Low-index nanopatterned barrier for hybrid oxide-free III-V silicon conductive bonding

: Oxide-free bonding of a III-V active stack emitting at 1300-1600 nm to a silicon-on-insulator wafer offers the capability to electrically inject lasers from the silicon side. However, a typical 500-nm-thick silicon layer notably attracts the fundamental guided mode of the silicon + III-V stack, a detrimental feature compared to established III-V Separate-Confinement Heterostructure (SCH) stacks. We experimentally probe with photoluminescence as an internal light source the guiding behavior for oxide-free bonding to a nanopatterned silicon wafer that acts as a low-index barrier. We use a sub-wavelength square array of small holes as an effective “low-index silicon” medium. It is weakly modulated along one dimension (superperiodic array) to outcouple the resulting guided modes to free space, where we use an angle-resolved spectroscopy study. Analysis of experimental branches confirms the capability to operate with a fundamental mode well localized in the III-V heterostructures.


Introduction
The interest of hybrid silicon photonics, with III-V active material bonded onto a platform such as Silicon-On-Insulator (SOI), is undisputed [1][2][3]. One first point was to get guidance from well mastered low-loss silicon structures, and to get active emission from the III-V epitaxial heterostructure stack (denoted EHS below), either in combination or for separate action, with a transfer in between. For these optical functions, bonding can make use of oxide, or polymer (e.g., benzocyclobutene BCB). Attempts have been pursued for bonding layer down to a few nm or of molecular nature [4]. GaAs-Si bonding without oxide is also sought actively [5,6] for the useful 1300 nm emission. Oxide-free bonding [7], is the only known technique to date that ensures not only a mechanical and thermal bond, but also an electrical bond, i.e. a low impedance non rectifying contact between silicon and EHS, as shown recently [8] for InP-Si bonding (Refs [5,6] show a non-rectifying Si-GaAs contact). Echoing the numerous novel uses of subwavelength structures in silicon photonics [9], oxide-free bonding of nanostructured silicon to III-V EHS was also demonstrated successfully [10], with positive photonics tests [11]. Besides a possible ohmic use, nanopatterns offer the capability of lateral guidance combined with additionally lateral thermal sinking in the upper silicon slab [12].
In this work, we argue that when an electrical contact across the bonded Si/EHS interface is desired, the natural vertical modal engineering practiced with the oxide (SiO 2 ) or polymer barrier is absent. Then, the stack's fundamental mode (FM) may widely overlap the Si slab, hence a penalty for a laser structure, as the confinement factor Γ FM of the active quantum wells (QWs) is reduced. To circumvent this penalty, we show that a nanopatterned silicon consisting of an array of holes with a modest air fraction f acts as a low-index barrier between Si and EHS, and thus greatly assists laser operation in the established regime of III-V's Separate Confinement Heterostructures (SCH). To evidence this, we analyze the dispersion of the Si-EHS stack: we excite guided modes by the localized QWs photoluminescence and outcouple them thanks to a superperiodic modulation of the basic sub-wavelength nanopattern. We first detail the sample, the experiment, and illustrate the TE polarized mode distribution undergoing confinement by the low-index barrier. We finally present the main experimental results.

