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) A GaInAsP/InP quantum-wire distributed feedback (Q-Wire
DFB) laser with the active region width of 24 nm in the period
of 240 nm was realized by an electron beam lithography, CH4/H2-reactive
ion etching and two-step organometallic vapor-phase-epitaxial
growth processes for the first time. A threshold current as
low as 2.7 mA (threshold current density = 270 A/cm2)
and differential quantum efficiency of 19 %/facet were achieved
for the stripe width of 3.0 mm
and the cavity length of 330 mm
under RT-CW condition. A single-mode operation with the sub-mode
suppression ratio (SMSR) as high as 51 dB (injection current
is twice the threshold) was also obtained in the lasing wavelength
of 1541 nm. From the lasing spectrum, the stopband width was
observed to be 4.8 nm which corresponds to the index-coupling
coefficient (ki)
of 180 cm-1.
(2) A GaInAsP/InP Q-Wire DFB laser with the active region
width of 30 nm in the period of 240 nm was realized by an
electron beam lithography, CH4/H2-reactive
ion etching and two-step organometallic vapor-phase- epitaxial
growth processes. High-mesa structure of this laser was fabricated
by using wet chemical etching to obtain low-damage interfaces
at the sidewalls. The spontaneous emission efficiency of this
Q-Wire DFB laser was almost comparable to that of a quantum-film
laser fabricated by one-step growth. This indicates not only
that this laser had a low-damage feature at the ultra fine
structures but also there was little nonradiative recombination
of the etched/regrown interfaces. By adopting low-damage fabrication
processes for high-mesa stripe structures, a threshold current
as low as 2.1 mA, which corresponds to a threshold current
density of 176 A/cm2, and
a differential quantum efficiency of 16 %/facet were obtained
for the stripe width of 3.4 mm
and the cavity length of 350 mm
under RT-CW condition. A sub-mode suppression-ratio (SMSR)
of 50 dB at a bias current of twice the threshold was also
achieved.
(3) A quantum-wire distributed feedback laser with GaInAsP/InP
strain- compensated single-quantum-well substrate for reducing
lateral size distribution of multiple-quantum-wire structures
was realized by an electron beam lithography, CH4/H2-reactive
ion etching and two-step organometallic vapor-phase-epitaxial
growth processes. This laser had the active region width of
30 nm in the period of 240 nm with the size fluctuation as
low as 8.7 %, which was much smaller than that in multiple-
quantum-wire laser of 18 %. A threshold current density as
low as 155 A/cm2 was achieved
for the stripe width of 3.5 mm
and the cavity length of 1120 mm
under a RT-CW condition.
(4) High characteristic temperature operation of 1590nm GaInAsP/InP
quantum-wire DFB lasers was obtained by the Bragg wavelength
detuning from the gain peak wavelength. The characteristic
temperature for threshold current of 95 K and that for the
differential quantum efficiency of 243 K were obtained for
20 to 80 ℃. These results indicated that the Bragg wavelength
detuning with a Q-WireDFB laser was very attractive for temperature-independent
and low threshold current operation.
(5) Investigations on polarization anisotropy for compressively
strained GaInAsP/InP quantum-wire (Q-Wire) structures fabricated
by electron beam lithography, dry etching and double-step
organometallic vapor- phase-epitaxial growth processes were
carried out via experimental evaluation of photoluminescence
(PL) and lasing characteristics. From PL measurement, parallel
transverse-electric field (TE) peak intensity to the Q-Wire
direction was measured to be 1.4-1.6 times stronger than perpendicular
TE peak intensity to the Q-Wire direction for the wire widths
of 24-45 nm. Furthermore, 2-types of Q-Wire laser with the
wire width of 35 nm were fabricated, i.e., quantum-wire directions
are perpendicular and parallel to the laser cavity; denoted
by Q-Wire⊥ and Q-Wire//,
respectively. As a result, although the spontaneous emission
efficiency of both lasers was almost the same, the threshold
current density of Q-Wire⊥was
much lower compared with that of Q-Wire//.
From the gain spectral measurement by Hakki-Paoli method,
it was demonstrated that the differential gain for Q-Wire⊥
was 5 times higher than that for Q-Wire//.
(6) Distributed reflector (DR) laser, which consists of the
active DFB and passive DBR sections with quantum-wire structure,
was studied. DFB and DBR sections are integrated by using
energy blue shift due to a lateral quantum confinement effect.
For DR laser with low-threshold and high-efficiency operation,
a high reflection DBR is required. From the theoretical and
experimental investigations of DBR reflectivity, DBR section
with the reflectivity of over 90% was confirmed. DR lasers
with low threshold, high efficiency and stable single-mode
operation have been realized using this high-reflective DBR
section. For further improvement in lower threshold current
operation, a DR laser with phase-shifted DFB section was realized.
