Low-operating-voltage polymer thin-film transistors based on poly(3-hexylthiophene) with hafnium oxide as the gate dielectric

The effects of hafnium oxide $(hbox{HfO}-{2})$ gate dielectric annealing treatment in oxygen $(hbox{O}-{2})$ and ammonia $(hbox{NH}-{3})$ ambient on the electrical performance of polymer thin-film transistors (PTFTs) based on poly(3-hexylthiophene) are investigated. The PTFTs with $hbox{HfO}-{2}$ gate dielectric and also octadecyltrichlorosilane surface modification, prepared by spin-coating process, exhibit good performance, such as a small threshold voltage of $-$0.5 V and an operating voltage as low as $-$4 V. Results indicate that the PTFT with $hbox{NH}-{3}$-annealed $hbox{HfO}-{2}$ shows higher carrier mobility, larger on/off current ratio, smaller subthreshold swing, and lower threshold voltage than the PTFT with $hbox{O}-{2}$-annealed $hbox{HfO}-{2}$. Capacitancevoltage analysis for metal-polymer-oxide-silicon structures indicates that the better electrical performance of the PTFT with $hbox{NH}-{3}$ -annealed $hbox{HfO}-{2}$ is attributed to improved dielectric/polymer interface and reduced series resistance in the transistor. © 2006 IEEE.published_or_final_versio

Bias-stress-induced instability of polymer thin-film transistor based on poly(3-hexylthiophene)

A polymer thin-film transistor (PTFT) based on poly(3-hexylthiophene) (P3HT) is fabricated by a spin-coating process and characterized. Its bias-stress-induced instability during operation is investigated as a function of time and temperature. For negative gate-bias stress, the carrier mobility remains unchanged, the off-state current decreases, and the threshold voltage shifts toward the negative direction. On the other hand, for negative drain-bias stress, the carrier mobility decreases slightly, the off-state current increases, and the threshold voltage shifts toward the positive direction. The threshold shifts under gate- and drain-bias stresses are observed to be logarithmically dependent on time, and the decay rate of the threshold-voltage shift is independent of temperature. The results suggest that the origin of the threshold-voltage shift upon negative gate-bias stress is predominantly associated with holes trapped within the SiO 2 gate dielectric or at the P3HT/SiO 2 interface, while time-dependent charge trapping in the deep trap states and creation of defect states in the channel region are responsible for the drain-bias stress effect on the PTFT. © 2011 IEEE.published_or_final_versio

Improved performance of pentacene OTFTs with HfLaO gate dielectric by using fluorination and nitridation

Pentacene organic thin-film transistors (OTFTs) with fluorinated high-κ HfLaO as gate insulator were fabricated. The dielectrics were prepared by sputtering method and then annealed in N 2 or NH 3 at 400 °C. Subsequently, the dielectrics were treated by fluorine plasma for different durations (100, 300, and 900 s). The N 2 and NH 3-annealed OTFTs with a 100-s plasma treatment achieve a carrier mobility of 0.62 and 0.66 cm 2V̇s, respectively, which are higher than those of the OTFTs without plasma treatment (0.22 and 0.41 cm 2V̇s). Moreover, the plasma-treated OTFTs realize better 1/f noise characteristics than those without plasma treatment. The improved performance is due to passivation of the dielectric surface by plasma-induced fluorine incorporation. However, for longer time (300 and 900 s) of plasma treatment, the performance of the OTFTs deteriorates in terms of carrier mobility and 1/f noise characteristics due to increased plasma-induced damage of the dielectric surface. The morphology of the pentacene film grown on the HfLaO gate insulator was characterized by SEM. It reveals that the pentacene film has larger grain size and smoother surface on the HfLaO dielectric (for both annealing gases) with 100-s plasma treatment than the others (0, 300, and 900 s). Finally, AFM characterization of the HfLaO film also confirms the damaging effect of excessive plasma treatment on the dielectric. © 2012 IEEE.published_or_final_versio

Low-voltage polymer thin-film transistors with high-k HfTiO gate dielectric annealed in NH3 or N2

