Conductive Atomic Force Microscopy Images Research Articles

Microanalyses of the reverse-bias leakage current increase in the laser lift off GaN-based light emitting diodes

Microanalyses of the reverse-bias leakage current increase in the laser lift off (LLO) GaN-based light emitting diodes were performed. It was found that the amount of dislocations did not obviously change after LLO process in our experiments. The conductive-atomic force microscopy images and transmission electron microscope (TEM) images results revealed that almost all screw dislocations became to be related with leakage current and the current intensity increased over 100 times after the LLO process; however, only nanopipes corresponded to leakage current in the sample without the laser irradiation. Scanning TEM images indicated microstructure changes induced by LLO process. Amount of point defects around dislocations might be responsible for the increase in leakage current by providing more levels for tunneling.

Imaging of Recording Marks and Their Jitters With Different Writing Strategy and Terminal Resistance of Optical Output

The relation between recording mark formation and jitter value with radial orient spot (ROS) type laser spot recording was investigated by using a new imaging method of conductive-atomic force microscopy (C-AFM). The readout performances of recording marks were studied by changing the terminal resistance of optical pickup head and writing strategies. The results of clear C-AFM images showed that is possible to adjust the conditions of terminal resistance and writing strategies to have better readout signal performance.

Surface characterization of ultrathin La 0.75Sr 0.25MnO 3 epitaxial films on SrTiO 3 substrate

The surface characteristics of ultrathin La 0.75Sr 0.25MnO 3 (LSMO) films epitaxially grown on SrTiO 3 (0 0 1) substrate were studied as a function of film thickness. The X-ray photoelectron spectroscopy (XPS) analysis displays that the ratio of Mn 3+/Mn 4+ in LSMO largely decreases with decreasing film thickness, which reduces the amount of itinerant electrons for electrical conduction. The conductive atomic force microscopy images also reveal that the fraction of conductive phase drastically decreases with decreasing film thickness to 5 nm, i.e. an insulative dead layer is formed. From the above observations, it is believed that, besides the strain effect, the formation of dead layer at the interface may also be related to the large reduction of Mn 3+ ions in the LSMO adjacent to the SrTiO 3 substrate.

Effect of P incorporation on aggregation of nanocrystallites in amorphous and nanocrystalline mixed-phase silicon thin films

We report the effects of P incorporation on the nanometer-scale structural and electrical properties of amorphous and nanocrystalline mixed-phase Si:H films. In the intrinsic and weakly P-doped (3×1018at/cm3) films, the nanocrystallites aggregate to cone-shaped structures. Conductive atomic force microscopy images showed high current flows through the nanocrystalline cones and a distinct two-phase structure in the micrometer range. Adding PH3 into the processing gas moved the amorphous/nanocrystalline transition to a higher hydrogen dilution ratio required for achieving a similar Raman crystallinity. In a heavily P-doped (2×1021at/cm3) film, the nanocrystalline aggregation disappeared, where isolated grains of nanometer sizes were distributed throughout the amorphous matrix. The heavily doped mixed-phase film with 5–10% crystal volume fraction showed a dramatic increase in conductivity. We offer an explanation for the nanocrystalline cone formation based on atomic hydrogen enhanced surface diffusion model, and propose that the coverage of P-related radicals on the existing nanocrystalline surface during film growth and the P segregation in grain boundaries are responsible for preventing new nucleation on the surface of the existing nanocrystallites, resulting in nanocrystallites dispersed throughout the amorphous matrix.

Near-interface magnetotransport in La0.75Sr0.25MnO3 epitaxial films on SrTiO3 substrate

The magnetotransport properties of La0.75Sr0.25MnO3 (LSMO) films epitaxially grown on SrTiO3 (0 0 1) substrate were studied. The Curie temperature (Tc) of LSMO films decreases from 300 to 105 K with a reduction in the film thickness to 5 nm. A clear metal–insulator transition is observed at a temperature consistent with Tc for films thicker than 10 nm. However, the 5 nm thick film displays an insulative characteristic along with a high magnetoresistence effect in a wide range of temperatures below and above Tc. A phase-separation phenomenon in the 5 nm thick film was demonstrated by thermally activated hopping transport between ferromagnetic metallic domains embedded in an insulative matrix at temperatures below Tc, but between paramagnetic metallic domains at temperatures above Tc. It was also confirmed by conductive atomic force microscopy images.

Study of Nanoscale Recorded Marks on Phase-Change Recording Layers and the Interactions With Surroundings

Conductive-atomic force microscopy (C-AFM) has been used for studying nanoscale recorded marks with different length on the phase-change recording layer of optical disks. Through C-AFM images, a comparison of nanoscale recorded marks on phase-change recording layer under different writing strategies and writing power has been taken. The comparison can help analyze the combination of writing strategy, writing power and laser pulse width. The various lengths of recorded marks for high density data storage have also been found out. The interactions between phase-change recording layers and their surroundings have also been studied. This study opens up a possibility to improve the capacity of data storage in today's commercial optical disks

Local current flow in amorphous and nanocrystalline mixed-phase silicon solar cells

Local current flow in amorphous and nanocrystalline mixed-phase n-i-p silicon solar cells is measured using conductive atomic force microscopy (C-AFM) and correlated to the material structure from Raman measurement. The C-AFM images show that the current is very low over the entire surface of the fully amorphous region. High current spikes are observed in the mixed-phase region, where the current spike corresponds to aggregations of nanocrystallites. The size of the nanocrystalline aggregations is on the order of a half micrometer in diameter. The density of the current spike increases from the mixed phase to the heavily nanocrystalline regions. A thick amorphous silicon buffer layer inserted between the p and i layers significantly reduced the magnitude of the current spike. The C-AFM measurements suggest that the mixed-phase cells can be considered as a two-phase parallel-connected diode structure, consistent with our previously proposed model.

