Investigation of L1_0 FePt-based soft/hard composite bit-patterned media by micromagnetic simulation

The soft/hard composite patterned media have potential to be the next generation of magnetic recording,but the composing modes of soft and hard materials have not been investigated systematically.L10 Fe Pt-based soft/hard composite patterned media with an anisotropic constant distribution are studied by micromagnetic simulation.Square arrays and hexagonal arrays with various pitch sizes are simulated for two composing types:the soft layer that encloses the hard dots and the soft layer that covers the whole surface.It is found that the soft material can reduce the switching fields of bits effectively for all models.Compared with the first type,the second type of models possess low switching fields,narrow switching field distributions,and high gain factors due to the introduction of inter-bit exchange coupling.Furthermore,the readout waveforms of the second type are not deteriorated by the inter-bit soft layers.Since the recording density of hexagonal arrays are higher than that of square arrays with the same center-to-center distances,the readout waveforms of hexagonal arrays are a little worse,although other simulation results are similar for these two arrays.     

Growth of three-dimensional quantum dot crystals on patterned GaAs (0 0 1) substrates

The growth of a three-dimensional (3D) InAs quantum dot (QD) crystal on a patterned GaAs (0 0 1) substrate is demonstrated. The morphology of QDs grown on a surface patterned with shallow holes is studied as a function of the amount of deposited InAs. We observe that the QDs form in the patterned holes close to each other forming lateral QD bimolecules for InAs coverages below the commonly observed critical thickness of 1.6 monolayers. When the coverage increases, the QD bimolecules coalesce to form larger single QDs. The QDs in the holes are then capped with a Ga(Al)As spacer. The buried QD array serves as a strain template for controlling the formation site of the QDs in the second layer. By tuning the growth conditions for the second and subsequent layers, we achieve a 3D InAs QD crystal with a high degree of perfection. A detail investigation of the growth on hole patterns with different periodicities is presented.     

Morphology response to strain field interferences in stacks of highly ordered quantum dot arrays

Twofold stacked InGaAs/GaAs quantum dot (QD) layers are grown on GaAs(001) substrates patterned with square arrays of shallow holes. We study the surface morphology of the second InGaAs QD layer as a function of pattern periodicity. Comparing our experimental results with a realistic simulation of the strain energy density E(str) distribution, we find that the second InGaAs QD layer sensitively responds to the lateral strain-field interferences generated by the buried periodic QD array. This response includes the well-known formation of vertically aligned QDs but also the occurrence of QDs on satellite strain energy density minima. Our calculations show that base size and shape as well as lateral orientation of both QD types are predefined by the E(str) distribution on the underlying surface.     

Structure and Dynamics of Ethanol Adsorbed on a Mica Surface

The structural and dynamic properties of nanoscale ethanol film on a mica surface are investigated via molecular dynamics simulations.We observe a dense,almost flat ethanol bilayer formed in the vicinity of the mica surface,with the hydrophobic alkyl groups pointing outward from the surface.Remarkably,such ethanol bilayer is laterally well-ordered with patterned adsorption sites.Each ethanol molecule in the first layer donates one hydrogen bond to the surface basal oxygen atoms and accepts one hydrogen bond from that in the second layer.The ethanol molecules within the bilayer exhibit constrained lateral mobility and delayed dynamics as compared with bulk ethanol,whereas those on top of the bilayer have bulk-like characteristics.     

Structure and polarity of 8CB films evaporated onto solid substrates

Brewster-angle reflection ellipsometry and surface optical second harmonic generation were used to study the growth of 4'-n-octyl-4-cyanobiphenyl (8CB) films evaporated in air onto polymeric and quartz glass substrates. The layer-by-layer growth of the films terminates after formation of two distinctive interfacial layers. Both of these two layers are polar and tilted. In the first layer the molecules lie nearly flat on the surface, while in the second layer they point on average about 50° toward the surface normal. The dipole moment of the second layer has a lower magnitude and an opposite direction with respect to the dipole moment of the first layer.     

