2. Photoreflectance of Semiconductors Tyler A. Niebuhr

3. Overview  Properties of Semiconductors  Structures  Defects  Optical Properties  Spectroscopy of Semiconductors  Modulation Spectroscopy  Equipment and Technology  Example of Experiment

4. Important Semiconductor Properties

5. Crystal Structure

6. P-Type Doping 444 44 444 3

7. N-Type Doping 444 44 444 5

8. Defects in Semiconductors  Point Defects  Vacancy  Interstitial  Substitutional Impurity  Interstitial Impurity

9. 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

10. Temperature and Photon Energy

11. 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

12. Absorption Spectroscopy

13. 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.

14. Modulation Spectroscopy  Affects due to impurities  Electric field created  Field causes change in structure  Electrons need less energy to “tunnel” to Conduction Band

15. Modulation Spectroscopy - - - - - - + + + + + +  Laser provides energy to electrons to jump to other bands  Modulation to offset field affects

16. Modulation Spectroscopy  Measure change in reflectance  Take the difference Laser on Laser off

17. Modulation Spectroscopy  Equipment used  Laser  Monochromator  Sample  Photodetector  Chopper  Frequency Generator  Lock-in Amplifier  Computer LaserSampleMonochromator Detector Lock-In CPU Freq. Gen.

18. Modulation Spectroscopy

20. 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

21. 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”

25. Output Data

26. Additional Measurements  Other measurements through this process  Energies of other transitions  Charge Density of Defects

27. Summary  Properties of Semiconductors  Structures  Defects  Optical Properties  Spectroscopy  Modulation Spectroscopy  Equipment and Technology  Example of Experiment