Vapor Transport Equilibration Treatment Research Articles

Near-stoichiometric Er3+ doped LiNbO3 thin films prepared by vapor transport equilibration treatment

A post-growth Li-rich vapor transport equilibration (VTE) technique was used to prepare near-stoichiometric Er3+ doped LiNbO3 thin films. Er3+ fluorescence property, defect feature and Li2O content of the films were characterized. The results show that the visible and near-infrared fluorescence intensity increases remarkably by ∼10 times the VTE time is increased from 0 to 5 h. The enhancement can be explained from viewpoint of Nb4+ defects concentration. X-ray photoelectron spectrometer study confirms presence of antisite NbLi4+ in the films and NbLi4+ is major defect. Its concentration decreases with increased VTE time. The Li2O content in the films is evaluated to be 48.8–49.7 mol% from the birefringence results. It increases with a rise in VTE time, matching with the characteristic of antisite defects. As defects concentration decreases, the non-radiative transition weakens , which is beneficial to improve fluorescence properties.

Note: Electro-optic coefficients of Li-deficient MgO-doped LiNbO3 crystal

A number of Li-deficient MgO-doped LiNbO3 (LN) crystals with different Li2O contents ranging from 43.4 mol. % to 44.5 mol. % were prepared by carrying out the Li-poor vapor transport equilibration treatment on 5 mol. % (in growth melt) MgO-doped LN crystals. Unclamped electro-optic (EO) coefficients γ13 and γ33 of these crystals were measured by Mach-Zehnder interferometry. The results show that γ13 (γ33) increases linearly by ∼14% (11%) as the Li2O content decreases from 44.5 mol. % of the as-grown state to 43.4 mol. % of the Li-deficient state. This feature is desired for the EO application of the Li-deficient MgO:LN crystal.

Notice of Retraction: Strip Waveguide With High Diffusion-Doped Concentration

We report a Ti:Er:LiNbO3 strip waveguide with high diffusion-doped surface Er3+ concentration. The waveguide was fabricated with a technological process in sequence of preparation of noncongruent, Li-deficient LiNbO3 substrate by performing Li-poor vapor transport equilibration treatment on a congruent Z-cut LiNbO3 plate, diffusion of 40-nm-thick Er metal film, and fabrication of 8-μm-wide Ti-diffused strip waveguide. The waveguide retains the LiNbO3 phase and shows the waveguiding characteristics similar to the conventional Ti:LiNbO3 waveguide. Secondary ion mass spectrometry study shows that the Er3+ diffusion reservoir was exhausted and the profile is Gaussian with a surface concentration two times larger than that of the conventional Ti:Er:LiNbO3 waveguide. The waveguide shows stable 1547-nm small-signal enhancement under the 1480-nm pumping without serious optical damage observed, and a 5-dB signal enhancement is obtained for the available coupled pump power of only 90 mW. A saturated net gain as much as 5 dB/cm is predicted theoretically.

Notice of Retraction: Optical-Damage-Resistant Highly Er$^{\bf {3+}}$ -Doped Ti:Er:LiNbO$_{\bf 3}$ Strip Waveguide

Retracted.

Notice of Retraction: Emission and Absorption Cross Sections of Photorefractive-Damage-Resistant Locally Er-Mg-Doped Near-Stoichiometric ${\rm Ti}:{\rm Mg}:{\rm Er}:{\rm LiNbO}_{3}$ Strip Waveguides

We have measured the polarized visible and near-infrared, and unpolarized mid-infrared (2.7 μm) emission spectra of photorefractive-damage-resistant locally Er-Mg-doped near-stoichiometric (NS) Ti:Mg:Er:LiNbO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> strip waveguide, fabricated on an <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">X</i> -cut initially congruent LiNbO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> substrate in sequence by local Er doping in air, Mg-Ti pre-diffusion in wet O <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> and post Li-rich vapor transport equilibration treatment. From the measured emission spectra, the emission and absorption cross section spectra were calculated based upon McCumber theory. The spectroscopic features are discussed in comparison with the spectra recorded from the area outside the waveguide, and with the previously reported results of bulk-doped NS Er:Mg:LiNbO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> crystals and congruent Er:LiNbO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> bulk material or Ti:Er:LiNbO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> waveguide structure. The results show that the spectra of the NS waveguide are traditional and reveal small differences from those spectra outside the waveguide in spectral shape, polarization dependence, as well as cross section values. In contrast, the cross section values of the NS waveguide show considerable differences from those of bulk-doped NS material and congruent bulk material or waveguide structure. The 552, 673, 996, and 1531 nm emission lifetimes of the Er <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3+</sup> ions outside the waveguide were also measured, and found to be comparable to the results of the bulk-doped NS crystal and the congruent bulk material or waveguide structure.

