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Optics Express

Optics Express

  • Editor: C. Martijn de Sterke
  • Vol. 19, Iss. 6 — Mar. 14, 2011
  • pp: 5040–5046
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Novel evanescent-coupled germanium electro-absorption modulator featuring monolithic integration with germanium p-i-n photodetector

Andy Eu-Jin Lim, Tsung-Yang Liow, Fang Qing, Ning Duan, Liang Ding, Mingbin Yu, Guo-Qiang Lo, and Dim-Lee Kwong  »View Author Affiliations


Optics Express, Vol. 19, Issue 6, pp. 5040-5046 (2011)
http://dx.doi.org/10.1364/OE.19.005040


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Abstract

We report a novel evanescent-coupled germanium (Ge) electro-absorption (EA) modulator with a small active area of 16 μm2 giving an extinction ratio of at least 10 dB for a wavelength range of 1580 – 1610 nm. The modulation efficiency of the modulator at this wavelength range was ~2 dB/V. In addition, monolithic integration of both evanescent-coupled Ge EA modulator and Ge p-i-n photodetector is demonstrated for the first time.

© 2011 OSA

1. Introduction

2. Design and fabrication

The evanescent EA modulator design comprises of a Ge rib that sits on a Si slab. The optical signal is evanescently-coupled into the Ge active region via a crystalline silicon waveguide (WG). The schematic and cross-section of the Ge EA modulator is shown in Fig. 1(a)
Fig. 1 (a) Schematic and (b) cross-section cut of the evanescent-coupled Ge EA modulator structure. The metal electrodes are not shown in the schematic, but are included in the cross-sectional cut. (c) Key process steps for monolithic integration of EA modulator and photodetector are illustrated (Dimensions are not to scale).
and 1(b), respectively. 8-inch silicon-on-insulator (SOI) substrates with Si and buried oxide (BOX) thicknesses of 220 nm and 2 μm, respectively, were used. Firstly, arsenic and boron were implanted into the Si substrate to form the n and p regions. Substrate and contact implantation using these two species were done with a dosage of 5 × 1014 cm2 and 1 × 1015 cm2, respectively. After implantation and activation, a silicon etch step was done for Si slab and Si WG formation. The Si WG width was 500 nm with a nanotaper width of 200 nm at the ends to allow efficient fibre-to-WG coupling and vice versa. Next, Ge selective epitaxy growth (SEG) was achieved using a single wafer cold wall ultra-high vacuum chemical vapour deposition (UHV-CVD) system. The Ge SEG process consisted of growing a SiGe buffer layer and Ge seed at 370°C, followed by a cyclic Ge growth at 550°C [9

9. K.-W. Ang, T.-Y. Liow, M.-B. Yu, Q. Fang, J. Song, G.-Q. Lo, and D.-L. Kwong, “Low thermal budget monolithic integration of evanescent-coupled Ge-on-SOI photodetector on Si CMOS platform,” IEEE J. Sel. Top. Quantum Electron. 16(1), 106–113 (2010). [CrossRef]

,10

10. T.-Y. Liow, K.-W. Ang, Q. Fang, J.-F. Song, Y.-Z. Xiong, M.-B. Yu, G.-Q. Lo, and D.-L. Kwong, “Silicon modulators and germanium photodetectors on SOI: Monolithic integration, compatibility, and performance optimization,” IEEE J. Sel. Top. Quantum Electron. 16(1), 307–315 (2010). [CrossRef]

]. For this work, a 400 nm thick intrinsic Ge was grown on both the photodetector and modulator regions. Reactive ion etching (RIE) with a Cl2-based chemistry was used to form the Ge rib at the modulator region with the photodetector region was masked. Subsequently, the Ge modulator received phosphorous and boron ion implantation with a dosage of 1 × 1015 cm2 and tilt 15° at the sidewalls, while the top of the Ge photodetector was boron-doped with a doping level of 4 × 1015 cm2. Finally, interlayer dielectric deposition, contact hole formation and metallization was done to complete the fabrication. Metal electrodes were located at the sides of the EA modulator to apply a lateral electric field for modulation. The key process steps are shown schematically in Fig. 1(c).

