Deposition pressure effect on chemical, morphological and optical properties ofbinaryAl nitrides

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Deposition pressure effect on chemical, morphological and optical

properties 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, Colombia

b_{Functional Nanoscale Devices for Energy Recovery Group, Institute of Microelectronics of Madrid, Spain} c

Tribology, Powder Metallurgy and Processing of Solid Recycled Research Group, Universidad del Valle, Cali, Colombia d

Universidad De la Amazonia, Colombia e

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 2014 Received in revised form 3 December 2014 Accepted 8 December 2014

Keywords:

Pulsed laser deposition Aluminum nitride

Morphology and optical properties

a b s t r a c t

Aluminum nitridefilms (AlN) were produced by Nd:YAG pulsed laser (PLD), with repetition rate of 10 Hz. The laser interaction on Al target under nitrogen gas atmosphere generates plasma which is produced at room temperature with variation in the pressure work from 0.39 Pa to 1.5 Pa thus producing different AlNfilms. In this sense the dependency of optical properties with the pressure of deposition was studied. The plasma generated at different pressures was characterized by optical emission spectroscopy (OES). Additionally ionic and atomic species from the emission spectra obtained were 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 with Boltzmann plot from relative intensities of spectral lines. The morphology and composition of thefilms were studied using atomic force microscopy (AFM), scanning electron microscopy (SEM), X-ray photoelectron spectroscopy analysis (XPS) and Raman Spectroscopy. The optical reflectance spectra and color coordinates of thefilms were obtained by optical spectral reflectometry technique in the range 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 novel application in optoelectronic industry.

1. Introduction

Aluminum nitride (AlN) thin_ﬁlms are applied widespread because

they have some excellent properties such as chemical stability, high thermal conductivity, low electric conductivity and wide band gap

(6.2 eV). Moreover, it presents a thermal expansion coefﬁcient similar

to that of GaAs, and a higher acoustic velocity, making it excellent for optical devices in the ultraviolet spectral region, acoustic optic devices,

and surface acoustic wave (SAW) devices. Polycrystallineﬁlms exhibit

piezoelectric properties and can be used for the transduction of both bulk and surface acoustic waves. Pulsed laser deposition (PLD) growth

of AlNﬁlms is rather critical because of its tendency to present

micro-cracking. This tendency is more evident with increasing the thickness

of theﬁlm and when using silicon substrates, particularly in the (100)

orientation, while using silicon substrates has been shown to improve

theﬁlms' growth. Pulsed laser deposition (PLD) using nanosecond

pulses is considered to be one of the most promising techniques for

the synthesis and deposition of thin _ﬁlms[1_–4]. This method has

advantages such as high reproducibility, control of theﬁlm growth

rate and stoichiometry and low impurity concentration in the

compo-sition of depositedﬁlms. On the other hand aluminum nitride (AlN)

exhibits attractive properties such as thermal and chemical stability, high thermal conductivity, high dielectric permittivity, breakdown

ﬁeld, high-speed piezoacoustic wave and mechanical hardness[1].

Many authors in the literature have discussed the effect of

growth conditions of AlN thinﬁlms deposited by PLD related to

the crystallinity, morphology and optical response[5–8]. Clearly,

the growth characteristics inﬂuence the ﬁnal properties of the

materials in a thin layer, but there is a deﬁciency in the discussion

of the effect produced by the variation of the pressure tank to the variation in color purity layered AlN obtained by PLD.

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

Contents lists available atScienceDirect

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

Optics & Laser Technology

http://dx.doi.org/10.1016/j.optlastec.2014.12.009

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

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on the rapidly evolving component species. These parameters are highly dependent on the irradiation conditions, laser intensity, pulse duration, wavelength, composition and atmosphere. Taking into account that the relationship between plasma and

morpho-logical quality in theﬁlms is very important, in this sense the AlN

ﬁlms are used as substrates for SAW sensors where the surface

quality is a decisive factor in the sensors performance[9,10].

So, the goal of this work is to study the effect of the applied deposition pressure on the chemical, morphological properties

and optical properties of binary AlN_ﬁlms deposited by PLD on Si

(100) for use in optical and electronic applications. Here, using

nitrogen as working gas, results on AlNﬁlms deposited from Al

targets, their characterization by X-ray photoelectron spectroscopy (XPS), Raman Spectroscopy and scanning electron microscopy (SEM) as well as investigations associated to changes in optical

response such as reﬂectance and color purity as function of

pressure deposition values were reported.

2. Experimental

In this research the experiments were carried out in usual PLD

con_ﬁguration consisting of a laser system into the multiport

stainless steel vacuum chamber equipped with a gas inlet, a rotating target and a heated substrate holder. The Nd:YAG laser that provides pulses at the wavelength of 1064 nm with 9 ns pulse duration and a repetition rate of 10 Hz was used. The laser beam

was focused with anf¼23 cm glass lens on the target at the angle

of 451, with respect to the normal. The target was rotated to

2.2 rpm to avoid fast drilling. The distance between the target and the substrate was 6.5 cm. The vacuum chamber was evacuated

down to 106Pa before deposition by using a turbo-molecular

pump backed with a rotary pump. The AlN thin ﬁlms were

deposited in nitrogen atmosphere as working gas, in an atmo-sphere of nitrogen reactive, the nitrogen gas pressure varied between 0.39 Pa and 1.5 Pa and aluminum target (99.99%). The

