bluuebunny/crossref_metadata_2025 huggingface dataset:
Structural and Ferroelectric Properties of Epitaxial PbZr0.52Ti0.48O3 and BaTiO3 Thin Films Prepared on SrRuO3/SrTiO3(100) Substrates
Authors: Contreras J. Rodríguez , Schubert J. , Poppe U. , Trithaveesak O. , Szot K. , Buchal Ch. , Kohlstedt H. , Waser R.
Published: April 2011

We have prepared single crystalline epitaxial PbZr0.52Ti0.48O3 (PZT) and BaTiO3 (BTO) thin films on single crystalline epitaxial SrRuO3 (SRO) thin films grown on SrTiO3 (100) (STO) substrates. PZT and SRO thin films were grown using high-pressure on-axis sputtering and BTO using pulsed laser deposition (PLD). The film thickness ranged between 12 to 165 nm. Their excellent structural properties, surface smoothness and interface sharpness were demonstrated by X-Ray Diffraction measurements (XRD), High Resolution Transmission Electron Microscopy (HRTEM) and Atomic Force Microscopy (AFM). Rutherford Backscattering Spectrometry and Channeling measurements (RBS/C) were used to analyze stoichiometry and crystalline quality. Ferroelectric hysteresis loops were obtained for all films of a thickness down to 12 nm showing a decrease in the remanent polarization Pr and an increase in the coercive field Ec towards thinner film thicknesses. Furthermore we have prepared tunneling junctions with a PZT or BTO barrier thickness of 3-6 nm. Reproducible bi-stable I-V-curves and bias dependence of the conductance were obtained suggesting an influence of the ferroelectric properties of the barrier material on the tunnel current.
Growth and Electrical Properties of Fe doped (Ba,Sr)TiO3 Thin Films Deposited by Pulsed Laser Deposition
Authors: Yonezawa Yoshiyuki , Kato Megumi , Konishi Yoshinori , Yoshida Shizuyasu , Okuda Nobuhiro , Maeda Takahiko , Tanuma Ryohei , Ohsawa Michio , Matsuyama Hideaki , Ogino Shinji , Fujishima Naoto , Matsuda Akinori , Furusyo Noboru , Chikyow Toyohiro , Kawasaki Masashi , Koinuma Hideomi
Published: April 2011

Impurity effects were investigated in (Ba,Sr)TiO3 (BST) systems in order to suppress leakage currents under relatively low oxygen pressure conditions by Pulsed Laser Deposition (PLD). We tried to dope transition metals, such as Mo, Mn, Cr, W and Fe into the BST target and used the targets to fabricate the films. By measuring electrical properties, we found Fe-doping had a significant effect on suppressing leakage current. Subsequently, we changed the amount of Fe doping from 0.1mol% to 6%. As a result, with post annealing, the sample with Fe:4% showed the lowest leakage current among those analyzed. Even without post annealing, the sample with Fe:6% showed the lowest leakage current. As for the dielectric constants, they decreased as the doping increased. At most, a 30% reduction was observed, compared with non-doped BST. XANES (X-ray Absorption Near Edge Structures) results indicated that the valency of the Fe ion was 3+ and located at the B-site of BST.
Erbium-Doped Barium Titanate Thin FilmWaveguides For Integrated Optical Amplifiers
Authors: Teren Andrew R. , Kim Seong-Soo , Ho Seng-Tiong , Wessels Bruce W.
Published: April 2011

The factors affecting optical gain were studied for Er-doped BaTiO3 thin film waveguides. Er-doped BaTiO3 with dopant concentrations of 0.3 – 9 at.% was deposited by metal-organic chemical vapor deposition. The luminescence efficiency was maximized by optimizing the growth temperature and erbium concentration as well as by post-deposition annealing. Stimulated emission was studied using the pump-probe technique over the spectral range of 1,520-1,550 nm. A maximum differential gain of 3 dB/cm was measured at 1,540 nm in an 8 mm long, 8 μm wide ridge waveguide.
Spin Polarization and Injection in ZnMnSe/ZnCdSe Heterostructures
Authors: Buyanova I.A. , Chen W.M. , Ivanov I.G. , Monemar B. , Toropov A.A. , Terent'ev Ya.V. , Sorokin S.V. , Lebedev A.V. , Ivanov S.V. , Kop'ev P.S.
Published: April 2011

Magneto-optical spectroscopy in combination with tunable laser excitation spectroscopy is employed to carry out a detailed study of spin alignment and spin injection in II-VI wide-bandgap semiconductor heterostructures, aiming at optimization of structural design for nano-scale spintronic applications. The use of tunable excitation is shown to provide a valuable opportunity to monitor separately spin relaxation and spin injection processes in the structures. Efficient spin alignment is achieved by using a diluted magnetic semiconductor (DMS) (a layer of ZnMnSe or a ZnMnSe/CdSe superlattice) as thin as 10 nm. The spin alignment efficiency is shown to depend critically on the ratio between the rates of spin relaxation and spin transport within the DMS layer. This allows the realization of spin alignment and spin switching functions by varying the structural design.
Spin Properties of Quantum wells Incorporating Semimagnetic Semiconductors
Authors: Malkova N. , Ekenberg U. , Thylen L.
Published: April 2011

The electronic band-edge spectrum of magnetic semiconductor quantumwells containing a diluted magnetic semiconductor as one of the constituents is studied within the envelope-function formalism. Quantum wells with normal and mutually inverted band arrangements are considered. The sp – d hydribization between the bare sp-electron states and the d-states of the Mn atoms is shown to lead to a spin-splitting effect. The spin-splitting effect is studied as a function of external magnetic field, well width, valence band offset and fraction of magnetic atoms. The results have bearing on the perspective for using the magnetic semicondutor structures in spin electronics.
Spin-Memory Loss in Metallic Multilayers
Authors: Pratt W. P. , Gu J.Y. , Bass J.
Published: April 2011

