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We present a comprehensive study of free-charge carrier and structural properties of two sets of InN films grown by molecular beam epitaxy and systematically doped with Mg from 1.0×1018 cm-3 to 3.9×1021 cm-3. The free electron and hole concentration, mobility, and plasmon broadening parameters are determined by infrared spectroscopic ellipsometry. The lattice parameters, microstructure, and surface morphology are determined by high-resolution X-ray diffraction and atomic force microscopy. Consistent results on the free-charge carrier type are found in the two sets of InN films and it is inferred that p-type conductivity could be achieved for 1.0×1018 cm-3≲[Mg]≲9.0×1019 cm-3. The systematic change of free-charge carrier properties with Mg concentration is discussed in relation to the evolution of extended defect density and growth mode. A comparison between the structural characteristics and free electron concentrations in the films provides insights in the role of extended and point defects for the n-type conductivity in InN. It further allows to suggest pathways for achieving compensated InN material with relatively high electron mobility and low defect densities. The critical values of Mg concentration for which polarity inversion and formation of zinc-blende InN occurred are determined. Finally, the effect of Mg doping on the lattice parameters is established and different contributions to the strain in the films are discussed. [ABSTRACT FROM AUTHOR]

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 (100) γ-LiAlO2, respectively, and semipolar (1011) InN grown on r-plane sapphire. The on-axis rocking curve (RC) widths were found to exhibit anisotropic dependence on the azimuth angle with minima at InN [0001] for the a-plane films, and maxima at InN [0001] 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 InN(111)A and InN(111)B 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. [ABSTRACT FROM AUTHOR]

The correlation between threading dislocations (TDs) and nonradiative recombination centers in InN films was investigated by infrared cathodoluminescence (CL). Samples were grown on nitridated (0001) sapphire substrates with a low-temperature-grown InN buffer layer by radio frequency molecular-beam epitaxy (RF-MBE). Panchromatic CL images of the InN films showed a high density of dark spots in a range of 108 cm-2 to 109 cm-2. The sample with a higher density of TDs had a higher density of CL dark spots. A depth-dependent CL measurement confirmed that CL dark spots aligned almost vertically in the film like TDs. Reasonable correlation between TDs and the nonradiative regions was also observed by a cross-sectional CL image of the InN film regrown on a microfaceted InN template, in which the TD density was dramatically reduced in part. These results suggest that threading dislocations act as nonradiative recombination centers in InN. [ABSTRACT FROM AUTHOR]

In-rich InxAl1-xN films (0.47 ≤ x ≤ 1) were grown directly on nitridated (0001) sapphire substrates without employing a buffer layer by radiofrequency molecular-beam epitaxy. Both photoluminescence peak energy and optical absorption-edge energy of the InxAl1-xN films decreased monotonically with increasing In composition, which was consistent with the recently reported InN bandgap energies of ~0.7 eV. The bowing parameter of this alloy was estimated to lie between 2.9 eV and 6.2 eV. Substrate nitridation with lower temperature and longer period conditions not only reduced misoriented InxAl1-xN phases remarkably but also produced narrower tilt distribution in the c-axis-oriented InxAl1-xN matrix. [ABSTRACT FROM AUTHOR]

We report a systematic investigation on Mg doped InN epilayers grown by radio-frequency plasma-assisted molecular beam epitaxy. Electrolyte capacitance voltage (ECV) combined with thermopower measurements find p-type conduction over an Mg concentration range. For InN:Mg in this p-type 'window' the Seebeck coefficients dramatically change their signs from negative to positive when the thickness of undoped InN interlayer decreases to zero. This notable sign change of Seebeck coefficient explains the previous inconsistency between ECV and thermopower results and confirms the existence of mobile holes in the InN:Mg. Taking into account the undoped InN interlayer, the hole density and mobility are extracted. [ABSTRACT FROM AUTHOR]

We study the unintentional H impurities in relation to the free electron properties of state-of-the-art InN films grown by molecular beam epitaxy (MBE). Enhanced concentrations of H are revealed in the near surface regions of the films, indicating postgrowth surface contamination by H. The near surface hydrogen could not be removed upon thermal annealing and may have significant implications for the surface and bulk free electron properties of InN. The bulk free electron concentrations were found to scale with the bulk H concentrations while no distinct correlation with dislocation density could be inferred, indicating a major role of hydrogen for the unintentional conductivity in MBE InN. [ABSTRACT FROM AUTHOR]

A systematic investigation on Mg doped and undoped InGaN epilayers grown by plasma-assisted molecular beam epitaxy has been conducted. Single phase InGaN alloys across the entire composition range were synthesized and Mg was doped into InxGa1-xN (0.1 ≤ x ≤ 0.88) epilayers up to ∼1020/cm3. Hall effect, thermopower, and electrochemical capacitance voltage experimental results demonstrate the realization of p-type InGaN across the entire alloy composition range for properly Mg doped InGaN. Hole densities have been measured or estimated to be in the lower ∼1018/cm3 range when the net acceptor concentrations are in the lower ∼1019/cm3 range across the composition range. [ABSTRACT FROM AUTHOR]

