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[S. Nakagawa](https://orcid.org/0000-0003-4539-6953), T. Shizu, T. Imai, M. Nakayama, J. Kim, [H. Minami](https://orcid.org/0000-0003-4884-3560), [K. Kadowaki](https://orcid.org/0000-0001-9391-1150), [M. Tsujimoto](https://orcid.org/0000-0003-4296-5137), [H. Nakao](https://orcid.org/0000-0003-4020-537X), [H. Eisaki](https://orcid.org/0000-0002-8299-6416), [S. Ishida](https://orcid.org/0000-0001-9445-8079), [T. Mochiku](https://orcid.org/0000-0003-2208-4279), [Y. Hasegawa](https://orcid.org/0000-0002-6674-4745), [T. Kashiwagi](https://orcid.org/0000-0002-4839-9247)

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[Study of device characteristics of intrinsic Josephson junction terahertz emitters related to annealing conditions of the crystals](https://mdr.nims.go.jp/datasets/0f05df4d-85df-4fa3-8516-1a7ff6dde5a5)

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Study of device characteristics of intrinsic Josephson junction terahertz emitters related to annealing conditions of the crystalsViewOnlineExportCitationCrossMarkRESEARCH ARTICLE |  APRIL 24 2023Study of device characteristics of intrinsic Josephsonjunction terahertz emitters related to annealing conditions ofthe crystalsS. Nakagawa ; T. Shizu; T. Imai; ... et. alJournal of Applied Physics 133, 163904 (2023)https://doi.org/10.1063/5.0137830Articles You May Be Interested InHigh-Tc superconducting THz emitters fabricated by wet etchingAIP Advances (January 2019)Plausibility of antiferromagnetism in and around the vortex cores of Bi2212 and Tl2223Journal of Applied Physics (May 2004)Quasi‐particles dynamics in underdoped Bi2212 under strong optical perturbationAIP Conference Proceedings (August 2009)Downloaded from http://pubs.aip.org/aip/jap/article-pdf/doi/10.1063/5.0137830/17269032/163904_1_5.0137830.pdfhttps://pubs.aip.org/aip/jap/article/133/16/163904/2885247/Study-of-device-characteristics-of-intrinsichttps://pubs.aip.org/aip/jap/article/133/16/163904/2885247/Study-of-device-characteristics-of-intrinsic?pdfCoverIconEvent=citehttps://pubs.aip.org/aip/jap/article/133/16/163904/2885247/Study-of-device-characteristics-of-intrinsic?pdfCoverIconEvent=crossmarkjavascript:;javascript:;javascript:;javascript:;https://doi.org/10.1063/5.0137830https://pubs.aip.org/aip/adv/article/9/1/015116/1069192/High-Tc-superconducting-THz-emitters-fabricated-byhttps://pubs.aip.org/aip/jap/article/95/11/6906/483862/Plausibility-of-antiferromagnetism-in-and-aroundhttps://pubs.aip.org/aip/acp/article/1162/1/177/819230/Quasi-particles-dynamics-in-underdoped-Bi2212https://servedbyadbutler.com/redirect.spark?MID=176720&plid=2061396&setID=592934&channelID=0&CID=740896&banID=520944490&PID=0&textadID=0&tc=1&adSize=1640x440&matches=%5B%22inurl%3A%5C%2Fjap%22%5D&mt=1684231192738024&spr=1&referrer=http%3A%2F%2Fpubs.aip.org%2Faip%2Fjap%2Farticle-pdf%2Fdoi%2F10.1063%2F5.0137830%2F17269032%2F163904_1_5.0137830.pdf&hc=efe27da23f896ded107f342512fa0af4e0490f8e&location=Study of device characteristics of intrinsicJosephson junction terahertz emitters related toannealing conditions of the crystalsCite as: J. Appl. Phys. 133, 163904 (2023); doi: 10.1063/5.0137830View Online Export Citation CrossMarkSubmitted: 6 December 2022 · Accepted: 6 April 2023 ·Published Online: 24 April 2023 · Corrected: 2 May 2023S. Nakagawa,1,2,a) T. Shizu,1 T. Imai,1 M. Nakayama,1,2 J. Kim,1 H. Minami,1,3 K. Kadowaki,1 M. Tsujimoto,4H. Nakao,5 H. Eisaki,2 S. Ishida,2 T. Mochiku,6 Y. Hasegawa,7 and T. Kashiwagi1,3AFFILIATIONS1Graduate School of Pure and Applied Sciences, University of Tsukuba, 1-1-1 Tennodai, Tsukuba, Ibaraki 305-8573, Japan2Research Institute for Advanced Electronics and Photonics, National Institute of Advanced Industrial Science and Technology(AIST), 1-1-1 Umezono, Tsukuba, Ibaraki 305-8568, Japan3Division of Materials Science, Institute of Pure and Applied Sciences, University of Tsukuba, 1-1-1, Tennodai, Tsukuba,Ibaraki 305-8573, Japan4Research Center for Emerging Computing Technologies, National Institute of Advanced Industrial Science and Technology(AIST), Central2, 1-1-1 Umezono, Tsukuba, Ibaraki 305-8568, Japan5Photon Factory, Institute of Materials Structure Science, High Energy Accelerator