Heterogeneously integrated long-wavelength VCSEL using silicon high contrast grating on an SOI substrate

We report an electrically pumped hybrid cavity AlGaInAssilicon long-wavelength VCSEL using a high contrast grating (HCG) reflector on a silicon-on-insulator (SOI) substrate. The VCSEL operates at silicon transparent wavelengths ~1.57 μm with >1 mW CW power outcoupled from the semiconductor DBR, and single-mode operation up to 65 °C. The thermal resistance of our device is measured to be 1.46 K/mW. We demonstrate >2.5 GHz 3-dB direct modulation bandwidth, and show error-free transmission over 2.5 km single mode fiber under 5 Gb/s direct modulation. We show a theoretical design of SOI-HCG serving both as a VCSEL reflector as well as waveguide coupler for an in-plane SOI waveguide, facilitating integration of VCSEL with in-plane silicon photonic circuits. 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Introduction
An integrated light source on silicon will play a key role in addressing the swelling demands of interconnect bandwidth for computing [1][2][3][4][5].VCSELs are particularly promising candidates as light sources for silicon photonic circuits because of their low manufacturing costs, testing scalability, low power consumption, facilitation of 2D array, and high data transmission rates [6][7][8][9][10][11][12][13][14].Although several structures have been reported with vertical emission light sources on silicon-on-insulator (SOI) substrates [15,16], electrically pumped, uncooled continuous-wave (CW) devices have not been achieved.A major challenge to integrate a VCSEL onto an SOI substrate lies in the very large thickness of a typical distributed Bragg reflector (DBR) required for long-wavelength VCSELs, which can introduce a challenging topography for integration with silicon processing.In addition, standalone DBR VCSELs lack the diffractive mechanism to directly provide phase-matching with in-plane waveguides.Grating couplers used in nanophotonic waveguides are often detuned to off-normal angles to avoid reflection [17].Integrating a typical VCSEL at a small angle would pose a daunting processing challenge.Recently, we reported a novel high reflectivity mirror which uses one single ultra-thin layer (~15% lambda) of high index contrast gratings (HCG) and its use in a VCSEL structure [18][19][20].In addition, we showed that the HCG can be designed to provide >95% coupling efficiency between a surface-normal input and an inplane SOI waveguide [21].
In this paper, we report an electrically pumped AlGaInAs-silicon VCSEL using a silicon HCG mirror on an SOI substrate.The VCSEL operates at silicon transparent wavelengths with >1 mW CW power outcoupled from the semiconductor DBR, and single-mode operation up to 65 °C.The thermal resistance of our device is measured to be 1.46 K/mW.Although not optimized for high-speed operation, the VCSEL is capable of >2.5 GHz 3-dB direct modulation bandwidth, and we show 5 Gb/s direct modulation through a fiber link of 2.5 km single mode fiber (SMF).In addition, we show that a Si-HCG can be designed to provide high reflection (>99%) as well as waveguide coupler for an in-plane SOI waveguide, facilitating integration of VCSEL with in-plane silicon photonic circuits.We believe this presents a promising approach for scalable, low cost integrated photonic circuits including active components.

