Decrease In Electron Mobility Research Articles

Effect of chlorine doping on electrical and optical properties of ZnO thin films

Chlorine doped ZnO thin films were grown by metal-organic chemical vapour deposition (MOCVD) on sapphire and fused silica substrates. Chlorine is incorporated by substitution of oxygen and acts as a donor, leading to an increase of electron concentration. Transport properties were studied for ZnO thin films with different chlorine content. Hall effect measurements show an increase of electron carrier concentration and a decrease of electron mobility upon increasing the amount of chlorine incorporated in ZnO. The lowest resistivity ρ = 3.6 × 10 − 3 Ω cm was obtained for layers deposited on sapphire substrate. UV–VIS–NIR spectroscopy has been used for the study of optical properties. For all samples, the optical transmittance in the visible range is greater than 80%. First principles computations were applied in order to examine the change in the band gap of ZnO with Cl doping.

Electrical characterization of CMOS transistors subject to externally applied mechanical stress

Hole and electron mobilities in CMOS structures are significantly influenced by a mechanical strain state. In the present work a new experimental device has been designed, able to apply a uniaxial in-plane strain along different crystallographic orientations. A hole mobility enhancement of +10% and an electron mobility decrease of −5% have been demonstrated with the application of a 0.05% compressive 〈110〉 strain; a hole mobility enhancement of +2% and an electron mobility decrease of −3% have been induced into the material with the application of a 0.05% compressive 〈100〉 strain.

The unoccupied electronic structure of the semi-conducting room temperature molecular magnet V(TCNE) 2

The unoccupied electronic structure of the organic-based magnet V(TCNE) x (TCNE = tetracyanoethylene, x ∼ 2) has been studied with near edge X-ray absorption fine structure (NEXAFS) and with photoelectron spectroscopy (PES). By studying V(TCNE) x upon sodium-doping, the electron-accepting state, i.e., the lowest unoccupied molecular orbital (LUMO) of V(TCNE) x was shown mainly to be localized on (TCNE) −-units in contrast to the hole-accepting state, i.e., the highest occupied molecular orbital (HOMO), which previously was assigned primarily to be V(3d)-derived. This study also showed that there are trap states for electron transport located below the (TCNE) 2− level, likely leading to decreased electron mobility.

Study on electrical properties of n-GaN grown at low temperature by metal-organic vapor phase epitaxy

Electrical properties of n-GaN grown at 740℃ and 900℃ by metal-organic vapor phase epitaxy were studied in detail. According to the electrochemical capacitance-voltage measurement, it is helpful to reduce the background impurity concentration in unintendedly-doped n-GaN grown at low temperature using TEGa as Ga precursor rather than TMGa. The results of Hall effect measurement of heavily Si-doped n-GaN show that, as Si concentration increases, the electron concentration increases almost linearly, which exhibits the dopant band conducting effect, while the electron mobility decreases. Atomic force microscope and X-ray diffraction measurements show that the surface morphology and crystal quality of most samples were determined by growth temperature and Si doping level. The surface morphology of all samples had a visible change only when the doping level was much higher at the same growth temperature. On the other hand, after the samples were annealed in oxygen, the electron mobility of all the samples remained almost constant, but the electron concentration decreased when the Si concentration was larger than 6×1019 cm-3. This result indicates that in the heavily Si-doped n-GaN grown at low temperature, the annealing process could result in the formation of acceptors due to the replacement of N by Si atoms.

Experimental study on electron mobility in ultrathin-body silicon-on-insulator metal-oxide-semiconductor field-effect transistors

The electron mobility in ultrathin-body (UTB) silicon-on-insulator (SOI) metal-oxide-semiconductor field-effect transistors (MOSFETs) with SOI thicknesses from 2.3to60nm is measured in a wide temperature range from 25to300K. In UTB SOI MOSFETs with SOI thickness ranging from 5to20nm, a monotonic decrease of electron mobility with a decrease in SOI thickness is observed at room temperature. At extremely low temperature (25K), however, the electron mobility of UTB SOI MOSFETs with SOI thickness of 5.7nm is not degraded but is almost the same as that of thick-body SOI MOSFETs. On the other hand, in UTB SOI MOSFETs with SOI thickness ranging from 3.5to4.5nm, an increase of electron mobility with a decrease in SOI thickness is clearly demonstrated at the effective normal field of around 0.3MV∕cm at 300K. It is concluded that the decrease of electron mobility in SOI thickness ranging from 5to20nm is due to the increased phonon scattering in thinner SOI films. The observed enhancement of electron mobility with decreasing SOI thickness from 4.5to3.5nm is ascribed to the subband structure modulation induced by quantum confinement effects in nanoscale SOI films. The impact of SOI thickness fluctuations on electron mobility and the methods to evaluate the thickness of ultrathin SOI films are also discussed.

