Ultra-shallow junction formation by excimer laser annealing and low energy (<1 keV) B implantation: A two-dimensional analysis

Nuclear Instruments and Methods in Physics Research B 186 (2002) 401–408 www.elsevier.com/locate/nimb

Ultra-shallow junction formation by excimer laser annealing and low energy (530 mJ/cm2 . The present characteristics suggest that two different types of defects are involved: a first type of defects, responsible of the SRH G–R component, which tends to be reduced as the energy density increases; a second type of defects, responsible of the field enhanced component and likely related to the cavities, appearing at energies >530 mJ/cm2 . The results presented above indicate that in order to reduce the residual defect density following ELA it is fundamental to increase the melt depth above the a-Si thickness, thus removing the advantages of the ‘‘two-layer’’ approach. Indeed, considerable structural improvement is observed at high energy densities (Ed ¼ 730 mJ=cm2 ). In fact, by melting the strained region of the substrate, where some of the end-of-range defects are present, the re-growth can be initiated from a less defected region of the substrate. On the other hand, when inducing a deeper melt depth, microcavities appear, caused by F out-diffusion from the melted layer [4]. Concomitantly, trap states, likely associated to dangling bonds present on the surface of the micro-cavities, also appear, as marked by the carrier depletion observed in the SRP or by the field enhanced component of the diode reverse current, thus limiting the application of the ‘‘twolayer’’ approach. It should be pointed out that, although most of the micro-cavities are located to a depth around 30–50 nm (see Fig. 1(c)), the car-

rier depletion observed in the SRP carrier profiles is located around 100 nm from the surface (see Fig. 2). This suggests that the carrier trapping, induced by the micro-cavities, can extend over a wide region, thus producing major variations in the low concentration region of the carrier profile.

4. Junction formation by low energy B implantation and ELA Some of the above discussed problems can be overcome by combining low energy B-ions implantation (