Material engineering in hybrid spintronic devices

The main objectives of this thesis is to achieve an overall progress of the quality of hybrid organic-inorganic devices for spintronic applications by improving the material quality of the main device components. By engineering the devices interfaces a huge improvement of the magnetic, ctructural, electric properties of the devices was achieved. New protocols have been developed for the growth of manganite films via Electron Beam Ablation technique (channel spark version, CSA). At the same time the films maintain high and robust surface magnetic properties as ascertained by spectroscopic and transport investigations. A radical modification of the top interface in vertical devices, that is the Alq3/Co interface, was performed by inserting an insulating tunnel barrier of Al2O3. This interfacial buffer layer was found to provide a considerably improved reproducibility and improved interfacial and bulk ferromagnetism in the top electrodes. The growth and properties of the buffer layers and of the subsequent metallic layers were investigated in detail by different experimental techniques. The performed engineering of the interfaces and device components allowed to achieve stable room temperature operation of LSMO/Alq3/Co vertical devices. The growth mechanism of pentacene organic semiconductor thin film on top of manganite layers was studied in a wide temperature interval and for different thicknesses

Ultra-high thermoelectric power factors in narrow gap materials with asymmetric bands

We theoretically unveil the unconventional possibility of achieving extremely high thermoelectric power factors in lightly doped narrow gap semiconductors with asymmetric conduction/valence bands operated in the bipolar transport regime. Specifically, using Boltzmann transport simulations, we show that narrow band gap materials, rather than suffering from performance degradation due to bipolar conduction, if they possess highly asymmetric conduction and valence bands in terms of either effective masses, density of states, or phonon scattering rates, then they can deliver very high power factors. We show that this is achieved because, under these conditions, electronic transport becomes phonon scattering-limited, rather than ionized impurity scattering-limited, which allows large conductivities. We explain why this effect has not been observed so far in the known narrow-gap semiconductors, interpret some recent related experimental findings, and propose a few examples from the half-Heusler materials family, for which this effect can be observed and power factors even up to 50 mW/mK2 can be reached

Electron and hole mobility of SnO2 from full-band electron-phonon and ionized impurity scattering computations

Mobility is a key parameter for SnO2, which is extensively studied as a practical transparent oxide n-type semiconductor. In experiments, the mobility of electrons in bulk SnO2 single crystals varies from 70 to 260 cm2V−1s−1 at room temperature. Here, we calculate the mobility as limited by electron–phonon and ionized impurity scattering by coupling the Boltzmann transport equation with density functional theory electronic structures. The linearized Boltzmann transport equation is solved numerically beyond the commonly employed constant relaxation-time approximation by taking into account all energy and momentum dependencies of the scattering rates. Acoustic deformation potential and polar optical phonons are considered for electron–phonon scattering, where polar optical phonon scattering is found to be the main factor which determines the mobility of both electrons and holes at room temperature. The calculated phonon-limited electron mobility is found to be 265 cm2V−1s−1, whereas that of holes is found to be 7.6 cm2V−1s−1. We present the mobility as a function of the carrier concentration, which shows the upper mobility limit. The large difference between the mobilities of n-type and p-type SnO2 is a result of the different effective masses between electrons and holes

Bipolar conduction asymmetries lead to ultra-high thermoelectric power factor

Low bandgap thermoelectric materials suffer from bipolar effects at high temperatures, with increased electronic thermal conductivity and reduced Seebeck coefficient, leading to a reduced power factor and a low ZT figure of merit. In this work we show that the presence of strong transport asymmetries between the conduction and valence bands can allow high phonon-limited electronic conductivity at finite Seebeck coefficient values, leading to largely enhanced power factors. The power factors that can be achieved can be significantly larger compared to their maximum unipolar counterparts, allowing for doubling of the ZT figure of merit. We identify this behavior in low-bandgap cases from the half-Heusler material family. Using both advanced electronic Boltzmann transport calculations for realistic material band structures and model parabolic electronic bands, we elaborate on the parameters that determine this effect. We then develop a series of descriptors that can guide machine learning studies in identifying such classes of materials with extraordinary power factors at nearly undoped conditions. For this we test more than 3000 analytical band structures and their features, and more than 120 possible descriptors, to identify the most promising ones that contain: (i) only band structure features for easy identification from material databases and (ii) band structure and transport parameters that provide much higher correlations, but for which parameter availability can be somewhat more scarce

Deformation potential extraction and computationally efficient mobility calculations in silicon from first principles

We present a first-principles framework to extract deformation potentials in silicon based on density-functional theory (DFT) and density-functional perturbation theory (DFPT). We compute the electronic band structures, phonon dispersion relations, and electron-phonon matrix elements to extract deformation potentials for acoustic and optical phonons for all possible processes. The matrix elements clearly show the separation between intra- and intervalley scattering in the conduction band, and quantify the strength of the scattering events in the degenerate bands of the valence band. We then use an advanced numerical Boltzmann transport equation (BTE) simulator that couples DFT electronic structures and energy/momentum-dependent scattering rates to compute the transport properties for electrons and holes. By incorporating ionized impurity scattering as well, we calculate the n-type and p-type mobility versus carrier density and make comparisons to experiments, indicating excellent agreement. The fact that the method we present uses well-established theoretical tools and requires the extraction of only a limited number of matrix elements, makes it generally computationally very attractive, especially for semiconductors with a large unit cell and lower symmetry

Material descriptors for the discovery of efficient thermoelectrics

The predictive performance screening of novel compounds can significantly promote the discovery of efficient, cheap, and nontoxic thermoelectric (TE) materials. Large efforts to implement machine-learning techniques coupled to materials databases are currently being undertaken, but the adopted computational methods can dramatically affect the outcome. With regards to electronic transport and power factor (PF) calculations, the most widely adopted and computationally efficient method is the constant relaxation time approximation (CRT). This work goes beyond the CRT and adopts the proper, full energy and momentum dependencies of electron–phonon and ionized impurity scattering to compute the electronic transport and perform PF optimization for a group of half-Heusler alloys. Then, the material parameters that determine the optimal PF based on this more advanced treatment are identified. This enables the development of a set of significantly improved descriptors that can be used in material screening studies, which offer deeper insights into the underlying nature of high-performance TE materials. We have identified nvεr/Do2mcond as the most useful and generic descriptor, a combination of the number of valleys, the dielectric constant, the conductivity effective mass, and the deformation potential for the dominant electron–phonon process. The proposed descriptors can accelerate the discovery of new efficient and environment-friendly TE materials in a much more accurate and reliable manner, and some predictions for very high-performance materials are presented