Crystal growth, VTE treatment, and characterizations of Nd-doped LiTaO3

Congruent 0.2 and 0.5mol% Nd-doped LiTaO3 single crystals were grown by the conventional Czochralski technique. Near-stoichiometric lithium-tantalate wafers were also produced from these crystals by the vapor-transport equilibration (VTE) process. The crystals were characterized by the measurements of composition (by inductively coupled plasma atomic emission spectroscopy (ICP-AES)), UV absorption edge, Curie temperature, and thermal diffusivity. From the ICP-AES study, we found that Li/Ta ratio increased by the VTE treatment, while Nd/Ta ratio was found to decrease with crystal length. Blue-shift in the absorption edge was observed in the VTE treated crystals as compared with the congruent ones. The Curie temperature decreased with increasing Nd concentration, while thermal diffusivity was found to increase after the VTE treatment.

Locally Er-Doped High-Solubility LiNbO3 Crystal Prepared by Li-Poor Vapor Transport Equilibration and Er Codiffusion

We demonstrate the feasibility of preparing locally Er-doped high-solubility LiNbO3 (LN) crystal for the use of integrated optics by simultaneous work of Er in-diffusion and Li-poor vapor transport equilibration (VTE) treatment. Four X-cut and four Z-cut congruent LN substrates locally coated with 30–44 nm thick Er-metal films underwent a Li-poor VTE treatment of 1030/240°, 1050/180°, 1070/180°, and 1050/180°C/h, respectively. After the VTE procedure, the Li2O content in the crystal, determined from the measured birefringence, decreases from original 48.5±0.1 mol% to (48.0–48.2)±0.1 mol% with an approximately homogeneous profile and shows less effect of the cut of plate. Secondary ion mass spectrometry was used to study the Er diffusion properties. The results show that all of the Er diffusion reservoirs were not exhausted. The diffused Er ions in all of the samples studied follow the complementary error function profile. From the measured Er profiles, characteristic diffusion parameters such as diffusion depth, diffusivity, diffusion constant, activation energy, and solubility were determined and discussed in comparison with the case of the traditional diffusion in the argon or O2 atmosphere. It is found that the diffusion in the Li-poor atmosphere shows an anisotropy similar to that in the traditional argon or O2 atmosphere. Under the same temperature, the Er diffusivity in the Li-poor atmosphere is considerably larger than in the O2. From the logarithmic plot of diffusivity versus 1/T and the Arrhenius law, the diffusion constant and activation energy are determined to be (1.2±0.2) × 107/(2.2±0.3) × 104μm2/h and (2.32±0.02)/(1.51±0.02) eV for the diffusion in the X-/Z-cut plate. The solubility in the X-/Z-cut plate is evaluated to be (1.42±0.24)/(1.14±0.15), (2.18±0.27)/(2.01±0.19), (2.62±0.26)/(2.68±0.19), and (2.54±0.20)/(2.30±0.16) × 1020 ions/cm3 for the VTE temperature of 1030°, 1050°, 1070°, and 1100°C, respectively. As expected, it depends on both the VTE temperature and the Li2O content in crystal. Under the same VTE temperature, the solubilities in the X- and Z-cut plates can be regarded as same within the error range. Under the same temperature, the solubility in the Li-poor atmosphere case is considerably higher than that in either the argon or the O2 atmosphere case.

