67 research outputs found
Quantitative Assessment of Carrier Density by Cathodoluminescence (2): GaAs nanowires
Precise control of doping in single nanowires (NWs) is essential for the
development of NW-based devices. Here, we investigate a series of MBE-grown
GaAs NWs with Be (p-type) and Si (n-type) doping using high-resolution
cathodoluminescence (CL) mapping at low- and room-temperature. CL spectra are
analyzed selectively in different regions of the NWs. Room-temperature
luminescence is fitted with the generalized Planck's law and an absorption
model, and the bandgap and band tail width are extracted. For Be-doped GaAs
NWs, the bandgap narrowing provides a quantitative determination of the hole
concentration ranging from about to
cm, in good agreement with the targeted doping levels. For Si-doped GaAs
NWs, the electron Fermi level and the full-width at half maximum of
low-temperature CL spectra are used to assess the electron concentration to
approximately to cm. These findings
confirm the difficulty to reach highly-doped n-type GaAs NWs, may be due to
doping compensation. Notably, signatures of high concentration (5-9 cm) at the very top of NWs are unveiled
Quantitative Assessment of Carrier Density by Cathodoluminescence (1): GaAs thin films and modeling
Doping is a fundamental property of semiconductors and constitutes the basis
of modern microelectronic and optoelectronic devices. Their miniaturization
requires contactless characterization of doping with nanometer scale
resolution. Here, we use low- and room-temperature cathodoluminescence (CL)
measurements to analyze p-type and n-type GaAs thin films over a wide range of
carrier densities ( to cm). The
spectral shift and broadening of CL spectra induced by shallow dopant states
and band filling are the signature doping. We fit the whole spectral lineshapes
with the generalized Planck's law and refined absorption models to extract the
bandgap narrowing (BGN) and the band tail for both doping types, and the
electron Fermi level for n doping. This work provides a rigorous method for the
quantitative assessment of p-type and n-type carrier density using CL. Taking
advantage of the high spatial resolution of CL, it can be used to map the
doping in GaAs nanostructures, and it could be extended to other semiconductor
materials.Comment: Supplemental Materia
Non-polar (11-20) InGaN quantum dots with short exciton lifetimes grown by metal-organic vapor phase epitaxy
We report on the optical characterization of non-polar a-plane InGaN quantum
dots (QDs) grown by metal-organic vapor phase epitaxy using a short nitrogen
anneal treatment at the growth temperature. Spatial and spectral mapping of
sub-surface QDs have been achieved by cathodoluminescence at 8 K.
Microphotoluminescence studies of the QDs reveal resolution limited sharp peaks
with typical linewidth of 1 meV at 4.2 K. Time-resolved photoluminescence
studies suggest the excitons in these QDs have a typical lifetime of 538 ps,
much shorter than that of the c-plane QDs, which is strong evidence of the
significant suppression of the internal electric fields.Comment: 4 figures, submitte
Local carrier recombination and associated dynamics in m-plane InGaN/GaN quantum wells probed by picosecond cathodoluminescence
Research data in support of the publication "Local carrier recombination and associated dynamics in m-plane InGaN/GaN quantum wells probed by picosecond cathodoluminescence". We have included the original data (tab-separated text files) as plotted for the quantum wells, measured by spatially- and time-resolved cathodoluminescence
Nanoscale electrical analyses of axial-junction GaAsP nanowires for solar cell applications
Axial p-n and p-i-n junctions in GaAs0.7P0.3 nanowires are demonstrated and analyzed using electron beam induced current microscopy. Organized self-catalyzed nanowire arrays are grown by molecular beam epitaxy on nanopatterned Si substrates. The nanowires are doped using Be and Si impurities to obtain p- and n-type conductivity, respectively. A method to determine the doping type by analyzing the induced current in the vicinity of a Schottky contact is proposed. It is demonstrated that for the applied growth conditions using Ga as a catalyst, Si doping induces an n-type conductivity contrary to the GaAs self-catalyzed nanowire case, where Si was reported to yield a p-type doping. Active axial nanowire p-n junctions having a homogeneous composition along the axis are synthesized and the carrier concentration and minority carrier diffusion lengths are measured. To the best of our knowledge, this is the first report of axial p-n junctions in self-catalyzed GaAsP nanowires
Intrinsic defects and mid-gap states in quasi-one-dimensional Indium Telluride
Recently, intriguing physical properties have been unraveled in anisotropic
semiconductors, in which the in-plane electronic band structure anisotropy
often originates from the low crystallographic symmetry. The atomic chain is
the ultimate limit in material downscaling for electronics, a frontier for
establishing an entirely new field of one-dimensional quantum materials.
