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IEEE JOURNAL OF SELECTED TOPICS IN QUANTUM ELECTRONICS, VOL. 13, NO. 5, SEPTEMBER/OCTOBER 2007
Deep-Etched Native-Oxide-Confined
High-Index-Contrast AlGaAs Heterostructure Lasers
With 1.3 µm Dilute-Nitride Quantum Wells
Di Liang, Student Member, IEEE, Jusong Wang, Student Member, IEEE, Juno Yu-Ting Huang, Student Member, IEEE,
Jeng-Ya Yeh, Luke J. Mawst, Senior Member, IEEE, and Douglas C. Hall, Senior Member, IEEE
Abstract—Using a modified, O2 -enhanced nonselective wet thermal oxidation process, deep-etched ridge waveguides in AlGaAs
heterostructures containing λ = 808 nm InAlGaAs single quantum
well or aluminum-free λ = 1.3 µm GaAsP/InGaAsN dilute nitride
multi-quantum-well active regions have been directly oxidized to
effectively provide simultaneous electrical isolation, interface state
passivation, and sidewall roughness reduction. The resulting highindex-contrast (HIC) ridge waveguide (RWG) diode lasers show
improved performance relative to conventional shallow-etched devices owing to both strong optical confinement and the complete
elimination of current spreading, with 5 µm stripe width dilutenitride devices showing up to a 2.3 times threshold reduction and
strong index guiding for kink-free operation. Oxidation of an
AlGaAs graded-index separate confinement heterostructure is
studied for varying O2 concentrations, and the interface passivation effectiveness of the native oxide is studied through comparison
with deposited SiO2 and via a study of the stripe-width dependence
of internal quantum efficiency and modal loss. The HIC RWG
structure is shown to enable the operation of half-racetrack-ringresonator lasers with a bend radius as small as r = 6 µm.
Index Terms—Integrated optoelectronics, materials processing,
semiconductor lasers, semiconductor waveguides.
I. INTRODUCTION
HE EMISSION wavelength of GaAs-based diode lasers
may be extended to 1.3 and 1.55 µm fiber-optic telecommunications bands through the use of quantum dot active regions [1], [2] or through the incorporation of dilute amounts
of nitrogen into the active region (yielding “dilute nitride” alloys) [3]–[7]. In each of these cases, the lower gains available
relative to conventional quantum well (QW) lasers make having a low-loss index guiding structure with a high lateral optical
confinement factor ΓL (i.e., strong lateral overlap of the in-plane
optical field and gain) of paramount importance for achieving
good laser performance. To realize a high ΓL requires both
T
Manuscript received on November 9, 2006; revised July 2, 2007. This work
was supported by the National Science Foundation under Grant ECS-0123501
and Grant ECS-0601702.
D. Liang was with the University of Notre Dame. He is now with the University of California at Santa Barbara, Santa Barbara, CA 93106 USA.
J. Wang and D. C. Hall are with the Department of Electrical Engineering, University of Notre Dame, Notre Dame, IN 46556–5637 USA (e-mail:
dhall@nd.edu).
J. Y.-T. Huang and L. J. Mawst are with the Department of Electrical and
Computer Engineering, University of Wisconsin, Madison, WI 53706–1691
USA (e-mail: yuting@cae.wisc.edu; mawst@engr.wisc.edu).
J.-Y. Yeh was with the University of Wisconsin, Madison, WI 53706–1691
USA. He is now with Intel, Santa Clara, CA 95052 USA.
Digital Object Identifier 10.1109/JSTQE.2007.905097
a large lateral refractive index step and minimization of lateral
current spreading. Index-guided laser structures are conventionally realized through etching into the upper cladding layer, or
through a more complex etching and selective area regrowth
process. Low-threshold-current InGaAsN QW ridge waveguide
(RWG) lasers have also been fabricated by pulsed anodic oxidization of an Alx Ga1−x As upper cladding layer [8]. Conceptually, improved optical and electrical confinement could be
provided by a deeply etched ridge waveguide (i.e., etched to
below the active region/heterostructure waveguide core), but,
in practice, the device performance with such a process is often degraded by nonradiative recombination due to etch defects
and surface states at the exposed active region sidewall, and
passivating these defects has proven to be difficult.
Although wet thermal oxidation has conventionally been
limited to high Al-content III–V alloys [9], [10], we have
elsewhere demonstrated a nonselective, O2 -enhanced wet
thermal oxidation process for forming a native oxide directly on
the deep-etch-exposed low-Al-content active region/waveguide
core of an AlGaAs QW heterostructure [11]. The oxide is of
sufficiently high quality to effectively passivate the sidewall
surface in a deep-etched high-index contrast (HIC) RWG, which
through both strong optical confinement and the complete
elimination of current spreading, has enabled high-efficiency
single-mode lasers to be fabricated from an AlGaAs gradedindex separate confinement heterostructure (GRINSCH) [12].
