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Radiation Effects and Defectsin Solids: Incorporating PlasmaScience and Plasma TechnologyPublication details, including instructions forauthors and subscription information:http://www.tandfonline.com/loi/grad20

Compartive study of defectsinduced by proton and heliumimplantation in LiNbO3 crystalS. M. Kostritskii a & P. Moretti ba Physics Dept , Kemerovo State University , 650043,Kemerovo, Russiab Laboratoire de Physicochimie des MateriauxLuminescents, Universite Claude-Bernard , 69622,Villeurbanne Cedex, FrancePublished online: 19 Aug 2006.

To cite this article: S. M. Kostritskii & P. Moretti (1999) Compartive study of defectsinduced by proton and helium implantation in LiNbO3 crystal, Radiation Effects andDefects in Solids: Incorporating Plasma Science and Plasma Technology, 150:1-4,151-156, DOI: 10.1080/10420159908226222

To link to this article: http://dx.doi.org/10.1080/10420159908226222

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Rodiarion E / / k l s & Dq&% m Sohds, Vol 150. pp 151-156 Reprints available directly from the publisher Photocopying permitted by license only

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COMPARATIVE STUDY OF DEFECTS INDUCED BY PROTON AND HELIUM IMPLANTATION IN

LiNb03 CRYSTAL

S.M. KOSTRITSKIIa and P. MORETTIb9*

a Physics Dept, Keinerovo State University. 650043 Kemerovo, Russia; bLahoratoire de Physicoclziinie des Materiaux Luininescerrts, Universite

Clriude-Bernard, 69622 Villeurbanne Cedes. France

(Received 6 JulJl 1998: 6 1 j h a l .form 20 September 1998)

The defects structure of Hef- and H+-implanted LiNb03 crystals have been investigated by micro-Raman scattering and IR-reflection spectroscopy. The analysis of these data allows us to conclude that: ( I ) the defects induced by He' implantation have a non-point nature and might be assigned to large-dimension clusters, having a new crystalline structure with a very specific lattice vibration spectrum. (2) in case of the H '-implanted samples the predominant lattice defects are found as to be of point nature, inducing weak crystal disorder only.

Keword.7: Lithium niobate: Implantation; Defects

1. INTRODUCTION

Planar optical waveguides have been formed in a wide range of crystals by using ion implantation. Light ions (He' and H+) accelerated with an energy in the MeV range can lead indeed to the formation of an efficient optical barrier at few microns depth beneath the irradiated surface [ 1,2]. The index change is related to the lattice defects induced at the end of the ion track by nuclear collisions. However the exact nature of the defects is not clear regarding the used ions [3,4]. In this work we report on the

*Corresponding author.

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I52/[544] S.M. KOSTRITSKII AND P. MORETTI

lattice dynamics of He+ and Hf implanted layers in LiNb03 crystals, investigated by the most efficient methods of vibration spectroscopy: Raman scattering and IR-reflection.

2. EXPERIMENT

We used commercially available Z- and Y-cut substrates congruently grown. The largest face of samples (with dimensions of 5 x 5 x 1 mm3) have been exposed at room temperature to either a He+ or H+ beam flux by using a van de Graaff accelerator. The energies used were 0.3, 0.65, 1, 2MeV and 0.35, 0.6, 1, 1.5MeV with doses of 1-2-4-x 1016ions/ cm2 and from 10l6 to 7 x 10'6ions/cm2 for helium and proton implan- tation, respectively. The projected ranges (i.e., the ion implantation areas) were deduced from TRIM simulations [5 ] .

Infrared (IR) reflection spectroscopy has been performed with a spectrometer "Specord M80" with the standard attachment, allowing to make measurements at the two incident angles cr of 20" and 70". Polarized IR-radiation, corresponding to the ordinary and extra- ordinary waves in the crystal, were used.

In order to scan the damaged layer below the irradiated face, micro- Raman measurements were performed on the polished edge of the samples, with a spatial resolution of 0.5 pm, by a "DILOR XY" spec- trometer. These measurements wcre conducted with an argon laser at a wavelength of 514.5 nm, in a backscattering scheme framework.

3. RESULTS AND DISCUSSION

In the IR-reflection spectra, measured at frequencies corresponding to the lattice vibrations, intensive new bands add to the known [6] IR- reflection spectrum of LiNb03 after helium implantation, as seen in Fig. I . These bands have the following pronounced bending points, corresponding to frequencies of either TO or LO phonons: (1) 350 (TO) and 380(LO)cm-', (2) 445 and 480cm-', (3) 510 and 550cm- ', (4) 739 and 745cmp', (5) 775 and 800cm ', (6) 820 and 860cm- I . In contrast, hydrogen implantation induces only a slight decrease of the total reflectivity in the region of the lattice vibrations and no new bands

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H+ A N D He IMPLANTATION I N LiNh03 [545], 153

1100 900 700 500 300 h (cm-1)

FIGURE 1 IR-reflection spectra measured at an incident angle of ru=20" (El) Z polarization) in LiNb03: (1) before implantation; (2) after He' implantation at 300 keV with a fluence of 2 x 10'6ions/cm'; (3) after H+ implantation at 350keV with 6.7 x 1 0 ' ~ ions/cm2.

merge in the IR-reflection spectrum (see Fig. I). Therefore, an impor- tant qualitative difference is established between the He+- and Hf- implanted LiNbO, layers concerning their lattice dynamics. Moreover, we have found that the reflectivity at the frequencies of the new bands in helium-implanted samples depends on the following various factors:

(a) The IR-light polarization. For E ( 1 Z, when A modes are observed, the spectra changes are the most evident. For E l Z, these changes are smaller, but they still have significant magnitudes.

