Songyan Chen

Epitaxial Growth of Germanium on Silicon for Light Emitters

This paper describes the role of Ge as an enabler for light emitters on a Si platform. In spite of the large lattice mismatch of ~4.2% between Ge and Si, high-quality Ge layers can be epitaxially grown on Si by ultrahigh-vacuum chemical vapor deposition. Applications of the Ge layers to near-infrared light emitters with various structures are reviewed, including the tensile-strained Ge epilayer, the Ge epilayer with a delta-doping SiGe layer, and the Ge/SiGe multiple quantum wells on Si. The fundamentals of photoluminescence physics in the different Ge structures are discussed briefly.

Plasmon-enhanced upconversion fluorescence in NaYF4:Yb/Er/Gd nanorods coated with Au nanoparticles or nanoshells

We investigate plasmon-enhanced upconversion (UC) fluorescence in Yb3+-Er3+-Gd+3 codoped sodium yttrium fluoride (NaYF4:Yb/Er/Gd) nanorods using gold nanoparticles or nanoshells. A simple method was proposed for the preparation of core/shell NaYF4/Au structures, with dispersed Au nanoparticles or uniform Au coating on the surface of the UC nanorod. Pure hexagonal-phase NaYF4:Yb/Er/Gd nanorods were synthesized via a liquid-solid reaction in oleic acid and ethanol solvent. A one-step approach was introduced to modify the hydrophobic surfaces of the as-deposited NaYF4:Yb/Er/Gd nanorods. After this surface modification, Au nanoparticles or nanoshells were successfully attached on the surfaces of NaYF4:Yb/Er/Gd nanorods. The as-deposited UC nanorods showed a strong UC emission in green and red bands under 980 nm laser excitation. The attachment of Au nanoparticles onto NaYF4:Yb/Er/Gd nanorods resulted in a more than three-fold increase in UC emissions, whereas the formation of continuous and compact Au shells ...

Room temperature photoluminescence of tensile-strained Ge/Si0.13Ge0.87 quantum wells grown on silicon-based germanium virtual substrate

National Basic Research Program of China [2007CB613404]; National Natural Science Foundation of China [60676027, 50672079]; Key Projects of Fujian Science and Technology [2006H0036]; Program for New Century Excellent Talents in University

Quantum-confined direct band transitions in tensile strained Ge/SiGe quantum wells on silicon substrates.

We directly demonstrate quantum-confined direct band transitions in the tensile strained Ge/SiGe multiple quantum wells grown on silicon substrates by room temperature photoluminescence. The tensile strained Ge/SiGe multiple quantum wells with various thicknesses of Ge well layers are grown on silicon substrates with a low temperature Ge buffer layer by ultrahigh vacuum chemical vapor deposition. The strain status, crystallographic, and surface morphology are systematically characterized by high-resolution transmission electron microscopy, atomic force microscopy, x-ray diffraction, and Raman spectroscopy. It is indicated that the photoluminescence peak energy of the tensile strained Ge/SiGe quantum wells shifts to higher energy with the reduction in thickness of Ge well layers. This blue shift of the luminescence peak energy can be quantitatively explained by the direct band transitions due to the quantum confinement effect at the Gamma point of the conduction band.

Enhanced photoluminescence of strained Ge with a delta-doping SiGe layer on silicon and silicon-on-insulator

National Basic Research Program of China (973 Program) [2007CB613404]; National Natural Science Foundation of China [60676027, 50672079, 60837001]; Program for New Century Excellent Talents in University; Open Project of State Key Laboratory of Functional

