Enhanced Electron Mobility Research Articles

Thick In x Ga1−x N Films Prepared by Reactive Sputtering with Single Cermet Targets

InxGa1−xN films with x = 0, 0.25, and 0.5 were grown on SiO2/Si(100) substrates by reactive sputtering at 200°C for 90 min with single cermet targets made by hot pressing a powder mixture of metallic indium and gallium and gallium nitride. After alloying with In, InGaN showed preferential (10\( \mathop 1\limits^{ - } \)0) diffraction, smooth surface with roughness less than 1.80 nm, reduced mismatch with the Si substrate, enhanced electron mobility above 7 cm2 V−1 s−1, and blue and green light-emitting capabilities.

Uniaxial Stress Induced Electron Mobility Enhancement in Silicon

The conduction band structure of Si under uniaxial [100] stress is investigated by using the two band k·p perturbation theory. The conduction band parameters, including the conduction band minimum energy shift, split and electron effective mass change are quantitative described. Based on these band parameters, uniaxial stress induced electron mobility enhancment is systematically studied through Boltzmann transport theory. The results show that, when uniaxial [100] stress is applied to Si crystal, a gain of more than 50 % in electron mobility can be obtained. The electron mobility along the [100] increase with tensile stress and decrease with compressive stress, while does the opposite for the electron mobility along the [010] and [001]. The electron mobility along the [100], [010] and [001] tend to saturate for stress beyond a certain level. The stress induced the reduction of conductivity effective mass and the suppression of inter-valley scattering rate are responsible for the enhancement of electron mobility. The calculated results provide valuable reference for the optimum stress and orientation of the conduction channel in the uniaxial strained Si nMOS devices design. The electron mobility calculated model used in the present work is suitable for implementation in TCAD simulators.

Enhancement of electron mobility and photovoltaic performance based on the P3HT:SPRGO organic photovoltaic

The time-of-flight method has been used to study the effect of P3HT:SPRGO composite films on charge mobility. Hole mobility was observed to remain constant at 7–10×10−5cm2/Vs as SPRGO concentration was increased from 0 to 20%, but then electron mobility had enhanced from 1.5 to 35×10−5cm2/Vs as SPRGO concentration was increased from 0 to 20%. The enhancement electron mobility was accompanied by an improved exciton dissociation of P3HT:SPRGO surface. Doping solution processable reduced graphene oxide (SPRGO) into P3HT that improved the organic photovoltaic (OPV) performance and the electron transported ability of P3HT:SPRGO composite films. The lower SPRGO concentration had improved OPV performance. The TOF (time of flight) curve of devices had shown that the electron mobility was enhanced along with increase of the SPRGO concentration. The beat OPV performance had reached 2.28% of power conversion efficiency. At the over high SPRGO concentration, the OPV performance had decreased along with an increase in SPRGO concentration. So the SPRGO is used for enhancing the electron mobility and OPV performance of P3HT:SPRGO.

Impact of Different Barrier Layers and Indium Content of the Channel on the Analog Performance of InGaAs MOSFETs

A barrier layer in an InGaAs MOSFET, which shows promise for high-performance logic applications due to enhanced electron mobility, is known to further improve the electron mobility. In this paper, a detailed investigation of the impact of different barrier layers on the analog performance of an InGaAs MOSFET is reported for the first time. The device parameters for analog applications, such as transconductance (gm), transconductance-to-drive current ratio (gm/IDS), drain conductance (gd), intrinsic gain (gm/gd), and unity-gain cutoff frequency (fT) are studied with the help of a device simulator. A barrier layer is found to improve the analog performance of such a device in general; with a double-barrier layer showing the best performance. An investigation on the impact of varying the indium content in the channel on the analog performance of an InGaAs MOSFET with a double-barrier layer is also reported in this paper. It is found that a higher In content results in better analog performance of such devices.

Impact of Deformation Potential Increase at Si/SiO2 Interfaces on Stress-Induced Electron Mobility Enhancement in Metal–Oxide–Semiconductor Field-Effect Transistors

The impact of deformation potential increase at metal–oxide–semiconductor (MOS) interfaces on stress effects is thoroughly studied. In our previous study, we revealed that the deformation potential (Dac) of Si increases at MOS interfaces. The energy split between two- and four-fold valleys is proportional to Dac. Therefore, it is considered that the Dac increase at MOS interfaces has an affect on strain effects. Dac effectively changes by adjusting Si-on-insulator (SOI) thickness and carrier distribution at MOS interfaces. Therefore, the SOI thickness dependence and carrier distribution dependence of electron mobility enhancement ratio (Δµe/µe) under strain are investigated. Experimental results are explained by the model including the Dac increase at MOS interfaces. In addition, experimental data are well reproduced by calculation using the position-dependent-Dac model. By applying uniaxial strain, effective mass, subband occupation, and intervalley scattering rate are also changed. Their effects on Δµe/µe are also discussed in this paper.

