High-sensitivity photodiodes using black silicon and induced junction

For decades, the core technologies behind silicon photodiodes have remained the same: anti-reflective coating is used to suppress reflections and the photogenerated charge carriers are collected using a pn-junction formed by doping silicon. Unfortunately, these technologies set fundamental limitations to the photodiode performance. AR coatings are effective only at a single wavelength and incidence angle, and dopants are associated with a dead layer where some carriers are lost due to recombination. This thesis introduces a new photodiode concept overcoming these limitations. Instead of AR coating, black silicon (b-Si), i.e., nanostructured silicon, is utilized to effectively suppress reflections at a wide range of wavelengths and incidence angles. The use of dopants is avoided by inducing the junction with negatively charged atomic layer deposited (ALD) aluminium oxide (Al2O3) film conformally covering the b-Si. The fabricated photodiodes exhibit close to ideal responsivity at wavelengths between 200 nm - 1100 nm, reaching >99% external quantum efficiency (EQE) at visible wavelengths and maintaining the performance up to an incidence angle of 60˚. At ultraviolet (UV), EQE even exceeds 100% due to effective harnessing of carrier multiplication. Thorough characterization measurements and simulations reveal that the exceptionally high EQE can be mainly explained by i) minimal reflections from b-Si, ii) efficient surface passivation by ALD Al2O3 and iii) lack of Auger recombination in the induced junction. High EQE is also seen at near-infrared (NIR) wavelengths. This is shown to result from scattering caused by b-Si which increases the effective optical path length up to 43% compared to a conventional photodiode. In addition to responsivity, dark current of the photodiode is also investigated. The dark current is seen to behave similarly than in conventional silicon photodiodes with respect to substrate resistivity and temperature. This indicates that the improved responsivity should translate directly into higher signal-to-noise ratio. Finally, the radiation hardness of the photodiodes against proton and electron irradiation is studied. The results suggest that induced junction photodiodes are more stable against ionizing electron irradiation than conventional pn-junction photodiodes. This is most likely due to Al2O3 used for induced junction having better tolerance against charging than silicon dioxide present in the conventional photodiodes. Other degradation effects caused by the irradiation are found to be independent on the photodiode surface and junction type suggesting that, in addition to better sensitivity, b-Si induced junction photodiodes can achieve better durability and longer operation lifetime in harsh conditions.