The free hole carriers in GaN have been limited to concentrations in the low 1018 cm−3 range due to the deep activation energy, lower solubility, and compensation from defects, therefore, limiting doping efficiency to about 1%. Herein, we report an enhanced doping efficiency up to ~10% in GaN by a periodic doping, metal modulation epitaxy growth technique. The hole concentrations grown by periodically modulating Ga atoms and Mg dopants were over ~1.5 x 1019 cm−3.
© 2008 American Institute of Physics

Doolittle, W. Alan

Lee, Kyung Keun

Look, David C.

Moseley, Michael

Namkoong, Gon

Trybus, Elaissa

English

Old Dominion University

Old Dominion UniversityODU Digital CommonsApplied Research Center Publications Applied Research Center2007Metal Modulation Epitaxy Growth for ExtremelyHigh Hole Concentrations Above 10(19) cm(-3)in GaNGon NamkoongOld Dominion University, gnamkoon@odu.eduElaissa TrybusKyung Keun LeeMichael MoseleyW. Alan DoolittleSee next page for additional authorsFollow this and additional works at: https://digitalcommons.odu.edu/arc_pubsPart of the Electrical and Computer Engineering Commons, and the Physics CommonsThis Article is brought to you for free and open access by the Applied Research Center at ODU Digital Commons. It has been accepted for inclusion inApplied Research Center Publications by an authorized administrator of ODU Digital Commons. For more information, please contactdigitalcommons@odu.edu.Repository CitationNamkoong, Gon; Trybus, Elaissa; Lee, Kyung Keun; Moseley, Michael; Doolittle, W. Alan; and Look, David C., "Metal ModulationEpitaxy Growth for Extremely High Hole Concentrations Above 10(19) cm(-3) in GaN" (2007). Applied Research Center Publications.4.https://digitalcommons.odu.edu/arc_pubs/4Original Publication CitationNamkoong, G., Trybus, E., Lee, K. K., Moseley, M., Doolittle, W. A., & Look, D. C. (2008). Metal modulation epitaxy growth forextremely high hole concentrations above 1019 cm-3 in GaN. Applied Physics Letters, 93(17), 172112. doi:10.1063/1.3005640AuthorsGon Namkoong, Elaissa Trybus, Kyung Keun Lee, Michael Moseley, W. Alan Doolittle, and David C. LookThis article is available at ODU Digital Commons: https://digitalcommons.odu.edu/arc_pubs/4Metal modulation epitaxy growth for extremely high hole concentrationsabove 1019 cm−3 in GaNGon Namkoong,1,a Elaissa Trybus,2 Kyung Keun Lee,2 Michael Moseley,2W. Alan Doolittle,2 and David C. Look31Applied Research Center, Old Dominion University, Newport News, Virginia 23606, USA2Department of Electrical and Computer Engineering, Georgia Institute of Technology,Atlanta, Georgia 30332-0250, USA3Semiconductor Research Center, Wright State University, Dayton, Ohio 45435, USAReceived 1 September 2008; accepted 2 October 2008; published online 31 October 2008The free hole carriers in GaN have been limited to concentrations in the low 1018 cm−3 range dueto the deep activation energy, lower solubility, and compensation from defects, therefore, limitingdoping efficiency to about 1%. Herein, we report an enhanced doping efficiency up to 10% inGaN by a periodic doping, metal modulation epitaxy growth technique. The hole concentrationsgrown by periodically modulating Ga atoms and Mg dopants were over 1.51019 cm−3. © 2008American Institute of Physics. DOI: 10.1063/1.3005640The attainment of high hole concentration1–3 has been amajor road block in nitride based solid state lighting tech-nologies. The major challenges related to p-type doping in-clude the following: 1 deep acceptor activation energies, aslarge as 170 meV for Mg in GaN,1 which activates onlyabout 1% of the Mg acceptors at room temperature; 2 solu-bility limits for Mg, in the low 1020 cm−3 range;4 3 com-pensation by native defects such as nitrogen vacancies,5which have a low formation energy in p-type GaN; and 4pyramidal extended defects due to inversion domains3,6which also lead to compensation in p-type material. Numer-ous growth techniques over the last 2 decades have beendeveloped for p-type doping. Representative growth tech-niques developed to overcome the technical barriers men-tioned above include Mg -doping,7 codoping,8 superlatticedoping,9 and doping with an alternative acceptor, Be.10 How-ever, none of the above growth techniques have provided areliable growth technology able to overcome the current lowhole concentration in III-nitride materials. As a result, thepresent maximum hole concentration in GaN that is consis-tently obtainable by molecular beam epitaxy MBE ormetal-organic chemical vapor deposition, with Mg doping, isgenerally limited to 1–21018 cm−3.In this letter, we report on the