Since semiconductor lasers were realized in 1962, various efforts have been made to enrich human life thorough novel
equipments and services. Among them optical fiber communications in global communications have brought out
marvelous information technology age represented by the internet. In this paper, emerging topics made on GaInAsP/InP
based long-wavelength lasers toward ultra-low power consumption semiconductor lasers for optical interconnects in
supercomputers as well as in future LSIs are presented.
The relationship between the propagation loss and roughness on multi-layered Si waveguides fabricated up to a 3rd layer was investigated. By reducing the surface and sidewall roughness of the waveguides, a low propagation loss of 3.7 dB/cm for the 3 layer a-Si:H waveguides was demonstrated. Furthermore, vertical coupling between multilayer
waveguides was demonstrated by use of a grating-type vertical coupler. A coupling efficiency of 22% was obtained for
10 pairs of gratings with a period of 640 nm, even with a layer distance of 1μm.
Wire-length dependences of In-place polarization anisotropy in GaInAsP/InP quantum-wire (Q-wire) structures
fabricated by dry-etching and regrowth processes were investigated using a photo luminescence (PL) measurement. The
reduction of polarization anisotropy of Q-wires is expected in the shorter Q-Wires. A strain-compensated GaInAsP/InP
single-quantum-well initial wafer was prepared by an organometallic-vapor-phase-epitaxy (OMVPE) system. Using
electron beam lithography, Ti-mask lift-off, CH4/H2 reactive-ion-etching and OMVPE regrowth processes, various
lengths (L) of the Q-wires were realized for wire-widths (W) of 11-, 14- and 18 nm. The Q-wires were measured the
polarization property in normal and parallel to wire-length direction at room temperature. As a result, stronger
polarization anisotropy was observed in narrower Q-Wires and reduced in shorter length of Q-Wires. Furthermore,
polarization anisotropy of strained Q-Wires was predicted by taking in account of the dipole moment interaction between
conduction and heavy-hole subbands optical transition. A 5-nm narrowed wire-width calculation results shows a good
agreement with experimental results. This could be considered that a strain distribution in the Q-Wire induced the energy
band deformation at the edge of the Q-Wire, which reduced the effective wire-width to much narrower than the actual
size observed by an SEM image.
In order to realize low damage fine structuring processes for the low-dimensional quantum structures, we investigated a
process for reducing the degradations of optical properties, which was induced during a reactive-ion-etching (RIE)
process with CH4/H2 gas mixture in the quantum-well (QW) structures. Quantitative studies of optical degradation were
carried out by photoluminescence (PL) and electroluminescence (EL) measurements. We introduced a thicker upper
optical confinement layer (OCL) to protect the QWs from the RIE-plasma. In practical, for the PL measurement, twotypes
of strain-compensated single-quantum-well (SC-SQW) structures were prepared for 40-nm-thick- and 80-nmthick-
upper OCL wafers and covered by 20-nm-thick SiO2. After the samples were exposed to CH4/H2-RIE for 5-
minutes, a relatively stronger suppression of integral PL intensity as well as a spectral broadening was observed in the
sample with 40-nm-thick OCL, while those did not change in the sample with 80-nm-thick OCL. For the EL
measurements, using two types of SC-DQW structures, samples were exposed to CH4/H2-RIE plasma for 5-minute and
then re-grown for other layers to form high-mesa stripe laser structures (Ws=1.5μm). As a result, the spontaneous
emission efficiency of the lasers with 80-nm-thick OCL was almost 2 times higher than that of the lasers with 40-nmthick
OCL. In addition, a lower threshold current as well as a higher differential quantum efficiency was obtained for the
lasers with 80-nm-thick OCL , while that in lasers with 40-nm-thick OCL indicated poor efficiency and a slightly higher
threshold.
For high performance operation of semiconductor lasers consisting of low-dimensional electron systems as the active medium, a good size uniformity and a formation of low-damage interfaces are essentially required. In this paper, we review our recent results obtained by GaInAsP/InP based long-wavelength lasers consisting of quantum-wire structures fabricated by electron beam lithography, CH4/H2-reactive ion etching and 2-step organo-metallic vapor-phase-epitaxial growth processes.
Good size uniformity of vertically-stacked multiple-quantum-wire structures was obtained with a standard deviation of less than ±2 nm. By using a strain-compensated quantum-well structure as an initial wafer, non-radiative recombinations at etched/regrown interfaces were fairly reduced, which resulted in a room-temperature continuous-wave (RT-CW) operation of a quantum-wire laser for more than 15,000 hours. By taking an advantage of this fabrication method, 1540 nm wavelength quantum-wire distributed feedback (DFB) lasers were also realized for the first time with relatively low threshold current and high differential quantum efficiency under a RT-CW condition.
Moreover, by utilizing the energy blue shift due to the lateral quantum confinement effect, a low threshold current operation with a stable single-mode property has been successfully demonstrated for distributed-reflector (DR) lasers consisting of a DFB section with wirelike active regions and a passive distributed-Bragg-reflector (DBR) section with narrow quantum-wire active regions.
Recent progress in dry etching technology of GaInAsP/InP compounds enabled realizations of fine vertical groove structures with a high aspect ratio. Since this technology enables the formation of etched mirrors in a wafer batch process, it leads to a low-cost production of not only simple Fabry-Perot lasers but also high performance and functional lasers for optical communications by integrating functional elements such as gratings for wavelength selection and high-reflectivity/low-reflectivity facets, while improvements in the size controllability with precision are still required.
In order to keep an initial mask width condition during reactive-ion-etching (RIE) with methane/hydrogen mixture gas, we developed a sequential etching process of the RIE followed by oxygen ashing for relatively short period (for the etching depth of around 240 nm) and repeated the process for required etching depth. The tilt angle of the etched facet was controlled to be less than 1 degree from the normal to the wafer, and the aspect ratio of a narrow groove (140 nm) as high as 17 was obtained. By using this technique we could realize high-reflectivity distributed Bragg reflector (DBR) consisting of semiconductor/polymer pillars and DBR lasers with low threshold current (less than 10 mA for the stripe width of 5 micronmeter) and high differential quantum efficiency of 50 % from the front facet, while the emission spectrum showed a multi-mode operation due to poor wavelength selectivity. A preliminary aging test was carried out at room temperature CW condition, and no degradation was observed after 5,000 hours.
With aiming at high performance single-wavelength lasers, we realized a novel distributed feedback (DFB) laser consisting of the first-order corrugations on the sidewalls of the stripe mesa, named ?gVertical Grating (VG),?h as well as a distributed reflector (DR) laser consisting of the VG-DFB structure and above mentioned DBR on the rear side. A stable single-mode operation with a sub-mode suppression ratio of 36dB, thre
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