We studied optical manifestation of structural defects in unintentionally doped GaN. A series of sharp peaks (labeled

as Yi with i=1,2,y,11) was observed in the low-temperature photoluminescence spectrum in the photon energy range

between 2.6 and 3.46 eV. We attribute the majority of these peaks to excitons bound to yet unidentified structural

defects. A preliminary transmission electron microscopy study in one of the samples exhibiting strong Yi lines revealed

numerous inclusions, presumably inversion domains, which may be responsible for at least some of the Yi lines

D. HUANG

H.MORKOC

J.JASINSKI

M.A. RESHCHIKOV

P. VISCONTI

Z. LILIENTAL-WEBER

We studied optical manifestation of structural defects in unintentionally doped GaN. A series of sharp peaks (labeled
as Yi with i=1,2,y,11) was observed in the low-temperature photoluminescence spectrum in the photon energy range
between 2.6 and 3.46 eV. We attribute the majority of these peaks to excitons bound to yet unidentified structural
defects. A preliminary transmission electron microscopy study in one of the samples exhibiting strong Yi lines revealed
numerous inclusions, presumably inversion domains, which may be responsible for at least some of the Yi lines

M. A. RESHCHIKOV

J. JASINSKI

Z. LILIENTAL WEBER

H. MORKOC

VISCONTI, Paolo

Archivio Istituzionale della Ricerca- Università del Salento

ARTICLE IN PRESSPhysica B 340–342 (2003) 440–443*Corresping, VirginiRichmond,+804-828-4E-mail a0921-4526/$doi:10.1016Photoluminescence from structural defects in GaNM.A. Reshchikova,e,*, J. Jasinskib, Z. Liliental-Weberb, D. Huanga,c, L. Hea,P. Viscontia,d, H. Morko@aaDepartment of Electrical Engineering, Virginia Commonwealth University, 601 West Main Street, Richmond, VA 23284, USAbLawrence Berkeley National Laboratory, Berkeley, CA 94720, USAcPhysics Department, Fudan University, Shanghai 200433, Chinad Instituto per lo Studio di Nuovi Materiali per l’Elettronica, CNR, Via Arnesano, 73100 Lecce, ItalyePhysics Department, Virginia Commonwealth University, 601 West Main Street, Richmond, VA 23284, USAAbstractWe studied optical manifestation of structural defects in unintentionally doped GaN. A series of sharp peaks (labeledas Yi with i=1,2,y,11) was observed in the low-temperature photoluminescence spectrum in the photon energy rangebetween 2.6 and 3.46 eV. We attribute the majority of these peaks to excitons bound to yet unidentified structuraldefects. A preliminary transmission electron microscopy study in one of the samples exhibiting strong Yi lines revealednumerous inclusions, presumably inversion domains, which may be responsible for at least some of the Yi lines.r 2003 Elsevier B.V. All rights reserved.PACS: 61.72.Nn; 78.55.Cr; 73.20.rKeywords: GaN; Photoluminescence; Structural defects1. IntroductionSharp unidentified photoluminescence (PL) peaksare often observed in GaN and commonly attrib-uted to structural defects [1–4]. Among severalhundred undoped GaN samples investigated, weselected some 40 exhibiting pronounced sharp PLpeaks in the photon range between 2.6 and 3.46 eV[5]. Since the properties of the studied PL peaksresemble those of the Y lines in II–VI compounds[6], or D lines in silicon [7], we labeled the ‘‘unusual’’onding author. Department of Electrical Engineer-a Commonwealth University, 601 West Main Street,VA 23284, USA. Tel.: +804-827-7000 ext. 358; fax:269.ddress: mreshchi@vcu.edu (M.A. Reshchikov).- see front matter r 2003 Elsevier B.V. All rights reserve/j.physb.2003.09.030lines in GaN as Yi [5] with i starting from 1 for thepeak with the highest photon energy and classifiedthem as shown in Table 1. As we noted in Ref. [4],the Yi lines should not be confused with artificiallines originating from oil contamination.An extensive but preliminary study of the Yilines can be found in Refs. [3–5]. In this work, wepresent an analysis of structural properties andtheir possible relation to the Yi lines in one of theGaN samples.2. ExperimentThe GaN layer investigated (thickness 1 mm)was grown on a 2-in c-plane sapphire substrate byd.ARTICLE IN PRESSTable 1Classification and typical characteristics of the