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1 May 2001

Volume 89, Issue 9, pp. 4689-5231

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Polaron hopping conduction and thermoelectric power in LaMnO3+δ

Sudipta Pal, Aritra Banerjee, E. Rozenberg, and B. K. Chaudhuri

J. Appl. Phys. 89, 4955 (2001); http://dx.doi.org/10.1063/1.1362411 (7 pages) | Cited 34 times

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Two different phases of LaMnO3+δ [one showing a metal–insulator transition (MIT), referred to as LaMn–C, and the other not showing a MIT, referred to as LaMn-S] have been clearly observed to follow two different conduction mechanisms. Interestingly, small polaron hopping models of Mott, Schnakenberg, and Emin are found to fit the conductivity data of all the samples above the corresponding MIT temperature. The conductivity data of the insulating (semiconducting) LaMn–S followed a nonadiabatic hopping conduction mechanism while LaMn–C and the Pb doped samples viz. La1−xPbxMnO3 (x=0.05–0.5) showed a similar type of MIT and followed an adiabatic small polaron hopping conduction mechanism in the high temperature paramagnetic phase (above the respective MIT temperature). Activation energy (W), density of states at the Fermi level N(EF), Debye temperature (θD), electron–phonon interaction constant (γP), etc. of LaMn–S showed appreciable differences from those of LaMn–C and La1−xPbxMnO3, which show a MIT. Polaron hopping conduction is also supported by thermoelectric power (TEP) measurements. An observed small but appreciable magnetic field dependence of the TEP data (measured at B=1.5 T) is considered to be associated with magnetic polarons. © 2001 American Institute of Physics.
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72.15.Jf Thermoelectric and thermomagnetic effects
72.20.Pa Thermoelectric and thermomagnetic effects
71.30.+h Metal-insulator transitions and other electronic transitions
71.38.Ht Self-trapped or small polarons
73.50.Dn Low-field transport and mobility; piezoresistance
63.70.+h Statistical mechanics of lattice vibrations and displacive phase transitions
75.47.Gk Colossal magnetoresistance
72.20.Fr Low-field transport and mobility; piezoresistance

Electronic structure of molecular crystals containing edge dislocations

Maija M. Kuklja and A. Barry Kunz

J. Appl. Phys. 89, 4962 (2001); http://dx.doi.org/10.1063/1.1359171 (9 pages) | Cited 12 times

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An attempt to model the electronic structure of molecular crystals containing an edge dislocation at the ab initio Hartree–Fock level is performed. The experimentally determined configurations for edge-type dislocations with the Burgers vector [001] in crystalline cyclotrimethylene trinitramine (RDX) and pentaetythritol tetranitrate (PETN) are theoretically simulated. It is shown that a shear stress, induced by the dislocations, produces local electronic states in the fundamental band gap of the crystal. These states are mainly formed by molecular orbitals of critical bonds (which are the N–NO2 group in RDX and the O–NO2 group in PETN) responsible for the stability of the materials. Optical absorption attributed to these electronic states is predicted and compared to the available experimental data. Properties of the defective solids are compared with those of the perfect crystals. Correlation of the electronic structure and sensitivity of the materials to initiation of a chemical reaction as well as some practical applications of the obtained results are discussed. © 2001 American Institute of Physics.
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61.72.Lk Linear defects: dislocations, disclinations
71.15.Ap Basis sets (LCAO, plane-wave, APW, etc.) and related methodology (scattering methods, ASA, linearized methods, etc.)
71.55.Ht Other nonmetals

Hole drift mobility in μc-Si:H

G. Juška, M. Viliūnas, K. Arlauskas, N. Nekrašas, N. Wyrsch, and L. Feitknecht

J. Appl. Phys. 89, 4971 (2001); http://dx.doi.org/10.1063/1.1359436 (4 pages) | Cited 18 times

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In microcrystalline hydrogenated silicon (μc-Si:H), the drift mobility dependencies of holes on electric field and temperature have been measured by using a method of equilibrium charge extraction by linearly increasing voltage. At room temperature the estimated value of the drift mobility of holes is much lower than in crystalline silicon and slightly higher than in amorphous hydrogenated silicon (a-Si:H). In the case of stochastic transport of charge carriers with energetically distributed localized states, the numerical model of this method gives insight into the mobility dependence on electric field. From the numerical modeling and experimental measurement results, it follows that the hole drift mobility dependence on electric field is predetermined by electric field stimulated release from localized states. © 2001 American Institute of Physics.
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72.80.Ng Disordered solids
72.20.Ee Mobility edges; hopping transport
72.20.Jv Charge carriers: generation, recombination, lifetime, and trapping