The concept of high contrast gratings
A high contrast grating structure consists of a single near-wavelength grating made from an ultra-thin high-refractive index material embedded in a low-index medium, as shown in Fig. 1(a).Detailed descriptions of HCG properties and the analytic formulism can be found in [18,19].Here we provide an overview of the underlying physics of the HCG.The grating bars can be considered as a periodic array of waveguides along the zdirection.Upon plane wave incidence, only a few waveguide array modes are excited.Due to a large index contrast and near-wavelength dimensions, there exists a wide wavelength range where only two modes have real propagation constants in the z-direction and, hence, carry energy.This is the regime of interest, and is referred as the dual-mode regime.The two modes then depart from the grating input plane (z = 0) and propagate downward ( + z-direction) to the grating output plane (z = t g ), and then reflect back up.The higher order modes are typically below cutoff and have the form of evanescent surface-bound waves.After propagating through the HCG thickness, each propagating mode accumulates a different phase.At the exiting plane, due to a strong mismatch to the exiting plane wave, the waveguide modes not only reflect back to themselves but also couple into each other.As the modes propagate and return to the input plane, similar mode coupling occurs.Following the modes through one round trip, the reflectivity solution can be attained.
For an HCG infinite in the x-y plane, due to its subwavelength period in air, only the 0th diffraction order, i.e. plane wave, carries energy in reflection and transmission.The HCG thickness determines the phase accumulated by the modes and controls their interference, making it one of the most important design parameters.
To obtain high reflection, the HCG thickness should be chosen such that a destructive interference is obtained at the exit plane, which cancels transmission.For full transmission, on the other hand, the thickness should be chosen such that the interference is well matched with the input plane wave at the input plane.Here, destructive interference does not mean that the fields are zero everywhere.Rather, it means that the spatial mode-overlap with the transmitted plane wave is 0, yielding a zero transmission coefficient.This prevents optical power from being launched into a transmissive propagating plane wave, and thus causes full reflection.Figure 1(b) shows surface normal reflectivity spectra contour as a function of HCG thickness.It can be seen that reflectivity assumes a well-behaved checker-board pattern as a function of thickness and wavelength, both normalized to grating period (Λ).This somewhat periodic dependence on thickness attests to the mode interference effect.
A unique phenomenon in HCG is that a high-Q resonator with surface-normal output can be obtained when constructive interferences are achieved at both input and exit planes.In this case, the HCG array modes couple strongly with each other and become self-sustaining after each round trip.Hence a resonator can thus reach high Q without mirrors.Figure 1(c) shows the intensity profile of an HCG designed to achieve a very significant (10 8 -fold) intensity buildup within the grating.
While the above analysis and simulation is for an infinite HCG, it is clear that the boundary conditions of a finite size HCG can lead to reduced reflectivity and Q value.Nevertheless, reasonably high values have been reported for surface-normal reflection reflection in VCSELs with as few as 4 periods and resonators with Q value in excess of 10,000, respectively [22,23].A VCSEL was demonstrated with 3 × 3 µm 2 HCG with merely 4 periods [22].The boundary conditions due to the finite size can, on the other hand, be leveraged into coupling to the in-plane propagation direction by matching the k-value in the x or y direction with the propagation constant of an in-plane waveguide.In Fig. 2(a), we show a design that can provide lateral coupling on the same silicon (high index) layer.The FDTD simulation in Fig. 2(b) shows that this HCG design provides both high out-of-plane reflectivity for a VCSEL, as well as efficient coupling into a silicon in-plane waveguide.We define total output as the power that is not reflected from the grating.At 1550 nm, the HCG reflects >99.4% of the light, and couples 47% of the output power into two opposite traveling in-plane waveguides.The remaining 53% output power is transmitted through the HCG as vertical output, as shown in Fig. 2(c), which can be suitable for off-chip applications such as integrated 3D imaging systems [24].>99.4% reflectivity for VCSEL, 0.6% total output power; 47% of the output power is coupled to waveguide, 53% is transmitted through the HCG.

