Subband Mobility Research Articles

Enhancement of electron transport mobility in GaAs/InGaAs asymmetrically doped narrow quantum well pHEMT structure

We study the effect of asymmetric doping concentrations on the electron mobility μ in GaAs/InGaAs-based single quantum well (SQW) as well as double quantum well (DQW) pseudomorphic high electron mobility transistor (pHEMT) structures. Unequal doping in the substrate and surface barriers (n d1 and n d2 ) causes asymmetric distributions of subband wave functions, ψ 0 and ψ 1, which influence the subband scattering rate matrix elements (SSRME), thereby affecting the subband mobility μ n . For narrow well widths (w w ), in SQW structures, mostly a single subband is occupied. We show that an increase in n d2 , keeping n d1 fixed, enhances μ nonlinearly. The interface roughness (ir-) scattering mostly dominates μ in thin wells (w w < 70 Å), while generally, μ is determined by ionized impurity (ii-) scattering and to some extent by alloy disorder (ad-) scattering. The influence of ir-scattering enhances, while ad-scattering diminishes, by reducing n d2 . For DQW, a double subband is occupied. In a symmetric DQW structure at resonance, n d1 = n d2 , ψ 0 and ψ 1 equally extend into both the wells. For a minor variation, say n d1 > n d2 , ψ 0 mostly lies in one well while ψ 1 is in the other well. In the case of n d1 < n d2 , the distribution reverts. The substantial changes in ψ 0 and ψ 1 influence the intra- and inter-SSRME differently through intersubband effects, leading to nonlinear μ n as a function of n d2 . Taking n d1 + n d2 = 3 × 1018 cm−3, we show that for w w1 = w w2 = 80 Å, a shallow dip in μ occurs at n d1 = n d2 = 1.5 × 1018 cm−3. Whereas, for w w1 = 60 Å and w w2 = 100 Å, the dip in μ occurs near the corresponding resonance, n d1 = 2.3 × 1018 cm−3 and n d2 = 0.7 × 1018 cm−3. Our results of nonlinear μ can be utilized for performance analysis of pHEMT.

Effect of structure parameters on electron mobility in asymmetric V-shaped double quantum well structure

Here, we analyse the effect of structure parameters like well width (w), central barrier width (b), and alloy concentration (x) on multisubband electron mobility μ in a GaAs-AlxGa1-xAs based modulation doped asymmetric V-shaped-double-quantum-well (VDQW) structure. The asymmetry in the structure potential is generated through the difference in the doping concentrations (N d ) in the side barriers i.e., N d1 (0 to 4 × 1018 cm−3) and N d2 (2 × 1018 cm−3). The mobility μ is calculated by considering ionised impurity (imp) and alloy disorder (ad) scattering mechanisms. The continuous variation of x inside the well makes μ ad < μ imp resulting in the dominance of ad-scattering on μ as a function of N d1. As a result, at the interface an increase in x from 0.1 to 0.3 reduces μ around 40%. However, an increase in w symmetrically (w 1 = w 2) enhances μ. Further, the introduction of non-symmetric well profile (w 1 ≠ w 2) not only causes asymmetric redistribution of subband wave functions ψ 0 and ψ 1 in the wells, but also changes the position and hence occupation of subband energy levels, thereby influencing the subband mobility. As the difference in w 1 and w 2 increases, the system becomes more and more single subband occupied as a function of N d1 and hence the mobility enhances due to the absence of intersubband scattering. Our results also reflected that an increase in b from 20 Å to 80 Å has a marginal effect on μ during single subband occupancy but improves μ during double subband occupancy through intersubband interaction.

Study of nonmonotonic electron mobility due to influence of asymmetric structure parameters in pseudomorphic heterojunction field effect transistors

Nonmonotonic electron mobility is obtained in InxGa1-xAs/GaAs double quantum well pseudomorphic heterostructure field effect transistor by changing the structure parameters. We show that a rapid drop in mobility occurs at the point of resonance of sub-band states due to asymmetric variation of doping concentrations. The sub-band wave function distributions change significantly near the resonance and influence the sub-band mobilities through the scattering potentials, there by causing the dip in μ. The depth of nonlinearity in μ enhances by increasing the central barrier width and the difference between the well widths. On the other hand, variation of μ as a function of asymmetric change of well widths leads to a hump like raise in μ under unequal doping concentrations. Our results of nonlinear mobility can be utilized in low temperature transistor applications.

