Steven R. Schofield

Quantum engineering at the silicon surface using dangling bonds

Individual atoms and ions are now routinely manipulated using scanning tunnelling microscopes or electromagnetic traps for the creation and control of artificial quantum states. For applications such as quantum information processing, the ability to introduce multiple atomic-scale defects deterministically in a semiconductor is highly desirable. Here we use a scanning tunnelling microscope to fabricate interacting chains of dangling bond defects on the hydrogen-passivated silicon (001) surface. We image both the ground-state and the excited-state probability distributions of the resulting artificial molecular orbitals, using the scanning tunnelling microscope tip bias and tip-sample separation as gates to control which states contribute to the image. Our results demonstrate that atomically precise quantum states can be fabricated on silicon, and suggest a general model of quantum-state fabrication using other chemically passivated semiconductor surfaces where single-atom depassivation can be achieved using scanning tunnelling microscopy.

Encapsulation of phosphorus dopants in silicon for the fabrication of a quantum computer

The incorporation of phosphorus in silicon is studied by analyzing phosphorus δ-doped layers using a combination of scanning tunneling microscopy, secondary ion mass spectrometry, and Hall effect measurements. The samples are prepared by phosphine saturation dosing of a Si(100) surface at room temperature, a critical annealing step to incorporate phosphorus atoms, and subsequent epitaxial silicon overgrowth. We observe minimal dopant segregation (∼5 nm), complete electrical activation at a silicon growth temperature of 250 °C and a high two-dimensional electron mobility of ∼102 cm2/V s at a temperature of 4.2 K. These results, along with preliminary studies aimed at further minimizing dopant diffusion, bode well for the fabrication of atomically precise dopant arrays in silicon such as those found in recent solid-state quantum computer architectures.

Scanning probe microscopy for silicon device fabrication

We present a review of a detailed fabrication strategy for the realisation of nano and atomic-scale devices in silicon using phosphorus as a dopant and a combination of ultra-high vacuum scanning probe microscopy and silicon molecular beam epitaxy (MBE). In this work we have been able to overcome some of the key fabrication challenges to the realisation of atomic-scale devices including the identification of single P dopants in silicon, the controlled incorporation of P atoms in silicon with atomic precision and the minimisation of P segregation and diffusion during Si encapsulation. Recently, we have combined these results with a novel registration technique to fabricate robust electrical devices in silicon that can be contacted and measured outside the ultra-high vacuum environment. We discuss the importance of our results for the future fabrication of atomic-scale devices in silicon.

STM characterization of the Si-P heterodimer

We use scanning tunneling microscopy (STM) and Auger electron spectroscopy to study the behavior of adsorbed phosphine (PH) on Si(001), as a function of annealing temperature, paying particular attention to the formation of the Si-P heterodimer. Dosing the Si(001) surface with ∼0.002 langmuirs of PH results in the adsorption of PH (x=2,3) onto the surface and etching of Si to form individual Si ad-dimers. Annealing to 350 °C results in the incorporation of P into the surface layer to form Si-P heterodimers and the formation of short one-dimensional Si dimer chains and monohydrides. In filled state STM images, isolated Si-P heterodimers appear as zigzag features on the surface due to the static dimer buckling induced by the heterodimer. In the presence of a moderate coverage of monohydrides this static buckling is lifted, rending the Si-P heterodimers invisible in filled state images. However, we find that we can image the heterodimer at all H coverages using empty state imaging. The ability to identify single P atoms incorporated into Si(001) will be invaluable in the development of nanoscale electronic devices based on controlled atomic-scale doping of Si.

Imaging charged defects on clean Si(100)-(2×1) with scanning tunneling microscopy

Scanning tunneling microscopy (STM) has been used to image charged defects on the clean Si(100)-(2×1) surface. Previous studies have shown that, in the absence of “C”-type defects, the surface does not pin the Fermi level, allowing near surface charge to influence the state density contributing to the tunneling current. As in the case of cleavage faces of III–V semiconductor crystals, the charge-induced band bending produces long-range enhancements superimposed on the periodic surface lattice. This is observed in empty-state STM images taken on n-type material. No band bending signature is seen in the filled-state images. This can be understood by considering the band structure at the surface, which has surface states within the band gap. The charged defects observed in this work are of the types commonly observed in clean Si(100)-(2×1) STM studies, however, not all defects of a given type appear charged. This would indicate subtle differences in structure or the influence of impurities. Predictions for p...

Investigating individual arsenic dopant atoms in silicon using low-temperature scanning tunnelling microscopy.

