In this paper we propose a new model for the growth of GaN thin films and other III-N compounds and alloys by the plasma-assisted MBE method. In the existing literature it has explicitly assumed that plasma-assisted MBE growth of GaN is a physical vapor phase deposition method, involving the reaction on a heated substrate of Ga-vapor with molecular nitrogen, activated first by a radiofrequency (RF) or microwave plasma. However, contrary to the MBE growth of traditional III-V compounds, which takes place under group-V rich conditions, the growth of atomically smooth GaN takes place under group-III rich conditions of growth. This is because the nitrogen activated species are highly reactive and as a result under N-rich conditions of growth the activated nitrogen species react with gallium on the substrate and limit their lateral mobility, leading to films with rough and faceted surface morphology.

We propose that under extreme Ga-rich conditions the GaN growth takes place through the supersaturation of the metallic Ga at the growing surface with nitrogen. According to this model the growth process is a liquid phase epitaxy (LPE) rather than vapor phase epitaxy. Of course the solubility of molecular nitrogen in Ga at the usual growth temperatures of 800 °C is very small but the solubility of atomic nitrogen and other activated nitrogen species is expected to be relatively high. In this model, the GaN growth rate at high temperatures is not limited by the thermal decomposition of GaN but instead by the solubility of the active nitrogen species in Ga at the growth temperature. Since the various nitrogen species (atomic nitrogen, nitrogen radicals etc) are expected to have different solubility in Ga at certain temperatures, the growth rate should depend sensitively on the nature of species produced by the various forms of plasma activation.

Experimental evidence will be presented that the growth rate of Ga- polar GaN under extreme Ga-rich conditions does not decrease at growth temperature in excess of 800 °C, a result attributed to higher solubility at higher growth temperatures of active nitrogen in the surface accumulated Ga. In other words, at these high temperatures the decomposition of GaN is overcompensated by the higher solubility of the active nitrogen species in Ga. This is to be contrasted with films grown under nitrogen rich or under Ga / N ratio approximately equal to one. In this case the growth rate was found to decrease abruptly above 750°C for Ga- polar GaN. Such a mechanism can also account for the efficient incorporation of Mg in GaN at high growth temperatures (750-800°C). At such temperatures the vapor pressure of Mg is more than 10 Torr and thus its efficient incorporation in the GaN lattice can only be accounted for by the high solubility of Mg in Ga and its incorporation in the GaN lattice from the liquid phase [1].

[1]. A. Bhattacharyya, W. Li, J. Cabalu, T. D. Moustakas, D. J. Smith and R. L. Hervig, Appl. Phys. Lett., 85, 4956 (2004).

The nanocomposite-film/p-Si junction properties and structural characteristics of the self-assembled ZnO nanoparticles in polystyrene-acrylic acid diblock copolymer on p-type (100) Si substrates are investigated and reported. The particles were developed in [PS]_m/[PAA]_n with four different block repeat unit ratios, m/n, of 159/63, 139/17,106/17, and 106/4, respectively, to determine the correlation between the particle parameters and the copolymer block lengths (m, n). The ability of the diblock copolymers to microphase separate and form self-assembled spherical nanodomains is used to template the ZnO nanoparticles onto the substrates. AFM images showed particles of spherical morphology with average size and density of about 250nm and 3.5x10⁷/cm², 60nm and 7x10⁸/cm², 60nm and 1x10¹⁰/cm² and 20nm and 1x10¹⁰/cm², were developed with block repeat unit ratios of 159/63, 139/17, 106/17 and 106/4, respectively. We established from AFM evaluations that the nanoparticle average size increased linearly with minority block length n, while the average density decreased exponentially with majority block length m. The x-ray diffraction θ-2θ scan showed the particles to have a wurtzite crystal structure with the (100) diffraction peak being the dominant orientation. Due to smaller dimensions and large separation between the particles, the broad diffraction peaks of ZnO nanoparticles could be detected only when θ was misaligned by 6^o with the substrate to remove the stronger Si peaks from the substrate. Room temperature I-V characteristics of the ZnO nanocomposite film on p-type (100) Si, measured for the first time, exhibited rectifying junction properties with an ideality factor n=1.7, and the transport mechanism in this system was evaluated from the electrical studies and discussed.

The support of NSF through grant #ECS0302494 is gratefully acknowledged.

Amorphous diamond-like carbon films are well known for their excellent tribological properties. In this paper we will discuss the sensoric properties of multifunctional a-C, a-C:H and a-C:H:Me films with respect to force/load/pressure and temperature measurement. It turned out, that nano structured amorphous carbon films showed an impressive piezorestive effect, which can be used for load measurement on tribological treated surfaces of different machine parts. The advantage over well known sensor techniques are: direct measurement in contact zones, high system stiffness and excellent adaptability to different base materials due to low deposition temperatures.

Unlike well known strain gages and piezoresistive sensors, which detect a deformation of the base substrate, this novel sensor can be used in a complete stiff arrangement without any elastic joint. For film preparation rf-plasma CVD processes were used. Substrates have been polished plates of steel 100Cr6 and Si-wafer. Precursor gases have been C₂H₂ and Ar. Besides gas pressure a variation of the substrate power and bias potential was performed. It turned out, that the tribological parameters varied slightly (hardness: 20-30GPa; friction coefficient: 0.10 to 0.15), whereas the electrical parameters showed remarkable differences with respect to the electrical resistivity and piezoresitivity.

The paper will also present different applications of amorphous carbon thin film sensors, e.g. smart washers to be used for actual force measurement in screws, ball bearings with local load measurement in the race track of the balls and arrays of piezoresitive pads for measurement of load distribution in mechanically contacts.