Deposition pressure effect on chemical, morphological and opticalproperties of binary Al-nitrides

Jaime Andrés Pérez Taborda a,b,n, J.C. Caicedo c, M. Grisales d, W. Saldarriaga e, H. Riascos a

a Universidad Tecnológica de Pereira, Grupo Plasma Láser y Aplicaciones, Colombiab Functional Nanoscale Devices for Energy Recovery Group, Institute of Microelectronics of Madrid, Spainc Tribology, Powder Metallurgy and Processing of Solid Recycled Research Group, Universidad del Valle, Cali, Colombiad Universidad De la Amazonia, Colombiae Laboratorio de Materiales Cerámicos y Vítreos, Universidad Nacional de Colombia, Sede Medellín, A.A. 568, Medellín, Colombia

a r t i c l e i n f o

Article history:Received 17 August 2014Received in revised form3 December 2014Accepted 8 December 2014

Keywords:Pulsed laser depositionAluminum nitrideMorphology and optical properties

a b s t r a c t

Aluminum nitride films (AlN) were produced by Nd:YAG pulsed laser (PLD), with repetition rate of10 Hz. The laser interaction on Al target under nitrogen gas atmosphere generates plasma which isproduced at room temperature with variation in the pressure work from 0.39 Pa to 1.5 Pa thusproducing different AlN films. In this sense the dependency of optical properties with the pressure ofdeposition was studied. The plasma generated at different pressures was characterized by opticalemission spectroscopy (OES). Additionally ionic and atomic species from the emission spectra obtainedwere observed. The plume electronic temperature has been determined by assuming a local thermo-dynamic equilibrium of the emitting species. Finally the electronic temperature was calculated withBoltzmann plot from relative intensities of spectral lines. The morphology and composition of the filmswere studied using atomic force microscopy (AFM), scanning electron microscopy (SEM), X-rayphotoelectron spectroscopy analysis (XPS) and Raman Spectroscopy. The optical reflectance spectraand color coordinates of the films were obtained by optical spectral reflectometry technique in therange from 400 nm to 900 nm. A clear dependence in morphological properties and optical properties,as a function of the applied deposition pressure, was found in this work which offers a novelapplication in optoelectronic industry.

& 2014 Elsevier Ltd. All rights reserved.

1. Introduction

Aluminum nitride (AlN) thin films are applied widespread becausethey have some excellent properties such as chemical stability, highthermal conductivity, low electric conductivity and wide band gap(6.2 eV). Moreover, it presents a thermal expansion coefficient similarto that of GaAs, and a higher acoustic velocity, making it excellent foroptical devices in the ultraviolet spectral region, acoustic optic devices,and surface acoustic wave (SAW) devices. Polycrystalline films exhibitpiezoelectric properties and can be used for the transduction of bothbulk and surface acoustic waves. Pulsed laser deposition (PLD) growthof AlN films is rather critical because of its tendency to present micro-cracking. This tendency is more evident with increasing the thicknessof the film and when using silicon substrates, particularly in the (100)orientation, while using silicon substrates has been shown to improve

the films' growth. Pulsed laser deposition (PLD) using nanosecondpulses is considered to be one of the most promising techniques forthe synthesis and deposition of thin films [1–4]. This method hasadvantages such as high reproducibility, control of the film growthrate and stoichiometry and low impurity concentration in the compo-sition of deposited films. On the other hand aluminum nitride (AlN)exhibits attractive properties such as thermal and chemical stability,high thermal conductivity, high dielectric permittivity, breakdownfield, high-speed piezoacoustic wave and mechanical hardness [1].

Many authors in the literature have discussed the effect ofgrowth conditions of AlN thin films deposited by PLD related tothe crystallinity, morphology and optical response [5–8]. Clearly,the growth characteristics influence the final properties of thematerials in a thin layer, but there is a deficiency in the discussionof the effect produced by the variation of the pressure tank to thevariation in color purity layered AlN obtained by PLD.

The study of pulsed laser ablation plumes has increased theattention recently due to its importance in laser deposition. Theplasma state is often called the fourth state of matter and transientphenomenon in nature with characteristic parameters dependent

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Optics & Laser Technology

http://dx.doi.org/10.1016/j.optlastec.2014.12.0090030-3992/& 2014 Elsevier Ltd. All rights reserved.

n Corresponding author. Present address: Instituto de Microelectrónica de Madrid(IMM-CSIC), Calle de Isaac Newton 8, Tres Cantos, 28760 Madrid, Spain.

E-mail address: jaimeandres@ingenieros.com (J.A. Pérez Taborda).

Optics & Laser Technology 69 (2015) 92–103

on the rapidly evolving component species. These parameters arehighly dependent on the irradiation conditions, laser intensity,pulse duration, wavelength, composition and atmosphere. Takinginto account that the relationship between plasma and morpho-logical quality in the films is very important, in this sense the AlNfilms are used as substrates for SAW sensors where the surfacequality is a decisive factor in the sensors performance [9,10].

So, the goal of this work is to study the effect of the applieddeposition pressure on the chemical, morphological propertiesand optical properties of binary AlN films deposited by PLD on Si(100) for use in optical and electronic applications. Here, usingnitrogen as working gas, results on AlN films deposited from Altargets, their characterization by X-ray photoelectron spectroscopy(XPS), Raman Spectroscopy and scanning electron microscopy(SEM) as well as investigations associated to changes in opticalresponse such as reflectance and color purity as function ofpressure deposition values were reported.

