Results 2005



This is an annual report on the research activities in the field of optical communications and high-speed electron devices for 2005 at the Graduate School of Science and Engineering, the Interdisciplinary Graduate School of Science and Engineering, and Quantum Nanoelectronics Research Center, Tokyo Institute of Technology.

These activities are initiated by Professor Yasuharu Suematsu (emeritus, the former president), and Professor Shigehisa Arai [Group A], mainly in the field of Low- Dimensional Quantum-Structure Lasers, Advanced Lasers for Photonic Integrations, and also in the fabrication of ultra-fine structures.

This report consists of a brief introduction of the research activities and a collection of the research papers published in 2005.

Publication List

Group members

  • Professor Emeritus
    • Yasuharu SUEMATSU D.E.
  • Professor
    • Shigehisa ARAI D.E.
  • Visiting Researcher
    • Anisul HAQUE1 Ph.D. -Apr.
  • Research Assistant
    • Takeo MARUYAMA D.E.
  • Post-Doctoral Research Fellow
    • Hideki YAGI2 D.E. -Sep.
    • Kazuya OHIRA D.E. Oct.-
  • Secretaries
    • Kyoko KASUKAWA B.A.
  • Graduate Students
    (Doctor Course)
    • Takeshi OKAMOTO3 M.E. -Mar.
    • Kazuya OHIRAI M.E. -Sep.
    • Dhanorm PLUMWONGROT M.E. Apr.-
    • Shinichi SAKAMOTO M.E. Apr.-
    • Saeed Mahmud ULLAH M.E. Apr.-
    • Hiromi OOHASHI M.E. Oct.-
  • Graduate Students
    (Master Course)
    • Tomonori MURAYAMA3 B.E. -Mar.
    • Dhanorm PLUMWONGROT B.E. -Mar.
    • Shinichi SAKAMOTO B.E. -Mar.
    • Tatsuya YAMAZAKI4 B.E. -Mar.
    • Hiroyuki KAWASHIMA B.S.
    • Koji MIURA B.E.
    • Yoshifumi NISHIMOTO B.E. Apr.-
    • Yosuke TAMURA B.E. Apr.-
    • Ryo SUEMITSU B.E. Apr.-
  • Undergraduate Students
    • Yoshifumi NISHIMOTO -Mar.
    • Jing-Long TANG5 -Mar.
    • Sung Hun LEE Apr.-
    • Hideyuki NAITOH Apr.-
    • Tadashi OKUMURA Apr.-
  • Research Student
    • Saeed Mahmud ULLAH M.E. -Mar.
  • Associate Visiting Researchert
    • Klaus MATHWIG6 B.E. Apr.-Jul.

Present Address

  • 1) East West University, Bangladesh
  • 2) Sumitomo Electric Industries, Ltd.
  • 3) NEC Corporation
  • 4) Hitachi Ltd.
  • 5) Prof. Hotate’s Lab., Department of Electrical Engineering, University of Tokyo
  • 6) Prof. Forchel’s Lab., Wurzburg University


Low Dimensional Quantum Structure Lasers

Staffs: Y. Suematsu, S. Arai, T. Maruyama, S. Tamura
Visiting Researcher: A. Haque
Post-Doctoral Research Fellow: H. Yagi, K. Ohira
Students: T. Murayama, D. Plumwongrot, S. M. Ullah, K. Miura, Y. Nishimoto, R. Suemitsu, Y. Tamura, S.H. Lee

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

Results obtained in this research are as follows:

(1) A GaInAsP/InP quantum-wire distributed feedback (Q-Wire DFB) laser with the active region width of 24 nm in the period of 240 nm was realized by an electron beam lithography, CH4/H2-reactive ion etching and two-step organometallic vapor-phase-epitaxial growth processes for the first time. A threshold current as low as 2.7 mA (threshold current density = 270 A/cm2) and differential quantum efficiency of 19 %/facet were achieved for the stripe width of 3.0 mm and the cavity length of 330 mm under RT-CW condition. A single-mode operation with the sub-mode suppression ratio (SMSR) as high as 51 dB (injection current is twice the threshold) was also obtained in the lasing wavelength of 1541 nm. From the lasing spectrum, the stopband width was observed to be 4.8 nm which corresponds to the index-coupling coefficient (ki) of 180 cm-1.

