s CWDM wafer-fused VCSELs grown by MOVPE

Journal of Crystal Growth 414 (2015) 210–214

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Journal of Crystal Growth journal homepage: www.elsevier.com/locate/jcrysgro

Fabrication and performance of 1.3-μm 10-Gb/s CWDM wafer-fused VCSELs grown by MOVPE A. Mereuta a,n, A. Sirbu b, A. Caliman a, G. Suruceanu a, V. Iakovlev a,b, Z. Mickovic b, E. Kapon a,b a b

BeamExpress SA, Lausanne, CH-1015, Switzerland Ecole Polytechnique Fédérale de Lausanne (EPFL), CH-1015, Switzerland

art ic l e i nf o

a b s t r a c t

Available online 13 November 2014

Recent progress in the fabrication and performance of 1.3-μm 10-Gb/s, low power consumption waferfused VCSELs grown by MOVPE under nitrogen atmosphere are presented and discussed. By optimization of the growth conditions and implementation of a cavity adjustment technique, the wavelength yield was increased to up to 70% for all four coarse wavelength division multiplexing (CWDM) channels implemented. It was demonstrated that single-mode devices can have threshold and operation currents below 1 and 7 mA, respectively, in the full 0–80 1C temperature range. The reproducibility of the fabrication process and prospects for enhancing performance and yield of such VCSELs are also discussed. & 2014 Elsevier B.V. All rights reserved.

Keywords: MOVPE Tunnel junction Vertical cavity surface emitting laser (VCSEL)

1. Introduction Long wavelength vertical cavity surface emitting lasers (VCSELs) operating in the 1.3-μm waveband are an attractive alternative to conventional edge emitting lasers for high speed and low cost optical interconnects. State of the art, long wavelength VCSELs with 4 1 mW single mode output power typically have GaAs, InP, and/or dielectric distributed Bragg reflectors (DBRs) for high reflectivity combined with InP/InAlGaAs quantum well (QW) active regions for high optical gain at elevated temperatures [1–4]. Compared with standard distributed feedback (DFB) edge-emitting lasers, VCSELs offer advantages of symmetric beam pattern as well as low operating currents and power consumption. However, still much more work has to be done in attaining reasonable yields at the very narrow coarse wavelength division multiplexing (CWDM) wavelength grids for applications in high bandwidth optical transmitters. This is because the VCSEL emission wavelength is set by the thickness of the optical cavity that includes the InP-based active region and part of top and bottom DBRs. Consequently, the VCSEL emission wavelength is dependent on epitaxial growth thickness tolerances of the active and DBR stacks as well as on thickness non-uniformities across the wafer.

n

Corresponding author. Tel.: þ 41 787275312; fax: þ 41 21 69 35480. E-mail address: alexandru.mereuta@epfl.ch (A. Mereuta).

http://dx.doi.org/10.1016/j.jcrysgro.2014.11.012 0022-0248/& 2014 Elsevier B.V. All rights reserved.

For particularly useful CWDM wavelengths at the 1271/1291/ 1311/1331-nm grid [3,5], typical values of 1% tolerance in growth and 1% thickness non-uniformity may reduce the yield in spectral matching to below 50%. In case of 1% non-uniformity of the DBR stop band center across the wafer, the emission wavelength nonuniformity of the fabricated devices is considerably larger than the accepted CWDM wavelength variation slot of 6.5-nm at room temperature. To solve these problems we optimized the growth process for better on-wafer thickness and composition uniformities with reproducible parameters from run to run. Also, we applied the cavity adjustment technique before the fusion to reach wavelength setting yield for a VCSEL wafer of up to 80% [6]. In addition, as the VCSEL structure contains a highly doped tunnel junction (TJ) structure, a further development was done in order to grow low absorption TJ layers, especially for 1271 and 1291-nm wavelengths, for achieving lower threshold and operation currents.

