ZnO Nanowire Field Effect Transistors Research Articles

Temperature Dependence of Electrical Characteristics of ZnO Nanowire Field-Effect Transistors with AZO and Aluminium Source/Drain Contact

In this study, ZnO nanowire field-effect transistor (FET) with an aluminium-doped ZnO (AZO) and an aluminium (Al) dual layer source and drain contact are fabricated and temperature dependent characteristics in the range of 200 – 300 K are analyzed through experimental measurements. The effect of temperature on threshold voltage, subthreshold slope, transconductance, and field effect mobility are analysed. The transfer curve exhibits a parallel shift toward a negative gate voltage direction with a negative shift of the threshold voltage, an increase in the subthreshold slope, and a field-effect mobility as the temperature rises. The electrical properties of the transistors demonstrate typical behaviour at various temperatures.

Simulation and Comparison of Current-Voltage Characteristics in Double Gate Nanowire FET Using ZnO and SiO2 as Gate Oxide Materials

The aim of the project is to simulate and compare current-voltage characteristics in double gate nanowire field effect transistors made of ZnO and SiO2 gate oxides with thicknesses ranging from 1 to 30 nm. Materials and methods: The drain characteristics of double gate nanowire FET for the two gate oxides were considered for analysis. Altogether 147 drain current samples were considered for each group. Results: The two groups are subjected to an independent sample test using SPSS software, which calculates the significant mean value. The SPSS tool's chart builder is used to create a statistical graph. The mean for those in the highest quartile of drain current of ZnO based DG NW FET (drain current=0.461 mA, 95% CI:0.232-0.458 mA, p<0.05), but in SiO2 based DG NW FET (drain current=0.115 mA, 95% CI:0.232-0.458 mA, p<0.05). Conclusion: ZnO nanowire field-effect transistors have better switching characteristics and higher drain current than SiO2. The drain current is altered by varying the oxide thickness of the material. The drain current of ZnO is significantly higher than that of SiO2.

Multichannel ZnO nanowire field effect transistors by lift-off process

This paper describes a new, low-cost, top-down fabrication process, which makes it possible to define nanowire field effect transistor arrays with different numbers of nanowires simultaneously and systematically comparing their electrical performance. The main feature of this process is a developed bilayer photoresist pattern with a retrograde profile, which enables the modification of the nanowire in width, length, height and the number of transistor channels. The approach is compatible with low-cost manufacture without electron beam lithography, and benefits from process temperatures below 190 °C. Process reliability has been investigated by scanning electron microscopy, transmission electron microscopy and atomic force microscopy. Electrical measurements demonstrate enhancement mode transistors, which show a scalable correlation between the number of nanowires and the electrical characteristics. Devices with 100 nanowires exhibit the best performance with a high field effect mobility of 11.0 cm2 Vs–1, on/off current ratio of 3.97 × 107 and subthreshold swing of 0.66 V dec–1.

Photo-assisted hysteresis of electronic transport for ZnO nanowire transistors

Recently, ZnO nanowire field effect transistors (FETs) have received renewed interest due to their extraordinary low dimensionality and high sensitivity to external chemical environments and illumination conditions. These prominent properties have promising potential in nanoscale chemical and photo-sensors. In this article, we have fabricated ZnO nanowire FETs and have found hysteresis behavior in their transfer characteristics. The mechanism and dynamics of the hysteresis phenomena have been investigated in detail by varying the sweeping rate and range of the gate bias with and without light irradiation. Significantly, light irradiation is of great importance on charge trapping by regulating adsorption and desorption of oxygen at the interface of ZnO/SiO2. Carriers excited by light irradiation can dramatically promote trapping/detrapping processes. With the assistance of light illumination, we have demonstrated a photon-assisted nonvolatile memory which employs the ZnO nanowire FET. The device exhibits reliable programming/erasing operations and a large on/off ratio. The proposed proto-type memory has thus provided a possible novel path for creating a memory functionality to other low-dimensional material systems.

Effect of Phosphate Buffered Saline Solutions on Top-Down Fabricated ZnO Nanowire Field Effect Transistor

ZnO based nanowire FETs have been fabricated by implementing a top-down approach, which uses optical photolithography, atomic layer deposition (ALD) of ZnO thin film, and anisotropic plasma etching. The effects of Phosphate Buffered Saline (PBS) solution on the surface of ZnO nanowire were investigated by measuring the FET characteristics at different PBS dilutions. The drain current, ION, exhibited an increase of 39 times in the highest PBS solution concentration compared to measurement in air. From the measured transfer characteristics and output characteristics in various PBS dilutions, the device was found to maintain n-type behaviour. These results indicate that the device can be effectively used for biomolecules sensing.

