Mi-Hwa Lim

Room temperature fabricated ZnO thin film transistor using high-K Bi1.5Zn1.0Nb1.5O7 gate insulator prepared by sputtering

In this letter, we report on dielectric and leakage current characteristics of room temperature deposited Bi1.5Zn1.0Nb1.5O7 (BZN) films on the basis of crystallographic structure and suitability of BZN gate insulators as key building blocks in the fabrication of low voltage operating ZnO based thin-film transistors (TFTs). The BZN films exhibited a high dielectric constant (∼51) and good leakage current characteristics (∼10−7A∕cm2) at an applied voltage of 5V. All room temperature processed ZnO based TFTs using BZN gate insulator exhibited field effect mobility of 1.13cm2∕Vs and low voltage device performance less than 4V.

Mn-doped Ba0.6Sr0.4TiO3 high-K gate dielectrics for low voltage organic transistor on polymer substrate

In this letter, we report on the role of Mn doping in markedly reducing leakage currents in Ba0.6Sr0.4TiO3 (BST) high-K gate dielectrics utilized in organic thin film transistors (OTFTs) fabricated on plastic substrates. Undoped and 3% Mn-doped BST thin films, deposited by rf magnetron sputtering at room temperature on Pt∕Ti∕SiO2∕Si substrates, exhibited relative dielectric constants of ∼24–28. At an applied electric field of 250kV∕cm, the 3% Mn-doped BST films exhibited leakage current densities below 2×10−8A∕cm2 compared to the much higher value of 5×10−4A∕cm2 characteristic of undoped BST films. Pentacene based OTFTs using 3% Mn-doped BST gate dielectrics exhibited low voltage operation of <10V. This demonstrates the potential use of Mn-doped BST films as high-K gate dielectrics for stable and low operating voltage OTFTs.

Characterization of Ni-Doped BST Thin Films on LSCO Buffer Layers Prepared by Pulsed Laser Deposition

We report on the effect of Ni doping on the tunability and dielectric loss of perovskite Ba 0 . 6 Sr 0 . 4 TiO 3 (BST) thin films with the change of Ni doping concentration. BST thin films were deposited on Pt/Ti/SiO 2 /Si and La 0 . 5 Sr 0 . 5 CoO 3 (LSCO)/Pt/SiO 2 /Si substrates by pulsed laser deposition. At an applied electric field of 143 kV/cm, the tunability values of BST films grown on Pt and LSCO/Pt were 12.6 and 62.8% at 100 kHz, respectively. The Ni doping effects were studied for BST thin films grown on LSCO buffer layer. With 1% Ni-doped BST thin films, results gave a tunability of 54.2%, a loss tangent as low as 0.0183, and a figure of merit of about 30. This work demonstrates a potential use for 1 mol % Ni-doped BST thin films in tunable microwave devices.

High field-effect mobility ZnO thin-film transistors with Mg-doped Ba0.6Sr0.4TiO3 gate insulator on plastic substrates

The authors report on the influence of Mg acceptor doping on the markedly reduced leakage current characteristics (<5×10−8A∕cm2 at 2MV∕cm) of Ba0.6Sr0.4TiO3 thin films. The suitability of room temperature deposited Mg-doped Ba0.6Sr0.4TiO3 films as gate insulators for low-voltage ZnO thin-film transistors (TFTs) (<6V) was investigated. All room temperature processed ZnO-TFTs on plastic substrates exhibited a high field-effect mobility of 16.3cm2∕Vs and a current on/off ratio of 6.4×104. The threshold voltage and subthreshold swing were 2.8V and 400mV/decade, respectively.

Low leakage current—stacked MgO∕Bi1.5Zn1.0Nb1.5O7 gate insulator— for low voltage ZnO thin film transistors

The authors report on the role of MgO capping layers in notably reducing leakage currents and improving mobility in ZnO thin film transistors (TFTs) utilizing compatible high-k Bi1.5Zn1.0Nb1.5O7 (BZN) gate insulators. All room temperature processed ZnO based TFTs with stacked MgO/BZN gate insulator exhibited a much enhanced field effect mobility of 5.4cm2∕Vs with excellent saturation characteristics as compared to that (μFE=1.13cm2∕Vs) of ZnO based TFTs with BZN gate insulator. This work demonstrates the suitability of MgO/BZN stacked gate insulators in the fabrication of low voltage ZnO based TFTs on plastic substrates.