OTFTs with P3HT as organic semiconductor and HfTiO as gate dielectric have been studied in this work. The HfTiO dielectric film was prepared by RF sputtering of Hf and DC sputtering of Ti at room temperature. Subsequently, the dielectric film was annealed in an NH3 or N2 ambient at 200 °C. Then a layer of OTS was deposited by spin-coating method to improve the surface characteristics of the gate dielectric. Afterwards, P3HT was deposited by spin-coating method. The OTFTs were characterized by I-V measurement and 1/f noise measurement. The OTFT with gate dielectric annealed in NH3 displays higher carrier mobility, smaller threshold voltage, smaller sub-threshold swing, and lower 1/f noise level than the OTFT annealed in N2. Moreover, the HfTiO dielectric film annealed in NH3 shows higher dielectric constant. In summary, HfTiO film annealed in NH 3 at low temperature is a promising candidate to act as the gate dielectric of high-quality low-voltage OTFTs. ©2009 IEEE.published_or_final_versionThe IEEE International Conference of Electron Devices and Solid-State Circuits (EDSSC 2009), Xi'an, China, 25-27 December 2009. In Proceedings of EDSSC, 2009, p. 201-20

β-Ga2O3 solar-blind deep-ultraviolet photodetector based on a four-terminal structure with or without Zener diodes

A four-terminal photodetector was fabricated on the (2⎯⎯⎯012¯01)-dominant β-Ga2O3 thin film which was deposited in a plasma-assisted molecular beam epitaxy system. The suitability of this film for solar-blind DUV detection was proved by its transmission spectra. Moreover, the device operating in a specific voltage-current mode can accurately detect the DUV radiation both qualitatively and quantitatively. Accordingly, a dark/photo voltage ratio of 15 was achieved, which is comparable to that of previously-reported β-Ga2O3 interdigital metal-semiconductor-metal photoconductor. More importantly, the aperture ratio of our proposed device exceeds 80%, nearly doubling that of the conventional interdigital metal-semiconductor-metal devices including photoconductor and Schottky-type photodiode, which can intensively benefit the detection efficiency. Furthermore, it was found the dark/photo voltage ratio was nearly trebled with the assistance of two Zener diodes, and further enhancement can be expected by increasing the operating current and/or adopting Zener diodes with smaller Zener voltage. Therefore, this work provides a promising alternative for solar-blind DUV detection.published_or_final_versio

Improved electrical properties of metal-oxide-semiconductor capacitor with HfTiON gate dielectric by using HfSiON interlayer

Metal-oxide-semiconductor (MOS) capacitor with HfTiONHfSiON stack structure as high- k gate dielectric is fabricated, and its electrical properties are compared with those of a similar device with HfTiON only as gate dielectric. Experimental results show that the device with HfTiONHfSiON gate dielectric exhibits better interface properties, lower gate leakage current, and enhanced high-field reliability. All these improvements should be attributed to the fact that the HfSiON buffer layer effectively blocks the diffusion of Ti atoms to the Si substrate, thus resulting in a Si O2 Si -like HfSiONSi interface. © 2007 American Institute of Physics.published_or_final_versio

Effects of annealing temperature and gas on pentacene OTFTs with HfLaO as gate dielectric

Pentacene organic thin-film transistors (OTFTs) with high-κ HfLaO as gate insulator were fabricated. HfLaO film was prepared by sputtering method. To improve the film quality, the dielectric was annealed in N 2, NH 3, or O 2 at two temperatures, i.e., 200 °C and 400 °C, respectively. The I-V characteristics of the OTFTs and C-V characteristics of corresponding organic capacitors were measured. The OTFTs could operate at a low operating voltage of below 5 V, and the dielectric constant of the HfLaO film could be above ten. For all the annealing gases, the OTFTs annealed at 400 °C achieved higher carrier mobility than their counterparts annealed at 200 °C (with the one annealed in NH 3 at 400 °C showing the highest carrier mobility of 0.45 cm 2/ V·s), which could be supported by SEM images which indicate that pentacene tended to form larger grains on HfLaO annealed at 400 °C than on that annealed at 200 °C. The C-V measurement of the organic capacitors indicated that the localized charge density in the organic semiconductor/oxide was lower for the 400 °C annealing than for the 200 °C annealing. Furthermore, through the characterization of gate current leakage, HfLaO film annealed at 400 °C achieved much smaller leakage than that annealed at 200 °C. Since the maximum processing temperature of ITO glass substrates is around 400 °C , this study shows that 400 °C is suitable for the annealing of HfLaO film in high-performance OTFTs on glass substrate. © 2011 IEEE.published_or_final_versio

