Shigeichi Yamamoto

Negative light‐modulation effect of boron‐doped hydrogenated amorphous silicon

Laser intensity dependence of transmission and the light‐modulation effect of boron‐doped hydrogenated amorphous silicon (a‐Si:H) were investigated with 780 nm laser diodes. Negative intensity dependence, which causes the transmission to decrease as the laser intensity is increased, was observed for B‐doped, but not for undoped a‐Si:H. Moreover, a negative light‐modulation effect was discovered where the constant signal light decreases with increasing control light. To explain these phenomena, a double‐absorption model, which has two absorption processes from a level associated with the impurity and from a valence band, was considered.

High-mobility thin-film transistor fabricated using hydrogenated amorphous silicon deposited by discharge of disilane

Plasma-enhanced chemical vapor deposition of hydrogenated amorphous silicon (a-Si:H) film was investigated with emphasis on the effect of disilane flow rate. A coplanar thin-film transistor (TFT) was fabricated using this a-Si:H film. Silicon-hydrogen bond content in the a-Si:H film was measured by infrared absorption spectroscopy. With decrease in the disilane flow rate from 3.0 cm3/min to 1.5 cm3/min, the maximum field-effect electron mobility (µ FE) of the TFT which depends on the gate voltage increased from 3.3 cm2/( Vs) to 4.9 cm2/( Vs), accompanied by a reduction in the silicon-hydrogen bond content. There was a negative correlation between µ FE and the silicon-hydrogen bond content in the a-Si:H film. The improvement mechanism of µ FE was discussed in terms of the chemical structure of the a-Si:H film.

Silicon Nitride Film for High-Mobility Thin-Film Transistor by Hybrid-Excitation Chemical Vapor Deposition

A hybrid-excitation technique was applied to nitrify amorphous silicon (a-Si:H) film surface using ammonia (NH3) gas and to deposit silicon nitride (SiNx) film using disilane (Si2H6) and NH3 gases for an a-Si:H thin-film transistor (TFT) at 280°C. With the use of this SiNx film as a gate insulator, a coplanar a-Si:H TFT with high field-effect electron mobility (5.3 cm2/(Vs)) was fabricated. This high electron mobility is considered to be due to three features as follows: (1) a good SiNx/a-Si:H interface created by the hybrid-excited nitrification of the a-Si:H film surface, (2) an ion-bombardment-free nitrogen-rich SiNx film fabricated by hybrid-excitation chemical vapor deposition (CVD) and (3) an a-Si:H film with less ion-bombardment damage. This hybrid-excited nitrification and the CVD effects on electrical properties of SiNx/(100)Si interface are also discussed.

Hybrid-Excitation Chemical Vapor Deposition of Silicon Nitride on (100)Si –The Film and Interface Properties-

Silicon nitride (SiNx) films are deposited on (100)Si substrates using silane (SiH4) and ammonia (NH3) gases by a hybrid-excitation chemical vapor deposition (hybrid-CVD) method at 280°C. In the deposition, the NH3 gas is decomposed by a radio frequency (13.56 MHz) discharge plasma, and is irradiated by 184.9-nm and 253.7-nm ultraviolet rays with SiH4 gas. The depositions are performed with emphasis on the gas flow ratio, NH3/SiH4, ranging from 3.3 to 29. The effective trapped carrier density at the interface between the SiNx film and the (100)Si substrate is reduced to 2.5×1010 cm-2 by the hybrid-CVD. The films show, moreover, high electrical resistance (e.g. 7×1015 Ω cm). These properties are better than those of the conventional plasma-enhanced CVD and photo-enhanced CVD films. The improvement mechanism in the bulk and interface properties is discussed.

Total Pressure Effects on the Properties of Silicon Nitride Films Fabricated by Photoenhanced Chemical Vapor Deposition

Silicon nitride ( SiNx ) films were deposited on (100)Si substrates using silane and ammonia gases by the direct-photolysis photoenhanced chemical vapor deposition method at 280° C. The depositions were performed with emphasis on the total pressures, which ranged from 0.51 Torr to 4.04 Torr. As the total pressure increased, the film resistivity decreased from 3.7×1015 Ω cm to 4.8×109 Ω cm. The effective trapped carrier density at the SiNx /(100)Si interface reached a minimum value (2.1×1010 cm-2) at 1.52 Torr. The film included silicon bonded with silicon (Si–Si) components as well as nitrified silicon (Si–N) components. The Si–Si components increased and the Si–N components decreased in number as the total pressure increased. Based on these results and gas analysis findings, two important reactions to characterize the film properties were discussed.

Hybrid-excitation chemical vapor deposition of a silicon nitride film

Abstract Gate-grade silicon nitride (SiN x ) films were deposited on Si(100) substrates using a hybrid-excitation chemical vapor deposition technique at 280°C. In the depositions, decomposition of ammonia (NH 3 ) gas was first accelerated using a glow discharge plasma. Then the reaction products were irradiated by 184.9 and 253.7 nm UV light with silane (SiH 4 or Si 2 H 6 ) gas over the substrate. The effective trapped carrier density at the SiN x / Si(100) interface was reduced to below 2 × 10 10 cm −2 . The films showed a high resistivity (> 1 × 10 15 Ω. cm). From these results and the gas analysis findings with respect to the reaction products, the mechanism of improvement of the tilm properties is discussed.

A LOW DEFECT DENSITY SiN x /(100)Si INTERFACE MADE BY HYBRID-EXCITATION CVD

A new technology for preparing silicon nitride(SiN x ) films has been developed. Interface charge density at the SiN x /(100)silicon is reduced to 1.5 × 10 10 cm −2 by a hybrid-excitation chemical vapor deposition technique using plasma- and photo-excitation processes.