bluuebunny/crossref_metadata_2025 huggingface dataset:
Details of the improvement of crystalline quality of a-plane GaN using one-step lateral growth
Authors: Iida Daisuke , Nagai Tetsuya , Kawashima Takeshi , Miura Aya , Okadome Yoshizane , Tsuchiya Yosuke , Iwaya Motoaki , Kamiyama Satoshi , Amano Hiroshi , Akasaki Isamu
Published: April 2011

Low defect density a-plane GaN films were successfully grown by sidewall epitaxial lateral overgrowth (SELO) technique. Control of V/III ratio during the growth of GaN by metalorganic vapor phase epitaxy (MOVPE) was found to be very important to achieve a complete overgrowth on the SiO2 mask regions and atomically flat surface. The threading dislocation and stacking fault densities in the overgrown regions were lower than 106 cm−2 and 103 cm−1, respectively. The root mean square roughness was 0.09 nm. We also fabricated and characterized a-plane-GaN-based-light-emitting diodes (LEDs) using SELO technique. The light output power of the blue-green LED steeply increased with the decrease of threading dislocation density from 1010 cm−2 to 108 cm−2 and tended to saturate at lower dislocation densities.
Comparison of the Incorporation of Various Transition Metals into GaN by MOCVD
Authors: Kane Matthew H , Fenwick William , Li Nola , Gupta Shalini , Park Eun Hyun , Ferguson Ian T
Published: April 2011

The incorporation of transition metals in GaN has long been of interest in spintronics due to theoretical predictions of room temperature ferromagnetism in these materials. However, the mechanism of the observed ferromagnetism of the nitride-based DMS is still controversial, and may originate from a carrier-mediated, defect-related or nanoscale clustering mechanism. In this work, we present a comparative study of the incorporation of various transition metals and their effect on the optical, structural, and magnetic properties of GaN. Metal-organic chemical vapor deposition (MOCVD) has been employed to produce epitaxial films of varying thickness and manganese and iron doping using bis-cyclopentyldienyl(magnanese,iron) as the transition metal sources. High-resolution X-ray diffraction reveals no secondary phases under optimized growth conditions. Magnetic hysteresis is observed at room temperature in both GaMnN and GaFeN, though the strength of the magnetic ordering is roughly an order of magnitude weaker in the Fe-alloyed samples. Increasing Mn concentrations significantly affect long-range lattice ordering, and the observation of local vibrational modes (LVMs) supports the formation of nitrogen vacancies, even under optimized MOCVD growth conditions. Such vacancies form shallow donor complexes and thus contribute to self-compensation. A disorder-induced mode at 300 cm−1 and a LVM due to vacancies at 669 cm-1 were revealed by Raman spectroscopy.
Influence of Growth Conditions on Phase Separation of InGaN Bulk Material Grown by MOCVD
Authors: Huang Yong , Jani Omkar , Park Eun Hyun , Ferguson Ian
Published: April 2011

Different types of phase separation in thick InGaN layers were studied using photoluminescence (PL) and x-ray diffraction (XRD). InGaN films of 100 nm in thickness were grown on 2 μm GaN templates with an In molar fraction ranging from 0% to 20% by metal organic chemical phase deposition (MOCVD). It is shown that suppression of the phase separation in InGaN can be made possible by increasing the TMIn flow rate, decreasing the layer thickness and decreasing the growth rate. Based on the results, two types of phase separation, microscopic quantum dots and macroscopic domains, are proposed accordingly. The influence of the growth conditions on each type is summarized respectively in this paper.
Deep centers in GaN layers grown on epitaxial lateral overgrowth templates by metalorganic chemical vapor deposition
Authors: Chevtchenko Serguei A , Xie J. , Fu Y. , Ni X. , Morkoç H.
Published: April 2011

The dependence of traps and their concentration in GaN on the quality of templates, on which the layers are grown, has been studied by deep-level transient spectroscopy (DLTS). Thin GaN layers studied were grown on GaN templates employing conventional epitaxial lateral overgrowth (ELO) and nano-ELO with SiNx nanonetwork. The concentrations of traps found in these layers were compared with a reference sample grown on a standard GaN template not utilizing ELO. Two traps A (0.55 eV – 0.58 eV) and B (0.20 eV – 0.23 eV) were detected in the temperature range from 80 K to 400 K. A reduction of traps in layers grown on the ELO and nano-ELO templates was noted. We attribute this reduction to the reduction of threading dislocation density and as a result reduced capture of point defects and complexes as part of dislocation core structure and/or reduced formation of defects and complexes in the vicinity of line defects where the formation can be energetically favorable.
Characterization of Non-Polar Surfaces in HVPE Grown Gallium Nitride
Authors: Lai Kun-Yu , Grenko Judith A. , Wheeler V. D. , Johnson Mark , Preble E. A. , Williams N. Mark , Hanser A. D.
Published: April 2011

Non-polar surfaces of HVPE grown GaN were characterized by cathodoluminescence (CL), scanning electron microscopy (SEM), and secondary ion mass spectrometry (SIMS). Both of a- and m-plane GaN were prepared by growing thick GaN along the c-axis, and cutting in transverse orientations. The exposed non-polar surfaces were prepared by mechanical polishing (MP) and chemically mechanical polishing (CMP). Non-uniform luminescent characteristics on a- and m-plane GaN were observed in CL images, indicating a higher concentration of impurities in the area of more luminescence. CL spectra from the bulk samples revealed two peaks: 364 nm and 510 nm, related to band edge and impurity defects respectively. The detection by SIMS confirmed that oxygen was inhomogeneously incorporated during the growth of thick GaN layers. Surface qualities of a- and m-plane GaN were also investigated. The lower optical intensities from a-plane GaN at low acceleration voltages indicated more surface damages were introduced during polish. The optical intensity difference from the two samples was reduced at higher acceleration voltages. Similar CL intensities at low acceleration voltages from a- and m-plane GaN substrates prepared by CMP indicated improved surface qualities.
Thermodynamic Analysis of Impurities in the Sublimation Growth of AlN Single Crystals
Authors: Du Li , Edgar James H
Published: April 2011

