Photoluminescence

Related products: MFLI

Photoluminescence is a common technique used to characterize the optoelectronic properties of semiconductors and other materials. The principle behind the measurement is simple: electrons are excited from the valence to the conductance band of the material by a laser whose energy is larger than the bandgap. The photoexcited carriers will then relax and finally spontaneously recombine with holes in the conduction band. In the case of direct semiconductors, the excess energy is emitted in the form of light (spontaneous emission). By analyzing the emitted light's spectrum, we can measure the material's response in terms of intensity vs. wavelength. From this, information can be gained on the band structure such as the bandgap width, relative light generation efficiency, quality of the material (inhomogeneous broadening) etc. More information can be gained by controlling the sample's environment for instance adding a magnetic field or changing its temperature.

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Measurement Strategies

A basic photoluminescence (PL) setup looks like the diagram in this page: a continuous wave (CW) laser has its light modulated by an optical chopper (or other light-modulating device) at up to a few kHz. The modulated beam is directed on the sample, where it excites the electrons from the valence to the conductance band. The spontaneous emission from the sample is collected and sent to a monochromator or a spectrometer where the light intensity is measured against its wavelength. Since the laser light is also collected and usually it has a significantly higher intensity, it is good practice to use optical filters to block it.

Ambient light can seriously interfere with the measurement, especially in open table-top setups. For this reason, the laser light, and consequently the emitted light, need to be modulated and measured with a lock-in amplifier to maximize the rejection of spurious light components.

Your benefits measuring with Zurich Instruments

The MFLI Lock-in Amplifier is an ideal tool for Photoluminescence. In addition to its 500 kHz input bandwidth that covers the most common modulation frequencies, it has several characteristics that make it a great choice:

  • The MFLI has a very low input noise level of only 2.5 nV/√Hz, so you can measure very small features in your spectra with a reasonable integration time.
  • The LabOne toolset features tools such as the Plotter that make setting up the experiment and optimising the signals very easy, for instance by showing the time trace of the signal's amplitude to assist you in the beam alignment.
    • Connecting it to a WiFi-enabled network, the MFLI can also be controlled through a tablet or even a smartphone, so you can bring the time trace with you wherever the alignment controls are located
  • Fast demodulators allow the measurement of short transients
  • The current input with 8 gain levels allows the direct measurement of photo-generated current from photodiodes without the need for an intermediate transimpedance amplifier
  • Fast digital data transfer through the USB or GbE connections ensure that you don't need a digitizer card to record your measurements. The data can be accessed and recorded in the LabOne User interface or through the programming libraries provided (MATLAB, LabVIEW, Python, C/C++, .NET)
  • The compact form factor of the MFLI makes it very easy to position it close to the measurement setup

Get in touch with us to see how the MFLI Lock-in Amplifier can improve your photoluminescence experiments.

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