It is demonstrated that the silicon atom dangling bond (DB) state serves as a quantum dot. Coulomb repulsion causes DBs separated by less, similar2 nm to exhibit reduced localized charge, which enables electron tunnel coupling of DBs. Scanning tunneling microscopy measurements and theoretical modeling reveal that fabrication geometry of multi-DB assemblies determines net occupation and tunnel coupling strength among dots. Electron occupation of DB assemblies can be controlled at room temperature. Electrostatic control over charge distribution within assemblies is demonstrated.

Structural and electronic properties of stoichiometric single-phase CrN(001) thin films grown on MgO(001) substrates by radio-frequency N plasma-assisted molecular-beam epitaxy, are investigated. In situ room-temperature scanning tunneling microscopy clearly shows the 1 ϫ 1 atomic periodicity of the crystal structure as well as long-range topographic distortions which are characteristic of a semiconductor surface. This semiconductor behavior is consistent with ex situ resistivity measurements over the range 285 K and higher, whereas below 260 K, metallic behavior is observed. The resistivity-derived band gap for the high-temperature region, 71 meV, is consistent with the tunneling spectroscopy results. The observed electronic (semiconductor/metal) transition temperature coincides with the temperature of the known coincident magnetic (para-antiferro) and structural (cubic-orthorhombic) phase transitions.For many years, CrN has received a lot of attention due to its high hardness and corrosion resistance. 1,2 In addition, CrN is also interesting due to its magnetic, optical, and electronic properties. It is known that CrN is paramagnetic (PM) with a B1 NaCl crystal structure at room temperature; at T Néel , reported in the range of 273-283 K, the material undergoes a phase transition to antiferromagnetic (aFM) with an orthorhombic P nma crystal structure. [3][4][5] found theoretically that the magnetic stress is the driving force for the lattice distortions, and thus linked the magnetic and structural transitions. So far, however, the electronic properties have been less understood.Filippetti et al. 5 reported, based on theoretical calculations, that CrN is a metal in its PM state but a weak metal in the aFM state. This agrees well with an earlier experimental work, based on CrN polycrystalline powders, which found metallic behavior with, however, an abrupt increase of the resistivity with increasing temperature at ϳ286 K. 7 On the other hand, Herle et al., 8 based on synthesized CrN powders, reported that CrN is a semiconductor with a band gap of 90 meV as measured by resistivity. Recently, Gall et al. 9 grew crystalline thin films of CrN by sputtering and reported that CrN behaves as a semiconductor (Mott-type insulator) with an optical gap of 0.7 eV. Although the electronic behavior at room temperature has become more clear, the value of the band gap as well as its detailed nature, remain to be determined; moreover, the discrepant results reported for low temperatures, remain to be resolved.In this letter, we present results concerning the unique structural and electronic properties of CrN(001). We show that high-quality epitaxial layers are grown by molecularbeam epitaxy (MBE), having crystalline (001) orientation and atomically smooth surface [as measured by scanning tunneling microscopy (STM)] and semiconducting behavior at 300 K. Bulk ex situ measurements confirm the semiconducting behavior at 300 K, but furthermore a transition to the metallic state is observed at ϳ285 K. This transition coincides with the ...

Alloy formation in ScGaN is explored using rf molecular beam epitaxy over the Sc fraction range x = 0-100%. Optical and structural analysis show separate regimes of growth, namely I) wurtzite-like but having local lattice distortions in the vicinity of the Sc Ga substitutions for small x (x ≤ 0.17), II) a transitional regime for intermediate x, and III) cubic, rocksalt-like for large x (x ≥ 0.54). In regimes I and III, the direct optical transition decreases approximately linearly with increasing x but with an offset over region II. Importantly, it is found that for regime I, an anisotropic lattice expansion occurs with increasing x in which a increases much more than c. These observations support the prediction of Farrer and Bellaiche [Phys. Rev. B 66, 201203-1 (2002)] of a metastable layered hexagonal phase of ScN, denoted h-ScN.

We study both experimentally and theoretically the electronic behavior of dangling bonds (DBs) at a hydrogen terminated Si(100)-2×1 surface. Dangling bonds behave as quantum dots and, depending on their separation, can be tunnel coupled with each other or completely isolated. On n-type highly doped silicon, the latter have a net charge of -1e, while coupled DBs exhibit altered but predictable filling behavior derived from an interplay between interdot tunneling and Coulomb repulsion. We found good correlation between many scanning tunneling micrographs of dangling bond structures and our theoretical results of a corresponding extended Hubbard model. We also demonstrated chemical methods to prevent tunnel coupling and isolate charge on a single dangling bond.

Experimental and theoretical results are presented regarding the incorporation of scandium into wurtzite GaN. Variation of the a and c lattice constants with Sc fraction in the low Sc concentration regime ͑0%-17%͒ are found that can be well explained by the predictions of first-principles theory. The calculations allow a statistical analysis of the variations of the bond lengths and bond angles as functions of Sc concentration. The results are compared to predictions from both a prior experimental study ͓Constantin et al., Phys. Rev. B 70, 193309 ͑2004͔͒ and a prior theoretical study ͓Farrer and Bellaiche et al. Phys. Rev. B 66, 201203͑R͒ ͑2002͔͒. It is found that the ScGaN lattice can be very well modeled as being wurtzitelike but with local lattice distortions arising from the incorporation of the Sc atoms. Effects of the addition of Sc on the stacking order for a large Sc fraction is also studied by high resolution transmission electron microscopy. The results show the existence of stacking faults, and induced stacking disorder. The explanation for the lattice constant variations is based on the effects of local lattice distortions and not related to the stacking faults.