C. García Núñez

On the true optical properties of zinc nitride

Refractive index (n) and extinction coefficient (k) of Zn3N2 layers deposited by radio-frequency magnetron sputtering at temperatures (Ts) between 298 and 523 K were determined by spectroscopic ellipsometry. Results showed strong variations of the apparent optical constants with Ts and time attributed to surface effects. Resonant Rutherford backscattering and spectroscopic ellipsometry confirmed the formation of a ZnO surface layer provoked by the ambient exposure. Samples grown at low Ts presented the lowest surface roughness and exhibited 2.0 < n < 2.8 and 0.6 < k < 1.0 in the 1.5–4.5 eV energy range. The extracted n and k values accurately reproduced the reflectance properties.

Thin film transistors based on zinc nitride as a channel layer for optoelectronic devices

Zinc nitride films were used as an active layer in thin film transistors to assess its performance in optoelectronic applications. Those nitride layers were grown by radio-frequency magnetron sputtering in Ar/N2 ambient using a Zn target. Bottom- and top-gate thin film transistors were fabricated by photolithography processes. Transmission measurements for these particular layers showed an absorption edge around 1.3 eV. Normally off transistor characteristics with a threshold voltage of 6 V were obtained in the bottom-gate configuration without post-growth annealing. In the saturation region, those transistors produced enhanced output characteristics under illumination with infrared/visible light.

Surface optical phonons in GaAs nanowires grown by Ga-assisted chemical beam epitaxy

Surface optical (SO) phonons were studied by Raman spectroscopy in GaAs nanowires (NWs) grown by Ga-assisted chemical beam epitaxy on oxidized Si(111) substrates. NW diameters and lengths ranging between 40 and 65 nm and between 0.3 and 1.3 μm, respectively, were observed under different growth conditions. The analysis of the Raman peak shape associated to either longitudinal or surface optical modes gave important information about the crystal quality of grown NWs. Phonon confinement model was used to calculate the density of defects as a function of the NW diameter resulting in values between 0.02 and 0.03 defects/nm, indicating the high uniformity obtained on NWs cross section size during growth. SO mode shows frequency downshifting as NW diameter decreases, this shift being sensitive to NW sidewall oxidation. The wavevector necessary to activate SO phonon was used to estimate the NW facet roughness responsible for SO shift.

Fast response ZnO:Al/CuO nanowire/ZnO:Al heterostructure light sensors fabricated by dielectrophoresis

Metal oxide nanowire (NW) photoconductors tend to exhibit high photoconductive gains and long recovery times mainly due to surface effects. In this work, p-type CuO NWs are synthesized by direct oxidation of copper and deposited on n-type ZnO:Al electrodes by dielectrophoresis. The heterostructure is electro-optically characterized showing recovery times in the 10 μs range, mainly limited by the resistance-capacitance product of the equivalent circuit, without signs of persistent effects. The fast response is attributed to short transit times across space charge regions built between CuO and ZnO:Al materials and fast carrier recombination at neutral regions.

Effects of Hydroxylation and Silanization on the Surface Properties of ZnO Nanowires

Silanization is commonly used to form bonds between inorganic materials and biomolecules as a step in the surface preparation of solid-state biosensors. This work investigates the effects of silanization with amino-propyldiethoxymethylsilane on hydroxylated sidewalls of zinc oxide (ZnO) nanowires (NWs). The surface properties and electrical characteristics of NWs are analyzed by different techniques after their hydroxylation and later silanization. Contact angle measurements reveal a stronger hydrophobic behavior after silanization, and X-ray photoelectron spectroscopy (XPS) results show a reduction of the surface dipole induced by the replacement of the hydroxyl group with the amine terminal group. The lower work function obtained after silanization in contact potential measurements corroborates the attenuation of the surface dipole observed in XPS. Furthermore, the surface band bending of NWs is determined from surface photovoltage measurements upon irradiation with UV light, yielding a 0.5 eV energy in hydroxylated NWs, and 0.18 eV, after silanization. From those results, a reduction in the surface state density of 3.1 × 10(11) cm(-2) is estimated after silanization. The current-voltage (I-V) characteristics measured in a silanized single NW device show a reduction of the resistance, due to the enhancement of the conductive volume inside the NW, which also improves the linearity of the I-V characteristic.

