1. INTRODUCTION

Thanks to the high-quality Ge epitaxial growth on Si, rapid progress in research into a high-performance Ge-on-Si photodetector (PD) has been seen in the past decades [1–3]. Because of the ease of integration, the most popular architecture of a Ge PD on silicon-on-insulator (SOI) substrate should be that a piece of epitaxial Ge is selectively grown on the top crystalline Si surface and converts light evanescently coupled from a Si waveguide into electron–hole pairs [4,5]. A vertical pin diode is formed with the epitaxial Ge as an intrinsic region and usually reversely biased to sweep out the light-injected carriers for current generation purpose.

An ideal PD should have a high responsivity, low dark current, and high electrical bandwidth. For a well-defined epitaxial Ge layer and its doping profile, both the dark current and the electrical bandwidth benefit from area reduction of the absorption region. The dark current is proportional to the cross-sectional area that current flows through. The serial capacitance contributing to the RC constant also has linear dependence on the pin junction area, which fundamentally constrains the electrical bandwidth. However, in the optical domain, the volume reduction of the PD absorption region goes against light-induced carrier generation and thus leads to low responsivity. Given a typical absorption coefficient of intrinsic Ge of $46{\mathrm{mm}}^{-1}$ at 1550 nm, a propagation length of 87 μm is necessary in bulky Ge to absorb incident light by a percentage of 90%.

The way of improving PD responsivity in the optical domain is mainly through increasing the external quantum efficiency, such as coupling more light into the Ge absorption region or preventing power leakage from other lossy mechanisms. For example, thinning down the Si membrane was reported to be good for light coupling into the Ge absorption region [6,7]. Using multiple isolated vias rather than a single $z$-invariant one for metal contact on Ge top can efficiently lower the metallic losses [8]. This paper proposes a new approach to responsivity enhancement in compact Ge-on-Si detectors by using photonic crystal (PC) on an Si slab. An ultra-compact and efficient PD is demonstrated. The design of a photonic bandgap (PBG) over the operation band enables cycle utilization of outgoing light power.

2. DEVICE DESIGN AND SIMULATION

Figure 1 illustrates the 3D bird view of a PC-surrounded Ge-on-Si PD. The PC holes in Si are formed together with the channel waveguides by the same lithography and etching steps. The same model is used for numerical simulation hereafter. The main purpose of surrounding photonic crystal is to reflect the outgoing waves back to the Ge absorption regions so that the to-be-wasted power experiences its second absorption. This work focuses on TE-like waves only in the Si slab waveguide and thus a triangular lattice is favorable to create a large bandgap [9].

 

Fig. 1. Schematics of the PC-surrounded PD, in which passivation layers are not shown.

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The structural parameters of the PC are optimized through examining the band diagram of a unit cell of the hexagonal PC slab, which is calculated by the plane-wave expansion method using BandSOLVE (RSoft Design Group, Inc., USA). For an SOI wafer with 220 nm thick Si overlay ($h=220\mathrm{nm}$), we calculated the band diagram for PC slabs with lattice constant $a$ ranging from 400 nm to 440 nm and related the PBG to the hole radius $r$. Figure 2(a) plotted the band diagram in the first Brillouin zone for the case of $a=420\mathrm{nm}$ and $r=115\mathrm{nm}$, which are selected to carry out full-wave simulation and experiment. The reason to choose these parameters is double-fold. Figure 2(b) shows that a larger air filling factor ($\propto r^2$) enables a wider PBG but makes modes of our interest close to the upper band edge and thus more leaky. On the other hand, both large and small $r$ challenges the lithography resolution limit. Therefore, we choose a lattice constant $a$ of 420 nm and a moderate $r$ of 115 nm to target a PBG with a central wavelength of 1550 nm and a bandgap width of nearly 400 nm. A large bandgap enables the PBG effect to be effective for broad operation band.

 

Fig. 2. (a) Dispersion curves under the PC slab light cone and PBGs for the case of $a=420\mathrm{nm}$ and $r=115\mathrm{nm}$. By compressing the lower PBG in (a) horizontally to a vertical line, a reduced band structure representing the change of PBG against hole radius is obtained in (b). The lines are intentionally separated by 1 nm for visualization purpose.

