53 research outputs found
Limitations of In2O3 as a transparent conducting oxide
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. Lett. 115, 082105 (2019); doi: 10.1063/1.5109569 and may be found at https://aip.scitation.org/doi/full/10.1063/1.5109569.Sn-doped In2O3 or ITO is the most widely used transparent conducting oxide. We use first-principles calculations to investigate the limitations to its transparency due to free-carrier absorption mediated by phonons or charged defects. We find that the main contribution to the phonon-assisted indirect absorption is due to emission (as opposed to absorption) of phonons, which explains why the process is relatively insensitive to temperature. The wavelength dependence of this indirect absorption process can be described by a power law. Indirect absorption mediated by charged defects or impurities is also unavoidable since doping is required to obtain conductivity. At high carrier concentrations, screening by the free carriers becomes important. We find that charged-impurity-assisted absorption becomes larger than phonon-assisted absorption for impurity concentrations above 1020 cm–3. The differences in the photon-energy dependence of the two processes can be explained by band structure effects
Spatially-resolved electronic and vibronic properties of single diamondoid molecules
Diamondoids are a unique form of carbon nanostructure best described as
hydrogen-terminated diamond molecules. Their diamond-cage structures and
tetrahedral sp3 hybrid bonding create new possibilities for tuning electronic
band gaps, optical properties, thermal transport, and mechanical strength at
the nanoscale. The recently-discovered higher diamondoids (each containing more
than three diamond cells) have thus generated much excitement in regards to
their potential versatility as nanoscale devices. Despite this excitement,
however, very little is known about the properties of isolated diamondoids on
metal surfaces, a very relevant system for molecular electronics. Here we
report the first molecular scale study of individual tetramantane diamondoids
on Au(111) using scanning tunneling microscopy and spectroscopy. We find that
both the diamondoid electronic structure and electron-vibrational coupling
exhibit unique spatial distributions characterized by pronounced line nodes
across the molecular surfaces. Ab-initio pseudopotential density functional
calculations reveal that the observed dominant electronic and vibronic
properties of diamondoids are determined by surface hydrogen terminations, a
feature having important implications for designing diamondoid-based molecular
devices.Comment: 16 pages, 4 figures. to appear in Nature Material
Impact of the stacking sequence on the bandgap and luminescence properties of bulk, bilayer, and monolayer hexagonal boron nitride
We examine the effects of stacking sequence and number of layers on the electronic and luminescence properties of hexagonal boron nitride (h-BN) structures with first-principles calculations based on density functional and many-body perturbation theory. We explored the variations of the magnitude and character (direct or indirect) of the quasiparticle bandgap and interband optical matrix elements for bulk, bilayer, and monolayer stacking polytypes. Although the fundamental gap for most structures is indirect, phonon-assisted transitions are strong (typically 600 times stronger than bulk Si) and enable efficient deep-ultraviolet (UV) luminescence. The polarization of the emitted light is transverse electric, which facilitates light extraction perpendicularly to the h-BN basal plane. Random stacking in turbostratic BN breaks the crystal symmetry and enables optical transitions across the quasi-direct bandgap, albeit with a weak matrix element. Our results demonstrate that h-BN is a promising material for efficient deep-UV light emitters
Energy Conversion: Solid-State Lighting
This chapter discusses recent developments in first-principles computational methods for the study of nitride materials employed for solid-state lighting. The chapter also presents examples that show the wide range of applications of first-principles calculations in this field, ranging from the basic structural and electronic properties of the nitride materials to the effects of strain, defects, and nonradiative recombination on the optoelectronic device performance. First-principles methods are a powerful explanatory and predictive computational tool that can assist and guide the experimental development of efficient solid-state optoelectronic devices and can help reduce the impact of general lighting on the world's energy resources
First-principles calculations of indirect Auger recombination in nitride semiconductors
Auger recombination is an important nonradiative carrier recombination mechanism in many classes of optoelectronic devices. The microscopic Auger processes can be either direct or indirect, mediated by an additional scattering mechanism such as the electron-phonon interaction and alloy disorder scattering. Indirect Auger recombination is particularly strong in nitride materials and affects the efficiency of nitride optoelectronic devices at high powers. Here, we present a first-principles computational formalism for the study of direct and indirect Auger recombination in direct-band-gap semiconductors and apply it to the case of nitride materials. We show that direct Auger recombination is weak in the nitrides and cannot account for experimental measurements. On the other hand, carrier scattering by phonons and alloy disorder enables indirect Auger processes that can explain the observed loss in devices. We analyze the dominant phonon contributions to the Auger recombination rate and the influence of temperature and strain on the values of the Auger coefficients. Auger processes assisted by charged-defect scattering are much weaker than the phonon-assisted ones for realistic defect densities and not important for the device performance. The computational formalism is general and can be applied to the calculation of the Auger coefficient in other classes of optoelectronic materials.Peer reviewe
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