Carrier Transport Phenomena Research Articles

Dispersive transport in the temperature dependent transient photoresponse of organic photodiodes and solar cells

The nanosecond transient photoresponse of organic solar cells and photodiodes based on a conjugated polymer (poly(3-hexylthiophene-2,5-diyl)) blended with a fullerene derivative ([6,6]-phenyl C61-butyric acid methyl ester) exhibits a strong temperature dependence, whose origin can be traced back to charge carrier transport phenomena. In the framework of a drift-diffusion model including multiple-trapping, the temperature dependence of effective mobilities arises naturally without the need of using a temperature dependent parameterization of the mobilities. Furthermore, the extended drift-diffusion simulation reproduces the measured change of slope of the transient current density from j(t)∼t(−1+α) to j(t)∼t(−1−α), indicating dispersive charge carrier transport influenced by an exponential trap distribution characterized by the dimensionless parameter α. A second kink is identified to be the point in time of the crossover from electron to hole dominated charge carrier transport, enabling for the determination of the donor and acceptor transport properties independent of each other.

Influence of gate-source/drain misalignment on the performance of bulk FinFETs by a 3D full band Monte Carlo simulation

We investigate the influence of gate-source/drain (G—S/D) misalignment on the performance of bulk fin field effect transistors (FinFETs) through the three-dimensional (3D) full band Monte Carlo simulator. Several scattering mechanisms, such as acoustic and optical phonon scattering, ionized impurity scattering, impact ionization scattering and surface roughness scattering are considered in our simulator. The influence of G—S/D overlap and underlap on the on-states performance and carrier transport of bulk FinFETs are mainly discussed in our work. Our results show that the on-states currents increase with the increment of G—D/S overlap length and the positions of a potential barrier and average electron energy maximum vary with the G—D/S overlap length. The carrier transport phenomena in bulk FinFETs are due to the effect of scattering and the electric field in the overlap/underlap regime.

Characterizing the Influence of TOPO on Exciton Recombination Dynamics in Colloidal CdSe Quantum Dots

Surface ligands play a significant role in the carrier recombination dynamics of colloidal quantum dots (QDs). Trioctylphosphine oxide (TOPO) is a ligand used during CdSe QD synthesis, which binds to surface cadmium atoms, but, despite its widespread use, its effect on exciton relaxation rates is poorly understood. In this work, we extract TOPO–CdSe binding equilibria from isothermal titration calorimetry experiments and use them to study the effect of TOPO concentration on CdSe time-resolved photoluminescence. We develop a quantitative kinetic model of CdSe QD exciton relaxation that is based upon an ensemble probability distribution of bound ligands and show that bound TOPO gives rise to band-edge states that participate in carrier recombination dynamics. Optoelectronic applications of QDs necessitate a good understanding of interfacial dynamics so that charge separation, carrier transport, and other phenomena can be optimized. This technique is a promising step toward that goal that is useful for any l...

Diffusion Length in Nanoporous Photoelectrodes of Dye-Sensitized Solar Cells under Operating Conditions Measured by Photocurrent Microscopy.

We determined the carrier diffusion lengths in nanoporous layers of dye-sensitized solar cells by using scanning photocurrent microscopy. The diffusion lengths were found to be 60-100 μm for the conventional cells. In addition, we found a correlation between the carrier diffusion lengths and the cell efficiency, which proved that improvement in the diffusion length is one of the crucial factors for optimizing device performance. The diffusion length was measured for various operating conditions by varying parameters such as solar light intensity and applied electrical voltage. In particular, we observed electric-field-driven, carrier transport phenomena (i.e., drift current) in modified cells. Fitting with the drift-diffusion model enabled us to extract the electric field strengths present in the TiO2 nanoporous layer.

Exploring carrier transport phenomena in a CVD-assembled graphene FET on hexagonal boron nitride

The supporting substrate plays a crucial role in preserving the superb electrical characteristics of an atomically thin 2D carbon system. We explore carrier transport behavior in a chemical-vapor-deposition- (CVD-) assembled graphene monolayer on hexagonal boron nitride (h-BN) substrate. Graphene-channel field-effect transistors (GFETs) were fabricated on ultra-thin h-BN multilayers to screen out carrier scattering from the underlying SiO2 substrate. To explore the transport phenomena, we use three different approaches to extract carrier mobility, namely, effective carrier mobility (μeff), intrinsic carrier mobility (μ), and field-effect mobility (μFE). A comparative study has been conducted based on the electrical characterization results, uncovering the impacts of supporting substrate material and device geometry scaling on carrier mobility in GFETs with CVD-assembled graphene as the active channel.

