Electron Drift Mobility Research Articles

Cd$_{0.9}$Zn$_{0.1}$Te Crystal Growth and Fabrication of Large Volume Single-Polarity Charge Sensing Gamma Detectors

Detector grade Cd <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">0.9</sub> Zn <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">0.1</sub> Te single crystals were grown using a tellurium solvent method. Single crystal blocks of volume ~1 cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sup> were prepared for detector fabrication and characterization. The grown crystals were characterized using infra-red transmission imaging and Pockel's effect measurements. Two detectors in single-polarity charge sensing configurations viz., small pixel, and virtual Frisch grid were fabricated on two crystals obtained from the same section of the ingot. Current-voltage measurements performed in planar configuration exhibited a very low leakage current of ~5 nA at 1000 V and resistivities of the order of 8.5×10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">10</sup> Ω·cm. Electron drift mobilities of the order of 840 cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> /V.s and electron mobility-lifetime products of the order of 2.7×10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-3</sup> cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> /V were calculated from alpha spectroscopy using detectors in planar configuration. The small pixel and the virtual Frisch grid detector showed similar energy resolution of 3.7% for 662 keV gamma rays however, the virtual Frisch grid configuration revealed a better overall performance with a peak-to-Compton ratio of 2.8. A digital spectrometer and related software has been developed using a digitizer card and used to employ offline correction schemes to compensate for the charge loss effects, resulting in significant improvement of the 662 keV peak resolution (1.8% as compared to 3.7% without correction) obtained in the case of small pixel detector.

Electron-transporting naphthalimide-substituted derivatives of fluorene

Naphthalimide-substituted derivatives of fluorene were synthesized by Suzuki–Miyaura coupling reactions. Most of the synthesized materials were found to constitute glasses with glass transition temperatures ranging from 30 to 76 °C as characterized by differential scanning calorimetry. Their initial weight loss temperatures range from 315 to 466 °C. Dilute solutions of the naphthalimide-substituted derivatives of fluorene in nonpolar solvents were found to emit in the blue region with high quantum yield ranging from 0.47 to 0.69, while in the solid state fluorescence quantum yields were found to be in the range of 0.06–0.25. Fluorenes possessing two naphthalimide moieties exhibited higher values of the fluorescence quantum yield due to enhanced oscillator strength of the transition. The derivatives of fluorene containing naphthalimide moieties exhibited pronounced positive solvatochromic behaviour in polar solvents indicating on charge-transfer character of the lowest excited states. Fluorescence red shift in dimethylsulfoxide is found of 94 nm, while the fluorescence quantum yield is increasing with polarity from 0.47 in cyclohexane to 0.84 in dimethylsulfoxide. Such untypical enhancement of fluorescence quantum yield is accounted to reduced intersystem crossing in polar solvents, which is proved by almost 10 fold increase in the values of nonradiative decay rate for compounds dissolved in polar solvents. Time-of-flight electron drift mobilities of the layer of 2,7-di((N-octyl-1,8-naphthalimide)-4-yl)-9,9-dioctyl-9H-fluorene in air at 25 °C appproach 1.2 × 10−3 cm2 V−1 s−1 at an eletric field of 6.4 × 105 Vcm−1.

Gamma Ray Detection with Cd0.9Zn0.1Te Based Detectors Grown Using a Te Solvent Method

ABSTRACTCd0.9Zn0.1Te (CZT) single crystal has been grown using a tellurium solvent method. Two CZT crystals have been chosen from two different locations of the grown ingot. The two crystals were characterized using infrared transmission (IR) imaging and radiation detectors in planar geometry were fabricated on them. Current-voltage characteristics (I-V) revealed a resistivity of ∼8.6×1010 Ω−cm for detector A (6.9×6.9×4.8 mm3) and 6.7×1010 Ω−cm for detector B (11.5×11.7×2.6 mm3). IR imaging showed a lesser concentration of Te inclusions/precipitates in detector A. The transport properties viz., electron drift-mobility and electron mobility-lifetime product were measured using alpha spectroscopy in these detectors in a planar configuration. Detector A showed better charge transport properties compared to detector B. The superior transport properties of crystal A were reflected in the spectroscopic properties of the detectors. Gamma pulse height measurements using a 241Am isotope revealed an energy resolution of ∼5 % for detector A and ∼7 % for detector B. A digital spectrometer and a biparametric correction scheme was incorporated to recover the pulse height spectrum of high energy gamma rays (137Cs source) from the effect of poor hole movement.

