OSA's Digital Library

Optics Express

Optics Express

  • Editor: C. Martijn de Sterke
  • Vol. 17, Iss. 5 — Mar. 2, 2009
  • pp: 3390–3395
« Show journal navigation

Propagation loss study of very compact GaAs/AlGaAs substrate removed waveguides

JaeHyuk Shin, Yu-Chia Chang, and Nadir Dagli  »View Author Affiliations


Optics Express, Vol. 17, Issue 5, pp. 3390-3395 (2009)
http://dx.doi.org/10.1364/OE.17.003390


View Full Text Article

Acrobat PDF (658 KB)





Browse Journals / Lookup Meetings

Browse by Journal and Year


   


Lookup Conference Papers

Close Browse Journals / Lookup Meetings

Article Tools

Share
Citations

Abstract

Very compact GaAs/AlGaAs optical waveguides with propagation loss as low as 0.9 dB/cm at λ=1.55 μm were demonstrated using substrate removal and evaporated Si for index loading. Loss components were identified and minimized through process and waveguide design. Process induced roughness contributed significantly to overall propagation loss. Therefore a low damage process in addition to proper waveguide design is needed for loss minimization.

© 2009 Optical Society of America

1. Introduction

2. Optical waveguide design

Figure 1(a) shows the cross sectional schematic of the optical waveguide used. It consists of a GaAs/AlGaAs epilayer removed from its growth substrate and glued onto a transfer substrate using the polymer Benzocyclobutane (BCB). Substrate removal provides a very high vertical index step due to high index of the semiconductor (n ≈ 3.4) and low index of BCB (n = 1.53) and air (n = 1). This high index contrast is very similar to that of a SOI waveguide and a submicron epilayer is enough for good optical confinement usually at or above 50% level. Lateral confinement is provided using either a 60 nm Al0.3Ga0.7As or Si layer. Figure 1(b) shows an example of an optical mode for a 1.0 μm wide waveguide with Si loading. The epilayer is grown using molecular beam epitaxy (MBE) and is unintentionally doped. It contains two GaAs quantum wells (QWs). If these QWs are doped, ohmic contacts can be made to them on the sides of the rib away from optical mode and work as buried electrodes that bring external voltages right to the core of the waveguide. This approach is used to fabricate very efficient modulators reported earlier [2–3

2. J. Shin, Y.-C. Chang, and N. Dagli, “Highly Efficient GaAs/AlGaAs Substrate Removed Nanowire Phase Modulators Based on Current Injection,” in Integrated Photonics and Nanophotonics Research and Applications (Optical Society of America, 2007), paper IMA2.

]. In such waveguides the main loss components are scattering and absorption. At λ = 1.55 μm absorption is mainly due to free carriers in the QWs if they are doped. In this study doping is not used and we focus on the identification and minimization of scattering loss components.

Fig. 1. (a) Cross sectional schematic of the substrate removed waveguide (not to scale) and (b) fundamental TE mode power profile of 1.0 μm wide waveguide with Si loading.

To estimate scattering loss due to surface roughness, we adapt the 2D analysis reported in [6

6. F. P. Payne and J. P. R. Lacey, “A Theoretical-Analysis of Scattering Loss from Planar Optical Wave-Guides,” Opt. Quantum Electron. 26, 977–986 (1994). [CrossRef]

] to 3D waveguides. This allows us to estimate the loss of an interface as

αi=Γi(nci2nsi2)2k038πnciSi
(1)

A factor of 1/2 was included in (1) to consider only one interface at a time. nci and nsi are the refractive indices of the core and clad material, k0 is the free space wave number. For example, nci and nsi of interface 1 shown in the inset of Fig. 3(a) are those of Si and BCB respectively. Γ = ∫∣U2 dli is the overlap of the normalized optical power with an interface where U is the normalized optical mode field distribution. The integration is taken along the length of the interface. Si is the integrated spectral density function detailed in [6

6. F. P. Payne and J. P. R. Lacey, “A Theoretical-Analysis of Scattering Loss from Planar Optical Wave-Guides,” Opt. Quantum Electron. 26, 977–986 (1994). [CrossRef]

