Al2O3 Buffer Research Articles

Growth of vertical carbon nanotubes according to the Al2O3 buffer layer preparation

Carbon nanotubes (CNTs) have been extensively studied for several decades due to its promising properties. In this study, effects of the Al2O3 buffer layer on the growth of the wellorganized CNTs forest were studied. The buffer layers were prepared by the two methods, direct sputtering using Al with oxygen feeding and annealing the sputtered pure Al films to obtain Al2O3 films. FE-SEM and AFM results showed that the surface of the buffer layer by the first method was more clean and smooth. The CNTs forest using the first buffer layer showed better results both in the growth length and CNTs quality.

Improvement of Capacitance Equivalent Thickness, Leakage Current, and Interfacial State Density Based on Crystallized High-K Dielectrics/Nitrided Buffer Layer Gate Stacks

The gate stack composed of tetragonal/cubic ZrO2 high-K dielectrics and nitrided Al2O3 buffer layer was investigated to reduce the capacitance equivalent thickness (CET), leakage current density (Jg), and interfacial state density (Dit). The high-K tetragonal/cubic phase of ZrO2 was formed by post metallization annealing at a low temperature of 450°C. The Jg was suppressed by the insertion of the Al2O3 buffer layer between ZrO2 and Si. The remote NH3 plasma nitridation on the Al2O3 buffer layer further leads to the deactivation of the oxygen vacancies and the well passivation of the Si dangling bonds. Accordingly, a suppressed Jg of 8.12 × 10−6 A/cm2 and Dit of 2.77 × 1011 cm−2 eV−1 were achieved in the crystallized ZrO2/nitrided Al2O3 gate stack with a low CET of 1.2 nm. The gate stack was also optically probed through the photoluminescence from Si, revealing that the hydrogen passivation and depassivation effects caused by the remote NH3 plasma treatment are highly correlated with the Dit. The results indicate that the crystallized high-K dielectrics/nitrided buffer layer is a promising gate stack structure to improve the electrical characteristics in the advanced metal-oxide-semiconductor devices.

Factors Affecting the Growth of Carbon Nanofibers on Titanium Substrates and Their Electrical Properties

The goal of this work is to investigate the growth mechanism of carbon nanofibers (CNFs) on titanium (Ti) substrates and the factors that affect the growth the CNFs. Ti substrates were chosen because they are a biocompatible material and the results of this work have potential application for electrochemical biosensors and/or supercapacitors. CNFs were synthesized via water-assisted chemical vapor deposition at 800 degrees C and at atmospheric pressure utilizing iron (Fe) nanoparticles as catalysts, ethylene (C2H4) as the precursor gas, and argon (Ar) and hydrogen (H2) as the carrier gases. The introduction of an Al2O3 buffer layer and the thickness as well as the roughness of the Ti substrates was found to affect the morphology and distribution of the Fe nanoparticles, and thus the morphology of the CNFs. The sputtered buffer layer leads to fewer catalyst nanoparticles that diffuse into the underlying Ti layer which results in a more uniform and denser distribution of the Fe nanoparticles. Stronger catalyst-substrate interaction results in a larger and sparser distribution of the Fe nanoparticles. On the other hand, when the roughness of the substrate exceeds the thickness of the Fe catalyst layer, the nanoparticles tend to form into large sized particles. The longest and densest CNFs (10.7 microm in length) were grown on a Ti layer of 10 nm in thickness with the introduction of the buffer layer. The characterization of CNFs was carried out using scanning electron microscopy (SEM), electron dispersive spectroscopy (EDS), atomic force microscopy (AFM), transmission electron microscopy (TEM). The electrical properties of the CNF films were investigated via the four-point probe method that showed an ohmic behavior. The sheet resistances of the CNF films on Ti substrates of different thickness were also reported.

