Nuclear Instruments and Methods in Physics Research Bprojects.itn.pt/KLorenz/Nanomod/papers/NIMB_2013_Rodrigues.pdf · advantage that numerous combinations of materials can be

Nuclear Instruments and Methods in Physics Research B xxx (2013) xxx–xxx

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Nuclear Instruments and Methods in Physics Research B

journal homepage: www.elsevier .com/locate /n imb

A comparative study of photo-, cathodo- and ionoluminescence of GaNnanowires implanted with rare earth ions

J. Rodrigues a,⇑, S.M.C. Miranda b, M. Peres a, E. Nogales d, L.C. Alves b,c, E. Alves b,c, G. Tourbot e, B. Daudin e,B. Méndez d, K. Lorenz b,c, T. Monteiro a

a Departamento de Física e I3N, Universidade de Aveiro, Aveiro 3810-193, Portugalb IST/CFN, Instituto Superior Técnico, Campus de Física Nuclear, Universidade Técnica de Lisboa, EN10, Sacavém 2686-953, Portugalc Centro de Física Nuclear da Universidade de Lisboa, Av. Prof. Gama Pinto, Lisboa 1649-003, Portugald Departamento de Física de Materiales, Universidad Complutense, Madrid 28040, Spaine CEA/CNRS Group,‘‘Nanophysique et Semiconducteurs’’, INAC, CEA/Grenoble, 17 rue des Martyrs, 38054 Grenoble Cedex 9, France

a r t i c l e i n f o a b s t r a c t

Article history:Received 27 July 2012Received in revised form 10 December 2012Available online xxxx

Keywords:GaNNanowiresRare-earthIon implantationPLCLIL

0168-583X/$ - see front matter � 2013 Elsevier B.V.http://dx.doi.org/10.1016/j.nimb.2012.12.028

⇑ Corresponding author.E-mail address: [email protected] (J. Rodrigues

Please cite this article in press as: J. Rodrigues etearth ions, Nucl. Instr. Meth. B (2013), http://dx

GaN nanowires (NWs) implanted with Europium, Praseodymium and Erbium ions were analysed by Pho-toluminescence (PL), Cathodoluminescence (CL) and Ionoluminescence (IL). The red 5D0 ?

7F2 and3P0 ?

3F2 luminescence of the Eu3+ (4f6) and Pr3+ (4f2) ions, respectively, was optically activated afterthe lattice damage was recovered by thermal annealing. On the contrary, for the case of the erbiumimplanted NWs no intra-4f11 transitions were identified in the visible and infra-red spectral range.Besides the lanthanide luminescence, the heat treated GaN NWs exhibit the band edge recombinationand a deep level emission in the yellow spectral range when the samples are excited by photons, elec-trons and protons with energies of 3.8 eV, 5.0 keV and 2.0 MeV, respectively. At RT, the dependence ofGaN NW luminescence intensity with the illumination/irradiation time was analysed using PL, CL andIL. The effects of the different excitation mechanisms are discussed to explain the observation that thebroad emission bands suffer a luminescence quenching for the GaN NWs irradiated with energetic par-ticles and photons. The influence of the irradiation on the optical properties of the GaN NWs is discussedand models for the recombination processes are established.

� 2013 Elsevier B.V. All rights reserved.

1. Introduction

Semiconductor nanowires (NWs) have been widely studiedsince they constitute ideal building blocks for functional nanoscaleelectronic and photonic devices [1]. NW heteroepitaxy has theadvantage that numerous combinations of materials can beachieved since the NW synthesis prevents formation of disloca-tions originating from lattice mismatch [2]. The growth of thesenanostructures has been reported for several materials, includingnitride compounds [3]. Among the various techniques that canbe employed in the growth of gallium nitride (GaN) nanowiresthe most used ones are molecular beam epitaxy (MBE), metal or-ganic chemical vapour deposition (MOCVD) and hydride vapourphase epitaxy (HVPE) [4–7].

