Scanning Nonlinear Dielectric Microscopy Research Articles

Local capacitance-voltage profiling and deep level transient spectroscopy of SiO2/SiC interfaces by scanning nonlinear dielectric microscopy

We extend our local capacitance-voltage profiling method for the real-space nanoscale investigation of the SiO2/SiC interface, which is still an issue in the application of SiC to power electronics devices. Our technique is based on time-resolved scanning nonlinear dielectric microscopy and can be used for collecting local capacitance-voltage and its voltage-derivative profiles at each measurement point. This technique can also be combined with local deep level transient spectroscopy for simultaneously imaging the spatial fluctuations of interface defect density in real-space. Here we use the proposed technique to measure SiC wafers with a thermal oxide layer and investigate the effect of interface nitridation. From the data volume obtained by our technique, various images are extracted and their correlation is analyzed to characterize and discuss the spatial fluctuations and hysteresis of the local capacitance-voltage profiles. As expected, the nitridation of the SiO2/SiC interface after thermal oxidation is effective for the reduction of hysteresis and fluctuations in local capacitance-voltage profiles. However, significant spatial fluctuations remain even after the nitridation treatment, which suggests the SiO2/SiC interface has intrinsic non-uniformity generating high surface potential fluctuations.

Microscopic Evaluation of Al<sub>2</sub>O<sub>3</sub>/p-Type Diamond (111) Interfaces Using Scanning Nonlinear Dielectric Microscopy

Improvement of channel mobility is required to improve the performance of the inversion channel MOSFETs using diamond. The previous studies have suggested that high interface defect density (Dit) at the Al2O3/diamond (111) interface has a significant impact on the carrier transport property on a channel region. To investigate the physical origins of the high Dit, especially from microscopic point of view, here we investigate Al2O3/p-type diamond (111) interfaces using scanning nonlinear dielectric microscopy (SNDM). We find the high spatial fluctuations of Al2O3/hydroxyl (OH)-terminated diamond (111) interface properties and their difference by the flatness of the diamond surface.

Surface Potential Fluctuations of SiO<sub>2</sub>/SiC Interfaces Investigated by Local Capacitance-Voltage Profiling Based on Time-Resolved Scanning Nonlinear Dielectric Microscopy

We investigate surface potential fluctuations on SiO2/SiC interfaces by local capacitance-voltage profiling based on time-resolved scanning nonlinear dielectric microscopy. As experimental indicators of surface potential fluctuations, we measured the spatial fluctuations of local capacitance-voltage and its first derivative profiles through the detection of the voltages at the infection points of the profiles. We show that, even for a sample with a nitrided interface with low interface defect density, the fluctuations of the measured voltages are much higher than the thermal energy at room temperature. This indicates the existence of high potential fluctuations, which can give the significant impacts on the carrier transport at the SiO2/SiC interface of SiC metal-oxide-semiconductor field effect transistors.

Boxcar Averaging Scanning Nonlinear Dielectric Microscopy.

Scanning nonlinear dielectric microscopy (SNDM) is a near-field microwave-based scanning probe microscopy method with a wide variety of applications, especially in the fields of dielectrics and semiconductors. This microscopy method has often been combined with contact-mode atomic force microscopy (AFM) for simultaneous topography imaging and contact force regulation. The combination SNDM with intermittent contact AFM is also beneficial for imaging a sample prone to damage and using a sharp microscopy tip for improving spatial resolution. However, SNDM with intermittent contact AFM can suffer from a lower signal-to-noise (S/N) ratio than that with contact-mode AFM because of the shorter contact time for a given measurement time. In order to improve the S/N ratio, we apply boxcar averaging based signal acquisition suitable for SNDM with intermittent contact AFM. We develop a theory for the S/N ratio of SNDM and experimentally demonstrate the enhancement of the S/N ratio in SNDM combined with peak-force tapping (a trademark of Bruker) AFM. In addition, we apply the proposed method to the carrier concentration distribution imaging of atomically thin van der Waals semiconductors. The proposed method clearly visualizes an anomalous electron doping effect on few-layer Nb-doped MoS2. The proposed method is also applicable to other scanning near-field microwave microscopes combined with peak-force tapping AFM such as scanning microwave impedance microscopy. Our results indicate the possibility of simultaneous nanoscale topographic, electrical, and mechanical imaging even on delicate samples.

