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 330 A/cm2 (66 A/cm2/well, @L=860 mm) was obtained for 1.55 mm wavelength five-quantum-well DFB laser consisting of periodic wire active regions. It was the lowest value reported for 1.55 mm 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 21 nm in the period of 100 nm were realized by CH4/H2-RIE followed by slight wet chemical etching and embedding growth by OMVPE. A threshold current density as low as 1.45 kA/cm2 was obtained with the cavity length of 980 mm. To our knowledge, this is the lowest value reported for 1.55 mm 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 47 meV 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 42 nm, period of 120 nm). A threshold current density as low as 540 A/cm2 was obtained with the cavity length of 1.38 mm. 

(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 (550 A/cm2) was obtained at room temperature.

(6)      Low threshold 1.5 mm 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 94 A/cm2 was obtained for a cavity length of 600 mm and the mesa-stripe width of 19.5 mm, while threshold current of 7 mA 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 1.55 mm GaInAsP/InP DFB lasers with deeply etched wire-like active regions was successfully obtained. Threshold current of as low as Ith = 0.7 mA (Jth = 150A/cm2) was obtained with a stable single-mode operation for the cavity length of 200 mm and the stripe width of 2.3 mm.

 

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. Yasumot                    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 30 mA was obtained at 200 K for the micro-cavity length lH=5.1 mm (groove width lL=183 nm, pitch L=5.3 mm, total cavity length L=300 mm, stripe width Ws=5 mm) while also showing a stable single-wavelength operation. Room temperature operation of an MMC laser consisting of 3l/4-BCB buried grooves (=0.70 mm) was also obtained with a threshold current as low as 18 mA for the total cavity length of 200 mm (L=20 mm, 10 elements, Ws=5 mm), 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 0.3o, 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)  1.5 mm 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 13.5 mA and differential quantum efficiency of 28% for a cavity length of 330 mm and a stripe width of 5mm was demonstrated.

(5)  Highly uniform 1.55 mm wavelength GaInAsP lasers with high reflective deeply etched semiconductor/Benzocyclobutene (BCB) DBR showing low threshold current as low as 7.2 mA and high differential quantum efficiency 50% from the front cleaved facet of 160 mm-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 5.2 mA for a active length of La=100 mm and a stripe width of 5 mm.

(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 11 mA (LC1 =150 mm and LC2 =40 mm) with a sub-mode suppression ratio of 36 dB (at I = 1.8 Ith) was achieved for a 5 mm wide stripe laser.

Quantum Coherent Electron Devices

 

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

Visiting Researcher:               B. Zhang

Students:     N. Kikegawa        N. Machida     M. Nagase         T. Arai                  Y.Ikeda               K. Ooshima          A. Kokubo             K. Satoh                M.Kurahashi               N.Sakai               H. Inazawa      H. Oguchi         S. Karasawa          M. Shioyama       M.Kurita              T. Konishi              K. Satoh                H.Tamura               R. Yamamoto

 

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) The solid-state biprism is solid-state version of vacuum biprism that have been realized, and is also proposed as a test ground for electron interference observation and a new conceptual device. Toward realization of the device, we have formulated and analyzed electron incoherence effects on the characteristics of solid-state biprism devices. It was shown that contrasts of interference fringes were largely degraded by the effect of increasing Fermi energy and we proposed a detecting method that could overcome this difficulty. On the other hand, using the state-of-the-art fabrication technology could reduce effects due to relaxation in metal wire and LO-phonon scattering in semiconductor active region.

(2) A new type of resonant-tunneling transistor with a tungsten grating placed 30nm above a GaAs/GaInP double-barrier resonant-tunneling semiconductor heterostructure has been fabricated by epitaxial overgrowth technique. The Schottky depletion around the buried metal contacts controls the current to a vertical transistor channel. The lateral extension of this channel is defined by a square opening in the grating with a side length of 1.4 mm, which corresponds to a sub-mm electrical width. The transport properties at 20 K showed a fine structure in the resonant tunneling characteristics, and it was affected by the gate bias. These effects were discussed in terms of lateral quantum confinement in the transistor channel defined.

(3) Scanning Hot Electron Microscopy (SHEM), which can be used to study the hot electron transmission mechanisms under the sample surface, has been set up in our lab. Varied hot electron emitters with metal/insulator/semiconductor (MIS) or semiconductor heterostructure structures are designed and fabricated. The hot electron signal for a single point is detected with the non-stationary noise reduction technique in the SHEM experiment. The transmission mechanisms of the hot electron under the sample surface and through the air gap areas have been analyzed and compared between the theory and experiment. Also the detection sensitivity and the spatial resolution are theoretically studied constructing a model of the SHEM process.

(4)    Current-Voltage (I-V) characteristics of triple-barrier resonant-tunneling diodes (TBRTD) were studied both theoretically and experimentally toward evaluation of phase coherence of hot electrons in semiconductors. Using non-equilibrium Green’s function formalism theoretically revealed relationship between phase relaxation time and peak width in I-V characteristics of TBRTD. Furthermore, we also developed a theory which could take effects of structural inhomogeneity on peak width into account. We estimated that the phase relaxation time was larger than 0.2 ps and the fluctuation of well width in TBRTD grown by metalorganic vapor phase epitaxy was a few atomic layers.

