A Multiple Regrowth Process for Monolithically ...

A Multiple Regrowth Process for Monolithically-Integrated InP-Based Mode-Locked Laser Diodes with Uni-Travelling Carrier Absorber Hans-J¨org Lohea , Riccardo Scolloa , Werner Vogtb , Emilio Ginib , Franck Robina , Daniel Ernia,c , Rik Harbersc and Heinz J¨ackela

c

a Electronics

Laboratory, ETH Zurich, CH-8092 Zurich; Center for Micro- and Nanoscience, ETH Zurich, CH-8093 Zurich; Laboratory for Electromagnetic Fields and Microwave Electronics, ETH Zurich, CH-8092 Zurich; b FIRST

ABSTRACT Progress in photonics by monolithic integration for higher functional density, performance and reduced cost faces challenging hurdles due to technological and functional heterogeneities. Advanced local material growth techniques are enabling concepts towards high-density photonic integration, unprecedented performance and multi-functionality and ultimately optical systems-on-a-chip. For example mode-locked laser diodes (MLLDs) are key devices for ultra-short pulse generation for all-optical Tbit/s communication networks. MLLDs suffer from material compromises and will benefit from the possibility to design the gain, absorber and passive-waveguiding sections independently. We have proposed and demonstrated the integration of a saturable absorber with a fast absorption recovery time based on an InP/InGaAsP unitraveling-carrier structure (UTC) to achieve pulses below 1 ps with repetition rates up to 40 GHz. The use of the UTC absorber instead of the commonly employed reverse-biased gain material requires however the heterogeneous growth of multiple layer stacks on the same chip with the butt-coupled regrowth technique. Critical for the MLLD performance are the reflections and the optical coupling between the different monolithic integrated layer structures of passive, absorbing and amplifying sections. 2D FDTD simulations of the optical waveguides demonstrate that to minimize reflections an angled interface between the different structures is preferable and can lead to reflection coefficients as low as 10−6 . To obtain an angled interface we used a wet chemical etching process sequence of selective and non-selective etchants, which is sensitive to crystal orientation and yields a 55◦ tilted interface. In addition we can conclude from our simulations that in order to minimize both, insertion loss and reflections, a bending of the light guiding layers has to be prevented. Bendings can lead to measured losses of 5-7 dB per interface whereas correctly aligned light guiding layers results in losses of 1.5 dB and intensity reflections below 10−5 per interface. The bendings originate from different growth rates near and far away from masked areas during regrowth due to reactants diffusion on the SiO2 mask. The bending can be minimized by optimizing the mask under etch of the SiO2 mask and low pressure MOVPE growth. We demonstrate operation of mode-locked laser diodes with an integrated UTC absorber and pulse durations below 1 ps. Keywords: mode-locked laser diode, monolithic integration, InP, MOVPE

1. INTRODUCTION The optical communication market was very volatile during the past five years mainly due to the overestimation of market growth. Nevertheless there is a slowly but continuously growing demand on network bandwidth. Currently several international telecommunication companies invest in fiber-to-the-home networks to offer new services like video on demand and to provide all three communication services, phone, television and Internet access over a single physical connection. This will increase the bandwidth demands even further. To exploit the multi-THz bandwidth of optical fiber networks, optical pulse sources with sub-ps pulse duration Further author information: Send correspondence to H.-J. L.: E-mail: [email protected], Telephone: +41 44 6320693 Integrated Optics, Silicon Photonics, and Photonic Integrated Circuits, edited by Giancarlo C. Righini, Proc. of SPIE Vol. 6183, 61831K, (2006) · 0277-786X/06/$15 · doi: 10.1117/12.662412

Proc. of SPIE Vol. 6183 61831K-1

pulse width [psec]