Proposal of contact potential promoted oxide semiconductor gas sensor

Contact between two oxide semiconductor grains different in work function (hetero-contact) is characterized by the generation of contact potential, which is sensitive to gases. The resistance of the hetero-contact is influenced by ambient gases dually, through a change in the surface density of carriers (receptor function of key grain) and through a change in drift mobility of electrons (contact potential effect), and hence not only it is more sensitive to gases than that of contact between grains of the same kind (homo-contact) but also it obeys different power laws. It is shown through explorations using device models that a novel composite oxides gas sensor making use of contact potential will be available if the two kinds of grains to be combined are selected properly and the structure the grains are packed in is designed properly. It is especially important to achieve such particular packing structure that allows hetero-contacts to be included in the main paths of electron transport.

Electron and hole transport in an anthracene-based conjugated polymer

The bulk transport properties of an anthracene-containing poly(p-phenylene-ethynylene)-alt-poly(p-phenylene-vinylene) (AnE-PVstat) were investigated by small-signal time-of-flight. AnE-PVstat exhibited a very good ambipolar behaviour, consistent with the high delocalization of both frontier molecular orbitals. The transport of negative carriers was found to be even faster than that of positive carriers, with an electron drift mobility of about six times higher than that evaluated for holes, suggesting that conjugated polymers with carbon-carbon triple bonds could represent a promising platform toward high performance ambipolar materials for plastic electronics.

Carrier mobility enhancement versus channel direction in highly-doped Si

Carrier mobilities (μ) versus Si channel directions have been studied for boron-, arsenic-, and phosphorus-implanted highly-doped Si (100) which is equivalent to a source and drain (SD) region of MOSFET by using a continuous anodic oxidation technique/differential Hall effect (CAOT/DHE) technique. The results for B-doped Si show that the hole drift mobility along the Si <100> channel direction can be enhanced relative to the Si <110> channel direction by an average of 12%, and nearly 115% near the surface. For As-doped Si the electron drift mobility along the Si <100> channel direction has little to no enhancement compared to the Si <110> channel direction. For P-doped Si, the electron drift mobility along the Si <100> channel direction can be enhanced relative to the Si <110> channel direction by an average of 10%, and as much as 18% near the surface. This study shows for the first time that electron mobility in P-doped Si is enhanced in the Si <100> channel direction, although the enhancement is not as significant as in B-doped Si. The results are qualitatively and semi-quantitatively consistent with the implant species-dependent effective mobility (μeff) behaviors of CMOS device I–V characteristics.

Electron drift-mobility measurements in polycrystalline CuIn1−xGaxSe2 solar cells

We report photocarrier time-of-flight measurements of electron drift mobilities for the p-type CuIn1−xGaxSe2 films incorporated in solar cells. The electron mobilities range from 0.02 to 0.05 cm2/Vs and are weakly temperature-dependent from 100–300 K. These values are lower than the range of electron Hall mobilities (2-1100 cm2/Vs) reported for n-type polycrystalline thin films and single crystals. We propose that the electron drift mobilities are properties of disorder-induced mobility edges and discuss how this disorder could increase cell efficiencies.

Enhancement-Mode Antimonide Quantum-Well MOSFETs With High Electron Mobility and Gigahertz Small-Signal Switching Performance

This letter demonstrates, for the first time, enhancement-mode (e-mode) antimonide MOSFETs by integrating a composite high-κ gate stack (3 nm Al2O3 -1 nm GaSb) with an ultrathin InAs0.7Sb0.3 quantum well (7.5 nm). The MOSFET exhibits record high electron drift mobility of 5200 cm2/V · s at carrier density (Ns) of 1.8 × 1012 cm-2, subthreshold slope of 150 mV/dec, ION/IOFF ratio of ~4000× within a voltage window of ~1 V, high ION of 40 μA/μm at VDS of 0.5 V for a 5-μm gate length (LG) device. The device exhibits excellent pinchoff in the output characteristics with no evidence of impact ionization enabled by enhanced quantization and e-mode operation. RF characterization allows extraction of the intrinsic device metrics (Cgs, Cgd, gm, veff and ft) and the parasitic resistive and capacitive elements limiting the short-channel device performance.