We review the results of measurements of spin-memory loss (spin-flipping) at cryogenic temperatures in sputtered or electrodeposited ferromagnetic and non-magnetic metals and alloys, and at metallic interfaces, using the current perpendicular to plane (CPP) geometry.
Transport in Metallic Multilayers with Both Specular and Diffuse Scattering at Interfaces
Authors: Shpiro Asya , levy Peter M. , Zhang Shufeng
Published: April 2011

By using a rather simple model potential we calculate the influence of specular and diffuse scattering at interfaces on the resistance for currents perpendicular to the plane of the layers (CPP) in multilayered metallic structures. We find that if one maintains the same interfaces between layers, their contribution to the total resistance of the system depends on the thickness of the intervening layers. Their contribution is constant only if the distance between interface is larger than the electron mean free path in the bulk of the layers. The error incurred by neglecting the dependence of the interface resistance on layer thickness depends on the amount of diffuse scattering at interfaces and the height of the potential step at the interface. For a Fe-Cr or Co-Cu multilayer consisting of three or five layers with realistic interfaces where both specular and diffuse scattering may occur, the error reaches 15-20%; this exceeds the experimental error. We suggest systems where this effect may be observed.
Circular Photogalvanic Effect in SiGe Semiconductor Quantum Wells
Authors: Ganichev Sergey D. , Kalz Franz-Peter , Rössler Ulrich , Prettl Wilhelm , Ivchenko Eugenius L. , Bel'kov Vasily V. , Neumann Robert , Brunner Karl , Abstreiter Gerhard
Published: April 2011

The photogalvanic effects, which require a system lacking inversion symmetry, become possible in SiGe based quantum well (QW) structures due to their built-in asymmetry. We report on observations of the circular and linear photogalvanic effects induced by infrared radiation in (001)-and (113)-orientedp–Si/Si1–xGex QW structures and analyse these observations in view of the possible symmetry of these structures. The circular photogalvanic effect arises due to optical spin orientation of free carriers in QWs with band splitting in k-space which results in a directed motion of free carriers in the plane of the QW. We discuss possible mechanisms that give rise to spin-splitting of the electronic subband states for different symmetries.
The use of NiMnSb Heusler alloy in MBE Grown Tunnel Junction
Authors: Turban Pascal , Andrieu Stéphane , Tagliaferri Alberto , Nadai Céline De , Brookes Nike
Published: April 2011

In this paper, the structural, magnetic and electronic properties of single-crystalline NiMnSb thin films grown by molecular beam epitaxy are presented. The growth process and surface morphology were studied by RHEED and STM. The structural quality of the films was investigated by EXAFS and HRTEM. For the optimized growth conditions, the expected NiMnSb C1b structure was obtained. Moreover, the most stable surface was found to be 4x1 reconstructed. The magnetic and electronic properties were studied by X-ray magnetic circular dichroïsm and spin-resolved x-ray photoemission spectroscopy experiments. The magnetization was observed to be essentially due to Mn, in agreement with theory. The surface was not found to be fully polarized but a polarization near 50% at room temperature was observed. Finally, the preparation of fully epitaxial NiMnSb/MgO/NiMnSb(001) trilayers is presented.
Photo-Assisted MOVPE Growth of ZnMgS on (100) Si
Authors: Rodriguez Angel , Shattuck Jeremy , Zhang Xiaoguang , Li Peng , Parent David , Ayers John , Jain Faquir
Published: April 2011

This paper presents for the first time photo-assisted Metalorganic vapor phase epitaxial (MOVPE) growth of ZnMgS on Si (100) substrates. The growth was done using dimethylzinc (DMZn), bismethylcyclo-pentadienyl-magnesium ((MeCP)2Mg), and diethylsulfhide (DES) as zinc, magnesium, and sulfur precursors. Epitaxial characterization by X-ray Photoelectron Spectroscopy (XPS), and low - angle X-ray Diffraction (XRD) results are presented. Mg solid phase incorporation is estimated to vary from 0 to 60 percent. The epitaxial nature of the ZnMgS layers has been verified using the low-angle X-ray diffraction eliminating any interference from the Si substrate. It can be shown with this technique that the change in the ZnMgS peak position changes from 27.35 degrees to 26.5 degrees with an increase in Mg incorporation, compared with a Si control sample peak at 27.4 degrees. XRD results obtained have been verified with XPS data. Chlorine doping of the ZnMgS layer was also studied. Concentrations up to 3 × 1015 cm−3 were observed in the ZnMgS layer. Results of the n (ZnMgS:Cl) – p (Si) diodes fabricated are also presented.
Optical Vibrational and Structural Properties of Ge1−xSn x alloys by UHV-CVD
Authors: Taraci Jennifer , Zollner S. , McCartney M. R. , Menendez Jose , Smith D. J. , Tolle John , Bauer M. , Duda Erika , Edwards N. V. , Kouvetakis J.
Published: April 2011

UHV-CVD growth based on a deuterium stabilized Sn hydride and digermane produces Ge-Sn alloys with tunable bandgaps. The Ge1−xSnx (x=2–20 at.%) alloys are deposited on Si (100) and exhibit superior crystallinity and thermal stability compared with MBE grown films. Composition, crystal and electronic structure, and optical and vibrational properties are characterized by RBS, low energy SIMS, high resolution electron microscopy TEM, x-ray diffraction, as well as Raman and IR spectroscopies. TEM studies reveal epitaxial films with lattice constants between those of Ge and Sn. X-ray diffraction shows well-defined (004) peaks and rocking curves indicate a tightly aligned spread of the crystal mosaics. Resonance Raman indicate a E1 bandgap reduction relative to Ge, consistent with a decrease of the E2 critical point observed in spectroscopic ellipsometry. IR transmission spectra indicate an increase in absorption with increasing Sn content consistent with a decrease of the direct bandgap.
Near-field photoluminescence spectroscopy of localized states in InGaAsN alloys
Authors: Mintairov A. M. , Blagnov P. A. , Kosel T. , Merz J. L. , Ustinov V. M. , Vlasov A. S. , Cook. R. E.
Published: April 2011

