bluuebunny/crossref_metadata_2025 huggingface dataset:
Photolumineseence, Electroluminescence, and Cathodoluminescence of ZnO:Zn Phosphor Films Prepared by MOCVD
Authors: Li Y. , Forsythe E. , Tompa G. S. , Liu J. , Morton D. C.
Published: April 2011

Cathode ray tube (CRT) technology remains the major display component for today's display technologies. The improvements from monochrome displays to today's full color displays have always been accompanied by improvements in the phosphors. The CRT type displays operate at very high voltages (over 10 keV) at relatively low currents, a few microamps/cm2. The phosphors and phosphor coating technology used in CRT based displays have been optimized for operation with these excitation conditions. However, the developments of field emitter displays based on microtip technology, or negative electron affinity technology require phosphors that operate at lower voltages, preferably 10 to 1500 eV at relatively higher current densities. Zn-rich zinc oxide (ZnO:Zn) powders have shown improved low voltage cathodoluminescence (CL) as compared to conventional ZnS based phosphors. ZnO:Zn thin film phosphors for cathodoluminescent displays, compared to conventional powder phosphors, can have the advantages of high electrical and thermal conductivity, high energy saturation limit, and high screen resolution. The photoluminescence (PL), Electroluminescence (EL), and CL from thin ZnO:Zn films were studied. The samples were prepared by Low Pressure Metal Organic Chemical Vapor Deposition (LP MOCVD) and post-annealed at temperatures from 700 °C to 1000 ° C. The PL, EL, and CL spectra have a peak centered at 590 nm that increases with annealing temperatures up to 1000 °C. The CL efficiencies are 0.12 Lm/W at electron voltages and currents as low as 500 V and 64 mA/cm2. The ZnO films have been characterized by X-ray Diffiractometry (XRD), and Sweep Electron Microscope (SEM). These PL, EL, and CL results from ZnO:Zn films show the promise for improved phosphors to meet Field Emitting Device (FED) challenges.
Morphology and Quantitative Nitrogen Impurity Measurements in Homoepitaxial Chemical Vapor Deposited Diamond
Authors: Catledge S. A. , Vohra Y. K. , Yan C. , Tohver H. T.
Published: April 2011

A thick (130 im) homoepitaxial diamond film was grown on a polished, single crystal {100} oriented natural type Ila diamond circular plate using high density microwave plasma chemical vapor deposition (MPCVD). The as-grown film shows facets around its perimeter, approaching the form of a cubo-octahedral crystal. The plate dimensions increased from 1.50 mm diameter and 0.25 mm thickness to 1.64 mm diameter and 0.38 mm thickness, corresponding to a growth rate of nearly 11 μm per hour. The substrate temperature during growth was approximately 900 C. The film was characterized by optical microscopy, Raman spectroscopy, photoluminescence spectroscopy (PL), and electron paramagnetic resonance (EPR) before and after deposition. The surface morphology of the as-grown film shows large growth steps and a few scattered penetration twins. The diamond film is transparent except for localized regions containing the twin defects. Raman spectroscopy shows a strong diamond peak at 1332 cm−1 superimposed on a background fluorescence. A broad Raman band at 1550 cm−1 is present on the surface defects and is indicative of an amorphous network of sp2-bonded carbon. The luminescence of the film reveals the presence of a nitrogen-vacancy pair. The homoepitaxially-grown diamond layer also exhibits the P1 substitutional nitrogen impurity with a concentration of 8 parts per million (ppm) as determined by EPR. No nitrogen impurities were detected in the original substrate before deposition. EPR is also used to confirm the single crystal nature of the grown diamond.
Growth of Cubic SiC Thin Films on Silicon from Single Source Precursors by Supersonic Jet Epitaxy
Authors: Boo Jin-Hyo , Ustin Scott A. , Ho Wilson , Maruska H. Paul , Norris Peter E. , Kim Ig-Hyeon , Sung Changmo
Published: April 2011

Cubic SiC thin films have been grown by supersonic jet epitaxy of single molecular precursors on Si(100), Si(111) and Separation by IMplanted OXygen (SIMOX) silicon on insulator (SOI) substrates at temperatures in the range 780 - 1000 °C. Real-time, in situ optical reflectivity was used to monitor the film growth. Films were characterized by ellipsometry, x-ray diffraction (XRD), and transmission electron microscopy (TEM). Monocrystalline, crack-free epitaxial cubic SiC thin films were successfully grown at 830 °C on carbonized Si(111) substrates using supersonic molecular jets of dimethylisopropylsilane, (CH3)2CHSiH(CH3)2, and diethylmethylsilane, (CH3CH2)2SiHCH3. Highly oriented cubic SiC thin films in the [100] direction were obtained on SIMOX(100) at 900 °C with dimethylisopropylsilane and on Si(100) at 1000 °C with diethylmethylsilane. A carbonized Si(100) surface was found to enhance SiC deposition from diethylmethylsilane at a growth temperature of 950 °C.
Measurement of Quasi-Fermi Levels in Quantum Well Lasers
Authors: Therrien J. , Mil'shtein S. , Chin A.
Published: April 2011

