High responsivity, low dark current, heterogeneously integrated thin film Si photodetectors on rigid and flexible substrates Sulochana Dhar,1,2,* David M. Miller,1 and Nan M. Jokerst1 1
Department of Electrical and Computer Engineering, Duke University, Durham, North Carolina 27708, USA 2 Currently at Intel Corporation, Elam Young Parkway, Hillsboro, Oregon 97124, USA * [email protected]
Abstract: We report thin film single crystal silicon photodetectors (PDs), composed of 13- 25 μm thick silicon, heterogeneously bonded to transparent Pyrex® and flexible Kapton® substrates. The measured responsivity and dark current density of the PDs on pyrex is 0.19 A/W – 0.34 A/W (λ = 470 nm – 600 nm) and 0.63 nA/cm2, respectively, at ~0V bias. The measured responsivity and dark current density of the flexible PDs is 0.16 A/W – 0.26 A/W (λ = 470 nm – 600 nm) and 0.42 nA/cm2, respectively, at a ~0V bias. The resulting responsivity-to-dark current density ratios for the reported rigid and flexible PDs are 0.3-0.54 cm2/nW and 0.38-0.62 cm2/nW, respectively. These are the highest reported responsivity-to-dark current density ratios for heterogeneously bonded thin film single crystal Si PDs, to the best of our knowledge. These PDs are customized for applications in biomedical imaging and integrated biochemical sensing. ©2014 Optical Society of America OCIS codes: (040.5160) Photodetectors; (040.6040) Silicon.
References and links 1.
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Received 17 Dec 2013; revised 15 Feb 2014; accepted 17 Feb 2014; published 25 Feb 2014 10 March 2014 | Vol. 22, No. 5 | DOI:10.1364/OE.22.005052 | OPTICS EXPRESS 5052
11. L. Luan, M. W. Royal, R. Evans, R. B. Fair, and N. M. Jokerst, “Chip scale optical microresonator sensors integrated with embedded thin film photodetectors on electrowetting digital microfluidics platforms,” IEEE Sens. J. 12(6), 1794–1800 (2012). 12. D S. Duun, R. G. Haahr, O. Hansen, K. Birkelund, and E. V. Thomsen, “High quantum efficiency annular backside silicon photodiodes for reflectance pulse oximetry in wearable wireless body sensors,” J. Micromech. Microeng. 20(7), 075020 (2010). 13. S. Dhar and N. M. Jokerst, “High responsivity, low dark current, large area, heterogenously bonded annular thinfilm silicon photodetectors,” IEEE Photonics Conference (2012). 14. S. Joo and D. F. Baldwin, “Adhesion mechanisms of nanoparticle silver to substrate materials: identification,” Nanotechnology 21(5), 055204 (2010). 15. X. Xu, H. Subbaraman, D. Kwong, A. Hosseini, Y. Zhang, and R. T. Chen, “Large area silicon nanomembrane photonic devices on unconventional substrates,” IEEE Photon. Technol. Lett. 25(16), 1601–1604 (2013). 16. S. Saha, M. M. Hilali, E. U. Onyegam, D. Sarkar, D. Jawarani, R. A. Rao, L. Mathew, R. S. Smith, D. Xu, U. K. Das, B. Sopori, and S. K. Banerjee, “Single heterojunction solar cells on exfoliated flexible 25 μm thick monocrystalline silicon substrates,” Appl. Phys. Lett. 102(16), 163904 (2013). 17. M. A. Meitl, Z. T. Zhu, V. Kumar, K. J. Lee, X. Feng, Y. Y. Huang, I. Adesida, R. G. Nuzzo, and J. A. Rogers, “Transfer printing by kinetic control of adhesion to an elastomeric stamp,” Nat. Mater. 5(1), 33–38 (2006). 18. N. M. Jokerst, M. A. Brooke, O. Vendier, S. Wilkinson, S. Fike, M. Lee, E. Twyford, J. Cross, B. Buchanan, and S. Wills, “Thin-film multimaterial optoelectronic integrated circuits,” IEEE Trans. Compon., Packag., Manuf. Technol., Part B 19(1), 97–106 (1996). 19. S. M. Fike, B. Buchanan, N. M. Jokerst, M. A. Brooke, T. G. Morris, and S. P. DeWeerth, “8 x 8 array of thinfilm photodetectors vertically electrically interconnected to silicon circuitry,” IEEE Photon. Technol. Lett. 7(10), 1168–1170 (1995). 20. D. H. Kim, N. Lu, R. Ma, Y. S. Kim, R. H. Kim, S. Wang, J. Wu, S. M. Won, H. Tao, A. Islam, K. J. Yu, T. I. Kim, R. Chowdhury, M. Ying, L. Xu, M. Li, H. J. Chung, H. Keum, M. McCormick, P. Liu, Y. W. Zhang, F. G. Omenetto, Y. Huang, T. Coleman, and J. A. Rogers, “Epidermal electronics,” Science 333(6044), 838–843 (2011). 21. S. Y. Lo, D. S. Wuu, C. H. Chang, C. C. Wang, S. Y. Lien, and R. H. Horng, “Fabrication of flexible amorphous-Si thin-film solar cells on a parylene template using a direct separation process,” IEEE Trans. Electron. Dev. 58(5), 1433–1439 (2011). 22. D. H. Kim, J. Viventi, J. J. Amsden, J. Xiao, L. Vigeland, Y. S. Kim, J. A. Blanco, B. Panilaitis, E. S. Frechette, D. Contreras, D. L. Kaplan, F. G. Omenetto, Y. Huang, K. C. Hwang, M. R. Zakin, B. Litt, and J. A. Rogers, “Dissolvable films of silk fibroin for ultrathin conformal bio-integrated electronics,” Nat. Mater. 9(6), 511–517 (2010). 23. Emulsitone, http://www.emulsitone.com/psif5x10_20.html. 24. T. A. Kwa, P. M. Sarro, and R. F. Wolffenbuttel, “Backside-illuminated silicon photodiode array for an integrated spectrometer,” IEEE Trans. Electron. Dev. 44(5), 761–765 (1997). 