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==Introduction==
==Introduction==
Photoluminescence is the process where a substance absorbs photons and reemits photons.  The incident photons excite electrons from a lower energy level to a higher energy level.  As the electrons relax from the higher energy level to the lower energy level a photon is released.  For a semiconductor, the electrons are excited from a set of energy levels called the valence band to a higher set of energy levels called the conduction band.  The energy area between the valence band and the conduction band is called the band gap, and no electrons can be energetically found here.
Photoluminescence{{w|Photoluminescence}} is the process where a substance absorbs photons and re-emits photons.  The incident photons excite electrons from a lower energy level to a higher energy level.  As the electrons relax from the higher energy level to the lower energy level a photon is released.  For a [[semiconductor]], the electrons are excited from a set of energy levels called the [[valence band]] to a higher set of energy levels called the [[conduction band]].  The energy area between the valence band and the conduction band is called the [[band gap]], and no electrons can be energetically found here.
'''
Photoluminescence (PL) Spectroscopy''' is a type of contactless and nondestructive probing method for determining the electronic structure of a sample.  In PL Spectroscopy, a [[laser]] is fired at a sample and the resulting fluorescence is measured by a [[spectrometer]].  The measured fluorescence can be used to determine the band gap of a semiconductor sample, impurity levels and possible defects in the sample, and the recombination mechanisms within the sample [http://www.nrel.gov/pv/measurements/photoluminescence_spectroscopy.html].  The determination of the electronic band gap for a semiconductor is the primary focus of this work.


Photoluminescence (PL) Spectroscopy is a type of contactless and nondestructive probing method for determining the electronic structure of a sample.  In PL Spectroscopy, a laser is fired at a sample and the resulting fluorescence is measured by a spectrometer.  The measured fluorescence can be used to determine the bandgap of a semiconductor sample, impurity levels and possible defects in the sample, and the recombination mechanisms within the sample [http://www.nrel.gov/pv/measurements/photoluminescence_spectroscopy.html].  The determination of the electronic bandgap for a semiconductor is the primary focus of this work.
As previously stated, in a semiconductor the electrons are excited from the valence to the conduction band.  When electrons relax from the conduction band back down to the valence band, photons will be released.  '''These photons will have energy that matches the transition from the conduction band to the valence band.  Measuring the emitted photons’ wavelength with a spectrometer will reveal the energy of the photon and therefore the size of the bandgap.'''
 
As previously stated, in a semiconductor the electrons are excited from the valence to the conduction band.  When electrons relax from the conduction band back down to the valence band, photons will be released.  These photons will have energy that matches the transition from the conduction band to the valence band.  Measuring the emitted photons’ wavelength with a spectrometer will reveal the energy of the photon and therefore the size of the bandgap.




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===Design Considerations and Explanations===
===Design Considerations and Explanations===
The first step to using of PL spectroscopy is to have a powerful enough laser to excite the electrons from the valence band to the conduction band for all the different samples you wish to examine.  We used at 450 nm laser to test samples of InGaN.  This works well since a wavelength of 450 nm is equal to an energy of 2.755 eV which is how big the bandgap of InGaN can get. When choosing a laser, it is the wavelength that is important with intensity being a secondary concern since it is the wavelength that determines if the electrons jump the band gap.
The first step to using of PL spectroscopy is to have a powerful enough laser to excite the electrons from the valence band to the conduction band for all the different samples you wish to examine.  We used at 450 nm (2.755 eV) laser to test samples.  This will only work for semiconductors with bandgaps smaller than 2.755eV. When choosing a laser, it is the wavelength that is important with intensity being a secondary concern since it is the wavelength that determines if the electrons jump the band gap.
 
