Bulletin of the American Physical Society
APS March Meeting 2014
Volume 59, Number 1
Monday–Friday, March 3–7, 2014; Denver, Colorado
Session S45: Semiconductors: Thermodynamic & Transport Properties I |
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Sponsoring Units: FIAP Chair: Lucas Lindsay, U.S. Naval Research Laboratory Room: Mile High Ballroom 4D |
Thursday, March 6, 2014 8:00AM - 8:12AM |
S45.00001: First principles study of lattice thermal conductivity and large isotope effect in materials David Broido, Lucas Lindsay, Tom Reinecke The isotope effect---the percent enhancement to a material's lattice thermal conductivity, $k$, with isotopic purification---depends on the interplay between phonon-isotope and phonon-phonon scattering. Diamond is known to have the largest measured room temperature (RT) isotope effect of any bulk crystal, achieving a $k$ enhancement of 50{\%}. Using an \textit{ab initio }Boltzmann transport equation approach, we have identified several other materials with far larger RT isotope effects [1]. In particular, we find that germanium carbide (GeC) and beryllium selenide (BeSe) have RT isotope effects of 450{\%}, almost an order of magnitude higher than that in diamond. Isotopic purification in these materials gives surprisingly high intrinsic RT $k$ values, over 1500 Wm$^{-1}$K$^{-1}$ for GeC and over 600 Wm$^{-1}$ K$^{-1}$ for BeSe, well above those of the best metals. These large values stem from fundamental material properties that give both enhanced phonon scattering by isotopes and weak anharmonic phonon-phonon scattering. The physical insights discussed in this work provide guidance for efficient manipulation of thermal transport properties of compound semiconductors through isotopic modification. \\[4pt] [1] L. Lindsay, D. A. Broido and T. L. Reinecke, Phys. Rev. B 88, 144306 (2013). [Preview Abstract] |
Thursday, March 6, 2014 8:12AM - 8:24AM |
S45.00002: First principles determination of ultra-high thermal conductivity fo Boron Arsenide: A competitor for diamond? Lucas Lindsay, David Broido, Tom Reinecke We have calculated the thermal conductivities ($k)$ of cubic III-V boron compounds using a predictive first principles approach. Boron Arsenide (BAs) is found to have a remarkable room temperature $k$ over 2000Wm$^{-1}$K$^{-1}$; this is comparable to those in diamond and graphite, which are the highest bulk values known. We trace this behavior in BAs to an interplay of certain basic vibrational properties that lie outside of the conventional guidelines in searching for high $k$ materials. We also find that cubic BN and BSb will have high $k$ with isotopic purification. This work provides new insight into the nature of thermal transport at a quantitative level and predicts a new ultra-high $k$ material of potential interest for passive cooling applications. \\[4pt] [1] L. Lindsay, D. A. Broido, and T. L. Reinecke, Phys. Rev. Lett. 111, 025901 (2013). [Preview Abstract] |
Thursday, March 6, 2014 8:24AM - 8:36AM |
S45.00003: May the character of the metal-insulator transition of disordered materials be determined by how one looks at it? Arnulf Moebius In a recent experiment, Siegrist et al. studied the metal-insulator transition (MIT) of phase-change materials [1]. They conclude that these substances exhibit a finite minimum metallic conductivity. The striking contrast to reports on other disordered substances motivates the present study of the influence of the MIT criterion used on the character of the MIT obtained [2]. First, we discuss inherent biases of various approaches to locating the MIT. Second, reanalyzing GeSb$_2$Te$_4$ data from [1], we show that this solid strongly resembles other disordered materials: The data may also be interpreted in terms of a continuous MIT. Checking the justification of these fits, however, uncovers data inconsistencies preventing an unambiguous interpretation. Third, comparing with previous experiments on crystalline Si:As, Si:P, Si:B, Ge:Ga, disordered Gd, and nano-granular Pt-C, we show that such an inconclusive behavior occurs frequently: The logarithmic temperature derivative of the conductivity highlights serious inconsistencies in the original interpretations in terms of a continuous MIT. Thus, the question for the character of the MIT of these materials has to be considered as yet open. [1] T. Siegrist et al., Nature Materials 10 (2011) 202. [2] A. Moebius, arxiv.org/abs/1308.1538. [Preview Abstract] |
Thursday, March 6, 2014 8:36AM - 8:48AM |
