J. C. She

Electrical and Photosensitive Characteristics of a-IGZO TFTs Related to Oxygen Vacancy

The electrical and photosensitive characteristics of amorphous indium-gallium-zinc-oxide (a-IGZO) thin-film transistors (TFTs) related to the oxygen vacancies V̈_O are discussed. With the filling of V̈_O of ratio from 14 to 8, the electron density of the a-IGZO channel decreases from 7.5 to 3.8 ( ×10¹⁶ cm^-3); the saturation mobility of the TFT decreases from 3.1 to 1.4 cm^-2/(V · s); the threshold voltage increases from 7 to 11 V for the TFT with a lower on-current; and the subthreshold slope increases from 2.4 to 4.4 V/dec for the TFT with a higher interface defect density of 4.9 × 10¹¹ cm^-2, the worst electrical stability of V_th ~ 10 V, and a hysteresis-voltage decrease from 3.5 to 2 V. The photoreaction properties of a-IGZO TFTs are also sensitive to the oxygen-content-related absorption of the a-IGZO channel. With the lowest content of oxygen in the channel, the TFT has the largest photocurrent gain of 50 μA (V_g = 30 V; V_d = 10 V) and decrease in V_th ( V_th V) at a high light intensity. The light-induced change of TFT characteristics is totally reversible with the time constant for recovery of about 2.5 h.

Correlation between resistance and field emission performance of individual ZnO one-dimensional nanostructures.

Both electrical and field emission measurements were carried out to study the correlation between resistance and field emission performance of individual one-dimensional (1D) ZnO nanostructures. Three types of 1D ZnO nanostructures were investigated (i.e., agave-like shape, pencil-like shape, and hierarchical structure) and were prepared by thermal chemical vapor transport and condensation without using any catalyst. The 1D ZnO nanostructures have obvious differences in resistance and thus conductivity from type to type. In addition, in the same type of 1D ZnO nanostructure, each individual emitter may also have variation in resistance and thus in conductivity. The field emission performance of the ZnO emitters was found to be strongly correlated with the resistance of each individual ZnO nanostructure: (i) a ZnO emitter with low resistance will have better emission; (ii) a high resistance region in a ZnO nanostructure is liable to the initiation of a vacuum breakdown event. The results indicate that, besides the uniformity in the geometrical structure, the uniformity in conductivity of the emitters in an array should be ensured, in order to meet the requirement of device application.

Field Electron Emission Characteristics and Physical Mechanism of Individual Single-Layer Graphene

Due to its difficulty, experimental measurement of field emission from a single-layer graphene has not been reported, although field emission from a two-dimensional (2D) regime has been an attractive topic. The open surface and sharp edge of graphene are beneficial for field electron emission. A 2D geometrical effect, such as massless Dirac fermion, can lead to new mechanisms in field emission. Here, we report our findings from in situ field electron emission characterization on an individual singe-layer graphene and the understanding of the related mechanism. The measurement of field emission from the edges was done using a microanode probe equipped in a scanning electron microscope. We show that repeatable stable field emission current can be obtained after a careful conditioning process. This enables us to examine experimentally the typical features of the field emission from a 2D regime. We plot current versus applied field data, respectively, in ln(I/E(3/2)) ∼ 1/E and ln(I/E(3)) ∼ 1/E(2) coordinates, which have recently been proposed for field emission from graphene in high- and low-field regimes. It is observed that the plots all exhibit an upward bending feature, revealing that the field emission processes undergo from a low- to high-field transition. We discuss with theoretical analysis the physical mechanism responsible for the new phenomena.

Synthesis and field-emission properties of aligned MoO3 nanowires

Aligned MoO3 nanowires have been grown on silicon substrates without using any catalyst. They were prepared in a two-step process: first by thermal evaporation and then further processed by oxidation. The MoO3 nanowires are of crystalline and have an orthorhombic structure. They also have high purity. Field-emission measurement showed that, typically, their turn-on field and threshold field were about 3.5 and 7.65 MV/m, respectively. Furthermore, the spatial distribution of emission sites was studied using transparent anode technique and the emission current of the sites was relatively uniform. These may be attributed to very good uniformity in the height and diameter of the nanowires, and to the separation between nanowires. Finally, the stability of the emission current over time was found to be within 10%. These findings indicate that MoO3 nanowires as a cold cathode have a potential future.

