T. Hopf

Controlled shallow single-ion implantation in silicon using an active substrate for sub-20-keV ions

We demonstrate a method for the controlled implantation of single ions into a silicon substrate with energy of sub-20‐keV. The method is based on the collection of electron-hole pairs generated in the substrate by the impact of a single ion. We have used the method to implant single 14‐keV P31 ions through nanoscale masks into silicon as a route to the fabrication of devices based on single donors in silicon.

Single-Ion Implantation for the Development of Si-Based MOSFET Devices with Quantum Functionalities

Interest in single-ion implantation is driven in part by research into development of solid-state devices that exhibit quantum behaviour in their electronic or optical characteristics. Here, we provide an overview of international research work on single ion implantation and single ion detection for development of electronic devices for quantum computing. The scope of international research into single ion implantation is presented in the context of our own research in the Centre for Quantum Computation and Communication Technology in Australia. Various single ion detection schemes are presented, and limitations on dopant placement accuracy due to ion straggling are discussed together with pathways for scale-up to multiple quantum devices on the one chip. Possible future directions for ion implantation in quantum computing and communications are also discussed.

The response of silicon detectors to low-energy ion implantation

Studies of electrical transients in single-crystal silicon induced by discrete low-energy (sub-20 keV) ions have been carried out at 90 K, with ionization measurements and damage accumulation in the sample being investigated. Ionization studies reveal a discrepancy between experimental results and predictions from the widely used SRIM (stopping and range of ions in matter) code, one which increases with decreasing energy: a result which has previously been suggested from studies with continuous ion beams. Damage accumulation studies of the sample also demonstrate that current models of damage build-up in silicon are inadequate at such low energies, with experiments indicating that individual ions create a much larger region of decreased charge collection efficiency outside of the small amorphous cores known to be formed by such impacts.

Integration of Single Ion Implantation Method in Focused Ion Beam System for Nanofabrication

A method of single ion implantation based on the online detection of individual ion impacts on a pure silicon substrate has been implemented in a focused ion beam (FIB) system. The optimized silicon detector integrated with a state-of-art low noise electronic system and operated at a low temperature makes it possible to achieve single ion detection with a minimum energy detection limit about 1 to 3.5 keV in a FIB chamber. The method of single ion implantation is compatible with a nanofabrication process. The lateral positioning of the implantation sites are controlled to nanometer accuracy (~5 nm) using nanofabricated PMMA masks. The implantation depth is controlled by tuning the single ion energy to a certain energy level (5-30 keV). The system has been successfully tested in the detection of 30 keV Si+ single ions. The counting of single ion implantation in each site is achieved by the detection of e-h pairs (an outcome of ionization energy) produced by the ion-solid interaction; each 30 keV Si+ ion implanting through a 5 nm SiO2 surface layer and stopping at a pure silicon substrate produces an average ionization energy about 7.0 keV. A further development for improving a detection limit down to less than 1 keV in FIB for low energy phosphorus implantation and detection is outlined. Fabrication of nanometer-scaled phosphorus arrays for the application of qubits construction is discussed.

Optimization of single keV ion implantation for the construction of single P-donor devices.

We report recent progress in single keV ion implantation and online detection for the controlled implantation of single donors in silicon. When integrated with silicon nanofabrication technology this forms the “top down” strategy for the construction of prototype solid state quantum computer devices based on phosphorus donors in silicon. We have developed a method of single ion implantation and online registration that employs detector electrodes adjacent to the area into which the donors are to be implanted. The implantation sites are positioned with nanometer accuracy using an electron beam lithography patterned PMMA mask. Control of the implantation depth of 20 nm is achieved by tuning the phosphorus ion energy to 14 keV. The counting of single ion implantation in each site is achieved by the detection of e-/h+ pairs produced by the implanted phosphorus ion in the substrate. The system is calibrated by use of Mn K-line x-rays (5.9 and 6.4 keV) and we find the ionization energy of the 14 keV phosphorus ions in silicon to be about 3.5-4.0 keV for implants through a 5 nm SiO2 surface layer. This paper describes the development of an improved PIN detector structure that provides more reliable performance of the earlier MOS structure. With the new structure, the energy noise threshold has been minimized to 1 keV or less. Unambiguous detection/counting of single keV ion implantation events were achieved with a confidence level greater than 98% with a reliable and reproducible fabrication process.

