Grain Size Control by Means of Solid Phase Crystallization of Amorphous Silicon
- PDF / 139,245 Bytes
- 6 Pages / 612 x 792 pts (letter) Page_size
- 26 Downloads / 210 Views
0989-A06-17
Grain Size Control by Means of Solid Phase Crystallization of Amorphous Silicon Jordi Farjas1, Pere Roura1, and Pere Roca i Cabarrocas2 1 GRMT, Physics department, University of Girona, Campus Montilivi, Girona, E-17071, Spain 2 LPICM (UMR 7647 CNRS), Ecole Polytechnique, Palaiseau Cedex, F-91128, France ABSTRACT The grain size of thermally crystallized a-Si films is controlled by the rates of nucleation, rN, and growth, rG, according to the standard Avrami theory. Despite this evidence, most papers devoted to improving the crystallized grain size analyze their results with a qualitative reference to this theory. In this paper, we will show that it is possible to identify the standard set of rN and rG values for a-Si and that experiments reveal that deviations from these values always result in a smaller grain size. It is also shown that no substantial improvement with non-conventional heat treatments can be expected. Finally, it is argued that a larger grain size is expected from a-Si films containing, in their as-grown state, a controlled density of embedded nanocrystals. I. INTRODUCTION Solid phase crystallization (SPC) of amorphous silicon (a-Si) thin films results in a material with improved transport properties and stability against Staebler-Wronski effect [1]. The present shortage in the worldwide production of silicon [2] makes the crystallized thin films an interesting alternative to monocrystalline or multicrystalline solar cells [3]. On the one hand, it takes advantage of the much lower material consumption, characteristic of thin film technology, and on the other, it has the stability of crystalline silicon. The main drawback of crystallized thin films comes from the deleterious effect of grain boundaries on the minority carrier lifetime which results in a lower photovoltaic conversion efficiency [4]. Although extensive research has been carried out for more than twenty years, no significant improvement has been achieved. In this paper, we first review a number of significant papers that allow us to identify the ëstandardí value of the kinetic parameters governing the SPC process. We will show that all experiments indicate that the observed deviations from the standard values never result in a larger grain size. Section III is devoted to analyzing if the final grain size can be improved after thermal treatments other than the conventional single isothermal annealing. Finally, in Section IV it is shown that it is possible to obtain a larger grain size through the crystallization of a-Si layers containing silicon nanocrystals embedded in the amorphous matrix. II. REVIEW OF THE LITERATURE The standard kinetic parameters and grain size The crystallization of a-Si begins with the spontaneous formation of nanometric ordered regions: the nuclei. They are usually revealed by TEM. The incubation time, t0, and the nucleation rate, rN are obtained from the density of nuclei as a function of the annealing time.
800
Temperature (∫C)
750
700
650
600
550
-1
800
750
Temperature (∫C)
700
650
6
