Nonselective oxidation of GaAs-based III-V compound semiconductor

heterostructures for in-plane semiconductor lasers

Di Liang, Jusong Wang, Douglas C. Hall*

Dept. of Electrical Engineering, Univ. of Notre Dame, Notre Dame, IN USA 46556-5637

ABSTRACT

A nonselective wet thermal oxidation technique for AlGaAs-containing heterostructures has been shown to enable the

fabrication of a variety of novel high-efficiency, high-power GaAs-based in-plane laser devices. Applied in conjunction

with a deep anisotropic dry etch, nonselective oxidation yields a simple, self-aligned high-index-contrast (HIC) ridge

waveguide (RWG) structure. The native oxide grown directly on the waveguide ridge simultaneously provides excellent

electrical insulation, passivation of the etch-exposed bipolar active region, and a low refractive index cladding, leading

to numerous laser performance benefits. The resulting strong lateral optical confinement at the semiconductor/oxide

interface (with refractive index contrast !n~1.7) enables half-racetrack ring resonator lasers with a record small 6 µm

bend radius. A nearly circularly-symmetric output beam is demonstrated on narrow w=1.4 µm aperture width straight

stripe-geometry lasers with single spatial and longitudinal mode total power output of ~180 mW at 228 mA (9x

threshold). With the complete structural elimination of lateral current spreading, the excellent overlap of the optical field

with the gain region provides high slope efficiency performance (ranging from >1.0 W/A at w=1.4 !m to 1.3 W/A for

w=150 !m broad area stripes) for 300 K cw operation of unbonded, p-side up 808 nm InAlGaAs graded-index separate

confinement heterostructure (GRINSCH) active region lasers. Using the direct thermal oxidation of a dilute nitride

GaAsP/InGaAsN MQW active region, 1.3 µm emission GaAs-based HIC RWG lasers exhibit a >2X threshold reduction

and kink-free operation relative to conventional low-confinement devices. Other recent progress on the application of

nonselective oxidation to GaAs-based semiconductor lasers will be reported.

Keywords: high-index-contrast, ridge waveguide laser, thermal oxidation, half-racetrack ring resonator, surface

passivation, dilute-nitride lasers, circular output beam

1. INTRODUCTION

The well-known problem of beam asymmetry in the output from edge-emitting semiconductor diode lasers illustrates the

challenges of forming a structure capable of providing adequate and dimensionally-comparable confinement of the

optical field and the gain-providing carriers both in-plane and out-of-plane. While the evolution of epitaxially-grown

heterostructures has provided the means for achieving a good optical confinement factor (i.e., overlap of optical field and

gain) in the vertical growth direction, poor in-plane current and optical confinement has remained a chronic issue

limiting diode-laser spatial mode quality and stability. Buried heterostructures employing layer disordering or etching

and regrowth techniques have improved laser beam quality but not eliminated beam asymmetry. Vertical cavity surface

emitting lasers can produce symmetric beams, but with much lower output power relative to edge-emitting devices. In

this work, we review and further discuss our recent successful efforts to fabricate a high-index-contrast (HIC) ridge

waveguide (RWG) structure for GaAs-based in-plane semiconductor lasers. Realized through the combination of a deep

anisotropic etch through the heterostructure waveguide followed by a non-selective O2-enhanced wet thermal oxidation

process, the HIC RWG completely eliminates lateral current spreading and provides a high lateral optical confinement

factor. Efficient, high-power edge-emitting lasers, including devices with nearly-symmetric output beams, have been

demonstrated. Benefiting from the strong optical confinement offered by a HIC interface, in-plane waveguides can also

be sharply bent to route optical signals or form ring oscillators with negligible bending loss.

There has been growing interest recently in HIC waveguides with !n >1 for integrated optics because of their potential

to enable several orders of magnitude of growth in device integration density and complexity [1]. A major obstacle has

been the greatly increased susceptibility of HIC waveguides to scattering loss, "s, directly proportional to the product

"2(#n)

2 where " is the root-mean-square (RMS) sidewall surface roughness of a waveguide with core cladding effective

index contrast #n; a more rigorous model predicts that "s increases in proportional to (#n)3 [2]. In a deep-etched

structure which exposes the bipolar active region sidewall, passivation of non-radiative recombination centers from

surface states or etch-damaged material is also critical to achieving good active semiconductor device performance. In

this work, we review the demonstrated utility of using non-selective oxidation to effectively smooth and passivate the

sidewalls of a deep-etched heterostructure ridge waveguide, overcoming the challenging issues of scattering loss and

interface recombination. We demonstrate the improved optical mode quality and performance of HIC RWG lasers,

including those with Al-free !~1.3 µm dilute nitride multi quantum well active regions, and present further data on the

reduced current spreading and oxide thickness dependence of these devices.

