Second harmonic generation from Ge doped SiO2 (Gex(SiO2)1x) thin films grown by
sputtering
Ibuki Kawamura, Kenji Imakita, Minoru Fujii, and Shinji Hayashi
Citation: Applied Physics Letters 103, 201117 (2013); doi: 10.1063/1.4831984
View online: http://dx.doi.org/10.1063/1.4831984
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APPLIED PHYSICS LETTERS 103, 201117 (2013)

Second harmonic generation from Ge doped SiO2 (Gex(SiO2)12x) thin films
grown by sputtering
Ibuki Kawamura, Kenji Imakita,a) Minoru Fujii, and Shinji Hayashi
Department of Electrical and Electronic Engineering, Graduate School of Engineering, Kobe University,
Rokkodai, Nada, Kobe 657-8501, Japan(Received 11 August 2013; accepted 1 November 2013; published online 15 November 2013)
Second-order nonlinear optical properties of sputter-deposited Ge-doped SiO2 thin films were
investigated. It was shown that the second-order nonlinearity of SiO2, which vanishes in theelectric-dipole approximation due to the centrosymmetric structure, can be significantly enhancedby Ge doping. The observed maximum value of d33 was 8.2 pm/V, which is 4 times larger thand22 of b-BaB2O4 crystal. Strong correlation was observed between the deff values and the electron
spin resonance signals arising from GePb centers, suggesting that GePb centers are the mostC 2013 AIP Publishing LLC.
probable origin of the large second-order nonlinearity. V[http://dx.doi.org/10.1063/1.4831984]
In recent years, nonlinear optical materials for silicon(Si) photonics have attracted significant attention. In particular, second order nonlinear optical material is of great interest toward the realization of CMOS-compatible electric
optical modulators and frequency converters.1–7 A lot of
researches to induce second order nonlinearity to CMOScompatible materials have been reported. For example,
strong second harmonic generation (SHG) was observed
from a Si waveguide on which a silicon nitride (SiN) layer
was deposited.8 The strain induced by the SiN layer to the
waveguide causes the breakdown of the centrosymmetry of
Si and brings about the large second-order nonlinearity.
Ultraviolet (UV)-poled GeO2 doped SiO2 thin films are
known to have large second order nonlinearity.9–11 The origin is preferentially orientated polarization of oxygen
vacancy defects, induced by the UV-poling process.
Furthermore, a few kinds of amorphous thin films preparedby conventional deposition systems have been reported to
exhibit large second order nonlinearity. For example, strong
SHG signals were observed from SiN thin films prepared byplasma enhanced chemical vapor deposition, and siliconmonoxide (SiO) thin films prepared by an electron beam
deposition.12,13 In these amorphous thin films, the origin of
the second order nonlinearity has not been elucidated.
In this work, we propose Ge doped SiO2 (Gex(SiO2)1x)
thin films as CMOS-compatible amorphous thin films thathave large second-order nonlinearity. We show that thesecond-order nonlinearity of SiO2, which vanishes in theelectric-dipole approximation due to the centrosymmetricstructure, can be significantly enhanced by the Ge doping(Gex(SiO2)1x). The observed effective second-order nonlinear coefficient (deff) reaches as high as 5.48 pm/V. This value
is more than twice larger than that of b-BaB2O4 (BBO).
Electron spin resonance (ESR) measurements reveal that thethin films possess two kinds of oxygen vacancy defects, i.e.,
SiE0 and GePb centers. The comparison between the deff values and the ESR signal intensities demonstrates that GePb
centers are the most possible origin of the large second-ordernonlinearity of Gex(SiO2)1x thin films.
a)Electronic mail: imakita@eedept.kobe-u.ac.jp0003-6951/2013/103(20)/201117/4/$30.00

