# Fileset

[s41699-019-0124-4.pdf](https://mdr.nims.go.jp/filesets/d9297680-f439-4ca8-bff2-4ceb985b2364/download)

## Creator

[Kenji Watanabe](https://orcid.org/0000-0003-3701-8119), [Takashi Taniguchi](https://orcid.org/0000-0002-1467-3105)

## Rights

[Creative Commons BY Attribution 4.0 International](https://creativecommons.org/licenses/by/4.0/)

## Other metadata

[Far-UV photoluminescence microscope for impurity domain in hexagonal-boron-nitride single crystals by high-pressure, high-temperature synthesis](https://mdr.nims.go.jp/datasets/78310fdf-41b8-481a-abd6-83dc5cca4b73)

## Fulltext

Far-UV photoluminescence microscope for impurity domain in hexagonal-boron-nitride single crystals by high-pressure, high-temperature synthesisARTICLE OPENFar-UV photoluminescence microscope for impurity domainin hexagonal-boron-nitride single crystals by high-pressure,high-temperature synthesisKenji Watanabe 1,2* and Takashi Taniguchi1,2Hexagonal-boron-nitride single crystals grown by high-pressure, high-temperature (HPHT) synthesis are commonly used as theinsulated substrate dielectric for two-dimensional (2D) atomic-layered materials like graphene and transition metal dichalcogenides(TMDs) to improve the flatness of the 2D materials atomically without disturbing the 2D electronic characteristics. However, HPHTsingle crystals often contain impure regions, which can hold subtle clues in regard to the 2D atomic-layered materials for newdiscoveries in the physics of 2D materials. To identify the position of the impure domains and to avoid them when the 2D devicesare prepared, a far-ultraviolet photoluminescence microscope was developed. This microscope makes it possible to visualize theimpure-growth region with ease in a no-contact and non-destructive manner.npj 2D Materials and Applications            (2019) 3:40 ; https://doi.org/10.1038/s41699-019-0124-4INTRODUCTIONSince the discovery of the exfoliation method to obtaingraphene,1 two-dimensional (2D) atomic-layer materials havebeen extensively studied in terms of the 2D physics of quantumeffects, superconductivity, photonics, and topology, and interest in2D materials is expanding to transition metal dichalcogenides(TMDs), oxides, and other materials.2 From the early stage ofresearch on graphene, hexagonal-boron nitride (h-BN) has beenknown as the best substrate dielectric for studying 2D physics ofgraphene.3 A single crystal of h-BN has a van der Waals layeredstructure in which each layer is atomically flat with sp2 bondingbetween boron and nitrogen atoms, and the interlayers areweekly coupled via van der Waals interaction.4 The surface of thecleaved layer is thus almost free of dangling bonds and chargetraps, which would scatter charged carriers and cause inhomoge-neous fluctuations in potential.Currently, our single crystals grown by high-pressure, high-temperature (HPHT) synthesis5 are most commonly used fordevices used in research of 2D physics.3,6 Although the size of thecrystals is limited to the order of square millimeters (in surfacearea), the HPHT single crystals formed by exfoliation usingadhesive tape make it possible to obtain a thin-layer substratedielectric with size of a few-hundred microns in area. Most studiesemploy them for demonstrating 2D devices, and a variety ofoutstanding properties and applications of atomically thin 2Dmaterials have been reported.7 For all the success of the HPHTsingle crystals for 2D physics, there is still room for improvementof the HPHT crystals. One issue is the potential problem of aninvisible impurity domain.According to a study using secondary ion mass spectrometry(SIMS), carbon and oxygen atoms are easily incorporated into thesingle crystals.5 Such incorporation of impurities affects theluminous properties of h-BN. Examples of cathodoluminescence(CL) spectra at room temperature for pure and impure samples areshown in Fig. 1. Similar spectra for the impure sample are alsoobtained by a photoluminescence (PL) method.8 The spectrum forthe pure sample is governed by the intense exciton luminescenceat 215 nm. In contrast, the impure sample, which contains a subtleamount of carbon and oxygen,5 shows additional luminescentstructures in the region between 300 and 400 nm (the origin ofwhich is still controversial.5,8–13) Moreover, owing to the relaxationprocess of excited carriers through excitons to the impurity bands,the intensity of the exciton luminescence for