Post-Growth Laser Annealing for High-Performance Ge Photodiodes on Si
Authors: Kawamata Yuta , Nagatomo Sho , Fukuda Kotaro , Izawa Yusaku , Hoshino Satohiko , Ishikawa Yasuhiko 
Published: February 2020

Ge epitaxial layers on Si have been applied to near-infrared (NIR) photodiodes (PDs) in Si photonics. Post-growth annealing at high temperatures (> ~800°C) effectively reduces the density of threading dislocations generated due to the 4% lattice mismatch between Ge and Si, leading to high-performance Ge PDs with a reduced dark current [1]. However, such a high-temperature annealing would degrade pre-processed Si devices, being an obstacle to incorporate Ge into Si CMOS process, particularly into the back end of line (BEOL). In this paper, an NIR laser annealing is proposed for selective post-growth annealing of Ge layers. Si is transparent in the NIR range with the wavelength λ above ~1.0 µm, while Ge shows a strong optical absorption up to λ ~ 1.6 µm. This indicates that Ge is selectively heated by irradiating NIR light (1.0 - 1.6 µm). In fact, Ge pin PDs on Si, fabricated using Ge layers after the irradiation of NIR laser, show a significant reduction of dark current without degradation of photodetection efficiencies.
In experiments, ultrahigh-vacuum chemical vapor deposition (600°C) was used for the epitaxial growth of Ge (600 nm) on p+-Si(001) wafer, followed by the growth of thin Si cap (70 nm). Then, the samples were irradiated with a scanned CW laser light (λ = 1.07 µm, 1 mm in diameter) in the air ambience. The penetration depths are ~0.8 µm for Ge and ~1 mm for Si, respectively, suggesting the selective annealing of Ge. The laser power was typically 63 W, and the duration of irradiation was typically 12 msec. The irradiation was repeated up to 10 cycles. In order to form pin structure, phosphorous ions were implanted as n-type dopants in the Si cap and the top region of Ge, followed by an activation annealing at 600°C in a furnace. Finally, Al electrodes were formed.
Figure 1 shows typical current-voltage (I-V) curves obtained at room temperature under dark. The dark current was reduced by the irradiation of laser light: at the reverse bias of 1 V, the current density was 51.3±0.7 mA/cm2 for the as-grown Ge, while those for the laser-irradiated (63 W, 12 msec) Ge were reduced to be 23.7±1.4 mA/cm2 for the repetition of 3 cycles and to 20.4±1.7 mA/cm2 for the increased repetition of 10 cycles. The reduced dark current is ascribed to the reduction of dislocation density due to the annealing induced by the laser irradiation. The obtained values are comparable to that for Ge annealed at 800°C in a furnace. Further reduction of dark current would be achieved by optimizing the condition.
Free-space responsivity spectra are shown in Fig. 2. All the spectra shifted towards longer wavelengths in comparison with theoretical one for unstrained Ge, reflecting the narrowing of direct gap induced by the tensile strain in Ge, which is generated due to the thermal expansion mismatch between Ge and Si [2]. For λ < 1.55 µm, the responsivity is as large as 0.15 - 0.22 A/W for the laser-annealed samples, which is comparable to the unannealed one, while for λ > 1.55 µm, the responsivity was slightly enhanced by the annealing. This probably results from the increase of tensile strain in Ge after the laser annealing, where Ge experiences higher temperatures than that in the growth (600°C).
In summary, Ge pin PDs on Si, fabricated after the NIR laser annealing, showed a significant reduction of dark current without degradation of photodetection efficiencies, being a promising way to incorporate Ge into BEOL.
[1] H. -C. Luan et al., Appl. Phys. Lett. 75, 2909 (1999). [2] Y. Ishikawa et al., Appl. Phys. Lett. 82, 2044 (2003).