High quality ZnO crystals with the sharp band-edge excitonic emission and very weak green emission were implanted by nitrogen ions. An additional red emission band was observed in the as-implanted ZnO crystal and investigated as a function of temperature. By employing the underdamped multimode Brownian oscillator model for the general electron-phonon coupling system, both the original green and nitrogen-implantation induced red emission bands were theoretically reproduced at different temperatures. Excellent agreement between the theory and the experiment enables us determine the energetic positions of the pure electronic levels associated with the green and red emission bands, respectively. The determined energy level of the red emission band is in good agreement with the data obtained from the deep-level transient spectroscopy measurements. © 2012 American Institute of Physics.published_or_final_versio

Anwand, W

Brauer, G

Dai, XM

Ling, CC

Skorupa, W

Xu, SJ

Journal of Applied Physics

HKU Scholars Hub

Title Emission bands of nitrogen-implantation induced luminescentcenters in ZnO crystals: experiment and theoryAuthor(s) Dai, XM; Xu, SJ; Ling, CC; Brauer, G; Anwand, W; Skorupa, WCitation Journal of Applied Physics, 2012, v. 112 n. 4, article no. 046102Issued Date 2012URL http://hdl.handle.net/10722/164515Rights Creative Commons: Attribution 3.0 Hong Kong LicenseEmission bands of nitrogen-implantation induced luminescent centersin ZnO crystals: Experiment and theoryX. M. Dai,1 S. J. Xu,1,a) C. C. Ling,1 G. Brauer,2 W. Anwand,2 and W. Skorupa21Department of Physics, The University of Hong Kong, Pokfulam Road, Hong Kong, China2Institut f€ur Ionenstrahlphysik und Materialforschung, Forschungszentrum Dresden-Rossendorf,Postfach 510119, D-01314 Dresden, Germany(Received 24 May 2012; accepted 9 August 2012; published online 30 August 2012)High quality ZnO crystals with the sharp band-edge excitonic emission and very weak greenemission were implanted by nitrogen ions. An additional red emission band was observed in the as-implanted ZnO crystal and investigated as a function of temperature. By employing theunderdamped multimode Brownian oscillator model for the general electron-phonon couplingsystem, both the original green and nitrogen-implantation induced red emission bands weretheoretically reproduced at different temperatures. Excellent agreement between the theory and theexperiment enables us determine the energetic positions of the pure electronic levels associatedwith the green and red emission bands, respectively. The determined energy level of the redemission band is in good agreement with the data obtained from the deep-level transientspectroscopy measurements.VC 2012 American Institute of Physics.[http://dx.doi.org/10.1063/1.4749402]Zinc oxide (ZnO) has been a technologically importantmaterial in a lot of fields due to its unique optical and electri-cal properties. Recently, it has attracted a renewed consider-able interest due to its potential applications in photonics andoptoelectronics.1 However, there are still some unsettledproblems that stand in the way of realizing these uses.Among them are the difficulty of p-type doping and theinsufficient understanding of the various defects.2,3 As-grown ZnO always exhibits n-type electrical conductivity inthe absence of intentional doping, and it is difficult to get re-producible and concentration-controllable p-type ZnO.Nitrogen has been used as the p-type dopant in ZnO andsome progress has been achieved along this direction.4Defect-related broad emissions are commonly observed inthe luminescence spectra of N-doped ZnO,4 but the origin ofthe defects is far from full understanding.5 Getting a betterunderstanding to these defects in ZnO is highly desirable.For nitrogen-implanted ZnO, previous deep-level transientspectroscopy (DLTS) measurements show that there are twomain deep levels located at 0.31 eV (E3) and 0.95 eV (D1)below the conduction band.6 The former is commonly foundin as-grown ZnO single crystals,6,7 while the latter is con-firmed to be created by the nitrogen implantation.6 On theother hand, deep centers in semiconductors and polar crystalsare often optically active, which offers us the chance toinvestigate them with optical methods such as the photolumi-nescence (PL) technique.In the present work, we attempt to obtain a better under-standing of optically active deep centers in nitrogen-implantedZnO. In our study, a number of pre- and nitrogen-implantedZnO samples were studied by the variable-temperature PLtechnique. In addition to the weak original green emission, ared emission band from the implanted samples