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impurities{}

Specifications that define imputities

impurities{

   donor{    name = "n-P-in-Si"       energy = 0.045   degeneracy = 2 }
   donor{    name = "n-As-in-Si"      energy = 0.054   degeneracy = 2 }
   acceptor{ name = "p-B-in-Si"       energy = 0.045   degeneracy = 4 }

   Note: To fully ionize the impurity atoms, just specify a large negative number for the energy level.
   donor{    name = "fully-ionized"   energy = -1000   degeneracy = 2 }


   charge{ name = "positive-charge"   type = positive } # can be used to put positive charges into the device (e.g. to describe interface charges)
   charge{ name = "negative-charge"   type = negative } # can be used to put negative charges into the device (e.g. to describe interface charges)
}

Donors/acceptors

The energy separation from the conduction or valence band edge is given in units of [eV].
These energies are meant as ionization energies, e.g. a donor with an energy level right below the conduction band edge would be specified by a small positive energy level.
When impurity levels are relatively deep compared to the thermal energy k_BT/e at room temperature, incomplete ionization must be considered. (This is done automatically).
(Cheat parameter: energy = -1000 (for instance), that means, all electrons are fully ionized from the donors (similar for holes/acceptors). This might be useful for low temperatures like 4 K where usually the degree of ionization is very small. By using -1000 one can force them to be completely ionized.)

Degeneracy of the specified energy levels

Degeneracy of impuritiy levels affects their degree of ionization.
The degeneracy of donors is usually assumed to be equal to 2, for acceptors it is equal to 4.
shallow donors: degeneracy factor 2
Outer s orbital is onefold occupied (neutral state). There is one possibility to get rid of one electron but there are two to incorporate one (spin up, spin down).
shallow acceptors: degeneracy factor 4
The sp³ orbital is threefold occupied. Thus, one possibility to incorporate an electron, four possibilities to get rid of one.
More details on degenerate impurity levels can be found in e.g. "Physics of Optoelectronic Devices" by Shun Lien Chuang.
Note that in nitride semiconductors crystallizing in the wurtzite structure the degeneracy factor may vary from 4 to 6 because of a small valence band splitting.

If full ionization is assumed, i.e. energy = -1000, then the degeneracy factor effectively becomes irrelevant. This can be seen from eqs. (1.4) - (1.7) in PhD thesis of S. Birner.

Donor levels (n-type) in units of [eV] relative to conduction band edge

n-As-in-Si             = 0.054  # DESSIS
n-As-in-Si             = 0.049  # American Institute of Physics Handbook, 3rd ed., McGraw-Hill, New York (1972)
n-P-in-Si              = 0.045  # DESSIS, American Institute of Physics Handbook, 3rd ed., McGraw-Hill, New York (1972)
n-Sb-in-Si             = 0.039  # DESSIS
n-N-in-Si              = 0.045  # DESSIS
n-As-in-Ge             = 0.013  # American Institute of Physics Handbook, 3rd ed., McGraw-Hill, New York (1972)
n-P-in-Ge              = 0.012  # American Institute of Physics Handbook, 3rd ed., McGraw-Hill, New York (1972)
n-N-in-SiC             = 0.10   # DESSIS
n-Si-in-GaAs           = 0.0058 #
n-Si-in-AlAs           = 0.007  # 300 K, Landolt-Boernstein
n-Si-in-Al0.27Ga0.73As = 0.006  # Landolt-Boernstein

More parameters can be found in the nextnano³ database file database.in or at this website: http://www.ioffe.ru/SVA/NSM/Semicond/

Acceptor levels (p-type) in units of [eV] relative to valence band edge

p-In-in-Si             = 0.16   # DESSIS
p-B-in-Si              = 0.045  # DESSIS, American Institute of Physics Handbook, 3rd ed., McGraw-Hill, New York (1972)
p-Al-in-Si             = 0.057  # American Institute of Physics Handbook, 3rd ed., McGraw-Hill, New York (1972)
p-B-in-Ge              = 0.010  # American Institute of Physics Handbook, 3rd ed., McGraw-Hill, New York (1972)
p-Al-in-Ge             = 0.010  # American Institute of Physics Handbook, 3rd ed., McGraw-Hill, New York (1972)
p-Al-in-SiC            = 0.20   # DESSIS
p-C-in-GaAs            = 0.027  # Landolt-Boernstein 1982

More parameters can be found in the nextnano³ database file database.in or at this website: http://www.ioffe.ru/SVA/NSM/Semicond/