Symposium : T
|09:00||COST MP0805 meeting|
|InN : Chair to be confirmed|
|11:00||Superconducting properties of InN with the carrier density close to Mott transition|
Authors : Takashi Inushima
Affiliations : Department of Electronics, Tokai University
Resume : InN is a key material for blue light emitting devices. However, there is considerable controversy concerning its crystal quality. The most serious problem for the crystal quality is the presence of superconductivity. A plausible explanation for the superconductivity is done considering the presence of a segregated In phase in InN samples. However, there are many experimental evidences which are not favorable for this mechanism. In order to understand whether the superconductivity is intrinsic, we have accumulated the large array of experimental results on a number of samples from different laboratories around the world, and found that the samples with their carrier density close to the Mott transition explain the superconducting properties of InN most clearly. The ac susceptibility provides us with the volume of inclusions with different superconducting transition temperatures, the type of superconductivity, and the vortex structure of superconducting InN. The temperature dependence of ac susceptibility-onset suggests two types of superconductivity. From the results of magneto-resistance, Shubnikov de Haas oscillation and magneto-plasma reflection measurements, we show that there are two types of electrons at the Fermi surface. Then we present a possible mechanism for the existence of the two types of electrons in the conduction band as unexpected residual carriers, which explains the two different types of superconductivity in InN.
|11:30||Free–standing InN and the Effect of the Substrate on its Growth|
Authors : R. Garcia, M. Barboza-Flores, F. A. Ponce
Affiliations : Department of Research in Physics, University of Sonora, Hermosillo, Sonora, 83000, México; Department of Research in Physics, University of Sonora, Hermosillo, Sonora, 83000, México; Department of Physics, Arizona State University, Tempe, AZ 85287, USA
Resume : InN is predicted to have low effective mass for electrons which should lead to high mobility and high saturation velocity. Applications for InN is a current research topic. The main applications can embrace field effect transistors, InN might become the successor of GaN for high-power high-temperature microwave and millimeter wave applications. In optoelectronics, ternary and quaternary alloys of InN with GaN and AlN can be used to make tunable band-gap devices such as LEDs, LDs, PVCs. In this work we present several suitable methods to synthesize InN. Chemical vapor deposition technique was used to grow InN micro-columns on different substrates (Si, SiO2 and Au/SiO2) and the effect on the growth was studied. This technique consists in a horizontal quartz tube reactor that uses ultra high purity InCl3 and UHP ammonia as precursors. The morphology, composition and structure of the InN columns were studied by Electron microscopy and X-ray Diffraction. It was found that organized wurtzite–type InN columns growth is favored when Au-droplets are used in silica substrates as nucleation sites. The optoelectonic properties were analyzed by Thermoluminescence and IR spectroscopy, by means of this technique a band gap of approximately 0.7 eV was measured in the InN columns grown in this research.
|11:45||Oxygen-induced changes of the InN(0001) and InN(000-1) surface electronic properties|
Authors : A. Eisenhardt, M. Himmerlich, S. Krischok
Affiliations : Institute of Physics and Institute of Micro- and Nanotechnologies, TU Ilmenau, PF 100565, 98684 Ilmenau, Germany
Resume : We investigated the surface electronic properties of as-grown In- and N-polar InN films and their changes upon the exposure to oxygen. The measurements were performed by photoelectron spectroscopy (XPS, UPS) in an UHV surface analysis chamber that is directly connected to the preparation chamber, where thin InN films were grown by plasma-assisted molecular beam epitaxy. The InN surfaces show polarity-dependent differences in the formation of surface reconstructions, surface states and surface band bending. The electron binding energies are higher for In-polar InN compared to N-polar InN surfaces confirming the commonly observed downward band bending for InN(0001), while as-grown N-polar InN surfaces seem to have almost flatband conditions. In order to evaluate the influence of specific adsorbates on the electronic properties, the as-grown surfaces were exposed to O2 under vacuum conditions. For InN(0001) a strong reduction of the downward band bending by 0.3-0.4 eV is found after adsorption of oxygen species, revealing a decrease in the surface electron concentration. This is accompanied by an increase of the work function Φ and the formation of an adsorbate-induced dipole. In contrast, InN(000-1) surfaces exhibit only a slight increase in the downward band bending (< 0.1 eV) as well as a slight increase of Φ. These model experiments reveal that oxygen alone cannot explain the strong surface electron accumulation normally observed at polar InN stored in the ambient.
