Molecular Beam Epitaxial Nitride Semiconductors: Epitaxy, Properties, and Solar Cell Applications

III-Nitride semiconductors (InN, GaN, AlN and their alloys) offer wide direct bandgaps that can be tuned over the complete spectral range from deep ultraviolet (6.2 eV) to near infrared (0.7 eV). They have wide applications for optoelectronic and electronic devices. Historically MOCVD has played the major role in III-nitride devices. However, MBE (molecular beam epitaxy) has advantages in the case of high-In-content InGaN and InN, and when monolayer level control of sharp interfaces and layer thickness is required. The ultra high vacuum environment of a MBE system enables powerful in-situ diagnostic tools to understand the epitaxial growth mechanisms, and precisely control epitaxial processes. In this presentation, I will first discuss the MBE growth of InN and InGaN thin layers and nano-columns. By using various in-situ monitoring tools, i.e., RHEED, laser reflection, and spectroscopic ellipsometry (SE), In adlayer assisted epitaxy of InGaN and InN, and the surface kinetics of In, Ga and Al adatoms have been thoroughly investigated. Special attentions have been paid to reduce the threading dislocations and residual electron density. P-type InN and InGaN across the entire alloy composition range have been achieved, despite of the notorious surface electron accumulation on high-In-content InGaN surfaces. Some important physical properties of InGaN will be discussed as well.Thanks to the powerful in-situ monitoring tools, especially the unique in-situ SE, monolayer level short period superlattices of (InN)n/(GaN)m and (GaN)n/(AlN)m grown by MBE have been demonstrated. They can be regarded as digital alloys, in which the alloy disorder scattering is expected to be much reduced compared to usual ternary alloys. Their potential applications for optoelectronic and electronic devices will be discussed. Finally, I will present our attempt to fabricate high-In-content (>20%) InGaN solar cells. The major challenge is how to reduce the large leakage current due to defective mismatched active layers. Technologies for in-situ leak path passivation by inserting wide band gap Al-N and AlOx layers have been proposed and developed. A N2O plasma source has been installed into the MBE system to supply oxygen species. This special MBE system enables integration of oxides into III-nitrides.