bluuebunny/crossref_metadata_2025 huggingface dataset:
Resonant Energy Transfer due to Exciton Coupling in Hybrid Persovskites Conjugated to GaN Semiconductors
Authors: Li Jianyou , Neogi Arup , Ishihara Teruya
Published: April 2011

Exciton-Exciton coupling in hybrid persovskites conjugated to GaN Semiconductors system has been studied. Excitons are strongly coupled in (C6H5C2H4NH3)2PbI4[bis(phenethyl-ammonium)tetraiodoplumbatel] (PEPI) is a hybrid inorganic-organic layered, with a perovskite structure quantum well (QW). The inorganic PbI4 monolayer is sandwiched between organic layers. The interaction between electron and hole forming the bound exciton is significantly stronger due to dielectric confinement. The lowest exciton binding energy is 220 meV, which is 20 times in comparison to GaAs. Gallium nitride (GaN) semiconductor has defect bound excitons with absorption band, which overlaps with the emission spectra of the PEPI system. This facilitates resonant energy transfer (RET) from the GaN defect bound exciton states to the excitons confined in the PEPI layer. We investigated the interaction by photoluminescence (PL) and found that 1s exciton in the PEPI layer strongly couples with the GaN defect level exciton, which is pronounced at lower temperature (< ∼ 100 K).
Electrical Properties, Deep Levels Spectra and Luminescence of Undoped GaN/InGaN Multi-quantum-well Structures as Affected by Electron Irradiation
Authors: Polyakov Alexander Y , Smirnov Nikolai B , Govorkov Anatoliy V , Markov Alexander V , Lee Cheul-Ro , Lee In-Hwan , Kolin Nikolai G , Merkurisov Denis I , Boiko Vladimir M , Wright James S
Published: April 2011

Effects of 10 MeV electron and fast reactor neutron irradiations on carrier removal rate and deep traps spectra were compared for undoped n-GaN samples. It is shown that for electron irradiation the carrier removal rate is well accounted for by the difference in introduction rates of nitrogen-vacancy-related donors with activation energy 0.2 eV and of nitrogen-interstitial-related acceptors at Ec-1.2 eV. In the case of neutron irradiation the introduction rate of all deep traps was much lower than the carrier removal rate indicating that the main contribution to electron removal was due to disordered regions. These regions give rise to a marked persistent photocapacitance signal and a hole-trap-like feature in deep traps spectra. The Fermi level position in the core of disordered regions is located near Ec-(0.85-1) eV.
Loss of Quantum Efficiency in Green Light Emitting Diode Dies at Low Temperature
Authors: Li Yufeng , Zhao Wei , Xia Yong , Zhu Mingwei , Senawiratne Jayantha , Detchprohm Theeradetch , Schubert E. Fred , Wetzel Christian M
Published: April 2011

The electroluminescence, photoluminescence and cathodoluminescence of GaInN/GaN multiple quantum well light emitting diode dies are analyzed at variable low temperature. Three dies of nominally identical structure but strongly different RT performance from 510 nm to 525 nm have been studied. The electroluminescence peak energy exhibits a blue shift from RT to 158 K followed by a red shift for lower temperature. In the same low-temperature range, a secondary emission peak appears near 390 nm (3.18 eV) that resembles a donor-acceptor pair transition from GaN. Depth profiling spectroscopy of this transition at 77 K reveals its location either in the unintentionally doped quantum barriers or within the n-GaN layer, rather than the commonly believed Mg doped p-type GaN layers. The external quantum efficiency of each die increases as the temperature is lowered. A maximum is reached for all near 158 K while for lower temperature as low as 7.7 K, the efficiency continuously drops. The pronounced efficiency maximum is tentatively assigned to a combination of temperature dependent mobility and shift of the actual pn-junction location.
Low-voltage Avalanche Breakdown in AlGaN Multi-quantum Wells
Authors: Zhang Shengkun , Zhou X. , Wang Wubao , Alfano R. R. , Dabiran A. M. , Osinky A. , Wowchak A. M. , Hertog B. , Plaut C. , Chow P. P.
Published: April 2011

In this work, electro-luminescence (EL) of a AlGaN p-i-n diode have been investigated in both avalanche and injection modes. The active i-region of the diode consists of Al0.1Ga0.9N/Al0.15Ga0.85N MQWs. Strong interband luminescence from the Al0.1Ga0.9N active layers was observed when operating the device in both avalanche and injection modes. The threshold voltage for avalanche breakdown is as low as 9 V. This indicates that the impact ionization coefficient of electrons is greatly enhanced in these Al0.1Ga0.9N/Al0.15Ga0.85N MQWs comparing to AlGaN bulk materials. Polarization-induced electric fields in the Al0.1Ga0.9N well layers are believed to be responsible for the enhancement of the ionization coefficient. In a control sample that has higher defect density, the electroluminescence was dominated by long-wavelength emissions, which results from impact ionizations of the defect levels.
Persistent Photoconductivity in High-mobility AlxGa1−xN/AlN/GaN Heterostructures Grown by Metal-organic Vapor-phase Epitaxy
Authors: Biyikli Necmi , Ozgur Umit , Ni Xianfeng , Fu Yi , Morkoc Hadis , Kurdak Cagliyan
Published: April 2011