Because the scattering loss in HIC waveguides is much more
strongly affected by sidewall roughness [13], another important
benefit of the AlGaAs sidewall oxidation process is the
significant smoothing of surface roughness (down to ≤ 5 nm)
attained with O2 -enhanced wet oxidation [14].
In Section II, we review the details of our HIC RWG laser
fabrication process. Then, in Section III, we show that a devicequality thermal oxide can be grown on a deep-etched dilute
nitride laser heterostructure not only on the Al0.65 Ga0.35 As
cladding layers, but also on the Al-free GaAs waveguide
and GaAsP/InGaAsN active region layers. With the high ΓL
provided by the resulting HIC RWG, enhanced laser performance with stable spatial mode behavior is achieved. Relative
to conventional shallow-etched index-guided RWG lasers
fabricated out of the same material, HIC RWG narrow-stripe
lasers show approximately two times lower lasing threshold
current densities with kink-free operation. In Section IV, we
report additional studies of the nonselective oxidation process
as applied to an AlGaAs GRINSCH, and show that the resulting
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Fig. 1. Processing schematic of dry etching and non-selective oxidation steps
in fabricating oxide-confined HIC RWG lasers.
HIC RWG enables lasing in e-beam-lithography-defined
half-racetrack-ring resonators with a bend radius as small
as r = 6 µm. The ability to realize low-bend loss curved
waveguides [15] is particularly promising for future photonic
integrated circuits, particularly those that will utilize “longwave on GaAs” materials for telecommunications applications.
Finally, Section V presents further data demonstrating the
efficacy of the nonselective wet thermal oxide in passivating
the active region of deeply etched AlGaAs GRINSCH lasers.
II. HIC RWG LASER FABRICATION PROCESS
Laser fabrication typically starts with a ∼200 nm plasmaenhanced chemical vapor deposition (PECVD) SiNx deposition
to protect the p+ -GaAs cap layer from later oxidation. The
waveguide stripe is then patterned through conventional photolithography followed by two successive reactive ion etching
(RIE) steps in CF4 /O2 and BCl3 /Cl2 /Ar plasma to translate the
photoresist pattern to the SiNx layer and semiconductor epilayers, forming a ridge as shown in Fig. 1(a). Unlike conventional
dry etching, which is stopped above the active layer so that
defects introduced by etching are kept away from the active region, dry etching in this case reaches the lower cladding layer
to form a waveguide with lateral dimension close to that of the
photoresist mask. Nonradiative recombination centers formed
during this initial etching process are largely reduced during
the following thermal oxidation process, typically at 450 ◦ C.
The O2 -enhanced nonselective wet thermal oxidation [11] of
the waveguide sidewalls (and base) [Fig. 1(b)] under conditions
given later provides a high-quality native oxide to serve as an
insulating dielectric, while simultaneously providing lateral optical confinement via the HIC (∆n ∼ 1.7) semiconductor/oxide
interface, enabling the realization of a HIC RWG capable of
supporting very sharp bending [15], [16].
Instead of depositing PECVD SiO2 or SiNx for electrical
confinement and surface passivation, the use of the native oxide as the dielectric layer also results in a self-aligned process,
which eliminates the potential alignment errors and the narrowing of the top contact area unavoidably resulting from a second
lithography step to open a current window in a conventional
RWG fabrication process. A final dry etching procedure in RIE
with a CF4 /O2 plasma then selectively removes the dielectric
stripe mask, using special care to prevent etch damage to the
p+ -GaAs cap layer. After standard lapping (to ∼100 µm thickness) and polishing, the wafer is metallized and cleaved into laser
bars. In this paper, the total p-side metallization (Ti/Au) thickness is ∼320 nm, and the device facets are uncoated. For ease
of characterization, unbonded devices are probe tested (junc-
Fig. 2. SEM cross-section image of w = 7 µm GaAsP/InGaAsN MQW structure, wet etched and nonselectively wet oxidized at 450 ◦ C for 30 min with
7000 ppm added O2 . The conduction band overlay schematically highlights
the epitaxial structure. (Inset): Closeup view showing ∼115 and 40 nm oxide
growth on GaAs waveguide core and GaAsP/InGaAsN MQW active region,
respectively.
tion side up) under both pulsed (2 µs pulse, 1% duty cycle) and
continuous-wave (CW) conditions at 300 K using a Keithley
model 2520 pulsed laser diode test system.
To further highlight the advantages of this fabrication process, we note that the conventional ridge structure formed by
removing or oxidizing the upper cladding layer can yield only
a small lateral effective index step (∆n ≤ 0.1), providing relatively weak optical mode confinement in the horizontal direction and leading to two undesirable effects: current spreading
and output beam asymmetry. The significant current spreading
(tens of microns) that plagues conventional RWG laser designs
is prevented in our new process as current flow is effectively
restrained to a vertical channel defined by the insulating oxide.