Research Organization (KEK), Tsukuba,Ibaraki 305-0801, Japan6National Institute for Materials Science (NIMS), 1-2-1 Sengen, Tsukuba, Ibaraki 305-0047, Japan7The Institute for Solid State Physics, The University of Tokyo, 5-1-5 Kashiwa-no-ha, Kashiwa, Chiba 277-8581, Japana)Author to whom correspondence should be addressed: nakagawa.shungo.su@alumni.tsukuba.ac.jpABSTRACTWe fabricated terahertz (THz) wave emitters from high-temperature superconductor Bi2Sr2CaCu2O8þδ (Bi2212) single crystals annealedunder oxygen gas (O2) flow and nitrogen gas (N2) flow conditions. To better understand the annealing effects of the crystal for the device,we evaluated both device properties and a c-axis lattice constant using x-ray diffraction. Compared to the N2-annealed sample, theO2-annealed sample shows higher critical current in the current–voltage characteristics and no clear emission. In addition, multiple hystere-sis loops were observed above 75 K. Based on the x-ray diffraction measurements, it is suggested that the presence of multiple hysteresisloops observed in the I–V characteristics of the O2-annealed sample is caused by the existence of layers that have varying levels of oxygencontent along the c-axis direction of the crystal. The formation of these layers is attributed to the deposition process of metallic thin filmsduring the device fabrication procedure. This result indicates that the Bi2212 crystal surface of the O2-annealed sample is more sensitivethan that of the N2-annealed one. The information is useful for preparing the Bi2212 crystals for THz-wave emitting devices.Published under an exclusive license by AIP Publishing. https://doi.org/10.1063/5.0137830I. INTRODUCTIONHigh-frequency wireless connection tools, such as cellularphones, have become necessities in our recent lives. These commu-nication tools have been developed step by step so as to achievehigher-speed communication with a larger amount of data, lowerdelay time, multi-connection, etc. Toward these objectives, THzwaves are regarded as promising frequency regions. Furthermore,numerous applications that utilize THz waves, including non-destructive imaging and sensing, astronomy, biological researchstudies, and medical diagnosis, are currently under development.1–3As for the THz emitters, semiconducting solid-state devices,such as resonant tunneling diodes (RTDs)4–7 and quantum cascadelasers (QCLs),8–12 have been extensively developed. As for RTDs,the sub-mW level of output power is achieved. However, there areJournal ofApplied Physics ARTICLE scitation.org/journal/japJ. Appl. Phys. 133, 163904 (2023); doi: 10.1063/5.0137830 133, 163904-1Published under an exclusive license by AIP PublishingDownloaded from http://pubs.aip.org/aip/jap/article-pdf/doi/10.1063/5.0137830/17269032/163904_1_5.0137830.pdfhttps://doi.org/10.1063/5.0137830https://doi.org/10.1063/5.0137830https://www.scitation.org/action/showCitFormats?type=show&doi=10.1063/5.0137830http://crossmark.crossref.org/dialog/?doi=10.1063/5.0137830&domain=pdf&date_stamp=2023-04-24http://orcid.org/0000-0003-4539-6953http://orcid.org/0000-0003-4884-3560http://orcid.org/0000-0001-9391-1150http://orcid.org/0000-0003-4296-5137http://orcid.org/0000-0003-4020-537Xhttp://orcid.org/0000-0002-8299-6416http://orcid.org/0000-0001-9445-8079http://orcid.org/0000-0003-2208-4279http://orcid.org/0000-0002-6674-4745http://orcid.org/0000-0002-4839-9247mailto:nakagawa.shungo.su@alumni.tsukuba.ac.jphttps://doi.org/10.1063/5.0137830https://aip.scitation.org/journal/japtechnical difficulties to generate higher output power above1 THz.13–15 QCLs have good output power characteristics reaching ashigh as 138mW at 4.4 THz9 and can generate frequencies rangingfrom 1.2 to 5.4 THz. However, since the operating maximum tem-peratures of QCLs are restricted by Tmax(K) = (h/kB)f � 50f (THz),where h is Planck’s constant and kB is Boltzmann’s constant,8 a low-temperature operation is required in order to produce the THzwaves with frequency around 1 THz.10 Recently, for the room tem-perature operation of QCLs, difference-frequency generation QCLtechniques have been developed.16–18We have developed THz emitters by using single crystals of ahigh-temperature superconductor Bi2Sr2CaCu2O8þδ (Bi2212).Bi2212 is composed of alternating stacks of the insulating Bi2O2layers and the superconducting CuO2 layers along the c-axis. Thisstacking structure results in forming intrinsic Josephson junctions(IJJs).19–21 Accordingly, when a dc voltage is applied across a smallmesa structure made of a Bi2212 single crystal, THz waves areemitted (Bi2212-THz emitter)22 due to the AC Josephson effect.23The mesa structure also functions as an electromagnetic (EM)cavity, with the standing wave modes depending upon its geometri-cal shape. When the frequency of the generated THz currentmatches the resonance frequency of a cavity mode, EM waves inthe THz region are strongly enhanced.22,24–26According to previous studies, the Bi2212-THz emittersproduce THz waves with radiation frequencies ranging from 0.3 to2.4 THz.27–30 The maximum output power was approximately30–100 μW from a single mesa device.27,29,31–33 It was reportedthat a 0.6 mW level of output power was achieved by synchronizingthree mesa structures.33 In addition, emissions ranging from1 � 11 THz were reported, from thinner and narrower rectangularBi2212 devices.34 Recently, it was shown that the polarization ofthe EM waves can be controlled.35–37 Information about the devel-opments of the Bi2212-THz emitters can be obtained from reviewarticles.38–42To date, extensive research has significantly improved devicestructures and their fabrication processes. On the other hand, whileit is known that material properties of crystals [superconductingtransition temperature (Tc), chemical composition, hole-doping,etc.] have an influence on device properties,43 the detailed relevanceof these properties is not yet understood. For example, it is empiri-cally believed that slightly under-doped crystals [e.g., annealed at600�C with 0.1% oxygen gas (O2)/99.9% argon gas flow] are betterfor device use.In this work, in order to obtain a better understanding of thedevice characteristics from a materials point of view, we attemptedto evaluate the electric and material properties of the Bi2212crystal chips (Bi-chips). Specifically, Bi-chips were fabricated fromcrystals annealed under two typical conditions, under N2 flow andunder O2 flow. The c-axis lattice constants were evaluated usingx-ray diffraction experiments with synchrotron radiation, depend-ing on the annealing conditions. We evaluated the device charac-teristics of the samples, based on the current–voltagecharacteristics and emission characteristics, depending on theapplied bias voltages. As a result, the material properties havehelped to gain a better understanding of the device’s electricalcharacteristics. The information is useful for preparing the Bi2212crystals for THz-wave emitting devices.II. SAMPLE PREPARATIONS AND EXPERIMENTALMETHODSBi2212 single crystals were grown using the floating zonemethod. The details of the crystal growth are described in Ref. 44.The crystals were cut into �3 mm2 squares. Then, to adjust theexcess oxygen content of these crystals, they were annealed for4–6 days either under O2 gas flow at 400�C with a flow rate of0.10 l/min or under N2 gas flow at 600�C with a flow rate of 1.0 l/min.In the case of an N2 gas flow, oxygen content was monitored using anoxygen analyzer (YOKOGAWA, OX400) and was found to be lessthan 1 ppm. At the end of annealing, the crystals were quenched. Themain purpose of the post-annealing is to tune the hole-doping.45,46To fabricate the Bi-chips from the crystals, the following wetetching method, established in our previous study,47,48 was used.First, both sides of the crystals were cleaved to prepare thin single-crystal plates with a thickness of 3–5 μm. Then, silver and gold weredeposited on both sides of the crystals. The total thickness of thismetal film was about 10 nm. Then, the photolithography techniqueswere used to create a mask pattern in the shape of 80� 200 μm2rectangles on the surface of the crystals. The crystals were then pro-cessed into the shape of the Bi-chip by the wet etching method.In the following, the Bi-chips prepared using O2- andN2-annealed crystals are referred to as Bi-chip-O2 and Bi-chip-N2,respectively. The dimensions of the obtained Bi-chips were about190� 70 μm2 for Bi-chip-O2 and about 180� 60 μm2 forBi-chip-N2.