Lecture6_AbsorptionAndPhotoGeneration

Lecture6_AbsorptionAndPhotoGeneration - Lecture Lecture 6...

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ecture 6 Lecture 6 Absorption and Photo-generation ECE 4833 - Dr. Alan Doolittle Georgia Tech
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Photogeneration Light with photon energy, h < Eg is not easily absorbed. A convenient expression for the energy of light is E=1.24/ where is the wavelength of the light in um. h < Eg h > Eg Light with energy, h > Eg is absorbed with the “unabsorbed” light intensity as a function of depth into the semiconductor is I(x) = I o e - x here Io is the initial light where Io is the initial light intensity, x is distance and is the absorption coefficient [1/cm]. ECE 4833 - Dr. Alan Doolittle Georgia Tech
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Absorption The absorption depth is given by the inverse of the absorption pp g y p coefficient, or α -1 . The absorption depth gives the distance into the material at which the light at that wavelength drops to out 36% of its original intensity (by a factor of 1/e) ECE 4833 - Dr. Alan Doolittle Georgia Tech about 36% of its original intensity (by a factor of 1/e).
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Absorption ECE 4833 - Dr. Alan Doolittle Georgia Tech
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Real Energy band Diagrams: irect verses Indirect Bandgaps Direct verses Indirect Bandgaps The energy required to liberate an electron from the atom (the energy bandgap) is the same in all “escape directions” (directions that an electron can leave the atom). Example: Electrons directed toward a neighboring atom would have a high escape energy, while electrons directed toward a channel in the crystal (a hole pg y , y ( through the crystal) would have a lower escape energy. Thus, the energy band diagram is actually a function of ECE 4833 - Dr. Alan Doolittle Georgia Tech momentum. Additionally, both energy and momentum (directed mass motion) must be conserved during any transition.
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Real Energy band Diagrams: irect verses Indirect Bandgaps Direct verses Indirect Bandgaps Probability of a “direct transition” from valence band to conduction band robability of a Indirect Bandgap Direct Bandgap is low but if the valence electron is on an atom vibrating in a direction (I.e. has momentum) Probability of a “direct transition” from valence that lowers the energy required, the probability increases! band to conduction band is high! ECE 4833 - Dr. Alan Doolittle Georgia Tech
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Real Energy band Diagrams: irect verses Indirect Bandgap variations in Light Absorption Direct verses Indirect Bandgap variations in Light Absorption Electric field resonates (fields are ditive) with the atomic dipole and thus + + + + + additive) with the atomic dipole and thus is absorbed strongly! Polar materials like GaAs, InP, GaN etc… tend to be better at absorbing light. No lattice ibration is - - - - - vibration is needed to absorb the ght=direct Magnitude of Electric Field of a light direct gap. ECE 4833 - Dr. Alan Doolittle Georgia Tech Photon of light
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Real Energy band Diagrams: Direct verses Indirect Bandgap variations in Light Absorption After the atoms move apart from their equilibrium positions, the core is displaced from the electron cloud. The photon’s electric field then resonates (fields are additive) with the atom core - + 0 0 0 0 electron cloud dipole and thus is absorbed.
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This note was uploaded on 08/23/2011 for the course ECE 4833 taught by Professor Doolittle during the Spring '10 term at Georgia Institute of Technology.

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Lecture6_AbsorptionAndPhotoGeneration - Lecture Lecture 6...

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