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Photoreflectance of Semiconductors Tyler A. Niebuhr.

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Presentation on theme: "Photoreflectance of Semiconductors Tyler A. Niebuhr."— Presentation transcript:

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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

7 N-Type Doping

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

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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”

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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


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