Carrier Transport Behavior Research Articles

Isotype heterojunction between organic crystalline semiconductors

We report the effect of n-n isotype organic heterojunction consisting of copper hexadecafluorophthalocyanine (F16CuPc) and phthalocyanatotin(IV) dichloride (SnCl2Pc). Their interfacial electronic structure was observed by Kelvin probe force microscopy (KPFM), and there is band bending in two materials, resulting in an electron accumulation region in F16CuPc layer and an electron depletion region in SnCl2Pc layer. The forming of organic heterojunction was explained by carriers flowing through the interface due to thermal emission of electrons. Furthermore, the carrier transport behavior parallel and vertical to heterojunction interface was also revealed by their heterojunction field-effect transistor with normally on operation mode and heterojunction diodes with rectifying property.

Nanoscale morphology and performance of molecular-weight-dependent poly(3-hexylthiophene)/TiO2 nanorod hybrid solar cells

We have investigated the effect of polymer molecular weight on the morphology and performance of poly(3-hexylthiophene)/TiO2nanorod hybrid photovoltaic devices by using scanning near field optical microscopy (SNOM), atomic force microscopy (AFM) and confocal Raman microscopy. From the topography and absorption mapping images, it is found that the rod-like structure of the low molecular weight polymer hybrid film consists of a large amount of grain boundaries and has a less continuous absorption mapping image. In contrast, the larger domain structure of the high molecular weight polymer hybrid film exhibits a continuous absorption mapping image, as a result of enhanced polymer stacking and electronic delocalization. The nanoscale morphology of the hybrid samples with different molecular weights also reveals the nature of photovoltaic performance and carrier transport behavior investigated by the time-of-flight technique.

Fabrication of Zn/Cd–Se micro heterostructures by electrochemical deposition in the pores of polycarbonate track-etch membranes and their characterization

We report on the fabrication of Zn–Se and Cd–Se micro heterostructures using template synthesis via electrochemical deposition in pores of polycarbonate track-etch membrane. The morphology of these heterostructures was studied by SEM and transport behavior of charge carriers was examined by recording their I– V curves. SEM results show high-resolution images of these structures and I– V characteristics show a prominent feature of negative differential resistance likewise found in resonant tunneling diodes (RTDs). This technique can be used to fabricate an array of multilayered heterostructures.

Electrical Uniformity of Direct Silicon Bonded Wafer Interfaces

This paper describes a series of electrical measurements and sample modifications that enabled the electrical properties of hybrid-orientation direct silicon bonded wafer interfaces to be determined. It is shown that the carrier transport across this near-surface (110)Si/(100)Si boundary is dictated by the defects present at the bond interface. These interface states are believed to pin the Fermi-level, producing a conduction barrier with a thermal activation energy Ea = 0.56eV. The defect band has been identified by deep-level transient spectroscopy and associated with the defect states typically observed in plastically deformed silicon. The carrier transport behavior across the bonding interface, as well as the observed interface trap levels are therefore attributed to the dislocation network present at the bonding interface. The spatial uniformity of the interface properties have been evaluated by TEM, electron-beam induced current microscopy, photoconductive decay and conduction measurements.

Charge Carrier Mobility in Films of Carbon-Nanotube- Polymer Composites

Electron and hole mobility in a sandwich-like structure ITO/polymer composite/Al based on a blend of single-wall carbon nanotubes (SWNT) and poly-N-vinylcarbazole (PVK) was studied with conventional time-of-flight (TOF) techniques and current-voltage characteristics in the steady-state mode. In SWNT (0.26 wt.%)- PVK films, transient currents exhibit dispersive behavior of the charge carrier transport for both electrons and holes. Within the range of electric field applied in the TOF experiments, electron mobility is ranged between (1.2 - 4.5) × 10−6 cm2/(V × s) and is higher than hole mobility by a factor of 5. Minor increasing of the SWNT concentration in the composite leads to formation of the more efficient charge transport network. Conductivity of the SWNT-PVK composite including 0.43 wt.% of SWNTs is found to be two orders of magnitude large than that of the PVK-SWNT containing 0.26 wt.% of of SWNTs. A simple model describing charge carrier transport through the SWNT-PVK blend is proposed.

