Ni Nanodots Research Articles

Magneto-optical Kerr effect (MOKE) and magnetic force microscopy (MFM) studies on Cr/Ni nanodot arrays deposited using innovative nano-stencil method

AbstractNanodot arrays of Nickel (Ni) on Chromium (Cr), [Cr in two forms i. e. (Cr dot) and Cr (layer)] were fabricated by electron beam evaporation technique using nano-stencil. The deposition of a periodic dot pattern was confirmed through atomic force microscopy (AFM) imaging. The nano magnetic properties of the Cr/Ni nanodot arrays were probed using both Magnetic Force Microscopy (MFM) and Magneto-Optical Kerr Effect (MOKE) magnetometer. MOKE measurements unveiled saturated hysteresis with the exchange bias effect for the in-plane configuration and a minor hysteresis loop for the out-of-plane configuration in the Cr/Ni nanodot system. The experimental results were found to be in good agreement with the theoretical Stoner-Wohlfarth (SW) model. Additionally, the exchange bias effect was also observed in the Cr (thin film)/Ni (nanodot) system for both the in-plane and out-of-plane configurations, showing the dominating behavior of the Cr layer over the Ni dots due to its continuous layer and higher thickness. The observation of exchange bias properties was also verified through MFM measurements conducted with both positive and negative in-plane magnetic fields. Hence, this straightforward nano-stencil method holds significant promise for the streamlined production of patterned AFM/FM arrays enabling the comprehensive study of exchange bias phenomena. The magnetic properties of the Cr/Ni structure can be manipulated by depositing Cr either in nanodot form or in continuous layer form.

Heteroepitaxial growth of Ga2O3 thin films on nickel-nanodot-induced buffer layers for solar-blind ultraviolet photodetector applications

Heteroepitaxial growth of gallium oxide (Ga2O3) thin films has been conducted on nickel (Ni)-nanodot-induced buffer layers using a mist chemical vapor deposition. The separation degree and droplet size of the Ni nanodots on sapphire substrates were controlled through a thermal annealing process, thereby being used as templates for the Ga2O3 thin-film growth. The growth of the inverted cone-shaped ε-Ga2O3 polymorph was observed on the nanodots while the α-Ga2O3 polymorph was grown on the sapphire substrate without nanodots, ultimately a thin film with a mixture of ε and α polymorphs can be achieved. By comparing photoresponses of metal–semiconductor–metal photodetectors fabricated on the thin film with the mixed polymorphs and on a single-crystalline α-Ga2O3 film, a promising template for using solar-blind photo-detecting applications has been verified.

WC-Ni cemented carbides prepared from Ni nano-dot coated powders

This study presents a novel approach for the synthesis of WC-Ni cemented carbides with enhanced mechanical properties. A low-cost solution-based route was used to coat WC powders with well-distributed metallic nickel dots measuring between 17 nm and 39 nm in diameter. Varying compositions with loadings of 2, 6, and 14 vol% Ni were consolidated using spark plasma sintering (SPS) at 1350 °C under 50 MPa of uniaxial pressure giving relative densities of 99 ± 1 %. The sintered WC-Ni cemented carbides had an even distribution of the Ni binder phase in all compositions, with retained ultrafine WC grain sizes of 0.5 ± 0.1 μm from the starting powder. The enhanced sinterability of the coated powders allowed for consolidation to near theoretical densities, with a binder content as low as 2 vol%. This is attributed to the uniform distribution of nickel and an extensive Ni-WC interface existing prior to sintering. The small size of the Ni dots likely also contributed to the solid-state sintering starting temperatures of as low as 800 °C. The mechanical performance of the resulting cemented carbides was evaluated by measuring the hardness at temperatures between 20 °C and 700 °C and estimating toughness at room temperature using Vickers indentations. These results showed that the mechanical properties of the WC-Ni cemented carbides synthesised by our method were comparable to conventionally prepared WC-Co cemented carbides with similar grain sizes and binder contents and superior to conventionally prepared WC-Ni cemented carbides. In particular, the 2 vol% Ni composition had excellent hardness at room temperature of up to 2210HV10, while still having an indentation fracture toughness of 7 MPa·m0.5. Therefore, WC-Ni cemented carbides processed by this novel approach are a promising alternative to conventional WC-Co cemented carbides for a wide range of applications.