The setup of the PL system was designed from two separate works from the University of Colorado [http://www.physics.ohio-state.edu/~reu/99reu/final_reports/paper_hall.PDF] and from the University of Ohio [http://www.physics.ohio-state.edu/~reu/99reu/final_reports/paper_hall.PDF].  While researching the design of a PL system, it became obvious that researchers in general do not share their exact PL system design.  Both of the works feature explicit diagrams of their photoluminescence spectroscopy setups. This open source PL spectroscopy setup is largely based on their previous work.


The setup of the PL system was designed from two separate works from the University of Colorado [http://www.physics.ohio-state.edu/~reu/99reu/final_reports/paper_hall.PDF] and from the University of Ohio [http://www.physics.ohio-state.edu/~reu/99reu/final_reports/paper_hall.PDF]While researching the design of a PL system, it became obvious that not many people share their exact PL system designBoth of the works feature explicit diagrams of their photoluminescence spectroscopy setupsOur spectroscopy setup is largely based on theirs.
Both of the setups from Colorado and OSU used a neutral density filter. We do not use one here because our laser has a variable intensity dial on it. The use of a mirror to bounce the laser onto the sample instead of direct illumination from the laser offers more flexibility. It is much easier to adjust the mirror than the laser. The laser is incident on the sample at a glancing angle so that specular reflection from the sample does not go near the fiber optic cable. This will reduce the laser signal read on the spectrometer and thus the noise. The spectrometer will still read some of the laser signal from diffuse reflectionThe focusing lens is used to gather the fluorescence from the sample and focus it onto the fiber optic cableThus the fiber optic cable must be placed away from the lens at a distance equal to the focal length of the lensThe whole PL experiment is done within a dark box to limit the effect of outside light on the spectrometer.


Both of the setups from Colorado and OSU used a neutral density filter.  We do not use one because our laser has a variable intensity dial on it.  The use of a mirror to bounce the laser onto the sample instead of direct illumination from the laser offers more flexibility.  It is much easier to adjust the mirror than the laser.  The laser is incident on the sample at a glancing angle so that specular reflection from the sample does not go near the fiber optic cable.  This will reduce the laser signal read on the spectrometer.  The spectrometer will still read some of the laser signal from diffuse reflection.  The focusing lens is used to gather the fluorescence from the sample and focus it onto the fiber optic cable.  Thus the fiber optic cable must be placed away from the lens at a distance equal to the focal length of the lens.  The whole PL experiment is done within a dark box to limit the effect of outside light on the spectrometer.
The mirror, sample, lens, and fiber optic cable are held in place with [[open source optics]] holders, which can be printed on an [[open source 3D printer]].  These open source optic holders allow us greater flexibility of placement than an optical bread board or lab bench would.  However, it does lower the overall stability of the system, but such fined tuned stability is not needed for a simple PL system like this.
The mirror, sample, lens, and fiber optic cable are held in place with open source optics holders.  These open source optic holders allow us greater flexibility of placement than an optical bread board or lab bench would.  However, it does lower the overall stability of the system, but such fined tuned stability is not needed for a simple PL system like this.


===Future Improvements===
===Future Improvements===
There many improvements to this simple system that could be implemented. One improved is an improved sample holder which makes swapping out samples easier and has a component for a low temperature liquid to cool the sample during data taking. Another improvement is the addition of an optical rail for the fiber optic cable, focusing lens, and sample to put all of them in a more definite straight line. The addition of a optical chopper and lock-in amplifier would improve signal to noise ratio and a monochromator would allow the system to focus on a small range of wavelengths at a time instead of looking a wide range of wavelengths.
There many improvements to this simple system that could be implemented.  
# An improved sample holder which makes swapping out samples easier.
# A component for a low temperature liquid to cool the sample during data taking.
# The addition of an optical rail for the fiber optic cable, focusing lens, and sample to put all of them in a more definite straight line.  
# The addition of a optical chopper and lock-in amplifier would improve signal to noise ratio
# A monochromator would allow the system to focus on a small range of wavelengths at a time instead of looking a wide range of wavelengths.
 