Local capacitance-voltage profiling and high voltage stress effect study of SiO2/SiC structures by time-resolved scanning nonlinear dielectric microscopy

We investigate SiO2/SiC structures by a nanoscale capacitance-voltage (CV) profiling method based on time-resolved scanning nonlinear dielectric microscopy (tr-SNDM). Owing to the high sensitivity to the variation in the tip-sample capacitance, wide bandwidth of the measurement, and high flexibility of offline processing, tr-SNDM permits the more accurate acquisition of the microscopic CV characteristics localized below the scanning tip. We demonstrate that tr-SNDM based local CV profiling is able to distinguish the different features of untreated and nitrided SiO2/SiC structures and image intrinsic spatial non-uniformity of the interfaces. We also investigate the nanoscale impacts of high voltage stress on the SiO2/SiC structures. By inspecting the local CV characteristics under high voltage stress, we suggest that the interface defect density locally increases during the stress application.

Nanoscale capacitance-voltage profiling of DC bias induced stress on a high-κ/SiO2/Si gate stack

We investigate DC bias stress effects on a HfOx/SiO2/p-Si gate stack using time-resolved scanning nonlinear dielectric microscopy. Our microscopy permits localized capacitance-voltage profiling and deep level transient spectroscopy on a particular area stressed prior to the measurement. We observed the increase of interface defect density and the drastic changes of local capacitance-voltage characteristics on the stressed area. Our numerical simulations explain that both fixed charges and interface defect density increased by the DC bias stress and caused the change of local characteristics. Because local capacitance-voltage characteristics is particularly important in device performance, time-resolved scanning nonlinear dielectric microscopy will be useful for the evaluation of local stress effects on miniaturized Si devices such as bias temperature instability.

High resolution front-side visualization of charge stored in EEPROM with scanning nonlinear dielectric microscopy (SNDM)

By exposing floating gates of EEPROM memory cells with frontside sample preparation, scanning nonlinear dielectric microscopy (SNDM) succeeded in reading back the data stored in the memory cells with a 250 nm node size. At an optimized voltage bias of AC = 3 V and DC = 1 V, a clear signal contrast between programmed and erased cells is obtained. The high resolution SNDM signal reveals the details of bowling-pin shape structure of memory cells, providing high confidence in data assignment during forensic applications. Such high resolution also makes SNDM a promising technique for newer generation devices with smaller node size.

High-precision local C–V mapping for ferroelectrics using principal component analysis

Local C–V mapping based on scanning nonlinear dielectric microscopy (SNDM) is a useful tool for characterizing ferroelectric domain dynamics at the nanoscale. In this study, we realized high-precision C–V mapping through an improved measurement system that introduces a digitizer and post-signal processing. Coupled with the high capacitance detection sensitivity of SNDM, nontrivial patterns corresponding to the polarization response were observed even in harmonic images with an order as high as thirty. Using these high-order harmonic components for the C–V curve resynthesis led to improved measurement accuracy. In addition, further improvement was achieved by introducing noise reduction based on principal component analysis. From the C–V curves resynthesized in this manner, parameters that represent the curve features could be extracted and subsequently displayed in two-dimensional maps. Detailed analyses of the datasets obtained with a ferroelectric HfO2 film revealed interesting distributions concerning local polarization switching dynamics.

Material Design Strategy for Enhancement of Readback Signal Intensity in Ferroelectric Probe Data Storage.

Ferroelectric probe data storage (FPDS) based on scanning nonlinear dielectric microscopy is expected as a next-generation data storage method with its large potential for improvement of the recording density. However, this novel method has a problem of low reading speed. To overcome this problem, a novel ferroelectric recording medium with large nonlinear permittivity is required because this data storage method uses the nonlinear dielectric response induced by small-amplitude ac bias to detect the bit data recorded in the form of polarization direction. Therefore, this article discusses nonlinear permittivity enhancement from the viewpoint of data storage application in the framework of the phenomenological theory. We reveal that the Curie-point control is one of the key techniques in material design for FPDS because nonlinear permittivity increases precipitously as the Curie temperature is approached, as with the linear permittivity and piezoelectric constants. A similar conclusion is also obtained through actual measurements of nonlinear permittivity in LiTaO3 single crystals. On the other hand, we also reveal that there is a tradeoff relationship between nonlinear permittivity and polarization stability. To avoid this undesirable situation in data storage applications, pinning-site control will also be important. We also propose to employ a double-layer structure in the ferroelectric recording media for further improvement.

