Naser Sedghi

Conduction processes in conjugated, highly regio-regular, high molecular mass, poly(3-hexylthiophene) thin-film transistors

Highly regio-regular poly(3-hexylthiophenes) (P3HT) thin films have been produced using the Tzrnadel method. They have head to tail counts approaching very close to 100% and high molecular mass. Thin-film transistors and Schottky diodes have been used to study the effects of counter ion and carrier density on field-effect and bulk mobility, respectively. The density of counter ions was increased using 2,3-dichloro-5,6-dicyano-1,4-benzoquinone (DDQ) and had a profound effect on both the field effect and bulk mobility with a ratio between the two of 102, in a similar way to DDQ in poly(β′-dodecyloxy-α,α′,-α′,α″terthienyl) (polyDOT3), but with substantially higher drift mobilities. A method is described, using Schottky barriers, of simply and accurately determining carrier drift mobility. The effect of carrier density on hole mobility is believed to be indirect, filling traps in the regions separating the highly ordered domains until trap free conduction and hence high mobility is reached.

The properties of MOS structures using conjugated polymers as the semiconductor

Data is presented from electrical measurements on MOS structures using the conjugated polymer regioregular poly-3-hexyl-thiophene (P3HT), a derivative of polythiophene. Improvement of the molecular mass and chain length is achieved using a process of fractionation. This is expected to produce an improvement in the electronic properties of the polymer. Comparison between fractionated and unfractionated P3HT on both MOS and TFT devices is presented. Metal contacts to P3HT are also studied with the formation of Schottky barriers readily observed. Current is found to fit standard theory and allows calculation of dopant levels and barrier heights all of which show good agreement with the ideal theory.

'Ge interface engineering using ultra-thin La2O3 and Y2O3 films: A study into the effect of deposition temperature'

A study into the optimal deposition temperature for ultra-thin La2O3/Ge and Y2O3/Ge gate stacks has been conducted in this paper with the aim to tailor the interfacial layer for effective passivation of the Ge interface. A detailed comparison between the two lanthanide oxides (La2O3 and Y2O3) in terms of band line-up, interfacial features, and reactivity to Ge using medium energy ion scattering, vacuum ultra-violet variable angle spectroscopic ellipsometry (VUV-VASE), X-ray photoelectron spectroscopy, and X-ray diffraction is shown. La2O3 has been found to be more reactive to Ge than Y2O3, forming LaGeOx and a Ge sub-oxide at the interface for all deposition temperature studied, in the range from 44 °C to 400 °C. In contrast, Y2O3/Ge deposited at 400 °C allows for an ultra-thin GeO2 layer at the interface, which can be eliminated during annealing at temperatures higher than 525 °C leaving a pristine YGeOx/Ge interface. The Y2O3/Ge gate stack deposited at lower temperature shows a sub-band gap absorption f...

Critical Considerations in Polymer Thin-Film Transistor (TFT) Dielectrics

We present a study of aqueous and plasma anodised aluminium oxide (Al2O3) and its performance in thin film transistors (TFTs). The current through the oxide was measured with aluminium electrodes and with one of the electrode replaced by poly(3-hexylthiophene)(P3HT). The current increased by up to 2 orders of magnitude with P3HT. The current increased further when the polymer was doped with different percentages of 2,3-dichloro-5,6-dicyano-1,4-benzoquinone (DDQ). It was also found to be dependent on the thickness of the polymer film. Surprisingly, the oxide current fell to its initial value when the polymer film was removed. Two mechanisms may explain the behaviour in these devices: charge injection and/or displacement. C-V plots were obtained from the MOS capacitors and were frequency dependent. They also showed substantial hysteresis, with a lateral shift along the voltage axis. This indicates the presence of a mobile species that increases with the concentration of dopant. We deduce that much of the increased gate current is associated with displacement currents induced by ion motion.