We report on the persistent photoconductivity (PPC) effect in AlxGa1−xN/AlN/GaN heterostructures with two different Al compositions (x=0.15 and 0.25). The two-dimensional electron gas (2DEG) was characterized by Shubnikov-de Haas and Hall measurements. At cryogenic temperatures under optical illumination, the 2DEG carrier density and mobility was enhanced. The persistent photocurrent in both samples exhibited a strong dependence on illumination wavelength.
Schottky I-V Characteristics of Au/Ni/GaN/SiNx nanonework/sapphire structures
Authors: Xie Jinqiao , Fu Yi , Morkoç Hadis
Published: April 2011

GaN layers on sapphire substrates were grown by metalorganic chemical vapor deposition using in situ porous SiNx nano-network. Crystalline quality of epilayers was characterized by X-ray rocking curve scans, and the full width at hall maximum values for (002) and (102) diffractions were improved from 252 arc sec and 405 arc sec, respectively, in control samples to 216 arc sec and 196 arc sec when SiNx was used. Ni/Au Schottky diodes (SDs) were fabricated and the SD performance was found to be critically dependent on the SiNx coverage (fewer and farther the pores the better the results) which is consistent with the trends of XRD and photoluminescence data. A 1.13eV barrier height was achieved when 5min SiNx layer was used compared with 0.78 eV without any SiNx nanonetwork. Furthermore, the breakdown voltage improved from 76 V to 250V when SiNx nanonetwork was used in otherwise identical structures.
Recessed Gate Processing for GaN/AlGaN-HEMTs
Authors: Pletschen Wilfried , Kiefer Rudolf , Raynor Brian , Mueller Stefan , Benkhelifa Foud , Quay Ruediger , Mikulla Michael , Schlechtweg Michael , Weimann Guenter
Published: April 2011

A dry etch process based on Cl2/SF6 has been developed to selectively remove GaN over AlGaN for the fabrication of recessed gate GaN/AlGaN HEMTs. Using this etching process recessed and non-recessed FETs were fabricated side by side on the same wafer to provide a fair comparision of data. Recessed gate FETs with a gatelength of 0.15μm show cutoff frequencies of 83 and more than 200 GHz for fT and fmax, respectively. Furthermore, gate-drain breakdown as high as 84V has been obtained which is more than twice as much compared to their non-recessed counterparts.
AlGaN/GaN High Electron Mobility Transistors on Si/SiO2/poly-SiC Substrates
Authors: Anderson Travis , Ren Fan , Voss Lars , Hlad Mark , Gila Brent P , Pearton Stephen , Covert Lance , Lin Jenshan , Thuret Julien , Lahreche H , Bove P
Published: April 2011

The dc and rf performance of AlGaN/GaN High Electron Mobility Transistors (HEMTs) grown by Molecular Beam Epitaxy on Si-on-poly (SopSiC) substrates is reported. The HEMT structure incorporated a 7 period GaN/AlN superlattice between the AlGaN barrier and GaN channel for improved carrier confinement. The knee voltage of devices with 2 μm gate-drain spacing was 2.12 V and increased to 3 V at 8 μm spacing. The maximum frequency of oscillation, fMAX, was ∼40 GHz for devices with 0.5 μm gate length and 2 μm gate-drain spacing. Parameter extraction from the measured rf characteristics showed a maximum intrinsic transconductance of 143 mS.mm−1.
Enhancing Semiconductors with Diamond
Authors: Zimmer Jerry
Published: April 2011

In the continuing thrust to extend Moore's law, silicon is beginning to confront several issues that require innovative materials solutions to increase transistor and interconnect speeds while dealing with the increasing thermal loads of advanced microprocessors. The unsurpassed thermal and mechanical properties of diamond can be profitably exploited to enhance the performance characteristics of silicon based circuits. The discussion focuses primarily on the integration of diamond with silicon for the manufacture of silicon on diamond (SOD) wafers for spreading heat from hot spots in microprocessors. Other interesting property combinations are also presented as food for thought regarding other possible enhancements to silicon device performance. Recent data and prognosis for the future will be presented.
Comparing the Young's Modulus of Near-Frictionless Carbon Films Obtained From Different Methods
Authors: Zhou Bo , Prorok Barton C. , Erdemir Ali , Eryilmaz O.
Published: April 2011

The near-frictionless carbon (NFC) thin films grown at Argonne National Lab are excellent candidates for mitigating friction and wear issues in rotating and sliding MEMS applications. In this study, NFC films with Si/Si3N4 substrates were tested with static and CSM (continuous stiffness measurement) nanoindentation methods. Furthermore, the same NFC films with 900 nm thickness were fabricated into freestanding structures for the membrane deflection experiment (MDE). The Young's modulus obtained from these three methods were compared and contrasted. The E values obtained from the nanoindentation methods were nearly constant when the indent depth was less than 100 nm (1/10 of the film thickness). Both the static and CSM indentation data indicated a clear increasing trend of the Young's modulus with the indent depth until the elastic response from the substrate started to dominate the values (depth larger than 800 nm). The average modulus obtained from static and CSM nanoindentations were 49.5 ± 4.3 GPa and 50.2 ± 1.8 GPa for as-deposited films, respectively. The values changed to 46.9 ± 5.4 GPa and 46.8 ± 2.9 GPa after microfabrication processing. The larger standard deviation values indicated the affect of the increased surface roughness to the indentation tests. The E value determined from MDE tests was 35.1 ± 2.3 GPa, which was measurably smaller those from the indentation tests. As the AFM analysis showed little/no pile-up during indentation, the difference might come from the residual stress relief, which influences the measurement, when the NFC membranes were released from the substrate.
Attempts to p-Dope Ultrananocrystalline Diamond Films in a Hot Filament Reactor
Authors: May Paul William , Hannaway Matthew
Published: April 2011