]. Calculations indicate that for Lc less than 50 nm, Si ≃ 5.42σ 2 Lc where σ and Lc are the rms roughness and autocorrelation length of the interface. Then the loss coefficient for an interface is proportional to αi ∝ Γi σ 2 Lc. Hence we have two handles to minimize scattering loss. One is to improve the growth, lithography and etching processes to reduce σ and Lc. If we can eliminate etching, σ and Lc directly correlate with lithographic quality and as grown interface quality. Otherwise very smooth etching condition or some way of smoothing an etched surface should be found. Hence high quality lithography and growth are essential for loss reduction. In addition we have control over Γi by changing the waveguide design and minimizing the overlap of the optical mode with the interfaces. This is not easy to do in a high index contrast structure shown in Fig. 1(a). A fair amount of power approaching 11 % resides outside the semiconductor for the waveguides discussed in this paper. In addition, for TE modes the main optical electric field component is perpendicular to the rib sidewalls. Due to boundary condition εSCESC = εBCBEBCB at the Si/BCB sidewall EBCB ≈ 5ESC since εSC ≈ 5εBCB. Hence field strength at the Si/BCB interface on the BCB side is enhanced and a significant fraction of the optical power overlaps with the semiconductor side walls. This overlap can be reduced by increasing the optical confinement which requires increasing the rib width. The maximum rib width would be limited to single mode operation.

3. Device fabrication

In the initial studies we used patterning and etching of a 60 nm thick Al0.3Ga0.7As layer on top of Al0.5Ga0.5As layer for waveguide definition. The patterning was done using an i-line projection aligner. Figure 2(a) shows the top view of such a waveguide. In this case there is significant sidewall roughness varying between 20 nm and 25 nm. The roughness was due to difficulties both in lithography and etching. Standing wave formation within the photoresist in the i-line stepper created excessive line width variation and poor sidewall definition. Due to the shallow etch depth and etch uniformity requirements, dry etching good quality waveguides was difficult and selective wet etching in citric acid:H2O2 was used. This resulted in nonuniform etching of the Al0.3Ga0.7As index loading layer. For waveguides fabricated on SOI wafers, one could perform an oxidation to smoothen the side walls [4

4. D. K. Sparacin, S. J. Spector, and L. C. Kimerling, “Silicon waveguide sidewall smoothing by wet chemical oxidation,” J. Lightwave Technol. 23, 2455–2461 (2005). [CrossRef]

]. However, this is difficult for GaAs/AlGaAs waveguides. To eliminate these difficulties we used electron beam lithography to improve pattern definition and eliminated the etching altogether. Fabrication of the optical waveguides started with blanket etching the original 60 nm Al0.3Ga0.7As index loading layer in citric acid:H2O2 (6:1). Waveguides with width between 1.0 μm and 2.2 μm were written with a JEOL JBX-5DII(U) electron beam lithography system on a photoresist ZEP520. After e-beam evaporating 60 nm thick Si, the patterns were lifted off by immersing the sample in a photoresist stripper 1165. Figure 2(b) shows the top view of such a waveguide. In this case 60 nm lifted off Si layer provides the index loading rather than 60 nm etched Al0.3Ga0.7As layer. The improvement in the sidewall quality is obvious and rms sidewall roughness was reduced to 4 nm. Liftoff has an added benefit that dimensions can be accurately controlled when compared to wet etching. Cross sectional profiles showing the details of the sidewalls and the BCB/semiconductor interface are also shown in Fig. 2(c) and (d). As seen AlGaAs sidewall is sloped due to selective chemical etch used in its fabrication whereas Si sidewall is more vertical. The BCB/semiconductor interface is free of voids. Once the waveguides are defined a mesa etch was performed to isolate individual waveguides. The sample was then bonded upside down to a transfer substrate with BCB and the epitaxial growth substrate was removed.

Fig. 2. Top down SEM view of optical waveguide fabricated by (a) etching Al0.3Ga0.7As defined by stepper (σ = 20 nm) and (b) lifting off Si defined by electron beam lithography (σ = 4 nm). (c) and (d) shows the cross sectional SEM view for each case.

4. Experimental results and discussion

Measurements were made by coupling the output of a tunable laser into the waveguides with a lensed fiber. Output light from the waveguides were collimated with a microscope objective and projected to a photodetector. Single mode operation was confirmed by checking if the transmission as a function of wavelength fit well to the expected Airy function shape. For waveguides wider than 1.8 μm, multi-mode behavior was observed. For these waveguides measurements were made within the wavelength range where the transmission fitted well to the Airy function dependence [7

7. A. De Rossi, V. Ortiz, M. Calligaro, L. Lanco, S. Ducci, V. Berger, and I. Sagnes, “Measuring propagation loss in a multimode semiconductor waveguide,” J. Appl. Phys. 97, 073105 (2005). [CrossRef]

].

The propagation loss can be found by measuring the ratio of maximum to minimum transmission, KT = Tmax /Tmin of waveguides with different lengths and fitting them to the equation below.

αtot(dB)=αL10logR=10log(KT+1KT1)
(2)

R is the facet reflectivity and L is the length of the waveguide. After each cleave, the facets were inspected under an optical microscope. Only the waveguides with clean facets were measured.