2. Experimental The InAlGaAs/InP QW active structure was grown by low pressure metallorganic vapor phase epitaxy (MOVPE) using nitrogen as carrier gas [7]. Growth was carried out on (1 0 0) exactly oriented InP substrates using trimethylindium, trimethylgallium, trimethylaluminum, arsine and phosphine as main precursors, and disilane and CBr4 as dopant precursors. In order to obtain a better thickness and composition uniformity in AIXTRON 200/4 reactor

A. Mereuta et al. / Journal of Crystal Growth 414 (2015) 210–214

the geometry of quartz liner deflectors was adapted for the epitaxial growth under nitrogen. By further optimization of total nitrogen flow in the reactor from 7 to 5.1-slpm the layer thickness uniformity was improved from  1 to less than 0.5% of standard deviation on 2" wafer with the difference in average thickness among three wafers in the same run of less than 0.1–0.2%. To ensure reproducible thickness and layer parameters control with low “memory effect” from run to run on 3  2" wafers, the quartz liner was cleaned before each growth of InP-based active cavities or GaAs-based DBRs. The growth of the active VCSEL cavities was performed in two steps. In the first step, the n-InP current spreading layer, InAlGaAs QWs, p-n and tunnel junction layers were grown. The 5-straincompensated InAlGaAs QWs (with typically  1–1.3% strain) are placed at the maximum of the electromagnetic field, while the tunnel junction layers are placed in a node. The growth of the InAlGaAs QW structure with the InP spacer, InAlGaAs QWs and the InP-based regrowth process were done at 80 mBars reactor pressure and 650 1C substrate temperature. The tunnel junction layers consist of highly doped p þ/n þ InAlGaAs layers with approximately 20% aluminum content in order to further minimize the absorption. The n-type InAlGaAs is doped with silicon and was grown at the same temperature as all n-type InP-based layers. The p-type InAlGaAs was doped with carbon; the growth was made at lower temperatures and V/III ratios, for optimal carbon incorporation [8–10]. With growth temperature as low as 510 1C, a hole concentration value of  5n1018 cm  3 was measured in the InGaAs layers [11]. By reducing the V/III ration from  25 to  10, a hole concentration of about 1.2n1019 cm  3 was reached (Fig. 1) while maintaining resonable quality of the surface morphology. This could be explained by published results [12] suggesting a higher As surface coverage as a consequence of the better AsH3 cracking in the nitrogen gas phase due to the heavier N2 molecules compared to H2. The high structural quality of the epi-layers was attested to by x-ray diffraction (XRD) and photoluminescence (PL) measurements. The C-V and secondary ion mass spectroscopy (SIMS) profiles of a test InGaAs tunnel junction structure with 100-nm silicon doped and 90-nm carbon doped layers are presented on Figs. 1 and 2, respectively. Good agreement between Polaron and SIMS measurements is achieved, thus demonstrating that all incorporated carbon in the InGaAs layer is activated. The interface betweeen p þ and n þ layers is very well defined in the C-V profile, but not so well in the SIMS one, probably due to the nonuniform etching during SIMS profiling. One can notice also on the SIMS profile a high concentration of Si in the p þ-InGaAs layer. The current – voltage (I-V) measurements of the processed TJ structure based on such layers showed a reverse biased resistivity of 10–5 Ωn cm  2. By adding a small content of aluminum in the InGaAs alloy, it is rendered transparent at the 1.5 and 1.3-μm telecom wavelengths. In addition, Al incorporation is favorable for enhancing the carbon doping. But a trade-off needs to be found between growth temperature, layer morphology, aliminum composition and carbon concentration. At relatively high aluminum contents, the InAlGaAs carbon doping, growth rate and layer morphology strongly depend on growth temperature. Therefore, InAlAs was mainly grown at 550–570 1C, while InAlGaAs (xAl ¼ 0.1–0.2) was grown at 520– 540 1C. Growth under nitrogen allows to use very low V/III ratios (10–30) while preserving high crystalline quality. Thus, in case of InAlAs grown at 560 1C and V/III ¼30, the maximum measured value of hole concentration was  8  1018 cm  3. The highest carrier concentration of  2.5  1019 cm  3 for InAlGaAs alloy grown with arsine was obtained at 540 1C and V/III ¼10 with very smooth surface. The carbon activation in the VCSEL active structures is done at  650 1C during the n þ -InAlGaAs and n-InP layers growth and regrowth.

