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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 3 4 4 4 4 N-Type Doping 4 4 4 4 5 4 4 4 4 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: E = h Absorption is the relative decrease in light intensity along it’s propagation path 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 Laser on Laser off Take the difference Modulation Spectroscopy Equipment used Laser Laser Monochromator Sample Photodetector Freq. Gen. Chopper Frequency Generator Lock-in Amplifier Computer Sample Detector Lock-In CPU Monochromator 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