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Photonics Research

| A joint OSA/Chinese Laser Press publication

  • Editor: Zhiping (James) Zhou
  • Vol. 1, Iss. 3 — Oct. 1, 2013
  • pp: 140–147
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High-performance waveguide-integrated germanium PIN photodiodes for optical communication applications [Invited]

Léopold Virot, Laurent Vivien, Jean-Marc Fédéli, Yann Bogumilowicz, Jean-Michel Hartmann, Frédéric Bœuf, Paul Crozat, Delphine Marris-Morini, and Eric Cassan  »View Author Affiliations


Photonics Research, Vol. 1, Issue 3, pp. 140-147 (2013)
http://dx.doi.org/10.1364/PRJ.1.000140


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Abstract

This paper reports on high-performance waveguide-integrated germanium photodiodes for optical communications applications. 200 mm wafers and production tools were used to fabricate the devices. Yields over 97% were obtained for three different compact photodiodes ( 10 × 10 μm and intrinsic region width of 0.5, 0.7, and 1 μm) within the same batch of three wafers. Those photodiodes exhibit low dark currents under reverse bias with median values of 74, 62, and 61 nA for intrinsic widths of 0.5, 0.7, and 1 μm, respectively, over a full wafer. Responsivities up to 0.78 A / W at 1550 nm and zero bias were measured. Zero bias operation is possible for 25 and 40 Gbps with receiver sensitivity estimated to 13.9 and 12.3 dBm , respectively.

© 2013 Chinese Laser Press

1. INTRODUCTION

This paper reports on the development of lateral PIN germanium photodiodes integrated with silicon waveguides on 200 mm SOI substrate using standard microelectronic tools and processes. Our aim was to develop the most efficient photodiodes in terms of raw performance (bandwidth, responsivity) and power consumption, at the lowest cost. Those devices exhibit performance compliant with 40 Gbps optical communication systems. Very high optical bandwidth (over 50 GHz), as well as responsivity up to 0.78A/W at a wavelength of 1550 nm, have been achieved at zero bias. Those photodiodes also present a very low dark current (typically 25 nA at 1V reverse bias), while offering very simple integration within a standard CMOS process flow. The design and fabrication will be detailed as well as the DC and AC characteristics, complemented by simulations and data transmission performance evaluations.

2. DEVICE DESIGN AND FABRICATION

A. Photodiode Integration Scheme

The integration of the photodiode with Si waveguides must remain as simple as possible, to minimize the costs and increase the robustness, without neglecting the performance of the device. The main considerations that have been taken into account in order to fabricate such photodiodes are presented hereafter.

First, we need to couple the light from a SOI waveguide to the photodiode. Two schemes are possible and led to very good coupling efficiency and resulted in high performance in terms of bandwidth, responsivity, and/or dark current. The light can be coupled either evanescently from the waveguide to the upper Ge layer or directly from the waveguide to Ge by butt coupling, as shown in Figs. 1(a) and 1(b), respectively. The Si waveguide is designed to be single mode at wavelength of 1550 nm. The chosen waveguide height and width are 220 and 500 nm, respectively.

Fig. 1. Waveguide-coupling configurations: (a) butt coupling and (b) evanescent coupling. PIN junction configuration: (c) lateral and (d) vertical.

Using the RSOFT beam propagation method (BPM), we have simulated the absorption efficiency at a wavelength of 1550 nm in a Ge layer with the following parameters: 10 μm width, 20 μm length, and 350 nm thickness. For the butt-coupling scheme, we have included a 50 nm thick Si seed layer, mandatory for Ge growth. For the same absorption efficiency of 96%, lengths of 10 and 17 μm are required for butt coupling and evanescent coupling, respectively, as shown in Fig. 2. The absorption length can be reduced for evanescent coupling by optimizing the Ge layer thickness, whereas it has little or no influence in the case of butt coupling. For a fixed Ge thickness of 350 nm, at a given absorption efficiency of 96%, the device capacitance is then 1.7 times higher for evanescent coupling, increasing the RC (resistance-capacitance product) time constant of the device.

Fig. 2. Absorption efficiency and device capacitance for evanescent and butt-coupling configurations, assuming intrinsic region width of 1 μm and Ge height of 350 nm.

