Mark J. Dalberth

Improved low frequency and microwave dielectric response in strontium titanate thin films grown by pulsed laser ablation

We have grown epitaxial strontium titanate films on lanthanum aluminate substrates at a range of oxygen pressures and substrate temperatures. The complex dielectric function was measured as a function of temperature and electric field bias using a microwave ring resonator and a flip-chip technique. The films having the highest dielectric constant were grown with an oxygen pressure of 600 mTorr and showed large grains in the plane of the film. The small-signal dielectric constant of these films could be changed by a factor of 4 by applying an electric field. The films with the highest dielectric constant showed increased losses, but an improved figure of merit for application to tunable circuits.

Tunable SrTiO/sub 3/ varactors using parallel plate and interdigital structures

The dielectric properties of SrTiO/sub 3/ (STO)/YBCO and STO films deposited on LaAlO/sub 3/ substrates were evaluated, for potential varactor applications wherein the capacitance is altered via a dc voltage applied across the STO film. The optimal growth temperature for the STO films in the STO/YBCO structures was 800/spl deg/C; higher temperatures resulted in interfacial degradation and poor film quality, and lower temperatures resulted in films with lower dielectric constants (/spl epsi//sub r/) and tunabilities. Interdigital varactors comprised of STO films deposited directly on LaAlO/sub 3/ displayed higher tunabilities and significantly lower losses.

Effect of SrTiO3 deposition temperature on the dielectric properties of SrTiO3/YBa2Cu3O7-δ/LaAlO3 structures

Abstract We report on the effect of the deposition temperature of SrTiO3 on the dielectric properties of SrTiO3/YBa2Cu3O7-δ/LaAlO3 thin film multilayer structures. In these structures, the YBa2Cu3O7-δ (YBCO) films were deposited at 800°C by laser ablation, followed by the in-situ deposition of the SrTiO3 (STO) layer at one of the following temperatures: 750°C, 650°C, 550°C, 450°C, 350°C, and 250°C. Gold (Au) films were deposited and patterned on top of the STO layer to form planar Au/STO/YBCO capacitor structures. The electrical response was studied by measuring the dielectric constant (er) and loss tangent (tanδ) of the ferroelectric film from 300–40 K, at 1.0 MHz, and at electric fields up to 100 kV/cm. Our results show 750°C to be a deposition temperature which allows for large variations of er with applied field, and with limited enhancement of tanδ. Lower deposition temperatures cause a reduction of the induced change in er and an increase in tanδ with applied field.

1.3- μ m InGaAsN VCSELs for telecom and datacom applications

VCSELs operating at 1.3 microns are the ideal laser source for meeting the exploding demand for bandwidth in local area and metro area networks. NCSELs will eventually replace the vast majority of 1.3 micron FP and DFB edge emitting lasers currently used in these applications, since they offer lower manufacturing cost, on wafer testability, extremely narrow linewidth, a circular output beam, high speed direct modulation, and the ability to be integrated into arrays. The primary challenge in 1.3 micron VCSELs has been to find an active region material that can be grown directly on high thermal conductivity and high reflectivity GaAs/AlGaAs distributed Bragg reflectors DBRs. In this work, we have developed MBE grown InGaAsN quantum wells that can be grown directly on GaAs substrates and integrated directly into high performance oxide VCSEL structures. We have demonstrated record room temperature CW single mode output powers in excess of 1 mW at an emission wavelength of 1287 nm. CW lasing has been observed as high as 125 degree(s)C, illustrating the excellent thermal performance of both the InGaAsN quantum wells and the GaAs/AlGaAs DBRs. Open eye diagrams were observed at 10 Gb/s, paving the way for OC-192 SONET and 10 Gb/s Ethernet applications.

Microwave Dielectric Tuning and Losses in Epitaxial Lift-Off Thin Films of Strontium Titanate

We report on the complex dielectric response function for SrTiO 3 (STO) thin films that have been removed from the original growth substrate via an epitaxial lift-off process. We measured the dielectric response as a function of frequency (10 kHz to above 2 GHz), as a function of temperature (4 K to 300 K), and as a function of applied electric field (between +/− 10 V/µm). The dielectric properties are well described by a model based on a dilute set of thermally activated two-level dipoles in a matrix of bulk STO. Activation energies for the dipoles are found to cluster around two central values near 50 meV and 100 meV. Attempt frequencies are generally around 10 THz. Assuming that the dipole moment of a typical two-level system is equal to that of a unit cell of polarized bulk STO, we find that a volume fraction of roughly 4% of a typical film is involved in the dipole response.

Infrared characterization of SrTiO/sub 3/ thin films using attenuated total reflectance

Attenuated Total Reflectance was used to measure the phonon vibration frequencies over the range 425-800 cm/sup -1/ of SrTiO/sub 3/ thin films deposited either directly on LaAlO/sub 3/, or on YBCO-coated LaAlO/sub 3/ single crystal substrates. In the s-polarized spectra, the transverse optic Ti-O stretching vibration shifted to lower frequencies as the film thickness increased, which was attributed to damping caused by the generation of lower frequency phonon modes. The transverse Ti-O stretching vibration was also observed in the p-polarized spectra, but was more heavily damped, which indicates the damping is more pronounced when the electric field has a component perpendicular to the film surface. Damping of the transverse modes was attributed to coupling of these modes to other phonon modes, and may be a source of the high losses at microwave frequencies.