Measured Carrier Density Research Articles

Carrier density and delocalization signatures in doped carbon nanotubes from quantitative magnetic resonance.

High-performance semiconductor materials and devices are needed to supply the growing energy and computing demand. Organic semiconductors (OSCs) are attractive options for opto-electronic devices, due to their low cost, extensive tunability, easy fabrication, and flexibility. Semiconducting single-walled carbon nanotubes (s-SWCNTs) have been extensively studied due to their high carrier mobility, stability and opto-electronic tunability. Although molecular charge transfer doping affords widely tunable carrier density and conductivity in s-SWCNTs (and OSCs in general), a pervasive challenge for such systems is reliable measurement of charge carrier density and mobility. In this work we demonstrate a direct quantification of charge carrier density, and by extension carrier mobility, in chemically doped s-SWCNTs by a nuclear magnetic resonance approach. The experimental results are verified by a phase-space filling doping model, and we suggest this approach should be broadly applicable for OSCs. Our results show that hole mobility in doped s-SWCNT networks increases with increasing charge carrier density, a finding that is contrary to that expected for mobility limited by ionized impurity scattering. We discuss the implications of this important finding for additional tunability and applicability of s-SWCNT and OSC devices.

Charge Carrier Dynamics in Planar Heterojunction Organic Solar Cells

The ionization energy (IE) offset of a donor–acceptor pair provides the driving force for hole transfer and subsequent free charge carrier generation in low‐bandgap nonfullerene organic solar cells (OSCs). However, the interfacial energetic landscape in bulk heterojunction OSCs is determined by the materials’ electronic structure and intermolecular interactions at the donor/acceptor interface, causing local energy‐level shifts and disorder. Herein, the impact of the IE offset on the charge transfer efficiency and charge carrier dynamics is systematically evaluated by characterizing PM6/ITIC, PM6/IT‐2Cl, and PM6/IT‐4Cl planar heterojunction (PHJ) solar cells. Ultrafast spectroscopy and time‐resolved charge carrier density measurements reveal that an IE offset of about ≈0.5 eV leads to efficient hole transfer and subsequent free charge generation. Furthermore, bimolecular charge recombination and consequently triplet generation are significantly reduced in systems with high IE offset. This work underlines the importance of sizeable donor–acceptor IE offsets in PHJ nonfullerene OSCs as critical for high‐efficiency donor/acceptor material and device design.

Impact of Large Gate Voltages and Ultrathin Polymer Electrolytes on Carrier Density in Electric-Double-Layer-Gated Two-Dimensional Crystal Transistors.

Electric-double-layer (EDL) gating can induce large capacitance densities (∼1-10 μF cm-2) in two-dimensional (2D) semiconductors; however, several properties of the electrolyte limit performance. One property is the electrochemical activity which limits the gate voltage (VG) that can be applied and therefore the maximum extent to which carriers can be modulated. A second property is electrolyte thickness, which sets the response speed of the EDL gate and therefore the time scale over which the channel can be doped. Typical thicknesses are on the order of micrometers, but thinner electrolytes (nanometers) are needed for very-large-scale-integration (VLSI) in terms of both physical thickness and the speed that accompanies scaling. In this study, finite element modeling of an EDL-gated field-effect transistor (FET) is used to self-consistently couple ion transport in the electrolyte to carrier transport in the semiconductor, in which density of states, and therefore quantum capacitance, is included. The model reveals that 50 to 65% of the applied potential drops across the semiconductor, leaving 35 to 50% to drop across the two EDLs. Accounting for the potential drop in the channel suggests that higher carrier densities can be achieved at larger applied VG without concern for inducing electrochemical reactions. This insight is tested experimentally via Hall measurements of graphene FETs for which VG is extended from ±3 to ±6 V. Doubling the gate voltage increases the sheet carrier density by an additional 2.3 × 1013 cm-2 for electrons and 1.4 × 1013 cm-2 for holes without inducing electrochemistry. To address the need for thickness scaling, the thickness of the solid polymer electrolyte, poly(ethylene oxide) (PEO):CsClO4, is decreased from 1 μm to 10 nm and used to EDL gate graphene FETs. Sheet carrier density measurements on graphene Hall bars prove that the carrier densities remain constant throughout the measured thickness range (10 nm-1 μm). The results indicate promise for overcoming the physical and electrical limitations to VLSI while taking advantage of the ultrahigh carrier densities induced by EDL gating.

