Low Dimensional
Quantum Structure Lasers
Visiting Researcher: A.
Haque
Post-Doctoral Research Fellow: N.
Nunoya
Students: H. Yagi K. Muranushi K.
Ohira A.
Onomura T.
Sano
T. Murayama P. Dhanorm
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) 1.5-mm-wavelength
partially strain-compensated GaInAsP/InP 5-layered quantum-wire lasers with the
wire width of 23 nm in the period of 80 nm were realized for the first time by
electron beam lithography, CH4/H2-reactive ion etching
and organometallic vapor-phase-epitaxial regrowth. The threshold current
density of 774 A/cm2 and differential quantum efficiency of 40 %
were obtained under a pulsed condition at room temperature. From measurement of
spontaneous emission spectra, the blue shift at the peak wavelength was 38 meV,
which was much larger than a calculated value. The spontaneous emission
spectral width was almost constant at temperatures between 103 K and 263 K,
indicating a lateral quantum confinement effect. Finally, the spontaneous
emission efficiency below the threshold was comparable to that of the Q-Film
lasers up to 85 °C, which revealed the low-damage property of the etched/regrown
interfaces.
(2) GaInAsP/InP
partially strain-compensated multiple-quantum-wire lasers 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
organometallic vapor-phase-epitaxial regrowth. Size distributions of these
quantum-wire structures were measured by scanning electron microscope views and
the standard deviation was obtained to be less than 2 nm. The differential
quantum efficiencies of these quantum-wire lasers were almost the same as those
of the 5-quantum-well lasers at room temperature. From EL spectra of various
wire width lasers, a larger energy blue shift than that predicted by a simple
analysis model was observed.
(3)
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.
(4) A
RT-CW operation of GaInAsP/InP quantum-wire lasers (23 nm wide, 5 stacked
quantum-wires) and quantum-wirelike lasers (43 nm wide, 5 stacked wires)
fabricated by electron beam lithography, CH4/H2-reactive
ion etching and 2-step organometallic vapor-phase-epitaxial growth processes
was realized for the first time. Lifetime measurement of this quantum-wire
laser was also carried out at RT-CW condition, and no noticeable degradation in
light output was observed even after 2,000 hours.
(5) High-performance
1.55 mm wavelength
GaInAsP/InP strongly index-coupled and gain-matched distributed feedback lasers
with periodic wirelike active regions were fabricated by electron beam
lithography, CH4/H2-reactive ion etching, and
organometallic vapor-phase epitaxial regrowth. This type of DFB laser with
wirelike active regions can operate at very low threshold because of its strong
index coupling and the reduction of the active medium volume. However, this
type of DFB laser consists of a larger portion of etched/regrowth interfaces
than conventional DFB laser. Therefore the reliability test of lasing
characteristics is very important. A CW life test was carried out. No
degradations in lasing characteristics were observed after an aging time of
8200 hours at a bias current of around 10 times the threshold.
(6) By
using a lateral quantum confinement effect, a new type of distributed reflector
laser consisting of a wirelike active section and a passive DBR section with
quantum-wire structure was demonstrated for the first time. In theoretical
analysis, a waveguide loss of a DBR structure increases by only 1 cm-1
compared with the value in case of no active layers. Using this waveguide
structure as a passive DBR section, a maximum reflectivity of 97 % would be obtained
for the wire width of 40 nm and DBR section length of 200 mm.
Threshold current of 15.4 mA, which corresponds to the threshold current
density of 320 A/cm2, was obtained for the active section length of
240 mm, the passive DBR
section length of 440 mm
and the stripe width of 20 mm
with both facets cleaved. The differential quantum efficiency from the front
facet was 16.2 % and the rear facet was 0.57 %, hence an asymmetric output
ratio of 28 was realized. This strong asymmetric output characteristic is a
specific property of the DR laser. For a lower threshold and a single-mode
operation, narrow stripe DR laser was also fabricated. As a result, Threshold
current of 7.6 mA and differential quantum efficiency from the front facet of
5.1 % were obtained under RT-CW condition for the active section length of 310 mm,
the passive section length of 270 mm
and the stripe width of 3 mm.
A single-mode operation with sub-mode suppression ratio (SMSR) of 40 dB was
achieved at relatively low bias level (I=1.2Ith).
