Nonselective oxidation of GaAs-based III-V compound semiconductor heterostructures for in-plane semiconductor lasers

Transcription

1 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 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 O 2 -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

2 [ 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 Al x Ga 1-x As (substantially reducing the oxidation rate selectivity to Al content) through the controlled addition of trace amounts of O 2 B ppm (1%) relative to N 2 B] to the process gas stream (N 2 +H 2 O 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) SiN x 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 SiN x 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 SiO 2 or SiN x 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) 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. (b)

3 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, nonselective oxidation is shown to be well-controllable for the formation of submicron structures simply by the tuning of several process parameters (e.g., temperature, O 2 concentration and flow rate of N 2 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 narrowstripe (w=5 µm) device with a cavity length of 361 µm under both pulsed current injection (1% duty cycle) and a <1 sec, fast direct current (dc) sweep (called fast-dc here). A low threshold current of I th =20.62 ma (corresponding to J th = A/cm 2 ) is achieved under both pulsed (1% duty cycle) and fast-dc modes, while the slope efficiency only drops slightly from the values (1.3 W/A) of the w=90!m broad-area device on the same bar to 1.1 W/A, corresponding to an external differential quantum efficiency of! d =72.68% based on a lasing wavelength of!=812 nm measured at 40 ma steady-on dc (i.e., true cw) injection current of 40 ma. The overlap of the pulsed and fast-dc L-I curves up to a 60 mw output power reveals the extraordinary thermal 250 performance of the native oxide-confined HIC RWG p-side up, unbonded, 300 K structure where the junction heat can be efficiently dissipated through the thin oxide cladding layer and 5 µm 90 µm into the high thermal conductivity Au p-metal contact. 200 L A near-field measurement is conducted by imaging the laser facet onto a silicon charge-coupled device (CCD) linear detector array. The line camera signal is displayed on a digital oscilloscope where the near-field profile is captured with 100# averaging to minimize the background noise. A far-field measurement is made using a rotation stage and slit-covered single element detector. Fig. 3 (a) and (b) show typical horizontal near-field patterns of two narrow-stripe oxide-confined straight lasers with effective apertures of 8.9 µm and 5 µm measured at two different above-threshold true cw current levels of 60 ma and 90 ma, and 70 ma and 100 ma, respectively. As expected from mode simulations, multi-mode operation is observed on the w=8.9 µm device as shown in Fig. 3(a). For example, an additional intensity spot can be seen rising up when the cw current is increased from 60 ma to 90 ma (dash vertical line arrow). The intensity spot on the left also grows to be as strong as the central spot of the fundamental mode at I=90 ma. The full width at half Total Output Power (mw) pulsed w=5 µm 1.1 W/A " =72.68% d fast-dc pulsed w=90 µm 1.3 W/A " d =85.13% cw, 40 ma! =812 nm p Wavelength (nm) Injection Current (ma) Fig. 2. L-I characteristics of straight HIC RWG lasers, showing a threshold current of I th =21.5 ma and a differential quantum efficiency of! d =72.68% for a narrow stripe laser (w=5µm, L=361 µm) in both pulsed (a) and cw (b) modes, and I th =110 ma and! d =72.68% for a broad-area laser (w=90 µm, L=361 µm). Insets: Spectrum of laser operating at cw of 40 ma (" peak =812 nm).

4 maximum (FWHM) and 1/e 2 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/e 2 width values measured at I=100 ma. (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/e 2 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/e 2 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 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 higherorder 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 I th =25.2 ma and laser cavity length of L= µm, the threshold current density is J th =1644 A/cm 2. This higher current density causes high junction temperature and a more noticeable red shift of the peak wavelength (# p = nm) measured at 189 ma true cw [Fig. 5 inset (a)]. At a true cw current

5 Fig. 5. Pulsed and cw 300 K light-current characteristic of a HIC RWG straight with I th =25.2 ma, active width w=1.39!m, and laser cavity length L=1107.1!m, leading to the threshold current density J th =1644 A/cm 2. The differential slope and quantum efficiencies at I=100 ma are R d =1.02 W/A and! d =68%, respectively. Inset (a): Linear spectrum measured at 189 ma true cw shows a peak wavelength of nm. Inset (b): Near-field profile measured at 180 ma true cw shows a 0.55 µm FWHM and 0.99 µm 1/e 2 width. Inset (c): SEM cross-section image of w=1.39!m HIC RWG structure after etching (with 200 nm-thick PECVD SiN x 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 oxidedefined AlGaAs/GaAs passive waveguide structures. of 180 ma and an output power close to 150 mw, the measured nearfield pattern in Fig. 5 inset (b) still shows a smooth single-mode profile with a FWHM of 0.55 µm and 1/e 2 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 R d =1.02 W/A slope efficiency in Fig. 5, corresponding to an! d =68% external quantum efficiency at a wavelength of 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

