Thermodynamic Study on Diffusion Growth of Ga3+-Doped LiNbO3

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Thermodynamic study on diffusion-growth of Ga3+-doped LiNbO3 single crystal thin film for photonic application De-Long Zhang, Qun Zhang, Jian Kang, Wing-Han Wong, Dao-Yin Yu, and Edwin Yue-Bun Pun Cryst. Growth Des., Just Accepted Manuscript • DOI: 10.1021/acs.cgd.5b01415 • Publication Date (Web): 19 Jan 2016 Downloaded from http://pubs.acs.org on January 21, 2016

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Crystal Growth & Design

Thermodynamic study on diffusion-growth of Ga3+-doped LiNbO3 single crystal thin film for photonic application De-Long Zhang,*,†,‡ Qun Zhang,† Jian Kang, † Wing-Han Wong,†,‡ Dao-Yin Yu† and Edwin Yue-Bun Pun‡ †

Department of Opto-electronics and Information Engineering, School of Precision

Instruments and Opto-electronics Engineering, Tianjin University, Tianjin 300072, People’s Republic of China, and Key Laboratory of Optoelectronic Information Technology (Ministry of Education),Tianjin University, Tianjin 300072, China ‡

Department of Electronic Engineering and State Key Laboratory of Millimeter Waves, City

University of Hong Kong, 83 Tat Chee Avenue, Kowloon, Hong Kong SAR, China

A thermodynamic study was performed on growth of Ga3+-doped LiNbO3(LN) thin film. Some Ga3+-doped LN thin films were grown on LN surface by thermal diffusion of Ga2O3 film in the temperature range of 1000-1100 oC. After growth, the Ga3+ ion in the grown thin film was profiled and its diffusion-growth characteristics were studied. The temperature dependences of diffusion-growth coefficient and solubility were quantified. These dependences are crucial to the design and growth of a Ga3+-doped LN thin film for various photonic applications. A comparison with the case of Ti4+ diffusion-growth, which induces increase of LN refractive index and hence formation of an optical waveguide, shows that Ga3+ grows considerably faster than Ti4+ because its smaller ionic radius. In addition, Ga3+ doping contribution to LN refractive index was studied by measuring and comparing the refractive indices at Ga3+-grown and Ga3+-grown parts of crystal surface. The results show that the contribution is small.

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I. Introduction Lithium niobate (LN) crystal has larger electro-optic, acousto-optic and nonlinear optical coefficients. It is an important host material used for integrated optics. However, the material suffers from serious optical damage, which impedes the development of novel devices. To solve the problem, some optical-damage-resistant dopants have been reported that include Mg2+, 1 Zn2+, 2 Sc3+, 3, 4 Tm3+, 5 In3+ 6, Ga3+, 7 Hf4+,

8

Zr4+,

9, 10

and Sn4+.11 For the congruent LN, the threshold concentration of

optical damage, named Cth, is 5.0, 7.5, 2.0, 3.0, 4.0, 2.0 and 2.5 mol% for Mg2+, Zn2+, Sc3+, In3+, Hf4+, Zr4+ and Sn4+, respectively. As the crystal composition approaches the stoichiometry, the Cth lowers significantly. It is only 0.78 mol% for Mg2+, 12 0.5 mol% for Ga3+, 7 0.4 mol% for Sc3+, 4 and 0.085 mol% for Zr4+.

10

The lower Cth is desired

to improve the material homogeneity and the optical quality of crystal when codoped with rare earth ions such as Er3+, and to increase the diffusion-growth rate and solubility of codoped rare-earth ions. In addition, a near-stoichiometric (NS) LN exhibits other advantages over the congruent LN such as lower domain poling electric field, larger electro-optic and nonlinear coefficients. Thus, an NS lithium niobate doped with > 0.78 mol% Mg2+, 0.5 mol% Ga3+, 0.4 mol% Sc3+ or 0.085 mol% Zr4+ would be more promising than the corresponding congruent material. The realization of an NS Ti-diffused LN (Ti:LN) waveguide doped with Dn+ (= Mg2+, Ga3+, Sc3+ or Zr4+) would open up some new applications and construct a technical platform for the development of active and passive devices. Towards this long-term goal, one should firstly explore a feasible way to grow a Dn+-doped LN thin film, which, as a substrate material of optical waveguide device, has a thickness of several tens of micrometers. One merit of LN is easy doping. Almost all the metals and nonmetals on the periodic table of chemical elements can be doped into it. Diffusion-growth doping is a practical and effective method to improve the property of LN, and the method ensures 2


