bluuebunny/crossref_metadata_2025_split huggingface dataset:
“Transport limiting mechanisms of amorphous (a-Si:H) and microcrystalline(mc-Si:H) thin film heterostructures for photovoltaic application”
Authors: Khan Shahrukh A. , Anderson W.A. 
Published: April 2011

AbstractDirect conversion of sunlight into electricity employing amorphous (a-Si:H) and microcrystalline (mc-Si) silicon solar cell structures provides cheap, continuous, large-area device capability. However, the inherent instability due to light induced degradation of such cells poses a challenge for practical photovoltaic applications. To work around this problem, we have made use of a-Si:H/c-Si (crystalline) and mc-Si/c-Si heterostructures. Plasma decomposition of 2% silane gas (SiH4/He) in an ECR (electron cyclotron resonance)-CVD configuration forms the basis of the deposition technique. Electro-optical properties exhibit photoconductivity (σp) of 6.5×10−6 S/cm and a low dark conductivity (σd) of 1.4×10−9 S/cm for a-Si:H films and a relatively high σp of 2.1×10−4 S/cm and a high σd of 1.2×10−7 S/cm for mc-Si films. A possible multi-tunneling transport mechanism is evident for a low forward bias with the current conduction becoming space charge limited with increasing bias. The domination of this latter mechanism may be responsible for non-ideal junction behavior.
Assembly of CdSe/CdS Quantum Dots on Au Surfaces for Photoreception
Authors: Tang Jing , Birkedal Henrik , McFarland Eric W. , Stucky Galen D. 
Published: April 2011

ABSTRACTCdSe/CdS core/shell quantum dots have been synthesized and assembled onto pre-functionalized gold surfaces by either hydrogen bonding or covalent bonds through different functional groups. Control of the conditions during the deposition process allows producing a high coverage of quantum dots via molecular linkages. The quantum-dot surface is highly photoactive and is used in a surface sensitized Schottky barrier photovoltaic structure as the photoreception component. Atomic force microscopy (AFM) and X-ray photoelectron Spectroscopy (XPS) are used to characterize and confirm the morphology and linkage of the assemblies on Au surfaces. The electron transfer from the quantum-dot layer to the Schottky barrier device is examined by measuring the current-voltage (IV) curve of such photovoltaic devices under simulated sun light.
Enhancement of Photoelectric Effect in Organic Dye Thin Film Cells by Surface Plasmon Excitation
Authors: Kato Keizo , Hirano Masaru , Shinbo Kazunari , Kaneko Futao , Wakamatsu Takashi 
Published: April 2011

ABSTRACTPhotoelectric properties have been investigated for the organic dye thin film cells utilizing surface plasmon (SP) excitation. The cells fabricated in this work had a prism/Al thin film/copper phtalocyanine (CuPc) thin film/Ag thin film structure, which was the Kretschmann configuration in the attenuated total reflection (ATR) method. Since the CuPc thin film exhibits p-type conduction, the Schottky and Ohmic contacts can be obtained at the interfaces between the CuPc and Al thin films and between the CuPc and Ag thin films, respectively, and the cells used in this work show photoelectric properties. The ATR and short-circuit photocurrent properties have been simultaneously measured as a function of the incident angles of the laser beams. The experimental and calculated results of the ATR and short-circuit photocurrent properties and the optical absorptions of the organic dye layers were discussed. It was found that the photoelectric effect in the organic dye thin film cells the Kretschmann configuration in the ATR method was enhanced by the SP excitation.
Solvent Processible Composite Carbon Nanotube Cathode for Polymer LED Applications
Authors: Liu Gao , Johnson Steve , Kerr John B. 
Published: April 2011

ABSTRACTComposites of single wall carbon nanotubes (SWNT) and conductive polymers were studied as potential cathode materials for application in polymer light emitting devices. A new conductive poly(2,7–9,9 (di(oxy-2,5,8-trioxadecane))fluorene) (PFO) possessing surfactant properties was used to stabilize SWNTs in solutions. The rigid PFO backbones act as a template while the ethyleneoxide side chains appear to wrap around the SWNTs. Up to 0.02% (by weight) of SWNTs are stabilized in the solution phase. The current vs. voltage behavior of the SWNT/PFO composite film (2% SWNT in PFO by weight) shows that most of the current is carried by SWNTs.
High Performance Solar Blind Detectors based on AlGaN grown by MBE and MOCVD
Authors: DUBOZ Jean-Yves , Reverchon Jean-Luc , Mosca Mauro , Grandjean Nicolas , Omnes Franck 
Published: April 2011