The modulator dimensions included lengths (LGe) of 20, 40 and 60 μm, and effective widths (WGe) of 0.6, 0.8 and 1 μm. The vertical p-i-n photodetector had a width and length of 5 μm and 25 μm, respectively. Figure 2(a)
Fig. 2 SEM images showing monolithic integration of evanescent-coupled Ge EA modulator (right) and Ge p-i-n photodetector (left) after Ge epitaxy and etch steps. (b) TEM cross-sectional image of the Ge rib whereby vertical sidewalls are clearly seen.
shows the scanning electron microscope (SEM) images of both the EA modulator and photodetector after Ge epitaxy and etch steps. Transmission electron microscope (TEM) cross-sectional image [Fig. 2(b)] confirmed that vertical sidewalls were obtained by dry etching. This enabled a uniform electric field to be applied laterally across the Ge rib. The optical measurement setup uses a Photonic Dispersion and Loss Analyzer (PDLA) with a Tunable Laser Source (TLS) module as the optical input. Light is coupled from the PDLA, into and out of the EA modulator using single-mode lensed fibers. When the optical signal is input from the Si WG into the Si slab, it is evanescently-coupled up into the Ge rib because of refractive index difference between the two materials. The optical output from the EA modulator is then fed back to the PDLA to measure the insertion loss at different reverse biases. Before the actual device was measured, a normalization fiber-to-fiber measurement was conducted to capture the loss of the measurement system and test path. Ultimately, the EA modulator insertion loss calculated by the PLDA was obtained through subtraction of the normalization loss.

3. Results and discussion

Direct bandgap absorption for bulk Ge occurs for wavelengths less than 1550 nm, and the absorption coefficient drops significantly for wavelengths higher than 1550 nm. However, tensile strain present in the Ge-on-Si grown layers results in bandgap shrinkage, and shifts the absorption edge by ~50 nm [11

11. M. Rouvière, M. Halbwax, J. L. Cercus, E. Cassan, L. Vivien, D. Pascal, M. Heitzmann, J.-M. Hartmann, and S. Laval, “Integration of germanium waveguide photodetectors for intrachip optical interconnects,” Opt. Eng. 44(7), 075402 (2005). [CrossRef]

]. The bandgap energy of intrinsic Ge can further be decreased via the FK effect by applying an electric field to enhance absorption for larger wavelengths [12

12. H. Shen and F. H. Pollak, “Generalized Franz-Keldysh theory of electromodulation,” Phys. Rev. B Condens. Matter 42(11), 7097–7102 (1990). [CrossRef] [PubMed]

]. The transmittance spectra of a 40 μm long, 0.6 μm wide Ge EA modulator for a range of wavelengths at different reverse biases is shown in Fig. 3(a)
Fig. 3 (a) Transmission spectra of Ge EA modulator (L Ge = 40 μm, W Ge = 0.6 μm) for a range of wavelengths at different reverse bias voltages. (b) The corresponding extinction ratio as a function of wavelength at different reverse biases.
. At 0 V reverse bias, the lowest level of transmittance was observed for wavelengths less than 1560 nm due to direct bandgap absorption (not shown). For wavelengths beyond 1560 nm at 0V bias, the transmittance increases abruptly when Ge absorption coefficient starts to roll-off. When a reverse bias is applied, the transmittance decreases for wavelengths beyond 1560 nm. This decrease in transmittance was attributed to the FK effect whereby bandgap shrinkage enhances optical absorption beyond the absorption edge. It should be noted that the applied electric field across the Ge rib should be less than 100 kV/cm to prevent breakdown of the Ge intrinsic region. The absorption edge of Ge can easily be shifted towards the C-band by using a Ge-rich Ge1- xSix alloy film to tune the modulation wavelength [8

8. J. Liu, M. Beals, A. Pomerene, S. Bernardis, R. Sun, J. Cheng, L. C. Kimerling, and J. Michel, “Waveguide-integrated, ultralow-energy GeSi electroabsorption modulators,” Nat. Photonics 2(7), 433–437 (2008). [CrossRef]

]. The series resistance of the device was small (~100 Ω) and optical modulation was observed even at 1 V reverse bias. The extinction ratio (ER) was calculated using Eq. (1), where IL(0) and IL(V) is the insertion loss in decibel (dB) at zero bias and a reverse bias of V, respectively.

