| Staffs: | Y. Suematsu | S. Arai | M. Asada | M. Watanabe | S. Tamura |
| Visiting Researcher: | G. Bacher |
| Students: | M. Tamura | T. Kojima | M. Madhan Raj | N. Serizawa | X. Y. Jia |
| T. Ando | S. Peng | H. Nakaya | K. Numata | S. Tanaka |
| N. Nunoya | S.Toyoshima | T. Numaguchi | Y. Hayafune | H. Yasumoto |
| M. Nakamura |
GaInAsP/InP strained-quantum-film, -wire, and -box lasers have been studied both theoretically and experimentally.
Results obtained in this research are as follows:
(1) Drive current of a directly modulated semiconductor laser for optical parallel interconnection was investigated by taking into account of both the threshold current and the differential quantum efficiency. As the results, it was found that the laser cavity design which yields the minimum drive current is almost the same condition to achieve the minimum threshold current whereas the cavity design for higher differential quantum efficiency is required when the total system loss is much higher. When the cavity length is 100μm and the rear mirror reflectivity Rr is 0.99, the front mirror reflectivity Rf which gives the minimum drive current was obtained to be 0.94, 0.91, and 0.8 for the total system loss of 6dB, 10dB, and 20dB, respectively.
(2) The photon recycling effect in quantum film (Q-film), quantum wire (Q-wire), and a quantum box (Q-box) has been theoretically investigated using rate equation analysis and the density-matrix method. The threshold reductions due to photon recycling in Q-film, Q-wire, and Q-box, in the case of lattice matched (LM) Ga0.47In0.53As/InP are 40%, 24%, and 0%, respectively, for a fixed cavity loss of lasing mode normalized by the optical confinement factor αL/ξL=50 cm-1. The estimation also shows that photon recycling is more effective in compressively-strained (CS) Ga0.18In0.82As0.73P0.27/InP quantum structures than in LM quantum structures. In both cases, the threshold reduction due to photon recycling is larger in the Q-film than in Q-wire and Q-box.
(3) Polarization dependence of 1.5μm wavelength GaInAsP/InP quantum-wire structures fabricated by EB lithography and two-step OMVPE growth were measured and compared with quantum-film structures fabricated on the same wafer. As the result, clear anisotropic PL properties of the quantum-wire structure corresponding to the wire width (25nm and 35nm) were confirmed. This anisotropy was observed to be a little stronger in 1% compressively-strained quantum-wire structure than that in lattice-matched one with the same wire width (35nm).
(4) Temperature dependence of GaInAsP/InP compressively-strained single-quantum-well lasers with quantum-wire (Q-Wire) size active region were measured and compared with those of quantum-film lasers. Lower threshold current as well as higher differential quantum efficiency operation of Q-Wire laser than those of Q-Film laser at a temperature below 200K were obtained. Threshold current density of Q-Wire laser was 34A/cm2 which was almost a half that of Q-Film laser being 68A/cm2 at T=90K.
(5) Gain spectra of 1.5μm wavelength GaInAsP/InP quantum-wire lasers (
=20, 25nm) were measured and compared with quantum-film lasers prepared on the same wafer. As a result, narrower gain spectra of the quantum-wire lasers were obtained at T=100K.
(6) 50nm-period GaInAsP/InP quantum-wire lasers (～20nm) were fabricated using EB lithography and wet etching followed by OMVPE embedding growth, and measured the cavity length dependence of the differential quantum efficiency. As the result, ηi～1.0 and αWG～4.5cm-1 were obtained at T=90K.
(7) Realization of room temperature operation of Multiple Microcavity laser consisting of λ/4-air gap (=0.39μm) and semiconductor reflectors was fabricated using two steps organometallic vapor-phase epitaxy (OMVPE) and two steps wet chemical etching. The threshold current was measured to be 78mA (Jth= 1.25kA/cm2) for a total cavity length 164μm (Pitch: 20μm; 8 elements) and stripe width of 40μm, and an effective power reflectivity of this cavity structure was estimated to be higher than 92%.
| Staffs: | S. Arai | | | | |
| Students: | T. Takizawa | A. Uchino | T. Shimizu | Y. Takeuchi | S. Yamazaki |
Semiconductor directional-coupler-type all-optical switching devices with tapered-shape structures were proposed and analyzed by numerical simulations. Moreover a GaInAs/InP multiple-quantum-well directional-coupler-type all-optical switch was fabricated and its low switching power operation was demonstrated.
