Cz-growth of ferroelectric LaBGeO<inf>5</inf> single crystals

Abstract

FerroelecricLaBGeO 5 (LBGO) has a trigonal symmetry belonging to the space group C 3 2 below ~530°C and it consists of {BO 4 } 5tetrahedra [1]. Then, according to the anionic group theory proposed by Chen [2], LBGO is expected to have an optical absorption edge in ultra-violet region. Studies on crystal structure, dielectric properties and some optical properties of LBGO appeared elsewhere, but a detail of its growth of bulk single crystal has not been reported. A few report on the Cz-pulling growth denoted two major problems [3]; a relatively large supercooling of the melt and a violent evaporation of B 2 O 3 during the pulling, which make difficult the pulling-growth from the melt.A large supercooling sometimes results in uncontrollable crystallization in Czockralski (Cz)-pulling. In this paper, we will focus on a single crystal growth of LBGO by Cz-pulling from a melt.As for an evaporation of B 2 O 3 , we set up a double-crucible configuration for the hot-zone of Cz-pulling furnace so as to suppress the evaporation or make it lower, where the inner-crucible contains a melt and the outer-crucible acts as a heating element coupled with rf-induction. The usefulness of this double-crucible configuration for suppressing an evaporation from a melt contained volatile constituents was already confirmed in the case of Bi 12 TiO 20 [4].The crystal growth was carried out along the [0001]-axis with pulling rates ranged in 0.5~2.5mm/h and crystal rotation rates of 15~25 rpm in N 2 -, air-and O 2 -flowing. A N 2 -and air-flowing resulted in crystals with misoriented subgrains, while an O 2 -flowing was successful for growing colorless, transparent and defects-free single crystals. Figure 1 shows the as-grown single crystals. The as-grown single crystals exhibited a three-fold cross-section showing the trigonal crystal symmetry.An ω-scanned X-ray rocking curve revealed that an FWHM corresponding to the (0003) reflection was as narrow as 14.4 arcsec., indicating a high crystallinity. Up to now, the single crystals of about 10mm in diameter were obtained. Figure 2 shows an optical transmittance of (0001) plate prepared from the O 2 -grown boule. The measured optical absorption edge lies at 193.5nm. This shorter absorption edge is unique among many ferroelectric oxide crystals. The spectrum proves to be transparent from ~225nm up to 3.3µm, as shown in the inserted figure in Fig.2. This wide transparency is comparable to that of β-BaB 2 O 4 .Since LBGO has also an optical nonlinear property and is highly resistive to water or humidity, practical SHG devices can be realized when the so-called periodically poled (PP) structure is formed.The PP-LBGO crystal is expected to replace usually used β-BaB 2 O 4 . Fig. 1 As-grown LBGO single crystals (1mm/div.) Fig. 2 An optical transmittance close to an absorptyion edge. The inserted graph shows a transmittance spectrum. (0.65mm thick (0001) plate).