2. Experimental

In this research the experiments were carried out in usual PLDconfiguration consisting of a laser system into the multiportstainless steel vacuum chamber equipped with a gas inlet, arotating target and a heated substrate holder. The Nd:YAG laserthat provides pulses at the wavelength of 1064 nmwith 9 ns pulseduration and a repetition rate of 10 Hz was used. The laser beamwas focused with an f¼23 cm glass lens on the target at the angleof 451, with respect to the normal. The target was rotated to2.2 rpm to avoid fast drilling. The distance between the target andthe substrate was 6.5 cm. The vacuum chamber was evacuateddown to 10�6 Pa before deposition by using a turbo-molecularpump backed with a rotary pump. The AlN thin films weredeposited in nitrogen atmosphere as working gas, in an atmo-sphere of nitrogen reactive, the nitrogen gas pressure variedbetween 0.39 Pa and 1.5 Pa and aluminum target (99.99%). Thefilms were deposited with a laser fluence of 7 J/cm2 for 15 min onsilicon (100) substrates. So, the plasma characterization wasperformed by optical emission spectroscopy (OES) by using aspectrometer model Jobin Yvon Triax 550 of 0.55 m, f¼6.4equipped with two gratings of 1200 l/mm and 150 l/mm, coupledto a CCD camera model 3000 air-cooled multi-channel and512�512 pixels. The crystal structure of the coating was deter-mined by using a D8 Advance Bruker X-ray diffractometer withCu-Ka (λ¼1.5405 Å) radiation. For the surface study a scanningelectron microscope Philips XL 30 was used . The AlN layersthickness around 150 nm was determined by the design of a stepbetween the substrate and the film. A profiler was used to performcontinuous scanning surface that takes into account the film andthe substrate area. A Dektak 8000 profilometer device with a tipdiameter of 1270.04 μm, scan length range (X) of 5070.1 μm–

20070.1 mm, scan height range (Y) from 10071 nm to100070.1 μm, measurement range 50 A–2.520 kA, vertical reso-lution (max.) of 1 A, sample thickness (max.) of 63.5 mm, hor-izontal resolution of 0.0033 um, stylus force from 1 to 100 mg andsample stage theta rotation of 3601 was used.

Chemical composition analysis of the coatings was done with aPhilips XL 30 FEG scanning electron microscope, an X-ray detectorand secondary electrons detector of Lithium Beryllium inside thechamber with the purpose of amplifying the signal in the EDSanalysis. Moreover, the XPS also was used on AlN samples todetermine the chemical composition and the bonding of alumi-num and nitrogen atoms using ESCA-PHI 5500 monochromatic Al-Kα radiation and a passing energy of 0.1 eV. The surface sensitivityof this technique is so high that any contamination can producedeviations from the real chemical composition; therefore, the XPS

analysis is typically performed under ultra-high vacuum condi-tions with a sputter cleaning source to remove any undesiredcontaminants. Morphological characteristics of the coatings likegrain size and roughness were obtained using an atomic forcemicroscope (AFM) from Asylum Research MFP-3Dr and calculatedby a scanning probe image processor (SPIP) which is the standardprogram for processing and presenting AFM data, therefore, thissoftware has become the de-facto standard for image processing innanoscale. The Al–N bond was verified by infrared spectroscopyand Raman and Fourier transform infrared spectroscopy (FTIR)characteristics of Al–N vibrational modes were found. Opticalreflectance spectra and color coordinates of the samples wereobtained by spectral reflectometry in the range of 400–900 nm bymeans of an Ocean Optics 2000 spectrophotometer. The coatedsamples received the white light from a halogen lamp illuminatorthrough a bundle of six optical fibers, and the light reflected on thesamples was collected by a single optical fiber and analyzed in thespectrophotometer. The fiber was fixed in perpendicular directionto the sample surface. An aluminum deposited by rapid thermalevaporation in high vacuumwas used as the reference sample, andthe experimental spectra were normalized to 100% reflectance ofthe reference sample. The morphology on AlN surface films wasanalyzed by SEM (Leika 360 Cambridge Instruments).

3. Results and discussion

3.1. Optical emission for the AlN plume

For the plasma generated by AlN materials a large number ofemission lines attributed to emission bands of aluminum nitridewas identified. In Fig. 1a, the most intense lines are emission ofaluminum species, apparently the main species emitted in theablation of aluminum species, being once ionized aluminum (Al II).The strongest lines in the spectrum of XII in plasma are at631.337 nm, for electron configuration 1s3p–1s3d. Atomic spectrallines are also indicating the presence of Al and atomic N2 (Fig. 1band c). The oxygen presence was observed in optical emission forthe AlN with 0.53 Pa and 0.66 Pa, which is a product of contam-ination in the vacuum chamber. All atomic emission lines wereidentified through the database of the National Institute ofStandards and Technology-NIST. Also emission lines of nitrogenspecies (neutral and multiply ionized) with most intense peaks at618.909 nm (N I), 644.902 nm (N III), 740.359 nm (N III) wereobserved. The emission peak of atomic nitrogen was dominantcompared to the emission peaks of atomic aluminum.

The oxygen presence is also attributed to low flow of nitrogengas during the degassing processes. The oxygen species observedare O II (762.882 nm), O III (751.325 nm) and O V (676.585 nm and743.153 nm). Shown in 509.985 nm an emission band of AlN (0.0)[11] is observed. A second emission band, weaker, is analyzed at523.060 nm for AlN (1.0) [8]. In this work the oxygen bands onlyare evidenced for a working pressure of 0.53 Pa. Moreover, inthis work participle density in Debye sphere Nd¼2.46�10�1 mwas found.

3.2. Local thermodynamic equilibrium for AlN films

In the local thermodynamic equilibrium for AlN it is possible totake into account that the plume is in local thermodynamicequilibrium (LTE) [12], therefore, the emission line intensity (I)in a specific wavelength (λm) may be expressed by

LnImnλmn

Amngmn

� �¼ Ln

NZ

� �� Emn

κTe

� �ð1Þ

J.A. Pérez Taborda et al. / Optics & Laser Technology 69 (2015) 92–103 93

where λmn is the transition wavelength, Imn is the intensity linetransition observed, Amn is the transition probability, gmn is thedegeneracy of the upper level, N is the total density of the exitedstate, Z is the partition function, Emn is the energy of the emit-ting level, k is the Boltzmann constant and Te is the electronictemperature. A typical plot is reported in Fig. 2 for the emission ofthe AlN plume. The higher temperature calculated in the presenceof N2 under 0.53 Pa can be associated to recombination phenom-ena which occurs during plume expansion and the thin filmsdeposition, in relation to local thermodynamic equilibrium of theelectron density, as shown in the following equation [12]:

neZ1:4� 1014T12e ðΔEmnÞ3cm�3 ð2Þ

where ne is the electron density, Te is the electronic temperature,ΔEmn is the transition from the upper energy level (Em) to thelower energy level (En).