The vapor phase species responsible for the transport of impurities in the sublimation-recondensation growth of bulk AlN crystals was predicted by thermodynamic analysis. AlN powder containing oxygen was investigated in Al-O-N system for an inert reactor. Dialuminum monoxide (Al2O) is strongly favored over all other possible oxygen containing species including NO and NO2. For AlN crystal growth in a graphite furnace, the Al-O-C-N system was studied. CO is the main species containing carbon and oxygen, and has a partial pressure more than one hundred times higher than all other carbon or oxygen containing species. Its partial pressure even exceeds that of Al vapor. Pure AlN growth on SiC seed was represented in the Al-N-Si-C system. SiC is not stable at high temperatures, the presence of nitrogen accelerates the decomposition of the SiC, and the most probable volatile silicon and carbon species originating from the SiC seed are Si, CN and C2N2.
Optical Properties of Nearly Lattice-matched AlInN/GaN Single Quantum Wells with Varying Well-widths
Authors: Tan Lay-Theng , Martin Robert W , Watson Ian M , O'Donnell Kevin P
Published: April 2011

Single GaN quantum wells in the nearly lattice-matched GaN/Al1−xInxN system have been studied using photoluminescence (PL) and PL excitation (PLE) spectroscopy. The structures were grown on free-standing GaN and sapphire substrates. Selectively excited PL is able to distinguish luminescence originating from the wells, barriers and the underlying GaN buffer layers. The PL spectra show that the quantum well transition energy decreases as the well-width increases. This manifestation of the quantum confined Stark effect (QCSE) results from intense spontaneous polarization fields, which are present even in the absence of strain in the QW layer. Power dependent PL measurements provide information on the screening of the internal fields. PLE data provide an estimation of the band gap and enable one to determine the energy shift between emission and absorption in the Al1−xInxN barriers.
Nanoparticle-loaded encapsulation materials for light-emitting diode applications
Authors: Mont Frank Wilhelm , Luo H. , Schubert M. F. , Kim J. K. , Schubert E. F. , Siegel R. W.
Published: April 2011

Nanoparticle-loaded encapsulants provide unique optical and material properties for the enhancement of light extraction efficiency in light-emitting diodes (LEDs). We report on the uniform dispersion of TiO2 nanoparticles with average diameter of 40 nm in epoxy, and the demonstration of a refractive index (n)of 1.68 at 400 nm wavelength, higher than that of pure epoxy (n = 1.53). It is found that proper chemical surfactants and nanoparticle preparation are critical to eliminate agglomeration of nanoparticles. Theoretical analysis of optical scattering in nanoparticle-loaded encapsulation materials reveals that although the size and loading factor of nanoparticles greatly influence scattering, specular transparency of the encapsulant film occurs if the thicknesses of the films are kept below the optical scattering length. Furthermore, the encapsulants benefit from an optimized scattering coefficient as demonstrated by three-dimensional ray-tracing simulations showing light extraction efficiency enhancements greater than 50%.
Comparison between the Photoluminescence Mappings Under Selective and Non-selective Excitation and the Electroluminescence Mappings of the Epitaxial Wafers with InGaN-LED Structure
Authors: Tadatomo Kazuyuki , Shimoike Osamu , Noda Hiromichi , Hiraoka Masahiro , Yoshimura Kazumasa , Hoshino Katsuyuki
Published: April 2011

We compared the photoluminescence (PL) mappings of epitaxial wafers for light emitting diodes (LEDs) by using a He-Cd laser (325 nm line) and a laser diode (LD) with peak wavelength of 405 nm as excitation sources and the electroluminescence (EL) mappings of the same wafers. The samples were epitaxial wafers for blue and green InGaN-LEDs obtained in commercial. The wafers were fabricated into LEDs with a Ni/Au transparent p-type electrode and a Ti/Ni n-type electrode after the PL mapping measurements. The He-Cd laser performed the band to band excitation of (Al)GaN cladding and contact layers (non-selective excitation). Because the photo-excited carriers at the cladding and contact layers diffused into the multi-quantum wells (MQWs) and contributed the PL emission by radiative recombination in the MQWs, the PL mapping under the influence of the (Al)GaN cladding and contact layers was obtained. On the other hand, the LD (405 nm) enable us to obtain the PL mapping under selective excitation of the MQWs without the influence of the cladding and contact layers. The PL mapping measurements were carried out at room temperature (RT) at the excitation power density of 310 W/cm2 under non-selective excitation (by the He-Cd laser) and at that of 11.5 W/cm2 under selective excitation (by the LD). The EL mapping was measured at a forward current of 20 mA at RT. The area of the wafer with high EL intensity was coincident with the area with the high PL intensity under selective excitation. Therefore, the PL mapping measurement under selective excitation of MQWs is recommended to characterize the epitaxial wafers and to estimate the device performance of InGaN-LEDs.
500 K operation AlGaN/GaN HFETs with a large current and a high breakdown voltage
Authors: Kambayashi Hiroshi , Li Jiang , Ikeda Nariaki , Yoshida Seikoh
Published: April 2011