Heterojunction Quantum Well Lasers were tested under forward and reverse bias, by Scanning Electron Microscopy (SEM) working in Differential Voltage Contrast(DVC) mode. DVC utilizes the impact of electrical potential across a device on the emission of secondary electrons. DVC consists of storing and subtracting two digitized images of the tested device under zero bias and in an operational regime. Calibration of the resultant image provides for quantitative measurements of the potential across the device. The question of whether the DVC technique measures the electron affinity of a material or its work function or the thermodynamic potential has been addressed in recent papers, however questions remained regarding discrepancies between the expected Quasi Fermi Energy (QFE) and the measured values. The experimental part of our work concentrated on taking (QFE) profiles by DVC across a single quantum well laser operating in inverse population, threshold, and high power emission modes of photoemission. The intensity was measured by a calibrated photodetector aligned with the laser in the SEM chamber. The cleaved and yet operational lasers have as a central part a layer of In0.2Ga0.8As 600nm thick between two undoped layers of Al0.3Ga0.7As 0.1 μm thick each with cladding layer of n-type Al0.6Ga0.4As on one side and a similar p-type layer on the other. The shape of QFE indicated that eletrons and photons contribute to total energy over the intrinsic area. This QFE profiling across the laser reveals areas of electron and photon confinement.
Rapid Cure Of Liquid Encapsulants And Structural Adhesives For Electronics Packaging Using Variable Frequency Microwave (Vfm) Energy
Authors: Fathi Zak , Tucker Denise A. , Wei Billy J. , Garard Richard S. , Mead Patricia F. , Balaji P. , Hutchins Charles L.
Published: April 2011

This paper reports on the use of an emerging process technique for curing of polymer encapsulants as used in the electronic packaging industry. Previous work performed in the area of materials processing has demonstrated the usefulness of sweeping operating frequencies in order to achieve high levels of electric field uniformity and process control. The use of controlled variable frequency microwave energy has been evaluated as a process technique compatible with electronic packaging requirements. The heating of a series of integrated circuits (ICs) and their subsequent characterization was performed. IC integrity was investigated using X‐Ray, Acoustic Microscopy, Decapsulation and Bond Pull. Processing of liquid encapsulants, underfills and glob‐tops, used in Flip Chip and Chip On Board (COB) applications, was performed. Differential Scanning Calorimetry was used to study cure extent. Further studies show that variable frequency microwave processing leads to fast curing of encapsulants. A reduction in cycle times from 15 to 20 times over conventional curing has been observed. Also, results have showed a reduction in the stresses induced by mismatches in coefficient of thermal expansion.
Viscoelastic Behavior of Polymer Thin‐Films Under Thermal Stresses
Authors: Bluestein Stephen D. , Bramono Dewi P. Y. , Miaoulis Ioannis N. , Wong Peter Y.
Published: April 2011

Stresses and deformation in microelectronic packaging are affected by the viscoelastic behavior of polymer materials during manufacture or operation. Predicting and measuring these thermo‐mechanical effects is important for new devices, components, and materials. The viscoelastic response of Nycoa 851 polyimide thin‐films during thermal loading is investigated. The time‐dependent relaxation of polyimide films was measured in‐situ, focusing on the change in thermo‐mechanical properties based on the thickness of the polyimide layer. The curvature change of the multilayer structure (silver‐polyimide‐quartz heterostructure) was obtained for different temperatures and polymer film thicknesses. The polyimide relaxation time constant and activation energy were determined. Results indicate that the thermo‐mechanical properties of polyimide thin films are dependent on the thickness of the polymer layer.
Novel Technique to Improve Adhesion Between Metal‐Polymer Interfaces
Authors: Menezes Marlon , Robertson I.M. , Birnbaum H.K.
Published: April 2011

Partially embedded copper clusters within a polyimide substrate have been utilized to act as “nano‐nails” to anchor the bulk metal overlayer to the underlying polyimide substrate resulting in increased adhesion between the metal and substrate. Isolated nano‐sized copper clusters can be formed on a polyimide surface under UHV conditions using very slow deposition rates and temperatures that are high enough to induce mobility of copper atoms on the polyimide surface. Because copper has a high surface energy with respect to the copper/polyimide interface and the polymer surface, the copper clusters exert an embedding force on the polyimide substrate. These clusters can be made to embed in the polymer by annealing in vacuum at temperatures close to the glass transition temperature of the polymer. By regulating the time and temperature of post‐deposition anneals, the degree of embedding of the copper clusters can be controlled. Cross‐sectional TEM results will illustrate the various degrees of cluster embedding, while adhesion testing results will be presented to show the improved adhesion obtained from these interfaces.
Creep Study for Fatigue Life Assessment of Two Lead-Free High Temperature Solder Alloys
Authors: Liang J. , Lee P. S. , Gollhardt N. , Schroeder S. , Morris W.
Published: April 2011

Creep of two lead-free high temperature solder alloys, 95Sn-5Ag and 99Sn-1.0Cu was studied in this investigation. Room and high temperature creep tests were performed to examine deformation mechanisms and to establish mathematical models of creep deformation for the alloys. A state variable creep model was introduced to model both primary and secondary creep deformation of these two alloys which show a very significant primary creep. Fatigue life models of the alloys were established based on an energy-based failure criteria, which was deduced from variable strain amplitude tests at a constant strain rate of 0.003/sec, and from variable strain rates tests with two constant strain amplitudes of 0.005 and 0.01. Applications of the creep model includes determination of peak loads and hysteresis strain energy density in strain-controlled fatigue tests. It is demonstrated that the creep model can be very helpful to establish fatigue failure criteria and to assess fatigue lives of these two alloys.
Thermodynamic and Kinetic Study of Phase Transformations in Solder/Metal Systems
Authors: Chromik R.R. , Cotts E.J.
Published: April 2011