In this paper a value of 5.90�1013 cm�1 was reported for theLTE approximation, which agrees with the literature [11–13].

On the other hand nitrogen elements are characteristic for thefirst and second positive system that occur between 250 nm and400 nm; although doing different variations in the spectra, such asintegration time and the width of the entrance slit, no prominent

lines were observed in this range. The relaxation of the excitedstate of nitrogen in the plasma emission is given by transitionsbetween atomic energy levels or through state transitions of theionized molecule and not by transitions of the neutral molecule.This suggests that the relaxation process is the recombination ofunpaired electrons with the ionized molecules (Fig. 2b).

3.3. Chemical composition

3.3.1. EDS analysis in AlN filmsThe EDS results from the AlN films surface showed the

presence of (Al, N, O) element, which is characteristic of thosematerials. The areas of the peaks were used to calculate thecomposition of both coatings; thus, the values from Fig. 3 indi-cated that AlN films were substoichiometrics. On the other hand, acareful correction has to be done in all stoichiometric analysesbecause EDS has low reliability for nitrogen concentration. In thissense, EDS elemental concentrations were obtained using the ZAFcorrection method; because certain factors related to the samplecomposition, called matrix effects associated with (atomic number(Z), absorption (A) and fluorescence (F)), can affect the X-rayspectrum produced during the analysis of electron microprobe

Fig. 1. Optical emission for the AlN plume with different values of nitrogen pressure: (a) emissivity for N2 1st positive system between 600 nm and 800 nm in AlN films asfunction of deposition pressure, (b) optical emission for the AlN with 0.53 Pa, and (c) optical emission for the AlN with 0. 66 Pa.

J.A. Pérez Taborda et al. / Optics & Laser Technology 69 (2015) 92–10394

and therefore, these effects should be corrected to ensure thedevelopment of an appropriate analysis.

The correction factors for a standard specimen of knowncompositions were determined initially by the ZAF routine. Therelative intensity of the peak K was determined by dead timecorrections and a referent correction for the X-ray measured. So,before each quantitative analysis of an EDS spectrum for AlNdeposited with 9.3�10�1 Pa, a manual background correction andan automated ZAF correctionwas carried out [14]. Thus, Fig. 3a showsthe energy-dispersive X-ray spectroscopy (EDS) values of AlN filmsdeposited with different work pressures. All samples were observedvia SEM and chemical analyses were done with an amplification of20,000� . The presence of oxygen has been often found during theproduction of AlN, which is normally associated to residual oxygen inthe chamber [11]. Fig. 3b shown the dependence of work pressure ondecrease of Al content; moreover a little increase in the N concen-tration in relation to the increase of deposition pressure has beenobserved. This effect can be associated to high sensitivity of ionicexchange in nitrogen under pressure changes.

3.3.2. XPS analysis in AlN filmsThe chemical relation between EDS results and XPS was

explored in the current research. Thus, the survey spectra of

Al 2p, and N 1s in Fig. 4 were recorded from AlN films, as shownin Fig. 5. From Fig. 5(a), the Al 2p peak is composed of a shoulderseparated by 1.7 eV with intense peak. The XPS spectrum of Al

23 24 25 26 27 28 29 30 31-26

-24

-22

-20

-18

-16

-14

ln(I

λ/ g

A)

Energy (eV)500 502 504 506 508 510 512 514 516 518 520

0.0

7.0x103

1.4x104

2.1x104

2.8x104

3.5x104

4.2x104

4.9x104

5.6x104

Inte

nsity

(a.u

.)

λ (nm)

Fig. 2. Local thermodynamic equilibrium: (a) determination of electronic temperature Te¼8.832.3 K from AlN plume emission in 0.533 Pa N2 by using the line N II and(b) the plasma electron density ne calculated from Stark broadening, for N II transition (2s2p2–4p) 3p at the line 504.871 N II, and the ne¼201�1019 cm�3. The error barsindicate the standard deviation values of the measurements for all AlN materials films.

2 4 6 8 10 12 14 1615

20

25

30

35

40

45

50 Al O

Deposition Pressure (10-1Pa)

Al c

once

ntra

tion

(%)

20

25

30

35

40

45

50

55

60

65N

N c

once

ntra

tion

(%)

Stoichiometric Concentration

Fig. 3. Chemical composition by EDS: (a) energy-dispersive X-ray spectroscopy (EDS) values and SEM surface images of AlN films and (b) correlation between work pressurein the Al–N plasma and aluminum, nitrogen and oxygen contents for deposited AlN thin films. The error bars indicate the standard deviation values of the measurements forall AlN systems.

1400 1200 1000 800 600 400 200 00

1x104

2x104

3x104

4x104

Inte

nsity

(c/s

)

Binding energy (eV)

Fig. 4. XPS spectrum of Al 2p–N 1s from Al–N film as a function of appliedpressure.

J.A. Pérez Taborda et al. / Optics & Laser Technology 69 (2015) 92–103 95

2p can be fitted well by two Gaussian functions. The value ofbinding energies obtained for the Al 2p peak was 73.9 eV and thehigher value for Al 2p was 75.9 eV, respectively. According to theliterature [15–17] for the Al 2p peak, the first one (73.9 eV) and thesecond one (75.9 eV) can be assigned to Al–N and Al–O bondsrespectively. The appearance of the peak at 73.9 eV clearly showsthat Al has reacted with N; therefore, it can be assigned to AlN[16,17]. In Fig. 4 N 1s peak is composed of spin doublets, eachseparated by 2.9 eV. The XPS spectrum of N 1s can be fitted well bytwo Gaussian functions which depicts the N 1s spectrum withvalues at 397.3 eV and 400.2 eV characteristic for N–N and Al–Nbonds, respectively [18,19].

