Future development

Both the doping concentrations of the charge sheet and multiplication layer have been found to be critical aspects of the structure design. Regarding the charge sheet region, it was very difficult to accurately control its doping concentration due to different problems in the heteroepitaxial growth apparatus. SIMS measurements showed a much higher doping concentration than the designed value, and this in turn led to a device which cannot be used to detect infrared light, as demonstrated by experimental measurements and simulations. Different solutions have been proposed to overcome this problem:

1. To balance the high doping concentration of this layer, its thickness should be reduced to have the same integrated charge as an identical layer with lower doping and greater thickness. However, only small adjustments can be made by reducing the thickness of the charge layer. It is not possible to use a doping concentration of the order of 1018 cm-3 because the thickness of the charge sheet layer will be reduced to less than 10 nm. This might cause further problems such as tunnelling through this thin layer which could also be changed to a conductive layer due to the high doping concentration. This might result in a structure unable to detect infrared light.

2. It could be possible to introduce an intrinsic spacer layer between the Si charge sheet and the Ge absorber layer to perform a step graded dopant diffusion to obtain the designed doping concentration of ~1.5 × 1017 cm-3. However, this

should be carefully calibrated during the growth and might add further complexity to the structure.

3. To better control the doping concentration of the charge sheet layer, ion implantation could be used. However, this process could possibly introduce further defects at the hetero-interface with the Ge absorber layer and hence this might enhance the threading dislocation density.

Regarding the multiplication layer, several efforts have been performed by the grower to reduce the doping diffusion tail. In particular, using an intrinsic silicon substrate instead of a highly doped substrate for future generations of Ge-on-Si SPAD device was suggested. Then, a highly doped Si layer, with a well-established thickness, was epitaxially grown on top of the intrinsic substrate followed by a 1.5 m-thick intrinsic multiplication layer. The wafer ID, for this test sample was 14-253. This approach demonstrated a reduction of the Si diffusion tail into the intrinsic Si layer (multiplication region), as shown by a SIMS measurement performed on a test sample (Figure 4.28).

Another solutions to this problem was proposed by IQE (a company based in UK

performing epitaxial growth) who suggested to use a highly antimony (Sb) doped (~1018

cm-3) Si substrate. A test sample was also made by growing a 1 m-thick intrinsic multiplication layer on top of the substrate. The wafer ID, for this test sample, was 22- 003. SIMS measurements for both wafers are also shown in Figure 4.28.

Figure 4.28. Comparison of the SIMS measured doping profile for structure 14-

Results from wafers 14-253 (black) and 22-003 (magenta), shown in Figure 4.28, were also compared with the P doping profile from wafer 12-028 (red), as illustrated in the previous sections. The doping profile from wafer 14-253 showed a slight improvement compared to the previous result obtained from wafer 12-028. The Sb doping profile showed a very small diffusion tail, and the designed level for the intrinsic region (~1015 cm-3) was obtained after ~300 nm from the substrate (Figure 4.28).

Electric field profile simulations, at 95% of VBD, were carried out with the modelling software to better evaluate the impact of these three different doping concentrations on the designed Ge-on-Si structure. The charge sheet doping concentration was kept fixed at 1.5 × 1017 cm-3 for all simulations. This is illustrated in Figure 4.29 where the colour code used is the same as in Figure 4.28 (black: P profile 14-253; red: P profile 12-028; magenta: Sb profile 22-003).

Figure 4.29. Simulated electric field profile for a 3rd Generation Ge-on-Si

structure. In particular, the SIMS measured doping profile reported in Figure 4.28 were used for Si multiplication layer: P profile 14-253 (black), P profile 12-028 (red), and Sb profile 22-003 (magenta). The doping concentration of the charge sheet layer was kept fixed at 1.5 × 1017 cm-3 for all the simulated structures.

It is clear from Figure 4.29 that the Sb doping profile (magenta line) produced the most uniform electric field profile across the multiplication layer. This improvement was more evident when this profile is compared with the simulated electric field profile

obtained by using the P doping profile of wafer 12-028 (red). Regarding to the electric field profile obtained by using the P doping profile of wafer 14-253 (black), this could be considered as acceptable for the designed structure.

Finally, the choice of the right doping profile for the Si multiplication layer must consider the reproducibility of the doping profile obtainable during the epitaxial growth. 4.12 Conclusion

In this chapter, the main advantages of a SACM structure have been described. The Ge- on-Si SPAD structure proposed in this work is similar to the structure of an InGaAs/InP SPAD, but the material system is completely different, thereby different design rules have been introduced.

The early design of the Ge-on-Si SPAD was based on three main aspects: electric field, doping concentration and temperature. Simulations have been performed to introduce the concepts of punch-through and breakdown voltage. In addition, the effects of the charge sheet doping concentration on the simulated structure were considered to evaluate the simulated electric field profile at 95% of the breakdown voltage. It has been underlined how is important to tailor the electric field between the Ge absorber layer and the Si multiplication layer to guarantee the detection of the infrared light with a high probability of triggering an avalanche of carriers.

Different SPAD generations have been analysed. Experimental data obtained from capacitance-voltage and SIMS measurements have been used in the modelling software to evaluate their impact on the designed structure. Therefore, comparisons between experimental and simulated data were evaluated. Although different problems with the epitaxial growth apparatus were experienced, their impact on the designed structure was helpful to better understand the behaviour on the structure itself. In particular, the high boron doping concentration of the charge sheet layer resulted in a device which could not be used to detect infrared light. However, the designed structure has been adjusted to take into account the doping diffusion tail from the Si substrate in the intrinsic Si multiplication layer. Furthermore, different dopants (P, As, and Sb) were studied and then introduced in the model to evaluate their impact on the electric field profile. Results of the measured Sb doping profile were found to show the most uniform electric field profile across the multiplication region, and this result suggested that this design

solution might be used in future research the growth of the next Ge-on-Si SPAD generations.

4.13 Reference

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Chapter 5 - Ge-on-Si Single-Photon Avalanche Diodes: Device