We used near-field magneto-photoluminescence scanning microscopy to study structural and optical properties of quantum-dot-like compositional fluctuations in GaAsN and InGaAsN alloys. We show that these fluctuations manifest themselves by the appearance of narrow emission lines (halfwidth 0.5−2 meV) at temperatures below 70K. We estimated the size, density, and nitrogen excess of individual compositional fluctuations (clusters), revealing phaseseparation effects in the distribution of nitrogen in GaAsN and InGaAsN. We found a dramatic difference in the Zeeman splitting of cluster lines between GaAsN and InGaAsN, indicating a strong effect of In on the exciton g-factor.
Quantum dots of InAs/GaSb type II superlattice for infrared sensing
Authors: Razeghi M. , Wei Y. , Gin A. , Brown G. J.
Published: April 2011

Throughout the past years, significant progress has been made in Type II (InAs/GaSb) photovoltaic detectors in both LWIR and VLWIR ranges. BLIP performance at 60K for 16μm photovoltaic type II detectors has been successfully demonstrated for the first time. The detectors had a 50% cut-off wavelength of 18.8 μm and a peak current responsivity of 4 A/W at 80K. A peak detectivity of 4.5×1010cm·Hz1/2/W was achieved at 80K at a reverse bias of 110mV. Detectors of cutoff wavelength up to 25μm have been demonstrated at 77K. The great performance of single element detectors appeals us to lower dimensional structures for both higher temperature performance and possible wavelength tunability. Simple calculations show that quantum effects will become significant when the lateral confinement is within tens of nanometers. The variation of applied gate voltage will move the electron and hole energy levels unevenly. The cutoff wavelength of the superlattice will vary accordingly. Auger recombination will also decrease and higher temperature operation becomes possible. In this talk, the latest results will be discussed.
Electroluminescence and Spectral Shift of CdS Nanoparticles on Si Wafer
Authors: Liang Eih-Zhe , Lin Ching-Fuh , Shih Sheng-Ming , Su Wei-Fang
Published: April 2011

Preparation of CdS nanoparticles, devices fabrication, and electroluminescence properties at room temperature of CdS nanoparticles on silicon substrates are reported. A spectral shift of 86-meV of free exciton transition was observed that was due to the passivation of p-hydroxyl thiophenol molecules around nanoparticles. Controlled process conditions such as heat treatment and/or with oxygen-rich environment are experimented and found to have significant influences on emission spectra. Radiative recombination corresponding to oxygen-impurity level, 273 meV below bandgap energy, presents in samples prepared in oxygen-rich environment. In addition to such mechanism, coalescence of nanoparticles into bulk form also exists and contributes to enhanced luminescence.
InAs quantum dots in AlAs/GaAs short period superlattices: structure, optical characteristics and laser diodes
Authors: Tokranov Vadim , Yakimov M. , Katsnelson A. , Dovidenko K. , Todt R. , Oktyabrsky S.
Published: April 2011

The influence of two monolayer - thick AlAs under- and overlayers on the formation and properties of self-assembled InAs quantum dots (QDs) has been studied using transmission electron microscopy (TEM) and photoluminescence (PL). Single sheets of InAs QDs were grown inside a 2ML/8ML AlAs/GaAs short-period superlattice with various combinations of under- and overlayers. It was found that 2.4ML InAs QDs with GaAs underlayer and 2ML AlAs overlayer exhibited the lowest QD surface density of 4.2×1010 cm-2 and the largest QD lateral size of about 19 nm as compared to the other combinations of cladding layers. This InAs QD ensemble has also shown the highest room temperature PL intensity with a peak at 1210 nm and the narrowest linewidth, 34 meV. Fabricated edge-emitting lasers using triple layers of InAs QDs with AlAs overlayer demonstrated 120 A/cm2 threshold current density and 1230 nm emission wavelength at room temperature. Excited state QD lasers have shown high thermal stability of threshold current up to 130°C.
Radiative recombination processes of thermal donors in silicon
Authors: Pizzini S. , Binetti S. , Leoni E. , Donne A. Le , Acciarri M. , Castaldini A.
Published: April 2011

There is a recent, renewed attention on the possible development of optical emitters compatible with silicon microelectronic technology and it has been recently shown that light emitting diodes could be manufactured on dislocated silicon, where dislocations were generated by plastic deformation or ion implantation. Among other potential sources of room temperature light emission, compatible with standard silicon-based ULSI technology, we have studied old thermal donors (OTD), as the origin of their luminescence is still matter of controversy and demands further investigation.

In this work we discuss the results of a spectroscopical study of OTD using photoluminescence (PL) and Deep Level Transient Spectroscopy (DLTS) on standard Czochralsky (Cz) silicon samples and on carbon-doped samples.

