研究実績 2010年度

An optical interconnection has advantages in terms of the delay and consumption power for very high-speed signals, i.e. exceeding 10 Gbit/s, because it is independent on the wiring capacity. Photonic integrated circuits (PICs) composed of functional optical devices on silicon-on-insulator (SOI) platforms have been attracting much attention for future LSIs. III-V/Si heterogeneous photonic integrated devices, such as lasers and waveguide devices have been investigated.

Results obtained in this research are as follows:

(1) Ultra low-power consumption light sources are essential to utilize the advantage of optical system in the short length optical interconnection. Toward low threshold and high efficiency performance, GaInAsP/InP membrane DFB laser with thin semiconductor core layer and low refractive-index cladding layers has been studied. A lateral current injection (LCI) type laser composed of 400 nm thin core layer including strain compensated five quantum wells, was investigated. Threshold current of 11 mA (threshold current density was 900 A/cm²) and an external differential quantum efficiency of 33% were obtained for the device with 720 μm-long cavity and 1.7 μm wide stripe under room temperature continuous-wave (RT-CW) condition.

(2) Dynamic characteristics of the LCI laser were studied for future high performance membrane laser. The modulation efficiency f_r was estimated to be 0.43 GHz/mA^1/2 from relative intensity noise (RIN) measurements. Eye opening up to 2.5 Gbps was confirmed at 37.5 mA bias and 0.8 Vpp modulation voltage with the PRBS word length of 2⁷-1.

(3) Fundamental properties of a lateral junction type photodiode, which has the same structure as that of above mentioned LCI laser, were studied. The responsivity of 0.27 A/W, 3 dB bandwidth of 6 GHz and 7.5 GHz at a bias voltage of 0 V and -2 V, respectively, were obtained for a stripe width of 1.4 μm and a device length of 220 μm. An error free performance up to 6 Gbps at 0 V was confirmed.

(4) A LCI type membrane distributed feedback (DFB) laser consisting of a 470-nm-thick semiconductor core and bonded by a benzocyclobutene (BCB) polymer on a SOI substrate was demonstrated for the first time by adopting a LCI structure. Room-temperature pulsed operation was achieved with a threshold current of 83 mA for a stripe width of 3.2 μm and a cavity length of 420 μm.

(5) An integrated-twin-guide (ITG) laser with high index-contrast waveguide structure was theoretically analyzed to achieve lower threshold current and high output efficiency compared with those of evanescently coupled structure on a SOI substrate. Better lasing characteristics could be expected with 250 nm-thick GaInAsP membrane and silicon waveguide ITG structure separated by about 200 nm-thick SiO₂ layer while accurate control of the SiO₂ separation layer as well as the length of the active section is essentially required.

(6) GaInAsP/InP LCI DFB laser with amorphous silicon (a-Si) surface grating on semi-insulating InP substrate was realized. A moderately low threshold current of 7.0 mA as well as a high differential quantum efficiency of 43% from the front facet were obtained under RT-CW condition for uniform grating LCI-DFB laser with a stripe width of 2.0 μm and a cavity length of 300 μm. A small-signal modulation bandwidth of 4.8 GHz was obtained at a bias current of 30 mA with a modulation current efficiency factor of 1.0 GHz/mA^1/2.

(7) A wirelike active region structure was introduced into a LCI-DFB laser to achieve a stable single-mode operation and low threshold due to its small active region volume and strong index-coupling effect. A relatively low threshold current of 11 mA and a differential quantum efficiency of 26% from the front facet were obtained. The lasing wavelenghth of 1560 nm and a sub-mode suppression-ratio (SMSR) of 30 dB were obtained at a bias current of two times the threshold.

(8) Toward demonstration of high performance LCI membrane lasers, the lateral carrier injection characteristics into thin semiconductor core layers were analyzed and experimentally confirmed. Narrow waveguide structure showed better static and dynamic lasing performance due to more efficient carrier consumption and less electrical response delay time.

(9) An athermal Si-slot-waveguide ring resonator embedded with BCB, a low-k material used in electronics, was proposed. By controlling the width of the BCB-filled gap to about 95 nm, the temperature coefficient of an equivalent-index of the Si-slot-waveguide can be reduced to almost zero while an accurate control of the width is essentially required. The dependences of peak wavelength shift on temperature in the fabricated devices of a conventional Si wire waveguide with SiO₂ cladding, Si wire waveguide with BCB cladding, and Si-slot-waveguide with BCB cladding were 26 pm/K, 17 pm/K, and -0.6 pm/K, respectively.