Temperature- and Doping-Dependent Anisotropic Stationary Electron Velocity in Wurtzite GaN

The influences of temperature, dopant density, free-carrier density, and field orientation on the electron drift mobility and velocity-field relationship in wurtzite <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">c</i> -axis GaN are quantified by means of theoretical investigation. Electron velocity perpendicular to the growth plane is uniformly lower than that parallel to the growth plane for field strengths below 500 kV/cm, although anisotropy within the basal plane itself is found to be insignificant. The calculated low-field electron mobility is demonstrated to be consistent with recent Hall measurements over a range of dopant densities. Low-field mobility is enhanced under the influence of free-carrier densities above the background doping due to both increased screening of ionized impurities and a reduction in A1-LO phonon lifetime through the plasmon-phonon interaction.

Laser ablated ZnO layers for ALGaN/GaN HEMT passivation

ZnO layer in a role of passivation of the AlGaN/GaN-based high electron mobility transistors (HEMTs) is presented. The thin layer is deposited by pulsed laser deposition technique. It is fully compatible with the process technology of high electron mobility transistors prepared on AlGaN/GaN heterostructures due to its physical properties similar to the GaN. We have succeeded to (1) suppress the gate leakage current; (2) increase the maximum of the drain current and the electron drift mobility, and (3) ensure the threshold voltage to be unaltered by employment of the thin ZnO layer to the channel area of the HEMT.

Numerical Analysis of Forward-Current/Voltage Characteristics of Vertical GaN Schottky-Barrier Diodes and p-n Diodes on Free-Standing GaN Substrates

Forward-current-density <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">J</i> <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">F</sub> /forward-voltage <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">V</i> <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">F</sub> characteristics of vertical gallium-nitride (GaN) Schottky-barrier diodes (SBDs) and p-n diodes on free-standing GaN substrates were computationally, as well as experimentally, investigated. Based on the thermionic emission model, electron-drift mobility μ <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">n</i> was used as a parameter to fit the <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">J</i> <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">F</sub> - <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">V</i> <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">F</sub> characteristics of both reported and fabricated GaN SBDs. At 300 K, μ <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">n</i> was fitted to be 600 cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> /V ·s when electron concentration <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">n</i> was 1 × 10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">16</sup> cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-3</sup> and 750 cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> /V ·s when <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">n</i> was 5 ×10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">15</sup> cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-3</sup> . Accordingly, the theoretical μ <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">n</i> - <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">n</i> curve for a carrier compensation ratio of 0.90 was applied in the case of n-GaN layers on GaN substrates. With respect to GaN p-n diodes, the reported <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">J</i> <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">F</sub> - <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">V</i> <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">F</sub> characteristics were successfully fitted with dislocation-mediated carrier lifetimes in the high-injection region and with Shockley-Read-Hall lifetimes in the generation-recombination current region.

Bipolar polymer semiconductor blends of C60‐end‐capped poly(4‐diphenylaminostyrene) and poly(4‐diphenylaminostyrene): One‐pot synthesis and charge‐transport properties

AbstractThree types of fullerene (C60)‐end‐capped poly(4‐diphenylaminostyrene) (C60–PDAS) and poly(4‐diphenylaminostyrene) (PDAS) blends were prepared to investigate their potential as bipolar polymer semiconductors. The concentration of C60 in the C60–PDAS/PDAS blends strongly affected the hole and electron drift mobility values; the hole drift mobility decreased with an increase in the C60 concentration. However, the electron drift mobility increased with an increase in the C60 concentration. The hole and electron drift mobility values were almost the same for the 1/2 C60–PDAS/PDAS blend; therefore, this polymer blend was thought to be a bipolar polymer semiconductor. © 2011 Wiley Periodicals, Inc. J Appl Polym Sci, 2011

Electroactive Twin Compounds Containing Trioxothioxanthene Electron Accepting Moieties