Si-HCG VCSEL design
In this work, we employ a eutectic thin film to improve the thermal performance of the HCG VCSEL. Figure 3(a) shows a schematic of a HCG VCSEL structure using an ultra-thin silicon HCG optimized as a reflector on a silicon-on-insulator (SOI) substrate.The active layer contains AlGaInAs compressively-strained quantum wells (QWs), and employs a proton implant-defined aperture for current confinement, as illustrated by Fig. 3(b).The Fabry-Pérot cavity, indicated by circulating red arrows, is formed by a III-V DBR top mirror and the Si-HCG bottom mirror.The two material systems are heterogeneously integrated via a AuSn eutectic thin film bonding process that provides electrical contact, as well as excellent thermal heat-sinking properties.Using the analytical tool from [19], the HCG parameter space is scanned for a fixed silicon grating thickness t g to generate a contour map of favorable designs, shown in Fig. 4. The HCG design is chosen to have a reflectivity in excess of 99.5%.Since the VCSEL has a hermetically-sealed cavity gap of controllable length L within the cavity, the design must also have a phase response that maintains resonance.The round-trip phase condition is given by: where HCG φ is the phase response of the silicon HCG, and L is the air gap length.An HCG design with 660 nm period and 275 nm air gap has a phase response of 1.2 HCG ϕ π = .As a result, the cavity gap thickness must be set to L = 1.8 μm at λ = 1.55 μm to achieve resonance.The length L can be controlled during fabrication by tailoring the thickness of the eutectic metal thin film used for bonding.
The Transfer Matrix Method (TMM) is used to verify the existence of an optical cavity within the VCSEL structure, as shown by the multi-color traces in Fig. 5. Red indicates the refractive index of the epitaxial structure; green are energy bands; and blue is the optical field intensity within the cavity.The confinement factor between the cavity and the active region is 2.1%.In order to meet the round-trip phase requirements established by Eq. ( 1), the emerging wave A from the semiconductor must be in phase with the reflected wave B returning from the silicon HCG, as illustrated in Fig. 5. Using COMSOL, a Finite Element Method (FEM) simulation tool, the heat distribution of the HCG VCSEL structure was modeled by an 8-μm aperture with 25, 45, and 65 mW heat sources in the active region.The boundary conditions are set as thermal insulation at the semiconductor-air interface to provide a worse-case scenario for heat dissipation.The heat transfer in the vertical and radial directions can be very different in a multilayer system such as a DBR [25], where the thermal resistance of each layer adds up in series and in parallel, respectively.Moreover, the heat flow is strongly affected by the alloy impurities and layer interfaces, due to the restriction of the phonon mean free path [26].In conventional standalone VCSELs, the heat dissipation relies on the bottom DBR in between the heat source and heat sink.In flip-chip-bonded VCSELs, the heat can be carried away through the AuSn bonding layer.
In Fig. 6, we show a comparison between a flip-chip bonded VCSEL on SOI using AuSn, and a standalone III-V VCSEL structure, with the DBR thermal conductivity modeled after the approach in [27].The maximum temperature generated is 125 °C for the VCSEL on SOI, whereas the standalone III-V structure reached 183 °C, shown in Figs.6(a) and 6(b), respectively.Although the flip-chip VCSEL is resting on insulator (SOI), the graph in Fig. 6(c) shows that, at a given input thermal power, the average temperature in the active region has a stronger dependence on the vertical thermal conductivity of the DBR for standalone VCSELs (red) than flip-chip-bonded VCSELs (blue).This means the flip-chip-bonded VCSELs can overcome the restriction of heat flow caused by the DBR alloy impurities and interfaces.The thermal conductivity of AuSn is 57 W/Km, roughly an order of magnitude better than for a standard DBR quaternary alloy, i.e. 5 W/Km for AlGaInAs [28].For alloy compositions with thermal conductivities below 7.2 W/mK, the bonded device exhibits superior thermal performance.

VCSEL fabrication
The silicon HCG is fabricated on a 6" SOI wafer using a 248nm DUV ASML lithography stepper and a Cl 2 -HBr dry etching process.A 1.3 μm thin film metallic stack is deposited with electron-beam evaporation at a hyper-eutectic composition for the AuSn material system.In Fig. 7(a) and 7(b) we show a tilted-view colorized SEM of the Si-HCG reflector on an SOI substrate, surrounded by the AuSn thin film (yellow) ready for bonding.The flip-chip thermocompression bonding process is performed in inert nitrogen ambient at 340 °C with 10 N of force.InP substrate removal is done via wet chemical etching using an HCl:H 3 PO 4 1:1 mixture.Figure 7(c) shows a 3D-scanned confocal microscope image of a Si-HCG VCSEL.

LIV
Figure 8 shows the temperature dependent light-current-voltage (LIV) characteristic of a VCSEL with CW operation achieved up to 60° C, and >1 mW CW power outcoupled from the semiconductor DBR at 15 °C.The device temperature is controlled with a thermoselectric cooler on a copper chuck.A Keithley current source is used to bias the VCSEL via electrical probing.Light is collected by a large-area germanium photodetector placed above the VCSEL.The inset shows the near field intensity below and above threshold taken with a Xenics InGaAs CCD camera through a 100-X objective at high-gain sensitivity.The mode distortion is due to speckle interference visible at high-gain camera settings.Single transverse and longitudinal mode emission was observed over the entire current and temperature range.
The VCSELs exhibit thermal rollover with increasing current bias due to gain spectrum redshifting more rapidly than the resonant cavity spectrum, an effect typical in VCSELs [6].The devices have slope efficiencies of ~0.3 mW/mA and threshold current i th as low as of 7 mA.The ripples on the LI curve are due to residue reflection from the back of the silicon substrate/air interface, as verified by the output spectra measured at various currents corresponding to the peaks and valleys of the L-I curve and the substrate thickness.In the future, the L-I ripples can be eliminated with roughening the backside of the substrate.

