Low Dimensional Quantum Structure Lasers
Post-Doctoral Research Fellow: B.
Chen
N. Nunoya (from Oct.)
Students: N.
Nunoya M.
Morshed H. Yagi K.
Muranushi K. Ohira A.
Onomura T. Sano
GaInAsP/InP strained-quantum-film, -wire and -box lasers have been
studied. Distributed feedback (DFB) lasers consisting of 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-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, and the spontaneous emission
spectral width was almost constant at temperatures between 103 K and 253 K,
indicating a lateral quantum confinement effect. Finally, the spontaneous
emission efficiency below the threshold was almost comparable to that of the
Q-Film lasers up to 85 °C, that revealed 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 also realized. Size fluctuations of these
quantum-wire structures were measured by scanning electron microscope views, from
which 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 that of the 5-quantum-well lasers at room temperature. From EL spectra
of various wire widths lasers, a larger energy blue shift than that from a
simple analysis model was observed, which can be attributed to residual
compressive strain between the active region and surrounding InP layer.
(3)
High-performance operation of
1.55 mm
wavelength GaInAsP/InP strongly index-coupled and gain-matched distributed
feedback lasers with periodic wirelike active regions was realized, whose
index-coupling coefficient was more than 300 cm-1. In order to
design lasers for low threshold current operation, threshold current
dependences on the number of quantum wells and the wire width were investigated
both theoretically and experimentally. A record low threshold current of 0.7 mA
was realized at room temperature CW condition for a 2.3-mm-wide buried heterostructure
with a 200-mm-long cavity. We also confirmed stable single-mode operation due
to a gain matching effect between the standing-wave profile and the wirelike
active region.
(4)
A CW life test of 1550 nm
gain-matched DFB laser, which consists of wire-like active regions and exhibits
sub-mA threshold, was carried out. No degradation was observed in the output
and the spectral characteristics after 8500 hrs operation at a bias current
around 10 times the threshold.
(5)
A distributed reflector (DR)
laser consisting of wirelike active regions with asymmetric output
characteristic was realized for the first time. To realize an asymmetric output
property while maintaining low threshold current operation, a l/4 shifted grating and
modulated active region widths were introduced into the grating structure.
Threshold current as low as 1.8 mA, asymmetric output ratio of 8, and a sub-mode
suppression-ratio (SMSR) of 33 dB at I=1.2Ith were
obtained for the cavity length of 200 mm and the stripe width of 2.3 mm under a RT-CW condition.
New Types of Semiconductor Lasers for Photonic
Integration
Post-Doctoral Research Fellow: B.
Chen
N. Nunoya (from Oct.)
Students: J. Wiedmann N. Nunoya H.-C.
Kim K.
Ebihara K.
Matsui T.
Okamoto M.
Ohta Y.
Onodera H.
Kanjo
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) and
vertical-grating distributed feedback (VG-DFB) lasers as well as
vertical-grating distributed-reflector (VG-DR) lasers have been studied both
theoretically and experimentally.
Membrane lasers
consisting of very thin semiconductor core layer sandwiched by polymer/SiO2
cladding layers have been also studied.
Results obtained in this research are as follows:
(1)
High-reflectivity
semiconductor/BCB reflectors were fabricated by multiple sequential steps of CH4/H2
RIE etching and O2 plasma ashing. The reflectivity was estimated to
be as high as 95%. Using these reflectors, highly uniform 1.55-mm-wavelength lasers with low
threshold and high differential quantum efficiency were demonstrated. In
addition, the reliability of such polymer-buried DBR lasers was investigated
for the first time. The technology employed in this work is highly promising
for the monolithic integration and batch processing of edge emitting lasers
with other photonic devices through low-loss polymer waveguides.
(2)
The novel design for
obtaining single-mode operation by combing a DBR facet with multiple cavities
was analyzed in theory and experiment. It was shown that the loss per groove is
an important parameter for the best choice of the cavity number. Single-mode
operation was obtained for different number of cavities. Increasing the number
of cavities will decrease the efficiency drastically. The threshold current is
lowest for two or three cavities. Therefore, it can be concluded that a CC
laser is best for laser operation according to high efficiency and low
threshold. For the CC laser an SMSR of 36dB was achieved at 1.8 Ith.
