J. Stoemenos

Excimer laser annealing of amorphous and solid-phase-crystallized silicon films

Very thin (25–50-nm-thick) amorphous silicon (a-Si) films were crystallized into polycrystalline silicon (polysilicon) films by the combination of low temperature solid phase crystallization (SPC) and subsequent excimer laser annealing (ELA). These films are, then, subjected to a standard low temperature process (<600 °C) of thin film transistor (TFT) fabrication. The performance of resultant TFTs was compared to those fabricated on polysilicon films obtained by simple excimer laser annealing of amorphous silicon films. The electrical characteristics of the TFTs were correlated with the structural characteristics of the polysilicon films, using transmission electron microscopy and x-ray diffraction as analytical tools. The polysilicon films obtained by the SPC process consist of large and heavily defected crystalline grains. These defects, however, could be eliminated by melting and solidifying the polysilicon films during the ELA process. As a result, the electrical properties of the 50-nm-thick polysili...

Amorphization and recrystallization of 6H‐SiC by ion‐beam irradiation

Amorphization of 6H‐SiC with 200 keV Ge+ ions at room temperature and subsequent ion‐beam‐induced epitaxial crystallization (IBIEC) with 300 keV Si+ ions at 480 °C have been studied by Rutherford backscattering spectrometry/channeling and transmission electron microscopy analysis. Experimental results on amorphous layer thicknesses have been compared with trim calculations in association with the critical energy density model. Density changes during amorphization have been observed by step height measurements. Particular attention has been directed to the crystal quality and a possible polytype transformation during the IBIEC regrowth. The IBIEC process consists of two stages and results in a multilayer structure. In the initial phase an epitaxial growth of 6H‐SiC has been obtained. With increasing IBIEC dose the epitaxial growth changes to columnar growth and is stopped by polycrystallization of 3C polytype in the near‐surface region.

Mode of growth and microstructure of polycrystalline silicon obtained by solid‐phase crystallization of an amorphous silicon film

The structure and the morphology of crystallized amorphous silicon (α‐Si) films which were deposited on glass and annealed in a conventional furnace or by rapid thermal process (RTP) are studied using transmission electron microscopy (TEM). The ellipsoidal shape of the grains is attributed to the fast solid‐state crystallization along the two mutually perpendicular 〈112〉 and 〈110〉 crystallographic directions. The growth is solely based on the twin formation. The stability of the microtwins was studied by RTP and in situ TEM heating experiments. The effect of the film thickness on the preferred orientation of the grains is discussed. Very thin films exhibit (111) preferred orientation due to the strongly anisotropic rate of growth of the nuclei, which imposes an orientation filtering due to a growth velocity competition. The mode of growth of these films is compared with poly‐Si films grown by low‐pressure chemical‐vapor deposition.

Microstructure of silicon implanted with high dose oxygen ions

Buried implanted oxide layers have been formed by high dose implantation of oxygen ions (3×1018 ions cm−2) into 〈100〉 silicon wafers, at a constant temperature of 500 °C. The implanted layers were studied by cross‐sectional transmission electron microscopy and secondary ion mass spectroscopy. The defects at both the top Si/SiO2 and the SiO2/bulk Si interfaces are shown to be SiO2 precipitates. The precipitates are unstable and can be eliminated by heat treatment, and a homogeneous top silicon layer with a low density of dislocations can be obtained.

In situ observation of nickel metal-induced lateral crystallization of amorphous silicon thin films

The lateral crystallization of amorphous silicon thin films induced by nickel was studied in detail, performing in situ annealing experiments with a transmission electron microscope. The nickel-induced crystallization starts with the fast growth of thin needle-like crystallites of [110] orientation, which advance along the 〈111〉 directions within the film plane. The fast growth rate and the small probability of the crystallite exhibiting the [110] orientation result in large crystalline grains. These grains are, however, composed of many small misorientated subgrains. It is thought that this is because the needle-like crystallite does not grow continuously but grows by successive jumps. Our model is that after the nickel disilicide precipitate grows a thin crystalline slice epitaxially at the leading edge of the needle-like crystallite, the nickel moves to the new leading edge and forms the new nickel disilicide precipitates to maintain the needle-like crystalline growth.

