Approaching conversion limit with all-dielectric solar cell reflectors.

Metallic back reflectors has been used for thin-film and wafer-based solar cells for very long time. Nonetheless, the metallic mirrors might not be the best choices for photovoltaics. In this work, we show that solar cells with all-dielectric reflectors can surpass the best-configured metal-backed devices. Theoretical and experimental results all show that superior large-angle light scattering capability can be achieved by the diffuse medium reflectors, and the solar cell J-V enhancement is higher for solar cells using all-dielectric reflectors. Specifically, the measured diffused scattering efficiency (D.S.E.) of a diffuse medium reflector is >0.8 for the light trapping spectral range (600nm-1000nm), and the measured reflectance of a diffuse medium can be as high as silver if the geometry of embedded titanium oxide(TiO(2)) nanoparticles is optimized. Moreover, the diffuse medium reflectors have the additional advantage of room-temperature processing, low cost, and very high throughput. We believe that using all-dielectric solar cell reflectors is a way to approach the thermodynamic conversion limit by completely excluding metallic dissipation.

Conductivity enhancement of multiwalled carbon nanotube thin film via thermal compression method.

For the first time, the thermal compression method is applied to effectively enhance the electrical conductivity of carbon nanotube thin films (CNTFs). With the assistance of heat and pressure on the CNTFs, the neighbor multiwalled carbon nanotubes (CNTs) start to link with each other, and then these separated CNTs are twined into a continuous film while the compression force, duration, and temperature are quite enough for the reaction. Under the compression temperature of 400°C and the compression force of 100 N for 50 min, the sheet resistance can be reduced from 17 to 0.9 k Ω/sq for the CNTFs with a thickness of 230 nm. Moreover, the effects of compression temperature and the duration of thermal compression on the conductivity of CNTF are also discussed in this work.

A novel polysilicon field-enhanced nanowire thin-film transistor with the TiN-hafnia-nitride-vacuum-silicon (THNVAS) structure for nonvolatile memory applications.

A novel poly-Si field-enhanced nanowire (FEN) TFT memory with the TiN-hafnia-nitride-vacuum-silicon (THNVAS) structure fabricated simply via a sidewall spacer formation has been presented. The THNVAS devices with superior memory performance were demonstrated by introducing the hafnia as blocking oxide and the vacuum, the lowest-k in nature, as tunneling layer. According to the simulation results, the memory device with oxide/nitride/vacuum gate dielectric exhibited a higher local electric-field of 4.72 x 10(7) V/cm as compared to 2.55 x 10(7) V/cm for the conventional oxide/nitride/oxide counterpart. In addition, the electric-field of tunneling layer could be further increased to 7.06 x 10(7) V/cm while the blocking oxide was substituted for hafnia. The experimental data showed that THNVAS possessed a greater threshold voltage shift of 3.75 V in 10 ms at V(GS) = 12 V, whereas the shift only 2.5 V for THNOS ones. These considerable improvements for THNVAS devices could be attributed to the evident field enhancement across the vacuum tunneling layer. Furthermore, owing to the empty feature of vacuum tunneling layer, the THNVAS demonstrated much-improved endurance performance and preferable data retention property. Hence, such excellent characteristics of THNVAS will be an attractive nonvolatile memory for future system-on-panel and 3-D Flash applications.

High-performance bottom-gate poly-Si polysilicon-oxide-nitride-oxide-silicon thin film transistors crystallized by excimer laser irradiation for two-bit nonvolatile memory applications.

High-performance bottom-gate (BG) poly-Si polysilicon-oxide-nitride-oxide-silicon (SONOS) TFTs with single grain boundary perpendicular to the channel direction have been demonstrated via simple excimer-laser-crystallization (ELC) method. Under an appropriate laser irradiation energy density, the silicon grain growth started from the thicker sidewalls intrinsically caused by the bottom-gate structure and impinged in the center of the channel. Therefore, the proposed ELC BG SONOS TFTs exhibited superior transistor characteristics than the conventional solid-phase-crystallized ones, such as higher field effect mobility of 393 cm2/V-s and steeper subthreshold swing of 0.296 V/dec. Due to the high field effect mobility, the electron velocity, impact ionization, and conduction current density could be enhanced effectively, thus improving the memory performance. Based on this mobility-enhanced scheme, the proposed ELC BG SONOS TFTs exhibited better performance in terms of relatively large memory window, high program/erase speed, long retention time, and 2-bit operation. Such an ELC BG SONOS TFT with single-grain boundary in the channel is compatible with the conventional a-Si TFT process and therefore very promising for the embedded memory in the system-on-panel applications.

The pH sensing characteristics of the extended-gate field-effect transistors of multi-walled carbon-nanotube thin film using low-temperature ultrasonic spray method.

