研究実績 2008年度

An Optical signal system has advantages in terms of the delay because it is independent on the wiring capacity. Optical functional active and passive devices on SOI platforms get much attention for photonic integrated circuit. III-V/Si Heterogeneous photonic integration devices, such as lasers and waveguide devices have been studied.

Results obtained in this research are as follows:

(1) Silicon based optical devices have a potential to realize ultracompact functional photonic integrated circuits. The development of active photonic devices such as lasers and optical amplifiers fabricated on SOI platforms has attracted considerable attention. Injection type DFB lasers directly bonded on an SOI substrate were successfully realized for the first time. A threshold current as low as 104 mA was obtained under a room-temperature pulsed condition for the stripe width of 25 mm and the cavity length of 1 mm.

(2) Ultra low-power consumption light source 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 was demonstrated for electrically pumped operation of the membrane laser. Threshold current of 105 mA with the stripe width of 5.4 mm and the cavity length of 1.47 mm were obtained.

(3) GaInAsP/InP LCI DFB laser on semi-insulating substrate was realized. A room temperature-pulsed operation was achieved for the cavity length of 300 mm length. The threshold current of 27 mA and the threshold current density of 2.6 kA/cm² were obtained. This device oscillated at an emission wavelength of 1540.7 nm and a side-mode suppression ratio (SMSR) of 35 dB at 2.0 I_th was obtained.

(4) A room temperature continuous wave operation of a LCI laser was achieved. A threshold current of 12 mA and external differential quantum efficiency of 12.5% was obtained with a cavity length of 490 mm and a stripe width of 1.4 mm. An internal quantum efficiency of about 19% and waveguide loss of 6.2 cm^-1 were estimated.

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. Furthermore novel photonic devices, such as a THz modulator and a laser transistor have been studied both theoretically and experimentally.

Results obtained in this research are as follows:

(1) 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.

(2) The origin and model of the time dependence of RIE-plasma induced optical property degradation of GaInAsP/InP quantum-well structures were investigated. We determined the origin of this damage was either from CH₄/H₂-plasma or O₂-plasma. We performed the plasma-exposure and PL measurements by using only O₂ gas and H₂ gas to determine the detail of the origin. After O₂ Plasma of various exposure times of 5, 10, and 20 min, PL intensity did not show noticeable degradation (>95% of the initial PL intensity). On the contrary, the wafer which was exposed to H₂ plasma showed dramatic reduction in PL intensity. Hence the origin of the degradation in the CH₄/H₂-RIE process could be attributed to the exposure to H₂-plasma. This is attributed to the H ion which was accelerated by RIE plasma and pass-through SiO₂ mask.
The suspended H ions are in unstable condition, which can be recovered by time or annealing, while, the dislocation-H complexes are more complicated. However, high temperature annealing can be used to remove such complexes. The effect of high temperature annealing during the OMVPE regrowth process using the same temperature steps for Q-wire lasers under a PH₃ rich atmosphere (1st : 250°C, 30 min, 2nd : 650°C, 60 min). The PL peak intensity of the wafer with 80-nm-thick OCL has about an 87% recovery after the annealing. On the contrary, the wafer with 40-nm-thick OCL has about a 75% recovery after the annealing.
In conclusion, the origins of plasma-induced PL degradation can be attributed to dislocation-H complexes and the suspended H ions between atoms in the crystal. And non-radiative recombination was found to be recovered during high temperature annealing in the embedding growth by organometallic vapor-phase-epitaxy.

(3) Size uniformity is an important issue to improve lasing characteristics of a quantum-wire (Q-Wire) laser. However, a relatively large wire-width fluctuation, DW, of 4 nm was obtained due to width-fluctuation in the resist pattern, which is caused by electronic dose fluctuation, or shot-noise, during EB exposure. Recently, we report a method for improving size uniformity by low temperature development. In the experiment, after an 80-nm-thick ZEP520 mixed with 10% of C₆₀ was spun on the InP substrate and prebaked on a hot-plate at 200oC for 2 min, the Q-Wire EB resist patterns were exposed in a pitch (D) of 100 nm with various dose conditions to obtain different wire-widths. The resist patterns were developed in ZED-N50 developer under room-temperature (RT) and -9oC conditions. As a result, under -9oC, over 50% reduction of DW (0.9 nm) was obtained compared with the results of RT. In conclusion, low temperature development reduced the sensitivity of EB resist, especially for the backward scattering electron, which is the origin of size-fluctuation in Q-Wire structures.

