Design and Modeling of GeSn-Based Heterojunction Phototransistors for Communication Applications

We propose the use of Ge 1-x Sn x heterojunction phototransistors (HPTs) as efficient optical receivers on Si substrates and analyze their performance. Our designs use n-Ge/p-Ge 1-x Sn x /n-Ge 1-x Sn x layers pseudomorphically grown on Si wafers via a Ge virtual substrate, which offers compatibility with complementary metal-oxide-semiconductor (CMOS) technology. By incorporating Sn into the Ge photon-absorbing layer to shrink the bandgap, the photodetection range can be significantly extended to the mid-infrared (MIR) region with a considerably enhanced optical response. The use of HPT structures provides optical conversion gain to further enhance the optical responsivity, thereby enabling efficient photodetection in the short-wave infrared region. We develop theoretical models to calculate the composition-dependent band alignments, the band structures (by taking into account the nonparabolic effect), the absorption coefficient, and the optical responsivity for the proposed GeSn HPTs. As the Sn content increases, the conduction band nonparabolicity becomes increasingly significant and considerably impacts the optical absorption coefficient. Moreover, analysis of the spectral response for the Ge 1-x Sn x HPTs shows that efficient photodetection covering the entirety of the fiber-optic telecommunication bands, as well as the emerging 2-μm MIR communication band, can be achieved. These results indicate that the proposed Ge 1-x Sn x HPTs are attractive for use as high-responsivity CMOS-compatible photodetectors in communication applications.