θ–2θ scans using x-ray diffraction were performed to studythe distribution of the c-axis lattice constants of the crystals thatwere processed into Bi-chips. A four-circle diffractometer was usedat BL-4C in Photon Factory, High Energy Accelerator ResearchOrganization (KEK). The beam shape at the sample position iselliptic with 0.6 mm vertical and 0.8 mm horizontal. The incidentx-ray energy was set to be 8.8 keV with a Si(1 1 1) double-crystalmonochromator. In the present measurements, we are able to eval-uate the precise distribution of the lattice constants on the order of10�2 Å for micrometer-sized samples.Some of the fabricated Bi-chips were assembled as Bi2212-THzemitters using a sandwich structure developed by our group.29,40 Inthis structure, the Bi-chips are sandwiched between two sapphire sub-strates. The diameter and thickness of the substrate are 7 and0.5mm, respectively. The electrodes were constructed on the topsurface of the substrates through sputtering of Cr and Au. Thedevices were mounted on a cold finger of a helium-flowing cryostat(Oxford Instruments, CF1104) equipped with an optical window.The electrical characteristics of the devices were measured using theconventional two-terminal method. An InSb thermal electron bolom-eter (HEB, QMC Instruments, QFI/2BI) was used to detect the elec-tromagnetic waves from the devices.29,40,49It is noted that the Bi-chips used for the x-ray diffraction mea-surements and the device characteristics measurements were differ-ent; however, they were made simultaneously from the same singlecrystal fragment.III. RESULTS AND DISCUSSIONWe measured the temperature dependencies of the magneticsusceptibility of the prepared samples to evaluate the amount ofJournal ofApplied Physics ARTICLE scitation.org/journal/japJ. Appl. Phys. 133, 163904 (2023); doi: 10.1063/5.0137830 133, 163904-2Published under an exclusive license by AIP PublishingDownloaded from http://pubs.aip.org/aip/jap/article-pdf/doi/10.1063/5.0137830/17269032/163904_1_5.0137830.pdfhttps://aip.scitation.org/journal/japhole-doping. Figure 1 shows the temperature dependence of thenormalized susceptibility of the samples annealed under two differ-ent conditions. For the measurements, the single crystals with�2 mm2 were used. The magnetic fields of 5 Oe were applied paral-lel to the c-axis of the crystals.The transition temperature onset, Tc onset [defined by a cross-ing point between two extrapolation lines of the normal state andthe diamagnetic transition curve in the field cooling (FC) measure-ment] was 84.2 K for the N2-annealed crystal and 83.1 K for theO2-annealed crystal, respectively. It is noted that other Bi2212single crystals prepared in the same way exhibited maximum Tc ofapproximately 89 K after specific heat treatment. From theseresults, it was determined that the N2 crystal is under-doped andthe O2 crystal is over-doped.Figure 2 shows the θ–2θ scans of two Bi-chips at around 0 020 reflections. Optical photographs of the Bi-chips glued on metal-lic plates by varnish are also displayed in Fig. 2. The θ–2θ scansshown in Fig. 2 have two obvious differences. First, the θ–2θ scanof the Bi-chip-O2 shows two peaks at 2θ ¼ 54:36� and 54:51�,while the θ–2θ scan of the Bi-chip-N2 shows a single peak at2θ ¼ 54:30�. Second, the positions of the two peaks observed inthe Bi-chip-O2 are higher than the peak position observed in theBi-chip-N2. According to the peak positions of the θ–2θ scan ofthe Bi-chip-O2, the estimated lattice constants of the c-axis are30.77 Å (at 2θ ¼ 54:51�) and 30.84 Å (at 2θ ¼ 54:36�). In the caseof the Bi-chip-N2, the estimated lattice constant of the c-axis is30.88 Å.It is known that the c-axis lattice constant of Bi2212 shrinkswith increasing oxygen content.50,51 The lattice constant values of30.88 Å and 30.77 Å are generally consistent with previous studiesfor under-doped and over-doped crystals, respectively.52 It is notedthat nonstoichiometric compositions of Bi and Sr in the crystalsalso affect the lattice constant.53 The present results indicate thatBi-chip-O2 contains two kinds of layers with different hole concen-trations, and both layers are more hole-doped compared withBi-chip-N2.To clarify the origin of the double