R. I. Johnson

LASER DEHYDROGENATION/CRYSTALLIZATION OF PLASMA-ENHANCED CHEMICAL VAPOR DEPOSITED AMORPHOUS SILICON FOR HYBRID THIN FILM TRANSISTORS

A low temperature process for laser dehydrogenation and crystallization of hydrogenated amorphous silicon (a‐Si:H) has been developed. This process removes hydrogen by laser irradiations at three energy steps. Studies of hydrogen out‐diffusion and microstructure show that hydrogen out‐diffusion depends strongly on film structure and the laser energy density. Both high quality and low leakage bottom gate polycrystalline silicon and a‐Si:H thin film transistors were monolithically fabricated on the same Corning 7059 glass substrate with a maximum process temperature of only 350 °C.

Preparation of oriented Bi-Ca-Sr-Cu-O thin films using pulsed laser deposition

Oriented c‐axis thin films of Bi‐Ca‐Sr‐Cu‐O on [100] SrTiO3 substrates have been fabricated using the pulsed excimer laser evaporation technique. Deposition at room temperature in 1 mTorr oxygen followed by an 875 °C anneal in oxygen yields superconducting films with zero resistance at 80 K and a resistivity drop near 110 K, hinting at the presence of another superconducting phase. Transmission electron microscopy shows that the films are epitaxial with the substrate, with an abrupt and planar interface boundary. The observed crystal structure is consistent with diffraction results on bulk materials.

Excimer‐laser‐induced crystallization of hydrogenated amorphous silicon

The electronic transport properties and structural morphology of fast‐pulse excimer‐laser‐ crystallized hydrogenated amorphous silicon (a‐Si:H) thin films have been measured. The room‐temperature dark dc conductivities and Hall mobilities increase by several orders of magnitude at well‐defined laser energy density thresholds which decrease as the impurity concentration in the films increases. The structural morphology of the films suggests an impurity‐induced reduction of the a‐Si:H melt temperature as the origin of this behavior.

GRAIN GROWTH IN LASER DEHYDROGENATED AND CRYSTALLIZED POLYCRYSTALLINE SILICON FOR THIN FILM TRANSISTORS

Selective dehydrogenation and crystallization are realized by a three‐step incremental increase in laser energy density. X‐ray diffraction and transmission electron microscopy show that the polycrystalline grains formed with this three‐step process are similar to those after a conventional one‐step laser crystallization of unhydrogenated amorphous silicon. The grain size increases with increasing laser energy density up to a peak value of a few micrometers. The grain size decreases with further increases in laser energy density. The transistor field effect mobility is correlated to the material properties, increasing gradually with laser energy density until reaching its maximum value. Thereafter, the transistors suffer from leakage through the gate insulators. A dual dielectric gate insulator has been developed for these bottom‐gate thin film transistors. Our structure simplifies fabrication of both high quality amorphous and polycrystalline thin film transistors on the same glass substrate. We discuss t...

Critical Laser Fluence Observed in (111) Texture, Grain Size and Mobility of Laser Crystallized Amorphous Silicon

This paper describes new results on the relationship between the grain size, mobility, and Si (111) x-ray peak intensity of laser crystallized amorphous silicon as a function of the laser fluence, shot density, substrate temperature, and film thickness. These observations include an unexpected narrow peak found in the silicon (111) x- ray peak intensity, which occurs at a specific laser fluence for a given film thickness and substrate temperature. Amorphous silicon materials processed at laser energy densities defined by this peak exhibit exceptionally large grain sizes and electron mobilities that cannot be obtained at any other energy and shot density combination above or below the energy at which the Si (111) x-ray peak intensity maximum occurs.

Characterization of the Substrate Interface of Excimer Laser Crystallized Polycrystalline Silicon Thin Films

Excimer laser crystallized Si thin films on fused silica substrates exhibit a peak in the average grain size as a function of laser energy density. The average grain size increases with increasing laser fluence until a maximum value , approximately 10 microns for a 100 nm thick Si film, is achieved. The peak in grain size is accompanied by a peak in the electron Hall mobility. Further increases in the laser fluence result in a decrease in the Si grain size and an increase in the intragranular defects. A small energy range of 40 mJ/cm 2 exists in which this peak in grain size can be achieved. Cross section TEM has shown that when the peak laser fluence is exceeded, the fused silica substrate can be as rough as 17 nm. Atomic force microscopy. performed on the substrate surface after the Si has been etched off, also shows that the magnitude and spatial frequency of the roughness increases when the critical laser fluence is exceeded. This degradation of the interface may also produce sites for stacking faults to form during the solidification of the Si. This result and results of simulations of the temperature of the interface during crystallization suggests that the peak energy range exists after the complete melting of the Si thin film and before the silica substrate starts to soften.

Laser Dehydrogenation of PECVD Amorphous Silicon

A low temperature process for laser dehydrogenation and crystallization of hydrogenated amorphous silicon has been studied. The key feature of this process is the removal of hydrogen from the amorphous silicon thin films while crystallizing the films at the same time. Studies of transient phenomena, hydrogen loss, and crystallinity, using transient reflectivity analyses, transmission electron microscopy and quadrupole mass spectrometry, find that hydrogen out-diffusion depends strongly on film structure and the melt duration controlled by the laser energy density. Utilizing this process, for which the maximum temperature is 350 °C, both high quality polycrystalline and amorphous silicon TFTs have been fabricated on the same Corning 7059 glass substrate.

Pulsed Laser Crystallization of Amorphous Silicon Films: Effects of Substrate Temperature and Laser Shot Density

A recent report on pulsed laser crystallization of a-Si thin films concluded that substrate bias temperatures up to 400°C in combination with laser fluences below 500 mJ/cm 2 had little effect on grain size and transport properties. The current report describes the effects of substrate bias temperature up to 500°C and laser fluence up to 540 mJ/cm 2 on grain size, mobility and Si (111) x-ray peak intensities. Results indicate that substrate bias temperatures above 400°C, in combination with high laser shot densities and large laser beam spot energies (> 500 mJ/cm 2 ), are a factor in Improving these film properties.

Excimer Laser Crystallized Amorphous Silicon Films: Effects of Shot Density and Substrate Temperature

Laser crystallization of a-Si thin films has been shown to produce materials with enhanced electrical properties and devices that are faster and capable of carrying higher currents. The quality of these polycrystalline films depends on a number of parameters such as laser energy density, shot density, substrate temperature, and the quality of the starting material. We find that the average grain size and transport properties of laser crystallized amorphous silicon films increase substantially with laser energy density, increase only slightly with laser shot density, and are unaffected by substrate temperatures of up to 400°C. The best films are those processed in vacuum but films of fair quality can also be obtained in air and nitrogen atmospheres.

Effect of thermal history on the transport properties and the location of Fe in YBa2Cu3O7 thin films

Epitaxial Fe‐substituted YBa2Cu3O7 films on LaAlO3 substrates prepared by pulsed‐laser deposition exhibit a large variation in Tc with changes in the cooldown rate of the films in an O2 environment immediately after deposition. A rapid cooldown rate (200 °C/min) yields surprisingly high‐Tc films with Tc≳75 K for 13% of the Cu replaced by Fe, while a slow cooldown of 10 °C/min results in low‐Tc films with Tc→0 at 11% Fe, a greater suppression than reported for bulk materials. In addition, post‐deposition anneals in O2 affect Tc significantly, even for anneal temperatures as low as 370 °C. These results are interpreted in terms of isolated‐ and clustered‐Fe atoms substituted on the Cu chains sites.