25. J. Muller, “Thin silicon film p-i-n photodiodes with internal reflection,” IEEE J. Solid-State Circuits 13(1), 173– 179 (1978). 26. M. Purica, E. Budianu, and M. Elena, “Blue/ultraviolet sensitive detector on silicon membrane,” Proceedings of International Semiconductor Conference (2002). 27. Y. Li, X. Mi, M. Sasaki, and K. Hane, “Precision optical displacement sensor based on ultra-thin film photodiode type optical interferometers,” Meas. Sci. Technol. 14(4), 479–483 (2003). 28. H. Zimmermann and B. Muller, “Ultralow-capacitance lateral p-i-n photodiode in a thin c-Si film,” IEEE Trans. Nucl. Sci. 49(4), 2032–2036 (2002). 29. H. Zimmermann, B. Muller, A. Hammer, K. Herzog, and P. Seegebrecht, “Large-area lateral p-i-n photodiode on SOI,” IEEE Trans. Electron. Dev. 49(2), 334–336 (2002). 30. M. J. Kerr, J. Schmidt, A. Cuevas, and J. H. Bultman, “Surface recombination velocity of phosphorus-diffused silicon solar cell emitters passivated with plasma enhanced chemical vapor deposited silicon nitride and thermal silicon oxide,” J. Appl. Phys. 89(7), 3821–3826 (2001).
1. Introduction The heterogeneous integration of thin film photonic devices onto host substrates, and in particular, flexible substrates, enables the development of new integrated structures, including bendable and conformal electronic and photonic systems. Example of conformal systems include flexible optical interconnects with integrated laser sources and waveguides [1, 2], large area flexible silicon solar cells [3], and photodetection systems [4–6]. The integration of thin film flexible single crystal material, such as Ge and Si, can result in high performance (high mobility, high responsivity, low noise) photonic devices critical for the development of high signal-to-noise (SNR), low power, conformal, portable systems [4, 5].
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Received 17 Dec 2013; revised 15 Feb 2014; accepted 17 Feb 2014; published 25 Feb 2014 10 March 2014 | Vol. 22, No. 5 | DOI:10.1364/OE.22.005052 | OPTICS EXPRESS 5053
High SNR, portable photodetection systems have applications in integrated biomedical imaging systems, and chemical sensing systems [7–12], many of which use the wavelength range λ = 400 nm – 700 nm to detect absorption and light scattering in biological tissue (from hemoglobin, de-oxy hemoglobin, β-carotene), and biochemical and fluorescence reactions. The absorption coefficient of silicon in this wavelength range makes it an excellent candidate for the development of photodetection systems customized for integrated sensing and imaging. Previously, our group has developed a diffuse reflectance spectroscopy (DRS) imaging probe for breast cancer margin assessment, composed of an array of custom bulk Si 600 μm thick annular photodetectors, as shown in Fig. 1(a) [7]. An optical analysis of the reported DRS system, which was back illuminated through the hole in the photodetector annulus, showed that the system would have higher throughput and less back illumination noise if the detector were in a thin film format. Towards the development of an integrated, portable sensing and imaging probe, our group reported preliminary results on highresponsivity, low dark current, heterogeneously bonded, annular, thin-film Si photodetectors developed for low light level biochemical sensing and biomedical imaging systems where portable light sources, low power consumption and high SNR requirements drive the development of high responsivity, low dark current photodetectors [13]. This paper focuses on the development of high responsivity, low dark-current, large-area, single-crystal, rigid and flexible thin film silicon photodetectors for high performance, low operating voltage detection schemes for conformal tissue imaging. The PDs have a 5.2 mm outer diameter (O.D.), and 1 mm inner diameter (I.D.). The rigid and flexible PDs are integrated on to a 500 μm thick transparent Pyrex® substrate, and a 127 μm thick flexible Kapton® substrate, respectively. Figure 1(b) shows a cross-section schematic of the reported PDs. The fabrication and characterization of the rigid and flexible devices are discussed herein. The responsivity and dark current performance is compared to previously reported rigid and flexible photodetectors in the literature to indicate that, to the best of our knowledge, the devices reported in this paper have the highest responsivity to dark-current ratio reported to date for single crystal thin film Si photodetectors.
Fig. 1. (a) Schematic of the previously developed diffuse reflectance spectroscopy probe for breast cancer margin assessment [7] (b) Cross-section view of the thin-film annular PDs reported herein.