improvement in hole con-ductivity in GaN by the metal modulation epitaxy MMEgrowth technique.11 MME is a technique wherein only themetal fluxes Ga and Mg dopants are modulated, in a shortperiodic fashion in a rf MBE system, while maintaining acontinuous nitrogen plasma flux. This technique has demon-strated dramatic improvements in hole concentrations,4.51018 cm−3 published previously11 and over 1.51019 cm−3 in the present report.To understand why MME provides such a significantenhancement in hole concentration, we first must review Mgincorporation behavior and the formation of compensatingdonor-type defects. The widely accepted underlying dynamicmechanism of Mg incorporation involves “surface segrega-tion and accumulation at the surface,” which forces the Mgto remain at high surface concentration levels, and createsMg-related defects at the growing surface.6,12,13 The degreeof the surface segregation and accumulation is dependentupon the substrate temperature, Mg flux, and III/V ratios.14Under N-rich conditions, Mg will preferentially incorpo-rate into readily available Ga substitutional sites; however,these substitutional Mg can then easily exchange with im-pinging Ga, forcing the Mg to largely remain segregated onthe surface. When the Mg concentration reaches more than1.2 ML monolayer,3 polarity inversion domains3,15 areformed and can trap free holes. Additionally, nitrogen vacan-cies, which have been proposed as the dominant compensat-ing defects in Mg-doped GaN,16 have higher formationenergies under N-rich conditions than under Ga-richconditions.17 Therefore, the useful aspects of N-rich growthare i Mg dopants can be incorporated into Ga substitutionalsites; and ii compensating N-vacancy concentrations arereduced due to their higher formation energy. However, adetrimental aspect is iii an increase in surface faceting thatcan create extended defects grain boundaries and disloca-tions. Therefore, effective Mg doping under N-rich condi-tions is a balance of the beneficial aspects of i and iiversus the detrimental effects of iii.Next we consider slight Ga-rich conditions, in which theGa coverage or Ga bilayers slightly exceeds the N cover-age; in this case, more Mg accumulates on the surface due tothe limited availability of Ga substitutional sites, and less isincorporated into the epitaxial layer.18 Attempts to solve thisproblem with higher Mg fluxes cause the local polarity toinvert from Ga-polar to N-polar,3,15 and results in reducedMg incorporation. Furthermore, the formation energy of ni-trogen vacancies decreases under slightly Ga-rich conditionsand as a result more nitrogen vacancies are available to com-pensate the free holes.17Even though slight Ga-rich conditions reduce Mg incor-poration into III-nitride materials, it is found that extremeGa-rich conditions, i.e., metal droplet growth conditions,lead to enhanced Mg incorporation.19 Under such conditions,the enhancement of Mg incorporation is attributed to the factthat the Mg dissolves into the metallic Ga and consequentlyis prevented from re-evaporating.19 Therefore, extreme Ga-rich conditions enhance the incorporation of Mg. Moreover,polarity inversion can be avoided by completely covering theaAuthor to whom correspondence should be addressed. Electronic mail:gnamkoon@odu.edu.APPLIED PHYSICS LETTERS 93, 172112 20080003-6951/2008/9317/172112/3/$23.00 © 2008 American Institute of Physics93, 172112-1growing surface with metallic Ga bilayers.15 Unfortunately,however, the formation energy of nitrogen vacancies is low-est in this condition17 and as mentioned above, these defectswill compensate free holes.Based on the considerations discussed above, p-typegrowth of GaN is not ideal under either Ga-rich or N-richconditions alone. The best strategy is a periodic growth withabrupt transitions between Ga-rich and N-rich conditions,Ga-rich to smooth the surface and to prevent compensatingextended defects resulting from faceting, and N-rich to pro-mote Mg incorporation onto the Ga-substitutional site with-out the production of compensating N-vacancies. Such an“oscillation” of growth conditions can be achieved by peri-odic modulation of the metal fluxes while maintaining a con-stant nitrogen flux. During the growth, metallic buildup oc-curs under Ga-rich conditions but depletion of both themetallic Ga bilayers and the surface-accumulated Mg dop-ants occur during the transition from Ga-rich to N-rich con-ditions. A detailed schematic is shown in Fig. 1a andgrowth parameters can be adjusted according to the GaNepitaxy phase diagram.20 However, as indicated in Fig. 1b,conventional growth or even