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Science Forum Index » Optics Forum » Question about strain...
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 Posted: Thu Jun 19, 2008 9:17 pm |
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I'm studying strain effects on semiconductor materials and i've found
that strain remove degeneracy in valence band: tensile with lh upper
to hh, compressive with heavy-holes upper to light-holes. I don't
understand the effects on bandgap: for example tensile compression
increases or decreases the bandgap (respect to unstrained material)?
Or this effect depends on physical propriety of semiconductor?
thanks |
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| whit3rd... |
| Posted: Fri Jun 27, 2008 12:41 pm |
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On Jun 20, 12:17 am, lionelgreenstr... at (no spam) gmail.com wrote:
Quote: I'm studying strain effects on semiconductor materials and i've found
that strain remove degeneracy in valence band: tensile with lh upper
to hh, compressive with heavy-holes upper to light-holes. I don't
understand the effects on bandgap: for example tensile compression
increases or decreases the bandgap (respect to unstrained material)?
Or this effect depends on physical propriety of semiconductor?
thanks
It's certainly a physical property: the bands in any solid
(semiconductor or other) are broader than the 'lines' of
gaseous atoms and ions, and this is a consequence of
the effect called 'pressure broadening'.
In general, bands get wider under pressure (and thus
the gaps get narrower). Neville Mott predicted that at
extremely high pressure, all materials become metals
(and this Mott transition in hydrogen is a very hot
research topic). When the bands of an insulator or
semiconductor merge (gap becomes zero), the
filled valence band and unfilled conduction band become
a single half-filled metallic conduction band. |
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| Phil Hobbs... |
| Posted: Fri Jun 27, 2008 8:07 pm |
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whit3rd wrote:
Quote: On Jun 20, 12:17 am, lionelgreenstr... at (no spam) gmail.com wrote:
I'm studying strain effects on semiconductor materials and i've found
that strain remove degeneracy in valence band: tensile with lh upper
to hh, compressive with heavy-holes upper to light-holes. I don't
understand the effects on bandgap: for example tensile compression
increases or decreases the bandgap (respect to unstrained material)?
Or this effect depends on physical propriety of semiconductor?
thanks
It's certainly a physical property: the bands in any solid
(semiconductor or other) are broader than the 'lines' of
gaseous atoms and ions, and this is a consequence of
the effect called 'pressure broadening'.
snip
This is probably a terminological point mainly, but band structure has
nothing to do with pressure broadening. It's a purely quantum
mechanical effect that exists at zero temperature and zero pressure,
whereas pressure broadening is due to dephasing collisions between
molecules, and so doesn't exist at zero temperature.
Cheers,
Phil Hobbs |
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| wy... |
| Posted: Fri Jul 11, 2008 11:44 am |
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On Jun 27, 9:07 pm, Phil Hobbs
<pcdhSpamMeSensel... at (no spam) electrooptical.net> wrote:
Quote: whit3rd wrote:
On Jun 20, 12:17 am, lionelgreenstr... at (no spam) gmail.com wrote:
I'm studying strain effects on semiconductor materials and i've found
that strain remove degeneracy in valence band: tensile with lh upper
to hh, compressive with heavy-holes upper to light-holes. I don't
understand the effects on bandgap: for example tensile compression
increases or decreases the bandgap (respect to unstrained material)?
Or this effect depends on physical propriety of semiconductor?
thanks
It's certainly a physical property: the bands in any solid
(semiconductor or other) are broader than the 'lines' of
gaseous atoms and ions, and this is a consequence of
the effect called 'pressure broadening'.
snip
This is probably a terminological point mainly, but band structure has
nothing to do with pressure broadening. It's a purely quantum
mechanical effect that exists at zero temperature and zero pressure,
whereas pressure broadening is due to dephasing collisions between
molecules, and so doesn't exist at zero temperature.
Cheers,
Phil Hobbs
I think the simplest physical picture is like this: when calculating
band gap, an essential parameter is the lattice constant of that
crystal. Pressure or temperature or field changes that constant.
A single atom has discrete energy levels. But when you put a lot of
them together in a crystal, these discrete levels bacome (or merge
into) bands. The bands' locations depend on the original atom energy
levels and how the atoms are put together. In 1D model, you have a
chain of atoms (consider single species first), and an atom just pulls
or sqeezes 2 heighbors' electron clouds. When balancing, the clouds
are of the same shape. If a strain presents, the clouds' shape get
distorted. (Meaning, the distance between adjacent atoms, the lattice
constant, changes. The joint Coulomb field of 2 neighboring atoms,
which an electon moves in, is changed. Therefore, cloud shape
changes.) Shape reflects orbits, so, the enegy levels will shift, and
directionally. In a real crystal, straining may have an effect on the
band-gap.
Wei |
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