S45.00004: Measuring Nanoscale Heat Transfer for Gold--(Gallium Oxide)--Gallium Nitride Interfaces as a Function Chester Szwejkowski, kai Sun, Costel Constantin, Ashutosh Giri, Christopher Saltonstall, Patrick Hopkins Gallium nitride (GaN) is considered the most important semiconductor after the discovery of Silicon. Understanding the properties of GaN is imperative in determining the utility and applicability of this class of materials to devices. We present results of time domain thermoreflectance (TDTR) measurements as a function of surface root mean square (RMS) roughness. We used commercially available 5mm x 5mm, single-side polished GaN (3-7 $\mu $m)/Sapphire (430 $\mu $m) substrates that have a Wurtzite crystal structure and are slightly n-type doped. The GaN substrates were annealed in the open atmosphere for 10 minutes (900-1000 $^{\circ}$C). This high-temperature treatment produced RMS values from 1-60 nm and growth of gallium oxide (GaO) as measured with an atomic force microscopy and transmission electron microscopy respectively. A gold film (80nm) was deposited on the GaN surface using electron beam physical vapor deposition which was verified using ellipsometry and profilometry. The TDTR measurements suggest that the thermal conductivity decays exponentially with RMS roughness and that there is a minimum value for thermal boundary conductance at a roughness of 15nm. [Preview Abstract] |
Thursday, March 6, 2014 8:48AM - 9:00AM |
S45.00005: Thermionic charge transport in CMOS nano-transistors Andreas Betz, M. Fernando Gonzalez Zalba, Sylvain Barraud, Quentin Wilmart, Bernard Placais, David A. Williams We report on DC and microwave electrical transport measurements in silicon-on-insulator CMOS nano-transistors at low and room temperature. At low source-drain voltage, the DC current and AC response show signs of quantization with an additional dependence on back-gate bias. We attribute the quantization to Coulomb blockade resulting from barriers formed under the spacer regions of the chip. We show that at high bias transport occurs thermionic over the highest barrier: Transconductance traces obtained from microwave scattering parameter measurements can be accurately fitted by a thermionic model. From this we deduce the ratio of gate capacitance and quantum capacitance $C_g/C_q = C_{ox}/(C_{ox}+C_q)$, as well as the electron temperature $T_e$. We show that transport in our devices remains thermionic at high bias up to room temperature. [Preview Abstract] |
Thursday, March 6, 2014 9:00AM - 9:12AM |
S45.00006: Lattice-matched heterostructure envelope functions and band-to-band transmission through broken-gap heterostructures Bart Soree, Maarten Van de Put, Wim Magnus, William Vandenberghe We developed an envelope function formalism capable of describing the electronic structure in lattice matched heterostructures. The formalism takes into account the different nature of the materials involved by using matrix elements in the basis formed by the solutions of their respective bulk Hamiltonian. A transformation between these basis sets has been devised to allow for expansion in one consistent and complete set. This transformation is described without full knowledge of the basis function, as this would defeat the purpose of the envelope function method. We employ only the known interband momentum matrix elements to obtain the transformation coefficients. With this method it is not only possible to describe the electronic structure in heterostructures in a more rigorous way, it is also possible to describe band-to-band transitions through these heterostructures. In particular, we studied the transmission coefficients through broken-gap heterostructure. A large discrepancy was found with the effective mass approach, which predicts full transmission at a certain energy. Our method correctly predicts additional reflections due to the interface betweeen the two materials. [Preview Abstract] |
Thursday, March 6, 2014 9:12AM - 9:24AM |
S45.00007: An envelope function expansion of the Wigner transport equation Maarten Van de Put, Bart Soree, Wim Magnus The Wigner function approach to quantum transport is well suited for application to nanoscaled electronic devices. However, the Wigner-Liouville equation is often formulated within the framework of the effective mass approximation. As the envelope function formalism based on k.p theory offers a more accurate description of the band structure, we have expanded the electron field operators in the corresponding envelope functions and rederived the Wigner transport equation accordingly. We obtain a set of coupled envelope-Wigner functions which enable us also to treat band- to-band transitions (BTBT) within the Wigner formalism. This way, we can provide a rigorous quantum mechanical treatment of BTBT events in phase space. Finally, we have extended this approach to the classical Boltzmann transport equation which introduces BTBT by invoking additional coupling terms on top of the classical drift-diffusion instead of ad-hoc generation and recombination terms. [Preview Abstract] |