2. Fabrication and integration processes for rigid and flexible annular thin-film Si PDs The rigid thin film PDs were fabricated from a p-type silicon-on-insulator (SOI) sample: 10 μm – 13 μm Si (30 Ω-cm – 60 Ω-cm, boron)/ 500 nm SiO2/670 μm Si substrate (30 Ω-cm – 60 Ω-cm, boron). Fabrication began with the spin-coating of a phosphorous doped spin-on glass (Emulsitone, Phosphorosilica film 5x1020 cm−3) on the device layer of the SOI substrate. A dopant diffusion anneal was performed at 1000 °C for 15 minutes to form the n-type region of the pn-junction. Dual-ring top contacts composed of Ti (1000 Å)/Ni (500 Å)/Au (2000 Å) were patterned using negative UV-lithography and lift-off. Individual annular PD mesas were patterned using positive UV lithography, and etched using deep reactive ion etching (DRIE, #203062 - $15.00 USD (C) 2014 OSA
Received 17 Dec 2013; revised 15 Feb 2014; accepted 17 Feb 2014; published 25 Feb 2014 10 March 2014 | Vol. 22, No. 5 | DOI:10.1364/OE.22.005052 | OPTICS EXPRESS 5054
SPTS Pegasus) which selectively stopped at the buried oxide layer (BOX) of the SOI substrate. Next, the individual PD mesas were protected in a spin coated layer of transparent bonding adhesive Waferbond®, and thermo-compressively bonded to a transparent temporary carrier substrate at 180°C in a vacuum oven (Fisher Scientific Isotemp 282A). The Si handle of the SOI substrate was removed using DRIE (Bosch process) which was selective to the BOX layer. Next, the BOX layer was etched using buffered oxide etchant, to expose the back of the Si PD mesas. Broad area back contacts, composed of Al (1000 Å)/Ti (800 Å)/Ni (500 Å)/Au (2000 Å), were patterned using negative UV lithography and lift-off. Metal pads consisting of 1 mm apertures and composed of Cr (50 Å)/Au (2000 Å) were deposited and patterned on the transparent Pyrex® host substrate. The thin-film PDs, supported on the temporary carrier substrate, were then thermo-compressively bonded to the host Pyrex® substrate using an Au-Au bond. The sample was then heated to 250 °C, the de-bonding temperature of the bonding adhesive Waferbond® to remove the temporary carrier substrate to complete the transfer of the thin-film PDs. The residual Waferbond® on the bonded PDs was removed using oxygen plasma ashing. Finally, a 52 nm SiN anti-reflection (AR) and passivation coating was deposited on the PDs using PECVD, and contact openings were etched using reactive ion etching. Figure 2(a) shows a transferred and bonded 2x1 array of rigid thin film annular Si pn junction PDs. The flexible thin-film annular Si PDs were fabricated from a p-type SOI sample: 25 μm Si (37.5 Ω-cm – 62.5 Ω-cm, boron)/ 1 μm SiO2/576 μm Si substrate (37.5 Ω-cm – 62.5 Ω-cm, boron). Fabrication of the flexible PDs began with the spin-coating of a phosphorous doped spin-on glass (Emulsitone, Phosphorosilica film 5x1020 cm−3) on the device layer of the SOI substrate. A dopant diffusion anneal was performed at 950 °C for 15 minutes to form the ntype region of the pn-junction. The front and back contact geometry and patterning, mesa patterning, and substrate removal process was identical to the rigid PDs. The 127 μm thick Kapton® host substrate was prepared for high temperature thermo-compression bonding process by a 250 °C, 20 min. thermal cure to release residual stress developed in the film during the manufacturing process. The surface roughness of the Kapton substrate is estimated to be ~330 Å [14], so a blanket Cr (50 Å)/Au (5000 Å) layer was deposited on the substrate, to ensure continuous metal coverage on the rough surface. The thin-film PD array, supported on a temporary pyrex carrier substrate, was then transferred and bonded to the prepared Kapton substrate using an Au-Au bond formed using thermo-compressive bonding. The sample was then heated to 250 °C to remove the temporary carrier substrate. The residual Waferbond® on the bonded PDs was removed using Waferbond Remover® and oxygen plasma ashing. Figure 2(b) shows a 2x1 array of 25 μm thick thin-film annular Si pn-junction PDs bonded to a flexible Kapton substrate. Figure 2(c) and 2(d) demonstrate inward and outward bending of the flexible PD array, respectively. It should be noted that no AR coating or passivation layer was deposited on these PDs. The development of larger arrays composed of the PD elements reported herein is possible given the scalability of the wafer bonding and DRIE processes for formation and transfer of thin film Si. Patterned thin film Si waveguide arrays, 4 cm2 in area, have been previously developed using large area wafer bonding and DRIE processing for thin film transfer and substrate removal [15]. Substrate heating due to large area substrate removal using dry etching can be addressed by using mechanical polishing and wet etching in conjunction with DRIE [15]. The host substrates utilized in this report are required to withstand up to 250 °C due to the use of Waferbond for thin-film transfer and handling. However, there are many other low thermal budget processes such as kerf-less exfoliation [16], methods utilizing elastomeric stamps [17] and transfer diaphragms [18] that enable fabrication, transfer, and bonding of thin-film devices, and thin-film device arrays to foreign substrates [3, 19]. These methods would allow integration of the devices reported herein on to substrates such as PET [5, 6], polyester [20], Parylene [21], and silk fibroin [22], and other inexpensive, eco-friendly and
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Received 17 Dec 2013; revised 15 Feb 2014; accepted 17 Feb 2014; published 25 Feb 2014 10 March 2014 | Vol. 22, No. 5 | DOI:10.1364/OE.22.005052 | OPTICS EXPRESS 5055
bio-compatible substrates. However, contact annealing temperatures affect contact resistance and dark currents, and processes which enable contact annealing (not necessarily after bonding to the host substrate) are an important consideration for high performance devices.