As part of the open source optics project we will be designing all of these components.
 
[[category:open source optics]]


== See also ==
== See also ==

Revision as of 11:41, 16 August 2012

Introduction

PhotoluminescenceW is the process where a substance absorbs photons and re-emits photons. The incident photons excite electrons from a lower energy level to a higher energy level. As the electrons relax from the higher energy level to the lower energy level a photon is released. For a semiconductor, the electrons are excited from a set of energy levels called the valence band to a higher set of energy levels called the conduction band. The energy area between the valence band and the conduction band is called the band gap, and no electrons can be energetically found here. Photoluminescence (PL) Spectroscopy is a type of contactless and nondestructive probing method for determining the electronic structure of a sample. In PL Spectroscopy, a laser is fired at a sample and the resulting fluorescence is measured by a spectrometer. The measured fluorescence can be used to determine the band gap of a semiconductor sample, impurity levels and possible defects in the sample, and the recombination mechanisms within the sample [1]. The determination of the electronic band gap for a semiconductor is the primary focus of this work.

As previously stated, in a semiconductor the electrons are excited from the valence to the conduction band. When electrons relax from the conduction band back down to the valence band, photons will be released. These photons will have energy that matches the transition from the conduction band to the valence band. Measuring the emitted photons’ wavelength with a spectrometer will reveal the energy of the photon and therefore the size of the bandgap.


Design

Equipment List

Design Schematic

Open-source-PL setup.png

Fig 1. The PL Test Bench
Fig 2. The PL Setup

Design Considerations and Explanations

The first step to using of PL spectroscopy is to have a powerful enough laser to excite the electrons from the valence band to the conduction band for all the different samples you wish to examine. We used at 450 nm (2.755 eV) laser to test samples. This will only work for semiconductors with bandgaps smaller than 2.755eV. When choosing a laser, it is the wavelength that is important with intensity being a secondary concern since it is the wavelength that determines if the electrons jump the band gap.

The setup of the PL system was designed from two separate works from the University of Colorado [2] and from the University of Ohio [3]. While researching the design of a PL system, it became obvious that researchers in general do not share their exact PL system design. Both of the works feature explicit diagrams of their photoluminescence spectroscopy setups. This open source PL spectroscopy setup is largely based on their previous work.

Both of the setups from Colorado and OSU used a neutral density filter. We do not use one here because our laser has a variable intensity dial on it. The use of a mirror to bounce the laser onto the sample instead of direct illumination from the laser offers more flexibility. It is much easier to adjust the mirror than the laser. The laser is incident on the sample at a glancing angle so that specular reflection from the sample does not go near the fiber optic cable. This will reduce the laser signal read on the spectrometer and thus the noise. The spectrometer will still read some of the laser signal from diffuse reflection. The focusing lens is used to gather the fluorescence from the sample and focus it onto the fiber optic cable. Thus the fiber optic cable must be placed away from the lens at a distance equal to the focal length of the lens. The whole PL experiment is done within a dark box to limit the effect of outside light on the spectrometer.

The mirror, sample, lens, and fiber optic cable are held in place with open source optics holders, which can be printed on an open source 3D printer. These open source optic holders allow us greater flexibility of placement than an optical bread board or lab bench would. However, it does lower the overall stability of the system, but such fined tuned stability is not needed for a simple PL system like this.

Future Improvements

There many improvements to this simple system that could be implemented.

  1. An improved sample holder which makes swapping out samples easier.
  2. A component for a low temperature liquid to cool the sample during data taking.
  3. The addition of an optical rail for the fiber optic cable, focusing lens, and sample to put all of them in a more definite straight line.
  4. The addition of a optical chopper and lock-in amplifier would improve signal to noise ratio
  5. A monochromator would allow the system to focus on a small range of wavelengths at a time instead of looking a wide range of wavelengths.

As part of the open source optics project we will be designing all of these components.

See also

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