Phase correlations of elemental maps using nuclear microscopy

In complex multi-elemental samples it is often necessary to determine the presence of various chemical phases. More complexity arises if it is also necessary to determine the spatial distribution of these phases. Here we present a new technique, based on the elemental maps, for the study of the phase distribution of multi-elemental samples. This technique uses the elemental maps obtained with nuclear microscopy to extract the spatially distributed phase information. We will explain the basic technique of phase correlation mapping, and then provide simulated and experimental results to demonstrate its capability in materials analysis. The simulations show the effect of the beam spatial resolution on the correlation maps and the experimental results show the phase correlation maps of elements in an array of phosphors from a video tube.

Measuring the Charge and Spin States of Electrons on Individual Dopant Atoms in Silicon

We review an ongoing effort to demonstrate technologies required for quantum computing with phosphorus donors in silicon. The main aspect of our research is to achieve control over charge and spin states of individual dopant atoms. This work has required the development of new techniques for engineering silicon nanodevices at the atomic level. We follow an approach for implanting single phosphorus ions into silicon substrates with integrated p–i–n detectors. Configuring our devices with radio-frequency single-electron transistors (RF-SETs) allows for charge sensing at low temperatures. In this context, we perform measurements of single-electron charge transfer between individual phosphorus donors. In a parallel effort, we employ nanoscale Schottky contacts for populating and depopulating individual dopant atoms. Of particular interest is coherent manipulation of single-electron charge and spin states on individual dopant atoms. Charge manipulation between coupled donor states may be achieved by either external microwave pumping or intrinsic tunnel coupling. Spin manipulation, on the other hand, involves magnetic resonance. In this context, we pursue electrically detected spin resonance in phosphorus-doped devices with a decreasing number of dopant atoms.

Nanofabrication of charge-based Si:P quantum computer devices using single-ion implantation

We report on progress towards a charge-based qubit using phosphorus atoms implanted in a silicon substrate. Prototype devices have been fabricated using standard lithographic techniques together with a new method of controlled single ion implantation using on-chip detector electrodes. Positional accuracy of the implanted ions was achieved using a nanoaperture mask defined using electron beam lithography. The two implanted phosphorus atoms are positioned ~50 nm apart, to form a qubit test device. A series of process steps has been developed to repair implant damage, define surface control gates, and to define single electron transistors used for qubit readout via the detection of sub-electron charge transfer signals. Preliminary electrical measurements on these devices show single charge transfer events that are resilient to thermal cycling.

Single-electron transistor coupled to a silicon nano-MOSFET

By capactively coupling sensitive charge detectors (i.e. single-electron transistors - SETs) to nanostructures such as quantum dots and two-dimensional systems, it is possible to investigate charge transport properties in extremely low conduction regimes where direct transport measurements are increasingly difficult. Ion-implanted nano-MOSFETs coupled to aluminium SETs have been constructed in order to study charge transport between locally doped regions in Si at mK temperatures. This configuration allows for direct source-drain measurement as well as non-invasive charge detection. Of particular interest are the effects of material defects and gate control on charge transport, which is of relevance to Si-based quantum computing.

Development of a single ion detection system for the implantation of donors with nanoscale precision

The authors have developed a technique which enables the implantation and detection of single low-energy (<15 kev) ions in a silicon substrate with nanoscale precision, and with a detection efficiency approaching 100%. The process is based on the collection of electron-hole pairs generated in the substrate by the ion impacts, and is currently being utilized for the construction of prototype quantum computer devices in the solid state.