Transition-metal impurities in semiconductors and heterojunction band lineups

Assessment of interstitial potentials for rapid prediction of absolute band energies in crystals

Electronic band alignment is a demanding process for first-principles simulations, but an important factor in materials selection for applications including electrocatalysis and photoelectrochemistry. Here, we revisit a bulk alignment procedure, originally developed by Frensley and Kroemer, using modern computational tools. The electrostatic potential in the interstitial region, obtained from density functional theory, with four exchange correlation functionals, is used to predict the valence band offsets of 27 zinc blende semiconductors. The results are found to be in qualitative agreement with Frensley and Kroemer's original data. In addition to absolute electron energies, the possibility of extracting effective ionic charges is investigated and compared to Bader partial charges. With further developments, such a procedure may support rapid screening of the bulk ionization potential and electron affinity of crystals, as we illustrate with an extension to rock salt and perovskite structure types.

Electrochemical Reduction of Undoped and Cobalt-Doped BiFeO3 Induced by Water Exposure: Quantitative Determination of Reduction Potentials and Defect Energy Levels Using Photoelectron Spectroscopy

The interaction of BiFeO₃ and Co-doped BiFeO₃ thin-film surfaces with water vapor is examined using photoelectron spectroscopy. Water exposure results in an upward shift of the Fermi energy, which is limited by the reduction of Bi and Fe in undoped BiFeO₃ and by the reduction of Co in oxidized Co-doped BiFeO₃. The results highlight the importance of surface potential changes induced by the interaction of solid surfaces with water and the ability of photoelectron spectroscopy to quantitatively determine electrochemical reduction potentials and defect energy levels.

Influence of oxygen vacancies on magnetic and transport properties of thin Zn1-xCoxOy films

The features of the structural, transport and magnetic properties of thin Zn1-xCoxOy films (x=0-0.45), fabricated on С-sapphire substrates by the pulsed laser deposition method are studied. It is found that the transport and ferromagnetic properties of the wurtzite Zn1-xCoxOy films nonmonotonously depend on Co concentration at room temperature. For the Zn0.87Co0.13Oy film, the strongest ferromagnetic signal is observed that is caused by formation of the greatest number of metallic Co clusters. A further increase of doping impurity concentration in the films leads to the oxidation of metallic Co and formation of the paramagnetic Co3O4 phase, in consequence of which the ferromagnetic signal subsides.

3d Transition Metals in II-VI Semiconductors

The magnetic and electronic properties of some I-VI semiconductors with 3d transition metals other than Mn are presented. For example, the nonzero orbital moment in Fe2+ leads to more complex electronic energy levels than for Mnt ions. In (CdFe)Se, inelastic light scattering experiments demonstrate that the ground state is nonmagnetic (Van Vleck ion), and directly measures the energy spacing between the three lowest levels. For Sc2+ ions the donor level lies above the bottom of the conduction band in the Cd-based materials.

Energetic pinning of magnetic impurity levels in quantum-confined semiconductors

Donor- and acceptor-type (D/A) impurities play central roles in controlling the physical properties of semiconductors. With continued miniaturization of information processing devices, the relationship between quantum confinement and D/A ionization energies becomes increasingly important. Here, we provide direct spectroscopic evidence that impurity D/A levels in doped semiconductor nanostructures are energetically pinned, resulting in variations in D/A binding energies with increasing quantum confinement. Using magnetic circular dichroism spectroscopy, the donor binding energies of Co2+ ions in colloidal ZnSe quantum dots have been measured as a function of quantum confinement and analyzed in conjunction with ab initio density functional theory calculations. The resulting experimental demonstration of pinned impurity levels in quantum dots has far-reaching implications for physical phenomena involving impurity-carrier interactions in doped semiconductor nanostructures, including in the emerging field of semiconductor spintronics where magnetic-dopant-carrier exchange interactions define the functionally relevant properties of diluted magnetic semiconductors.

Electronic structure origins of polarity-dependent high-TC ferromagnetism in oxide-diluted magnetic semiconductors

Future spintronics technologies based on diluted magnetic semiconductors (DMSs) will rely heavily on a sound understanding of the microscopic origins of ferromagnetism in such materials. Discoveries of room-temperature ferromagnetism in wide-bandgap DMSs hold great promise, but this ferromagnetism remains poorly understood. Here we demonstrate a close link between the electronic structures and polarity-dependent high-TC ferromagnetism of TM(2+):ZnO DMSs, where TM(2+) denotes 3d transition metal ions. Trends in ferromagnetism across the 3d series of TM(2+):ZnO DMSs predicted from the energies of donor- and acceptor-type excited states reproduce experimental trends well. These results provide a unified basis for understanding both n- and p-type ferromagnetic oxide DMSs.