bluuebunny/crossref_metadata_2025 huggingface dataset:
Design and calculation of advanced microstructural patterns by laser interference metallurgy
Authors: Lasagni Andres Fabian , Holzapfel Christian , Mücklich Frank
Published: April 2011

Laser Interference Metallurgy is a recently developed method for the laser material surface modification by which various interference patterns can be transformed directly, permanently, and efficiently to the surfaces of different kinds of materials. By using of this technique, different metallurgical effects such as melting, recrystallization, quenching, recovery, defect or phase formation can be exploited. In this work, advanced microstructural patterns are designed by means of the two dimensional Fourier Transformation. After that, the geometrical configuration for the laser interference experiments is calculated. Finally, thin metallic films are irradiated in order to demonstrate the viability of the presented method. The resulted structures were studied by means of Focus Ion Beam and Transmission Electron Microscopy. These surfaces represent a new type of long ordered topographical as well chemical patterns which might be applicable for well defined surface functionalization.
Ultra sensitive Bio-Chemical sensors Based on Optical Resonance
Authors: Prasad Shalini
Published: April 2011

With the focus on bio terrorism and environmental sensing, there is need for development of smart integrated bio-chemical sensors. We investigate Whispering Gallery Mode (WGM) based High Q micro cavity as a biosensor. We integrate the lab-on-a-chip approach with High Q technology to develop highly selective and smart sensors for in-situ detection of aqueous media based agents. We have investigated the ring based micro-cavity structure as a sensor. We identify the detection of agent through the changes observed in the wavelength shift and Q factor as a result of enzyme-substrate interactions. We report an experimental demonstration of a compact micro-cavity based biochemical sensor based on the micro ring structure. The micro-cavity, fabricated on a silica-on-silicon substrate, is designed to have a resonant wavelength (l) near 1.34 μm. The transmission spectrum of the sensor is measured with different ambient refractive indices ranging from n =1.0 to 1.5. Chemical detection was achieved by measuring the shift in resonant wavelength, and variation to the quality factor
Oscillatory Temperature-driven Morphological Relaxation of Surface Ripple Using Weak Pulsed Laser
Authors: Khenner Mikhail
Published: April 2011

A continuum (Mullins-type) model is proposed for the non-isothermal, isotropic evolution of a crystal surface on which mass transport occurs by surface diffusion. The departure from constant temperature is assumed induced by low-energy incident pulsed radiation. It has been previously shown experimentally and theoretically that such heating mode gives rise to the quasistationary regime, in which the surface temperature of a thick solid film oscillates about the mean value with the pulse repetition frequency. The implications of oscillatory driving with high frequency values on relaxation of surface ripple are examined; in particular, the traveling wave solutions with decreasing amplitude are detected numerically. Pulsed heating also results in faster smoothing of the ripple, compared to the case when the surface is at constant temperature which is same as the mean temperature in the pulsed heating mode. Impact on ripple shape is minor for ripple amplitudes considered.
Real time measurement of photochemical surface reactions by using a total reflection prism
Authors: Sato Yuki , Sato Yuji , Murahara Masataka
Published: April 2011

The photochemical reaction processes, i.e., photo-dissociation of a reaction solution, defluorination of fluorocarbon [FEP], and substitution of a hydrophilic group, were measured in real time, when irradiating a Xe2 excimer lamplight on the water or ammonia solution as a reaction solution and FEP placed on the attenuated total reflectance [ATR] prism. In case of ammonia solution, with the increase in irradiation time, the absorption peaks of 3300 cm−1 and 1650 cm−1 gradually decreased, and the new absorption of -CF2-NH2 gradually appeared at 1410 cm−1, which presented the strongest absorption after UV irradiation for 25 minutes. The results indicate that the ammonia water has turned to amino groups. In addition, the contact angle of water was measured. When compared to the untreated sample, whose contact angle was 110 degrees, the contact angle of the sample treated with water became 31 degrees after the Xe2 lamp irradiation for 25 minutes, and that of the sample modified by ammonia water further improved to 27 degrees after the irradiation for 25 minutes.
Electrowetting–induced photochemical surface modification onto fluorocarbon
Authors: Anai Hiroyuki , Sato Yuji , Murahara Masataka
Published: April 2011

The PTFE was modified into hydrophilic with 1/100 of the shots number required to obtain the same contact angle with water by the laser irradiation alone, when irradiating an ArF excimer laser on the sample surface at the moment of applying a 6 kV to the water placed on the PTFE surface to decrease the contact angle with water.

A plasma treatment method is widely used for plastic surface modification, but the hydrophilic property generated by this method fades away soon. On the other hand, we have previously reported that the ArF excimer laser light was applied on a sample surface in the presence of water to substitute hydrophilic groups, which was modified to have a long–lasting hydrophilic property. This method, however, needed 3000 to 10000 shots of the laser irradiation, and it is less economical compared with the plasma processing that requires only one–minute irradiation. There is an electro–wetting method, in which the contact angle with water decreases temporarily when a high voltage is applied between the water and the sample, but the contact angle is restored to its original position when stopping the voltage application.

Thus, we demonstrated the surface modification of PTFE maintaining the hydrophilic property for a long period with only 100 shots, by irradiating the ArF excimer laser on the sample at the moment when the wettability became high by the electro–wetting method. Water was placed in the gap between the silica glass and the PTFE to create a thin liquid layer with capillary phenomenon. A high voltage (6 kV) of direct current (DC) or alternating current (AC) was applied on the gap, and the ArF excimer laser was vertically irradiated on the sample surface. The water was photo–dissociated to produce H and OH. At the same time, the C—F bond of the PTFE was also photo–dissociated, and the F atom bonded to the H atom to produce HF. The OH group united with the dangling bond of C, which resulted in modifying the PTFE surface to be hydrophilic.

