Hamad Al-Brithen

Molecular beam epitaxy control of the structural, optical, and electronic properties of ScN(001)

Scandium nitride (001) oriented layers have been grown on magnesium oxide (001) substrates by molecular beam epitaxy using a rf-plasma source and a scandium effusion cell. The Sc/N flux ratio is found to be critical in determining the structural, optical, and electronic properties of the grown epitaxial layers. A distinct transition occurs at the point where the Sc/N flux ratio equals 1, which defines the line between N-rich and Sc-rich growth. Under N-rich conditions, the growth is epitaxial, and the surface morphology is characterized by a densely packed array of square-shaped plateaus and four-faced pyramids with the terraces between steps being atomically smooth. The films are stoichiometric and transparent with a direct optical transition at 2.15 eV. Under Sc-rich conditions, the growth is also epitaxial, but the morphology is dominated by spiral growth mounds. The morphology change is consistent with increased surface diffusion due to a Sc-rich surface. Excess Sc leads to understoichiometric layers with N vacancies which act as donors. The increased carrier density results in an optical reflection edge at 1 eV, absorption below the 2.15 eV band gap, and a drop in electrical resistivity.

Crystalline phase and orientation control of manganese nitride grown on MgO(001) by molecular beam epitaxy

The phase and orientation of manganese nitride grown on MgO(001) using molecular beam epitaxy are shown to be controllable by the manganese/nitrogen flux ratio as well as the substrate temperature. The most N-rich phase, θ phase (MnN), is obtained at very low Mn/N flux ratio. At increased Mn/N flux ratio, the next most N-rich phase, the η phase (Mn3N2), is obtained having its c axis normal to the surface plane. Further increasing the Mn/N flux ratio, the η phase (Mn3N2) having its c axis in the surface plane is obtained. Finally, the e phase (Mn4N) is obtained at yet higher Mn/N flux ratio. The structural phase variation with Mn/N flux ratio is due to the kinetic control of the surface chemical composition, which determines the energetically most favorable phase. For a given Mn/N flux ratio, the phase is also found to be a function of the substrate temperature, with the less N-rich phase occurring at the higher substrate temperature. The change of phase with temperature is attributed to the change in the ...

Molecular beam epitaxial growth of atomically smooth scandium nitride films

High quality scandium nitride films have been grown on magnesium oxide (001) substrates by molecular beam epitaxy using a rf plasma source for nitrogen. Both reflection high energy electron diffraction and x-ray diffraction confirm that these films have (001)-orientation. Atomic force microscopy reveals a surface morphology consisting of large plateaus and pyramids. The plateaus are found to be atomically smooth and have a 1×1 surface structure, as revealed by in situ scanning tunneling microscopy.

Structural and magnetic properties of η-phase manganese nitride films grown by molecular-beam epitaxy

Face-centered tetragonal (fct) η-phase manganese nitride films have been grown on magnesium oxide (001) substrates by molecular-beam epitaxy. For growth conditions described here, reflection high energy electron diffraction and neutron scattering show primarily two types of domains rotated by 90° to each other with their c axes in the surface plane. Scanning tunneling microscopy images reveal surface domains consisting of row structures which correspond directly to the bulk domains. Neutron diffraction data confirm that the Mn moments are aligned in a layered antiferromagnetic structure. The data are consistent with the fct model of G. Kreiner and H. Jacobs for bulk Mn3N2 [J. Alloys Compd. 183, 345 (1992)].

ScGaN alloy growth by molecular beam epitaxy: Evidence for a metastable layered hexagonal phase

Alloy formation in ScGaN is explored using rf molecular beam epitaxy over the Sc fraction range x=0-100%. Optical and structural analysis show separate regimes of growth, namely (I) wurtzitelike but having local lattice distortions in the vicinity of the Sc{sub Ga} substitutions for small x (x{ =}0.54). In regimes I and III, the direct optical transition decreases approximately linearly with increasing x but with an offset over region II. Importantly, it is found that for regime I, an anisotropic lattice expansion occurs with increasing x in which a increases much more than c. These observations support the prediction of Farrer and Bellaiche [Phys. Rev. B 66, 201203-1 (2002)] of a metastable layered hexagonal phase of ScN, denoted h-ScN.

