Journal of Luminescence 131 (2011) 2311–2316

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Journal of Luminescence

0022-23

doi:10.1

n Corr

Grenobl

E-m

journal homepage: www.elsevier.com/locate/jlumin

Influence of deposition conditions on the optical properties of erbium-dopedyttrium oxide films grown by aerosol–UV assisted MOCVD

Rached Salhi a,b,n, Ramzi Maalej c, Mohieddine Fourati b, Yannick Guyot d, Odette Chaix-Pluchery a,Laetitia Rapenne a, Carmen Jimenez a, Jean-Luc Deschanvres a

a Laboratoire des Materiaux et du Genie Physique, Grenoble INP, CNRS, Minatec, 3 Parvis Louis Neel, 38016 Grenoble, Franceb Laboratoire de Chimie Industrielle, Ecole Nationale d’Ingenieur de Sfax, BP W 3038 SFAX, Tunisiac Laboratoire de Physique Appliquee, Departement de Physique, Faculte de Sciences de Sfax, 3018 Sfax, Tunisiad LPCML-UMR 5620 CNRS/UCBL Universite Claude Bernard Lyon 1 10 Rue Ada Byron 69622 Villeurbanne Cedex, France

a r t i c l e i n f o

Article history:

Received 22 March 2011

Received in revised form

18 May 2011

Accepted 30 May 2011Available online 6 June 2011

Keywords:

Y2O3

CVD process

Films

Luminescence properties

13/$ - see front matter & 2011 Elsevier B.V. A

016/j.jlumin.2011.05.058

esponding author at: Laboratoire des Mate

e INP, CNRS, Minatec, 3 Parvis Louis Neel, 38

ail address: salhi_rached@yahoo.fr (R. Salhi).

a b s t r a c t

Erbium-doped Y2O3 films were prepared by aerosol–UV assisted metal-organic chemical vapour

deposition (MOCVD) at 410 1C. The effects of humidity of carrier gas and UV-assistance on their

structure and optical properties were investigated on the as-deposited and thermal annealed films

using infrared spectroscopy, X-ray diffraction and transmission electron microscopy. It was found that

the as-deposited Er:Y2O3 films crystallise in the Y2O3 cubic structure and present a very low organic

contamination when the deposition takes place under high air humidity and, even better, with

UV-assistance. After annealing, two different structural phases are observed corresponding to the cubic

and the monoclinic structures of Y2O3. The Er3þ luminescence analysed in the visible and IR regions,

shows the classical green transitions. The best optical properties were obtained with as-deposited and

annealed Er:Y2O3 films grown under high air humidity with UV-assistance. Under such deposition

conditions, 4I13/2 lifetimes was found to be 3.07 and 6.1 ms for films annealed at 800 and 1000 1C,

respectively, and up-conversion phenomena were underlined. This indicates that the deposition

conditions, in particular air humidity, play an important role in the luminescent properties even after

annealing.

& 2011 Elsevier B.V. All rights reserved.

1. Introduction

The development of the optical telecommunication compo-nents requires the synthesis of active optical materials. Thin filmsare especially required in order to integrate on the same chip bothoptical and electronic functions. As amplification media, rare-earth doped crystalline host materials such as yttria [1] areconsidered as an alternative to the classical silicate or phosphateglasses. Erbium-doped yttria is one type of luminescent materials,which has received extensive research attention due to theunique luminescent characteristics of erbium dopants and theirstability [2]. Yttria has been widely studied because of its highchemical resistance, thermal stability and optical compatibil-ity [3]. Moreover, the similar bixbyite type crystal structure ofY2O3 and Er2O3 (a¼10.604 and 10.547 A, respectively) allows theincorporation of a large amount of Er3þ ions in Y2O3 [4]. Differentroutes, such as the sol–gel process [5], molecular beam epitaxy

ll rights reserved.

riaux et du Genie Physique,

016 Grenoble, France.

[6], injection chemical vapour deposition [7], aerosol assistedmetal-organic chemical vapour deposition [8], have been used tosynthesise erbium-doped yttria films.