the impure sample islower than that for the pure sample.14In fact, such an impurity-incorporated domain is oftencontained in HPHT single crystals. A microphotograph (a),scanning electron microscope (SEM) image (b), and filtered CLimages (c) and (d) for a relatively pure HPHT h-BN single crystal areshown in Fig. 2. In Fig. 2d, a hexagonal- or pentagonal-shapeddomain appears near the center of the crystals, which haverelatively high concentrations of impurities and show impurity CLbands at around 340 nm. In Fig. 2c, the surrounding region of thedomain is brighter and shows 215-nm excitonic CL. The impuritiesin h-BN layers could potentially cause serious adverse effectswithin the adjacent 2D atomic layers in delicate phenomena.15Thus, in consideration of the rapidly progressing research on 2Dmaterials, a simple and user-friendly method for locating andidentifying impurity distributions in the h-BN substrate dielectric isnecessary. Hopefully, to control the device quality, the methodcan be incorporated into the exfoliation and stacking processesfor 2D materials.Since h-BN has a wide band gap and is highly transparent in thevisible region, the impure domain, which shows luminescence inthe ultraviolet (UV) region as described, is also invisible to thenaked eye. It is impossible to distinguish the impure domain witha microscope (Fig. 2a). Thus, as shown in Fig. 2c, d, CL imaging isthe most useful method for observing impure regions in crystals.However, CL imaging uses a vacuum chamber with electronirradiation, and h-BN shows insulation properties even in theimpurity domain; therefore, during CL measurement, the samplebecomes charged, and contamination (primarily residual hydro-carbon from the pumping system) collects on its surface.16 Thus,1National Institute for Materials Science, Namiki 1-1 Tsukuba, Ibaraki 305-0044, Japan. 2The authors contributed equally: Kenji Watanabe, Takashi Taniguchi. *email: WATANABE.Kenji.AML@nims.go.jpwww.nature.com/npj2dmaterialsPublished in partnership with FCT NOVA with the support of E-MRS1234567890():,;http://orcid.org/0000-0003-3701-8119http://orcid.org/0000-0003-3701-8119http://orcid.org/0000-0003-3701-8119http://orcid.org/0000-0003-3701-8119http://orcid.org/0000-0003-3701-8119https://doi.org/10.1038/s41699-019-0124-4mailto:WATANABE.Kenji.AML@nims.go.jpmailto:WATANABE.Kenji.AML@nims.go.jpwww.nature.com/npj2dmaterialsto use CL imaging to locate and identify the impure domains inpreparing 2D devices requires an additional cleaning step forthese contaminated surfaces prior to stacking 2D atomic layers.This contamination can be difficult to remove completely, evenwith wet chemical cleaning processes, which may, on the contrary,cause additional contamination.On the other hand, a PL method using a weak far-UV excitationsource can avoid the contamination problem. The impure-growthdomain can be found in the small h-BN fragments resulting fromexfoliation by visualizing the intensity of impurity PL band. Duringthis visualization in a non-destructive manner, charging thesample is almost negligible in practice. In this study, weconstructed a far-UV PL microscope for identifying and markingthe impurity region of both bulk crystals and exfoliated thinsamples on a substrate. In general, using a fluorescencemicroscope is an easy means to observe sample quality. However,for wide-band-gap materials such as h-BN, a unique design for theoptical system is needed. This paper aims to further study 2Dmaterials by (i) describing a simple yet powerful method forevaluating h-BN and (ii) drawing attention to the invisibleimpurity-incorporated domain in HPHT single crystals.RESULTS AND DISCUSSIONSFar-UV PL microscopeThe far-UV PL microscope is shown schematically in Fig. 3. It iscomposed of an excitation-light source, an objective for focusingthe excitation light and collecting the luminescence from thesample, a filter box for exchanging filters, and a lens for focusingPL images on a charge coupled device (CCD) detector in theoptical path. The sample is irradiated by the excitation lightfocused with the objective, and a luminescence image of thesample is collected, filtered, and then focused on the CCDdetector. Designing the microscope there faced concerningdealing with the wide wavelength range of UV light (from 200to 400 nm): (1) selecting an excitation-light source at far-UVwavelength, (2) suppressing chromatic aberration in the UV range,(3) choosing