GaN NWs have been doped in order to modify their electrical,optical and magnetic properties. However doping these NWs is adifficult task due to low diffusion coefficients inhibiting ex-situdoping by diffusion [1] and the ‘‘self purification’’ effect posing

All rights reserved.

).

al., A comparative study of pho.doi.org/10.1016/j.nimb.2012.1

challenges for in situ doping during growth [8]. One approach todoping these nanostructures is by ion implantation [1]. This meth-od is a key technology in the semiconductor industry and allowsthe introduction of dopants in a controlled way and without thelimitations of the solubility of the dopant in the matrix [9]. The ma-jor disadvantage of this approach is the fact that defects are createdduring the implantation and thus post-implant thermal treatmentsare usually required in order to recover the crystal, which is diffi-cult in nanostructures since they exhibit a low thermal stabilitywhen compared to bulk materials [1].

Nitride systems proved to be important technological materialsfor lighting applications and one approach to tune visible lightemission is based on the incorporation of rare-earth (RE) ions inthe nitride hosts [9,10]. For further RE based optical device devel-opments the understanding of the luminescence stability shouldbe tested under harsh ambient conditions. A worthy method to ex-plore the produced effects is by using ion beam excitation imping-ing on the doped NWs. In a general way, the ion irradiation isexpected to produce damage and modifications in the lattice struc-ture leading to changes in the optical centres luminescence inten-sity [11].

to-, cathodo- and ionoluminescence of GaN nanowires implanted with rare2.028

http://dx.doi.org/10.1016/j.nimb.2012.12.028

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2 J. Rodrigues et al. / Nuclear Instruments and Methods in Physics Research B xxx (2013) xxx–xxx

In this work we report a comparison between photo-, cathodo-and ionoluminescence properties of GaN NW samples doped withdifferent RE ions. In all the cases, the excitation sources provideband to band excitation with 3.8 eV photons, 5.0 keV electronsand 2.0 MeV protons.

NWs were implanted with europium, praseodymium and er-bium ions, followed by thermal annealing treatments in order torecover the lattice damage and promote the RE ion activation. Inaddition, the mechanisms responsible for the observation of theyellow band in the implanted NWs are also discussed.

2. Experimental details

The GaN NWs were grown by radio frequency plasma-assistedmolecular beam epitaxy (MBE) on (111) Si substrates, as reportedelsewhere [5]. A thin (2–3 nm) AlN buffer layer was deposited priorto the NW growth which was performed for 2 h at 880 �C in nitro-gen rich conditions. The samples were implanted to a fluence of1 � 1013 at/cm2 with Er, Pr or Eu ions at 150 keV. The implantationwas carried out at room temperature and along the c-axis of thevertically aligned NWs. Post-implant rapid thermal annealing(RTA) was performed for 30 s at 1000 �C in flowing N2 using a hal-ogen lamp furnace.

Room temperature (RT) Raman spectroscopy (Horiba JobinYvon HR800) was measured in backscattering configuration byexciting the samples with the 532 nm laser line of a Ventus-LP-50085 (Material Laser Quantum).

Steady state photoluminescence (PL), performed with an angleof 90� between the laser beam and the sample surface, was gener-ated using the 325 nm light from a cw He–Cd laser and an excita-tion power density less than 0.6 W cm�2. The samples weremounted in a cold finger of a closed-cycle helium cryostat andthe sample temperature was controlled in a range from 14 K toRT. The luminescence was measured using a dispersive systemSPEX 1704 monochromator (1 m, 1200 mm�1) fitted with a cooledHamamatsu R928 photomultiplier tube.

RT Cathodoluminescence (CL) studies were performed in a Hit-achi S2500 SEM using a Hamamatsu PMA-11 charge coupling de-vice (CCD) camera, with an acceleration voltage Vacc = 5 kV. Thebeam was defocused to a diameter of several hundreds of microm-eters and the sample was tilted by 25�, thus the electron beam hitsboth the top surface and parts of the side facets of the NWs.

