Henrique L. Gomes

Bias-induced threshold voltages shifts in thin-film organic transistors

An investigation into the stability of metal-insulator-semiconductor (MIS) transistors based on α-sexithiophene is reported. In particular, the kinetics of the threshold voltage shift upon application of a gate bias has been determined. The kinetics follow stretched-hyperbola-type behavior, in agreement with the formalism developed to explain metastability in amorphous-silicon thin-film transistors. Using this model, quantification of device stability is possible. Temperature-dependent measurements show that there are two processes involved in the threshold voltage shift, one occurring at T≈220 K and the other at T≈300 K. The latter process is found to be sample dependent. This suggests a relation between device stability and processing parameters.

Gate-bias stress in amorphous oxide semiconductors thin-film transistors

A quantitative study of the dynamics of threshold-voltage shifts with time in gallium-indium zinc oxide amorphous thin-film transistors is presented using standard analysis based on the stretched exponential relaxation. For devices using thermal silicon oxide as gate dielectric, the relaxation time is 3×105 s at room temperature with activation energy of 0.68 eV. These transistors approach the stability of the amorphous silicon transistors. The threshold voltage shift is faster after water vapor exposure suggesting that the origin of this instability is charge trapping at residual-water-related trap sites.

Reproducible resistive switching in nonvolatile organic memories

Resistive switching in nonvolatile, two terminal organic memories can be due to the presence of a native oxide layer at an aluminum electrode. Reproducible solid state memories can be realized by deliberately adding a thin sputtered Al2O3 layer to nominal electron-only, hole-only, and bipolar organic diodes. Before memory operation, the devices have to be formed at an electric field of 109V∕m, corresponding to soft breakdown of Al2O3. After forming, the structures show pronounced negative differential resistance and the local maximum in the current scales with the thickness of the oxide layer. The polymer acts as a current limiting series resistance.

Effect of oxygen on the electrical characteristics of field effect transistors formed from electrochemically deposited films of poly(3-methylthiophene)

Field effect devices have been formed in which the active layer is a thin film of poly(3-methylthiophene) grown electrochemically onto preformed source and drain electrodes. Although a field effect is present after electrochemical undoping, stable device characteristics with a high modulation ratio are obtained only after vacuum annealing at an elevated temperature, and only then if the devices are held in vacuo. The polymer is shown to be p type and the devices operate in accumulation only. The hole mobility in devices thermally annealed under vacuum is around 10-3 cm2 V-1 s-1. On exposure to ambient laboratory air, the device conductance increases by several orders of magnitude. This increase may be reversed by subjecting the device to a further high-temperature anneal under vacuum. Subsidiary experiments show that these effects are caused by the reversible doping of the polymer by gaseous oxygen.

Electrical characterization of the rectifying contact between aluminium and electrodeposited poly(3-methylthiophene)

A detailed investigation both of the DC and of the AC electrical properties of the Schottky barrier formed between aluminium and electrodeposited poly(3-methylthiophene) is reported. The devices show rectification ratios up to 2*104 which can be increased further after post-metal annealing. The reverse characteristics of the devices follow predictions based on the image-force lowering of the Schottky barrier, from which the doping density can be estimated. As the forward voltage increases, the device current is limited by the bulk resistance of the polymer with some evidence for injection limitation at the gold counter-electrode at high bias. In the bulk-limited regime, the device current is thermally activated near room temperature with an activation energy in the range 0.2-0.3 eV. Below about 150 K the device current is almost independent of temperature. Capacitance voltage plots obtained at frequencies well below the device relaxation frequency indicate the presence of two distinct acceptor states. A set of shallow acceptor states are active in forward bias and are believed to determine the bulk conductivity of the polymer. A set of deeper acceptors are active only for very small forward voltages and for all reverse voltages, namely when band banding causes the Fermi energy to cross these states. The density of these deeper states is approximately an order of magnitude greater than that of the shallow states. Evidence is presented also for the influence of fabrication conditions on the formation of an insulating interfacial layer at the rectifying interface. The presence of such a layer leads to inversion at the polymer surface and a modification of the I-V characteristics.

Electronic transport in field-effect transistors of sexithiophene

The electronic conduction of thin-film field-effect-transistors (FETs) of sexithiophene was studied. In most cases the transfer curves deviate from standard FET theory; they are not linear, but follow a power law instead. These results are compared to conduction models of “variable-range hopping” and “multi-trap-and-release”. The accompanying IV curves follow a Poole-Frenkel (exponential) dependence on the drain voltage. The results are explained assuming a huge density of traps. Below 200 K, the activation energy for conduction was found to be ca. 0.17 eV. The activation energies of the mobility follow the Meyer-Neldel rule. A sharp transition is seen in the behavior of the devices at around 200 K. The difference in behavior of a micro-FET and a submicron FET is shown.

Interface state mapping in a Schottky barrier of the organic semiconductor terrylene

In this work we quantitatively map interface states in energy in a Schottky barrier between aluminum and the vacuum sublimed organic semiconductor terrylene. The density map of these interface states was extracted from the admittance spectroscopy data. They revealed an interface state density of � 2 � 10 12 cm � 2 eV � 1 close to the valence band which decreases slightly towards midgap. Additional dc measurements show that the semiconductor bulk activation energy is 0.33 eV which may correspond to an acceptor level. 2002 Elsevier Science B.V. All rights reserved.

Analysis of deep levels in a phenylenevinylene polymer by transient capacitance methods

Transient capacitance methods were applied to the depletion region of an abrupt asymmetric n+−p junction of silicon and unintentionally doped poly[2-methoxy, 5 ethyl (2′ hexyloxy) paraphenylenevinylene] (MEH-PPV). Studies in the temperature range 100–300 K show the presence of a majority-carrier trap at 1.0 eV and two minority traps at 0.7 and 1.3 eV, respectively. There is an indication for more levels for which the activation energy could not be determined. Furthermore, admittance data reveal a bulk activation energy for conduction of 0.12 eV, suggesting the presence of an additional shallow acceptor state.

Electronic levels in MEH-PPV

pn-Junctions of MEH-PPV on top of heavily doped n-type silicon were used in electrical measurements. Through deep-level transient-spectroscopy (DLTS)-like measurements, four traps (two majority and two minority traps) could be identified on top of the shallow acceptor level responsible for conduction. Furthermore, evidence is found for interface states.

Voltage-and light-induced hysteresis effects at the high-k dielectric-poly(3-hexylthiophene) interface

Capacitance-voltage (C-V) measurements have been undertaken on metal-insulator-semiconductor capacitors formed from atomic-layer-deposited films of aluminium titanium oxide as the insulator and poly(3-hexylthiophene) as the insulator. Upon cycling from −30to+30V in the dark, the C-V plots show large, temperature-dependent, reversible shifts in the flatband voltage to more negative voltages consistent with reversible, shallow hole trapping at or near the insulator-semiconductor interface. When illuminated with photons of energy exceeding the polymer band gap, even larger shifts to positive voltages are observed accompanied by inversion layer formation. This latter effect has potential applications in optical sensing.