Double Subband Occupancy Research Articles

Asymmetric Doping‐Dependent Electron Transport Mobility in InxGa1–xAs/GaAs Quantum Well Field‐Effect Transistor Structure

We analyze the asymmetric doping‐dependent electron mobility μ of GaAs/InGaAs/GaAs quantum well field‐effect transistor (QWFET) structure. We consider doping concentrations, nd1 and nd2, in the substrate and surface barriers, respectively, and study μ as a function of nd2, taking (nd1 + nd2) unchanged. An increase in nd2 decreases nd1, yielding interesting changes in the occupation of subbands. For well width W < 164 Å, μ is due to single subband occupancy (SSO). Around W = 164 Å, there occurs first SSO, then double subband occupancy (DSO), and again SSO with an increase in nd2. Near the transition of subbands, abrupt discontinuities in μ arise due to inter‐subband effects. Thus, high to low and then high values of μ are obtained, displaying almost flat‐like variations, symmetric about |nd2 − nd1| = 0. As W becomes wider, complete DSO occurs throughout the range of nd2 having reduced μ. Alternatively, keeping nd1 unchanged and by increasing nd2, μ raises due to enhanced N s , with a drop near the transition from SSO to DSO. Under SSO, μ is controlled by the ionized impurity and alloy disorder scatterings, while under DSO, the impurity scattering determines μ. Our analysis on μ can help to examine the inter‐subband effects on device characteristics of the QWFET system.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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 parabolic potential well profile on multisubband electron mobility in a coupled quantum well in presence of an external electric field

We study the effect of parabolic potential well profile on the electric field dependent multisubband electron mobility in a double quantum well structure. We consider a barrier delta doped GaAs–AlxGa1−xAs parabolic coupled double quantum well subject to an external electric field perpendicular to the interface plane. Variation of the electric field transfers the system from double subband occupancy to single subband occupancy at which an enhancement of mobility is obtained due curtailment of the intersubband effects. It is gratifying to show that enhancement of curvature of the potential profile i.e., replacement of a square quantum well into a parabolic quantum well structure enhances the mobility through the intersubband effects. We vary the structure parameters such as well width, barrier width, doping concentration and height of the parabolic potential to analyze the effect of the parabolic well structure on the field dependent multisubband electron mobility, which shows interesting features.

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.

Effect of strain on multisubband electron transport in GaAs/InxGa1-xAs coupled quantum well structures

We analyze the strain induced changes in the low temperature multisubband electron mobility mediated through the intersubband interactions in a pseudomorphic GaAs/InxGa1−xAs coupled double quantum well structure. We consider the non-phonon scattering mechanisms and study the effect of strain on them. We show that strain reduces the mobility due to ionized impurity (imp-) scattering μimp but enhances the mobility due to interface roughness (IR-) scattering μIR. For alloy disorder (AL-) scattering as long as the lowest subband is occupied, the effect of strain enhances the mobility μAL. However, once the second subband is occupied, there is almost no change, rather decrease in μAL for larger well widths. It is gratifying to note that for single subband occupancy, the effect of strain enhances the total mobility μ. On the other hand, for double subband occupancy, initially there is almost no change, but with increase in well width the total mobility reduces. We vary the In composition x from 0.15 to 0.2 and 0.25 and the barrier width between the two wells to analyze their effects on the mobility which shows interesting results. Our study of multisubband mobility can be utilized for the low temperature device applications.

Magnetotransport properties of lattice-matched In0.18Al0.82N/AlN/GaN heterostructures

Magnetotransport properties of the two-dimensional electron gas (2DEG) in lattice-matched In0.18Al0.82N/AlN/GaN heterostructures have been studied at low temperatures and high magnetic fields. The double subband occupancy of the 2DEG in the triangular quantum well at the heterointerface is observed. The 2DEG density is determined to be 2.09×1013 cm−2 and the energy separation between the first and the second subbands is 191 meV. Both of them are significantly higher than those in AlxGa1−xN/AlN/GaN heterostructures owing to the stronger spontaneous polarization effect. The evident difference of the quantum scattering times in the two subbands of the 2DEG indicates that the interface roughness scattering plays an important role in the transport properties of the 2DEG in InxAl1−xN/AlN/GaN heterostructures.

Hot electron energy relaxation via acoustic phonon emission in modulation-dopedIn0.53Ga0.47As/In0.52Al0.48Asheterojunctions with double-subband occupancy

Hot electron energy relaxation via acoustic phonon emission in modulation-doped<mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline"><mml:mrow><mml:msub><mml:mrow><mml:mi mathvariant="normal">In</mml:mi></mml:mrow><mml:mrow><mml:mn>0.53</mml:mn></mml:mrow></mml:msub></mml:mrow><mml:mrow><mml:msub><mml:mrow><mml:mi mathvariant="normal">Ga</mml:mi></mml:mrow><mml:mrow><mml:mn>0.47</mml:mn></mml:mrow></mml:msub></mml:mrow><mml:mi mathvariant="normal">As</mml:mi><mml:mo>/</mml:mo><mml:mrow><mml:msub><mml:mrow><mml:mi mathvariant="normal">In</mml:mi></mml:mrow><mml:mrow><mml:mn>0.52</mml:mn></mml:mrow></mml:msub></mml:mrow><mml:mrow><mml:msub><mml:mrow><mml:mi mathvariant="normal">Al</mml:mi></mml:mrow><mml:mrow><mml:mn>0.48</mml:mn></mml:mrow></mml:msub></mml:mrow><mml:mi mathvariant="normal">As</mml:mi></mml:math>heterojunctions with double-subband occupancy