Shu-Tong Chang

Electron mobility enhancement in strained-germanium n-channel metal-oxide-semiconductor field-effect transistors

The dependence of electron mobility on strain, channel direction, and substrate orientation is theoretically studied for the germanium n-channel metal-oxide-semiconductor field-effect transistors. For the unstrained channel, (111) substrate can provide the highest mobility among the three orientations, mainly due to its largest quantization mass and smallest conductivity mass in L valley. The tensile strain parallel to the [1¯10] channel direction on (111) substrate gives 4.1 times mobility of Si at 1MV∕cm, and the mobility enhancement starts to saturate for the strain larger than 0.5%. The compressive strain of ∼1.5% transverse to [1¯10] on (111) substrate yields 2.9 times mobility enhancement at 1MV∕cm.

Comprehensive study of the Raman shifts of strained silicon and germanium

Raman shifts are investigated on silicon and germanium substrates under the uniaxial tensile strain on various substrate orientations. The strain splits the triply degenerate optical (LO, TO) phonons at the zone center (k≈0). The redshifts of Si Raman peaks induced by the tensile strain on all substrate orientations are observed. With the specific polarization of the incident light, however, the unusual blueshifts of Ge Raman peaks induced by the tensile strain are observed on (110) and (111) Ge substrates. By using the suitable phenomenological constants and taking the Raman selection rules into consideration, the experimental results are in reasonable agreement with the lattice dynamical theory.

A high efficient 820 nm MOS Ge quantum dot photodetector

A Ge quantum dot photodetector has been demonstrated using a metal-oxide-semiconductor (MOS) tunneling structure. The oxide film was grown by liquid phase deposition (LPD) at 50/spl deg/C. The photodetector with five-period Ge quantum dot has responsivity of 130, 0.16, and 0.08 mA/W at wavelengths of 820 nm, 1300 nm, and 1550 nm, respectively. The device with 20-period Ge quantum dot shows responsivity of 600 mA/W at the wavelength of 850 nm. The room temperature dark current density is as low as 0.06 mA/cm/sup 2/. The high performance of the photodetectors at 820 nm makes it feasible to integrate electrooptical devices into Si chips for short-range optical communication.

Package-strain-enhanced device and circuit performance

The hole mobility enhancement can be as high as /spl sim/18% for SiO/sub 2/ and /spl sim/20% for high-k HfO/sub 2/ gate stack dielectrics with the uniaxial compressive strain (0.2%) parallel to the channel. The highest drain current of /spl sim/22% at saturation and /spl sim/30% at linear region is observed for the bulk Si PMOS with high-k gate stacks. The drain current and hole mobility of bulk Si PMOS are degraded under the small biaxial tensile strain, while substrate-strained Si device displays the opposite. The nonoptimized ring oscillator has the speed enhancement of /spl sim/7% under the uniaxial tensile strain parallel to NMOS channel. Proper package strain also gives the drive-current as well as mobility enhancement at 100/spl deg/C.

Numerical Simulation on the Photovoltaic Behavior of an Amorphous-Silicon Nanowire-Array Solar Cell

In this letter, we propose an amorphous-silicon (a-Si) solar cell with a nanowire-array structure. The proposed structure has photon absorption and carrier transport that are perpendicular to each other, which could overcome the efficiency limit of an a-Si solar cell. This nanowire structure has an n-type a-Si nanowire array in which the i-layer and the p-layer a-Si are sequentially grown along the surface of the nanowire. Under illumination, light is absorbed along the axial direction of the nanowire, and carrier transport is along the radial direction. Numerical simulations show that the photocurrent of the a-Si solar cell with a 4000-nm-long nanowire is nearly 40% more than that of a planar a-Si solar cell. A conversion efficiency of 11.6% was obtained, which is around 32% enhancement.

Driving Current Enhancement of Strained Ge (110) p-Type Tunnel FETs and Anisotropic Effect

The experimental investigation carried out the strained Ge (110) p-type tunneling field-effect transistor, and it resulted in the current enhancement of ×2.9 BTBT in the 112 direction, as compared with Si 110/(100) due to a small band gap. In addition, the high on/ off current ratio, with an on current ~1 μA/μm and an off current ~10 pA/μm, and the well control for leakage current without SOI substrate were obtained. The anisotropic effect of tunneling directions for strained Ge on (110) orientation was discussed and explained as due to effective reduced mass.

3-D simulation of strained Si/SiGe heterojunction FinFETs

The 3D structure and simulation of strained Si/SiGe FinFETs is proposed in this paper. A channel doping of 10/sup 16/ cm/sup -3/, dual polysilicon gate 1.5 nm gate oxide, abrupt source/drain-to-channel junctions, and a Si/sub 0.8/Ge/sub 0.2/ body with fixed 5 nm surrounding Si are used in the 3D simulation. Strained Si/SiGe device at the threshold voltage (V/sub T/) yields a better gate control, smaller roll-off characteristics, smaller subthreshold swing, and significant g/sub m/ enhancement. This novel strained Si/SiGe FinFET with the enhanced carrier mobility and heterojunction confinement is demonstrated with greatly improved performance for NMOS by 3-D simulation.

Chemical-mechanical polishing of low-dielectric-constant spin-on-glasses: film chemistries, slurry formulation and polish selectivity

Abstract Alkyl siloxane-based low-dielectric-constant (low-k) spin-on-glass (SOG) thin films with varying amounts of organic content were subjected to polishing experiments using silica- and ZrO 2 -based slurries with a variety of additives. As the amount of organic content in SOG increases, the chemical-mechanical polishing (CMP) removal rate decreases with silica-based potassium hydroxide-added slurry. On the other hand, zirconia-based slurry resulted in higher removal rates for both SOG (>400 nm/min) and thermal oxide and an adjustment in polish selectivity (related to thermal oxide) ranging from 1.2 to 9.1 can be achieved by adding various amounts of tetra-alkyl substituted ammonium hydroxide. Post-CMP materials characterization by Fourier transform infrared spectroscopy (FTIR) and atomic force microscopy (AFM) shows the chemical stability and CMP compatibility of SOG thin films.

Novel MIS Ge-Si quantum-dot infrared photodetectors

The metal-insulator-semiconductor (MIS) Ge-Si quantum-dot infrared photodetectors (QDIPs) are successfully demonstrated. Using oxynitride as gate dielectric instead of oxide, the operating temperature reaches 140 and 200 K for 3-10 and 2-3 /spl mu/m detection, respectively. From the photoluminescence spectrum, the quantum-dot structures are responsible for the 2-3 /spl mu/m response with high operation temperature, and the wetting layer structures may be responsible for the 3-10 /spl mu/m response. This novel MIS Ge-Si QDIP can increase the functionality of Si chip such as noncontact temperature sensing and is compatible with ultra-large scale integration technology.

Energy band structure of strained Si1−xCx alloys on Si (001) substrate

We report the energy band structures of strained Si1−xCx alloys on Si (001) substrates. All calculations are based on a 20×20 Hamiltonian matrix constructed from the linear combination of atomic orbital approximation with spin–orbit interaction, strain effect, and lattice disorder effect taken into account. The lattice disorder parameter is obtained from fittings with the experimental band gap of strained Si1−xCx alloy with small carbon concentration and reflects the initial reduction of band gap of relaxed Si1−xCx alloy, while simple virtual crystal approximation does not. The effect of strain on band structure is incorporated in terms of the interatomic interaction parameters, which are functions of bond length and bond angle. The strained Si1−xCx alloy becomes metallic when x=28%. All the directional effective masses are affected by the strain. Overall agreements are found between our theoretical calculations and recent experimental results.