New Types of
Semiconductor Lasers for Photonic Integration
Post-Doctoral Research Fellow: N.
Nunoya
Students: H. –C. Kim T.
Okamoto Y. Onodera H.
Kanjo T.
Hasegawa
T.
Yamazaki
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 3.6mA for the active region length of 210mm and an SMSR=35dB were
obtained.
(2) By use of vertical grating DFB structure, it is
clarified that structural birefringence can be drastically reduced. The grating
coupling coefficient can also be made polarization insensitive by adjusting the
grating depth.
(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.
Quantum
Coherent Electron Devices
Post-Doctoral Research Fellow: M.
Nagase
Students: T. Hirata T. Okada K. Mae H. Nagatsuka Y. Ninomiya M.
Fukasawa
H.
Kanoh D. Miyamoto U. Shirai T.
Kaminosono S.
Sato Y. Toriumi
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) A
new experimental method using ballistic electron emission microscope (BEEM) for
observation of electron wave diffraction in semiconductor is proposed and
numerical simulations of the experiment were performed in order to clarify
conditions for the observation. A material combination of GaInAs/InP showed a
clear diffraction pattern due to a phase shift structure as a cause of
wavefront modulation. The diffraction pattern still appeared when an energy
spread of 200 meV of electrons in BEEM current were assumed. This energy spread
allows BEEM current to be over 1 pA when the tunneling current is set to 10 nA
and the current level is large enough to be detected by ordinal BEEM system.
(2) We have analyzed experimental data of hot-electron current
modulation by magnetic field measured for fabricated semiconductor double-slit
device. Electron motions through double-slit and single slit were numerically
calculated by the quantum beam propagation method and classical particle
dynamics. The experimental results could not be explained by double-slit
interference. However, quantum wave diffraction and classical particle picture
could explain the experimental results if the double slit worked effectively as
single slit. This can be considered to occur either when the double-slit was
collapsed to be single slit or when wavefront spread of emitted electron beam
were smaller than the slit spacing.
(3) At sufficiently low temperatures, e.g., below 10 K, current in I-V
characteristics of double-barrier resonant-tunneling diodes (DBRTDs) in the
low-current and low-voltage region is inversely proportional to the phase
coherence length. However, with increasing temperature, this proportional
behavior no longer holds due to thermal energy spread around the Fermi energy
in the emitter. In this study, the temperature range in which current amplitude
is inversely proportional to the phase coherence length is theoretically
derived. The temperature range is dependent on the structural parameters of
DBRTDs. By choosing appropriate device parameters, we can make the current
amplitude inversely proportional to the phase coherence length even at
relatively high temperatures, e.g., 77 K. These results are highly useful for
estimating electron phase coherence length over a wide temperature range and
for studying the temperature dependence of the phase coherence length using
DBRTDs.
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 T.
Abe T.
Morita R.
Yamamoto
K.
Kashima H.
Maeda K.
Matsuda N.
Orihashi K. Sasao
K.
Arai N.
Kaneda T.
Ozono H.
Satoh K.
Takeuchi
K. Yokoyama E. Hase T. Iguchi R. Nakagawa T. Nonaka
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.
Results
obtained up to now are as follows:
(1) In a novel transistor where hot electrons travel through undoped
semiconductor region by adopting a combination of double-barrier
resonant-tunneling structure and metal wire, we fabricated new structure where
emitter was fabricated partially on the top of the device to improve the ratio
between collector current and gate current. As a result, high gain over 100 was
observed at room temperature.
(2) In buried metal heterojunction bipolar transistors (BM-HBTs),
dependence of the thickness of overgrown layer on number of buried metal wires
were studied, moreover, wet-etching process for 0.1μm-wide emitter was improved
to get higher reproducibility. As a result, we fabricated BMHBT with 0.1μm-wide
emitter and confirmed current gain of 20.