To evaluate the wettability of the modified sample, the contact angle with water was measured. Improving the contact angle with water from 110 degrees for the untreated sample to 50 degrees for the treated sample had needed 10000 shots at the laser fluence of 5 mJ/cm2. By combined high voltage application and ArF excimer laser irradiation treatments, however, the 50–degree contact angle was yielded with 500 shots, 1/20 of 10000, when applying the DC of 6 kV, and with 100 shots only, 1/100 of 10000, when applying the AC of 6 kV. Moreover, the modified sample was observed for a change in contact angle with passage of time. The contact angle was 60 degrees after applying the high voltage, and 110 degrees when stopped. On the other hand, the sample modified by combined the high voltage application and ArF excimer laser irradiation maintained the 50–degree contact angle for one month after stopping the voltage application.
Conformal deposition of a silicon-oxide and a fluorocarbon on a nano-shaped material in supercritical carbon dioxide
Authors: Shimizu Takashi , Ishii Kenichi , Suzuki Eiichi
Published: April 2011

A novel surface engineering of nano-shaped materials, i.e., surface coating in supercritical fluid is proposed. Conformal deposition of a silicon-oxide thin film in supercritical carbon dioxide (scCO2) where O2 gas is miscible as an oxidizing agent is demonstrated. A uniform silicon-oxide film has been deposited even on backside of a nano-shaped SiO2, which is hard in the standard chemical vapor deposition. Filling of a Si-oxide into the horizontal gap has also been successfully achieved. Deposition of a fluorocarbon in scCO2 were tried to investigate surface modification using scCO2 and a hydrophilic SiO2 surface became water-repellent after the surface modification. These results indicate that the developed surface engineering in the supercritical fluid is promising technology in nanoscale device fabrication and micro (nano-) electromechanical systems.
Melt Fracture of Polymer Thin Film at Strongly Attractive Surfaces
Authors: Li Chunhua , Jiang Jun , Rafailovich Miriam H. , Sokolov Jonathan C.
Published: April 2011

In this study, we report an interesting phenomenon of “melt fracture” which was observed when a high viscosity film dewets from a film of lower viscosity. We propose that this phenomena is similar to the “melt fracture” or “shark skin” that is observed when extruding bulk polymer. We hypothesized that the “melt fracture” occurs as a result of shear which is imposed by the dewetting layer on the visco-elastic lower layer. The dewetting layer is adhered to the lower layer via entanglements across the polymer/polymer interface. When the other interface of the liquid film is adsorbed to an attractive substrate interface, a velocity gradient occurs in the film and therefore can result in the shear gradient. We proposed that if this shear rate exceeds the natural reptation time, melt fracture of thin film resulted. Screening the substrate interaction by first deposition a very thin layer of immiscible polymer such as poly (vinyl-pyridine) PVP reduced the degree of melt fracture. A DI 3000 Atomic Force Microscopy (AFM) was used to quantify the depth and the dynamics of the melt facture process.
Mid-IR Interband Cascade Lasers
Authors: Yang Rui Q. , Hill Cory J. , Qiu Yueming
Published: April 2011

Efficient mid-IR interband cascade (IC) lasers have been developed based on III-V semiconductor materials with lasing emission covering a wavelength range from 2.7 to 5.6 microns. These IC lasers reuse injected electrons in cascade stages for photon generation with high quantum efficiency to achieve high output powers. Also, IC lasers have a low threshold current density with a very efficient use of applied voltage, resulting in reduced power consumption. Single-mode distributed feedback lasers have been made, and integrated into aircraft and balloon instruments which made atmospheric measurements of CH4 and HCl. In this work, the characteristics of IC lasers and their recent developments are reviewed.
Analysis of the Front Facet Temperature in Laser Diode With Non Absorbing Mirror
Authors: Ochalski Tomasz Jan , Pierscinska Dorota , Malag Andrzej
Published: April 2011

Non-absorbing mirror (NAM) lasers, also known as window lasers, were developed to increase catastrophic optical damage level of the devices. In this work we present the analysis of the laser diodes (LD) front facet temperature distribution. Micro-thermoreflectance technique is used to perform a detailed temperature maps of the operating LDs. Such a technique gives us temperature maps with a high spatial resolution equal 0.6μm. We demonstrate micro thermoreflectance as a perfect tool to determine real temperature distribution of the operating laser diode front facet.
InAs/In0.15Ga0.85As1−xNx quantum dots for 1.5 μm laser applications
Authors: Richter Mirja , Damilano Benjamin , Massies Jean , Duboz Jean-Yves , Wieck Andreas D.
Published: April 2011

Self-assembled InAs quantum dots (QDs) on GaAs substrate encapsulated with In0.15Ga0.85As1−xNx (GINA) have been grown by solid source molecular beam epitaxy, using optimal growth temperatures and rates for each of the two layers. The photoluminescence (PL) intensity increases for annealed samples. The optimum annealing temperature depends on the nitrogen composition. InAs QDs overgrown with larger nitrogen mole fraction GINA show a smaller PL peak blue-shift after annealing. Intense and narrow PL emission in the 1.5 µm range is achieved for samples annealed at optimal temperature.
Electron emission mechanism of diamond characterized by combined XPS/UPS/FES
Authors: Yamaguchi Hisato , Yamada Takatoshi , Pate Bradford B. , Kudo Masato , Takakuwa Yuji , Okano Ken
Published: April 2011