Ultrananocrystalline diamond (UNCD) films have been deposited using hot filament chemical vapour deposition using Ar/CH4/H2 gas mixtures plus additions of B2H6 in an attempt to make p-type semiconducting films. With increasing additions of B2H6 from 0 to 40,000 ppm with respect to C, the film growth rate was found to decrease substantially, whilst the individual grain sizes increased from nm to μm. With 40,000 ppm of B2H6, crystals of boric oxide were found on the substrate surface, which slowly hydrolysed to boric acid on exposure to air. These results are rationalised using a model for UNCD growth based on competition for surface radical sites between CH3 and C atoms.
Diamond Surface Modifications with Diazonium Salt
Authors: Uetsuka Hiroshi , Shin Dongchan , Tokuda Norio , Saeki Kazuhiko , Nebel Christoph E.
Published: April 2011

The growth of covalently bonded nitrophenyl layers on atomically smooth boron-doped single crystalline diamond surfaces is characterized, using cyclic voltammetric attachment and constant potential grafting by electrochemical reduction of aryl diazonium salts. We apply atomic force microscopy (AFM) to characterize nitrophenyl layer-growth and thickness. Angle-resolved X-ray photoelectron spectroscopy is applied to reveal bonding arrangement of phenyl molecules and transient current measurements during the grafting is used to investigate the dynamics of chemical bonding. Nitrophenyl groups at an initial stage of attachment grown three-dimensional forming layers with varying heights and densities. Layer thicknesses of up to 80 Å are detected for cyclic voltammetry attachment after 5 cycles, whereas the layer becomes denser and only about 25 Å thick in case of constant potential attachment. No monomolecular closed layer could be detected. The data are discussed taking into account established growth models.
Irradiation-induced structural modifications in multifunctional nanocarbons
Authors: Gupta Sanju
Published: April 2011

Severe environmental tolerability is the prime factor in the development of novel space materials exhibiting excellent physical properties accompanied by lightweight, reusability, and multifunctional capabilities. Diamond is known for its reputation being radiation hard besides a range of outstanding properties (electronic, optical, mechanical, and chemical) and hence it is preferable in harsh environments. Carbon nanotubes are also of great interest because of several unsurpassable physical properties and it needs to be shown that they are physically stable and structurally unaltered when subjected to irradiation. Therefore, a family of novel nanocarbons (nanodiamond and nanotubes) films deposited by microwave plasma-assisted chemical vapor deposition (MWCVD) technique was subjected to gamma radiation (1, 5, and 20 Mrads) and to medium energy electron-beam irradiation to study their effects on the microscopic structure and corresponding physical properties to establish property-structure correlation. Microstructural and physical properties characterizations prior to and post-irradiation include scanning electron microscopy (SEM), atomic force microscopy (AFM), Raman spectroscopy (RS), X-ray diffraction (XRD), field emission (FE), and high-resolution transmission electron microscopy (HRTEM). Dramatic improvement in the field emission properties for microcrystalline diamond and relatively small but systematic behavior for nanocrystalline diamond with increasing radiation dose is discussed in terms of the critical role of defects which tends to form clusters. The results also indicate that nanocrystalline carbon tends to reach a state of damage saturation they are discussed in terms of sp3, sp2+δ ←→ sp2 C inter-conversion. The effects of electron beam irradiation on the carbon nanotube show that multi¡Vwalled nanotubes tend to be relatively more robust than those of single–walled. This is because increased exposure on an individual bundle of single-walled nanotubes promoted graphitization, pinching, and cross-linking similar to polymers forming intra-molecular junction (IMJ) within the area of electron beam focus, possibly through aggregates of amorphous carbon. Formation of novel nanostructures (nano– ring and helix– V like) due to irradiation is also observed. These studies gleam on the dynamics of nano-manufacturing and a regime of possible relevance to these materials for a) short-term space missions; b) radiation hard programmable logic circuits; and c) radiation pressure sensors.
Lift -Off Process to get Free-Standing High Quality Single Crystal Diamond Films and Suspended Single Crystal Diamond Devices
Authors: Yang Jie , Wang C. F. , Hu E. L. , Butler James E.
Published: April 2011

Freestanding and suspended single crystal diamond devices, micro disks and beam structures, have been fabricated on single crystal diamond substrates using a lift-off process employing ion implantation followed by electrochemical etching. The ion implantation created subsurface damage in the diamond while the top surface was sufficiently undamaged that a subsequent homo-epitaxial diamond layer could be grown by chemical vapor deposition (CVD). After the CVD growth and patterning by lithography and reactive ion etching, the underlying damage layer was etched/removed by an electrochemical etch. Different implant ions and energies were simulated and tested to optimize the process. The electrochemical etching process was monitored by an optical video technique. The electrochemical etching process used both ac and dc applied electrical potentials. Photoluminescence (PL), Raman spectra, and polarized light transmission microscopy have been used to characterize the implanted substrate and lift-off films. AFM has been used to monitor the surface changes after mechanical polishing, ion implantation, CVD growth and the lift-off process. This research has revealed that the parameters of ion implantation (implant species, dose and energy) dramatically affect the lift-off process. The etching mechanism and critical parameters are discussed in this work. PL spectroscopy indicated differences between the uppermost layers of the homo-epitaxial film and the lift-off interface. Three principal classes of defects have been observed: growth defects inherent in the diamond substrates (type Ib, HPHT), defects induced by the polishing process and associated stress, and point defects.
Development of Thin Film and Nanorod ZnO-Based LEDs and Sensors
Authors: Pearton Stephen J. , Tien L C , Kim H S , Norton D P , Chen J J , Wang H T , Kang B S , Ren F , Lim W T , Wright J , Khanna R , Voss L F , Stafford L , Jun J , Lin Jenshan
Published: April 2011

