Bulletin of the American Physical Society
APS March Meeting 2018
Volume 63, Number 1
Monday–Friday, March 5–9, 2018; Los Angeles, California
Session S07: Electronic and Thermodynamic Properties of Semiconductors |
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Sponsoring Units: FIAP Chair: Katerina Nikolaidou, Univ of California - Merced Room: LACC 153B |
Thursday, March 8, 2018 11:15AM - 11:27AM |
S07.00001: Investigation of Intrinsic Physical Properties for High Quality FeSi Single Crystals Yuankan Fang, Sheng Ran, M Brian Maple, Weiwei Xie We report physical properties of single-crystalline FeSi over a wide temperature range from 1.8 K to 400 K. The FeSi crystals were evaluated by means of X-ray diffraction, specific heat, and magnetization measurements and found to have a very low concentration of magnetic impurities ~100 ppm. In the temperature range 150-67 K, the electrical transport behavior is consistent with activated behavior with an energy gap of 57 meV, below which the resistivity is significantly smaller together with a sign change of the Hall coefficient and magneto-resistivity. Further decrease of the temperature results in a sharp reversible transition from a negative slope to a positive slope of ρ(T) at 19 K. Corresponding magnetization and magneto-resistivity measurements suggest that there is no magnetic order associated with this transition. Furthermore, no feature can be observed in specific heat measurements. The results suggest complicated electron transport properties and provide evidence of a conducting surface state in FeSi at low temperatures. |
Thursday, March 8, 2018 11:27AM - 11:39AM |
S07.00002: Boron nitride encapsulated structure as efficient heat dissipation for nano-electronics Xian Zhang, Fan Ye, Philip Feng Atomically thin two dimensional materials, such as graphene, moly-disulfide (MoS2) and hexagonal boron nitride (h-BN), attract increasing attention owing to their unique physical properties and applications. Fully h-BN encapsulated structure emerges as a novel platform with significant performance enhancement, owing to strong, in-plane bonding of the planar hexagonal lattice structure. Though significant efforts have been achieved in h-BN encapsulated 2D devices, thermal properties remain unexplored. In this work, we investigate lateral and interfacial thermal transport in fully h-BN encapsulated MoS2 devices, fabricated by the ultra-clean van der Waals method. Utilizing the refined optothermal Raman technique, the interfacial thermal conductance in BN/MoS2/BN interface is 60±27MW/m2K, which is significantly higher than MoS2/SiO2 structure. Lateral thermal conductivity of BN is 131±27W/mK. We also found the BN encapsulated MoS2 FET has significantly lower performance temperature than basic MoS2 FET. This demonstrates that the BN encapsulated structure offers a great heat dissipation for MoS2, opens new opportunities for studying thermal transport mechanism in 2D heterostructure devices and sheds light on engineering high-performance 2D FETs with low energy dissipation. |
Thursday, March 8, 2018 11:39AM - 11:51AM |
S07.00003: Induced Persistent Photoconductivity in Strontium Titanate Harry Gordon-Moys, Christopher Bell Strontium titanate (STO) is a wide, direct, bandgap semiconductor which can be doped from the insulating state to a high mobility semiconductor through to a metallic state which at temperatures lower than ~ 300 mK becomes one of the lowest carrier density superconductors. As well as traditional chemical doping, carriers can also be introduced by using band-gap illumination which generates long lived electron-hole pairs, where the electron mobility is much higher than that of the holes. The density of carriers can be controlled by intensity of the light. Furthermore by varying the wavelength of the incident light near the band edge the optical penetration depth, and therefore the sheet thickness of the photo-generated electrons, can also be tuned. Photocarrier doping, therefore, offers continuous, real-time tuning of both sheet carrier density and thickness. In undoped STO these photo-generated carriers recombine rapidly, however the presence of defects can dramatically enhance their lifetime leading to photoconductivity which persists for many hours after illumination has ceased. Here we present our work on inducing tuneable persistent photoconductivity at the surface of STO by large doses of ultra-violet light at low temperatures. |