Doping-Dependent Nonlinear Electron Mobility in GaAs|InxGa1 –xAs Coupled Quantum-Well Pseudo-Morphic MODFET Structure

We analyze the asymmetric delta-doping dependence of nonlinear electron mobility μ of GaAs|InxGa1 –xAs double quantum-well pseudo-morphic modulation doped field-effect transistor structure. We solve the Schrodinger and Poisson’s equations self-consistently to obtain the sub-band energy levels and wave functions. We consider scatterings due to the ionized impurities (IMP), alloy disorder (AL), and interface roughness (IR) to calculate μ for a system having double sub-band occupancy, in which the inter-sub-band effects play an important role. Considering the doping concentrations in the barriers towards the substrate and surface sides as Nd1 and Nd2, respectively, we show that variation of Nd1 leads to a dip in μ near Nd1 = Nd2, at which the resonance of the sub-band states occurs. A similar dip in μ as a function of Nd1 is also obtained at Nd1 = Nd2 by keeping (Nd1 + Nd2) unchanged. By increasing the central barrier width and well width, the dip in μ becomes sharp. We note that even though the overall μ is governed by the IMP- and AL-scatterings, the dip in μ is mostly affected through substantial variation of the sub-band mobilities due to IR-scattering near the resonance. Our results of nonlinear electron mobility near the resonance of sub-band states can be utilized for the performance analysis of GaAs|InGaAs pseudo-morphic quantum-well field-effect transistors.

Structural asymmetry induced nonmonotonic electron mobility in pseudomorphic double quantum well high electron mobility transistor structure

Non-monotonic mobility μ of electrons is obtained in a pseudomorphic GaAs/InxGa1−xAs high electron mobility transistor having double quantum well structure with asymmetric doping concentrations in the outer barriers. A dip in μ occurs close to the resonance of subband states because of the quantum mechanical transfer of the wave functions between the wells. Near resonance, the asymmetry in subband wave functions and anticrossing of energy levels as a function of doping concentration influence the intra- and intersubband scattering rate matrix elements. Near resonance, the subband wave functions spread asymmetrically and the energy levels exhibit anticrossing as a function of doping concentration thereby influencing the intra and intersubband scattering rate matrix elements. The dip in μ, which amplifies with raise in the asymmetry of the width of wells, is basically due to the subband mobilities limited by the interface roughness scattering. We also show that an appropriate selection of the structure parameters leads to a hump in mobility dominated by the intersubband effects on the screened ionized impurity scattering potential under double subband occupancy. The nonlinearity in μ can be utilized to study the characteristics improvement of devices such as high electron mobility transistors.

Doping-Dependent Nonlinear Electron Mobility in GaAs|In-=SUB=-x-=/SUB=-Ga-=SUB=-1-x-=/SUB=-As Coupled Quantum-Well Pseudo-Morphic MODFET Structure

We analyze the asymmetric delta-doping dependence of nonlinear electron mobility &amp;mu; of GaAs|InxGa1-xAs double quantum-well pseudo-morphic modulation doped field-effect transistor structure. We solve the Schrodinger and Poisson's equations self-consistently to obtain the sub-band energy levels and wave functions. We consider scatterings due to the ionized impurities (IMP), alloy disorder (AL), and interface roughness (IR) to calculate &amp;mu; for a system having double sub-band occupancy, in which the inter-sub-band effects play an important role. Considering the doping concentrations in the barriers towards the substrate and surface sides as Nd1 and Nd2, respectively, we show that variation of Nd1 leads to a dip in &amp;mu; near Nd1=Nd2, at which the resonance of the sub-band states occurs. A similar dip in &amp;mu; as a function of Nd1 is also obtained at Nd1=Nd2 by keeping (Nd1+Nd2) unchanged. By increasing the central barrier width and well width, the dip in &amp;mu; becomes sharp. We note that even though the overall &amp;mu; is governed by the IMP- and AL-scatterings, the dip in &amp;mu; is mostly affected through substantial variation of the sub-band mobilities due to IR-scattering near the resonance. Our results of nonlinear electron mobility near the resonance of sub-band states can be utilized for the performance analysis of GaAs|InGaAs pseudo-morphic quantum-well field-effect transistors. Keywords: asymmetric double quantum wells, GaAs|InxGa1-xAs structures, nonlinear electron mobility, pseudo-morphic HEMT structures, resonance of sub-band states.

Temperature-dependent subband mobility characteristics in n-doped silicon junctionless nanowire transistor**Project supported by the National Key Research and Development Program of China (Grant No. 2016YFA0200503).

