RESEARCH ACTIVITIES
Low Dimensional
Quantum Structure Lasers
Visiting
Researcher: A. Haque
Students: H. Yagi K.
Ohira T.
Sano T.
Murayama D. Plumwongrot
M.
Hirose K.
Miura
GaInAsP/InP
strained-quantum-film, -wire, and -box lasers have been studied both theoretically
and experimentally. A new type of DR (distributed reflector) lasers fabricated
by the same fabrication process as that of quantum-wire lasers and distributed
feedback (DFB) lasers with wirelike active regions have been also studied.
Results obtained in this research are as follows:
(1) GaInAsP/InP
multiple-quantum-wire structures with the wire widths of 18 nm and 27 nm in the
period of 80 nm were fabricated by electron beam lithography, CH4/H2-reactive
ion etching and two-step organometallic vapor-phase-epitaxial growth. Size distributions
of these quantum-wire structures were measured by scanning electron microscope
views and the standard deviation was estimated to be less than ±2 nm. From EL
spectra at 103 K, the full-width at half maximum of these quantum-wire
structures was almost comparable to that of the quantum-film structure
fabricated from the same initial quantum-well.
(2)
Wire width
dependence of the large energy blue shift in GaInAsP/InP partially strain-compensated
vertically-stacked multiple-quantum-wire structures is accurately explained for
the first time using an 8 band k•p theory without any fitting parameter.
Variations of energy levels due to a non-uniform strain profile in stacked
quantum-wires are calculated to be less than 2.4 meV. It is found that unlike
quantum films, the energy-band structures of strained quantum-wires depend on
the amount of strain-compensation in barrier regions and on the number of wire
layers in the vertical stack.
(3) A
RT-CW operation of GaInAsP/InP quantum-wire lasers (23 nm wide, 5 stacked
quantum-wires) and wirelike lasers (43 nm wide, 5 stacked wires) fabricated by electron
beam lithography, CH4/H2-reactive ion etching and
organometallic vapor-phase-epitaxial regrowth was realized for the first time.
Lifetime measurement of this quantum-wire laser was also carried out at RT-CW
condition, and no noticeable performance degradation was observed even after
9,500 hours.
(4) GaInAsP/InP
strain-compensated 5-stacked compressively strained quantum-wire lasers with
the wire width of 14 nm in the period of 80 nm were realized by electron beam
lithography, CH4/H2-reactive ion etching and organometallic
vapor-phase-epitaxial regrowth. By adopting completely strain-compensating
barriers, a smaller energy blue shift at the peak wavelength in EL spectra than
that in the case of a partial strain-compensation was observed that indicates
the suppression of the strain relaxation effect during etching and InP regrowth
process. Lateral quantum confinement effect in this quantum-wire laser could be
also observed via sharper shape of the EL spectrum than that of quantum-film
lasers in the higher transition energy region.
(5)
GaInAsP/InP
multiple-quantum-wire lasers (wire widths of 19 nm and 27 nm in a period of 100
nm) with SiO2/semiconductor reflector by electron beam lithography,
CH4/H2-reactive ion etching and two-step organometallic
vapor-phase-epitaxial growth. As a result, oscillations from the ground levels
could be obtained at RT. In addition, the threshold current densities of these
quantum-wire lasers were lower than and differential quantum efficiencies were
comparable to those of quantum-film lasers at RT.
(6) Low
threshold operation of 1.55 mm
wavelength GaInAsP/InP strongly index-coupled and gain-matched distributed
feedback lasers with periodic wirelike active regions, which was fabricated by
electron beam lithography, CH4/H2-reactive ion etching,
and organometallic vapor-phase epitaxial regrowth, were demonstrated and their
reliability was also investigated to date. As a result, no degradations in
lasing characteristics were observed after an aging time of 8200 hours at a
bias current of around 10 times the threshold. In the experiment, all measured
device of this laser oscillated on the long-wavelength side of the stopband. We
investigated the single-mode operation of DFB laser with wirelike active regions
by using coupled-mode theory. As a result, it was theoretically demonstrated
that DFB lasers with wirelike active regions oscillated in the long-wavelength
side mode of the stopband by considering the gain-matching effect. The facet
phases of two cleaved facets were also investigated. Lasing modes exist on the
long-wavelength side for any facet combination.
