In-plane Mobilities Research Articles

Morphological design strategies to tailor out-of-plane charge transport in conjugated polymer systems for device applications.

The transport of charge carriers throughout an active conjugated polymer (CP) host, characterized by a heterogeneous morphology of locally varying degrees of order and disorder, profoundly influences the performance of CP-based electronic devices, including diodes, photovoltaics, sensors, and supercapacitors. Out-of-plane charge carrier mobilities (μout-of-plane) across the bulk of the active material host and in-plane mobilities (μin-plane) parallel to a substrate are highly sensitive to local morphological features along their migration pathways. In general, the magnitudes of μout-of-plane and μin-plane are very different, in part because these carriers experience different morphological environments along their migration pathways. Suppressing the impact of variations in the morphological order/disorder on carrier migration remains an important challenge. While much is known about μin-plane and its optimization for devices, the current challenges are associated with μout-of-plane and its optimization for device performance. Therefore, this review is devoted to strategies for improving μout-of-plane in neat CP films and the implications for more complex systems, such as D:A blends which are relevant to OPV devices. The specific strategies discussed for improving μout-of-plane include solvent/field processing methods, chemical modification, thickness confinement, chemical additives, and different post-annealing strategies, including annealing with supercritical fluids. This review leverages the most recent fundamental understanding of mechanisms of charge transport and connections to morphology, identifying robust design strategies for targeted improvements of μout-of-plane.

Bidisperse Nanospheres Jammed on a Liquid Surface.

Jammed packings of bidisperse nanospheres were assembled on a nonvolatile liquid surface and visualized to the single-particle scale by using an in situ scanning electron microscopy method. The PEGylated silica nanospheres, mixed at different number fractions and size ratios, had large enough in-plane mobilities prior to jamming to form uniform monolayers reproducibly. From the collected nanometer-resolution images, local order and degree of mixing were assessed by standard metrics. For equimolar mixtures, a large-to-small size ratio of about 1.5 minimized correlated metrics for local orientational and positional order, as previously predicted in simulations of bidisperse disk jamming. Despite monolayer uniformity, structural and depletion interactions caused spheres of a similar size to cluster, a feature evident at size ratios above 2. Uniform nanoparticle monolayers of high packing disorder are sought in many liquid interface technologies, and these experiments outlined key design principles, buttressing extensive theory/simulation literature on the topic.

Boron arsenide heterostructures: lattice-matched heterointerfaces and strain effects on band alignments and mobility

BAs is a III–V semiconductor with ultra-high thermal conductivity, but many of its electronic properties are unknown. This work applies predictive atomistic calculations to investigate the properties of BAs heterostructures, such as strain effects on band alignments and carrier mobility, considering BAs as both a thin film and a substrate for lattice-matched materials. The results show that isotropic biaxial in-plane strain decreases the band gap independent of sign or direction. In addition, 1% biaxial tensile strain increases the in-plane electron and hole mobilities at 300 K by >60% compared to the unstrained values due to a reduction of the electron effective mass and of hole interband scattering. Moreover, BAs is shown to be nearly lattice-matched with InGaN and ZnSnN2, two important optoelectronic semiconductors with tunable band gaps by alloying and cation disorder, respectively. The results predict type-II band alignments and determine the absolute band offsets of these two materials with BAs. The combination of the ultra-high thermal conductivity and intrinsic p-type character of BAs, with its high electron and hole mobilities that can be further increased by tensile strain, as well as the lattice-match and the type-II band alignment with intrinsically n-type InGaN and ZnSnN2 demonstrate the potential of BAs heterostructures for electronic and optoelectronic devices.

C568: A new two-dimensional sp2-sp3 hybridized allotrope of carbon

Using first-principles calculations, we designed a new two-dimensional sp2−sp3 hybridized allotrope of carbon. This allotrope is made up of 5-6-8 carbon rings thus, known as C568. C568 is an indirect semiconductor with a band gap of 1.13 eV (HSE06) at Γ-M symmetric point in the Brillouin zone. The possibility of the existence of C568 is evident from its energetic, dynamical, mechanical, and thermal stabilities. C568 maintains its structural integrity up to 800 K, which we confirmed by employing ab-initio molecular dynamics (AIMD) simulations. C568 shows extraordinary room temperature in-plane electronic mobility of order ∼104 cm2V−1s−1, which is significantly higher than the black phosphorus (∼103 cm2V−1s−1) monolayer. Energetically, this sheet is more stable than the other sp2−sp3-hybridized carbon allotropes such as pentagraphene and tetrahexcarbon. Moreover, C568 can be stacked into three different configurations, which also exhibit interesting electronic as well as transport properties. These configurations are also dynamically stable and show in-plane mobilities of order ∼104 cm2V−1s−1.