Ionoluminescence measurements were performed at theOM150 microprobe type end-station installed in IST/CTN using a2 MeV proton beam and a beam current of �1 nA. The ion beam in-duced luminescence is focused on an optical fibre, guided into amonochromator (Jobyn-Yvon/Horiba Triax 190), with a triple dif-fraction grating Turret (a 1200 or a 300 grooves/mm diffractiongrating can be chosen), and detected with a Peltier cooled CCD de-vice. The whole system sensitivity covers the radiation wavelengthrange between 300 and 1000 nm with a resolution between 0.3

Fig. 1. (a) Side view and (b) plane view SEM imag

Please cite this article in press as: J. Rodrigues et al., A comparative study of phoearth ions, Nucl. Instr. Meth. B (2013), http://dx.doi.org/10.1016/j.nimb.2012.1

and 2 nm depending on the used diffraction grating and entranceslit aperture. A more complete description may be found elsewhere[12]. The measurements were carried out at an incidence angle of5� between the beam and the sample surface and the quadrupolestriplet system used to adjust the focused beam to irradiate an areaof �250 � 250 lm2 at the surface. Although inhibiting beam scan-ning information this geometry increases the sensitivity to the thin(�70 nm) implanted top layer of the NWs. Several consecutivemeasurements (acquisition time of �20 s) were taken at each pointin order to study the effect of the proton irradiation. All the pre-sented luminescence spectra are uncorrected for the optics spec-tral response.

3. Results and discussion

3.1. Samples morphology and structural properties

SEM images of the NWs can be seen in Fig. 1 showing their goodvertical alignment (Fig. 1a). In Fig. 1b it is possible to observe thatthe NWs are randomly spaced. These NWs were characterized byRaman spectroscopy and the Raman spectrum indicates that theGaN NWs have high-crystalline quality. This structure belongs tothe C4

6v space group and taking into account the considerationsof group theory it was reported that eight sets of phonon modesare allowed at the C point: 2A1 + 2E1 + 2B1 + 2E2 [13]. One of theA1 and E1 modes are acoustic whereas the others are optical modes[13]. Among these vibrational modes only A1, E1 and E2 are Ramanactive. Since GaN is a noncentrosymmetric material the A1 and E2

modes are split into longitudinal optical (LO) and transversal opti-cal (TO) [14]. Fig. 2 shows a typical Raman spectrum of the hexag-onal GaN NWs where the A1 TO and LO modes and one of the E2

modes are identified. E2H denotes the higher frequency E2 mode.

These three modes appear at 538, 567 and 734 cm�1, respectively,which is in good agreement with the values found in literature[13]. Additional modes appear at 521, 673 and 890 cm�1. Thesemodes can be attributed to defect related phonons [15] or to therelaxation of the q = 0 selection rule due to surface disorder of fi-nite crystalline size allowing new modes that correspond toq – 0 phonons as reported elsewhere [14].

3.2. Rare earth luminescence in GaN NWs

Fig. 3 shows the 14 K PL (Fig. 3a, b and d) spectra upon excita-tion above the bandgap of the GaN NWs implanted with europiumand praseodymium ions along with the RT CL (Fig. 3c and e) and IL(Fig. 3f) spectra of Eu implanted samples. After annealing rareearth optical activation was achieved for europium and praseo-dymium ions. The samples exhibit the trivalent Eu3+ 5D0 ?

7F2

and Pr3+ 3P0 ?3F2 intra-4f transitions, overlapped with broad yel-

low and red emission bands. In the case of the Eu implanted sam-ple, the 5D0 ?

7F2 transitions appear at 620.6, 621.6 and 622.6 nm

es of the GaN nanowires before implantation.



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20000

25000

30000

35000

A1(TO) E2H

A1(LO)

Inte

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Fig. 2. RT Raman spectrum of the RE-doped GaN nanowires.

J. Rodrigues et al. / Nuclear Instruments and Methods in Physics Research B xxx (2013) xxx–xxx 3

(Fig. 3a) with the last two lines showing the strongest intensity.The main lines of the 3P0 ?

