1. Introduction

For several years, there has been a lot of research attention on optical interconnects as a potential replacement of copper wires. The projected applications range from data-storage centers, to board to board, chip to chip and even to on-chip data links [1, 2]. However, to be a viable candidate at the chip scale, optical interconnects have to meet aggressive requirements in terms of power consumption, data density, and preferably monolithic integration with silicon. For example, it has been suggested that to effectively replace copper wire, optical output devices with an energy consumption of less than 100 fJ/bit will be required by 2020 [3]. Moreover, the devices should operate over a wide spectral range to facilitate wavelength-division multiplexing (WDM). Such low power consumption and wideband devices are not obtainable with conventional silicon modulators based on a Mach–Zehnder interferometer (MZI) or a resonator [3, 4]. In this aspect, the electro-absorption (EA) modulator is one of the best approaches to obtain low energy consumption and high speed modulation due to its small footprint (low capacitance) and subpicosecond operation time for a wide spectral range [5]. Strong electroabsorption can be obtained from III-V direct-gap materials based on quantum confined Stark effect (QCSE); epitaxial growth, wafer bonding, or die bonding of III-V materials on Si are currently under investigation by several research teams [6]. On the other hand, Ge, a group IV material like Si, has been successfully monolithically integrated into CMOS fabrication processes by several semiconductor companies [7, 8]. Despite being an indirect-gap semiconductor, the direct-gap transition in bulk Ge has been shown to have light modulation, detection, and emission capabilities within the telecommunication wavelengths [8–13]. Particularly for light modulation, bulk Ge or SiGe EA modulators [11–13] based on the Franz-Keldysh effect (FKE) have been demonstrated with low energy consumption [11] and large 3 dB bandwidth [13]. Interestingly, Ge quantum wells (QWs) [14–17] have demonstrated a QCSE as strong as that of III-V semiconductor QWs conventionally used for high performance photonic devices, and the possibility for light detection and emission have also been demonstrated [18, 19]. A strong light modulation from Ge/SiGe MQWs with a voltage swing of 1 V has been shown from a modulator with side-entry configuration [20]. So far, high speed modulation using Ge/SiGe MQWs has been demonstrated up to 3.5 GHz using a surface illuminated p-i-n diode structure [21] and transmission at 7 Gbps has also been demonstrated in waveguide configuration based on butt-coupling between the MQW structure and a Silicon-on-Insulator (SOI) waveguide [22]. In this paper, we demonstrate a modulator with a modulation bandwidth of 23 GHz obtained from a Ge/SiGe MQW waveguide. The modulator also exhibits high extinction ratio over a wide spectral range with an estimated energy consumption of as low as 108 fJ per bit.

2. Ge/SiGe MQWs and device fabrication

Ge/SiGe MQWs were grown by low-energy plasma-enhanced chemical vapor deposition (LEPECVD) [23]. On a 100 mm Si(001) substrate, a 13 µm Si_1−yGe_y graded buffer was deposited with linearly increasing Ge concentration y at a rate of 7%/μm from Si to Si_0.1 Ge_0.9 and capped with a 2 µm Si_0.1Ge_0.9 layer forming a fully relaxed virtual substrate (VS). A 500 nm boron-doped (~1 × 10¹⁸ cm⁻³) Si_0.1Ge_0.9 layer was grown to serve as a p-type contact, and followed by a 50 nm Si_0.1Ge_0.9 spacer. The MQW itself consisted of twenty nominally 10 nm Ge QWs sandwiched between 15 nm Si_0.15Ge_0.85 barriers. The average Ge concentration in the Ge/SiGe MQWs was designed to be equal to that of the Si_0.1Ge_0.9 VS in order to achieve strain compensation. Finally, a 50 nm Si_0.1Ge_0.9 cap layer and a 100 nm phosphorus-doped (~1 × 10¹⁸ cm⁻³) Si_0.1 Ge_0.9 n-type contact were added.

