Nanosecond Laser Annealing of In-Situ Boron-Doped Ge Layers for Dopant Activation
Authors: Frauenrath Marvin , Acosta Alba Pablo , Papon Anne-Marie , Hartmann Jean-Michel 
Published: November 2022

Doped Ge can be employed in the sources and drains of Ge pMOS transistors 1 or as p-type films in Ge p-i-n Photo-Detectors (PDs) 2. If the growth temperature is low enough, they can even be used in GeSn PDs and Light Emitting Devices 3. Those Short and Mid Wavelength Infra-Red devices are rapidly gaining interest especially since the demonstration of electrically pumped lasing in GeSn at temperatures up to 100K by the University of Arkansas 4.

The electrically active carrier concentration in binary Si:P was significantly increased thanks to a dissolution with nanosecond laser annealing (NLA) of P clusters 5. Ge:B alloys with metastable, ultra-high substitutional concentrations of B can also be obtained thanks to in-situ doping 6. However, electrical activation in such Ge:B binaries was limited by the formation of electrically inactive clusters 7. In this work, we evaluated whether or not they could be dissolved by NLA.

The melt threshold shifted from 0.875 Jcm-2 to 0.85 Jcm-2 as the substitutional B concentration increased (from Time Resolved Reflectivity maps during Ge:B NLA; Figure 1 (a)). This was likely due to differences in terms of crystalline quality, surface roughness and/or B concentration itself.

Figure 1 (b) shows normalized ω-2ϴ scans around the (0 0 4) X-Ray Diffraction order. The Ge:B peak did not significantly change below the melt threshold at 0.825 Jcm-2. Above it, the Ge:B peak became less and less intense and vanished at the full melt threshold (1.05 Jcm-2). NLA at even higher Energy Densities (ED, 2.00 Jcm-2) did not result in a recovery of the Ge:B peak.

At the melt threshold, localized surface structures melted, as shown by Atomic Force Microscopy in Figure 1 (c) with a 30 nm z-scale. Surface structures were rectangular with sides along the <110> directions, as for the cross-hatch of the as-grown layer. The size of all surface structures was quite constant. Comparable surface structures were previously seen for SiGe 8.

A High Resolution Transmission Electron Microscopy image of the Ge:B layer after NLA with an ED of 0.85 Jcm-2 is shown in Figure 1 (d). The Ge:B layer had a thickness of 33 nm, which was slightly lower than that of the as-grown layer, i.e. 39 nm. It might be that material agglomerated in the 12 nm high surface structures 8. Surface structures otherwise had a brighter contrast, which might be due to the accumulation of lighter B atoms in those surface structures. Well defined Fast Fourier Transform features outlined the somewhat good crystalline quality of NLA Ge:B. However, they were less bright than for a superior quality epitaxial layer.

Laser annealing resulted in a redistribution of B, with the formation of electrically inactive clusters that did not contribute to strain. Accordingly, the sheet resistance increased by 70%, from 39.82 Ω/□ up to 68.62 Ω/□, when the layer melted. This corresponded to an electrically active carrier loss of around 50%, from 8.1x1020 cm-3 down to 3.8x1020cm-3, shown in Figure 1 (e). This behavior was independent of the as-grown substitutional B concentration. Even multiple shots with various energy densities at the same position, shown in Figure 1 (f), were not able to improve electrical activation. However, there was some slight improvements of the sheet resistance in the sub melt regime, which would need to be confirmed in future experiments.

We have seen above that single shot NLA resulted in the formation of (i) surface structures becoming larger for high EDs and (ii) electrically inactive clusters not contributing to strain. With the use of multiple shots, surface structures merged and formed even larger surface structures. This led to more B redistribution and likely to the formation of more electrically inactive boron-interstitial clusters in the melt regime. In the sub melt regime, multi shot NLA might improve contact resistance. NLA in the melt regime therefore seemed not to be able to improve electrical activation in heavily in-situ boron-doped Ge layers as it did for heavily in-situ phosphorous doped Si.

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Figure 1