The nitrogen bonding configurations in GaNxAs1–x alloys grown by molecular beam epitaxy with 0.07=0.03, the nitrogen is found to exist in a single bonding configuration – the Ga–N bond; no interstitial nitrogen complexes are present. The amount of nitrogen in the alloys is estimated from the XPS using the N  1s photoelectron and Ga  LMM Auger lines and is found to be in agreement with the composition determined by x-ray diffraction

C. F. McConville

I. Mahboob

L. F. J. Piper

M. Hopkinson

Moulder J. F.

T. D. Veal

English

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Core-level photoemission spectroscopy of nitrogen bonding in GaNxAs1−x alloys

Veal, T. D. (Tim D.)

Mahboob, I.

Piper, L. F. J.

McConville, C. F. (Chris F.)

Hopkinson, M.

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  University of Warwick institutional repository: http://go.warwick.ac.uk/wrapThis paper is made available online in accordance with publisher policies. Please scroll down to view the document itself. Please refer to the repository record for this item and our policy information available from the repository home page for further information.  To see the final version of this paper please visit the publisher’s website. Access to the published version may require a subscription.  Author(s): T. D. Veal, I. Mahboob, L. F. J. Piper, C. F. McConville, and M. HopkinsonArticle Title: Core-level photoemission spectroscopy of nitrogen bonding in GaNxAs1–x alloys Year of publication: 2004 Link to published version: http://dx.doi.org/10.1063/1.1784886 Publisher statement: None     APPLIED PHYSICS LETTERS VOLUME 85, NUMBER 9 30 AUGUST 2004Core-level photoemission spectroscopy of nitrogen bondingin GaNxAs1−x alloysT. D. Veal, I. Mahboob, L. F. J. Piper, and C. F. McConvillea)Department of Physics, University of Warwick, Coventry, CV4 7AL, United KingdomM. HopkinsonDepartment of Electronic and Electrical Engineering, University of Sheffield, Mappin Street,Sheffield S1 3JD, United Kingdom(Received 18 February 2004; accepted 21 June 2004)The nitrogen bonding configurations in GaNxAs1−x alloys grown by molecular beam epitaxy with0.07,x,0.11 have been studied using x-ray photoelectron spectroscopy (XPS). In contrast toprevious studies of alloys with xø0.03, the nitrogen is found to exist in a single bondingconfiguration – the Ga–N bond; no interstitial nitrogen complexes are present. The amount ofnitrogen in the alloys is estimated from the XPS using the N 1s photoelectron and Ga LMM Augerlines and is found to be in agreement with the composition determined by x-ray diffraction. © 2004American Institute of Physics. [DOI: 10.1063/1.1784886]Dilute nitride alloys, III–V semiconductors with a fewper cent of the anions replaced by nitrogen, are promisingmaterials for optoelectronic devices in 1.3 and 1.55 mm tele-communications. Most importantly, these applications relyupon the nitrogen-induced dramatic reduction of the bandgap to obtain the technologically important emission wave-lengths. Additionally, the fact that GaInNAs, with a compo-sition to give the appropriate band gap, can be latticematched to GaAs substrates enables high quality epitaxialgrowth and incorporation into the existing electronic technol-ogy infrastructure.1Post-growth annealing is normally required to producedevice quality dilute nitride material, enhancing lumines-cence efficiency by removing interstitial nitrogen complexes,but also causing a blueshift of the emission due to out-diffusion of nitrogen2 or diffusion of nitrogen inside thematerial.3 Evidence of non-substitutional nitrogen inGaNxAs1−x has been provided by comparison of data fromhigh-resolution x-ray diffraction (HRXRD) and secondaryion mass spectrometry (SIMS). While SIMS measures thetotal nitrogen content irrespective of the location of the ni-trogen atoms, HRXRD gives the change in the lattice con-stant caused by substitution of nitrogen on anion lattice sites.Nitrogen concentrations measured by SIMS have been foundto differ from those determined using HRXRD by