75 research outputs found
Transmission electron microscopy of indium gallium nitride nanorods grown by molecular beam epitaxy
Crystal and Electronic Structure of Bismuth Thiophosphate, BiPS4:An Earth Abundant Solar Absorber
The optoelectronic properties of crystalline BiPS4 have been described for the first time for solar energy conversion. Detailed structural analysis is extracted from XRD refinement of powders synthesized by the solid-state method. BiPS4 exhibits a rather unusual 3-dimensional orthorhombic structure with two distinctive Bi sites with octahedral coordination distorted by 6s2 lone pairs. High-resolution TEM imaging clearly shows the two Bi–Bi interatomic distances in close agreement with the XRD analysis. BiPS4 displays a complex Raman spectrum under near-resonant conditions which is rationalized by density functional perturbation theory. Hybrid-functional-DFT calculations show significant spin–orbit coupling effects in Bi 6p bands, not only affecting the band dispersion but also lowering the conduction band minimum by approximately 0.5 eV. The optical properties of BiPS4 powders are dominated by a direct transition at 1.72 eV, closely matching the calculated band gap. Electrochemical experiments revealed n-type conductivity with a flat band potential located at 0.16 V vs RHE. We also show a remarkable agreement between the position of the band edges estimated from first-principles calculations and electrochemical measurements. The time-resolved photoluminescence transient revealed a carrier lifetime of approximately 1 ns, manifesting as strong potential- and wavelength-dependent photocurrent responses. Finally, the nature of the structural defects responsible for the relatively short lifetime is briefly discussed
Pulsed laser deposition of single phase n- and p-type Cu2O thin films with low resistivity
Low resistivity (~3-24 mOhm.cm) with tunable n- and p-type phase pure Cu2O
thin films have been grown by pulsed laser deposition at 25-200 0C by varying
the background oxygen partial pressure (O2pp). Capacitance data obtained by
electrochemical impedance spectroscopy was used to determine the conductivity
(n- or p-type), carrier density, and flat band potentials for samples grown on
indium tin oxide (ITO) at 25 0C. The Hall mobility of the n- and p-type Cu2O
was estimated to be ~ 0.85 cm2.V-1s-1 and ~ 4.78 cm2.V-1s-1 respectively for
samples grown on quartz substrate at 25 0C. An elevated substrate temperature ~
200 0C with O2pp = 2 - 3 mTorr yielded p-type Cu2O films with six orders of
magnitude higher resistivities in the range ~ 9 - 49 kOhm.cm and mobilities in
the range ~ 13.5 - 22.2 cm2.V-1s-1. UV-Vis-NIR diffuse reflectance spectroscopy
showed optical bandgaps of Cu2O films in the range of 1.76 to 2.15 eV depending
on O2pp. Thin films grown at oxygen-rich conditions O2pp > 7 mTorr yielded
mixed-phase copper oxide irrespective of the substrate temperatures and upon
air annealing at 550 0C for 1 hour completely converted to CuO phase with
n-type semiconducting properties (~12 Ohm.cm, ~1.50 cm2.V-1s-1). The as-grown
p- and n-type Cu2O showed rectification and a photovoltaic (PV) response in
solid junctions with n-ZnO and p-Si electrodes respectively. Our findings may
create new opportunities for devising Cu2O based junctions requiring low
process temperatures.Comment: 41 pages(including suppl. material), 7 figures (14 suppl. figures
Towards predictive modelling of near-edge structures in electron energy loss spectra of AlN based ternary alloys
Although electron energy loss near edge structure analysis provides a tool
for experimentally probing unoccupied density of states, a detailed comparison
with simulations is necessary in order to understand the origin of individual
peaks. This paper presents a density functional theory based technique for
predicting the N K-edge for ternary (quasi-binary) nitrogen alloys by adopting
a core hole approach, a methodology that has been successful for binary nitride
compounds. It is demonstrated that using the spectra of binary compounds for
optimising the core hole charge ( for cubic TiAlN
and for wurtzite AlGaN), the predicted spectra
evolutions of the ternary alloys agree well with the experiments. The spectral
features are subsequently discussed in terms of the electronic structure and
bonding of the alloys.Comment: 11 pages, 9 figures, 1 tabl
Observation of antisite domain boundaries in Cu<sub>2</sub>ZnSnS<sub>4</sub> by atomic-resolution transmission electron microscopy
Atomic resolution transmission electron microscopy has been used to examine antisite defects in Cu2ZnSnS4 (CZTS) kesterite crystals grown by a hot injection method.</p
Distinguishing cubic and hexagonal phases within InGaN/GaN microstructures using electron energy loss spectroscopy
3D InGaN/GaN microstructures grown by metal organic vapor phase epitaxy (MOVPE) and molecular beam epitaxy (MBE) have been extensively studied using a range of electron microscopy techniques. The growth of material by MBE has led to the growth of cubic GaN material. The changes in these crystal phases has been investigated by Electron Energy Loss Spectroscopy, where the variations in the fine structure of the N K‐edge shows a clear difference allowing the mapping of the phases to take place. GaN layers grown for light emitting devices sometimes have cubic inclusions in the normally hexagonal wurtzite structures, which can influence the device electronic properties. Differences in the fine structure of the N K‐edge between cubic and hexagonal material in electron energy loss spectra are used to map cubic and hexagonal regions in a GaN/InGaN microcolumnar device. The method of mapping is explained, and the factors limiting spatial resolution are discussed
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