We implanted ultra low doses (2×10 11 cm -2 ) of 121 Sb ions into isotopically enriched 28 Si and find high degrees of electrical activation and low levels of dopant diffusion after rapid thermal annealing. Pulsed Electron Spin Resonance shows that spin echo decay is sensitive to the dopant depths, and the interface quality. At 5.2 K, a spin decoherence time, T 2 , of 0.3 ms is found for profiles peaking 50 nm below a Si/SiO 2 interface, increasing to 0.75 ms when the surface is passivated with hydrogen. These measurements provide benchmark data for the development of devices in which quantum information is encoded in donor electron spins.* Email: T_Schenkel@LBL.gov 1 cond-mat/0507318Spins of electrons bound to donor atoms in silicon at low temperature are promising candidates for the development of quantum information processing devices [1][2][3]. This is due to their long decoherence times, and the potential to leverage fabrication finesse in a silicon transistor paradigm. Recently, relatively long transverse relaxation times (T 2 ) were determined for electron spins in pulsed electron spin resonance (ESR) studies of phosphorous donors in isotopically enriched silicon. Here, donors were present as a random background doping across 28 Si epi layers and T 2 extrapolated to 60 ms for isolated donors [3]. Formation of test devices for quantum information processing requires the integration of individual dopant atoms with a control and readout infrastructure. Donor array fabrication is being addressed by ion implantation [4][5][6] and scanning probe based hydrogen lithography [7,8]. Dopant spacing depends on the choice of entangling interactions between quantum bits (qubits) and ranges from 20 to over 100 nm, corresponding to ultra low ion implantation doses of <10 10 to 2.5×10 11 cm -2 . In this letter, we report on depth profiles and electrical activation following rapid thermal annealing (RTA) of ultra low dose 121 Sb implants and correlate electron spin relaxation times with the dopant distribution below an interface and with the interface quality.We processed wafers with 10 μm thick, 28 Si enriched epi layers (500 ppm 29 Si) on p-type natural silicon (100) and natural silicon control wafers (100), both with impurity concentrations ≤10 14 cm -3 . Standard CMOS processes were followed for formation of 5-10 nm thick thermal SiO 2 . Typical densities of trapped charges and interface traps were 1 to 2×10 11 cm -2 for the thermal oxides. 121 Sb-ion implantation with a dose of 2×10 11 cm -2 was conducted with implant energies of 120 keV and 400 keV. 121 Sb was used to avoid any ambiguity of results due to 31 P background in 28 Si epi layers. RTA for repair of 2 cond-mat/0507318 implant damage and substitutional incorporation of dopants into the silicon lattice, i. e., electrical activation, was performed with an AGA Heatpulse 610. Following annealing, carrier depth profiles were probed with Spreading Resistance Analysis (SRA) [9]. Secondary Ion Mass Spectrometry (SIMS) [9] was used to characterize elemental depth ...

Spawned by the finding of efficient quantum algorithms, the development of a scalable quantum computer has emerged as a premiere challenge for nanoscience and nanotechnology in the last years. Spins of electrons and nuclei in P31 atoms embedded in silicon are promising quantum bit (qubit) candidates. In this article we describe single atom doping strategies and the status of our development of single atom qubit arrays integrated with control gates and readout structures in a “top down” approach. We discuss requirements for P31 qubit array formation by single ion implantation, and integration with semiconductor processing.

When nanometer-scale holes (diameters of 50 to a few hundred nm) are imaged in a scanning electron microscope (SEM) at pressures in the 10−5 to 10−6 Torr range, hydrocarbon deposits build up and result in the closing of holes within minutes of imaging. Additionally, electron or ion beam assisted deposition of material from a gas source allows the closing of holes with films of platinum or tetraethylorthosilicate oxide. In an instrument equipped both with a focused ion beam, and a SEM, holes can be formed and then covered with a thin film to form nanopores with controlled openings, ranging down to only a few nanometers, well below resolution limits of primary beams.

