Enhanced Performance Designs of Group-IV Light Emitting Diodes for Mid IR Photonic Applications

The optical emission properties of Ge_1-ySn_y and Ge_1-x-ySi_xSn_y LEDs on Si(100) substrates are investigated by room temperature electroluminescence (EL). In the case of Ge_1-ySn_y LEDs, the basic device architecture consists of a thick (1.0-1.5 μm) highly doped (n = 1-3x10¹⁹ cm^-3) n-Ge layer grown on a Si wafer to act simultaneously as a buffer and bottom contact. This is followed by the growth of the active i-Ge_1-ySn_y layer (400-700 nm) with compositions that span from pure Ge (y = 0) to levels beyond the crossover to direct gap semiconductors for this alloy (y = 0.12). The device is capped by a p-Ge_1-y’Sn_y’ layer (100-200 nm) grown pseudomorphically to the intrinsic layer containing y’ < y to promote carrier confinement in the active region. These devices feature a single interface between the n-Ge and i-Ge_1-ySn_y layers where strain relaxation may occur to accommodate the lattice mismatch by generating defects. The EL spectra from these devices show a monotonic redshift of the direct and indirect gap signals with the separate peaks merging into a single peak as the active layer materials are tuned into the direct gap composition range through higher Sn incorporation. Additionally, analysis of the EL intensities shows evidence for a significant increase in the non-radiative recombination rate due to the generation of relaxation-induced misfit dislocations at the n-Ge/i-Ge_1-ySn_y interface. The emission properties of devices with non-relaxed interfaces are then investigated by replacing the n-Ge bottom contact with a n-Ge_1-y’’Sn_y’’ material containing y’’ < y such that pseudomorphic growth takes place between all three active components of the device while maintaining carrier confinement to the active layer. These devices exhibit remarkably stronger EL than the counterparts with a single defected interface. This result can be traced to at least a 3x longer non-radiative recombination lifetime in the lattice-matched devices due to the elimination of strain relaxation induced defects at both interfaces. The room temperature EL from an Esaki-like diode formed from a pn junction of heavily doped (n, p ~ 10¹⁹) Ge_0.91Sn_0.09 layers at the indirect to direct gap crossover grown on a Ge buffer is also presented. This device represents the viability of electrically injected Ge_1-ySn_y devices for IR lasers on Si. Finally EL from Ge_1-x-ySi_xSn_y LEDs is discussed. These devices are grown on Si substrates through the same n-Ge bottom contact buffer layer that was applied in the Ge_1-ySn_y case. The Si composition in the active intrinsic region of these devices is held nearly constant (x = 0.02-0.03) while the Sn concentration is varied between y = 0.03-0.10. These devices are then capped with p-Ge_1-x’-y’Si_x’Sn_y’ layers grown pseudomorphically to the intrinsic layer to prevent the formation of strain relaxation defects. In this case, to promote carrier confinement we choose x’ > x and y’ ~ y. The EL spectra from these devices show the same redshift of the direct and indirect band gaps to lower energies due to the compositional tuning associated with Sn substitution analogous to that observed in the Ge_1-ySn_y devices. This includes a merging of the direct and indirect gap peak signals for devices containing 7-10% Sn in the active layer due to the separation between the direct and indirect band gaps narrowing. This indicates that electrically pumped direct-gap Ge_1-x-ySi_xSn_y devices are feasible and thus may present a more thermally robust alternative to Ge_1-ySn_y for similar Si-based IR photonic applications.