Ultrahigh-Quality Ge-on-Glass with a 3.7% Uniaxial Tensile Strain.

While there have been notable advancements in the quality of epitaxial Ge on Si, the crystal quality of bulk Ge remains much superior, which provides an effective method to study the performance potentials of Ge-based semiconductor devices. This study showcases the development of ultrahigh-quality Ge/poly-Si/SiO2 on glass with a Ge thickness reduced to ≤100 nm (10 µm width) through wafer bonding, thinning, and polishing processes. The minority lifetimes measured for the Ge thin films range between 200 and 1000 ns, surpassing those achieved with epi-Ge on Si by at least 20 to 100 times. The wafer bonding process introduces a desirable tensile strain of 0.1%, attributed to thermal expansion mismatch. A Ge microbridge structure was employed to amplify the tensile strain, reaching a maximum uniaxial tensile strain of 3.7%. The much longer minority carrier lifetime together with the strain-induced band gap engineering holds promise for improving light emission efficiency. This work establishes an economical and convenient method for producing high-quality tensile-strained Ge thin films, a pivotal step in exploring the potential of Ge in light emission applications.

Monolithically integrated 940†nm VCSELs on bulk Ge substrates.

This research successfully developed an independent Ge-based VCSEL epitaxy and fabrication technology route, which set the stage for integrating AlGaAs-based semiconductor devices on bulk Ge substrates. This is the second successful Ge-based VCSEL technology reported worldwide and the first Ge-based VCSEL technology with key details disclosed, including Ge substrate specification, transition layer structure and composition, and fabrication process. Compared with the GaAs counterparts, after epitaxy optimization, the Ge-based VCSEL wafer has a 40% lower surface root-mean-square roughness and 72% lower average bow-warp. After device fabrication, the Ge-based VCSEL has a 10% lower threshold current density and 19% higher maximum optical differential efficiency than the GaAs-based VCSEL.

25 Gb/s NRZ transmission at 85°C using a high-speed 940 nm AlGaAs oxide-confined VCSEL grown on a Ge substrate.

In this Letter, we present a comprehensive analysis of the high-speed performance of 940 nm oxide-confined AlGaAs vertical-cavity surface-emitting lasers (VCSELs) grown on Ge substrates. Our demonstration reveals a pronounced superiority of Ge-based VCSELs in terms of thermal stability. The presented Ge-VCSEL has a maximum modulation bandwidth of 16.1 GHz and successfully realizes a 25 Gb/s NRZ transmission at 85 ∘C. The experimental results underscore the significance and potential of Ge-VCSELs for applications requiring robust performance in high-temperature environments, laying the cornerstone for the future development of VCSEL devices.

Sub-10 µm-Thick Ge Thin Film Fabrication from Bulk-Ge Substrates via a Wet Etching Method.

Low-defect density Ge thin films are crucial for studying the impact of defect density on the performance limits of Ge-based optical devices (optical detectors, LEDs, and lasers). Ge thinning is also important for Ge-based multijunction solar cells. In this work, Ge wet etching using three acidic H2O2 solutions (HF, HCl, and H2SO4) was studied in terms of etching rate, surface morphology, and surface roughness. HCl-H2O2-H2O (1:1:5) was demonstrated to wet-etch 535 µm-thick bulk-Ge substrates to 4.1 µm with a corresponding RMS surface roughness of 10 nm, which was the thinnest Ge film from bulk-Ge via a wet etching method to the best of our knowledge. The good quality of pre-etched bulk-Ge was preserved, and the low threading dislocation density of 6000-7000 cm-2 was maintained after the etching process. This approach provides an inexpensive and convenient way for accurate Ge substrate thinning in applications such as multijunction solar cells and sub-10 µm-thick Ge thin film preparation, which enables future studies of low-defect density Ge-based devices such as photodetectors, LEDs, and lasers.

Interdiffusion in group IV semiconductor material systems: applications, research methods and discoveries.

Group IV semiconductor alloys and heterostructures such as SiGe, GeSn, Ge/Si and SiGe:C have been widely used and under extensive research for applications in major microelectronic and photonic devices. In the growth and processing of these materials, nanometer scale interdiffusion is common, which is generally undesirable for device performance. With higher Ge molar fractions and higher compressive strains, Si-Ge interdiffusion can be much faster than dopant diffusion. However, Si-Ge interdiffusion behaviors have not been well understood until recent years. Much less studies are available for GeSn. This review starts with basic properties and the applications of major group IV semiconductors, and then reviews the progress made so far on Si-Ge and Ge-Sn interdiffusion behaviors. Theories, experimental methods, design and practical considerations are discussed together with the key findings in this field.

Thermal sublimation: a scalable and controllable thinning method for the fabrication of few-layer black phosphorus.

We report uniform layer-by-layer sublimation of black phosphorus under heating below 600 K. The uniformity and crystallinity of BP samples after thermal thinning were confirmed by Raman spectra and Raman mapping. The sublimation rate of BP was around 0.18 nm min-1 at 500 K and 1.15 nm min-1 at 550 K. Both room and high temperature Raman peak intensity ratio [Formula: see text] as functions of BP thickness were established for in situ thickness determination and control. Uniform and crystalline 2 to 4-layer BP flakes with areas from 10 to 1000 µm2 were prepared with this method. No micron scale defects were observed. The sublimation thinning method was shown to be a controllable and scalable approach to prepare high-quality few-layer black phosphorus.