Results obtained in this research are as follows:
(1) A semiconductor directional-coupler-type all-optical switch with tapered-shape structures, based on the nonlinear refractive index variation induced by photoexcited carriers, was proposed and its taper-shape dependence was investigated. By the numerical simulation, it is shown that an introduction of tapered-shape structures is effective for the improvement of the extinction ratio. It is also found that this device can drive the signal light of about 17dB higher power than the switching power.
(2) A GaInAs/InP multiple-quantum-well (MQW) directional-coupler-type all-optical switch utilizing the carrier-induced nonlinearity was fabricated and its low switching power operation was demonstrated at 1.55μm wavelength. Extinction ratios at each output port of 2.3 dB and 2.6 dB were obtained with low switching light power of 5.9 mW at the input fiber end.
(3) Multiple-quantum-well directional-coupler-type all-optical switches having various waveguide widths were fabricated using CH4/H2 RIE dry etching and FIB (Focused Ion Beam) techniques. Comparison between the theory and experimental results were carried out by Crosstalk measurement on the waveguide. The result shows that the permissible fabrication error of the waveguide should be less than ±160nm and also a high value of Crosstalk up to 29dB were experimentally confirmed.
| Staffs: | K. Furuya | Y. Miyamoto | M. Suhara | S. Tamura | |
| Visiting Researcher: | F.Vazquez |
| Research Student: | B. Hansson |
| Students: | H. Hongo | N. Kikegawa | N. Machida | D. Kobayashi | R. Takemura |
| J. Yoshinaga | H. Honji | T. Oobo | H. Toda | T. Hattori |
| N. Matsumoto | T. Arai | Y. Ikeda | A. Kokubo | K. Ooshima |
| K. Sato | H. Nakamura | A. Yamaguchi | H. Goto | M. Kurahashi |
| M. Nakamura | H. Tobita | E. Zhang | X. X. Zhou |
Ballistic transport of hot electron has a possibility of new high-speed devices using wave property of electron. We studied wave properties of hot electron for new principle of electron devices.
Results obtained in this research are as follows:
(1) OMVPE grown resonant tunneling diodes (RTD) were studied towards the evaluation of phase coherent length of hot electrons in semiconductors. We demonstrated a possibility of high temperature estimation of the coherent length from I-V characteristics in appropriately designed GaInAs/InP triple barrier RTDs. The coherent length was tentatively evaluated as 90nm and 55nm at 4K and 77K, respectively. Effect of heterointerface roughness and/or remote impurity on resonant energy width was investigated by using GaInAs/InP double barrier RTDs and theoretically analyzed.
(2) We have proposed Scanning Hot Electron Microscopy (SHEM) which is the technique of the hot electron observation. After demonstrating the theoretical possibility of SHEM, we could obtain the hot electron current of 4pA. It is necessary for the observation of the spatial distribution to detect such low hot electron current. Therefore noise reduction is required. We clarified the noise property. Moreover the noise can be reduced.
(3) Sub-micron hot electron transistors (HETs) by electron beam lithography were fabricated. The key process to fabricate small HETs is the small opening of the polyimide layer by dry-etching with bi-layer (PMMA/LS-SOG) resist. In this etching, removals of scum by slight wet etching and release of stress are essential to make a small opening (160 x 350 nm2). The smallest emitter size of fabricated HET was (0.3x1.5+0.6x1)μm2. The size dependence of current gain in HETs is also studied.
| Staffs: | M. Asada | M. Watanabe | | | |
| Students: | Y. Kohno | W. Saitoh | K. Mori | H. Sugiura | K. Yamazaki |
| K. Osada | M. Tsutsui | K. Yoshida | A. Itoh | T. Sugiyama |
| T. Funayama | K. Okamoto |
Superlattices and ultrathin layers with the combination of metal and insulator were proposed as one of the candidates of the material for ultrahigh-speed electronic devices and optical devices because of the low resistivity of metals, low dielectric constant and wide band gap of insulators and high conduction band offset at metal/insulator heterointerface. A novel transistor using quantum interference in metal/insulator heterostructure has been proposed and it was shown theoretically that sub-pico second response could be expected in such devices.
Results obtained up to now are as follows:
(1) A very short channel tunneling field effect transistor using CoSi2/Si/CdF2/CaF2 heterostructures lattice-matched to Si substrate was proposed. Theoretical analysis shows that this transistor has characteristics similar to those of conventional MOS-FET even with the channel length of 5 nm and the possibility of subpicosecond response.
