Low Dimensional Quantum Structure Lasers


Staffs:      Y. Suematsu     S. Arai     S. Tamura

Post-Doctoral Research Fellow:         Q. Yang       B. Chen

Visiting Researcher:                           J. Shim

Students:      N. Nunoya     M. Nakamura       H. Yasumoto       M. Morshed     H. Midorikawa          K. Fukuda        K. Muranushi K. Ohira


GaInAsP/InP strained-quantum-film, -wire, and -box lasers have been studied both theoretically and experimentally. Distributed feedback (DFB) lasers consisting of narrow 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)      1.5 mm GaInAsP/InP lasers with narrow wirelike (43 nm and 70 nm) active regions, which consist of strain-compensated five-quantum-well structure, were realized for the first time by using EB lithography, CH4/H2 reactive-ion-etching (RIE), and organo-metallic-vapor-phase-epitaxy (OMVPE) embedding growth. As the result, lower threshold current density (318 A/cm2) than that of planar 5MQW lasers (550 A/cm2) prepared on the same wafer was obtained at room temperature. Moreover, lower threshold current density and higher differential quantum efficiency than those of quantum-film lasers were obtained up to 85°C. The occurrence of non-radiative recombination traps at the etched/regrown interfaces was successfully suppressed.

(2)      In order to evaluate the quality of etched/regrown interfaces of above mentioned GaInAsP/InP wirelike lasers, the temperature dependence of the spontaneous emission efficiency of samples with wire widths of 43 nm and 70 nm was compared to that of quantum-well lasers. As the result, the product of the surface recombination velocity and the carrier lifetime S×t at the etched/regrown interfaces was evaluated to be less than 2 nm at room temperature. No degradation in the spontaneous emission efficiency was observed in the temperature range between 25 °C and 85 °C for both samples. Therefore, these results indicate that high quality etched/regrown interfaces can be obtained with GaInAsP/InP fine structures.

(3)      By using the same fabrication method mentioned above, low threshold current density 1.5 mm wavelength DFB lasers with deeply etched wirelike active regions have been demonstrated. A high index-coupling coefficient of 360 cm-1 was obtained and a record low threshold current density Jth = 94 A/cm2 operation was achieved with DFB lasers consisting of double layered wirelike active regions.

(4)      A submilliampare operation of 1.55 mm GaInAsP/InP BH-DFB lasers with deeply etched wire-like active regions was successfully obtained. Threshold current as low as Ith = 0.7 mA (Jth = 150 A/cm2) and an external differential quantum efficiency of hd=23 %/facet were obtained with a stable single-mode operation (SMSR = 36 dB @I=2.6Ith) for the cavity length of 200 mm and the stripe width of 2.3 mm under a RT-CW condition. Concerning the reliability of these lasers, for the 240-mm-long BH-DFB laser having a threshold current of Ith = 0.85 mA, a RT-CW test is being done without bonding on a heatsink (just pin-clipped). No degradation is observed after 800 hours at an output power of 1 mW (@I = 8.75 mA = 10.3Ith).

(5)      Single mode operation characteristics of DFB lasers with deeply etched wirelike active regions have been studied both theoretically and experimentally.  Experimentally, lasing modes for all measured samples were observed at a longer-wavelength-side of the stop-band of the grating. It was theoretically explained in terms of so called “a gain matching effect,” where the standing wave profile of longer-wavelength-side modes match gain regions while those of shorter-wavelength-side modes take peaks between gain regions.



New Types of Semiconductor Lasers for Photonic Integration


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

Post-Doctoral Research Fellow:       M. M. Raj          B. Chen

Visiting Researcher:                        J.-I. Shim

Students:      J. Wiedmann     N. Nunoya    Y. Saka            H. Yasumoto     K. Matsui                    K. Ebihara       T. Okamoto       A. Umeshima M. Ohta           Y. Onodera

Research Students:   H.-C. Kim


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, Coupled Cavity (CC) lasers, Distributed Reflector (DR) lasers, and Membrane lasers have been studied both theoretically and experimentally.

Results obtained in this research are as follows:


(1)      MMC lasers consisting of narrow and deep grooves were analyzed by an improved perturbation feedback theory and transfer matrix method. It was shown that the attainable effective reflectivity is limited by the diffraction loss in the grooves. By filling the grooves with the polymer Benzocyclobutene (BCB), the diffraction loss can be reduced. An increasing number of periods will lead to a larger reflectivity. For 8 elements of l/(4nL)-long BCB filled grooves the reflectivity is estimated to be 98 % and a threshold current can be reduced to 1 mA for a microcavity length of 4.9 mm and stripe width of 1 mm.