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that the grown surface is of good optical quality. In case that a homogeneously Ga3+, Sc3+ or Zr4+-doped LN is commercially attained not as easily as the congruent pure LN, a Dn+-doped LN thin film can be alternatively produced by diffusion-growth method, and an NS Ti:LN waveguide doped with Dn+ can be further produced by Ti4+ diffusion and post Li-rich vapor transport equilibration (VTE) treatment. To develop a device, it is essential to have the knowledge of the thermodynamic diffusion-growth properties. In the previous papers, people has reported the diffusiongrowth properties of H+, 13 Li+, 14-15 Mg2+, 16, 17 Zn2+, 18 Sc3+, 19 Er3+, 20, 21 Pr3+, 22 Tm3+, 23

Ti4+ 24-26 and Zr4+ in LN. 27 The relevant knowledge could not be found yet for Ga3+

ion. The major advantage of Ga3+-doped LN is that the doped crystal has a lower Cth value, only 0.5 mol% in an NS LN. Moreover, in recent years, the epitaxially grown GaN on LN has been successfully used for monolithic integration of electronic and electro-optic devices. 28, 29 As Ga3+ is different from the above-mentioned diffusers in atomic weight, ionic size or chemical valence, it must display different diffusion-growth characteristics. It is therefore essential to perform an independent investigation on the thermodynamic properties of Ga3+ diffusion-growth in LN. Present paper concentrates on the study. The results are compared with the case of Ti4+ diffusion, which induces increase of refractive index and hence formation of an optical waveguide. In addition, the influence of Ga3+ doping on the LN refractive index is also investigated.

II. Experiment Four congruent lithium niobate crystal plates (Z-cut) with a Li2O content of 48.5 ± 0.1 mol% and the optically polished surfaces were employed here. At first, Ga2O3 (99.995%) film was coated onto part surface of each plate (the other surface part remained uncoated). All of the plates were coated with the same thickness of 155 ± 2 nm. After the Ga2O3 film deposition, Ga3+-doped LN thin film was then grown in air 3



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at different temperatures of 1000, 1030, 1060 and 1100 oC, all of which are lower than the Curie point of the LN (~1142 oC at the congruent point) so as to maintain the conformability of ferroelectric domain (annealing at a temperature above the Curie point results in orientation disorder of originally conformable, uniform ferroelectric domain in crystal). To get a profile of error function of complement (erfc), from which the Ga3+ solubility data can be obtained, two effective measures have been taken. First, thick Ga2O3 film was coated. Second, the diffusion-growth duration was chosen as short as possible. By further taking into account the cost issue of expensive secondary ion mass spectrometry (SIMS) analysis, a growth duration range from 10 minutes to 2 h was chosen here. Table 1 summarizes the diffusion-growth condition taken for each sample plate. After diffusion-growth, the refractive indices at Ga3+-grown and Ga3+-free surface parts of each sample were measured by prism coupling technique (Metricon 2010 prism coupler). Figure 1(a) shows the schematic of the measurement principle. The system is equipped with two laser sources with the output wavelengths of 1.311 and 1.553 µm. One surface of sample plate to be measured is tightly contacted with the base of the rutile prism by the aid of a coupling head, through which a pressure is applied to the sample to realize the tight contact. A transverse electric (TE)- or transverse magnetic (TM)-polarized laser beam is incident onto the sample surface through the coupling prism. The light reflected from the prism/sample interface is detected by a photodetector. The intensity of the reflected light is measured versus the angle of incidence θp inside prism. As an example, Figure 1(b) shows the measured pattern of TE-polarized 1.553 µm light reflected from a Z-cut congruent LN. As the θp exceeds the critical angle θc, the beam is totally reflected at the interface between the prism and sample [see the red beam in Figure 1(a)]. In this case, a constant light intensity appears on the pattern as shown in Figure 1(b). As θp is smaller than θc, the 4


