Bulletin of the American Physical Society
APS March Meeting 2018
Volume 63, Number 1
Monday–Friday, March 5–9, 2018; Los Angeles, California
Session H11: Dopants and Defects in Semiconductors - 2D, Nano, and Novel MaterialsFocus
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Sponsoring Units: DMP DCOMP FIAP Chair: Cyrus Dreyer, Rutgers Univ Room: LACC 303A |
Tuesday, March 6, 2018 2:30PM - 3:06PM |
H11.00001: Engineering point and extended defects in transition metal dichalcogenides Invited Speaker: Hannu-Pekka Komsa Two-dimensional (2D) materials such as graphene, hexagonal boron nitride, and transition metal dichalcogenides have recently received lots of attention due to their unique material properties and numerous potential applications. The 2D atomic structure can also facilitate distinct defect formation mechanisms and offer new possibilities for defect engineering. |
Tuesday, March 6, 2018 3:06PM - 3:18PM |
H11.00002: Soft-Doping: A Strategy for 2D Materials Electronics Doron Naveh, Hadas Alon, chen stern, Moshe Kirshner, Ofer Sinai, Michal Wasserman, Ryan Selhorst, Raymond Gasper, Todd Emrick, Ashwin Ramasubramaniam One of the remaining challenges of 2D materials electronics is to achieve high lateral resolution of their chemical potential by doping. Doping of atomic layers by traditional methods such as ion implantation suffer from several deficiencies including their low cross-section and the vulnerability of 2D materials to impurities. In this work, we overcome this challenge by physical adsorption of polar zwitterion dopants, establishing a soft-hard matter interface. We demonstrate a high-resolution effective doping of graphene with a zwitterion co-polymer photoresist that can easily be patterned to arbitrarily predetermined structures. As an example, we demonstrate that this method preserves all structural and electronic properties of graphene and modifies its surface potential exclusively. |
Tuesday, March 6, 2018 3:18PM - 3:30PM |
H11.00003: Unveiling the Defect Structure of Localized Excitons in WSe2 Monolayer Shuai Zhang, Chen-Guang Wang, Ming-Yang Li, Di Huang, Lain-Jong Li, Wei Ji, Shiwei Wu Understanding and controlling defects in crystalline materials is an everlasting theme in material science and engineering. For two dimensional materials at atomic limit, defects are more influential and have come to the forefront in the pursuit of electronic and optoelectronic applications. In particular, intrinsic defects in semiconducting transition metal dichalcogenide (TMD) monolayers lower the carrier mobility and photoluminescence quantum yield despite of their direct semiconducting bandgap. Furthermore, defects in WSe2 monolayer host localized excitons that could behave as single quantum emitters (SQEs) at low temperature. However, the exact nature of these defects remains elusive. In this talk, I will introduce our recent progress in study of the point defect in WSe2 by using low temperature scanning tunneling microscopy and spectroscopy, in corroboration with density functional theory calculations. We observed the tungsten vacancy is the dominant defect in WSe2 and can form localized exciton. In the end of the talk, I will also discuss the relationship between the localized exciton trapped by the tungsten vacancy and the SQEs. We conjecture that the tungsten vacancy defect is a likely candidate, at least a precursor, for SQEs in the WSe2 monolayer. |
Tuesday, March 6, 2018 3:30PM - 3:42PM |
H11.00004: Impact of External Perturbations on Carbon Nanotube Electronic Structure Benjamen Taber, Jon Mills, Christian Gervasi, Madison Wright, George Nazin While single-walled carbon nanotubes (SWCNTs) can exhibit ballistic charge transport, their electronic behavior is highly sensitive to environmental effects. Environmental inhomgeneities, such as defects in the gate dielectric of a SWCNT transistor, can perturb the local electronic structure of the SWCNT, leading to localized states. These localized states may trap charge carriers, significantly degrading device performance. In this work, we use scanning tunneling microscopy and spectroscopy to investigate the impact of inhomegeneities in a dielectric on the local density of states (LDOS) of SWCNTs. We use a monolayer of RbI on Au(111) as our model dielectric, and find that anti-phase defects in the RbI lead to perturbations in the RbI LDOS, which generate spatially-confined mid-gap states in the SWCNT. We will also discuss recent density functional theory results examining the impact of external charges on SWCNT LDOS. |
Tuesday, March 6, 2018 3:42PM - 3:54PM |
H11.00005: Magneto-optical measurements of semiconducting nanocrystals doped with rare-earth ions Joseph Murphy, Subash Kattel, Ying-Hao Chien, Lance Kailey, Brian Leonard, William Rice The magneto-optical properties of semiconducting nanocrystals doped with transition metals, exhibiting strong spd spin-exchange effects, have been thoroughly studied; using rare earth elements as dopants offers a pathway to understanding spin-exchange interactions with electrons in f orbitals, instead. To study this interaction, there must be strong overlap between carrier and 4f dopant wavefunctions, while this is not possible in 2D or bulk semiconductors, it is ideal in 0D nanocrystals. This study uses Tb-doped CdSe nanocrystals which are characterized using three primary techniques to extract information about the spf interaction: magneto-photoluminescence, magnetic circular dichroism and time-resolved differential transmission. MCD measurements, a direct measure of the Zeeman splitting of excitonic states, exhibit a sign-flip, indicating the presence of a spf interaction. Magneto-PL measurements support this evidence through complementary data by instead studying the degree of circular polarization of the emitted light. Finally, time-resolved differential transmission measurements show a difference in carrier lifetimes between doped and undoped nanocrystals, further supporting the presence of spin-exchange interaction. |
