Low-Threshold Current-Injection Single-Mode Lasing
in T-shaped GaAs/AlGaAs Quantum Wires
Shu-man LIU, Masahiro YOSHITA, Makoto OKANO, Toshiyuki IHARA,
Hirotake ITOH and Hidefumi AKIYAMA
Institute for Solid State Physics, University of Tokyo, and CREST, JST, 5-1-5 Kashiwanoha, Kashiwa, Chiba 277-8581, Japan
Loren N. PFEIFFER, Ken W. WEST, and Kirk W. BALDWIN
Bell Laboratories, Lucent Technologies, 600 Mountain Avenue, Murray Hill, New Jersey 07974
E-mail address: lsm@issp.u-tokyo.ac.jp
Abstract: Current injection lasers incorporating 20 periods of T-shaped GaAs/AlGaAs quantum
wires as the active region have been fabricated by a cleaved-edge overgrowth method with a
molecular beam epitaxy. Continuous single-mode operation at the photon energy of ~1.5eV
from ground subband of quantum wires has been demonstrated between 30K up to 70K in laser
diodes with high-reflectivity coatings on both cleaved facets. The lowest threshold current (Ith)
of 0.27 mA, the typical output power of 0.13 mW at 3Ith and the external differential quantum
efficiency of 17% have been achieved at 30 K.
Quantum-wire lasers are expected to operate efficiently with low injection currents [1-3].
Intensive experimental efforts have been made to verify and understand low threshold currents
in quantum-wire lasers [4-11]. The first quantum-wire lasers using GaAs V-groove wires with 9
nm x 80-100 nm sizes grown with MOCVD by Kapon and coworkers in 1989 showed
continuous-wave (cw) room-temperature lasing with about 50 mA threshold current [4]. Tiwari
and coworkers reported, in 1994, 3-period 10 nm x 35 nm InGaAs V-groove wire lasers grown
by molecular beam epitaxy (MBE) showing significantly low threshold current of 0.19 mA at
room temperatures [6]. Wegscheider and coworkers in 1994 reported current-injection
15-period 7 nm x 7 nm GaAs T-shaped quantum-wire lasers formed by cleaved-edge
overgrowth method with MBE with the atomic-layer precision, in which 0.4-0.8 mA threshold
currents for multi-mode cw lasing were measured at 4 K [7]. In 2005, Yagi and coworkers
reported 9 nm x 24 nm InGaAsP rectangular etching-regrowth quantum-wire lasers fabricated
with electron-beam lithography, dry-wet etching, and MOCVD growth and achieved cw lasing
with low thresholds of 2.7 mA at room temperature and 1.2 mA at 180K [11].
Nevertheless, physics arguments on low threshold current in quantum wire lasers are still
unclear, because threshold currents are determined totally by the optical and material properties
of quantum wires, the design of optical waveguide and cavity, and the electrical properties in
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transport, injection, and leakage. Further experiments on laser samples with high controllability
in material, optical, and electrical structures are necessary.
In this work, we study current-injection 20-period 14nm x 6nm T-wire lasers by using a
new simple carrier injection scheme. This injection scheme uses a symmetric doping structure
for p-type and n-type layers, and the current-injection paths are well defined. In this device, we
demonstrated low-threshold single-mode lasing in the wire ground subband at 30-70 K. At 30 K,
a low threshold current of 0.27 mA has been demonstrated under cw operation condition with
the external differential quantum efficiency of 17%.
