研究実績 2007年度

GaInAsP/InP strained-quantum-film, -wire, and -box lasers have been studied both theoretically and experimentally. A new type of distributed reflector (DR) laser, fabricated by the same fabrication processes as those of quantum-wire lasers and distributed feedback (DFB) lasers with wirelike active regions, has also been studied.

Results obtained in this research are as follows:

(1) 1590 nm GaInAsP/InP quantum-wire distributed feedback lasers incorporating Bragg wavelength detuning from the gain peak wavelength of active regions were fabricated by electron beam lithography, CH₄/H₂ reactive ion etching, and organometallic vapor-phase-epitaxy regrowth. A stable single-mode operation was obtained over a wide temperature range for devices with a detuning amount of about 30-40 nm. In particular, a submode suppression ratio of 42 dB was achieved at 1.2 times the threshold. The characteristic temperature for the threshold current density at 95 K and that for the differential quantum efficiency at 243 K were obtained over a 20 to 80 °C temperature range.

(2) By adopting a relatively large amount (54 nm) of Bragg wavelength detuning from the gain peak wavelength of the active regions, temperature dependences of threshold current density and differential quantum efficiency were markedly improved in a GaInAsP/InP distributed feedback laser with wirelike active regions emitting at 1600 nm. A stable single-longitudinal-mode operation with a sub-mode suppression ratio of 51 dB (at a bias current of 2 times the threshold) was obtained at room temperature. Variations of threshold current density and differential quantum efficiency between 10 and 85 °C were reduced to as low as ±19 and ±24%, respectively; hence, the variation of the operating current required for a certain output power in this temperature range was reduced from 1/2 to 1/3 of that without Bragg wavelength detuning.

(3) A Distributed reflector (DR) laser, which consists of the active DFB and passive DBR sections with a quantum-wire structure, was studied. DFB and DBR sections are integrated by using the energy blue shift due to the lateral quantum confinement effect. For a DR laser with low-threshold and high-efficiency operation, a high reflection DBR mirror is required. From the theoretical and experimental investigations of DBR reflectivity, a DBR section with the reflectivity of over 90% was confirmed. DR lasers with low threshold, high efficiency and stable single-mode operation have been realized using this high-reflective DBR section. For further improvement in lower threshold current operation, a DR laser with a phase-shifted DFB section was realized. Phase-shifted grating can be fabricated easily by changing the EB lithography patterns. From the theoretical investigation of the grating structure, it was found that the lowest threshold current can be obtained by adopting a, l/8-shifted grating. As a result, threshold current as low as 1.2 mA and an external differential quantum efficiency of 13% from the front facet were obtained under RT-CW conditions. A Lasing mode exists inside the stopband due to the phase shift. A stable single-mode operation with an SMSR of 49 dB was obtained at a bias current of twice the threshold. The maximum differential quantum efficiency at the front facet was realized to be 36%. Recently, sub-mA operation of a DR laser has been realized for the first time with a higher index coupling coefficient of 570 cm utilizing a deep DFB grating region in the active section. A minimum threshold current of 0.8 mA (threshold current density of 180 A/cm²) has been realized. Good single mode characteristics were preserved with a SMSR, value of 41 dB. The differential quantum efficiency was 20%/facet.

(4) Direct modulation characteristics of a DR laser have been investigated experimentally for the first time. A Clear eye opening was found up to 4.976 Gbps for back-to-back transmission, and up to 3.125 Gbps for a 10 km standard fiber transmission network. However, a BER test showed error free transmission up to 9.953 Gbps for a back-to-back as well as 10 km dispersion shifted fiber. The small signal modulation response gave a modulation bandwidth of 4.5 GHz.

(5) Monolithic integration of a DR laser with front side power monitor (PM) has been fabricated for the first time utilizing quantum wire like structures using the fabrication method including EB lithography, CH4/H2-reactive ion etching and OMVPE regrowth,. The front side power monitor is a new concept where a low absorption waveguide section is placed in front of an active or another passive device to monitor the real time power. Good linear characteristics have been observed. However, the absorption was high (26%), which was due to a long device length. It can be reduced by using a narrower wirewidth as well as proper device length. It can be used at any position of in a PIC with minimum interference with the signal.

(6) The isolation resistance between the active DFB LD region and passive device (PM) region requires high electrical isolation which was realized by deep groove etching beyond active layer. A high isolation resistance of 60 MW, was realized with a 500 nm wide and 3.8 mm deep groove where the optical coupling was estimated to be 95% for a groove width of 500 to 530 nm.

