bluuebunny/crossref_metadata_2025 huggingface dataset:
Searching for Smart Durable Coatings to Promote Bone Marrow Stromal Cell Growth While Preventing Biofilm Formation
Authors: Namavar Fereydoon , Jackson John D. , Sharp J. Graham , Mann Ethan E. , Bayles Kenneth , Cheung Barry Chin Li , Feschuk Connie A , Varma Shailaja , Haider Hani , Garvin Kevin L.
Published: April 2011

There is a great need to develop methods to regulate cellular growth in order to enhance or prevent cell proliferation as needed, to either improve health or prevent disease. In this work we evaluated the adhesion, survival and growth of bone marrow stromal cells on the surface of several new ion beam engineered nano-crystals of ceramic hard coatings such as zirconium, titanium, tantalum and cerium oxides. Cell adhesion and growth on the ceramic coatings were compared to adhesion and growth on a nano-crystalline silver coating which is known to possess antibacterial properties. The initial results of a study to determine the effect of nanocrystalline titanium and silver coating onstaphylococcus aureusbiofilm growth is also discussed.
Drop-by-drop Polymer Deposition by Acoustic Picoliter Droplet Generators for Applications in Semiconductor Industry and Biotechnology
Authors: Lee Grace C. , Cohen Jeremiah R , Demirci Utkan
Published: April 2011

Photoresist droplets are ejected onto a wafer surface by an acoustic two dimensional micromachined ejector array. The spread of single droplets on a silicon wafer surface at varying droplet speeds is studied. Series of photoresist droplets are printed periodically drop-on-demand on a silicon wafer surface and profiles of a single droplet and two droplets overlapping with varying distances of 25 μm and 1 μm on a silicon wafer are demonstrated. Moreover, 3.4 μm thick spinless full coverage of a 4 inch wafer with photoresist is demonstrated which indicates a potential for coating wafers in less than a few seconds.
Al rich AlN/AlGaN Quantum Wells
Authors: Al tahtamouni Talal Mohammed Ahmad , Nepal Neeraj , Lin Jingyu , Jiang Hongxing
Published: April 2011

Two sets of AlN/AlxGa1−xN quantum wells (QW) have been grown by metalorganic chemical vapor deposition (MOCVD). The first set consists of five samples of AlN/AlxGa1−xN QWs with (x ∼ 0.65) with well width, Lw, varying from 1 to 3 nm. The second set consists of four samples of AlN/AlxGa1−xN with (Lw = 1.5 nm) with Al composition, x, varying from 0.70 to 0.85. Low temperature photoluminescence (PL) spectroscopy has been employed to study the Lw dependence of the PL spectral peak position, emission efficiency, and line width. Our results have shown that these AlN/AlGaN QW structures exhibit polarization fields of ∼ 4 MV/cm. Due to effects of quantum confinement and polarization fields, AlN/AlGaN QWs with Lw between 2 and 2.5 nm exhibit the highest quantum efficiency. The dependence of the emission linewidth on Lw yielded a linear relationship. The implications of our results on deep ultraviolet (UV) optoelectronic device applications are also discussed.
Strain-free Low-defect-density Bulk GaN with Nonpolar Orientations
Authors: Paskova Tanya , Paskov Plamen P. , Darakchieva Vanya , Kroeger Roland , Hommel Detlef , Monemar Bo , Lourdudoss Sebastian , Preble Edward , Hanser Andrew , Williams Mark N. , Tutor Michael
Published: April 2011

Bulk GaN sliced in bars along (11-20) and (1-100) planes from a boule grown in the [0001] direction by HVPE was confirmed as strain free material with a low dislocation density by using several characterization techniques. The high-structural quality of the material allows photoluminescence studies of free excitons, principal donor bound excitons and their two-electron satellites with regard to the optical selection rules. Raman scattering study of the bulk GaN with nonpolar orientations allows a direct access to the active phonon modes and a direct determination of their strain-free positions.
Study of Defects in p-type Layers in III-nitride Laser Diode Structures Grown by Molecular Beam Epitaxy
Authors: Xiu Huixin , Costa Pedro MFJ , Kauer Matthias , Smeeton Tim M , Hooper Stewart E , Heffernan Jonathan , Humphreys Colin J
Published: April 2011

This paper reports on the study of defects in p-type layers in III-nitride laser structures grown by molecular beam epitaxy. Characterization of the heterostructures was carried out using atomic force microscopy and transmission electron microscopy. The results show that a high density of extended defects – possibly inversion domains – exist in the p-type cladding layers of as-grown structures with either AlGaN/GaN superlattices or bulk AlGaN cladding layers. TEM analysis of operated and aged devices does not reveal any significant structural modification of the p-type material which might be the cause of deterioration in the lasing performance or failure.
Ultraviolet light emitting diodes using non-polar A-plane AlGaN multiple quantum wells
Authors: Chandrasekaran Ramya , Bhattacharyya Anirban , France Ryan , Thomidis Christos , Williams Adrian , Moustakas Theodore
Published: April 2011

In this paper, we report the growth and fabrication of non-polar A-plane AlGaN multiple quantum well based ultraviolet light emitting diodes (UV-LEDs). The LEDs were grown on R-plane sapphire substrates using molecular beam epitaxy (MBE). The Current-voltage characteristics of the fabricated devices demonstrated rectifying behavior with a series resistance of 38 ohms. An electro-luminescence emission at 338 nm was obtained.
Low-dislocation-density Nonpolar AlN Grown on 4H-SiC (11-20) Substrates
Authors: Suda Jun , Horita Masahiro , Kimoto Tsunenobu
Published: April 2011