Manipulation of Graphene Properties by Interface Engineering

Graphene is generally regarded as an ideal candidate material for post-CMOS nanoelectronics. In contrast to traditional semiconductors, the unique two dimensional structure of graphene offers the necessity and possibility of studying the interface characteristics for its sensitivity to the top surface and interface between graphene and the substrate. In this paper, we provide preliminary studies in understanding graphene surface and interface issues in electronic structure, carrier transport and related phenomena down to nano-scale.

Influence of carrier transport on Raman amplification in silicon waveguides

This paper analyze for the first time the impact of the charge carrier transport on the continuously pumped stimulated Raman amplification in silicon waveguides. A novel analytical model is developed using which the coupled differential equations of the pump and the probe optical signals and those of carrier transport are solved concurrently. The simulation and analysis suggest that the neglect of the carrier transport phenomenon, reported in the previously published works, is approximately justified only if the effective carrier lifetime is comparable to the carrier transit time, otherwise it can result in substantial overestimation of the free carrier density at the optical mode center.

Electrical, Optical, and Morphological Properties of P3HT-MWNT Nanocomposites Prepared by in Situ Polymerization

In situ polymerization of thiophene to poly(3-hexylthiophene) (P3HT) was carried out in the presence of different loadings of multiwall carbon nanotubes (MWNTs) (0.1−10 wt %). It was found that the nanotubes are dispersed uniformly in the polymer matrix and the polymer chains wrap around the nanotube walls. NMR analysis indicated the presence of both CH−π and π−π interaction between P3HT and MWNTs, thereby enhancing the optical, thermal, and electrical properties of the nanocomposite for certain loadings of MWNTs. With an increase in MWNT concentration the head−tail (H-T) regioregularity of the composites was found to decrease. The UV−vis spectra of the composite films show a red shift of the π−π* transition band with increasing MWNT concentration, which was attributed to the uncoiling of the P3HT chain on the MWNT surface. Additionally, the charge carrier transport phenomenon was studied for these nanocomposites. The temperature-dependent conductivity measurement revealed that the addition of MWNTs into P3HT polymer shifted the main conduction mechanism from variable-range hopping to fluctuation-assisted tunneling, and the polymer acted as a barrier in bundle-to-bundle hopping.

Modeling of the Potential Profile for the Annealed Polycrystalline PbSe Film

A new model for the mechanism of photoconductivity in annealed polycrystalline PbSe fllm is presented. The combined mechanism with respect to double heterojunction due to oxidation layer, dopant segregation and carrier trapping at grain-boundary is proposed. This letter focuses on characterizing the potential proflle, which is extremely important from the viewpoint of carrier transport phenomena. A potential proflle adjacent to the boundaries was calculated, and the efiect of biased voltage was in detail discussed, which shows that the mech- anism of photoconductivity of annealed polycrystalline PbSe fllm depends on properties of the grain boundaries.

Carrier transport simulation of anomalous temperature dependence in nematic liquid crystals

We investigated the carrier transport phenomena in model liquid crystalline systems, which were constructed on the basis of the Gay-Berne potential and Monte Carlo calculation. The carrier transport was analyzed under the condition that the molecular arrangement in the system was fixed and thermally activated carriers were transported by hopping in the system. The carrier transport simulation was performed by Monte Carlo method using Miller-Abrahams hopping ratio. By these calculations, we reproduced the experimental results of the electronic conduction in nematic liquid crystals.

Soluble terthiophene-labeled cruciform molecule as a semiconductor for organic field-effect transistor

Cruciform conjugated molecule, 4(HP3T)-benzene bearing terthiophene moieties has been synthesized through Heck coupling reaction using 5-hexyl-5″-vinyl-[2,2′,5′,2″]terthiophene as dendrons and 1,2,4,5-tetrabromo-benzene as the core unit; this molecule has been fully characterized. This terthiophene-based molecule exhibits good solubility in common organic solvents and good self-film forming property. They are intrinsically crystalline as they exhibit well-defined X-ray diffraction patterns from uniform orientations of molecules. Thus, intermolecular interaction can be enhanced to affect the carrier transport phenomena after annealing at 160 °C. The semiconducting property of 4(HP3T)-benzene has been evaluated in organic field-effect transistors. 4(HP3T)-benzene exhibit carrier mobility as high as (3.7 ± 0.5) × 10 −4 cm 2 V −1 s −1 with an on/off ratio of about 1 × 10 3.

