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Application Description

Photoluminescence is a common technique used to characterize the optoelectronic properties of semiconductors and other materials. Its principle is simple: electrons are excited from the valence to the conductance band of the material by a laser with an energy larger than the bandgap. As a consequence, the photoexcited carriers relax and then 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 spectrum of the emitted light, it is possible to measure the material's response in terms of intensity as a function of wavelength. This gives access to information about the band structure – the bandgap width, the relative light generation efficiency, the quality of the material (inhomogeneous broadening), etc. Additional information can be gained by controlling the sample's environment, e.g., adding a magnetic field or changing the sample's temperature.

Measurement Strategies

Photoluminescence Application Setup using the Zurich Instruments MFLI Lock-in Amplifier

The figure illustrates a basic photoluminescence (PL) setup: the light from a continuous-wave (CW) laser is modulated by an optical chopper (or another light-modulating device) at up to a few kHz. The modulated beam impinges 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 it usually 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 the emitted light need to be modulated and measured with a lock-in amplifier to maximize the rejection of spurious light components.

Product Highlights

MFLI 500 kHz / 5 MHz Lock-in Amplifier

  • DC - 500kHz/5MHz 16 bit Current and Voltage Inputs
  • Ultra-low and flat Input Voltage Noise: < 2.5 nV/√Hz (> 1kHz)
  • Short time constants: 337 ns to 83 s
  • High Dynamic Reserve: 120 dB
  • API programming support for Python, MATLAB, LabVIEW, C, .NET

The Benefits of Choosing Zurich Instruments

With its 500 kHz input bandwidth covering the most common modulation frequencies, the MFLI Lock-in Amplifier is an ideal fit for PL experiments:

  • The MFLI has a very low input noise level of only 2.5 nV/√Hz, so that you can measure very small features in your spectra within a reasonable integration time.
  • LabOne® features tools such as the Plotter, which displays the time trace of the signal's amplitude to assist you during beam alignment.
  • Connecting it to a WiFi-enabled network, the MFLI can also be controlled through a tablet or even a smartphone: you can bring the time trace with you wherever the alignment controls are located.
  • Fast demodulators enable the measurement of short transients.
  • Thanks to the current input with 8 gain levels it is possible to measure directly 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 available programming interfaces (for Python, C, MATLAB®, LabVIEW and .NET).
  • The compact form factor of the MFLI makes it easy to position close to the measurement setup.

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