Materials Science and Engineering B105 (2003) 101–105

Er-defect complexes and isolated Er centerspectroscopy in Er-implanted GaN

A. Brauda,∗, J.L. Doualana, R. Moncorgea, B. Pipeleersb, A. Vantommeb

a CIRIL—ISMRA, 6 Boulevard Maréchal Juin, 14050 Caen Cedex, Franceb Departement Natuurkunde, Instituut voor Kern-en Stralingsfysica, Celestijnenlaan 200 D, 3001 Leuven, Belgium

Abstract

Photoluminescence (PL), photoluminescence excitation (PLE) spectra and luminescence decay of the Er3+ 4I13/2 →4I15/2 transition areinvestigated at 7 K in Er-implanted GaN samples. Under below-gap excitation, PL and PLE spectra reveal the existence of two types of Ercenters. One type of Er center which can only be excited by resonant intra 4f shell transition is predominant while other Er centers are clearlyexcited via local defects. Evolution of Er luminescence as a function of implantation dose and implantation geometry is presented for bothtypes of Er centers. Luminescence dynamics study shows that the4I13/2 manifold has a shorter lifetime (τ ∼ 1 ms) when Er ions are part ofEr-defect complexes than when Er ions are isolated from any defect (τ = 3.8 ms). This result indicates the existence of non-radiative energytransfers in Er-defect complexes from Er ions towards defects or impurities. Er decay in Er-defect complexes is then successfully comparedwith classical energy transfer models.© 2003 Elsevier B.V. All rights reserved.

PACS: 78.55; 78.20

Keywords: Erbium; GaN; Photoluminescence excitation; Energy transfer

1. Introduction

Rare-earth doped GaN is being widely studied for itsvarious applications in optoelectronics[1–4]. This interestis motivated by the unique optical properties of rare-earthdoped materials. Optically active 4f electrons of rare earthions are shielded from the influence of the local crystal fieldby outer 5s and 5p shells of electrons. Therefore, energylevels of the 4fn configurations are only slightly differentfrom free ion energy levels. Rare-earth emission in rare-earthdoped semiconductors such as InP, GaAs or Si is limited bystrong emission quenching processes[5]. In contrast GaN isa wide-bandgap semiconductor and leads thus to a reducedrare-earth emission quenching.

Questions still remain about the fundamental understand-ing of the mechanisms underlying the excitation of rare-earthions in this host. In order to improve the performance ofrare-earth doped GaN devices, it is important to deepen our

∗ Corresponding author. Tel.:+33-2-31-45-25-60;fax: +33-2-31-45-25-57.

E-mail address: alain.braud@ismra.fr (A. Braud).

understanding of the incorporation of rare-earth ions in GaNand the excitation processes of the 4f-shell. We present inthis paper new spectroscopic properties of different types ofEr centers in Er-implanted GaN samples.

2. Experimental details

The GaN wafer prepared by metalorganic chemical vapordeposition (MOCVD) was mounted on a two-axis goniome-ter and subsequently implanted with 80 keV Er ions at roomtemperature with a dose of either 2.5 × 1014 or 5 × 1014

Er/cm2. During implantation, the GaN〈0 0 0 1〉 axis was ei-ther aligned with the ion beam (‘channeled’ implantation)or tilted by 10◦ (‘random’ implantation). Subsequently, thesamples were annealed in a tube furnace at 950◦C for 30 minunder flowing nitrogen using unimplanted GaN as a prox-imity cap.

The Er-implanted samples were mounted in a APDliquid helium cryostat and cooled down to 7 K. PL andPLE studies were performed by exciting the GaN:Er sam-ples with a CW tunable Ti:Sapphire laser or Ar laser. For

0921-5107/$ – see front matter © 2003 Elsevier B.V. All rights reserved.doi:10.1016/j.mseb.2003.08.024

102 A. Braud et al. / Materials Science and Engineering B105 (2003) 101–105

1520 1540 1560 1580 1600 16200,0

0,2

0,4

0,6

0,8

1,0

λex citation

= 800 nm

Wavelength (nm)

PL

inte

nsi

ty (

a.u

.)

1520 1540 1560 1580 1600 16200,0

0,2

0,4

0,6

0,8

1,0

λex citation

= 809 nm

(a)

(b)

Fig. 1. 1.54�m PL spectra recorded at 7 K for different excitation wavelengths (a) resonant excitation and (b) non-resonant excitation.

luminescence decay measurements the laser source wasmodulated by means of an acousto-optic modulator. Infraredluminescence was recorded using a 0.75 m-monochromatorequipped with a thermo-electric cooled InGaAs photodiode.The monochromator resolution was kept below 0.6 nm forall spectra. The PL and PLE signals were processed usinglock-in techniques.

800 802 804 806 808 810 812 814 816 818 820 822 8240,0

0,5

1,0

1,5

2,0

2,5

λdetect ion

=1542nm

λdetect ion

=1538nm

PL

E i

nte

nsi

ty (

a.u

)

Excitation wavelength (nm)

Fig. 2. PLE spectra around 800 nm recorded at 7 K for different detection wavelengths.