Diffusion in both the Pd‐Sn and Cu‐Sn systems has been investigated using differential scanning calorimetry. Averaged interdiffusion coefficients for the PdSn4, PdSn2 and Cu3Sn intermetallics have been calculated, where equilibrium concentrations in the diffusion couples are assumed. There is an obvious hierarchy within the Pd‐Sn system where diffusion is fastest in the most Sn‐rich intermetallic. Comparisons within each system, including consideration of the solute diffusion coefficients in pure Sn, provide evidence that in the most Sn rich phase (e.g. PdSn4) the interstitial diffusion of metal atoms is the dominant reaction mechanism. In contrast, the diffusion coefficient for the Cu‐rich phase Cu3Sn was found to be five orders of magnitude smaller than the solute diffusion coefficient for Cu in pure Sn.
Electron Emission from Cold Cathodes
Authors: Pryor R. W. , Li Lihua , Busta H. H.
Published: April 2011

New observations are presented on the emission of electrons from n-type polycrystalline diamond and n-type boron nitride (BN) cold cathode films, both as synthesized and after post-synthesis annealing. The films have been observed to show an increase in electron emission after annealing by one to several orders of magnitude, depending upon the type of emitter and the specific surface treatment. Observations from both plasma and laser annealing treatments will be presented.

The annealed BN cold cathodes have been observed to yield stable emission currents as high as 2 A cm−2 (∼4 mA total current) at fields as low as ∼30 V μm−1. This is believed to be the highest reported current density yet observed from a planar film cold cathode emitter.
Design Consideration of GaN-Based Surface Emitting Lasers
Authors: Honda T. , Koyama F. , Iga K.
Published: April 2011

The threshold current density of GaN-based vertical cavity surface emitting lasers (VCSELs) has been estimated. It is clarified that the introduction of a quantum well structure as an active layer is very effective for a low threshold operation and that high reflective mirrors are required for low threshold GaN-based VCSELs. Also, attempts on micro-fabrication process of GaN is presented.
Realization and Characterization of Optically-Pumped GaInN-DFB Lasers
Authors: Hofmann R. , Wagner V. , Gauggel H-P. , Adler F. , Ernst P. , Sohmer A. , Bolay H. , Scholz F. , Schweizer H.
Published: April 2011

We demonstrate room temperature laser activity of optically pumped GaN/GalnN-DFB-lasers. The ridge-waveguide DFB lasers were realized on GaN/GalnN double heterostructures grown by low-pressure metal-organic vapor phase epitaxy. The best laser threshold we achieved is 1.9 MW/cm2.

By varying the grating period, the laser emission wavelength could be tuned from 399 to 415 nm. This allows to determine the dispersion relation of the effective refractive index neff(λ) and the spectral dependence of the pump power density at the laser threshold Pth(λ) over the whole emission range. Furthermore, the shift of the emission wavelength with temperature of the DFB-lasers is investigated and is found to be small compared to the emission wavelength shift of the gain maximum.
Study of IBAD Deposited AlN Films for Vacuum Diode Electron Emission
Authors: Forsythe E.W. , Sprague J.A. , Khan B.A. , Metha S. , Smith D.A. , Murzin I.H. , Ahern B. , Weyburne D. W. , Tompa G. S.
Published: April 2011

We have demonstrated the basic operation of a vacuum diode based on the negative electron affinity polycrystalline AlN thin film emitters. The AlN films, both undoped and Ge doped, were deposited by ion beam assisted deposition (IBAD). The IBAD process utilizes thermal evaporation from either electron-beam or resistance heated sources with ion bombardment from Kaufman-type ion sources at energies from 50 to 1500 eV. Films were post-annealed by rapid thermal annealing and long-time tube furnace annealing in a N2 atmosphere to test improvements in crystallinity. The electrical and transport properties of the films were tested by DC l-V measurements. The structure of the AlN films was investigated by TEM, SIMS, optical absorption, and RBS as a function of growth parameters and annealing. The field emission was tested for films with different Ge doping concentrations, film thickness, diode voltage, and post annealing conditions. Field emission was observed for the undoped AlN films with a thickness of approximately 10 nm.
Pressure-Controlled GaN MBE Growth Using a Hollow Anode Nitrogen Ion Source
Authors: Leung M. S. H. , Klockenbrink R. , Kisielowski C. , Fujii H. , Krüger J. , Sudhir G. S. , Anders A. , Liliental-Weber Z. , Rubin M. , Weber E. R.
Published: April 2011

GaN films were grown on sapphire substrates at temperatures below 1000 K utilizing a Hollow Anode nitrogen ion source. A Ga flux limited growth rate of ~ 0.5 µm/h is demonstrated. Active utilization of strain and the assistance of a nitrogen partial pressure during buffer layer growth are found to be crucial issues that can improve the film quality. The best films exhibit a full width at half maximum of the x-ray rocking curves of 80 arcsec and 1.85 meV for the excitonic photoluminescence measured at 4 K. A Volmer-Weber three dimensional growth mode and the spontaneous formation of cubic GaN inclusions in the hexagonal matrix are observed in the investigated growth temperature range. It is argued that this growth mode contributes to a limitation of the carrier mobility in these films that did not exceed 120 cm2/Vs though a minimum carrier concentration of ~ 1015 cm−3 was achieved.
MOVPE Growth and Optical Characterization of GaPN Metastable Alloy Semiconductor
Authors: Onabe K.
Published: April 2011

