Materials science and design for germanium monolithic light source on silicon

[摘要] Germanium (Ge) is an optically active material with the advantages of Si-CMOS compatibility and monolithic integration. It has great potential to be used as the light emitter for Si photonics. Tensile strain and n-type doping are two key properties in Ge to achieve optical gain. This thesis mainly focuses on: (1) physical understandings of the threshold behavior of Ge-on-Si bulk laser and the temperature dependent performance; (2) process developments to grow and planarize the epitaxial Ge on Si in oxide trenches and corners; (3) introduction of n-type dopant into Ge-on-Si thin films while studying the threading dislocation behavior in n-Ge during annealing; (4) Design an external cavity Ge laser integrated with Si waveguides for a low threshold current and single mode operation. Heavy n-type doping was observed to change the Ge electronic band structure by band gap narrowing effect. We also found a failure of using a simple Drude model to explain free carrier absorption in n-Ge. We modified the optical gain simulation based on the above two observations in Ge. We found a broad gain bandwidth of ~ 200 nm from 1550 nm to 1750 nm and a higher net materials gain. We predicted a theoretical lasing threshold current density of 5~10 kA/cm² in the bulk Ge laser device with the n-type doping of mid-10¹⁹ cm-³ at room temperature. We also predicted the Ge laser device would have better temperature stability regarding the threshold current compared to the III-V laser. Single crystalline Ge was epitaxial grown on Si in oxide trenches using ultra high vacuum chemical vapor deposition. The selective growth lead to the faceting in Ge because of the different growth rates of crystal orientations. We developed a suitable photolithography and oxide etch process to get the vertical oxide sidewall for Ge trench filling. We also tested the Ge growth in the T-shape corners to improve the reflectivity at the waveguide end. The T-shape structure was also useful for the Ge/Si waveguide coupling in the external cavity laser. Furthermore, we developed a chemical mechanical polishing (CMP) process for the over-grown Ge waveguides. The Ge CMP process was selective to oxide, flexible to change in the CMP rate by DI water dilution and controllable for a minimum dishing of Ge in the oxide trenches. N-type doping helped to increase the direct band transition in Ge for light emission. We developed a delta-doping method to grow a dopant source called ;;delta doping layer;; on the single crystalline Ge layer without introducing extra defects. We then used rapid thermal annealing to drive the dopant into the underlying Ge layer. The dopant enhanced diffusion was discovered to speed up the drive-in process. The active n-type concentration in Ge could reach up to 5×10¹⁹ cm-3 using the delta doping source and annealing process. Since the dopant source layer had a disrupted Ge growth, we used the developed CMP process to remove it after the dopant drive-in. A comprehensive dopant diffusion simulation was developed to predict the annealing temperature and time to achieve high n-type doping and uniform distribution. We used plan-view transmission electron microscopy to examine the threading dislocation density (TDD) in n-Ge for both blanket films and trench grown waveguides. We found a high TDD of ~ 1×10⁸cm-² in 1 [mu]m thick blanket Ge with doping of 3×10¹⁸ cm-³ after high temperature annealing at 850 °C for 40 min. The TDD is 1×10⁹ cm-² in the 300 nm thick and 1 [mu]m wide Ge waveguide. We examined the effects of annealing temperature, Ge thickness, Si/Ge inter-diffusion and trench width on the threading dislocation behavior. However, we have not found the exact reason causing the high TDD and therefore, further study is required on the TDD reduction for the Ge waveguide. Finally, we designed an external cavity Ge laser using distributed Bragg reflector (DBR) gratings on Si waveguides. A detailed discussion on the cross section design was presented to mitigate the internal optical loss from claddings and metal layers and to improve the current injection uniformity across the Ge waveguide. The aim of the DBR grating design was to achieve a single mode operation by controlling the full width half maximum of the grating reflectance spectrum. We also discussed the coupling between Ge and Si waveguides and different designs were presented to increase the coupling efficiency.

[发布机构] Massachusetts Institute of Technology