Figure 1 shows a schematic view of a laser structure. 20 T-wires are formed at the right
angle T shaped intersections of 20-period 14-nm Al0.07Ga0.93As multiple quantum wells
(MQWs) and a 6-nm GaAs quantum well. The cleaved-edge overgrowth method has been
described in details elsewhere[12]. Briefly, the first MBE growth takes place on a (001) oriented
n+-GaAs substrate beginning with a 1-m n-type GaAs buffer layer, a 0.3-m n-type GaAs layer,
20 periods of the n-type GaAs (6 nm)/Al0.35Ga0.65As(9 nm) MQW injection layer, a 1.35-m
lower cladding layer of 50% digital alloy (GaAl)4(AlAs)4, a stem MQW layer composed of 20
periods of 14-nm Al0.07Ga0.093As QWs (stem well) and 39-nm Al0.35Ga0.65As barriers, a 1.35-m
upper cladding layer of 50% digital alloy (GaAl)4(AlAs)4, 100 periods of p-type GaAs (6
nm)/Al0.35Ga0.65As (9 nm) MQW injection layer, and a 10-nm p-type GaAs cap layer. Si and C
were used as n-type and p-type dopants, respectively. The nominal doping level in both p and n
type injection layers is 1x1018 cm-3. The cleaved-edge overgrowth on the in situ cleaved (110)
edge consists of a 6-nm GaAs QW (arm well) layer, a 11-nm Al0.5Ga0.5As barrier layer, a
178-nm Al0.1Ga0.9As core layer, a 1.0-m side cladding layer of 50% digital alloy
(GaAs)6(AlAs)6, and a 10-nm GaAs cap layer. After the MBE growth, the upper layers were
partial etched away as shown schematically in Fig. 1 so as to minimize leakage current and also
to contact the n-type MQWs. AuBe and Ni/Ge/Au/Ni/Au were used for the p-type and n-type
contacts, respectively. After evaporation of metal ohmic contacts, the wafer was annealed at
450oC for 30 min for alloying the contacts.
In this structure, the symmetrically doped injection layers are arranged sandwiching the
stem MQWs, but electrically isolated from the latter by two undoped cladding layers (see Fig.
1). The single arm well is divided into three parts: the intersection of stem MQWs with the arm
well (solid circle), i.e. T-wire active region; the intersections of upper and lower cladding layers
with arm well, referred as cladding arm; and the intersections of doped MQWs with arm well
(dashed circle), referred as injection arm for simplification. Three cladding layers bound the
T-wire gain region to produce a T-shaped waveguide confining an optical mode in the vicinity of
T-wires. Under a forward bias, both electrons and holes are injected into the arm well through
the parallel p-n junction as shown by the crude arrows in Fig.1, then travel to the active region
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of T-wires along the arm well. Thus the current is expected to be confined in the thin arm well
during laser operation.
Laser bars of 500-m cavity length were cut from the wafer by cleavage with the cleaved
facets perpendicular to the axis of the QWRs, i.e. [ıī0] direction. After deposition of 100 nm
SiO2 insulating layer with plasma-assisted CVD, the cavity facets were high reflection (HR)
coated by 50-nm Au-layers on the front and 300 nm on the rear. The laser device was attached
p-type side up to a copper block with silver-epoxy glue, and set on the cold finger in a
helium-flow-type cryostat. The laser diode was driven by a DC-voltage-current source-monitor
unit (Advantest R6240A), and studied under the cw operation mode. For the measurement of
lasing characteristics, light emission from the laser diode was dispersed in a 0.75-m
monochromator and detected with a charge-coupled-device (CCD) camera. A Si photodiode
power meter was used for the measurement of output power. Photoluminescence (PL)
measurement was performed with the excitation light of 1.67 eV photon energy from a cw
titanium-sapphire laser in the backward-scattering geometry via the overgrown (110) surface
under point excitation into a 0.8-m spot with a 0.5 numerical aperture objective lens.
The bottom panel in Fig. 2 shows a PL spectrum of the laser diode at 30 K. An emission
peak at 1.564 eV is attributed to the ground exciton recombination in T-wires [13,14]. The
splitting of the peak is due to thickness fluctuation of the arm QW. The second strong peak at
1.642 eV in PL spectrum is the emission from the stem wells. The weak emission centered at
1.582eV near the T-wire peak is due to the emissions from the injection arm, the arm well and
the doped MQW layers, according to our separate spatial resolved imaging measurements.
