Toshio Tanaka

Shift of the Gate Threshold Voltage of MOS Transistors dne to the Introduction of Shallow Impurities

It is possible to shift the gate threshold voltage of MOS transistors with little sacrifice of other device performances by introducing shallow level impurities only into a region near the semiconductor surface under the gate. Some theoretical information is given which may be useful for designing MOS transistors in an attempt to realize such a shift. The effects of the impurity introduction on the characteristics of MOS diode structures are also described.

Interface Properties of Al2O3-Ge Structure and Characteristics of Al2O3-Ge MOS Transistors

The interface properties of Al2O3-Ge structure formed on germanium wafers by DC reactive sputtering are evaluated. The fast interface states are diminished by subjecting the sample to heat treatment in an inert gas flow. The interface state density is estimated to be typically less than 4×1011 state/cm2-eV from the shift of the flat-band voltage with temperature as originally adopted by Gray and Brown. From the measurement of the parallel conductance of the MOS structure, the electron capture cross section of the trap located at the interface is estimated to be about 0.9×10-17 cm2 using the tunnelling model of Preier, and about 1×10-18 cm2 using the surface potential fluctuation model of Nicollian and Goetzberger. With reactively sputtered Al2O3 films as a gate insulator, n-channel germanium MOS transistors are made with a high effective mobility. The effective mobility is typically more than 2000 cm2/V-sec and the gate threshold voltage is about 1 volt or less.

Solid-Solid Diffusion of Boron in Silicon Using Reactive Sputtering

An investigation on the solid-solid diffusion of boron into silicon from doped oxide layer deposited by the reactive sputtering method is reported here. The surface concentrations are controlled over a wide range of low concentrations of from 1015 atoms cm-3 to 1017 atoms cm-3 and are almost independent of the diffusion temperatures and diffusion times. The diffusion profile obeys mostly the complementary error function and the diffusion coefficient obtained is expressed as D=0.15exp (-4.25(eV)/kT) cm2 sec-1 at a concentration of about 1016 atoms cm-3 over a range of temperature of 1100°C to 1270°C. This value is smaller than those observed at high concentrations. Furthermore the surface concentrations depend linearly on the target concentrations.

The 1/ f Noise MOS Transistors

The low-frequency noise characteristics of MOS transistors with SiO2 films formed by reactive sputtering as gate insulators were investigated. An attempt is made to analyze the characteristics on the basis of the tunneling model. Reasonable agreement of the experimental result with the theorectical characteristics is found. Some informations on the noise generation centers are obtained.

Interface Characteristics of the Reactively Sputtered Al2O3-Si Structure

The interface characteristics of Al2O3-Si structures formed on silicon wafers by dcreactive-sputtering of aluminum in an oxygen ambient are evaluated. The presence of a dispersion of interface state time constants was found by measuring the parallel conductance of the MOS structures. An attempt is made to analyze the experimental results by using the tunneling model. We conclude that traps are distributed in the Al2O3 to a depth of 4.8A from the interface with densities of the order of 1011/eVcm2 or less. These densities were further reduced by hydrogen treatment. The electron capture cross section of the traps located at the interface is about 1.5×10-14 cm2 and that for holes is smaller by about a factor of 10.

Effect of Heat Treatment on the Interface Characteristics in Reactively Sputtered Al2O3-Si Structures

Aluminum oxide (Al2O3) silicon MOS structures formed by DC reactive sputtering of pure aluminum in a low pressure oxygen ambient are subjected to heat treatments in various environments, and the interface characteristics of these structures are evaluated. The surface state density at the Al2O3-Si interface is found to be diminished by annealing in a nitrogen ambient, and be reduced further by additional heat treatment in a hydrogen ambient over a comparatively wide energy range.

The Effects of Traps in the Semiconductor on the Characteristics of MOS Transistors

The influences of hypothetic and real deep lying centers introduced intentionally or not in the semiconductor on the characteristics of MOS transistors are formulated and are numerically computed for silicon silicon-dioxide structures, on the assumption that the centers are distributed uniformly and the mobility of carriers in the channel is constant and independent of the existence of the centers. A deterioration or an improvement due to an introduction of the centers, depending on the position of levels associated with the center in the forbidden gap and the values of their capture cross sections for electrons and holes, is resulted.

Characteristics of Silicon Silicon-Dioxide Structures Formed by DC Reactive Sputtering

The C-V characteristics of silicon silicon-dioxide structures fabricated using DC reactive sputtering method were investigated. The fast interface states were able to be eliminated by subjecting the sample to high temperature treatment in inert gas flow and subsequent Al-treatment. The surface state charge Qss was of the order of 1010 qcm-2 in the sample treated in nitric acid prior to oxide deposition. The substitution of a hydrofillic pretreatment for the nitric pretreatment resulted in a negative Qss of approximately -5×1011 q cm-2. This small Qss was stable under bias-temperature stressing at 300°C even without additional processing, and, on the other hand, the negative Qss could be stabilized with comparable ease by forming the usual phosphate glass layer on the oxide. This stable negative Qss is of interest from the viewpoint of practical application such as making normally off n-channel MOS transistors.