Karol Jarolimek

The Optical Spectra of a-Si:H and a-SiC:H Thin Films Measured by the Absolute Photothermal Deflection Spectroscopy (PDS)

The new absolute PDS setup allows to measure simultaneously the absolute values of the optical transmittance T, reflectance R and absorptance A spectra in the spectral range 280 2000 nm with the typical spectral resolution 10 nm in ultraviolet and visible spectral range and 20 nm in the near infrared region. The PDS setup provides the dynamic detection range in the optical absorptance up to 4 orders of magnitude using non-toxic liquid perfluorohexane Fluorinert FC72. Here we demonstrate the usability of this setup on a series of intrinsic as well as doped a-Si:H and a-SiC:H thin films deposited on glass substrates by radio frequency (RF) plasma enhanced chemical vapor deposition (CVD) from hydrogen, silane and methane under various conditions. The increase of the Tauc gap with increasing carbon concentration in intrinsic a-SiC:H was observed. The defect-induced localized states in the energy gap were observed in doped a-Si:H as well as undoped a-SiC:H below the Urbach absorption edge.

Band Offsets at the Interface Between Crystalline and Amorphous Silicon from First Principles

The band offsets between crystalline and hydrogenated amorphous silicon (a-Si:H) are key parameters governing the charge transport in modern silicon hetrojunction solar cells. They are an important input for macroscopic simulators that are used to further optimize the solar cell. Past experimental studies, using X-ray photoelectron spectroscopy (XPS) and capacitance-voltage measurements, have yielded conflicting results on the band offset. Here we present a computational study on the band offsets. It is based on atomistic models and density-functional theory (DFT). The amorphous part of the interface is obtained by relatively long DFT first-principles moleculardynamics (MD) runs at an elevated temperature on 30 statistically independent samples. In order to obtain a realistic conduction band position the electronic structure of the interface is calculated with a hybrid functional. We find a slight asymmetry in the band offsets, where the offset in the valence band (0.30 eV) is larger than in the conduction band (0.17 eV). Our results are in agreement with the latest XPS measurements that report a valence band offset of 0.3 eV [M. Liebhaber et al., Appl. Phys. Lett. 106, 031601 (2015)].

Nanocrystal size distribution analysis from transmission electron microscopy images

We propose a method, with minimal bias caused by user input, to quickly detect and measure the nanocrystal size distribution from transmission electron microscopy (TEM) images using a combination of Laplacian of Gaussian filters and non-maximum suppression. We demonstrate the proposed method on bright-field TEM images of an a-SiC:H sample containing embedded silicon nanocrystals with varying magnifications and we compare the accuracy and speed with size distributions obtained by manual measurements, a thresholding method and PEBBLES. Finally, we analytically consider the error induced by slicing nanocrystals during TEM sample preparation on the measured nanocrystal size distribution and formulate an equation to correct this effect.

Amorphous Semiconductors Studied by First-principles Simulations: Structure and Electronic Properties

Atomistic models of amorphous solids enable us to study properties that are difficult to address with experimental methods. We present a study of two amorphous semiconductors with a great technological importance, namely a- Si:H and a-SiN:H. We use first-principles density functional theory (DFT), i.e. the interatomic forces are derived from basic quantum mechanics, as only that provides accurate interactions between the atoms for a wide range of chemical environments (e.g. brought about by composition changes). This type of precision is necessary for obtaining the correct short range order. Our amorphous samples are prepared by a cooling from liquid approach. As DFT calculations are very demanding, typically only short simulations can be carried out. Therefore most studies suffer from a substantial amount of defects, making them less useful for modeling purposes. We varied the cooling rate during the thermalization process and found it has a considerable impact on the quality of the resulting structure. A rate of 0.02 K/fs proves to be sufficient to prepare realistic samples with low defect concentrations. To our knowledge these are the first calculations that are entirely based on first-principles and at the same time are able to produce defect-free samples. Because of the high computational load also the size of the systems has to remain modest. The samples of a-Si:H and a-SiN:H contain 72 and 110 atoms, respectively. To examine the convergence with cells size, we utilize a large cell of a-Si:H with a total of 243 atoms. As we obtain essentially the same structure as with the smaller sample, we conclude that the use of smaller cells is justified. Although creating structures without any defects is important, on the other hand a small number of defects can give valuable information about the structure and electronic properties of defects in a-Si:H and a-SiN:H. In our samples we observe the presence of both the dangling bond (undercoordinated atom) and the floating bond (over-coordinated atom). We relate structural defects to electronic defect states within the band gap. In a-SiN:H the silicon-silicon bonds induce states at the valence and conduction band edges, thus decreasing the band gap energy. This finding is in agreement with measurements of the optical band gap, where increasing the nitrogen content increases the band gap.