A 3 µm wide 90 µm long Ge/SiGe MQW p-i-n diode was fabricated in order to investigate the modulation performance of Ge QWs in waveguide configuration. The mesa was patterned by ultraviolet (UV) lithography and dry etching. A passivation stack of SiO₂/Si₃N₄ was deposited by plasma enhanced chemical vapor deposition (PECVD). The bottom and top contacts were defined by UV lithography, reactive ion etching, and wet etching of the passivation layer. An Al layer was evaporated and lifted-off for both top and bottom contacts. The schematic view and scanning electron microscope (SEM) images of the fabricated device are shown in Fig. 1(a) and (b) respectively. Mode calculations were performed using a film mode matching complex solver assuming a linear variation of refractive index in the graded buffer as discussed in Ref. 24. Light is guided within the Ge/SiGe MQWs and the 2 µm relaxed buffer thanks to the light confinement of the graded buffer. Assuming that 10 nm-Ge / 15 nm-Si_0.15Ge_0.85 MQW region is equivalent to a Si_0.09Ge_0.91 homogeneous region, the overlap factor of the first TE mode with the MQW region was calculated to be 12%. The fundamental optical mode of the waveguide and its detailed cross section are shown in Fig. 1(c) and (d).

 

Fig. 1 (a) Schematic view and (b) scanning electron microscope (SEM) images of the fabricated device; (c) The fundamental optical mode of the waveguide; (d) Detailed cross section of the fabricated diode.

Download Full Size | PDF

3. Device characterization

Transmission measurements of the Ge/SiGe MQW waveguides at several reverse bias voltages were performed at room temperature with a spectral resolution of 0.1 nm from 1400 to 1460 nm, covering the absorption edge of the material system. Light from a tunable laser with transverse-electric (TE) polarization [24] was butt coupled into the waveguide using a taper-lensed fiber. Objective lenses were used to couple output light into a single mode fiber connected to a digital photodetector recording the output power. The coupling loss at the input and output facets was subtracted from the spectra by using a cut-back technique. Transmission measurements of 3 µm wide Ge/SiGe MQW waveguides with different lengths were conducted at a particular wavelength within the measurement range; then, by extrapolating the data to a zero waveguide length, the coupling loss was estimated.

The absorption spectra of the waveguide at different reverse bias voltages are reported in Fig. 2(a) . The wavelength separation of around 3 nm between each transmission peaks is consistent with the Fabry-Perot resonance between the input and the output facets of the waveguide. The absorption edge is shifted from the 0.8 eV of bulk Ge due to both the confinement effect in the QWs and the strain between the Ge QWs and the VS. By increasing the reverse bias voltages, red shift of the absorption spectra, which is a characteristic of the QCSE, is observed. The spectra have rather constant values of absorption at wavelength below 1410 nm and an increase in absorption with increasing reverse bias at longer wavelength. This was also reported in our previous work with a waveguide of comparable size [18], and was explained to be due to the long absorption length of the waveguide configuration. From Fig. 2(b), an extinction ratio (ER) higher than 6 dB for a wide spectral range between 1425 and 1446 nm is obtained, with a maximal ER value of around 10 dB achieved between 1433 and 1442 nm for a reverse bias of 5 V and between 1433 and 1437 nm for a reverse bias of 4 V. Moreover, from Fig. 2.(c) with a voltage swing of 1 V between 3 and 4 V and of 2 V between 3 and 5 V, an ER higher than 6 dB is obtained from 1433 to 1437 nm and from 1433 to 1444 nm respectively. As seen in Fig. 2.(a), the absorption loss at 0 V of the Ge/SiGe waveguide used in this work is around 5.5 – 12 dB at the wavelength region between 1433 and 1444 nm. This absorption loss could be decreased by optimizing the light confinement in the MQW region. Indeed the overlap of the first optical mode with the MQW region is only 12% as shown in Fig. 1(c), and part of the optical loss also comes from the overlap between the optical mode and the 2µm thick Si_0.1Ge_0.9 relaxed buffer and the doped region. With an optimized optical overlap in the QW region, it is theoretically predicted that a device only 20 µm long would be enough to obtain a sufficiently high value of ER from Ge/SiGe MQW waveguide [25] with a low optical loss.

