Minigaps in the Plasmon Dispersion of a Two-Dimensional Electron Gas with Spatially Modulated Charge Density

Abstract

In a two-dimensional electron gas with a spatially modulated charge density we observe a splitting of the two-dimensional-plasmon dispersion. The charge-density modulation is induced in a metal-oxide-silicon capacitor with a modulated oxide thickness of submicrometer periodicity. The splitting is caused by the superlattice effect of the charge-density modulation on the collective excitation spectrum and depends strongly on the Fourier expansion coefficients of the charge-density profile.PACS numbers: 71.45. Gm, 73.40.Qv In metal-oxide-semiconductor (MOS) structures electrons can be confined in a narrow potential well. These space-charge layers exhibit two-dimensional (2D) electronic properties which can be varied via the gate potential. 1 One interesting aspect of the 2D-MOS system is the possibility to lower the dimensionality by periodic lateral potential variation which may induce superlattice effects. Such lateral potential variation has been shown to arise naturally on high-index surfaces of semiconductors. 2,3 By use of submicron technology we have fabricated MOS structures with a lateral charge-density modulation which leads to new artificially created electronic properties. In these structures we observe that the 2D-plasmon frequency co p , which is known to vary continuously with the square root of the plasmon wave vector q in a 2D system with constant charge density,^8 exhibits gaps and separated plasmon bands.The sample geometry is shown schematically in Fig. 1. We use large-area (diameter, 5 mm) MOS capacitors on /?-Si (100) substrates (substrate resistivity, 20 n/n; peak channel mobility, 3000 cm 2 /V-s). The thickness of the insulating oxide varies with a periodicity a. The linear periodic FIG. 1. Schematic geometry of oxide-modulated MOS capacitors.structures are prepared by holographic lithography and dry etching techniques. Samples with periodicities from 300 to 1200 nm have been fabricated. A thin layer of 3 nm NiCr is evaporated with varying angles onto the structures to achieve a continuous semitransparent gate. If a gate voltage V g is applied between gate and substrate, stripes of different charge density are induced at the semiconductor interface. The widths of the stripes, t\ and t 2 = i a -t u are controlled by the holographically prepared mask, and the oxide thickness d x by the etching process. For the dimensions used here the charge density N si in the regime /, can be approximated by N S i sss € 0X (V g -V tI )/d h / = 1,2. e ox is the static dielectric constant of the oxide. The threshold voltage V t is determined from the onset of the integrated dynamic conductivity, which is measured in the frequency regime 20 cm -1 to 400 cm -1 and averaged over the gate area. This threshold agrees with results from Shubnikov-de Haas and from CV experiments. A difference in the threshold voltages in the regimes t\ and t 2 has not been resolved, and so we take V n = V t2 = V t in the following. The excitation of plasmons is investigated by transmission spectroscopy. We measure the relative chang...

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