We have investigated the growth of three-dimensional Ag particles at atomic steps on the surface of highly oriented pyrolytic graphite using a scanning electron microscope. By controlling the growth parameters the cluster growth was confined to the steps avoiding terrace nucleation. In this way quasi-one-dimensional chains of Ag nanoclusters of approximately 10 nm diam were produced. The results suggest the viability of an important new route to the creation of controlled nanoscale structures. A comprehensive surface study indicates that cluster mobility and coalescence play an important role in the growth mechanism on the steps. Evidence was also found that the graphite surface has several different types of surface steps. A quantitative analysis of the cluster distribution on the steps was performed, to investigate the nucleation and growth processes at temperatures from 50 to 205 °C.

The decay of spin memory in a 2D electron gas is found to be suppressed close to the metal-insulator transition. By dynamically probing the device using ultrafast spectroscopy, relaxation of optically excited electron spin is directly measured as a function of the carrier density. Motional narrowing favors spin preservation in the maximally scattered but nonlocalized electronic states. This implies that the spinrelaxation rate can be both tuned in situ and specifically engineered in appropriate device geometries. DOI: 10.1103/PhysRevLett.86.2150 Emerging technologies for generating and manipulating electronic spins are heralded as the precursor to a new generation of spin-functional devices [1]. One of the key requirements is to find a way to control electron spin in semiconductors which can be independently gated. Much work has been devoted to generating spins in semiconductors either optically [2], by injection through magnetic semiconductors [3][4][5], or within ferromagnetic semiconductors [6], while the transport of spin has also been shown to be feasible [7,8]. However, the relaxation of carrier spin is ascribed to several competing mechanisms and particularly at low temperatures has been found to vary widely between different samples [9][10][11][12][13]. One reason for this is that spin relaxation can be particularly sample dependent since it is strongly influenced by impurities and defects. It is therefore desirable that the same sample be used within a study. However, circumstances may prevent this, as in investigations of the dependence of spin relaxation on background doping level, where different wafers must be used [14]. In addition, such structures have no means for tuning the spin relaxation in a prescribed way.In this Letter, we show an advantageous method which avoids the use of multiple samples to vary the number of carriers in a semiconductor by applying a reverse bias to a semitransparent Schottky contact situated above a charge transport layer. The effect of the gate is to transform the quasi-2D electron gas (2DEG) from a high-density metallic state to a low-density insulating state. At low temperatures it is possible to use this technique to reduce the conductivity by several orders of magnitude. This large change in conductivity is, however, accompanied by a relatively small change in electron density, typically less than an order of magnitude. It has been realized that the remote ionized donors that provide the electrons for the 2DEG play an important role in this drop in conductivity [15][16][17]. Spatial variations in the density of these donors result in random fluctuations in the potential in the plane of the 2DEG. In the conducting regime, the 2DEG acts to screen these fluctuations. However, as the density is reduced the screening becomes less effective and the fluctuations grow. This manifests itself as electron localization and corresponds to a large decrease in conductivity. In this Letter, the spinrelaxation dynamics of photoexcited carriers are investigated as we pass through th...

Electrically pumped single-photon sources using semiconductor quantum dots are of interest as they can be integrated with other semiconductor devices, using standard processing techniques. In this letter, we report electroluminescence from single quantum dots in a lateral p-i-n junction. Exciton and biexciton emission from a single quantum dot can be achieved under different electrical bias conditions. Antibunching effects from exciton and biexciton emission are observed using cw and pulsed electrical injection, indicating single-photon emission; this can be used for quantum information processing.

