Al(Ga)N is a candidate material for an optoelectronic device such as a deep UV-LED (Light Emitting Diode). In order to fabricate the Al(Ga)N device with low dislocation density, it is indispensable to develop a high quality AlN substrate. Recently, bulk AlN with low dislocation density was produced by PVT (Physical Vapor Transport). However, the growth process requires high temperatures exceed 2000ºC. We consider an another possibility, i.e., solution growth, to produce AlN template. Solution growth was generally performed under low temperatures, thus it may reduce the production cost of the crystal. In the present study, we discuss the crystalline quality of AlN grown by our original solution growth method.

 In 2011, we have developed a solid-source solution growth (3SG) method which used Li₃N as a nitrogen source instead of N₂ gas [1]. In this method, we can control a nitrogen composition of the source materials, i.e., Al-Li₃N mixture. We confirmed that a 5mm-thick AlN epitaxial layer grew on AlN seed crystal along [1-100] under the condition of Al:Li₃N = 4:1. The TD density in the seed crystal was below the detection limit of TEM analysis. Though dense threading dislocations (TDs) were generated in the initial stage of 3SG (TD density = 10⁹ cm^-2), it was decreased to 10⁸ cm^-2 at 5mm-thick region. Next, we grew AlN by 3SG on the AlN(0001) substrate which was grown by PVT. In this experiment, the TD density was decreased from 10⁹ to 10⁸ cm^-2 at 2mm-thick region. That is, an annihilation ratio of TDs during the growth along [0001] was larger than that in the case of [1-100]. This is due to the difference in annihilation mechanisms of TDs. The TDs were annihilated by forming half-loops when the growth direction was [1-100]. On the other hand, the TDs were annihilated by forming bundle structures in the latter case. These results imply that the growth along [0001] is better than that along [1-100] to obtain a low dislocation density crystal by 3SG.

Acknowledgement

This work was supported by a JST PRESTO program.

References

[1] Y. Kangawa et al., Appl. Phys. Express 4 (2011) 095501.

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