Conductive atomic force microscopy studies of thin SiO2 layer degradation

The dielectric degradation of ultrathin (∼2nm) silicon dioxide (SiO2) layers has been investigated by constant and ramped voltage stresses with the conductive atomic force microscopy (CAFM). CAFM imaging shows clearly the lateral degradation propagation and its saturation. Current-voltage characteristics, performed at nanometer scale, show the trap creation rate in function of the stress condition. The critical trap density has been found.

Defects in nitride semiconductors: From nanoscale imaging to macroscopic device behavior

Scanning capacitance microscopy (SCM), atomic force microscopy (AFM), and conductive AFM are used to image the spatial distribution and electronic properties of threading dislocations in Al x Ga 1− x N/GaN epitaxial layers grown by molecular-beam epitaxy. SCM imaging reveals that GaN growth directly on SiC substrates leads to clustering of negatively charged dislocations at nucleation island boundaries, while incorporation of an AlN buffer leads to a random spatial distribution of negatively charged dislocations. Numerical simulations demonstrate that clustered dislocations are less effective in depleting mobile carriers. AFM and conductive AFM imaging reveal the presence of highly conductive threading dislocations which lead to excessive reverse-bias leakage current flow in Schottky diodes. Temperature-dependent current–voltage spectroscopy is used to develop a model for current flow via these dislocations based on a Frenkel–Poole emission process. On the basis of this model, heterostructures are designed to suppress this emission mechanism. Conductive AFM imaging and electrical measurements then confirm the expected suppression of leakage current and specifically of the Frenkel–Poole emission process, demonstrating the validity of both the proposed mechanism of current flow and the approach for leakage current suppression.

Local Current Flow in Mixed-Phase Silicon Solar Cells and Correlation to Light-Induced Open-Circuit Voltage Enhancement

AbstractWe use conductive atomic force microscopy (C-AFM) to measure the local current flow in the mixed-phase hydrogenated silicon n-i-p solar cell structure without the top ITO contact. The forward biased C-AFM images reveal that for the fully amorphous region the current is very low on the entire surface. However, high current spikes appear in the mixed-phase region, where the current spikes are correlated to the formation of nanocrystallite aggregations with a diameter of ~500 nm. Furthermore, the density of the current spikes increases from the mixed-phase to the substantially nanocrystalline regions. The nanocrystallite aggregation supports our previously proposed parallel-connected two-diode model for Voc drops with crystalline volume fraction and light-induced Voc increase in the mixed-phase solar cells. Adding a 50-nm thick a-Si:H buffer layer between the p and i layers significantly reduces the magnitude of the high current spikes, even the top morphology appears unaffected. This result is also consistent with the previously proposed two-diode model for explaining the carrier transport in the mixed-phase solar cells.

Local conductivity and surface photovoltage variations due to magnesium segregation in p-type GaN

Conductive atomic force microscopy (C-AFM) and surface photovoltage (SPV) microscopy were used to investigate local electronic structure in p-type GaN. C-AFM imaging revealed locally reduced forward- and reverse-bias conductivity near threading dislocations. In addition, regions near threading dislocations demonstrated significantly enhanced surface photovoltage response when compared to regions away from dislocations. Analytical treatment of the surface photovoltage as a function of pertinent material properties indicated that reduced background dopant concentration is the most likely cause for the increased SPV. Both reduced conductivity and enhanced surface photovoltage are shown to be consistent with Mg segregation to dislocation cores that results in regions of locally decreased electrically active Mg concentration surrounding the dislocations.

Correlated scanning Kelvin probe and conductive atomic force microscopy studies of dislocations in gallium nitride

Scanning Kelvin probe microscopy (SKPM) and conductive atomic force microscopy (C-AFM) have been used to image surfaces of GaN grown by molecular beam epitaxy. Detailed analysis of the same area using both techniques allowed imaging and comparison of both surface potential variations arising from the presence of negatively charged threading dislocations and localized current leakage paths associated with dislocations. Correlations between the charge state of dislocations, conductivity of current leakage paths, and dislocation type were thereby established. Analysis of correlated SKPM and C-AFM images revealed a density of negatively charged features of ∼3×108 cm−2 and a localized current leakage path density of ∼3×107 cm−2, with approximately 25% of the leakage paths spatially correlated with negatively charged dislocation features. Based on correlated topography and previous studies quantifying the densities of edge, screw, and mixed dislocations, our results suggested that dislocations having an edge component behave as though negatively charged while pure screw dislocations are solely responsible for the observed leakage paths and are uncharged.