Thursday, March 8, 2018 11:51AM - 12:03PM |
S07.00004: High efficiency, solution processed quantum dot light emitting diode with inorganic charge transport layers Ramesh Vasan, Omar Manasreh High efficiency all-inorganic quantum dot light emitting diodes with metal oxide charge transport layers are fabricated and characterized. The device is solution processed and active layers are fabricated by using spin coating technique. It consists of patterned FTO anode, nickel oxide thin film hole transport layer, green emitting alloyed CdSe/ZnS quantum dot emissive layer, zinc oxide nanoparticle electron transport layer and Al cathode. The device turn on voltage is 5V with an emission peak at 555 nm. The FWHM of the electroluminescence spectrum is ~42 nm, indicating a highly saturated pure green color emission. The device performance is evaluated by measuring the IV characteristics, luminance, current efficiency, luminous efficiency and quantum efficiency. The maximum current and luminous efficiencies of the device are 144 cd/m2 and 14.2 lm/W, respectively. The luminance of the device increases with input voltage to a maximum of ~ 120,000 cd/m2 and saturates at 10.5V. The peak external and internal quantum efficiencies of the device are 11.4% and 44.6%, respectively. The superior light output characteristics of the device can be attributed to the efficient charge transport properties of the metal oxide transport layers. |
Thursday, March 8, 2018 12:03PM - 12:15PM |
S07.00005: Evidence for an Intermediate Phase below the Critical Density for the MI transition in silicon MOSFETs Shiqi Li, Qing Zhang, Myriam Sarachik The magnetoconductivity of the strongly interacting 2D electron system in low disorder silicon MOSFETs is found to scale with B/T throughout the insulating phase, confirming earlier measurements for densities near the metal-insulator transition nc [1, 2]. We report further that the magnetoconductivity can be collapsed onto a single curve deep in the insulator for all densities up to approximately n* ~ 0.8 nc. The shape of the scaled curves varies between n* and nc, where nc signals the entry into the metallic state. This suggests the presence of an intermediate phase between the deep insulator and the metal. |
Thursday, March 8, 2018 12:15PM - 12:27PM |
S07.00006: Abstract Withdrawn
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Thursday, March 8, 2018 12:27PM - 12:39PM |
S07.00007: Thermal Conductivity of amorphous and nanocrystalline silicon films prepared by hot-wire chemical-vapor deposition* Xiao Liu, Battogtokh Jugdersuren, Brian Kearney, Daniel Queen, Thomas Metcalf, James Culbertson, Paul Desario, Rhonda Stroud, William Nemeth, Qi Wang We measured thermal conductivity of amorphous and nanocrystalline silicon (a-Si and nc-Si) thin films. The films were prepared by hot-wire chemical-vapor deposition. The crystallinity of the films is controlled by the hydrogen dilution during growth. For a-Si, the result is in good agreement with those of previous reports. The thermal conductivity of the as-grown nc-Si is 70% higher and increases 35% more after annealing at 600οC. The results of all a-Si and nc-Si films have similarly weak temperature dependence from 85 to 300 K. Structural analysis shows that the as-grown nc-Si is approximately 60% crystalline. The nanograins, averaging 9.1 nm, are embedded in an amorphous matrix. The grain size increases to 9.7 nm upon annealing, accompanied by the disappearance of the amorphous phase. We extend the models of grain boundary scattering of phonons with two different non-Debye dispersion relations to explain our result of nc-Si, confirming that the strong grain size dependent heat transport mechanism still applies to grain sizes in the 10-nm range. However, the similarity in thermal conductivity between a-Si and nc-Si suggests the heat transport mechanisms in both structures may not be as dissimilar as we currently understand. |
Thursday, March 8, 2018 12:39PM - 12:51PM |