2. DEVICE FABRICATION

We have elsewhere reported a simple process modification which enables a significantly enhanced oxidation rate for low

Al-ratio AlxGa1-xAs (substantially reducing the oxidation rate selectivity to Al content) through the controlled addition of

trace amounts of O2B [0-10000 ppm (1%) relative to N2B] to the process gas stream (N2+H2O vapor) [3, 4]. Low Al-ratio

AlGaAs and even Al-free III-As waveguide core regions can now be oxidized laterally through this non-selective wet

thermal oxidation technique without fully oxidizing the higher Al-ratio cladding layer, allowing a much higher, real

lateral index step (!n~1.7) to be achieved. We have demonstrated a new self-aligned deep etch plus non-selective

oxidation process for GaAs-based RWG laser fabrication, overcoming the typical limitations of deep-etched structures

[5]. The laser fabrication process typically starts with the growth of a ~200 nm plasma-enhanced chemical vapor

deposition (PECVD) SiNx mask layer to protect the p+-GaAs cap layer from a subsequent oxidation step. The

waveguide stripe is patterned through conventional photolithography followed by two successive dry etching steps to

translate the photoresist (PR) pattern to the SiNx layer and semiconductor epilayers. 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 dimensions close to that of the PR

mask. Non-radiative recombination centers formed during this initial etching process are eliminated or largely reduced

during the following non-selective thermal oxidation process, typically at 450 °C, through conversion to oxide or

annealing, respectively. As shown in the schematic of Fig. 1(a) and the SEM cross-section image of Fig. 1(b), the oxide

grown on the waveguide sidewalls (and base) results in a HIC (!n~1.7) semiconductor/oxide interface, enabling the

realization of a HIC RWG providing strong optical confinement and capable of supporting very sharp bending, while

simultaneously providing scaling from a conventional-lithography-defined ridge dimension ("1 "m) to the submicron

dimensions required for HIC waveguide single-mode operation. Furthermore, instead of depositing PECVD SiO2 or

SiNx for electrical confinement and surface passivation, the native oxide itself acts directly as the dielectric layer,

providing a self-aligned process which eliminates the potential alignment errors, which unavoidably result from a second

lithography step needed to open the contact window in the conventional fabrication process flow. A final dry etching

procedure then selectively removes the dielectric masking stripe, using special care to prevent etch damage to the p+-

GaAs cap layer, and the wafer is then thinned, metallized and cleaved into bars for laser characterization.

(a) (b)

Fig. 1. Schematic and SEM image of high-index-contrast ridge waveguide structure. Inset conduction band schematic shows location of graded-index waveguide core and quantum well.

To further highlight the advantages of this process, we note that the shallow etch in the conventional process flow 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) which plagues conventional index-guided RWG laser designs is

prevented in this new device structure as current flow is effectively restrained to a vertical channel defined by the

insulating oxide [6]. Uniform carrier distribution in the oxide-confined active region also substantially minimizes spatial

mode-hopping, resulting in kink-free operation (i.e., output power vs. current device characteristics which are linear and

free of kinks at currents above threshold) [6]. Strong optical mode confinement from the vertical oxide walls also offers

a potential for overcoming the limitation of the asymmetric optical mode profile and corresponding output beam in-plane

vs. out-of-plane far-field divergence angle asymmetry in edge-emitting lasers [7], a well-known disadvantage for

numerous applications. We have elsewhere shown that the non-selective wet thermal oxidation process enables a

significant reduction of semiconductor waveguide scattering loss through an effect known as oxidation smoothing

where a thermal oxidation process smoothes the sidewall roughness as the oxidation front progresses inward [8].

Compared with the lithography and etching processes typically required to achieve submicron feature sizes, non-

selective oxidation is shown to be well-controllable for the formation of submicron structures simply by the tuning of

several process parameters (e.g., temperature, O2 concentration and flow rate of N2 carrier gas), and can be carried out

with much lower cost equipment. The proposed HIC process clearly can provide a significant improvement in the

device performance/cost ratio.

3. DEVICE PERFORMANCE

The light output power vs. current (L-I) characteristics in Fig. 2 show the good kink-free laser performance for a narrow-

stripe (w=5 µm) device with a cavity length of 361 µm under both pulsed current injection (1% duty cycle) and a

maximum (FWHM) and 1/e2 power width measured on the pattern at

I=90 mA are 4.03 µm and 5.51 µm, respectively. These two values,

both significantly smaller than the 8.9 µm aperture size, demonstrate

the strong lateral optical confinement provided by the HIC interface.