6 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): ad /! d /! " = # +! = +! ad ( / ) /1.2 1/ a ( d/ ) /1.2 where a=0.41# 2 (n 1 2 -n 2 2 ), d, #, n 1 and n 2 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 n 1 for the 1.39 µm laser is smaller than that of the 15 µm laser. On the other hand, the cladding index n 2 does not vary much since heat generation occurs mostly in the active region (within the waveguide) where, because the doping is the lowest, Joule (I 2 R) heating is the greatest and where nonrecombination 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. (1) 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 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.

7 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 don t contribute to the photon generation at all, or generate spontaneous emission in directions not amplified by the guidedmode 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 farfield 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 Al x Ga 1-x As layer (1! x! 0.9) placed above the waveguide core layer [24-26], inspired from oxideconfined VCSELs, show a strong capability for suppressing the escape of carriers due to current spreading. This approach, however, doesn t 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 O 2 at 450 C is then applied to grow about 200 nm of oxide, leaving a residual Al 0.6 Ga 0.4 As 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 A/cm 2 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 and A/cm 2 threshold current density at the same cavity length, 2.85X and 2.53X higher, respectively, than their HIC counterparts.

8 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. The stripe-width dependent J th 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 nonradiative 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 240 order to minimize the influence of the distributed mirror loss. Fig. 8 also shows that even broad-area devices (w>50 r=8 µm µm) fabricated with a HIC RWG structure consistently 200 show improved performance (~24% lower J th ) due to the complete and significant elimination of current spreading. Output Power (mw) (a) r=25 µm (d) r=6 µm (b) r=10 µm (c) r=8 µm Pulsed, 300 K (unbonded, p-side up) Current (ma) 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). 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 device s 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

9 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 O 2, respectively. Total Ouput Power, P (mw) Total Output Power, P (mw) (a) pulsed, 300 K 350 p-side up, unbonded 300 QW 104 nm L=934 µm 3 oxide w=120 µm 250 w=90 µm w=60 µm V w=40 µm w=25 µm w=15 µm w=10 µm w=7 µm w=5 µm Injection Current, I (ma) 400 R : 0.92~1.28 W/A pulsed, 300 K (b) 350 d p-side up, unbonded Ave. R d =1.05 W/A L=915 µm 300 w=120 µm w=90 µm w=60 µm 546 nm oxide w=40 µm 150 w=25 µm QW w=15 µm w=10 µm w=7 µm w=5 µm Injection Current, I (ma) 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 O 2 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 O 2 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. Voltage, V (V) 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 (R d < 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