Simple fabrication of Ni nanodots decorated Ni/Ketjen black composite for sulfur host in lithium‐sulfur battery

Ni nanomaterials exhibit excellent adsorption and catalytic ability in the redox reaction of Li-S batteries. Hence, Ni nanodots are embedded in the micropores and mesopores of commercial Ketjen black (KB) to obtain Ni/KB composite nanomaterials by an in situ preparation method with low cost and high yield. The prepared Ni nanodots with 5 to 30 nm diameter act as the active sites in KB matrix and enhance the intrinsic activity; they can not only adsorb lithium polysulfides (LiPSs) by the Ni-S bond but also promote the conversion rate of LiPSs. Consequently, with 3.1 mg cm−2 S loading on positive electrode, the initial discharge capacity of Ni/KB/S electrode at 0.05 C reaches 1253 mAh g−1 and the initial discharge capacity at 0.5 C reaches 818.4 mAh g−1, with the stable cycle exceeding over 500 cycles.

Nonvolatile switching of magnetostrictive nanodot from single-domain to vortex states by voltage at room temperature

We report the voltage regulation of electrodeposited elliptical magnetostrictive Ni nanodot arrays from single-domain to nonvolatile vortex state at room temperature. On the piezoelectric substrate, isolated elliptical Ni nanodots are fabricated between a pair of square electrodes, with the long axis parallel to the joint line of the electrodes. By applying a voltage to the surface electrodes pair, local stress is generated to induce the magnetization of the nanodots from single-domain to vortex state. The magnetization state of the nanodots is characterized by a magnetic force microscope. Even after the voltage is removed or applying another voltage, the nanodots maintain a stable vortex magnetization state, which shows that the vortex state after regulation is nonvolatile. These results are of great significance for the study of the low-energy-consumption regulation of the nano-dimensional magnetic material and vortex state-based nonvolatile memory.

Micromagnetic Simulations of Fe and Ni Nanodot Arrays Surrounded by Magnetic or Non-Magnetic Matrices.

Combining clusters of magnetic materials with a matrix of other magnetic materials is very interesting for basic research because new, possibly technologically applicable magnetic properties or magnetization reversal processes may be found. Here we report on different arrays combining iron and nickel, for example, by surrounding circular nanodots of one material with a matrix of the other or by combining iron and nickel nanodots in air. Micromagnetic simulations were performed using the OOMMF (Object Oriented MicroMagnetic Framework). Our results show that magnetization reversal processes are strongly influenced by neighboring nanodots and the magnetic matrix by which the nanodots are surrounded, respectively, which becomes macroscopically visible by several steps along the slopes of the hysteresis loops. Such material combinations allow for preparing quaternary memory systems, and are thus highly relevant for applications in data storage and processing.

Prompt Electrodeposition of Ni Nanodots on Ni Foam to Construct a High-Performance Water-Splitting Electrode: Efficient, Scalable, and Recyclable

HighlightsFacile electrodeposition for fabricating active Ni nanodots (NiNDs) on Ni foam (NF) is shown.Binder- and heteroatom-free recyclable NiO/NiNDs@NF electrodes are efficiently made.NiO/NiNDs@NF bifunctional catalytic electrodes are used for water splitting.

Hybrid Top-Down/Bottom-Up Fabrication of a Highly Uniform and Organized Faceted AlN Nanorod Scaffold.

As a route to the formation of regular arrays of AlN nanorods, in contrast to other III-V materials, the use of selective area growth via metal organic vapor phase epitaxy (MOVPE) has so far not been successful. Therefore, in this work we report the fabrication of a highly uniform and ordered AlN nanorod scaffold using an alternative hybrid top-down etching and bottom-up regrowth approach. The nanorods are created across a full 2-inch AlN template by combining Displacement Talbot Lithography and lift-off to create a Ni nanodot mask, followed by chlorine-based dry etching. Additional KOH-based wet etching is used to tune the morphology and the diameter of the nanorods. The resulting smooth and straight morphology of the nanorods after the two-step dry-wet etching process is used as a template to recover the AlN facets of the nanorods via MOVPE regrowth. The facet recovery is performed for various growth times to investigate the growth mechanism and the change in morphology of the AlN nanorods. Structural characterization highlights, first, an efficient dislocation filtering resulting from the ~130 nm diameter nanorods achieved after the two-step dry-wet etching process, and second, a dislocation bending induced by the AlN facet regrowth. A strong AlN near band edge emission is observed from the nanorods both before and after regrowth. The achievement of a highly uniform and organized faceted AlN nanorod scaffold having smooth and straight non-polar facets and improved structural and optical quality is a major stepping stone toward the fabrication of deep UV core-shell-based AlN or AlxGa1-xN templates.