Quantum decoherence of nanodevices under convective and radiative boundary conditions

Low dimensional nanostructures under thermal transport conditions have been traditionally treated as closed quantum systems. Many emerging operational nanodevices, however, will interact with their environment through convection and radiation processes. The realistic treatment of boundary conditions apart from internal conduction must now be quantified. Starting with a nanostructure operating in the ballistic or quasiballistic transport regime, the behavior of coherent carrier transport is investigated with the onset of convection and radiation interactions. Quantum decoherence is found to occur due to nanostructure heat carrier correlations to the fluid molecules or photons. Diagonalization of the quantum density matrix follows and serves as a mechanism to induce classicality on the heat carrier transport regime. Numerical experimentation has been performed with a silicon nanowire modeled under convective and radiative boundary conditions. Decoherence time scales of the nanowire are characterized for varying gaseous flow speeds, temperatures, and radiation environments. The magnitude of environmental coupling is also investigated to determine the decoherence behavior. Decoherence time scales are found to range from 300nsto7ms with gaseous convection producing the shortest temporal evolution. The loss of coherence has been found in every case analyzed, thus suggesting the transport regime to lie in classical phase space for this type of open quantum system. The transformation of heat carrier coherent quantum transport to the classical structure of phase space with the onset of environmental thermal interactions such as gas/liquid molecules or photons represents the major result of this work. The predictable outcome of preferred pointer states is discussed with the possibility of fabricating nanostructures that exhibit predetermined engineered thermal transport behavior under a specific set of boundary conditions.

Carrier Transport Mechanisms in Organic Light-Emitting Devices with a N,N′-diphenyl-N,N′-bis(1-naphthyl)-1,1′-biphenyl-4,4′-diamine:tris-(8-hydroxyquinoline) Aluminum Mixed Layer Acting as an Emitting Layer

The current density-voltage (J-V) measurements on organic light-emitting devices (OLEDs) with and without a N,N′-diphenyl-N,N′-bis(1-naphthyl)-1,1′-biphenyl-4,4′-diamine (NPB):tris-(8-hydroxyquinoline) aluminum (Alq3) mixed layer were performed to investigate the carrier transport mechanism. The slopes of the double logarithmic J-V curves for the OLEDs with and without the mixed layer in the space-charge-limited current (SCLC) region were 6.5 and 2, respectively. The dramatic increase in the slope of the double logarithmic J-V curve for the OLEDs with the mixed layer was attributed to the decrease in the mobility difference of the charge carriers at the NPB/Alq3 heterointerface resulting from the minimization of the energy barriers, and the carrier transport behaviors in the SCLC region were significantly affected by the unintentional virtual electrode, serving as a charge carrier reservoir, that formed at the heterointerface.

Development of a TOF measurement system of charge carrier dynamics in diamond thin films using a UV pulsed laser

A fast TOF measurement system with 150 ps time resolution for transport behavior of free charge carriers in an intrinsic diamond film by using a UV pulsed laser was developed. The 213 nm UV laser light narrowed to approximately 80 μm widths could locally create hole–electron pairs in selected locations on a diamond film between two parallel electrodes on the surface. This system measured accurate charge transport characteristics in a diamond film, because created charge carriers moved in a part of the diamond film where they did not get any influence from the laser irradiation. Diamond samples used for verification of the TOF system were intrinsic CVD diamond films with thickness between 4 and 10 μm grown on HP/HT diamond substrates. Transit time of holes for one diamond film was 4.7 ns with a traverse distance of 250 μm. The local irradiation of laser made it possible to measure transport characteristics of electrons and holes separately. In addition, it substantially reduced the influence of photoelectron, because the laser beam did not irradiate electrodes. Through several examinations, excellent reliability of the TOF system was confirmed.

Carrier carrying capacity of one-step grown suspended carbon nanotube bridge with carbon nanotube contact electrodes: For practical one-dimensional electronics

The authors report the carrier transport and electrical breakdown behaviors of micron-long-channel suspended carbon nanotube (CNT) of carbon-based one-dimensional junction with CNTs as drain and source electrodes. The structure consisted of bundle-type CNT electrode–a CNT channel–bundle-type CNT electrode, produced by one-step in situ direct growth via a unique diluted magnetic nanothick film catalyst at low temperature. The unique suspended all-one-dimensional CNT-based junction provides some insights into recent reports that an electrical breakdown of CNTs can be induced not by the contact problem but by the nonhomogeneously Joule heating along nanotube without release of heat into contacts by the observed breakdown at midlength of a 1–2μm long channel. The high current carrying all-CNT junction with bridging yield of 90% and stable operation at fixed voltage level can contribute into realization of practical integrated nanoelectronics such as interconnector and transistor via junction formation of one-step final process.