Three dimeric molecules based on trioxothioxanthene and benzotriazole moieties were synthesized by alkylation and esterification reactions. The chemical structures of the compounds were confirmed by 1HNMR, IR spectroscopies and mass spectrometry. Dilute solutions of trioxothioxanthene and benzotriazole-based compounds in THF absorb electromagnetic radiation in the region of 230–320 nm. 1,10-Bis-(9,10,10-trioxothioxanthen-3-yl-carboxy)decane can be transformed into the glassy state with the glass transition temperature of 35°C. It is capable of transporting negative charges. Electron drift mobility in the layers of trioxothioxanthene-based twin compound molecularly dispersed in PC-Z approaches 10−6 cm2/Vs at an electric field of 1 × 106 V/cm.

Synthesis of poly(p -phenylene)-poly(4-diphenylaminostyrene) bipolar block copolymers with a well-controlled and defined polymer chain structure

A poly(p-phenylene) (PPP)-poly(4-diphenylaminostyrene) (PDAS) bipolar block copolymer was synthesized for the first time. A prerequisite prepolymer, poly(1,3-cyclohexadiene) (PCHD)-PDAS binary block copolymer, in which the PCHD block consisted solely of 1,4-cyclohexadiene (1,4-CHD) units, was synthesized by living anionic block copolymerization of 1,3-cyclohexadiene and 4-diphenylaminostyrene. To obtain the PPP-PDAS bipolar block copolymer, the dehydrogenation of this prepolymer with quinones was examined, and tetrachloro-1,2-(o)-benzoquinone was found to be an appropriate dehydrogenation reagent. This dehydrogenation reaction was remarkably accelerated by ultrasonic irradiation, effectively yielding the target PPP-PDAS bipolar block copolymer. The hole and electron drift mobilities for PPP-PDAS bipolar block copolymer were both on the order of 10−3 to 10−4 cm2/V·s, with a negative slope when plotted against the square root of the applied field. Therefore, this bipolar block copolymer was found to act as a bipolar semi-conducting copolymer. © 2011 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem, 2011

Measurement of the drift mobilities and the mobility-lifetime products of charge carriers in a CdZnTe crystal by using a transient pulse technique

In this work we present results on the measurement of the drift mobility and the mobility-lifetime product of charge carriers in a 16-pixellated CdZnTe detector. For the determination of an interaction position based on the pulse rise-time method in a CZT detector, it is necessary to characterize the transport properties governed by drift mobility and lifetime for electrons and holes. In order to extract the transport properties of an electron and a hole, we bombarded 5.5-MeV alpha particles from a 241Am source and 81-keV gamma rays emitted from a 133Ba source on the negatively biased contact of the CZT detector. A time-of-flight (TOF) method was used to measure the electron drift mobility at room temperature whose value turned out to be 906.4 cm2/V⋅s. With the Hecht's equation, the electron mobility-lifetime product was also determined from the bias-dependent alpha response and was equal to (9.88 ± 2.33) × 10−3 cm2/V. On the other hand, the hole mobility-lifetime product was evaluated by a model based on the average charge collection efficiency which accounts for the absorption probability with a given photon energy. By using a single parameter fitting of the model, we obtained the hole mobility-lifetime product of (8.28 ± 0.17) × 10−4 cm2/V.

Time-of-Flight Characterization of Single-crystalline CVD Diamond with Different Surface Passivation Layers

ABSTRACTThe electronic properties of diamond, e.g. a high band-gap and high carrier mobilities, together with material properties such as a very high thermal conductivity, chemical inertness and a high radiation resistance makes diamond a unique material for many extreme electronic applications out of reach for silicon devices. This includes, e.g. microwave power devices, power devices and high temperature electronics. It is important to have an effective passivation of the surface of such devices since the passivation determines the ability of the device to withstand high surface electric fields. In addition, the passivation is used to control the surface charge which can strongly influence the electric field in the bulk of the device. It is possible to measure sample parameters such as electron and hole drift mobilities, charge carrier lifetimes or saturation velocities using Time-of-flight (ToF) method. The ToF technique has also been adapted for probing the electric field distribution and the distribution of trapped charge. In this paper we present new data from lateral ToF studies of high-purity single crystalline diamond with different surface passivations. Silicon oxide and silicon nitride are used as passivation layers in the current study. The effect of the passivation on charge transport is studied, and the results of different passivation materials are compared experimentally.

