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Journal of Crystal Growth 311 (2009) 4402–4407

Contents lists available at ScienceDirect

Journal of Crystal Growth

0022-02

doi:10.1

� Corr

E-m

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

Purification and mechanical nanosizing of Eu-doped GaN

Tiju Thomas a,�, Xiaomei Guo b, MVS Chandrashekhar a, Carl B. Poitras a, William Shaff a,Mark Dreibelbis c, Jesse Reiherzer c, Kewen Li b, Francis J. DiSalvo c, Michal Lipson a, M.G. Spencer a

a School of Electrical and Computer Engineering, Cornell University, Ithaca, NY, USAb Boston Applied Technologies Inc., 6F Gill Street, Woburn, MA 02132, USAc Department of Chemistry and Chemical Biology, Cornell University, Ithaca, NY, USA

a r t i c l e i n f o

Article history:

Received 29 May 2009

Received in revised form

7 July 2009

Accepted 20 July 2009

Communicated by R. Kernused for making Eu:GaN nanoparticles of sizes 30–50 nm using a soft ball-milling technique. The

Available online 29 July 2009

PACS:

81.05.Ea

81.07.Bc

81.07.Wx

81.20.Wk

Keywords:

A1. Doping

A1. Purification

B1. Gallium compounds

B1. Nitrides

B1. Nanomaterials

B2. Phosphors

48/$ - see front matter & 2009 Elsevier B.V. A

016/j.jcrysgro.2009.07.028

esponding author. Tel.: +1607 3518081.

ail address: tt332@cornell.edu (T. Thomas).

a b s t r a c t

Eu:GaN powder synthesized using a high temperature ammonothermal process is known to be dark in

appearance due to presence of Eu-containing absorbing particles. Improvement of the visual quality of

the Eu:GaN powder is achieved by rinsing in dilute acids. Acid-rinsed Eu:GaN has photoluminescence

(PL) enhanced by a factor of 3 when compared to as-prepared Eu:GaN. Such visually clear powders are

particle size was determined using X-ray diffraction, scanning electron microscopy and a dynamic light

scattering system. Longer durations of a ‘‘soft’’ ball-milling technique results in particle size reduction.

These nanopowders show significant photoluminescence intensity with no yellow luminescence, and

have a reduced PL intensity with increasing ball-milling time. Eu:GaN nanopowder embedded in a KBr

matrix shows at least a 10� improvement in transmittance when compared to as-prepared powders.

The improvement of transmittance depends on both the concentration and particle size. This improved

transmittance suggests that such a transparent matrix could be used as a laser gain medium.

& 2009 Elsevier B.V. All rights reserved.

1. Introduction

The demonstration of emission of the three primary visiblecolors – blue, green and red – from rare-earth-doped GaN(RE:GaN) has given rise to significant interest in this class ofmaterials due to applications as next-generation phosphors andmaterials in optoelectronic devices [1,2]. The first report ofRE:GaN powder [3] showed green emission similar to bulkRE:GaN grown by Steckl and Zavada [1,2]. Interest in RE:GaNpowder has increased due to the demonstration of RE:GaNpowder based thin film transistors (TFT’s), success in incorporat-ing red light emitting Eu centers in GaN matrix and demonstra-tion of its complementary metal oxide semiconductor (CMOS)compatibility [3,4]. In this paper we solve critical problems withEu:GaN powder, that of the poor visual clarity due to presence ofdark absorbing particles and that of nanosizing these powderswhile retaining luminescence.

ll rights reserved.

Typically, the Eu:GaN produced using a high temperatureammonothermal process is observed to have a much darkerappearance than pure GaN [3,5]. Due to the nature of this darkerappearance there is an obvious impediment in its practicalapplication as a laser material or as a phosphor since itcontributes to reduced luminescence. It is reasonable to expectthat the visual appearance of Eu:GaN be similar to that of pureGaN because of the relatively dilute concentration of Eu atoms(�0.5 at%) [6]. We demonstrate a chemical process for Eu:GaNparticles, which removes the dark particles that contribute to thisdarker visual appearance of the material and improves thephotoluminescence (PL) of these particles by 300% compared tothe powder previously reported [6]. The visual clarity approachesthat of pure GaN (Fig. 1). In this paper, we will discuss themechanism of the photoluminescence intensity improvementusing X-ray photoelectron spectroscopy (XPS), X-ray diffraction(XRD) and PL measurements. GaN-based optical devices likelasers are currently being pursued [7].

