Jiabao Sun

Mechanical tensile strain induced gate and substrate currents change in n and p-channel metal-oxide-semiconductor field-effect transistors

To investigate and understand the reliability behavior of strained silicon devices, the changes of gate currents (Ig) and substrate currents (Isub) in n and p-channel metal-oxide-semiconductor field-transistors (MOSFETs) under different types of mechanically applied tensile stresses have been studied. It has been observed that, under the uniaxial tensile stress, both Ig and Isub of pMOSFETs increase with the increase of applied stress under the inversion and the accumulation conditions. However, an opposite stress dependence in nMOSFETs has been observed for Ig and Isub in both the inversion and the accumulation regimes. Similar changes have been found for Ig and Isub of nMOSFETs under biaxial tensile stress. The observations are explained by the strain induced band structure modulation and the repopulation of carriers.

Experimental Study on NBTI Degradation Behaviors in Si pMOSFETs Under Compressive and Tensile Strains

In this letter, we experimentally investigated the effects of four types of strains, uniaxial tensile strain, uniaxial compressive strain, biaxial tensile strain, and biaxial compressive strain, on the negative bias temperature instability (NBTI) degradation behaviors of Si pMOSFETs. The strains were applied using a wafer bending system to avoid processing effects on the NBTI characteristics as a result of strain engineering. We confirm experimentally, for the first time, that both uniaxial and biaxial compressive strains are advantageous in terms of the NBTI improvement in Si pMOSFETs. However, the NBTI reliability was degraded under both uniaxial and biaxial tensile strains. These results could not be explained by considering only the gate leakage current change due to the strain. The strain-induced modulation of the interaction between the carriers and Si-H bonds at the interface must also be considered.

Comparative study on strain induced electrical properties modulation of Si p-n junctions

Understanding the p-n junctions is of great importance because p-n junctions are the essential elements of semiconductor devices. In this study, we experimentally investigated the strain induced electrical properties modulation of Si p+-n and n+-p junctions. It is found that, under the uniaxial tensile stress, the current in the large-forward-bias region increases significantly, while a rather small current increase is observed in the diffusion-current-dominant region. Besides, the ideality factors in the diffusion-current-dominant region and the large-forward-bias region decrease when the amount of the applied stress increases. The observations are explained by the strain induced variations in energy band structure, their effect on minority carrier concentrations, and the piezoresistance effect.

Experimental Investigation on Alloy Scattering in sSi/

Alloy scattering in a sSi/Si_0.5Ge_0.5/strained Silicon on Insulator (SOI) (sSOI) quantum-well (QW) p-MOSFET is investigated by hole density modulation through applying back-gate biases. The hole mobility under negative back-gate biases is found degraded by intensified alloy scattering at low electrical field because more holes are distributed in the bulk Si_0.5Ge_0.5. At higher electrical field, the higher density of holes populated at the Si/ Si_0.5Ge_0.5 interface and less holes in the bulk Si_0.5Ge_0.5 result in less pronounced alloy scattering, leading to mobility enhancement under negative back-gate biases. This confirms experimentally that alloy scattering does not play a significant role in the hole mobility of sSi/ Si_0.5Ge_0.5/sSOI QW p-MOSFETs under normal operating mode.

{\rm Si}_{0.5}{\rm Ge}_{0.5}

Al2O3 films deposited as gate dielectrics on germanium (Ge) by atomic layer deposition (ALD) were post annealed in an ozone atmosphere at 450°C for 15, 25 and 35 minutes. The structure and chemical compound of Al2O3/Ge gate stacks were detected by X-ray photoelectron spectroscopy (XPS) measurements after ozone post annealing (OPA) treatments. It is confirmed by XPS measurements that this OPA treatment could significantly decreases the amount of oxygen deficiencies in Al2O3 films, resulting in better insulating property. By using these Al2O3/Ge gate stacks, the Ge pMOSFETs were successfully demonstrated. The results show that OPA treatment could enhance the low field mobility and meanwhile improve the breakdown behaviors of Ge pMOSFETs.