was observed.By employing the underdamped multimode Brownian oscilla-tor (MBO) model recently developed for electron-phonon cou-pling systems,8,9 both the green and red emission bands inZnO were theoretically reproduced. The pure electronic levelsof the green and red emissions are, thus, determined and foundto be consistent with the one of the DLTS detected levels(0.9–1.0 eV below EC) very well, which indicates that thisN-implantation induced deep centers are optically and electri-cally active.The ZnO (0001) samples used in the experiments werecommercially obtained from Cermet Inc., USA. The studiedsamples include one pre-implantation control sample and twoimplanted with different nitrogen ion doses: 1013 cm2 and1014 cm2. During nitrogen implantation, the accelerationvoltage of the nitrogen ion beam was 150 keV while the sam-ple temperature was set at 300 C. The implantation processand the ZnO samples used were identical to those employed inthe previous DLTS study.6 Variable-temperature photolumi-nescence experiments were carried out on a PL system with aJanis closed cycle cryostat providing a temperature range from4 to 300K. The 325 nm line of a Kimmon He-Cd continuous-wave laser with an output power of 30 mW was employed asthe excitation light source. Standard lock-in amplifier technol-ogy was also used to enhance the signal-to-noise ratio. Otherdetails of the PL setup have been described elsewhere.10Figure 1 shows the PL spectra in the visible spectralrange measured from the pre-implantation sample at differ-ent temperatures. The inset illustrates the 4K band-edgeemissions of the sample in the ultraviolet (UV) spectralregion. As reported in the literature,11 our pre-implantationsample emits dominant band-edge excitonic transitions and avery weak green band centered at about 500 nm when it wasexcited by the 325 nm UV laser, which indicates the highquality of the sample used in this study. Under the identicalexperimental conditions, PL spectra of the N-implanted sam-ples were also measured at different temperatures. Unlikea)Author to whom correspondence should be addressed. Electronic mail:sjxu@hku.hk.0021-8979/2012/112(4)/046102/3/$30.00 VC 2012 American Institute of Physics112, 046102-1JOURNAL OF APPLIED PHYSICS 112, 046102 (2012)Downloaded 19 Nov 2012 to 147.8.230.79. Redistribution subject to AIP license or copyright; see http://jap.aip.org/about/rights_and_permissionsthe pre-implantation sample, the two implanted samples atlow temperatures emit a noticeable red light band centered at620 nm instead of the weak green emission, as shown inFig. 2. Similar red emission band was also recently observedin N-implanted ZnO bulk by Pan et al.12 As the temperatureincreases, the intensity of the red emission band goes downaccompanied by a blueshift of the emission peak at highertemperatures above 200K. In the mean time, the relative in-tensity of the green luminescence band increases andbecomes comparable to the red luminescence band at hightemperatures. These phenomena indicate that the defectinteractions may exist in the N-implanted ZnO and lead tocomplex luminescence mechanisms of defects.It is known that the color centers in wide gap polar crys-tals such as ZnO are usually treated as strong electron-phononcoupling systems.13 In the pioneer theoretical models, onlycoupling between electron and primary phonon such as longi-tudinal optical (LO) phonon was considered.14 Dissipationfrom an acoustical phonon bath was not taken into account. Inthe newly developed MBO model,8,9 some primary nuclearcoordinates couple linearly to an electronic two-level systemand simultaneously to a harmonic bath. Recently, the MBOmodel has been employed to reproduce PL spectra of InGaNquantum dots spontaneously formed in GaN matrix15 and thestructured green emission band in ZnO,13 respectively. Here,we use the model to calculate both the red and green emissionbands observed in the N-implanted ZnO. Following the previ-ous study,13 71.2meV or 574.2 cm1 (the characteristicenergy of the LO phonon in wurzite ZnO) was adopted as theenergy of the primary oscillator in our calculations. The fre-quency xLO of the primary oscillator, the damping coefficientc characterizing the coupling strength of the primary oscillatorand the bath modes, and Huang-Rhys factor S revealing thestrength of the electron-LO phonon coupling, are the threekey parameters of the MBO model.In Fig. 2, the solid lines are the theoretical PL spectra of1013 cm2 N-implanted sample at different temperatures byusing the MBO model. For this sample, considering that thegreen emission band is relatively very weak at temperaturesbelow 100K, we took it into account in the theoretical calcula-tions only at temperatures higher than 100K. Such treatmentleads to some discrepancy between theory and experiment atlow temperatures for wavelengths below 575 nm. FromFig. 