Employing electrostatic confinement with a dual-gate device we realize periodic arrays of electron dots on Si widely tunable in diameter and electron number. From far-infrared transmission studies of dimensional resonances, we deduce dot diameters down to 40 nm for as little as 20 electrons in quantum states spaced by more than 5 meV. Excitation energies as well as mode dispersions in finite magnetic fields are found to strongly depend on the strength and the shape of the lateral confining potential. A detailed analysis of the oscillator strengths indicates a direct effect of strong quantum confinement.The high-frequency response of laterally bound electron layers has been investigated both in classically confined quasi-two-dimensional (2D) systems' and, more recently, in quantum confined systems that exhibit either quasi-one-dimensional 's (1D) or quasi-zero-dimensional (OD) properties. In particular, 1D inversion channels realized on GaAs, InSb, and Si have been extensively studied. 4 s Very recently electrons have been confined to OD quantum dots on GaAs (Refs. 8-10) and InSb. s Here we study the high-frequency response of periodic arrays of dots on Si containing few (20-350) inversion electrons.Significant quantum confinement on silicon can only be expected with dot diameters IY well below 100 nm. To meet this requirement we prepare a dual-gate structure which allows us to electrostatically define a wide range of dot diameters between 40 and 150 nm. Moreover, the main advantage of such a device is the continuous and nearly independent tunability of the depth of the lateral confining potential, the dot diameter W, and the electron number Np. This enables us to study the high-frequency response of dot arrays on Si in the transitory regime between classical and quantum confinement. Figure 1 shows a schematic cross section of our dualgate metal-oxide-silicon device. The bottom gate is a semitransparent NiCr mesh sandwiched between a thermal Si02 layer, grown on (100) p-type Si with a specific resistivity of 20 Acm at 300 K, and a plasmaenhanced chemical-vapor deposition (PECVD) Sio2 layer with a thin continuous NiCr layer of Re= 1 kQll:i on top.The scanning electron micrograph shows a top view of the bottom gate. The mesh has a periodicity of a 400 nm with circularly shaped openings of diameter t =150 nm. Different voltages Vs, and Vsb are applied between a substrate contact and the top and bottom gates, respectively.In the presence of band-gap radiation electron dots are induced at the Si-Si02 interface underneath the openings of the bottom gate via V~, . While Vg& essentially determines Np, Vgb serves to isolate the dots and to continuously vary the depth of the lateral confining potential from zero to a value exceeding the band gap of Si. As demonstrated below at fixed Vg& and Vgb the electronic diameter of the dots &can be further reduced via a substrate-bias voltage Vsg which is added to V~& in the dark and does not significantly change %p. This enhances the depletion Vgl Vgb «I d pECVD Si02 Si02 p-Si (1QO) ...

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