It is reported that we demonstrated a large current operation AlGaN/GaN HFET with a low-on state resistance and a high breakdown voltage operation at room temperature and 500 K. We developed our unique ohmic electrode using Ti/AlSi/Mo. In addition, we investigated the dependence between the distance from the gate electrode to the drain electrode and the off-state breakdown voltage. As a result, the breakdown voltage of a unit HFET was over 1100 V. Furthermore, on the large scale HFET with the gate width of 240 mm, the maximum drain current of over 50 A was obtained at room temperature and also, that of over 25 A was obtained at 500 K. The off-state breakdown voltage was obtained about 800 V at room temperature and about 600 V at 500 K, although Si-based FETs can not operate in such a high temperature.
Influence of Overheating Effect on Transport Properties of AlGaN/GaN Heterostructures
Authors: Kurakin Andriy , Vitusevich Svetlana , Petrychuk Mykhailo , Hardtdegen Hilde , Danylyuk Serhiy , Belyaev Alexander , Klein Norbert
Published: April 2011

AlGaN/GaN heterostructures can operate at very high power in dc and rf regimes due to the superior properties of III-nitride materials. At the same time, the higher level of dissipated power causes higher overheating in the structures. Considering the fact that the temperature is a driving force for aging processes in devices, it is clear that the investigations of thermal effects in 2DEG of GaN-based structures are of great importance. Additionally, such studies result in a better understanding of the transport features which affect device performance, and allow us to determine the conditions under which hot-electron effects play the dominant role in electron transport. In this report we focus on thermal effects in transmission line model (TLM) patterns. The experimental results and numerical simulations of self-heating effects in TLM patterns processed on heterostructures with different layer design (different Al mole fraction of barrier layer, different substrates) are analyzed. The GaN-based heterostructures were grown by MOCVD. One of the investigated samples consists of a nucleation layer of AlGaN (16% Al), an undoped GaN buffer layer and an AlGaN (33%) undoped barrier layer. This type of structure was formed on sapphire and SiC substrates. Additionally, another two types of structures were grown on sapphire substrate: with wide bandgap barrier layer (with Al mole fraction of 75%), and with a thin AlN spacer layer (5nm) in between the AlGaN barrier layer (23 nm) and the GaN buffer layer (1300 nm). All samples were processed with TLM patterns with standard Ti/Al/Ti/Au contact metallization and annealed for 40s at 800° C. The conducting channel of the devices has a width W = 100 μm with intercontact lengths L of 1, 5, 10, 15, 20, 25, 30, 35 μm. The average overheating temperature over the active channel area for each structure type is estimated. The strong influence of the Al mole fraction of the barrier layer and also the substrate type of the structure on noise and transport properties is revealed and analyzed. It was shown that the implementation of wide bandgap barriers and high thermal conductance substrates allows us to improve the overall performance of the structures significantly. Optimal conditions for the observation of hot-electron effects are determined.
Fully Unstrained GaN on Thick AlN Layers for MEMS Application
Authors: Tonisch Katja , Niebelschuetz Florentina , Cimalla Volker , Romanus Henry , Ambacher Oliver
Published: April 2011

Usually, the fabrication of microelectromechanical systems (MEMS) requires unstrained or tensile strained active layers on a selectively removable sacrificial layer. Compressive strain would lead to instabilities due to buckling effects. For group III-nitride based MEMS, AlN is a promising material for sacrificial layers since it can be epitaxially overgrown and etched selectively to GaN. However, due to the larger lattice constants GaN is growing compressively strained on AlN. Nanoheteroepitaxy opens a way to yield unstrained, high quality epitaxial GaN layers on nanocrystalline AlN thin film by means of a 3D strain relaxation mechanism. For this purpose sputtered nanocrystalline AlN films were overgrown with single crystalline GaN and AlGaN/GaN layers by metalorganic chemical vapor deposition. The high quality of the layers is proven by an atomically flat surface and a 2D electron gas at the interface of the AlGaN/GaN heterostructure.
Assembling the Building Blocks for Diamond Electronics
Authors: Yost William Joseph
Published: April 2011

The desire to exploit the extreme properties which differentiate diamond from other, more mature wide bandgap technologies has recently been given further impetus by the development of high quality single crystal CVD diamond material [1].

To realise the significant potential of diamond devices over existing device technology depends on completing a number of key objectives, in particular providing:

(a). access in volume to high quality, ultra-high purity, single crystal material,

(b). the capability to provide carriers by doping the material in a controlled manner,

(c). the ability to process thin layers and structures.

Providing access to bulk single crystal diamond (albeit not electronic grade material) has already been largely achieved and plates are commercially available for cutting applications [2]. Routes to providing suitable charge carriers are being widely investigated. Although intrinsic diamond can have exceptional electronic properties [1], in reports of both p-type and n-type diamond [3,4] the dopants are very deep (0.37 eV and 0.6 eV for boron and phosphorous respectively), which limits the realisation of conventional electronic devices operating at ambient temperatures.

A series of novel devices undergone preliminary experimental evaluation. Devices made up of boron and intrinsic layers, where the boron concentration exceeds the limit of metallic conduction (>1×1020 cm−3), offer carrier diffusion at room temperature from the highly doped regions with low mobility, into adjacent regions of intrinsic material with high carrier mobility [5]. To provide the required device performance, the interface between the doped and intrinsic layers needs to be defect free and to change doping levels by several orders of magnitude in a few atomic layers.