(3)
A self-assembled
monolayer (SAM) molecule is attractive as an active region of an electron
device because of its inherent small thickness (~1-2 nm) between each
electrode. We reported processes to fabricate small Au/SAM/Au junctions by
using electron beam lithography. As a SAM molecule, we used benzene-1,4-dithiol
on Au. To obtain an atomically flat Au electrode without deformation of shape,
lower deposition rate, lower sample temperature, and adequate annealing
temperature were required. By using a SiO2 pattern as a shadow mask,
twice oblique evaporations made small Au/SAM/Au junctions. A minimum feature
size of slit of a SiO2 pattern was 60 nm by using electron beam
lithography. Si substrate isolated by SiO2 works as a gate electrode
of three terminal devices by the Au/SAM/Au junctions. Observed I-V
characteristics showed modulation of drain current by gate bias. Reproducible
current jump and step-like current modulation were also observed.
(4) 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.
(5) 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.
(6) Superradiance from photon-assisted tunneling electrons is analyzed
quantum-mechanically, and an amplifier device for the terahertz range utilizing
this phenomenon is discussed. In this device, electrons which are 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 from the
analysis that the gain up to a few terahertz is possible by optimizing the
device structure and the impedances at the input and output ports.
Light Emitting Devices
Using Advanced Structures and Materials
Staff: M.
Watanabe
Students: T. Maruyama T.
Ishikawa N.
Nakamura M.
Matsusda
H.
Fujioka Y.
Niiyama T.
Yokoyama T.
Kanazawa
K.
Jinen S.
Tamura R.
Ideue
New
type of light emitting devices using quantum-nano structures are studied. Intersubband
quantum cascade lasers using CdF2-CaF2 heterostructures
have been proposed and novel crystal growth technique named
Nanoarea-Local-Epitaxy has been proposed. Sharp 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 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.
(3)
Memory effect of CdF2/CaF2
triple barrier resonant tunneling diode structure has been demonstrated using
p-type Si substrate. Strong hysteresis of negative differential resistance
characteristics were observed and reproduced many times during the bias sweep
due to electron charge and discharge. In resonant tunneling, electrons are
trapped in CdF2/CaF2 quantum well and discharged by
applying reverse bias voltage. Asymmetry of polarity of carrier reservers (top
metal electrode and p-type Si substrate) was found to be essential for reproducible
hysteresis. This phenomenon strongly suggests the potential of CdF2/CaF2
heterostructures for memory device application.
(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, epitaxial growth of high-Mg-content (12-15%)
BeMgZnSe lattice matched to GaP (001) substrate has been demonstrated using BeZnSe
buffer layer with molecular beam epitaxy (MBE). BeZnSe buffer layers were
introduced to suppress phase separation on GaP substrate even at high-Mg and Be
content region maintaining lattice match. Full-width at half-maximum (FWHM) (w
scan (004)) of Be0.47Mg0.15Zn0.38Se was 662.4 arcsec
and that for Be0.48Mg0.12Zn0.40Se was 172
arcsec by X-ray diffraction (XRD). BeZnSe buffer layer was found to be
promising for the growth of energy bandgap (~3.90 eV) BeMgZnSe heterostructures
on GaP substrate.
Processing for Nanometer Structures
Staffs: K. Furuya S.
Arai Y. Miyamoto M.
Watanabe S.
Tamura
Visiting
Researcher: T.
Tanaka
Students: T. Arai T.
Morita R. Yamamoto H. Nakamura H.
Maeda K.
Matsuda
Y. Ninomiya K.
Sasao N. Kaneda U.
Shirai K. Takeuchi K.
Yokoyama
E. Hase R.
Nakagawa T.
Nonaka
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:
An AlInAs layer and high electron mobility transistor (HEMT) structures were grown using low-oxygen-content metalorganic precursors by metalorganic vapor phase epitaxy (MOVPE). The oxygen concentration in the AlInAs layer measured by secondary ion mass spectrometry (SIMS) was 7x1015 cm-3. Moreover, the mobility and sheet carrier concentrations of the AlInAs/InP HEMT structure in which a 2.5 mm-thick AlInAs buffer layer was inserted to reduce the diffusion of impurities from the substrate surface, were 5,500 cm2/Vs and 1.0x1012 cm-2 at 300 K, and 110,000 cm2/Vs and 8.7x1011 cm-2 at 77 K, respectively. For the AlInAs/GaInAs HEMT structure with the same buffer layer, mobility and sheet carrier concentrations were 12,000 cm2/Vs and 1.2x1012 cm-2 at 300 K, and 92,000 cm2/Vs and 1.2x1012 cm-2 at 77 K, respectively.