Diamond has various advantages as an electron emitter in addition to the low-threshold voltage, negative electron affinity (NEA), high thermal conductivity, high mechanical hardness, and high chemical stability. The difficulty in clarification of electron emission mechanism is preventing diamond from being used in the practical use. It is extremely difficult to identify the surface potential of the emitting diamond from conventional Emission current (I)- Anode voltage (V) characteristics. If one could measure the potential of the emitting surface, the band diagram of emitting diamond can be completed. The combined spectroscopy of XPS/UPS/FES specially built for this study, is one of the most powerful tool, which could identify the potential of the emitting surface. In this study, we have succeeded in observing the origin of field-emitted electrons using our combined XPS/UPS/FES system.
Fabrication of planar SiON optical waveguide and its characterization
Authors: Cho Yu-Jeong , Kim Yeong-Cheol
Published: April 2011

Silicon oxynitride (SiON) was deposited as a core layer on a silica (SiO2) under-cladding layer by using plasma enhanced chemical vapor deposition (PECVD). The refractive index of the SiON core layer was varied between 1.45 and 1.78 by changing the gas flow ratio of SiH4, N2O and NH3. Etching experiments were performed using a dry etching equipment to fabricate the SiON core. An optical planar waveguide with a core and under-cladding thicknesses of 6 μm and 8 μm, respectively, and a refractive index contrast (Δn) of 7 % has been fabricated.
Photoluminescence investigations on a native donor in ZnO
Authors: Meyer Bruno K. , Lautenschläger S. , Graubner S. , Neumann C. , Sann J.
Published: April 2011

The shallow donor impurities in ZnO with binding energies between 46 and 56 meV have been studied in great detail in the recent years. They give rise to neutral donor bound exciton recombinations with the A- and B-valence bands, show rotator states and two-electron-satellite transitions. These properties allowed to establish the excited state splittings of the donors as well as confirming Hayne's rule in ZnO. So far they all seem to be of extrinsic origin, hydrogen, aluminum, gallium and indium in order of increasing binding energy. For many years it was common sense that intrinsic defects would dominate the n-type conductivity of ZnO. Interstitial zinc as well as oxygen vacancies should be double donors, and in order to contribute to the n-type-conduction they should have shallow levels, and low formation energies to be abundant. In PL-measurements at T∼100 K on various ZnO samples, single crystals as well as thin films, a luminescence around 3.31 eV was detected. Due to its line shape and temperature behaviour it is identified as bound-to-free recombination. If we assume that the 3.31 eV band with its level at EC ∼ 130 meV is the ++/+ level of the zinc interstitial we calculate for the binding energy of the +/0 level ∼ 130 meV, i.e. around 33 meV. Undoped Zn-rich epitaxial films grown by CVD show a dominant I3 recombination at 3.367 eV which according to Haynes rule is consistent with a shallow donor level at 33 meV. Moreover, they have free n-type carrier densities of 2×1019 cm−3 and as revealed by SIMS the common donor impurities (Al, Ga, In) cannot account for the high carrier densities.
Laser Assisted Molecular Beam Deposition of High Mobility Zinc Oxide
Authors: Li Meiya , Chokshi Nehal , DeLeon Robert L. , Tompa Gary , Anderson Wayne
Published: April 2011

We have utilized laser assisted molecular beam deposition (LAMBD) to deposite zinc oxide (ZnO) at room temperature. The production of ZnO films via LAMBD utilizes an excimer laser induced ablation plume of zinc in combination with pulsed oxygen gases to create a molecular beam of ZnO clusters. The deposited films have been characterized structually and electrically in an as deposited state as well as after post deposition excimer laser annealing. The films undergo a clear structural change from a nanoparticle like film to either a microgranular film or a smooth continuous film depending upon the laser annealing power.

Al ohmic contacts were made to both as-deposited and laser-annealed ZnO thin films. Thickness and refractive index, chemical composition, and surface morphology of the thin films were analyzed by using ellipsometry, electron spectroscopy for chemical analysic (ESCA), and feild emission scanning electron microscopy (FESEM), respectively. Dark current-voltage (I-V), DC photo I-V, Hall-effect, and photoluminescence measurements were employed for testing the device performance.

In some cases, the ZnO thin film was changed from n-type into p-type after laser annealing, and photoconductive behavior was clearly seen on the laser-annealed samples, with values of 2.8 × 10−4 Ω−1. Also, both samples with and without laser annealing show near-band emission at ∼3.3 eV. Furthermore, the full width at half maximum (FWHW) values of the laser-annealed LAMB ZnO film was reduced from 20.02 nm or 21.68 nm to 18.2 nm when compared with that of non-laser annealed samples. This indicates that the laser annealing provided an improved stochiometric quality and crystallinity to the ZnO thin films.
Czochralski Crystal Growth of Zinc Oxide-Tellurium Oxide System
Authors: Nawash Jalal M. , Lynn Kelvin G.
Published: April 2011