Although progress over the last few years has been rapid, there remain substantial technical challenges ahead for the realisation of large scale diamond active electronics. This paper will identify and review progress against these key issues.
Color Centers in Diamond as Practical Single-Photon Source to Illustrate Quantum Complementarity
Authors: Jacques Vincent , Regnnie Steven , Chauvat Dominique , Roch Jean-François
Published: April 2011

A recent experiment performed by S. S. Afshar [reviewed in M. Chown, New Scientist183, 30 (2004)] has been interpreted as a possible violation of the complementarity principle of quantum mechanics. Starting from a single-photon wavefront-splitting interference experiment, we propose a new scheme for Afshar's experiment, and we show that Afshar's interpretation is incorrect. Furthermore, this design is well suited to illustrate the complementarity inequality in the interesting intermediate regimes with partial fringe visibility and partial which-path information.
Improving Diamond Device Performances for Pulsed Mode Detection
Authors: Hamrita Hassen , Tromson Dominique , Descamps Caroline , Mer Christine , Nesladek Milos , Bergonzo Philippe
Published: April 2011

CVD diamond combines attractive properties for the fabrication of detection devices operating in specific environments. One inherent problem however with diamond is the presence of defect levels that are altering the detection characteristics. They result in unstable responses and carrier losses. One of the issues can be to operate the devices in the pulsed mode regime, when transitory effects are less impacted by defective levels evolution than for steady state currents. This therefore implies the use of materials that are faster, but also of lower carrier lifetime, and therefore generally agreed to be of poorer quality. This has therefore to be compromised with respect to the desired performances.
High Efficiency Heat Sinks from Polycrystalline Diamond Grown by CVD Method
Authors: Voronov Oleg A. , Tompa Gary S. , Veress Veronika
Published: April 2011

While absolute power levels in microelectronic devices are relatively modest (a few tens to a few hundred watts), heat fluxes can be significant (through 50 W/cm2 in current electronic chips and up to 2000 W/cm2 in semiconductor lasers). Diamond heat sinks enable heat transfer rates well above what is possible with standard thermal management devices. We have fabricated heat sinks using diamond, which has the highest temperature thermal conductivity of any known material. Polycrystalline diamonds manufactured by chemical vapor deposition (CVD) are machined by laser and combined with metallic or ceramic tiles. Cooling by fluid flow through micro-channels enhances heat removal. These unique attributes make diamond based heat sinks prime contenders for the next generation of high heat load sinks. Such devices could be utilized for efficient cooling in a variety of applications requiring high heat transfer capability, including semiconductor lasers, microprocessors, multi-chip modules in computers, laser-diode arrays, radar systems, and high-flux optics, among other applications. This paper will review test designs, heat flux measuring system, and measured heat removal values.
Low Threshold Field Emission from Amorphous Carbon Films Grown by Electrochemical Deposition
Authors: Kiyota Hideo , Higashi Mikiteru , Kurosu Tateki , Iida Masamori
Published: April 2011

Electrochemical deposition of amorphous carbon (a-C) film is performed by applying a direct-current potential to substrates immersed in methanol. Both scanning electron microscopy and Raman results indicate that smooth and homogeneous a-C films are grown on specific substrate materials such as Si, Ti, and Al. Field emission measurements demonstrate excellent emission properties such as threshold fields lower than 5 V/μm. Field enhancement factors are estimated to be 1300–1500; these are attributed to local field enhancements around sp2 carbon clusters that are embedded in the a-C films. Emission properties of a-C films grown on Si exhibit a current saturation under higher applied fields. These saturation behaviors are explained by an effect of a potential barrier formed at the interface between a-C films and substrates. Since the interface barrier is reduced by formation of the Ti interfacial layer, an approach to use carbide formation at the interface is verified as useful to improve the emission properties of a-C films.
Surface Bio-functionalization of boron doped diamond
Authors: Bonnauron Mathias , Agnes Charles , Mailley Pascal , Arnault Jean-Charles , de Sanoit Jacques , Mer Christine , Rocha Licinio , Bergonzo Philippe , Nesladek Milos , Omnes Frank , Elmazria Omar
Published: April 2011

Biotin was grafted onto boron doped nanocrystaline diamond electrode through a four-step-chemical process. First step of grafting was electro-chemical reduction of p-4,nitro diazonium salts. The grafting was characterized using X-ray Photoelectron Spectroscopy (XPS), Atomic Force Microscopy (AFM) and fluorescence analysis. XPS analysis and curve fitting procedure lead to the identification of different bond states of carbon present in the grafted film and highlights difference of coverage rate between samples. AFM highlights the non-mononuclear aspect of the diazonium electrografting. Fluorescence measurements with avidin recognition prove the efficient of the grafting.
Energetics, Electronic Properties, and Geometries of B-Doped Diamond: A First-Principles Study
Authors: Saito Susumu , Maeda Tomohisa , Miyake Takashi
Published: April 2011

