High-quality Substrates Research Articles

P-type doping of ZnO films and growth of tenary ZnMgO and ZnCdO: application to light emitting diodes and laser diodes

ZnO and the related ZnMgO and ZnCdO ternary solid solutions are possible alternatives to GaN-based compounds for the fabrication of ultraviolet (UV)/blue light emitting diodes (LEDs) and injection laser diodes. The ZnO materials system has advantages in terms of the higher binding energy of excitons (60 meV), which leads to efficient electron-hole pair-to-photon conversion at the elevated operating temperatures likely for such devices, the availability of commercially available high-quality ZnO substrates, lower growth temperatures (by at least 200°C) and simplicity of selective wet etching processes for mesa formation in devices. Progress in development of ZnO LEDs has been disappointing due to the difficulty of achieving robust p-type doping and the low crystal quality of heterojunctions and quantum wells. We critically review reports of p-type doping using group V impurities as well as progress in growing ternary ZnMgO and ZnCdO films. We also summarise recent progress and prospects for further advancement of ZnO-based light emitters.

Anisotropic strain relaxation and the resulting degree of polarization by one- and two-step growth in nonpolar a-plane GaN grown on r-sapphire substrate

Anisotropic strain relaxation and the resulting degree of polarization of the electronic transition in nonpolar a-plane GaN using one- and two-step growth are studied. By using two-step growth, a slower coalescence and a longer roughening-recovery process lead to larger anisotropic strain relaxation, a less striated surface, and lower densities of basal stacking fault (BSF) and prismatic stacking fault (PSF). It is suggested that anisotropic in-plane strains, surface striation, and BSF and PSF densities in nonpolar a-GaN are consequences of the rate of coalescence, the period of roughening-recovery process, and the degree of anisotropic strain relaxation. In addition, the two-step growth mode can enhance the degree of polarization of the electronic transition. The simulation results of the k⋅p perturbation approach show that the oscillator strength and degree of polarization of the electronic transition strongly depend on the in-plane strains upon anisotropic in-plane strain relaxation. The research results provide important information for optimized growth of nonpolar III-nitrides. By using two-step growth and by fabricating the devices on the high-quality nonpolar free-standing GaN substrates, high-efficiency nonpolar a-plane InGaN LEDs can be realized. Nonpolar a-plane InGaN/GaN LEDs can exhibit a strongly polarized light to improve the contrast, glare, eye discomfort and eye strain, and efficiency in display application.

Substrate crystal recovery for homoepitaxial diamond synthesis

Abstract Homoepitaxial chemical vapor deposition (CVD) of diamond requires high quality substrate crystals. This paper describes the process of diamond substrate crystal recovery so that the original substrate can be reused for multiple synthesis processes. A three-stage treatment is applied after homoepitaxial CVD growth. First the original substrate is separated by laser cutting, then the cut surface is mechanically polished, and finally polycrystalline material at the edges of the recovered seed plate is laser trimmed. This recovery process yields reusable diamond substrates that do not differ appreciably from their original state in terms of stresses and impurity concentrations. While the recovery process was demonstrated using HPHT seed substrates the process can also be applied to the as-grown CVD diamond plates. Infrared absorption spectral analysis, surface profilometry, birefringence imaging and Raman spectroscopy are performed after each processing step to monitor crystal quality. The nitrogen concentration in the substrate crystal remains constant throughout CVD and recovery processes. When using HPHT type Ib substrates the detected nitrogen concentration is 110–180 ppm. The nitrogen is mainly incorporated in form of C center defects and no transformation to other forms of defect centers occurs during the CVD process. Birefringence imaging showed a low level of internal stress within the HPHT crystals. No change is observed during CVD growth and recovery processes. It is shown that the polycrystalline rim removal is essential for repeatable CVD deposition on the same seed substrate. Substrate crystal recovery allows growth of up to 20 crystals from one original seed.

