Activities
Quantum-Film, Quantum-Wire, and Quantum-Box Lasers

Staffs:　Y. Suematsu　S. Arai　S. Tamura
Visiting Researcher:　Jong-In Shim
Post-Doctoral Research Fellow:　Q. Yang　R. Winterhoff　Bo Chen
Students:　T. Kojima　N. Nunoya　S. Tanaka　M. Nakamura　H. Yasumoto　I. Fukushi　M. Morshed　H. Midorikawa　K. Fukuda
　
GaInAsP/InP strained-quantum-film, -wire, and -box lasers have been studied both theoretically and experimentally. Distributed feedback (DFB) lasers with wirelike active regions fabricated by the same fabrication process as Quantum-Wire lasers have been also studied.
Results obtained in this research are as follows:
　
(1) The temperature deoendence of the internal quantum efficiency of 1.5-mm-wavelength GaInAsP/InP compressively strained 20-nm-wide quantum-wire lasers was evaluated from the cavity length dependence of the differential quantum efficiency. As a result, high internal quantum efficiency hi ~ 1.0 was obtained at T < 200 K, which decreased with an increase temperature.

(2) The gain spectral characteristics of 1.5-mm-wavelength GaInAsP/InP compressively quantum-wire lasers with wire widths of 20 nm and 25 nm, fabricated by electron-beam lithography and two-step organometallic vapor phase epitaxial (OMVPE) growth, were measured at a temperature of 100 K and were compared with those of quantum-film lasers fabricated on the same wafer. It was found, for the first time, that the material gain spectrum of quantum-wire lasers was theoretically investigated and explained in terms of the twofold longer intraband relaxation time in the quantum-wire structure.

(3) Low damage GaInAsP/InP narrow wire structures with vertical mesa shape were realized by CH4/H2 reactive ion etching (RIE) followed by a slight wet chemical etching and an embedding growth by OMVPE. By using this fabrication process, a threshold current density as low as 330A/cm2 (66A/cm2/well, @L=860mm) was obtained for 1.55mm wavelength five-quantum-well DFB laser consisting of periodic wire active regions. It was the lowest value reported for 1.55mm GaInAsP/InP DFB lasers fabricated by the dry etching process at that time.

(4) GaInAsP/InP multiple-layered quantum-wire lasers with the wire width of 21nm in the period of 100nm were realized by CH4/H2-RIE followed by slight wet chemical etching and embedding growth by OMVPE. A threshold current density as low as 1.45kA/cm2 was obtained with the cavity length of 980mm. To our knowledge, this is the lowest value reported for 1.55mm GaInAsP/InP quantum wire lasers fabricated by the etching and regrowth method. Because of the temperature dependence of the lasing wavelength, a relatively large blue shift of 47meV in the quantum-wire laser was observed, which can be attributed to not only a lateral quantum confinement effect but also a three-dimensional compressive strain effect. Finally, we improved the initial wafer structure in order to suppress over-etching of the active region, and obtained lasers consisting of a five-layered wirelike active region with good size uniformity (wire width of 42nm, period of 120nm). A threshold current density as low as 540A/cm2 was obtained with the cavity length of 1.38mm.

(5) In order to reduce non-radiative recombinations due to a large lattice mismatch at the etched/regrowth interfaces, 1.5mm GaInAsP/InP lasers with narrow wirelike (42nm) active regions, which consist of partially strain-compensated 5MQW structure, were realized for the first time. As the result, lower threshold current density (318/cm2) than that of planar 5MQW lasers (550A/cm2) was obtained at room temperature.

(6) Low threshold 1.5mm wavelength GaInAsP/InP double-quantum-well DFB lasers with deeply etched active regions were successfully obtained by EB lithography, CH4/H2-RIE, and OMVPE regrowth. A record low threshold current density of 94A/cm2 was obtained for a cavity length of 600mm and the mesa-stripe width of 19.5mm, while threshold current of 7mA was obtained for a cavity length of 280 mm. The threshold current dependence on both the cavity length and the active region width well agreed with theoretical results.

(7) A submiliampare operaton of 1.55mm GaInAsP/InP DFB lasers with deeply etched wire-like active regions was successfully obtained. Threshold current of as low as Ith = 0.7mA (Jth = 150A/cm2) was obtained with a stable single-mode operation for the cavity length of 200mm and the stripe width of 2.3mm.
　
New Types of Semiconductor Lasers for Photonic Integration
Staffs:　Y. Suematsu　S. Arai　Y. Miyamoto　S. Tamura
Students:　M. Madhan Raj　J. Wiedmann　S. Toyoshima　Y. Saka　H. Yasumoto　K. Matsui　K. Ebihara　M. Oyake　T. Okamoto　A. Umeshima
　
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 Multiple-Micro-Cavity (MMC) lasers, Deeply Etched Distributed Bragg Reflector (DBR) lasers and Couple Cavity lasers with corrugated active region have been studied both theoretically and experimentally.