Large Local‐Compressive Stress‐Induced Improvements in Piezoelectric Characteristics of Lead Zirconate Titanate Thin Films on a Ni Nanodots Array

AbstractInterfacial stress present in ferroelectric thin films is known to affect largely their piezoelectric properties by modifying crystal orientation and domain structure. Here, a nonconventional way is proposed to substantially improve the polarization and piezoelectric characteristics of Pb(Zr,Ti)O3 thin films by modifying the surface of the typical Si substrate into an embossed structure with Ni nanodots. Uniform Ni nanodot arrays are successfully produced by an external magnetic field through the consolidation process of thin Ni layer. The exceptionally large thermal expansion mismatch of ≈94% between the thin films and the Ni nanodots is believed to induce a large local‐compressive stress around the nanodot region and thus the intensified orientation toward c‐axis. For this demonstration, the in situ sputtering processing enabled by combining heavily 12 mol% Nb‐doping with a bottom electrode structure of Ir/TiW is used. As a highlight of the improvement, a significant increase of ≈33% in effective piezoelectric coefficient is observed for the 90 nm Ni nanodot case. An apparent shift of the polarization–electric field curve suggests the existence of internal field, as an evidence of the in situ domain formation.

Voltage induced artificial ferromagnetic-antiferromagnetic ordering in synthetic multiferroics

This paper presents numerical and experimental data for dipole-dipole coupled Ni nanodots on a piezoelectric [Pb(Mg1/3Nb2/3)O3]0.68[PbTiO3]0.32 substrate. Simulation results show that the dipole coupling produces artificial ferromagnetic (parallel magnetization alignment in the nanodot arrays) behavior that can be modified to artificial antiferromagnetic behavior with an applied voltage. Experimental results show the trends in Mr and Hc predicted by the model, but discrepancies arise due to geometric defects present in the fabricated samples. Geometric defects are introduced into the Ni nanodot models, thus dramatically improving the correlation between experiments and analysis. This work shows, through numerical simulations, that artificial multiferroic nanostructures can be designed to produce switching from parallel (artificial ferromagnetic) to antiparallel (artificial antiferromagnetic) magnetization ordering by leveraging dipole coupling with voltage induced changes in magnetic anisotropy.

Comparative study on plasmonic properties of a Ni film and 2D Ni nanodot array fabricated on an ITO substrate

A two-dimensional (2D) Ni nanodot array (NDA), made of transition metals with partially filled d-bands, was fabricated on an indium tin oxide (ITO) substrate with good transparency using a porous anodic aluminum oxide mask. A Ni film with the thickness of 50nm exhibits a broad dip at 437nm in the extinction spectrum, while a Ni NDA with the diameter of 45nm shows a broad localized surface plasmon resonance (LSPR) peak at 400nm. Both the broad-shaped dip and peak are due to the surface plasmon resonance and the d-band related absorption. We confirmed that the size dependence of transition metals is weak in the LSPR spectra of Ni NDAs, with diameters of 45nm and 90nm.