2, it can be seen that a good agreement between theexperiment and the theory is achieved in the whole interestedtemperature range. The parameter values used in a typical cal-culation (i.e., T¼ 200K) were hxoeg1 ¼ 2:92 eV, S1 ¼ 7:00,c1 ¼ 65, and hxoeg2 ¼ 2:38 eV, S2 ¼ 5:25, c2 ¼ 150. The sub-script 1(2) is for the green(red) emission. The weights for thegreen and red bands are 0.61 and 0.39, respectively.The first point that we would like to address in this sec-tion is the determination of the deep center levels from theirbroadening luminescence spectra due to the electron-phononcoupling. According to the electron-phonon coupling theoryof color centers in wide gap crystals and semiconductors,14strong electron-phonon coupling, i.e., Huang-Rhys factorS 1, induces a red-shift (ShxLO) of the emission spectralcenter (peak) from the zero-phonon line (ZPL). In otherwords, only ZPL gives the real energetic position of the colorcenter (defect) in these cases. From the ZPL values adoptedin our calculations, the energetic positions of deep centersproducing the green and red emission bands are determinedto be 2.92 and 2.38 eV, respectively. Considering 3.30 eVband gap of ZnO at room temperature, the defect levels ofthe green and red emission bands could be located at around0.38 eV and 0.92 eV below the bottom of conduction band.The 0.92 eV level coincides well with the D1 deep trap asdetected in the previous DLTS study6 which involved theidentical N-implantation and ZnO samples. It is worthy topoint out that the D1 deep traps were also observed in the N-implanted ZnO sample after the identical annealing processat 300 C.6 It is thus plausible to conclude that the D1 trap isan intrinsic defect.It is known that kinetic ions implanted into target mate-rial would collide with host atoms and subsequently pushFIG. 1. PL spectra of the control sample at different temperatures. A weakgreen emission band centered at 500 nm can be found. The inset shows4K band-edge emissions of the sample.FIG. 2. Experimental and theoretical PL spectra at different temperaturesfor the 1013 cm2 N-implanted sample. The open symbols represent the ex-perimental data while the solid lines are the theoretical spectra. Appropriateoffset shifts in the vertical axis and the magnifications (times were marked)in the intensity were done to the curves for the simple clarity.046102-2 Dai et al. J. Appl. Phys. 112, 046102 (2012)Downloaded 19 Nov 2012 to 147.8.230.79. Redistribution subject to AIP license or copyright; see http://jap.aip.org/about/rights_and_permissionsthem away from their lattice sites. Therefore, the implanta-tion always produces some intrinsic point defects such asvacancies and local lattice distortion in the targeted crystal.It is also known that oxygen vacancies have a low defect for-mation energy in the ZnO material, which has been demon-strated by first-principles calculations.16–19 Along with thisline, the dominant electron deep levels (D1) in as-implantedn-ZnO could be tentatively assigned to the oxygen vacancyVO.6 The positron annihilation spectroscopy study carriedout by Tuomisto et al. showed that the primary defect in theas-electron-irradiated ZnO (with electron energy of 2 MeV)was also identified as VO whose concentration is 10 timeslarger than that of VZn.20 Furthermore, the VO was also iden-tified to be the dominant donor-type defect in the 2.5 MeVelectron irradiated ZnO by using optical detection of electronparamagnetic resonance21 and in Pþ-implanted ZnO byRaman study.22 Vlasenko and Watkins suggested that theZPL of the defect emission centered at 600 nm of theelectron-irradiated ZnO was at 500 nm (2.48 eV),21 whichwas close to the ZPL of the red emission band in our N-implanted sample. Before the electron-irradiation, their con-trol ZnO samples emit a typical structureless green lumines-cence band when excited by an UV laser. Observations ofthe similar red emission bands in our N-implanted ZnO andin Vlasenko et al. electron-irradiated samples indicate thatone possible origin of the defects producing the red emissionband is one kind of intrinsic defect, most likely oxygen va-cancy. When nitrogen ions were injected into the ZnO crys-tal, they collide with the oxygen host atoms and make someoxygen atoms away from their equilibrium lattice sites. Suchcollision thus leads to generation of more oxygen vacancies.Furthermore, the charge particle implantation likely altersthe charge state of