High sensitivity hexagonal boron nitride lateral neutron detectors

Hexagonal boron nitride (h-BN) thermal neutron detectors have demonstrated the highest detection efficiency among all solid-state detectors (at 58% for a detection area of 1 mm2 and 53% for a detection area of 9 mm2). However, scaling up the detector size of vertical h-BN detectors is challenging due to increased dark current, capacitance, and surface recombination with the increasing detection area. Here, we report the demonstration of a 29 mm2 thermal neutron detector fabricated from a freestanding 10B enriched h-BN epilayer of 90 μm in thickness with a detection efficiency of 50% by employing a lateral device geometry. The lateral detector geometry takes advantage of the unique layered structure of h-BN which naturally provides higher in-plane carrier mobilities than those in the vertical direction. Moreover, due to the reduced area of metals in contact with the h-BN material, the detrimental effects associated with the surface recombination at the metal contacts and device capacitance were reduced, which resulted in improved charge collection efficiency and signal to noise ratios. This work laid the ground work for scaling up to large size neutron detectors based on h-BN.

Crystallization Mechanism and Charge Carrier Transport in MAPLE-Deposited Conjugated Polymer Thin Films.

Although spin casting and chemical surface reactions are the most common methods used for fabricating functional polymer films onto substrates, they are limited with regard to producing films of certain morphological characteristics on different wetting and nonwetting substrates. The matrix-assisted pulsed laser evaporation (MAPLE) technique offers advantages with regard to producing films of different morphologies on different types of substrates. Here, we provide a quantitative characterization, using X-ray diffraction and optical methods, to elucidate the additive growth mechanism of MAPLE-deposited poly(3-hexylthiophene) (P3HT) films on substrates that have undergone different surface treatments, enabling them to possess different wettabilities. We show that MAPLE-deposited films are composed of crystalline phases, wherein the overall P3HT aggregate size and crystallite coherence length increase with deposition time. A complete pole figure constructed from X-ray diffraction measurements reveals that in these MAPLE-deposited films, there exist two distinct crystallite populations: (i) highly oriented crystals that grow from the flat dielectric substrate and (ii) misoriented crystals that preferentially grow on top of the existing polymer layers. The growth of the highly oriented crystals is highly sensitive to the chemistry of the substrate, whereas the effect of substrate chemistry on misoriented crystal growth is weaker. The use of a self-assembled monolayer to treat the substrate greatly enhances the population and crystallite coherence length at the buried interfaces, particularly during the early stage of deposition. The evolution of the in-plane carrier mobilities during the course of deposition is consistent with the development of highly oriented crystals at the buried interface, suggesting that this interface plays a key role toward determining carrier transport in organic thin-film transistors.

Stable ultra-thin CdTe crystal: a robust direct gap semiconductor

Employing density functional theory based calculations, we investigate structural, vibrational and strain-dependent electronic properties of an ultra-thin CdTe crystal structure that can be derived from its bulk counterpart. It is found that this ultra-thin crystal has an 8-atom primitive unit cell with considerable surface reconstructions. Dynamic stability of the structure is predicted based on its calculated vibrational spectrum. Electronic band structure calculations reveal that both electrons and holes in single layer CdTe possess anisotropic in-plane masses and mobilities. Moreover, we show that the ultra-thin CdTe has some interesting electromechanical features, such as strain-dependent anisotropic variation of the band gap value, and its rapid increase under perpendicular compression. The direct band gap semiconducting nature of the ultra-thin CdTe crystal remains unchanged under all types of applied strain. With a robust and moderate direct band gap, single-layer CdTe is a promising material for nanoscale strain dependent device applications.

Titanium trisulfide (TiS3): a 2D semiconductor with quasi-1D optical and electronic properties.