ﬁlms were deposited with a laserﬂuence of 7 J/cm2_{for 15 min on}

silicon (100) substrates. So, the plasma characterization was performed by optical emission spectroscopy (OES) by using a

spectrometer model Jobin Yvon Triax 550 of 0.55 m, f¼6.4

equipped with two gratings of 1200 l/mm and 150 l/mm, coupled to a CCD camera model 3000 air-cooled multi-channel and

512512 pixels. The crystal structure of the coating was

deter-mined by using a D8 Advance Bruker X-ray diffractometer with

Cu-Ka (

λ¼

1.5405 Å) radiation. For the surface study a scanning

electron microscope Philips XL 30 was used . The AlN layers thickness around 150 nm was determined by the design of a step

between the substrate and theﬁlm. A proﬁler was used to perform

continuous scanning surface that takes into account theﬁlm and

the substrate area. A Dektak 8000 proﬁlometer device with a tip

diameter of 1270.04

μ

m, scan length range (X) of 5070.1

μ

m–

200₇0.1 mm, scan height range (Y) from 100₇1 nm to

100070.1

μ

m, 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 and

sample stage theta rotation of 3601was used.

Chemical composition analysis of the coatings was done with a Philips XL 30 FEG scanning electron microscope, an X-ray detector and secondary electrons detector of Lithium Beryllium inside the chamber with the purpose of amplifying the signal in the EDS analysis. Moreover, the XPS also was used on AlN samples to determine the chemical composition and the bonding of alumi-num and nitrogen atoms using ESCA-PHI 5500 monochromatic

Al-K

α

radiation and a passing energy of 0.1 eV. The surface sensitivity

of this technique is so high that any contamination can produce deviations 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 undesired contaminants. Morphological characteristics of the coatings like grain size and roughness were obtained using an atomic force microscope (AFM) from Asylum Research MFP-3Dr and calculated by a scanning probe image processor (SPIP) which is the standard program for processing and presenting AFM data, therefore, this software has become the de-facto standard for image processing in

nanoscale. The Al–N bond was veriﬁed by infrared spectroscopy

and Raman and Fourier transform infrared spectroscopy (FTIR)

characteristics of Al–N vibrational modes were found. Optical

reﬂectance spectra and color coordinates of the samples were

obtained by spectral reﬂectometry in the range of 400–900 nm by

means of an Ocean Optics 2000 spectrophotometer. The coated samples received the white light from a halogen lamp illuminator

through a bundle of six opticalﬁbers, and the light reﬂected on the

samples was collected by a single opticalﬁber and analyzed in the

spectrophotometer. Theﬁber wasﬁxed in perpendicular direction

to the sample surface. An aluminum deposited by rapid thermal evaporation in high vacuum was used as the reference sample, and

the experimental spectra were normalized to 100% reﬂectance of

the reference sample. The morphology on AlN surface ﬁlms was

analyzed 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 of emission lines attributed to emission bands of aluminum nitride

was identiﬁed. In Fig. 1a, the most intense lines are emission of

aluminum species, apparently the main species emitted in the ablation of aluminum species, being once ionized aluminum (Al II). The strongest lines in the spectrum of XII in plasma are at

631.337 nm, for electron conﬁguration 1s3_p_–_1s3_{d. Atomic spectral}

lines are also indicating the presence of Al and atomic N2(Fig. 1b

and c). The oxygen presence was observed in optical emission for the AlN with 0.53 Pa and 0.66 Pa, which is a product of contam-ination in the vacuum chamber. All atomic emission lines were

identiﬁed through the database of the National Institute of

Standards and Technology-NIST. Also emission lines of nitrogen species (neutral and multiply ionized) with most intense peaks at 618.909 nm (N I), 644.902 nm (N III), 740.359 nm (N III) were observed. The emission peak of atomic nitrogen was dominant compared to the emission peaks of atomic aluminum.

The oxygen presence is also attributed to low_ﬂow of nitrogen

gas during the degassing processes. The oxygen species observed are O II (762.882 nm), O III (751.325 nm) and O V (676.585 nm and 743.153 nm). Shown in 509.985 nm an emission band of AlN (0.0)

[11]is observed. A second emission band, weaker, is analyzed at

523.060 nm for AlN (1.0)[8]. In this work the oxygen bands only

are evidenced for a working pressure of 0.53 Pa. Moreover, in

this work participle density in Debye sphere Nd¼2.46101_m

was found.

3.2. Local thermodynamic equilibrium for AlNﬁlms

In the local thermodynamic equilibrium for AlN it is possible to take into account that the plume is in local thermodynamic

equilibrium (LTE) [12], therefore, the emission line intensity (I)

in a speci_ﬁc wavelength (

λ

m) may be expressed by

Ln Imn

λ

mn

Amngmn

¼Ln N

Z

Emn

κ

Te

(3)

where

λ

mn is the transition wavelength, Imn is the intensity line

transition observed, Amn is the transition probability, gmn is the

degeneracy of the upper level,Nis the total density of the exited

state, Z is the partition function,Emn is the energy of the

emit-ting level, k is the Boltzmann constant and Te is the electronic

temperature. A typical plot is reported inFig. 2for the emission of

the AlN plume. The higher temperature calculated in the presence

of N2under 0.53 Pa can be associated to recombination

phenom-ena which occurs during plume expansion and the thin ﬁlms

deposition, in relation to local thermodynamic equilibrium of the

electron density, as shown in the following equation[12]:

neZ1:41014Te12ðΔEmnÞ3cm3 ð2Þ

whereneis the electron density,Teis the electronic temperature,

Δ

Emn is the transition from the upper energy level (Em) to the

lower energy level (En).

