Optical, surface, and structural studies of InN thin films grown on sapphire by molecular beam epitaxy

Abstract

A series of indium nitride (InN) thin films have been grown on sapphire substrates by molecular beam epitaxy (MBE) technology under different growth conditions of temperature and plasma power. Their structural, surface, and optical properties are studied by a variety of techniques of scanning electron microscopy, Hall effect, x-ray diffraction, photoluminescence (PL), Raman scattering, x-ray photoelectron spectroscopy (XPS), synchrotron radiation x-ray absorption near edge structure (XANES), and so on. The lower carrier concentration in InN can be obtained with a higher MBE growth temperature and a lower plasma power. As the plasma power increases, the PL peak energy is observed to shift toward the higher energy side and the Raman E2 (high) and A1 (LO) modes are shifted to the lower frequency. The residual compressive strain in epitaxial InN is relaxed. The lower plasma power and the higher growth temperature are preferred for the MBE growth of high-quality InN films. The influencing factors on the InN PL peak and band gap Eg have been revealed. It is evidenced that the InN PL peak and Eg can be shifted from high down to ∼0.65 eV with the carrier concentration down to low E19 cm−3 and the plasma power down to ∼80 W. Both the XPS and N K-edge XANES revealed the antisite defect of N on the In site, NIn. The XANES In L-edge measurements on the In L3-edge of InN films with various carrier concentrations has indicated the fourfold InN intermediate crystal structures. These obtained results are significant and useful to deepen the understanding and promote further investigation in InN and III-N materials.