Laakso, A.

Myers, T. H.

Oila, J.

Ptak, A. J.

Rummukainen, M.

Saarinen, K.

English

The Research Repository @ WVU (West Virginia University)

Faculty Scholarship2004Vacancy Defects In O-Doped Gan Grown ByMolecular-Beam Epitaxy: The Role Of GrowthPolarity And StoichiometryM. RummukainenJ. OilaA. LaaksoK. SaarinenA. J. PtakSee next page for additional authorsFollow this and additional works at: https://researchrepository.wvu.edu/faculty_publicationsThis Article is brought to you for free and open access by The Research Repository @ WVU. It has been accepted for inclusion in Faculty Scholarshipby an authorized administrator of The Research Repository @ WVU. For more information, please contact ian.harmon@mail.wvu.edu.Digital Commons CitationRummukainen, M.; Oila, J.; Laakso, A.; Saarinen, K.; Ptak, A. J.; and Myers, T. H., "Vacancy Defects In O-Doped Gan Grown ByMolecular-Beam Epitaxy: The Role Of Growth Polarity And Stoichiometry" (2004). Faculty Scholarship. 724.https://researchrepository.wvu.edu/faculty_publications/724AuthorsM. Rummukainen, J. Oila, A. Laakso, K. Saarinen, A. J. Ptak, and T. H. MyersThis article is available at The Research Repository @ WVU: https://researchrepository.wvu.edu/faculty_publications/724Vacancy defects in O-doped GaN grown by molecular-beam epitaxy:The role of growth polarity and stoichiometryM. Rummukainen,a) J. Oila, A. Laakso, and K. SaarinenLaboratory of Physics, Helsinki University of Technology, P.O. Box 1100 FIN-02015 HUT, FinlandA. J. Ptak and T. H. MyersDepartment of Physics, West Virginia University, Morgantown, West Virginia 26506~Received 29 December 2003; accepted 19 April 2004; published online 25 May 2004!Positron annihilation spectroscopy is used to study vacancy defects in GaN grown bymolecular-beam epitaxy due to different polar directions and varying stoichiometry conditionsduring oxygen doping. We show that Ga-polar material is free of compensating Ga vacancies up to@O#51018 cm23 in Ga stable growth, but high concentrations of VGa are formed in N-stableconditions. We also show that vacancy clusters are formed in N-polar material grown in Ga stableconditions, which may be related to the higher reactivity of the N-polar surface. These clusters haveno apparent influence on the electrical properties of the material. We thus infer that their charge stateis neutral. © 2004 American Institute of Physics. @DOI: 10.1063/1.1762984#Properties of gallium nitride depend strongly on thecrystal orientation and the polarity of the growth direction.Growth in the Ga-polar ~0001! direction generally leads tosmooth surface morphology and high quality material, whileN-polar (0001¯) growth is much more difficult, though it maybe desirable since it is more easily etched.1,2 The stoichiom-etry of Ga and N overpressures during the growth is alsoimportant.1 Generally molecular-beam epitaxy ~MBE! ofGaN takes place in Ga stable conditions, where the diffusiv-ity of Ga adatoms is high at the surface.1,3 Approaching Nstable growth, however, the surface mobility of Ga adatomsis reduced due to Ga–N bonds1,3 and surface reactivity isincreased enhancing oxygen incorporation by orders ofmagnitude.4,5Experimental and theoretical studies demonstrate the in-fluence of growth polarity and stoichiometry on the impurityincorporation.4–6 The epitaxial N-polar GaN contains typi-cally an order of magnitude more oxygen than Ga-polar lay-ers, and approaching N-stable conditions the oxygen incor-poration is strongly enhanced for both polarities.5 In Gapolar samples, controlled oxygen doping without significantelectrical compensation can be achieved up to @O#>1018 cm23, but at higher doping levels electrically com-pensating defects are formed.5 The effect of growth polarityand stoichiometry on the formation of point defects is muchless studied and thus less well understood than on the impu-rity incorporation.In this work we apply positron annihilation spectroscopyto study the formation of vacancy defects in GaN grown indifferent polar orientations and under different stoichiometryconditions. We confirm that Ga-polar material is free of com-pensating Ga vacancies at moderate O-doping levels for Gastable growth, but a high concentration of VGa is formed inN-stable conditions. We also show that vacancy clusters areformed in N-polar GaN grown in Ga stable stoichiometry,but these clusters are likely to be electrically inactive.The O-doped GaN layers ~Table I! were grown at WestVirginia University ~WVU! by rf plasma-assisted MBE, asdescribed earlier.5,7 The N-polarity GaN was grown bynucleating GaN buffer layers directly on sapphire in highlyGa-rich conditions. The Ga-polarity GaN was prepared byepitaxy on ~0001! GaN templates grown by metalorganicchemical vapor deposition ~MOCVD! on A-plane sapphire atthe Naval Research Laboratory. The stoichiometry was var-ied by changing the beam-equivalent pressure ~BEP! of Ga,as shown in Table I.The positron experiments were performed with a mo-noenergetic beam by measuring the Doppler broadening ofthe 511 keV annihilation radiation.8 The Doppler spectrawere analyzed with the conventional S and W parameters,describing positron annihilations with valence and core elec-trons, respectively. When thermalized positrons are trappedat vacancy defects, the reduced electron density leads to anincrease in S and decrease in W parameters.8In the MBE layers, the S parameter is high at low posi-tron implantation energies, E,5 keV, due to positron anni-hilations on the sample surface ~Fig. 1!. When the energy isincreased the annihilations take place inside the GaN layer,a!Electronic mail: mru@fyslab.hut.fiTABLE I. The polarity, thickness, Ga-beam equivalent pressure, O-partialpressure, and dopant concentration in the studied MBE-grown layers.Polarity andthickness ~mm!BEP ~Ga!(1026 Torr)pO2~Torr!@O#(cm23)n(cm23)#1 N 1 1.6 4.0310212 1.031015#2 N 1 0.96 5.0310210 1.731018#3 N 1 1.4 4.0310211 3.031016 6.031016#4 N 1 1.6 4.0310211 3.031016 6.631016#5 N 1 0.5 1.031027 6.831019#6 Ga 1.8 0.49 1.531027 2.531022 3.031020#7 Ga 1.8 0.46 5.531028 9.031019#8 Ga 1 0.48 5.031028 2.631021 4.031017#9 Ga 1.36 0.49 2.031027 1.531022 7.431019#10 Ga 0.62 0.95 1.131027 1.531018 1.431018#11 Ga 1.9 1.2 4.0310212 ,131016 3.831017#12 Ga 0.9 0.93 1.031029 5.031016 2.831016#13 Ga 1 1.6 3.0310211 3.031017#14 Ga 1 0.73 2.0310210 1.031016 1.231016#15 Ga 1 1 2.031028 3.031017 3.031017APPLIED PHYSICS LETTERS VOLUME 84, NUMBER 24 14 JUNE 200448870003-6951/2004/84(24)/4887/3/$22.00 © 2004 American Institute of Physicsseen as a constant level in the S parameter. At E.17 keVpositrons reach the underlying Al2O3 substrate layer and theS parameter decreases (SAl2O3’0.43). Annihilation param-eters in the p-type Mg-doped GaN reference layer corre-spond to vacancy-free GaN lattice.9 In the N-polar samples1–4 the S parameters characterizing the MBE layer are ap-proximately the same while in the N-polar sample 5 the Sparameter is clearly lower. However, the S parameters in allthe N-polar layers are clearly higher than in a vacancy-freeGaN lattice, Sb’0.463, indicating positron trapping to va-cancy type defects.The S parameter curves in the Ga-polar samples ~Fig. 2!show two kinds of behavior. A plateau characterizing theMBE layer is clearly distinguishable