Abhishek Singh Dahiya

Fabrication of field-effect transistors and functional nanogenerators using hydrothermally grown ZnO nanowires

The present work demonstrates the production of single crystalline ZnO nanowires (NWs) using the low temperature hydrothermal process and their integration as the active channel material and piezoelectric elements in single NW field-effect transistors (FETs) and functional nanogenerators (NGs), respectively. Even though hydrothermally grown ZnO NWs show high levels of excess free carriers ≫1018 cm−3, we show that an optimized thermal annealing process at just 450 °C in atmospheric air sufficiently reduces this level to around ∼3.7 × 1017 cm−3. The excess free carrier suppression is verified by assessing the field-effect transport behaviour in a single NW FET. The single device is found to exhibit good performance metrics, including low off-state current (pA range), high on-state current (in the 10 s of μA range) and moderate effective mobility (∼10 cm2 V−1 s−1). The functional NGs are based on vertically grown ZnO NWs with ∼7 μm thick polydimethylsiloxane (PDMS) polymer matrix. We show that a NG incorporating annealed ZnO NWs can continuously generate higher output voltages and power compared to a reference device based on as-grown ZnO NWs. This included peak output voltage of ∼109 mV and an output power density of ∼16 μW cm−3. We envisage that this approach of thermal annealing may find practical applications in other areas of hydrothermal ZnO NW research, including high performance NW FETs and piezoelectric energy harvesters.

Fabrication of high performance field-effect transistors and practical Schottky contacts using hydrothermal ZnO nanowires

The production of large quantities of single crystalline semiconducting ZnO nanowires (NWs) at low cost can offer practical solutions to realizing several novel electronic/optoelectronic and sensor applications on an industrial scale. The present work demonstrates high-density single crystalline NWs synthesized by a multiple cycle hydrothermal process at ∼100 °C. The high carrier concentration in such ZnO NWs is greatly suppressed by a simple low cost thermal annealing step in ambient air at ∼450 °C. Single ZnO NW FETs incorporating these modified NWs are characterized, revealing strong metal work function-dependent charge transport, unobtainable with as-grown hydrothermal ZnO NWs. Single ZnO NW FETs with Al as source and drain (s/d) contacts show excellent performance metrics, including low off-state currents (fA range), high on/off ratio (10(5)-10(7)), steep subthreshold slope (<600 mV/dec) and excellent field-effect carrier mobility (5-11 cm(2)/V-s). Modified ZnO NWs with platinum s/d contacts demonstrate excellent Schottky transport characteristics, markedly different from a reference ZnO NW device with Al contacts. This included abrupt reverse bias current-voltage saturation characteristics and positive temperature coefficient (∼0.18 eV to 0.13 eV). This work is envisaged to benefit many areas of hydrothermal ZnO NW research, such as NW FETs, piezoelectric energy recovery, piezotronics and Schottky diodes.

Zinc oxide sheet field-effect transistors

The present work investigates charge carrier transport in back-gated field-effect transistors based on ZnO sheets (BG ZS-FETs). The ZSs used in this work have been synthesized via the catalytic-assisted vapor-liquid-solid process inside a horizontal quartz tube furnace at around 950 °C. The BG ZS-FETs were constructed as bottom-gate top-contact structures using suspended and non-suspended ZS as the active channel material. Assessment of key device performance metrics revealed excellent n-channel behavior with low off-state current in the femtoamp range, high on-state current (∼2 μA/μm), high on-to-off current ratio (>107), a steep sub-threshold swing of around 190 mV/dec, and field-effect carrier mobility of around 60 cm2/Vs. Temperature dependent charge transport studies reveal excessive mobility degradation in the non-suspended device while the same parameter in the suspended case appeared fairly stable. The present work is envisaged to benefit ongoing research towards the development of high performanc...

Single-crystalline ZnO sheet Source-Gated Transistors.