S07.00008: Ultrafast Carrier Dynamics in Nitride-based wide band gap semiconductors: Hexagonal Boron Nitride and Aluminum Nitride Ioannis Chatzakis, Roderick B. Davidson II, Adam Dunkelberger, Daniel Ratchford, Joshua Caldwell, Scott Katzer, David Storm, Alexander Giles, Chase Ellis, Joseph Tischler, Jeffrey Owrutsky We used time-resolved pump-probe spectroscopy with sub-picosecond resolution to observe free carrier dynamics in hexagonal boron nitride (h-BN) and aluminum nitride (AlN). The samples were excited by two-photon excitation with a pump pulse at 267 nm and probed by an infrared beam spectrally tuned to the edge of the Reststrahlen band (e.g. 1650 cm-1for h-BN). We observed strong Reststrahlen band softening with the injection of carriers due to the (LO)- phonon free-carrier plasma coupling effect. Lifetimes were observed from a few picoseconds to nanoseconds in h-BN and AlN. |
Thursday, March 8, 2018 12:51PM - 1:03PM |
S07.00009: Intrinsic Photoconductivity Imaging of Monolayer-Bilayer WSe2 Flakes Zhaodong Chu, Ali Han, Chao Lei, Di Wu, Xiaoqin (Elaine) Li, Lain-Jong Li, Keji Lai The spatial variation of photo response in few-layer transition metal dichalcogenides (TMDCs) is of critical importance for their applications in optoelectronic devices. In conventional microscopy experiments, the local distribution of photocurrent across source and drain electrodes is measured, which may be complicated by the diffusion of photo-induced carriers, the Schottky contact, and other extrinsic effects. Using a microwave impedance microscope (MIM) with light stimulation, we demonstrate the intrinsic photoconductivity mapping of CVD-grown WSe2 flakes without the need of contact electrodes. The measured photoconductivity as a function of the above-gap laser intensity in the bilayer WSe2 region is around one order of magnitude higher than that in the monolayer area, indicative of a significant enhancement of the carrier lifetime due to interlayer coupling. The noninvasive MIM imaging is expected to provide important insights on the remarkable electrical and optical properties of TMDCs. |
Thursday, March 8, 2018 1:03PM - 1:15PM |
S07.00010: Exploring resonant interaction between hybrid lead-halide perovskite quantum dots and plasmonic metal nanoparticles Benaz Mendewala, Sayantani Ghosh The field of nanoplasmonics has been rapidly advancing due to its many applications in photovoltaics, displays and sensing. When excited at their resonance frequency, metallic nanoparticles can generate localized electric fields that can enhance optical properties of, or transfer energy to, surrounding media, which in our case is semiconductor quantum dots (QDs). The coupling between plasmonic nanoparticles and QD excitons has been extensively observed, displaying favorable characteristics such as reduced lifetime and suppressed blinking. While traditional CdSe QDs have most commonly been used for these studies, the interaction between hybrid lead-halide perovskite quantum dots (PQDs) and plasmonic nanoparticles remains to be investigated. PQDs have distinct advantages over conventional QDs, including high quantum yield and ease of compositional tunability. Utilizing wavelength-dependent photoluminescence spectroscopy and time-resolved photoluminescence spectroscopy, we study the nature of the plasmon-exciton coupling and its effect on PQD optical properties. |
Thursday, March 8, 2018 1:15PM - 1:27PM |
S07.00011: On the thermal stability and optical properties of extreme wide-bandgap MgZnO semiconductor alloys Dinesh Thapa, Jesse Huso, John Morrison, Matthew McCluskey, Leah Bergman Mg(x)Zn(1-x)O provide by-design alloys with varying UV-bandgaps, 3.3 to 7eV, depending on the composition x. For an intermediate composition-range the different crystal structures of the end-members may result in a mixed-phase alloy, which renders the system not viable in that range. By growing the alloys away from their thermodynamic-equilibrium, the solubility limit can be extended, and single-phase alloys may be realized. However, a key issue addressed here is the thermal stability of such alloys. Alloys up to x=0.72 were grown at a low-temperature 250C. The as-grown alloys were found via XRD and transmission studies to possess a single-phased wurtzite structure with a bandgap range 3.3 to 4.42eV. The extent of their thermal stability was studied via post-growth annealing up to 900C. Alloys with low-Mg up to x=0.34 were found to be highly stable and retain their optical and material properties. Alloys with higher Mg, up to x=0.72, were found to be unstable and were phase-separated into cubic and wurtzite phases with bandgap ~ 6 and 3.5eV, respectively. It was found that the solubility-limit at x~0.30 is much higher than expected. |