Carrier mobility characteristics in GaInNAs dilute nitride films grown by atomic hydrogen-assisted molecular beam epitaxy

We have investigated the electrical properties of GaInNAs dilute nitride films grown by atomic hydrogen-assisted molecular beam epitaxy. We found that although the hole mobilities in Be-doped p-GaInNAs films exhibit a temperature dependence nearly identical to that for the homoepitaxial p-GaAs films, the electron mobilities in Si-doped n-GaInNAs films are strongly affected by the introduction of nitrogen into Ga(In)As. Further, the degree of scattering by the ionized impurity-like centers generated by N atoms decreased with increasing Si doping, while neutral impurity-like scattering became more dominant with increasing Si doping. These results suggest that the decrease of electron mobility and carrier concentration in Si-doped n-GaInNAs films is strongly correlated with the presence of N and Si atoms.

Synthesis and characterization of a novel electroluminescent polymer based on phenoxazine and fluorene derivatives

A new electroluminescent polymer was prepared by oxidative coupling copolymerization of N-(4- n-butylphenyl)phenoxazine (PPX) and 9,9-di- n-butylfluorene (DBF) using iron(III) chloride. The obtained polymers were soluble in common organic solvents and had number-average molecular weight as high as 10,000. The UV–vis absorption maximum and photoluminescence peak shifted to longer wavelengths with the increase of PPX content. The increase of PPX content also caused increase of hole mobility and decrease of electron mobility. A simple single-layer light-emitting-diode device having a structure of indium tin oxide/polymer/Ca/Al was fabricated. The polymer containing 30% of PPX unit showed highest electroluminescent properties with a maximum luminance of 1084 cd/m 2 at 15.5 V operating voltage.

An analysis of the increase in sheet resistance with the lapse of time of Al 0.3 Ga 0.7 N/GaN HEMT structure wafers

We have observed the increase in sheet resistance with the lapse of time for some Al0.3Ga0.7N/GaN HEMT structure wafers, and this was due to the decrease in electron mobility rather than the decrease in carrier concentration. In the wafers, a lot of triangular micro-cracks extending to 〈110〉 directions were observed by AFM, which were a few tiny lines initially. On the other hand, a wafer that was kept in the nitrogen atmosphere was little increased in sheet resistance. It is plausible that oxygen decreases surface energy of cleaved planes, and micro-cracks progress. The reciprocal lattice map of XRD showed no lattice relaxation of the AlGaN/GaN system. Monochromatic CL image of 359 nm wavelength showed those micro-cracks as dark lines, which means that the micro-cracks work as non-radiative centers, and the charge of trapped electrons might deplete or scatter 2DEG beneath the micro-cracks and cause the increase in sheet resistance. (© 2007 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Optical and kinetic properties of cathodically deposited amorphous tungsten oxide films

Optical properties and the coloration–decoloration kinetics of electrochromic films of amorphous tungsten oxide (a-WO 3), produced by cathodic deposition from a sodium tungstate based aqueous peroxide electrolyte, have been investigated. As films color in 1 N H 2SO 4, sequential appearance of bands with maxima at ∼1 eV, 1.6 eV, 2 eV, and 2.4 eV is observed in their optical absorption and electrosorption spectra, is the same as in the case of reduction of nanosized hydrated-WO 3 colloids with a gradual decrease in their size to that of 12-tungsten polyanions with Keggin structure, indicating the presence of such polytungstates in cathodically deposited a-WO 3, too. When polytungstate is reduced by one electron, an absorption band with a maximum at ∼1.6 eV appears in the optical spectrum of the film. This band corresponds to the optical excitation of charge transfer of the W 5+ → W 6+ type between two adjacent tungsten atoms. The reduction of polytungstate by a second electron with potential shift towards more negative values is accompanied by the appearance of an analogous band with a maximum at ∼2 eV. The reduction of such polytungstates involves participation of the bulk of injected electrons, indicating their dominate role in the nanostructure of the films investigated. The effective co-diffusion coefficient of electrons and protons in cathodically deposited a-WO 3 exhibits a potential dependence with a maximum at 0.1 V against a silver-chloride electrode, where its value is ∼10 −8 cm 2/s. It has been shown that the decrease in this coefficient at potential values of over 0.1 V is caused by a decrease in electron mobility.