Dielectric Properties of Mn-Doped BST Thin Films for Microwave Application

In this letter, we report on the effect of Mn doping on the tunability and dielectric loss of Ba0.6Sr0.4TiO3 thin films at low frequency (100 kHz) and at microwave frequency (3.5 GHz), respectively. Perovskite Ba0.6Sr0.4TiO3 thin films were deposited on Pt/TiO2/SiO2/Si and MgO single crystal substrates by pulsed laser deposition. At an applied electric field of 150 kV/cm, the BST films (0, 1, 3, 5 mol% Mn doped) grown on Pt/TiO2/SiO2/Si substrate showed the tunability value within the range from 46% to 49%. With 3% Mn doped BST thin films, results gave a tunability of 49% and a loss tangent as low as 0.0158 and the highest figure of merit of about 31. The microwave properties of BST films grown on MgO substrate were measured using interdigital capacitors (IDC) structure at 3.5 GHz. 3 mol% Mn doped BST also showed better tunability (27.4%) compared to that (11.6%) of undoped BST film. It is suggested that 3 mol% Mn doped BST film is effective candidate for high performance tunable device applications.

Ferroelectric Thin-Film-Based Electrically Tunable Bandpass Filters

We report on gold/Ba 0 . 5 Sr 0 . 5 TiO 3 (BST) ferroelectric thin film C-band tunable bandpass filters (BPFs) designed and fabricated on magnesium oxide substrates using three different structures: microstrip, coplanar waveguide (CPW), and conductor backed coplanar waveguide (CBCPW). The two-pole filters were designed for a center frequency of 5.89, 5.88, or 5.69 GHz with bandwidths of 3, 9, or 7%, respectively. The BST-based CPW filter offers a higher sensitivity parameter as well as a lower loss parameter compared to the BST tunable two-pole microstrip filter and CBCPW filter. With an applied voltage of 40 V, the tuning range for the BPF with CPW structure was determined to be 170 MHz.

Design of Tunable Band-pass Filters and Phase Shifters Using BST Thin Film

In this paper, the band-pass filters (BPFs) and phase shifters using BST thin film are presented. Filters are consisted of Microstrip, Coplanar Waveguide (CPW), and conductor backed CPW (CBCPW) structures and phase shifters are consisted of CPW line and CPW with slit structures to improve phase shift effect. From the measurement results, the Microstrip, CPW, and CBCPW bandpass filters show the center frequencies of 6.61 GHz, 6.31 GHz, and 6.21 GHz with the insertion losses of 7.33 dB, 5.83 dB, and 6.83 dB, respectively at the bias voltage of 40 V. The tuning range for the bandpass filters is determined as 3%~8%. From the experiments, the CPW line and the CPW with slit phase shifters show the insertion losses of 1.9 dB and 2.4 dB at the bias voltage of 30 and the phase shifts of 9.8° and 10.6° at 16 ~ 18 GHz, respectively.

Influence of narrow transverse slit in ferroelectric based voltage tunable phase shifter

We report on gold/Ba0.5Sr0.5TiO3 (BST) ferroelectric thin film Ku-band phase shifters designed and fabricated on MgO substrates using two different structures; the coplanar waveguide (CPW) meander line phase shifter and the new phase shifter with narrow transverse slits in the ground plane of the CPW structure. The ability of these devices to provide a variable phase shift is based on the change in dielectric constant of BST with an applied dc bias. The phase shifter with narrow transverse slits showed larger phase shift capability than that of the conventional meander line phase shifter. In the former phase shifter, the phase shift of about 10.6 as a maximum value for the frequency range from 16 GHz to 18 GHz was obtained. In the latter, the phase shift of 9.7 deg at 18 GHz and at an applied voltage of 30 V was measured.

Fabrication and characterization of glass-ceramic+Al/sub 2/O/sub 3/ composition for LTCC(low temperature co-fired ceramic)

The glass+ceramic compositions for LTCC dielectric materials have been investigated. The glass+ceramic mixtures consist of cordierite based crystallizable glass (MgO-Al/sub 2/O/sub 3/-SiO/sub 2/-GeO/sub 2/), Al/sub 2/O/sub 3/ as a filler and zinc borate as a sintering additive. The GAZ series had superior characteristics to GA series. Densification was almost finished in GAZ series (glass+Al/sub 2/O/sub 3/+zinc borate) after sintering at 850/spl deg/C, while in GA series (glass+Al/sub 2/O/sub 3/) it was finished after sintering at 900/spl deg/C. Especially the loss tangent of GAZ series was much lower than that of GA series. GAZl sintered at 900/spl deg/C showed promising microwave properties having the Qf (quality factor/spl times/frequency in GHz) of 5594.9 and a dielectric constant of 5.92 at 8.4 GHz.