Electronic and structural properties of chain-like InTe are essential for
better understanding of device applications such as thermoelectrics. Here, we
use scanning tunneling microscopy/spectroscopy (STM/STS) measurements and
density functional theory (DFT) calculations to directly image the in-plane
structural anisotropy in tetragonal Indium Telluride (InTe). As results, we
report the direct observation of one-dimensional In1+ chains in InTe. We
demonstrate that InTe exhibits a band gap of about 0.40 +-0.02 eV located at
the M point of the Brillouin zone. Additionally, line defects are observed in
our sample, were attributed to In1+ chain vacancy along the c-axis, a general
feature in many other TlSe-like compounds. Our STS and DFT results prove that
the presence of In1+ induces localized gap state, located near the valence band
maximum (VBM). This acceptor state is responsible for the high intrinsic p-type
doping of InTe that we also confirm using angle-resolved photoemission
spectroscopy.Comment: n
Quantum Confinement and Electronic Structure at the Surface of van der Waals Ferroelectric {\alpha}-InSe
Two-dimensional (2D) ferroelectric (FE) materials are promising compounds for
next-generation nonvolatile memories, due to their low energy consumption and
high endurance. Among them, {\alpha}-InSe has drawn particular
attention due to its in- and out-of-plane ferroelectricity, whose robustness
has been demonstrated down to the monolayer limit. This is a relatively
uncommon behavior since most bulk FE materials lose their ferroelectric
character at the 2D limit due to depolarization field. Using angle resolved
photoemission spectroscopy (ARPES), we unveil another unusual 2D phenomena
appearing in 2H \alpha-InSe single crystals, the occurrence of a
highly metallic two-dimensional electron gas (2DEG) at the surface of
vacuum-cleaved crystals. This 2DEG exhibits two confined states which
correspond to an electron density of approximatively 10
electrons/cm, also confirmed by thermoelectric measurements. Combination
of ARPES and density functional theory (DFT) calculations reveals a direct band
gap of energy equal to 1.3 +/- 0.1 eV, with the bottom of the conduction band
localized at the center of the Brillouin zone, just below the Fermi level. Such
strong n-type doping further supports the quantum confinement of electrons and
the formation of the 2DEG.Comment: 20 pages, 12 figure
The microstructure of non-polar a-plane (11 2 0) InGaN quantum wells
Atom probe tomography and quantitative scanning transmission electron microscopy are used to assess the composition of non-polar a-plane (11-20) InGaN quantum wells for applications in optoelectronics. The average quantum well composition measured by atom probe tomography and quantitative scanning transmission electron microscopy quantitatively agrees with measurements by X-ray diffraction. Atom probe tomography is further applied to study the distribution of indium atoms in non-polar a-plane (11-20) InGaN quantum wells. An inhomogeneous indium distribution is observed by frequency distribution analysis of the atom probe tomography measurements. The optical properties of non-polar (11-20) InGaN quantum wells with indium compositions varying from 7.9% to 20.6% are studied. In contrast to non-polar m-plane (1-100) InGaN quantum wells, the non-polar a-plane (11-20) InGaN quantum wells emit at longer emission wavelengths at the equivalent indium composition. The non-polar a-plane (11-20) quantum wells also show broader spectral linewidths. The longer emission wavelengths and broader spectral linewidths may be related to the observed inhomogeneous indium distribution.This work was carried out with the support of the United Kingdom Engineering and Physical Sciences Research Council under Grants Nos. EP\J001627\1, EP/I012591/1, and EP\J003603\1. The European Research Council has also provided financial support under the European Community's Seventh Framework Programme (FP7/2007-2013)/ERC Grant Agreement No. 279361 (MACONS). J. Etheridge and S. D. Findlay acknowledge funding from the Australian Research Council (ARC) (Project Nos. DP110104734 and DP110101570, respectively). The Titan3 80-300 TEM/STEM at the Monash Centre for Electron Microscopy was supported by the ARC Grant No. LE0454166.This is the final version of the article. It first appeared from the American Institute of Physics via http://dx.doi.org/10.1063/1.494829
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