Understanding electron-withdrawing substituent effect on structural, electronic and charge transport properties of perylene bisimide derivatives

A series of n-type perylene bisimide (PBI) derivatives with electron-withdrawing substituents at both bay and imide nitrogen positions were investigated. The effects of these substituents on internal energy relaxation, molecular orbitals, air stability, electronic properties and charge transport behaviors were systematically discussed with density functional theory (DFT) which has been demonstrated reliable for organic semiconductor study. The investigated derivatives with electron-withdrawing substituents show favorable performances in terms of these properties. The LUMO levels are greatly stabilized by at least 0.3 eV and these derivatives show the strong absorption from 400 to 700 nm which match with the solar spectra very well. The charge transport mainly happens between molecules in the same organic molecular layer and electronic couplings between layer-to-layer molecules are very weak, thus the mobility along layer-to-layer direction is less efficient. The variation of molecular packings and intermolecular interactions produce the highly anisotropic mobilities. The derivative with two fluorine atoms at bay positions and –CH₂C₃F₇ at imide group has broad and strong absorption in the UV-Visible region and the electron mobility could get to 0.514 cm² V⁻¹ s⁻¹ which is greatly encouraging for molecular and material design in organic solar-cell devices. These calculated results are in good agreement with the experimental data. However, not all the functionalization with halogen substituents would induce the increase of the electronic coupling and electron mobility. The derivatives with four halogen substituents at the bay positions could not show advantages in terms of electron mobility which is induced by the distorted conjugated structures. The theoretical understanding of these n-type organic semiconductors figures out the importance of tuning the molecular geometry to get high performance semiconductor materials.     

Modeling of charge transport across disordered organic heterojunctions

Organic electronic devices often consist of a sandwich structure containing several layers of disordered organic semiconductors. In the modeling of such devices it is essential that the charge transport across the organic heterojunctions is properly described. The presence of energetic disorder and of strong gradients in both the charge density and the electric field at the heterojunction complicates the use of continuum drift-diffusion approaches to calculate the electrical current, because of the discrete positions of the sites involved in the hopping transport of charges. We use the results of three-dimensional Monte Carlo simulations to construct boundary conditions in a one-dimensional continuum drift-diffusion approach that accurately describe the charge transport across the junction. The important effects of both short- and long-range Coulomb interactions at the junction are fully accounted for. The developed approach is expected to have a general validity.     

Influence of equilibrium charge reservoir formation on photo generated charge transport in TiO2/organic devices

Charge transport measurements have been performed using the photo induced charge extraction by linearly increasing voltage (photo-CELIV) technique on indium tin oxide/titanium dioxide/poly(3-hexylthiophene):[6,6]-phenyl C61 butyric acid methyl ester/copper (ITO/TiO₂/P3HT:PCBM/Cu) devices. By adjusting the offset voltage such that holes are accumulated at the ITO/TiO₂ contact we obtain space charge limited current (SCLC) extraction in the dark. Using photo-generation the current response is limited by SCLC extraction at low carrier concentrations but becomes purely recombination limited at high photo-generated carrier concentration. A 1-D drift diffusion model has been developed to simulate our results and we show that the hole blocking ITO/TiO₂ contact is responsible for the SCLC behavior. The highly reduced recombination of charges seen in P3HT:PCBM devices is necessary to obtain the large extraction current transients that are seen in the experimental measurements. By comparing the simulated dark CELIV and photo-CELIV we show that photo-generated extraction is more sensitive towards changes in the surface recombination velocity.     

Effects of disorder on charge transport in semico++ nducting polymers

Polaron transport process in semiconducting polymers with disordered structure and morphology is simulated using a nonadiabatic evolution method. The simulations are performed within the framework of an extended version of the Su–Schrieffer–Heeger model modified to include diagonal disorder, off-diagonal disorder and an external electric field. It is found that the polaron transport mechanism is determined by the competition between the disorder and the electric field. For the case of pure diagonal disorder, the polaron transport undergoes a crossover from adiabatic drift to nonadiabatic hopping under a large scope of electric field (E < 2.0 mV/Å), whereas the field-assisted tunneling dominates charge transport for a higher electric field. For the combined disorder, it is demonstrated that the competition effects and the synergetic effects between the diagonal and off-diagonal disorder are equally important. The effects of diagonal disorder are negative in most cases, and the energetically easier pathways for charge transport are mainly opened by the off-diagonal disorder.     