3F2 transitions of the Pr3+ ion werefound at 650.25 and 652.15 nm (Fig. 3b). These observations arein line with previous results measured for GaN layers doped withboth ions [9,10]. Since the peak position and spectral shape ofthe ions luminescence agrees with those observed in GaN layerswe can exclude their consignment from the AlN buffer and inferthat the RE ions were effectively incorporated into the GaN NWs,occupying Ga sites as also recently suggested by RT CL measure-ments for the praseodymium ions [16]. The temperature depen-dence of the Eu3+ and Pr3+ PL was analysed in the range between14 K and RT and it was found that due to nonradiative competitiveprocesses, the PL intensity of the lines decreased with increasingtemperature (only shown for Eu3+ ions in Fig. 3d). At RT the overallluminescence intensity of the ions luminescence corresponds to�40% and �43% of the measured values at 14 K for the Eu3+ andPr3+ ions, respectively, also in good agreement with previous mea-surements on thick GaN layers [17,18]. The use of energetic parti-cles instead of photons as excitation sources evidences that theintra-4f6 Eu3+ transitions are observed by CL and IL, as shown inFig. 3c, e and f. The rare-earth ion luminescence is overlapped witha broad emission band with a peak position dependent on the usedexcitation conditions (source and spot incidence and area). For thecase of the PL probed over the entire sample, the dominantluminescence is the yellow band (YB) with maximum at �530–560 nm (see Section 3.3). However, due to the PL band shapeasymmetry on the low energy side we cannot exclude that a smallcontribution of a red band could occur (see also Fig. 4b). Using elec-trons or protons as excitation sources, causes changes in the rela-tive intensity of the detected yellow and red bands with maximashifting to the red spectral region, in particular for the IL spectra(cf. Fig. 3d and f). It is well established that the proton irradiationprocess is expected to generate point defects [19–21]. In GaNlayers, the red luminescence band has been associated to nativedefect-complexes and can similarly be generated in neutron irradi-ated samples [22]. At this stage, we cannot rule out that the defectconcentration responsible for the red band could increase due tothe irradiation with high energetic electrons and protons.However, no increase of the red band is observed with increasingirradiation time (see Section 3.3). Also the probing depth is differ-ent for the three techniques but difficult to interpret for the case ofNW samples. The defect distribution in the NWs after implantationwill vary with depth. Furthermore, it is possible that even the deep,nominally unimplanted, regions of the NWs were damaged duringthe implantation due to sputtered ions from the side facets or thesubstrate. While IL, with a penetration depth of several microme-


tres, excites the entire NWs with relatively constant excitationdensity, PL and CL are more sensitive to the surface regions. Inthe case of CL, the electron beam impinges on the sample obliquelyso also the side facets will be excited. In the case of PL, with normalincidence, excitation is probably strongest from the implanted tipsof the NWs but excitation of deeper layers and light scatteringeffects cannot be excluded. A likely explanation for the observeddifferences is the variation of excitation density from the threetechniques. In the case of IL, the strong ionization processes occur-ring when the energetic proton penetrates the material may lead toa saturation of the yellow band putting in evidence the red band(with maximum at ca. 690 nm). The stability of the broad lumines-cence bands will be discussed in the next section.

Concerning the luminescence stability of the Eu3+ emission theresults of the CL analysis (Fig. 3c) indicate that the emission inten-sity is practically constant during 8 min of irradiation time. Thesame trend was found for the PL (not shown). For the IL, the lumi-nescence intensity quenches to �40% of the initial value after only3 min of irradiation (see inset in Fig. 3c). This behaviour for theproton irradiation exposure means that the Eu3+ luminescence isaffected by the generation of new defects by irradiation which de-stroys the stability of the optically active ions environment. It isalso worth mentioning that the Eu-luminescence line width isstrongly broadened in the IL spectra suggesting that different Eu-centres are excited simultaneously which are not excited in thePL and CL measurements.

In contrast to europium and praseodymium doped GaN NWs,no intraionic Er3+ luminescence (PL, CL or IL) was detected eitherin the visible (green) or infrared (�1.54 lm) spectral region. Forthis case the luminescence is dominated by an overlap of the broadyellow and red bands which will be matter of further discussion inthe next section.

