Ba0.6Sr0.4TiO3 Thin Films Research Articles

Point defect distributions and their electrical effects on (Ba,Sr)TiO3/Pt thin films

Deuterium annealing effects on the electrical properties of (Ba,Sr)TiO3 (BST) thin films were investigated using electrical measurements and x-ray photoelectron spectroscopy (XPS) band bending analysis as a function of Pt surface coverage. In combination with the results from secondary ion mass spectroscopy (SIMS) depth profiling, these results suggest that the majority of deuterium defects incorporated during low temperature D2/N2 anneals is electrically inactive. Therefore, there are quite likely other contributors, in addition to charged interstitial deuterium defects, to the observed leakage current degradation after deuterium/hydrogen forming gas annealing, such as oxygen vacancies formed under reducing D2 annealing conditions and annealing-induced changes in the BST/Pt interface state density. The XPS band bending analysis provides information on the electrostatic potential difference across BST thin films with a Pt bottom electrode. With this boundary condition as input, simulations based on a previously developed equilibrium point defect model and the static (annealing time independent) deuterium depth profiles obtained by SIMS analysis have been performed to predict equilibrium defect distributions across BST thin films.

Low dielectric loss and enhanced tunability of Ba0.6Sr0.4TiO3 based thin films via material compositional design and optimized film processing methods

Low dielectric loss in conjunction with high tunability are simultaneous requirements for tunable device applications which have proved problematic to achieve. In this work, material compositional design and optimized film processing methods were employed to simultaneously lower the dielectric loss and enhance the dielectric tunability of Ba0.6Sr0.4TiO3 (BST) based thin films without compromising the device impedance matching (εr<500) and control voltage (<10 V) requirements. The films compositional design was achieved by Mg doping BST from 3 to 10 mol %. The Mg doped thin films were fabricated via the metalorganic solution deposition technique using carboxylate-alkoxide precursors and postdeposition annealing in an oxygen ambience at 750 °C for 60 min. The films dielectric loss at these doping levels was identical, tan δ∼0.007. In contrast, the films permittivity (339–220), leakage current density (2.55×10−8–2.43×10−9 A/cm2), tunability (21.3%–5.7%), and grain size (75.2–61.55 nm) were observed to decrease with increasing Mg concentration levels from 3 to 10 mol %, respectively. Device quality values of tunability, 40% and 32%, for the 3 and 7 mol % doped BST films, respectively, were achieved by elevating the applied bias from 237 to 474 kV/cm. This device quality tuning is compatible with voltage requirements of current semiconductor based systems. Our results suggest that the low level acceptor doping from 3 to 7 mol %, optimized precursor solution concentration (0.43 M), and oxygenated postdeposition thermal processing were found to work in concert to lower dielectric loss, limit defect density concentration, optimize film microstructure, and eliminate undesirable film/electrode interfacial phases. As a result, high quality BST thin films, with their resultant high breakdown fields, were realized. It is the excellent film quality (i.e., optimized composition and microstructure), which allowed the enhanced tunability at elevated dc bias to be achieved without risk of dielectric breakdown. The enhanced dielectric and insulating properties of the 3–7 mol % Mg doped BST thin films make them excellent candidates for integration into tunable devices.

Lateral Epitaxial Growth of Ba0.6Sr0.4TiO3 Thin Films

We report a novel growth technique for epitaxial thin films by combination of selective heteroepitaxial growth and lateral homoepitaxial growth. Ba0.6Sr0.4TiO3 (BST) thin films were deposited on the substrates having patterned SiOx layers at 450°C using pulsed laser deposition. Post annealing was carried out thereafter for lateral epitaxial growth.The difference in the crystallization temperature of BST thin film on the amorphous masking layers and lattice-matched single crystalline substrate enables selective nucleation and heteroepitaxial growth from the regions of single crystalline substrates during the film deposition. Lateral homoepitaxial growth is expected from the crystallized BST thin film toward the amorphous BST on SiOx during the post annealing process.In this paper, a study on the difference in nucleation and growth behavior of BST thin films on the amorphous masking layers and lattice-matched single crystal substrates is presented.