Results obtained in this research are as follows:
　
(1) Multiple Micro-Cavity (MMC) lasers consisting of narrow and deep grooves buried with a Benzocyclobutene (BCB) polymer, were realized by the CH4/H2-reactive ion etching (RIE) process. A threshold current of 30mA was obtained at 200K for the micro-cavity length LH = 5.1 µm (groove width LL = 183nm, pitch L = 5.3mm, total cavity length L = 300mm, stripe width Ws = 5mm) while also showing a stable single-wavelength operation. Room temperature operation of an MMC laser consisting of 3l/4-BCB buried grooves (= 0.70µm) was also obtained with a threshold current as low as 18mA for the total cavity length of 200mm (L = 20 mm, 10 elements, Ws = 5mm), and the effective reflectivity of the MMC structure was estimated to be 94 %.

(2) Low temperature operation of l/4-groove (filled with BCB) MMC laser was achieved. For a temperature range of 100K to 150K, the threshold current as low as 10mA to 16mA (L=200mm, pitch L=20mm, =0.23mm,  and Ws = 5mm) was obtained. A stable single-mode operation was confirmed for a wide temperature range (100K to 200K) with the temperature coefficient of 0.06nm/K.

(3) A narrow vertical groove with high aspect ratio was fabricated using Electron Beam (EB) lithography and CH4/H2-RIE followed by O2 ashing. The groove width lL and the facet angle were measured to be 147nm and 0.3°, respectively. The groove depth was 2.6mm and an aspect ratio reached to 17.7. The roughness of the etched facet was measured using a field emission electron probe surface roughness analyzer and found to be same as cleaved.

(4) 1.5mm wavelength GaInAsP/InP lasers with high reflectivity deeply etched DBR was demonstrated. The reflectivity was estimated to be 90 % from the measurement of the threshold current dependence on the cavity length and the output power ratio from the front to the rear facets. And also room temperature CW operation of this deeply etched third-order Bragg reflector lasers was obtained for the first time. A threshold current of 13.5mA and differential quantum efficiency of 28% for a cavity length of 330mm and a stripe width of 5mm was demonstrated.

(5) Highly uniform 1.55mm wavelength GaInAsP lasers with high reflective deeply etched semiconductor/Benzocyclobutene (BCB) DBR showing low threshold current as low as 7.2mA and high differential quantum efficiency 50% from the front cleaved facet of 160mm-long DBR lasers with 15-DBR reflectors (5l/4-thick semiconductor with 3l/4-BCB groove) on the rear side were successfully obtained with high uniformity. A double sided-DBR laser having 15-DBRs on the rear and 3-DBRs on the front side of the cavity, fabricated by the same method showed a threshold current as low as 5.2mA for a active length of La=100mm and a stripe width of 5mm.

(6) A new type of a single-mode laser consisting of a deeply etched DBR and a coupled-cavity was fabricated. A threshold current under room temperature CW condition as low as 11mA (LC1 = 150mm and LC2 = 40mm) with a sub-mode suppression ratio of 36 dB (at I = 1.8Ith) was achieved for a 5mm wide stripe laser.
　
Processing for Nanometer Structures
Staffs: K. Furuya S. Arai Y. Miyamoto M. Watanabe M. Suhara S. Tamura

Students: W. Saito T. Kojima T. Arai A.Kokubo K. Sato S. Tanaka N. Nunoya A. Ito M. Kurahashi H. Tobita Y. Harada M.Nakamura H. Yasumoto H. Oguchi S. Karasawa M. Saito I. Fukushi M. Morshed S. Yamagami Y. Okuda K. Sato K. Fukuda R. Yamamoto
　

Study of nanometer structure fabrication technology is important for the realization of quantum effect devices such as quantum-wire, or -box devices and ballistic electron devices based on wave characteristics of electrons.

Results obtained in this research are as follows:

　
(1) The etching damage induced by electron-cycrotron-resonance reactive- ion-etching (ECR-RIBE) using pure Cl2 gas and Cl2/H2 mixture gas was characterized by photoluminescence (PL) intensity at 77K of GaInAs/InP heterostructure. As a result, it was indicated that nonradiative recombination traps induced by ECR-RIBE were reduced to one order of magnitude smaller by adopting a negative bias voltage to the sample, Cl2/H2 mixture gas rather than pure Cl2 gas and a lower substrate temperature.

(2) GaInAsP wire structures of various sizes were fabricated by electron-beam lithography, CH4/H2 reactive ion etching and organometallic vapor phase epitaxy (OMVPE) embedding growth and their photoluminescence intensity dependence on the wire width was measured. As the result, the product of etched sidewall recombination velocity and carrier lifetime (S・t) was estimated to be about 30 nm at room temperature.

(3) Fabrication techniques for 80-100-nm-period fine electrodes with 30-40 nm thicknesses were developed. To obtain a resist pattern suitable for the lift-off process, we used a double-layer resist with ZEP-520 and PMMA. The mixing of C60 into both layers and rinsing by perfluorohexane prevented pattern collapse. As a result, a Au/Cr pattern with 80-nm period over 30 nm steps was obtained.

(4) Toward nano-metal buried in InP structure, fabrication process of nano-tungsten wire and InP buried growth of tungsten stripes were studied. Tungsten wire with 20 nm width was formed by a novel metal-stencil liftoff. Tungsten stripes with 1 µm width and 2 µm pitch were embedded with flat InP layer of 1.1 µm thickness.