We were able to show that their main optical activity, which consists of a narrow band at 0.767 eV ( P line), is correlated to a transition from a shallow donor level of OTD to a deep level at EV+0.37 eV which is tentatively associated to C-O complexes. As we have shown that the P line emission persists at room temperature, we discuss about its potentialities to silicon in optoelectronic applications.
Field emission enhancement of DLC films using triple-junction type emission structure
Authors: Paik Namwoong , Martin Michael , Kim Daeil , Kim Sungjin , Kim Steven , Song Kie Moon
Published: April 2011

Negative Electron Affinity (NEA) of Diamond-like-Carbon (DLC) films has made DLC films a favorable candidate for field emission display (FED). It was suggested that triple-junction type structure could enhance the field emission characteristics. A triple junction is defined as the intersection of a semiconductor surface with a metal substrate in vacuum. In this study, field emission enhancement in triple junction type structures was investigated. As a metal substrate 5000 of Mo films were deposited. Then, 3000–4000 of DLC film was deposited as a semiconductor material. Thin film layers were made using a negative ion beam source. After the deposition, using an excimer laser, we removed the DLC layer and made circular shaped triple junction trenches with a diameter of 25–250 μm. The field emission characteristics such as I–V characteristics turn on voltage and emission lifetime data were obtained for a diode type field emission measurement system. Overall results show significantly enhanced performance of field emission characteristics such as uniform emission over patterned area, reduced turn on voltages and longer lifetimes can be achieved.
Depth Profiling of SiC Lattice Damage Using Micro-Raman Spectroscopy
Authors: Muntele Iulia C. , Ila Daryush , Muntele Claudiu I. , Poker David B. , Hensley Dale K.
Published: April 2011

Depth profiling for the amount of lattice damage using a Confocal Micro-Raman (CMR) spectrometer is demonstrated in this paper. Samples of n-type silicon carbide were implanted with 2 MeV He and O ions at both room temperature and 500 °C, and fluences between 1015 and 1017 ions/cm2. Post-implantation annealing at 1000 °C was also performed in order to study the damage evolution. Optical Absorption Spectrophotometry (OAS) was used for establishing the opacity (and therefore the probing depth) of the damaged layer to the 632.8 nm wavelength of the He-Ne laser used for CMR throughout this study. The methodology used and the results obtained are presented herein. Total dissipation of amorphous carbon islands was observed even at low annealing temperatures of the RT implanted samples, along with an increase in the size of the amorphous silicon islands.
High-Performance InAs/GaAs Quantum Dots Infrared Photodetector With/Without Al0.2Ga0.8As Blocking Layers
Authors: Ye Zhengmao , Campbell Joe C. , Chen Zhonghui , Baklenov O. , Kim E. T. , Mukhametzhanov I. , Tie J. , Madhukar A.
Published: April 2011

InAs/AlGaAs quantum dot infrared photodetectors based on bound-to-bound intraband transitions in undoped InAs quantum dots are reported. AlGaAs blocking layers were employed to achieve low dark current. The photoresponse peaked at 6.2 μm. At 77 K and –0.7 V bias the responsivity was 14 mA/W and the detectivtiy, D*, was 1010 cmHz1/2/W.
Effects of Ion Bombarding and Nitrogenation on the Properties of Photovoltaic a-CNx Thin Films
Authors: Zhou Z. B. , Cui R. Q. , Hadi G. M. , Pang Q. J. , Jin C. Y. , Ding Z. M.
Published: April 2011

Amorphous carbon nitride films (a-CNx) were synthesized by using single ion beam sputtering of a graphite target in argon and nitrogen sputtering gases. This thin film could be used as a novel photovoltaic material. The films were characterized with the technique of laser Raman, spectroscopic ellipsometry and electron spin resonance spectrometer (ESR). In this paper we report the effects of ion impacting and nitrogenation on the microstructure, density of defect states, bonding character, optical and photovoltaic properties. Effective decreasing of intensity of the ESR signal and formation of C-N bonding were observed, which could be attributed to the increment of the impinging ions on the growing films. The nitrogenation of a-CNx films could decrease the Tauc optical gap (0.62∼0.86eV) and the intensity of ESR signal, increase photon absorption coefficient of the films (106∼104cm−1).

The primary photovoltaic values of the devices having Schottky structure of ITO/CNx/Al are Isc 1.56 μ A/cm2 and Voc 250 mV, respectively, when exposed to AM1.5 illumination ( 100mW/cm2, 25°C).
Third Order Mode Optically Pumped Semiconductor Laser for an Integrated Twin Photon Source in Quantum Optics
Authors: Semaltianos N. G. , De Rossi A. , Berger V. , Vinter B. , Chirlias E. , Ortiz V.
Published: April 2011

Lasing action on a third order waveguide mode is demonstrated at room temperature under optical pumping, in a specifically designed quantum well laser structure. The AlGaAs heterostructure involves barriers which ensure that the third order mode has a higher overlap with the single quantum well emitter than the fundamental mode. Third order mode operation of a laser structure opens the way to modal phase matched parametric down conversion inside the semiconductor laser itself. It is a first step towards the realization of semiconductor twin photon laser sources, needed for quantum information experiments.
Preparation of CdS/ZnO Core/shell Structured Nanoparticles by Hydrothermal Method
Authors: Yan Chunhua , Sun Lingdong , Fu Xuefeng , Liao Chunsheng
Published: April 2011

CdS and ZnO capped CdS (CdS/ZnO) semiconductor nanoparticles were synthesized via hydrothermal method by thermal decomposition of the cysteine-cadmium and Zn(OH)42− complex precursors. Both of the photoluminescence properties and structure characterization confirmed the core/shell structure as expected. Compared to CdS nanoparticles, the band-gap emission of CdS/ZnO was greatly improved, that means the capping layer of ZnO modified the surface of CdS and reduced the surface defects effectively. ED and XRD confirmed the formation of hexagon phased CdS and the TEM image indicated the size of CdS/ZnO was about 20 nm.
Oxidation Kinetics and Microstructure of Wet-Oxidized MBE-Grown Short-Period AlGaAs Superlattices
Authors: Todt René , Dovidenko Katharine , Katsnelson Alexei , Tokranov Vadim , Yakimov Michael , Oktyabrsky Serge
Published: April 2011