A novel, simple and low-temperature ultrasonic spray method was developed to fabricate the multi-walled carbon-nanotubes (MWCNTs) based extended-gate field-effect transistors (EGFETs) as the pH sensor. With an acid-treated process, the chemically functionalized two-dimensional MWCNT network could provide plenty of functional groups which exhibit hydrophilic property and serve as hydrogen sensing sites. For the first time, the EGFET using a MWCNT structure could achieve a wide sensing rage from pH = 1 to pH = 13. Furthermore, the pH sensitivity and linearity values of the CNT pH-EGFET devices were enhanced to 51.74 mV/pH and 0.9948 from pH = 1 to pH = 13 while the sprayed deposition reached 50 times. The sensing properties of hydrogen and hydroxyl ions show significantly dependent on the sprayed deposition times, morphologies, crystalline and chemical bonding of acid-treated MWCNT. These results demonstrate that the MWCNT-EGFETs are very promising for the applications in the pH and biomedical sensors.

High-performance polycrystalline silicon thin-film transistors with two-dimensional location control of the grain boundary via excimer laser crystallization.

High-performance low-temperature polycrystalline silicon (Poly-Si) thin-film transistors (TFTs) have been fabricated with two-dimensional (2-D) location-controlled grain boundaries using excimer laser crystallization (ELC). By locally increased thickness of the amorphous silicon (a-Si) film that was served as the seed crystals with a partial-melting crystallization scheme, the cross-shaped grain boundary structures were produced between the thicker a-Si grids. The Poly-Si TFTs with one parallel and one perpendicular grain boundary along the channel direction could therefore be fabricated to reach excellent field-effect mobility of 530 cm2/V-s while the conventional ones exhibited field-effect mobility of 198 cm2/V-s. Furthermore, the proposed TFTs achieved not only superior electric properties but also improved uniformity as compared with the conventional ones owing to the artificially controlled locations of grain boundaries.

Effect of the annealing ambient on the electrical characteristics of the amorphous InGaZnO thin film transistors.

The influence of the thermal annealing on the amorphous indium gallium zinc oxide (a-IGZO) thin-film transistors (TFTs) under different ambient gases has been systematically addressed. The chemical bonding states and transfer characteristics of a-IGZO TFTs show evident dependence on the annealing ambient gas. For the a-IGZO TFTs in the oxygen ambient annealing at 250 degrees C for 30 mins exhibited a maximum field effect mobility (max muFE) of 9.36 cm2/V x s, on/off current ratio of 6.12 x 10(10), and a subthreshold slope (SS) of 0.21 V/decade. Respectively, the as-deposited ones without annealing possess a max muFE of 6.61 cm2/V x s, on/off current ratio of 4.58 x 10(8), and a SS of 0.46 V/decade. In contrast, the a-IGZO TFTs annealed at 250 degrees C for 30 mins in the nitrogen ambient would be degraded to have a max muFE of 0.18 cm2/V x s, on/off current ratio of 2.22 x 10(4), and a SS of 7.37 V/decade, corresponding. It is attributed to the content of the oxygen vacancies, according the x-ray photoelectron spectroscopy (XPS) analyze of the three different samples.

Field emission properties of carbon nanotube arrays on the thickness-controlled flexible substrate by the pattern transfer process.

A technigal with the polydimethylsiloxane (PDMS) solution infiltrated into the SiOx-coated CNTAs has been utilized to directly transfer the CNTAs away from the silicon substrate. The oxide coating layer was utilized to protect the morpholgy of as-grown patterned vertical aligmed carbon nanotube (CNTs) arrays. The high density plasma reactive ions etching (HDP-RIE) system was used to make the CNTs emerge from the surface of the flexible substrate and modify the crystallines of CNTs. After the protecting oxide was HDP-RIE-processed for 8 min, the emission current properties were enhanced to be 1.03 V/microm and 1.43 V/microm, respectively, for the turn-on field and the threshold field, as compared with 1.25 V/microm and 1.59 V/microm for the as-grown CNTs, accordingly. The Field Emission (FE) enhancement after dry etching could be attributed to the open-ended structures and better crystalline.

High-performance ZnO thin-film transistors with location-controlled crystal grains fabricated by low-temperature hydrothermal method.

In this paper, high-performance bottom-gate (BG) thin-film transistors (TFTs) with zinc oxide (ZnO) artificially location-controlled lateral grain growth have been prepared via low-temperature hydrothermal method. For the proper design of source/drain structure of ZnO/Ti/Pt thin films, the grains can be laterally grown from the under-cut ZnO beneath the Ti/Pt layer. Consequently, the single one vertical grain boundary perpendicular to the current flow will be produced in the channel region as the grown grains from the source/drain both sides are impinged. As compared with the conventional sputtered ZnO BG-TFTs, the proposed location-controlled hydrothermal ZnO BG-TFTs (W/L = 250 microm/10 microm) demonstrated the higher field-effect mobility of 6.09 cm2/V x s, lower threshold voltage of 3.67 V, higher on/off current ratio above 10(6), and superior current drivability, reflecting the high-quality ZnO thin films with less grain boundary effect in the channel region.