Strong optical mode confinement from the vertical oxide walls
also offers a potential means for overcoming the asymmetry in
the optical mode profile and output beam in-plane versus outof-plane far-field divergence in edge-emitting lasers [17]. The
oxidation can also provide scaling from an optical lithography
defined ridge dimension (≥1 µm) to the submicron dimensions
required for both HIC waveguide single-mode operation and
to realize a symmetric output beam laser device. However, we
have observed in practice that multimode HIC RWG devices
with widths of even 7 µm exhibit kink-free operation to high
output powers in a stable single mode due to the excellent optical
confinement, lack of current spreading, and apparent preference
for lasing in the lowest loss fundamental mode [12].
III. DILUTE NITRIDE HIC RWG LASERS
Both deeply etched HIC-type and conventional (shallow
etched) index-guided RWG laser diodes are fabricated in a
λ ∼ 1250–1270 nm large optical cavity, multiple quantum well
(MQW) heterostructure grown by metal–organic chemical vapor deposition. Three 8 nm InGaAsN (In = 40%, N = 0.5%)
QWs are alternately embedded in four 10 nm GaAs0.85 P0.15
barriers, which are sandwiched in a 300 nm GaAs separate confinement heterostructure (SCH) formed with 1.1 µm
Al0.65 Ga0.35 As cladding layers [3]. Prior to RWG laser fabrication, wet-etched stripes are used to study the nonselective
oxidation of the GaAsP/InGaAsN MQW active region. Fig. 2
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IEEE JOURNAL OF SELECTED TOPICS IN QUANTUM ELECTRONICS, VOL. 13, NO. 5, SEPTEMBER/OCTOBER 2007
Fig. 3. Pulsed (1% duty cycle) LI characteristic of an L = 307 µm HIC-type
broad-area device with w = 85 µm effective laser aperture showing Ith =
169.6 mA and R d = 0.51 W/A. Insets show spectrum of the device operating at
300 mA (peak λ 1.234 µm) and SEM of a different dry etch-exposed waveguide
sidewall nonselectively oxidized at 450 ◦ C for 1 h.
shows a scanning electron microscope (SEM) image of a 7-µmwide stripe-masked ridge wet etched in a H3 PO4 :H2 O2 :H2 O
solution for 90 s, and then, wet oxidized for 30 min at 450 ◦ C
with the addition of 7000 ppm O2 (relative to the N2 carrier gas
bubbled through 95 ◦ C H2 O). The higher magnification SEM
image inset clearly demonstrates ≥40 nm of oxide growth in
the Al-free active region with 115 nm of oxide formed in the
GaAs waveguide core layer. While there is a possibility that
the InGaAsN layers may contain trace amounts of Al due to
the interaction of the nitrogen source (DMHy) and Al in the
MOCVD reactor, we believe this effect is very small. We note
that the Al-free GaAs layer does not have this potential issue, nor
does InGaAs QWs for which we have observed oxide growth
(data not shown). We have shown elsewhere that the addition
of O2 significantly enhances the oxidation rates of an undoped
Al0.20 Ga0.8 As waveguide core containing a single 10 nm GaAs
QW [11]. Fig. 2 demonstrates that substantially thicker GaAs
layers and even a dilute-nitride MQW structure can also be
nonselectively oxidized.
For HIC RWG laser fabrication, devices are deeply etched
via RIE with a BCl3 /Cl2 /Ar plasma for 12 min to form a
1.8-µm-high ridge. A 2 h nonselective oxidation at 450 ◦ C with
the addition of 7000 ppm O2 is then used to grow ∼2.5 µm of
oxide (measured at the etch-exposed GaAsP/InGaAsN MQW
active region), resulting in an effective laser active aperture that
is, on all devices, 5 µm smaller than the optically patterned laser
stripe width. For comparison, conventional index-guided RWG
lasers are fabricated with a shallow etch for 8 min under the
same dry etch conditions to a 0.75 µm depth, followed by a
short 30 min nonselective oxidation at 450 ◦ C with 7000 ppm
O2 to convert part of the Al0.65 Ga0.35 As upper cladding layer
to a 200 nm native oxide for device isolation.
Fig. 3 shows the pulsed (1% duty cycle) light output power
versus current (LI) characteristic of a cavity length L = 307 µm,
w = 85 µm (effective laser aperture dimension) broad area device having a low threshold current of 169.6 mA and a high slope
efficiency of 0.51 W/A, corresponding to a threshold current
density of 650 A/cm2 and an external differential quantum effi-
Fig. 4. Pulsed LI characteristics of typical w = 10 µm stripe geometry lasers.