Czochralski technique was employed in an attempt to grow a single crystal of the system ZnO-TeO2. A good quality grown crystal is expected to be transparent with a very light yellow color. The crystals exhibit a high resistivity of the order of 1013 Ω-cm. Different mole percentages have been tested for growth. Several attempts were performed to pull a single crystal. It was found that the best mole percentage of ZnO-TeO2 is 35.5:64.5. The pulled material grows uniformly, such that necking and conning are noticed. The pulled material contained multiple single crystals which were isolated and studied. Each one of them was transparent. Some properties will be presented. The pulled material of the 40:60 ZnO-TeO2 mole percentage returned four phases. Two of these phases are very uncommon.
Manipulation of ZnO Nanowire by Low-Temperature Solution Approach
Authors: Lin Chia-Hsin , Cheng Syh-Yuh , Lin Ren-Jay , Wang Yi-Hui
Published: April 2011

A catalyst free, structure-induced heterogeneous nucleation and direct growth of ZnO nanowires on organic and inorganic substrates was prepared by low-temperature solution approach process. The experimental results showed that ZnO nanowires could be directly synthesized upon the concave of substrate without any pre-seeding. In this work ZnO nanowires were grown on both polystyrene bead layer and physical-grinded wafer substrate. ZnO nanowires with a broad aspect ratio of 10−2 ∼ 102 was controlled mainly by adjusting of reactant concentration and pH state of solution. A needle-like ZnO nanotip were also prepared by a two-step limited growth condition as a result that tip diameter is several nanometers only, which may be highly in favor of the field emission. Structure-induced heterogeneous nucleation and growth facilitates the fabrication of ZnO nanowires as the potential photoeletronic units in field-emission displays.
Sn-based Group-IV Semiconductors on Si: New Infrared Materials and New Templates for Mismatched Epitaxy
Authors: Tolle John , Roucka Radek , D'Costa Vijay , Menendez Jose , Chizmeshya Andrew , Kouvetakis John
Published: April 2011

We report growth and properties of GeSn and SiGeSn alloys on Si (100). These materials are prepared using a novel CVD approach based on reactions of Si-Ge hydrides and SnD4. High quality GeSn films with Sn contents up to 20%, and strain free microstructures have been obtained. The lattice mismatch between the films and Si is relieved by Lomer edge dislocations located at the interface. This material is of interest due to the predicted cross-over to a direct gap semiconductor for moderate Sn concentrations. We find that the direct band gap, and, consequently, the main absorption edge, shifts monotonically to lower energies as the Sn concentration is increased. The compositional dependence of the direct band gap shows a strong bowing, such that the direct band gap is reduced to 0.4 eV (from 0.8 eV for pure Ge) for a concentration of 14% Sn. The ternary SiGeSn alloy has been grown for the first time on GeSn buffer layers. This material opens up entirely new opportunities for strain and band gap engineering using group-IV materials via decoupling of strain and composition. Our SiGeSn layers have lattice constants above and below that of pure Ge, and depending on the thickness and composition of the underlying buffer layer they can be grown relaxed, with compressive, or with tensile strain. In addition to acting as a buffer layer for the growth of SiGeSn, we have found that GeSn can act as a template for the subsequent growth of a variety of materials, including III-V semiconductors.
Deep Ultraviolet Light Emitting Diodes with Emission below 300 nm
Authors: Khan M. Asif
Published: April 2011

In this paper we will describe the problems in growth and fabrication of deep UV LED devices and the approaches that we have used to grow AlGaN-based multiple quantum well deep UV LED structures and to overcome issues of doping efficiency, cracking, and slow growth rates both for the n- and the p-type layers of the device structures. Several innovations in structure growth, device structure design and fabrication and packaging have led to the fabrication of devices with emission from 250-300 nm and cw-milliwatt powers at pump currents of only 20 mA (Vf ≤ 6 V). Record wall plug efficiencies above 1.5 % are now achievable for devices with emission at 280 nm. Thermal management and a proper device design are not only key factors in achieving these record performance numbers but are also crucial to device reliability. We will also discuss some of our initial research to clarify the factors influencing the lifetime of the deep UV LEDs. In addition to our own work, we will review the results from the excellent research carried out at several other laboratories worldwide.
Effects of the high-refractive index SiNx passivation on AlGaN/GaN HFETs with a very low gate-leakage current
Authors: Kambayashi Hiroshi , Wada Takahiro , Ikeda Nariaki , Yoshida Seikoh
Published: April 2011

We have reported on the stress effects of the SiNx passivation film on AlGaN/GaN heterojunction field effect transistors (HFETs). The AlGaN/AlN/GaN heterostructure was grown using a metalorganic chemical vapor deposition (MOCVD). We first investigated the dependences between SiNx films and the refractive index of these. As a result, the stress type of the SiNx films was all Tensile. Moreover, the refractive index of SiNx increased, the stress was decreasing and almost invariable over refractive index 2.1. Furthermore, we fabricated some kinds of AlGaN/GaN HFETs and estimated the gate-leakage current and the current collapse of HFET. As a result, we confirmed the relationship between the stress of passivation film and gate-leakage current, and that the low stress SiNx film with a high-refractive index can suppress both a gate-leakage current and a current collapse.
Properties of InN grown by High-Pressure CVD
Authors: Alevli Mustafa , Durkaya Goksel , Woods Vincent , Habeck Ute , Kang Hun , Senawiratne Jayantha , Strassburg Martin , Ferguson Ian T. , Hoffmann Axel , Dietz Nikolaus
Published: April 2011