Synthesis and Characterization of Vertically Aligned Carbon Nanotube Forest for Solid State Fiber Spinning

Continuous carbon nanotubes (CNT) fibers were directly spun from a vertically aligned CNT forest grown by a plasma-enhanced chemical vapor deposition (PECVD) process. The correlation of the CNT structure with Fe catalyst coarsening, reaction time, and the CNTs bundling phenomenon was investigated. We controlled the diameters and walls of the CNTs and minimized the amorphous carbon deposition on the CNTs for favorable bundling and spinning of the CNT fibers. The CNT fibers were fabricated with an as-grown vertically aligned CNT forest by a PECVD process using nanocatalyst an Al2O3 buffer layer, followed by a dry spinning process. Well-aligned CNT fibers were successfully manufactured using a dry spinning process and a surface tension-based densification process by ethanol. The mechanical properties were characterized for the CNT fibers spun from different lengths of a vertically aligned CNT forest. Highly oriented CNT fibers from the dry spinning process were characterized with high strength, high modulus, and high electrical as well as thermal conductivities for possible application as ultralight, highly strong structural materials. Examples of structural materials include space elevator cables, artificial muscle, and armor material, while multifunctional materials include E-textile, touch panels, biosensors, and super capacitors.

Surface photovoltage and Auger electron spectromicroscopy studies of HfO2/SiO2/4H-SiC and HfO2/Al2O3/4H-SiC structures

The electronic and chemical properties of the interface region in the structures obtained by the passivation of epitaxial n-type 4H-SiC layers with bilayers consisting of a 5nm-thick SiO2 or Al2O3 buffer film and high-κ HfO2 layer were investigated. The main aim was to estimate the influence of the passivation approach on the interface effective charge density (Qeff) from the surface photovoltage (SPV) method and, in addition to determine the in-depth element distribution in the interface region from the Auger electron spectroscopy (AES) combined with Ar+ ion profiling. The structure HfO2/SiO2/4H-SiC exhibited slightly superior electronic properties in terms of Qeff (in the range of −1011qcm−2).

Growth, optical, and electrical properties of nonpolar m-plane ZnO on p-Si substrates with Al2O3 buffer layers

Nonploar m-plane ZnO films were grown on p-Si (111) substrates by using atomic layer deposition. X-ray diffraction and high resolution tunneling electron microscopy measurements showed that the ZnO films were 〈101¯0〉 oriented, and the crystalline quality of ZnO films was improved with an Al2O3 buffer layer, which significantly enhances the photoluminescence and reduces the reverse leakage current of ZnO/Si heterojuction.

Ultra simple catalyst layer preparation for the growth of vertically aligned CNTs and CNT-based nanostructures

A simple and rapid route for the preparation of iron-based catalyst layers for CVD growth of carbon nanotubes is introduced. The catalyst was deposited by a microwave-assisted nonaqueous sol–gel process onto silicon wafers with or without an Al2O3 buffer layer and onto colloidal silica spheres. High quality vertically aligned carbon nanotube (VACNT) mats were grown on planar substrates. The versatility of the catalyst preparation is highlighted by the growth of CNTs onto non-planar surfaces such as the silica spheres.

Atomic layer deposited TiO2 for implantable brain-chip interfacing devices

In this paper we investigated atomic layer deposition (ALD) TiO2 thin films deposited on implantable neuro-chips based on electrolyte-oxide-semiconductor (EOS) junctions, implementing both efficient capacitive neuron-silicon coupling and biocompatibility for long-term implantable functionality.The ALD process was performed at 295°C using titanium tetraisopropoxide and ozone as precursors on needle-shaped silicon substrates. Engineering of the capacitance of the EOS junctions introducing a thin Al2O3 buffer layer between TiO2 and silicon resulted in a further increase of the specific capacitance. Biocompatibility for long-term implantable neuroprosthetic systems was checked upon in-vitro treatment.