nonoptimal modulation growth,leads to incomplete depletion of the surface-accumulated Gaand Mg when the modulation growth occurs under Ga-richconditions. It is found that nonoptimal modulation growth ofMg-doped GaN limits the hole concentration to the low1018 cm−3 range.In this work, both the Ga and Mg shutters are modulatedthe Mg being delivered from a Veeco corrosive seriesvalved cracker with a flux of 4–510−10 Torr beamequivalent pressure BEP at a growth temperature of500 °C.11,18 The Ga fluxes used are large enough that drop-lets rapidly form when the Ga shutter opens and are subse-quently depleted when the Ga shutter closes. Figure 2ashows Mg concentrations in alternating Mg-doped/undopedGaN with different periodic open/close duty cycles ofmetal shutters while maintaining the same Ga fluxes. Theopening cycles for the Mg and Ga shutters are changed fromno modulation to 30 s while the closing cycles are main-tained for 10 s. The resultant Mg concentrations from sec-ondary ion mass spectrometry SIMS measurements showconstant values of 1–21020 cm−3 within factors of 2regardless of different shutter duty cycles, supporting the as-sumption of saturated Mg through surface accumulation. Itshould be noted that the oxygen concentrations are found tobe about 31017 cm−3 in the Mg-doped GaN except forthe initial growth, which contains about 11018 cm−3. Indi-vidual Mg-doped samples grown with different periodic dutycycles of the metal shutters are also confirmed to have simi-lar Mg concentrations of 1–21020 cm−3.However, it is found that the hole concentration is in-creased, from 21018 to 1.41019 cm−3, by employingshorter shutter opening times more N-rich average condi-tions, as shown in Fig. 2b. One of the key observations isthat the doping efficiency increases up to 10% for the high-est hole concentration of 1.41019 cm−3. Since the SIMS-determined Mg concentrations are almost the same within afactor of 2 for all samples grown under a variety of differentgrowth conditions, the increased hole concentrations can beattributed to optimal incorporation of surface-accumulatedMg dopants into Ga substitutional sites with minimal forma-tion of compensating defects i.e., point defects, mainlyN-vacancies, and extended defects, mainly faceting. Dopingefficiency around 10% is also reported by Bhattacharyyaet al.19 with extreme Ga-rich growth conditions at muchhigher growth temperature of 770 °C, producing hole con-centrations varying from 21017 to 31018 cm−3 and cor-responding mobilities varying from 30 to 2 cm2 /V s. How-ever, the higher growth temperature limited the incorporationof Mg concentrations to low 1019 cm−3, resulting in low1018 cm−3 hole concentration. The MME approach at the lowgrowth temperatures is the only approach reported to en-hance hole concentration over 1019 cm−3.To better understand the role of the buildup and deple-tion process in Mg-doped MME growth, while maintaining aFIG. 1. Color online Schematic of the MME growth technique: the peri-odic cycling of buildup and depletion of surface-accumulated Ga and Mgdopants a during the transitions between Ga- and N-rich conditions and bunder Ga-rich growth conditions.FIG. 2. Color online a SIMS profile of alternating Mg-doped/undoped GaN with different periodic open/close duty cyclings with constant Ga flux=6.510−7 Torr, and b individual hole concentrations and resistivities grown under different periodic duty cyclings.172112-2 Namkoong et al. Appl. Phys. Lett. 93, 172112 2008constant duty cycle of shutter opening 5 s and closing10 s the Ga fluxes were increased from 5.2 to 8.210−7 Torr BEP. The resultant hole concentrations areshown in Fig. 3a and it is seen that in this BEP range, thehighest hole concentrations are obtained with the lowest Gafluxes most N-rich average conditions. It is also found thatthe reflection high-energy electron diffraction RHEED im-ages depend strongly on Ga fluxes and are closely correlatedto the resulting hole concentration. As shown in Fig. 3b, asample with high hole concentration 1019 cm−3 shows thespotty patterns with streaky lines, indicating progressiveN-rich growth. In contrast, as seen in Fig. 3c, a GaNsample with a hole concentration of 1018 cm−3 shows a morestreaky RHEED pattern. Further study on RHEED intensityon this sample indicates that modulation growth under higherGa flux 8.210−7 Torr occurs in the Ga-rich region, indi-cated in Fig. 1b. More detail studies on RHEED intensitywill be presented later.Based on these observations, the extremely high holeconcentrations over 1019 cm−3 can be obtained only when thesurface depletion process occurs during the transitions