Dual-purpose single-cavity oxide-confined VCSEL-photodetector C. Bringer, V. Bardinal, C. Fontaine, L. Averseng, T. Camps, P. Dubreuil, A. Muñoz-Yagüe LAAS-CNRS 7, Avenue du Colonel Roche F-31077 Toulouse Cedex, France Tel: (33/0)5-61-33-78-36 Fax: (33/0)5-61-33-62-08 e-mail: [email protected], [email protected], [email protected]

ABSTRACT The increasing interest for high-speed, compact and low cost devices for optoelectronic applications such as bidirectional optical interconnects, optical imaging or telemetry has recently led to focus on the ability for the verticalcavity surface-emitting lasers to be used as resonant cavity enhanced photodetectors for dual-purpose applications. Here we present results on design, fabrication and characterization of an oxide-confined 830nm top-emitting laser for selfaligned emission and photodetection. In this single-cavity GaAs-based device, submitted alternatively to forward and reverse bias, the oxide layer is not only used to obtain a single mode emission but also to enable decoupling between a small surface emission and a large surface detection. However the optical path is observed to change because of the refractive index difference between the oxidized and non-oxidized zones of the structure. This leads to a detrimental blue-shift on the wavelength of the Fabry-Pérot cavity mode. In this work, we demonstrate this effect in photodetection by the means of spatially localized photocurrent and reflectance spectra measurements. These results show that the photocurrent is correctly collected in the whole device despite of the presence of an oxide layer. The results obtained on selective etching for optimisation of this dual-purpose device are presented. Keywords : III-V semiconductors, microcavity devices, detection quantum efficiency, reflectivity, VCSEL, RCE detector, oxide modes, selective etching, optical interconnects, telemetry

1. INTRODUCTION Vertical semiconductor microcavities are very attractive nanoscale artificial structures due to their high internal optical confinement. The increasing need for high-speed, compact and low cost devices for optoelectronic applications such as bi-directional optical interconnects, has recently led to focus on the ability for the VCSELs (Vertical-Cavity Surface-Emitting Lasers) to be used as Resonant Cavity Enhanced (RCE) photodetectors for dual purpose operation1 . Indeed, despite a narrow spectral detection sensitivity due to the high finesse of their cavity, VCSELs diodes are able to efficiently detect light at their resonance wavelength only by changing their applied voltage polarity. Here we present results on design, fabrication and characterization of an oxide-confined top-emitting VCSEL for both light emission and photo-detection at 830nm in a single-cavity GaAs-based device. We show that oxide modes are present in photodetection, as already reported in emission operation, leading to a detrimental shift on the wavelength of the FabryPérot cavity mode2 . We present a chemical etching procedure that can be applied to the upper VCSEL Distributed Bragg Reflector (DBR) in order to lower this detrimental effect. Finally, we demonstrate its efficiency on a resonant cavity enhanced photodetector.

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2. DESIGN OF A DUAL PURPOSE VCSEL-RESONANT PHOTODETECTOR It is now well established that the best performance VCSELs are oxide-confined ones : indeed the use of a buried oxide aperture close to the cavity allows for the definition of a very low active volume, leading to ultra-low threshold current and single mode emission. In our case (Fig. 1), the buried oxide layer is not only used to obtain a high optical quality laser beam but also to enable decoupling between the surface emission, limited by the oxide aperture (diameter300) at the mandatory condition that its pH is strictly controlled between 8.35 and 8.4 and the ammoniac is weakly diluted7 . For AlAs, a diluted fluorhydric acid based solution (HF:H2 O) was used. Indeed, it is known to be extremely selective for AlGaAs compounds with an aluminium composition higher than 40% relative to Ga-rich compounds. This ensures in our case (selective etching of AlAs relative to Al0,2 Ga0,8 As) that a complete Bragg reflector period will systematically been etched. Our tests on (Al,Ga)As ternary compounds unfortunately showed that the diluted ammoniac based solution dramatically damages their surface. For this reason, we had to use very diluted solutions which were observed to exhibit much lower selectivity. Etch rates were measured on thick layers and on alternative thin layers for several dilution ratios and for several Al compositions. A visual control of surface colour changes along etching ensured that the ternary layer was completely removed. The quality of the final surface was controlled with an optical microscope and the etched depth was measured by profilometry. The etching procedure optimised for the Al0,2 Ga0,8 As (~60 nm) and AlAs (~70 nm) DBR thin layers consists of : a 40 s-etching in (NH4 OH:H2 O2 :H2 O) (1:1:300), a 2 min-rinse followed by a 15s-etching in (HF:H2 O) (1:20) and a final 2min- rinse.

6.2 Application to RCE-detectors The above procedure was applied to resonant asymmetric GaAs-massive cavity detectors. Eight periods for the upper DBR have been intentionally grown instead of five (ideal geometry, as indicated by modelling) in order to remove the periods in excess. Etching was carried out on samples just after their epitaxy but also on processed devices. Reflectivity measurements achieved on step-by-step etched samples (Fig. 6) showed that the selective etching provides the expected effect. The slight difference observed between the theoretical and experimental reflectivity values at resonance can be attributed to the GaAs absorption dispersion model used, which has to be further refined. Sensitivity measurements performed with the experimental set up described in Fig. 3 on processed samples showed that the etch of three periods actually leads to a sensitivity increase from 0.42 to 0.57A/W (cf. Fig 7). This implies a change in the quantum efficiency from 64 to 88%, which corresponds to a 24% improvement, and in the FWHM from 8nm to 11nm. Moreover the surface quality after several successive etchings has been found to be preserved.

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Fig. 7: Measured sensitivity spectra before (dotted lines) and after (solid lines) the etch of three periods of the RCE detector

7. CONCLUSIONS We have theoretically and experimentally studied the optical detection behaviour of oxide-confined VCSELs used as RCE detectors. Simultaneous measurements of spatially localised photocurrent and reflectivity spectra have demonstrated the presence of oxide modes in the lateral zones of the device in detection. The spectral split observed between oxidized and non-oxidized zones has been correlated to the reflectivity spectrum. It accounts for an optical cavity length change. A perfect spectral overlapping between emission and detection on the whole active surface can only be achieved by locally degrading the finesse of the cavity, by etching periods of the top Bragg reflector in the oxidized zones.

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For this purpose, we have developed a multilayer selective etching procedure which can be used to optimise the sensitivity performance of the microcavity detectors by decreasing the upper DBR reflectivity, and thereby the cavity finesse. This technique has been successfully applied to RCE asymmetric microcavity detectors. It will be soon applied on dual-purpose oxide-confined VCSELs. This would allow for an optimised emission/detection operation of these devices.

ACKNOWLEDGMENTS The authors wish to thank Michel Fraces and Bruno Duchenne from ONERA, Département d’Optique Théorique et Appliquée (DOTA) in Toulouse, France for financial support and technical assistance.

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