Effect of Oxide Buffer Layers on the Optical and Electrical Properties of Ga-Doped Zinc Oxide Thin Film

The effects of oxide buffer layers on the optical and electrical properties of sputtered Gallium-doped zinc oxide (GZO) films were intensively investigated for developing the electrodes of photonic devices, which demand high optical transmittance and low resistivity. The use of Al2O3 and SiO2 buffer layers could increase transmittance of GZO films to 90.69% around the wavelength of 550 nm by controlling the optical spectrum. The resistivity of the deposited GZO films was much dependent on the deposition condition of O2 ratio during buffer layer deposition. It is considered that the carrier mobility of GZO films on SiO2 buffer is highly related to the lattice crystallinity of SiO2 and GZO films. In contrast, on the Al2O3 buffer layer, rough surface morphology could increase the resistivity of GZO films due to the doping effect of diffused Al atoms.

The Microstructure, Resistivity and Infrared Emissivity of ITO Film with O/Ar Ratio Variation in Al<sub>2</sub>O<sub>3</sub> Buffer Layer

ITO films with different oxygen-argon ratios varies from 0 % to 8 % for Al2O3 buffer layer have been fabricated by magnetron sputtering on soft PET substrate. The microstructure, resistivity, transmittance in visible light range and infrared emissivity were measured by XRD, four-point probe technology, UV-Vis spectrophotometer and Fourier Transform Infrared Spectroscopy (FTIR) as a function of different oxygen-argon ratios of Al2O3 buffer layers, respectively. It can be found that the (222) plane crystallization improves with the increase of O/Ar ratio, (622） peak under 2% O/Ar ratio and (440), (211) peaks under 8% O/Ar ratio appear, respectively. The resistivity is also influenced significantly by the O/Ar ratio, though the variation of resistivity is not sensitive to O/Ar ratio higher than 2%. The results reveal that the best ITO film performance under the oxygen-argon ratio 2% for Al2O3 buffer layer. It is also found that surface roughness and interface state play an important role in the variation of transmittance and emissivity. The emissivity of all the ITO films deposited on Al2O3 buffer layer is dominated by both sheet resistance and surface morphology.

Influence of Al<sub>2</sub>O<sub>3</sub> Buffer Layers on the Properties of Indium–Tin Oxide Films on PET Substrate by RF-Magnetron Sputtering

Indium Tin Oxide (ITO) films on PET substrate sandwiching Al2O3 buffer layers with different thickness have been prepared by magnetron sputtering at low deposition temperature. The crystal structures, electrical and optical properties of ITO films have been investigated by XRD, four-point probe technology and UV-Vis spectrophotometer as a function of different Al2O3 buffer layers thickness, respectively. XRD reveals that there is an amorphous structure in ITO films with no buffer layer. However, ITO films became crystalline after sandwiching the buffer layer. It can be found that there are two major peaks, (222) and (400) of ITO film. A smallest resistivity of 3.53×10-4 Ω.cm was obtained for ITO film with Al2O3 buffer layers thickness 75nm. The average transmittance of ITO/Al2O3/PET films in the visible range of 400-760nm wavelength was around 80%. It can be conclude that the (222) orientation of ITO film is more in favor of low resistivity.

Piezoelectric Pb(Zr0.52Ti0.48)O3 thin films on single crystal diamond: Structural, electrical, dielectric, and field-effect-transistor properties

The combination of piezoelectric materials and single crystal diamond offers the opportunity for the development of multifunctional micromachined devices under extreme conditions. In this work, the authors report the structural, electrical, optical, and dielectric properties of Pb(Zr0.52,Ti0.48)O3 (PZT) thin films integrated on single crystal diamond (100) substrates. The corresponding field effect transistor based on the metal-piezoelectric-insulator-semiconductor (MPIS) structure was fabricated on a homoepitaxial p-type diamond layer grown on a type-Ib diamond substrate. Different intermediate layers were deposited on single diamond substrates prior to the PZT films growth in order to achieve the best polarization properties. It was observed that the utilization of an Al2O3 buffer layer followed by a SrTiO3 seed layer favored the formation of a single perovskite PZT phase. Transmission electron diffraction patterns revealed that the PZT films included an initial layer at the SrTiO3/PZT interface followed by a well crystallized layer. The PZT film grown on SrTiO3/Al2O3/diamond exhibited much better in-plane polarization than that of the PZT film on Al2O3/diamond. The photoresponse behavior revealed that carriers trapping effect was trivial in the PZT film. The channel electrical conductivity of the MPIS field effect transistor was successfully modulated by the gate bias.

