Drift mobility of holes and electrons in perdeuterated anthracene single crystals.
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Mey, W.↗
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Results for “HOLE MOBILITY”
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Density and mobility effects of the majority carriers in organic semiconductors under light excitation
This study demonstrates that the effect of light excitation on the density and the mobility of the majority carriers can be explored in organic semiconductors by modulated photocurrent spectroscopy. The spectra of phase and amplitude of the modulated photocurrent of pentacene films indicate a significant increase in the density of the photogenerated mobile holes (majority carriers). This increase is accompanied by a comparatively much smaller increase of the steady state photocurrent response which can be reconciled with a decrease in the mobility (μ) of holes. The decrease of μ is supported from an unusual increase of the Y/μ ratio of the out-of-phase modulated photocurrent (Y) signal to the mobility under light excitation. It is proposed that the mobile holes, which are generated from the dissociation of the light-created excitons more likely near the pentacene-substrate interface by electron trapping, populate grain boundaries charging them and producing a downward band bending. As a result, potential energy barriers are build up which limit the transport of holes interacting through trapping-detrapping with deep partially occupied traps in the charged grain boundaries. On the other hand, the transport of holes interacting through trapping-detrapping with empty traps is found unaffected.
75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
The importance of holes in aluminium tris-8-hydroxyquinoline (Alq{sub 3}) devices with Fe and NiFe contacts
To study the dominant charge carrier polarity in aluminium tris-8-hydroxyquinoline (Alq{sub 3}) based spin valves, single Alq{sub 3} layer devices with NiFe, ITO, Fe, and aluminium electrodes were fabricated and characterised by Time of Flight (ToF) and Dark Injection (DI) techniques, yielding a lower hole mobility compared to electron mobility. We compare the mobility measured by DI for the dominant carrier injected from NiFe and Fe electrodes into Alq{sub 3}, to that of holes measured by ToF. This comparison leads us to conclude that the dominant charge carriers in Alq{sub 3} based spin valves with NiFe or Fe electrodes are holes.
36 MATERIALS SCIENCE↗
Charge transport in single CuO nanowires
Charge transport in single crystal, p-type cupric oxide (CuO) nanowire (NW) was studied through temperature based (120 K–400 K) current-voltage measurements. CuO NW with a diameter of 85 nm was attached to Au electrodes 2.25 μm apart, using dielectrophoresis. At low electrical field (<0.89 × 103 V/cm), an ohmic conduction is observed with an activation energy of 272 meV. The injected electrons fill traps with an average energy, ET = 26.6 meV and trap density, NT = 3.4 × 1015 cm−3. After the traps are saturated, space charge limited current mechanism becomes dominant. For 120 K ≤ T ≤ 210 K phonon scattering limits mobility. For T ≥ 220 K, a thermally activated mobility is observed and is attributed to small polaron hopping with an activation energy of 44 meV. This mechanism yields a hole mobility of 0.0015 cm2/V s and an effective hole concentration of 4 × 1018 cm−3 at 250 K.
Wu, Junnan↗
Method for extracting relevant electrical parameters from graphene field-effect transistors using a physical model
Due to its intrinsic high mobility, graphene has proved to be a suitable material for high-speed electronics, where graphene field-effect transistor (GFET) has shown excellent properties. In this work, we present a method for extracting relevant electrical parameters from GFET devices using a simple electrical characterization and a model fitting. With experimental data from the device output characteristics, the method allows to calculate parameters such as the mobility, the contact resistance, and the fixed charge. Differentiated electron and hole mobilities and direct connection with intrinsic material properties are some of the key aspects of this method. Moreover, the method output values can be correlated with several issues during key fabrication steps such as the graphene growth and transfer, the lithographic steps, or the metalization processes, providing a flexible tool for quality control in GFET fabrication, as well as a valuable feedback for improving the material-growth process.
