The concentration of the acceptor responsible for the yellow luminescence (YL) band at about 2.2eV in GaN is determined from photoluminescence. The YL band intensity increases linearly with excitation power density and partially saturates above some critical value. The dependence is quantitatively described within a phenomenological model accounting for recombination statistics in GaN layer and saturation of acceptors with photogenerated holes. The incomplete saturation of the YL intensity at high excitation intensities is explained by gradual saturation of acceptors at different distances from the sample surface. The identity of deep and shallow acceptors in GaN is discussed

Reshchikov, Michael A.

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Virginia Commonwealth UniversityVCU Scholars CompassPhysics Publications Dept. of Physics2006Determination of acceptor concentration in GaNfrom photoluminescenceMichael A. ReshchikovVirginia Commonwealth University, mreshchi@vcu.eduFollow this and additional works at: http://scholarscompass.vcu.edu/phys_pubsPart of the Physics CommonsReshchikov, M.A. Determination of acceptor concentration in GaN from photoluminescence. Applied Physics Letters,88, 202104 (2006). Copyright © 2006 AIP Publishing LLC.This Article is brought to you for free and open access by the Dept. of Physics at VCU Scholars Compass. It has been accepted for inclusion in PhysicsPublications by an authorized administrator of VCU Scholars Compass. For more information, please contact libcompass@vcu.edu.Downloaded fromhttp://scholarscompass.vcu.edu/phys_pubs/39Determination of acceptor concentration in GaN from photoluminescenceM. A. ReshchikovaDepartment of Physics, Virginia Commonwealth University, Richmond, Virginia 23284Received 5 January 2006; accepted 31 March 2006; published online 15 May 2006The concentration of the acceptor responsible for the yellow luminescence YL band at about2.2 eV in GaN is determined from photoluminescence. The YL band intensity increases linearlywith excitation power density and partially saturates above some critical value. The dependence isquantitatively described within a phenomenological model accounting for recombination statistics inGaN layer and saturation of acceptors with photogenerated holes. The incomplete saturation of theYL intensity at high excitation intensities is explained by gradual saturation of acceptors at differentdistances from the sample surface. The identity of deep and shallow acceptors in GaN isdiscussed. © 2006 American Institute of Physics. DOI: 10.1063/1.2204835Gallium nitride has gained unprecedented attention dueto its applications in emitters green, blue, and ultravioletlight emitting diodes LEDs and laser diodes, optical detec-tors, and high-power amplifiers. However, even the best-quality GaN contains many point defects reducing the effi-ciency and lifetime of electronic and optical devices. To date,the majority of point defects in GaN remain unidentified andinsufficiently studied.1 In particular, in undoped GaN grownby any technique a yellow luminescence YL band is com-monly observed near 2.2 eV that is attributed to transitionsfrom the shallow donors or conduction band to an unidenti-fied deep acceptor.1 The main candidate for the deep acceptoris the gallium vacancy-oxygen complex VGaON that isabundantly formed in n-type GaN during growth.2,3 In par-ticular, the YL and green luminescence bands in high-qualityfreestanding GaN templates were assigned respectively to2− /− and −/0 charge states of VGaON.4 However, there re-mains a controversy over the identification of the YL band inless pure material.1 In particular, Armitage et al.5 proposedthat the carbon impurity in C-doped GaN and VGa-relateddefects in undoped GaN cause YL bands very similar inshape and position.The main difficulty in the identification of the YL bandin GaN is associated with the common failure to determinethe concentration of the related acceptor and compare it withthe concentrations