(7) We
have been studying a new type of distributed reflector laser consisting of a
wirelike active section and a passive DBR section with quantum-wire structure
by using a lateral quantum confinement effect. To evaluate the reflectivity of the
DBR, we fabricated Fabry-Perot type lasers with DBR section on a single side.
The reflectivity was estimated from the output ratio from the front to the rear
facet. The reflectivity higher than 90 % was achieved at a DBR length of 200 mm
for the wire width of 60 nm and 350 mm
for 30 nm, where cleaved facet reflectivity of Rf = 0.3 and
the refractive index difference of Dn
= 0.03 were assumed. Using this DBR structure, DR laser was fabricated with
planer type structure by BCB polymer. As a result, threshold current of 3.2 mA
was obtained for active section length of 260 mm,
passive DBR section length of 130 mm
and stripe width of 4.3 mm
with both facets cleaved. The differential quantum efficiency from the front
facet was 19.2 % and the rear facet was 1.7 %, hence an asymmetric output ratio
of 11 was realized. A single-mode operation with sub-mode suppression ratio
(SMSR) of 46.6 dB was achieved at a bias current of 3.75 times the threshold.
(8) The
device mentioned above, however, could not operate with high performance
theoretically indicated. This result was caused by damage of dry etching to
form the stripe. For higher efficiency, wet chemical etching was used to form
the laser stripe instead of dry etching. As a result of fabricating the DR
laser by wet chemical etching, threshold current of 2.5 mA, which corresponds
to the threshold current density of 189 A/cm2, was obtained for the
active section length of 300 mm,
the passive DBR section length of 210 mm
and the stripe width of 4.4 mm.
The active and passive sections consist of 90-nm-wire with the period of 240.0
nm and 40-nm-wire with the period of 241.25 nm, respectively. The differential
quantum efficiency from the front facet was 35.6 % and the rear facet was 0.54
%. An asymmetric output ratio of 66 was realized. A single-mode operation with
sub-mode suppression ratio (SMSR) of 53.7 dB was achieved at a bias current of
twice the threshold.
New Types of
Semiconductor Lasers for Photonic Integration
Students: H. –C. Kim T. Okamoto Y. Onodera H. Kanjo T.
Hasegawa
S.
Sakamoto T. Yamazaki M.
Hirose K.
Miura
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 Distributed Reflector (DR) lasers and Membrane lasers have been studied both theoretically and experimentally.
Results obtained in this research are as follows:
(1) 1.3mm-wide
narrow mesa stripe DR lasers consisting of first-order vertical grating
(VG)-DFB and first-order deeply etched DBR mirrors were realized for the first
time by one-step epitaxy and fine vertical etching processes. A threshold
current of 2.8mA for the active region length of 150mm
and an SMSR=44dB were obtained.
(2) In
case of TE mode, a narrow stripe waveguide exhibits relatively low coupling
coefficient even for deep grating. 1.5mm
vertical grating distributed feedback lasers with tensile quantum well were
realized single mode operation of TM polarized VG-DFB lasers. A threshold
current of 2.6mA and SMSR=50dB were obtained.
(3) Novel
semiconductor laser structure, such as, membrane laser which has the
Benzocyclobutene (BCB) cladding layers, enables to increase optical confinement
into active layer due to a large refractive index difference between active
layer and cladding layers. A room temperature continuous wave operation of
membrane DFB laser consisting of deeply etched single-quantum-well wirelike
active regions was already demonstrated. In order to realize single mode and
low threshold operation of membrane DFB laser, buried heterostructure (BH) was
innovated by slightly changing the fabrication process. A threshold pump power
of 1.5 mW and a sub-mode suppression-ratio of 42 dB were obtained for a 142 nm-thick
semiconductor membrane core layer with a cavity length of 120 mm
and a stripe width of 2 mm
under room-temperature continuous wave optical pumping. The corresponding
threshold for current injection was roughly estimated to be 27 mA.
