Ruguan Li

Dielectric properties and tunability of cubic pyrochlore Bi1.5MgNb1.5O7 thin films

The Bi1.5MgNb1.5O7 thin films with cubic pyrochlore structure were prepared onto Pt-coated sapphire substrates by rf magnetron sputtering deposition from a stoichiometric target. Dielectric measurements indicated that the Bi1.5MgNb1.5O7 thin films exhibited low dielectric loss of ∼0.0018–0.004, medium dielectric constant of ∼86, and superior tunable dielectric properties at room temperature. A bias field of 1.6 MV/cm resulted in the maximum voltage tunability of 39%. A brief discussion is given on the enhanced tunability compared to Bi1.5ZnNb1.5O7 thin films. The low loss and superior tunability make Bi1.5MgNb1.5O7 thin films promising for potential tunable capacitor applications.

Dielectric properties of Ba0.5Sr0.5TiO3/SiN bilayered thin films grown on Pt-coated sapphire substrates

The Ba0.5Sr0.5TiO3(BST)/SiN bilayered thin films with a SiN layer serving as a buffer layer between the top electrode and the BST layer have been prepared onto Pt-coated c-plane sapphire substrates. The dielectric measurements show that the loss tangent has been significantly lowered. The dielectric properties of the BST/SiN bilayered thin films are strongly dependent on the SiN thickness. The BST/SiN bilayered thin films at a SiN/BST thickness ratio of 0.2 give the largest figure of merit of 50.1. The thickness effect was discussed as well with a series connection model of multilayered capacitors, and the favorable simulation was obtained.

Tunable capacitors employing BZN/BST thin films for RF applications

Tunable parallel-plate capacitors employing Bi1.5Zn1.0Nb1.5O7/Ba0.5Sr0.5TiO3 (BZN/BST) thin films for RF applications are reported. The intermediate frequency measurements indicate that the BZN/BST-based varactors demonstrate large tunability of 39% at 40 V and high device quality factor of 300 at 1 MHz. The devices maintain quite low leakage current density even under a high applied bias. The quality factor analysis shows that the device quality factor is highly dependent on conductor loss of electrodes at frequencies above 1 MHz. The phase shifter employing BZN/BST-based varactors exhibits lower insertion loss than does employing semiconductor diodes at a designed frequency of 445 MHz, demonstrating the potential of tunable capacitors employing BZN/BST thin films for RF applications.

Enhanced leakage current performance and conduction mechanisms of Bi1.5Zn1.0Nb1.5O7/Ba0.5Sr0.5TiO3 bilayered thin films

The Bi1.5Zn1.0Nb1.5O7 (BZN)/Ba0.5Sr0.5TiO3 (BST) bilayered films and BST thin films were prepared via radio frequency (RF) magnetron sputtering, and the leakage current behaviors of the films were investigated. Lower leakage currents of BZN/BST bilayered films were achieved compared with that of BST thin films, especially in the high field region. The potential barrier height of top contact is increased by insertion of a BZN layer in between BST and top Pt-electrodes, which might be responsible for the current reduction of BZN/BST under negative biases (positive voltage applied to bottom electrodes). For the case of positive biases, BZN/BST showed a significant increase in the threshold electric field of Poole-Frenkel emission compared with that of BST, resulting in a current decrease in BZN/BST than that in BST. For the BZN/BST bilayered films at 300 K, since the Schottky emission, Fowler-Nordheim tunneling and Poole-Frenkel emission occur in an extremely high electric field, the leakage current is gover...

Structure and Dielectric Properties of Bi1.5MgNb1.5O7 Thin Films Sputtered on Pt-Coated Sapphire Substrates

The cubic pyrochlore Bi 1.5 MgNb 1.5 O 7 thin films were prepared onto Pt-coated sapphire substrates by rf magnetron sputtering from a stoichiometric target. The phase evolution, morphologies, dielectric properties and voltage tunable properties of thin films with sputtering parameters was investigated. The structure and dielectric properties were dependent on substrate temperature, overall sputter gas pressure and oxygen content. The Bi 1.5 MgNb 1.5 O 7 thin films were found to be electric field tunable and the maximum voltage tunability of 29.2% at bias field 1.2 MV/cm was obtained.

Study of dielectric loss mechanisms in Bi1.5MgNb1.5O7 thin film varactors

The dielectric loss of parallel-plate Pt/Bi1.5MgNb1.5O7 (BMN)/Pt thin film varactors were investigated. The effects of four extrinsic loss mechanisms were discussed. The results showed that the loss mechanism of conduction electrons, local polar regions, and universal relaxation contributed little to the BMN thin film varactors. However, the losses owing to charged defects were found to be the mainly loss mechanism at low frequency (below 1 MHz). The experimental data were in good agreement with the ω1/3 power law. We studied the effect of charged defects mechanism using BMN thin films annealing in different air conditions. It is clearly seen that the charged defects were associated with oxygen vacancies. In the frequency range 100 MHz-6 GHz, the losses of BMN thin film varactors did no fit the charged defects mechanism very well, which were due to the additional losses induced by electrodes, parasitic capacitances, and test equipments. The study of extrinsic loss mechanisms provided useful information fo...

Parallel Plate Varactors Employing Bi1.5Zn1.0Nb1.5O7/Ba0.5Sr0.5TiO3 Bilayered Thin Films

Parallel plate capacitors based on Bi1.5Zn1.0Nb1.5O7 (BZN) / Ba0.5Sr0.5TiO3 (BST) bilayered thin films were fabricated on sapphire substrates. The BZN/BST bilayered thin films were deposited by radio frequency magnetron sputtering, from two individual Bi1.5Zn1.0Nb1.5O7 and Ba0.5Sr0.5TiO3 stoichiometric ceramic targets. Dielectric measurements indicated that the capacitors exhibited device quality-factor of up to ∼ 200, with a relative tunability of ∼ 40% at a bias field of 1.6 MV/cm.The figure of merit was about 80. Compared with the parallel plate capacitors with single BST thin film configuration, the leakage current of the BZN/BST capacitors was observed to be much lower. It was also found that the capacitance was frequency independent in the range from 100 kHz to 100 MHz, while the device quality-factor dropped down as frequency increasing. The decrease of device quality-factor accompanied with frequency independent capacitance could be attributed to the conductor loss of electrodes.