Synthesis of crystalline alumina nanowires and nanotrees

Abstract Crystalline alumina nanowires were synthesized at elevated temperatures in a catalyst-assisted process using iron as catalyst. Nanotrees that formed by alumina nanowires were also found. SEM shows that typical nanowires are around 50 nm in diameter and around 2 μ m in length. The tree trunk of the nanotrees are around 100 nm in diameter and around 10 μ m in length. HRTEM with electron diffraction study reveals that the nanostructures are crystalline. The EDX confirms that the nanostructures contain only elements of Al and O. The XRD study shows that the nanowires are α-Al2O3. The results are explained in the light of growth mechanism based on a vapor–liquid–solid (VLS) process.

Field emission study of SiC nanowires/nanorods directly grown on SiC ceramic substrate

Single crystalline silicon carbide (SiC) nanowires were grown directly on the surface of bulk SiC ceramic substrate in a catalyst-assisted thermal heating process. The morphology of the nanowire film and the diameter of nanowires were found to be sensitive to the thickness of catalyst film and both of them had a strong effect on field emission performance. Very low turn-on and threshold fields for electron emission were observed with SiC nanowires of small diameter. A model is proposed to qualitatively explain the field emission findings, which assumes the occurrence of an insulator-to-metal-like transition in a field emitting nanowire.

Fabrication of vertically aligned Si nanowires and their application in a gated field emission device

A technique involving a combination of using self-assembled nanomask and anisotropic plasma etching is developed for fabricating vertically aligned single-crystalline Si nanowires (SiNWs). The SiNWs are shown to have excellent field emission performance with the turn-on field as low as 0.8MV∕m and the threshold field being 5.0MV∕m. In addition, an emission current density of 442mA∕cm2 can be obtained at an applied field of ∼14MV∕m. The technique is easily employed to fabricate arrays of SiNW-based field emission microtriodes. Mechanisms are proposed to explain the formation of the SiNWs and the observed field emission properties.

Vacuum breakdown of carbon-nanotube field emitters on a silicon tip

Findings are given from the experimental observation of the vacuum breakdown of carbon-nanotube (CNT) field emitters on a Si tip. The CNTs were grown on the apex of a Si microtip by microwave plasma-enhanced chemical vapor deposition. The electrical contact of the CNT-Si junction was shown to be of ohmic type. A fine tungsten microprobe in combination with a scanning electron microscopy (SEM) system was employed for both the field emission and the contact conductivity measurements. This arrangement allows to precisely measure the characteristics of individual CNT and to in situ inspect the morphology of the CNT emitters on Si tips before and after vacuum breakdown events. An upper limit in emission current density of ∼103 A/m2 from the CNT emitters was recorded before a vacuum breakdown event is initiated. Clear evidence was found to show that the vacuum breakdown of the CNTs results in melting of the Si tip. These findings enhance the understanding of the failure mechanism of CNT emitters. It also has im...

Field emission display device structure based on double-gate driving principle for achieving high brightness using a variety of field emission nanoemitters

In order to apply various cold cathode nanoemitters in a field emission display (FED) and to achieve high brightness, a FED device structure with double gates and corresponding driving method have been proposed. Individual pixel addressing can be achieved by applying proper sequence of positive or negative voltage to the lower gate and upper gate, respectively. The feasibility of the device has been demonstrated by using carbon nanotube and tungsten oxide nanowire cold emitters. Display of moving images has been demonstrated and high luminance up to 2500cd∕m2 was obtained. The reported device structure is versatile for nanoemitters regardless of substrate or preparation temperature. The results are of significance to the development of FED using nanoemitters.

Enhancing electron emission from silicon tip arrays by using thin amorphous diamond coating

A thin (∼2 nm thick) amorphous diamond coating was prepared on single crystal silicon tip arrays by using a filtered vacuum arc plasma deposition technique. The coating has a microscopically uniform morphology. As compared to uncoated tips, the electron emission of the coated tip arrays is enhanced, showing an increase in the total current, lower turn-on field and a lower-slope Fowler–Nordheim plot. We propose that field-emitted electrons could tunnel through such a thin coating with few scattering events. It is shown that the low potential barrier at the interface is the major cause of the enhancing effects instead of the negative surface electron affinity of the coating.