Structural studies of low-temperature low-pressure chemical deposited polycrystalline silicon

Structural studies were performed on Si films formed by chemical‐vapor deposition on SiO2 films using silane under pressures below 150 mTorr and temperatures below 640 °C. The mode of growth of these films was found pressure dependent. Films grown at pressures below 10 mTorr were found to have a columnar structure with a (001) preferred orientation ending at a curved surface. The radius of the crystallites increases and the radius of curvature of their free surface decreases as the pressure decreases, while the converse is true for the temperature dependence. Transition from this mode is associated with diminishing of the capillarity effects. For pressures above 10 mTorr the structure is striated with a 〈111〉 twin texture almost perpendicular to the substrate. At pressures above 100 mTorr the structure is similar to the previous one but with a tufty appearance. This structure is associated with compressive and dilatational strain. The size of initial crystallites was found also pressure dependent increasi...

Effect of excimer laser annealing on the structural and electrical properties of polycrystalline silicon thin-film transistors

The structural and electrical properties of excimer laser annealed polycrystalline silicon thin-film transistors (polysilicon TFTs) are investigated in relation to the laser energy density. The devices were fabricated on 50 nm thick polysilicon films prepared by excimer laser crystallization (ELA) of amorphous silicon or by a combined solid phase crystallization (SPC) and ELA process. The structural properties of the polysilicon films have been investigated by transmission electron microscopy analysis. The effective density of states distributions in the polysilicon films and in the oxide traps near the oxide/polysilicon interface have been determined from low frequency noise measurements. The TFT performance parameters are compared with respect to their correlation with the structural properties of the polysilicon films and their electrically active defects, the basic variables being the starting material (amorphous silicon or SPC polysilicon) and the laser energy density.

New conditions for synthesizing SOI structures by high dose oxygen implantation

Abstract High doses (1.5x1018 ions cm−2) of oxygen ions were implanted at 200 keV into a silicon wafer kept at 700°C throughout the implantation. After annealing at 1150°C for 2 h the top silicon layer contains dislocations with a density of about 5x108 cm−2 and a large density of SiO2 precipitates. Annealing at 1300°C for 6 h dissolves all the precipitates and leads to a 310 nm thick silicon layer free of precipitates and with a density of dislocation of about 107cm−2. The SiO2 layer is 320 nm thick and the Si/SiO2 interfaces are very sharp.

Effect of pressure on the growth of crystallites of low-pressure chemical-vapor-deposited polycrystalline silicon films and the effective electron mobility under high normal field in thin-film transistors

The morphology of polycrystalline films grown by low‐pressure chemical‐vapor deposition (LPCVD) is investigated by transmission electron microscopy (TEM) as a function of the film thickness, the deposition pressure, and the level of contamination. An orientation filtering mechanism, due to the growth‐velocity competition in the early stage of growth, is responsible for the preferred orientation of the films. The size of the crystallites, the surface roughness, and the type of the structural defects are investigated by combined cross‐sectional and plane‐view TEM analysis. In polycrystalline silicon thin‐film transistors (TFTs), the influence of surface roughness scattering on the mobility is investigated by measuring the effective electron mobility under high effective normal field at 295 and 77 K. Although the surface curvature is increased when the deposition pressure is decreased, the surface roughness scattering is constant in the deposition pressure range from 40 to 0.5 mTorr. By decreasing the deposi...

Achievements and limitations in optimized GaAs films grown on Si by molecular‐beam epitaxy

A systematic study of the growth of high‐quality films of GaAs on Si substrates has been performed for applications in devices, particularly in optoelectronic devices for cointegration in optical interconnects. The effort for optimized active layers was approached through the separate optimization of substrate preparation, growth time parameters, and postgrowth treatment. In particular, the study of growth involved the investigation of the effect of silicon substrate orientation, post‐growth treatment, as well as multilayer and, especially, silicon buffer layers. For quantification of film quality, a number of characterization methods were used both in situ: reflected high‐energy electron diffraction (RHEED); and ex situ: optical, electrical [current versus voltage (I‐V), capacitance versus voltage (C‐V), deep‐level transient spectroscopy (DLTS), Hall], transmission electron microscopy (TEM), scanning electron microscopy (SEM), electron channeling patterns, x‐ray double‐crystal diffractometry (DDX). Schot...