We have grown a variety of isolated GaN nanowires using gas-source molecular beam epitaxy (MBE) and characterized their structural and optical properties. The nanowires have demonstrated a number of promising materials characteristics, including low defect density and high luminescence intensity. Well separated nanowires formed spontaneously on Si(111) substrates after deposition of a thin AlN buffer layer. Metal catalysts were not used. X-ray diffraction indicates that the c and a lattice parameters are within 0.01 % of the lattice parameters of bulk GaN. Transmission electron microscopy (TEM) revealed the nanowires to be free of dislocations and stacking faults, although a GaN matrix layer growing at the base of the wires was found to have a high density of basal plane stacking faults. The room temperature photoluminescence (PL) intensity compared favorably with a free-standing film of high quality GaN. Several features of the low temperature PL spectra also indicated that the nanowires had few structural defects or chemical impurities. Finally, electrical characterization of dispersed nanowires demonstrated that efficient electrical contacts could be made and that the resistivity of the nanowires was comparable to that of bulk material.

We present a new contact resonance force microscopy (CRFM) imaging technique, isomorphic contact resonance (iso-CR), that acquires data at a constant contact resonance (CR) frequency, and hence constant tip-sample contact stiffness across the scan area. Constant CR frequency is obtained by performing force versus distance measurements to vary the applied force at each pixel (i.e. force-volume mapping mode). The CR frequency increases with increasing applied force; thus, a carefully selected target frequency will be reached for most pixels at some point in the force versus distance curve. In the iso-CR mode, the cantilever maintains an invariant vibrational shape and a constant environmental damping, thus simplifying interpretation of amplitude and quality factor contrast compared to conventional CRFM. Iso-CR imaging of a piezoelectric AlN thin film sample is demonstrated. Iso-CRFM images were obtained by mechanically driving the base of the cantilever, and iso-CR piezoresponse force microscopy (iso-CR-PFM) images were obtained by electrically biasing the tip. The PFM phase images reveal that the sample contains nanoscale Al-polar (or ‘up’) and N-polar (or ‘down’) domains, with ≈180° phase contrast between oppositely polarized domains. The PFM amplitude and Q-factor images also show ‘up’ vs. ‘down’ domain contrast, which decreases with increasing CR frequency. The frequency-dependent amplitude and Q contrast is ascribed to a frequency-dependent electrostatic contribution to the signal. Domain contrast is not observed in the CRFM (mechanically driven) images. To summarize, the iso-CR capability to control the resonance frequency across multiple excitation schemes helps elucidate the origin of the electromechanical and nanomechanical image contrast.

The contact resonance (CR) of a surface coupled atomic force microscope (AFM) cantilever can act as an amplifier of AC surface motion for piezoresponse force microscopy and related methods. However, the amplifier properties of the CR vary depending on tip-sample boundary conditions, leading to the appearance of displacement amplitude contrast when only stiffness contrast exists. It was recently proposed that the shape of the vibrating cantilever as a function of CR frequency could be analytically modeled and a shape factor calibration could be applied. Here, we demonstrate an experimental reconstruction of the contact resonance shape factor that can be used to quantify surface displacements in AFM measurements, without reliance on analytical models with uncertain input parameters. We demonstrate accurate quantification of surface displacement in periodically poled lithium niobate and pave the way for quantification of extremely small surface strains in the future.

The carrier concentration in as-grown ensembles of n-type GaN nanowires was determined by Raman spectroscopy of the coupled longitudinal phonon–plasmon (LPP+) mode and modeling of the carrier concentration dependence of the LPP+ frequency. The Raman measurements and analyses enabled estimation of the carrier concentration in single-nanowire devices fabricated from the as-grown ensembles. The nanowires were grown by plasma-assisted molecular beam epitaxy in either of the two growth systems. Twelve samples were examined, of which 11 samples were Si-doped and one was undoped. The Raman-measured carrier concentrations in the Si-doped samples ranged from (5.28 ± 1.19) × 1016 cm−3 to (6.16 ± 0.35) × 1017 cm−3. For a subset of samples grown with varying Si cell temperature, from 1125 °C to 1175 °C, the carrier concentration was found to be an Arrhenius function of Si cell temperature, with activation energy of 6.281±0.011 eV. Co-illumination by an above band gap UV laser (325 nm, excitation intensity = 0.7 W/cm2 or 4.5 W/cm2) induced small increases in carrier concentration, relative to illumination by the Raman excitation laser alone (633 nm, excitation intensity ≈100 kW/cm2). The lowest Si-doped sample showed the largest increase in carrier concentration, (6.3 ± 4.8) × 1015 cm−3 with UV excitation intensity of 0.7 W/cm2. These results imply that, even in the absence of UV illumination, surface depletion does not have a significant effect on the Raman carrier concentration measurements. Immersion in a high-dielectric-constant oil (ε = 2.24) caused downshifts of similar magnitude in the LPP+ frequencies of undoped and doped nanowires. This result implies that the LPP+ mode has bulk plasmon rather than surface plasmon character, because immersion in a high-dielectric-constant medium is predicted to cause a large decrease in the surface plasmon frequency, which would induce a larger LPP+ downshift in doped than undoped nanowires. A surface optical (SO) phonon peak was observed in each sample in air at ≈96.4% of the LPP+ frequency. The SO frequency decreased to ≈93.1% of the LPP+ frequency upon oil immersion, as predicted by a simple dielectric model.