Shunt screening, size effects and I /V analysis in thin-film photovoltaics

V. G. Karpov, G. Rich, A. V. Subashiev, and G. Dorer

J. Appl. Phys. 89, 4975 (2001); http://dx.doi.org/10.1063/1.1359158 (11 pages) | Cited 12 times

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We present an analytical model that quantitatively describes the physics behind shunting in thin film photovoltaics and predicts size-dependent effects in the I/V characteristics of solar cells. The model consists of an array of microdiodes and a shunt in parallel between the two electrodes, one of which mimics the transparent conductive oxide and has a finite resistance. We introduce the concept of the screening length L, over which the shunt affects the system electric potential. The nature of this screening is that the system generates currents in response to the point perturbation caused by the shunt. L is expressed explicitly in the terms of the system parameters. We find the spatial distribution of the electric potential in the system and its I/V characteristics. The measured I/V characteristics depend on the relationship between the cell size l and L, being markedly different for the cases of small (l≪L) and large (l≫L) cells. We introduce a new regime of the large photovoltaic cell where all the characteristics are calculated analytically. Our model is verified both numerically and experimentally: good agreement is obtained. © 2001 American Institute of Physics.
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84.60.Jt Photoelectric conversion
85.60.Dw Photodiodes; phototransistors; photoresistors

Lithium doping of semiconducting organic charge transport materials

G. Parthasarathy, C. Shen, A. Kahn, and S. R. Forrest

J. Appl. Phys. 89, 4986 (2001); http://dx.doi.org/10.1063/1.1359161 (7 pages) | Cited 25 times

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We study the effects of lithium (Li) incorporation in the cathodes of organic light-emitting devices. A thermally evaporated surface layer of metallic Li is found to diffuse through, and subsequently dope, the electron transporting organic semiconducting thin films immediately below the cathode, forming an Ohmic contact. A diffusion length of ∼700 Å is inferred from analyses of the current–voltage and secondary ion mass spectrometry data. The conductivity of the Li-doped organic films is ∼3×10−5 S/cm. Photoemission spectroscopy suggests that Li lowers the barrier to injection at the organic/cathode interface, introduces gap states in the bulk of the organic semiconductor, and dopes the bulk to facilitate efficient charge transport. © 2001 American Institute of Physics.
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85.60.Jb Light-emitting devices
79.20.Rf Atomic, molecular, and ion beam impact and interactions with surfaces
79.60.Jv Interfaces; heterostructures; nanostructures

Electrostatic potential and quantum transport in a one-dimensional channel of an induced two-dimensional electron gas

O. A. Tkachenko, V. A. Tkachenko, D. G. Baksheyev, K. S. Pyshkin, R. H. Harrell, E. H. Linfield, D. A. Ritchie, and C. J. B. Ford

J. Appl. Phys. 89, 4993 (2001); http://dx.doi.org/10.1063/1.1352024 (8 pages) | Cited 12 times

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We discuss quantization of the conductance in short, ultraclean one-dimensional quantum wires of a design where an electron gas is induced electrostatically. Two-level sets of gates allow independent control of electron density in the constriction and in the reservoirs, thus varying the conductance G as a function of the bias on the gates G(Vtg,Vsg). Up to 12 clean well-resolved conductance G(Vsg) plateaus confirm the high quality of the constriction. The experimental curves are modeled using three-dimensional self-consistent calculations in the Thomas–Fermi approximation of the electrostatic potential of the constriction and solution of the two-dimensional problem of electron transport in the calculated potential. Our calculations are in qualitative agreement with the experiment. © 2001 American Institute of Physics.
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73.63.Nm Quantum wires
73.21.Hb Quantum wires
73.23.-b Electronic transport in mesoscopic systems

Analysis of multiphase clocked electron pumps consisting of single-electron transistors

Shuhei Amakawa, Hiroshi Mizuta, and Kazuo Nakazato

J. Appl. Phys. 89, 5001 (2001); http://dx.doi.org/10.1063/1.1358314 (8 pages) | Cited 3 times

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Pump circuits consisting of single-electron transistors are analyzed in which electrons are pumped by multiphase clock pulses. An optimal low-temperature operation condition is presented where pumped current is maximized, yet the power consumption is not. Approximate formulas for the number of electrons transferred per clock cycle and the power consumption are derived for that condition, which clearly show the advantage of the pump circuits for low-power applications. The power consumption becomes even less at higher temperatures. However, the relatively large island capacitance between transistors limits the operation temperature. © 2001 American Institute of Physics.
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85.35.Gv Single electron devices
84.30.Sk Pulse and digital circuits

Transport properties of V–VI semiconducting thermoelectric BiSbTe alloy thin films and their application to micromodule Peltier devices

