Average Electron Mobility Research Articles

The influence of gate dielectrics on a high-mobility n-type conjugated polymer in organic thin-film transistors

Organic thin-film transistors based on a high mobility n-type semiconductor poly{[n,n9-bis(2-octyldodecyl)-naphthalene-1,4,5,8-bis(dicarboximide)-2,6-diyl]-alt-5,59-(2,29-bithiophene)} P(NDI2OD-T2) and different polymer gate dielectrics are fabricated. The average electron mobility decreases from 0.76 to 0.08 cm2/Vs with the increase of the gate dielectric constant from 2.6 to 7.8. The P(NDI2OD-T2) film shows unconventional face-on molecular packing, which results in short distances and pronounced interactions between electrons and gate dielectric. Therefore, the decrease of the electron mobility with the increasing dielectric constant is attributed to the Fröhlich polaron effect for the interaction between electrons in the channel and ionic polarization cloud in the gate dielectric.

Electrical characterization of wafer-bonded Ge(111)-on-insulator substrates using four-point-probe pseudo-metal-oxide-semiconductor field-effect transistor method

The electrical properties of wafer-bonded n-type Ge(111)-on-insulator (Ge(111)-OI) substrates were characterized using a four-point-probe pseudo-metal-oxide-semiconductor field-effect transistor (pseudo-MOSFET) method. Average electron and hole mobilities in the Ge(111)-OI channel were measured to be ~1000cm2/Vs in accumulation mode and ~310cm2/Vs in inversion mode, respectively. The measured mobility strongly depended on the sample position, due to the spatially inhomogeneous distribution of the interface states. Despite the existence of interface states, the carrier mobility exhibited a high value demonstrating the prospect of wafer-bonded Ge(111)-OI as a channel material in MOSFETs.

Direct UV Patterning of Electronically Active Fullerene Films

AbstractWe utilize UV light for the attainment of high‐resolution, electronically active patterns in [6,6]‐phenyl C61‐butyric acid methyl ester (PCBM) films. The patterns are created by directly exposing selected parts of a solution‐cast PCBM film to UV light, and thereafter developing the film by immersing it in a tuned developer solution. We demonstrate that it is possible to attain complex, large‐area PCBM structures with a smallest demonstrated‐feature size of 1 μm by this method, and that the patterned PCBM material exhibits a high average electron mobility (1.2 × 10−2 cm2 V−1 s−1) in transistor experiments. The employment of UV light for direct patterning of PCBM for electronic applications is attractive, because PCBM exhibits high absorption in the UV range, and no sacrificial photoresist is needed. The patterning is achieved through the transformation by UV light of the soluble PCBM monomers into insoluble dimers with retained attractive electronic properties.

Synthesis and Characterization of Solution-Processable Ladderized n-Type Naphthalene Bisimide Copolymers for OFET Applications

Solution-processable n-type ladder-based polymers are highly desirable due to their potential capability to form strong π–π interactions. A series of 5 highly soluble naphthalene diimide (NDI) polymers are presented, differing in the degree to which they are able to form imine-bridged ladder polymer structures. Average electron mobilities as high as 0.0026 cm2 V–1 s–1, which show an electron-mobility improvement of 4 orders of magnitude following ladderization, and on/off current ratios on the order of 104 are reported for the novel material PNDI-2BocL, an alkyl-substituted poly(benzoquinolinophenanthrolinedione). The structure–property relationship of the aforementioned series of copolymers is presented and discussed as it pertains to organic field-effect transistor (OFET) performance.

Charge transport, carrier balance, and blue electrophosphorescence in diphenyl[4-(triphenylsilyl)phenyl]phosphine oxide devices

Diphenyl[4-(triphenylsilyl)phenyl]phosphine oxide (EMPA1) displays a wide highest occupied molecular orbital–lowest unoccupied molecular orbital gap (4.1 eV), singlet (4.3 eV) and triplet (3.4 eV), and an electron-dominated charge transport that follows a trap-free space charge limited model with an average electron mobility of 5.7×10−6 cm2 V−1 s−1 and a hole mobility of 1.1×10−6 cm2 V−1 s−1. At high driving voltages (>6 V), ambipolar charge transport is observed, resulting in a balanced charge density in the active layer. Highly efficient blue phosphorescent organic light-emitting diodes were realized, showing a high external quantum efficiency (21%) and a luminance efficiency of 45 cd/A using a bis[2-(4′,6′-difluorophenyl)-pyridinato-N,C2′]iridium(III) picolinate dopant.