(2) A GaInAsP/InP Q-Wire DFB laser with the active region width of 30 nm in the period of 240 nm was realized by an electron beam lithography, CH4/H2-reactive ion etching and two-step organometallic vapor-phase- epitaxial growth processes. High-mesa structure of this laser was fabricated by using wet chemical etching to obtain low-damage interfaces at the sidewalls. The spontaneous emission efficiency of this Q-Wire DFB laser was almost comparable to that of a quantum-film laser fabricated by one-step growth. This indicates not only that this laser had a low-damage feature at the ultra fine structures but also there was little nonradiative recombination of the etched/regrown interfaces. By adopting low-damage fabrication processes for high-mesa stripe structures, a threshold current as low as 2.1 mA, which corresponds to a threshold current density of 176 A/cm2, and a differential quantum efficiency of 16 %/facet were obtained for the stripe width of 3.4 mm and the cavity length of 350 mm under RT-CW condition. A sub-mode suppression-ratio (SMSR) of 50 dB at a bias current of twice the threshold was also achieved.

(3) A quantum-wire distributed feedback laser with GaInAsP/InP strain- compensated single-quantum-well substrate for reducing lateral size distribution of multiple-quantum-wire structures was realized by an electron beam lithography, CH4/H2-reactive ion etching and two-step organometallic vapor-phase-epitaxial growth processes. This laser had the active region width of 30 nm in the period of 240 nm with the size fluctuation as low as 8.7 %, which was much smaller than that in multiple- quantum-wire laser of 18 %. A threshold current density as low as 155 A/cm2 was achieved for the stripe width of 3.5 mm and the cavity length of 1120 mm under a RT-CW condition.

(4) High characteristic temperature operation of 1590nm GaInAsP/InP quantum-wire DFB lasers was obtained by the Bragg wavelength detuning from the gain peak wavelength. The characteristic temperature for threshold current of 95 K and that for the differential quantum efficiency of 243 K were obtained for 20 to 80 °C. These results indicated that the Bragg wavelength detuning with a Q-WireDFB laser was very attractive for temperature-independent and low threshold current operation.

(5) Investigations on polarization anisotropy for compressively strained GaInAsP/InP quantum-wire (Q-Wire) structures fabricated by electron beam lithography, dry etching and double-step organometallic vapor- phase-epitaxial growth processes were carried out via experimental evaluation of photoluminescence (PL) and lasing characteristics. From PL measurement, parallel transverse-electric field (TE) peak intensity to the Q-Wire direction was measured to be 1.4-1.6 times stronger than perpendicular TE peak intensity to the Q-Wire direction for the wire widths of 24-45 nm. Furthermore, 2-types of Q-Wire laser with the wire width of 35 nm were fabricated, i.e., quantum-wire directions are perpendicular and parallel to the laser cavity; denoted by Q-Wire and Q-Wire//, respectively. As a result, although the spontaneous emission efficiency of both lasers was almost the same, the threshold current density of Q-Wirewas much lower compared with that of Q-Wire//. From the gain spectral measurement by Hakki-Paoli method, it was demonstrated that the differential gain for Q-Wire was 5 times higher than that for Q-Wire//.

(6) Distributed reflector (DR) laser, which consists of the active DFB and passive DBR sections with quantum-wire structure, was studied. DFB and DBR sections are integrated by using energy blue shift due to a lateral quantum confinement effect. For DR laser with low-threshold and high-efficiency operation, a high reflection DBR is required. From the theoretical and experimental investigations of DBR reflectivity, 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 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 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 condition. 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.

(7) Antireflection coating has been performed on the fabricated DR laser. Al2O3 (n=1.74) has been used for low residual reflectivity to improve the differential quantum efficiency and single-mode operation. As a result, threshold current was reduced and differential quantum efficiency was increased reproducibly which can be attributed from the facet phase of the DFB section. A reduction of threshold current from 4 mA to 1.4 mA and increase of differential quantum efficiency from 19%/facet to 27%/facet have been obtained.

(8) For even lower threshold current operation of DR laser, narrow stripe buried heterostructure (BH) has been proposed to adopt. Mass transport technique has been taken into consideration to cover the sidewall of the active grating region of the stripe with InP so that surface recombination leakage current can be reduced and as a consequence, threshold current will be reduced. At the same time narrower stripe width around 1 mm will be fabricated.

(9) Utilizing the DR laser fabrication process which includes EB lithography, CH4/H2-reactive ion etching and OMVPE regrowth, monolithic integration of DR laser with electroabsorption modulator (EAM) and detector has been proposed. The large energy blue shift of quantum-wire has been utilized to achieve different functional devices. The wire width has been modulated at the different sections of the device and thereby controlling the transition energy. This fabrication technique possesses the simultaneous fabrication of multiple devices without further complication of the established process.