The high resolution X-ray photoelectron spectroscopy (XPS)results for AlN deposited with 9.3�10�1 Pa demonstrate that Alatoms bonded to N in the form of nitride, because the elementalconcentration of the Al–N film was obtained by adjusting the laserincidence on Al target and N2 was the working gas in this research;it was discovered that amounts of Al–N in the AlN film weremaximum in the current establishment of process conditions andthe ratio of Al to N in the film was about 2:1. Generally, formativeAl–N phase indicates that the aluminum and nitrogen activity andactivation energy provided by the present deposition conditionsare enough for the formation of AlN thin film. Although the surfacetemperature of the substrate during deposition of AlN film isaround 300 1C, the substrate lies in a high-density plasma regionand a high ion-to-atom ratio of aluminum and nitrogen can bepropitious to the formation of AlN phase at the low temperaturebelow 330 1C. All aluminum-nitride films with all Al 2p peakswere fitted as one or more pairs of spin–orbit split sub-peaks witha separation of 0.4 eV between the Al 2p3/2 and Al 2p1/2 compo-nents (Fig. 5a). The ratio of the area of the 2p3/2 component to thearea of the 2p1/2 component was fixed at 2:1. Moreover all N 1speaks were fitted as one or more pairs of spin–orbit split sub-peaks with a separation of 0.2 eV, sowing the N–Al boundcentered in 399.66 eV and N–N centered in 402.23 eV (Fig. 5b).

All Al 2p sub-peaks were fitted as 95% Gaussian. For this study,the binding energy of a fitted Al 2p spin–orbit sub-peak pair isreported as the centroid of the pair. The centroid of the spin–orbitpair in eV was calculated as shown

Al2p Centroid ðeVÞ ¼AAl 2p3=2

� EAl 2p3=2þAAl 2p1=2

� EAl 2p1=2

AAl 2p3=2þAAl 2p1=2

ð3Þ

where A is the sub-peak integrated area and E is the adjusted sub-peak binding energy in eV. Therefore, calculation of the peak

areas without O 1s contribution gives an atomic ratio of Al:N¼0.392:0.588, which is similar to the stoichiometry ofAl0.40N0.60 [20] and close to EDS results showed in Fig. 3. So, inFig. 6, it is a clear dependence on the concentration of nitrogenand oxygen in AlN thin films.

3.4. Structural characterization by XRD results

The x-ray diffraction patterns for AlN deposited with9.3�10�1 Pa observed in Fig. 7 are at 37.91 and 44.291, corre-sponding to AlN c-axis (0002) and AlN (200) orientations respec-tively. As it can be observed, strong preferential c-axis orientationis obtained for the lower nitrogen–aluminum ratio. The crystal-lographic orientation of the grains in the film is determined bythe preferential growth of certain crystal planes over others. Themechanism of preferential orientation of AlN films can beexplained by the crystalline lattice structure generated by AlNconfiguration materials which is in agreement with optical emis-sion results (Fig. 1), EDS (Fig. 3) and XPS results (Fig. 5). To furtherobtain information regarding bond formation and structure thepolycrystalline hexagonal structure of wurtzite type (file no. 25-1133 of form JCPDS-ICDD diffraction database) was detected in all

80 79 78 77 76 75 74 73 72 71 700.0

5.0x103

1.0x104

1.5x104

2.0x104

2.5x104

Al-O76.27 eV

Al2p

Inte

nsity

(a.u

.)

Binding energy (eV)

Al-N73.78 eV

406 404 402 400 398 396 394

0.0

4.0x103

8.0x103

1.2x104

1.6x104

Inte

nsity

(a.u

.)

Binding energy (eV)

N1s

N-Al399.66 eV

N-N402.23 eV

Fig. 5. High-resolution XPS spectrum of: (a) Al 2p and (b) N 1s, where the few formation of oxy-nitride N–Al–O and Al–N bonds are observed to occur at differenttemperatures.

4 6 8 10 12 14 1630

35

40

45

50

55

60

65

70C

ompo

sitio

n (a

t.%)

Deposition Pressure (10-1Pa)

NO

25

30

35

40

45

50

55

60

65

70

75

Com

posi

tion

(at.%

)

Fig. 6. Compositions results from XPS analysis showing the pressure dependenceat the concentrations of N and O deposited on Si at 300 1C. Oxygen is the onlyobserved contamination in these films. The error bars indicate the standarddeviation values of the measurements for all AlN layers.

J.A. Pérez Taborda et al. / Optics & Laser Technology 69 (2015) 92–10396

films. Such one axial hexagonal texture with c-axis perpendicularto the Silicon substrate has been detected in AlN films. As the (103)planes make a large angle with the (200) ones, the (103) diffrac-tion is competitive to the (0002) one in terms of texture and theratio is thus directly related to the contribution of the hexagonal(0002) texture component.

3.5. Vibrational characterization by FTIR and Raman results

FTIR spectrometry measurements were carried out for thesame films previously analyzed by XRD. It was reported thatcrystalline AlN exhibits characteristic transverse optical (TO) andlongitudinal optical (LO) modes. Fig. 8a shows FTIR spectra of aspectrum for AlN deposited with 9.3�10�1 Pa in the range of468–800 cm�1; after deconvolution the respective modes areactive in the infrared observed, especially a narrow band centeredat 680 cm�1 which may be attributed to the contribution of thephonon mode E1 (TO) of the w-AlN as well as the presence of fivebands around 485 cm�1, 520 cm�1, 615 cm�1, 655 cm�1 and691 cm�1, associated with Al–O bonds characteristic of a sym-metric stretching, Al–N non-stoichiometric phases (AlxNy), thephonon mode A1 (TO) of hexagonal AlN, the LO phonon mode ofAlN hexagonal and hexagonal Al–N, respectively. Inside the

deposited films there is residual stress that induces the shift ofthe FTIR peaks from their characteristic positions. It can be due tothe non-equilibrium nature of PLD [20–22].

In this sense the AlN normally crystallize in the hexagonalwurtzite structure (space group C46v-P63mc) with four atoms inthe unit cell. Then, from Raman results, in Fig. 8b for AlN depositedwith 9.3�10�1 Pa it was possible to observe that the k¼0 pointgroup theory predicts the following eight sets of modes: 2A1þ2Bþ2E1þ 2E2 of which one Al, one El, and two E2 are Ramanactive. One set of A1 and one of E1 correspond to acoustic phonons.The B modes are silent [1]. Note that phonons with E1 and E2symmetry, respectively, are twofold degenerate. The modes withA1 and E1 symmetry are also infrared active. The frequencies are allmeasured with an error of 71 cm�1. Moreover Table 1 shows theFTIR and Raman active modes associated to Al–N vibrations[7–10]. The FTIR and Raman spectra are in good agreement withthe optical emission results (Fig. 1), EDS (Fig. 3), XPS results (Fig. 5)and XRD results (Fig. 7), which confirm the formation of largehexagonal AlN material films.

