When transmission and absorption spectra of metal oxide thin films are not smooth and exhibit fluctuations or irregularities, several factors typically contribute:

1. Thin Film Thickness Variations (Interference Effects):

• Variations or non-uniformity in film thickness can lead to interference fringes in the spectra, causing oscillations (fringes).
• Even minor variations in thickness can cause constructive and destructive interference patterns, known as Fabry-Perot fringes, especially evident in transparent films.

2. Surface Roughness and Defects:

• Surface irregularities, cracks, pores, or scratches cause scattering and diffraction, making spectra appear rough or irregular.
• Uneven coating or particle agglomeration can also create local variations in refractive index or thickness.

3. Instrumentation Noise and Experimental Errors:

• Spectrometer noise, detector sensitivity fluctuations, or improper baseline corrections can introduce irregularities.
• Misalignment of the sample or instability in the experimental setup can further distort spectral data.

4. Interference Effects:

• Thin-film interference: Interference between reflected waves from the top and bottom interfaces of the thin film generates oscillatory patterns (fringes) in the spectra.
• These interference fringes are common and expected, especially if the film thickness is on the order of the wavelength of incident light.

5. Non-uniform Material Composition:

• Variations in chemical composition, such as gradients or impurity distribution, lead to changes in optical properties within the film, affecting absorption and transmission uniformity.

6. Instrumental Resolution and Noise:

• Low resolution or noisy instrumentation leads to less smooth spectra.
• Improper calibration, instrument instability, or detector sensitivity issues contribute to rough spectra.

6. Film Porosity and Grain Boundaries:

• High porosity or significant grain boundary regions in polycrystalline metal oxide films scatter incident photons, producing irregularities.
• Grain boundaries alter optical paths and refractive indices locally.

How to Improve Spectral Smoothness:

• Control Thickness: Ensure uniform thickness during deposition through better control of deposition conditions (e.g., optimized spin coating, sputtering conditions).
• Reduce Surface Roughness: Use polishing, annealing, or controlled deposition techniques.
• Refine Measurement Techniques: Improve the accuracy and stability of spectroscopic measurements and equipment calibration.
• Material Processing: Optimize deposition parameters (temperature, vacuum, rate) to ensure material homogeneity, smoother surfaces, and lower defect density.

Perhaps the film is not smooth, but rough. This causes scattering of the incident radiation, and may lead to not smooth spectra.

The transmission and absorption spectra of metal oxide thin films can appear non-smooth due to factors like surface roughness, defects, and variations in crystallinity.