In Fig. 3 (b), single-mode operation is achieved on a w=5 µm device

and well maintained at both current levels of 70 mA and 100 mA,

with 1.99 µm FWHM and 3.41 µm 1/e2 width values measured at

I=100 mA.

Spatial single-mode operation in the directions both parallel and

perpendicular to the junction plane is also further confirmed in the

far-field pattern of the same w=5 µm device at different output

power levels [Fig. 3(c)]. Mode simulations show that the higher-

order modes of the same waveguide structure are cut-off at a width

around w=1 µm so a passive RWG with w=5 µm is not expected to

operate in the single-mode regime. However, single-mode operation

for HIC waveguides in active laser devices are largely affected by

mode competition where the fundamental mode with the lowest loss

reaches stimulated emission first and consumes most of the carriers,

suppressing the lasing probability for higher-order transverse modes.

Higher order modes also penetrate further into the oxide cladding

and can thus experience greater loss due to absorption by the metal

on the outside of the ridge.

4. REALIZATION OF SYMMETRIC OUTPUT BEAM

The asymmetry of the divergence angles, defined as the ratio of

!///!", is only 2.73 in Fig. 3(c), smaller than the typical values of !4

observed in conventional shallow-etched index-guided RWG lasers.

The strong dependence of the optical mode on the waveguide

dimension offers a promising path to the further reduction of the

output beam asymmetry by further scaling the aperture dimension.

An asymmetric mode profile is clearly shown in the mode simulation

of Fig. 4 (a) for a passive AlGaAs rib waveguide structure (w=4 µm)

commonly employed for a conventional shallow-etched RWG laser.

Due to the 1:2.4 ratio compression of the horizontal scale in the top

of Fig. 4, the asymmetry for this representative conventional design

is much worse (~27:1) than it appears. Reducing the rib waveguide

width actually inversely increases the lateral dimension of the optical

mode due to a loss of effective optical confinement as shown in Fig.

4(b). By using a slightly broadened active region and squeezing the

mode laterally with the low index (n~1.6) native oxide, however, a

circular mode (1:1 aspect ratio) can be obtained in a HIC RWG [Fig.

4(c)].

Diode lasers with a stripe dimension "5 µm are fabricated to

experimentally demonstrate the possibility of achieving a perfectly

circular output beam. Fig. 5 shows the L-I characteristic of a HIC

RWG straight laser showing a small aperture size of 1.39 µm. With

threshold current of Ith=25.2 mA and laser cavity length of L=1107.1

µm, the threshold current density is Jth=1644 A/cm2. This higher

current density causes high junction temperature and a more

noticeable red shift of the peak wavelength (#p=826.27 nm)

measured at 189 mA true cw [Fig. 5 inset (a)]. At a true cw current

(a)

(b)

(c)

Fig. 3. Near-field patterns of 8.9 µm (b) and 5 µm (a)

wide HIC RWG lasers at true cw currents of 60 and 90

mA, and 70 and 100 mA, respectively. Multi-mode

operation is observed on the w=8.9 µm device as

additional intensity spots which appears when the

current is increased from 60 mA to 90 mA. FWHM

and power width at the 1/e2 level are 4.03 µm and 5.51

µm, respectively. Single-mode operation is well

maintained on the w=5 µm device at both current

levels with 1.99 µm FWHM and 3.41 µm 1/e2 width

values. Single-lobe far-field patterns at 20 mW output

power in both directions parallel and perpendicular to

the epilayers are shown in (c), indicating a FWHM

beam asymmetry of 2.64.

of 180 mA and an output power close to 150 mW, the measured near-

field pattern in Fig. 5 inset (b) still shows a smooth single-mode profile

with a FWHM of 0.55 µm and 1/e2 width of 0.99 µm, demonstrating

strong mode stability. The fast-dc LI measurement, limited to a 100

mA injection current, shows no signs of thermal roll-over.

The Rd=1.02 W/A slope efficiency in Fig. 5, corresponding to an

!d=68% external quantum efficiency at a wavelength of 826.27 nm, is

remarkably high considering the laser aperture is only 1.39 µm wide. An extrapolation of the efficiency data of devices

with 90 µm, 40 µm, 10 µm, 7 µm and 5 µm aperture dimensions predicts an external quantum efficiency of only ~40%

for a 1.39 µm wide aperture laser [9]. The unexpectedly high efficiency can be attributed to stopping of the dry etch in

this device before penetration through the waveguide core and SQW active region, leading to a reduction in the number

of etching-induced defects introduced into the active region. As shown by the dashed line in Fig. 4(c) which marks the

level of the SQW, the oxide growth here still penetrates through a large part of waveguide core layer to achieve a HIC

RWG. This thermal oxidation process converts much of the etch-damaged material to an amorphous oxide, eliminating

a substantial numbers of the non-radiative recombination centers. Other remaining etch-induced defects in the

semiconductor may also be annealed out during the next thermal oxidation process when considering that 350 °C anneals

have been shown to promote some recovery from RIE etch damage [10].