Scanned electrical probe characterization of carrier transport behavior in InAs nanowires

Conductive atomic force microscopy combined with detailed calculations of electronic subband structure has been used to analyze electron transport behavior across the transition between ballistic and drift/diffusive motion in InAs nanowires grown by metal organic chemical vapor deposition. Using a conducting tip in an atomic force microscope as a local, positionable electrical probe, InAs nanowire resistance as a function of electron transport distance within the nanowire has been measured. For distance of ∼200nm or less, this resistance is observed to be nearly independent of distance, while for larger distances the resistance increases linearly with distance. Analytical calculations indicate that a resistance only weakly dependent on distance should be observed for distances up to a few times the electron mean free path, and calculations of the mean free path using a variety of approaches yield values in the range of ∼50nm, very consistent with the observation of distance-independent resistance for transport distances up to ∼200nm.

Differential magnetoresistance technique for mobility extraction in ultra-short channel FDSOI transistors

Ultra-thin Silicon-on-Insulator (SOI) transistor has proved to offer advantages over bulk MOSFETs for high-speed, low power applications. However, there is still a strong need to obtain an accurate understanding of carrier transport and mobility behaviour in these advanced devices. In this work, magnetoresistance technique is used to perform mobility measurements in Fully-Depleted Silicon-on-Insulator (FDSOI) MOSFETs. This technique has the advantage of allowing channel mobility measurement from weak to strong inversion without requiring the knowledge of the transistor’s effective channel length. The influence of different scattering mechanisms in the channel is investigated in details by obtaining mobility values at low temperatures. A new differential method enabling mobility extraction from pure channel magnetoresistance corrected for source–drain series resistance is presented. After the correction, in devices with large series resistance an increase in the extracted mobility is obtained in strong inversion, where the channel resistance is small and conventional mobility extraction is most affected by the impact of series resistance.

Characterization for Organic Solid Solution and Formation of Organic Electronics

AbstractRecent two decades have seen the rapid development of organic electronics and much attention has been paid to carrier transport behavior. However, other characteristics, such as material compatibility, may be overlooked. We propose a new doping method taking advantage of fused organic solid solution, which is prepared by high-pressure and high temperature processing. In this method, the stable material systems can be selected and high performance organic light-emitting diodes with different colors have been demonstrated.

Influence of measuring environment on the electrical characteristics of pentacene-based thin film transistors

This work concerns the electrical properties and carrier transport behavior of polycrystalline pentacene-based thin film transistors (TFTs) in air and high-vacuum environments. The transistor in a high vacuum outperforms that in air. The dependence of the field-effect mobility on the gate voltage and the drain voltage is considered. A potential barrier model is applied to estimate the trap density of the pentacene transistor. The determined trap density at the grain boundaries of the pentacene film in air exceeds that in a high vacuum. These results show that the increased trap concentration at the grain boundaries in ambient air limits carrier transport.

A three-dimensional MOS transistor formation technique with crystallographic orientation-dependent TMAH etchant

Along with continuous scaling of MOS devices on LSI’s since early 1970s, it is widely recognized that three-dimensional (3-D) transistor structures will increasingly be important to extend further scaling in future. In response to the requirement, tetra methyl ammonium hydroxide (TMAH) etchant has been utilized to obtain 3-D MOS transistors named corrugated-channel transistor, CCT with vertical channels. The transistor channel is formed on steep vertical walls of almost exact {1 1 1} Miller index planes of single crystal {1 1 0} silicon substrate. Obtained performance of the CCT with 900 nm high and 54 nm wide beams is satisfactory resulting in almost five times more drivability as compared with planar transistor with the same planar area. Any adverse effect due to possible damages on etched surface has not been detected with respect to inversion carrier transport behavior. In this study, fundamental etching characteristics of TMAH are evaluated and it is clarified that the etch rate strongly depends on small deviation from the exact [1 1 2] direction of mask edge even if orientation selectivity exceeds 100 in appropriate conditions.