Influence of post-grown Li-rich and Li-poor vapor transport equilibration on composition, OH− absorption and optical-damage threshold of Mg (5 mol%) : LiNbO3 crystals

Li-rich (Li-poor) vapor transport equilibration (VTE) treatments on a number of Z-cut 0.47 mm thick congruent MgO (5 mol% in melt) : LiNbO3 crystals were carried out at 1100°C over different durations ranging in 1–172 (40–395) h. Neutron activation analysis shows that neither Li-rich nor Li-poor VTE-induced Mg and Nb loss from the crystal occurred. The Li2O content in the crystal was measured as a function of VTE duration by the gravimetric method. The Li-rich/Li-poor VTE effects on OH− absorption were studied in comparison with the as-grown crystal. The study shows that the Li-rich VTE results in OH− absorption band annihilation. After further oxidation treatment the band reemerges and peaks at the same wavenumber as that of the as-grown crystal (∼3535.6 cm−1), showing that the MgO concentration in the Li-rich VTE crystal is still above the optical-damage threshold. The Li-poor VTE causes OH− band shift to 3486.3–3491.6 cm−1, indicating that the MgO concentration in all Li-poor VTE crystals is all below the optical-damage threshold. Further successive Li-rich VTE and oxidation treatments on the Li-poor VTE-treated crystal lead the band to shift back to 3535.6 cm−1, showing that the post Li-rich VTE brought the Li-poor VTE-treated crystal above the optical-damage threshold again. It is found that the peaking position, band width, peaking absorption and band area of the absorption at ∼3486 cm−1 all increase monotonously with the decrease of the Li2O content arising from prolonged Li-poor VTE, and quantitative relationships to the Li2O content are established for the latter two parameters. The VTE effects on the OH− absorption are conducted with the VTE-induced OH− content alteration and charge redistribution.

OH − absorption in off-congruent LiNbO 3 crystals prepared by Li-poor vapor transport equilibration

An X-cut and a Z-cut Li-deficient, non-congruent LiNbO 3 crystal were prepared by carrying out Li-poor vapor transport equilibration (VTE) treatment on one mm thick congruent plates at 1100 °C for 170 h. From the measured optical absorption edges (OAE), the Li 2O content is evaluated to be 47.74 ± 0.13 mol% in the X-cut plate and 47.61 ± 0.13 mol% in the Z-cut VTE crystal. The composition homogeneity is verified by measuring the OAE as a function of the crystal thickness. The visible optical absorption and powder X-ray diffraction studies allow to conclude that both VTE crystals retain still the LiNbO 3 phase. OH − absorption characteristics of the two VTE crystals were investigated in comparison with those of the as-grown crystals. The results show that the Li-poor VTE treatment only slightly modifies the area, spectral width, spectral shape and background absorption of the OH − band. Comparison shows that the Li-poor VTE effects on the OH − absorption have large differences from the Li-rich VTE effects in bandwidth and areas of three component peaks at 3466, 3481 and 3488 cm −1. These spectral differences are explained on the basis of the H + site occupation and crystal defect models taking into account the VTE effect on crystal defect concentration. The Li-poor VTE effects are attributed to the VTE inducing the redistribution of OH − ions in different sites.

Growth, etching morphology and spectra of LiAlO2 crystal

Abstractγ‐LiAlO2 single crystal was successfully grown by Czochralski method. The crystal quality was characterized by X‐ray rocking curve and chemical etching. The effects of air‐annealing and vapor transport equilibration (VTE) on the crystal quality, etch pits and absorption spectra of LiAlO2 were also investigated in detail. The results show that the as‐grown crystal has very high quality with the full width at half maximum (FWHM) of 17.7‐22.6 arcsec. Dislocation density in the middle part of the crystal is as low as about 3.0×103 cm–2. The VTE‐treated slice has larger FWHM value, etch pits density and absorption coefficient as compared with those of untreated and air‐annealed slices, which indicates that the crystal quality became inferior after VTE treatment. (© 2008 WILEY‐VCH Verlag GmbH &amp; Co. KGaA, Weinheim)

Locally Er-Doped Near-Stoichiometric LiNbO3 Crystal for Integrated Optics

X- and Z-cut locally Er-doped near-stoichiometric LiNbO3 crystals have been fabricated using two methods: direct vapor transport equilibration (VTE) treatment on congruent crystals coated with erbium (Er) metal, and a two-step process with standard thermal in-diffusion of Er metal followed by VTE treatment. Detailed studies on the crystalline phase, the Li composition, the diffusivity and the spectroscopic property of Er3+ ion, and the diffused surface roughness indicate that there is a limitation on the initial Er metal film thickness, and the near-stoichiometric Er:LiNbO3 crystals prepared by both methods are promising substrates for integrated optics applications.

