We report on the fabrication of GaN co-doped with silicon and zinc by metalorganic vapor phase epitaxy and a detailed study of photoluminescence in this material. We observe an exceptionally high absolute internal quantum efficiency of blue photoluminescence in GaN:Zn,Si. The value of0.93±0.04 has been obtained from several approaches based on rate equations

Bakin, A.

Behrends, A.

Reshchikov, Michael A.

Waag, A.

Willyard, A. G.

VCU Scholars Compass

Virginia Commonwealth UniversityVCU Scholars CompassPhysics Publications Dept. of Physics2011Extremely high absolute internal quantumefficiency of photoluminescence in co-dopedGaN:Zn,SiMichael A. ReshchikovVirginia Commonwealth University, mreshchi@vcu.eduA. G. WillyardVirginia Commonwealth UniversityA. BehrendsInstitute of Semiconductor TechnologyA. BakinInstitute of Semiconductor TechnologyA. WaagInstitute of Semiconductor TechnologyFollow this and additional works at: http://scholarscompass.vcu.edu/phys_pubsPart of the Physics CommonsReshchikov, M.A., Willyard, A.G., Behrends, A., et al. Extremely high absolute internal quantum efficiency ofphotoluminescence in co-doped GaN:Zn,Si. Applied Physics Letters, 99, 171110 (2011). Copyright © 2011 AIPPublishing 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/27Extremely high absolute internal quantum efficiency of photoluminescencein co-doped GaN:Zn,SiM. A. Reshchikov,1,a) A. G. Willyard,1 A. Behrends,2 A. Bakin,2 and A. Waag21Physics Department, Virginia Commonwealth University, Richmond, Virginia 23284, USA2Institute of Semiconductor Technology, Braunschweig University of Technology, Braunschweig, Germany(Received 27 July 2011; accepted 3 October 2011; published online 27 October 2011)We report on the fabrication of GaN co-doped with silicon and zinc by metalorganic vapor phaseepitaxy and a detailed study of photoluminescence in this material. We observe an exceptionallyhigh absolute internal quantum efficiency of blue photoluminescence in GaN:Zn,Si. The value of0:9360:04 has been obtained from several approaches based on rate equations. VC 2011 AmericanInstitute of Physics. [doi:10.1063/1.3655678]Co-doping GaN by Si and Zn leads to bright emission ata photon energy of about 2.9 eV.1 Since this is in the bluespectral range, such a co-doping has been used in early LEDstructures for creating blue light emission.2 However, afterthe “invention” of the InGaN quantum well, the Zn-relatedemission has no longer been used in LEDs. New interest inthis material could arise from the field of single photon emit-ters, and high internal quantum efficiency (IQE) is a prereq-uisite in this case.Whereas the external quantum efficiency can be deter-mined by merely measuring the number of photons emittedby the LED chip, the determination of the IQE of electrolu-minescence or photoluminescence (PL) is much more indi-rect. Often, the ratio of room-temperature and low-temperature PL intensity is used as a rough estimate of therelative IQE. There is no consensus in literature on how todetermine the absolute IQE of PL in GaN. High values ofthe absolute IQE of PL in GaN or InGaN were reported, inthe range of 30%-70%.3–6 However, assumptions for themethod used in these publications to estimate the absoluteIQE require better justification.In this work, we report on extremely high absolute IQEof PL in degenerate n-type GaN co-doped with Zn and Si.The IQE was determined from analysis of the temperature andexcitation intensity dependences of PL. The absolute IQE ofthe blue luminescence (BL) band in GaN:Zn,Si exceeds 90%.Samples for this work were grown by metalorganicvapor phase epitaxy (MOVPE) on c-plane sapphire sub-strates. After growing 2 lm-thick undoped GaN, a 0.4 lm-thick AlGaN barrier was grown, followed by a 0.2 lm-thickGaN:Zn,Si layer and capped with a 40 nm-thick AlGaN bar-rier. The concentration of free electrons in the GaN:Zn,Silayer has been estimated as 3 1018 cm3 from electrochem-ical C-V (ECV) profiling measurements and 5 1018 cm3from time-resolved PL experiments. The PL lifetime of theBL band (s1 ¼ 0:5 ls) was independent of temperature atT < 250K, i.e., the layer is degenerate. Steady-state PL wasexcited with a 50 mW He-Cd laser. Other details of the ex-perimental set-up as well as detailed information on themethod of estimating the IQE of PL can be found in Ref. 