Tuesday, March 6, 2018 3:54PM - 4:06PM |
H11.00006: Defect Formation on the Surface of Giant Rashba Systems Wenhan Zhang, Damien West, Lunyong Zhang, Sang-Wook Cheong, Shengbai Zhang, Weida Wu Polar discontinuity at interfaces of different materials may give nontrivial atomic or electronic properties. A well-known case is the interface between insulating LaAlO3 and SrTiO3 perovskites, which possesses electron gas with extremely high carrier mobility. No consensus has been reached regarding the origin of this conductive layer, but many studies indicate polar discontinuity plays a significant role. Recently, the family of BiTeX (X= I, Br, Cl) is reported as polar semiconductors with giant Rashba-type spin splitting effect. They have drawn intensive research interests since then because of the great potentials for practical spintronic functions. The bulk polar atomic corrugations give rise to significant polar discontinuities near the surface. Via scanning tunneling microscopy (STM), we found that the cleaving temperature can dramatically influence the defect formation on the surface of giant Rashba semiconductor BiTeCl. Here, we will present the detailed STM and DFT studies to understand the underlying mechanism. |
Tuesday, March 6, 2018 4:06PM - 4:18PM |
H11.00007: Perovskite-type Metal Halides as Luminescent Materials Koushik Biswas, Qingguo Feng, Byungkyun Kang Halide semiconductors and insulators broadly belonging to perovskite-type crystal structures have received considerable attention due to their range of properties which are suitable for a variety of applications. If electronic band gap is taken as one metric, these materials span values that are relevant for solar absorbers, semiconductor radiation detectors and scintillators, obtainable by virtue of different metal-halogen combinations. Here, we concern ourselves with luminescent materials covering 3-dimensional (3-D) ABX3-type compounds and the 0-D perovskite, Cs4PbBr6. The former consist of corner or edge-sharing octahedra such as in CsCaI3 and KCaI3, while the latter is obtained from almost disjointed octahedra connected only by Cs-Br bridges. Employing density functional methods, we study their structural differences which is reflected in distinctive electronic properties and formation of self-trapped carriers. We discuss these properties as they relate to radiative decay and light yield in gamma scintillator applications. We also speculate on the observed green emission in CsPbBr3/Cs4PbBr6 composites which may be related to excitonic luminescence. |
Tuesday, March 6, 2018 4:18PM - 4:30PM |
H11.00008: Atomic Models for Surface Stabilization of Cesium Lead Halide Perovskite Nanocrystals within Chemical Effect of Metal Halides Dongsuk Yoo, Sun Won Kim, Jae-Hyeon Ko, Yong-Hyun Kim Colloidal cesium lead halide perovskite nanocrystals (NCs) have been emerged as promising optoelectronic materials due to their unique optical properties and low-temperature solution processability. Despite the advantages, structural instability of the perovskite NCs leads to size and shape changes as well as phase transition, resulting in uncontrollable their physicochemical properties. Recent synthetic approach using metal halides made great progress on enhancing structural stability of the NCs. However, the influence of the additional metal halides on the perovskite NCs, especially on their surfaces, is not fully understood. |
Tuesday, March 6, 2018 4:30PM - 4:42PM |
H11.00009: Towards Quantitative Conductivity Measurements of Doped Silicon Nanowires with Infrared Near-Field Optical Microscopy Earl Ritchie, David Hill, Clayton Casper, Xiao You, James Cahoon, Joanna Atkin Performance in semiconducting structures, such as silicon nanowires (SiNWs), depends on nanometer-scale electronic and structural properties, including surface roughness, mobility, or dopant density. We report the use of mid-infrared scattering-type scanning near-field optical microscopy (s-SNOM) as a non-destructive optical method to extract nanoscale conductivity maps in semiconductors. Combining atomic force microscope (AFM) characterization with mid-infrared spectroscopic analysis, we can extract quantitative local dielectric properties. Using this technique, we can detect local changes in the electrically-active doping concentration from the free-carrier absorption in both n-type and p-type doped SiNWs. The high spatial resolution (< 20 nm) allows us to directly measure free-carrier concentrations arising from B and P dopants in single and multi-junction SiNWs. Combined with finite element analysis, we can analyze the local carrier concentration and mobility, and correlate nanoscale variations in doping, strain, crystallinity and structure with growth conditions. This capability will enable the use of s-SNOM as an advanced platform for exploratory research and practical characterization of semiconducting structures. |