(3)
A novel DR laser including a
vertically etched grating was successfully fabricated. In case of a mesa width
of WS = 6 mm and mesa width variation of DWS = 0.5 mm, a low threshold current of Ith = 12.4 mA and
a high differential quantum efficiency of hd = 42% were achieved with an SMSR of 33 dB.
(4)
Distributed feedback lasers
with a deeply etched first order vertical grating were realized for the first
time. It was shown that we could obtain an effective coupling by reducing the
stripe width. The sample with the cavity length of 430 mm, 1.8 mm stripe width and 0.2 mm grating depth on each
lateral side exhibited a 12.5 mA threshold current, 37 % total differential
quantum efficiency and an SMSR of 35 dB at a bias current of two times the
threshold.
(5)
By
use of VG-DFB structure, it was clarified that structural birefringence can be
completely eliminated. The grating coupling coefficient can also be
polarization independent by adjusting grating depth.
(6)
Novel semiconductor laser
structure, that is, 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 the active and cladding layers.
A RT-CW operation of membrane DFB laser consisting of deeply etched
single-quantum-well wirelike active regions was demonstrated for the first time
under optical pumping. A threshold power of 38 mW was obtained for 10.7 mm-wide and 40 mm-long device. From
spontaneous emission spectrum, a large stop-band width of 65 nm and a low
equivalent refractive index of 2.30, which are peculiar to a thin membrane
waveguide structure, were observed.
(7)
In order to realize single
mode and low threshold operation of 1.5 mm-wavelength GaInAsP/InP
membrane DFB laser, buried heterostructure (BH) was innovated by slightly
changing the fabrication process. A threshold pump power of 4.8 mW and an SMSR of
39 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 RT-CW optical
pumping. The corresponding threshold for current injection was roughly
estimated to be 88 mA.
Quantum
Coherent Electron Devices
Staffs: K.
Furuya Y.Miyamoto N.Machida S. Tamura
Visiting
Researcher: B. Zhang
Post-Doctoral Research Fellow: M.
Nagase (from Oct.)
Students: M. Nagase S.
Karasawa M. Kurahashi H. Oguchi
T.
Hirata R. Yamamoto H. Nakamura T. Okada
K.
Mae H. Maeda H. Nagatsuka Y.
Ninomiya
M.
Fukasawa D. Miyamoto K. Takeuchi
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, consecutive
numerical simulation essential to discuss observation of hot electron
interference in solid-state biprism has been performed. Two dimensional quantum
beam propagation method was used to calculate electron wavefunction when there
exists magnetic field. As results, it was found that magnetic field in a range
from 0 to 0.3 T shifted interference pattern of hot electron in space and
application of magnetic field in this range enables us to detect hot electron
interference when transverse energy spread of incident electron flux is 0.3
meV.
(2) To
eject electron wave with high lateral coherence into an active device region, a
coherent emitter has been proposed. By introducing an additional electrode, the
gate, to a double-barrier resonant-tunneling emitter, an electron flux with a
single wavelength, a wide wavefront spread, and high current density can be
achieved. This emitter can also be used for various types of solid-state
spectroscopy.
(3) We have proposed a novel
transistor where hot electrons are generated and travel through undoped
semiconductor region by adopting a combination of double-barrier
resonant-tunneling structure and metal wire, and we have attempted to
demonstrate attractive potential formation required in operation of the biprism
by our proposed transistor. After fundamental studies of crystal growth for
resonant-tunneling structure and burying metal in semiconductors, we have
fabricated the device by re-growth to bury Tungsten in GaAs using OMVPE. By
analysis of measured data, for the first time, a formation of attractive
potential around the buried metal wire has been demonstrated. Furthermore, to
reduce emitter-gate leakage current, we have fabricated transistors on InP
semi-insulating substrate and wired by free-standing wire. From measured I-V
characteristics, it has been shown that buried metal wire gate has enabled to
produce attractive potential and leakage current has been reduced by examining
value of peak current at negative differential resistance. Therefore, we have
been able to show the potential of our device.
(4) Based
on reciprocity theorem of quantum mechanics, a new experimental method using
ballistic electron emission microscopy (BEEM) for observation of electron wave
diffraction in semiconductor has been proposed and numerical simulations of the
experiment have been 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 energy spread of 200 meV of electrons
in BEEM current was assumed. This energy spread allows BEEM current to be over
1 pA when tunneling current is set to 10 nA and the current level is large
enough to be detected by ordinal BEEM system.
