Intersubband Effects Research Articles

Effect of non-square potential profile on three subband electron mobility in AlGaAs quantum well structures

This work analyses the effect of the non-square structure potentials, such as V-shaped (V), parabolic (P), cubic (C), semi-V (SV), semi-parabolic (SP), and semi-cubic (SC) on the low temperature electron mobility (μ) as a function of doping concentrations (N d = 0.1 to 3.0 × 1018 cm−3) in modulation δ-doped quantum well (QW) structures. We calculate μ by adopting screened ionized impurity (ii-) and alloy disorder (al-) scatterings. We consider higher subband occupancy up to three and show that the intersubband effects influence the screened scattering potentials differently, such that μ ii increases while μ al decreases, leading to nonlinear enhancement of μ. Further, there are sudden drops in μ, near the transition of occupation of subbands due to the intersubband effects, and the magnitude of the drop is reduced at the third subband occupancy. The number of occupied subbands (nos), for the considered range of N d , differs with NSQW structures, e.g., nos = 3 in the case of VQW, CQW, and PQW, nos = 2 in SCQW and SPQW, and nos = 1, in SVQW structures. Interestingly, in VQW, the occupation of the second subband starts at a higher N d , compared to CQW and PQW, while, the third subband occupancy shows an opposite trend. Furthermore, the dissimilarity in electron charge distributions in the NSQW structures influences the ii-scattering potential differently, causing μ ii (VQW) > μ ii (PQW) > μ ii (CQW), while for al-scattering the order of μ al reverses.

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.

Transport and quantum lifetimes of electrons in modulation doped Al0.3Ga0.7As/In0.15Ga0.85As double quantum well structure

The enhancement of transport and quantum lifetimes of electrons in barrier delta-doped Al0.3Ga0.7As/In0.15Ga0.85As pseudomorphic Double-Quantum-Well structure is analysed as a function of applied electric field and well width. We consider screened ionised-impurity, interface-roughness, and alloy-disorder scattering potentials and show that the quantum lifetime is dominated by ionised-impurity scattering while the transport lifetime is decided by most of the scattering mechanisms. In the absence of electric field, there is double subband occupancy, and therefore, the transport as well as quantum lifetimes are influenced by the intersubband effects. Application of electric field perpendicular to the interface of the quantum well structure enhances the transport lifetime whereas reduces quantum lifetime non-monotonically resulting in a rapid enhancement of their ratio through ionised impurity scattering. Further, as the well width increases, the effect of ionised-impurity scattering decreases leading to the enhancement of both the transport and quantum lifetimes as well as their ratio. In fact, for well width of 150 Å, an improvement in the ratio of transport and quantum lifetimes can be achieved from 9.8 to 90 by increasing the electric field from 0 to 7.6 kV/cm.

Electric field induced non-monotonic electron transport in pseudomorphic Al0.3Ga0.7As/In0.15Ga0.85As coupled quantum well structure

In this article, the non-monotonic electron mobility (μ) is modulated by applying an external electric field (Fext) in pseudomorphic Al0.3Ga0.7As/In0.15Ga0.85As asymmetric coupled quantum well (ACQW) structure. The structural asymmetry is achieved through dissimilar doping in the side barriers. Application of a specific Fext (here, Fext = 6.66 kV/cm) makes the proposed structure potentially symmetric causing resonance of subband states (RSS) between the wells. Near resonance, the drastic change in the distribution of subband wave functions leads to cusp like variation of the scattering rate matrix elements resulting non-monotonic nature of μ. We show that the variation of μ is mostly due to the ionized impurity scattering mediated by intersubband effects. However, there is a sudden drop in mobility due to interface roughness scattering i.e., μir at RSS and subsequently, it reduces the overall magnitude of μ. We show that μ can be enhanced with a sharp drop at the resonance point by increasing the well width, central barrier width, spacer width, or doping region width of the proposed structure. The results obtained can be utilized to improve the performance of negative resistance based low dimensional device structures.

Asymmetric doping induced nonlinear electron mobility in double V-shaped quantum well based FET structure under external electric field

The effect of asymmetric doping profile on the electron mobility μ is studied in a Field Effect Transistor (FET) structure based on Al x Ga 1−x As double V-shaped quantum well (D-VQW) in presence of an external electric field F. We introduce asymmetry in the structure by considering different doping concentrations in the side barriers along the substrate and surface. The resultant asymmetric potential is varied as a function of F. Accordingly the energy levels E l and wave functions ξ l within the coupled structure alter. By varying F suitably the resonance of subband states can be realized between the wells. Around the point of resonance, ξ l rearranges significantly. The change in ξ l influences the screened scattering potentials through intersubband effects leading to nonlinear μ. We consider ionized impurity (Imp-) scattering and alloy disorder (Al-) scattering for the D-VQW structure and show that under double subband occupancy, μ has an oscillatory behavior, basically due to Imp-scattering. The oscillatory μ enhances by increasing the well width and central barrier width and also by decreasing the height of the V-shaped well. Our results of nonlinear electron mobility can be employed to tune the transistor performance with gate electric field by adopting an optimized set of structure parameters.

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.

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.

