The phenomenon of severe dopant loss during rapid thermal annealing of phosphorus-implanted germanium has been investigated. Dopant activation improves for temperatures above 500°C and reaches 100% activation for samples annealed at 600°C. However, a heavily defective junction with approximately 50% dopant loss is recorded. Although surface passivation of the implanted germanium using plasma-enhanced chemical vapor deposited silicon dioxide did not prevent the dose loss, it assisted in the achievement of defect-free, single-crystal germanium with improved electrical characteristics at a reduced thermal budget. Phosphorus introduced into germanium via solid-state diffusion from phosphosilicate glass did not exhibit dose loss upon rapid thermal annealing, suggesting that dose loss could be an effect of implant damage.The saturation of the silicon drain current upon dimension shrinkage could be a showstopper for the continued scaling of silicon metal-oxide-semiconductor field-effect transistors ͑MOSFETs͒. One method to overcome this limit is to introduce new substrate materials such as germanium, which has a higher carrier mobility compared to silicon, to allow for a larger drive current. Other attractive properties of germanium are the smaller mobility bandgap for supply voltage scaling and the smaller optical bandgap for the broadening of the absorption wavelength spectrum. A broader wavelength spectrum would allow optoelectronic integration to enhance complementary metal-oxide-semiconductor ͑CMOS͒ industry functionality. 1 In the past, the lack of knowledge of dopant incorporation in germanium has hindered the realization of germanium MOSFET structures. In a literature survey done by Jasper et al., 2 it was noted that some of the most recent publications are almost 15 years old. Well-activated n + /p source/drain junctions, in particular, are expected to pose a fabrication challenge because of the low solubility and fast diffusion of n-dopants in germanium. 3 Research works have reported that for near surface implants, a severe loss of dopant has been observed during the annealing process. 2,3 However, the understanding of this dopant loss phenomenon is still vague.In this article, we investigate the dopant loss phenomenon using phosphorus-implanted germanium, some of which has been passivated with plasma-enhanced chemical vapor deposited ͑PECVD͒ silicon dioxide. The main objective of this passivation is to understand the effectiveness of silicon dioxide in the prevention of phosphorus dose loss in germanium upon rapid thermal annealing ͑RTA͒.To understand the impact of implantation damage on the dopant loss mechanism during RTA, the phosphorus dopant profiles were compared with those of samples prepared by solid-state diffusion from phosphosilicate glass ͑PSG͒, which were free from implantationinduced damage. ExperimentalCzochralski-grown p-type ͑100͒ germanium wafers with starting resistivity of 3.9-5.1 ⍀ cm were used in this study. No special surface preparation or cleaning was done prior to implantation. The ...

Advantages of multiple-pulse laser annealing with a moderate energy fluence over a single-pulse annealing with a high energy fluence are demonstrated on the formation of shallow p ϩ /n junction. When the silicon surface is preamorphized, the multiple-pulse laser annealing with a fluence adjusted to a value which can melt the amorphous layer but not crystal silicon shows that the successive pulses do not increase junction depth further but decrease sheet resistance significantly. Under this condition, the junction depth is still controlled by the depth of the preamorphized layer. However, when the laser fluence is high enough to melt the crystal silicon, the successive pulses result in the deepening of junction depth. This is attributed to the increase of surface roughness by the successive pulses, thereby increasing the total absorbed energy.

The narrowing or broadening of the boron profile during annealing of laser-processed samples is observed to occur depending on which of two competing mechanisms, uphill diffusion of boron due to a highly defective single-crystal layer near the surface or transient-enhanced diffusion due to end-of-range defects, dominates during the post-laser processing anneal. The results show that uphill diffusion of boron is found to dominate during annealing of a single-pulse laser-processed sample because the defects near the surface cannot be efficiently removed with a single laser pulse adjusted to a value that can melt the amorphous silicon but not the underlying crystalline substrate. Junctions thus become shallower with the post-laser processing anneal. However, with successive laser pulses, the dopants are observed to move deeper into the silicon with subsequent rapid thermal annealing cycles. This could be due to the reduced contribution of uphill boron diffusion when the defects near the surface decrease with successive pulses. The end-of-range defects, which cannot be sufficiently annealed because the melt depth is not beyond the amorphous layer, thus play the key role in broadening the boron concentration profile for multiple-pulse laser-annealed silicon.

For preamorphized boron-implanted samples subjected to nonmelt laser spike annealing (LSA), increasing the LSA temperature at temperatures below 1250 °C results in negligible sheet resistance changes due to the formation of inactive boron-interstitial clusters (BICs). These clusters, which are evidenced as a kink in the boron profile beyond the amorphous/crystalline interface, result chiefly from the inadequate removal of end-of-range (EOR) defects. When the LSA temperature is elevated beyond 1250 °C, sheet resistance improvement takes place due to the increase in active boron dose from the dissolution of the BIC at higher temperatures. Cluster dissolution also gives rise to a supersaturation of silicon interstitials that deepen the junctions as a result of transient enhanced diffusion (TED). With an additional post-LSA treatment, severe deactivation, especially at lower LSA temperatures, and further TED is observed. Two concurrent mechanisms, namely, boron clustering (which gives rise to deactivation and sheet resistance degradation) and dissolution of the BIC (which gives rise to TED) formed during the LSA step, are believed to take place during the post-LSA thermal budget. As the LSA temperature increases, TED from the as-LSA profile upon rapid thermal annealing (RTA) is significantly reduced as a result of the improved effectiveness of the EOR defect dissolution during the higher temperature LSA step. When carbon co-implantation is performed, deactivation and TED is successfully suppressed with the reduction in free silicon interstitial concentration due to the formation of complexes of carbon and silicon interstitials. The amount of deactivation upon RTA becomes independent of LSA temperature for the carbon-implanted samples, largely because boron clustering becomes limited by the small concentration of free silicon interstitials present instead of the LSA temperatures used.

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