Development of highly sensitive Ge NIR photodiode utilizing nanotexturing and charged oxides

Near-infrared (NIR) photodetectors play a critical role in many applications such as telecommunications, medical imaging, and spectroscopy, where high responsivity and low noise are desired. Germanium (Ge), with its appropriate bandgap for NIR absorption and compatibility with complementary metal-oxide-semiconductor (CMOS) processes, is a promising material for such devices. However, conventional Ge photodiodes are hindered by high reflectance, limited quantum efficiency, and elevated dark current, which compromise performance. This thesis presents a Ge-based NIR photodiode featuring three novel aspects. Firstly, to reduce optical losses, a nanostructured active area is realized with inductively coupled plasma reactive ion etching. Secondly, the charge separating electric field is induced by the high negative thin film charge (Qtot) using atomic layer deposited (ALD) aluminium oxide (Al2O3). This layer is pivotal in inverting the surface of lightly doped n-type Ge substrate to p-type, thereby forming a lateral conductive layer. The nanostructure enhances this phenomenon, affecting the depth of the conductive layer and related electric field. Thirdly, to effectively passivate the surface outside the active area, various Ge surface passivation schemes are investigated to incorporate positive Qtot, which is needed also to isolate the active area, thereby minimizing dark current collection by the anode under reverse bias. Unexpectedly, plasma-enhanced chemical vapor deposited silicon nitride is found to possess a high negative Qtot on Ge, despite being positive on silicon. Nevertheless, plasma-enhanced ALD silicon oxide (SiO2) demonstrates a positive Qtot, and an optimized SiO2/Al2O3 stack provides planar surface passivation achieving an effective surface recombination velocity of 1.3 cm/s. The fabricated Ge nanostructured photodiode achieves an external quantum efficiency (EQE) above 90% across the 1.2–1.6 μm wavelength range at zero bias voltage and room temperature. Its spectral responsivity extends into the visible and ultraviolet regions, with EQE exceeding 100% below 300 nm due to carrier multiplication. The high responsivity is attributed to minimized optical and electrical losses. The graded refractive interface enables absorptance exceeding 99% across the 300 to 1600 nm range. Effective surface passivation and a defect-free charge-separating region provided by ALD Al2O3 enable a low dark current density of 72 μA/cm² at 1 V reverse bias at room temperature, which further decreases to 2.8 nA/cm² at 93 K. At the critical telecommunication wavelength of 1550 nm, the specific detectivity reaches 1.71×1011 Jones at room temperature, increasing to 2.45×1013 Jones at 93 K. This proof-of-concept photodiode offers a promising solution for enhancing NIR detection across diverse applications.