Optimization of heat shield for single silicon crystal growth by using numerical simulation

In integrated circuit-grade single silicon Czochralski growth, the position and material of heat shield are main parameters affecting the heat exchange and crystal growth condition. By optimizing the above parameters, we attempted to increase the growth rate and crystal quality. Numerical simulation proved to verify the results before and after optimization. Through analyses of the temperature and microdefect distribution, it is found that the optimized heat shield can further increase the pulling rate and decrease the melt/crystal interface deflection, increase the average velocity of argon flow from ~2 to ~5 m&middots-1, which is in favor of the transportation of SiO, and obtain the low defects concentration crystal and that the average temperature along the melt-free surface is 8 °C higher than before avoiding supercooled melt effectively. ? The Nonferrous Metals Society of China and Springer-Verlag Berlin Heidelberg 2012.

Bifurcation of thermocapillary convection in a shallow annular pool of silicon melt

In order to understand the nature of surface patterns on silicon melts in industrial Czochralski furnaces, we conducted a series of unsteady threedimensional numerical simulations of thermocapillary convections in thin silicon melt pools in an annular container. The pool is heated from the outer cylindrical wall and cooled at the inner wall. Bottom and top surfaces are adiabatic. The results show that the flow is steady and axisymmetric at small temperature difference in the radial direction. When the temperature difference exceeds a certain threshold value, hydrothermal waves appear and bifurcation occurs. In this case, the flow is unsteady and there are two possible groups of hydrothermal waves with different number of waves, which are characterized by spoke patterns traveling in the clockwise and counter-clockwise directions. Details of the flow and temperature disturbances are discussed and number of waves and traveling velocity of the hydro- thermal wave are determined.

Low threshold voltage light-emitting diode in silicon-based standard CMOS technology

Low-voltage silicon （Si）-based light-emitting diode （LED） is designed based on the former research of LED in Si-based standard complementary metal oxide semiconductor （CMOS） technology. The low-voltage LED is designed under the research of cross-finger structure LEDs and sophisticated structure enhanced LEDs for high efficiency and stable light source of monolithic chip integration. The device size of low-voltage LED is 45.85x38.4 （#m）, threshold voltage is 2.2 V in common condition, and temperature is 27 ~C. The external quantum efficiency is about 10-6 at stable operating state of 5 V and 177 mA.

Evolution of stress in evaporated silicon dioxide thin films

The evolution of stress in evaporated SiO2, used as optical coatings, is investigated experimentally through in situ stress measurement. A typical evolution pattern consisting of five subprocedures （thin film deposition, stopping deposition, cooling, venting the vacuum chamber, and exposing coated optics to the atmosphere） is put forward. Further investigations into the subprocedures reveal their features. During the deposition stage, the stresses are usually compressive and reach a stable state when the deposited film is thicker than 100 nm. An increment of compressive stress value is observed with the decrease of residual gas pressure or deposition rate. A very low stress of-20 MPa is formed in SiO2 films deposited at 3×10^-2 Pa. After deposition, the stress increases slightly in the compressive direction and is subject to the stabilization in subsequent tens of minutes. In the process of venting and exposure, the compressive component increases rapidly with the admission of room air and then reaches saturation, followed by a logarithmic decrement of the compressive state in the succeeding hours. An initial discussion of these behaviors is given.

Limits on light emission from silicon

Although silicon is an indirect semiconductor, light emission from silicon is governed by the same gener- alized Planck＇s radiation law as the emission from direct semiconductors. The emission intensity is given by the absorptance of the volume in which there is a difference of the quasi Fermi energies. A difference of the Fermi energies may result h＇om the absorption of external light （photoluminescence） or from the injection of electrons and holes via selective contacts （electroluminescence）. The quantum efficiency may be larger than 0.5 for carrier densities below 10^15 cm^-3. At larger densities, non-radiative recombination, in particular Auger recombination dominates. At all carrier densities, the relation between emission intensity and difference of the quasi Fermi energies is maintained. Since this difference is equal to the voltage of a properly designed solar cell, luminescence is the key indicator of material quality for solar cells.

Silicon electro-optic modulator with high-permittivity gate dielectric layer

A high-permittivity （high-k） material is applied as the gate dielectric layer in a silicon metal-oxide- semiconductor （MOS） capacitor to form a special electro-optic （EO） modulator. Both induced charge density and modulation efficiency in the proposed modulator are improved due to the special structure design and the application of the high-k material. The device has an ultra-compact dimension of 691 μm in length.

Thermal stress cleaving of silicon wafer by pulsed Nd:YAG laser

The applied laser energy absorbed in a local area in laser thermal stress cleaving of brittle materials using a controlled fracture technique produces tensile thermal stress that causes the material to separate along the moving direction of the laser beam. The material separation is similar to crack extension, but the fracture growth is controllable. Using heat transfer theory, we establish a three-dimensional （3D） mathematical thermoelastic calculational model containing a pre-existing crack for a two-point pulsed Nd：YAG laser cleaving silicon wafer. The temperature field and thermal stress field in the silicon wafer are obtained by using the finite element method （FEM）. The distribution of the tensile stress and changes in stress intensity factor around the crack tip are analyzed during the pulse duration. Meanwhile, the mechanism of crack propagation is investigated by analyzing the development of the thermal stress field during the cleaving process.

Fabrication and characteristics of silicon-rich oxide thin films with controllable compositions

Silicon-rich oxide films with controllable optical constants and properties are deposited by the reactive magnet- ton sputtering method on a Si target. The O/Si atomic ratio x of SiOx is tuned from 0.12 to 1.84 by adjusting the oxygen flow rate, which is found to be a more effective way to obtain SiOx films compared with changing the oxygen content [O2/（Ar ＋ O2） ratio]. The optical properties of SiOx films can be tuned from semiconductor to dielectric as a function of ratio x. The structures and components are also investigated by an x ray photoelectron spectroscopy analysis of the Si 2p core levels, the results of which exhibit that the structures of SiOx can be thoroughly described by the random bonding model.

Enhanced electroluminescence from nanocrystallite Si based MOSLED by interfacial Si nanopyramids

The interracial Si nano-pyralnid-enhanced electroluminescence （EL） of an ITO/SiOx/p-Si/Al metal-oxidesemiconductor （MOS） diode with turn-on voltage of 50 V, threshold current of 1.23 mA/cm^2, output power of 16 nW, and lifetime of 10 h is reported.

Optoelectronic characterization of ZnS/PS systems

ZnS thin films are deposited on porous silicon （PS） substrates with different porosities by pulsed laser deposition （PLD）. The photoluminescence （PL） spectra of the samples are measured at room temperature. The results show that the PL intensity of PS after deposition of ZnS increases and is associated with a blue shift. With the increase of PS porosity, a green emission at about 550 nm is observed in the PL spectra of ZnS/PS systems, which may be ascribed to the defect-center luminescence of ZnS films. Junction current- voltage （I-V） characteristics were studied. The rectifying behavior of I-V characteristics indicates the formation of ZnS/PS heterojunctions, and the forward current is seen to increase when the PS porosity is increased.

Photonic crystal waveguides and their applications Invited Paper

Two-dimensional （2D） slab photonic crystal waveguides （PCWGs） on silicon-on-insulator （SOI） wafer were designed and fabricated. Full photonic band gap, band gap guided mode, and index guided mode were observed by measuring the transmission spectra. Mini-stop-bands in the PCWG were simulated with different structure parameters. Coupling characteristics of PCWG were investigated theoretically considering the imperfections during the fabrication process. It was found that suppressing power reservation effect can realize both short coupling length and high coupling efficiency.