Amorphous Buffer Research Articles

Kinetics of self-induced nucleation and optical properties of GaN nanowires grown by plasma-assisted molecular beam epitaxy on amorphous AlxOy

Nucleation kinetics of GaN nanowires (NWs) by molecular beam epitaxy on amorphous AlxOy buffers deposited at low temperature by atomic layer deposition is analyzed. We found that the growth processes on a-AlxOy are very similar to those observed on standard Si(111) substrates, although the presence of the buffer significantly enhances nucleation rate of GaN NWs, which we attribute to a microstructure of the buffer. The nucleation rate was studied vs. the growth temperature in the range of 720–790 °C, which allowed determination of nucleation energy of the NWs on a-AlxOy equal to 6 eV. This value is smaller than 10.2 eV we found under the same conditions on nitridized Si(111) substrates. Optical properties of GaN NWs on a-AlxOy are analyzed as a function of the growth temperature and compared with those on Si(111) substrates. A significant increase of photoluminescence intensity and much longer PL decay times, close to those on silicon substrates, are found for NWs grown at the highest temperature proving their high quality. The samples grown at high temperature have very narrow PL lines. This allowed observation that positions of donor-bound exciton PL line in the NWs grown on a-AlxOy are regularly lower than in samples grown directly on silicon suggesting that oxygen, instead of silicon, is the dominant donor. Moreover, PL spectra suggest that total concentration of donors in GaN NWs grown on a-AlxOy is lower than in those grown under similar conditions on bare Si. This shows that the a-AlxOy buffer efficiently acts as a barrier preventing uptake of silicon from the substrate to GaN.

Thickness effects in barium hexaferrite films deposited by magnetron sputtering

In this work, barium ferrite films were deposited onto sapphire substrates by radio frequency magnetron sputtering. A thin amorphous barium ferrite film of about 20 nm was deposited at low temperature as a buffer layer. Barium ferrite films with thicknesses of 100 nm, 1 μm and 8 μm have been prepared. The results show that the 8-μm-thick barium ferrite films have no cracks whereas the barium ferrite films without amorphous buffer layer break and peel off. The barium ferrite films with a thickness of 100 nm show good c-axis orientation and strong magnetic anisotropy. The c-axis orientation deteriorates, and the magnetic anisotropy disappears gradually with the increase of the thickness of the barium ferrite films. The barium ferrite films with a thickness of 8 μm orientate randomly and show magnetic isotropy. The thickness effects may be explained by the increase of the random nucleation sites.

Magnetostatic spin wave in a very thin CoFeB film grown on an amorphous FeZr buffer layer

The transmission of a magnetostatic spin wave in a very thin CoFeB film (2.3 nm) grown on an amorphous paramagnetic FeZr layer was demonstrated for the first time by using an antenna method with coplanar waveguides. The attenuation length, determined by using the difference in intensities due to a change in the distance between the antennas, was estimated to be 0.7 µm. We discuss why we are able to observe spin-wave propagation in such a thin magnetic layer, taking the low damping constant (α = 0.0027) of the CoFeB films grown on the amorphous FeZr layer and the estimated large group velocity in the CoFeB films into account.

A kinetic Monte Carlo study of defect assisted transport in silicon heterojunction solar cells

AbstractThe device performance of an amorphous silicon (a‐Si)/crystalline silicon (c‐Si) solar cell depends strongly on the interfacial transport properties of the device. The energy of the photogenerated carriers at the barrier strongly depends on the strength of the inversion at the heterointerface and their collection requires interaction with the defects present in the intrinsic amorphous silicon buffer layer. In this work we present a theoretical model to study the defect assisted transport of photogenerated carriers through the intrinsic amorphous silicon barrier. We implement the kinetic Monte Carlo (KMC) method which allows us to simulate the interaction of discrete carriers with discrete defects. This method allows us to study defect transport which takes place on a time scale which is too long for traditional ensemble Monte Carlo's to analyze. We analyze the injection and extraction of carriers via defects by calculating transition rates, i.e. probability of transition to defect states within the intrinsic amorphous silicon barrier. The KMC results allow us to quantitatively study the properties of the heterointerface barrier in terms of how it affects transport. (© 2015 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Enhanced tunable properties of Bi1.5MgNb1.5O7 thin films grown on Pt–Si substrates using amorphous TiOx buffer layers

