**Nano Express**

# Electrostatically Shielded Quantum Confined Stark Effect Inside Polar Nanostructures

## Authors

- First online:
- Received:
- Accepted:

DOI: 10.1007/s11671-009-9347-1

## Abstract

The effect of electrostatic shielding of the polarization fields in nanostructures at high carrier densities is studied. A simplified analytical model, employing screened, exponentially decaying polarization potentials, localized at the edges of a QW, is introduced for the ES-shielded quantum confined Stark effect (QCSE). Wave function trapping within the Debye-length edge-potential causes blue shifting of energy levels and gradual elimination of the QCSE red-shifting with increasing carrier density. The increase in the *e*−*h* wave function overlap and the decrease of the radiative emission time are, however, delayed until the “edge-localization” energy exceeds the peak-voltage of the charged layer. Then the wave function center shifts to the middle of the QW, and behavior becomes similar to that of an unbiased square QW. Our theoretical estimates of the radiative emission time show a complete elimination of the QCSE at doping densities ≥10^{20} cm^{−3}, in quantitative agreement with experimental measurements.

## Introduction

*E*-field, reduces mutual overlap and the related emission probability. The lowering of the confined energy levels, relative to the unperturbed square QW, causes red-shifting of the emitted radiation during electron-hole recombination. This effect has been the subject of extensive perturbative [5] as well as non-perturbative analytic treatments [6–9], including excitonic effects [10–14]. In general earlier analytic theories neglected the modifications to the (intrinsic polar or externally applied)

*E*-field caused by the charge separation and the resulting dielectric shielding, assuming in effect very low carrier densities.

At high carrier densities, charge separation and dipole field formation is sufficient to cause shielding of the intrinsic polarization *E*-field [15]. The resulting potential gradient across the QW is not uniform, and most of the potential drop is localized across charged layers formed at the edges of the QW (Fig. 1b). The electric gradient scale is of the order of the Debye length. For densities near 10^{19} cm^{−3} the Debye length shrinks down to nm-scale (Fig. 1c), and the potential drop is mostly localized at the QW edges while the QW interior is nearly field-free (shielding of the intrinsic *E*-field). This constitutes the ES-shielded QCSE. It has been anticipated [16] that the shielding of the interior *E*-field would reduce or even eliminate the QCSE at densities 10^{19} cm^{−3}. Detailed numerical simulations, employing the self-consistent Poisson–Schrodinger equations [17] have showed that a much higher than expected carrier density, near 10^{20} cm^{−3}, is required to eliminate the QCSE for QWs wider than 5 nm. This has been attributed to the persistence of carrier confinement in the potential dips at the QW edges, even when the electric field is screened out from the middle. However, an analytic treatment examining the carrier behavior in the ES-shielded QCSE is so far lacking.

This study focuses in finding solutions for the confined carrier wave functions by solving the one-particle Schrodingers’ equation. To gain insight the following simplifying assumptions are used: (a) The shielded potential has exponentially decaying profile on the Debye length ∼λ_{D} scale; (b) the peak-to-peak shielded voltage is a given function of the carrier density and the intrinsic polarization strength
and (c) excitonic effects are ignored.

The shielded potential results from a self-consistent solution of Poisson’s equation for point-like charges obeying Fermi statistics [15]. Neglecting the charge spreading of the carrier wave function is not too severe when the carrier localization length ∼λ_{D} is much smaller than the QW width *L*. When the Fermi level separation from the lowest occupied levels is much larger than κ*T*, i.e., for nearly Maxwellian distributions, the shielded potential is well approximated by a symmetric profile
The exponentially decaying profiles remain a reasonable approximation for Fermi–Dirac distributions in general.

We obtain results based on: (a) a second order perturbative expansion; (b) non-perturbative series expansion; and (c) a numerical solution of Scrodinger’s equation for the carrier envelope wave function. The analytic expressions for the energy levels from (a) are evaluated against numerical the results from (c). The infinite λ_{
D
}, zero shielding limit reverts to the original (unshielded) QCSE results.

