Ferroelectric Na 0.5 K 0.5 NbO 3 Films for Voltage Tunable Microwave Devices
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Ferroelectric Na0.5K0.5NbO3 Films for Voltage Tunable Microwave Devices Choong-Rae Cho 1, Alex Grishin 1, Johanna Andrèasson 2, Ture Lindbäck 2, Saeed Abadei 3, and Spartak Gevorgian 3 1
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Department of Condensed Matter Physics, Royal Institute of Technology, S-100 44 Stockholm, Sweden; Department of Materials and Manufacturing Engineering, Luleå University of Technology, S-971 87 Luleå, Sweden; Department of Microelectronics, Chalmers University of Technology, S-412 96 Göteborg, Sweden.
ABSTRACT Single phase Na0.5K0.5NbO3 (NKN) thin films have been pulsed laser deposited on Al2O3(01 1 2), LaAlO3(001), and MgO(001) single crystal substrates as well as on the SiO2/Si(001) wafers to demonstrate films feasibility for voltage tunable microwave device applications. NKN film texture has been found to be quite different on three different single crystals: highly c-axis oriented on Al2O3, “cube-on-cube” epitaxial quality on LaAlO3, biaxial textures on MgO, while NKN films grown on Si substrate with various thickness of SiO2 buffer layer possess highly c-axis oriented quadrupled structure. NKN film interdigital capacitors fabricated onto single crystal oxide substrates showed tunability of 30-40 % and dissipation factor of 0.01-0.02 at 1 MHz and applied electric field of 100kV/cm. Microwave frequency measurements for NKN/Si varactors yield 13 % tunability and dielectric loss tanδ as low as 0.012 at 40 GHz under 200 kV/cm applied bias.
INTRODUCTION An intensive research effort is undertaken to study ferroelectric thin films for voltage tunable device applications. Recent results on magnetron sputtered Na0.5K0.5NbO3 (NKN) films on LaAlO3 [1] and pulsed laser deposited NKN films on SiO2/Si wafers [2] have demonstrated high tunability and low dielectric loss, tanδ < 0.01, thus attract our attention to NKN as a candidate for microwave applications. It is generally accepted that the microstructure of ferroelectric thin films is closely related to microwave dielectric properties [3] while the demands to fabricate ferroelectric films on not expensive and/or semiconductor substrates, such as Al2O3 and Si, are increasing with the demands from commercial applications and Microwave Monolithic Integrated Circuit (MMIC) technologies. In this paper, we report on crystalline properties of NKN films on various substrates and their dielectric properties both at radio and microwave frequencies.
EXPERIMENTAL PROCEDURES A KrF excimer laser (Lambda Physik-300, λ = 248 nm, pulse duration of 25 ns) was used to ablate stoichiometric Na0.5K0.5NbO3 target at the energy density of 4-5 J/cm2 and DD3.2.1
repetition rate of 15 Hz. Al2O3(01 1 2), LaAlO3(001) and MgO(001) single crystals and Si(001) wafers with various thickness of thermally grown SiO2 buffer layer have been used as substrates. All NKN films have been grown at 650 oC in an ambient oxygen partial pressure around 400 mTorr. The deposition was followed by in-situ annealing at 400 oC in 500 Torr oxygen for 30 min. To measure dielectric properties, interdigital capacitor (IDC) structur
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