APPLIED PHYSICS LETTERS 93, 261111 共2008兲

Low-loss nonselectively oxidized AlxGa1−xAs heterostructure waveguides Y. Lou and D. C. Halla兲 Department of Electrical Engineering, University of Notre Dame, Notre Dame, Indiana 46556, USA

共Received 4 November 2008; accepted 5 December 2008; published online 29 December 2008兲 The use of nonselective AlGaAs oxidation 共i.e., via the use of controlled, dilute O2 addition during wet thermal oxidation兲 enables a significant propagation loss reduction in fully oxidized Al0.3Ga0.7As/ Al0.85Ga0.15As planar oxide single heterostructure waveguides. Prism coupling measurements are utilized to characterize the oxidized waveguide cladding and core layer thicknesses, refractive indices, and propagation loss. At a wavelength of 633 nm, above the Al0.3Ga0.7As core’s bandgap energy, the strongly absorbing semiconductor heterostructure waveguide is converted by nonselective oxidation to a transparent oxide heterostructure waveguide with a propagation loss of only 5.0 dB/cm 共1.15 cm−1兲. A low loss of 3.6 dB/cm 共0.83 cm−1兲 is obtained at 1.3 ␮m. © 2008 American Institute of Physics. 关DOI: 10.1063/1.3058709兴 For the integration of photonic circuits, nonabsorbing passive waveguide sections are required for signal routing and the interconnection of active devices 共lasers, amplifiers, modulators, and detectors兲. Presently, a semiconductor waveguide absorption edge may be blueshifted for passive regions by quantum well intermixing 共QWI兲 共typically ⬃50–100 meV兲1–3 or selective-area epitaxial regrowth.4 We have previously demonstrated that with the large refractive index variation in wet-thermally oxidized AlxGa1−xAs with Al composition 共x兲,5 fully oxidized AlGaAs heterostructures can themselves support waveguiding.6 As the insulating native oxides of AlGaAs7,8 have a substantially larger bandgap than that of any semiconductor alloy, these oxide heterostructure waveguides could allow integration of passive waveguides with very broad transmission bands, potentially useful, for example, for waveguide sensors based on spectral analysis. Another application of interest for oxide waveguides is a GaAs-based Er-doped waveguide amplifier in which pump and/or signal lasers could be monolithically integrated with an Er-doped native oxide gain medium. We have shown in studies reported elsewhere that AlGaAs native oxides are a viable host for optically active Er3+, supporting strong room temperature photoluminescence.9–11 In our first demonstration of fully oxidized single-heterostructure 共1.4 ␮m Al0.4Ga0.6As/ 1.0 ␮m Al0.8Ga0.2As兲 planar waveguides, an optical loss of ␣ = 2.5 cm−1 共10.9 dB/cm兲 was obtained at ␭0 = 1.55 ␮m, while absorption of the external field by the substrate prevented characterization above the GaAs bandgap energy.6 Conversion of a semiconductor heterostructure to an oxide heterostructure may also provide a means for realizing a mode expander.12 It is important to further reduce the propagation loss if transparent oxide heterostructure waveguides are to be practical. In this work, we describe significant loss reductions in oxide heterostructure waveguides, with realization of transparency well above the GaAs band edge, achieved through both heterostructure modifications and process improvements. In particular, the use of an O2-enhanced, nonselective wet oxidation process13 leads to both a higher index contrast for improved optical guiding and a significant decrease in surface roughness for reduced scattering loss. a兲

Electronic mail: [email protected].

