Anomalous crystallization of hydrogenated amorphous silicon during fast heating ramps

Abstract

Thermal crystallization experiments carried out using calorimetry on several a-Si:H materials with different microstructures are reported. The samples were crystallized during heating ramps at constant heating rates up to 100 K/min. Under these conditions, crystallization takes place above 700°C and progressively deviates from the standard kinetics. In particular, two crystallization processes were detected in conventional a-Si:H, which reveal an enhancement of the crystallization rate. At 100 K/min, such enhancement is consistent with a diminution of the crystallization time by a factor of 7. In contrast, no systematic variation of the resulting grain size was observed. Similar behavior was also detected in polymorphous silicon and silicon nanoparticles, thus showing that it is characteristic of a variety of hydrogenated amorphous silicon materials.Crystallization of amorphous silicon (a-Si) is a subject that has been studied extensively during the last three decades. It improves the electrical conductivity substantially, 1 making the resulting polycrystalline silicon useful for a number of applications, such as thin film transistors (TFT), liquid crystal displays (LCD), and image sensors.2 The kinetics of this process follows the nucleation and growth steps with rate constants characterized by transmission electron microscopy. [3][4][5][6] The relative values of nucleation and growth rates determine the final grain size that can be controlled through the crystallization temperature or by ion irradiation during crystallization. 4 The temperature range in the published results seldom extends above 700°C because of the difficulty in reaching the isothermal conditions without any incipient crystallization taking place during the heating ramp. This difficulty is partially overcome by calorimetric experiments where the material is heated at a constant rate.From the heat that evolves the crystallization rate can be obtained. Kinetic studies using calorimetry have only been performed during the epitaxial crystallization of thin a-Si films obtained from rapid solidification.7 With the exception of our preliminary work, 8 no systematic studies on hydrogenated a-Si (a-Si:H) have been published. In this paper we extend the temperature range during the heating ramps above 700°C up to 100 K/min. Under these conditions, an unexpected and substantial enhancement of the crystallization rate was observed. Besides the intrinsic interest of characterizing a new phenomenon, our study can be useful to understand the crystallization that occurs during rapid thermal annealing (RTA) processes.

Calorimetry of dehydrogenation and dangling-bond recombination in several hydrogenated amorphous silicon materials

“…Although appealing, this explanation should be carefully complemented by independent crystallization experiments. However, our own experiments on pm-Si: H indicate that, at the usual heating rates of the EGA thermograms ͑Ͼ10 K / min͒, its crystallization kinetics is identical to that of conventional a-Si: H films 31,56 and no incipient crystallization related to the nanocrystals of its structure is detected.…”