Optical properties of uniform, porous, amorphous Ta2O5 coatings on silica: temperature effects

We present spectroscopic ellipsometry (SE) results, in the 0.75–5 eV spectral range, obtained on highly uniform Ta2O5 coatings deposited on high-quality silica substrates by ion sputtering. The study was motivated mainly by issues related to the exploitation of Ta2O5–SiO2 λ/4 multilayers in detectors of gravitational waves. Two sets of samples with nominal thicknesses of 40 and 500 nm were considered. A sub-set of samples was treated with post-growth annealing in air for several hours at temperatures Tann up to 600 °C. The SE data were complemented with photothermal common-path interferometry measurements at 1064 nm providing data about absorption losses in the 1–4 ppm range. SE data, taken at room temperature, were analysed by exploiting three different three-phase (substrate/film/surface) models (Cody–Lorentz, Tauc–Lorentz and Herzinger–Johs) of the fundamental absorption edge. Following the literature (Stenzel 2009 J. Phys. D: Appl. Phys. 42 055312) the simulations exploited a graded nano-porosity inside the coating, testing both the shape and composition of the pores. The best simulation of data was obtained using the Cody–Lorentz approach and a quasi-uniform density (6–7.5%) of empty spherical pores, slowly degrading from the substrate/film interface towards the film/ambient interface. A comparison with the literature indicated a high stoichiometric quality of the coatings. The analysis of samples annealed to increasingly higher Tann showed (i) a slight blue-shift of the energy gap (ii) an increase in the pore volume fraction, (iii) an increase (1–2%) in the coating thickness, (iv) a small (less than 1%) reduction in the index of refraction in the transparency region and (v) a limited increase in absorption losses. These findings were interpreted in terms of a release of the compressive strain inherent to the deposition process.

Development of a Titanium Dioxide-Coated Microfluidic-Based Photocatalyst-Assisted Reduction Device to Couple High-Performance Liquid Chromatography with Inductively Coupled Plasma-Mass Spectrometry for Determination of Inorganic Selenium Species

We developed a selective and sensitive hyphenated system employing a microfluidic-based vapor generation (VG) system in conjunction with high-performance liquid chromatography (HPLC) separation and inductively coupled plasma-mass spectrometry (ICPMS) detection for the determination of trace inorganic selenium (Se) species. The VG system exploited poly(methyl methacrylate) (PMMA) substrates of high optical quality to fabricate a microfluidic-based photocatalyst-assisted reduction device (microfluidic-based PCARD). Moreover, to reduce the consumption of photocatalysts during analytical procedures, a microfluidic-based PCARD coated with titanium dioxide nanoparticles (nano-TiO2) was employed to avoid consecutive loading. Notably, to simplify the coating procedure and improve the stability of the coating materials, a dynamic coating method was utilized. Under the optimized conditions for the selenicals of interest, the online HPLC/TiO2-coated microfluidic-based PCARD/ICPMS system enabled us to achieve detection limits (based on 3σ) of 0.043 and 0.042 μg L(-1) for Se(IV) and Se(VI), respectively. Both Se(IV) and Se(VI) could be efficiently vaporized within 15 s, while a series of validation experiments indicated that our proposed method could be satisfactorily applied to the determination of inorganic Se species in the environmental water samples.

Growth of High-Quality GaN Template from Nanometer-Size Lattice Channels by Hydride Vapor Phase Epitaxy

GaN are an attractive material in power devices for energy-saving measures in consumer products, automobiles, and industrial machines. To realize such GaN devices, however, high-quality GaN substrates are indispensable. In this paper, we describe the growth of high-crystalline-quality GaN template by HVPE with the nano-FIELO technique, where GaN growth starts by forming facet structures from nanometer-size channels which are opened on SiO2 layer deposited on GaN/sapphire substrate. The lattice pattern consisting of 500 nm × 500 nm square SiO2 masks surrounded by 80-nm-wide channels is used in this study. A nanoimprinting technique is applied followed by dry etching to fabricate lattice channels. From cross-sectional TEM observation, it is shown that the dislocation density is significantly reduced by the lattice pattern. Uniform GaN growth over 2-inch-diameter wafer is realized.

