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

Organic field effect transistors (OFETs) of poly(p-phenylenevinylene) fabricated by chemical vapor deposition (CVD) with improved hole mobility

Investigation of non-linear dependence of exciton recombination efficiency on PCBM concentration in P3HT:PCBM blends

A major loss mechanism in high intensity organic light-emitting devices is the quenching of excitons in the presence of polarons. In this work, the interaction of excitons in N,N′-di(1-naphthyl)-N,...

Spatial Extent of Interaction between Excitons and Polarons in a Bilayer Organic Field-Effect Transistor

4,4′-bis[(N-carbazole) styryl] biphenyl (BSB4 or BSBCz) is one of the widely studied organic fluorescent materials for blue organic electroluminescent devices in the recent times. In this work, BSB4 is used as a guest material to construct the host-guest matrix for the emissive layer (EML) of a pure blue fluorescent organic light-emitting diode (OLED). A pure blue emission suitable for display application with a Commission Internationale de l’Eclairage coordinate of (0.147,0.070) is achieved by the blue-shift of the emission spectrum of the host-guest matrix from that of the pristine guest (BSB4) molecules. The optimization of OLED structures is carried out by considering (a) charge balance in the EML for high exciton density, and (b) optical interference of generated light in the organic layers for increased light outcoupling. A thorough comparative study on the use of different combinations of widely used hole and electron transport layers to obtain charge balance in the EML of the OLED, thereby enhancing the external quantum efficiency (EQE) is shown. Optical interference effects in the fabricated OLEDs are analyzed by optical simulation of each device structure by transfer matrix method. With the optimized device structures, we are able to overcome the 2% EQE limit that has been reported so far for blue fluorescent OLEDs with BSB4 as light emitting material and achieve a maximum EQE of 4.08%, which is near to the theoretical limit of EQE for fluorescent OLEDs.

Investigation of 4,4′-bis[(N- carbazole) styryl] biphenyl (BSB4) for a pure blue fluorescent OLED with enhanced efficiency nearing the theoretical limit

In an organic semiconductor optoelectronic device, the built-in field within the active layer is typically determined by the difference in contact potentials of the device. However, the presence of space charges and trap states contribute to the electric field within the thin film. Depending on the maximum applied forward voltage, the trap states can be charged, inducing hysteresis in the optoelectronic response of the system. In this work, we investigate the electric fields inside organic photovoltaic device structures, in the presence of traps, using electroabsorption (EA) spectroscopy. Comparing simulations with our experimental results, we explained the origin of hysteresis in the electroabsorption signal as a function of applied DC bias. We solved Poisson’s equation to estimate the densities of trapped carriers in the active layers. The filled trap densities in poly(3-hexylthiophene-2,5-diyl) (P3HT) and poly[(2,5-bis(2-hexyldecyloxy)phenylene)-alt-(5,6-difluoro-4,7-di(thiophen-2-yl)benzo[c]-[1,2,5]thiadiazole)] (PPDT2FBT) were found to be ∼1×1017 and ∼6×1016 cm −3, respectively. From the transient EA measurements, the estimated values of energies of the trap states with respect to the HOMO level were 0.82 and 0.76 eV in P3HT and 0.70 and 0.64 eV in PPDT2FBT, which indicated the presence of midgap traps in these organic semiconductor thin films. Such trap induced changes in the internal fields within the active layers, affect the mobility and carrier transport in the organic optoelectronic devices. The midgap traps lead to exciton quenching and also act as non-radiative recombination centers, resulting in reduction in luminescence efficiency of the active layers.

Jayeeta Bhattacharyya

Papers

Organic field effect transistors (OFETs) of poly(p-phenylenevinylene) fabricated by chemical vapor deposition (CVD) with improved hole mobility

Investigation of non-linear dependence of exciton recombination efficiency on PCBM concentration in P3HT:PCBM blends

Spatial Extent of Interaction between Excitons and Polarons in a Bilayer Organic Field-Effect Transistor

Investigation of 4,4′-bis[(N- carbazole) styryl] biphenyl (BSB4) for a pure blue fluorescent OLED with enhanced efficiency nearing the theoretical limit

Investigation of internal fields in organic semiconductors in the presence of traps