Patricia A. Morris

Reduction of the ionic conductivity of flux grown KTiOPO4 crystals

Abstract A mechanism and method for reducing the ionic conductivity of KTP crystals grown by the flux technique utilizing the doping of trivalent ions (Ga and A1) on the Ti site and tetravalent ions (Si) on the P site are described. Results are presented showing that, by using appropriate concentrations of these dopants added to the growth solutions, crystals can be grown over a range of growth temperatures which have ionic conductivities that are 1.5 to 3 orders of magnitude lower than undoped KTP crystals grown over similar temperatures. The dopant ion concentrations obtained in the crystals are consistent with the concentrations calculated, using the defect mechanism described, to achieve the observed ionic conductivity reductions. Distribution coefficients have been calculated for the dopants over a wide range of growth temperatures. The distribution coefficient for Ga in KTP appears to be relatively insensitive to concentration or temperature over a range of doping levels and growth temperatures. The use of doping is shown to also be effective in reducing the ionic conductivity of KTA crystals grown by the flux technique.

Ion exchange of Rb, Ba, and Sr in KTiOPO4

The potassium ions of potassium titanyl phosphate (KTiOPO4, or KTP) are relatively mobile and may be exchanged with other ions from molten salts. Distribution coefficients for potassium, rubidium, barium, and strontium between nitrate melts and titanyl phosphate were measured between 350 and 425 °C. At 425 °C, using melts containing K and a dopant chosen from Rb, Ba, Sr, and with dopant cation mole fractions in the melt of 0.1, Rb is incorporated in the crystal at a site fraction of 0.014, Ba at 0.007, and Sr at 0.002. Introducing 0.008 fraction of Ba increases the low‐frequency dielectric constant and loss of KTP several orders of magnitude, giving conductivities (ωe0e‘) of 4×10−5 Ω−1 cm−1 at 200 kHz. The results support a potassium vacancy mechanism for ionic conductivity in KTP. Using low concentrations of divalent ions in the melt, flux‐grown KTP has a lower uptake of divalent ions than does hydrothermally grown KTP, an effect attributed to the presence of potassium vacancies created in the high‐tempe...

Impurities in nonlinear optical oxide crystals

Impurities in nonlinear optical oxide crystals can affect many of the properties for device applications. The structures of typical crystals are tolerant with respect to occupancy and are nonstoichiometric on the cation sublattices (e.g. the A sublattice in crystals with the general formula AMO3). This may, at least in part, be due to the presence of the relatively strong covalent nature of the acentric oxide groups determining the nonlinear optical properties. These circumstances make the incorporation of impurities into the lattice relatively easy and result in large distribution coefficients for many impurities. Generally, little purification during growth will occur with respect to these impurities and therefore, it is usually necessary to purify the starting materials of any unwanted ions. Chemical or powder processing and firing procedures can be used to prevent any contamination of the crystal growth precursors by common impurities (e.g. Si, Al, Fe, Ca, Na, K, Mg, Cl, and S) at a level of <10 parts per million total concentration. A combination of analytical techniques, including those which require little or no sample preparation (e.g. secondary ion mass spectrometry, neutron activation analysis, or laser microprobe mass spectrometry), should be used to determine the impurities present in a material. For example, the effects of protons incorporated (OH-) in the lattice of these crystals can be very detrimental and can be detected using infrared spectroscopy. The growth of many of these crystals requires flux techniques, but the temperature dependence of any nonstoichiometry present and of the distribution coefficients make the use of slow cooling techniques generally not recommended when uniformity of properties is required.