In this work, we present the effect of humidity of carrier gas andUV-assistance on the structural and optical properties of as-depositedand annealed Er:Y2O3 films prepared by aerosol–UV assisted metal-organic chemical vapour deposition (MOCVD). The effect of theUV-assistance on MOCVD process has been already reported for otheroxides [25]. The interaction between the radiation and the species ofthe reaction was mainly characterised by increasing the depositionrate. Nevertheless, this interaction depends on the wavelength, on thesource power and on the chemical bonding of the precursors.

We have previously described the effects of humidity of carriergas and UV-assistance on the kinetics and on the chemical,structural and morphological characteristics of the film [8]. Weestablished that humidity in the carrier gas allows to decrease theorganic contamination of the films and to increase the crystal-linity of the as-deposited films. We will present in this work theeffect of thermal annealing on the optical properties of Er:Y2O3

films as a function of the humidity (25% RH or 93% RH) andUV-assistance during deposition and we correlate these proper-ties to the structural characteristics of the films.

Fig. 1. FT-IR spectra from as-deposited and annealed Er:Y2O3 films obtained for

different annealing temperatures. L0 samples refer to films deposited under low

air humidity without UV-assistance, H1 samples refer to films deposited under

high air humidity with UV-assistance.

R. Salhi et al. / Journal of Luminescence 131 (2011) 2311–23162312

2. Experimental details

2.1. Deposition process

Erbium-doped yttrium oxide films (Er:Y2O3) were depositedby means of the UV-assisted MOCVD technique using yttriumacetylacetonate Y(acac)3 (Y(C5H7O2)3) and erbium(TMHD)3

(Er-tris(2,2,6,6-tetramethyl-3,5-heptanedionate)) as precursorsin butanol at a total concentration of 0.03 mol/l with 5 mol% ofEr. All these products were purchased from Strem Chemicals.Precursors were selected for their low-toxicity, good stability atroom temperature, easy handling and low cost [9]. The substrateused in these experiments was the (1 0 0) oriented silicon wafer.

The UV-assisted MOCVD process is based on the pyrolysis ofan aerosol on a heated substrate at atmospheric pressure [10].The aerosol is produced by means of a flat piezoelectric transdu-cer, which generates an ultrasonic beam in a solution containingthe reactants of the material to be deposited. This ultrasonicspraying system guarantees a narrow dispersion of droplet size(4–10 mm). So, a heterogeneous vapour/solid reaction occurs atthe substrate heated surface, leading to the CVD process [11].The aerosol was then conveyed by air as a carrier gas with twodifferent air relative humidity values (RH %); the first one (low airhumidity) was close to 25% and the second one (high airhumidity) close to 93%. The air flow source was parallel to thesubstrate surface, and the ultraviolet (UV) light (250–450 nm)emitted from high-pressure Hg lamps (EXFO Omni Cure S1000,type 100 W Mercury Vapour Short Arc) irradiated the substratesurface perpendicularly through a synthesised quartz window(UV grade). In this work, Er:Y2O3 films were prepared at 410 1Csubstrate temperature. In these conditions the Er atomic concen-tration in the layer was close to 2%. The effects of humidity ofcarrier gas and UV-assistance on the chemical, structural andmorphological characteristics of the films, together with theoptimisation of the deposition conditions have been presentedin a previous work [8].

In the following, the samples will be labelled as L0 (Er:Y2O3

deposited under low air humidity (RH 25%) without UV-assis-tance), L1 (Er:Y2O3 deposited under low air humidity (RH 25%)with UV-assistance), H0 (Er:Y2O3 deposited under high air humid-ity (RH 93%) without UV-assistance) and H1 (Er:Y2O3 depositedunder high air humidity (RH 93%) with UV-assistance).

After deposition, a thermal annealing in air was performed at atemperature ranging between 500 and 1000 1C. The as-depositedand annealed films were studied as a function of depositionconditions (air humidity and UV-assistance) and annealingtemperature.