Inverted organic solar cells with ITO electrodes modified with an ultrathin Al2O3 buffer layer deposited by atomic layer deposition

We report on the properties of inverted polymer solar cells using an ultrathin Al2O3 buffer layer on indium tin oxide (ITO). The ultrathin Al2O3 layer, deposited by the atomic layer deposition method, was found to reduce the work function of ITO and turns ITO into an electron-collecting electrode. The current density–voltage characteristics of unexposed devices showed an s-shape kink. The kink was eliminated upon exposure to ultraviolet (UV) illumination. Inverted solar cells based on P3HT:PC60BM yielded a fill factor of 0.64 and a power conversion efficiency of about 2.8% estimated for 100 mW cm−2 simulated AM 1.5 illumination.

Channel/ferroelectric interface modification in ZnO non-volatile memoryTFT with P(VDF-TrFE)polymer

We report on the fabrication of ZnO non-volatile memory thin-film transistors (NVM-TFTs) with 200 nm-thick poly(vinylidene fluoride/trifluoroethylene)[P(VDF-TrFE)] ferroelectric layer. The NVM-TFTs have been tested for the most optimum properties in respect of their memory windows and memory retention properties, as prepared with the modified channel/ferroelectric interfaces inserted by respective thin buffer layers: 1, 3, 5, 10, and 20 nm-thin Al2O3, or 1 nm-thin inorganic–organic hybrid dielectrics of a AlOx-(or TiOx-) self assembled monolayer (SAM). All our NVM-TFTs operated on glass substrates under the low-voltage WR-ER pulses of ±20 V with a maximum field effect mobility of ∼1 cm2/V s and memory window of 12∼16 V. Among all the NVM-TFTs, the device with the 5 nm-thin Al2O3 buffer demonstrated the longest retention time of more than 104 s without much reduction of write-to-erase (WR/ER) current ratio, keeping a good memory window of ∼16 V and WR/ER ratio of ∼40.

Atomic Layer Deposited Al2O3 Film on Si 100 as Buffer Layer for HfxTi1-xO2 Deposition

Ultra-thin films of Al2O3 were deposited on Si(100) by atomic layer deposition (ALD) using tris(diethylamino)aluminium (TDEAA) with water vapor as the oxidizer. It is planned to deposit nano-laminated composite films of HfxTi1-xO2 on top of this ultra-thin Al2O3 buffer layer using tetrakis(diethylamino)titanium (TDEAT) and tetrakis(diethylamino)hafnium (TDEAH). The introduction of the Al2O3 buffer layer will improve the interfacial quality and help to retain amorphous structure of HfxTi1-xO2/Al2O3/Si. Discussion of the addition of the new precursor, TDEAA, to the existing ALD system is discussed along with the optimization of process parameters for ALD of Al2O3 films. An Al2O3 film growth rate of 1 cycle was achieved in the ALD temperature window of 200-275o C.

Enhancing the retention properties of ZnO memory transistor by modifying the channel/ferroelectric polymer interface

We report on the fabrication of ZnO nonvolatile memory thin-film transistors (NVM-TFTs) with thin poly(vinylidene fluoride/trifluoroethylene) [P(VDF-TrFE)] ferroelectric layer. Our NVM-TFT operates on glass substrates under low voltage write-erase (WR-ER) pulse of ±20 V with a maximum field effect mobility of ∼1 cm2/V s, maximum memory window of ∼20 V, and WR-ER current ratio of 4×102. When the NVM-TFT has a modified channel/ferroelectric interface with an inserted thin Al2O3 buffer layer, our device shows long retention time of more than 104 s, which is much enhanced compared to that of the other device without the buffer. The dynamic response of our devices with or without the buffer was clear enough to distinguish the WR and ER states as performed with 300 ms pulse.