Ga/N flux ratio influence on Mn incorporation, surface morphology, and lattice polarity during radio frequency molecular beam epitaxy of (Ga,Mn)N

The effect of the Ga/N flux ratio on the Mn incorporation, surface morphology, and lattice polarity during growth by rf molecular beam epitaxy of (Ga,Mn)N at a sample temperature of 550 °C is presented. Three regimes of growth, N-rich, metal-rich, and Ga-rich, are clearly distinguished by reflection high-energy electron diffraction and atomic force microscopy. Using energy dispersive x-ray spectroscopy, it is found that Mn incorporation occurs only for N-rich and metal-rich conditions. For these conditions, although x-ray diffraction in third order does not reveal any significant peak splitting or broadening, Rutherford backscattering clearly shows that Mn is not only incorporated but also substitutional on the Ga sites. Hence, we conclude that a MnxGa1−xN alloy is formed (in this case x∼5%), but there is no observable change in the c-axis lattice constant. We also find that the surface morphology is dramatically improved when growth is just slightly metal rich. When growth is highly metal-rich, but not G...

Phase stability, nitrogen vacancies, growth mode, and surface structure of ScN(0 0 1) under Sc-rich conditions

Rocksalt structure scandium nitride films have been grown on magnesium oxide (0 0 1) substrates by molecular beam epitaxy using a radio frequency plasma source for nitrogen. The case of Sc-rich growth conditions, which occurs when the scandiumflux JSc exceeds the nitrogen flux JN; is discussed. Despite the excess Sc during growth, reflection highenergy electron diffraction and X-ray diffraction (XRD) show that these films have only a single orientation which is (0 0 1), and ion channeling confirms the good crystallinity. Rutherford backscattering shows that these films are offstoichiometric, and this is found to be directly related to variations in the nitrogen, not the scandium, content by secondary ion mass spectrometry. High-resolution XRD reciprocal lattice mapping shows that these variations in the nitrogen content are related to the existence of the N-vacancies. It is concluded that Sc-rich growth leads to the incorporation of N-vacancies into the crystal structure, the concentration of which depends on the Sc/N flux ratio. Additionally, excess scandiumconditions at the surface are explored by in situ scanning tunneling m icroscopy. The observed wider terrace widths as compared to N-rich growth are due to an increased surface diffusion which is attributed to a Sc-rich, metallic surface structure. Combined with the large dislocation density, the enhanced diffusion results in a predominant spiral growth mode. r 2002 Elsevier Science B.V. All rights reserved.

Incorporation of manganese into semiconducting ScN using radio frequency molecular beam epitaxy

The incorporation of manganese into semiconducting ScN, using radio frequency molecular beam epitaxy, has been investigated. X-ray diffraction and reflection high energy electron diffraction measurements show the face-centered tetragonal rocksalt-type crystal structure with Sc and Mn cations and N anions. In addition to the solute incorporation into the lattice, which is clear from the positions of the diffraction peaks, atomic force microscopy images show that the surface of the alloy grown at TS⩽518°C contains dot-like features, indicating surface accumulation. The areal dot density is found to decrease as the growth temperature increases, whereas the Mn incorporation increases at 518 °C. This behavior is suggestive of a thermally activated process, and it is well explained by an Arrhenius law, giving an activation energy (diffusion barrier) of 0.67 eV. Increasing the growth temperature to 612 °C leads to an increased desorption rate, resulting in little Mn incorporation. It has been found that the grow...

Mixing Rocksalt and Wurtzite Structure Binary Nitrides to Form Novel Ternary Alloys: ScGaN and MnGaN

Combination of tetrahedral and octahedral based nitrides are explored. The two cases of MnGaN and ScGaN with low Mn and Sc fractions are examined. It is found that for the MnGaN case, the Mn is incorporated under N rich conditions with little lattice change. However, for the ScGaN case, the Sc is incorporated onto the Ga sites but with a local bond angle distortion.

Surface reconstructions of cubic gallium nitride (001) grown by radio frequency nitrogen plasma molecular beam epitaxy under gallium-rich conditions

Cubic GaN has been grown under gallium (Ga)-rich growth conditions using radio frequency nitrogen plasma molecular beam epitaxy on MgO(001) substrates. Reflection high energy electron diffraction patterns indicate the smoothness of the c-GaN surface and show 2× and even 8× periodicities after the growth at sample temperature Ts<200°C and 1×1 at higher temperatures. Scanning tunneling microscopy images reveal a sequence of variant surface reconstructions including c(4×12), 4×7, c(4×16), 4×9, c(4×20), and 4×11. These variant reconstructions correspond to slightly different Ga adatom coverages all less than 1∕4 ML, with 4×11 having the highest, and c(4×12) the lowest Ga coverage. The electronic properties of these six variant reconstructions are investigated, and they are found to have a metallic nature.