Preparation and characterization of nanometer‐thin freestanding polymer foils for laser‐ion acceleration

ABSTRACTWe report on the production and characterization of polymer‐based ultra‐thin (sub 10 nm) foils suited for experiments on laser‐ion acceleration in the regime of radiation pressure acceleration. Beside the remarkable mechanical stability compared with commonly used diamond‐like‐carbon foils, a very homogeneous layer thickness and a small surface roughness have been achieved. We describe the technical issues of the production process as well as detailed studies of the mechanical stability and surface roughness tests. The capability of producing uniform targets of large area is essential for advanced laser‐ion acceleration projects which are dealing with high repetition rate and extended measurement series, but might also be useful for other applications which require ultra‐thin and freestanding substrates of high quality. © 2013 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2013, 51, 1355–1360

K16200 窒化物半導体デバイスの今後の展望(【K16200】情報・知能・精密機器部門企画,基調講演)

Market trend of GaN-based white LEDs and RF HFET is surveyed. Key points for the future white light source for general lighting and high power electron devices are discussed. Advantage of using high-quality GaN substrates grown by Na flux method on the performance of high-current density operation LEDs and also high power electron devices will be shown. Also, merit of using high-density nitrogen radical source on the growth of thick InGaN by plasma-assisted (PA-) molecular beam epitaxy (MBE) is presented.

Double-Pattern Textured ZnO:Ga Thin Films Fabricated by an APPJ and an DC Sputtering

Transparent conductive oxides (TCO) are essential for superstrate solar cells due to its outstanding optical and electrical properties. 1 In ordertoimprovetheefficiencyofsolarcells,itisimportanttofabricate high-quality TCO substrates. Recently, zinc oxide (ZnO) based TCO have attracted significant attention because of high abundance, low cost and non-toxicity. Moreover, ZnO films doped with group III elements are more stable than tin-doped indium oxides (ITO) and fluorine-doped tin oxides (FTO) in the hydrogen and silan (SiH4) plasma discharge. 2,3 Hence, they are very suitable for the process of microcrystalline silicon (μc-Si) and amorphous silicon (a-Si) solar cells. For silicon based thin film solar cells, textured ZnO electrodes are necessary to increase light absorption in μc-Si absorbers and to reduce the degradation in a-Si absorbers. This is because that textured ZnO films can scatter the incident light efficiently and enhance the light trapping effect. To date, many efforts have been made to achieve superior textured ZnO films. Post wet etching by diluted acids for aluminum-doped zinc oxides (AZO) and gallium-doped zinc oxides (GZO) is the well-known method. 4,5 However, this process creates a lot of waste in ZnO and is not fit for large-scale production. Recently, pyramid-like textured zinc oxides doped with boron (BZO) have been successfully fabricated by a low pressure chemical vapor deposition (LPCVD) or a metal organic chemical vapor deposition (MOCVD) without any post treatments. 6,7 Nevertheless, disadvantages of these techniques are expensive equipments and the usage of toxic precursors. Here, we developed a novel, safe, and low-cost technique to produce textured GZO films. A bi-layer structure of organosilicon/ GZO was conducted to fabricate double-pattern textured GZO films on glass substrates. This bi-layer structure was achieved by an atmospheric pressure plasma jet (APPJ) and a dc magnetron sputtering. The surface morphology, optical characteristics and conductivity of double-pattern textured GZO films can be effectively controlled by the haze of organosilicon underlayers. After a post-annealing treatment in high vacuum, double-pattern textured GZO films can reveal the excellent optical transmittance and electrical conductivity.