Neutral defects in SrTiO3 studied with screened hybrid density functional theory

The properties of neutral defects in SrTiO3 are calculated using the screened hybrid density functional of Heyd, Scuseria, and Ernzerhof. The formation energies, the crystal field splittings affecting the SrTiO3 band structure, and the relaxation geometries around each defect are discussed. Oxygen vacancies introduced in SrTiO3 are found to cause a small tetragonal elongation of the lattice along the z-axis. The resulting conduction band minimum electron effective masses deviate from the bulk values and support the proposal of enhanced electron mobility along the direction of the compressive strain. The locations of the various defect bands within the SrTiO3 gap are estimated without introducing any post hoc corrections, thus allowing a more reliable comparison with experiment.

Enhancement of multisubband electron mobility in parabolic AlxGa1−xAs-GaAs double quantum well structures

We analyze the low temperature multisubband electron mobility in AlxGa1−xAs–GaAs parabolic double quantum well structures in which the outer barriers are delta doped with Si. The structural parabolic potential, obtained from gradual variation of the alloy fraction x (from 0 to xp), partly compensates the triangular like potential profile near the outer interfaces inducing the electrons to move towards the centre of the wells. We study the effect of interplay of ionized impurity (II) scattering and alloy disorder (AD) scattering on the subband mobility. We show that when single subband is occupied both II- and AD-scatterings govern the mobility. However, once second subband is occupied, the mobility is influenced by II-scattering mediated by intersubband effects. It is gratifying to show that the mobility is considerably enhanced in parabolic double quantum wells (0.3 ≥ xp > 0) compared to the square double quantum well structures (xp = 0) at large well widths where double subband is occupied. By increasing the electron density (Ns), the enhancement increases further. We also show that in case of a parabolic single quantum well structure large enhancement in mobility is obtained compared to that of square single quantum well structure as long as single subband is occupied, unlike the double quantum well systems. Our results of mobility in parabolic double quantum wells can be utilized for low temperature device applications.

Gate-all-around buckled dual Si nanowire nMOSFETs on bulk Si for transport enhancement and digital logic

In this paper, we report formation of GAA buckled dual Si nanowire MOSFETs including two sub-80nm Si cores on bulk Si using [email protected] optical lithography and local oxidation for the first time. 0.833GPa uniaxial tensile stress is measured in the buckled suspended dual Si nanowires using micro-Raman spectroscopy. The array of GAA buckled dual Si nanowire MOSFETs at VDS=50mV shows 64mV/dec. subthreshold slope and 61% stress-based low-field electron mobility enhancement in comparison to the omega-gate relaxed reference device. Finally, digital logic implementation is demonstrated using multi-gate nanowires on bulk Si.

Enhancement of Electron Mobility and Photoconductivity in Quantum Well In0.52Al0.48As/In0.53Ga0.47As/In0.52Al0.48As οn InP Substrate

Isomorphic In0.52Al0.48As/In0.53Ga0.47As/In0.52Al0.48As quantum well structure on InP substrate were grown by molecular beam epitaxy. We investigated the electron transport properties and mobility enhancement in the structures by changing of doping level, the width d of quantum well In0.53Ga0.47As or by illumination using light with λ = 668 nm. Persistent photoconductivity was observed in all samples due to spatial separation of carriers. We used the Shubnikov de Haas e ect to analyze subband electron concentration and mobility. The maximal mobility was observed for quantum well width d = 16 nm.

Electron mobility enhancement in ZnO thin films via surface modification by carboxylic acids

Modifying the surface of polycrystalline ZnO films using a monolayer of organic molecules with carboxylic acid attachment groups increases the field-effect electron mobility and zero-bias conductivity, resulting in improved transistors and transparent conductors. The improvement is consistent with the passivation of defects via covalent bonding of the carboxylic acid and is reversible by exposure to a UV-ozone lamp. The properties of the solvent used for the attachment are crucial because solvents with high acid dissociation constants (Ka) for carboxylic acids lead to high proton activities and etching of the nanometers-thick ZnO films, masking the electronic effect.

