Electrical transport across Au/Nb:SrTiO3 Schottky interface with different Nb doping

We have investigated electron transport in Nb doped SrTiO$_3$ single crystals for two doping densities. We find that the resistivity and mobility are temperature dependent in both whereas the carrier concentration is almost temperature invariant. We rationalize this using the hydrogenic theory for shallow donors. Further, we probe electrical transport across Schottky interfaces of Au on TiO$_2$ terminated n-type SrTiO$_3$. Quantitative analysis of macroscopic I-V measurements reveal thermionic emission dominated transport for the low doped substrate whereas it deviates from such behavior for the high doped substrate. This work is relevant for designing devices to study electronic transport using oxide-semiconductors.Comment: 10 Pages, 3 Figure

High magnetoresistance at room temperature in p-i-n graphene nanoribbons due to band-to-band tunneling effects

A large magnetoresistance effect is obtained at room-temperature by using p-i-n armchair-graphene-nanoribbon (GNR) heterostructures. The key advantage is the virtual elimination of thermal currents due to the presence of band gaps in the contacts. The current at B=0T is greatly decreased while the current at B>0T is relatively large due to the band-to-band tunneling effects, resulting in a high magnetoresistance ratio, even at room-temperature. Moreover, we explore the effects of edge-roughness, length, and width of GNR channels on device performance. An increase in edge-roughness and channel length enhances the magnetoresistance ratio while increased channel width can reduce the operating bias.Comment: http://dx.doi.org/10.1063/1.362445

Electric field induced charge injection or exhaustion in organic thin film transistor

The conductivity of organic semiconductors is measured {\it in-situ} and continuously with a bottom contact configuration, as a function of film thickness at various gate voltages. The depletion layer thickness can be directly determined as a shift of the threshold thickness at which electric current began to flow. The {\it in-situ} and continuous measurement can also determine qualitatively the accumulation layer thickness together with the distribution function of injected carriers. The accumulation layer thickness is a few mono layers, and it does not depend on gate voltages, rather depends on the chemical species.Comment: 4 figures, to be published in Phys. Rev.

Selective Dynamic Nuclear Spin Polarization in Spin-Blocked Double-Dot

We study the mechanism of dynamical nuclear spin polarization by hyperfine interaction in spin-blocked double quantum dot system. We calculate the hyperfine transition rates and solve the master equations for the nuclear spins. Specifically, we incorporate the effects of the nuclear quadrupole coupling due to the doping-induced local lattice distortion and strain. Our results show that nuclear quadrupole coupling induced by the 5% indium substitution can be used to explain the recent experimental observation of missing arsenic NMR signal in the spin-blocked double dots.Comment: 4 pages, 3 figure

Absence of an intrinsic value for the surface recombination velocity in doped semiconductors

A self-consistent expression for the surface recombination velocity $S$ and the surface Fermi level unpinning energy as a function of light excitation power ($P$) is presented for n- and p-type semiconductors doped above the 10$^{16}$ cm$^{-3}$ range. Measurements of $S$ on p-type GaAs films using a novel polarized microluminescence technique are used to illustrate two limiting cases of the model. For a naturally oxidized surface $S$ is described by a power law in $P$ whereas for a passivated surface $S^{-1}$ varies logarithmically with $P$. Furthermore, the variation in $S$ with surface state density and bulk doping level is found to be the result of Fermi level unpinning rather than a change in the intrinsic surface recombination velocity. It is concluded that $S$ depends on $P$ throughout the experimentally accessible range of excitation powers and therefore that no instrinsic value can be determined. Previously reported values of $S$ on a range of semiconducting materials are thus only valid for a specific excitation power.Comment: 10 pages, 7 figure

Deep levels in a-plane, high Mg-content MgxZn1-xO epitaxial layers grown by molecular beam epitaxy