10 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 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 J L <4.2 na/cm 2.5 V measured for a ~140 nm oxide of Al 0.3 Ga 0.7 As grown under similar conditions (450 C, 28 min, 2000 ppm O 2 ; data not shown). However, the major defect planting process (BCl 3 /Cl 2 /Ar plasma dry etching at 20 mtorr base pressure and 100 W RF power) was not applied to the sample measured, raising the possibility of leakage problems due to etch-related defects. Semiconductor damage during dry etching is well-known, especially for compound semiconductors with direct bandgaps [10, 31-33]. Wong et al. studied the Ar sputtering etch and BCl 3 /CCl 2 F 2 RIE damage to a GaAs/AlGaAs quantum well structure through comparing the cathodoluminescence spectra for damaged and undamaged areas and reported noticeable etch-induced damage extending from the surface to about 100 nm at 80 V bias voltage and as deep as nm at 350 V bias voltage [10]. The BCl 3 /Cl 2 /Ar RIE process in this work utilized a bias voltage of about 250 V, which may cause damage extending to ~200 nm beneath the etch surface or more. As noted above, it is possible that the subsequent thermal oxidation process may act as an annealing step to repair some of the etch damage [10], but the temperature and time requirements for our system and material have not been explored. We hypothesize that residual dry etch-induced defects in the semiconductor active region are responsible for much of the device degradation observed in Fig. 10 (a). It is also likely that a thicker oxide provides better device passivation by converting more of the etchdamaged semiconductor to an amorphous insulating oxide. Comparing with the performance (threshold current density and efficiency) of devices passivated by a 240 nm oxide in Fig. 2 (20 min non-selective oxidation with addition of 4000 ppm O 2 ), devices in Fig. 10 (b) with a thicker 546 nm oxide demonstrate a similarly good performance, indicating the establishment of satisfactory passivation mechanism and insulating capability, a smooth interface and adequate optical confinement with only ~240 nm of oxide. The possibility that a thicker oxide growth may have other desirable or undesirable consequences is an open issue for further studies. 7. HIGH-INDEX-CONTRAST DILUTE NITRIDE LASERS The emission wavelength of GaAs-based diode lasers may be extended to the!=1.3 and 1.55 µm fiber-optic telecommunications bands through the incorporation of dilute amounts of nitrogen into the active region (yielding dilute nitride alloys) [34-38]. Low-threshold-current InGaAsN quantum well ridge waveguide (RWG) lasers have also been fabricated by pulsed anodic oxidization of an Al x Ga 1-x As upper cladding layer [39]. Conceptually, improved optical and electrical confinement could be provided by a deeply-etched ridge waveguide. We show that a devicequality thermal oxide can be grown on a deep-etched dilute nitride laser heterostructure at not only the Al 0.65 Ga 0.35 As cladding layer, but also at the Al-free GaAs waveguide and GaAsP/InGaAsN active region layers. We show that with the resulting high lateral optical confinement factor, " L (i.e., strong lateral overlap of the in-plane optical field and gain), enhanced laser performance with stable spatial-mode behavior is achieved. Prior to the fabrication of diode lasers, oxidation studies on an Al-free, dilute-nitride active region laser structure grown by MOCVD were conducted. The structure is a!~ 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%) quantum wells are alternately embedded in four 10 nm GaAs 0.67 P 0.15 barriers, all sandwiched in a 300 nm GaAs separate confinement heterostructure (SCH) formed with 1.1 µm Al 0.65 Ga 0.35 As cladding layers [36]. In order to expose the Alfree GaAs waveguide and GaAsP/InGaAsN MQW active region with a smooth and clean interface for non-selective oxidation studies, wet-etched stripes are used. Fig. 11 shows scanning electron microscope (SEM) images of a 7 µm wide stripe-masked ridge wet etched in a H 3 PO 4 :H 2 O 2 :H 2 O solution for 90 sec and then wet oxidized for 30 min at 450 C with the addition of 7000 ppm O 2 (relative to the N 2 carrier gas bubbled through 95 C H 2 O). The higher magnification SEM image in the bottom 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, this effect is believed to be very small. We note that the Al-free GaAs layer does not have this potential issue, nor do

11 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 O 2 significantly enhances the oxidation rates of an undoped Al 0.20 Ga 0.8 As 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 BCl 3 /Cl 2 /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 O 2 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 O 2 to convert part of the Al 0.65 Ga 0.35 As 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 ma and a high slope efficiency of 0.51 W/A, corresponding to a threshold current density of 650 A/cm 2 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 O 2. 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, I th =45.73 ma, J th =871 A/cm 2, and the differential slope efficiency at 2I th is R d =0.451 W/A. For (b) conventional device with L=520!m, I th =86.18 ma and J th = A/cm 2, 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 I th =169.6 ma and R d =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.

12 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 (I th =45.7 ma) and high slope efficiency (R d =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!I th ), 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 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 edgeemitting 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 and ECS REFERENCES [1] M. Smit, "Photonic integration," Telektronikk, vol , pp , [2] S. Suzuki, M. Yanagisawa, Y. Hibino, and K. Oda, "High-density integrated planar lightwave circuits using SiO 2 /GeO 2 waveguides with a high refractive index difference," IEEE Journal of Lightwave Technology, vol. 12, pp , [3] Y. Luo, "Properties of AlGaAs native oxides for integrated photonics and optoelectronics applications," Ph.D. Dissertation, Department of Electrical Engineering: University of Notre Dame, Indiana, [4] Y. Luo and D. C. Hall, "Nonselective Wet Oxidation of AlGaAs Heterostructure Waveguides Through Controlled Addition of Oxygen," IEEE Journal of Selected Topics in Quantum Electronics, vol. 11, pp , [5] D. Liang, J. Wang, and D. C. Hall, "High-efficiency native-oxide-passivated high-index-contrast ridge waveguide lasers," Electronics Letters, vol. 42, pp , 2006.

