1. Introduction

The silicon photonics has attracted great attention from both academia and industry during the past decade, due to the low cost and easy compatibility with complementary metal oxide semiconductor (CMOS) technology. Various kinds of silicon based devices, including the germanium (Ge) photodetector [1–8 ], the silicon modulator [9,10 ] and different passive devices [11,12 ], had been reported and demonstrated. Among different kinds of Ge photodetector structures, the waveguide coupling structure is the most widely adopted one, and it can be divided into vertical and lateral PIN junction structures [7]. Although the bandwidth has achieved 120 GHz for lateral PIN photodetector [6], it is hard to exceed 30 GHz for vertical structure due to the high parasitic parameter. There are two main methods to improve the bandwidth of the vertical PIN structure Ge photodetector. The first one is reducing the parasitic parameter and carrier transit time by reducing intrinsic region width or area [4,8 ]. However, this method depends on the advanced fabrication technology [4] and on the other hand degrades other performance parameters obviously [8]. Alternatively, the gain peaking technique was proposed to extend the bandwidth [13]. Although a 60 GHz bandwidth had been successfully demonstrated by introducing an on chip spiral inductor into the electrode [14], the design and fabrication of the complicated electrode restrict its practicability.

In this paper, we propose and demonstrate, for the first time to our best knowledge, a new and effective method for increasing the bandwidth of a conventional waveguide coupling vertical PIN structure Ge photodetector by introducing an off-chip inductor (i.e. a span of gold wire). The gold wire connects the discrete ground electrodes and its size is specially chosen according to the given parasitic parameter of the total device. Thanks to the standard wire bonding technique used for device packaging, the proposed technique requires no additional fabrication process. For demonstration, a conventional Ge photodetector with 3dB bandwidth of less than 30 GHz can be extended to over 60 GHz.

2. Principle

The two main factors that limit the frequency response of the Ge photodetector are carrier transit time and RC parasitic parameter. The 3dB bandwidth can be described as Eq. (1) [15].

To analyze the relationship between ${\text{f}}_{\text{RC}}$ and the RC parasitic parameter, the Ge photodetector can be modeled as a simple RC circuit, as Fig. 1(a) shown. In this circuit, $C_{PD}$ is the junction capacitance, $R_{PD}$ is the junction resistance and $Z_{load}$ is the equivalent load resistance. The cut off frequency of this equivalent RC circuit is usually described as Eq. (2).

 

Fig. 1 Equivalent circuit model of the Ge photodetector (a) without inductor and (b) with inductor.

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A small ${\text{f}}_{\text{RC}}$ can be expected by reducing the $R_{PD}$ and $C_{PD}$. The first approach strongly depends on the fabrication conditions, and it is usually hard to change the fabrication flow in the foundry. Reducing the intrinsic region will certainly lead to small junction capacitance and less carrier transit time, thus results in a high bandwidth [4,8 ]. The other parameters such as responsivity will be inevitably degraded obviously on the other hand. Considering the trade-off of all the factors, the intrinsic region width is usually adopted as 0.4 µm with about 0.1 µm N + + region [16, 17 ]. Under such a condition, the bandwidth of the Ge photodetector is normally less than 30 GHz.

To extend the bandwidth of the Ge photodetector, the gain peaking is proposed theoretically [13]. The principle is based on introducing an inductor into the photodetector equivalent circuit to form an RLC circuit, as shown in Fig. 1(b). When the photodetector operates at high frequency condition, part of capacitance influence can be counteracted by the introduced inductance. This leads to an elevation of the bandwidth curve at high frequency, and thus the bandwidth is extended. The quantitative relationship between the inductance value and the bandwidth can be obtained from the transfer function of the RLC circuit, which is usually described as Eq. (3) [14].

Although the on chip spiral inductor had been introduced into the electrode of Ge photodetector by leveraging the available CMOS technology [14], a very complicated and careful design for electrode should be performed and double layer metals are needed. By contrast, an off chip inductor in virtue of the standard wire bonding technique is simple and easy, achieving the comparable performance compared with the on-chip gain peaking.

The schematic layout of the proposed scheme is shown in Fig. 2(b) . The ground (G) electrode for the proposed Ge photodetector is designed to be three separate parts, instead of the conventional one integral electrode, as shown in Fig. 2(a). This did not change the basic structure of the coplanar waveguide (CPW), which is composed of the center signal conductor and the parallel ground conductors. Furthermore, such change brings negligible parasitic parameter variation since the reduced electrode area is very small. Two gold wires are then introduced to connect the discrete ground electrodes. The inductance value induced by the gold wires can be varied through controlling its size, such as the length and the diameter. For demonstration, the total length of the two gold wires used in Fig. 2(b) is about 450 µm with a diameter of 25 µm. According to the previous result [18], the equivalent inductance and resistance for such gold wire are about 1 nH/mm and 2 ohm/mm within a wide frequency range. Thus, the total equivalent inductance and resistance are approximately considered as constants and their values are about 450 pH and 0.9 ohm, respectively. According to the Eq. (2) to (4) , the bandwidth of the Ge photodetector before and after the wire bonding can be calculated, as shown in Fig. 3 . The fit parameters used in the calculation is obtained by simulation and measurement and is shown in Table 1 . Part of the fit values such as $C_{PD}$ vary between photodetectors may be due to either farbrication variation of the PIN junction or an inexact fit of the Ge photodetector response [14]. Figure 3 shows that the bandwidth can be effectively extended from less than 30 GHz to over 60 GHz.

 

Fig. 2 Schematic layout for the Ge photodetector (a) without wire bonded and (b) with wire bonded.

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Fig. 3 Simulated S₂₁ of Ge photodetector with and without inductor.