Growth of AlN on 4H-SiC (11-20) substrates by plasma-assisted molecular-beam epitaxy is presented. Very high-quality AlN can be grown under a slightly Al-rich condition. Transmission electron microscopy revealed that the AlN layer has 4H crystalline structure with high-phase purity (stacking-fault density is 5×106cm−1) and the density of threading dislocations is as small as 8×107cm−2.
Optical Transmission Measurements on MOCVD Grown GaMnN Films on Sapphire
Authors: Arkun Fevzi Erdem , El-Masry Nadia A , Muth John , Zhang Xiyao , Mahrouse Amr , Zavada John , Bedair Salah M
Published: April 2011

We demonstrate optical transmission measurements performed on 1.2 μm thick GaMnN films grown by metalorganic chemical vapor deposition on (0001) sapphire substrates. According to the data acquired from these measurements, Mn forms a deep acceptor band at 1.4 eV above the valance band of GaMnN. Full width at half maximum of this absorption band increases from 107 meV to 198meV as the Mn concentration increases from 0.3% to 1.6 %; which indicates that this band becomes wider as the concentration of Mn increases in the lattice. A broad absorption band starting at 1.9eV and extending to the band edge of GaMnN was also determined. This was attributed to the transition from the Mn energy band to the conduction band edge of GaMnN. Absorption at both of these bands scales with the Mn concentration and thickness of the films. The effect of co-doping of GaMnN films with magnesium on the transmission spectra was also investigated. The absorption band initially observed at 1.4 eV was shifted to 1.6 eV as a result of introduction of Magnesium into the lattice of GaMnN. From these results we conclude that Mn is incorporated in the lattice and forms an energy band in the bandgap of GaMnN. The width of this energy band is also a function of the Mn concentration in GaMnN.
Wet Etching of Bulk AlN Crystals
Authors: Zhuang Dejin , Herro Ziad G. , Li Xianglin , Schlesser Raoul , Sitar Zlatko
Published: April 2011

AlN bulk crystals of different crystallographic orientations were etched in aqueous KOH solution (general etchant) and molten KOH/NaOH eutectic alloy (defect selective etchant). In the general etchant, the aluminum polar surface (0001) was found to be stable while the nitrogen polar (0001) surface and non-polar m-plane (1010) surfaces showed significant etching. Etch patterns on nitrogen polar surfaces consisted of complete coverage of hexagonal pyramids while the m-plane surfaces were etched in a layer-by-layer mode. TEM studies revealed that the observed etch hillocks on nitrogen polar surfaces were not associated with extended structural defects. When etched in a defect-selective etchant, the aluminum polar surface showed well-defined hexagonal etch pits, with an etch pit density (EPD) of 2×104 cm−2. The nitrogen polar surfaces and m-plane surfaces showed etch features similar to the one obtained during general etching, however, the etch feature density was significantly lower. The etch mechanisms, etch kinetics, as well as the correlation between the observed etch features and structural defects in bulk crystals will be discussed.
Compound-source Molecular Beam Epitaxy of GaN on Si at Low Temperature Using GaN Powder and Ammonia as Sources
Authors: Honda Tohru , Sawada Masaru , Yamamoto Hiromi , Sawadaishi Masashi
Published: April 2011

Low-temperature growth of GaN is very attracting for the application to light emitting devices grown on Si substrates because it prevents the melt-back reaction between Ga and Si substrates. The low-temperature growth of GaN by compound-source molecular beam epitaxy (CS-MBE) has been reported. In the previous report, GaN powders were used as a source and no additional nitrogen source was introduced during the growth. At present, its growth mechanism is unclear. In this paper, CS-MBE of GaN layers using GaN and ammonia as sources is discussed. Especially, the reduction of excess Ga in GaN layers by introducing ammonia supply is discussed based on their refraction high-energy electron diffraction (RHEED) patterns and x-ray photoelectron spectroscopy (XPS) spectra.
Investigation of Low Angle Grain Boundaries in Hexagonal Silicon Carbide
Authors: Chen Yi , Chen Hui , Zhang Ning , Dudley Michael , Ma Ronghui
Published: April 2011

Interaction between basal plane dislocations and single or well-spaced threading dislocations is discussed based on synchrotron white beam X-ray topographic studies carried out on physical vapor transport grown hexagonal silicon carbide single crystals. The basal plane dislocations are able to cut through single or well-spaced threading edge dislocations even if the formation of kinks/jogs is energetically unfavorable while threading screw dislocations were mostly observed to act as effective pinning points. However, basal plane dislocations can sometimes cut through a threading screw dislocation, forming a superjog and which subsequently migrates on the prismatic plane via a cross-slip process. Threading edge dislocation walls act as obstacles for the glide of basal plane dislocations and the mechanism by which this occurs is discussed. The character of low angle grain boundaries and their dislocation content are discussed.
MBE Growth of Cubic InN
Authors: Schörmann Jörg , As Donat Josef , Lischka Klaus
Published: April 2011

Cubic InN films were grown on top of a c-GaN buffer layer by rf-plasma assisted MBE at different growth temperatures. X-Ray diffraction investigations show that the c-InN layers consist of a nearly phase-pure zinc blende (cubic) structure with a small fraction of the wurtzite (hexagonal) phase grown on the (111) facets of the cubic layer. The content of hexagonal inclusions is decreasing with decreasing growth temperature. The full-width at half-maximum (FWHM) of c-InN (002) rocking curve is about 50 arcmin. Low temperature photoluminescence measurements reveal a band gap of about 0.61eV for cubic InN.
High Resolution Transmission Electron Microscopy Study of Thermal Oxidation of Single Crystalline Aluminum Nitride
Authors: Chaudhuri Jharna , Lee Rac Gyu , Nyakiti Luke Owuor , Gu Zheng , Edgar James H , Li Peng
Published: April 2011