Fig. 2. (a) Photomicrograph of a 2x1 array of rigid annular thin-film Si PDs bonded to a Au coated Pyrex® substrate; (b) Photomicrograph of a 2x1 array of flexible annular thin-film PDs bonded to a Au coated Kapton® substrate; (c) Demonstration of inward bending of flexible PDs; (d) demonstration of outward bending of flexible PDs.
3. Device characterization and analysis The surface normal spectral response and dark current of the fabricated 2x1 PD array samples were measured using a Keithley source-measurement unit (SMU-4200). All spectral response and dark current measurements were performed at near 0 V (35 μV – 176 μV) bias. A fibercoupled Xenon arc lamp (MAX-302, Asahi Spectra) with eight wavelength filters, each with 10 nm FWHM (λ = 470 nm, 480 nm, 490 nm, 500 nm, 510 nm, 560 nm, 580 nm, 600 nm), was used as the source for spectral response measurements, which is driven by the DRS application. The responsivity for each PD was calculated using the photocurrent measured for each of the eight wavelengths by illuminating each PD with a fiber bundle in the surface normal configuration, and the power measured at the output of the source fiber bundle. The surface normal spectral response of the rigid PDs was modeled using a 3D simulation in Silvaco Atlas®. The simulated model used the dopant concentration of the starting wafer used to fabricate the devices (3x1014 cm−3). The simulated structure consisted of a 10 μm x 10 μm x 10 μm volume of p-type silicon. The simulated area was restricted to 10 μm x 10 μm to save computational time since the doping contours (and therefore electric field contours) varied only as a function of the depth of the semiconductor. A complementary error-function profile was used for phosphorous diffusion with a phosphorous surface concentration equal to 5x1020 cm−3. The dopant profile and surface concentration corresponded to the phosphorous doped spin-on-glass used for fabricating the pn junction in the devices [23]. The top and bottom contacts were assumed to be ideal ohmic contacts. A 52 nm thick SiN AR coating was included in the model. A 3D optical source with a beam intensity of 1 W/cm2 was swept across a 470 nm – 600 nm wavelength range to estimate the photocurrent. The photocurrent values calculated in Silvaco were divided by the power incident on the illuminated front
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Received 17 Dec 2013; revised 15 Feb 2014; accepted 17 Feb 2014; published 25 Feb 2014 10 March 2014 | Vol. 22, No. 5 | DOI:10.1364/OE.22.005052 | OPTICS EXPRESS 5056
surface (10 μm x 10 μm) of the simulated structure to calculate the spectral response. Figure 3(a) shows the measured responsivity for the fabricated 2x1 array of thin film PDs with a junction anneal of 1000 °C with and without an AR coating. It was observed that the measured responsivity (with and without the AR coating) was in good agreement with the responsivity simulated in Silvaco for a junction depth of 750 nm. The dark-current density for the rigid array was measured to be 0.63 nA/cm2.This yielded a responsivity to dark current density ratio of 0.3 cm2/nW – 0.54 cm2/nW (λ = 470 nm-600 nm) for the rigid PD array. Figure 3(b) shows the measured spectral response for the fabricated flexible PD array, before any flexing was performed on the array. The measured responsivity was 0.16 A/W – 0.26 A/W for λ = 470 nm – 600 nm. Also shown in Fig. 3(b) is the measured spectral response for a 2x1 PD array bonded to a rigid Pyrex substrate (without AR coating), for comparison. The junction doping and anneal conditions for the flexible and rigid devices were identical (Emulsitone, Phosphorosilica film 5x1020 cm−3 annealed at 950°C for 15 minutes). The resistivity and thickness of the flexible and rigid devices were 37.5 Ω-cm – 62.5 Ω-cm and 25 µm, and 30 Ω-cm – 60 Ω-cm and 10 µm, respectively. The similarity in responsivity values for the two sets of devices indicated that there was no severe degradation in PD spectral response by bonding and handling PDs on a flexible substrate. The dark current measured for flexible devices was 60 pA – 111.39 pA, i.e. a dark current density of 0.293 0.545 nA/cm2. This translates to an average responsivity to dark current ratio of 0.38 – 0.62 cm2/nW.
Fig. 3. (a) Measured and simulated spectral response of a 2x1 array of rigid thin-film Si PDs with junction anneal temperature = 1000°C; (b) Measured spectral response of flexible and rigid thin-film Si PDs with junction anneal temperature = 950°C. +/−σ error for the photoresponse measurements is more than 2 orders of magnitude lower than the signal measured.
4. Bending fatigue characterization of flexible annular thin-film Si PDs Next, the effect of repeated bending on the performance of the fabricated flexible PD array was assessed. Repeated measurements of the surface normal spectral response and the dark IV curve of a fabricated PD in the array were made after outward bending of the array 20 times on tubes of four different radii (in order of testing): 6.43 cm, 5 cm, 3.45 cm, and 2.82 cm, and inward bending of the array 20 times on the following radii (in order of testing): 5.73 cm, 4.4 cm, 3.45 cm, and 2.4 cm. One bending cycle was defined as the bending of devices on the tube and releasing them back to their original configuration. The O.D. and I.D. of the flexible PDs are 5.2 mm and 1 mm, respectively, and the center-to-center spacing is 6 mm. Figure 4(a) and 4(c) show the surface normal spectral response at the start of the experiment, and after 20 outward and inward bending cycles on each tube, respectively. Figure 4(b) and #203062 - $15.00 USD (C) 2014 OSA
Received 17 Dec 2013; revised 15 Feb 2014; accepted 17 Feb 2014; published 25 Feb 2014 10 March 2014 | Vol. 22, No. 5 | DOI:10.1364/OE.22.005052 | OPTICS EXPRESS 5057
4(d) show the dark I-V curves measured at the start of the experiment, and after each set of outward and inward bending cycles, respectively. No significant change in the surface normal spectral response, or dark current at 0V bias was observed after subjecting the fabricated flexible PD array to multiple outward bending cycles down to a radius of 2.82 cm, and multiple inward bending cycles down to a radius of 2.4 cm.
