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[Zhao Ma](https://orcid.org/0000-0003-0151-8592), [Takaaki Mano](https://orcid.org/0000-0002-6955-260X), [Akihiro Ohtake](https://orcid.org/0000-0002-3519-4613), [Takashi Kuroda](https://orcid.org/0000-0001-6445-7673)

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[Enhanced photoluminescence intensity of buried InGaAs/GaAs(001) quantum wells by sulfur termination](https://mdr.nims.go.jp/datasets/98e8cac5-023c-4abe-a447-34dd4492fad0)

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Enhanced photoluminescence intensity of buried InGaAs/GaAs(001) quantum wells by sulfur terminationJapanese Journal ofApplied Physics      REGULAR PAPER • OPEN ACCESSEnhanced photoluminescence intensity of buriedInGaAs/GaAs(001) quantum wells by sulfurterminationTo cite this article: Zhao Ma et al 2024 Jpn. J. Appl. Phys. 63 121002 View the article online for updates and enhancements.You may also likeRecent developments on plasma basedneutron sources from microscopicinnovations to meter-scale applicationsS. R. Mirfayzi, M. Gryaznevich, O.Lonsdale et al.-Enhanced performance of CaZrS3-basedchalcogenide perovskite solar cells(CPSC): a theoretical study onoptimization of hole transport materialHend I. Alkhammash and Md. Ariful Islam-Disk array-type plasmonic chip forfluorescence enhancementYasunori Nawa, Riku Shimosaka, AtsushiShimizu et al.-This content was downloaded from IP address 144.213.253.16 on 05/02/2025 at 08:58https://doi.org/10.35848/1347-4065/ad9802https://iopscience.iop.org/article/10.35848/1347-4065/ad9f00https://iopscience.iop.org/article/10.35848/1347-4065/ad9f00https://iopscience.iop.org/article/10.35848/1347-4065/ad9f00https://iopscience.iop.org/article/10.35848/1347-4065/ad9977https://iopscience.iop.org/article/10.35848/1347-4065/ad9977https://iopscience.iop.org/article/10.35848/1347-4065/ad9977https://iopscience.iop.org/article/10.35848/1347-4065/ad9977https://iopscience.iop.org/article/10.35848/1347-4065/ad9977https://iopscience.iop.org/article/10.35848/1347-4065/ad9c82https://iopscience.iop.org/article/10.35848/1347-4065/ad9c82https://pagead2.googlesyndication.com/pcs/click?xai=AKAOjstSjl5V7kw-PvCndQW2sLuSGxuaAVu5fsv8bIYjxAG_Ba5UUVj7emFGIj_pO6MLAoHH-VmxkkmEaa5Er8cY-afxPe47b41o9XF5noCYCIyG-6ce_7zL2Di6-5etJhyCSn8PxmU8vZfdMb785swkO7gzeXC7xCfxJVE6lsuDGK43q8cv7LAgjrjTl_hbTxBh6FH_H63w9PRRVNUJKMZBEeUf2X9_ZHNGMi60H0C1ETdAJvbYtj5o23UPFNcSLV4l5wMfHI-UzFqyCJn22tGvIRhleABitj_nxR3eJuyw1cNdqvGt2IVKUWsXzqvJEohDttzdHZGVUlR8EOEvfiC2x4FeLusQrLsLh8w4KDZcAtA-&sig=Cg0ArKJSzDGhPpHIMKBV&fbs_aeid=%5Bgw_fbsaeid%5D&adurl=https://ecs.confex.com/ecs/248/cfp.cgi%3Futm_source%3DIOP%26utm_medium%3Dbanner%26utm_campaign%3DIOP_248_abstract_submission%26utm_id%3DIOP%2B248%2BAbstract%2BSubmissionEnhanced photoluminescence intensity of buried InGaAs/GaAs(001) quantumwells by sulfur terminationZhao Ma1,2* , Takaaki Mano1* , Akihiro Ohtake1 , and Takashi Kuroda1,21National Institute for Materials Science, Tsukuba, Ibaraki 305-0044, Japan2Department of Applied Chemistry, Graduate School of Engineering, Kyushu University, Fukuoka 819-0395, Japan*E-mail: MA.Zhao@nims.go.jp and MANO.Takaaki@nims.go.jpReceived September 12, 2024; revised November 25, 2024; accepted November 26, 2024; published online December 16, 2024Sulfur (S) termination of III–V semiconductor surfaces is an effective technique for passivating surfaces to prevent oxidation. In this study, wesystematically investigated the effects of S termination by (NH4)2Sx treatment on enhancement of the photoluminescence (PL) properties of buriedInGaAs/GaAs(001) quantum wells (QWs). X-ray photoelectron spectroscopy (XPS) and PL measurements revealed that the (NH4)2Sx treatmentsuppresses the formation of surface oxides, especially arsenic oxides, and enhances the PL intensity of QWs. Clear correlation between the PLand XPS results suggests that greater PL intensity is attributable to a reduction in the number of nonradiative recombination centers at the surfacecaused by arsenic oxide formation. In addition, from the observed temporal changes in PL intensities, we found that the S-terminated surfacesexhibit long-term and high resistance to surface oxidation by air exposure. 