S.M. Hearne

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.

Ion-beam-induced-charge characterisation of particle detectors

Abstract Ion-beam-induced-charge collection (IBIC) in a nuclear microprobe has been used to characterise detectors for the measurement of particles over a median energy range (100 keV–1 MeV). Three standard detector devices have been studied: a PIPS detector with a buried (ion-implanted) junction structure, a Schottky barrier junction device and a PN-junction photodiode. A 2.0 MeV focussed helium ion beam was used to probe the active area of each device with a spatial resolution ∼1–2 μm, to quantify the thickness of the dead layer, the charge collection response and the reduction in charge collection efficiency induced by ion-beam damage.

Temperature-dependent emptying of grain-boundary charge traps in chemical vapor deposited diamond

We have used the technique of ion beam induced charge with a 2 MeV He+ microprobe to image particle detectors fabricated from polycrystalline chemical vapor deposited diamond as a function of temperature. We find that detectors which display a thermally stimulated current peak at 190 °C display increased charge collection efficiency when heated above that temperature. The probability of detecting the impact of a single ion at room temperature was less than 2%, but this probability rises to over 80% at 170 °C. We model this effect by showing that charge trapped at grain boundaries is liberated at elevated temperatures and this results in an increased electric field within the detector volume and hence a raised charge collection efficiency.

The role of charge trapping at grain boundaries on charge transport in polycrystalline chemical vapor deposited diamond based detectors

We report a detailed investigation of the trapping and release of charge carriers from grain boundaries in polycrystalline diamond grown by chemical vapor deposition (poly-CVD). A model for charge trapping and release is presented for samples which display very different bulk characteristics as determined by photoluminescence, dark conductivity, and thermally stimulated current measurements. Experimental studies were performed as a function of temperature and applied electric field using ion beam induced charge to map the charge collection efficiency of charge induced by a scanned, focused, 2MeV He+ microprobe. Even though the carrier velocity and charge collection efficiency should begin to saturate at electric fields above 1×104V∕cm, the efficiency was found to increase by a factor of 3 when the electric field is increased to greater than 1×105V∕cm. A model based on the localized enhancement of the electric field caused by trapped charge at grain boundaries is found to account for this unexpected result...

Charge trap levels in sulfur-doped chemical-vapor-deposited diamond with applications to ultraviolet dosimetry

Electrically active defects and traps in sulfur-doped polycrystalline diamond films synthesized by microwave-assisted chemical-vapor deposition are evaluated using thermally stimulated conductivity measurements after ultraviolet (UV) illumination. The measurements are found to be consistent with the latest theoretical predictions for the role of sulfur dopants in diamond. The suitability of S-doped diamond as a UV dosimeter is discussed.

IBIC characterisation of novel detectors for single atom doping of quantum computer devices

Abstract Single ion implantation and online detection is highly desirable for the emerging application, in which single 31P ions need to be inserted in prefabricated silicon cells to construct solid-state quantum bits (qubits). In order to fabricate qubit arrays, we have developed novel detectors that employ detector electrodes adjacent to the prefabricated cells that can detect single keV ion strikes appropriate for the fabrication of shallow phosphorus arrays. The method utilises a high purity silicon substrate with very high resistivity, a thin SiO2 surface layer, nanometer masks for the lateral positioning single phosphorus implantation, biased electrodes applied to the surface of the silicon and sensitive electronics that can detect the charge transient from single keV ion strikes. A TCAD (Technology Computer Aided Design) software package was applied in the optimisation of the device design and simulation of the detector performance. Here we show the characterisation of these detectors using ion beam induced charge (IBIC) with a focused 2 MeV He ions in a nuclear microprobe. The IBIC imaging method in a nuclear microprobe allowed us to measure the dead-layer thickness of the detector structure (required to be very thin for successful detection of keV ions), and the spatial distribution of the charge collection efficiency around the entire region of the detector. We show that our detectors have near 100% charge collection efficiency for MeV ions, extremely thin dead-layer thickness (about 7 nm) and a wide active region extending laterally from the electrodes (10–20 μm) where qubit arrays can be constructed. We demonstrate that the device can be successfully applied in the detection of keV ionisation energy from single events of keV X-rays and keV 31P ions.

Electrical characteristics of proton irradiated AlGaN devices

The growth of high quality GaN films offers the possibility of making this wide band gap material available for the fabrication of novel microelectronic devices with unique properties. However, growth defects that arise from the growth process result in high leakage currents that may be detrimental for some practical applications. It has been shown that these leakage currents can be compensated by the deliberate introduction of new defects by ion beam irradiation. The phenomenon has been investigated by use of a focused ion beam in a nuclear microprobe. AlGaN/GaN films were deposited on sapphire and then an interdigitated electrode pattern was deposited on the surface. Irradiation of the film through the electrodes allowed the resistivity to be monitored on-line. Irradiated areas displayed greatly improved signal to noise ratio when employed as a UV detector.

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.

Ion beam lithography with single ions

Although sub-micron structures have been fabricated with ion beam lithography using focused MeV ions, the best resolution of the method has not yet been approached. The best resolution is potentially around 10 nm which is the diameter of latent damage produced by the passage of a single fast ion through sensitive materials where the ion range could be tens of micrometres. In principle, the latent damage can be developed to create very high aspect ratio nanostructures. We call this technique single ion nanolithography. In order to approach the ultimate resolution of lithography with single ions we investigate the resist material, the exposure as a function of ion type and development parameters. To implement the technique we have developed a novel strategy that employs a resist film on an active substrate that functions as a detector sensitive to single ion impacts. Together with a focused microbeam, the precise control of ion fluence attained by counting ion impacts allows us to perform a convenient systematic study of the track formation and seek conditions where single ion tracks can be produced. We report here the current status of the investigations using PMMA and CR-39 resists which are shown to be sensitive to single ions. A key issue is also the post-development imaging method.

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.