Prashun Gorai

Material descriptors for predicting thermoelectric performance

In the context of materials design and high-throughput computational searches for new thermoelectric materials, the need to compute electron and phonon transport properties renders direct assessment of the thermoelectric figure of merit (zT) for large numbers of compounds untenable. Here we develop a semi-empirical approach rooted in first-principles calculations that allows relatively simple computational assessment of the intrinsic bulk material properties which govern zT. These include carrier mobility, effective mass, and lattice thermal conductivity, which combine to form a semi-empirical metric (descriptor) termed βSE. We assess the predictive power of βSE against a range of known thermoelectric materials, as well as demonstrate its use in high-throughput screening for promising candidate materials.

Surface-based manipulation of point defects in rutile TiO2

Through isotopic self-diffusion measurements, the present work resolves a discrepancy in the literature about the primary oxygen-related point defect in rutile TiO2 by showing that suitably prepared surfaces can controllably inject large numbers of an exceptionally mobile defect. Results strongly suggest that this defect is the oxygen interstitial, whose existence in TiO2 has been predicted computationally but never experimentally confirmed. The surface pathway offers an approach for replacing donor oxygen vacancies with acceptor oxygen interstitials facilitating manipulation of near-surface electronic bands.

A computational framework for automation of point defect calculations

Abstract A complete and rigorously validated open-source Python framework to automate point defect calculations using density functional theory has been developed. The framework provides an effective and efficient method for defect structure generation, and creation of simple yet customizable workflows to analyze defect calculations. The package provides the capability to compute widely-accepted correction schemes to overcome finite-size effects, including (1) potential alignment, (2) image-charge correction, and (3) band filling correction to shallow defects. Using Si, ZnO and In2O3 as test examples, we demonstrate the package capabilities and validate the methodology.

Electrostatic drift effects on near-surface defect distribution in TiO2

The present work employs a combination of isotopic self-diffusion measurements and diffusion-drift modeling to identify a unique mechanism for defect accumulation in surface space-charge layers of TiO2. During oxygen gas-exchange experiments at elevated temperatures, rutile (110) surfaces inject charged oxygen interstitials into the underlying bulk. Yet near-surface electric fields attract the injected defects back toward the surface, retarding their diffusional migration and leading to longer residence times within the space-charge layers. The extended residence time enhances kick-in reactions, resulting in measureable pile-up of the isotope. Related effects probably generalize to other related semiconductors.

Potential for high thermoelectric performance in n-type Zintl compounds: a case study of Ba doped KAlSb4

High-throughput calculations (first-principles density functional theory and semi-empirical transport models) have the potential to guide the discovery of new thermoelectric materials. Herein we have computationally assessed the potential for thermoelectric performance of 145 complex Zintl pnictides. Of the 145 Zintl compounds assessed, 17% show promising n-type transport properties, compared with only 6% showing promising p-type transport. We predict that n-type Zintl compounds should exhibit high mobility μn while maintaining the low thermal conductivity κL typical of Zintl phases. Thus, not only do candidate n-type Zintls outnumber their p-type counterparts, but they may also exhibit improved thermoelectric performance. From the computational search, we have selected n-type KAlSb4 as a promising thermoelectric material. Synthesis and characterization of polycrystalline KAlSb4 reveals non-degenerate n-type transport. With Ba substitution, the carrier concentration is tuned between 1018 and 1019 e− cm−3 with a maximum Ba solubility of 0.7% on the K site. High temperature transport measurements confirm a high μn (50 cm2 V−1 s−1) coupled with a near minimum κL (0.5 W m−1 K−1) at 370 °C. Together, these properties yield a zT of 0.7 at 370 °C for the composition K0.99Ba0.01AlSb4. Based on the theoretical predictions and subsequent experimental validation, we find significant motivation for the exploration of n-type thermoelectric performance in other Zintl pnictides.