We have investigated the temperature-dependent effective mobility characteristics in impurity band and conduction subbands of n-doped silicon junctionless nanowire transistors. It is found that the electron effective mobility of the first subband in 2-fold valleys is higher than that of the second subband in 4-fold valleys. There exists a maximum value for the effective subband mobilities at low temperatures, which is attributed to the increase of thermally activated electrons from the ionized donors in the impurity band. The experimental results indicate that the effective subband mobility is temperature-dependent on the electron interactions by thermal activation, impurity scattering, and intersubband scattering.

Electron transport in AlxGa1−xAs δ-MIGFETs: Conductivity enhancement induced by magnetic field effects

The electronic structure and the transport phenomena of δ-MIGFETs have been studied in an AlxGa1−xAs host matrix. The subband structure and mobility calculations were performed within the effective mass approximation and relative mobility formula, respectively. Both the electronic structure and the transport properties are calculated as dependent on the applied magnetic field (B), the aluminum molar fraction (x) and the contact voltage in one of the gates (VC1). It was found that the mobility and conductivity are enhanced by increasing the magnetic field for appropriate aluminum molar fraction and contact voltage. In particular, the mobility (conductivity) is improved almost 26% (32%) for VC1 = 900 meV (850 meV), x = 0.2, and B = 20 T.

Nonmonotonous electron mobility due to structurally induced resonant coupling of subband states in an asymmetric double quantum well

We show that sharp nonmonotic variation of low temperature electron mobility μ can be achieved in GaAs/AlxGa1-xAs barrier delta-doped double quantum well structure due to quantum mechanical transfer of subband electron wave functions within the wells. We vary the potential profile of the coupled structure as a function of the doping concentration in order to bring the subbands into resonance such that the subband energy levels anticross and the eigen states of the coupled structure equally share both the wells thereby giving rise to a dip in mobility. When the wells are of equal widths, the dip in mobility occurs under symmetric doping of the side barriers. In case of unequal well widths, the resonance can be obtained by suitable asymmetric variation of the doping concentrations. The dip in mobility becomes sharp and also the wavy nature of mobility takes a rectangular shape by increasing the barrier width. We show that the dip in mobility at resonance is governed by the interface roughness scattering through step like changes in the subband mobilities. It is also gratifying to show that the drop in mobility at the onset of occupation of second subband is substantially supressed through the quantum mechanical transfer of subband wave functions between the wells. Our results can be utilized for performance enhancement of coupled quantum well devices.

Structural asymmetry induced size quantized nonmonotonous electron mobility in GaAs/AlxGa1−xAs double quantum well structure

We show that nonmonotonic enhancement of low temperature electron mobility μ can be achieved in GaAs/AlxGa1−xAs barrier delta doped double quantum well structure under double subband occupancy. We vary the well widths asymmetrically by shifting the position of the central barrier and analyze its effect on the multisubband electron mobility taking different doping profiles. It is gratifying to show that when there is delta doping in one of the side barriers, a hump in μ is achieved as a function of the well width which enhances with increase in the width of the central barrier. On the other hand, when both the side barriers are symmetrically doped, a dip in μ is exhibitted which increases with increase in the doping concentration. In the former case, the mobility is mostly contributed from the higher subband limited by the ionized impurity scattering. Whereas, in the later case, the wavy nature of mobility is due to the drastic variation of subband mobilities through the interface roughness scattering.

Oscillation of electron mobility in parabolic double quantum well structure due to applied electric field

We show that oscillation of low temperature electron mobility μ can be obtained by applying an electric field F along the growth direction of the asymmetrically barrier delta doped AlxGa1-xAs parabolic double quantum well structure. The drastic changes in the subband Fermi energies and distributions of subband wave functions as a function of F yield nonmonotonic intra- and intersubband scattering rate matrix elements mediated by intersubband effects. The oscillatory enhancement of μ, which is attributed to the subband mobilities governed by the ionized impurity scattering, magnifies with increase in well width and decrease in height of the parabolic structure potential. The results can be utilized for nanoscale low temperature device applications.

Oscillating electron mobility in GaAs/AlxGa1−xAs double quantum well structure under applied electric field

We show that oscillation of low temperature electron mobility μ can be achieved in a single side barrier delta doped asymmetric GaAs/AlxGa1−xAs double quantum well structure. By applying an external electric field F the potential structure can be varied. Accordingly the system undergoes changes from single subband occupancy to double and then again to single subband occupancy. Under double subband occupancy the subband Fermi energies and wave functions exhibit nonmonotonous behavior leading to drastic cusp like changes in the intra- and intersubband scattering rate matrix elements as a function of F. We show that the resultant subband mobilities mediated by the intersubband effects are responsible for the oscillatory enhancement of μ. The oscillation of μ is mostly governed by the ionized impurity scattering and becomes prominent by increasing the well width and surface electron density. Our results can be utilized for low temperature nanoscale device applications.

