Wide-band-gap GaN and Ga-rich InGaN alloys, with energy gaps covering the blue and near-ultraviolet parts of the electromagnetic spectrum, are one group of the dominant materials for solid state lighting and lasing technologies and consequently, have been studied very well. Much less effort has been devoted to InN and In-rich InGaN alloys. A major breakthrough in 2002, stemming from much improved quality of InN films grown using molecular beam epitaxy, resulted in the bandgap of InN being revised from 1.9 eV to a much narrower value of 0.64 eV. This finding triggered a worldwide research thrust into the area of narrow-band-gap group-III nitrides. The low value of the InN bandgap provides a basis for a consistent description of the electronic structure of InGaN and InAlN alloys with all compositions. It extends the fundamental bandgap of the group III-nitride alloy system over a wider spectral region, ranging from the near infrared at ∼1.9 μm (0.64 eV for InN) to the ultraviolet at ∼0.36 μm (3.4 eV for GaN...

When group-III nitrides go infrared: New properties and perspectives

We have derived consistent sets of band parameters band gaps, crystal field splittings, band-gap deformation potentials, effective masses, and Luttinger and EP parameters for AlN, GaN, and InN in the zinc-blende and wurtzite phases employing many-body perturbation theory in the G0W0 approximation. The G0W0 method has been combined with density-functional theory DFT calculations in the exact-exchange optimized effective potential approach to overcome the limitations of local-density or gradient-corrected DFT functionals. The band structures in the vicinity of the point have been used to directly parametrize a 44 k·p Hamiltonian to capture nonparabolicities in the conduction bands and the more complex valence-band structure of the wurtzite phases. We demonstrate that the band parameters derived in this fashion are in very good agreement with the available experimental data and provide reliable predictions for all parameters, which have not been determined experimentally so far.

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Consistent set of band parameters for the group-III nitrides AlN, GaN, and InN

III-nitride ultraviolet emitters : technology and applications

The authors demonstrate ultrabroadband time-resolved THz spectroscopy on a single InAs nanowire with 10 nm spatial resolution and sub-100 fs time resolution. Phase-locked ultrashort pulses in the rich terahertz spectral range1,2,3,4,5,6,7,8,9,10,11,12,13,14,15,16,17,18 have provided key insights into phenomena as diverse as quantum confinement7, first-order phase transitions8,12, high-temperature superconductivity11 and carrier transport in nanomaterials1,6,13,14,15. Ultrabroadband electro-optic sampling of few-cycle field transients1 can even reveal novel dynamics that occur faster than a single oscillation cycle of light4,8,10. However, conventional terahertz spectroscopy is intrinsically restricted to ensemble measurements by the diffraction limit. As a result, it measures dielectric functions averaged over the size, structure, orientation and density of nanoparticles, nanocrystals or nanodomains. Here, we extend ultrabroadband time-resolved terahertz spectroscopy to the sub-nanoparticle scale (10 nm) by combining sub-cycle, field-resolved detection (10 fs) with scattering-type near-field scanning optical microscopy (s-NSOM)16,17,18,19,20,21,22,23,24,25,26. We trace the time-dependent dielectric function at the surface of a single photoexcited InAs nanowire in all three spatial dimensions and reveal the ultrafast (<50 fs) formation of a local carrier depletion layer.

Ultrafast multi-terahertz nano-spectroscopy with sub-cycle temporal resolution

Infrared and optical spectroscopy represents one of the most informative methods in advanced materials research. As an important branch of modern optical techniques that has blossomed in the past decade, scattering-type scanning near-field optical microscopy (s-SNOM) promises deterministic characterization of optical properties over a broad spectral range at the nanoscale. It allows ultrabroadband optical (0.5-3000 µm) nanoimaging, and nanospectroscopy with fine spatial (<10 nm), spectral (<1 cm-1 ), and temporal (<10 fs) resolution. The history of s-SNOM is briefly introduced and recent advances which broaden the horizons of this technique in novel material research are summarized. In particular, this includes the pioneering efforts to study the nanoscale electrodynamic properties of plasmonic metamaterials, strongly correlated quantum materials, and polaritonic systems at room or cryogenic temperatures. Technical details, theoretical modeling, and new experimental methods are also discussed extensively, aiming to identify clear technology trends and unsolved challenges in this exciting field of research.