In this paper a value of 5.901013_cm1_{was reported for the}

LTE approximation, which agrees with the literature[11–13].

On the other hand nitrogen elements are characteristic for the

ﬁrst and second positive system that occur between 250 nm and

400 nm; although doing different variations in the spectra, such as integration time and the width of the entrance slit, no prominent

lines were observed in this range. The relaxation of the excited state of nitrogen in the plasma emission is given by transitions between atomic energy levels or through state transitions of the ionized molecule and not by transitions of the neutral molecule. This suggests that the relaxation process is the recombination of

unpaired electrons with the ionized molecules (Fig. 2b).

3.3. Chemical composition

3.3.1. EDS analysis in AlNﬁlms

The EDS results from the AlN ﬁlms surface showed the

presence of (Al, N, O) element, which is characteristic of those materials. The areas of the peaks were used to calculate the

composition of both coatings; thus, the values fromFig. 3

indi-cated that AlNﬁlms were substoichiometrics. On the other hand, a

careful correction has to be done in all stoichiometric analyses because EDS has low reliability for nitrogen concentration. In this

sense, EDS elemental concentrations were obtained using theZAF

correction method; because certain factors related to the sample composition, called matrix effects associated with (atomic number

(Z), absorption (A) and ﬂuorescence (F)), can affect the X-ray

spectrum produced during the analysis of electron microprobe Fig. 1.Optical emission for the AlN plume with different values of nitrogen pressure: (a) emissivity for N21st positive system between 600 nm and 800 nm in AlNﬁlms as function of deposition pressure, (b) optical emission for the AlN with 0.53 Pa, and (c) optical emission for the AlN with 0. 66 Pa.

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

The correction factors for a standard specimen of known compositions were determined initially by the ZAF routine. The

relative intensity of the peak K was determined by dead time

corrections and a referent correction for the X-ray measured. So, before each quantitative analysis of an EDS spectrum for AlN

deposited with 9.3101_{Pa, a manual background correction and}

an automatedZAFcorrection was carried out[14]. Thus,Fig. 3a shows

the energy-dispersive X-ray spectroscopy (EDS) values of AlN_ﬁlms

deposited with different work pressures. All samples were observed

via SEM and chemical analyses were done with an ampliﬁcation of

20,000. The presence of oxygen has been often found during the

production of AlN, which is normally associated to residual oxygen in

the chamber[11].Fig. 3b shown the dependence of work pressure on

decrease of Al content; moreover a little increase in the N concen-tration in relation to the increase of deposition pressure has been observed. This effect can be associated to high sensitivity of ionic exchange in nitrogen under pressure changes.

3.3.2. XPS analysis in AlNﬁlms

The chemical relation between EDS results and XPS was explored in the current research. Thus, the survey spectra of

Al 2p, and N 1s inFig. 4were recorded from AlNﬁlms, as shown

inFig. 5. FromFig. 5(a), the Al 2p peak is composed of a shoulder separated 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

Intensity (a.u.)

λ

(nm)

Fig. 2.Local thermodynamic equilibrium: (a) determination of electronic temperatureTe¼8.832.3 K from AlN plume emission in 0.533 Pa N2by using the line N II and (b) the plasma electron densitynecalculated fromStark broadening, for N II transition (2s2

p2_–

4p) 3p at the line 504.871 N II, and thene¼2011019 cm3

. The error bars indicate the standard deviation values of the measurements for all AlN materialsﬁlms.

2 4 6 8 10 12 14 16

15 20 25 30 35 40 45 50

Al O

Deposition Pressure (10-1Pa)

Al concentration (%)

20 25 30 35 40 45 50 55 60 65 N

N concentration (%)

Stoichiometric Concentration

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

1400 1200 1000

800

600

400

200

0

0 1x10

4

2x10

4

3x10

4

4x10

4

In

te

n

s

it

y

(c

/s

)

Binding energy (eV)

Fig. 4. XPS spectrum of Al 2p–N 1s from Al–Nﬁlm as a function of applied pressure.

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2p can be ﬁtted well by two Gaussian functions. The value of binding energies obtained for the Al 2p peak was 73.9 eV and the higher value for Al 2p was 75.9 eV, respectively. According to the

literature[15–17]for the Al 2p peak, theﬁrst one (73.9 eV) and the

second one (75.9 eV) can be assigned to Al–N and Al–O bonds

respectively. The appearance of the peak at 73.9 eV clearly shows that 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, each

separated by 2.9 eV. The XPS spectrum of N 1s can beﬁtted well by

two Gaussian functions which depicts the N 1s spectrum with

values at 397.3 eV and 400.2 eV characteristic for N–N and Al–N

bonds, respectively[18,19].

The high resolution X-ray photoelectron spectroscopy (XPS)

results for AlN deposited with 9.3101_{Pa demonstrate that Al}

atoms bonded to N in the form of nitride, because the elemental

concentration of the Al–Nﬁlm was obtained by adjusting the laser

incidence on Al target and N2was the working gas in this research;

it was discovered that amounts of Al–N in the AlN ﬁlm were

maximum in the current establishment of process conditions and

the ratio of Al to N in theﬁlm was about 2:1. Generally, formative

Al–N phase indicates that the aluminum and nitrogen activity and

activation energy provided by the present deposition conditions

are enough for the formation of AlN thin_ﬁlm. Although the surface

temperature of the substrate during deposition of AlN ﬁlm is

around 3001C, the substrate lies in a high-density plasma region

and a high ion-to-atom ratio of aluminum and nitrogen can be propitious to the formation of AlN phase at the low temperature

