We employ the vibrating tip of an atomic force microscope as a lithographic tool to mechanically pattern a thin photoresist layer covering a GaAs–AlGaAs heterostructure. High aspect ratio electron beam deposited tips, additionally sharpened in an oxygen plasma, are used to minimize the dimensions of the fabricated quantum electronic devices. The fabrication parameters of the tips and the sharpening process are investigated. With these ultrasharp tips we are able to produce lines and holes with periods down to 9 nm in photoresist. In addition, the very sharp tips yield substantial improvements in the imaging mode.
We investigate the transport properties of insulating phases in the 2D electron system of highmobility A16aAs/GaAs heterostructures of Corbino geometry at very low temperatures.We find that the nonlinear current-voltage characteristics for insulating phases in the integer and fractional quantum Hall regime and for a low-density insulating phase are very similar. The behavior of these characteristics with changing temperature and filling factor unambiguously points to the percolation metal-insulator transition as the cause for all insulating phases investigated.We propose a metalinsulator phase diagram in the (B,N, ) plane based on our experimental data. PACS numbers: 71.30.+h, 73.20.Dx, 73.40.Kp The metal-insulator transition at low filling factors in a 2D electron system of high-mobility A1GaAs/GaAs heterostructures has been investigated in a number of studies (see, e. g., [1 -5]) by using various experimental techniques. In the majority of the reports the transition into an insulating phase is attributed to the formation of a pinned Wigner crystal. However, some doubts in this interpretation were expressed, e.g. , in Ref. [6]. Optical investigations strongly suggest that in high magnetic fields the 2D electron system becomes strongly inhomogeneous; in photoluminescence spectra there coexist two lines, one of which is caused by radiative recombination of 2D electrons from metallic regions [4,7,8] and the other proves the existence of insulating islands in the electron system. The conduction of an inhomogeneous macroscopic system is determined by the coverage of the sample with metallic and insulating areas, respectively. In this case the metalinsulator transition must be discussed as a percolation problem. Here we introduce a method based on investigations of the current-voltage characteristics which makes it possible to compare the transport properties of insulating phases realized in the integer and fractional quantum Hall regime and at low electron densities in A1GaAs/GaAs heterostructure samples. Our experimental results provide strong evidence for the percolation nature of all metalinsulator transitions studied.Let us first consider the definition of insulating phases in a 2D system. As known, in the integer and fractional quantum Hall regime the Hall conductivity o-Y is quantized and the dissipative conductivity cr " tends to zero at low temperatures.The Hall conductivity is finite due to the existence of extended states below the Fermi level that are able to carry dissipationless Hall current, as has been shown in experiments on charge transfer in Corbino samples [9]. It is reasonable to call such a state of the 2D electron system an insulator because the Fermi level lies in localized states and electron transport in the direction of the electric field is absent. Obviously, a so-defined insulating phase can be characterized by the value of the Hall conductivity. The insulating phase at low electron densities corresponds to o. "Y = 0 since in this case the extended states below the Fermi level are not ...
We describe a novel technique using an atomic force microscope (AFM) for integrated nanometer-scale lithography on various mask materials such as photoresist or gold covering a mesa-etched GaAs-AlGaAs heterostructure at ambient conditions. The generated patterns can be transferred to the two-dimensional electron gas by wet chemical etching or by ion beam irradiation. We succeed in fabricating hole arrays with a periodicity down to 35 nm and a hole diameter of only a few nanometers. In magnetoresistance studies on so-called antidot devices with 95 nm period at T=4.2 K we can clearly observe commensurability oscillations, demonstrating the successful pattern transfer to the electron system. With the AFM we can also pattern lines of varying width and depth into prefabricated devices.
The need for high-quality aspheres is rapidly growing, necessitating increased accuracy in their measurement. A reliable uncertainty assessment of asphere form measurement techniques is difficult due to their complexity. In order to explore the accuracy of current asphere form measurement techniques, an interlaboratory comparison was carried out in which four aspheres were measured by eight laboratories using tactile measurements, optical point measurements, and optical areal measurements. Altogether, 12 different devices were employed. The measurement results were analysed after subtracting the design topography and subsequently a best-fit sphere from the measurements. The surface reduced in this way was compared to a reference topography that was obtained by taking the pointwise median across the ensemble of reduced topographies on a Cartesian grid. The deviations of the reduced topographies from the reference topography were analysed in terms of several characteristics including peak-to-valley and root-mean-square deviations. Root-mean-square deviations of the reduced topographies from the reference topographies were found to be on the order of some tens of nanometres up to 89 nm, with most of the deviations being smaller than 20 nm. Our results give an indication of the accuracy that can currently be expected in form measurements of aspheres.
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