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Applied Physics Reviews — 2007

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Making waves: Kinetic processes controlling surface evolution during low energy ion sputtering

Wai Lun Chan and Eric Chason

J. Appl. Phys. 101, 121301 (2007); http://dx.doi.org/10.1063/1.2749198 (46 pages)

Online Publication Date: 20 June 2007

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When collimated beams of low energy ions are used to bombard materials, the surface often develops a periodic pattern or “ripple” structure. Different types of patterns are observed to develop under different conditions, with characteristic features that depend on the substrate material, the ion beam parameters, and the processing conditions. Because the patterns develop spontaneously, without applying any external mask or template, their formation is the expression of a dynamic balance among fundamental surface kinetic processes, e.g., erosion of material from the surface, ion-induced defect creation, and defect-mediated evolution of the surface morphology. In recent years, a comprehensive picture of the different kinetic mechanisms that control the different types of patterns that form has begun to emerge. In this article, we provide a review of different mechanisms that have been proposed and how they fit together in terms of the kinetic regimes in which they dominate. These are grouped into regions of behavior dominated by the directionality of the ion beam, the crystallinity of the surface, the barriers to surface roughening, and nonlinear effects. In sections devoted to each type of behavior, we relate experimental observations of patterning in these regimes to predictions of continuum models and to computer simulations. A comparison between theory and experiment is used to highlight strengths and weaknesses in our understanding. We also discuss the patterning behavior that falls outside the scope of the current understanding and opportunities for advancement.
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79.20.Rf Atomic, molecular, and ion beam impact and interactions with surfaces
61.80.Jh Ion radiation effects
68.35.Fx Diffusion; interface formation
66.30.Lw Diffusion of other defects
01.30.Rr Surveys and tutorial papers; resource letters

Perpendicular recording media for hard disk drives

S. N. Piramanayagam

J. Appl. Phys. 102, 011301 (2007); http://dx.doi.org/10.1063/1.2750414 (22 pages)

Online Publication Date: 6 July 2007

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Perpendicular recording technology has recently been introduced in hard disk drives for computer and consumer electronics applications. Although conceptualized in the late 1970s, making a product with perpendicular recording that has competing performance, reliability, and price advantage over the prevalent longitudinal recording technology has taken about three decades. One reason for the late entry of perpendicular recording is that the longitudinal recording technology was quite successful in overcoming many of its problems and in staying competitive. Other reasons are the risks, problems, and investment needed in making a successful transition to perpendicular recording technology. Iwasaki and co-workers came up with many inventions in the late 1970s, such as single-pole head, CoCr alloy media with a perpendicular anisotropy, and recording media with soft magnetic underlayers [S. Iwasaki and K. Takemura, IEEE Trans. Magn. 11, 1173 (1975); S. Iwasaki and Y. Nakamura, ibid. 14, 436 (1978); S. Iwasaki, Y. Nakamura, and K. Ouchi, ibid. 15, 1456 (1979)]. Nevertheless, the research on perpendicular recording media has been intense only in the past five years or so. The main reason for the current interest comes from the need to find an alternative technology to get away from the superparamagnetic limit faced by the longitudinal recording. Out of the several recording media materials investigated in the past, oxide based CoCrPt media have been considered a blessing. The media developed with CoCrPt-oxide or CoCrPt–SiO2 have shown much smaller grain sizes, lower noise, and larger thermal stability than the perpendicular recording media of the past, which is one of the reasons for the success of perpendicular recording. Moreover, oxide-based perpendicular media have also overtaken the current longitudinal recording media in terms of better recording performance. Several issues that were faced with the soft underlayers have also been solved by the use of antiferromagnetically coupled soft underlayers and soft underlayers that are exchange coupled with an antiferromagnetic layer. Significant improvements have also been made in the head design. All these factors now make perpendicular recording more competitive. It is expected that the current materials could theoretically support areal densities of up to 500–600 Gbits/in.2. In this paper, the technologies associated with perpendicular recording media are reviewed. A brief background of magnetic recording and the challenges faced by longitudinal recording technology are presented first, followed by the discussions on perpendicular recording media. Detailed discussions on various layers in the perpendicular recording media and the recent advances in these layers have been made. Some of the future technologies that might help the industry beyond the conventional perpendicular recording technology are discussed at the end of the paper.
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75.50.Ss Magnetic recording materials
75.50.Ee Antiferromagnetics
75.30.Gw Magnetic anisotropy
85.70.Li Other magnetic recording and storage devices (including tapes, disks, and drums)
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Small-angle neutron scattering in materials science: Recent practical applications