Gate-Tunable Spin Exchange Interactions and Inversion of Magnetoresistance in Single Ferromagnetic ZnO Nanowires.

Electrical control of ferromagnetism in semiconductor nanostructures offers the promise of nonvolatile functionality in future semiconductor spintronics. Here, we demonstrate a dramatic gate-induced change of ferromagnetism in ZnO nanowire (NW) field-effect transistors (FETs). Ferromagnetism in our ZnO NWs arose from oxygen vacancies, which constitute deep levels hosting unpaired electron spins. The magnetic transition temperature of the studied ZnO NWs was estimated to be well above room temperature. The in situ UV confocal photoluminescence (PL) study confirmed oxygen vacancy mediated ferromagnetism in the studied ZnO NW FET devices. Both the estimated carrier concentration and temperature-dependent conductivity reveal the studied ZnO NWs are at the crossover of the metal-insulator transition. In particular, gate-induced modulation of the carrier concentration in the ZnO NW FET significantly alters carrier-mediated exchange interactions, which causes even inversion of magnetoresistance (MR) from negative to positive values. Upon sweeping the gate bias from -40 to +50 V, the MRs estimated at 2 K and 2 T were changed from -11.3% to +4.1%. Detailed analysis on the gate-dependent MR behavior clearly showed enhanced spin splitting energy with increasing carrier concentration. Gate-voltage-dependent PL spectra of an individual NW device confirmed the localization of oxygen vacancy-induced spins, indicating that gate-tunable indirect exchange coupling between localized magnetic moments played an important role in the remarkable change of the MR.

Coulomb blockade and plasmonic nanoantenna effect in back gated ZnO nanorod FET

Fabrication and characterization of ZnO nanorod field effect transistor (FET) is reported. Back-gated ZnO nanowire FETs were fabricated on SiO2 covered p-type Si substrate using lithographically patterned Ti/Au contacts. The ZnO nanorods were synthesized by chemical bath deposition method using the precursors zinc nitrate hexahydrate (Zn (NO3)2·6H2O) and hexamethylenetetramine (HMT) (C6H12N4). Fabricated FET has shown clear gate response. I–V characterization done on the fabricated FET has shown Coulomb blockade and plasmonic nanoantenna like effects. The observation of Coulomb blockade and plasmonic effect of the fabricated FET structure have been explained.

High-performance ZnO nanowire field-effect transistor with forming gas treated SiO2 gate dielectrics

The SiO2 films thermally grown on Si wafer have been annealed in forming atmosphere (N2:H2 = 9:1) prior to use as gate insulators in ZnO nanowire field effect transistors (ZnO NW-FETs). Without the annealing process, ZnO NW-FETs exhibit very poor performance, and most of them even cannot be depleted under a high gate voltage of −100 V; however, with the annealing process in forming atmosphere, the device characteristics can be significantly improved, exhibiting a large turn on-off ratio of ∼104 and a low sub-threshold swing ∼1 V/decade. The pre-annealing treatment of SiO2 (300 nm)/p-Si in N2/H2 ambient may significantly reduce the number of non-bridging oxygen atoms, which blocks the interaction between ZnO nanowires and SiO2 surface, and finally enhances the electrical characteristics of the back-gated ZnO NW-FETs. In addition, the FET electrode fabrication process introduced in this paper is much simpler than the traditional photo-lithography and lift-off method, which has potential applications in futu...

Hydrogen plasma-mediated modification of the electrical transport properties of ZnO nanowire field effect transistors

We investigated the effects of hydrogen plasma treatment on the electrical transport properties of ZnO nanowire field effect transistors (FETs) with a back gate configuration. After hydrogen plasma treatment of the FET devices, the effective carrier density and mobility of the nanowire FETs increased with a threshold voltage shift toward a negative gate bias direction. This can be attributed to the desorption of oxygen molecules adsorbed on the surface of the nanowire channel, to passivation and to doping effects due to the incorporation of energetic hydrogen ions generated in plasma.

High-gain subnanowatt power consumption hybrid complementary logic inverter with WSe2 nanosheet and ZnO nanowire transistors on glass.

A 1D-2D hybrid complementary logic inverter comprising of ZnO nanowire and WSe2 nanosheet field-effect transistors (FETs) is fabricated on glass, which shows excellent static and dynamic electrical performances with a voltage gain of ≈60, sub-nanowatt power consumption, and at least 1 kHz inverting speed.