Nanomaterials processing for flexible electronics

Inorganic nanomaterials such as nanowires (NWs) and nanotubes (NTs) are explored for future flexible electronics applications due to their attributes such as high aspect ratio, enhanced surface-to-volume ratio, prominent mobility and ability to integrate on non-conventional substrates. Device performance of semiconducting NWs are demonstrated to be superior compared to the organic counterparts. Among the synthesis methods, bottom-up vapour-liquid-solid (VLS) growth mechanism playing central role for preparing wide variety of high crystal quality semiconducting NWs. However, the high temperature synthesis process prevents fabrication of NW devices directly over flexible substrates which imply the investigation of efficient transfer techniques such as dry contact printing and electric field assisted assembly. Currently, many efforts are directed to study the integration techniques of NWs from growth substrates to non-conventional receiver substrates and parameters such as transfer-yield, alignment and density. These efforts will help to utlilize NWs as building blocks in future flexible electronic devices and circuits. This work focuses on VLS growth of semiconducting NWs and their transfer-printing over large area substrate to fabricate flexible electronics.

Zinc nitride thin films: basic properties and applications

Zinc nitride films can be deposited by radio frequency magnetron sputtering using a Zn target at substrate temperatures lower than 250°C. This low deposition temperature makes the material compatible with flexible substrates. The asgrown layers present a black color, polycrystalline structures, large conductivities, and large visible light absorption. Different studies have reported about the severe oxidation of the layers in ambient conditions. Different compositional, structural and optical characterization techniques have shown that the films turn into ZnO polycrystalline layers, showing visible transparency and semi-insulating properties after total transformation. The oxidation rate is fairly constant as a function of time and depends on environmental parameters such as relative humidity or temperature. Taking advantage of those properties, potential applications of zinc nitride films in environmental sensing have been studied in the recent years. This work reviews the state-of-the-art of the zinc nitride technology and the development of several devices such as humidity indicators, thin film (photo)transistors and sweat monitoring sensors.

On the growth mechanisms of GaAs nanowires by Ga-assisted chemical beam epitaxy

GaAs nanowires (NWs) growth kinetics by Ga-assisted chemical beam epitaxy on Si(111) substrates is studied as a function of the initial Ga catalyst dimensions and growth parameters such as substrate temperature and V/III flux ratio. The preparation method for substrates is optimized in order to obtain a surface oxide with a thickness around 0.5 nm, allowing the decomposition of Ga metalorganic precursor and the preferential growth of GaAs NWs at the oxide pinholes. The successful self-formation of Ga droplets over the slightly oxidized Si surface has been observed by scanning electron microscopy (SEM), whose initial size is demonstrated to affect both the NW growth rate and the resultant NW aspect ratio. NW morphology is thoroughly analyzed by SEM, showing a self-organized array of vertically aligned match-shaped GaAs NWs with a hexagonal footprint. In addition, the crystalline structure of NWs is monitored in-situ by reflection high-energy diffraction (RHEED), showing pure zincblende phase along the whole NW stem.

Continuous-flow system and monitoring tools for the dielectrophoretic integration of nanowires in light sensor arrays

Although nanowires (NWs) may improve the performance of many optoelectronic devices such as light emitters and photodetectors, the mass commercialization of these devices is limited by the difficult task of finding reliable and reproducible methods to integrate the NWs on foreign substrates. This work shows the fabrication of zinc oxide NWs photodetectors on conventional glass using transparent conductive electrodes to effectively integrate the NWs at specific locations by dielectrophoresis (DEP). The paper describes the careful preparation of NW dispersions by sedimentation and the dielectrophoretic alignment of NWs in a home-made system. This system includes an impedance technique for the assessment of the alignment quality in real time. Following this procedure, ultraviolet photodetectors based on the electrical contacts formed by the DEP process on the transparent electrodes are fabricated. This cost-effective mean of contacting NWs enables front-and back-illumination operation modes, the latter eliminating shadowing effects caused by the deposition of metals. The electro-optical characterization of the devices shows uniform responsivities in the order of 106 A W(-1) below 390 nm under both modes, as well as, time responses of a few seconds.

Fabrication and Characterization of Multiband Solar Cells Based on Highly Mismatched Alloys

Multiband solar cells are one type of third generation photovoltaic devices in which an increase of the power conversion efficiency is achieved through the absorption of low energy photons while preserving a large band gap that determines the open circuit voltage. The ability to absorb photons from different parts of the solar spectrum originates from the presence of an intermediate energy band located within the band gap of the material. This intermediate band, acting as a stepping stone allows the absorption of low energy photons to transfer electrons from the valence band to the conduction band by a sequential two photons absorption process. It has been demonstrated that highly mismatched alloys offer a potential to be used as a model material system for practical realization of multiband solar cells. Dilute nitride GaAs1-xNx highly mismatched alloy with low mole fraction of N is a prototypical multiband semiconductor with a well-defined intermediate band. Currently, we are using chemical beam epitaxy to synthesize dilute nitride highly mismatched alloys. The materials are characterized by a variety of structural and optical methods to optimize their properties for multiband photovoltaic devices.