up to 30%for xø2.5%, indicating the presence of significant concen-trations of non-substitutional nitrogen.4,5 Using XPS and ionchanneling experiments, Spruytte et al. confirmed the exis-tence of interstitial nitrogen in GaNxAs1−x grown by molecu-lar beam epitaxy (MBE) for xø2.9%.2 Upon rapid thermalannealing (60 s at 760°C), all of the nitrogen was found tobe located substitutionally on anion lattice sites.In this letter, the nitrogen bonding configurations inGaNxAs1−x grown by MBE are investigated using x-ray pho-toelectron spectroscopy. The nitrogen is found to exist in asingle bonding configuration; the Ga–N bond. No interstitialnitrogen is observed, in contrast to previous studies of highnitrogen content GaNxAs1−x, where up to 30% of the nitro-a)Author to whom correspondence should be addressed; electronic mail:c.f.mcconville@warwick.ac.uk0003-6951/2004/85(9)/1550/3/$22.00 1550Downloaded 06 Jul 2009 to 137.205.202.8. Redistribution subject to Agen was found to be located non-substitutionally.2,4,5The GaNxAs1−x epilayers were grown to a thickness of300 nm on semi-insulating GaAss001d substrates by molecu-lar beam epitaxy using a turbo-pumped Vacuum GeneratorsV80 system equipped with an Oxford Applied Research(OAR) HD25 RF plasma nitrogen source. A regulated supplyof purified nitrogen gas was provided to the RF source bymeans of a piezoelectric-controlled leak valve. The sampleswere all grown with a substrate temperature of 375°C. Thenitrogen content of the three GaNxAs1−x samples used in theXPS study was determined by x-ray diffraction using Cu Kasl=1.54178 Åd as the radiation source. X-ray diffractioncurves around the (004) Bragg reflections, recorded to mea-sure the lattice constants of the epilayers, are shown in Fig.1. The N contents, estimated from the epilayer lattice con-stants assuming Vegard’s law, are x=0.073, 0.083, and 0.104.The variations in XRD peak intensity for the differentsamples correspond to different crystalline qualities. Fromthis small set of samples no trend relating crystalline qualityto nitrogen content is apparent. The XRD indicates that theFIG. 1. (004) XRD spectra of GaNAs layers MBE-grown on GaAss001d at375°C, indicating N contents of 7.3%, 8.3%, and 10.4%.© 2004 American Institute of PhysicsIP license or copyright; see http://apl.aip.org/apl/copyright.jspAppl. Phys. Lett., Vol. 85, No. 9, 30 August 2004 Veal et al. 1551sample with x=0.083 has better crystalline quality than thesample with x=0.104, which in turn is better than the samplewith x=0.073.XPS analysis of the Ga 2p, Ga 3d, As 2p, As 3d, andN 1s photoelectron spectra and the Ga LMM Auger spectrawas performed using a Scienta ESCA300 spectrometer at theNational Centre for Electron Spectroscopy and SurfaceAnalysis, Daresbury Laboratory, UK. This incorporates a ro-tating anode Al Ka x-ray source shn=1486.6 eVd, x-raymonochromator and 300 mm mean radius spherical-sectorelectron energy analyzer and parallel elecron detection sys-tem. The analyzer was operated with 0.8 mm slits and at apass energy of 150 eV. Gaussian convolution of the analyzerbroadening with an effective line width of the 0.27 eV forthe x-ray source gives an effective instrument resolution of0.45 eV. XPS analysis of samples that contain Ga and N iscomplicated by the fact that, for both commonly used x-raysource anode materials (Al and Mg), Ga Auger peaks occurin the same region of the spectrum as the N 1s photoelectronpeaks. A GaAss001d reference sample was also analyzed sothat the nitrogen-related features in the GaNAs spectra couldbe distinguished from the Ga Auger lines of GaAs.The surfaces of the GaNxAs1−x epilayers and GaAs ref-erence sample were prepared in situ for the XPS analysis bya two-stage atomic hydrogen cleaning process consisting of a10 kL dose of H2 at room temperature followed by a 10 kLdose at a sample temperature of 350°C. Each 10 kL dosetook 40 min using a hydrogen gas pressure of ,5310−6 mbar. The molecular hydrogen was thermallycracked with ,50% efficiency using an OAR TC-50 thermalgas cracker. The samples were radiatively annealed using atungsten filament mounted behind them. The removal of at-mospheric contamination was confirmed using XPS by theabsence of C 1s and O 1s photoelectron peaks and the lackof oxide-related chemically shifted components in the As 3d,As 2p, Ga 3d, and Ga 2p spectra.A series of normalized core-level photoelectron spectrarecorded from the GaNxAs1−x alloys are shown in Fig. 2. TheFIG. 