David C. Ingram

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 ...

Metal/semiconductor phase transition in chromium nitride(001) grown by rf-plasma-assisted molecular-beam epitaxy

Structural and electronic properties of stoichiometric single-phase CrN(001) thin films grown on MgO(001) substrates by radio-frequency N plasma-assisted molecular-beam epitaxy, are investigated. In situ room-temperature scanning tunneling microscopy clearly shows the 1×1 atomic periodicity of the crystal structure as well as long-range topographic distortions which are characteristic of a semiconductor surface. This semiconductor behavior is consistent with ex situ resistivity measurements over the range 285 K and higher, whereas below 260 K, metallic behavior is observed. The resistivity-derived band gap for the high-temperature region, 71 meV, is consistent with the tunneling spectroscopy results. The observed electronic (semiconductor/metal) transition temperature coincides with the temperature of the known coincident magnetic (para-antiferro) and structural (cubic-orthorhombic) phase transitions.

Composition-dependent structural properties in ScGaN alloy films: A combined experimental and theoretical study

Experimental and theoretical results are presented regarding the incorporation of scandium into wurtzite GaN. Variation of the a and c lattice constants with Sc fraction in the low Sc concentration regime (0%–17%) are found that can be well explained by the predictions of first-principles theory. The calculations allow a statistical analysis of the variations of the bond lengths and bond angles as functions of Sc concentration. The results are compared to predictions from both a prior experimental study [Constantin et al., Phys. Rev. B 70, 193309 (2004)] and a prior theoretical study [Farrer and Bellaiche et al. Phys. Rev. B 66, 201203(R) (2002)]. It is found that the ScGaN lattice can be very well modeled as being wurtzitelike but with local lattice distortions arising from the incorporation of the Sc atoms. Effects of the addition of Sc on the stacking order for a large Sc fraction is also studied by high resolution transmission electron microscopy. The results show the existence of stacking faults, and...

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.

Hydrogen analysis as a function of depth for hydrogenous films and polymers by proton recoil detection

Abstract A quantitative analysis of the hydrogen content of a material, as a function of depth, may be achieved by energy analysis of protons recoiled from a sample of the material subject to MeV helium ion bombardment. The work presented includes an experimental determination of the cross-section for recoil production which is compared to theoretical predictions. Data is presented from the analysis of hydrogenous films and polymers. For the latter, the analysis has been performed on MeV heavy ion implanted material as well as pristine samples. The particular advantages of this technique are that it is compatible with and complementary to Rutherford backscattering (RBS) analysis for heavy element analysis, and it is less destructive and has a higher cross section than other nuclear reaction techniques.

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.

Ellipsometric measurements of molecular‐beam‐epitaxy‐grown semiconductor multilayer thicknesses: A comparative study

Variable angle of incidence spectroscopic ellipsometry, cross‐sectional transmission electron microscopy, and Rutherford backscattering are used to measure heterojunction layer thicknesses in the same AlGaAs/GaAs sample. All three techniques yield the same thickness values within error limits. Two additional samples were implanted with 750‐keV Ga ions to fluences of 5×1015 and 1016 cm−2, respectively, and results of diagnostics measurements by the three techniques compared. The three techniques are found to complement each other in providing useful information.

The effect of MeV ion irradiation on the hydrogen content and resistivity of direct ion beam deposited diamondlike carbon

Abstract Diamondlike carbon films fabricated by direct ion beam deposition have been irradiated with 6.4 MeV fluorine and 1 MeV gold ions. Both beams reduce the hydrogen content with the fluorine beam being much more efficient than the gold beam. The resistivity of the materials is also reduced by both beams with the fluorine beam producing a much larger drop in resistivity and a lower fluence than the gold beam. It is concluded that defects produced by electronic energy loss of the bombarding ions are responsible for both the loss of hydrogen and the change in resistivity. The magnitude of both of these effects is reduced with increasing ion mass as the proportion of nuclear to electronic energy loss is increased. This is due to the production of retrapping centers in the case of hydrogen and ion induced annealing in the case of resistivity. There is a threshold in fluence for both effects and this may be associated with fluctuations in the rate of deposition of the electronic energy loss along an individual ion track.