Pseudomorphic stabilization of rocksalt GaN in TiN/GaN multilayers and superlattices

Gallium nitride (GaN) in its stable wurtzite phase has proven its utility in light-emitting diodes and diode lasers as well as high-temperature and high-power electronic devices. In addition to its equilibrium wurtzite phase, GaN exhibits two cubic polymorphs, a zinc-blende phase and a high-pressure rocksalt phase. Here, we report the pseudomorphic stabilization of the high-pressure rocksalt phase of GaN within TiN/GaN multilayers as verified using x-ray diffraction and high-resolution transmission electron microscopy. High-resolution lattice imaging confirmed that the lattice parameter of the rocksalt GaN phase is 0.41 nm. The critical thickness of the GaN film that can be pseudomophically stabilized in rocksalt phase within TiN/GaN superlattices is determined to be less than 2 nm.

Figure 1

(Color online) (a) X-ray diffraction patterns from six TiN/GaN multilayers with varying period thicknesses showing the presence of the wz-GaN phase in the sample with the largest period. (b) A high-resolution diffraction pattern from sample D shows satellite peaks with their spacing matching the period thickness of the multilayer.

Figure 2

(Color online) High-resolution cross-sectional TEM image of TiN (12.2 nm)/GaN (6.2 nm) multilayer grown on MgO (100) substrate. (a) The lattice image shows the cube-on-cube epitaxy of GaN layers on TiN layers; (b) inset in the middle shows a magnified view of the lattice image; (right), the comparison of the FFT patterns obtained from the GaN layers [(c) and (d)] with the one obtained from TiN buffer layer (e) confirms the cubic crystal structure of the epitaxial GaN phase.

Figure 3

(Color online) High-resolution cross-sectional TEM image of TiN(3.7 nm)/GaN(1.6 nm) superlattice grown on an MgO(100) substrate. (a) The lattice image obtained from the middle of the sample confirming the existence of a superlattice structure; (b) a magnified view of the lattice image showing the cube-on-cube epitaxy throughout the superlattice; and (c) the FFT pattern obtained from the leftmost image confirming the cubic symmetry throughout the cross section of the multilayer sample.

Figure 4

Cross-sectional TEM image of a TiN(20 nm)/GaN(variable thickness) multilayer grown on MgO(100). Cross-sectional TEM image of a TiN/GaN multilayer in which the TiN-layer thickness is kept constant at 20 nm while the GaN-layer thickness is systematically increased from 1.5 to 50 nm in order to determine the critical thickness at which the rocksalt-to-wurtzite transformation takes place in the GaN layers. The image shows that the interface between the first TiN and the first GaN layer is sharp and the interface roughness increases with increase in thickness of GaN layer. (Inset) Selected area diffraction pattern obtained from the multilayer and the MgO substrate showing that the multilayer is composed of polycrystalline layers.

Figure 5

(Color online) Microdiffraction patterns obtained from different layers of the TiN (20 nm)/GaN (variable thickness) multilayer. (a) Cross-sectional TEM image showing the bottom layers of TiN(20 nm)/GaN(variable thickness) multilayer grown on MgO (100) substrate. $⟨010⟩$ zone axis electron-beam microdiffraction pattern obtained from the (b) MgO substrate, (c) TiN buffer layer, (d) the first GaN layer $\left(\sim 1.5–2\text{nm}\right)$, and (e) the second GaN layer $\left(\sim 1.5–2\text{nm}\right)$. The microdiffraction patterns confirm that the cubic symmetry of the MgO substrate is retained in the TiN buffer layer and in the first two GaN layers.