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Table 1. Fit Parameters Used in the Simulation

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As the wire bonding technology is the standard and necessary process during the Ge photodetector packaging, this method adds no extra fabrication process, and thus it is a simple method to realize ultrahigh speed Ge photodetector. Furthermore, the proposed method is quite flexible, considering the bonded wires can be easily removed and re-bonded with changeable length and diameter.

3. Fabrication

The Ge photodetector is fabricated using 0.18 μm CMOS technology at the Institute of Microelectronics (IME) in Singapore. The silicon on insulator (SOI) wafer used is 220 nm thick with 2 µm buried oxide (BOX). The grating coupler with etching depth of 70 nm is used for vertical coupling. Fully etched channel waveguide with 500 nm width is designed for single mode operation. A 500 nm thick germanium is grown on the P + doping region of the silicon, and about 100 nm N + + doping region is on top of germanium to form the vertical PIN junction and N type ohmic contact. Both two sides next to germanium are P + + doping region and the distance between P + + and germanium is about 700 nm. The double-layer metals are then deposited on the heavy doping area to form the electrode. To be noted, the double layer metals were utilized according to the specific foundary processing flow, it is not obligatory for the proposed scheme. The cross section of the Ge photodetector is shown in Fig. 4(a) , and the microscopic image of the germanium is shown in Fig. 4(b).

 

Fig. 4 (a) Cross section of the Ge photodetector; (b) Microscopic image of the germanium; (c) Microscopic image for the Ge photodetector with wire bonded.

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A post fabrication wire bonding was carried out. The model of the machine used for wire bonding is West Bond 7700 and the model of the gold wire is Heraeus with diameter of 25 µm. The microscopic image of the Ge photodetector with wire bonded is shown in Fig. 4(c). The middle G electrode was first connected with the right G electrode, and the left G electrode was then connected with the right one. The total length of the two pieces of gold wires is about 450 µm. It is worth mentioning that the bonding pattern in Fig. 4(c) is not the only way for the proposed scheme, an alternative way is first connecting left G electrode with middle one and then connectting middle G electrode with right one. Furthermore, as the proposed mothed is based on the wire bonding technology, which is a common and standard packing technology, it can be thus implemented for many devices at the same time. The mass production will be possible if the bonding technology is stable in the industrial production.

4. Experimental results

The DC characteristic measurements are performed based on the Keithley 2400 source meter. Figure 5 shows the measured dark current and photocurrent for the Ge photodetectors before and after the wire bonding with reverse bias voltage range from 0 to −3.5 V. The dark current is about 64 and 61 nA for the cases before and after wire bonding when the bias voltage is −0.5 V. According to the measured photocurrent, the responsivities are calculated to be about 1 and 0.85 A/W before and after wire bonding at 1550 nm under a bias voltage of −3 V. The reasonable responsivity degradation may be attributed to the photocurrent consumption by the gold wires, which are not ideal conductor without resistance.

 

Fig. 5 Measured dark current and photocurrent for the Ge photodetector without wire bonded and with wire bonded.

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The frequency response of the Ge photodetector is measured based on the lightwave component analyzer (LCA), which can operate at frequency range from 10 MHz to 67 GHz. The output modulated optical signal at 1550 nm with power of about 5 dBm is coupled into the Ge photodetector through a cleaved fiber. A polarization controller is used to maximize the coupling efficiency. Then the converted electrical signal is measured by the LCA through an RF cable. Meanwhile, the reverse biased voltage from the Keithley 2400 source meter is applied to the Ge photodetector through a bias-tee. Under a −4 V biased voltage, the measured and fitted S₂₁ parameters are shown in Fig. 6(a) for both the Ge photodetectors with and without wire bonded. To be noted, the jitter at high frequency in the measured curve is due to the limited bandwidth (40 GHz) of the cable and connectors utilized in the measurement, even if a carefully calibration had been performed. However, it still can be seen that the bandwidth can be extended from less than 30 GHz to over 60 GHz from both the measured and fitted S₂₁ curve. It is worth mentioning that the good performance for multiple devices can be expected since the fabrication technology used in our method is common and standard. A very similar performance had been found in our experiment, provided the bonding technology is stable.

 

Fig. 6 (a) Measured S₂₁ for the Ge photodetector without wire bonded and with wire bonded when the bias voltage is −4V; Measured 32 Gb/s NRZ data transmission results for the Ge photodetector (b) without wire bonded and (c) with wire bonded.

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Finally, the eye diagrams are measured using the proposed photodetector. The highest bit rate available is 32 Gb/s, and non-return-to zero format is used with pseudo random bit sequence (PRBS) of 2⁷-1. The detected electrical signal is measured through a Digital Communication Analyzer. The measured eye diagrams for two cases are shown in Fig. 6(b) and 6(c). It is obvious that a more open eye diagram can be observed for the photodetector with wire bonded. According to Ref [13], the small equivalent resistance of the gold wire inductor will not introduce obvious noise for such a circuit. Both the measured dark current and photocurrent before and after bonding indicate the negligible noise. To be noted the eye diagram improvement will be more significant if a higher bitrate signal was utilized and the impedance mismatch between Ge photodetector and probe can be reduced further [1].

5. Conclusion

We propose and demonstrate a new scheme for increasing the bandwidth of a conventional Ge photodetector. By introducing the gold wires into the discrete ground electrodes, the parasitic parameter can be engineered, and thus the bandwidth can be increased. Simulation and experimental results show that the bandwidth of the Ge photodetector can be extended from less than 30 GHz to over 60 GHz, owing to the two introduced gold wires of with 450 µm length. The proposed off chip parasitic parameter engineering provides an effective, low cost and flexible approach to improve the conventional Ge photodetector, without increasing the complexity of the fabrication process.