The Gap1–x Nx, alloy semiconductor has been grown with the N concentration as high as 6.3% by inetalorganic vapor phase epitaxy (MOVPE) using 1,1-dimethylhydrazine (DMHy) as the N source. The growth characteristics show the key role of the non-equilibrium circumstances during the growth, where the N desorption from the surface limits the N incorporaton. The band-edge states have been studied using time-resolved PL and PL excitation spectroscopies. The PL takes place via the tail states below the absorption edge. The band edge, which shifts to lower energies with increasing N concentration, originates from the A-line energy (i.e. isolated N states) rather than the indirect-gap energy of GaP in the limit x = 0.
Large Area Supersonic Jet Epitaxy of AlN, GaN, and SiC on Silicon
Authors: Lauhon L.J. , Ustin S. A. , Ho W.
Published: April 2011

AlN, GaN, and SiC thin films were grown on 100 mm diameter Si(111) and Si(100) substrates using Supersonic Jet Epitaxy (SJE). Precursor gases were seeded in lighter mass carrier gases and free jets were formed using novel slit-jet apertures. The jet design, combined with substrate rotation, allowed for a uniform flux distribution over a large area of a 100 mm wafer at growth pressures of 1–20 mTorr. Triethylaluminum, triethylgailium, and ammonia were used for nitride growth, while disilane, acetylene, and methylsilane were used for SiC growth. The films were characterized by in situ optical reflectivity, x-ray diffraction (XRD), atomic force microscopy (AFM), and spectroscopic ellipsometry (SE).
Effect of Growth Parameters and Local Gas-Phase Concentrations on the Uniformity and Material Properties of GaN/Sapphire Grown by Hydride Vapor-Phase Epitaxy
Authors: Safvi S. A. , Perkins N. R. , Horton M. N. , Kuech T. F.
Published: April 2011

The effects of flowrate variation and geometry on the growth rate, growth uniformity and crystal quality were investigated in a horizontal Gallium Nitride vapor phase epitaxy reactor. To better understand the effects of these parameters, numerical model predictions are compared to experimentally observed values. Parasitic gas phase reactions between group III and group V sources and deposition of material on the wall are shown to lead to reduced overall growth rates and may be responsible for inferior crystal quality. A low ammonia concentration is correlated with the deposition of polycrystalline films. A low V/III ratio and an ammonia concentration lead to poor crystalline quality and increased yellow luminescence. An optimum HVPE growth process requires selection of reactor geometry and operating conditions to minimize these parasitic reactions and wall deposition while providing a uniform reactant distribution across the substrate.
New Pathways to Heteroepitaxial GaN by Inorganic CVD Synthesis and Characterization of Related Ga-C-N Novel Systems
Authors: Kouvetakis J. , O’Keeffe M. , Brouseau Louis , McMurran J. , Williams Darrick , Smith D. J.
Published: April 2011

We describe the development of a new deposition method for thin oriented films of GaN on basal plane sapphire using an exclusively inorganic single-source precursor free of carbon and hydrogen, Cl2GaN3. The films have been characterized by Rutherford backscattering spectroscopy (RBS) and cross sectional transmission electron microscopy (TEM) for composition morphology and structure. RBS analysis confirmed stoichiometric GaN and TEM observations of the highly conformal films revealed heteroepitaxial columnar growth of crystalline wurrtzite material on sapphire. Auger and RBS oxygen and carbon resonance profiles indicated that the films were pure and highly homogeneous. We also report the reactions of Cl2GaN3 with organometallic nitriles to yield a crystalline, novel gallium carbon nitride of composition GaC3N3. Quantitative X-ray powder diffraction has been used to refine the cubic structure of this material which consists of Ga atoms octahedrally surrounded by on the average three C and three N atoms. The structurally analogous LiGaC4N4 phase has also been prepared and characterized.
Thin Film Growth of Group III Nitrides by Mass Separated Ion Beam Deposition
Authors: Ronning C. , Dreher E. , Feldermann H. , Sebastian M. , Zweck J. , Fischer R. , HofsÄss H.
Published: April 2011

We have grown gallium nitride (GaN), aluminum nitride (AIN) and boron nitride (BN) thin films by mass separated ion beam deposition. All deposited films were found to be almost stoichiometric. AIN and GaN films are crystalline even after room temperature deposition whereas for the formation of crystalline boron nitride temperatures above 150°C are necessary. The influence on the phase formation and the film structure of ion energy and substrate temperature on the one hand, and bond ionicity on the other hand, was investigated for these three systems.
Nanopipes and Inversion Domains in High-Quality GaN Epitaxial Layers
Authors: Ponce F. A. , Young W. T. , Cherns D. , Steeds J. W. , Nakamura S.
Published: April 2011

In this paper we report that, in addition to dislocations, two other types of defects are observed in high quality GaN thin films. These defects have a filamentary nature, are oriented along the <0001> direction. and may not be easily distinguished from the pure dislocations. Using a combination of conventional electron microscopy with convergent beam electron diffraction techniques we show that one of these types of dislocations consist of nanopipes, which are coreless dislocations with Burgers vectors <0001>. The other type of observed defects consist of inversion domains with [0001 ] orientation within the [0001] matrix. The origin of the inversion domains and nanopipes is discussed.
Structural and Optical Characterization of High-Quality Cubic GaN Epilayers Grown on GaAs and 3C-SiC Substrates by Gas-Source MBE Using RHEED In Situ Monitoring
Authors: Okumura H. , Balakrishnan K. , Feuillet G. , Ohta K. , Hamaguchi H. , Chichibu S. , Ishida Y. , Yoshida S.
Published: April 2011