Fig. 4. (a) Outward bending spectral response characterization; (b) Outward bending dark current characterization; (c) Inward bending spectral response characterization; (d) Inward bending dark current characterization.
5. Conclusion and discussion of results There have been several previous reports of thin film monocrystalline Si PDs [3, 4, 6, 24–29], some of which have been heterogeneously bonded to alternative host substrates [3, 4, 6, 24, 25, 27, 28]. One report of 500μm x 500 μm area thin film Si pn junction PD arrays heterogeneously bonded to a flexible membrane demonstrates imaging capability; however, the performance of the reported PDs has not been characterized or optimized [4]. Flexible thin film Si MSM PDs, 3mm x 3 mm in area, integrated onto Polyethylene terephthalate (PET) substrates have been explored for flexible imaging systems; however, the thickness of the fabricated PDs (260 nm) limited the responsivity to 7.4x10−8 A/W at 0V bias, and 7.42x10−5 A/W at 4V bias [6]. The annular devices in this report, each 20.44 mm2 in area, demonstrate a higher responsivity to dark current ratio than previous reports, to the best of our knowledge. The peak responsivity vs. dark current density for previous reports of heterogeneously bonded
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Received 17 Dec 2013; revised 15 Feb 2014; accepted 17 Feb 2014; published 25 Feb 2014 10 March 2014 | Vol. 22, No. 5 | DOI:10.1364/OE.22.005052 | OPTICS EXPRESS 5058
thin-film monocrystalline Si PDs is shown in Fig. 5, demonstrating graphically that the PDs developed in this work achieve higher peak responsivity to dark current density ratio.
Fig. 5. Comparison of annular thin-film Si PDs in this report to previously reported thin-film single crystal Si PDs.
The high responsivity of these devices was attributed to the use of a shallow junction depth for efficient carrier collection, and the use of a SiN AR and passivation layer for rigid PDs that reduced front surface reflectivity and the front surface recombination velocity [30], thereby reducing the recombination rate of the photocarriers generated close to the surface of the PDs. The low dark current density is attributed to the use of a low bias voltage (enabled by the high responsivity achieved in an unbiased condition), a SiN passivation layer (for rigid PDs), the absence of semiconductor material outside of the detection volume which prevented thermally generated carriers in the substrate from diffusing to the pn-junction, and the development of a fabrication process that does not degrade the electronic properties of the semiconductor material. Acknowledgment This research was supported by the National Institutes of Health through the Bioengineering Research Partnership award 1 RO1 EB011574-01A1.
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Received 17 Dec 2013; revised 15 Feb 2014; accepted 17 Feb 2014; published 25 Feb 2014 10 March 2014 | Vol. 22, No. 5 | DOI:10.1364/OE.22.005052 | OPTICS EXPRESS 5059
Department of Electrical and Computer Engineering, Duke University, Durham, North Carolina 27708, USA 2 Currently at Intel Corporation, Elam Young Parkway, Hillsboro, Oregon 97124, USA * [email protected]
Abstract: We report thin film single crystal silicon photodetectors (PDs), composed of 13- 25 μm thick silicon, heterogeneously bonded to transparent Pyrex® and flexible Kapton® substrates. The measured responsivity and dark current density of the PDs on pyrex is 0.19 A/W – 0.34 A/W (λ = 470 nm – 600 nm) and 0.63 nA/cm2, respectively, at ~0V bias. The measured responsivity and dark current density of the flexible PDs is 0.16 A/W – 0.26 A/W (λ = 470 nm – 600 nm) and 0.42 nA/cm2, respectively, at a ~0V bias. The resulting responsivity-to-dark current density ratios for the reported rigid and flexible PDs are 0.3-0.54 cm2/nW and 0.38-0.62 cm2/nW, respectively. These are the highest reported responsivity-to-dark current density ratios for heterogeneously bonded thin film single crystal Si PDs, to the best of our knowledge. These PDs are customized for applications in biomedical imaging and integrated biochemical sensing. ©2014 Optical Society of America OCIS codes: (040.5160) Photodetectors; (040.6040) Silicon.
References and links 1.
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1. Introduction The heterogeneous integration of thin film photonic devices onto host substrates, and in particular, flexible substrates, enables the development of new integrated structures, including bendable and conformal electronic and photonic systems. Example of conformal systems include flexible optical interconnects with integrated laser sources and waveguides [1, 2], large area flexible silicon solar cells [3], and photodetection systems [4–6]. The integration of thin film flexible single crystal material, such as Ge and Si, can result in high performance (high mobility, high responsivity, low noise) photonic devices critical for the development of high signal-to-noise (SNR), low power, conformal, portable systems [4, 5].
