GaSb based midinfrared equilateral-triangle-resonator semiconductor lasers
S.-Q. Yu, Y. Cao, S. R. Johnson, Y.-H. Zhang, and Y.-Z. Huang
Citation: Journal of Vacuum Science & Technology B 26, 56 (2008); doi: 10.1116/1.2819260
View online: http://dx.doi.org/10.1116/1.2819260
View Table of Contents: http://scitation.aip.org/content/avs/journal/jvstb/26/1?ver=pdfcov
Published by the AVS: Science & Technology of Materials, Interfaces, and Processing
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GaSb based midinfrared equilateral-triangle-resonator
semiconductor lasers
S.-Q. Yu,a兲 Y. Cao, S. R. Johnson, and Y.-H. Zhang
Center for Solid State Electronics Research and Department of Electrical Engineering, Arizona State
University, Tempe, Arizona 85287-6206

Y.-Z. Huang
State Key Laboratory on Integrated Optoelectronics, Institute of Semiconductors,
Chinese Academy of Sciences, Beijing 100083, People’s Republic of China

共Received 21 September 2007; accepted 5 November 2007; published 3 January 2008兲
Theoretical calculations of the mode characteristics of an equilateral-triangle resonator 共ETR兲 with
a 10 ␮m cavity side length show that the fundamental mode, with longitudinal mode index of 25,
has a wavelength of 2.185 ␮m and a longitudinal mode separation of 100 nm. This mode has a
quality factor 共⬃2 ⫻ 105兲 that is much larger than the first 共⬃5 ⫻ 104兲 and second 共⬃3 ⫻ 104兲 order
modes, indicating that single fundamental mode lasing should be accessible over a broad
wavelength tuning range. An electrically injected ETR based on this design is fabricated from an
InGaAsSb/ AlGaAsSb/ GaSb, graded-index separate-confinement heterostructure, laser diode wafer
with a 2.1 ␮m emission wavelength. This device achieved single mode, continuous wave operation
at 77 K with a threshold current of 0.5 mA and a single mode wavelength tuning range of 3.25 nm,
which is accomplished by varying the injection current from 0.5 to 6.0 mA. © 2008 American
Vacuum Society. 关DOI: 10.1116/1.2819260兴

I. INTRODUCTION
Spectroscopic gas detection techniques have many applications in environmental protection such as air pollution
monitoring.1–3 Even though Fourier transform infrared
共FTIR兲 spectroscopy has been widely adapted in these areas
because of its broad spectrum coverage, high resolution, and
quick response, tunable laser absorption spectroscopy
共TLAS兲 is gaining more and more acceptance due to its potential to achieve integrated, compact, and in situ monitoring
capabilities.4,5 The key component of a TLAS system for air
pollution monitoring is a single mode midinfrared 共2 – 3 ␮m兲
laser diode with a broad wavelength tuning range.
Semiconductor
lasers
incorporating
InGaAsSb/
AlGaAsSb quantum well 共QW兲 active regions grown on
GaSb substrates are ideal candidates for this application.
This active material is well developed for light emission over
the 2 – 3 ␮m range, and high performance GaSb based edgeemitting lasers 共EELs兲 with low threshold and high power
output have been demonstrated.6,7 Obtaining a single mode
device with low fabrication costs is nontrivial. Currently,
distributed-feedback lasers have to be fabricated using costly
electron-beam-lithography technology to define the grating.8
Semiconductor single mode microdisk lasers are easy to fabricate but have the major drawback of low laser light extraction efficiency.9 Short cavity Fabry-Pérot lasers can have longitudinal mode spacing wider than the gain spectrum, and
therefore, can achieve single mode operation when one mode
overlaps the gain spectrum. However, forming a laser cavity
a兲

Author to whom correspondence should be addressed. Electronic mail:
yushq@asu.edu