The as-prepared Eu:GaN powder produced in our laboratoryhas been shown to be a prospective laser medium [8]. The highoptical refractive index [9] of GaN (n ¼ 2.3) makes it as an

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Fig. 1. (a) and (b) is pure GaN; (c) and (d) is Eu:GaN powder, before and after acid rinse, respectively. The acid rinse does not change the visual appearance of pure GaN

powder, but renders the dark as-prepared Eu:GaN powder almost as visually clear as pure-GaN powder.

T. Thomas et al. / Journal of Crystal Growth 311 (2009) 4402–4407 4403

attractive material for ceramic laser gain medium. While suchtransparent ceramics have been realized using oxides, [10] this isnot so for nitrides. The chemical and thermal stability of RE:GaNalong with its optical properties make it attractive for laserapplications [6]. The recent demonstration of emission fromRE:GaN heteroepitaxially grown on silicon led to the firstobserved visible lasing action on silicon [11]. The as-preparedEu:GaN powder are micron-sized particles that are highlycrystalline and luminescent. Such large particles would lead tosignificant scattering of transmitted light in a ceramic gainmedium. Therefore, in order to make a RE:GaN powder suitablefor such applications, the particle size must be reduced to muchless than the wavelength of visible light, typically o100 nm. Arecent work demonstrated the synthesis of Eu:GaN micron-sizedagglomerates made up of nano grains [12]. This work used Eu2O3

and Ga2O3 as the starting materials in an ammonothermalsynthesis. The particles thus obtained showed significant agglom-eration. The observed photoluminescence properties were similarto bulk Eu:GaN, but showed significant yellow luminescence (YL)which is believed to be due to impurities such as oxygen or carbon[13]. In this paper will demonstrate an alternate process toachieve nanosized Eu:GaN particles with sizes as low as �30 nm,with significantly suppressed YL.

2. Experiments

The powder is synthesized using a Ga–Eu–Bi alloy (composi-tion: 95.75, 1.25 and 3 at%, respectively) as the starting material.This alloy is heated in a quartz tube reactor in an argonatmosphere to a temperature of 650 1C. The heating is continuedbeyond 650 1C in an ammonia ambient at the reaction tempera-ture of 950 1C which is kept constant for 5 h while maintaining asteady partial pressure of ammonia in the reaction chamber. High-purity Eu:GaN with approximately 0.5 at% Eu doping is obtained.The details of the mechanisms of the reaction are reported

elsewhere [14]. In this study, we use the powder samples formedusing this method [6].

The powder prepared using this method is known to have astrong emission in the red [14]. Despite its strong luminescence,this powder is dark in appearance on the account of severalabsorbing dark particles in it. The removal of the absorbingparticles from the powder was done by chemical rinsing in diluteacid solutions. For acid rinsing 1 M HNO3 and 1 M HCl were usedover varying durations (0–16 h) and temperatures (room tem-perature and 100 1C). The effect of the rinse on the luminescencewas quantified using PL (see Figs. 2 and 3). PL measurements wereperformed using a CW He–Cd laser (325 nm wavelength) as theexcitation source. It was observed that ball milling for 20 and 40 hreduces the PL intensity by a factor of 4 and 5, respectively. Bestcleansing was achieved with a room temperature rinsing for 4 husing 1 M HNO3. While this chemical rinse clears the powder of alldark particles as verified by XRD, PL measurements also show thatthe acid-rinsed powder has a 300% higher luminescence intensitythan the as-prepared powder. Since the acid rinsing of Eu:GaNpowders were done at room temperature for a short times, weexpect changes only at the surface of the particles. Comparableluminescence enhancement was obtained by HNO3 rinsing at100 1C. However, this last process tends to be more aggressive, aslonger rinse times lead to irrecoverable quenching ofluminescence. We believe that this is due to cannibalisticoxidation of the entire GaN particles [15]. This is discussed ingreater detail later in the results section of this paper. In short, aroom temperature rinse for 4 h using 1 M HNO3 was determinedto be the optimal way to achieve the most luminescenceenhancement in a controllable fashion.

For nanosizing a roll-milling machine (US Stoneware Company,model: 784) was utilized for ball-milling the acid-rinsed Eu:GaNpowder, which is free from mixed oxide impurities now due to theacid-rinsing technique discussed above. Yttria-stabilized zirconiaballs with mixed sizes were used with a total ball-to-powderweight ratio of approximately 20:1, and the ball milling wasperformed in ethanol. The as-milled powders were noticeably

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Fig. 2. (a) PL intensity as a function of rinse time, for various rinse solutions. The graph shows that the HNO3 rinses result in improvements in PL intensities when compared

to HCl rinse. Results for 15 min rinses in boiling HNO3 and HCl are also shown. All values are normalized to the peak intensity of the as-prepared powder. (b) PL spectra for

unrinsed, 4 and 16 h HNO3 rinse at 100 1C samples. All of the samples came from the same starting batch. Boiling in HNO3 for 16 h quenches the PL. Optimized rinsing

results in a substantial increase of the photoluminescence intensity, accompanied by increased visual clarity of the powder.