By monitoring RHEED reconstruction patterns during gas source molecular beam epitaxy growth, the optimization of the growth for cubic GaN was carried out successfully. Cubic GaN epilayer having a X-ray diffraction width of 16min and a low temperature photoluminescence emission width of 19meV was obtained on a 3C-SiC substrate by adjusting the effective III/V ratioin-situduring the growth, which can be inferred from the surface reconstruction transitions. It was found that the surface reconstructions of cubic GaN surfaces are good indices for the optimization of growth parameters.
Comparison of the Microstructure of AlN Films Grown by MOCVD and by PLD on Sapphire Substrates
Authors: Li Yun-Xin , Salamanca-Riba Lourdes , Talyan V. , Venkatesan T. , Wongchigul C. , Zhou P. , Tang X. , Spencer M. G.
Published: April 2011

(0001) aluminium nitride thin films were grown epitaxially on (0001) Sapphire substrates by MOCVD at 1200° C and PLD at 800° C. Both films have the same epitaxial growth relationship: (0001)AlN//(0001)Sap, and the same in-plane relationship which shows a 30° rotation between A1N and Sapphire: [ 12 10]AlN//[0 110]Sap and [10 10]AlN //[ 2110]Sap. The full width at half maximum (FWHM) of x-ray rocking curve of the MOCVD A1N film was 0.16° and PLD A1N film was 0.2°. Films grown by both MOCVD and PLD showed high crystalline quality. HRTEM images showed that these films are single crystalline with very low density of defects.

Dislocations in the film parallel to the film / substrate interface were observed in both A1N films. Atomic force microscopy images showed that the MOCVD films have flatter and larger terraces than the PLD films. The PLD technique for A1N growth needs to be improved further. But both films have a surface roughness of approximately 100nm.
Characterization of OMVPE-Grown AlGaInN Heterostructures
Authors: Bour D. P. , Chung H. F. , Götz W. , Romano L. , Krusor B. S. , Hofstetter D. , Rudaz S. , Kuo C. P. , Ponce F. A. , Johnson N. M. , Craford M. G. , Bringans R. D.
Published: April 2011

We report on the OMVPE growth and characterization of AlGalnN and its heterostructures, including measurements of electrical properties (Hall), optical properties (photo- and cathodo- luminescence), structural characteristics (x-ray diffraction and TEM); and also the emission of InGaN/AlGaN heterostructures subject to optical and electrical pumping.
Thickness Dependence of Electronic Properties of GaN Epi-layers
Authors: Götz W. , Walker J. , Romano L.T. , Johnson N.M. , Molnar R.J.
Published: April 2011

The electronic properties of heteroepitaxial GaN were investigated for unintentionally doped, n-type films grown by hydride vapor phase epitaxy on sapphire substrates. The GaN layers were characterized by variable temperature Hall-effect measurement, capacitance-voltage (C-V) measurements, and deep level transient spectroscopy (DLTS). The measurements were performed on as-grown, 13 μn thick films and repeated after thinning by mechanical polishing to 7 μm and 1.2 μm. The room temperature electron concentrations as determined by the Hall-effect measurements were found to increase from ∼1017 cm−3 (13 μm) to ∼1020 cm−3 (1.2 μm) with decreasing film thickness. However, the C-V and DLTS measurements revealed that the ionized, effective donor and deep level concentrations, respectively, remained unchanged in regions close to the top surface of the films. These findings are consistent with the presence of a thin, highly conductive near interface layer which acts as a parasitic, parallel conduction path. Possible sources of such a shunt near the GaN/sapphire interface include oxygen contamination from the sapphire substrate or a structurally highly defective, 300 nm thick interface layer.
Persistent Photoconductivity in p-Type GaN Epilayers and n-Type AlGaN/GaN Heterostructures
Authors: Li J. Z. , Lin J. Y. , Jiang H. X. , Khan M. A. , Chen Q. , Salvador A. , Botchkarev A. , Morkoc H.
Published: April 2011

Persistent photoconductivity (PPC) effect has been observed in p-type GaN epilayers grown both by metal-organic chemical vapor deposition (MOCVD) and reactive molecular beam epitaxy (MBE) as well as in a two-dimensional electron gas (2DEG) system formed by an AlGaN/GaN heterostructure grown by MOCVD. Its properties have been investigated at different conditions.
P-Type Conductivity with a High Hole Mobility in Cubic GaN/GaAs Epilayers
Authors: As D. J. , Rüther A. , Lübbers M. , Mimkes J. , Lischka K. , Schikora D.
Published: April 2011

Temperature dependent Hall-Effect-measurements on unintentionally doped cubic GaN epilayers grown by molecular beam epitaxy (MBE) are reported. The cubic GaN layers have been deposited on semiinsulating (001) GaAs-substrates under N-stabilized growth conditions which were controlled by in-situ reflection high energy electron diffraction (RHEED) measurements. GaN-layers, which were fabricated under N-stabilized conditions have a (2×2) surface reconstruction during growth and show p-type conductivity. At room temperature the measured hole concentrations and mobilities are p = 9.7* 1012 cm-3, μp ≅ 350 cm2/Vs, respectively. Temperature dependent measurements of the carrier concentration yield an acceptor activation energy of EA = 0.445 eV. The nature of these defects will be discussed in view of intrinsic defects proposed by theoretical calculations already published in literature. The temperature dependence of the mobility is dominated by polar optical phonon scattering in the investigated temperature range.
Characterization of GaN Films on Sapphire by Cathodoluminescence
Authors: Chao L.-L. , Cargill G. S. , Kothandaraman C.
Published: April 2011

