Cutback measurements on arsenic triselenide thin films

Sample preparation

As2Se3 films were deposited on Corning glass slides by thermal evaporation due to the robust stoichiometry of bonding in this glass (Pressure < 2.10 -5 Torr, deposition rate ~ 100 Å/s). They were annealed for 2 hours at 180 oC under vacuum before measurements. The thickness of each film was precisely measured by spectroscopic ellipsometry.

Measurement and experimental set up

Transmission measurements were performed with a HeNe laser (633 nm P = 10 mW) and an Ar+ laser (514 nm, P = 40 mW) on 7 thin films: 201 nm, 362 nm, 591 nm, 959 nm, 973 nm, 1227 nm and 3076 nm thick films. Because of the photodarkening effect exhibited by As2Se3, the transmission decreased with time until reaching saturation. To take this effect into account, the transmitted intensity was recorded versus time until relatively stable for four different locations on each film. These four curves were averaged and the resulting curve was fitted with a stretched exponential function to extract the characteristic time and the transmission at t=0 and at saturation.




Results and analysis

The experimental data obtained for the average transmission at t = 0s is shown on the graph below. The green dots correspond to illumination with the Argon laser while the red dots correspond to illumination with the HeNe laser. This data was fitted using the Transfer Matrix Method 1 to determine the refractive index of As2Se3 at 514 nm and 633 nm. The corresponding calculated data points are plotted as black crosses on the same graph. The annealed refractive index values obtained at 514 nm and 633 nm appear to be close to the one measured by spectroscopic ellipsometry within an error of 1%.



These measurements also revealed that there exists an optimal thickness ~200 nm for which the characteristic time (i.e. time to saturation, defined as 6tau) is minimum. This is the result of a trade-off between high losses for thick films where photodarkening is slow and quenching of the photodarkening effect for small thicknesses due to interface interactions.



References:
1. Ajoy K. Ghatak, K. T., M.R. Shenoy, "Numerical Analysis of Planar Optical Waveguides Using Matrix Approach" Journal of Ligthwave Technology LT-5, 660-667 (1987).