The second point is the PIN junction configuration. It can be either vertical or lateral [Figs. 1(c) and 1(d)]. Both have yielded similar results. The intrinsic region width or height can be controlled by ion implantations or by Ge growth with in situ doping, for the lateral and vertical configurations, respectively.

B. Intrinsic Region Design

Fig. 3. BPM simulation of light absorption in the intrinsic region of a lateral PIN photodiode for a 350 nm thick Ge layer in a 10×10μm cavity, butt coupled to a Si waveguide (220nm×500nm). The intrinsic region width is set at 0.5, 0.7, and 1 μm. The inset reports the maximum responsivity for those conditions at 1.55 μm wavelength.

The frequency response of PIN photodiodes is mainly driven by the transit time across the depletion layer and the RC time constant induced by the global capacitance and resistance of the device. The transit time is directly obtained by the ratio of the depletion length lD to the carrier drift velocity vdrift:
τtr=lDvdrift,
(1)
and the RC time constant is defined by
τRC=RC,
(2)
where R is the equivalent resistance of the photodiode circuit and Cj is the junction capacitance assuming no “package” capacitance, which is defined as follows:
Cj=εAlD,
(3)
with ε the Ge dielectric constant and A the area of the junction. To get a rough estimate of the photodiode maximum theoretical bandwidth, we will assume at first that the depletion length is equal to wi: the intrinsic region is supposed to be fully depleted, and the active dopant concentration is high enough so that the space charge region will not be extended under a few volts reverse bias. Moreover the p-type and n-type doped region are assumed to have constant doping level with abrupt profiles at the edges of the intrinsic region. Following the methodology presented in [37

37. G. Lucovsky, R. F. Schwarz, and R. B. Emmons, “Transit-time considerations in p–i–n diodes,” J. Appl. Phys. 35, 622–628 (1964). [CrossRef]

], the transfer function of the photodiode can be analytically calculated to obtain the frequency response with contributions from both the RC and transit time parts. The sheet resistance of the photodiode has not been taken into account, and only a 50 Ω load resistance is considered. Taking into consideration the dimensions of the Ge layer (which is 10 μm long and 350 nm thick), the 3dB maximum opto-electric bandwidth is reported in Fig. 4 as a function of the intrinsic region width. As seen from this graph, the bandwidth is dominated by the transit time if the intrinsic region width remains over 150 nm. The highest reachable bandwidth, without degrading too much absorption efficiency, is obtained when the intrinsic region matches the waveguide width (here 500 nm) and is estimated to be around 70 GHz.

Fig. 4. Photodiode maximum theoretical 3dB bandwidth.

Another point essential in the achievement of low power consumption and highly sensitive receivers is the dark current. Germanium has very narrow (direct and indirect) bandgaps. Thus, reverse biasing PIN junctions leads to an increase of the dark current, mainly because of trap-assisted tunneling (low electric field) or band-to-band tunneling (high electric field). To maintain high signal-to-noise ratio (SNR), independently of the bandwidth, dark current has to be reduced to its minimum. This can be achieved with very good crystalline quality germanium as well as with a low electric field inside the junction (by applying a low reverse bias or increasing the intrinsic region width). However, the latter can lead to a reduction of the drift velocity of the photogenerated carriers, degrading the bandwidth. Yet, only good epitaxy process conditions control can allow very low dark current, and no particular care in design is necessary. Three photodiode designs were chosen with different intrinsic region widths. Those devices will be called A, B, and C for wi equal to 0.5, 0.7, and 1 μm, respectively, and are described in Table 1 with their expected performances.