of the dominant impurities and vacancy-related defects. The omnipresent method to determine therelative concentration of the acceptor from the relative inten-sity of the YL band in different samples is unacceptable sincethe photoluminescence PL intensity from a particular de-fect depends on many factors.6 Previously we proposed amethod of how to determine the concentration of radiativeacceptors in GaN from the analysis of the dependence of PLintensity on excitation intensity.6 However, at that time wecould not explain satisfactorily the incomplete saturation ofthis dependence at excitation power densities correspondingto the saturation of the radiative acceptors with photogener-ated holes. In this work we give an explanation of the incom-plete saturation and determine the concentration of radiativeacceptors in a GaN sample.A 1.9-m-thick unintentionally doped GaN layer wasgrown by metal organic chemical vapor deposition methodon c-plane sapphire substrate by EMCORE Corporationand studied by several research groups within the Wood-WittInitiative “Defects in GaN” Sample No. EM1256 for inter-nal reference. Concentration of Si, C, and O in this samplehas been estimated as about 31016, 41016, and51016 cm−3, respectively, by the secondary ion massspectrometry.7 Concentrations of free electrons at room tem-perature, n0, shallow donors, ND, and all compensating ac-ceptors, NA, have been estimated as 21016, 61016, and41016 cm−3, respectively, from the analysis of thetemperature-dependent Hall effect.8 Steady-state PL was ex-cited with unfocused He–Cd laser 55 mW, 325 nm, dis-persed by a 1200 rules/mm grating in a 0.3 m monochro-mator, and detected by a cooled photomultiplier tube.Calibrated neutral-density filters were used to attenuatethe excitation power density Pexc over the range of 110−5–0.3 W/cm2. Time-resolved PL was excited with apulsed nitrogen laser 337 nm and analyzed with an oscil-loscope. A closed-cycle optical cryostat was used for tem-peratures between 10 and 320 K.PL spectra of the GaN sample at different temperaturesare shown in Fig. 1. At 10 K the spectrum contains a sharpline at 3.484 eV attributed to the annihilation of an excitonbound to a neutral shallow donor DBE, followed by twoLO phonon replicas, and a broad YL band with a maximumat 2.2 eV. With increasing temperature the DBE emissionaElectronic mail: mreshchi@vcu.eduFIG. 1. PL spectrum from a GaN layer at different temperatures. The QE ofdifferent PL bands: exciton=8% at 10 K; YL=2% at 10 K and 4% at100–300 K; UVL=0.01% at 100 K.APPLIED PHYSICS LETTERS 88, 202104 20060003-6951/2006/8820/202104/3/$23.00 © 2006 American Institute of Physics88, 202104-1 This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to IP:128.172.48.59 On: Tue, 14 Apr 2015 17:04:10quenches, giving way to the free exciton FE lines shiftedby 6 meV to higher energies. At temperatures close to100 K, when the FE emission significantly quenches, a veryweak ultraviolet luminescence UVL band could be de-tected, representing a peak at 3.27 eV followed by two LOphonon replicas Fig. 1. At higher temperatures the UVLband quenches, and only the strong YL band and weak exci-ton band peaking at 3.428 eV could be seen at room tem-perature Fig. 1.With increasing excitation intensity, P0, up to1015photons/cm2 s, the YL intensity, IYL, increases lin-early, and at higher excitation intensities it partially saturatesFig. 