Interplay between temperature effects and surface recombination process in UV photoresponse of ZnO nanowires

We demonstrate the interplay between surface recombination dynamics and environmental temperature in UV photoresponse of ZnO nanowire field effect transistors (FETs) with and without a passivation layer. We find that optoelectronic performance and photoresponse mechanism can be significantly altered by temperature, influencing surface chemical interaction and reaction associated with charge trapping. Particularly, regardless of surface passivation on nanowires, photocurrent gains and decay rate are drastically enhanced with increasing temperature. Furthermore, the temperature dependence of photoresponse behavior in nanowire FETs is discussed in terms of surface recombination rates, carrier concentration, surface potential barrier height, and charge trapping rates on the surface.

Effect of atomic layer deposition temperature on the performance of top-down ZnO nanowire transistors.

This paper studies the effect of atomic layer deposition (ALD) temperature on the performance of top-down ZnO nanowire transistors. Electrical characteristics are presented for 10-μm ZnO nanowire field-effect transistors (FETs) and for deposition temperatures in the range 120°C to 210°C. Well-behaved transistor output characteristics are obtained for all deposition temperatures. It is shown that the maximum field-effect mobility occurs for an ALD temperature of 190°C. This maximum field-effect mobility corresponds with a maximum Hall effect bulk mobility and with a ZnO film that is stoichiometric. The optimized transistors have a field-effect mobility of 10 cm2/V.s, which is approximately ten times higher than can typically be achieved in thin-film amorphous silicon transistors. Furthermore, simulations indicate that the drain current and field-effect mobility extraction are limited by the contact resistance. When the effects of contact resistance are de-embedded, a field-effect mobility of 129 cm2/V.s is obtained. This excellent result demonstrates the promise of top-down ZnO nanowire technology for a wide variety of applications such as high-performance thin-film electronics, flexible electronics, and biosensing.

Gate modulation of below-band-gap photoconductivity in ZnO nanowire field-effect-transistors

We investigated the modulation of the photoconductivity under below-band gap excitation in single ZnO nanowire field effect transistors. Light excitation at 550 nm does not induce any change in the drain–source current when the gate voltage is kept at Vgs = 0 V, but results in a current increase when it is set to Vgs = −50 V. At this negative value of the gate voltage we further investigated the photo-reaction in the below-band-gap range 400–800 nm, observing a qualitative similar profile for all the photo-current curves. These results were attributed to a local effect, suggesting that the change in conductivity is due to the release of electrons from interface states located between the ZnO nanowire active channel and the gate dielectric SiO2.

ZnO nanowire field-effect transistors with Pt nanocrystals fabricated on a flexible plastic substrate for a non-volatile memory application

In this work, a single ZnO nanowire field-effect transistor (NWFET) with discrete-trap storage nodes of Pt nanocrystals (NCs) embedded within Al2O3, a high-κ tunneling/control dielectric material, was successfully fabricated on a flexible plastic substrate for a non-volatile memory application. The ZnO NWFET with embedded Pt NCs exhibits a clear hysteresis loop with a threshold voltage shift of 4.16 V under the ±15 V gate–source voltage sweep ranges, implying that the charging and discharging phenomena occur during the program/erase operations. The ZnO NWFET achieves a reasonable value of approximately 105 for the on/off current ratio for read-out of storage data. Furthermore, memory features, such as programming efficiency and on-cell current, are compared between the flat and convexly bent states in order to investigate the feasibility of flexible plastic devices as transistors.

Vertical stacking of ZnO nanowire devices with different functionalities on plastic substrates

In this paper, we demonstrate the vertical stacking of ZnO nanowire (NW) field-effect transistors (FETs), a ZnO NW-based nanofloating gate memory (NFGM) device, and a ZnO NW-based inverter on a flexible plastic substrate. For the vertical stacking, four ZnO NW devices are sequentially constructed on each of four layers, and these devices are isolated from each other using Al2O3 material. Each of the stacked ZnO NW devices exhibits the good electrical characteristics with n-type depletion modes under both the flat and bent states. Moreover, the switching, memory, and inverting characteristics of the stacked ZnO NW devices are examined.

Electric-Field Control of Ferromagnetism in Mn-Doped ZnO Nanowires

In this Letter, the electric-field control of ferromagnetism was demonstrated in a back-gated Mn-doped ZnO (Mn-ZnO) nanowire (NW) field-effect transistor (FET). The ZnO NWs were synthesized by a thermal evaporation method, and the Mn doping of 1 atom % was subsequently carried out in a MBE system using a gas-phase surface diffusion process. Detailed structural analysis confirmed the single crystallinity of Mn-ZnO NWs and excluded the presence of any precipitates or secondary phases. For the transistor, the field-effect mobility and n-type carrier concentration were estimated to be 0.65 cm(2)/V·s and 6.82 × 10(18) cm(-3), respectively. The magnetic hysteresis curves measured under different temperatures (T = 10-350 K) clearly demonstrate the presence of ferromagnetism above room temperature. It suggests that the effect of quantum confinements in NWs improves Tc, and meanwhile minimizes crystalline defects. The magnetoresistace (MR) of a single Mn-ZnO NW was observed up to 50 K. Most importantly, the gate modulation of the MR ratio was up to 2.5 % at 1.9 K, which implies the electric-field control of ferromagnetism in a single Mn-ZnO NW.

