Multisubband Electron Research Articles

Improvement of electron mobility mediated by interface roughness scattering in pseudomorphic GaAs/In0.15Ga0.85As asymmetric double quantum well structure

We analyse low temperature multisubband electron mobility mediated by interface roughness (ir-) scattering i.e., µirs as a function of electric field F in a pseudomorphic modulation δ-doped GaAs/In0.15Ga0.85As asymmetry-double-quantum-well (ADQW) structure. The asymmetry is obtained by considering different (I) well widths (w1 = 100 Å and w2 = 150 Å) and (II) doping concentrations (Nd1 = 1.0 × 1018 cm−3 and Nd2 = 1.5 × 1018 cm−3). We adopt random phase approximation and calculated µirs by considering screened ir-scattering potential. The cusp like behaviour of Fermi Energy levels at resonance field Fr makes µirs non-monotonic through intersubband interaction. We show that there is an enhancement of µirs and shifting of the resonance position with increasing central barrier width b in both cases. However, in case I, the increase in Nd enhances µirs but Fr doesn’t change. Whereas, in case II, as w increases, not only µirs but also Fr changes. The effect of the structure parameters on µirs can be utilized to improve the surface roughness effect in new emerging opto-electronic devices.

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.

Magnetoconductivity oscillations induced by intersubband excitation in a degenerate 2D electron gas

Magnetoconductivity oscillations and absolute negative conductivity induced by nonequilibrium populations of excited subbands in a degenerate multisubband two-dimensional electron system are studied theoretically. The displacement from equilibrium, which can be caused by resonant microwave excitation or by any other reason, is assumed to be such that electron distributions can no longer be described by a single Fermi level. In this case, in addition to the well-known conductivity peaks occurring at the Shubnikov-de Haas conditions and small peaks of normal intersubband scattering, sign-changing oscillations with a different shape are shown to be possible. We found also that even a small fraction of electrons transferred to the excited subband can lead to negative conductivity effects.

Effect of external electric field on multisubband electron mobility in n-V-shaped double quantum well HEMT structure

In this work, we present theoretically the effect of external electric field Fe on low temperature multisubband electron mobility μ in V-shaped double quantum well (V-DQW) HEMT structure. We consider the impact of ionised impurity and alloy disorder scatterings for the calculation of μ. We show that, in the proposed structure, when Fe, is absent, there are two subbands occupied below the Fermi levels. However, as Fe increases, there is an alteration of the potential profile, which changes the energy levels and wave function distributions leading to variation of occupation of subband states, i.e. from double to single. During double subband occupancy, initially, μ enhances with Fe, attains a peak value and then decreases. Whereas, for Fe where the transition from double to single subband occupancy occurs, there is a sudden rise in μ due to the cease of inter-subband interaction. It is interesting to note that different structure parameters, e.g. well widths Ww, central barrier width BC, doping concentrations ND, alloy concentrations xv at the well edges of the V-DQW have a fascinating impact on μ. We show that increasing Ww, and BC and decreasing ND, and xv enhances μ.

Electric field induced non-linear multisubband electron mobility in V-shaped asymmetric double quantum well structure

ABSTRACTWe study the effect of the external electric field Fext on the low-temperature electron mobility μ in an asymmetrically doped AlxGa1-xAs based V-shaped double quantum well (VDQW) structure. We show that nonlinearity of µ occurs under double subband occupancy on account of intersubband effects. The field Fext alters the VDQW potential leading to transfer of subband wave functions between the wells, which affects the scattering potentials and hence μ. In the VDQW structure, due to the alloy channel layer, the alloy disorder (Al-) scattering happens to be significant along with the ionised impurity (Imp-) scattering. The non-linear behaviour of μ is because of μImp, while the overall magnitude of μ is mostly due to μAl. The increase of difference in the doping concentrations of the outer barriers increases the nonlinearity of μ. The oscillatory character of μ is amended by varying the width of the well and barrier and also the height of the VDQW. Our results can be used to study VDQW based nanoscale field effect transistor structures.