Electrical and Thermal Transport Properties in Layer-Structured (ZnO)mIn2O3 (m=5 and 9) Ceramics

The electrical and thermal transport properties are characterized for layer-structured (ZnO)mIn2O3 (m=5 and 9) (ZmIO) ceramics of near theoretical densities. The results of the low temperature Hall effect study suggested that the InO2- layers, which have oxygen defects, determine the carrier scattering mechanism of the material, and explain the previously demonstrated two-dimensional character of the carrier transport behavior. From an adaptation of the Wiedemann–Franz law, the lattice thermal conductivity (κL) of ZmIOs at 300 K is estimated to be ∼2.6 W/mK; this value is nearly one fifteenth of that for Al-doped ZnO ceramics (∼40 W/mK). These results suggest that the two-dimensional structure consisting of the InZnmOm+1+ and InO2- sub-lattices gives rise to the strong electrical anisotropy and the low κL which is attributed to the reduced mean free path of the phonon, similar to artificial super-lattice materials.

Requirements for low intermodulation distortion in GaN-Al/sub x/Ga/sub 1-x/N high electron mobility transistors: a model assessment

A model analysis of the large-signal characteristics of GaN-Al/sub x/Ga/sub 1-x/N high-electron mobility transistors (HEMTs) with particular emphasis on intermodulation distortion (IMD) and the third-order intercept point. Since the nonlinearity depends critically on the carrier transport behavior, a Monte Carlo (MC) based numerical simulation scheme has been employed. The focus is to identify parameters and their interdependencies with a view of setting optimal limits for enhanced microwave performance. A case is made for increased mole fraction for the barrier layer, reducing the transit length, and introducing a thin AlN interfacial layer for suppressing real space transfer for enhancing the device performance. Finally, high-temperature predictions of the nonlinear behavior and IMD have been made, by carrying out the MC simulations at 600 K. In a process a favorable case is made for the GaN system as a potential candidate for microwave and RF applications at elevated temperatures.

Modification of a post-implantation defect activity for photovoltaic conversion

The problem of an eventual usefulness of defects and traps in solar cells (and especially in implanted Si material) has been investigated since the 1980s. Usually in place of an increase, the efficiency actually diminishes because of the non-radiative recombination on numerous defects. At the beginning of the 1990s a new approach was proposed. The fundamental difference concerns the thermodynamic aspects of devices: microelectronic and photovoltaic (PV) which differentiates a receiver from a generator. In the solar cell, there is an additional dimension, i.e. optoelectronic properties where the electronic behaviour (recombination) can be completed/modified by an optical activity (generation). PV characteristics depend simultaneously on optical (absorption, conversion) and electronic (carrier transport and collection) behaviour. We have analyzed theoretically and experimentally some possible modifications of the post-implantation defect activity on single-crystal Si in view of a very- and ultra-high PV conversion efficiency. The applied techniques can be classified as bandgap, defect and stress engineering. The results of the local crystalline modification have been compared with those obtained by spectral response. We show that an adequate implantation and annealing allow an important transformation of conversion, transport and collecting characteristics.

Electrical properties in arsenic-ion-implanted GaAs

A model of effective surface potential energy is proposed to explain unknown behavior of carrier transport in film and leakage currents in Schottky diodes. X-ray photoelectron spectroscopy is employed to investigate a series of band diagrams of arsenic-ion-implanted GaAs films. It is shown that the effective surface potential energies, affected by the shift of core levels, are due to different microstructures and atomic environments in these films at different annealing temperatures. Moreover, the effective barrier heights on the surfaces of these films are changed by the different effective surface potential energies which increase from 0.49 to 0.75 eV in films annealed from 200 to 600 °C, but slightly decrease to 0.7 eV in films annealed up to 800 °C.

Various routes to chaos in n-GaAs compensated with Ni impurities

We consider the dynamical behavior of non-linear carrier transport for n-GaAs compensated with Ni deep impurities at room temperature. When both dc and ac external biases are applied simultaneously, the system exhibits a richness of bifurcation features as the amplitude of the ac bias varies continuously while the dc bias is kept fixed at a suitable value. These features include period-doubling and period-halving cascades, type-I and type-III intermittencies, frequency-locked behavior, and the coexistence of different attractors.

Nonlinear response and driven chaos in n-type GaAs compensated with Ni impurities.

We consider the dynamical behavior of nonlinear carrier transport for n-type GaAs compensated with Ni deep impurities at room temperature under the application of both dc and ac biases. The stability properties of the fixed points of the system are studied when only a dc bias is applied. When both dc and ac biases are applied simultaneously, the system exhibits richness of bifurcation features such as the amplitude of the ac bias varies continuously while the dc bias is kept fixed at a suitable value. These features include period-doubling cascade, type-I and -III intermittencies, period-3 and -5 windows, and the coexistence of different attractors in the same positive differential conductivity region.