|12:00||Analysis of the band alignment and surface states of clean In-polar, N-polar, m-plane and a-plane InN surfaces by in-situ photoelectron spectroscopy|
Authors : M. Himmerlich, A. Eisenhardt, S. Krischok
Affiliations : Institute of Physics and Institute of Micro- and Nanotechnologies, TU Ilmenau, PF 100565, 98684 Ilmenau, Germany
Resume : The chemical and electronic properties of (0001), (000-1), (1-100) and (11-20) oriented InN surfaces have been characterized under UHV conditions after plasma-assisted molecular beam epitaxy growth. For this purpose, thin InN films were grown on GaN/Al2O3(0001), GaN/6H-SiC(000-1) and m-plane as well as a-plane bulk GaN substrates, respectively. Subsequently the core levels and valence band density of states were in-situ characterized using XPS and UPS. The energy distance between surface valence band maximum and Fermi level as well as the work function are dependent on crystal orientation. For clean, stoichiometric (000-1), (1-100) and (11-20) InN surfaces, the downward band bending is strongly reduced compared to the In-polar configuration, pointing to nearly flat band conditions. The InN(0001) surface exhibits an In-induced (2×2) reconstruction and a surface state at the Fermi edge, while for the growth on the other crystallographic orientations, no superstructure was observed in RHEED experiments and the occupied surface states are located at the valence band edge. The trends of variation of the surface band alignment as well as the observed occupied states are in good agreement with results of calculations based on density functional theory [1-3].  D. Segev, C. G. Van de Walle, Europhys. Lett. 76, 305 (2006)  C. G. Van de Walle, D. Segev, J. Appl. Phys. 101, 081704 (2007)  A. Belabbes, J. Furthmüller, F. Bechstedt, Phys. Rev. B 84, 205304 (2011)
|12:15||Vacancy clusters in n-type and p-type InN|
Authors : Filip Tuomisto, Floris Reurings, Christian Rauch
Affiliations : Department of Applied Physics, Aalto University, Finland
Resume : In vacancies are abundant in most InN thin films. These in-grown defects are typically complexed with a donor-type defect that may in principle be a residual impurity such as O or H, an n-type dopant such as Si, or an intrinsic defect such as the N vacancy. The In vacancies and their complexes are generally deep acceptors generating radiative and non-radiative deep levels in the gap, they compensate for the n-type conductivity, and add to the scattering centers limiting the carrier mobility in these materials. We present results obtained in nominally undoped, Si-doped and Mg-doped InN thin films grown by molecular beam epitaxy. We show how various In vacancy – donor complexes can be selectively identified with positron annihilation spectroscopy. The results indicate that In vacancy – N vacancy pairs are the dominant form of in-grown cation vacancy complexes, while there is strong clustering of more N vacancies around the In vacancies at high Si doping levels near the layer/substrate interface. Moderate Mg doping strongly reduces the apparent concentration of In vacancy defects in line with the lowering of the Fermi level (the bulk of the layer is p-type in these cases), but at high doping levels In-N vacancy clusters reappear. We interpret these observations in the light of extended defect and impurity distributions.
|12:30||Carrier dynamics characteristics of 1.55 μm InN quantum well lasers on GaAs substrate|
Authors : Steven Lampard
Affiliations : electrical and electronic engineering,university of leeds
Resume : A model for carrier dynamics study on InN Quantum well laser structure was investigated using rate equation approach. Temperature dependence on the dynamics is also added in the the model. At low tempertures, carriers filling follows non-equlibrium distribution however at high tempertures carriers filling is more fermi-dirac like. We also found S-shaped thereshold current thoughout the tempertaure range from 77K to 300K experimently.
|Application I : Chair to be confirmed|
|14:15||Dilute Nitride III-V Quantum Wells for Solar Cell Applications|
Authors : Alex Freundlich, Gopi Krishna Vijaya, Andenet Alemu
Affiliations : Photovoltaics and Nanostructures Laboratory, Center for Advanced Materials and Physics Department, University of Houston, Houston TX 77204-5004 USA
Resume : Existing multi-junction III-V solar cells have demonstrated concentrated sunlight efficiencies in excess of 43% (under 500 suns). For over a decade various schemes, involving the use of bulk-like 1.0-1.2 eV dilute nitride (i.e. GaInNAs) subcells, have been contemplated to further enhance the efficiency of triple and quadruple junction solar cells. Nevertheless, thus far poor minority carrier diffusion lengths (20-40 nm) encountered in dilute nitrides has led to severe degradations of device open circuit voltage and has been a major impediment toward the development of efficient solar cells. In order to alleviate issues encountered in bulk-like InGaAsN devices, we consider an alternate path, where dilute nitride quantum wells (well width << minority diffusion length) are inserted within the intrinsic region of a conventional GaAs p-i-n sub- cell. Realization of such single junction device has been shown to significantly improve the open circuit voltages compared to bulk-counterparts and appears as a viable route toward multi-junction devices with practical conversion efficiencies approaching 50%. Furthermore it is shown that unique properties of dilute nitrides (increased electron effective masses, and negligible valence band offset) provide a pathway toward the implementation of novel quantum engineered designs that could enable practical conversion efficiencies well beyond the theoretical limits of bulk-like semiconductor devices.