Yuri B. Melnichenko and George D. Wignall

J. Appl. Phys. 102, 021101 (2007); http://dx.doi.org/10.1063/1.2759200 (24 pages)

Online Publication Date: 25 July 2007

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Modern materials science and engineering relies increasingly on detailed knowledge of the structure and interactions in “soft” and “hard” materials, but there have been surprisingly few microscopic techniques for probing the structures of bulk samples of these substances. Small-angle neutron scattering (SANS) was first recognized in Europe as a major technique for this purpose and, over the past several decades, has been a growth area in both academic and industrial materials research to provide structural information on length scales ∼ 10–1000 math (or 1–100 nm). The technique of ultrahigh resolution small-angle neutron scattering (USANS) raises the upper resolution limit for structural studies by more than two orders of magnitude and (up to ∼ 30 μm) and hence overlaps with light scattering and microscopy. This review illustrates the ongoing vitality of SANS and USANS in materials research via a range of current practical applications from both soft and hard matter nanostructured systems.
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61.05.fg Neutron scattering (including small-angle scattering)

Review of zincblende ZnO: Stability of metastable ZnO phases

A. Ashrafi and C. Jagadish

J. Appl. Phys. 102, 071101 (2007); http://dx.doi.org/10.1063/1.2787957 (12 pages)

Online Publication Date: 1 October 2007

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Common II-VI compound semiconducting materials are stable thermodynamically with zincblende phase, while the II-O materials such as zinc oxide (ZnO) and beryllium oxide (BeO) are stable with wurtzite phase, and cadmium oxide (CdO) and magnesium oxide (MgO) are stable in rocksalt phase. This phase disharmony in the same material family laid a challenge for the basic physics and in practical applications in optoelectronic devices, where ternary and quaternary compounds are employed. Thermodynamically the zincblende ZnO is a metastable phase which is free from the giant internal electric fields in the [001] directions and has an easy cleavage facet in the 〈110〉 directions for laser cavity fabrication that combined with evidence for the higher optical gain. The zincblende materials also have lower ionicity that leads to the lower carrier scattering and higher doping efficiencies. Even with these outstanding features in the zincblende materials, the growth of zincblende ZnO and its fundamental properties are still limited. In this paper, recent progress in growth and fundamental properties of zincblende ZnO material has been reviewed.
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73.61.Ga II-VI semiconductors
78.55.Et II-VI semiconductors
71.55.Gs II-VI semiconductors
72.10.Fk Scattering by point defects, dislocations, surfaces, and other imperfections (including Kondo effect)
61.66.Fn Inorganic compounds
61.50.Ks Crystallographic aspects of phase transformations; pressure effects
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Internal photoemission at interfaces of high-κ insulators with semiconductors and metals