(4) We
have realized membrane BH-DFB laser arrays by arranging the laser cavities (10 mm
spaced 15 elements with 5 different grating periods). A total wavelength span
of 72 nm was achieved with a small lasing wavelength fluctuation of up to ±1.2
nm at RT-CW condition under optical pumping. From this value, membrane
thickness fluctuation was estimated to be ±0.4 nm. Threshold pump power of 3.4
mW and SMSR of 45 dB were achieved in a typical device.
(5) Membrane
BH-DFB laser arrays with different grating periods and different stripe width were
successfully fabricated using EB lithography, CH4/H2-RIE and
OMVPE. The
possibility for a laser array covering a wide wavelength range of 51 nm with a
wavelength controllability of less than 0.8 nm (100GHz) was
demonstrated. This multi-wavelength laser array may be a
candidate for a coarse WDM system or a wavelength conversion device between LAN
and MAN.
(6) Low threshold operation of membrane buried heterostructure distributed
feedback (BH-DFB) laser arrays. 45 devices were fabricated on a wafer covering
a wide wavelength range of 75 nm. The lowest threshold pump power as low as
0.64 mW along with the sub-mode suppression-ratio (SMSR) of over 30 dB at 2
times the threshold were successfully obtained at RT-CW condition.
(7) Phase shifted
membrane BH-DFB laser with short cavity length of 50 mm was fabricated and characterized. Threshold
pump power of 3.1mW and SMSR of 35dB were achieved with RT-CW condition under
optical pumping. Laser cavities were operated under optical pumping at RT-CW
using micro PL setup. Stripe width was 2 mm, cavity length was 50 mm, and
grating period was 310 nm, respectively. Threshold pump power Pth
was 3.1 mW which corresponded to a threshold current Ith of
around 70 mA (estimated
by assuming an absorption coefficient as 10000 cm-1). An emission wavelength
of 1549 nm was observed. Considering the lasing mode as the Bragg wavelength,
equivalent refractive index was estimated to be around 2.50 which agreed well
with the value calculated from the cross sectional waveguide structure. From
the stop band width of 38 nm, index coupling coefficient was estimated to be
710 cm-1.
(8) Benzosyclobutene
(BCB) used for cladding layer of membrane laser structure has negative
temperature coefficient of refractive index which is opposing value of
semiconductor material. So athermal waveguide can be designed with controlling
the thickness of membrane core layer. Membrane BH-DFB lasers with membrane core
thickness of 150nm and 65nm were fabricated. Slope of lasing wavelength
dependences on temperature were measured to be 0.0526nm/K and 0.0245nm/K,
respectively. The value for membrane thickness of 65nm was one in five of
typical semiconductor DFB lasers.
Quantum
Coherent Electron Devices
Students: K. Mae H.
Nagatsuka Y. Ninomiya H. Kanoh D.
Miyamoto
U.
Shirai T.
Kaminosono S.
Sato Y.
Toriumi T.
Arai
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) We have carried out Young’s double-slit
experiment for the hot-electron wave in man-made semiconductor structures with
a 25-nm-space double slit in an InP layer buried within GaInAs, a 190-nm-thick
GaInAsP hot-electron wave propagation layer, and a collector array of 80 nm
pitch. At 4.2 K, dependences of the collector current on the magnetic field
were measured and found to agree clearly with the double-slit interference
theory. The present results show evidence for the wave front spread of hot
electrons using the three-dimensional state in materials, for the first time,
and the possibility of using top-down fabrication techniques to achieve quantum
wave front control in materials.