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The AlxGa1−xAs heterostructures employed in this work are grown by metal organic chemical vapor deposition 共MOCVD兲 at a growth temperature of 750 ° C. All layers are unintentionally doped. Above a 0.25 ␮m GaAs buffer layer, an AlxGa1−xAs layer of continuously graded alloy composition is inserted from x = 0.05 to x = 0.85 over a distance of 0.09 ␮m in order to minimize potential oxide adhesion issues, followed by a 1.5 ␮m Al0.85Ga0.15As lower waveguide cladding layer. Above this cladding layer is an Al0.3Ga0.7As guiding layer with thickness of either 1 or 1.5 ␮m and a 50 nm GaAs cap layer. Because the addition of O2 during wet oxidation significantly enhances the oxidation rates of low Al-content AlGaAs, the AlGaAs waveguide core layer can be oxidized using reduced process times and temperatures, yielding a higher quality oxide with lower surface roughness. In addition, the use here of the larger core/cladding Al ratio of Al0.3Ga0.7As/ Al0.85Ga0.15As compared with the Al0.4Ga0.6As/ Al0.8Ga0.2As heterostructure of Ref. 6 provides a higher refractive index contrast for an oxide waveguide. In particular, the presence of dilute O2 during wet oxidation significantly increases the refractive index of Al0.3Ga0.7As from n ⬃ 1.48 共no O2 added兲 to n ⬃ 1.68 共4000–7000 ppm O2兲.5,13 After cap layer removal, both the Al0.3Ga0.7As and Al0.85Ga0.15As layers are fully oxidized from the surface, resulting in an oxide single heterostructure planar waveguide. Details of the nonselective O2-enhanced wet oxidation process used in this work can be found in Ref. 13, along with data showing that nonselective oxidation increases the refractive index of the core layer Al0.3Ga0.7As oxide while having negligible effect on the cladding layer Al0.85Ga0.15As oxide. In this work, the heterostructure waveguide layer thicknesses, refractive indices, and propagation loss are characterized using a Metricon 2010 prism coupling system and technique described in detail elsewhere.5,6 Figure 1 shows a transverse electric 共TE兲 prism coupling mode profile measured at 633 nm for the 1 ␮m guiding layer structure after oxidizing for 88 min at 450 ° C with the addition of 7000 ppm O2 共relative to the N2 carrier gas兲. The waveguide structure schematic inset in Fig. 1 gives the refractive indices and thicknesses as determined by the mode profile for both guiding layer 共n1 = 1.645, T1 = 0.712 ␮m兲 and cladding layer 共n2

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© 2008 American Institute of Physics

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Y. Lou and D. C. Hall

FIG. 1. Prism coupler TE mode profile 共at ␭0 = 633 nm兲 for a 1 ␮m Al0.3Ga0.7As/ 1.5 ␮m Al0.85Ga0.15As waveguide oxidized at 450 ° C for 88 min with 7000 ppm O2 / N2 in the carrier gas. Two guided modes are marked with arrows. The inset gives the oxide guiding and cladding layer indices and thicknesses as determined from the mode positions. The dashed vertical line marks the core index at which waveguiding transitions from total internal reflection in the core to guiding by substrate modes.