56

J. Vac. Sci. Technol. B 26„1…, Jan/Feb 2008

shorter than 100 ␮m, which is still not short enough to obtain single mode operation, is difficult using conventional
cleaving technology.
Recent theoretical work shows that semiconductor lasers
utilizing an equilateral-triangle-resonator 共ETR兲 integrated
with a waveguide connected to one vertex of the resonator
can provide both large mode separation and directional light
emission.10–12 Therefore, a GaSb based ETR laser offers a
low cost solution as a TLAS light source. In this paper the
growth, fabrication, and characterization are reported for
four
high-quality
EEL
devices
utilizing
the
InGaAsSb/ AlGaAsSb/ GaSb material system with emission
wavelengths from 2.1 to 3.1 ␮m. The laser material with
2.1 ␮m emission is chosen for the fabrication of electrically
injected ETR lasers that have a 10 ␮m side length.
II. InGaAsSb/AlGaAsSb QW LASER MATERIAL
GROWTH AND CHARACTERIZATION
The materials were grown on n-type 共100兲 GaSb substrates using a VG V80H solid source molecular beam epitaxy machine equipped with Ga, In, and Al group-III
sources, Si, Be, and Te dopant sources, and As, Sb, and P
valved cracker group-V sources. The bulk zone and the
cracking zone of group-V sources are held at constant temperatures and automated valve controllers are used to control
the group-V fluxes.
The EEL structure consists of a graded-index separateconfinement
heterostructure
with
a
triple
InGaAsSb/ AlGaAsSb QW active region. The laser structure
starts with a 300 nm thick Te-doped 共n = 5 ⫻ 1017 cm−3兲
GaSb buffer layer, followed by a 200 nm thick Te-doped
共n = 5 ⫻ 1017 cm−3兲 layer graded from GaSb to
Al0.90Ga0.10As0.08Sb0.92, and a 1.8 ␮m thick Te-doped 共n = 5

1071-1023/2008/26„1…/56/6/$23.00

©2008 American Vacuum Society

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Yu et al.: GaSb based midinfrared equilateral-triangle-resonator semiconductor lasers

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TABLE I. A summary of active region structures and characterization results for devices A, B, C, and D; all
measurements were done under pulsed operation. 共Notation: Eg is the band gap energy of the QW material;
⌬EC / ⌬EV is the conduction/valence band discontinuity between the QW barrier and the QW; L / W is the device
cavity length/stripe width; Ith is the threshold current density; Tmax is the maximum lasing temperature; and T0
is the characteristic temperature.兲
Device
QW
QW
Barrier
Eg 共meV兲
⌬EC
共meV兲
⌬EV
共meV兲
␭ 共␮m兲
L / W 共␮m兲
Jth
共A / cm2兲
Tmax 共°C兲
T0 共K兲
cw 共duty
cycle兲

A

B

C

D

In0.15Ga0.85As0.06Sb0.94 In0.50Ga0.50As0.25Sb0.75 In0.50Ga0.50As0.25Sb0.75 In0.60Ga0.40As0.35Sb0.65
Al0.25Ga0.75As0.02Sb0.98 Al0.25Ga0.75As0.02Sb0.98 Al0.40Ga0.60As0.03Sb0.97 Al0.40Ga0.60As0.03Sb0.97
590
387

440
630.9

440
756.9

400
848.7

86

15.5

82.1

30.3

2.1
830/ 50
181

2.8
825/ 50
533

2.8
805/ 50
497

3.1
1800/ 50
4370

125
76
Yes

40
33
No 共12.5%兲

65
62
Yes

25
26
No 共1%兲

⫻ 1017 cm−3兲 Al0.90Ga0.10As0.08Sb0.92 lower cladding layer.
The triple QW active regions are comprised of three compressively strained 8 – 12 nm thick InGaAsSb QWs separated
by 20 nm thick low Al-content AlGaAsSb barriers and are
sandwiched between two 250– 350 nm thick low Al-content
AlGaAsSb waveguide layers. There are two 100 nm thick,
undoped, linearly graded-index regions on either side of the
waveguide layers. The In and Al concentrations in the active
region are varied to achieve the various target emission
wavelengths. The waveguide region is followed by a 1.8 ␮m
thick Be-doped 共p = 5 ⫻ 1017 cm−3兲 Al0.90Ga0.10As0.08Sb0.92
upper cladding layer, a 200 nm thick Be-doped 共p = 5
⫻ 1017 cm−3兲 layer graded from Al0.90Ga0.10As0.08Sb0.92 to
GaSb, and a 100 nm thick Be-doped 共p+兲 GaSb contact layer.
A 20 nm thick undoped sacrificial InAs cap layer is grown
on top of the p+ GaSb contact layer to prevent oxidation of
the layers underneath. The InAs layer is selectively etched
away with citric acid during device processing.
The growth temperatures are 525 ° C for the bottom cladding layer and 485 ° C for the active region and the top cladding layer; relatively low growth temperatures are employed
to minimize In and Sb interdiffusion in the InGaAsSb QWs.
The Sb/III flux ratio was kept constant at 1.25, while the
As/III flux ratio was varied to control the group-V mole fraction in the QWs and the thick lattice-matched AlGaAsSb
layers. The composition of the cladding layers, QWs, and
QW barriers was calibrated using high resolution x-ray diffraction and photoluminescence 共PL兲 measurements prior to
the growth of laser structures.
Broad contact EEL devices were fabricated using standard
photolithography, wet etching, and metal deposition; after
which device performance was measured to characterize the
material quality. The as-cleaved devices were mounted p side
up on a thermoelectrically controlled metal stage for tem-

