PACS 81.05. Ea, 81.15.Kk Refractive indices of B x Al 1-x N and B y Ga 1-y N epitaxial layers were determined in the ultraviolet (UV) wavelength range from 230 nm to 600 nm by the light reflectance spectrum. The relative refractive index differences of (AlN/B 0.01 Al 0.99 N) and (GaN/B 0.02 Ga 0.98 N) heterostructures were 17% at 250 nm and 11% at 370 nm, respectively. These values indicate that an optical waveguide can be organized in the UV to deep-UV spectral region using the nitride semiconductors which include a small amount of boron. The relative refractive index difference of 6% was achieved with only 0.2% and 1.1% of boron for (AlN/B x Al 1-x N) and (GaN/B y Ga 1-y N) heterostructures at 250 nm and 370 nm, respectively. 1 Introduction Recent studies on light-emitting devices focus on high-power and highly efficient UVlight-emitting diodes and laser diodes. However power exchange efficiency from electrical to optical power must be improved in the UV-light-emitting devices. The reasons for this small power exchange efficiency was believed to be related to the crystal quality, for example, residual strain and large amount of dislocation density in the epitaxial layer. The residual strain originates from thermal strain and lattice mismatch in heterostructure. The thermal stress is caused by the difference of thermal expansion coefficients. A (GaN/AlN) multi-buffer-layer structure was proposed as a residual-strain-controlling technique in the GaN and AlN grown on a SiC substrate [1][2][3].The BAlGaN quaternary system was suggested as a new lattice-matched system to the SiC and AlN substrates [4]. Estimated band-gap energy (Eg) of the BAlGaN quaternary system are between 6.3 eV (190 nm for the B 0.05 Al 0.95 N) and 3.8 eV (340 nm for the B 0.17 Ga 0.83 N) [4]. Therefore, this quaternary system is a promising material for the light-emitting devices of the UV to deep-UV spectral region.Possible epitaxial growth of the BAlN ternary and the BN binary system were first demonstrated on the 6H-SiC substrate by low-pressure metalorganic vapor phase epitaxy (LP-MOVPE) [4]. Epitaxial growth of BGaN was demonstrated by molecular beam epitaxy (MBE) and VPE [5,6]. The maximum boron contents up to 13% and 9% were experimentally demonstrated for BAlN and BGaN, respectively. The maximum boron contents are sufficient to lattice-match the BAlGaN quaternary system to AlN [7]. The first photoluminescence (PL) emission from BGaN was demonstrated at a low temperature [8]. The bandoffset and refractive indices of BGaN and BAlGaN, and the band-gap energy, effective mass, and optical gain of BGaN were estimated theoretically [9][10][11]. Room-temperature PL emission from the (BGaN/AlGaN) multi-quantum-well (MQW) structure was first reported at 360.4 nm [12]. Then, improved PL intensity and characteristics of the strain-controlled (BAlGaN/AlN) MQW structure were also reported at 250.0 nm by adopting the (GaN/AlN) multi-buffer-layer structure [13,14]. However, there is no experimental data on the refractive indices of BAlN and BGaN.