Confocal Raman Microscopy: Performance, Pitfalls, and Best Practice

Experimental Characterization of the Thermal Time Constants of GaN HEMTs Via Micro-Raman Thermometry

“…However, (2a) and (2b) cannot capture the effect of the inverse piezoelectric stress and electric field along the -axis due to the inability of cut off the current in an ungated HEMT [28]. The 100×, NA = 0.8 microscope objective used in our measurements resulted in a laser spot size of ≈0.8 µm with a depth of field of ≈3.3 µm [25] such that the temperature and thermoelastic stress values represent the volumetric average value of these quantities over a ≈0.8 µm cylinder through the depth of the GaN buffer.…”

“…Due to the fact that phonon frequencies and lifetimes vary with temperature, micro-Raman spectroscopy can be used to measure the local temperature in GaN HEMTs provided that the relationship between Raman peak position and temperature is known by calibration [3]- [5]. The lateral (spot size) and axial (depth) spatial resolution depend on the wavelength of the laser excitation and numerical aperture (NA) of the microscope objective with typical values of ≈1 µm and ≈5 µm [25]. The temperature resolution, typically ±5 °C or better, depends on the optical characteristics and stability of the spectrometer and the signal to noise ratio of the Raman spectrum.…”

The Role of ConfocalRaman Spectroscopy in Food Science

“…For a typical setup of NA = 0.9 and sample refractive index of n = 1.5, the correction has been calculated to give a depth scale depression of ∼2. 45 This, however, does not take into account the diffraction and the effect of the confocal aperture, which has been calculated by various authors to different levels. 41,42,46,47 The work of Sourisseau and Maraval, 48 generally thought to be the most complete model, reduces the compression to ×1.7.…”

“…This apparently confusing result is caused by a small amount of the incident laser light penetrating beyond the focal position, so that the intensity gradually decays as you move away from the focus rather than stopping abruptly. 35,45,49,50 This out-of-focus intensity will generate Raman photons, some of which will subsequently pass through the focus. These photons, which then pass through the focus, will be indistinguishable from those generated at the actual focus.…”

“…For the interested reader, a good review of all these issues has been recently published by Everall,45 who has been instrumental in examining many of these effects. It may seem from the discussion above that it is impossible to get good results from confocal Raman spectroscopy.…”