Off-Congruent LiNbO3 Crystals Prepared by Li-Poor Vapor Transport Equilibration for Integrated Optics

X- and Z-cut off-congruent LiNbO3 (LN) crystals for use for integrated optics were prepared by carrying out Li-poor vapor transport equilibration (VTE) treatment on 1-mm-thick congruent plates at 1100 °C for durations ranging from 65 to 170 h. The Li2O contents and crystalline phase in these Li-poor VTE-treated crystals were characterized by using Raman scattering, optical absorption, and powder X-ray diffraction spectroscopy. The studies of optical absorption in the ultraviolet region show that the Li-poor VTE treatments result in different extents of red-shifts of the optical absorption edges (OAEs), and the amount of the red-shift increases with the prolonged VTE duration. The mean Li2O contents in all VTE samples were evaluated from the measured OAEs. The results show that the VTE-induced reduction of the mean Li2O content can reach as much as ∼1 mol% for both cases of X-cut and Z-cut, and the mean Li2O content and the VTE duration follows the linear relations: cLi = (48.62 ± 0.06) − (0.0052 ± 0.0004)t for the X-cut crystals and cLi = (48.53 ± 0.08) − (0.0063 ± 0.0005)t for the Z-cut crystals. The out-diffusion of Li shows a slight anisotropy. The diffusivity in a Z-cut crystal is slightly larger. The results of visible absorption, Raman scattering, and powder X-ray diffraction show that all the VTE crystals still retain the LiNbO3 phase, consistent with the result predicted from the LiNbO3 phase diagram. The critical VTE duration, longer than which the VTE will induce LiNb3O8 phase in the crystal, is roughly evaluated to be about 577 and 476 h for an X-cut and a Z-cut plate, respectively.

Emission characteristics of Er 3+ in vapor-transport-equilibrated Er/Zn-codoped LiNbO 3 crystals

Polarized visible and infrared emission characteristics of Er 3+ ions in vapor-transport-equilibration (VTE)-treated LiNbO 3 crystals codoped with different concentrations of Zn and Er were investigated in comparison with corresponding as-grown crystals. The results show that the VTE treatment leads to substantial spectral changes of Er 3+ emissions at 0.65, 0.98 and 1.5 μm regions, and the spectral changes in the 0.98 and 1.5 μm regions appear to be Zn-concentration-dependent. It is concluded in combination with X-ray powder diffraction results and optical absorption characteristics reported previously that the VTE treatment resulted in crystalline phase transformation with respect to Er 3+ ions from original LiNbO 3 to ErNbO 4 phase in all crystals studied. The formation of the ErNbO 4 phase and the Zn 2+ codopants are responsible for the VTE-induced substantial spectral changes. The emission characteristics of the ErNbO 4 precipitates in the Zn/Er-codoped crystals are found to be very different from those of the ErNbO 4 precipitates in the only Er-doped crystal in the infrared region, and the difference is attributed to the influence of the Zn 2+ codopant on the Er 3+ ion environment. The mechanism of the crystalline phase transformation is qualitatively explained from the viewpoint of the declined solubility of Er 3+ ion in a Li-rich LiNbO 3 crystal and from the phase diagram of Li 2O–Nb 2O 5 system.

Properties of Ru-doped near-stoichiometric lithium niobate crystals produced by vapor transport equilibration

Ruthenium (Ru)-doped near-stoichiometric lithium niobate crystals (Ru:SLN) were prepared by the vapor transport equilibration (VTE) technique from Ru-doped congruent lithium niobate (CLN) grown using the Czochralski method. Increasing the duration time of the VTE treatment caused the ratio of [Li]/[Nb] in the crystal to increase, which in turn caused the absorption edges to shift to violet. The absorption coefficient in the UV/VIS absorption spectra at 530 nm decreased because of the change in the valence of the Ru ions cause by the VTE treatment. The OH − absorption spectra showed that the broad band located at 3482 cm −1 gradually decreased while another band located at 3466 cm −1 gradually increased and the shape became sharper as the VTE treatment time increased. When the VTE treatment time reached 200 h, Ru-doped near-stoichiometric lithium niobate crystals were obtained. Two-beam coupling examination with a 532 nm laser showed that the Ru-doped near-stoichiometric lithium niobate crystals had a larger exponential gain coefficient, lower diffraction efficiency, faster response time, and higher sensitivity than did the Ru-doped CLN crystals.

