Toshihiro Kamei

Effects of embedded crystallites in amorphous silicon on light-induced defect creation

We investigate effects of embedded crystallites in hydrogenated amorphous silicon on light-induced metastable dangling-bond defect creation in a systematic manner. Inclusion of a small volume fraction of crystallites into the amorphous matrix significantly suppresses defect creation against moderate light illumination. Excess carriers generated in the amorphous matrix tend to recombine in the embedded crystallites, which suppresses nonradiative recombination within the amorphous matrix and the subsequent defect creation. The presence of a small volume fraction of crystallites, however, is no longer effective to improve the stability against strong light exposure such as pulsed laser irradiation. In this case, the higher carrier concentration favors bimolecular direct carrier recombination within the amorphous matrix.

A Significant Reduction of Impurity Contents in Hydrogenated Microcrystalline Silicon Films for Increased Grain Size and Reduced Defect Density

We have prepared high-purity hydrogenated microcrystalline silicon films (µc-Si:H) on quartz substrates from a mixture of silane and hydrogen using a new type of ultrahigh vacuum plasma enhanced chemical vapor deposition system. In substrate temperatures ranging from 25 to 350°C, we have produced high-crystallinity µc-Si:H, as observed by Raman spectroscopy. At the highest temperature (~350°C), we obtain larger crystalline Si grains, ~1000 A, estimated using Scherrers formula. Transmission electron microscopy micrographs show that these crystalline grains are conical and extend 7000~8000 A in the film growth direction with a lateral size of ~1000 A. At a mid-range temperature (~200°C), a spin density as low as ~5×1015 cm-3 and the midgap position of the Fermi level imply a substantial reduction of the density of defects states in this pure film. Moreover, this pure film is stable against prolonged light exposure. Implications of these results for the role of impurities in the growth process and optoelectric properties of µc-Si:H are discussed.

Deposition and extensive light soaking of highly pure hydrogenated amorphous silicon

We have developed an ultrahigh vacuum plasma‐enhanced chemical‐vapor deposition system, and deposited high‐purity device‐quality hydrogenated amorphous silicon films. High sensitivity secondary ion mass spectrometry measurements show that impurity contents in the bulk of the present films are reduced to 2×1015 cm−3 for O, 7–10×1015 cm−3 for C, and 5×1014 cm−3 for N; these impurities are normally present at fairly high levels. Nevertheless, extensive light soaking of the films resulted in a defect density as high as 5×1017 cm−3, which is well above the impurity content. This result excludes those models of photoinduced degradation that postulate one‐to‐one correlation between light‐induced defects and O, C, or N impurity atoms.

DEPOSITION OF ULTRAPURE HYDROGENATED AMORPHOUS SILICON

We have succeeded in a drastic reduction of impurity contents in hydrogenated amorphous silicon (a-Si:H) films by a newly developed ultrahigh vacuum plasma-enhanced chemical vapor deposition (UHV/PECVD) system. High sensitivity secondary ion mass spectrometry shows that impurity contents in the films are as low as 1015 cm−3 both for oxygen and carbon, and 1014 cm−3 for nitrogen. These values represent the lowest concentrations of atmospheric contaminants for a-Si:H films reported so far. In particular, oxygen content has not been reduced below 1×1018 cm−3 using conventional UHV/PECVD techniques, and not below 5×1017 cm−3 even by high growth rate process of very high frequency plasma. The essential features of the present UHV/PECVD system are an extremely low outgassing rate of 8×10−9 Torr l/s, extremely low partial pressure of contaminant gas species <10−12 Torr, and purification of feed gas SiH4 at “point of use.” The specific origins of impurities in the films are discussed: outgassing of the reactor wa...

Defect determination kinetics during the growth of a-Si:H

Abstract The deposition-rate dependence of the defect density in a-Si:H films, grown at low substrate temperatures, was experimentally investigated by using a triode RF-discharge reactor. The triode system enabled us to select only SiH3 radicals, with a long lifetime, as a growth precursor, and to suppress ion bombardment of the growing surface. The defect density and photoconductivity of the films were drastically improved with a decrease of deposition rate. Surface dangling bond termination processes which do not involve the precursor were invoked in order to explain this deposition-rate dependence of the defect density. Results on intermittent deposition, where the growth and intermission process were repeated in cycles, showed an increase of the photoconductivity in the resulting film, which supports the existence of the dangling bond termination process on the surface.