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To investigate the advantages of using PC, we used the Rsoft FULLWAVE package to implement 3D finite-difference time-domain (FDTD) simulation [10]. The snapshots of the $H_y$ fields are visualized in Figs. 3(a) and 3(b) for the cases with and without PC holes, respectively. Owing to the high refractive index and the graded (311) face of Ge, the transition loss from the Si slab waveguide to the Ge absorption region is negligible. Without PC holes, light passing through the Ge section is coupled down to the Si slab waveguide and radiates out. With the PBG, the radiation modes are prohibited so that the outgoing waves are reflected back to Ge and experience a second round of absorption. By normalizing the field intensity to the input power, it is clearly seen that PDs with PC holes have stronger absorption. For quantitative analysis, in Fig. 4, we calculate the averaged time-varying power absorption inside the lossy medium Ge by using the equation of $\frac{1}{T}{\int }_t^{t+\mathrm{\Delta }T}{\int }_{\mathrm{Ge}}{\epsilon }_i{\mathit{E}}^2\mathrm{d}V\mathrm{d}t$, where $T$ is an optical cycle, ${\epsilon }_i$ is the imaginary part of permittivity, $\mathit{E}$ is the complex field, an $V$ is the volume of Ge. In reality, there are other loss mechanisms in the Ge PD, such as the highly doped Si mesa, the Ge $N++$ region, and the metallic absorption from the cathode contact. For simplicity, we assume that the Ge absorption is dominant and efficiently converted to current with an ideal quantum efficiency of unity. The Ge epitaxy window area is $3\mathrm{\mu m}\times 5\mathrm{\mu m}$. The time for the PD with PC to get its steady state is nearly doubled compared to the one without PC. Be reminded that this light traveling time is at a scale of $<1\mathrm{ps}$ and much shorter than the typical PD response time. The existence of PC enables an increment of light absorption by 57%. Apparently, the merit of using PC is more embodied for compact PDs. Those with long Ge absorption regions have less power radiated into the Si slab so that PC is not badly needed.

 

Fig. 3. Snapshots of field evolution in PDs (left) with and (right) without PC simulated by using the FDTD method. The upper and lower panels are for the central $y$-cut and $x$-cut planes of the PC slab, respectively. All the panels are normalized to the excitation power and captured at the same time step.

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Fig. 4. Time-varying absorption in Ge under continuous-wave excitation.

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3. EXPERIMENT AND CHARACTERIZATION

The devices were fabricated with the standardized process flow for multiple projects wafers (MPW) runs in the FAB line of the Institute of Microelectronics, Singapore. It started from an 8 in. (20.32 cm) SOI wafer with a 220 nm thick Si overlay. The lithography condition was optimized to target a hole radius of 115 nm. Boron was moderately implanted (with a concentration of around $1\mathrm{e}19{\mathrm{cm}}^{-3}$) into the full Si mesa by balancing low series resistance and low impact to Ge epitaxy. A 120 nm thick sacrificial oxide was deposited by plasma-enhanced chemical vapor desposition (PECVD) and carefully etched to expose the Si surface. Subsequently, in an ultra-high vacuum chemical vapor deposition epitaxy reactor, the Ge epitaxy started from a 20 nm pseudo-graded SiGe buffer layer and a 30 nm Ge seed layer and ends with a targeted thickness of 500 nm. The selective growth was assured by a cyclical deposition and etch-back method. The top Ge surface was heavily but shallowly doped with a concentration of nearly $1\mathrm{e}19{\mathrm{cm}}^{-3}$ to act as the cathode. Then the sacrificial oxide was removed by wet etching and another inter-dielectric layer was deposited to cover the whole device. Finally, contact holes were opened by using a highly selective dry-etching recipe. TaN/Al layers were deposited and etched to form the metallization.