The role of local potential minima on charge transport in thin organic semiconductor layers

We have performed a systematic study of dependence of time-resolved photocurrent on the point of charge excitation within the organic semiconductor channel formed by two coplanar metal electrodes. The results confirm that spatial variation of electric field between the electrodes crucially determines transport of photogenerated charge carriers through the organic layer. Time-of-flight measurements of photocurrent demonstrate that the transit time of photogenerated charge carrier packets drifting between the two electrodes decreases with increasing travelling distance. Such counterintuitive result cannot be reconciled with the spatial distribution of electric field between coplanar electrodes, alone. It is also in contrast to expected role of space-charge screening of external electric field. Supported by Monte Carlo simulations of hopping transport in disordered organic semiconductor layer, we submit that the space-charge screens the external electric field and captures slower charge carriers from the photogenerated charge carrier packet. The remaining faster carriers, exhibit velocity distribution with significantly higher mean value and shorter transit time.     

A theoretical study on the charge transport properties of DNA

The charge transport properties of DNA are studied by the first-principle simulation to discuss the possibility of applying DNA to molecular wire. Both the hopping model and band-like model are introduced. By using hopping model, the theoretical hole mobilities calculated by Marcus theory between the same bases in DNA are 5.6 × 10⁻³, 4.1 × 10⁻², 2.0 × 10⁻² and 1.2 × 10⁻⁴ cm²V⁻¹s⁻¹ for T-T, A-A, C-C and G-G; and the calculated electron mobilities are 5.3 × 10⁻⁸, 1.5 × 10⁻⁴, 8.1 × 10⁻⁷ and 7.5 × 10⁻¹⁰ cm²V⁻¹s⁻¹ for T-T, A-A, C-C and G-G, respectively. And the charge transport for both holes and electrons between different bases exhibits directivity. By using band-like model, we calculated the band width of DNA with double helix structure and bilinear structure to investigate which structure will facilitate to the charge transport. We found that the band width of DNA increased when DNA transforming from the double helix structure to the bilinear structure, which means DNA with the bilinear structure possesses better charge transport properties. This research sheds a light on the molecular design for the molecule serving as the molecular wire.     

Analysis of trap effect on reliability using the charge pumping technology in La-incorporated high-k dielectrics

In this paper, reliability characteristics of nMOSFETs with La-incorporated HfSiON and HfON and metal gate have been studied. HfLaSiON shows greater device degradation by hot carrier (HC) stress than by positive bias temperature (PBT) stress, while HfLaON exhibits similar degradation during HC stress and PBT stress. To evaluate the contribution of bulk trap during PBT stress, a novel charge pumping (CP) technique is applied to extract the distribution of bulk trap (N_bt) before and after PBT stress. To evaluate permanent damage during HC stress, an appropriate selection of frequency range in CP method is considered. The initial interface trap density of HfLaSiON and HfLaON is similar, while the near-interface trap (N_IT) density of HfLaSiON after HC stress is equal or greater than that of HfLaON.     

Effect of dihydropyrazine on structures and charge transport properties of N-heteropentacenes matters: A theoretical investigation

A family of N-heteropentacenes acted as promising candidates for organic semiconductor materials is of immense interest. It should be attributed to the following reasons that (1) the positions, numbers and valence-states of N atom in N-heteropentacenes can effectively tune their electronic structure, stability, solubility, and molecular stacking; (2) diverse intermolecular interaction and π-stacking motifs appear in their crystals. The effect of the position and number of the 6-π-pyrazine on their structures and charge-transport properties has been systematically investigated in our previous work (J. Phys. Chem. C 115 (2011) 21416). Therefore, in this work, the study on the role of 8-π-dihydropyrazine with another valence-state N atoms is our focus. Density functional theory, Marcus electron transfer theory and Brownian diffusion assumption coupled with kinetic Monte-Carlo simulation are applied into this investigation. Our theoretical results indicate that in contrast with pyrazine, dihydropyrazine introduced is more helpful for promoting p-type organic semiconductor materials. For molecule 4, hole mobility of its single crystal theoretically reach 0.71 cm² V⁻¹ s⁻¹, and coupled with its fine hole-injection ability, it should be a promising candidate for p-type organic semiconductor materials. Although the lowest triplet-state energies of the molecules studied are very small, introduction of dihydropyrazine is very helpful for increasing the energies.     