(7) Fabrication processes of GalnAsP/InP arbitrary shaped low dimensional quantum structures using electron beam (EB) lithography, Ti-mask lift-off and reactive ion etching (RIE)-dry etching is reported. Scanning electron microscopy is used to characterize the samples. Various patterns were successfully achieved, such as quantum wires and quantum dots, with better dimensional and positional controllability than the self-assemble growth process.

(8) 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 liftoff, 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 by the polarization property in the 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 lengths of Q-Wires. Furthermore, polarization anisotropy of strained Q-Wires was predicted by taking into account of the dipole moment interaction between conduction and heavy-hole subbands optical transition. The results of a 5-nm narrowed wire-width calculation results showed a good agreement with experimental results. It 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.

(9) 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 a 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 practice, for the PL measurement, two-types of strain-compensated single-quantum-well (SC-SQW) structures were prepared for 40-nm-thick- and 80-nm-thick- upper OCL wafers and covered by a 20-nm-thick SiO2 layers. 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-minutes and then re-grown for other layers to form high-mesa stripe laser structures (Ws=1.5 mm). As a result, the spontaneous emission efficiency of the lasers with an 80-nm-thick OCL was almost 2 times higher than that of the lasers with a 40-nm-thick OCL. In addition, a lower threshold current as well as a higher differential quantum efficiency was obtained for the lasers with an 80-nm-thick OCL , while that in lasers with a 40-nm-thick OCL indicated poor efficiency and a slightly higher threshold.

Semiconductor lasers with low threshold current, high efficiency, and single wavelength operation are very attractive for optical interconnection and a number of optoelectronics applications. New types of semiconductor lasers, such as membrane lasers have been studied both theoretically and experimentally.

Results obtained in this research are as follows:

(1) Novel semiconductor laser structures, such as a membrane laser, which has the Benzocyclobutene (BCB) cladding layers, enables one to increase optical confinement into the active layer due to a large refractive-index difference between the active layer and cladding layers. A RT-CW operation of a membrane BH-DFB laser, consisting of deeply etched single-quantum-well wirelike active regions, was already demonstrated. In order to realize a high reflective cavity, a surface corrugation structure was investigated. A narrow stripe membrane BH-DFB laser array using surface corrugation was fabricated. A single-transverse mode and 68 nm large stop-band operation was realized with 0.6 mm-stripe the membrane BH-DFB laser using surface corrugation. This stopband width corresponds to the index-coupling coefficient of k_i=2950 cm^-1, which is two times larger than a conventional (flat surface) membrane laser with a of 2.0, mm stripe width. In addition, we fabricated a short-cavity membrane DFB laser with a 40-nm-deep surface corrugation structure. A threshold optical pump power of as low as 0.34 mW was realized for a 2.0-mm-wide and 80-mm-long device under RT-CW conditions.

(2) Though thermal characteristics of a membrane laser were considered to be disadvantageous due to the thermal conductivity of BCB which is about 200 times lower than InP, high temperature (85 °C) continuous wave operation of an optically pumped membrane BH-DFB laser using polymer cladding was obtained using the Bragg wavelength detuning technique for its low threshold operation. The thermal resistance of this laser was estimated to be 2.3×10⁴ K/W.

(3) We fabricated novel membrane BH-DFB lasers with an air-bridge structure which is more robust than the conventional one and is suitable for large scale wafer fabrication. The minimum threshold pump power P_th of 4.3 mW was obtained at 20 °C. Continuous wave operations up to a moderately high temperature (80 °C) were achieved under an optical pumping. The thermal resistance was estimated to be 11 K/mW, which is half of that of membrane BH-DFB lasers fabricated by bonding on a BCB coated InP substrate.

(4) A silicon on insulator (SOI) substrate can realize ultracompact photonic circuits at the low-loss fiber communication wavelength due to the high index contrast between the silicon core (n=3.45) and the oxide cladding layer (n=1.45). A room-temperature continuous-wave operation under optical pumping was demonstrated with GaInAsP/InP membrane DFB lasers directly bonded on an SOI substrate formed with a rib-waveguide structure. A threshold pump power of 11.3 mW and a sub-mode suppression ratio of 29 dB were obtained with a cavity length of 140, mm and a stripe width of 1.5, mm. Light output was obtained through a 500 mm long SOI waveguide.

(5) Membrane lasers on an SOI substrate have low thermal conductivity layers like SiO₂ and poor thermal characteristics. We fabricated GaInAsP/InP membrane DFB lasers on SOI rib-waveguides. The SOI rib-waveguides work as a heat release layer. Continuous wave operation under optical pumping was obtained up to 85 °C and the characteristic temperature T₀ of 64K was obtained.

(6) We fabricated an injection membrane LED. A GaInAsP/InP substrate with n-type and p-type InP cladding layers were bonded onto an SOI substrate. The rectifying property and light emitted from the quantum well were observed.