Recursos de colección
Sapientia Repositório Institucional Universidade do Algarve (30.790 recursos)
O Repositório Institucional da Universidade do Algarve desenvolvido no âmbito do projecto RCAAP.
O Repositório Institucional da Universidade do Algarve desenvolvido no âmbito do projecto RCAAP.
Lancaster, J; Taylor, DM; Sayers, P; Gomes, Henrique L.
Capacitance-voltage (C-V) measurements have been undertaken on metal-insulator- semiconductor capacitors fabricated using the high-k material aluminium titanium oxide (ATO) as the dielectric and regio-regular poly(3-hexylthiophene), (P3HT), as the active semiconductor layer. When the gate voltage is cycled between accumulation and depletion in the dark, the C-V plots undergo a large, reversible, anticlockwise hysteresis consistent with the reversible trapping of holes at the insulator-semiconductor interface. Evidence is also obtained that electron injection from the gold electrode into the semiconductor is triggered under positive bias and may be responsible for neutralization of the trapped holes. When illuminated with light of energy greater...
Stallinga, Peter; Gomes, Henrique L.
The effects of metal contacts on the electrical characteristics in thin-film transistors are discussed. It is found that the effects of these contacts are twofold. First, a constant potential that can range from zero to some volts (half the bandgap) is added to the entire channel. Second, a residual barrier is formed with a height that depends on the bias, and is in the order of tens of meV when a current is present. It is shown that these predicted effects are in agreement with experimental observations. (c) 2006 Elsevier B.V. All rights reserved.
Gomes, Henrique L.; Taylor, D. M.
Schottky barrier diodes based on Al/poly(3-methylthiophene)/Au have been fabricated and their electrical behaviour investigated. I-V characteristics revealed a dependence on the fabrication conditions, specifically on the time under vacuum prior to evaporation of the rectifying contact and post-metal annealing at elevated temperature. The available evidence is consistent with the formation of a thin insulating layer between the metal and the polymer following these procedures. Long periods under vacuum prior to deposition of the aluminium electrode reduced the likelihood of such a layer forming. Capacitance-voltage plots of the devices were stable to voltage cycling, so long as the forward voltage did...
Pereira, L.; Rodrigues, A.; Gomes, Henrique L.; Pereira, E.
The present work reports some experimental results on the electrical AC behaviour of metal-undoped diamond Schottky diodes fabricated with a free-standing MPCVD diamond film (5 mum thick). The metals are gold for the ohmic contact and aluminium for the rectifier. The capacitance and loss tangent vs, frequency shows that capacitance presents a relaxation maximum at frequencies near 10 kHz at room temperature. Although the simple model (small equivalent circuit) can justify the values for the relaxation, it cannot justify the departure from the Debye model, also verified in the Cole-Cole plot. Taking into account the existence of traps in the...
Gomes, Henrique L.; Stallinga, Peter; Murgia, M; Biscarini, F; Muck, T; Wagner, V; Smits, E; De Leeuw, DM
The electrical stability of metal insulator semiconductor (MIS) capacitors and field effect transistor structures based in organic semiconductors were investigated. The device characteristics were studied using steady state measurements AC admittance measurements as well as techniques for addressing trap states. Temperature-dependent measurements show clear evidence that an electrical instability occurs above 200 K. and is caused by an electronic trapping process. It is suggested that the trapping sites are created by a change in the organic conjugated chain, a process similar to a phase transition.
Mathijssen, SGJ; Colle, M; Gomes, Henrique L.; Smits, ECP; de Boer, B; McCulloch, I; Bobbert, PA; de Leeuw, DM
The electrical instability of organic field-effect transistors is investigated. We observe that the threshold-voltage shift (see figure) shows a stretched-exponential time dependence under an applied gate bias. The activation energy of 0.6 eV is common for our and all other organic transistors reported so far. The constant activation energy supports charge trapping by residual water as the common origin.
Kiazadeh, A; Rocha, PR; Chen, Q; Gomes, Henrique L.
It is demonstrated that planar structures based on silver nanoparticleshosted in a polymer matrix show reliable and reproducible switching properties attractive for non-volatile memory applications. These systems can be programmed between a low conductance (off-state) and high conductance (on-state) with an on/off ratio of 3 orders of magnitude, large retention times and good cycle endurance. The planar structure design offers a series of advantages discussed in this contribution, which make it an ideal tool to elucidate the resistive switching phenomena.
Pereira, L; Gomes, Henrique L.; Rodrigues, A; Pereira, E
The interesting properties of polycrystalline diamond thin film have led to an increasing amount of research on this material, especially on its electrical applications. In this work, diodes based on free-standing Al/diamond/Au structures are fabricated using diamond grown on silicon and copper substrates, and their DC and AC electrical behavior investigated. Studies of the frequency dependence of the impedance of the diodes allow modeling of the diodes by an equivalent circuit. A tentative explanation of the experimentally observed behavior due to different defects and structures is made, in order to establish the different transport phenomena.
Stallinga, Peter; Gomes, Henrique L.; Charas, A.; Morgado, J.; Alcacer, L.
Poly(phenylene vinylene) (PPV) grown via the precursor route, deposited on top of heavily doped n-type silicon, was studied using electrical measurement techniques. The results are compared to PPV grown via deposition of soluble derivative (MEH-PPV). The two types are very similar. They have comparable free carrier densities and both show minority-carrier effects. The activation energy found via the loss tangent is 0.13 eV. The effect of exposure to oxygen is visible in the capacitance and the current.
Jones, GW; Taylor, DM; Gomes, Henrique L.