ER=IL(0)IL(V).
(1)

The ER for the range of wavelengths from 1560 to 1640 nm was extracted at different reverse bias voltages using Eq. (1) and plotted in Fig. 3(b). For a reverse bias of 5 V, the ER of 10 dB or more was achieved for the wavelength range of 1580 – 1610 nm.

The impact of device dimensions on optical modulation was also investigated. Figure 4(a)
Fig. 4 The variation of extinction ratio with (a) modulator length and (b) modulator width is shown at 3 V bias for λ = 1600 nm. (c) The insertion loss for the Ge EA modulator after subtracting contributions from fiber-to-Si WG and Si WG propagation losses by using a reference Si WG.
shows that the ER increases from 3 to 9 dB linearly at 3 V bias for 1600 nm as the modulator length increases from 20 to 60 μm (at a fixed W Ge of 0.8 μm) due to a larger active area for optical modulation. Correspondingly, the ER decreases from 6 to 4 dB with increasing modulator width from 0.6 to 1 μm (at a fixed L Ge of 40 μm) because the EA effect reduces as the electric field across the modulator gets smaller at the same reverse bias [Fig. 4(b)]. The IL of the EA modulator was obtained by excluding contributions of fiber-to-WG coupling loss and Si WG propagation loss from the transmission spectra by using a reference Si WG [Fig. 4(c)]. The IL consists of the Si WG-to-EA modulator coupling and Ge absorption loss. At wavelengths smaller than 1560 nm, the IL was the largest due to direct bandgap absorption by Ge. As the Ge absorption coefficient rolls off at the absorption edge, the IL reduces at larger wavelengths when optical absorption weakens. An IL of less than 10 dB was obtained for wavelengths larger than 1600 nm for a 40 μm long, 0.6 μm wide EA modulator. A modulator with a larger width (W Ge = 0.8 μm) was found to give a lower insertion loss, indicating that the IL can be lowered by optimizing device design. The ER/IL ratio is one of the parameters to determine the EA modulator’s operating range. This ratio is representative of the modulator’s signal-to-noise performance. Therefore, a high ER/IL ratio should be chosen to determine the modulator’s operating range. For the EA modulator with L Ge = 40 μm, and W Ge = 0.6 μm, the ER/IL ratio at 5 V is > 1 for wavelength range of 1593 – 1638 nm with the optimum point at ~1625 nm (Note: Maximum λ was 1638 nm). For a wider modulator (W Ge = 0.8 μm) of the same length, the ER/IL ratio was similar in trend, albeit lower due to a weaker FK effect in the modulator at the same bias. In addition to a high ER/IL ratio, the absolute ER of the modulator at its operating wavelength should also be large. In this work, we decide on the operating range by ensuring the ER is >10 dB and the ER/IL ratio is > 1. As such, the operating wavelength range for the device is defined from 1593 to 1610 nm.

Figure 5
Fig. 5 A linear relationship was observed between applied reverse bias and extinction ratio giving a modulation efficiency of 2 dB/V. Inset shows a 1.25 Gbps modulated waveform from a pseudorandom bit sequence (PRBS) source with a pattern length of [27 1] bits.
shows that the relationship of ER versus reverse bias (i.e. modulation efficiency) for a range of wavelengths is linear which is desirable for analog applications. The modulation efficiency was observed to be ~2 dB/V. This linearity is similarly observed in Ref. [8

8. J. Liu, M. Beals, A. Pomerene, S. Bernardis, R. Sun, J. Cheng, L. C. Kimerling, and J. Michel, “Waveguide-integrated, ultralow-energy GeSi electroabsorption modulators,” Nat. Photonics 2(7), 433–437 (2008). [CrossRef]

] when the EA modulator is operating in the pseudo-linear regime. The highest ER was observed to be around wavelengths of 1590 to 1600 nm. The inset gives the modulated optical signal from the output of the EA modulator (L Ge = 20 μm, W Ge = 0.8 μm) at a wavelength of 1600 nm for a bit rate of 1.25 Gbps. The RF signal was from a pseudorandom bit sequence (PRBS) source with a pattern length of [27 1] bits. Due to larger insertion loss and sensitivity of our measurement set-up, we were unable to obtain RF measurements for the longer EA modulators. In our future work, optimization to the design will be done to reduce insertion loss for such measurements.

