Session E51: Dopants and Defects in Semiconductors: Theory

Beyond Graphene: Electronic and Mechanical Properties of Defective 2-D Materials

One of the challenges in the production of 2-D materials is the synthesis of defect free systems which can achieve the desired properties for novel applications. However, the reality so far indicates that we need to deal with defective systems and understand their main features in order to perform defect engineering in such a way that we can engineer a new material. In this talk I discuss first, the introduction of defects in a hierarchic way starting from 2-D graphene to form giant Schwarzites or graphene foams, which also can exhibit further defects, thus we can have several levels of defectiveness. In this context, it will be shown that giant Schwarzites, depending on their symmetry, can exhibit Dirac-Fermion behavior and further, possess protected topological states as shown by other authors. Regarding the mechanical properties of these systems, it is possible to tune the Poisson Ratio by the addition of defects, thus shedding light to the explanation of the almost zero Poisson ratios in experimentally obtained graphene foams. Second, the idea of Haeckelites, a planar sp2 graphene-like structure with heptagons and pentagons, can be extended to transition metal dichalcogenides (TMDs) with square and octagonal-like defects, finding semi-metallic behaviors with Dirac-Fermions, and even topological insulating properties.   

Scalable, Composable Operators for Defect Analysis and Design

Defect structures in semiconductors can profoundly affect electronic properties through electron-phonon interactions. Knowledge of the changes to phonon properties induced by defects is vital for understanding phonon effects on electronic behavior. Defects can cause the dominant phonon peaks in the spectrum to split and shift resulting in carriers that can scatter in energy levels not foreseen in the bulk. We have developed a novel mathematical and computational framework for estimating the phonon spectra in the harmonic approximation for lattices containing arbitrary defect structures. Linear operators are used to calculate defective phonon spectra directly from the spectrum of a pristine reference lattice. The primary benefit is that the full eigensolve must only be performed once for the reference defect-free crystal. The operators can be adjusted to vary the defect concentration, defective mass ratio, and/or defective potential subsequently without having to re-evaluate the eigensystem. In the talk, we will present the theory behind our methodology and initial results about the sensitivity of electro-thermal properties of semiconductors to various point defects.   

First principle study of the role of oxygen non-stoichiometry in the structure and properties of amorphous InO and InGaO.

Ab-initio molecular dynamics liquid-quench simulations of amorphous In-O and In-Ga-O are performed to investigate the structural, electronic and optical properties of these oxides. A new approach is developed to study the formation of oxygen defects in the amorphous oxides and their role in carrier generation and transport. First, the effect of oxygen non-stoichiometry on the local structure (i.e, the average Metal-Oxygen bond length and coordination) as well as on the long-range structural characteristics (i.e., the average M-M distance and the M-O-M angle) is discussed. The latter determines how the MO polyhedra are connected into a continuous network. Presence of Ga has a significant effect on the long-range structural correlations in non-stoichiometric structures. Further, the structural properties of oxygen defects as well as their effect on the electronic band structure is investigated. The results reveal charge accumulation on metal-metal bond(s) near the Fermi level suggesting the existence of two types of oxygen defect. Finally, strongly localized states near the valence band maximum originate from O$_{\mathrm{2}}$ bonds and from charge imbalance associated with low coordinated oxygen atom surrounded by highly coordinated metal atoms.   

Polarization effects in silver delafossite systems

Delafossites are a promising class of materials which has applications in catalysis and optoelectronic devices. Even though much work has been carried out on the cuprate family of delafossites, little is known about the structural and electronic properties of it's silver counterpart. In this work, we present a computational study for two delafossite oxides of the form $AgB_{1-x}Fe_{x}O_{2}$(For B = Al,Ga). A large number of structures are studied by varying the Fe alloying percentage(x) from 0 to 5 and by choosing the impurity sites randomly. We find that the local structural changes occurring at the vicinity of Fe atoms in these two systems have opposite trends with regard to the O-O distance. The reason for this difference in the trends is identified as the polarization effects on the inter-atomic distances caused by the displacements in O atoms resulting from the incorporation of Fe in sites, previously occupied by either Al or Ga. We believe that these effects are mediated by the differences in the atomic radii of Fe, Al and Ga. Higher alloying levels coupled with nearest neighbor Fe atoms can intensify these distortions in the structure creating deformations in the O-Ag-O bonds, which are directly related to the formation of the conduction band edge in these systems.   