Variable intensity photoconductivity (PC) performed under vacuum at 325 nm was used to estimate drift mobility (μ) and density (σs) of negative surface charge for c-axis oriented Si-doped GaN nanowires (NWs). In this approach, we assumed that σs was responsible for the equilibrium surface band bending (∅) and surface depletion in the absence of illumination. The NWs were grown by molecular beam epitaxy to a length of approximately 10 μm and exhibited negligible taper. The free carrier concentration (N) was separately measured using Raman scattering which yielded N = (2.5 ± 0.3) × 1017 cm−3 for the growth batch studied under 325 nm excitation. Saturation of the PC was interpreted as a flatband condition whereby ∅ was eliminated via the injection of photogenerated holes. Measurements of dark and saturated photocurrents, N, NW dimensions, and dimensional uncertainties, were used as input to a temperature-dependent cylindrical Poisson equation based model, yielding σs in the range of (3.5 to 7.5) × 1011 cm−2 and μ in the range of (850 to 2100) cm2/(V s) across the (75 to 194) nm span of individual NW diameters examined. Data illustrating the spectral dependence and polarization dependence of the PC are also presented. Back-gating these devices, and devices from other growth batches, as field effect transistors (FETs) was found to not be a reliable means to estimate transport parameters (e.g., μ and σs) due to long-term current drift. The current drift was ascribed to screening of the FET back gate by injected positive charge. We describe how these gate charging effects can be exploited as a means to hasten the otherwise long recovery time of NW devices used as photoconductive detectors. Additionally, we present data illustrating comparative drift effects under vacuum, room air, and dry air for both back-gated NW FETs and top-gated NW MESFETs.

The recent announcement of the synthesis of C3N4 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, SiO2, 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.

Low electron energy cathodoluminescence (LEECL) was used to examine polishing-induced damage in a bulk high-pressure grown GaN single-crystal platelet. The Ga-polarity face of the platelet was mechanically polished; chemically-assisted ion-beam etching (CAIBE) to a depth of 200 nm was performed on a portion of this face. Low-temperature (15 K) CL spectra of the polished-only and polished+CAIBE regions of the Ga-face were taken at 2.8 kV, 5.4 kV, and 10.6 kV (corresponding to average electron penetration depths of 19 nm, 56 nm, and 170 nm). The low-temperature CL spectrum of the unpolished, N-polarity face was taken at 10.6 kV. In the near-band-edge region, all the CL spectra from the Ga-polarity face show a narrow peak near 3.47 eV, ascribed to donor-bound exciton recombination, and several overlapping peaks at lower energy (3.1 eV to 3.4 eV), ascribed to defect-related levels or to donor-acceptor pair recombination. Functional curve-fitting analysis enabled deconvolution of the spectra into the sum of an asymmetric peak (the donor-bound exciton) and several symmetric Gaussian peaks (the lower energy, defect-related or donor-acceptor peaks). The linewidth of the donor-bound exciton peak decreased with increasing penetration depth, and also decreased on going from the polished-only to the polished+CAIBE region. The relative intensity of a defect-related peak at ≈3.325 eV showed a similar decreasing trend with increasing penetration depth or with CAIBE treatment. The LEECL results suggest that the thickness of the polishing damage layer is approximately 400 nm; the 200 nm CAIBE step is thus insufficient to completely remove the damage.