A. Boulouz, S. Chakraborty, A. Giani, F. Pascal Delannoy, A. Boyer, and J. Schumann

J. Appl. Phys. 89, 5009 (2001); http://dx.doi.org/10.1063/1.1360701 (6 pages) | Cited 30 times

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Thin semiconducting thermoelectric films with narrow energy band gaps are considered to be very promising for future microdevice applications (sensors and generators). The polycrystalline BiSbTe alloys (V–VI semiconductors) are examples. In this report, the detailed temperature dependence of electrical resistivity [ρ(T)], n- and p-type carrier concentration [n(T) and p(T)], and Hall mobility [μ(T)] of n-type Bi2Te3, p-type Sb2Te3, and p-type (Bi1−xSbx)2Te3 (x=0.73 and 0.77) alloy films prepared by metalorganic chemical vapor deposition are presented in the range of 100–500 K. From the room temperature measurement of the Seebeck coefficient (α), the values of α for Bi2Te3, Sb2Te3, and (Bi1−xSbx)2Te3 with x=0.73 and 0.77 are found to be −220, +110, +240, and +210 μV/K, respectively, which are optimal in these types of film materials. The carrier concentration of these films at 300 K is found to be around (1019–1020) cm−3. The ρ(T) data show an exponential increase with increasing temperature irrespective of the carrier types. For the temperature dependence of the Hall mobility, the lattice contribution is found to be predominant for all the films. Also, we have fabricated a simple micromodule Peltier device (MMP) using the n-type Bi2Te3 and the p-type (Bi1−xSbx)2Te3 (x=0.77) films where a maximum cooling of 2.6 °C was obtained with a low input current of 2.5 mA. © 2001 American Institute of Physics.
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73.61.Le Other inorganic semiconductors
73.50.Lw Thermoelectric effects
73.50.Jt Galvanomagnetic and other magnetotransport effects (including thermomagnetic effects)
85.80.Fi Thermoelectric devices

Anode hole injection, defect generation, and breakdown in ultrathin silicon dioxide films

D. J. DiMaria and J. H. Stathis

J. Appl. Phys. 89, 5015 (2001); http://dx.doi.org/10.1063/1.1363680 (10 pages) | Cited 40 times

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Using a variety of experimental techniques, hot holes are demonstrated to produce bulk and interfacial defect sites in silicon dioxide layers of metal–oxide–semiconductor structures. Similar to defect production by hot electrons, hot holes are shown to generate these sites by the energy they deposit in contacting silicon layers near the oxide interface. This deposited energy is believed to release hydrogenic species which can move into and through the oxide layer producing defects. The buildup of these defect sites is related to the destructive breakdown of ultrathin gate oxides in p-channel field-effect transistors under inversion conditions where direct tunneling of energetic holes to the gate electrode would occur and dominate the current in the external circuit at low gate voltages. However, the results presented here are inconsistent with current reliability models which use anode hole injection to explain destructive breakdown of the oxide layer in n-channel field-effect transistors where hole currents are small relative to electron currents. © 2001 American Institute of Physics.
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73.61.Ng Insulators
77.22.Jp Dielectric breakdown and space-charge effects
73.40.Qv Metal-insulator-semiconductor structures (including semiconductor-to-insulator)
85.30.Tv Field effect devices
85.30.De Semiconductor-device characterization, design, and modeling
77.55.-g Dielectric thin films
73.20.Hb Impurity and defect levels; energy states of adsorbed species

Effect of hydrostatic pressure on degradation of CdTe/CdMgTe heterostructures grown by molecular beam epitaxy on GaAs substrates

D. Wasik, M. Baj, J. Siwiec-Matuszyk, J. Gronkowski, J. Jasiński, and G. Karczewski

J. Appl. Phys. 89, 5025 (2001); http://dx.doi.org/10.1063/1.1360217 (6 pages) | Cited 2 times

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We have shown that external hydrostatic pressure leads to the creation of structural defects, mainly in the vicinity of the II–VI/GaAs interface in the CdTe/Cd1−xMgxTe heterostructures grown by the molecular beam epitaxy method on GaAs substrates. These defects propagating across the epilayer cause permanent damage to the samples from the point of view of their electrical properties. In contrast, photoluminescence spectra are only weakly influenced by pressure. Our results shed light on the degradation process observed even without pressure in II–VI-based heterostructures. © 2001 American Institute of Physics.
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81.05.Dz II-VI semiconductors
81.15.Hi Molecular, atomic, ion, and chemical beam epitaxy
68.55.-a Thin film structure and morphology
68.35.Ct Interface structure and roughness
78.55.Et II-VI semiconductors
78.66.Hf II-VI semiconductors
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