Due to their fabrication simplicity, fully compatible with low-cost large-area device assembly strategies, source-gated transistors (SGTs) have received significant research attention in the area of high-performance electronics over large area low-cost substrates. While usually based on either amorphous or polycrystalline silicon (α-Si and poly-Si, respectively) thin-film technologies, the present work demonstrate the assembly of SGTs based on single-crystalline ZnO sheet (ZS) with asymmetric ohmic drain and Schottky source contacts. Electrical transport studies of the fabricated devices show excellent field-effect transport behaviour with abrupt drain current saturation (IDSSAT) at low drain voltages well below 2 V, even at very large gate voltages. The performance of a ZS based SGT is compared with a similar device with ohmic source contacts. The ZS SGT is found to exhibit much higher intrinsic gain, comparable on/off ratio and low off currents in the sub-picoamp range. This approach of device assembly may form the technological basis for highly efficient low-power analog and digital electronics using ZnO and/or other semiconducting nanomaterial.

A facile hydrothermal approach for the density tunable growth of ZnO nanowires and their electrical characterizations

Controlling properties of one-dimensional (1D) semiconducting nanostructures is essential for the advancement of electronic devices. In this work, we present a low-temperature hydrothermal growth process enabling density control of aligned high aspect ratio ZnO nanowires (NWs) on seedless Au surface. A two order of magnitude change in ZnO NW density is demonstrated via careful control of the ammonium hydroxide concentration (NH4OH) in the solution. Based on the experimental observations, we further, hypothesized the growth mechanism leading to the density controlled growth of ZnO NWs. Moreover, the effect of NH4OH on the electrical properties of ZnO NWs, such as doping and field-effect mobility, is thoroughly investigated by fabricating single nanowire field-effect transistors. The electrical study shows the increase of free charge density while decrease of mobility in ZnO NWs with the increase of NH4OH concentration in the growth solution. These findings show that NH4OH can be used for simultaneous tuning of the NW density and electrical properties of the ZnO NWs grown by hydrothermal approach. The present work will guide the engineers and researchers to produce low-temperature density controlled aligned 1D ZnO NWs over wide range of substrates, including plastics, with tunable electrical properties.

Double buffer circuit for the characterization of piezoelectric nanogenerators based on ZnO nanowires

The accurate and precise measurements of voltage and current output generated by a nanogenerator (NG) are crucial to design the rectifying/harvesting circuit and to evaluate correctly the amount of energy provided by a NG. High internal impedance of the NGs (several MΩ) is the main limiting factor for designing circuits to measure the open circuit voltage. In this paper, we present the influence of the characterization circuit used to measure the generated voltage of piezoelectric NGs. The proposed circuit consists of a differential amplifier which permits us to measure the voltage provided by the NG without applying any parasitic bias to it. The proposed circuit is compared to a commercial electrometer and a homemade buffer circuit based on a voltage follower circuit to show its interest. For the proposed double buffer circuit, no asymmetric behavior has been noticed contrary to the measurements made using a simple buffer circuit and a Keithley electrometer. The proposed double buffer circuit is thus sui...

Flexible Organic/Inorganic Hybrid Field-Effect Transistors with High Performance and Operational Stability

The production of high-quality semiconducting nanostructures with optimized electrical, optical, and electromechanical properties is important for the advancement of next-generation technologies. In this context, we herein report on highly obliquely aligned single-crystalline zinc oxide nanosheets (ZnO NSs) grown via the vapor-liquid-solid approach using r-plane (01-12) sapphire as the template surface. The high structural and optical quality of as-grown ZnO NSs has been confirmed using high-resolution transmission electron microscopy and temperature-dependent photoluminescence, respectively. To assess the potential of our NSs as effective building materials in high-performance flexible electronics, we fabricate organic (parylene C)/inorganic (ZnO NS) hybrid field-effect transistor (FET) devices on flexible substrates using room-temperature assembly processes. Extraction of key FET performance parameters suggests that as-grown ZnO NSs can successfully function as excellent n-type semiconducting modules. Such devices are found to consistently show very high on-state currents (Ion) > 40 μA, high field-effect mobility (μeff) > 200 cm2/(V s), exceptionally high on/off current modulation ratio (Ion/off) of around 109, steep subthreshold swing (s-s) < 200 mV/decade, very low hysteresis, and negligible threshold voltage shifts with prolonged electrical stressing (up to 340 min). The present study delivers a concept of integrating high-quality ZnO NS as active semiconducting elements in flexible electronic circuits.