Tuesday, March 6, 2018 4:42PM - 4:54PM |
H11.00010: Self-interaction correction calculations of band edges and alignments in low-dimensional transition metal chalcogenides and their heterostructure Hyo Seok Kim, Yong-Hoon Kim In view of realizing advanced devices based on newly emerging low-dimensional materials and their heterostructures, prediction of band edges and their alignment is currently a primary target of first-principles calculations. Because of the self-interaction error, the conventional LDA and GGA within density functional theory (DFT) perform poorly in predicting band lineup and defect states. Although adopting hybrid DFT functionals or the many-body GW method could be in principle a solution, their computational cost is too high to be routinely employed. Self-interaction correction approaches could provide a practical solution for this problem, and we particularly consider the atomic self-interaction correction (ASIC) method [1]. While previous ASIC studies typically used a globally fixed ASIC parameter (α), we recently showed that selecting different α according to atomic species could provide more reliable defect level locations for transition metal dichalcogenide (TMDC)/graphene interfaces [2]. Here, we extend the study and apply the method to various TMDC/TMDC and TMDC/graphene heterostructures, as well as low-dimensional Cd and Zn chalcogenides. |
Tuesday, March 6, 2018 4:54PM - 5:06PM |
H11.00011: Effect of Impurities on Properties of Tl6SeI4 Semiconductor for Hard Radiation Detection Oleg Kontsevoi, Wenwen Lin, Bruce Wessels, Mercouri Kanatzidis The semiconductor Tl6SeI4 is a promising candidate material for room temperature X-ray and γ-ray detection. The carrier transport and detection performance strongly depend on concentration of impurity levels acting as carrier trapping and scattering centers. The main impurities in Tl6SeI4 were identified as O, C, Si, Sn, Al, Pb, Bi, Cl, Br, S and Te. To assess the effect of impurities on detector performance, theoretical calculations were performed based on first-principles density functional theory (DFT) including nonlocal exchange-correlation functionals. The results show that Al and Bi introduce deep defect levels in the band gap and can be detrimental to the detector performance of Tl6SeI4. Si impurities can act as deep donors if Tl6SeI4 is grown in Tl-rich/Se-poor conditions, but become electrically benign in Tl-poor/Se-rich conditions, Pb and Sn are shallow donors, and Cl, Br, S, Te impurities are inactive. Interstitial O introduces trapping levels near the middle of the gap, but will become electrically inactive if its concentration is reduced below the level of intrinsic Se vacancies, in which case O atoms will occupy Se sites. The effect of various chemical environments on defect properties was examined and the optimal conditions for material synthesis were suggested. |
Tuesday, March 6, 2018 5:06PM - 5:18PM |
H11.00012: The Origin of Localization in Ti-Doped Silicon Yi Zhang, Ryky Nelson, Ka-Ming Tam, Tom Berlijn, Wei Ku, Un jong Yu, N S Vidhyadhiraja, Juana Moreno, Mark Jarrell Intermediate band semiconductors hold the promise to significantly improve the efficiency of solar cells, but only if the impurity band is metallic. We apply our recently developed first principles method [1-4] to investigate the nature of localization in Ti-doped Si, a promising candidate for intermediate band solar cells. Our results show that, contrary to common belief, Anderson localization plays a more important role than Mott localization in the metal-insulator transition. Implications for the theory of intermediate band solar cells will be discussed. |
Tuesday, March 6, 2018 5:18PM - 5:30PM |
H11.00013: Influence of Defects and Dopants on the Photovoltaic Performance of Bi2S3 Dan Han, Deyan Sun, Maohua Du, Shiyou Chen Bi2S3 attracted intensive attention recently as light-absorber, sensitizer or electron acceptor materials in various solar cells. Using first-principles calculations, we find that the photovoltaic efficiency of Bi2S3 solar cells is limited by its intrinsic point defects, i.e., both S vacancy and S interstitial can have high concentration and produce deep defect levels in the bandgap. Unexpectedly most of the intrinsic defects in Bi2S3, including even the S interstitial, act as donor defects, explaining the observed n-type conductivity and also causing the high p-type conductivity impossible thermodynamically. The doping in Bi2S3 by a series of extrinsic elements is studied, showing that most of dopant elements such as Cu, Br and Cl make the material even more n-type and only the Pb doping makes it weakly p-type. Based on this, we propose that the surface region of n-type Bi2S3 nanocrystals in p-PbS/n-Bi2S3 nano-heterojunction solar cells may be type-inverted into p-type due to the Pb doping, with a buried p-n junction formed in the Bi2S3 nanocrystals, which provides a new explanation to the observed longer carrier lifetime and higher efficiency. |
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