Table 1. Theoretical Performances of the PIN Ge Photodiodes

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C. Device Fabrication

The photodiodes and passive structures (fiber couplers and waveguides) have been fabricated on 200 mm SOI wafers with 2 μm buried oxide (BOX) and 220 nm upper Si layer using CEA-Leti clean room facilities. The single-mode strip waveguides (220nm×500nm) as well as the grating couplers were fabricated by 193 nm deep-UV lithography followed by dry etching of the Si layer down to the BOX. An 800 nm thick SiO2 layer was then deposited and polished down to 700 nm by chemical–mechanical polishing (CMP). 10×10μm cavities were defined and SiO2 and Si layers were etched at the end of the waveguide, leaving roughly 50 nm of Si at the bottom of the cavity, as illustrated in Fig. 5. Ge was selectively grown (with GeH4) into the Si cavity by the reduced-pressure chemical vapor deposition (RPCVD) technique. In order to accommodate the lattice mismatch (4.2%) between Ge and Si, a thin Ge buffer layer was grown at low temperature (400°C) followed by a thicker layer grown at higher temperature (750°C), yielding a higher growth rate, a smoother surface, and some defect curing. To avoid faceting of the Ge layer inside the Si cavity, the Ge was overgrown outside of the cavity, which also has the benefit of reducing the threading dislocation density (TDD) [32

32. J. M. Hartmann, J.-F. Damlencourt, Y. Bogumilowicz, P. Holliger, G. Rolland, and T. Billon, “Reduced pressure-chemical vapor deposition of intrinsic and doped Ge layers on Si(001) for microelectronics and optoelectronics purposes,” J. Cryst. Growth 274, 90–99 (2005). [CrossRef]

]. When the photodiode will be reversed biased, the threading dislocations will serve as generation centers, thus increasing the dark current. To further reduce the TDD, the selective epitaxial growth was followed by short thermal cycling under H2 [38

38. S. Kobayashi, Y. Nishi, and K. C. Saraswat, “Effect of isochronal hydrogen annealing on surface roughness and threading dislocation density of epitaxial Ge films grown on Si,” Thin Solid Films 518, S136–S139 (2010). [CrossRef]

]. Finally, a CMP step was used to recover a flat surface and reduce the Ge layer thickness down to around 350 nm, as shown by the cross-sectional transmission electron microscopy (TEM) image in Fig. 6. A SiO2 layer was deposited prior to the ion implantation step to protect the Ge layer and limit dose loss during activation anneals. A self-alignment process was used to ensure a good definition of the intrinsic region centered on the Si waveguide. Intrinsic regions with three widths (0.5, 0.7, and 1 μm) have been fabricated. n-type and p-type regions were implanted with phosphorus and boron, respectively. Rapid thermal annealing (RTA) was used to activate the dopants. A SiO2 capping layer was deposited, and the contacts on the doped Ge regions were fabricated by etching 0.4×0.4μm via holes down to the Ge-doped regions. Ti/TiN was then deposited prior to filling with W and a last CMP step. Electrodes were fabricated by depositing and then patterning a Ti/TiN/AlCu metal stack. Three 200 mm wafers were processed within the same batch with the same conditions. This allows analysis of homogeneity over a wafer as well as wafer-to-wafer uniformity. Some statistical results and the best device performances are then reported.

Fig. 5. Schematic cross-sectional view of the final photodiode structure. Light coming from the waveguide is injected into the intrinsic region of the Ge photodiode perpendicularly to the schematics.
Fig. 6. Cross-sectional TEM image of the cavity after Ge epitaxy and CMP steps.

3. DEVICE PERFORMANCE: RESULTS AND DISCUSSION

In this section, characterization of the three types of photodiodes (A, B, and C) will be presented. The experimental results will be compared to theoretical expected characteristics, and we will discuss and try to explain the discrepancies with the help of simulations.

A. DC Measurements

Dark current was first measured as a function of the applied bias for the three PIN photodiodes described previously (A, B, and C), this over the full wafers. Yield results are provided in Table 2.

Table 2. Dark Current Median Values over Full Wafers (373 dies per wafer) for the Three Types of Photodiodes

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The yield displayed is the ratio of the number of photodiodes exhibiting the current-voltage characteristic of a diode to the total number of photodiodes. The median is more representative than the average since the latter will be degraded by very leaky photodiodes (with dark current in the range of several microampere). The best photodiodes have then been selected for detailed measurements. Table 3 shows the best dark current and corresponding current densities obtained for the three types of photodiodes. Such dark currents are in the range of the best reported values for Ge PIN photodiodes, and close to their III–V counterparts. The lowest dark current value is obtained for C-type photodiodes and was equal to 6 nA under 1V.