One of the advantages of the EA modulator is the low power consumption during operation. This is crucial as integrated circuits are becoming denser which results in increased power consumption and on-chip heating. An energy consumption of less than 100 fJ per bit is achievable at 3V reverse bias with our current EA modulator design (assuming a 1Gbps bit rate) [8

8. J. Liu, M. Beals, A. Pomerene, S. Bernardis, R. Sun, J. Cheng, L. C. Kimerling, and J. Michel, “Waveguide-integrated, ultralow-energy GeSi electroabsorption modulators,” Nat. Photonics 2(7), 433–437 (2008). [CrossRef]

]. However, at bias of larger than 3 V, the reverse leakage current increases and contributes to higher energy consumption. This leakage current at higher reverse bias was attributed to Ge film quality and/or implant conditions which can be minimized by process optimization [10

10. T.-Y. Liow, K.-W. Ang, Q. Fang, J.-F. Song, Y.-Z. Xiong, M.-B. Yu, G.-Q. Lo, and D.-L. Kwong, “Silicon modulators and germanium photodetectors on SOI: Monolithic integration, compatibility, and performance optimization,” IEEE J. Sel. Top. Quantum Electron. 16(1), 307–315 (2010). [CrossRef]

]. Even so, the energy consumption is still less than 1 pJ/bit at 5 V operation bias. Figure 6
Fig. 6 The responsivity of the monolithically-integrated Ge photodetector (L Ge = 25 μm, W Ge = 5 μm) at modulation wavelength range of 1580 – 1610 nm.
shows the responsivity of the integrated photodetector for wavelength range of 1580 – 1610 nm at a reverse bias of 1 – 3 V. The lower responsivity (~0.4 A/W) obtained for this range of wavelengths at 1 V, as compared to a wavelength of 1550 nm (~0.9 A/W) can be improved by strain engineering of the Ge layer [13

13. J. Liu, J. Michel, W. Giziewicz, D. Pan, K. Wada, D. D. Cannon, S. Jongthammanurak, D. T. Danielson, L. C. Kimerling, J. Chen, F. Ö. Ilday, F. X. Kärtner, and J. Yasaitis, “High-performance, tensile-strained Ge p-i-n photodetectors on a Si platform,” Appl. Phys. Lett. 87(10), 103501 (2005). [CrossRef]

]. We also observe that the responsivity increases with reverse bias which is a clear indication of the FK effect is also occurring in the Ge p-i-n photodetector. This is regardless of whether the electric field is applied laterally in the Ge EA modulator, or vertically in the Ge p-i-n photodetector. In addition, this could be leveraged to improve the responsivity at the weaker absorption wavelength range of the Ge p-i-n photodetector. Finally, a comparison of the key results in this work with other Ge-based EA modulators in literature is presented in Table 1

Table 1. Key Results of this Work are Compared with Other Ge-Based EA Modulators a

table-icon
View This Table
.

4. Summary

For the first time, an evanescent-coupled Ge EA modulator was monolithically integrated with Ge photodetector. High modulation efficiency, RF signal modulation and low power consumption are demonstrated in this work. This work provides a route to realize high speed low power electronic-photonic integrated circuits with Ge-based modulators and photodetectors.

References and links

1.

L. Liao, D. Samara-Rubio, M. Morse, A. Liu, D. Hodge, D. Rubin, U. D. Keil, and T. Franck, “High speed silicon Mach-Zehnder modulator,” Opt. Express 13(8), 3129–3135 (2005). [CrossRef] [PubMed]

2.

A. Liu, L. Liao, D. Rubin, H. Nguyen, B. Ciftcioglu, Y. Chetrit, N. Izhaky, and M. Paniccia, “High-speed optical modulation based on carrier depletion in a silicon waveguide,” Opt. Express 15(2), 660–668 (2007). [CrossRef] [PubMed]

3.

M. K. Chin and W. S. C. Chang, “Theoretical design optimization of multiple-quantum-well electroabsorption waveguide modulators,” J. Quantum Electron. 29(9), 2476–2488 (1993). [CrossRef]

4.

Y.-H. Kuo, Y. K. Lee, Y. Ge, S. Ren, J. E. Roth, T. I. Kamins, D. A. B. Miller, and J. S. Harris, “Strong quantum-confined Stark effect in germanium quantum-well structures on silicon,” Nature 437(7063), 1334–1336 (2005). [CrossRef] [PubMed]

5.

