Guido Mula

Dynamically stable gallium surface coverages during plasma-assisted molecular-beam epitaxy of (0001) GaN

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.

Gallium adsorption on (0001) GaN surfaces

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, finiteequilibrium 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 60.2 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!.#.

Self-assembled zinc blende GaN quantum dots grown by molecular-beam epitaxy

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 1.3×1011 cm−2. Embedded ZB GaN quantum dots show strong ultraviolet photoluminescence without any thermal quenching up to room temperature.

Growth and characterisation of self-assembled cubic GaN quantum dots

Abstract Self-assembled cubic GaN quantum dots have been grown by plasma-assisted molecular-beam epitaxy on cubic AlN. Atomic force microscopy and transmission electron microscopy reveal islands of a mean height of 1.6 nm and a mean diameter of 13 nm. The influence of stacking faults on island nucleation is discussed. The quantum dots show ultraviolet photo- and cathodo-luminescence with no thermal quenching up to room temperature.

Epitaxial Growth of GaN, AlN and InN: 2D/3D Transition and Surfactant Effects

The influence of foreign species on the growth mode has been studied, with special emphasis on the role of In. It is demonstrated that the growth mode, i.e. 2D or 3D, first depends on the competing kinetics of Ga and N. In this view, we show that the surfactants modify both the Ga droplet formation process and the N desorption rate. Next, we discuss the role of strain relaxation in nitride heterostructures. In particular, we show that GaN and InGaN can experience a Stranski-Krastanow growth mode leading to the formation of quantum dots. A mechanism of quantum dot nucleation is proposed in the case of GaN on AlN.

Laser emission in HgCdTe in the 2-3.5 μm range

Abstract Stimulated emission from HgCdTe separate confinement heterostructure waveguides embedding quantum wells as gain medium is studied. We explore the effect of composition grading in the barriers as well as that of using strained quantum wells on the temperature evolution of the laser threshold. The main effect of barrier grading is to suppress carrier trapping at low temperatures, leading to an improved excitation transfer from the barriers to the quantum wells, and to an intrinsic exponential dependence of the threshold on the temperature. For the strained quantum well structures, a two-fold reduction of the threshold is obtained, as compared with similar band gap unstrained structures. The threshold dependence on photon energy points out the importance of Auger type nonradiative recombinations. We determine the Auger constant in our structures, and compare them with published data. An exponential dependence of the Auger constants on the band gap, valid for type I heterostructures, is shown. A comparison of the published operating temperatures in competing devices is done. It appears that III–V type II and unipolar lasers are largely ahead of II–VI lasers. The future of HgCdTe could lie in vertical cavity, surface emitting lasers.

Structure and ordering of GaN quantum dot multilayers

Grazing incidence x-ray techniques are used to characterize the structure of multilayered GaN quantum dots in an AlN matrix. For a dot lateral size of 170 A, the values of the interdot vertical and lateral correlation lengths are 1500 and 250 A, respectively. The presence of smaller quantum dots is observed only in the layers deposited first. The strain distribution in the multilayer is also investigated as a function of depth. Along the dot columns, the crystal lattice remains coherent, with elastic relaxation from the bottom to the top of the multilayer.

Atomic-layer epitaxy of GaN quantum wells and quantum dots on (0001) AlN

We show that a dynamically stable Ga film is formed on (0001) AlN in a large range of Ga fluxes at a substrate temperature of 740 °C. This feature allows for atomic layer epitaxy (ALE) of GaN on AlN by alternate exposure to Ga and N flux. We show that, at a growth temperature of 740 °C, one ALE cycle leads to the formation of a two-dimensional GaN layer, whereas further cycles lead to the formation of GaN quantum dots following a Stranski–Krastanov growth mode. This behavior is confirmed by atomic force microscopy, transmission electron microscopy, and cathodoluminescence.

Microscopic control of ZnSe–GaAs heterojunction band offsets

Abstract ZnSe-GaAs heterostructures were synthesized by molecular beam epitaxy at room temperature on GaAs(1 1 0) substrates cleaved in situ, and at 290–320°C (Zn/Se beam pressure ratio = 1) on epitaxial GaAs(1 0 0) layers grown on GaAs(1 0 0) wafers. The epitaxial structures were characterized in situ by photoemission spectroscopy and reflection high energy electron diffraction. The photoemission-determined valence band offset was ΔEv = 1.10 ± 0.05 eV for ZnSe- GaAs(1 1 0) heterojunctions and ΔEv = 0.78−0.83 ± 0.07 eV for ZnSe-GaAs(1 0 0) heterojunctions. This germanium layers (4–6 monolayers thick) fabricated at the interface prior to ZnSe deposition yield a decrease in the overall valence band discontinuity for both ZnSe-Ge-GaAs(1 1 0) and ZnSe-Ge-GaAs(1 0 0) heterojunctions.

Self-Assembled GaN Quantum Dots Grown by Plasma-Assisted Molecular Beam Epitaxy

It is shown that under specific growth conditions the strain relaxation of GaN deposited on AlN obeys the Stranski-Krastanov mechanism. As a consequence, it is demonstrated that, for both cubic and hexagonal phases, the growth of self-organized three-dimensional islands can be achieved. These islands behave as quantum dots, exhibiting optical properties dominated by localization effects.