Ba<Subscript>0.1</Subscript>Sr<Subscript>0.9</Subscript>TiO<Subscript>3</S ubscript>-BaTi<Subscript>4</Subscript>O<Subscript>9</Subscript> composite thin films with different BaTi<Subscript>4</Subscript>O<Subscript>9</Subscript> concentrations were deposited on (100) LaAlO<Subscript>3</Subscript> (LAO) single crystal substrates via a pulsed laser deposition (PLD) using a combined target configuration of Ba<Subscript>0.1</Subscript>Sr<Subscript>0.9</Subscript>TiO<Subscript>3</S ubscript> and BaTi<Subscript>4</Subscript>O<Subscript>9</Subscript> ceramics. The Ba<Subscript>0.1</Subscript>Sr<Subscript>0.9</Subscript>TiO<Subscript>3</S ubscript>-BaTi<Subscript>4</Subscript>O<Subscript>9</Subscript> thin films showed good crystal growth with c-orientation on (100) LAO substrates, which were characterized by X-ray diffraction (XRD). From scanning electron microscopy (SEM) measurements, we could see that the crystal grains of the Ba<Subscript>0.1</Subscript>Sr<Subscript>0.9</Subscript>TiO<Subscript>3</S ubscript>-BaTi<Subscript>4</Subscript>O<Subscript>9</Subscript> thin films changed from small ordered clusters to big triangle clusters with the increase of BaTi<Subscript>4</Subscript>O<Subscript>9</Subscript> concentration. The microwave dielectric properties of the Ba<Subscript>0.1</Subscript>Sr<Subscript>0.9</Subscript>TiO<Subscript>3</S ubscript>-BaTi<Subscript>4</Subscript>O<Subscript>9</Subscript> thin films under low temperature range from 65 to 120 K were measured at 7.7 Ghz and the results showed that all the thin films were in the paraelectric state. The dielectric constant and loss tangent of the Ba<Subscript>0.1</Subscript>Sr<Subscript>0.9</Subscript>TiO<Subscript>3</S ubscript>-BaTi<Subscript>4</Subscript>O<Subscript>9</Subscript> thin films were modified by the addition of BaTi<Subscript>4</Subscript>O<Subscript>9</Subscript>. Specifically, the loss tangent of the composite thin films was greatly decreased from 0.025 for pure Ba<Subscript>0.1</Subscript>Sr<Subscript>0.9</Subscript>TiO<Subscript>3</S ubscript> to 0.020 for the 4.7% BaTi<Subscript>4</Subscript>O<Subscript>9</Subscript> doped Ba<Subscript>0.1</Subscript>Sr<Subscript>0.9</Subscript>TiO<Subscript>3</S ubscript>, and finally to 0.010 for the 15.9% BaTi<Subscript>4</Subscript>O<Subscript>9</Subscript> doped Ba<Subscript>0.1</Subscript>Sr<Subscript>0.9</Subscript>TiO<Subscript>3</S ubscript> sample. Copyright Springer-Verlag Berlin/Heidelberg 2004