only in samples 6–9.The high layer specific S parameter, S’0.485, indicates trap-ping to vacancy type defects. In the other Ga-polar samples,Nos. 10–15, the S parameter continues to decrease without aclear plateau from the surface value toward S’0.465, whichwas recorded in the MOCVD GaN below the MBE layer.The annihilation parameters specific to the MBE layer can-not thus be estimated in these samples.The number of different vacancy type defects can bestudied by plotting the S parameters as a function of W pa-rameters ~Fig. 3!.8 The data points from N-polar samples1–4 are clearly separate from the Ga-polar samples 6–9 andthe N-polar sample 5, indicating the vacancy defects in thesetwo sample groups are different. Positron studies have pre-viously identified two types of vacancy defects: Gavacancies,10 with characteristic parameters SVGa /Sb’1.046and WVGa /Wb’0.80, and vacancy clusters with an open vol-ume of a divacancy or bigger,11,12 characterized bySclust /Sb’1.10 and Wclust /Wb’0.75. In Fig. 3 the (S ,W)values from the Ga-polar samples 6–9 and the N-polarsample 5 fall very close to the point characteristic of the Gavacancy, suggesting a high VGa concentration. On the otherhand, the slope of the line between the (S ,W) points of theN-polar samples 1–4 and the vacancy-free lattice (Sb ,Wb)coincides very well with that characteristic to the vacancycluster. The positron trapping at the vacancy clusters is alsosuggested by the very high S parameter, S/Sb’1.08.Interestingly, the Ga vacancies are detected in very highconcentrations both in Ga-polar and the N-polar sampleswhere the gallium BEP is low, ;0.531026 Torr. This meansthe growth approaches N-stable conditions.5 Note that thisalso significantly enhances oxygen incorporation. The S andW values do not significantly differ from SVGa and WVGa, i.e.,positron trapping is in saturation. Therefore, it is only pos-sible to give a lower limit estimate, @VGa#*531018 cm23.8It is likely that these vacancies are charged, forming thecompensating defects that limit the carrier concentration ob-tainable from the oxygen. The charged nature is reflected inthe extremely low mobility, <20 cm2/V s at 300 K, in thesamples 5–9.The presence of the high VGa concentration in thesamples with lower gallium BEP is very reasonable. It agreeswith the previous results, where the VGa concentration wasfound to increase in undoped (n-type! MOCVD GaN layerswith the increasing V/III-molar ratio.13 According to Ref. 5,the incorporation of oxygen is significantly increased both inGa-polar and N-polar growth direction when the growth isFIG. 1. The S parameter as a function of positron incident energy in thestudied N-polar GaN layers and in the p-type GaN:Mg layer. The meanpenetration depth is indicated by the top axis.FIG. 2. The S parameter as a function of positron incident energy in twoGa-polar GaN layers. The S parameter characterizing the MBE layer isdistinguishable in sample 6, while in sample 10 annihilations in the MBElayer are mixed with annihilations on the surface and the MOCVD-GaNsubstrate.FIG. 3. The S – W plot of (S ,W) values characterizing the MBE layers. The(S ,W) values corresponding to vacancy free GaN (Sb ,Wb), Ga vacancy(SVGa,WVGa), and vacancy cluster (Sclust ,Wclust) are also shown.4888 Appl. Phys. Lett., Vol. 84, No. 24, 14 June 2004 Rummukainen et al.N-stable. The observed high VGa concentration in theN-stable samples thus agrees with the theoretical calcula-tions, predicting the formation of compensating VGa shouldbe enhanced in the strongly n-type material.14,15The positron annihilation experiments in irradiated GaNhave