Impurity-mediated near-infrared (NIR) photoresponse in silicon is of great interest for photovoltaics and photodetectors. In this paper, we have fabricated a series of n+/p photodetectors with hyperdoped silicon prepared by ion-implantation and femtosecond pulsed laser. These devices showed a remarkable enhancement on absorption and photoresponse at NIR wavelengths. The device fabricated with implantation dose of 1014 ions/cm2 has exhibited the best performance. The proposed method offers an approach to fabricate low-cost broadband silicon-based photodetectors

Ahmad, Waseem

Chen, Zhi David

Li, Shibin

Liu, Bohan

Liu, Zhijun

Xuan, Yaoyu

Ying, Xiangxiao

Zhang, Ting

English

University of Kentucky

University of KentuckyUKnowledgeElectrical and Computer Engineering FacultyPublications Electrical and Computer Engineering9-4-2017Optical and Electronic Properties of FemtosecondLaser-Induced Sulfur-Hyperdoped Silicon N+/PPhotodiodesTing ZhangUniversity of Electronic Science and Technology of China, ChinaBohan LiuUniversity of Electronic Science and Technology of China, ChinaWaseem AhmadUniversity of Electronic Science and Technology of China, ChinaYaoyu XuanUniversity of Electronic Science and Technology of China, ChinaXiangxiao YingUniversity of Electronic Science and Technology of China, ChinaSee next page for additional authorsRight click to open a feedback form in a new tab to let us know how this document benefits you.Follow this and additional works at: https://uknowledge.uky.edu/ece_facpubPart of the Electrical and Computer Engineering Commons, and the Nanoscience andNanotechnology CommonsThis Article is brought to you for free and open access by the Electrical and Computer Engineering at UKnowledge. It has been accepted for inclusionin Electrical and Computer Engineering Faculty Publications by an authorized administrator of UKnowledge. For more information, please contactUKnowledge@lsv.uky.edu.Repository CitationZhang, Ting; Liu, Bohan; Ahmad, Waseem; Xuan, Yaoyu; Ying, Xiangxiao; Liu, Zhijun; Chen, Zhi David; and Li, Shibin, "Optical andElectronic Properties of Femtosecond Laser-Induced Sulfur-Hyperdoped Silicon N+/P Photodiodes" (2017). Electrical and ComputerEngineering Faculty Publications. 16.https://uknowledge.uky.edu/ece_facpub/16AuthorsTing Zhang, Bohan Liu, Waseem Ahmad, Yaoyu Xuan, Xiangxiao Ying, Zhijun Liu, Zhi David Chen, andShibin LiOptical and Electronic Properties of Femtosecond Laser-Induced Sulfur-Hyperdoped Silicon N+/P PhotodiodesNotes/Citation InformationPublished in Nanoscale Research Letters, v. 12, issue 1, 522, p. 1-4.© The Author(s). 2017This article is distributed under the terms of the Creative Commons Attribution 4.0 International License(http://creativecommons.org/licenses/by/4.0/), which permits unrestricted use, distribution, andreproduction in any medium, provided you give appropriate credit to the original author(s) and the source,provide a link to the Creative Commons license, and indicate if changes were made.Digital Object Identifier (DOI)https://doi.org/10.1186/s11671-017-2287-2This article is available at UKnowledge: https://uknowledge.uky.edu/ece_facpub/16NANO EXPRESS Open AccessOptical and Electronic Properties ofFemtosecond Laser-Induced Sulfur-Hyperdoped Silicon N+/P PhotodiodesTing Zhang1, Bohan Liu1, Waseem Ahmad1, Yaoyu Xuan1, Xiangxiao Ying1, Zhijun Liu1, Zhi Chen1,2,3and Shibin Li1*AbstractImpurity-mediated near-infrared (NIR) photoresponse in silicon is of great interest for photovoltaics andphotodetectors. In this paper, we have fabricated a series of n+/p photodetectors with hyperdoped siliconprepared by ion-implantation and femtosecond pulsed laser. These devices showed a remarkable enhancement onabsorption and photoresponse at NIR wavelengths. The device fabricated with implantation dose of 1014 ions/cm2has exhibited the best performance. The proposed method offers an approach to fabricate low-cost broadbandsilicon-based photodetectors.Keywords: Ion-implantation, Hyperdoped silicon, NIR