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Fig. 1. C-V profile of the p þ/nþ InGaAs tunnel junction structure.

Fig. 2. SIMS profile of the pþ /nþ InAlGaAs tunnel junction structure.

In the second step, an InP/InGaAsP layer regrowth over the patterned and etched TJ mesas  6–7 μm wide was carried out. The accuracy of fixing the emission wavelength in the wafer fused VCSELs is set mainly by the thickness control of both the first n-InP/InAlGaAs half-cavity and the overgrown InP/InGaAsP layers on top of the TJ mesas. Therefore, during the growth of the InPbased layers, a constant TMIn source concentration in the nitrogen flow was maintained by an EPISON controller.

3. VCSEL fabrication and results The VCSEL structures were assembled by wafer fusion by using one InP-based active cavity and 2 GaAs-based top and bottom DBRs for each CWDM wavelength [13,14]. After the top-DBR substrate removal, the VCSEL cavity is mapped by PL spectroscopy in order to characterize the emission wavelength. In the active structure design, 2 InGaAsP/InP thin layers (  10–15-nm thick) were added at the optical field nodes on both sides of active cavity for better fusion quality. The cavity adjustment is done by chemical etching either of part of the GaAs or the InGaAsP layers before the fusion. The fabrication procedure employed for improving the device wavelength yield is based on the flexibility of the wafer fusion approach. The procedure includes in particular (i) selecting the DBR components to fit the active region emission properties determined during the wafer assembly process; and (ii) the use of wet chemical etching with 1-nm accuracy based

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on oxidation/etching cycles of the cavity layer. Applying different variations of this technique we reach wavelength setting yield of devices from a VCSEL wafer of up to 90% [5]. In Fig.3 the cavity mapping, peak wavelength profile and a cavity spectrum of a 1310-nm VCSEL 2"-wafer are presented. The structure has two regions with different emission wavelengths – one at  1304-nm, and the other at  1298-nm, were a cavity adjustment was performed before the fusion. An offset of approximately 7-nm can be seen from the peak wavelength profile across the 2" wafer. This offset is due to the etching  6–7-nm of InGaAsP from the InP-based cavity before the fusion in order to increase the chances to obtain devices at target wavelength. By applying the cavity adjustment technique, the probability to obtain the devices at the targeted CWDM wavelength increases, even on the first fused test batch of the wafers. Here it is important to mention that the mapped PL spectrum does not correspond to the emission spectrum of the electrically pumped VCSELs subsequently processed from the same wafer. In fact, the device emission wavelength is 5–10-nm longer, depending on the VCSEL design and pumping current, caused by active region heating. The correction action taken after the first fusion considerably increases the wavelength yield, as we can adjust it by etching either on full wafer or on specific parts of it. In Fig. 4, as an example, is presented the cavity mapping of a 1271-nm VCSEL fused wafer. In this batch the cavity was adjusted by partial removal of a InGaAsP layer by etching of a ring in the wafer center. The average cavity wavelength is 1263-nm with relative standard deviation (Std Dev) of 0.1%, which corresponds to Std Dev in cavity length of  1.5 nm. The room temperature VCSEL device emission wavelength at room temperature (RT), for this design at working currents of