(10) In order to improve the property of the athermal ring resonator with Si-slot -waveguide, a reduction of the propagation loss of a slot-waveguide and an introduction of a spot-size converter were achieved. The propagation loss of the Si-slot-waveguide of 5.6 dB/cm, that is the lowest value to our knowledge, was obtained by introducing an inductively-coupled-plasma reactive-ion-etching (ICP-RIE). For spectral characteristics of ring resonator, a Q factor was improved from 13200 to 31000, and the whole loss of the ring resonator was improved by about 8 dB.

(11) The relationship between the propagation loss and the surface roughness in multilayered a-Si waveguides was studied. The propagation loss of 1st layer crystal (c)-Si, the 2nd and 3rd layer a-Si waveguides were 6.0 dB/cm, 10.2 dB/cm, and 12.0 dB/cm, respectively. The propagation loss was significantly affected by the top and bottom surface roughness rather than the material absorption of a-Si.

(12) A surface-activated-bonding (SAB) technique is expected to have advantages of low process temperature. Si-to-Si direct bonding strength of 1.6 MPa was obtained by using plasma pretreatment prior to the heating and weighting. 1.4 MPa of InP-to-Si direct bonding strength was obtained by improving chemical cleaning process. Photoluminescence (PL) properties of GaInAs quantum-wells (QWs) bonded on a Si substrate were studied. An introduction of a 30-nm-thick superlattice buffer layer on the top of the wafer greatly suppressed photoluminescence intensity degradation near the bonded interface.

(13) The SAB technology was investigated to realize InP-based active photonic devices on a SOI substrate for compact PICs. A wide area GaInAsP/InP membrane structure was successfully bonded onto a SOI waveguide using a low-temperature (150°C) N2 plasma SAB method. The full width at half maximum (FWHM) of the PL intensity of 5QWs membrane structure bonded on a SOI substrate was comparable to that of as grown QWs on an InP substrate. PL-mapping results showed that the PL intensity distribution and peak wavelength shift of the GaInAsP 5QWs structure around the Si waveguides were small.

(14) A successful bonding of 2 inches diameter Si and GaInAsP/InP wafers was achieved by using BCB bonding technique. Air voids were eliminated by choosing appropriate pre-cure conditions of BCB. A TE mode propagation loss of 17 dB/cm was obtained with 150-nm-thick and 460-nm-wide GaInAsP/InP waveguides bonded on Si substrates.

To meet demands of high-speed data transmission, optical communication systems are widely used. It is promising that optical communications will spread more into the interconnections between computers, boards, and even chips. In such systems, high-speed light sources as well as multifunctional optical devices are necessary. As a new device, distributed reflector (DR) laser with wirelike active regions has been studied. A 1.3 μm AlGaInAs/InP transistor laser (TL) with high quality buried-heterostructure (BH) has also been studied both theoretically and experimentally.

Results obtained in this research are as follows:

(1) The enhancement of direct modulation bandwidth of distributed reflector (DR) lasers with wirelike active regions utilizing optical injection locking was demonstrated. The small-signal bandwidth was increased to >15 GHz at a bias current of 5 mA, which is 4 times smaller bias current than that for conventional edge-emitting lasers. Clear eye diagram with 20% eye margin at 10 Gbps was also achieved at the bias current of 5 mA and voltage swing of 0.4 V_pp. By adopting 5-quantum-well wirelike active regions and thin optical confinement layers (40 nm), a mask test of 10 GbE with a 20% margin was passed with a low bias current of 10 mA and the maximum 3 dB bandwidth of 15 GHz was obtained.

(2) For a high-reflectivity DBR section in DR lasers, small active volume attributed to the energy blue shift of wirelike active regions was utilized. Transmission and reflection spectra of the DBR with periodically etched active regions were measured for the first time. Obtained stopband width was 13.2 nm from which the index-coupling coefficient κ_i was estimated to be 446 cm^-1. These spectra were moderately in good agreement with theoretical ones.