Vapor transport equilibration-induced formation of micron-sized ErNbO 4 precipitates in Zn/Er-codoped LiNbO 3 crystals

Congruent LiNbO 3 crystals with Zn/Er-doping levels of 5.5/1, 6/0.15 and 7/0.8 mol%/mol% were thermally treated at 1100 °C over 101 h using vapor transport equilibration (VTE) technique. It is shown that the VTE treatment has brought these crystals closer to stoichiometric composition. X-ray powder diffraction and Er 3+ spectroscopic characterizations show that the VTE treatment induced formation of micron-sized ErNbO 4 precipitates on the surface and in the bulk of all crystals studied. It was observed using an optical microscope that the precipitates grow preferably along X, Y and Z axes of the host matrix, and show different morphologies, densities and dimensions, depending on the Er 3+ and Zn 3+ doping level. The formation origin of the precipitate is qualitatively explained.

X‐Ray, Micro‐Raman, Optical Absorption/Emission Studies of ErNbO4 Grown by Vapor Transport Equilibration

Vapor transport equilibration (VTE) treatments were performed on a Y‐cut bulk Er (1.6 mol%)‐doped congruent LiNbO3 crystal and an X‐cut pure congruent crystal, on one surface of which a 40 nm‐thick film of erbium metal was coated before the VTE treatment. Scanning electron microscope, powder or single‐crystal X‐ray diffraction (XRD), polarized infrared absorption/emission of Er3+ as well as micro‐Raman spectroscopy were used to study the two VTE crystals. The results are discussed in comparison with a corresponding as‐grown bulk Er‐doped crystal, calcined ErNbO4 powder, and a locally Er‐doped congruent LiNbO3 crystal prepared by using the standard Er‐diffusion technique. The experimental results show that the VTE treatment induces the formation of micrometer‐sized ErNbO4 precipitates with the crystallographic morphology of a flat polyhedron not only on the surfaces of both crystals but also in the bulk of the homogeneously Er‐doped one. The optical absorption and emission studies show that the formation of the precipitates results in substantial spectral changes in both the 0.98 and 1.5 μm regions. The micro‐Raman studies allow to resolve four additional Raman peaks around 800 cm−1 in the E(TO) spectra of the two VTE crystals. These additional Raman peaks are associated with the characteristic vibrations with respect to the NbO43− group. Characteristic XRD, optical absorption, and emission and Raman peaks for identifying the ErNbO4 phase are proposed. Finally, the formation mechanism and light‐scattering effect of the precipitates are discussed.

Er 3 + diffusion in congruent LiNbO3 crystal doped with 4.5 mol % MgO

Standard thermal diffusion of Er3+ into X-cut congruent LiNbO3 crystal with the MgO doping level close to the antiphotorefractive threshold concentration, 4.5 mol%, was attempted at the temperature close to Curie point of the crystal. Single-crystal x-ray diffraction, photoluminescence spectroscopy and secondary ion mass spectrometry (SIMS) were used to study the crystalline phase with respect to Er3+ ions and the depth profile of the diffused Er3+ ions. The results show that the thickness of the Er metal film coating should not be thicker than 4.6 nm. In this case, the diffusion is complete, the diffused surface is clean, and the Er3+ ions in the diffused layer still retain the LiNbO3 phase. On the other hand, the diffusion will be incomplete if the initial thickness of the Er metal film is thicker than 4.6 nm. The residual Er3+ ions form ErNbO4 grains on the surface of the crystal. SIMS analysis shows that the diffused Er3+ ions follow a monoexponential decay profile. The experimental results also show that the diffusivity of Er3+ in the X-cut congruent Mg(4.5 mol %):LiNbO3 is similar to 4.69×10−3 μm2/h at 1130 °C. Further vapor transport equilibration treatment following the standard diffusion procedure results in transformation of the diffused Er3+ ions from LiNbO3 into the ErNbO4 phase and, hence, substantial spectral change of the 1.5 μm emission.

