Optical Properties of Semiconductor Thin Films

Semiconductor thin films exhibit a rich array of optical behaviours governed by their electronic structure, thickness and microstructure. At the core lies the bandgap, which dictates the onset of absorption and emission. Variations in refractive index and extinction coefficient across the visible and infrared spectrum give rise to interference fringes, optical confinement and light-trapping effects. Excitonic resonances near the band edge enhance absorption and influence photoluminescence, while structural disorder introduces sub-bandgap states manifesting as an exponential Urbach tail. As film thickness is reduced to the nanometre scale, quantum confinement alters energy levels and oscillator strengths. Surface and interface reflections can be tuned through anti-reflection coatings or dielectric overlay, enabling precise control over transmission and reflection spectra. These phenomena underpin applications in photovoltaics, light-emitting diodes, photodetectors and optical coatings, where optimisation of absorption, emission efficiency and carrier dynamics is essential. Advances in deposition techniques, such as atomic layer deposition and pulsed laser deposition, have refined control over film uniformity and crystallinity, further enhancing optical performance. The global push for sustainable energy and high-speed optical communication continues to drive innovation in both fundamental understanding and practical engineering of semiconductor thin films.

Research from Nature Portfolio

Recent studies have proposed a unified methodology for determining the optical bandgap of semiconductors by fitting a sigmoidal (Boltzmann) function to their absorption spectra. This approach applies equally to direct-gap, indirect-gap and amorphous films without the need for extrapolative data processing, thereby minimising errors arising from spectral acquisition and baseline correction. Beyond yielding precise bandgap values, the method also quantifies the degree of structural disorder, offering insight into the (dis)order of crystalline and amorphous films. By standardising bandgap measurement across a wide range of materials, this framework streamlines comparative studies and accelerates the design of thin-film devices with optimised optical performance.

Optical Properties of Semiconductor Thin Films publication trend

The graph below shows the total number of articles in optical properties of semiconductor thin films across all publications each year (not limited to Nature Index journals).

Technical terms

Bandgap (Egap): The energy difference between the valence and conduction bands; determines the threshold for photon absorption and emission.

External quantum efficiency (EQE): The ratio of collected charge carriers to incident photons as a function of wavelength; reflects device loss mechanisms and defect states.

Urbach energy: A parameter characterising the exponential tail of sub-bandgap absorption; indicative of structural and thermal disorder.

Reabsorption: The process by which emitted photons are reabsorbed within the film, affecting measured emission spectra and quantitative analysis.

Photoluminescence: Light emission following optical excitation; used to probe electronic states, bandgap and recombination dynamics.

References

1. Below the Urbach Edge: Solar Cell Loss Analysis Based on Full External Quantum Efficiency Spectra. ACS Energy Letters (2023).
2. The impact of reabsorption effect on composition analysis of organic semiconductors. Science China Materials (2024).
3. Revisiting the optical bandgap of semiconductors and the proposal of a unified methodology to its determination. Scientific Reports (2019).
4. Static Disorder in Lead Halide Perovskites. The Journal of Physical Chemistry Letters (2022).