(b) The He+-ions energy ( E ) , i.e. the position (6) of ion stopping region bclow the surface, which is deduced from TRIM simulations (see Fig. 2). Thus, we have:

A R N 116 N I / E , (1)

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154/[546] S.M. KOSTRITSKII AND P. MORETTI

0..25

0.20

* 0.15 a

0.10

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FIGURE 2 Dependence of the normalized IR-reflectivity of the new 480cm-’ band on implantation energy (fixed fluence: 2 x 10l6 ions/cm’) recorded for two incident angles a, ( 1 ) cy = 20” and (2) a = 70”. Lines are merely guides for eyes. The projected ions ranges calculated by TRIM simulation are - 1, 1.8, 2.5 and 4.8pm for 0.3, 0.65, 1 and 2 MeV. respectively.

where AR is a reflectivity increment induced by He+ implantation at the LO-TO gaps of the new bands.

(c) The implantation dose [Hcf]. The AR increment is indeed roughly proportional to the dose: AR N [He+].

(d) The incident angle (a) of IR-radiation on the sample surface, as seen on Fig. 2, i.e. the value of the average photometred depth r at IR-reflection measurements. The dependence o f t on a is:

t - XCOS(CY)/[27rITJ&’], (2)

where X is wavelength of IR-radiation, E’ is the real part of dielectric constant in range of the LO-TO gap related to the new bands.

Thus, it is very important to compare the probed depth ( t ) to the expected barrier layer position (6). For a = 70°, we have t N 1 pm, hence

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H' AND He IMPLANTATION IN LiNb03 [547]/155

r g 6 in the samples implanted at He+-ions energy of 300 keV, and t << 6 in the all samples irradiated a t the other energies. Note that the new bands are not indeed observed in the latter cases. For Q = 20°, we have t g 2.8 pm. Therefore, t > 6, t E 6, t cc 6, for the samples implanted at 300 and 650 keV, 1 MeV, 2 MeV, respectively. Here also no new bands are observed for t << 6.

These results indicate clearly structural changes, inducing the appearance of new phonon modes detected by IR-reflection spectro- scopy, and which seem to be localized in the ion stopping region only, i.e., in the nuclear damage layer, serving as an optical isolation barrier in implanted waveguides [1,2]. In order to confirm this assumption, micro- Raman spectroscopy investigation was performed.

The micro-Raman spectra of He-implanted samples also exhibit new bands at the following frequencies: 476, 540, 551, 689, 745, 838 and 910 cm-', which have favourably the polarization, corresponding to the A ; phonon modes. The frequencies of four of these new Raman-bands coincide with the ones of the new bands in IR-reflection spectra. These bands have a maximum magnitude at a depth corresponding to the barrier layer position, as illustrated in Fig. 3 for the 689 cm-' band. As the laser beam diameter is much larger than the thickness of barrier layer (roughly 4 and 0.5 pm, respectively), we used a simple standard procedure to extract the depth profile of the band intensity from the raw data obtained by micro-Raman scanning. In contrast, no new bands are observed in the micro-Raman spectra of H-implanted samples, only weak depolarization and broadening of the characteristic LiNb03 bands can be noticed.

The analysis of our experimental data, according to the theory of defect-induced changes in Raman scattering [7] and IR-reflection [8], allows us to conclude that: (1) the defects induced by He+-implantation have a non-point nature and might be assigned to large-dimension clusters [3], having a new crystalline structure with a very specific lattice vibration spectrum; (2) in H+-implanted samples the predominant lat- tice defects are found, in agreement with previous results [4], as to be of point nature, inducing weak crystal disorder only.

These results are of prime interest, since the optical phonons give thc dominating contributions in the spontaneous polarization, electrooptical effect and other related phenomena [6,9], in LiNb03 crystals.

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156/[548] S.M. KOSTRITSKII AND P. MORETTI

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0 1 2 3 4 Beam shift (km)

FIGURE 3 Micro-Raman band intensity (at 689cm-') versus the shift of the probe beam scanned at the polished edge of a sample implanted with Hef ions at 1 MeV and at a dose of 4 x 10'6ionsjcm2. From TRIM simulations the ion-implantation areas expected in this case extend from 2.3 to 2.8 pm.

References

[l] P.D. Townsend, Nucl. Imtrunl and Meih. B 46, 18 (1990). [2] P. Moretti, P. Thevenard, K. Wirl, P. Hertel, H. Hesse, E. Kratzig and G. Godefroy,

[3] Y. Avrahami and E. Zolotoyabko, Nucl. Instr. and Meih. B 120, 84 (1996). [4] S.O. Salem, B. Canut. P. Moretti, J. Meddeb, S.M.M. Ramos and P. Thevenard,

[S] J.F. Ziegler, J.P. Biersack and U. Littmark, The Stopping and Ranges of Ions in Solids,

[6] A.S. Barker and R. Loudon, Phys. Rev. 158, 433 (1968). [7] H. Vogt, J . Phys.: Condens. Mutter 3, 3697 (1991). [X I K.A. Muller. Y. Luspin, J.L. Servoin and F. Gervais, J . Physique-Lettrrs 43, L537

[Y] W.D. Johnston, J . Appl. Phys. 41, 3279 (1970).

Ferroelecirics 128, 13 (1992).

Radiuf. EjJ 133-134, 103 (1995).

Pergamon Press, New York. 1988.

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