Optical Constants of Annealed a-Si:H from Transmittance at Normal Incidence
- PDF / 304,926 Bytes
- 6 Pages / 595 x 842 pts (A4) Page_size
- 58 Downloads / 162 Views
Optical Constants of Annealed a-Si:H from Transmittance at Normal Incidence Atsutoshi Doi and Yoshiyuki Matsumoto Department of Electrical Engineering, Faculty of Science and Engineering, Kinki University, Kowakae, Higashiosaka, Osaka 577-8502, Japan ABSTRACT We study changes in the optical constants of a-Si:H films caused by the thermal annealing involved in solid phase crystallization. The aim is to examine the growth mechanism, since changes in refractive index are most probably caused by a change in the network structure. The refractive index change was studied from interference fringes in transmitted light at normal incidence, and shows differing dependence on temperature in different thermal ranges. DSC measurement was also performed to examine changes in the network structure with temperature. Changes in optical and thermal properties induced by an increase of temperature reveal frequent network changes of a-Si:H below 470 ◦ C and of a-Si in the range 470 to 570 ◦ C. We also found crystallization at about 570 ◦ C, and grain growth above the crystallization temperature. Knowledge of network changes in a-Si film allows orientation control by an external seed. INTRODUCTION Solid phase crystallization of amorphous silicon films deposited on glass substrates is a potentially useful process that has received considerable attention [1,2]. However, little is known about network changes caused by the thermal annealing that takes place with crystallization. Changes in the network structure are most likely to change the refractive index. Accordingly, we investigate changes in the optical constants of annealed a-Si:H films with a view to understanding changes in crystallization of the mother network. Optical properties of a-Si:H films have been studied by ellipsometric measurement. Standard ellipsometry measures two relative properties of orthogonally polarized radiation. Determination of the three quantities, refractive index n, absorption coefficient α, and film thickness d, therefore requires a further independent measurement that may introduce additional errors as a result of any irregularity in the thickness. In our work, optical constants were obtained by analyzing interference fringes in transmitted light at normal incidence [3,4]. Thin a-Si:H films were deposited onto quartz substrate by PECVD. Measurements were made after isochronous annealing for 16 hours, at temperatures up to 1000 ◦ C. The refractive index depends differently on temperature in distinct ranges; the transition temperatures between these ranges are 240, 340, 450, 570, and 680 ◦ C. The transitions at the lowest three temperatures probably involve network changes in the a-Si:H film; the transition at 570 ◦ C is due to crystallization of a-Si; and the transition observed above the crystallization temperature is due to grain growth. Differential scanning calorimetry (DSC) is performed to examine changes in the network structure as the temperature varies. Our demonstration that external seeding can control crystalline orientation verifies the value of transmittance
Data Loading...