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High SNR, portable photodetection systems have applications in integrated biomedical imaging systems, and chemical sensing systems [7–12], many of which use the wavelength range λ = 400 nm – 700 nm to detect absorption and light scattering in biological tissue (from hemoglobin, de-oxy hemoglobin, β-carotene), and biochemical and fluorescence reactions. The absorption coefficient of silicon in this wavelength range makes it an excellent candidate for the development of photodetection systems customized for integrated sensing and imaging. Previously, our group has developed a diffuse reflectance spectroscopy (DRS) imaging probe for breast cancer margin assessment, composed of an array of custom bulk Si 600 μm thick annular photodetectors, as shown in Fig. 1(a) [7]. An optical analysis of the reported DRS system, which was back illuminated through the hole in the photodetector annulus, showed that the system would have higher throughput and less back illumination noise if the detector were in a thin film format. Towards the development of an integrated, portable sensing and imaging probe, our group reported preliminary results on highresponsivity, low dark current, heterogeneously bonded, annular, thin-film Si photodetectors developed for low light level biochemical sensing and biomedical imaging systems where portable light sources, low power consumption and high SNR requirements drive the development of high responsivity, low dark current photodetectors [13]. This paper focuses on the development of high responsivity, low dark-current, large-area, single-crystal, rigid and flexible thin film silicon photodetectors for high performance, low operating voltage detection schemes for conformal tissue imaging. The PDs have a 5.2 mm outer diameter (O.D.), and 1 mm inner diameter (I.D.). The rigid and flexible PDs are integrated on to a 500 μm thick transparent Pyrex® substrate, and a 127 μm thick flexible Kapton® substrate, respectively. Figure 1(b) shows a cross-section schematic of the reported PDs. The fabrication and characterization of the rigid and flexible devices are discussed herein. The responsivity and dark current performance is compared to previously reported rigid and flexible photodetectors in the literature to indicate that, to the best of our knowledge, the devices reported in this paper have the highest responsivity to dark-current ratio reported to date for single crystal thin film Si photodetectors.
Fig. 1. (a) Schematic of the previously developed diffuse reflectance spectroscopy probe for breast cancer margin assessment [7] (b) Cross-section view of the thin-film annular PDs reported herein.
2. Fabrication and integration processes for rigid and flexible annular thin-film Si PDs The rigid thin film PDs were fabricated from a p-type silicon-on-insulator (SOI) sample: 10 μm – 13 μm Si (30 Ω-cm – 60 Ω-cm, boron)/ 500 nm SiO2/670 μm Si substrate (30 Ω-cm – 60 Ω-cm, boron). Fabrication began with the spin-coating of a phosphorous doped spin-on glass (Emulsitone, Phosphorosilica film 5x1020 cm−3) on the device layer of the SOI substrate. A dopant diffusion anneal was performed at 1000 °C for 15 minutes to form the n-type region of the pn-junction. Dual-ring top contacts composed of Ti (1000 Å)/Ni (500 Å)/Au (2000 Å) were patterned using negative UV-lithography and lift-off. Individual annular PD mesas were patterned using positive UV lithography, and etched using deep reactive ion etching (DRIE, #203062 - $15.00 USD (C) 2014 OSA
Received 17 Dec 2013; revised 15 Feb 2014; accepted 17 Feb 2014; published 25 Feb 2014 10 March 2014 | Vol. 22, No. 5 | DOI:10.1364/OE.22.005052 | OPTICS EXPRESS 5054
SPTS Pegasus) which selectively stopped at the buried oxide layer (BOX) of the SOI substrate. Next, the individual PD mesas were protected in a spin coated layer of transparent bonding adhesive Waferbond®, and thermo-compressively bonded to a transparent temporary carrier substrate at 180°C in a vacuum oven (Fisher Scientific Isotemp 282A). The Si handle of the SOI substrate was removed using DRIE (Bosch process) which was selective to the BOX layer. Next, the BOX layer was etched using buffered oxide etchant, to expose the back of the Si PD mesas. Broad area back contacts, composed of Al (1000 Å)/Ti (800 Å)/Ni (500 Å)/Au (2000 Å), were patterned using negative UV lithography and lift-off. Metal pads consisting of 1 mm apertures and composed of Cr (50 Å)/Au (2000 Å) were deposited and patterned on the transparent Pyrex® host substrate. The thin-film PDs, supported on the temporary carrier substrate, were then thermo-compressively bonded to the host Pyrex® substrate using an Au-Au bond. The sample was then heated to 250 °C, the de-bonding temperature of the bonding adhesive Waferbond® to remove the temporary carrier substrate to complete the transfer of the thin-film PDs. The residual Waferbond® on the bonded PDs was removed using oxygen plasma ashing. Finally, a 52 nm SiN anti-reflection (AR) and passivation coating was deposited on the PDs using PECVD, and contact openings were etched using reactive ion etching. Figure 2(a) shows a transferred and bonded 2x1 array of rigid thin film annular Si pn junction PDs. The flexible thin-film annular Si PDs were fabricated from a p-type SOI sample: 25 μm Si (37.5 Ω-cm – 62.5 Ω-cm, boron)/ 1 μm SiO2/576 μm Si substrate (37.5 Ω-cm – 62.5 Ω-cm, boron). Fabrication of the flexible PDs began with the spin-coating of a phosphorous doped spin-on glass (Emulsitone, Phosphorosilica film 5x1020 cm−3) on the device layer of the SOI substrate. A dopant diffusion anneal was performed at 950 °C for 15 minutes to form the ntype region of the pn-junction. The front and back contact geometry and patterning, mesa patterning, and substrate removal process was identical to the rigid PDs. The 127 μm thick Kapton® host substrate was prepared for high temperature thermo-compression bonding process by a 250 °C, 20 min. thermal cure to release residual stress developed in the film during the manufacturing process. The surface roughness of the Kapton substrate is estimated to be ~330 Å [14], so a blanket Cr (50 Å)/Au (5000 Å) layer was deposited on the substrate, to ensure continuous metal coverage on the rough surface. The