ABSTRACTSolar blind detectors based on AlGaN grown by Molecular Beam Epitaxy and Metal Organic Vapor Phase Epitaxy have been fabricated and characterized. Metal Semiconductor Metal (MSM) detectors and vertical Schottky detectors have been realized, with a design that allows back side illumination. The growth was optimized in order to improve the layer quality, avoid crack formation, and provide the best detector performance. The technological process was also optimized in order to reduce the dark currents and improve the spectral rejection ratio, which is a key factor for solar blind detection. As a result, a rejection ratio of 5 decades between the UV (below 300 nm) and 400 nm, and a steep cut off limited by alloy fluctuations have been obtained. A noise equivalent power below 10 fW is obtained in MSM detectors.
GaN quantum dot UV light emitting diode
Authors: Lee Jeong-Sik , Tanaka Satoru , Ramvall Peter , Okagawa Hiroaki 
Published: April 2011

ABSTRACTThe fabrication and evaluation of a UV light-emitting diode (LED) incorporating GaN quantum dots as the active layer is demonstrated. The GaN quantum dots were fabricated on an AlxGa1-xN (x∼0.1) surface using Si as an antisurfactant. Exposing the AlxGa1-xN surface to the Si antisurfactant prior to GaN growth enabled the formation of quantum dots on a surface where growth by the Stranski-Krastanov mode would not be possible. A fairly high density of dots (1010-1011 cm-2) with controllable dot sizes was achieved. Room temperature luminescence at 360 nm was clearly observed during current injection (cw) into an LED structure including the GaN quantum dots. The origin of the electroluminescence is discussed by comparing it to photoluminescence measurements.
LP-MOCVD growth of GaAlN/GaN heterostructures on Silicon Carbide. Application to HEMT's devices.
Authors: di Forte Poisson M-A. , Magis M. , Tordjman M. , Aubry R. , Peschang M. , Delage S. L. , di Persio J. , Grimbert B. , Hoel V. , Delos E. , Ducatteau D. , Gaquiere C. 
Published: April 2011

ABSTRACTThis paper reports on the LP-MOCVD growth optimisation of GaAlN/GaN heterostructures grown on Silicon Carbide substrates for HEMT applications, and on the first device performances obtained with these structures. The critical impact of some growth parameters on the physical properties of the GaAlN/GaN epilayers has been identified and studied using High Resolution X-Ray diffraction (HR-XRD), AFM, C-V and sonogauge measurements. The SiC substrate surface preparation (both ex-situ and in-situ) and the nucleation layer growth conditions (growth temperature, thickness, composition and strain) have been found to be key steps of the GaAlN/GaN/SiC growth process. SiC substrates from different suppliers have been evaluated and their influence on the physical properties of the GaAlN/GaN HEMT structures investigated. Static characteristics of the devices such as maximum drain current Idss or pinch-off voltage have been correlated with the nucleation layer composition of the HEMT structure and the defect density of the SiC substrate. First devices measured at 10 GHz using a load pull system exhibited CW output power in excess of 2.8 W/mm for a gate length of 0.5 μm. Under static measurements, we found an Idss around 1 A/mm and a pinch-off voltage of –5 V.
Thermal Conductivity of GaN Grown on Silicon Substrates
Authors: Mion C. , Chang Y. C. , Muth J. F. , Rajagopal P. , Brown J. D. 
Published: April 2011

ABSTRACTOne of the principle problems of high power electronic devices is the extraction of heat from the active region of the device. The thermal conductivity of the substrate is a crucial parameter affecting the thermal dissipation capability of a device. In this study we investigated the thermal conductivity at room temperature of five GaN samples grown on silicon, employing the 3 omega thermal conductivity method. The thickness of the GaN layers varied from 0 to 700 nm. The effective thermal conductivity for the GaN layers was found to range from 130 to 140 W/m·K which is comparable to thermal conductivity of the silicon substrate. This result indicates that the heat transfer of GaN on silicon is as good as either GaN or silicon alone, and that no substantial thermal degradation attributable to the GaN/Silicon interface is observed.
Arsenic Incorporation Behavior in Nitrogen-rich GaNAs Alloys Synthesized by Metalorganic Chemical Vapor Deposition (MOCVD)
Authors: Gherasimova M. , Wheeler R. G. , Guido L. J. , Chang K. L. , Hsieh K. C. 
Published: April 2011

