ELS EVI E R Thin Solid Films 380 (2000) 252-255 www.elsevier.com/locate/tsf

Raman spectra of TiN/AlN superlattices

M. Bernarda, A. Deneuville">*, 0. Thomasb, P. Gergaudb, P. Sandstrom", J. Birch"

aLEPES-CNRS, BP 166, 38042 Grenoble Cidex 9, France bMTOP-CNRS, Faculte' des Sciences et Techniques de Saint Erome, 13397Marseille Cidex 20, France

'Department of Physics, Linkoping University, S-58183 Linkoping, Sweden

Abstract

TIN (4.5 nm)/AlN (3, 6, 22 nm) superlattices deposited by DC magnetron sputtering on MgO(001) at a temperature of 850°C exhibit Raman signals. They indicate N and Ti vacancies (as in thick TIN) in TIN,-, layers (x = 3 f 2%). x is higher for the sample with 3-nm thick AlN layers, which is ascribed to N diffusion from AlN (standing close to the TIN interfaces) to TIN. In comparison to Raman peaks of thick AlN, there are split signals of wurzite AlN phase, and a signal from another phase, which might be defective rocksalt AlN standing close to the TIN interfaces. The Raman signals clearly show interactions between A N and TIN layers. 0 2000 Elsevier Science B.V. All rights reserved.

Keywords: SC/metal superlattices; Nanometric layers; Raman; Vacancies; TiN, A1N

1. Introduction

For nm-size TiN/AIN superlattices grown on Mg0(001), the AIN phase transition from wurzite to rocksalt cubic was recently seen, when the AIN thick- ness became I 2 nm [1,2]. These results need to be confirmed and understood from the characteristics of the AIN and TiN layers. While the Raman signals of wurzite AIN are well known [3], the selection rules forbid first-order Raman signals from any cubic phase. In fact, rocksalt cubic TiN [4-6] and zinc-blende AIN [7] give Raman signals. On nm-size TiN (4.5 nm)/AlN (3, 6, 22 nm) superlattices, with a constant TiN thick- ness, we identify from Raman spectroscopy: (i) several AIN phases; and (ii) sub-stoichiometric TiN, -n layers containing N and Ti vacancies. The significance of these results will be discussed.

2. Experimental details

The samples were deposited by DC magnetron sput- tering on MgO(001) substrates at 850"C, beginning with the TiN deposition from elemental Ti (99.98%) and AI (99.999%) targets using N, of N 70 purity as a sputter- ing gas at a total pressure of 10 mtorr. The thickness of the layers was determined by fitting X-ray reflectivity curves with GIXA software. The TiN thickness was kept constant at 4.5 f 0.2 nm while the AIN thickness increased from 3 f 0.5 to 6 f 1 to 22 f 4 nm, with 22, 17 and 10 periods, respectively. The micro Raman spectra were recorded at room temperature in a backscattering geometry with a Dilor Labram appara- tus. They were excited at energies E, of 1.96, 2.41, 2.71 and 3.4 eV (632,514,458 and 363 nm, respectively) by HeeNe and Ar lasers.

3. Results and discussion

*Corresponding author. Tel.: + 33-476-881009; fax: + 33-476-

E-mail address: deneuvil@lepes.polycnrs-gre.fr (A. Deneuville). 887988. All the properties of the samples vary systematically

with their preparation conditions. The Raman signals

0040-6090/00/$ - see front matter 0 2000 Elsevier Science B.V. All rights reserved PII: S 0 0 4 0 - 6 0 9 O ( 0 0 ) 0 1 5 3 1 - 5

M. Bernard et al. /Thin Solid Films 380 (2000) 252-255 253

are highly reproducible. The Raman contributions from the different layers of a sample are cumulative. There- fore, hereafter for each sample, the AIN contribution to the Raman spectrum will be obtained by subtracting the TiN contribution to this Raman spectrum from the whole Raman signal of the sample.