2. By fitting the IYLP0 dependence with the theoret-ical expression proposed in Ref. 6,IYL = P0 P0YLLeffNYL + 1YL−1, 1where YL and YL are the lifetime and quantum efficiencyQE of the YL, NYL is the concentration of acceptors re-sponsible for the YL band, and Leff is “the effective thicknessof the layer subject to excitation,”6 we obtain NYL	81015 cm−3 by ignoring the discrepancy between the theo-retical and experimental dependencies above P0	11016 cm−2 s−1 Fig. 2. Note that for this evaluation the QEof the YL band YL=0.04 has been estimated from thedirect measurement of PL power with correction for the ge-ometry of the PL registration optics.6 However, it is wellknown that the upward band bending of about 0.8 eV existsnear the surface due to the presence of negative charge at thesurface of GaN.9 In the sample with the concentration ofuncompensated donors of 21016 cm−3 this band bendingshould create a 0.2-m-thick depletion region near the sur-face from which the photogenerated carriers will be quicklyswept out due to strong electric field, and the layer would bedead for PL. Therefore, the determined QE may be underes-timated about ten times since the excitation intensity beyondthe 0.2-m-thick depletion region is reduced about ten timesif the absorption coefficient  of GaN at 325 nm is about105 cm−1.10 Note also that the depletion region remains al-most unchanged in the whole range of the excitation inten-sities used in this work. Indeed, under continuous illumina-tion with our He–Cd laser the band bending in GaNdecreases by not more than 25% as compared to the dark.11Below we develop a phenomenological model for calculatingthe concentration of acceptors in n-type semiconductors ac-counting for the depletion region near the surface andgradual saturation of defects located at different depths of thelayer with arbitrary thickness L instead of using the effectivethickness Leff.Let us consider a GaN layer with thickness L and deple-tion region width x0 Fig. 3. Laser intensity P0 decreasesinside the layer as exp −x. At distance x, the followingintensity will be absorbed within a thin layer x:Px = P0e−xx . 2The number of acceptors related to the YL band in the thinlayer of unit area is NYL=NYLx. Then the total intensityof the YL band emitted in the region from x0 to L in alldirections isIYL = x0L P0e−xYLP0e−x/NYL + 1/YLdx=NYLYLlnP0 + P1P0 + P2 , 3where P1=NYLYLYLe−x0−1 and P2=NYLYLYLe−L−1. At P0P1 the IYLP0 dependence islinear,IYLP0  P1 = YLe−x0P0 = YLextP0, 4where YLext is the QE which can be found by direct measure-ments at low excitation intensity as YLext= IYL/ P0. AtP0P2,IYLP0  P2 =NiYLL − x0 . 5Between P1 and P2 the IYLP0 dependence is sublinear withgradually reducing slope. In typical GaN layers with thick-FIG. 2. Dependence of the YL intensity on excitation intensity at 300 K.Points—experimental data. Solid lines—calculated by using Eq. 6 with thefollowing parameters: =105 cm−1, =0.4 ms, YLexc=0.04, NYL=11015,3.31015 and 101016 cm−3.FIG. 3. Schematic of PL process. a Decrease of the excitation intensityinside GaN layer; b band diagram of the GaN layer having a depletionregion between x=0 and x=x0 due to upward band bending near the layersurface; c cross section of the GaN layer with selected thin layer x whereacceptors with the density N=Nix are shown as dots which emit PL in alldirections. A fraction 	 of the total PL intensity emitted from the layer iscollected with a lens from the front side of the GaN layer.202104-2 M. A. Reshchikov Appl. Phys. Lett. 88, 202104 2006 This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to IP:128.172.48.59 On: Tue, 14 Apr 2015 17:04:10ness of more than 1 m the transition region between P1 andP2 extends on several orders of magnitude of P0, and thecomplete saturation cannot be observed in the typical rangeof excitation intensities. Moreover, at very high excitationintensity the IYLP0 dependence should become a squareroot dependence when the exciton emission becomes thedominant mechanism of recombination.6 Thus, for not verythin GaN layers L1 m and not very high excitationpower densities Pexc100 W/cm2, Eq. 3 may be simpli-fied toIYL =NYLYLln1 + P0P1 , 6consisting of the linear region at P0P1 and incompletesaturation region at P0P1.The experimental IYLP0 dependence is fitted with Eq.6 in Fig. 3. The lifetime of the YL at room temperature inthe studied sample has been evaluated as 0.4 ms from thetime-resolved PL study where a nearly exponential decay ofthe YL band intensity has been observed after pulse excita-tion. The value of YLext 0.04 was estimated from directmeasurement