We investigate coherent transport in Si:MOSFETs with nominal gate lengths 50 to 100nm and various widths at very low temperature. Independent of the geometry, localized states appear when G ≃ e 2 /h and transport is dominated by resonant tunnelling through a single quantum dot formed by an impurity potential. We find that the typical size of the relevant impurity quantum dot is comparable to the channel length and that the periodicity of the observed Coulomb blockade oscillations is roughly inversely proportional to the channel length. The spectrum of resonances and the nonlinear I-V curves allow to measure the charging energy and the mean level energy spacing for electrons in the localized state. Furthermore, we find that in the dielectric regime the variance var(lng) of the logarithmic conductance lng is proportional to its average value < lng > consistent with one-electron scaling models.After the pioneering work of Scott-Thomas et al [1], Coulomb blockade in quantum dots formed by an impurity potential has been studied in quasi 1D wires or pointcontact geometries [2,3]. In comparison with lithographically defined lateral quantum dots, impurity quantum dots (IQD) contain typically fewer electrons. Downscaling the size of the IQD allows to operate a silicon based single electron quantum dot transistor even at room temperature [4] [5]. In the opposite case of wires, i.e. in disordered thin and wide insulating barriers, no Coulomb blockade oscillations have been reported up to now. Resonant tunneling through single ionized donor potentials is responsible for electron transport in disordered thin insulating barriers. This has been studied in the deeply insulating regime of large thin barriers formed by depleting electrostatically a semiconductor under a gate [6], or in thin amorphous silicon tunnel barriers [7]. Interaction between distant impurity states in the channel have been revealed by peculiarities of the nonlinear transport. However, a single ionized donor potential cannot accomodate many electrons without becoming screened.We report for the first time Coulomb oscillations in very short MOSFETs with source drain distance d SD less than 0.05µm and width much larger than d SD . Contrarily to quasi 1D wires, where the size of the IQD is somewhat arbitrary, we will show that the diameter of the IQD is comparable to the source drain distance and that on resonance the conductance g in quantum units e 2 /h is close to one. Furthermore we demonstrate that the fluctuations of the conductance characterized by var(lng) are proportional to the mean value < lng >, rather independent of geometry. Such an observation is consistent with one-parameter noninteracting scaling models of the metal-insulator transition. Our experimental findings suggest that interactions do not destroy this oneparameter description.The devices are MOSFETs on the (100) surface of silicon doped to a level of 7 10 12 Borons/cm −2 for the 100nm series and of 3 1013 Borons/cm −2 for the 50nm series. The gate oxyde thickness is only d SiO2 = 3.8nm f...
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