Gex(SiO2)1x thin films were prepared by a sputtering
method. Small pieces of Ge tips (4  4 mm2) and a SiO2 wafer (10 cm in diameter) were used as sputtering targets. Ge
concentration was controlled by the areal ratio of Ge to SiO2
targets. Energy dispersive X-ray spectroscopy (EDS) measurements confirm that the Ge concentration ranges from 0 to
15.2 at. %. Fused silica plates were used as substrates. The
film thickness was about 1 lm. X-Ray photoelectron spectroscopy (XPS) measurements were carried out with a fully
automated XPS microprobe (PHI-Xtool, Ulvac-Phi, Japan)
using Al Ka X-ray beam (1486.7 eV). Annealing was performed in a N2 gas atmosphere. deff was estimated by analyzing SHG signals based on a Maker-fringe method. Figure 1
shows the optical setup. A mode-locked Ti:sapphire femtosecond laser with the wavelength of 800 nm and the full
width of maximum (FWHM) of about 10 nm was used forthe excitation. The pulse width and repetition frequency
were 70 fs and 82 MHz, respectively. A monochromatorequipped with a photomultiplier tube (PMT) was used todetect the SHG signal. The polarization of the excitation
light was controlled by a Glanprism (GP) and a half-wave
plate (HWP). A polarizer was placed in front of the monochromator to resolve the polarization of the SHG signal.
Samples were set on a motorized rotation stage to change theincident angle (h). Both the excitation and detection werep-polarized for the measurements of SHG spectra andMaker-fringe patterns. h was set to 50 for the estimationof deff. The refractive index (n) and extinction coefficient (k),which are necessary for the analysis of the SHG signal, were
estimated from the transmittance (T) and reflectance (R)spectra. k at the wavelength of k was obtained by using therelation k ¼ (k/4pL)ln(T/(1  R)), where L is the thicknessof the thin films.14 n was calculated by the relationn ¼ 1/2d(1/kn  1/knþ1), where kn is the peak wavelength of
the nth peak in the interference fringe in the reflectance spectra of thin films.15 BBO crystal was used as a reference sample whose deff is about 2.0 pm/V.16 The second ordernonlinear tensor components (dij) were estimated by analyzing the polarization dependence of SHG signals.17,18
Figures 2(a) and 2(b) show the Ge concentration dependence of Ge 3d and Si 2p XPS spectra for as-deposited

103, 201117-1

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Appl. Phys. Lett. 103, 201117 (2013)

FIG. 1. Optical setup used for SHG measurements. GP, Glan prism; HWP,
half-wave plate; VIS, visible-blocking filter; IR, infrared-blocking filter;
PMT, photomultiplier tube.

Gex(SiO2)1x. In Fig. 2(a), two peaks appear at 29.4 and33.0 eV. These peaks correspond to Ge0 and Ge4þ, respectively, indicating that Ge clusters and GeO2 are simultaneously formed. In Fig. 2(b), a peak assigned to Si4þ can beseen at 103.2 eV. The peak energy is independent of Ge concentration, meaning that there are no Si-Ge bonds. Note thatthese results are very similar to those reported previously byother authors.19 Figure 2(c) shows the Ge concentration dependence of the transmittance spectra. With increasing Ge
concentration, the transmittance in the visible region
decreases due to absorption by Ge clusters. On the other
hand, high-transmittance of the near infrared region, which
is important for applications in optical telecommunication,is preserved even for the sample with the highest Ge
concentration.
Figure 3(a) shows Ge concentration dependence of the
SHG spectra. For all the samples, we can see a peak at
400 nm with the FWHM of 4.5 nm. These values are thehalves of those of the excitation light, confirming that theobserved signals are the SHG of the excitation light. With
increasing the Ge concentration, the SHG signal increases
until the Ge concentration reaches 5.2 at. % and then

FIG. 2. Ge concentration dependence of XPS spectra in the (a) Ge 3d and(b) Si 2p regions for as-deposited Gex(SiO2)1x thin films with different Ge
concentration. (c) Transmittance spectra of the as-deposited Gex(SiO2)1x
thin films with different Ge concentration.

