ZnO Transistors Research Articles

Design Considerations for ZnO Transistors Made Using Spatial ALD

We have used spatial atomic layer deposition (SALD) to fabricate a variety of thin film transistors using zinc oxide as the semiconductor and aluminum oxide as the insulator. The conductive electrodes are formed either from aluminum-doped zinc oxide, or from thermally evaporated metals. Spatial ALD can be performed at atmospheric pressure, and can grow coatings very quickly. Deposition occurs without a requirement for valve timing, by moving the substrate relative to the precursors flowing from a coating head. The proximity of the substrate to the coating head is controlled via a gas bearing design – allowing SALD to operate with purge times that are equivalent to the precursor exposure times (sub-second) unlike traditional chamber-based systems. In this talk we will discuss examples of thin film electronics with architectures that are enabled by ALD, with a particular focus on spatial ALD. We use the well known conformality of ALD to build self-aligned, sub-micron channel length, vertical transistors. The vertical transistor electrical performance is impressive compared to amorphous silicon TFTs, for example, yet the alignment tolerances inherent in the fabrication process are large.Our second approach to fabricating devices uses selective area deposition, with inkjet-printed inhibitor, to pattern all the active layers. We build full transistors and circuits using one inhibitor ink, the SALD system, and an oxygen plasma to clean the substrate. Performance of the resulting transistors and circuits match photolithographically patterned ones.

Self-assembled nanocrystalline ZnO thin film transistor performance optimization for high speed applications

ZnO nanocrystals grown at relatively low temperatures using various vacuum deposition techniques can yield semiconducting thin films of self-assembled nanocolumns 20-50 nm in diameter. Such films are suitable for the fabrication of high speed and transparent thin film transistors (TFTs). Unlike amorphous TFTs, the performance of ZnO transistors depends both on the crystal quality of nanocrystals and the electrical properties of boundary layers between them. We investigated the use of radio frequency sputtering, atomic layer deposition, and pulsed laser deposition techniques to fabricate self-assembled nanocrystalline thin films and determined the influence of deposition conditions on the performance of transistors. Device design and fabrication parameters were also optimized to demonstrate TFTs with high current density and high speed performance comparable to single crystalline-based transistors.

Aqueous electrolyte-gated ZnO transistors for environmental and biological sensing

An aqueous electrolyte-gated transistor based on a nanostructured ZnO channel is a simple, versatile and low-cost biosensor.

Organic–inorganic hybrid gate dielectric for solution-processed ZnO thin film transistors

The preparation of a hybrid dielectric film was carried out by blending sol–gel-derived sodium beta alumina (SBA) and poly(4-vinylphenol) (PVP) to enhance the capacitance of a gate dielectric film. PVP-SBA was cured at a temperature sufficiently low enough to apply to a plastic substrate, while maintaining good electrical properties and uniformity. Addition of sol–gel-derived SBA improved the film density, resulting in good PVP-SBA thermal stability. The prepared PVP-SBA was used for high-performance aqueous solution-based ZnO transistors at 200 °C.

High‐Performance ZnO Transistors Processed Via an Aqueous Carbon‐Free Metal Oxide Precursor Route at Temperatures Between 80–180 °C

An aqueous and carbon-free metal-oxide precursor route is used in combination with a UV irradiation-assisted low-temperature conversion method to fabricate low-voltage ZnO transistors with electron mobilities exceeding 10 cm(2) /Vs at temperatures <180 °C. Because of its low temperature requirements the method allows processing of high-performance transistors onto temperature sensitive substrates such as plastic.

Hybrid ZnO nanowire networked field-effect transistor with solution-processed InGaZnO film

We examined the effect on the transistor properties of the spatial gaps between nanowires that may randomly exist in the nanonet-structured, ZnO transistor. A hybrid-type, ZnO nanowire-based transistor was fabricated by combining the nanonet-structured ZnO nanowire arrays with a solution-deposited InGaZnO (IGZO) film and its performance was compared with that of the device without the IGZO film. By filling the disconnected carrier paths (gaps) between the arrayed ZnO nanowires with IGZO solution coating, much improved transistor characteristics, such as a narrow threshold voltage distribution and minimized multiple turn-on behavior, were obtained, which highlighted the importance of the gap filling in the nanonet transistors.