The impact of process conditions and crystal properties on the structure of thermal oxides formed on AlN were determined by high resolution transmission electron microscopy (HRTEM). Oxidation for 2 hours at both 800 ° and 1000 °C produced mostly amorphous oxide layers whereas oxidation for 4 and 6 hours at 1000 °C produced partly crystalline and epitaxial oxide layers. The crystalline oxide was mostly single phase á-Al2O3 except at the surface where it was a mixture of γ-Al2O3 and á-Al2O3. The amorphous oxide layer first transformed to γ-Al2O3 and then to the stable á-Al2O3 as evidenced by the non-uniform thickness of the oxide and the existence of the γ-Al2O3 at the surface. The AlN crystal contained a high density of defects at the interface at 800 °C but it was nearly defect- and oxygen-free at 1000 °C. This could be due to the rapid diffusion of the nitrogen and aluminum interstitials at high temperatures leading to a point defect equilibrium throughout the nitride. A faceted interface between Al2O3 and AlN could be attributed to non-uniform out diffusion of aluminum.
A Study of Defects and Surface Properties in AlGaN/GaN High Electron Mobility Transistors (HEMTs) Grown by MOCVD on Semi-Insulating SiC Substrates
Authors: Sin Yongkun , Kim Hyun I , Adams Paul , Stupian Gary
Published: April 2011

AlGaN/GaN HEMTs (High Electron Mobility Transistors) grown on semi-insulating (SI) SiC substrates are very promising for high power, high speed, and high temperature operation with great potential for both military and commercial applications. These high performance characteristics are possible due to presence of high two-dimensional electron gas (2 DEG) charge sheet density maintaining a high Hall mobility at the AlGaN barrier/GaN buffer hetero-interface. However, reliability of AlGaN HEMTs still remains a major concern because of the large number of defects and traps present both in the bulk as well as at the surface leading to current collapse. We report on the study of defects and surface properties in MOCVD-grown Al0.27Ga0.73N HEMT structures on SI SiC substrates. Our HEMT structures consist of a 25nm thick undoped AlGaN barrier layer and a 3μm thick undoped GaN buffer layer grown on a 100nm thick AlN nucleation layer. Hall measurements showed a charge sheet density of ∼1013/cm2 and a Hall mobility of ∼1500cm2/V·sec. Both cross-sectional and plan view TEMs were employed to study defects in the heterostructures and XPS (X-ray Photoelectron Spectroscopy) and AES (Auger Electron Spectroscopy) employed to study surface properties in both GaN and AlGaN layers. DC characterization results from AlGaN Schottky diodes with Pt/Au Schottky contacts are also reported along with results from AlGaN/GaN HEMT devices.
Piezoelectric Fields in Tilted GaInN Quantum Wells
Authors: Feneberg Martin , Lipski Frank , Sauer Rolf , Thonke Klaus , Wunderer Thomas , Brückner Peter , Scholz Ferdinand
Published: April 2011

GaInN/GaN quantum wells (QWs) grown on different crystal facets have been investigated by field-dependent photoluminescence and electroluminescence experiments. Externally applied voltage changes the total field strength and direction of the electric fields inside the quantum wells, consisting of piezoelectric and built-in fields. Electroluminescence with increasing current results in a peak shift due to screening of the field by the injected carriers. By modeling the peak shifts of the photoluminescence (PL) and electroluminescence (EL) signals we found strong piezoelectric fields for a {0001} sample and nearly vanishing fields for a sample grown on the {1-101} plane of GaN.
Optical Properties of Undoped, n-Doped and p-Doped GaN/AlN Superlattices
Authors: Paskov Plamen , Monemar Bo , Kamyiama Satoshi , Amano Hiroshi , Akasaki Isamu
Published: April 2011

We report a photoluminescence study of near-surface GaN/AlN superlattices grown by metalorganic chemical vapor deposition (MOCVD) on a thick GaN layer. Undoped, Si-doped and Mg-doped structures with the well/barrier thickness ratio 3:1 and different periods are investigated. It is found that the energy position, intensity and linewidth of the emission are determined by the interplay of the built-in polarization field, depletion field and screening by the free carriers. In n-type structures an electron accumulation at the bottom interface is evidenced by the observed recombination of the two-electron gas with the photo-excited holes.
Phonon Decay in GaN and AlN and Self-Heating in III-N Devices
Authors: Holtz Mark , Song D. Y. , Nikishin S. A. , Soukhoveev V. , Usikov A. , Dmitriev V. , Mokhov E. , Makarov U. , Helava H.
Published: April 2011

We report studies of the temperature dependence of Raman lines in high quality GaN and AlN. The temperature dependence of the phonon energies and linewidths are used to produce consistent phonon decay properties of zone center optic phonons. In GaN we observe the E22 phonon to decay into three phonons, while the A1(LO) phonon is well described according to the so-called Ridley process – one TO and one LA phonon. For AlN the E22 phonon decays by two phonon emission and the A1(LO) line also exhibits a dependence consistent with the Ridley process. Along with the phonon decay processes, it is important in each case to take into account the contribution of the thermal expansion, including the temperature dependence, to describe observed temperature shifts in the phonon properties.
1.37 - 2.90 Micron Intersubband Transitions in GaN/AlN Superlattices
Authors: DeCuir Eric Anthony , Fred Emil , Manasreh Omar , Xie Jinqiao , Morkoc Hadis , Baumann Esther , Hofstetter Daniel
Published: April 2011