Group III-nitride compound semiconductors (e.g. AlN-GaN-InN) have generated considerable interest for use in advanced optoelectronic device structures. The fabrication of multi-tandem solar cells, high-speed optoelectronics and solid state lasers operating at higher energy wavelengths will be made possible using (Ga1-y-xAlyInx)N heterostructures due to their robustness against radiation and the wide spectral application range. To date, the growth of indium rich (In1-xGax)N films and heterostructures remains a challenge, primarily due to the large thermal decomposition pressures in indium rich group III-nitride alloys at the optimum growth temperatures. In order to control the partial pressures during the growth process of InN and related alloys, a unique high-pressure chemical vapor deposition (HPCVD) system with integrated real-time optical monitoring capabilities has been developed. We report initial results on InN layers grown at temperatures as high as ∼850°C with reactor pressures around 15 bar. Such process conditions are a major step towards the fabrication of indium rich group III-nitride heterostructures that are embedded in wide band gap group III-nitrides. Real-time optical characterization techniques are applied in order to study the gas phase kinetics and surface chemistry processes during the growth process.

For an ammonia to TMI precursor flow ratio below 500, multiple phases with sharp XRD features are observed. Structural analysis perform by Raman scattering techniques indicates that the E2 high mode improves as NH3:TMI ratio is decreased to below 500. Optical characterization of these InN layers indicates that the absorption edge shifts from down from 1.85 eV to 0.7 eV. This shift seems to be caused by a series of localized absorption centers that appear as the indium to nitrogen stoichiometry varies. This contribution will correlate the process parameters to results obtained by XRD, Raman spectroscopy and optical spectroscopy, in order to assess the InN film properties.
Growth and characterization of semipolar InGaN/GaN multiple quantum wells and light-emitting diodes on (10 11) GaN templates
Authors: Chakraborty Arpan
Published: April 2011

InGaN/GaN MQW samples were grown by metal organic chemical vapor deposition (MOCVD) on (10-1-1) oriented GaN templates. Effects of growth temperature and reactor pressure on the photoluminescence (PL) properties were investigated. The emission intensity improved significantly when the QWs were grown at 100 Torr, compared to higher pressure growths. The effect of well-width on the luminescence properties was investigated and an optimum well width of 40 Å was determined. Excitation dependent PL measurements revealed no shift in the PL emission wavelength suggesting the absence of electric field in the quantum wells. Furthermore, LEDs fabricated on (1011) GaN templates, emitting at 439 nm, showed no shift in the EL emission wavelength with the increase in drive current, reconfirming the absence of polarization.
Origins of Parasitic Emissions from 353 nm AlGaN-based UV LEDs over SiC Substrates
Authors: Park Ji-Soo , Fothergill Daryl W , Wellenius Patrick , Bishop Seann M. , Muth John F. , Davis Robert F.
Published: April 2011

The effects of p-GaN capping layers and p-type carrier-blocking layers on the occurrence of parasitic emissions from 353 nm AlGaN-based LEDs have been investigated. LEDs without a p-type Al0.25Ga0.75N carrier-blocking layer showed a shoulder peak at ∼370 nm due to electron overflow into the p-Al0.10Ga0.90N cladding layer and subsequent electron-hole recombination in the acceptor levels. Broad emission between 380 and 450 nm from LEDs having a p-GaN capping layer was caused by 420 nm luminescence from the p-GaN capping layer, which was optically pumped by 353 nm UV emission from the quantum wells. Broad, defect-related luminescence at ∼520 nm was emitted from the AlGaN layers within the quantum wells.
Comparison of the Electroluminescence of Blue and Deep-UV Light-Emitting Diodes at Elevated Temperatures
Authors: Cao Xian-An , Stecher T. , LeBoeuf S.
Published: April 2011

The performance of InGaN and AlGaN-based blue (465nm) and deep ultraviolet (UV) (280 nm) light-emitting diodes (LEDs) at elevated temperatures (25-175 °C) were investigated. As a result of uniform high-Al content AlGaN alloys yielded by migration-enhanced metalorganic chemical vapor deposition, the deep-UV LED showed dominant band-edge emission, much smaller alloy broadening and weaker localization effects as compared to the InGaN LED. Strong carrier localization was retained in the blue LED up to 175 °C, leading to temperature-independent emission intensity at low-energy tails. The UV LED, however, showed a much more rapid decrease in light output with increasing temperature. The characteristic temperature was 37 K, compared to 270 K for the blue LED. These findings implicate the lack of localization effects in AlGaN alloys as one of the causal factors in the poor thermal performance of the deep UV LED and suggest that increasing carrier confining potentials will provide a critical means to improve its thermal stability.
Epitaxal growth of InGaN quantum dots grown by MOVPE: Effect of capping process on the structural and optical properties
Authors: Yamaguchi Tomohiro , Sebald Kathrin , Gutowski Juergen , Figge Stephan , Hommel Detlef
Published: April 2011

The surface morphology of thin InxGa1-xN layers in uncapped structures and the overgrowth using GaN or InxGa1-xN with a lower In content than the InxGa1-xN layer as the protection layer of the InxGa1-xN layer were investigated in addition to their dependence on growth temperature during the growth by metalorganic vapor phase epitaxy (MOVPE). It was necessary to decrease the growth temperature of the InxGa1-xN layer in order to realize homogeneous dots. In order to obtain a high luminescence efficiency, on the other hand, it was essential to increase the growth temperature of the protection layer. Sharp lines related to the localized electronic states, which are attributed to InGaN quantum dots, were observed by micro-photoluminescence (μ-PL) for the samples with the InxGa1-xN layers protected by the InxGa1-xN layers grown at 700∼740°C.
Raman scattering of self-assembled gallium nitride nanorods synthesized by plasma-assisted molecular beam epitaxy
Authors: Wang D. , Tin C.-C. , Williams J. R. , Park M. , Park Y. S. , Park C. M. , Kang T. W. , Yang W.-C.
Published: April 2011

