Engineering Papers⌕ Search

SEARCH · Engineering Papers

Results for “HOLE MOBILITY”

Search indexed NASA NTRS and DOE OSTI research on propulsion, heat transfer, battery materials and energy systems. Follow report and document links to the original sources.

Quote a phrase for an exact phrase match. Source license links do not imply unrestricted reuse.

At least 37 records · Page 2
All sourcesNASA NTRSDOE OSTI
DOE OSTI2020

Electron and hole mobility of rutile GeO 2 from first principles: An ultrawide-bandgap semiconductor for power electronics

Rutile germanium dioxide (r-GeO 2 ) is a recently predicted ultrawide-bandgap semiconductor with potential applications in high-power electronic devices, for which the carrier mobility is an important material parameter that controls the device efficiency. We apply first-principles calculations based on density functional and density functional perturbation theory to investigate carrier-phonon coupling in r-GeO 2 and predict its phonon-limited electron and hole mobilities as a function of temperature and crystallographic orientation. The calculated carrier mobilities at 300 K are μ elec , ⊥ c → = 244 cm 2 V –1 s –1 , μ elec , ∥ c → = 377 cm 2 V –1 s –1 , μ hole , ⊥ c → = 27 cm 2 V –1 s –1 , and μ hole , ∥ c → = 29 cm 2 V –1 s –1 . At room temperature, carrier scattering is dominated by the low-frequency polar-optical phonon modes. The predicted Baliga figure of merit of n -type r-GeO 2 surpasses several incumbent semiconductors such as Si, SiC, GaN, and β -Ga 2 O 3 , demonstrating its superior performance in high-power electronic devices.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2020

Evaluating the Ratio of Electron and Hole Mobilities from a Single Bulk Sample Using Photo-Seebeck Effect

When a semiconductor is under photoexcitation, the voltage response to a temperature gradient is the photo-Seebeck effect. In this work, we study this effect, focusing on the contribution from transport of photo-excited carriers. We demonstrate that by combining photo-Seebeck with photoconductivity measurements, one can determine the ratio between electron and hole mobilities, and hence both of them when one is known. This is found for the case of defect-free samples, where no detail on the absorbance, carrier lifetime or recombination is necessary. Our method reported here does not require chemical doping, which could introduce defects and is often not feasible. It applies to both thin film and bulk samples. Experiment wise, photo-Seebeck effect is relatively easy to implement, or added to existing systems. In a broader context, for semiconductors with significant influence from defects, our result suggests that the photo-Seebeck behavior can still be understood. In this case another photo-transport property is necessary, in order to identify the mobilities of carriers and information regarding the defects. This framework integrates the information from photoexcitation and thermal gradients to provide a general method to determine fundamental electronic properties of materials.

36 MATERIALS SCIENCE↗
DOE OSTI2018

High Hole Mobility and Nonsaturating Giant Magnetoresistance in the New 2D Metal NaCu 4 Se 4 Synthesized by a Unique Pathway.

The new compound NaCu 4 Se 4 forms by the reaction of CuO and Cu in a molten sodium polyselenide flux, with the existence of CuO being unexpectedly critical to its synthesis. It adopts a layered hexagonal structure (space group P6 3 / mmc with cell parameters a = 3.9931(6) Å and c = 25.167(5) Å), consisting of infinite two-dimensional [Cu 4 Se 4 ] - slabs separated by Na + cations. X-ray photoelectron spectroscopy suggests that NaCu 4 Se 4 is mixed-valent with the formula (Na + )-(Cu + ) 4 (Se 2- )(Se - )(Se 2 ) 2- . NaCu 4 Se 4 is a p-type metal with a carrier density of similar to ~10 21 cm -3 and a high hole mobility of similar to ~808 cm 2 s -1 at 2 K based on electronic transport measurements. First-principles calculations suggest the density of states around the Fermi level are composed of Cu-d and Se-p orbitals. At 2 K, a very large transverse magnetoresistance of similar to 1400% was observed, with a nonsaturating, linear dependence on field up to 9 T. Furthermore, our results indicate that the use of metal oxide chemical precursors can open reaction paths to new low-dimensional compounds.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2025

Green solvent-processable poly(3-hexylthiophene) derivative maintains high hole mobility in diodes