perature dependent continuous wave 共cw兲 and pulse measurements. The output power from one facet was monitored
using a calibrated PbS photodetector, and the lasing spectra
were measured using a Nicolet 760 FTIR spectrometer
equipped with a CaF2 beam splitter, a liquid nitrogen cooled
InSb photodetector, and 0.125 cm−1 spectra resolution.
The four devices 共A, B, C, and D with respective lasing
wavelengths of 2.0, 2.8, 2.8, and 3.1 ␮m兲 were measured
under pulsed operation at room temperature. A complete
summary of the device active region structures and characteristics is given in Table I. Among the four devices, device
A 共with In0.15Ga0.85As0.06Sb0.94 / Al0.25Ga0.75As0.02Sb0.98 QWs
and Al0.25Ga0.75As0.02Sb0.98 waveguide layers兲 demonstrated
the lowest threshold current density 共Jth = 181 A / cm2兲, the
highest lasing temperature 共Tmax = 125 ° C兲, and highest characteristic temperature 共T0 = 76 ° C兲, all measured under
pulsed operation. This device also achieved room temperature cw operation with peak output powers up to 8.5 mW
and a maximum operating temperature of 50 ° C. The cw
operation L − I curve and multimode lasing spectra 共at three
times threshold兲 are shown in Fig. 1共a兲; Jth = 313 A / cm2 and
the peak wavelength is 2.105 ␮m.
Both devices B and C are designed to lase at 2.8 ␮m and
both utilize In0.50Ga0.50As0.25Sb0.75 QWs in the active region.
The Al composition of the barrier/waveguide layer is lower
in device B 共Al0.25Ga0.75As0.02Sb0.98兲 and higher in device C
共Al0.40Ga0.60As0.03Sb0.97兲. The pulsed operation characteristics of device B are Jth = 533 A / cm2, Tmax = 40 ° C, and
T0 = 33 K, with lasing maintained up to a duty cycle of
12.5%, while for device C, Jth = 497 A / cm2, Tmax = 65 ° C,
and T0 = 62 K, with lasing maintained all the way up to cw
operation. Device D contains In0.60Ga0.40As0.35Sb0.65 QWs
with Al0.40Ga0.60As0.03Sb0.97 barrier/waveguide layers. By introducing additional In into the QW, the lasing wavelength is

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Yu et al.: GaSb based midinfrared equilateral-triangle-resonator semiconductor lasers

58

FIG. 2. 共a兲 Schematic drawing of an ETR with an outcoupling waveguide.
The arrows indicate the propagation of longitudinal modes in the folded
cavity. 共b兲 Schematic diagram of the corresponding Fabry-Pérot cavity with
an unfolded light ray; both wave vectors ␤l and km in the longitudinal and
lateral directions are shown.

