TEM Investigation of the Microstructure in Laser-Crystallized Ge Films
- PDF / 2,089,729 Bytes
- 6 Pages / 417.6 x 639 pts Page_size
- 96 Downloads / 187 Views
439
TEM INVESTIGATION OF THE MICROSTRUCTURE IN LASER-CRYSTALLIZED Ge FILMS RONALD P. GALE, JOHN C. C. FAN, RALPH L. CHAPMAN, AND HERBERT J. Lincoln Laboratory, Massachusetts Institute of Technology Lexington, Massachusetts 02173
ZEIGER
ABSTRACT The crystallite morphology and defect structure of laser-crystallized Ge films have been characterized by transmission (TEM) and scanning (SEM) electron microscopy. The elongated crystallites present in these films have their long axis in the [001] direction. In regions where these crystallites are well aligned with each other, competitive lateral growth and (110) texture are observed. The defects most frequently found are J1111 microtwins. A growth model that accounts for the texture and defects is proposed.
INTRODUCTION We have previously reported [1] the crystallization of thin amorphous Ge films on fused-silica substrates (without surface relief structures) by scanning with the slit image of a cw Nd:YAG laser. The crystallized films contain well-aligned elongated crystallites with lateral dimensions up to 2-3 x 100 pm arranged in periodic structures with spacings of 40 to about 250 pm along the direction of laser motion. A quantitative theoretical model that predicts the periodic structures has been developed for the dynamics of the crystallization process [2], but the growth mechanism that produces the aligned crystallites is not well understood. Our model for scanned laser crystallization attributes the periodic structural features to the cyclic motion of the crystallization front. In brief, crystallization begins when the laser heats an amorphous region of the film to a critical temperature, Tc. Due to the resulting release of the latent heat of crystallization, the crystallization front initially moves forward at a velocity much greater than the laser scan rate. The front eventually outdistances the laser so far that its temperature falls below the critical value, and it abruptly comes to rest. When the laser moves far enough for the temperature at the new location of the front to reach the critical value, the cycle is repeated. By fitting this model to the observed 0 periodicity, a value of 700 C has been obtained for Tc. We refer to each cycle as a crystallization event; the structure produced by such an event is unaffected by another laser scan. The cyclic motion of the front has been confirmed experimentally by optical transmission measurements, and the growth velocity has been measured as about 200 cm/sec [3]. In this investigation we have used transmission (TEM) and scanning (SEM) electron microscopy to characterize the crystallite and defect morphologies of laser-crystallized Ge films. On the basis of the results we propose a model for grain growth in these films.
440 CRYSTALLITE AND DEFECT MORPHOLOGY Figure 1 is a transmission optical micrograph of a section of a crystallized Ge film, showing periodic features with 40 pm spacing. The wide lighter bands consist of elongated crystallites that fan out to produce a chevron-like pattern. Outside the center a
Data Loading...
