N. J. Curson

Atomically precise placement of single dopants in si.

We demonstrate the controlled incorporation of P dopant atoms in Si(001), presenting a new path toward the creation of atomic-scale electronic devices. We present a detailed study of the interaction of PH3 with Si(001) and show that it is possible to thermally incorporate P atoms into Si(001) below the H-desorption temperature. Control over the precise spatial location at which P atoms are incorporated was achieved using STM H lithography. We demonstrate the positioning of single P atoms in Si with approximately 1 nm accuracy and the creation of nanometer wide lines of incorporated P atoms.

Towards the fabrication of phosphorus qubits for a silicon quantum computer

The quest to build a quantum computer has been inspired by the recognition of the formidable computational power such a device could offer. In particular silicon-based proposals, using the nuclear or electron spin of dopants as qubits, are attractive due to the long spin relaxation times involved, their scalability, and the ease of integration with existing silicon technology. Fabrication of such devices, however, requires atomic scale manipulation-an immense technological challenge. We demonstrate that it is possible to fabricate an atomically precise linear array of single phosphorus bearing molecules on a silicon surface with the required dimensions for the fabrication of a silicon-based quantum computer. We also discuss strategies for the encapsulation of these phosphorus atoms by subsequent silicon crystal growth.

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.

Progress in silicon-based quantum computing

We review progress at the Australian Centre for Quantum Computer Technology towards the fabrication and demonstration of spin qubits and charge qubits based on phosphorus donor atoms embedded in intrinsic silicon. Fabrication is being pursued via two complementary pathways: a ‘top–down’ approach for near–term production of few–qubit demonstration devices and a ‘bottom–up’ approach for large–scale qubit arrays with sub–nanometre precision. The ‘top–down’ approach employs a low–energy (keV) ion beam to implant the phosphorus atoms. Single–atom control during implantation is achieved by monitoring on–chip detector electrodes, integrated within the device structure. In contrast, the ‘bottom–up’ approach uses scanning tunnelling microscope lithography and epitaxial silicon overgrowth to construct devices at an atomic scale. In both cases, surface electrodes control the qubit using voltage pulses, and dual single–electron transistors operating near the quantum limit provide fast read–out with spurious–signal rejection.

Single Photon Detection with a Quantum Dot Transistor

We propose and demonstrate a type of GaAs/AlGaAs modulation-doped field effect transistor (FET) which is sensitive to single photons. The FET contains a layer of InAs quantum dots formed using an in-situ, self-organising method, adjacent to the channel and separated from it by a thin AlGaAs barrier. Capture of a single photo-excited carrier by a quantum dot leads to a sizeable change in the source-drain current through the transistor, allowing the detection of a single photon. We show this is because the mobility of the electron channel is extremely sensitive to the charge trapped in the dots. This discovery may allow a new type of single photon detector to be developed which does not rely upon avalanche processes.

Measurement of phosphorus segregation in silicon at the atomic scale using scanning tunneling microscopy

In order to fabricate precise atomic-scale devices in silicon using a combination of scanning tunneling microscopy (STM) to position dopant atoms and molecular beam epitaxy to encapsulate the dopants it is necessary to minimize the segregation∕diffusion of dopant atoms during silicon encapsulation. We characterize the surface segregation∕diffusion of phosphorus atoms from a δ-doped layer in silicon after encapsulation at 250°C and room temperature using secondary ion mass spectrometry (SIMS) and STM. We show that the surface phosphorus density can be reduced to a few percent of the initial δ-doped density if the phosphorus atoms are encapsulated with 5 monolayers of epitaxial silicon at room temperature. We highlight the limitations of SIMS to determine phosphorus segregation at the atomic scale and the advantage of using STM directly.

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.

Scanning tunneling microscope based fabrication of nano- and atomic scale dopant devices in silicon: The crucial step of hydrogen removal

The use of a scanning tunneling microscope (STM) to pattern a hydrogen resist on the Si(001) surface has recently become a viable route for the fabrication of nanoscale planar doped devices in silicon. A crucial step in this fabrication process is the removal of the hydrogen resist after STM patterning before Si encapsulation of the dopants via molecular beam epitaxy. We compare thermal and STM-stimulated hydrogen desorptions in terms of surface morphology and integrity of dopant nanostructures embedded in the surface. We find that the boundaries of STM patterned P-in-Si nanostructures are maintained by STM-stimulated hydrogen desorption. In comparison, for an optimized thermal annealing at 470°C for 15s to remove the hydrogen there is a lateral diffusion out of the nanostructured region of up to ∼7–8nm. Our results demonstrate the advantages of nonthermal hydrogen desorption for the preservation of atomic scale dopant patterns in silicon.

Modification of a shallow 2DEG by AFM lithography

A conducting tip of an atomic force microscope (AFM) is used to induce ultra-small oxide patterns on metallic (Ti) thin film and semiconducting (GaAs) surfaces. The oxide is used to deplete a shallow two-dimensional electron gas (2DEG) formed at a GaAs/AlGaAs interface, 274 A beneath the surface. The depleted portion of the 2DEG is rendered highly resistive and remains insulating over a wide voltage range at low temperatures. Furthermore, in-plane side gates, defined by oxide lines, are used to constrict the 2DEG into a narrow one-dimensional (1D) electron channel which exhibits quantized conductance at low temperatures. A relatively high sub-band energy spacing (∼5.5 meV) is achieved between the first two sub-bands, demonstrating the steep lateral confining potential provided by side gates through oxide walls.