We present characterizations of few-layer titanium trisulfide (TiS3) flakes which, due to their reduced in-plane structural symmetry, display strong anisotropy in their electrical and optical properties. Exfoliated few-layer flakes show marked anisotropy of their in-plane mobilities reaching ratios as high as 7.6 at low temperatures. Based on the preferential growth axis of TiS3 nanoribbons, we develop a simple method to identify the in-plane crystalline axes of exfoliated few-layer flakes through angle resolved polarization Raman spectroscopy. Optical transmission measurements show that TiS3 flakes display strong linear dichroism with a magnitude (transmission ratios up to 30) much greater than that observed for other anisotropic two-dimensional (2D) materials. Finally, we calculate the absorption and transmittance spectra of TiS3 in the random-phase-approximation (RPA) and find that the calculations are in qualitative agreement with the observed experimental optical transmittance.

Effect of Thickness-Dependent Microstructure on the Out-of-Plane Hole Mobility in Poly(3-Hexylthiophene) Films

Regioregular poly(3-hexylthiophene) (RR-P3HT) is a widely used donor material for bulk heterojunction polymer solar cells. While much is known about the structure and properties of RR-P3HT films, important questions regarding hole mobilities in this material remain unresolved. Measurements of the out-of-plane hole mobilities, μ, of RR-P3HT films have been restricted to films in the thickness regime on the order of micrometers, beyond that generally used in solar cells, where the film thicknesses are typically 100 to 200 nm. Studies of in-plane carrier mobilities have been conducted in thinner films, in the thickness range 100-200 nm. However, the in-plane and out-of-plane hole mobilities in RR-P3HT can be significantly different. We show here that the out-of-plane hole mobilities in neat RR-P3HT films increase by an order of magnitude, from 10(-4) cm(2)/V·s, for a 80 nm thick film, to a value of 10(-3) cm(2)/V·s for films thicker than 700 nm. Through a combination of morphological characterization and simulations, we show that the thickness dependent mobilities are not only associated with the differences between the average morphologies of thick films and thin films, but specifically associated with changes in the local morphology of films as a function of distance from the interfaces.

Carrier mobility as a function of carrier density in type-II InAs/GaSb superlattices

We report on a study of the in-plane carrier mobility in InAs/GaSb superlattices as a function of carrier density. Instead of using a number of differently doped samples, we use the persistent-photoconductivity effect to vary the carrier density over a wide range from n- to p-type in single samples and perform Hall effect measurements. Hence, our data are not obscured by sample to sample nonuniformities. We demonstrate that low-temperature in-plane mobilities are limited by screened interface roughness scattering (IRS), although present models of two-dimensional carrier screening of IRS lead to a limited agreement with our data.

Demonstration of interface-scattering-limited electron mobilities in InAs∕GaSb superlattices

The in-plane transport in InAs∕GaSb type-II superlattices (SLs) is a sensitive indicator of SL growth quality and of the eventual performance of devices made from these materials. The in-plane mobility of electrons that move predominantly in the InAs layer is affected by a number of intrinsic and extrinsic scattering mechanisms, including interface roughness scattering (IRS). The hallmark of classic IRS-limited transport in SLs and quantum wells is the sixth power dependence of mobility on layer width. While IRS-limited transport was demonstrated in a number of SL and quantum well systems, it has never been demonstrated in the important InAs∕GaSb SL material. In this paper, we perform temperature dependent Hall effect measurements on a series of InAs∕GaSb SLs with a fixed GaSb layer width and a variable InAs layer width d. The low temperature (10K) in-plane electron mobilities μ as a function of d behave as μ∝d6.20, which follows the classic sixth power dependence expected from theory. At the same time, the dominance of the IRS-limited transport indicates that our samples are less affected by other scattering mechanisms, so that mobility measurements are another indicator of sample quality.

Molecular-beam epitaxy of p-type m-plane GaN

We report on the plasma-assisted molecular-beam epitaxy of Mg-doped (101¯0) GaN on (101¯0) 6H–SiC. Secondary ion mass spectroscopy measurements show the incorporation of Mg into the GaN films with an enhanced Mg incorporation under N-rich conditions relative to Ga-rich growth. Transport measurements of Mg-doped layers grown under Ga-rich conditions show hole concentrations in the range of p=1×1018 to p=7×1018cm−3 and a dependence between hole concentration and Mg beam equivalent pressure. An anisotropy in in-plane hole mobilities was observed, with the hole mobility parallel to [112¯0] being higher than that parallel to [0001] for the same hole concentration. Mobilities parallel to [112¯0] were as high as ∼11.5cm2∕Vs (at p∼1.8×1018cm−3).