(2) We
gave numerical foundations for previously proposed experiment of hot-electron
diffraction observation in solid based on Ballistic Electron Emission
Microscope (BEEM). It was found that hot-electron wave function in
semiconductor injected from a scanning probe became quasi-spherical wave because
of large refraction effect at base metal/semiconductor interface due to large
discontinuities of conduction band bottoms and effective masses between each
material. Thus, it has been made clear that the quantum reciprocity, our
measurement principle, can be applied to our proposed experimental
configuration by using BEEM. We have also carried out numerical simulations for
our experiment with a p-phase
shifter structure as a wave front modulator by Finite Difference Time Domain
(FDTD) method. The results showed diffraction patterns with high contrast and
our scenario for hot-electron diffraction observation came into existence.
High-Speed Electron Devices Using Advanced Structures and
Materials
Staffs: K. Furuya M. Asada Y. Miyamoto M.
Watanabe S. Tamura
Post-Doctoral Research Fellow: Qui Weibin Z. Y. Zhang
Students: K. Nishihori K. Kashima N. Orihashi H.
Maeda K. Matsuda
K. Sasao N. Kaneda K.
Arai T. Ozono H. Satoh
K.
Takeuchi K. Yokoyama S. Gotoh E.
Hase S. Hattori
H.
Imai R. Nakagawa T. Nonaka Y. Yamada T.
Fujisaki
M. Ishida H. Kamata Y. Sugiyama
Research Student: I. Medugorac
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.
The terahertz frequency range is remaining undeveloped, in spite of
various wide applications, due to the lack of compact solid state amplifiers
and oscillators. Terahertz response of quantum nano-structures and its
application to terahertz devices are studied. Quantum effect in metal/insulator/semiconductor
heterostructures, which are suitable for ultra-high frequency operation, are also
studied as well as in semiconductor heterostructures.
Results
obtained up to now are as follows:
(1) We
proposed and fabricated a hot electron transistor in which an electron
propagates only through an intrinsic semiconductor. In this transistor, an
emitter mesa was fabricated between gate electrodes to reduce the gate leakage
current from the emitter to the gate. To suppress the current leakage from the
emitter and the gate pads, free-standing tungsten wires were also fabricated. The
measured I-V characteristics
at 20K showed effective control of collector current by gate bias. When the
device was operated, it was confirmed that the gate-leakage current from the
emitter to the gate was smaller than the collector current. The calculated transconductance gm was approximately 10 mS/mm.
(2) GaInAs/InP
heterojunction bipolar transistors (HBTs) with a single 0.1-mm-wide tungsten wire were fabricated. The
tungsten wire was buried in an undoped InP collector
layer and functioned as a collector electrode. In a previous process of
fabricating HBTs
with buried wires, we could not achieve a device operation with only a single
wire. One of the possible reasons for this failure was the insufficient growth
of the buried wire, because this wire requires a long growth time. The
nonuniformity of facet formation in a previous wet etching process used to
fabricate an InP emitter also contributed to the failure. By increasing the
growth time of the buried wire and changing the wet etching solution used to
enable uniform fabrication of emitter mesas, we observed a transistor operation
with a current gain of 20 in the fabricated HBT. The emitter area of the device
was 0.1 x 0.5 mm2.
(3) The modulation of drain current by gate
voltage in a self-assembled monolayer (SAM) of benzene-1,4-dithiol was
confirmed on the basis of the transistor structure. A device was fabricated by
the shadow mask technique with an air-bridge structure. By electron beam
lithography, the top area of the Au/SAM/Au junction was fabricated to be 370 nm
by 230 nm. The measured current-voltage characteristics showed an exponential
increase in drain current when drain voltage was increased and a decrease in
drain current when gate bias was increased. Because the modulated drain current
was greater than the gate leakage current, modulation by the gate bias was
confirmed. However, no bonding was expected in the upper SAM/Au junction because
the magnitude of the drain current was less than 100 pA.
(4) Superradiance from photon-assisted tunneling electrons is analyzed
quantum-mechanically, and an amplifier device for the terahertz range utilizing
this phenomenon is proposed. In this device, electrons transported with the
photon-assisted tunneling at the input port generate the amplified output due
to the superradiance at the output port. It is shown theoretically that the
gain up to a few terahertz is possible by optimizing the device structure.