shown that the isolated VGa are mobile and recover bymigration already at 300 °C.16 Thus the observed vacanciesare most probably VGa-donor-impurity pairs (VGa–ON in thiscase!, which can survive the growth temperature. The pres-ence of the Ga vacancies only in the samples with the highoxygen content ~N-stable growth! manifests the essential roleof oxygen both in decreasing the formation energy of Gavacancies and enhancing their stability.9In Ga-polar layers grown in Ga-stable conditions~samples 10–15! the annihilation parameters ~Fig. 2! show asmooth transition from the surface values toward those ofGaN lattice, indicating a positron diffusion length of about100 nm. The long positron diffusion length and the absenceof apparent signs of vacancies indicate that at the most @VGa#is in the 1016 cm23 range. This is in good agreement withthe electrical properties which show very littlecompensation.5 Controlled oxygen doping can thus be doneup to the level of 1018 cm23 without significant formation ofcompensating Ga vacancies.The observation of the vacancy clusters in the N-polarlayers grown in Ga-stable conditions is very interesting. Ac-cording to the depth scan of Fig. 1, the vacancy clusters arefound throughout the MBE layer. The concentration of theclusters can be estimated to be in the 1017 cm23 range. Thepositron data do not give evidence of the charge state of thevacancy cluster. However, two results suggest the observedvacancies may be electrically neutral. First, there is a closecorrespondence between the oxygen as measured by second-ary ion mass spectroscopy ~SIMS! and the carrier concentra-tion measured by Hall experiments for these and otherN-polar samples,5 particularly below the 1018 cm23 range.Second there is little difference between the measured Hallmobilities for samples 3 and 4 with the high vacancy clustercontent, and samples 12 and 14 with no detectable vacancycluster content. Room temperature mobilities for thesesamples were between 350 and 400 cm2/V s, and all fourmobilities peaking at ;800 cm2/V s at lower temperature.The dislocation scattering likely plays a significant role inlimiting the mobility.17Positron trapping at similar vacancy clusters has beenobserved earlier in MBE GaN layers ~unknown growth po-larity! grown on Si~111! substrate12 and in Si-doped MBElayers grown on HVPE-GaN substrate11 ~Ga-polar, but inver-sion boundaries were seen by electron microscopy!. AlsoUedono et al. have reported positron trapping at larger va-cancy clusters in N-polar MBE-grown GaN layer.18 It thusseems that the formation of clusters is related to the N-polargrowth direction. Though obtaining high quality growth onthe N-polar surface is harder, the layers of this work had acomparable surface morphology and electrical quality to Ga-polar growth. The higher reactivity of the (0001¯) surfaceenhances the incorporation of oxygen impurities,5 but alsoenables the formation of vacancy clusters. This may be oneof the limiting factors for N-polar MBE-growth of GaN.The formation of vacancy cluster requires empty Ga andN lattice sites. In addition to VGa , which always has a lowformation energy in n-GaN,14,15 perhaps N vacancies couldform under Ga stable condition ~i.e., favorable stoichiom-etry! especially in N polar growth since this has a largernumber of N sites that can be left empty. VGa and VN canform cluster by mobility of at least one of the species at thegrowth temperature, especially since VGa and VN are at-tracted by their opposite charges.In conclusion, we observe positron trapping at two kindsof vacancy type defects in MBE-grown O-doped GaN; a Gavacancy, or more