1. IntroductionIII–V semiconductor-based heterostructures are widely used inthe field of high-performance optoelectronic devices, includinghigh-electron mobility transistors,1,2) heterojunction bipolartransistors,3,4) laser and light-emitting diodes, photodetectors,and solar cells.5) During the crystal growth or etchingprocesses of these heterostructures, their surfaces are keptmore or less clean, with only small amounts of attachedforeign atoms. When the structures are exposed to ambient airafter processing, the surfaces come into contact with oxygenand other foreign contaminants. The surfaces of GaAs-basedstructures are then gradually oxidized, with a high density ofsurface states being formed on these surfaces, resulting instrong Fermi-level pinning effects near the midgap ofGaAs.6,7) Around 1990, various surface-termination processesusing VI-group element atoms, such as sulfur (S) and selenium(Se), were actively investigated with the aim of suppressingthese surface oxidation processes, chiefly to improve thedevices’ Schottky and other properties for use in metal-insulator-semiconductor devices.7–9) To form terminated sur-faces, wet processes using Na2S·9H2O,10,11) (NH4)2S,12–14)(NH4)2Sx,7,8,15–17) Na2Se in NH4OH,9) (NH4)2S+ Se,18) andvacuum deposition of elemental sulfur19,20) or selenium21–31)in ultra-high vacuum (UHV) chambers were attempted,demonstrating their effectiveness and potential for practicalapplication.Surface-termination techniques for passivating semicon-ductor nanostructures such as nanowires, nanopillars, andquantum dots are increasingly coming back into thelimelight.32–35) In these nanostructures, the core regions areoften exposed to air and/or located very close to the surface.The oxidized surfaces therefore act as nonradiative-recombi-nation centers and/or charge trapping sites that degrade theoptical properties of the nanostructures by decreasing theoptical emission intensity10) and/or broadening the opticalemission linewidth.32–35) Suppression of surface oxidation istherefore a critical challenge for improving the opticalproperties of these nanostructures. In more recent studies,sulfur termination followed by coating with a dielectricmaterial such as Al2O3 or SiOx are often used for surfacepassivation.35–38) Enhanced photoluminescence (PL) emis-sion and narrowing of the single quantum dot spectra havebeen reported. However, the effects of S termination on theoptical properties have not yet been elucidated. For thefurther improvement of the optical properties of nanostruc-tures by using surface termination techniques, it is necessaryto understand in greater detail the surface-termination effectson surface oxidation and their impact on optical properties.In this study, we systematically investigated the impact ofan (NH4)2Sx-based passivation process on the optical proper-ties of buried quantum wells (QWs) and the chemicalcomposition of the GaAs (001) surfaces by means of PLspectroscopy and X-ray photoelectron spectroscopy (XPS)measurements. The inhibitory effects of S termination onsurface oxidation were clearly shown. Suppressing surfaceoxidation clearly enhanced PL intensity.2. Experimental methodsOur experimental procedures are shown schematically inFig. 1. As starting material, we used an identical singleInGaAs QW structure grown on GaAs (001) substrate usingmolecular beam epitaxy. After growth of a 300 nm GaAsbuffer layer at 580 °C, 10 nm In0.12Ga0.88As QW was grown at490 °C. The QW was then capped with 100 nm GaAs at 490 °C. The surface was finally capped with amorphous As bysupplying As2 flux (1× 10−5 Torr beam equivalent pressure)at room temperature for 30min to protect the sample fromoxidation after exposure to ambient air. Before performingeach PL and XPS measurement the amorphous As wasremoved by dipping the samples in ∼30% H2O2 solution.This procedure enabled us to study identical surfaces for eachexperiment. This sample is referred to as the “Reference”sample in this paper. It should be noted that the H2O2 solutionoxidizes the GaAs surface. To remove the surface oxides, weimmersed the samples in buffered potassium hydroxide (KOH)solution for 1 min,17) followed by rinsing in ultra-pure waterand N2 gas blowing. The sample at this step is referred to“KOH-treated”. For sulfur termination, the KOH-treatedsample was dipped in ammonium sulfide solution with excesssulfur [(NH4)2Sx] at 60 °C for 15min. The sample was thentaken out from the solution and dried by N2 blow without121002-1Japanese Journal of Applied Physics 63, 121002 (2024) REGULAR PAPERhttps://doi.org/10.35848/1347-4065/ad9802Content from this work may be used under the terms of the Creative Commons Attribution 4.0 license. Any further distribution of thiswork must maintain attribution to the author(s) and the title of the work, journal citation and DOI.