Low-temperature formation of crystalline Si:H/Ge:H heterostructures by plasma-enhanced CVD in combination with Ni-nanodots seeding nucleation

Hydrogenated microcrystalline (µc) Si/Ge heterostructures were prepared on quartz substrates by plasma-enhanced chemical vapor deposition (CVD) from VHF inductively coupled plasma of SiH4 just after GeH4 employing Ni nanodots (NDs) as seeds for crystalline nucleation. The crystallinity of the films and the progress of grain growth were characterized by Raman scattering spectroscopy and atomic force microscopy (AFM), respectively. When the Ge films were grown on Ni-NDs at 250 °C, the growth of µc-Ge films with crystallinity as high as 80% was realized without an amorphous phase near the Ge film/quartz substrate interface. After the subsequent Si film deposition at 250 °C, fine grains were formed in the early stages of film growth on µc-Ge films with compositional mixing (µc-Si0.85Ge0.15:H) caused by the release of large lattice mismatch between c-Si and c-Ge. With further increase in Si:H film thickness, the formation of large grain structures accompanied by fine grains was promoted. These results suggest that crystalline Si/Ge heterojunctions can be used for efficient carrier collection in solar cell application.

Simple and scalable preparation of master mold for nanoimprint lithography

Nanoimprint lithography (NIL) is one of the most prominent bottom-up techniques for duplicating nanostructures with a high throughput. However, fabrication of starting master mold commonly requires expensive equipment of top-down techniques, or additional steps to transfer the fabricated patterns from bottom-up methods. Here we demonstrate that a SiO2 nanostructure manufactured from a self-assembled block copolymer, polystyrene-b-polydimethylsiloxane (PS-b-PDMS), directly serves as a master mold for NIL without further modification. A hexagonally aligned pattern over the entire substrate is established using a simple technique; solvent annealing and etching. Etching also plays an important role in endowing fluorine on the surface of SiO2, thus promoting smooth demolding upon imprinting. The obtained pattern of the SiO2 nanostructure is transferred to a polymer surface using UV nanoimprint. Identical patterns of the SiO2 nanostructure are elaborately reproduced on Ni and Cu nanodot arrays via electroplating on the polymer transcript, which was verified by morphological observations. The uniformity of the replicated Ni nanodot array is evaluated using spectroscopic ellipsometry. The measured optical response of the Ni nanodot is validated by electromagnetically simulated results, indicating that the pattern transfer is not limited to a small local area. In addition, the durability of the SiO2 mold pattern is corroborated after the imprinting process, thus guaranteeing the reusability of the fabricated nanostructure as a master mold. The proposed approach does not require any high-end lithographic techniques; this may result in significant cost and time reductions in future nanofabrication.

Binary information propagation in circular magnetic nanodot arrays using strain induced magnetic anisotropy

Nanomagnetic logic has emerged as a potential replacement for traditional Complementary Metal Oxide Semiconductor (CMOS) based logic because of superior energy-efficiency (Salahuddin and Datta 2007 Appl. Phys. Lett. 90 093503, Cowburn and Welland 2000 Science 287 1466–68). One implementation of nanomagnetic logic employs shape-anisotropic (e.g. elliptical) ferromagnets (with two stable magnetization orientations) as binary switches that rely on dipole−dipole interaction to communicate binary information (Cowburn and Welland 2000 Science 287 1466–8, Csaba et al 2002 IEEE Trans. Nanotechnol. 1 209–13, Carlton et al 2008 Nano Lett. 8 4173–8, Atulasimha and Bandyopadhyay 2010 Appl. Phys. Lett. 97 173105, Roy et al 2011 Appl. Phys. Lett. 99 063108, Fashami et al 2011 Nanotechnology 22 155201, Tiercelin et al 2011 Appl. Phys. Lett. 99, Alam et al 2010 IEEE Trans. Nanotechnol. 9 348–51 and Bhowmik et al 2013 Nat. Nanotechnol. 9 59–63). Normally, circular nanomagnets are incompatible with this approach since they lack distinct stable in-plane magnetization orientations to encode bits. However, circular magnetoelastic nanomagnets can be made bi-stable with a voltage induced anisotropic strain, which provides two significant advantages for nanomagnetic logic applications. First, the shape-anisotropy energy barrier is eliminated which reduces the amount of energy required to reorient the magnetization. Second, the in-plane size can be reduced (∼20 nm) which was previously not possible due to thermal stability issues. In circular magnetoelastic nanomagnets, a voltage induced strain stabilizes the magnetization even at this size overcoming the thermal stability issue. In this paper, we analytically demonstrate the feasibility of a binary ‘logic wire’ implemented with an array of circular nanomagnets that are clocked with voltage-induced strain applied by an underlying piezoelectric substrate. This leads to an energy-efficient logic paradigm orders of magnitude superior to existing CMOS-based logic that is scalable to dimensions substantially smaller than those for existing nanomagnetic logic approaches. The analytical approach is validated with experimental measurements conducted on dipole coupled Nickel (Ni) nanodots fabricated on a PMN-PT (Lead Magnesium Niobate-Lead Titanate) sample.