Intersubband transitions in the spectral range of 1.37-2.90 °Cm is observed in molecular beam epitaxy grown Si-doped GaN/AlN multiple quantum wells using a Fourier-transform spectroscopy technique. A blue shift in the peak position of the intersubband transition is observed as the well width is decreased. A sample with a well width in the order of 2.4 nm exhibited the presence of three bound states in the GaN well. The bound state energy levels are calculated using a transfer matrix method. An electrochemical capacitance voltage technique is used to obtain the three dimensional carrier concentrations in these samples which further enable the calculation of the Fermi energy level position. Devices fabricated from these GaN/AlN quantum wells are found to operate in the photovoltaic mode.
Evaluation on Crystal and Optical Properties of AlN:Er Prepared by RF magnetron sputtering method
Authors: Uekusa Shin-ichiro , Ohno Takahiko , Arai Tomoyuki , Miura Hiroshi
Published: April 2011

The Er that is rare earth element causes luminescence at 1540 nm, which is a low loss transmission window in silica-based fibers used in optical communications. Moreover, it is known that the temperature quenching of the luminescence of Er becomes smaller with the wide band gap of the host material. We report on the photoluminescence (PL) characteristic and X-ray diffraction (XRD) of AlN:Er thin films which are deposited by RF reactive magnetron co-sputtering method. All the thin films were deposited by RF reactive magnetron co-sputtering method from a 99.999 % Al target and 99.99 % Er chips in a nitrogen gas (99.9998 %) atmosphere. The thin films were achieved with a discharge power of 250 W in a total pressure of 5.0∼10-3 Torr, and the deposition times were one hour. After the deposited, these samples were annealed for 30 minutes in the temperature range from 400 to 900° in a nitrogen gas (99.999 %) atmosphere with an infrared lamp heater. PL spectra of AlN:Er were measured using the 325 nm line of a He-Cd laser at 15 K. Consequently, we observed PL spectra around 1500 nm. The strong luminescence of peak wavelength at 1538 nm is based on the intra-4f emitting centers of Er. We report systematically the experimental results of PL and XRD.
Ion-Implanted GaN/AlGaN/GaN HEMTs with Extremely Low Gate Leakage Current
Authors: Nomoto Kazuki , Tajima Taku , Mishima Tomoyoshi , Satoh Masataka , Nakamura Tohru
Published: April 2011

We demonstrate the realization of compatibility of extremely low gate leakage current and low source resistance with Si ion-implanted (I/I) GaN/AlGaN/GaN surface-stabilized high-electron mobility transistor (HEMT) without any recess etching process. The source/drain regions were formed using Si ion implantation into undoped GaN/AlGaN/GaN on sapphire substrate. Using ion implantation into source/drain (S/D) regions with energy of 80 keV, the performances were significantly improved. On-resistance (Ron) reduced from 105 to 9.2 Ω·mm. Saturation drain current (Idss) and maximum transconductance (gmMAX) increased from 49 to 527 mA/mm and from 13 to 84 mS/mm (Vg=+1V).
Low-Temperature Cathodoluminescence Mapping of Green, Blue, and UV GaInN/GaN LED Dies
Authors: Xia Yong , Detchprohm Theeradetch , Senawiratne Jayantha , Li Yufeng , Zhao Wei , Zhu Mingwei , Wetzel Christian
Published: April 2011

GaInN based light emitting diodes (LEDs) play an important role as energy efficient light sources in solid state lighting. A controversial discussion addresses the origin of lateral light emission variations and their correlation with either of the identified defects, e.g., threading dislocations and V-defects. In order to establish any possible correlation of defects and luminescence centers, we analyze three UV, blue and green LED dies by microscopic mapping of spectroscopic cathodoluminescence and secondary electrons at variable low temperature from 7 K to room temperature. Particular effort is being placed on a quantitative analysis of the luminescence signal. Image intensities are not being scaled and offset for highest contrast as otherwise typical for imaging mode. In standard configuration, we analyze image areas of (0.037 mm)2 with pixel resolution of 72 nm. Following regions of strong and weak emission we find that remain bright and dark respectively even at low temperature. Those variations increase with the mean emission wavelength of the LEDs and with temperature. The largest peak wavelength variation associated with the intensity contrast was observed in the green LEDs and amounts to 5 nm. Here the peak wavelength is higher in the dark spots than in the bright ones. This finding corresponds to the general trend when comparing the lower efficiency in longer wavelength green emitters to the blue ones.
Fabrication of AlGaN/GaN HFET with a High Breakdown Voltage on 4-inch Si (111) Substrate by MOVPE
Authors: Niiyama Yuki , Kato Sadahiro , Sato Yoshihiro , Iwami Masayuki , Li Jiang , Takehara Hironari , Kambayashi Hiroshi , Ikeda Nariaki , Yoshida Seikoh
Published: April 2011

We investigated an AlGaN/GaN heterostructure field effect transistor (HFET) on Si substrates using a multi-wafer metalorganic vapor phase epitaxy (MOVPE) system. It was confirmed that a GaN film with smooth surface and without any crack was obtained. To increase a resistance of a GaN buffer layer, the carbon (C) -doping was carried out by controlling the V/III ratio and the growth pressure. The breakdown voltage of the buffer layer was dramatically improved by introducing the C. As a result, the breakdown voltage was about 900 V when the C concentration was about ∼8×1018 cm−3. After while, an AlGaN/GaN heterojunction FET (HFET) on a C-doped GaN buffer layer was fabricated. We achieved the breakdown voltage of over 1000 V and the maximum drain current of over 150 mA/mm, respectively. It was found that the C doped buffer layer is very effective for improving the breakdown voltage of AlGaN/GaN HFETs.
Electronic Structures of Heavily Boron-doped Superconducting Diamond Films
Authors: Yokoya Takayoshi , Okazaki Hiroyuki , Nakamura Tetsuya , Matsushita Tomohiro , Muro Takayuki , Ikenaga Eiji , Kobata Masaaki , Kobayashi Keisuke , Takeuchi Akihisa , Awaji Akihiro , Takano Yoshihiko , Nagao Masanori , Takenouchi Tomohiro , Kawarada Hiroshi , Oguchi Tamio
Published: April 2011