Effect of non-square structure potential on the multisubband electron mobility in double quantum well structure

We study the impact of non-square potential well structure on the electron mobility μ of double quantum well (DQW) based field effect transistors carved out of the AlxGa1-xAs alloy. The barriers lying towards the substrate and surface sides of the DQW are delta doped with Si. We consider DQWs having V-shaped (VDQW), parabolic (PDQW), cubic (CDQW) and square (SDQW) potential wells to obtain the low temperature double subband electron mobility μ. We consider ionized impurity (Imp-) and alloy disorder (Al-) scatterings to calculate μ as a function of well width w and surface electron density NS. We show that the changes in the structure potentials influence the interplay of intersubband effects on the scattering mechanisms differently causing μ(VDQW) < μ(PDQW) ≅ μ(CDQW) as a function of w whereas μ(VDQW) < μ(PDQW) < μ(CDQW) as a function of Ns. We show that μ increases not only with increase in the width of the central barrier b but also decrease with the height of the non-square potentials. Our results can be utilized to analyze the effect of non-square quantum well potentials on the channel conductivity of the quantum well modulation doped field effect transistors.

Nonlinear electron transport mobility in asymmetric wide quantum well structure

The nonlinearity of multisubband electron mobility µ in a GaAs/AlxGa1−xAs wide quantum well structure is studied by varying the well width w and doping concentration Ndb (Ndt) lying in the bottom (top) barrier. The electrons diffuse into the well and accumulate near the interfaces forming two sheets of coupled two dimensional electron gases equivalent to a double quantum well structure. We show that interchange of doping concentrations Ndb and Ndt lead to the enhancement of µ as a function of w as long as Ndt > Ndb, even though the surface electron density remains unaltered. Further, keeping Ndb unchanged, variation of Ndt leads to nonlinearity in µ near the resonance of subband states at Ndt = Ndb at which the subband energy levels exhibit anticrossing. The variation of µ becomes prominent by increasing the well width and resonant doping concentration. The nonlinearity in µ is mostly because of the change in the interface roughness scattering potential through intersubband effects due to the substantial changes in the distributions of the subband wave functions around resonance. Our results of nonmonotonic variation of µ can be utilized for low temperature coupled quantum well devices.

Polar charges effect on multisubband electron mobility in the semiparabolic quantum wells based on AlN/AlGaN/AlN

This work presents multisubband electron mobility in the polarization semi-parabolic quantum wells (SPQWs) AlN/AlxGa1−xN/AlN. First, a theory of multisubband mobility between the lowest two subbands under a uniform external electric field is studied. By using the variation method, the one-dimensional Poisson and Schrödinger equations have been solved within a finite potential barrier model and a bent band figured by all confinement sources (realistic model). Then, we computed and discussed about the effective confining potential profile, the wave function and multisubband mobility. Our result shows that the positive interface polarization charges effect on the distribution of the two-dimensional electron gas (2DEG) in SPQWs so it has great influences on multisubband mobility.

Surface Potential Equation for Low Effective Mass Channel Common Double-Gate MOSFET

Formulation of accurate yet computationally efficient surface potential equation (SPE) is the fundamental step toward developing compact models for low effective mass channel quantum well MOSFETs. In this paper, we propose a new SPE for such devices considering multisubband electron occupancy and oxide thickness asymmetry. Unlike the previous attempts, here, we adopt purely physical modeling approaches (such as without mixing the solutions from finite and infinite potential wells or using any empirical model parameter), while preserving the mathematical complexity almost at the same level. Gate capacitances calculated from the proposed SPE are shown to be in good agreement with numerical device simulation for wide range of channel thickness, effective mass, oxide thickness asymmetry, and bias voltages.

