Bulletin of the American Physical Society
APS March Meeting 2014
Volume 59, Number 1
Monday–Friday, March 3–7, 2014; Denver, Colorado
Session A44: Focus Session: Defects in Semiconductors: PV Materials |
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Sponsoring Units: DMP FIAP Chair: Shiyou Chen, China Eastern Normal University Room: Mile High Ballroom 4C |
Monday, March 3, 2014 8:00AM - 8:36AM |
A44.00001: Band alignment of zinc-blende and chalcopyrite semiconductors: Effects of misfit dislocations Invited Speaker: Fumiyasu Oba The band offset is a key quantity that largely determines electrical transport across the heterointerfaces in electronic devices and photovoltaic cells. Its accurate determination has therefore been one of the central issues in computational materials science. The band offset by nature depends on the atomistic and electronic structure of heterointerfaces. Assuming such dependences to be weak at interfaces composed of structurally and chemically similar materials, a band alignment diagram, where relevant materials are aligned using a common reference level, carries information of the band offsets. Quantities such as branch points from bulk calculations and ionization potentials from surface calculations, as well as band offsets explicitly obtained from interface calculations, have been used for the alignment, but the effects of misfit dislocations at semicoherent interfaces have been neglected. In this talk I will revisit the band alignment of zinc-blende and chalcopyrite semiconductors using semilocal and hybrid density functional calculations [1-4]. In particular, the effects of misfit dislocations on the band offsets are discussed for selected zinc-blende heterointerfaces via explicitly treating edge dislocation arrays in the calculations [1]. The variation in the electrostatic potential associated with the presence of misfit dislocations is found to be localized around the dislocation cores. The misfit dislocations typically affect the band offsets by only about 0.1 eV at a distance of 1 nm from the interfaces.\\[4pt] [1] Y. Hinuma, F. Oba, and I. Tanaka, Phys. Rev. B 88, 075319 (2013).\\[0pt] [2] Y. Hinuma, F. Oba, Y. Kumagai, and I. Tanaka, Phys. Rev. B 88, 035305 (2013).\\[0pt] [3] Y. Hinuma, F. Oba, Y. Nose, and I. Tanaka, J. Appl. Phys. 114, 043718 (2013).\\[0pt] [4] Y. Hinuma, F. Oba, Y. Kumagai, and I. Tanaka, Phys. Rev. B 86, 245433 (2012). [Preview Abstract] |
Monday, March 3, 2014 8:36AM - 8:48AM |
A44.00002: Stacking faults and lamellar twins with intrinsic point defects in poly-crystalline CdTe analyzed by density functional theory Christopher Buurma, Maria Chan, Tadas Pauluaskas, Robert Klie, Sivalingam Sivananthan Polycrystalline CdTe is a prominent photovoltaic material with proven industry success. To develop the next generation of thin film CdTe solar cells, higher open-circuit voltages and longer minority carrier lifetimes must be achieved. Playing a major role in doping, defect migration, recombination, and current transport are grain boundaries and other extended defects within grains of poly-crystalline CdTe. Commonly observed with STEM in CdTe are twins and stacking faults that extend throughout the entire grain. These twins can appear as lamellar repeating twins, or as single column stacking faults occurring in repetition near that of a Wurtzite structure. In this talk, we will use first principles density functional theory to investigate the thermodynamics and electronic structures such structures observed in STEM. The interaction energetics between adjacent twins and sets of twins are investigated. We will also investigate the likelihood of formation of neutral and charged native point defects in and near these extended defect structures. Binding energies of multiple point defects near such structures are also revealed. Implications towards PV efficiencies are discussed. [Preview Abstract] |
Monday, March 3, 2014 8:48AM - 9:00AM |
A44.00003: Origin of reduced efficiency in high Ga concentration Cu(In,Ga)Se$_{2}$ solar cell S.-H. Wei, B. Huang, H. Deng, M.A. Contreras, R. Noufi, S. Chen, L.W. Wang CuInSe$_{2}$ (CIS) is one of the most attractive thin-film materials for solar cells. It is well know that alloying Ga into CIS forming Cu(In,Ga)Se$_{2}$ (CIGS) alloy is crucial to achieve the high efficiency, but adding too much Ga will lead to a decline of the solar cell efficiency. The exact origin of this puzzling phenomenon is currently still under debate. Using first-principles method, we have systemically studied the structural and electronic properties of CIGS alloys. Our phase diagram calculations suggest that increasing growth temperature may not be a critical factor in enhancing the cell performance of CIGS under equilibrium growth condition. On the other hand, our defect calculations identify that high concentration of antisite