Matching conflicting oxidation conditions and strain accommodation in perovskite epitaxial thin-film ferroelectric varactors

Abstract

Perovskite oxide materials of the general chemical formula ABO3 are a rich playground for epitaxial stacks of different functional layers for novel device applications. In the example of a tunable metal–insulator–metal ferroelectric varactor (tunable capacitor) made from the highest conducting perovskite SrMoO3 as an electrode and the tunable dielectric Ba0.5Sr0.5TiO3 (BST), we show how the extremely conflicting oxidation potentials can be conciliated in a fully functional heterostructure. Controlling the growth kinetics by the substrate temperature, oxygen pressure, and oxidation time, the formation of the non-conducting Mo6+ states can be effectively suppressed and the BST cation stoichiometry can be tuned. The cumulative impact of the cation nonstoichiometry, epitaxial strain, and oxygen deficiency in the BST films leads to the expansion of their c-axis lattice parameter via the formation of point defects. The dielectric permittivity of 440, the high tunability of 3.5, and the quality factor of 50 are achieved for the varactors at the frequency of 1 GHz. It turns out that the varactor performance is anti-correlated to the tetragonal lattice distortion of BST, which itself is interrelated to the oxidation conditions. The mechanism of the leakage current through oxygen deficient BST layers of the varactors is analyzed within the combined scenarios of the space-charge limited current and Poole–Frenkel field-assisted emission from traps. The achieved high capacitance per unit area of 0.04 pF/μm2 and moderate leakage currents of 0.025 μA/pF make these varactors suitable for applications in microwave microelectronic devices.

Impact of Strain Engineering on Antiferroelectricity in NaNbO3 Thin Films

“…The origin of the high leakage for untreated films can thus be related to the presence of oxygen vacancies. A possible mechanism leading to the formation of oxygen vacancies during film growth could be related to an intermixing of the Na and Nb cations, which has been observed for similar perovskites …”

“…A possible mechanism leading to the formation of oxygen vacancies during film growth could be related to an intermixing of the Na and Nb cations, which has been observed for similar perovskites. 40 Structural characterization of the samples before and after heat treatment revealed neither a change in the crystal structure nor in the strain state (see the Supporting Information).…”

High performance all-oxide thin film tunable capacitors on Si substrates for agile microwave applications

“…4a) for the ascending and descending positive V b could be related to the relaxation current in the dielectric layer. 20 The larger hysteresis for the ascending and descending negative V b has probably a different origin and is associated with the corresponding difference in the leakage current of the varactors J Leak (V b ) (see Fig. 4b).…”

“…4b). 20 The direct injection of the electrons at the BaSrTiO 3 /SrMoO 3 interface results in a space charge limiting current (SCLC) mechanism for positive V b (negatively biased bottom electrode). However, at the negative V b (negatively biased top electrode), the Pt/BaSrTiO 3 top interface dominates the leakage current of the varactor, which is determined either by the SCLC (increase of the negative V b ) in Fig.…”

“…Here the height of the Pt/BST Schottky barrier is a function of the concentration of oxygen vacancies in the BST in the vicinity of the interface, which depends on V b . 20 As a capacitance of about 1 pF is typically desired in microwave circuits, the areas of ∼ 100 µm 2 are required for the varactors with the obtained dielectric permittivity of 120 of the functional epitaxial BST films on Si substrates. Thus, the corresponding moderate leakage currents below 150 nA are expected for the varactors despite of the relatively high leakage current density of 10 3 A/m 2 at ±10 V.…”