be-tween Ga-rich and N-rich regions.Preliminary analysis of temperature-dependent Hall-effect measurements has been performed on Mg-doped GaNsamples grown with different shutter duty cycles. It is foundthat the donor/acceptor compensation ratio is about 0.3 for asample with a high hole concentration of 1.451019 cm−3.An accurate acceptor activation energy cannot be measuredbecause the Mg ions are not isolated, as would be the caseat lower concentrations. At a concentration of 1–21020 cm−3, the average Mg separation is about 1.1 nm,which is close to the expected Bohr radius of Mg about0.8 nm for a 200 meV acceptor in GaN. Therefore, therewill be considerable wavefunction overlap, and the conduc-tion will be highly dependent on interimpurity tunneling, aprocess not describable by the usual thermal activation en-ergy. A detailed discussion of these results, including identi-fication of the compensating donors and temperature-dependent Hall effect, will be presented separately.In summary, a doping growth technique has been devel-oped by effectively establishing the periodic buildup anddepletion process to facilitate the incorporation of Mg dop-ants into Ga substitutional sites while suppressing the forma-tion of compensating defects. The highest hole concentra-tions obtained with MME doping are 1.51019 cm−3 atroom temperature, with a resistivity of 0.9  cm and mobil-ity of 0.5 cm2 /V s. RHEED characteristics needed to repro-duce these results have been described. The demonstratedcapability of high hole concentrations in p-type GaN willexpedite the development of future nitride based electronicand optical devices.This work was supported by the Office of Naval Re-search Basic Science Grants monitored by Dr. Paul Maki, theAir Force Office of Scientific Research monitored by Dr.Donald Silversmith, the Defense Advanced ResearchProjects Agency monitored by Dr. Brian Pierce and Dr. Ste-fanie Tompkins and partially supported through NSF-EECGrant No. 0824311.1T. D. Moustakas and R. J. Molnar, MRS Symposia Proceedings No. 281Material Research Society, Pittsburgh, 1993, p. 753.2E. Haus, I. P. Smorchkova, B. Heying, P. Fini, C. Poblenz, T. Mates, U. K.Mishra, and J. S. Speck, J. Cryst. Growth 246, 55 2002.3V. Ramachandra, R. M. Feenstra, W. L. Sarney, L. Salamanca-Riba, J. E.Northrup, L. T. Romano, and D. W. Greve, Appl. Phys. Lett. 75, 8081999.4G. Namkoong, W. A. Doolittle, and A. S. Brown, Appl. Phys. Lett. 77,4386 2000.5C. Stample and C. G. Van de Walle, Appl. Phys. Lett. 72, 459 1998.6S. Figge, R. Kroger, T. Bottcher, P. L. Ryder, and D. Hommel, Appl. Phys.Lett. 81, 4748 2002.7M. L. Nakarmi, K. H. Kim, J. Li, J. Y. Lin, and H. X. Jiang, Appl. Phys.Lett. 82, 3041 2003.8H. Katayama-Yoshida, T. Nishimatsu, T. Yamamoto, and N. Orita, J.Phys.: Condens. Matter 13, 8901 2001.9P. Kozodoy, Y. P. Smorchkova, M. Hansen, H. Xing, S. P. DenBaars, U. K.Mishra, A. W. Saxler, R. Perrin, and W. C. Mitchell, Appl. Phys. Lett. 75,2444 1999.10A. J. Ptak, L. Wang, N. C. Giles, T. H. Myers, L. T. Romano, C. Tian, R.A. Hockett, S. Mitha, and P. Van Lierde, Appl. Phys. Lett. 79, 45242001.11S. D. Burnham, G. Namkoong, D. C. Look, B. Clafin, and W. AlanDoolittle, J. Appl. Phys. 104, 024902 2008.12T. S. Cheng, S. V. Novikov, C. T. Foxon, and J. W. Orton, Solid StateCommun. 109, 439 1999.13Q. A. Sun, A. Selloni, T. H. Myers, and W. A. Doolittle, Phys. Rev. B 73,155337 2006.14N. Sipe and R. Venkat, MRS Internet J. Nitride Semicond. Res. 7, 12002.15D. S. Green, E. Haus, F. Wu, L. Chen, U. K. Mishra, and J. S. Speck, J.Vac. Sci. Technol. B 21, 1804 2003.16C. Stample and C. G. Van de Walle, Appl. Phys. Lett. 72, 459 1998.17C. G. Van de Walle and J. Neugebauer, Braz. J. Phys. 26, 163 1996.18S. D. Burnham, W. A. Doolittle, G. Namkoong, and W. Henderson, MRSSymposia Proceedings No. 798 Materials Research Society, Pittsburgh,2003, p. Y8.11.19A. Bhattacharyya, W. Li, J. Cabalu, T. D. Moustakas, and D. J. Smith,Appl. Phys. Lett. 85, 4956 2004.20B. Heying, R. Averbeck, L. F. Chen, and E. Haus, J. Appl. Phys. 88, 18552000.FIG. 3. Color online a Hall-effect data for Mg-doped GaN with differentGa fluxes with a duty cycle 5 /10 s; b RHEED images recorded forMg-doped GaN with a Ga flux of 6.510−7 Torr; and c with a Ga flux of8.210−7 Torr.172112-3 Namkoong et al. Appl. Phys. Lett. 93, 172112 2008

Metal Modulation Epitaxy Growth for Extremely High Hole Concentrations Above 10(19) cm(-3) in GaN

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Metal Modulation Epitaxy Growth for Extremely High Hole Concentrations Above 10(19) cm(-3) in GaN

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