Keisaku Yamada

Oxygen Vacancy Induced Substantial Threshold Voltage Shifts in the Hf-based High-K MISFET with p+poly-Si Gates -A Theoretical Approach

A theoretical investigation has been made of the origin of substantial threshold voltage (Vth) shifts observed in p+poly-Si gate Hf-based metal insulator semiconductor field effect transistors (MISFETs), by focusing on the effect of oxygen vacancy (VO) formation in HfO2. It has been found that VO formation and subsequent electron transfer across the interface definitely causes substantial Vth shifts, especially in p+poly-Si gate MISFETs. Moreover, the theory also systematically reproduces recent experimental reports that large flat band (Vfb) shifts are observed, even in intrinsic poly-Si gates, and that the Vfb shifts exhibit a high dependence on HfSiOx thickness.

Effective-Work-Function Control by Varying the TiN Thickness in Poly-Si/TiN Gate Electrodes for Scaled High-

We have investigated the controllability of the effective work function (phim,eff) of TiN as a work-function-determining metal (WFM) for various gate-electrode structures in HfSiON MOSFETs. phim,eff was controllable from 4.7 to 4.44 eV by changing the TiN thickness from 30 to 2 nm in poly-Si/TiN gate electrodes, without any distinct increase in EOT. Therefore, thin-TiN and thick-TiN WFMs are preferred for the reduction in threshold voltage in nMOSFETs and pMOSFETs with poly-Si/TiN gate electrodes, respectively. A similar controllability was not observed with W/TiN gate electrodes but was evident with W/TaSiN/TiN gate electrodes. This means that controllability is a characteristic of metal gate electrodes with a structure including a Si-rich layer (such as poly-Si and TaSiN)/TiN. It is considered that Ti suboxides, which increase phim,eff as a thin insulator with negative fixed charges, or interface dipoles in the TiN/HfSiON interface, are reduced by oxidation of the Si-rich layer, producing the required result of phim,eff decrease when the TiN thickness becomes as thin as 2 nm.

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HfO2∕SiO2∕Si(001) structures were annealed in dry oxygen, and compositional depth profiles were measured by high-resolution Rutherford backscattering spectroscopy. Growth of the interfacial SiO2 layer and simultaneous surface accumulation of Si were observed. The observed result indicates that silicon species are emitted from the SiO2∕Si interface to release the stress induced by oxidation as was predicted by recent theoretical studies.

CMOSFETs

Metal-insulator-semiconductor field-effect transistors (MISFETs) with aluminum oxide as a gate insulator have been fabricated on a hydrogen-terminated diamond surface using its surface conductive layer. The aluminum oxide gate insulator was deposited on the diamond surface by the pulsed laser deposition method. The on-off ratio measured by dc was greater than five orders of magnitude, one of the best results reported for diamond FETs. The gate leak current of aluminum oxide MISFETs is three orders of magnitude less than that of conventional CaF2 MISFETs. These characteristics indicate that aluminum oxide gate insulators are suitable for high reliability power device applications of diamond MISFETs.

Si emission from the SiO2/Si interface during the growth of SiO2 in the HfO2/SiO2/Si structure

The impact of nitridation on open volumes in thin HfSiOx films fabricated on Si substrates by atomic layer deposition was studied using monoenergetic positron beams. For HfSiOx, positrons were found to annihilate from the trapped state due to open volumes which exist intrinsically in an amorphous structure. After plasma nitridation, the size of open volumes decreased at a nitrogen concentration of about 20at.%. An expansion of open volumes, however, was observed after postnitridation annealing (PNA) (1050°C, 5s). We found that the size of open volumes increased with increasing nitrogen concentration in HfSiOx. The change in the size of open volumes was attributed to the trapping of nitrogen by open volumes, and an incorporation of nitrogen into the amorphous matrix of HfSiOx during PNA. We also examined the role of nitrogen in HfSiOx using x-ray photoelectron spectroscopy and first principles calculations.

Characterization of diamond metal-insulator-semiconductor field-effect transistors with aluminum oxide gate insulator

Thin Hf0.6Si0.4Ox and Hf0.3Al0.7Ox films fabricated by metal-organic chemical-vapor deposition and atomic-layer-deposition techniques were characterized using monoenergetic positron beams. Measurements of the Doppler broadening spectra of annihilation radiation and the lifetime spectra of positions indicated that positrons annihilated from the trapped state by open volumes that exist intrinsically in amorphous structures of the films. For HfSiOx, the mean size of the open volumes and their size distribution decreased with increasing postdeposition annealing (PDA) temperature. For HfAlOx, although the overall behavior of the open volumes in response to annealing was similar to that for HfSiOx, PDA caused a separation of the mean size of the open volumes. When this separation occurred, the value of the line-shape parameter S increased, suggesting an oxygen deficiency in the amorphous matrix. This fragmentation of the amorphous matrix can be suppressed by decreasing the annealing time.

Characterization of HfSiON gate dielectrics using monoenergetic positron beams

Effects of nitrogen incorporation on suppression of electron charge traps in Hf-based high- kappa gate dielectrics have been studied by first-principles calculations, focusing on interactions between N atoms and electrons trapped at oxygen vacancies (V_Os). Our total energy calculations revealed that the formation energy of a doubly occupied state of V_O is significantly increased in HfO_xN_y compared to that in HfO₂ . This clearly indicates that the electron charge traps at V_Os are considerably suppressed by N incorporation

Annealing properties of open volumes in HfSiOx and HfAlOx gate dielectrics studied using monoenergetic positron beams

We studied effects of nitrogen incorporation into Hf-based high-k gate dielectrics on local insulating properties by conductive atomic force microscopy. Nitrogen-incorporated HfSiO/HfO2/SiO2 gate stacks exhibited excellent dielectric reliability, whereas we observed the creation of local leakage sites for untreated gate stacks, i.e., without nitridation. Both types of high-k dielectric layers were crystallized, and there was no relationship between the current leakage sites and surface morphology. These findings indicate that grain boundaries of the high-k films do not act as the leakage sites. Instead, we propose nitrogen incorporation as an important method for terminating the current leakage paths and discuss detailed mechanisms based on first-principles calculations.

Role of Nitrogen Atoms in Reduction of Electron Charge Traps in Hf-Based High-

Gate semi-around silicon nanowire (SiNW) FETs have been fabricated and their electrical characteristics, especially on the drivability, have been assessed for future high performance devices. Among different wire size, a SiNW FET with a cross-section of 12×19 nm2 has shown an improvement in the on-current (ION) when normalized by the channel peripheral length. A high ION over 1600 µA/µm at an overdrive voltage of 1 V has been achieved with a gate length and an oxide thickness of 65 and 3 nm, respectively. The origin of the high drivability has been speculated by higher carrier density, improved carrier mobility and the reduction in the series resistance.

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Density functional calculations are performed for lanthanum-oxide clusters in order to study the local dielectric properties of such clusters using the dielectric constant defined at local points. An increase in coordination number brings about an increase in electron population on the central lanthanum atom, leading to an increase in the local dielectric constant.