Our analytic models find that increasing the carrier density causes an increase (blue shifting) of the energy levels relative to the unshielded (red-shifted) QCSE values. The confined energy levels asymptote to the values for a flat square QW, and the red shift is effectively eliminated, for densities ≥ 10^{19}cm^{−3}. The perturbative energy levels agree with the numerical values at low*V*_{p}, and become inaccurate when the polarization voltage
exceeds the energy of the fundamental confined mode in a square QW. Numerical solutions of the Schrodinger equation for high polarization, relevant to GaN parameters, show that at high *V*_{p} the perturbation results overestimate the energy level shifts by a factor of 2, but they provide the correct trends over the entire range.

The dependence of the characteristic emission time on the carrier density is computed based on the numerically evaluated eigenfunctions. Despite the adopted simplifications these results reproduce the three order of magnitude increase in the emission rate between densities 10^{19} and 10^{21}, leading to a complete rectification of the QCSE, as was reported from experimental and detailed computations in Ref. [17].

Interestingly, it is found that elimination of the QCSE-related energy red-shift clearly precedes the recovery of the radiative emission time: the energy red-shifting is gradually eliminated between densities 10^{17}cm^{−3} and 10^{19}cm^{−3} while the emission probability is restored at higher densities between 10^{19}cm^{−3} and 10^{20}cm^{−3}. The first result agrees with the energy recovery behavior obtained in [16] while the emission probability behavior agrees with the results in [17]. The delay in the restoration of the emission probability is explained in terms of carrier trapping at the QW edge.

## QW Eigen Modes with ES-shielded Polar Potential

_{D}=

*a*/λ

_{D}scales as the inverse Debye length and

*a*is of order unity. The peak amplitude

*V*

_{o}here is taken equal to half the intrinsic "polarization voltage" The value Φ

_{p}(0) = 0 at mid-point equals the bottom energy for a polarization-free square well (Fig. 2), and serves as the reference point for electron energy levels. Hole levels are measured from the bottom of the valence well. The above symmetric potential applies for low carrier density and a Fermi level near the mid bandgap. For high doping the reference point

*x*

_{o}defined by Φ

_{p}(

*x*

_{o}) = 0 moves closer to the left (right), with unequal edge potentials −

*V*

_{p}(−

*L*/2) >

*V*

_{p}(

*L*/2) (−

*V*

_{p}(−

*L*/2) >

*V*

_{p}(

*L*/2)) for

*N*-doped (

*P*-doped) materials. For analytic simplicity this study will retain the symmetric potential.

*x*as

*k*

_{ y },

*k*

_{ z }correspond to the continuous spectrum along the QW. Analytic solutions of (2) are obtained from second order perturbation theory, in terms of an expansion in unperturbed square well modes

The mode energy *E*_{
n
} is always measured relative to the middle of the well; the latter always coincides with the bottom energy for the square (un-biased) QW, as shown in Fig. 1.

The shift in energy levels relative to the square QW eigen values, obtained from (7), is plotted in Fig. 2a versus the ratio κ_{D}*L* ≡ *L*/λ_{D} for the lowest three modes. The chosen parameters are peak-to-peak sheath potential 2*V*_{o} = 50 meV, QW width *L* = 8 nm and *m*_{e}^{*}/*m*_{e} = 0.19 for GaN. For λ_{D} ≫ *L*/2 the polarization field is nearly unshielded, the potential profile nearly linear, and the red-shifting hovers near the maximum value, characterizing the ordinary QCSE. Red shifting is however reduced rapidly as the screening range becomes equal or shorter than half the QW width, λ_{D} ≤ *L*/2, becoming completely negligible at λ_{D} < *L*/4. Beyond this point the energy levels revert to the square QW eigen values and the QCSE is completely "rectified". Using the scaling
with the value
for the GaN dielectric constant recasts energy shift Fig. 2a in terms of the carrier density *N*_{e}, Fig. 2b. Complete shielding of the QCSE occurs at *N*_{e} ≥ 10^{20} cm^{−3}. This value agrees well quantitatively with similar results obtained in [17], based on the observed decrease in the radiative emission time.

