PACS: 81.05.Ea; 81.15.HiWe have investigated the growth temperature dependence of InN crystalline quality grown by RF-MBE. It was confirmed that the crystalline quality improved by increasing the growth temperature within the dissociation limit of InN. We obtained FWHM as narrow as 3.7 cm --1 for E 2 (high frequency) phonon mode peak of Raman spectroscopy and 24 meV for the band-edge luminescence at 77 K. These values prove excellent quality of our samples. In this study, we obtained optical band-gap energy of around 0.8 eV at room temperature from the high-quality InN samples.Introduction The nitride semiconductors, GaN, AlN and InN, show many interesting physical properties. Among them, InN is considered to have the smallest effective mass, 0.14m 0 [1], and the highest electron drift velocity, 4.2 Â 10 7 cm/s [2]. Therefore, InN is very promising materials for channel layers in high-speed and high-frequency electronic devices such as hetero-structure field-effect-transistors (HFET). Compared with other compounds, however, InN has many unknown properties. For example, reported bandgap energy has a large discrepancy between $0.8 eV [3-7] and $1.9 eV [8][9][10]. From this viewpoint, it is now very important to grow high-quality single crystals to establish the fundamental properties of InN.We have previously reported that InN films with high-electron mobility were successfully grown by radio frequency plasma-excited molecular beam epitaxy (RF-MBE) using a low temperature InN buffer layer [11]. The growth temperature was optimized by in-situ observation of reflection high-energy electron diffraction (RHEED). In this paper, InN films have been grown at different temperatures without InN buffer layer. We tried to optimize the growth temperature by characterizing the crystalline quality by RHEED, Raman scattering, X-ray diffraction (XRD), Hall-effect, optical absorption, and photoluminescence (PL) studies.

Growth of InN by radio frequency plasma-excited molecular beam epitaxy on (0001) sapphire was systematically studied. To improve the crystalline quality of InN, the following growth conditions were found to be essential: (1) nitridation of sapphire, (2) two-step growth, (3) precise control of V/III ratio, and (4) selection of optimum growth temperature. Results of structural characterization using x-ray diffraction, transmission electron microscopy, and extended x-ray absorption fine structure have clearly demonstrated that InN grown in this study had single crystalline with ideal hexagonal wurtzite structure. It is confirmed, however, that the InN had a threading dislocation density on the order of 1010/cm2 and large twist distribution. Photoluminescence studies on these well-characterized InN clearly demonstrated that band-gap energy of InN should be less than 0.67 eV at room temperature.

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A novel two-step growth method for creating InGaN quantum dots (QDs) was developed by using a combination of an In x Ga 1-x N nucleation layer with a platelet structure and an In y Ga 1-y N formation layer with an indium content lower than that of the In x Ga 1-x N nucleation layer. The realized QDs were investigated by micro-photoluminescence measurements. We observed sharp emission lines at 4 K with a spectral width down to the spectral resolution limit of the experimental setup of 0.17 meV. This growth concept is discussed in comparison with conventional growth methods.

We present a detailed study of the structural characteristics of molecular beam epitaxy grown nonpolar InN films with a-and m-plane surface orientations on r-plane sapphire and ͑100͒ ␥-LiAlO 2 , respectively, and semipolar ͑1011͒ InN grown on r-plane sapphire. The on-axis rocking curve ͑RC͒ widths were found to exhibit anisotropic dependence on the azimuth angle with minima at InN ͓0001͔ for the a-plane films, and maxima at InN ͓0001͔ for the m-plane and semipolar films. The different contributions to the RC broadening are analyzed and discussed. The finite size of the crystallites and extended defects are suggested to be the dominant factors determining the RC anisotropy in a-plane InN, while surface roughness and curvature could not play a major role. Furthermore, strategy to reduce the anisotropy and magnitude of the tilt and minimize defect densities in a-plane InN films is suggested. In contrast to the nonpolar films, the semipolar InN was found to contain two domains nucleating on zinc-blende InN͑111͒A and InN͑111͒B faces. These two wurtzite domains develop with different growth rates, which was suggested to be a consequence of their different polarity. Both, a-and m-plane InN films have basal stacking fault densities similar or even lower compared to nonpolar InN grown on free-standing GaN substrates, indicating good prospects of heteroepitaxy on foreign substrates for the growth of InN-based devices.