Usha Philipose

Influence of growth temperature on the stoichiometry of InSb nanowires grown by vapor phase transport

We report on the influence of growth parameters on the stoichiometry of indium antimonide nanowires grown by vapor phase transport. Using electron microscopy and composition analysis, we show that there is an optimum growth temperature window for growing stoichiometric indium antimonide (InSb) nanowires. The choice of the metal catalyst, evaporation and growth temperature are all critical parameters affecting the morphology and stoichiometry of the growing crystal. By controlling the growth temperature, it was possible to grow either stoichiometric InSb nanowires or In nanowires that contained no Sb within detectable limits. Electrical transport measurements of single InSb nanowires with two ohmic contacts demonstrate n-type conduction persisting from room temperature to 15 K.

Digitally tunable holographic lithography using a spatial light modulator as a programmable phase mask

In this paper, we study tunable holographic lithography using an electrically addressable spatial light modulator as a programmable phase mask. We control the phases of interfering beams diffracted from the phase pattern displayed in the spatial light modulator. We present a calculation method for the assignment of phases in the laser beams and validate the phases of the interfering beams in phase-sensitive, dual-lattice, and two-dimensional patterns formed by a rotationally non-symmetrical configuration. A good agreement has been observed between fabricated holographic structures and simulated interference patterns. The presented method can potentially help design a gradient phase mask for the fabrication of graded photonic crystals or metamaterials.

Holographic fabrication of 3D photonic crystals through interference of multi-beams with 4 + 1, 5 + 1 and 6 + 1 configurations

In this paper, we are able to fabricate 3D photonic crystals or quasi-crystals through single beam and single optical element based holographic lithography. The reflective optical elements are used to generate multiple side beams with s-polarization and one central beam with circular polarization which in turn are used for interference based holographic lithography without the need of any other bulk optics. These optical elements have been used to fabricate 3D photonic crystals with 4, 5 or 6-fold symmetry. A good agreement has been observed between fabricated holographic structures and simulated interference patterns.

Holographic fabrication of functionally graded photonic lattices through spatially specified phase patterns

In this paper, we present a method for the mathematically formulated phase engineering of interfering laser beams through a spatial light modulator for a holographic fabrication of graded photonic lattices. The desired phases can be programmed at specific locations by assigning gray levels in cellular structures. The method is demonstrated by embedding single-lattice structures or missing lattices in dual-lattice periodic photonic structures. The demonstrated method can be potentially combined with the coordinate transformation technique in transformation optics for the fabrication of graded photonic devices.

Stoichiometry dependent electron transport and gas sensing properties of indium oxide nanowires

The effect of stoichiometry of single crystalline In2O3 nanowires on electrical transport and gas sensing was investigated. The nanowires were synthesized by vapor phase transport and had diameters ranging from 80 to 100 nm and lengths between 10 and 20 μm, with a growth direction of [001]. Transport measurements revealed n-type conduction, attributed to the presence of oxygen vacancies in the crystal lattice. As-grown In2O3 nanowires were shown to have a carrier concentration of ≈5 × 10(17) cm(-3), while nanowires that were annealed in wet O2 showed a reduced carrier concentration of less than 10(16) cm(-3). Temperature dependent conductivity measurements on the as-grown nanowires and analysis of the thermally activated Arrhenius conduction for the temperature range of 77-350 K yielded an activation energy of 0.12 eV. This is explained on the basis of carrier exchange that occurs between the surface states and the bulk of the nanowire, resulting in a depleted surface layer of thickness of the order of the Debye length (LD), estimated to be about 3-4 nm for the as-grown nanowires and about 10 times higher for the more stoichiometric nanowires. Significant changes in the electrical conductance of individual In2O3 nanowires were also observed within several seconds of exposure to NH3 and O2 gas molecules at room temperature, thus demonstrating the potential use of In2O3 nanowires as efficient miniaturized chemical sensors. The sensing mechanism is dominated by the nanowire channel conductance, and a simple energy band diagram is used to explain the change in conductivity when gas molecules adsorbed on the nanowire surface influence its electrical properties. Less stoichiometric nanowires were found to be more sensitive to oxidizing gases while more stoichiometric nanowires showed significantly enhanced response to reducing gases.

