ISSN 0020-1685, Inorganic Materials, 2007, Vol. 43, No. 9, pp. 980–987. © Pleiades Publishing, Inc., 2007. Original Russian Text © E.A. Volkova, N.I. Leonyuk, A.V. Mokhov, 2007, published in Neorganicheskie Materialy, 2007, Vol. 43, No. 9, pp. 1094–1101. 980 INTRODUCTION There is considerable interest in rare-earth alumi- num borate crystals with the general formula RAl 3 (BO 3 ) 4 (R = yttrium or lanthanide), motivated by their attractive functional (nonlinear-optical, lasing, nonlinear-active, and other) properties in combination with their high thermal, chemical, and mechanical sta- bility and unique thermal conductivity, 14–15 W/(m K) [1]. An important practical aspect is the possibility of extensive isomorphous substitutions on the rare- earth site, which makes it possible to radically mod- ify the properties of these materials. They can be used in engineering high-efficiency miniature lasers and other electronoptical and acoustoelectronic devices and, more importantly, new high-tech non- linear-active devices. In particular, (Nd,Y)Al 3 (BO 3 ) 4 crystals were used to create a cw green laser [2]. There is considerable inter- est in NdAl 3 (BO 3 ) 4 〈Cr〉, which shows an insignificant concentration quenching of luminescence and offers the largest gain coefficient among the existing minia- ture solid-state lasers [3]. A key feature of NdAl 3 (BO 3 ) 4 (NAB) crystals is the extremely high activator (neody- mium) concentration, which makes them potential can- didates for the fabrication of disk-shaped gain elements with an effective pump absorption depth in the range 100–200 µm. YAl 3 (BO 3 ) 4 (YAB) crystals doped with Cr 3+ and Yb 3+ are being studied intensely as potential materials for advanced compact optoelectronic devices [4, 5]. YAB〈Yb〉 nonlinear-active crystals about 3 × 3 × 3 mm in dimensions can be pumped by light-emitting diodes, e.g., InGaAs [5]. Single-crystal YAB〈Yb〉 films are potentially attractive as components of planar waveguides. Therefore, small crystals and thin layers of YAB〈Yb〉 and NAB offer considerable potential for use in next-generation devices for scientific, medical, industrial, and other applications. Since no studies in this area have been conducted since the report by Lutz et al. [6], we made an attempt to bridge this gap. In this paper, we report the growth of YAB〈Yb〉 and NAB epil- ayers from high-temperature borate–molybdate solu- tions and analyze the effect of growth conditions on the composition, structure, and morphology of the epilayers. CRYSTAL-CHEMICAL ANALYSIS The selection of substrates is a critical issue in the growth of epitaxial films and is determined primarily by crystallographic and physicochemical factors. We considered a number of well-known single crystals, taking into account their composition, crystal struc- ture (in particular, interatomic distances), melting point, thermal expansion coefficient, and refractive index, as substrates for subsequent epitaxial growth of rare-earth aluminum borate layers. In the fabrica- tion of planar waveguides, it is of key importance that their refractive index slightly exceed that of the substrate. Detailed analysis of all the above factors leads us to conclude that only undoped YAB crystals are capable of fully meeting all of these require- ments. The principal quantitative characteristic for control- ling the perfection and composition of growing layers is the growth rate. In liquid-phase epitaxy (LPE), it can be determined on a real time scale of a particular exper- iment. It is well known that diffusion-controlled mass transport to the crystal surface leads to diffusion-con- Liquid-Phase Epitaxy of NdAl 3 (BO 3 ) 4 and Yb-Doped YAl 3 (BO 3 ) 4 E. A. Volkova a , N. I. Leonyuk a , and A. V. Mokhov b a Moscow State University, Vorob’evy gory 1, Moscow, 119899 Russia b Institute of Geology of Ore Deposits, Petrography, Mineralogy, and Geochemistry (IGEM), Russian Academy of Sciences, Staromonetnyi per. 35, Moscow, 119017 Russia e-mail: volkova@geol.msu.ru Received October 17, 2006 Abstract—Epitaxial layers of NdAl 3 (BO 3 ) 4 (NAB) and YAl 3 (BO 3 ) 4 〈Yb〉 (YAB〈Yb〉) containing up to 10 at % Yb have been grown by liquid-phase epitaxy on YAB substrates. Their growth kinetics have been studied at rel- ative supersaturations of the high-temperature solution from 2 × 10 –2 to 16 × 10 –2 . The ytterbium concentration in YAB〈Yb〉 has been shown to vary little during the epitaxial process. Near the edges of the substrate, the sur- face morphology of the layers is complicated by vicinals, which have a spiral form in the case of YAB〈Yb〉. On {10 1} YAB substrates, homogeneous single-crystal NAB films have been grown. DOI: 10.1134/S0020168507090130 1