3.3. The yellow band in implanted and annealed GaN NWs

As mentioned above, all the implanted and annealed samplesexhibit a broad YB PL centred at �530–560 nm with a tail extend-ing to the red spectral region. Fig. 4 shows a comparison betweenthe 14 K PL spectra of the as-grown GaN NWs and the Er implantedand annealed GaN NWs. The high optical quality of the as-grownGaN NWs is evidenced by a strong near band edge recombination,due to free and bound excitons [23] as shown in the inset of Fig. 4a.As expected no deep level yellow and red bands are observed. Con-trarily, after implantation and annealing the spectrum noticeablyevidences a broadening of the near band edge emission and thepresence of the broad bands. The existence of these bands suggeststhat the defects produced during the implantation and annealingare responsible for that emission. A YB is commonly observed inbulk GaN and GaN layers being one of the most studied opticallyactive defects in the gallium nitride system [24–27]. The chemicalnature of the defect from where the YB PL originates is still in de-bate being the VGaON and CN defects among the most mentioned inthe literature [28,29]. Besides the chemical nature of the defectalso the type of the transitions involved and the location of the de-fects in depth or at the surface of the GaN structures have beenmatter of study [30]. In particular, this identification assumes animportant role when dealing with nanostructures such as those ex-plored in this work. As shown in Fig. 4b the PL intensity of the YB isseen to decrease as the temperature rises, being its internal quan-tum efficiency �40% (determined as the ratio of PL intensities at RTand 14 K), in line with the other reports [31]. However, care mustbe taken by considering this value for the YB in the implanted andannealed GaN NWs since photodegradation of the emitting defectoccurs as a function of increasing photon illumination time (Fig. 5aand b), in contrast to the PL behaviour of the YB in bulk and GaNlayers. The luminescence degradation was also observed under



616 618 620 622 624 626

Eu3+ 5D0→7F

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PL

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T=14 K

Pr3+3P

0→3F

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600 605 610 615 620 625 630

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Eu

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IL in

ten

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(ar

b. u

nit

s)

RTEu3+ 5D0→7F

2

(f)

Fig. 3. 14 K PL spectra of (a) Eu3+ and (b) Pr implanted and annealed GaN nanowires obtained with 325 nm excitation. (c) Exposure time dependence of the Eu3+ CL. The insetshows the integrated CL and IL Eu3+ emission intensity as a function of the exposure time, (d) 14 K and RT PL and (e) and (f) RT CL and IL spectra of the Eu-doped sample.

350 400 450 500 550 600

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Fig. 4. (a) 14 K PL spectra of as-grown and (b) temperature dependent PL spectra of Er-implanted and annealed GaN nanowires obtained with 325 nm excitation.


particle irradiation as indicated by the CL and IL spectra shown inFig. 5c and d, respectively.

The normalized broad luminescence intensity as a function ofthe exposure time is shown in Fig. 6 for the PL, CL and IL cases.


The measurements presented in Fig. 6 were performed at RT intwo experimental environments considering the luminescenceevolution in air (PL) and vacuum (PL, CL and IL). Under photon illu-mination the band photodegradation is higher for the case of the



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(d)

Fig. 5. Evolution of the yellow and red luminescence bands with the exposition time for (a) PL performed in air and (b) vacuum, (c) CL and (d) IL measurements.

0 20 40 60 80 1001201401601802002202400.0

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Fig. 6. Maximum peak (a) PL, (b) CL and (c) IL intensity of the yellow band for the Er-implanted and annealed GaN nanowires as a function of the exposition time.