= 1.506, T2 = 1.52 ␮m兲, for an index contrast of ⌬n = 0.139. For the 1.5 ␮m guiding layer structure oxidized for 164 min at 450 ° C with 7000 ppm O2 added, the mode profile 共not shown兲 has three guided modes and gives n1 = 1.650, T1 = 1.27 ␮m guiding layer and n2 = 1.510, T2 = 1.48 ␮m cladding layer parameters, for an index contrast of ⌬n = 0.140. For comparison, the 1.4 ␮m Al0.4Ga0.6As waveguide core layer in Ref. 6, oxidized using the conventional wet thermal process, required 262 min at 493 ° C. For both structures in this work, the guided modes are much sharper and deeper than those observed in Ref. 6, a characteristic of reduced propagation loss. The higher resulting refractive index contrast between oxides of different Al content AlGaAs, and the lower oxide surface roughness for the lower Al content guiding layer material,13 both results of the addition of O2 during wet thermal oxidation, together enable the substantial reduction in the propagation loss. With the higher index contrast between the core and the cladding layer, the electric field of the guided mode is better confined. This, in turn, reduces interface scattering and radiation loss, and absorption by the GaAs substrate for photon energies above the bandgap. For waveguide loss measurements, a fiber-bundle probe is scanned along the guided beam away from the coupling prism to collect the scattered light intensity, directly proportional to the power propagating in the guided mode. Figure 2 shows the propagation losses of the lowest order TE mode for several oxide heterostructures at the wavelengths of ␭0 = 633 nm, 830 nm, 1.3 ␮m, and 1.55 ␮m, obtained with a ⫾20% accuracy from fitting the measured scattered power versus fiber position curve with a decaying exponential function in the manner shown in Ref. 6. The data point of Fig. 2共a兲 indicates the loss of 2.5 cm−1 共10.7 dB/cm兲 at 1.55 ␮m for the Al0.4Ga0.6As/ Al0.8Ga0.2As heterostructure of Ref. 6 with an index contrast of ⌬n = 0.064 after conventional 共more Al selective兲 wet thermal oxidization with ultrahigh purity 共i.e., O2-free兲 N2 carrier gas for 262 min at 493 ° C. For this waveguide, a loss of 3.0 cm−1 共12.9 dB/cm兲 was measured at ␭0 = 1.3 ␮m 共not shown兲, but the losses at 633 and 830 nm could not be characterized due to attenuations above the detection limit of 7 cm−1 共30 dB/cm兲. In Fig. 2共b兲, we show a 34% reduction in the loss at ␭0 = 1.55 ␮m to 7.1 dB/cm for the same Al0.4Ga0.6As/ Al0.8Ga0.2As heterostructure, now oxidized with the addition of 6000 ppm O2 for 100 min at

Appl. Phys. Lett. 93, 261111 共2008兲

FIG. 2. Propagation losses for lowest order TE modes of oxide waveguides prepared by wet oxidizing: 共a兲 a 1.4 ␮m Al0.4Ga0.6As/ 1.0 ␮m Al0.8Ga0.2As single heterostructure 共SH兲 in ultra high purity N2 at 493 ° C for 262 min 共10.7 dB/cm at ␭0 = 1.55 ␮m, Ref. 6兲 and 共b兲 in 6000 ppm O2 in N2 共at 475 ° C for 100 min, 7.1 dB/cm at ␭0 = 1.55 ␮m兲; and 共c兲 a 1.0 ␮m Al0.3Ga0.7As/ 1.5 ␮m Al0.8Ga0.2As SH 共88 min兲 or 共d兲 1.5 ␮m Al0.3Ga0.7As/ 1.5 ␮m Al0.8Ga0.2As SH 共164 min兲, both oxidized at 450 ° C with 7000 ppm O2 in the N2 carrier gas. The loss measurement error is ⬃⫾20%.