defects M$_{Cu} $(M$=$In, Ga) rather than anion defects are the key deep-trap centers in CIGS. The more the Ga concentration in CIGS, the more harmful the deep-trap is. Self-compensation in CIGS, which forms 2V$_{Cu}+$M$_{Cu} $defect complexes, is found to be beneficial to quench the deep-trap levels induced by M$_{Cu}$ in CIGS, especially at low Ga concentration. Unfortunately, the density of isolated M$_{Cu}$ is quite high and cannot be largely converted into 2V$_{Cu}+$M$_{Cu}$ complexes under thermal equilibrium condition. Thus, nonequilibrium growth conditions or low growth temperature that can suppress the formation of the deep-trap centers M$_{Cu}$ may be necessary for improving the efficiency of CIGS solar cells with high Ga concentrations. [Preview Abstract] |
Monday, March 3, 2014 9:00AM - 9:12AM |
A44.00004: Defect Segregations at Grain Boundaries of CuInSe2 and Cu2ZnSnSe4 and Its Impact on Photovoltaic Performance Wanjian Yin, Yelong Wu, Su-Huai Wei, Rommel Noufi, Mowafak Al-Jassim, Yanfa Yan Grain boundaries (GBs) in absorber layers of polycrystalline thin-film solar cells play important roles in cell performance. In this presentation, we will review our recent results of density functional theory (DFT) study on the GB properties in solar cell materials including CuInSe2 (CIS) and Cu2ZnSnSe4 (CZTSe). We found that intrinsic GBs in these semiconductors are detrimental, probably due to the formation of deep gap states caused by wrong bonds. However, intrinsic defects and some extrinsic impurities have the tendency to segregate to grain boundaries. The segregations lead to two major effects: (1) passivating the deep defect states in the band gap by breaking or weakening the wrong bonding at GBs and (2) creating neutral hole barriers. The existence of Na$^{\mathrm{+}}_{\mathrm{i}}$ further induces the band bending and increases the hole barrier. Our results suggest benign GB properties in CIS. We further propose approaches to engineer GBs in CZTSe to improve its cell performance. [Preview Abstract] |
Monday, March 3, 2014 9:12AM - 9:24AM |
A44.00005: Stability and Electronic Structure of $Cu_{2} ZnSnS_{4} $ Surfaces: a First-Principles Study Peng Xu, Shiyou Chen, Bing Huang, Hong-Jun Xiang, Xin-Gao Gong, Su-Huai Wei Through the surface energy first-principles calculations, we studied the possible surface structures of the frequently observed cation-terminated (112) and anion-terminated ($\overline {112} )$ surfaces in various sample grown conditions. We found that the polar surfaces are stabilized by the charge-compensating defects, such as vacancies ($V_{Cu} $,$V_{Zn} )$, antisites ($Zn_{Cu} $,$Zn_{Sn} $ , $Sn_{Zn} )$ and defect clusters ($Cu_{Zn} +Cu_{Sn} $,$2Zn_{Cu} +V_{Sn} $ ). In stoichiometric single-phase CZTS samples, Cu-enriched defects are favored on (112) surfaces and Cu--depleted defects are favored on ($\overline {112}$) surfaces, while in non-stoichiometric samples grown under Cu poor and Zn rich conditions, both surfaces favor the Cu-depleted defects, which explains the observed Cu-deficiency on the surfaces of the synthesized CZTS thin films. The electronic structure analysis shows that Cu-enriched surfaces produce detrimental states in the band gap, while Cu-depleted surfaces produce no gap states and are thus benign to the solar cell performance. The calculated surface properties are consistent with experimental observation that Cu-poor and Zn-rich CZTS solar cells have higher efficiency. [Preview Abstract] |
Monday, March 3, 2014 9:24AM - 9:36AM |
A44.00006: Understanding the effects of dopant impurities on quaternary chalcogenide system properties by investigating and modeling local vibrational modes and Raman lineshapes Prashant Sarswat, Michael Free Cu$_{\mathrm{2}}$ZnSnS$_{\mathrm{x}}$Se$_{\mathrm{4-x}}$ (CZTSSe) has gained attention as a p-type absorber layer due to its attractive properties such as optimum band gap, high absorption coefficient, and use of low cost elements. However, impurities in CZTSSe produce detrimental effects, which limit the device performance. Phonon dispersion in most of the semiconductors is found to be susceptible to the pairing between atoms within the lattice. Hence, a change in phonon dispersion can be used to investigate the effects of foreign impurities on such pairing. Thus a series of experiments were conducted to investigate the effect of free holes on the optical phonons of doped CZTSSe system as well as to evaluate asymmetry in the Raman lineshape. When irradiated with photons, doped CZTS possibly produces a continuum of inter-valence band electronic excitations, which can envelop the Raman-active phonon energy. Such overlap between the electronic continuum and discrete state can cause interference effects in CZTSSe. It was observed that Raman lineshape becomes more asymmetric, wider, and shifts towards lower frequency when laser power density increased. All these observations were found for Raman A mode as well as E (TO, LO) mode for doped CZTSSe samples. [Preview Abstract] |