of the QE by using a standard sample.6NYL wasthe only fitting parameter, and the value of NYL=3.31015 cm−3 was found. Note that the proposed method forestimation of the concentration of the radiative defects isrelatively accurate see the fits for NYL=1015 and 1016 cm−3in Fig. 2 and does not require knowing the width of thedepletion region. Note also that about twofold increase of theYL intensity with increasing temperature from 10 to 100 KFig. 1 favors the assumption that the QE of exciton emis-sion is about 50% at 10 K and the real internal QE of theYL YL is close to 25% at 300 K. The difference betweenYLext=4% and YL=25% is consistent with an assumption ofa depletion region of about 0.1–0.2 m in the studied GaNsample.Unfortunately we are still unable to identify the defectresponsible for the YL band in GaN. The value of NYL=3.31015 cm−3 is about an order of magnitude smallerthan the total concentration of acceptors or major impurities,including C and O, in the studied sample. Remarkably, wecan estimate also the concentration of the shallow acceptorresponsible for the UVL band in this sample, NUVL. Indeed,6NUVL = NYLIUVLIYLCYLCUVL, 7where the hole capture coefficients for two acceptors canbe taken as CYL=310−7 and CUVL=110−6 cm3 s−1 andthe ratio of the integrated PL intensities between the YLand UVL bands at 100 K is 400. This gives NUVL	31012 cm−3. The result calls into question the possibility forCN to be likely formed in GaN Ref. 12 and be responsiblefor the UVL band.13,14In conclusion, we have developed a new method forestimation of the radiative defects in semiconductors fromthe analysis of photoluminescence. Application of thismethod to unintentionally doped n-type GaN allowed us toestimate the concentration of deep acceptors responsible forthe strong yellow luminescence band in this sample as 3.31015 cm−3. Concentration of the shallow acceptors in thissample is found to be 1000 times lower. The method is con-ceptually insensitive to the presence of depletion region nearthe surface of the semiconductor layer, which may signifi-cantly reduce the observed luminescence intensity.The author thanks Dr. D. C. Look from Wright StateUniversity and Dr. S. Guo and Dr. D. Gotthold fromEMCORE Corporation for providing GaN samples withinthe Wood-Witt initiative.1M. A. Reshchikov and H. Morkoç, J. Appl. Phys. 97, 061301 2005.2J. Neugebauer and C. G. Van de Walle, Appl. Phys. Lett. 69, 503 1996.3T. Mattila and R. M. Nieminen, Phys. Rev. B 55, 9571 1997.4M. A. Reshchikov, H. Morkoç, S. S. Park, and K. Y. Lee, Appl. Phys. Lett.81, 4970 2002.5R. Armitage, W. Hong, Q. Yang, H. Feick, J. Gebauer, E. R. Weber, S.Hautakangas, and K. Saarinen, Appl. Phys. Lett. 82, 3457 2003.6M. A. Reshchikov and R. Y. Korotkov, Phys. Rev. B 64, 115205 2001.7The SIMS data were provided by S-P. Guo and D.W. Gotthold EM-CORE.8The electrical properties were provided by D.C. Look and Z.-C FangWright State University9S. Sabuktagin, M. A. Reshchikov, D. K. Johnstone, and H. Morkoç, Mater.Res. Soc. Symp. Proc. 798, Y5.39 2004.10J. F. Muth, J. H. Lee, I. K. Shmagin, R. M. Kolbas, H. C. CaseyJr., B. P.Keller, U. K. Mishra, and S. P. DenBaars, Appl. Phys. Lett. 71, 25721997.11M. A. Reshchikov, S. Sabuktagin, D. K. Johnstone, and H. Morkoç, J.Appl. Phys. 96, 2556 2004.12A. F. Wright, J. Appl. Phys. 92, 2575 2002.13U. Birkle, M. Fehrer, V. Kirchner, S. Einfeldt, D. Hommel, S. Strauf, P.Michler, and J. Gutowski, MRS Internet J. Nitride Semicond. Res. 4S1,G5.6 1999.14E. R. Glaser, J. A. FreitasJr., B. V. Shanabrook, and D. D. Koleske, Phys.Rev. B 68, 195201 2003.202104-3 M. A. Reshchikov Appl. Phys. Lett. 88, 202104 2006 This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to IP:128.172.48.59 On: Tue, 14 Apr 2015 17:04:10

Determination of acceptor concentration in GaN from photoluminescence

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Determination of acceptor concentration in GaN from photoluminescence

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