most oxygen vacancies. According to Vande Walle’s theoretical calculations,23,24 the appearing of atransition level E (2þ/0) offers the opportunity to emit a redlight. The electron-LO phonon coupling and dissipation ofcontinuous acoustic phonon bath also affect this lumines-cence process. Therefore, a broad red emission band is gen-erated. From these available theoretical and experimentalstudies, the oxygen vacancy is the most possible microstruc-tural origin causing the red emission band observed inN-implanted ZnO. As for the commonly observed greenemission band without the periodic fine structures in ZnO, theoxygen vacancy, but having the neutral charge state, is stillthe possible origin.25 From the electronic configurations ofoxygen and zinc atoms as well as the unit crystalline structureof wurtzite ZnO, an oxygen vacancy could have three differ-ent charge states: the neutral, 1þ, and 2þ, corresponding tothe states of losing 0, 1, and 2 electrons, respectively.23 Forthe structured green emission band in ZnO, copper impuritiesin ZnO are widely thought to be the resource because it isalways observed in Cu-doped ZnO.13,26,27 However, thedefect characterization and firm identification in ZnO are stilla challenging issue.2,3,5A set of ZnO bulk crystals, including the as-grown andthe ones implanted with different doses, have been studied inour present work. Two different colorful emissions, namelythe common green band and the N-implantation-caused redband, have been investigated both experimentally and theo-retically. DLTS have detected two signals, namely D1 andE3 in the samples. The pure electronic level location of theimplantation-induced red emission band was determined tobe 0.92 eV below the conduction band, which is in goodagreement with the D1 center (0.93 eV) detected by DLTS.Combining with the recent first-principles calculations andthe other available experimental results, we suggest that theoxygen vacancy is the most possible origin of the N-implan-tation-caused red emission band in ZnO.This work was financially supported by the Hong KongRGC-GRF Grants (Grant Nos. HKU705606P and 703612P),the Joint Research Fund for Overseas Chinese, Hong Kongand Macau Scientists of NSFC (Grant No. 61028012) andthe HKU matching fund. One of the authors, SJX, would liketo thank C. C. Zheng for his technical assistance in the re-plotting and format conversion of Fig. 2.1€U. Ozg€ur, Y. I. Alivov, C. Liu, A. Teke, M. A. Reshchikov, S. Dogan, V.Avrutin, S.-J. Cho, and H. Morkoc¸, J. Appl. Phys. 98, 041301 (2005).2D. C. Look, Mater. Sci. Eng. B 80, 383 (2001).3M. D. McCluskey and S. J. Jokela, J. Appl. Phys. 106, 071101 (2009).4A. Tsukazaki, A. Ohtomo, T. Onuma, M. Ohtani, T. Makino, M. Sumiya,K. Ohtani, S. F. Chichibu, S. Fuke, Y. Segawa, H. Ohno, H. Koinuma, andM. Kawasaki, Nature Mater. 4, 42 (2005).5A. Janotti and C. G. Van de Walle, Rep. Prog. Phys. 72, 126501 (2009).6Q. L. Gu, C. C. Ling, G. Brauer, W. Anwand, W. Skorupa, Y. F. Hsu,A. B. Djurisic´, C. Y. Zhu, S. Fung, and L. W. Lu, Appl. Phys. Lett. 92,222109 (2008).7J. C. Simpson and J. F. Cordaro, J. Appl. Phys. 63, 1781 (1988).8S. Mukamel, Principles of Nonlinear Optical Spectroscopy (Oxford Uni-versity Press, New York, 1995), p. 226.9Y. Zhao and R. S. Knox, J. Phys. Chem. A 104, 7751 (2000).10S. J. Xu, W. 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Vlasenko and G. D. Watkins, Phys. Rev. B 71, 125210 (2005).22Z. Q. Chen, A. Kawasuso, Y. Xu, H. Naramoto, X. L. Yuan, T. Sekiguchi,R. Suzuki, and T. Ohdaira, J. Appl. Phys. 97, 13528 (2005).23C. G. Van de Walle, Physica B 308, 899 (2001).24A. Janotti and C. G. Van de Walle, Appl. Phys. Lett. 87, 122102 (2005).25Y. W. Heo, D. P. Norton, and S. J. Pearton, J. Appl. Phys. 98, 0735023(2005).26R. Dingle, Phys. Rev. Lett. 23, 579 (1969).27N. Y. Graces, L. Wang, L. Bai, N. C. Giles, L. E. Halliburton, and G. Can-twell, Appl. Phys. Lett. 81, 622 (2002).046102-3 Dai et al. J. Appl. Phys. 112, 046102 (2012)Downloaded 19 Nov 2012 to 147.8.230.79. Redistribution subject to AIP license or copyright; see http://jap.aip.org/about/rights_and_permissions

Emission bands of nitrogen-implantation induced luminescent centers in ZnO crystals: experiment and theory

X. M. Dai

S. J. Xu

C. C. Ling

G. Brauer

W. Anwand

W. Skorupa

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Emission bands of nitrogen-implantation induced luminescent centers in ZnO crystals: Experiment and theory

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Emission bands of nitrogen-implantation induced luminescent centers in ZnO crystals: experiment and theory

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