thin-film PD array, supported on a temporary pyrex carrier substrate, was then transferred and bonded to the prepared Kapton substrate using an Au-Au bond formed using thermo-compressive bonding. The sample was then heated to 250 °C to remove the temporary carrier substrate. The residual Waferbond® on the bonded PDs was removed using Waferbond Remover® and oxygen plasma ashing. Figure 2(b) shows a 2x1 array of 25 μm thick thin-film annular Si pn-junction PDs bonded to a flexible Kapton substrate. Figure 2(c) and 2(d) demonstrate inward and outward bending of the flexible PD array, respectively. It should be noted that no AR coating or passivation layer was deposited on these PDs. The development of larger arrays composed of the PD elements reported herein is possible given the scalability of the wafer bonding and DRIE processes for formation and transfer of thin film Si. Patterned thin film Si waveguide arrays, 4 cm2 in area, have been previously developed using large area wafer bonding and DRIE processing for thin film transfer and substrate removal [15]. Substrate heating due to large area substrate removal using dry etching can be addressed by using mechanical polishing and wet etching in conjunction with DRIE [15]. The host substrates utilized in this report are required to withstand up to 250 °C due to the use of Waferbond for thin-film transfer and handling. However, there are many other low thermal budget processes such as kerf-less exfoliation [16], methods utilizing elastomeric stamps [17] and transfer diaphragms [18] that enable fabrication, transfer, and bonding of thin-film devices, and thin-film device arrays to foreign substrates [3, 19]. These methods would allow integration of the devices reported herein on to substrates such as PET [5, 6], polyester [20], Parylene [21], and silk fibroin [22], and other inexpensive, eco-friendly and
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bio-compatible substrates. However, contact annealing temperatures affect contact resistance and dark currents, and processes which enable contact annealing (not necessarily after bonding to the host substrate) are an important consideration for high performance devices.
Fig. 2. (a) Photomicrograph of a 2x1 array of rigid annular thin-film Si PDs bonded to a Au coated Pyrex® substrate; (b) Photomicrograph of a 2x1 array of flexible annular thin-film PDs bonded to a Au coated Kapton® substrate; (c) Demonstration of inward bending of flexible PDs; (d) demonstration of outward bending of flexible PDs.
3. Device characterization and analysis The surface normal spectral response and dark current of the fabricated 2x1 PD array samples were measured using a Keithley source-measurement unit (SMU-4200). All spectral response and dark current measurements were performed at near 0 V (35 μV – 176 μV) bias. A fibercoupled Xenon arc lamp (MAX-302, Asahi Spectra) with eight wavelength filters, each with 10 nm FWHM (λ = 470 nm, 480 nm, 490 nm, 500 nm, 510 nm, 560 nm, 580 nm, 600 nm), was used as the source for spectral response measurements, which is driven by the DRS application. The responsivity for each PD was calculated using the photocurrent measured for each of the eight wavelengths by illuminating each PD with a fiber bundle in the surface normal configuration, and the power measured at the output of the source fiber bundle. The surface normal spectral response of the rigid PDs was modeled using a 3D simulation in Silvaco Atlas®. The simulated model used the dopant concentration of the starting wafer used to fabricate the devices (3x1014 cm−3). The simulated structure consisted of a 10 μm x 10 μm x 10 μm volume of p-type silicon. The simulated area was restricted to 10 μm x 10 μm to save computational time since the doping contours (and therefore electric field contours) varied only as a function of the depth of the semiconductor. A complementary error-function profile was used for phosphorous diffusion with a phosphorous surface concentration equal to 5x1020 cm−3. The dopant profile and surface concentration corresponded to the phosphorous doped spin-on-glass used for fabricating the pn junction in the devices [23]. The top and bottom contacts were assumed to be ideal ohmic contacts. A 52 nm thick SiN AR coating was included in the model. A 3D optical source with a beam intensity of 1 W/cm2 was swept across a 470 nm – 600 nm wavelength range to estimate the photocurrent. The photocurrent values calculated in Silvaco were divided by the power incident on the illuminated front
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surface (10 μm x 10 μm) of the simulated structure to calculate the spectral response. Figure 3(a) shows the measured responsivity for the fabricated 2x1 array of thin film PDs with a junction anneal of 1000 °C with and without an AR coating. It was observed that the measured responsivity (with and without the AR coating) was in good agreement with the responsivity simulated in Silvaco for a junction depth of 750 nm. The dark-current density for the rigid array was measured to be 0.63 nA/cm2.This yielded a responsivity to dark current density ratio of 0.3 cm2/nW – 0.54 cm2/nW (λ = 470 nm-600 nm) for the rigid PD array. Figure 3(b) shows the measured spectral response for the fabricated flexible PD array, before any flexing was performed on the array. The measured responsivity was 0.16 A/W – 0.26 A/W for λ = 470 nm – 600 nm. Also shown in Fig. 3(b) is the measured spectral response for a 2x1 PD array bonded to a rigid Pyrex substrate (without AR coating), for comparison. The junction doping and anneal conditions for the flexible and rigid devices were identical (Emulsitone, Phosphorosilica film 5x1020 cm−3 annealed at 950°C for 15 minutes). The resistivity and thickness of the flexible and rigid devices were 37.5 Ω-cm – 62.5 Ω-cm and 25 µm, and 30 Ω-cm – 60 Ω-cm and 10 µm, respectively. The similarity in responsivity values for the two sets of devices indicated that there was no severe degradation in PD spectral response by bonding and handling PDs on a flexible substrate. The dark current measured for flexible devices was 60 pA – 111.39 pA, i.e. a dark current density of 0.293 0.545 nA/cm2. This translates to an average responsivity to dark current ratio of 0.38 – 0.62 cm2/nW.