InN nanocolumns (NCs) without vertical diameter variation have been grown on GaN templates by molecular beam epitaxy. Growth temperature is the key parameter to obtain such shaped InN NCs. The density and surface distribution can be controlled by the V / III ratio and initial nucleation process. Intense photoluminescence (PL) is observed for the optimal InN NCs, about two orders of magnitude stronger than the InN epilayers. Superior excitation power and temperature dependence of the PL indicate significantly reduced defect density and residual electron density, non-degenerate property, and possibly suggest that there is a small or negligible level of electron accumulation at the smooth non-polar lateral surfaces. [ABSTRACT FROM AUTHOR]

Abstract: Droplet elimination by radical-beam irradiation (DERI), which is suitable for the growth of InN and periodic InN/InGaN structures, was developed for the growth of thick InGaN films by radio-frequency plasma-assisted molecular beam epitaxy (RF-MBE). In this method, Ga-to-nitrogen radical (N ) beam flux ratio was fixed in the entire growth process, and In beam supply was modulated under the condition in which excess In of more than 2–3 monolayer wetting layers always existed on the surface, that is, no precise control of In beam flux during growth was needed. Thick and uniform InGaN films were successfully grown by the developed method. In addition, InGaN films with an entire alloy composition could be simply grown by controlling Ga-to-N beam flux ratio. [Copyright &y& Elsevier]

Abstract: InN nanocolumns were grown on (0001) sapphire substrates by radio-frequency plasma-assisted molecular-beam epitaxy, simply with substrate nitridation, InN nucleation, and high V/III growth conditions. Here, the InN nucleation was conducted by forming In droplets on the nitridated substrates and subsequently nitriding these In droplets. We investigated the growth evolution of InN nanocolumns and the growth-temperature dependence of the morphology. X-ray rocking curves (XRC) for the (0002) reflection of the samples basically consisted of two components: a broad peak and a very sharp one. We propose that the broad peak came from the lower part of relatively short nanocolumns and the near-interface region of relatively long nanocolumns. In contrast, the sharp peak came from the (strain-free) upper part of relatively short nanocolumns and the high-crystal-quality region apart from the interface in relatively long InN nanocolumns. The relative intensity of the latter to the former increased with growth time. The shape of the nanocolumns varied with growth temperature: nanocolumns grown at 380 and 420°C had a taper-like appearance, but the top broadened with increasing growth temperature, becoming broader at the top than the base at 470 and 500°C. When we grew InN nanocolumns under the same conditions, but without surface nitridation, the subsequent columns were not all along a c-axis. If In droplets were not used, then columns did not form. Thus, both the surface nitridation and In droplets were needed to form -axis aligned nanocolumns. [Copyright &y& Elsevier]

GaN films were grown at various V/III ratios on sapphire (0 0 0 1) substrates with and without nitridation process by radio-frequency plasma-excited molecular beam epitaxy. Based on the results of X-ray diffraction pole-figure and φ scan measurements, the GaN film grown without nitridation process has a tendency to form the metastable in-plane rotation domain by ∼11° from the substrate, in addition to the main domain rotated by 30° from the substrate. In suppressing the formation of the metastable in-plane rotation domain, it is found to be very effective to perform the nitridation process. In addition, even in GaN films grown without nitridation process, decrease in V/III ratio enables us to suppress the formation of the metastable domain. The rotation domains are considered to have the epitaxial relationship of [1 2 &3macr; 0]GaN||[1 1 &2macr; 0]sapphire and [&1macr; &2macr; 3 0]GaN||[1 1 &2macr; 0]sapphire. [Copyright &y& Elsevier]

Abstract: The structural and electrical properties of N- and In-polar InN films grown directly on substrates were investigated. The twist contribution of directly grown InN films, which was defined by the estimated X-ray rocking curve''s full-width at half-maximum (XRC''s FWHM) of InN (101¯0) reflection, varied considerably by at least 40arcmin for both N- and In-polar InN films, depending on growth conditions. In contrast, the tilt contribution, defined by the XRC''s FWHM of InN (0002) reflection, was roughly fixed at 1.5–2.5arcmin for N-polar InN films and 8–12arcmin for In-polar InN films. For samples with the same twist contribution, In-polar InN films had larger tilt contribution than N-polar InN films. Nevertheless, electrical properties of In-polar InN had much better properties than those of N-polar InN films. [Copyright &y& Elsevier]

Abstract: High-quality epitaxial InN films have been employed as underlying templates for the growth of In-rich In x Ga1− x N (0.71⩽x⩽0.90) films by radio-frequency plasma assisted molecular beam epitaxy. The epitaxial InN films (InN templates) with the thickness of 500nm were grown on (0001) sapphire substrates with a low-temperature deposited 10nm thick InN (LT-InN) buffer layer, and then In-rich In x Ga1− x N films with the thickness of 250nm were grown on these InN templates. As compared with In-rich In x Ga1− x N films grown directly on the LT-InN buffer layers, the In-rich In x Ga1− x N films grown on the InN templates showed a decrease in full-width at half-maximum of the (0002) X-ray rocking curves. It was found that the insertion of the InN template was very effective in improving the crystalline quality of In-rich In x Ga1− x N. Optical properties were also investigated by photoluminescence. Lattice distortion of the In-rich In x Ga1− x N films induced by the underlying InN templates is also discussed. [Copyright &y& Elsevier]