Synthesis and Characterization of Nanostructured BSTO Thin-Films for Microwave Applications

Nanophase synthesis of ferroelectric thin-films of Ba0.6Sr0.4TiO3 (BSTO) was studied systematically for applications in tunable microwave components. Synthesis of nanostructured BSTO was performed using a pulsed-laser deposition system with real-time in-situ process control. The main research goal was to utilize the pulsed laser deposition parameters to control the grain growth for low microwave loss nanostructured BSTO thin-films on crystalline substrates such as LaAlO3. These parameters include the energy density of the laser pulses, wavelength, oxygen partial pressure, distance between the target and the substrate, and the substrate temperature. The nanostructural characterization was performed using XRD, SEM and AFM. Microwave characterization was done using coplanar waveguide lines to characterize the frequency dependent dielectric properties (ϵr and tan δ). BSTO films were grown at the same measured temperature and energy density but in different oxygen ambient pressures from 19 mTorr through 300 mTorr. Using contact mode AFM, the grain size was found to decrease as the oxygen ambient pressure was reduced from 150 mTorr to 38 mTorr. The growth process changed when the pressure was increased above 150 mTorr. Nanocluster structures rather than nanoparticles were found at 225 mTorr. Average grain sizes less than 100 nm were obtained to oxygen pressures below 75 mTorr. The XRD spectra indicate the highly crystalline nature of the film. Microwave measurements, performed between 9–18 GHz, suggest the nano-structured BSTO thin-films on LaAlO3 (LAO) substrates are highly tunable (up to 25%).

Microwave Performance of Distributed Analog Phase Shifter Using Ferroelectric (Ba,Sr)TiO3 Thin Films

This work presents the design, fabrication and microwave performance of distributed analog phase shifter (DAPS) fabricated on (Ba,Sr)TiO3 (BST) thin films for X-band applications. Ferroelectric BST thin films were deposited on MgO substrates by pulsed laser deposition. The DAPS consists of high impedance coplanar waveguide (CPW) and periodically loaded tunable BST interdigitated capacitors (IDC). In order to reduce the insertion loss of DAPS and to remove the alteration of unloaded CPW properties according to an applied dc bias voltage, BST layer under transmission lines were removed by photolithography and RF-ion milling. The measured results are in good agreement with the simulated results at the frequencies of interest. The measured differential phase shift based on BST thin films was 24° and the insertion loss decreased from −1.1 dB to −0.7 dB with increasing the bias voltage from 0 to 40 V at 10 GHz.

Effects of Post-Oxygen Plasma Treatment on Pt/(Ba,Sr)TiO3/Pt Capacitors at Low Substrate Temperatures

We investigated how oxygen plasma post-treatment improves leakage characteristics of Pt/(Ba,Sr)TiO3 (BST)/Pt capacitors prepared by the radio-frequency (RF) cosputtering technique. Experimental results indicate that oxygen plasma treatment can effectively passivate the oxygen vacancies of (Ba,Sr)TiO3 (BST) films, thus decreasing the electric conduction paths of leakage currents. By this low-temperature (250°C), and short-duration (∼5 min) process, the leakage current is reduced by as many as two orders of magnitude. The reliability characteristics of time-dependent dielectric breakdown (TDDB) are improved as well. However, the usage of oxygen plasma treatment is not unrestricted. Plasma treatment for an extended period (more than 10 min) degrades both the leakage and reliability characteristics, due to plasma damage. Excellent electrical characteristics, including low leakage current (1.5 ×10-8 A/cm2) under 0.1 MV/cm, high dielectric constant (288), and a lifetime longer than 10 years under 2 MV/cm can be achieved. Therefore, the proposed technique for as-deposited BST films is a highly promising means of improving the electrical characteristics of BST thin films.

Leakage Current and Dielectric Properties of Ba0.5Sr0.5TiO3 Films Deposited by RF Sputtering at Low Substrate Temperature

AbstractMost studies of Ba0.5Sr0.5TiO3 (BST) thin film deposition have focused on chemical vapor deposition or spin-on techniques. Both these techniques require high substrate temperature (greater than 600 °C), either during the deposition or during an anneal after deposition. A few groups have reported on sputtered films, but most of these studies also used high-temperature processes. While such temperatures are compatible with poly-Si plug DRAM and related technologies, they are far above the limits for technologies that require the deposition of non-refractory metals before the deposition of the ceramic film. For example, the use of Al metalization before the deposition of BST would limit the BST processing temperature to about 450 °C. A process compatible with such a temperature limit is reported. Such a process makes fabrication of high quality BST thin films difficult, primarily due to the need for oxidation and grain growth in the ceramic. The leakage current and dielectric properties of BST films deposited in such a process are reported and are shown to be sufficient for practical device applications.