Monday, March 3, 2014 9:36AM - 9:48AM |
A44.00007: Defect formation enthalpy of Cu2ZnSnS4 and Cu2ZnSnSe4 revisited: ab initio insights into the limitations of CZTS technology Julien Vidal, Pawel Zawadzki, Vladan Stevanovic, Stephan Lany The defect physics of earth abundant quaternary compound Cu2ZnSnS4 (CZTS) and Cu2ZnSnSe4 (CZTSe) is extremely complex not only because of the many competing phases but also because of the many possible cationic substitutions. Previous theoretical studies have indicated that CZT(S,Se) has a quite high hole concentration originating from intrinsic defects such as Cu vacancies and Cu-on-Zn antisites. In this study, we have carried out state-of-the-art defect calculations including thermochemical corrections to the phase diagram and specific correction to the formation enthalpy of shallow defects. The latter was found to be a critical point in the analysis of intrinsic defects in both CZTS and CZTSe. Indeed, and at variance with previous studies, our GW-corrected ab initio defect calculation reveals that both Cu-on-Zn and Zn-on-Cu antisites have comparable formation enthalpy, which results in the pinning of the Fermi level in the mid-band gap region. The latter has two important consequences: the relatively low carrier concentration in CZT(S,Se) and the limitation of the open circuit voltage. It is also found that higher carrier concentrations are achievable under growth conditions where CZT(S,Se) is only marginally stable and may decompose into binary or ternary competing phases. [Preview Abstract] |
Monday, March 3, 2014 9:48AM - 10:00AM |
A44.00008: Origins of varying carrier concentration in Cu$_2$SnS$_3$ photovoltaic absorbers Lauryn Baranowski, Pawel Zawadzki, Stephan Lany, William Tumas, David Ginley, Eric Toberer, Andriy Zakutayev Within the Cu-Sn-S family of earth abundant photovoltaic absorbers, the Cu$_2$SnS$_3$ phase is predicted to be the most promising absorber material [P. Zawadzki, et al.]. To date there has been limited synthetic work on the Cu$_2$SnS$_3$ phase, particularly the carrier concentration. In this study, we develop an understanding of the effects of RF sputtering growth conditions on the hole concentrations of Cu$_2$SnS$_3$ absorber films, and use these results to identify the underlying causes of the observed variations in carrier concentration. Two effects are identified that control the carrier concentration in Cu$_2$SnS$_3$ films. The first effect, which occurs during Cu-rich growth, is isostructural alloying with a metallic Cu$_3$SnS$_4$ phase, which gives rise to hole concentrations above 10$^{19}$ cm$^{-3}$. The second effect is that, when the Cu$_2$SnS$_3$ films are grown under Sn-rich conditions, varying the S chemical potential during film deposition gives 10$^{18}$-10$^{19}$ cm$^{-3}$ holes. This variation in carrier concentration with S chemical potential can be explained by a Cu vacancy defect model. Understanding the origins of the varying doping density in Cu$_2$SnS$_3$ films allows for targeted growth to achieve desired carrier concentrations for device integration. [Preview Abstract] |
Monday, March 3, 2014 10:00AM - 10:12AM |
A44.00009: Observation of Coulomb repulsion between Cu intercalants in Cu$_{\mathrm{x}}$Bi$_{2}$Se$_{3}$ Christopher Mann, Damien West, Ireneusz Miotkowski, Yong Chen, Shengbai Zhang, Chih-Kang Shih Using scanning tunneling microscopy and \textit{ab initio }simulations, we have identified several configurations for Cu-dopants in Cu$_{\mathrm{x}}$Bi$_{2}$Se$_{3}$, with Cu intercalants being the most abundant. Through statistical analysis, we show strong short-range repulsive interactions between Cu intercalants. At intermediate range (\textgreater 5nm), the pair distribution function shows oscillatory structure along the \textless 1 0 -1\textgreater\ directions which appears to be influenced by different diffusion barriers along the \textless 1 0 -1\textgreater\ and \textless 2 -1 -1\textgreater\ directions. [Preview Abstract] |
Monday, March 3, 2014 10:12AM - 10:24AM |
