1.6 Dopants
L1 Damage-resistant dopants
Most foreign ions in LiNbO3 take a lithium site, whatever their charge, because the lithium octahedron is the larger one P-comp. Iron is the famous troublemaker: it switches between Fe2+ and Fe3+, donates electrons under visible light and holds them in the dark, which writes light patterns into the refractive index (Fe doping, Chapter 04).
Magnesium does the opposite. Above about 5 mol% MgO in congruent crystal, optical damage drops by more than a hundredfold P-comp. The magnesium displaces the niobium antisites that trap charge P-comp and raises the photoconductivity a hundredfold, so light-induced fields leak away before they can grow P(abs). Zinc, indium, scandium, hafnium and zirconium act the same way, each above its own threshold P-comp.
mol% 0 1 2 3 4 5 6 7 8 9 CLN +---------+---------+---------+---------+---------+---------+---------+---------+---------+ ^^===^====^ ab c d melt 1.2 +---------+---------+---------+---------+---------+---------+---------+---------+---------+ ^ a NSLN +---------+---------+---------+---------+---------+---------+---------+---------+---------+ ^====^^=^=========^ a bc de
mol% 0 1 2 3 4 5 6 7 8 9 CLN +----+----+----+----+----+----+----+----+----+ ^=^==^ abc d m1.2 +----+----+----+----+----+----+----+----+----+ ^ a NSLN +----+----+----+----+----+----+----+----+----+ ^=^^^====^ a bcde
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| below < 4.5 % MgO | above > 4.5 % MgO |
| 3484 cm−1 | 3534 cm−1 |
| 2.87 µm | 2.83 µm |
- Setting
- MgO 2.5 mol% host CLN
- Against printed thresholds
- below all sourced (4.5-5.5)
- OH band P-comp
- 3484 cm⁻¹, below 4.5 % MgO
- Li sites P-comp
- [Li0.95Nb0.01V0.04]NbO3
- Shown
- 0 % state shown; no data between 0 and 3 %
Basis: MgO/(Li2O + MgO + Nb2O5) or MgO/(LiNbO3 + MgO); the second is about 2× the first (approximate). No exact conversion is sourced, so values sit on the axis as printed. P-compZn in crystal vs melt: ccrys = 1.649 cmelt − 0.082 cmelt2; distribution coefficient about 1 at 5–6 mol%. P-comp2 mol% In2O3 carries 4 at.% In (two cations per oxide); the value is a melt concentration. P-comp2 mol% Sc2O3 carries 4 at.% Sc (two cations per oxide); the value is a melt concentration. P-compMelt and crystal concentrations differ through the distribution coefficient: 1.2–1.3 at low concentration. P-compMelt and crystal concentrations differ through the distribution coefficient: 1.2–1.3 at low concentration. P-comp
Pick a dopant and a host, then move the content slider; markers are printed values on the basis each source printed. Cross 4.5 % MgO in congruent crystal and the OH band steps from 3484 to 3534 cm−1. In the one proposed site scheme the Nb antisites are already gone at 3 % MgO, below the damage threshold. Switch to near-stoichiometric and the markers fall to 0.2–2 %, with no measurable damage above 0.78 mol%.
L2 Thresholds and sites
Damage-resistant dopants
| Dopant | Threshold | Status |
|---|---|---|
| MgO, first report | damage reduced at 4.6 % Mg | P-comp |
| MgO | ≥ 4.5 at.% MgO; photoconductivity up 100×, Glass current unchanged; activation energy 0.1 eV vs 0.5 eV | P(abs) |
| MgO | about 5 mol% MgO in the congruent melt; 5.5 % “close to the NbLi content” | P-comp |
| MgO, melt [Li]/[Nb] = 1.2 | lowered to 3 % | P-comp |
| MgO, near-stoichiometric ([Li]/[Nb] = 0.993) | > 0.78 mol%: no measurable photorefractive damage at 532 nm up to 2 MW/cm2 | P(abs) |
| MgO, near-stoichiometric (other reports) | 1–2 %; about 1 %; 0.67 mol% (TSSG); 0.2 mol% | P-comp |
| ZnO | about 7 mol% (6 mol% in one report); 7.5 % “the largest threshold” | P-comp |
| In2O3, Sc2O3 | below 2 mol% oxide (4 at.%) in the melt | P-comp |
| HfO2, ZrO2 | below 2 mol% | P-comp |
Site occupancy with Mg
Li-site vacancy compensation, one proposed scheme P-comp. The 0 % formula has [Li]/[Nb] = 0.941 derived.
