Scanning Capacitance Microscopy Research Articles

Dielectric thickness dependence of capacitive behavior in graphene deposited on silicon dioxide

Scanning capacitance microscopy (SCM) is used to probe on nanoscale the capacitive behavior of graphene deposited on a SiO2∕Si n+ substrate (with SiO2 thickness of 300 or 100nm). The SCM tip provides the contact on graphene, while the Si n+ substrate acts as the backgate contact in the graphene/SiO2∕Si capacitor. The authors studied the screening by the graphene two-dimensional electron gas on the modulating potential applied between the backgate and the SCM tip. In particular, they determined the effect of the oxide thickness on the lateral distribution (i.e., screening length) and the density of the screening charge in graphene. Experimental results indicate that thinner oxide leads to higher screening charge density in the graphene sheet and to higher effectively biased area on graphene. This correspondingly increases the total capacitance (Ctot) of the graphene/SiO2∕Si capacitor. We evaluated the dependence on the dielectric thickness of the classical metal-oxide-semiconductor (MOS) capacitance (CMOS) and of the quantum capacitance (Cq) contributions to Ctot.

Impact of boron-interstitial clusters on Hall scattering factor in high-dose boron-implanted ultrashallow junctions

The Hall scattering factor rH has been determined for holes in high-dose boron-implanted ultrashallow junctions containing high concentrations of boron-interstitial clusters (BICs), combining scanning capacitance microscopy, nanospreading resistance, Hall effect, and secondary ion mass spectroscopy measurements. A value of rH=0.74±0.1 has been found in reference defect-free fully activated junctions, in good agreement with the existing literature. In the case of junctions containing high concentrations of immobile and electrically inactive BICs, and independently of the implant or the annealing process, the rH value has been found to be equal to 0.95±0.1. The increase in the rH value is explained in terms of the additional scattering centers associated to the presence of high concentrations of BICs.

Experimental observation of FIB induced lateral damage on silicon samples

Scanning spreading resistance microscopy (SSRM) and scanning capacitance microscopy (SCM) were used to analyze focused ion beam (FIB) induced lateral damage around milled structures on silicon. For this purpose, circular shaped structures were realized, and the influence of the implanted Ga dose (ranging from 10 15 to 10 17 cm −2) and of the patterned area dimension (diameters ranging from 1 to 4 μm) on the damage were examined. It is shown that the extension of the lateral damage around FIB milled structures is much larger than the area of the purposely irradiated regions (up to a factor of 5) and increases with both, Ga dose and pattern diameter. Besides, the unique capability of SCM and SSRM techniques for the detection of FIB induced damage is demonstrated. In particular, it is shown that their high sensitivity to low densities of defects allows detecting larger damaged areas compared to Atomic Force Microscopy (AFM) topography maps.

Implanted and irradiated SiO2∕Si structure electrical properties at the nanoscale

In this work, a conductive atomic force microscope (C-AFM), a scanning capacitance microscope (SCM), and a kelvin probe force microscope (KPFM) have been used to qualitatively study at the nanoscale the electrical properties of irradiated and implanted gate oxides of metal-oxide-semiconductor structures. These techniques have allowed to investigate the electrical conduction (C-AFM) and the presence of charge (SCM and KPFM) in the oxide of the analyzed structures. The impact of the energy of the impinging ions has also been qualitatively evaluated.

Scanning capacitance characterization of potential screening in InAs nanowire devices

We have used scanning capacitance microscopy and spectroscopy to examine the effects of micron-scale metal contacts, typically present in nanowire-based electronic devices, on carrier modulation and electrostatic behavior in InAs semiconductor nanowires. We observe a pronounced dependence of scanning capacitance images and spectra on distance between the scanning capacitance probe tip and nanowire contact up to distances of 3–4 μm. Based on the comparison of these data with results of finite-element electromagnetic simulations, we interpret these results as a consequence of electrostatic screening of the tip-nanowire potential difference by the large metal contact. These results provide direct experimental verification of contact screening effects on the electronic behavior of nanowire devices and are indicative of the importance of assessing and accounting for the effect of large-scale contact and circuit elements on the characteristics of nanoscale electronic devices generally.