peaks in the O2-annealedcrystals, we also measured the θ–2θ scans of bulk (�2 mm2 size)Bi2212 single crystals annealed with O2. Figure 3 shows the resultsof an x-ray diffraction experiment of the samples. The measure-ment system was the same as the one used for the Bi-chips.As shown in Fig. 3, the surface of the sample with the depos-ited metal film was cleaved and removed. θ–2θ scans were per-formed both before and after cleaving to evaluate the crystal state.As with the Bi-chip, the total thickness of the Ag and Au depositedmetal thin film is about 10 nm. The intensity of the scan data wasnormalized by the maximum value and plotted on a logarithmicscale.A low-angle side peak is present in the crystals before cleav-age. The peak is not naturally observed in crystals before metaldeposition, but it can be obtained with high reproducibility indeposited O2-annealed crystals. Furthermore, the peak is observednot only in Ag and Au deposition but also in Au- or Ag-only depo-sition, Au sputtering, and Ar ion sputtering. On the other hand,FIG. 1. Temperature dependence of normalized magnetic susceptibilities fortwo single crystals annealed under different atmospheric conditions. Theobserved data were normalized to be �1 by using the susceptibility dataobtained at the lowest temperature. The filled and open symbols indicate thezero field cooling and field cooling measurements, respectively. The inset dis-plays a magnification plot around Tc for the field cooling measurements. Thedata shown in the inset were also normalized to be �1 by using the susceptibil-ity data obtained at the lowest temperature of the field cooling measurements.FIG. 2. The θ–2θ scan plots for the Bi-chip-O2 and the Bi-chip-N2 around 0 020. The optical photographs of the Bi-chips are also displayed near the curves.Journal ofApplied Physics ARTICLE scitation.org/journal/japJ. Appl. Phys. 133, 163904 (2023); doi: 10.1063/5.0137830 133, 163904-3Published under an exclusive license by AIP PublishingDownloaded from http://pubs.aip.org/aip/jap/article-pdf/doi/10.1063/5.0137830/17269032/163904_1_5.0137830.pdfhttps://aip.scitation.org/journal/japafter removing the metal deposited top surface of the crystal bycleaving, the low-angle side peak disappeared as displayed in Fig. 3.The slight change in the shape and position of the main peak maybe due to the fact that the sample is larger than the x-ray beamdiameter, causing the x-ray irradiation point on the sample to beslightly altered during the second sample setting.This result strongly suggests that the origin of the low-angleside peak is due to processing treatment, such as deposition.Evaporation and sputtering are performed under a vacuum.Therefore, the crystal surfaces are prone to a decrease in oxygenconcentration. This feature is observed as two peaks in the θ–2θscan.Figure 4(a) shows current–voltage characteristics (IVCs) ofthe Bi-chip-N2 measured at various bath temperatures TB. Thevoltage is low when the current starts to ramp up, and then whenthe applied current exceeds the critical Josephson current, Ic, itsuddenly jumps to a large value, indicating that the system turnsto the voltage state. Then, when the current is reduced, thevoltage state persists down to a certain value of the current, thusforming a hysteresis loop as indicated by the arrows in Fig. 4. At35 K, Ic is �15 mA, and the maximum applied bias voltage Vc is�7 V. According to the size of Bi-chips, the estimated Jc is�0:14 kA/cm2. As seen in Fig. 4(a), the hysteresis loop shrinkswith increasing TB.In Fig. 4(b), the radiation intensities detected by the HEB (Vbol)are plotted as a function of the applied voltage to the Bi-chip-N2.These data were obtained at the same time during the measurementof the IVCs shown in Fig. 4(a). EM waves are observed in the lowcurrent region of the IVCs from TB ¼ 35–65 K, with strong radiantintensity observed at TB ¼ 45 K. According to the previous studies,the maximum emission power observed here is estimated to beabout a few μW. The observed results are consistent with the trendsin device characteristics from previous studies.27,29,31,54Figure 5(a) shows the IVC of the Bi-chip-O2 observedbetween TB � 35 K and 85 K. In the case of Bi-chip-O2, IVCsexhibit large