below 3301C. All aluminum-nitride ﬁlms with all Al 2p peaks

wereﬁtted as one or more pairs of spin–orbit split sub-peaks with

a separation of 0.4 eV between the Al 2p3/2and Al 2p1/2

compo-nents (Fig. 5a). The ratio of the area of the 2p3/2component to the

area of the 2p1/2component wasﬁxed at 2:1. Moreover all N 1s

peaks were ﬁtted as one or more pairs of spin–orbit split

sub-peaks with a separation of 0.2 eV, sowing the N–Al bound

centered 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 aﬁtted Al 2p spin–orbit sub-peak pair is

reported as the centroid of the pair. The centroid of the spin–orbit

pair in eV was calculated as shown

Al2p CentroidðeVÞ ¼AAl 2p3=2EAl 2p3=2þAAl 2p1=2EAl 2p1=2

AAl 2p3=2þAAl 2p1=2

ð3Þ

whereAis the sub-peak integrated area andEis the adjusted

sub-peak binding energy in eV. Therefore, calculation of the sub-peak

areas without O 1s contribution gives an atomic ratio of Al:

N¼0.392:0.588, which is similar to the stoichiometry of

Al0.40N0.60[20]and close to EDS results showed in Fig. 3. So, in

Fig. 6, it is a clear dependence on the concentration of nitrogen

and oxygen in AlN thinﬁlms.

3.4. Structural characterization by XRD results

The x-ray diffraction patterns for AlN deposited with

9.3101Pa observed in Fig. 7 are at 37.91 and 44.291,

corre-sponding to AlNc-axis (0002) and AlN (200) orientations

respec-tively. As it can be observed, strong preferentialc-axis orientation

is obtained for the lower nitrogen–aluminum ratio. The

crystal-lographic orientation of the grains in theﬁlm is determined by

the preferential growth of certain crystal planes over others. The

mechanism of preferential orientation of AlN ﬁlms can be

explained by the crystalline lattice structure generated by AlN

conﬁguration materials which is in agreement with optical

emis-sion results (Fig. 1), EDS (Fig. 3) and XPS results (Fig. 5). To further

obtain information regarding bond formation and structure the

polycrystalline 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 70 0.0

5.0x103 1.0x104 1.5x104 2.0x104 2.5x104

Al-O 76.27 eV

Al2p

Intensity (a.u.)

Binding energy (eV)

Al-N 73.78 eV

406 404 402 400 398 396 394 0.0

4.0x103 8.0x103 1.2x104 1.6x104

Intensity (a.u.)

Binding energy (eV)

N1s

N-Al 399.66 eV

N-N 402.23 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 different temperatures.

4 6 8 10 12 14 16

30 35 40 45 50 55 60 65 70

Composition (at.%)

Deposition Pressure (10

-1

Pa)

N O

25 30 35 40 45 50 55 60 65 70 75

Composition (at.%)

Fig. 6.Compositions results from XPS analysis showing the pressure dependence at the concentrations of N and O deposited on Si at 3001C. Oxygen is the only observed contamination in these ﬁlms. The error bars indicate the standard deviation values of the measurements for all AlN layers.

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ﬁlms. Such one axial hexagonal texture withc-axis perpendicular

to the Silicon substrate has been detected in AlNﬁlms. 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 the ratio 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 the

same ﬁlms previously analyzed by XRD. It was reported that

crystalline AlN exhibits characteristic transverse optical (TO) and

longitudinal optical (LO) modes.Fig. 8a shows FTIR spectra of a

spectrum for AlN deposited with 9.3101_{Pa in the range of}

468–800 cm1_{; after deconvolution the respective modes are}

active in the infrared observed, especially a narrow band centered

at 680 cm1_{which may be attributed to the contribution of the}

phonon modeE1(TO) of the w-AlN as well as the presence ofﬁve

bands around 485 cm1_{, 520 cm}1_{, 615 cm}1_{, 655 cm}1 _and

691 cm1_{, associated with Al}_–_{O bonds characteristic of a}

sym-metric stretching, Al–N non-stoichiometric phases (AlxNy), the

phonon mode A1 (TO) of hexagonal AlN, the LO phonon mode of

AlN hexagonal and hexagonal Al–N, respectively. Inside the

depositedﬁlms there is residual stress that induces the shift of

the FTIR peaks from their characteristic positions. It can be due to

the non-equilibrium nature of PLD[20–22].

In this sense the AlN normally crystallize in the hexagonal

wurtzite structure (space group C46v-P63mc) with four atoms in

the unit cell. Then, from Raman results, inFig. 8b for AlN deposited

with 9.3101_{Pa it was possible to observe that the}_k_¼_{0 point}

group theory predicts the following eight sets of modes: 2A1þ

2Bþ2E1þ 2E2 of which one Al, one El, and two E2 are Raman

active. One set ofA1and one ofE1correspond to acoustic phonons.

The B modes are silent [1]. Note that phonons with E1 and E2

symmetry, respectively, are twofold degenerate. The modes with

A1andE1symmetry are also infrared active. The frequencies are all

measured with an error of71 cm1_{. Moreover}_{Table 1}_{shows the}

FTIR and Raman active modes associated to Al–N vibrations

[7–10]. The FTIR and Raman spectra are in good agreement with

the optical emission results (Fig. 1), EDS (Fig. 3), XPS results (Fig. 5)

and XRD results (Fig. 7), which conﬁrm the formation of large

hexagonal AlN materialﬁlms.

