Vertically Conductive Single-Crystal SiC-Based Bragg Reflector Grown on Si Wafer

Growth of single-crystal silicon carbide on silicon substrate (SiC-on-Si) is seen as a very attractive approach to combine the excellent properties of SiC with the low cost, large wafer size and well-developed micro-machining of Si wafers. Despite their large lattice and thermal expansion mismatches, both around 20%, uniform and crack-free single-crystal SiC-on-Si templates can be obtained with a relatively good crystal quality1. Consequently, SiC-on-Si pseudo-substrate are now investigated for a broad range of applications including photonic2,3, gallium nitride based (GaN) devices on Si4,5, micro-electro-mechanical systems (MEMS)6 and graphene epitaxial growth7. We propose and demonstrate for the first time the use of the SiC-on-Si technology to fabricate a vertically conductive single-crystal distributed Bragg reflector (DBR) on Si substrate. Such SiC-based DBRs enable the monolithic integration of efficient GaN-based optoelectronic devices on large Si wafers. SiC is indeed commonly used as growth substrate for commercial high power GaN devices as it has the smallest lattice mismatch amongst all foreign substrates for the hetero-epitaxy of III-nitride compound semiconductors, typically less than 4%. However, as SiC substrates are smaller and much more expensive than Si substrates, the SiC-on-Si technology is economically very attractive for the monolithic integration of GaN devices on large Si wafers. Several demonstrations of GaN light emitting diodes (GaN-LEDs) and GaN power devices grown on SiC-on-Si substrates have already been reported4,5. By using Si as a platform technology, GaN devices can also be directly integrated with CMOS devices and depreciated CMOS factory plants can be utilized, which lead to substantial cost saving in capital equipment investment and device fabrication costs. Thanks to its high electrical and thermal conductivities, combined with a large refractive index (RI) and a low absorption in the visible spectrum8, SiC is also a material of choice for the fabrication of single-crystal DBRs on Si operating in the visible or the infrared (IR) spectra. Light extraction (or absorption) efficiency of an optoelectronic device can be greatly improved with a DBR when used as a rear mirror sandwiched between the Si substrate and the device structure9. It is therefore very attractive to develop a SiC-based DBR for GaN-LEDs on Si. Another advantage of monolithic DBR-LEDs on Si would be to greatly simplify the device processing, and thus to reduce the manufacturing cost, compared to the standard GaN-LED on Si technology. Indeed, because of the strong optical absorption occurring in the Si substrate, manufacturing of high brightness (HB) LEDs on Si currently requires the removal of the Si growth substrate followed by the transfer of the III-N epilayers to a new high reflective carrier10. This process is particularly difficult and expensive to apply on large substrates as it requires low wafer bowing and often expensive gold-based bonding layer, hence lowers the process yield and induces a high manufacturing cost. A DBR consists of multiple transparent layers with alternative high and low RI, and with each layer thickness carefully chosen to create an optical resonance effect at the desired wavelength11. DBRs are fundamental for the fabrication of many photonic and optoelectronic devices using optical resonance effects in a microcavity12. Such devices include Fabry-Perot filters and modulators, resonant cavity (RC)-LEDs and vertical cavity surface emitting lasers (VCSELs). Monolithic growth of a DBR for GaN devices on Si requires the use of transparent materials which are compatible with both Si and III-N semiconductors, strongly limiting the number of suitable candidates. Most of the demonstrations of DBR for GaN-devices (mainly targeting LED applications) on Si have been made using only III-N semiconductors as the constitutive layers13. Reflectance as high as 95% was achieved, but as many as 20 pairs AlInN/GaN were needed because of the small RI contrast achievable between those III-N semiconductor layers14. Growth of such thick stack of layers implies also the use of complex stress management during the heteroepitaxy on Si because of the large lattice mismatch, making it challenging to grow crack-free DBR-LEDs. So far, the only successful report of such DBR-LED on Si was achieved only by using DBRs with a small number of pairs and thus with a relatively low reflectance15. Another drawback with III-N DBRs comes from their weak thermal and electrical conductivities which strongly limit their attractiveness for HB-LEDs. Rare-earth-oxides based DBRs paired with Si thin-films have also been investigated as the high RI contrast allows high reflectivity and large stopband in the visible with just few pairs16,17. However, their detrimental thermal and electrical properties strongly limit their potential for HB-LEDs as well18. In this paper, vertically conductive DBRs, using single-crystal SiC thin-films paired with doped or undoped Al(Ga)N layers, were heteroepitaxially grown on large Si substrates. High uniformity over 100 mm Si wafers, with a typical average peak reflectance of ~55% centered in the blue wavelengths and a stopband of 100 nm, is demonstrated using 1.5 DBR pairs. Furthermore, the DBR structure using Si-doped AlGaN shows very good vertical electrical conductivity, with current density as high as 70 A/cm2 at 1.5 V, without visible degradation of the optical performance compared to its non-conductive counterpart. Such DBR structures with high electrical conductivity of materials with high thermal conductivity are ideal candidates for the monolithic integration of SiC-based and GaN-based high power optoelectronic devices on Si.