The development of new etching and contact metallurgies for the ZnO/ZnMgO/ZnCdO materials system and various approaches for realizing ZnO LEDs are reviewed. ZnO nanorod MOSFETs and pH sensors have been demonstrated. In addition, selective detection of hydrogen with Pt-coated single ZnO nanorods is discussed discussed. The Pt-coated single nanorods show a current response approximately a factor of three larger at room temperature upon exposure to 500ppm H2 in N2 than thin films of ZnO. The power consumption of these sensors can be very small (in the nW range) when using discontinuous coatings of Pt. Once the Pt coating becomes continuous, the current required to operate the sensors increases to the μW range. The ZnO nanorods are insensitive to oxygen in the measurement ambient.
Electrical and Magnetic Properties of Doped ZnO Nanowires
Authors: Panin Gennady N , Baranov Andrey N. , Kang Tae Won , Kononenko Oleg V. , Dubonos Sergey V. , Min S. K. , Kim H. J.
Published: April 2011

ZnO nanowires doped with Mn, Fe, Sn, and Li during the thermal growth following direct chemical synthesis were investigated using electric and magnetic measurements. Current-voltage characteristics of individual nanowires configured as a two-terminal device with Al electrodes show apparent rectify behavior indicating the Schottky-like barrier formation and resistivity being less 3 Ω·cm. Reproducible resistance modulation by a dc voltage at room temperature is observed. Magnetic susceptibility of the doped nanowires as a function of temperature demonstrates Curie–Weiss behavior. Magnetization versus field curves show hysteresis with the coercive field of about 200 Oe. The spatially-resolved magnetic force measurements of individual nanowires revealed the magnetic domain structure. The domains align perpendicular to c-axis and can be polarized in the external magnetic field.
The Growth of ZnO on CrN Buffer Layer Using Surface Phase Control by Plasma Assisted Molecular-beam Epitaxy
Authors: Park Jinsub , Minegishi Tsutomu , Park Seunghwan , Im Inho , Hanada Takahasi , Hong Soonku , Goto Takenari , Cho Meoungwhan , Yao Takafumi
Published: April 2011

Epitaxial ZnO films are successfully grown on Al2O3 substrates with phase controlled CrN buffer layer using Zn and O-plasma pre-exposures on CrN layers by plasma assisted molecular beam epitaxy (P-MBE). The Zn exposures on CrN layers prior to ZnO film growth result in the formation of rocksalt CrN without surface oxidation. On the other hand, the surface of the initially deposited CrN layers with rocksalt structure changes into hexagonal structured Cr2O3 after O-plasma exposure as confirmed by reflection high-energy electron diffraction (RHEED) and high resolution transmission electron microscopy (HR TEM). Etching studies show that the ZnO films grown on CrN have +C polarity, while the polarity of ZnO on Cr2O3/CrN double buffer is -C polarity. The interdiffusion of Zn and Cr occurs at the ZnO/CrN interface, while the interdiffusion is negligible at the ZnO/ Cr2O3 interface. The interdiffusion of Cr and Zn can be suppressed by inserting a low-temperature ZnO buffer layer in between ZnO and CrN layers, which helps improve the crystal quality of ZnO layers grown with CrN buffer.
Structural and Optical Studies of Multi Shape ZnO Nanostructures Grown by Direct Vapor Phase Technique
Authors: Hullavarad Shiva S , Karulkar P C , Vispute R D , Heng R , Venkatesan T
Published: April 2011

In this paper we report the growth of ZnO nanowires (12-60 nm) and nanorods (500 nm) by a method of Catalysis Free Direct Vapor Phase (DVP) technique. The nanowires were grown on c-Al2O3 and pulsed laser deposited ZnO nucleation layer on Al2O3 substrates at 800 °C without employing any metal catalysts that are conventionally used in MOCVD or Vapor-Liquid-Solid phase techniques. The ZnO nanowires are found to emit UV light at 386 nm with considerably lower green band emission.
Controllable Synthesis of ZnO Nanonails by Vapor-Solid Process: Growth Mechanism and Structural and Optical Properties
Authors: Umar Ahmad , Haq Q. Ahsanul , Kim Sang Hoon , Im Yeon Ho , Hahn Yoon Bong
Published: April 2011

Single-crystalline with good optical properties aligned ZnO nanonails were grown on steel alloy substrate without the use of metal catalyst or additives by the thermal evaporation process using high purity metallic zinc powder and oxygen as source materials for zinc and oxygen, respectively. Detailed morphological studies by FESEM revealed that the obtained nanonails are grown in a high density over the whole substrate surface and are exhibiting perfect hexagonal-shaped caps. The diameters of the nanonails at their tops and bases are ranges from 120∼160nm and 50∼70 nm, respectively. The detailed structural characterizations confirmed that the synthesized nanostructures are single-crystalline and grown along the c-axis direction. Raman scattering and room-temperature photoluminescence studies demonstrated the wurtzite hexagonal phase and good optical properties, respectively for the grown nanonails.
A Comparative Study of MOCVD Produced ZnO Films Doped with N, As, P and Sb
Authors: Tompa Gary S. , Sun S. , Rice C. E. , Provost L. G. , Mentel D.
Published: April 2011