ABSTRACTHomogeneous GaNAs alloy films containing 3 to 4 percent of arsenic anion fraction were synthesized by metalorganic chemical vapor deposition (MOCVD). Post-growth annealing resulted in the formation of GaAs precipitates of several nanometers in size embedded in the nitrogen-rich GaN(As) matrix, while as-grown ternary films exhibited no evidence of phase segregation. Band gap reduction due to alloying was observed by the optical transmission measurements, leading to an estimate for the bowing parameter of 25 eV in the films with the arsenic content of 3.5 %.
Investigation of molecular co-doping for low ionization energy p-type centers in (Ga,Al)N
Authors: Feng Zhe Chuan , Payne Adam M. , Nicol David , Helm Paul D. , Ferguson Ian , Senawiratne Jayantha , Strassburg Martin , Dietz Nikolaus , Hoffmann Axel , Hums Christoph 
Published: April 2011

ABSTRACTThis work initiates an investigation of molecular co-doping to produce p-type centers in (Ga,Al)N with ionization energies lower than Mg. Dopant complexes can be formed between a doubly ionized acceptor such as (Cu, Li or Ag) and a singly ionized donor (silicon). Ion implantation of Cu, Li and Ag into silicon doped GaN films grown by Metalorganic Chemical Vapor Deposition (MOCVD) has been performed. Secondary ion mass spectroscopy (SIMS) data confirmed the simulated depth profile. High resolution X-ray diffraction and Raman spectroscopy were used to characterize the crystalline damage and subsequent recovery upon anneal. A complete recovery was observed after high temperature (700–900°C) annealing. Low temperature (6K) photoluminescence (PL) for Cu-implanted GaN showed bands identified with crystalline lattice damage due to the Cu-implantation. The annealed samples showed recovery of standard crystalline GaN features. Additional donor-acceptor pair features are observed below 3.35 eV indicating the existence of an acceptor state.
Novel Method for the Activation of Acceptor Dopant in AlN Introducing Localized Band by Isoelectronic Dopant
Authors: Takizawa Toshiyuki 
Published: April 2011

ABSTRACTIn this study we propose a novel method to increase hole concentration introducing isovalent substitutional dopant into a p-type nitride semiconductor. Acceptor dopant in nitride semiconductors makes deep acceptor level (>100 meV) and generates few hole carriers into the valence band because of large electron affinity of N.In contrast to this, substitution of isovalent group-V atoms (P, As and Sb) that has smaller affinity than N makes a localized group-V band upward the valence band maximum (VBM). When both acceptor and isovalent group-V atoms are incorporated into nitride semiconductor, holes can be drastically activated by isovalent atoms, and can easily move in the group-V band. We have also investigated this material, Mg-doped AlN:V (V=P, As or Sb), using first-principles pseudopotential method. As a calculation result, substitiution of P and As makes localized group-V band upward the VBM of AlN, and moreover this can be adjusted the VBM of GaN. The Mg incorporation into AlN:V as an acceptor dopant drastically decreases the Fermi level (ΔEF=-0.10 eV), that is, hole concentration can be drastically raised by the group-V band. Consequently novel p-type material with isovalent dopant can be a candidate to efficiently inject hole current into the VBM of GaN.
Fabrication of Thermoelectric Devices Using AlInN and InON Films prepared by reactive radio-frequency sputtering
Authors: Yamaguchi S. , Izaki R. , Kaiwa N. , Sugimura S. , Yamamoto A. 
Published: April 2011

ABSTRACTWe have studied the thermoelectric properties of AlInN and InON films prepared by reactive radio-frequency (RF) sputtering. We have fabricated thermoelectric devices which are composed of 20-pair nitride or oxynitride films with Chromel metal. For a AlInN device, the maximum output power was 7.6×10-7 W at ΔT = 257K, and for a InON device, that was 6.5×10-8 W at ΔT = 214K.
Synthesis and characterization of high quality ferromagnetic Cr-doped GaN and AlN thin films with Curie temperatures above 900K
Authors: Wu Stephen Y. , Liu H. X. , Gu Lin , Singh R. K. , van Schilfgaarde M. , Smith David J. , Dilley N. R. , Montes L. , Simmonds M. B. , Newman N. 
Published: April 2011

