Yasuyoshi Mishima

Silicon Thin-Film Formation by Direct Photochemical Decomposition of Disilane

Silicon thin-films have been deposited by the direct photolysis of disilane at a substrate temperature below 300°C. The growth rate depends on irradiation intensity of a low pressure mercury-lamp, and a typical rate of 15 A/min has been obtained under ~0.08 watts/cm2 illumination, regardless of substrate temperature. The deposited films are composed of an amorphous network containing bonded-hydrogen in the range 6–9 at.%. The bonding configurations of SiH groups varied from silicon dihydride to monohydride with increasing substrate temperature, and correspondingly the dark conductivity decreased from 10-7 to 10-11 Ω-1cm-1. A broad photoluminescence peak at 1.4 eV was observed for a specimen grown at 200°C.

POLYCRYSTALLINE SILICON FORMED BY ULTRAHIGH-VACUUM SPUTTERING SYSTEM

Using an ultrahigh‐vacuum (UHV) sputtering system, we could grow two new methods of polycrystalline silicon films. The one is as‐deposited polycrystalline silicon on glass at substrate temperatures under 500 °C. The other is solid‐phase‐crystallization by thermal annealing of as‐deposited amorphous silicon films in a UHV. As‐deposited polycrystalline silicon films were oriented to (220) and grain sizes were determined from half‐width of x‐ray diffraction to be about 40 nm. From the deposition temperature dependence of the x‐ray diffraction peak intensity, the activation energy of the crystalline growth was calculated to be about 0.6 eV. Hydrogen atoms in the sputtering gas lower the reproducibility of as‐deposited poly‐Si. Polycrystalline silicon films produced by thermal annealing of as‐deposited amorphous silicon films at 550 °C in UHV have a (111) orientation. Field‐effect mobilities of the as‐deposited polycrystalline silicon film and the polycrystalline silicon film by UHV thermal annealing were 5 an...

Investigation of the bubble formation mechanism in a‐Si:H films by Fourier‐transform infrared microspectroscopy

The formation of bubbles and holes in hydrogenated amorphous silicon (a‐Si:H) films after isochronal annealing has been investigated. The internal stress of the bubble in an a‐Si:H film has been determined by the relationship between the diameter and height of a bubble. In addition, the internal stress of the bubble has also been investigated based on the difference in hydrogen concentration between the flat and bubble areas determined by Fourier‐transform infrared microspectroscopy. Consistent results are obtained from both methods regarding the magnitude of the internal stress. These results indicate that the bubble is formed by molecular hydrogen evolving from the a‐Si:H film.

High-performance CMOS circuits fabricated by excimer-laser-annealed poly-Si TFTs on glass substrates

High-performance CMOS circuits are fabricated from excimer-laser-annealed poly-Si TFTs on a glass substrate (300/spl times/300 mm). The propagation delay time of the 121 stage CMOS ring oscillators with 0.5 /spl mu/m gate length is 0.18 nsec at 5 V supply voltage. The maximum operating frequency of the 40-stage shift registers with 1 /spl mu/m gate length is 133 MHz at 5 V supply voltage. This value is high enough for peripheral CMOS circuits with line-at-a-time addressing.

Internal Photoemission in a-Si:H Schottky-Barrier and MOS Structures

Internal photoemission of metal/a-Si: H and metal/SiO2/a-Si:H systems have been measured in the temperature range 86 to 300 K. The height of the potential barriers at the Pd/a-Si:H and SiO2/a-Si:H interfaces have been determined to be 0.98 and 3.03 eV, respectively. The temperature coefficients of the Schottky barrier height and the optical band gap of a-Si:H have been obtained to be 3.3×10-4 and 2.7×10-4 eV/K, respectively. Appreciable quantum yield below the photoemission threshold for an MOS structure is found to be dependent on preparation conditions of a-Si:H , being attributed to electron emission from the filled gap-states in the film.

Nucleation of Microcrystallites in Phosphorus-Doped Si:H Films

Structural properties of partially crystallized Si:H films prepared by glow discharge technique have been studied by Raman scattering and optical absorption spectra. The structure of the Si:H films is found to be a mixture of microcrystalline and amorphous phases. The volume fraction of a microcrystalline part in the Si:H network was determined to be 0.37–0.58 on the basis of the effective-medium theory.

NON-MASS-SEPARATED ION SHOWER DOPING OF POLYCRYSTALLINE SILICON

Practical polycrystalline silicon thin‐film transistors need a low‐temperature doping technique. Ion shower doping with a main ion source of P2Hx (x=1,2,...) was studied. This technique implants a molecule in the polycrystalline silicon surface with a low acceleration voltage. A critical impurity density from polycrystalline phase to amorphous phase for phosphorus in polycrystalline silicon of 2.0×1020 ions/cm3was found. Sheet resistance with ion shower doping was lower than with conventional ion implantation at low temperature. This is a result of an increase in sheet carriers, because the low‐temperature recovering of defects is done by molecular implantation and hydrogen atoms compensate defects. Implanted polycrystalline Si with a Hall mobility of 5 cm2/V s was obtained.

Current Transport Mechanism of Hydrogenated Amorphous Silicon Schottky Barrier Diodes

We report detailed studies on the forward current-voltage characteristics of Pd/a-Si:H Schottky barrier diodes. Exact values of the barrier height and its temperature coefficient have been determined by the internal photoemission technique. Based upon this result, the current transport mechanism through the a-Si:H Schottky barrier has been quantitatively examined. The fundamental characteristics of the Schottky barrier are considered to be dominated by diffusion theory rather than by thermionic emission theory.

Improved lifetime of poly-Si TFTs with a self-aligned gate-overlapped LDD structure

We investigated the lifetimes for various poly-Si thin film transistor (TFT) structures. A gate-overlapped lightly doped drain (GOLDD) structure was self-aligned by the side etching of Al-Nd in an Al-Nd/Mo gate electrode. The dopant activation process in the LDD regions of GOLDD TFTs was performed by using a H/sub 2/ ion-doping technique. We also observed the effect of lifetime on the source/drain activation process. The thermal annealing of the source/drain region was found to extend the lifetime. The predicted lifetime of our GOLDD poly-Si TFT is superior to those of non-lightly doped drain (non-LDD) and lightly-doped drain (LDD) poly-Si TFTs. The trapped-electron density at the drain junction after bias-stressing was also investigated using a two-dimensional (2-D) simulation.

P‐3: The Effect of a Laser Annealing Ambient on the Morphology and TFT Performance of Poly‐Si Films

We have examined the effect of a laser annealing ambient on the morphology and TFT performance of poly-Si films. Oxygen in the ambient prevents the relaxation of surface roughness because of a thin oxide formed on the poly-Si surface during laser irradiation. Poly-Si films that crystallize in air (oxygen ambient) have larger gains, higher mobility, and rougher surfaces than films crystallized in a vacuum. To reduce this surface roughness, the surface oxide that formed during laser irradiation in air was removed and poly-Si films were irradiated by the excimer laser in a vacuum again. The surface roughness drastically decreased from 13 nm to 1 nm while the grain size remained the same.