Bi1.5MgNb1.5O7 (BMN) thin films were prepared on TiOx buffered Pt–Si substrates by RF magnetron sputtering. The experiments revealed that the BMN thin films with amorphous TiOx buffer layers had better tunable properties than without it. The leakage current density and dielectric loss tangent of the thin films were significantly reduced by insetting amorphous TiOx buffer layers without degradation of the tunability. The mechanism of the enhancement of tunable properties for the BMN thin films with amorphous TiOx buffer layers was proposed. And, the BMN thin films with amorphous TiOx buffer layers has a large figure of merit of 87, being a potential candidate for tunable device applications.

Perpendicular magnetization of CoFeB on top of an amorphous buffer layer

Perpendicular magnetic anisotropy was observed in sputtered FeZr/CoFeB/MgO multilayers. A thin paramagnetic amorphous FeZr layer was used as a buffer layer and perpendicular anisotropy was obtained by annealing the samples without an external magnetic field. The critical CoFeB thickness for perpendicular anisotropy was 1.8nm; the anisotropy changes from out-of-plane to in-plane as the CoFeB thickness increases beyond this point. Perpendicular anisotropy was also enhanced when a Ta layer was capped on top of the MgO layer. The amorphous buffer provided better perpendicular anisotropy than previously reported Ta buffer, and it may be applied to perpendicular magnetization MRAM devices where good uniformity of tunnel junctions is required.

Efficient inverted bulk-heterojunction solar cells from low-temperature processing of amorphous ZnO buffer layers

In this report, we demonstrate that solution-processed amorphous zinc oxide (a-ZnO) interlayers prepared at low temperatures (∼100 °C) can yield inverted bulk-heterojunction (BHJ) solar cells that are as efficient as nanoparticle-based ZnO requiring comparably more complex synthesis or polycrystalline ZnO films prepared at substantially higher temperatures (150–400 °C). Low-temperature, facile solution-processing approaches are required in the fabrication of BHJ solar cells on flexible plastic substrates, such as PET. Here, we achieve efficient inverted solar cells with a-ZnO buffer layers by carefully examining the correlations between the thin film morphology and the figures of merit of optimized BHJ devices with various polymer donors and PCBM as the fullerene acceptor. We find that the most effective a-ZnO morphology consists of a compact, thin layer with continuous substrate coverage. In parallel, we emphasize the detrimental effect of forming rippled surface morphologies of a-ZnO, an observation which contrasts with results obtained in polycrystalline ZnO thin films, where rippled morphologies have been reported to improve efficiency. After optimizing the a-ZnO morphology at low processing temperature for inverted P3HT:PCBM devices, achieving a power conversion efficiency (PCE) of ca. 4.1%, we demonstrate inverted solar cells with low bandgap polymer donors on glass/flexible PET substrates: PTB7:PC71BM (PCE: 6.5% (glass)/5.6% (PET)) and PBDTTPD:PC71BM (PCE: 6.7% (glass)/5.9% (PET)). Finally, we show that a-ZnO based inverted P3HT:PCBM BHJ solar cells maintain ca. 90–95% of their initial PCE even after a full year without encapsulation in a nitrogen dry box, thus demonstrating excellent shelf stability. The insight we have gained into the importance of surface morphology in amorphous zinc oxide buffer layers should help in the development of other low-temperature solution-processed metal oxide interlayers for efficient flexible solar cells.