ABSTRACTReactive MBE growth was used to synthesize ferromagnetic Cr-doped GaN and AlN thin films with Curie temperatures above 900K. 2% Cr-doped GaN and 7% Cr-doped AlN were found to have a saturation magnetization moment of 0.42 and 0.6 μB/Cr atom, indicating that ∼14% and ∼20% of the Cr, respectively, are magnetically active. Structural characterization using X-ray diffraction (XRD) and transmission electron microscopy (TEM) did not find evidence of a ferromagnetic secondary phase. Electrical characterization indicate that the resistivity of the Cr-doped GaN films depends exponentially on temperature as R=Roexp[(To/T)1/2], characteristic of variable range hopping. In contrast, Cr-doped AlN films are highly resistive. Local spin density functional calculations predict that Cr forms a deep level defect in both systems and the t2 level falls approximately at midgap. Our theoretical and experimental results indicate that ferromagnetism in Cr-doped GaN and AlN arises as a result of the double exchange mechanism within the partially filled Cr t2 band.
GaN Epitaxial Growth Process at High Growth Temperature by NH3 Source Molecular Beam Epitaxy
Authors: Ohshima Naoki , Sugihara Akihiro , Yoshida Naoya , Okabe Naohiko 
Published: April 2011

ABSTRACTWe have investigated in detail dependence of annealing GaN buffer layer and GaN growth processes on a sapphire substrate at a high temperature of 1000 degree C. The GaN layers are grown by NH3 gas source molecular beam epitaxy. The behavior of GaN buffer and epitaxial layer has been observed by in-situ reflection high-energy electron diffraction and the surface morphologies of as-grown and chemically etched GaN layers by atomic force microscopy. It is found that there is distinct difference in the surface morphology of epitaxial GaN layer between at growth temperatures of below 950 degree C and that of 1000 degree C. It has been considered that the growth kinetics of GaN epitaxial layer extremely depends on the growth temperature.
Structural Characterization of Low-Temperature InN Buffer Layer Grown by RF-MBE
Authors: Araki T. , Nanishi Y. 
Published: April 2011

ABSTRACTThe microstructure of an InN buffer layer grown on (0001) sapphire at low temperature by radio-frequency molecular beam epitaxy (RF-MBE) is characterized by transmission electron microscopy. The low-temperature InN buffer layer is found to contain local inhomogeneous regions of island-like grains surrounded by misoriented InN grains and inclusions of cubic phase. The generation of such anti-phase InN nuclei near the island-like grains is expected to give rise to defects at the interface. It is considered that these anti-phase InN nuclei are formed by local fluctuations of stoichiometry due to inadequate surface migration during the growth of the InN buffer layer, indicating the important of controlling the surface stoichiometry during InN growth.
Synthesis, Structure and Luminescence of High Brightness Gallium Nitride Powder
Authors: Garcia R. , Thomas A. , Bell A. , Stevens M. , Ponce F. A. 
Published: April 2011

ABSTRACTHighly luminescent GaN powders have been synthesized by reacting high purity gallium metal with ultra-high purity ammonia in a horizontal quartz reactor at 1100 °C. The powders produced in this reactor consist of light grey micro-crystals with wurtzite structure. Elemental analysis indicates that the powders obtained by this method have a high nitrogen concentration (more than stoichiometric GaN, 16.73 weight %). Powder X-ray diffraction demonstrates that the material has a high purity and single crystalline structure. Electron microscopy shows that the powders consist of at least two kinds of particles, small sized platelets and large sized needles. The resulting GaN powders are thousands of times more cathode- and photo-luminescent than other GaN powders including commercially available material. Their luminescence intensities are comparable to that of GaN thin films grown by hydride vapor phase epitaxy and metal organic chemical vapor deposition.
Temperature Dependence of the Optical Properties for InN Films Grown by RF-MBE
Authors: Ishitani Y. , Xu K. , Terashima W. , Masuyama H. , Yoshitani M. , Hashimoto N. , Che S. B. , Yoshikawa A. 
Published: April 2011

ABSTRACTInN epitaxial layers are grown on sapphire substrates. The investigated samples have electron concentration in a range of 1.8 ×1018 − 1.1 ×1019 cm-3. Optical reflection and transmission measurements are performed. The plasma edge energy position in the spectra is constant in a measurement temperature range of 5 – 300 K. The reflection and transmission spectra are calculated on the basis of the LO phonon-electron coupling scheme and non-parabolic conduction band structure. From this analysis we find that the observed absorption edge is attributed to valence band to conduction band transition rather than the valence band to defect (impurity) band transition, and the intrinsic bandgap energy of 0.64 (±0.03) eV. This bandgap energy increases by 40 – 50 meV as the temperature decreases from 295 to 10 K.
Study of the growth mechanism and properties of InN films grown by MOCVD
Authors: Jain Abhishek , Redwing Joan M. 
Published: April 2011

