Heterogeneous Integration of Microscale GaN Light‐Emitting Diodes and Their Electrical, Optical, and Thermal Characteristics on Flexible Substrates

Abstract

LEDs that can be utilized as implantable light sources have been playing increasingly important roles in neuroscience research, along with the development of genetically encoded actuators and indicators. [3,4] In particular, gallium nitride (GaN)/indium gallium nitride (InGaN) based blue LEDs are utilized as implantable light sources for optogenetic stimulation and/or exciting fluorophores for neural signal sensing. [5,6] High performance GaN blue LEDs are typically grown on rigid, single crystalline substrates including sapphire, [7,8] silicon (Si) [1] and silicon carbide (SiC), [9] and novel strategies on growing and releasing GaN devices on unusual substrates like zinc dioxide coated graphene, [10] boron nitride (BN), [11] amorphous glasses, [12] nanovoid-mediated substrates, [13] etc. are also actively explored. [14] Although diced bare LED chips have found their uses in wearable and implantable systems by flip-chip bonding, [15,16] thinfilm LEDs (with thicknesses less than 10 µm) with various emission wavelengths that are released from original growth substrates and integrated onto flexible and stretchable substrates are more desirable for biomedical applications. [17] Recent results have successfully demonstrated that released, thinfilm LEDs are integrated with flexible, stretchable, and even biodegradable substrates with better biocompatibilities like improved skin conformance and reduced lesion during implantation. [5,18,19] Thin-film, freestanding red and infrared (IR) LEDs based on gallium arsenide can be easily formed by selective sacrificial etching; [20] however, conventional techniques for thinfilm GaN based purple/blue/green LED release and integration typically rely on sophisticated process steps including laser liftoff (LLO) (for GaN on sapphire), [21,22] chemical etching (for GaN on Si), [1,23,24] wafer bonding, [25][26][27] layer transfer, [11] device pick and place, [28,29] etc., [30,31] thus limiting their use. While GaN LED epitaxial liftoff and integration with flexible substrates have been extensively exploited for various planar device architectures like GaN LEDs on graphene, [7,10] BN, [11] glasses with nanovoids, [12,13] Si, [32] SiC-on-insulator, [9] the performance of these devices is still inferior to their counterparts on conventional sapphire substrates, in terms of their current-voltage characteristics, quantum efficiencies, etc. Therefore, it is highly desirable to develop simple and reliable technologies to implement the mainstream, state-of-the-art GaN LEDs (for example,

A wireless, implantable optoelectrochemical probe for optogenetic stimulation and dopamine detection

“…The utility of freestanding InGaN based micro-LEDs for optogenetic stimulations has been demonstrated in previous works 36 . Here we evaluate optical and thermal properties of the micro-LED before and after integrating with the PEDOT:PSS coated diamond film for electrochemical sensing, with results depicted in Fig.…”

“…The LED is fabricated on sapphire and the diamond film is grown on silicon. They are released from native substrates respectively by laser liftoff and wet etching, forming freestanding films transferred onto PI using polydimethylsiloxane (PDMS) stamping methods 35,36 . On the diamond layer, the PEDOT:PSS film is conformally coated via spin casting and lithographically patterned, with a thickness of ~100 nm (Fig.…”

High Performance, Biocompatible Dielectric Thin‐Film Optical Filters Integrated with Flexible Substrates and Microscale Optoelectronic Devices

“…The thin‐film microscale blue LED structure (from bottom to top) consisted of a GaN buffer layer, a n‐GaN layer, an InGaN/GaN multiple‐quantum‐well layer, and a p‐GaN layer, with a total thickness of about 7.1 µm. The GaN‐based micro‐LEDs were grown on flat sapphire substrates and released after laser liftoff . The thin‐film, microscale GaAs‐based PV detector structure (from bottom to top) included a p‐GaAs contact layer, an Al 0.3 Ga 0.7 As back surface field (BSF), a p‐GaAs base layer, a n‐GaAs emitter layer, and an InGaP window layer, with a total thickness of about 3.7 µm.…”

“…Therefore, it can be utilized as a perfect mirror to improve the emission intensity of thin‐film blue LEDs, as shown in Figure . The microscale GaN‐based blue LEDs used here have an emission peak at 470 nm and a size of 125 µm × 180 µm, and the details of their fabrication process and performance are described in previous work . To match with the dimension of the LED, the filter illustrated in Figure b is first patterned into grid forms (size ≈ 150 µm × 200 µm) down to the sacrificial layer by laser milling, and then released by undercutting the sacrificial layer (Figure a).…”

Design of Silicon Photonic Structures for Multi-Site, Multi-Spectral Optogenetics in the Deep Brain

“…Alternative strategies are also proposed, based on polymer waveguides, which exhibit better biocompatibility and even dissolvability within the body [9][10][11].Recently developed thin-film, microscale optoelectronic devices that can be directly implanted into the animal brain provide alternative promising solutions to advanced optical neural interfaces. By integrating microscale light-emitting diodes (LEDs), photodetectors, and various electronic and chemical sensors onto flexible substrates, as well as wirelessly operated circult modules, untethered, multifunctional neural probes can be realized [12][13][14][15]. Nevertheless, challenges associated with electrical heating and device's long-term stability within the tissue remain daunting for these directly implanted active emitters and sensors [16].…”