Effect of large strain on dielectric and ferroelectric properties of Ba0.5Sr0.5TiO3 thin films

BaxSr1−xTiO3 is ideally suited as a tunable medium for radio frequency passive component. In this context we have studied the effect of biaxial strain on the dielectric and ferroelectricproperties of Ba0.5Sr0.5TiO3thin filmsgrown epitaxially on SrTiO3 (001) substrates. The lattice parameters of the films determined by high-resolution x-ray diffraction with the thickness varying from 160 to 1000 nm indicated large biaxial compressive strain which decreased from 2.54% to 1.14% with increasing film thickness. Temperature-dependent measurements of the dielectric constant in our strained Ba0.5Sr0.5TiO3thin films revealed a significant increase in the Curie temperature as the film thickness is below 500 nm. Enhanced ferroelectric behavior was observed for highly strained films with a remanent polarization of 15 μC/cm2 in the 160-nm-thick layer. However, the thick films(≥500 nm) exhibited weak temperature dependence of the dielectric constant without any pronounced peak corresponding to the Curie temperature, which may suggest inhomogeneous strain distribution in the thick films

Epitaxial growth of (001)-oriented Ba0.5Sr0.5TiO3 thin films on a-plane sapphire with an MgO/ZnO bridge layer

High quality (001)-oriented Ba0.5Sr0.5TiO3 (BST) thin films have been grown on a-plane sapphire(112¯0) by rf magnetron sputtering using a double bridge layer consisting of (0001)-oriented ZnO (50 nm) and (001)-oriented MgO (10 nm) prepared by plasma-assisted molecular beam epitaxy. X-ray diffraction revealed the formation of three sets of in-plane BST domains, offset from one another by 30°, which is consistent with the in-plane symmetry of the MgO layer observed by in situ reflective high electron energy diffraction. The in-plane epitaxial relationship of BST, MgO, and ZnO has been determined to be BST [110]//MgO [110]//ZnO [112¯0] and BST [110]/MgO [110]//ZnO [11¯00]. Capacitance-voltage measurements performed on BST coplanar interdigitated capacitor structures revealed a high dielectric tunability of up to 84% at 1 MHz

High pressure measurements on visible spectrum AlxGa1−xAs heterostructure lasers: 7100–6750-Å 300-K operation

Pressure applied to high performance cw 300-K bulk-limit (Lz ~600 A) single quantum well heterostructure Alx Gal _ x As (x ~ 0.28, A ~ 7100 A) laser diodes is used to simulate composition change and determine the threshold increase at shorter wavelength. Unless small quantum well sizes are employed in more sophisticated designs it is unlikely that A (for cw 300-K operation) can be made much less than 6900 A

High pressure measurements on AlxGa1−xAs-GaAs (x = 0.5 and 1) superlattices and quantum well heterostructure lasers

Absorption data on AIAs-GaAs and Alx Gal _ x As-GaAs superlattices (SL's) and emission data on Alx Gal _ x As-GaAs quantum-well heterostructure (QWH) laser diodes subjected to hydrostatic pressure (0-10 kbar) at 300 K are presented. Superlattice absorption data show that the confined-particle transitions, which partition and "label" the r energy band high above the band edge, all move with the same pressure coefficient of 11.5 meV /kbar. (For bulk GaAs, the pressure coefficient is 12.5 me V /kbar. ) The effect of the L indirect minima on the highest observed confined-particle transitions is small; the effect of the X minima is large. At lower pressures, QWH diodes exhibit a pressure dependence similar to that of the free (unconstrained) SL's. The data on QWH diodes demonstrate, however, a size-dependent [Lz (GaAs) < 500 A] shift in slope to a lower (8.5 meV /kbar) energy gap versus pressure coefficient at higher pressures. This change in slope can be explained by considering the effect on the light- and heavy-hole subbands of shear stresses generated within the p-n diode heterostructure

Room-temperature ferroelectricity in strained SrTiO3

Systems with a ferroelectric to paraelectric transition in the vicinity of room temperature are useful for devices. Adjusting the ferroelectric transition temperature (T-c) is traditionally accomplished by chemical substitution - as in BaxSr1-xTiO3, the material widely investigated for microwave devices in which the dielectric constant (epsilon(r)) at GHz frequencies is tuned by applying a quasi-static electric field(1,2). Heterogeneity associated with chemical substitution in such films, however, can broaden this phase transition by hundreds of degrees(3), which is detrimental to tunability and microwave device performance. An alternative way to adjust Tc in ferroelectric films is strain(4-8). Here we show that epitaxial strain from a newly developed substrate can be harnessed to increase Tc by hundreds of degrees and produce room-temperature ferroelectricity in strontium titanate, a material that is not normally ferroelectric at any temperature. This strain-induced enhancement in T-c is the largest ever reported. Spatially resolved images of the local polarization state reveal a uniformity that far exceeds films tailored by chemical substitution. The high er at room temperature in these films ( nearly 7,000 at 10 GHz) and its sharp dependence on electric field are promising for device applications(1,2).Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/62658/1/nature02773.pd