We study electronic and geometrical properties of B-doped diamond in the framework of the density functional theory. The systems studied are simple-cubic (sc) BC7 and face-centered-cubic (fcc) BC15 and B2C14. In the case of BC7, we also study the effect of the tetragonal and rhombohedral distortions of the unit cell to be introduced to the B-doped diamond grown homoepitaxially on the diamond (100) and (111) films, respectively. In the case of fcc B2C14, on the other hand, the inhomogeneous doping effects are discussed from the viewpoint of the energetics. It is confirmed that in the homogeneous doping case, B doping gives rise to hole doping into the valence band, which should be responsible for the metallic and superconducting transport properties of the material. In the case of the inhomogeneous doping, two B atoms on the first-nearest neighbor sites as well as on the second-nearest neighbor sites are found to lower the Fermi-level density of states, which is unfavorable for superconductivity. Although tetragonal and rhombohedral distortions are found to give a sizable change to the electronic states near the Fermi level, the value of the Fermi-level density of states itself is found to remain almost unchanged. Finally, in order to study the fundamental-gap value of the heavily B-doped diamond, we study the electronic structure of the sc BC7 and fcc BC15 in the GW approximation. A large reduction of the gap value upon doping is confirmed.
Simple and Effective Way to Improve the Stability of Titanium Based Boron Doped Diamond Film Electrode
Authors: Guo Liang , Chen Guohua
Published: April 2011

Boron-doped diamond film coated titanium (Ti/BDD) becomes increasingly attractive because of the combined properties of these two unique materials. The challenge for the composite material is the stability especially when it is used as an electrode. In order to meet this challenge, two temperature (2-temp) staged hot filament chemical vapor deposition method was employed in this study. The accelerated working life time was significantly increased to 804 hours for the 2-temp electrode, compared with 244 hours for the diamond film electrode fabricated under one temperature (1-temp) stage method. With the characterization of micro-Raman, XRD, and cross-sectional SEM, a multilayer of Ti/TiC/(Diamond+Amorphous Carbon)/Diamond can be found in the 2-temp sample and the structure of Ti/TiC/Diamond in the 1-temp sample. There was less void space observed in the interlayer of 2-temp sample. The multi-layered compact structure plays an important role in improving the adhesion of diamond film to the titanium substrate which in turn increases the electrode working life time by over 3 times.
Diamond Based Ion-Sensitive Field Effect Transistors for Cellular Biosensing
Authors: Bennett Andrew , Gaudin Olivier , Williams Oliver A. , Foord John S. , Jackman Richard B.
Published: April 2011

The use of a chemically inert and essentially biocompatible material for cellular biosensing is an attractive idea. In this context we have studied the operation of diamond-based ion-sensitive field effect transistors (ISFETs) within solutions of varying pH, and alkali-halide concentrations. In particular, we report the use of an inexpensive diamond substrate material, often referred to as ‘black’ diamond. pH sensitivity was observed when devices based on hydrogen-terminated p-type surfaces where employed, provide some surface oxidation was performed prior to their use. Variation in the threshold voltage for ISFET operation of the order of 20mV/pH unit was determined. In terms of operation in potassium iodide solution, we have shown that the device is shows selective sensitivity to the iodide species, despite the equi-molar presence of both K+ and I- species. The origin of this selectivity is discussed.
Persistent Photoinduced Changes in Charge States of Donors and Acceptors in Hydrothermally Grown ZnO
Authors: Giles Nancy C. , Jiang Yongquan , Yang Xiaocheng , Evans S. M. , Halliburton L. E.
Published: April 2011

Bulk ZnO grown by the hydrothermal technique was investigated using electron paramagnetic resonance (EPR), photoluminescence (PL), and infrared absorption (FTIR) techniques. Isolated subsitutional lithium is the dominant acceptor and could be detected using EPR or PL. A large concentration of neutral Li+-OH− centers were observed using FTIR data. EPR spectra assigned to Mn, Co, Ni, Fe, and Group III (Al, Ga) donors were also observed. Photoinduced changes in the charge states of the different deep and shallow centers were produced using 325 nm light, and the stability of these changes were monitored with EPR during pulsed thermal anneals. The charge-state changes for some defects were persistent and remained up to 300 K. These impurities, when present in device structures, may act as stable charge trapping sites.
Transition Metal-Doped ZnO: A Comparison of Optical, Magnetic, and Structural Behavior of Bulk and Thin Films
Authors: Fenwick William E. , Kane Matthew H. , Fang Zaili , Zaidi Tahir , Li Nola , Rengarajan Varatharajan , Nause Jeff , Ferguson Ian T.
Published: April 2011

Transition metal-doped ZnO bulk crystals and thin films have been investigated to determine the effects of transition metal incorporation on optical, magnetic, and structural properties of ZnO. A modified melt growth technique was used to grow bulk Zn1-xMnxO, Zn1-xCoxO, and Zn1-xFexO. Optical transmission measurements show an apparent shift in absorption edge with increasing transition metal incorporation. Raman spectroscopy also shows increasing lattice disorder with increasing transition metal concentration. ZnO thin films doped with Ni, Co, and Gd were grown by metalorganic chemical vapor deposition (MOCVD). While the Co-doped thin films showed antiferromagnetic behavior, magnetic hysteresis was observed in the Ni-doped and Gd-doped thin films. Structural quality was verified with X-ray diffraction (XRD), and optical properties were investigated using room temperature photoluminescence (PL) and optical transmission measurements. Properties of ZnO:TM bulk crystals and thin films are compared and used to discuss possible origins of ferromagnetism in these materials.
Growth and Characterization of Homoepitaxial ZnO Thin Films Grown by CVD
Authors: Neumann Christian , Sann Joachim , Lautenschläger Stefan , Bertram Frank , Christen Jürgen , Bläsing Jürgen , Krost Alois , Meyer Bruno K.
Published: April 2011

ZnO as a direct wide-band-semiconductor with its band gap of 3.3 eV at room temperature is a promising optoelectronic material. The main obstacle in the ZnO system is its lack of achieving reproducible p-type conductivity. The main reasons for this are the high residual intrinsic and extrinsic defect concentrations which are still not completely understood.