Raman scattering experiment was carried out to study material and electronic properties of the aligned GaN nanorods grown by plasma-assisted molecular beam epitaxy. Due to the small size of the nanorods and the relatively high power density of the laser illumination used in this investigation, local heating of the sample cannot be neglected. Special care has been taken to identify the frequency shift in the optical phonon peaks induced by local heating prior to any spectral analysis. The residual stress in the GaN nanorods was estimated by analyzing the frequency shift of its E2 Raman mode. It was found that the frequency of E2 mode is shifted by only 0.1 cm-1 comparing with the stress-free frequency, indicating a negligible residual stress in nanorods. Owing to aligned geometry of the nanorods and the back-scattering scattering geometry used in the experiment, A1(LO) Raman mode was collected. The free carrier concentration as well as electron mobility of the GaN nanorods were obtained by the line shape analysis of the coupled A1(LO) phonon-plasmon mode. The electron concentration and mobility of electron obtained from line shape analysis are 3.3×1017 cm-3 and 140 cm2/Vs, respectively. We presented a general method of better determining the electronic parameters of the GaN nanostructures via Raman scattering.
Visible Light-Emitting Diodes Grown by Plasma Assisted Molecular Beam Epitaxy on Hydride Vapor-Phase Epitaxy GaN Templates and the Development of Dichromatic (Phosphorless) White LEDs
Authors: Cabalu Jasper S. , Williams Adrian , Chen Tai-Chou P. , France Ryan , Moustakas Theodore D.
Published: April 2011

Much of the work on III-Nitride-based LEDs that has been published and applied commercially has been done using metal-organic chemical vapor deposition (MOCVD) as a method of film growth. We report on the growth and fabrication of visible light emitting diodes, by combining hydride vapor-phase epitaxy (HVPE) and rf plasma-assisted MBE (PAMBE) methods. Thick (∼7 μm to 10 μm) HVPE n+-GaN smooth and textured templates, were used as substrates for the growth of LED structures by rf-PAMBE. The active regions of the LED structures, which consist of InGaN/GaN MQWs, were grown using the pulsed nitrogen plasma technique leading to abrupt well and barrier interfaces as confirmed by x-ray diffraction (XRD) measurements. Using this method, we obtained InGaN/GaN MQWs whose room temperature photoluminescence (PL) spectra have a full width at half maximum (FWHM) of 12 nm (105 meV). Visible LEDs on smooth GaN templates emitting in the blue to green were produced with EL spectrum FWHM as narrow as 27 nm. On the other hand, white LEDs without the use of phosphor have been produced utilizing textured MQWs as the active region, a phenomenon we initially attribute to different incorporation of In on the different QW-planes. The growth and fabrication of these devices was preceded by detailed growth and doping studies of the various layers of the LED structure. These include detailed nucleation studies on (0001) sapphire substrates as well as identification of kinetic factors which lead to good crystalline-quality InGaN alloys and InGaN/GaN MQWs.
Normally Off-Mode AlGaN/GaN HFET with p-Type Gate Contact
Authors: Tsuyukuchi Norio , Nagamatsu Kentaro , Hirose Yoshikazu , Iwaya Motoaki , Kamiyama Satoshi , Amano Hiroshi , Akasaki Isamu
Published: April 2011

A normally off-mode AlGaN/GaN heterostructure field effect transistor (HFET) using a p-type GaN gate was fabricated and their static properties were compared with those of HFET having a Schottky gate. HFET having a p-GaN gate contact shows a very low leakage current density of 18.2 μA/mm at VGS and VDS of 0 V and 20 V, respectively.
Flip Chip Mounting for Improved Thermal Management of AlGaN/GaN HFETs
Authors: Ji Hangfeng , Sarua Andrei , Kuball Martin , Das Jo , Ruythooren Wouter , Germain Marianne , Borghs Gustaaf
Published: April 2011

We report on a temperature study of flip-chip mounted multi-finger AlGaN/GaN heterostructure field effect transistors (HFETs) using micro-Raman spectroscopy and infrared (IR) thermography. Flip-chip mounting can be used to improve thermal device management, in particular, for devices grown on low thermal conductivity substrates such as sapphire. In this study, we compare two flip-chip mounted HFETs of different flip-chip contact bump layout designs and a non flip-chip mounted HFET. Both temperature measurements and 3D temperature simulations are presented. The results show that minimizing the distance between the bumps and the active area of the HFET is essential for obtaining a low device operating temperature.
Perturbation of charges in AlGaN/GaN heterostructures studied by nanoscale capacitance-voltage technique
Authors: Koley Goutam , Lakshmanan Lakshminarayanan
Published: April 2011

Perturbation of charges at the surface and interface of AlGaN/GaN heterostructures has been studied by quantitative nanoscale capacitance-voltage (C-V) measurements. The nanoscale C-V curves were found to have different slopes in the forward and reverse directions. These measurements indicate a change in confinement of the two-dimensional electron gas (2DEG) at the AlGaN/GaN interface depending on the direction of the dc voltage sweep during C-V measurements, which can be explained by surface state charging and discharging during the bias sweep. Under UV illumination, the density of the 2DEG increased significantly as inferred from the increase in threshold voltage of the nanoscale C-V scans, and no change in 2DEG confinement, depending on the direction of the bias sweep, was observed.
Weak Antilocalization in Polarization-Doped AlGaN/GaN Heterostructures
Authors: Thillosen Nicolas Henri , Schäpers Thomas , Kaluza Nicoleta , Hardtdegen Hilde , Guzenko Vitaliy
Published: April 2011