2.2. Characterisation

Fourier Transform Infrared spectroscopy (FT-IR) was used tostudy the structural evolution of the films as a function of thedeposition conditions. Absorbance spectra were measuredbetween 250 and 4000 cm�1 with 4 cm�1 resolution using aBio-Rad FT-IR spectrometer FTS165. Baseline correction and Sisubstrate spectrum subtraction were performed. X-ray diffractionpatterns were obtained using a Siemens D8 y/2y diffractometerwith Cu Ka1 radiation. Fluorescence spectra of Er3þ ions werecollected in the visible region using a Jobin–Yvon Horiba LabRamspectrometer equipped with a liquid nitrogen cooled CDD detec-tor. The pump wavelength used was the blue radiation of anargon ion laser at 488 nm. Experiments were conducted at roomtemperature in a backscattering geometry using an OlympusBX41 microscope. Transmission electron microscopy (TEM) andelectron diffraction were carried out on a JEOL 2011 microscopeoperating at 200 kV with a 0.19 nm point to point resolution.

Cross-section samples were obtained by the tripod method.Samples were polished on both sides using diamond impregnatedfilms with grain size from 15 mm sequentially down to 0.5 mmuntil films were less than 15 mm thick. Low angle argon ion beammilling was used for final perforation of the samples, variableenergy milling varying from 3.5 KeV to 100 eV was applied tominimise radiation damage.

To measure the 4I13/2 level lifetime, the samples were excitedunder 525 and 978 nm pulsed radiation (repetition rate: 10 Hz)from a dye laser pumped by a frequency doubled YAG:Nd3þ laser.Emissions were separated by a Jobin–Yvon monochromator, andthe output signal was detected using a cooled Ge diode. Thefluorescence decay times were monitored by using a Lecroymodel 9410 oscilloscope interfaced with a computer. For acquisi-tion of the emission spectra, these signals were integrated using aboxcar average and a gated integrator (Princeton AppliedResearch Model 162 and 164).

3. Results and discussion

The FT-IR absorption spectra of L0 and H1 Er:Y2O3 films arepresented in Fig. 1. In the first case, the absorbance spectrum of theas-deposited film shows bands located between 250 and 700 cm�1

attributed to amorphous yttria [12]. It also exhibits a large band inthe 1300–1700 cm�1 region attributed to C¼O, C¼C, CH3 andC–CH3 vibrations [13–24]. We have previously described that thisband and the traces of acetylacetonate ligands are mainly respon-sible for this organic contamination [8]. In the case of the H1 as-deposited films, the signal from the organic residues decreases.At the same time, IR bands corresponding to cubic yttria appears at300, 330, 370, 460 and 560 cm�1, with a weak one at 410 cm�1.These values are close to those referred in the literature [12]. Afterannealing at 500 1C in air, the organic bands tend to disappear buta temperature as high as 800 1C is required to eliminate completelythis contamination. Furthermore, the increase in the annealingtemperature tends to increase the intensity of the characteristiclines of yttria in H1 films in comparison with to L0 films. The bandclose to 1200 cm�1 corresponds to the formation of SiO2, due tothe substrate oxidation during the thermal annealing.

The XRD patterns corresponding to L0 and H1 films are shownin Fig. 2. The as-deposited L0 film is amorphous (Fig. 2a).

Fig. 2. XRD patterns from (a) as-deposited and (b) annealed Er:Y2O3 films at

different annealing temperatures. The L0 and H1 film labels denote the deposition

conditions as in Fig. 1 (M¼Y2O3 Monoclinic, C¼Y2O3 cubic).

R. Salhi et al. / Journal of Luminescence 131 (2011) 2311–2316 2313

The crystallisation of the L0 film in the Y2O3 cubic phase isobserved after annealing at 500 1C, as shown in Fig. 2b. Annealingat higher temperatures (up to 1000 1C) leads to the coexistence ofboth cubic and monoclinic Y2O3 phases (Fig. 2b).