Nanoscale Chemical Patterns on Gold Microplates

To generate nanoscale biochemical patterns for fundamental biophysical studies as well as practical biosensors, there remains a need for a high quality and versatile substrate. We show that chemically synthesized gold microplates on indium tin oxide are an ideal substrate that combines several desirable characteristics, including low cost, single crystallinity, optical transparency, electrical conductivity, and ease in chemical functionalization. We have developed a convenient one-pot method that allows us to synthesize plates of desired dimensions and surface coverage directly on indium tin oxide. We have used electrochemical desorption to strip the capping agents, allowing reliable functionalization with alkanethiol self-assembled monolayers. These plates can serve as nanoscale “lab benches” that allow high-resolution scanning probe lithography, high-resolution imaging, and electrical manipulation. Two applications are demonstrated here: nanoshaved self-assembled monolayers (SAMs) on the single crystalline microplates serve as a high-resolution etching resist; AFM nanografting on the plates generates SAM patterns with tailored terminal chemical functionalities. [The original abstract for this article contains (characters/images) that cannot be displayed here. Please click on the link below to read the full abstract and article.]

Fungal diversity associated with Hawaiian Drosophila host plants.

Hawaiian Drosophila depend primarily, sometimes exclusively, on specific host plants for oviposition and larval development, and most specialize further on a particular decomposing part of that plant. Differences in fungal community between host plants and substrate types may establish the basis for host specificity in Hawaiian Drosophila. Fungi mediate decomposition, releasing plant micronutrients and volatiles that can indicate high quality substrates and serve as cues to stimulate oviposition. This study addresses major gaps in our knowledge by providing the first culture-free, DNA-based survey of fungal diversity associated with four ecologically important tree genera in the Hawaiian Islands. Three genera, Cheirodendron, Clermontia, and Pisonia, are important host plants for Drosophila. The fourth, Acacia, is not an important drosophilid host but is a dominant forest tree. We sampled fresh and rotting leaves from all four taxa, plus rotting stems from Clermontia and Pisonia. Based on sequences from the D1/D2 domain of the 26S rDNA gene, we identified by BLAST search representatives from 113 genera in 13 fungal classes. A total of 160 operational taxonomic units, defined on the basis of ≥97% genetic similarity, were identified in these samples, but sampling curves show this is an underestimate of the total fungal diversity present on these substrates. Shannon diversity indices ranged from 2.0 to 3.5 among the Hawaiian samples, a slight reduction compared to continental surveys. We detected very little sharing of fungal taxa among the substrates, and tests of community composition confirmed that the structure of the fungal community differed significantly among the substrates and host plants. Based on these results, we hypothesize that fungal community structure plays a central role in the establishment of host preference in the Hawaiian Drosophila radiation.

Architecture, microstructure and Jc anisotropy of highly oriented biaxially textured Co-doped BaFe2As2 on Fe/IBAD-MgO-buffered metal tapes

Optimized, biaxially textured BaFe1.8Co0.2As2 thin films with an in-plane alignment of 1.7° have been realized on high-quality IBAD-textured MgO-coated technical substrates utilizing additional Fe buffer layers. High critical current densities (Jc) were achieved, comparable to films on single crystalline MgO (Jc ≥ 1 MA cm−2 at 4 K, self-field). Transmission electron microscopy investigations reveal a small number of c-axis correlated defects introduced by the MgO template. The effect of these defects on the Jc anisotropy was determined in angular-dependent electronic transport measurements.