The effects of nanometal-induced crystallization on the electrical characteristics of bottom-gate poly-Si thin-film transistors.

The effects of active layer thickness and device dimensions on nanometal-induced crystallization (nano-MIC) were studied to determine the electrical characteristics of the polycrystalline silicon (poly-Si) thin-film transistors (TFTs) with bottom-gate structures. The nano-MIC poly-Si film was obtained via deposition of a 0.4-nm-thick Ni film on the amorphous silicon layer and subsequent annealing at 550 degrees C for 0.5 to 8 h. The EDS revealed a approximately 0.1% Ni concentration in the poly-Si film. The cross-sectional TEM image shows the vertical-grain growth mechanism, where the bottom side of the grain exhibits a larger crytalline area than the top side. Therefore, the field effect mobility of the bottom-gate poly-Si TFTs increases with increased active-amorphous-silicon (a-Si) thickness. Furthermore, the mobility increases when the device dimensions are scaled down. A mechanism for explaining such phenomenon in relation to the nano-MIC bottom-gate poly-Si TFTs was also proposed.

Effect of oxygen annealing on the ultraviolet photoresponse of P-NiO-nanoflower/n-ZnO-nanowire heterostructures.

A transparent ultraviolet (UV) sensor using nanoheterojunctions (NHJs) composed of p-type NiO nanoflowers (NFs) and n-type ZnO nanowires (NWs) was prepared through a sequential low-temperature hydrothermal-growth process. The devices that were annealed in an oxygen (O2) ambient exhibited better rectification behavior (I forward/I reverse = 427), a lower forward threshold voltage (V(th) = 0.98 V), a lower leakage current (1.68 x 10(-5) A/cm2), and superior sensitivity (I uv/I dark = 57.8; I visible/I dark = 1.25) to UV light (lambda = 325 nm) than the unannealed devices. The remarkably improved device performances and optoelectronic characteristics of the annealed p-NiO-NF/n-ZnO-NW NHJs can be associated with their fewer structural defects, fewer interfacial defects, and better crystallinity. A stable and repeatable operation of dynamic photoresponse was also observed in the annealed devices. The excellent sensitivity and repeatable photoresponse to UV light of the hydrothermally grown p-NiO-NF/n-ZnO-NW NHJs annealed in a suitable O2 ambient indicate that they can be applied to nano-integrated optoelectronic devices.

Field-emission characteristics of Al-doped ZnO nanostructures hydrothermally synthesized at low temperature.

The aluminum-doped ZnO (AZO) nanostructures with different Al concentrations were synthesized on AZO/glass substrate via a simple hydrothermal growth method at a temperature as low as 85 degrees C. The morphologies, crystallinity, optical emission properties, and chemical bonding states of AZO nanostructures show evident dependence on the aluminum dosage. The morphologies of AZO nanostructures were changed from vertically aligned nanowires (NWs), and NWs coexisted with nanosheets (NSs), to complete NSs in respect of the Al-dosages of 0-3 at.%, 5 at.%, and 7 at.%, correspondingly. The undoped ZnO and lightly Al-doped AZO (< or = 3 at.%) NWs are single-crystalline wurtzite structure. In contrast, heavily Al-doped AZO sample is polycrystalline. The AZO nanostructure with 3 at.% Al-dosages reveals the optimal crystallinity and less structural defects, reflecting the longest carrier lifetime and highest conductivity. Consequently, the field-emission characteristics of such an AZO emitter can exhibit the higher current density, larger field-enhancement factor (beta) of 3131, lower turn-on field of 2.17 V/microm, and lower threshold field of 3.43 V/microm.

Study on fatigue and breakdown properties of Pt/(Pb,Sr)TiO(3)/Pt capacitors.

Pulsed-laser deposited (Pb,Sr)TiO(3) (PSrT) films on Pt/SiO(2)/Si substrate at various ambient oxygen pressures (P(O(2))) are investigated in this work. Films deposited at P(O(2)) below 100 mTorr exhibit the (100) preferred orientation and a tetragonal structure with larger tetragonality. In addition, films deposited at 80 mTorr exhibit the most apparent ferroelectric properties in contrast to those deposited at 200 mTorr. Moreover, films deposited at higher P(O(2)) also exhibit longer lifetimes and higher breakdown fields due to their smaller leakage current density, in terms of the reduction of defects, compensation of oxygen vacancies (OVs), an improved interface and small cluster sizes. An energy band model reveals that fatigue properties of PSrT films are dominated by interfacial states at low P(O(2)) and by deep trapping states at high P(O(2)), which could be ascribed to OVs located at the interfaces and inside films, respectively.