Adsorption compression in surface layers

A recently elucidated aspect of adsorption, compression in confined phases, is discussed. Grand Canonical Monte Carlo simulations were performed for the adsorption of Lennard–Jones molecules and new details of intermolecular interactions in adsorbed layers are analysed. It is shown that a strong attraction to a surface can cause adsorption compression not only in the first layer, but also in higher layers. Compression of the first layer creates a pattern of active sites; the second layer tends to be commensurate with this pattern and has density higher than that of a ‘free’ layer. This pattern propagates to higher layers. However, there is a wide range of chemical potentials where the first layer is compressed and the second layer is not yet formed. It was found that transition to adsorption compression results in oscillations of the isosteric heat of adsorption. These oscillations are determined by a combination of (a) changes in adsorbed layers’ structure and (b) exchange of molecules between layers. In particular, at high affinity to adsorbent, the adsorption isotherm for the first layer has a slight maximum because an increase of the chemical potential causes molecules to leave the compressed first layer and move to the second layer. For this reason, the isosteric heat of adsorption decreases and can become negative. Analysis of adsorption compression mechanisms in the context of theory and emerging experimental results indicates that the significance of this phenomenon is not limited to fundamental aspects of adsorption and capillarity. These mechanisms play a crucial role in various applications, such as heterogeneous catalysis, membrane separations, and self-assembly on surfaces. Results are discussed in a broader context of theory, experiments and previous simulations.     

Structural properties and diffusion processes of the Cu3Au (0 0 1) surface

The surface relaxation and surface energy of both the mixed AuCu and pure Cu terminated Cu₃Au (0 0 1) surfaces are simulated and calculated by using the modified analytical embedded-atom method. We find that the mixed AuCu termination is energetically preferred over the pure Cu termination thereby the mono-vacancy diffusion is also investigated in the topmost few layers of the mixed AuCu terminated Cu₃Au (0 0 1) surface. In the mixed AuCu terminated surface the relaxed Au atoms are raised above Cu atoms for 0.13 Å in the topmost layer. All the surface atoms displace outwards, this effect occurs in the first three layers and changes the first two inter-layer spacing. For mono-vacancy migration in the first layer, the migration energies of Au and Cu mono-vacancy via two-type in-plane displace: the nearest neighbor jump (NNJ) and the second nearest neighbor jump (2NNJ), are calculated and the results show that the NNJ requires a much lower energy than 2NNJ. For the evolution of the energy requirements for successive nearest neighbor jumps (SNNJ) along three different paths: circularity, zigzag and beeline, we find that the circularity path is preferred over the other two paths due to its minimum energy barriers and final energies. In the second layer, the NN jumps in intra- and inter-layer of the Cu mono-vacancy are investigated. The calculated energy barriers and final energies show that the vacancy prefer jump up to a proximate Cu site. This replacement between the Cu vacancy in the second layer and Cu atom in the first layer is remunerative for the Au atoms enrichment in the topmost layer.     

Thermal stress reduction of GaAs epitaxial growth on V-groove patterned Si substrates

We investigate the thermal stresses for GaAs layers grown on V-groove patterned Si substrates by the finite-element method. The results show that the thermal stress distribution near the interface in a patterned substrate is nonuniform,which is far different from that in a planar substrate. Comparing with the planar substrate, the thermal stress is significantly reduced for the Ga As layer on the patterned substrate. The effects of the width of the V-groove, the thickness, and the width of the SiO₂ mask on the thermal stress are studied. It is found that the SiO₂ mask and V-groove play a crucial role in the stress of the Ga As layer on Si substrate. The results indicate that when the width of V-groove is 50 nm, the width and the thickness of the SiO₂ mask are both 100 nm, the Ga As layer is subjected to the minimum stress. Furthermore,Comparing with the planar substrate, the average stress of the Ga As epitaxial layer in the growth window region of the patterned substrate is reduced by 90%. These findings are useful in the optimal designing of growing high-quality Ga As films on patterned Si substrates.     

Atomistic simulation of the self-diffusion in Fe (1 1 1) surface

The formation energies, the intra- and inter-layer self-diffusion activation energies of a single vacancy in Fe (1 1 1) surface have been investigated with the modified analytical embedded-atom method (MAEAM). The results show that the effect of the surface is down to the sixth layer for the formation and intra-layer migration of the vacancy. It is easier for a vacancy to form and to migrate in intra-layer in the first (especially), the second and the third layer. For inter-layer migration, a single vacancy in each of the first six layers is favorable to migrate to the upper layers. On the contrary, a single vacancy in the seventh, the eighth and the ninth layers is favorable to migrate to the lower layers.     