likely the VGa–ON complex, and a vacancycluster. When the growth is done in Ga-stable conditions,leading to a low or moderate O-doping, vacancy clusters areformed in N-polar GaN, while Ga-polar layers show no sig-nificant vacancy concentrations. The absence of vacancies inthe low-doped Ga-polar samples is consistent with electricaldata indicating negligible compensation. However, when thegrowth is N-stable, leading to a high oxygen doping, the Gavacancy becomes the dominating defect in the both growthdirections, suggesting that oxygen impurities have a crucialrole in the formation of Ga vacancies.The authors would like to thank A. E. Wickenden, D. D.Koleske, and R. L. Henry at the Laboratory of AdvancedMaterial Synthesis, Naval Research Laboratory for providingthe MOCVD GaN templates. Work at WVU was supportedby ONR Grant Nos. N00014-02-1-0974 and N00014-01-0571, monitored by Colin Wood.1 E. J. Tarsa, B. Heying, X. H. Wu, S. P. DenBaars, and J. S. Speck, J. Appl.Phys. 82, 5472 ~1997!.2 M. Seelmann-Eggebert, J. L. Weyher, H. Obloh, H. Zimmermann, A. Rar,and S. Porowski, Appl. Phys. Lett. 71, 2635 ~1997!.3 T. Zywietz, J. Neugebauer, and M. Scheffler, Appl. Phys. Lett. 73, 487~1998!.4 T. Zywietz, J. Neugebauer, and M. Scheffler, Appl. Phys. Lett. 74, 1695~1999!.5 A. J. Ptak, L. J. Holbert, L. Ting, C. H. Swartz, M. Moldovan, N. C. Giles,T. H. Myers, P. Van Lierde, C. Tian, R. A. Hockett, S. Mitha, A. E.Wickenden, D. D. Koleske, and R. L. Henry, Appl. Phys. Lett. 79, 2740~2001!.6 A. L. Rosa, J. Neugebauer, J. E. Northrup, C. D. Lee, and R. M. Feenstra,Appl. Phys. Lett. 80, 2008 ~2002!.7 T. H. Myers, L. S. Hirsch, L. T. Romano, and M. R. Richards-Babb, J.Vac. Sci. Technol. B 16, 2261 ~1998!.8 K. Saarinen, P. Hautoja¨rvi, and C. Corbel, in Identification of Defects inSemiconductors, edited by M. Stavola ~Academic, New York, 1998!.9 J. Oila, V. Ranki, J. Kivioja, K. Saarinen, P. Hautoja¨rvi, J. Likonen, J. M.Baranowski, K. Pakula, T. Suski, M. Leszczynski, and I. Grzegory, Phys.Rev. B 63, 045205 ~2001!.10 K. Saarinen, T. Laine, S. Kuisma, J. Nissila¨, P. Hautoja¨rvi, L. Dobrzyniski,J. M. Baranowski, K. Pakula, R. Stepniewski, M. Wojdak, A. Wysmolek,T. Suski, M. Leszczynski, I. Grzegory, and S. Porowski, Phys. Rev. Lett.79, 3030 ~1997!.11 P. Laukkanen, S. Lehkonen, P. Uusimaa, M. Pessa, J. Oila, S. Hautakan-gas, K. Saarinen, J. Likonen, and J. Kera¨nen, J. Appl. Phys. 92, 786~2002!.12 E. Calleja, M. A. Sanchez-Garcia, D. Basak, F. J. Sanchez, F. Calle, P.Youinou, E. Munoz, J. J. Serrano, J. M. Blanco, C. Villar, T. Laine, J. Oila,K. Saarinen, P. Hautoja¨rvi, C. H. Molloy, D. J. Somerford, and I. Harrison,Phys. Rev. B 58, 1550 ~1998!.13 K. Saarinen, P. Seppa¨la¨, J. Oila, P. Hautoja¨rvi, C. Corbel, O. Briot, and R.L. Aulombard, Appl. Phys. Lett. 73, 3253 ~1998!.14 T. Mattila and R. M. Nieminen, Phys. Rev. B 55, 9571 ~1997!.15 J. Neugebauer and C. Van de Walle, Appl. Phys. Lett. 69, 503 ~1996!.16 K. Saarinen, T. Suski, I. Grzegory, and D. C. Look, Phys. Rev. B 64,233201 ~2001!.17 D. C. Look and J. R. Sizelove, Phys. Rev. Lett. 82, 1237 ~1999!.18 A. Uedono, S. F. Chichibu, Z. Q. Chen, M. Sumiya, R. Suzuki, T. Ohdaira,T. Mikado, T. Mukai, and S. Nakamura, J. Appl. Phys. 90, 181 ~2001!.4889Appl. Phys. Lett., Vol. 84, No. 24, 14 June 2004 Rummukainen et al.

Vacancy Defects In O-Doped Gan Grown By Molecular-Beam Epitaxy: The Role Of Growth Polarity And Stoichiometry

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Vacancy Defects In O-Doped Gan Grown By Molecular-Beam Epitaxy: The Role Of Growth Polarity And Stoichiometry

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