ABSTRACTThin films of InN were grown on (0001) Sapphire by MOCVD. The effect of growth conditions and buffer layer on the film morphology was studied. Growth temperature and TMI flow rate were important factors in the growth of InN. The use of a low temperature AlN buffer layer was also found to improve the morphology and crystal quality of the films. Thin (<40Å) AlN buffer layers produced the best results while polycrystalline InN was obtained when the buffer layer thickness exceeded 60Å. Delamination of the InN films was observed to occur at growth temperature, which limited the thickness of the films to less than 300 nm. A room temperature mobility of 792 cm2/Vs and an electron concentration of 2.1×1019 cm-3 were measured in an approximately 200 nm thick InN layer grown on sapphire.
Growth and Fabrication of 2 inch Free-standing GaN Substrates via the Boule Growth Method
Authors: Hanser Drew , Liu Lianghong , Preble Edward A. , Thomas Darin , Williams Mark 
Published: April 2011

ABSTRACTHigh quality, single crystal GaN substrates have been demonstrated using a boule growth process. Here we report on the crystalline boules that were formed during the growth process and their material characterization. Using hydride vapor phase epitaxy process, GaN crystals were grown at growth rates greater than 200 μm/hr. Boules greater than 3 mm thick were grown and processed into free-standing substrates. Rocking curve measurements using high-resolution X-ray diffraction were performed on the substrates with FWHM values of 92 and 146 arcsec for the (002) and (102) reflections, respectively. Atomic force microscope images, etch pit studies, and transmission electron micrographs of the GaN material show high quality material quality with a dislocation density in the range of 5×106 to 1×107 cm-2.
Growth of GaN crystals under ammonothermal conditions
Authors: Callahan Michael J. , Wang Buguo , Bouthillette Lionel O. , Wang Sheng-Qi , Kolis Joseph W. , Bliss David F. 
Published: April 2011

ABSTRACTGrowth of GaN bulk crystals under ammonothermal conditions has been developed. The experiments were performed in ammono-basic solutions in high nickel content autoclaves for up to 3 weeks. Nutrients were crystalline GaN made from vapor phase transport growth. Single crystal clusters of GaN on the order of 500 μm – 1 mm long were obtained. These crystals were spontaneously nucleated on the walls of the autoclave. Transport growth on polycrystalline GaN seeds and single crystal HVPE seeds was also achieved. GaN has a high solubility in ammono-basic solutions, on the order of several weight percent. The ammonothermal crystals were characterized by photoluminescence (PL), X-ray diffraction and SEM.
Vapor phase transport of AlN in an RF heated reactor: Low and high temperature studies
Authors: Noveski V. , Schlesser R. , Freitas J. , Mahajan S. , Beaudoin S. , Sitar Z. 
Published: April 2011

ABSTRACTAlN crystals were grown from the vapor phase in an RF heated AlN sublimation reactor. The studies were performed with the following goals: 1) to optimize the growth rate by investigating mass transfer effects, and 2) to establish a process for epitaxial growth on AlN seeds. A one-dimensional mass transfer model based on equilibrium sublimation and gas-phase diffusion was developed. Model parameters were estimated and the model was validated from growth experiments carried out in a 600 Torr nitrogen atmosphere and temperatures ranging from 2000 to 2400°C. Continuous growth on AlN seed crystals was accomplished as a result of optimizing the initial stage of growth and achieving a delicate balance between the rate of mass transfer and the rate of surface rearrangement. During this experimental study, centimeter-size single crystals of AlN were obtained within the 1.25” diameter boule that was grown at a predicted growth rate of 0.1–0.3 mm/hr, at 500 Torr of nitrogen, short source-to-seed distance, low supersaturation and growth temperatures of 2110–2140°C. Chemical analysis of impurities in the grown AlN boules confirmed a very low oxygen contamination of 100 ppm wt. Cathodoluminescence studies showed well defined near band edge emission peak located slightly above 6 eV.
HNO3 treatment of sapphire for management of GaN polarity in MOCVD method: Comparison of the properties of +c and –c GaN region
Authors: Takabe Motoki , Sumiya Masatomo , Fuke Shunro 
Published: April 2011

ABSTRACTGaN film with Ga- (+c) and N- (-c) face polarities was simultaneously grown on c-plane sapphire substrate by using only metalorganic chemical vapor deposition. Although several kinds of processes were examined, we achieved the simultaneous growth by using the patterned AlN nucleation layer or selective treatment of sapphire surface in HNO3 solution. The latter process improved not only the quality of the sample but also the boundary between +c and –c GaN region, because the treat of substrate itself for controlling the polarity of GaN was carried out at lower temperature. The PL and Raman spectra of the simultaneous growth samples indicated larger amount of impurities in –c GaN side, which was consistent with our previous work [Sumiya et al. Appl. Phys. Lett. 76, 2098 (2000)].