High‐Mobility Low‐Voltage ZnO and Li‐Doped ZnO Transistors Based on ZrO2 High‐k Dielectric Grown by Spray Pyrolysis in Ambient Air

Sequential layers of the high-k dielectric ZrO2 and the electron transporting semiconductors ZnO and Li-doped ZnO are deposited onto conductive indium tin oxide electrodes using spray pyrolysis. With these structures, thin-film transistors are fabricated with operating voltages below 6 V and maximum electron mobilities on the order of 85 cm(2) V-1 s(-1).

Low-voltage ZnO thin-film transistors based on Y2O3 and Al2O3 high-k dielectrics deposited by spray pyrolysis in air

We report the application of ambient spray pyrolysis for the deposition of high-k polycrystalline Y2O3 and amorphous Al2O3 dielectrics and their use in low-voltage ZnO thin-film transistors. The films are studied by means of atomic force microscopy, UV-visible absorption spectroscopy, impedance spectroscopy, and field-effect measurements. ZnO transistors based on spray pyrolysed Y2O3 and Al2O3 dielectrics show low leakage currents, and hysteresis-free operation with a maximum electron mobility of 34 cm2/V s and current on/off ratio on the order of 105. This work is a significant step toward high-performance oxide electronics manufactured using simple and scalable processing methods.

Influences of pH and Ligand Type on the Performance of Inorganic Aqueous Precursor-Derived ZnO Thin Film Transistors

The aqueous precursor-derived ZnO semiconductor is a promising alternative to organic semiconductors and amorphous silicon materials in applications requiring transparent thin-film transistors at low temperatures. The pH in the aqueous solution is an important factor in determining the device performance of ZnO-TFTs. Using a basic aqueous solution, the ZnO transistor annealed at 150 °C exhibited a high field-effect mobility (0.42 cm(2) V(-1) s(-1)) and an excellent on/off ratio (10(6)). In contrast, the ZnO layer annealed at 150 °C prepared from an acidic solution was inactive. Chemical and structural analyses confirmed that the variation of the device characteristics originates from the existing state difference of Zn in solution. The hydroxyo ligand is stable in basic conditions, which involves a lower energy pathway for the solution-to-solid conversion, whereas the hydrated zinc cation undergoes more complex reactions that occur at a higher temperature. Our results suggest that the pH and ligand type play critical roles in the preparation of aqueous precursor-based ZnO-TFTs which demonstrate high performance at low temperatures.

Development of ZnO Transistors and Their Application to Bio-Sensors

Development of ZnO Transistors and Their Application to Bio-Sensors

Properties of SiO2 and Si3N4 as gate dielectrics for printed ZnO transistors

In this article, the authors report on thin film transistors based on gas phase synthesized ZnO nanoparticles using low temperature deposited silicon dioxide and silicon nitride as gate dielectrics. For bottom gate transistors, the devices using silicon nitride as gate insulator show the lowest off-current for a given induced charge and the steepest subthreshold slope. The charge carrier mobility of around 3×10−3 cm2/V s and an Ion/Ioff ratio of around 105 are almost independent of the insulator material. In a double gated thin film transistor using low stress silicon nitride as a top gate insulator, transistor parameters are extracted for the identical semiconducting layer when the bottom and top gates are used. It is shown that the extracted charge carrier mobility is not inherent for the ZnO nanoparticle layer but rather an effective value for the device under investigation.

Direct photopatternable organic–inorganic hybrid gate dielectric for solution-processed flexible ZnO thin film transistors

A direct photopatternable organosiloxane-based organic–inorganic hybrid gate dielectric was synthesized. The sol–gel derived hybrid dielectric could be cured at a temperature sufficiently low enough to apply to temperature-sensitive polymeric substrates, whilst maintaining good electrical properties. The addition of hexa(methoxymethyl)melamine minimizes the polar silanol group by enhancing the cross-linking reaction and improves the film density, so that the resulting hybrid dielectric retains both thermal and chemical stability against the highly basic aqueous semiconductor precursor, forming a coherent interface between the dielectric and the semiconductor. The synthesized hybrid materials allow for a high-performance solution-processed flexible ZnO transistor on polymeric substrate at 150 °C.

