Preparation And Surface Evolution Of Cubic Aln Thin Films Deposited By Laser Molecular Beam Epitaxy

The metastable cubic structure of A1N thin film semiconductor possesses unique properties compared with the stable hexagonal phase. For example, decreased phonon scattering and easier doping are expected due to its higher crystallographic symmetry; there does not exist polarization electrical fields in the (001) growth direction, etc. These properties make cubic A1N a promising optoelectronic and microelectronic material. However, the epitaxial growth of cubic A1N film is quite difficult due to its metastable nature. Laser molecular beam epitaxy (LMBE) is an excellent nonequilibrium technique to deposit thin films, and so it is desirable for the synthesis of metastable cubic A1N. In this paper, cubic A1N films were prepared on MgO substrates by LMBE technique, and the effects of substrate temperature, laser energy, laser frequency and N2pressure on the crystal structure and surface morphology of cubic A1N films were analyzed. The energy structure, optical and electrical properties, and the surface evolution of cubic A1N films were also investigated. The results are as follows:1. Cubic A1N films with the single (200) preferred orientation were obtained on the MgO(100) substrates under the substrate temperature range of 650～750℃and the laser energy range of80-150mJ/p. Cubic AIN films show the excellent crystallinity and surface morphology at the substrate temperature of700℃and the laser energy of150mJ/p.2. AIN film illustrates the amorphous structure at the N2pressure of0.01Pa. While at the range of0.1～10Pa, AIN films show the cubic (200) preferred orientation, and the crystallinity become worse with the increasing of N2pressure. For the laser frequency, cubic AIN films also exhibit the single (200) preferred orientation in the range of5-9Hz. As the increasing of laser frequency, the transverse vibration absorption of films becomes broaden. At lOHz, AIN films exhibits the wurtzite structure with hexagonal structure. Thus, the optimizing processing parameters for cubic AIN films on MgO(100) substrates are as follows:substrate temperature of700℃, laser energy of150mJ/p, N2pressure of0.1Pa and laser frequency of5Hz.3. The in-plane epitaxial relationship of cubic AIN films and MgO substrates under the optimizing processing parameters is c-AIN (100)[100] II MgO (100)[100]. The bandgap energy of cubic AIN film is5.06eV, and the dielectric constant is8.2.4. The surface morphology evolution of cubic AIN films with increasing growth time shows the characteristic of the Stranski-Krastanov growth mode. AIN adatoms prefer to settle along the substrate steps in the initial stage, and the surface appears to be slightly rough. Subsequently, an undulate surface, and the nucleation and early growth of3D islands are observed. A rapid increase of roughness occurs in the following islands growth, and apparent gaps on the top of islands are observed, which gradually close up with the increasing of growth time. At the growth time of2h, a smooth surface with a roughness of1.06nm is obtained due to the spread and coalescence of islands.