New n-type polymer semiconductors based on naphthalene diimide and selenophene derivatives for organic field-effect transistors

n-Type conjugated polymers based on naphthalene diimide (NDI) and various selenophene derivatives have been synthesized, characterized and evaluated as semiconductors for n-channel organic field-effect transistors (OFETs). The new poly(naphthalene diimides) (PNDIs) have weight average molecular weights of 12–107 kDa with a polydispersity of 1.1–2.6. These PNDIs combined a constant electron affinity of 3.9 eV with an optical band gap that varies from 1.7 eV in PNDISS to 1.4 eV in ePNDIBS due to intramolecular charge transfer. X-ray diffraction analysis of films of the polymers revealed a lamellar crystalline structure in which the lamellar interchain distance varied from 2.45 nm in PNDISS to 2.75 nm in PNDIBDS while the π-stacking distance varied from 0.397 nm in PNDIBDS to 0.443 nm in PNDISS. Average field-effect electron mobility of 0.008 to 0.24 cm2 V−1 s−1 with high on/off current ratio (104 to 106) was observed from the bottom gate/top contact n-channel OFETs. A 3.4-fold enhancement in electron mobility was observed in phenyl end-capped and high molecular weight ePNDIBS whereas such end-capping had no effect on the electron mobility in PNDISS and PNDIBDS.

Atomic layer deposition of Ti-doped ZnO films with enhanced electron mobility

Ti is introduced as a dopant during the atomic layer deposition (ALD) growth of ZnO for use as a transparent electrode. ALD-grown Ti-doped ZnO (TZO) films are deposited via alternate stacking of ZnO and TiOx atomic-doping layers. Their growth behavior, structural, electrical and optical properties are investigated. Macroscopic film growth and doping concentration characterization show that both diethylzinc and titanium tetrakis(isopropoxide) exhibit enhanced adsorption during the ALD of TZO films. Contrary to conventional homogeneous compounds, atomic-layer Ti doping by ALD results in a much higher electrical conductivity and doping efficiency compared to its Al counterpart. Specifically, the ALD-grown TZO films show an electrical conductivity of 951 S cm−1, nearly twice that of AZO films (591 S cm−1), thanks to the high doping efficiency of Ti (41%) and its extraordinary high mobility (>20 cm2 V−1 s−1). Such high electron mobility is likely due to a smaller concentration of inactivated dopants as scattering centers.

Electronic conductivity effective masses along arbitrary directional channel in uniaxial strained Si(001)

Electronic conductivity effective mass is one of the key parameters studing electron mobility enhancement in unixial strained Si material. Its in-depth study has the significant theoretical and practical values. In this paper, we first establish the E-k relation for conduction band in a unixial strained Si material. And the model of electronic conductivity effective mass along an arbitrary directional channel in the uniaxial strained Si (001) is obtained. Our concluding results are described as follows. 1) Tensile stress should be used to enhance electron mobility for unixial trained Si. 2) In the case of tensile stress application, both [110]/(001) and [100]/(001) directions are the desirable ones from the evaluation of electronic conductivity effective mass. And [110]/(001) direction should be preferable when the density of state effective mass is taken into consideration. 3) If [100] direction becomes the channel direction under [110]/(001) uniaxial strain, the further electron mobility enhancement will occur. The results above can provide valuable reference for the conduction channel design related to stress and orientation in the strained Si nMOS device.

Characterization of silicon–carbon alloy materials for future strained Si metal oxide semiconductor field effect transistors

N type metal oxide semiconductor field effect transistors (NMOSFETs) using strained silicon–carbon alloy (Si1−xCx) surface channels are reported in this work. Tensile-strained Si1−xCx layers with substitutional carbon content up to ~1% were epitaxially grown on (100) Si substrates by ultra-high vacuum chemical vapor deposition, using silane and methylsilane as the silicon and carbon sources, respectively. NMOSFETs were fabricated using standard MOS processing with reduced thermal treatment in order to minimize the possibility of strain relaxation. A reciprocal space mapping method was used to analyze strain distribution in the silicon–carbon alloy thin films on Si substrates. The election inversion layer mobilities of the Si1−xCx and Si control devices at room temperature are comparable. This is in contrast to the electron mobility enhancement observed in NMOSFETs fabricated using tensile-strained Si grown on relaxed Si1−xGex layers. At low temperatures, the electron inversion layer mobility of Si1−xCx devices is lower than that of the Si controls, and appears to be affected by Coulomb as well as possibly by random alloy scattering.

Electric field induced enhancement of multisubband electron mobility in strained GaAs/InGaAs coupled quantum well structures

We analyze the enhancement of multisubband electron mobility due to an external electric field in a pseudomorphic GaAs/InxGa1−xAs coupled double quantum well structure. An electric field, applied perpendicularly to the interface plane, changes the potential energy profile of the structure. This change alters the energy level, wave function as well as the occupation of a subband. By varying the field, the system can be transformed from double subband occupancy to single subband occupancy resulting in an enhancement of the mobility due to the suppression of the intersubband effects. We consider scatterings due to the ionized impurities, interface roughness and alloy disorder and study the variations in the intrasubband and intersubband scattering matrix elements as a function of the electric field. The novelty of our work is the analysis of the effect of structure parameters like well width, barrier width and doping concentration on the field dependent multisubband electron mobility. We show that a large enhancement in mobility can be achieved through application of an external electric field by suitably choosing the material parameters.