3. Results and discussion

Erbium ions are excited resonantly at 809 nm within theEr3+ 4I15/2 →4I9/2 transition. The PL spectrum recordedaround 1.54�m at 7 K is the same for all samples and shownin Fig. 1a. In this specific case the PL spectrum resolu-tion is 0.1 nm. The structure composed of sharp lines is

A. Braud et al. / Materials Science and Engineering B105 (2003) 101–105 103

characteristic of intra 4f shell transitions. Each line corre-sponds to a Stark to Stark sublevel transition. The typicallinewidth of these lines which is about 0.3 nm indicates thatthe Er local site symmetry is not distorted.

Excitation of Er-implanted GaN by a non-resonant wave-length of 800 nm leads to a different PL spectrum. This PLspectrum presented inFig. 1b is made of a large numberof lines indicating that different subsets of Er centers withvarious local site symmetries contribute to this spectrum.Er luminescence in case of non-resonant excitation has al-ready been observed[6,7]. The excitation of these Er cen-ters involves broad, below-gap optical absorption by defects,impurities or defect–impurity complexes with subsequentnon-radiative transfer of the energy to nearby Er emittingions rather than direct intra-4f shell absorption.

In order to verify this assumption PLE spectra around800 nm are recorded and presented inFig. 2. For a detec-tion wavelength of 1538 nm the PLE spectrum exhibits twocomponents. The first component consists in sharp lines cor-responding to inter Stark sublevel transitions involved inthe4I15/2 →4I9/2 transition. The second component of thisPLE spectrum is a broad absorption band which is part ofthe absorption spectrum of the above-mentioned local de-fect. Thus, the comparison of PL and PLE spectra enablesus to identify two types of Er centers: Er ions in a sitewith a well-defined symmetry which will be referred to as“regular” Er ions and Er ions in a distorted local site associ-ated with local defects or impurities which will be referredto as Er-defect complexes.

In case of resonant excitation the PL spectrum is thusa mixture of luminescence from “regular” Er and fromEr-defect complexes. It is possible with the PLE spectrumrecorded at 1538 nm to separate the luminescence contribu-tions of “regular” Er and Er-defect complexes. For instancewith an excitation wavelength of 809 nm, for which theluminescence contribution of “regular” Er is the largest,the PLE spectrum shows that 17% of the luminescence at1538 nm comes from Er-defect complexes. Therefore, thePL spectrum changes according to the excitation wave-length since the luminescence contribution of both types ofEr centers depends on the excitation wavelength.

A second PLE spectrum is displayed inFig. 2 for aslightly different detection wavelength of 1542 nm. Thiswavelength is characteristic of the Er-defect complexes PLspectrum. In this case the PLE spectrum only exhibits abroad absorption band related to local defects. The fact thatthere is no sharp lines characteristic of intra 4f shell tran-sitions indicates that the number of Er-defect complexes isvery small compared to the number of “regular” Er ions.RBS measurements performed on these samples show thata large percentage (from 92 to 70% depending on the sam-ple) of Er ions occupies a Ga substitutional site. Therefore,“regular” Er ions are likely to occupy a Ga substitutionalsite. Despite the small number of Er-defect complexes, theluminescence from these Er-defect complexes is not neg-ligible which implies that the absorption cross-section of

0 1 2 3 4 50,0

0,5

1,0

1,5

2,0

2,5

3,0

3,5

4,0

Dose (1014at/cm2)

PL

E i

nte

nsi

ty (

a.u

.)

Er-defect (Channeled)

Er-defect (Random)

0 1 2 3 4 50,0

0,5

1,0

1,5

2,0

2,5

3,0

Regular Er (Channeled)

Regular Er (Random)

Fig. 3. Evolution of integrated PLE intensity as a function of implantationdose and implantation geometry for “regular” Er ions and Er-defectcomplexes.

these local defects is larger than the intra-4f-shell absorptioncross-section.

It is to be noted that these conclusions only concern op-tically active Er-defect complexes and not the overall pop-ulation of Er-defect complexes which may be different incase of strong luminescence quenching. The PLE spectrumrecorded at 1538 nm enables the discrimination between theluminescence contribution of either type of Er center. Theintegrated luminescence intensity of “regular” Er ions iscompared using PLE spectra of four samples with differ-ent implantation doses and implantation geometry as pre-sented inFig. 3. The luminescence intensity of “regular”Er ions is slightly larger in case of channeled implanta-tion. This is consistent with RBS measurements which showthat the Er substitutional fraction is larger for the chan-neled implantation. BesidesFig. 3 shows that the lumines-cence intensity of “regular” Er ions is linearly dependenton the Er dose for both random and channeled implanta-tion geometry. This tends to show that the incorporation ofEr ions in a Ga substitutional site is not drastically per-turbed up to a dose of 5×1014Er/cm2 for both implantationgeometry.