photoresponseBackgroundTraditional silicon-based devices could not show desir-able NIR photoresponse due to limitation of opticalbandgap (1.12 eV) of silicon [1], and many attempts havebeen made to enhance the absorptance of silicon material,especially at NIR wavelengths [2–9]. The discovery ofchalcogen-supersaturated silicon fabricated by laser irradi-ation in SF6 atmosphere demonstrated an approach to en-hance the sub-bandgap absorption [10, 11]. In thisprocess, the material can be doped beyond the solubilitylimit [12]. Besides, light trapping effect caused by theunique pointed cone structure on silicon surface also in-creases the efficiency of light absorption [13]. In thispaper, we have fabricated hyperdoped silicon prepared byion-implantation and femtosecond pulsed laser. Hallmeasurement was carried out to measure the electricalproperties of hyperdoped silicon. Photodetectors based onn+/p junction demonstrated high performances on bothNIR absorption and photoresponse.MethodsOne-side polished p-type silicon [100] wafers (300 μm)with resistivity 8–12 Ω cm were ion-implanted with1.2 keV 32S+ into a depth of approximately 40 nm atroom temperature. The implantation doses were 1 ×1014, 1 × 1015, and 1 × 1016 ions/cm2. Pulsed laser melt-ing (PLM) was carried out by 1 kHz train of 100 fs,800 nm femtosecond laser pulses with a fluence of0.5 J/cm2. Then, laser spot of 200 μm diameter is fo-cused on the silicon and patterned square areas up to10 mm × 10 mm. Rapid thermal annealing (RTA) wasimplemented at 600 °C for 30 min in a N2 atmosphere.We determined the absorptance (A) of the samples bymeasuring reflectance (R) and transmittance (T) byusing a UV-Vis-NIR spectrophotometer (UV3600, Shi-madzu, Tokyo, Japan) equipped with an integratingsphere detector [3]. The absorptance was calculated byA = 1-R-T. The concentration and mobility of carrierswere measured by Hall Effect measurement system atroom temperature (via van der Pauw technique) [14]. Toinvestigate whether the impurity/intermediate band(IB) formed by sulfur impurities in silicon enhances thesub-bandgap photoresponse, we employed a Fourier-transform photocurrent spectroscopy method as de-scribed in Ref. [15, 16], where the chopped FTIR globarlight source is focused onto the sample, and the* Correspondence: shibinli@uestc.edu.cn1School of Optoelectronic Information, University of Electronic Science andTechnology of China, Chengdu, Sichuan 610054, ChinaFull list of author information is available at the end of the article© The Author(s). 2017 Open Access This article is distributed under the terms of the Creative Commons Attribution 4.0International License (http://creativecommons.org/licenses/by/4.0/), which permits unrestricted use, distribution, andreproduction in any medium, provided you give appropriate credit to the original author(s) and the source, provide a link tothe Creative Commons license, and indicate if changes were made.Zhang et al. Nanoscale Research Letters  (2017) 12:522 DOI 10.1186/s11671-017-2287-2generated photocurrent is then demodulated by anexternal lock-in amplifier and finally fed back to theexternal port of the FTIR.Results and DiscussionFigure 1 shows the absorptance of silicon samples im-planted at different doses. The samples processed withPLM showed highest absorptance at visible and NIRwavelengths while as-implanted samples showed lowestabsorptance. However, the annealing process reduces theabsorption in NIR region of spectra. The high Vis-NIRabsorptance of microstructured silicon is ascribed to thefollowing reasons: hyperdoping-induced impurity bandand microstructured surface-generated light trapping ef-fect. As illustrated in Fig. 1d, an impurity band inducedby dopants is formed in silicon, which is responsible forsub-bandgap absorption [17]. Consequently, the hyper-doped silicon shows high absorptance in NIR range.Meanwhile, laser melting reconstructs the silicon surfaceand produces an array of cones that leads to multiple re-flection and absorption [13], as displayed in Fig. 1e, f.The processed