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beam refracts into the bulk of substrate material and the detector intensity drops abruptly. Accordingly, a knee appears on the pattern and corresponds to the critical angle θc. The prism coupler equips with a program to search the knee location and hence determine the critical angle θc. From the measured θc, the refractive index of the sample can be evaluated on the basis of the law of total reflection. The Ga3+ ions in the grown thin film was profiled by SIMS (ION-TOF TOF SIMS V). A detailed experimental description for the SIMS analysis has been given in Refs. 19 and 23.

III. Results and Discussion For the conventional Ti4+-diffused LN waveguide, it is usually fabricated by in-diffusion of a 6-8-µm-wide, 90-110-nm-thick Ti-strip at 1030-1060 oC for 9 h. The Ti4+ concentration at the waveguide surface is around 1.2 × 1021 ions/cm3. The Ti4+-induced ordinary/extraordinary index increment at the waveguide surface is ~ 0.005/0.012 at the 1.5 µm wavelength.30 It is unclear if Ga3+-doping affects the LN refractive index, and if so, if the effect is comparable to the Ti4+-induced increment. This effect can be known by measuring and comparing the refractive indices at Ga3+-grown and Ga3+-free parts of sample surface. Table 1 brings together the measured ordinary (no) and extraordinary (ne) refractive index values (at the 1.553 µm wavelength) at the Ga3+-grown (data labeled by a) and Ga3+-free (data labeled by b) surface parts of each sample. For convenience, the difference of the no or ne values at the Ga3+-grown and Ga3+-free surface parts, named ∆no or ∆ne, which represents Ga3+-doping effect on the LN index, are also given in Table 1. One can see that the Ga3+-doping effect on the surface index is on the order 10-4 for both cases of no and ne, and is neglectable within the error 1 × 10-3. Identical result is obtained from the index values measured at the 1.311 µm wavelength. It is evident that the Ti4+-induced index increment is much larger than the 5



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contribution of Ga3+ dopant. Therefore, the Ga3+ doping contribution to the refractive index increment in the Ti-diffused LN waveguide is small relative to the Ti4+ contribution. This is similar to the case of transition metal Zr4+ and rare-earth Er3+, Tm3+, Sc3+ doping, but is different from the case of H+, Zn2+, Ti4+, Li+, Mg2+, In3+ doping, among which the former three ions induce the index increase noticeably while the latter three induce the index decrease measurably. It is also crucial to know the Li2O contents at the Ga3+-grown and Ga3+-free surface parts of the samples studied. So that one can judge the extent of Li2O out-diffusion, which usually takes place as an ion diffuses into LN. Based on the Li-composition-dependent Sellmeier expression,

31

the Li2O-contents at the

Ga3+-grown and Ga3+-free parts of sample surface can be quantified from the refractive indices measured there. The results are shown in Table 1. We note that the Li2O-contents at the Ga3+-grown and Ga3+-free parts of sample surface are similar, and are the same as that of the congruent plate. This implies that Li out-diffusion is not serious for all the studied four sample plates. 3.1 Ga3+ diffusion-growth coefficient Next, we emphasizes on the important thermodynamic parameter: Ga3+ diffusion-growth coefficient in LN. Suppose that the Ga3+ planar thermodynamic diffusion-growth process is described by a one-dimensional Fick-type model with a Li2O-content-dependent Ga3+ diffusion-growth coefficient D. 19, 23 As Li out-diffusion is not measurable for all the studied samples, D is a constant here. The diffusion equation has two possible forms of solution: erfc and Gaussian function. When the diffusion reservoir is not depleted, it has an erfc solution with a constant surface concentration, which, as mentioned above, is regarded as the solid solubility. More detailed description for the two solutions has been given in Refs. 19 and 23. Figure 2 shows the profiles of the substrate constituents and diffused Ga3+ ions in 6