Hard X-ray multilayer mirror round-robin on the wavefront preservation capabilities of W/B4C coatings

A round-robin between the multilayer deposition laboratories of the Advanced Photon Source, the European Synchrotron Radiation Facility and the National Synchrotron Light Source II has been initiated in order to study standard W/B4C multilayer mirrors produced by the different facilities. The use of such multilayer mirrors for hard X-ray monochromatisation represents an important alternative to crystal-based devices when greater photon flux density is desirable for, e.g., X-ray imaging applications and other photon-intensive techniques. Currently, knowledge about the potential degradation of the wavefront in terms of beam profile distortion and coherence properties due to reflection on a multilayer mirror is limited. In order to address this issue, the beam profile and coherence properties of a monochromatic synchrotron beam reflected by the individual mirrors were studied at the Advanced Photon Source insertion device beamline 32-ID. The results indicate that by using the same coating material, commercially available high quality substrates and a similar coating technique, mirrors with comparable performance can be produced with quite different multilayer deposition facilities. Furthermore, no wave-optical formalism is available at this time which relates the influence of a multilayer reflection on the wavefront to the structural quality of the mirror. Hence, the experimental studies presented are highly targeted in order to identify parameters which have a potential influence on the wavefront preservation properties of a multilayer.

Integrating functional oxides with graphene

Graphene–oxide hybrid structures offer the opportunity to combine the versatile functionalities of oxides with the excellent electronic transport in graphene. Understanding and controlling how the dielectric environment affects the intrinsic properties of graphene is also critical to fundamental studies and technological development of graphene. Here we review our recent effort on understanding the transport properties of graphene interfaced with ferroelectric Pb(Zr,Ti)O3 (PZT) and high-κ HfO2. Graphene field effect devices prepared on high-quality single crystal PZT substrates exhibit up to tenfold increases in mobility compared to SiO2-gated devices. An unusual and robust resistance hysteresis is observed in these samples, which is attributed to the complex surface chemistry of the ferroelectric. Surface polar optical phonons of oxides in graphene transistors play an important role in the device performance. We review their effects on mobility and the high source-drain bias saturation current of graphene, which are crucial for developing graphene-based room temperature high-speed amplifiers. Oxides also introduce scattering sources that limit the low temperature electron mobility in graphene. We present a comprehensive study of the transport and quantum scattering times to differentiate various scattering scenarios and quantitatively evaluate the density and distribution of charged impurities and the effect of dielectric screening. Our results can facilitate the design of multifunctional nano-devices utilizing graphene–oxide hybrid structures.

White light-emitting diode based on fluorescent SiC

A monolithic white light-emitting diode (LED,) comprising a combination of a fluorescent-SiC (f-SiC) substrate and a nitride-based near-UV LED stack, is proposed. On the basis of the recombination of donor and acceptor pairs, the f-SiC substrate works as a phosphor for visible light emission. By employing the Fast Sublimation Growth Process method, the high-quality f-SiC substrate doped with N and B exhibited a nonradiative carrier lifetime of 55μs and an internal quantum efficiency (IQE) of 40%. With increasing donor and acceptor doping concentrations, a high IQE was estimated even at a high excitation level.

Materials and growth issues for high-performance nonpolar and semipolar light-emitting devices

Growth of InGaN/GaN light-emitting devices on nonpolar or semipolar planes offers a viable approach to reducing or eliminating the issues associated with polarization-related electric fields present in c-plane III-nitride heterostructures. Although progress in device performance has been rapid since the introduction of high-quality free-standing nonpolar and semipolar GaN substrates, a full appreciation of the materials challenges unique to nonpolar and semipolar III-nitride semiconductors has been slower to emerge. Only recently have researchers begun to understand issues such as the origins of the pyramidal hillocks typically observed on nominally on-axis m-plane GaN films, the effects of m-plane substrate misorientation on surface morphology and device performance, the mechanics of anisotropic cracking in tensile strained m-plane AlGaN films, the formation of basal-plane stacking faults in long-wavelength m-plane InGaN quantum wells, and the mechanisms for stress relaxation in semipolar AlGaN and InGaN films. In this paper, we review the materials and growth issues unique to high-performance nonpolar and semipolar light-emitting devices grown on high-quality free-standing GaN substrates and provide an outlook for the opportunities and challenges that lie ahead.