ZnO thin films are of interest for an array of applications; including: light emitters, photovoltaics, sensors and transparent contacts among others. Of particular interest is the potential to produce p-type layers from which p-n junction devices could be routinely produced. While it is fairly routine for MOCVD to produce n-type films with doping concentrations in the 10E20 cm-3 range and resistivities below 10E-3 ohm-cm; it is very difficult to produce measurable p-type ZnO. We report on our efforts with doping films p-type using N gas sources and metalorganic sources of P, As, and Sb. Films showing acceptor bands by photoluminescence have been demonstrated; however reliable electrical measurements remain difficult. Specific problems include achieving low resistance ohmic contacts, accounting for the photo-responsiveness of ZnO films and sensitivity limits in Hall measurements of low-doped and compensated materials. The presentation will review deposition parameters, produced and processed films and material characteristics.
Properties of p-Type ZnO Grown by Oxidation of Zn-Group-V Compounds
Authors: Kaminska Eliana , Przezdziecka Ewa , Piotrowska Anna , Kossut Jacek , Boguslawski Piotr , Pasternak Iwona , Jakiela Rafal , Dynowska Elzbieta
Published: April 2011

ZnO:N and ZnO:Sb layers were obtained by thermal oxidation of Zn compounds with group-V elements. The films are polycrystalline, with the acceptor concentration in the range 1020 cm-3, and the background concentration of H of 5×1019 cm-3. Transport measurements reveal the p-type conductivity with the hole concentrations exceeding 1017 cm-3. Rich photoluminescence spectra involve excitons bound to neutral acceptors and donor-to-acceptor transitions. p-ZnO:N and p-ZnO:Sb films show transparency of about 85 % in the visible wavelength range, making these materials very promising for transparent electronics.
Analysis and Applications of ZnO Semiconductor Films Deposited by Laser and Sputtering Techniques
Authors: Yen Tingfang , Li Meiya , Chokshi Nehal , Kim Sung Jin , Cartwright Alexander N. , Jeong Yongwoo , Anderson Wayne A.
Published: April 2011

In this paper, ZnO thin films deposited by two methods have been studied. Specifically, the films were grown using i) Laser Assisted Molecular Beam Deposition (LAMBD) and ii) RF sputtering. Subsequent to film deposition, a subset of samples deposited using LAMBD were laser annealed. An additional set of samples (from LAMBD and RF sputtering) were annealed with N2 or forming gas at 600°C for 30mins.

After deposition, optical and electrical properties of ZnO thin films have been studied. The application of ZnO to optical devices, including Metal-Semiconductor-Metal Photodetectors (MSM-PD) and solar cells, has been made. Several deposition experiments recently demonstrated that the thin films of RF-ZnO and LAMBD-ZnO have near ZnO parameters including refractive index close to 2, 1:1 stoichiometry ZnO, and 3.3 eV ZnO bandgap. Mixtures of single crystal and polycrystal grains were observed by Transmission Electron Microscopy (TEM) from LAMBD ZnO thin films. MSM current-voltage data show symmetrical photo current behavior. High ratio of photocurrent to dark current, good responsivity and fast pulse response of LAMBD-ZnO MSM were observed. ZnO/Si heterojunction solar cell result has been demonstrated and improvement in the ultraviolet light spectrum of spectral response has been shown in this paper.
Optical Properties of Porous ZnO Nanorods Grown by Aqueous Solution Method
Authors: Lee Sang Hyun , Lee Hyun Jung , Goto Takenari , Cho Meoung-Whan , Yao Takafumi
Published: April 2011

ZnO nanorods were synthesized by chemical solution method at low temperature. The surface of nanorods was changed to porous by using thermal annealing and chemical etching. Surface morphology and their optical properties were changed according to annealing and etching condition. Photoluminescence from as-grown ZnO nanorods almost showed defect related emission in wide range from 450∼900 nm. After annealing at 500°C, the band-edge emission of ZnO was observed and emission at visible range was changed to green with decreasing red-orange. The surface morphology of ZnO nanorods was transformed to porous by chemical etching and it led to increase the intensity of band-edge emission about three times. The internal quantum efficiency for porous ZnO nanorods, which was calculated from ratio PL intensity at 10K and 300K, is about 21%. Also, the random lasing at porous ZnO nanorods was occurred at high optical excitation by a photon with traveling inside or outside of porous ZnO nanorod gets amplified by injection second photon before leaving porous ZnO nanorods.
Synthesis and Characterization of Nickel-Doped ZnO Nanocrystals
Authors: Zhang Xiao Li , Qiao Ru , Li Yan , Qiu Ri , Kang Young Soo
Published: April 2011

The structure and magnetism of ZnO-based solid solutions, dilute magnetic semiconductors, with nickel solute were obtained via a solvothermal method. Compared with previous methods for solid solution DMSs, our synthesis method was really facile and economical. The one-dimensional solid solution of Zn1-xNixO nanostructures were grown in a alcoholic solution. Moreover, the percentage of doped nickel can be easily controlled. The X-ray diffraction, transmission electron micrograph and magnetization hysteresis loops of nickel-doped ZnO nanocrystals were presented to confirm that the nickel impurities are embedded inside the nanocrystal.
Transport Studies of Transition Metal Ion Doped ZnO: Bulk and Thin Films
Authors: Singh Shubra , Rama N , Ramachandra Rao M.S.
Published: April 2011