Universal model for defect-related visible luminescence in ZnO nanorods

We study the optical properties of ZnO nanorods (NRs) fabricated by chemical bath deposition, hydrothermal, and the vapour–liquid–solid method (VLS). Scanning electron microscopy demonstrates differences in the structural properties for the various samples. The optical emission properties are studied by photoluminescence (PL) spectroscopy where all samples are characterized by a UV and visible emission band. The visible emission band is due to defects in the nanorods. VLS samples show a blue shift in the visible region of the PL spectra with respect to the other samples, however, all three samples are fitted with the same three visible Gaussian components under varying percent contributions due to the structural differences. The visible defect components are characterized by blue (B), green (G), and orange (O) states with energies at 2.52 eV, 2.23 eV, and 2.03 eV, respectively. The applicability of this universal model for visible B–G–O defects is tested against the literature and successfully fitted regardless of the fabrication method. Differences in the percentage contribution for the visible B–G–O defects is explained by variations in the fabrication method. This model indicates how defects can be controlled based on the fabrication method. Furthermore, the B state, which is associated with the ‘green luminescence band’, results from a transition from the defect level to the valence band, or possibly a shallow-acceptor, according to photoluminescence excitation measurements. The role of the B state in sensing applications is discussed.

Measurement of Spurious Voltages in ZnO Piezoelectric Nanogenerators

The recent development and commercialization of microelectromechanical systems (MEMS)/nanoelectromechanical systems (NEMS) has brought the related challenge of independently powering such systems. The concept of the nanogenerator (NG) has shown potential for harvesting energy from the ambient environment to power MEMS/NEMS. Kinetic energy harvesting NGs based on the piezoelectric properties of ZnO nanowires have attracted much interest. In this paper, we have fabricated hydrothermally synthesized ZnO-based NGs following the procedures standardized in the published literature. Likewise, reference NGs without ZnO piezoelectric material were fabricated in parallel with the ZnO NGs. The voltage output of both the ZnO NG and the reference NG was measured given a 10-Hz cyclic vertical load. Unexpectedly, both the ZnO and the reference NG were found to produce 3 mV with 0.451 N of load. A finite-element model was created to determine that the voltage potential of the NG should be about 1 mV with the given load. A possible explanation for this unexpected behavior is that the measured signals are not entirely piezoelectric in nature. Rather, the signals recorded from the NGs may be some alternate phenomenon, such as the triboelectric, flexoelectric, or electret effect.

Photoluminescence Study of the Influence of Additive Ammonium Hydroxide in Hydrothermally Grown ZnO Nanowires

We report the influence of ammonium hydroxide (NH4OH), as growth additive, on zinc oxide nanomaterial through the optical response obtained by photoluminescence (PL). A low-temperature hydrothermal process is employed for the growth of ZnO nanowires (NWs) on seedless Au surface. A more than two order of magnitude change in ZnO NW density is demonstrated via careful addition of NH4OH in the growth solution. Further, we show by systematic experimental study and PL characterization data that the addition of NH4OH can degrade the optical response of ZnO NWs produced. The increase of growth solution basicity with the addition of NH4OH may slowly degrade the optical response of NWs by slowly etching its surfaces, increasing the point defects in ZnO NWs. The present study demonstrates the importance of growth nutrients to obtain quality controlled density tunable ZnO NWs on seedless conducting substrates.