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.

Near-infrared electroluminescence from a single-period (CdF2/CaF2) inter-subband quantum cascade structure on a Si substrate is reported. The CdF2/CaF2 heterostructure is a good candidate for realizing a Si-based short-wavelength quantum cascade laser because of its large conduction band discontinuity of 2.9 eV and small lattice mismatch with the Si substrate. In the experiment, an active region consisting of (CdF2/CaF2) heterostructures was grown epitaxially on the Si substrate by molecular beam epitaxy, and an Au/Al electrode was evaporated on the active region. The wafer was polished mechanically and cleaved. Electroluminescence from the device was observed in the near-infrared region at room temperature for the first time.

An improvement of the electroluminescence (EL) intensity of the CdF 2 /CaF 2 intersubband transition (ISBT) light-emitting structure is reported. In the ISBT active region, the precise control of crystal growth is strongly required. In this work, the hydrogenannealing surface flattening of Si substrates was employed to obtain a high quality active region to suppress non-radiative leakage current. A markedly flat surface was obtained in the 30-µm-wide trench bottom after hydrogen annealing. In the EL measurement of the device fabricated in the trench, the EL intensity was approximately 40 times stronger than that of the device without patterning and hydrogen annealing.

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