W. S. Lau

Mechanism of leakage current reduction of tantalum oxide capacitors by titanium doping

In this letter, the authors will point out that defect states related to oxygen vacancies in tantalum oxide capacitors can be suppressed by titanium doping, resulting in significant leakage current reduction. The theory is that titanium forms an acceptor which can move at high temperature and neutralize other donors. However, defect states which cannot be suppressed by titanium doping were detected. These are explained by H2O-related contamination occurring at low temperature (<400°C) during the cooling down period.

General theory of acceptor-oxygen-vacancy complex single donor in high-dielectric-constant metallic oxide insulators

Previously, we showed experimentally that Si∕O-vacancy complex single donors and C∕O-vacancy complex single donors in tantalum oxide films have smaller ionization energies than the first ionization energy of O-vacancy double donors [W. S. Lau, L. L. Leong, T. Han, and N. P. Sandler, Appl. Phys. Lett. 83, 2835 (2003)]. In this letter, a theory based on a larger electron orbit for acceptor O-vacancy complex single donors compared to the O-vacancy double donors is proposed to explain the physics behind our previously reported experimental observation and why Si or C may cause an increase in leakage current in tantalum oxide films.

Formation of Ohmic contacts in AlGaN/GaN HEMT structures at 500 °C by Ohmic contact recess etching.

Good Ti/Al/Ti/Au Ohmic contacts were achieved in undoped-AlGaN/GaN HEMT structures at 500 °C (measured by a thermocouple) by using an Ohmic contact recess etch. The Ohmic recess etch is deeper than the undoped AlGaN layer and hence reaches the two-dimensional electron gas. Good morphology and well-defined edge profile are also achieved. Because of the low temperature, this process can tolerate quite some water vapor related issue during Ohmic contact anneal.

Effective channel length measurement of metal-oxide-semiconductor transistors with pocket implant using the subthreshold current-voltage characteristics based on remote Coulomb scattering

For deep submicron metal-oxide-semiconductor (MOS) transistors with pocket implant, the electron mobility of the short channel transistor is always smaller than that of the long channel transistor in the above-threshold regime, resulting in serious error in effective channel length (Leff) measurement based on the assumption that the mobility is independent of gate length. Our theory predicts that when the gate oxide is very thin the electron mobility is approximately equal for short and long channel MOS transistors in the subthreshold regime. Thus it is possible to perform Leff measurement in the subthreshold region of MOS transistors with ultrathin gate oxide.

Similarity between the first ionized state of the oxygen vacancy double donor in tantalum oxide and the first ionized state of the cadmium vacancy double acceptor in cadmium sulfide

The author suggests that the first ionized state of the oxygen vacancy double donor (VO+) in Ta2O5 behaves like an electron trap with an electron-repulsive energy barrier, resulting in a small electron capture cross section, especially at low temperature. This is similar to the first ionized state of the cadmium vacancy double acceptor (VCd−) in CdS. Single donors or acceptors do not have such a problem. With the help of this theory, the author proposes a two-scan zero-bias thermally stimulated current method: scan 1 for the detection of VO+ and scan 2 for other defect states.

Mechanism of leakage current reduction of tantalum oxide capacitors by postmetallization annealing

In this letter, the authors will point out that defect states related to impurities or structural defects in tantalum oxide capacitors can be passivated by hydrogen during postmetallization anneal (PMA) while oxygen vacancies are enhanced by PMA such that some will observe a decrease while other may observe an increase in the leakage current after PMA. The PMA process can be tuned such that the hydrogen passivation of defect states dominates over the enhancement of oxygen vacancies, resulting in significant leakage current reduction.

An Improved Shift-and-Ratio Effective Channel Length Extraction Method for Metal Oxide Silicon Transistors with Halo/Pocket Implants

The original shift-and-ratio method tends to significantly over-estimate the effective channel length Leff of metal–oxide–silicon (MOS) transistors with halo/pocket implants because the carrier mobility of the short transistor tends to be smaller than that of the long transistor. A modification of the original method has been tested on both n-channel metal–oxide–silicon (NMOS) and p-channel metal–oxide–silicon (PMOS) transistors fabricated by state-of-the-art complementary metal–oxide–silicon (CMOS) technology. The values of Leff generated by this method are more reasonable than the original shift-and-ratio method with much less computation time involved. The theoretical basis of our method is that the carrier mobility of the short transistor is approximately equal to that of the long transistor when the gate voltage is close to the threshold voltage for state-of-the-art MOS transistors with <2 nm gate oxide.

Anomalous narrow width effect in p-channel metal-oxide-semiconductor surface channel transistors using shallow trench isolation technology

Abstract For PMOS (p-channel metal–oxide–semiconductor) transistors isolated by shallow trench isolation (STI) technology, reverse narrow width effect (RNWE) was observed for large gate lengths such that the magnitude of the threshold voltage becomes smaller when the channel width decreases. However, PMOS transistors with small gate lengths show up a strong anomalous narrow width effect such that the magnitude of the threshold voltage becomes larger when the channel width decreases. We attribute such an anomalous narrow width effect to an enhancement of phosphorus and arsenic transient enhanced diffusion (TED) due to Si interstitials generated by the deep boron source/drain (S/D) implant towards the gate/STI edge.

Application of zero-temperature-gradient zero-bias thermally stimulated current spectroscopy to ultrathin high-dielectric-constant insulator film characterization

Previously, we have reported our application of the zero-bias thermally stimulated current (ZBTSC) spectroscopy technique to study defect states in high-dielectric-constant insulator films such as tantalum oxide with much less parasitic current which can be a serious limitation for the conventional thermally stimulated current method. However, a parasitic current can still be observed for ZBTSC because of a small parasitic temperature gradient across the sample. The thermal design of the ZBTSC system can be improved, resulting in zero-temperature-gradient ZBTSC which can be used to detect deeper traps than those by ZBTSC.

Mechanism for improvement of n-channel metal-oxide-semiconductor transistors by tensile stress

This paper investigates the physics behind the overall on-current (Ion) improvement of n-channel metal-oxide-semiconductor (NMOS) transistors by uniaxial tensile stress. The strain-induced change in subthreshold off-current (Ioff) is related to the following strain-induced effects: (i) mobility enhancement, (ii) reduction in the saturation threshold voltage (Vth,sat), and (iii) improvement in subthreshold swing (Sts). By selecting transistors whose Ioff is less sensitive to the statistical variation in gate length, we studied the effects of process-induced tensile stress on Ion and Ioff of NMOS transistors. We found that both externally applied tensile stress and process-induced tensile stress led to a bigger percentage increase in subthreshold Ioff compared to the percentage increase in Ion. Our explanation is that the increase in Ioff is mainly due to an increase in mobility and a decrease in Vth,sat by tensile stress. The improvement of subthreshold swing by tensile stress can lead to a decrease in sub...