FIG. 3. (a) SHG spectra at h of 50 and (b) Maker-fringe patterns, as afunction of Ge concentration. (c) Ge concentration dependence of intensityof SHG (left-side) and deff (right-side).

decreases. When the Ge concentration is 0 at. %, the SHG
signal is below the detection limit. Figure 3(b) shows Makerfringe patterns as a function of Ge concentration. In all thepatterns, the SHG intensity is negligible at 0 , and takes the
maximum at 50 and 50 . This symmetric pattern is very
similar to those observed for thin films with in-plane isotropy
such as thermally poled optical glasses.20,21 The left axis of
Figure 3(c) shows the intensity of the observed SHG as afunction of Ge concentration. It is the highest at 5.2 at. %.
The decrease in the high Ge concentration region is partly
due to large absorption at the SHG and excitation wavelengths. To eliminate the effect of the absorption, we estimated deff by using a formula taking into account theabsorption.17 The right axis of Figure 3(c) shows the deff. It
increases with increasing Ge concentration, takes the maximum at 11.2 at. % and then decreases. The maximum valueis 5.48 pm/V. This value is more than twice larger than thatof BBO crystal.
Figure 4(a) shows the annealing temperature dependence of the transmittance spectra. The transmittance doesn’tstrongly depend on the annealing temperature, although itslightly decreases when the annealing temperature is 200  Cor 400  C. Figure 4(b) shows the annealing temperature dependence of the SHG spectra. The SHG intensity decreases
with increasing annealing temperature, and completely vanishes at 850  C. Figure 4(c) shows the SHG and deff as afunction of annealing temperature. Both the SHG intensityand deff decrease with increasing the annealing temperature.
Similar results have been reported for SiN thin films.18
In general, there are two kinds of second-order nonlinearity, i.e., the bulk and interface nonlinearity. To determine them, we measured the thickness dependence of SHG
spectra at the Ge concentration of 2.6 at. %. Figure 5(a)shows the result. The SHG intensity strongly depends on thefilm thickness. Figure 5(b) shows the SHG intensity as afunction of the thickness. The dots and the solid line are theexperimental and calculated results, respectively. The calculation assumes the bulk-type SHG response of absorptive

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Appl. Phys. Lett. 103, 201117 (2013)

FIG. 4. Annealing temperature dependence of (a) transmittance spectra, (b)SHG spectra at h of 50 , and (c) intensity of SHG (left-side) and deff(right-side) at the Ge concentration of 5.2 at. %.

thin films.17 The SHG intensity increases with increasing thefilm thickness, and the experimental result is well reproducedby the calculation. This suggests that the bulk second ordernonlinearity is responsible for the observed SHG. There areseveral possible origins as bulk second order nonlinearity of
our samples. One of them is Ge clusters. If the polarizationof the Ge clusters is preferentially oriented, they can induce
the second order nonlinearity. Similar discussion can be seenin the previous report on the SHG from Si-rich SiN, in which
Si clusters are suggested as the possible origin.12 However,this model cannot explain the decrease of deff in the high Ge
concentration region in Fig. 3(c). The second one is oxygen
vacancy defects. In case of UV-poled GeO2 doped SiO2 thinfilms, preferentially oriented oxygen vacancy defects (GeE0centers) induce the second order nonlinearity. Figure 6(a)shows the ESR signals as a function of Ge concentration.
The solid curves are the results of the fitting by two Gaussianfunctions. The g values of the two peaks are 2.004 and2.019, which can be assigned to SiE0 and GePb centers,
respectively.22–24 Note that the signal from GeE0 center, gvalue of which is 1.995 (Ref. 25) (351.9 mT in Fig. 6(a)), is
not observed. This is in sharp contrast with UV-poled GeO2
doped SiO2 (germanosilicate glass), in which GeE0 centers
are the dominant oxygen vacancy defects.9,26 Figure 6(b)FIG. 5. (a) Film thickness dependence of SHG spectra at h of 50 at the
Ge concentration of 2.6 at. %. (b) Experimental (dots) and calculated (solidline) SHG intensity as a function of film thickness.