We study subsurface arsenic dopants in a hydrogen-terminated Si(001) sample at 77 K, using scanning tunnelling microscopy and spectroscopy. We observe a number of different dopant-related features that fall into two classes, which we call As1 and As2. When imaged in occupied states, the As1 features appear as anisotropic protrusions superimposed on the silicon surface topography and have maximum intensities lying along particular crystallographic orientations. In empty-state images the features all exhibit long-range circular protrusions. The images are consistent with buried dopants that are in the electrically neutral (D0) charge state when imaged in filled states, but become positively charged (D+) through electrostatic ionization when imaged under empty-state conditions, similar to previous observations of acceptors in GaAs. Density functional theory calculations predict that As dopants in the third layer of the sample induce two states lying just below the conduction-band edge, which hybridize with the surface structure creating features with the surface symmetry consistent with our STM images. The As2 features have the surprising characteristic of appearing as a protrusion in filled-state images and an isotropic depression in empty-state images, suggesting they are negatively charged at all biases. We discuss the possible origins of this feature.

Towards the atomic-scale fabrication of a silicon-based solid state quantum computer

Abstract The construction of a scalable quantum computer in silicon, using single phosphorus atoms as qubits, presents a significant technological challenge. This paper describes recent results from a ‘bottom-up’ strategy to incorporate individual phosphorus atoms in silicon with atomic precision using a combination of advanced scanning tunnelling lithography techniques followed by low temperature silicon molecular beam epitaxial overgrowth. To date we have demonstrated (i) placement of individual phosphorus molecules at predetermined sites in the silicon surface using a hydrogen resist strategy, (ii) spatially controlled phosphorus incorporation into the silicon surface, (iii) minimisation of surface segregation by low temperature silicon encapsulation and (iv) complete electrical activation of the donors. Whilst these results bode well for the fabrication of silicon devices with atomically precise dopant profiles, we discuss the challenges that remain before a few qubit P in Si quantum computer prototype can be realised.

Interface and nanostructure evolution of cobalt germanides on Ge(001)

Cobalt germanide (CoxGey) is a candidate system for low resistance contact modules in future Ge devices in Si-based micro and nanoelectronics. In this paper, we present a detailed structural, morphological, and compositional study on CoxGey formation on Ge(001) at room temperature metal deposition and subsequent annealing. Scanning tunneling microscopy and low energy electron diffraction clearly demonstrate that room temperature deposition of approximately four monolayers of Co on Ge(001) results in the Volmer Weber growth mode, while subsequent thermal annealing leads to the formation of a Co-germanide continuous wetting layer which evolves gradually towards the growth of elongated CoxGey nanostructures. Two types of CoxGey nanostructures, namely, flattop- and ridge-type, were observed and a systematic study on their evolution as a function of temperature is presented. Additional transmission electron microscopy and x-ray photoemission spectroscopy measurements allowed us to monitor the reaction between ...

Studying atomic scale structural and electronic properties of ion implanted silicon samples using cross-sectional scanning tunneling microscopy

The atomic scale structural and electronic characteristics of a silicon sample implanted with bismuth atoms are investigated using cross-sectional scanning tunneling microscopy (XSTM) and scanning tunneling spectroscopy (STS). We demonstrate that cleaving ion implanted samples provides an effective room temperature route for the preparation of atomically flat silicon surfaces with low defect density, preventing the diffusion of volatile impurities such as dopants. This enables atomic resolution STM studies of solitary implanted impurity atoms in their intrinsic silicon crystal sites and further allows us to map out a depth profile of the band-structure of the implanted area using STS.

Nondestructive imaging of atomically thin nanostructures buried in silicon

Microwave microscopy enables three-dimensional characterization of atomically thin semiconductor structures with nanometer precision. It is now possible to create atomically thin regions of dopant atoms in silicon patterned with lateral dimensions ranging from the atomic scale (angstroms) to micrometers. These structures are building blocks of quantum devices for physics research and they are likely also to serve as key components of devices for next-generation classical and quantum information processing. Until now, the characteristics of buried dopant nanostructures could only be inferred from destructive techniques and/or the performance of the final electronic device; this severely limits engineering and manufacture of real-world devices based on atomic-scale lithography. Here, we use scanning microwave microscopy (SMM) to image and electronically characterize three-dimensional phosphorus nanostructures fabricated via scanning tunneling microscope–based lithography. The SMM measurements, which are completely nondestructive and sensitive to as few as 1900 to 4200 densely packed P atoms 4 to 15 nm below a silicon surface, yield electrical and geometric properties in agreement with those obtained from electrical transport and secondary ion mass spectroscopy for unpatterned phosphorus δ layers containing ~1013 P atoms. The imaging resolution was 37 ± 1 nm in lateral and 4 ± 1 nm in vertical directions, both values depending on SMM tip size and depth of dopant layers. In addition, finite element modeling indicates that resolution can be substantially improved using further optimized tips and microwave gradient detection. Our results on three-dimensional dopant structures reveal reduced carrier mobility for shallow dopant layers and suggest that SMM could aid the development of fabrication processes for surface code quantum computers.