annealing evidently reduces absorptanceat NIR wavelengths range, which mainly caused by thetwo aspects: (1) annihilate the nanostructures on the sili-con surface, decreasing the light trapping effect [18]; and(2) result in the bond rearrangement within siliconmatrix, which optically inactivate sulfur impurities [11].Because of the similar surface structure created bysame laser parameters, the intensity of absorption inNIR range mainly depends on the dopant’s impuritylevels [19]. In the past, we have illustrated the possibleS-related energy levels corresponding to the photo-response spectral features [20]. It showed the large en-hancement observed at NIR region dependently resultedfrom the S-related energy level (~ 614 meV), whichgreatly enhanced the sub-bandgap absorptance. Prior toannealing process, absorption has no dramatic changewith respect to the doping dose as shown in Fig. 2a. Themicrostructured silicon with 1016 and 1015 ion/cm2implantation dose show similar absorptance, and thesample implanted at 1014 ions/cm2 shows unnoticeabledecrease. We consider the lower absorptance forannealed samples in NIR range can be ascribed to thetwo aspects. M. A. Sheehy et al. [21] proposed the ab-sorption decrease of below bandgap after annealingprocess is attributed to the diffusion out of the crystal-line grains to the grain boundaries of the supersaturateddopants and defects. These defects include vacancies,dangling bonds, and floating bonds. Once the defectsdiffuse to the grain boundaries, they would no longermake a contribution to impurity bands in the Si, thus re-ducing the absorption of below bandgap radiation.Moreover, the literature [22] reported that no remark-able redistribution of S occurred until the annealingtemperature reached at 650 °C. During this process, theFig. 1 a–c Dependence of absorptance on different fabrication process with various implantation doses. d Impurity band located within bandgap of Sifacilitates generation of carriers which participate in absorption of lower energy photons. e Scanning electron micrograph of silicon spikes. f Illustration ofoptical path on microstructured surfaceZhang et al. Nanoscale Research Letters  (2017) 12:522 Page 2 of 4S appears to complex with defect clusters, which meansthe S atoms will combine with each other at the Si wafersurface. This phenomenon leads to a reduction of theactive doping concentration.The carrier density and mobility of microstructuredsilicon with different ion-implantation doses are shownin Fig. 2b. It is evident that sheet density increases withion-implantation dose, and mobility decreases with in-creasing ion-implantation dose. According to Shockley-Read-Hall (SRH) recombination effect, in an indirectbandgap semiconductor such as Si and Ge, the carrierlifetime decreases with the increase of dopant concentra-tion [23, 24]. The decrease of mobility leads to an in-crease of recombination probability, so the decrease ofmobility results in a decrease on electron lifetime andthe decrease on mobility with increasing doping dose isconsistent with SRH recombination effect. After annealing,the sheet carrier density decreases dramatically due to ther-mal diffusion effect as we discussed previously.Figure 3 shows the photoresponse with different dopingdose, and the inset shows the diagram of n+/p photo-detector. The photoresponse at NIR range indicates the ap-pearance of impurity-mediated band. The prominent peakat approximately 960 nm corresponds to the generation ofelectron-hole pairs in silicon substrate, which are separatedby the built-in potential of n+/p junction and collected atthe top and bottom Al contacts. This phenomenon is wellknown as the heterojunction theory in Si devices [25].The observed photoresponse in NIR is ascribed to thesulfur impurity levels in hyperdoped silicon. Such impur-ity levels facilitate the below bandgap absorption asmentioned above. The absorbed NIR light is convertedinto electron-hole pairs, resulting in the enhancement ofphotoresponse in NIR range (1100 ~ 1600 