Numerical simulation of transconductance of AlGaN/GaN heterojunction field effect transistors at high temperatures

Based on the investigation of the influence of temperatures on parameters, including polarization, electron mobility, thermal conductivity and conduction band discontinuity at the interface between AlGaN and GaN, the temperature dependence of transconductance for AlGaN/GaN heterojunction field effect transistors (HFETs) has been obtained by using a quasi-two-dimensional approach and the calculated results are in good agreement with the experimental data. The reduction in transconductance at high temperatures is primarily due to the decrease in electron mobility in the channel. Calculations also demonstrate that the self-heating effect becomes serious as environment temperature increases.

A thermal model for static current characteristics of AlGaN∕GaN high electron mobility transistors including self-heating effect

A thermal model of AlGaN∕GaN high electron mobility transistors (HEMTs) has been developed based on a quasi-two-dimensional numerical solution of Schrödinger’s equation coupled with Poisson’s equation. The static current characteristics of HEMT devices have been obtained with the consideration of the self-heating effect on related parameters including polarization, electron mobility, saturation velocity, thermal conductivity, drain and source resistance, and conduction-band discontinuity at the interface between AlGaN and GaN. The simulation results agree well with our experimental data. It has also been demonstrated that the reduction of the saturation drain current at high power dissipation is primarily due to the decrease of electron mobility in the channel. The proposed model is valuable for predicting and evaluating the performance of different device structures and packages for various applications.

Investigation of Electrical Properties in Si Ion Implanted GaN Layer as A Function of Dose and Energy

ABSTRACTThe sheet resistance and sheet carrier concentration for Si ion implanted GaN have been investigated as a function of Si ion dosages and ion's energy using van der Pauw method and Hall effect measurement. Si ion implanted GaN is annealed at 1200 °C for 10 sec in N2 gas flow with 50 nm-thick SiNx cap layer to avoid dissociation of GaN. For Si ion energy of 30 keV, the sheet resistance is decreased from 103 to 56 ohm/sq. for the dose ranging from 1 × 1014 to 2 × 1015/cm2. For the Si dose larger than 2 × 1015/cm2, the sheet carrier concentration is saturated around 1 ×s 1015/cm2. Si ion implanted GaN with energy of 50, 80, and 120 keV at a dose of 2 × 1015/cm2 also reveal the sheet carrier concentration of about 1 × 1015/cm2 with the decrease of electron mobility. It is suggested that the implanted Si donors are strongly compensated by the residual implantation-induced defects.

Effects of stoichiometry on electrical, optical, and structural properties of indium nitride

A series of indium nitride (InN) epilayers with different excess indium (In) concentration are grown by plasma-assisted molecular-beam epitaxy on (0001) sapphire substrates. The increasing excess In concentration of the epilayers correlates with an increasing free-electron concentration and a decreasing electron mobility. Photoluminescence (PL) illustrates a 0.77–0.84eV transition for all samples with a redshift in the peak energy with increasing In concentration (for the highest free-electron concentration of 4×1021cm−3). This suggests that the ∼0.8eV PL transition is not consistent with the band-edge transition in InN. Moreover, an additional PL transition at 0.75eV along with the In clusters observed in transmission electron microscopy analysis are found only in the 29% excess In sample. This implies a relationship between the new PL transition and the presence of In clusters. Finally, secondary-ion mass spectrometry is used to verify that the contamination, especially hydrogen (H) and oxygen (O) impurities, has no influence on the redshift of the ∼0.8eV PL peaks and the existence of the additional 0.75eV peak in the sample containing In clusters.

Specific features of Hall effect in capped multilayer pHEMT structure of AlxGa1 − xAs/InxGa1−xAs/GaAs

Electrical properties of capped multilayer pHEMT (pseudomorphic High Electron Mobility Transistor) structure of AlxGa1−xAs/InxGa1−xAs/GaAs have been studied using a Hall effect method. In the device structure InxGa1−xAs layer was prepared as a channel layer and AlxGa1−xAs as the supply and spacer layers. Mole fraction, x, for the alloys varied from 19% to 20% (for InxGa1−xAs) and 20% to 24% (for AlxGa1−xAs). From the measurement carried out at room temperature, it was found that increasing In content in the channel layer will be followed by decreasing electron mobility of multilayer properties of pHEMT structure. This phenomenon is found to be in contradiction with the calculation for the alloy concerned. This phenomenon supports the consideration that electrical properties of supply and spacer layer affect the channel layer properties. From the Hall measurement, relationships between Al content in the supply and spacer with respect to the electron mobility and sheet resistance were obtained.

