1.Background

GaN-based semiconductors have a wide range of applications such as high-power electronic devices for wireless communications, visible light sources for illumination and signal, and UV light sources for sterilization.
At present, those devices are fabricated by processing the GaN-based semiconductor thin films together with single-crystalline substrates. However, only limited materials can be used as the substrates since they must be stable at the GaN thin film growth temperature (typically as high as 1000°C) and have similar crystal structure to GaN. In addition, some potential applications of the thin-film devices have been hampered by the substrates, whose typical thickness is 100 times as large as that of the thin-film devices. If the thin-film devices alone can be released from the original substrates and transferred onto any other substrates (Fig. 1 ), the thin-film devices may have a wider field of application (Fig. 2 ). Accordingly, a tremendous worldwide effort is underway to develop such detachment processes (e.g., Fig. 3 ).
We, NTT Labs., have been investigating growth of layered BN thin films toward a view to utilizing it as a release layer between the substrates and the GaN-based thin-film devices. The key point is that the BN takes a layered structure, same as Graphite, which is unique among the nitride semiconductors.

2.Summary of the development

NTT has newly developed a technique of growing high-quality layered BN thin films on sapphire substrates and also growing high-quality GaN-based thin films on BN. Since, in this structure, the cleavable BN plays the role of a release layer, one can easily detach the GaN-based thin-film devices from the sapphire substrates and transfer them to other substrates [MeTRe (Mechanical Transfer using a Release layer) method: Fig. 4 ].
With this technology, the devices can be transferred without any laser beam machining or chemical treatment, either of which is required in the conventional methods (Fig. 3 ). Additionally, surface flattening processes also become unnecessary after the removal of the thin-film devices. Furthermore, MeTRe method is better suited for a large-area processing, hopefully up to the range of one meter. Accordingly, a significant reduction of the fabrication cost is expected.
We have demonstrated a very thin LED, in which a detached thin-film device is sandwiched by two laminate films (total thickness is as small as 0.2 mm, Fig. 5 ), indicating that this technology has opened a door for GaN-based flexible devices. A solar-cell application also looks promising since power generation efficiency can be improved by attaching UV-sensitive GaN-based solar cells onto conventional Si-based ones. The characteristic feature of GaN, transparent but UV-sensitive, makes it feasible to attach the GaN-based solar cells on windows, which can cut hazardous UV light while generating power.

3.Technological key points

(1) Growth of high-quality BN thin films (Fig. 6 )

It has been difficult to grow high-quality BN films on single crystal sapphire substrates mainly because of their substantially different crystal structures. However, by systematic optimization of the growth parameters in Metal-Organic Chemical Vapor Deposition (MOCVD)^*2 process, including the substrate temperature and growth rate, we have succeeded in preparing high-quality epitaxial BN thin films on sapphire substrates.

(2) Growth of high-quality GaN thin films on layered BN (Fig. 7 )

We have found that high-quality GaN thin films can be grown on the layered BN thin films by inserting a buffer layer of Al_1-xGa_xN. This is most likely due to a higher affinity of the Al-contained nitrides for BN. Eventually, our wish of growing high-quality GaN thin films on sapphire with a BN release layer in between them was accomplished.

4.Prospects

Now the GaN-based thin-films devices can be detached from substrates much easier than they could previously. Further investigation is currently underway to improving the performance of the conventional solar cells (Fig. 8 ) and the functionality of the CMOS-based devices as well as increasing the area of the detachable devices.

Terminology

*1:GaN-based thin-film devices

We generically call GaN, AlN, InN, and their alloys (Al_1-xGa_xN, In_1-xGa_xN) GaN-based semiconductors. In order to create electronic devices (e.g., transistor) and optical devices (e.g., LED), stacking thin film layers of GaN-based semiconductors and microfabrication are necessary. We call such functional devices GaN-based thin-film devices.

*2:Metal-Organic Chemical Vapor Deposition (MOCVD)

MOCVD is a widely used chemical vapor deposition method of epitaxial growth of materials, especially compound semiconductors such as GaAs, InP, and GaN. By supplying source gases of the constituent elements into a reactor and giving rise to chemical reaction on substrate surfaces, single-crystal thin films of target materials can be grown on the substrates.