30 35 40 45 50 55 60 65 70

Si -

Subs

trat

e

AlN

(200

)

AlN

(103

)

AlN

(000

2)

Inte

nsity

(u.a

)

2θ (Degrees)

P = 0.93 Pa

Fig. 7. XRD results showing the polycrystalline hexagonal structure of AlN wurtzite(file no. 25-1133 of form JCPDS-ICDD diffraction database) deposited on Si (100)substrates at 300 1C and a pressure of 0.93 Pa nitrogen.

480 560 640 720 80084

88

92

96

100

Tran

smitt

ance

(%)

Wavenumber (Cm-1) Wavenumber (Cm-1) 600 650 700 750 800 850 900

Ram

an In

tens

ity

Fig. 8. Vibrational analysis for AlN materials films: (a) FTIR spectroscopy of AlN films deposited on Si (100) substrates at 300 1C and a pressure of 0.93 Pa nitrogen with E2(high) phonon mode and (b) Raman shift measures of AlN films deposited with 9.3�10�1 Pa where it was possible to observe that the k¼0 point group theory predicts thefollowing eight sets of modes.

Table 1Vibrational modes reported in the literature for hexagonal aluminum nitride films.

Mode symmetry Frequency (cm�1) Reference

E2 665 [7]303 [10]426 [10]

A1 (TO) 667 [7]659 [8–9]

E1 (TO) 667 [7]672 [8]671 [9]614 [10]

A1 (LO) 910 [7]897 [8]888 [9]663 [10]

E1 (LO) 910 [7]912 [8]895 [9]821 [10]

J.A. Pérez Taborda et al. / Optics & Laser Technology 69 (2015) 92–103 97

3.6. Surface topography and morphological results analyzed by AFMand SEM

The observed dependence of the AlN films surface morphologyunder nitrogen pressure during deposition is closely related withthe film growth mechanism, associated to the surface diffusionlength (L) which is given by [20]

L� Dτð Þ1=2 ð4Þwhere D is the diffusion coefficient and τ is the residence time ofadatoms. Larger values of diffusion length imply more time for theadatoms to find energetically favorable lattice positions, thus,reducing the density of surface defects and improving the crystalquality.

Associating Eq. (4), Table 1 and Fig. 9 it is possible to show thesurface morphology of AlN films. Therefore, the changes onmorphological surface as functions of increase in the depositionpressures were studied by recording AFM images along with SEMmicrographs. These results evidence the random distribution ofmicro-particles or micro-droplets on these surfaces as a functionof deposition pressure (0.66 Pa and 0.53 Pa). Thus, the depositionpressure affects clearly the increase of grain size, roughness andmicro-drops; this can be possible due to low surface mobilitywhen the pressure was varied from 0.39 Pa to 1.5 Pa. This surfacemobility reduces the possibility that the micro-drops are anchoredon the surface when arriving with high energy on AlN film. Otherpossible reason can be associated with the mean free path thatproduces surface diffusion of nano-drops or micro-drops whichcan decrease the overall number of particles, also affecting theboundaries sizes. In this sense it was presented in Table 2 thesurface roughness, grain size for AlN films grown at 0.39 and0.53 Pa, and their optical emission lines due to optical emissionspectrometry signals from AlN plasma.

3.7. Optical reflectance analysis of AlN films

The reflectivity measure is the fractional amplitude of thereflected electromagnetic field, while reflectance refers to thefraction of incident electromagnetic power that is reflected at aninterface. The reflectance is thus the square of the magnitude ofthe reflectivity. The reflectivity can be expressed as a complexnumber as determined by Fresnel's equations for a single layer,whereas the reflectance is always a positive real number. In certainfields, reflectivity is distinguished from reflectance by the fact thatreflectivity is a value that applies to thick reflecting objects. Whenreflection occurs from thin layers of material, internal reflectioneffects can cause the reflectance to vary with surface thickness.

Reflectivity is the limit value of reflectance as the surface becomesthick; it is the intrinsic reflectance of the surface, hence irrespec-tive of other parameters such as the reflectance of the rear surface.On the other hand, the dominant wavelength of a color stimulus isdefined as the wavelength of the monochromatic stimulus that,when additively mixed in suitable proportions with the specifiedachromatic stimulus, matches the color stimulus considered [21].

Taking into account the above, an example in thin filmcalculator in OptiScan is given to calculate the reflectance andtransmittance of Krestchmann configuration which generate sur-face plasma resonance at a certain incident angle. Unfortunatelythe other properties of the surfaces, such as reflectance, transmit-tance, or phase change, are rarely satisfied. However the thin filmsare commonly used to modify these properties without alteringthe specular behavior. In an optical coating, the films, togetherwith their support, or substrate, are generally solid [22]. Theparticular materials used for the AlN films vary with the applica-tions. It is possible to construct assemblies of thin films which willreduce the reflectance of a surface and hence increase thetransmittance of a component, or increase the reflectance of asurface, or which will give high reflectance and low transmittanceover part of a region and low reflectance and high transmittanceover the remainder and so on. In this sense in the current work forAlN films the reflectance, R was taken as the ratio of the irradianceof the reflected beam to that of the incident beam, and transmit-tance, T, as the ratio of the irradiance of the transmitted beam tothat of the incident beam, and defined as follows [22,23]:

R¼ η0�Yη0þY

� �Uη0�Yη0þY

� �n

; T ¼ 4η0ReðYÞðη0þYÞU ðη0þYÞn ð5Þ

where R is the reflectance, T is the transmittance, η0 is the surfaceadmittance for incident medium, and Y is the surface admittanceof the thin films and substrate; moreover the effects of multiplefilms are included in the surface admittance. Each layer generatesa matrix in the equation which will change the electric andmagnetic fields [22,23].

Fig. 9. Deposition conditions as functions of morphological properties: (a) Relationship among surface roughness, grain size with plasma pressure and (b) SEM micrographand AM images for AlN thin films deposited with a pressure of 0.66 Pa and 0.53 Pa.