(a) HIC and (b) conventional shallow-etched index-guided RWG structures. (a)
HIC diode laser with L = 525 µm, Ith = 45.73 mA, Jth = 871 A/cm2 , and
the differential slope efficiency at 2 Ith is R d = 0.451 W/A. (b) Conventional
device with L = 520 µm, Ith = 86.18 mA, and Jth = 1657.3 A/cm2 , and a
kink indicative of mode hopping occurs (typical of most of the conventional
devices). (Inset): spectrum of HIC diode laser operating at 100 mA pulsed
current, showing a peak wavelength of 1.23 µm.
ciency of 50.8% (at λpeak = 1.234 µm), respectively. Extrapolation of threshold current density versus inverse cavity length
data (not shown) to 1/L = 0 gives 416.1 A/cm2 at infinite cavity
length, a low value indicative of the high quality of the laser material and heterostructure. For L = 1 mm devices, a 16% threshold current density reduction is obtained for HIC broad-area
lasers (Jth = 502 A/cm2 , w = 85 µm) relative to shallow-etch
broad-area devices (Jth = 598 A/cm2 , w = 90 µm), demonstrating a benefit due to the elimination of current spreading
even in wide emitter devices. The inset in Fig. 3 shows a SEM
cross-sectional image of the etch-exposed RWG sidewall oxidized under the same conditions but for a shorter time period
of 1 h, resulting in about 430–1220 nm of oxide growth in the
MQW active region. An apparent superlinear lateral oxidation
rate at the GaAsP/InGaAsN MQW active region observed from
three samples oxidized for 30 min, 1 and 2 h to thicknesses
of 40, 1220, and 2500 nm, respectively, can be attributed to the
additional effect of inward oxidation of this more slowly oxidizing region from the surrounding faster-oxidizing GaAs and AlGaAs layers. The nonuniform oxidation observed in the AlGaAs
cladding layers and GaAs waveguide p–n junction layer may be
attributed to doping-related effects [18] and interface-enhanced
oxidation [19] observed in other heterostructures. The spectrum
in Fig. 3, measured at 300 mA (∼1.7 × Ith ) with 1% duty cycle pulses, shows a peak wavelength of 1.234 µm. We have
shown elsewhere that growth modifications not incorporated in
the structures used in this paper can extend the wavelength out
to close to λ = 1300 nm [4], [5].
In narrow-stripe lasers, where optical and current confinement
become more critical, a much more significant performance advantage is achieved by employing the HIC RWG structure of
Fig. 1(b). Fig. 4 compares typical output power versus current
characteristics for: 1) HIC and 2) conventional RWG lasers
with a w = 10µm laser effective active stripe width. It is well
known that both poor optical confinement and carrier leakage
via current spreading [20] can lead to mode hopping in weakly
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Fig. 5. Threshold current density versus laser stripe width for two laser types.
(a) HIC RWG lasers with cavity length of 707 µm. (b) Conventional indexguided lasers with cavity length of 711.3 µm. A 2.3 × threshold current reduction
is achieved for w = 5 µm HIC laser due to the complete elimination of current
spreading effects.
guided narrow-stripe lasers that, in turn, causes kinks in the
LI characteristics. Such behavior is observed for the device of
Fig. 4(b), and is typical in most of our conventional devices. In
contrast, the HIC RWG laser of Fig. 4(a) shows kink-free operation suggesting stable spatial mode behavior. As observed for
AlGaAs GRINSCH lasers [12], the vertical channel formed after
nonselective oxidation completely eliminates current spreading
and also provides strong index guiding. As shown in Fig. 4(a),
low threshold (Ith = 45.7 mA) and high slope efficiency (Rd =
0.45 W/A) operation is obtained without visible mode-hoppinginduced LI kinks. The inset spectrum is measured at a pulsed
injection current of 100 mA (∼ 2.2Ith ), showing a similar peak
wavelength of 1.23 µm as the broad-area device of Fig. 3.
To further study current spreading effects, Fig. 5 plots the
threshold current density versus laser stripe width for comparable cavity length: 1) HIC and 2) conventional, shallow-etched
RWG devices. The shallow-etched devices especially show dramatically increasing threshold current density with decreasing
stripe width, with a highest value of Jth = 2587 A/cm2 for the
w = 5 µm conventional device, which is more than 2.3 × higher
than that (1103.3 A/cm2 ) of an HIC RWG device with the same
effective active stripe width. Notably, the threshold current density of the w = 5 µm HIC structure laser is merely 2 × higher
than that of broad area (w > 90 µm) HIC devices, indicating
not only excellent optical and electrical confinement, but also
negligible sidewall nonradiative recombination even though the
native oxide is grown in direct contact with the active layer. As
conventional lasers fabricated from similar material and bonded
to heat sinks operate CW at temperatures up to 100 ◦ C [5], we
believe that these improved performance HIC RWG lasers will
also operate CW when soldered to heat sinks.
Finally, we note that oxide growth on the GaAsP/InGaAsN
active region has also recently been observed with conventional
(non-O2 enhanced) wet thermal oxidation, although to a lesser
extent (e.g., 50–130 nm in 1 h at 450 ◦ C, i.e., 8–9 × slower
oxidation rate; data not shown). Preliminary analysis of HIC
RWG lasers made with such non-O2 -enhanced oxidation appear to have comparable performance to those reported earlier. It is known that relative to conventional wet oxidation,
Fig. 6. SEM cross-section views of AlGaAs/InAlGaAs/GaAs GRINSCH
ridge geometry laser oxidized laterally under conditions noted. (a) UHP N2
at 450 ◦ C for 100 min (with conduction band overlay to schematically show
location of InAlGaAs single QW sandwiched in GRINSCH). (b) Mixed 2000
ppm O2 + N2 at 450 ◦ C for 45 min. (c) Mixed 4000 ppm O2 + N2 at 450 ◦ C
for 40 min. (d) Mixed 7000 ppm O2 + N2 at 450 ◦ C for 35 min.
