A combination of scanning tunneling microscopy and spectroscopy and density functional theory is used to characterize excess electrons in TiO2 rutile and anatase, two prototypical materials with identical chemical composition but different crystal lattices. In rutile, excess electrons can localize at any lattice Ti atom, forming a small polaron, which can easily hop to neighboring sites. In contrast, electrons in anatase prefer a free-carrier state, and can only be trapped near oxygen vacancies or form shallow donor states bound to Nb dopants. The present study conclusively explains the differences between the two polymorphs and indicates that even small structural variations in the crystal lattice can lead to a very different behavior.

Hexagonal BN (h-BN) is attracting a lot of attention for two-dimensional electronics and as a host for single-photon emitters. We study the properties of native defects and impurities in h-BN using density functional theory with a hybrid functional. Native vacancy and antisite defects have high formation energies, and are unlikely to form under thermodynamic equilibrium for typical growth conditions. Self-interstitials can have low formation energies when the Fermi level is near the band edges, and may form as charge compensating centers; however, their low migration barriers render them highly mobile, and they are unlikely to be present as isolated defects. The defect chemistry of h-BN is most likely dominated by defects involving carbon, oxygen, and hydrogen impurities. Substitutional carbon and oxygen, as well as interstitial hydrogen and boron vacancy-hydrogen complexes, are low-energy defects in h-BN. Based on our results, we can rule out several proposed sources for defect-related luminescence in h-BN. In particular, we find that the frequently observed 4.1 eV emission cannot be associated with recombination at CN, as has been commonly assumed. We suggest alternative assignments for the origins of this emission, with CB as a candidate. We also discuss possible defect origins for the recently observed single-photon emission in h-BN, identifying interstitials or their complexes as plausible centers.

Basal-plane stacking faults in wurtzite AlN, GaN, and InN are studied using density-functionalpseudopotential calculations. The formation energies follow the trend exhibited for the zinc-blende/wurtzite energy differences in the bulk materials, namely, lowest energy for GaN and highest for AlN. Type-I stacking faults have the lowest energy, followed by type-II stacking faults, and finally extrinsic stacking faults. We also examine a type of intrinsic stacking fault that has not, to the best of our knowledge, been previously discussed; its energy is slightly lower than the type-II faults. Investigations of the electronic structure reveal that there are no localized states in the band gap. However, stacking faults can bound a quantum-well-like region of zincblende material surrounded by the wurtzite host, giving rise to a luminescence line below the wurtzite band gap. ͓S0163-1829͑98͒50224-X͔The III-nitrides have great potential for technological applications in the area of optoelectronic devices operating in the green, blue, and ultraviolet region of the spectrum, as well as for high-power and high-temperature electronics. A number of materials problems still exist, however, many of which are related to the lack of suitable substrates. Growth on mismatched substrates causes the epilayers to contain a very high concentration of extended defects, up to five orders of magnitude higher than in other materials used for optoelectronic devices. 1 Light-emitting diodes and lasers can be fabricated in this material in spite of the high defect concentrations. However, the extent to which the extended defects affect quantum efficiencies and device lifetime is still unknown. Fundamental studies of the effect of extended defects on the electronic and optical properties of the material are necessary to address these issues. In particular, it is essential to know whether the defects give rise to electrically active levels in the band gap.Stacking faults are one of the main types of extended defects occurring in epitaxial III-V nitrides. Typically they are terminated at each end by partial dislocations, and transmission electron microscopy studies have reported a higher density of stacking faults near the film/substrate interface. [2][3][4][5] In this paper we use first-principles calculations to investigate the atomic and electronic structure and the formation energy of stacking faults along the ͓0001͔ direction in wurtzite ͑WZ͒ AlN, GaN, and InN. Our main conclusions are that ͑1͒ type-I stacking faults always have the lowest energies; ͑2͒ the formation energy increases going from GaN to InN to AlN; and ͑3͒ stacking faults introduce no localized states in the band gap, although they can give rise to a zinc-blendelike region that can trap electrons, as suggested by Albrecht et al. 6 We also introduce a type of intrinsic stacking fault, which has an energy higher than type-I faults but lower than the other types. Estimates of stacking-fault energies based on bulk calculations for polytypes were recently derived by Wright; 7 to our ...

We systematically measure the dielectric function of atomically thin MoS2 films with different layer numbers and demonstrate that excitonic effects play a dominant role in the dielectric function when the films are less than 5–7 layers thick. The dielectric function shows an anomalous dependence on the layer number. It decreases with the layer number increasing when the films are less than 5–7 layers thick but turns to increase with the layer number for thicker films. We show that this is because the excitonic effect is very strong in the thin MoS2 films and its contribution to the dielectric function may dominate over the contribution of the band structure. We also extract the value of layer-dependent exciton binding energy and Bohr radius in the films by fitting the experimental results with an intuitive model. The dominance of excitonic effects is in stark contrast with what reported at conventional materials whose dielectric functions are usually dictated by band structures. The knowledge of the dielectric function may enable capabilities to engineer the light-matter interactions of atomically thin MoS2 films for the development of novel photonic devices, such as metamaterials, waveguides, light absorbers, and light emitters.

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