Analysis of surface states in ZnO nanowire field effect transistors

Nanowires (NWs) have attracted considerable interests for scaled electronic and optoelectronic device applications. However, NW based semiconductor devices normally suffer from surface states due to the existence of dangling bonds or surface reconstruction. Because of their large surface-to-volume ratio, surface states in NWs can easily affect the metallic contacts to NWs and electron transport in NW. Here, we present ZnO NW surface analysis by performing current–voltage characterization on ZnO NW Schottky barrier field effect transistors with different metal contacts (Ti, Al, Au) at both room temperature and cryogenic temperature. Our results show that three metal contacts are all Schottky contacts to ZnO NWs due to surface states. Our further study reveals: (a) the surface states related Schottky barrier height (SBH) can be extracted from a back to back Schottky diodes model and the SBH values are in the range of 0.34–0.37eV for three metal contacts; (b) the trap activation energy determined from the Arrhenius plots of different Schottky metal contacts is in the range of 0.23–0.29eV, which is oxygen vacancies related; and (c) based on the space-charge-limited model, the surface state density of ZnO NW is in the range of 1.04×1010–1.24×1010/cm2.

Temperature Dependence of Electron Transport in ZnO Nanowire Field Effect Transistors

Nanowires (NWs) have attracted considerable interests for electronic and optoelectronic device applications. However, the carrier transport mechanisms in these NW devices have not been well understood. Here we present the electron transport of ZnO NWs field effect transistors (FETs). Our results show that the electron transport of ZnO NW FETs is governed by the space charge limited model at temperatures below a trap temperature. Above the trap temperature, the electron transport is thermionic emission dominated. Based on the space charge limited model, an accurate method is developed for field effect mobility extraction. The extracted electron carrier mobility is strongly dependent on temperature with a peak value of 51 cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> /Vs at 167 K. Under the space-charge limited transport, the field mobility is lower in comparison with the values extracted from the thermionic emission model. The electron mobility due to space charge scattering has a value of 483 cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> /Vs at the trap temperature with temperature dependence of T <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">4 ~ 5</sup> . The interface state density between the back gate dielectric and ZnO NWs is in the range 9.03×10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">6</sup> /cm-8.72×10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">7</sup> /cm at temperatures ranging from 77 to 227 K.

Characteristics of light-induced electron transport from P3HT to ZnO-nanowire field-effect transistors

We fabricated ZnO-nanowire (NW) field-effect transistors (FETs) coated with poly(3-hexylthiophene) (P3HT) and characterized the electron-transfer characteristics from the P3HT to the ZnO NWs. Under irradiation by laser light with a wavelength of 532 nm, photo-induced electrons were created in the P3HT and then transported to the ZnO NWs, constituting a source-drain current in the initially enhancement-mode P3HT-coated ZnO-NW FETs. As the intensity of the light increased, the current increased, and its threshold voltage shifted to the negative gate-bias direction. We estimated the photo-induced electron density and the electron-transfer characteristics, which will be helpful for understanding organic-inorganic hybrid optoelectronic devices.

Solution processable multi-channel ZnO nanowire field-effect transistors with organic gate dielectric

The present work focuses on nanowire (NW) applications as semiconducting elements in solution processable field-effect transistors (FETs) targeting large-area low-cost electronics. We address one of the main challenges related to NW deposition and alignment by using dielectrophoresis (DEP) to select multiple ZnO nanowires with the correct length, and to attract, orientate and position them in predefined substrate locations. High-performance top-gate ZnO NW FETs are demonstrated on glass substrates with organic gate dielectric layers and surround source–drain contacts. Such devices are hybrids, in which inorganic multiple single-crystal ZnO NWs and organic gate dielectric are synergic in a single system. Current–voltage (I–V) measurements of a representative hybrid device demonstrate excellent device performance with high on/off ratio of ∼107, steep subthreshold swing (s-s) of ∼400 mV/dec and high electron mobility of ∼35 cm2 V−1 s−1 in N2 ambient. Stable device operation is demonstrated after 3 months of air exposure, where similar device parameters are extracted including on/off ratio of ∼4 × 106, s-s ∼500 mV/dec and field-effect mobility of ∼28 cm2 V−1 s−1. These results demonstrate that DEP can be used to assemble multiples of NWs from solvent formulations to enable low-temperature hybrid transistor fabrication for large-area inexpensive electronics.