Прыжковая проводимость Мотта и Эфроса-Шкловского в пленках из наночастц Si, легированных фосфором и бором

The electrical characteristics of thin films formed from Si nanoparticles (nc-Si) with various degrees of doping are studied. To exclude the influence of ionic conductivity, the current parameters of the films were recorded in an ultrahigh vacuum (P ~ 3 – 5∙10–9 Torr) with preliminary high-temperature (9500C) annealing. An analysis of the temperature dependences of the conductivity showed that in nc-Si films formed from heavily doped nanoparticles (the concentration of free electrons ne is greater than 1019 cm-3), the transport is determined by variable-length hopping (VRH). In these samples, the Mott conductivity prevails at temperatures above 300C and at lower temperatures, the Efros-Shklovskii type variable range hopping conduction is dominate. In films with a medium level of doping of nanoparticles (ne <1019 cm-3), transport is realized by the Mott, Efros - Shklovskii and thermally activated conductivities. At the same time, thermally activated conductivity is dominated at temperatures above 560K. In nc-Si films formed from undoped nanoparticles, the transport parameters are determined by thermally activated conductivity and Mott's conductivity. Conductivity of Efros - Shklovskii is not observed in such films. From the analysis of the parameters corresponding to the Mott and Efros - Shklovsky conductivities, the localization lengths of wave functions, the density of states at the Fermi level (g (EF)), and average hopping lengths are found. The average hopping lengths in nc-Si films from nanoparticles pre-etched in HF are in the range 56 - 86 nm, which indicates that hopping in such films occurs via intermediate nanoparticles.

INVESTIGATION OF POSSIBILITY OF THE MISFIT DISLOCATION DENSITY REDUCTION IN GE / SI FILMS WITH A BUFFER LAYER

В работе исследована возможность улучшения качества гетероэпитаксиальных структур Ge / Si с буферным слоем. Показано, что при использовании подготовительного слоя, состоящего из наноостровков, зарощенных низкотемпературным буферным слоем, возможно проявление так называемого эффекта аннигиляции дислокаций несоответствия в объеме буферного слоя Buf, что значительно улучшает приборное качество получаемых структур. Представлена зависимость морфологии поверхности слоя чистого Ge на буфере от времени роста наноостровков в интерфейсе Si / Buf . Выявлены оптимальные технологические параметры роста наноостровков для получения слоя Ge с минимальной значением шероховатости. Наилучших результатов удалось достичь при времени осаждения наноостровков 2 мин. При этом была достигнута минимальное значение шероховатости поверхности, равное 78 нм. Показано, что при дальнейшем увеличении размеров наноостровков, процесс аннигиляции дефектов замедляется, и рост низкотемпературного буферного слоя сменяется трехмерным островковым ростом, что увеличивает перепады рельефа поверхности выращиваемого слоя. The possibility of improving the quality of Ge / Si heteroepitaxial structures with a buffer layer is investigated. It is shown that when using a preparatory layer consisting of nanostructures overgrown with a low-temperature buffer layer, it is possible to manifest the so-called effect of annihilation of the misfit dislocations in the bulk of the buffer layer Buf , which significantly improves the quality of the resulting structures. The dependence of the morphology of the surface of the pure Ge layer on the buffer on the growth time of nanostructures in the Si / Buf interface is presented. The optimal technological parameters of the growth of nanostructures for obtaining a Ge layer with a minimum roughness value are revealed. The best results were achieved when the deposition time of nanostructures was 2 min. At the same time, the minimum surface roughness value of 78 nm was achieved. It is shown that with a further increase in the size of the nanostructures, the process of annihilation of defects slows down, and the growth of the low-temperature buffer layer is replaced by a three-dimensional island growth, which increases the differences in the relief of the surface of the grown layer.

Comparative Study on the Quality of Microcrystalline and Epitaxial Silicon Films Produced by PECVD Using Identical SiF4 Based Process Conditions

Hydrogenated microcrystalline silicon (µc-Si:H) and epitaxial silicon (epi-Si) films have been produced from SiF4, H2 and Ar mixtures by plasma enhanced chemical vapor deposition (PECVD) at 200 °C. Here, both films were produced using identical deposition conditions, to determine if the conditions for producing µc-Si with the largest crystalline fraction (XC), will also result in epi-Si films that encompass the best quality and largest crystalline silicon (c-Si) fraction. Both characteristics are of importance for the development of thin film transistors (TFTs), thin film solar cells and novel 3D devices since epi-Si films can be grown or etched in a selective manner. Therefore, we have distinguished that the H2/SiF4 ratio affects the XC of µc-Si, the c-Si fraction in epi-Si films, and the structure of the epi-Si/c-Si interface. Raman and UV-Vis ellipsometry were used to evaluate the crystalline volume fraction (Xc) and composition of the deposited layers, while the structure of the films were inspected by high resolution transmission electron microscopy (HRTEM). Notably, the conditions for producing µc-Si with the largest XC are different in comparison to the fabrication conditions of epi-Si films with the best quality and largest c-Si fraction.

A Multiphysics Peridynamic Model for Simulation of Fracture in Si Thin Films during Lithiation/Delithiation Cycles

Material failure is the main obstacle in fulfilling the potential of electrodes in lithium batteries. To date, different failure phenomena observed experimentally in various structures have become challenging to model in numerical simulations. Moreover, their mechanisms are not well understood. To fill the gap, here we develop a coupled chemo-mechanical model based on peridynamics, a particle method that is suitable for simulating spontaneous crack growth, to solve the fracture problems in silicon thin films due to lithiation/delithiation. The model solves mechanical and lithium diffusion problems, respectively, and uses a coupling technique to deal with the interaction between them. The numerical examples of different types of Si films show the advantage of the model in this category and well reproduce the fracture patterns observed in the experiments, demonstrating that it is a promising tool in simulating material failure in electrodes.

Photothermal Effects and Heat Conduction in Nanogranular Silicon Films

We present results on the photothermal (PT) and heat conductive properties of nanogranular silicon (Si) films synthesized by evaporation of colloidal droplets (drop-casting) of 100 ± 50 nm-sized crystalline Si nanoparticles (NP) deposited on glass substrates. Simulations of the absorbed light intensity and photo-induced temperature distribution across the Si NP films were carried out by using the Finite difference time domain (FDTD) and finite element mesh (FEM) modeling and the obtained data were compared with the local temperatures measured by micro-Raman spectroscopy and then was used for determining the heat conductivities k in the films of various thicknesses. The cubic-to-hexagonal phase transition in Si NP films caused by laser-induced heating was found to be heavily influenced by the film thickness and heat-conductive properties of glass substrate, on which the films were deposited. The k values in drop-casted Si nanogranular films were found to be in the range of lowest k of other types of nanostructurely voided Si films due to enhanced phonon scattering across inherently voided topology, weak NP-NP and NP-substrate interface bonding within nanogranular Si films.