Magnetotransport in modulation-doped In0.53Ga0.47As/In0.52Al0.48As heterojunctions: in-plane effective mass, quantum and transport mobilities of 2D electrons

Shubnikov–de Haas (SdH) and Hall effect measurements, performed in the temperature range between 3.3 and 20 K and at magnetic fields up to 2.3 T, have been used to investigate the electronic transport properties of lattice-matched In0.53Ga0.47As/In0.52Al0.48As heterojunctions. The spacer layer thickness (tS) in modulation-doped samples was in the range between 0 and 400 Å. SdH oscillations indicate that two subbands are already occupied for all samples except for that withtS= 400 Å. The carrier density in each subband, Fermi energy and subband separation have been determined from the periods of the SdH oscillations. The in-plane effective mass (m*) and the quantum lifetime (τq) of 2D electrons in each subband have been obtained from the temperature and magnetic field dependences of the amplitude of SdH oscillations, respectively. The 2D carrier density (N1) in the first subband decreases rapidly with increasing spacer thickness, while that (N2) in the second subband, which is much smaller thanN1 , decreases slightly with increasing spacer thickness from 0 to 200 Å. The in-plane effective mass of 2D electrons is similar to that of electrons in bulk In0.53Ga0.47As and show no dependence on spacer thickness. The quantum mobility of 2D electrons is essentially independent of the thickness of the spacer layer in the range between 0 and 200 Å. It is, however, markedly higher for the samples with a 400 Å thick spacer layer. The quantum mobility of 2D electrons is substantially smaller than the transport mobility which is obtained from the Hall effect measurements at low magnetic fields. The transport mobility of 2D electrons in the first subband is substantially higher than that of electrons in the second subband for all samples with double subband occupancy. The results obtained for transport-to-quantum lifetime ratios suggest that the scattering of electrons in the first subband is, on average, forward displaced in momentum space, while the electrons in the second subband undergo mainly large-angle scattering.

Full-band Monte Carlo model of electron and hole transport in strained Si including inelastic acoustic phonon scattering

Full-band Monte Carlo simulations of electron and hole transport in strained Si on Si0.7Ge0.3 have been performed with a transport model which includes a wave-vector-dependent inelastic acoustic phonon scattering rate. Only two unambiguously determined deformation potentials are needed to achieve excellent agreement with experimental drift velocity versus electric field data for unstrained Si over the very wide temperature range from 20 to 430 K. For strained Si, this model yields in-plane lattice mobilities of 3490 cm2/(V s) for electrons and 1760 cm2/(V s) for holes at 300 K. Drift velocity and energy versus electric field characteristics are given as reference for conventional device simulations. In contrast to simpler transport models, we do not find a pronounced Gunn effect at 77 K.

Hole mobility measurements in heavily doped Si/sub 1-x/Ge/sub x/ strained layers

Both Hall and drift in-plane mobilities have been measured in compressively strained p-type Si/sub 1-x/Ge/sub x/ layers grown by both chemical vapor deposition (CVD) and molecular beam epitaxy (MBE). Measurements were taken over the boron doping range of 10/sup 18/ cm/sup -3/ to 10/sup 20/ cm/sup -3/ with Ge contents of 0/spl les/x/spl les/0.22. The apparent drift mobility is found to increase with increasing Ge content, whereas the Hall mobility decreases for the same samples at all doping levels studied. The Hall factor decreases with increasing Ge content, which may be due to additional scattering mechanisms introduced by Ge along with changes in the valence band structure as a result of strain. In this study we also provide the first report of Hall mobility measurements of deuterium-passivated, heavily doped Si.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

Low-field hole mobility of strained Si on (100) Si1−xGex substrate

Strain Hamiltonian and k⋅p theory are employed to calculate low-field hole mobility of strained Si layers on (100)Si1−xGex substrate. Nonparabolicity and the warped nature of the valence bands are included. At room temperature, in-plane hole mobilities of strained Si are found to be 1103 and 2747 cm 2 V−1 s−1 for x equal to 0.1 and 0.2, respectively. These hole mobilities are, respectively, 2.4 and 6 times higher than that of bulk Si. This improvement in the mobility results is mainly due to the large splitting energy between the occupied light-hole band and the empty heavy-hole band and smaller effective mass. The effect of p-type doping on mobility is also presented.

