Jpn. J. Appl. Phys. Vol. 40 (2001) pp. 5338–5339
Part 1, No. 9A, September 2001c2001
The Japan Society of Applied Physics

Short Note

GaInAsP Microcylinder (Microdisk) Injection Laser with AlInAs(O x ) Claddings
Masayuki F UJITA, Reona U SHIGOME, Toshihiko BABA, Akihiro M ATSUTANI1 , Fumio KOYAMA1 and Kenichi I GA1Faculty of Electrical and Computer Engineering, Yokohama National University, 79-5 Tokiwadai, Hodogaya-ku, Yokohama 240-8501, Japan
1 Precision and Intelligence Laboratory, Microsystem Research Center, Tokyo Institute of Technology, 4259 Nagatsuta, Midori-ku, Yokohama 226-8503, Japan(Received May 23, 2001; accepted for publication June 6, 2001)

We propose and demonstrate a 1.5-µm-GaInAsP/AlInAs microcylinder laser which improves the fragility of the microdisk
laser. The threshold current density was comparable to that reported for the 0.98-µm-GaInAs microcylinder laser. The selective
oxidation of the AlInAs claddings will further reduce the threshold by strong optical confinement in the disk like structure andwill simplify the large-scale integration of this type of device.
KEYWORDS: AlInAs, GaInAsP, microcavity, microcylinder, microdisk, selective oxidation semiconductor laser

flow rate is higher than 3 sccm, the optimum value for InP.
It is effective for increasing the etch rate of AlInAs, which
is nearly 60% of that for InP, and the etching selectivity ofAlInAs against the EB resist. Figure 1 shows the scanningelectron micrograph (SEM) of the fabricated microcylinder
laser. The side-wall angle of formed mesas was 87◦ against
the substrate plane, and the side-wall roughness was less than
10 nm. As the injection device, it has an AuZn electrode forthe top p-side and a AuGe one for the n-side.
Figure 2 shows lasing characteristics at room temperature under the pulsed condition. The lasing is observed atλ = 1507 nm and 1520 nm. The mode space of 13 nm isnarrower than 19 nm, which was calculated for two differ-AuZnp-GaInAs
Contact
p-AlInAs Cladding
GaInAsP
Active Layer
n-AlInAs Cladding
n-InP Substrate
Fig. 1. SEM of fabricated 12-µm-diameter GaInAsP/AlInAs microcylinder laser.

293 K, Pulsed (100 ns, 10 kHz)
Resolution 0.1 nm

Intensity [arb. units]

Microcavity semiconductor lasers are suitable for largescale integration due to their ultrasmall volume and ultralow
threshold. In particular, microdisk lasers1–5) have simple
geometry and can easily realize a high Q whispering galley (WG) mode. This is because of the strong optical confinement in both vertical and lateral directions by the total internal reflection at semiconductor and air boundaries.
In the GaInAsP/InP system, we have demonstrated roomtemperature continuous-wave operation with a threshold current of 40 µA4) and the smallest injection device with a disk
diameter of 2 µm.5) However, the structure of injection devices is fragile, since the disk active layer is exposed tothe air and supported by posts of submicron width. As analternative, a microcylinder laser having a simple circularmesa cavity has been investigated.6) However, the opticalconfinement is lower and therefore, the threshold is higher
than those in microdisk lasers. In this study, we propose aGaInAsP/AlInAs microcylinder laser to solve these problems.
An Al0.48 In0.52 As layer lattice-matched to an InP substratehas a refractive index of 3.16 at a wavelength λ of 1.55 µm,
which is slightly lower than that of InP. The use of this layer
as cladding is expected to improves the optical confinement
factor into quantum wells (QWs) by 10%, compared with thatfor InP claddings. In addition to this, it is expected to achievea much higher optical confinement, as in microdisk lasers, byselective oxidation. In this paper, we first describe the fabrication and evaluation of the microcylinder laser. Then, wediscuss the possibility of the oxide cladding structure.
In the experiment, we prepared an epitaxial wafer by
low-pressure metal-organic vapor-phase-epitaxy. It had aGaInAsP (Q) active layer composed of six 1% compressively strained (CS) QWs each of 4 nm thickness, 1.2-µmQ barrier layers each of 10 nm thickness, and 1.2-µm-Q and1.1-µm-Q cover layers of 50 nm and 30 nm thickness, respectively. The spontaneous emission peak wavelength was
λ = 1.525 µm. The thickness of the upper p-type and lower
n-type Al0.48 In0.52 As claddings was 1.5 µm, and the highly pdoped GaInAs top contact layer was 0.3 µm. For this wafer,
circular mesas of 12 µm in diameter and 4.8 µm in depth wereformed by electron-beam (EB) lithography and Cl2 inductively coupled plasma (ICP) etching using SAMCO International Inc. RIE-101ip.7) For GaInAsP/InP, we sometimes use
CH4 -based plasma etching, since it achieves a smooth etching
profile.1) However, the etch rate of this plasma against AlInAs
is too low to form fine mesas using an EB resist mask. Cl2ICP etching successfully solves this problem. We employeda gas pressure of 0.5 Pa and a Cl2 flow rate of 10 sccm. This26 mA