Spin-filtering effect of thin Al2O3 barrier on tunneling magnetoresistance

Tunneling magnetoresistance (TMR) dependence on the Al2O3 barrier thickness was investigated for CoFe/Al2O3/CoFe magnetic tunnel junctions (MTJs). MTJs with very thin Al2O3 layers were grown by inserting an amorphous FeZr buffer layer whose role is only to reduce the roughness of bottom electrode. The TMR decreased as the thickness of the Al2O3 layer was reduced. The results are analyzed with the dependence of the spin-filtering effect on the Al2O3 thickness. It was found that a simple model of separating sp- and d-like electrons does not work, and it may suggest that the tunneling electrons are in rather hybridized state.

Enhanced catalyst-free nucleation of GaN nanowires on amorphous Al2O3 by plasma-assisted molecular beam epitaxy

We report on plasma-assisted molecular beam epitaxial growth of GaN nanowires (NWs) on Si(111) substrates with a thin amorphous Al2O3 buffer layer deposited by atomic layer deposition. Comparison of nucleation kinetics shows that presence of amorphous Al2O3 buffer significantly enhances spontaneous nucleation of GaN NWs. Slower nucleation was observed on partially amorphous silicon nitride films. No growth of NWs was found on sapphire substrate under the same growth conditions which we explain by a low density of defects on monocrystalline substrate surface where NWs may nucleate. Our finding shows that tuning of substrate microstructure is an efficient tool to control rate of self-induced nucleation of GaN NWs.

Arrangement of GaN nanowires grown by plasma-assisted molecular beam epitaxy on silicon substrates with amorphous Al2O3 buffers

We report on catalyst-free growth of GaN nanowires (NWs) by plasma-assisted molecular beam epitaxy on Si(111) substrates with a thin amorphous Al2O3 buffer layers deposited at low temperature by atomic layer deposition. We show that nanowires obtained are homogenously distributed and well aligned with the c-axis perpendicular to the substrate in a similar way as their counterparts grown without the Al2O3 buffer. However, NWs are twisted nearly randomly and do not show any in-plane alignment with the substrate even for the thinnest (~2nm) buffer used, which is quite opposite to the behavior observed for NWs grown on a bare silicon.

Oxygen pressure dependent growth of pulsed laser deposited LiNbO3 films on diamond for surface acoustic wave device application

LiNbO3 films were grown on nanocrystalline diamond (NCD)/Si substrates with amorphous SiO2 buffer layer by pulsed laser deposition technique at various oxygen pressures ranging from 10Pa to 80Pa. The stoichiometric LiNbO3 ceramic target was used. Significant effects of oxygen pressure on c-axis orientation, stoichiometry, crystallinity and surface morphology of LiNbO3 films were investigated. It was found that the above properties are strongly dependent on the oxygen pressure. Completely c-axis oriented LiNbO3 films with the average surface roughness of 9.0nm could be achieved under an optimum oxygen pressure of 40Pa. Surface acoustic wave devices were fabricated and characterized utilizing the structure of LiNbO3/SiO2/NCD/Si.

Optimized amorphous silicon oxide buffer layers for silicon heterojunction solar cells with microcrystalline silicon oxide contact layers

We report on the systematic optimization of the intrinsic amorphous silicon oxide buffer layer in interplay with doped microcrystalline silicon oxide contact layers for silicon heterojunction solar cells using all silicon oxide based functional layers on flat p-type float-zone wafers. While the surface passivation quality is comparably good within a wide range of low oxygen contents, the optical band gap increases and the dark conductivity decreases with increasing oxygen content, giving rise to an inevitable trade-off between optical transparency and electrical conductivity. On the cell level, fill factor FF and short circuit current density Jsc losses compete with the open circuit voltage Voc gains resulting from a thickness increase of the front buffer layers, whereas Jsc and Voc gains compete with FF losses resulting from increasing thickness of the rear buffer layers. We obtained the highest active area efficiency of ηact = 18.5% with Voc = 664 mV, Jsc = 35.7 mA/cm2, and FF = 78.0% using 4 nm front and 8 nm rear buffer layer with an oxygen content of 5%.

