R. Mallory

Analysis of the transport process providing spin injection through an Fe/AlGaAs Schottky barrier

Electron-spin polarizations of 32% are obtained in a GaAs quantum well via electrical injection through a reverse-biased Fe/AlGaAs Schottky contact. An analysis of the transport data using the Rowell criteria demonstrates that single-step tunneling is the dominant transport mechanism. The current–voltage data show a clear zero-bias anomaly and phonon signatures corresponding to the GaAs-like and AlAs-like LO phonon modes of the AlGaAs barrier, providing further evidence for tunneling. These results provide experimental confirmation of several theoretical analyses, indicating that tunneling enables significant spin injection from a metal into a semiconductor.

Comparison of Fe/Schottky and Fe/Al2O3 tunnel barrier contacts for electrical spin injection into GaAs

We compare electrical spin injection from Fe films into identical GaAs-based light-emitting diodes (LEDs) using different tunnel barriers—a reverse-biased Fe/AlGaAs Schottky contact and an Fe/Al2O3 barrier. Both types of structures are formed in situ using a multichamber molecular-beam epitaxy system. A detailed analysis of the transport data confirms that tunneling occurs in each case. We find that the spin polarization achieved in the GaAs using the Al2O3 barrier is 40% (best case; 30% typical), but the electrical efficiency is significantly lower than that of the Fe Schottky contact.

Templated synthesis of ZnSe nanostructures using lyotropic liquid crystals.

We report a technique for controlled synthesis of zero-, one-, and two-dimensional compound semiconductor nanostructures by using cubic, hexagonal, and lamellar lyotropic liquid crystals as templates, respectively. The liquid crystals were formed by self-assembly in a ternary system consisting of a poly(ethylene oxide)-poly(propylene oxide)-poly(ethylene oxide) amphiphilic block copolymer as the surfactant, heptane as the non-polar dispersed phase, and formamide as the polar continuous phase. ZnSe quantum dots and nanowires with diameters smaller than 10 nm, as well as free-standing, disc-like quantum wells, were grown inside the spherical, cylindrical, and planar nanodomains, respectively, by reacting diethylzinc that was dissolved in the heptane domains with hydrogen selenide gas that was brought into contact with the liquid crystal in a sealed chamber at room temperature and atmospheric pressure. The shape and size of the resulting nanostructures can be manipulated by selecting the templating phase of the liquid crystal, the size of the dispersed nanodomains that is controlled by the composition of the template, and the concentration of diethylzinc in them.

Electrical spin pumping of quantum dots at room temperature

We report on electrical control of the spin polarization of InAs∕GaAs self-assembled quantum dots (QDs) at room temperature. This is achieved by electrical injection of spin-polarized electrons from an Fe Schottky contact. The circular polarization of the QD electroluminescence shows that a 5% electron spin polarization is obtained in the InAs QDs at 300K, which is remarkably insensitive to temperature. This is attributed to suppression of the spin-relaxation mechanisms in the QDs due to reduced dimensionality. These results demonstrate that practical regimes of spin-based operation are clearly attainable in solid-state semiconductor devices.

Water-based synthesis of ZnSe nanostructures using amphiphilic block copolymer stabilized lyotropic liquid crystals as templates

Simultaneous size and shape control of zinc selenide (ZnSe) nanostructures was achieved using lyotropic liquid crystal templates formed by the self-assembly of an amphiphilic block copolymer in the presence of water and a less polar organic solvent. ZnSe quantum dots, hollow nanospheres, nanotubes, and nanolaminates (parallel nanoplates or free-standing quantum wells) were grown at room temperature in the aqueous phase of a poly(ethylene oxide)–poly(propylene oxide)–poly(ethylene oxide) block copolymer/water/p-xylene ternary system. The nucleation of ZnSe was facilitated by an irreversible reaction between zinc acetate dissolved in water and hydrogen selenide gas that was allowed to diffuse into the self-assembled template. The nanostructures exhibit good crystallinity under high-resolution transmission electron miscroscopy and quantum confinement effects in their photoluminescence spectra. The shape and size of nanocrystals can be precisely controlled by altering the structure of the templating phase, by tuning the size of the nanodomains, and by changing the concentration of the zinc precursor.

Spin injection across (110) interfaces: Fe/GaAs(110) spin-light-emitting diodes

We report electrical spin injection from an Fe contact into a (110)-oriented light-emitting diode (LED) structure, and compare results with data obtained from (001)-oriented structures to address the dependence of spin injection on interface and orientation. Fe∕AlGaAs∕GaAs LEDs were grown by molecular-beam epitaxy, and processed to form surface emitting structures. Electroluminescence results obtained using a reverse-biased Fe Schottky tunnel barrier injector show that a 13% electron spin polarization is achieved in the GaAs(110) quantum well due to injection across the Fe∕AlGaAs(110) interface. Analysis of the transport data indicates that tunneling is a significant transport mechanism at low temperatures. The temperature dependence of the spin polarization is similar to that of (001)-oriented spin LEDs, and is dominated by the GaAs electron spin lifetime.

Photoluminescence studies of type-II diluted magnetic semiconductor ZnMnTe/ZnSe quantum dots

Type-II diluted magnetic semiconductor ZnMnTe quantum dots (QDs) in ZnSe matrix grown by molecular beam epitaxy were investigated by conventional and magnetophotoluminescence (PL) spectroscopy. The QD emission exhibits a type-II characteristic in excitation power dependence of PL peak energy. A nonzero circular polarization of PL at the absence of magnetic field was observed. This phenomenon is attributed to the accumulation of interface charges confined in adjacent layers. The magneto-optical measurement demonstrates a magnetic-induced degree of circular polarization in the PL spectra, indicating the Mn incorporation into the QD system.

Spin injection studies into GaAs quantum wells in the presence of confined electrons

We compare the electroluminescence spectra (spectral composition and polarization characteristics) of two types of Fe-based AlGaAs/GaAs n-i-p spin light emitting diodes (spin LEDs). In type A spin LEDs the GaAs quantum well (QW) does not contain any confined carriers, while in type B LEDs the GaAs QW is occupied by confined electrons generated by excess n-type doping in the AlGaAs(n) barrier. Type B LEDs show a significantly smaller circular polarization at the e1h1 feature than type A devices. Other differences include the presence of the e1l1 exciton as well as excitonic phonon replicas in type B LEDs. Possible mechanisms for these differences are discussed.