3.1. R a m a n signal j?om the TiN layers

Fig. 1 shows the Raman spectra of the three samples excited by the 458-nm laser light. All spectra have the same square-like shape at 190-350 cm- ' and a triangu- lar-like shape at approximately 555 cm-', and finally wide maxima centered at approximately 810 and 1110 cm-'. The same structures are found in thin films and crystals of cubic rocksalt TiN [4-61. The square- and triangular-like signals are defect-induced first-order Raman scattering, because, like thicker TiN films [4] and TiN crystals [5,61, these 4.5-nm TiN films contain Ti and N vacancies. The square-like (acoustical phonons with TA and LA modes, respectively, at approx. 200 and 306 cm-'1 and the triangular (TO modes up to a maximum at 555 cm-' and weak contribution of the LO modes up to 620 cm-'1 signals originate from Ti atoms surrounding N vacancies and N atoms surround- ing Ti vacancies, respectively [5,6,8]. The small 414 cm-' (2 TA) structure and the wide maxima around 810 (LA + TO) and 1110 (2 TO) cm-' originate from overtones. From previous works [6], when the ratio of N/Ti vacancy concentrations (and therefore x) in- creases: (i) the intensities of these second order signals decrease rapidly; (ii) the first order signals shift rapidly to higher wavenumbers; and (iii) the signal from the acoustical phonons increases. From the comparison of our spectra to these previous spectra, the overall N sub-stoichiometry in our samples can be estimated as x = 3 f 2 % .

The signals from the acoustical and optical phonons are nearly identical for the samples with 6 and 22 nm AIN thickness. The optical phonon signal from the sample with the 3-nm AIN layers is nearly identical to those of the two other samples, but shifted to approxi- mately 563 cm-'. Its acoustical phonon signal is more

200 400 600 800 1000 Wavenumber (cm-1)

Fig. 1. Raman spectra of the three samples excited by the 458-nm laser light.

+ Y i .- c fn S

C

S (R

K

B - E

400 440 480 520 560 600 640 680 Wavenumber (cm-1)

Fig. 2. Raman spectra of Fig. 1, normalized at 555 and 700 cm-'. In the inset, the solid and the mixed curve are the A1N contribution to the Raman spectra for the samples with 22 and 6 nm of A N , respectively.

intense and the mean wavenumber of its TA phonons appears at a higher (212 instead of 200 cm-'1 wavenumber. From these features, according to [6], the concentration of N vacancies has to be higher in this last sample. We suggest a diffusion of N from AIN (standing at a distance < 6 nm from the TiN inter- faces) to TiN, which partially fills the N vacancies in the TiN layers up to a saturated value, reached here in the samples with AIN thickness 2 6 nm.

3.2. R a m a n signal f r o m the AIN layers

From coherent-inelastic neutron-scattering experi- ments and the calculated dispersion curves, the first- order Raman spectrum of TiN has to spread up to 620 cm-' 181. The more intense Raman signal (E,) from wurzite AIN [3] is at approximately 660 cm-'. There- fore, the structures at a wavenumber of approximately 658 cm-' in Fig. 1 for the samples with AIN thickness 2 6 nm originate from AIN. Hereafter, we will sepa- rate the contributions of the AIN and TiN layers in the Raman signal of the samples.

After shifting the curve of the sample with 3-nm AIN by - 8 cm-' (to correct for its higher x), the Raman signals of all samples were normalized at 555 and 700 cm-', Fig. 2. While (except for the second-order con- tribution from 400 to 470 cm-') the curves are identi- cal on the low wavenumber (TO) side, systematic varia- tions appear on the high wavenumber side. The inset of Fig. 2 shows the two differential Raman spectra (which are expected to contain first-order AIN and second- order TiN contributions) obtained by subtracting the normalized spectrum of the sample with 3-nm AIN from those of the two samples with thicker AIN layers between 620 and 700 cm-'. As the penetration depth of the light is limited by the absorption of the TiN layers, the same thickness of TiN, but increasing thick- ness of AIN as the thickness of the AIN layers in- creases in the different samples, is probed. Therefore, in the differential spectra, the common component at 680-700 cm-' has to be ascribed to second-order TiN

254 M. Bernard et al. /Thin Solid Films 380 (2000) 252-255

Wavenumber (cm-1)

Fig. 3. Raman spectra of the 22-nm sample at different excitation energies.

and the central 650-680 cm-' and the 620-650 cm-' parts (which vary with the AIN thickness) to AIN.

