Measurement of surface and bulk temperature in ZnSe by femtosecond laser-excited photoluminescence

This work presents temperature-dependent photoluminescence (PL) behavior of polycrystalline optical quality ZnSe at temperatures between 300 − 480 K. We propose the use of PL to subsequently measure the material surface and bulk temperature. Electron-hole pairs (EHP) were excited via single and two-photon excitation by a 1 -kHz femtosecond laser, and PL from the EHP region was collected. The material was heated and cooled in discrete temperature steps while emission spectra were collected at each step. With increasing temperature, the emission peak wavelength showed a mostly linear red-shift whose slope was nearly identical for both single photon and two-photon excitation cases. Single-photon excitation of EHP exhibited a blue-shifted peak, offset by about 5 nm from that of two-photon excitation, as well as a broader high-energy emission spectrum. All emission spectra showed significant broadening with increased temperature, attributed to increased electron-phonon interactions. The emission peak sensitivity to temperature changes and measurement uncertainty indicate the ability to measure changes in surface temperature as small as 3.1 K, and bulk temperature as small as 1.6 K. The linearity of the peak wavelength shift with temperature is consistent with theoretical predictions and the data agree quantitatively well with the Varshni and Passler models from the chosen parameters, with some slope deviation of the Varshni model present. The data also exhibit a similar slope to the measured band gap wavelength via absorption spectra and the Tauc method, but with an offset due to low emission from low density of states at the conduction band minimum.