Enhanced Dielectric Properties Of Compositionally Modified BST Based Thin Films For Voltage Tunable Microwave Devices

ABSTRACTIn this work, material compositional design and optimized film processing methods, were employed to simultaneously lower the dielectric loss and enhance the dielectric tunability of Ba0.6Sr0.4TiO3 (BST) based thin films without compromising the device impedance matching (εr<500) and device control voltage (<10 V) requirements. The films compositional design was achieved by Mg doping BST from 3 to 10 mol % via the metalorganic solution deposition (MOSD) technique and post-deposition annealing in an oxygen ambience. The films dielectric loss at these doping levels was identical, tanδ ∼0.007 and the permittivity values ranged from 339 to 220. Device quality values of tunability, 40 and 32 %, for the 3 and 7 mol% doped BST films, respectively, were achieved by elevating the applied bias to 474 kV/cm. This device quality tuning is compatible with voltage requirements of current semiconductor based systems. The results suggest that the low level acceptor doping from 3 to 7 mol%, optimized precursor solution concentration (0.43 M), and oxygenated post-deposition thermal processing were found to work in concert to lower dielectric loss, limit defect density concentration, optimize film microstructure, and eliminate undesirable film/electrode interfacial phases. The enhanced dielectric and insulating properties of the 3–7 mol% Mg doped BST thin films make them excellent candidates for integration into tunable devices.

MgO-mixed Ba0.6Sr0.4TiO3 bulk ceramics and thin films for tunable microwave applications

The dielectric properties of Ba0.6Sr0.4TiO3 (BST) and 1.0, 20.0, 40.0, and 60.0 wt % MgO-mixed BST bulk ceramics and thin films were investigated for tunable microwave applications. The effects of MgO mixing with BST on BST phase transitions were examined. It is observed that Mg substitution into BST causes a shift in the cubic-tetragonal BST phase transition peak to a lower temperature. Mg-substituted BST and MgO-mixed phases exhibit depressed and broadened BST phase transition peaks, resulting in decreased dielectric constants and dielectric losses at room temperature. BST and 1.0, 20.0, 40.0, and 60.0 wt % MgO-mixed BST thin films (∼0.3 μm thick) were deposited on (100) MgO single-crystal substrates at 750°C in an oxygen ambient pressure of 200 mTorr by pulsed laser deposition. The MgO–BST composite thin films showed a relatively high dielectric Q (=1/tan δ) compared to the pure BST and Mg-substituted BST thin films while retaining useful dielectric tuning.

Dependence of luminescence efficiency on dopant concentration and sintering temperature in the erbium-doped Ba0.7Sr0.3TiO3 thin films

We found the dependence of luminescence efficiency on Er3+ concentration and sintering temperature in the Er-doped Ba0.7Sr0.3TiO3 (BST) thin films is governed by crystallinity and ion–ion interaction. X-ray diffraction and Raman studies of the sol-gel prepared samples show that the BST polycrystalline phase occurred when the sintering temperature reaches 700 °C, whereas, it becomes worse for temperature above 700 °C resulting from phase separation and the Er3+ concentration exceeding 3 mol % due to charge compensation mechanism. The observed green emission reaches maximum at sintering temperature 700 °C and 3 mol % Er3+ ions concentration. We also showed the Er dopant does not affect the dielectric property of BST thin films in C–V measurement and the Ba0.7Sr0.3TiO3 films doped with Er3+ ions may have potential use for electroluminescence devices.

Strain-relieved Ba0.6Sr0.4TiO3 thin films for tunable microwave applications

This article discusses tunable dielectric thin films, with particularly emphasis on strain-relieved and defect-reduced tunable dielectric thin films that significantly reduce dielectric loss at microwave frequencies. Ba0.6Sr0.4TiO3 (BST) thin films (∼0.3 μm thick) were deposited onto (100) MgO single crystalline substrates by pulsed laser deposition at 750 °C and 200 mTorr O2 with a nominal 600-Å-thick BST buffer layer. These films were observed to be strain relieved and show better dielectric properties by exhibiting a significantly high dielectric Q(=1/tan δ>100) while retaining a useful dielectric tuning (=[C(0)–C(23 V/μm)]/C(0)>20%, where C is the film capacitance) at 8 GHz compared to strained BST thin films. The BST buffer layer could be composed of any porous BST phase with randomly oriented grains between a nearly amorphous phase and a fully crystalline phase. A further increase in the dielectric Q was observed in strain-relieved BST thin films mixed with MgO and strain-relieved BST thin films doped with W.