The Raman spectra of the same samples were recorded with light excitation E,, of 1.96, 2.41, 2.71 and 3.4 eV. Fig. 3 shows spectra obtained for the sample with the thicker (22 nm) AIN layers. In addition to the predominant signals from TiN, structures at a wavenumber of approximately 658 cm- ' appear more and more clearly as E,, increases. After normalizing each spectrum at 550 and 700 cm-' at each value of E,, the subtraction of the (shifted) spectra from the sample with the 3-nm AIN layers from those shown in Fig. 3 at the corresponding E, gives the AIN contribu- tions to the Raman spectra of Fig. 4. Between 650 and 680 cm-', the AIN contribution to the Raman spectra increases when E, increases, due to a resonant effect. As in the inset of Fig. 2, the central part 650-680 cm-' is too wide to be fitted by a single Lorentzian function. Although there is some noise in the AlN contributions to the Raman spectra, and poor wavenumber resolu- tion when E, = 3.4 eV, the same wavenumbers (645, 655, 659, 666 and 672 cm-' appear for the Lorentzian components in all the fits of the AIN contributions to the Raman spectra of the samples. Therefore, the existence and the wavenumbers of these components can be trusted.

The known AIN phonon modes in this wavenumber range are E, (660 cm-' and EITO (673 cm-' for the (relaxed) wurzite phase [3] and TO (655 cm- '1 for the zinc-blende phase [7]. The 659 and 672 cm-' compo- nents are ascribed to a nearly relaxed wurzite phase.

630 650 670 690

Fig. 4. A1N contribution to the Raman spectra of the sample with 22-nm AIN layers at different light excitation energies.

While the 655 cm-' signal might be ascribed to an AIN zinc-blende phase, because of the neighboring ratios of the intensities of the 666/655 cm-' and 672/659 cm-' components, we also ascribed the 655-666 cm-' pair to wurzite. The origin of the shift remains unclear. The wide component at approximately 645 cm-' is too wide and too far away from the modes of the relaxed wurzite or zinc-blende phases to be ascribed to either of these phases. Only the rocksalt AIN phase was otherwise reported [1,21. It is shown in the inset of Fig. 2 that the relative ratio of the 645 cm-' signal increases as the AIN thickness decreases. The existence of N vacancies in AIN near the TiN interface was shown in Section 3.1. Therefore, the 645 cm-' signal might be ascribed to a defective AIN rocksalt phase standing near the TiN interfaces. As the defective TiN rocksalt phase, such a defective AIN rocksalt phase would exhibit a defect-induced fist-order Raman scattering. As the wurzite and cubic phases, which have neighboring structures, exhibit Raman signals in the same wavenumber range [7], this defective AIN rocksalt phase might exhibit a Raman signal at approximately 645 cm-'. As the defect-induced first-order Raman signals look like the total distribution of phonons [4-61, they have to be wide. The 645 cm-' signal obtained here presents all the characteristics of a defect-induced first-order Raman signal from a defective rocksalt AIN phase near the TiN interface.

4. Conclusion

Some characteristics of the TiN and AIN layers in nm-size TiN (4.5 nm)/AIN (3, 6, 22 nm) superlattices were derived from Raman measurements. In particular, in this thickness range, the chemical composition of the TiN and AIN layers depends on the thickness of the AIN layers. Actually, as in all other known rocksalt TiN materials, our TiN layers contain both N and Ti vacan- cies, with an overall N sub-stoichiometry TiN,-,. In our superlattices, x = 3 f 2%, but with a higher x in the sample with the lowest AIN thickness (3 nm). This difference is ascribed to N diffusion from the AIN standing in the vicinity of the TiN interfaces to the TiN layers. The AIN Raman signals are different to those of thicker films. Besides signals ascribed to a nearly re- laxed wurzite AIN phase and another wurzite AIN phase, a signal from another phase appears. It could possibly be ascribed to a defect-induced first-order Raman signal from a defective rocksalt cubic phase of AIN near the TiN interfaces.

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