J. Hanna

Control of nucleation and growth in the preparation of crystals by plasma-enhanced chemical vapour deposition

Abstract Both microcrystalline silicon (μc-Si) and epitaxial silicon (epi-Si) have been prepared by plasma-enhanced chemical vapour deposition from SiF4 with the assistance of atomie hydrogen. Precursors of the form SiF n H m (n + m = 3) were made as a result of successive reduction of fluorinated fragments SiF n (n≤3) with atomie hydrogen in the gas phase. Either μe-Si or epi-Si was grown at a deposition rate of 10 a s−1 or more from these precursors by selecting conditions of atomie hydrogen flow rate and substrate temperature Ts. The appropriate condition for growing crystals with large grains including epi-Si is distinguished from that for making μc-Si where the formation of nuclei is dominant. An attempt was made to control the surface reactions by adding the species SiH n and atomie hydrogen for the purpose of modulating the silicon network.

Structural and electrical properties of poly-SiGe thin films prepared by reactive thermal CVD

Abstract Polycrystalline thin films of Si × Ge (0.95≥×≥0.05, poly-SiGe) were prepared on SiO 2 and glass substrates by thermal chemical vapor deposition (CVD) from Si 2 H 6 and GeF 4 at 450°C. The Ge-rich films had a high crystallinity owing to the direct nucleation of crystallites on the substrates. The crystallinity of Si-rich films, however, depended on the growth rate, and the direct nucleation was achieved at smaller growth rates, leading to improvement of the crystallinity. All the films were p-type regardless of film composition, whose carrier concentration decreased from 10 18 cm −3 to 10 12 cm −3 , and the Hall mobility from 100 cm 2 /Vs to 1 cm 2 /Vs with an increase in the Si content in the films. The Hall mobility was improved up to 8 cm 2 /Vs in the film prepared at a growth rate of 3.6 nm/min.

High crystallinity poly-SixGe1-x at 450°C on amorphous substrates

The composition variation and strutural properties of poly-SiGe thin films prepared by Reactive Thermal chemical vapor deposition (CVD) with Si2H6 and GeF4 were investigated. Deposition of the films at 450° C was carried out with various gas flow ratios of Si2H6 to GeF4 on amorphous substrates such as glass plates and oxidized silicon wafers. The structural profiles of films were characterized by X-ray diffraction (XRD) and Raman scattering spectroscopies, scanning electron microscopy (SEM) and transmission electron microscopy (TEM). All films show high crystallinity and (220) prefered orientation. The mole fractions of Si in the Six Ge1-x films were estimated to be from 0.95 to 0.05 for x, using Vegards law for the XRD peaks. Crystallinity in the Si-rich films was affected by growth rate and was greatly improved at lower growth rates. TEM observation revealed that high crystallinity was well established, even in poly- Si0.95Ge0.05 film, owing to direct polycrystalline layer growth on the substrate surface.

Low temperature poly-SixGe1−x deposited by reactive thermal CVD for thin film transistor application

Abstract We have prepared a highly silicon-rich polycrystalline silicon–germanium (poly-SixGe1−x, x=0.98) film at 450 °C by the reactive thermal chemical vapor deposition (RTCVD) from a gaseous mixture of disilane (Si2H6) and germanium tetrafluoride (GeF4) under He dilution. We fabricated inverted-staggered bottom-gate n-channel thin-film transistors (TFTs) with films of 200 nm thickness deposited directly on SiO2/Si substrates. The TFT characteristics are influenced by the grain size of the poly-SixGe1−x films and carrier mobility was increased significantly after hydrogenation. The fairly high mobility of 36 cm 2 / V s and a low threshold voltage of 1.8 V have been achieved in the TFT prepared with the film having the largest grain size of 100 nm. We discuss the effect of crystallinity and defects in the films in conjunction with the TFT characteristics.

Growth of amorphous, microcrystalline and epitaxial silicon at the same substrate temperature under control of atomic hydrogen

Abstract Investigations were made into deposition mechanisms in film preparation through reduction of SiF4 with atomic hydrogen (HR-CVD) and that through oxidation of SiH4 with F2 (spontaneous deposition) to clarify similarities and differences between these two methods. It was found that films can be changed in structure from amorphous to microcrystalline or further to two-dimensional epitaxial by controlling degrees of the oxidation or the reduction. It was also confirmed that ordering of the Si-network is accelerated mainly by supply of atomic hydrogen in the former method, while in the latter the structure is considerable changed by chemical reactions accelerated thermally near the surface.

Atomic hydrogen improves the rigidity of Si-network and tail states at the valence band in a-Si:H and a-Si:H(F) films

Abstract The Si-network structure and properties related to the the tail states at the valence band were investigated systematically in a-Si:H prepared under a concentration control of atomic hydrogen during the film growth. Raman and IR studies indicated that hydrogen contents and the fluctuation of the Si-networks were reduced with a contribution of atomic hydrogen, leading to the improvement of the tail states to such an extent that the analysis of transient photocurrents in the non-dispersive transport could be applied. The effect of atomic hydrogen on the propagation of Si-network may explain in terms of “chemical annealing”.

P- and n-type doping in spontaneous chemical deposition

We have investigated the substitutional doping and effects of the dopants on the film growth in a new preparation technique for Si thin films termed “Spontaneous Chemical deposition”, featuring spontaneous chemical reactions of SiH4 with F2 at the reduced pressure. It was found that boron and phosphorus were successfully incorporated in the film when PH3 and B2H6 were used as a dopant material, resulting in Fermi energy shifts up to 0.24eV for n-type films and 0.3eV for p-type films. The dopants affected the growth process even in the very small amount of the impurity level: in phosphorous doping, the propagation of Si-network was promoted remarkably with an increase in the dopant concentration; on the other hand, in the boron doping it was retarded very much, while this could be solved by promoting the surface reactions.

Material Design by Structural Modulation of Amorphous Semiconductors

A new concept in material design based on the structural modulation of amorphous semiconductors is described. Near-infrared sensitive photoconductors with high photoresponse are designed by employing stacked multilayers of a-Si:H(F)/a-SiGex:H(F) prepared by hydrogen radical enhanced chemical vapor deposition. Measurements of conductivity perpendicular to the layers have indicated that structural modulation suppresses dark conductivity without sacrificing photoconductivity. Studies of transient photocurrent suggest the presence of artificially-produced shallow trapping states in the modulated structure.