Optics and Photonics Journal, 2013, 3, 21-24
doi:10.4236/opj.2013.32B005 Published Online June 2013 (http://www.scirp.org/journal/opj)
A Comparative Study of Fabrication of Long Wavelength
Diode Lasers Using CCl2F2/O2 and H2/CH4
B. Cakmak1,2, M. Biber3, T. Kar a ca l i 2, C. Duman4
1Erzurum Technical University, Vice-President, Erzurum, Turkey
2Department of Electrical and Electronics Engineering, Atatürk University, Erzurum, Turkey
3Ağrı İbrahim Çeçen University, Dean of the Faculty of Art and Science, Ağrı, Turkey
4Erzurum Vocational School, Atatürk University, Erzurum, Turkey
Received 2013
ABSTRACT
We report comparatively on fabrication of two-section ridge-waveguide tapered 3 quantum well (QW) InGaAsP/InP
(1300 nm) and 5 QW AlGaInAs/InP (1550 nm) diode lasers. Gas mixtures of CCl2F2/O2 and H2/CH4 were used to form
ridge-waveguide on the lasers with InP-based material structures. As known, chlorine- and hydro-carbon based gases
are used to fabricate ridge-waveguide structures. Here, we show the difference between the structures obtained by using
the both gas mixtures in which surface and sidewall structures as well as performance of the lasers were analysed using
scanning electron microscopy. It is demonstrated that gas mixtures of CCl2F2/O2 highly deteriorated the etched struc-
tures although different flow rates, rf powers and base pressures were tried. We also show that the structures etched
with H2/CH4 gas mixtures produced much better results that led to the successful fabrication of two-section devices with
ridge-waveguide. The lasers fabricated using H2/CH4 were characterized using output power-current (P-I) and spectral
results.
Keywords: Diode Lasers; Fabrication; Two-Section; Ridge-Waveguide; CCl2F2/O2 and H2/CH4
1. Introduction
InP-based devices have started to dominate opto-electronics
because lasers and related devices with InGaAsP/InP and
AlGaInAs/InP heterostructures are suitable for low-loss
fibre communications and integrated optics. The fabrica-
tion of integrated optoelectronic devices necessitates
pattern transfer techniques with a high degree of preci-
sion and a variable anisotropy, which is not achievable
with wet etching process. Various dry etching techniques,
such as plasma etching [1-2], reactive ion etching (RIE)
[3-5], ion beam etching (IBE) [6-7], reactive ion beam
etching (RIBE) [8-9], chemically assisted ion beam
etching (CAIBE) [10-11] and inductively coupled plasma
(ICP or RIE/ICP) etching [12] have been successfully
used to fabricate InP-based devices to date. Of these
techniques, RIE and ICP, which are well known and
widely used dry-etching methods, provide higher anisot-
ropy and better surface morphology when compared with
other techniques. RIE of InP has been reported by using
Cl2-based (halogen) chemistries [13-14] and methane
(CH4)/hydrogen (H2) mixtures [15-16].
In this study, we have fabricated two-section In-
GaAsP/InP and AlGaInAs/InP laser devices with ridge
waveguide using chlorine- and hydrocarbon based gas
mixtures and reported the results comparatively.
2. Fabrication of the Lasers
The epitaxial structures used for the fabrication of two-
contact devices are InGaAsP/InP (1300 nm) with three
quantum wells (QW) and AlGaInAs/InP (1550 nm) five
quantum wells (QW). These material structures are MBE
grown at the IQE (Europe) Ltd [17]. The epitaxial layers
were grown on a Si doped (3 x 1018) InP substrate. The
material system contained an 0.8 m n-type InP lower
cladding layer, a 226 nm waveguide layer, a 25 nm In0.85
GaAs0.33P quaternary etch stop layer, a 1.6 m p-type InP
upper cladding layer, a 50 nm In0.71GaAs0.62P transition
layer and finally an 0.2 m In0.53GaAs contact layer with
Zn doped at a concentration of > 1.5 x 1019 cm-3.
The active layer in AlGaInAs structures contains five
6 nm Al0.24GaIn0.71As quantum wells sandwiched by 10
nm Al0.44GaIn0.49As barriers. The wells are surrounded in
both directions with a 60 nm Al0.9GaIn0.53As buffer layer
and a 60 nm intrinsic AlGaInAs step graded index region.
Because AlGaInAs quaternary materials have a larger
conduction band offset (Ec /Eg = 0.7) compared to
InGaAsP ((Ec/Eg = 0.4), termal stability and electron
confinement in Al-quaternary quantum wells are better