[7]. Donor electron and nuclear spins are promising candidates for implementation of quantum bits in silicon [7]. The detection of low energy single ion impacts for device integration has been accomplished via detection of secondary electrons [2,4,9], or by collection of electron holepairs in optimized diodes [3]. It is also well known that high energy (MeV) single ion impacts can upset device currents [10], and an extension of this approach to low energy ions was recently outlined in Ref. 11. Also, random telegraph noise due to switching occupancies of single Coulomb scattering centers [12] has long been observed in sub-micron transistors, and it can thus be expected that the impact of lower energy (<100 keV) single ions, which is accompanied by the generation of multiple charged defects, can also be sensed in FETs.In this letter we report on the detection of low energy (50 to 70 keV) antimony and xenon ion impacts in FETs with channel areas of 4 μm 2 at room temperature. FETs were formed for development of single donor spin readout techniques, and spin dependent neutral donor scattering was recently observed in transport studies with similar devices used here [13]. Single ions change transistor channel mobilities through formation of defects upon impact, enabling precision placement of defined numbers of dopants into transistor channels. Upon further reduction of the beam current to ~0.1 ions/s, pulses contain mostly no ions, and current steps from single ion hits are recorded (Figure 2 c)). The probability for multiple ion hits in one pulse under these conditions of reduced beam current was less then 3%.During exposures with ions of different impact energies and charge states we found that the sensitivity to ion impacts, i. e. the magnitude of current steps, was gradually reduced with increasing implant dose. Further, variations in step heights at the given noise level did not allow us to confidently discriminate multiple hits from single ion hits based on the step heights. Due to the degrading sensitivity, it was also difficult to investigate charge state effects on the single ion induced current step height. It can be expected that the localized deposition of potential energy of multiply and highly charged ions [21] contributes significantly to the formation of defects in the gate oxide and at the Si-SiO 2 interface, and future work aims at quantifying this effect.Following a series of exposures with an accumulated dose of ~10 11 cm -2 , devices were annealed for damage repair and dopant activation. Rapid thermal annealing (RTA) was performed in an AGA Heatpulse at 900º C for 20 s in Argon, followed by another 30 min. N 2 /H 2 -forming gas anneal at 400º C. In figure 3, we show a series of I-V curves of a pristine A-FET 5 (Fig. 3 a) and NMOS-FET (Fig. 3 b), after FIB processing and forming gas anneal, and then after monitored implantation with noble gas and Sb ions and the consecutive anneals, demonstrating that devices were functional transistors after the full process sequence. The threshold voltages, V...

The ability to inject dopant atoms with high spatial resolution, flexibility in dopant species, and high single ion detection fidelity opens opportunities for the study of dopant fluctuation effects and the development of devices in which function is based on the manipulation of quantum states in single atoms, such as proposed quantum computers. The authors describe a single atom injector, in which the imaging and alignment capabilities of a scanning force microscope ͑SFM͒ are integrated with ion beams from a series of ion sources and with sensitive detection of current transients induced by incident ions. Ion beams are collimated by a small hole in the SFM tip and current changes induced by single ion impacts in transistor channels enable reliable detection of single ion hits. They discuss resolution limiting factors in ion placement and processing and paths to single atom ͑and color center͒ array formation for systematic testing of quantum computer architectures in silicon and diamond.

Several solid state quantum computer schemes are based on the manipulation of electron and nuclear spins of single donor atoms in a solid matrix. The fabrication of qubit arrays requires the placement of individual atoms with nanometer precision and high efficiency. In this article we describe first results from low dose, low energy implantations and our development of a low energy (<10 keV), single ion implantation scheme for 31 P q+ ions. When 31 P q+ ions impinge on a wafer surface, their potential energy (9.3 keV for P 15+ ) is released, and about 20 secondary electrons are emitted. The emission of multiple secondary electrons allows detection of each ion impact with 100% efficiency. The beam spot on target is controlled by beam focusing and collimation. Exactly one ion is implanted into a selected area avoiding a Poissonian distribution of implanted ions.

We report the integration of a scanning force microscope with ion beams. The scanning probe images surface structures non-invasively and aligns the ion beam to regions of interest. The ion beam is transported through a hole in the scanning probe tip. Piezoresistive force sensors allow placement of micromachined cantilevers close to the ion beam lens. Scanning probe imaging and alignment is demonstrated in a vacuum chamber coupled to the ion beam line. Dot arrays are formed by ion implantation in resist layers on silicon samples with dot diameters limited by the hole size in the probe tips of a few hundred nm.

We describe a scanning probe instrument which integrates ion beams with the imaging and alignment function of a piezo-resistive scanning probe in high vacuum. The beam passes through several apertures and is finally collimated by a hole in the cantilever of the scanning probe. The ion beam spot size is limited by the size of the last aperture. Highly charged ions are used to show hits of single ions in resist, and we discuss the issues for implantation of single ions.