Table 3. Best Dark Current and Dark Current Density Values under 1V Reverse Bias for the Three Wafers and the Three Photodiodes

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A full current-voltage characteristic is shown in Fig. 7 for the three photodiodes measured on the same die. They exhibit very good rectifying characteristics. Series resistances RS were extracted from the 0.8–1.5 V region of the curves; values ranged from 18Ω to 36Ω. It is clear from I–V measurements that our photodiode leakage shows a strong dependence on the electric field. Indeed, the dark current directly increases with increasing bias and/or reducing intrinsic region width (the smaller wi, the stronger the electric field). To guarantee the best SNR, the dark current must remain as low as possible. A low reverse bias is thus preferred as long as it does not degrade the responsivity and the frequency response of the photodiode.

Fig. 7. Typical dark current characteristics of photodiodes A, B, and C (wi=0.5, 0.7, and 1 μm, respectively).
Fig. 8. Device capacitance as a function of reverse bias. A, B, and C type photodiodes have wi=0.5, 0.7, and 1 μm, respectively.

In order to evaluate the responsivity of the photodiodes, the coupling losses of the grating couplers were measured using a test structure composed of two grating couplers connected by a wide Si waveguide (2 μm width and 220 nm height) assuming no propagation losses along the waveguide. Since the grating couplers were optimized for TE polarization, a polarization controller was used at the output of the laser, and the optical power at the output of the polarization controller was measured to be 544 μW. The coupling losses at a wavelength of 1550 nm were extracted, and the optical power at the input of the photodiode was then estimated to be around 110 μW. The photocurrent does not increase with increasing reverse bias. The electric field inside the depletion region is high enough to extract all the photogenerated carriers even at zero bias. The responsivity at 1550 nm increases from 0.52 to 0.78A/W when wi goes from 0.5 to 1 μm, with 0.62A/W for the wi=0.7μm diode.

B. AC Measurements

The capacitances of the photodiodes were measured at 1 MHz using a calibrated LCR meter. The capacitance evolution with applied reverse bias is shown in A, B, and C, respectively. From for the three types of photodiodes. At 3V, capacitances were measured to be 4fF, 3fF, and 2.5fF for photodiodes A, B, and C, respectively Fig. (8). From PIN junction simple theory, it is estimated that if the doping concentrations are high enough and symmetric in the p-type and n-type doped regions, with abrupt profiles, the space charge region will not extend that much (few nanometers) with reverse bias. However, clear voltage dependence is shown here, meaning that the intrinsic region is not fully depleted under low reverse bias, which is also due to the nonabrupt doping profiles.

Fig. 9. (a) Photodiode frequency response at 3V bias. For clarity purposes, the curves for photodiodes A and C were artificially shifted by +3 and 3dB, respectively. (b) 3dB opto-electrical bandwidth function of the applied reverse bias for the three photodiodes. A, B, and C type photodiodes have wi=0.5, 0.7, and 1 μm, respectively.

Table 4. AC Measured and Extrapolated Results

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The maximum bandwidth is greater and the responsivity lower than the theoretical ones. It corresponds to a reduction of the effective intrinsic region width, leading to a smaller transit time and also to a decrease of the absorption efficiency due to higher overlapping between the optical modes and the doped regions. To investigate this effect, process simulations were run, in particular ion implantation and annealing steps. Monte Carlo ion implantation simulations were performed with Silvaco ATLAS software. Those simulations were calibrated with secondary ion mass spectrometry (SIMS) profiles to ensure the validity of the results. Good agreements were found between the simulated and measured 1D profiles. Using those calibrated profiles, it was possible to simulate the ion implantation in 2D for the p-type and n-type doped region definition, for the three types of photodiodes, and the doping profiles along the junction are shown in Fig. 10. Besides, phosphorus is known to be diffusing into germanium [39

39. C. O. Chui, K. Gopalakrishnan, P. B. Griffin, J. D. Plummer, and K. C. Saraswat, “Activation and diffusion studies of ion-implanted p and n dopants in germanium,” Appl. Phys. Lett. 83, 3275–3277 (2003). [CrossRef]

42

42. S. Koffel, R. J. Kaiser, A. J. Bauer, B. Amon, P. Pichler, J. Lorenz, L. Frey, P. Scheiblin, V. Mazzocchi, J.-P. Barnes, and A. Claverie, “Experiments and simulation of the diffusion and activation of the n-type dopants P, As, and Sb implanted into germanium,” Microelectron. Eng. 88, 458–461 (2011). [CrossRef]

]. With our annealing conditions, phosphorus diffused during the activation annealing step, further reducing the effective intrinsic region width. Diffusion mechanisms of phosphorous are still not yet well understood, and the lack of parameters in Silvaco ATHENA prevented us from accurately matching SIMS profiles after annealing.