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Figure 1: (a) Simulated pulse width versus gain bandwidth for MLLDs with two different absorber recovery times. (b) 3D schematic drawing (not to scale) of a MLLD with different layer stacks for absorber, gain and passive waveguide sections. are required. Monolithically integrated mode locked laser diodes (MLLDs) are promising candidates for this purpose. MLLDs are compact, electrically pumped and can be easily electronically and optically synchronized to other network components and data streams. Reported monolithically-integrated laser diodes exhibit repetition frequencies from less than 10 GHz to several hundreds of GHz1 and even up to the THz regime.2, 3 Various materials such as bulk,4, 5 quantum wells and quantum dots6, 7 were used for MLLDs to obtain emission at the telecommunication wavelength at 1300 nm and 1550 nm. Quantum dots (QD) show very fast absorber recovery times and a low saturation energy when used as an absorber. These properties favor this material for use in MLLDs. Two-sections QD laser diodes demonstrated sub pico second pulse width with several mW of output power.7 The main drawback so far is that this material did not reach an emission wavelength of 1.55µm. Quantum wells have been used extensively because of several advantages for mode locking lasers such as a lower line-width enhancement factor, better saturation energies ratio between absorber and gain section and a less pronounced temperature dependency. Uncompressed pulses of reported MLLDs with repetition rates used in actual optical networks (10−40 GHz) are however limited to 1−2 ps pulse duration and are suffering from small mode-locking ranges.1, 8 The main structural reason for the limited pulse duration of current MLLDs is the use of the same material and layer structure for the forward-biased gain section and the reversed biased absorber section. However, the design requirements for best mode-locking operation are contradictory for these elements.9 For the absorber and the gain sections, a low and high saturation energy are required. Also, whereas a good carrier confinement is needed for the gain section the absorber has to show a fast carrier extraction. To assess the important absorber parameters for mode-locking we investigated MLLDs with a distributed time domain model including ultrafast intra band relaxation processes like carrier heating and spectral hole burning. We simulated MLLDs with different absorber recovery times, saturation energies and absorber lengths. The conclusion was, that the absorber recovery time is the dominant parameter to obtain short pulse durations from MLLDs due to the fast intraband effects in the gain section.10 Fig. 1a shows the simulated pulse duration as a function of the gain bandwidth for two different absorber recovery times. For currently used reversed-biased gain-material absorbers a recovery times around 10 ps11 can be achieved. In agreement with the reported results we can identify a pulse-width limitation around 2 ps due to the complex interplay of the fast intraband gain relaxation and the slow absorber recovery.10 For the absorber with a recovery time of 2 ps the pulses remain Fourier-limited down to 100 fs. Considering the reduction of bandwidth in semiconductor optical amplifiers with respect to the material gain bandwidth we can expect pulse widths around 500 fs. For instance, optically-pumped semiconductor vertical external-cavity surface emitting lasers with a fast, low-temperature grown semiconductor saturable absorber mirror and a quantum-well

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Figure 2: Processes for PICs: butt-coupled regrowth (BR), selective-area growth (SAG), quantum-well intermixing (QWI) and twin waveguide coupling (TWC). gain structure already demonstrated pulse durations of 480 fs.12 To overcome the pulse-duration limitations in monolithically-integrated MLLDs we have proposed and realized a uni-traveling carrier absorber structure (UTC) with a fast recovery time below 3ps.10, 13 This new absorber requires the integration of three different sections as shown schematically in Fig.1b. In addition to absorber and gain section, the third passive waveguide section is needed to limit the pumping current and the noise due to spontaneous emission.14 Several integration-process schemes were developed during the last years.15 We can distinguish four different approaches (see Fig. 2): butt-coupled regrowth (BR), selective-area growth (SAG), quantum-well intermixing (QWI) and twin waveguide coupling (TWC) with its derivatives like offset quantum wells. For the TWC process the active-layer stack is grown on top of a passive waveguide. Then, the active layer is selectively etched away and mode couplers for the transition from the upper into the lower waveguide are formed. So far only gain sections had been integrated into passive waveguides with this technique. The QWI process consists of the growth of quantum wells whose optical transition energy is shifted by material interdiffusion which alters the QWs composition. Several techniques like laser-induced or ion implantationinduced diffusion make this process area selective. Even though great progress has been achieved in the QWI process,16 the sections for different functions can not be designed independently. Thus the the absorber recoverytime problem can not be solved with this technology. The SAG process involves a patterned SiO2 mask on the wafer during epitaxy. Due to the diffusion effects of the grown species on the mask the growth rate and material composition depends on the local mask topology and the distance to the mask edge. Therefore the transition energies of the quantum wells can be shifted to integrate passive and active sections. The SAG suffers from the same constraints as the QWI process. Finally, only the BR process offers the full freedom to design the different layer structures independently. For the BR process, the initially grown layer structure is locally removed down to the substrate or a previously grown etch stop and replaced by a successively gown new layer structure. In principle the regrowth can be repeated until all needed structures are integrated on a single wafer. The drawback is a more complex etching process and the MOVPE regrowth process has to be optimized to yield interfaces with low reflections and losses. With a BR process the integration of up to four different layer structures was already demonstrated.17 In the following we will present the use of the BR integration process for a mode-locked laser diode with three different layer structures.