7.A PL spectrum from GaN:Zn,Si at three representativetemperatures is shown in Fig. 1. It includes a strong BL bandwith a maximum at 2.9 eV, a weak exciton peak at higher pho-ton energies, and a broad defect-related band with a maximumat about 2.1 eV. The latter will be called here the yellow lumi-nescence (YL) band, although its identity and relation to theYL bands reported in literature1 were not established. The BLband is attributed to transitions from the conduction band (orshallow donor at very low temperature) to the ZnGaacceptor.1,7 The shape of this band at different temperatures iswell known,1,7 and this information was used to deconvolutethe bands before finding their integrated intensities.Figure 2 shows the temperature dependence of the inte-grated PL intensity for three PL bands in relative units (rightscale). The BL band quenches at T > 250K with the activa-tion energy of about 300meV. The characteristic tempera-ture of the quenching T0 can be defined as the point wherethe low-temperature and high-temperature parts of the de-pendence cross when extrapolated as straight lines in theArrhenius plot (see Fig. 2). This value in the studied samplesis unusually high (T0 ¼ 300K) as compared to the valuesreported earlier (T0 ¼ 200 220 K).7,8 The exciton and YLband intensities slowly decrease with increasing temperaturefrom 15 to 250K (not shown for T < 100K). However,simultaneously with the quenching of the BL band, namely,at T ¼ 250 350K, they both exhibit a sharp rise by anFIG. 1. (Color online) PL spectra at three representative temperatures.Pexc ¼ 0:0015W/cm2. Weak oscillations are due to interference effect.Black dashed line shows the low-energy tail of the BL band in a Zn-dopedGaN sample where no YL band was observed.a)Electronic mail: mreshchi@vcu.edu.0003-6951/2011/99(17)/171110/3/$30.00 VC 2011 American Institute of Physics99, 171110-1APPLIED PHYSICS LETTERS 99, 171110 (2011) 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, 31 Mar 2015 19:28:30order of magnitude and then decrease again at higher temper-atures. Such a behavior clearly indicates a very high IQE ofthe BL band.8 Below we will explain the results and describehow the absolute IQE for the PL bands (shown on the leftscale of Fig. 2) was determined.With increasing temperature from 15 to 250K, the BLband intensity remained nearly constant, in agreement withthe model for the BL band in n-type GaN:Zn developed inRefs. 7 and 8. The temperature dependence of the BL bandIQE g1ðTÞ in n-type GaN can be described with a formula7,8g1ðTÞ ¼g101þ ð1 g10Þs1Q1; (1)where g10 is the IQE of the BL band at low temperatures(T < 250K), s1 is the BL band lifetime determined from time-resolved PL measurements, Q1 is the hole-emission coeffi-cient (Q1 ¼ CpAg1NveEA=kT), CpA is the hole-capture coeffi-cient for the Zn-related BL band in GaN (CpA ¼ 106 cm3/s),1Nv is the density of states in the valence band, g is the degener-acy factor of the acceptor level, EA is the ionization energy ofthe ZnGa acceptor (0.40 eV at 15K and decreases to 0.35 eV atT  200K),1 and k is Boltzmann’s constant. According to aphenomenological model for an n-type semiconductor,7,8 thethermal quenching of the strongest PL band results in simulta-neous increase of recombination rate (or PL intensity) via allother recombination channels.There are four main recombination channels in GaN:Zn,Si,each with absolute IQE gi. These include the BL band with g1defined with Eq. (1), exciton or near-bandedge emission withg2 ¼ Bnp=G, the YL band with g3 ¼ Cp3N3p=G, and recombi-nation via nonradiative centers with g4 ¼ Cp4N4p=G, where Gis the electron-hole generation rate, n and p are concentrationsof free electrons and holes, respectively, B is the exciton ornear-bandedge recombination rate coefficient, and Cpi and Niare hole-capture coefficients and concentrations of differentdefects participating in carrier recombination. The PL intensityintegrated over a spectral region of a particular PL band, IPLi , isproportional to the absolute IQE of this PL band, and it can beexpressed as IPLi ¼ bgiG, where b is a constant determined bythe total collection efficiency of luminescence (same for allchannels and independent of temperature).Next, we define gi0 and IPLi0 (IPLi0 ¼ bgi0G) as thetemperature-dependent absolute IQE and corresponding inte-grated PL