nm) [20]. Thedevice with implantation dose of 1014 ions/cm2 showsthe highest photoresponse in the wavelength range of1010–1100 nm. The broad peak has been investigatedowning to deep sulfur levels in femtosecond laser-processed silicon [20, 26]. In addition, we found thatthe device with 1014 ions/cm2 has showed higherphotoresponse than those with 1015 and 1016 ions/cm2.And the Hall measurement indicated that the sampleimplanted at 1014 ions/cm2 had a bulk concentrationof 1019 ions/cm3. As demonstrated by SRH recombin-ation effect, carrier lifetime depends on dopant con-centration in silicon. E. Mazur has concluded that thesample with 1019 ions/cm3 dopant concentration wasexpected to show longer carrier lifetime than 1020 and1021 ions/cm3 [23]. Our Hall measurement results,sample implanted at 1014 ions/cm2 shows the highestmobility, are in agreement with the conclusion. Basedon this theory, although a sample with higher dopingdose shows greater absorptance, there is still a balancebetween optical absorption and carrier mobility. Aspresented in Fig. 3, the device with 1014 ions/cm2 ismost probable to show the highest photoresponse,which is consistent with the conclusion reported inRef. [23].Fig. 2 a Dependence of absorptance on different ion-implantation dose. All samples were microstructured by PLM. b Electronic properties ofreference silicon and microstructured silicon for different ion-implantation dose before annealing and one after annealingFig. 3 Photoresponse of n+/p detectors with different ion-implantationdose. Inset shows the top view and sectional view of the device. Lightgray shows the patterns of interdigitated contact on microstructuredsurface and all standing contact on backsideZhang et al. Nanoscale Research Letters  (2017) 12:522 Page 3 of 4ConclusionsWe have measured the response of photodetectors basedon microstructured silicon with different ion-implantationdose. The incorporation of impurities leads to a remark-able enhancement on absorptance and photoresponse atNIR wavelengths. And device implanted at 1014 ions/cm2exhibits the highest photoresponse. PLM combined withion-implantation demonstrates a considerable techniquefor the fabrication of NIR detectors. This technique mayoffer a feasible approach to fabricate low-cost broadbandsilicon-based photodetectors.AcknowledgementsThis work was supported by the National Natural Science Foundation ofChina under grant nos. 61421002, 61574029, and 61371046. This work wasalso partially supported by the University of Kentucky.Authors’ ContributionsTZ, BL, and YX carried out the experiments and analyzed the data. XY and ZLparticipated in the work to measure the responsivity (a.u.). WA analyzed thetheory and polished the English. SL and ZC gave equipment support. All authorsread and approved the final manuscript.Competing InterestsThe authors declare that they have no competing interests.Publisher’s NoteSpringer Nature remains neutral with regard to jurisdictional claims inpublished maps and institutional affiliations.Author details1School of Optoelectronic Information, University of Electronic Science andTechnology of China, Chengdu, Sichuan 610054, China. 2Department ofElectrical and Computer Engineering, University of Kentucky, Lexington, KY40506, USA. 3Center for Nanoscale Science and Engineering, University ofKentucky, Lexington, KY, USA.Received: 20 April 2017 Accepted: 23 August 2017References1. Wu C, Crouch CH, Zhao L et al (2001) Near-unity below-band-gapabsorption by microstructured silicon. Appl Phys Lett 78(13):1850–18522. Jiang J, Li S, Jiang Y et al (2013) Mechanism of optical absorptionenhancement of surface textured black silicon. 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Optical and Electronic Properties of Femtosecond Laser-Induced Sulfur-Hyperdoped Silicon N+/P Photodiodes

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Optical and Electronic Properties of Femtosecond Laser-Induced Sulfur-Hyperdoped Silicon N+/P Photodiodes

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