The lower the sample temperature, the sharper the absorption bands
This holds true for many spectroscopic applications. Partially, because excited states are either less occupied or not occupied at all. This is why measurements at low temperatures may enhance the outcome and result in higher sensitivities. However, integrating a cryostat into the spectrograph can be quite a hassle.
Chosing the right hardware can well ease the task, particularly when a proprietary spectrograph is to be used.
One application for FTIR-spectroscopy at low temperatures is the determination of concentration levels of doping materials on the basis of the absorption spectrum. For precision measurements which are required with weakly doped materials, the sample must be cooled down to only a few kelvin.
The CiS Research Institute for Microsensor Systems and Photovoltaics in Erfurt, Germany, has peformed this type of measurement on aluminum-doped, multi-crystal silicon at 10 K.
By correlation with four-point resistance measurements, the calibration of the FTIR-absorption bands on aluminum concentration could be expanded to > 1015 cm3. This allowed measurements of the aluminum concentration in compensated silicon (codoped with boron, phosphorus and aluminum) 1.
For the experiment, a Cryostation was inserted in the sample chamber of the proprietary FTIR-spectrograph Tensor 27 from Bruker. The compact Cryostation features a closed helium cycle and the sample chamber has various windows to facilitate the combination with a variety of spectrometers. In addition, the sample chamber can easily be accessed, which makes sample mounting a piece of cake.
- K. Lauer, C. Möller, T. Bartel, and F. Kirscht, “Low-temperature FTIR investigation of aluminum-doped solar-grade silicon,” Energy Procedia, vol. 55, pp. 545-551 (2014)