Ying-Jhe Yang

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.

Stress-Induced Hump Effects of p-Channel Polycrystalline Silicon Thin-Film Transistors

Positive bias temperature instability in p-channel polycrystalline silicon thin-film transistors is investigated. The stress-induced hump in the subthreshold region is observed and is attributed to the edge transistor along the channel width direction. The electric field at the corner is higher than that at the channel due to thinner gate insulator and larger electric flux density at the corner. The current of edge transistor is independent of the channel width. The electron trapping in the gate insulator via the Fowler-Nordheim tunneling yields the positive voltage shift. As compared to the channel transistor, more trapped electrons at the edge lead to more positive voltage shift and create the hump. The hump is less significant at high temperature due to the thermal excitation of trapped elections via the Frenkel-Poole emission.

Photoconductivity in single AlN nanowires by subband gap excitation

Photoconductivity of individual aluminum nitride (AlN) nanowires has been characterized using different subband gap excitation sources. It is interesting that both positive (under 1.53 and 2.33 eV excitations) and negative (under 3.06 and 3.81 eV excitations) photocurrent responses are observed from the wide band gap nitride nanowires. The negative photoconductivity, which is attributed to the presence of electron trap and recombination center in the bulk of AlN, is capable to be inversed by a strong positive photoconductive mechanism of surface while changes the ambience from the atmosphere to the vacuum. An oxygen molecular sensitization effect is proposed to be the reason resulting in the enhancement of positive photocurrent and the inversion of negative photoresponse in the vacuum. Understanding of the diverse photoconductivity and its molecular effect is of great importance in the development of energy-selective and highly sensitive nanowire photodetector of AlN in the visible and ultraviolet ranges.

Mechanical strain effect of n-channel polycrystalline silicon thin-film transistors

The current change of n-channel polycrystalline silicon thin-film transistors is analyzed experimentally and theoretically under different strain conditions. Under the uniaxial strain parallel to the channel, the +6.7% and +5.3% drain current enhancements are achieved in linear and saturation regions, respectively. There are −4.4% (linear) and −4.6% (saturation) drain current degradations when the uniaxial strain is applied perpendicular to the channel. The polycrystalline silicon is mainly composed of (111)-oriented grains, measured by electron diffraction pattern. Phonon-limited mobility is theoretically calculated. There is a qualitative agreement between experiments and theoretical analysis.

Photoconduction mechanism of oxygen sensitization in InN nanowires

The photoconduction (PC) mechanism in indium nitride (InN) nanowires (NWs) has been investigated via environment-, temperature-, and power-dependent measurements. The adsorbed oxygen-induced modulation of the surface state is proposed to be the leading factor in the long lifetime or high gain transport and in sensitizing photocurrent generation in the InN NWs. The electron trapping effect by adsorbed oxygen can be verified by the increased activation energy from 33 ± 4 (in vacuum) to 58 ± 2 meV (in oxygen). The observed supralinear power dependence of photocurrent also suggests the presence of acceptor states that influence the carrier recombination behavior and compensate the thermal carriers in the InN NWs. The potential influence of native oxide on the molecule-sensitive PC in this nitride nanomaterial is also inferred.

Dynamic Bias Instability of p-Channel Polycrystalline-Silicon Thin-Film Transistors Induced by Impact Ionization

The dynamic stress switching of p-channel polycrystalline-silicon (poly-Si) thin-film transistors from full depletion to accumulation bias creates the high electric field near source/drain (S/D) junctions due to the slow formation of the accumulated electrons at the SiO2/poly -Si interface. The high electric field causes impact ionization near the S/D, where the secondary electrons surmount the SiO2 barrier and are trapped near the interface. The channel region near the S/D is inverted to p-type by the trapped electrons, and the effective channel length is reduced. The drain current increases with the stress time, particularly for short-channel devices.

Effects of Applied Mechanical Uniaxial and Biaxial Tensile Strain on the Flatband Voltage of (001), (110), and (111) Metal–Oxide–Silicon Capacitors

The flatband-voltage shift of metal-oxide-silicon capacitors is investigated under the application of low-level stress (up to 220 MPa of biaxial stress and 380 MPa of uniaxial stress) to different substrate orientations. We propose that the flatband-voltage shift be modeled as the net effect of silicon-band-edge shifts and modulation of the separation between the band edge and the Fermi level under low levels of applied mechanical strain. For the (001) n-type substrate, a negative flatband-voltage shift is observed due mainly to the downward shift of the conduction-band edge, while a positive flatband-voltage shift is observed for the (001) p-type substrate due to the upward shift of the valence-band edge. For the uniaxial tensile strain on n-substrate capacitors for (110) and (111) substrates, the modulation of band-edge and Fermi-level separation by the conduction-band density of states exceeds the downward shift of the conduction band, which induces a positive flatband shift that is distinct from that observed in the (001) n-substrate. The shift of the band edges is determined by the proposed model and compared with theoretical calculations.

SiGe/Si Quantum-Dot Infrared Photodetectors With

The multicolor absorption of MOS SiGe/Si quantum-dot (QD) infrared photodetectors is demonstrated using the boron delta-doping in Si spacers. The energy-dispersive X-ray spectroscopy shows that the Ge concentration in the wetting layers is much smaller than that in QDs. Most holes stay at the ground state in QDs instead of wetting layers. The energy band structure in QDs is calculated to understand the absorption spectrum. The absorption at 3.7-6 mum is due to the intersubband transition in the SiGe QDs. The other absorption at 6-16 mu m mainly comes from the intraband transition in the boron delta-doping wells. Since the broadband spectrum covers most of the atmospheric transmission windows for infrared, the broadband detection is feasible using this device.

{\bm \delta}

The optimal combinations of strain and channel on different substrate orientations for Ge NMOSFETs are reported. Applying biaxial tensile stress on (111) wafer with [-110] channel direction can reach highest mobility value (4.1x) and largest ballistic saturation current (2.6x). For both tensile and compressive strain, the mobility and Jsat enhancement can be found for all substrate orientations if strain condition and channel direction are optimized.

Doping

In this article we present detailed stress simulation characterization of the 3-D boundary effects and show that the high-performance n FET can be achieved by the ultra-high-stress CESL stressor and optimal geometric structure design. A symmetric structure which results in the biaxial- like stress is favored for n FET in terms of Ion, Bsat rsat, and vnj. The comprehensive study helps the future device circuit design and remains valid for future technology node of 22 nm.