The kinetics of the wet oxidation process of MBE-grown high-Al-content AlAs/Al0.6Ga0.4As short-period superlattices (SPSLs) was investigated and compared to AlGaAs alloys and pure AlAs. We found that alloys and superlattices (SLs) have different oxidation characteristics. These differences were attributed to traces of the superlattice structure in the oxidized material. The microstructure and chemistry of SPSLs with an equivalent composition of Al0.98Ga0.02As was studied, using transmission electron microscopy, energy-dispersive x-ray spectroscopy, Rutherford backscattering, and nuclear reaction analysis for hydrogen-profiling. We also report on the mechanical stability of oxidized SPSL layers in optoelectronic device structures.
Atomistic Modeling of III-V Semiconductors: Thermodynamic Equilibrium and Growth Kinetics
Authors: Grosse Frank , Carter-Barvosa William , Zinck Jennifer J. , Gyure Mark F.
Published: April 2011

Growth kinetics and thermodynamic equilibrium can both be determining factors at different stages of III-V semiconductor heteroepitaxy. We study their interplay, employing kinetic Monte Carlo simulations for the InAs(001) surface. The simulation contains atomistic details of both species, including the stability of different reconstructions and their kinetics. The behavior of the surface in thermodynamic equilibrium, including different reconstructions, is determined exclusively by extensive total energy calculations employing ab initio density functional theory. The continuous phase transition between the α?(2×4) and β?(2×4), predicted by theory, is confirmed by experiment. At full layer coverage, a recovery of the stable reconstruction is observed. The different time scales associated with As2 and In are discussed with respect to equilibrium and kinetics.
Optical properties of GaN with Ga and N polarity
Authors: Reshchikov M. A. , Huang D. , Yun F. , Visconti P. , King T. , Morkoç H. , Jasinski J. , Liliental-Weber Z.
Published: April 2011

We compared photoluminescence (PL) and cross-sectional transmission electron microscopy (TEM) characteristics of GaN samples with Ga and N polarities grown by molecular beam epitaxy (MBE) on sapphire substrates. Ga-polar films grown at low temperature typically have very smooth surfaces, which are extremely difficult to etch with acids or bases. In contrast, the N-polar films have rougher surfaces and can be easily etched in hot H3PO4 or KOH. The quality of the X-ray diffraction spectra is also much better in case of Ga-polar films. Surprisingly, PL efficiency is always much higher in the N-polar GaN, yet the features and shape of the PL spectra are comparable for both polarities. We concluded that, despite the excellent quality of the surface, MBE-grown Ga-polar GaN layers contain higher concentration of nonradiative defects. From the analyses of cross-sectional TEM investigations, we have found that Ga-polar films have high density of threading dislocations (5x109 cm-2) and low density of inversion domains (1x107 cm-2). For N-polar GaN the situation is the reverse: the density of dislocations and inversion domains are 5x108 and ~1x1011 cm-2, respectively. One of the important conclusions derived from the combined PL and TEM study is that inversion domains do not seem to affect the radiative efficiency very adversly, whereas dislocations reduce it significantly.
Growth of High Al Concentration AlGaN for Solar Blind Photodetector Applications
Authors: Guo Shiping , Pophristic Milan , Ferguson Ian , Peres Boris , Lamarre Phil , Tobin Steve , Wong Kwok , Reine Marion , Sood Ashok
Published: April 2011

The development of a metalorganic chemical vapor deposition growth process for AlxGa1-xN materials with high aluminum composition (x∼0.40-0.60) on sapphire substrates is reported. Room temperature Hall measurements of Si-doped AlGaN epilayers with x∼0.40 show a narrow window for efficient doping with a carrier concentration of ∼1.5x1018 cm-3 and a mobility of ∼35 cm2/V-s obtained under optimum growth conditions. AlxGa1-xN-based solar-blind p-i-n device structures were grown and diode I-V curves were obtained with a high R0A of <2.9×1010 ohm-cm2. Secondary ion mass spectroscopy measurements show a sharp transition between regions of high concentration of Mg (2×1020 cm-3) and Si (1×1019 cm-3) in p-GaN and n-AlGaN layers, respectively.
Fast Proton Damage Effects on the Luminescence Properties of High-Quality GaN
Authors: Yang Qing , Feick Henning , Armitage Rob , Weber Eicke R.
Published: April 2011

High-quality GaN layers grown by hydride-vapor phase epitaxy (HVPE) and homoepitaxial layers grown thereon by molecular beam epitaxy (MBE) are studied after fast proton irradiation. A radiation-induced decrease of the band edge luminescence is observed. Time-resolved photoluminescence (PL) reveals a significant reduction of the carrier lifetime. A lifetime degradation constant of Kt = 4×10-15cm2/ns is reported for very thick HVPE GaN, and large variations are observed for the different layers. Partial recovery of the carrier lifetime and PL intensity with annealing is observed for thin HVPE GaN but not for the MBE overgrowth layer.
Effects of the Schottky electrode structure in GaN based UV-VUV (50-360 nm) photodetector
Authors: Motogaito Atsushi , Ohta Keiichi , Hiramatsu Kazumasa , Ohuchi Youichiro , Tadatomo Kazuyuki , Hamamura Yutaka , Fukui Kazutoshi
Published: April 2011

Characterization of Schottky barrier UV detectors with a comb-shaped electrode and a transparent electrode for VUV region using synchrotron radiation was carried out. These Schottky type detectors are effective to operate in UV and VUV light (50360 nm). In particular, the latter realizes high responsivity (0.01 A/W for VUV light) and no photoemission of Au and GaN. The penetration depth for VUV light was estimated to 0.01 m. So the VUV light was absorbed in near the surface of i-GaN layer or the interface of Au and i-GaN layer.
Electrical Characterization of GaN Metal Oxide Semiconductor Diode using Sc2O3 as the Gate Oxide
Authors: Mehandru R. , Gila B.P. , Kim J. , Johnson J.W. , Lee K.P. , Luo B. , Onstine A.H. , Abernathy C. R. , Pearton S.J. , Ren F.
Published: April 2011

