Surface control by low-temperature deposited buffer layer enables the growth of high-quality GaN on a sapphire substrate the lattice mismatch of which is as large as 14%. With use of such highly mismatched systems, novel electronic devices such as new lighting source which replaces conventional light bulb, super-high-density optical storage systems, high-power and high-speed transistors and UV image sensors for remote sensing and flame detection will be realized. This paper describes the details of the mechanism of this surface control technology.
Atomic structures of Ga-polar GaN(0001) reconstructed surfaces have been investigated by using first-principles total energy calculations as well as scanning tunneling microscopy measurements. It is found that the 2×2, 4×4, and 5×5 reconstructed structures of Ga-polar GaN(0001) surfaces consist of Ga adatoms adsorbed on the Ga-terminated surface bilayer.
Nondestructive determination of the polarity of GaN has been achieved by use of coaxial impact collision ion scattering spectroscopy. GaN films were deposited on c-plane sapphire substrates by a two-step atmospheric pressure metalorganic chemical vapor deposition using GaN buffer layers. The correlation between the samples prepared by interrupting the growth sequence at the various stage and their polarity was systematically investigated. It has been found that the polar direction of GaN growth is influenced by the polarity at the interface prior to the deposition of GaN epitaxial layer. We describe the mechanism of determining the polar direction by addressing our recent research related to substrate nitridation, buffer layer, and annealing of buffer layer.
Applications of in-situ reflectance monitoring to epitaxial growth are presented by an example of monitoring GaN metalorganic vapor phase epitaxy. Shallow-angle reflectance using ultraviolet light was used to compare the different types of growth on sapphire and on 6H-SiC substrates. By this method, stable monitoring is possible without being influenced by a strong and visible black-body radiation from the substrate heated to high temperatures. The growth-rate was successfully monitored by optical interference, and the morphological change was detected by the reflectivity change due to Rayleigh scattering. The use of p-polarized light in measuring shallow-angle reflectance that is called surface photoabsorption was applied to monitor the chemical stoichiometry of GaN surface during growth.
Our recent research on the dislocation behavior in GaN thin layers analyzed by means of cross sectional transmission electron microscopy (TEM) is briefly described. Layers of GaN and InxGa1-xN (x∼0.2) were grown by different vapor phase epitaxial methods. (1) On the surface of InGaN (T = 100 nm) grown on GaN, a pit was produced on the end of each threading dislocation. A thicker layer of InGaN has a two-story structure originated from the pits. This demonstrates that dislocations have strong influences on the growth process and the resultant layer-structure. (2) Microstructures in the epitaxial lateral overgrown (ELO) layers of GaN depend upon the growth conditions: When nitrogen (N2) is used as a carrier gas, the c-axis of ELO-GaN tilts by 40° at most. The c-axis tilting is due to the generation of horizontal dislocations during the growth. The degree of tilting depends also on the width of mask-terrace. In case of a H2-carrier gas, on the other hand, there is no tilting of c-axis, and regions of few dislocations are left over the terraces.
GaInN/GaN layers with low-temperature (LT) AlN buffer layers were grown on sapphire substrates by OMVPE. Two types of sample series were investigated by X-ray CTR and X-ray reflectivity measurements. One of them was grown on a sapphire substrate with nitridation process before LT-AlN buffer layer growth. The other was grown without the nitridation process. The X-ray CTR and X-ray reflectivity measurements show that 1) an amorphous layer was formed on the sapphire substrate by annealing at 1150oC in H2, 2) when sapphire was exposed to NH3 at temperatures lower than 800oC, formation of the amorphous and/or crystalline AlN layer did not occur, 3) nitridation process caused a crystalline AlN layer growth on the sapphire substrate and an amorphous AlN layer of 100∼50Å on it, 4) the crystalline AlN layer formed by the nitridation process worked as a seed crystal to make the LT-AlN layer high crystalline, 5) since the high crystalline AlN layer formed by the nitridation process did not work well as buffer layer, the crystalline quality of GaInN and GaN layers was poor, 6) since the poor crystalline LT-AlN layer obtained without the nitridation process worked well as a buffer layer, the crystalline quality of GaInN and GaN layers was good.
Surface reconstruction and their transition have been examined for MBE-grown GaN epitaxial layers. Several types of reconstruction were observed depending on the growth condition, crystal structure etc., and the reconstruction transitions were found to be related to effective III/V stoichiometry on the growing surface. It is shown that the surface reconstruction phase diagram and its phase transition line are useful for the optimization of MBE growth of GaN. Little amount of As4 residual pressure was found to affect the structure of GaN growing surfaces. These As surfactant effects are discussed and the growth of cubic GaN under small amount of As4 pressure is reported. The correspondence between the lattice polarity of hexagonal GaN epilayers and the surface reconstruction was confirmed by CAICISS technique. The quality of the epilayers which are grown so that the Ga-polarity is achieved is much improved.