The effect of doping of transition metal ions (Fe and Co) on transport properties of ZnO has been studied in both bulk and thin films. The solubility limit of these ions have been found to be higher in thin films compared to bulk. Optical measurements reveal the presence of Fe in both 2+ and 3+ state. Co is believed to be in 2+ states. Electrical resistivity measurements show that while for bulk Fe doped ZnO samples there is a decrease in resistivity compared to undoped ZnO, it increases for bulk Co doped ZnO samples. However, thin film samples of both types of doped compounds show a decrease in resistivity compared to undoped ZnO. This difference in bulk and thin film behaviour has been explained on the basis of experimental results.
The Impact of Fermi Pinning on Thermal Properties of the Instabilities in ZnO TFTs
Authors: Cross R.B.M. , De Souza M. M.
Published: April 2011

In this paper we describe gate bias and temperature induced device instabilities of inverted-staggered ZnO-TFTs. It is shown that low positive and negative gate bias results in the transfer characteristics shifting in a positive and negative direction respectively. It is suggested that this is a consequence of temporary charge trapping at or close to the channel/insulator interface. The degradation of device parameters such as the threshold voltage, subthreshold slope and effective channel mobility is demonstrated at elevated measurement temperatures, suggesting the generation of defects and/or trap states in the interfacial region. In addition, it is postulated from the extracted activation energy of the drain current that the Fermi-level is pinned during the operation of the devices due to the high level of states close to the conduction band edge. These results highlight the relatively ease with which defects could be created at the interface, indicating a high concentration of weak or strained bonds. Both charge trapping and defect creation-induced instabilities appear to be reversible, as all devices regain their original characteristics after a period of relaxation at room temperature.
Multipolar Photonic Interactions between Quantum Dots of Different Sizes
Authors: Matsueda Hideaki
Published: April 2011

Multipolar interactions between quantum dots involving real and virtual photons are compared on the basis of our (micro-photoluminescence) spectra of coupled QDs (quantum dots), and suggested as the principle of intra-device, inter-device, and inter-chip interconnections and logical operations, with the device concepts for solid state nanometer networks and a quantum computer.
Optical Properties of Colloidal CdSe/ZnS core/shell Nanocrystals Embedded in a UV Curable Resin
Authors: Joshi Abhishek , Davis Edwin , Narsingi Kaushik , Manasreh Omar , Weaver B. D.
Published: April 2011

Optical absorption and photoluminescence techniques were used to investigate the band gap of colloidal CdSe/ZnS core/shell nanocrystals matrixed in a UV curable resin. The band gap was measured for several nanocrystals with size ranging between 1.9 and 4.0 nm. The band gap (Eg) was determined from the first exciton peaks observed in the optical absorption spectra. Both Debye and Einstein temperatures were estimated from fitting the energy band gap vs. temperature using two different empirical expressions.
Optical Properties of Heavily Doped n-type CdSe Quantum Dots for Intersubband Device Applications
Authors: Zhang Shengkun , Zhou Xuecong , Shen Aidong , Wang Wubao , Alfano Robert , Lu Hong , Charles William , Yokomizo I , Tamargo Maria , Franz K , Gmachl C
Published: April 2011

In this research, interband and intersubband optical properties of heavily doped n-type CdSe quantum dots were investigated by temperature dependent photoluminescence (PL) spectroscopy, picosecond time-resolved PL spectroscopy and Fourier transform infrared (FTIR) spectroscopy. Two doped and one undoped CdSe quantum dot samples with multiple QD layers were grown over ZnCdMgSe barrier layers on InP (001) substrate by molecular beam epitaxy. Heavy doping leads to decreasing of activation energy of nonradiative recombination centers, however, does not affect the luminescence efficiency of doped quantum wells. Time resolved PL experiments show that the PL decay times of the doped samples have weak dependence on well width and are much longer than that of the undoped sample. The two doped CdSe QD samples show strong Intersubband IR absorption that peaked at 2.54 μm, 2.69 μm and 3.51 μm. The ISB absorption is found to be strongly polarization dependent due to the large size of the QDs.
Novel Morphologies of InAs Quantum Dot Growth on GaAs Surfaces Containing Nanostructures Formed by Droplet Epitaxy
Authors: Lee Jihoon , Wang Zh. M. , Liang B.L. , Sablon K. , Strom N. W. , Salamo G. J.
Published: April 2011

Self-assembled InAs quantum dot clusters (QDCs) and InGaAs QD molecules (QDMs) have been demonstrated through a growth technique called “droplet epitaxy” by molecular beam epitaxy (MBE). For QDCs, the size and density of QDs can be controlled with variation of InAs monolayer coverages. For QDMs, Ga contribution from GaAs mound with the interaction of InAs deposition resulted in various number of InGaAs QDs per GaAs mound, ranging from 2 to 6 (bi-QDMs to hexa-QDMs) depending on the specific InAs monolayer deposition. High step density on sidewall of GaAs mound and anisotropy of surface diffusion gave a rise to preferential formation of InAs and InGaAs QDs around GaAs mounds. This hybrid growth approach combining droplet epitaxy and typical QD growth is relatively simple and flexible and doesn't require further ex-situ surface preparation. This approach of QD arrangement can find applications in optoelectronics as well as physical study of QD interaction.
Isotropic Lateral Ordering of III-V Quantum Dots Over GaAs (001) By Self-Assembly
Authors: Hussein Mohammad Lutfi , Marega Euclydes , Salamo Gregory
Published: April 2011