13 [6] D. Liang, D. C. Hall, J. Y.-T. Huang, J.-Y. Yeh, and L. J. Mawst, "High-Index-Contrast Oxide-Confined GaAsP/InGaAsN Multi-Quantum-Well Ridge Waveguide Lasers," presented at The 20th IEEE International Semiconductor Laser Conference, Big Island, Hawaii, USA, [7] D. Liang, J. Kulick, and D. C. Hall, "High-Efficiency Oxide-Confined Ridge Waveguide Laser with Nearly Symmetric Output Beam," presented at The 19th Annual Lasers and Electro Optics Society Meeting, Montreal, Canada, [8] D. Liang, D. C. Hall, and G. M. Peake, "Oxidation Smoothing of Sidewall Roughness in AlGaAs Heterostructure Waveguides," presented at The 18th Annual Lasers and Electro Optics Society Meeting, Sydney, Australia, [9] D. Liang, J. Wang, J. Y.-T. Huang, J.-Y. Yeh, L. J. Mawst, and D. C. Hall, "Deep-Etched Native-Oxide- Confined High-Index-Contrast AlGaAs Heterostructure Lasers With 1.3!m Dilute-Nitride Quantum Wells," Submitted to IEEE Journal of Selected Topics in Quantum Electronics, [10] H. F. Wong, D. L. Green, T. Y. Liu, D. G. Lishan, M. Bellis, E. L. Hu, P. M. Petroff, P. O. Holtz, and J. L. Merz, "Investigation of reactive ion etching induced damage in GaAs/AlGaAs quantum well structures," Journal of Vacuum Science & Technology B: Microelectronics and Nanometer Structures, vol. 6, pp , [11] H. C. Casey, Jr., M. B. Panish, and J. L. Merz, "Beam divergence of the emission from double-heterostructure injection lasers," Journal of Applied Physics, vol. 44, pp , [12] W. Dumke, "The angular beam divergence in double-heterojunction lasers with very thin active regions," IEEE Journal of Quantum Electronics, vol. 11, pp , [13] G. Ru, X. Yu, J. Cai, J. Yan, and F. S. Choa, "Robust slab-coupled buried-rib semiconductor laser with high fibre coupling efficiency," Electronics Letters, vol. 40, pp , [14] R. K. Huang, J. P. Donnelly, L. J. Missaggia, C. T. Harris, J. Plant, D. E. Mull, and W. D. Goodhue, "Highpower nearly diffraction-limited AlGaAs-InGaAs semiconductor slab-coupled optical waveguide laser," IEEE Photonics Technology Letters, vol. 15, pp , [15] A. Martin and H. Amos, "Lateral current spreading in ridge waveguide laser diodes," Applied Physics Letters, vol. 74, pp , [16] J. Piprek, J. K. White, and A. J. SpringThorpe, "What limits the maximum output power of long-wavelength AlGaInAs/InP laser diodes?," IEEE Journal of Quantum Electronics, vol. 38, pp , [17] W. T. Tsang, "The effects of lateral current spreading, carrier out-diffusion, and optical mode losses on the threshold current density of GaAs-Al[sub chi ]Ga[sub 1 - chi ]As stripe-geometry DH lasers," Journal of Applied Physics, vol. 49, pp , [18] S. Y. Hu, D. B. Young, A. C. Gossard, and L. A. Coldren, "The effect of lateral leakage current on the experimental gain/current-density curve in quantum-well ridge-waveguide lasers," IEEE Journal of Quantum Electronics, vol. 30, pp , [19] S. Y. Hu, S. W. Corzine, K.-K. Law, D. B. Young, A. C. Gossard, L. A. Coldren, and J. L. Merz, "Lateral carrier diffusion and surface recombination in InGaAs/AlGaAs quantum-well ridge-waveguide lasers," Journal of Applied Physics, vol. 76, pp , [20] G. J. Letal, J. G. Simmons, J. D. Evans, and G. P. Li, "Determination of active-region leakage currents in ridgewaveguide strained-layer quantum-well lasers by varying the ridge width," IEEE Journal of Quantum Electronics, vol. 34, pp , [21] D. Ban, E. H. Sargent, K. Hinzer, J. D.-W. St, A. J. SpringThorpe, and J. K. White, "Direct observation of lateral current spreading in ridge waveguide lasers using scanning voltage microscopy," Applied Physics Letters, vol. 82, pp , [22] R. K. Price, V. B. Verma, K. E. Tobin, K. C. Hsieh, V. C. Elarde, and J. J. Coleman, "Intrinsic parameter and modal characteristics of asymmetric cladding ridge waveguide lasers," presented at The 19th Annual Laser & Electro-Optics Society Meeting, Montreal, Canada, [23] G. Belenky, L. Shterengas, C. L. J. Reynolds, M. W. Focht, M. S. Hybertsen, and B. Witzigmann, "Direct measurement of lateral carrier leakage in 1.3-!m InGaAsP multiple-quantum-well capped mesa buried heterostructure lasers," IEEE Journal of Quantum Electronics, vol. 38, pp , [24] S. Furst, C. Farmer, L. Hobbs, R. D. L. Rue, and M. Sorel, "Native oxidation of aluminum-containing III-V compound layers for increased current and optical confinement in semiconductor lasers," presented at The 19th Annual Lasers and Electro Optics Society Meeting, Montreal, Canada, 2006.