Temperature measurement in AlGaN/GaN High-Electron-Mobility Transistors using micro-Raman scattering spectroscopy

High power RF device performance decreases as operation temperature increases (e.g. decreasing electron mobility affects cut-off frequencies and degrades device reliability). Therefore determination of device temperature is a key issue for device topology optimisation. In this work the temperature variation of AlGaN/GaN high-electron-mobility transistors grown either on silicon or sapphire substrate under bias operation was measured by micro Raman scattering spectroscopy. Temperature measurements up to power dissipation of 16 W for 4 mm development devices were carried out and a peak temperature of 650 K was determined. The difference of thermal resistance for similar devices grown on the two different substrates was assessed. The thermal resistances of different device topologies were compared to optimise the component design.

Effect of Deuterium Diffusion on the Electrical Properties of AlGaN/GaN Heterostructures

AbstractWe have studied the influence of a deuterium diffusion on the electrical characteristics of the 2D gas present in AlGaN/GaN heterostructures. The deuterium diffusion is performed by exposing the structures to a rf remote deuterium plasma. We find that both the sheet carrier concentration and the electron mobility decrease and that these effects are partly reversible under thermal annealing. These results suggest that deuterium behave as acceptors in the 2D gas region. The negatively charged deuterium act as additional scattering centers for electrons.

Image and exchange-correlation effects in double gate silicon-on-insulator transistors

We have studied the influence of the image and exchange-correlation effects in double-gate silicon on insulator (DGSOI) devices, in the calculation of both charge distribution and of electron mobility. We find that the image and exchange correlation potentials produce a greater confinement of the carriers and, according to the uncertainty principle, a greater phonon scattering rate, which produces a decrease in electron mobility. We have also observed that the influence of image and exchange-correlation potentials on electron mobility, while almost negligible for bulk silicon inversion layers, becomes increasingly important as the silicon thickness decreases, due to the effect of volume inversion in DGSOI inversion layers. It is thus essential to take these effects into account in order to correctly evaluate the inversion charge distribution and the electron mobility in DGSOI inversion layers.

Influence of image force and many-body correction on electron mobility in ultrathin double gate silicon on insulator inversion layers

We study the influence of the image and exchange-correlation effects in double-gate silicon-on-insulator (DGSOI) devices, in the calculation of both charge distribution and electron mobility. The image and exchange correlation potentials produce a greater confinement of the carriers and, according to the uncertainty principle, a greater phonon scattering rate, which produces a decrease in electron mobility. Moreover, the influence of image and exchange-correlation potentials on electron mobility, while almost negligible for bulk silicon inversion layers, becomes increasingly important as the silicon thickness decreases, due to the effect of volume inversion in DGSOI inversion layers. These effects must then be taken into account in order to achieve a correct evaluation of the charge distribution and of mobility in DGSOI inversion layers.

Electrical activity of dislocations and point defects of deformation origin in CdxHg1−x Te crystals

The generation of dislocations with even a relatively low density (Ndis≤107cm−2) leads to significant variations in the kinetic coefficients of CdxHg1−xTe (x=0.20–0.21) crystals. In n-type crystals, a substantial decrease in electron mobility takes place along with a marked growth in electron concentration. For p-type crystals, the transition from the activation conductivity to the metal one is observed in the low-temperature range of 4.2–40 K, as is the alternating-sign behavior of the Hall coefficient R H depending on temperature and magnetic-field strength. A dominant role in the observed modifications is played by electronic states of point defects formed during the dislocation motion rather than the dislocations themselves. The totality of the data can be explained in terms of the formation of connected channels of an opposite-type conductivity in the form of a three-dimensional dislocation network in the matrix of the main crystal.

Electrical properties of carbon nitride thin films: Role of morphology and hydrogen content

The influence of hydrogen content and ambient humidity on the electrical properties of carbon nitride (CNx) films deposited by reactive magnetron sputtering from a graphite target in Ar discharges mixed with N2 and H2 at a substrate temperature of 350°C have been investigated. Carbon films deposited in pure Ar exhibit a dark resistivity at room temperature of ∼4 × 10-2 Ωcm, while the resistivity is one order of magnitude lower for CN0.25 films deposited in pure N2, due to their denser morphology. The increasing H2 fraction in the discharge gas leads to an increased resistivity for all gas mixtures. This is most pronounced for the nitrogen-free films deposited in an Ar/H2 mixture, where the resistivity increases by over four orders of magnitude. This can be related to a decreased electron mobility as H inhibits the formation of double bonds. After exposure to air, the resistivity increases with time through two different diffusion regimes. The measured electrical properties of the films are related to the apparent film microstructure, bonding nature, and ambient humidity.