Spectra of electroluminescence (EL) and laser emissions from the front facet of the cavity
of the laser diode for various current levels at 30 K are shown in upper panels of Fig. 2. The
insets are the corresponding near-field patterns. Emission from the T-wires dominates the EL
spectra. No emission from stem wells was observed in the EL and lasing spectra at all injection
currents investigated here, which indicates that current leakage into stem wells has been
effectively prevented and the current was confined in the arm well, as expected. The very weak
EL emission centered at 1.582eV from the arm well indicates that only a small amount of
carriers passed through the T-wire active region and recombined outside under the present
operation condition.
Even at a low current level of 0.05 mA the EL spectrum exhibits clear developed 0.3-meV
spaced Fabry-Pérot (FP) oscillations. The small deviation of the FP oscillation center from the
PL peak is probably due to absorption of the high-energy photons within this spectral region. As
the current increases to 0.25 mA, the contrast ratio of the FP oscillations grows significantly and
finally turns into a single lasing mode at 1.554 eV after the current reaches the threshold. The
corresponding near-field pattern turns from a spreading heart shape at low current levels into a
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circular one, indicating a tight optical confinement above threshold. Single-mode lasing has
been similarly observed at the cryostat temperatures between 30 K and 70 K.
The corresponding single facet light output characteristics versus current (I-L curve) of a
500-m cavity length device operating under the cw condition measured at temperatures
between 30 K and 70 K is shown in Fig.3a. At 30 K we observe the lowest threshold current (Ith)
of 0.27 mA and the highest external differential quantum efficiency (d) of 17% near Ith. I-L
curves above 40 K exhibit good linearity but at 30 K the slope decreases when the current is
higher than 0.5 mA. The reason is unclear at present. At 30-40K, a single-mode output power of
0.13 mW was obtained for a 1 mA input current, or 3Ith. Taking into account the light absorption
in the Au-coatings, d can be expressed by d = i [T/(Lcav+Acoating+T)], where i is internal
quantum efficiency and is assumed to be unity here, T is transparency of the front HR coating,
Acoating is the total absorption of photons by HR coatings on both facets, and Lcav = 2L is the
amount of the photons lost due to internal loss where L = 500 m is cavity length. By using the
measured d = 0.17 and T = 0.054, Acoating = Afront + Arear = 0.104 calculated from the optical
constant of Au in Ref[17], we obtain  = 1.6 cm-1, indicating the high quality of our T-wires.
Figure 3b shows the current versus voltage (I-V) characteristic of the laser diode,
exhibiting typical diode I-V curve. The turn–on voltage of the device at temperatures between
30 K and 70 K is similar and near 1.55 V. The leakage current below 1.55 V is smaller than 1
A. The series resistance is estimated to be 400-600 ohm at temperature form 70 K to 40 K,
while about 1000 ohm at 30 K. The soft turn-on of the I-V curve at 30 K is probably due to
carrier freezing in doped MQW and buffer layers. Figure 3c depicts the measured threshold
current and the corresponding measured external quantum efficiency as a function of the
cryostat temperature. It can be seen that the threshold current increases slowly at first and then
quickly above 50 K. We suspect that some carriers in the small active volume may escape into
the other parts of the arm well outside the QWR active region at the high temperature region.
In summary, we have presented a low-threshold current-injection laser using 20 periods of
T-shaped GaAs/AlGaAs quantum wires as gain medium with a simple current-injection scheme.
We have demonstrated cw single-mode lasing at 1.554 eV with threshold of 0.27 mA and
differential quantum efficiency of 17% at 30 K.
This work was partly supported by a Grant-in-Aid from the Ministry of Education, Culture,
Sports, Science, and Technology, Japan.
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Figure captions
Figure 1 Schematic structure of a T-shaped quantum-wire laser sample with 20 wires.
Figure 2 From bottom to top: PL spectrum measured in the geometry of point excitation, EL
spectrum of 500-m-long T-wire laser structure at 0.05mA, 0.25mA and lasing spectrum at
0.42mA. Inset is the corresponding near-field pattern.
Figure 3 (a) Light output vs current (b) current vs voltage characteristic curves of the T-wire
laser operating at various temperatures. (c) Variations of the threshold current and external
quantum efficiency with increasing temperature.
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