Deep Level Transient Spectroscopy (DLTS) has been used to investigate hole traps in the depletion region of Schottky barrier diodes formed from electropolymerised poly(3-methylthiophene). The capacitance transients appear to be composed of a fast and a slow component Analysis of the slower component using the "rate window" technique yields isochronal differential capacitance curves that depend on temperature in the manner predicted by theory. © 1996 The Institution of Electrical Engineers. Printed and Published by the IEE.
Verbakel, F; Meskers, SCJ; Janssen, RAJ; Gomes, Henrique L.; Coelle, M; Buechel, M; de Leeuw, DM
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 10(9) V/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...
Pereira, L.; Pereira, E.; Gomes, Henrique L.
The electrical and photoconductive features of as-grown microwave-plasma-assisted chemical-vapour deposition (MPCVD) diamond films are studied in correlation with magnetic results obtained from electron paramagnetic resonance (EPR). Also, the morphology is analysed by atomic force microscopy (AFM) showing  crystals with a good uniformity of the deposit. The photoresponse as well the current-voltage features observed show an efficient photogeneration of carriers while the optoelectronic characteristics of the metal-diamond junction have an ideality factor of 1.6 together with a rectification ratio of about 10(4) at +/-2.5 V. The nature of the mechanisms responsible for the conduction is discussed. (C) 1998 Elsevier Science...
Stallinga, Peter; Gomes, Henrique L.; Murgia, M.; Mullen, K.
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 x 10(12). cm(-2)eV(-1) close to the valence band which decreases slightly towards midgap. Additional do measurements show that the semiconductor bulk activation energy is 0.33 eV which may correspond to an acceptor level. (C) 2002 Elsevier Science B.V. All rights reserved.
Muck, T; Leufgen, M; Lebib, A; Borzenko, T; Geurts, J; Schmidt, G; Molenkamp, LW; Wagner, V; Gomes, Henrique L.
We present organic field-effect transistors with dihexylquaterthiophene (DH4T) as active material, a derivative of the oligothiophene α-4T with two hexyl chains as end groups. This substitution makes this molecule suitable not only for vacuum sublimation but also for solution processing which enables cheaper production. Additionally, the layer ordering is improved. We compare vacuum deposited and solution processed OFETs based on DH4T. The former ones show nearly ideal I-V characteristics. The latter ones show deviations from ideal behavior and lower currents. Furthermore, temperature dependent measurements of drain-source current in vacuum deposited DH4T-OFETs shows an unusual variation of the OFET apparent mobility....
Stallinga, P.; Gomes, Henrique L.; Jones, G. W.; Taylor, D. M.
Schottky diodes resulting from an intimate contact of aluminum on electrodeposited poly(3-methylthiopene) were studied by admittance spectroscopy, capacitance-voltage measurements and voltaic and optically-induced current and capacitance transients. The loss tangents show the existence of interface states that can be removed by vacuum annealing. Furthermore, the C-V curves contradict the idea of movement of the dopant ions.
Gomes, Henrique L.; Leite, R. B.; Afonso, R.; Stallinga, Peter; Cancela, Leonor
An impedance method was developed to determine how immune system cells (hemocyte) interact with intruder cells (parasites). When the hemocyte cells interact with the parasites, they cause a defensive reaction and the parasites start to aggregate in clusters. The level of aggregation is a measure of the host-parasite interaction, and provides information about the efficiency of the immune system response. The cell aggregation is monitored using a set of microelectrodes. The impedance spectrum is measured between each individual microelectrode and a large reference electrode. As the cells starts to aggregate and settle down towards the microelectrode array the impedance of...
Rocha, PRF; Gomes, Henrique L.; Asadi, K; Katsouras, I; Bory, B; Verbakel, F; van de Weijer, P; de Leeuw, DM; Meskers, SCJ
The degradation in light output of an Organic Light Emitting Diode (OLED) has been studied extensively and has been explained by different mechanisms, such as formation of chemical defects or electrical traps and by thermally induced inter-diffusion of dopants. However, there is an overlooked type of degradation, where the light output decreases rapidly with time. This catastrophic failure can often be attributed to a hard electrical short due to local defects. Here, we show that this "sudden death" can also occur in the absence of a hard electrical short. We investigate this phenomenon by current-voltage characteristics and small-signal impedance measurements...
Bory, B. F.; Rocha, P. R. F.; Janssen, R. A. J.; Gomes, Henrique L.; de Leeuw, D. M.; Meskers, S. C. J.
Thin LiF interlayers are typically used in organic light-emitting diodes to enhance the electron injection. Here, we show that the effective work function of a contact with a LiF interlayer can be either raised or lowered depending on the history of the applied bias. Formation of quasi-Ohmic contacts for both electrons and holes is demonstrated by electroluminescence from symmetric LiF/polymer/LiF diodes in both bias polarities. The origin of the dynamic switching is charging of electrically induced Frenkel defects. The current density-electroluminescence-voltage characteristics can qualitatively be explained. The interpretation is corroborated by unipolar memristive switching and by bias dependent reflection measurements....
Gomes, Henrique L.; Stallinga, Peter; Rost, H.; Holmes, A. B.; Harrison, M. G.; Friend, R. H.
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. (C) 1999 American Institute of Physics. [S0003-6951(99)02308-6].
Bentes, E.; Gomes, Henrique L.; Stallinga, Peter; Moura, L.
Field effect transistors (FETs) based on organic materials were investigated as sensors for detecting 2,4,6-trinitrotoluene (TNT) vapors. Several FET devices were fabricated using two types of semiconducting organic materials, solution processed polymers deposited by spin coating and, oligomers (or small molecules) deposited by vacuum sublimation. When vapors of nitroaromatic compounds bind to thin films of organic materials which form the transistor channel, the conductivity of the thin film increases and changes the transistor electrical characteristic. The use of the amplifying properties of the transistor represents a major advantage over conventional techniques based on simple changes of resistance in polymers frequently...