Recent photoemission studies on heavily boron-doped superconducting diamond films, reporting the electronic structure evolution as a function of boron concentrations, are reviewed. From soft X-ray angle-resolved photoemission spectroscopy, which directly measures electronic band dispersions, depopulation of electrons (or formation of hole pockets) at the top of the valence band were clearly observed. This indicates that the holes at the top of the valence bands are responsible for the metallic properties and hence superconductivity at lower temperatures. Hard X-ray photoemission spectroscopy observed shift of the main C 1s core level and intensity evolution of a lower binding energy additional structure, suggesting chemical potential shift, carrier doping efficiency by boron doping, and possibility of boron-related cluster formations.
Electrical Transport in Heavily B-Doped Epitaxial Diamond and NCD
Authors: Nesladek Milos , Bergonzo Phillipe , Mares Jiri , Hubik Pavel
Published: April 2011

This paper deals with a LT transport study in B-doped diamond. To understand the electrical transport in heavily B-doped diamond and the influence of disorder onto the electrical transport, we have prepared nanocrystalline and epitaxial B-doped diamond films at CEA Saclay. The transport properties of these layers have been studied at the Institute of Physics Czech Academy of Sciences in Prague. It has been found that our B-doped nanocrystalline diamond exhibits also the SC transition, similarly as the original Russian work done on HPHT polycrystals or single crystal diamond. Additionally, the properties of (111) epitaxial films are discussed.
Biological Applications of Nanocrystalline Diamond
Authors: Williams Oliver A. , Daenen Michael , Haenen Ken
Published: April 2011

Nanocrystalline diamond films have generated substantial interest in recent years due to their low cost, extreme properties and wide application arena. Diamond is chemically inert, has a wide electrochemical window and is stable in numerous harsh environments. Nanocrystalline diamond has the advantage of being readily grown on a variety of substrates at very low thickness, resulting in smooth conformal coatings with high transparency. These films can be doped from highly insulating to metallically conductive and at very high concentrations become superconducting.
The effects of Methane Concentration on diamond nucleation and growth during bias enhanced nucleation on 3C-SiC(100) surfaces
Authors: Arnault Jean-Charles , Saada Samuel , Intiso Luciana , Bergonzo Philippe
Published: April 2011

The methane effects on nucleation and growth of diamond during bias enhanced nucleation treatment have been studied on 3C-SiC (100) surfaces. At low methane concentration of 1%, no diamond nucleation was observed, whether at 3 %, nucleation density values as high as 4×1010/cm2 were reached. A further increase of the methane concentration up to 5% induces a significant enhancement of the diamond nucleation density that was observed only slightly higher at 7×1010/cm2. Moreover, the Field Emission Gun Scanning Electron Microscopy (FEG-SEM) pictures well emphasized that the methane content affects both the nucleation and growth mechanisms.
Nano Crystalline Diamond Synthesized by Hot Filament Chemical Vapor Deposition
Authors: Hirai Takayuki , Kanno Yoshinori , Takagi Yoshiki
Published: April 2011

Due to its extreme hardness, chemical and mechanical stability, large band gap and highest thermal conductivity, poly-crystalline diamond is expected to be an excellent packaging material for biomedical and environmental MEMS devices. Recently nano crystalline diamond (NCD) has been synthesized by microwave plasma chemical vapor deposition (MPCVD) technique using a gas mixture of methane-hydrogen or methane-hydrogen and inert gas, argon mixture. Diamond synthesis from liquid carbon source has a relatively high growth rate among various CVD methods. Hot filament chemical vapor deposition (HFCVD) is popular method in order to grow the diamond particles or films. The equipment of HFCVD in present paper is simple and the easy operating. Therefore we tried to NCD synthesis by HFCVD from liquid carbon source such as methanol, ethanol etc. The structure, surface morphology, and grain size of the diamond were examined with field emission scanning electron microscopy (FE-SEM) and Raman spectroscopy. We confirmed our diamond particles as NCD with typical Raman peak of NCD. And we observed 100nm under in diameter with FE-SEM. We will refer to NCD films synthesis by HFCVD in the paper.
Highly Oriented Diamond Films Grown at High Growth Rate
Authors: Li Xianglin , Collazo Ramon , Sitar Zlatko
Published: April 2011

Highly oriented diamond (HOD) films were grown at a high growth rate on (100) silicon substrates by microwave plasma chemical vapor deposition (MPCVD), following the standard bias-enhanced nucleation (BEN) process. The growth rate and diamond quality were investigated as a function of methane concentration in hydrogen (2-6%), and N2/CH4 ratio (0 to 0.12). A four-fold increase in the growth rate of HOD films and a three times faster expansion and coalescence of (100) facets was observed within the above process window. The films with the best quality were grown under an N2/CH4 ratio of 0.08 at methane concentration of 3.5%. The ratio of x-ray intensity between the first order twins and {111} poles was only 1%. A detailed study of the crystalline quality and phase purity as a function of methane concentration and nitrogen addition is presented.
Photoelectron Emission Mechanism From Hydrogen Terminated Nano-Crystalline Diamond
Authors: Takeuchi Daisuke , Saeki Kazuhiko , Nebel Christoph Erwin , Yamasaki Satoshi , Williams Oliver Aneurin
Published: April 2011