Low-temperature and high magnetic field dynamic scanning capacitance microscope

We demonstrate a dynamic scanning capacitance microscope (DSCM) that operates at large bandwidths, cryogenic temperatures, and high magnetic fields. The setup is based on a noncontact atomic force microscope (AFM) with a quartz tuning fork sensor for the nonoptical excitation and readout in topography, force, and dissipation measurements. The metallic AFM tip forms part of a rf resonator with a transmission characteristics modulated by the sample properties and the tip-sample capacitance. The tip motion gives rise to a modulation of the capacitance at the frequency of the AFM sensor and its harmonics, which can be recorded simultaneously with the AFM data. We use an intuitive model to describe and analyze the resonator transmission and show that for most experimental conditions it is proportional to the complex tip-sample conductance, which depends on both the tip-sample capacitance and the sample resistivity. We demonstrate the performance of the DSCM on metal disks buried under a polymer layer and we discuss images recorded on a two-dimensional electron gas in the quantum Hall effect regime, i.e. at cryogenic temperatures and in high magnetic fields, where we directly image the formation of compressible stripes at the physical edge of the sample.

Study on channel depletion in metal‐oxide‐semiconductor field effect transistor using top‐view imaging through scanning capacitance microscopy

AbstractCarrier profiles within the near‐surface channel region of n‐type metal‐oxide‐semiconductor field‐effect transistors (n‐MOSFETs) have been examined using scanning capacitance microscopy (SCM). After the removal of a poly‐Si gate electrode, we were able to assess the qualitative local carrier concentration within specified regions of an n‐MOSFET using static capacitance (dC/dZ) measurement with its bias dependence (dC/dZ–V spectra). We found that the dC/dZ‐signal at the center of the channel region lowers as the gate length is reduced. This result is attributed to the carrier depletion within the channel, which is consistent with the threshold voltage (Vth) characteristics of the device properties. Copyright © 2008 John Wiley & Sons, Ltd.

The influence of coalescence time on unintentional doping in GaN/sapphire

Unintentional n-type doping is commonly observed to occur during the growth of nominally undoped GaN on sapphire. Scanning capacitance microscopy reveals that a layer of GaN adjacent to the GaN/sapphire interface is n-type, whilst the remainder of the epilayer appears insulating. If two-dimensional GaN growth is commenced immediately after the growth of the nucleation layer, then the unintentionally doped layer is not present and the material is highly resistive. However, such samples have a high threading dislocation density (ca. 5×10 9 cm −2). To reduce the threading dislocation density, the GaN nucleation layer is annealed and growth is then continued at a low V:III ratio to promote three-dimensional island formation. These islands are eventually coalesced using an increased V:III ratio and temperature. Longer coalescence times lead to lower threading dislocation densities. The width of the unintentionally n-doped layer is found to increase as the coalescence time increases, whilst the carrier concentration in the layer is not observed to change. The dopant in the unintentionally conductive layer is shown, using secondary ion mass spectrometry, to be oxygen.

Annealing Temperature Dependence of the Electrically Active Profiles and Surface Roughness in Multiple Al Implanted 4H-SiC

In the present work, we systematically studied the effect of the annealing temperature (from 1400 °C to 1650 °C) on the electrical activation of 4H-SiC implanted with multiple energy (from 40 to 550 keV) and medium dose (1×1013 cm-2) Al ions. The evolution of the acceptor (NA) and compensating donor (ND) depth profiles was monitored by the combined use of scanning capacitance microscopy (SCM) and scanning spreading resistance microscopy (SSRM). We demonstrated that the electrical activation of the implanted layer with increasing annealing temperature is the result of the increase in the acceptor concentration and of the decrease in the ND/NA ratio. Atomic force microscopy (AFM) morphological analyses indicated that the surface quality is preserved even after the 1650 °C annealing process.

Investigation of local electrical properties of coincidence-site-lattice boundaries in location-controlled silicon islands using scanning capacitance microscopy

We used scanning capacitance microscopy (SCM) to investigate the electrical activity of grain boundaries consisting of random and coincidence-site-lattice (CSL) boundaries in location-controlled silicon islands, which were fabricated using the μ-Czochralski process with an excimer laser. The SCM results suggest that the electrical activity of the {122}∑9 CSL boundary is much smaller than that of a random boundary, and the {111}∑3 CSL boundary is negligible. This is consistent with previous theoretical predictions and experimental results for thin-film transistors.

