Radiation detecting-structures and fabrications methods thereof are presented. The methods include, for instance: providing a substrate, the substrate including at least one trench extending into the substrate from an upper surface thereof; and epitaxially forming a radiation-responsive semiconductor material layer from one or more sidewalls of the at least one trench of the substrate, the radiation-responsive semiconductor material layer responding to incident radiation by generating charge carriers therein. In one embodiment, the sidewalls of the at least one trench of the substrate include a (111) surface of the substrate, which facilitates epitaxially forming the radiation-responsive semiconductor material layer. In another embodiment, the radiation-responsive semiconductor material layer includes hexagonal boron nitride, and the epitaxially forming includes providing the hexagonal boron nitride with an a-axis aligned parallel to the sidewalls of the trench.