2. BUTT-COUPLED REGROWTH PROCESS Fig. 3 shows the three vertical layer structures for the gain, absorber and passive sections, which have to be integrated into a single device. The MQW gain structure, with an emission wavelength around 1.5 µm, consists of three lattice-matched 6.8 nm thick InGaAs wells embbeded in InGaAsP barriers with a photoluminescence peak at 1.2 µm (Q1.2). The UTC absorber structure is optimized for low saturation energies and fast carrier

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WG 100nm Q1.3 nid

10nm Q1.1 Zn 3 · 1017 40nm InP Zn 3 · 1017 10nm InP nid 80nm Q1.1 nid 50nm Q1.2 nid 65nm MQW nid 50nm Q1.2 nid 5nm InP nid 490nm InP Si 2 · 1018 8nm Q1.1 Si 2 · 1018 100nm InP Si 2 · 1018 8nm Q1.1 Si 2 · 1018 50nm InP Si 2 · 1018

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430nm InP Si 2 · 1018 300nm InP nid ⇐ grown on this etch stop

Figure 3: The three layer stacks for the multiple quantum well gain (MQW), the uni-traveling carrier absorber (UTC), and the passive waveguide (WG) section to be integrated into a single device. All layers are latticematched to the InP substrate. (Q1.x denotes an InGaAsP layer with a photoluminescence peak at 1.x µm, MQW: 7nm thick InGaAs quantum wells with 12 nm thick Q1.2 barriers. Zn, Si: Zn and Si doping level in cm−3 ) extraction.10 Different thicknesses of the lower InP buffer layers are required to match the optical modes of each layer stack. We optimized the layer thicknesses to achieve a mode overlap better than 99.9% with a finite element mode solver. Due to the low refractive-index contrast between the sections we estimated intensity reflection coefficients around 10−4 from effective index calculations. For the butt-coupled regrowth process the total mask coverage of the wafer is only a few percent and so we can neglect selective area growth effects for the most part of the wafer. Especially the core layer stacks as shown in the lower half of Fig. 3 will be affected because their thicknesses and material compositions determine the electrical and optical properties of the design. At the mask edges the growth homogeneity will suffer from material piling due to growth reactants diffusion on the SiO2 mask. To obtain a planar regrown surface at the interfaces we investigated the influence of the mask under-etch and the MOVPE chamber pressure on the material growth rates at the mask edge (Fig. 4a). Therefore we measured the maximum growth rate at the mask edge R1 normalized to the intended growth rate R0 as a function of the chamber pressure and the mask under etch. R0 was verified at a distance of 1mm from masked areas. The results are summarized in Fig. 4b. In agreement with the observation for selective-area growth reported by T. Fujii et. al.,18 the growth rate and hence the material piling at the mask edge decreases with decreasing chamber pressure. This dependence can be explained by the longer free mean path of the growth species at lower pressure. At higher pressure the growth species accumulate closer to the mask edges and are incorporated into the semiconductor lattice. The importance of the mask under-etch also is obvious from Fig. 4b. The layers thickness decreases continuously with increasing under-etch. For very large under-etch lengths we get a ratio of 1 at 30 mbar chamber pressure but the material does not fill the area under the mask completely, which may result in increased optical reflections. Generally, the InGaAsP layers however suffer from a poor crystalline quality for a 30 mbar chamber pressure. For 160 mbar chamber pressure a good crystalline quality of all layers was obtained. However the material piling at the mask edge overshoots the mask before the area under the mask is filled. Hence a hole under the mask arises before a ratio R1 /R0 =1 can be reached. If we grow and regrow the three MLLD layer stacks in only three