O2 -enhanced wet oxidation of Al0.3 Ga0.7 As produces a denser
oxide with much higher refractive index [11] and greater etch
resistance [14]. However, further study is required for dilute
nitride material systems to understand the role and benefit of
adding O2 during wet thermal oxidation.
IV. OXIDATION OF AlGaAs GRINSCH
Our studies of HIC RWG laser diodes [12], [15]–[17] have
mostly focused on devices fabricated in a λ = 808 nm highpower, large optical cavity, single-strained InAlGaAs QW
GRINSCH with Al0.65 Ga0.35 As waveguide cladding layers,
grown via metal–organic chemical vapor deposition by EpiWorks, Inc., to closely match the design in [21]. Since the Al
ratio of the AlGaAs waveguide core region is not constant,
but graded toward the InAlGaAs single QW as shown by the
schematic conduction band (Ec ) inset in Fig. 6(a), the oxidation
rate selectivity that mainly depends on Al-ratio is likely to result
in variations in the depth of the oxidation front. Here, we further
explore the oxidation profile of a GRINSCH waveguide to optimize oxidation conditions for the best control of the dimensions
for electrical and optical confinement.
Fig. 6 shows the SEM cross-section images for samples wet
oxidized with: 1) ultrahigh purity (UHP) N2 carrier gas at 450 ◦ C
for 100 min; 2) mixed 2000 ppm O2 + N2 at 450 ◦ C for 45 min;
3) mixed 4000 ppm O2 + N2 at 450 ◦ C for 40 min; and 4)
mixed 7000 ppm O2 + N2 at 450 ◦ C for 35 min, respectively.
All samples are etch-stained for 5 s in 1:1:10 HCl:H2 O2 :H2 O for
enhancing SEM image contrast. To provide the best comparison,
oxidation times were adjusted for each case to obtain a native
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Fig. 7. Total output power versus current characteristics for w = 10-µm-wide
HIC RWG lasers in half-racetrack-ring geometry with 4 different bend radii.
(a) r = 25 µm. (b) r = 10 µm. (c) r = 8 µm. (d) r = 6 µm. Total resonator
cavity lengths are ∼1 mm for (a)–(c) and 636 µm for (d).
oxide of approximately 400 nm thickness in the upper and lower
AlGaAs cladding layers. A noticeable difference clearly exhibited in the SEM images is that the oxide growth in the GRINSCH waveguide region is “catching up” to that in the upper and
lower cladding layers as the O2 content in the reaction gases
is increased. For the case 1) of the conventional wet oxidation,
a fairly long oxidation time (100 min) is required to achieve
the same thickness cladding layer oxide as the nonselective (O2
added) oxidation achieves in 35–45 min. The oxidation rate
selectivity for different Al-ratio AlGaAs is also shown by the
“protruded” oxidation front in the waveguide region for case
1). Here, the minimum thickness oxide (∼160 nm) is grown at
the center of the waveguide region where the InAlGaAs QW
is located, making it the region of weakest lateral carrier and
optical confinement. The oxide is also formed directly beneath
the p+ -GaAs cap layer in case 1), due to enhanced lateral oxidant diffusion along the GaAs/AlGaAs interface [19], which,
for narrower stripes, could block the path for current injection
needed for laser operation.
In contrast, the oxidation front in the waveguide region becomes progressively more uniform with increasing O2 content
due to the enhancement of the oxidation rate for low Al-ratio
AlGaAs [11] and the lateral diffusion of oxidant through the
oxide in the cladding layers [Fig. 6 (b)–(d)]. A similar thickness of oxides in the waveguide core and cladding regions is
observed when 4000–7000 ppm O2 is added into the oxidation
stream, giving optimum lateral dimension control and electrical
confinement. Based on this study, 4000 ppm O2 was chosen as
the optimal content for the laser diodes fabricated in this paper.
The HIC RWG achieved through this process enables very
tight waveguide bends with low loss. This has been demonstrated through the fabrication of half-racetrack-ring resonator
lasers with a bend radius as low as r = 6 µm, as shown
here in Fig. 7 [16]. In addition to lasing for e-beam lithography defined devices with r = 25, 10, and 8 µm reported
in [16], Fig. 7(d) shows lasing with comparable performance
Fig. 8. Total output power versus current. Solid curves: native oxide-confined
laser (w = 7.7 µm, L = 590 µm). (a) Pulsed mode. (b) Fast-CW mode.