Air stable electron-transporting and ambipolar bay substituted perylene bisimides

Four new perylene bisimides containing carbazolyl and triphenylamino electron-donor groups in the bay region have been designed, synthesized and characterized. The materials possess high thermal stability and form uniform films. They display a wide absorption window extending to the near infrared region of the spectrum and demonstrate efficient photoinduced intramolecular electron transfer. Ionization potential values of these perylene bisimide derivatives measured by photoelectron spectroscopy range from 5.8 eV to 6 eV. Charge-transporting properties were investigated by the xerographic time of flight (XTOF) technique. Complementary ambipolar charge-transport was observed in differently linked carbazolyl substituted perylene bisimides while the triphenylamino substituted material exhibited competent electron drift mobility (>10−3 cm2 V−1 cm−1) under ambient conditions. Density functional theory (DFT) calculations were performed for carbazolyl bay substituted perylene bisimides in order to understand the complementary ambipolar charge transport as well as the difference in the optical properties.

Ultrasound‐accelerated dehydrogenation of poly(1,3‐cyclohexadiene): efficient synthesis of poly(para‐phenylene)

AbstractThe effect of ultrasonication on the dehydrogenation of poly(1,3‐cyclohexadiene) (PCHD) with benzoquinones was examined with the aim of improving the rate of reaction at moderate temperature. The type of solvent and the ultrasound treatment strongly affected the dehydrogenation of PCHD. The rate of reaction of the dehydrogenation of PCHD with 2,3‐dichloro‐5, 6‐dicyano‐1,4‐benzoquinone (DDQ) or 3,4,5,6‐tetrachloro‐1,2‐(o)‐benzoquinone (TOQ) was markedly improved by the use of ultrasound, and poly(para‐phenylene) (PPP) and PPP–TOQ complex, respectively, were successfully obtained. The electron drift mobility for PPP was of the order of 10−4 cm2 V−1 s−1 with a negative slope, while that for PPP–TOQ complex was of the order of 10−3 to 10−4 cm2 V−1 s−1 with a negative slope. The dehydrogenation of PCHD with benzoquinones under ultrasonication is thus an effective method to obtain soluble PPP with a well‐defined polymer chain structure. Copyright © 2010 Society of Chemical Industry

Effect of fluorine interface redistribution on performance of AlGaN/GaN HEMTs

The effect of fluorine interface redistribution on dc and microwave performances of SF 6 plasma-treated AlGaN/GaN high-electron mobility transistors (HEMTs) was investigated. Selective SF 6 plasma treatment of the AlGaN/GaN HEMT gate interface yielded increases in the current gain cut-off frequency ( f T) and maximum frequency of oscillation ( f max) of almost 60%. Annealing induced fluorine interface redistribution showed a low impact on the electron drift mobility and a negligible impact on the peak transconductance of the HEMTs. A large impact of the fluorine interface redistribution was observed for the threshold voltage and sheet carrier concentration of two-dimensional electron gas (2DEG). Consequently, it led to a decrease in the f T and f max values, but the values were still higher than those of conventional reference HEMTs.

Numerical simulation of evolution of electron-hole avalanches and streamers in silicon in a uniform electric field

Numerical simulation of origination and evolution of streamers in Si is performed for the first time. It is assumed that an external electric field E 0 is constant and uniform, the avalanche and streamer are axially symmetric, and background electrons and holes are absent. The calculations have been performed in the context of the diffusion-drift approximation with impact and tunneling ionization, Auger recombination, and electron-hole scattering taken into account. The most realistic values of the ionization and recombination rates, diffusion coefficients, and drift mobilities of electrons and holes have been used. It is shown that the features of evolution of avalanches and streamers are generally consistent with the result obtained previously for a hypothetic semiconductor with equal kinetic coefficients for electrons and holes. Asymmetry of these coefficients (mostly, the impact-ionization coefficients) manifests itself only at the initial stage of evolution. However, with time, two exponentially self-similar streamers are formed, differing only in the sign of charge of fronts and directions of their propagation. Empirical dependences of the main parameters of streamers on E 0 in the range of 0.34–0.75 MV/cm have been derived for this most important stage of evolution.