GaN metal oxide semiconductor diodes were demonstrated utilizing Sc2O3 as the gate oxide. Sc2O3 was grown at 100°C on MOCVD grown n-GaN layers in a molecular beam epitaxy (MBE) system, using a scandium elemental source and an Electron Cyclotron Resonance (ECR) oxygen plasma. Ar/Cl2 based discharges was used to remove Sc2O3, in order to expose the underlying n-GaN for ohmic metal deposition in an Inductively Coupled Plasma system. Electron beam deposited Ti/Al/Pt/Au and Pt/Au were utilized as ohmic and gate metallizations, respectively. An interface trap density of 5 × 1011 eV-1cm-2was obtained with the Terman method. Conductance-voltage measurements were also used to estimate the interface trap density and a slightly higher number was obtained as compared to the Terman method. Results of capacitance measurements at elevated temperature (up to 300°C) indicated the presence of deep states near the interface.
Transparent ZnO-Based Ohmic Contact to p-GaN
Authors: Kaminska E. , Piotrowska A. , Golaszewska K. , Guziewicz M. , Kruszka R. , Kudla A. , Ochalski T. , Barcz A. , Dietl T. , Matsukura F. , Sawicki M. , Wawro A. , Zielinski M. , Jasinski J. , Liliental-Weber Z.
Published: April 2011

Highly conductive ZnO films were fabricated on p-GaN in a two-step process. First, zinc was thermally evaporated on p-GaN. Next, zinc film was oxidized in oxygen flow. To increase the conductivity of ZnO, nitrogen was introduced into zinc during its deposition. The above procedure proved successful in fabricating ZnO of the resistivity of ~1x10-3Ωcm and resulted in ohmic contacts of resistivity ~1x10-2Ωcm2 to low-doped p-GaN, and light transmittance of ~75% in the wavelength range of 400-700 nm.
Photoluminescence of Zn-doped GaN grown by HVPE
Authors: Reshchikov M. A. , Huang D. , Morkoç H. , Molnar R. J.
Published: April 2011

We investigated the behavior of photoluminescence (PL) in a wide range of temperatures and excitation intensities from Zn-doped GaN grown on c-plane of sapphire by hydride vapor phase epitaxy (HVPE). The low-temperature PL spectrum exhibited sharp peaks in the excitonic part with the full width at half maximum (FWHM) of about 5 meV. The peaks were attributed to free excitons and excitons bound to shallow donors and to Zn-related acceptors. A blue band with a maximum at 2.9 eV and FWHM of 0.33 eV dominated in the PL spectrum at low temperatures. We observed a significant UV shift (about 50 meV) of this band when the excitation density was increased from 10-6 to 100 W/cm2 or when the temperature of the sample was increased from 15 to 300 K. With increasing temperature, the blue band intensity quenched by 104 times in the range 180 - 250 K with the activation energy of about 0.64 eV. When the sample was illuminated from the substrate side, the blue band intensity dropped only 30 times in the temperature range of 180 - 340 K, and the activation energy did not exceed 0.25 eV. The PL spectrum of the GaN:Zn samples under investigation also contained broad red and green bands with maxima at about 1.8 and 2.4 eV, respectively. In PL from the substrate side, the blue and yellow (2.2 eV) luminescence dominated in the defect-related region of PL spectrum. Time-resolved PL measurements revealed nonexponential decay of the defect-related bands.
Raman Studies on Oxygen Doped GaN Grown by Molecular Beam Epitaxy
Authors: Papadimitriou D. , Ptak A.J. , Korakakis D. , Giles N.C. , Myers T.H.
Published: April 2011

High quality Ga-polarity GaN films were grown by plasma-assisted molecular beam epitaxy to study strain effects due to oxygen incorporation. Oxygen concentrations up to 2 × 1022 cm-3 were studied. Layers containing oxygen at levels above 1022 cm-3 exhibit severe cracking while oxygen concentrations less than 1021 cm-3 apparently does not introduce significant strain. Raman spectra of O2-doped films were evaluated with respect to spectra of unintentionally doped GaN films (n=4x1014 cm-3) grown under the same conditions except for the O2-flux. Analysis of the E2 (high frequency phonon mode near 570 cm-1) Raman band indicated the heaviest doped samples exhibit compressive strain. Frequency-shifts between 0.7 and 0.9 cm-1 were observed as a function of the distance from the crack edges in the heavily doped samples, implying a strain (da/a) of the order of 5x10-4.
Optical Properties of Carbon Doped Cubic GaN Epilayers Grown on GaAs (001) Substrate by Molecular Beam Epitaxy
Authors: As D. J. , Köhler U. , Lischka K.
Published: April 2011

The optical properties of Carbon doped cubic GaN epilayers have been investigated by temperature and intensity dependent photoluminescence measurements. RF-plasma assisted molecular beam epitaxy equipped with an e-beam-evaporation source for carbon doping is used to grow the cubic GaN layers on GaAs (001) substrates. With increasing Carbon flux a new photoluminescence line at 3.08 eV appeared at 2K. This line is attributed to a donor acceptor transistion, which involves the shallow CN acceptor. From the spectral position the binding energy of the C acceptor is estimated to be about EC = 0.215 eV. Our experiments demonstrate that C indeed introduces a shallow acceptor in cubic GaN with an acceptor binding energy, which is about 15 meV lower than that observed for the Mg acceptor in cubic GaN. However, at high C fluxes a deep red luminescence band appeared at 2.1 eV, indicating compensation effects.
Vibrational Spectroscopy of GaN:Mg Under Pressure
Authors: McCluskey M.D. , Zhuravlev K.K. , Kneissl M. , Wong W. , Treat D. , Limpijumnong S. , Walle C.G. Van de , Johnson N.M.
Published: April 2011