Fabrication and photoelectrical characteristics of ZnO nanowire field-effect transistors

The fabrication and photoelectrical characteristics of suspended ZnO nanowire (NW) field-effect transistors (FETs) are presented. Single-crystal ZnO NWs are synthesized by a hydrothermal method. The fabricated FETs exhibit excellent performance. When Vds = 2.5 V, the peak transconductance of the FETs is 0.396 μS, the average electron mobility is 50.17 cm2/(V ṁ s), the resistivity is 0.96 × 102 Ω.cm at Vgs = 0 V, and the current on/off ratio (Ion/Ioff) is approximately 105. ZnO NW-FET devices exposed to ultraviolet radiation (2.5 μW/cm2) exhibit punch-through and threshold voltage (Vth) shift (from −0.6 V to +0.7 V) and a decrease by almost half of the source-drain current (Ids, from 560 nA to 320 nA) due to drain-induced barrier lowering. Continued work is underway to reveal the intrinsic properties of suspended ZnO nanowires and to explore their device applications.

Raman scattering of phonon-plasmon coupled modes in self-assembled GaN nanowires

We report the determination of free-electron concentration and mobility of free-standing GaN nanowires (NWs) by line shape analysis of the coupled longitudinal optical phonon-plasmon Raman modes (L+). The E2high phonon mode at 566.9 cm−1 with a sharp linewidth of 2.8 cm−1 indicates strain free NWs with high crystalline perfection. The lattice temperature of the NWs was varied between 313 and 472 K by varying the excitation laser beam power. For unintentionally doped samples at room temperature, an average electron concentration and mobility of strain free NWs were found to be ∼2×1017 cm−3 and 460 cm2/V s, respectively. We have shown that the electron concentration does not change significantly over a temperature range between 313 and 472 K. The electron mobility decreases at high temperatures, in agreement with literature data for compact layers. For Si-doped NWs, the L+ phonon peak is strongly upshifted indicating a higher free-carrier concentration of about 1×1018 cm−3. Asymmetric broadening observed at the lower frequency side of the L+ phonon peak might be ascribed to the enhancement in surface optical modes due to the high surface-to-volume ratio of NWs.

A Novel Alternating Phenylenevinylene Copolymer with Perylene Bisimide Units: Synthesis, Photophysical, Electrochemical, and Photovoltaic Properties

An alternating phenylenevinylene copolymer P bearing perylene bisimide moieties along the backbone was synthesized by Heck coupling. The tert-butyl and hexyloxy side groups enhanced the solubility of the copolymer in common organic solvents. It had glass transition and decomposition temperatures of 72 and 370 °C, respectively. The absorption of P was broad, with longer wavelength maximum around 500 nm and optical band gap of 1.66 eV. The solution of P emitted yellow−orange light with photoluminescence (PL) maximum at 555 nm. Moreover, efficient interaction and charge/energy transfer between the phenylenevinylene−donor and the perylene bisimide−acceptor led to PL quenching in the thin film of P. The current−voltage characteristics of ITO/copolymer P/Al indicated that copolymer P behaved as an n type organic semiconductor. The electron current was found to be a space charge limited current (SCLC), providing a direct measure of mobility as a function of temperature and field. The average electron mobility fo...

DC characterization of accumulation layer and bulk layer electron mobility in 4H–SiC depletion mode MOSFET

4H–SiC depletion mode metal oxide semiconductor field effect transistors (MOSFETs) have been fabricated and characterized. The device has a structure similar to that of a metal semiconductor FET (MESFET) where a physical conducting channel already exists. A MOS gate is used to replace the Schottky gate and this allows the device to operate in depletion mode as well as accumulation mode. The oxide layer was formed using thermal oxidation without any nitridation. The average electron mobility of the accumulation layer has been previously deduced to be about 17.5 cm 2/V s. In this work, we further analyzed the DC current–voltage characteristics of the devices and deduced the electron mobility of the accumulation layer as a function of the gate voltage ( V GS). A model was developed from which the mobility of the bulk channel layer was estimated as a function of V GS. The doping density in the channel layer was also deduced and the result is consistent with that obtained from secondary ions mass spectroscopy.