Kinetics of oxygen interstitial injection and lattice exchange in rutile TiO2

The existence of a facile surface pathway for generation of O interstitials (Oi) in rutile that can facilitate annihilation of O undesirable vacancies has been demonstrated recently. Through isotopic self-diffusion experiments, the present work determines a value of approximately 1.8 eV for the activation energy of Oi injection from TiO2 (110). The mean path length for Oi diffusion decreases by nearly an order of magnitude upon adsorption of 0.1 monolayer of sulfur. Sulfur apparently inhibits the surface annihilation rate of Ti interstitials, lowering their bulk concentration and the corresponding catalytic effect they seem to exert upon Oi exchange with the lattice.

Surface-assisted defect engineering of point defects in ZnO

Semiconductor surfaces facilitate the injection of highly mobile point defects into the underlying bulk, thereby offering a special means to manipulate bulk defect concentrations. The present work combines diffusion experiments and first-principles calculations for polar ZnO (0001) surface to demonstrate such manipulation. The rate behavior of oxygen interstitial injection varies dramatically between the Zn- and O-terminated ZnO surfaces. A specific injection pathway for the Zn-terminated surface is identified, and activation barrier determined from the first-principles calculations agrees closely with the experimental activation energy of 1.7 eV.

Mechanism and kinetics of near-surface dopant pile-up during post-implant annealing

Dopant pile-up within 1-2 nm of Si/SiO2 interfaces during post-implant annealing can influence the performance of microelectronic devices using silicon-on-insulator technology or super-steep retrograde channels. Pile-up results from changes in the dopant interstitial charge state induced by band bending at the interface. But, there exists little mechanistic understanding of the specific conditions needed for pile-up or of the kinetics of temporal evolution. The present work uses continuum simulations coupled with experiments in the case of B implanted into Si to show that pile-up requires a zone near the interface wherein the Fermi level exceeds the ionization level for dopant interstitials to change their charge state. The spatial extent of pile-up corresponds closely to the width of this zone unless the annihilation probability of defects at the interface is large. The time and temperature dependences of pile-up closely track those of the free dopant interstitials concentration.

Mechanism and energetics of O and O2 adsorption on polar and non-polar ZnO surfaces

Polar surfaces of semiconducting metal oxides can exhibit structures and chemical reactivities that are distinct from their non-polar surfaces. Using first-principles calculations, we examine O adatom and O2 molecule adsorption on 8 different known ZnO reconstructions including Zn-terminated (Zn-ZnO) and O-terminated (O-ZnO) polar surfaces, and non-polar surfaces. We find that adsorption tendencies are largely governed by the thermodynamic environment, but exhibit variations due to the different surface chemistries of various reconstructions. The Zn-ZnO surface reconstructions which appear under O-rich and H-poor environments are found to be most amenable to O and O2 adsorption. We attribute this to the fact that on Zn-ZnO, the O-rich environments that promote O adsorption also simultaneously favor reconstructions that involve adsorbed O species. On these Zn-ZnO surfaces, O2 dissociatively adsorbs to form O adatoms. By contrast, on O-ZnO surfaces, the O-rich conditions required for O or O2 adsorption tend to promote reconstructions involving adsorbed H species, making further O species adsorption more difficult. These insights about O2 adsorption on ZnO surfaces suggest possible design rules to understand the adsorption properties of semiconductor polar surfaces.

Kinetic model for electric-field induced point defect redistribution near semiconductor surfaces

The spatial distribution of point defects near semiconductor surfaces affects the efficiency of devices. Near-surface band bending generates electric fields that influence the spatial redistribution of charged mobile defects that exchange infrequently with the lattice, as recently demonstrated for pile-up of isotopic oxygen near rutile TiO2 (110). The present work derives a mathematical model to describe such redistribution and establishes its temporal dependence on defect injection rate and band bending. The model shows that band bending of only a few meV induces significant redistribution, and that the direction of the electric field governs formation of either a valley or a pile-up.