated by point defects such as vacancies or interstitial atoms, whose formation and migration enthalpies determine the kinetics of this thermally activated process. Self-diffusion studies also contribute to the understanding of the diffusion of impurities, and a quantitative understanding of self- and foreign-atom diffusion in semiconductors is central to the development of advanced electronic devices. In the past few years, self-diffusion studies have been performed successfully with isotopically controlled semiconductor heterostructures of germanium, silicon, gallium arsenide and gallium phosphide. Self-diffusion studies with isotopically controlled GaAs and GaP have been restricted to Ga self-diffusion, as only Ga has two stable isotopes, 69Ga and 71Ga. Here we report self-diffusion studies with an isotopically controlled multilayer structure of crystalline GaSb. Two stable isotopes exist for both Ga and Sb, allowing the simultaneous study of diffusion on both sublattices. Our experiments show that near the melting temperature, Ga diffuses more rapidly than Sb by over three orders of magnitude. This surprisingly large difference in atomic mobility requires a physical explanation going beyond standard diffusion models. Combining our data for Ga and Sb diffusion with related results for foreign-atom diffusion in GaSb (refs 8, 9), we conclude that the unusually slow Sb diffusion in GaSb is a consequence of reactions between defects on the Ga and Sb sublattices, which suppress the defects that are required for Sb diffusion.

A New Hydrogen Sensor Based on a Pt / GaAs Schottky Diode

A new hydrogen-sensitive detector based oii a PffGaAs Scliottlry diode has been fabricated. The dcvices have been characterized sy dark current-voltage aiid capacitance-voltage measurtrnents, as a function of temperature and gas phase composition. At 150°C, the detection limit for liydrogen is 6 ppm iil a nitrogeii environment and 200 ppm in air.

Transition from self‐organized InSb quantum‐dots to quantum dashes

We have grown self‐organized InSb quantum dots on semi‐insulating InP (001) substrates by molecular beam epitaxy. We studied the size dependency of the uncapped InSb quantum dots on the nominal thickness of the deposited InSb by atomic force microscopy. The dot sizes have a pronounced minimum at about 2.2 monolayers of InSb. After a nominal thickness of 3.2 monolayers we observe a drastic change of the dot shape, from quantum dots to quantum dashes. From there on the dots grow in a quasicylindric shape aligned in the (110) direction.We have grown self‐organized InSb quantum dots on semi‐insulating InP (001) substrates by molecular beam epitaxy. We studied the size dependency of the uncapped InSb quantum dots on the nominal thickness of the deposited InSb by atomic force microscopy. The dot sizes have a pronounced minimum at about 2.2 monolayers of InSb. After a nominal thickness of 3.2 monolayers we observe a drastic change of the dot shape, from quantum dots to quantum dashes. From there on the dots grow in a quasicylindric shape aligned in the (110) direction.

HF/H2O vapor etching of SiO2 sacrificial layer for large-area surface-micromachined membranes

Abstract An HF/H 2 O vapor etching technique has been applied as a sacrificial oxide etching process step in surface-micromachining technology. This technique does not suffer from the notorious problem known as stiction, i.e., permanent attachment of movable structures to the underlying substrate during drying after a conventional wet etch process is used. Vapor condensation has been controlled by adjusting the temperature difference between the substrate and the HF/H 2 O liquid source of vapor. Optical modulator devices have been fabricated to demonstrate the large possibilities of the vapor etching technique. Movable polysilicon membranes with a surface area of 10 mm 2 and a thickness of 0.73 μm over a 1.65 μm air gap have been routinely obtained with a 100% yield.

Ferromagnetism in bulk Co-Zn-O

The origin of ferromagnetism in diluted magnetic semiconductors is still an open question, yielding a great deal of research across the world. This work focuses on the Co-Zn-O system. Room-temperature ferromagnetism is observed after a partial reaction of Co3O4 and ZnO, which can be ascribed neither to carrier mediation nor segregated cobalt metallic clusters. Another mechanism is yielding room-temperature ferromagnetism. This mechanism is associated with a partial reaction of ZnO and Co3O4 grains, and always appears when the starting phases (Co3O4 and ZnO) are present in the sample, suggesting that interfaces are involved in the origin of the observed ferromagnetism.The origin of ferromagnetism in diluted magnetic semiconductors is still an open question, yielding a great deal of research across the world. This work focuses on the Co-Zn-O system. Room-temperature ferromagnetism is observed after a partial reaction of Co3O4 and ZnO, which can be ascribed neither to carrier mediation nor segregated cobalt metallic clusters. Another mechanism is yielding room-temperature ferromagnetism. This mechanism is associated with a partial reaction of ZnO and Co3O4 grains, and always appears when the starting phases (Co3O4 and ZnO) are present in the sample, suggesting that interfaces are involved in the origin of the observed ferromagnetism.