The propagation loss of the waveguides fabricated using i-line projection aligner and Al0.3Ga0.7As etching was about 21 dB/cm. Excessive rms sidewall roughness observed in Fig. 2(a) was the main reason. Such high loss made this fabrication technique impractical. The open and crossed squares in Fig. 3(b) show the measured propagation loss as a function of waveguide width for the waveguides fabricated with e-beam lithography and Si liftoff. The two sets of measurements were from devices fabricated from the identical MBE wafer using the identical fabrication steps except for the time of immersion in 1165 for Si liftoff. Single mode behavior was observed for waveguides as wide as 1.78 μm. For the first sample shown in open squares an overnight immersion in 1165 was used. For this sample the propagation loss was 12 dB/cm at 1.0 μm and reduced to 4.3 dB/cm for 1.78 μm wide waveguides. For the second sample shown in crossed squares immersion in 1165 was only 30 minutes. For this sample loss dropped significantly. The propagation loss was on average 0.9±0.4 dB/cm over a wide waveguide width range making such waveguides very attractive for device applications. The measurement result also suggested that the sample surface may have undergone some roughening when immersed in 1165.

Fig.3. (a) Calculated optical power overlap with interfaces, and (b) calculated and measured propagation loss as a function of waveguide width. Open and crossed squares are measurements for long and short immersion time in the photoresist stripper 1165. Dashed and solid curves are calculations for each case with σ values in the legend.

Table 1. Measured Root Mean Square Roughness σ and Autocorrelation Length Lc of Surfaces shown in Fig. 3(a).

table-icon
View This Table

Figure 3(a) shows the fraction of optical power overlap with each interface Γi from optical modes found with commercial software. The overlap is highest for the bottom and top interfaces since the length along which the optical mode overlaps with these interfaces are much longer when compared with others. The top Si surface labeled 1 in Fig. 3(a) has the highest overlap since Si has the highest refractive index and pulls the optical mode towards itself. The overlap factor for interfaces 1 and 4 does not change much with waveguide width. The overlap factor with interfaces labeled 2 and 3 decreases as waveguide widens since the fundamental mode becomes more confined laterally and less energy resides on the sidewalls and outside the rib.

Figure 3(b) also shows the calculated propagation loss using the formulas provided earlier. According to these calculations the maximum loss contribution from the evaporated Si surface and bottom Al0.5Ga0.5As labeled 1 and 4 in the inset of Fig. 3(a) were less than 0.1 dB/cm. Calculations for the sample fabricated with shorter 1165 immersion shown as the solid curve agrees well with measurements. For the longer 1165 immersion sample, we find that we need to increase the rms roughness of interface 2 from 4 nm to 12 nm to accurately predict the measured loss coefficient. Loss prediction with σ 2 = 12 nm is in very good agreement with the data as seen in Fig. 3(b). The main reason for this artificial increase in rms roughness is the additional surface roughening near the edge of the Si layer during liftoff due to pitting on the top Al0.5Ga0.5As. While the edge may look fine under top inspection there could be a lot of voids and pits under the edge which contributes to scattering. Given these results, it is reasonable to conclude that significant portion of the loss originates from scattering due to the roughness introduced by pitting of Al0.5Ga0.5As in 1165. We could reduce the fabrication induced roughness by using a photoresist that requires less aggressive chemicals for liftoff, such as acetone, or modify the epilayer structure. AFM measurements showed that the surface roughness of the original epilayer which was capped with 5 nm thick GaAs to prevent oxidation of the Al0.3Ga0.7As layer underneath remains unchanged after an 8 hour bath 1165. Hence, utilizing an epilayer with 5nm – 10nm thick GaAs cap layer directly on top of the Al0.5Ga0.5As layer could prevent pitting. Doing so would reduce the surface roughness and associated scattering.

5. Conclusions

Acknowledgments

This work is supported by NSF grants ECS-0501355, ECS- 0702087 and a UCSB academic senate grant. JaeHyuk Shin thanks Dr. Jiyun Byun for help in analyzing the waveguide sidewall roughness from both SEM and AFM images.

References and Links

1.

M. A. Webster, R. M. Pafchek, G. Sukumaran, and T. L. Koch, “Low-loss quasi-planar ridge waveguides formed on thin silicon-on-insulator,” Appl. Phys. Lett. 87, 231108 (2005). [CrossRef]

2.

J. Shin, Y.-C. Chang, and N. Dagli, “Highly Efficient GaAs/AlGaAs Substrate Removed Nanowire Phase Modulators Based on Current Injection,” in Integrated Photonics and Nanophotonics Research and Applications (Optical Society of America, 2007), paper IMA2.

3.