optical filters usable in the UV range, and (4)detecting the weak luminescence of h-BN. The solutions to each ofthose challenges are described in detail in the following.The wavelength of the excitation-light source must be shorterthan that corresponding to the band-gap energy of h-BN, that is,about 6 eV (206 nm), to excite the exciton PL band.17,18 Theoptimal excitation wavelength must be sufficiently different fromthe wavelength of exciton luminescence at 215 nm to separatethe PL component from the excitation by a far-UV interferencefilter. Designing the filter for a suitable transmission characteristicin this wavelength range is extremely difficult because thecombinations of optical materials in the far-UV region (wavelength<230 nm) are limited. Although a synchrotron source is one of themost powerful tunable sources in the far-UV region,19 using asynchrotron radiation facility is generally limited to authorizedusers. It is thus unsuitable for general use such as evaluating h-BNused in the stacking process of 2D materials. A laser is one of themost plausible light sources. However, commercially availablelasers emitting wavelengths shorter than 206 nm are generallyexpensive, and many need pulse operation with a low repetitionrate, which is likely to cause damage to optical components,including mirrors, filters, and the h-BN sample.20 In view of thissituation, we employed a deuterium (D2) lamp filtered with adouble monochromator (SOL instruments Ltd., DM160) with asubtractive configuration (zero dispersion mode), and we set thefiltered excitation wavelength at 198 nm (denoted by (1) in Fig. 3).This wavelength is the shortest UV wavelength that can propagatein the atmosphere. Using filtered light from the broaderbandwidth has the advantage that excitation wavelength can befreely selected. The wavelength of 198 nm is relatively far from theband-gap energy of h-BN, so it is sufficient for separating the PLcomponent. The excitation power at the sample position wasbelow a few µW on average. Although this lower power excitationminimized the potential for damage to the optics and samples,Fig. 1 Cathodoluminescence (CL) spectra of exciton and impuritybands at room temperature. The impurity domain of the singlecrystal shows the impurity bands from 300 to 400 nm with the weakexciton band at 215 nmFig. 2 Single h-BN crystal images: a microphotograph, b scanningelectron microscope (SEM), c CL (exciton), and d CL (impurity). TheSEM image is blurred by charging up of the insulating h-BN sample.No metal coat (to avoid electrification) was formed so that CLimages could be observed. Images b–d were measured by using afar-UV CL system, in which the output images have low resolution of640 × 480 pixels, although the impure domain near the center of thecrystal is clearly discernible in images c and d. The scale bar is500 µmK. Watanabe and T. Taniguchi2npj 2D Materials and Applications (2019)    40 Published in partnership with FCT NOVA with the support of E-MRS1234567890():,;the reduced signal intensity made it difficult to obtain PL images.To detect low-PL signals, a Peltier-cooled CCD camera (Andor,Apogee Alta, denoted by (4) in Fig. 3), which can capture thesubtle intensity of images with small dark-current noise overlonger exposure times, was used. An exposure time of 60 s wassufficient to obtain the PL images, even for flakes with thickness ofa few microns. Note that to gain the sensitivity in terms ofluminescence, the CCD pixels were merged a block of 2 × 2 pixels(4 pixels) for each channel. In Figs. 4c, d and 5, each imagetherefore has a maximum of 512 × 512 channels (merged pixels).In addition, in Fig. 6, the edges of the PL images are trimmed, andthe number of pixels is less than that of the other images. The PLimages thus show low resolution and poor quality, although it isenough to locate and identify impurity distributions in h-BN.An infinity-corrected optical system was incorporated in themicroscope. It was thus possible to install auxiliary opticalcomponents in the parallel optical path between the objectivelens and the condenser lens for CCD imaging with minimal error inFig. 3 Schematic diagram for far-UV photoluminescence microscope. The two insets show characteristic diagrams of the filter transmittancefor the excitation source and the luminescence signal, respectively. The labels numbered from (1) to (4) denote the four technical issuesexplained in the textFig. 