(4) 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. For further threshold current reduction, a DR laser with a phase-shifted DFB section was studied. Phase-shifted grating can be fabricated easily by changing the EB lithography patterns. From the theoretical analysis, it was found that threshold current can be reduced to half by adopting a l/8-shifted grating.
Experimentally, sub-mA operation of a DR laser with phase-shifted DFB section as low as 0.9 mA and an external differential quantum efficiency of 18% from the front facet were obtained under RT-CW conditions. The reduction of threshold current from the previous phase-shifted DR lasers was achieved by increased index-contrast in grating (15% increase in groove depth) and shortened cavity length (50%). A stable single-mode operation with an SMSR of 45 dB was obtained at a bias current of 2.2 times the threshold current. A Lasing mode exists inside the stopband due to the phase shift.

(5) Direct modulation characteristics of a DR laser have been investigated experimentally. 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.

(6) Direct modulation of semiconductor lasers is the most powerful scheme for cost-effective low power consumptive light sources. Therefore injection-locking or passive feedback techniques became very attractive issues in order to increase the direct modulation speed for over 40 Gb/s applications such as very-short-reach (VSR) optical link. Theoretical analysis of modulation bandwidth enhancement in self-injection-locked DR lasers was carried out. The proposed laser structure included the DR lasers and the front feedback section for self-injection-locking and the front DBR section. We investigated 3 dB bandwidths for various coupling efficiencies and phases using rate equations based small signal analysis. When the phase of reflected light was chosen properly, a high modulation bandwidth over 30 GHz was expected with a bias current of 30 mA, with which the solitary laser had 16 GHz bandwidth. The required bias current for 30 GHz in the solitary laser is about 100 mA if the maximum modulation bandwidth determined by the K-factor was ignored and, therefore, the advantage of self-injection locking was confirmed.

(7) Optical circuits and optical networks are being installed in everywhere from long-haul to inter/intra chip networks due to the high capacity of their data rate. On the other hand, because of wide bandwidth capability and high directivity compared with microwaves used in conventional wireless communication systems, sub-THz and THz waves are expected to be crucial frequency-bands in next generation wireless communication systems. Therefore, it is very important to realize the direct signal conversion method between THz wave signals and optical signals. Recently we proposed and realized a novel direct conversion method using photon-generated free-carriers. Free carriers which generated by photon absorption absorb THz waves due to the skin effect and free-carrier absorption. By changing light power irradiated into the semiconductor, THz power passing through the semiconductor can be changed. Using this phenomenon, intensity modulation of THz waves can be realized. Experimentally the intensity change of sub-THz waves by the intensity change of optical input was observed using GaInAs modulator on an InP substrate. Using 96 GHz waves, an absorption coefficient of THz waves aT = 850 cm^-1 and a modulation depth of 15.6% were observed at an input optical power of 30 mW. By using > 1 THz waves, higher extinction ratio should be obtained.

(8) A novel method of signal media conversion from optical signal to Terahertz (THz) signal by the skin effect and the free-carrier absorption using photon generated free-carriers in semiconductor was demonstrated. The modulator 2mm thickness intrinsic GaInAs grown on semi-insulating InP substrate and 1.55 mm wavelength light was used. Using 192 GHz continuous sub-THz wave and 1.55 mm optical signal, the modulation depth of 45% and the modulated speed up to 2 MHz were demonstrated. The low modulation speed was attributed to the large rise time of THz signal due to the carrier spreading in the GaInAs modulator. In order to confine the photon-generated carriers, the GaInAs modulator was etched and a 1-mm-diameter disk was formed. As a result the rise time of sub-THz signal was reduced from 600 ns to 200 ns.