W. R. Clarke

Exploring the limits of N-type ultra-shallow junction formation.

Low resistivity, near-surface doping in silicon represents a formidable challenge for both the microelectronics industry and future quantum electronic devices. Here we employ an ultra-high vacuum strategy to create highly abrupt doping profiles in silicon, which we characterize in situ using a four point probe scanning tunnelling microscope. Using a small molecule gaseous dopant source (PH3) which densely packs on a reconstructed silicon surface, followed by encapsulation in epitaxial silicon, we form highly conductive dopant sheets with subnanometer control of the depth profiles. This approach allows us to test the limits of ultra-shallow junction formation, with room temperature resistivities of 780 Ω/□ at an encapsulation depth of 4.3 nm, increasing to 23 kΩ/□ at an encapsulation depth of only 0.5 nm. We show that this depth-dependent resistivity can be accounted for by a combination of dopant segregation and surface scattering.

Investigating the regrowth surface of Si:P δ-layers toward vertically stacked three dimensional devices

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.

Microscopic four-point-probe resistivity measurements of shallow, high density doping layers in silicon

We present room temperature resistivity measurements of shallow, monolayer doped phosphorus in silicon, a material system of interest for both conventional microelectronic manufacturing, and future quantum electronic devices. Using an in-situ variable spacing microscopic four-probe system, we demonstrate the ability to separate the conductivity of the substrate and the doping layer. We show that the obtained sensitivity to the dopant layer derives from a combination of the nanoscale contacting areas and the conductivity difference between the highly doped 2D layer and the substrate. At an encapsulation depth of only 4 nm, we demonstrate a room temperature resistivity of 1.4 kΩ/◻.

Fabrication of induced two-dimensional hole systems on (311)A GaAs

We demonstrate a method for fabricating induced two-dimensional hole devices in (311)A GaAs. The method uses a metallic p+‐GaAs capping layer as an in situ top gate that pins the Fermi energy close to the valence band, thereby allowing very small gate biases to be used to induce a two-dimensional hole system at a AlGaAs∕GaAs interface. We present transport data from devices with different levels of background impurities. Modeling the mobility as a function of hole density gives a quantitative measure of the level of disorder and indicates that these systems can be used for a systematic study of the effects of disorder in strongly interacting low-dimensional systems.

Evolution of the Bilayer ν =1 Quantum Hall State Under Charge Imbalance

We use high-mobility bilayer hole systems with negligible tunneling to examine how the bilayer nu=1 quantum Hall state evolves as charge is transferred from one layer to the other at constant total density. We map bilayer nu=1 state stability versus imbalance for five total densities spanning the range from strongly interlayer coherent to incoherent. We observe competition between single-layer correlations and interlayer coherence. Most significantly, we find that bilayer systems that are incoherent at balance can develop spontaneous interlayer coherence with imbalance, in agreement with recent theoretical predictions.

Interaction correction to the longitudinal conductivity and Hall resistivity in high-quality two-dimensional GaAs electron and hole systems

We study the corrections in the low temperature limit to both the longitudinal conductivity and Hall resistivity due to electron-electron interactions in high-quality GaAs systems. Using the recent theory of Zala we find that the interaction corrections to the conductivity and Hall resistivity are consistent with each other in n-GaAs, although the agreement is not as good in p-GaAs. This suggests that interaction effects can explain the metallic drop in resistivity at B=0 in n-GaAs systems, but more work is required to understand p-GaAs systems.

Comparison of nickel silicide and aluminium ohmic contact metallizations for low-temperature quantum transport measurements

We examine nickel silicide as a viable ohmic contact metallization for low-temperature, low-magnetic-field transport measurements of atomic-scale devices in silicon. In particular, we compare a nickel silicide metallization with aluminium, a common ohmic contact for silicon devices. Nickel silicide can be formed at the low temperatures (<400°C) required for maintaining atomic precision placement in donor-based devices, and it avoids the complications found with aluminium contacts which become superconducting at cryogenic measurement temperatures. Importantly, we show that the use of nickel silicide as an ohmic contact at low temperatures does not affect the thermal equilibration of carriers nor contribute to hysteresis in a magnetic field.

The Reduced Effective Interaction Parameter in Closely Spaced Two‐dimensional Hole Systems

We have developed a technique utilizing a double quantum well heterostructure that allows us to study the effect of a nearby ground‐plane of the metallic behavior in a GaAs two‐dimensional hole system (2DHS). Here, we explore the effective reduction in the interaction parameter rs due to a nearby ground‐plane, and how it depends on the 2D hole density p and the distance d between the ground‐plane and the 2DHS.