Sample and experimental set-up
We show a schematic cross-section of our stack in Fig. 1(a) [7]. The silicon slab of the SOI is 500-nm-thick, the EHS has a 380-nm-thick SCH with QWs emitting at 1505 nm, sandwiched between 100 nm InP on the Si side, and 820 nm InP on the top (air) side to avoid that the FM visits the top surface. A representative cross-section of an oxide-free bonded SOI-EHS with nanopatterned silicon (here about 200nm) and QW revelation is shown in Fig. 1(b). In our photonic study, the etch-depth is 270 nm, leaving 230 nm un-etched Si.
In Fig. 1(c), we sketch an injected laser diode exploiting silicon to spread the current below the QWs, thus reducing Joule heating near the active junction. High doping is then needed in silicon. Without nanopattern, this doping would cause residual absorption in Si as the FM would overlap this region (usually a TE mode, fitted to exploit e-hh transitions in QWs). It is therefore welcome to repel the FM profile up into the EHS, a feature that, in addition, increases Γ FM for this mode. The nanopattern lowers the Si index ~3.48 to an homogenized in-plane effective index n hom,xy ~2.8-3.3, well suited for this purpose. We will denote n eff,z the effective indices of guided modes (pertaining to index profile in the zdirection).   The chosen nanopattern is a square array of holes. For the sole use as a low-index layer, any sub-wavelength period a < λ/2n eff,z ~215 nm suffices, but no easy diagnostic of its photonic role at device scale (say > 50 µm) can be made [11]. We therefore add a weak onedimensional periodicity along x, by enlarging every third hole. As shown in Fig. 1(d), there are three hole diameters used in three pair combinations, lithography and etching technologies being optimized to deliver vertical walls. The result is a set of three designs, labelled f 1 (82,66), f 2 (82,54) and f 3 (66,54), (d i ,D i ) being the two diameters in nm, with decreasing air fractions f i (thus, increasing n eff,xy ). These modulated patterns are essentially equivalent to a superposition of (i) arrays of uniform diameters 72 nm, 65 nm, 58 nm for f 1 , f 2 , f 3 respectively (based on the same average hole area, e.g. 72 2 ≈[82 2 + 66 2 + 66 2 ]/3) with (ii) a 3a-period modulation. The diffraction efficiency of this modulation is given by the relevant Fourier component A G of the xy dielectric map at wavevector G = G x = 2π/3a along x (actually not needed here).
Typically, we operate to extract modes of indices 3.0-3.2 at λ = 1500 nm around normal incidence, hence 3a ~480 nm. The air-filling factors are then f 1 = 0.158, f 2 = 0.128, f 3 = 0.104. Applied to a uniform medium of index 3.48 (Si) with infinite air cylinders in the microscopic limit (d << λ), they have homogenized indices n hom,xy of about 3.04, 3.13 and 3.19, calculated from a standard two-dimensional plane-wave expansion. We note that ellipsometry could seem a suitable technique for establishing the index sequence, but for transparent substrates, thick top layers, small patterns (~50 µm) and small InP bonded pieces (large sizes are not routine yet), it seems too awkward. In Fig. 1(e,f) the setup used for measurement is explained. It resembles that of [13] used for guided mode extraction studies. A red laser excites a photoluminescent spot inducing radially expanding guided waves. Those waves launched along x meet the effective medium (70 × 30 µm patches) in which the nanopattern modulation causes diffraction and outcoupling. Behind the objective and the beam splitter, a slit is translated to select the beam waves with a given in-plane direction k || ≡ k x according to the Ewald construction of Fig. 1(e), k || = k g -G, k g = n eff,z k o being a guided mode wavevector, and denoting k o = 2π/λ. A second splitter provides imaging with an infrared camera as well as a fibered path to a spectrometer equipped with an InGaAs cooled array.

Dispersion analysis
We first calculated the several TE and TM guided modes of the SOI + EHS stack using tabulated Si, SiO 2 and InP dispersion laws n(λ) without absorption. In Fig. 2(a), we vary n hom,xy ("effective medium", Fig. 2(b)) and plot all branches at a fixed wavelength λ = 1505 nm. We replaced the whole SCH by a single 380 nm-thick purely dispersive layer with typical Sellmeier parameters [14], Fig. 2(c), chosen to fit experimental data for all modes of the stack.
The most salient feature around our range, n hom,xy ~3.0, is the TE1/TE2 anticrossing. The presence of such an anticrossing is typical of a mode residing or not on one side of the low index barrier upon interaction with the mode on the other side. As for the fundamental mode, there is a broad smeared anticrossing immediately at n hom,xy ~3.4, which is indicative of the Si attraction for the mode when it is pure. We hope to mitigate this feature thanks to the low index effective medium (it would be less smeared with a first InP layer of >100 nm). Nevertheless, assessing the small shifts of n eff,z for TE0 with our set-up is not very reliable. Instead the TE1/TE2 anticrossing offers a remarkable signature of the role of n hom,xy .
To understand more explicitly the role of the low-index barrier, we plot on Fig. 2(d-g) the modal profiles of the three first TE modes for illustrative situations close to the four actual cases: no holes, f 3 , f 2 , f 1 . In this order the index n hom,xy decreases. We push the lower bound to 2.93, instead of the actual one (~3.00) to better visualize the fate of higher modes.
We first see that for pure silicon, the FM mostly lies within the silicon layer. This pertains also to the choice of a rather thick silicon slab (500 nm). It is a good typical value, and could be useful, either to carry intense current, or to sink substantial heat sideways off a structure's active junction. But we also see that as soon as air is introduced, for the f 3 sample and n hom,xy~3 .23, the FM is clearly centered in the SCH, and its overlap with the nanopattern is quite low. As more air is introduced, going from f 3 to f 2 and to f 1 , the overlap of the SCH guided mode with the pattern still diminishes, and all parts of the mode profile are pushed toward the SCH.
The origin of the anticrossing of modes TE1 and TE2 can also be inferred from the profiles: for f 3 , TE1 occupies the silicon and TE2 lies rather in the III-V stack (the EHS), whereas for f 1 , we observe almost the contrary: TE2 now manages to be in the silicon region, while TE1 has a substantial lobe in the EHS. These considerations will be useful to explain experimental data that will be presented in the following section.   23; (f) Same with n hom,xy = n Si -0.40 ~3.08; (g) Same with n hom,xy = n Si -0.55 ~2.93. The fundamental mode is repelled in the III-V stack for lower n hom,xy . Its overlap with the nanostructured layer decreases. The anticrossing behavior of modes TE1 and TE2 clearly appears as Si guidance is driven to lower effective indices by the diminishing n hom,xy .