Growth and optical properties of Ge/Si quantum dots formed on patterned SiO2/Si(0 0 1) substrates

Single and stacked layers of Ge/Si quantum dots were grown in SiO₂ windows patterned by electron-beam lithography on oxidized Si (0 0 1) substrates. The growth of a silicon buffer layer prior to Ge deposition is found to be an additional parameter for adjusting the Ge-dot nucleation process. We show that the silicon buffer layer evolves towards [1 1 3]-faceted pyramids, which reduces the area of the topmost (0 0 1) surface available for Ge nucleation. By controlling the top facet area of the Si buffer layers, only one dot per circular window and a high cooperative arrangement of dots on a striped window can be achieved. In stacked layers, the dot homogeneity can be improved through the adjustment of the Ge deposited amount in the upper layers. The optical properties of these structures measured by photoluminescence spectroscopy are also reported. In comparison with self-assembled quantum dots, we observed, both in single and stacked layers, the absence of the wetting-layer component and an energy blue shift, confirming therefore the dot formation by selective growth.     

Patterned growth of ZnO nanofibers

A simple method is adopted to grow ZnO nanofibers laterally among the patterned seeds designed in advance on silicon substrate. The preparation of seed lattices is carried out by lithographing the metal zinc film evaporated on the substrate. A layer of aluminum is covered on the zinc layer to prevent the ZnO nanorods vertically growing on the top surface. After oxidation, the patterned ZnO/Al2O3 spots are formed at the sites for the horizontal growth of ZnO nanofibers by the vapor phase transportation （VPT） method using the zinc powders as source material.     

Screen printing for producing ferroelectret systems with polymer-electret films and well-defined cavities

We report a process for preparing polymer ferroelectrets by means of screen printing—a technology that is widely used for the two-dimensional patterning of printed layers. In order to produce polymer-film systems with cavities that are suitable for bipolar electric charging, a screen-printing paste is deposited through a screen with a pre-designed pattern onto the surface of a polymer electret film. Another such polymer film is placed on top of the printed pattern, and well-defined cavities are formed in-between. During heating and curing, the polymer films are tightly bonded to the patterned paste layer so that a stable three-layer system is obtained. In the present work, polycarbonate (PC) films have been employed as electret layers. Screen printing, curing and charging led to PC ferroelectret systems with a piezoelectric d ₃₃ coefficient of about 28 pC/N that is stable up to 100 ^°C. Due to the rather soft patterned layer, d ₃₃ strongly decreases already for static pressures of tens of kPa. The results demonstrate the suitability of screen printing for the preparation of ferroelectret systems.     

Patterning of polypyrrole using a fluoropolymer as an adsorption-protecting molecule

Patterning of the conducting polymer polypyrrole (PPy) was achieved using perfluoropolyether (PFPE) as a mask material. The fluoropolymer PFPE has both hydrophobic and oleophobic properties that allowed the generation of passivated patterns against PPy deposition. We exploited these properties to achieve the selective micropattern deposition of PPy, by simple chemical oxidation in an aqueous solution. Using a microcontact printing method, circle patterns with exposed carboxyl groups were prepared, while other region was protected by PFPE. Chemical oxidation of PPy on the patterned substrate resulted in selective deposition of PPy onto only the carboxylate-terminated regions, with little deposition on the PFPE layer. Cross-sectional analysis of the pattern revealed that the PFPE layer would form a hole-like structure around the carboxylate-terminated surfaces, with PPy deposition only in the holes. The PFPE layer had little influence on surface smoothness, compared to other self-assembled monolayers. These results suggest that PFPE can be used as a protective material for the surface modification and patterning of various materials.     

Unusual Multilayer Surface Alloy: Al(100)-c(2 x 2)-2Li

An unusual multilayer surface alloy is formed by adsorption of one monolayer Li on Al(100). It is shown by low energy electron diffraction that the first three layers consist of a mixed Al-Li layer, a pure Al layer, and a second Al-Li layer. Thus the alloy has the same layer stoichiometry as the (100) surface of the metastable Al(3)Li bulk alloy. However, the relative orientation of the two mixed layers is the same as that in the Al(3)Ti-type structure. These findings are confirmed by total-energy calculations, which lead further to the prediction that the bulk Al(3)Li alloy has a faulted, Al(3)Ti-type surface.     

Growth and modification of thin a-Si:H/a-Ge:H bi-layers to sacrificial c-SiGe alloys through ArF-Excimer laser assisted processing

A combination of ArF-Excimer laser assisted techniques has been used for depositing and modifying ultra thin amorphous Si/Ge bi-layer structures. The first step consisted in producing, at low substrate temperatures, thin bi-layer coatings through Laser induced Chemical Vapour Deposition (LCVD) in both, large areas as well as in small regions of Si(1 0 0) wafers. In the second step, these bi-layer structures have been modified through Pulsed Laser Induced Epitaxy (PLIE) for obtaining heteroepitaxial SiGe alloys with a thin buried Ge rich layer, while keeping a shallow upper Si rich surface with good crystalline quality. Threshold for epitaxial alloy formation has been determined by Raman spectroscopy and estimated to be above 200 mJ/cm². Optical profilometry has been used for evaluating the thickness of the structures and the lateral dimensions of patterned features. SEM, TOF-SIMS and XPS have been used to corroborate the results. For testing IC compatibility, some samples have been overgrown with epitaxial Si and etched through conventional IC processing techniques, revealing that the laser processed layers are suitable to be used as sacrificial layers for producing Micro-Electro-Mechanical Systems (MEMSs) or Silicon-on-Nothing (SON) devices.     