30

35

40

45

50

55

60

65

70

Si - Su

b

s

trate

AlN (200) AlN (103)

Al

N (0002)

Intensity (u.a)

2 θ

(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 3001C and a pressure of 0.93 Pa nitrogen.

480 560 640 720 800

84 88 92 96 100

Transmittance (%)

Wavenumber (Cm

-1

_{) Wavenumber}

_(Cm

-1

₎

600 650 700 750 800 850 900

Raman Intensity

Fig. 8.Vibrational analysis for AlN materialsfilms: (a) FTIR spectroscopy of AlNfilms deposited on Si (100) substrates at 3001C and a pressure of 0.93 Pa nitrogen with E2 (high) phonon mode and (b) Raman shift measures of AlNfilms deposited with 9.3101

Pa where it was possible to observe that thek¼0 point group theory predicts the following eight sets of modes.

Table 1

Vibrational modes reported in the literature for hexagonal aluminum nitrideﬁlms.

Mode symmetry Frequency (cm1₎ _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]

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3.6. Surface topography and morphological results analyzed by AFM and SEM

The observed dependence of the AlNﬁlms surface morphology

under nitrogen pressure during deposition is closely related with

theﬁlm growth mechanism, associated to the surface diffusion

length (L) which is given by[20]

Lð ÞD

τ

1=2

ð4Þ

whereDis the diffusion coefﬁcient and

τ

is the residence time of

adatoms. Larger values of diffusion length imply more time for the

adatoms to ﬁnd energetically favorable lattice positions, thus,

reducing the density of surface defects and improving the crystal quality.

Associating Eq.(4),Table 1andFig. 9it is possible to show the

surface morphology of AlN ﬁlms. Therefore, the changes on

morphological surface as functions of increase in the deposition pressures were studied by recording AFM images along with SEM micrographs. These results evidence the random distribution of micro-particles or micro-droplets on these surfaces as a function of deposition pressure (0.66 Pa and 0.53 Pa). Thus, the deposition pressure affects clearly the increase of grain size, roughness and micro-drops; this can be possible due to low surface mobility when the pressure was varied from 0.39 Pa to 1.5 Pa. This surface mobility reduces the possibility that the micro-drops are anchored

on the surface when arriving with high energy on AlNﬁlm. Other

possible reason can be associated with the mean free path that produces surface diffusion of nano-drops or micro-drops which can decrease the overall number of particles, also affecting the

boundaries sizes. In this sense it was presented in Table 2 the

surface roughness, grain size for AlN ﬁlms grown at 0.39 and

0.53 Pa, and their optical emission lines due to optical emission spectrometry signals from AlN plasma.

3.7. Optical reﬂectance analysis of AlNﬁlms

The reﬂectivity measure is the fractional amplitude of the

reflected electromagnetic field, while reflectance refers to the

fraction of incident electromagnetic power that is reﬂected at an

interface. The reﬂectance is thus the square of the magnitude of

the reﬂectivity. The reﬂectivity can be expressed as a complex

number as determined by Fresnel's equations for a single layer,

whereas the reﬂectance is always a positive real number. In certain

fields, reflectivity is distinguished from reflectance by the fact that

re_ﬂectivity is a value that applies to thick re_ﬂecting objects. When

reﬂection occurs from thin layers of material, internal reﬂection

effects can cause the reﬂectance to vary with surface thickness.

Reﬂectivity is the limit value of reﬂectance as the surface becomes

thick; 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 is

deﬁned as the wavelength of the monochromatic stimulus that,

when additively mixed in suitable proportions with the speciﬁed

achromatic stimulus, matches the color stimulus considered[21].

Taking into account the above, an example in thin ﬁlm

calculator in OptiScan is given to calculate the reﬂectance and

transmittance of Krestchmann con_ﬁguration which generate

sur-face plasma resonance at a certain incident angle. Unfortunately

the other properties of the surfaces, such as reﬂectance,

transmit-tance, or phase change, are rarely satisﬁed. However the thinﬁlms

are commonly used to modify these properties without altering

the specular behavior. In an optical coating, theﬁlms, together

with their support, or substrate, are generally solid [22]. The

particular materials used for the AlNﬁlms vary with the

applica-tions. It is possible to construct assemblies of thinﬁlms which will

reduce the reﬂectance of a surface and hence increase the

transmittance of a component, or increase the reﬂectance of a

surface, or which will give high reﬂectance and low transmittance

over part of a region and low reﬂectance and high transmittance

over the remainder and so on. In this sense in the current work for

AlNﬁlms the reﬂectance,Rwas taken as the ratio of the irradiance

of the reﬂected beam to that of the incident beam, and

transmit-tance,T, as the ratio of the irradiance of the transmitted beam to

that of the incident beam, and deﬁned as follows[22,23]:

R_¼

η

0Y

η

0þY

U

η

0Y

η

0þY

n

;T_¼ 4

η

0ReðYÞ

ðη0þYÞUðη0þYÞn ð

5_Þ

whereRis the reﬂectance,Tis the transmittance,

η

0is the surface

admittance for incident medium, andYis the surface admittance

of the thinﬁlms and substrate; moreover the effects of multiple

ﬁlms are included in the surface admittance. Each layer generates

a matrix in the equation which will change the electric and

magneticﬁelds[22,23].

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

Table 2

Surface roughness, grain size for AlNﬁlms grown at 0.39 and 0.53 Pa, and their optical emission lines due to optical emission spectrometry signals from AlN plasma.