Method for noncontact measurement of 2DEG mobility and carrier density in AlGaN/GaN on semi-insulating wafers

We present a charge-assisted sheet resistance technique for noncontact wafer level determination of two-dimensional electron gas (2DEG) mobility versus sheet carrier density without any test structures or gates. Instead, the electrical biasing of the 2DEG is provided by surface charge deposition, using a corona charging method. Analysis of the sheet resistance versus deposited charge identifies the 2DEG full depletion condition and enables calculation of the 2DEG sheet carrier density required for the mobility. Results for AlGaN/GaN heterostructures on semi-insulating SiC and sapphire substrates show good agreement with Hall results at a zero-bias condition.

Observation of high carrier density, ohmic contact, and metallic conductivity down to 5 K in aluminum-contacted multilayer MoS2 flakes

Molybdenum disulfide (MoS2) has attracted considerable attention, because the monolayers or multilayers of MoS2 may become building blocks of electronic/optoelectronic devices in post-silicon technology. The contact metals have significant effects on the transport properties of MoS2 devices. We previously reported that unconventional transport properties of back-gated MoS2 flakes with Al contact may be attributed to a contact-induced doping effect. However, measurements of carrier density, which can confirm the doping effect, were not performed. In this paper, we present the experimental results of carrier density estimated via Hall effect measurements. The observed carrier densities of the Al-contacted MoS2 flakes ((3–8) × 1013 cm−2) are ∼10 times greater than those reported for Au-contacted MoS2 flakes, indicating the doping effect due to the Al contact. We also observe ohmic contact and metallic conductivity down to 5 K for the Al-contacted flakes, which are distinct from the properties of Au-contacted MoS2 flakes.

1D topological systems for next-generation electronics

1D topological systems for next-generation electronics

Metal-insulator transitions in epitaxial rocksalt-structure Cr1−x/2N1−xOx (001)

Substitutional replacement of N with O in epitaxial CrN(001) layers is achieved by reactive sputter deposition in a mixed ${\mathrm{N}}_{2}$ and $\mathrm{Ar}+{\mathrm{O}}_{2}$ atmosphere. O incorporation facilitates Cr vacancies, yielding a rocksalt-structure solid solution ${\mathrm{Cr}}_{1\ensuremath{-}x/2}{\mathrm{N}}_{1\ensuremath{-}x}{\mathrm{O}}_{x}$ with a single compositional parameter $x$ and a measured lattice constant that decreases from $a=0.4175$ to 0.4116 nm for $x=0$ to 0.59. First-principles calculations predict $da/dx=+0.0200$, \ensuremath{-}0.0018, and \ensuremath{-}0.0087 nm for $\mathrm{Cr}{\mathrm{N}}_{1\ensuremath{-}x}{\mathrm{O}}_{x}, {\mathrm{Cr}}_{1\ensuremath{-}x/3}{\mathrm{N}}_{1\ensuremath{-}x}{\mathrm{O}}_{x}$, and ${\mathrm{Cr}}_{1\ensuremath{-}x/2}{\mathrm{N}}_{1\ensuremath{-}x}{\mathrm{O}}_{x}$, respectively, and confirm, in combination with ion-beam compositional analyses and x-ray diffraction results, a vacancy concentration of $x/2$ per cation site. The room-temperature resistivity decreases by over two orders of magnitudes from $\ensuremath{\rho}=1.5\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}3}$ to $7.7\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}6}\phantom{\rule{0.16em}{0ex}}\mathrm{\ensuremath{\Omega}}\phantom{\rule{0.16em}{0ex}}\mathrm{m}$ for $x=0--0.26$. This is accompanied by a transition from a negative to a positive temperature coefficient of resistivity, an increase in the Hall mobility from ${\ensuremath{\mu}}_{\mathrm{H}}=0.37$ to $1.7\phantom{\rule{0.16em}{0ex}}{\mathrm{cm}}^{2}/\mathrm{Vs}$, an increase in the carrier concentration $n=1.1\ifmmode\times\else\texttimes\fi{}{10}^{20}$ to $4.9\ifmmode\times\else\texttimes\fi{}{10}^{21}\phantom{\rule{0.16em}{0ex}}{\mathrm{cm}}^{\ensuremath{-}3}$, and a decrease in the calculated band gap from 0.54 to 0 eV. However, $\ensuremath{\rho}$ increases again to $6.9\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}5}\phantom{\rule{0.16em}{0ex}}\mathrm{\ensuremath{\Omega}}\phantom{\rule{0.16em}{0ex}}\mathrm{m}$ for $x=0.34$ and to $g10\phantom{\rule{0.16em}{0ex}}\mathrm{\ensuremath{\Omega}}\phantom{\rule{0.16em}{0ex}}\mathrm{m}$ for $x\ensuremath{\ge}0.59$, with a corresponding drop in ${\ensuremath{\mu}}_{\mathrm{H}}$ and the opening of a gap. The maximum in the measured carrier density and conductivity at $x=0.26$ is attributed to a maximum in the density of states near the Fermi level, in perfect agreement with calculations using supercells with randomly distributed O substitutions and Cr vacancies that predict a maximum at $x=0.25\ifmmode\pm\else\textpm\fi{}0.05$. The measurements indicate insulator-to-metal and subsequent metal-to-insulator transitions at $x=0.04\ifmmode\pm\else\textpm\fi{}0.03$ and $0.30\ifmmode\pm\else\textpm\fi{}0.03$, respectively. The transition to metallic conduction is attributed to substitutional O on anion sites acting as donors, while the measured $n$ for $x=0.01--0.26$ quantitatively indicates charge compensation by cation vacancies that act as acceptors. The insulating properties at large $x$ are likely caused by increased electron correlation effects.