(c) True CW mode. Dashed curves: PECVD SiO2 -confined laser (w = 7 µm,
L = 335 µm). (d) Pulsed mode. (e) Fast-CW mode. (Inset): Spectrum of the
native-oxide-confined laser operating at 85 mA CW current, showing a peak
wavelength of 816.7 nm.
for r = 6 µm, w = 10 µm ridge-width device. The slightly improved threshold current and efficiency is due to r = 6 µm device’s shorter cavity length (636 µm versus 1 mm for the others).
V. INTERFACE PASSIVATION STUDY
Interface passivation is a critical factor affecting semiconductor device performance, particularly for GaAs-based devices
that can have high surface recombination velocities [22]. As the
dimension of devices shrinks, the increasing surface-to-volume
ratio may further degrade the device performance. For HIC
RWG lasers in this paper, the direct contact of the oxide with
the active region could be quite problematic if it does not form a
high-quality, low-defect interface with the semiconductor [23].
In order to compare the passivation capacity of a thermally
grown native oxide with conventionally used deposited dielectric films, deep-etched lasers passivated by similar thicknesses
(∼150 nm) of PECVD SiO2 versus a grown nonselective wet
thermal native oxide are compared. The index of the PECVD
SiO2 for the recipe used was measured to be ∼1.456 at 808 nm
indicating good stoichiometry.
Fig. 8 shows that the typical LI characteristics of narrow stripe
(effective aperture width w = 7.7 µm) lasers passivated by the
native oxide are much better than PECVD SiO2 -passivated devices. Due to its shorter cavity length of 335 µm, the PECVD
SiO2 -confined laser should have a higher slope efficiency and a
lower threshold current than the 590 µm native oxide-confined
laser if it had provided better or comparable passivation relative
to the native oxide. This, however, is clearly not the case; compared with a threshold current of 24 mA and a slope efficiency
of 1.1 W/A achieved on a native-oxide-confined laser under
pulsed operation (1% duty cycle), the PECVD SiO2 -confined
laser needs a higher current to reach threshold (Ith = 40 mA)
and exhibits a much lower slope efficiency (Rd = 0.65 W/A),
indicating that the PECVD SiO2 is not nearly as good as the
nonselective native oxide in passivating defects and minimizing
waveguide loss.
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Fig. 10. Inverse external differential quantum efficiency versus cavity length
for native oxide-confined HIC GRINSCH RWG lasers with stripe widths of 5,
7, 10, 40, and 90 µm (measured pulsed with a 1% duty cycle at 300 K).
Fig. 9. Threshold current density versus stripe width for HIC RWG lasers
measured under pulsed mode with a 1% duty cycle. (a) Native oxide confined
(w = 5 µm, L = 333.7 µm). (b) PECVD SiO2 -confined (w = 5 µm, L =
335 µm) (Inset): Corresponding structural schematics of the devices.
Without any heat sink, under a fast-CW condition (a fast
dc current sweep time of ∼0.34 s), Fig. 8(b) shows that the
native-oxide-confined laser has a comparably low threshold
current and follows the pulsed LI curve without rolling over
until I ∼ 160 mA. In contrast, under fast-CW operation,
the PECVD SiO2 -confined laser [Fig. 8 (e)] experiences a
higher threshold and lower efficiency with a “rollover” of
output beginning at I ∼ 120 mA. The earlier onset of rollover,
usually associated with heating, may suggest a poorer thermal
performance of PECVD SiO2 -confined devices. Surprisingly,
the native oxide-confined laser tested p-side up without a
heat sink shows a true CW (steady state dc) threshold of only
Ith = 26 mA, just 2 mA higher than the pulsed threshold
current, with no LI curve roll over until I = 125 mA. A linear
spectrum with a peak lasing wavelength at 816.7 nm is obtained
for a dc injection current of 85 mA. No data were taken above
125 mA to prevent possible thermal damage to the device.
In addition to interface passivation differences, the oxidation
smoothing discussed in [14] should result in a lower scattering
loss than that of the PECVD SiO2 -confined devices, contributing
to improved laser performance. The reduction of defects associated with etching during the oxidation process (both through
thermal annealing and via conversion of etch-damaged semiconductor near the surface to amorphous oxide) is an additional
benefit of the oxide-confined lasers not afforded by the deposited
oxide-confined devices.
To further compare the efficacy of deposited versus native oxides while eliminating the impact of the distributed mirror loss,
inversely proportional to laser cavity length, further analysis is
conducted by selecting laser bars of both PECVD SiO2 - and
native-oxide-confined HIC RWG lasers of almost identical cavity length containing devices with varying stripe width. Results
of this stripe width-dependent study are shown in Fig. 9, where
the threshold current densities of: 1) native oxide-confined and
2) PECVD SiO2 -confined lasers with nearly identical structure dimensions are plotted as a function of the laser stripe
width. As the laser stripe width decreases, the lasing threshold current densities increase rapidly but at different rates for
both laser types. Native oxide-confined lasers clearly demonstrate a smaller increase, especially in the narrow stripe range
(w < 15 µm). For a native oxide-confined laser, the threshold
current density at w = 5 µm is 978 A/cm2 , 3.4 × higher than
that of a laser with w = 40 µm. For a w = 5 µm PECVD SiO2 confined device, the value of 1590 A/cm2 is 3.8 × higher than
at w = 40 µm. At w = 15 µm, the native-oxide devices have
1.96 × lower threshold current densities than the SiO2 -insulated
devices, and narrower native-oxide devices all maintain more
than 1.5 × lower threshold current density than their SiO2 insulated counterparts. An overall lower threshold current density of native oxide-confined lasers further proves the superior
interface passivation of the native oxide relative to a deposited
dielectric.
