Olivia M. Berengue

Structural characterization of indium oxide nanostructures: a Raman analysis

In this work we report on structural and Raman spectroscopy measurements of pure and Sn-doped In2O3 nanowires. Both samples were found to be cubic and high quality single crystals. Raman analysis was performed to obtain the phonon modes of the nanowires and to confirm the compositional and structural information given by structural characterization. Cubic-like phonon modes were detected in both samples and their distinct phase was evidenced by the presence of tin doping. As a consequence, disorder effects were detected evidenced by the break of the Raman selection rules.

Back-to-back Schottky diodes: the generalization of the diode theory in analysis and extraction of electrical parameters of nanodevices

We report on the analysis of nonlinear current-voltage characteristics exhibited by a set of blocking metal/SnO(2)/metal. Schottky barrier heights in both interfaces were independently extracted and their dependence on the metal work function was analyzed. The disorder-induced interface states effectively pinned the Fermi level at the SnO(2) surface, leading to the observed Schottky barriers. The model is useful for any two-terminal device which cannot be described by a conventional diode configuration.

Semiconducting Sn3O4 nanobelts: Growth and electronic structure

The study of structures based on nonstoichiometric SnO2−x compounds, besides experimentally observed, is a challenging task taking into account their instabilities. In this paper, we report on single crystal Sn3O4 nanobelts, which were successfully grown by a carbothermal evaporation process of SnO2 powder in association with the well known vapor-solid mechanism. By combining the structural data and transport properties, the samples were investigated. The results showed a triclinic semiconductor structure with a fundamental gap of 2.9 eV. The semiconductor behavior was confirmed by the electron transport data, which pointed to the variable range hopping process as the main conduction mechanism, thus giving consistent support to the mechanisms underlying the observed semiconducting character.

Electron transport properties of undoped SnO2 monocrystals

Using low-resistance indium contacts, we measured some transport properties of undoped vapor-liquid-solid grown tin oxide monocrystals with a belt shape. From the transport measurements, the two following conduction mechanisms were investigated: thermal activation and variable range hopping. An energy gap of 3.8 eV was found. The energy gap was confirmed by thermally activated measurements in the range between 10 and 300 K. For high temperatures (T>300 K), the influence of the disorder caused by the superficial ions layer is measurable. The electron transport in this case was found to be governed by the well known variable range hopping mechanism and the spatial extension of carrier’s wavelength was calculated to be 4 nm.

Magnetoresistance in Sn-Doped In2O3Nanowires

In this work, we present transport measurements of individual Sn-doped In2O3nanowires as a function of temperature and magnetic field. The results showed a localized character of the resistivity at low temperatures as evidenced by the presence of a negative temperature coefficient resistance in temperatures lower than 77 K. The weak localization was pointed as the mechanism responsible by the negative temperature coefficient of the resistance at low temperatures.

Oxygen-induced metal-insulator-transition on single crystalline metal oxide wires

In this work we report on the transition from metal to insulator conduction of individual single crystalline In2 O3 wires induced by different oxygen concentration during their growth. The transport measurements revealed that the metallic conduction was mainly governed by the acoustic phonon scattering and the insulating character was addressed by the variable range hopping mechanism, which in turn can be considered as a first evidence of the occurrence of an Anderson-like metal-insulator-transition (MIT). The experimental data provided the critical carrier density to be 8×1018 cm-3 corresponding to a critical impurities spacing of 2.5 nm, which was found to be in agreement with previous reported data on polycrystalline indium oxide samples and with our recent finding on In2 O3 semiconducting samples. The approach presented here can be used to grow other metal oxide systems in which oxygen vacancies play a fundamental role for the electron transport features.

Measuring the mobility of single crystalline wires and its dependence on temperature and carrier density

Kinetic transport parameters are fundamental for the development of electronic nanodevices. We present new results for the temperature dependence of mobility and carrier density in single crystalline In(2)O(3) samples and the method of extraction of these parameters which can be extended to similar systems. The data were obtained using a conventional Hall geometry and were quantitatively described by the semiconductor transport theory characterizing the electron transport as being controlled by the variable range hopping mechanism. A comprehensive analysis is provided showing the contribution of ionized impurities (low temperatures) and acoustic phonon (high temperatures) scattering mechanisms to the electron mobility. The approach presented here avoids common errors in kinetic parameter extraction from field effect data, serving as a versatile platform for direct investigation of any nanoscale electronic materials.

Activation energies in diamond films evaluated using admittance spectroscopy and resistivity measurements

This article reports on the study of high-quality boron-doped diamond films using admittance techniques. We have found two well-defined energy states at 74 and 340 meV, indicating that the doping procedure has induced defects and consequently provoked the localization of carriers. This is a direct indication that there are different coexisting conduction mechanisms for the transport of carriers. Additionally, we perform complementary resistivity experiments showing the presence of the variable range hopping as the dominant transport mechanism.

The study of electron scattering mechanisms in single crystal oxide nanowires

We report on transport measurements of individual Sn doped In2O3 nanowires. From these measurements we point out that spin–orbit and boundary scattering mechanisms seem to give a negligible contribution to the transport of electrons in these nanowires. In fact, these results can be extended to other oxide systems: the presence of a weak disorder arising from the random potential at the boundaries screen electrons away from the surface into the nanowire. Electrons travelling through the nanowire in inner conducting channels are not directly influenced by the surfaces and the boundary scattering is decreased. These findings were also supported by calculations of the electron distribution in the cross-section of the nanowires when some disorder is taken into account.

Direct evidence of traps controlling the carriers transport in SnO 2 nanobelts

This work reports on direct evidence of localized states in undoped SnO2 nanobelts. Effects of disorder and electron localization were observed in Schottky barrier dependence on the temperature and in thermally stimulated currents. A transition from thermal activation to hopping transport mechanisms was also observed. The energy levels found by thermally stimulated current experiments were in close agreement with transport data confirming the role of localization in determining the properties of devices.