Multigate Buckled Self-Aligned Dual Si Nanowire MOSFETs on Bulk Si for High Electron Mobility

In this paper, we report for the first time making multi-gate buckled self-aligned dual Si nanowires including two sub-100 nm cross-sectional cores on bulk Si substrate using optical lithography, hard mask/spacer technology, and local oxidation. ≈0.8 GPa uniaxial tensile stress was measured on the buckled dual nanowires using micro-Raman spectroscopy. The buckled multigate dual Si nanowires show excellent electrical characteristics, e.g., 62 mV/decade and 42% low-field electron mobility enhancement due to uniaxial tensile stress in comparison to the non-strained device, all at VDS = 50 mV and 293 K.

Physical insight into reduced surface roughness scattering in strained silicon inversion layers

A seemingly anomalous enhancement of electron mobility in strained silicon inversion layers at high sheet densities has exposed a conspicuous gap between device physics theory and experiment in recent years. We show that the root of this discrepancy is due to a bulging effect in the electron \Delta 4 wavefunction at the silicon surface. This renders \Delta 4 electrons more susceptible to perturbations in surface structure thereby increasing surface roughness scattering for these states. Strain engineering utilized by the CMOS industry reduces the relative occupancy of the \Delta 4 states resulting in less overall surface roughness scattering in the channel. We show that the origin of this effect can be explained by moving beyond the effective mass approximation and contrasting the properties of the \Delta 2 and \Delta 4 wavefunctions in a representation that comprehends full crystal and Bloch state symmetry.

Effects of phonon confinement on high-electric field electron transport in an InGaAs/InAlAs quantum well with an inserted InAs barrier

The alternating change of electron mobility values in the modulation doped InAlAs/InGaAs/InAlAs quantum well (QW) dependently on a thickness of the InAs layer inserted in the center of the QW is theoretically predicted and experimentally observed. The electron mobility enhancement by a factor of 1.5–2 takes place when the 4 nm-thick InAs layer is inserted into the 17 nm-width QW. The experimental maximal value of the electron drift velocity at the threshold electric field for intervalley electron scattering achieves (1.8−2)×107 cm/s and does not nearly depend on the thickness of the InAs insert. The high value of maximal drift velocity is conserved at the additional doping of the InAs insert up to electron density of 4×1012 cm−2 in the QW.

Self-Encapsulated Doping of n-Type Graphene Transistors with Extended Air Stability

This paper presents an innovative approach to fabricating controllable n-type doping graphene transistors with extended air stability by using self-encapsulated doping layers of titanium suboxide (TiOx) thin films, which are an amorphous phase of crystalline TiO(2) and can be solution processed. The nonstoichiometry TiOx thin films consisting of a large number of oxygen vacancies exhibit several unique functions simultaneously in the n-type doping of graphene as an efficient electron-donating agent, an effective dielectric screening medium, and also an encapsulated layer. A novel device structure consisting of both top and bottom coverage of TiOx thin layers on a graphene transistor exhibited strong n-type transport characteristics with its Dirac point shifted up to -80 V and an enhanced electron mobility with doping. Most interestingly, an extended stability of the device without rapid degradation after doping was observed when it was exposed to ambient air for several days, which is not usually observed in other n-type doping methods in graphene. Density functional theory calculations were also employed to explain the observed unique n-type doping characteristics of graphene using TiOx thin films. The technique of using an "active" encapsulated layer with controllable and substantial electron doping on graphene provides a new route to modulate electronic transport behavior of graphene and has considerable potential for the future development of air-stable and large-area graphene-based nanoelectronics.

Investigation of AlInN HEMT structures with different AlGaN buffer layers grown on sapphire substrates by MOCVD

We investigate the structural and electrical properties of AlxIn1–xN/AlN/GaN heterostructures with AlGaN buffers grown by MOCVD, which can be used as an alternative to AlInN HEMT structures with GaN buffer. The effects of the GaN channel thickness and the addition of a content graded AlGaN layer to the structural and electrical characteristics were studied through variable temperature Hall effect measurements, high resolution XRD, and AFM measurements. Enhancement in electron mobility was observed in two of the suggested AlxIn1−xN/AlN/GaN/Al0.04Ga0.96N heterostructures when compared to the standard AlxIn1–xN/AlN/GaN heterostructure. This improvement was attributed to better electron confinement in the channel due to electric field arising from piezoelectric polarization charge at the Al0.04Ga0.96N/GaN heterointerface and by the conduction band discontinuity formed at the same interface. If the growth conditions and design parameters of the AlxIn1−xN HEMT structures with AlGaN buffers can be modified further, the electron spillover from the GaN channel can be significantly limited and even higher electron mobilities, which result in lower two-dimensional sheet resistances, would be possible.