intensity for the ith channel (i 6¼ 1) which wouldbe observed if the BL band did not quench at all, as opposedto the values gi and IPLi which include the quenching. Inpractice, the temperature dependence of IPLi0 can be found bymodeling the experimentally found IPLi ðTÞ dependence attemperatures below the quenching of the BL band with thefollowing expression:IPLi0 ðTÞ ¼IPLi0 ð0Þ1þ C1eE1=kT þ C2eE2=kT (2)and extrapolating this dependence into higher temperatures.Here, IPLi0 ð0Þ¼ bgi0ð0ÞG is the integrated PL intensity in thelow-temperature limit and C1, C2, E1, and E2 are fitting pa-rameters that determine positions and slopes of lines in theArrhenius plot. The IPLi0 ðTÞ [or gi0ðTÞ] dependences for theexciton (i ¼ 2) and YL (i ¼ 3) bands are shown in Fig. 2with dashed and dash-dotted curves, respectively.SinceP4i¼1 gi ¼ 1 and each term gi with i > 1 containsthe hole concentration p as a factor, we can express p asp ¼ Gð1 g1ÞBnþ Cp3N3 þ Cp4N4 (3)and substitute this into expressions for the IQE of any chan-nel with i > 1. In the region of substantial thermal quenchingof the BL band (T  250 350K), intensities of other PLbands increase because g1 in Eq. (3) decreases. If the BLband did not quench, concentration of holes would bedescribed by the same Eq. (3) with g1 ¼ g10. Thus, by divid-ing gi on gi0 and cancelling out the terms that have the sametemperature dependence, we can express the relative changeof the integrated PL intensity IPLi =IPLi0 due to the quenchingof the BL band asIPLiIPLi0¼ gigi0¼ 1 g11 g10: (4)Note that the IPLi =IPLi0 ratios and their temperature depend-ence are identical for all recombination channels with i > 1.Recalling that g1 ¼ ðIPL1 =IPL10 Þg10, we see that g10 is the onlyfitting parameter in the fit of the experimentally found IPLi ðTÞdependences with Eq. (4) for channels with i > 1, providedthat we have the IPL1 ðTÞ=IPL10 ðTÞ dependence and simulatedthe IPLi0 ðTÞ (i > 1) dependences.The quenching of the BL band begins at much highertemperatures (T0 ¼ 300 K) than it is commonly observed(200–220K).1,8 This can be explained by the very smallPL lifetime (which is due to a high concentration of freeFIG. 2. (Color online) Temperature dependence of the absolute internal quan-tum efficiency of PL in GaN:Zn,Si at Pexc ¼ 0:0015W/cm2. Points are theexperimental data. Solid curve for the BL band shows g1 calculated by usingEq. (1) with the following parameters: g10 ¼ 0:97, g ¼ 2, CpA ¼ 106 cm3/s,s1 ¼ 0:5 ls, and EA ¼ 300meV. Dotted lines are extrapolations of the low-temperature and high-temperature parts of the g1ðTÞ dependence with cross-ing at T0. Dashed and dash-dotted lines show g20ðTÞ and g30ðTÞ calculated byusing Eq. (2) with the following parameters: g20ð0Þ ¼ 3:7 103, C1 ¼ 10,C2 ¼ 13, E1 ¼ 12, E2 ¼ 50meV (for the exciton band), g30ð0Þ ¼ 0:03,C1 ¼ 8, C2 ¼ 103, E1 ¼ 30, and E2 ¼ 200meV (for the YL band).G ¼ 2:4 1020 cm3s1 and b ¼ 4:2 1017 cm s for all the bands. Solidlines for the exciton and YL bands show g2ðTÞ and g3ðTÞ calculated by usingEq. (4) with g10 ¼ 0:92 and g10 ¼ 0:93, respectively.171110-2 Reshchikov et al. Appl. Phys. Lett. 99, 171110 (2011) 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, 31 Mar 2015 19:28:30electrons) and exceptionally high IQE of the BL band[g10 ¼ 0:97 from the fit of the experimental data for the BLband with Eq. (1)]. The best fits of the IPLi ðTÞ dependenceswith Eq. (4) for the exciton and YL bands are shown in Fig. 2with solid curves. We obtained g10 ¼ 0:92 and g10 ¼ 0:93 byfitting the IPLi ðTÞ dependences for the exciton and YL bands,respectively. Very close values of g10 obtained from analysisof two recombination channels provide confidence in validityof the results.In order to verify the exceptionally high IQE of the BLband by an independent method, we measured dependencesof the integrated PL intensities on excitation intensity at200K. We used a simple relation between the excitationpower density, Pexc, and the carrier generation rate:G ¼ 1:6 1023Pexc.7 Since the YL band intensity starts satu-rating at relatively low excitation intensity,1 only the excitonand BL bands were analyzed (Fig. 3). As expected, at low ex-citation intensity, the IQE for both bands is constant, i.e., theirintensities increase linearly with the excitation intensity. Athigh excitation intensity, the ZnGa acceptors become