© 2024 The Author(s). Published on behalf of The Japan Society of Applied Physics by IOP Publishing Ltd © 2024 The Author(s). Published on behalf of The Japan Society of Applied Physics by IOP Publishing Ltd https://crossmark.crossref.org/dialog/?doi=10.35848/1347-4065/ad9802&domain=pdf&date_stamp=2024-12-16https://orcid.org/0000-0003-0151-8592https://orcid.org/0000-0003-0151-8592https://orcid.org/0000-0002-6955-260Xhttps://orcid.org/0000-0002-6955-260Xhttps://orcid.org/0000-0002-3519-4613https://orcid.org/0000-0002-3519-4613https://orcid.org/0000-0001-6445-7673https://orcid.org/0000-0001-6445-7673mailto:MA.Zhao@nims.go.jpmailto:MANO.Takaaki@nims.go.jphttps://doi.org/10.35848/1347-4065/ad9802https://creativecommons.org/licenses/by/4.0/rinsing in water. Finally, we annealed the sample in vacuumchamber at 150 °C for 30min using a ramp heater to evaporatethe excess sulfur atoms on the surface. The heating rate was10 °Cmin−1. The annealing temperature was carefully cali-brated by using thermo-couples set at the sample position. Thebackground pressure during the annealing was around1× 10–3 Pa. The sample at this step is referred to “S-terminated”.After each chemical treatment, the samples were stored invacuum chamber to avoid further oxidation. To investigatethe temporal change of PL and XPS signals with air exposuretime, we took the samples out of the vacuum chamber andexposed them to the air for different periods from less than30 min to 30 d. For the PL measurement, we used anNd:YVO4 based laser emitting at a wavelength of 532 nm.The PL signals were fed into a 50 cm spectrometer anddetected by a cooled charge-coupled device camera. The PLmeasurement was performed mostly at 4 K, where the carrierthermal escape from the QW was efficiently suppressed. Inaddition, we measured the PL spectra at room temperature forthe samples after long-term air exposure (7 month) to clarifythe thermal impact on nonradiative recombination.The surface chemical conditions were analyzed using anXPS measurement system (Surface Science InstrumentsM-Probe). XPS measurements were performed using mono-chromatic Al Kα radiation (1486.6 eV) at room temperature.Photoelectrons were detected at an angle of 35˚ from thesurface. The As 3d and Ga 3d spectra were measured andfitted using a Voigt function with the ratio of Gaussian toLorentzian components fixed at 2.5. Peak separations of0.70 eV and 0.45 eV, respectively, were assumed for the 5/2and 3/2 spin–orbit components of As 3d and Ga 3d.3. Results and discussionFigures 2(a)–2(c) show the low-temperature PL spectra of theReference, KOH-treated, and S-terminated samples, imme-diately after the chemical treatment (air exposure time of lessthan 30 min). While all the samples exhibited a clear PLemission peak at around 900 nm from the InGaAs QWs, therewere significant differences in their intensity. The integratedPL intensities of the KOH-treated and S-terminated sampleswere 1.2- and 1.4-fold larger than that of the Referencesample, respectively, as seen in Fig. 2(d). The results revealthat both the KOH treatment and S termination are effectivefor enhancing the PL emission efficiency. We attribute theenhanced PL intensity to the decreased number of nonradia-tive recombination centers at the surfaces.Next, we investigated the time-dependent change in the PLintensity of these samples with exposure to air. Figure 3 showsthe changes in the integrated PL intensity of the three samplesafter air exposure for certain periods of time. As expected, thePL intensity of the Reference sample, where the surface oxideshad already been formed by dipping in the H2O2 solution,exhibits marginal change: even after 30 d, the intensitydecreased by only ∼10% from the initial value. On the otherhand, the PL intensity of the KOH-treated sample changedrapidly: after 5 d, the intensity decreased by more than 20%,reaching nearly the same level as that of the Reference sample,suggesting that the