Table 2Surface roughness, grain size for AlN films grown at 0.39 and 0.53 Pa, and theiroptical emission lines due to optical emission spectrometry signals from AlNplasma.

Pressure(Pa)

Roughness(nm)

Grain size(nm)

%At(Al)

%At(N2)

OES

0.39 3.8 85.2 26.30 50.85 Al XII—N III—O V AlN(0.0) AlN (1.0)

0.53 3.6 62.9 23.70 58.80 Al XII—N II—O III

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In this work the AlN films have been compared with purealuminum because the aluminum mirror finish has the highestreflectance of any metal in the 200–500 nm range and the 3000–10,000 nm range (far IR) regions, while in the 500–700 nm visiblerange it is slightly outdone by AlN and silver, but in the 700–3000 nm range (near IR) it is slightly outdone by gold and coppermaterials [24]. Fig. 10 shows the optical reflectance spectra of theAlN single layers obtained at different deposition pressures. Thereflectance of the aluminum and the eye sensibility are shown forcomparison. The spectra of the samples show high reflectance forlong wavelengths, near to 62% for the AlN films deposited with4�10�1 Pa and close to 28% for AlN films deposited with 1.5 Pa.These values of reflectances at these wavenumbers agree well withprevious reports in the literature for AlN films [25,26]. However inthis work it was observed that the minimum of reflectance at550 nm is due to interference effect in reflected light.

However a clear decrease in reflectivity for short wavelengthsis seen, characteristic of a system with high free electron densitywith a reflectance edge below 530 nm due to a screened plasmaresonance [27]. The white and aluminum colors of the AlN filmsare a result of the steep plasma reflection edge that occurs in the

visible region where the reflectivity minimum is around 540 nm.The reflectance values for AlN films are in good agreement withthe optical emission results (Fig. 1), EDS (Fig. 3), XPS results(Fig. 5), XRD results (Fig. 7), FTIR-Raman spectra (Fig. 8), andAFM-SEM results (Fig. 9). The reflectance values for AlN films arein good agreement with the optical emission results (Fig. 1) by thespectral signals Al XII (631.337 nm) and NI (618.909 nm), charac-teristics of AlN, EDS (Fig. 3) with XPS (Fig. 5) results because of theAlN adjusted stoichiometry strapped with bounds from Al 2p andN 1s spectral signals (Al–N 73.78 eV), XRD results (Fig. 7) asso-ciated with polycrystalline hexagonal structure of wurtzite type(0002) adjusted to high reflectance, FTIR-Raman spectra (Fig. 8)due to vibrational signals typical of AlN with suitable opticalwhining (Al–N E1(TO) 678.8 cm�1 and Al–N A1(TO) 692.5 cm�1),and AFM-SEM results (Fig. 9) associated with superficial morphol-ogy and adjusted to the other physical and chemical characteristicsdiscussed above absorbs and disperses spectral lines of lightincident at the rank of major optical reflectance [28].

Moreover in this research high dependency on reflectancepercentages in this work was found when the deposition pressurewas varied from 0.39 Pa to 1.5 Pa, therefore, the changes in opticalproperties can be related not only with changes on temperaturedeposition but also with changes generated on morphology sur-face films due to variation of deposition pressure (Fig. 11a). Fig. 11aexhibits one constant region for wavelength of 760–800 nm; inthose regions it is possible to appreciate the effect of the deposi-tion pressure on reflectance of the films. Fig. 11b shows thedecrease of reflectance when the deposition pressure is increased,which indicates that pressure also promotes the absorbance in theAlN deposited via PLD.

Moreover the error bars of the values presented in Fig. 10 wereobtained at the base of the uncertainty in adjusting of reflectancecurves as a function of wavelength, to assign values to theparameters determined by fitting (reflectance or transmittancefor AlN films). Therefore, the reflectance values shown in Fig. 11b(filled triangles) are the average of data obtained in each run andthus have their respective error bars. In this sense the error barsindicate the standard deviation values of the measurements for allAlN films [28].

From the reflectance spectra a weak but clear effect of deposi-tion pressure on the optical properties is seen. As the pressure isdecreased, the reflectance of the films tends to be higher in thenear infrared region, while the minimum in reflectance, between562 nm and 571 nm for all AlN films.

400 500 600 700 800 9000

102030405060708090

100

Ref

lect

ance

(%)

Wavelengeth (nm)

Fig. 10. Dominant wavelength and color purity results: (a) Optical reflectanceof AlN films deposited onto Si (100) substrates at different deposition pressures(0.39–1.5 Pa); aluminum optical reflectance and eye sensibility were also plotted asreferences.

760 765 770 775 780 785 790 795 8002025303540455055606570

Ref

lect

ance

(%)

Wavelength (nm)0.4 0.6 0.8 1.0 1.2 1.4 1.6

25

30

35

40

45

50

55

60

65

70

Ref

lect

ance

(%)

Deposition pressure (Pa)

Fig. 11. Reflectance dependency: (a) Reflectance of AlN films deposited onto Si (100) substrates at different deposition pressures of 760–800 nm wavelength and(b) reflectance as function of deposition pressure. The error bars indicate the standard deviation values of the measurements for all AlN films.

J.A. Pérez Taborda et al. / Optics & Laser Technology 69 (2015) 92–103 99

3.8. Dominant wavelength and color purity analysis of AlN films

3.8.1. Dominant wavelengthIn order to calculate dominant wavelength for AlN films, it was

necessary first introduce the identification of a color by its “x–ychromaticity coordinates” as plotted in the Chromaticity Diagram(Fig. 12). The diagram enables to quantitatively graph the hue andsaturation of a particular color by its dominant wavelength andexcitation purity, respectively [29].

