Germanium implantation into substrates for integrated optics

Nuclear Instruments and Methods in Physics Research B 86 (1994) 279-287 North-Holland

Germanium B. Poumellec a CNRS UA 446,

implantation

into substrates for integrated

NIUMI B

Beam Intenttions with Materials & Atoms

optics

ap*,A. Traverse b, S. Artigaud ’ and J. Hervo ’

UPS Orsay, Bit. 415, F-91405 Orsay Cedex, France b Centre de Spectrom’ttie Nucliaire et de Spectro&rie de Masse, CNRS-IN2P3, Bit. 108, 91405 Orsay, France ’ Alcatel Abthom Recherche / DFP, Route de Nozay, F-91460 Marcoussis, France

Received 16 July 1993 and in revised form 3 December 1993

Germanium and helium implantations have been performed in LiNbO,, SiO, quartz and silica. The agreement between calculated and experimental doping profiles is excellent. The index profiles coincide with the calculated collision profiles but we have observed a surface effect in quartz and LiNbO,. In the first material, Ge implantation yields a larger decrease of the refractive index at the surface than He, as it is predicted by calculation if we assume the refractive index and the disorder profile to be connected. In contrast, in LiNbO, a reverse observation is made with respect to the refractive index. It is accompanied by chemical perturbation which interferes with the structural modification at the origin of the refractive index change. One advantage of the method is that implanted Ge is in a reduced state.

1. Introduction Ion implantation is a promising tool to write optical wave-guides for integrated optics. It is possible to use this procedure either for building a waveguide (by He+ implantation for instance) or for implanting active species (like rare earths for instance). The refractive index profile (An(z)) is a priori related to an implantation profile, either the ion concentration profile or the nuclear disorder profile. Zhang et al. [l] claim that the modification of the refractive index in the He+ implanted waveguides is actually primarily due to collision damage. It has been previously noticed [2] that implantation leads to opposite effects depending on the amorphous or crystalline state of the matrix. This is schematically described in Fig. 1. Implantation into crystalline insulating material leads to a decrease of the refractive index probably because of the disorder induced by the incoming ion (Fig. la). At high dose, the decrease of the refractive index is limited by the appearance of an amorphous phase [3,4]. Finally, a waveguide is localized at the surface and the implantation region is the cladding. Fig. lb describes the case of an amorphous target. In this case, implantation leads to compaction 151 and to an increase of the refractive index until a saturation is reached at high dose. The waveguide is hence directly made by implantation and therefore buried. * Corresponding 69855484.

author,

tel. +33

1 69416351, fax +33 1

It has been shown [6,7] that soft annealing (250°C 30 min) clears part of the defects without significant modification of the index profile and leads to acceptable attenuation i.e. less than 1 dB/cm. In this paper, we describe germanium implantation in targets which are potential substrates for integrated optics, i.e. silica, quartz and lithium niobate. Germanium is the key in the elaboration of photorefractive gratings into waveguides [8] or to efficient second harmonic generation [9,3]. Because there are many papers available on He+ implantation (see Table 11, we have also performed such experiments for comparison. Dopant implantation profiles and the nature of resulting defects were studied in order to correlate them with the measured refractive indices.

2. Implantation: experimental details Using the facilities in CSNSM [lo] two sets of experiments were performed: 1) He ions were implanted into SiO,, quartz and LiNbO,, the energy and fluence conditions being chosen close to those described in ref. [ll]. 2) Ge ions were also implanted into SiO,, quartz and LiNbO, in the same run. The incident energy (5 MeV) was chosen so that the mean range of Ge was approximately the same as in the He implantation experiments, i.e. 2 to 3 km. The TRIM code [12] was used to determine the implantation parameters reported in Table 2: mean range R, and straggling AR,. The code assumes amorphous targets, which is the case

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280

B. Poumellec et al. / Nucl. Instr. and Meth. in Phys. Res. B 86 (1994) 279-287

Table 1 Implantation experiments Substrate

Particle

Energy [MeV] (depth [pm])

Fluence

0.9-2

10’5-10’6

[311

[321

Ana

Optical fiber 080pm

H+

LiNbO, z cut LiNbO, x cut LiNbO,

e-

0.012

10”

Tizf

0.360 (0.35)

2.5 x 10”

LiNbO, x or y cut LiNbO, y cut WQ3

ffb MB/cm1

[ions/cm21

Ref.