 6–10 mA, is higher by 7–9-nm as compared to the PL peak of the cavity. For this wafer, the wavelength yield (at 1271 þ/  5 nm) was expected to be  70%, as confirmed by the room temperature emission spectra of the processed VCSEL devices (Fig. 4c). The processing of the double-fused wafer into VCSEL devices is performed using standard VCSEL processing steps: reactive ion etching of the top DBR, selective chemical etching of the InAlGaAs/InP active region, dielectric film deposition, dry etching and e-beam deposition of metals for contacts and electroplating for bond-pads. Examples of the VCSEL chip optical image and device schematic cross-section are presented in Fig. 5. The light is emitted from the top side of the wafer and the P- and N-contact pads are placed on the same surface. The current flows only through the InP layers, QWs and tunnel junction, which provides both optical and electrical confinement. Fig. 6 shows the typical emission spectra at all four wavelengths and 10-Gbps modulation response of 4 CWDM VCSELs fabricated with the wafer fusion technology. Vertical lines indicate the borders of the 1271/1291/1311/1331-nm CWDM channels windows. For all VCSELs, single mode optical emission is observed with side-mode suppression ratio (SMSR) of 440 dB in the 5–11-mA range of pumping current. For these devices, four sets of active cavities, top and bottom DBR 2"-wafers designed for corresponding wavelengths have been grown. By optimization of growth and employing the cavity adjustment technique the device wavelength yield was improved significantly on wafer. As part of further optimization work, the VCSEL performance for one particular CWDM wavelength was further improved by design optimization of the active region and the device layout. In particular, the strain in the InAlGaAs QWs was increased to 1.3%

Fig. 3. An example of a 1310-nm wavelength VCSEL 2"-wafer cavity PL mapping with 2 distinct emission wavelength regions (a), wavelength peak profile across the wafer (b) and the PL spectrum in the center of the un-etched region (c).

Fig. 4. PL mapping of a 1270-nm wavelength VCSEL 2"-wafer (a), cavity wavelength histogram on the mapped VCSEL wafer (b), and the 2" wafer RT distribution of VCSEL emission spectra at 7-mA of drive current (c).

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Fig. 5. VCSEL chip optical micrograph (a) and schematic cross section of a typical wafer fused VCSEL (b).

Fig. 6. 20 1C Single mode emission spectra at different currents and 10.3125-Gb/s eye diagrams (back-to back transmission) of the 4 CWDM VCSELs. Vertical lines indicate the borders of the CWDM channels windows.

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Frequency, GHz Fig. 7. LIV characteristics in the 0–80 1C temperature range of 1.3-μm VCSELs with 1.3% of strain in QWs (a), s21 characteristics at 20 and 70 1C for different diode currents (b) and eye diagrams representing 10 Gb/s modulation response at 20 and 80 1C at I bias ¼ 5 mA, ER ¼ 6.4 dB (c).

for reaching higher optical gain. As a result, improved 10-Gb/s performance up to high temperatures and with reduced power consumption was achieved despite non-optimized values of the parasitics. In Fig. 7a, we present the light-current-voltage (LIV) characteristics in the 0–80 1C temperature range of a 1.3 mm VCSEL thus obtained. The threshold current and the operation current for 1mW emission power in the full temperature range are below

1 and 5 mA, respectively. The s21 characteristics at 20 and 70 1C for different diode currents (Fig. 7b) show that the 3-dB cut-off is about 6 GHz for currents above 5 mA, denoting a very good potential for high frequency operation. The eye diagrams at 10 Gb/s modulation response are clearly open at both 20 and 80 1C (Fig. 7c). Optical data transmissions (at Ibias ¼5 mA) through 10- km of single mode fiber (SMF) link with bit error rate (BER)