The first step in calculating the chromaticity coordinates is tocompute the “tristimulus values” of a particular filter with aparticular illumination for AlN films. Loosely, the tristimulusvalues can be thought of as the amount of red, green, and bluein the filter, but the correspondence is not quite that simple. Thesevalues are based on the empirically determined CIE Color Match-ing Functions X�(λ), �Y(λ), and Z�(λ) [29]. Note that we typicallyuse the CIE 1931 2-degree field-of-view standard, as this one is themost well-known and widely used. It is useful to note that bydefinition the y�(λ) Color Matching Function is identical to thePhotopic Curve V(λ), or y�(λ)� V(λ). The Color Matching Func-tions can also be obtained from numerous sources presented inthe literature [29,30]. In this sense the tristimulus values X, Y, andZ are then given by

X ¼Z 1

0IðλÞTðλÞχðλÞdλ ð6Þ

Y ¼Z 1

0IðλÞTðλÞyðλÞdλ ð7Þ

Z ¼Z 1

0IðλÞTðλÞzðλÞdλ ð8Þ

For determination of dominant wavelength in AlN films (Table 3),one simply constructs a line between the chromaticity coordinates ofthe reference white point on the diagram (for instance, CIE-E, CIEC,etc.) and the chromaticity coordinates of the filter and then extra-polates the line from the end that terminates at the filter point. Thewavelength associated with the point on the horseshoe-shaped curveat which the extrapolated line intersects is the dominant wavelength[30].

Dominant wavelength and color purity have been determinedfrom the reflectance spectra of all films. The color purity changeswith the deposition pressure, in this study ranges from 0.39 Pa to1.5 Pa for AlN films, away from the color purity of pure aluminum(0.90), confirming that the films are less white compared with thealuminum. So, the results of color measurement indicate that allfilms reflect a hue slightly shifted far to the white as compared toaluminum reflectivity and with similar color purity (Table 3). Thedominant wavelength was varied for all the samples, from 562 nmto 570 nm. AlN films deposited with lower pressure are situatedlittle close to that of pure aluminum reference (574 nm).

3.8.2. Color purityOnce the line is constructed to determine the dominant

wavelength, it is very straightforward to calculate the excitationpurity (color purity) of a color that represents the filter transmis-sion. The excitation purity applied for AlN films is defined to be theratio of the length of the line segment that connects the chroma-ticity coordinates of the reference white point and the color ofinterest to the length of the line segment that connects thereference white point to the dominant wavelength [31]. Theseline segments are illustrated in Fig. 12. As pointed out above, theexcitation purity is a well-defined quantitative measure of thesaturation of a particular color. The larger the excitation purity, themore saturated the color appears, or the more similar the color isto its spectrally pure color at the dominant wavelength. Thesmaller the excitation purity, the less saturated the color appears,or the more white it is. Pastel colors are very poorly saturated, forexample. When there is no well-defined dominant wavelength,the excitation purity is still defined as described above, except thatthe denominator should be taken as the length of the line segmentbetween the reference white point and the point at which thedominant wavelength construction line intersects the line thatcontains the purple colors near the bottom of the diagram. Forevery wavelength in the spectrum (e.g. AlN films), is possible tocalculate (X,Y,Z) from CIE color matching functions. Therefore thePlot (x,y) for all wavelengths in the spectrum generates a horse-shoe shaped diagram thus, all physical colors lie inside the horse-shoe. Finally the color, as determined from the tristimulus values,can now be represented graphically on a two-dimensional graphcalled the Chromaticity Diagram with x–y coordinates on thisgraph which are given by [31–33]

χ ¼ XXþYþZ

; y¼ YXþYþZ

ð9Þ

Taking into account the last discussion, the excitation purity(purity for short or color purity) of a stimulus is the differencefrom the illuminant's white point to the furthest point on thechromaticity diagram with the same hue (dominant wavelengthfor monochromatic sources); using the CIE 1931 color space givenby [31]

pe¼ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiðx�xnÞ2þðy�ynÞ2ðx1�xnÞ2þðy1�ynÞ2

sð10Þ

0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.70.0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9Y

Axi

s

X Axis

Fig. 12. Chromatic diagram, in the x, y coordinates, of the reflectivity for AlN films.White coordinates of achromatic point are located at (1/3, 1/3).

Table 3Optical characteristics for AlN films deposited at 300 1C onto Si substrate.

Deposition pressure(Pa)

Dominant wavelength(nm)

Colorpurity

Axis X Axis Y

4.0�10�1 571 0.71 0.3329 0.3415.3�10�1 570 0.66 0.343 0.3629.3�10�1 567 0.63 0.242 0.1231.5 562 0.62 0.210 0.119Reference aluminum 574 0.90 0.358 0.371

J.A. Pérez Taborda et al. / Optics & Laser Technology 69 (2015) 92–103100

where (xn, yn) is the chromaticity of the white point and (xI, yI) is thepoint on the perimeter whose line segment to the white pointcontains the chromaticity of the stimulus. Different color spaces, suchas CIELAB or CIELUV may be used, and will yield different results.

Taking into account the last discussion in the Fig. 13a areobserved the differences in the dominant wavelength for all AlNfilms deposited with different deposition pressures. In Fig. 13b theinfluence of deposition pressure on color purity can be observed.This graph shows the increase of purity values towards color graypurity. The wavelength is an important optical characteristic fordifferent materials in relation with the changes observed in AlNplasma (Figs. 1–3).

Moreover the dominant wavelength and color purity values forAlN films are in good agreement with the optical emission results(Fig. 1), EDS (Fig. 3), XPS results (Fig. 5), XRD results (Fig. 7), FTIR-Raman spectra (Fig. 8), AFM-SEM results (Fig. 9) and the reflec-tance values (Figs. 10 and 11) which confirm the susceptibility thatpresent the AlN films in terms of dominant wavelength and colorwith changes in deposition pressure. So, when the wavelength inthe AlN layers is changed it is possible to observe that natural coloris changed. In this sense the purity color dependence and otheroptical constant dependence in AlN films with pressure obtainedin this work demonstrate the possibility of some purity colorcontrol. The last discussion can be proved observing the changesin the optical energy gap, plasma frequency and refractive index asa function of deposition pressure (Table 4).

In this sense the error bars of the values presented in Fig. 12were obtained at the base of the uncertainty in adjusting chro-matic diagram for different disposition pressures, to assign valuesto the parameters determined by fitting (dominant wavelengthand color purity for AlN layers). Therefore, these optical constantvalues shown in Fig. 13 (filled, circles and squares) are the averageof data obtained in each run and thus have their respective errorbars. In this sense the error bars indicate the standard deviationvalues of the measurements for all AlN films.