Carrier transport and related phenomena in MOS devices

Silicon-based devices are currently the most attractive group because they are functioning at room temperature and can be easily integrated into conventional silicon microelectronics. There are many models and simulation programs available to compute CV curves with quantum correction [Choi C-H, Wu Y, Goo JS, Yu Z, Dutton RW. IEEE Trans on Electron Devi 2000; 47(10): 1843; Croci S, Plossu C, Burignat S. J Mater Sci Mater Electron 2003; 14: 311; Soliman L, Duval E, Benzohra M, Lheurette E, Ketata K, Ketata M. Mater Sci Semicond Process 2001; 4: 163]. This work deals with the simulation of electron transfer through SiO 2 barrier of metal–oxide–semiconductor structure (MOS). The carrier density is given by a self consistent resolution of Schrödinger and Poisson equations and then the MOS capacitance is deduced and compared with results available in literature. As it is well known, the MOS capacitance–voltage profiling provides a simple determination of structure parameters. The extracted tunnel oxide thickness and substrate doping are compared with those used in the simulation. For the purpose to investigate the electron tunnelling through the barrier, we have used the transfer matrix approach. Using I– V simulations, we have shown that the traps in SiO 2 matrix have a drastic influence on electron tunnelling through the barrier. The trap-assisted contribution to the tunnelling current is included in many models [Maserjian J, Zamani N. J Appl Phys 1982; 53(1): 559; Houssa M, Stesmans A, Heyns MM. Semicond Sci Technol 2001; 16: 427; Aziz A, Kassmi K, Kassmi Ka, Olivie F. Semicond Sci Technol 2004; 19: 877; Wu You-Lin, Lin Shi-Tin. IEEE Trans Dev Mater Reliab 2006; 6(1): 75; Larcher L. IEEE Trans Electron Dev 2003; 50(5): 1246]; this is the basis for the interpretation of stress induced leakage current (SILC) and breakdown events. Memory effect becomes typical for this structure. We have studied the I– V dependence with trap parameters.

Subpicosecond Raman studies of electric-field-induced optical phonon instability in anIn0.53Ga0.47As-based semiconductor nanostructure

Transient carrier transport phenomena in anIn0.53Ga0.47As-based p–i–n semiconductor nanostructure have been studied by using subpicosecondtransient/time-resolved Raman spectroscopy. We observe an instability of the GaAs-likeoptical phonon population in this nanostructure semiconductor that occurs when electronsare accelerated to very high velocities by the application of intense electric fields. Theresults open up a new channel for creating coherent THz frequency that can be used inTHz electronic devices. We suggest that the observed phenomena will have enormousimpact on the carrier dynamics and carrier transport in nanoscale semiconductor electronicdevices.

Anomalous charge carrier transport phenomena in highly aluminum doped SiC

Temperature dependant Hall effect measurements were performed on aluminum doped SiC samples with specific resistivities between 8 Ωcm and 0.1 Ωcm. In all samples indications for impurity conduction at low temperatures were found. In low doped samples, a sharp transition between the impurity conduction transport regime and the hole conduction regime is visible in the temperature dependence of specific resistivity, charge carrier concentration and mobility. In highly doped samples (ρ < 0.2 Ωcm), this transition is no longer confined to a small temperature range and much less abrupt. We conclude that the impurity conduction is still present at high temperatures in highly aluminum doped SiC so that at least two competing transport mechanisms are present simultaneously in these samples. (© 2006 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Anomalous nonlinear photoresponse in a InGaN/GaN heterostructure

The nonlinear (third to fourth order) as well as linear photoconductivity in a Gallium nitride/Indium-Gallium nitride (GaN/InGaN) heterostructure is investigated using femtosecond pulses in the infrared (IR) and near ultraviolet (UV). An anomalous IR photoresponse is explained by a four level model for the GaN region including defect density fluctuations and nonlinear carrier transport phenomena. The same model also explains the observed subpicosecond noninstantaneous IR response of the photodetector. The linear UV photoresponse originates in the InGaN region. Design guidelines for GaN-based nonlinear photodetectors used in autocorrelation measurements are suggested.