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better than 10  12 have been demonstrated at 20 1C and 80 1C with energy-to-data distance ratio (EDDR) value of less than 80 fJ/ (bitnkm). 4. Conclusion In conclusion, we present the result of the fabrication and performance characterization of low power consumption, 1.3-μm 4  10-Gbps CWDM wafer-fused VCSELs based on MOVPE grown epitaxial structures. The developed devices are single mode and have very low threshold and working currents below 1 and 7-mA, respectively, in the full 0–80 1C temperature range. We demonstrate that by proper optimization of growth and using the flexibility of the wafer fusion approach it is possible to achieve high yields of VCSEL devices compatible with requirements of applications in CWDM transmitters. In particular, cavity adjustment procedures were developed to allow 70% yield in VCSEL devices fitting the specifications of CWDM channels in the 1310-nm waveband. This progress suggests that this technology has significant potential as reproducible manufacturing platform for long wavelength VCSELs for optical fiber data communications. Acknowledgement This work was partially supported by CTI Project 12874.1 PFNM-NM References [1] E. Kapon, A. Sirbu, Long-wavelength VCSELs: power-efficient answer, Nat. Photon. 3, 2009, 27-29. [2] M. Müller, et al., 1.3 μm short-cavity VCSELs enabling error-free transmission at 25 Gbit/s over 25 km fibre link, Electron. Lett. 48 (2012) 1487–1489 (November). [3] V. Iakovlev, G. Suruceanu, A. Caliman, A. Mereuta, A. Mircea, C.A. Berseth, A. Syrbu, A. Rudra, E. Kapon, High-performance single mode VCSELs in the 1310-nm waveband, IEEE Photonic Tech. L. 17 (5) (2005) 947–949 (May).

[4] M.-R. Park, O.-K. Kwon, W.-S. Han, K.-H. Lee, S.-J. Park, B.-S. Yoo, Allepitaxial InAlGaAs–InP VCSELs in the 1.3–1.6-mm wavelength range for CWDMB and applications, IEEE Photon. Technol. Lett. 18 (17) (2006) 1717–1719, 〈www.raycan.com〉. [5] International Telecommunication Union, ITU-T Recomendation G.694.2-200312, Spectral Grids for WDM Applications: CWDM Wavelength Grid, 2003. [6] A. Sirbu, A. Mereuta, A. Caliman, V. Iakovlev, G. Suruceanu, D. Ellafi, Z. Mickovic, E. Kapon, Wavelength Controlled VCSELs Emitting in the 1310nm Waveband (April 2014), SPIE Photonic Europe, Brussels (2014) 14–17. [7] A. Mereuta, A. Sirbu, V. Iakovlev, A. Rudra, A. Caliman, G. Suruceanu, C.A. Berseth, E. Deichsel, E. Kapon, 1.5 mm VCSEL structure optimization for high-power and high temperature operation, J. Cryst. Growth 27 (2004) 520–525. [8] K. Kurihara, N. Arai, R. Uedab, M. Takashimab, K. Sakatab, M. Takaharab, K. Shimoyama. 16th IPRM 31,May - 4, June 2004 Kagoshima, Japan. [9] Hiroshi Ito, Haruki Yokohama, Carbon doping in InAlAs grown by MOCVD, J. Crys. Growth 173 (1997) 315–320. [10] Abdallah Ougazzaden, J.a.y. Holavanahalli, Michael Geva, Lawrence E. Smith, Carbon doping of InAlAs in LP-MOVPE using CBr4, J. Cryst. Growth 221 (2000) 66–69. [11] A.Mereuta, A.Mircea, A.Rudra, V.Iakovlev, A.Caliman, A.Syrbu, E.Kapon, Carbon doping of In(Al)GaAs and InAlAs grown by MOVPE in nitrogen. Proceedings of EW-MOVPE Workshop, Bratislava, June 3-6, 2007, A15, pp.69-72. [12] S. Jochum, E. Kuphal, V. Piataev, H. Burkhard, Very high compositional homogeneity 0.1.55 um strain-compensated InGaAsP MQW structures by MOVPE under N-2 atm, J. Cryst. Growth 195 (1998) 637. [13] A. Sirbu, A. Mircea, A. Mereuta, A. Caliman, C.A. Berseth, G. Suruceanu, V. Iakovlev, M. Achtenhagen, A. Rudra, E. Kapon, 1.5 mW single-mode operation of wafer-fused 1550 nm VCSELs, IEEE Photonic Tech. L. 14 (2004) 738–740. [14] A. Sirbu, V. Iakovelv, A. Mereuta, et al., Wafer-fused heterostructures: application to vertical cavity surface-emitting lasers emitting in the 1310 nm band, Semicond. Sci. Tech. 26 (2011) 014–016.