D. Paget

Preparation of clean reconstructed InAs(001) surfaces using HCl/isopropanol wet treatments

A simple treatment method using HCl/isopropanol solutions is given for preparation of high-quality InAs(001) surfaces. The surface structure and chemistry were characterized using low-energy electron diffraction and photoemission spectroscopy as a function of UHV temperature. The treatment removes the natural oxide and leaves on the surface a physisorbed overlayer containing arsenic and small amounts of InClx. Annealing at 330 °C induces desorption of this overlayer and reveals a clean arsenic-rich (2×4)/c(2×8) surface. The indium-rich (4×2)/c(8×2) reconstruction is obtained upon further annealing to 410 °C.

Origin of the optical anisotropy of GaAs (001)

We present a detailed experimental investigation of the room temperature optical anisotropy of clean GaAs(001) surfaces, and of the change of this anisotropy after exposure to oxygen. On the As-rich (2×4) surface, this anisotropy consists of two main signals, situated, respectively, near 3 and 4.5 eV. The results, together with the analysis of the anisotropy of Ga1−xAlxAs, lead us to the conclusion that the anisotropy near 3 eV originates mainly from localized states related to surface dimers. However, the anisotropy near 4.5 eV is related to bulk-like optical transitions, since this anisotropy is chemically insensitive, and since its spectral shape is similar to that of the bulk dielectric constant.

Circularly polarized luminescence microscopy for the imaging of charge and spin diffusion in semiconductors

Room temperature electronic diffusion is studied in 3 μm thick epitaxial p(+) GaAs lift-off films using a novel circularly polarized photoluminescence microscope. The method is equivalent to using a standard optical microscope and provides a contactless means to measure both the charge (L) and spin (L(s)) diffusion lengths simultaneously. The measured values of L and L(s) are in excellent agreement with the spatially averaged polarization and a sharp reduction in these two quantities (L from 21.3 to 1.2 μm and L(s) from 1.3 to 0.8 μm) is found with increasing surface recombination velocity. Outward diffusion results in a factor of 10 increase in the polarization at the excitation spot. The range of materials to which the technique can be applied, as well as a comparison with other existing methods for the measurement of spin diffusion, is discussed.Room temperature electronic diffusion is studied in 3 mum thick epitaxial p+ GaAs lift-off films using a novel circularly polarized photoluminescence microscope. The method is equivalent to using a standard optical microscope and provides a contactless means to measure charge (L) and spin (L_s) diffusion lengths. The measured values of L and L_s are in excellent agreement with the spatially averaged polarization and a sharp reduction in these two quantities (L from 21.3 mum to 1.2 mum and L_s from 1.3 mum to 0.8 mum) is measured with increasing surface recombination. Outwards diffusion results in a factor of 10 increase in the polarization at the excitation spot.

Soft nitridation of GaAs(100) by hydrazine sulfide solutions: Effect on surface recombination and surface barrier

The effect of nitridation of GaAs(100) by hydrazine sulfide solutions on the surface recombination velocity and surface barrier has been studied using photoluminescence and photoreflectance spectroscopies. Nitridation produces a decrease of surface recombination velocity by a factor of 26. After three years of air exposure, the recombination velocity is still smaller than for the naturally oxidized surface by a factor of 11. The observed effect is caused by a continuous nitride monolayer bonded with the GaAs substrate. The surface Fermi level is still pinned near midgap, which is attributed to residual unpassivated surface defects.

Fluidic assembly of hybrid MEMS: a GaAs-based microcantilever spin injector

A proof-of-concept fluidic assembly of a hybrid MEMS GaAs microcantilever spin injector is presented here. Instead of monolithically forming MEMS from pre-deposited layers, we fabricate a hybrid MEMS by assembling pre-fabricated parts. Sub-millimetre sized patches of GaAs having a thickness of 3 µm are pre-fabricated, as is a metalized fused silica support layer. The GaAs patches are manipulated and assembled onto the silica support using capillary forces; the resultant hybrid MEMS comprises a GaAs microcantilever on a robust fused silica support. A novel ohmic contact is demonstrated by bonding a GaAs patch (p-type carbon doped to 1 × 1018 cm−3) onto the pre-metalized silica support layer prior to annealing; measurements revealed ohmic behaviour and a specific contact resistivity of ~10−5 Ω cm. Preliminary investigations show that, when contacting the cantilever against a metallic or magnetic surface, injected photocurrents as large as several tens of nA can be obtained, for which the spin polarization is equal to 16%.