Cathodoluminescence (CL) spectroscopy and microscopy were used to study the luminescent properties of a variety of GaN films, both Si-doped and unintentionally-doped, grown on sapphire substrates. A narrow and intense near band-edge emission was found in the CL spectrum of each film examined, and deep-level emission was also observed for some of the films. The luminescence efficiency of near band-edge emission increased with a faster rate than that of deep-level emission when the pumping current was increased. Spatial nonuniformities of luminescence were observed in monochromatic CL microscopy, and microstructures were observed in scanning electron microscopy. No correlations between luminescence features and microstructural features were seen. Degradation of near band-edge luminescence was observed, accompanied by growth of deep-level emission.
Initial Growth Stage of AlGaN Grown Directly on (0001) 6H-SiC by MOVPE
Authors: Horino K. , Kuramata A. , Tanahashi T.
Published: April 2011

We investigated the growth process of AlGaN films grown directly on 6H-SiC (0001) substrates by metalorganic vapor phase epitaxy (MOVPE). We focused on the initial growth stage to clarify the mechanism of nitride growth on SiC. From Energy Dispersive X-ray (EDX) analysis we found that an Al-rich region generated naturally at the AlGaN/SiC interface. We also found that Al flux determined the density of grain which generated during the initial growth stage, and this grain density reflected the surface morphology.
Optical Signature of the GaN (1010) Surface
Authors: Noguez C. , Esquivel-Sirvent R. , Alfonso D. R. , Ulloa S. E. , Drabold D. A.
Published: April 2011

We present a theoretical study of the optical properties of the GaN (1010) surface. We employed a semi-empirical tight-binding method to calculate the surface electronic structure. The parameters were adjusted to reproduce the correct band structure of the bulk wurzite GaN. These parameters were interpolated to the surface using Harrison’s rule. From the surface electronic structure the surface dielectric response was obtained. The dielectric response is analized in terms of surface-surface, and surface-bulk electronic transitions.
Stability and Band Offsets of SiC/GaN, SiC/AlN, and AlN/GaN Heterostructures
Authors: Majewski J. A. , Städele M. , Vogl P.
Published: April 2011

We present first-principles calculations of structural and electronic properties of heterova-lent SiC/GaN, SiC/AIN, and isovalent AIN/GaN heterostructures that are grown pseudo-morphically on (001) or (110) SiC substrates. For the polar interfaces, we have investigated reconstructed stoichiometric interfaces consisting of one and two mixed layers with lateral c(2 × 2), 2 × 1, 1 × 2, and 2 × 2 arrangements. The preferred bonding configurations of the reconstructed interfaces are found to be Si-N and Ga-C. With respect to vacuum, the valence band maximum is found to be highest in SiC and lowest in A1N. In these systems, the valence band offsets deviate substantially from the transitivity rule and depend sensitively on the microscopic details of the interface geometry. The SiC/AIN and AIN/GaN heterostructures are predicted to be of type I, whereas SiC/GaN heterostructure can be of type I or II.
Chlorine-Based Plasma Etching of GaN
Authors: Shul R. J. , Briggs R. D. , Pearton S. J. , Vartuli C. B. , Abernathy C. R. , Lee J. W. , Constantine C. , Barratt C.
Published: April 2011

The wide band gap group-III nitride materials continue to generate interest in the semiconductor community with the fabrication of green, blue, and ultraviolet light emitting diodes (LEDs), blue lasers, and high temperature transistors. Realization of more advanced devices requires pattern transfer processes which are well controlled, smooth, highly anisotropic and have etch rates exceeding 0.5 μm/min. The utilization of high-density chlorine-based plasmas including electron cyclotron resonance (ECR) and inductively coupled plasma (ICP) systems has resulted in improved etch quality of the group-III nitrides over more conventional reactive ion etch (RIE) systems.
Ion Implantation and Annealing Studies in III–V Nitrides
Authors: Zolper J. C. , Pearton S. J. , Williams J. S. , Tan H. H. , Karlicek R. J. , Stall R. A.
Published: April 2011

Ion implantation doping and isolation is expected to play an enabling role for the realization of advanced Ill-Nitride based devices. In fact, implantation has already been used to demonstrate n- and p-type doping of GaN with Si and Mg or Ca, respectively, as well as to fabricate the first GaN junction field effect transistor.1-4 Although these initial implantation studies demonstrated the feasibility of this technique for the Ill-Nitride materials, further work is needed to realize its full potential.

After reviewing some of the initial studies in this field, we present new results for improved annealing sequences and defect studies in GaN. First, sputtered A1N is shown by electrical characterization of Schottky and Ohmic contacts to be an effective encapsulant of GaN during the 1100 °C implant activation anneal. The A1N suppresses N-loss from the GaN surface and the formation of a degenerate n+-surface region that would prohibit Schottky barrier formation after the implant activation anneal. Second, we examine the nature of the defect generation and annealing sequence following implantation using both Rutherford Backscattering (RBS) and Hall characterization. We show that for a Si-dose of l × l016 cm-2 50% electrical donor activation is achieved despite a significant amount of residual implantation-induced damage in the material.
Molecular Dynamic Simulations of Semiconductor Clusters
Authors: Dubey Girlta S. , Gumbs Godfrey
Published: April 2011

Molecular dynamics simulations are carried out for silicon clusters to analyze the effect of heating and cooling on their structural properties. The calculations are based on the Kaxiras and Pandey potential which has been derived from a microscopic calculation with the density functional method. Results for the cluster formation are presented for different rates of cooling and heating. Our simulations clearly show the dependence of the patterns on the cooling and heating rates as well as the way in which these clusters evolve. Our results indicate that the structures are most stable when the rate of loss of thermal energy is slow and the potential energy is lowest.
Porous Microcrystalline Silicon Solar Cells
Authors: Duttagupta S. P. , Kurinec S. K. , Fauchet P. M.
Published: April 2011