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the studied four samples that were grown at the temperatures of (a) 1000, (b) 1030, (c) 1060 and (d) 1100 oC. The red-ball curve represents the profile of the diffused Ga3+ ions and the other plots show the signals of the substrate ions. The profiles shown in Figure 2 reflect the practical one-dimensional geometry because each erosion crater has a flat bottom area (> 40 × 55 µm2) definitely greater than the maximum detecting area 22 × 22 µm2. More detailed justification for this argument is given in Refs. 19. It is evident from Figure 2 that majority of Ga3+ ions residue within a ~300 nm thick layer, i. e. the diffusion reservoir near crystal surface. Under the growth condition adopted, the reservoir, which is detailed below, was not depleted for all studied samples. On the other hand, some Ga3+ ions grow into the bulk with a penetration depth > 10 µm and a Ga3+-doped thin film was formed on sample surface. The Ga3+ ions grown in all of the studied four samples follow an erfc profile, given by

I ( z ) = I ( z 0 )erfc[( z − z 0 ) / d z ] + I 0N ,

(1)

where I(z), I0N and z0 denotes the Ga3+ yield, background noise and diffusion reservoir thickness, respectively, and dz is the Ga3+ growth depth parameter. The green curves in Figure 2 represent the fitting results. Table 1 brings together the fitting parameters for all of the studied samples. The dz value is 9.2 ± 0.2, 10.3 ± 0.2, 10.4 ± 0.2 and 8.2 ± 0.2 µm for the 1000, 1030, 1060 and 1100 oC samples, respectively. In words, the grown Ga3+ ions in the studied samples follow an erfc profile. Since the thermodynamic diffusion-growth equation has a solution of the erfc, the diffusion-growth model is verified by the experiment. It is worthy of mentioning that significant feature of substrate signals is observed near the sample surface (see Figure 2). This is related to the diffusion reservoir formation. In point of view of diffusion-growth mechanism, 20, 32 in the initial stage of growth the Li, Nb and O ions in the substrate enter into the Ga2O3 layer. As the Ga3+

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diffusion reservoir, a temporary GaxNby-oxide thin layer is formed on crystal surface. 20, 32

Consequently, substantial changes of the substrate signals are observed near the

crystal surface. As the diffusion-growth continues, the Ga3+ ions continually enter into the bulk of sample. The charge centrality demands that some substrate ions inside the reservoir return to the bulk. As a result, the thickness of the temporarily formed reservoir layer decreases as the diffusion-growth is prolonged. During the growth, the Ga3+ concentration in the reservoir holds the constant solubility all the time. At the time moment of reservoir depletion, the temporary layer vanishes because all the substrate ions return to the bulk of crystal. In this case, the signal intensity of substrate ions no longer changes with the depth and tends to the constants. As the background noise is zero for all the studied samples, the depth profile of Ga3+ concentration can be considered as C ( z ) = C ( z 0 )erfc[( z − z 0 ) / d z ] ,

(2)

where C(z0) represents the surface Ga3+ concentration and denotes the solid solubility here. It is noted that dz does not mean the 1/e diffusion-growth depth. To quantify the Ga3+ diffusion-growth coefficient, it is essential to consider the 1/e diffusion depth [≈ 0.635dz, obtained by a fit to the plot of 1/e depth against dz]. The Ga3+ diffusion-growth coefficient is evaluated as 4.3 ± 0.2, 8.0 ± 0.3, 16.4 ± 0.6 and 40.7 ± 2.0 µm2/h at the temperature 1000, 1030, 1060 and 1100 oC, respectively. In Figure 3 the Ga3+ diffusion-growth coefficient D (red balls) is plotted against the reciprocal absolute temperature, in order to derive the diffusion constant and chemical activation energy. The red solid-line represents the linear fit on the semilogarithmic scale. The fitting expression for the logarithmic D(T) as a function of 1/T is indicated in Figure 3. Based on this fitting expression and the Arrhenius law [D = D0exp(-Ea/kBT)], one obtain the diffusion constant D0 = (1.29 ± 0.15) × 1014 µm2/h