Thursday, March 6, 2014 9:24AM - 9:36AM |
S45.00008: Mobility--Lifetime Measurements of Amorphous Hydrogenated Boron Carbide Using the Steady-State Photoconductivity Method Justin Hurley, Mahbube Siddiki, Christopher Keck, Bradley Nordell, Thuong Nguyen, Anthony Caruso, Michelle Caruso As a p-type semiconductor with a high band gap (\textgreater 2.5 eV) and high electrical resistivity (\textgreater 10\textasciicircum 12 $\Omega $\textbullet cm), ortho-carborane-based amorphous hydrogenated boron carbide (a-BxC:Hy), grown by plasma-enhanced chemical vapor deposition, is one of a handful of materials suitable for direct-conversion solid-state neutron detection. Traditionally, there has been minimal investigation into the boron carbide class of solids outside of its mechanical uses, and the basic knowledge of electrical transport properties needed to optimize a-BxC:Hy for detector applications is lacking. In particular, the mobility--lifetime product ($\mu \tau )$, a measure of the ability to extract and transport charges within a material, is an important figure of merit for detector devices. Herein we will describe our implementation of the steady-state photoconductivity method, which provides a straightforward determination of $\mu \tau $ in a-BxC:Hy films. Values of $\mu \tau $ as a function of wavelengths spanning the UV-Vis range have been determined for a range of a-BxC:Hy samples. We will describe how thin-film growth conditions can be adjusted to optimize $\mu \tau $. [Preview Abstract] |
Thursday, March 6, 2014 9:36AM - 9:48AM |
S45.00009: Density functional calculations of multiphonon capture cross sections at defects in semiconductors Georgios D. Barmparis, Yevgeniy S. Puzyrev, X.-G. Zhang, Sokrates T. Pantelides The theory of electron capture cross sections by multiphonon processes in semiconductors has a long and controversial history. Here we present a comprehensive theory and describe its implementation for realistic calculations. The Born-Oppenheimer and the Frank-Condon approximations are employed. The transition probability of an incoming electron is written as a product of an instantaneous electronic transition in the initial defect configuration and the line shape function (LSF) that describes the multiphonon processes that lead to lattice relaxation. The electronic matrix elements are calculated using the Projector Augmented Wave (PAW) method which yields the true wave functions while still employing a plane-wave basis. The LSF is calculated by employing a Monte Carlo method and the real phonon modes of the defect, calculated using density functional theory in the PAW scheme. Initial results of the capture cross section for a prototype system, namely a triply hydrogenated vacancy in Si are presented. The results are relevant for modeling device degradation by hot electron effects. [Preview Abstract] |
Thursday, March 6, 2014 9:48AM - 10:00AM |
S45.00010: Nonlinear magneto-transport in two dimensional electron system with anisotropic mobility William Mayer, Sergey Vitkalov, Alexey Bykov, Andrey Goran Nonlinear magnetotransport of 2D electrons in GaAs/AlAs heterostructures with anisotropic mobility $\mu$ placed in quantizing magnetic fields is studied in Hall bar geometry. It is found, that for an electric current flowing in the direction corresponding to the low mobility, the transition of electron systems in the state with zero differential resistance occurs at considerably smaller value of the electric current than for a current flowing in the direction of the high mobility. The obtained results indicate the importance of the anisotropy of the scattering potential for the electron quantal heating. [Preview Abstract] |
Thursday, March 6, 2014 10:00AM - 10:12AM |