Characterization of ultra-thin SiO2 by capacitance-voltage and charge pumping measurements

In this paper, we present results on electrical measurements of ultra thin SiO₂ layers (from 3.5 nm down to 1.7 nm), used as gate dielectric in metal-oxide-semiconductors (MOS) devices. Capacitance-voltage (C-V) measurements and simulations on MOS capacitors have been used for extracting the electrical oxide thickness. The SiO₂/Si interface and oxide quality have been analyzed by charge pumping (CP) measurements. The mean interface traps density is measured by 2-level CP, and the energy distribution within the semiconductor bandgap of these traps are investigated by 3-level charge pumping measurements. A comparison of the energy distribution of the SiO₂/Si interface traps is made using classical and quantum simulations to extract the surface potential as a function of the gate signal. When the gate oxide thickness <3.5 nm, we prove that it is mandatory to take into account the quantum effects to obtain a more accurate energy distribution of the SiO₂/Si interface traps. We also explain the increase of the apparent interface traps density measured by 2-levels CP with the increase of the oxide thickness, for transistors made from the same technological process.     

Characterization of various alloying metal oxide nanoparticles embedded in HfOxNy as charge trapping nodes in nonvolatile memory devices

This work compares Co_xMo_yO, Co_xFe_yO and Fe_xMo_yO alloying metal oxide nanoparticles (AMONs) that were individually embedded in HfO_xN_y high-k dielectric as charge trapping nodes. They were formed by chemical vapor deposition using Co/Mo, Co/Fe and Fe/Mo acetate, respectively, calcined and reduced in Ar/NH₃ ambient. The effects of various pre-treatments on Co_xMo_yO, Co_xFe_yO and Fe_xMo_yO AMONs preparation were investigated. The results indicate that the larger charge trap density, larger memory window and better programming characteristics of Co_xMo_yO AMONs are attributable to their higher surface density and smaller diameter. The average collected charge in each Co_xMo_yO AMON is the smallest among three AMONs, revealing that a local leakage path is associated with the least charge loss. The main mechanism that governs the programming characteristics involves the trapping of holes.     

Electrical transport properties in electroless-etched Si nanowire field-effect transistors

We report the electrical transport of the Si nanowires in a field-effect transistor (FET) configuration, which were synthesized from B-doped p-type Si(1 1 1) wafer by an aqueous electroless etching method based on the galvanic displacement of Si by the reduction of Ag⁺ ions on the wafer surface. The FET performance of the as-synthesized Si nanowires was investigated and compared with Ag-nanoparticles-removed Si nanowires. In addition, high-k HfO₂ gate dielectric was applied to the Si nanowires FETs, leading to the enhanced performance such as higher drain current and lower subthreshold swing.     

基于量子模型的碳纳米管场效应晶体管电子输运特性

采用量子输运模型和NEGF理论,自洽求解薛定谔方程和泊松方程,对类MOS-碳纳米管场效应晶体管的电子输运特性建模。考察了沟道长度Lg为5~25 nm时,其对器件的导通电流、阈值电压、关态泄漏电流、电流开关比、亚阈值摆幅等性质的影响。结果表明:当Lg≥15 nm时,MOS-CNTFET没有量子尺寸效应;当Lg<15 nm时,器件出现短沟道效应;Lg<10 nm时短沟道效应更加明显。     

The role of the electrode configuration on the electrical properties of small-molecule semiconductor thin-films

This paper presents a systematic analysis of the electrode configuration influence on the electrical properties of organic semiconductor (OSC) thin-film devices. We have fabricated and electrically characterized a set of planar two-terminal devices. The differences in I-V characteristics between the top and bottom contact structures are presented and analyzed. Top-contact configurations have a linear current vs. electric field behavior, while the bottom-electrode devices display a transition from ohmic to space-charge-limited conduction regime. The transition is temperature- and thickness-dependent. Finite-element calculations show that when the OSC film is connected using top electrodes, the current flows through the OSC bulk region. On the other hand, the bottom-electrode configuration allows most of the current to flow near the OSC/substrate interface. The current probes interfacial states resulting in a space-charge conduction regime. The results shed some light on the so-called “contact effects” commonly observed in organic thin-film transistors. The findings presented here have implications for both the understanding of the charge transport in OSC films and the design of organic semiconductor devices.     