Analysis of carrier transport phenomena in high band gap II–VI-based MIS photovoltaic devices

High efficiency devices with band gaps in the 1.6–2.0 eV range are needed to develop high efficiency compound semiconductor tandem devices. CdSe with a band gap of 1.7 eV and Cd x Zn 1− x Te with a band gap that is tunable over this range show promise. Some success has been achieved with CdSe metal–insulator–semiconductor device structures. Using AMPS simulations we demonstrate that tunneling is not needed to achieve effective transport of light generated carriers across the intrinsic layer and good device performance. Fabrication of Cu/ZnSe/CdSe/transparent conductor devices has been guided by the simulation results, and measured performance data for these devices are found to be consistent with predictions. Guided by these insights short circuit current densities of 14.7 mA/cm 2 have been achieved without correction for Cu absorption. Open circuit voltages in excess of 400 mV have been achieved for Cu contacts; higher open circuit voltages are attained using Au, but losses in short circuit current occur. The key to further improvements in performance is development of p contacts with larger work function.

Carrier transport and related phenomena in nanosize periodic silicon/insulator structures

Experimental facts and theoretical models describing mechanisms of carrier transport across silicon/insulator quantum wells are reviewed. Single and multi quantum wells (MQW) formed in periodic Si/CaF 2 and Si/SiO 2 nanostructures are mainly covered. Carrier traps in the barrier material (CaF 2, SiO 2) are shown to have dramatic influence on electron and hole tunneling through the barriers. When the carrier energy coincides with local trap levels, the carrier transport gets the resonant character, otherwise the effects caused by charging of the traps dominate. As a result, a region of negative differential resistance (NDR) and memory (hysteresis) effect become typical for I– V curves of the structures. Intensity of electroluminescence is shown to be mediated by the competition between radiative recombination and non-radiative Auger processes in the wells. Novel applications of periodic quantum well nanostructures are considered.

Effect of substitutionally dissolved Ce in Si on the magnetic and electric properties of magnetic semiconductor Si1−xCex films

A magnetic semiconductor Si:Ce thin film was prepared using a vacuum evaporation system with electron-beam guns. The as-deposited thin film was amorphous and exhibited n-type conduction. It showed temperature dependence of resistivity (ρ–T), as in a normal semiconductor, and a diamagnetic property. By annealing at 973 K, however, the film epitaxially crystallizes and the conduction changes to the p-type. The resistivity of the film abruptly decreases by three orders of magnitude below 30 K, unlike that of the as-deposited film. The magnetic susceptibility measured at a low magnetic field (750 Oe) also decreases around the same temperature in ρ–T curves. These magnetic and carrier transport phenomena are responsible for the substitutionally dissolved Ce in Si.

Microelectrical characterisation of diamond films: an attempt to understand the structural influence on electrical transport phenomena

Abstract The properties of microelectrical conduction in microwave plasma assisted chemical vapour deposition (MPCVD) diamond films were investigated using an atomic force microscopy probe, giving a morphological map of the electrical conduction with a spatial resolution better than 500 nm. Also, a cathodoluminescence map with a spatial resolution of about 1 μm was obtained, giving the possibility of correlating the defects involved in the different carrier transport phenomena. Using micro-Raman analysis several bands could be identified. It is found that the defects responsible for the cathodoluminescence (CL) blue band are responsible for the major part of the electrical conduction in diamond films, while the defects localised in 〈111〉 surfaces, responsible for the green CL emission, could be involved in a less conductive process.

Analysis of hot carrier transport in AlGaAs/InGaAs pseudomorphic HEMTs by means of electroluminescence

The carrier transport phenomena occurring in pseudomorphic AlGaAs/InGaAs HEMTs biased in the on-state impact-ionization regime is analyzed in this paper. We confirm the presence, in the electroluminescence spectra of pseudomorphic HEMTs, of a dominant contribution due to electron-hole recombination and we identify a composite peak due to recombination of cold carriers. We analyze the recombination peak using a high-resolution monochromator, which reveals the fine structure due to transitions between electron and hole subbands in the channel quantum well, thus providing useful data concerning the properties of the InGaAs HEMT channel. We also demonstrate that recombination between nonenergetic electrons and holes occurs in the gate-source region, as already observed in InAlAs/InGaAs HEMT's on InP. This recombination emission is superimposed to a less intense contribution mostly coming from the gate drain region. This contribution has a nearly Maxwellian distribution which extends to fairly high energies (>3 eV) and has equivalent temperatures in the 1000-3000 K range. Finally we show evidence of recombination in the AlGaAs layers (observed at high electric field), which demonstrates, in these devices, real space transfer of both electrons and holes.