The integrated luminescence intensity of Er-defect com-plexes for the four samples is presented inFig. 3. The

104 A. Braud et al. / Materials Science and Engineering B105 (2003) 101–105

0 2 4 6 8 10

0,01

0,1

1

"regular" Er

Exponential decay

Inte

nsi

ty (

a.u

)

Time (ms)

Er-defect complexes

Theory

Fig. 4. 1.54�m luminescence decay at 7 K for “regular” Er ions and Er-defect complexes.

luminescence is much stronger in case of channeled implan-tation and appears to saturate with a dose of 5×1014Er/cm2.The reason for this result could be that the number ofEr-defect complexes is larger in case of channeled im-plantation which would be in apparent contradiction withRBS results which show a lower level of damage in chan-neled implantation. But the PL results only show theoptically active Er-defect complexes. Therefore, the expla-nation of the larger luminescence in channeled implantedsamples may be that the channeled implantation createsEr-defect-complexes where luminescence quenching pro-cesses are less efficient. In order to have a better descriptionof these quenching processes, the4I13/2 level lumines-cence decay is recorded for both resonant and non-resonantexcitation. As shown inFig. 4, the 4I13/2 level lifetime(τ ∼ 1 ms) is shorter for a non-resonant excitation, i.e. forEr ions in Er-defect complexes. Moreover, this lumines-cence decay is non-exponential. This indicates that energytransfers towards other centers take place which quench theEr infrared luminescence.

In case of resonant excitation both types of Er centersare simultaneously excited. It is possible to minimize theEr-defect complexes part of the decay by fixing the ex-citation wavelength at 809 nm as explained previously.The Er decay is then exponential which confirms that theEr-defect complexes luminescence contribution is weak.For “regular” Er, the4I13/2 level lifetime is about 3.8 ms.The luminescence quenching taking place within Er-defectcomplexes can be modeled using classical energy transfertheories. Er ions play here the role of donors while theenergy is transferred towards acceptors, namely, defects orimpurities. Energy transfers between donors and acceptorsare assumed to be of two kinds. They are either direct

[8] or assisted by migration[9] in which case successiveenergy transfers among donors occur before the final en-ergy transfer from the donor to the acceptor. Assuming adipole–dipole interaction between donors and acceptors,the luminescence decay of donors can be described by theexpression:

I(t) = I0 exp(−γ

√t − t

τ

), with

1

τ= 1

τD+ 1

τ0(1)

whereτ0 represents the donor intrinsic lifetime,γ the di-rect energy transfer parameter and 1/τD the migration as-sisted energy transfer rate. Er decay of Er-defect complexesis successfully described byEq. (1). The fitting procedureleads toγ = 1.3 ms−1/2 andτ = 3.25 ms. This value ofτis very close toτ0 = 3.8 ms obtained for “regular” Er ions.This result shows that the migration assisted energy trans-fer rate 1/τD is very small and that energy transfers betweenEr ions and defects within Er-defect complexes are predom-inantly direct. This conclusion seems reasonable since theenergy transfer from the defect towards the associated Erions is very efficient as shown by the PLE spectra. Besidesthe migration of energy among Er ions depends on the spec-tral overlap between the donor absorption spectrum and theacceptor emission spectrum[10]. This spectral overlap issmall at low temperature and the energy migration is thususually limited at low temperature.

4. Conclusion

Photoluminescence (PL), photoluminescence excita-tion (PLE) spectra and luminescence decay of the Er3+4I13/2 →4 I15/2 transition are investigated at 7 K in Er-

A. Braud et al. / Materials Science and Engineering B105 (2003) 101–105 105

implanted GaN samples. Under below-gap excitation, PLand PLE spectra reveal the existence of two types of Ercenters. “Regular” Er ions appear to occupy the Ga sub-stitutional site. The local site symmetry of this Er centerdoes not seem to be perturbed by local strains. Besides,this Er center which can only be excited by resonant intra4f shell transition is predominant while other Er centersare clearly excited via local defects. PLE spectra show onone hand that the number of these Er-defect complexes issmall compared to the number of “Regular” Er ions andon the other hand that the absorption cross-section of thedefect itself is much larger than the typical Er absorptioncross-section. Evolution of Er luminescence as a functionof implantation geometry shows that the luminescence in-tensity is larger in case of channeled implantation for bothtypes of Er centers. Luminescence dynamics study showsthat the4I13/2 manifold has a shorter lifetime (τ ∼ 1 ms)when Er ions are part of Er-defect complexes than whenEr ions are isolated from any defect (τ = 3.8 ms). Thisresult indicates the existence of non-radiative energy trans-fers in Er-defect complexes from Er ions towards defectsor impurities. The comparison of the Er decay in Er-defectcomplexes with classical energy transfer models clearlyshows that this luminescence quenching is based on direct

energy transfers without significant energy migration amongEr ions.

Acknowledgements

This work is supported by the EU under contract no.HPRN-CT-2001-00297 (RENIBEL).

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