Ultrathin AAO Membrane as a Generic Template for Sub-100 nm Nanostructure Fabrication

Anodic aluminum oxide (AAO) templates are emerging as a platform for simple, cost-effective, high-throughput top-down nanofabrication of regular arrays of nanostructures. Thus far, however, AAO pattern transfer has largely been restricted to smooth and chemically inert surfaces, mostly Silicon substrates. Here, we present a more generalizable strategy for preparing free-standing through-hole ultrathin alumina membranes (UTAMs) and transferring them to both smooth and rough substrates, thereby enabling the fabrication of centimeter-scale arrays of nanostructures with sub-100 nm feature diameters on almost arbitrary substrates. To validate the utility of our procedures, we transferred UTAMs to surfaces relevant for photocatalytic applications and prepared plasmonic photocathodes consisting of dense arrays of size-controlled sub-100 nm Au and Ni nanodots on top of chemically noninert NiOx thin films. To demonstrate the functionality of the fabricated structures, we used a plasmonic photocathode consisting of...

Resistive Switching Characteristics of Si-Rich Oxides with Embedding Ti Nanodots

Resistive random access memories (ReRAMs) have been intensively studied as potential candidates for the next generation of memory devices due to their simplicity in structure. Among various switching materials, Si-rich oxide (SiOx) has been received much attention from the viewpoint of the good compatibility to the current Si-ULSI technology. So far, we have studied resistance switching behaviors of SiOx sandwiched with Pt and/or Ni electrodes caused by formation of conducting filament path presumably due to the oxygen deficiency [1, 2]. In addition, from local I-V measurements performed by contacting a single Ni-nanodots (NDs) as a top electrode with a Rh coated-AFM tip, we have confirmed the size scalability of SiOx as a resistive switching material. However, to realize low power switching operation, one of the major concerns is the control of the formation of the conductive filament paths in SiOx. In this work, we have expected that NDs can trigger the formation of the conductive filament path in SiOx, and evaluated an impact of NDs embedding into SiOxon the switching characteristics. MIM diodes consisting of Ni electrodes and SiOx in which Ti-NDs were embedded were fabricated on SiO2/Si(100) by electron beam (EB) deposition and surface treatment using a remote plasma of pure H2 (H2-RP) as follows: At first, a ~15 nm-thick Ni layer was formed as a bottom electrode and followed by the EB deposition of a ~1 nm-thick SiOx layer. And then, Ti-NDs were formed by a combination of a Ge(20 nm)/Ti(3 nm) stacked layer formation and subsequent H2-RP exposure without external heating [3], where Ge layer was used as a barrier layer against oxidation of the ultrathin Ti layer. During the H2-RP exposure, H2 gas pressure and VHF power of a 60 MHz generator were maintained at 24 Pa and 500 W, respectively. After the H2-RP exposure, a ~8 nm-thick SiOx layer and then Ni top electrodes were formed by EB deposition. We also fabricated Ni/SiOx/Ni-MIM diodes without NDs as references. AFM observations showed the formation of Ti-NDs with an areal density as high as 3.3×1011cm-2 was taken after H2-RP exposure. XPS analysis confirmed that the Ge layer was completely etched away by H2-RP exposure and surface oxidation of Ti-NDs proceeds during air exposure. Resistive switching characteristics of SiOx with and without NDs were measured from I-V curves in voltage sweep mode (Figs. 1(a) and (b)). In both cases, a distinct unipolar type resistive-switching was repeatedly observed. However, SET and RESET voltages were well separated by embedding NDs. These results indicate that the NDs in SiOx are likely to act as a trigger for the formation of conductive path in a reproducible fashion. Also, the resistances in high and low resistance states (HRS and LRS) calculated from the current levels at -0.1 V were summarized as functions of the switching cycle (Fig. 2). Obviously, for the diodes with NDs, a stable resistive switching was repeatedly observed in comparison with the diodes without NDs. With embedding NDs, the variations in the resistance of both ON and OFF states became large markedly, and the ON/OFF ratio in resistance was increased from 45 to 150, which indicates that the Ti oxide formed at embedded Ti-NDs surface reduces the current levels through the SiOx. In summary, the embedding of Ti-based NDs in SiOxis very effective to separate the operation voltages between SET and RESET voltages and to increase in the ON/OFF ratio in resistance. Acknowledgements This work was supported in part by Young Scientists (A) No. 15H05520 from the Ministry of Education, Culture, Sports, Science and Technology, Japan and by JSPS Core-to-Core Program of International Collaborative Research Center on Atomically Controlled Processing for Ultra large Scale Integration. References 1) A. Ohta, et al, Jpn. J. Appl. Phys., 52(2012) 06FF02. 2) A. Ohta, et al, IEICE TRANSACTIONS on Electronics, E96-C(2013) 702. 3) K. Makihara, et. al, J. Optoelectronics and Adv. Materials, 12 (2010) 626. Figure 1