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and the chemical activation energy Ea = 3.41 ± 0.08 eV. It is crucial and useful to compare the diffusion-growth coefficient of Ga3+ with that of Ti4+. For convenience of comparison, the Ti4+ diffusion-growth coefficient, which was reported in Refs. 24 and 25, is also plotted in Figure 3 (see the green and magenta plots). One can see that the Ga3+ diffusion-growth is one order of magnitude faster than the Ti4+ diffusion-growth. One may ask, why does Ga3+ in the LN crystal grow considerably faster than Ti4+? As we know, the diffusion-growth of an ion relies also on the physical and chemical properties of the ion itself, such as the ionic diameter and atomic mass, besides the host material properties and diffusion temperature. In principle, the smaller atomic mass and ionic size mean the higher diffusion rate. Regarding Ga3+ and Ti4+, the two diffusers possess different atomic weights, which are 70 g/mol for Ga and 48 g/mol for Ti, and different ionic diameters, which are ~1.2 Ǻ for the Ga3+ and ~1.4 Ǻ for the Ti4+. The origin is not so direct and clear why Ga3+ grows faster as it has a smaller size but an atomic weight 40% larger. It was shown in the case of molecule diffusion in gas that the molecule diffusion coefficient is in reciprocal proportion to the square-root of the molecular weight but to the square of the molecular size. In other words, the diffusion coefficient of a molecule in gas relies more on the dimension of the molecule. This should be also the case for the diffusion-growth of an ion in a solid: different ionic sizes are mainly responsible for the difference in diffusion rate. It is thus comprehensible that Ga3+ grows faster than Ti4+. Due to similar reason, the diffusion-growth coefficient of Ga3+ (~40 µm2/h @ 1100 oC) is also different from those of other diffusers such as H+ [on order of µm2/h@300 oC (a proton-exchanged LN waveguide is usually fabricated at 100-300 oC)], 13 Li+ (on order 103 µm2/h@1100 oC), 14-15 Mg2+ (on order a few 16 or 30 17

µm2/h@1100 oC), Zn2+ [0.6 µm2/h@700 oC (a Zn-vapor in-diffused LN waveguide

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is usually fabricated at 600-700 oC)], 18 Sc3+ (0.6 µm2/h@1100 oC), 19 Er3+ (Er3+, Pr3+ and Tm3+ are all on the order 10-2 µm2/h@1100 oC),

20, 21

Pr3+,

22

Tm3+,

23

and Zr4+

(0.3 µm2/h@1050 oC). 27 3.2 Ga3+ solubility In this subsection, attention is paid to another important thermodynamic parameter: Ga3+ solubility, which, as pointed out above, is thought as the surface Ga3+ concentration in the case of erfc profile. It can be calculated on the basis of Eq. (2) and the law of mass conservation.

23

From each Ga3+ profile in Figure 2, one can

determine the corresponding input parameter τ, which denotes the Ga2O3 film thickness in relation to the erfc part of the Ga3+ profile. 23 It has a value of 55.2, 70.4, 76.0 and 65.3 nm for the 1000, 1030, 1060 and 1100 oC samples, respectively. Table 1 brings together the evaluated values of Ga3+ solubility. For convenience, these solubility data have been also normalized according to the Nb concentration, ~ 1.85 × 1026 ions/m3. The results are also given in Table 1. One can see that the solubility increases by one and half times as the temperature goes from 1000 to 1100 oC. The largest molar solubility is ~1.5 mol%. In Figure 4 the solubility is logarithmic-plotted as a function of 1/T. From the logarithmic plot, the solubility constant S and enthalpy of solution ∆Hs can be derived by correlating with the Arrhenius law [C(T) = Sexp(-∆Hs/kBT)]. The red solid-line plot in Figure 4 denotes the linear fitting result on the semilogarithmic scale. The trial expression for the best fit is log{10-20C(T) [ions/cm3]} = (1.917 ± 0.04) - (0.20 ± 0.01)104/T [K]. By addressing this fitting expression and the Arrhenius law, one can find the solubility constant S = (82.7 ± 7.7) × 1020 ions/cm3 and the enthalpy of solution ∆Hs = 0.41 ± 0.02 eV.

