Khaled Khamchane

Silicon substrate integrated high Q-factor parallel-plate ferroelectric varactors for microwave/millimeterwave applications

Parallel-plate Ba0.25Sr0.75TiO3 (BST) varactors with a record high Q factor are fabricated on Si substrate. At 45 GHz the Q factor is about 40, and the tuneability at 25 V is more than 40% in the measured frequency range 0.045–45 GHz. The improvement in the Q factor is achieved by using a thick bottom electrode consisting of Pt (50 nm)/Au (0.5 μm) allowing us to reduce the microwave losses associated with metal layers. The BST films exhibit relatively high permittivity (150) at zero bias and high resistivity (1010 Ω cm) at fields up to 700 kV/cm.

Microwave loss mechanisms in Ba0.25Sr0.75TiO3 thin film varactors

Parallel-plate Au(Pt)∕Ba0.25Sr0.75TiO3∕(Pt)Au thin film varactors were fabricated on high resistance Si substrates and characterized at dc, rf, and microwave frequencies. In the frequency range 10–45 GHz the varactors show relatively low losses, with loss tangent less than 0.025 at 45 GHz. Due to the thick and highly conductive Pt/Au electrodes the metal losses are less than 10%. However, the loss tangent of the ferroelectric film is still three to five times higher than that in Ba0.27Sr0.73TiO3 single crystal. The analysis of the dc field dependences of loss tangent and permittivity in a wide frequency range shows that these additional losses are mainly due to the charged defects. Extrapolation of measured low frequency (1 MHz) loss tangents to the microwave region using the power law ω1∕3 is in good agreement with experiment. The dc current through the varactor is found to be controlled by Schottky emission and Poole-Frenkel mechanisms depending on the polarity. The Poole-Frenkel mode is associated with...

Non-Destructive Microwave Characterization of Ferroelectric Films on Conductive Substrates

Limitations, and detailed accuracy analysis are provided for microwave dielectric characterization of ferroelectric films, sandwiched between two thin film electrodes, and under DC electric bias field. Electrodynamic simulations are carried out, and test structures with different electrode shape/size are fabricated and measured to check the accuracy of simple analytic formulas. It is shown that using this simple method the permittivity and loss tangent of ferroelectric films maybe measured at frequencies up to 50 GHz and above. The accuracy of measurement is rather high and in extreme cases not worse than 20%.

Ba0.25Sr0.75TiO3 thin-film varactors on SrRuO3 bottom electrode

Ba0.25Sr0.75TiO3∕SrRuO3 (BSTO/SRO) thin-film varactors were fabricated on (001)LaAlO3 substrates and characterized at rf and microwave frequencies of up to 10 GHz in the temperature range of 50–300 K. X-ray analysis reveals epitaxial growth of (001)-oriented BSTO films with extended out-of-plane lattice parameter b=3.925A. The tensile out-of-plane strain (u33=0.3%) of BSTO films results in an out-of-plane component of the relative permittivity e33=490. The renormalized Curie-Weiss constant and Curie temperature are C33=8.0×104K and T33=140K, respectively. An interfacial layer presumably forms at the Au∕Pt top electrode with a thickness-to-relative permittivity ratio of di∕ei=0.3–0.4nm and reduces the apparent relative permittivity to ea=250. The relative tunability of the varactor (at 10 V dc bias) increases from 40% (295 K) up to 80% (125 K) as the temperature decreases in the whole frequency range. The varactor loss tangent increases with increasing frequency and decreasing temperature (from tanδ=0.01 a...

dc and microwave resistivities of SrRuO3 films deposited on SrTiO3

Epitaxial SrRuO3 thin films on (001) SrTiO3 substrates grown by pulsed laser ablation at deposition temperatures ranging between 690° and 810 °C have been examined. The correlations between dc and microwave resistivity, microstructure, and surface morphology are obtained. According to the x-ray measurements the structure of all films is a mixture of highly oriented domains of strained SrRuO3 phases with different lattice parameters. The films grown at 780 °C consist mainly of orthorhombic phase I with a bulk lattice parameter close to the orthorhombic single crystal and reveal same resistivity values of 270 μΩ cm for both dc and microwave. Films obtained at other deposition temperatures contain a larger amount of orthorhombic phase II, with extended bulk lattice parameters and dc resistivity increases up to 1700 μΩ cm for the lowest deposition temperature. The corresponding microwave resistivity values are 50% higher than the dc ones. The dc resistivity of SrRuO3 films is in inverse proportion to the inte...

