Lawrence H. Robins

Steady-state and time-resolved photoluminescence from relaxed and strained GaN nanowires grown by catalyst-free molecular-beam epitaxy

We report steady-state and time-resolved photoluminescence (TRPL) measurements on individual GaN nanowires (6–20 μm in length, 30–940 nm in diameter) grown by a nitrogen-plasma-assisted, catalyst-free molecular-beam epitaxy on Si(111) and dispersed onto fused quartz substrates. Induced tensile strain for nanowires bonded to fused silica and compressive strain for nanowires coated with atomic-layer-deposition alumina led to redshifts and blueshifts of the dominant steady-state PL emission peak, respectively. Unperturbed nanowires exhibited spectra associated with high-quality, strain-free material. The TRPL lifetimes, which were similar for both relaxed and strained nanowires of similar size, ranged from 200 ps to over 2 ns, compared well with those of low-defect bulk GaN, and depended linearly on nanowire diameter. The diameter-dependent lifetimes yielded a room-temperature surface recombination velocity S of 9×103 cm/s for our silicon-doped GaN nanowires.

Investigation of the structure of barium titanate thin films by Raman spectroscopy

Raman spectroscopy was used to examine the structure of barium titanium oxide thin films grown by metal‐organic chemical vapor deposition (MOCVD) and laser‐assisted deposition. The spectra were compared with the spectra of a ceramic specimen and a single crystal. Raman peaks specific to the tetragonal ferroelectric phase of BaTiO3 were seen in the spectra of several films. Other Raman peaks were ascribed to impurity (non‐BaTiO3) phases in the films or to the substrates (fused quartz, MgO). Some of the Raman peaks showed a strong polarization dependence. The MOCVD films were also characterized by x‐ray diffraction, energy‐dispersive x‐ray spectroscopy, and transmission electron microscopy. The film‐to‐film variation of the strength of BaTiO3 features in the Raman spectrum, relative to impurity‐phase features, was qualitatively consistent with the x‐ray diffraction and electron microscopy results. Spatially resolved Raman measurements showed that the structure of the laser‐deposited film varies significantl...

Growth defects in diamond films

Abstract : Growth defects in diamond films grown by plasma-assisted chemical vapor deposition (CVD) were studied by high resolution electron microscopy. Several features of the microstructure were resolved and their importance to the growth of the diamond film was evaluated. The observations included various twin boundaries of the type sigma=3 as well as sigma=9, sigma=27 and sigma=81, which form by an interaction of lower order twins. These higher order boundaries, are loci of intersection points of growing planes on two adjacent twins and can serve as an indicator for the local crystal growth direction. The central nucleation site for the growing planes in many cases can be traced back to a quintuplet twin point. A twin quintuplet has five reentrant angles and thus serves as a preferred nucleation site for new planes as the crystal grows.

Optical band gap dependence on composition and thickness of InxGa1−xN (0<x<0.25) grown on GaN

Band gap measurements have been carried out in strained and relaxed InxGa1−xN epilayers with x<0.25. Values of x were determined from x-ray diffraction of relaxed films. The lowest energy absorption threshold, measured by transmittance, was found to occur at the same energy as the peak of the photoluminescence spectrum. Bowing parameters for both strained and relaxed films were determined to be 3.42 and 4.11 eV, respectively. The dependence of the band gap shift, ΔEg, on strain is presented.

Refractive index study of AlxGa1−xN films grown on sapphire substrates

A prism coupling method was used to measure the ordinary (no) and extraordinary (ne) refractive indices of AlxGa1−xN films, grown by hydride vapor phase epitaxy (HVPE) and metalorganic chemical vapor deposition (MOCVD) on sapphire, at several discrete wavelengths from 442 nm to 1064 nm. In addition, spectroscopic transmittance and reflectance, correlated with the prism coupling results, were used to measure no as a continuous function of wavelength between the band gap of each sample (255 nm to 364 nm, depending on Al fraction) and 2500 nm. The Al mole fractions (x), determined by energy dispersive x-ray spectroscopy (EDS), were x=0.144, 0.234, 0.279, 0.363, 0.593, and 0.657 for the HVPE-grown samples, and x=0.000, 0.419, 0.507, 0.618, 0.660, and 0.666 for the MOCVD-grown samples. The maximum standard uncertainty in the EDS-determined value of x was ±0.02. The maximum standard uncertainty in the refractive indices measured by prism coupling was ±0.005 and a one-Sellmeier-term equation was adequate to fit ...