Fig. 10. TCAD simulated implantation profiles for the three designed intrinsic region widths, obtained by cross section along the dashed line shown in the inset of the figure.

In the following analysis, the BER is fixed to 1e12 and the corresponding Q value is 7, evaluated from Eq. (5). For 25 Gbps applications, the type B photodiodes exhibit the highest responsivity at 25 GHz under zero bias; consequently they will have the greatest sensitivity. The sensitivity of the receiver using photodiode B is computed as a function of the global rms noise current (including both the photodiode and TIA contributions). The sensitivity will be limited by the TIA noise, giving a maximum sensitivity of 13.9dBm. At 1V reverse bias, type C photodiodes exhibit the highest responsivity at 25 GHz. Including the dark current to the shot noise with a typical value of 61 nA at 1V, the sensitivity is still limited by the TIA but increases to 15.4dBm. Further increase of the reverse bias leads to 15.5dBm sensitivity at 2V with photodiode C. In any case, the front end noise will limit the performance of the photodiodes, and a very low noise amplifier will be needed to increase the sensitivity of the receiver.

For 40 Gbps operation, type A photodiodes yield the highest responsivity at zero bias, leading to 12.3dBm sensitivity. At 1V reverse bias, type C photodiodes will offer the greatest sensitivity, i.e., 14dBm. Pushing the reverse bias to 2V will only increase sensitivity to 14.1dBm.

The lack of low noise TIA for data rates over 10 Gbps limits the maximum sensitivity of Ge PIN-based receivers. Assuming an ideal PIN photodiode with maximum theoretical responsivity at 1550 nm of 1.24A/W at zero bias (without dark current), maximum TIA limited sensitivity will be around 17.9dBm. To achieve sensitivities over this value, with actual TIA, the only way is to increase the use of avalanche photodiodes (APDs). However APDs require high bias, thus increasing the power consumption, but also limiting its integration with CMOS-based drivers.

4. CONCLUSION

We have demonstrated the integration of Ge photodiodes on Si at the wafer scale using selective epitaxial growth of Ge with very good yields over three 200 mm wafers. The lateral butt-coupling configuration of the PIN photodiodes allowed simple process integration with limited steps and good robustness. The fabricated photodiodes exhibited responsivity ranging from 0.52 to 0.78A/W at 1550 nm. The dark currents reported here are the lowest achieved for such a photodiode configuration, proving the good crystalline quality of the epitaxial Ge layer, with only 6 nA for the best device, and 61 nA for the median value over a wafer, under 1V. Zero bias operation led to optical bandwidth over 50 GHz, enabling 40 Gbps data transmission with very low power consumption. Using commercially available TIA characteristics, the sensitivity of a receiver based on presented photodiodes, biased at 1V, has been estimated to be 15.4 and 14dBm for data rates of 25 and 40 Gbps, respectively, for a BER of 1e12.

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D. Choi, Y. Ge, J. S. Harris, J. Cagnon, and S. Stemmer, “Low surface roughness and threading dislocation density Ge growth on Si (001),” J. Cryst. Growth 310, 4273–4279 (2008). [CrossRef]

34.

K. Toko, T. Tanaka, Y. Ohta, T. Sadoh, and M. Miyao, “Defect-free Ge-on-insulator with (100), (110), and (111) orientations by growth-direction-selected rapid-melting growth,” Appl. Phys. Lett. 97, 152101 (2010). [CrossRef]

35.

K. Toko, Y. Ohta, T. Sakane, T. Sadoh, I. Mizushima, and M. Miyao, “Single-crystalline (100) Ge networks on insulators by rapid-melting growth along hexagonal mesh-pattern,” Appl. Phys. Lett. 98, 042101 (2011). [CrossRef]

36.

Y. H. Tan and C. S. Tan, “Growth and characterization of germanium epitaxial film on silicon (001) using reduced pressure chemical vapor deposition,” Thin Solid Films 520, 2711–2716 (2012). [CrossRef]

37.