Infrared study of OH− defects in KTiOPO4 crystals

Variations in the concentrations and distributions of the OH− defects present in flux and hydrothermal KTiOPO4 (KTP) crystals, measured by infrared spectroscopy of single crystals, are attributed to differences in the growth environments and other nonhydrogenic defects present in the crystals. The concentrations of OH− have been estimated from the infrared data to be approximately 400 ppma (parts per million atomic) (3.0×1019 cm−3) in the flux crystals, 1100–1500 ppma (0.74–1.1×1020 cm−3) in the high‐temperature hydrothermal and 600 ppma (4.3×1019 cm−3) in the low‐temperature hydrothermal crystals. A 3566 cm−1 peak and a 3575 cm−1 band are observed in all crystals. The integrated intensity of the OH− absorption band at 3566 cm−1 increases at the expense of the 3575 cm−1 band at higher temperatures in the high‐temperature hydrothermal crystals. Several OH− peaks (3490, 3455, 3428, 3420, and 3333 cm−1), which have strongly temperature‐dependent linewidths, are present in the hydrothermally grown KTP crystal...

Ionic conductivity and damage mechanisms in KTiOPO4 crystals

The ionic conductivity and damage susceptibilities of KTP crystals are related to the defects present in the crystals, which result from the conditions of growth by the flux, high, and low temperature hydrothermal techniques. The effects of Ba impurities on the ionic conductivity and damage are also discussed.

Proton Effects in KTiOPO 4

Evidence supporting the temperature dependent defect mechanism of nonstoichiometry on the potassium and oxygen sublattices in KTP is presented. The primary compensating defects for the formation of vacant potassium sites in typical flux grown KTP are vacant oxygen sites. Protons (OH - ) are the principal defect compensating for the formation of vacant potassium sites in high temperature hydrothermal KTP. A model of the ionic conductivity in high temperature hydrothermal KTP is proposed in which specific protons participate in cooperative motion over a limited distance with the potassium vacancies migrating along the “channels” in the structure in the Z-direction. The higher activation energy measured for ionic conductivity in flux grown KTP (0.5 eV) relative to high temperature hydrothermal (0.3 eV) is suggested to be due to the energy required to dissociate from a defect complex, such as a (VO - VK). The correlation of ionic conductivity to damage susceptibility appears to be due to the levels of compensating defects for vacant potassium sites in KTP, which are related to the concentrations of Ti 3+ formed in the crystals. Further study is ongoing to understand the specific mechanisms involved in the ionic conductivity and damage in KTP grown by the flux and hydrothermal techniques.

Growth, Microstructures and Optical Properties of KNbO 3 Thin Films

Potassium niobate, KNbO 3 , possesses high nonlinear optical coefficients making it a promising material for frequency conversion into the visible wavelength range. While epitaxial thin films of KNbO 3 have been reported [1,2], only limited data exists concerning the optical loss mechanisms and nonlinear optical properties of these films. In this study, epitaxial thin films of KNbO 3 have been grown using ion beam sputter deposition and evaluated in terms of their microstructures and optical properties. Characterization of the microstructures of these films includes the in-plane epitaxial relationship to the substrate. The relationships between the growth parameters and microstructures developed to the indices of refraction and the optical losses (absorption and scattering) are discussed.

Gray-track damage in KTiOPO4 crystals

A correlation is observed between the laser and electric-field damage susceptibilities of most KTiOPO4 (KTP) crystals. The observed gray track damage susceptibilities are found to be correlated to the concentrations of defects in the crystals which can stabilize the Ti3+ damage defect sites in KTP. These stabilizing defects are those which can have an effective positive charge, such as oxygen vacancies, protons, and Ba and F impurities. The defects present in KTP crystals depend on the technique (hydrothermal or flux) used for growth. Post-growth processing techniques have been developed which reduce the laser gray track and electric-field damage susceptibilities of both hydrothermal and flux KTP.

Infrared Spectroscopy of Protons in KTiOPO 4

We report infrared (IR) measurements of protons (H + ), present as OH − , in KTiOPO 4 grown by various techniques. The IR spectra are sensitive to the growth technique used. In high temperature hydrothermally grown crystals we observe IR bands which have strongly temperature dependent linewidths. The Arrhenius-type activation energies for these linewidths are small (6–15 meV). The IR bands are polarized and this information can aid in making OH − site assignments.