We report the fabrication of photovoltaic devices by the anodization of microcrystalline silicon films on single crystal silicon substrates. The porosity of the films was varied from 20% to 60% by changing the anodization conditions. An unetched μc-Si based device was used for reference. The influence of the porosity on the series resistance (Rs), the reflectance, and the spectral response of the devices was studied in detail. In order to determine Rs, the current-voltage characteristics were analyzed, both in the dark and under illumination. We observed that the value of Rs increased from 3.1 Ω to 97 Ω and the value of the reflectance decreased from 24% to 7% when the porosity increased from 20% to 60%. Initially, an optimum device performance (fill factor of 0.53 and efficiency of 7.2%) was achieved for a porosity of 40%, which was about a 40% improvement as compared to the reference (unetched) μc-si based device. Due to a further reduction in Rs by using an intermediate ITO layer and a superior grid-contact architecture, a device efficiency of 10% has been recently achieved.
Electroluminescence Device Perspectives of Si+-Implanted SiO2
Authors: Kozlowski F. , Porteanu H. E. , Petrova-Koch V. , Koch F.
Published: April 2011

Thermal oxide layers on Si which have been implanted with Si conduct electrical current and emit light. The electroluminescence effect (EL) has an efficiency which is comparable to the best values that have been reported for the porous Si based devices. Generally, the EL spectrum differs substantially from the photoexcited luminescence. It is linked with that excited by external high energy electrons in cathodoluminescence (CL) experiments. We suggest to consider the effect as internal CL. Based on studies of transport and EL characteristic we evaluate the possibilities of an electroluminescent device based on such layers.
Electroluminescent Devices Based on Zero- and One- Dimensional Silicon Structures
Authors: Nassiopoulos A. G. , Photopoulos P. , Ioannou-Sougleridis V. , Grigoropoulos S. , Papadimitriou D.
Published: April 2011

Stable solid state electroluminescent devices, based on one- and two- dimensional silicon structures were fabricated. One- dimensional structures were in the form of silicon nanopillars on a silicon substrate, fabricated by using lithography and etching techniques. Isolation and planarization of nanopillars was made by using polymethyl methacrylate (PMMA), which is a non conducting polymer, totally transparent in the visible region. Gold (Au) was used as top contact metal and the back ohmic contact was based on aluminum. Zero-dimensional structures were in the form of silicon nanocrystallites, deposited by low pressure chemical vapour deposition (LPCVD) on a thin SiO2 layer, thermally grown on silicon. Gold and aluminum were also used as contact metals. In both cases the devices showed stable electroluminescence at room temperature, visible with the naked eye. Devices based on nanocrystallites showed more uniform emission. The onset of light emission was between 5 and 7 Volts. The devices were checked for several hours of continuous operation and no degradation was observed.
Preparation and Characterization of the Active Layer for an Led Based on Oxidized Porous Silicon
Authors: Tsybeskov L. , Hiirschman K. D. , Moore L. F. , Fauchet P. M. , Calcott P. D. J.
Published: April 2011

We have studied the photoluminescence (PL) in oxidized porous silicon (PSi), prepared from anodized crystalline Si followed by annealing at temperatures ranging from 700 to 1000°C. It has been found that two PL bands with spectral peaks at 1.6 e V (near-IR band) and near 2 eV (red band) exist with a strong dependence on preparation (annealing) conditions. Recent experimental results show a correlation between the intensity of the near-IR band and the level of leakage current in the diode-like structure. The suppression of the near-IR emission results in improved carrier transport, and the enhancement of the red band emission maximizes the electroluminescence (EL) efficiency. The PL study of thermally oxidized PSi indicates different recombination mechanisms. The red PL band is associated with a mechanism similar to band-tail-recombination within the quasi-bandgap of Si nanograins, whereas the near infra-red PL is associated with recombination via defect centers. These mechanisms will be discussed.
Substrate-Surface Effect on Initial Growth Process of Microcrystalline Silicon Films
Authors: Ikuta K. , Park J. W. , Kuo L. H. , Yasuda T. , Yamasaki S. , Tanaka K.
Published: April 2011

Initial growth processes of hydrogenated microcrystalline silicon (μc-Si:H) films have been investigated by scanning tunneling microscopy (STM), high-resolution transmission electron microscopy (HRTEM), and reflection high energy electron diffraction (RHEED). The μc-Si:H films were prepared by plasma enhanced chemical vapor deposition (PECVD) on H-terminated Si(111) and plasma-oxidized SiO2/Si(111) surfaces that were made atomically-flat by a careful wet processing. On H-terminated Si(111) the initial growth was epitaxial as evidenced by HRTEM and RHEED, while on SiO2/Si(111) the initial process was nucleation of amorphous Si followed by formation of randomly oriented μc-Si:H structure. STM observation revealed that, on both H-terminated and SiO2-terminated surfaces, initial growth processes proceed through the nucleation-and-coalescence mechanism.
Theory of Pressure Effects on Silicon Nanocrystallites
Authors: Allan G. , Delerue C. , Lannoo M.
Published: April 2011

Pressure effects on silicon nanocrystallites are calculated using semi-empirical tight-binding and ab-initio local density calculations. Using the confinement model in porous silicon a red shift of the luminescence energy with increasing pressure is obtained. Quantum confinement in BC8 phase silicon nanocrystallites obtained after release of high pressure are also studied. It increases the cluster gap and also enhances the electron-hole radiative recombination rate.
Growth Mechanism of Microcrystalline Silicon Deposited by ECRCVD
Authors: Beckers I. , Conrad E. , Müller P. , Nickel N. H. , Sieber I. , Fuhs W.
Published: April 2011