Effects of different annealing gases on pentacene OTFT with HfLaO gate dielectric

Pentacene organic thin-film transistors (OTFTs) with HfLaO high-kappa gate dielectric were fabricated. The dielectric was prepared by a sputtering method and then annealed in N2,NH3,O2, or NO at 400°C. The carrier mobility of the NH3-annealed OTFT could reach 0.59 cm2/V̇s, which is higher than those of the other three devices. Moreover, the NH3-annealed OTFT obtained the smallest subthreshold swing of 0.26 V/dec among them. Furthermore,1/f noise measurement indicated that the NH3-annealed OTFT achieved the smallest 1/f noise. All these should be attributed to the improved interface between the gate dielectric and the organic semiconductor associated with the passivation effects of the NH3 annealing on the dielectric surface. © 2010 IEEE.published_or_final_versio

The effectiveness of the 13-valent pneumococcal conjugate vaccine against hypoxic pneumonia in children in Lao People's Democratic Republic: An observational hospital-based test-negative study

Background: Pneumococcal pneumonia is a leading cause of childhood mortality. Pneumococcal conjugate vaccines (PCVs) have been shown to reduce hypoxic pneumonia in children. However, there are no studies from Asia examining the effectiveness of PCVs on hypoxic pneumonia. We describe a novel approach to determine the effectiveness of the 13-valent PCV (PCV13) against hypoxia in children admitted with pneumonia in the Lao People's Democratic Republic. Methods: A prospective hospital-based, test-negative observational study of children aged up to 59 months admitted with pneumonia to a single tertiary hospital in Vientiane was undertaken over 54 months. Pneumonia was defined using the 2013 WHO definition. Hypoxia was defined as oxygen saturation <90% in room air or requiring oxygen supplementation during hospitalisation. Test-negative cases and controls were children with hypoxic and non-hypoxic pneumonia, respectively. PCV13 status was determined by written record. Vaccine effectiveness was calculated using logistic regression. Propensity score and multiple imputation analyses were used to handle confounding and missing data. Findings: There were 826 children admitted with pneumonia, 285 had hypoxic pneumonia and 377 were PCV13-vaccinated. The unadjusted, propensity-score adjusted and multiple-imputation adjusted estimates of vaccine effectiveness against hypoxic pneumonia were 23% (95% confidence interval: -9, 46%; p=0•14); 37% (6, 57%; p=0•02) and 35% (7, 55%; p=0•02) respectively. Interpretation: PCV13 is effective against hypoxic pneumonia in Asia, and should be prioritised for inclusion in national immunisation programs. This single hospital-based, test-negative approach can be used to assess vaccine effectiveness in other similar settings. Funding: Funded by the Bill & Melinda Gates Foundation

Investigations on DNA damage and frequency of micronuclei in occupational exposure to electromagnetic fields (EMFs) emitted from video display terminals (VDTs)

The potential effect of electromagnetic fields (EMFs) emitted from video display terminals (VDTs) to elicit biological response is a major concern for the public. The software professionals are subjected to cumulative EMFs in their occupational environments. This study was undertaken to evaluate DNA damage and incidences of micronuclei in such professionals. To the best of our knowledge, the present study is the first attempt to carry out cytogenetic investigations on assessing bioeffects in personal computer users. The study subjects (n = 138) included software professionals using VDTs for more than 2 years with age, gender, socioeconomic status matched controls (n = 151). DNA damage and frequency of micronuclei were evaluated using alkaline comet assay and cytochalasin blocked micronucleus assay respectively. Overall DNA damage and incidence of micronuclei showed no significant differences between the exposed and control subjects. With exposure characteristics, such as total duration (years) and frequency of use (minutes/day) sub-groups were assessed for such parameters. Although cumulative frequency of use showed no significant changes in the DNA integrity of the classified sub-groups, the long-term users (> 10 years) showed higher induction of DNA damage and increased frequency of micronuclei and micro nucleated cells