Strain induced changes in magnetization of amorphous Co95Zr5 based multiferroic heterostructures

A clear change in the magnetic anisotropy in a layer of amorphous Co95Zr5 is obtained at the orthorhombic phase transition of the BaTiO3 substrate. The use of an amorphous buffer layer between the ferroelectric substrate and amorphous magnetic film shows that bulk strain governs the change in the magnetic anisotropy of our ferromagnetic-ferroelectric heterostructure. Moreover, we show that the thermal magnetization curves exhibit anisotropic behavior.

Effect of laser fluence on c‐axis orientation of LiNbO3 piezoelectric films on nanocrystalline diamond by pulsed laser deposition

AbstractCompletely c‐axis oriented LiNbO3 piezoelectric films have been deposited on nanocrystalline diamond (NCD)/Si substrates with SiO2 buffer layer by pulsed laser deposition. The amorphous SiO2 buffer layer was formed on NCD/Si substrates by sol‐gel method. The c‐axis orientation and crystallinity of LiNbO3 films are strongly dependent on the laser fluence, and the laser fluence 3.6 J/cm2 is found to be the optimal value for the growth of oriented LiNbO3 film, which has a smooth surface with composed of a large mount of uniform grains. The average surface roughness of LiNbO3 films is about 6.7 nm.

Fractal analysis and atomic force microscopy measurements of surface roughness for Hastelloy C276 substrates and amorphous alumina buffer layers in coated conductors

In coated conductors, surface roughness of metallic substrates and buffer layers could significantly affect the texture of subsequently deposited buffer layers and the critical current density of superconductor layer. Atomic force microscopy (AFM) is usually utilized to measure surface roughness. However, the roughness values are actually relevant to scan scale. Fractal geometry could be exerted to analyze the scaling performance of surface roughness. In this study, four samples were prepared, which were electro polished Hastelloy C276 substrate, mechanically polished Hastelloy C276 substrate and the amorphous alumina buffer layers deposited on both the substrates by ion beam deposition. The surface roughness, described by root mean squared (RMS) and arithmetic average (Ra) values, was analyzed considering the scan scale of AFM measurements. The surfaces of amorphous alumina layers were found to be fractal in nature because of the scaling performance of roughness, while the surfaces of Hastelloy substrates were not. The flatten modification of AFM images was discussed. And the calculation of surface roughness in smaller parts divided from the whole AFM images was studied, compared with the results of actual AFM measurements of the same scan scales.

Hot-wire chemical vapor deposition and characterization of p-type nanocrystalline SiC films and their use in Si heterojunction solar cells

Wide optical bandgap p-type nanocrystalline silicon carbide (p-nc-SiC) films deposited by hot-wire chemical vapor deposition were used as window layers in n-type crystalline Si heterojunction (HJ) solar cells. The effect of H 2 flow rates on the material properties of p-nc-SiC films was investigated by X-ray diffractometer and Raman spectroscopy. Moreover, the optical and electrical properties, such as optical bandgap (E g), dark conductivity, and activation energy (E a), of p-nc-SiC films were also measured. It was found that H 2 flow rates played an important role in forming of p-nc-SiC films and increasing the E g and decreasing the E a of p-nc-SiC films. Moreover, the effect of hydrogenation process of the amorphous Si buffer layer on solar cell characteristics was investigated. After the deposition and hydrogenation parameters were optimized, the Si HJ solar cells with the open-circuit voltage of 0.59 V, short-circuit current density of 38.06 mA/cm 2, fill factor of 62.03%, and the conversion efficiency of 14.09% could be obtained.