75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
Enhanced performance of polymer:fullerene bulk heterojunction solar cells upon graphene addition
Graphene has potential for applications in solar cells. We show that the short circuit current density of P3HT (Poly(3-hexylthiophene-2,5-diyl):PCBM((6,6)-Phenyl C61 butyric acid methyl ester) solar cells is enhanced by 10% upon the addition of graphene, with a 15% increase in the photon to electric conversion efficiency. We discuss the performance enhancement by studying the crystallization of P3HT, as well as the electrical transport properties. We show that graphene improves the balance between electron and hole mobilities with respect to a standard P3HT:PCBM solar cell.
75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
High carrier mobility in orientation-controlled large-grain (≥50 μm) Ge directly formed on flexible plastic by nucleation-controlled gold-induced-crystallization
High-carrier-mobility semiconductors on flexible-plastic are essential to realize flexible electronics. For this purpose, electrical properties of orientation-controlled large-grain Ge crystals on flexible-plastic directly formed by nucleation-controlled gold-induced-crystallization (GIC) are examined, and compared with those obtained by aluminum-induced-crystallization (AIC). The Ge crystals show p-type conductions. Here, hole concentrations are 2.2 × 10{sup 17} and 5.8 × 10{sup 20} cm{sup −3} for GIC-Ge and AIC-Ge, respectively, which are explained on the basis of the solubility of Au and Al in Ge. Thanks to the low hole concentration, GIC-Ge shows high hole mobility (160 cm{sup 2} V{sup −1} s{sup −1}) compared with AIC-Ge (37 cm{sup 2} V{sup −1} s{sup −1}). These demonstrate significant advantage of GIC to realize high-performance flexible-electronics.
75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
Phonon-limited low-field mobility in silicon: Quantum transport vs. linearized Boltzmann Transport Equation
We propose to check and validate the approximations made in dissipative quantum transport (QT) simulations solved in the Non-equilibrium Green's Function formalism by comparing them with the exact solution of the linearized Boltzmann Transport Equation (LB) in the stationary regime. For that purpose, we calculate the phonon-limited electron and hole mobility in bulk Si and ultra-scaled Si nanowires for different crystal orientations 〈100〉, 〈110〉, and 〈111〉. In both QT and LB simulations, we use the same sp{sup 3}d{sup 5}s{sup *} tight-binding model to describe the electron/hole properties and the same valence-force-field approach to account for the phonon properties. It is found that the QT simplifications work well for electrons, but are less accurate for holes, where a renormalization of the phonon scattering strength is proved useful to improve the results.
71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
Analysis of defect structure in silicon. Effect of grain boundary density on carrier mobility in UCP material
The relationships between hole mobility and grain boundary density were studied. Mobility was measured using the van der Pauw technique, and grain boundary density was measured using a quantitative microscopy technique. Mobility was found to decrease with increasing grain boundary density.
Dunn, J.↗
Ultrathin body GaSb-on-insulator p-channel metal-oxide-semiconductor field-effect transistors on Si fabricated by direct wafer bonding
We have realized ultrathin body GaSb-on-insulator (GaSb-OI) on Si wafers by direct wafer bonding technology using atomic-layer deposition (ALD) Al{sub 2}O{sub 3} and have demonstrated GaSb-OI p-channel metal-oxide-semiconductor field-effect transistors (p-MOSFETs) on Si. A 23-nm-thick GaSb-OI p-MOSFET exhibits the peak effective mobility of ∼76 cm{sup 2}/V s. We have found that the effective hole mobility of the thin-body GaSb-OI p-MOSFETs decreases with a decrease in the GaSb-OI thickness or with an increase in Al{sub 2}O{sub 3} ALD temperature. The InAs passivation of GaSb-OI MOS interfaces can enhance the peak effective mobility up to 159 cm{sup 2}/V s for GaSb-OI p-MOSFETs with the 20-nm-thick GaSb layer.
75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
Perovskite CsPbBr 3 Single Crystal Detector for Alpha-Particle Spectroscopy.