(40)

He+ ;5, 2

He+ He+

0.012

t331

1.2x 10’6

1

1111

10’6


ill1

4x10’6

0.025

[I61

10’6

0.1

[341

5 x lo’s_ 1.6x 10” 5 x 10’52 x 10’6 10’8-1020

0.05

3

[151

0.02

2-3

[ill

10-3-10-2

> 0.3

[51

t;; He+

Quartz

He+

:5, 0.7-2.2

NbLiO,

He+

0.8-2.2

Silica

e-

0.025 (2-7)

a An is the index difference between the core and the cladding of the waveguide. b (Yis the attenuation measured around 1.5 km.

for silica but not for the other substrates: quartz and LiNbO, 001 oriented. However, during implantation the samples were tilted with respect to the ion beam by a few degrees to avoid channelling. So, the crystalline materials behaved for the incoming ions like amorphous targets. The ionic flux was adjusted in order to prevent heating during implantation. The lower fluence of Ge (5 x 1014 Ge/cm*) was chosen so that the number of vacancies created per cm2 as estimated by TRIM [12] (see Table 2), is of the same order of magnitude as that created by 2 X 10 l6 He/cm2. The higher Ge fluence creates a number of vacancies twice as large (not

shown in Table 2). In this case, the maximum Ge/Si content is 3 X 10m3. The profile estimations, the calculated number of vacancies created per incoming ion and the implantation parameters are summarized in Table 2. During Ge implantation quartz gives rise to a pink luminescence which is due to nonbridging oxygen hole centers, while silica gives rise to blue luminescence due to oxygen vacancies [13]. Antonini et al. [14] noticed that implantation induced absorption at 244 nm, quoted B,, is much more intense for heavy particles than for lighter ones. Knowing that B, is connected to the blue fluorescence in silica, we deduced that the blue lumi-

Table 2 Implantation parameters Runs Implanted ion type Fluence [ions/cm’] Current [p.A/cm2] Energy [MeV] Target R, [&ml

AR, [km1

a 4He+ 2x 10’6 0.25 1.25 LiNbO, 2.75 0.15

SiO, 4.10 0.20

quartz 4.10 0.20

Number of vacancies /incoming ion

157

169

169

b 4He+ 2 x 10’6 0.31 1.0 LiNbO, 2.25 0.15

;4

Si02 3.20 0.15

quartz 3.20 0.15

5 %4 0.08 5.1 LiNbO, SiO, 2.08 3.08 0.33 0.40 for 5 X 1014 ions/cm2 17 104

14792

d 74Ge+ 1 x 1015 0.08 5.1 quartz 3.08 0.40 14792

B. Poumellec et al. / Nucl. Instr. and Meth. in Phys. Res. B 86 (1994) 279-287

@

For

crystalline target

281

2) A modification of the matrix atom profiles, similar for Nb and Li, is observed at the implantation depth. The oxygen profile was not measured. 3) There is a Li depletion at the sample surface. 4) Hydrogen has diffused into the target, the concentration profile ends approximately at the end of the Ge profile. 3.2. He implanted NbLiO,

@

For amorphous target

There is also a modification profiles at the He implantation but indicated by vertical lines in with an 0 modification profile at ness. No hydrogen was detected. at the surface.

of the Nb and Li depth (not measured Fig. 2b as calculated) slightly smaller thickThere is Li depletion

Ge implanted SO,

Fig. 1. Schematic index profiles obtained by implantation (a) into a crystal, (b) into an amorphous material.

