EUROPEAN CONGRESS AND EXHIBITION ON ADVANCED MATERIALS AND PROCESSES - EUROMAT 2021

ΖnO is a very promising material for blue/UV optoelectronics as a low-cost, transparent, and conductive semiconductor with a wide direct energy bandgap (3.3 eV) and large exciton binding energy (60 meV) at room temperature. However, being difficult to introduce reproducible and stable p-type impurities in ZnO, which exhibits intrinsically n-type conductivity, it is necessary to rely on heterojunctions with other materials, most commonly silicon, for electronic applications. Additionally, it has been shown that laser-microstructured silicon in SF6 gas demonstrates increased light absorption, even for photon energies below the silicon bandgap. Therefore, we deposit thin ZnO films on laser-microstructured silicon substrates for the development of ZnO/Si photodetectors with enhanced and broad spectral responsivity across the UV-Vis-NIR, compared to planar ZnO/Si heterojunctions.

In this work, silicon substrates of p- and n-type conductivity were irradiated with nanosecond laser pulses in SF6 gas. A thin film of ZnO was conformally deposited on microstructured and flat silicon substrates by ALD. Top and bottom electrodes were deposited by thermal evaporation, followed by thermal annealing. PL measurements indicate high crystalline quality of the annealed ZnO films while optical measurements show increased light absorption for the microstructured heterojunctions. In the case of ZnO/p-Si heterojunctions, dark I-V measurements show a non-linear behavior with higher current values for the microstructured device due to the increased active area. Photocurrent is observed under reverse bias, even for wavelengths below the silicon bandgap in the case of microstructured ZnO/p-Si. Photoconductivity measurements show increased responsivity across the UV-Vis-NIR spectral range for the mictrostructured device due to its large active area and enhanced light absorption. On the other hand, the responsivity of the ZnO/n-Si device varies depending on the illumination wavelength and the application of external bias voltage, allowing for wavelength-selective operation.