Mikko Ruoho

Enhancement of the third-order optical nonlinearity in ZnO/Al2O3 nanolaminates fabricated by atomic layer deposition

We investigate the third-order optical nonlinearity in ZnO/Al2O3 nanolaminates fabricated by atomic layer deposition and show that the third-order optical nonlinearity can be enhanced by nanoscale engineering of the thin film structure. The grain size of the polycrystalline ZnO film is controlled by varying the thickness of the ZnO layers in the nanolaminate in which thin (∼2 nm) amorphous Al2O3 layers work as stopping layers for ZnO crystal growth. Nanoscale engineering enables us to achieve a third harmonic generated signal enhancement of ∼13 times from the optimized nanolaminate structure compared to a ZnO reference film of comparable thickness.

Influence of aluminium doping on thermoelectric performance of atomic layer deposited ZnO thin films

We study the effect of Al doping on thermoelectric power factor of ZnO films grown using atomic layer deposition method. The overall doping level is tuned by either varying the precursor pulsing sequence or by varying the number of precursor pulses while keeping the sequence unchanged. We observe that commonly utilized doping approach when periodic dopant layers are densely packed results in reduced power factor. At the same time, we find that thermoelectric performance can be improved by clustering the dopants. In addition, the clustering was found to tune the preferred crystal orientation of the polycrystalline film.

ALD-Assisted Multiorder Dispersion Engineering of Nanophotonic Strip Waveguides

We propose a new technique for the multiorder dispersion engineering of nanophotonic strip waveguides. Unlike other techniques, the method does not require wafers with customized parameters and is fully compatible with standard wafers used in nanophotonics. The dispersion management is based on the application of nanometer-thick TiO2 layer formed by atomic layer deposition. The method is simple and reliable and allows good control of dispersion up to the fourth-order terms. The additional advantages are the reduction of propagation losses and partial compensation of fabrication tolerances.

Measurement of thin film thermal conductivity using the laser flash method

We present a method to measure the in-plane thermal conductivity of thin films by the laser flash technique. The method uses a well-defined structure for the analysis. We have realized the structure by conformal deposition of ZnO films of different thicknesses using atomic layer deposition onto a 20 μm thick ion track etched polycarbonate membrane as substrate. By using this procedure we could determine the thermal conductivity of the deposited thin film from the total thermal diffusivity of the nanocomposite structures. The method has been used to obtain the in-plane thermal conductivity of the deposited ZnO layers within the thickness range of less than 100 nm.

GaAs nanowires grown on Al-doped ZnO buffer layer

We report a pathway to grow GaAs nanowires on a variety of substrates using a combination of atomic layer deposition and metallo-organic vapor phase epitaxy (MOVPE). GaAs nanowires were grown via MOVPE at 430–540 °C on an atomic-layer-deposited Al:ZnO buffer layer. The resulting nanowires were affected only by the properties of the buffer layer, allowing nanowire growth on a number of substrates that withstand ∼400 °C. The growth occurred in two phases: initial in-plane growth and subsequent out-plane growth. The nanowires grown exhibited a strong photoluminescence signal both at room temperature and at 12 K. The 12 K photoluminescence peak was at 1.47 eV, which was attributed to Zn autodoping from the buffer layer. The crystal structure was zincblende plagued with either twin planes or diagonal defect planes, which were related to perturbations in the seed particle during the growth. The used method combines substrates with variable properties to nanowire growth on a transparent and conductive Al:ZnO buf...

Optimization of Cuprous Oxides Thin Films to be used as Thermoelectric Touch Detectors

The electronic and optical properties of p-type copper oxides (CO) strongly depend on the production technique as it influences the obtained phases: cuprous oxide (Cu2O) or cupric oxide (CuO), the most common ones. Cu films deposited by thermal evaporation have been annealed in air atmosphere, with temperature between 225 and 375 °C and time between 1 and 4 h. The resultant CO films have been studied to understand the influence of processing parameters in the thermoelectric, electrical, optical, morphological, and structural properties. Films with a Cu2O single phase are formed when annealing at 225 °C, while CuO single phase films can be obtained at 375 °C. In between, both phases are obtained in proportions that depend on the film thickness and annealing time. The positive sign of the Seebeck coefficient (S), measured at room temperature (RT), confirms the p-type behavior of both oxides, showing values up to 1.2 mV·°C-1 and conductivity up to 2.9 (Ω·m)-1. A simple detector using Cu2O have been fabricated and tested with fast finger touch events.