Measurement of a Threshold Initiation Carrier Density for a Reduction in Gas Br eakdown Voltage

Measurement of a Threshold Initiation Carrier Density for a Reduction in Gas Br eakdown Voltage

Extracting Carrier Mobility in Conducting Polymers Using a Photoinduced Charge Transfer Reaction

Recently, the electrical conductivity of conducting polymers has been significantly improved. Unfortunately, there are nearly no reports on the carrier mobility and carrier density measurements at this stage because traditional measurement approaches are difficult to apply in highly doped disordered systems. In this study, we developed a method to extract the carrier mobility in a highly doped disordered system using a solid state photoinduced charge transfer reaction. Combining the number of the charges transferred from photobase generators to the conducting polymers, the carrier mobility can be extracted from the conductivity change. This approach was successfully applied to conducting polymers with different conductivities. Potentially, this method could also be used to measure the carrier mobility of other disordered systems such as p-type and n-type carbon nanotube films.

In-situ terahertz optical Hall effect measurements of ambient effects on free charge carrier properties of epitaxial graphene

Unraveling the doping-related charge carrier scattering mechanisms in two-dimensional materials such as graphene is vital for limiting parasitic electrical conductivity losses in future electronic applications. While electric field doping is well understood, assessment of mobility and density as a function of chemical doping remained a challenge thus far. In this work, we investigate the effects of cyclically exposing epitaxial graphene to controlled inert gases and ambient humidity conditions, while measuring the Lorentz force-induced birefringence in graphene at Terahertz frequencies in magnetic fields. This technique, previously identified as the optical analogue of the electrical Hall effect, permits here measurement of charge carrier type, density, and mobility in epitaxial graphene on silicon-face silicon carbide. We observe a distinct, nearly linear relationship between mobility and electron charge density, similar to field-effect induced changes measured in electrical Hall bar devices previously. The observed doping process is completely reversible and independent of the type of inert gas exposure.

Measuring Carrier Density in InP‐based Materials by Using EDTA Solution in Electrochemical Capacitor Voltage Testing

The quick test and evaluation of carrier density and its distribution is an important part in the fabrication of photonic integrated devices. Ammonium tartrate is commonly used as the corrosion fluid when electricity chemistry capacitance voltage analysis (ECV) was used to measure the carrier density of InP‐based epitaxial wafer. However, it has the shortcomings of trivial preparation process, volatilization, slow etching rate, which seriously influence the efficiency of the test. Hence, we have adopted a new solution, which is called as ethylenediamine tetraacetic acid disodium (EDTA). We experimentally demonstrated that the EDTA solution has the advantages of convenience, high efficiency and good stability. And, it can significantly reduce the etching time, thus improving the ECV test efficiency and reducing the production cycle and device fabrication cost.

Defect-Induced Luminescence Quenching vs. Charge Carrier Generation of Phosphorus Incorporated in Silicon Nanocrystals as Function of Size

Phosphorus doping of silicon nanostructures is a non-trivial task due to problems with confinement, self-purification and statistics of small numbers. Although P-atoms incorporated in Si nanostructures influence their optical and electrical properties, the existence of free majority carriers, as required to control electronic properties, is controversial. Here, we correlate structural, optical and electrical results of size-controlled, P-incorporating Si nanocrystals with simulation data to address the role of interstitial and substitutional P-atoms. Whereas atom probe tomography proves that P-incorporation scales with nanocrystal size, luminescence spectra indicate that even nanocrystals with several P-atoms still emit light. Current-voltage measurements demonstrate that majority carriers must be generated by field emission to overcome the P-ionization energies of 110–260 meV. In absence of electrical fields at room temperature, no significant free carrier densities are present, which disproves the concept of luminescence quenching via Auger recombination. Instead, we propose non-radiative recombination via interstitial-P induced states as quenching mechanism. Since only substitutional-P provides occupied states near the Si conduction band, we use the electrically measured carrier density to derive formation energies of ~400 meV for P-atoms on Si nanocrystal lattice sites. Based on these results we conclude that ultrasmall Si nanovolumes cannot be efficiently P-doped.