Quantitative Mobility Spectrum Analysis of AlGaN/GaN Heterostructures Using Variable-Field Hall Measurements

Carrier transport properties of AlGaN/GaN heterostructures have been analyzed with the quantitative mobility spectrum analysis (QMSA) technique. The nominally-undoped Al0.15Ga0.85N/GaN sample was grown by metal-organic vapor phase epitaxy. Variable-magneticfield Hall measurements were carried out in the temperature range of 4-160 K and magnetic field range of 0-6.6 T. QMSA was applied to the experimental variable-field data to extract the concentrations and mobilities associated with the high-mobility 2DEG and the relatively lowmobility bulk electrons for the temperature range investigated. For temperatures below 100 K the calculated mobility and carrier density values were close to the experimental results. No bulk conduction was observed in this temperature range. At 160 K, QMSA results show that parallel conduction in 3 mm thick GaN layer started to affect the average electron mobility.

Electrical Conductivity and Oxygen Nonstoichiometry of La0.2Sr0.8Fe0.55Ti0.45O3-δ

The electrical conductivity of (LSFTO) was measured in the temperature range and at oxygen partial pressures of . The conductivity was measured on both increasing and decreasing to ensure that equilibrium was reached. Long equilibration times were required at intermediate oxygen partial pressures between and . The pressure dependence of the conductivity in this range differs from that previously reported for related compositions. The mobilities of the charge carriers were derived from the stoichiometry and conductivity data. The average hole and electron mobilities of LSFTO at are 0.015 and , respectively. The oxygen nonstoichiometry as a function of was determined by solid-state coulometric titration. Data were obtained in the temperature range, , and in the range, . A point defect model was used to fit the dependence of δ on . The partial molar thermodynamic quantities for oxidation were calculated and the oxygen nonstoichiometry data were related to the electrical conductivity data.

Optical absorption and Hall effect in (220) and (400) oriented polycrystalline silicon films

Carrier transport properties were investigated for polycrystalline silicon (poly-Si:H:F) films fabricated at 300 °C by 100 MHz plasma enhanced chemical vapor deposition from gaseous mixture of SiF4 and H2. Analysis of free carrier optical absorption (FCA) revealed that 1 μm thick (400) oriented phosphorus-doped poly-Si:H:F films with a carrier concentration of 5×1019 cm−3 had the average electron mobility in crystalline grains at 40 cm2/V s, while the electron mobility of the (220) oriented phosphorus-doped poly-Si:H:F films was only 12 cm2/V s. These results indicated that (400) oriented poly-Si:H:F films had excellent quality crystalline grains. Analyses of the FCA combined with Hall effect current measurements revealed that the electrical conductivity at grain boundaries of top doped films increased as the underlying film thickness increased from 0 to 280 nm for (400) oriented phosphorus-doped/undoped double layered samples, but grain boundaries still acted as large resistive regions limiting the effective conductivity.

Time of flight experimental studies of CdZnTe radiation detectors

A time of flight technique was used to study the carrier trapping time, τ, and mobility, μ, in CdZnTe (CZT) and CdTe radiation detectors. Carriers were generated near the surface of the detector by a nitrogen-pumped pulsed dye laser with wavelength ∼500 nm. Signals from generated electrons or holes were measured by a fast oscilloscope and analyzed to determine the trapping time and mobility of carriers. Electron mobility was observed to change with temperature from 1200 cm2/Vs to 2400 cm2/Vs between 293 K and 138 K, respectively. Electron mobilities were observed between 900 cm2/Vs and 1350 cm2/Vs at room temperature for various CZT detectors. Electron mobilities in various CdTe detectors at room temperature were observed between 740 cm2/Vs and 1260 cm2/Vs. Average electron mobility was calculated to be 1120 cm2/Vs and 945 cm2/Vs for CZT and CdTe, respectively. Hole mobilities in both CZT and CdTe were found to vary between 27 cm2/Vs and 66 cm2/Vs. Electron trapping times in CZT at room temperature varied from 1.60 µs to 4.18 µs with an average value of about 2.5 µs. Electron trapping time in CdTe at room temperature varied between 1.7 µs and 4.15 µs with an average value of about 3.1 µs.

Helical Resonator Plasma Oxidation of Amorphous and Polycrystalline Silicon for Flat Panel Displays

AbstractA low temperature process for the oxidation of amorphous and polycrystalline silicon was studied using a helical resonator plasma source. Constant current anodization of amorphous silicon was performed at 350°C and 30 mtorr in an oxygen plasma. Oxidation rate data were analyzed by both parabolic and power law expressions: parabolic rate coefficient B = 23.45 nm2/min and power law rate coefficients α = 0.3807 and C* = 7.3 nm/minα. Oxidation at substrate floating conditions yielded thin oxides; the growth rate saturated after 20 mins, at a thickness of about 10 nm. For TFr fabrication, a two layer oxide was used. The first layer was grown by plasma oxidation to a thickness of about 10 nm to ensure a good interface, and the second layer deposited by PECVD to the final thickness of 35 nm. Electrical properties of control oxides grown on c-Si at the same conditions were promising, with a fixed oxide charge of 1.98 × 1011 charges/cm2 and a mean breakdown field of 5.3 MV/cm after post metallization anneal. The TF's had an average effective electron mobility of 31.1 V/cm2 and an average threshold voltage of 4.6 V.