The development of environmentally sustainable fabrication methods for organic solar cells (OSCs) is critical to enable their large-scale adoption. Conventional solution-processed OSCs often rely on halogenated, toxic solvents that pose health and environmental risks, limiting their scalability. This is because π-conjugated polymers tend to have a low solubility in non-halogenated solvents. A common strategy to enhance solubility in alternative solvents is through the incorporation of polar solubilizing groups, often on every repeat unit of at least one monomer. However, too many polar side chains tend to disrupt favorable morphology and decrease charge transport properties. In this study, we demonstrate that a minimal degree of side chain functionalization can improve green solvent processability while preserving electrical performance. We tested this hypothesis using our previously reported random copolymer derivative of poly(3-hexylthiophene) (P3HT) where ∼10 mol% of the side chains are 6-pentanoatehexyl side chains. We find that this low-level functionalization significantly broadens the solvents that can be used to process P3HT to less toxic solvents such as o-xylene and anisole. Furthermore, the copolymer processed from greener solvents showed high hole mobilities (∼10⁻ 3 cm 2 /Vs by the Space-Charge Limiting Current method in diodes), comparable to P3HT films cast from toxic chloroform. These findings suggest that minimal side-chain modification is a viable strategy for expanding green solvent compatibility while preserving electrical performance, paving the way toward more sustainable organic semiconductor designs.

36 MATERIALS SCIENCE↗
DOE OSTI2014

Anisotropy in the hole mobility measured along the [110] and [1{sup ¯}10] orientations in a strained Ge quantum well

In this paper, we report on anisotropic transport properties of strained germanium (sGe) quantum wells grown on Si (001) substrates with p-type doping beneath the sGe channel. Mobility measurements were made along orthogonal [110] directions. The level of measured resistivity anisotropy in the [110] and [1{sup ¯}10] orientations was found to vary between 2 and 9 for different samples. This corresponds to an actual mobility anisotropy ratio of between 1.3 and 2, values that are significantly higher than previously found for sGe. From modeling of the low temperature (12 K) mobility, using the relaxation time approach, the anisotropy in mobility was accounted for by a difference in interface roughness scattering between the two orientations. For the [110] orientation, a step height of Δ = 0.28 nm and interface roughness periodicity of λ = 7 nm were found while for the [1{sup ¯}10] orientation, λ reduced to 4 nm and Δ increased to 0.42 nm. High-resolution X-ray diffraction and transmission electron microscopy confirmed a 1° off-cut in the wafer towards the [1{sup ¯}10] direction.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2017

Three-Dimensional Morphology Control Yielding Enhanced Hole Mobility in Air-Processed Organic Photovoltaics: Demonstration with Grazing-Incidence Wide-Angle X-ray Scattering

Polymer organic photovoltaic (OPV) device performance is defined by the three-dimensional morphology of the phase-separated domains in the active layer. In this work, we determine the evolution of morphology through different stages of tailored solvent vapor and thermal annealing techniques in air-processed poly(3-hexylthiophene-2,5-diyl)/phenyl-C61-butyric acid methyl ester-based OPV blends. A comparative evaluation of the effect of solvent type used for vapor annealing was performed using grazing-incidence wide-angle X-ray scattering, atomic force microscopy, and UV–vis spectroscopy to probe the active-layer morphology. A nonhalogenated orthogonal solvent was found to impart controlled morphological features within the exciton diffusion length scales, enhanced absorbance, greater crystallinity, increased paracrystalline disorder, and improved charge-carrier mobility. Low-boiling, fast-diffusing isopropanol allowed the greatest control over the nanoscale structure of the solvents evaluated and yielded a cocontinuous morphology with narrowed domains and enhanced paths for the charge carrier to reach the anode.

36 MATERIALS SCIENCE↗
DOE OSTI2019

Sharp Drop in the Mobility of Holes with a Decrease in Their Two-Dimensional Concentration by an External Voltage in Boron δ-Doped Diamond Layers