Optical and structural study of GaN nanowires grown by catalyst-free molecular beam epitaxy. I. Near-band-edge luminescence and strain effects

GaN nanowires with diameters of 50–250 nm, grown by catalyst-free molecular beam epitaxy, were characterized by photoluminescence (PL) and cathodoluminescence (CL) spectroscopy at temperatures from 3 to 297 K, and high-resolution x-ray diffraction (HRXRD) at ≈297 K. The lattice parameters of the nanowires, determined by HRXRD, are in good agreement with recent measurements of freestanding quasisubstrates; the relative variation of the lattice parameters between the nanowires and quasisubstrates is ≤2×10−4. Both as-grown samples, which contained nanowires oriented normal to the substrate as well as a rough, faceted matrix layer, and dispersions of the nanowires onto other substrates, were examined by PL and CL. The D0XA line at 3.472 eV, ascribed to excitons bound to shallow donors, was observed in low-temperature PL and CL; free-exciton lines (XA at ≈3.479 eV, XB at ≈3.484 eV) were observed in PL at temperatures between 20 and 80 K. The linewidth of the D0XA peak was larger in PL spectra of the nanowires ...

Extended x-ray absorption fine structure study of AlxGa(1−x)N films

Extended x-ray absorption fine structure above the Ga–K edge has been used to study the local structure of AlxGa1−xN films grown by metal organic chemical vapor deposition. With increasing Al content, x, the Ga–N bond length decreases, but much less than the average bond length. On the other hand, the x dependence of the Ga–Ga and Ga–Al distances does follow the variation of the average cation–cation distance. We conclude that bond angle distortions accommodate the differences between the Ga–N and Al–N bond lengths.

Sputtered amorphous carbon nitride films

The recent announcement of the synthesis of C 3 N 4 has increased interest in this unique material. Carbon nitride may have several useful applications as wear and corrosion resistant coatings, electrical insulators, and optical coatings. We have produced amorphous carbon nitride coatings containing up to 40% nitrogen using planar magnetron RF sputtering with and without an ion beam in a nitrogen atmosphere. Both wavelength dispersive x-ray spectrometry (WDX) and x-ray photoelectron spectroscopy (XPS) indicate this composition. Coatings up to 2 μm thick were produced on alumina, silicon, SiO 2 , and glass substrates using a graphite target. Films with transparency greater than 95% in the visible wavelengths and harder than silicon have been produced. The properties of these films are correlated with composition, fabrication, conditions, and subsequent heat treatments. A scanning tunneling microscope (STM) and transmission electron microscopy (TEM) were used to characterize the morphology of the films. XPS studies confirm the stability of a carbon nitrogen phase up to 600 °C. Compositional variations were determined with secondary ion mass spectrometry (SIMS) depth profiling, and the Raman spectra are compared with those of carbon and carbon nitride films prepared by other methods.

Optical and structural study of GaN nanowires grown by catalyst-free molecular beam epitaxy. II. Sub-band-gap luminescence and electron irradiation effects

GaN nanowires with diameters of 50–250 nm, grown by catalyst-free molecular beam epitaxy, were characterized by photoluminescence (PL) and cathodoluminescence (CL) spectroscopy at temperatures from 3 to 297 K. Both as-grown samples and dispersions of the nanowires onto other substrates were examined. The properties of the near-band-edge PL and CL spectra were discussed in Part I of this study by [Robins et al. [L. H. Robins, K. A. Bertness, J. M. Barker, N. A. Sanford, and J. B. Schlager, J. Appl. Phys. 101,113505 (2007)]. Spectral features below the band gap, and the effect of extended electron irradiation on the CL, are discussed in Part II. The observed sub-band-gap PL and CL peaks are identified as phonon replicas of the free-exciton transitions, or excitons bound to structural defects or surface states. The defect-related peaks in the nanowires are correlated with luminescence lines previously reported in GaN films, denoted the Y lines [M. A. Reshchikov and H. Morkoc, J. Appl. Phys. 97, 061301 (2005)...

Cathodoluminescence measurement of strained alumina single crystals

Abstract Alumina single crystals indented with a Vickers hardness indenter were investigated by cathodoluminescence (CL) imaging and spectroscopy in a scanning electron microscope. The spatial resolution of the CL images was approximately 0·4 μm. CL spectra were measured with wavelength resolution of 1·0 nm in the wavelength range of 200 to 900 nm; in the immediate region of the ruby lines (693 nm), spectra were measured with a resolution of 0·15 nm. Stress-induced frequency shifts of the ruby lines were utilized to measure the residual stresses in the region surrounding the indentation. Both hydrostatic and non-hydrostatic stresses were measured. The magnitude of the stresses was observed to vary with distance from the center of the indent and also with crystallographic orientation. After the samples were annealed at 1600°C for 10 h, no measurable stresses remained either in the center of the indent or in the surrounding area.