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Wang, Liming

University of British Columbia

High-quality Ge thin films are essential for enhancing Ge-based optical devices like transistors, photodetectors, LEDs, and lasers. Epitaxial Ge (epi-Ge) thin films on Si are preferred for their optimal micron or sub-micron thickness, necessary for overcoming issues like self-absorption and high lasing thresholds in Ge lasers. However, epi-Ge on Si has a high threading dislocation density (TDD) of 10⁶ to 10¹⁰ cm⁻². In contrast, bulk Ge, typically a few hundred microns thick, has superior crystal quality (TDD ≤ 10⁴ cm⁻²). We developed wet etching methods to thin bulk Ge wafers to micron-scale free-standing films while maintaining high crystal quality. We successfully etched 535-μm thick bulk Ge substrates down to 2-μm, the thinnest Ge film from bulk Ge via wet etching reported. The photoluminescence (PL) peak intensity of the Ge sample (TDD = 6000 cm⁻², n-doping = 10¹⁶ cm⁻³) increased 32-fold when reduced from 535 µm to 2 µm, surpassing the PL peak intensity of 0.75 µm thick epi-Ge on Si (n-doping = 10¹⁹ cm⁻³) by 2.5 times. A 405 µm thick zero-TDD bulk Ge sample (n-doping = 5 × 10¹⁷ cm⁻³) showed a PL peak intensity ten times that of epi-Ge, rising to twelve times when thinned to 1 µm. This ultra-low TDD method for bulk-Ge provides higher PL with much lower n-doping, reducing the negative effects of high optical absorption, non-radiative recombination, bandgap narrowing, and large footprints. To use bulk-Ge thin films for light-emitting diodes and lasers, bonding to substrates and introducing tensile strain are crucial. We created ultra-high-quality Ge/poly-Si/SiO₂ thin films on glass with sufficient bonding strength and smooth surfaces. The minority lifetimes for these bonded films ranged from 200 to 1000 ns, far exceeding those of epi-Ge on Si, indicating their ultra-high quality. Ge microbridges were then fabricated to amplify tensile strain, reaching a maximum uniaxial tensile strain of 3.7%, crucial for improving light emission by narrowing the gap between Ge's direct and indirect energy bands. This work established an economical and efficient method for producing high-crystal-quality, high-tensile-strained Ge thin films, advancing their potential in light emission applications, particularly for on-chip optical interconnect Ge lasers.

Text

2024-07-25

Attribution-NonCommercial-NoDerivatives 4.0 International

http://hdl.handle.net/2429/88701

Materials Engineering

University of British Columbia

UBCV

http://creativecommons.org/licenses/by-nc-nd/4.0/