In silica (Fig. 2c), the Ge implantation profile is again coinciding with the TRIM calculation. In contrast to LiNbO,, there is no structure in the profiles of the other constituent elements: Si and 0. Hydrogen contamination occurs from the surface but the level is quite low in comparison with the case of LiNbO,. In quartz (Fig. 2d), the implantation profile is also in agreement with the calculation. It is worth noting that the bulk, crystalline or amorphous, is perturbed in a different way in the atomic arrangement (because the implantation induced index change is opposite due to an amorphization in one case and a compaction in the other case) but not in the chemical composition.

4. Refractive index change nescence is especially produced by heavier ions, as it is observed experimentally. In contrast, we have observed no fluorescence from lithium niobate.

3. Concentration

profiles

Measurements of the implantation profiles have been made by means of secondary ion mass spectroscopy (SIMS) with cesium particles and negative secondary electrons. Surface charge suppression has been achieved by using an auxiliary electron gun. Results are shown in Fig. 2. 3.1. Ge implanted LiNbO, We note that: 1) The experimental Ge profile is in agreement with the calculations (see Fig. 2a and Table 2, compare full and dashed lines).

We have used the dark line method for measuring the waveguide properties. The principle is to couple the light into the waveguide by means of a prism (see appendix). By varying the incident angle of the beam, the intensity of the light coupled to the waveguide exhibits shining lines and the reflected beam exhibits dark lines. The shape of the curve of the refracted intensity versus the incident angle or the effective index is characteristic of the waveguide. We have used two wavelengths to perform the measurements: A= 0.5435 and 0.6328 pm. The results are shown in Table 3. It is sometimes difficult to measure the incident angle (ei in Fig. 6 in the appendix) except in SiO,. Particularly in He implanted LiNbO,, we had to anneal the sample (244°C 30 mitt) in order to couple light into the waveguide. It is shown in ref. [l] that this yields a partial decrease of the refractive index (from 5 to 2%) due to bleaching of defect complexes but improves the attenuation greatly. For silica samples, the number of coupled modes is

B. Pownellec et al. /NucL Instr. and Meth. in Phys. Res. B 86 (1994) 279-287

282

6

Nb 93 Li 6

/// L/J :

‘0

la) 1

033

n Ill

,

,

,

2

3

4

Depth (~0 6r-

1ICI

I

I

I

I

I

I

,

1

2

3

4

5

6 Depth (~1

5F ‘; * 4Ge 74

(4

I

I

I

1

2

3

I

I

4 5 Depth @m)

t

B. Poumellec et al. /Nucl. Table 3 Effective

Quartz

LiNbO,

283

mode index (at 0.6328 pm) Ge 5.1 MeV 5 X 1014 ions cm-*

Ge 5.1 MeV

2 X 1CP ions cm-* 1.25 MeV max = 1.4693 mode 1 = 1.4661

1.4738 1.4690

1.4740 1.4696

mode 2 = 1.4622

1.4633

1.4640

mode 3 = 1.4565 substrate mode = 1.4562 1.00 MeV

1.4549 1.4455-1.4467

1.4561 1.4558

surface = 1.5499

1.4746

1.4760

mode 1 = 1.5369 mode 2 = 1.5232 mode 3 = 1.5012

1.4695 1.4631

1.4702 1.4630

1.4611-1.4614

1.4549-1.4562

He SiO,

Instr. and Meth. in Phys. Res. B 86 (1994) 279-287

mode 4 = 1.4731 substrate mode = 1.4709-17 1.25 MeV annealed a

1015 ions cmm2

mode 1 = 2.0812 mode 2 = 2.0676

2.1293 2.1182

mode = 2.0894 2.0511 surface3 = mode 4 = 2.0321

2.1048 2.1371

a 30 min., 244°C. b For Ge implantation.

larger, as the refractive index change is larger. The value of the refractive index for the substrate mode shows that there is no change of index near the surface. Whatever the experiment, the maximum index change remains largely below the saturation, which can be estimated to be close to the index of quartz, i.e. 1.55. For quartz, the behaviour is completely different. The index change is negative and very large, limited to a value of about 1.46. It seems that the effect of He or Ge implantation is different. There is no disturbance of the surface for He implantation since the surface index obtained by reconstruction of the profile is = 1.55, i.e. close to the original one (1.552). However, for Ge implantation the number of modes is very limited because of a large decrease of the surface index and the limitation of the minimum of index in the material. The resultant waveguide has a maximum index difference of about 2 X lo-‘. These results are in agreement with those of Zhang et al. [15].

