Ziqing Duan

Effects of Mg on the electrical characteristics and thermal stability of MgxZn1−xO thin film transistors

The effects of the Mg composition (x=0, 0.06, and 0.10) on the electrical characteristics and thermal stability of MgxZn1−xO thin film transistors (TFTs) are investigated. The Mg0.06Zn0.94O TFT shows the smallest subthreshold slope and highest field effect mobility. The O1s spectra of x-ray photoelectron spectroscopy measurements indicate that the oxygen vacancies are reduced in Mg0.06Zn0.94O relative to a pure ZnO channel device. Mg0.06Zn0.94O TFTs also show higher thermal stability compared to the pure ZnO TFTs, which is mainly attributed to the suppression of oxygen vacancies in the channel.

ZnO thin film transistor immunosensor with high sensitivity and selectivity

A zinc oxide thin film transistor-based immunosensor (ZnO-bioTFT) is presented. The back-gate TFT has an on-off ratio of 108 and a threshold voltage of 4.25 V. The ZnO channel surface is biofunctionalized with primary monoclonal antibodies that selectively bind with epidermal growth factor receptor (EGFR). Detection of the antibody-antigen reaction is achieved through channel carrier modulation via pseudo double-gating field effect caused by the biochemical reaction. The sensitivity of 10 fM detection of pure EGFR proteins is achieved. The ZnO-bioTFT immunosensor also enables selectively detecting 10 fM of EGFR in a 5 mg/ml goat serum solution containing various other proteins.

Reduction of persistent photoconductivity in ZnO thin film transistor-based UV photodetector

We report a ZnO-based thin film transistor UV photodetector with a back gate configuration. The thin-film transistor (TFT) aspect ratio W/L is 150 μm/5 μm and has a current on-off ratio of 1010. The detector shows UV-visible rejection ratio of 104 and cut-off wavelength of 376 nm. The device has low dark current of 5 × 10−14 A. The persistent photoconductivity is suppressed through oxygen plasma treatment of the channel surface which significantly reduces the density of oxygen vacancy confirmed by XPS measurements. The proper gate bias-control further reduces recovery time. The UV-TFT configuration is particularly suitable for making large-area imaging arrays.

A ZnO Nanostructure-Based Quartz Crystal Microbalance Device for Biochemical Sensing

We report a ZnO-nanostructure-based quartz crystal microbalance (nano-QCM) device for biosensing applications. ZnO nanotips are directly grown on the sensing area of a conventional QCM by metalorganic chemical vapor deposition (MOCVD). Scanning electron microscopy (SEM) shows that the ZnO nanotips are dense and uniformly aligned along the normal to the substrate surface. By using superhydrophilic nano-ZnO surface, more than tenfold increase in mass loading sensitivity of the nano-QCM device is achieved over the conventional QCM. The ZnO nanotip arrays on the nano-QCM are functionalized. The selective immobilization and hybridization of DNA oligonucleotide molecules are confirmed by fluorescence microscopy of the nano-QCM sensing areas.

Vertically integrated ZnO-Based 1D1R structure for resistive switching

We report a ZnO-based 1D1R structure, which is formed by a vertical integration of a FeZnO/MgO switching resistor (1R) and an Ag/MgZnO Schottky diode (1D). The multifunctional ZnO and its compounds are grown through MOCVD with in situ doping. For the R element, the current ratio of the high-resistance state (HRS) over the low-resistance state (LRS) at 1 V is 2.4 × 106. The conduction mechanisms of the HRS and LRS are Poole–Frenkel emission and resistive conduction, respectively. The D element shows the forward/reverse current ratio at ±1 V to be 2.4 × 107. This 1D1R structure exhibits high RHRS/RLRS ratio, excellent rectifying characteristics and robust retention.

Improvement of Negative Bias Stress Stability in Mg 0.03 Zn 0.97 O Thin-Film Transistors

A small amount of Mg is introduced into ZnO to form the ternary compound Mg0.03Zn0.97O (MZO), which serves as the channel layer of thin-film transistors (TFTs). The MZO TFT shows smaller subthreshold swing of 0.57 V/decade and much better negative bias stress (NBS) stability, which shifts only by -1.21 V in the threshold voltage over its ZnO TFT counterpart (subthreshold swing of 0.79 V/decade; -3.56 V shifts in the threshold voltage after NBS). The superior stability against NBS is mainly attributed to the reduction of donor-like defects associated with ionized oxygen vacancies in the MZO TFT channel.

Structural, electrical, and piezoelectric properties of ZnO films on SiC-6H substrates

Epitaxial ZnO films were grown on c-plane SiC-6H substrates using metal-organic chemical vapor deposition. X-ray diffraction coupled θ-2θ and ϕ-scans show that the n-type ZnO films have c-axis orientation and in-plane registry with the n-type 6H-SiC substrates. This isotype ZnO∕SiC heterojunction shows rectifying characteristics. Electrical measurements exhibit that the reverse current is in the picoampere (10−12–10−10A) range under the reverse bias of less than 5V, the on-off current ratio is ∼107, and the ideality factor is ∼1.23. The surface acoustic wave characteristics in the structure consisting of a piezoelectric ZnO and a semi-insulating SiC-6H substrate were also studied. The structure shows promise for high frequency and low loss rf applications.