Homoepitaxial growth of ZnO and thus minimization of intrinsic defects due to lattice mismatch and incorporation of residual substrate species could be a solution to overcome these problems. Despite the availability of ZnO bulk single crystals reports regarding ZnO homoepitaxy are still quite rare. In this paper we report on a successful homoepitaxial growth of ZnO thin films by chemical vapor deposition (CVD).

Atomic force microscopy shows that two-dimensional epitaxial growth was achieved without any additional buffer layer. With a rocking curve full width at half maximum (FWHM) of 17 arcsec the deposited films show a superior crystalline quality compared to its substrate. The optical quality of the epitaxial films has been characterized laterally by cathodoluminescence and spectrally by photoluminescence. Excitonic emissions at 4K are as narrow as 110 μeV. A dependence of the appearance of excitonic emissions from the growth polarity can be shown which is attributed to different incorporation rates of extrinsic defects.
Orders of Magnitude Reduction in Threading Dislocations in ZnO Grown on Facet-Controlled GaN
Authors: Chua Soo Jin , Zhou Hai Long , PAN Hui , Osipowicz Thomas
Published: April 2011

ZnO is grown by chemical vapour deposition on {1 1 -2 2} GaN planes formed by epitaxial layer overgrowth. Window stripes in a SiO2 mask are oriented in the <1 -1 0 0> direction of the GaN film. Triangular GaN ridge are formed during ELO growth by metal organic chemical vapour deposition. A flat (0 0 0 1) ZnO plane is grown on each triangular cross-section ridge and it is found that the ZnO film has dislocation density reduced by two orders of magnitude compared to that of the GaN substrate.
Cathodoluminescence Study of Indented ZnO Crystals
Authors: Jiménez Juan , Avella Manuel , Jiménez Juan , Rodríguez Tomás , Callahan Michael , Grant E. , Rakes K. , Bliss David , Wang Buguo
Published: April 2011

Vickers indentations of ZnO crystals grown by te hydrothermal method wer studied by cathodoluminescence. The plastic deformation induced defects, which have influence on the luminescence spectrum. The main changes in the luminescence response concern the generation of non radiative recombination centers, but also a band close to the first order phonon replica of the free exciton is observed to appear in the regions deformed by the indentation. Besides, deep levels are also generated. The result of this is a relative increase of the defect related bands respect to the dominant bound exciton emission.
Electrical Characterization of Isotype n-ZnO/n-GaN Heterostructures
Authors: Alivov Yahya , Bo Xiao , Akarca-Biyikli Sena , Qian Fan , Johnstone Daniel , Lopatiuk-Tirpak Olena , Chernyak Leonid , Litton Cole , Morkoç Hadis
Published: April 2011

Electrical properties of n-ZnO/n-GaN isotype heterostructures obtained by rf-sputtering of ZnO films on GaN layers grown by metal-organic vapour phase epitaxy are discussed. Current-voltage (I-V) characteristics of the n-ZnO/n-GaN diodes revealed highly rectifying behavior with forward and reverse currents ∼1.43×10-2 A/cm2 and ∼2.4×10-4 A/cm2, respectively, at ±5 V. From the Arrhenius plot built using temperature dependent current-voltage characteristics (I-V-T) an activation energy 0.125 eV was derived for the reverse bias leakage current path, and 0.62 eV for the band offset from forward bias measurements. From electron-beam induced current measurements the minority carrier diffusion length in ZnO was estimated in the range 0.125-0.175 mm, depending on excitation conditions. The temperature dependent EBIC measurements yielded an activation energy of 0.462 ± 0.073 V.
Cathodoluminescence Study of Hydrothermal Zn1-xMgxO Alloy Crystals
Authors: Mass Julio , Avella Manuel , Jiménez Juan , Callahan Michael , Grant E. , Rakes K. , Bliss David , Wang Buguo
Published: April 2011

Hydrothermal ZnMgO crystals were studied by cathodoluminescence. The high energy shift of the excitonic luminescence demonstrates the Mg incorporation in the ZnO lattice in at a few percent. The spectral parameters of the luminescence emission show a marked dependence of the incorporation of defects and Mg on the growth facet. This growth sector selectivity shows similar trends to those observed in hydrothermal ZnO crystals. The in depth distribution of Mg was studied varying the acceleration voltage of the excitation e-beam, showing a slight accumulation of Mg close to the surface.
Effect of Cryogenic Temperature Deposition of Various Metal Contacts to Bulk, Single-Crystal n-type ZnO
Authors: Wright Jon , Stafford L. , Gila B. P. , Norton D. P. , Pearton S. J. , Wang Hung-Ta , Ren F.
Published: April 2011

The development of reliable and thermally stable Ohmic and Schottky contacts to ZnO is one of the critical issues related to the fabrication of ZnO-based UV light emitters/detectors and field effect transistors. To date, a number of different metallization schemes and surface cleaning procedures prior to metal deposition have been examined for rectifying contacts on n-ZnO. While these reports have shown that low reactive metals such as Au, Ag and Pd form rectifying contacts with Schottky barrier heights in the 0.6-0.8 eV range, the thermal stability of these contacts is usually extremely poor, with degradation occurring even at 60°C for Au/n-ZnO. One approach to achieving increased barrier heights that has proven successful for GaAs, InP, InGaAs and other compound semiconductors is the use of cryogenic deposition temperatures. In this context, we report in this work on the effect of cryogenic temperature metal deposition on the contact properties of Pd, Pt, Ti, and Ni on single-crystal n-type ZnO. Deposition at both room and low temperature produced contacts with Ohmic characteristics for Ti and Ni metallizations. In comparison, both Pd and Pt contacts showed rectifying characteristics after deposition. All rectifying contacts exhibited barrier heights around 1-2 eV and idealities between 1 and 2. Low temperature deposition gave higher resistances in comparison to room temperature deposition for all cases. Larger contacts also corresponded to an increase in resistance. Changes in contact behavior were measured on Pd to anneal temperatures of ∼300°C, showing an increase in barrier height along with a decrease in ideality with increasing temperature. This difference with annealing temperature is in sharp contrast to previous results for Au contacts to ZnO. There were no differences in near-surface stoichiometry for the different deposition temperatures; however low temperature contacts demonstrated some cracking in Pt and Pd, probably due to surface stress.
Microstructure and Electrical Property Correlations in Ga:ZnO Transparent Conducting Thin Films
Authors: Bhosle Vikram , Narayan Jagdish
Published: April 2011