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

(8)

In this work the AlN ﬁlms have been compared with pure

aluminum because the aluminum mirrorﬁnish has the highest

reﬂectance 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 visible

range it is slightly outdone by AlN and silver, but in the 700–

3000 nm range (near IR) it is slightly outdone by gold and copper

materials[24].Fig. 10shows the optical reﬂectance spectra of the

AlN single layers obtained at different deposition pressures. The

reﬂectance of the aluminum and the eye sensibility are shown for

comparison. The spectra of the samples show high re_ﬂectance for

long wavelengths, near to 62% for the AlNﬁlms deposited with

4101_{Pa and close to 28% for AlN}_ﬁ_{lms deposited with 1.5 Pa.}

These values of reﬂectances at these wavenumbers agree well with

previous reports in the literature for AlNﬁlms[25,26]. However in

this work it was observed that the minimum of reﬂectance at

550 nm is due to interference effect in reﬂected light.

However a clear decrease in reﬂectivity for short wavelengths

is seen, characteristic of a system with high free electron density

with a reﬂectance edge below 530 nm due to a screened plasma

resonance[27]. The white and aluminum colors of the AlNﬁlms

are 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 AlNﬁlms 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), and

AFM-SEM results (Fig. 9). The reﬂectance values for AlNﬁlms are

in good agreement with the optical emission results (Fig. 1) by the

spectral signals AlXII (631.337 nm) and NI (618.909 nm),

charac-teristics of AlN, EDS (Fig. 3) with XPS (Fig. 5) results because of the

AlN adjusted stoichiometry strapped with bounds from Al 2p and

N 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 optical

whining (Al–N E1(TO) 678.8 cm1and Al–N A1(TO) 692.5 cm1),

and AFM-SEM results (Fig. 9) associated with superﬁcial

morphol-ogy and adjusted to the other physical and chemical characteristics discussed above absorbs and disperses spectral lines of light

incident at the rank of major optical reﬂectance[28].

Moreover in this research high dependency on reﬂectance

percentages in this work was found when the deposition pressure was varied from 0.39 Pa to 1.5 Pa, therefore, the changes in optical properties can be related not only with changes on temperature deposition but also with changes generated on morphology

sur-faceﬁlms due to variation of deposition pressure (Fig. 11a).Fig. 11a

exhibits one constant region for wavelength of 760–800 nm; in

those regions it is possible to appreciate the effect of the

deposi-tion pressure on reﬂectance of the ﬁlms. Fig. 11b shows the

decrease of reﬂectance when the deposition pressure is increased,

which indicates that pressure also promotes the absorbance in the AlN deposited via PLD.

Moreover the error bars of the values presented inFig. 10were

obtained at the base of the uncertainty in adjusting of reﬂectance

curves as a function of wavelength, to assign values to the

parameters determined by ﬁtting (reﬂectance or transmittance

for AlNﬁlms). Therefore, the reﬂectance values shown inFig. 11b

(_ﬁlled triangles) are the average of data obtained in each run and

thus have their respective error bars. In this sense the error bars indicate the standard deviation values of the measurements for all

AlN_ﬁlms[28].

From the reﬂectance spectra a weak but clear effect of

deposi-tion pressure on the optical properties is seen. As the pressure is

decreased, the reﬂectance of theﬁlms tends to be higher in the

near infrared region, while the minimum in reﬂectance, between

562 nm and 571 nm for all AlNﬁlms.

400

500

600

700

800

900

0

10

20

30

40

50

60

70

80

90

100 Reflectance (%)

Wavelengeth (nm)

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

760 765 770 775 780 785 790 795 800 20

25 30 35 40 45 50 55 60 65 70

Reflectance (%)

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

Reflectance (%)

Deposition pressure (Pa)

Fig. 11.Reflectance dependency: (a) Reflectance of AlNfilms 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 AlNfilms.

(9)

3.8. Dominant wavelength and color purity analysis of AlN_ﬁlms

3.8.1. Dominant wavelength

In order to calculate dominant wavelength for AlNﬁlms, it was

necessary ﬁrst introduce the identiﬁcation of a color by its“x–y

chromaticity coordinates”as plotted in the Chromaticity Diagram

(Fig. 12). The diagram enables to quantitatively graph the hue and saturation of a particular color by its dominant wavelength and

excitation purity, respectively[29].

Theﬁrst step in calculating the chromaticity coordinates is to

compute the _“tristimulus values_” of a particular _ﬁlter with a

particular illumination for AlN ﬁlms. Loosely, the tristimulus

values can be thought of as the amount of red, green, and blue

in the_ﬁlter, but the correspondence is not quite that simple. These

values are based on the empirically determined CIE Color

Match-ing FunctionsX(

λ

),Y(

λ

), andZ(

λ

)[29]. Note that we typically

use the CIE 1931 2-degreeﬁeld-of-view standard, as this one is the

most well-known and widely used. It is useful to note that by

deﬁnition the y(

λ

) Color Matching Function is identical to the

Photopic Curve V(

λ

), or y(

λ

)V(

λ

). The Color Matching

Func-tions can also be obtained from numerous sources presented in

the literature[29,30]. In this sense the tristimulus valuesX,Y, and

Zare then given by

X¼Z 1

0

IðλÞTðλÞχðλÞd

λ

ð6Þ

Y¼Z 1

0

IðλÞTðλÞyðλÞd

λ

ð7Þ

Z¼

Z 1

0

IðλÞTðλÞzðλÞd

λ

ð8Þ

For determination of dominant wavelength in AlNﬁlms (Table 3),

one simply constructs a line between the chromaticity coordinates of the reference white point on the diagram (for instance, CIE-E, CIEC,

etc.) and the chromaticity coordinates of theﬁlter and then

extra-polates the line from the end that terminates at theﬁlter point. The

wavelength associated with the point on the horseshoe-shaped curve at which the extrapolated line intersects is the dominant wavelength

[30].

