Nadia K. Pervez

Impact of thermal strain on the dielectric constant of sputtered barium strontium titanate thin films

Barium strontium titanate thin films were deposited by sputtering on Pt/SiO2 structures using five different host substrates: magnesium oxide, strontium titanate, sapphire, silicon, and vycor glass. These substrates were chosen to provide a systematic change in thermal strain while maintaining the same film microstructure. All films have a weakly textured microstructure. Temperature dependent dielectric measurements from 100–500 K determined that decreasing thermal expansion coefficient of the host substrate (i.e., larger tensile thermal strain) reduced the film dielectric permittivity. The experimentally determined Curie–Weiss temperature decreased with increasing tensile thermal strain and the Curie–Weiss constant increased with tensile strain as predicted by Pertsev et al. [J. Appl. Phys. 85, 1698 (1999)].

High tunability barium strontium titanate thin films for rf circuit applications

Large variations in the permittivity of rf magnetron sputtered thin-film barium strontium titanate have been obtained through optimization of growth conditions for maximum dielectric strength and zero-field permittivity in a parallel-plate capacitor structure. Using nominal target compositions of Ba0.5Sr0.5TiO3, and Pt electrodes on c-plane sapphire substrates, adjustment of the O2 partial pressure during deposition was used to vary the excess Ti incorporation into the films, which influenced the low-field permittivity, loss tangent, and dielectric strength. By balancing the benefits of a high permittivity with dielectric strength and loss, we have produced films capable of sustaining short-duration fields greater than 4MV∕cm with over 13:1 (>90%) change in dielectric constant, and greater than 5:1 tunability in bias fields under 1MV∕cm.

Low-loss tunable capacitors fabricated directly on gold bottom electrodes

At microwave frequencies, conductor losses due to the bottom electrode resistance severely limit the performance of metal-insulator-metal capacitors that employ tunable dielectric thin films. Here we demonstrate that a novel tunable dielectric, bismuth zinc niobate (BZN), can be integrated directly with low-resistivity Au bottom electrodes. The favorable crystallization kinetics allowed for a low thermal budget process compatible with Au electrodes. BZN thin films on Au bottom electrodes showed low dielectric loss tangents of ∼0.0005 and high dielectric tunabilities of ∼50%. The Au/BZN interface was abrupt and free of reaction phases. At high frequencies (>1MHz) the total Au/BZN capacitor device loss was reduced compared to capacitors with Pt bottom electrodes. The low device losses of Au/BZN capacitors revealed a device geometry-dependent loss mechanism that contributed significantly to the device loss at high frequencies.

A high-resolution spectrometer based on a compact planar two dimensional photonic crystal cavity array

We demonstrate a compact spectrometer based on an array of high-quality-factor photonic crystal nanocavities, coupled via a planar two-dimensional waveguide. This architecture enables spectral analysis of incident light with resolution as high as the bandwidth of the cavity mode–0.3 nm at 840 nm for our device. The design is easily extended to the visible and deep-infrared spectral ranges. The two-dimensional cavity array can be mated to commercial two-dimensional optical detector arrays, creating a compact and high-resolution spectrometer suitable for a range of applications including materials and chemical analysis.

Photonic crystal spectrometer

We demonstrate a new kind of optical spectrometer employing photonic crystal patterns to outcouple waveguided light from a transparent substrate. This spectrometer consists of an array of photonic crystal patterns, nanofabricated in a polymer on a glass substrate, combined with a camera. The camera captures an image of the light outcoupled from the patterned substrate; the array of patterns produces a spatially resolved map of intensities for different wavelength bands. The intensity map of the image is converted into a spectrum using the photonic crystal pattern response functions. We present a proof of concept by characterizing a white LED with our photonic crystal spectrometer.