Time-resolved DFWM, excite/probe and transient grating studies of InxGa1-xAs/GaAs superlattices

Low-temperature time-resolved degenerate four-wave mixing (DFWM), single-beam excite/probe and transient grating measurements have been performed on InxGa1-xAs/GaAs superlattices with thin ( approximately 6 nm) InxGa1-xAs wells. We find weak dephasing of heavy-hole (hh) excitons by acoustic phonons. The hh exciton homogeneous linewidth temperature coefficient is 1.9+or-1 mu eV K-1. The low-temperature hh exciton optical nonlinearity is successfully modelled in terms of exciton phase-space filling and exchange interactions in the hh exciton gas. From low-temperature transient grating measurements we find hh exciton in-plane mobilities as high as 6.7*104 cm2 V-1 s-1 and mobilities which are greater in superlattices with thinner barriers.

Determination of the parallel and perpendicular intermolecular tunnelling rates in two Langmuir-Blodgett quantum well systems

The first experimental determination of the anisotropy of the electron transfer rate in two Langmuir-Blodgett insulator multilayer systems is presented. A key feature of these two multilayer systems is the presence of the planar conjugated phthalocyanine ring. The first system is composed of tetra-tertiary-butyl phthalocyanine monosulphonic acid. In it the ring planes are approximately normal to the layer plane. The second system is composed of the two-ring phthalocyanine HOSiPcOSiPcOSI( nC 6H 13) 3. In it the rings lie in the layer plane. The anisotropy of the systems is determined from a model of the recombination rate, at high density, of photoelectrons and photoholes on the same layer. The ratio of the intralayer to interlayer transfer rates between adjacent rings is found to be 1500 and 570 respectively. Coupled with a previous determination of the interlayer transfer time of 1.6 and 0.30 ns, the intralayer transfer times are then determined as 1.07 and 0.56 ps respectively. The in-plane mobilities can then be determined as 0.34 and 1.2 cm 2 s −1 V −1 respectively. The results justify regarding these systems as organic multiple-quantum wells.

Determination of anisotropic electron transport properties of two Langmuir-Blodgett organic multiple quantum wells

The first experimental determination of the anisotropy of the electron transfer rate in two- Langmuir-Blodgett insulator multilayer systems is presented. A key feature of these two multilayer systems is the presence of the planar-conjugated phthalocyanine ring. The first system is composed of tetra-tertiary-butyl phthalocyanine monosulphonic acid. In it the ring planes are approximately normal to the layer plane. The second system is composed of the two-ring phthalocyanine, HOSiPcOSiPcOSi(n-C 6H 13) 3. In it the rings lie in the layer plane. The anisotropy of the systems is determined from a model of the recombination rate, at high density, of photoelectrons and photoholes on the same layer. The ratio of the intralayer to interlayer transfer rate between adjacent rings is found to be 1500 and 570 respectively. Coupled with a previous determination of the interlayer transfer time of 1.6 and 0.15 ns, the intralayer transfer times are then determined as 1.07 and 0.28 ps respectively. The in-plane mobilities can then be determined as 0.34 and 2.4 cm 2 s -1 V -1 respectively. The results justify regarding these systems as organic multiple quantum wells.

Ambipolar diffusion and carrier lifetime measurements in all-binary (InAs)2(GaAs)5 strained quantum wells grown on GaAs

The in-well ambipolar diffusion coefficients and carrier lifetimes in ordered all-binary quantum wells, composed of (InAs)2(GaAs)5 short-period strained-layer superlattices (SPSLSs) grown on [001]-oriented GaAs substrates, are measured using picosecond optically induced transient grating and pump-probe techniques. Quantum wells containing SPSLSs may be expected to exhibit higher in-plane hole mobilities compared to InGaAs ternary alloy quantum wells because of a larger average indium content and reduced disorder scattering in the SPSLS structures. The results obtained in the SPSLSs are compared to those obtained in InGaAs alloy quantum wells of comparable well width and confinement energies. This indicates that, for a given SPSLS and its equivalent alloy, the ambipolar diffusion coefficients are comparable while the carrier lifetimes in the SPSLSs are longer. The longer carrier lifetimes in the SPSLSs suggest that these binary structures possess fewer nonradiative recombination centers than the alloys. The absence of a dramatic improvement in the transport properties, however, may indicate that imperfect interfaces in the SPSLSs may be offsetting their possible advantages, in spite of the fact that optical measurements in the SPSLSs indicate high quality and x-ray and transmission electron microscopic measurements demonstrate the binary nature of these structures.