2. X-ray photoelectron spectra of N 1s photoelectron and Ga LMMAuger lines from GaNAs samples with x=0.073, 0.083, and 0.104. Eachspectrum is normalized to the intensity of the Ga L2M45M45 Auger line at393.7 eV.spectral region shown includes the low binding energy tail ofDownloaded 06 Jul 2009 to 137.205.202.8. Redistribution subject to Aa Ga LMM Auger peak centered at 393.7 eV, another GaLMM Auger peak at 398.9 eV (Ref. 6), and the N 1s corelevel photoemission peak at a binding energy of 397.6 eV.2The variation of the intensity of the N 1s peak with respectto the Ga LMM peaks reflects the different nitrogen contentsof the samples.To determine the different nitrogen bonding configura-tions, the N 1s and Ga LMM peaks have been analyzed inmore detail to resolve any chemically shifted components.For this, the same spectral region was also analyzed for aGaAss001d sample prepared in the same way. This GaAsspectrum is shown in Fig. 3, along with that of the mostnitrogen-rich sample. The Ga LMM Auger peak at 393.7 eVreferred to above is shown in full in this figure. The nitrogen-related contributions to the GaNAs spectra are then revealedat the bottom of this figure by subtracting the normalizedGaAs spectrum from that of GaNAs. The original spectraand this difference spectrum are also shown with an ex-panded intensity scale in the inset of Fig. 3. The N 1s peak at397.6 eV is now clearly resolved. However, two furtherpeaks at 394.2 and 399.4 eV are also revealed. These arechemically shifted components of the Ga LMM lines associ-ated with the Ga–N bonding.Curve fitting of the difference spectra has been per-formed using a Shirley background and Voigt functions, eachwith a full width at half-maximum of 0.9 eV and consistingof 20% Lorentzian and 80% Gaussian line shapes. The curvefit of the difference spectrum for GaN0.104As0.896 is shown inFig. 4. The ratio of the areas of the two Ga LMM peaksassociated with the Ga–N bonding was set in the curve fit-ting, to be equal to that of the Ga LMM peaks from theGa–As bonding in the spectra recorded from GaAs (see Fig.3). This provides a good fit to the peaks at 394.2 and399.4 eV in the difference spectrum, confirming that thesespectral features are due solely to Ga LMM lines chemicallyshifted (by 0.5 eV to higher binding energy with respect tothe GaAs LMM lines) as a result of the Ga being bonded toN. The remaining peak in the difference spectrum is alsoFIG. 3. XPS spectra from the GaNAs sample with x=0.104 (light solid line)and GaAs (light dashed line). The difference spectrum (the result of sub-tracting the GaAs spectrum from the GaNAs spectrum) is also shown (heavysolid line).well fitted by a single component, indicating that all of theIP license or copyright; see http://apl.aip.org/apl/copyright.jsp1552 Appl. Phys. Lett., Vol. 85, No. 9, 30 August 2004 Veal et al.nitrogen is bonded to Ga and is located on group V latticesites. The photoemission spectra can also be used to estimatethe composition of the GaNAs epilayers. For example, thearea of the Ga–N component at 394.2 eV in Fig. 4 is 10.6%of the total Ga LMM Auger peak area. This value is in ex-cellent agreement with the alloy composition of x=0.104 de-termined from the lattice constant obtained from the XRDspectrum, assuming Vegard’s law. The same analysis wasalso applied to the photoemission spectra from the other twosamples. The compositions determined from the peak areaswere again found to be in agreement (within the error of±0.3%) with the values obtained from XRD. The recent re-sults of Bisognin et al. found that Vegard’s law is obeyed inGaNxAs1−x up to at least x=0.045.7 Our