Temperature dependence of electron density and electron–electron interactions in monolayer epitaxial graphene grown on SiC

We report carrier density measurements and electron–electron (e–e) interactions in monolayer epitaxial graphene grown on SiC. The temperature (T)-independent carrier density determined from the Shubnikov–de Haas (SdH) oscillations clearly demonstrates that the observed logarithmic temperature dependence of the Hall slope in our system must be due to e–e interactions. Since the electron density determined from conventional SdH measurements does not depend on e–e interactions based on Kohn’s theorem, SdH experiments appear to be more reliable compared with the classical Hall effect when one studies the T dependence of the carrier density in the low T regime. On the other hand, the logarithmic T dependence of the Hall slope δRxy/δB can be used to probe e–e interactions even when the conventional conductivity method is not applicable due to strong electron–phonon scattering.

InGaAs Inversion Layer Mobility and Interface Trap Density from Gated Hall Measurements.

In this work, we use gated Hall method for direct measurement of free carrier density and electron mobility in inversion InGaAs MOSFET channels. At room temperature, the highest Hall mobility of 1800 cm2/Vs is observed at electron density in the channel ≈1×1012 cm-2. A comparison with mobility values estimated from transistor characteristics reveals a significant underestimation of mobility which arises from overestimation of channel density obtained from C-V measurements. Temperature dependence of the electron mobility provides the evidence that remote Coulomb scattering dominates at electron density < 3×1011 cm-2. Contrary to the capacitance-based methods commonly used for extraction of interface trap density, gated Hall method can separate the contributions of fast-trapped charges and free carriers in the channel to the total charge of a MOS capacitor. This allows for reliable estimation of trap density at the III-V/high-k interface including border traps. The results illustrate that even high-quality interfaces providing high-mobility transport suffer from fast border traps above the conduction band. In contrast with Si where the effect of border traps is negligible, in low density-of-state InGaAs channel material as much as half of the channel electrons can be trapped and excluded from transport, increasing switching energy and dissipated power.

Optical and electrical properties of gallium doped indium tin oxide optimized for low deposition temperature applications

In optoelectronic and photovoltaic devices, transparent conductive oxides are important in establishing a good electrical contact while minimizing optical losses over a broad range of wavelengths (400–1200nm). To date, research has focused on In2O3 - SnO2 (ITO) films. In this paper, we report on a study of Ga-doped ITO (GITO) films, which in contrast to standard ITO 90/10 (i.e. In:Sn=90:10) films contain less In. Initially, we describe the development of a multicomponent Ga-In-Sn oxide target with a Ga:In:Sn ratio of 4:64:32, which was used in a radio-frequency sputtering system to deposit GITO thin films on glass substrates. Furthermore, we describe the microstructural/structural (scanning electron microscopy and X-ray diffraction spectroscopy), optical (wavelength dependent complex refractive indices) and electrical (resistivity, mobility, free carrier density) measurements used to optimize sputtering conditions and post-annealing processing. As well as achieving an optimized/improved GITO thin film deposited at high substrate and annealing temperatures, we obtained promising thin GITO films with excellent optical properties and with relatively low resistivity (1.7mΩcm) deposited and annealed at temperatures around 200°C.

Integrating 2D electron gas oxide heterostructures on silicon using rare-earth titanates

ABSTRACTIntegrating oxide heterostructures on silicon has the potential to leverage the multifunctionalities of oxide systems into semiconductor device technology. We present the growth and characterization of two-dimensional electron gas (2DEG) oxide systems LaTiO3/SrTiO3 (LTO/STO) and GdTiO3/SrTiO3 (GTO/STO) on Si(001). We show interface-based conductivity in the oxide films and measure high electron densities ranging from ∼9 × 1013 cm-2 interface-1 in GTO/STO/Si to ∼9 × 1014 cm-2 interface-1 in LTO/STO/Si. We attribute the higher measured carrier density in the LTO/STO films to a higher concentration of interface-bound oxygen vacancies arising from a lower oxygen partial pressure during growth. These vacancies donate conduction electrons and result in an increased measured carrier density. The integration of such 2DEG oxide systems with silicon provides a bridge between the diverse electronic properties of oxide systems and the established semiconductor platform and points toward new devices and functionalities.

