Precision few-electron silicon quantum dots

We demonstrate the successful down-scaling of donor-based silicon quantum dot structures to the single donor limit. These planar devices are realized in ultra high vacuum (UHV) by means of scanning tunneling microscope (STM) based hydrogen lithography which – in combination with a gaseous dopant source and a thermal silicon source – allows for the patterning of highly-doped planar Si:P structures with sub-nm precision encapsulated in a single-crystal environment. We present advancements of the alignment strategy for patterning ex-situ metallic contacts and top gates over the buried dopant devices. Here, we use a hierarchical array of etched registration markers. A key feature of the alignment process is the controlled formation of atomically flat plateaus several hundred nanometers in diameter that allows the active region of the device to be patterned on a single atomic Si(100) plane at a precisely known position. We present a multiterminal Si:P quantum dot device in the many-electron regime. Coplanar regions of highly doped silicon are used to gate the quantum dot potential resulting in highly stable Coulomb blockade oscillations. We compare the use of these all epitaxial in-plane gates with conventional metallic surface gates and find superior stability of the former. We highlight the challenges of down-scaling within a planar architecture and show how capacitance modeling can be used to optimize the tunability of quantum dot devices. Based on these results, we demonstrate the fabrication of an in-plane gated few-donor quantum dot device which shows highly stable Coulomb blockade oscillations as well as a surprisingly dense excitation spectrum on the scale of 100 μeV. We explain how these low energy resonances arise from transport through valley-split states of the silicon quantum dot providing extensive effective mass calculations to support our findings. Finally, we describe how STM H-lithography can be used to incorporate individual impurities at precisely known positions within a gated device and demonstrate transport through a single phosphorus donor. We find a bulk-like charging energy as well as clear indications for bulk-like excited states. We highlight the potential of this technology to realize elementary building blocks for future donor-based quantum computation applications in silicon

Probing the quantum states of a single atom transistor at microwave frequencies

The ability to apply gigahertz frequencies to control the quantum state of a single P atom is an essential requirement for the fast gate pulsing needed for qubit control in donor-based silicon quantum computation. Here, we demonstrate this with nanosecond accuracy in an all epitaxial single atom transistor by applying excitation signals at frequencies up to ≈13 GHz to heavily phosphorus-doped silicon leads. These measurements allow the differentiation between the excited states of the single atom and the density of states in the one-dimensional leads. Our pulse spectroscopy experiments confirm the presence of an excited state at an energy ≈9 meV, consistent with the first excited state of a single P donor in silicon. The relaxation rate of this first excited state to the ground state is estimated to be larger than 2.5 GHz, consistent with theoretical predictions. These results represent a systematic investigation of how an atomically precise single atom transistor device behaves under radio frequency excitations

A surface code quantum computer in silicon

The exceptionally long quantum coherence times of phosphorus donor nuclear spin qubits in silicon, coupled with the proven scalability of silicon-based nano-electronics, make them attractive candidates for large-scale quantum computing. However, the high threshold of topological quantum error correction can only be captured in a two-dimensional array of qubits operating synchronously and in parallel—posing formidable fabrication and control challenges. We present an architecture that addresses these problems through a novel shared-control paradigm that is particularly suited to the natural uniformity of the phosphorus donor nuclear spin qubit states and electronic confinement. The architecture comprises a two-dimensional lattice of donor qubits sandwiched between two vertically separated control layers forming a mutually perpendicular crisscross gate array. Shared-control lines facilitate loading/unloading of single electrons to specific donors, thereby activating multiple qubits in parallel across the array on which the required operations for surface code quantum error correction are carried out by global spin control. The complexities of independent qubit control, wave function engineering, and ad hoc quantum interconnects are explicitly avoided. With many of the basic elements of fabrication and control based on demonstrated techniques and with simulated quantum operation below the surface code error threshold, the architecture represents a new pathway for large-scale quantum information processing in silicon and potentially in other qubit systems where uniformity can be exploited

Molecule-by-Molecule Writing Using a Focused Electron Beam

The resolution of lithography techniques needs to be extended beyond their current limits to continue the trend of miniaturization and enable new applications. But what is the ultimate spatial resolution? It is known that single atoms can be imaged with a highly focused electron beam. Can single atoms also be written with an electron beam? We verify this with focused electron-beam-induced deposition (FEBID), a direct-write technique that has the current record for the smallest feature written by (electron) optical lithography. We show that the deposition of an organometallic precursor on graphene can be followed molecule-by-molecule with FEBID. The results show that mechanisms that are inherent to the process inhibit a further increase in control over the process. Hence, our results present the resolution limit of (electron) optical lithography techniques. The writing of isolated, subnanometer features with nanometer precision can be used, for instance, for the local modification of graphene and for catalysis.</p

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A Single-Atom Transistor

The ability to control matter at the atomic scale and build devices with atomic precision is central to nanotechnology. The scanning tunneling microscope can manipulate individual atoms and molecules on surfaces, but the manipulation of silicon to make atomic-scale logic circuits has been hampered by the covalent nature of its bonds. Resist-based strategies have allowed the formation of atomic-scale structures on silicon surfaces, but the fabrication of working devices—such as transistors with extremely short gate lengths, spin-based quantum computers and solitary dopant optoelectronic devices—requires the ability to position individual atoms in a silicon crystal with atomic precision. Here, we use a combination of scanning tunnelling microscopy and hydrogen-resist lithography to demonstrate a single-atom transistor in which an individual phosphorus dopant atom has been deterministically placed within an epitaxial silicon device architecture with a spatial accuracy of one lattice site. The transistor operates at liquid helium temperatures, and millikelvin electron transport measurements confirm the presence of discrete quantum levels in the energy spectrum of the phosphorus atom. We find a charging energy that is close to the bulk value, previously only observed by optical spectroscopy