(2)      Highly uniform 1.55 mm wavelength lasers with high reflective deeply etched semiconductor/BCB DBR structures were realized. Low threshold current of 7.2 mA and high differential quantum efficiency of 50 % from the front facet were demonstrated (L = 160 mm, WS = 5 mm) with rather high yield. The reliability of such polymer-buried DBR lasers was tested for the first time and a lifetime over 5,000 hours at CW condition and constant output power of 2.5 mW was achieved. Double sided-DBR laser having 15-DBRs on the rear and 5-DBRs on the front side showed a threshold current as low as 5.0 mA (L = 160 mm, WS = 5 mm).

(3)      A new type of a single-mode laser consisting of a deeply etched DBR and several small cavities was fabricated and analyzed. Single-mode operation was achieved for different number of cavities. However, increasing the number of cavities will decrease the quantum efficiency drastically. By analysis with the transfer matrix method and by experiment it was found that the CC laser with only two cavities is the best according to high efficiency and low threshold current. A threshold current under room temperature CW condition as low as 11 mA (L1 =150 mm and L2 =40 mm) with a submode-suppression-ratio (SMSR) of 36 dB (at I = 1.8 Ith) was achieved for a 5-mm-wide stripe laser.

(4)      A new type of DR laser consisting of high reflective DBR and a laser cavity with vertical grating at the sidewalls was proposed and demonstrated. Single-mode operation (SMSR = 35 dB) with high differential quantum efficiency (hd = 42 % from the front facet) was achieved.

(5)      In order to realize high performance semiconductor lasers, we proposed membrane DFB lasers with wirelike active regions. For the cladding layer, the polymer Benzocyclobutene (BCB) was used to achieve a high optical confinement. Our model calculation reveals that threshold current as low as 10 mA for a 50 mm-long and 1 mm-wide device can be expected.

(6)      Membrane DFB laser strucutres were fabricated. Photoluminescence (PL) spectra successfully showed the stop-band due to DFB structure. The stop-band width was measured to be about 30 nm, and the index-coupling coefficient ki was estimated to be 965 cm-1. The equivalent refractive index of the membrane waveguide was estimated to be 2.96, which is approximately 10% lower than that of conventional laser structure.



















Quantum Coherent Electron Devices


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

Visiting Researcher:           B. Zhang

Students:        M. Nagase        N. Sakai          H. Oguchi       M. Kurahashi                       

                     S. Karasawa      T. Okada         T. Konishi        H. Tamura    

                     H. Nakamura     T. Hirata          R. Yamamoto    H. Nagatsuka                       

                     Y. Ninomiya        K. Mae                                   


Ballistic transport of hot electron has a possibility of new high-speed and functional 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 a solid-state version of the vacuum biprism that has been realized, and is also proposed as a test ground for electron interference observation and a new conceptual device. Toward the device, the electron wave propagation was simulated to reveal conditions for high contrast of the interference pattern. The coherent hot electron emitter was proposed where the Fermi energy could be adjusted within 0.3 meV. This emitter generates the electron wave with the wavefront spread more than 100 nm, that is, high lateral coherence.

(2) Scanning Hot Electron Microscopy (SHEM) for the observation of electron wave diffraction has been studied. Using the sphere/plane model, the jelium model and the image force potential, the hot and the thermal equilibrium electron currents flown through the probe, and their ratio were revealed in relation with parameters. Parasitic effects including of capacitive coupling current, the residual series resistance induced current, the phase shift and response suppression induced by the gap control system, have been made clear to be eliminated. AlAs/GaInAs hot electron emitters were designed and fabricated. Using the emitter and eliminating of parasitic effects, detection experiments of the hot electron from the semiconductor emitter have been executed.

(3) Current-Voltage (I-V) characteristics of double-barrier resonant-tunneling diodes were analyzed toward evaluation of phase coherence of hot electrons in semiconductors. Taking into account the phase-breaking effect and structural inhomogeneity by using the correlation function theory of phase breaking could fit the experimental data over four orders of magnitude. The phase-breaking effect appears in the current region of less than one-hundredth of the peak in I-V characteristics, in addition to the peak of the second derivative curve. The well width fluctuation, if any, influences the latter but not the former. Using the theory, the phase-coherent length and the well width fluctuations were estimated to be 500 nm and 0.6 nm.


High-Speed Electron Devices Using Advanced Structures and Materials


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

Students:        T.Arai                M.Tsutsui         Y. Harada        H. Tobita       Y. Iketani        

                      A. Itoh              Y. Oguma        S.Yamagami  M. Saitoh      K. Hoshina     

N. Shashinaka   Y. Okuda         R. Yamamoto  T. Morita      T. Abe             

H. Satoh             T. Sakai           H. Nagatsuka Y. Kontani     T. Nagai


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)    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 25 nm-long gate p-type devices, comparable drivability to conventional MOSFETs was archived with the drain current of -293 mA/mm and the transconductance of 431mS/mm at VDS = VGS = -1.5 V. The on/off ration was improved by using a very thin 35nm-thick SOI structure.

(2)    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.

