Zinc oxide thin film transistors using MgO-Bi1.5Zn1.0Nb1.5O7 composite gate insulator on glass substrate

We report on high mobility ZnO thin film transistors (TFTs) (< 5 V), utilizing a room temperature grown MgO-Bi_1.5Zn_1.0Nb_1.5O₇ (BZN) composite gate insulator on a glass substrate. 30 mol% MgO added BZN composite gate insulators exhibited greatly enhanced leakage current characteristics (~< 2 × 10^− 8 A/cm² at 0.3 MV/cm) due to the high breakdown strength of MgO, while retaining an appropriate high-k dielectric constant of 32. The ZnO-TFTs with MgO-BZN composite gate insulators showed a high field-effect mobility of 37.2 cm²/Vs, a reasonable on-off ratio of 1.54 × 10⁵, a subthreshold swing of 460 mV/dec, and a low threshold voltage of 1.7 V.     

Influences of post-annealing on structural and electrical properties of Bi1.5Zn1.0Nb1.5O7 thin films prepared by pulsed laser deposition

Bi_1.5Zn_1.0Nb_1.5O₇ (BZN) thin films were prepared on Pt/TiO₂/SiO₂/Si(100) substrates at 650 °C under an oxygen pressure of 10 Pa by using pulsed laser deposition process. The crystallinity, microstructure and electrical properties of BZN thin films were investigated to verify the influences of post-annealing thermal process on them. The X-ray diffractometer (XRD) results indicate that all Bi_1.5Zn_1.0Nb_1.5O₇ thin films without post-annealing process or with post-annealing in situ vacuum chamber and in oxygen ambient exhibit a cubic pyrochlore structure. The improved crystallinity of BZN thin films through post-annealing was confirmed by XRD and scanning electron microscope (SEM) analysis. Dielectric constant and loss tangent of the as-deposited BZN thin films are 160 and 0.002 at 10 kHz, respectively. After annealing, dielectric properties of thin films are significantly improved. Dielectric constant and loss tangent of the in situ annealed films are 181 and 0.0005 at 10 kHz, respectively. But the films post-annealed in O₂ oven show the largest dielectric constant of 202 and the lowest loss tangent of 0.0002, which may attribute to the increase in grain size and the elimination of oxygen vacancies. Compared with the as-deposited BZN thin films, the post-annealed films also show the larger dielectric tunability and the lower leakage current density.     

Structure and microwave dielectric properties of Bi1.5Zn1.0Nb1.5O7 thin films deposited on alumina substrates by pulsed laser deposition

Bi_1.5Zn_1.0Nb_1.5O₇ (BZN) thin films were deposited on polycrystalline alumina substrates by pulsed laser deposition at different substrate temperatures. The phase structure and surface morphology were characterized using X-ray diffractometer (XRD) and atomic force microscopy. Microwave dielectric properties were performed using split-post dielectric resonator method at spot frequencies of 10, 15 and 19 GHz, respectively. The XRD results indicate that the as-deposited Bi_1.5Zn_1.0Nb_1.5O₇ thin films deposited at 650 °C are amorphous in nature. The dielectric permittivity and loss tangent of the amorphous BZN thin films are 75.5 and 0.013 at 10 GHz, respectively. As the measure frequency increased to 19 GHz, the dielectric permittivity slightly decreases and loss tangent slightly increases. BZN thin films were crystallized after the post-annealing by a rapid thermal annealing in air for 30 min. The crystallized BZN thin films exhibit the excellent dielectric properties and frequency responses. The dielectric permittivity and loss tangent of the crystallized BZN thin films are 154 and 0.038 at 10 GHz, respectively.     

Ferroelectric and dielectric multilayer heterostructures based on KTa0.65Nb0.35O3 and Bi1.5-xZn0.92-yNb1.5O6.92-1.5x-y grown by pulsed laser deposition and chemical solution deposition for high frequency tunable devices

