At first, we simulated the spectral response of p–i–n photodetector with single material as the active region. Meanwhile, to compare with the result of our structure as shown in Fig. 1(a), the top layer of each photodetector was set as n+-type Si at 200 nm, both layers of p+-type and n+-type doping are set to be 1 × 1019 cm-3. The spectral response was shown in Figs. 2(a)–2(d), respectively. The material in the p+-substrate is consistent with that in the active region, as shown in Fig. 3(c). It is found that when the thickness of the active region increases gradually, the peak of responsivity shows different degrees of redshift. For example, when the thickness of the active region of Si photodetector is up to several microns, the absorption peak can be redshifted to around 800 nm. However, because we use multi-layer materials to fully combine the detection ranges, the active region of the detector becomes much thicker and this results in a greatly reduced response rate. Thus, the calculation range of each active region is set within 1000 nm.
To determine the variation of Ge content in graded-SiGe, the samples of Ge epitaxial growth on Si by two-step method were annealed and performed auger electron spectroscopy (AES) characterization. The phenomenon of atomic interdiffusion appears at the interface between Si and Ge[26, 27], forming a graded-SiGe layer. Due to the changing of Ge content, the spectral response of graded-SiGe photodetector changes with the thickness of the active region. The low-temperature (LT) layer of Ge is about 100 nm, due to the high dislocation and defect density, the diffusion of Si element is mainly concentrated in the LT layer. The annealing process can minimize the dislocation in Ge layer and it is beneficial to the epitaxial growth of GeSn. From Figs. 3(a) and 3(b), we find that the content of Ge in graded-SiGe rapidly increase to about 0.8 before entering the high-temperature (HT) Ge layer, and then slowly go to 1.0 in the HT Ge layer (determined by annealing conditions). Therefore, when the thickness of graded-SiGe active region is very thick, the absorption characteristic tends to be more and more similar to the characteristic of Ge, as shown in Figs. 2(b) and 2(c). With the change of thickness of absorption layer from 200 to 1000 nm, the absorption peak of Si photodetectors shifted from 400 to 500 nm. The response peak of graded-SiGe shifted from 700 to 1000 nm, and the response peak of Ge shifted from 800 to 1000 nm. The response peak of Ge0.9Sn0.1 was basically stable at 1800 nm as shown in Fig. 2(d). Note that the responsivity of longer wavelength optical signal needs a thicker active region to lead the redshift phenomenon.
There are two main sources of dark current density (Jdark) in the photodetector: the bulk leakage current density (Jbulk) and the surface leakage current density (Jsurf). The main source of Jsurf is the interface traps around the mesa. The interface traps can introduce energy levels in the bandgap and eventually increase the Jsurf. Both doping and TDD have a significant effect on the minority carrier lifetime (MCL) and ultimately affect the Jdark. The TDD of Si material in substrate and active region is much lower than that of epitaxial Ge. The MCL of Si is mainly affected by the doping. We set the MCL of n+-Si to 1 × 10–8 s. The MCL of Si in active region is set to 1 × 10–4 s. For the MCL of Ge film in active region, the effect of TDD cannot be ignored. We set the MCL of Ge material to 5 × 10–5 s. The MCLs of SiGe and GeSn are consistent with that of Ge material. To take into account the effect of surface recombination, we compared the effect of SRV on the Jdark. It is observed in Fig. 3(d) that the effect on the Jdark is not significant when the SRV is 1 × 106 cm/s or below, and the Jdark begin to increase when the SRV is higher than this order. Therefore, under such settings, the contribution of Jbulk to Jdark is more significant. Different from the simulation of spectral response, considering that Jsurf varies from different materials, in the simulation of Jdark, the p+-region, i-region and n+-region of each photodetector is set as the same material. The Jdark of the p–i–n photodetector with singlesingle material and Ge0.9Sn0.1, as shown in Figs. 4(a)–4(d), respectively. We used relatively ideal materials for the simulation, the Jdark is generally smaller than measured in the experiment. The Jdark of graded-SiGe and Ge are of the same order of magnitude. The Jdark of the Ge0.9Sn0.1 is much larger mainly due to the small bandgap of Ge0.9Sn0.1 material, which is two orders of magnitude higher than that in Ge material. This is consistent with former reports[12, 21].
Based on this analysis, to keep the absorption peak of each material at different positions and control the thickness of the active region within a reasonable range, we superimposed Si, Ge and Ge0.9Sn0.1 from top to bottom in the active region. The thickness of each layer in the active region was set to 600 nm. We simulated the spectral response of the device as shown in Fig. 5(a). One can see that the cut-off wavelength is about 2300 nm. To flatten the response, we reduced the thickness of Ge0.9Sn0.1 from 600 to 100 nm. The responsivity at 1800 nm is reduced to about 0.57 A/W, it is still an acceptable value. To maximize the range of the flat response, we set the thickness of Ge0.9Sn0.1 to 100 nm and reduced the thickness of the Ge layer, as shown in Fig. 5(b). We find that when the thickness of Ge layer is between 100 and 200 nm, it is consistent with the tendency of flat response. Considering the contribution of the Si and graded-SiGeabsorption layers in the Vis optical signal, we set the thickness of Ge layer to 100 nm. It can be observed from Fig. 5(c) that when the thickness of the Si layer is between 0 and 600 nm, there is no significant effect on the spectral response. This happens because that the absorption coefficient of Ge is much larger than that of Si in the Vis range. A thin Si layer in active region cannot improve the responsivity of the photodetector. However, in the process of growth and annealing, i-Si plays an important role as a pure diffusion source and a barrier to resist the diffusion of doped elements from p+-substrates to the Ge layer. To match the conditions of the experiment, we set the thickness of Si layer to 100 nm. Finally, we simulated the effect of graded-SiGe layer on the spectral response when the thickness is 200 to 300 nm, as shown in Fig. 5(d). To maintain the coefficient characteristics of graded-SiGe, the thickness is controlled in some range. This suggests that when the thickness of the graded-SiGe layer is set to 240 nm, the spectral response is relatively flat. The responsivity of this photodetector is about 0.57 A/W in the range of 700 to 1800 nm.
In the procedure of regulating the thickness of different materials in the active region, when the thickness of the Ge0.9Sn0.1 layer is reduced from 600 to 100 nm, the Jdark is reduced from 0.22 to 0.17 mA/cm2, as shown in Fig. 6(a). When the thickness of other materials is changed, the magnitude of the Jdark remains the same. This is due to the photodetectors of pure Si, graded-SiGe, and Ge have at least two orders of magnitude lower than that in Ge0.9Sn0.1. This structure wraps a thin Ge0.9Sn0.1 layer inside the device to reduce the Jsurf and Jbulk of Ge0.9Sn0.1. This leads to a reduction in Jdark. The spectral response of the designed photodetector and the Jdark changes between 0–6 V reverse bias was shown in Fig. 6(b). This indicates that when the reverse bias is below 5 V, the Jdark remains stable.