We report the correlations between processing, microstructure and electrical properties of Ga doped ZnO films. Films with varying grain size were grown on amorphous glass by changing the substrate and pulsed laser deposition variables. The results corresponding to these films were compared with those from epitaxial single crystal films grown on (0001) sapphire. Microstructural characteristics were analyzed in detail by using X-ray diffraction and transmission electron microscopy. Electrical properties were evaluated by resistivity measurements in the temperature range of 15-300K and Hall measurements at room temperature. It was observed that the grain boundaries and orientation of grains (texture characteristics) affected the carrier concentration and the mobility considerably in nanocrystalline films deposited on glass substrates. This effect is envisaged to occur as a result of trapping of electrons and build up of a potential barrier across the grain boundaries. However, the resistivity in nanocrystalline films could be decreased significantly by carefully controlling the deposition conditions. For a film deposited on glass at 2000C and 1 mtorr of oxygen partial pressure, we attained a minimum resistivity value of 1.8 × 10-4Ω-cm. As a comparison, the epitaxial films on sapphire substrates showed a resistivity of 1.4 × 10-4 Ω-cm, deposited at 4000C and pressure of 2.4 × 10-2 torr. Role of grain boundaries and defects in controlling the carrier generation and transport is considered in detail and possible mechanisms limiting the electrical conductivity in films with different microstructures are identified.
Opto-Electronic Properties and Stability of Artificial Zinc Oxide Molecules
Authors: Pozhar Liudmila A , Brown Gail J.
Published: April 2011

The Hartree-Fock (HF), restricted open shell HF (ROHF), and multiconfiguration self-consistent field (CI/CASSCF/MCSCF) approximations are used to study computationally the electronic properties of zinc oxide artificial molecules whose structure and composition have been derived from those of the symmetry elements of the wurtzite bulk lattice of zinc oxide. Such molecules may provide realistic models for small ZnO quantum dots (QDs) synthesized in “vacuum” or quantum confinement (such as that of well-defined nanopore arrays of silica and alumina membranes) using variety of methods in particular, supercritical fluid deposition. The computational direct optical transition energy (OTE) of the confined molecule appears to be several times smaller than that of the corresponding vacuum cluster. The charge and spin density distributions of these molecules (CDDs and SDDs, respectively) differ significantly, revealing dramatic effects of quantum confinement on electronic properties of Zn-O clusters. The obtained results suggest that manipulations with the electronic properties of the confined clusters by sophisticated design of their quantum confinement may provide means for synthesis of Zn-O – based electronic materials that combine a wide, tunable band gap with large, tunable exciton binding energy.
Epitaxial Growth of ZnO Thin Films on SiC Prepared by Chemical Solution Deposition
Authors: Park Young-Sik , Jeon Young-Sun , Jeon Kyung-Ok , Kang Bo-An , Hwang Kyu-Seog , Jeong Ju-Hyun , Lee Young-Hwan
Published: April 2011

Zinc oxide (ZnO) thin films have emerged as one of the most promising oxide materials owing to their optical and electrical properties, together with their high chemical and mechanical stability. Chemical solution deposition (CSD) is attractive technique for obtaining ZnO thin films and has the advantages of easy control of the film composition and easy fabrication of a larger-area thin film at low cost. In this work, epitaxial ZnO thin films on SiC substrate were prepared by using a CSD method with a zinc naphthenate precursor. Precursor films were pyrolyzed at 500°C for 10 min in air and finally annealed at 600°C, 700°C, 800°C and 900°C for 30 min in air. Crystallinity and in-plane alignment of the films were investigated by X-ray diffraction theta-2 theta scan and pole-figure analysis. Scanning electron microscope, scanning probe microscope, and He-Cd laser (325 nm) are used to detect the surface morphology and photoluminescence of the films. The effects of annealing temperature on crystallinity and epitaxy of the films will be fully discussed on the basis of the results of X-ray diffraction analysis.
Influence of the Particle Size on Acoustic Phonon Modes of ZnO Nanocrystals
Authors: Yadav Harish Kumar , Sreenivas K. , Gupta Vinay , Singh S. P. , Sundarakannan B. , Katiyar R. S.
Published: April 2011

Size dependence of the low frequency vibrational spectra of ZnO nanocrystals prepared using chemical method has been investigated. Optical transmission spectra of the ZnO colloid solution exhibit a shift in the onset of the absorption band edge from 332 to 350 nm due to particle growth. X-ray diffraction analysis of the prepared ZnO nanocrystals exhibit peaks corresponding to the hexagonal wurtzite structure. Two peaks with unusually very high intensity were observed in the low frequency (∼ 10- 25 cm-1) Raman spectra of these nanocrystals. The peak position of these phonon modes shifted towards lower frequencies as the size of the nanocrystals increases and assigned to the confinement of acoustic phonons in ZnO nanocrystals.
Enhancement of Field Emission Current from ZnO Nanorods Fabricated by Two Step Chemical Vapor Deposition with Laser Ablation of ZnO
Authors: Hirate Takashi , Kimpara Takashi , Takizawa Kazumoto , Satoh Tomomasa
Published: April 2011