S45.00011: Two-dimensional charge transport in polycrystalline black phosphorus Xuxu Bai, Yanru Song, Bei Bao, Lilin Sun, Shun Wang, Ying Liu Black phosphorus, a narrow band gap semiconductor, is the only elemental layered material other than graphene. Theoretical calculations indicate the electronic state of single layer black phosphorus is different from that of the bulk, similar to graphene, which has attracted attentions in the condensed matter physics community. Here we report preliminary electrical transport measurements in bulk polycrystalline black phosphorus. Our results of the temperature dependence of resistivity reveal 2D variable range hopping transport behavior below 10 K. In the hopping regime, the magnetoresistance is negative at weak magnetic field, due to quantum interference of the hopping wave functions in two dimensions, and positive at stronger field. Hall effect and the anisotropy of the magnetoresistance will also be discussed. [Preview Abstract] |
Thursday, March 6, 2014 10:12AM - 10:24AM |
S45.00012: ABSTRACT WITHDRAWN |
Thursday, March 6, 2014 10:24AM - 10:36AM |
S45.00013: Mobility Determination of Amorphous Hydrogenated Boron Carbide from Dark-Injection Space-Charge-Limited Current Method Christopher Keck, Bradley Nordell, Thuong Nguyen, Justin Hurley, Anthony Caruso, Michelle Paquette There has been particular interest in thin-film amorphous hydrogenated boron carbide (a-BxC:Hy) for solid-state direct-conversion neutron detection because it has a high cross-section for neutron capture and demonstrates a high electrical resistivity (on the order of 10$^{12}$ $\Omega \cdot$ cm). Rigorous studies into the electrical transport properties of the material are yet to be done. The experimental determination of the material's mobility is complicated by the fact that it is likely below the noise floor of conventional measurements such as the DC Hall Effect method, wherein the measured Hall Voltage is directly proportional to the mobility. One way to circumvent this problem is to utilize a drift mobility measurement technique such as the dark-injection space-charge-limited current (DI-SCLC) method, where the mobility is calculated using the transit time of the charge carrier; as transit time and mobility are inversely proportional, this method is ideal for low mobility materials. The implementation of the DI-SCLC method for mobility measurements in a-BxC:Hy will be described, and the relationship of mobility, resistivity, and carrier concentration as a function of thin-film growth conditions will be discussed. [Preview Abstract] |
Thursday, March 6, 2014 10:36AM - 10:48AM |
S45.00014: First-principles calculation of mobility in silicon Yuning Wu, X.-G. Zhang, Sokrates T. Pantelides We introduce a new first-principles method to calculate Coulomb-scattering-limited electron mobility in silicon. The lifetime of a Bloch state due to scattering can be interpreted as arising from an additional imaginary part of electron self-energy. By introducing an artificial imaginary potential, the electron self-energy can be extracted from the complex band structure of a periodic system while eliminating the interference effect due to multiple scattering between impurities. This allows an implementation using density functional theory within the Quantum-Espresso package. The calculated electron mobility agrees with the experimental data. [Preview Abstract] |
Thursday, March 6, 2014 10:48AM - 11:00AM |
S45.00015: Accumulation mode GaAs/AlGaAs 2D electron system with independent control of the channel and contact resistance S.J. MacLeod, A.M. See, I. Farrer, D.A. Ritchie, A. Ludwig, A. Wieck, A. Hamilton Semiconductor-insulator-semiconductor FETs (SISFETs) are an attractive alternative to modulation doped (MD) GaAs/AlGaAs systems. GaAs SISFETs consist of epitaxially grown layers of GaAs then AlGaAs capped with a degenerately doped GaAs layer. The cap acts as an in situ, over-all top-gate which attracts carriers to the GaAs/AlGaAs interface. The MBE grown top-gate eliminates scattering and charge-noise from surface states and unlike Schottky gates, there is no strain between the gate and insulating AlGaAs layer due to similar thermal expansion rates. The absence of a doping layer improves carrier mobility ($\mu$) at low densities ($n_s$) since in shallow MD devices the doping layer creates an additional long-range random impurity potential. For this reason SISFET devices improve $\mu$ at low $n_s$. However in the low $n_s$ regime the high contact resistance dominates the device resistance, which can limit electrical transport measurements. We fabricate a GaAs 2D electron SISFET with dual-gate architecture to independently control the contact and channel resistance. We characterize our device using standard low-temperature electrical transport measurements. The 2D $n_s$ could be varied from $0.1-3\times 10^{11}\,cm^{-2}$ with a $\mu$ of up to $9\times 10^{6}\,cm^{2}V^{-1}s^{-1}$. [Preview Abstract] |
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