Madalina Furis

Electrical Detection of Spin Accumulation at a Ferromagnet-Semiconductor Interface

We show that the accumulation of spin-polarized electrons at a forward-biased Schottky tunnel barrier between Fe and -GaAs can be detected electrically. The spin accumulation leads to an additional voltage drop across the barrier that is suppressed by a small transverse magnetic field, which depolarizes the spins in the semiconductor. The dependence of the electrical accumulation signal on magnetic field, bias current, and temperature is in good agreement with the predictions of a drift-diffusion model for spin-polarized transport.

Excitons in carbon nanotubes with broken time-reversal symmetry.

Near-infrared magneto-optical spectroscopy of single-walled carbon nanotubes reveals two absorption peaks with an equal strength at high magnetic fields (>55 T). We show that the peak separation is determined by the Aharonov-Bohm phase due to the tube-threading magnetic flux, which breaks the time-reversal symmetry and lifts the valley degeneracy. This field-induced symmetry breaking thus overcomes the Coulomb-induced intervalley mixing which is predicted to make the lowest exciton state optically inactive (or dark).

Quantifying electrical spin injection: Component-resolved electroluminescence from spin-polarized light-emitting diodes

The spin-polarized light-emitting diode (spin-LED) is a very effective tool for accurately quantifying electrical spin injection in a model independent manner. We resolve and identify various components which occur in the electroluminescence (EL) spectra of GaAs quantum-well-based spin-LEDs, and examine the circular polarization of each. While a number of components exhibit significant circular polarization, the values do not necessarily reflect the electrical spin injection efficiency. We show that a reliable measure of spin injection efficiency can be obtained only if one takes care to spectroscopically resolve and accurately identify the free exciton or free carrier components of the EL spectrum, and exclude other components.

Electrical spin injection across air-exposed epitaxially regrown semiconductor interfaces

We have fabricated spin-polarized light-emitting diode structures via epitaxial regrowth of Zn1−xMnxSe on air-exposed surfaces of AlyGa1−yAs/GaAs quantum wells. No passivation procedures were used to protect or prepare the III–V surface. The electroluminescence is strongly circularly polarized due to the electrical injection of spin-polarized electrons from the ZnMnSe contact into the GaAs quantum well. An analysis of the optical polarization yields a lower bound of 65% for the spin injection efficiency. These results demonstrate the robustness of the spin injection process in the diffusive transport regime, and attest to the practicality of manufacturing semiconductor-based spin injection devices.

Local Hanle-effect studies of spin drift and diffusion in n:GaAs epilayers and spin-transport devices

In electron-doped GaAs, we use scanning Kerr-rotation microscopy to locally probe and spatially resolve the depolarization of electron spin distributions by transverse magnetic fields. The shape of these local Hanle- effect curves provides a measure of the spin lifetime as well as spin transport parameters including drift velocity, mobility and diffusion length. Asymmetries in the local Hanle data can be used to reveal and map out the effective magnetic fields due to spin-orbit coupling. Finally, using both spin imaging and local Hanle effect studies, we investigate the drift and diffusion of electrically-injected spins in lateral Fe/GaAs spin-detection devices, both within and outside the current path.

Anomalous circular polarization of photoluminescence spectra of individual CdSe nanocrystals in an applied magnetic field.

A Zeeman splitting between otherwise degenerate spin eigenstates under applied magnetic fields is a fundamental quantum mechanical effect that has been studied in many different quantum systems ranging from atoms and molecules to electronhole excitations (excitons) in both bulk and nanostructured semiconductors. Recently, a significant research effort has focused on field-dependent spin phenomena in semiconductor quantum dots (QDs) motivated by a desire to understand the fundamental spin structure of QD electronic states and by potential technological applications in solid-state spintronics and quantum information processing.

Bright-exciton fine structure and anisotropic exchange in CdSe nanocrystal quantum dots

We report on polarization-resolved resonant photoluminescence (PL) spectroscopy of bright (spin-1) and dark (spin-2) excitons in colloidal CdSe nanocrystal quantum dots. Using high magnetic fields to 33 T, we resonantly excite (and selectively analyze PL from) spin-up or spin-down excitons. At low temperatures (<4K) and above ~10 T, the spectra develop a narrow, circularly polarized peak due to spin-flipped bright excitons. Its evolution with magnetic field directly reveals a large (1-2 meV), intrinsic fine structure splitting of bright excitons, due to anisotropic exchange. These findings are supported by time-resolved PL studies and polarization-resolved PL from single nanocrystals.

Bias-dependent electron spin lifetimes in n-GaAs and the role of donor impact ionization

In bulk n-GaAs epilayers doped near the metal-insulator transition, the authors study the evolution of electron spin lifetime τs as a function of applied lateral electrical bias Ex. τs is measured via the Hanle effect using magneto-optical Kerr rotation. At low temperatures (T 100ns at zero bias), a marked collapse of τs is observed when Ex exceeds the donor impact ionization threshold at ∼10V∕cm. A steep increase in the concentration of warm delocalized electrons—subject to Dyakonov-Perel spin relaxation [Sov. Phys. Solid State 13, 3023 (1972)]—accounts for the rapid collapse of τs and strongly influences electron spin transport in this regime.

Room-temperature ultraviolet emission from GaN/AlN multiple-quantum-well heterostructures

We investigate the photoluminescence (PL) properties of GaN/AlN multiple-quantum-well structures grown by plasma-induced molecular-beam epitaxy by time-resolved PL spectroscopy. Despite the large strain induced by the lattice mismatch between GaN and AlN, the samples exhibit strong room-temperature UV emission characterized by a nonexponential decay that varies across the PL feature. The energy corresponding to the peak of the PL spectra varies as a function of the well width, in agreement with a calculation of the electron–hole (e1h1) transition energy that includes the large piezoelectric and spontaneous polarizations existing inside the wells. The thermal quenching activation energies of the emission intensity can be identified as the donor and acceptor binding energies.

Si doping of high-Al-mole fraction AlxGa1−xN alloys with rf plasma-induced molecular-beam-epitaxy

Very high levels of n-type doping of AlxGa1−xN alloys were recently achieved by rf plasma-induced molecular-beam epitaxy on sapphire substrates and Si as a dopant. Electron concentrations were obtained up to 1.25×1020 cm−3 when the Al mole fraction was 50%, and 8.5×1019 cm−3 electrons were measured even when the Al mole fraction was 80%. Other material properties were determined by optical absorption, photoluminescence, cathodoluminescence, x-ray diffraction, and atomic force microscopy measurements and high optical and morphological qualities were shown.