We investigated the electron affinity on hydrogen-terminated nano- (NCD) and ultranano-crystalline diamond (UNCD) films to reveal influences of defects, sp2 and grain boundaries on photoemission properties. To compare properties, single crystalline IIa diamond, optical grade poly-, nano-, ultranano-crystalline diamond films, a diamond-like carbon film, a highly oriented pyrolytic graphite have been characterized. All hydrogen terminated SCD, PCD and NCD samples show continuous sub-band yield, arising at about 4.4 eV, regardless of their crystal and/or grain sizes, while UNCD, DLC, and HOPG do not show the specific photoemission in the regime hν < 6 eV. Note that the UNCD film grown with some amount of hydrogen in the gas phase (UNCD-H) shows a comparable yield spectrum as hydrogen-terminated SCD. According to the normalized spectra, obviously, the electron emission mechanism on hydrogen-terminated NCD films is the same as that of SCD with a NEA of -1.1 eV. The sub-band yield from NCD is even larger than that of SCD, which is attributed to surface enlargement.
Ultraviolet Photodetection Properties of ZnO Microtubes
Authors: Cheng Jiping , Fu Ming , Zhang Yunjin , Guo Ruyan
Published: April 2011

Photodetectors based on wide-bandgap semiconductors have demonstrated several advantages over traditional ultraviolet (UV) detectors (photomultiplier tubes and Si-based UV detectors) such as low power consumption, high stability, and no need of other optical filters. ZnO stands a good chance of being a candidate material for solar-blind UV detection because of its direct bandgap of 3.37eV and high photoresponse. In this work, single crystal ZnO microtubes synthesized using a microwave-heating growth method and their UV photodetection properties were studied. The ZnO microtubes exhibited relatively fast UV photoresponse with a cut-off wavelength ∼370 nm, indicating their potential applications as high efficient and low cost UV detectors.
Modification of ZnO Layers by Molecular Adsorbates During Electrochemical Deposition
Authors: Loewenstein Thomas , Neumann Christian , Meyer Bruno K. , Yoshida Tsukasa , Schlettwein Derck
Published: April 2011

ZnO and ZnO/EosinY hybrid materials were electrodeposited from aqueous zinc salt solutions on (0001) GaN and on (0001) ZnO. Crystalline ZnO was deposited as proven by X-ray diffraction (XRD). The intensity pattern for ZnO/EosinY showed a preferential orientation with the c- plane of ZnO parallel to GaN (0001) or ZnO (0001). XRD rocking curves with FWHM=0.25° indicated a high level of in-plane orientation of the deposited ZnO crystalline domains. The peak position of (0002) ZnO was shifted by 2Θ=1.3°. This difference and the corresponding simultaneous shift of (0004) ZnO were explained by a lattice expansion by 3.6 % in the c- direction. This clearly indicated the strong influence of the Eosin Y molecules adsorbed during the electrodeposition of ZnO. Scanning electron microscopy (SEM) revealed the formation of domains with different crystal sizes pointing at a varying density of nucleation sites on the substrate.
Growth and Characterization of Optical and Electrical Properties of ZnO Nano- and Microwires
Authors: Grundmann Marius , Rahm Andreas , Nobis Thomas , von Wenckstern Holger , Czekalla Christian , Lenzner Jörg , Lorenz Michael
Published: April 2011

ZnO nanowires are readily fabricated with a novel and unique, high-pressure pulsed laser deposition process developed recently by us. It allows the flexible and reproducible control of morphology, diameter, lenght and composition. Furthermore, we report on the electrical characterization as well as stimulated emission from a zinc oxide (ZnO) microcrystals grown by carbothermal evaporation observed by spatially resolved photoluminescence (PL) and high excitation spectroscopy (HES).
Surface Luminescence of Polycrystalline Zinc Oxide Excited by Hydrogen Atoms
Authors: Sushchikh Michael , Styrov Vladislav , Tyutunnikov Vladimir , Cordella Nick
Published: April 2011

Excitation of a luminescence by highly exothermic chemical reaction on the surface of a luminophore provides a unique opportunity to separate surface luminescence from the bulk luminescence. This enables studies of the electronic properties of the semiconductor surfaces even if the surfaces are of complicate shapes. We have studied heterogeneous chemiluminescence (HCL) of ZnO powders. The luminescence was excited by a release of chemical energy, namely by catalytic recombination of hydrogen atoms. The HCL spectra were compared to the photoluminescence (PL) spectra. The HCL spectra were sensitive to the details of preparation and treatment whereas PL spectra almost did not change. HCL spectra of powder samples pretreated for enhancing “green” luminescence exhibited long-wavelength tail (up to 800 nm) and their maximum was blue-shifted as compared with PL spectra. Different HCL bands forming long-wavelength tail were isolated by changing the temperature of the samples. Additional milling of ZnO led to amplification of the HCL-specific surface bands. Pure ZnO showed neither PL nor HCL; however we were able to observe HCL surface bands with maxima at 610 nm and 730 nm after treatment of the sample in atomic hydrogen atmosphere at 570 K. Remarkably, such treatment did not cause appearance of the PL. The HCL in the presence of atomic hydrogen was steady in time and was caused by an abstraction of adsorbed hydrogen by incident hydrogen atoms, i.e. the reaction followed Eley-Rideal mechanism. The HCL can be utilized for in situ monitoring of the growth and evolution of ZnO in controlled atmosphere.
Controllable Synthesis of ZnO Nanorod Arrays via Simple Solution-Based Method
Authors: Charoensirithavorn Patcharee , Yoshikawa Susumu
Published: April 2011