4 A comparison between CL and PL images: PL images for bulk single crystal for c exciton band and d impurity band with reference CLimages a exciton and b impurity bands (an explanatory drawing is also shown). The streak lines in the PL images (denoted by pink brokenlines in explanatory drawing) are scattered PL light at the striae inside the crystal. The scale bar is 200 µm for c, dK. Watanabe and T. Taniguchi3Published in partnership with FCT NOVA with the support of E-MRS npj 2D Materials and Applications (2019)    40 focus. The microscope was thus highly flexible in regard toconfiguring optical components such as an epi-illuminator andfilters.Chromatic aberration of optics between near 200 and 400 nm isthe most serious problem affecting imaging with the developedmicroscope. It is caused by the widely varying refractive indices atwavelengths from 200 to 400 nm of lens components for focusing.Accordingly, a Schwarzschild objective,21 which is composed oftwo opposing concave and convex mirrors for collimation(denoted by (2) in Fig. 3) was introduced. As a result, the objectivesuffers virtually no chromatic aberration. Two objectives, one formagnification of ×15 and one for that of ×40, were useddepending on the sample size. An achromatic-lens unit (designedfor wavelengths of 215 and 340 nm) was used to collimate thesignal image on the CCD detector. The lens unit is composed oftwo convex (CaF2) lenses and one concave (quartz) lensconfigured for a focal length of 360mm. Note that it is possibleto design an achromatic lens with a wavelength range for thislonger focal length.The exchanging filters selecting luminescence bands are placedin the filter box (denoted by (3) in Fig. 3). The filter for the impurityband is a commercially available U-340 filter glass (HOYA CandeoOptronics Corporation), which transmits an ideal wavelengthrange (300–350 nm) covering the impurity PL bands. For theexciton PL image, the combination of a 220-nm narrow bandpassfilter (Acton Optics & Coatings 220 nm) and a reject filter forexcitation light (Asahi Spectra, custom-made filter) was used. The220-nm filter transmits the exciton PL signal at ~215 nm andblocks the impurity PL signal at ~340 nm. The Asahi Spectra’s filterblocks the 198-nm excitation light and transmits the excitonPL band.Measuring bulk samplesExample PL images of the HPHT single crystal are shown in Fig. 4.As shown in Fig. 4c, similar to the exciton CL image (Fig. 4a), the PLimage of the exciton band is darker at the center of the crystalindicating the impure domain; conversely, the PL and CL imagesof the impurity band (Fig. 4b, d) show brighter at the domain. Theimages obtained by CL and PL imaging are essentially the same.More specifically, striae in the crystal appeared as bright lines inthe PL image, which can be attributed to the scatter of the PLcomponents propagating inside the crystal. Since the CL imagesare synchronously obtained by scanning the excitation electronbeam, such scattering is weaker than that due to direct excitationby the electron beam, and it is considered negligible. Employingthis PL property makes it possible to visualize structural defects ofexcitation-wavelength size and characterize them by this PLimaging method. For exfoliation to obtain a larger single-domainflake, PL could also be used to select a bulk crystal with fewerstriae.Measuring exfoliated samplesExample PL images for exfoliated h-BN on a Si substrate are shownin Fig. 5. Unlike the PL images of bulk samples in Fig. 4, theintensity of these PL images is distributed throughout the flakes,and it is difficult to distinguish the proportion of flakes showingintense exciton or impurity PL bands. This distribution of theintensity is attributed to the exfoliated flakes have a variety ofthicknesses and crystallinity characteristics, caused by differingpositions of single crystals or by entirely different crystals,resulting in each flake showing a variety of intensities of theexciton and impurity bands.To overcome the above-described difficulty, the intensity ratiosof exciton images and impurity images were compared, and amask image, in which the intensity of the impurity image waslarger than that of the exciton image by a threshold value wasformed. The threshold value was determined by using theintensity ratio obtained from the bulk crystals, as shown inFig. 