Single-crystal AlN (0001)/diamond (111) Heterostructure
Kazuyuki Hirama, Yoshitaka Taniyasu, and Makoto Kasu
Materials Science Laboratory
Both aluminum nitride (AlN) and diamond have wide bandgaps: 6.0 eV for AlN and 5.5 eV for diamond. On the other hand, they have opposite doping characteristics. For AlN, n-type doping is easier than p-type doping, while for diamond, p-type doping is easier than n-type doping. The AlN/diamond heterostructure is expected to combine the features of both materials and appears promising for achieving high-efficiency deep-ultraviolet light-emitting diodes and high-power electron devices. The key to realizing these devices is single-crystal growth of AlN on diamond. However, to date, due to the difference in the crystal structures (hexagonal structure for AlN; cubic structure for diamond), AlN layers grown on diamond substrates have had multi-domain structures .
Here, using diamond (111) plane, we grew the AlN layer on the diamond substrate because the atomic bonding configuration of the hexagonal AlN (0001) plane is similar to that of the cubic diamond (111) plane. Figure 1 shows the X-ray pole figures of AlN (0002) and (10
11) planes. In Figure 1(a), a strong diffraction peak was only observed at Ψ = 0º. This indicates that the AlN  direction was oriented normal to the diamond (111) surface. In Figure 1(b), at Ψ = 62º, we only observed six diffraction peaks, which are attributed to six equivalent (10 11) planes for hexagonal AlN. These results indicate single-crystal growth of the AlN (0001) layer on the diamond (111) substrate.
In the cross-section TEM image at the heterointerface (Fig. 2), an abrupt AlN/diamond interface was observed without any interface layer. This confirms that the single-crystal AlN (0001) layer epitaxially grows from the nucleation step just on the diamond (111) surface. At the AlN(0001)/diamond(111) interface, there are two possible bonds, C-N or C-Al bonds. Because the bond energy of the C-N bond (3.1 eV/bond) is stronger than that of the C-Al bond (2.6 eV/bond), the C-N bond is preferentially formed at the AlN/diamond interface. As a result, the AlN layer grown on diamond has Al polarity.
For the AlN(0001)/diamond(111) heterostructure, the in-plane epitaxial relationship is [10
10]AlN || [1 10]diamond, although the lattice mismatch for [10 10]AlN || [1 10]diamond is larger than that for [11 20]AlN || [1 10]diamond. The in-plane epitaxial relationship can be explained in terms of interface energy. The bond densities at the interface for [10 10]AlN || [1 10]diamond and [11 20]AlN || [1 10]diamond are 2.7×1014 cm-2 and 2.2×1014 cm-2, respectively. Because [10 10]AlN || [1 10]diamond has a higher bond density and therefore lower interfacial energy, [10 10]AlN || [1 10]diamond is energetically preferred for the single-crystal AlN (0001) layer on diamond (111) surface.
 K. Hirama, Y. Taniyasu, and M. Kasu, Jpn. J. Appl. Phys. 49 (2010) 04DH01.
 Y. Taniyasu and M. Kasu, J. Cryst. Growth 311 (2009) 2825.
Fig. 1. XRD pole figures of (a) AlN (0002) and (b) AlN (10 11)
planes of the AlN grown on diamond (111).
Fig. 2. TEM image at the AlN (0001)/
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