J. Rodrigues et al. / Nuclear Instruments and Methods in Physics Research B xxx (2013) xxx–xxx 5

GaN NWs in air than for the vacuum case (Fig. 6a). In the first case,the YB PL peak intensity decreased about 70% in 240 min while invacuum environment a minimal degradation occurs (the PL inten-sity suffers a decrease of less than 30% for the same exposure time).The luminescence quenching is well described by long (hundredsof minutes) exponential and bi-exponential decay times for theGaN NWs in air and vacuum, respectively. The formation of latticedefects acting as nonradiative centres such as aggregates and dis-location loops have been pointed out as one of the possible mech-anisms to explain the luminescence intensity decay insemiconductor nanostructures [32,33]. Dissociation of defect com-plexes can also be responsible for this behaviour. Since the PLrecombination degradation was found to be sensitive to the sampleenvironment other surface mediated processes may also accountfor the instability of the YB PL. A higher level of luminescence deg-radation is observed with particle irradiation exposure time, asshown in Fig. 6b and c for CL and IL measurements, respectively.In the case of CL, the intensity decreases to about 80% after the9 min of exposure time exhibiting a linear trend. For the case ofthe IL, the YB luminescence is totally quenched after �3 min of


exposure. The behaviour is well accounted for by a bi-exponentialdecay with time constants of 35 s and 3.5 min. The irradiation withhigh energetic particles can induce more structural damages in theNWs leading to a higher content of nonradiative paths, conductingto the fast decrease of the luminescence intensity. The high energybeam may also cause an increase of the local temperature in thesample during the exposure time which could lead to a quenchingdue to competitive nonradiative pathways as those observed dur-ing temperature dependent PL measurements. However, for all theused excitation processes no recovery of the luminescence wasfound after interruption of the excitation source for large periodsof time excluding such thermal quenching as decisive factor.

3.4. Ionoluminescence as a characterization tool for NW-based opticalmaterials

The results presented above show distinct luminescence char-acteristics of defect bands as well as rare earth luminescencedepending on the excitation conditions used in IL, PL and CL mea-surements. While PL and CL are often combined in order to assess




the properties of optical materials, IL analysis is still relativelyscarce. Albeit preliminary, our results demonstrate the feasibilityof IL characterization even on nanostructured materials withdimensions well below the beam diameter. Besides the possibilityfor in situ studies of irradiation effects on optical properties, it wasshown that the distinct excitation conditions (in particular excita-tion density and depth) allow discrimination between differentfeatures in the luminescence spectra. In future work, it would beinteresting to increase the lateral resolution of the measurementsusing micro- or even nano-beams. For some nano- or micro-struc-tures, like this it would be possible to measure single structures. Itwould also open the possibility to measure full light emitting de-vice (LED) structures with lateral resolution. In this sense, microp-robes have been previously used to study the degradationmechanisms of GaN-based LEDs using the IBIC technique [34,35]and simultaneous IL measurements would add significantly tosuch analysis.

4. Conclusions

High optical quality MBE GaN NWs were implanted with rareearth ions followed by subsequent high temperature thermalannealing treatments. This process favoured on the one hand theeuropium and praseodymium optical activation and on the otherhand promoted the appearing of broad luminescence bands inthe yellow and red spectral regions. Under photon and electronexcitation the europium emission was found to be stable withtime, however with proton irradiation the intraionic luminescencedecreased by �60% in a few minutes.

The yellow luminescence observed in the implanted and an-nealed GaN NWs was found to exhibit a distinct behaviour withthe illumination/irradiation time when compared with the oneusually observed in both GaN bulk crystals and layers. The lumi-nescence degradation depends on the environment media andexcitation source meaning that both surface adsorbed species (asrevealed from the in-vacuum and in-air PL spectra) and defects(CL and IL spectra) must be considered has potential sources ofluminescence killers.

Acknowledgments

Funding by FCT Portugal (Ciência 2007, PTDC/CTM/100756/2008, PTDC/CTM-NAN/2156/2012, PEst-C/CTM/LA0025/2011, RECI/FIS-NAN/0183/2012 and the Portugese - French bilateral collabo-ration program PESSOA) is gratefully acknowledged. J. Rodriguesand M. Peres thank FCT for their PhD Grant, SFRH/BD/76300/2011 and SFRH/BD/45774/2008, respectively.


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