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

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.