The Ga surface coverage during the growth of GaN by plasma-assisted molecular-beam epitaxy (PAMBE) has been systematically studied by reflection high-energy electron diffraction as a function of the Ga flux and the substrate temperature. As a consequence, a diagram is depicted, which describes the Ga surface coverage during PAMBE as function of growth conditions. In particular, we show that a region exists in this diagram, in which the Ga surface coverage is independent of fluctuations in the Ga flux or the substrate temperature and which forms a “growth window” for GaN growth. The influence of the Ga surface coverage on the GaN surface morphology and the growth kinetics is discussed.

We study the adsorption behavior of Ga on (0001) GaN surfaces combining experimental specular reflection high-energy electron diffraction with theoretical investigations in the framework of a kinetic model for adsorption and ab initio calculations of energy parameters. Based on the experimental results we find that for substrate temperatures and Ga fluxes typically used in molecular-beam epitaxy of GaN, finite equilibrium Ga surface coverages can be obtained. The measurement of a Ga/GaN adsorption isotherm allows the quantification of the equilibrium Ga surface coverage as a function of the impinging Ga flux. In particular, we show that a large range of Ga fluxes exists, where $2.5\pm 0.2$ monolayers (in terms of the GaN surface site density) of Ga are adsorbed on the GaN surface. We further demonstrate that the structure of this adsorbed Ga film is in good agreement with the laterally contracted Ga bilayer model predicted to be most stable for strongly Ga-rich surfaces [Northrup et al., Phys. Rev. B $61,$ 9932 (2000)]. For lower Ga fluxes, a discontinuous transition to Ga monolayer equilibrium coverage is found, followed by a continuous decrease towards zero coverage; for higher Ga fluxes, Ga droplet formation is found, similar to what has been observed during Ga-rich GaN growth. The boundary fluxes limiting the region of 2.5 monolayers equilibrium Ga adsorption have been measured as a function of the GaN substrate temperature giving rise to a Ga/GaN adsorption phase diagram. The temperature dependence is discussed within an ab initio based growth model for adsorption taking into account the nucleation of Ga clusters. This model consistently explains recent contradictory results of the activation energy describing the critical Ga flux for the onset of Ga droplet formation during Ga-rich GaN growth [Heying et al., J. Appl. Phys. $88,$ 1855 (2000); Adelmann et al., J. Appl. Phys. $91,$ 9638 (2002).].

The local Zn/Se relative concentration at the interface in ZnSe-GaAs(001) heterostructures synthesized by molecular beam epitaxy was found to be controlled by the Zn/Se flux ratio employed during the early growth stage of ZnSe on GaAs. Correspondingly, the valence band discontinuity varies from 1.20 eV (Zn-rich interface) to 0.58 eV (Se-rich interface). Comparison with the results of first-principles calculations suggests that the observed trend in band offsets is related to the establishment of neutral interfaces with different atomic configurations.

Zinc blende (ZB) GaN quantum dots have been grown by plasma-assisted molecular-beam epitaxy on AlN buffer layers using 3C-SiC(001) substrates. The two- to three-dimensional growth mode transition is studied by following the evolution of the reflection high-energy electron diffraction pattern. ZB GaN island layers are further examined by atomic force microscopy and transmission electron microscopy, extracting a mean island height of 1.6 nm and a mean diameter of 13 nm at a density of ${\text{1.3×10}}^{11} {\mathrm{cm}}^{\mathrm{-2}}.$ Embedded ZB GaN quantum dots show strong ultraviolet photoluminescence without any thermal quenching up to room temperature.

Magnetoluminescence spectroscopy of the photopumped stimulated emission, together with systematic photoluminescence studies as a function of the photogeneration rate, were used to identify the main recombination mechanisms occurring in shallow ${\mathrm{Zn}}_{0.10}$ ${\mathrm{Cd}}_{0.90}$Se/ZnSe multiple quantum wells for blue-green laser applications. Although the main spontaneous recombination processes have excitonic character, we found that lasing originates from free-carrier transitions.