J. E. Roth, O. Fidaner, R. K. Schaevitz, Y.-H. Kuo, T. I. Kamins, J. S. Harris, and D. A. Miller, “Optical modulator on silicon employing germanium quantum wells,” Opt. Express 15(9), 5851–5859 (2007). [CrossRef] [PubMed]

6.

Y. Rong, Y. Ge, Y. Huo, M. Fiorentino, M. R. T. Tan, T. I. T. Kamins, T. J. Ochalski, G. Huyet, and J. S. Harris, Jr., “Quantum-confined stark effect in Ge/SiGe quantum wells on Si,” IEEE J. Sel. Top. Quantum Electron. 16(1), 85–92 (2010). [CrossRef]

7.

S. Jongthammanurak, J. Liu, K. Wada, D. D. Cannon, D. T. Danielson, D. Pan, L. C. Kimerling, and J. Michel, “Large electro-optic effect in tensile strained Ge-on-Si films,” Appl. Phys. Lett. 89(16), 161115 (2006). [CrossRef]

8.

J. Liu, M. Beals, A. Pomerene, S. Bernardis, R. Sun, J. Cheng, L. C. Kimerling, and J. Michel, “Waveguide-integrated, ultralow-energy GeSi electroabsorption modulators,” Nat. Photonics 2(7), 433–437 (2008). [CrossRef]

9.

K.-W. Ang, T.-Y. Liow, M.-B. Yu, Q. Fang, J. Song, G.-Q. Lo, and D.-L. Kwong, “Low thermal budget monolithic integration of evanescent-coupled Ge-on-SOI photodetector on Si CMOS platform,” IEEE J. Sel. Top. Quantum Electron. 16(1), 106–113 (2010). [CrossRef]

10.

T.-Y. Liow, K.-W. Ang, Q. Fang, J.-F. Song, Y.-Z. Xiong, M.-B. Yu, G.-Q. Lo, and D.-L. Kwong, “Silicon modulators and germanium photodetectors on SOI: Monolithic integration, compatibility, and performance optimization,” IEEE J. Sel. Top. Quantum Electron. 16(1), 307–315 (2010). [CrossRef]

11.

M. Rouvière, M. Halbwax, J. L. Cercus, E. Cassan, L. Vivien, D. Pascal, M. Heitzmann, J.-M. Hartmann, and S. Laval, “Integration of germanium waveguide photodetectors for intrachip optical interconnects,” Opt. Eng. 44(7), 075402 (2005). [CrossRef]

12.

H. Shen and F. H. Pollak, “Generalized Franz-Keldysh theory of electromodulation,” Phys. Rev. B Condens. Matter 42(11), 7097–7102 (1990). [CrossRef] [PubMed]

13.

J. Liu, J. Michel, W. Giziewicz, D. Pan, K. Wada, D. D. Cannon, S. Jongthammanurak, D. T. Danielson, L. C. Kimerling, J. Chen, F. Ö. Ilday, F. X. Kärtner, and J. Yasaitis, “High-performance, tensile-strained Ge p-i-n photodetectors on a Si platform,” Appl. Phys. Lett. 87(10), 103501 (2005). [CrossRef]

OCIS Codes
(160.2100) Materials : Electro-optical materials
(230.2090) Optical devices : Electro-optical devices
(230.4110) Optical devices : Modulators
(250.3140) Optoelectronics : Integrated optoelectronic circuits

ToC Category:
Optical Devices

History
Original Manuscript: December 6, 2010
Revised Manuscript: February 1, 2011
Manuscript Accepted: February 3, 2011
Published: March 2, 2011

Citation
Andy Eu-Jin Lim, Tsung-Yang Liow, Fang Qing, Ning Duan, Liang Ding, Mingbin Yu, Guo-Qiang Lo, and Dim-Lee Kwong, "Novel evanescent-coupled germanium electro-absorption modulator featuring monolithic integration with germanium p-i-n photodetector," Opt. Express 19, 5040-5046 (2011)
http://www.opticsinfobase.org/oe/abstract.cfm?URI=oe-19-6-5040


