T. Schenkel

Interaction of slow, very highly charged ions with surfaces

The present article reviews recent advances in the studies of the interaction of slow (v < vBohr), very highly charged ions (such as Xe 52+ and Au 69+ ) with surfaces of metals, semiconductors, and insulators (including biological materials). After a brief summary of past developments, we describe key experimental techniques for studies of secondary particle emission and the de-excitation dynamics of the highly charged ions. Recent progress in measurement and determination of the mechanisms leading to secondary electron yields, secondary ion yields and total sputtering yields will be discussed. The deexcitation dynamics are addressed in experiments on projectile neutralization and energy loss in thin films of material. We review the theoretical concepts briefly and introduce theoretical models in the discussion of experimental results. Following the presentation of fundamental studies we will address emerging applications of slow, very highly charged ions in surface analysis and surface modification. # 1999 Published by Elsevier Science Ltd. All rights reserved.

Reaching the quantum limit of sensitivity in electron spin resonance

The detection and characterization of paramagnetic species by electron spin resonance (ESR) spectroscopy is widely used throughout chemistry, biology and materials science, from in vivo imaging to distance measurements in spin-labelled proteins. ESR relies on the inductive detection of microwave signals emitted by the spins into a coupled microwave resonator during their Larmor precession. However, such signals can be very small, prohibiting the application of ESR at the nanoscale (for example, at the single-cell level or on individual nanoparticles). Here, using a Josephson parametric microwave amplifier combined with high-quality-factor superconducting microresonators cooled at millikelvin temperatures, we improve the state-of-the-art sensitivity of inductive ESR detection by nearly four orders of magnitude. We demonstrate the detection of 1,700 bismuth donor spins in silicon within a single Hahn echo with unit signal-to-noise ratio, reduced to 150 spins by averaging a single Carr-Purcell-Meiboom-Gill sequence. This unprecedented sensitivity reaches the limit set by quantum fluctuations of the electromagnetic field instead of thermal or technical noise, which constitutes a novel regime for magnetic resonance. The detection volume of our resonator is ∼ 0.02 nl, and our approach can be readily scaled down further to improve sensitivity, providing a new versatile toolbox for ESR at the nanoscale.

Formation of a few nanometer wide holes in membranes with a dual beam focused ion beam system

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 10−6u2009Torr 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.

Single atom doping for quantum device development in diamond and silicon

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 (SFM) 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 center) array formation for systematic testing of quantum computer architectures in silicon and diamond.

Controlling spin relaxation with a cavity

Spontaneous emission of radiation is one of the fundamental mechanisms by which an excited quantum system returns to equilibrium. For spins, however, spontaneous emission is generally negligible compared to other non-radiative relaxation processes because of the weak coupling between the magnetic dipole and the electromagnetic field. In 1946, Purcell realized that the rate of spontaneous emission can be greatly enhanced by placing the quantum system in a resonant cavity. This effect has since been used extensively to control the lifetime of atoms and semiconducting heterostructures coupled to microwave or optical cavities, and is essential for the realization of high-efficiency single-photon sources. Here we report the application of this idea to spins in solids. By coupling donor spins in silicon to a superconducting microwave cavity with a high quality factor and a small mode volume, we reach the regime in which spontaneous emission constitutes the dominant mechanism of spin relaxation. The relaxation rate is increased by three orders of magnitude as the spins are tuned to the cavity resonance, demonstrating that energy relaxation can be controlled on demand. Our results provide a general way to initialize spin systems into their ground state and therefore have applications in magnetic resonance and quantum information processing. They also demonstrate that the coupling between the magnetic dipole of a spin and the electromagnetic field can be enhanced up to the point at which quantum fluctuations have a marked effect on the spin dynamics; as such, they represent an important step towards the coherent magnetic coupling of individual spins to microwave photons.

Single ion implantation for solid state quantum computer development

Several solid state quantum computer schemes are based on the manipulation of electron and/or nuclear spins of single 31P atoms in a solid matrix. The fabrication of qubit arrays requires the placement of individual atoms with nanometer precision and high efficiency. We describe the status of our development of a low energy, single ion implantation scheme for 31Pq+ ions. High ion charge states enable registration of single ion impacts with unity efficiency through the detection of secondary electrons. Imaging contrast in secondary electron emission allows alignment of the implantation and integration with consecutive lithography steps. Critical issues of process integration and resolution limiting factors are discussed.

Electrical activation and electron spin resonance measurements of implanted bismuth in isotopically enriched silicon-28

We have performed continuous wave and pulsed electron spin resonance measurements of implanted bismuth donors in isotopically enriched silicon-28. Donors are electrically activated via thermal annealing with minimal diffusion. Damage from bismuth ion implantation is repaired during thermal annealing as evidenced by narrow spin resonance linewidths (Bpp=12μT) and long spin coherence times (T2=0.7 ms, at temperature T=8 K). The results qualify ion implanted bismuth as a promising candidate for spin qubit integration in silicon.

Electronic desorption of alkyl monolayers from silicon by very highly charged ions

Self-assembled alkyl monolayers on Si (111) were exposed to low doses of slow (v≈6.6×105u2002m/s≈0.3vBohr), highly charged ions, like Xe41+ and Th73+. Atomic force microscope images show craters from single ion impacts with diameters of 50–63 nm. Emission of secondary ions by highly charged projectiles was monitored by time-of-flight secondary ion mass spectrometry (TOF-SIMS). TOF-SIMS data give insights into the dependence of electronic desorption effects on the projectile charge state. We discuss the potential of highly charged projectiles as tools for materials modification on a nanometer scale.

Light-emitting nanostructures formed by intense, ultrafast electronic excitation in silicon (100)

The intense, ultrafast electronic excitation of clean silicon (100)–(2×1) surfaces leads to the formation of silicon nanostructures embedded in silicon, which photoluminescence at ∼560 nm wavelength (∼2 eV band gap). The silicon surfaces were irradiated with slow, highly charged ions (e.g., Xe44+ and Au53+) to produce the electronic excitation. The observation of excitonic features in the luminescence is particularly unusual for silicon nanostructures. The temperature dependence and the measurement of the triplet–singlet splitting of the emission strongly support the excitonic assignment.

Influence of hydrogen on the stability of positively charged silicon dioxide clusters

Spectra of positively charged secondary ions from thermally grown SiO2 films were recorded in a time-of-flight secondary ion mass spectrometry scheme. Ablation of cluster ions was induced by the impact of slow (4 keV/u) Au69+ projectiles. The intensities of SixOyHz+, (x=1–22, y=1–44, z=0–7) clusters are found to depend sensitively on the oxygen to silicon ratio and also on the hydrogen content. We find that oxygen rich clusters, y=2x+1, and, in one case, y=2x+2, can be stabilized by the incorporation of two additional hydrogen atoms in the cluster.