A44.00010: Interface effects on calculated defect levels for oxide defects Arthur Edwards, Hugh Barnaby, Peter Schultz, Andrew Pineda Density functional theory (DFT) has had impressive recent success predicting defect levels in insulators and semiconductors [Schultz and von Lillienfeld, 2009]. Such success requires care in accounting for long-range electrostatic effects. Recently, Komsa and Pasquarello have started to address this problem in systems with interfaces. We report a multiscale technique for calculating electrostatic energies for charged defects in oxide of the metal-oxide-silicon (MOS) system, but where account is taken of substrate doping density, oxide thickness, and gate bias. We use device modeling to calculate electric fields for a point charge a fixed distance from the interface, and used the field to numerically calculate the long-range electrostatic interactions. We find, for example, that defect levels in the oxide do depend on both the magnitude and the polarity the substrate doping density. Furthermore, below 20 {\AA}, oxide thickness also has significant effects. So, transferring results directly from bulk calculations leads to inaccuracies up to 0.5 eV-- half of the silicon band gap. We will present trends in defect levels as a function of device parameters. We show that these results explain previous experimental results, and we comment on their potential impact on models for NBTI. [Preview Abstract] |
Monday, March 3, 2014 10:24AM - 10:36AM |
A44.00011: A comprehensive ab initio study of doping in bulk ZnO with group V elements Guido Petretto, Fabien Bruneval Zinc-oxyde, despite being a promising candidate for several electronic applications, up to now has provided several challenges to the scientific community, both from an experimental and theoretical point of view [1]. In fact, a reliable p-type doping still has not been achieved and standard density functional theory (DFT) calculations has often provided unsatisfactory results and failed to help in the search for better configurations to obtain such property. To solve the band gap underestimation problem we have made use of the HSE hybrid functional[2], tuning the admixing parameter to match the experimental band gap. Within this framework, we extensively studied the formation and transition energies of group V elements related defects. These include simple substitutional defects X$_\textrm{O}$, X$_\textrm{Zn}$ (X=N, P, As, Sb) and complexes of the form X$_\textrm{Zn}$-2V$_\textrm{Zn}$ and X$_\textrm{Zn}$-V$_\textrm{Zn}$. The stability of these complexes is also addressed. We show that it is unlikely to obtain good acceptor states from these elements due to deep transition energies and the presence of donor-like defects. [1]Avrutin, V. et al., Proceedings of the IEEE, 98, 1269 (2010) [2]Heyd, J. et al., Journal of chemical physics 118, 8207 (2003) [Preview Abstract] |
Monday, March 3, 2014 10:36AM - 10:48AM |
A44.00012: Identifying microscopic mechanisms for hole traps in nitride heterostructures John Lyons, Luke Gordon, Anderson Janotti, Chris G. Van de Walle Some recent designs of nitride semiconductor devices employ heterostructures (such as N-face high-electron-mobility transistors) in which the electronic Fermi level is established near the valence-band maximum due to the influence of polarization fields. In many of these heterostructures, the presence of hole-trapping centers is thought to adversely affect device performance. This behavior has been observed in many different types of devices, and its physical origin remains unknown. Using first-principles calculations based on a hybrid functional, we investigate possible origins for this phenomenon. We explore both intrinsic defect candidates as well as impurities. With Schr\"{o}dinger-Poisson simulations, we then investigate how the behavior of these species and their spatial distribution within the heterostructure layers is reflected in the performance of nitride semiconductor devices. [Preview Abstract] |
Monday, March 3, 2014 10:48AM - 11:00AM |
A44.00013: Electronic and optical properties of GaSb:N from first principles Priyamvada Jadaun, Hari Nair, Vincenzo Lordi, Seth Bank, Sanjay Banerjee We present an ab-initio study of dilute nitride III-Vs, focusing on dilute nitride GaSb (GaSb:N). GaSb:N displays promise towards realization of optoelectronic devices accessing the mid-infrared wavelength regime. Theoretical and experimental results on its electronic and optical properties are however few. To address this, we present a first principles, density functional theory study using the hybrid HSE06 exchange-correlation functional of GaSb doped with 1.6\% nitrogen. We conduct a comparative study on GaAs:N, also with 1.6\% nitrogen mole fraction, and find that GaSb:N has a smaller band gap and displays more band gap bowing than GaAs:N. In addition we examine the orbital character of the bands, finding the lowest conduction band to be quasi-delocalized, with a large N-3s contribution. At high concentrations, the N atoms interact via the host matrix, forming a dispersive band of their own which governs optoelectronic properties and dominates band gap bowing. While this band drives the optical and electronic properties of GaSb:N, its physics is not captured by traditional models for dilute-nitrides. We thus propose that a complete theory of dilute-nitrides should incorporate orbital character examination, especially at high N concentrations. [Preview Abstract] |
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