Low-Threshold Current-Injection Single-Mode Lasing inT-shaped Quantum Wires with Parallel Doping Layer
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: CW single-mode lasing from 30K to 70K with the lowest threshold current of 0.27mA has
been demonstrated in 20-period T-shaped quantum-wire laser diodes with parallel p and n doping layers.
©2007 Optical Society of America
OCIS codes: (140.5960) Semiconductor lasers

1. Introduction
Quantum wire (QWR) lasers are expected to exhibit improved performance including low threshold currents, narrowspectral linewidths and reduced temperature sensitivity due to the particular one-dimensional active region[1]. Thus
quantum wire laser diodes are one subject of intensive research and have been realized by several groups during thepast decade[2-4]. However, the expected superior performances have not yet been practically realized because of thedifficulty in preparation of highly homogenous QWR structure. T-shaped quantum wires (T-wire) can be producedwith a quality comparable to that of quantum wells by the cleave-edge-overgrowth (CEO) method in molecular beam
epitaxy (MBE), and allows size less than the Bohr radius of the exciton and small thickness fluctuations, hence
exhibits confinement in the 1D quantum limit[5-8]. In 1994, Wegscheider et al. [9] reported low threshold currents of
0.4-0.8 mA at 4 K for multimode emission from 15-period T-wire laser diodes with perpendicular p and n dopinglayers. Here, we report the lowest-threshold T-wire lasers based on a new injection scheme introducing parallel p andn doping layers. We also present detailed experimental studies of the current-injection T-wire lasers via
micro-photoluminescence (microPL) spectroscopy and imaging, and show that through proper design of current
injection path it is possible to achieve low threshold lasing.
2. Fabrication
Figure1 shows a schematic view of a laser structure.
20-period GaAs/AlGaAs quantum wires formed at
T-shaped intersections of 14-nm-thick stem wells and a
6-nm-thick arm well are fabricated by the CEO methodwith MBE. The quantum wires are embedded in thecore of the T-shaped optical waveguide surrounded by
the three cladding layers. Si-doped (C-doped) multiple
quantum wells (MQWs) as an n-type (p-type) injection
layer were grown at the bottom (top) of the slab
waveguide during the first growth. By this scheme, bothelectrons and holes are injected into the arm well and Fig.1 Schematic structure of a T-shaped quantum-wire laser sample
with 20 wires of 14 nm by 6 nm size.
then flow into the T-wires, which is different fromRef.[4]. A laser bar with 500 µm cavity length was cut from the wafer bycleavage and the cavity facets were coated by 50-nm-thick (front facet) and
200-nm-thick gold films.
3. Results
Figure 2 shows typical microPL spectra obtained by scanning across anentire 7-µm-thick sample. A simplified schematic cross-section of thesample is shown in the inset. From the microPL spectra, we can determine
the recombination energy in 1D T-wire, 2D stem wells, C-doped andSi-doped injection as 1.565, 1.644, 1.586 and 1.600 eV, respectively. The
narrow FWHM of ~ 0.9 meV and the small energy split of ~1.8 meV of thedominant 1D exciton peak indicate small thickness fluctuation of theFig.2 MicroPL spectra with the schematiccross-section

overgrown arm well.
Under cw current injection operation, single mode lasing wasobserved between 30 and 70 K with the threshold current from 0.27
to 0.70 mA and the differential quantum efficiency of from 12% to8%. A typical light-current-voltage characteristic at 30 K is shown inFig. 3a. High-resolution electroluminescence (EL) and lasing spectra
below and above threshold at 30 K are shown in Fig.3b. The
modulation in the EL intensity below lasing threshold shows
Fabry-Perot cavity modes with the mode spacing of 0.28 meV (0.16nm). Above threshold, a narrow peak develops at 1.555eV in the Fig.3 a) A typical voltage-current-outputat 30 K; b) High-resolution emission
center of the spontaneous emission band. The largest shift as the characteristic
spectra at three input currents.
current increases to 1.5 mA is 2 meV, showing the stability of thelasing energy from our T-wire laser diodes. No emissions from stem wells have been observed in all the EL spectra
below and above threshold. The threshold injection current of 0.27 mA for 20-period 500-µm-long T-wires represents
14 µA per single-wire, which is to our knowledge, the lowest lasing threshold current reported for GaAs quantumwires.