Deep level defects in n-type unintentionally doped a-plane MgxZn1−xO, grown by molecular beam epitaxy on r-plane sapphire were fully characterized using deep level optical spectroscopy (DLOS) and related methods. Four compositions of MgxZn1−xO were examined with x = 0.31, 0.44, 0.52, and 0.56 together with a control ZnO sample. DLOS measurements revealed the presence of five deep levels in each Mg-containing sample, having energy levels of Ec − 1.4 eV, 2.1 eV, 2.6 V, and Ev + 0.3 eV and 0.6 eV. For all Mg compositions, the activation energies of the first three states were constant with respect to the conduction band edge, whereas the latter two revealed constant activation energies with respect to the valence band edge. In contrast to the ternary materials, only three levels, at Ec − 2.1 eV, Ev + 0.3 eV, and 0.6 eV, were observed for the ZnO control sample in this systematically grown series of samples. Substantially higher concentrations of the deep levels at Ev + 0.3 eV and Ec − 2.1 eV were observed in ZnO compared to the Mg alloyed samples. Moreover, there is a general invariance of trap concentration of the Ev + 0.3 eV and 0.6 eV levels on Mg content, while at least and order of magnitude dependency of the Ec − 1.4 eV and Ec − 2.6 eV levels in Mg alloyed samples

Temperature dependent transport characteristics of graphene/n-Si diodes

Realizing an optimal Schottky interface of graphene on Si is challenging, as the electrical transport strongly depends on the graphene quality and the fabrication processes. Such interfaces are of increasing research interest for integration in diverse electronic devices as they are thermally and chemically stable in all environments, unlike standard metal/semiconductor interfaces. We fabricate such interfaces with n-type Si at ambient conditions and find their electrical characteristics to be highly rectifying, with minimal reverse leakage current ($<$10$^{-10}$ A) and rectification of more than $10^6$. We extract Schottky barrier height of 0.69 eV for the exfoliated graphene and 0.83 eV for the CVD graphene devices at room temperature. The temperature dependent electrical characteristics suggest the influence of inhomogeneities at the graphene/n-Si interface. A quantitative analysis of the inhomogeneity in Schottky barrier heights is presented using the potential fluctuation model proposed by Werner and G\"{u}ttler.Comment: 5 pages, 5 figure

Quantitative analysis of electronic transport through weakly-coupled metal/organic interfaces

Using single-crystal transistors, we have performed a systematic experimental study of electronic transport through oxidized copper/rubrene interfaces as a function of temperature and bias. We find that the measurements can be reproduced quantitatively in terms of the thermionic emission theory for Schottky diodes, if the effect of the bias-induced barrier lowering is included. Our analysis emphasizes the role of the coupling between metal and molecules, which in our devices is weak due to the presence of an oxide layer at the surface of the copper electrodes.Comment: 4 pages, 3 figure

Characterization of the Schottky Barrier in SrRuO3/Nb:SrTiO3 Junctions

Internal photoemission spectroscopy was used to determine the Schottky barrier height in rectifying SrRuO3/Nb-doped SrTiO3 junctions for 0.01 wt % and 0.5 wt % Nb concentrations. Good agreement was obtained with the barrier height deduced from capacitance-voltage measurements, provided that a model of the nonlinear permittivity of SrTiO3 was incorporated in extrapolating the built-in potential, particularly for high Nb concentrations. Given the generic polarizability of perovskites under internal/external electric fields, internal photoemission provides a valuable independent probe of the interface electronic structure.Comment: 15 pages, 3 figure

Magnetodielectric Coupling in Nonmagnetic Au/GaAs:Si Schottky Barriers

We report on a heretofore unnoted giant negative magnetocapacitance (>20%) in non-magnetic Au/GaAs:Si Schottky barriers that we attribute to a magnetic field in-duced increase in the binding energy of the shallow donor Si impurity atoms. Depletion capacitance (Cdep) dispersion identifies the impurity ionization and capture processes that give rise to a magnetic field dependent density of ionized impurities. Internal photoemission experiments confirm that the large field-induced shifts in the built-in potential, inferred from 1/Cdep^2 vs voltage measurements, are not due to a field-dependent Schottky barrier height, thus requiring a modification of the abrupt junction approximation that accounts for the observed magnetodielectric coupling.Comment: 25 pages, 5 figures, submitted to Phys. Rev.