FIG. 1. 共a兲 Typical L-I characteristic for device A measured under cw operation, where the cavity length is L = 830 ␮m and stripe width is
W = 50 ␮m. The inset gives the lasing spectra just above threshold. 共b兲
Room temperature PL and lasing spectra for device D. Inset gives pulsed
threshold current vs temperature from 4 to 24 ° C, which is used to extract
T0 = 26 K.

pushed out to 3.1 ␮m. However, performance is reduced and
the device only lased under pulsed operation with
Jth = 4370 A / cm2, Tmax = 25 ° C, and T0 = 26 K, with lasing
maintained up to a duty cycle of 1% for a long cavity device
共1800 ␮m兲. The room temperature PL and lasing spectrum
of device D with peak wavelengths of 3057 and 3118 nm,
respectively, are given in Fig. 1共b兲. The inset shows the lasing threshold current versus temperature measured from
4 to 24 ° C for the purpose of extracting T0. The 3.1 ␮m lasing wavelength is close to the reported longest wavelength
achieved by devices using this material system.13,14
The substantial increase in threshold current density 共Jth兲
from device A to devices B and C is attributed to the increasing Auger recombination with decreasing band gap energy.
The improved device temperature performance from device
B to C is due to improved carrier confinement in the active
region. For device B, even though the band gap of

Al0.25Ga0.75As0.02Sb0.98 共1.086 eV兲 is much larger than that
of In0.50Ga0.50As0.25Sb0.75 共0.440 eV兲, over 97% of the band
offset is in the conduction band. This leaves a valence band
offset of only 15.5 meV which is smaller than the thermal
energy 共25.8 meV兲 at room temperature. Since the shallow
hole well can only provide weak confinement, poor thermal
performance is expected due to carrier leakage followed by
nonradiative recombination in the cladding layer. When the
Al composition in the QW barrier is increased from 0.25 to
0.40 共device C兲, the valence band discontinuity increases to
83 meV, providing much better confinement of the holes and
hence improving thermal performance. This could also explain why device C achieved room temperature cw operation, while device B only achieved pulsed operation up to a
duty cycle of 12.5%.
In order to extend the laser emission to even longer wavelengths in device D, additional In was incorporated into the
active region. However, besides material quality degradation
due to higher strain and increased narrow-band-gap Auger
recombination, poor hole confinement may also be a major
contributing factor to poor performance 共high Jth, low Tmax
and T0兲; the In0.60Ga0.40AsSb/ Al0.40Ga0.60As0.03Sb0.97 QWs
only have a 30.3 meV valence band discontinuity, which is
similar to that of device B.
III. ETR CAVITY DESIGN
A schematic drawing of the ETR with an output coupling
waveguide on one vertex is given in Fig. 2共a兲. Based on
Snell’s law, a light ray propagating parallel to the side of the
ETR cavity will experience total internal reflection if the
refractive index of the semiconductor is grater than 2.0 as is

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Yu et al.: GaSb based midinfrared equilateral-triangle-resonator semiconductor lasers

the case for all common semiconductors. As shown in Fig.
2共a兲, a particular given light ray can return to the starting
point after a total of six reflections, resulting in a laser cavity
length equal to the perimeter of the ETR cavity. By unfolding the light rays inside the ETR cavity as shown in
Fig. 2共b兲, the ETR cavity can be modeled as a Fabry-Pérot
cavity with two tilted facets, which provide 100% total reflection for laser oscillation in an ideal case. The optical field
transmission is represented by the wave vectors ␤l and km in
the longitudinal and lateral directions, respectively.11 The incident angle on the sides of the ETR for the fundamental
transverse mode is larger than that for the higher order transverse modes. Therefore, the ETR cavity has a strong mode
selection for the fundamental mode over high order modes.11
Based on the cross section of the InGaAsSb/ AlGaAsSb
EELs described above, the equivalent refractive index n in
the transverse direction is determined to be 3.4. The longitudinal mode separations of the fundamental, first, and second
order lateral modes in the ETR cavity are calculated11 and
shown in Fig. 3共a兲. The different phase shifts for even and
odd transverse modes induced by the total reflections at the
ETR sides result in a large wavelength difference between
the even and odd transverse modes. For a 10 ␮m ETR cavity, the 25th fundamental longitudinal mode has a mode
separation of 100 nm at a wavelength of 2185 nm. This is
coincident with the lasing wavelength of the 2.1 ␮m EEL
structure utilized above, making it a promising structure for a
single longitudinal mode ETR device operating at 2.1 ␮m.
Another important quantity that characterizes optical cavities is the quality factor Q, which is defined as the ratio of
the amount of energy stored in the cavity in the form of a
standing wave to the amount of energy lost during a round
trip of the cavity. In laser cavities, the optical mode with the
highest quality factor is favored for laser emission because of
its low cavity loss. Q factors for modes in an ETR cavity
with a side length of 10 ␮m are calculated based on the
analytical field distribution and far-field emission
关see Fig. 3共b兲兴.11 These calculations show that the fundamental mode has much higher quality factors 共⬃2 ⫻ 105兲 than the
first 共⬃5 ⫻ 104兲 and second 共⬃3 ⫻ 104兲 order modes. Higher
order transverse modes in an ETR cavity experience higher
cavity losses because they are only partially reflected from
the cavity sidewall.
IV. ETR LASER DEVICE FABRICATIONS AND
DEVICE CHARACTERIZATIONS
The ETR laser processing started with wet etching to remove the InAs cap layer, followed by the deposition of a
300 nm thick Si3N4 layer on the wafer to function as a hard
mask for dry etching. The ETR pattern with a 10 ␮m long
side length and 1 ␮m wide outcoupling waveguide is transferred onto the Si3N4 layer using standard photolithography
and reactive-ion etching 共RIE兲. The patterned hard mask defined the ETR geometry for subsequent inductively coupled
plasma 共ICP兲 etching. The ICP etching recipe was optimized
to attain a smooth surface and a vertical sidewall profile, and
the ICP etching depth was 4 ␮m, close to the vertical pen-