The properties of AlGaN/GaN heterostructures have been a subject of great activity because of their application in high frequency, high power, and high temperature devices. Magnetotransport measurements give the possibility to study the properties of a two-dimensional electron gas. Indeed, Shubnikov-de Haas oscillations of a two-dimensional electron gas can be observed at high magnetic fields. Moreover, magnetoresistance measurements close to zero magnetic field give the possibility to investigate the weak localization and weak antilocalization arising from AlGaN/GaN heterostructures. The latter is related to the spin-orbit interaction on the spin of the carriers present in these heterostructures and is a key-feature of the spin-transistor proposed by Datta and Das [1]. As a matter of fact, the spin orientation between the electrodes of this novel device should be manipulated by the controllable strength of the spin-orbit interaction in the two-dimensional electron gas. In order to obtain information on spin-orbit effects in AlGaN/GaN heterostructures we therefore analyzed the weak antilocalization observed in the magnetoresistance.

Polarization-doped AlGaN/GaN heterostructures were grown by metalorganic vapor phase epitaxy (MOVPE) on the (0001) surface of sapphire substrates. First a 3 mm-thick GaN buffer layer was grown, followed by a Al0.3Ga0.7N layer with a thickness of 20 nm. Magnetotransport measurements were performed over a magnetic field range from –50 mT to +50 mT at various temperatures between 0.1 and 18 K. The Hall-bars were prepared by optical lithography and Ar+-ion-beam-etching technique. The metals used for the ohmic contacts were Ti/Al/Ni/Au. The mobility and carrier concentration in the single occupied subband were obtained from the Shubnikov-de Haas oscillations with the respective values of 9100 cm2/Vs and 6.2×1012 cm-2.

In our case, the Shubnikov-de Haas oscillations of the two-dimensional electron gas reveal the occupation of a single subband in the nearly triangular quantum well. At low magnetic field, weak localization as well as weak antilocalization were observed, showing that strong spin-orbit interaction is present in our structures. A previous report [2] explained the weak-antilocalization as being related to the intersubband scattering due to the occupation of a second subband in a modulation-doped quantum well. We show that weak antilocalization is also present in a polarization-doped quantum well with a single subband being occupied. In this perspective, temperature-dependent weak antilocalization measurements will be presented and analyzed using adequate theoretical models. Finally, the relevant scattering times like elastic scattering time, dephasing time as well as spin-orbit scattering time have been extracted. [1] S. Datta and B. Das, Appl. Phys. Lett. Vol. 56, pp. 665-667, 1990. [2] J. Lu et al., Appl. Phys. Lett., Vol. 85, pp. 3125-3127, 2004
Polarization-Induced 3-Dimensional Electron Slabs in Graded AlGaN Layers
Authors: Simon John , Wang Kejia , Xing Huili , Jena Debdeep , Rajan Siddharth
Published: April 2011

By compositionally grading AlGaN layers over different thicknesses, high mobility electron gases are produced by polarization-induced doping. Temperature dependent Hall and capacitance-voltage measurements were performed on these AlGaN layers, and two degrees of freedom are found for choosing the carrier concentration of these slabs. Carrier mobilities determined from Hall measurements are observed to be much higher than impurity doped structures of similar carrier densities. Alloy and phonon scattering are determined to be the major contributors limiting the mobility of the electron in the graded layers. This form of polarization-induced doping offers an attractive alternative to the traditional doping techniques, and may be used for highly conductive AlGaN layers with high Al composition, both for lateral and vertical transport.
ZnCdO/ZnMgO and ZnO/AlGaN Heterostructures for UV and Visible Light Emitters
Authors: Osinsky Andrei , Dong Jianwei , Xie J. Q. , Hertog B. , Dabiran A. M. , Chow P. P. , Pearton S. J. , Norton D. P. , Look D. C. , Schoenfeld W. , Lopatiuk O. , Chernyak L. , Cheung M. , Cartwright A.N. , Gerhold M.
Published: April 2011

This paper reviews of some of the progress made in the development of ZnO-based light emitting diodes (LEDs). n-ZnO/p-AlGaN-based heterostructures have been successfully for the fabrication of UV emitting LEDs that have operated at temperatures up to 650K, suggesting an excitonic origin for the optical transitions. RF-plasma-assisted molecular beam epitaxy has been used to grow epitaxial CdxZn1-xO films on GaN/sapphire structure. These films have a single-crystal wurtzite structure as demonstrated by structural and compositional analysis. High quality CdxZn1-xO films were grown with up to x=0.78 mole fraction as determined by RBS and SIMS techniques. Optical emission ranging from purple (Cd0.05Zn0.95O) to yellow (Cd0.29Zn0.71O) was observed. Compositional fluctuations in a Cd0.16Zn0.84O films were not detected by spatially resolved CL measurements, although intensity fluctuation with features of ∼0.5 μm diameter were seen on the intensity maps. Time resolved photoluminescence shows multi-exponential decay with 21 psec. and 49±3 psec. lifetimes, suggesting that composition micro-fluctuations may be present in Cd0.16Zn0.84O film.
Time-resolved Spectroscopy of Excitons Bound at Shallow Neutral Donors in HVPE GaN
Authors: Monemar Bo , Paskov P. P. , Bergman J. P. , Malinauskas T. , Jarasiunas K. , Toropov A. A. , Shubina T. V. , Usui A.
Published: April 2011

