Low
Dimensional Quantum Structure Lasers
Post-Doctoral Research
Fellow: Q.
Yang B.
Chen
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
Post-Doctoral Research
Fellow: M.
M. Raj B.
Chen
Students: J. Wiedmann N. Nunoya Y. Saka H.
Yasumoto K. Matsui K. Ebihara T. Okamoto A. Umeshima M. Ohta Y.
Onodera
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.