Liquid Ga droplets play a double role in the self-catalyzed growth of GaAs nanowires on Si(111) substrates covered with a native SiO x layer: they induce the formation of nanosized holes in SiO x and then drive the uniaxial nanowire growth directly onto the underlying Si. The independent control of the two mechanisms is a prerequisite for mastering the growth of nanowires, but it is challenging in a conventional growth procedure where they both take place under the same droplets. To that end, we have developed an in situ procedure where the Ga droplets used for the formation of SiO x holes are removed before new Ga droplets drive the growth of GaAs nanowires. In that way, it was possible to study the interaction between Ga droplets and SiO x , to create holes in SiO x with controlled number density and size, and, finally, to grow GaAs nanowires only within those holes. Our results show unprecedented control of the nanowire nucleation with unique possibilities: (1) deliberate control of the number density of nanowires within 3 orders of magnitude (106–109 cm–2) without patterning the substrate and without changing the growth conditions, (2) highly synchronous nucleation events and, thus, exceptionally narrow nanowire length distributions (standard deviation <1% for 3 μm long nanowires), (3) high yield of vertical nanowires up to 80% (against GaAs islands), (4) highly reproducible results, and (5) independent control of the nanowire diameter from the number density. We anticipate that our methodology could be also exploited for different materials or other types of nanostructures.
We demonstrate a simple route to grow ensembles of self-catalyzed GaAs nanowires with a remarkably narrow statistical distribution of lengths on natively oxidized Si(111) substrates. The fitting of the nanowire length distribution (LD) with a theoretical model reveals that the key requirements for narrow LDs are the synchronized nucleation of all nanowires on the substrate and the absence of beam shadowing from adjacent nanowires. Both requirements are fulfilled by controlling the size and number density of the openings in SiO x , where the nanowires nucleate. This is achieved by using a pre-growth treatment of the substrate with Ga droplets and two annealing cycles. The narrowest nanowire LDs are markedly sub-Poissonian, which validates the theoretical predictions about temporally anti-correlated nucleation events in individual nanowires, the so-called nucleation antibunching. Finally, the reproducibility of sub-Poissonian LDs attests the reliability of our growth method.
We introduce droplet-confined alternate pulsed epitaxy for the self-catalyzed growth of GaAs nanowires on Si(111) substrates in the temperature range from 550 °C down to 450 °C. This unconventional growth mode is a modification of the migration-enhanced epitaxy, where alternating pulses of Ga and As4 are employed instead of a continuous supply. The enhancement of the diffusion length of Ga adatoms on the {11̅0} nanowire sidewalls allows for their targeted delivery to the Ga droplets at the top of the nanowires and, thus, for a highly directional growth along the nanowire axis even at temperatures as low as 450 °C. We demonstrate that the axial growth can be simply and abruptly interrupted at any time without the formation of any defects, whereas the growth rate can be controlled with high accuracy down to the monolayer scale, being limited only by the stochastic nature of nucleation. Taking advantage of these unique possibilities, we were able to probe and describe quantitatively the population dynamics of As inside the Ga droplets in specially designed experiments. After all, our growth method combines all necessary elements for precise growth control, in-depth investigation of the growth mechanisms and compatibility with fully processed Si-CMOS substrates.
ZrO2 is of very high interest for various applications in semiconductor industry especially as high-k dielectric in metal–insulator–metal (MIM) capacitor devices. Further improvement of deposition processes, of material properties, and of integration schemes is essential in order to meet the strict requirements of future devices. In this paper, the authors describe a solution to solve one of the key challenges by reducing the process time of the bottle neck high-k atomic layer deposition (ALD). The authors extensively optimized the most common ALD process used for the ZrO2 deposition (TEMAZ/O3) resulting now in a doubled growth rate compared to the published growth rates of maximum 1 Å/cycle. Chemical reactions explaining the origin of the high growth rate are proposed by theoretical process modelling. At the same time, the outstanding electrical properties of ZrO2 thin films could be preserved. Finally, the integration of the ZrO2 process in MIM capacitor devices with TiN electrodes was evaluated. Thereby, the known effect of TiN bottom electrode oxidation by the O3 process was analyzed and significantly reduced by different integration approaches including wet chemical treatments and ALD process variations. The resulting MIM capacitors show low leakage current and high polarity symmetry.
Thin films based on dodecylamine stabilized gold nanoparticles interlinked with different organic molecules are prepared by automatic layer-by-layer self-assembly in a microfluidic quartz crystal microbalance (QCM) cell, to obtain an in situ insight on the film formation by ligand/linker exchange reactions. The influence of interlinking functional groups and the length of the organic linker molecule on the assembly behavior is investigated. Alkyldithiols with different lengths are compared to alkyldiamines and alkylbisdithiocarbamates with a C8 alkylic molecular backbone. The stepwise layer-by-layer assembly occurs independently of the linker molecule, while the largest frequency changes always correspond to the gold nanoparticle step. During the solvent rinsing and ligand/linker exchange reaction step, the frequency is almost constant with slight increases or decreases dependent on the molar mass of the linker compared to the exchanged ligand. The assembly efficiency is higher for shorter molecules and for molecules with stronger interacting functional groups. The densities of the composite films are calculated from QCM data and independent thickness measurements. They reflect the higher fraction of organic material in the films comprising longer organic linkers. The plasmon resonance band of the gold nanoparticles in the final assemblies is measured with UV/vis spectroscopy. Band positions in films prepared from dithiols and diamines of comparable lengths are very similar, while the spectrum of the bisdithiocarbamate film exhibits a distinct blue-shift. This observation is explained by the longer molecular structure of the linker due to a larger binding group, in conjunction with a delocalization of particle charge on the organic molecule. Obtained results play an essential role in the understanding of thin film layer-by-layer self-assembly processes, and enable the formation of new gold nanoparticle networks with organic diamine and bisdithiocarbamate molecules.
Chemiresistive composites of gold (Au) nanoparticles interlinked with different types of organic molecules were prepared automatically by layer-by-layer self-assembly using a microfluidic cell. For the assembly process, dodecylamine-stabilized Au nanoparticles with an average size of 3.7 nm as well as alkyl dithiols, alkyl diamines, and alkyl bisdithiocarbamates with different alkyl chain length (C6 and C8) were used. X-ray photoelectron spectroscopy was applied on prepared nanoparticle composites to study the film composition and the degree of interlinkage. For the measurement of electrical and vapor-sensing properties, silicon dies equipped with gold interdigitated electrodes were used. All films show linear current-voltage characteristics and conductivities in the range of 10–2 and 10–4 Ω−1 cm−1 at room temperature. The sensitivity of the film is investigated by dosing them with vapors of toluene, 1-propanol, 4-methyl-2-pentanone, and water in the concentration range from 100 to 5,000 ppm at 0% relative humidity. All composite films respond with an increase in their electrical resistance to the analytes. The sensors show a high signal-to-noise ratio which indicates a detection limit below 100 ppm for all test vapors. The response dynamics demonstrate a high reversibility and a fast sensing mechanism especially for dithiols and diamines with response and recovery times from 2 to 10 s. The dithiol sensors exhibit a high selectivity to toluene and 4-methyl-2-pentanone whereas the bisdithiocarbamate composites are suitable for the detection of water and 1-propanol. All materials are stable for (at least) several months.
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