Post-transition-metal oxides: In2O3, Cdo, ZnO

The electronic structure of post-transition metal-oxide semiconductors (such as CdO, In2O3 , SnO2 , and Sn-doped In2O3 ,) has been a subject of enduring interest due to the potential use of such oxides as transparent electrical conductors. One important feature of the electronic structure of transition-metal oxides is the hybridization between metal d orbitals and ligand oxygen p orbitals. Band structure theories indicate the strength of the hybridization that can occur in such systems, but observation of such hybridization has only been possible indirectly, via spectroscopies such as photoemission. Soft x-ray emission spectroscopy (SXE) has
recently been shown to be a technique that allows the direct measurement of the hybridization of metal d orbitals with ligand p orbitals.

Here we report a comprehensive SXE and SXA study of the valence and conduction band electronic structure of ZnO, CdO, In2O3 , SnO2 and Sn-doped In2O3 (Sn-doped In2O3 is
commonly referred to as ITO). For each oxide, we have measured the O 2p valence and conduction band PDOS. We also directly observed shallow core-level hybridization in each of these oxides. The results of our measurements are compared to theory.


ZNO

ZnO crystallizes in the wurtzite structure (1) and has a band gap of 3.4 eV.14 Many first principles calculations of ZnO have been published, focusing in particular on the electronic and structural properties. It is clear from both the calculations
and results of photoemission experiments that the zinc d electrons interact strongly with the oxygen p electrons...

...In general for these oxides, the O 2p valence band feature observed at higher emission energy is that of oxygen 2p bands of nonbonding character, while the lower emission energy peak is those bands formed from oxygen 2p bonding orbitals mixing with the metal s states.

....There is less uniform agreement between the published calculations for the O 2p conduction band PDOS and our SXA measurements than we found between our valence band
SXE measurements and the same calculations. In particular, the calculations of Yang and Dy predict a gap in the conduction band PDOS between 7.5 and 10 eV above the VBM.
This is in contrast with both the observed SXA spectrum representing the O 2p conduction band PDOS and also the calculations of Schroder and Xu which both show a continuous density of states



Cdo

CdO is unusual among the IIB-VI binary compound semiconductors as it exists solely in a face-centered-cubic rocksalt structure which is more typical of the ionic insulators. All other Cd and Zn oxides and chalcogenides crystallize in either the cubic zinc-blende or the hexagonal wurtzite structure in which the metal is tetrahedally coordinated. In CdO the metal cation is octahedrally coordinated.The magnitude of the band gap in CdO is an issue of some controversy. The lowest energy indirect band gap of CdO is usually quoted to be 0.55 eV at room temperature and 0.84 eV at 100 K, while the direct band gap is 2. eV.28 However, previous photoemission experiments indicate that the gap at room temperature must be more than 1 eV. This assertion is based on the observation that the O 2p valence band onset in photoemission from oxygen deficient (vacuum annealed) CdO is, at 1.53 eV, relative to the Fermi-Dirac-like onset associated with electrons in the conduction
band. These electrons in the conduction band arise from oxygen vacancies in the vacuum annealed CdO, and have a carrier concentration of approximately 0.8531020 cm23.


In2O3

Indium oxide crystallizes in the bixbyite crystal structure and is a wide band gap semiconductor with a direct gap of 3.75 eV and an indirect gap of 2.6 eV.33 Relatively few band structure or tight binding calculations have been reported for
indium oxide, due in part to the 40-atom unit cell.


R. G. Egdell; Inorganic Chemistry Laboratory, Oxford University, Cormac McGuinnessl Boston University; Influence of shallow core-level hybridization on the electronic structure of post-transition-metal oxides studied using soft X-ray emission and absorption; PHYSICAL REVIEW B 68, 2003





(1) The wurtzite crystal structure, named after the mineral wurtzite, is a crystal structure for various binary compounds. It is an example of a hexagonal crystal system.

Among the compounds that can take the wurtzite structure are wurtzite itself, AgI, ZnO, CdS, CdSe, α-SiC, GaN, AlN, and other semiconductors. In most of these compounds, wurtzite is not the favored form of the bulk crystal, but the structure can be favored in some nanocrystal forms of the material.

Each of the two individual atom types forms a sublattice which is HCP-type (short for "hexagonal close-pack"). When viewed altogether, the atomic positions are the same as in lonsdaleite (hexagonal diamond). Each atom is tetrahedrally coordinated.

The wurtzite structure is non-centrosymmetric (i.e., lacks inversion symmetry). Due to this, wurtzite crystals can (and generally do) have properties such as piezoelectricity and pyroelectricity, which centrosymmetric crystals cannot.




Piezoelectricity
From Wikipedia


Piezoelectricity is the ability of some materials (notably crystals and certain ceramics, including bone) to generate an electric potential[1] in response to applied mechanical stress. This may take the form of a separation of electric charge across the crystal lattice. If the material is not short-circuited, the applied charge induces a voltage across the material. The word is derived from the Greek piezein, which means to squeeze or press.

The piezoelectric effect is reversible in that materials exhibiting the direct piezoelectric effect (the production of electricity when stress is applied) also exhibit the converse piezoelectric effect (the production of stress and/or strain when an electric field is applied). For example, lead zirconate titanate crystals will exhibit a maximum shape change of about 0.1% of the original dimension.

The effect finds useful applications such as the production and detection of sound, generation of high voltages, electronic frequency generation, microbalances, and ultra fine focusing of optical assemblies. It is also the basis of a number of scientific instrumental techniques with atomic resolution, the scanning probe microscopies such as STM, AFM, MTA, SNOM etc, as well as more mundane uses including acting as the ignition source for cigarette lighters.

Megasonic cleaning uses the piezoelectric effect to enable removal of submicrometre particles from substrates. A ceramic piezoelectric crystal is excited by high-frequency AC voltage, causing it to vibrate. This vibration generates an acoustic wave that is transmitted through a cleaning fluid, producing controlled cavitation. As the wave passes across the surface of an object, it causes particles to be removed from the materials being cleaned.


Pyroelectricity

Pyroelectricity is the ability of certain materials to generate an electrical potential when they are heated or cooled. As a result of this change in temperature, positive and negative charges move to opposite ends through migration (i.e. the material becomes polarized) and hence, an electrical potential is established.


CRYSTAL CLASSES

Of the thirty-two crystal classes, twenty-one are non-centrosymmetric (not having a centre of symmetry), and of these, twenty exhibit direct piezoelectricity (the 21st is the cubic class 432). Ten of these are polar (i.e. spontaneously polarize), having a dipole in their unit cell, and exhibit pyroelectricity. If this dipole can be reversed by the application of an electric field, the material is said to be ferroelectric.