*e*

*V*

_{o}≥

*E*

_{1}

^{(0)}∼31 meV. Since the combined inherent and strain-induced polarization fields can reach values up to 5 MeV/cm [18] and up to 2.5 V over a 10 nm QW, numerical solutions of Schrodinger Equation are required for realistic polarization values. For comparison Fig. 3 plots the lowest energy levels obtained from numerical solutions (points) and perturbation theory (curves) versus the ratio

*L*/2λ

_{D}for

*V*

_{o}= 0.250 V. For unshielded or partially shielded QCSE with λ

_{D}≤

*L*/4 the perturbation theory overestimates the red-shift by a factor of 2. Good agreement occurs for λ

_{D}<

*L*/8 when the charged layer thickness is much smaller than the QW thickness, and thus the size of the perturbation, parameterized by becomes negligible.

*V*

_{o}and λ

_{D}. To that end the eigenfunctions of Eq. 2are obtained in terms of an infinite power series expansion a la Frobenius, Appendix1. The fast convergence of the series solutions allows the calculation of the expectation values of the kinetic energy potential energy 〈

*e*Φ(

*x*)〉 and the total energy expectation value, yielding

where *K*_{
n
}, *W*_{
n
} are functions of *eV*_{o}/κ*T* and the quantum number *n*, and *C*_{o} is the wave function normalization constant. The kinetic energy
increases with decreasing λ_{D}, while the potential (“edge-binding”) energy is fixed. For *eV*_{o} > 5κ*T* the ratio *W*_{1}/*K*_{1} for the fundamental mode is nearly constant and hovers close to 1/2, Appendix 1.

The reduction of the red shift with increasing ES shielding and decreasing shielding distance λ_{D}, manifested experimentally as a blue shift relative to the unscreened QCSE, is qualitatively understood as following. For λ_{D} < *L*/2 the sin *h*(*x*/λ_{D}) potential behaves like an edge-well inside the square well, instead of a tilted QW floor. If confinement within the edge-well occurs, the lowest energy level must satisfy 〈*E*_{1}〉 ≤ 0. As long as the confined “kinetic energy”
is less than the edge-binding energy*eV*_{o}*W*_{1} then *E*_{1} < 0 and the wave function is trapped at the QW edge. Edge-confinement within a range shorter than the well width, λ_{D} < *L*/2, increases the mode energy relative to that for a tilted QW bottom and causes blue shift relative to the unshielded QCSE. The blue-shift increases with increasing carrier density, meaning shorter confinement length λ_{D}. Eventually, for large enough density with
the kinetic energy exceeds the edge-binding energy and 〈*E*_{1}〉 > 0, edge confinement ceases, and the wave function shifts to the center to occupy the full QW width. At the same time most of the well bottom becomes nearly as flat as in a square well, since
is excluded from most of the interior. Full “rectification” of the QCSE occurs and the eigen values and eigen modes approach that of a square QW.

*V*

_{o}<

*V*

_{th}or where is the threshold under given λ

_{D}, and the threshold under given

*V*

_{o}. This transition is shown in Fig. 4a and b, plotting the fundamental mode profiles Ψ(

*x*) for various values of λ

_{D}/

*L*, for low and high voltages, respectively

*V*

_{o}= 0.250 V and

*V*

_{o}= 2.05 V. As the screening distance decreases, the center of the wave function moves from the left edge towards the center of the well. The transition to full QW occupancy occurs at shorter screening length λ

_{D}for higher

*V*

_{o}(Fig. 4b).

_{D}=

*L*/8. The fundamental

*E*

_{1}becomes positive at about For

*V*

_{o}<

*V*

_{th}the value

*E*

_{1}increases and tends to the square well limit as

*V*

_{o}≃ 0. Figure 5b shows the fundamental eigen value

*E*

_{1}versus

*L*/λ

_{D}for two different voltages

*V*

_{o}. The eigen values asymptote to the square QW limit at shorter screening distance for the case of higher polarization

*V*

_{o}.