Oscillation of Electron Mobility in V‐Shaped Double Quantum Well Structure Under Applied Electric Field

Herein, it is shown that the oscillation of low temperature electron mobility μ can be obtained as a function of external electric field F in AlxGa1−xAs based V‐shaped double quantum well (VQW) structure. The oscillation of μ can be enhanced by increasing the well width and barrier width as well as decreasing the doping concentration and height of the V‐shaped potential. The mobility due to the ionized impurity (II‐) scattering μII is responsible for the oscillation of μ through intersubband effects within double subband occupancy. On the other hand, the mobility due to the alloy disorder (AD‐) scattering, μAD, which shows almost a flat‐like character, governs the overall magnitude of μ. It is further gratifying to show that the drop in μ at the transition from single to double subband occupancy minimizes and even alters to a rise in μ by varying the AD‐scattering potential through the structure parameters of the VQW. These results can be utilized to analyze the non‐square quantum well based field effect transistors.

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.

Enhancement of electron mobility in GaAs/AlxGa1-xAs square-parabolic double quantum well HEMT structure

We show that the enhancement of electron mobility μ as function of well width w can be achieved in a GaAs/AlxGa1-xAs square-parabolic double quantum well (SPDQW) high electron mobility transistor (HEMT) structure. We consider the structures, in which one of the square well lies towards the substrate (SPDQW), and in another towards the surface (PSDQW). We also consider a symmetric square double quantum well (SDQW). In all the structures the wells are of equal width w and the side barriers are symmetrically delta doped with Si. We calculate μ by considering ionized impurity (ii-), alloy disorder (al-) and interface roughness (ir-) scatterings. We show that for lower w, single subband is occupied and μ is governed by ir-scattering. Near the transition from single subband to double subband occupancy, the drop in μ occurs and thereafter μ is governed by ii-scattering mediated by intersubband effects. We analyze the effect of coupling of subband wave functions through the central barrier width b and change in the potential profile through an external electric field F on μ. We show that for all values of w, μ (PSDQW) > μ (SPDQW) > μ (SDQW). We also calculate μ for a parabolic double quantum well (PDQW) and analyze the effect of change in the curvature of the parabolic potential which shows that μ (SPDQW) > μ (PDQW) at larger well widths. Our results can be utilized for the enhancement of the hybrid quantum well HEMT devices.

Nonlinear electron transport in GaAs/InGaAs asymmetric double-quantum-well pseudomorphic high-electron-mobility transistor structure

We analyze the structural-asymmetry-induced nonlinear enhancement of the electron mobility μ in a GaAs/InxGa1−xAs double-quantum-well pseudomorphic high-electron-mobility transistor (p-HEMT). We consider the well widths wi and ws and the doping concentrations ndi and nds in the barriers along the substrate (inverted doping) and surface (surface doping), respectively. We show that for a suitable choice of ws and nds, the variation of wi leads to interesting changes in the occupation of subbands, i.e., first, single-subband, then double-subband, and again, single-subband occupancy. By increasing ndi, the range of double-subband occupancy is enhanced. However, the lowering of μ due to the inter-subband effects is minimized. In the case of a single-channel HEMT, i.e., say, for ndi = 0, only a single subband is occupied. The mobility exhibits a cusplike behavior with a minimum at a certain value of wi. If one takes a higher value of nds, the occupation of subbands shows a different trend, i.e., from single- to double-subband occupancy as a function of wi for different ndi values with nonmonotonic variations of μ. Our results of the nonlinear enhancement of mobility can be utilized for the analysis of the coupled GaAs/InxGa1−xAs double-quantum-well p-HEMT structure.

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.

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.

Structural asymmetry induced enhancement of electron mobility in coupled double quantum well structures

We study the enhancement of electron mobility through structural asymmetry of a delta doped GaAs/AlxGa1−xAs coupled quantum well structure. We obtain the subband energy levels and wave functions through selfconsistent solution of the Schrodinger equation and the Poisson’s equation. We calculate the low temperature electron mobility by considering screened scattering potentials due to ionized impurities, interface roughness and alloy disorder by adopting random phase approximation. We show that the mobility increases with increase in the asymmetry of the doping concentrations. The change in mobility due to interchange of doping concentrations of the barriers is because of asymmetry of the interface roughness scattering potential. The barrier dependent enhancement in mobility is controlled by the variation of the subband Fermi wave vector through splitting in the subband energy levels. We show that the range of well width, up to which the interface roughness scattering dominates, depends on the doping concentration through intersubband effects.

Electric field induced oscillating electron mobility in asymmetric wide GaAs/AlxGa1−xAs quantum well structure

For the first time, we show that an oscillatory enhancement of low temperature electron mobility μ can be achieved in an asymmetric GaAs/AlxGa1−xAs wide quantum well structure by applying an external electric field F perpendicular to the interface plane. We show that the oscillating nature of μ as a function of F is mostly due to the contribution of the higher subband through intersubband effects when there is double subband occupancy. We further show that the wavy nature of μ can be enhanced by increasing the well width, surface electron density, and also by considering a double quantum well structure. Our results can be utilized for low temperature device applications.

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.

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.