FIG. 6. X-band ESR signals of (a) as-deposited Gex(SiO2)1x thin films as afunction of Ge concentration and (b) annealing temperature at room temperature at the Ge concentration of 5.2 at. %. The intensity of SiE0 center (leftside) and GePb center (right-side) as a function of (c) Ge concentration and(d) annealing temperature at the Ge concentration of 5.2 at.%. deff as a function of the ESR signal intensity of (e) GePb centers and (f) Si E0 centers.

shows X-band ESR signals as a function of annealing temperature. We can see that the ESR signal intensity depends
strongly on the annealing temperature. Figure 6(c) plots theintensity of the ESR peaks as a function of Ge concentration.
With increasing Ge concentration, the intensities of GePb
and SiE0 centers increase and then decrease. The intensity of
GePb centers takes the maximum at 11.2 at. %, while that of
SiE0 centers does at 7.3 at. %. Figure 6(d) shows the annealing temperature dependence of the ESR signal intensities of
GePb and SiE0 centers. Both of them decrease with increasing the annealing temperature. In Figs. 6(e) and 6(f), deff areplotted as a function of the ESR signal intensities of GePb
and SiE0 centers, respectively. We can see correlations
between the deff value and the ESR signal intensities in Figs.
6(e) and 6(f). Furthermore, the correlation coefficients ofFigs. 6(e) and 6(f) are 0.83 and 0.45, respectively. The
results suggest that the GePb centers are more probable origin of the second-order nonlinearity of Gex(SiO2)1x thinfilms. It should be stressed here that the second-order nonlinearity should not appear if the material possesses inversion symmetry. The observed large second ordernonlinearity suggests that the inversion symmetry of theGex(SiO2)1x thin films is broken during the deposition process. The mechanism of the preferential orientation of GePb
centers is not clear.

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Appl. Phys. Lett. 103, 201117 (2013)

In conclusion, we observed strong SHG from
Gex(SiO2)1x thin films prepared by sputtering. The maximum value of d33 was 8.2 pm/V, which is about 4 times
larger than d22 of BBO crystal. The comparison between thedeff values and the ESR signal intensities suggests that thepossible origin of the large second-order nonlinearity is
GePb centers. Our results suggest that Gex(SiO2)1x thinfilms are a promising material for second-order nonlinear optical devices which can be used in silicon photonics.
This work was supported by KAKENHI (23310077).
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FIG. 7. (a) p- and (b) s-polarized SHG signal as a function of rotation angleof HWP. The rotation angles of 0 (or 90 , 180 ) and 45 (or 135 ) correspond to the P- and S-polarization of the excitation light. The h was 50 .
The tensor components of d15, d31, and d33 as a function of (c) Ge concentration and (d) annealing temperature at the Ge concentration of 5.2 at. %.

Figures 7(a) and 7(b) show the p- and s-polarized SHG
intensity as a function of the rotation angle of the HWP in
Fig. 1. The rotation angles of 0 (or 90 , 180 ) and 45 (or135 ) correspond to the p- and s-polarization of the excitation
light, respectively. We find that both p- and s-polarized SHG
signal intensities strongly depend on the polarization of theexcitation light. In Fig. 7(a), we see that the p-polarized SHG
intensity takes a maximum (minimum) when the excitation
light is p (s) -polarized. On the other hand, in Fig. 7(b), the spolarized SHG signals become zero in the case of p- and spolarization of the excitation light. To analyze these data, we
assume that the thin films possess in-plane symmetry (C1v
symmetry group). The assumption is plausible considering thefact that the thin films are vertically deposited on isotropic
silica substrates. In fact, The Maker-fringe patterns in Fig.
3(a) and the polarization dependence in Fig. 7(b) are consistent with the results predicted for thin films belonging to theC1v symmetry group.12,27 In this symmetry group, thesecond-order nonlinear tensors have three independent tensor
components of d15, d31, and d33.28 Figures 7(c) and 7(d) show
the tensor components as a function of Ge concentration andannealing temperature. All the tensor components show dependence similar to that of deff shown in Figs. 3(c) and 4(c).
For all the samples, d33 is the largest and d15 is the smallest.
This result is similar to those previously reported for thin films
belonging to the C1v symmetry group and implies the breakdown of the inversion symmetry in the z-axis.12,25,29 The maximum value of d33 is 8.2 pm/V at 11.2 at. %. This is about 4times larger than d22 of BBO crystal, and comparable to d33 of
UV-poled GeO2 doped SiO2 thin films (12.5 pm/V).9

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