G. Lucovsky, R. F. Schwarz, and R. B. Emmons, “Transit-time considerations in p–i–n diodes,” J. Appl. Phys. 35, 622–628 (1964). [CrossRef]

38.

S. Kobayashi, Y. Nishi, and K. C. Saraswat, “Effect of isochronal hydrogen annealing on surface roughness and threading dislocation density of epitaxial Ge films grown on Si,” Thin Solid Films 518, S136–S139 (2010). [CrossRef]

39.

C. O. Chui, K. Gopalakrishnan, P. B. Griffin, J. D. Plummer, and K. C. Saraswat, “Activation and diffusion studies of ion-implanted p and n dopants in germanium,” Appl. Phys. Lett. 83, 3275–3277 (2003). [CrossRef]

40.

M. Koike, Y. Kamata, T. Ino, D. Hagishima, K. Tatsumura, M. Koyama, and A. Nishiyama, “Diffusion and activation of n-type dopants in germanium,” J. Appl. Phys. 104, 023523 (2008). [CrossRef]

41.

H. Bracht, S. Schneider, and R. Kube, “Diffusion and doping issues in germanium,” Microelectron. Eng. 88, 452–457 (2011). [CrossRef]

42.

S. Koffel, R. J. Kaiser, A. J. Bauer, B. Amon, P. Pichler, J. Lorenz, L. Frey, P. Scheiblin, V. Mazzocchi, J.-P. Barnes, and A. Claverie, “Experiments and simulation of the diffusion and activation of the n-type dopants P, As, and Sb implanted into germanium,” Microelectron. Eng. 88, 458–461 (2011). [CrossRef]

43.

W. T. Tsang, ed., Semiconductors and Semimetals, Lightwave Communication Technology, Part D. Photodetectors (Academic, 1985), Vol. 22, pp.  1–451.

OCIS Codes
(040.5160) Detectors : Photodetectors
(130.0250) Integrated optics : Optoelectronics
(130.3120) Integrated optics : Integrated optics devices
(230.5160) Optical devices : Photodetectors
(230.5170) Optical devices : Photodiodes

ToC Category:
Integrated Optics

History
Original Manuscript: June 29, 2013
Revised Manuscript: August 29, 2013
Manuscript Accepted: August 29, 2013
Published: October 2, 2013

Citation
Léopold Virot, Laurent Vivien, Jean-Marc Fédéli, Yann Bogumilowicz, Jean-Michel Hartmann, Frédéric Bœuf, Paul Crozat, Delphine Marris-Morini, and Eric Cassan, "High-performance waveguide-integrated germanium PIN photodiodes for optical communication applications [Invited]," Photon. Res. 1, 140-147 (2013)
http://www.opticsinfobase.org/prj/abstract.cfm?URI=prj-1-3-140


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  39. C. O. Chui, K. Gopalakrishnan, P. B. Griffin, J. D. Plummer, and K. C. Saraswat, “Activation and diffusion studies of ion-implanted p and n dopants in germanium,” Appl. Phys. Lett. 83, 3275–3277 (2003). [CrossRef]
  40. M. Koike, Y. Kamata, T. Ino, D. Hagishima, K. Tatsumura, M. Koyama, and A. Nishiyama, “Diffusion and activation of n-type dopants in germanium,” J. Appl. Phys. 104, 023523 (2008). [CrossRef]
  41. H. Bracht, S. Schneider, and R. Kube, “Diffusion and doping issues in germanium,” Microelectron. Eng. 88, 452–457 (2011). [CrossRef]
  42. S. Koffel, R. J. Kaiser, A. J. Bauer, B. Amon, P. Pichler, J. Lorenz, L. Frey, P. Scheiblin, V. Mazzocchi, J.-P. Barnes, and A. Claverie, “Experiments and simulation of the diffusion and activation of the n-type dopants P, As, and Sb implanted into germanium,” Microelectron. Eng. 88, 458–461 (2011). [CrossRef]
  43. W. T. Tsang, ed., Semiconductors and Semimetals, Lightwave Communication Technology, Part D. Photodetectors (Academic, 1985), Vol. 22, pp.  1–451.

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