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Figure 4: (a) Schematic drawing (not to scale) of a etched MQW layer structure with SiO2 etching and regrowth mask indicating the important regrowth parameters: mask under-etch and MOVPE pressure. The growth rate at the mask edge is denoted as R1 . (b) Normalized maximum growth rate R1 /R0 at the mask edge as a function of the mask under-etch for 160mbar and 30mbar MOVPE chamber pressures. steps as indicated in Fig 1b, the absolute material piling for a 160 mbar chamber pressure will be so severe that the resulting wafer unevenness is not suitable for further device processing. To solve this material-piling issue we had to resort to the splitting up of the epitaxy into two main parts as indicated by the horizontal line in Fig. 3. In a first step, the core layer structures are grown and in a second step the thick upper cladding layers for the optical confinement. The overall epitaxial process thus consists of five growth steps as shown in Fig. 5: 1. Because of its high sensitivity to non-planar surfaces we first grow the core layers of the MQW structure as presented in Fig. 3. Additionally, a 90 nm-thick InP non-intentionally doped (nid) and a 100 nm-thick InGaAs nid sacrificial layers are grown on top of the structure. The photo luminescence emission peak wavelength of the quantum wells is intentionally red-shifted by 40nm compared to the desired laser wavelength to compensate for group V element interdiffusion and a resulting blue shift during the subsequent growth steps.19 2. The laser gain sections are then masked with a SiO2 layer for selective-area regrowth of the UTC absorber butt-coupled to the gain section. The gain material is wet-etched with a sequence of selective and non selective etchants. Then, the UTC core layers are regrown on the wafer. On top of the layers shown in Fig. 3 sacrificial 120 nm-thick InP and a 100 nm-thick InGaAs nid layers are grown. These two layers are removed during the subsequent steps. 3. For the second regrowth all active sections (UTC-absorber and MQW-gain) of the lasers are covered with a SiO2 mask and the UTC material is wet-etched using the same combination of selective and non-selective etchant as for the MQW layer stack. The core layers of the passive waveguide are then regrown with an additional 100 nm-thick InGaAs sacrificial layer on top. 4. After removal of the SiO2 mask the sacrificial InGaAs nid layers on top of the core layers are etched selectively. Thus, a very clean surface is provided for the growth of the nid upper cladding for passive waveguide sections on top of passive and active cores. Thus, the remaining unevenness of the core-layer growths are flattened out. 5. The wafer is patterned a last time using a SiO2 mask. Because the undoped InP upper cladding on top of the UTC and gain sections is composed only of two different materials, it can be wet-chemically removed with material selective etchants. During this etching the sacrificial InP layers of the MQW and UTC growths are removed to grow the p-doped cladding on top of a clean surface.