4c, d, in such a way that the mask image properly matchedthe impure region. As a result, computer-aided overlay of themask image yielded the position of the impurity domain. Theresult of comparing Fig. 5a, b overlaid on a white-light image isshown in Fig. 5c. In this case the impure regions denoted by greenmasks are scattered over the observation area. As a more specificexample, the exfoliated h-BN on a SiO2 substrate is shown in Fig. 6.At a glance, the flake at the center of the microphotograph in Fig. 6aappears to have a uniform surface; however the PL microscopejudged that over 80% of the region is covered by the impuritydomain, as shown in Fig. 6b, where the green area indicates theimpurity PL intensity is relatively greater than that of the exciton.Thus, the flake shown in Fig. 6 is likely to have originated from thebulk sample near the center impurity domain, as shown in Fig. 4.Fig. 5 PL images for exfoliated single crystal layers on Si substrate for a exciton band and b impurity band. It is difficult to judge which regionsare the impurity domains. c Green masked image indicating the impurity domain overlaid on a white-light image. The scale bar is 200 µmFig. 6 Example of evaluation for exfoliated single crystal on SiO2substrate: a microphotograph image and b overlay image withgreen mask corresponding to the area in which the impurity band isbrighter than the exciton band. Dark-grey shapes are metal patternsfor indicating the position of the whole substrate. The scale bar is50 µmK. Watanabe and T. Taniguchi4npj 2D Materials and Applications (2019)    40 Published in partnership with FCT NOVA with the support of E-MRSIn summary the use of the developed far-UV fluorescencemicroscope provides an easy means to observe crystal samplequality of wide-band-gap materials such as h-BN. This microscopeis in fact a powerful tool for evaluating the stacking process of 2Dmaterials by distinguishing impurity-incorporated regions withoutcontact in a non-destructive manner. It also makes it possible toobtain almost-ideal interfaces for h-BN and 2D atomic layers. Inaddition, incorporating this microscope evaluation into theautonomous robotic identification and assembly of 2D crystals22would greatly enhance processing high-quality van der Waalssuperlattices.This simple microscope using PL images is also promising forother 2D materials, such as TMDs, as well as other wide-band-gapmaterials, such as AlGaN-based materials, in addition to h-BNbecause of the widely tunable excitation-light source and smallchromatic aberration of the microscope.METHODSSample preparationh-BN single crystals were synthesized by HPHT method at 4.5 GPa and1500 °C using barium boron nitride as a solvent. SIMS was employed toestimate concentration of carbon and oxygen impurities. Details aredescribed in ref. 5.h-BN flakes were mechanically cleaved and exfoliated on cleaned Sisubstrate (the sample represented in Fig. 5) and SiO2/Si substrate (thesample represented in Fig. 6). Both substrates did not show remarkableluminescence in the UV region which affected the PL imaging of h-BNsamples.DATA AVAILABILITYThe data supporting the findings of this work are available from the correspondingauthor upon reasonable request.CODE AVAILABILITYThe code used to analyze the data are available from the corresponding author uponreasonable request.Received: 4 July 2019; Accepted: 4 October 2019;REFERENCES1. Novoselov, K. S. et al. Electric field effect in atomically thin carbon films. Science306, 666–669 (2004).2. Geim, A. K. & Grigorieva, I. V. Van der Waals heterostructures. Nature 499,419–425 (2013).3. Dean, C. R. et al. Boron nitride substrates for high-quality graphene electronics.Nat. Nanotechnol. 5, 722–726 (2010).4. Mishima, O. & Era, K. In Electric Refractory Materials (ed. Kumashiro, Y.) 495–556(Marcel Dekker, Inc., New York, Basel, 2000).5. Taniguchi, T. & Watanabe, K. Synthesis of high-purity boron nitride single crystalsunder high pressure by using Ba-BN Solvent. J. Cryst. Growth 303, 525–529 (2007).6. Wang, L. et al. One-dimensional electrical contact to a two-dimensional material.Science 342, 614–617 (2013).7. Yankowitz, M., Ma, Q., Jarillo-Herrero, P. & LeRoy, B. J. van der Waals hetero-structures combining graphene and hexagonal boron nitride. Nat. Rev. Phys. 1,112–125 (2019).8. Era, K., Minami, F. & Kuzuba, T. Fast luminescence from carbon-related defects ofhexagonal boron nitride. J. Lumin. 