Optically pumped lasing at 3 µm from compositionally graded GeSn with tin up to 22.3.

The recent demonstration of the GeSn laser opened a promising route towards the monolithic integration of light sources on the Si platform. A GeSn laser with higher Sn content is highly desirable to enhance the emission efficiency and to cover longer wavelength. This Letter reports optically pumped edge-emitting GeSn lasers operating at 3 µm, whose device structure featured Sn compositionally graded with a maximum Sn content of 22.3%. By using a 1950-nm laser pumping in comparison with a 1064-nm pumping, the local heating and quantum defect were effectively reduced, which improved laser performance in terms of higher maximum lasing temperature and lower threshold.

Study of direct bandgap type-I GeSn/GeSn double quantum well with improved carrier confinement.

The GeSn-based quantum wells (QWs) have been investigated recently for the development of efficient GeSn emitters. Although our previous study indicated that the direct bandgap well with type-I band alignment was achieved, the demonstrated QW still has insufficient carrier confinement. In this work, we report the systematic study of light emission from the Ge0.91Sn0.09/Ge0.85Sn0.15/Ge0.91Sn0.09 double QW structure. Two double QW samples, with the thicknesses of Ge0.85Sn0.15 well of 6 and 19 nm, were investigated. Band structure calculations revealed that both samples feature type-I band alignment. Compared with our previous study, by increasing the Sn composition in GeSn barrier and well, the QW layer featured increased energy separation between the indirect and direct bandgaps towards a better direct gap semiconductor. Moreover, the thicker well sample exhibited improved carrier confinement compared to the thinner well sample due to lowered first quantized energy level in the Γ valley. To identify the optical transition characteristics, photoluminescence (PL) study using three pump lasers with different penetration depths and photon energies was performed. The PL spectra confirmed the direct bandgap well feature and the improved carrier confinement, as significantly enhanced QW emission from the thicker well sample was observed.

Study of a SiGeSn/GeSn/SiGeSn structure toward direct bandgap type-I quantum well for all group-IV optoelectronics.

A SiGeSn/GeSn/SiGeSn single quantum well structure was grown using an industry standard chemical vapor deposition reactor with low-cost commercially available precursors. The material characterization revealed the precisely controlled material growth process. Temperature-dependent photoluminescence spectra were correlated with band structure calculation for a structure accurately determined by high-resolution x-ray diffraction and transmission electron microscopy. Based on the result, a systematic study of SiGeSn and GeSn bandgap energy separation and barrier heights versus material compositions and strain was conducted, leading to a practical design of a type-I direct bandgap quantum well.

Investigation of electrically active defects in InGaAs quantum wire intermediate-band solar cells using deep-level transient spectroscopy technique.

InGaAs quantum wire (QWr) intermediate-band solar cell-based nanostructures grown by molecular beam epitaxy are studied. The electrical and interface properties of these solar cell devices, as determined by current-voltage (I-V) and capacitance-voltage (C-V) techniques, were found to change with temperature over a wide range of 20-340 K. The electron and hole traps present in these devices have been investigated using deep-level transient spectroscopy (DLTS). The DLTS results showed that the traps detected in the QWr-doped devices are directly or indirectly related to the insertion of the Si δ-layer used to dope the wires. In addition, in the QWr-doped devices, the decrease of the solar conversion efficiencies at low temperatures and the associated decrease of the integrated external quantum efficiency through InGaAs could be attributed to detected traps E1QWR_D, E2QWR_D, and E3QWR_D with activation energies of 0.0037, 0.0053, and 0.041 eV, respectively.

Progress and Application of Halide Perovskite Materials for Solar Cells and Light Emitting Devices.

Halide perovskite materials have attracted worldwide attention in the photovoltaic area due to the rapid improvement in efficiency, from less than 4% in 2009 to 26.1% in 2023 with only a nanometer lever photo-active layer. Meanwhile, this nova star found applications in many other areas, such as light emitting, sensor, etc. This review started with the fundamentals of physics and chemistry behind the excellent performance of halide perovskite materials for photovoltaic/light emitting and the methods for preparing them. Then, it described the basic principles for solar cells and light emitting devices. It summarized the strategies including nanotechnology to improve the performance and the application of halide perovskite materials in these two areas: from structure-property relation to how each component in the devices affects the overall performance. Moreover, this review listed the challenges for the future applications of halide perovskite materials.

Probing anharmonic phonons in WS2 van der Waals crystal by Raman spectroscopy and machine learning.

Understanding the optothermal physics of quantum materials will enable the efficient design of next-generation photonic and superconducting circuits. Anharmonic phonon dynamics is central to strongly interacting optothermal physics. This is because the pressure of a gas of anharmonic phonons is temperature dependent. Phonon-phonon and electron-phonon quantum interactions contribute to the anharmonic phonon effect. Here we have studied the optothermal properties of physically exfoliated WS2 van der Waals crystal via temperature-dependent Raman spectroscopy and machine learning strategies. This fundamental investigation will lead to unveiling the dependence of temperature on in-plane and out-of-plane Raman shifts (Raman thermometry) of WS2 to study the thermal conductivity, hot carrier diffusion coefficient, and thermal expansion coefficient.

Investigation of GeSn Strain Relaxation and Spontaneous Composition Gradient for Low-Defect and High-Sn Alloy Growth.

Recent development of group-IV alloy GeSn indicates its bright future for the application of mid-infrared Si photonics. Relaxed GeSn with high material quality and high Sn composition is highly desirable to cover mid-infrared wavelength. However, its crystal growth remains a great challenge. In this work, a systematic study of GeSn strain relaxation mechanism and its effects on Sn incorporation during the material growth via chemical vapor deposition was conducted. It was discovered that Sn incorporation into Ge lattice sites is limited by high compressive strain rather than historically acknowledged chemical reaction dynamics, which was also confirmed by Gibbs free energy calculation. In-depth material characterizations revealed that: (i) the generation of dislocations at Ge/GeSn interface eases the compressive strain, which offers a favorably increased Sn incorporation; (ii) the formation of dislocation loop near Ge/GeSn interface effectively localizes defects, leading to the subsequent low-defect grown GeSn. Following the discovered growth mechanism, a world-record Sn content of 22.3% was achieved. The experiment result shows that even higher Sn content could be obtained if further continuous growth with the same recipe is conducted. This report offers an essential guidance for the growth of high quality high Sn composition GeSn for future GeSn based optoelectronics.