Low nonradiative recombination can also be reflected by a
high internal quantum (or injection) efficiency η i as defined in
Ref. [25]. From the relationship of 1/η d versus 2L/ln(1/R1 R2 ),
where η d , R1 , R2 , respectively, represent the external differential quantum efficiency, front and rear facet reflectivity (defined
as R1 = R2 = 0.32 in this paper), η i can be obtained by extrapolating the external differential quantum efficiency to the
point of zero cavity length (L = 0), and the internal modal loss
αi can be found from the slope for the data shown in Fig. 10
for three different cavity lengths of each of five different width
native-oxide-confined HIC RWG lasers [w = 5, 7, 10, 40, and
90 µm (BA)]. Nonradiative recombination at the sidewall can
reduce η i in the case where the carrier density outside the active region QW remains unclamped above laser threshold [25].
Fig. 11, which plots the resulting η i and αi values versus laser
stripe width w resulting from the linear fits in Fig. 10, shows
that all of the devices achieve η i > 80%. This indicates that the
nonradiative recombination at the ridge sidewall does not cause
a large performance penalty although narrow stripe lasers do
exhibit some degradation in efficiency.
The total internal loss αi as a function of laser stripe width,
also shown in Fig. 11, illustrates a relationship similar to that
proposed by Lee et al. [13]: the narrower the waveguide width,
the higher the scattering loss due to the increasing interaction of
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IEEE JOURNAL OF SELECTED TOPICS IN QUANTUM ELECTRONICS, VOL. 13, NO. 5, SEPTEMBER/OCTOBER 2007
exhibit desirable kink-free operation due to the uniform carrier
distribution in the active region and up to 2.3 × reduction of
threshold current density for narrow-stripe devices primarily
owing to the elimination of current spreading.
For HIC RWG lasers fabricated from an AIGaAs GRINSCH,
native-oxide-confined devices realized through the direct O2
enhanced wet oxidation of the etched sidewall exhibit higher
efficiency and up to 2 × lower threshold current density for
narrow stripe widhts than those employing a deposited PECVD
dielectric. This improvement interface recombination and the
sidewall-roughness-induced scattering loss achieved with the
high-quality nonselective wet-thermal native oxide.
Fig. 11. Internal differential quantum efficiency η i and internal loss α i versus
laser stripe width w for native-oxide-confined laser data of Fig. 10.
the propagating light with the sidewall roughness. In particular,
HIC waveguides typically suffer greatly from scattering loss,
shown to be inversely proportional to waveguide width to the
fourth power (∝ 1/w4 ) at the same roughness level [13]. Price
et al. have recently studied the mode confinement and loss
mechanism of conventional index-guided lasers by varying
the shallow etch depth in the upper cladding layers [24] and
observed a more than 4 × loss increase from 5 cm−1 to about
21 cm−1 on the best w = 3 µm devices (∆n = 0.004) when the
etch stops at 400 and 100 nm above the GaAs waveguide core
layer, respectively. The fast increase of the loss is attributed to
the scattering from the interaction of the optical mode with etch
imperfections [24], indicating the strong effect of scattering loss
even on conventional index-guided narrow-stripe devices of
fairly low lateral index contrast. Although the total laser internal
modal loss is composed of not only waveguide scattering loss
but also material and free carrier absorption losses, the very low
total waveguide loss seen in Fig. 11 for these HIC structures,
and particularly, the increase of only 2.2 × in loss (from 4.49
to 9.89 cm−1 ) as the width of these lasers decreases from 90
to 5 µm, point to a definite performance benefit achieved here
from oxidation smoothing [14] of the deep-etched sidewall
roughness.
VI. CONCLUSION
Motivated by the desirable enhancements in photonic
integrated circuit (PIC) design flexibility and chip functional
density that a HIC waveguide structure can provide, we have
implemented a new deep etch plus nonselective, roughnesssmoothing oxidation process to achieve an HIC RWG structure
with a simplified fabrication process that significantly improves
the performance of laser devices and enables the formation of
sharply bent waveguides required for integrated ring resonator
lasers and routing waveguides. Several desirable features including self-aligned processing and effective optical and carrier
confinement have been found to result from the application
of nonselective oxidation to AlGaAs/GaAs heterostructures
containing both 808 nm InAlGaAs single QW and 1.23 µm
GaAsP/InGaAsN MQW active regions. Compared with conventional index-guided lasers, HIC RWG dilute nitride lasers
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Di Liang (S’02) was born in Kunming, China, in 1980. He received the B.S.
degree in optical engineering from Zhejiang University, Hangzhou, China, in
2002, the M.S.E.E. and Ph.D. degrees in electrical engineering from the University of Notre Dame, Notre Dame, IN, in 2004 and 2007, respectively.