findings extend thisresult, allowing us to conclude that, in the absence of non-substitutional nitrogen, Vegard’s law is valid up to at least,10.5% nitrogen.In previous photoemission spectroscopy of MBE-grownGaNAs, an additional component in the N 1s spectrum hasbeen observed at a higher binding energy of ,398.5 eV.2This was attributed to the presence of a nitrogen complex inwhich N is less strongly bonded to Ga. The absence of sucha component in Fig. 4 indicates that, inspite of the high Ncontent of these samples, the concentration of interstitial N isbelow the detection limit of XPS.FIG. 4. Curve fit (heavy solid line) of the difference spectrum (points)shown in Fig. 3. The two Gas−Nd LMM Auger peaks and the single com-ponent of the N 1s peak (light solid lines) are vertically offset.Downloaded 06 Jul 2009 to 137.205.202.8. Redistribution subject to AWhile in the majority of cases incorporating more than3% N in GaNAs epilayers leads to the existence of intersti-tial N complexes,2,4,5 alloys with a high N content but free ofnon-substitutional N have been achieved in at least two pre-vious studies. First, Qiu et al. used metal-organic MBE togrow GaNxAs1−x alloys with x=0.10, as determined by bothSIMS and XRD.8 Second, Bisognin et al. have very recentlyfound all of the N to be substitutional in GaNAs sampleswith x up to 0.045. This was determined using a combinationof nuclear reaction analysis, Rutherford backscattering spec-troscopy, and HRXRD.7 The high nitrogen contents of ouralloys and those of both Bisognin et al. and Qiu et al. with xup to ,0.10 were achieved by growing at the relative lowtemperatures of 375 and ,400°C, respectively; GaNAs istypically grown at between 450 and 550°C. However, thereason for the lack of interstitial nitrogen in these high nitro-gen content alloys is unclear.Photoemission spectroscopy has been used to determinethe nitrogen bonding configurations in GaNxAs1−x alloys (x=0.073, 0.083, and 0.104) grown by molecular beam epitaxy.All of the nitrogen is bonded to gallium and is located sub-stitutionally on group V lattice sites. In contrast to previousstudies of GaNxAs1−x alloys with xø0.003, no component inthe N 1s spectrum associated with interstitial nitrogen wasobserved. The alloy compositions determined from the pho-toemission spectra are in good agreement with those ob-tained from XRD analysis.The authors would like to thank Dr. D. S.-L. Law for hisassistance with the XPS experiments at the National Centrefor Electron Spectroscopy and Surface Analysis, DaresburyLaboratories, UK. The Engineering and Physical SciencesResearch Council, UK is acknowledged for financial supportunder Grant Nos. GR/S56030/01 and GR/S14252/01, and forthe provision of research studentships for I.M. and L.F.J.P.1M. Kondow, K. Uomi, A. Niva, T. Kitatani, and Y. Yazawa, Jpn. J. Appl.Phys., Part 1 35, 1273 (1996).2S. G. Spruytte, C. W. Coldren, J. S. Harris, W. Wampler, P. Krispin, K.Ploog, and M. C. Larson, J. Appl. Phys. 89, 4401 (2001).3L. Grenouillet, C. Bru-Chevallier, G. Guillot, P. Gilet, P. Ballet, P. Duvaut,G. Rolland, and A. Million, J. Appl. Phys. 91, 5902 (2002).4W. Li, M. Pessa, and J. Likonen, Appl. Phys. Lett. 78, 2864 (2001).5W. J. Fan, S. F. Yoon, T. K. Ng, S. Z. Wang, W. K. Loke, T. Liu, and A.Wee, Appl. Phys. Lett. 80, 4136 (2002).6J. F. Moulder, W. F. Stickle, P. E. Sobol, and K. D. Bomben, Handbook ofX-Ray Photoelectron Spectroscopy (Perkin-Elmer, Eden Prairie, MN,1992).7G. Bisognin, D. D. Salvador, C. Mattevi, M. Berti, A. V. Drigo, G. Ciatto,L. Grenouillet, P. Duvaut, P. Gilet, and H. Mariette, J. Appl. Phys. 95, 48(2004).8Y. Qui, S. A. Nikishin, H. Temkin, N. N. Faleev, and Y. A. Kudriavtsev,Appl. Phys. Lett. 70, 3242 (1997).IP license or copyright; see http://apl.aip.org/apl/copyright.jsp

Core-level photoemission spectroscopy of nitrogen bonding in GaNxAs1–x alloys

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Core-level photoemission spectroscopy of nitrogen bonding in GaNxAs1–x alloys

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