Mechanically Transferred GaN-based Optical and Electronic Devices - A method for Lifting Thin-Film Devices from Substrates using Hexagonal BN -

GaN-based optical and electrical devices are attracted much attention in fields of solid state lighting, high-power and high-temperature electronics. If the thin-film devices can be easily set apart from their host substrates and transferred onto any other substrates, without any laser beam machining or chemical processing, they may have a wider field of application. Recently, we proposed the use of a hexagonal BN (h-BN) grown directly on a sapphire substrate as a release layer [1]. We have also demonstrated transfer of the GaN-based devices from the sapphire substrates to foreign materials, e.g., metals or flexible sheets [1-3]. In this study, we fabricated light-emitting diodes (LEDs) and high electron mobility transistors (HEMTs) on the BN/sapphire substrates, and investigated their properties before and after the transfer to foreign materials. We grew a single crystal h-BN layer (3 nm) on a c-plane sapphire substrate by metalorganic chemical vapor deposition. Source gases were triethylboron and ammonia. The orientation relationship between the substrate and the h-BN is (0001)h-BN || (0001)sapphire, where the plane of boron and nitrogen bonded with sp 2 hybridization is parallel to the substrate surface. On the h-BN/sapphire substrate, we grew an InGaN/GaN LED structure, consisting of a 0.3-μm-thick Si-doped Al0.1Ga0.9N layer, a 3-μmthick Si-doped GaN layer, a 10-period InGaN/GaN multiquantum-well (MQW) structure, and a 0.1-μmthick Mg-doped GaN layer. One period of the MQW structure consists of an 8.6-nm-thick GaN barrier layer and a 1.8-nm-thick In0.15Ga0.85N well layer. We also grew a HEMT structure, consisting of a 0.3μm-thick AlN layer, a 1.5-μm-thick GaN layer, and a 20-nm-thick Al0.36Ga0.64N barrier layer. After fabricating the LEDs by conventional photolithography, dry etching, and lift-off techniques, we mechanically released the LEDs from the substrate and formed an Al/Au electrode on the backside of them by e-beam evaporation. We subsequently put the LEDs on a foreign substrate via an adhesive sheet (an indium sheet). Finally, we heated whole the structure to a temperature (~ 170°C) sufficient to heatseal the indium to the LEDs. Figure 1 shows the schematic illustration of the transferred LED. Figure 2 shows the photograph of blue-light electroluminescence from the transferred vertical-type LED. We confirmed that no degradation occurred during and after the transfer process. In a similar way, we also transferred an LED to a piece of Scotch tape, which was sealed by the other tape (Fig. 3). This very thin (< 0.1 mm) LED also emits blue-light under a forward bias condition. We applied the same approach to a transfer of the HEMTs. After released from the substrate, an In metal was electroplated on the backside of the HEMTs, followed by a thermal fusion bonding to a copper plate (Fig. 4). We measured dc characteristics of the HEMTs before the release from the sapphire substrates and after the transfer to the copper plates. The transconductance was 80 mS/mm, and good pinch-off characteristics (Fig. 5) were obtained even for the transferred HEMT. No degradation due to the release and transfer process was observed in the ID-VDS and transfer characteristics. Our newly developed method of mechanical release and transfer will open up avenues of fabricating novel devices, e.g., thin flexible LEDs as well as improving the performance of high-power GaN-based transistors, which has been limited by self-heating effect.