Optical Constants Determination of Thermally-Evaporated Undoped Lead Iodide Films from Transmission Spectra Using the PUMA Method

0.7-mu m thick lead iodide (PbI2) films thermally-evaporated on glass substrates held at different temperatures T-s (35 - 195 degrees C) are studied. Typical observed X-ray diffraction (XRD) patterns and scanning electron microscope (SEM) micrographs of such PbI2 films prepared at high substrate temperatures T-s (> 100 degrees C) were found to be crystalline with hexagonal 2H-polytypic structure with the c-axis perpendicular to the surface. The room-temperature normal-incidence transmittance T-exp(lambda) of the PbI2 films has been measured as a function of spectral wavelength lambda in the range 300 - 1100 nm and was used to retrieve the spectral dependence of their optical constants n(lambda) and kappa(lambda) using the Pointwise Unconstrained Minimization Approach (PUMA) method. The energy variation of the absorption coefficient alpha(lambda)(= 4 pi kappa(lambda)/lambda) of the PbI2 films in the region of strong optical absorption was analyzed using various interband transition models and was found to be reasonably described by an approximate power-law relation ahv proportional to (hv - E)(m), with m = 2 and E-g(opt) congruent to 2.2 eV(+/- 2%) (Tauc interband dielectric model, where E-g(opt) is the optical bandgap energy). But, it is more remarkable for m = 1/2 and E-g congruent to 2.45 eV(+/- 2%) (direct interband transition model) over a broader spectral range. For T-s > 100 degrees C and in the transparent and weak absorption regions, the PUMA-retrieved n (lambda) - lambda data of the PbI2 films was found to fit the Wemple-DiDomenico (WDD) dispersion formula, with bandgap energy parameter E-o congruent to 3.9 eV congruent to 2 E-g(opt), single-oscillator energy strength E-d congruent to 19 eV and static index of refraction n(o) congruent to 2.5. Analysis of the data in the absorption tail to Urbach formula yielded an Urbach-tail parameter Gamma(U) that decreased with increasing substrate temperature to a value around 75 meV at the high substrate temperature side. These results indicate that using film growth temperatures beyond 100 degrees C leads to an enhancement in the crystallinity of the PbI2 films and reduces band tailing.