The first Ruddlesden-Popper with out-of-plane ferroelectricity, a candidate millimeter-wave tunable dielectric

While paraelectric (Ba,Sr)TiO3 films were once used as tunable dielectrics in radio frequency (RF) circuits, dielectric loss above 10 GHz renders (Ba,Sr)TiO3 incompatible with the high frequency future of RF electronics. The related Ruddlesden-Popper titanates—(ATiO3)nAO with A = (Ba,Sr)—have demonstrated low loss up to 100 GHz, but these experiments have used interdigitated capacitors leveraging in-plane dielectric tunability, rather than commercially viable metal-insulator-metal (MIM) capacitors requiring out-of-plane dielectric tunability. To achieve out-of-plane tunability in a Ruddlesden-Popper film, first-principles calculations indicate the concentration of barium and the series member, n, should both be maximized, but synthesizing such films is extremely challenging. Here, we refine existing synthesis techniques to grow the highest n Ruddleden-Popper ever reported (n = 20), containing the highest concentration of barium ever accomplished in a Ruddlesden-Popper (A = Ba0.6Sr0.4). With a firm grasp on the synthesis, we have demonstrated epitaxially strained heterostructures of (ATiO3)nAO dielectric layers with metallic SrRuO3 electrodes. Measurements confirm that such Ruddlesden-Popper films are, in fact, ferroelectric and that the dielectric constant is highly tunable at room temperature. To assess their relevance to the future of tunable dielectrics for GHz electronics, it remains to evaluate the dielectric loss of these new phases at frequencies greater than 10 GHz.