A method to characterize the dielectric and interfacial properties of metal–insulator-semiconductor structures by microwave measurement

We have developed a method to investigate the dielectric and interfacial properties of gate dielectric thin films by microwave measurement. Ba0.5Sr0.5TiO3 (BST) thin films were deposited on 10 Ω cm (normal) and 10 k Ω cm [high-resistivity, (HR)] silicon substrates at the same time by rf magnetron sputtering. For the BST/HR-silicon, coplanar waveguides (CPW) were fabricated and measured at microwave frequencies with thru-reflect-line calibration while capacitance (C–V) measurements were carried out for BST/normal silicon. From the phase change of CPW transmission line and the maximum capacitance in C–V measurement, the dielectric constants of both the BST thin film and interface layer can be determined. Furthermore, the behaviors of insertion loss versus bias voltage were investigated. The results indicate that our method can provide useful information to study the dielectric and interfacial properties of metal–insulator–semiconductor structures.

Electrical and dielectric behavior of MgO doped Ba 0.7Sr0.3TiO 3 thin films on Al2 O3 substrate

In this letter, we present the results of the fabrication and characterization of 5 mol % MgO doped Ba0.7Sr0.3TiO3 (BST) films grown on Pt/TiN/SiO2 coated on Al2O3 substrates using the rf magnetron sputtering technique. The dielectric and electrical properties of Ba0.7Sr0.3TiO3 thin film were found to improve obviously by means of MgO doping. The leakage current density of BST thin film decreased about 1 order of magnitude on MgO doping, while BST film with MgO doping had a higher dielectric constant than that without MgO doping. The dielectric constant of the films increased with increasing annealing temperature due to the consistent increase in grain size and crystallinity. The 750 °C annealed, 100 nm thick film indicated a high dielectric constant of 440 at 100 kHz and the lattice constant of 3.986 Å. The improvement of the electrical properties of BST films was associated with the reduced oxygen vacancies due to improved oxygenation of BST films in the presence of MgO. The MgO doped BST films exhibited a high tunability of 25% and dc resistivity of 6×1010 Ω cm at an applied electric field of 200 kV/cm. The time-dependent dielectric breakdown studies indicated that the films had a longer lifetime of over 10 yrs on operation at the electric field of 0.4 MV/cm which is better than undoped BST film.

Strain induced (Ba,Sr)TiO3 thin films for Metal-Ferroelectric-Insulator-Semiconductor memory devices

Strain induced Metal-Ferroelectric-Insulator-Semiconductor (MFIS) has been proposed for non-volatile ferroelectric random access memory (NV-FRAM). This MFIS structure employs (Ba,Sr)TiO3 (BST) thin films as a ferroelectric layer. These BST thin films were at paraelectric composition and were transformed to have a tetragonality due to the strain induced into the BST films. X-ray diffraction study on the BST layer revealed that the in-plane and plane normal lattice parameters of BST films were different. This suggests that the strain induced into the BST thin films distorts the BST lattices. In order to obtain the epitaxial nature of the BST thin films as well as to prevent charge injection from silicon to the BST film, yttria stabilized zirconia (YSZ) thin films was used as a buffer layer. The MFIS structures with the strained BST thin films exhibited memory window, i.e., non-volatility of 0.6 V in C-V measurements at the maximum bias voltage of 5 V. We have reported the ferroelectric and structural properties in the strain-induced MFIS structure.

Epitaxial Ba0.5Sr0.5TiO3 thin films for microwave phase shifters

A tunable phase shifter was fabricated with epitaxial Ba0.5Sr0.5TiO3 (BST) thin film and gold coplanar waveguide. BST thin film of the thickness ∼0.5 μm was deposited by laser ablation on the MgO(OOl) single crystalline substrate. Gold electrode of the thickness ∼2 μm was prepared by the sequence of thermal evaporation, electroplating, and wet etching. Epitaxial quality of the BST thin film was confirmed by X-ray diffraction. The microwave performance of phase shifter was measured at room temperature in the frequency range of 8–12 GHz, and with applied bias voltage of up to 30 V. Effect of Mn dopant in the epitaxial films was also considered.

Contribution of dielectric and metallic losses in RF/microwave tunable varactors using (Ba,Sr)TiO3 thin films

Recently, there has been significant interest in use of (Ba,Sr)TiO3 (BST) thin films for tunable high frequency (RF and microwave) components. In a previous work we have shown that BST thin films grown by metalorganic chemical vapor deposition (MOCVD) exhibit films very low losses (as low as 0.003–0.004) and tunabilities over 50% at low operation voltages. In order to integrate BST thin films in tunable devices, the objectives of this work are : (i) study the effect of bottom and top electrode on the performance of thin film based capacitors, (ii) correlate low frequency (10 kHz) and high frequency (45 MHz - 1 GHz) measurements, (iii) separate the contribution of dielectric losses and metallic losses at both low and high frequency and finally (iv) show the potential usefulness of series capacitors structures.

