50 research outputs found
Formation and annealing of dislocation loops induced by nitrogen implantation of ZnO
Although zinc oxide is a promising material for the fabrication of short
wavelength optoelectronic devices, p-type doping is a step that remains
challenging for the realization of diodes. Out of equilibrium methods such as
ion implantation are expected to dope ZnO successfully provided that the
non-radiative defects introduced by implantation can be annealed out. In this
study, ZnO substrates are implanted with nitrogen ions, and the extended
defects induced by implantation are studied by transmission electron microscopy
and X-ray diffraction (XRD), before and after annealing at 900^{\circ}C. Before
annealing, these defects are identified to be dislocation loops lying either in
basal planes in high N concentration regions, or in prismatic planes in low N
concentration regions, together with linear dislocations. An uniaxial
deformation of 0.4% along the c axis, caused by the predominant basal loops, is
measured by XRD in the implanted layer. After annealing, prismatic loops
disappear while the density of basal loops decreases and their diameter
increases. Moreover, dislocation loops disappear completely from the
sub-surface region. XRD measurements show a residual deformation of only 0.05%
in the implanted and annealed layer. The fact that basal loops are favoured
against prismatic ones at high N concentration or high temperature is
attributed to a lower stacking fault energy in these conditions. The
coalescence of loops and their disappearance in the sub-surface region are
ascribed to point defect diffusion. Finally, the electrical and optical
properties of nitrogen-implanted ZnO are correlated with the observed
structural features.Comment: 8 page
Dual-Frequency VSOP Observations of AO 0235+164
AO 0235+164 is a very compact, flat spectrum radio source identified as a BL
Lac object at a redshift of z=0.94. It is one of the most violently variable
extragalactic objects at both optical and radio wavelengths. The radio
structure of the source revealed by various ground-based VLBI observations is
dominated by a nearly unresolved compact component at almost all available
frequencies.
Dual-frequency space VLBI observations of AO 0235+164 were made with the VSOP
mission in January-February 1999. The array of the Japanese HALCA satellite and
co-observing ground radio telescopes in Australia, Japan, China and South
Africa allowed us to study AO 0235+164 with an unprecedented angular resolution
at frequencies of 1.6 and 5 GHz. We report on the sub-milliarcsecond structural
properties of the source. The 5-GHz observations led to an estimate of T_B >
5.8 x 10^{13} K for the rest-frame brightness temperature of the core, which is
the highest value measured with VSOP to date.Comment: 8 pages, 8 figures, to appear in Publ. Astron. Soc. Japa
Structural recovery of ion implanted ZnO nanowires
5 pagesInternational audienceIon implantation is an interesting method to dope semiconducting materials such as zinc oxide provided that the implantation-induced defects can be subsequently removed. Nitrogen implantation followed by anneals under O2 were carried out on zinc oxide nanowires in the same conditions as in a previous study on bulk ZnO [J. Appl.Phys. 109, 023513 (2011)], allowing a direct comparison of the defect recovery mechanisms. Transmission electron microscopy and cathodoluminescence were carried out to assess the effects of nitrogen implantation and of subsequent anneals on the structural and optical properties of ZnO nanowires. Defect recovery is shown to be more effective in nanowires compared with bulk material due to the proximity of free surfaces. Nevertheless, the optical emission of implanted and annealed nanowires deteriorated compared to as-grown nanowires, as also observed for unimplanted and annealed nanowires. This is tentatively attributed to the dissociation of excitons in the space charge region induced by O2 adsorption on the nanowire surface
Polarity in GaN and ZnO: Theory, measurement, growth, and devices
This article may be downloaded for personal use only. Any other use requires prior permission of the author and AIP Publishing. This article appeared in Appl. Phys. Rev. 3, 041303 (2016) and may be found at https://doi.org/10.1063/1.4963919.The polar nature of the wurtzite crystalline structure of GaN and ZnO results in the existence of a spontaneous electric polarization within these materials and their associated alloys (Ga,Al,In)N and (Zn,Mg,Cd)O. The polarity has also important consequences on the stability of the different crystallographic surfaces, and this becomes especially important when considering epitaxial growth. Furthermore, the internal polarization fields may adversely affect the properties of optoelectronic devices but is also used as a potential advantage for advanced electronic devices. In this article, polarity-related issues in GaN and ZnO are reviewed, going from theoretical considerations to electronic and optoelectronic devices, through thin film, and nanostructure growth. The necessary theoretical background is first introduced and the stability of the cation and anion polarity surfaces is discussed. For assessing the polarity, one has to make use of specific characterization methods, which are described in detail. Subsequently, the nucleation and growth mechanisms of thin films and nanostructures, including nanowires, are presented, reviewing the specific growth conditions that allow controlling the polarity of such objects. Eventually, the demonstrated and/or expected effects of polarity on the properties and performances of optoelectronic and electronic devices are reported. The present review is intended to yield an in-depth view of some of the hot topics related to polarity in GaN and ZnO, a fast growing subject over the last decade
The Arecibo Legacy Fast ALFA Survey: II. Results of Precursor Observations
In preparation for the full Arecibo Legacy Fast ALFA extragalactic HI survey,
precursor observations were carried out in Aug--Sep 2004 with the 7-beam
Arecibo L-band feed array (ALFA) receiver system and the WAPP spectral
processors. While these observations were geared mainly at testing and
debugging survey strategy, hardware and software, approximately 36 hours of
telescope time yielded science--quality data. From those observations, an
initial list of 730 tentative detections of varying degree of reliability was
extracted. Ninety--eight high signal-to-noise candidates were deemed to be bona
fide HI line detections. To test our ability to discriminate cosmic signals
from RFI and noise, 165 candidates ranging in reliability likelihood were
re--observed with the single beam L--band wide system at Arecibo in Jan--Feb
2005. Of those, 41% were confirmed as real. We present the results of both the
ALFA and single beam observations for the sample of 166 confirmed HI sources,
as well as our assessment of their optical counterparts. Of the 166 sources, 62
coincide with previously known HI sources, while optical redshifts were
available for an additional 18 galaxies; thus, 52% of the redshifts reported
here were previously unknown. Of the 166 HI detections, 115 are identified with
previously cataloged galaxies, of either known or unknown redshift, leaving 51
objects identified for the first time. Because of the higher sensitivity of the
Arecibo system, fewer than 10% of the 166 HI sources would have been detected
by a HIPASS--like survey of the same region. Three of the objects have HI
masses less than 10^7 solar masses. The full ALFALFA survey which commenced in
February 2005 should detect more than 100 times as many objects of similarly
low HI mass over the next 5 years.Comment: 40 pages, including 4 tables and 8 figures; to appear in Astron. J.;
see http://egg.astro.cornell.edu/alfalfa/pubs.ph
Discovery of New Faint Radio Emission on 8\ub0 to 3' Scales in the Coma Field, and Some Galactic and Extragalactic Implications
NRC publication: Ye