The Y2O3 cubic phase is already detected in the XRD pattern ofthe H1 as-deposited film (Fig. 2a). After annealing at 500 1C,diffraction peaks assigned to the monoclinic Y2O3 phase [15] arealso present. Annealing at higher temperatures (up to 1000 1C)leads to the coexistence of both Y2O3 phases, as shown in Fig. 2b.It is to be noted that the number of diffraction peaks and theirintensity are higher for the H1 film annealed at 800 1C incomparison with the L0 film annealed at 1000 1C. This indicatesa higher degree of crystallisation of the H1 film and givesevidence of the strong influence of the deposition conditions oncrystal structure transformations.

TEM images obtained for L0 and H1 as-deposited films arereported in Fig. 3. The electron diffraction patterns indicate thatboth films are crystallised in the Y2O3 cubic structure, even in theas-deposited state and are polycrystalline. However, their micro-structure is dependant on the deposition conditions: the L0 film

(Fig. 3a,b) is made of nanocrystallites as shown by the thickdiffraction rings whereas the H1 film has a columnar structureand larger crystallites (Fig. 3c,d). The grain sizes determined fromthe TEM image are comprised between 5 and 10 nm in L0 filmsand close to 20 nm in H1 films. The TEM images of the annealedfilms (not shown here) are very similar to those obtained with H1as-deposited film.

The optical characterisation was performed by measuringthe fluorescence spectra of the as-deposited films in the visibleregion at room temperature after excitation at 488 nm into the4F7/2 state. The spectra, normalised to the green 4S3/2-

4I15/2

transition, have a predominantly green emission from the ther-malised 2H11/2-

4I15/2 (Fi Zj) and 4S3/2-4I15/2 (Ei Zj) transitions in

the region comprised between 525 and 575 nm. The fluorescencespectra of the Er:Y2O3 films deposited under different conditionsare shown in Fig. 4.

As-deposited L0 and L1 Er:Y2O3 films are not luminescent(Fig. 4). The low response of these films can be explained by thepresence of the high vibrational energies associated with thetraces of acetylacetonate ligands detected by FT-IR. The organiccontamination increases the rate of multiphonon relaxation: theenergy gap between 4F7/2 and the next lower level 2H11/2 isapproximately 1535 cm�1, which corresponds to the energy ofthe stretching vibration of the organic species. However, whenthe films are deposited under high air humidity, they presentluminescent properties (Fig. 4 H0). This fact is emphasised in thecase of UV assisted deposition (Fig. 4 H1). These films have noorganic contamination, which would require more than twointrinsic yttria phonons (600 cm�1) [16] and makes the multi-phonon relaxation less probable. As a result, there is a largerpopulation of Er3þ ions in the 4F7/2 state making the cross-relaxation mechanism more efficient.

An accurate analysis of the fluorescence spectrum of the as-deposited H1 Er:Y2O3 film is reported in Fig. 5. Using the relevantenergy scheme established by Kisliuk et al. [17] for Er3þ ions in C2

symmetry centre of Y2O3, all emission lines have been identifiedin the spectrum with some reassignments. The two classic greentransitions from the excited 2H11/2 and 4S3/2 to the ground state4I15/2 are observed around 530 and 550 nm, respectively. All thecorresponding lines are assigned in Fig. 5a and b, respectively.Two other transitions corresponding to the 2P3/2-

4I9/2 (Pi-Bj)and 2H9/2-

4I13/2 (Ki-Yj) transitions, are also detected around528 nm and between 553 and 571 nm, respectively. These latteremissions allow to put in evidence the existence of up-conversionphenomena [18]. The intensities of these emission lines aregenerally smaller than those of the classical emissions. Theseemission spectra are in good agreement with another workinvolving Y2O3 crystals [19], indicating the incorporation of therare-earth ions into the crystal structure.

The room temperature emission spectra of the Er:Y2O3 filmsannealed in the 500–1000 1C temperature range are not changed.The effect of the annealing temperature for different film deposi-tion conditions on the relative intensity of the transitions E2-Z8,E2-Z4, Z8 and E1-Z7, Z8 is shown in Table 1. The relativeintensity increases when raising the annealing temperature from500 to 1000 1C. For all annealing temperatures, H1 films showhigher relative intensities than L0 films. This has been attributedto a more random distribution of the activator centres throughthese films and to an improved crystallisation state [20].