475 ° C, giving n1 = 1.646, T1 = 1.20 ␮m guiding layer and n2 = 1.558, T2 = 1.11 ␮m cladding layer parameters, for an increased index contrast of ⌬n = 0.088. Figures 2共c兲 and 2共d兲 shows the propagation loss in the lowest order guided mode at all four wavelengths for the Al0.3Ga0.7As/ Al0.85Ga0.15As waveguides with 共c兲 1 ␮m and 共d兲 1.5 ␮m guiding layers, both oxidized with the addition of 7000 ppm O2 at 450 ° C, for 88 min and 164 min, respectively 共as described above兲. For 共c兲, the losses 共in dB/cm兲 versus wavelength are 8.5 共␭0 = 633 nm兲, 6.2 共830 nm兲, 5.2 共1.3 ␮m兲 and 4.0 共1.55 ␮m兲. For 共d兲, the losses are 5.0 共␭0 = 633 nm兲, 4.3 共830 nm兲, 3.6 共1.3 ␮m兲 and 4.0 共1.55 ␮m兲. With the waveguide improvements made possible with O2 addition during oxidation, specifically the reduced scattering loss afforded by reduced oxide surface roughness, and enhanced index contrast for improved mode confinement to reduce the evanescent 共external兲 field strength and thus above-gap absorption by the GaAs substrate, we are able to reduce the loss at 830 and 633 nm to measurable levels. In effect, we can see that the absorbing semiconductor waveguide has been converted to a wide bandgap 共Eg ⬃ 4 – 5 eV兲 oxide waveguide that is relatively transparent for photons of wavelengths well beyond the absorption edge of both GaAs or Al0.3Ga0.7As. These values compare favorably to the 8.5 dB/cm losses reported in passive bandgap-tuned AlGaAs/GaAs waveguides fabricated via quantum-well intermixing at the lasing wavelength of 860 nm of the as-grown material, climbing rapidly to ⬎20 dB/ cm below 850 nm due to the limited absorption shift achievable via the QWI technique.2 Although a higherloss 3 ␮m wide dry-etched single-mode ridge structure is employed, most of the loss in Ref. 2 is attributed to free carrier absorption from the cladding layers and residual corematerial absorption. The losses of Fig. 2 may be further reduced by use of an upper cladding layer, omitted here to make possible prism coupling loss measurements. We have previously demonstrated waveguiding in single-mode double-heterostructure oxide waveguides,6 though loss measurements in these structures are more complicated. The propagation loss at 1.55 ␮m for both 共c兲 and 共d兲 is about 4 dB/ cm= 0.92 cm−1. This additional 44% reduction

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Appl. Phys. Lett. 93, 261111 共2008兲

Y. Lou and D. C. Hall

in propagation loss compared with 共b兲 can be attributed primarily to the higher index contrast of the higher Al ratio heterostructure. An overall loss reduction of 63% relative to 共a兲 is achieved. The loss of an oxide waveguide includes surface scattering loss, bulk volume scattering loss, radiation loss, and absorption loss by substrate for wavelengths ␭ ⬍ ␭g共GaAs兲. Radiation loss can be caused by microscopic variations in curvature along the propagation path, and may occur if there are nonuniformities in the oxide thickness shrinkage versus position. Like the attenuations at 1.3 and 1.55 ␮m for the first planar oxide waveguide, the attenuation versus wavelength cannot be described well by Rayleigh’s 1 / ␭40 law for volume scattering or by 1 / ␭20 theories for interface scattering.14 It is impossible here to separate the surface scattering loss, bulk volume scattering loss, radiation loss, and absorption loss by the substrate for shorter wavelengths without other characterization techniques. The larger loss at shorter wavelengths for 共c兲 the 1 ␮m core versus 共d兲 the 1.5 ␮m core can be attributed in general to reduced mode confinement by the smaller core. This leads to a higher field strength both at the core/cladding interfaces 共for increased scattering兲 and in the substrate 关for increased absorption loss when ␭ ⬍ ␭g共GaAs兲 ⬃ 0.87 ␮m兴. Some increased absorption at 1.55 ␮m due to the presence of OH groups, verified by Fourier transform infrared spectroscopy, is probable, as shown previously.6 Waveguide loss measurements repeated after exposure for 2 weeks to 100% relative humidity indicate an increased loss of only 10% 共data not shown兲, from which we conclude that the hygroscopic behavior observed for AlGaAs native oxides5 is not a major issue for potential oxide waveguide applications. In conclusion, the resulting higher index contrast and reduced scattering achieved through nonselective AlGaAs oxidation 共i.e., by O2 addition during wet thermal oxidation兲 enables a significant reduction in propagation loss in fully

oxidized AlGaAs heterostructure planar waveguides, with losses as low as 3.6 dB/cm at 1.3 ␮m and 5.0 dB/cm above the GaAs bandgap at 633 nm, achieved. The authors are grateful to A. Allerman and O. Blum at Sandia National Laboratory for providing the heterostructures used in this work. This work was supported in part by National Science Foundation Grant Nos. ECS-9502705, ECS-0123501, and ECCS-0824187. 1

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