Nuclear magnetization in gallium arsenide quantum dots at zero magnetic field.

Optical and electrical control of the nuclear spin system allows enhancing the sensitivity of NMR applications and spin-based information storage and processing. Dynamic nuclear polarization in semiconductors is commonly achieved in the presence of a stabilizing external magnetic field. Here we report efficient optical pumping of nuclear spins at zero magnetic field in strain-free GaAs quantum dots. The strong interaction of a single, optically injected electron spin with the nuclear spins acts as a stabilizing, effective magnetic field (Knight field) on the nuclei. We optically tune the Knight field amplitude and direction. In combination with a small transverse magnetic field, we are able to control the longitudinal and transverse components of the nuclear spin polarization in the absence of lattice strain—that is, in dots with strongly reduced static nuclear quadrupole effects, as reproduced by our model calculations.

Light-induced nuclear quadrupolar relaxation in semiconductors

Light excitation of a semiconductor, known to dynamically-polarize the nuclear spins by hyperfine contact interaction with the photoelectrons, also generates an intrinsic nuclear depolarization mechanism. This novel relaxation process arises from the modulation of the nuclear quadrupolar Hamiltonian by photoelectron trapping and recombination at nearby localized states. For nuclei near shallow donors, the usual diffusion radius is replaced by a smaller, quadrupolar, radius. If the light excitation conditions correspond to partial donor occupation by photoelectrons, the nuclear magnetization and the nuclear field can be decreased by more than one order of magnitude.

Chemistry of Wet Treatment of GaAs(111)B and GaAs(111)A in Hydrazine-Sulfide Solutions

Chemical treatment by hydrazine-sulfide solutions, known to produce surface nitridation of GaAs(100), was applied to GaAs(111)A and B surfaces. The chemistries of these treatments for the Ga-terminated A surface and the As-terminated B one were investigated using synchrotron radiation photoemission and Auger electron spectroscopies. For the B surface, such treatment was found to produce an effective surface nitridation, via substitution of surface arsenic with nitrogen atoms from hydrazine molecules. The process automatically stops after formation of one monolayer. In contrast, the A surface is covered by sulfur bonded to underlying gallium. This extreme dependence on surface polarity is explained by competitive adsorption processes of HS- and OH- anions and of hydrazine molecules, on Ga-adsorption sites, which have distinct configurations on the A and B surfaces.

Surface recombination in doped semiconductors: Effect of light excitation power and of surface passivation

A self-consistent expression for the surface recombination velocity S and the surface Fermi level unpinning energy as a function of light excitation power (P ) is presented for nand p-type semiconductors doped above the 10 cm range. Measurements of S on p-type GaAs films using a novel polarized microluminescence technique are used to illustrate two limiting cases of the model. For a naturally oxidized surface S is described by a power law in P whereas for a passivated surface S varies logarithmically with P . Furthermore, the variation in S with surface state density and bulk doping level is found to be the result of Fermi level unpinning rather than a change in the intrinsic surface recombination velocity. It is concluded that S depends on P throughout the experimentally accessible range of excitation powers and therefore that no instrinsic value can be determined. Previously reported values of S on a range of semiconducting materials are thus only valid for a specific excitation power.For n- and p-type semiconductors doped above the 1016 cm−3 range, simple analytical expressions for the surface recombination velocity S have been obtained as a function of excitation power P and surface state density NT. These predictions are in excellent agreement with measurements on p-type GaAs films, using a novel polarized microluminescence technique. The effect on S of surface passivation is a combination of the changes of three factors, each of which depends on NT: (i) a power-independent factor which is inversely proportional to NT and (ii) two factors which reveal the effect of photovoltage and the shift of the electron surface quasi Fermi level, respectively. In the whole range of accessible excitation powers, these two factors play a significant role so that S always depends on power. Three physical regimes are outlined. In the first regime, illustrated experimentally by the oxidized GaAs surface, S depends on P as a power law of exponent determined by NT. A decrease of S such as the one induc...

Imaging ambipolar diffusion of photocarriers in GaAs thin films

Images of the steady-state luminescence of passivated GaAs self-standing films under excitation by a tightly focussed laser are analyzed as a function of light excitation power. While unipolar diffusion of photoelectrons is dominant at very low light excitation power, an increased power results in a decrease of the diffusion constant near the center of the image due to the onset of ambipolar diffusion. The results are in agreement with a numerical solution of the diffusion equations and with a physical analysis of the luminescence intensity at the centre of the image, which permits the determination of the ambipolar diffusion constant as a function of electron concentration.