Structural properties of InN on GaN grown by metalorganic vapor-phase epitaxy

InN has been expected to be a suitable material for electronic devices such as high mobility transistors because of its small effective mass compared to other nitrides. Heteroepitaxial InN films were grown by metalorganic vapor-phase epitaxy. The films have been structurally characterized by triple-axis x-ray diffraction (XRD) analysis in terms of lattice-mismatch dependence and InN film thickness dependence, and Hall measurements have been performed. In the XRD measurement, omega and omega-2 theta scans were used, and the degree of tilting (the linewidth of x-ray signal, Delta omega(c)) [(0002) reflection] and that of twisting (Delta omega(a)) [(10 (1) over bar 0) reflection] have been separated. In addition, the degree of distribution of lattice constant c (Delta 2 theta(c)) [(0002) reflection] of InN films has been assessed. For study of the lattice-mismatch dependence, growth of InN films on GaN, AlN and directly on sapphire substrates was performed, and accordingly, Delta omega(c) was found to range from about 500 to 4000 arcsec, and Delta 2 theta(c) from about 400 to 700 arcsec. Among those three kinds of samples, InN films grown on GaN showed the smallest Delta omega(c) and Delta 2 theta(c) values. Observation of c- and a-lattice parameters has shown that the InN on GaN is affected by the residual strain. On the other hand, InN thickness dependence of XRD showed that Delta omega(c) was changed from about 700 to 500 arcsec, and Delta 2 theta(c) from about 600 to 300 arcsec with increasing InN thickness from 400 to 2400 Angstrom. In accordance with the thickness of InN, Delta omega(a) was found to change from about 2500 to 1700 arcsec. Moreover, it was found that the InN film less than 1200 Angstrom thick is composed of grain islands with different crystalline orientation and that the growth mode changes at a thickness of about 1200 Angstrom-and screw dislocations occur. It is found that the residual strain in InN films over 1200 Angstrom thick is gradually released, resulting in almost the same orientation. This is reflected in the reduction of the mosaicity, the proceeding of relaxation and the surface morphology. Selection of GaN for the underlying layer of the InN film has been shown to lead to structural improvement of the epitaxial InN film. In fact, InN film with a thickness of 2400 Angstrom grown on GaN has a Hall mobility of about 700 cm(2)/V s even at an electron carrier concentration of 5X10(19) cm(-3). This value corresponds to that for GaAs at the same impurity concentration. (C) 1999 American Institute of Physics. [S0021-8979(99)01311-0].