surface chemical condition of the KOH-treated sample converges with that of the Reference sampledue to oxidation. The PL intensity of the S-terminated samplealso decreased by 16% in the first five days, then more slowlylater. It is important here to note that the intensity of the S-terminated sample is always more than 20% higher than thatof the other two samples, suggesting that the S termination isstable even after exposure to air for one month.We also measured the PL spectra at room temperature forall the samples after long-term storage in air (7 months).Figures 4(a)–4(c) show the room temperature spectra of thethree samples, together with their low temperature spectrameasured at 4 K. The PL signals at room temperature arearound three orders of magnitude weaker than those mea-sured at 4 K due to the impact of carrier thermal escape fromthe shallow QW. Remarkably, the intensity contrast betweenthe three samples is more significant at room temperaturethan 4 K. At room temperature, the S-terminated sampleshows intensities twice as strong as the others. However, at4 K, the S-terminated sample shows intensities around 1.2times higher than the other samples, the signature of which issimilar with what we observed in Figs. 2 and 3. We attributethe enhanced contrast to increased probability that carriersreach the surface before being recombined in the QW.Fig. 1. Schematic drawing of experimental procedures.121002-2Jpn. J. Appl. Phys. 63, 121002 (2024) Z. Ma et al.© 2024 The Author(s). Published on behalf of The Japan Society of Applied Physics by IOP Publishing Ltd XPS measurements were carried out to study the chemicalstates of the three samples and their relation to their opticalproperties. Figures 5 and 6 respectively show the As 3d andGa 3d core-level spectra. The spectra in (a) were measuredfrom the sample within 30 min after the treatment, and thosein (b) were measured from the sample that had been exposedto air for 14 d.The As 3d spectra were fitted with components corre-sponding to Ga–As, AsO, As2O3, As2O5, and GaAsO4, andthe Ga 3d spectra were fitted with Ga–As, Ga2O3, andGaAsO4 components. The quantities of Ga- and As-oxidesmeasured from the Reference sample [top panels of Figs. 5(a)and 6(a)] are more than two-fold those measured on thecleaned (2× 4)-reconstructed GaAs(001) surface after ex-posure to air for 24 h (data not shown). It is thereforeconfirmed that H2O2 treatment strongly facilitates the oxida-tion of the GaAs surface immediately after the removal of theamorphous-As cap layers. In the KOH-treated sample[middle panels of Figs. 5(a) and 6(a)], the oxide-relatedcomponents are significantly lower than seen in the referencesample [Figs. 5(c) and 6(c)], indicating that the Ga- and As-oxides had been effectively removed by the KOH treatment.When the Reference sample had been exposed to air for14 d, the intensities of oxide-related components increasedslightly [top panels of Figs. 5(a), 5(b) and 6(a), 6(b)]. On theother hand, for the KOH-treated sample, the oxide-relatedcomponents had drastically increased after 14 d, such thattheir intensities were comparable with those of the Referencesample [middle panels of Figs. 5(a), 5(b) and 6(a), 6(b)]. Thismeans that the KOH treatment was able to remove most ofthe oxides, but is not effective in suppressing oxide formationthat takes place after long periods of exposure to air.The As 3d spectrum measured immediately after the Streatment is shown in the bottom panel of Fig. 5(a). There islittle evidence of components that would indicate thepresence of As-oxides. While the formation of As-oxides isconfirmed after 14 d, as shown in the bottom panel ofFig. 5(b), the amount of oxides is one order of magnitudesmaller than those seen on the KOH-treated sample[Fig. 5(c)]. On the other hand, Ga-oxides components can(a) (b) (c)(d)Fig. 2. Low-temperature PL spectra of (a) Reference, (b) KOH-treated, and (c) S-terminated samples right after the chemical treatment (Day 0). Theintegrated intensity of these spectra is plotted in (d).Fig. 3. Time-dependent change in the integrated PL intensity of the threesamples after exposure to air for certain periods of time. They were measuredat 4 K.121002-3Jpn. J. Appl. Phys. 63, 121002 (2024) Z. Ma et al.