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

Staffs: Y. Suematsu S. Arai M. Asada M. Watanabe S. Tamura

Visiting Researcher: Q. Yang

Students: M. Tamura T. Kojima T. Ando H. Nakaya N. Nunoya S. Tanaka M. Nakamura H. Yasumoto I. Fukushi M. Morshed

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) 1.5μm-wavelength GaInAsP/InP quantum-wire lasers with the wire width of 20nm in 50nm period and 25nm in 70nm period were fabricated by electron-beam lithography, wet-chemical etching and 2-step organometallic vapor phase epitaxial growth. Temperature dependencies of various characteristics of these lasers were measured and compared with those of quantum-film lasers fabricated on the same wafer. As a result, better lasing properties of the quantum-wire laser over the quantum-film laser, i.e., lower threshold current and higher differential quantum efficiency operation, were confirmed for the first time at temperatures below 200K. An internal quantum efficiency of the quantum-wire laser was evaluated to be almost 100% up to 200K from the cavity length dependence of differential quantum efficiency. The possibility of a complex-coupled distributed feedback laser consisting of quantum-wire active region as the grating structure was also confirmed.

(2) Anisotropic polarization properties of photoluminescence intensity from GaInAsP/InP quantum-wire structures due to the lateral quantum confinement effect were observed. Furthermore, gain spectra of quantum-wire lasers as well as quantum-film lasers were measured. As the result, narrower material gain spectrum of quantum-wire lasers was observed at T=100K.

(3) Synchrotron x-ray diffractometry has been used to investigate GaInAsP quantum-wire structures on InP with a quantum-well layer between the substrate and the wire. The lateral periodicity was determined with high accuracy. An elastic stress relaxation, which occurs near the free surface of the sidewalls, was observed. It results in deformation gradients in the wires, which influence the distribution of the diffracted intensity in reciprocal space.

(4) 1.5μm-wavelength GaInAsP/InP multiple-quantum-wire (2-layers) lasers with the wire width of 20nm in 100nm period were fabricated by using CH4/H2 RIE dry etching and wet-chemical cleaning process. Fundamental lasing characteristics of these lasers were almost the same as those fabricated by wet-chemical etching process.

New Types of Semiconductor Lasers

Staffs: Y. Suematsu S. Arai Y. Miyamoto S. Tamura

Students: M. Madhan Raj T. Kojima J. Wiedmann K. Numata S. Peng S. Toyoshima N. Nunoya S. Tanaka Y. Saka M. Nakamura H. Yasumoto K. Matsui M. Oyake I. Fukushi

　

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 and Distributed Feedback (DFB) lasers with corrugated active region, have been studied both theoretically and experimentally.

Results obtained in this research are as follows:

  1. A room temperature operation of an MMC laser consisting of λ/4-air gap (=0.39μm) and semiconductor reflectors was fabricated using two steps organometallic vapour-phase epitaxy (OMVPE) and two steps wet chemical etching. The threshold current as low as 78mA (Jth= 1.25kA/cm2) was obtained for a total cavity length 164μm (PitchΛ:20μm; 8 elements) and stripe width of 40μm. The effective power reflectivity of this cavity structure was estimated to be higher than 92%.

  2. A room temperature operation of an MMC laser having a groove width of 3λ/4 (=0.70μm, filled with BCB) was fabricated using CH4/H2-RIE process. The threshold current was measured to be as low as 18mA for a total cavity length of L=200μm (pitch: Λ=20λm; 10 elements) for a stripe width of Ws=5μm, and effective reflectivity was estimated to be 94%.

  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 0.3o, respectively. The groove depth was 2.6μm 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. Low temperature operation of λ/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=200μm, pitch Λ=20μm, =0.23μm, and Ws=5μm) 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.

  5. Fairly low threshold current density operation (330A/cm2) of 1.55μm wavelength GaInAsP/InP MQW-DFB (5-quantum-well) lasers with rectangular-shaped periodic active regions was achieved by CH4/H2 RIE and OMVPE regrowth process.

Semiconductor Photonic Devices

Staffs: Y. Suematsu S. Arai

Students: T. Shimizu S. Yamazaki

Semiconductor directional-coupler type optical switching device was investigated both theoretically and experimentally.