3.9. Optical energy gap, plasma frequency and refractive index

3.9.1. Optical energy gapIn crystalline semiconductors, equation (11) has been obtained

to relate the optical energy gap (Egap) with absorption coefficientfrom reflectance results given by [34]

αðvÞhv¼ Bðhv�EgapÞm ð11Þwhere Egap is the optical energy gap, B and hυ are the optical gapconstant, and incident photon energy, respectively; α(v) is the

absorption coefficient defined by Beer–Lambert's law as α(v)¼2.302�Abs(λ)/d where d and Abs are the film thickness and filmabsorbance, respectively. For more precise determination of α, it isnecessary to perform corrections to the absorption due to reflec-tion; also, m is the index which can have different values of 1/2,3/2, 2, and 3 [34]. In this sense the optical energy gap as functionof deposition pressures for AlN films has been presented in Table 4.

3.9.2. Plasma frequencyThe plasma frequency (ωp) is the most fundamental time-scale

in plasma physics. Clearly, there is a different plasma frequency foreach species. However, the relatively fast electron frequency is, byfar, the most important, and references. So, it is easily seen that ωp

corresponds to the typical electrostatic oscillation frequency of agiven species in response to a small charge separation. Forinstance, consider a one-dimensional situation in which a slabconsisting entirely of one charge species is displaced from itsquasi-neutral position by an infinitesimal distance, associated tohigher charge carrier concentration with the higher the plasmafrequency. Therefore, plasmon oscillations for different materialsare excited in different spectral regions. Thus, in many semicon-ductors like (AlN), the plasma reflection edge can be found in thereflectance as functions of wavelength. The spectral position of theplasma reflection edge depends on the charge carrier concentra-tion according to Eq. (12) [35]. Taking into account the above theplasma frequency (ωp) as a function of deposition pressures forAlN films [36] has been presented in Table 4

ωp ¼1

2πc4πNe2

mnε1

� �1=2

ð12Þ

where ωp is the plasma frequency, N is the free electron density, e isthe electron charge, mn is the effective mass of electrons, and ε1 isthe high frequency dielectric constant and c is the velocity of light.

0.4 0.6 0.8 1.0 1.2 1.4 1.6560

562

564

566

568

570

572 Aluminum 574 (nm)

Dom

inan

t wav

elen

gth

(nm

)

Deposition pressure (Pa)

Aluminium (0.90) AlN 300 °C

0.4 0.6 0.8 1.0 1.2 1.4 1.6

0.60

0.62

0.64

0.66

0.68

0.70

0.72

0.74

Aluminium (0.90)

Col

or p

urity

Deposition pressure (Pa)

AlN 300 °C

Fig. 13. Color results for AlN films deposited with 0.93 Pa: (a) dominant wavelength as function of deposition pressure and (b) color purity as a function of depositionpressure. The error bars indicate the standard deviation values of the measurements for all AlN films.

Table 4Optical constants (energy gap, plasma frequency and refractive index) for all AlNfilms as function of deposition pressure.

Depositionpressures (Pa)

Optical energygap (eV)

Plasma frequency(cm�1)

Real refractiveindex

4.0�10�1 6.2 4125 2.1945.3�10�1 5.9 3873 2.1759.3�10�1 5.5 3220 2.1541.5�10�1 5.3 3210 2.149

J.A. Pérez Taborda et al. / Optics & Laser Technology 69 (2015) 92–103 101

3.9.3. Refractive indexIt is known that several widely used methods of analyzing

reflectance (R) and transmittance (T) for a supported thin filmneglect the effect of the rear surface of the substrate. Equations aregiven which relate R and T to the complex refractive index (n-ik)and thickness of the thin film, and a method for their solution hasbeen described. This relies on Powell's technique, and permitschanges to be made to the equations relating R, T to (n-ik). Thisflexibility has allowed the calculation of the effect of the neglect ofthe rear of the substrate. An example is given of the use of themethod for the determination of refractive index (n-ik). In thisresearch aluminum nitride (AlN) with wurtzite phase (w-AlN)with a wide band gap (6.2 eV) for semiconductor material wasused, giving it potential application for deep ultraviolet optoelec-tronics. The refractive index for all AlN films is shown in Table 4.The refraction index has been calculated from relation which holdsfor the reflectance measurements [37]

2nd¼Nλ ð13Þ

where n is the refraction index, d is the sample thickness, and N isthe interference order. The interference order was determinedgraphically. In the region where the refraction index is weaklydependent on λ (λ44500 Å) the dependence of N on 1/λ ispractically linear, and in the case of sufficiently thin sample Nmay be determined precisely from the intersection of this linewith the y-axis.

4. Conclusions

A dependency in relation to nitrogen concentration, roughnessand grain size in the AlN films with the nitrogen work pressurewas found in this work, increasing in this sense the nitrogenconcentration and the roughness on AlN films. The plasmapressure affects the stoichiometry and the morphological naturein the AlN films. The variation of nitrogen work pressure exhibitslow effect on intensity of the spectral lines emitted. The electrontemperature value (Te¼8832.3 K) presented in the aluminumnitride plasma is similar to previous works. Structural and com-positional results show that the thin films deposited at a tem-perature of 300 1C improved orientation of hexagonal AlNincreases as the reflectance values for the lower depositionpressures. X-ray photoelectron spectroscopy (XPS) confirmed theformation of the binary films AlN.

For AlN films the pressure deposition has a marked influenceon the optical properties. A decrease in the reflectance of 55%, areduction of color purity about 13% and decrease in the dominantwavelength around 1.6% was found with deposition pressurebetween 4.0�10�1 Pa and 1.5 Pa. This conclusion can be provedobserving the changes in the optical energy gap, plasma frequencyand refractive index as function of deposition pressure. So, thepurity color dependence in AlN films with pressure obtained inthis work demonstrates the possibility of some purity colorcontrol.

Acknowledgments

J.A. Pérez acknowledges projects: Nano-structured High-efficiencyThermo-Electric Converters (nanoHITEC) and Photoacoustic Mea-surements of Nanostructures for Thermoelectric Applications (PHO-MENTA) from MINECO and Santander bank and your scholarshipprogram “Young Professors and Researchers Latin America, Santan-der Universities” (Spain, 2013)

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