hysteresis loops along the current direction. Forexample, Ic and Vc at 35 K are �40 mA and �3 V, respectively.From the value of Ic, Jc can be estimated to be �0:30 kA/cm2. Thehysteresis loops shrink rapidly with increasing TB. At 70 K, a singlehysteresis loop is observed at �30 mA. Notably, at 75 K, a secondhysteresis loop appears at �80 mA. This result suggests not onlythe existence of crystal inhomogeneity in the Bi-chip-O2 but alsothe existence of the two junctions with different Ic’s.FIG. 3. (a) The θ–2θ scans around 0 0 26 of a Bi2212 bulk single crystal. Thiscrystal has a total of about 10 nm of Ag and Au deposited on the surface.Measurements were taken before and after the surface cleavage. (b) Schematicimage and optical photographs of the samples evaluated.FIG. 4. (a) Typical temperature dependencies of the IVCs and (b) the radiationintensities detected by the HEB. Vbol is plotted as a function of the appliedvoltage to the Bi-chip-N2. The data of Vbol are shifted vertically with 50 mV. Theinset of the upper panel displays temperature dependencies of the c-axis resis-tivity. The temperature attached to the data indicates TB of the sample.Journal ofApplied Physics ARTICLE scitation.org/journal/japJ. Appl. Phys. 133, 163904 (2023); doi: 10.1063/5.0137830 133, 163904-4Published under an exclusive license by AIP PublishingDownloaded from http://pubs.aip.org/aip/jap/article-pdf/doi/10.1063/5.0137830/17269032/163904_1_5.0137830.pdfhttps://aip.scitation.org/journal/japThe Jc’s observed at lower temperatures are higher than that ofBi-chip-N2. The increase in Jc observed for over-doped crystalsseems to be consistent with previous studies.55–57 However, theobtained value of Jc is one order of magnitude smaller than that ofthe over-doped crystals.56,58 Therefore, the second hysteresis at�80 mA (Jc � 0:62 kA/cm2) appeared around 75 K is likely to haveoriginated from the over-doped phase of the sample. This pointwill be discussed later.Figure 5(b) shows the bias voltage dependence of the radiantintensity of the Bi-chip-O2. As distinct from Bi-chip-N2, apparentradiation was not observed. The device characteristics ofBi-chip-O2 are completely different from those observed in theBi-chip-N2 in terms of the shape and size of the hysteresis loops, aswell as the EM radiation capability. As for IVC at low temperatures,Bi-chip-N2 has a lower Ic and a larger Vc, whereas Bi-chip-O2 hasa higher Ic and a smaller Vc.The insets in Figs. 4(a) and 5(a) show the temperature depen-dence of the c-axis resistance. In both cases, the c-axis resistanceshows an upturn from room temperature to low temperatures, withsignificant drops at about 85 K, which correspond to the supercon-ducting transitions. The finite resistance below Tc is primarilyattributed to the resistance of the thin film on the sapphire sub-strate, which serves as the power feed path, and the contact resis-tance measured by the two-terminal method. Therefore, theincrease in resistivity near Tc in Bi-chip-O2 includes the effect ofcontact resistance. Taking this into account, the resistivity upturnof Bi-chip-O2 was much smaller than that of Bi-chip-N2. This alsoconfirms that Bi-chip-O2 is more hole-doped.45,46,50Here, we discuss the relationship between the crystals and thedevice characteristics based on the above experimental results. First,our annealing conditions clearly change the amount of hole-dopingp of the crystals as shown in Fig. 1. According to the transitiontemperatures of the samples and Tallon’s rule,59 these are estimatedto be p � 0:134 (N2-annealed) and p � 0:188 (O2-annealed).These correspond to so-called under-doped and over-dopedsamples.The fabricated Bi-chips using those crystals clearly show achange in the c-axis lattice constants depending on the differencein hole-doping of the samples as seen in Fig. 2. As with the previ-ous studies,50,51 the c-axis lattice constant of Bi2212 shrinks withincreasing oxygen content. In addition to the change in the latticeconstant, the data obtained from θ–2θ scans clearly indicate theexistence of the two crystalline layers with different lattice constantsin the Bi-chip-O2, while the Bi-chip-N2 is expected to have a singledomain. It means that the two layers would have differenthole-doping.As