Judd-Ofelt analysis of spectroscopic property of Er3+ in congruent and near-stoichiometric Zn∕Er-codoped LiNbO3 crystals

Congruent LiNbO3 crystals doped with 5.5∕1, 6∕0.15, and 7∕0.8mol%∕mol% of Zn∕Er were thermally treated at 1120°C over 101h using vapor transport equilibration (VTE) technique. Unpolarized visible and infrared absorption spectra of the VTE-treated and corresponding as-grown crystals were recorded at room temperature. It is shown that VTE treatment has brought these crystals closer to stoichiometric composition. X-ray powder diffraction and optical absorption results show that the VTE treatment induces the crystalline phase transformation with respect to Er3+ from the original Er3+:LiNbO3 to the ErNbO4 in all crystals studied. The formation of the ErNbO4 phase results in substantial absorption changes that include the drop of the transparency in the visible region, the reduction of Er3+ absorption, and the Zn-doping-level-dependent changes in spectral shape and peaking position at 0.98 and 1.5μm regions. Based upon the measured absorption spectra, the influences of Zn-doping level and VTE treatment on the Er3+ spectroscopic properties were studied by using the Judd-Ofelt (JO) theory. The results show that the VTE treatment has a little effect on the fluorescence branch ratio, while a large effect on the JO parameters, oscillator strength, and electron transition probability. The Zn doping level affects the JO parameters, oscillator strength, and transition probabilities of some individual manifolds. In addition, the VTE treatment leads to the increases of the theoretical radiative lifetimes of almost all manifolds considered. For the emission at 1.5μm, the lifetime increase is as much as 30% and the theoretical results are in well agreement with the experimental data. The VTE-induced lifetime increase is qualitatively explained.

Simulation of Ti diffusion into LiNbO3 in Li-rich atmosphere

A model is proposed for describing two-dimensional diffusion of Ti into an initially congruent LiNbO3 crystal under a Li-enriched atmosphere created by a mixed two-phase (Li3NbO4 and LiNbO3) powder at elevated temperature [vapor transport equilibration (VTE)]. The influence of VTE treatment on Ti diffusivity is considered in the model. To solve the model, four key input parameters including Li-concentration-dependent Li and Ti diffusivities and two switching times t1 and t2 were determined. Prior to solve the Ti-diffusion model, a one-dimensional VTE model is solved at first to obtain the dynamic Li2O concentration depth profile. Both the Li-diffusion and Ti-diffusion models were solved by using finite difference method. Based on secondary-ion-mass spectrometry analysis, the validity of the VTE and Ti-diffusion models as well as the numerical method employed are confirmed. After that, diffusion at 1100°C of an 8-μm-wide, 100-nm-thick Ti strip defined on the surface of a Z-cut or an X-cut substrate was simulated for the VTE duration up to 130h. Based on the numerical results, the Ti-(Li-)diffusion characteristics are discussed in the aspects of (1) the relation of depth and width profile function of Ti concentration to the VTE duration, (2) the substrate cut effect on both the Ti and Li diffusions, (3) the relation of the 1∕e Ti-concentration depth and half-width to the VTE duration, and (4) the VTE duration dependence of the mean [Li]∕[Nb] ratio within the Ti-diffused layer.

Fabrication of β-BaB 2O 4 layers on (0 0 1) Sr 2+-doped α-BaB 2O 4 by vapor transport equilibration (VTE)

Crystalline β-BBO layers have been successfully prepared on (0 0 1)-oriented Sr 2+-doped α-BBO substrates using vapor transport equilibration technique. The layers were characterized by X-ray diffraction, X-ray rocking curve and transmission spectra. The present results manifest that the VTE treatment time and powder ratio are important factors on the preparation of β-BBO layers. β-BBO layers with a highly (0 0 l) preferred orientation were obtained according to XRD profiles. The full width at half-maximum of the rocking curve for the layer is as low as about 1000 in., which shows the high crystallinity of the layer. These results reveal the possibility of fabricating β-BBO (0 0 1) layers on (0 0 1)-oriented Sr 2+-doped α-BBO substrates by VTE.