Time-resolved photoluminescence (TRPL) spectroscopy has been performed in the temperature range 2 K to 300 K on thick (300 μm – 1 mm) nominally undoped bulk HVPE grown GaN layers. The PL spectra are dominated by the free excitons (FEs) and the two neutral donor bound excitons (DBEs) at about 3.4712 eV and 3.4721 eV at 2 K, assigned to the residual O and Si donors, respectively. The zero-phonon decay curves for both FEs and DBEs are nonexponential, with a fast initial decay and a slower tail at longer times. The initial decay time of the FE is about 130 ps at 2 K in such samples, the corresponding initial decay time for the DBEs for O and Si is close to 300 ps. The different lines corresponding to the so called two-electron transitions for the DBE show a different behaviour, with a longer decay time than the corresponding principal DBE states. The LO phonon replicas of the DBEs also have a decay time in excess of 1 ns in lightly doped samples.
Growth of Large AlN Single Crystals Along the [0001] Direction
Authors: Herro Ziad Georges , Zhuang Dejin , Schlesser Raoul , Collazo Ramon , Sitar Zlatko
Published: April 2011

We have demonstrated growth of large AlN single crystals using (0001)-oriented AlN seeds. Boules with a diameter of 15 mm and length up to 12 mm were obtained from 5 mm seeds. Step flow growth was observed on both Al and N-polar surfaces. N-polar face was suitable for growth within a large window of growth parameters while the Al-polar seeds yielded high-quality crystals only at low supersaturation.
Dislocation Reduction and Structural Properties of GaN layers Grown on N+-implanted AlN/Si (111) Substrates
Authors: Jamil Muhammad , Grandusky James R , Jindal Vibhu , Tripathi Neeraj , Sandvik Fatemeh Shahedipour
Published: April 2011

An alternative scheme to the growth of crack free, dislocation reduced III-Nitride layers on Silicon substrate has been previously introduced that relies on formation of an ion implanted defective layer in the substrate with implantation taking place in the presence of AlN buffer layer. Here, the effects of N+ ion implantation of AlN/Si (111) substrate on the structural and optical properties of the overgrown GaN epilayers have been investigated. Temperature dependent photoluminescence has been used to investigate the impact of the implantation conditions (energy and dose) on optical and structural quality of the GaN overgrown layers. A correlation between PL and high resolution x-ray diffraction (XRD) of the overgrown GaN layers show that the lowest FWHM of bandedge, the highest bandedge to deep defect blue luminescence band ratio, and the lowest symmetric rocking curve FWHM are achieved for the optimized implantation conditions. This correlates well with the results of etch pit density measurement showing an order of magnitude reduction in threading dislocation defect density
Structural and optical properties of MOCVD InAlN epilayers
Authors: Hernández S. , Wang K. , Amabile D. , Nogales E. , Pastor D. , Cuscó R. , Artús L. , Martin R.W. , O'Donnell K.P. , Watson I.M. , Network RENiBEl
Published: April 2011

We have studied the structural and optical properties of InAlN alloys with compositions nearly lattice-matched to GaN. Scanning electron microscopy measurements reveals a good overall surface quality, with some defect structures distributed across the surface whose density increases with the InN concentration. On the other hand, Raman scattering experiments show three peaks in the frequency range between 500 and 900 cm-1, which have been assigned to InN-like and AlN-like E2 modes and A1(LO) mode of the InAlN. These results agree with theoretical calculations previously reported where two-mode and one-mode behavior was predicted for the E and A(LO) modes, respectively. Photoluminescence and photoluminescence excitation allowed us to determine the emission and absorption energies of the InAlN epilayers. Both energies display a redshift as the InN fraction increases. We find a roughly linear increase of the Stokes shift with InN fraction, with Stokes shift values of ≈0.5 eV in the composition range close to the lattice-matched condition.
SiH4 exposure of GaN surfaces: A useful tool for highlighting dislocations
Authors: Oliver Rachel , Kappers Menno J. , Sumner Joy , Datta Ranjan , Humphreys Colin J.
Published: April 2011

Fast-turnaround, accurate methods for the assessment of threading dislocation densities in GaN are an essential research tool. Here, we present an in-situ surface treatment for use in MOVPE (metal-organic vapour phase epitaxy) growth, in which GaN is exposed to a SiH4 flux at 860 °C in the presence of NH3. Subsequent characterisation by atomic force microscopy shows that the treatment is effective in increasing edge and mixed/screw dislocation pit sizes on both n- and p-type material, and on partially coalesced GaN layers.
Comparative investigation of quantum-dot-like localization centers in InGaN quantum well and quantum dot structures
Authors: Sebald Kathrin , Lohmeyer Henning , Gutowski Jürgen , Yamaguchi Tomohiro , Hommel Detlef
Published: April 2011

We present comparative micro-photoluminescence measurements on InGaN/GaN quantum well and quantum dot samples. Single sharp emission lines were investigated for both kinds of samples as a function of temperature and excitation density. For the sharp emission lines of the quantum dots and the strong localization centers in the quantum well samples comparable experimental findings were obtained such as the independence of their spectral position of the excitation density and the observation of binding and antibinding multiexcitonic states giving clear evidence for the quantum dot nature of localization centers.