Figure 5 shows the scanning electron microscopy (SEM) image of the fabricated device after Ge epitaxy. In fabrication, a triangle region was added in front of the rectangle photon collection region, so that a knife-edge shape was obtained, which led to adiabatic light transition from the Si channel waveguide to the Ge absorption region. In addition, only one column of holes was kept to isolate the Ge epitaxy region and the contact region. Those holes underneath the anode contact were filled to maintain a smooth membrane for the anode contact. One may change this if the PBG effect is really desired in lateral directions.

 

Fig. 5. SEM images captured after finishing the front-end process. The lateral dimension of the Ge bottom is 5 μm.

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The dark current was measured for a set of PC-surrounded PDs with different lengths. A linear dependence can be observed from the inset in Fig. 6(a). The dark current at $-1\mathrm{V}$ reverse bias was measured to be 160 nA for a 5 μm long PD. In Fig. 6(b) we also examined the change of dark current against the environment temperature by mounting the chip on an electrical temperature controller and found that 1 μA dark current can be maintained up to 70°C. These values are nearly 1 order of magnitude better than our baseline design, with an Ge epitaxy area of $8\mathrm{\mu m}\times 25\mathrm{\mu m}$ on the same chip. This is in good agreement with our expectations stated previously.

 

Fig. 6. I-V characteristics of photodetectors (a) with different active region lengths and (b) at temperatures ranging from 21°C to 78°C.

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The electrical bandwidth of the 5 μm long PD with PC is plotted in Fig. 7(a) against the applied voltage. The zero-biased 3 dB bandwidth is around 1 GHz. The bandwidth goes to the limit while raising the voltage to 4 V, at which the 3 dB bandwidth reaches 17 GHz. The bandwidths of PDs with various lengths of Ge absorption regions on the same chip are plotted in Fig. 7(b) at a biasing voltage of 4 V. The bandwidth is insensitive to the Ge length though the absorption length decreases from 25 μm to 5 μm. This is reasonable because, as a matter of fact, the RC constant that limits the bandwidth has little dependence on the length of the absorption region. A short absorption region may decrease the capacitance but increase the series resistance and thus make no change to the RC constant. For the PC surrounding case, the missing holes may also increase the series resistance to some extent. This adverse effect may become severe if many columns of PC holes are drilled between the absorption region and the contact regions. To circumvent this issue, one may consider designing a U-shape contact regions to surround three sides of the absorption region so that smaller series resistance could be expected.

 

Fig. 7. Electrical bandwidth against (a) biasing voltage and (b) the length of Ge absorption region.

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Finally, we examined the broadband operation of the device to extract the light responsivity. As shown in Fig. 8(a), in order to calibrate the fiber coupling loss and waveguide propagation loss, the PDs are placed in the center of the 3 mm long chip center. A directional coupler with a splitting ratio of 10:90 is designed before the PD so that a small fraction of power can be used for the auto-alignment purpose. A dummy device with a cascaded direction coupler was fabricated nearby as a reference to acquire the system losses without any device loaded. A tunable light source is used for the measurement of the broadband responsivity spectrum. The results in Fig. 8(b) show that such a 5 μm long PD with PC has a peak responsivity of 0.75 A/W at a wavelength 1.55 μm and maintains a high-level responsivity of 0.6 A/W over a broad operation band of 120 nm. These specifications are very comparable to those obtained from a baseline device with an Ge epitaxy area of $8\mathrm{\mu m}\times 25\mathrm{\mu m}$, which are expected to have a sufficiently long absorption region for saturated light detection [11].

 

Fig. 8. (a) Layout of a PD and a reference coupling structure for power calibration; (b) responsivity of the 5 μm long PD with surrounding PC holes.

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4. CONCLUSION

In summary, we have numerically verified the benefits of using photonic crystal to enhance the optical responsivity and reduce the dark current of a compact Ge-on-Si photodetector. We have also demonstrated an ultra-compact PD with high performance in dark current, electric bandwidth, and responsivity. This work provides an example of using PC for PD enhancement and we believe that more compact PDs can be realized in ultra-small cavities with PBG effect.