Intermittent-contact scanning capacitance microscopy versus contact mode SCM applied to 2D dopant profiling

This study compares two different methods of scanning capacitance microscopy (SCM). The first and approved one operates in contact mode and the second novel one in intermittent-contact (IC) mode. Measurements were performed on several samples and the results are compared. New technical expertises on the novel intermittent-contact method are shown and in conclusion assets and drawbacks of this SCM method are emphasized.

Two-dimensional electron gas insulation by local surface thin thermal oxidation in AlGaN∕GaN heterostructures

In this letter, the physical effects of the local surface thin thermal oxidation on the current transport in AlGaN∕GaN heterostructures are reported. Current-voltage measurements performed on appropriate test patterns demonstrated that a selective oxidation process at 900°C enables the suppression of the current flow through the two-dimensional electron gas. The electrical insulation was achieved even though structural analysis showed that the formed oxide did not reach the depth of the AlGaN∕GaN heterointerface. The combination of capacitance-voltage measurements with depth-resolved scanning capacitance microscopy enabled to correlate the electrical results to the doping and the compositional stability of the material during oxidation.

Localized Charging Properties of Si Nanocrystals Embedded in a SiO2 Layer by Scanning Capacitance Microscopy

In this work, we characterized charging properties of silicon nanocrystals (NCs) using scanning capacitance microscopy (SCM). The size of the NCs produced by pyrolysis method is in the range of 10–50 nm with a density of around 1011/cm2. Using a conducting tip, a charge was injected directly into the NCs, and the charging characteristics of a NC were analyzed through the dC/dV–V curve shift and SCM images. Bias voltage stress to a NC cause the variation of capacitance due to the electron charging, which is reflected in SCM images. By measuring consecutive dC/dV–V curve shift after NC charging, we also analyzed retention characteristics of a NC. In temperature dependence experiment, NC reveals the characteristics of good chargeability at high temperature. Capacitance spectroscopy and related SCM images show that SCM can be an adequate tool for more detailed analysis of the charging and discharging effects of each NC.

Scanning Probe Microscopy for Dielectric and Metal Characterization

The properties of both insulators and metals can be characterized capacitively with scanning probe microscopy, though the techniques employed are different. Intermittent contact scanning capacitance microscopy (IC-SCM) is a useful technique for characterizing the dielectric constant of insulators. Scanning Kelvin force microscopy (SKFM) can image potential distributions and work functions within conductors. Both techniques depend on the capacitance between the tip and sample, which has an inverse square dependence on tip-sample separation, limiting spatial resolution. A differential data acquisition technique to improve spatial resolution is described.

Formation of Pentacene wetting layer on the SiO 2 surface and charge trap in the wetting layer

We studied the early-stage growth of vacuum-evaporated pentacene film on a native SiO 2 surface using atomic force microscopy and in-situ spectroscopic ellipsometry. Pentacene deposition prompted an immediate change in the ellipsometry spectra, but atomic force microscopy images of the early stage films did not show a pentacene-related morphology other than the decrease in the surface roughness. This suggested that a thin pentacene wetting layer was formed by pentacene molecules lying on the surface before the crystalline islands nucleated. Growth simulation based on the in situ spectroscopic ellipsometry spectra supported this conclusion. Scanning capacitance microscopy measurement indicated the existence of trapped charges in the SiO 2 and pentacene wetting layer.

Unintentional doping in GaN assessed by scanning capacitance microscopy

AbstractUsing scanning capacitance microscopy (SCM) we observe a layer of unintentional n‐type conductivity in GaN/sapphire epilayers in a region adjacent to the GaN/sapphire interface. It is shown to have a carrier concentration of (1.66 ± 0.08) × 1018 cm–3. Secondary ion mass spectroscopy (SIMS) data from a similar sample suggests that the n‐type conductivity in this region results from unintentional oxygen incorporation. Various doped regions are also observed in epitaxial lateral over‐growth (ELOG) samples. GaN initially grows through a SiNx mask forming faceted stripes, which exhibit unintended n‐type conductivity. Magnesium is used to enhance stripe coalescence, resulting in p‐type regions. The top layer, grown following coalescence, appears non‐conducting. The different electron concentrations found within the ELOG cross‐section could correspond to the different facets formed during growth which may indicate that the changes in carrier concentration reflect the rate of impurity incorporation on each facet. (© 2008 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Galvanic Corrosion and Localized Degradation of Aluminum-Matrix Composites Reinforced with Silicon Particulates