59

FIG. 3. 共a兲 Calculated wavelength of the fundamental, first, and second order
modes in a 10 ␮m side length ETR. 共b兲 Calculated Q factors of the fundamental, first, and second order modes.

etration depth of the optical field of the confined modes. The
equilateral triangle vertices were slightly rounded as a result
of pattern distortion during photolithography. Next the residual Si3N4 hard mask was removed using RIE, followed by
the deposition of a 100 nm SiO2 insulating layer, on which a
contact window was opened on top of the ETR structure
using wet etching. A top Ti/ Pt/ Au p contact was formed
using standard metal deposition, and the wafer was lapped
down to a thickness of 120 ␮m after which a bottom
AuSn/ Ti/ Pt/ Au n contact was deposited. These steps were
followed by annealing at 300 ° C for 30 s to form Ohmic
contacts.
The fabricated midinfrared ETR laser device was cleaved
across the outcoupling waveguide and characterized at 77 K.
Figure 4共a兲 shows the integrated spectrum intensity versus
the injection current measured under cw operation. The
current threshold is 0.5 mA, which corresponds to

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Yu et al.: GaSb based midinfrared equilateral-triangle-resonator semiconductor lasers

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FIG. 5. Lasing wavelength tuning vs temperature for a short cavity device
共L = 97 ␮m兲.

effective index of refraction and the thermal expansion of the
ETR cavity, both of which increase the optical path length of
the lasing mode. When the injection current is further increased to 8 mA, the spectrum starts to show multimode behavior 关see top-left inset of Fig. 4共a兲兴.
V. DISCUSSION

FIG. 4. 共a兲 The integrated laser spectrum intensity vs injection current for a
mid-IR ETR laser with a 10 ␮m side length; bottom-right inset shows the
laser spectrum just below threshold 共0.3 mA兲 and just above threshold
共0.7 mA兲; top-left inset shows the multimode laser spectrum under high
injection. 共b兲 Normalized laser spectra under different injection conditions;
inset gives lasing wavelength vs injection current.

Jth = 1.2⫻ 103 A / cm−2. Above threshold, the lasing intensity
increases monotonically and starts to saturate around 4 mA.
The spectrum below threshold 共0.3 mA兲 and above threshold
共0.7 mA兲 is given in the bottom-right inset. At 0.3 mA a
narrow electroluminescence spectrum is observed with a full
width at half maximum 共FWHM兲 of 53 nm. As the injection
current is increased to 0.7 mA, single mode emission at
1916.84 nm appears with a FWHM of 0.74 nm.
The single mode laser spectra under injection currents of
2.0, 4.0, and 6.0 mA are shown in Fig. 4共b兲. As the current
increases, the lasing wavelength is tuned from
1916.78 to 1920.03 nm, resulting in a continuous 3.25 nm
single mode tuning range. The lasing wavelength shifts linearly with injection current 共see inset兲 and is a consequence
of thermal heating of the cavity caused by nonradiative recombination. Thermal heating results in an increase of the

