Nm Of Al2O3 Research Articles

Steady-state and transient photoconductivity in c-axis GaN nanowires grown by nitrogen-plasma-assisted molecular beam epitaxy

Analysis of steady-state and transient photoconductivity measurements at room temperature performed on c-axis oriented GaN nanowires yielded estimates of free carrier concentration, drift mobility, surface band bending, and surface capture coefficient for electrons. Samples grown (unintentionally n-type) by nitrogen-plasma-assisted molecular beam epitaxy primarily from two separate growth runs were examined. The results revealed carrier concentration in the range of (3–6)×1016 cm−3 for one growth run, roughly 5×1014–1×1015 cm−3 for the second, and drift mobility in the range of 500–700 cm2/(V s) for both. Nanowires were dispersed onto insulating substrates and contacted forming single-wire, two-terminal structures with typical electrode gaps of ≈3–5 μm. When biased at 1 V bias and illuminated at 360 nm (3.6 mW/cm2) the thinner (≈100 nm diameter) nanowires with the higher background doping showed an abrupt increase in photocurrent from 5 pA (noise level) to 0.1–1 μA. Under the same conditions, thicker (151–320 nm) nanowires showed roughly ten times more photocurrent, with dark currents ranging from 2 nA to 1 μA. With the light blocked, the dark current was restored in a few minutes for the thinner samples and an hour or more for the thicker ones. The samples with lower carrier concentration showed similar trends. Excitation in the 360–550 nm range produced substantially weaker photocurrent with comparable decay rates. Nanowire photoconductivity arises from a reduction in the depletion layer via photogenerated holes drifting to the surface and compensating ionized surface acceptors. Simulations yielded (dark) surface band bending in the vicinity of 0.2–0.3 V and capture coefficient in the range of 10−23–10−19 cm2. Atomic layer deposition (ALD) was used to conformally deposit ≈10 nm of Al2O3 on several devices. Photoconductivity, persistent photoconductivity, and subgap photoconductivity of the coated nanowires were increased in all cases. TaN ALD coatings showed a reduced effect compared to the Al2O3 coated samples.

High-mobility enhancement-mode 4H-SiC lateral field-effect transistors utilizing atomic layer deposited Al2O3 gate dielectric

Lateral metal-oxide-semiconductor field-effect transistors (MOSFETs) have been fabricated on 4H-SiC utilizing deposited dielectrics and gate-last processing. The bilayer dielectric consists of thin nitrided SiO2 covered by 25 nm of Al2O3 deposited using atomic layer deposition. Field-effect mobility and threshold voltage (VT) vary with SiC nitric oxide (NO) anneal temperature. Peak mobility of 106 cm2/V⋅s was obtained with corresponding VT of 0.8 V. The peak mobility decreases to 61 cm2/V⋅s with a lower temperature NO anneal, while the VT increased to 1.4 V. Thus with proper gate engineering, high-mobility normally off MOSFET devices can be obtained, leading to higher-performance gate-controlled power devices.

Fabrication of low-loss channel waveguides in Al2O3 and Y2O3 layers by inductively coupled plasma reactive ion etching

Etching of amorphous Al2O3 and polycrystalline Y2O3 films has been investigated using an inductively coupled reactive ion etch system. The etch behaviour has been studied by applying various common process gases and combinations of these gases, including CF4/O2, BCl3, BCl3/HBr, Cl2, Cl2/Ar and Ar. The observed etch rates of Al2O3 films were much higher than Y2O3 for all process gases except for Ar, indicating a much stronger chemical etching component for the Al2O3 layers. Based on analysis of the film etch rates and an investigation of the selectivity and patterning feasibility of possible mask materials, optimized optical channel-waveguide structures were fabricated in both materials. In Al2O3, channel waveguides were fabricated with BCl3/HBr plasma and using a standard resist mask, while in Y2O3, channel waveguides were fabricated with Ar and using either a resist or a sputter deposited Al2O3 mask layer. The etched structures in both materials exhibit straight sidewalls with minimal roughness and sufficient etch depths (up to 530 nm for Al2O3 and 250 nm for Y2O3) for defining waveguides with strong optical confinement. Using the developed etch processes, low additional optical propagation losses (on the order of 0.1 dB/cm) were demonstrated in single-mode ridge waveguides in both Al2O3 and Y2O3 layers at 1550 nm.

Application of Atomic Layer Deposition for Gas Permeation Barrier Thin Films

We determined by a Ca-test with optical modeling that a 25 nm thick Al2O3 film, grown by atomic layer deposition on polyester, reduced the effective water vapor transmission rate to < 10-5 g-H2O/m2-day. This is a reduction of more than 105x compared to the bare polyester. The corresponding diffusion coefficient for water vapor through this film was calculated to be D=2.5x10-17 cm2/s, and the threshold thickness for barrier properties was ~ 5 nm of Al2O3. We further concluded that these values were likely at the lower limit of this Ca-test method, because of edge diffusion of gas through the epoxy seal in the test structure.

