The Role of Hydrogen in Laser Crystallized Polycrystalline Silicon
- PDF / 83,232 Bytes
- 6 Pages / 612 x 792 pts (letter) Page_size
- 17 Downloads / 260 Views
M5.23.1
The Role of Hydrogen in Laser Crystallized Polycrystalline Silicon N. H. Nickel and K. Brendel Hahn-Meitner-Institut Berlin Kekuléstr. 5, D-12489 Berlin,Germany.
ABSTRACT Polycrystalline silicon produced by laser crystallization of hydrogenated amorphous silicon contains large amounts of residual hydrogen. This reservoir of hydrogen can be used to passivate additional grain boundary defects by annealing the specimens at low temperatures in vacuum. Information on hydrogen bonding is obtained from hydrogen diffusion measurements. Laser crystallization results in a pronounced increase of the hydrogen binding energy in the resulting poly-Si samples compared to the amorphous precursor material. Fully crystallized poly-Si contains H concentrations of up to 17 at.%.
INTRODUCTION Polycrystalline silicon (poly-Si) produced by laser crystallization of amorphous silicon is of great interest for device applications such as thin-film transistors and solar cells. Commonly, amorphous silicon with a low hydrogen concentration is used for laser crystallization because the presence of large amounts of H causes difficulties during the crystallization process. In order to obtain large crystalline grains a laser energy density exceeding the melt-through threshold must be applied. This, however, releases large amounts of hydrogen in a very short time and the a-Si film ablates. A low temperature furnace anneal for several hours can prevent the destruction of the a-Si film during laser crystallization [1]. However, a low temperature furnace anneal is not practical for the fabrication of hybrid devices where amorphous and polycrystalline silicon based devices are placed next to each other on the same substrate. Furthermore, lowering the H content using a conventional furnace anneal is a very time-consuming processing step. While a-Si with a low H content can be crystallized with a single laser pulse a major drawback of the resulting poly-Si is the fact that grain-boundary defects have to be passivated with hydrogen to obtain device-grade material. On the other hand, hydrogenated amorphous silicon typically contains 8 – 15 at.% hydrogen. To avoid explosive out-diffusion of H during laser crystallization and thus, destruction of the silicon film a dehydrogenation and crystallization procedure is employed [2,3]. This technique takes advantage of the fact that the amount of H evolving from the sample increases with increasing laser fluence. Laser crystallization starts at a fluence of EL ≈ 130 mJ/cm2 and ends at the desired final laser fluence by increasing EL in steps of about 20 to 40 mJ/cm2 depending on the hydrogen content of the aSi:H starting material and the film thickness. However, little is known about the dehydrogenation process. Secondary-ion-mass spectrometry (SIMS) measurements showed that the H concentration decreases homogeneously over the depth of thin specimens (d ≈ 50 nm) [2]. In addition, Raman backscattering measurements of the Si−H local vibrational modes revealed that the dissociation rate of isolated Si−H is en
Data Loading...