We fabricated well-aligned ZnO nanorods with thin diameter by two-step chemical vapor deposition method combined with laser ablation of a sinterd ZnO target. Firstly, well-aligned ZnO nanorods with thick diameter of about 110 nm are grown on an n-Si (111) wafer. Next, a thin ZnO nanorod with about 30 nm diameter is grown on the center of the flat tip of each well-aligned and thick ZnO nanorod by controlling a flow rate of oxygen. Although thick ZnO nanorods do not emit a recognizable field emission current, thin ZnO nanorods with 1500 nm length show a field emission current of 500 μA under an electric field of 31 V/μm.
Synthesis of Water Soluble PbSe Quantum Dots
Authors: Etgar Lioz , Lifshitz Efrat , Tannenbaum Rina
Published: April 2011

Water-soluble PbSe semiconductor quantum dots (QDs) with near-infrared absorption of 1100-2520 nm (corresponding to a diameter of 3-13 nm) were synthesized using 2-aminoethanthiol. The oleic acid stabilizing ligands used in the traditional synthesis of PbSe were exchanged with the 2-aminoethanethiol (AET) ligands, which promoted the solubilization of the QDs in an aqueous medium. This occurred due to the attraction of the surrounding water molecules to the exposed amino-group, thus allowing the particles to reside in the water environment. The water-soluble PbSe QDs have very narrow size distribution (σ ≈ 4.5-5.5%). Transmission electron microscopy (TEM), spectrophotometric measurements, and Fourier transform infrared (FTIR) spectroscopy indicate that the morphology, size, size distribution and chemical composition of the PbSe QDs remained unchanged during the transfer to an aqueous medium. In conclusion, the ability to synthesize water soluble PbSe QDs with stable properties and uniform size distribution will allow them to have substantial advantages for biological applications such as biosensors and drug delivery.
Monoatomic Chains Deposited onto Silicon Steps Studied by Density Functional and Scattering Theories
Authors: Mazzone Anna
Published: April 2011

The purpose of this study is the assessment of the properties of deposited atomic chains. Therefore linear chains of covalent and metallic atoms, i.e. As and Ag,deposited onto monolayer SA steps onto Si(100) have been considered. Their study is based on a semi-empirical Hamiltonian, used for the evaluation of both the electronic structure and the conductance, and the calaculations analyze the binding energy of chains of variable length deposited onto steps in the light of the analogous energy of free-standing chains and of chains deposited onto the Si(100) surface. This comparison shows that the stability of the chains has a primary dependence on the type of the substrate, rather than on the chain length and composition, and increases in the order: free standing,deposited onto SA,deposited onto Si(100). The central result of the calculations of the conductance is that the dependence on the type of the substrate and on the chain composition parallels the one of the binding energy.
Control of Spontaneous Emission from Colloidal Quantum Dots in a Polymer Microcavity
Authors: Menon Vinod , Valappil Nikesh , Zeylikovich Iosef , Gayen Taposh , Das Bidyut , Alfano Robert R
Published: April 2011

We report the fabrication of a one dimensional microcavity structure embedded with colloidal CdSe/ZnS core/shell quantum dots using solution processing. The microcavity structures were fabricated by spin coating alternating layers of polymers of different refractive indices (poly-vinylcarbazole, and poly-acrylic acid) to form the distributed Bragg reflectors (DBRs). Greater than 90% reflectivity was obtained using ten periods of the structure. The one dimensional microcavity was formed by sandwiching a λ/n thick defect layer between two such DBRs. The microcavity demonstrated directionality in emission and well behaved dispersion characteristics. Room temperature time-resolved photoluminescence measurements carried out on this structure showed six fold enhancement of spontaneous emission rate. The photoluminescence decay time of the quantum dots was found to be ∼ 1 ns while for the quantum dots embedded in the microcavity it was ∼150 ps.
Selective-Area Growth of Self-Assembled InAs-QDs by Metal Mask Method for Optical Integrated Circuit Applications
Authors: Ozaki Nobuhiko , Takata Yoshiaki , Ohkouchi Shunsuke , Sugimoto Yoshimasa , Ikeda Naoki , Watanabe Yoshinori , Kitagawa Yoshinori , Mizutani Akio , Asakawa Kiyoshi
Published: April 2011

We have developed a selective-area-growth (SAG) method of self-assembled InAs quantum dots (QDs) using a metal-mask (MM) combined with molecular beam epitaxy for realizing photonic crystal (PC) based ultra-small and ultra-fast all-optical devices (PC-SMZ and PC-FF). Successful SAG of QDs was confirmed by atomic-force-microscopy observations and photoluminescence (PL) measurements. High density and high uniformity comparable to those of conventional QDs grown without the MM were achieved; the QD density was 4 × 1010cm-2 and a linewidth of the PL peak was around 30meV at room temperature. In addition, insertion of a strain-reducing layer on the grown QD was effective for varying the PL peak wavelength of the QD from 1240nm to 1320nm without any extra optical degradation. The MM method reported here is promising for achieving the all optical devices, PC-SMZ and PC-FF, which require SAG of QDs and a QD ensemble with a different absorption-peak wavelength in a different area.