Øystein Dahl

Ferroelectric stripe domains in PbTiO3 thin films: Depolarization field and domain randomness

Observation of stripe domains in PbTiO3 thin films using standard x-ray diffraction analysis at room temperature is discussed. High-quality c-axis oriented thin films of varying thickness, from 6 to 210 unit cells, were grown on buffered NH4–HF etched SrTiO3(001) and Nb:SrTiO3(001) substrates using off-axis radio frequency magnetron sputtering. High-resolution linear Qx scans reveal a superstructure around the specular Bragg peaks, consistent with the presence of ferroelectric stripe domains. For thin samples, the stripe width is found to be proportional to the square root of the film thickness, with random in-plane orientation of domains. For films with a thickness of more than ∼100 unit cells, both monodomain samples and stripe domains were observed. We present evidence for the presence of a threshold depolarization field, above which there is a monotonically decreasing relationship between the domain width and the depolarization field. Furthermore, simulations show that random variations in size of the...

Imaging of out-of-plane interfacial strain in epitaxial PbTiO3∕SrTiO3 thin films

In this study, we rely on low-angle annular dark-field scanning transmission electron microscopy to probe the interface strain profile in epitaxial PbTiO3∕SrTiO3 thin-films. All samples displayed a compressively strained layer at the PbTiO3∕SrTiO3 interface, with the strain vector parallel to the polarization direction. The width of the strained layer was found to be ∼15–30A, dependent on the electrode environment. Our findings open a perspective to use interface strain engineering in combination with control of electrostatic boundary conditions as a tool to monitor the effective interface polarization. These findings have implications for future use of ferroelectrics in electronic and mechanical devices.

Polarization direction and stability in ferroelectric lead titanate thin films

In this article, we examine the initial polarization of PbTiO3 thin films grown epitaxially on SrRuO3 electrodes. It is found that the as-grown predominant polarization is directed toward the SrRuO3 bottom electrode in films thinner than 20 nm and directed toward the top surface in thicker films. The data is interpreted in terms of a Landau–Ginzburg–Devonshire model for a semiconducting ferroelectric with asymmetric boundary conditions. Based on the measured hysteresis loops and the stability of the two polarization directions with time, it is concluded that charged defects serve to impose a preferential downward polarization in very thin films.

Crystalline and dielectric properties of sputter deposited PbTiO3 thin films

The crystalline and dielectric properties of sputter deposited PbTiO3 thin films were investigated as a function of the film thickness and growth temperature. The crystalline quality was found to be independent of the film thickness from 2.4to200nm. The capacitance of 0.12mm2 Pt∕PbTiO3∕SrRuO3 and Pt∕PbTiO3∕Nb:SrTiO3 capacitors was found to deviate from the ideal parallel plate capacitance for PbTiO3 films thinner than 10nm. The decrease in capacitance was consistent with field penetration into the electrodes. The surface Pb content, as determined from x-ray photoelectron spectroscopy, was found to decrease with increasing growth temperature from 700to760°C. However, no change could be observed in the crystalline quality.

Epilayer control of photodeposited materials during UV photocatalysis

Epilayer control of photoassisted oxidation and reduction on the ferroelectric PbTiO3 polar surface was investigated. Photo-oxidation of a AgNO3 electrolyte resulting in formation of Ag7NO11 particles was observed on the PbTiO3/Nb:SrTiO3 film surface; whereas PbTiO3/SrRuO3/SrTiO3 leads to AgNO3 reduction under UV illumination. The oxidation reaction resulting in the formation of Ag7NO11 is explained in terms of a rectifying interface between PbTiO3 and Nb:SrTiO3, controlling the charge transport during UV photocatalysis.

Initial Studies of 6082 Aluminium Thin Films

The microstructure of thin aluminium 6082 films, with a thickness from 0.1 μm to 1.7 μm, has been investigated. The films were deposited by sputtering on Si substrates. The resulting microstructure was studied using Scanning and Transmission Electron Microscopy. The grains grew columnarly and, depending on the growth conditions, the diameter varied from 20 to 100 nm. Heat treatment at 500°C for 4 h was performed in an attempt to increase the grain size, but no subsequent grain growth was observed, indicating that the grain boundaries are stable. After deposition, several different ageing temperatures ranging from 150°C to 300°C and varying ageing times from 0.5 h to 10 h were performed. No precipitation was observed in the films, which could be due to the small grain size which is in the order of the precipitation free zone dimensions in bulk alloys.

Nanosecond laser ablation and deposition of Ge films

In this work, nanosecond-pulsed from ultra-violet to infrared lasers: KrF (248 nm, 25 ns) and Nd:YAG (1064 nm, 532 nm, 355 nm, 5 ns) were employed for ablation and deposition of germanium films in background pressure of <10-6 Torr. Deposition was carried out at room temperature on Si, GaAs, sapphire and glass. The as-deposited films, characterized by using scanning electron microscopy (SEM) and atomic force microscopy (AFM), consist of nano to micron-sized droplets on nanostructured film. The dependence of film properties on laser wavelengths and fluence are discussed.