Growth of beta barium borate (β-BaB₂O₄) thin films for nonlinear optical applications

We have prepared crystalline beta barium borate, β-BaB₂O₄ (the so-called BBO), thin films on several kinds of substrates using pulsed laser deposition (PLD) and opposed-targets magnetron sputtering (OTMS) techniques. From the measurement of X-ray diffraction, we have found that the crystallinity of the BBO film grown by both techniques is highly dependent on the deposition temperature and the choice of the substrates. There is a different phase transition temperature for the BBO films grown on different substrates and by different deposition techniques. The best films obtained were the ones grown on a sapphire (0 0 0 1) substrate at a deposition temperature of 800°C by PLD and 700°C by OTMS, which show a single X-ray diffraction (0 0 6) peak normal to the surface of the film. The second harmonic generation (SHG) signal of these films reveals a 6-fold symmetry with a large magnitude contrast between its maximum (1.2 pm/V) and minimum (0.2 pm/V).