Here we report the first spectroscopic alpha particle detection based on CsPbBr 3 detectors with asymmetric contacts. The CsPbBr 3 single crystal was grown from melt using Bridgman method and then fabricated into detectors with different contacts. The In/CsPbBr 3 /Au detector presented a low dark current (~100 nA/cm 2 ) and temporal stable performance under high electric field (1000 V/cm). Such detector demonstrated excellent gamma ray resolving capability with a full-width at half maximum (FWHM) of ~5.9 keV for the 57 Co 122 keV γ ray. The CsPbBr 3 detector was capable of simultaneously resolving both the alpha particle (5.5 MeV) and γ ray (59.5 keV) peaks from 241 Am radiative isotope. The transport properties of CsPbBr 3 were then determined based on the alpha particle spectra and corresponding rise time distributions. The equivalent values of electron and hole mobilities were indicated as 63 and 49 cm 2 /(V∙s) respectively. The calculated electron and hole mobility-lifetime products were 4.5 x 10 -4 and 9.5 x 10 -4 cm 2 /V, respectively, demonstrating superior transport properties of holes over electrons in CsPbBr 3 . As a result, this work widens the scope of perovskite detectors to encompass charged radiation as well as high energy X/γ rays, and will significantly promote and guide further studies on perovskite materials for radiation detection applications.
46 INSTRUMENTATION RELATED TO NUCLEAR SCIENCE AND ↗
New non-linear photovoltaic effect in uniform bipolar semiconductor
A linear theory of the new non-linear photovoltaic effect in the closed circuit consisting of a non-uniformly illuminated uniform bipolar semiconductor with neutral impurities is developed. The non-uniform photo-excitation of impurities results in the position-dependant current carrier mobility that breaks the semiconductor homogeneity and induces the photo-electromotive force (emf). As both the electron (or hole) mobility gradient and the current carrier generation rate depend on the light intensity, the photo-emf and the short-circuit current prove to be non-linear functions of the incident light intensity at an arbitrarily low illumination. The influence of the sample size on the photovoltaic effect magnitude is studied. Physical relations and distinctions between the considered effect and the Dember and bulk photovoltaic effects are also discussed.
71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
THz cyclotron resonance of a 2D hole gas in a GaN/AlN heterostructure
The recent discovery of highly conducting two-dimensional hole gases (2DHGs) in GaN/AlN heterojunctions has opened the door to efficient complementary GaN electronics, a long-standing challenge in wide-bandgap semiconductor device physics. Electrical transport studies and simulations indicate that both heavy- and light-hole valence bands are occupied in these 2DHGs, but direct experimental characterization of the fundamental parameters of the mobile holes remains at an early stage. Here, we use time-domain terahertz spectroscopy and pulsed magnetic fields up to 31 T to directly measure cyclotron resonance of the mobile 2D holes in these GaN-based 2DHGs at low temperature (8 K), revealing key material properties including effective masses, densities, scattering times, and mobilities.
75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
Fabrication and characterization of Cu(In,Ga)Se{sub 2} p-channel thin film transistors
Cu(In,Ga)Se{sub 2} thin film transistors are demonstrated with the on-off ratio of ∼10{sup 3} and the saturation hole mobility of 1.8 cm{sup 2}/V-s. Due to the high hole concentration (∼5 × 10{sup 17 }cm{sup −3}), the channel needs to be etched to turn off for the accumulation mode operation. The Cu(In,Ga)Se{sub 2} film after etching reveals a larger mobility, and a narrower (112) X-ray diffraction line than the original thick layer, indicating the better crystallinity of the initial growth as compared to the subsequent Cu(In,Ga)Se{sub 2} layer. Both the hole concentration and the saturation mobility increase with the decreasing Cu/(In + Ga) ratio probably due to the effect of Cu vacancies.