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 &amp;gt; 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.

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 and hole mobilities in semimetallic bismuth nanowires

The Shubnikov de Haas effect is used to determine the electron and hole mobilities in a bismuth nanowire. We identify an excess hole density from a doping effect introduced during the on-film-formation-of-nanowires fabrication process. Three electron subbands and a single hole band contribute to the oscillatory magnetoresistance and these bands can be decomposed by fast Fourier transform analysis of xx into different orbits on an anisotropic Fermi surface. A nonharmonic Shubnikov de Haas oscillation from the hole band is due to variation in the carrier density with applied magnetic field. Electron and hole scattering is dominated by a short-range potential with a hole mobility of 5000 cm 2 V �1 s �1 and an electron mobility of 20 000 cm 2 V �1 s �1 at 1.6 K. Mobility analysis of the fast Fourier transform of xx is used to determine the individual three electron and single hole subband mobilities.

Band Structure Dependence of Electron Mobility in Modulation-Doped Lattice-Matched InAlAs/InGaAs/InAlAs Heterostructures

Variable-magnetic-field Hall measurements were performed on modulation-doped In0.52Al0.48As/In0.53Ga0.47As/In0.52Al0.48As double heterostructures to investigate the influence of channel layer thickness on the two dimensional electron gas mobility. Quantitative mobility spectrum analysis revealed that the first and second subband mobilities increased with the channel thickness. This enhancement is attributed to the decreased subband effective mass, subband carrier density, and alloy disorder scattering potential.

Effect of electric field on low temperature multisubband electron mobility in a coupled Ga0.5In0.5P/GaAs quantum well structure

The effect of uniform electric field on low temperature (T=0 K) multisubband electron mobility μi is analyzed by considering a barrier delta-doped Ga0.5In0.5P/GaAs coupled double quantum well structure. We consider ionized impurity scattering and interface roughness (IR) scattering. The screening of the scattering potentials is obtained by adopting the random phase approximation. Starting with a double-subband occupied system we have studied the changes in the intrasubband and intersubband scattering processes by varying the electric field F and highlight the influence of F on the intersubband effects which yields interesting results on μi. At a certain electric field, the system undergoes a transition from double subband to single subband occupancy leading to a large enhancement in mobility due to the suppression of the intersubband interactions. We show that by reversing the electric field a large change in mobility is obtained due to the asymmetric nature of the IR scattering potential. It is also gratifying to show that by varying the electric field the relative dominance of different scattering mechanisms on subband mobility changes through the intersubband interaction.

Low temperature electron magnetotransport in InxGa1-xAs/In0.52Al0.48As quantum wells with high electron density

We present the investigations of low temperature magnetotransport in the one- or two- side Si delta-doped quantum well structures In0.52Al0.48As/InxGa1-xAs/In0.52Al0.48As grown on InP with different InAs fraction in InxGa1-xAs and hence the different quantum well depth. Hall effect and Shubnikov- de Haas oscillations were measured in quantized magnetic field up to 7 T in the temperature interval 0.3<T<4.2 K. The oscillation analysis reveals the two pronounced frequencies with the different amplitudes from two filled subbands. The electron subband mobility is shown to be sensitive to composition and doping.

The impact of uniaxial stress on subband structure and mobility of strain Si NMOSFETs

An effect of stress distortion on the conduction band structure was derived by k.p method considering a second order perturbation. From k.p conduction band calculations, stress-induced band edge split and the change of effective mass are quantitatively evaluated. The physical reasons of warped subband structure and abnormal mobility enhancement by uniaxial stress are investigated. Variation rates of experimental electron mobility in the silicon n-channel metal-oxide-semiconductor field-effect-transistors under a [110] uniaxial stress as a function of channel direction is theoretically studied.

Positive magnetoresistance in In0.53Ga0.47As/In0.52Al0.48As quantum well

Magnetotransport measurements have been carried out on In0.53Ga0.47As/In0.52Al0.48As quantum wells in a temperature range between 1.5 and 77K. We have observed a large positive magnetoresistance in the low magnetic field range， but saturating in high magnetic fields. The magnetoresistance results from two occupied subbands in the two-dimensional electron gas. With the intersubband scattering considered， we obtained the subband mobility by analyzing the positive magnetoresistance. It is found that the second subband mobility is larger than that of the first due to the existence of the intersubband scattering.