Layered Boron Nitride as a Release Layer for Mechanical Transfer of GaN-Based Devices
Yasuyuki Kobayashi, Kazuhide Kumakura, Tetsuya Akasaka, and Toshiki Makimoto
Materials Science Laboratory
Nitride semiconductors are the materials of choice for a variety of device applications. One important practical goal is to realize such devices on large, flexible and affordable substrates, on which direct growth of nitride semiconductors of sufficient quality is problematic. Several techniques have been investigated to enable the transfer of nitride devices from one substrate to another, but existing methods still have some important disadvantage. Here we demonstrate that hexagonal boron nitride (h-BN) can form a release layer that enables the mechanical transfer of GaN-based device structures onto foreign substrates .
Photograph in Figure 1 shows the transferred AlGaN/GaN heterostructure, approximately 2 cm square, on an adhesive sheet (an indium sheet in this case) attached to a foreign sapphire substrate. We can see the surface of the indium sheet because the AlGaN is transparent. The size of the transferred area can be controlled by the adhesive sheet size. Some protrusions from the indium sheet are clearly visible, indicating the AlGaN/GaN heterostructure is mechanically released from the native sapphire substrate. No cracks were observed in the transparent AlGaN/GaN heterostructure up to maximum size of about 1 cm square, suggesting that mechanical release process using the h-BN layer ensures minimal crack formation. Next, we describe the electroluminescence emitted from the transferred multiple quantum well (MQW) light-emitting diode (LED) at room temperature. For comparison, the same MQW LED structure was grown on a typical low-temperature AlN buffer layer on a sapphire substrate and the conventional MQW LED was fabricated without lift-off. Current-voltage characteristics of the transferred LED show clear rectification. The electroluminescence intensities from the transferred LED were comparable to or higher than the intensities from the conventional LED on the low-temperature AlN buffer layer at the same current. The comparable intensities of the electroluminescence of the conventional and transferred LED indicate that the MQW preserves its original quality after the transfer. We further succeeded in transferring a verticaltype LED. This vertical-type LED emits blue light at room temperature (Figure 2).
The process we report here opens the way to releasing and transferring a wide range of nitride semiconductor devices to large-area, flexible and affordable substrates.
 Y. Kobayashi, K. Kumakura, T. Akasaka, and T. Makimoto, Nature 484 (2012) 223.
Fig. 1. Photograph of the transferred AlGaN/GaN
structure on the foreign sapphire substrate.
Fig. 2. Optical image of the blue-light EL from
the transferred vertical-type LED.
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