 

High-Speed Electron Devices Using Advanced Structures and Materials

 

Staffs:     K. Furuya              M. Asada Y. Miyamoto           M. Watanabe         

M. Suhara              S. Tamura

Students:         W. Saitoh            T.Arai               K.Ooshima        M.Tsutsui        K. Yoshida         Y.Harada   H.Tobita  Y. Iketani               A. Itoh    Y. Oguma

                     M.Kurita             S.Yamagami      M.Saitoh           K. Hoshina      N. Shashinaka    H.Inazawa  Y.Okuda  K.Kobayashi           Y.Kontani

 

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. A novel approach for improvement of high performance of heterojunction bipolar transistor has been proposed. In this device, metal buried in semiconductor structure could achieve high speed operation.

Results obtained up to now are as follows:

 

(1)    Room-temperature negative differential resistance (NDR) was observed in Si/CaF2 double-barrier resonant tunneling diodes fabricated by molecular beam epitaxy on Si substrate. The devices consist of Au/ CaF2(amorphous)/Si(amorphous)/CaF2(crystalline)/n+-Si(111)(substrate). In spite of the fact that the well layer (Si) and one of the barrier layers (CaF2) were amorphous, their interface was well distinguished in a transmission electron microscope image, and clear NDR was obtained with the peak-to-valley current ratio of 3.10 for a sample with a 2.8-nm-thick Si well. The voltage at the peak current varies with the thickness of the Si well, as predicted theoretically.

(2)    The Schottky source/drain (S/D) MOSFET has the potential for scaling into the nanometer regime, because low electrode resistances with very shallow extension can be realized using metal source/drain. Very short channel schottky S/D MOSFET with SOI structure were analyzed theoretically. The short channel effect can be suppressed even with a 15 nm-long channel at tOX = 1 nm and tSOI = 3 nm. The very short channel devices with metal gate (ErSi2 for n-type, and PtSi for p-type) were fabricated on SIMOX substrate, and the room temperature operation of sub 50nm devices was demonstrated. For 35 nm-long gate p-type devices, comparable drivability to conventional MOSFETs was archived with the drain current of -176 mA/mm and the transconductance of 390mS/mm at VDS = VGS = -1.5 V. The on/off ration was improved by using a very thin SOI structure.

(3)    Current change under irradiation of a terahertz electromagnetic wave was measured for a triple-barrier resonant tunneling diodes integrated with a planar patch antenna. Gradual change from the classical square-law detection to photon-assisted tunneling with increasing photon energy was observed.The intersubband THz gain due to electron transition between adjacent quantum wells was estimated using the measurement of current change. Estimated gain at room temperature was ~0.15cm-1 at 1.4THz for a sample with relatively thick separation layer.

(4)    The buried metal heterojunction bipolar transistor (BM-HBT) was proposed, in which buried metal in the collector layer could reduce the total base-collector capacitance (CBC). To confirm transistor operation of BM-HBT, we fabricated a device with buried tungsten mesh replacing the subcollector layer, where tungsten mesh works as a schottky collector electrode. DC current gain of 12 was measured from common-emitter collector I-V characteristics.     

 

Light Emitting Devices Using Advanced Structures and Materials

 

Staffs:          M. Watanabe

Students:       T. Maruyama           Y. Maeda               K. Osada               T. Funayama   M. Tsuganezawa     Y. Iketani               S. Okano               N. Nakamura

                    N. Sakamaki            M. Suzuki              D. Kuruma            T. Teraji

 

New type of light emitting devices on Si substrate are investigated using super-heterostructures, such as CdF2-CaF2 superlattices, nanocrystalline silicon or ZnO embedded in epitaxial CaF2 grown on Si(111) substrate. Intersubband quantum cascade lasers using CaF2-CdF2 and CaF2-Si superlattices have been proposed and analyzed theoretically. Crystal growth of multilayered heterostructures and device fabrication technique has been studied.

Results obtained in this research are as follows.

 

(1) In order to realize intersubband cascade lasers, it is essential to control energy levels of subbands in CdF2 or Si quantum wells confined by CaF2 energy barriers. This year, CdF2-CaF2 double barrier resonant tunneling diode structures have been fabricated using molecular beam epitaxy (MBE) combined with partially ionized CaF2 beam to improve coverage and surface morphology of CaF2 barrier layers. Contamination control technique was also essential to achieve pinhole-free ultra-thin layers. In I-V characteristics, negative differential resistance (NDR) with extremely high peak-to-valley ratio (PVR) greater than 105 has been obtained at room temperature for the first time. This result implies that pinholes in 1nm-thick CaF2 layer was perfectly suppressed so that leakage current was reduced to low level determined by tunneling.

(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 obtained using the current constriction structure fabricated by photolithography and wet chemical etching.

(3) Formation technique of epitaxial ZnO on thin CaF2 layer of less than 10nm in thickness was investigated. Thin barrier layer is strongly required for sufficient hole injection of EL devices. Doping technique of ZnO was also studied.

(4) Epitaxial growth of nanometer-thick Si-CaF2 multilayers on Si(111) substrate has been investigated. This year, appropriate pre-annealing temperature for silicon substrate with various misscut angle was studied. In order to realize ultra-thin Si epitaxy on CaF2, appropriate width of terraces and step height should be prepared. Pre-annealing temperature for Si substrate with misscut angle of 0.1°, 1° and 3.5° was optimized resulting in flatness and homogeneity of nanometer-thick Si epilayer on CaF2 has been improved.

 

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 mm width and 2 mm pitch were embedded with flat InP layer of 1.1 mm thickness.