'On the nature of the interfacial layer in ultra-thin TiN/LaLuO3 gate stacks'

We present a detailed investigation on the nature of the interfacial layer (IL) in ultra-thin TiN/LaLuO3 (LLO) gate stacks, which is of importance to facilitate CMOS scaling. The molecular beam deposited LaLuO3 films are found to be amorphous by high-resolution transmission electron microscopy. A similar to 9 angstrom thick LaLuO3/interlayer transition observed by medium energy ion scattering correlates with the presence of a dual silicate/SiO2-like interfacial layer derived from the analysis of photoelectron line positions and electron energy loss spectra. A theoretical model is used for the dielectric transition in a bi-layer LaLuO3/IL structure, linking physical and electrical characterization data. The obtained leakage current of 10(-3) A/cm(2) at 1.5 V and equivalent oxide thickness of 0.75 nm for TiN/LaLuO3 gate stacks are adequate for scaling in the 14-12 nm node.

Reliability studies on Ta2O5 high-κ dielectric metal-insulator-metal capacitors prepared by wet anodization

The temperature dependence of leakage current for tantalum oxide metal-insulator-metal capacitors has been investigated over the temperature range 20–160 °C. The leakage current shows an increase with temperature and the conduction mechanism at medium to high electric fields is in agreement with the modified Poole–Frenkel model. The activation energy of the dominant deep trapping center in the oxide is calculated using this model. Constant voltage and constant current stress have been applied to the devices and the effect of stress conditions on leakage current, breakdown voltage, and high frequency capacitance-voltage have been investigated. Early oxide breakdown or time-dependent dielectric breakdown was observed during constant voltage and constant current stress, in which the former is a function of stress time and applied voltage or current. There is an increase in leakage current with time during the constant voltage stress, presumably due to generation of positive defect states. This is also appare...

Functionalized Regioregular Polyalkylthiophenes for Biosensing Applications

A regioregular copolymer of 3–hexylthiophene and 3–(6–hydroxyhexan–1yl)thiophene has been functionalised with biotin hydrazide; binding of avidin to the biotin moieties causes drastic changes to the absorption spectrum of the polymer in solution, and to the electrochemistry and conductivity of the polymer in thin films.

The role of nitrogen doping in ALD Ta2O5 and its influence on multilevel cell switching in RRAM

The role of nitrogen doping on the stability and memory window of resistive state switching in N-doped Ta2O5 deposited by atomic layer deposition is elucidated. Nitrogen incorporation increases the stability of resistive memory states which is attributed to neutralization of electronic defect levels associated with oxygen vacancies. The density functional simulations with the screened exchange hybrid functional approximation show that the incorporation of nitrogen dopant atoms in the oxide network removes the O vacancy midgap defect states, thus nullifying excess defects and eliminating alternative conductive paths. By effectively reducing the density of vacancy-induced defect states through N doping, 3-bit multilevel cell switching is demonstrated, consisting of eight distinctive resistive memory states achieved by either controlling the set current compliance or the maximum voltage during reset. Nitrogen doping has a threefold effect: widening the switching memory window to accommodate the more intermed...

CV measurements on LaLuO3 stack metal-oxide-semiconductor capacitor using a new three-pulse technique

A new three-pulse CV measurement technique has been developed to investigate the trapping and detrapping of negative and positive charges in SiO2/LaLuO3 gate dielectric stacks on p-type silicon. Two types of negative and positive trapped charges have been observed in these devices which are deemed to be related to electron and hole trapping, respectively. The technique has the advantage that trapping and detrapping of both types of charges can be measured independently. The concentrations of trapped charge types, their release times, and their relationship with measurement parameters such as pulse charging/discharging time and pulse amplitude have been investigated. A logarithmic universal relationship was found between the flat-band voltage shifts due to detrapping of positive charges with the discharging time.

Electron trapping at the high-κ/GeO2 interface: The role of bound states

Passivation of the germanium interface using native oxide results in a potential well at the interface of GeO2 and high-κ dielectric for n-channel metal-oxide-semiconductor field effect transistors. The well forms a potential trapping site for electrons, which could result in threshold voltage instability. We present calculations for the bound states and their occupancy in the well. The significance of threshold voltage shift in an 8-nm node field effect transistor and those with larger dimensions is investigated. The main conclusion is that charge storage in the well is insignificant at the 8-nm technology node but can have a significant effect on larger devices.