Fig. 3. (a) Measured and simulated spectral response of a 2x1 array of rigid thin-film Si PDs with junction anneal temperature = 1000°C; (b) Measured spectral response of flexible and rigid thin-film Si PDs with junction anneal temperature = 950°C. +/−σ error for the photoresponse measurements is more than 2 orders of magnitude lower than the signal measured.
4. Bending fatigue characterization of flexible annular thin-film Si PDs Next, the effect of repeated bending on the performance of the fabricated flexible PD array was assessed. Repeated measurements of the surface normal spectral response and the dark IV curve of a fabricated PD in the array were made after outward bending of the array 20 times on tubes of four different radii (in order of testing): 6.43 cm, 5 cm, 3.45 cm, and 2.82 cm, and inward bending of the array 20 times on the following radii (in order of testing): 5.73 cm, 4.4 cm, 3.45 cm, and 2.4 cm. One bending cycle was defined as the bending of devices on the tube and releasing them back to their original configuration. The O.D. and I.D. of the flexible PDs are 5.2 mm and 1 mm, respectively, and the center-to-center spacing is 6 mm. Figure 4(a) and 4(c) show the surface normal spectral response at the start of the experiment, and after 20 outward and inward bending cycles on each tube, respectively. Figure 4(b) and #203062 - $15.00 USD (C) 2014 OSA
Received 17 Dec 2013; revised 15 Feb 2014; accepted 17 Feb 2014; published 25 Feb 2014 10 March 2014 | Vol. 22, No. 5 | DOI:10.1364/OE.22.005052 | OPTICS EXPRESS 5057
4(d) show the dark I-V curves measured at the start of the experiment, and after each set of outward and inward bending cycles, respectively. No significant change in the surface normal spectral response, or dark current at 0V bias was observed after subjecting the fabricated flexible PD array to multiple outward bending cycles down to a radius of 2.82 cm, and multiple inward bending cycles down to a radius of 2.4 cm.
Fig. 4. (a) Outward bending spectral response characterization; (b) Outward bending dark current characterization; (c) Inward bending spectral response characterization; (d) Inward bending dark current characterization.
5. Conclusion and discussion of results There have been several previous reports of thin film monocrystalline Si PDs [3, 4, 6, 24–29], some of which have been heterogeneously bonded to alternative host substrates [3, 4, 6, 24, 25, 27, 28]. One report of 500μm x 500 μm area thin film Si pn junction PD arrays heterogeneously bonded to a flexible membrane demonstrates imaging capability; however, the performance of the reported PDs has not been characterized or optimized [4]. Flexible thin film Si MSM PDs, 3mm x 3 mm in area, integrated onto Polyethylene terephthalate (PET) substrates have been explored for flexible imaging systems; however, the thickness of the fabricated PDs (260 nm) limited the responsivity to 7.4x10−8 A/W at 0V bias, and 7.42x10−5 A/W at 4V bias [6]. The annular devices in this report, each 20.44 mm2 in area, demonstrate a higher responsivity to dark current ratio than previous reports, to the best of our knowledge. The peak responsivity vs. dark current density for previous reports of heterogeneously bonded
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Received 17 Dec 2013; revised 15 Feb 2014; accepted 17 Feb 2014; published 25 Feb 2014 10 March 2014 | Vol. 22, No. 5 | DOI:10.1364/OE.22.005052 | OPTICS EXPRESS 5058
thin-film monocrystalline Si PDs is shown in Fig. 5, demonstrating graphically that the PDs developed in this work achieve higher peak responsivity to dark current density ratio.
Fig. 5. Comparison of annular thin-film Si PDs in this report to previously reported thin-film single crystal Si PDs.
The high responsivity of these devices was attributed to the use of a shallow junction depth for efficient carrier collection, and the use of a SiN AR and passivation layer for rigid PDs that reduced front surface reflectivity and the front surface recombination velocity [30], thereby reducing the recombination rate of the photocarriers generated close to the surface of the PDs. The low dark current density is attributed to the use of a low bias voltage (enabled by the high responsivity achieved in an unbiased condition), a SiN passivation layer (for rigid PDs), the absence of semiconductor material outside of the detection volume which prevented thermally generated carriers in the substrate from diffusing to the pn-junction, and the development of a fabrication process that does not degrade the electronic properties of the semiconductor material. Acknowledgment This research was supported by the National Institutes of Health through the Bioengineering Research Partnership award 1 RO1 EB011574-01A1.
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Received 17 Dec 2013; revised 15 Feb 2014; accepted 17 Feb 2014; published 25 Feb 2014 10 March 2014 | Vol. 22, No. 5 | DOI:10.1364/OE.22.005052 | OPTICS EXPRESS 5059