(5) A novel amplifier device with two-dimensional
electron gas in semiconductor heterostructures, which has a possibility of operating
in the THz range, is proposed and analyzed. The operation principle is
equivalent to that of the klystron tube in the low-frequency classical limit. Transconductance
and power gain are analyzed quantum mechanically including the influence of
electron scattering and electron energy distribution. Calculated results show
that the transconduntance has a peak in the sub-THz and THz ranges. An
oscillator using this device integrated with a coplanar line and a slot antenna
is proposed, and the possibility of THz oscillation is shown by the analysis of
oscillation conditions. Fabrication of this device is now in progress with the
layer structure of GaInAs/InAlAs HEMT (high-electron mobility transistor).
Fabrication process has been established, and the DC characteristics with
sufficient driving current for THz oscillation have been obtained.
(6) Millimeter and sub-millimeter oscillators with
RTDs (resonant tunneling diodes) integrated with stacked-layer slot antennas
are proposed, and oscillation at 250GHz is demonstrated with GaInAs/InAlAs
RTDs. Detailed electromagnetic analysis is also performed and shows that
oscillation up to 1.2THz is feasible by reducing the antenna size.
(7) Frequency
mixing characteristics are examined for 0.8-mm-diameter GaInAs/InAlAs triple-barrier RTDs in 100GHz
band. Obtained heterodyne signals by fundamental mixing show that the
conversion losses of these diodes are as low as the reported results of the
conventional Schottky barrier diodes when the bias voltage is set in the
negative differential regions. Epitaxial growth of
resonant tunneling structure with CdF2/CaF2
(insulator/insulator) and CoSi2/CaF2 (metal/insulator)
super-hetero- structures on Si substrates was established using nano-area local
epitaxy, and room-temperature negative differential resistances were obtained
with very high uniformity and reproducibility. Details are described in the
next category (Light Emitting Devices Using Advanced Structures and Materials).
Light Emitting Devices
Using Advanced Structures and Materials
Staff: M.
Watanabe
Students: M.
Matsusda Y. Niiyama H. Fujioka K. Jinen
T.
Kanazawa T. Yokoyama S. Tamura R. Ideue
H.
Murata T.
Murata T.
Tabira Y.
Fujisawa
New
type of light emitting devices using quantum-nano structures are studied. Intersubband
quantum cascade lasers using CdF2-CaF2 and CoSi2-CaF2
heterostructures have been proposed and novel crystal growth technique named
Nanoarea-Local-Epitaxy has been proposed. Energy subbands in the quantum wells
has been confirmed using resonant tunneling diode with double- and
triple-barrier structures. Novel candidate for lattice-matched, wide-bandgap
compound semiconductor Be-chalcogenides was proposed and epitaxial growth and
room temperature UV emission of high-Be-content BeZnMgSe has been demonstrated.
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,
Nanoarea-Local-Epitaxy (NLE) was proposed to grow CdF2-CaF2
double-barrier and triple-barrier resonant tunneling diode (RTD) structures.
The hole arrays of 50 nm in diameter was formed on a 15 nm-thick SiO2
layer on Si substrate with 0.1° off miscutt angle using electron beam
lithography. This yields atomically flat Si surface with no atomic steps in
growth region, which is essential to grow pinhole-free 1 nm-thick CaF2
for tunneling barrier layers. In the result, uniformity of the I-V
characteristics and stability of the device performance was dramatically
enhanced. And moreover, control of energy subband has been demonstrated for the
first time by changing CdF2-QW thickness even for triple-barrier
resonant tunneling diode structures.
