Band-edge thermometry (BET), also known as absorption band-edge spectroscopy (ABES), is a very precise method to determine the substrate temperature during MBE growth. At room temperature, many semiconductors are essentially transparent in the infra-red (IR) range of the spectrum (~900-1700nm). However, as the temperature increases, semiconductors start absorbing incident light in part of this wavelength range and the absorption edge shifts with temperature due to thermal expansion and other effects. BET works by measuring the IR radiation from an incident source (substrate heater or an external halogen lamp if heater power is not sufficient) that is transmitted by the substrate. The substrate temperature can be calculated by measuring the position of the transmitted light absorption edge.
Compared with reflectance corrected pyrometry, BET has the advantages of working well with very small substrates and not being affected by coating of viewports or scattered radiation from cells. However, BET measurements can cause local heating of the substrate and are not reliable for doped substrates. Therefore these two methods complement each other and the two measurements are often cross-calibrated. Our MBE tool is equipped with NIR spectrometer (Control Development Inc.) controlled by in-house made software (LabView), providing real-time temperature measurements.