Fig. 3. (a) Evolution of Ga 3d core energy levels (with Fermi level as the reference)

in Ga2O3. Inset shows the XPS spectra for the Ga 3d core levels for Eu:GaN after 1 h

HNO3 rinse. (b) The band diagram of GaN and Ga2O3 with experimentally

determined values for electron affinities (w), valence and conduction band offsets

and bandgaps.

T. Thomas et al. / Journal of Crystal Growth 311 (2009) 4402–44074404

lighter color and showed improved dispersibility in ethanolcompared to as-rinsed powders, a possible indication of reducedlevels of dark particles and aggregations. Dramatic broadening ofthe XRD peaks was observed (Fig. 4c) in the milled powders andcan be partially attributed to the reduction in the average particlesize and possible strain induced in the particles due to the ballmilling. Scanning electron micrographs also reveal that the ball-milled particles show a tendency to weakly agglomerate, likelydue to increased surface-to-volume ratio of the GaN nanoparticles[12]. Measurements of particle size made using dynamic lightscattering (DLS) [16] based Malvern Zetasizer Nano-ZS instrumentagree well with both SEM pictures (Fig. 4a and b) and Scherrerformula calculations from XRD.

3. Results and analysis

We know from the discussion above that the Eu-containingoxide particles are responsible for the dark appearance of as-prepared Eu:GaN. Since these particles were removed by acidrinsing, we believe that the dark impurities are separate particlesin the as-prepared Eu:GaN. We now turn our attention to thesurface chemistry associated with acid-rinsed Eu:GaN particles. Inorder to understand the changes occurring on GaN particlesresulting from acid rinse, XPS studies were undertaken. XPSmeasurements were performed using Al Ka X-rays with an energyof 1486 eV, and a spot size of 1 mm. The spectral resolution of theXPS system is �0.1 eV. As revealed by the XPS measurements, twopeaks corresponding to the Ga 3d level, at �20.7 and �19.7 eV,were observed in all of the samples. The two peaks are attributedto Ga–N and Ga–O bonds, respectively [15]. The oxide peakcorresponding to Ga2O3 also shows an additional shift from theGaN peak, attributed to band bending at the Ga2O3–GaN interface.This Ga2O3–Ga 3d level shifted continuously with respect to theduration over which the chemical rinse was performed beforereverting back to its nominal position for longer more aggressiverinses (see Fig. 3a). This observation indicates that the acid rinseprocess continuously oxidizes the GaN surface, forming a cleanerGaN-oxide interface with progressive rinsing. This consistently

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Fig. 4. (a) and (b) SEM of GaN powder before and after 40 h of ball milling, respectively. The average size (measured using XRD and DLS) is about 35 nm. However,

significant particle aggregation is observed. (c) XRD peaks of ball-milled GaN are broadened due to nanosizing. Table 1 uses minor shifts in these XRD peaks to determine

biaxial stresses in these ball-milled powders.

T. Thomas et al. / Journal of Crystal Growth 311 (2009) 4402–4407 4405

increasing additional shift is attributed to electrostatic potentialsat the interface arising from the valence band discontinuitybetween the GaN and the Ga2O3, in analogy with GaN–AlNinterfaces [17]. The oxide formed on the surface is also expected tobe responsible for the yellowish-green coloration of the Eu:GaNpowder after acid rinse. It must also be noted that because therinsing is done at room temperature and for short durations(�4 h), O or Cl contamination of the GaN matrix due to diffusion isnot expected.

The valence band offset was determined by using the followingexpression [17]:

DEv ¼ ðEv � ECLÞGaN � ðEv � ECLÞGa2O3 �DECL ð1Þ

where (Ev�ECL) is the difference between the valence bandmaximum and a conveniently identifiable core level (which inthis case is the 3d level), and DECL is the difference between thecharacteristic core levels in GaN and Ga2O3 in the case where theheterojunction is formed, i.e. the brightest sample. (Ev�ECL)GaN