Yi peaks in GaNGrouplabelSub-grouplabelNominalposition (eV)Huang–Rhys factorComments, properties Tentative attributionY1 Y10Y1003.453.460.02 Observed only in N-polar filmswith high density of inversiondomainsExcitons bound to inversiondomain interfaceY2 Y20Y2003.413.420.05 Disappears after hot wet etching.Shifts with excitation intensity insome samplesExciton bound to structuraldefect at the surfaceY3 Y30Y3003.373.38o0.2 Observed only in two samples Exciton bound to structuraldefect ?Y4 Y40Y4003.3503.3620.01 Evolves with UV exposure.Memory effect at 15K. Intensitydepends on history of the surfaceExciton bound to structuraldefect at the surfaceY5 3.34 Observed in 5 samples Exciton bound to structuraldefect ?Y6 3.32 B0.1 Evolves with UV exposure.Memory effect at 15K. Intensitydepends on history of the surface.In all samples shifts much withexcitation intensitySurface donor–acceptor pairY7 Y70Y7003.213.230.06 The most common Yi peak. Notaffected by etching or UVexposureExciton bound to structuraldefect (in bulk)Y8 3.08 B0.2 Observed only in one sample ?Y9 2.86 o0.3 Observed only in one sample ?Y10 2.80 Observed in four samples ?Y11 2.68 Observed only in one sample ?M.A. Reshchikov et al. / Physica B 340–342 (2003) 440–443 441molecular beam epitaxy with ammonia as thenitrogen source. A few AlN/GaN buffer layerswith total thickness of 200 nm were employed toreduce the strain due to the GaN/sapphire latticemismatch. The wafer was cut and one of the pieceswas briefly etched in hot H3PO4 to smoothen thesurface. High-resolution X-ray diffraction (XRD)established rather high crystal quality of the layer.The full-width at half-maximum (FWHM) inomega scans (rocking curves) is 1.6 and 2.5 arcminfor the (0 0 0 2) and (1 0 %1 4) directions, respec-tively. Structural defects were studied by cross-sectional transmission electron microscopy(TEM). Ga polarity of the sample was establishedfrom etching characteristics, surface morpholo-gies, TEM, and XRD data [8]. PL experimentswere carried out in the temperature range of15–300K using a closed cycle optical cryostat. Theluminescence was excited with the 325 nm line of aHe–Cd laser, dispersed using a 0.5m gratingmonochromator and detected with a Hamamatsuphotomultiplier tube.3. ResultsFig. 1 shows the low-temperature PL spectra ofas-grown and etched samples. As we reportedpreviously [3–5], after brief etching in H3PO4 theY2 peak disappears, indicating its surface-relatedorigin. Other characteristic features of the Yi peaks(evolution of the Y4 and Y6 peaks with UVexposure time, significant shift of the Y6 peakwith excitation power and negligible shifts forother peaks, Huang–Rhys factors of about 0.01and 0.02 for the Y4 and Y7 peaks, respectively,doublet structure of the Y4 and Y7 peaks) werealso observed in this sample.ARTICLE IN PRESSFig. 2. Cross-sectional TEM image for as-grown sample (a)and after etching in H3PO4 (b). Dislocation-free sapphiresubstrate and defect-rich buffer layer can be seen in the bottompart of each image.Fig. 3. Dark-field cross-sectional TEM image of as-grownsample. Numerous inclusions are indicated by arrows. The insetenlarges the area with one of the inclusions (this image wastaken under multi-beam conditions).10610710810910103.1 3.2 3.3 3.4 3.5as-grownetchedPL Intensity (rel. units)Photon Energy (eV)DBE (FE)Y2Y7Y6Y4T = 15 KFE-LOFig. 1. PL spectrum at 15K for as-grown GaN sample andafter etching for 1min in H3PO4 at 160C. FWHM is 11meV(DBE peak) and 10meV (Y4 peak). Excitation density is100Wcm2.M.A. Reshchikov et al. / Physica B 340–342 (2003) 440–443442Fig. 2 shows the TEM cross-sectional image ofthe studied sample before (a) and after etching inH3PO4 for 1min at 160C (b). The as-grownsample has columnar-like grain structure with arelatively rough surface. After etching, the surfacebecame flat, with large faceted pits. Note thatwithin the experimental error, the thickness of thesample has not changed after the 1-min etch. Afteretching for even more than one hour, the layerthickness still remained about the same, exceptthat the pits reached the sapphire surface andincreased in size. The density of threadingdislocations