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I.
INTRODUCTION
In recent years, there has been a sustained interest on III-nitrides driven by their wide-range band gaps suitable for optoelectronics applications especially in near-IR/near-UV devices. Among all the III-nitrides, InN has the shortest direct band gap1,2 and smallest effective mass for electrons,3 which leads to high mobility and high saturation velocity at room temperature. These advantages make it a highly competitive candidate for various optoelectronic devices, such as solarcell, high mobility transistors, light-emitting diodes, and so on.4 The growth of single crystalline and good quality InN films has been widely studied since the 1990s by metalorganic vapor-phase epitaxy (MOVPE). Because of the low InN dissociation temperature and extremely high equilibrium vapor pressure of nitrogen over the InN film,5 a low-growth temperature of about 500 °C is necessary for InN preparation. However, this work is extremely challenging due to a low decomposition rate of NH3 at low-growth temperature, which restricts the achievement of high quality InN films. Therefore, the knowledge on the fundamental properties, such as the structural, electrical, and optical properties, of InN material is of shortage.
In this work, a series of InN films were grown in different conditions using MOVPE to understand their structural properties. The crystalline structures of as-grown wafers were analyzed by x-ray diffraction (XRD) and Raman scattering measurements. Moreover, phonon dispersions were simulated by the first principles calculation to identify the experimental results. The relationship between the structural properties of InN films and growth conditions is systematically discussed.
II.
EXPERIMENTAL AND THEORETICAL DETAILS
A series of nominally undoped InN films were epitaxied on a 1.5-[mu]m-thick wurtzite GaN underlayer with (0001) sapphire substrate by MOVPE. During the growth of GaN underlayer, trimethylgallium (TMGa) and ammonia (NH3) were used as the source precursors, with high purity N2 as the carrier gas. Then, InN film was epitaxied for about 1 h on the top of GaN underlayer using trimethylindium (TMIn) instead of TMGa. Three sets of wafers of about 250-nm-thick InN films were chosen to examine the effects of growth condition. The growth conditions of the three sets of films are summarized in Table I. For the first set (T1P2R3