Synchrotron radiation induced chemical vapor deposition of thin films from metal hexacarbonyls* Lowtemperature chemical vapor deposition of tungsten from tungsten hexacarbonyl

We have studied focused electron beam induced deposition from W(CO)6 at beam primary energies between 20 and 0.06 keV. Submicrometer resolution with 4 nA beam current was maintained at very low primary energies using a retarding field configuration. Decomposition cross sections of W(CO)6 for primary energies below about 1 keV were found to be about a factor of 4 larger than those at 20 keV. Depending on the scan conditions, the resistivity of the deposits formed using low primary energies was found to be up to about a factor of 4 lower than at 20 keV implying a higher metallic content. These results form the basis of an improved method for repairing clear defects on x-ray masks and for making conducting tracks on semiconducting materials.

Gas-assisted etching with a finely focused ion beam has been studied. The presence of a reactive gas, in this case Cl2, results in an enhanced etch rate compared to the rate for sputtering for many materials, including Si, Al, and GaAs. Other advantages over sputtering are the absence of redeposited material and the high etch selectivity possible with some material combinations, which has been exploited in the etching of microstructures. In some applications of this technique, a protective layer of low etch rate material is used over the substrate to improve the quality of the etched structure. The characteristics of the etching process have been studied with variation in the scan speed, gas flux, and current density into the scanned area. In general, a high scan speed, high gas flux, and low current density were found to give the maximum enhancement in the etch rate over sputtering. The application of these results to etching over a wide range of experimental conditions is discussed, to give a basis for estimating the etching results that would occur under other conditions.

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GaAs microdisk lasers with holes pierced through the disk surface are investigated for their threshold characteristics. Disks are fabricated with either a single hole or two diametrically opposite holes at various distances from the disk outer edge. Even though the disk area is reduced by only 1%, we find that the lasing threshold for a disk with one hole is reduced by up to 50% compared to a disk with no hole. We attribute this reduction to the perturbation of nonlasing modes, which decreases the amplification of spontaneous emission in these modes and makes more carriers available to contribute to lasing. © 1999 American Institute of Physics. ͓S0003-6951͑99͒00301-0͔Semiconductor microdisk lasers are of interest for micron-scale low-threshold laser devices. Their laser modes approximate whispering-gallery ͑WG͒ modes, for which the high reflectivity at the curved disk boundary gives lowthreshold operation. The laser threshold is a function of the optical confinement, gain/absorption balance, scattering processes, and device dimensions. The optical confinement in these structures is high because of the large dielectric discontinuity between the disk and the surrounding air, while carrier absorption and scattering processes are difficult to modify. Threshold characteristics can be calculated, 1-3 and various research groups have sought to achieve lowthreshold operation by shrinking the device dimensions to reduce spontaneous emission into nonlasing modes; 4-6 the smallest microdisk laser reported having 1.6 m diameter. 5 Other groups have concentrated on optimizing the edge quality of their devices to improve the optical confinement, resulting in further reductions of the laser threshold. 7 In this letter we present an approach for decreasing the threshold in a large semiconductor disk (Rӷ ) based on reducing the amplified spontaneous emission ͑ASE͒ into competing nonlasing modes. We achieve this reduction by removing material from the interior of the disk, which reduces the photon lifetimes for modes with field in this region. Since the ASE depends on the photon lifetimes of the corresponding modes, fewer carriers emit into these modes, making more carriers available to contribute to lasing. The introduction of one small hole, comprising approximately 1% of the disk area, can reduce the laser threshold by a factor of 2. Figure 1 shows an electron micrograph of a typical microdisk structure with small holes etched through the top surface ͑Fig. 1͒. The disks contain four 10 nm GaAs quantum wells ͑QWs͒ separated by Al 0.28 Ga 0.72 As barriers, and are defined by electron-beam lithography and reactive ion etching. The pedestals ͑500 nm Al 0.65 Ga 0.35 As) are created by undercutting with a selective wet etch. Finally, the structures are passivated in ammonium sulphide and stabilized with a 30-nm-thick silicon nitride encapsulation. 8 The disks have a diameter of 12.6 m with 0.5 m square holes. Disks have been fabricated with one hole, or two holes placed opposite each other, separated from the disk outer edge by 0.5,...