Resistance-Switching Characteristics of Si-rich Oxide Evaluated by Using Ni Nanodots as Electrodes in Conductive AFM Measurements

Resistance-Switching Characteristics of Si-rich Oxide Evaluated by Using Ni Nanodots as Electrodes in Conductive AFM Measurements

In Situ TEM Study on Au Mediated Growth of NiSi2 in Si Nanowire: A Vapor-Liquid-Solid Analogy

The vapor-liquid-solid (VLS) mechanism is the prominent growth process for semiconductor nanowires (NWs), for example in Si NW growth. Supply atoms of Si are provided by vapor precursors to a supersaturated Au-Si liquid eutectic alloy that mediates the solid nanowire growth. We report here a new solid-liquid-solid (SLS) silicide phase growth mechanism in Si NWs that is analogous to the VLS mechanism in being a liquid-mediated growth, but is fundamentally different in terms of nucleation and mass transport.To observe the growth, we dispersed VLS grown <111> Si NWs on 30 nm TEM silicon nitride membranes with pre-evaporated Ni dots. In-situ TEM heating observations, including real-time video recording, of the interaction between the Au-terminated Si NW and the Ni nanodots were conducted at 700 ºC as a function of time. Figure (a) is a TEM image showing a segment of NiSi2 grown by the distinct gold mediation mechanism, and Figure (b) shows the HR image at the NiSi2/Au and Au/Si interfaces. At the very beginning of the process, the Au-Si tip forms a liquid alloy at the elevated temperature (Figure c). Then, Ni atoms dissolve from the Ni nanodots into the Si NWs and diffuse interstitially to and dissolve in the liquid alloy (Figure d). Upon super-saturation of Ni and Si inside the Au-Ni-Si alloy, a single {111} terminated octahedral NiSi2 nucleus forms in the middle of the liquid alloy (Figure e) and continues to grow until it is large enough to be pinned at the inner surface of the oxidized Si NW. Meanwhile, the Au-Ni-Si liquid alloy starts to move from the tip towards the other end of the Si NW, and catalytically transforms Si to NiSi2 along the moving path (Figure f). Here, two concurrent processes mediated by the moving Au-Ni-Si liquid layer occur: (1) progressive dissolution of the Si NW at the liquid alloy/Si (111) interface, and (2) continuing supply of Ni so that the precipitation of NiSi2 phase occurs at the liquid alloy/NiSi2 interface, which is closer to the tip of the NW. In a similar fashion to the VLS growth of Si, the NiSi2 nucleates at {110} type facets and propagates on its (111) planes. This constitutes a SLS growth process for the NiSi2 phase. With SLS growth, the NiSi2 phase occurs at lower temperatures when mediated by the liquid alloy, compared with the temperatures used in thin film reactions. In contrast NiSi2 is found to nucleate homogeneously in Si NWs without Au mediation only when the temperature is above 800 ºC.Figure captions: (a) TEM bright field image showing NiSi2 forms from the tip of the nanowire with a Au alloy layer moving towards the opposite end. Scale bar is 100 nm (b) HRTEM image of the area around Au. The reaction front is NiSi2 (111) plane that maintains a (111) interface with the solidified Au as demonstrated in the inset FFT patterns. Scale bar is 5 nm. SLS growth process: (c) Ni interstitial diffusion through a Si medium (NW) and accumulation in the Au-Si eutectic alloy. (d) Formation of a liquid Au-Ni-Si ternary alloy at the Si NW tip. (e) Nucleation and growth of a single octahedral NiSi2 crystallite inside the liquid ternary alloy. (f) Continuous dissolution of the Si NW by the eutectic ternary that moves down through the Si NW and concurrent growth of NiSi2 on the other side of the eutectic in the steady state.