Here we present a convenient solution-based method, which can afford a procedure to easily fabricate highly oriented ZnO nanorods on substrate at relatively low temperatures. The as-synthesized products have been characterized by scanning electron microscopy (SEM) and X-Ray diffraction (XRD). The results revealed that a densely packed and perpendicularly oriented single-crystalline ZnO nanorod arrays grew vertically on the fluorine-doped SnO2 transparent conducting oxide (FTO) glass substrates. In addition, we found that the length of the nanorod could be freely modified by controlling the solution temperature.
Visible Luminescence from ZnO Nanostructures
Authors: Cheng An-jen , Wang Dake , Seo Hee Won , Park Minseo , Tzeng Yonhua
Published: April 2011

Room temperature photoluminescence (PL) spectra from zinc oxide (ZnO) nanostructures were studied. ZnO samples were produced via thermal chemical vapor deposition (thermal-CVD) and a variety of ZnO nanostructures were synthesized by adjusting the oxygen content during the growth process. All samples exhibit a sharp and strong ultra-violet near-band-edge (NBE) emission at about 3.18 eV. The visible emission from the samples deposited under an oxygen-deficient condition were dominated by blue-green band emission at 2.34 eV. The intensity of the blue-green band was greatly reduced (so-called green band free) for the ZnO deposited at the center of the wafer while strong violet-blue emission bands and broad bands at yellow-orange-red range were collected from the ZnO grown along the edge of the wafer. We believe that the spatial inhomogeniety was caused by turbulent gas flow in the reaction chamber, which resulted in different local oxygen concentration. Origin of visible luminescence from ZnO nanostructures will be discussed and a model to explain the observed visible luminescence process will be presented.
Schottky Contact Behaviour as a Function of Metal and ZnO Surface Polarity
Authors: Allen M.W. , Miller P. , Metson J.B. , Reeves R.J. , Alkaisi M.M. , Durbin S.M.
Published: April 2011

Hall effect, photoluminescence (PL) and Schottky diode measurements were made on the Zn-polar , O-polar and m-plane faces of hydrothermally grown, bulk ZnO. Several polarity related differences were observed in the PL spectra. The most noticeable was increased emission from free excitons and from a triplet of emissions at 3.3725 - 3.3750 eV on the Zn-polar face. Polarity effects were also observed in the properties of highly rectifying, planar, silver oxide diodes fabricated by RF sputtering using an Ag target and an Ar/O2 plasma. The most significant of these was a consistent 130 meV larger barrier height for silver oxide diodes on the Zn-polar face compared to the O-polar face. These polarity effects are thought to result from the internal compensation of bound spontaneous polarization charges at the Zn-polar and O-polar faces. In addition, Au and Ag Schottky diodes with image-force-controlled ideality factors were achieved on the Zn-polar face of bulk ZnO without any special surface treatments.
Large Area Multi-wafer MOCVD of Transparent and Conducting ZnO Films
Authors: Tompa Gary S. , Sun S. , Provost L. G. , Mentel Dan , Sugrim D. , Chan Philip , Tong Keny , Wong Raymond , Lee A.
Published: April 2011

ZnO thin films are of interest for an array of applications, including: light emitters, photovoltaics, sensors and transparent contacts, among others. Production routes for ZnO include sputtering, MBE and MOCVD. This paper focuses on our efforts to produce a large scale MOCVD thin film production tool and the results obtained from the reactor. Specifically, we have constructed a tool with a 16” wafer carrier that uniformly deposits ZnO films on 38×2” wafers simultaneously. The reactor operates at low pressure (<0.1 Atmosphere) and through 700°C. High quality, uniform films have been deposited on an array of substrates. Al-doped films exhibited resisitivities in the 1×10-3 ohm-cm range and transmissivity greater than 80%. Film morphology and crystallinity are a function of process parameters. The large area oxide MOCVD reactor design challenges and results are summarized. Tool performance and ZnO thin film quality are reviewed, as well as preliminary ZnO contact performance on GaN LEDs.
Electroplating of ZnO Nanowires Using Nanohole Arrays of Anodized Aluminum Oxide and Effects of Thermal Annealing
Authors: Ogata Ken-Ichi , Shingubara Shoso , Yorozu Hiromi , Nakanishi Tadahiko
Published: April 2011

Electroplating of ZnO nanowires was conducted using gold embedded anodized aluminum oxide (AAO) films on Si substrates. For electroplating, insulating layers at the bottom of AAO nanohole structures need to be removed. After electroplating, hexagonal structure of vertical ZnO nanowires was observed, however, they were broken and lied down by thermal annealing process. Photoluminescence (PL) spectra were investigated and that of post annealed ZnO nanowires indicates that nitrogen atoms were incorporated as acceptor.
ZnO Nanofibers Doped with Ga, In and Er Fabricated with Electrospinning Technique
Authors: Khan Aurangzeb , Jadwisienczak Wojciech M. , Khan Saima N. , Kordesch Martin E.
Published: April 2011

ZnO nanofibers doped with Ga, In and Er metals have been fabricated by electrospinning technique. The diameter of the fibers was in the range of 0.5-2 μm and the length can be up to several feet. After spinning fabrication step the samples were dried out and annealed at 900 °C in air. Room temperature photoluminescence (PL) spectra measured for undoped and In- and Ga-doped ZnO fiber samples exhibit only a strong near band edge (NBE) emission at ∼380 nm with very weak green band at 525 nm. In contrary, the PL spectrum of Er-doped ZnO fibers shows a very weak NBE and strong green emission band at ∼550 nm at 300 K. The electrospinning mechanism used for the fabrication of nanofibers was found to be productive, simple and easy to implement disregarding of the doping type and concentration.
Lasing Characteristics of Nano-structured Zinc Oxide
Authors: Kuo Shou-Yi , Chen Wei-Chun , Lai Fang-I
Published: April 2011