Epitaxial growth of Bi_1.5-xZn_0.92-yNb_1.5O_6.92-1.5x-y (BZN) thin films was achieved on (100)_pc LaAlO₃ substrate by pulsed laser deposition (PLD) and by chemical solution deposition based on Pechini process. Effect of bismuth and zinc deficiency on the BZN thin films obtained by PLD was discussed, in relation with the starting target composition. Dielectric permittivity and bandgap values were determined from electrical and spectroscopic ellipsometry measurements performed on randomly oriented films grown on Pt/Si substrate. BZN thin films obtained by PLD exhibit, at 100 kHz, a dielectric constant of ε_r = 203 and quite low dielectric losses of tanδ = 5 × 10^− 2. Epitaxial ferroelectric − dielectric KTa_0.65Nb_0.35O₃ (KTN) − Bi_1.5-xZn_0.92-yNb_1.5O_6.92-1.5x-y (KTN on BZN and BZN on KTN) bilayers were obtained by PLD on (100)_pc LaAlO₃ with the insertion of a suitable buffer layer of KNbO₃ in the case of KTN on BZN. Such multilayer heterostructures with an epitaxial growth control of each layer are promising candidates for potential integration in microwave devices.     

Enhanced tunable dielectric properties of Ba0.5Sr0.5TiO3/Bi1.5Zn1.0Nb1.5O7 multilayer thin films by a sol-gel process

Ba_0.5Sr_0.5TiO₃(BST)/Bi_1.5Zn_1.0Nb_1.5O₇(BZN) multilayer thin films were prepared on Pt/Ti/SiO₂/Si substrates by a sol-gel method. The structures and morphologies of BST/BZN multilayer thin films were analyzed by X-ray diffraction (XRD) and field-emission scanning electron microscope. The XRD results showed that the perovskite BST and the cubic pyrochlore BZN phases can be observed in the multilayer thin films annealed at 700 °C and 750 °C. The surface of the multilayer thin films annealed at 750 °C was smooth and crack-free. The BST/BZN multilayer thin films annealed at 750 °C exhibited a medium dielectric constant of around 147, a low loss tangent of 0.0034, and a relative tunability of 12% measured with dc bias field of 580 kV/cm at 10 kHz.     

High performance self-aligned top-gate ZnO thin film transistors using sputtered Al2O3 gate dielectric

High performance self-aligned top-gate zinc oxide (ZnO) thin film transistors (TFTs) utilizing high-k Al₂O₃ thin film as gate dielectric are developed in this paper. Good quality Al₂O₃ thin film was deposited by reactive DC magnetron sputtering technique using aluminum target in a mixed argon and oxygen ambient at room temperature. The resulting transistor exhibits a field effect mobility of 27 cm²/V s, a threshold voltage of − 0.5 V, a subthreshold swing of 0.12 V/decade and an on/off current ratio of 9 × 10⁶. The proposed top-gate ZnO TFTs in this paper can act as driving devices in the next generation flat panel displays.     

Large nonlinear optical response of a Bi1.5Zn1.0Nb1.5O7 thin film fabricated by pulsed laser deposition

The Bi_1.5Zn_1.0Nb_1.5O₇ (BZN) thin film has been fabricated on MgO (001) substrate by pulsed laser deposition. The nonlinear optical properties of the BZN film were investigated using Z-scan technique at a wavelength of 532 nm with 25 ps pulse duration. The two-photon absorption coefficient and the nonlinear refractive index of the BZN film were obtained to be 4.2 × 10^− 6 cm/W and 1.6 × 10^− 10 cm²/W respectively, which are comparable with those of some representative nonlinear optical materials. The large and fast response optical nonlinearities indicated that the BZN film is a promising candidate for future photonics devices.     

Displacive disorder in three high-k bismuth oxide pyrochlores

We use time-of-flight neutron powder diffraction to examine static displacive disorder in three different pyrochlore A₂B₂O₆O′ compounds with Bi on the A site. The compounds (Bi_1.5Zn_0.5)(Nb_1.5Zn_0.5)O₆O′ (BZN), (Bi_1.5Zn_0.5)(Ta_1.5Zn_0.5)O₆O′ (BZT), and (Bi_1.5Zn_0.5)(Sb_1.5Zn_0.5)O₆O′ (BZS), are of interest – particularly BZN – for their high dielectric constants in the absence of any phase transition from the cubic high temperature phase. The local structures of the three compounds is characterized by displacive disorder from the ideal pyrochlore positions for both the A and O′ sites, with the precise nature of the disorder being quite similar. However the extent of displacive disorder is different, despite the B–O networks being nearly identical in the three compounds. The reported dielectric constants of the three compounds are related to the extent of local displacement, and BZN, with the largest extent of local atomic displacement of A and O′, is also reported to have the largest dielectric constant at 1 MHz. The dielectric constants are also related to the magnitudes of the thermal parameters of the different ions. The strongest correlation is found to involve the thermal parameter on the B site (Nb, Ta, or Sb).     