Proc. of SPIE Vol. 6183 61831K-5

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Figure 5: (a) Schematic drawing of the complete lateral and vertical layer structure on the wafer ready for device fabrication. (b) SEM micrograph of the grown passive waveguide-MQW gain interface with undoped and doped cladding interface corresponding to the left interface of Fig. 5a. (the layers were selectively etched to enhance the material contrast)

(a) 10GHz MLLD

(b) Test device

Figure 6: (a) Top-view micrograph of a 4 mm long 10 GHz laser chip with four lasers. Two different configurations were used. The outer lasers are arranged in an absorber, passive waveguide gain section order whereas the inner two have an absorber, gain and passive waveguide configuration. (b) Top view micrograph of a 1.5 mm long test device for loss measurements with 300 µm and 500 µm long gain sections embedded in passive-waveguide sections. The rib waveguide at the outer facets is tilted by 7◦ to reduce the out-coupling reflections. After growth of all designed sections a 2.5 µm wide rib for transverse single mode operation and a passivation with dielectric polyimide are processed. The n-contact and p-contact build a coplanar electrical waveguide on top of the chip. These are needed for high frequency tip contacting to supply the absorber sections with a 10−40 GHz modulation for hybrid mode locking. Finally the devices are cleaved and mounted on copper heat sinks for characterization. Fig. 6 shows a top view micrograph of a laser chip with 10 GHz repetition rate and four lasers with two different absorber, gain and passive waveguide section sequences. The four thin horizontal lines are the rib waveguides.

3. REGROWTH INTERFACE With the introduction of an additional structural interface in the MLLD, the detrimental effects of internal reflections must be considered. For external-cavity mode-locked lasers M. Schell et al20 have demonstrated, that the internal reflection must be lower than 10−5 to avoid multiple pulses in the actively mode locking regime. The butt-coupled interface can be fabricated with two geometries presented in Fig. 7. The vertical interface is obtained by selective wet etching21 or reactive ion etching.22 A combination of non-selective and selective wet etchants gives rise to an angled interface.23 We have investigated the impact of the two interface geometries with a two-dimensional FDTD code with regard to reflection and losses. For the vertical interface we found a reflection coefficient of 5 · 10−4 in accordance with

Proc. of SPIE Vol. 6183 61831K-6

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Figure 7: (a) Schematic cross-section and (b) 2D-FDTD simulation results for the vertical and angled buttcoupled interfaces

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Figure 8: (a) SEM Micrographs of waveguide-MQW interfaces. Top: a non ideal up-bending of the light guiding layer. Bottom: ideally aligned core layers after process optimization. (The layers were selectively etched to enhance the material contrast ) visible. (b) Simulation results for the interface shown in (a). Upper case leads to 5 dB loss and a reflectivity of 3 · 10−5 whereas the lower interface shows 0.04 dB loss and a reflectivity of 2 · 10−5 . our effective index calculations. In contrast to this high reflection value, angled interfaces reduce the reflections to 2 · 10−6 . This value is consistent with the upper reflection limit of 5 · 10−6 measured by Brenner et al.24 for an angled interface. It is therefore mandatory to use angled interfaces to suppress multiple pulse formation. Another important process requirement is the alignment of the core layers. Whereas the interface structure is relatively robust to simple vertical offsets up to 100 nm23 we identified a severe performance degradation due to the bending of the light-guiding layers as shown in Fig. 8a. The bending is due to material piling at the mask edges as described in the previous section. The 2D-FDTD simulations shown in Fig. 8b demonstrate that this bending will produce high losses in the order of 5 dB. This will prevent the use of low gain material and thus a high gain saturation energy, which is advantageous for MLLD operation.25 The resulting reflectivity for the deformed interface of 3 · 10−5 is above the multiple-pulse formation limit. Hence we must expect multiple pulse operation from MLLDs with these interfaces.

Proc. of SPIE Vol. 6183 61831K-7

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Figure 9: (a) Emission spectrum and (b) autocorrelation trace of a MLLD with 40 GHz repetition rate, 70 µm long UTC absorber and 930 µm long MQW gain section with strong internal reflections.

