S.E. Andresen

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.

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.

Spin-dependent quasiparticle transport in aluminum single-electron transistors.

We investigate the effect of Zeeman splitting on quasiparticle transport in normal-superconducting-normal (NSN) aluminum single-electron transistors (SETs). In the above-gap transport, the interplay of Coulomb blockade and Zeeman splitting leads to spin-dependence of the sequential tunneling. This creates regimes where either one or both spin species can tunnel onto or off the island. At lower biases, spin-dependence of the single quasiparticle state is studied, and operation of the device as a bipolar spin filter is suggested.

Low-Noise Detection System for the Counted Implantation of Single Ions in Silicon

A unique detection system has been developed which allows for the counted implantation of individual low-energy heavy ions into silicon. This system can ensure the placement of individual ions at precise locations within a wafer using an EBL-machined resist mask, and utilizes the generation of ionization within the silicon substrate to allow for the reliable detection of implants down to 14 keV. Due to the necessity for low-noise operation, it is important that both the capacitance of the detectors and their leakage current be reduced as much as possible. To this end, we have now created a detector architecture with a measured capacitance of 0.6 pF and sub-pA leakage current at liquid nitrogen temperature, which has allowed us to achieve a resolution of 410 eV (44.2 electrons RMS) when coupled to low-noise signal-processing electronics and operated at 90 K.

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.

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.

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.

Fabrication of single atom nanoscale devices by ion implantation

Fabrication of nanoscale devices that exploit the rules of quantum mechanics to process information presents a formidable technical challenge because it will be necessary to control quantum states at the level of individual atoms, electrons or photons. We have developed a pathway to the construction of quantum devices using ion implantation and demonstrate, using charge transport analysis, that the devices exhibit single electron effects. We construct devices that employ two P donors in Si by employing the technique of ion beam induced charge (IBIC) in which single 14 keV P ions can be implanted into ultra-pure silicon by monitoring on-substrate detector electrodes. We have used IBIC with a MeV nuclear microprobe to map and measure the charge collection efficiency in the development of the electrode structure and show that 100 % charge collection efficiency can be achieved leading to the fabrication of prototype devices that display quantum effects in the transport of single charge quanta between the islands of implanted donors.