The lifetime of the 4S3/2-4I15/2 transition in Er:Y2O3 was

obtained after excitation at 525 nm. The lifetimes measured fordifferent deposition conditions and different annealing tempera-tures are presented in Table 2. The as-deposited L0 and L1 Er:Y2O3

films obtained under low air humidity are not luminescent.The lifetime of Er3þ is 0.25 ms for the H1 film. It increases whenincreasing the annealing temperature from 500 to 1000 1C.

Fig. 3. Low magnification images electron diffraction patterns and HRTEM images of the interface obtained in cross-section of as-deposited L0 (a, b) and H1 (c, d) Er:Y2O3

films, respectively. (L0 and H1 labels as used in Fig. 1.)

Fig. 4. Fluorescence spectra of as-deposited Er:Y2O3 films obtained under differ-

ent deposition conditions (L0: low air humidity without UV-assistance, L1: low air

humidity with UV-assistance, H0: high air humidity without UV-assistance and

H1: high air humidity with UV-assistance).

R. Salhi et al. / Journal of Luminescence 131 (2011) 2311–23162314

The UV-assistance promotes a small increase in the lifetime; butthe use of higher air humidity provokes a strong increase inlifetime. A lifetime of 2.95 ms was obtained for H1 film annealedat 800 1C, but it is only 2.3 ms for L0 film annealed at 1000 1C.

The gain in the thermal budget when using high air humiditywith UV-assistance is significant. These conditions are clearly thebest conditions of deposition.

Fig. 6 shows an Er3þ emission spectrum, in the 1450–1650 nmregion, of a H1 Er:Y2O3 film excited at 978 nm. The maximum ofthe 4I13/2-

4I15/2 transition occurs at 1540 nm. In addition to thismaximum, the spectrum shows several Stark components.Comparison of these wavelengths with literature values [21]indicates that both 4I13/2 and 4I15/2 Stark levels may be involved.The Stark splitting provides emission over a relatively largewavelength range, which is important for application as a broad-band optical amplifier.

L0 films are luminescent only after annealing at 1000 1C.The lifetime of the infrared emission of the 4I13/2-

4I15/2 transitionwas measured on Er:Y2O3 H1 films annealed in the 700–1000 1Ctemperature range excited at 978 nm. It increases from 1.5 to 3.07and 6.1 ms when increasing the annealing temperature from 700to 800 and 1000 1C, respectively. For comparison, it is equal to2.1 ms in the case of L0 film annealed at 1000 1C.

The decay curves corresponding to this infrared emission infilms deposited under different conditions are presented in Fig. 7.For all investigated samples, the curves showed a deviation fromthe single exponential behaviour. There are two kinds of lifetimes(t): the short lifetimes are 2.1 and 2.7 ms in the case of L0 and L1films, respectively (Fig. 7); the long lifetimes are 5.5 and 6.1 msfor H0 and H1 films, respectively (Fig. 7). However, their devia-tions from the exponential behaviour were dependant on thedeposition conditions as evidenced in Fig. 7. The lifetime of the4I13/2-

4I15/2 transition in the films deposited under low airhumidity shows a much greater deviation from exponential

Fig. 5. Fluorescence spectra of as-deposited H1 Er:Y2O3 film obtained with a 488 nm

excitation of 40 kW/cm2 incident power. The assignment of the different transitions

is indicated: they correspond to 2H11/2-4I15/2 (Fi-Zj) in the 510–540 nm region (a),

to 4S3/2-4I15/2 (Ei-Zj) in the 545–575 nm region (b). (H1 label as used in Fig. 1.)

Table 1Relative intensity of the room temperature emission lines of as-deposited and

annealed L0 and H1 Er:Y2O3 films (excitation at 488 nm).