Galvanic corrosion of an Al-based metal-matrix composite (MMC) reinforced with Si particulates was examined by considering the semiconductor/electrolyte junction properties of the Si reinforcements. Scanning capacitance microscopy and photoelectrochemical experiments indicated that the Si reinforcements were slightly p-doped. At the open-circuit condition, a wide depletion layer was expected to be present at the slightly p-doped Si surface. The blocking characteristics of the depletion layer for electrochemical reactions would thus limit the galvanic interactions between the Si reinforcements and the Al matrix. The photoelectrochemical experiments also suggested that solar irradiation may promote galvanic corrosion because of the enhanced cathodic activity of the Si reinforcements under illuminated conditions. Corrosion was found to initiate from Fe-containing intermetallics, which served as effective cathodes. A chemical decoration method indicated that, while the bulk region of the Si reinforcements was not effective for cathodic reactions, the exposed Si surfaces at the interfaces were effective cathodic sites. It was hypothesized that during the processing of the MMC, interdiffusion caused the Si surfaces at the interfaces to be highly p-doped with Al, and upon propagation of localized corrosion from the sites adjacent to the Fe-containing intermetallics, the highly p-doped Si surfaces were exposed to solution and served as effective cathodic sites to induce galvanic corrosion. A scanning vibrating electrode technique revealed a net cathodic current over the localized corrosion regions, which were many times larger than the intermetallic particle or the Si reinforcement particle. Accordingly, the solution near the localized corrosion regions was alkalinized, as was revealed by the scanning ion–selective electrode technique.

Scanning capacitance microscopy study of carrier depletion within grains of n‐type polycrystalline silicon

AbstractThe local electrical properties and corresponding surface topography for phosphorus‐doped n‐type polycrystalline (n‐poly) silicon layers were investigated using scanning capacitance microscopy (SCM) of chemical mechanical polishing (CMP) and backside‐etched samples. Applying negative bias stress between a sample and a scanning SCM probe tip was found to induce a depleted region within the CMP n‐ poly‐Si sample. In contrast, bias stressing does not intrinsically change the backside‐etched n‐poly‐Si sample. By combining our results with the results of energy dispersive x‐ray spectroscopy measurement, we can conclude that the active dopant concentration within n‐poly‐Si grains is inhomogeneous. Copyright © 2008 John Wiley & Sons, Ltd.

The origin and reduction of dislocations in Gallium Nitride

Two methods for GaN growth on sapphire by metal-organic vapour phase epitaxy are discussed. The first involves only two-dimensional (2D) growth, and results in a high dislocation density, but also a high electrical resistivity. The second involves initial growth of three-dimensional (3D) islands employing a low V:III ratio, followed by island coalescence at a high V:III ratio. It is often assumed that threading dislocations (TDs) form via the coalescence of 3D islands, but detailed atomic force microscopy studies on partially coalesced samples find no evidence of an increased TD density at coalescence boundaries, suggesting that other possible origins for TDs should be considered. The 3D–2D growth method allows TD densities as low as 1.1 × 108 cm−2 to be achieved, but unlike the 2D growth samples these layers are not highly resistive. Scanning capacitance microscopy is used to demonstrate the presence of an unintentionally doped layer close to the GaN/sapphire interface. To simultaneously achieve a reduced TD density compared to 2D growth samples and a high resistivity, a high temperature AlN buffer layer may be employed.

Doping characterization of InAs∕GaAs quantum dot heterostructure by cross-sectional scanning capacitance microscopy

In order to better understand dopant incorporation in quantum dot infrared photodetectors, the application of cross-sectional scanning capacitance microscopy (SCM) has been used to investigate carrier occupation/distribution in a multilayer InAs∕GaAs quantum dot (QD) heterostructure for different doping techniques. The doping schemes in the QD structure include direct doping (in InAs QD layers) and remote doping (in GaAs barrier layers), each with different doping concentrations. The SCM image suggests that large band bending occurs due to highly doped, remote-doping layers, thereby causing electron redistribution in direct-doping layers. The experimental result is supported by a band structure calculation using the Schrödinger–Poisson method by NEXTNANO3.