Namrata Dewan

Growth of amorphous TeOx (2≤x≤3) thin film by radio frequency sputtering

Thin films of Tellurium oxide TeOx over a wide range of x (2 to 3) were prepared by radio frequency diode sputtering at room temperature on corning glass and quartz substrate. The deposited films are amorphous in nature and IR spectroscopy reveals the formation of Te-O bond. X-ray photoelectron spectroscopy shows the variation in the stoichiometry of TeOx film from x=2 to 3 with an increase in oxygen percentage (25 to 100%) in processing sputtering gas composition. Raman spectroscopy depicts the formation of TeO3 trigonal pyramid besides TeO4 disphenoid in the amorphous TeOx film with increase in the value of x. The varying stoichiometry of TeOx thin film (x=2 to 3) was found to influence the optical, electrical, and elastic properties. The optical band gap of film increases from 3.8 to 4.2 eV with increasing x and is attributed to the decrease in density. The elastic constants (C11 and C44) of the deposited films are lower than the corresponding value reported for TeO2 single crystal.

Temperature stable LiNbO3 surface acoustic wave device with diode sputtered amorphous TeO2 over-layer

Amorphous TeO2 thin film, sputtered in the O2+Ar(25%+75%) gas environment using a metallic tellurium target, has been identified as an attractive negative temperature coefficient of delay (TCD) material that can yield a temperature stable device when combined with a surface acoustic wave (SAW) device based on positive TCD material such as LiNbO3. The influence of amorphous TeO2 over-layer on the SAW propagation characteristics (velocity and temperature coefficient of delay) of the SAW filters (36 and 70 MHz) based on 128° rotated Y-cut X-propagating lithium niobate (128° Y‐XLiNbO3) single crystal has been studied. It is found that 0.042λ thick TeO2 over-layer on a prefabricated SAW device operating at 36 MHz centre frequency, reduces the TCD of the device from 76ppm°C−1 to almost zero (∼1.4ppm°C−1) without deteriorating its efficiency and could be considered as a suitable alternative for temperature stable devices in comparison to conventional SiO2 over-layer.

Theoretical studies on a TeO2/ZnO/diamond-layered structure for zero TCD SAW devices

High-frequency surface acoustic wave (SAW) devices based on diamond substrate are useful because of their very high SAW velocity. In the present work, SAW propagation characteristics, such as phase velocity, coupling coefficient and temperature coefficient of delay (TCD) of a TeO2/ZnO/diamond-layered structure, are examined using theoretical calculations. The ZnO/diamond bi-layer structure is found to exhibit a high positive TCD value. A zero TCD device structure is obtained after integration with a TeO2 over layer having a negative TCD value. Introduction of a non-piezoelectric TeO2 over layer on the bi-layer structure (ZnO/diamond) increases the coupling coefficient. A relatively low thickness of TeO2 thin film (~(1.6–3.1) × 10−3λ) is required to achieve temperature-stable SAW devices based on diamond.

Influence of γ-radiation doses on the properties of TeOx: (x=2–3) thin film

The influence of γ-ray doses (10–50 Gy) on the optical and electrical properties of radio-frequency sputtered tellurium dioxide (TeOx) thin film was studied. The composition of the as-deposited TeOx films deposited under 25% oxygen and 100% oxygen in the sputtering gas mixture (Ar+O2) was x=2 and 3, respectively. TeO3 films were found to be highly sensitive to the γ-radiation doses and the value of optical band gap decrease from 4.18 to 3.56 eV with increasing radiation dose from 10 to 50 Gy. Current-voltage characteristics of the films showed an increase in the value of conductivity with increasing radiation doses. Monotonic decrease in the values of dielectric constant for the deposited films with increase in radiation dose was observed. The effect of γ-ray doses on the properties of TeOx film has been correlated with the rearrangement of the bipyramidal structure of amorphous TeOx thin film.