seen in Fig. 3, the separation of the peaks would be origi-nated from the deposition processes of the metallic thin films byevaporation. For the deposition of the metallic thin films, smallpieces of Au and Ag are heated in boats under a vacuum. The con-dition affects the surface of the crystals. In particular, the surface ofO2-annealed crystals would be highly sensitive and easily reducedduring evaporation.These characteristics are clearly reflected in the IVCs of theBi-chips. In the case of the Bi-chip-N2, a temperature-dependentsingle hysteresis loop is observed due to the single domain of thesample, as discussed in the θ–2θ scan data. The observedJc � 0:14 kA/cm2 at 35 K is comparable to that of previous studies.For example, Ozyuzer et al.43 reported a value of 0.04–0.17 kA/cm2(at 12 K) for the under-doped crystal in the terahertz devices.For the case of the Bi-chip-O2, a single hysteresis loop of IVCswith Ic � 40 mA (estimated Jc � 0:3 kA/cm2) is observed around35 K. In addition, the two hysteresis loops in the IVCs are observedat higher TB. As discussed in the θ–2θ scan data, these two hystere-sis loops would be originated from the surface and inside parts ofthe Bi-chips. The surface part would have a smaller Jc compared tothe inside part of the sample, as Jc generally increases exponentiallywith an increase in hole-doping.55–58The observed Jc � 0:3 kA/cm2 at 35 K is comparable to previ-ous studies in which 0.36 kA/cm2 (at 4.2 K) for the over-dopedcrystal43 and 0.22 kA/cm2 (at 70 K) for optimally doped crystals52have been reported. However, these Jc’s are one order of magnitudesmaller than the reported values, such as 2 kA/cm2 at p ¼ 0:16,4.69 kA/cm2 at p ¼ 0:186, and 8 kA/cm2 at p ¼ 0:19.56,58 The dis-crepancy of Jc’s between our results and the previous studies56,58would be related to the preparation of the over-doped crystals. Inour samples, as mentioned above, the carrier contents of thesurface of the Bi-chip would be reduced by evaporation. This parthas a small Jc; however, the inside of the Bi-chip has larger Jc’s. Atleast, the inside part of the Bi-chip has Jc � 0:62 kA/cm2 at 75 K.In addition, as seen in the IVCs from 85 to 75 K in Fig. 5, the valueFIG. 5. (a) IVCs and (b) the bias voltage dependence of radiation intensities forthe Bi-chip-O2 observed from 35 to 85 K. Note that the data of the IVCs andVbol are shifted horizontally with 1 V to understand the change of IVCs withincreasing temperatures. The inset of the upper panel displays the temperaturedependence of the c-axis resistivity.Journal ofApplied Physics ARTICLE scitation.org/journal/japJ. Appl. Phys. 133, 163904 (2023); doi: 10.1063/5.0137830 133, 163904-5Published under an exclusive license by AIP PublishingDownloaded from http://pubs.aip.org/aip/jap/article-pdf/doi/10.1063/5.0137830/17269032/163904_1_5.0137830.pdfhttps://aip.scitation.org/journal/japof Jc of the inside part increases rapidly as Tbs decreases. Thus, theJc of the inside part of the Bi-chip is even larger at low tempera-tures. The Jc would be similar to the previously reported values56,58at lower temperatures. It is noted that in the case of the Bi2212THz device structures, the self-heating due to the application of dcbias voltage/current would be reflected in the reduction of thevalue of Jcs.Finally, we discuss the emission characteristics of the devices.According to the x-ray peak intensity ratio for the Bi-chip-O2, thevolume fraction of the less doped layers corresponding to thesurface part of the Bi-chip is small. At 35 K, these layers may meetthe voltage conditions for terahertz radiation; however, the smallvolume fraction may be the most significant reason for the lack ofclear emission from the sample. In addition, at high temperatures,the highly doped layers are also in a voltage state, but terahertzradiation was not observed here because the applied bias voltage tothe sample was small relative to the fundamental oscillation condi-tions determined by the chip width.On the other hand, the Bi-chip-N2 is expected to be homoge-neous crystalline characteristics from the θ–2θ scans as seen inFig. 