Distributed phase shifters using (Ba,Sr)TiO3 thin films on sapphire and glass substrates

In this paper, we present distributed phase shifter results employing voltage tunable (Ba,Sr)TiO3 (BST) thin film capacitors deposited by RF magnetron sputtering. Both parallel-plate and interdigital structures are investigated for different application requirements. The parallel-plate capacitor based circuit on glass (Vycor) substrate has a continuous 0–180? phase shift at 30 GHz with an insertion loss of 4dB and return loss better than 12 dB. A similar K-band phase shifter on a sapphire substrate was also demonstrated with a 0–270? differential phase shift and an insertion loss of 6.1dB at 20 GHz. The circuit using interdigital BST capacitors on sapphire substrate shows a continuous 0–360? phase shift at 8.2 GHz with an insertion loss of only 4.9 dB. These results show the feasibility of using BST thin film technology in microwave and millimeter wave phase shifter applications.

High nonlinearity of Ba0.6Sr0.4TiO3 films heteroepitaxially grown on MgO substrates

We have heteroepitaxially deposited Ba0.6Sr0.4TiO3 (BST) thin films on (001)-oriented MgO substrates using pulsed-laser deposition. By optimizing the deposition temperature and adjusting the film thickness, we have successfully increased the dielectric nonlinearity and decreased the dielectric loss of BST films. BST thin films grown at 750 °C with a thickness of 1.1 μm showed a dielectric constant tunability of greater than 65% and a tunability/loss of 43 at a surface electric field of 80 kV/cm at room temperature. X-ray diffraction and transmission electron microscopy analyses indicated that the tunability and dielectric loss were closely related to the crystallinity of the BST films.

ChemInform Abstract: Recrystallization of Oxygen Ion Implanted Ba0.7Sr0.3TiO3 Thin Films.

The crystallization behavior of chemical-solution-deposited and amorphous Ba0.7Sr0.3TiO3 (BST) thin films was analyzed with respect to the evolution of the structural and dielectric properties of the films as a function of the annealing temperature. The amorphous films were produced by oxygen ion implantation into crystalline BST thin films. In the amorphous thin films, the crystallization to the perovskite phase occurred at T 5 550°C, whereas the as-deposited CSD films showed the first crystalline XRD-reflex only after annealing at T 5 650°C. Here a carbon-rich intermediate phase delayed the crystallization process to higher temperatures.

Microstructure related influence on the electrical properties of pulsed laser ablated (Ba, Sr)TiO3 thin films

The microstructural dependence of electrical properties of (Ba, Sr)TiO3(BST) thin films were studied from the viewpoint of dc and ac electrical properties. The films were grown using a pulsed laser deposition technique in a temperature range of 300 to 600 °C, inducing changes in grain size, structure, and morphology. Consequently, two different types of films were realized, of which type I, was polycrystalline, multigrained, while type II was [100] oriented possessing a densely packed fibrous microstructure. Leakage current measurements were done at elevated temperatures to provide evidence of the conduction mechanism present in these films. The results revealed a contribution from both electronic and ionic conduction. In the case of type I films, two trapping levels were identified with energies around 0.5 and 2.73 eV, which possibly originate from oxygen vacancies VÖ and Ti3+ centers, respectively. These levels act as shallow and deep traps and are reflected in the current–voltage characteristics of the BST thin films. The activation energy associated with oxygen vacancy motion in this case was obtained as 1.28 eV. On the contrary, type II films showed no evidence of deep trap energy levels, while the identified activation energy associated with shallow traps was obtained as 0.38 eV. The activation energy obtained for oxygen vacancy motion in type II films was around 1.02 eV. The dc measurement results were further elucidated through ac impedance analysis, which revealed a grain boundary dominated response in type I in comparison to type II films where grain response is highlighted. A comparison of the mean relaxation time of the two films revealed three orders of magnitude higher relaxation time in the case of type I films. Due to smaller grain size in type I films the grains were considered to be completely depleted giving rise to only grain boundary response for the bulk of the film. The activation energy obtained from conductivity plots agree very well with that of dc measurements giving values 1.3 and 1.07 eV for type I and type II films, respectively. Since oxygen vacancy transport have been identified as the origin of resistance degradation in BST thin films, type I films with their higher value of activation energy for oxygen ion mobility explains the improvement in breakdown characteristics under constant high dc field stress. The role of microstructure in controlling the rate of degradation is found useful in this instance to enhance the film properties under high electric field stresses.