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# nextnano3 - Tutorial

## Triangular well

Author: Stefan Birner

If you want to obtain the input file that is used within this tutorial, please contact stefan.birner@nextnano.de.
`-> 1DGaAs_triangular_well_nn3.in / *_nnp.in - `input file for the nextnano3 and nextnano++ software

## Triangular well

A triangular well consists of a potential with a constant slope that is bound at one side by an infinite barrier.

For x < 0 nm we have an infinite barrier. In our case it is represented by a conduction band offset of 100 eV.

For x > 0 nm we have a linear potential of V(x) = e F x.

V(x) describes a charge e in an electric field F where the product e F is assumed to be positive.

The Schrödinger equation for the transverse component of the electronic wave function has the following form inside the well:

[ - hbar²/(2m*) d²/dx² + e F x ] psi(x) = E psi(x)

Usually one applies Dirichlet boundary conditions at x = 0 nm so that psi(x=0) = 0 in order to represent an infinite barrier, i.e. the high barrier prevents significant penetration of electrons into the barrier region.

In our case, we apply Neumann (or Dirichlet) boundary conditions at x = -10 nm and x = 150 nm and let the infinite barrier be represented by the conduction band offset of 100 eV. Both boundary conditions lead to the same eigenenergies for the lowest eigenstates.

The Schrödinger equation can be simplified by introducing suitable new variables and thus reduces to the Stokes or Airy equation. Its solutions, the so-called Airy functions, are discussed in most textbooks, see for example:

The Physics of Low-Dimensional Semiconductors - An Introduction
John H. Davies
Cambridge University Press (1998)

 The figure shows the conduction band edge (black line) which is represented by a triangular potential well V(x) = e F x. Also shown are the four lowest energy levels and corresponding wave functions. The electric field that has been applied is F = 5 MV/m, i.e. 0.05 V / 10 nm. The effective electron mass has the value 0.067m0 (GaAs).

One can see that the distance between the energy levels decreases with increasing n because the quantum well width gets larger for higher energies.
Note that in a parabolic well, the energy levels are equally spaced whereas in an infinitely deep square well, the energy level separation increases with increasing energy.

The eigenvalues of the Airy equation can be calculated using the formula:

En = cn [ (e F hbar)² / (2m*)]1/3   (The units of En in this equation are [J].)

The lowest eigenvalue has the value c1 = 2.338.

For large n, cn can be approximated by the following equation which can be derived from WKB theory (named after Wentzel, Kramers and Brillouin):

cn ~  [ 3/2 pi (n - 1/4) ]2/3

The nextnano³ eigenvalues for the lowest four eigenstates are in very good agreement with the analytic results:

 nextnano³ eigenvalue (eV) calculated eigenvalue (eV) cn (exact) cn (approximated) n = 1 `0.05644` `0.05664` c1 = `2.338` c1 = `2.320251` n = 2 `0.09882` `0.09889` c2 = c2 = `4.081810` n = 3 `0.13351` `0.13365` c3 = c3 = `5.517164` n = 4 `0.16416` `0.16435` c4 = c4 = `6.784455`

The triangular potential is not symmetric in x, thus the wave functions lack the even or odd symmetry that one obtains for the infinitely deep square well.

The triangular well model is useful because it can be used to approximate the (idealized) triangularlike shape near a heterojunction formed by the discontinuity of the conduction band and an electrostatic field of electrons or remote ionized impurities.

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