Spectral and thermal performance
Using a Si HCG and AuSn bonding layer-instead of a traditional quaternary alloy DBRresults in excellent thermal performance.As mentioned above, the thermal conductivity of AuSn is roughly an order of magnitude better than for a quaternary alloy such as AlGaInAs.
The thermal resistance of the laser is determined by the following ratio: .
The top and bottom terms in Eq. ( 3) can be extracted from the spectral properties of the laser.Figure 9 shows the wavelength shift of the device for varying injection bias (at a fixed heat sink temperature of 20 °C), as well as the wavelength shift versus temperature (at a fixed injection current of 1.5 th i ).The wavelength shift versus dissipated power is measured to be 0.143 nm/mW, and the shift versus temperature is 0.098 nm/K, as shown in Fig. 10, for an 8-µm radius protonimplant aperture VCSEL.The latter is determined by the index change of the VCSEL cavity versus temperature.Taking the ratio of these two numbers, the experimental thermal resistance of our device is 1.46 K/mW.  Figure 12 shows the BER waterfall curves and eye diagrams of a directly modulated HCG-VCSEL.The device has error free (BER < 10 −9 ) operation up to 5 Gb/s at 20 °C.Fiber transmission performance of the signal was assessed using bit-error-rate (BER) measurements before (back-to-back) and after transmission through a link of 2.5 km SMF.

Conclusion
An electrically pumped AlGaInAs-silicon VCSEL structure using a high contrast grating is reported for the first time.CW power >1 mW and operation up to 65° C is demonstrated.Direct modulation with bandwidth >2.5 GHz, and 5 Gb/s direct modulation was also realized with error free transmission over a 2.5 km fiber link.HCG-VCSELs on silicon offer a low cost, energy efficient, scalable solution for integrated laser sources for silicon photonics circuits.We show that a Si-HCG can be designed to provide high reflection (>99.4%) as well as waveguide coupler for an in-plane SOI waveguide.We believe this can be important for scalable, low cost integrated photonic circuits including lasers, optical amplifiers and other III-V based active components.

Fig. 1 .
Fig. 1.(a) Schematic of HCG, where Λ, HCG period; s, bar width; a, air gap width; tg, HCG thickness.(b) Reflectivity contour plot of HCG versus normalized wavelength (λ/Λ) and normalized thickness (t g /Λ).(c) Intensity profile inside a HCG designed to high a high Q-value for surface-normal incidence/output and the grating showing a very high (10 8 ) resonant energy buildup.

Fig. 2 .
Fig. 2. HCG reflector and coupler simulation with FDTD.(a) Schematic indicating incident beam as vertical input.Total output consists light coupled into opposite traveling in-plane waveguides and transmitted through HCG.The HCG layer consists of a 358-nm thick silicon layer on an SOI substrate.The HCG is designed to have 863 nm period and 0.6 duty cycle.(b)>99.4% reflectivity for VCSEL, 0.6% total output power; 47% of the output power is coupled to waveguide, 53% is transmitted through the HCG.

Fig. 3 .
Fig. 3. Schematic of VCSEL with silicon HCG as bottom mirror.(a) Tilted-view of VCSEL cross-section with circulating red arrows indicating optical cavity, drawn to scale.The two material systems are heterogeneously integrated via AuSn thin film, with a hermetically sealed air gap of length L within the cavity.(b) The VCSEL employs a proton implant-defined aperture for current confinement, indicated by red curved lines between contacts.

Fig. 4 .
Fig. 4. Parameter space of HCG designs using analytical method in [19].(a) Contour map for reflectivity >99%, highest possible reflectivity is 100%.(b) Corresponding phase response of HCG for same dimensions.An HCG design with 660 nm period and 275 nm air gap has a reflectivity >99.5% and a phase response of ~1.2π.

Fig. 5 .
Fig. 5. TMM simulation of the VCSEL cavity.Red trace: indicates the refractive index of the epitaxial structure; green trace: energy bands; and blue trace: optical field intensity within the cavity.(a) Illustrates the numerous alternating pairs of DBR and the decaying optical field into the III-V mirror.(b) Expanded view of region near semiconductor-air interface.The confinement factor is with the quantum well (QW) is 2.1%.The emerging wave A from the semiconductor must be in phase with the reflected wave B returning from silicon HCG to maintain resonance.

Fig. 8 .
Fig. 8. Temperature dependent CW LIV of Si HCG VCSEL from 15 to 60 °C.Inset shows near field intensity below and above threshold.

Fig. 9 .
Fig. 9. Spectrum at different bias currents and temperatures.Wavelength peaks are recorded to measure shift for varying injection biases, and heat sink temperatures.

Fig. 11 .
Fig. 11.(a) Small signal direct modulation (S21) for Si HCG VCSEL at 20 °C for different current biases.A resonance frequency f R = 2.5 GHz, parasitic pole frequency f p = 1.4 GHz, and damping factor γ = 9.6x10 9 s −1 , are extracted from the family of curves.(b) D-factor fitting of VCSEL exhibiting thermal damping at 5.4 GHz.