4. EXPERIMENTAL RESULTS To measure the loss and reflections of the interfaces we fabricated test devices with a gain section embedded between passive waveguides (Fig. 6b). In order to prevent reflections at the chip endings the passive waveguide are tilted 7◦ with regard to the cleaved facet. The interface losses were measured at a wavelength of 1630nm where both the passive waveguide and the gain sections are transparent. To deembed the waveguide and fiber-to-chip coupling losses, the same measurements were performed with devices without a gain section. For butt-coupled interfaces with a severe bending of the light-guiding layers (Fig. 8) we extracted a loss of 3−5 dB per interface, which is in good agreement with our simulation results. For well-aligned interfaces we deduce a loss of 1.5 dB per interface. This value is higher than our simulation results which could be explained by the fact that lateral waveguide inhomogeneities are not accounted for in the 2D simulation. But it is also 1 dB higher than previous reported results for similar interfaces.24 The main differences are the splitting of the regrowth process into the core layers and cladding layers as well as a higher growth pressure and hence a higher material piling at the interface. To quantify the reflections we investigated the optical spectrum of our test devices. For the well-aligned structures no residual spectrum modulation due to internal or out-coupling reflections could be resolved. Knowing the modal gain from performed Hakki-Paoli measurements, we can derive from the measurement noise an upper reflection limit of 1 · 10−5 . For devices with a bending of the core layers we observed a residual modulation of the gain spectrum corresponding to internal reflections between 1 · 10−4 and 1 · 10−3 . As mentioned mode locked lasers are very sensitive to internal reflections. Fig. 9 presents the lasing spectrum and the autocorrelation trace of a 1 mm-long MLLD from a wafer which suffered from bending of the core layers. The UTC absorber section was 70 µm-long and biased with -2 V. The MQW gain section was 930 µm and forward-biased with a pumping current of 300 mA. As expected from our simulations and reflection measurements the internal reflections are visible in the over modulation of the emission spectrum. The spectral distance of the over modulation corresponds to the subcavity length of the absorber section. The autocorrelation trace (Fig.9b) shows multiple pulse operation due to compound-cavity mode locking.13 The pulse width of the central pulse is 600 fs. On the other hand only a slight overmodulation is visible in the emission spectrum of a MLLD with optimized interfaces (Fig. 10). The UTC absorber section was 30 µm long and biased with -0.4 V. The MQW gain section was 970 µm long and forward-biased with a pumping current of 250 mA. The autocorrelation trace shows a much weaker compound cavity mode locking behavior. A pulse width of 950 fs was extracted from the autocorrelation trace.

Proc. of SPIE Vol. 6183 61831K-8

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Figure 10: (a) Emission spectrum and (b) autocorrelation trace of an optimized MLLD with 40 GHz repetition rate, 30 µm long UTC absorber and 970 µm long MQW gain section with a weak interface reflectivity.

5. CONCLUSION Future optical networks with THz bandwidth will require optical sources with sub-ps pulse durations. Monolithically-integrated mode-locked laser diodes will need a fast absorber structure to fulfill this requirement. Indeed, reversed-biased gain sections used so far as absorbers can not be optimized independently to obtain the best achievable mode-locking regime and the required absorber recovery times. To avoid compound-cavity mode-locking due to internal reflections, an angled interface has to be used. Such interfaces may lead to internal reflections as low as 1 · 10−6 . For these two reasons, i.e., the need for angled interface and independent optimization of the MLLD sections, the best suited technology for mode locked laser diodes is the butt-coupled regrowth technique. The regrowth process demands well controlled growth-mask under-etch and a low pressure MOVPE growth. The presented split growth technique is necessary to limit the absolute material piling at the butt coupled interfaces. Otherwise the resulting unevenness inhibit further device processing. Interface reflection coefficients lower than 1 · 10−5 and losses as low as 1.5 dB were measured. Processed modelocked laser diodes demonstrate low internal reflections and a sub ps pulse width at a repetition rate of 40GHz.

ACKNOWLEDGMENTS The authors greatfully acknowledge the Swiss National Science Foundation (SNF) for funding in the frame work of the National Center of Competence in Research-Quantum Photonics (NCCR-QP).

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