Libin Gao

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...

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...