For LiNbO,, there is no saturation effect at least in the case of He implantation [16]. However, for this material, the surface index in the case of He is strongly decreased by about 11 X lo-’ (note that the annealing conditions are rather soft, and that Chandler et al. [16] showed that the index profile is resistant up to 1000°C). For Ge implantation, it is smaller, reaching only 6 x 10-*.

5. Optical absorption

In order to know what are the defects resulting from implantation, we have performed UV-visible absorption experiments. In He and Ge implanted LiNbO,, we have seen neither a change of the shape of the absorption spectrum nor the appearance of new structures after implantation. The only difference we have detected, is a strong increase of absorption above 3.4 eV (or below

Fig 2. SIMS profiles. The vertical lines indicate R, and R, + Ap values as calculated by TRIM. (a) for Ge implantation in LiNbO,, 5.1 MeV, dose = 1Cl15ions/cm’. 1 = H, 6 = Li, 93 = Nb (there is no interference with the oxygen since we have checked that there is no structure for the oxygen profile). Dashed line shows the Ge profile as calculated by TRIM, the curve has been shifted vertically for comparison. (b) for He implantation in LiNbO,, 1.25 MeV, dose = 2X 1Cl’6ions/cm’. 6 = Li, 33 = 20 +H (there is no hydrogen profile in this sample, so this profile is for the oxygen) 94 = Nb + H = Nb (for the same reason). (c) for Ge implantation in silica, 5.1 MeV, dose = 1015 ions/cm’. 1 = H, 30 = Si, 32 = 20, 74 = Ge. (d) for Ge implantation in quartz, 5.1 MeV, dose = 1Ol5 cm*. 74 = Ge. The rapid decrease at the surface corresponds to a charge effect often seen in very insulating material.

B. Pomelkc et al./ Nucl. Instr. and Meth. in Phys.Res. B 86 (1994) 279-287

284

or 0.12. Similar defects i.e. GeE’ or Ge(2) have oscillator strengths of 0.7 or 0.6 respectively. We obtain a defect ~n~ntration of 3.9 X 10” or 1.3 X 1019 cmP3 and a relation of the concentration of Ge-related defects to the amount of implanted Ge of 0.53 or 1.75. Of course this value cannot exceed 1, but this indicates (taking into account that our precision on this estimation is of the order of 1%) that the defect is the one which has the oscillator strength equal to 0.4. It is an oxygen vacancy between two germanium atoms [22]. The proportion of Ge related defects to the total Ge amount is much higher than in material classically doped by thermal synthesis (i.e. 0.006).

Absorption f

Si02

.ol 190

II 220

,I 250

,I wavelergth 280

6. Discussion 6.1. Concentrationprofiles

310nm

Fig. 3. Optical absorption spectra for the silica before (dashed curve) and after He or Ge implantation (dash-dotted curve or full curve, respectively).