Characterization and performance of Schottky diode based on wide band gap semiconductor ZnO using a low-cost and simplified sol-gel spin coating technique

In this study, a Schottky diode based on wide band gap semiconductor ZnO was fabricated on p-type Si substrate using sol-gel spin coating method. Al/ZnO/p-Si diode indicates a rectification behavior. At lower voltages, the forward current of the diode was found to obey the intrinsic thermally generated charge carriers and at relatively higher voltages, the current mechanism of the diode is controlled by a space charge limited conduction mechanism. Under reverse bias conditions, the current-voltage characteristics of the diode exhibit the lower current as compared under forward bias, indicating the existence of a current limitation mechanism induced by two field lowering mechanisms which are Poole-Frenkel and Schottky mechanisms. The parameters, series resistance R_S, the ideality factor n and the barrier height φ_B0 of the diode were determined by performing different plots obtained from the experimental forward bias current-voltage. The capacitance measurements show that the values of capacitance were almost independent of the forward bias under various frequencies. The higher values of the capacitance at low frequencies were attributed to the excess capacitance resulting from the interface states in equilibrium with the ZnO.     

The impact of total ionizing radiation on body effect

A new phenomenon, for the first time, shows that radiation-induced body effect factor decrease in NMOS transistors is presented. The results indicate that body effect factor shift decreases as the total ionizing dose (TID) level increases in NMOS transistors, especially in the narrow-channel ones, which can be considered as one of the radiation-induced narrow-channel effect (RINCE). A first-order model is developed by applying charge conservation principle. Good agreement is obtained by comparing the modeling with experimental results. Finally, some implications to mitigate the RINCE effect are discussed.     

The effect of pulsed laser annealing on the nickel silicide formation

The pulsed laser annealing (PLA) is used to assist nickel silicide transformation for Schottky barrier height reduction and tensile strain enhancement and the effect of different laser power are investigated. In this report, a two-step annealing process which combine the conventional rapid thermal annealing with pulsed laser annealing is proposed to achieve a smooth silicon-rich NiSi_x interfacial layer on (1 0 0) silicon. With optimized laser energy, a 0.2 eV Schottky barrier height (SBH) modulation is observed from Schottky diode electrical characterization. Furthermore, PLA provides sufficient effective temperature during silicidation which also lead to increased tensile stress of silicide film than the two-step RTA silicide is also investigated. The SBH modulation and tensile stress enhancement benefits of PLA silicidation are considered as an alternative to the conventional rapid thermal annealing for ultra-scaled devices performance enhancement.     

Study of nitrogen impact on VFB-EOT roll-off by varying interfacial SiO2 thickness

Flat band voltage (V_FB) roll-off in long channel devices at thin equivalent oxide thickness (EOT) is studied on SiO₂/nitrided-HfSiO stacks. V_FB increases when SiO₂ interfacial layer thickness decreases, and charges pumping (CP) frequency sweep analysis shows higher trap density near Si/SiO₂ interface. Based on this observation, an atomic diffusion model is introduced. Higher concentration of nitrogen atom in the HfSiO(N) layer diffuses to the Si/SiO₂ interface through the SiO₂ layer in thinner SiO₂ device, and accumulates near Si/SiO₂ interface which can introduce higher density of interfacial traps. Lifetime extracted from negative bias temperature instability (NBTI), and mobility are also degraded in thinner SiO₂ devices due to the higher interfacial trap density.The V_FB roll-off can be improved by lowering nitrogen concentration in the HfSiO(N) layer from optimizing plasma nitridation pressure, decreasing post deposition anneal temperature, or using defect absorbing layer on the high-k oxide.     

Fabrication and characterization of p-n junctions based on ZnO and CuPc

p-n Junctions based on zinc oxide (ZnO) and copper-phthalocyanine (CuPc) were fabricated using pulsed laser deposition and thermal evaporator techniques, respectively. Current-voltage (I-V) characteristics of the ZnO-CuPc junction showed rectifying behavior. Various junction parameters such as barrier height and ideality factor were calculated using I-V data and observed to be 0.63 ± 0.02 eV and 4.0 ± 0.3, respectively. Cheung and Norde’s method were used to compare the junction parameters obtained by I-V characteristics.     

The protective effect of thin amorphous hydrogenated carbon a-C:H films during metallisation of metal-carbon-oxide-silicon (MCOS) diodes

Capacitance-voltage (C-V) characteristics of the as-grown metal(Al)-carbon-oxide(SiO₂)-semiconductor(Si) structures are examined at the frequency of 1 MHz and compared with the C-V characteristics of the conventional metal(Al)-SiO₂-Si (MOS) structures. The density of the oxide charge Q_o/q is extracted from the experimental results. Q_o/q was found to be 1×10¹² cm⁻² for the MOS structures and 7×10¹¹ cm⁻² for the metal-carbon-oxide-silicon structures. This difference can be attributed to the presence of the carbon layer which acts as a protective coating during metallisation of the wafers.