Masato Ueda

High‐rate deposition of amorphous hydrogenated silicon from a SiH4 plasma

An extremely high deposition rate of amorphous hydrogenated silicon has been achieved by employing a new rf discharge technique. The deposition rate has been increased to more than 50 A/s at a substrate temperature of 200 °C without accompanying any appreciable deterioration in the electronic and structural properties as compared to those of specimens prepared at a conventional deposition rate (∼1 A/s). Thermal stability of the high‐rate samples is improved with respect to that of low‐rate specimens.

Two-Phase Structure of a-Si1-xNx:H Fabricated by Microwave Glow-Discharge Technique

Amorphous Si1-xNx:H films (x \lesssim0.1) were made by the microwave discharge technique. At x less than 0.05, both the activation energy Ea of the conductivity σ and the optical energy bandgap Eop of a-Si1-xNx:H films were almost constant, while σ decreased rapidly with increasing x. It is predicted from the intensity variation of the Si-H stretching modes as a function of x that the film consists of mixed phases of a-Si and a-Si3N4, and that Eop and Ea are determined by the a-Si phase.

Preferential segregation of dopants in μc-Si:H

Abstract Electronic and structural properties of heavily B-doped μc-Si:H films prepared by rf glow discharge technique have been studied by Raman scattering, IR absorption, SIMS and conductivity measurements. It is found that boron atoms in μc-Si:H tend to segregate in the amorphous tissue. The remarkable difference in doping efficiency between B- and P-doped μc-Si:H was interpreted in terms of the different degree of dopant segregation in the amorphous phase.

New mode of plasma deposition in a capacitively coupled reactor

A new technique of radio frequency plasma‐enhanced chemical vapor deposition is developed to achieve extremely high growth rates exceeding 40 A/s. The two disk electrodes are surrounded by a stainless‐steel mesh at ground potential, and an external variable reactance is connected between the substrate electrode and the ground. When the resonance condition in the circuit composed of the variable reactance and the substrate sheath capacitance is satisfied, a very high deposition rate is obtained as a result of dramatic change in the potential distribution between the two electrodes.

Effect of hydrogen dilution on structure of a-Si:H prepared by substrate impedance tuning technique

The effect of hydrogen dilution of a source gas, SiH4, on structural inhomogeneity has been studied for hydrogenated amorphous silicon films prepared under the mode involving a high deposition rate by the substrate impedance tuning technique. As the silane fraction decreases from 100% to 1% with hydrogen dilution, the SiH bonding configuration gradually changes into the dihydride (SiH2) group; finally, the films become microcrystalline. On the other hand, the state density in the mid gap and the spin density, related to the structural disorder, have minima at silane fractions of 100% and 10%. These results seem to be consistent with a two-phase model suggested for high rate films, with an emphasis on a different medium range disorder between the films prepared at high and low deposition rates.

Amorphous Silicon Static Induction Transistor

A hydrogenated amorphous silicon (a-Si:H) Schottky-gate static induction transistor (SIT) is proposed and its characteristics are calculated for the case that the gap-state distribution in a-Si:H is constant near the Fermi level. It is found that an a-Si:H SIT with a gate spacing of about 5 µm can yield a sufficiently high ON/OFF current ratio of more than 108 when the gap-state density near the Fermi level is 5×l015 cm-3eV-1. The cut-off frequency of an a-Si:H SIT is expected to exceed a few MHz.

Influence of interface states on field effect and capacitance-voltage characteristics of metal/oxide/a-Si:H structures

Abstract Separate determination of the electronic density of states in bulk a-Si:H and at the oxide/a-Si:H interface of a MOS structure have been made by combining the field effect technique with the capacitance-voltage method. It is found that electronic states existing at the oxide/a-Si:H interface cause no serious errors in the determination of the bulk gap-state density. The discrepancy in the measured gap-state density between the present method and the DLTS or ICTS techniques does not arise from the presence of surface states, but from differences in the electronic properties of a-Si:H.

Amorphous silicon static induction transistor

Abstract The current-voltage characteristics of a new a-Si:H static induction transistor(SIT) are calculated by taking into account the gap state density. It is shown that SIT, in which the finger-plate Schottky gate electrode with a spacing of about 3∼5 μm is buried in the i layer of a-Si:H n + in + structure, can achieve a high on/off current ratio of about 10 8 in the gate voltage range less than ten volts for the gap state density near the Fermi level being 5×10 15 ∼ 1×10 16 cm −3 eV −1 . The operation frequency of SIT is expected to exceed a few MHz because the transconductance is about 400 times larger than that of a conventional a-Si:H thin-film transistor(TFT).

Growth kinetics of amorphous hydrogenated silicon studied by pulsed rf discharge

Abstract The growth rate and hydrogen bonding configuration of a-Si:H prepared by a pulsed rf discharge technique were measured as a function of repetition frequency. The result was compared with the case of the gas-phase polymerization of monomers C 2 H 2 , C 2 H 4 , and C 2 H 6 in a pulsed rf discharge. From distinct difference between the pulsed-plasma depositions of a-Si:H and C:H films, it is concluded that the growth of a-Si:H proceeds through the heterogeneous reactions among chemical species on the substrate surface.

Effect of Annealing on Hydrogenated Amorphous Silicon Prepared at High Deposition Rate

Structural and electronic stabilities in hydrogenated amorphous silicon (a-Si:H) prepared at deposition rates ranging 10–50 A/sec have systematically been investigated by observing annealing effects on the spectra of optical absorption, photoluminescence and infrared absorption. High-rate samples deposited at a substrate temperature of 200°C exhibited excellent thermal stability in electronic properties at annealing temperatures up to 465°C.