Temperature-Compensated Devices Using Thin

This letter evaluates the applicability of TeO₂ thin films as an attractive alternative over conventional SiO₂ layers in the field of temperature compensation of various devices. It is reported that the processing environment during TeO₂ growth plays an important role in defining its negative temperature coefficient of delay (TCD), and the magnitude of TCD increases continuously from -1.862times10³ ppmmiddotdegC^-1 to -9.883times10³ ppmmiddotdegC^-1 with an increase in the oxygen percentage from 25% to 100% in the processing gas mixture. A very low thickness of 0.008lambda for the TeO₂ over layer (in comparison to the 0.32lambda-thick SiO ₂ layer) is required for achieving a zero-TCD device without degrading its performance

hboxTeO_2

The anomalous elastic properties of TeO_2+x thin films deposited by rf diode sputtering on substrates at room temperature have been studied. The deposited films are amorphous, and IR spectroscopy reveals the formation of Te-O bond. X-ray photoelectron spectroscopy confirms the variation in the stoichiometry of TeO_2+x film from x = 0 to 1 with an increase in the oxygen percentage in processing gas composition. The elastic parameters of the films in comparison to the reported values for TeO_2+x single crystal are found to be low. However, the temperature coefficients of elastic parameters of all deposited films exhibit anomalous behavior showing positive values for TC(C₁₁) in the range (32.0 to 600.0) x 10^-40 C^-1 and TC(C¹¹) = (35.0 to 645.5) x 10^-4degC^-1 against the negative values TC(Cn) = -2.7 x 10^-4degC^-1 and TC(C₁₁) = -0.73 x 10^-4degC^-1 reported for TeO₂ single crystal. The variation in the elastic parameters and their temperature coefficients is correlated with the change in the three-dimensional network of Te-O bonding. The anomalous elastic properties of the TeO_2+x films grown in 100% O₂ are useful for potential application in the design of temperature stable surface acoustic wave devices.

Layer With Negative TCD

Recently, we reported that amorphous TeO2 thin films possess a high negative temperature coefficient of delay (TCD) contrary to its bulk counterpart, and are useful for the temperature stability of LiNbO3 based surface acoustic wave (SAW) devices possessing a positive TCD. In the present work TeO2 thin film deposited under 60% O2 and 40% Ar gas environment, is found to be most suitable as an over layer to yield a temperature stable device at reasonably low thickness. The SAW propagation and TCD characteristics of the TeO2/IDTs/128° Y-X LiNbO3 layered structure have been studied. It is found that ~ 0.01 thick TeO2 over-layer reduces the TCD of the device from 76 ppm °C -1 to almost zero (~ 2 ppm °C -1 ) without deteriorating its efficiency. Material parameters of the deposited TeO2 thin film are also determined. Metallic cap of the packaged device was removed and amorphous TeO2 film of increasing thickness (0.3 - 4.5 µm) was sputtered onto the SAW filters containing IDT electrodes and the SAW propagation and TCD characteristics were measured for each thickness. To protect the aluminium IDT electrodes on the SAW device, TeO2 over layer was deposited intentionally at room temperature. Thickness of the films was measured every time separately using a surface profiler (Dektak IIA) by depositing the films on quartz substrates simultaneously. TeO2 over layer was deposited under different sputtering gas composition (25%, 40% and 60% O2 in the Ar + O2 gas mixture). The typical film deposition parameters are presented in table I. Deposition rate of TeO2 thin film was found to be decreasing from 0.35 µm h -1 to 0.2 µm h -1 with increase in the O2 content in sputtering gas mixture from 25% to 60%.

Anomalous elastic properties of RF-sputtered amorphous TeO 2+x thin film for temperature-stable SAW device applications

The wide range temperature (150 K to 470 K) and frequency (100 Hz to 1 MHz) dependence of the dielectric constant and ac conductivity of rf-sputtered TeO2+y thin films (y = 0 to 1) deposited in metal–insulator–metal configuration is studied. A decrease in the value of the dc conductivity measured at room temperature was observed with increasing y, whereas the ac conductivity and dielectric constant show an increasing trend. The values of the dielectric constant for temperatures more than 340 K were found to depend intensely on the frequency because of the intrinsic behavior of the films. The dielectric distribution data have been analyzed in terms of (i) a mixed conduction model (with a spread of relaxation times) and (ii) a random free energy barrier model (which assumes that the conduction mechanism is hopping, and the dielectric spread is correlated with dc conductivity). The reason for the dielectric spread has been discussed on the basis of these models. It has been observed that the dominant transport mechanism in TeO2+y film is purely hopping in the low temperature region ( 340 K).