He is currently with the University of California at Santa-Barbara. His
current research interests include III–V compound semiconductor photonics,
silicon photonics, semiconductor fabrication, and photonic integrated circuits.
He is the author or coauthor of more than 16 published journal and conference
papers.
Mr. Liang is a member of the IEEE Laser and Electro-Optics Society, the
IEEE Electronic Device Society, the Optical Society of America, and the International Society for Optical Engineering.
Jusong Wang (S’01) was born in Beijing, China, in 1978. He received the B.S.
degree in electronics engineering from Tsinghua University, Beijing, in 2001,
and the M.S. degree in electrical engineering, in 2004 from the University of
Notre Dame, Notre Dame, IN, where he is currently working toward the Ph.D.
degree in electrical engineering.
His current research interests include GaAs-based photonics devices and
photonic integrated circuits, including high-index-contrast S-bend waveguides
and half-ring and full-ring resonator laser diodes with extremely small bend
curvature.
Mr. Wang is a member of the IEEE Laser and Electro-Optics Society, the Optical Society of America, and the International Society for Optical Engineering.
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Juno Yu-Ting Huang (S’06) received the B.S. and M.S. degrees in chemical
engineering and the Diploma in composite materials from the National Cheng
Kung University, Tainan City, Taiwan, R.O.C. She is currently working toward
the Ph.D. degree in electrical and computer engineering at the University of
Wisconsin-Madison, Madison.
Her current research interests include the development of semiconductor
lasers.
Jeng-Ya Yeh received the B.S. degree in physics from the National Tsing Hua
University, Hsinchu, Taiwan, R.O.C., in 1996, and the Ph.D. degree in electrical
and computer engineering from the University of Wisconsin-Madison, Madison,
in 2005.
He is currently with Intel, Santa Clara, CA. His current research interests
include developing high-performance long wavelength (1300-nm and beyond)
InGaAsN quantum well (QW) lasers by metalorganic chemical vapor deposition
(MOCVD) for optical communication, optimization, and physical understanding of the lasing characteristics of InGaAsN QW lasers.
Luke J. Mawst (M’88–SM’93) was born in Chicago, IL, in 1959. He received
the B.S. degree in engineering physics and the M.S. and Ph.D. degrees in electrical engineering from the University of Illinois at Urbana-Champaign, Urbana,
in 1982, 1984, and 1987, respectively.
In 1987, he was a Senior Scientist in the Research Center, Thompson Ramo
Wooldridge (TRW), Inc., Redondo Beach, CA, where he was engaged in design
and development of semiconductor lasers using metalorganic chemical vapor
deposition (MOCVD) crystal growth. He is currently a Professor in the Electrical and Computer Engineering Department, University of Wisconsin-Madison,
Madison, where he is involved in the development novel III/V compound semiconductor device structures, including vertical cavity surface emitters (VCSELs), active photonic lattice structures, InGaAsN lasers, and high-power Alfree diode lasers. He is the coinventor of the resonant optical waveguide (ROW)
antiguided array and has contributed to its development as a practical source of
high coherent power. He has also developed a novel single-mode edge-emitting
laser structure, the Arrow laser, as a source for coupling high powers into fibers.
His current research interests include low-temperature MOCVD grown highly
strained InGaAs and InGaAsN led to record low-threshold current density diode
lasers. He is the author or coauthor of more than 140 published technical papers
and is the holder of 18 patents.
Prof. Mawst is the recipient of the TRW Group Level Chairman’s Award.
Douglas C. Hall (S’86–M’91–SM’06) received the B.S. degree (summa cum
laude) in physics from Miami University, Oxford, OH, in 1985, and the M.S.
and Ph.D. degrees in electrical engineering from the University of Illinois at
Urbana-Champaign, Urbana, in 1988 and 1991, respectively.
From 1991 to 1994, he was with the U.S. Naval Research Laboratory,
Washington, DC, where he investigated high-power laser amplifiers and erbiumdoped fiber sources for fiber optic gyroscopes. He is currently an Associate
Professor in the Department of Electrical Engineering, University of Notre
Dame, Notre Dame, IN. His current research interests include understanding
and developing new applications of native oxides for optoelectronic, electronic,
and integrated photonic devices, particularly semiconductor lasers, GaAs-based
metal–oxide–semiconductor field-effect transistors (MOSFETs) and low-loss
high-index-contrast optical waveguides. He is the author or coauthor of more
than 95 published technical papers.
Prof. Hall is the corecipient of the Naval Research Laboratory Alan Berman
Research Publication Award in 1993.