was measured on the as-prepared powder, which did not undergoany rinsing. (Ev�ECL)Ga2O3 was obtained from the over-oxidizedsample, with quenched luminescence, attributed to Ga2O3 asmentioned above. We obtain that (ECL)GaN and (ECL)Ga2O3 are �17.1and �20.6 eV, respectively, and that DECL is 4.5 eV. Therefore, avalence band offset of 1.0 eV was extracted. This value agrees wellwith experimentally measured values of band parameters.Furthermore, experimentally determined values of electronaffinities of GaN and Ga2O3 are �3.6 and 3.2 eV, respectively,which results in a conduction band offset DEc of �0.4 eV,assuming bandgaps of 3.4 and 4.9 eV for GaN and Ga2O3,respectively [17–19]. This implies a valence band offset of 1.1 eV,which agrees very well with our measurements. This is illustratedin Fig. 3b. As the oxide layer grows uniformly around the GaNparticles during the rinse process, greater nitride–oxide interfacepotentials develop as the interface formation proceeds. We foundthat a well-defined GaN–Ga2O3 interface was formed by rinsing in1 M HNO3 for 4 h at room temperature (Fig. 3). The valence band

offset attains the ideal GaN/Ga2O3 value when the PL intensity ofthe powder is highest. For the optimized room temperature rinse,a Ga2O3 thickness of �3 nm is obtained, as measured by XPS(comparing the Ga2O3 and GaN 3d peaks), which is in agreementwith the limiting thickness in thermally oxidized planar GaN,measured by Wolter et al. [15]. An oxidation using a moreaggressive etch (e.g., boiling in HNO3 or higher temperaturethermal oxidation) is not self-limited and can completely oxidizethe GaN particles to Ga2O3, which explains the loss of lumines-cence (Fig. 2a).

To determine if the PL enhancement in acid-rinsed Eu:GaN isdue to the clean GaN/Ga2O3 interface, we performed similar acid-rinse experiments on undoped GaN. We observed no increase inPL enhancement in acid-rinsed GaN, when compared to as-prepared GaN. Hence a clean GaN/Ga2O3 interface cannot be usedto account for the PL enhancement in acid-rinsed Eu:GaN. Thisimplies that Eu-containing compounds, which are expected to bethe dark in appearance, are responsible for sub-optimal lumines-cence in as-prepared Eu:GaN. The likely candidates are mixednitrides/oxides of Ga and Eu in as-prepared Eu:GaN powders thatare soluble in acids. [20] Hence we conclude that the removal ofEu-containing absorbing particles (found mixed in the as-prepared powder along with Eu:GaN) contributes to the observedincrease in photoluminescence in acid-rinsed Eu:GaN. If the rinseis too short, the luminescence is not maximized due to incompleteremoval of absorbing particles. However, if the chemical rinse istoo long, all of the GaN gets consumed due to cannibalisticoxidation quenching the luminescence completely (Fig. 2a). Thisquenching is accompanied by a corresponding loss of interfacepotential, as observed by XPS (inset of Fig. 3a), indicating that allof the GaN has been consumed.

For the ball-milled powders, PL was used to determine theoptical properties (Fig. 5a). There is no significant YL observedeven in the ball-milled powders, apparently there were not manydefects (e.g., Ga vacancies) or impurities introduced in the crystal[9]. This field results in the holes moving towards the surface,

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T. Thomas et al. / Journal of Crystal Growth 311 (2009) 4402–44074406

while the electrons stay in the interior of the particle, this reducesthe probability of the recombination of the electrons and holes,hence, resulting in lower observed PL.

In order to determine the crystalline quality, induced stressand electron concentration of the ball-milled powders, Ramanspectroscopy measurements were performed (Fig. 5b). Thesemeasurements were performed using a Renishaw reflectionconfiguration, using an excitation wavelength of 488 nm and withspectral resolution of 0.1 cm�1 and spatial resolution of 1mm. Weobserve that 20 and 40 h of ball-milling introduce a red-shift in

Fig. 5. (a) The PL of as-prepared and ball-milled Eu:GaN shows that the 621 nm line is re

Raman spectra of Eu:GaN before and after ball-milling shows red-shifts in E2(high) and

the sample.

Table 1Measured strains and Eu incorporation in as-prepared and ball-milled Eu:GaN powder

Particle size(nm)

E2 (high) Raman shift(cm�1)

A1(LO) Raman(cm�1)

As-prepared Eu:GaN 2000–5000 0.8 0.8

After 20 h ball-milling

50–70 �12.9 �24.8

After 40 h ball-milling

30–50 �14.7 �27.3

The phonon frequency shift in all powders is reported with respect to pure GaN.

