Study On Defects&Strain And Defect Reduction Methods In GaN Epitaxial Materials

As one of the most important third-generation (wide band-gap) semiconductors,Gallium Nitride has got wider application. However, because of lacking of substrate forhomo-epitaxy, there is a high density of defects in normally hetero-epitaxial III-Nitrides,which limits device applications. For this reason, the influence and mechanism ofdefects enforcing on optical and electronic properties, and methods of defect reduction,always get enough attention in academia. Therefore, this study is stirred by these factsand could be divided into two parts. Firstly, the strain state in barrier layer would bealtered by cap layer and additional misfit dislocation could be introduced, whoseinfluence on the transport properties of two-dimensional electron gas (2DEG) isinvestigated. A characterization method of main defect in non-polar GaN, whoseimportance has been recognized recent years, is examined. The relationship betweendefects and strain state in non-polar GaN films is also studied. Then, for practicalapplication, the dislocation density is effectively reduced by homo-epitaxy and aninnovative method based on defect selective etching.The main work and conclusions are as follows:1. It is concluded that strain state and dislocation density could be modulated bydifferent kinds of cap layer materials, which would further influence the transportproperties of2DEG. Compared with AlGaN/GaN hetero-structure with no cap layer,GaN cap layer could make strain relaxation smaller in AlGaN barrier layer, whileAlN will introduce more severe relaxation. Therefore, the former will improve theelectrical properties of2DEG, while the latter will worsen it.2. A fast and non-destructive method to determine relative basal stacking fault (BSF)density has been adopted, whose authenticity has been proved. Based on thismethod, non-polar a-plane GaN layers grown by different strategies have beeninvestigated. It is shown that, on one hand, the anisotropic strain could be relaxed byBSF; on the other, impurity-like defect will also introduce so called hydrostatic ina-plane GaN, which should not be neglected.3. A defect selective etching system using molten KOH and hot phosphoric acid(H3PO4) has been set up. The best etching conditions for different purposes aredetermined via studying a series of etching time spans and temperatures. Thedifferent etching results by molten KOH and hot H3PO4have also beencomparatively investigated. 4. Low dislocation density AlGaN/GaN hetero-structures have been derived byhomo-epitaxy. It shows that2DEG mobility could be improved indeed with reduceddefect density. Besides, based on the above-mentioned etching method, lowdislocation epi-layers have also been achieved by epitaxial lateral overgrowth usingin-situ SiNxmask on etched GaN template and GaN re-growth on etchedAlGaN/GaN structure.