Discriminating a deep defect from shallow acceptors in supercell calculations: gallium antisite in GaAs

To make reliable first principles predictions of defect energies in semiconductors, it is crucial to discriminate between effective-mass-like defects---for which existing supercell methods fail---and deep defects---for which density functional theory calculations can yield reliable predictions of defect energy levels. The gallium antisite Ga$_{As}$ is often associated with the 78/203 meV shallow double acceptor in Ga-rich gallium arsenide. Within a framework of level occupation patterns, analyses of structure and spin stabilization can be used within a supercell approach to distinguish localized deep defect states from shallow acceptors such as B$_{As}$. This systematic analysis determines that the gallium antisite is inconsistent with a shallow state, and cannot be the 78/203 shallow double acceptor. The properties of the Ga antisite in GaAs are described, predicting that the Ga antisite is a deep double acceptor and has two donor states, one of which might be accidentally shallow. --- Sandia National Laboratories is a multi-program laboratory managed and operated by Sandia Corporation, a wholly owned subsidiary of Lockheed Martin Company, for the U.S. Department of Energy’s National Nuclear Security Administration under contract DE-AC04-94AL85000.   

{\em Ab Initio} Modeling of Transition-Metal Impurities in MgO

Fe- and Ni-doped MgO are promising materials for the catalytic conversion of methane and CO$_2$. However, theoretical studies of these materials are scarce. The self-interaction error (SIE) in approximate DFT leads to an incorrect description of the electron localization and hybridization between $d$ states of Ni or Fe and the oxide electronic bands. Replacing a fraction $\alpha$ of the (semi-)local exchange by the exact exchange reduces the SIE, but $\alpha$ remains a parameter depending on the target property. We explore the dependence of the formation energies of Ni$_{\rm Mg}$ and Fe$_{\rm Mg}$ substitutional defects in MgO on $\alpha$ in the Heyd-Scuseria-Ernzerhof hybrid functional (HSE), and compare the results to CCSD(T) embedded-cluster calculations. For Ni$_{\rm Mg}$ defects HSE($\alpha$ = 0.3) reproduces CCSD(T) formation energies and CO adsorption energies on Ni$_{\rm Mg}$. However, $\alpha $ = 0.48 is needed in the case of Fe$_{\rm Mg}$. For both Ni$_{\rm Mg}$ and Fe$_{\rm Mg}$, $\alpha$ = 0.44-0.50 satisifies best the exact DFT condition that the HOMO does not depend on occupation. Contrary to PBE and HSE06, HSE($\alpha $ $\approx$ 0.5) reproduces the experimentally observed $O_{h} \rightarrow D_{4h}$ (oblate) Jahn-Teller distortion for Fe$_{\rm Mg}$.   

Phase Transformations upon Doping in Tungsten Trioxide

Tungsten trioxide (WO$_3$) is an emerging semiconductor material, with a growing number of applications in Li-ion batteries, photocatalysis, gas sensors and electrochromic devices. As an electrochromic material, WO$_3$ turns from transparent to blue upon doping with monovalent species. Due to it having an empty A-site in the ABO$_3$ perovskite structure, high doping concentrations are possible through intercalation. Tungsten trioxide has been experimentally shown to transform from the ground-state monoclinic symmetry to cubic symmetry with increasing monovalent doping [1]. We use first-principles calculations to understand this transformation. Our calculations show that the addition of electrons to the conduction band is a primary driver of the phase transformation. We quantify the energetics and structural aspects of this transformation using density functional theory, allowing us to elucidate the mechanism. Comparison with experiment, role of the dopant species, and implications of structural changes for device applications will be discussed. [1] Q. Zhong, J. Dahn, K. Colbow. \textit{Phys. Rev. B} \textbf{46} 2554 (1992).   

Electronic structure of intrinsic defects in non-stoichiometric amorphous In-Ga-Zn-O semiconductors

Amorphous oxide semiconductors, such as amorphous In-Ga-Zn-O (a-IGZO), have attracted much attention because of their use as a channel material in thin-film transistors (TFTs). Despite many advantages such as flexibility, transparency, and high electron mobility, a-IGZO based TFTs suffer from defects which cause the instability of threshold voltage under negative bias illumination stress (NBIS) as well as positive bias stress (PBS). Recently, we have proposed that O-vacancy and O-interstitial defects are responsible for the NBIS and PBS instabilities, respectively. In the previous studies, O-related defects were intentionally introduced in stoichiometric a-IGZO. Since the composition ratio is likely to be deviated from the ideal stoichiometry during fabrication, it is important to understand the electronic structure of non-stoichiometric a-IGZO. Here we perform density functional calculations to investigate the electronic structure of O-related defects in various a-IGZO systems with non-stoichiometric chemical compositions, which are generated through melt-and-quench molecular dynamics simulations. We consder both O-abundant and O-deficient samples and discuss the role of intrinsic defects in the device instability.   