Results obtained in this research is as follows:

  1. Multiple-quantum-well directional-coupler-type all optical switches were fabricated by CH4/H2 RIE and FIB techniques. From the measurement of cross-talk characteristics of devices with various waveguide width, a permissible fabrication error of the width was obtained to be less than 160nm. A cross-talk up to 29dB was obtained with this device.

Quantum Coherent Electron Devices

Staffs: K. Furuya Y. Miyamoto M. Suhara S. Tamura

Visiting Researcher: B. ZHANG

Research Student: B. Hansson

Students: N. Kikegawa N. Machida M. Nagasei T. Oobo H. Toda T. Hattori N. Matsumoto T. Arai Y. Ikeda A. Kokubo K. Ooshima K. Sato H. Goto Y. Harada M. Kurahashi M. Nakamura N. Sakai 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) GaInAs/InP triple-barrier resonant-tunneling diodes (TBRTD) were studied towards the evaluation of phase coherence of hot electrons in semiconductors. We elucidated theoretically relationships between phase relaxation times and voltage widths in I-V characteristics of TBRTDs by using non-equilibrium Green function formalism. Experimental results showed that phase coherence was dependent on temperature.

(2) The Scanning Hot Electron Microscope (SHEM) is a tool to observe non-thermal-equibilrium electrons under the surface of the solid and enables us to study the hot electron diffraction pattern caused by a small structure in the propagation layer. To observe the hot electron spatial distribution by SHEM, the detection time should be shortened. The reduction of the noise current including the vibration noise is investigated comprehensively. Using these, in particular, the non-stationary noise reduction technique with the digital measurement, the hot electron current was detected in 30 s, 1/20 of the measurement time reported before.

(3) We proposed a new type of solid state electronic device with an operation principle similar to the electron beam biprism in vacuum. This device can primarily be used for studying the basic wave nature of hot electrons and in particular the wavefront spread. A simple theoretical analysis of the device was performed and the results displayed an interference pattern with regular spacings, which was the main characteristic of an electron beam biprism in vacuum. This result indicated that, in principle, the interference phenomena of hot electrons caused by biprism effects in solid state could be observed.

(4) Deflectron as sub-milimeter-wave amplifier was analyzed theoretically. As obstruction of the beam for modulation has a possibility of fatal damage, modulation is carried out by change of distance between parallel plates and electron beam. At sub-milimeter-wave region, large attenuation of waveguide limited length of waveguide and total current. When current density of 2 kA/square cm at 50 kV and 1 THz as modulated frequency were assumed, handling power of a deflectron was 275 mW, and power gain was 10.

(5) GaInAs/AlAs/InP resonant tunneling diodes with three different barrier thicknesses (3.5, 5.3, and 7 nm) were fabricated by metalorganic vapor phase epitaxy and the barrier thickness dependence of the peak current density was measured. The range of peak current was from 100 A/cm2 to 0.1 A/cm2. In the measurement of peak current density distribution, the deviations of peak current density became larger when the barrier became thicker. This fluctuation of peak current density can be explained by the thickness fluctuation of the barrier in the wafer's millimeter range.

High-Speed Electron Devices Using Nanometer-thick Metal/Insulator

Layered Heterostructures

Staffs: M. Asada M. Watanabe

Students: Y. Kohno W. Saitoh H. Sugiura K. Yamazaki

M. Tsutsui K. Yoshida Y. Iketani A. Itoh

Y. Oguma T. Sugiyama T. Funayama K. Hoshina

N. Shashinaka S. Yamagami

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 can be expected in such devices.

Results obtained up to now are as follows:

  1. One-hundred-nanometre-long channel field effect transistors using CdF2/CaF2 heterostructures on a Si substrate were fabricated by molecular beam epitaxy growth and electron beam lithography. In these transistors, the gate was a highly doped Si substrate, the gate insulator was a 5-nm-thick layer of CaF2, the channel was a 5-nm-thick layer of CdF2, and the source and drain were Au/Cr. Transistor action was observed at 16K. The drain current and the transconductance were 16nA and 28nS, respectively. The operating voltage was high and the drain current was very small due to the high potential barrier of the Schottky contact at the source and drain electrodes.