Highy-quality nano-structured ZnO samples have been synthesized by simple chemical solution and post-thermal treatment. The samples were characterized by x-ray diffraction (XRD), scanning electron microscope (SEM), transmission electron microscope (TEM), temperature-dependent photoluminescence (PL) spectra measurements. XRD patterns illustrated that there were no second phases in these ZnO samples, and the TEM results indicated that the ZnO samples are single crystalline with a hexagonal structure. Room-temperature PL spectra of ZnO thin films showed a strong UV near-band-edge (NBE) emission located at about 390 nm and a green defect-related (G) emissions, where the intensity ratio (INBE/IG) varies with the annealing temperatures. Meanwhile, the ZnO samples exhibited free exciton and very sharp exciton emissions at low temperatures. Particularly, room-temperature UV random lasing characteristic of ZnO films has been observed as well. It is shown that these nano-structured ZnO samples can exhibit random laser action depending on the growth condition. The threshold intensity for the lasing is comparable to earlier reported data. These results indicate that nano-structured ZnO samples prepared by simple techniques may be a promising material for further photonic devices. Possible lasing mechanism is discussed and further investigation to clarify the mechanism between the nano-structured ZnO samples is still underway.
Synthesis and Microstructural Analysis of Benzylthiol-functionalized Au Nanocrystals
Authors: Vitale Floriana , Mirenghi Luciana , Piscopiello Emanuela , Protopapa Maria Lucia , Tapfer Leander , Giannini Cinzia , Guagliardi Antonietta , Cervellino Antonio , Pellegrini Giovanni , Trave Enrico , Mattei Giovanni , Mazzoldi Paolo , Fratoddi Ilaria , Russo Maria Vittoria
Published: April 2011

We report on the synthesis and microstructural and microanalytical analysis of dodecanethiol and benzylthiol-stabilized Au nanoparticles (diameter range between 2 nm and 10 nm). Stable functionalized gold nanoparticles were synthesized by using two different routes: a two-phase liquid-liquid method and a one-phase method. The size, strain, shape and crystalline structure of the nanocrystals were determined by a full-pattern X-ray powder diffraction analysis and high-resolution electron microscopy. The chemical environment of the Au nanocrystals and their interaction with the thiols was investigated by X-ray photoelectron spectroscopy. Photoluminescence spectroscopy measurements show a characteristic IR emission at λ=960 nm of small sized Au nanocrystals (∼3 nm). The origin of this IR emission line seems to be correlated to the Au-S bonding and the size of the nanoparticles.
Iron Nanoparticles Embedded in Silica Glass: A Computational Study
Authors: Kroll Peter , Theuerkauf Jens , Wieland Thomas
Published: April 2011

We performed electronic structure calculations within density functional theory including ab initio molecular dynamic simulations of isolated Fe13 and Fe55 clusters. We observed the energy preference of icosahedral clusters over cuboctahedrons for both Fe13 and Fe55. The magnetic structure is ferromagnetic for Fe13, but anti-ferromagnetic for Fe55. But isolated clusters exhibit a HOMO-LUMO gap.
Subsequently, we embedded the clusters into models of silica glass of different size. For embedded clusters we observed the formation of an iron oxide interface between cluster and dielectric matrix. Though many different models were computed, they show a very homogeneous trend. The magnetization of optimized embedded models is close or larger than the magnetization of free clusters. The small clusters retain their electronic structure around the Fermi-level despite some major distortions.
Thermodynamic Modeling of Island Size Distributions for InGaAs/GaAs Self Assembled Quantum Dots: A Quantitative Effort to Understand Ensemble Size Nonuniformity
Authors: Cederberg Jeff , Roshko Alexana , Hyland Brit , Coltrin Michael
Published: April 2011

Experimental island count histograms as a function of SAQD volume have been evaluated using an established model. The experimental data was obtained for 51 mm wafers grown by MOCVD and analyzed over the center 26 × 26 mm square of the wafer with AFM. More than one distribution is required for all conditions investigated to obtain adequate representations of the experimental data. Consistent parameters are obtained for samples grown with a variable InAs thickness. Higher growth temperatures results in material being converted into relaxed islands. Extended annealing without AsH3 eliminates small islands, suggesting that they are not a stable distribution.
Microscopical Study of Au Nanocrystals Self-Assembled on (100)Si and SiO2/(100)Si Substrates
Authors: Piscopiello Emanuela , Tapfer Leander , Vittori Antisari Marco , Paiano Pasquale , Prete Paola , Lovergine Nicola
Published: April 2011

This work reports on the structural characterization of Au nanocrystals directly prepared on the surface of (100)Si and 150 nm-thick SiO2 deposited (100)Si substrates, by a physical self-assembly method, consisting in the UHV evaporation of a thin Au film and its successive high temperature annealing. The morphology, orientation, and crystalline structure of Au nanocrystals were characterized by scanning and high-resolution transmission electron microscopy and X-ray diffraction, respectively. Experimental results show that the nature of the substrate strongly influences the process of Au nanocrystal formation upon heat treatment, by affecting the interaction of deposited Au with the underlying material. In the case of clean (100)Si substrates the Au strongly interacts with Si, so that Au nanoislands are obtained with a well defined epitaxial relationships with the substrate, i.e. [100]AuÐÐ[110]Si and [110]AuÐÐ[311]Si. The nanoisland shape is affected by faceting at the Au/Si interface, the Au nanocrystal being limited by the {111}, {311}, {711} and {-111} planes of Si. In the case of SiO2/(100)Si substrates spherical shaped Au nanoparticles with random crystal orientation are instead, produced.