Efficient n-type doping of MoO$_3$

MoO$_3$ is a layered material that has shown great promise as a transparent contact to organic photovoltaics and organic light-emitting diodes. It also exhibits photo- and electrochromic properties, and can be used as catalyst or sensor material. Due to its capability to reversibly accommodate Li it can be used in Li batteries. Despite these interesting properties, remarkably little is known about the properties of native defects and the possibilities of controlling the properties by intentional doping. We have used advanced hybrid functional calculations within density functional theory to investigate efficient doping strategies for this material. In particular, we explored the role of native defects in the conductivity of this material. To control the n-type conductivity and to obtain a larger amount of free carriers, intentional dopants can be used. We show that substitutional Re (on a Mo site) is a shallow donor. In contrast, substitutional doping with Mn, which also has one additional electron compared to Mo, leads to compensating behavior. Halogen impurities on the O site also act as shallow donors.   

The Role of Interface Defects on Phase Selection in ZnO/MgO Core/Shell Nanowires

ZnO nanowires, coated with MgO, have potential for optical devices. The optical properties of the ZnO/MgO coreshell structures depend on the properties of the interface. Both epitaxial interface with wurtzite MgO on ZnO and non-epitaxial interface with rocksalt MgO have been observed. Differences in MgO interface structure poses questions to what type of defects must be present to achieve these different crystal structures. Density functional theory calculations are carried out to explore the role of surface oxygen vacancies and the step edges. The results from DFT calculations show that oxygen vacancies have no effect on the crystal structure. On the other hand, the calculations show that the step edges make rocksalt MgO structure more favorable. TEM images taken on the ZnO nanowires that has rocksalt MgO coating revealed steps in the ZnO Nanowire, confirming the theoretical prediction.   

Structure of self-assembled Mn atom chains on Si(001)

Mn has been found to self-assemble into atomic chains running perpendicular to the surface dimer reconstruction on Si(001). They differ from other atomic chains by a striking asymmetric appearance in filled state scanning tunneling microscopy (STM) images. This has prompted complicated structural models involving up to three Mn atoms per chain unit. Combining STM, atomic force microscopy and density functional theory we find that a simple necklace-like chain of single Mn atoms reproduces all their prominent features, including their asymmetry not captured by current models. The upshot is a remarkably simpler structure for modelling the electronic and magnetic properties of Mn atom chains on Si(001).   

Electronic structures in SiC/SiO2 interface from first-principles calculation -Roles of peculiar electron states floating in internal space-

Silicon carbide (SiC) is a promising material for power electronic devices. We have reported that the wavefunction at the conduction-band minimum (CBM) of SiC ``floats” in internal space with continuum-state character [1]. By considering the floating nature of the CBM, drastic energy-level changes of CBM observed in SiC polytypes can be explained naturally [2]. Moreover, we have clarified that floating nature of CBM varies the effective masses in SiC. In this study, we have investigated how the electronic structure of CBM is modified in SiC/SiO2 interfaces, where the internal space is severely deformed, and how the floating electron state affects the material properties. We have found that we can realize 1 dimensional electron channels in the interface, and that the effective masses of CBM strongly depend on the interface structures. [1] Y. –i. Matsushita, S. Furuya, and A. Oshiyama, PRL, 108, 246404 (2012). [2] Y. –i, Matsushita, and A. Oshiyama, PRL 112 136403 (2014).   

Ab initio study of the effect of vacancies on the thermal conductivity

Point defects and vacancies in particular can have a profound impact on phonon thermal transport. Examples are seen in diamond [1] and cubic boron arsenide [2, 3] where large C and As vacancy concentrations give much lower thermal conductivity than expected [2, 3]. Here, we calculate the phonon-vacancy scattering rates using an \textit{ab initio }Green's function approach [1], which treats the scattering to all orders in contrast to standard perturbation theory approaches. The lattice thermal conductivity, k, is calculated from first principles by solving the Boltzmann transport equation for phonons, with interatomic force constants determined using density functional theory. The reduction in k with vacancy defect density is assessed. The phonon-vacancy scattering can show significant differences using the Green's function method compared to what would be predicted from the perturbative Born approximation, consistent with previous findings for diamond [1]. [1] N. A. Katcho J. Carrete, Wu Li and N. Mingo, Phys. Rev. B 90, 094117 (2014). [2] L. Lindsay, D. A. Broido and T. L. Reinecke, Phys. Rev. Lett. 111, 025901 (2013). [3] Bing Lv, et. al. Appl. Phys. Lett. 106, 074105 (2015).