Lateral ordering of InGaAs quantum dots over GaAs (001) has been achieved in earlier reports resembling anisotropic pattern. We present in this letter a method of breaking the anisotropy of ordered QDs by changing the growth environment. We do show experimentally that using As2 molecules instead of As4 as a background flux is effective in controlling the diffusion of Ga adatoms in away to make it possible to produce isotropic ordering of InGaAs QDs over GaAs (001). Our results are consistent with reported experimental and theoretical studies on surface structure and diffusion mechanism over GaAs.
Modeling of Silicon Nanodots Nucleation and Growth Deposited by LPCVD on SiO2 : From Molecule/surface Interactions to Reactor Scale Simulations
Authors: Zahi Ilyes , Vergnes Hugues , Caussat Brigitte , Esteve Alain , Djafari Rouhani Mehdi , Mur Pierre , Blaise Philippe , Scheid Emmanuel
Published: April 2011

We present first results combining models at continuum and atomistic (DFT) levels to improve understanding of key mechanisms involved in silicon nanodots (NDs) synthesis on SiO2 by Low Pressure Chemical Vapor Deposition (LPCVD) from silane SiH4. In particular, by simulating an industrial LPCVD reactor using the CFD code Fluent, we find that the deposition time could be increased and then the reproducibility and uniformity of NDs deposition could be improved when highly diluting silane in a carrier gas. A consequence of this high dilution seems to be that the contribution to deposition of unsaturated species such as silylene SiH2 highly increases. This result is important since our first DFT calculations have shown that silicon chemisorption on silanol Si-OH or siloxane Si-O-Si bonds present on SiO2 substrates could only proceed from silylene (and probably from other unsaturated species). The silane saturated molecule could only contribute to NDs growth, i.e. silicon chemisorption on already deposited silicon bonds. Increasing silylene contribution to deposition in highly diluting silane could then also exalt silicon nucleation on SiO2 substrates and then increase NDs density.
Phonon Overheating in Quantum Dots: Low Electronic Densities
Authors: Král Karel
Published: April 2011

The electronic coupling to the longitudinal optical phonons has been recently used to interpret theoretically the electronic energy relaxation in quantum dots. In this theory the LO phonon modes served as a reservoir, on which the electron executes multiple scattering acts. Quite reasonably such a phonon subsystem is expected to be passive, namely, in a long-time limit of development, the whole system should be able to achieve such a stationary state, in which the statistical distributions of both subsystems, electron and phonons, do not change in time. We remind briefly that the recent approach to the relaxation in quantum dots has led to a non-passivity of such a reservoir. We remind as well the method of a partial elimination of the phonon overheating effect by using the Lang-Firsov transformation. Then we apply such a modified relaxation theory to the electronic relaxation at low electronic densities in quantum dots and come to conclusions concerning the role of e-LO scattering mechanism. We show that the modified theory of relaxation gives a plausible dependence of relaxation rate on temperature of sample. We also come to conclusions that at low levels of electronic excitation of quantum dot sample the electron-LO phonon mechanism can become relatively weak with respect to other relaxation mechanisms not supporting the electronic up-conversion effect.
Self-assembly of Semiconductor Quantum Dots by Droplet Epitaxy
Authors: Koguchi Nobuyuki
Published: April 2011

We have proposed a novel self-assembling growth method, termed Droplet Epitaxy, for the direct formation of QDs without using any lithography in 1990. Compared with the island formation based on the Stranski-Krastanow growth mode, the Droplet Epitaxy is applicable to the formation of quantum dots not only in lattice-mismatched but also in lattice-matched systems such as GaAs/AlGaAs. The process of the Droplet Epitaxy in MBE chamber consists of forming numerous III-column element droplets such as Ga or InGa with homogeneous size of around 10 nm on the substrate surface first by supplying their molecular beams, and then reacting the droplets with As molecular beam to produce GaAs or InGaAs epitaxial microcrystals.
Another advantage of the Droplet Epitaxy is the possibility of the fabrication of QDs structures without wetting layer by cotrolling the stoichiometry of the substrate surface just before the deposition of III-column element droplets.
Also we can control the shape of the QDs structure self-organizingly such as pyramidal shape, single-ring shape and concentric double-ring shape. These ring structures will provide excellent possibilities for the investigation of quantum topological phenomena.
Nanostructuring Rh(110) Surfaces by Ion Etching
Authors: Molle Alessandro , Toma Andrea , Boragno Corrado , Valbusa Ugo , de Mongeot Francesco Buatier
Published: April 2011

The ion irradiation of the Rh(110) surface results in the self-organised formation of various nano-structured morphologies like ripples, mounds, pyramids which have been thoroughly studied as a function of the incidence angle and of the impact energy of the impinging ions. A study of the evolution of the surface ripples at various impact energies above the hot-spot threshold, has been rationalized in terms of a contribution due to an ion-induced surface diffusion mechanism. In the very low ion incidence regime, where the formation of hot spots following ion impact is inhibited, the formation of a rhomboidal pyramid pattern is singled out and attributed to the predominant reorganization of surface adatom and vacancies produced in the topmost surface layers. The metastable rhomboidal pyramid pattern, was recently proven to have extraordinary chemical reactivity since it is endowed with a very high density of undercoordinated step sites runnin along the very open <1-12> azimuthal direction.
Characterization of Medium-range Order in Self-Assembled Organic-inorganic Hybrid by Fluctuation X-ray Microscopy
Authors: Fan Lixin , Paterson David , McNulty Ian , Treacy M. M. J. , Kumar Dushyant , Du Phong , Wiesner Ulrich , Gibson J. Murray
Published: April 2011