  2. The Schottky source/drain MOSFET has the potential for scaling into the nanometer regime, because low electrode resistances with very shallow extension can be realized using metal source/drains. Very short channel Schottky source/drain MOSFETs with SOI structure were analyzed theoretically and n-type devices were demonstrated experimentally. It is shown theoretically that drivability of Schottky source/drain MOSFET comparable to conventional MOSFETs can be realized by low Schottky barrier height. The short channel effect can be suppressed even with a 15nm-long channel at 1nm-thick gate oxide and 3nm-thick SOI layer. The room temperature operation of sub 50nm n-type ErSi2 Schottky source/drain MOSFETs on a SIMOX substrate was demonstrated.

  3. 35nm metal gate p-type MOSFETs with PtSi Schottky source/drain were fabricated on a SIMOX substrate by electron beam lithography and self-aligned silicide process. The drain current was ?176μA/μm and the transconductance was 390mS/mm at VDS = VGS = -1.5V. For these devices, comparable drivability to conventional MOSFETs was achieved. The on/off ratio was improved by using a very thin SOI structure.

  4. Terahertz detection properties were studied for resonant tunneling　structures integrated with planar patch antenna. Gradual change　from classical square-law detection to photon-assisted tunneling was　observed with increasing irradiated photon energy in triple-barrier　resonant tunneling diodes.

Light Emitting Devices Using semiconductor(Metal)/Insulator

Nanostructures

Staffs: M. Watanabe

Students: Y. Aoki A. Yamada T. Maruyama Y. Maeda K. Osada

M. Tsuganezawa T. Funayama, Y. Iketani S. Ikeda

S. Okano, D. Kuruma, N. Nakamura

New type of light emitting devices on Si substrate are investigated using 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 and CaF2-Si superlattices have been proposed and analyzed theoretically. At the heterointerface of these materials, strong quantum confinement can be expected because of the large conduction band discontinuity. Crystal growth and device fabrication technique has been also studied.

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).

  2. Intensity and uniformity of visible photoluminescence (PL) from nanocrystalline silicon (nc-Si) embedded in CaF2 has been dramatically improved using ex situ rapid thermal annealing (RTA). Electroluminescence has been also demonstrated using the current constriction structure fabricated by photolithography and wet chemica etching.

  3. Formation technique of epitaxial ZnO nanocrystals in single crystalline CaF2 on Si(111) substrate has been demonstrated using RF sputtering for ZnO, CaF2 regrowth and annealing in ultra high vacuum. 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 390nm (~Eg) was improved by annealing in vacuum (<10-8 Torr). Electroluminescence from band edge has been clearly observed at room temperature.

Processing for Nanometer Structures

Staffs: K. Furuya S. Arai M. Asada Y. Miyamoto M. Watanabe M. Suhara S. Tamura

Students: M. Tamura T. Kojima T. Ando H. Hattori T. Arai A. Kokubo N. Nunoya K. Sato Y. Harada M. Kurahashi M. Nakamura H. Tobita E. Zhang I. Fukushi S. Karasawa H. Oguchi

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 device based on wave characteristics of electrons.

Results obtained in this research are as follows:

  1. A 25 nm pitch InP pattern formed by using calixarene was buried in a GaInAs structure by organometallic vapor phase epitaxy (OMVPE). The introduction of tertiarybutylphosphine as the phosphorus source prevented from deforming the fine structure when the temperature was raised and a 25 nm pitch periodic structure was buried successfully.

  2. GaInAsP/InP wire structures was fabricated by CH4/H2-reactive ion etching and OMVPE regrowth. A product of sidewall recombination velocity and carrier lifetime was estimated from photoluminescence intensity dependence on the wire width. Low damage nano-structure fabrication was realized.

  3. The nonradiative recombination velocity at the sidewall of GaInAsP/InP quantum-well lasers with narrow wire-like active region, which were fabricated by wet etching and OMVPE regrowth, was estimated from the active region width dependence of spontaneous emission efficiency.

  4. To fabricate fine refractory metal structure, metal-stencil liftoff , in which gold/chromium and SiO2 replace conventional resist to prevent thermal deformation in liftoff process and 20 nm width tungsten wire was fabricated by proposed metal-stencil liftoff.

  5. For buried OMVPE growth of tungsten wires, growth conditions of OMVPE were studied. As a result, tungsten stripes with 1 ?m width could be buried by 1 ?m thick InP layer without void.

  6. To fabricate fine periodic electrode by lift-off, new double-layer resist consisting of ZEP-520 incorporating C60 and PMMA was developed and Au/Cr electrode with 80 nm width and 40 nm height was formed on InP substrate.