Lasers with other aperture dimensions are also fabricated and measured under the same conditions. Fig. 6 shows that for

shrinking laser lateral dimensions (from 15 !m to 7, 4.67, 2.67, 1.62, and 1.39 µm), the FWHM divergence angle "//

parallel to the junction plane increases due to increased light diffraction (from 5.5° to 8.8°, 15.0°, 15.94°, 22.05°, and

Fig. 5. Pulsed and cw 300 K light-current characteristic of a HIC RWG

straight with Ith=25.2 mA, active width w=1.39 !m, and laser cavity

length L=1107.1 !m, leading to the threshold current density Jth=1644

A/cm2. The differential slope and quantum efficiencies at I=100 mA are

Rd=1.02 W/A and !d=68%, respectively. Inset (a): Linear spectrum

measured at 189 mA true cw shows a peak wavelength of 826.27 nm.

Inset (b): Near-field profile measured at 180 mA true cw shows a 0.55 µm

FWHM and 0.99 µm 1/e2 width. Inset (c): SEM cross-section image of

w=1.39 !m HIC RWG structure after etching (with 200 nm-thick PECVD

SiNx mask layer on the ridge top) and 20 min, 450 °C nonselective

oxidation. Dashed line shows SQW location.

Fig. 4. (a,b) conventional and (c) native oxide-

defined AlGaAs/GaAs passive waveguide

structures.

28.39°, respectively). The divergence increase is not linear but accelerates as the stripe width gets smaller and smaller.

A weaker, opposite dependence of the divergence angle in the direction perpendicular to the junction plane on laser

stripe width is also observed in Fig. 6, resulting in a noticeable reduction of the beam asymmetry !"/!// from >8 (at w=15

µm) down to 1.23 (at w=1.39 µm). While the vertical dimension (i.e., thickness of the waveguide core layer) is not

changed, the divergence angle !" still decreases from 47.1° to 35° as the stripe width is reduced from 15 µm to 1.39 µm.

The variation of !" can be explained from an expression (Eqn. (1)) derived from Ref. [11] by Dumke [12], which is

applicable not only to the case of a thin waveguide layer [11], but also that of a thick waveguide laser (i.e., large optical

cavity):

2 2

/ /

1 ( / ) /1.2 1/ ( / ) /1.2

ad d

a d a d

! !"

! !#= =

+ + (1)

where a=0.41#2(n1

2-n2

2), d, #, n1 and n2 are the thickness of the waveguide core layer, free-space wavelength and

refractive indices of the waveguide core and cladding, respectively. Because Fig. 6 shows the divergence angle when all

lasers are well above threshold under true cw mode operation without a heatsink, heat can easily build up inside the

resonance cavity (i.e., the waveguide region here) but to a varying degree for lasers with stripe widths of 1.39, 1.62,

2.67, 4.67, 7 and 15 µm. As the data trend in Fig. 6 shows, narrower devices consume more injection current to

compensate the losses from non-radiative recombination and optical scattering, which results in a higher current density

and a higher junction temperature. The narrower stripe lasers, therefore, experience more heat buildup than the wider

ones since temperature is proportional to the current density. Because the material refractive index (real part) reduces

with rising material temperature, the waveguide core index n1 for the 1.39 µm laser is smaller than that of the 15 µm

laser. On the other hand, the cladding index n2 does not vary much since heat generation occurs mostly in the active

region (within the waveguide) where, because the doping is the lowest, Joule (I2R) heating is the greatest and where non-

recombination processes most likely occur due to bipolar activity. It thus turns out that the parameter a in Eqn (1)

decreases (i.e., 1/a increases) from thermal effects as the device dimension narrows, leading to a smaller and smaller fast

axis divergence angle !". Furthermore, the power area density at the laser emission facet is the highest for the w=1.39

µm laser since the >50 mW/facet output power is distributed over the smallest area (w!d). This high power area density

further increases the local temperature at the laser facet and consequently causes further narrowing of the fast axis

divergence angle.

The extrapolated curve fits in Fig. 6 predict that the

a perfectly circular beam will require a submicron

aperture (w=0.56 µm) device where the divergence

angle is 32.4° in directions both parallel and

normal to the junction plane. The passive

waveguide mode simulation based on the same

wafer and device structure (Fig. 6 inset) also shows

a circularly symmetric mode profile when w=0.5

µm. The small discrepancy may be due to the

passive nature of the mode simulation which

neglects carrier-dependent index variations present

in active devices. Utilizing the good controllability

of the relatively slow non-selective oxidation

process, submicron aperture devices can potentially

be patterned by conventional photolithography and

dry etching when followed by a longer oxidation to

achieve the target effective aperture size.