365 nm) independent of the implanted particle, i.e. above the optical gap. The increase of the corresponding absorption tail in the visible range gives rise to a brown coloration of the He implanted LiNbO, samples. No such coloration has been detected in Ge implanted samples. In quartz and silica, the absorption is modified in a similar way. The result is shown in Fig. 3. He implantation leads to an absorption increase in the whole W-visible range but does not give rise to B, defect absorption at 244 nm. We detect only the formation of the E’ defect absorption near 215 nm as already mentioned by Antonini [14] (for each proton of 1 MeV, two E’ defects are produced). In contrast, in the spectrum of the Ge implanted samples there is no detectable increase of the absorption in the UV-visible range but an increase of the B, band. This band is known to be due to several defect absorptions [17]. One part comes from the Si bulk and the other from germanium. A comparison of the He and Ge implantation experiments shows an increase of the absorption due to Ge. If we assume that this comes from the implanted layer, the thickness of which being 2AR, (i.e. 3 X IO-’ cm), we obtain an absorption coefficient of 1000 cm-‘. In comparison, Albert et al. [18,19] obtained = 100 cm-’ for a fluence of iO13 at. cm-* (i.e. 50 times lower than ours) at 3 MeV. Furthermore, we can use Smakula’s expression [20] and the data of Anoikin et al. [21] to compute the number of defects multiplied by the oscillator strength. This last parameter is not known precisely because there are several defects absorbing in the same range. From Hosono et al. [22] it is either 0.4

Whatever the target, the agreement is always quite good between the experimental and calculated implantation profiles of Ge. Therefore, it can be assumed that the experimental He implantation profile also agrees with that calculated by TRIM [12] although it was not possible to measure the He profile. Ge or He implantation perturb the elements of LiNbO,, as seen in the SIMS profile in Figs. 2a and 2b. In the case of He implantation, the pe~urbation coincides with the calculated vacancy profile (see Fig. 4b). This indicates that Nb, Li and 0 atoms are pushed away from their crystalline sites inducing the change of ionic emissivity seen as modulations on concentration profiles of this elements (Fig. 2b). This seems to be connected with cleavage initiations seen in the bulk after implantation revealing concentration of stresses in this region. In the case of Ge implantation, the modulations are coinciding rather with the Ge profile than with the vacancy profile (see Fig. Sb). Hence, an explanation similar to that proposed for He is not valid. We suggest that the ge~a~um itself creates an ove~helming disorder, which is of chemical nature. In the Ge implanted LiNbO,, there is also a hydrogen contamination which occurs only in this case. It seems that the implanted sample acts as a hydrogen pump. If we try to correlate the H profile (Fig. 2a) to a disorder profile, it appears to be closer to the electronic disorder than to the nuclear one. The first one has an arctan shape close to the hydrogen profile, whereas the other exhibits a bell shape. On the other hand, it is known that the surface of LiNbO, is perturbed when it is bombarded with heavy particles like Ti (23,241. Lithium diffuses to the surface. In the case of Ge implantation, the reverse behavior is observed. With Ti bombardment, the lithium migration accompanies probably the implantation of positive charges which

B. Poumellecet al. /Nucl. Instr. and Meth. in Phys. Res. B 86 (1994) 279-287

is compensated by Li migration. Hence, with Ge (which seems to be less offing) the situation is reversed, and this explains the pumping effect on H. The Li depletion at the surface comes from another process ~preferentia1 etching) already described by Tianhao et al. 12.51.This process is less effective in SiO,, perhaps

285

N Gel

‘IA lo-’ 4

N We)

5.o lO“ t 3.0 1 oq4

depth(p m)

1.0 1 o-4

1

-1.0 10”

0

I

I

t

I

,

1

2 (W-Jth (pm)

3

I

N (vacancies) 1‘4 100

1.0 loa

N (vacancies) 6.0 10-l

5.0 ro-2 2.0 10“ 3.0 1 o-2

t

“2.0 10”

0

1

t

I

,

2

1

b

3

depfh(~rn}

f.0 ioq2

Fig. 5. TRIM calculation of the germanium impIantat~on profile (a) and of disorder distribution fnuciear disorder) (b) in LiNbO,. -1.0

lo-*

’ 0

i

I

I

I

I

2

t

!

1

3

because it is more covalent and so less sensitive to the charge migration. 6.2. Refiactiue index change

Fig. 4. TRIM calculation of the helium ~pl~ntat~on profile (a) and of disorder distribution (nuclear disorder) fbf in LiNbO,.