New Types of Semiconductor Lasers for Photonic Integration

Staffs: Y. Suematsu, S. Arai, T. Maruyama, S. Tamura
Students: T. Okamoto, S. Sakamoto, T. Yamazaki, H. Kawashima, J.-L. Tang, H. Naitoh, T. Okumura

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 structure, such as a membrane laser, which has the Benzocyclobutene (BCB) cladding layers, enables 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 membrane DFB laser, consisting of deeply etched single-quantum-well wirelike active regions, was already demonstrated. We realized multiple- wavelength laser arrays based on a membrane buried-heterostructure distributed-feedback (BH-DFB) structure. Under optically pumped RT-CW operation, a wavelength fluctuation for a fixed DFB grating period was found to be less than ±1.2 nm for all 75 samples prepared on the same wafer. A total wavelength span of 64 nm and an average channel spacing of 3.4 nm were obtained in the DFB period modulation range of 311.25 – 335.00 nm with 1.25 nm steps. An average channel spacing of 4.1 nm was also obtained in the stripe modulation range of 1.0 – 2.8 mm with 0.2 mm steps.

(2) The lasing properties such as stripe-width dependence of index-coupling coefficient were evaluated using narrow stripe membrane BH-DFB lasers. The lowest threshold pump power was obtained with the stripe width of 1.0 mm. A single transverse-mode operation was obtained for the stripe width less than 1.0mm.

(3) Realization of high reflective cavity, asperity corrugation structure was investigated. Asperity corrugation structure was fabricated by controlling InP regrowth time. Membrane BH-DFB laser with asperity corrugation structure was realized with a cavity length of 80 mm and a threshold pump power of 1.3 mW at RT-CW condition. An index-coupling coefficient was estimated to be over 2000 cm-1.

(4) In order to realize low threshold and stable mode operation, 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 0.6mm-stripe membrane BH-DFB laser using surface corrugation. This stopband width corresponds to the index-coupling coefficient of ki=2950 cm-1, which is two times larger than conventional (flat surface) membrane laser of 2.0 mm stripe width.

(5) Wafer bonding technology was investigated to integrate active photonic devices on a silicon on insulator (SOI) wafer for highly compact photonic- integrated circuits. A single-quantum-well (SQW) GaInAsP/InP membrane structure bonded onto an SOI wafer was successfully obtained by a direct bonding method with a thermal annealing at 300-450 °C under H2 atmosphere. The PL intensity of the SQW membrane structure did not degrade after this direct bonding process and its spectral shape was not broadened.

Financial Support

1. Grant-in-Aid from the Ministry of Education, Culture, Sports, Science and Technology (MEXT), Japan.

Grant-in-Aid for Research Center for Ultra-high Speed Electronics
Grant-in-Aid for Quantum Nanoelectronics Research Center
Grant-in-aid for Nano-level foundry support, Nanotechnology Support Project
Grant-in-Aid for Scientific Research (A, B, C)
Grant-in-Aid for Exploratory Research
Grant-in-Aid for Encouragement of Young Scientists

2. Other Grant

Core Research for Evolutional Science and Technology (CREST), Japan Science and Technology Corporation
Fellowship of the JSPS for Japanese Junior Scientists
Grant-in-aid for Frequency resources development, Strategic Information and Communications R&D Promotion Scheme, Ministry of Public Management, Home Affairs, Posts and Telecommunications
Grant-in-aid for New function, minute technology (quantum, nano-technology, etc.), Strategic Information and Communications R&D Promotion Scheme, Ministry of Public Management, Home Affairs, Posts and Telecommunications
Precursory Research for Embryonic Science and Technology (PRESTO), Japan Science and Technology Corporation
Seki Memorial Foundation for the Promotion of Science and Technology

3. Companies & Others

Canon Co., Ltd.
Fujikura, Ltd.
Fujitsu Co., Ltd.
Furukawa Electric Industries Co., Ltd.
Hitachi Cable Co., Ltd.
Matsushita Electric Industrial Co., Ltd.
Taiyo Nippon Sanso Co., Ltd.
NTT Photonics Research Laboratories.
Sumitomo Electric Industries Co., Ltd.
Tosoh Finechem Co.

Nishiyama Laboratory
Quantum Nanoelectronics Research Core, Tokyo Institute of Technology

7F, S9-1, 2-12-1 O-okayama, Meguro-ku Tokyo 152-8552, Japan +81-3-5734-2555 ee.e titechnishiyama

Nishiyama lab. Student's room : South Bldg. 9 #701, #706, #707 | Measurement room : South Bldg. 9 #604, #502, #201 |
Clean room : South Bldg. 9 #202, B1F Exposure house | Research Laboratory of Ultra-High Speed Electronics