Ferroelectric domain wall relaxation in Ba0.25Sr0.75TiO3 films displaying Curie-Weiss behavior

Ferroelectric films may be used in integrated circuits for high frequency and memory applications. Losses and interfaces between films and electrodes are problematic. This work concerns the temperature and electric field response of the complex dielectric permittivity and the relaxation of domain walls in a ferroelectric layer that is of sufficient quality to show a Curie-Weiss behavior. Laser ablation was used to deposit 1200 nm thick Ba0.25Sr0.75TiO3 layers between metallic oxide, (100 nm) SrRuO3 and (120 nm) La0.67Ca0.33MnO3, films in epitaxial heterostructures. The electric field response (E⩽80kV∕cm) of the real ϵ′ and imaginary ϵ″ parts of the complex permittivity of the intermediate Ba0.25Sr0.75TiO3 layer in these parallel plane film capacitors was studied at temperatures above and below the phase transition point TCurie. The latter was determined from the temperature dependence of the inverse dielectric permittivity and its value, TCurie=145K, agrees well with that of bulk single crystal. ϵ′ of the...

Microwave properties of SrTiO3/SrRuO3/CeO2/YSZ heterostructure on low-resistivity silicon

Abstract Properties of thin film SrTiO 3 /SrRuO 3 /CeO 2 /YSZ heterostructures on low-resistivity (25 Ωcm) Si substrates are studied at microwave frequencies. Heterostructures are grown by laser ablation at different conditions and characterized by XRD. Coplanar capacitors with 2 μm gap are formed by a lift-off process of the Au/Ti electrodes deposited by e-beam evaporation on the surface of heterostructures. Microwave properties of the capacitors are measured at room temperature in the range of 1–50 GHz as a function of electric field. The measured tunability of the capacitors is in the range of 10–20%. The loss in capacitors is caused mainly by the Si substrate with maximum value around the dielectric relaxation frequency 6 GHz. The intrinsic loss tangent (0.08 at 50 GHz) of capacitors is calculated from equivalent circuit model by removing the substrate loss.

SrTiO 3 thin film varactors on Si for microwave applications

Dielectric properties of SrTiO 3 (STO) thin films in Au/Pt/STO/Pt/TiO 2 /SiO 2 /Si structure are studied at microwave frequencies. STO films with thickness of 300 nm were grown by laser ablation at different temperatures on templates of Pt/TiO 2 /SiO 2 on Si with resistivity of 5 kQ-cm. XRD measurements reveal highly textured (111)STO films with small amount of (100) and (110) domains. The full with at half maximum of the rocking curve of (11 1)STO domains is about 2 degrees. Circular Au/Pt top electrodes with diameters in the range of 10–60 μ m were formed by e-beam evaporation and lift-off process. The on chip microwave impedance of devices was measured in the frequency range of 45 MHz-45 GHz as a function of DC electric field in the range of 0–800 kV/cm. The loss tangent is less than 0.06 in the whole frequency range. Application of DC bias results in resonant absorption of microwave power at frequencies of 1.3, 3.0, 4.7, 6.4, and 9.7 GHz. The relative change of the dielectric permittivity under 800 kV/cm DC field is more than 20% in the whole frequency range. The results indicate that the varactors based on these STO films are suitable for tunable microwave applications.

Correlation between microstructure, DC resistivity and magnetoresistance of SrRuO 3 films

We have investigated the correlation between microstructure, DC resistivity and magnetoresistance of SrRuO 3 thin films. The films were epitaxially grown by pulsed laser deposition on (001) SrTiO 3 substrates in a temperature range of 690–810°C. According to x-ray measurements, the structure of all films is a mixture of highly oriented domains of strained orthorhombic phases ( ortho-I and ortho-II ) with different lattice parameters. Films deposited at 780°C show a minimum resistivity (270 μΩcm at 300 K) and a maximum magnetoresistance (8% at 5 K ). These films consist mainly of ortho-I phase (a=0.393 nm). Films deposited at 690°C (predominantly ortho-II ) have the highest resistivity (up to 1700 μΩcm at 300 K) and lowest magnetoresistance (3% at 5K).