We have grown AlN nanorods and AlN films using plasma-assisted molecular beam epitaxy by changing the Al source flux. Plasma-assisted molecular beam epitaxy of AlN was performed on c-plane Al2O3 substrates with different levels of aluminum (Al) flux but with the same nitrogen flux. Growth behavior of AlN was strongly affected by Al flux, as determined by in-situ reflection high energy electron diffraction. Prior to the growth, nitridation of the Al2O3 substrate was performed and a two-dimensionally grown AlN layer was formed by the nitridation process, in which the epitaxial relationship was determined to be [11-20]AlN//[10-10]Al2O3, and [10-10]AlN//[11-20]Al2O3. In the growth of AlN films after nitridation, vertically aligned nanorod-structured AlN was grown with a growth rate of 1.6μm/h, in which the growth direction was<0001>, for low Al flux. However, with high Al flux, Al droplets with diameters of about 8μm were found, which implies an Al-rich growth environment. With moderate Al flux conditions, epitaxial AlN films were grown. Growth was maintained in two-dimensional or three-dimensional growth mode depending on the Al flux during the growth; however, final growth occurred in three-dimensional growth mode. A lowest root mean square roughness of 0.6 nm (for 2μm×2μm area) was obtained, which indicates a very flat surface.

We report plasma-assisted molecular beam epitaxy of InXGa1-XN films on c-plane sapphire substrates. Prior to thegrowth of InXGa1-XN films, GaN film was grown on the nitride c-plane sapphire substrate by two-dimensional (2D) growthmode. For the growth of GaN, Ga flux of 3.7×10−8 torr as a beam equivalent pressure (BEP) and a plasma power of 150W with a nitrogen flow rate of 0.76 sccm were fixed. The growth of 2D GaN growth was confirmed by in-situ reflection high-energy electron diffraction (RHEED) by observing a streaky RHEED pattern with a strong specular spot. InN films showedlower growth rates even with the same growth conditions (same growth temperature, same plasma condition, and same BEPvalue of III element) than those of GaN films. It was observed that the growth rate of GaN is 1.7 times higher than that ofInN, which is probably caused by the higher vapor pressure of In. For the growth of InxGa1-xN films with different Incompositions, total III-element flux (Ga plus In BEPs) was set to 3.7×10−8 torr, which was the BEP value for the 2D growthof GaN. The In compositions of the InxGa1-xN films were determined to be 28, 41, 45, and 53% based on the peak positionof (0002) reflection in x-ray θ-2θ measurements. The growth of InxGa1-xN films did not show a streaky RHEED pattern butshowed spotty patterns with weak streaky lines. This means that the net sticking coefficients of In and Ga, considered basedon the growth rates of GaN and InN, are not the only factor governing the growth mode; another factor such as migrationvelocity should be considered. The sample with an In composition of 41% showed the lowest full width at half maximum valueof 0.20 degree from the x-ray (0002) omega rocking curve measurements and the lowest root mean square roughness valueof 0.71nm.

We report growth of epitaxial AlN thin films on c-plane sapphire substrates by plasma-assisted molecular beam epitaxy. To achieve two-dimensional growth the substrates were nitrided by nitrogen plasma prior to the AlN growth, which resulted in the formation of a two-dimensional single crystalline AlN layer. The formation of the two-dimensional AlN layer by the nitridation process was confirmed by the observation of streaky reflection high energy electron diffraction (RHEED) patterns. The growth of AlN thin films was performed on the nitrided AlN layer by changing the Al beam flux with the fixed nitrogen flux at 860˚C. The growth mode of AlN films was also affected by the beam flux. By increasing the Al beam flux, two-dimensional growth of AlN films was favored, and a very flat surface with a root mean square roughness of 0.196 nm (for the 2 μm × 2 μm area) was obtained. Interestingly, additional diffraction lines were observed for the two-dimensionally grown AlN films, which were probably caused by the Al adlayer, which was similar to a report of Ga adlayer in the two-dimensional growth of GaN. Al droplets were observed in the sample grown with a higher Al beam flux after cooling to room temperature, which resulted from the excessive Al flux.

We report the structural characterization of BixZn1-xO thin films grown on c-plane sapphire substrates by plasma-assisted molecular beam epitaxy. By increasing the Bi flux during the growth process, BixZn1-xO thin films with various Bi contents (x = 0~13.17 atomic %) were prepared. X-ray diffraction (XRD) measurements revealed the formation of Bi-oxide phase in (Bi)ZnO after increasing the Bi content. However, it was impossible to determine whether the formed Bi-oxide phase was the monoclinic structure α-Bi2O3 or the tetragonal structure β-Bi2O3 by means of XRD θ-2θ measurements, as the observed diffraction peaks of the 2θ value at ~28 were very close to reflection of the (012) plane for the monoclinic structure α-Bi2O3 at 28.064 and the reflection of the (201) plane for the tetragonal structure β-Bi2O3 at 27.946. By means of transmission electron microscopy (TEM) using a diffraction pattern analysis and a high-resolution lattice image, it was finally determined as the monoclinic structure α-Bi2O3 phase. To investigate the distribution of the Bi and Bi-oxide phases in BiZnO films, elemental mapping using energy dispersive spectroscopy equipped with TEM was performed. Considering both the XRD and the elemental mapping results, it was concluded that hexagonal-structure wurtzite BixZn1-xO thin films were grown at a low Bi content (x = ~2.37 atomic %) without the formation of α-Bi2O3. However, the increased Bi content (x = 4.63~13.17 atomic %) resulted in the formation of the α-Bi2O3 phase in the wurtzite (Bi)ZnO matrix.