was estimated as 1 1010 cm2, withrelative contribution of edge, screw, and mixeddislocations of about 99.8%, less than 0.1%, and0.1–0.2%, respectively. The columnar inversiondomains with diameters in the range of 10–20 nmand density of 3–4 1010 cm2, propagating fromthe buffer layer up to the film surface, were alsoobserved in the TEM images.An interesting observation is numerous inclu-sions with the characteristic faceted shapes,observed inside the studied layer (Fig. 3). Theyare not identified at the moment, but tentativelyattributed to another type of inversion domains.4. DiscussionY lines observed in GaN and other semiconduc-tors are commonly attributed to excitons bound tostructural defects [2–7]. However, the exact originof these defects is not yet sufficiently clear. In theinvestigated GaN sample, exhibiting several strongYi peaks, the density of the screw threadingARTICLE IN PRESSM.A. Reshchikov et al. / Physica B 340–342 (2003) 440–443 443dislocations is very low, while the edge threadingdislocations are abundant. One could assume thatthe edge threading dislocations are the reason ofthe Yi peaks, however we studied several GaNsamples with similar densities of the threadingdislocations that do not exhibit the Yi peaks at all.The columnar narrow inversion domains alsocannot be responsible for the Yi peaks except forthe Y1 line at 3.45 eV observed in the N-polar GaNwith the density of the columnar inversiondomains of about 1011 cm2 [5,9]. We attributethe Y2 line to excitons bound to some defects in thesurface layer, since it completely disappearsafter etching of the thin surface layer (compareFigs. 1 and 2). As, for the Y4, Y6, and Y7 lines,they might be related to inclusions shown inFig. 3. However extensive experimental data arerequired to confirm the validity of this assumption.We finally note that the Yi peaks may originatefrom excitons bound to point defects (impurities ornative defects) that are in turn trapped atthreading-edge dislocations or other structuraldefects in GaN.5. ConclusionsWe observed, studied, and classified the Yi linesin the low-temperature photoluminescence spec-trum of undoped GaN. These lines are tentativelyattributed to structural defects, however the exacttype of defects and mechanism of emission is notestablished yet. Cross-sectional TEM imagesrevealed inclusions, possibly inversion domains,which may be responsible for at least some of theYi lines.AcknowledgementsThe work was funded by AFOSR (Dr. G.L.Witt), NSF (Dr. U. Varshney), and ONR (Dr.C.E.C. Wood and Dr. Y.S. Park), and wasmotivated in part by the Wood Witt program.Work at LBNL was supported by AFOSR, OrderNo. FQ86710200852, through the U.S. DoE underContract No. DE-AC03-76SF0098. The TEMgroup (J.J. and Z.L.-W.) would like to acknowl-edge the use of the facilities at the National Centerfor Electron Microscopy at LBNL.References[1] N. Grandjean, M. Leroux, M. La .ugt, J. Massies, Appl.Phys. Lett. 71 (1997) 240.[2] E. Calleja, M.A. S!anchez-Garc!ıa, F.J. S!anchez, F. Calle,F.B. Naranjo, E. Munoz, U. Jahn, K. Ploog, Phys. Rev. B62 (2000) 16826.[3] M.A. Reshchikov, D. Huang, F. Yun, H. Morko@, R.J.Molnar, C.W. Litton, Mater. Res. Soc. Symp. Proc. 693(2002) I6.28.[4] M.A. Reshchikov, D. Huang, H. Morko@, Mater. Res. Soc.Symp. Proc. 743 (2003) L11.3.[5] M.A. Reshchikov, D. Huang, F. Yun, P. Visconti, L. He, H.Morko@, J. Jasinski, Z. Liliental-Weber, R.J. Molnar, S.S.Park, K.Y. Lee, J. Appl. Phys. 94 (2003) 5623.[6] P.J. Dean, Phys. Stat. Sol. A 81 (1984) 625;P.J. Dean, G.M. Williams, G. Blackmore, J. Phys. D 17(1984) 2291.[7] R. Sauer, Ch. Kisielowski-Kemmerich, H. Alexander, Phys.Rev. Lett. 57 (1986) 1472;Yu. Shreter, Yu.T. Rebane, A.R. Peaker, Phys. Stat. Sol. A138 (1993) 681.[8] D. Huang, P. Visconti, K.M. Jones, M.A. Reshchikov, F.Yun, A.A. Baski, T. King, H. Morko@, Appl. Phys. Lett. 78(2001) 4145.[9] M.A. Reshchikov, D. Huang, H. Morko@, Mater. Res. Soc.Symp. Proc. 693 (2003) L11.3.

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Photoluminescence from structural defects in GaN

Photoluminescence from structural defects in GaN

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