The influence of fabrication process on top-gate thin-film transistors

Amorphous indium zinc oxide (a-IZO) thin-film transistors (TFTs) with bottom- and top-gate structures were fabricated at room temperature by direct current (DC) magnetron sputter in this research. High dielectric constant (κ) hafnium oxide (HfO₂) films and a-IZO were deposited for the gate insulator and the semiconducting channel under a mixture of ambient argon and oxygen gas, respectively. The bottom-gate TFTs showed good TFT characteristics, but the top-gate TFTs did not display the same characteristics as the bottom-gate TFTs despite undergoing the same process of sputtering with identical conditions. The electrical characteristics of the top-gate a-IZO TFTs exhibited strong relationships with sputtering power as gate dielectric layer deposition in this study. The ion bombardment and incorporation of sputtering ions damaged the interface between the active layer and the gate insulator in top-gate TFTs. Hence, the sputtering power was reduced to decrease damage while depositing HfO₂ films. When using 50 W DC magnetron sputtering, the top-gate a-IZO TFTs showed the following results: a saturation mobility of 5.62 cm²/V-s; an on/off current ratio of 1 × 10⁵; a sub-threshold swing (SS) of 0.64 V/decade; and a threshold voltage (V_th) of 2.86 V.     

Silver cofirable Bi1.5Zn0.92Nb1.5O6.92 microwave ceramics containing CuO-based dopants

Bi₂O₃–ZnO–Nb₂O₅ system has emerged as a good low sintering (1050 °C) microwave material because it exhibits high dielectric constant and low temperature coefficient of resonance frequency (τ_f). We have lowered the sintering temperature of Bi_1.5Zn_0.92Nb_1.5O_6.92 (BZN) below 900 °C by using 3 wt.% of CuO-based dopants, such as 0.21BaCO₃–0.79CuO (BC) and 0.81MoO₃–0.19CuO (MC). The doped BZN exhibits high microwave dielectric constant at 2.3 GHz (k  120). The interfacial behavior between BZN and silver was investigated by using X-ray diffractometer, scanning electronic microscope, and electronic probe microanalyzer. The extent of silver migration of MC and BC dopants is reduced at least by one order of magnitude as compared with V₂O₅ dopant when the samples was prepared at 900 °C for 4 h. Thus, CuO-based dopants can replace V₂O₅ to lower the sintering temperature of BZN and to be cofired with silver.     

High-performance ZnO thin film transistors with sputtering SiO2/Ta2O5/SiO2 multilayer gate dielectric

A high-performance ZnO thin film transistor （ZnO-TFT） with SiO₂/Ta₂O₅/SiO₂ (STS) multilayer gate insulator is fabricated by sputtering at room temperature. Compared to ZnO-TFTs with sputtering SiO₂ gate insulator, its electrical characteristics are significantly improved, such as the field effect mobility enhanced from 11.2 to 52.4 cm²/V s, threshold voltage decreased from 4.2 to 2 V, and sub-threshold swing improved from 0.61 to 0.28 V/dec. The improvements are attributed to the high gate capacitance (from 50 to 150 nF/cm²) as well as nice surface morphology by using dielectric with high～k Ta₂O₅ sandwiched by SiO₂ layers. The capacitance-voltage characteristic of a metal-insulator-semiconductor capacitor with the structure of Indium Tin Oxide/STS/ZnO/Al was investigated and the trap charges at the interface or bulk is evaluated to be 2.24 × 10¹² cm⁻². From the slope of C⁻² versus gate voltage, the doping density N_D of ZnO is estimated to be 1.49 × 10¹⁶ cm⁻³.     

Structure and dielectric properties of sputtered bismuth magnesium niobate thin films

The Bi_1.5MgNb_1.5O₇ (BMN) thin films were prepared on platinum coated sapphire by rf magnetron sputter deposition. Effects of substrate temperature, sputter pressure and O₂/(O₂ + Ar) mixing ratio on phase structures and dielectric properties of thin films were investigated. The results indicated that sufficiently high substrate temperature and low sputter pressure would facilitate the formation of cubic pyrochlore in BMN thin films. Meanwhile, the appropriate O₂/(O₂ + Ar) mixing ratio of sputter atmosphere was required. The deposited Bi_1.5MgNb_1.5O₇ cubic pyrochlore thin films with (222) oriented texture exhibited large tunability of ~ 50% at a maximum applied bias field of 1.5 MV/cm, with low dielectric loss of ~ 0.007. The temperature and frequency dependent dielectric measurements indicated that no noticeable dielectric dispersion was detected in BMN cubic pyrochlore thin films.     

