Retrieval of Optical Constants of Undoped Flash-Evaporated Lead Iodide Films from an Analysis of Their Normal Incidence Transmission Spectra Using Swanepoel's Transmission Envelope Theory of Non-Uniform Films

Normal-incidence transmission-wavelength (T-exp(lambda) - lambda) spectra of 1 and 1.2.m thick flash-evaporated lead iodide (PbI2) films on 1.1 mm thick glass slides held at 200 degrees C display well-spaced several interference-fringe maxima and minima in the lambda-range 520-900 nm, without exhibiting a transparent region and with the maxima lying well below the substrate transmission. Below 520 nm, these T-exp(lambda) - lambda curves drop steeply to zero (at alpha less than or similar to 505 nm), signifying crystalline-like PbI2 film absorption. As corrections of measured (PbI2 film/substrate) transmittance data for substrate absorption and spectrometer slit-width effect were marginal over the studied lambda-range, the observed low transmittance of (PbI2 film/substrate) system was related to PbI2 film thickness non-uniformity (Delta d), which causes shrinkage of both maxima and minima and leads to significant film optical absorption that reduces both maxima and minima. The McClain ENVELOPE algorithm was utilized, with a minor modification, to construct maxima T-M (lambda(max)/lambda(min)) and minima T-m (lambda(max)/lambda(min)) envelope curves, which were analyzed by Swanepoel's envelope method of non-uniform films using an approach that takes account of dispersive substrate refractive index n(s)(lambda) and circumvents the non-availability of a high-lambda transparency region. In such analytical approach, Delta d was varied till a re-generated T-gen(lambda) - lambda curve matches the T-exp(lambda) - lambda curve. An average thickness (d) over bar of the film, besides its refractive index n(lambda) and absorption coefficient n(lambda) in the weak, medium and strong absorption regions, were then obtained. The energy-dependence of alpha(lambda) is discussed in view of interband electronic transition models. The obtained results are consistent with other literature studies on similar flash-evaporated PbI2 films.