(3)    To reduce total base-collector capacitance (CBCT), GaInAs/InP based buried metal heterojunction bipolar transistors (BM-HBTs) was fabricated, in which tungsten stripe with the same area as the emitter was buried with an intrinsic collector layer. CBCT of 10.3 fF was calculated from Y12 parameters and an effective base-collector junction area of the BM-HBT was estimated to be 22% that of conventional-HBT.

(4)    Fabrication process for narrow emitter along <010> direction in heterojunction bipolar transistor fully drawn by electron beam lithography was studied. Emitter structure of a 100 nm width was formed by using epitaxial structure with 30-nm-thick InP layer of emitter. Transistor operation of devices with 0.5-µm-wide emitter was confirmed.     


Light Emitting Devices Using Advanced Structures and Materials


Staffs:         M. Watanabe

Students:      T. Maruyama         T. Funayama       M. Tsuganezawa  S. Okano

                    N. Sakamaki          M. Suzuki            N. Nakamura       T. Ishikawa

                    T. Teraji                 D. Okamoto


New type of light emitting devices using quantum-nano structures are studied. Intersubband quantum cascade lasers using CdF2-CaF2 heterostructures have been proposed and formation of sharp energy subbands in the quantum wells has been confirmed using resonant tunneling diode structures. Epitaxial growth and ultra-violet light emission of high quality nc-ZnO embedded in CaF2 on Si has been demonstrated for the first time. Novel candidate for lattice-matched, wide-bandgap compound semiconductor Be-chalcogenides was proposed and epitaxial growth condition was 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 (RTD) 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. In I-V characteristics of RTD, negative differential resistance (NDR) with extremely high peak-to-valley ratio (PVR) near 106 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) Novel fabrication technique of CdF2/CaF2 resonant tunneling diode in sub-micrometer size patterned area was developed. The fine growth area was formed by electron beam lithography and wet etching of SiO2 mask layer formed on Si substrate. In the result, uniformity of the I-V characteristics and stability of the device performance was dramatically enhanced.

(3) 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.

(4) The epitaxial growth of zinc oxide (ZnO) nanocrystals embedded in a single-crystalline CaF2 layer on a Si(111) substrate has been demonstrated. Highly c-axis-oriented ZnO 4-10 nm thick was grown on a CaF2(111) layer using radio-frequency (RF) sputtering followed by annealing in ultrahigh vacuum, resulting in the formation of epitaxial self-organized ZnO nanocrystals on CaF2/Si(111). It was found that CaF2 can be grown epitaxially over ZnO/CaF2 by molecular beam epitaxy (MBE), thus the CaF2/ZnO/CaF2 heterostructure has been formed on a Si(111) substrate. Abrupt heterointerfaces between CaF2 and ZnO were confirmed on a transmission electron microscope (TEM) cross section, and ultraviolet (UV) photoluminescence (PL) corresponding to the band-gap energy of ZnO was dominant in PL spectra observed at room temperature.

(5) 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.

(6) Be-chalcogenides II-VI semiconductors is good candidate. for the materials of long life-time II-VI lasers because of the strong covalent bonding of Be. Moreover, BeMgZnSe quaternary compound semiconductor can be lattice matched to Si and its energy band gap covers UV region (290-345nm).This year, BexZn1-xSe (x=0.2-1.0) was grown on CaF2/Si(111) substrate by MBE at growth temperature of 300, thickness of 200 nm and growth rate of 2 nm/min. In-situ RHEED pattern indicated that epitaxial BeZnSe layer was obtained on the CaF2 interlayer. Lattice constant of the obtained layer was evaluated by XRD. (111) peaks of XRD results confirmed that the BeZnSe is lattice matched to Si when x=0.45, which is consistent with Vegard's law.


Processing for Nanometer Structures


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

Students:    T. Arai              H. Tobita              Y. Harada            H. Oguchi

                  S. Yamagam      Y. Okuda              K. Sato                 R. Yamamoto

                  T. Morita          H. Nakamura       T. Ninomiya


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)          Buried growth of a GaAs layer over a tungsten stripe by organometallic vapor phase epitaxy was studied. Triethylgallium (TEG) was compared with trimethylgallium (TMG) from the viewpoint of migration length. A 70-nm-wide tungsten stripe was buried by a 0.77-µm-thick layer of GaAs with a flat surface using TMG.

(2)          To realize ohmic contact to n-GaAs by very shallow doped layer, insertion of ultrathin Ga0.5In0.5As layer was evaluated theoretically and experimentally. Theoretical current-voltage characteristics by using self-consistent potential calculation and field emission current by WKB method shows 10-6Wcm2 as contact resistivity by 10-nm-thick GaInAs layer with 2×1019cm-3 as carrier concentration and 10-nm-thick GaAs layer with 8×1018cm-3 as carrier concentration. TLM measurement of fabricated structure by OMVPE shows 1.7×10-5Wcm2 as contact resistivity.