The Photorefractive Effect
    The Photorefractive (PR) Effect is a non-linear optical property by which high intensity light changes the optical properties of the material on which it shines.  The simplest geometry for examining the PReffect is Two-Wave Mixing, and it this geometry that the final experiment will use.
 

• The Photorefractive Process outlines the processes taking place in a PRmaterial for the Two-Wave Mixing geometry

The Experiment
    The goal of this project is to observe PReffects in bulk crystal Germanium.

• For most PRmaterials there is a linear electro-optic effect.  Because the quadratic electro-optic effect is usually very weak, the strength of this effect determines the strength of the PReffect.  Germanium, however, has a crystal structure having inversion symmetry.  Thus it lacks a linear electo-optic effect.  So current thought deems the material not to be photorefractive.  There are, however, resonant frequencies for which the quadratic electro-optic effect does become significant for other PRmaterials.  In fact the quadratic effect can even dwarf the linear effect at these frequencies.  The idea of Prof Nolte is to find the resonant frequencies in Germanium and to exploit the quadratic electro-optic effect to produce measurable PReffects.
• Some information on Germanium
• Germanium is an indirect semiconductor.  Thus to make transitions from the valence band to the conduction band with the least amount of energy an electron must absorb a photon and a phonon.
• Indirect Bandgap-  .70 eV  at 77K
• Direct Bandgap-   .881eV  at 77K
• Because the material needs to have a high concentration of defects yet also have a high resistivity, the samples when properly doped will still have to be cooled to liquid nitrogen temperatures.
• From an academic journal I was able to get electro-absorption data on Germanium at liquid nitrogen temperatures.  This data will have to be confirmed for my samples upon reaching high enough resistivities; however, this data indicates that it is possible to see PReffects at a photon energy corresponding to the direct band-gap.  This energy corresponds to a wavelength of 1.35 microns.
• After electro-absorption experiments to determine proper photon energy, the proper source must be found and the Two-Wave mixing apparatus must be constructed.

Progress

• Germanium samples were procured at the end of January.  The samples are doped with Antimony making them n-type.  The next few months will be devoted to getting a proper amount of Copper diffused into the samples compensating for the Antimony.
• Samples were cut on a dicing saw in Patty Metcalf's lab.  About ten pieces were cut each about 10mm x 1mm x 1mm
• Samples were electroplated with a Copper Sulfate solution and a 9V battery.
• Samples were placed in a diffusion furnace at 1000K and initially melted.  Lower temps were used ranging from 400K to 700K.  Copper readily diffuses into Germanium so diffusion time was not a concern.
• The annealing process began
• Initially so much Copper diffused into the Ge that the material became p-type and highly conductive.
• Started with anneals at 500ºC for only a brief time and let slowly cool.  This lead to a significant (3 orders of maginitude) increase in the resistance.
• Continuing along this reasoning.  I began slowly increasing anneal time and temperature trying to increase the resistance.  As expected the resistivity stayed relatively constant as copper was annealed out.
• Using a higher temperature one of the samples shows an increase of an order of magnitude at a 600ºC anneal time for 4 minutes.
• All samples have now melted believed due to residual Indium left from the attachment of contacts.
• New samples have been made and diffused at 500ºC.
• New anneal attemps and an attempt to find the boundary where all the Copper precipitates out leaving the material n-type again prove unsuccessful.
• Switch to different strategy where the amount of Copper diffused into the material is controlled instead of controlling the amount of Copper precipitated out.