© 2024 The Author(s). Published on behalf of The Japan Society of Applied Physics by IOP Publishing Ltd be clearly seen in the spectrum measured immediately afterthe S treatment [bottom panel of Fig. 6(a)]; the quantities ofGa-oxides are much greater than those on the KOH-treatedsample. However, the amount of Ga-oxides increasedonly minimally after 14 d, as shown in Fig. 6(c). The Streatment is therefore extremely effective in removing As-oxides, as well as in preventing further oxidation of both Asand Ga.A comparison of the the observed changes in the XPSspectra [Figs. 5(c) and 6(c)] with the PL results (Fig. 3),(a) (b) (c)Fig. 4. PL spectra of the three samples after being stored in air for 7 months for the (a) Reference, (b) KOH-treated, and (c) S-terminated samples. The solidlines are the spectra measured at room temperature and the broken lines are the spectra measured at 4 K.(a) (b)(c)Fig. 5. As 3d core-level spectra of the three samples. (a) immediately after preparation and (b) after exposure to air for 14 d. The ratios of the oxide-relatedcomponents (As–O/Ga–As) are plotted in (c).121002-4Jpn. J. Appl. Phys. 63, 121002 (2024) Z. Ma et al.© 2024 The Author(s). Published on behalf of The Japan Society of Applied Physics by IOP Publishing Ltd reveals a clear correlation between them. Shown in Figs. 7(a),7(b) are PL intensities plotted as a function of the XPSintensity ratio of As–O/Ga–As (Ga–O/Ga–As). The PLintensity monotonically decreases with increasing As–O/Ga–As ratio [Fig. 7(a)], while no clear correlation could befound for Ga-oxides [Fig. 7(b)]. It therefore appears that theformation of As-oxides increases the density of nonradiativerecombination centers, resulting in the observed decrease inPL intensity.It is important to note here that it is possible to maintainnearly As-oxide-free surfaces if the S-terminated samples aretransferred to another vacuum deposition system within(a) (b)(c)Fig. 6. Ga 3d core-level spectra of the three samples. (a) immediately after preparation and (b) after exposure to air for 14 d. The ratios of the oxide-relatedcomponents (Ga–O/Ga–As) are plotted in (c).(a) (b)Fig. 7. PL intensities plotted as a function of the XPS intensity ratio of As–O/Ga–As (a) and Ga–O/Ga–As (b). Red dots represent data points. Broken linerepresents fitting of the experimental data.121002-5Jpn. J. Appl. Phys. 63, 121002 (2024) Z. Ma et al.© 2024 The Author(s). Published on behalf of The Japan Society of Applied Physics by IOP Publishing Ltd 30 min. By coating these surfaces with dielectric materials,the surface properties are likely to endure.39) We have thusconfirmed the effectiveness of S termination followed bycoating with dielectric materials.4. ConclusionsWe studied the effects of sulfur termination using (NH4)2Sxtreatment on the photoluminescence properties of buriedInGaAs QWs in GaAs (001). Immediately after S termina-tion, we observed higher PL intensity than that seen in theH2O2-treated reference sample. After removing the surfaceoxides by KOH treatment, we observed an immediateenhancement. However, the PL intensity sharply decreasesand falls to nearly the same level as that of the Referencesample after just a few days, since KOH treatment is noteffective in suppressing oxide formation under conditions oflong exposure to air. In contrast, the intensity of the S-terminated samples remained brighter than that of theReference and KOH-treated samples even after 30 d. Thesurface chemical conditions as revealed by XPS showed theformation of As-oxides to be a major contributor to thenonradiative recombination process at the surface, since weobserved a clear correlation between PL intensity and As–O/Ga–As ratio. S termination thus effectively and durablysuppresses the formation of As-oxides at the surface.AcknowledgmentsThis work was supported by the Innovative Science andTechnology Initiative for Security, Grant No. JPJ004596,ATLA, Japan.ORCID iDsZhao Ma https://orcid.org/0000-0003-0151-8592Takaaki Mano https://orcid.org/0000-0002-6955-260XAkihiro Ohtake https://orcid.org/0000-0002-3519-4613Takashi Kuroda https://orcid.org/0000-0001-6445-76731) T. Mimura, S. Hiyamizu, T. Fujii, and K. Nanbu, “A new field-effecttransistor with selectively doped GaAs/n-AlxGa1−xAs heterojunctions,” Jpn.J. Appl. Phys. 19, L225 (1980).2) L. Pfeifer, K. W. West, H. L. Stormer, and K. W. 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