(2) A channel coupling tunneling field effect transistor with CoSi2/Si/CdF2/CaF2 heterostructures lattice-matched to Si substrate, was proposed and analyzed. This device has a negative differential resistance with the gate length of 20 nm at room temperature.
(3) Small area (1μm-diam.) metal/insulator resonant tunneling diode applicable to terahertz device was fabricated. The diode integrated with a planar patch antenna and a microlens is proposed for the observation of the terahertz photon assisted tunneling under relatively low power radiation. Negative differential resistance was observed at room temperature with the peak-to-valley ratio of ～2.8.
| Staffs: | M. Watanabe | | | | |
| Students: | T. Matsunuma | Y. Aoki | A. Yamada | T. Maruyama | Y. Isaka |
| Y. Maeda | J. Nishiyama | K. Osada | M. Tsuganezawa | Y. Iketani |
| S. Ikeda |
New type of light emitting devices on Si substrate are investigated using nanometer-size super-heterostructures, such as nanocrystal silicon, ZnO embedded in single crystal insulator (CaF2) formed on Si (111). And also, intersubband quantum cascade lasers using CaF2-CdF2 superlattices has been proposed and analyzed theoretically. Using these materials, strong quantum confinement can be expected and that may lead fascinating changes in the optical properties relative to those of the 3D bulk material. Crystal growth and device fabrication technique has been also developed.
Results obtained in this research are as follows:
(1) As a first step to achieve intersubband cascade lasers, CaF2-CdF2 triple barrier resonant tunneling diode structure has been fabricated. In I-V characteristics, negative differential resistance (NDR) was clearly observed even at room temperature. Maximum P/V ratio was around 6 (at 300K) among the I-V curves which was reproduced more than twice. NDR bias voltage agreed well with theoretical prediction.
(2) Intensity of visible photoluminescence (PL) from nanocrystal silicon (nc-Si) embedded in CaF2 has been increased using ex situ rapid thermal annealing (RTA) process. Uniformity of luminescence was also improved. Growth condition of nc-Si and RTA condition dependence (temperature, time, and atmosphere) was clarified.
(3) Formation technique of epitaxial ZnO nanocrystals in single crystalline CaF2 on Si (111) substrate has been developed using RF sputtering for ZnO, CaF2 regrowth and in situ annealing. Epitaxial growth of ZnO nanocrystals on CaF2 (111) were confirmed by TEM lattice image and size effect was clearly observed in photoluminescence (PL) peak shift. PL intensity at 380nm (～Eg) was improved by annealing in vacuum (<10-8Torr). Growth condition and annealing temperature dependence of PL spectra were clarified.
| Staffs: | K. Furuya | S. Arai | Y. Miyamoto | M. Watanabe | M. Suhara |
| S. Tamura |
| Visiting Researcher: | G. Bacher |
| Students: | H.Hongo | M. Tamura | T. Kojima | H. Honji | X. Y. Jia |
| T. Ando | H. Hattori | H. Nakaya | A. Kokubo | S. Tanaka |
| N. Nunoya | K.Sato | Y. Hayafune | H. Nakamura | H. Yasumoto |
| M. Nakamura | M.Nakamura | E. Zhang | | |
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) Sidewall recombination velocity of GaInAsP/InP wire structures fabricated by Cl2/H2-ECR-RIBE was estimated from PL intensity dependence on the wire width. As the result, a reduction of surface damage was observed.
(2) In EB lithography, the size distribution of dense multiple wire patterns (period: 50 nm) formed on EB-resist (ZEP-520) was measured and its deviation was reduced by proximity effect correction.
(3) A 25-nm pitch GaInAs/InP buried structure was fabricated by electron beam lithography with Calixarene resist and two-step wet chemical etching. The fabrication of 20nm pitch would be possible by reduction of stage instability of system that is in progress.
(4) Si delta-doping by organometallic vapor phase epitaxy (OMVPE) onequidistant monolayer steps was studied towards the dopant-ordering in semiconductors. Growth condition conserving monolayer steps after the delta-doping was investigated in InP with respect to disilane flow and growth temperature. Transport anisotropy in the delta-doped layer is studied by measuring sheet resistance along parallel and perpendicular to the monolayer step.
(5) Si delta-doping by organometallic vapor phase epitaxy (OMVPE) onequidistant monolayer steps was studied towards the dopant-ordering in semiconductors. Growth condition conserving monolayer steps after the delta-doping was investigated in InP with respect to disilane flow and growth temperature. Transport anisotropy in the delta-doped layer is studied by measuring sheet resistance along parallel and perpendicular to the monolayer step.