saturatedwith holes, and the BL band efficiency decreases.1 The de-pendence can be fitted with the model suggested in Ref. 9.More importantly, Eq. (4) can be used to fit the relativechange in the exciton band IQE and to find g10 with an inde-pendent method. The increase in g2 at G > 1024 cm3s1 iscaused by the decrease in the BL band IQE. From the fit, weobtain g10 ¼ 0:9060:05 for the BL band at 200K. Unfortu-nately, we were not able to determine g10 more accuratelywith this method since the excitation power densities above200W/cm2 were not available in our experiment. The advant-age of this method is that IPLi0 for the exciton emission is line-arly proportional to G in the entire range of the excitationintensities due to high concentration of free electrons in thestudied sample.Very high IQE of PL observed in this work does notcontradict to the relatively high density of threading disloca-tions, of the order of 109 cm2, expected for GaN layersgrown on sapphire by MOVPE. While the dislocations arecommonly considered as the main source of nonradiativerecombination in GaN, the Zn acceptors with concentrationabove 1018 cm3 and hole-capture cross-section of 1013 cm2(Ref. 1) can successfully compete with the nonradiativechannel for capture of minority carriers (holes), providedthat the doping does not create point nonradiative defects inlarge number.In summary, we have fabricated high quality MOVPEGaN layers co-doped with silicon and zinc and used threeapproaches to determine the absolute IQE of PL, namely, (1)the characteristic temperature T0 at which the thermalquenching of the BL band begins, (2) the value of theincrease in intensity of PL bands caused by the quenching ofthe BL band, and (3) the value of the rise of the relative IQEfor the exciton band caused by a decrease of the BL band ef-ficiency at high excitation intensities. With these independ-ent approaches, we have estimated the absolute IQE of theBL band as g10  0:9360:04 at T < 250K. The methodsused here are very accurate when the IQE approaches unitysince the size of the effect (e.g., an increase of PL intensitiesfor all PL bands caused by a quenching of a PL band withhigh IQE) becomes very large as compared to errors in mod-eling the IPLi0 ðTÞ or IPLi0 ðPexcÞ dependences. The nonradiativerecombination efficiency is less than 10% at 250K and 55%at room temperature.The work was supported by the BMBF under the project“Ephquam.”1M. A. Reshchikov and H. Morkoc¸, J. Appl. Phys. 97, 061301 (2005).2S. Nakamura, T. Mukai, and M. Senoh, Appl. Phys. Lett. 64, 1687 (1994).3H. Y. Ryu, K. H. Ha, J. H. Chae, K. S. Kim, J. S. Son, O. H. Nam, Y. J.Park, and J. I. Shim, Appl. Phys. Lett. 89, 171106 (2006).4Q. Dai, M. F. Schubert, M. H. Kim, J. K. Kim, E. F. Schubert, D. D.Koleske, M. H. Crawford, S. R. Lee, A. J. Fischer, G. Thaler, et al., Appl.Phys. Lett. 94, 111109 (2009).5X. Ni, J. Lee, M. Wu, X. Li, R. Shimada, U¨. O¨zgu¨r, A. A. Baski, H. Mor-koc¸, T. Paskova, G. Mulholland, et al., Appl. Phys. Lett. 95, 101106(2009).6H. Yoshida, M. Kuwabara, Y. Yamashita, K. Uchiyama, and H. Kan,Appl. Phys. Lett. 96, 211122 (2010).7M. A. Reshchikov, A. Kvasov, T. McMullen, M. F. Bishop, A. Usikov, V.Soukhoveev, and V. A. Dmitriev, Phys. Rev. B 84, 075212 (2011).8M. A. Reshchikov and R. Y. Korotkov, Phys. Rev. B 64, 115205 (2001).9M. A. Reshchikov, Appl. Phys. Lett. 88, 202104 (2006).FIG. 3. (Color online) Dependence of PL quantum efficiency on excitationintensity for the BL band and exciton band at T ¼ 200 K.G ¼ 1:6 1023  Pexc. Solid line for the exciton band shows g2ðGÞ calcu-lated by using Eq. (4) with g10 ¼ 0:90. Dashed lines show g2ðGÞ withg10 ¼ 0:95 (upper curve) and g10 ¼ 0:85 (lower curve).171110-3 Reshchikov et al. Appl. Phys. Lett. 99, 171110 (2011) This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. 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Extremely high absolute internal quantum efficiency of photoluminescence in co-doped GaN:Zn,Si

M. A. Reshchikov

A. G. Willyard

A. Behrends

A. Bakin

A. Waag

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Applied Physics Letters

http://scholarscompass.vcu.edu/cgi/viewcontent.cgi?article=1008&context=phys_pubs

Extremely high absolute internal quantum efficiency of photoluminescence in co-doped GaN:Zn,Si

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