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IV. Conclusion We have investigated thermodynamic growth characteristics of Ga3+-doped LN thin film by thermal diffusion method. It is shown that the growth of Ga3+ in LN obeys to a Fick-type thermodynamic diffusion model with a constant Ga3+ diffusion-growth coefficient. From measured Ga3+ profiles, the temperature dependences of diffusion-growth coefficient and solubility were quantified. With known these dependences one can design and grow a Ga3+-doped LN thin film for various photonic applications. The Ga3+ diffusion-growth coefficient/solubility increases by one order/1.5-fold as the temperature goes from 1000 to 1100 oC. Ga3+ grows one order of magnitude faster than Ti4+ because of its smaller radius. The diffusion-growth coefficient of Ga3+ is also different from those of other ions such as H+, Li+, Mg2+, Zn2+, Sc3+, Er3+, Pr3+, Tm3+, Zr4+. In addition, present work shows that Ga3+ dopant has small effect on the LN refractive index. Acknowledgement. This work was supported by the National Natural Science

Foundation of China under Project nos. 50872089, 61077039 and 61377060, by the Research Grants Council of the Hong Kong Special Administrative Region, China, under Project No 11211014, by the Key Program for Research on Fundamental to Application and Leading Technology, Tianjin Science and Technology Commission of China under Project no. 11JCZDJC15500, and by Specialized Research Fund for the Doctoral Program of Higher Education of China under Project no. 20100032110052.

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Zhang, D. L.; Qiu, C. X.; Zhang, W. Z.; Hua, P. R.; Yu D. Y. and Pun, E. Y. B. J. Amer. Ceram. Soc. 2013, 96, 2722-2724.

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Madison, S. M.; Henderson, W.; Namkoong, G.; Lee, K. K.; Patel, K. M.; Doolittle, Alan W. and Ralph, S. E. 2005 Conf. Laser and Electro-Optics (CLEO’05,

Baltimore,

MD,

May

22-27,

2005,

Paper

CThD7) 2005

1-3, 1605-1607. 29

Lee, K. K.; Namkoong, G.; Madison, S. M.; Ralph, S. E.; Doolittle, W. A.; Losurdo, M.; Bruno, G.; Cho, H. K. Mater. Sci. Eng. B-Adv. Funct. Solid-State Mater., 2007, 140, 203-211.

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Strake, E.; Bava, G. P. and Montrosset, I. J. Lightwave Technol. 1988, 6, 1126-1135.

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Schlarb, U.; Betzler, K. Phys. Rev. B 1993, 48, 15613-15620.

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Zolotoyabko, E.; Avrahami, Y.; Sauer, W.; Metzger, T. H. and Peisl, J. Appl. Phys. Lett. 1998, 73, 1352-1354.

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Table 1 Initial Ga2O3 film thickness, diffusion-growth condition, measured refractive index and evaluated Li2O content for Ga3+ diffusion-growth in initially congruent Z-cut LN plates in air at different temperatures of 1000, 1030,1060 and 1100 oC. Also given include the Ga3+ profile type, erfc fitting parameters [z0, I(z0), dz], diffusion-growth coefficient, diffusion constant, chemical activation energy, solubility, solubility constant and enthalpy of solution. Sample No. Ga2O3 thickness (nm) Diffusion condition no at 1.553 µm ∆no at 1.553 µm

1 155 ± 2 1000 oC 2h 2.2103a

2 155 ± 2 1030 oC 80 min 2.2102

3 155 ± 2 1060 oC 40 min 2.2101

4 155 ± 2 1100 oC 10 min 2.2103

2.2106b

2.2107

2.2104

2.2106

-4

-4

-4

-3×10-4

-3×10

ne at 1.553 µm

-5×10

-3×10

2.1357a

2.1355

2.1353

2.1356

b

2.1370

2.1362

2.1365

-4

-4

-9×10-4

2.1367

∆ne at 1.553 µm

0

Surface Li2O content

48.6a

48.6

48.6

48.6

b

48.6

48.6

48.6

48.6

Type of Ga profile

Erfc

Erfc

Erfc

Erfc

z0 (µm)

0.25

0.25

0.25

0.25

I(z0) (cps)

55

60

65

61

0.0

0.0

0.0

0.0

9.2 ± 0.2

10.3 ± 0.2

10.4 ± 0.2

8.2 ± 0.2

4.3 ± 0.2

8.0 ± 0.3

16.4 ± 0.6

40.7 ± 2.0

(± 0.1 mol%) 3+

I0N

(cps)

dz (µm)