A SANS Study of 3PEG−LiClO4−TiO2 Nanocomposite Polymer Electrolytes

Nanocomposite polymer electrolytes based on a fully amorphous polymer electrolyte (trihydroxypoly(ethylene oxide-co-propylene oxide), known as 3PEG, with LiClO4 salt) were studied by means of small-angle neutron scattering with the aim to clarify how the filler particles (nanosized TiO2 or Al2O3) are distributed in the polymer. The scattering data show that the particles are not well dispersed but aggregate in fractal, polymer-like structures. The structures, with a fractal dimension of ∼2 on length scales from 100 nm up to at least 400 nm, consist of branched chains which are about 1−2 primary particles wide (∼40 nm for TiO2 and ∼30 nm for Al2O3). The obtained results provide structural support for the hypothesis of earlier works that the ionic conductivity increase observed in nanocomposite polymer electrolytes is due to the formation of a percolating layer close to the filler particles with higher conductivity.

Equilibrium Amorphous Silicon–Calcium–Oxygen Films at Interfaces in Copper–Alumina Composites Prepared by Melt Infiltration

The microstructure of copper–alumina (Cu‐Al2O3) composites that have been prepared via the melt infiltration of liquid copper into porous alumina preforms was studied in detail, using various transmission electron microscopy (TEM) techniques. Two different samples—with open pore diameters of 0.2 and 0.8 μm—were investigated. For both specimens, a single crystalline copper network that extended throughout the open porosity of the alumina preform was observed. An amorphous glass phase that contained silicon and calcium was observed at the Al2O3/Cu/Al2O3 triple junctions. The diameters of these amorphous pockets, which were strongly faceted along the Al2O3 grains, were up to 20 and 100 nm for the initial pore sizes of 0.2 and 0.8 μm, respectively. A glass phase that contained silicon and calcium also was present at the Cu/Al2O3 interfaces, whereas the Al2O3 boundaries remained dry. Detailed high‐resolution transmission electron microscopy investigations have shown that the interfacial glass phase at the Cu/Al2O3 interfaces exhibited a uniform equilibrium film thickness along the interface region. However, the interfacial film thickness was dependent on the orientation of the Al2O3 grain, and its value varied from 0.4 nm for Al2O3 rhombohedral‐plane termination ((1¯012)) up to 1 nm for Al2O3 basal‐plane termination ((0001)).

Thermally induced structural modifications in sputtered C/Al2O3 multilayers

We have prepared C/Al2O3 multilayers by rf and rf magnetron sputtering on sapphire (112̄0) and silicon (100) substrates. We have analyzed their structure by x-ray reflectivity measurements in the as-deposited state and after annealing. The interface roughness is only 0.3 nm. After thermal treatment, the double-layer thickness increases, which is caused by an expansion of the carbon layers. Raman scattering experiments show that this behavior can be explained by a reduction of the number of sp3-coordinated carbon atoms. Conductivity measurements support the structural model. From in situ resistance measurements during the deposition process we derive a percolation threshold of 1.2 nm for carbon and 0.9 nm for Al2O3. Therefore, the smallest double-layer thickness possible would be slightly above 2.1 nm. Both materials remain amorphous during the annealing process up to 1100 °C. The thermal stability of this material combination is extremely high. Annealing at 1100 °C for 1 h not only enhances the reflectivity of the multilayer system, but also prevents further period expansion of the carbon layers during subsequent annealing steps. We also discuss possible applications as normal-incidence mirrors for the C K edge.

Spin-dependent tunneling between ferromagnetic metals in a new type of tunnel junction (abstract)

We report on tunneling experiments in tunnel junctions with ferromagnetic electrodes (cobalt or permalloy) separated by a relatively thick (15 nm) Al2O3 layer in which small cobalt clusters are embedded. Electron transport is, thus, by successive tunnelings from electrode to cluster and from cluster to cluster. This structure for multichannel and multistep tunneling is less liable to metallic bridging by pinholes than junctions with an unique thin layer of insulator, and also makes easier the control of the total tunnel resistance. Layers of Al2O3 and discontinuous ultrathin layers of cobalt (“cluster layers”) are alternately deposited by sputtering through masks. The structure is characterized by transmission electron microscopy (Co clusters of 3 nm typical diameter, with about 2 nm of Al2O3 between successive cluster layers and between clusters and electrode). The total tunnel resistance can reach 25 kΩ at 4.2 K. Its variation with magnetic field reflects the magnetization reversal of the electrodes and cobalt clusters, with a maximum fractional change of about 21% at room temperature.