| MgO (%) | Formula |
|---|---|
| 0 | [Li0.95 Nb0.01 V0.04] Nb O3 |
| 3.0 | [Li0.94 Mg0.03 V0.03] Nb O3 |
| 8.0 | [Li0.84 Mg0.08 V0.08] Nb O3 |
Threshold signatures
| Signature | Below | Above | Status |
|---|---|---|---|
| OH band, Mg ([Li]/[Nb] = 0.945) | 3484 cm−1 (< 4.5 % MgO) | 3534 cm−1 (> 4.5 % MgO) | P-comp |
| OH band, Zn | 3484 cm−1 (< 7 % Zn) | 3506, 3535 cm−1 | P-comp |
| OH line, Mg | 2.87 µm | 2.83 µm | P(abs) |
| Fe3+ EPR with Mg or Zn | Li-site axial spectrum | new isotropic line above 5.5 % MgO or 7 % ZnO; Fe3+ on Nb sites deduced | P-comp |
Other effects of damage-resistant dopants
Tc and density increase, the UV edge shifts to shorter wavelength, and the noncritical phase-matching temperature rises; no new absorption appears within 0.35–4 µm P-comp. Distribution coefficient: 1.2–1.3 at low concentration, about 1 for MgO and ZnO at 5–6 mol% P-comp. Zn in the crystal vs the melt: ccrys = 1.649 cmelt − 0.082 cmelt2 P-comp.
Photorefractive and spectroscopic dopants
| Ion | Site and states | Spectroscopy | Status |
|---|---|---|---|
| Fe | Fe2+ and Fe3+ on Li sites (congruent); [Fe2+]/[Fe3+] = 0.1–0.2 as grown, set by oxidation-reduction anneals | Fe2+ to Nb5+ intervalence band at 2.6 eV (oscillator strength 10−2); Fe2+ d-d at 1.1 eV (4 × 10−4); O to Fe3+ charge transfer at 3.1 eV | P-comp |
| Fe, near-stoichiometric | partial Fe3+ on Nb sites proposed from EPR; disputed | P-comp | |
| Fe, Cu | photovoltaic (Glass) constants 2.8 × 10−9 and 5.5 × 10−10 cm/V | saturated photovoltaic fields 104–105 V/cm | P-comp |
| Ti | Ti4+ substitutes for Li (congruent); Ti3+ EPR at Li sites independent of [Li]/[Nb]; with 6 % Mg, Ti3+ mainly on Nb sites | P-comp | |
| Er | Er3+ on Li sites; site-selective spectroscopy resolves at least 11 centres in SLN:Er and 13 in CLN:Er, narrower lines in SLN | lanthanides sit 30–50 pm off-centre in the Li octahedron | P-comp |
| Cr | Cr3+ on Li sites in low-doped congruent crystal; not photovoltaic-active | P-comp |
L3 Mechanism and units
Two mechanisms, both partly right. (1) Mg fills Li vacancies and replaces NbLi until the antisites are gone; the threshold scales with the antisite content and falls with Li enrichment P-comp. (2) Diffraction on Zn- and In-doped crystal shows that near and above threshold the dopant begins to occupy Nb sites; Volk & Wöhlecke 2008 calls this association of the threshold with partial Nb-site incorporation “unambiguous” P-comp. Whether antisites vanish at the threshold or at a lower “first threshold” is open; one energetic analysis finds the two-step scheme unfavourable and simultaneous substitution more likely P-comp.
Units trap. MgO content is written either as MgO/(Li2O + MgO + Nb2O5) or as MgO/(LiNbO3 + MgO); for usual concentrations the second is about twice the first P-comp. Melt and crystal concentrations also differ through the distribution coefficient (L2). No exact conversion between the two bases is given, so the threshold axes place each value on the basis its source printed.
Ti in-diffusion at 950–1020 °C inverts the polarization of a surface layer on the +c face only P(abs) P-comp; domains and their inversion: 1.8. Waveguide fabrication belongs to Chapter 05.
Sources
- Volk & Wöhlecke 2008T. Volk, M. Woehlecke, Lithium Niobate: Defects, Photorefraction and Ferroelectric Switching, Springer Series in Materials Science 115 (Springer, Berlin Heidelberg, 2008). doi:10.1007/978-3-540-70766-0
- Bryan et al. 1984D. A. Bryan, R. Gerson, H. E. Tomaschke, "Increased optical damage resistance in lithium niobate", Appl. Phys. Lett. 44, 847-849 (1984). doi:10.1063/1.94946
- Furukawa et al. 2000Y. Furukawa, K. Kitamura, S. Takekawa, A. Miyamoto, M. Terao, N. Suda, "Photorefraction in LiNbO3 as a function of [Li]/[Nb] and MgO concentrations", Appl. Phys. Lett. 77, 2494-2496 (2000). doi:10.1063/1.1318721
- Zhong et al. 1980G.-G. Zhong, J. Jin, Z.-K. Wu, conference abstract, 11th International Quantum Electronics Conference (IQEC), Washington DC, 1980-06, IEEE Cat. 80CH1561-0, p. 631; also cited as J. Opt. Soc. Am. 70, 631 (1980).
- Miyazawa 1979S. Miyazawa, "Ferroelectric domain inversion in Ti-diffused LiNbO3 optical waveguide", J. Appl. Phys. 50, 4599-4603 (1979). doi:10.1063/1.326568
- Iyi et al. 1995N. Iyi, K. Kitamura, Y. Yajima, S. Kimura, Y. Furukawa, M. Sato, "Defect structure model of MgO-doped LiNbO3", J. Solid State Chem. 118, 148-152 (1995). doi:10.1006/jssc.1995.1323