Thursday, March 8, 2018 1:27PM - 1:39PM |
S07.00012: Spectroscopic Characterization of Er Optical Center in GaN epilayers Brendan Ryan, Ho Vinh, Vinh Nguyen Er doped GaN material is known to result in the formation of luminescent centers suitable for applications in optoelectronic devices. We report here a significant enhancement of photoluminescence from the Er optical center in the visible region synthesized by metal organic chemical vapor deposition. Employing resonant excitation via the higher-lying inner 4f shell transitions and band-to-band excitation of the GaN host has resulted narrow emission lines from Er optical centers in green emission at room-temperature. The emission spectrum consists of PL lines centered at 537 and 558 nm identified as Er transitions from the 2H11/2 and 4S3/2 levels to the 4I15/2 ground state. Using the high resolution spectroscopy and time-resolved photoluminescence studies, we able to identify the crystal field spitting and the dynamics of Er optical centers in GaN in the visible region. The information has provided evidence for a defect-related trap level that may be involved in the energy transfer between the GaN host and the Er ions. |
Thursday, March 8, 2018 1:39PM - 1:51PM |
S07.00013: Wavelength selective Photoluminescence (PL) imaging for probing kinetic energy dependence of carrier diffusion in a GaAs epilayer Sunny Zhang, Liqin Su, Timothy Gfroerer, Jacob Kon, Mark Wanlass, Yong Zhang
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Thursday, March 8, 2018 1:51PM - 2:03PM |
S07.00014: Temperature Measurement by a Nanoscale Electron Probe using Energy Gain and Loss Spectroscopy Juan Idrobo, Andrew Lupini, Tianli Feng, Raymond Unocic, Franklin Walden, Daniel S. Gardiner, Tracy C. Lovejoy, Niklas Dellby, Sokrates Pantelides, Ondrej Krivanek Heat dissipation in integrated nanoscale devices is a major issue that requires the development of nanoscale temperature probes. Here, we report the implementation of a method that combines electron energy gain and loss spectroscopy to provide a direct measurement of the local temperature of BN nano-flakes. We find that a BN optical phonon presents a linear red shift behavior (towards lower energies) as the sample is heated up, from 50 to 1300 °C. First-principles calculations reveal that the shift is due to lattice thermal expansion and anharmonic phonon scattering, with the latter being the dominant factor to reduce the energy of the phonon as the temperature increases. The gain peak exhibits an increase of intensity as a function of temperature, in accordance with the occupation probability theory. The experiments and theory presented here open the doors to the study of anharmonic effects in materials by directly probing phonons in the electron microscope. |
Thursday, March 8, 2018 2:03PM - 2:15PM |
S07.00015: Sub-cycle control of optical properties by THz-induced dressed states Hideki Hirori, Kento Uchida, Tomohito Otobe, Koichiro Tanaka, Toshimitsu Mochizuki, Changsu Kim, Masahiro Yoshita, Hidefumi Akiyama, Loren Pfeiffer, K West The laser dressing eigenstates of electronic systems in atoms leads to important quantum optical phenomena, such as Rabi splitting, optical Stark effect, and electromagnetically induced transparency, for the applications of ultrafast optical switching, slowed and stored light, and quantum information processing. In principle, a similar situation should be encountered in solids for hydrogen-like quasi-particles of excitons (electron-hole pairs). The intraexcitonic transition energy lies in the terahertz (THz) frequency range with large dipole moments, which making them fascinating system for exploiting THz excitonic interactions in optical communication devices. Here, we probe the transient absorption changes of near-infrared (NIR) pulse in a GaAs quantum well under the presence of a multi-cycle THz wave. By changing the delay between the NIR probe and the THz wave the absorption strengths are modulated on the sub-cycle THz timescale, and the frequency analysis shows the formation of THz-induced dressed states of excitons. Besides the novel concept for sub-cycle control of optical properties, our findings reveal the importance of accounting for interference effects of THz-induced nonlinear susceptibilities in the interpretation of THz pump–optical probe experiments. |
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