J. Shin, Y. C. Chang, and N. Dagli, “0.3 V drive voltage GaAs/AlGaAs substrate removed Mach-Zehnder intensity modulators,” Appl. Phys. Lett. 92, 201103 (2008). [CrossRef]

4.

D. K. Sparacin, S. J. Spector, and L. C. Kimerling, “Silicon waveguide sidewall smoothing by wet chemical oxidation,” J. Lightwave Technol. 23, 2455–2461 (2005). [CrossRef]

5.

L. B. Soldano and E. C. M. Pennings, “Optical Multimode Interference Devices Based on Self-Imaging -Principles and Applications,” J. Lightwave Technol. 13, 615–627 (1995). [CrossRef]

6.

F. P. Payne and J. P. R. Lacey, “A Theoretical-Analysis of Scattering Loss from Planar Optical Wave-Guides,” Opt. Quantum Electron. 26, 977–986 (1994). [CrossRef]

7.

A. De Rossi, V. Ortiz, M. Calligaro, L. Lanco, S. Ducci, V. Berger, and I. Sagnes, “Measuring propagation loss in a multimode semiconductor waveguide,” J. Appl. Phys. 97, 073105 (2005). [CrossRef]

8.

J. Shin, Y.-C. Chang, and N. Dagli, “Low-Loss Ultra-Compact GaAs/AlGaAs Substrate Removed Waveguides,” in Integrated Photonics and Nanophotonics Research and Applications (Optical Society of America, 2008), paper IWE3.

OCIS Codes
(230.7370) Optical devices : Waveguides
(290.5880) Scattering : Scattering, rough surfaces

ToC Category:
Optical Devices

History
Original Manuscript: December 4, 2008
Revised Manuscript: February 12, 2009
Manuscript Accepted: February 17, 2009
Published: February 19, 2009

Citation
JaeHyuk Shin, Yu-Chia Chang, and Nadir Dagli, "Propagation loss study of very compact GaAs/AlGaAs substrate removed waveguides," Opt. Express 17, 3390-3395 (2009)
http://www.opticsinfobase.org/oe/abstract.cfm?URI=oe-17-5-3390


Sort:  Author  |  Year  |  Journal  |  Reset  

References

  1. M. A. Webster, R. M. Pafchek, G. Sukumaran, and T. L. Koch, "Low-loss quasi-planar ridge waveguides formed on thin silicon-on-insulator," Appl. Phys. Lett. 87, 231108 (2005). [CrossRef]
  2. J. Shin, Y.-C. Chang, and N. Dagli, "Highly Efficient GaAs/AlGaAs Substrate Removed Nanowire Phase Modulators Based on Current Injection," in Integrated Photonics and Nanophotonics Research and Applications (Optical Society of America, 2007), paper IMA2.
  3. J. Shin, Y. C. Chang, and N. Dagli, "0.3 V drive voltage GaAs/AlGaAs substrate removed Mach-Zehnder intensity modulators," Appl. Phys. Lett. 92, 201103 (2008). [CrossRef]
  4. D. K. Sparacin, S. J. Spector, and L. C. Kimerling, "Silicon waveguide sidewall smoothing by wet chemical oxidation," J. Lightwave Technol. 23, 2455-2461 (2005). [CrossRef]
  5. L. B. Soldano and E. C. M. Pennings, "Optical Multimode Interference Devices Based on Self-Imaging - Principles and Applications," J. Lightwave Technol. 13, 615-627 (1995). [CrossRef]
  6. F. P. Payne and J. P. R. Lacey, "A Theoretical-Analysis of Scattering Loss from Planar Optical Wave-Guides," Opt. Quantum Electron. 26, 977-986 (1994). [CrossRef]
  7. A. De Rossi, V. Ortiz, M. Calligaro, L. Lanco, S. Ducci, V. Berger, and I. Sagnes, "Measuring propagation loss in a multimode semiconductor waveguide," J. Appl. Phys. 97, 073105 (2005). [CrossRef]
  8. J. Shin, Y.-C. Chang, and N. Dagli, "Low-Loss Ultra-Compact GaAs/AlGaAs Substrate Removed Waveguides," in Integrated Photonics and Nanophotonics Research and Applications (Optical Society of America, 2008), paper IWE3.

Cited By

Alert me when this paper is cited

OSA is able to provide readers links to articles that cite this paper by participating in CrossRef's Cited-By Linking service. CrossRef includes content from more than 3000 publishers and societies. In addition to listing OSA journal articles that cite this paper, citing articles from other participating publishers will also be listed.

Figures

Fig. 1. Fig. 2. Fig.3.
 

« Previous Article  |  Next Article »

OSA is a member of CrossRef.

CrossCheck Deposited