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References

  1. L. Liao, D. Samara-Rubio, M. Morse, A. Liu, D. Hodge, D. Rubin, U. D. Keil, and T. Franck, “High speed silicon Mach-Zehnder modulator,” Opt. Express 13(8), 3129–3135 (2005). [CrossRef] [PubMed]
  2. A. Liu, L. Liao, D. Rubin, H. Nguyen, B. Ciftcioglu, Y. Chetrit, N. Izhaky, and M. Paniccia, “High-speed optical modulation based on carrier depletion in a silicon waveguide,” Opt. Express 15(2), 660–668 (2007). [CrossRef] [PubMed]
  3. M. K. Chin and W. S. C. Chang, “Theoretical design optimization of multiple-quantum-well electroabsorption waveguide modulators,” J. Quantum Electron. 29(9), 2476–2488 (1993). [CrossRef]
  4. Y.-H. Kuo, Y. K. Lee, Y. Ge, S. Ren, J. E. Roth, T. I. Kamins, D. A. B. Miller, and J. S. Harris, “Strong quantum-confined Stark effect in germanium quantum-well structures on silicon,” Nature 437(7063), 1334–1336 (2005). [CrossRef] [PubMed]
  5. J. E. Roth, O. Fidaner, R. K. Schaevitz, Y.-H. Kuo, T. I. Kamins, J. S. Harris, and D. A. Miller, “Optical modulator on silicon employing germanium quantum wells,” Opt. Express 15(9), 5851–5859 (2007). [CrossRef] [PubMed]
  6. Y. Rong, Y. Ge, Y. Huo, M. Fiorentino, M. R. T. Tan, T. I. T. Kamins, T. J. Ochalski, G. Huyet, and J. S. Harris, Jr., “Quantum-confined stark effect in Ge/SiGe quantum wells on Si,” IEEE J. Sel. Top. Quantum Electron. 16(1), 85–92 (2010). [CrossRef]
  7. S. Jongthammanurak, J. Liu, K. Wada, D. D. Cannon, D. T. Danielson, D. Pan, L. C. Kimerling, and J. Michel, “Large electro-optic effect in tensile strained Ge-on-Si films,” Appl. Phys. Lett. 89(16), 161115 (2006). [CrossRef]
  8. J. Liu, M. Beals, A. Pomerene, S. Bernardis, R. Sun, J. Cheng, L. C. Kimerling, and J. Michel, “Waveguide-integrated, ultralow-energy GeSi electroabsorption modulators,” Nat. Photonics 2(7), 433–437 (2008). [CrossRef]
  9. K.-W. Ang, T.-Y. Liow, M.-B. Yu, Q. Fang, J. Song, G.-Q. Lo, and D.-L. Kwong, “Low thermal budget monolithic integration of evanescent-coupled Ge-on-SOI photodetector on Si CMOS platform,” IEEE J. Sel. Top. Quantum Electron. 16(1), 106–113 (2010). [CrossRef]
  10. T.-Y. Liow, K.-W. Ang, Q. Fang, J.-F. Song, Y.-Z. Xiong, M.-B. Yu, G.-Q. Lo, and D.-L. Kwong, “Silicon modulators and germanium photodetectors on SOI: Monolithic integration, compatibility, and performance optimization,” IEEE J. Sel. Top. Quantum Electron. 16(1), 307–315 (2010). [CrossRef]
  11. M. Rouvière, M. Halbwax, J. L. Cercus, E. Cassan, L. Vivien, D. Pascal, M. Heitzmann, J.-M. Hartmann, and S. Laval, “Integration of germanium waveguide photodetectors for intrachip optical interconnects,” Opt. Eng. 44(7), 075402 (2005). [CrossRef]
  12. H. Shen and F. H. Pollak, “Generalized Franz-Keldysh theory of electromodulation,” Phys. Rev. B Condens. Matter 42(11), 7097–7102 (1990). [CrossRef] [PubMed]
  13. J. Liu, J. Michel, W. Giziewicz, D. Pan, K. Wada, D. D. Cannon, S. Jongthammanurak, D. T. Danielson, L. C. Kimerling, J. Chen, F. Ö. Ilday, F. X. Kärtner, and J. Yasaitis, “High-performance, tensile-strained Ge p-i-n photodetectors on a Si platform,” Appl. Phys. Lett. 87(10), 103501 (2005). [CrossRef]

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