As silicon electronics approaches the atomic scale, interconnects and circuitry become comparable in size to the active device components. Maintaining low electrical resistivity at this scale is challenging because of the presence of confining surfaces and interfaces. We report on the fabrication of wires in silicon--only one atom tall and four atoms wide--with exceptionally low resistivity (~0.3 milliohm-centimeters) and the current-carrying capabilities of copper. By embedding phosphorus atoms within a silicon crystal with an average spacing of less than 1 nanometer, we achieved a diameter-independent resistivity, which demonstrates ohmic scaling to the atomic limit. Atomistic tight-binding calculations confirm the metallicity of these atomic-scale wires, which pave the way for single-atom device architectures for both classical and quantum information processing.

Using density functional theory and guided by extensive scanning tunneling microscopy (STM) image data, we formulate a detailed mechanism for the dissociation of phosphine (PH3) molecules on the Si(001) surface at room temperature. We distinguish between a main sequence of dissociation that involves PH2+H, PH+2H, and P+3H as observable intermediates, and a secondary sequence that gives rise to PH+H, P+2H, and isolated phosphorus adatoms. The latter sequence arises because PH2 fragments are surprisingly mobile on Si(001) and can diffuse away from the third hydrogen atom that makes up the PH3 stoichiometry. Our calculated activation energies describe the competition between diffusion and dissociation pathways and hence provide a comprehensive model for the numerous adsorbate species observed in STM experiments.

Nanoscale control of doping profiles in semiconductor devices is becoming of critical importance as channel length and pitch in metal oxide semiconductor field effect transistors (MOSFETs) continue to shrink toward a few nanometers. Scanning tunneling microscope (STM) directed self-assembly of dopants is currently the only proven method for fabricating atomically precise electronic devices in silicon. To date this technology has realized individual components of a complete device with a major obstacle being the ability to electrically gate devices. Here we demonstrate a fully functional multiterminal quantum dot device with integrated donor based in-plane gates epitaxially assembled on a single atomic plane of a silicon (001) surface. We show that such in-plane regions of highly doped silicon can be used to gate nanostructures resulting in highly stable Coulomb blockade (CB) oscillations in a donor-based quantum dot. In particular, we compare the use of these all epitaxial in-plane gates with conventional surface gates and find superior stability of the former. These results show that in the absence of the randomizing influences of interface and surface defects the electronic stability of dots in silicon can be comparable or better than that of quantum dots defined in other material systems. We anticipate our experiments will open the door for controlled scaling of silicon devices toward the single donor limit.

No abstract

Molecular beam epitaxy and scanning tunneling microscopy (STM) patterning are combined to form highly doped, planar devices in silicon at the atomic level. The absolute device location is registered to microscopic markers (see image; scale bar: 50 μm) for the alignment of surface contacts, enabling the correlation of the electrical properties of atomically controlled devices such as nanowires, tunnel junctions, and nanodots to the dopant location, monitored using high‐resolution STM techniques.

We investigate the surface quality of encapsulated Si:P δ-layers for the fabrication of multilayer devices with the potential to create architectures with sub 20 nm resolution in all three spatial dimensions. We use scanning tunneling microscopy to investigate how the dopant incorporation chemistry of the first active layer strongly affects the quality of the Si encapsulation which serves as the regrowth interface for the second active layer. Low temperature Hall measurements of the encapsulated layers indicate full dopant activation for incorporation temperatures between 250–750 °C with 20% higher carrier densities than previously observed.

Chemisorption of a single hydrogen atom on the n-type Si͑001͒ surface is investigated by scanning tunneling microscopy ͑STM͒ and first-principles density functional theory ͑DFT͒ calculations. The STM experiments show that the formation of a hemihydride induces static buckling of the neighboring Si-Si dimers and suggest that different buckling configurations of these dimers are observed at negative and positive biases. They also show that the appearance of an isolated Si-Si-H hemihydride on Si͑001͒ exhibits a complex voltage dependence with the brightness of the dangling bond of the hemihydride changing significantly at negative sample bias. DFT calculations predict two stable, ground state atomic configurations for the hemihydride on Si͑001͒. These correspond to parallel and antiparallel configurations of the Si-Si-H hemihydride with respect to the neighboring bare Si-Si dimers. In order to understand the bias-dependent appearance in the STM images of the n-type Si͑001͒ surface, we include the effect of hemihydride charging due to tip-induced band bending. In filled state, the STM images are shown to result from the electronic and structural features that originate from the charge-dependent parallel configuration. In empty state, the energetics and STM measurements support the charge independent antiparallel configuration, while either structure can produce simulated images consistent with experiment.

In this paper, we present a method for a comprehensive analysis of the efficiency roll-off with current density in phosphorescent organic light-emitting diodes (OLEDs). By combining electrical and optical excitation in time-resolved spectroscopic experiments, we are able to measure the excited-state lifetime for different driving conditions. It is, thus, possible to correlate changes of the triplet lifetime with a decrease of the radiative quantum efficiency of the emitting system due to exciton quenching processes. As compared to the conventional analysis of the measured external quantum efficiency (EQE) in dependence of the applied current density, the lifetime analysis is not affected by changes of the charge-carrier balance with current, which can have a significant impact on the interpretation of the results. By performing timeresolved spectroscopy for a series of red phosphorescent OLEDs, triplet-polaron quenching (TPQ) is identified as the dominant mechanism behind the efficiency roll-off up to a current density of 100 mA=cm 2 , while the conventional EQE vs current plot rather suggests triplet-triplet annihilation as the main quenching mechanism. We show that this apparent discrepancy is caused by exciton quenching occurring already at very low current densities, where EQE measurements are not reliable due to significant changes of the charge-carrier balance in this region. In addition, we present evidence that the triplet-polaron quenching rate Γ TPQ is independent of the microcavity so that variations of the triplet lifetimes of a series of devices exhibiting different layer thicknesses can be described with a single parameter set.