Transcript Photoreflectance of Semiconductors

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