Wouter Woestenborghs

Lanthanide-Assisted Deposition of Strongly Electro-optic PZT Thin Films on Silicon: Toward Integrated Active Nanophotonic Devices

The electro-optical properties of lead zirconate titanate (PZT) thin films depend strongly on the quality and crystallographic orientation of the thin films. We demonstrate a novel method to grow highly textured PZT thin films on silicon using the chemical solution deposition (CSD) process. We report the use of ultrathin (5-15 nm) lanthanide (La, Pr, Nd, Sm) based intermediate layers for obtaining preferentially (100) oriented PZT thin films. X-ray diffraction measurements indicate preferentially oriented intermediate Ln2O2CO3 layers providing an excellent lattice match with the PZT thin films grown on top. The XRD and scanning electron microscopy measurements reveal that the annealed layers are dense, uniform, crack-free and highly oriented (>99.8%) without apparent defects or secondary phases. The EDX and HRTEM characterization confirm that the template layers act as an efficient diffusion barrier and form a sharp interface between the substrate and the PZT. The electrical measurements indicate a dielectric constant of ∼650, low dielectric loss of ∼0.02, coercive field of 70 kV/cm, remnant polarization of 25 μC/cm(2), and large breakdown electric field of 1000 kV/cm. Finally, the effective electro-optic coefficients of the films are estimated with a spectroscopic ellipsometer measurement, considering the electric field induced variations in the phase reflectance ratio. The electro-optic measurements reveal excellent linear effective pockels coefficients of 110 to 240 pm/V, which makes the CSD deposited PZT thin film an ideal candidate for Si-based active integrated nanophotonic devices.

Preferentially oriented BaTiO3 thin films deposited on silicon with thin intermediate buffer layers.

Barium titanate (BaTiO3) thin films are prepared by conventional 2-methoxy ethanol-based chemical solution deposition. We report highly c-axis-oriented BaTiO3 thin films grown on silicon substrates, coated with a lanthanum oxynitrate buffer layer of 8.9 nm. The influence of the intermediate buffer layer on the crystallization of BaTiO3 film is investigated. The annealing temperature and buffer layer sintering conditions are optimized to obtain good crystal growth. X-ray diffraction measurements show the growth of highly oriented BaTiO3 thin films having a single perovskite phase with tetragonal geometry. The scanning electron microscopy and atomic force microscopy studies indicate the presence of smooth, crack-free, uniform layers, with densely packed crystal grains on the silicon surface. A BaTiO3 film of 150-nm thickness, deposited on a buffer layer of 7.2 nm, shows a dielectric constant of 270, remnant polarization (2Pr) of 5 μC/cm2, and coercive field (Ec) of 60 kV/cm.

VCSEL With Photo-Aligned Liquid Crystal Overlay

A technological platform for a vertical cavity surface-emitting laser with a tunable polarization is presented. It is achieved by integrating an 850-nm vertical cavity surface-emitting laser chip in a liquid crystal cell that uses photo-alignment to orient the nematic liquid crystal. The vertical cavity surface-emitting laser and the liquid crystal layer can be electrically driven with separate electrodes. We demonstrate that the polarization state of the laser emission can be controlled by applying an appropriate voltage over the liquid crystal layer. The polarization of the emission depends on the emission angle, in particular, for voltages close to the liquid crystal threshold voltage, which is in agreement with expectations from the extended Jones calculus.

A two-dimensional model for an in-plane organic photo-conductive bilayer sensor

In this work we present a two-dimensional (2D) model for an organic thin film photo-conductive sensor containing a planar heterojunction and in-plane electrodes. The model simulates the flow of charge carriers based on the standard one-dimensional semiconductor transport and continuity equations, and combines this with a 2D model for the electric field. This procedure results in a hybrid differential/integral equation formulation. We present and analyse simulation results that resemble very well measured current–voltage characteristics of a real sensor under different illumination levels. We find that for currents below a critical value the sensor behaves like a resistor. Above this critical current the current increases much more slowly due to space charge accumulation close to the cathode. We explain the critical current as the maximum reverse current of the solar cell formed by the heterojunction covering the cathodic electrode.

Transient and local illumination of an organic photoconductive sensor

In this paper we investigate the performance of a transparent photoconductive sensor based on a double layer of organic materials (m-MTDAB / PTCBI) which are deposited on two interdigitated transparent ITO electrodes. The performance of the sensor is demonstrated with electro-optical measurements: the I(V) curves consist of two linear sections meeting at a knee voltage Vt. Linear regression performed on the I(V) curves below Vt show that the conductance is a power law of the luminance incident on the device. We present a model to describe the behaviour of the sensor below Vt. We present measurements of I(t) for a transient illumination of the sensor. Plotting the inverse of the current as a function of time we find that the transient is consistent with the model for voltages below Vt. For voltages above Vt we find that the sensor behaves like a resistor in series with a space charge (SC) region. We present a local illumination experiment that confirms the existence of a SC region between the electrodes of the photoconductive sensor for V

Vertical cavity surface emitting laser with nematic and chiral liquid crystals overlay

A technological platform for a vertical cavity surface emitting laser (VCSEL) with tunable polarization is presented. It is realized by integrating an 850nm VCSEL chip in a liquid crystal (LC) cell that uses photo-alignment (PA) to orient the LC. Two kinds of LC are filled in and form a thin layer over the emitter of the VCSEL: nematic LC or chiral nematic LC (cLC). The VCSEL and the nematic LC layer can be electrically driven with separate electrodes. The polarization state of the laser emission can be controlled by applying an appropriate voltage over the nematic LC layer. The chiral nematic LC has a reflection band that contains the VCSEL emission wavelength, so that one circular polarized mode of the laser emission is reflected as a feedback into the VCSEL. We found that the emission from the VCSEL with cLC overlay is circularly polarized.