Enhancement of multisubband electron mobility in square-parabolic asymmetric double quantum well structure

We show that higher multisubband electron mobility μ can be achieved in AlxGa1-xAs based hybrid type square-parabolic asymmetric double quantum well (HDQW) compared to that of the symmetric square double quantum well (SDQW) and parabolic double quantum well (PDQW) structures. We show that μ(HDQW) > μ(PDQW) > μ(SDQW). This trend in the enhancement of μ is mostly governed by the change in the ionized impurity scattering potential. The mobility of the HDQW can be enhanced by increasing the well width and also the curvature of the parabolic potential. In addition, μ(HDQW) becomes larger when the parabolic well lies towards the substrate side compared to that of the surface side due to the reduced effect of the interface roughness scattering. The mobility also increases with increase in the surface electron density Ns. The enhancement of μ in HDQW is because of the reduction of the intersubband scattering rate matrix elements due to the asymmetric distribution of the subband wave functions. Our results can be utilized to develop asymmetric double quantum well field effect transistor.

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.

Enhancement of multisubband electron mobility in asymmetrically doped coupled double quantum well structure

We study the effect of coupling of subband wave functions on the multisubband electron mobility in a barrier delta doped GaAs/AlxGa1−xAs asymmetric double quantum well structure. We use selfconsistent solution of the coupled Schrödinger equation and Poisson's equation to calculate the subband wave functions and energy levels. The low temperature mobility is considered by using scatterings due to ionized impurities, interface roughness and alloy disorder. We show that variation of the width of the central barrier considerably affect the interplay of different scattering mechanisms on electron mobility through intersubband effects. Under single subband occupancy, the mobility increases with decrease in the barrier width as functions of doping concentration as well as function of well width. However, in case of double subband occupancy, effect of intersubband interaction yields opposite trend, i.e., increase in mobility with increase in barrier width. It is gratifying to show that in case of asymmetric variation of well widths the mobility shows nonmonotonic behavior which varies with change in the width of the central barrier under double subband occupancy.

Enhancement of Multisubband Electron Mobility in Barrier Delta-Doped Asymmetric GaAs/AlGaAs Double Quantum Well Structure

ABSTRACTWe analyse the effect of asymmetry in the structure parameters on low-temperature multisubband electron mobility μ in a barrier delta-doped GaAs/AlGaAs double quantum well structure. We obtain the subband energy levels and wave functions through self-consistent solution of Schrodinger and Poisson's equations. We show that μ is enhanced through asymmetry in well widths and spacer widths as a function of doping concentration, mediated by intersubband effects. We analyse the interplay of different scattering mechanisms on μ and show that the nonlinear enhancement of μ arises due to interface roughness (ir-) scattering even though the mobility is mostly limited by ionized impurity (imp-) scattering. We further show that the range of μ up to which the ir-scattering dominates depends on the occupation of subbands. Our results can be utilized for low-temperature devices.

Enhancement of low temperature electron mobility due to an electric field in an InGaAs/InAlAs double quantum well structure

The effect of external electric field F on multisubband electron mobility μ in an In0.53Ga0.47As/In0.52Al48As double quantum well structure is analyzed. We consider scatterings due to ionized impurities, interface roughness and alloy disorder to analyze μ. The variation of scattering mechanisms as a function of F for different structure parameters shows interesting results through intersubband interactions. For small well widths, the mobility is governed by interface roughness scattering. When two subbands are occupied, the effect of impurity scattering gets enhanced through intersubband interactions. Our results of enhancement in mobility as a function of F, can be utilized for low temperature devices.

Cyclotron resonant photoresponse of a multisubband two-dimensional electron system on liquid helium

We report an experimental study of surface electrons on liquid helium-4 under cyclotron resonant excitation. Significant resonance-induced redistribution of electron density due to overheating was observed. Analysis of redistributed charges indicates an unusually large expansion of the electron system in a lateral direction, which cannot be understood in the framework of the generally accepted effective electron temperature approximation. To interpret the data we suggest that under resonant cyclotron pumping, a fraction of nonthermalized electrons is formed, in which the average energy associated with electron in-plane motion can significantly exceed the average energy associated with motion perpendicular to the helium surface.