|14:45||GAINNAS SOLAR CELLS WITH HIGH NITROGEN CONTENT|
Authors : Arto Aho, Antti Tukiainen, Ville Polojärvi, Joel Salmi and Mircea Guina
Affiliations : Tampere University of Technology/Optoelectronics Research Centre, +358 40 7400191, email@example.com, Tampere, Finland
Resume : GaInNAs alloys are very attractive for developing ultra-high efficiency multijunction solar cells; in particular, they can absorb solar spectrum down to 0.8-1.1eV and at the same time can be grown lattice matched on GaAs. We report the growth and characterization of GaInNAs/GaAs single junction p-i-n solar cells with N contents ranging from 0 to 6%. The structures were grown by RF-plasma assisted molecular beam epitaxy on n-GaAs(100) substrates at ~440oC using a V/III-ratio of ~10. The amount of In was adjusted according to N content to ensure lattice matching of the GaInNAs film to GaAs. AM0 measurements of uncoated samples reveal that the current (Jsc) increased when the N content was increased from 1% (13 mA/cm2) to 4% (22 mA/cm2). When N content was increased above 4% the material quality degrade significantly reducing the Jsc. Simultaneously, the open circuit voltage decreases from 0.66 V for 1% N to 0.16 V for 6% N. The reasons for material degradation will be discussed using atomic force microscopy, x-ray diffraction, quantum efficiency and photoluminescence measurements. Finally, the potential of GaInNAs with 4% nitrogen was assessed in a 3-junction GaInP/GaAs/GaInNAs solar cell. For measurements at AM1.5G the cell exhibited a current density of ~10 mA/cm.
|15:00||Effects of well depth on carrier transport dilute nitride III-V quantum well solar cells|
Authors : S.Jiang，O.Kengradomying，QY.Wang，J.M.Rorison
Affiliations : University of Bristol
Resume : The quantum well technology has been proved to be a promising solution to improve the efficiency of solar cells. By growing thin layers of material that has lower band gap energy in the bulk, confined band gap can be implemented. However, it is convinced that the depth of the well affects the carrier transport as it can be tailored so that the quantum well will have the best performance. The first section of the report illustrates the effect of well depth on carrier transport from the aspect of transport and carrier life time in GaAs/GaInAs quantum well. Then the model is implemented into dilute nitride system where a small quantity of nitrogen exists in the well material. Due to the properties of nitrogen, the conduction band of well is separated into two curves, which has unique effects on the quantum well performance. Similar schedule will be carried on to discover the carrier transport performance in GaInAs/GaInAsN quantum wells and further improvements on the model will be shown.
|15:15||Dilute nitride nipi Solar Cells|
Authors : B. Royall(1), R. Ketlhwaafetse(1), S. Mazzucato(1), N. Balkan(1),M. Guina(2)
Affiliations : (1)University of Essex, School of Computer Science and Electronic Engineering, Colchester, UK (2)Optoelectronics Research Centre, Tampere University of Technology FIN-33101 Tampere, Finland
Resume : In this paper dilute nitride nipi solar cells are modelled and demonstrated. Such devices consist of 3 or more horizontal layers whose doping alternates between n-type and p-type. Modelling is carried out using the drift diffusion conductivity for 1 eV GaInNAs nipi solar cells, both as single band gap solar cells and as the 3rd junction in 3 junction series connected tandem solar cells. Modelling results shows that 1 eV GaInNAs nipi structures could reach AM1.5G one sun efficiencies as high as 15 % alone and 35 % efficiency when incorporated into a GaInP / GaAs / 1eV cell. Prototype GaAs and GaInNAs nipi structure were grown by molecular beam epitaxy and selective contacts were formed by ion implantation of Si and Mg. Spectral response and I-V characteristics of devices under AM1.5G illumination are reported.
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