Structure and composition of the NiPd(1 1 0) surface

The NiPd(1 1 0) alloy surface was studied using low energy electron diffraction to measure the structure and composition of the first three atomic layers. The surface layer is highly enriched in Pd and has a significantly buckled structure. The second layer is also buckled, with displacements even larger than the surface layer. The second layer also exhibits intralayer segregation (chemical ordering), with alternate close-packed rows of atoms being Ni enriched and Pd enriched. The third layer has a structure and composition close to that of the bulk alloy. These results are compared with results for the other low index faces of NiPd, the extensive literature on NiPt alloy surfaces, and the growing body of theoretical literature for NiPd alloy surfaces.     

New relaxation phenomena in the outer atomic layers of Fe{211}

A low-energy electron diffraction analysis of a {211} surface of body-centered cubic iron reveals relaxations in the directions perpendicular and parallel to the surface plane. Both relaxations alternate in successive layers. The perpendicular relaxation goes from contraction of 10.5% to expansion of 5% to contraction of 1%. The parallel relaxation goes from a shift of the first layer of 0.24Å (10% of the nearest neighbor distance) toward more symmetrical registration with the second layer, to an opposite shift of 0.035 Å of the second layer with respect to the third.     

Silicon enhanced carbon nanotube growth on nickel films by chemical vapor deposition

Silicon enhances carbon nanotube growth on nickel films by chemical vapor deposition using methane and hydrogen. Nanotube growth characteristic is significantly improved on nickel films patterned by argon plasma etching on silicon oxide layers. Auger electron spectroscopy shows that a reduced silicon phase forms in the surface silicon oxide layer by Ar ion bombardment used for patterning. The enhanced growth of carbon nanotubes could be ascribed to an oxygen removal effect by silicon in the process of synthesis.     

宽沟道SI-GaAs衬底上适于光发射机的BH激光器

目前,光发射机中的激光器有采用MOCVD和MBE方法生长的多量子阱激光器,用LPE法生长的BH激光器。我们根据现有的实验条件,为了制作单片集成的光发射机,在沟道SI-GaAs衬底上采用两次液相外延生长BH激光器,实现了表面平面化。在800℃一次外延生长四层。第一层n⁺-GaAs缓冲层,第二层N-GaAlAs下限制层,第三层非掺杂构GaAs有源层,第四层为P-GaAlAs上限制层。采用适当的腐蚀条件刻蚀出有源区最窄的燕尾形隐埋条。在二次外延中,我们仅装一槽GaAlAs源液,在晶片上仅停留一次便生长出两个掩埋层,且层间界面与有源区自对准。上层为N-GaAlAs,载流子浓度为10¹⁶cm^-3,下层为高阻伴随生长层。由于高阻伴随层的存在对电流产生了有有效的侧向限制作用,因此避免了通常的SiO₂膜沉积等一系列工艺,提高了成品率,减化了工艺程序。利用n型掩埋层和隐埋条区P型上限制层之间铝组分及载流子类型、浓度的差异,虽然做一种宽接触电极,但由于隐埋条区上有良好的欧姆接触,而在掩埋层上为非良欧姆接触,所以起到了一定的电流外限制作用。n型电极是从n⁺-GaAs层引出的。 这种沟道SI-GaAs衬底正装GaAlAs/GaAs BH激光器室温连续工作阈值电流为55mA,P-I曲线在100℃仍有良好的线性关系。     

Electronic structure of epitaxial graphene layers on SiC: effect of the substrate

A strong substrate-graphite bond is found in the first all-carbon layer by density functional theory calculations and x-ray diffraction for few graphene layers grown epitaxially on SiC. This first layer is devoid of graphene electronic properties and acts as a buffer layer. The graphene nature of the film is recovered by the second carbon layer grown on both the (0001) and (0001[over]) 4H-SiC surfaces. We also present evidence of a charge transfer that depends on the interface geometry. Hence the graphene is doped and a gap opens at the Dirac point after three Bernal stacked carbon layers are formed.