(3) The effect of in-situ thermal cleaning on the regrowth interface quality of 1.3 μm AlGaInAs/InP BH lasers prepared by organo-metallic vapor-phase-epitaxy (OMVPE) was investigated in terms of the surface recombination velocity S determined from the electroluminescence property below the threshold. As the results, we could obtain the product of S and carrier lifetime τ, to be S•τ = 315 nm and an internal quantum efficiency of approximately 70% for the thermal cleaning conditions of 30-60 min. in a PH₃ atmosphere at a temperature of 450°C.

(4) High-performance 1.3 μm AlGaInAs/InP BH lasers were realized by adopting the above mentioned in-situ thermal cleaning. A threshold current as low as I_th=8.1 mA (threshold current density Jth=1.0 kA/cm2) as well as high external differential quantum efficiency η_d = 66% was obtained with a stripe width of 1.6 μm and a cavity length of 500 μm. An internal quantum efficiency η_i was evaluated to be 55%, 59%, and 76%, for the thermal cleaning temperature of 250, 450, and 650°C, respectively while the waveguide loss was almost the same (4 cm^-1).

(5) Small- and large-signal analyses of transistor lasers (TLs) were done for 0.98-μm wavelength GaInAs/GaAs and 1.3-μm AlGaInAs/InP systems. The modulation bandwidth of the TL was found to be larger than that of a laser diode (LD) due to a lower damping effect. From a small signal analysis higher maximum modulation bandwidth around 40 GHz could be realized for the TL whereas that of the LD was limited at around 25 GHz due to the damping effect. By a large signal analysis, a clear eye diagram under >40 Gbps was obtained.

(6) A CW operation of a long wavelength TL using 1.3 μm AlGaInAs/InP material system was achieved for the first time under p/n/p configuration. A threshold emitter current of 17 mA was obtained for a stripe width of 1.8 μm and a cavity length of 500 μm. By sweeping collector-base voltage, the threshold current and the slope efficiency could be controlled. These results indicate that the optical signal control by emitter current as well as collector voltage can be realized simultaneously, which is impossible by conventional LDs.

(7) We proposed hetero-bipolar-transistor-semiconductor-optical-amplifiers (HBT-SOAs) and successfully simulated their characteristics with AlGaInAs quantum-well (QW) active region. By a large signal analysis, much faster carrier recovery time of 165 ps (10%-90%) was obtained in the case of the HBT-SOA than that of the diode SOA (801 ps) due to the faster carrier supply time into the active region with a input power of 10 mW. The result indicates smaller pattern effect at high frequency and less input power dependence can be expected for HBT-SOAs compared with conventional SOAs.

Results obtained in this research are as follows:

(1) The relative permeability of every natural material is 1 at optical frequencies because the magnetization of natural materials does not follow the alternating magnetic field of light. If we can overcome this restriction and control both the permeability and the permittivity at optical frequencies, we will be able to establish a new field involving optical/photonic devices for future communication technologies.
To move one step closer to this goal, we demonstrated an InP-based optical multi-mode interferometer (MMI) combined with metamaterials consisting of minute split-ring resonators (SRRs) arrayed on the MMI. The MMI could operate at an optical-fiber-communication wavelength of 1.5 μm. In the device, the permeability exhibited a strong and sharp resonance at 200 THz, and the real part of the relative permeability changed from -0.4 to 2.4 around this frequency.

(2) Semiconductor lasers with nanometer-sized optical cavities are indispensable components for advanced optical applications, such as on-chip optical interconnects and dense photonic integration, that need ultrasmall coherent light sources.
To develop a more feasible device, we analyzed a novel structure, a semiconductor distributed-feedback (DFB) laser with strong confinement induced by SPPs, designed for use at 1.55-μm wavelength. The diffraction grating is formed by replacing the upper guiding GaInAsP with InP at regular intervals in the longitudinal direction. The waveguide is covered with metal (gold) through thin low-index dielectric layers (SiO₂); therefore SPP layers are excited on both sides of the waveguide. This structure is less sensitive to fabrication tolerances, because the metal region only has to confine light with its SPPs and is not concerned with longitudinal feedback of light, which is performed by the semiconductor DFB structure.
From the simulation, it was found that the threshold current of the plasmonic semiconductor DFB structure can reduced to 650 μA because of the small active region and the strong confinement of light beyond the diffraction limit.