Fig. 6. (a) Comparison of IR Transmittance of a KBr pellet with nano-Eu:GaN and as-pre

wavelengths. (b) IR transmittance of KBr pellet with Eu:GaN (at 1600 cm�1) shows signi

Eu:GaN is incorporated at 2 wt%. This wavelength was chosen since transmittance for

the E2 peak of approximately 12.1 and 13.9 cm�1, respectively(Table 1). With respect to as-prepared Eu:GaN, the (10 0) XRDpeak of the ball-milled powders is shifted to smaller angles(2Dy ¼ �0.12, �0.14 after 20, 40 h ball-milling ), while the (0 0 2)peak is shifted to a higher angle (2Dy ¼ 0.05, 0.1 after 20, 40 hball-milling). The relative shifts in the angles give a measure of thestrains in the a and c directions, respectively.

The relative shifts in the XRD peaks were used to calculate thestrains in both as-prepared and ball-milled powders. After 20 and40 h of ball-milling, the tensile strains in the a direction are 0.2%

tained even after ball-milling is performed, although the PL intensity is reduced. (b)

A1(LO) positions which are used to infer semiconducting properties and stresses in

s.

shift Strain along a-axis(%)

Strain along c-axis(%)

c/aratio

Eu conc.(at%)

0.09 0.13 1.628 0.5

0.2 �0.15 1.621 0.5

0.3 �0.3 1.617 0.5

pared Eu:GaN shows substantial improvement in transmittance over all measured

ficant improvement in transmission (410� ) of the pellet is seen when ball-milled

the nano-GaN:KBr pellet is at its peak value at this wavelength.

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T. Thomas et al. / Journal of Crystal Growth 311 (2009) 4402–4407 4407

and 0.3%, respectively, while the compressive strains in the c

direction are 0.15% and 0.3%, respectively. Incorporation of Euatoms in GaN using our synthesis method results in the expansionof both the a and c lattice constants in Eu:GaN and have beendetermined to be of 0.09% and 0.13%, respectively [7,21]. Thesestrains and Raman shifts were previously used in the Kisielowskimodel [22] to study Eu incorporation in GaN powder. In this study,this model revealed a Eu incorporation of �0.5 at% [7]. We observethat the lattice constant ratio (c/a) decreases with increasing ball-milling time, but the volume of a unit cell of as-prepared Eu:GaNand ball-milled Eu:GaN remains the same. This indicates that theconcentration of Eu atoms remains the same even after nanosiz-ing. The change in c/a ratio indicates increasing biaxial strain withincreasing ball-milling time.

The electron concentration of the GaN particles is known toinfluence the A1(LO) [23]. The A1 peak is observed to be blueshifted and has increasing widths with increasing electronconcentration. Beyond an electron concentration of around(�1019 cm�3) the peak disappears completely. The as-preparedpowder has a distinct A1 (LO) peak at 734.8 cm�1, which is veryclose to that of unstrained Eu:GaN with an electron concentrationof o1�1017 cm�3 [6,23]. However, ball-milling Eu:GaN for 20 and40 h shifts the A1(LO) to 710.8 and 708.3 cm�1, respectively. Theseshifts suggest that these powders continue to have semiconduct-ing properties with electron concentration below 1017 cm�3 [23].The reason for the anomalously large red-shift [21] in the A1(LO)peak in ball-milled powders is currently unknown, however.

To study the suitability of ball-milled nanosized Eu:GaN forlaser applications, infrared (IR) transmittance was investigated byembedding these powders in a KBr matrix commonly used forFourier transform infrared (FTIR) spectroscopy. The pellet wasformed by mixing 80 mg of KBr, with 2, 5, 7.5 and 10 wt% of as-prepared and ball-milled Eu:GaN. Use of ball-milled Eu:GaNresulted in increased transmission at all weight percentages,including a tenfold improvement at 2 wt% (Fig. 6a and b). Weattribute the higher transmittance to the reduced scattering dueto smaller GaN particles in the pellet made using ball-milledpowders.

In conclusion, we have demonstrated significant enhancementof the PL intensity of Eu:GaN using acid rinses to obtain visuallyclearer powders. We have shown the improvement of the PLenhancement is primarily due to removal of dark Eu-containingcompounds from the powder through 1 M HNO3 rinse. We havealso demonstrated that luminescent nano Eu:GaN, free from YL

can be obtained using a top-down ball-milling approach. Thenanoparticles obtained in this manner have significant agglom-eration possibly due to increased surface-to-volume ratio. Ramanstudies on the particles show that the ball-milled particles retainthe semiconducting properties. Longer ball-milling duration timesresults in further reduction of particle sizes and increasedagglomeration. Both Raman spectroscopy and XRD indicate thatthe ball-milled particles have tensile and compressive stresses inthe a and c directions, respectively, by virtue of the mechanicalprocess employed. We also show that this ball-milled Eu:GaNpowder when used in KBr matrix shows significantly improvedtransmittance suggesting possible applications in lasers.

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