Interfacial nitrogen stabilizes carbon-coated mesoporous silicon particle anodes

We report for the first time that the dehydrogenation process of PAN was suppressed and the silicon oxide of the MSP surface was reduced during annealing in Ar + H2. Consequently, the remaining –NH bonds of the carbon chain can interact with the fresh amorphous Si on the surface of the MSPs to form a Si–N–C layer, which improves the adhesion between Si and C and serves as a stable electrolyte blocking layer. In addition, based on micron-sized MSPs, the structural stability of the electrode is dramatically enhanced through in situ formation of Si nanocrystals of less than 5 nm. The low Li+ diffusion kinetics of the Si–N–C layer and self limiting inhomogeneous lithiation in MSPs jointly create unlithiated Si nanocrystals, acting as supporting frames to prevent pulverization of the anode material. Our nitriding MSP anode has exhibited for the first time a 100% capacity retention (394 mA h g−1) after 2000 cycles (10 cycles each at 0.1, 0.5, 1, 2, and 1 and then 1950 cycles at 0.5 A g−1) and a 100% capacity retention at 0.1 A g−1 (540 mA h g−1) after 400 cycles. Thus, our work proposes a novel avenue to engineer battery materials with large volume changes.

Germanium n+/p shallow junction with record rectification ratio formed by low-temperature preannealing and excimer laser annealing

A germanium n+/p shallow junction formed by a combination of low-temperature preannealing (LTPA) and excimer laser annealing at a low fluence of 150 mJ/cm2 for phosphorus-implanted germanium is demonstrated. The LTPA step plays a critical role in annihilating the implantation damages and significantly suppressing phosphorus diffusion during laser annealing process, resulting in a very small dopant diffusion length with high activation level of phosphorus. A well-behaved Ge n+/p shallow junction diode with a record rectification ratio of ~107 and low leakage current density of 8.3 × 10-5 A/cm2 is achieved, which is greatly beneficial to the scaled Ge MOSFET technology.

A CMOS-compatible approach to fabricate an ultra-thin germanium-on-insulator with large tensile strain for Si-based light emission

We present a method to introduce a large biaxial tensile strain in an ultra-thin germanium-on-insulator (GOI) using selective oxidation of SiGe epilayer on silicon-on-insulator (SOI) substrate. A circular patterned Si0.81Ge0.19 mesa on SOI substrate with the sidewall protected by Si3N4 or SiO2 is selectively oxidized to generate local 12 nm GOI with high crystal quality, which shows enhanced photoluminescence due to large tensile strain. Direct band photoluminescence peak significantly shifts to longer wavelength as compared to that from bulk Ge due to a combination of strain-induced band gap reduction and quantum confinement effect.

Modulation of Schottky Barrier Height of Metal/TaN/n-Ge Junctions by Varying TaN Thickness

This paper demonstrates experimentally the modulation of the Schottky barrier height (SBH) of metal/TaN/n-Ge junctions by varying the TaN thickness in a dual stacked metal layer. The effective SBH for the metal/TaN/n-Ge Schottky junctions, originally pinned at 0.53-0.61 eV, decays to 0.44 eV, the barrier height of TaN/n-Ge, with an increase in the TaN thickness from 0 to 10 nm, independent of the type of metal. Dipole formation at the TaN/Ge interface is proposed to alleviate the metal-induced gap states and shift the pinning level toward the conduction band. The modulation of the SBH can be ascribed to the interdiffusion between the cap metals (Al, Fe, and Ni) and TaN.

Thermal Stability of Nickel Germanide Formed on Tensile-Strained Ge Epilayer on Si Substrate

The thermal stability of a nickel germanide film formed on a tensile-strained Ge epilayer on a silicon substrate with a low-temperature Si0.77Ge0.23 (50-nm)/Ge (50-nm) buffer is investigated. A record temperature of 700°C for the stability of sheet resistance of nickel germanide is reported, which is increased by about 150°C compared to that on bulk Ge and comparable to the temperature for nickel silicide on the Si substrate. The improvement of the thermal stability is demonstrated due to the delay of the agglomeration of the nickel germanide film on Ge-on-Si, which is proposed to be attributed to the increase of the tensile strain in the Ge epilayer during thermal annealing due to the thermal mismatch between Si and Ge.