Optical Absorption Properties Of Semiconductor Ultra-thin Film

A trade-off exists in active layer thickness decision for applications in thin-film photoelectricity and surface photocatalysis. The thickness of the semiconductor film should increase to enhance optical absorption, but it should decrease to reduce carriers scattering and recombination which can benefit the effective transportation and collection of the photoexcited carriers. The ideal situation is to enhance the optical absorption without increasing the film thickness.In this work, a three-layer film, semiconductor ultra-thin film/lossless phase compensation layer/metal reflective layer, based on conventional asymmetrical Fabry-Perot (FP) interference structure is constructed. This structure can greatly boost the optical absorption. The light-matter interaction of the semiconductor ultra-thin film can be greatly enhanced based on strong interference when its thickness is much less than λ/4n, and the super absorption wavelength region of the ultra-thin film can also be tuned by changing the thickness of the dielectric layer.There are two structures in our samples, one is Ge/SiO2/Au with SiO2 losses dielectric layer, the other is Ge/Al2O3/Al with Al2O3 dielectric layer. We studied the theoretical simulations of two group samples and the absorption spectrum of experimental validation under different polarization incidence from visible to near infrared region, some conclusions are obtained. The ultra-thin film can achieve optical absorption enhancement which satisfies the asymmetrical FP resonator mechanism, and the dielectric layer can significantly increase the absorption intensity and strengthen the tunability of absorption wavelength. The function of the losses dielectric layer is to introduce the phase difference to meet the destructive interference condition, so different thickness of dielectric layer have different absorption gain. The absorption enhancement varies due to the employment of different metals:the semiconductor ultra-thin film with the Al reflective layer which has the smaller optical loss can gain more optical absorption than that with the Au reflective layer. The optical absorption spectrum of the samples show a good angular insensitivity under p-polarized and s-polarized incident light.Magnetron sputtering was employed to prepare semiconductor film and metallic back reflection layer. Electron beam evaporation was employed to prepare SiO2 layer, and atomic layer deposition was employed to prepare Al2O3. The lift-off process was employed to prepare the thin-film step, then we employed AFM (atomic force microscopy) and SEM (scanning electron microscopy) to characterize the film thickness and surface morphology. The extract thin-film optical constants and the experimental measurement of the absorption spectrum were achieved by the ellipsometer and n&k thin-film optical measurement system.The lossless phase compensation nanocavity structure realizes the significant optical absorption enhancement and spectrally tunable super absorption of the semiconductor ultra-thin film. This optical absorber can be applied in optoelectronic, energy harvesting/conversion and surface photocatalysis and enable the development of new thin-film active devices.