Modern Scattering-Type Scanning Near-Field Optical Microscopy for Advanced Material Research

Transitions between the 1s and 2p levels of the fundamental heavy-hole exciton in GaAs quantum wells, followed by scattering into the 2s state, are investigated by resonant THz excitations using a free-electron laser. We report on the external control of this intra-excitonic population transfer by an external magnetic field.

THz free-electron laser spectroscopy of magnetoexcitons in semiconductor quantum wells

We discuss recent experimental and theoretical results that report on the observation of dipole-forbidden intra-exciton transitions in semiconductors via terahertz excitation. Additional manipulation capabilities are gained through the application of a magnetic field.

/pdf/observation-and-manipulation-of-dipole-forbidden-exciton-2mf6dkxi00.pdf

Observation and manipulation of dipole-forbidden exciton transitions in semiconductors

The performance of organic semiconductor photodetectors depends on the choice of materials as the active layers. In this paper, we show that a 1:1 blend of PPDT2FBT-PC71BM blend gives high responsivity of about 0.33 A/W at 660 nm and is operable in the wavelength range of 350 to 720 nm. The causes for high efficiencies at low operating voltages are discussed.

Solution Processed UV-Visible Organic Photodetector with High Responsivity at Low Operating Voltage

Pentacene thin films grown on SiO2 substrates with few micron-spaced patterns were found to show anisotropy in their photoluminescence (PL) responses. PL emission was maximum for excitation polarization parallel to the groove direction. Detailed analysis of polarized Raman spectra gave insight on the intermolecular coupling. The anisotropy in PL was found to arise from preferential in-plane molecular alignment of pentacene, induced by the walls of the grooves. This was verified by spatial scan of the polarized micro-PL emission. Though earlier reports have shown anisotropy for nanoscale patterned substrates, to our knowledge, controlled orientation of molecules induced by micron spaced patterning has not been reported so far. This work reports observation of six fold increase in PL response for polarization parallel to the groove of widths 5 μm.

https://iopscience.iop.org/article/10.1088/2053-1591/aaf911/pdf

Optical polarization anisotropy induced by molecular alignment in pentacene films confined in micron sized grooves etched in SiO2 substrate

Electroabsorption (EA) measurements can be used to identify the type of excitons contributing to the absorption spectra of semiconductors which have applications in optoelectronics. However, the inferences from the EA measurement greatly depend on the method of fitting and extraction of parameters from the measured spectra. We deconstruct the absorption spectrum by fitting multiple Gaussians and obtain the relative contribution of first and second derivative of each absorption band in EA spectrum, which gives indication of the Frenkel, charge transfer or mixed nature of the excitons involved. We check the applicability of the method for pentacene which is widely used and well studied organic semiconductor. We report EA measurements of poly[(2,5-bis(2-hexyldecyloxy)-phenylene)-alt-(5,6-difluoro-4,7-di(thiophen-2-yl)benzo[c]-[1,2,5]-thiadiazole)] (PPDT2FBT). Our analysis shows that besides the feature around 3.07 eV, which is strongly Frenkel-like, most of the absorption bands for PPDT2FBT are mixed states, having relatively high charge transfer contributions. Since charge transfer excitons have higher dissociation efficiencies, we infer PPDT2FBT to be a promising candidate for photovoltaic applications.

Jayeeta Bhattacharyya

Papers

THz free-electron laser spectroscopy of magnetoexcitons in semiconductor quantum wells

Observation and manipulation of dipole-forbidden exciton transitions in semiconductors

Solution Processed UV-Visible Organic Photodetector with High Responsivity at Low Operating Voltage

Optical polarization anisotropy induced by molecular alignment in pentacene films confined in micron sized grooves etched in SiO2 substrate

Identification of the nature of excitons in PPDT2FBT using electroabsorption spectroscopy