Abstract
Plasma-assisted molecular-beam epitaxy (PA-MBE) is applied in this thesis work for III-nitride epitaxy. Due to lack of suitable substrates, III-nitride semiconductors are typically grown by heteroepitaxial growth. Low cost and excellent crystal quality Si wafer is a good choice as the substrate material for III-nitride epitaxy if the difficulty of lattice mismatch can be overcome. In this thesis, we demonstrate that III-nitride semiconductors can be heteroepitaxially grown as thin films as well as zero-, and one-dimensional structures on Si(111) substrates by PA-MBE. For growing III-nitride epilayers on Si, we utilize the concept of commensurate lattice match (CLM) to overcome the problem of large lattice mismatch. It was found in our group that high quality III-nitride epilayers can be grown on Si(111) substrates using a commensurately matched AlN/Si3N4 double buffer layer structure. In this technique, a single crystal Si3N4 layer is used as a diffusion barrier to prevent intermixing and autodoping effects. And, the AlN layer, which is nearly lattice match with GaN and commensurate lattice matched with InN, is used as the second buffer layer. Based on this growth technique, InGaN epilayers covering the entire InGaN composition have also been grown on Si(111) substrates by PA-MBE. For zero dimensional III-nitrides, we find that InN quantum dots (QDs) can be spontaneously formed on AlN and GaN surfaces by PA-MBE under the Stranski-Krastanow (S-K) mode. By using the technique of reflection high-energy electron diffraction (RHEED), we can observe the 2D-3D transition of S-K growth mode and the lattice constant varied can observe drastically at the 2D-3D transition point from AlN to InN lattice constant. For one-dimensional III-nitrides, we demonstrate that vertically aligned InN, GaN and InGaN nanorods can be grown on Si(111) by plasma-assisted molecular-beam epitaxy. For the case of InN nanorods, near-infrared photoluminescence (PL) can be clearly observed at room temperature. However, in comparison to the InN epitaxial films, the PL efficiency is significantly lower. Moreover, the variable-temperature PL measurements of InN nanorods exhibit anomalous temperature effects. We propose that these unusual PL properties are results of considerable structural disorder and strong surface electron accumulation effect. For the case of GaN nanorods, room temperature (300 K) high-intensity PL peak is at the 3.4 eV near-bandedge transition without yellow defect emission. GaN free and donor bound exicton peaks can be clearly observed in low-temperature PL spectra, indicative of high crystal quality of GaN nanorods.