Dominant wavelength and color purity have been determined

from the reﬂectance spectra of allﬁlms. The color purity changes

with the deposition pressure, in this study ranges from 0.39 Pa to

1.5 Pa for AlNﬁlms, away from the color purity of pure aluminum

(0.90), conﬁrming that theﬁlms are less white compared with the

aluminum. So, the results of color measurement indicate that all

ﬁlms reﬂect a hue slightly shifted far to the white as compared to

aluminum reﬂectivity and with similar color purity (Table 3). The

dominant wavelength was varied for all the samples, from 562 nm

to 570 nm. AlNﬁlms deposited with lower pressure are situated

little close to that of pure aluminum reference (574 nm).

3.8.2. Color purity

Once the line is constructed to determine the dominant wavelength, it is very straightforward to calculate the excitation

purity (color purity) of a color that represents theﬁlter

transmis-sion. The excitation purity applied for AlNﬁlms is deﬁned to be the

ratio of the length of the line segment that connects the chroma-ticity coordinates of the reference white point and the color of interest to the length of the line segment that connects the

reference white point to the dominant wavelength [31]. These

line segments are illustrated inFig. 12. As pointed out above, the

excitation purity is a well-deﬁned quantitative measure of the

saturation of a particular color. The larger the excitation purity, the more saturated the color appears, or the more similar the color is to its spectrally pure color at the dominant wavelength. The smaller the excitation purity, the less saturated the color appears, or the more white it is. Pastel colors are very poorly saturated, for

example. When there is no well-deﬁned dominant wavelength,

the excitation purity is still deﬁned as described above, except that

the denominator should be taken as the length of the line segment between the reference white point and the point at which the dominant wavelength construction line intersects the line that contains the purple colors near the bottom of the diagram. For

every wavelength in the spectrum (e.g. AlNﬁlms), is possible to

calculate (X,Y,Z) from CIE color matching functions. Therefore the

Plot (x,y) for all wavelengths in the spectrum generates a

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 graph

called the Chromaticity Diagram with x–y coordinates on this

graph which are given by[31–33]

χ

¼_X_þX_Y_þ_Z; y¼_X_þY_Y_þ_Z ð9Þ

Taking into account the last discussion, the excitation purity (purity for short or color purity) of a stimulus is the difference from the illuminant's white point to the furthest point on the chromaticity diagram with the same hue (dominant wavelength for monochromatic sources); using the CIE 1931 color space given

by[31]

pe¼

ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi

ðxxnÞ2þðyynÞ2 ðx1xnÞ2þðy1ynÞ2 s

ð10Þ

0.0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9 Y Axis

X Axis

Fig. 12.Chromatic diagram, in thex,ycoordinates, of the reﬂectivity for AlNﬁlms. White coordinates of achromatic point are located at (1/3, 1/3).

Table 3

Optical characteristics for AlNﬁlms deposited at 3001C onto Si substrate.

Deposition pressure (Pa)

Dominant wavelength (nm)

Color purity

AxisX AxisY

4.0101

571 0.71 0.3329 0.341 5.3101

570 0.66 0.343 0.362 9.3101

567 0.63 0.242 0.123

1.5 562 0.62 0.210 0.119

(10)

where (xn,yn) is the chromaticity of the white point and (xI,yI) is the

point on the perimeter whose line segment to the white point contains the chromaticity of the stimulus. Different color spaces, such as CIELAB or CIELUV may be used, and will yield different results.

Taking into account the last discussion in the Fig. 13a are

observed the differences in the dominant wavelength for all AlN

ﬁlms deposited with different deposition pressures. InFig. 13b the

inﬂuence of deposition pressure on color purity can be observed.

This graph shows the increase of purity values towards color gray purity. The wavelength is an important optical characteristic for different materials in relation with the changes observed in AlN

plasma (Figs. 1–3).

Moreover the dominant wavelength and color purity values for

AlNﬁlms 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 re_ﬂ

ec-tance values (Figs. 10 and 11) which conﬁrm the susceptibility that

present the AlNﬁlms in terms of dominant wavelength and color

with changes in deposition pressure. So, when the wavelength in the AlN layers is changed it is possible to observe that natural color is changed. In this sense the purity color dependence and other

optical constant dependence in AlNﬁlms with pressure obtained

in this work demonstrate the possibility of some purity color control. The last discussion can be proved observing the changes in the optical energy gap, plasma frequency and refractive index as

a function of deposition pressure (Table 4).

In this sense the error bars of the values presented inFig. 12

were obtained at the base of the uncertainty in adjusting chro-matic diagram for different disposition pressures, to assign values

to the parameters determined by ﬁtting (dominant wavelength

and color purity for AlN layers). Therefore, these optical constant

values shown inFig. 13(_ﬁlled, circles and squares) are the average

of data obtained in each run and thus have their respective error bars. In this sense the error bars indicate the standard deviation

values of the measurements for all AlNﬁlms.