Diffusion-growth coefficient D (µm2/h)

-5×10

-9×10

Diffusion constant D0 (µm2/h)

(1.29 ± 0.15) × 1014

Activation energy Ea (eV)

3.41 ± 0.08

τ (nm)

55.2

70.4

76.0

65.3

Ga solubility (×10 cm )

2.01±0.07

2.27±0.07

2.45± 0.08

2.67±0.10

Ga3+ solubility (mol%)

1.09±0.04

1.23±0.04

1.32±0.04

1.44±0.05

3+

20

-3

Solubility constant (×1020 cm-3)

82.7 ± 7.7

Enthalpy of solution (eV)

0.41 ± 0.02

a: data for Ga3+-grown surface part; b: data for Ga3+-free surface part.

15



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Captions for figures

Figure 1 (a) Schematic of Metricon 2010 prism coupler for measurement of refractive index at surface of LN crystal. (b) Pattern of relative intensity of TE-polarized 1.553 µm light (reflected from prism) measured as a function of incident angle inside prism for a Z-cut congruent LN.

Figure 2 Depth profiles, on a semilogarithmic scale, of 6Li, 93Nb, 16O and 70Ga SIMS signals detected from four LN sample plates grown under different temperatures of (a) 1000, (b) 1030, (c) 1060 and (d) 1100 oC. The green plots represent the erfc fits.

Figure 3 Temperature dependence of Ga3+ diffusion-growth coefficient in LN (red balls). The red line represents the linear fit to the red symbols on the semi-logarithmic scale. The fitting expression is indicated. Green and magenta lines represent the Ti4+ diffusion-growth coefficient reported in Refs. 24 and 25, respectively.

Figure 4 Solubility of Ga3+ in LN (red balls) versus 1/T on a semilogarithmic scale. The red line represents the linear fit to the red symbols on the semi-logarithmic scale. The fitting expression is indicated.

16


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For TOC use only Thermodynamic study on diffusion-growth of Ga3+-doped LiNbO3 single crystal thin film for photonic application De-Long Zhang, Qun Zhang, Jian Kang, Wing-Han Wong, Dao-Yin Yu and Edwin Yue-Bun Pun

A thermodynamic study was performed on diffusion-growth of Ga3+-doped LiNbO3 thin film. Some Ga3+-doped LiNbO3 thin films were grown on LiNbO3 surface by thermal diffusion-growth of Ga2O3 film in the temperature range of 1000-1100 oC. Temperature dependences of diffusivity and solubility were quantified, which are crucial to design and growth of a Ga3+-doped LiNbO3 thin film for various photonic applications.

17



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Figure 1 (a) Schematic of Metricon 2010 prism coupler for measurement of refractive index at surface of LN crystal. (b) Pattern of relative intensity of TE-polarized 1.553 µm light (reflected from prism) measured as a function of incident angle inside prism for a Z-cut congruent LN. 99x119mm (300 x 300 DPI)


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1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Figure 2 Depth profiles, on a semilogarithmic scale, of 6Li, 93Nb, 16O and 70Ga SIMS signals detected from four LN sample plates grown under different temperatures of (a) 1000, (b) 1030, (c) 1060 and (d) 1100 oC. The green plots represent the erfc fits. 60x41mm (300 x 300 DPI)



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Figure 3 Temperature dependence of Ga3+ diffusion-growth coefficient in LN (red balls). The red line represents the linear fit to the red symbols on the semi-logarithmic scale. The fitting expression is indicated. Green and magenta lines represent the Ti4+ diffusion-growth coefficient reported in Refs. 24 and 25, respectively. 58x40mm (300 x 300 DPI)


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Figure 4 Solubility of Ga3+ in LN (red balls) versus 1/T on a semilogarithmic scale. The red line represents the linear fit to the red symbols on the semi-logarithmic scale. The fitting expression is indicated. 58x40mm (300 x 300 DPI)


Thermodynamic Study on Diffusion Growth of Ga3+-Doped LiNbO3

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