Effects of oxygen impurity on microcrystalline silicon films

We have systematically studied the effects of oxygen impurities on the crystal quality and electrical properties of hydrogenated microcrystalline Si films (/spl mu/c-Si:H) grown at 200/spl deg/C. Two threshold oxygen concentrations [0]s are observed: crystal quality does not deteriorate below /spl ap/1/spl times/10/sup 20/ cm/sup -3/ of [0], while carrier density increase rapidly above /spl ap/1/spl times/10/sup 18/ cm/sup -3/ of [0]. The electron carrier density obeys 1.4 th power law of [0] in the range between 1/spl times/10/sup 18/ and 1/spl times/10/sup 20/ cm/sup -3/. This superlinear relationship implies that oxygen aggregates act as donors. Lower temperature (/spl ap/200/spl deg/C) formation of oxygen aggregates is discussed.

How do Impurities Affect the Growth of μc-Si:H?

Ultra clean plasma CVD process opens the doorway to clarify the role of impurities in the growth process of μc-Si:H. A reduction of impurity levels during the growth extends the temperature range for crystalline formation to lower side, i.e., high-crystallinity μc-Si:H formation even at room temperature, substantially reduces midgap defect density at 200°C, and enlarges crystalline grain size at 350°C. These results imply that impurities disrupt crystalline formation even on hydrogen covered surface. The crystalline-to-amorphous transition is induced by a loss of surface hydrogen coverage due to thermal hydrogen desorption at higher temperature of ∼450°C irrespective of the effect of oxygen impurity. Light-soaking experiments for the series of the films from a-Si:H to μc-Si:H films with different crystalline volume fraction indicate that the presence of small volume fraction of crystallite significantly suppresses light induced defect creation under the present light soaking condition of 3SUN 60°C 6hr. These results are explained in terms of preferential recombination of photo-excited carriers in the crystallite.

Integrated Amorphous Silicon Photodiode Detector for Microfabricated Capillary Electrophoresis Devices

We have demonstrated that hydrogenated amorphous Si (a-Si:H) photodiodes are sufficiently sensitive to be used as detectors for chemical and biological assays performed on microfabricated capillary electrophoresis devices. A limit of detection of 56 pM for fluorescein flowing in a 50-μrn deep microchannel is observed utilizing confocal optics. Moreover, the fluorescence has been successfully detected with a newly designed integrated a-Si:H detector, in which the detector and laser are placed on the same side of the microchip. This optical design is ideal for monolithic integration of the a-Si:H detector technology with VCSEL excitation.

A study of surface reactions during the growth of B-doped a-Si:H using the intermittent deposition technique

Abstract The intermittent deposition technique has been applied to the fabrication of B-doped a-Si:H. In this technique, growth and intermission processes (waiting time) are repeated in cycles by using a mechanical shutter, while continuously maintaining the discharge. The dark conductivities in B-doped films deposited at 150 and 200°C increase with waiting time, accompanied by optical gap narrowing except when the waiting time is short. In gas exchange experiments, where B2H6 doped SiH4 is replaced by Ar during the waiting time, the conductivity of the film grown at 150°C increases without the narrowing. The reactions determining the conductivity and optical gap during the waiting time are discussed.

A Highly Sensitive Integrated a-Si:H Fluorescence Detector for Microfluidic Devices

Most of micromachined and/or integrated fluorescence detectors suffer from high limit of detection (LOD) compared to conventional optical system that consists of discrete optical components, which is mainly due to higher laser light scattering of integrated optics rather than detector sensitivity. In this work, we have reduced background (BG) photocurrent of an integrated hydrogenated amorphous Si (a-Si:H) fluorescence detector due to laser light scattering by nearly one order magnitude, significantly improving a LOD. The detection platform comprises a microlens and the annular fluorescence detector where a thick SiO 2 /Ta 2 O 5 multilayer optical interference filter (>6 μm) is monolithically integrated on an a-Si:H pin photodiode. With a microfluidic capillary electrophoresis (CE) device mounted on the platform, the system is demonstrated to separate DNA restriction fragment digests (LOD: 58 pg/μL) as well as 2 nM of fluorescein-labeled oligomer (LOD: 240 pM) with high speed, high sensitivity and high separation efficiency. The integrated a-Si:H fluorescence detector exhibits high sensitivity for practical fluorescent labeling dyes as well as feasibility of monolithic integration with a laser diode, making it ideal for application to point-of-care microfluidic biochemical analysis.