The divergence angle of 32.4° projected for a

perfectly symmetric output beam device is not

optimal for butt-coupling to a commercial silica

glass fiber whose index step of no more than 0.01

gives an acceptance angle of only 11.9.° However,

through use of a micro-lens system or tapered lens

Fig. 6. A plot of the laser beam divergence angles and divergence angle

ratio (i.e., beam asymmetry) vs. laser stripe width. The point of

intersection of the curve fits predicts that a perfectly circular output beam

with 32.4° divergence angles could be achieved at a stripe width of w=0.56 µm. Inset: passive waveguide mode simulation when w=0.5 µm.

fiber, such a laser may offer improved matching of the laser output mode to the fiber mode. The methods reported here

for increasing the lateral optical confinement factor and eliminating current spreading are similar and complementary to

recently reported slab-coupled optical waveguide approaches [13, 14]. It may be possible to further minimize or

overcome the mode mismatch problem between a laser beam and optical fiber by employing an oxide-confined HIC

device with a different, optimized epilayer design. The symmetric beam lasers described here may also offer a low

astigmatism beam for high-brightness focused spot applications such as optical disk writing or laser printing.

5. STUDIES OF LATERAL CURRENT SPREADING AND OPTICAL CONFINEMENT

The unavoidable current spreading in conventional RWG lasers prevents the reduction of their lateral dimension while

still maintaining low threshold operation. Since the laser beam is not well guided laterally outside the waveguide region,

the carriers escaping out from the ridge prior to getting collected by the underlying quantum well region either dont

contribute to the photon generation at all, or generate spontaneous emission in directions not amplified by the guided-

mode stimulated emission. These lost photons essentially act as an electrical loss in the laser cavity and are likely to

cause increased heating. Furthermore, negative impacts also include spatial mode distortion (e.g., a more elliptical far-

field radiation pattern as in Fig. 4), mode hopping-induced kinks in the L-I curve, a perturbation of the longitudinal mode

via lateral mode instability [15] and a rise in intermodulation distortion under direct modulation [16]. Tsang has done

some early work to analyze the effects of lateral current spreading on stripe-geometry double-heterostructure lasers [17],

followed by Hu et al. giving experimental data and a modified theoretical model for RWG lasers [18, 19]. Letal et al.

have reported that up to 42% of the injection current escapes from the index-guided region at threshold in a w=2 µm

InGaAsP/InP MQW laser [20], emphasizing why conventional RWG lasers are normally limited to stripe widths of w!5

µm. A lateral spreading distance of up to 20 µm is reasonably estimated from simulations [15]. Direct observation of

the current spreading using scanning voltage microscopy has also been recently reported [21], enabling an intuitive

understanding from a visual perspective.

Numerous approaches to device structure optimization have been demonstrated to minimize current spreading, including

varying the residual upper cladding thickness (i.e., etch depth) [15, 22] and utilizing a buried heterostructure [23]. It is

obvious that a reduced residual upper cladding layer thickness improves the lateral carrier confinement, however, ~3-4"

higher optical scattering loss is also typically observed [22] since the guided light interacts with the waveguide sidewall

roughness more intensely. Higher internal optical loss plus the probable extension of etch defects into the active region

consequently leads to an overall higher threshold current [22]. A buried heterostructure employing an etch plus

regrowth process to confine the etch-defined p-waveguide region by an adjacent regrown n-cladding material has been

shown to reduce the current spreading through reverse biasing of the n-cladding/p-waveguide diode junction.

Nevertheless, up to 30% of the threshold current in such single-mode devices still does not contribute to active region

pumping [23]. Laser devices in which a buried current aperture is formed through selective lateral oxidation of a buried

high Al-ratio AlxGa1-xAs layer (1 ! x ! 0.9) placed above the waveguide core layer [24-26], inspired from oxide-

confined VCSELs, show a strong capability for suppressing the escape of carriers due to current spreading. This

approach, however, doesnt provide much increase in the lateral index step to increase optical confinement, and is thus

ill suited for achieving devices with smaller dimension or sharply bent features.

The HIC RWG laser structure described herein simultaneously tackles the two limitations of conventional RWG devices:

carrier confinement and device scalability. For comparative analysis, weak index-guided lasers employing only a

shallow-etch have been fabricated along with HIC RWG lasers from the same 808 nm GRINSCH material.

