Isamu Shimizu

Roles of Atomic Hydrogen in Chemical Annealing

A systematic study has been performed to reveal the role of atomic hydrogen in chemical annealing, where the deposition of a thin layer and treatment with atomic hydrogen are repeated alternately, for the fabrication of a stable structure. Structural relaxation resulting from impingement of atomic hydrogen on the growing surface is differentiated into two processes: the structural promoted relaxation on the surface and changes caused within the sub-surface depending on the conditions for deposition of the thin layer and the flux of atomic hydrogen. The structural changes within the sub-surface resulted in either the widening of the optical gap or crystallization at rather low substrate temperatures (T s: 100-150° C). High-quality a-Si:H with the optical gap of 1.87 eV exhibiting rather high stability against light soaking was successfully fabricated by this technique. The defect density of the film was 4×1015 cm-3 in its well annealed state and 6×1016 cm-3 in the saturated state. At high T s, on the other hand, the hydrogen treatment mainly enhanced chemical activities of the growing surface and resulted in either narrowing of the optical gap or promotion of the grain growth.

Highly Oriented ZnO Films Prepared by MOCVD from Diethylzinc and Alcohols

Alcohols have been shown to be useful as oxidizing agents of diethylzinc for the growth of ZnO films by MOCVD. C-axis oriented ZnO films have been deposited on glass substrates at 300°C. The problem of a premature reaction, which is observed between diethylzinc and oxygen or water vapor, is shown to be overcome when alcohols are used instead. The formation, stability and decomposition of the precursor of MOCVD, e.g., adducts, are discussed on the basis of a mass-spectroscopy analysis.

A-Si thin film as a photo-receptor for electrophotography

Charging with corona and its photo-discharging characteristics were investigated for a photo-receptor of a-Si thin film prepared by glow discharge of SiH4. A thin layer of P-doped Si (n-type) provided between the metal electrode (Ni/Cr) and a-Si (“intrinsic”) was adequate to give a sufficient charge-retentivity (td ∼ 30 sec) for negative corona. Excellent photosensitivity (4 erg/cm2 for half decay) and wide spectral sensitivity (< 750 nm) were attained.

Preparation of photoconductive a-SiGe alloy by glow discharge

Abstract Systematic investigations had been made of preparing a-SiGe alloy with high photoconductivity. The a-SiGe alloy prepared by glow discharge from gaseous mixture of GeF4SiF4H2 showed a satisfactory photoresponse and an effective doping feasibility to control its Fermi-levels, resulting from a great reduction in the amount of the dangling bonds.

Role of the hydrogen plasma treatment in layer-by-layer deposition of microcrystalline silicon

We have investigated the role of hydrogen in hydrogenated microcrystalline silicon (μc-Si:H) formation using hydrogen plasma treatments, in particular examining the possibility of subsurface reaction due to permeating hydrogen atoms, which leads to the crystallization of hydrogenated amorphous silicon (a-Si:H). It is demonstrated that the hydrogen plasma treatment of a-Si:H film on the anode using a cathode covered by a-Si:H film, which is inevitably coated during the deposition period, gives rise to the deposition of μc-Si:H over the a-Si:H layer, i.e., chemical transport takes place. It is also found that the pure hydrogen plasma treatment using a clean cathode induces only etching of the a-Si:H layer. These results imply that the present hydrogen plasma condition does not cause crystallization of a-Si:H but only etching, and that careful experimentation is required to determine the real subsurface reaction due to atomic hydrogen.

Role of Hydrogen Plasma during Growth of Hydrogenated Microcrystalline Silicon : In Situ UV-Visible and Infrared Ellipsometry Study

We have applied in situ UV-visible and infrared phase-modulated ellipsometry to investigate the role of hydrogen plasma during the growth of hydrogenated microcrystalline silicon (μc-Si:H) by plasma-enhanced chemical vapor deposition (PECVD). The results of the deposition of μc-Si:H from the SiH 2 highly diluted in H 2 , layer-by-layer (LbL) technique and post-hydrogenation experiments showed that the 3-dimensional cross-linking and relaxation of a Si network near the growing surface were essential for the formation of microcrystalline silicon. The major role of hydrogen plasma is the creation of the free volumes on the growing surface due to the inhomogeneous etching of the Si network and the promotion of the cross-linking reactions

Preparation of Polycrystalline Silicon by Hydrogen-Radical-Enhanced Chemical Vapor Deposition

The preparation of both amorphous and epitaxial crystalline silicon films by Hydrogen-Radical-Enhanced Chemical Vapor Deposition at variable hydrogen flow rates is discussed. The feasibility of fabricating polycrystalline Si at high growth rates and a low substrate temperature is demonstrated. Finally, the n-type characteristics of PH3 doping and p-type characteristic for BF3 doping are examined in terms of the conductivity and the Hall mobility of the films.

Discussion on the mechanism of photodoping

Abstract The photo-enhanced reaction between metallic silver and vitreous chalcogenides is known as “photodoping”. Based on a series of experimental results, a model for photodoping was proposed. It was assumed in this model, that a junction barrier at the silver-chalcogenide interface worked for separating photocarriers. Holes are captured by metallic silver, and electrons are trapped by active or loosely bound chalcogen atoms after travel toward the interior of a glass layer. The Coulomb attraction field between ions thus formed is large enough to overcome the kinetic barrier in the process of silver diffusion. The square root dependence for the growth of the photodoped depth with exposure time has been explained in the light of this proposed model.

In situ real time studies of the formation of polycrystalline silicon films on glass grown by a layer‐by‐layer technique

The effect of atomic hydrogen on thin deposited layers of amorphous silicon was studied. Amorphous silicon layers less than 10 nm thick were first deposited from fluorinated precursors. These layers were then exposed to an atomic hydrogen flux. The amorphous layers quickly relaxed to a crystalline structure. Thick films of high crystalline content were prepared through sequential repetition of the deposition and hydrogen exposure process (layer‐by‐layer technique). The relaxation process was studied by real time in situ ellipsometry and infrared measurements. The relationship between substrate temperature, amorphous layer thickness, hydrogen exposure time, and structure was determined. A new model in which hydrogen acts to ‘‘liquify’’ the subsurface region by breaking Si–Si bonds is suggested. From the ‘‘liquidlike’’ state the subsurface relaxes to its most thermodynamically stable constituents; during relaxation, crystalline silicon is formed with effluence of SiH4, SiFxH4−x, and SiF4 vapors.

Growth of Amorphous and Crystalline Silicon by HR-CVD (Hydrogen Radical Enhanced CVD)

Systematic studies have been made on preparation of Si thin films from SiF 4 under control over the flow of atomic hydrogens. The gas phase reactions taking place in the mixture of fragments (SiFn) resulting from plasma-induced dissociation and atomic hydrogens were widely investigated by a mass spectroscopy. Chemically active species,i.e., SiF 2 H and SiH 2 F were found as those related to the growth of films. The growth in the vicinity of substrates involves either endothermic or radical-enhanced reaction for the propagation of the three dimensional Si networks, accompaning release of terminators such as H and F. Accordingly, Si thin films with structures from amorphous to crystalline were obtained by controlling the flow of atomic hydrogen. A marked improvement in the hole-transport was established in the Si films containing hydrogen less than 5–6 at % due to the reduction in the tail states near the valence band.