Oleg Babchenko

Nanostructured three-dimensional thin film silicon solar cells with very high efficiency potential

We report on the experimental realization of amorphous/microcrystalline silicon tandem solar cells (Micromorph) based on our three-dimensional design. An enhancement is reached in the short-circuit current by 40%, with an excellent open-circuit voltage of 1.41V and a fill factor of 72%. We have used nanoholes or microholes dry etched into the ZnO front contact layer. Monte Carlo optical modeling shows that stable efficiency of amorphous silicon p-i-n solar cells in over 12% range is possible. For the Micromorph cells, efficiency over 15% with the thickness of amorphous Si below 200 nm and of microcrystalline Si around 500 nm is possible.

Osteogenic cell differentiation on H-terminated and O-terminated nanocrystalline diamond films.

Nanocrystalline diamond (NCD) films are promising materials for bone implant coatings because of their biocompatibility, chemical resistance, and mechanical hardness. Moreover, NCD wettability can be tailored by grafting specific atoms. The NCD films used in this study were grown on silicon substrates by microwave plasma-enhanced chemical vapor deposition and grafted by hydrogen atoms (H-termination) or oxygen atoms (O-termination). Human osteoblast-like Saos-2 cells were used for biological studies on H-terminated and O-terminated NCD films. The adhesion, growth, and subsequent differentiation of the osteoblasts on NCD films were examined, and the extracellular matrix production and composition were quantified. The osteoblasts that had been cultivated on the O-terminated NCD films exhibited a higher growth rate than those grown on the H-terminated NCD films. The mature collagen fibers were detected in Saos-2 cells on both the H-terminated and O-terminated NCD films; however, the quantity of total collagen in the extracellular matrix was higher on the O-terminated NCD films, as were the amounts of calcium deposition and alkaline phosphatase activity. Nevertheless, the expression of genes for osteogenic markers – type I collagen, alkaline phosphatase, and osteocalcin – was either comparable on the H-terminated and O-terminated films or even lower on the O-terminated films. In conclusion, the higher wettability of the O-terminated NCD films is promising for adhesion and growth of osteoblasts. In addition, the O-terminated surface also seems to support the deposition of extracellular matrix proteins and extracellular matrix mineralization, and this is promising for better osteoconductivity of potential bone implant coatings.

Effective Extraction of Photoluminescence from a Diamond Layer with a Photonic Crystal

Diamond-based materials possess many unique properties, one of them being a broad-band visible photoluminescence due to a variety of color centers. However, a high material refractive index makes the extraction of photoluminescence (PL) from a diamond layer inefficient. In this paper, we show that by periodical nanopatterning of the films surface into a form of two-dimensional photonic crystal, the extraction of PL can be strongly enhanced within the whole visible spectrum compared to the extraction of PL in a pristine or randomly nanopatterned film. On the basis of theoretical calculations, enhancement is shown to be due to the photonic crystal effect, including efficient coupling of an excitation laser into the diamond.

Diamond photonic crystal slab: leaky modes and modified photoluminescence emission of surface-deposited quantum dots.

Detailed analysis of a band diagram of a photonic crystal (PhC) slab prepared on a nano-diamond layer is presented. Even though the PhC is structurally imperfect, the existence of leaky modes, determined both theoretically and experimentally in the broad spectral region, implies that an efficient light interaction with a material periodicity occurs in the sample. It is shown that the luminescence emission spectrum of a light source placed directly on the PhC surface can be modified by employing the optical modes of the studied structure. We stress also the impact of intrinsic optical losses of the nano-diamond on this modification.

Enhanced photoluminescence extraction efficiency from a diamond photonic crystal via leaky modes

Two-dimensional photonic crystal can be exploited as the top part of a light source in order to increase its extraction efficiency. Here, we report on the room-temperature intrinsic photoluminescence (PL) behavior of a nanocrystalline diamond (NCD) layer with diamond columns prepared on the top and periodically ordered into the lattice with square symmetry. Angle-resolved far-field measurements in the Γ–X crystal direction of broadband visible PL revealed up to six-fold enhancement of extraction efficiency as compared to a smooth NCD layer. A photonic band diagram above the lightcone derived from these measurements is in agreement with the diagram obtained from transmission measurements and simulation, suggesting that the enhancement is primarily due to lights coupling to leaky modes.

Nanostructuring of diamond films using self-assembled nanoparticles

We report the use of gold, nickel and diamond nanoparticles as a masking material for realization of diamond nano-structures by applying the dry plasma etching process. Applying low power plasma (100 W) in a gas mixture of CF4/O2 for 5 minutes results in a formation of three different types of diamond nanostructures, depending on the mask type material and particle size. Using of the Ni mask results in realization of diamond nano-rods, applying of the Au mask brings cauliflower-like structures, and using the diamond powder allows the production of irregular nano-structures. The main advance of the presented etching procedure is use of a self-assembly strategy where no lithographic steps are implemented.