3.9. Optical energy gap, plasma frequency and refractive index

3.9.1. Optical energy gap

In crystalline semiconductors, equation(11)has been obtained

to relate the optical energy gap (Egap) with absorption coefﬁcient

from reﬂectance results given by[34]

αð

vÞhv¼BðhvEgapÞm ð11Þ

whereEgapis the optical energy gap,Bandh

υ

are the optical gap

constant, and incident photon energy, respectively;

α

(v) is the

absorption coefﬁcient deﬁned by Beer–Lambert's law as

α

(v)¼

2.302Abs(

λ

)/dwheredandAbsare theﬁlm thickness andﬁlm

absorbance, respectively. For more precise determination of

α

, it is

necessary to perform corrections to the absorption due to re_ﬂ

ec-tion; also,mis the index which can have different values of 1/2,

3/2, 2, and 3[34]. In this sense the optical energy gap as function

of deposition pressures for AlN_ﬁlms has been presented inTable 4.

3.9.2. Plasma frequency

The plasma frequency (

ω

p) is the most fundamental time-scale

in plasma physics. Clearly, there is a different plasma frequency for each species. However, the relatively fast electron frequency is, by

far, the most important, and references. So, it is easily seen that

ω

p

corresponds to the typical electrostatic oscillation frequency of a given species in response to a small charge separation. For instance, consider a one-dimensional situation in which a slab consisting entirely of one charge species is displaced from its

quasi-neutral position by an inﬁnitesimal distance, associated to

higher charge carrier concentration with the higher the plasma frequency. Therefore, plasmon oscillations for different materials are excited in different spectral regions. Thus, in many

semicon-ductors like (AlN), the plasma reﬂection edge can be found in the

reﬂectance as functions of wavelength. The spectral position of the

plasma reﬂection edge depends on the charge carrier

concentra-tion according to Eq.(12) [35]. Taking into account the above the

plasma frequency (

ω

p) as a function of deposition pressures for

AlNﬁlms[36]has been presented inTable 4

ω

p¼

1

2

π

c

4

π

Ne2

mn

ε

₁

1=2

ð12Þ

where

ω

pis the plasma frequency,Nis the free electron density,eis

the electron charge,mnis the effective mass of electrons, and

ε1

is

the high frequency dielectric constant andcis the velocity of light.

0.4 0.6 0.8 1.0 1.2 1.4 1.6 560

562 564 566 568 570

572 Aluminum 574 (nm)

Dominant wavelength (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

o

r pur

it

y

Deposition pressure (Pa)

AlN 300 °C

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

Table 4

Optical constants (energy gap, plasma frequency and refractive index) for all AlN

ﬁlms as function of deposition pressure.

Deposition pressures (Pa)

Optical energy gap (eV)

Plasma frequency (cm1

)

Real refractive index

4.0101

6.2 4125 2.194

5.3101

5.9 3873 2.175

9.3101 _5.5 ₃₂₂₀ _2.154

(11)

3.9.3. Refractive index

It is known that several widely used methods of analyzing

reﬂectance (R) and transmittance (T) for a supported thin ﬁlm

neglect the effect of the rear surface of the substrate. Equations are

given which relateRandTto the complex refractive index (n-ik)

and thickness of the thinﬁlm, and a method for their solution has

been described. This relies on Powell's technique, and permits

changes to be made to the equations relatingR,Tto (n-ik). This

ﬂexibility has allowed the calculation of the effect of the neglect of

the rear of the substrate. An example is given of the use of the method for the determination of refractive index (n-ik). In this research aluminum nitride (AlN) with wurtzite phase (w-AlN) with a wide band gap (6.2 eV) for semiconductor material was used, giving it potential application for deep ultraviolet

optoelec-tronics. The refractive index for all AlNﬁlms is shown inTable 4.

The refraction index has been calculated from relation which holds

for the reﬂectance measurements[37]

2nd¼N

λ

ð13Þ

wherenis the refraction index,dis the sample thickness, andNis

the interference order. The interference order was determined graphically. In the region where the refraction index is weakly

dependent on

λ

(

λ

44500 Å) the dependence of N on 1/

λ

is

practically linear, and in the case of sufﬁciently thin sample N

may be determined precisely from the intersection of this line

with they-axis.

4. Conclusions

A dependency in relation to nitrogen concentration, roughness

and grain size in the AlNﬁlms with the nitrogen work pressure

was found in this work, increasing in this sense the nitrogen

concentration and the roughness on AlN ﬁlms. The plasma

pressure affects the stoichiometry and the morphological nature

in the AlNﬁlms. The variation of nitrogen work pressure exhibits

low effect on intensity of the spectral lines emitted. The electron

temperature value (Te¼8832.3 K) presented in the aluminum

nitride plasma is similar to previous works. Structural and

com-positional results show that the thin ﬁlms deposited at a

tem-perature of 3001C improved orientation of hexagonal AlN

increases as the reﬂectance values for the lower deposition

pressures. X-ray photoelectron spectroscopy (XPS) conﬁrmed the

formation of the binaryﬁlms AlN.

For AlNﬁlms the pressure deposition has a marked inﬂuence

on the optical properties. A decrease in the reﬂectance of 55%, a

reduction of color purity about 13% and decrease in the dominant wavelength around 1.6% was found with deposition pressure

between 4.0101_{Pa and 1.5 Pa. This conclusion can be proved}

observing the changes in the optical energy gap, plasma frequency and refractive index as function of deposition pressure. So, the

purity color dependence in AlN ﬁlms with pressure obtained in

this work demonstrates the possibility of some purity color control.

Acknowledgments

J.A. Pérez acknowledges projects: Nano-structured High-efﬁciency

Thermo-Electric Converters (nanoHITEC) and Photoacoustic Mea-surements of Nanostructures for Thermoelectric Applications (PHO-MENTA) from MINECO and Santander bank and your scholarship

program“Young Professors and Researchers Latin America,

Santan-der Universities”(Spain, 2013)

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