B. CAKMAK ET AL.
22
than that of P-quaternary systems. Therefore, AlGaInAs
QWs and barriers were used in our material system, al-
lowing lasers to be operated without cooling. Our mask
design incorporated 2 and 4 m wide straight wave-
guides tapering out at angles of 2o, as shown in Figure 1.
The etch mask, formed using standard photolithogra-
phy process, contained 200 nm and 50 nm thick layers of
sputtered Si3N4 and evaporated Ni, respectively. The
mask pattern was then transferred to the substrate by RIE
with CHF3/O2 (40/3 sccm) and HF. The fabrication result
is shown in Figure 2.
2.1. Formation of Ridge-waveguides
The ridge waveguide with a 1.7 µm deep and 4 µm wide
was formed by dry etching InGaAs contact and InP upper
cladding layers with a gas mixture of H2:CH4 at a flow
rate of 20:10 sccm and CCl2F2/O2 at a flow rate of 19:1
sccm.
The stop-etch quaternary layer (In0.85GaAs0.33P) is ef-
ficient to provide a precise control of etch depth. Dry
etching was conducted in a parallel RIE at an RF power
of 400 W and a process pressure of 0.75 mTorr. A
maximum etch rate of 50 nm/min was observed in the
RIE. A polymer layer accumulated in the chamber, re-
quiring oxygen plasma (rf power 150 W, 50 sccm)
cleaning after each run. In RIE process, etching occurs
by both chemical due to the formation of volatile prod-
ucts and physical because of sputtering of surface by the
ions. Etching result in a gas mixture of CCl2F2/O2 is
shown in Figure 3.
Figure 1. Waveguide mask.
Ni/Si
3
N
4
mask
Si
3
N
4
Figure 2. Formation of Si3N4/Ni mask.
As shown in Figure 3, the surface and sidewall has a
sponge-like structure which proves that gas ions attacked
and deteriorated the structure. When used hydrocarbon
based gases (H2:CH4), the etched structure exhibits much
less roughness on the surface and sidewalls, as depicted
in Figure 4.
A SiO2 layer of 200 nm was then deposited using PECVD
(plasma enhanced chemical vapor deposition) followed
by the removal of the two-layered mask (SiO2/Ni) using
H2SO4 and HF. After applying contact window mask
using a second photolithography process, the SiO2 layer
on the waveguide was removed by HF to confine the
current injection to the ridge. Following a final photo-
lithography process to form two-contact on the p-side, a
20 nm titanium and 200 nm gold metal alloy was used in
the p-contact recipe. Then, the two contacts were defined
by lift-off. The two sections, both monolithically inte-
grated on a single laser chip, are called gain and absorber
sections. Finally, a 14 nm Au, 14 nm Ge, 14 nm Au, 11nm
Figure 3. Scanning electron microscope (SEM) photo of the
etched structure using CCl2F2/O2 gas mixtures.
Figure 4. SEM photo of the etched structure using H2/CH4
gas mixtures.

B. CAKMAK ET AL. 23
Ni and 200 nm Au was deposited on the n-side after
thinning the wafer to 100 µm. Annealing at 400℃ for 1
minute was the optimum condition to obtain the best
lasing characteristics since otherwise the devices gave
very poor characteristics.
Figure 5 shows the optical microscope image of the
two-section device fabricated. Separation was deposited
with SiO2 (green color) while the rest of the surface was
evaporated with an alloy of Au/Ti (yellow color).
3. Characterisation Measurement Results
The laser was driven by a pulse generator, output of
which was applied to a current probe to obtain current
from the pulse generator. A digital oscilloscope was used
to monitor the output of the current probe. Output power-
current (P-I) measurements were carried out using an
optical powermeter via an optical sensor. A single-mode
fiber and an optical spectrum analyser were used to ob-
tain spectral measurements from the laser.
Figure 6 shows P-I result of the laser fabricated using
CCl2F2/O2 gas mixtures. As seen in this figure, output
power is very low and the laser is operating like an light
emitting diode. P-I result of the device, in which H2/CH4
gases are used, is shown in Figure 7 that demonstrates
much better result. As seen, the threshold current is
around 40 mA.
Absorber
section
Separation
Tapered
waveguide
Gain section
Figure 5. Microscope image of two-section device.
0
0,1
0,2
0,3
0,4
0,5
0,6
020406080100 120 140 160 180 200
P(µW)
I(mA)
Figure 6. P-I result of the laser etched by using CCl2F2/O2
gas mixtures.
0
10
20
30
40
50
60
70
80
90
100
050100 150 200 250
P(μW)
I(mA)
Figure 7. P-I result of the laser etched by using H2/CH4 gas
mixtures.
Figure 8. Optical spectrum of the laser.
Figure 8 shows the spectrum of the laser fabricated
using AlGaInAs/InP (1550 nm) in which ridge-wave-
guide was fabricated using H2/CH4 gas mixtures.
4. Conclusions
We showed the fabrication of two section tapered wave-
guide lasers with InGaAsP/InP and AlGaAsP/InP structures
using gas mixtures of CCl2F2/O2 and H2/CH4. It is demons-
trated that smooth surface and sidewall structures were
obtained using H2/CH4 gas mixtures. However, it was
also observed that the use of gas mixtures of CCl2F2/O2
caused very rough structures that resulted in the failure of
the laser devices. The lasers fabricated using hydrocarbon
chemistries produced much better results with threshold
current of ~40 mA. It can be concluded that the fabri-
cated long wavelength diode lasers with 1550 nm wave-
length can be used in fiber optic communication systems.
5. Acknowledgements
We wish to acknowledge the financial support of TUB-

B. CAKMAK ET AL.
24
ITAK via Project 107E163.
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