Effect of doping profile on multisubband electron mobility in AlGaAs parabolic quantum well structures

We analyse the low temperature multisubband electron mobility in an AlxGa1−xAs parabolic quantum well in which the side barriers are delta doped with Si. We change the doping profile and other structure parameters to study their effect on the electron mobility μ through intersubband effects. We show that with increase in surface electron density Ns the mobility due to ionized impurity (Imp-) scattering μImp increases while due to alloy disorder (Al-) scattering μAl decreases. As long as single subband is occupied, μ is governed by both Imp- and Al-scatterings. Once double subband is occupied, μ is determined by Imp-scattering only through intersubband effects. We show that increase in μ with increase in spacer width at a certain well width can be enhanced by increasing Ns. Further we show that the mobility enhances by reducing the curvature of the parabolic potential. The mobility is more when there is doping in both of the side barriers compared to single side doping. Our results of electron mobility in a parabolic quantum well can be utilized for low temperature device application.

Enhancement of electron mobility in asymmetric coupled quantum well structures

We study the low temperature multisubband electron mobility in a structurally asymmetric GaAs/AlxGa1-xAs delta doped double quantum well. We calculate the subband energy levels and wave functions through selfconsistent solution of the coupled Schrodinger equation and Poisson's equation. We consider ionized impurity scattering, interface roughness scattering, and alloy disorder scattering to calculate the electron mobility. The screening of the scattering potentials is obtained by using static dielectric response function formalism within the random phase approximation. We analyze, for the first time, the effect of asymmetric structure parameters on the enhancement of multisubband electron mobility through intersubband interactions. We show that the asymmetric variation of well width, doping concentration, and spacer width considerably influences the interplay of scattering mechanisms on mobility. Our results of asymmetry induced enhancement of electron mobility can be utilized for low temperature device applications.

Multisubband electron mobility in a parabolic quantum well structure under the influence of an applied electric field

We study the multisubband electron mobility in a barrier delta doped AlxGa1−xAs parabolic quantum well structure under the influence of an applied electric field perpendicular to the interface plane. We consider the alloy fraction x = 0.3 for barriers and vary x from 0.0 to 0.1 for the parabolic well. Electrons diffuse into the well and confine within the triangular like potentials near the interfaces due to Coulomb interaction with ionized donors. The parabolic structure potential, being opposite in nature, partly compensates the Coulomb potential. The external electric field further amends the potential structure leading to an asymmetric potential profile. Accordingly the energy levels, wave functions and occupation of subbands change. We calculate low temperature electron mobility as a function of the electric field and show that when two subbands are occupied, the mobility is mostly dominated by ionised impurity scattering mediated by intersubband effects. As the field increases transition from double subband to single subband occupancy occurs. A sudden enhancement in mobility is obtained due to curtailment of intersubband effects. Thereafter the mobility is governed by both impurity and alloy disorder scatterings. Our analysis of mobility as a function of the electric field for different structural parameters shows interesting results.

Multisubband electron mobility in asymmetric GaAs/AlGaAs quantum well structures

The effect of structural asymmetry on multisubband electron mobility is studied by considering a barrier delta doped GaAs/AlxGa1−x As quantum well structure. The subband wave functions and energy levels are obtained by adopting selfconsistent solution of the coupled Schrodinger and Poisson’s equations. We consider scatterings due to ionized impurities, interface roughness and alloy disorder. The screening of the scattering potentials is obtained by adopting static dielectric response function formalism. We show that the interplay of different scattering mechanisms on low temperature electron mobility yields interesting results through intersubband interaction. Further, we show that asymmetry in structure parameters, such as, doping concentrations and spacer widths, influence in enhancing the mobility considerably.

Population inversion and novel magnetoconductivity oscillations induced by resonant microwave irradiation in multisubband two-dimensional electron systems

The possibility of a population inversion in a multisubband two-dimensional (2D) electron system exposed to resonant microwave (MW) radiation and a static magnetic field is shown. Theoretical analysis given here indicates that the resonant excitation of the third subband, necessary for the population inversion, is accompanied by novel magnetoconductivity oscillations in the 2D electron system formed on the surface of liquid helium.