Influence of Diamond CVD Growth Conditions and Interlayer Material on Diamond/GaN Interface

In this study we present the diamond deposition on AlGaN/GaN substrates focusing on the quality of the diamond/GaN interface. The growth of diamond films was performed using microwave chemical vapour deposition system in different gas mixtures: standard CH4/H2 (at low and high ratio of CH4 to H2) and addition of CO2 to CH4/H2 gas chemistry. The diamond films were grown directly on GaN films either without or with thin interlayer. As interlayer, 100 nm thick Si3N4 was used. Surprisingly, in the case of standard CH4/H2 gas mixture, no diamond film was observed on the GaN with SiN interlayer, while adding of CO2 resulted in diamond film formation of both samples with and without SiN interlayer. Moreover, adding of CO2 led to higher growth rate. The morphology of diamond films and the quality of the diamond/GaN interface was investigated from the cross-section images by scanning electron microscopy and the chemical character (i.e. sp3 versus sp2 carbon bonds) was measured by Raman spectroscopy.

Nanostructured diamond layers enhance the infrared spectroscopy of biomolecules.

We report on the fabrication and practical use of high-quality optical elements based on Au mirrors coated with diamond layers with flat, nanocolumnar, and nanoporous morphologies. Diamond layers (100 nm thickness) are grown at low temperatures (about 300 °C) from a methane, carbon dioxide, and hydrogen gas mixture by a pulsed microwave plasma system with linear antennas. Using grazing angle reflectance (GAR) Fourier transform infrared spectroscopy with p-polarized light, we compare the IR spectra of fetal bovine serum proteins adsorbed on diamond layers with oxidized (hydrophilic) surfaces. We show that the nanoporous diamond layers provide IR spectra with a signal gain of about 600% and a significantly improved sensitivity limit. This is attributed to its enhanced internal surface area. The improved sensitivity enabled us to distinguish weak infrared absorption peaks of <10-nm-thick protein layers and thereby to analyze the intimate diamond-molecule interface.

Diamond as functional material for bioelectronics and biotechnology

Understanding the interaction between the biological environment (tissues, cells, proteins, electrolytes, etc.) and a solid surface is crucial for biomedical applications such as bio-sensors, bio-electronics, tissue engineering and the optimization of implant materials. Cells, the cornerstones of living tissue, perceive their surroundings and subsequently modify it by producing extracellular matrix (ECM), which serves as a basis to simplify their adhesion, spreading and differentiation (Shakenraad & Busscher, 1989). This process is considerably complex, flexible and strongly depends on the cell cultivation conditions including the type of the substrate. Surface roughness of the substrate plays an important role (Babchenko et al., 2009; Kalbacova et al., 2009; Kromka et al., 2009; Zhao et al., 2006), other influential factors include both the porosity (Tanaka et al., 2007) and the wettability of the substrate, the latter influencing protein conformation (Browne et al., 2004; Rezek, Ukraintsev, Michalikova, Kromka, Zemek & Kalbacova, 2009) as well as the adsorption and viability of cells (Grausova et al., 2009; Kalbacova, Kalbac, Dunsch, Kromka, Vanecek, Rezek, Hempel & Kmoch, 2007). Materials which are commonly employed as substrates for in vitro testing are polystyrene and glass. In this context, diamond as a technological material can provide a relatively unique combination of excellent semiconducting, mechanical, chemical as well as biological properties (Nebel et al., 2007). Diamond also meets the basic requirements for large-scale industrial application, most notably, it can be prepared synthetically. Diamond can be synthesized either as a bulk material under high-pressure and high-temperature conditions, or in the form of thin films by chemical vapor deposition of methane and hydrogen on various substrates including glass and metal (Kromka et al., 2008; Potocky et al., 2007). Moreover, the application of selective nucleation makes it possible to directly grow conductive diamond microstructures, which operate e.g. as transistors or pH sensors (Kozak et al., 2010). Nowadays, it is possible to deposit diamond even on large areas (600 cm2 or more) using linear antennas (Kromka et al., 2011; Tsugawa et al., 2010). The excellent compatibility of diamond with biological materials and environment (Bajaj et al., 2007; Grausova et al., 2009; Diamond as Functional Material for Bioelectronics and Biotechnology

CHAPTER 13:Low Temperature Diamond Growth

Diamond thin films represent a class of multi-functional materials whose morphological, chemical, optical and electronic properties can be tailored on demand for specific applications. Nevertheless, this materials versatility inherently requires a high degree of control and understanding of the diamond growth technology. Here, especially, processes at low temperatures become important because of physical limitations regarding the intrinsic properties of typical target substrates (i.e., low melting temperature, high expansion coefficient, high thermal diffusion and chemical reactivity) and compatibility with standard semiconductor industrial technologies. However, low temperature diamond growth (LTDG) is still highly challenging, where novel phenomena are encountered that still remain to be understood. The present chapter focuses on low temperature diamond growth from technological and practical points of view. The LTDG process is divided in two strategies, which are based on i) the modification of the deposition systems and ii) the change of gas chemistry. The state of the art of each strategy and the fundamental growth processes that are involved are reviewed. Among the discussed diamond growth processes, microwave surface wave plasma in linear antenna configuration with oxygen-containing gas mixtures is shown as the most promising process for LTDG over large areas with high optical and electronic grade materials. The growth phenomena observed in linear antenna microwave plasma provide a simple way to control nano- and poly-crystalline diamond character. A practical comparison between focused and linear antenna microwave plasma is presented on several key studies, which utilize LTDG on amorphous silicon, glass, germanium and optical elements used for IR spectroscopy.