Annealing effect on dielectric and leakage current characteristics of Mn-doped Ba0.6Sr0.4TiO3 thin films as gate insulators for low voltage ZnO thin film transistor

We report on the dielectric properties and leakage current characteristics of 3 mol% Mn-doped Ba_0.6Sr_0.4TiO₃ (BST) thin films post-annealed up to 600 °C following room temperature deposition. The suitability of 3 mol% Mn-doped BST films as gate insulators for low voltage ZnO thin film transistors (TFTs) is investigated. The dielectric constant of 3 mol% Mn-doped BST films increased from 24 at in-situ deposition up to 260 at an annealing temperature of 600 °C due to increased crystallinity and the formation of perovskite phase. The measured leakage current density of 3 mol% Mn-doped BST films remained on the order of 5 × 10^− 9 to 10^− 8 A/cm² without further reduction as the annealing temperature increased, thereby demonstrating significant improvement in the leakage current characteristics of in-situ grown Mn-doped BST films as compared to that (5 × 10^− 4 A/cm² at 5 V) of pure BST films. All room temperature processed ZnO-TFTs using a 3 mol% Mn-doped BST gate insulator exhibited a field effect mobility of 1.0 cm²/Vs and low voltage device performance of less than 7 V.     

Thickness-ratio-dependent dielectric properties of Bi1.5Zn1.0Nb1.5O7/Ba0.5Sr0.5TiO3 bilayered thin films

Bi_1.5Zn_1.0Nb_1.5O₇ (BZN)/Ba_0.5Sr_0.5TiO₃ (BST) thin films were prepared on Pt/Ti-coated sapphire substrates by radio frequency magnetron sputtering. The specific relationship between the dielectric properties and the thickness ratio of the BZN thickness to the BST thickness was investigated. The presence of BZN films effectively reduced the dielectric loss of the thin films. The thickness-ratio-dependent dielectric constant and dielectric loss behaviors were in good accordance with the simulation results based on the series connection theory. The optimum thickness ratio was determined to be around 0.5, exhibiting a maximum commutation quality factor of about 16,000. The built-in electric field at the region near the BZN–BST interface may be responsible for the asymmetric characteristic of the electric-field-dependent dielectric properties of the BZN/BST thin films.     

Investigations on structural evolution and dielectric characteristics of high performance Bi-based dielectrics

A new system of (Bi_1.5Zn_1−x/3Ti_xNb_1.5−2x/3)O₇ (0 ≤x ≤ 1.5) ceramics have been successfully developed and the dielectric properties have been systematically studied. The results showed the formation of temperature compensation dielectrics over the wide range of Ti concentrations. The incorporation of Ti⁴⁺ into the bismuth zinc niobate ceramics induce the decrease of lattice constant linear while remaining cubic pyrochlore phase. The IR spectra confirm the formation of pyrochlore structure and give information about the distribution of ions between the A and B sites. It has been found that the effect of Ti substitution on dielectric properties of sintered ceramics intensifies with a higher x. The system exhibits the novel high permittivity and low dielectric loss: the permittivity values (?_r) saturate at 160-210, and loss values remain at low values (∼1 × 10⁻⁴). The temperature dependence of permittivity is strongly dependent with the compositions. Dielectric relaxation phenomena have been observed during low temperature. New high frequency and MW materials, differed by Ti content, with temperature compensation and controllable dielectric constant of 150-210 have been developed.     

