S. H. Tessmer

Subsurface charge accumulation imaging of a quantum Hall liquid

The unusual properties of two-dimensional electron systems that give rise to the quantum Hall effect have prompted the development of new microscopic models for electrical conduction. The bulk properties of the quantum Hall effect have also been studied experimentally using a variety of probes including transport,, photoluminescence,, magnetization and capacitance, measurements. However, the fact that two-dimensional electron systems typically exist some distance (about 1,000 Å) beneath the surface of the host semiconductor has presented an important obstacle to more direct measurements of microscopic electronic structure in the quantum Hall regime. Here we introduce a cryogenic scanning-probe technique—‘subsurface charge accumulation’ imaging—that permits very high resolution examination of systems of mobile electrons inside materials. We use this technique to image directly the nanometre-scale electronic structures that exist in the quantum Hall regime.

Scanning tunneling microscopy of defect states in the semiconductor Bi2Se3

Scanning tunneling spectroscopy images of Bi2Se3 doped with excess Bi reveal electronic defect states with a striking shape resembling clover leaves. With a simple tight-binding model, we show that the geometry of the defect states in Bi2Se3 can be directly related to the position of the originating impurities. Only the Bi defects at the Se sites five atomic layers below the surface are experimentally observed. We show that this effect can be explained by the interplay of defect and surface electronic structure. Understanding the electronic properties of defects and the ability to control them will be crucial for the performance of the future microelectronic devices. 1 Scanning tunneling microscopy ~STM! represents a unique tool for the studies of defects as it combines atomic scale resolution with local spectroscopic capability. However, STM observation and analysis of defect states in semiconductors are complicated by surface effects such as in-gap surface states and reconstruction. These effects are avoided at the ~110! surfaces of a number of III-V semiconducting systems, 2 attracting extensive research. 3‐ 8 A number of point defect types have been observed. However, the positions of these defects with respect to the surface plane could be inferred only from indirect observations. The interpretation of such observations is complicated by the drastic effect the surface proximity may have on the defect states. 9 Modeling STM measurements of defects in semiconductors is not straightforward: Approximation of the STM images by maps of the local surface electronic density of states 10 is justified only if the charge relaxation rates of defect states significantly exceed the tunneling rate of electrons between the tip and the sample. 11 Tip-induced effects also need to be taken into account. These may include both local band bending, 3 and charging of the defect states by the tunneling current, resulting in bias voltage-dependent lattice relaxation in the vicinity of the defect atoms. 8 Careful analysis is necessary to clearly separate these effects from the intrinsic defect properties, and the bulk features of the observed defect states from the surface effects.

Thermally activated charge transport in microbial protein nanowires.

The bacterium Geobacter sulfurreducens requires the expression of conductive protein filaments or pili to respire extracellular electron acceptors such as iron oxides and uranium and to wire electroactive biofilms, but the contribution of the protein fiber to charge transport has remained elusive. Here we demonstrate efficient long-range charge transport along individual pili purified free of metal and redox organic cofactors at rates high enough to satisfy the respiratory rates of the cell. Carrier characteristics were within the orders reported for organic semiconductors (mobility) and inorganic nanowires (concentration), and resistivity was within the lower ranges reported for moderately doped silicon nanowires. However, the pilus conductance and the carrier mobility decreased when one of the tyrosines of the predicted axial multistep hopping path was replaced with an alanine. Furthermore, low temperature scanning tunneling microscopy demonstrated the thermal dependence of the differential conductance at the low voltages that operate in biological systems. The results thus provide evidence for thermally activated multistep hopping as the mechanism that allows Geobacter pili to function as protein nanowires between the cell and extracellular electron acceptors.

Local atomic structure and discommensurations in the charge density wave of CeTe3.

The local structure of in the incommensurate charge density wave (IC-CDW) state has been obtained using atomic pair distribution function analysis of x-ray diffraction data. Local atomic distortions in the Te nets due to the CDW are larger than observed crystallographically, resulting in distinct short and long Te-Te bonds. Observation of different distortion amplitudes in the local and average structures is explained by the discommensurated nature of the CDW, since the pair distribution function is sensitive to the local displacements within the commensurate regions, whereas the crystallographic result averages over many discommensurated domains. The result is supported by STM data. This is the first quantitative local structural study within the commensurate domains in an IC-CDW system.

A simple low-dissipation amplifier for cryogenic STM

A current sensitive preamplifier designed for low-temperature scanning tunneling microscopy applications is presented. It combines the dc current measurement necessary for the feedback loop operation with a low noise ac measurement used for spectroscopy. The active device is a high electron mobility transistor which was chosen for its low input capacitance and excellent low-temperature performance. The power dissipation of the transistor can be kept at about 10 μW making it compatible with a variety of cryogenic systems. The ac current sensitivity is about 4 fA/√Hz at 4.2 K.

Scanning-probe spectroscopy of semiconductor donor molecules

Semiconductor devices continue to press into the nanoscale regime, and new applications have been proposed for which a single dopant atom acts as the functional part of the device1,2,3. Moreover, because shallow donors and acceptors are analogous to hydrogen atoms, experiments on small numbers of dopants have the potential to be a testing ground for fundamental questions of atomic and molecular physics4,5. Although dopant properties are well understood with respect to the bulk, the study of configurations of dopants in small numbers is an emerging field6,7. Here we present local capacitance measurements of electrons entering silicon donors in a gallium arsenide heterostructure. To the best of our knowledge, this study is the first example of single-electron capacitance spectroscopy carried out directly with a scanning probe tip8. The precise position with respect to tip voltage of the observed single-electron peaks varies with the location of the probe, reflecting a random distribution of silicon within the donor plane. In addition, three broad capacitance peaks are observed independent of the probe location, indicating clusters of electrons entering the system at approximately the same voltages. These broad peaks are consistent with the addition energy spectrum of donor molecules, effectively formed by nearest-neighbour pairs of silicon donors.

High-scan-range cryogenic scanning probe microscope

We have designed and constructed a scanning probe microscope operable at temperatures down to 260 mK within a top-loading helium-3 cryostat. It achieves a large scan range with the sample situated near the bottom of the scanning head—maximizing the cooling efficiency of the liquid helium. The scan head is completely thermally compensated, thus eliminating thermal expansion and contraction on cooling and warm-up, as well as thermal drift during operation. We demonstrate the performance using two distinct scanning probe methods: scanning tunneling microscopy and charge accumulation imaging.

Imaging of low-compressibility strips in the quantum Hall liquid

Using subsurface charge accumulation scanning microscopy, we image strips of low compressibility corresponding to several integer quantum Hall filling factors. We study in detail the strips at Landau level filling factors

Single-electron capacitance spectroscopy of individual dopants in silicon

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Modeling subsurface charge accumulation images of a quantum hall liquid

and 4. The observed strips appear significantly wider than predicted by theory. We present a model accounting for the discrepancy by considering a disorder-induced nonzero density of states in the cyclotron gap.