Random telegraph signals in deep submicron n-MOSFET's

Random telegraph signals (RTS) in the drain current of deep-submicron n-MOSFET's are investigated at low and high lateral electric fields. RTS are explained both by number and mobility fluctuations due to single electron trapping in the gate oxide. The role of the type of the trap (acceptor or donor), the distance of the trap from the Si-SiO/sub 2/ interface, the channel electron concentration (which is set by the gate bias) and the electron mobility (which is affected by the drain voltage) is demonstrated. The effect of capture and emission on average electron mobility is demonstrated for the first time. A simple theoretical model explains the observed effect of electron heating on electron capture. The mean capture time depends on the local velocity and the nonequilibrium temperature of channel electrons near the trap. The difference between the forward and reverse modes (source and drain exchanged) provides an estimate of the effective trap location along the channel.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

Multiquantum well structure with an average electron mobility of 4.0×106 cm2/V s

We report a modulation-doped multiquantum well structure which suppresses the usual ambient light effect associated with modulation doping. Ten GaAs quantum wells 300-Å wide are symmetrically modulation doped using Si δ doping at the center of 3600-Å-wide Al0.1Ga0.9As barriers. The low field mobility of each well is 4.0×106 cm/V s at a density of 6.4×1010 cm−2 measured at 0.3 K either in the dark, or during, or after, exposure to light. This mobility is an order of magnitude improvement over previous work on multiwells.

A new technique to determine the average low-field electron mobility in MESFET using C-V measurement

A method to determine the average low-field mobility using the number of electrons available for the conduction based on C-V measurement is proposed. This technique requires neither information of the doping profile in the channel, nor the exact value of the threshold voltage. For a D-mode MESFET, the average electron mobility magnitude is compared with that of the C. Chen and D.K. Arch (1989) method. The technique to determine the average electron mobility in the channel described is much simpler. Based on C-V measurement, good agreement is obtained between experimental data and simulation calculation for the electron density in the channel. Using the proposed method, the dependence of average electron mobility on the gate voltage is also proposed. Using the proposed method for determining the average electron mobility, the effect of a p-buried layer on the mobility was investigated, and is in good agreement with the physical phenomena.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

Real space transfer and modulation of electron mobility in GaAs/AlGaAs double quantum well structures

The electronic properties of a GaAs/AlGaAs double quantum well field-effect transistor (DQWFET) are studied at 77 and 4.2 K. The channel of the DQWFET consists of two quantum wells with different widths. The channel conductance as a function of the gate voltage indicates the switching of the conductive channel from one quantum well to the other, and the average electron mobility is modulated. In order to confirm the phenomena, photoluminescence (PL) spectra are examined as a function of the gate voltage, which shows a specific variation associated with the anti-crossing of the two quantized levels in the DQW structure.

Effects of substrate misorientation on anisotropic electron transport in InGaAs/GaAs heterostructures

The effects of substrate misorientation on anisotropic electrical properties of thick InGaAs layers grown on GaAs by low-pressure metalorganic vapor phase epitaxy are studied. Hall bar samples oriented along [110] and [1̄10] were cut from layers grown simultaneously on substrates oriented (a) exactly on (001) and (b) at 2° off (001) towards [011]. In layers grown on (001) oriented substrates a very large mobility anisotropy (μ[110]/μ[1̄10]≊ 1000) is observed and attributed mainly to a lattice mismatch induced anisotropy. For the misoriented substrates, the average electron mobility increases by an order of magnitude and shows only a small residual anisotropy in the opposite direction (μ[110]/μ[1̄10] ≊ 0.7).

The influence of dislocation density on electron mobility in InP films on Si

The average electron mobility and dislocation density have been measured as functions of the film thickness in InP films grown on Si substrates by gas-source molecular beam epitaxy. In a region extending about 2 μm from the Si interface, the density of dislocations was found to be very high and clustering of dislocations was observed by transmission electron microscopy. The corresponding mobility was very small. Beyond 2 μm, clustering was not observed, the density of dislocations decreased, and the average mobility increased with increasing film thickness. Thus, the threading dislocations created by the large InP/Si lattice mismatch can significantly degrade the free carrier mobility of the InP film.