Sample Wavelength

(nm)

Assignment Relative intensity

As-deposited 500 1C 800 1C 1000 1C

L0 553.7 E2-Z8 – 0.62 0.75 0.91

560.8 E2-Z4, Z8 – 0.56 0.62 0.84

563.8 E1-Z7, Z8 – 1 1 1

H1 553.7 E2-Z8 0.68 0.8 0.95 0.99

560.8 E2-Z4, Z8 0.65 0.75 0.84 0.89

563.8 E1-Z7, Z8 1 1 1 1

Table 2Lifetime t (ms) of the 4S3/2 state in as-deposited and annealed Er:Y2O3 films as a

function of deposition conditions (excitation at 525 nm).

Sample t (ms)

As-deposited 500 1C 800 1C 1000 1C

L0 – 1 1.45 2.3

L1 – 1.15 1.7 2.65

H0 0.19 2 2.75 4.57

H1 0.25 2.2 2.95 4.85

Fig. 6. Room temperature infrared emission spectrum of a H1 Er:Y2O3 film

annealed at 800 1C for 1 h in air obtained with a 978 nm excitation of 40 kW/cm2

incident power. This spectrum recorded in 1450–1650 nm regions corresponds to

the 4I13/2-4I15/2 (Yi-Zj) transitions. (H1 label as used in Fig. 1.)

Fig. 7. Decay curve of the infrared emission of the 4I13/2-4I15/2 transition under

978 nm excitation in Er:Y2O3 films deposited under different conditions after annealing

at 1000 1C. (L0, L1, H0 and H1 labels denote the deposition conditions as in Fig. 4.)

R. Salhi et al. / Journal of Luminescence 131 (2011) 2311–2316 2315

behaviour compared to the lifetime of the films prepared underhigh air humidity.

In H1 films, the increase in lifetimes with the annealedtemperature is attributed to the crystallisation of the films inthe monoclinic phase of Y2O3. This phase contains three crystal-lographically inequivalent Y3þ sites instead of two Y3þ sites in

the normal cubic phase [26]. They are potential sites for Er3þ ionsubstitution and thus, better optical properties can be expectedfor the monoclinic phase in comparison wit h the cubic phase.

Several studies have been focused on the lifetime of the4I13/2-

4I15/2 transition for Er:Y2O3 samples prepared under var-ious techniques. In 1968, Weber mentioned an observed value of8 ms in a 0.04 at% Er:Y203 monocrystal at 77 K [22]. In thin films,Hoekstra et al. [10] have reported 1 ms for as-deposited sample

R. Salhi et al. / Journal of Luminescence 131 (2011) 2311–23162316

containing 2 at% Er and elaborated at 500 1C using a spray-pyrolysis process although the same authors obtained 7.2 mswith films deposited by a PVD process containing 0.2 at% Er[23]. Using an aerosol assisted MOCVD under high air humiditywith UV-assistance, the present study demonstrates that it ispossible to obtain a long lifetime of 6.1 ms for 2 at% Er:Y2O3 afterannealing at 1000 1C.

The comparison between results obtained for films annealed at800 and 1000 1C indicates that the more interesting opticalproperties are relative to films deposited under high air humiditywith UV-assistance and annealed at temperature as low as 800 1C.Lastly, it is to be noted that the fluorescence spectra and decaytimes for samples obtained in these deposition conditions arewell correlated with FT-IR and XRD results.

4. Conclusion

The effects of humidity of carrier gas and UV-assistance on thestructure and optical properties of Er:Y2O3 films have been inves-tigated on as-deposited and post-deposition annealed films. It wasfound that the as-deposited Er:Y2O3 films under high air humiditywith UV-assistance present a very low organic contamination andhave the Y2O3 cubic structure. After annealing at 800 1C, the cubicand monoclinic phases of yttria coexist in the films. The Erþ3

luminescence was studied in the visible and IR regions. The resultsindicate that the deposition conditions play an important role in theluminescent properties after annealing and the best conditionscorrespond to a film deposition under high air humidity and withUV-assistance. Under these conditions, a 4I13/2 lifetime of 6.01 mswas found for film annealed at 1000 1C.

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