Role of oxygen vacancies in visible emission and transport properties of indium oxide nanowires

We report on the effect of oxygen vacancies on the defect-related emission and the electronic properties of In2O3 nanowires. The nanowires were synthesized by vapor phase transport and had diameters ranging from 80–100 nm and lengths over 10–20 μm, with a growth direction of [0 0 1]. The as-grown nanowires connected in an FET type of configuration show n-type conductivity, which is ascribed to the presence of intrinsic defects like oxygen vacancies in the nanowire. The resistivity, transconductance, field effect mobility and carrier concentration of the In2O3 nanowires were determined to be 1.82 × 10−2 Ω cm, 11.2 nS, 119 cm2 V−1 s−1 and 4.89 × 1017 cm−3, respectively. The presence of oxygen vacancies was also confirmed by photoluminescence measurements, which show a strong UV emission peak at 3.18 eV and defect peaks in the visible region at 2.85 eV, 2.66 eV and 2.5 eV. We present a technique of post-growth annealing in O2 environment and passivation with (NH4)2S to reduce the defect-induced emission.

Synthesis of metallic, semiconducting, and semi-metallic nanowires through control of InSb growth parameters

In this work we present a simple route to grow metallic, semiconducting or semi-metallic nanowires by chemical vapor deposition. Metallic indium (In), semiconducting indium antimonide (InSb) and semi-metallic antimony (Sb) nanowires were successfully synthesized by controlling temperature and hence Sb vapor pressure in the higher eutectic region of the InSb phase diagram. Semiconducting InSb nanowires were synthesized by direct antimonidization of In droplets at a temperature of 480 °C in an Sb-rich environment. I–V measurements on a single 50 nm thick InSb nanowire field-effect transistor show electrons to be the majority carriers with an electron concentration of ≈1018 cm−3. Thermally activated Arrhenius conduction was observed in the temperature range from 200–325 K, yielding an activation energy of 0.11 eV. Metallic In nanowires were grown at 600 °C, using a process similar to that for the growth of InSb nanowires. However, the higher growth temperature resulted in Sb re-evaporating from the growing nanowire crystal, leading to growth of In nanowires. The In nanowires were found to have an extremely high (≈1021 cm−3) electron concentration. Temperature dependent conductivity measurements show that at high temperatures the In nanowire conductivity varies as T−3/2, suggesting that acoustic phonons controlled electron transport. Antimony nanowire growth occurred at 400 °C by a self-catalyzed growth mechanism. Electron transport measurements on a single Sb nanowire reveal p-type conduction, with a hole concentration of ≈1019 cm−3. A higher hole mobility compared to electron mobility and the presence of surface states is the most likely cause of the hole-dominated conductivity in the Sb nanowires.

Electrically tunable diffraction efficiency from gratings in Al-doped ZnO

Transparent conducting aluminum-doped zinc oxide (AZO) can be used as part of an active plasmonic device due to its electrically tunable permittivity, which is accomplished by changing the carrier concentration with electrical biasing. In this letter, we report a continuous electrical tuning of diffraction efficiency from AZO gratings in the visible range (specifically 532 nm) when the AZO is under bias voltages between −1 V and −3.5 V. The carrier concentration in AZO under negative bias has been measured and simulated. The diffraction efficiency changes have been explained by the carrier concentration variation and induced complex refractive index change at the Al2O3 and AZO interface. The reported results can lead toward the application of post-fabrication tuning of optoelectronic devices using AZO.

Role of Electro-Deposition Parameters on Preparing Tailored Dimension Vertically-Aligned ZnO Nanowires

A systematic study of the growth of uniform vertically-aligned ZnO nanowires is presented. It is shown how, by careful control over the growth parameters, high quality nanowires of chosen morphology may be synthesized at low temperatures using electrodeposition. The key experimental parameters were the growth temperature, the ZnCl 2 concentration in the electrolyte, the applied voltage and the thickness of the catalytic Au film. As a result of optimizing these parameters, ZnO nanowires with tailored dimensions were realized - an essential prerequisite for the use of such nanowires in functional nanoscale devices.

Localized surface plasmon polariton resonance in holographically structured Al-doped ZnO

In this paper, we studied the localized surface plasmon polariton (SPP) resonance in hole arrays in transparent conducting aluminum-doped zinc oxide (AZO). CMOS-compatible fabrication process was demonstrated for the AZO devices. The localized SPP resonance was observed and confirmed by electromagnetic simulations. Using a standing wave model, the observed SPP was dominated by the standing-wave resonance along (1,1) direction in square lattices. This research lays the groundwork for a fabrication technique that can contribute to the core technology of future integrated photonics through its extension into tunable conductive materials.