(5) To explore a possibility of phase
coherence estimations of hot electron in semiconductors, current-voltage (I-V)
characteristics of triple-barrier resonant-tunneling diodes (TBRTDs) have been
studied. The current peak broadening caused by the well-well coupling and
well-electrode coupling was revealed. The electron phase breaking effect was
incorporated into the analysis to obtain the I-V curve of TBRTD using the phase
correlation method. For strong well-well coupling, the peak current was
independent of the phase breaking, while for weak coupling it is dependent on
it. Therefore, the electron coherence can be estimated using TBRTD designed
appropriately.
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 K. Hoshina M. Saitoh N.
Sashinakai
S. Yamagami T. Abe T. Morita H. Sato R.
Yamamoto
Y. Fujioka K.
Kashima Y. Kontani K. Matsuda T. Nagai
H.
Nagatsuka N. Orihashi T. Sakai H.
Sato K. Yokoyama
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)
A GaInAs/InP
buried metal heterojunction bipolar transistors (BM-HBTs) with a 0.3-µm-wide
emitter was fabricated by electron-beam lithography, in which tungsten stripe
with the same area as the emitter was buried with an intrinsic collector layer.
Total base collector capacitance was reduced to about 30% of that calculated
from physical dimensions of a conventional HBTs. Minimum total base collector
capacitance was less than 1fF..
(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 a
combination of dry etching and wet etching. .Current gain of 51 was observed in
the HBT with 0.1-μm-wide emitter was confirmed. To our knowledge, this emitter
width of HBT is the smallest.
Light Emitting Devices Using Advanced Structures and Materials
Staff: M.
Watanabe
Students: T. Maruyama S. Okano N. Sakamaki M.Suzuki
T.
Ishikawa N.
Nakamura Y. Niiyama M.
Matsusda
D.
Okamoto T.
Kanazawa
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.
Novel candidate for lattice-matched, wide-bandgap compound semiconductor Be-chalcogenides
was proposed and epitaxial growth of high-Be-content BeZnSe was 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 resonant tunneling diode (RTD)
structures. The fine hole arrays of 100 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, energy subband control by changing CdF2-QW thickness
was firstly demonstrated.
(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. To control inhomogeneous migration of
CaF2, hydrogen terminated Si(100) was prepared and CaF2
was gown at 300°C, which yields atomically flat CaF2 epilayers on
Si(100) substrate. Based on the CaF2, DBRTD structure was fabricated
and NDR with peak-to-valley current ratio (PVCR) = 4.3 was observed at room
temperature for the first time.
(3) 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.
(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, BexZn1-xSe
(x=0.2-1.0) was grown on CaF2/Si(111) and GaP(001) substrate by
MBE at growth temperature of 250oC. The crystalline quality and
surface morphology of BeZnSe/GaP(001) were better than the one of BeZnSe/CaF2/Si(111).
Single crystalline and atomically flat BeZnSe on GaP(001) was obtained at 250oC,
0.1 mm/h and
VI/II=1. UV photoluminescence was observed from band edge at 17K and room
temperature. The growth of BeMgZnSe on GaP(001) was also obtained. Crystalline
quality of BeMgZnSe was maintained until Mg content of around 15 %. This
result, which was contained in high Mg content, was obtained for the first
time.
Processing for Nanometer
Structures
Staffs: K. Furuya S.
Arai Y. Miyamoto M. Watanabe S. Tamura
Students: T. Arai H. Oguchi S.
Yamagami T. Morita H.
Nakamura
Y. Okuda R.
Yamamoto H. Maeda T.
Ninomiya K.
Takeuchi K. Yokoyama
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)
Contact characteristics and
isolation characteristics of 80-nm-period fine electrodes were reported. By
using Au/Ti as electrodes and 2×1019 cm-3 as a carrier
concentration of n-GaInAs, observed contact resistivity was less than 1.5×10-4
Wcm2.
By using etching of n-GaInAs layer and band discontinuity in undoped isolation
characteristics satisfied the condition to observe an interference pattern of
hot electron.