V. V. Afanas’ev and A. Stesmans

J. Appl. Phys. 102, 081301 (2007); http://dx.doi.org/10.1063/1.2799091 (28 pages)

Online Publication Date: 25 October 2007

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Internal photoemission spectroscopy provides the most straightforward way to characterize the relative energies of electron states at interfaces of insulators with metals and semiconductors by measuring the spectral onset of electron/hole photoemission from one solid into another. The article reviews the application of this technique for characterization of advanced nanometer-thin insulators prospected to be used in microelectronic devices. Fundamental aspects and technical features of the internal photoemission experiments are discussed together with basic electronic properties of a number of investigated high-permittivity insulating films and their interfaces in semiconductor heterostructures. Significant differences are found in the electronic properties of nanometer-thin amorphous insulating layers as compared to the known bulk phase characteristics. The band alignment at the interfaces of these insulators with metals is found to be highly sensitive to the surface preparation procedures. By contrast, at semiconductor/oxide interfaces the parameters of occurring interlayers affect the energy barriers only marginally at least in the case of studied oxides with close bandgap width (5.6–5.9 eV). The latter finding is in favor of the models describing the band offsets at semiconductor/insulator interfaces on the basis of the bulk density of electron states. Deviation of metal/oxide interfaces from this simple behavior is explained by (unintentional) formation of a polarization layer at the interface which may contain uncompensated charges and dipoles affecting the barrier height.
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79.60.Jv Interfaces; heterostructures; nanostructures
73.40.Qv Metal-insulator-semiconductor structures (including semiconductor-to-insulator)
73.40.Ns Metal-nonmetal contacts
73.20.At Surface states, band structure, electron density of states
73.22.-f Electronic structure of nanoscale materials and related systems
77.55.-g Dielectric thin films
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Recent progress in solution processable organic light emitting devices

Franky So, Benjamin Krummacher, Mathew K. Mathai, Dmitry Poplavskyy, Stelios A. Choulis, and Vi-En Choong

J. Appl. Phys. 102, 091101 (2007); http://dx.doi.org/10.1063/1.2804122 (21 pages)

Online Publication Date: 13 November 2007

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Organic light emitting devices (OLEDs) have been the subject of intense research because of their potential for flat panel display and solid state lighting applications. While small molecule OLEDs with very high efficiencies have been demonstrated, solution processable devices are more desirable for large size flat panel display and solid state applications because they are compatible with low cost, large area roll-to-roll manufacturing process. In this review paper, we will present the recent progress made in solution processable OLEDs. The paper will be divided into three parts. In the first part of the paper, we will focus on the recent development of fluorescent polymer OLEDs based on conjugated polyfluorene copolymers. Specifically, we will present results of carrier transport and injection measurements, and discuss how the charge transport and injection properties affect the device performance. In the second part of the paper, we will focus on the recent progress on phosphorescent dye-dispersed nonconjugated polymer OLEDs. Specifically, we will present our recent results on high efficiency green and blue emitting devices based on the dye-dispersed polymer approach. Similar to fluorescent conjugated polymer OLEDs, charge transport and injection properties in dye-dispersed polymer OLEDs also play an important role in the device performance. In the third part of this paper, we will present our results on white emitting phosphorescent OLEDs. Two approaches have been used to demonstrate white emitting OLEDs. First, white emitting OLEDs were made using blue emitting OLEDs with downconversion phosphors. Second, white emitting OLEDs were made by dispersing red, green, and blue phosphorescent dyes into the light emitting layer. High efficiency devices have been demonstrated with both approaches.
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85.60.Jb Light-emitting devices

Nanostructured ceramics by electrospinning

Ramakrishnan Ramaseshan, Subramanian Sundarrajan, Rajan Jose, and S. Ramakrishna

J. Appl. Phys. 102, 111101 (2007); http://dx.doi.org/10.1063/1.2815499 (17 pages)

Online Publication Date: 3 December 2007

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Nanostructured ceramics are attractive materials that find potential uses ranging from simple everyday applications like paints and pigments to sophisticated ones such as bioimaging, sensors, etc. The inability to economically synthesize nanoscale ceramic structures in a large scale and simultaneously achieve precise control of their size has restricted their real time application. Electrospinning is an efficient process that can fabricate nanofibers on an industrial scale. During the last 5 years, there has been remarkable progress in applying this process to the fabrication of ceramic nanorods and nanofibers. Ceramic nanofibers are becoming useful and niche materials in several applications owing to their surface dependant and size dependant properties. These advances are reviewed here. The various ceramic nanofiber systems that have been fabricated so far are presented. The physical and chemical property enhancements due to the nanosize have been discussed in detail and the various applications they fit into are outlined in this article.
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81.16.-c Methods of micro- and nanofabrication and processing
68.65.-k Low-dimensional, mesoscopic, nanoscale and other related systems: structure and nonelectronic properties
81.20.-n Methods of materials synthesis and materials processing
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