2118.513145015001550
Wavelength [nm]0 10 20 30 40 50
Current [mA]
Fig. 2. Lasing characteristics of 12-µm-diameter GaInAsP/AlInAs microcylinder laser.

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Jpn. J. Appl. Phys. Vol. 40 (2001) Pt. 1, No. 9A

M. F UJITA et al.

Contact
AlInAs Cladding
GaInAsP
Active Layer
AlInAs Cladding

AlInAsOxide

InP Substrate
Fig. 3. Schematic of GaInAsP microdisk laser with selectively oxidized
AlInAs claddings.

51.0Rough0.50450

0500
550
Oxidation Temperature [ C]

Relative PL Intensity [dB]

Oxidation Rate [ m/h]

Mirror-5600

Fig. 4. Temperature dependence of AlInAs oxidation. Open and closed
circles denote oxidation rate and relative PL intensity, respectively.

ent azimuthal modes by the formula λ2 /(2π an m ) with thedisk radius a and the modal index n m . The lasing modes
are considered to be different radial modes. The thresholdcurrent is 9 mA. The threshold current density Jth , which is
obtained as the threshold current divided by the mesa area,
is 8.0 kA/cm2 . This value is comparable to that reported fora microcylinder laser with a 0.98-µm-Ga0.8 In0.2 As CS-QWs
active layer, which is expected to have a higher gain than thatof GaInAsP.6) A reasonable explanation is the low surface recombination of GaInAsP and the smooth side-walls formedby the ICP etching. However, compared with the thresholdof microdisk lasers, these values are too large, even considering the large leakage current (∼ 70% of the total current)
passing through the center region. Clearly, this is causedby the low optical confinement of the GaInAsP/AlInAs orGaInAs/GaInP structure.
For the threshold reduction, we next discuss the possibility of the oxide cladding device. A schematic of the target
structure is shown in Fig. 3. It has the appearance of a microdisk laser buried by the oxide. It has been reported that theindex of AlInAs is decreased to 2.4 by the oxidation.8) The
estimated optical confinement factor in one QW will be enhanced to 2.3 times that by InP claddings and 0.8 times thatby air claddings. Thus, this structure achieves both the mechanical stability and the strong optical confinement simultaneously. The optimum oxidation depth for low light scattering
loss of the lasing mode and efficient current injection throughunoxidized claddings is nearly 0.7 µm.3) As a similar structure, a GaAs/GaInAs QW active layer with oxidized AlGaAs5339

cladding has been reported.9) A GaInAsP active layer bonded
on a GaAs/AlAs substrate has been investigated as anothercandidate.10) However, GaInAsP/AlInAsOx is more advantageous for the reduction in device size and threshold due to thelower surface recombination and/or the simple process that
does not involve bonding.
As a preliminary experiment for the oxidation of AlInAs,
we prepared a wafer without the top contact layer. The oxidation was carried out by using water vapor with N2 gas. Figure 4 shows the temperature dependence of the oxidation rate
and the relative photoluminescence (PL) intensity normalized
by that before the oxidation. The oxidation rate simply increases as the temperature increases. However, the surface
after oxidation was very rough due to the evaporation of As
at a temperature over 550◦ C. The oxidation rate was muchslower than those reported for thin films of less than 0.1 µmthickness.11–13) The PL was not degraded or was improved ata temperature less than 520◦ C due to an annealing effect. One
problem with the oxidation which we noted is the nonuniformity of the oxidation depth. This will be improved by controlling the vapor flow in the furnace, as has been done foroxide confinement vertical-cavity surface-emitting lasers.
In conclusion, we proposed novel schemes for themicrodisk-like laser, i.e., the GaInAsP/AlInAs microcylinder
laser and its oxidized device. We achieved lasing operationof the former device, and performed the preliminary experiment for the latter device. The strong optical confinement andthe mechanical stability expected for the oxidized device will
help the planarization of the device substrate by a polymer
and the formation of metal pads. Thus, this device will beeffective for the large-scale integration of this type of device.
The authors would like to thank Dr. N. Nishiyama and Mr.
M. Arai, Tokyo Institute of Technology, for help with the experiment.

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