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  • Atomic Energy levels between electrons The Fermi Energy Level lies halfway between the source of the electrons and their recipient. The focus of our experiments have been to find the band gap energy. There is a difference between calculated values and actual values.
  • Crystal Structure provides the periodicity which allows us to determine the properties of a semiconductor. It allows us to define the valence band, the conduction band, band gap Energy, Fermi energy, and if the crystal structure were to change everything else would be affected. For example Temperature affects the crystal structure, increasing the bond lengths while decreasing the Band Gap energy. Just imagine how these properties would change if the whole structure would change or perhaps a different atom were to be introduced.
  • Click to next slide when giving comparison: Compare these defects to stacking bricks. If one brick was different in size it would affect the whole building structure causing dips in some places, bulges in others and eventually weaken the overall structural integrity of the building.
  • If the surface pulls in an atom just floating around, the bonds themselves will be weaker, not to mention the extra electrons, or lack there of. Since there wouldn’t be a balance of protons and electrons, essentially an electric field would be produced due to those surface impurities.
  • The energy required to jump from the full valence band to the conduction band is the Gap Energy, E c -E v . If the electron does not receive energy equal to the Band Gap energy, then it will just fall back down to the valence band.
  • There must needs be a conservation of energy. So the energy to jump the band gap is received from the photon. We know that different states of electrons require a specific energy to make the transition to a higher energy level. So by observing the absorption of light, we can determine the energies that cause the exciton transitions, or in other words we can determine the band gap energy.
  • This graph contains a lot of information needing explanation. The electrons lie at different energy levels, for example the energy required for the jump of an electron from a donor atom is less then the jump from the valence band to the conduction band. The energy level required to jump from the donor atom, or to a donor atom is generally equal to the ionic energy level. We can take the Angular frequency to be proportional to Energy (E=h  ), and so we can see when a photon energy is absorbed; giving us further information about the crystal structure of the semiconductor.
  • This is what we are doing in our Research Lab. Modulating the semiconductor sample, and then observing the reflection of light and looking at where that reflection changes (the wavelength). From the wavelength we can determine the frequency (f=c/wavelength). This will give us the energy at which the electrons need to jump to the conduction band.
  • Energy due to the affects of the laser cancel the field due to the defects at the surface. With this added energy from the laser, the semiconductor now acts similar to a metal. A metal has no field inside because the electrons can move and counter the field. Shining the laser on the semiconductor causes it to be more like a metal (conductor). Now it can act as though there were no defects in the material.
  • The lock-in multiplies the signal by a pure sine wave at the reference frequency. All components of the input signal are multiplied by the reference simultaneously. The average of the product of two sine waves is zero, unless the frequencies are EXACTLY the same. The lock-in can measure signal that are 0.01% of the input. If the incoming signal is DC or has a frequency other than the reference frequency, the “Average” or final output of the lock-in amplifier will be zero.

photoreflectan of semicondutor photoreflectan of semicondutor Presentation Transcript

  • Photoreflectance of Semiconductors Tyler A. Niebuhr
  • Overview
    • Properties of Semiconductors
      • Structures
      • Defects
      • Optical Properties
    • Spectroscopy of Semiconductors
      • Modulation Spectroscopy
      • Equipment and Technology
    • Example of Experiment
  • Important Semiconductor Properties
  • Crystal Structure
  • P-Type Doping 4 4 4 4 4 4 4 4 3
  • N-Type Doping 4 4 4 4 4 4 4 4 5
  • Defects in Semiconductors
    • Point Defects
      • Vacancy
      • Interstitial
      • Substitutional Impurity
      • Interstitial Impurity
  • Majority of Defects on the Surface
    • Defects cause strain on the surface
      • Cracks form
      • Periodicity lost
    • Defects produce additional electric fields
    • Defects affect semiconductor performance
  • Temperature and Photon Energy
  • Optical Properties
    • Energy required to jump from the Valence Band to the Conduction Band
    • Photons provide energy where:
    • Absorption is the relative decrease in light intensity along it’s propagation path
    E = h 
  • Absorption Spectroscopy
  • Modulation Spectroscopy
    • Definition:
    The measurement and interpretation of changes in the optical response of a sample which are caused by modifying in some way the measurement conditions.
  • Modulation Spectroscopy
    • Affects due to impurities
      • Electric field created
    • Field causes change in structure
    • Electrons need less energy to “tunnel” to Conduction Band
  • Modulation Spectroscopy
    • Modulation to offset field affects
    - - - - - - + + + + + +
    • Laser provides energy to electrons to jump to other bands
  • Modulation Spectroscopy
    • Measure change in reflectance
    • Take the difference
    Laser on Laser off
  • Modulation Spectroscopy
    • Equipment used
      • Laser
      • Monochromator
      • Sample
      • Photodetector
      • Chopper
      • Frequency Generator
      • Lock-in Amplifier
      • Computer
    Laser Sample Monochromator Detector Lock-In CPU Freq. Gen.
  • Modulation Spectroscopy
  • Lock-In Amplifier
    • Detect and measure very small AC signals, as small as 0.01% of the input
    • Reference frequency required
      • Discards any signal/noise not at reference frequency
    • Result is a DC signal proportional to the signal amplitude
  • Computer Program
    • A specific program is required to acquire and process the data
    • Lab View provides a Graphical User Interface (GUI) to write such a program
      • Result is a “Virtual Instrument”
  • Output Data
  • Additional Measurements
    • Other measurements through this process
      • Energies of other transitions
      • Charge Density of Defects
  • Summary
    • Properties of Semiconductors
      • Structures
      • Defects
      • Optical Properties
    • Spectroscopy
      • Modulation Spectroscopy
      • Equipment and Technology
    • Example of Experiment