## Radiative Emission Probability

*e*−

*h*transition probability during radiative emission, proportional to the dipole moment overlap integral

*u*

_{ c }

*u*

_{ v }, and the slowly varying envelopes, valid for as long as

*L*, λ

_{D}≫

*a*, the above is approximated by

*u*

_{ c }

*u*

_{ v }was used in arriving at (10). The last integral over the unit lattice unit cell volume

*C*is independent of the polarization. For “vertical transitions” with (given that ) the dependence on the polarization voltage

*V*

_{o}and screening distance λ

_{D}is carried entirely in the overlapping between electron-hole envelopes

_{ h }(

*x*) = Ψ

_{ e }(

*L*−

*x*). Taking the transition probability for a flat QW with as reference, and since the emission time one has

*L*/λ

_{D}for various peak voltages

*V*

_{o}, using the wave function profiles obtained from numerical solutions. Characteristic emission times tend to increase with increasing applied polarization voltage

*V*

_{o}, and decrease with decreasing screening distance λ

_{D}. The results of Fig. 6a are plotted verusus the corresponding carrier density

*N*in Fig. 6b, for QW width 8 nm. These results reproduce the three order of magnitude emission increase between densities 10

^{19}and 10

^{21}, resulting in complete rectification of the QCSE, that was first obtained using detailed Poisson–Schrodinger simulations in Ref. [17] for a 7 nm QW.

^{17}cm

^{−3}and 10

^{19}cm

^{−3}, though the radiative emission time remains almost constant there. The emission probability is restored, rather abruptly, at higher densities between 10

^{19}cm

^{−3}and 10

^{20}cm

^{−3}. This lagging in restoring the emission probability is explained via edge-carrier trapping, mentioned in the previous discussion. As carrier density increases and the edge-potential range λ

_{D}narrows down, the increasing edge-confinement of the wave function causes the energy level to increase. As long as the “confinement energy” is smaller than the edge potential depth

*e*

*V*

_{o}electron and hole wave functions remain edge-localized and no significant change in overlap and in recombination time occurs. The abrupt decrease in the radiative emission time (increase in the radiative emission rate) occurs after since at this point the wave function moves from edge-confinement to full QW occupancy. Practically this means that the QCSE-related energy red-shift has already been eliminated before the radiative emission time recovers. This behavior agrees with the results in [17].

## Shielding of the Peak Polarization Voltage

It has so far been tacitly assumed that the charged layer peak-voltage *V*_{o} is independent of the screening carrier density *N*_{e, h} and the peak-to-peak voltage 2*V*_{o} was taken equal to the “polarization voltage”
for an unscreened QW, Fig. 2a. In other words the shielding only modified the potential profile across the QW. However, for given applied
and *L*, the shielded *V*_{o} does depend on the carrier density, and in fact *V*_{o} is reduced below *V*_{p} at high carrier densities. The shielding of the peak voltage is summarized below, based on results from earlier studies [15].

*L*increases well beyond λ

_{D}the peak-to-peak voltage asymptotes rapidly to a maximum saturation value Figure 8a plots 2

*V*

_{o}versus

*L*for various polarization strength values and shows the saturation for

*L*/λ

_{D}≫ 1. Clearly

*V*

_{s}increases with polarization strength The dependence of

*V*

_{s}on density is given in Fig. 8b. The fact that

*V*

_{s}decreases with increasing density stems from Gausses law: it takes a given amount of surface charge to screen a given field. Applying scaling arguments the charge layer thickness is (half of the electric field screened at each QW edge) and the sheath voltage Thus for given polarization the voltage

*V*

_{s}scales roughly as when

*L*> 2λ

_{ D }.