Phase-shifted grating can be fabricated easily by changing
the EB lithography patterns. From the theoretical investigation
of the grating structure, it was found that the lowest threshold
current can be obtained by adopting l/8-shifted
grating. As a result, threshold current as low as 1.2 mA and
an external differential quantum efficiency of 13% from the
front facet were obtained under RT-CW condition. Lasing mode
exists inside the stopband due to the phase shift. A stable
single-mode operation with an SMSR of 49 dB was obtained at
a bias current of twice the threshold.
(7) Antireflection coating has been performed on the fabricated
DR laser. Al2O3
(n=1.74) has been used for low residual reflectivity to improve
the differential quantum efficiency and single-mode operation.
As a result, threshold current was reduced and differential
quantum efficiency was increased reproducibly which can be
attributed from the facet phase of the DFB section. A reduction
of threshold current from 4 mA to 1.4 mA and increase of differential
quantum efficiency from 19%/facet to 27%/facet have been obtained.
(8) For even lower threshold current operation of DR laser,
narrow stripe buried heterostructure (BH) has been proposed
to adopt. Mass transport technique has been taken into consideration
to cover the sidewall of the active grating region of the
stripe with InP so that surface recombination leakage current
can be reduced and as a consequence, threshold current will
be reduced. At the same time narrower stripe width around
1 mm will be fabricated.
(9) Utilizing the DR laser fabrication process which includes
EB lithography, CH4/H2-reactive
ion etching and OMVPE regrowth, monolithic integration of
DR laser with electroabsorption modulator (EAM) and detector
has been proposed. The large energy blue shift of quantum-wire
has been utilized to achieve different functional devices.
The wire width has been modulated at the different sections
of the device and thereby controlling the transition energy.
This fabrication technique possesses the simultaneous fabrication
of multiple devices without further complication of the established
process.
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(1) Novel semiconductor laser structure, such as a membrane
laser, which has the Benzocyclobutene (BCB) cladding layers,
enables to increase optical confinement into the active layer
due to a large refractive-index difference between the active
layer and cladding layers. A RT-CW operation of membrane DFB
laser, consisting of deeply etched single-quantum-well wirelike
active regions, was already demonstrated. We realized multiple-
wavelength laser arrays based on a membrane buried-heterostructure
distributed-feedback (BH-DFB) structure. Under optically pumped
RT-CW operation, a wavelength fluctuation for a fixed DFB
grating period was found to be less than ±1.2 nm for all 75
samples prepared on the same wafer. A total wavelength span
of 64 nm and an average channel spacing of 3.4 nm were obtained
in the DFB period modulation range of 311.25 - 335.00 nm with
1.25 nm steps. An average channel spacing of 4.1 nm was also
obtained in the stripe modulation range of 1.0 - 2.8 mm
with 0.2 mm steps.
(2) The lasing properties such as stripe-width dependence
of index-coupling coefficient were evaluated using narrow
stripe membrane BH-DFB lasers. The lowest threshold pump power
was obtained with the stripe width of 1.0 mm.
A single transverse-mode operation was obtained for the stripe
width less than 1.0mm.
(3) Realization of high reflective cavity, asperity corrugation
structure was investigated. Asperity corrugation structure
was fabricated by controlling InP regrowth time. Membrane
BH-DFB laser with asperity corrugation structure was realized
with a cavity length of 80 mm and
a threshold pump power of 1.3 mW at RT-CW condition. An index-coupling
coefficient was estimated to be over 2000 cm-1.
(4) In order to realize low threshold and stable mode operation,
narrow stripe membrane BH-DFB laser array using surface corrugation
was fabricated. A single-transverse mode and 68 nm large stop-band
operation was realized with 0.6mm-stripe
membrane BH-DFB laser using surface corrugation. This stopband
width corresponds to the index-coupling coefficient of ki=2950
cm-1, which is two times
larger than conventional (flat surface) membrane laser of
2.0 mm stripe width.
(5) Wafer bonding technology was investigated to integrate
active photonic devices on a silicon on insulator (SOI) wafer
for highly compact photonic- integrated circuits. A single-quantum-well
(SQW) GaInAsP/InP membrane structure bonded onto an SOI wafer
was successfully obtained by a direct bonding method with
a thermal annealing at 300-450 ℃ under H2
atmosphere. The PL intensity of the SQW membrane structure
did not degrade after this direct bonding process and its
spectral shape was not broadened.
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