We studied transport through ultra-small Si quantum dot transistors fabricated from silicon-oninsulator wafers. At high temperatures, 4 K < T < 100 K, the devices show single-electron or single-hole transport through the lithographically defined dot. At T < 4 K, current through the devices is characterized by multidot transport. From the analysis of the transport in samples with double-dot characteristics, we conclude that extra dots are formed inside the thermally grown gate oxide which surrounds the lithographically defined dot.

Abstract. Numerical simulations have been carried out to study and compare the mechanisms of the formations of the topside and bottomside ionospheric irregularities. As is well-known, the topside ionospheric irregularities have always originated from the bottomside ionosphere due to Gravitational Rayleigh-Taylor (GRT) instability. From the penetration process we found that the primary bubble with a horizontal scale of tens of kilometers always has a smooth shape in the bottomside ionosphere and breaks into much smaller-scale structures after penetrating into deep topside ionosphere. Since the plasma bubble is unlikely to break into irregularities in the bottomside ionosphere, the bottomside irregularities probably result from the turbulence in the neutral atmosphere. Examples of simulations are presented to demonstrate that bottomside irregularities may exist in a region with a strong neutral wind jet and seeding gravity wave motion. A strong neutral jet stream always has a strong vertical wind shear, which provides a broad height range of dynamic instability. When the atmospheric gravity wave approaches its critical level, its amplitude will continually grow to break into smaller-scale waves and turbulence. Then all the waves of different scales as well as the turbulence will become the seeds of the bottomside irregularities. IntroductionEquatorial spread F is mainly driven by Gravitational Rayleigh- Taylor is to investigate the cascading process of the bubble from wave structure into irregularities.In contrast to the topside ionospheric irregularities, which are originated in the distant bottomside ionosphere, the bottomside ionospheric irregularities are originated from right where they are, without spreading to the nonperturbed region. Therefore the seeded perturbation is very probably limited to a finite region where the growth of the perturbation is so fast that the perturbation grows in a short time to break into many small-scale structures before it can spread to the nonperturbed region. The exact condition which can confine the bottomside irregularities is still unclear. So another one of the purposes of this study is to find such a possible condition by the method of numerical simulation. In this study we propose that the existence of a strong neutral wind jet superposed by seeding gravity waves in the bottomside ionosphere may generate and confine the irregularities in a limited height range.We will briefly review the numerical method in section 2 and present the simulation results in section 3. The discussions will be given in section 4. Numerical ModelThe model used in this study, which is the same as the model of Zalesak and Ossakow [1980], has been described by Chou and Kuo [1993, 1996]

The neutral wind field effects on the development of the equatorial plasma bubbles have been simulated by a two‐dimensional time‐dependent model similar to that developed by Zalessak and Ossakow. The results indicate that when there exists no neutral wind, any perturbation on the bottom‐side of the ionosphere density profile will be amplified by the gravitational Rayleigh‐Taylor (GRT) instability and an upwelling bubble will form as expected. When a zonal neutral wind field exists, the uplift velocity of the bubble will be enhanced by a uniform neutral wind, but suppressed by vertical shear of the wind field. Secondary structures called plumes will grow out from the side walls of the primary bubble if some secondary perturbation signal with vertical structure is seeded. We notice that patches and multiple plume structures have been observed in the mid‐latitude and low‐latitude ionosphere. So we have developed a two‐dimensional local theory of the generalized GRT instability to calculate the growth rate of a small‐scale perturbation signal in the vicinity of the primary bubble and found that the growth rate increases with time along with the growth of the bubble. It is proved that the irregularities generated by some of our simulations should be able to cascade into observable (e.g., by radar) turbulence in a reasonable period of time after their generation. Similar to the effect on the growth of the primary bubble, a strong uniform wind field may enhance the growth rate of a perturbation signal in the vicinity of the bubble, while a strong vertical wind shear will suppress the growth rate. The main point of this theory is that the growth rate of the GRT instability is controlled by the density gradients and the magnitudes of the velocity of the plasma motion relative to the neutral wind in both directions. In addition, our simulations have proved that a seeding wave with phase velocity matching the background wind speed will generate the fastest growing bubble, disapproving the theory of spatial resonance.

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