Self-assembly of C60 monolayer on epitaxially grown, nanostructured graphene on Ru(0001) surface

C60 molecules adsorbed on graphene/Ru(0001) substrate were investigated by scanning tunneling microscopy (STM) at 5 K. On high quality substrates, C60 molecules adopt a commensurate growth mode, leading to formation of a supramolecular structure with perfect periodicity and few defects. On under-annealed substrates with imperfections and domains, the molecules form the same closely packed hexagonal structures in spite of underlying corrugations, disorders or steps, indicating a weak molecule-substrate interaction—a conclusion that is also supported by DFT calculations. This system may be beneficial to the fabrication of carbon based devices and of other types of organic functional overlayers.

Orientation dependence of polarized Raman spectroscopy for nonpolar, semi-polar, and polar bulk GaN substrates

Polarized Raman spectroscopy has been performed on high-quality bulk gallium nitride substrates with various crystal orientations. The transverse-optic (TO) and longitudinal-optic (LO) phonons shifted to a lower frequency upon changing the observed crystal plane from polar to semi-polar to nonpolar. This result is well explained by the mixing of A1 and E1 optical phonons. In addition, we analyzed in detail the frequency of LO-phonon-plasmon-coupled (LOPC) modes by calculating the peak frequency as a function of carrier concentration and the propagation direction of the LOPC mode. Carrier densities deduced by the analysis showed excellent agreement with the results of Hall measurements. We provide a simple, quick, and nondestructive procedure for identifying nonpolar GaN planes as well as obtaining the carrier concentration.

As flat as it gets: ultrasmooth surfaces from template-stripping procedures

In an experimentally simple replica process, the natural flatness of mica or polished silicon wafers can be transferred to metal films, resulting in metal surfaces with topographic features in Angstrom dimensions over large areas. Two decades after its invention, the template-stripping process continues to appeal to scientists from diverse research backgrounds primarily due to its simplicity, cost-effectiveness and ability to yield high quality substrates and structures. This article introduces the basic construction process for template-stripped substrates, and reports on a variety of extensions of the process, including the generation of materials contrasts and the design of tailored topographies. It also highlights the use of such substrates in a variety of research fields in nanoscience and technology ranging from surface force measurement and high definition imaging to the self-assembly of model membranes and plasmonics.

Azide Functional Monolayers Grafted to a Germanium Surface: Model Substrates for ATR-IR Studies of Interfacial Click Reactions

High-quality azide-functional substrates are prepared by a low temperature reaction of 11-bromoundecyltrichlorosilane with UV-ozone-treated germanium ATR-IR plates followed by nucleophilic substitution of the terminal bromine by addition of sodium azide. The resulting monolayer films are characterized by atomic force microscopy (AFM), contact angle analysis, X-ray photoelectron spectroscopy (XPS), attenuated total reflectance infrared spectroscopy (ATR-IR), and ellipsometry. XPS and ellipsometric thickness data correspond well to the results of molecular model calculations confirming the formation of a densely packed azide-functional monolayer. These azide-functional substrates enable interfacial "click" reactions with complementary alkyne-functional molecules to be studied in situ by ATR-IR. To illustrate their potential utility for kinetic studies we show that, in the presence of copper(I) catalyst, the azide-modified surfaces react rapidly and quantitatively with 5-chloro-pentyne to form triazoles via a 1,3-dipolar cycloaddition reaction. Time-resolved ATR-IR measurements indicate that the interfacial click reaction is initially first order in azide concentration as expected from the reaction mechanism, with a rate constant of 0.034 min(-1), and then transitions to apparent second order dependence, with a rate constant of 0.017 min(-1)/(chains/nm(2)), when the surface azide and triazole concentrations become similar, as predicted by Oyama et al. The reaction achieves an ultimate conversion of 50% consistent with the limit expected due to steric hindrance of the 5-chloro-pentyne reactant at the surface.