Achieving high B:Si 2D surface concentrations for tunnelling devices

High dopant concentrations of both p-type and n-type dopants, exceeding 10²⁰ cm^-3, are needed for the development of silicon devices which rely on band to band tunneling, such as tunnel diodes and tunnel field effect transistors (TFETs). Conventional chemical vapor deposition suffers from adatom-mediated diffusion during the growth process, which must be high enough temperature to shed precursor ligands. Molecular beam epitaxy, by contrast, can produce highly crystalline material at lower growth temperatures, which limits diffusion and preserves dopant profiles. Previous studies have achieved n-type doping delta layers by incorporating phosphine (PH₃) into silicon homoepitaxy, producing material which has an electronic structure distinct from lightly doped silicon. In this work, we present a novel method for fabricating p-type delta layers utilizing solid boron in a high-temperature effusion cell within an ultra-high vacuum environment. We provide evidence of achieving high 2D dopant concentrations using electronic transport measurements, and use scanning tunneling microscopy to characterize the electronic structure of the material.