Characterization of dual floating gate memory devices constructed on glass

The performance of dual floating gate memory devices constructed on glass is examined in this study. The dual floating gate memory device is composed of a ZnO transistor with a bottom-gate oxide layer whose two laterally separated regions are embedded with Al nanoparticles. For the memory device, four different states are achieved through Fowler–Nordheim (F–N) tunneling and channel hot electron (CHE) injection. Each of these four different states is distinguished by a difference of about 0.5 V in the threshold voltage shift. A detailed description on the four-state operation is given in this paper.

Spray‐Deposited Li‐Doped ZnO Transistors with Electron Mobility Exceeding 50 cm2/Vs

Ambient spray pyrolysis is used for the deposition of high quality polycrystalline ZnO films utilizing blends of precursor solutions based on Zinc and Lithium acetates and the demonstration of n-channel thin-film transistors with electron mobility exceeding 50 cm(2)/Vs (see figure).

Fast, simple ZnO/organic CMOS integrated circuits

Hybrid organic–inorganic CMOS thin-film circuits are a simple, potentially low-cost, approach for large-area, low-power microelectronic applications. We have used atmospheric pressure processes to deposit inorganic ZnO and organic diF TES-ADT semiconductor layers and an Al 2O 3 gate dielectric. The organic semiconductor uses a contact-treatment-related microstructure that allows circuits to operate without directly patterning the organic layer. Using a simple 4-mask process with bifunctional Ti/Au contacts for both ZnO and organic transistors, 7-stage ring oscillators were fabricated and operated at >500 kHz corresponding to a propagation delay of <150 ns/stage at a supply bias of 35 V. These are the fastest organic–inorganic CMOS circuits reported to date.

High‐Performance Zinc Oxide Transistors and Circuits Fabricated by Spray Pyrolysis in Ambient Atmosphere

The use of a simple deposition technique, namely spray pyrolysis, for the fabrication of high-mobility, low-voltage ZnO transistors and simple integrated circuits is demonstrated. The method is compatible with large-area deposition and could potentially address both the issue of manufacturing cost and high operating voltages.

Fabrication of Active Light-Emitting Device Combined with ZnO Transistors

Organic light-emitting diodes (OLEDs) have received much attention owing to their flexibility, low cost, and ease of processing. Although the active matrix display is a suitable system for what transistors and OLEDs with a large aperture ratio should be fabricated in each pixel. In this work, as one of the methods of expanding effective light-emitting area for active matrix displays, a new type of active light-emitting device combined with a ZnO transistor is proposed. The low-temperature, low-damage sputtering method improves on current by two orders of magnitude. A luminance of the active light-emitting device driven by ZnO transparent field-effect transistor (FET) was obtained. We fabricated a device with ZnO transparent FET and reported the electrical properties of ZnO FET.

Maskless Patterning of Low-Temperature High-Mobility ZnO

ZnO is a material that has gained a great deal of interest in the low-temperature fabrication of high-mobility transistors. For rapid prototyping of device structures, as well as cycle time reduction for investigations of processing/geometry interactions, a maskless process with high spatial accuracy is desired. High-mobility ZnO transistors were fabricated on glass using a laser-thermal-based maskless patterning process to generate working circuits. All deposition temperatures were below 200 &#xB0;C, allowing this process to be compatible with polymeric supports. A variety of circuits using 5 &#x3BC;m design rules and various channel width and length dimensions (w/l) were fabricated and analyzed. The circuits demonstrated included ring oscillators and logic circuits, such as an exclusive OR made from NANDs and NORs. The ring oscillator frequency was demonstrated to be about 20 kHz at 20 V. An active-matrix backplane was fabricated and used to drive an OLED device. The operating characteristics of the transistors, circuits, and processes were found to be interrelated and need to be co-optimized for the best performance.

High performance ZnO nanowire field effect transistor using self-aligned nanogap gate electrodes

A field effect transistor (FET) using a zinc oxide nanowire with significantly enhanced performance is demonstrated. The device consists of single nanowire and self-aligned gate electrodes with well defined nanosize gaps separating them from the suspended nanowire. The fabricated FET exhibits excellent performance with a transconductance of 3.06μS, a field effect mobility of 928cm2∕Vs, and an on/off current ratio of 106. The electrical characteristics are the best obtained to date for a ZnO transistor. The FET has a n-type channel and operates in enhancement mode. The results are close to those reported previously for p-type carbon nanotube (CNT) FETs. This raises the possibility of using ZnO as the n-type FET with a CNT as the p-type FET in nanoscale complementary logic circuits.