(Invited) On the Improvement of Ge Lasers
Authors: Xia Guangrui , Wang Liming 
Published: December 2024

Electrically pumped Ge lasers were first demonstrated in 2012, which are considered a promising solution to Si-compatible on-chip lasers. Theoretical studies showed that laser structure design, stress engineering, and Ge material quality improvement are the three main methods for improving Ge laser performance. Epitaxial Ge thin films (epi-Ge) on Si have been the choice for Ge in photonic and microelectronic devices due to the suitable thickness in the micron or sub-micron scale. For Ge on-chip lasers, this thickness range is required to overcome the self-absorption, high lasing threshold, and coupling difficulty associated with larger thicknesses. However, the major problem of epi-Ge on Si is the high threading dislocation density (TDD) ranging from 106 to 1010 cm-2 despite the extensive research efforts to reduce the TDD. In contrast, bulk Ge, typically a few hundred microns thick, maintains superior crystal quality (TDD ≤ 104 cm-2).
To take advantage of this ultra-high crystal quality, we successfully developed wet etching recipes to thin bulk Ge wafer pieces down to micron-scale free-standing Ge thin films with good sample integrity and surface roughness while maintaining the ultra-high crystal quality of the starting bulk-Ge samples. Two bulk Ge wafers were chosen and thinned from 535 or 405 µm to 1-2 µm. As a result, the photoluminescence (PL) peak intensity of the thinned Ge samples increased significantly. The key material parameters are shown in the table below. The specification values beneficial for more PL intensity are shaded in green, and those not desired are in red. The benchmarking epi-Ge control sample has been compared to the epi-Ge from the best Ge lasers, which have similar specifications and PL intensity. Compared with the bulk Ge or bulk-Ge-based thin films, the epi-Ge has a higher tensile strain, higher n-type doping, and lower surface roughness, which all support much higher PL intensity except for the much higher TDD. However, the samples with much increased PL peak intensities are from bulk-Ge-based thin films with non-optimized strain, doping, and surface roughness, demonstrating the TDD reduction's high effectiveness in improving Ge PL. Measurements showed much longer minority carrier lifetimes in these high PL samples due to the very low TDDs. TDD reduction in Ge relaxes the common requirements of high n-doping and stress in enhancing Ge laser performance. It thus reduces the side effects of high optical absorption, high non-radiative recombination, bandgap narrowing, and large footprints associated with these two techniques.
After proving the bulk-Ge-based thin film concept, we also developed bonding, thinning, and polishing techniques to make thin Ge bonded on handle substrates and to introduce tensile strain from the bonding step. Ultra-high-quality Ge/poly-Si/SiO2 thin films on glass with sufficient bonding strength and surface smoothness were achieved. The minority lifetimes for these bonded Ge thin films range between 200 and 1000 ns, surpassing those achieved with epi-Ge on Si by at least 20 to 100 times. This is an important indication of the ultra-high quality of the bonded Ge thin films. Later, Ge microbridges were fabricated, which amplified the tensile strain introduced in the bonding step, reaching a maximum uniaxial tensile strain of 3.7%. This is crucial for reducing the difference between Ge's direct and indirect energy bands for much more efficient light emission.
In summary, this work established an economical and convenient fabrication technology for producing high-crystal-quality and high-tensile-strained Ge thin films bonded on substrates — a pivotal step in exploring Ge's potential in light emission applications, especially for Ge lasers for on-chip optical interconnects.

Figure 1