We also investigated the samples over a temperature range from 5 to 100 K by EL imaging measurement..
Figure 4 shows near-field EL images of a sample at 0.35 mA, 30 K and 0.38 mA, 5K, respectively. The signal is theunfiltered emission of the sample. It is clear that at 30 K, when the current injection is above threshold, the lasing
pattern is located at the center of T-wire region, while at 5 K under the current injection of 0.38 mA, the EL from thep-doped layer becomes distinct. Similar EL emission from p-doped layer has been observed at temperatures below
30 K.
We discuss further the carrier injection, recombination based on a schematic energy band diagram shown inFig.5. Once entering the arm well, electrons can move electrons to the T-wire active region with lowest energy.
However, lower mobility of hole in injection layer is believed tomake the injection of electron and hole into T-wires
nonequilibrium. When the forward bias is high enough, electrons
have large probability to pass T-wires. Therefore, strong
emission from the C-doped layer is observed at 5 K when thecurrent is 0.38 mA. To avoid such hole freezing, modulation
doping will help in obtaining enough hole mobility even at 5 K.
At high temperature gain reduction is usually caused by thermal
smearing of carrier population. The optical confinement factor isvery small (4.6x10-3) [10] in our T-shaped structure, thus it is
EL images at 30
possible that the peak gain can not compensate all the losses Fig.4
K, 0.35 mA and 5 K, Fig.5 Schematic energy band
even with high material quality. Increasing the number of 0.38mA.
diagram.
T-wires in the active region might help to increase the opticalconfinement factor.
4. Conclusions
We have presented a low-threshold current-injection laser using T-shaped GaAs/AlGaAs quantum wires as gain
medium with a simple current-injection scheme. We have demonstrated continuous-wave single mode lasing at
1.555eV with threshold of 0.27mA and differential quantum efficiency of 12% at 30K and discussed the carrier
injection and recombination at various temperature range based on EL imaging measurements.
References[1] Y. Arakawa and H. Sakaki, “Multidimensional quantum well laser and temperature dependence of its threshold current,” APL 40,939 (1982).
[2] S. Tiwari et al., “High efficiency and low threshold current strained V-groove quantum-wire lasers,”APL 64, 3536 (1994).
[3] S. Hara et al, “Self-organised InGaAs quantum wire lasers on GaAs multi-atomic steps,”Electron. Lett. 34, 894 (1998).
[4] H. Yagi et al., “Low-threshold-current operation of 1540 nm GaInAsP/InP distributed-feedback lasers with multiple-quantum-wire activeregions,” APL 87, 223120 (2005).
[5] H. Akiyama et al., “Photoluminescence study of lateral confinement energy in T-shaped InxGa1-xAs quantum wires” PRB 57, 3765 (1998).
[6] W. Langbein et al, “Optimization of the confinement energy of quantum-wire states in T-shaped GaAs/AlxGa1-xAs structures,” PRB 54,14595 (1996).
[7] J. Hasen et al., “Metamorphosis of a quantum wire into quantumdots,” Nature 390, 54 (1997)[8] H. Akiyama et al., “Coulomb-correlated electron-hole plasma and gain in a quantum-wire laser of high uniformity,” PRB 67, 041302R (2003).
[9] W. Wegscheider et al.,“Current injection GaAs/AlGaAs quantum wire lasers fabricated by cleaved edge overgrowth,” APL 65, 2510 (1994)[10] Y. Takahashi et al., “Room-temperature excitonic absorption in quantum wires,” APL 87, 223119 (2005).