Haofeng Li

High-performance solution-processed plasmonic Ni nanochain-Al2O3 selective solar thermal absorbers

Selective solar thermal absorber coating is an important component of concentrated solar power systems. It maximizes the absorption of solar spectrum and minimizes thermal radiation losses in the mid-infrared regime. In this letter, we demonstrate a solution-processed plasmonic Ni nanochain-Al2O3 selective solar thermal absorber with a high solar absorptance >90% and a low thermal emittance loss <10%. Unlike conventional graded-index cermet coatings, the spectral selectivity is tailored by the lengths of Ni nanochains, elimating the requirement of costly vacuum deposition for stringent thickness control. These results open a path to utilize plasmonics for low-cost, high-performance solar thermal systems.

Low temperature growth of high crystallinity GeSn on amorphous layers for advanced optoelectronics

High crystallinity GeSn substitutional alloy thin films with up to 8.7 at.% Sn are directly grown on amorphous SiO2 layers at low crystallization temperatures of 370~470 °C for potential applications in 3D electronic-photonic integration on Si as well as inexpensive virtual substrates for tandem solar cells. The optimal Ge0.913Sn0.087 thin film demonstrates a strong (111) texture and an average gain size of 10 μm, and its grain boundaries are mostly twin and low-angle boundaries with low densities of defect recombination centers. The 8.7 at.% Sn incorporated substitutionally into the Ge lattice far exceeds the ~1 at.% equilibrium solubility limit. Correspondingly, the direct band gap is significantly red-shifted from 0.8 eV for pure Ge to ~0.5 eV for crystalline Ge0.913Sn0.087, right at the verge of the indirect-to-direct gap transition that occurs at 8-10 at.% Sn alloying. Optoelectronic properties are greatly enhanced due to this transition.

Infrared absorption of n-type tensile-strained Ge-on-Si

We analyze the IR absorption of tensile-strained, n-type Ge for Si-compatible laser applications. A strong intervalley scattering from the indirect L valleys to the direct Γ valley in n+ Ge-on-Si is reported for the first time to our knowledge. The intervalley absorption edge is in good agreement with the theoretical value. On the other hand, we found that the classical λ2-dependent Drude model of intravalley free-carrier absorption (FCA) breaks down at λ<15 μm. A first-principle model has to be employed to reach a good agreement with the experimental data. The intravalley FCA loss is determined to be <20 cm(-1) for n=4×10(19) cm(-3) at λ=1.5-1.7 μm, an order lower than the results from Drude model. The strong L→Γ intervalley scattering favors electronic occupation of the direct Γ valley, thereby enhancing optical gain from the direct gap transition of Ge, while the low intravalley free-electron absorption at lasing wavelengths leads to low optical losses. These two factors explain why the first electrically pumped Ge-on-Si laser achieved a higher net gain than the theoretical prediction using λ2-dependent free-carrier losses of bulk Ge and indicate the great potential for further improvement of Ge-on-Si lasers.

Pseudo single crystal, direct-band-gap Ge0.89Sn0.11 on amorphous dielectric layers towards monolithic 3D photonic integration

We demonstrate pseudo single crystal, direct-band-gap Ge0.89Sn0.11 crystallized on amorphous layers at 18u2009μm long that forms optoelectronically benign twin boundaries with others grains. These pseudo single crystal, direct-band-gap Ge0.89Sn0.11 patterns are suitable for monolithic 3D integration of active photonic devices on Si.

Ultralarge transient optical gain from tensile-strained, n-doped germanium on silicon by spin-on dopant diffusion

The direct band gap optical gain of tensile-strained, highly n-doped germanium on silicon is investigated by femtosecond ultrafast transmittance spectroscopy. A germanium film with 0.22% tensile strain is grown on a silicon substrate by using molecular beam epitaxy. An activated doping concentration up to 4 × 1019 cm−3 is achieved by phosphorus diffusion from a spin-on dopant source. The transmittance of the germanium film is clearly increased upon increasing the pump power. A peak optical gain of up to 5300 cm−1 around 1.7 µm and a gain spectrum broader than 300 nm are obtained. These results show a simple yet promising way to realize gain medium for monolithic-integrated germanium lasers.

GeSn mid-IR materials and devices for 3D photonic integration

We present direct gap, pseudo-single-crystal GeSn on amorphous dielectric layers and flexible substrates for MIR active integrated photonics. The high transient gain and the strain engineering via flexible substrate deformation indicate promising device applications.

GeSn optical gain media towards monolithic 3D photonic integration

We present pseudo-single-crystal, direct band gap GeSn gain media fabricated at <;450 °C on dielectric layers towards monolithic 3D photonic integration. A high transient optical gain ~5000 cm-1 has been at λ=2100-2200 nm at 300K.

Direct-band-gap, Pseudo Single Crystal GeSn on Amorphous Layers towards 3D Integrated Photonics and Flexible Photonics

High-crystallinity, direct-band-gap Ge0.89Sn0.11 is achieved at <450°C on amorphous dielectric layers and flexible substrates. The geometrically confined growth of pseudo-single-crystal Ge0.89Sn0.11 enables monolithic, large-scale 3D Si photonics.

Low temperature crystallization of highly textured GeSn thin films on SiO 2 for 3D photonic integration

We report a highly (111)-textured Ge_0.9Sn_0.1 thin film fabricated on SiO₂ at 464°C. Incorporating Sn into Ge significantly reduces the crystallization temperature and red-shifts the direct band gap to ~0.5 eV, approaching indirect-to-direct gap transition.