Microcrystalline silicon (μc-Si) films were prepared by electron cyclotron resonance assisted chemical vapor deposition (ECRCVD) using helium, argon and hydrogen dilution. The crystalline fraction was estimated from Raman backscattering spectra and scanning electron-microscopy (SEM) was used to obtain information on roughness and homogeneity of the films. For hydrogen dilution the highest crystallinity (Xc = 85 %) occurs at a ratio of ΔH = [H2]/([H2]+[SiH4])= 0.98. At the same time the deposition rate decreases continuously with increasing H2 dilution. These results are consistent with the idea that H etching promotes the growth of μc-Si. At ΔH > 0.98 a Xc decreases due to a H mediated transition of small crystallites into amorphous tissue. The implications of these results for the growth mechanisms are discussed.
Photocarrier Recombination in Microcrystalline Silicon Studied by Light Induced Electron Spin Resonance Transients
Authors: Müller J. , Finger F. , Malten C. , Wagner H.
Published: April 2011

To get information on the density of states distribution and the photocarrier recombination in microcrystalline silicon (μc-Si:H), samples with various amounts of n- and p-type doping are studied with electron spin resonance (ESR) and stationary and time-resolved light induced ESR. The intensity of the dark ESR signals from dangling bonds (DB) and conduction electrons (CESR) is investigated as a function of the doping level. The DB signal has a flat distribution over a wide doping range while the CESR signal strongly increases with n-type doping. Upon illumination with white or infrared light both resonances are enhanced with an intensity that depends on the doping level. The decay of the light induced signal and the dependence in time and intensity of the residual signal on different initial excitation energies and dark/light -sequences is studied. The results are discussed with a schematic band diagram for μc-Si:H. The existence of a potential barrier is proposed which spatially separates photogenerated carriers. A large band-offset between crystalline and disordered regions is further suggested.
Injection Electroluminescence from Thin Film p-i-n Structures made from Nanocrystalline Hydrogenated Silicon
Authors: Andreev A. A. , Averbouch B. Y. , Mavlyanov P. , Aldabergenova S. B. , Albrecht M. , Stenkamp D. , Strunk H. P.
Published: April 2011

Nanocrystalline silicon films are prepared by plasma enhanced chemical vapour deposition of silane under the conditions of high hydrogen dilution (3:100). The film structure consists of nanoclusters 0.8 to 5 nm in size (volume fraction 30%) embedded in an amorphous matrix. The Taue gap of the amorphous matrix is 1.95 to 2.05 eV depending on deposition parameter. These films are characterized as regards photoluminescence (PL) and, prepared to p-i-n structures, electroluminescence (EL). The PL and EL agree in (i) luminescence peak at 1.9 eV, i.e. small Stokes shift, (ii) almost no temperature dependence between 77 K and 293 K, (iii) fast kinetics with time constant of a few 10−8 s. These data can be understood in terms of quantum confinement in Si nanocrystallites smaller than around 2 nm. The EL in addition exhibits a luminescence band extending up to 3 eV, which can be interpreted by interband transition due the hot carriers.
Synthesis of Polyvanadates from Solutions
Authors: Livage J. , Bouhedja L. , Bonhomme C. , Henry M.
Published: April 2011

A wide range of polyvanadates can be synthesized from aqueous solutions. Vanadium oxide gels V2O5nH2O are formed around the point of zero charge (pH≈2). They exhibit a ribbon-like structure. Weak interactions between these ribbons lead to the formation of mesophases in which vanadium oxide gels or sols behave as nematic liquid crystals. Organic species can be easily intercalated between these oxide ribbons leading to the formation of hybrid nanocomposites made of alternative layers of organic and inorganic components. Hybrid nanophases can also be formed above the point of zero charge, in the presence of large organic ions such as [N(CH3)4]+. They often exhibit layered structures in which organic cations lie between the polyvanadate planes. Cluster shell polyvanadates have been obtained in the presence of anions such as Cl− or I−. They are made of negatively charged polyvanadate hollow spheres in which the anion is encapsulated. Organic cations then behave as counter ions for the formation of the crystal network.
Magnetic Microstructure of a Nanocrystalline Ferromagnet - Micromagnetic Model and Small-Angle Neutron Scattering
Authors: Weissmüller J. , McMichael R. D. , Barker J. , Brown H. J. , Erb U. , Shull R. D.
Published: April 2011

We report on a combined theoretical and experimental study of the magnetic microstructure of a single component, single phase, Pore-free nanocrystalline ferromagnetic material. From the equations of micro-magnetics we conclude that the magnetic microstructure is the convolution product of an anisotropy field microstructure and of a response function with a correlation length lH that depends on the applied field Ha. We derive equations for small angle neutron scattering by such structures, and present experimental scattering data for electrodeposited nanocrystalline Ni, the first where for a wide range of Ha the dominant scattering contribution is from the purely magnetic microstructure, not from nuclear or magnetic contrast at pores or second phases. The variation of the scattering cross section with Ha is in excellent agreement with the theory, indicating that the underlying changes in the magnetic microstructure with Ha are not displacements of domain walls, but changes in lH and hence in the magnetic response to an entirely stationary anisotropy field microstructure. At 20K the anisotropy fields are dominated by magnetocrystalline anisotropy, but at 300K the perturbation is from a much stronger interaction which maintains some moments aligned antiparallel to the field direction at Ha as high as 1.4MA/m (18kOe).