 

Fig. 2 (a) Absorption spectra of the 3 µm wide 90 μm long Ge/SiGe MQW waveguide as a function of wavelength and photon energy for reverse bias voltages of 0, 1, 2, 3, 4, and 5 V corresponding to electric fields of approximately 0, 3, 4.5, 6, and 7.5 x 10⁴ V/cm. Extinction ratio of the waveguide (b) between 0 and 3, 4, 5 V and (c) for a voltage swing of 1 V between 3 and 4 V and of 2 V between 3 and 5 V.

Download Full Size | PDF

The frequency response of the modulator was evaluated at 1448 nm, where the signal level is larger. As shown in Fig. 3(a) , optical light was butt-coupled into the waveguide, and an ac signal generated by an opto-RF vector network analyzer (Agilent 86030A) coupled with a dc bias Tee was used to drive the modulator using coplanar electrodes. The modulated optical signal was coupled back to the opto-RF vector network analyzer by objective lenses. The normalized optical response at the dc reverse bias of - 4.5 V as a function of frequency is given in Fig. 3(b). A 3 dB cut-off frequency of 23 GHz was experimentally obtained from the waveguide modulator.

 

Fig. 3 (a) Diagram of the system for measuring frequency response of the Ge/SiGe MQW modulator; (b) Normalized optical response at the dc reverse bias of - 4.5 V as a function of the frequency.

Download Full Size | PDF

For energy consumption, the junction capacitance of the waveguide device with a size of 3 μm x 90 μm and 20 Ge QWs is calculated to be 62 fF, which is also consistent with the values extracted from S11 parameter measurement. The formula used in the device energy consumption calculation is energy/bit = 1/4 (CV_off²-CV_on²) [13] in which C is the junction capacitance and V_off and V_on are the bias voltages at the “off” and “on” states. The formula represents the main contribution of device energy consumption required to charge and discharge the capacitance in the case of non return to zero (NRZ) data, in which the probability of a transition between “on” and “off” states is 1/2 per bit. Therefore, the energy consumption per bit of the EA modulator in this paper can estimated to be 108 fJ/bit for a voltage swing of 1 V between 3 and 4 V biases, 248 fJ/bit for a voltage swing of 2 and 4 V between 3 and 5 V biases and 0 and 4 V biases respectively, and 388 fJ/bit for a voltage swing of 5 V between 0 and 5 V biases. These values are very competitive compared to silicon MZI modulators [4], and of the same order of magnitude as demonstrated bulk Ge modulators [11–13]. Furthermore, the energy consumption of Ge/SiGe MQW modulators could be further reduced by simply using devices with smaller width and shorter length [22] and increasing the overlap factor between the optical mode and the active region, for example by optimizing the thickness of the relaxed buffer and the doped layers, in order to retain a high ER. Additionally, it is worth mentioning that dark leakage and photogenerated current during the “on” and “off” states also contribute to the energy dissipation of the EA modulator. In our devices, relatively low dark leakage current values of 200 A/cm² at −1 V bias were demonstrated with a flat increase in reverse current up to a bias of - 8 V; therefore, this power dissipation is not expected to contribute dominantly in our structures. Photogenerated currents depend on the input power level and operating conditions and a detailed discussion of the corresponding energy dissipation can be found in Ref 25.

4. Conclusion

To summarize, we demonstrate the first high speed modulation up to 23 GHz using QCSE in Ge/SiGe MQWs. The modulator is in waveguide configuration. The Ge/SiGe MQWs exhibit a wide spectral range with an ER greater than 10 dB with the use of only 90 µm long device with an estimated energy consumption of 108 fJ per bit. By improving the overlap factor between the optical mode and the Ge/SiGe MQW region, smaller devices with even lower energy consumption and comparable ER can be envisioned.