Understanding nucleation and growth of crystalline Si films on glass is of prime importance in order to tailor and optimize semiconductor properties for electronic devices such as solar cells. Commercial glass limits the maximum processing temperature of Si films to 600 C. Solid phase and laser crystallization, or a combination of both techniques, are thus primarily used to crystallize Si films on glass. While random nucleation and growth processes always result in the formation of a log± normal size distribution, control of nucleation sites allows one to determine the location of grain growth or even the crystallographic orientation of grains. Via sequential lateral solidification using a copper vapor laser, we obtain Si crystallites on glass of several tens of mm in length.

Nucleation and Growth of Crystalline Silicon Films on Glass for Solar Cells

We report the fabrication of amorphous indium-gallium-zinc-oxide (a-IGZO) thin-film transistors (TFTs) with a bottom-gate inverted-staggered structure with an etch stopper formed by a self-aligned process using back-side UV exposure. In addition to reduction in process complexity, the gate-to-source capacitance of an a-IGZO TFT is significantly reduced, resulting in fast TFT circuits. The fabricated TFT exhibits a field-effect mobility value of 42.59 cm2/V·s , a threshold voltage of 6.1 V, and a gate voltage swing of 374 mV/dec. An 11-stage ring oscillator made of TFTs shows a propagation delay time of 56 ns/stage at 25 V.

High-Performance Amorphous Indium–Gallium–Zinc–Oxide Thin-Film Transistor With a Self-Aligned Etch Stopper Patterned by Back-Side UV Exposure

Large two dimensional amorphous silicon image sensor arrays offer a new approach to electronic document input and x-ray imaging. The sensor array technology is now capable of image capture at greater than 10 frames/sec and with resolution of 200–400 spi. We describe our new high resolution imaging system, comprising a page-sized sensor array with nearly 3 million pixels, and the accompanying high speed read out and processing electronics. The key technological issues of pixel resolution, sensor fill factor, leakage currents and noise are reviewed. Measurements of a new array architecture are described, in which the sensor is formed as a single continuous film on top of the matrix addressing components.

Two Dimensional Amorphous Silicon Image Sensor Arrays

A thorough investigation of polysilicon films crystallized from amorphous silicon using a XeCl excimer laser has been completed. Emphasis was placed on the development of large crystals, good uniformity, and low surface roughness, with an eye towards improving throughput. This was accomplished through use of SiO2 barrier layers, multiple passes, and careful consideration of annealing ambient. Thermal modeling was performed to help interpret the observed trends and aid in optimization. By controlling these process parameters, samples were produced with a high degree of uniformity and two-dimensional structural ordering, with grains nearly identical in size and stacked in rows and columns. However, a biproduct of the ordering was the creation of large topographical features at the intersection points of multiple grain boundaries, which lead to surface roughness in the range of 12 nm. By applying more shots to the same area, films with maximum grain sizes on the order of 3 μm and reduced surface roughness (6...

A systematic study and optimization of parameters affecting grain size and surface roughness in excimer laser annealed polysilicon thin films

We report the observation of crystallization and simultaneous formation of surface microstructures in hydrogenated amorphous silicon (a-Si:H) thin films as one step laser processing. Light trapping microstructures of around 300 nm in height were formed on a-Si:H films of thickness in the range of 1.5 μm to 2 μm deposited on soda lime glass after exposure to femtosecond laser pulses. Scanning electron microscope (SEM) images show the formation of spikes that are around 1 μm part and their heights could be controlled by the laser fluences. Atomic force microscope (AFM) images were taken to study the roughness created on the surface. The mean roughness of the textured surface increases with laser fluence at smaller power densities, and for power densities beyond 0.5 J/cm2 the film removal deteriorates the texturing. X-ray diffraction results indicate the formation of a nano-crystalline structure with (111) and (311) crystal orientation after the laser treatment. The observed black color and enhanced optical absorption in the near infrared region in laser treated films may be due to a combined effect of light trapping in the micro-structured silicon surface because of multiple total internal reflections, phase change in the film, possible defect sites induced after laser treatment and formation of SiOx. Demonstration of light trapping microstructures in thin a-Si:H films and simultaneous crystallization could provide new opportunities for optoelectronic devices.

Femtosecond-laser-induced-crystallization and simultaneous formation of light trapping microstructures in thin a-Si:H films

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

Grain growth in laser dehydrogenated and crystallized polycrystalline silicon for thin film transistors

Fast-pulse laser crystallization of amorphous silicon thin films on non-crystalline substrates provides a low-temperature process for generating polycrystalline silicon. This process can be augmented by including laser doping to reduce the number of process steps in the fabrication of thin-film polysilicon devices. We have studied the simultaneous laser crystallization and laser doping process, starting with amorphous silicon on fused silica substrates and using the gas immersion technique for the doping. n-type and p-type doping employed PF5 and BF3 gases, respectively. Films were characterized both structurally and electrically. The grain size increases with increasing laser energy density as the film becomes fully melted and reaches a peak value, similar to laser crystallization without doping. The dopant concentration increases with the number of laser shots and, with 100 shots, achieves a high dose with a low sheet resistance below 1000 ohms/square, appropriate for devices. The dopant profile extends to a depth comparable to the melt depth, beyond which it falls off to the background level. Therefore, the doping depth and concentration can be controlled with the laser parameters.

Laser Doping and Crystallization of Amorphous Silicon Thin Films

We studied the performance of amorphous silicon thin-film-transistor circuits, fabricated by a self-aligned process. The oscillation frequencies of ring oscillators built with self-aligned transistors are found to be consistently higher than the frequencies of similar circuits with traditional, non-self-aligned transistors. Performance improvements of up to 35% are typically observed. The performance improvement is consistent with the elimination of source/drain parasitic capacitance, usually observed in the traditional, non-self-aligned transistors. The output waveforms of these ring oscillators are compared to the simulation results and reasonable agreement is found.

P. Mei

Papers

Grain growth in laser dehydrogenated and crystallized polycrystalline silicon for thin film transistors

Laser Doping and Crystallization of Amorphous Silicon Thin Films

Performance study of self-aligned amorphous silicon thin-film transistor circuits