The microscopic structure of Mg-H complexes in GaN has been a subject of intense theoretical and experimental investigation. In order to probe the Mg-H structure, we have studied the effect of hydrostatic pressure on the local vibrational mode (LVM) frequency. At ambient pressure, the LVM frequency is 3125 cm-1, which corresponds to a N-H stretching mode. In this study, Fourier-transform spectroscopy was performed on free-standing GaN:Mg,H samples in a diamond-anvil cell, with nitrogen as a pressure-transmitting fluid. The samples had been removed from their sapphire substrate by the laser-liftoff technique. The LVM frequency was measured, at liquid helium temperatures, for pressures ranging from 0 to 5 GPa. The pressure dependence of the frequency is nonlinear: first it decreases with pressure, then it increases. Comparison with first-principles calculations allows us to derive information about the microscopic structure of the Mg-H complex. The calculated stable configuration indeed gives rise to a frequency shift consistent with experiment. Based on the comparison between theory and experiment, we can exclude the bond-center configuration, which would result in a much larger pressure derivative than experimentally observed.
Band-gap engineering in amorphous/microcrystalline ScxGa1-xN.
Authors: Little M. E. , Kordesch M. E.
Published: April 2011

Reactive sputtering was used to grow thin films of ScxGa1-xN with scandium concentrations of 20%-70% on quartz substrates at temperatures of 300-675 K. X-ray diffraction (XRD) of the films showed either weak or no structure, suggesting the films are amorphous or microcrystalline. Optical absorption spectra were taken of each sample and the optical band gap was determined. The band gap varied linearly with increasing Ga concentration between 2.0 and 3.5 eV. Ellipsometry was used to confirm the band gap measurements and provide optical constants in the range 250-1200 nm. ScN and GaN have different crystal structures (rocksalt and wurzite, respectively), and thus may form a heterogeneous mixture as opposed to an alloy. Since the XRD data were inconclusive, bilayers of ScN/GaN were grown and optical absorption spectra taken. A fundamental difference in the spectra between the bilayer films and alloy films was seen, suggesting the films are alloys, not herterogeneous mixtures.
Growth of Gallium Nitride Textured Films and Nanowires on Polycrystalline Substrates at sub-Atmospheric Pressures
Authors: Chandrasekaran Hari , Sunkara Mahendra K.
Published: April 2011

Textured gallium nitride (GaN) films were grown on polished, polycrystalline and amorphous substrates in sub-atmospheric pressures, by direct nitradation of a thin molten gallium films using electron cyclotron resonance (ECR) microwave nitrogen plasma. C-plane texturing was achieved, independent of the substrate crystallinity. Single crystal quality GaN nanowires with diameters ranging from 40-50 nm were also synthesized using direct nitridation of thin gallium films with nitrogen plasma. Scanning electron microscopy (SEM), X-ray Diffraction (XRD), Energy Dispersive Spectroscopy (EDS) and Cross-sectional transmission electron microscopy (CS-TEM), high-resolution TEM (HRTEM) and Micro-Raman spectroscopy were used to characterize the synthesized gallium nitride films and GaN nanowires.
Layer Thickness Dependence of Strain in GaN grown by HVPE
Authors: Sim Gyu Gwang , Yu P. W. , Reynolds D. C. , Look D. C. , Kim Sang Soo , Noh D.Y.
Published: April 2011

Wurzite GaN epilayers on sapphire substrates usually suffer from biaxial compressive strain due to the mismatches of the thermal expansion coefficients and the lattice constants between GaN layers and sapphire substrates. We have investigated the layer thickness effects on strain and transition energies by photoluminescence (PL), photoreflectance (PR) and X-ray diffraction (XRD). Samples used in this study are grown by hydride vapor phase epitaxy (HVPE) and have the layer thickness of 0.76, 2.6, 5.3 and 48 m. The PL and PR spectra showed the redshift of the transition energies with increasing layer thickness. This is attributed to strain-induced energy shift. The layer thickness dependence of strains is directly observed by XRD. The strain along the c -axis (εzz) decreased with increasing layer thickness. This indicates the strain is relaxed with layer thickness. From strain variation with layer thickness, we suggest that strain relaxation process is rapid at the initial stage of growth and becomes slower as the layer grows. The full width at half maximum (FWHM) of PL spectra and theta rocking curves decrease with increasing layer thickness. This indicates the crystal quality improves as the strain is reduced. Since the strain effect is very small at the layer thickness of 48 μm, we expect zero strain for thicker layers that can potentially be used as substrates for homoepitaxy.
Optical and Structural Properties of Mn-Implanted GaN Films
Authors: Xu J. , Li J. , Zhang R. , Xiu X.Q. , Lu D.Q. , Gu S.L. , Shen B. , Shi Y. , Zheng Y. D.
Published: April 2011

The optical and structural properties of Mn-implanted GaN films have been investigated. The films studied were grown by metal organic chemical vapor deposition (MOCVD), with Mn implanted in 150 KeV, which can offer many distinguished advantages compared with other doping methods. A new energy band with a minimum at 2.9 eV in the reflectance spectra has been observed. The yellow band emission was greatly decreased according to the result of photoluminescence. The structure analysis revealed that the Mn doped sample has good crystal quality as the pure GaN after annealed. And further discussions on the role of Mn in GaN: Mn films have been presented.
New buffer layer technique using underlying epitaxial AlN films for high-quality GaN growth
Authors: Shibata Tomohiko , Kida Yoshihiro , Miyake Hideto , Hiramatsu Kazumasa , Asai Keiichiro , Nagai Teruyo , Sumiya Shigeaki , Tanaka Mitsuhiro , Oda Osamu
Published: April 2011

