Direct Gap Ge1-ySny Alloys: Fabrication and Design of Mid-IR Photodiodes

Chemical vapor deposition methods were developed, using stoichiometric reactions of specialty Ge₃H₈ and SnD₄ hydrides, to fabricate Ge_1-ySn_y photodiodes with very high Sn concentrations in the 12%–16% range. A unique aspect of this approach is the compatible reactivity of the compounds at ultra-low temperatures, allowing efficient control and systematic tuning of the alloy composition beyond the direct gap threshold. This crucial property allows the formation of thick supersaturated layers with device-quality material properties. Diodes with composition up to 14% Sn were initially produced on Ge-buffered Si(100) featuring previously optimized n-Ge/i-Ge_1-ySn_y/p-Ge_1-zSn_z type structures with a single defected interface. The devices exhibited sizable electroluminescence and good rectifying behavior as evidenced by the low dark currents in the I-V measurements. The formation of working diodes with higher Sn content up to 16% Sn was implemented by using more advanced n-Ge_1-xSn_x/i-Ge_1-ySn_y/p-Ge_1-zSn_z architectures incorporating Ge_1-xSn_x intermediate layers (x ∼ 12% Sn) that served to mitigate the lattice mismatch with the Ge platform. This yielded fully coherent diode interfaces devoid of strain relaxation defects. The electrical measurements in this case revealed a sharp increase in reverse-bias dark currents by almost two orders of magnitude, in spite of the comparable crystallinity of the active layers. This observation is attributed to the enhancement of band-to-band tunneling when all the diode layers consist of direct gap materials and thus has implications for the design of light emitting diodes and lasers operating at desirable mid-IR wavelengths. Possible ways to engineer these diode characteristics and improve carrier confinement involve the incorporation of new barrier materials, in particular, ternary Ge_1-x-ySi_xSn_y alloys. The possibility of achieving type-I structures using binary and ternary alloy combinations is discussed in detail, taking into account the latest experimental and theoretical work on band offsets involving such materials.