Approximately 925 nm of the combined 200 nm GaAs p+ cap layer plus 1.5 µm upper cladding layer is removed by RIE

after patterning straight stripes with varying widths. A 20 min non-selective oxidation with the addition of 4000 ppm O2

at 450 °C is then applied to grow about 200 nm of oxide, leaving a residual Al0.6Ga0.4As upper cladding layer thickness

of 575 nm. A deeply etched sample with the same mask pattern is oxidized under the same conditions to closely match

device dimensions. Fig. 7 shows the relationship of the threshold current density of weak index-guided lasers vs.

reciprocal laser cavity length. Both narrow stripe (w=5, 7 µm) and broad-area devices (w=90 µm), usually immune to

current spreading, experience a higher threshold current density to varying degrees. At L=500 µm, for instance, weak

index-guided broad-area lasers need 492.75 A/cm2 to reach lasing threshold while the HIC broad-area devices require

only 48.6% of this amount. Narrow-stripe devices with stripe widths of 5 and 7 µm exhibit 2608.7 and 1855.1 A/cm2

threshold current density at the same cavity length, 2.85X and 2.53X higher, respectively, than their HIC counterparts.

The stripe-width dependent Jth data of Fig. 8 reveals a similar picture but further demonstrates that the threshold current

density of the weak index-guided structure climbs much faster than that of the HIC structure as the laser stripe width is

reduced, indicating a proportionately larger fraction of carriers are escaping from the ridge waveguide region in

conventional devices. Unlike the shallow-etched devices, narrow stripe HIC devices can suffer somewhat from non-

radiative recombination at the semiconductor/oxide interface, but the greater overall consequence of eliminating current

spreading leads to a more than 2! performance improvement overall on w " 15 µm devices. Two sets of devices with

similar cavity length are selected for comparison in Fig. 8 in

order to minimize the influence of the distributed mirror

loss. Fig. 8 also shows that even broad-area devices (w>50

µm) fabricated with a HIC RWG structure consistently

show improved performance (~24% lower Jth) due to the

complete and significant elimination of current spreading.

The HIC RWG achieved through this process enables very

tight waveguide bends with low loss [27]. This has been

demonstrated through the fabrication of half-racetrack-ring

resonator lasers with a bend radius as low as r=6 !m, as

shown here in Fig. 9 [28]. In addition to lasing for e-beam

lithography defined devices with r=25, 10 and 8 !m

reported in [28], Fig. 9 shows lasing with comparable

performance for an r=6 !m, w=10 !m ridge width device.

The slightly improved threshold current and efficiency is

due to the r=6 !m devices shorter cavity length (636 !m

vs. 1 mm for the others).

6. OXIDE THICKNESS DEPENDENCE

The native oxide formed at the etch-exposed active region

is the paramount feature in this HIC structure, particularly

for active devices, since it serves multiple key purposes

including interface passivation, sidewall roughness

Fig. 7. Comparison of weak index-guided and HIC RWG laser

threshold current densities vs. inverse laser cavity length 1/L,

showing the significantly higher threshold current density for (b)

w=7 and (c) 5 µm weak index-guided devices vs. (e) 7 and (f) 5

µm HIC RWG lasers. Even (a) w=90 µm weak index-guided

broad-area lasers show notable degradation due to current

spreading relative to (d) their HIC RWG counterpart.

Fig. 8. Threshold current density vs. laser stripe width for HIC

devices and weak index-contrast devices, both fabricated on

808 nm GRINSCH structure and probe tested in room

temperature and under pulsed mode (1% duty cycle) with p-

side up. Up to 2.38X threshold current density reduction is

achieved on w=5 µm HIC devices due to elimination of current

spreading.

0

40

80

120

160

200

240

0 300 600 900 1200 1500

Outp

ut

Po

wer

(mW

)

Current (mA)

(a) r=25 µm

(b) r=10 µm

(c) r=8 µm

Pulsed, 300 K (unbonded, p-side up)

r=8 µm

(d) r=6 µm

Fig. 9. Total output power vs. current characteristics for w=10 !m wide HIC RWG lasers in half-racetrack-ring geometry with bend radii of (a)

r=25 !m, (b) r=10 !m, (c) r=8 !m and (d) r=6 !m. Total resonator cavity lengths are ~1 mm for (a)-(c) and 636 !m for (d).

reduction, metal/semiconductor isolation and optical confinement. Each role is critical to the HIC RWG structure and

failure in any aspect could derail the whole enterprise. It is therefore of great importance to explore if there is a

minimum oxide thickness required for proper device operation, or even, perhaps, whether there is an optimal oxide

thickness.

HIC RWG lasers devices have been fabricated with two different oxide thicknesses. Fig. 10 exhibits significantly

different levels of performance for lasers distinguished only by their oxide thickness at the QW active region: (a) 104

and (b) 546 nm, corresponding to 10 and 40 min nonselective oxidations with addition of 4000 ppm O2, respectively.