We demonstrate high-quality epitaxial AlN films on C-plane sapphire as a new buffer layer technique for the growth of high-quality GaN. The obtained GaN films were atomically flat and the full width at half maximum (FWHM) values of the X-ray rocking curve (XRC) were 120arcsec and 510arcsec for (0004) and (10-10), respectively. XRC-FWHM values of the underlying AlN film were 90arcsec and 1800arcsec for (0004) and (10-10), respectively. The XRC results of the GaN indicated that tilted mosaicity of the GaN was much smaller than that of conventional GaN and that small tilted mosaicity coincided with small twisted mosaicity. It was characteristic that the twisted mosaicity drastically reduced from the AlN to the GaN. According to TEM results, it was clarified that the high-quality AlN layer had an effect in reducing the amount of screw-type threading dislocations and that the GaN/AlN interface played an important role in reducing the amount of edge-type dislocations.
Lateral Growth of InN on GaN/Sapphire
Authors: Yang Fuh-Hsiang , Hwang Jih-Hsien , Chen Kuei-Hsien , Yang Ying-Jay
Published: April 2011

The lateral growth of high quality InN on the stripe-pattern GaN/sapphire substrate with an OMVPE system was studied. The surface morphology and structural properties were investigated. Epitaxial films were achieved due to the greatly reduced strain by lateral growth. Two kinds of growth mode were observed due to different growth conditions of V/III ratios. X-ray rocking curve with FWHM of 700 arcsec shows the good quality of the film and E2 mode of Raman spectrum with FWHM of 3.5 cm-1 is among the best results ever reported in the literature.
Fabrication of p-n junction with Mg-doped wide bandgap InAlGaN for application to UV emitters
Authors: Hirayama H. , Yamanaka T. , Kinoshita A. , Hiraoka K. , Hirata A. , Aoyagi Y.
Published: April 2011

Mg-doped quaternary InAlGaN is very attractive for use as p-side layers of 300-nm band ultraviolet (UV) light-emitting diodes (LEDs) or laser diodes (LDs), because high hole conductivity is expected to obtain for wide bandgap (~4 eV) InAlGaN with Mg-doping. We fabricated p-n junction diode consisting of Mg-doped In0.02Al0.28Ga0.70N and Si-doped Al0.25Ga0.75N, and demonstrated intense UV emission under CW current injection at room temperature. The rising voltage in I-V curve was around 3.8 V and the breakdown voltage was as high as 10 V. Single peaked intense emission was observed at 340 nm from around InAlGaN/AlGaN p-n junction area without any deep level emission. Also we found that Ni/Au electrode directly fabricated on Mg-doped InAlGaN is useful. From these results, Mg-doped InAlGaN is considered to be very attractive for use as p-side layer of UV-LEDs or LDs.
Quaternary AlInGaN MQWs for Ultraviolet LEDs
Authors: Zhang J. P. , Yang J. W. , Adivarahan V. , Wang H. M. , Fareed Q. , Kuokstis E. , Chitnis A. , Shatalov M. , Simin G. , Khan M. Asif , Gaska R. , Shur M. S.
Published: April 2011

We report a pulsed atomic layer epitaxy (PALE) growth technique for quaternary AlInGaN films for ultraviolet optoelectronic applications. Using the PALE approach high quality quaternary AlInGaN/AlInGaN multiple quantum wells (MQWs) were successfully grown over sapphire substrates. From X-ray diffraction, atomic force microscopy, and photoluminescence study, a high structural and optical quality was established for the AlInGaN MQWs. Incorporating the PALE grown quaternary MQWs as the active layer of light emitting diode (LED) on sapphire or SiC substrates we also demonstrated room temperature deep ultraviolet electroluminescence under dc and pulsed electrical pumping. The peak emission wavelength can be tuned from 305 nm to 340 nm with spectrum FWHM of about 20 nm by varying the alloy compositions of the quaternary AlInGaN active layers using PALE. Comparative study of LEDs over sapphire and SiC substrates was also done in order to determine the influence of epilayer design on the performance parameters and the role of the substrate absorption.
Raman Studies of GaNP Alloy
Authors: Buyanova I. A. , Chen W. M. , Goldys E. M. , Xin H. P. , Tu C. W.
Published: April 2011

Raman scattering (RS) spectroscopy is employed to characterize the effect of nitrogen on structural properties of GaNxP1-x alloy with nitrogen composition up to 3 %. Two-mode behavior of the alloy is clearly shown. The frequency of the GaP-like LO phonons is found to decrease with N composition as –1.13 cm-1x. This dependence is proposed to be largely due to the biaxial strain in the GaNP epilayers, as a result of lattice mismatch to the GaP substrate. The frequency of the GaN-like phonons is found to be more sensitive to nitrogen content, increasing with the rate of +2.6 cm-1x. The addition of nitrogen is also found to cause a dramatic quenching of the two-phonon Raman scattering and an appearance of the zone edge GaP-like vibrations. These effects are suggested to reflect local distortion in the GaNP lattice induced by nitrogen, as well as possible clustering of N atoms.
Influence of growth temperature on emission efficiency of InGaN/GaN multiple quantum wells
Authors: Chen Fei , Cartwright A. N. , Sweeney Paul M. , Cheung M. C. , Flynn Jeffrey S. , Keogh David
Published: April 2011

A comparative study, using time-resolved and CW photoluminescence spectroscopy, of MOVPE grown InGaN/GaN multiple quantum wells deposited on HVPE GaN/Sapphire at different growth temperatures was undertaken. It was found that the PL linewidth increased and the peak emission energy decreased as the growth temperature was reduced. Moreover, the sample grown at an intermediate growth temperature exhibited total integrated luminescence intensity much greater than the samples grown at higher or lower growth temperatures. A phenomenological carrier recombination dynamics model based on the competition of quantum well-like radative recombination, spatially localized radiative recombination in potential minima and non-radiative recombination through defects is presented to provide an explanation of the observed emission dynamics and efficiency. In this model, the emission efficiency is determined by the relative area of defects and the number density of localized states in the potential minima, both of which are influenced by the growth temperature. Furthermore, the photon energy dependent lifetimes are well fitted with this model by assuming a Gaussian shape localized states distribution. The localized potential minima are consistent with nanoscale indium rich regions due to indium aggregation.