(2)
Epitaxial growth and room temperature
negative differential resistance (NDR) of CdF2-CaF2
double barrier resonant tunneling diode (RTD) on Si(100) substrate was
successfully achieved for the first time. Si(100) substrate with misscut angle
of 2° was used in order to form 2 mono layer silicon atomic steps, i.e. one
unit layer, which yields atomically flat CaF2 epilayers on Si(100)
substrate with excellent uniformity and reproducibility. Double-barrier RTD
structures were fabricated and NDR with peak-to-valley current ratio (PVCR) of
around 4 was observed at room temperature for the first time. Moreover, CdF2
quantum-well thickness dependence of applied bias voltage for NDR peak current
has been clearly demonstrated and it was well explained by Esaki-Tsu formula
assuming layer thickness fluctuation
(3)
CoSi2(Metal)/CaF2(Insulator)
triple barrier resonant tunneling diode structure has been demonstrated using
Nanoarea Local Epitaxy with hole arrays of 40 nm diameter. Negative
Differential Resistance with high peak-to-valley current ratio (PVCR) of 33 at
room temperature has been demonstrated for the first time. This value was the
highest value ever reported concerning CoSi2/CaF2
heterostructures. This result has strong impact for three-terminal device
application using metallic quantum-well as a control electrode.
(4) 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, the BeZnSe buffer layer by Migration
Enhancement Epitaxy (MEE) was introduced. In the result, that improved the
surface morphology, the crystalline quality and the luminescence property. The
40-nm-thick BeZnSe as buffer layer and the 200-nm-thick BeZnSe as epi-layer was
grown by MEE and conventional MBE growth mode, respectively. In the photoluminescence
(PL) measurements, the full-width at half-maximum (FWHM) at λ= 342 nm was 56
meV at 13K and 82 meV at room temperature, respectively. We can conclude that
MEE reduce stacking faults and/or dislocations because MEE suppress
agglomeration of Be.
(5)
Photoluminescence
of quantum-well (QW) structures on the GaP (001) substrate has been demonstrated.
The quantum well structure consists of Be0.15Zn0.85Se
(compressive) wells with different thickness of 2 nm and 5 nm. Each well was
separated by 50-nm-thick Be0.41Zn0.59Se (unstrained)
barrier layers. The nearly band-edge emission of Be0.15Zn0.85Se
and Be0.41Zn0.59Se were observed 3.42 eV and 3.57 eV at
13K, respectively. Those were assigned to quantum levels in each quantum well.
Processing for Nanometer
Structures
Staffs: K. Furuya S.
Arai Y. Miyamoto M. Watanabe S. Tamura
Visiting
Researcher: T. Tanaka
Students: M. Yoshizawa H. Maeda K.
Matsuda Y. Ninomiya K. Sasao
N. Kaneda U. Shirai K. Takeuchi K.
Yokoyama S. Gotoh
E. Hase R. Nakagawa T.
Nonaka Y.
Yamada
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)
We
investigated the effects of low-oxygen-content metalorganic precursors on
oxygen impurities and Hall mobility. The oxygen concentration in the AlInAs
layer was less than 2 x 1017 cm-3 under all growth conditions. We confirmed the high mobility of the
AlInAs/InP high electron mobility transistors (HEMT) structure with the AlInAs
buffer layer (5,500 cm2/Vs at
300 K, and 110,000 cm2/Vs at
77 K). For the AlInAs/GaInAs HEMT structure with the same buffer layer, we
obtained the high mobility (12,000 cm2/Vs at 300 K, and 92,000 cm2/Vs at 77 K)
(2) Line edge roughness (LER) of resist patterns becomes a concern because LER of gate patterns deteriorates the performance and uniformity of device characteristics. We have investigated LER of resist patterns using various electron beam (EB) lithography : 100 keV electron projection lithography (EPL), 50 keV variable-shaped electron beam (VSB) lithography, 50 keV point electron-beam lithography (PB), and 2 keV lowenergy electron-beam proximity projection lithography (LEEPL). LER of chemically-amplified resists and non-chemically-amplified resists are compared using a critical-dimension scanning electron microscope (CD-SEM) to measure LER. Independent of electron energy, LER strongly depended on latent image quality including aerial image quality, resist dissolution contrast, electron scattering blur, etc, even though increasing acid diffusion, i.e. blurring the latent image, monotonically decreased LER of an isolated pattern by smoothing effect of the acid diffusion. The LER differences among the lithography used are due to the difference of the latent image quality, not to the electron energy or resist sensitivity.