The control of formation and ordering of self assembled nanostructures, with medium- to long-range order, is a challenge that limits advances in many fields of nanotechnology. We have developed a technique, which we call fluctuation x-ray microscopy, that offers quantitative insight into medium-range correlations in disordered materials at nanometer- and larger-length scales. We examined the influence of sol-gel process variables on medium range order in PI-b-PEO/ aluminosilicate bulk using this technique. The nano-structuring of inorganic materials was directed by polymer self-assembly. The medium range correlation between the nanostructures in two hybrids was quantitatively examined and compared.
Grain Size Refinement in Cu bottom lead in CPP GMR Stack
Authors: Tadisina Zeenath Reddy , Gupta Subhadra , Papusoi Cristian , Fujiwara Hideo , Morris Robb , Thompson Gregory
Published: April 2011

GMR current-in-plane (CIP) spin valves are currently used in production of high density recording heads. For the next generation of heads, with recording densities higher than 100 billion bits per square inch (Gbpsi), confined-current-path (CCP), current- perpendicular-plane (CPP) GMR heads are the subject of intensive study. For the CPP stack, thick (150 nm) Cu leads are required to minimize the potential drop across the leads. This often leads to a rough film surface in the bottom lead which translates up into the GMR stack, thereby reducing the GMR effect because of roughening of the interfaces. Thick bottom leads also have large average grain sizes, leading to an increase in the coercivity of the ferromagnetic layers deposited on top of these leads. In order to decrease the grain size and roughness of the bottom Cu lead, lamination of the thick (150 nm) Cu with Ta interlayers was carried out, keeping the total Cu and Ta thicknesses constant. As the number of Ta-Cu bilayers was increased, the roughness decreased significantly. It was observed that the roughness decreased from over 1 nm for a single bilayer to about 0.3 nm for 10 bilayers. Atomic force microscopy (AFM) was used to determine the roughness of the samples and in-plane grain size. Scanning electron microscopy was used to confirm the in-plane grain size. X-ray diffraction was used to analyze the out-of-plane grain size of the Cu/Ta laminates. Cross-sectional transmission electron microscopy (TEM) was also utilized to analyze the interface quality. An increase in the resistance of the laminated lead was seen with increase in the number of laminations. This undesirable side effect can be minimized by improvement of the base vacuum of the sputter system, deposition at relatively high rates, and reduction of the latency time between successive film depositions. The in-plane film stress was also studied as a function of lamination frequency, and found constant for the same total thickness of Ta and Cu, as expected. The coercivity of Co90Fe10 layers deposited over laminated Cu/Ta leads was seen to decrease with increasing lamination frequency, also as expected. An even stronger effect on the CoFe coercivity was produced by using a 5nm Ta layer to cap the thick Cu lead, yielding significantly lower coercivity
Directed Self-Assembly of Thin Block Copolymer Films under Controlled Atmosphere
Authors: Kim Sangcheol , Briber R. M. , Karim Alamgir , Jones Ronald L. , Kim Ho-Cheol
Published: April 2011

We have developed a simple and fast methodology to control the orientation of cylindrical microdomains in thin block copolymer films under controlled atmosphere conditions. Asymmetric block copolymers of polystyrene and poly(ethylene oxide) (PS-b-PEO) were dissolved in toluene and spin coated with less than one minute exposure to different solvent vapors and controlled humidity. The specific choice of solvents was based on the polymer solubility parameters and the solvent vapor pressure. By controlling the spin coating environment we were able to produce cylinders with orientations ranging from parallel to perpendicular with respect to the substrate. Orientation was apparently controlled by the preferential affinity of the vapor atmosphere. A combinatorial gradient technique was employed to investigate the mechanism of solvent-induced microdomain orientation.
Magnetic Property of á-Fe Nanoparticles Prepared by Solventless Thermal Decomposition Method
Authors: Han Young Chul
Published: April 2011

Nano sized α-Fe particle was synthesized by modified thermal decomposition method. It resulted in the higher saturation magnetization (Ms) almost equivalent to the value of bulk Fe power (Mbulk = 210 emu/g). To prepare Fe nanoparticles, the Fe2+-(oleate)2 complex was annealed at 400 J in pyrex tube and the prepared Fe3O4 nanoparticle was reduced to Fe crystal structure at 700 J with NaCl under Ar+H2 gas atmosphere and annealed again under high vacuum system of 10−5 torr. The crystallinity and structure of the Fe nanoparticle was investigated by powder X-ray diffraction (XRD). The shape and size was confirmed by transmission electron microscope (TEM) images. The magnetic properties were characterized with coercivity and remanence from hysteresis loop by vibrating sample magnetometer (VSM)
Iron substituted SrRuO3 thin films
Authors: Ullah Aman , Yoo Young Zo , Chmaissem Omar , Kolesnik Stanislaw , Dabrowski Bogdan , Alan Genis
Published: April 2011

Fe-doped SrRuO3 thin films were grown on SrTiO3 (STO) substrates by using the Pulse Laser Deposition (PLD) method. Fe concentrations in the SrRuO3 thin films ranged from 0 % to 11 % to manifest the effect of Fe doping on their microstructure. Lattice constants of the films did not show obvious changes with the Fe doping whereas surface morphologies drastically changed from stepped to island structure with increasing Fe. X-ray reflectivity revealed that the surface and interfacial roughness decreased with increasing concentration of Fe. Surface roughness of the films was confirmed by atomic force microscopy. Therefore, a precise determination of film roughness as well as its growth mechanism using X-ray and AFM is exploited to correlate the Fe doping with structural disorder.