Influence of stoichiometry on the dielectric properties of sputtered strontium titanate thin films

The dielectric permittivity, dielectric quality factor (inverse dielectric loss), and lattice parameter of 140 nm sputtered SrTiO3 films were dependent on the oxygen partial pressure and total chamber pressure (O2+Ar) during film growth. Films were grown at 25 and 75 mTorr (mT) in an oxygen rich and oxygen deficient sputtering gas environment concurrently on (100) SrTiO3 and (111) Pt/(0001) Al2O3 substrates. Films were deposited on platinized sapphire for electrical characterization and the homoepitaxial films were used as a structural and chemical standard. High resolution triple axis x-ray diffraction results showed an increase in mismatch between the film and substrate (200) peak in homoepitaxial SrTiO3 films with higher total growth and lower oxygen pressures. Dielectric quality factors of the SrTiO3 films on platinized sapphire at 1 MHz for the 25 mT (50 sccm Ar/50 sccm O2), 25 mT (90 sccm Ar/10 sccm O2), 75 mT (50 sccm Ar/50 sccm O2), and 75 mT (90 sccm Ar/10 sccm O2) film growths were 320, 251, 209...

At the Crossroads of the World: Women of the Middle East

The authors offer a brief introduction to the history of women of the Middle East, with a focus on three major religions (Judaism, Christianity, and Islam). Schools are paying increased attention to teaching world history, but they are giving too little attention to incorporating women as part of world history. One of the major dividing lines within the Middle East has been religion, a fixed feature of the world history curriculum. The authors attempt to provide insights, based on new research about women in the region, into how religion and culture influence womens lives in this area of the world. They conclude with a brief consideration of how women are organizing for change in the Middle East.

Dielectric losses of SrTiO3 thin film capacitors with Pt bottom electrodes at frequencies up to 1GHz

The letter reports on the dielectric losses of differently textured SrTiO3 films on Pt bottom electrodes at frequencies between 1MHz and 1GHz. Device parasitic contributions to the measured device losses were partially removed by measuring shorted devices. Different dielectric loss mechanisms were identified. These included strongly frequency dependent loss peaks and a low temperature loss increase that showed a power-law dependence on the bulk permittivity of the films. Possible origins of the different loss mechanisms are discussed.

Waveguide-integrated photonic crystal spectrometer with camera readout

We demonstrate an infrared spectrometer based on waveguide-coupled nanocavity filters in a planar photonic crystal structure. The input light is coupled into the waveguide, from which spectral components are dropped into the cavities and radiated off-chip for detection on a commercial InGaAs camera. The spectrometer has a footprint of only 60 μm by 8 μm. The spectral resolution is about 1 nm in the operation bandwidth of 1522–1545 nm. By substituting the membrane material and structure parameters, this design can be easily extended into the visible regime and developed for a variety of highly efficient, miniature photonic applications.

Optimization of high tunability barium strontium titanate thin films grown by RF magnetron sputtering

Barium strontium titanate is a solid solution perovskite with a field-dependent permittivity. At microwave frequencies, its tunable dielectric constant and low loss make it a competitive choice for varactors and other tunable circuit elements. Much attention has been focused on the production of low-loss films for such applications, with little emphasis on how the film properties contribute to the circuit loss. When tunable elements are implemented in circuits, electrode loss dominates over film loss. In applications such as phase shifters, where cascaded tuning elements provide a predetermined amount of tuning, circuit designs using high tunability films minimize the number of tuning elements required, resulting in an overall reduction in circuit loss. When growth conditions are optimized for superior electrical properties, tunability and film loss are the two quantities of interest. By changing the oxygen partial pressure during growth, the amount of excess Ti incorporated into the film is changed. Films with higher excess Ti contents exhibit lower losses, higher breakdown voltages, and lower permittivities than more stoichiometric films. While all of the films approach the same high-field capacitance limit, the total tunability is determined not only by the zero-field permittivity but also by the breakdown voltage; a device must be able to tolerate sufficient applied bias to reach its high-field capacitance limit. By balancing these factors, we have produced capacitors with an unprecedented 13.71:1 (92.7%) tuning ratio at an applied field of 4.7 MV/cm.