Continuous wave lasing in GaInAsP
microdisk injection laser with threshold
current of 40 µ A
M. Fujita, R. Ushigome and T. Baba
A threshold current of 40 µA, nearly 1/4 of the previous lowest record,
has been obtained in a GaInAsP-InP microdisk injection laser. This
decrease was thought to be mainly due to the reduction of disk diameter
and symmetric post-claddings by Cl2/Xe inductively coupled plasma
etching.
Low threshold lasing is indispensable in an integrated photonic system
with a large number of light emitters. Microdisk lasers [1, 2] are promising devices for obtaining an ultra-low threshold due to their small cavity volume of < 1 µm3 and strong optical confinement owing to the
semiconductor/air boundaries. Previously, we have reported a record
low threshold current Ith of 150 µA under continuous-wave (CW) conditions at room temperature in a GaInAsP-InP device [3]. However, this Ith
is much higher than the theoretical value of 10 µA, which is expected
from the rate equation analysis [3]. In this study, we obtained a remarkable reduction in the threshold of devices fabricated by using the Cl2/Xe
inductively coupled plasma (ICP) etching [4], which allowed a smooth
and vertical etching profile to be realised.
The basic structure and fabrication process were the same as those
described in [3]. A GaInAsP-InP wafer with four compressively-strained
quantum wells (QWs) was vertically etched to form circular mesas
using ICP etching system SAMCO RIE-200ip with a Shipley SAL601
electron beam resist as a mask. The gas flow rates of the Cl2 and Xe
were 5 and 1 sccm, respectively, and the total gas pressure was 0.5 Pa.
The ICP and bias power were 300 and 100 W, respectively. This etching
condition enabled sufficient selectivity of the semiconductor to be
obtained against the resist mask. The sidewall roughness was much
lower than that obtained using Cl2 based electron cyclotron resonance
(ECR) plasma etching and almost comparable to that using CH4 based
ECR etching. The disk shape was formed by the selective wet etching of
the InP claddings using an HCl solution. The current flowed from the
top electrode via a Pr-Ir needle. The light was directly detected by a
multimode fibre and analysed by an optical spectrum analyser with a
resolution of 0.2 nm.
700A/cm2. But considering the gradual decrease in carrier concentration
from the post-claddings toward the disk edge, at which the whispering
gallery (WG) mode is localised, the effective Jth for the WG mode is
estimated to be 320A/cm2 (or 80A/cm2 per quantum well (QW)). This
value is comparable to the lowest recorded for a 1.5 µm GaInAsP stripe
laser with a 1500 µm-long cavity [5]. This result indicates that a high
quality microcavity was achieved by this fabrication process.
Fig. 2 shows the scanning electron micrograph (SEM) of the device
fabricated in this study. Mesas formed by the CH4-based ECR etching
have tilted sidewalls, the angle against the substrate plane of which is <
80° [2]. Therefore, p- and n-side posts formed by the wet etching
become asymmetric. This disturbs the diffusion of carriers, especially
holes, or causes a large scattering loss of the WG mode at the post
edges. In contrast to this, mesas formed by the ICP etching have almost
vertical sidewalls. This allows symmetrical posts to be formed, as seen
in Fig. 2.
Fig. 2 Scanning electron micrograph of device fabricated by Cl2/Xe ICP
etching, and schematic diagram of formed mesas and disks with posts
Fig. 3 Threshold current for 3 µm-diameter device calculated with excess
etch-depth of p-side cladding against n-side cladding
Half width of n-side post is fixed to 0.8 µm; D is diffusion constant
Fig. 1 Lasing spectra and laser mode peak intensity against current characteristic
CW, 287K
We observed CW lasing in 15 devices with a disk diameter of 2.7 µm.
Fig. 1 shows a lasing characteristic. The threshold Ith was 40 µA, and
the lasing wavelength was 1.563 µm. The intensity of the mode peak was
20dB higher than the background spontaneous emission level at a current of 1.5 × Ith. If a uniform carrier distribution in the disk active region
is assumed, then threshold current density Jth can be calculated to be
ELECTRONICS LETTERS
27th April 2000
Vol. 36
Fig. 3 shows the threshold current Ith against excess etch-depth of the
p-side cladding ∆wp against the n-side one. It was calculated by rate
equations including the carrier diffusion effect [3]. The scattering loss at
the posts was determined by the n-side post edges. By reducing ∆wp
from 0.3 µm to 0 µm, Ith can be reduced 0.5–0.9 times by the accelerated
carrier diffusion. We consider that the reduction in ∆wp and the reduction in disk diameter were the two major reasons for the remarkable
reduction in threshold. Another reason considered is the matching of the
lasing wavelength and the gain peak. We shall discuss this in another
Paper and include a measurement of the spontaneous emission factor.
In summary, we have obtained a threshold current of 40 µA and an
effective threshold current density of 320A/cm2 in a GaInAsP microdisk
injection laser. We expect to further reduce the threshold to < 10 µA by
reducing the disk diameter to < 2 µm.
No. 9
Acknowledgments: We would like to thank Y. Kokubun, Yokohama
National University, and K. Iga, S. Arai, F. Koyama, and T. Miyamoto,
of the Tokyo Institute of Technology, for helpful suggestions. We also
thank A. Kasukawa, The Furukawa Electric Co. Ltd., for help with the
experiment. This work was partly supported by The Grant-in-Aid
#10210203 by the Ministry of Education, Science, Sports and Culture,
and also supported by the Japan Society for the Promotion of Science,
Research for the Future, #JSPS-RFTF 97P00103 and Research Fellowship #05045.
© IEE 2000
Electronics Letters Online No: 20000609
DOI: 10.1049/el:20000609
1
2
3
7 March 2000
4
M. Fujita, R. Ushigome and T. Baba (Yokohama National University,
Division of Electrical and Computer Engineering, 79-5 Tokiwadai,
Hodogayaku, Yokohama, 240-8501 Japan)
E-mail: [email protected]
ELECTRONICS LETTERS
References
27th April 2000
Vol. 36
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MCCALL, S.L., LEVI, A.F.J., SLUSHER, R.E., PEARTON, S.J., and LOGAN, R.A.:
‘Whispering gallery mode microdisk lasers’, Appl. Phys. Lett., 1992, 60,
(3), pp. 289–291
BABA, T., FUJITA, M., SAKAI, A., KIHARA, M., and WATANABE, R.: ‘Lasing
characteristics of GaInAsP/InP strained quantum-well microdisk
injection lasers with diameter of 2–10 µm’, IEEE Photonics Technol.
Lett., 1997, 9, (7), pp. 878–880
FUJITA, M., SAKAI, A., and BABA, T.: ‘Ultra-small and ultra-low threshold
GaInAsP-InP microdisk injection lasers - design, fabrication, lasing
characteristics and spontaneous emission factor’, IEEE J. Sel. Topics
Quantum Electron., 1999, QE-5, (3), pp. 673–681
MATSUTANI, A., OHTSUKI, H., KOYAMA, F., and IGA, K.: ‘Vertical and smooth
etching of InP by Cl2/Xe inductively coupled plasma’, Jpn. J. Appl.
Phys., 1999, 38, (7A), pp. 4260–4261
OSINSKI, J.S., ZOU, Y., GRODZINSKI, P., MATHUR, A., and DAPKUS, P.D.: ‘Lowthreshold-current-density 1.5 µm lasers using compressive strained
InGaAsP quantum wells’, IEEE Photonics Technol. Lett., 1992, 4, (1),
pp. 10–13
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