Current-injection lasing in T-shaped GaAs/AlGaAsquantum-wire lasers
Shu-man Liu1, Masahiro Yoshita1, Makoto Okano1 and Hidefumi Akiyama1,2
Loren N. Pfeiffer2 and Ken W. West21Institute for Solid State Physics, University of Tokyo, and CREST, JST, Chiba 277-8581, Japan
Bell Laboratories, Lucent Technologies, 600 Mountain Avenue, Murray Hill, New Jersey 07974, USA

2Abstract. Current-injection T-shaped GaAs/AlGaAs quantum wires lasers have been fabricated by a cleaved-edge
overgrowth method with molecular beam epitaxy. Continuous single-mode operation at photon energy of ~1.5 eV has
been demonstrated between 30 K up to 70 K from laser diodes with high-reflectivity coating on both cleaved facets. Thelowest threshold current (Ith) of 0.27 mA has been achieved at 30K, which are attributed to the high quality of the 2D
confined structure and hence very low internal losses of the optical cavities.
Keywords: Quantum wire lasers, Low threshold current
PACS: 78.67.Lt, 42.55.Px

INTRODUCTION
The quantum-wire lasers are expected to operate
efficiently with low injection currents. Intensiveexperimental efforts have been made to verify andunderstand such low threshold current. Nevertheless,the physics arguments are still unclear, because
threshold currents are determined not only by the
optical and material properties of quantum wires, butalso by the design of optical waveguide and cavity,and by the electrical properties of the laser devices.
Further experiments are necessary on quantum-wirelaser samples with high controllability in material,
optical, and electrical structures. In this work, wedemonstrate low-threshold singe-mode lasing in 20period T-shaped quantum wire (T-wire) lasers withhigh material quality and a new efficient current
injection scheme.

EXPERIMENT
Current-injection T wire lasers were fabricated by
the cleaved-edge overgrowth method with molecularbeam epitaxy (MBE)[1]. Figure 1 shows a schematic
view of the laser structure. The quantum wires wereformed at the T-intersections of 20-period 14-nm-thick(001) Al0.07Ga0.97As multiple quantum wells (MQWs)(stem wells) and a 6-nm-thick (110) GaAs quantumwell (arm well). Laser bars of 500 µm cavity length
were cut from the wafer by cleavage with the cleaved

FIGURE 1. Schematic cross-sectional view of a Twire laser structure. The arrows at the upper panel
indicate the carrier injection from the doped layer to Twire active region via the arm well.
facets perpendicular to the axis of the T-wires afterMBE growth and metallization. The cavity facets werecoated by high reflectivity Au-layers after depositionof 70 nm SiO2 insulating layer with plasma-assisted
chemical vapor deposition.
By this injection scheme, both electrons and holes
are injected into the active region of 20 T-wires via
only the arm well, i.e. the current is well defined in thethin arm well, as shown by the arrows in the upperpanel. Light emission from the laser diode wasdispersed in a 0.75-m spectrometer and detected with acharge-coupled-device (CCD) camera. A photodiodedetector was used to measure the output power.

0.4 ns and the assumedη i= 1. This estimated result
agrees well with our separate experimental results on
optically pumped T-wire lasers [2-5].

RESULTS AND DISCUSSION
Figure 2 shows the current versus voltage (I-V)
curve of the laser diode at temperatures from 5K toroom temperature (r.t.), exhibiting typical diodecharacteristic. The soft turn-on of the I-V curve attemperatures below 30 K is probably due to carrier
freezing in doped MQW and buffer layers. Figure 3
shows the single facet light output power
characteristics for a 500 µm length laser diode as aFIGURE 2. Diode current-voltage characteristics of aT-wire laser at various temperatures from 5K to room
temperature (r.t.) indicated in the figure.
function of the bias current at 30 K, together with theoutput spectra at four different bias points below andabove threshold, respectively. The threshold current(Ith) of the laser device is 0.27 mA and the meandifferential quantum efficiency is 12% at 30K. The
optical spectrum is modulated by Fabry-Pérot
oscillations below the threshold current and turns intoa single lasing mode at 1.554 eV after the current
reaches the threshold current. The largest shift as thecurrent increases to 1.5 mA is 1.2 meV, showing thestability of the lasing energy from our T-wire lasers.
No emission or lasing from the stem wells has been
observed at all injection currents investigated,indicating high injection efficiency. Single-mode
lasing has been similarly observed at the cryostat
temperatures between 30 and 70 K. If we assume
internal quantum efficiency ηi= 1 on the basis of highinjection efficiency and negligibly small nonradiative
decay, the measured η d of 12% at 30 K gives anestimation of internal loss αi = 0.32 cm-1. The Ith of
0.27 mA at 30 K for 20 T-wires corresponds to 0.014
mA per 500-µm-long single-wire. Thus the threshold
carrier density is estimated to be 7x105 cm-1 per singlewire by using separately measured carrier lifetime ofFIGURE 3 Light power–current characteristics andemission spectra at different bias currents for a highreflection (HR) coated T-wire laser diode under c.w.
operation at 30K. The threshold current (Ith), theestimated differential quantum efficiency ( η d) andinternal loss (αi) has been indicated in the figure.

SUMMARY
We have presented low-threshold current-injectionlasers using 20-period T-wires of high material quality
as gain medium and a simple but efficient current
injection scheme, which contribute to the low
threshold current and hence low threshold current
density and low internal loss in our c.w. operated laser
diodes.

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