It is shown, both analytically and numerically, that the hole mobility in boron δ-doped (i.e., with a thickness on the order of several lattice constants) diamond layers drops with a decrease in their two-dimensional concentration during depletion of the δ-doped layer by an external voltage. This drop is most pronounced for the maximum initial two-dimensional hole concentrations ~ 3 × 10{sup 13} cm{sup –2} (limited from above by the condition for the possibility of their significant reduction without the electrical breakdown of diamond). This is explained by a decrease in the degree of screening of scattering Coulomb potentials of ionized boron atoms and an increase in the scattering efficiency of degenerate holes from these atoms due to a drop in the kinetic energy of holes. The corresponding calculations of the transport scattering cross section of holes are performed beyond the Born approximation (i.e., perturbation theory), which is inapplicable for boron δ-doped diamond layers. The predicted effect can be used to increase the efficiency of source-to-drain current modulation by a gate voltage in diamond field-effect transistors (FETs) with δ-doped conduction channels.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2019

Quasiparticle band structure and optical properties of rutile GeO 2 , an ultra-wide-band-gap semiconductor

Rutile GeO 2 is a visible and near-ultraviolet-transparent oxide that has not been researched for semiconducting applications in electronic and optoelectronic devices. We explore the electronic and optical properties of rutile GeO 2 with first-principles calculations based on density functional theory and many-body perturbation theory. Our band-structure calculations indicate a dipole-forbidden direct bandgap at Γ with an energy of 4.44 eV and effective masses equal to m* e⟂ = 0.43 m 0 , m* e∥ = 0.23 m 0 , m* h⟂ = 1.28 m 0 , and m* h∥ = 1.74 m 0 . In contrast to the self-trapped hole polarons by lattice distortions in other wide-bandgap oxides that reduce the hole mobility, holes in rutile GeO 2 are delocalized due to their small effective mass. The first allowed optical transitions at Γ occur at 5.04 eV ($\vec{E}$⟂$\vec{c}$) and 6.65 eV ($\vec{E}$∥$\vec{c}$). Furthermore, we evaluate the optical absorption coefficient and refractive index along both crystallographic directions. Our estimates for the exciton binding energies using the Bohr model are close to the reported experimental value. The ultrawide-bandgap and light carrier effective masses of rutile GeO 2 , coupled with its optical transparency in the visible and near UV, are promising for applications in UV-transparent conductors and solar-blind photodetectors.

36 MATERIALS SCIENCE↗
DOE OSTI2016

Electron and hole drift mobility measurements on methylammonium lead iodide perovskite solar cells

We report nanosecond domain time-of-flight measurements of electron and hole photocarriers in methylammonium lead iodide perovskite solar cells. The mobilities ranged from 0.06 to 1.4 cm{sup 2}/Vs at room temperature, but there is little systematic difference between the two carriers. We also find that the drift mobilities are dispersive (time-dependent). The dispersion parameters are in the range of 0.4–0.7, and they imply that terahertz domain mobilities will be much larger than nanosecond domain mobilities. The temperature-dependences of the dispersion parameters are consistent with confinement of electron and hole transport to fractal-like spatial networks within nanoseconds of their photogeneration.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2023

One-Particle and Excitonic Band Structure in Cubic Boron Arsenide

Cubic BAs has received recent attention for its large electron and hole mobilities and large thermal conductivity. This is a rare and much desired combination in semiconductor industry: commercial semiconductors typically have high electron mobilities, or hole mobilities, or large thermal conductivities, but not all of them together. Here, in this paper, predictions from an advanced self-consistent many-body perturbative theory are reported and it is shown that with respect to one-particle properties, BAs is strikingly similar to Si. There are some important differences, notably there is an unusually small variation in the valence band masses. With respect to two-particle properties, significant differences with Si appear. The excitonic spectrum for both q = 0 and finite q is reported, and it is shown that while the direct gap in cubic BAs is about 4 eV, dark excitons can be observed down to about ≈1.5 eV, which may play a crucial role in application of BAs in optoelectronics.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2014

A hole accelerator for InGaN/GaN light-emitting diodes

The quantum efficiency of InGaN/GaN light-emitting diodes (LEDs) has been significantly limited by the insufficient hole injection, and this is caused by the inefficient p-type doping and the low hole mobility. The low hole mobility makes the holes less energetic, which hinders the hole injection into the multiple quantum wells (MQWs) especially when a p-type AlGaN electron blocking layer (EBL) is adopted. In this work, we report a hole accelerator to accelerate the holes so that the holes can obtain adequate kinetic energy, travel across the p-type EBL, and then enter the MQWs more efficiently and smoothly. In addition to the numerical study, the effectiveness of the hole accelerator is experimentally shown through achieving improved optical output power and reduced efficiency droop for the proposed InGaN/GaN LED.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