Highly-Crystallized Ge:H Film Growth from GeH4 Very High Frequency Inductively-Coupled Plasma: Crystalline Nucleation Initiated by Ni Nanodots

We prepared Ni nanodots (Ni-NDs) by exposing ultrathin Ni films to very high frequency inductively-coupled plasma (VHF-ICP) of pure H2 without any external heating and demonstrated the feasibility of introducing Ni-NDs for subsequent Ge film growth with high crystallinity. A ∼2.0-nm-thick Ni film/quartz was exposed to H2 plasma generated by inductive coupling with an external single-turn antenna connected to a 60-MHz power generator, and a Ge film was subsequently grown from GeH4 VHF-ICP. When the Ge film was grown on Ni-NDs from GeH4 VHF-ICP at 250 °C, the growth of highly crystallized Ge films with crystallinity as high as 95% at a rate as high as ∼15 nm/s, being ∼2 times higher than the rate obtained by VHF-ICP without Ni-NDs, was realized without an amorphous phase near the Ge film/quartz substrate interface. This result is attributed to heat generation due to the efficient recombination of atomic hydrogen on the Ni surface and heat transfer from the NDs to the precursors for the Ge film adsorbed on the Ni-NDs at the initial stage of VHF-ICP from GeH4 diluted with H2 to initiate crystalline nucleation and growth.

다공성 알루미나 마스크를 이용한 니켈 나노점 구조 제작

본 연구에서는 인산 용액 하에서 2차 양극 산화 기법에 의해 제작된 양극 산화 알루미나 막을 마스크로 이용하여 정렬된 니켈 나노점 구조를 제작하였다. <TEX>$2{\mu}m$</TEX> 두께의 얇은 양극 산화 알루미나 막 표면에 평균 279 nm 크기의 기공구조를 형성하였으며 이를 얇은 니켈 박막의 열 증착 시 다공 구조 마스크로 이용하여 정렬된 니켈 나노점 구조를 제작하였다. 형성된 니켈 나노점의 크기는 평균 293 nm의 크기를 가지고 있으며 알루미나 막 표면상의 기공 구조의 형상을 따르고 있음을 볼 수 있었다. 제작된 나노점 구조의 자기적 특성을 상온에서 자기이력곡선의 측정을 통해 살펴보았으며 연속적인 니켈 박막과 비교하였을 때 고립된 나노점 구조로 인하여 자화용이축을 따라 각형비의 감소와 보자력의 증가가 나타남을 관찰할 수 있었다. 본 연구를 통해 양극 산화 막을 마스크로 이용한 박막 증착 과정을 통해 균일한 자기 나노점 구조를 제작할 수 있음을 확인할 수 있었다. We have fabricated an ordered Ni nanodot structure using an alumina mask prepared via 2-step anodization technique under phosphoric acid. We have formed a porous structure with average pore size of 279 nm on <TEX>$2{\mu}m$</TEX> thick alumina film and the thermal deposition of thin Ni film though the mask led to the formation of ordered Ni nanodot structure with an average dot size of 293 nm, following the pore structure on the mask. We further investigated the magnetic properties of the nanodot structure by measuring the hysteresis curve at room temperature. When compared to the magnetic properties of a continuous Ni film, we observed the decrease in the squareness and the increase in coercivity along the magnetization easy axis, due to the isolated nanodot structure. Our study suggests that the ordered nanodot structure can be easiy fabricated with thin film deposition technique using anodized alumina mask as a mask.