/sSOI Quantum-Well p-MOSFET

In this paper, we report, for the first time, that surface roughness scattering is not necessarily the dominant scattering mechanism in the high-normal-field region of Ge nMOSFETs. This statement is quite different from the well-recognized situation in Si MOSFETs. In Ge(100), phonon scattering is dominant in the high-field region. Thus, it is difficult to increase the high-normal-field mobility in Ge(100) nMOSFETs by controlling the interface roughness. In contrast, because Ge(111) and Ge(110) are free of intervalley phonon scattering, the high-field mobility in Ge(111) and Ge(110) nMOSFETs could be enhanced by the Ge interface engineering. Furthermore, different from that in Si nMOSFETs, mobility limited by surface roughness scattering in Ge nMOSFETs shows a strong temperature dependence due to the valley occupancy change of electrons. The results in this paper should facilitate efforts to increase the high-normal-field mobility in Ge nMOSFETs.

Reliability improvement of Ge pMOSFETs with Al2O3 dielectric by ozone post annealing

In this paper, we have experimentally investigated the effects of all types of strains, including uniaxial tensile strain, uniaxial compressive strain, biaxial tensile strain and biaxial compressive strain, on the negative bias temperature instability (NBTI) of Si pMOSFETs. Strain is applied by using a wafer bending system to avoid processing effects on the NBTI characteristics that result from strain engineering. We confirm experimentally, for the first time, that both uniaxial and biaxial compressive strain in Si pMOSFETs is advantageous as demonstrated by suppressed NBTI. On the other hand, NBTI is enhanced under both uniaxial and biaxial tensile strains. Differences in measured in gate current (Ig) can be attributed to the varying NBTI degradation under different types of strains. The experimental results are partly explained by strain induced band structure modulation and hole repopulation among the heavy hole and light hole subbands.

Comparison of Different Scattering Mechanisms in the Ge (111), (110), and (100) Inversion Layers of nMOSFETs With Si nMOSFETs Under High Normal Electric Fields

We have, for the first time, confirmed that in Ge nMOSFETs the dominant scattering mechanism in the high-field region is not necessarily the surface roughness scattering. We found that, in Ge(100) nMOSFETs, because the phonon scattering is still dominant in the high-field region, it is difficult to enhance the high-field mobility by controlling the interface roughness. Since Ge(111) and Ge(110) nMOSFETs are free of the intervalley phonon scattering, the high-field mobility could be enhanced by the Ge interface engineering. Furthermore, different from that in Si nMOSFETs, surface roughness scattering in Ge nMOSFETs shows a strong temperature dependence.

Comprehensive study of NBTI under compressive and tensile strain

In this paper, we review the recent progresses about the effect of the uniaxial tensile strain on the electrical properties of the Si p-n junctions and MOS capacitors. We found that the uniaxial tensile stress could increase the junction current in the large-forward-bias region significantly. However, only a slight current increase has been observed in the diffusion-current-dominant region. In nMOSFETs the uniaxial tensile strain could enhance Isub significantly, while decreasing Ig slightly. Furthermore, in pMOSFETs, the uniaxial tensile strain could enhance both Ig and Isub. All these results have been explained by taking the strain induced subband structure modulation, current components variation and the piezoresistance effect into consideration.

New findings on the scattering mechanisms in inversion layers of Ge (111), (110), and (100) nMOSFETs under high electric field—Differences with Si nMOSFETs

In this study, we experimentally examine the change of gate currents (I_g) and substrate currents (I_sub) in n and pMOSFETs under different types mechanically applied stress. It is found that, under the uniaxial tensile stress, both I_g and I_sub of pMOSFETs increase with the increase of the stress under the inversion condition. However, an opposite stress dependence in nMOSFETs could be observed for I_g and I_sub. Similar changes were found for I_g and I_sub of nMOSFETs under biaxial tensile stress. Furthermore, the results are explained by the strain altered band structure and the repopulation of carrier.