Dramatically Enhanced Performance of Flexible Micro‐VCSELs via Thermally Engineered Heterogeneous Composite Assemblies

Abstract

elevated temperatures. [ 13,19,20 ] Development of novel integration pathways to allow implementation of VCSELs in mechanically compliant formats without sacrifi cing their wafer-level performance is therefore critically important to realize the full potential of this technology for future applications in fl exible and wearable optoelectronics. In this Communication, we present approaches that can overcome aforementioned performance limitations of printed micro-VCSELs on fl exible substrates having low thermal conductivities by exploiting thermally engineered composite assemblies. Systematic studies on electrical, optical, and thermal properties of fl exible micro-VCSELs at various assembly confi gurations, together with thermal modeling based on 3D fi nite element analysis (FEA), highlight that the developed concept of materials design and device fabrication enables facile and effi cient heat removal from micro-VCSELs printed on plastics and therefore allows the realization of performance characteristics that are comparable to or even higher than those achievable on the source wafer, in ways that also preserve their critical benefi ts in mechanically fl exible and optically transparent constructions. Figure 1 a shows schematic illustration for printed arrays of micro-VCSELs in thermally engineered composite assemblies that heterogeneously incorporate materials of high thermal conductivity between the VCSEL and a polymeric substrate to facilitate an effi cient thermal management. Specifi cally, a thin (on the order of a few μm) layer of metal is introduced on the front surface of fl exible substrate, the bottom surface of the released micro-VCSEL, or on both, respectively, which we refer to as V-A-M-P, V-M-A-P, and V-M-A-M-P, respectively, where V, A, M, and P denote micro-VCSEL, organic adhesive, metal, and polymeric substrate, respectively. Figure 1 b shows schematic illustration and corresponding cross-sectional scanning electron microscope (SEM) images of the composite VCSEL assemblies on a polyethylene terephthalate (PET, ≈50 μm thickness), in which the thickness of adhesive and metal (Cr/Ag/Au: 10 nm/1000 nm/30 nm) layers is about ≈1.0 and ≈1.04 μm, respectively. Such advanced designs of fl exible composite assemblies for the effi cient thermal management of printed micro-VCSELs permitted dramatic improvement of output performance of devices operating on plastics while minimally affecting the mechanical fl exibility as well as the level of optical transparency in printed modules. Figure 1 c shows a photographic image of 5 × 5 arrays of interconnected microVCSELs integrated on a PET substrate mounted on a curved surface (bending radius: ≈12.0 mm), with comparable output characteristics to devices on the GaAs source wafer even under the fl exible and optically transparent format (Figures S1–S3, Supporting Information). This outcome was enabled by the Dramatically Enhanced Performance of Flexible MicroVCSELs via Thermally Engineered Heterogeneous Composite Assemblies