*V*

_{o}to

*V*

_{p}, versus sheath length, for given doping density

*N*

_{D}= 10

^{18}cm

^{−3}. For as long as

*L*≤ 2λ

_{D}one has unsaturated behavior Once saturation is reached for

*L*> 2λ

_{D}the peak-to-peak voltage is pinned at

*V*

_{s}, independent of

*L*. This is because when

*L*> 2λ

_{D}the polarization field is screened-out from the QW interior length

*L*− 2λ

_{D}that yields a negligible contribution to the voltage difference;

*V*

_{s}comes entirely from two charged layers of width λ

_{D}. Hence, for wide QWs the peak-to-peak voltage turns out much smaller than the polarization voltage, and the ratio 2

*V*

_{o}/

*V*

_{p}goes as 1/

*L*. Notice that the saturation length

*L*

_{s}where 2

*V*

_{o}dips below

*V*

_{p}depends also on the field strength; letting and yields thus saturation occurs at smaller QW thickness with increasing . According to Fig. 8c, one may apply unsaturated values for QW thickness

*L*< 10 nm and for up to doping densities 10

^{19}cm

^{−3}. This is illustrated in Fig. 9, plotting the ratio 2

*V*

_{o}/

*V*

_{p}versus doping density

*N*

_{D}for fixed QW

*L*= 8 nm and for various strengths

*L*= 8 nm, the values 2

*V*

_{o}assume their saturation values and the shielded voltage falls significantly below

*V*

_{p}when doping densities exceed ≥10

^{20}cm

^{−3}. This is illustrated in Fig. 10, showing the screened potential profiles, 10a, and electric fields, 10b, for various doping levels

*N*

_{D}across an 8-nm QW for The peak-to-peak voltage decreases well below

*V*

_{p}with increasing

*N*

_{D}. In addition, the electron and hole charged layers become asymmetric:

*V*

_{e}across the negative charged layer is different than

*V*

_{h}across the positive charged layer. In general, reduction of the peak-to-peak voltage, as well as asymmetric electron-hole profiles should be considered for a more accurate description of the ES shielded QCSE. In particular, the drop in

*V*

_{s}<

*V*

_{p}with increasing density could accelerate the cancellation of the QCSE and the blue shifting of the energy levels. For the relevant to our GaN experiments parameters, however, the red-shifting is all but cancelled out at density 10

^{19}cm

^{−3}, just before such effects become significant. Thus it appears that energy level blue-shifting caused by the sin

*h*effect in the potential profile cancels to a large degree the QCSE effect, before shielding of the peak amplitude itself becomes important.

## Conclusions

A simplified model employing ES-shielded, exponentially-decaying polarization potentials localized at the QW edges, was employed to study the QCSE at high doping densities. Blue shifting of energy levels relative to the unshielded QCSE occurs with increasing carrier density, due to the wave function constriction within scale length λ_{D} < *L*/2. When the “edge-localization energy”
exceeds the peak-voltage of the charged layer *eV*_{o} the wave function center shifts to the middle of the QW and behavior becomes similar to that of a square (unbiased) QW. In addition, at very high doping the shielded peak voltage is reduced well below the original unshielded “polarization voltage” *V*_{p}. Both effects cause gradual elimination of the QCSE red-shifting, an increase in the *e*−*h* wave function overlap and a decrease of the radiative emission time. A significant reduction of the peak polarization voltage requires higher carrier densities than most practical situations, and screening effects stem mainly from the interior-screening and the localization of the polarization voltage within QW edge-layers. Our theoretical estimates show that the elimination of the QCSE related red-shift in energy precedes the recovery in the radiative emission time, in quantitative agreement with experimental measurements in [17].

## Appendix-1: 1-D Edge-confined Modes - Asymptotic Polynomial Expansions

_{D}≪

*L/2*one may approximate the sinh potential for

*x*< 0, , as where ζ the distance from the edge The sin

*h*Schrodinger Equation 2 is then approximated by one for an exponential potential which has been analyzed elsewhere.

^{1}A dimensionless scaling measuring length in units of and energy in units of yields

*n*labels the energy quantum number A change of variable for ζ > 0 with removes the exponential term and reduces (13) to

*w*= 1, 0, and are given by A series expansion

making use of
The leading term goes as
and gives the asymptotic behavior at
For practical purposes is suffices to keep polynomial terms up to order *M* equal to twice the integer part
inside the infinite sum in (17).