75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
Charge transport studies in donor-acceptor block copolymer PDPP-TNT and PC71BM based inverted organic photovoltaic devices processed in room conditions
Diketopyrrolopyrole-naphthalene polymer (PDPP-TNT), a donor-acceptor co-polymer, has shown versatile behavior demonstrating high performances in organic field-effect transistors (OFETs) and organic photovoltaic (OPV) devices. In this paper we report investigation of charge carrier dynamics in PDPP-TNT, and [6,6]-phenyl C{sub 71} butyric acid methyl ester (PC71BM) bulk-heterojunction based inverted OPV devices using current density-voltage (J-V) characteristics, space charge limited current (SCLC) measurements, capacitance-voltage (C-V) characteristics, and impedance spectroscopy (IS). OPV devices in inverted architecture, ITO/ZnO/PDPP-TNT:PC71BM/MoO{sub 3}/Ag, are processed and characterized at room conditions. The power conversion efficiency (PCE) of these devices are measured ∼3.8%, with reasonably good fill-factor 54.6%. The analysis of impedance spectra exhibits electron’s mobility ∼2 × 10{sup −3} cm{sup 2}V{sup −1}s{sup −1}, and lifetime in the range of 0.03-0.23 ms. SCLC measurements give hole mobility of 1.12 × 10{sup −5} cm{sup 2}V{sup −1}s{sup −1}, and electron mobility of 8.7 × 10{sup −4} cm{sup 2}V{sup −1}s{sup −1}.
36 MATERIALS SCIENCE↗
Rapid Charge-Transfer Cascade through SWCNT Composites Enabling Low-Voltage Losses for Perovskite Solar Cells
Good charge selective contacts are vital to achieving efficient perovskite solar cells. The selectivity of such a contact is determined by its energetic offset or alignment with the bands of the perovskite absorber and, most importantly, by its interfacial recombination losses. This study uses single-walled carbon nanotubes (SWCNTs) as a high-mobility hole extraction layer at the interface with a perovskite absorber. When employed as a charge-transfer cascade-like structure with a low-mobility hole-selective contact, this configuration can suppress the interfacial recombination through the rapid removal of holes from the interface region. Reducing these recombination losses in turn results in very low voltage losses of less than 340 mV in n-i-p perovskite solar cells.
14 SOLAR ENERGY↗
Non-contact, non-destructive, quantitative probing of interfacial trap sites for charge carrier transport at semiconductor-insulator boundary
The density of traps at semiconductor–insulator interfaces was successfully estimated using microwave dielectric loss spectroscopy with model thin-film organic field-effect transistors. The non-contact, non-destructive analysis technique is referred to as field-induced time-resolved microwave conductivity (FI-TRMC) at interfaces. Kinetic traces of FI-TRMC transients clearly distinguished the mobile charge carriers at the interfaces from the immobile charges trapped at defects, allowing both the mobility of charge carriers and the number density of trap sites to be determined at the semiconductor-insulator interfaces. The number density of defects at the interface between evaporated pentacene on a poly(methylmethacrylate) insulating layer was determined to be 10{sup 12 }cm{sup −2}, and the hole mobility was up to 6.5 cm{sup 2} V{sup −1} s{sup −1} after filling the defects with trapped carriers. The FI-TRMC at interfaces technique has the potential to provide rapid screening for the assessment of interfacial electronic states in a variety of semiconductor devices.
75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
Effects of localization on hot carriers in InAs/AlAs{sub x}Sb{sub 1–x} quantum wells
The temperature dependence of a InAs/AlAs{sub 0.84}Sb{sub 0.16} multi-quantum-well sample is studied using continuous wave photoluminescence. An “s-shape” shift in peak energy is observed and attributed to low energy localization states. High incident power density photoluminescence measurements were performed to probe the nature of such localization. The results opposed the possibility of a type-II band structure and supported the idea of low energy localization states. The effect of such localization on hot carriers in our system was studied and an improvement in their stability due to hole mobility at elevated temperature is presented.
77 NANOSCIENCE AND NANOTECHNOLOGY↗