Role of high-k gate insulators for oxide thin film transistors

In conventional TFTs, SiO₂ or SiN_x have been used as gate insulators. But they could not induce the high on-current due to their low-capacitance. Since they have low-capacitance that originated from low dielectric constant, on-current of TFTs with low-k insulated are limited by low-capacitance. We have investigated high-k materials, such as HfO₂, ZrO₂ and modified structures for the use of gate insulators in oxide thin film transistors. ZrO₂ and HfO₂ are the most attractive materials with their superior properties, such as high breakdown field intensity (~ 15 MV/cm), high dielectric constant (~ 25), and the capability of room-temperature process. Since they have high-capacitance due to high dielectric constant, it can be easily expected to result in high on-current. In this work, we demonstrated the comparison of oxide thin film transistors with HfO₂, ZrO₂ and SiO₂ and the roles of gate insulators are analyzed. In the result, oxide thin film transistors with SiO₂, HfO₂ and ZrO₂ have on-currents of ~100 μA, ~500 μA, and ~3 mA, respectively. Especially oxide thin film transistor with ZrO₂ has larger on-current than oxide thin film transistor with HfO₂. The result means that ZrO₂ is more suitable than HfO₂ for the gate-dielectric material which can be fabricated at room temperature.     

Ni-Al-O diffusion barrier layer for high-κ metal-oxide-semiconductor capacitor

Using amorphous Ni-Al-O (a-Ni-Al-O) thin film as the intermediate layer, poly-crystalline Er₂O₃ thin film was grown on a-Ni-Al-O/Si (p-type) via laser molecular beam epitaxy, forming the Er₂O₃/Ni-Al-O gate stack. It was found that the mean dielectric constant of the Er₂O₃/Ni-Al-O gate stack with an equivalent oxide thickness of 1.5 nm is about 17-23, the interfacial states density is about 3.16 × 10¹² cm⁻² and the stack gate leakage current density is as small as 4.1 × 10^− 6 A/cm². Furthermore, The insertion of the Ni-Al-O thin film between the Er₂O₃ gate dielectric and p-Si substrate prevents the oxygen from being out-diffused, which significantly improved the stability of gate stack, showing that the Er₂O₃/Ni-Al-O gate stack thin film could be used as an ideal gate oxide layer for the future Metal Oxide Semiconductor Field Effect Transistors.     

Color emission and dielectric properties of Eu-doped Gd2O3 gate oxide thin films

High-k Gd₂O₃ used for thin film transistor (TFT) gate insulators has been synthesized via a simple solution process. Phase analysis and capacitive performance reveal that a high dielectric constant of ~ 20 and a low leakage current level of < 10⁻⁸ A/cm² at 1 MV/cm with a good transparency under the visible wavelength region are readily produced by the sol-gel method. Eu³⁺ doping leads to an increased dielectric constant induced by the additional electric dipole transition, which is evidently visualized by the photoluminescence behavior and/or by the defect-controlled thin film microstructures. Thus, the solution-processed (Gd,Eu)₂O₃ film is a viable gate insulator to be considered for the proposed “color emissive” switching devices as well as for the low power-driven TFT devices.     

Characteristics of laser-annealed ZnO thin film transistors

We investigated the effects of laser annealing on ZnO thin film transistors (TFTs). ZnO layers were deposited on a bottom-gate patterned Si substrate by radio-frequency sputtering at room temperature. Laser annealing of the ZnO films reduced the full width at half maximum of the ZnO (002) diffraction peak from 0.49° to 0.1°. It reveals that the crystalline quality is improved by annealing effect. A SiO₂ formed in low temperature was used as the gate dielectric. Unannealed ZnO-TFTs were operated in enhancement mode with a threshold voltage of 21.6 V. They had a field-effect mobility of 0.004 cm²/Vs and an on/off current ratio of 134. Laser annealing of the ZnO-TFTs by 200 laser pulses reduced their threshold voltage to 0.6 V and increased their field-effect mobility to 5.08 cm²/Vs. The increase of mobility is originated from the crystallization enhancement of ZnO films after laser annealing.     

Characteristics of transparent ZnO based thin film transistors with amorphous HfO2 gate insulators and Ga doped ZnO electrodes

Coplanar type transparent thin film transistors (TFTs) have been fabricated on the glass substrates. The devices consist of intrinsic ZnO, Ga doped ZnO (GZO), and amorphous HfO₂ for the semiconductor active channel layer, electrode, and gate insulator, respectively. GZO and HfO₂ layers were prepared by using a pulsed laser deposition (PLD) and intrinsic ZnO layers were fabricated by using an rf-magnetron sputtering. The transparent TFT exhibits n-channel, enhancement mode behavior. The field effect mobility, threshold voltage, and a drain current on-to-off ratio were measured to be 14.7 cm²/Vs, 2 V, and 10⁵, respectively. High optical transmittance (> 85%) in visible region makes ZnO TFTs attractive for transparent electronics.