*c*

_{ o }from

*E*

_{ n }〉 is

*E*

_{ 1 }〉 > 0 occurs for Both

*K*and

*W*depend on and on the energy eigen value -ε

_{1}where ε

_{1}= ξ

_{1}. The ratio

*W*

_{1}/

*K*

_{1}is plotted in Fig. 11 versus the peak voltage (normalized in units of κ

*T*) using the lowest mode energy

*n*= 1 inside (20) and (21). Note that for

*V*

_{o}> 5κ

*T*the ratio hovers near 1/2 and thus detrapping occurs at

## Appendix 2: Charged Layer Potential

*x*) on the Fermi-Dirac occupation number

*f*in determining the local carrier density is

*N*

_{o}is a reference carrier density and the corresponding Debye length which includes the dielectric shielding ε from core (bound) electrons. The sum of the electron, hole and charged donor charge densities (N-doping is assumed without loss of generality) on the right-hand side follows from the equilibrium Fermi-Dirac occupation numbers,

with *E*_{C}, *E*_{V}, *F* being respectively the conduction, valence, and Fermi levels, *G*_{
e,h
}(*E*) the electron (hole) density of states and *N*_{D} the dopant density (normalized to *N*_{o}), and
The Fermi level *F* is obtained from the condition ρ[χ_{o}|_{Φ=0}] = 0 at the neutral point Φ(x_{o}) = 0. This automatically guarantees total charge neutrality over the QW as follows. The point *x*_{o} where ρ(*x*_{o}) = 0 is also the location of the minimum of the screened electric field, since
there. Now, from
and Gausses law follows
and *Q*_{-}=-*Q*_{+}. The sheath Eqs. 23 and 24 yield the free carrier dielectric shielding inside a plasma-filled QW capacitor of plate charge
under the nonlinear response ρ[Φ].

*N*

_{D}= 0. Since the Fermi level in this case lies close to mid-bandgap and the non-degenerate Maxwellian limit applies for the carrier statistics. The carrier density is simply given by where is the zero polarization electron and hold density. Three dimensional density of states is assumed for large enough QW width with small energy spacing Poisson's equation is then simplified to

*x*=

*X*(Φ) is given in terms of elliptic integrals of complex argument, and hence Φ(

*x*) follows in terms of the elliptic amplitude (Jacobi ) function,

where
is the potential drop over half the QW length *L* and
(Different profiles apply for given applied voltages [19] across the sheaths.) The field and voltage profiles have respectively even/odd symmetry about the middle of the QW,
reflecting the opposite electron and hole densities for an undoped material. The opposite polarity electron and hole sheath potentials *V*_{
e
} = -*V*_{
h
} = *V*_{
o
} are respectively defined by *V*_{
e
} ≡ Φ(0) - Φ(*L*/2) and *V*_{
h
} ≡ Φ(*L*/2) - Φ(*L*). The corresponding nominal sheath lengths are *L*_{
e
} = *L*_{
h
} = *L*/2. However, when *L*_{
e, h
} ≫ λ_{D}, the field in each sheath is essentially localized within a few λ_{D} while the rest of the length is almost field-free.

Solutions and shielded voltage profiles for both Maxwellian, Eq. 26, as well as Fermi-Dirac distributions in general, Eqs. 23, 24, have been given in [15]. Maxwellian profiles are reasonably well fitted with sin*h*-profiles employed in the present analysis, such as the bottom of the QW Fig. 2a. The screened profiles remain essentially similar for Fermi-Dirac distributions in general, as shown in Fig. 9a, with one difference: the symmetry between the electron and hole charged-layers is broken, *V*_{
e
} ≠ -*V*_{
h
}. In addition, F-D statistics yields higher saturation voltages V_{S} under given parameters. The saturation values shown in Fig. 7 correspond to general F-D solutions. Finally, for sufficiently small potentials
any sheath profiles, including (26), are reduced to exponential profiles [15]
, solutions of the linear differential equation

## Footnotes

^{1} The solutions with
are the odd-symmetry eigenfunctions of the general attractive potential