Stress Engineering and Optimization of Thick Garnet Crystal Films Grown by Pulsed Laser Deposition

We present here results indicating that stress in films grown by pulsed laser deposition (PLD) may be engineered simply by altering the growth parameters of substrate temperature and laser fluence to balance tensile and compressive stresses. Compositional and structural analysis of Gd3Ga5O12 (GGG) films grown on Y3Al5O12 (YAG) substrates, using three different PLD setups and two different ablating lasers, reveals the effects of different growth parameters on residual stress. Some stress reduction strategies were investigated, including slower heating and cooling ramp rate, and amorphous buffer layers, but changing the growth parameters of substrate temperature and laser fluence was found to have a more significant effect. To characterize the evolution of film stress as thickness increases for different laser fluences, three films were grown in stages to allow substrate curvature measurements and X-ray diffraction analysis to be performed every time the thickness had doubled (from 1 to 16 μm in thickness). The results from these experiments reveal a compressive stress that relaxes with thickness, thought to be due to lattice mismatch, and leads to the conclusion that stress in PLD grown films of GGG on YAG is a balance between lattice mismatch, thermal expansion mismatch, and ion-bombardment.

From metastable to stable: possible mechanisms for the evolution of W18O49 nanostructures

The evolution of nanoscale W18O49 from nanowires into nanosheets is investigated. FESEM and HRTEM images of self-synthesized W18O49 nanostructures obtained from sputter-deposited tungsten films after thermal annealing at various temperatures (550–900 °C) and times (0.5–3 h) are presented and discussed. W18O49 nanostructures obtained from annealed samples with sufficient thermal budgets exhibit Moire fringes with various periodicities along the growth direction. Experimental results confirm that the evolution of nanostructures from nanowires into nanosheets can be attributed to two mechanisms. One is the formation of an amorphous buffer layer on the (02) planes between two neighboring nanowires that facilitates twisting and/or rotation of the two wires and re-crystallization of the buffer layer. The other is the assembly of two neighboring nanowires into a quasi-single crystal by coherent their (010) planes, followed by the rolling of subgrain boundaries to form single-crystalline nanosheets.

Detection of carrier information in heterojunctions of nanocrystalline/crystalline Si

Hall effect measurements on heterojunctions of nanocrystalline Si:H (nc-Si:H) film with crystalline Si wafer fabricated by plasma enhanced chemical vapor deposition technique were made as a function of temperature (20–300 K) and magnetic field (0–15 T). Magnetic field-dependent resistivity and Hall data were interpreted with the quantitative mobility spectrum analysis (QMSA) method, which successfully separated the two-dimensional electron gases (2DEGs) at the nc-Si:H/c-Si interface from 3D carriers appearing in the films and substrates. Mobilities and densities of 2DEGs for the specimens were measured. Detail analyses about influences of interface and epitaxial layer quality including doping, film thickness and mean size of nanocrystals on the mobilities and densities of 2DEGs at interfaces were carried out. The important role of the amorphous buffer layer within the junction was identified. Origin of high mobility observed in the prepared films was revealed. Forward and reverse current mechanisms as well as the dependence of breakdown voltages on temperature in the operated nc-Si:H/c-Si heterostructure were elucidated.

Stabilization of Protein Structure in Freeze-Dried Amorphous Organic Acid Buffer Salts

The purpose of this study was to elucidate the physical properties and protein-stabilizing effects of some pH-adjusting excipients (carboxylic acids and their sodium salts) in frozen solutions and in freeze-dried solids. Thermal and powder X-ray diffraction (XRD) analysis indicated a high propensity of sodium citrates to form glass-state amorphous solids upon freeze-drying. Some salts (e.g., sodium succinate) crystallized in the single-solute frozen solutions. FT-IR analysis of bovine serum albumin (BSA) and bovine immunoglobulin G (IgG) in the aqueous solutions and the freeze-dried solids showed that some glass-forming salts (e.g., monosodium citrate) protected the secondary structure from lyophilization-induced perturbation. Freeze-drying of BSA at different concentrations indicated retention of the secondary structure at similar monosodium citrate/protein concentration ratios, suggesting stabilization through direct interaction that substitute water molecules inevitable for the conformation integrity. The carboxylic acid salts should provide rigid hydrogen bonds and electrostatic interactions that raise the glass transition temperature of the amorphous solids and stabilize protein structure. The relevance of the structural stabilization to the protein formulation design was discussed.