The mechanism for the index change is entirely dependent on the collisional process [261. So the re-

286

B. Poumellec et al. /Nucl. Instr. and Meth. in Phys. Res. B 86 (1994) 279-287

fractive index profile coincides with the nuclear disorder profile [6]. The maximum of the implantation profile is almost coinciding with the nuclear disorder profile in the case of heavy elements like Ge (see Monte Carlo calculation of Figs. 4 and 5) but the main difference between both profiles is the existence of a tail to the surface on the disorder profile. This explains that the maximum index difference in crystal is always smaller than we can expect. For quartz, the decrease of the surface index in the case of Ge implantation is due to the nuclear disorder tail which is, contrary to He implantation, not negligible for Ge implantation. For LiNbO,, this observation agrees with the change of index with the oxido-reduction state in the region of electronic disorder [16,27,28] and leads to difficulties to obtain a good waveguide just after implantation. A short and soft annealing stage yields a good waveguide (probably by washing out the charges trapped at various centers), revealing that the decrease of the surface index in this material is much larger than in the others.

material for the core. This applies to He implantation. But, ion implantation can be used also to insert active species for manufacturing optical action. In this field, germanium implantation into SiO, is interesting because the waveguide is directly made by the active species. As implantation allows to obtain a high density of germanium atoms in reduced valence state; this can be interesting either in second harmonic generation or photorefractivity because the 244 nm band is bleachable [29]. Recent publications [18,19], appeared after our experiments, confirm this interest.

Acknowledgements

We are surements, Hubert for Tissot and

grateful to Mr. Bourigeaut for index meato Mrs. C. Hervo for SIMS, to Mrs. S. optical absorption, to Mr. Fortuna, Miss F. the SEMINARIS team for implantation.

7. Conclusions

Appendix

Comparison of refractive indices in silica, quartz and LiNbO, implanted with Ge or He yields the following conclusions: - The index change is much larger in crystalline than in amorphous materials. - The calculated profiles are consistent with the experimental implantation profile, but the refractive index profiles are subject to more complex phenomena. Heavy particle implantation like germanium leads to a decrease of the surface refractive index in quartz and LiNbO,. However, for the last material, hydrogen contamination and/or germanium in a reduced valence state lead to some compensation. LiNbO, appears to be more sensitive to nuclear disorder than quartz. - The use of crystalline material in waveguide elaboration is motivated by the fact that the absorbing species introduced by the implantation are produced in the cladding zone, so we expect a more transparent

A.1. Method for measuring the index profile

Fig. 6. Coupling

We have used the dark line method, the principles of which are published in ref. [30] A.l.1. Calculation of the reflectivity versus the effective refractive index profile

Because of the two-dimensional problem, the propagation equation in the waveguide is

z, t) = E(z)ei(Bx-or). Let p(z) = sin8(z) be the effective propagation vector equal to the projection of the wave vector onto the propagation direction. e(z) is the propagation angle of the beam in the waveguide (quoted 0, in Fig. 6).

with

4(x,

((2~n(z))/(h))

scheme of light into the waveguide

with a prism.

B. Poumellec et al. / Nucl. Insrr. and Meth. in Phys. Res. B 86 (1994) 279-287

The effective index neff = P/(27r/h) = /3/k = sin@(z), provided that the Descartes law does not depend on z. neff is an input in our problem. By replacing into Eq. (l), we get

n(z)

[

-$- p(z)jE(z)

= -k%‘(z)E(z),

or

;-$

+n2(z)

[

I

E(z)

=&E(z).

This is the non-time-dependent Schrijdinger equation with --n’(z) playing the role of a potential. Therefore if neff is smaller than n(z) for all z then E(z) is not localized in z. A.1.2.

Experimental

setup

The

relationship between the effective mode index and the incident angle Bi is hff

=

sin(A)/-

+ cos(A)sin

(ei).

The sensitivity on the incident angle is l/100’ so the absolute precision on the effective index is 10e4 taking into account A = 55.34” and NP = 1.6539 at He-Ne wavelength for silica and quartz or A = 45.00” and NP = 2.589 for He-Ne or NP = 2.655 for Green Ar+ or NbLiO,.

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