Two bars with comparable bar

length are selected for a legitimate

comparison and both of them

represent typical performance (i.e.,

neither the best or the worse)

among all devices tested. In Fig.

10(a), only broad-area devices with

stripe widths of 120, 90, 60 and 40

µm operate as lasers, albeit with

fairly poor performance. The w=25

µm device behaves strangely,

followed by only spontaneous light

emission on devices of w=15 and

10 µm. Narrow w=7 and 5 µm

stripe devices are still diodes but

emit no light at all. In contrast,

devices in Fig. 10 (b) show

consistently high slope efficiency

on broad-area devices with a

maximum of 1.28 W/A observed

on w=120 µm devices. The lower

average slope efficiency for all

widths of 1.05 W/A is attributed to

the comparatively low slope

efficiency (Rd < 1 W/A) of w=7 and

5 µm narrow stripe devices, which

sit near the edge of the bar and are

thus also subject to poor facet

cleaves. It is noted that the stripe

widths in Fig. 10 refers to the photo

mask dimension instead of the

actual laser aperture size obtained

by deducting the oxide thickness.

Clearly, a 104 nm thin oxide is

inadequate for achieving desirable

device performance, due to a

combination of insufficient

interface passivation, inadequate

oxidation smoothing, poor

insulating property, or high optical

loss owing to the evanescent wave

penetration through the thin oxide

layer to the absorbing sidewall

metal. The failure of w=5 and 7

µm devices in Fig. 10 (a) to emit

0

50

100

150

200

250

300

350

400

-1

0

1

2

3

4

0 200 400 600 800 1000 1200 1400

w=120 µm

w=90 µm

w=60 µm

w=40 µm

w=25 µm

w=15 µm

w=10 µm

w=7 µm

w=5 µm

To

tal O

up

ut

Po

we

r, P

(m

W)

Vo

ltag

e, V

(V)

Injection Current, I (mA)

pulsed, 300 Kp-side up, unbonded

L=934 µm

0.78 V

QW 104 nm oxide

(a)

0

50

100

150

200

250

300

350

400

0 200 400 600 800 1000 1200 1400

w=120 µm

w=90 µm

w=60 µm

w=40 µm

w=25 µm

w=15 µm

w=10 µm

w=7 µm

w=5 µm

To

tal O

utp

ut

Po

we

r, P

(m

W)

Injection Current, I (mA)

Rd: 0.92~1.28 W/A

Ave. Rd=1.05 W/A

pulsed, 300 Kp-side up, unbonded

L=915 µm

QW

546 nm oxide

(b)

Fig. 10. Light-current characteristic of (a) a L=934 µm long bar with 104 nm non-selective

oxide formed at the active region (inset) in 10 min, 450 °C oxidation with 4000 ppm O2

participation, showing poor device performance, and (b) a L=915 µm long bar with 546 nm

non-selective oxide grown at the active region (inset) in 40 min oxidation with 4000 ppm

O2 participation, showing decent device performance with a slope efficiency as high as 1.28

W/A on a w= 120 µm broad-area device and an average of 1.05 W/A for all devices. The

current-voltage characteristic of the 5 µm wide device (not working) in (a) indicates

substantial current leakage with a low turn-on voltage.

light altogether spotlights the factors of interface passivation and oxide insulating property, as there should be some

weak, spontaneous emission even if the metal absorption or scattering was too high to achieve stimulated emission. The

current-voltage characteristic of the w=5 µm device shown in Fig. 10 (a) indicates an anomalously low turn-on voltage

(0.78 V) compared to the normal range of 1.6-1.7 V. An early turn-on may result from a high leakage current through

the thin AlGaAs oxide, whose insulating property is just fair when compared [29] with other III-V compound

semiconductor (InAlP) native oxides [30]. Preliminary studies on the electrical properties of non-selective oxides do

show a negligible leakage of JL

InGaAs quantum wells for which we have

also observed oxide growth (data not

shown). It has been shown by Luo et al. [4]

that the addition of O2 significantly

enhances the oxidation rates of an undoped

Al0.20Ga0.8As waveguide core containing a

single 10 nm GaAs quantum well. Fig. 11

now demonstrates that substantially thicker

GaAs layers and even a dilute-nitride MQW

structure can also be non-selectively

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 hour non-selective oxidation at

450 °C with the addition of 7000 ppm O2 is

then used to grow ~2.5 !m of oxide

(measured at the etch-exposed

GaAsP/InGaAsN MQW active region),

resulting in an effective laser active aperture

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

0.75 µm deep etch (requiring 8 min under

the same dry etch conditions), followed by a

short 30 min nonselective oxidation at 450

°C with 7000 ppm O2 to convert part of the

Al0.65Ga0.35As upper cladding layer to a 200

nm native oxide for device isolation.

