The electrical conductivity dependence on bonding temperature seen in does not show monotonic behavior, attributed to the trade-off between conductivity increase and decrease by formation of covalent bonds and thermal expansion mismatch between GaAs and Si at higher temperature, respectively. Interfacial oxide formation might also be a cause of higher interfacial resistivity at higher temperature for our wafer bonding process in ambient air. It should be also noted that the wafer bonding process basically contains some randomness in reproducibility for the bonded interfacial properties degradable for example even by a single particle accidental incorporation into the interface. The conductivity enhancement seen in can be explained through an analysis of the heterojunction band offset at the GaAs/Si interfaces. One-dimensional simulations of the heterojunction bandbending (PC1D software, University of New South Wales) indicate thinning of the potential barrier at the valence band edge due to the change of the doping concentration in GaAs from 9×1018 cm−3 to 5×1019 cm−3 for the p-type/p-type pairs as seen in , leading to the interfacial electrical conductivity enhancement. On the other hand, simulations indicate tunnel junction formation due to the same change in doping concentration in GaAs for the p-type/n-type pairs as seen in . This valence-band-edge rising on the GaAs side enables tunnelling carrier transport, leading to higher conductivity and ohmic characteristics across the heterojunction interfaces. These p-type/p-type and p-type/n-type GaAs/Si ohmic heterojunctions are very suitable for next-generation III-V/Si hybrid optoelectronic devices that will enable both optical and electrical interconnections.
We have fabricated hundreds of lasers in a single wafer bonding step demonstrating the advantage of this approach for high volume, low cost integration over the conventional pick-and-place scheme18,40
. Evanescent optical coupling to underneath waveguides to fabricate so-called hybrid Si lasers10,11,12,13,14,15,16,41
could be realized by preparing rib structures on commercially available silicon-on-insulator substrates in advance of wafer bonding. In contrast to oxide-mediated bonding used for hybrid laser fabrication to date10,11,12,13,14,15
, conductive wafer-bonded heterointerfaces enable vertical carrier injection that prevents current spreading towards cavity stripe edges. Therefore, direct-bonded hybrid lasers have the advantages of higher quantum efficiencies and simpler fabrication without mesa etching or ion implantation for carrier confinement that was required in the fabrication of earlier lateral-current-injection III-V/Si hybrid lasers41
The low FF
seen in is likely due to the large series resistance. However, the wafer-bonded GaAs/Si heterojunction interfacial resistance with exactly same doping concentrations in GaAs and Si to those used for the bonding surfaces in the dual-junction solar cell seen in is far lower than the total series resistance of the dual-junction solar cell estimated from the light I–V
characteristics. We therefore attribute the low FF
principally to insufficient optimization of our front metal contact grids. Very high efficiency, over 30% under 1 sun, seems quite realistic simply through a contact redesign and would be expected based on the Jsc
values obtained at this preliminary research stage. To the best of our knowledge, while there have been two reports for all-III–V bonded multijunction solar cells42,43
, this is the first bonded multijunction solar cell with a Si subcell. Our monolithic AlGaAs/Si dual-junction solar cell (overcoming a 4% lattice-mismatch between AlGaAs and Si) has demonstrated a proof-of-principle for the viability of direct wafer bonding for solar cell applications. This wafer-bonding interconnecting approach is extendable to ultrahigh efficiency multijunction solar cells, such as InGaN/AlGaAs/Si/Ge four-junction solar cells, with optimal subcell bandgap sequences free from the lattice-matching restriction required in conventional heteroepitaxy. In this work, we adopted an etch-back method to detach the GaAs growth substrate to simplify the fabrication process. Alternatively, the incorporation of an epitaxial lift-off6,44,45
technique would enable the reuse of the GaAs substrates to reduce the production costs.
In conclusion, we have investigated GaAs/Si direct wafer bonding for electrically conductive, optically transparent materials interconnection in conjunction with heterointerfacial energy band alignment calculations in relation to doping concentrations. Heavy, degenerating doping at the GaAs and Si surfaces to be bonded is found to be significant for enhancing the GaAs/Si interfacial conductivity and results in ohmic GaAs/Si heterointerfaces even for bonding temperatures of as low as 300°C for both p-type/p-type and p-type/n-type combinations. Utilizing the p+-GaAs/p+-Si and p+-GaAs/n+-Si direct bonding, we have demonstrated a low threshold III–V laser on a Si substrate and a high-efficiency III–V/Si multijunction solar cell, respectively. Our low-temperature direct semiconductor bonding technique opens up a new pathway for realizing high-performance III–V/Si hybrid optoelectronics.