24-25, 71–74 (1981).9. Museur, L., Feldbach, E. & Kanaev, A. Defect-related photoluminescence of hex-agonal boron nitride. Phys. Rev. B 78, 155204 (2008).10. Du, X. Z., Li, J., Lin, J. Y. & Jiang, H. X. The origin of deep-level impurity transitionsin hexagonal boron nitride. Appl. Phys. Lett. 106, 021110 (2015).11. Weston, L., Wickramaratne, D., Mackoit, M., Alkauskas, A. & Van de Walle, C. G.Native point defects and impurities in hexagonal boron nitride. Phys. Rev. B 97,214104 (2018).12. Grenadier, S. J., Maity, A., Li, J., Lin, J. Y. & Jiang, H. X. Origin and roles of oxygenimpurities in hexagonal boron nitride epilayers. Appl. Phys. Lett. 112, 162103(2018).13. Vokhmintsev, A., Weinstein, I. & Zamyatin, D. Electron–phonon interactions insubband excited photoluminescence of hexagonal boron nitride. J. Lumin. 208,363–370 (2019).14. Watanabe, K. & Taniguchi, T. Hexagonal boron nitride as a new ultravioletluminescent material and its application. Int. J. Appl. Ceram. Technol. 8, 977–989(2011).15. Wong, D. et al. Characterization and manipulation of individual defects in insu-lating hexagonal boron nitride using scanning tunnelling microscopy. Nat.Nanotechol. 10, 949–953 (2015).16. Wanzenboeck, H. D., Roediger, P., Hochleitner, G., Bertagnolli, E. & Buehler, W.Novel method for cleaning a vacuum chamber from hydrocarbon contamination.J. Vac. Sci. Technol. A 28, 1413–1420 (2010).17. Cassabois, G., Valvin, P. & Gil, B. Hexagonal boron nitride is an indirect bandgapsemiconductor. Nat. Photonics 10, 262–266 (2016).18. Schué, L. et al. Bright luminescence from indirect and strongly bound excitons inh-BN. Phys. Rev. Lett. 122, 067401 (2019).19. Hua, Li, L. et al. Photoluminescence of boron nitride nanosheets exfoliated by ballmilling. Appl. Phys. Lett. 100, 261108 (2012).20. Watanabe, K. & Taniguchi, T. Jahn–Teller effect on exciton states in hexagonalboron nitride single crystal. Phys. Rev. B 79, 193104 (2009).21. Riedl, M. J. Optical Design Fundamentals for Infrared Systems 2nd edn (SPIE Press,Bellingham, WA, 2001).22. Masubuchi, S. et al. Autonomous robotic searching and assembly of two-dimensional crystals to build van der Waals superlattices. Nat. Commun. 9, 1413(2018).ACKNOWLEDGEMENTSWe would like to thank Prof. T. Machida and Ms. M. Onodera for granting uspermission to use their sample photos. We also thank Mr. K. Kainuma, Atago Giken forhis help in constructing the system. This research was partially supported by JSPSKAKENHI Grant Number 26286025, JP25107004, JP17H02748, the Elemental StrategyInitiative (MEXT, JAPAN), and CREST (JST, JPMJCR15F3).COMPETING INTERESTSThe authors declare no competing interests.ADDITIONAL INFORMATIONCorrespondence and requests for materials should be addressed to K.W.Reprints and permission information is available at http://www.nature.com/reprintsPublisher’s note Springer Nature remains neutral with regard to jurisdictional claimsin published maps and institutional affiliations.Open Access This article is licensed under a Creative CommonsAttribution 4.0 International License, which permits use, sharing,adaptation, distribution and reproduction in anymedium or format, as long as you giveappropriate credit to the original author(s) and the source, provide a link to the CreativeCommons license, and indicate if changes were made. The images or other third partymaterial in this article are included in the article’s Creative Commons license, unlessindicated otherwise in a credit line to the material. If material is not included in thearticle’s Creative Commons license and your intended use is not permitted by statutoryregulation or exceeds the permitted use, you will need to obtain permission directlyfrom the copyright holder. To view a copy of this license, visit http://creativecommons.org/licenses/by/4.0/.© The Author(s) 2019K. Watanabe and T. Taniguchi5Published in partnership with FCT NOVA with the support of E-MRS npj 2D Materials and Applications (2019)    40 http://www.nature.com/reprintshttp://www.nature.com/reprintshttp://creativecommons.org/licenses/by/4.0/http://creativecommons.org/licenses/by/4.0/ Far-UV photoluminescence microscope for impurity domain in hexagonal-boron-nitride single crystals by high-pressure, high-temperature synthesis Introduction Results and discussions Far-UV PL microscope Measuring bulk samples Measuring exfoliated samples Methods Sample preparation References References References Acknowledgements Competing interests ADDITIONAL INFORMATION