Cathodoluminescence (CL) spectroscopy in a scanning electron microscope was used to identify and to map the spatial distribution of luminescent defects in a synthetic diamond single crystal. Several defect CL bands were observed in the 1.5-3.5 eV region: (i) a band with a zero-phonon line at 2.156 eV, attributed to a center containing nitrogen and atomic vacancies; (ii) a broadband centered at ∼2.2 eV, tentatively attributed to a boron-containing center; (iii) a doublet line at 2.33 eV, attributed to a nitrogen-containing center; (iv) a zero-phonon line at 2.555 eV, attributed to a nickel-containing center; (v) a broadband centered at ∼2.85 eV, attributed to a dislocation-related center; and (vi) a zero-phonon line at 3.188 eV, attributed to a center containing nitrogen and a carbon interstitial. Lines due to free and acceptor-bound excitons were observed in the 5.0-5.4 eV region. The spatial variation of the CL was examined in the vicinity of regions of relatively high dislocation density (∼106 dislocations cm−2), which had been found in a previous x-ray diffraction imaging experiment. A quantitative analysis was made of the spatial variation of the band intensities. Upon moving from a relatively defect-free region to the center of a high dislocation density region, the intensities of defect bands (i) and (v) increased by very large factors (these bands were observed only within the high dislocation density regions); the intensity of defect band (vi) increased by a factor of ∼2; the acceptor-bound exciton intensity increased by a factor of 1.3; the intensities of defect bands (ii)-(iv) decreased by a factor of ∼2; and the free exciton intensity decreased by a factor of ∼7.5.

Understanding the source of variation of the optical properties within an ensemble of nanowires (NWs) is an important step toward fabrication of NWs with more uniform and better-controlled properties. This in turn will facilitate the development of complex architectures for device applications. Analysis of gold- and copper-catalyzed zinc oxide (ZnO) NWs grown in a high temperature tube furnace on a sapphire substrate shows that the structural and optical properties vary with NW growth sites on the substrate. Results show that there are systematic changes in the optical properties of the ZnO NWs from the center to the edge of the substrate, and also from "upstream" to "downstream" along the gas flow direction, implying changes in the availability of the source material and catalyst over the substrate surface during the growth. Photoluminescence microscopy has been used to map out these changes and unravel the growth patterns. We observe two distinct trends that are labeled the "edge" trend and the "gas flow" trend, respectively, and are linked to two growth phases. The first phase starts while the growth tube temperature and gas flow have not yet reached their final steady states. In this phase, due to the limited availability of the source material in gas phase, only metal nanodroplets that are located at the substrate edges have the advantage of early growth. While the edge NWs continue their growth, the second phase starts when the growth tube temperature and gas flow reach reasonable stabilities. Our results show that these two phases are more pronounced in the case of copper-catalyzed NWs. In the first phase, NWs have a better chance to grow at the substrate edges. In the second phase, copper diffuses downstream, causing an interesting variation in the optical properties of NWs. Numerous morphology examinations of NWs show that the variation in emission is related to the change in the ratio of the surface area to the bulk volume of the NWs within an ensemble of NWs. The patterns of the ZnO NW optical properties can be used as an indicator to follow the growth patterns of the NWs across the substrate.

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.

We designed experiments to investigate the role of dislocation density on the performance of Schottky diodes fabricated on a GaN material grown conventionally and by pendeo-epitaxy. Devices of varying geometries were fabricated on low defect density GaN regions grown selectively via pendeo-epitaxy. In addition, corresponding devices were fabricated on the conventional GaN material with a high density of dislocations. Schottky diodes fabricated on pendeo-material showed nearly two orders of magnitude lower leakage current and displayed improved ideality factor, while diodes built on a conventional material displayed nonideal characteristics.

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 than in quasisubstrates. The broadening of the D0XA peak in PL of the nanowires is tentatively ascribed to inhomogeneous stress/strain. In addition, the D0XA peak was significantly broader in CL than in PL spectra of the same nanowire samples. The further large broadening of the CL peak (as compared to PL) is tentatively ascribed to Stark effect broadening, induced by the electric fields of trapped charges that are created in the CL excitation process.

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)]. The CL was partially quenched by electron beam irradiation for an extended time; the quenching was stronger for the free and shallow-donor-bound exciton peaks than for the defect-related peaks. The quenching appeared to saturate at high irradiation dose (with final intensity ≈30% of initial intensity) and was reversible on thermal cycling to room temperature. The electron irradiation-induced quenching of the CL is ascribed to charge injection and trapping phenomena.

Inductively coupled plasma optical emission spectroscopy is shown to confirm a recent correlation between photoluminescence (PL) peak energy for AlGaAs epitaxial films and the Al mole fraction x of those films. These two methods also agree within their expanded uncertainties with the Al composition as determined by growth rate measurements using reflection high energy electron diffraction intensity at the time of specimen growth. No systematic variations between the three methods as a function of Al mole fraction were observed. The lowest uncertainty was found in the PL measurements, allowing certification of Al mole fraction x in standard reference materials to an expanded uncertainty of 0.003 for x<0.35. Details of the uncertainty analysis, as well as possible improvements in those uncertainties, are discussed.