Above room temperature ferromagnetic behavior is achieved in Si through Mn ion implantation. Threehundred-keV Mn + ions were implanted to 0.1% and 0.8% peak atomic concentrations, yielding a saturation magnetization of 0.3 emu/ g at 300 K for the highest concentration as measured using a SQUID magnetometer. The saturation magnetization increased by ϳ2ϫ after annealing at 800°C for 5 min. The Curie temperature for all samples was found to be greater than 400 K. A significant difference in the temperature-dependent remnant magnetization between the implanted p-type and n-type Si is observed, giving strong evidence that a Si-based diluted magnetic semiconductor can be achieved.

The atomic arrangement of the technologically important As-rich GaAs͑001͒-͑2 3 4͒ reconstructed surface is determined using bias-dependent scanning tunneling microscopy (STM) and first-principles electronic structure calculations. The STM images reveal the relative position and depth of the atomicscale features within the trenches between the top-layer As dimers, which are in agreement with the b2͑2 3 4͒ structural model. The bias-dependent simulated STM images reveal that a retraction of the topmost dangling bond orbitals is the novel electronic mechanism that enables the STM tip to image the trench structure.

We report a large spin-polarized current injection from a ferromagnetic metal into a nonferromagnetic semiconductor, at a temperature of 100 Kelvin. The modification of the spin-injection process by a nanoscale step edge was observed. On flat gallium arsenide [GaAs(110)] terraces, the injection efficiency was 92%, whereas in a 10-nanometer-wide region around a [111]-oriented step the injection efficiency is reduced by a factor of 6. Alternatively, the spin-relaxation lifetime was reduced by a factor of 12. This reduction is associated with the metallic nature of the step edge. This study advances the realization of using both the charge and spin of the electron in future semiconductor devices.

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A reversible 2D critical transition is observed on the GaAs(001) surface and modeled as a lattice-gas Ising system. Without depositing any material, 2D GaAs islands spontaneously form. The order parameter, four critical exponents, and coupling energies are measured from scanning tunneling microscope images of the microscopic domain structure and correlation functions as a function of temperature and pressure. Unprecedented insight into the domain structure of a 2D Ising system through the critical point and a complete Hamiltonian for modeling the GaAs(001) surface are presented.

The topologically protected surface states of three-dimensional (3D) topological insulators have the potential to be transformative for high-performance logic and memory devices by exploiting their specific properties such as spin-polarized current transport and defect tolerance due to suppressed backscattering. However, topological insulator based devices have been underwhelming to date primarily due to the presence of parasitic issues. An important example is the challenge of suppressing bulk conduction in BiSe and achieving Fermi levels ( E) that reside in between the bulk valence and conduction bands so that the topologically protected surface states dominate the transport. The overwhelming majority of the BiSe studies in the literature report strongly n-type materials with E in the bulk conduction band due to the presence of a high concentration of selenium vacancies. In contrast, here we report the growth of near-intrinsic BiSe with a minimal Se vacancy concentration providing a Fermi level near midgap with no extrinsic counter-doping required. We also demonstrate the crucial ability to tune E from below midgap into the upper half of the gap near the conduction band edge by controlling the Se vacancy concentration using post-growth anneals. Additionally, we demonstrate the ability to maintain this Fermi level control following the careful, low-temperature removal of a protective Se cap, which allows samples to be transported in air for device fabrication. Thus, we provide detailed guidance for E control that will finally enable researchers to fabricate high-performance devices that take advantage of transport through the topologically protected surface states of BiSe.

Ballistic-electron-emission microscopy measurements have been performed on n-type Au/Si͑100͒ interfaces for injection energies up to 1.2 eV over a range of Au overlayer thicknesses from ϳ65 to ϳ340 Å at both room temperature and 77 K. Hot-electron attenuation lengths in the Au overlayer have been determined to be 133Ϯ2 Å at room temperature and 147Ϯ6 Å at 77 K over the energy range of 0.92-1.20 eV above the Fermi level. The lack of energy dependence and the relatively small temperature-dependent change in the attenuation lengths that have been measured indicate that electron scattering with defects is the dominant mechanism affecting hot-electron transport in these Au overlayers. The ratio of the zero-thickness collection current at 77 K to that at room temperature has been measured to be 1.79Ϯ0.09. This large increase in the collection efficiency at 77 K is attributed primarily to the large temperature dependence of the transverse acoustic-phonon population in Si. Images with significant reductions in the collection current at topographic locations that have a large surface gradient have been obtained at room temperature. Calculations, which assume that the probability of transmission across the interface is independent of the transverse momentum of the electron, correlate well with the experimentally observed reductions. This result indicates that the injected electrons remain forward focused with little broadening as they pass through the Au overlayer, which implies that elastic scattering at the Au/Si interface accounts for the observation from previous Au/Si ballistic-electron-emission microscopy studies that transverse momentum is not conserved.

The deposition of Mn onto Si͑001͒ in the submonolayer regime has been studied with scanning tunneling microscopy to gain insight into the bonding and energetics of Mn with Si. The as-deposited Mn films at room temperature are unstructured. Upon annealing to 300-700°C three-dimensional islands of Mn or Mn x Si y form while between the islands the Si͑001͒-͑2 ϫ 1͒ reconstruction becomes visible. With increasing annealing time the density of islands per surface area decreases while the average height of the remaining islands increases. The large islands grow in size at the expense of the small ones, which can be understood in the context of Ostwald ͓Z. Phys. Chem. 34, 495 ͑1900͔͒ ripening theory. The average island height shows a time dependence of H ϳ t 1/4 , indicating that surface diffusion is the growth limiting process.