The role of aluminum oxide buffer layer in organic spin-valves performance

The electronic structures of the 8-hydroxyquinoline-aluminum (Alq3)/Al2O3/Co interfaces were studied by photoelectron spectroscopy. A strong interface dipole was observed, which leads to a reduction in the electron injection barrier. The x-ray photoelectron spectroscopy spectra further indicate that the Al2O3 buffer layer prevents the chemical interaction between Alq3 molecules and Co atoms. X-ray magnetic circular dichroism results demonstrate that a Co layer deposited on an Al2O3 buffered Alq3 layer shows better magnetic ordering in the interface region than directly deposited Co, which suggests a better performance of spin valves with such a buffer layer. This is consistent with the recent results from [Dediu et al., Phys. Rev. B 78, 115203 (2008)].

Al2O3 buffer in a ZnO thin film transistor with poly-4-vinylphenol dielectric

We compared the characteristics of bottom-gate ZnO-thin film transistors using poly-4-vinylphenol (PVP) and PVP/Al2O3 dielectrics. The PVP dielectric is more hydrophobic than the PVP/Al2O3 dielectric and is not useful for TFT devices because of its high leakage current density, but this leakage current density can be significantly reduced by inserting Al2O3. We deposited ZnO and Al2O3 films by atomic layer deposition (ALD) because it is a low-temperature process. The ZnO-TFTs with either a PVP or a PVP/Al2O3 dielectric exhibit typical field-effect transistor characteristics with n-channel properties. The ZnO-TFT containing PVP/Al2O3 exhibits clear pinch-off and excellent saturation with an enhanced mode operation. The on/off ratio of 7.9 × 104 for the device containing the hybrid dielectric is about three orders of magnitude higher than the ratio of 47 for the device containing PVP. The subthreshold gate swings are 12 V/decade for the TFT containing PVP and 1.2 V/decade for the TFT containing PVP/Al2O3. The density of the interface trap state is significantly lower in the device containing PVP/Al2O3 than in the ZnO-TFT containing PVP. The saturation mobility was 0.05 and 0.8 cm2 V−1 s−1, respectively, in the TFTs containing PVP and PVP/Al2O3.

Microstructural, Magnetic and Microwave Properties of Large Area BaFe$_{12}$O$_{19}$ Thick Films ($>\!100\ \mu$m) Deposited on /a-SiO$_{2}$/Si and /a-Al$_{2}$O$_{3}$/Si Substrates

We have demonstrated the feasibility of thick BaFe12O19 ceramic films fabricated on silicon (100) substrates. We obtained large area (1-in diameter) thick films (100-200 mum) by using a screen printing technique and amorphous SiO2 or Al2O3 buffer layers. To characterize the microstructural, magnetic, and microwave properties of ferrite films, we employed X-ray diffractometry, scanning electron microscopy, energy dispersive X-ray spectroscopy, dc magnetometry, and ferromagnetic resonance (FMR). We placed our emphasis on investigating silicon diffusion in terms of a depth analysis of composition for different buffer layers. We present and discuss FMR measurements for two polycrystalline samples with different buffer layers. We find that a-Al2O3 is superior to a-SiO2 as a buffer layer in achieving a tradeoff between magnetic and mechanical properties.

Efficient suppression of charge trapping in ZnO-based transparent thin film transistors with novel Al2O3∕HfO2∕Al2O3 structure

Charge trapping is dramatically suppressed in ZnO transparent thin film transistors (TFTs) employing a multilayered gate insulator with HfO2 layer sandwiched by Al2O3 layers. In spite of its high dielectric constant, HfO2 has critical drawbacks including huge charge trap density in interfaces. We suggest and demonstrate an elegant solution to minimize the charge trapping introducing Al2O3 buffer layers. The operation of Al2O3∕HfO2∕Al2O3 multilayered gate-insulator structure in the ZnO transparent TFT is evaluated to ensure the voltage difference in the hysteresis loop as low as 0.2V, and the immunization to the threshold voltage shift induced by repeated sweeps of gate voltage.