Is NiCo2S4 Really a Semiconductor?

NiCo2S4 is a technologically important electrode material that has recently achieved remarkable performance in pseudocapacitor, catalysis, and dye-synthesized solar cell applications.1−5 Essentially, all reports on this material have presumed it to be semiconducting, like many of the chalcogenides, with a reported band gap in the range of 1.2–1.7 eV.6,7 In this report, we have conducted detailed experimental and theoretical studies, most of which done for the first time, which overwhelmingly show that NiCo2S4 is in fact a metal. We have also calculated the Raman spectrum of this material and experimentally verified it for the first time, hence clarifying inconsistent Raman spectra reports. Some of the key results that support our conclusions include: (1) the measured carrier density in NiCo2S4 is 3.18 × 1022 cm–3, (2) NiCo2S4 has a room temperature resistivity of around 103 μΩ cm which increases with temperature, (3) NiCo2S4 exhibits a quadratic dependence of the magnetoresistance on magnetic field, (4) t...

Temperature-dependent Hall effect measurements on Cz-grown silicon pulled from compensated and recycled feedstock materials

In this work, temperature-dependent Hall effect measurements in the temperature range 88–350K were carried out to investigate the electrical properties of three solar grade p-type Czochralski (Cz) silicon ingots, pulled from recycled p-type multi-crystalline silicon top cuts and compensated solar grade (SoG) feedstock. Material bulk properties including Hall mobility, carrier density and resistivity as functions of temperature were studied to evaluate the influence of compensation and impurities.Recycled top cut replacing poly-silicon as feedstock leads to a more uniform resistivity. In addition, higher concentrations of O and C, give rise to oxygen related defects, which act as neutral scattering centers displaying only a slight influence on the electrical properties at low temperature compared to the dominant compensation effect.The electrical performances of all samples are shown to be strongly dependent on compensation level, especially at the lowest temperature (~88K). A significant presence of incompletely ionized phosphorus was deduced through the measured carrier density. The temperature-dependent Hall effect measurements fit Klaassen׳s mobility model very well at low temperatures (<150K), showing consistency with the explanation of a reduced screening effect on ionized dopants for lightly doped silicon, while the deviation at the high temperature probably may be accounted for by the presence of as-grown defects, such as oxygen related defects and phosphorus clusters, which are usually neglected in most mobility models.

The effect of substrate temperature on the microstructural, electrical and optical properties of Sn-doped indium oxide thin films

In this work, Sn doping In 2 O 3 (ITO) thin films with a thickness of 200 nm were deposited on glass substrates by electron beam evaporation (EBE) method at different substrate temperatures. The crystal structure of these films was studied by X-ray diffraction technique. The sheet resistance was measured by a four-point probe. Van der Pauw method was used to measure carrier density and mobility of ITO films. The optical transmittance spectra were recorded in the wavelength region of 300–800 nm. Scanning electron microscope (SEM) has been used for the surface morphology analysis. The prepared ITO films exhibited body-centered cubic (BCC) structure with preferred orientation of growth along the (2 2 2) crystalline plane. The grain size of the films increases by rising the substrate temperature. Transparency of the films, over the visible light region, is increased with increasing the substrate temperature. It is found that the electrical properties of ITO films are significantly affected by substrate temperature. The electrical resistivity decreases with increasing substrate temperature, whereas the carrier density and mobility are enhanced with an increase in substrate temperature. The evaluated values of energy band gap E g for ITO films were increase from 3.84 eV to 3.91 eV with increasing the substrate temperatures from 200 °C to 500 °C. The SEM micrographs of the films revealed a homogeneous growth without perceptible cracks with particles which are well covered on the substrate.

Transient measurements of carrier relaxation time and density in the P3HT:PCBM organic photovoltaic cell

The authors have used transient photovoltage measurements to evaluate carrier relaxation times (τ) in P3HT:PCBM based photocells over a wide range of open circuit voltages. Satisfactory agreement is found with data obtained by low frequency impedance measurements. The authors find the differential capacitance measurements yield data consistent with the theoretical value expected based on Langevin recombination. The Langevin coefficient is three orders of magnitude smaller than the theoretical one. For the low light levels, the relaxation time variation is determined by the RC time constant behavior of the photodiode.