Fig. 12 shows the pulsed (1% duty cycle) L-

I characteristic of a broad-area device (c)

with L=307 µm cavity length, w=85 µm

(effective laser aperture dimension). 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 a differential quantum efficiency of

50.8% (at !peak=1.234 µm), respectively, are

demonstrated. Fig. 12 also shows typical L-

I characteristics for (a) HIC and (b)

conventional RWG lasers with a w~10 µm

narrow stripe laser effective active stripe

width. It is well known that both poor

optical confinement and carrier leakage via

current spreading can lead to mode hopping

in weakly-guided narrow-stripe lasers which

in turn causes kinks in the L-I

characteristics, as was above demonstrated

for conventional shallow-etched 808 nm

GRINSCH devices (Fig. 7). Such behavior

is observed for the device of Fig. 12 as well,

Fig. 11. SEM cross-section image of 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 in the image, showing ~115 nm and 40 nm oxide growth on GaAs

waveguide core and GaAsP/InGaAsN MQW active region, respectively.

Fig. 12. Pulsed L-I characteristics of typical w~10 !m stripe geometry lasers with

(a) HIC and (b) conventional shallow-etched index-guided RWG structures. For

(a) HIC diode laser with L=525 !m, Ith=45.73 mA, Jth=871 A/cm2, and the

differential slope efficiency at 2Ith is Rd=0.451 W/A. For (b) conventional device

with L=520 !m, 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). For (c) a

L=307 µm HIC-type broad-area device with w=85 µm effective laser aperture,

pulsed (1% duty cycle) L-I characteristic of showing Ith=169.6 mA and Rd=0.51

W/A. Inset: spectrum of HIC diode laser operating at 100 mA pulsed current,

showing a peak wavelength of 1.23 !m and SEM of a different dry etch-exposed

waveguide sidewall nonselectively oxidized at 450 °C for 1 hour.

and is typical in most of our conventional devices. In contrast, the HIC RWG laser of Fig. 12 shows kink-free operation

suggesting stable spatial-mode behavior. As shown in Fig. 12, laser operation with a low-threshold current (Ith=45.7

mA) and high slope efficiency (Rd=0.45 W/A) is obtained without visible mode-hopping induced L-I kinks. The inset

spectrum is measured at a pulsed injection current of 100 mA (~2.2!Ith), showing a peak wavelength of 1.23 µm. It has

been shown elsewhere that growth modifications not incorporated in the structures used in this work can extend the

wavelength to close to 1300 nm [35, 37]. The inset SEM cross-sectional image of the etch-exposed RWG sidewall

oxidized under the same conditions but for a shorter time period of 1 hour, 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 hours to thicknesses of 40 nm, 1220 nm 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 non-uniform oxidation observed in the AlGaAs cladding

layers and GaAs waveguide p-n junction layer may be attributed to doping-related effects [40] and interface-enhanced

oxidation [41] observed in other heterostructures.

Furthermore, up to 2.3! lower threshold current density is achieved on native oxide-confined HIC w=5 µm narrow stripe

devices relative to conventional, shallow-etched RWG devices with comparable cavity length because of the total

elimination of current spreading (data not shown) [9]. A similar 2.38! threshold current density reduction on 808 nm

GRINSCH HIC RWG lasers with a 5 µm stripe width was shown above in Fig. 8, indicating a comparable performance

gain in both material structures.

8. CONCLUSION

In summary, we have reviewed recent progress in high-index-contrast ridge waveguide lasers offering improved spatial

mode behavior and capability for a high-power symmetric output beam from edge-emitting laser devices. The

performance benefits achieved through the complete elimination of current spreading are highlighted through a

comparative analysis of weak-index-guided and HIC RWG devices. A comparison of device performance for different

oxide thicknesses is reported. Application of the deep-etch plus non-selective oxidation fabrication process has been

demonstrated for !=808 nm AlGaAs-GaAs GRINSCH and !~1.3 !m Al-free InGaAsN/GaAsP/GaAs active region

lasers. In addition to providing a high lateral optical confinement factor for efficient performance of conventional edge-

emitting lasers, the HIC RWG is shown to enable very-small radius waveguide bends with low bend loss, opening

potential new vistas for dense photonic integration.

ACKNOWLEDGEMENTS

We gratefully acknowledge J. Y.-T. Huang, J.-Y. Yeh, and L. J. Mawst of the University of Wisconsin at Madison for

growing the dilute nitride active region heterostructures used in this work. This work was supported in part by the

National Science Foundation under Grants ECS-0123501 and ECS-0601702.

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