2. In fact, the IVCs of the sample have single hysteresis loopswith lower Ic’s compared to the Bi-chip-O2. As shown in Fig. 4,clear THz radiation was observed from the sample since the wholeIJJs of the Bi-chip-N2 would contribute to the radiation.As discussed above, a part of the device characteristics can beexplained from a material perspective. While we need further workon device characteristics related to the hole-doping of the crystals,the relationships among hole-doping, lattice constants, and IVCscan be understood. In addition, it is evident that the surface of theover-doped crystals is very sensitive to the heat treatment processes,such as electrode evaporation on the crystal surface. This informa-tion is important for preparing the crystals for the THz emittingdevices.IV. CONCLUSIONSThe device characteristics of Bi2212-THz emitters were ana-lyzed based on the properties of the crystal. Bi2212 single crystalsannealed under N2 and O2 gas flow were fabricated into Bi-chipsby the wet etching method. X-ray diffraction was performed on thechips to investigate the difference in the c-axis lattice constant withrespect to the excess oxygen content. As a result, it was found thatonly in O2-annealed crystals, the excess oxygen near the chipsurface is reduced by metal deposition. This change in the latticeconstant corresponds well to the difference in Jc’s in device proper-ties, especially reflected in the two hysteresis loops on the IVC ofthe O2-annealed sample. This understanding of the crystal proper-ties also offers a reasonable explanation for why clear emission wasonly obtained from the N2-annealed sample. The results of thisstudy reveal processing issues in Bi2212 single crystals with highoxygen content and provide useful information for the develop-ment of Bi2212-THz emitters.ACKNOWLEDGMENTSThis study was supported by the Japan Society for thePromotion of Science Grant-in-Aid for Scientific Research(No. 15H01996). T.K. was supported by the Japan Society for thePromotion of Science Grant-in-Aid for Scientific Research (Nos.17K05018 and 20H02590). This study was also supported byTIA-Kakehashi grants (Nos. 2018-43 and 2019-47). The x-ray dif-fraction measurements were performed under the approval of thePhoton Factory Program Advisory Committee (Proposal No.2019G634).AUTHOR DECLARATIONSConflict of InterestThe authors have no conflicts to disclose.Author ContributionsS. Nakagawa: Conceptualization (equal); Data curation (equal);Formal analysis (equal); Investigation (lead); Visualization (lead);Writing – original draft (lead); Writing – review & editing (lead).T. Shizu: Formal analysis (equal); Investigation (equal). T. Imai:Investigation (equal). M. Nakayama: Investigation (equal). J. Kim:Investigation (equal). H. Minami: Resources (equal); Writing –review & editing (equal). K. Kadowaki: Resources (equal); Writing –review & editing (equal). M. Tsujimoto: Writing – review & editing(equal). H. Nakao: Investigation (supporting); Methodology (equal);Software (equal); Supervision (equal); Writing – review & editing(equal). H. Eisaki: Supervision (equal); Writing – original draft(equal); Writing – review & editing (equal). S. Ishida: Writing –review & editing (equal). T. Mochiku: Writing – review & editing(equal). Y. Hasagawa: Writing – review & editing (equal).T. Kashiwagi: Conceptualization (equal); Data curation (equal);Funding acquisition (equal); Investigation (equal); Project adminis-tration (equal); Resources (equal); Supervision (equal); Writing –original draft (lead); Writing – review & editing (lead).DATA AVAILABILITYThe data that support the findings of this study are availablewithin the article.REFERENCES1M. Tonouchi, “Cutting-edge terahertz technology,” Nat. Photonics 1, 97–105(2007).2B. Ferguson and X.-C. Zhang, “Materials for terahertz science and technology,”Nat. Mater. 1, 26–33 (2002).3S. Dhillon, M. Vitiello, E. Linfield, A. Davies, M. C. Hoffmann, J. Booske,C. Paoloni, M. Gensch, P. Weightman, G. Williams, and E. Castro-Camus, “The2017 terahertz science and technology roadmap,” J. Phys. D: Appl. Phys. 50,043001 (2017).4Y. Koyama, R. Sekiguchi, and T. Ouchi, “Oscillations up to 1.40 THz fromresonant-tunneling-diode-based oscillators with integrated patch antennas,”Appl. Phys. Express 6, 064102 (2013).5M. Asada and S. 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