In order to compare the wavelength tuning capability of
an ETR laser with a short cavity laser, device A was cleaved
to a short cavity length of 93 ␮m and measured under
quasi-cw operation 共10% duty cycle兲. At room temperature,
the lasing spectrum near the threshold exhibited a single longitudinal mode with a FWHM of 0.78 nm. The threshold
current density of 850 A / cm2 is much higher compared with
a longer cavity device due to increased mirror loss and Auger
recombination. The temperature dependent wavelength tuning 共see Fig. 5兲 shows that this device has a free tuning range
of 1.6 nm, which is only half of that of the ETR laser.
The ETR lasers exhibit much higher threshold current
densities compared to conventional EELs. The high threshold current density is mainly attributed to carrier losses at the
ETR cavity facet interface which was formed using ICP etching. Studies reveal that devices fabricated on GaSb exhibit
large leakage current, which is attributed to a residual native
oxide layer on the GaSb surface as this material is highly
chemically reactive with atmospheric oxygen, which can
form native oxides several nanometers thick. An additional
consequence of surface oxidation is the formation of elemental antimony at the oxide-GaSb interface, which creates a
conduction channel parallel to the interface and leads to high
surface leakage currents. Hence, suitable surface passivation
is an important step for further development of GaSb midinfrared ETR lasers. Another type of carrier loss is caused by
material damage at the surface. During ICP etching, semiconductor material is removed by the bombardment of high
energy ions, which can potentially induce surface damage to
the semiconductor material. These defects become carrier re-

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Yu et al.: GaSb based midinfrared equilateral-triangle-resonator semiconductor lasers

combination centers during laser operation. Surface damage
is inevitable during ICP etching. However, it can be reduced
by balancing the chemical and physical etching components
with an optimized ICP etch recipe.
The high threshold current density is also attributed to the
less than perfect geometric ETR cavity shape induced during
the fabrication process. The operation of the ETR laser is
based on the ETR geometric shape to provide total reflection
and form a cavity. Because of processing limitations, it is
difficult to get a perfect ETR cavity with smooth and vertical
sidewalls. During fabrication, pattern distortions round the
ETR vertices and leave less than perfectly straight sidewalls,
which introduce additional optical losses in the ETR cavity.
These pattern distortions are generated mainly in the photolithography process. The quality of the ETR cavity pattern
can be further improved by utilizing advanced lithography
techniques such as electron beam lithography.
The continuous, single mode tuning range of 3.25 nm is
much smaller than the theoretically predicated 100 nm fundamental mode spacing. The high order transverse mode
starts to appear when injection is only 16 times of threshold
共8 mA兲. This may be due to the reduced optical loss for
higher order modes when the metal contact is deposited on
the ETR sidewall. A simple solution for this problem would
be to use a low refractive index polyamide to surround the
ETR for planarization, while exposing the ETR top surface
for metallization to maintain a metal-free sidewall as well as
a uniform electrical injection.
VI. CONCLUSIONS
High-quality GaSb based InGaAsSb/ AlGaAsSb QW
EELs with emission wavelengths from 2.1 to 3.1 ␮m were
grown, fabricated, and characterized. The 2.1 ␮m device
achieved cw operation up to 8.5 mW and a maximum operating temperature of 50 ° C. By increasing the QW barrier Al
to
mole
fraction
from
Al0.25Ga0.75As0.02Sb0.98

61

Al0.40Ga0.60As0.03Sb0.97 in the 2.8 ␮m devices, the hole confinement energy was increased from 15 to 82 meV, resulting
in improved thermal performance and cw operation.
Calculations of the mode characteristics of a 10 ␮m side
length ETR cavity show that the 25th fundamental longitudinal mode has a mode separation of 100 nm at a wavelength
of 2185 nm and that the fundamental mode has a much
larger quality factor 共⬃2 ⫻ 105兲 than those of the first 共⬃5
⫻ 104兲 and second 共⬃3 ⫻ 104兲 order modes. Electrically injected, 10 ␮m side length ETR lasers with 2.1 ␮m emission
achieved single mode, cw operation at 77 K with a 0.5 mA
threshold current and a single mode tuning range of 3.25 nm.
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JVST B - Microelectronics and Nanometer Structures

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