We investigated the role of temperature and In∕N flux ratios to determine suitable growth windows for the plasma-assisted molecular beam epitaxy of In-face (0001) InN. Under vacuum, InN starts decomposing at 435°C as defined by the release of N2 from the InN crystal and a buildup of an In adlayer and liquid In droplets on the sample surface. At temperatures greater than 470°C, InN decomposition was characterized by a release of both In vapor and N2 in the absence of a significant accumulation of an In adlayer. No growth was observed at substrate temperatures above 500°C or at temperatures in which the decomposition rates were higher than the growth rates. A growth diagram was then constructed consisting of two growth regimes: the “In-droplet regime” characterized by step-flow growth and relatively flat surfaces and the “N-rich regime” characterized by rough, three-dimensional surfaces. The growth diagram can then be used to predict the surface structure of films grown at varying substrate temperatures and In fluxes. A 2.5 monolayer In adlayer was observed during In-droplet growth, suggesting that an In wetting layer was necessary for step-flow growth.

We study the effect of different deposition conditions on the properties of In-polar InN grown by plasma-assisted molecular beam epitaxy. GaN buffer layers grown in the Ga-droplet regime prior to the InN deposition significantly improved the surface morphology of InN films grown with excess In flux. Using this approach, In-polar InN films have been realized with room temperature electron mobilities as high as 2250cm2∕Vs. We correlate electron concentrations in our InN films with the unintentionally incorporated impurities, oxygen and hydrogen. A surface electron accumulation layer of 5.11×1013cm−2 is measured for In-polar InN. Analysis of optical absorption data provides a band gap energy of ∼0.65eV for the thickest InN films.

We have investigated the adsorption and subsequent desorption of Ga on (0001) GaN using simultaneous line-of-sight quadrupole mass spectrometry (QMS) and reflection high-energy electron diffraction (RHEED). The in situ QMS and RHEED desorption transient measurements demonstrate the Ga flux dependent accumulation of the theoretically predicted laterally contracted Ga bilayer [J. E. Northrup et al., Phys. Rev. B 61, 9932 (2000)] under conditions similar to those used during GaN growth by rf-plasma molecular beam epitaxy. We correlated bioscillatory RHEED desorption transients [C. Adelmann et al., J. Appl. Phys. 91, 9638 (2002)] to QMS-measured Ga-adsorbate coverage and found both to be consistent with layer-by-layer desorption of the Ga-adsorbate bilayer. The QMS-measured steady-state Ga-adlayer coverage exhibited a continuous increase from 0 to 2.4 ML (monolayer) with respect to impinging Ga flux at substrate temperatures of 640–700°C. We observed an exponential dependence of the Ga flux corresponding to 1.0 ML Ga-adsorbate coverage on substrate temperature and we measured an apparent activation energy of 2.43±0.11eV and an attempt prefactor of 6.77×1012nm∕min (4.36×1011Hz) for this transition.

We report on the growth of GaN quantum dots and the control of their density in the StranskiKrastanov mode on AlN ͑0001͒ by rf-plasma molecular beam epitaxy at 750°C. After depositing the equivalent of 2-3 ML GaN coverage, as limited by N fluence under Ga-droplet growth conditions, excess Ga was desorbed and Stranski-Krastanov islands formed under vacuum. We present the dependence of island density as a function of GaN coverage ͑for two growth rates: 0.10 and 0.23 ML/s͒, as estimated from atomic force microscopy and cross-sectional transmission electron microscopy. With a GaN growth rate of 0.23 ML/s, the island density was found to vary from less than 3.0ϫ10 8 -9.2ϫ10 10 cm Ϫ2 as the GaN coverage was varied from 2.2 ͑critical thickness͒ to 3.0 ML. For a GaN growth rate of 0.10 ML/s, the island density varied from 2.0ϫ10 10 to 7.0 ϫ10 10 cm Ϫ2 over a GaN coverage range of 2.0-3.0 ML. For each growth rate, the GaN islands were found to be of nearly uniform size, independent of the quantum dot density.

The impact of the Ga adlayer coverage onto the surface morphologies and pit densities of GaN (0001) films grown by plasmaassisted molecular beam epitaxy (PAMBE) has been studied using quantitative in situ quadrupole mass spectrometry (QMS). As the equilibrium Ga adlayer coverages rise continuously from 0 to 2.5 monolayers (ML) the surface pit densities decrease from $2 Â 10 9 cm À2 to zero, yielding characteristic step-flow and spiral growth hillock features. These results show that there is a direct and quantitative link between Ga adlayer coverage, adatom diffusion and surface defect structure without any discontinuities.