A gated probe for scanning tunnelling microscopy (STM) has been developed. b The probe extends normal STM operations by means of an additional electrode fabricated next to the tunnelling tip. The extra electrode does not make contact with the sample and can be used as a gate. We report on the recipe used for fabricating the tunnelling tip and the gate electrode on a silicon nitride cantilever. We demonstrate the functioning of the scanning gate probes by performing single-electron tunnelling spectroscopy on 20-nm gold clusters for different gate voltages.The constant pace in miniaturisation of electronic chips is thought 1 to eventually lead to circuit elements consisting of just a few molecules. For this reason a great effort is currently addressed to studying the transport properties of nanostructures such as quantum dots, fullerenes, nanotubes, metal nanoclusters and macromolecules. For such applications it is important to characterise the electronic energy spectrum of the nanostructures for different sizes and arrangements. At present, the interfacing of small objects with macroscopic electrical contacts presents the main experimental challenge, which has been successfully addressed only in some particular cases 2 . Conversely, scanning probe techniques, and scanning tunnelling microscopy (STM) in particular, do not suffer from interfacing problems. Typically the STM is used to image and locate nanostructures deposited on a conducting substrate. Afterwards the sharp STM tip can be selectively positioned on top of specific structures to acquire their spectroscopic characteristics 3 . This type of measurement can give valuable information on the electronic states of the molecule. A clearer interpretation, however, becomes possible when one is able to control and change the electronic states with the use of a gate electrode (i.e. perform three-terminal measurements). The gate can also be used to avoid the extra level smearing caused by high bias voltages and currents, by shifting the electronic levels in the nanostructure towards the Fermi energy of the leads 4 . Threeterminal electrical measurements have already been proven very fruitful in the field of SET transistors and semiconductor quantum dots 5 . Figure 1a illustrates the scheme of a typical STM measurement on a metallic cluster (or more generally on a nanostructure): electrons can tunnel from the STM tip onto the cluster and then across an insulating tunnel barrier into the conducting substrate. Inside the cluster the extra electron can occupy the ground state or, when a voltage bias is applied, one of the excited states. No current can flow if the tunneling electrons have energy less than the charging energy of the cluster (E c = e 2 /C, where C is the capacitance of the cluster). E c can be large for small clusters. The charging energy strongly regulates transport, causing electrons to tunnel one-byone, which is known as single electron tunneling 5 . So far, the gate electrode has been absent in all STM studies. In this Letter we present the design a...
The properties of InAs (110) surfaces have been investigated by means of low-temperature scanning tunneling microscopy and spectroscopy. A technique for ex-situ sulphur passivation has been developed to form an accumulation layer on such a surface. Tunneling spectroscopy at 4.2 K shows the presence of 2D subbands in the accumulation layer. Measurements in high-magnetic field demonstrate Landau quantization of the energy spectrum, both in the 2D subbands and the 3D bulk conduction band. PACS: 73.20.Dx, 61.16.Ch, 71.70.Di The properties of 2D electron gases (2DEG) subject to high magnetic fields have generated a large amount of theoretical and experimental work. In particular, the study of edge channels and electron-electron interactions [1] is currently a field of great interest. Spatially resolved imaging of edge channels was performed by van Haren et al. [2], using a special technique to obtain macroscopically wide channels. For higher spatial resolution measurements one would like to exploit the potential of STM techniques. However, this requires a semiconductor system having a 2DEG at the surface. InAs is a good candidate material for this, since it is known that it can have a surface 2DEG [3]. Wildöer et al. [4] have performed low-temperature scanning tunneling microscopy and spectroscopy on InAs (110) surfaces that were cleaved in situ at 4.2 K. A clean surface was obtained and Landau quantization was observed. In this study, however, no clear evidence was found for the formation of a surface 2DEG.In this article we report on the development of a suitable system for STM studies on a 2DEG: the sulphur-passivated InAs (110) surface. We demonstrate the presence of 2D subbands and of Landau quantization in high magnetic fields.From the work of Tsui [3], it is known that an oxide layer at the surface of ntype InAs pins the Fermi energy above the bottom of the conduction band, thereby forming a surface accumulation layer. We have found that oxidation of the InAs surface is not a practical ex-situ technique for an STM study, since the oxide grows too thick after just a few minutes of exposure to air. We have therefore studied different passivation techniques for InAs taking advantage of its chemical similarity with GaAs, for which a vast literature of passivation methods is available. Those techniques mainly use reactive S-containing solutions and/or gases. In practice, we have focused on two among the most practical and relatively less toxic wet techniques: one employing a solution of (NH 4 ) 2 S as in Ref. 5, the other one using a solution CH 3 CSNH 2 /NH 4 OH at 90 o C as in Ref. 6.
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