Local C–V mapping for ferroelectrics using scanning nonlinear dielectric microscopy

Detailed analysis of local polarization switching will promote the further development of a wide range of applications using ferroelectrics. Here, we propose a local C–V mapping technique using scanning nonlinear dielectric microscopy (SNDM) that enables visualization of dynamic ferroelectric switching behavior in real space. Using this method, C–V butterfly curves characteristic of ferroelectrics can be measured on a scanning probe microscopy platform with nanoscale resolution by virtue of the high capacitance-detection sensitivity of SNDM. This provides real-space mapping of the net switchable polarization, the switching voltage, and the local imprint with a short measurement time (e.g., 10 min or less for 256×256 pixels). Furthermore, the proposed method will be useful for study of the electric-field response of domain walls. In this paper, we present some examples of experiments with LiTaO3 single crystals and HfO2-based ferroelectric thin films and give an overview of what kind of evaluation is possible with the local C–V mapping technique.

Profiling of carriers in a 3D flash memory cell with nanometer-level resolution using scanning nonlinear dielectric microscopy

This work used scanning nonlinear dielectric microscopy to profile the distributions of carriers in channels and floating gate structures less than 10 nm in size within a three dimensional flash memory cell. An exceptionally sharp diamond probe tip (having a radius of less than 5 nm) was employed so as to obtain extremely high spatial resolution, and this technique was found to provide high contrast images of floating gates. The minimum spatial resolution obtainable from this apparatus was determined to be less than 1.9 nm. In addition, the present study demonstrated that variations in the diffusion lengths of N-type impurities between channels were less than 21 nm. The present study establishes an extremely helpful means of optimizing the performance and failure analysis of flash memory cells and similar devices.

Effects of deposition conditions on the ferroelectric properties of (Al1−xScx)N thin films

The ferroelectricity of (Al1−xScx)N (x = 0–0.34) thin films with various thicknesses was investigated. (Al1−xScx)N films were prepared at 400 °C on (111)Pt/TiOx/SiO2/(001)Si substrates by the radio frequency dual-source reactive magnetron sputtering method using Al and Sc targets under pure N2 gas or a mixture of N2 and Ar gases. The film deposited under N2 gas showed larger remanent polarization than those under N2 + Ar mixture. Ferroelectricity was observed for films with x = 0.10–0.34 for about 140-nm-thick films deposited under N2 gas. The x = 0.22 films showed ferroelectricity down to 48 nm in thickness from the polarization–electric field curves and the positive-up-negative-down measurement. The ferroelectricity of the 9 nm-thick film also was ascertained from scanning nonlinear dielectric microscopy measurement. These results reveal that ferroelectric polarization can switch for films with much broader compositions and thicknesses than those in the previous study.

Nanoscale characterization of unintentional doping of atomically thin layered semiconductors by scanning nonlinear dielectric microscopy

We use scanning nonlinear dielectric microscopy (SNDM) to visualize unintentional carrier doping of few-layer Nb-doped MoS2 mechanically exfoliated on SiO2. SNDM enables imaging of the majority carrier distribution in as-exfoliated samples at the nanoscale. We show that, unlike thick MoS2 layers, atomically thin layers exhibit a p- to n-type transition as the thickness decreases. The level of the observed unintentional n-doping is estimated to be 1×1013 cm−2, in agreement with the results of previous independent studies. In addition, the influence of ultraviolet–ozone treatment on the majority carrier distribution is also investigated. The n-type doping is observed to progress with increasing processing time. SNDM can be readily applied to atomically thin layered semiconductors and will advance understanding of and the ability to predict device characteristics even at an early stage of the fabrication process.

A scanning nonlinear dielectric microscopic investigation of potential-induced degradation in monocrystalline silicon solar cells

Carrier distributions in monocrystalline silicon solar cells affected by potential-induced degradation (PID) were investigated using scanning nonlinear dielectric microscopy (SNDM), employing three samples with different PID levels. These observations clearly demonstrated that reductions in the carrier density were correlated with the extent of PID. Depth profile measurements showed that the solar cells were affected by PID to a significant depth of approximately 90 μm, equal to almost half the cell thickness. This result suggests that the Na+ migration is readily induced by the application of a high voltage. Super-higher-order-SNDM was also employed to obtain a more precise analysis of the depletion layer distributions in monocrystalline silicon solar cells with and without the application of a high voltage. The data showed that the depletion layer of a sample without PID was thinner than that of a sample exhibiting PID, indicating that the carrier concentration in the former sample was higher. This finding was in good agreement with the results produced by direct quantitative dopant measurements using SNDM.

Carrier Depletion near the Grain Boundary of a SiC Bicrystal

Silicon carbide (SiC) bicrystals were prepared by diffusion bonding, and their grain boundary was observed using scanning transmission electron microscopy. The n-type electrical conductivity of a SiC single crystal was confirmed by scanning nonlinear dielectric microscopy (SNDM). Dopant profiling of the sample by SNDM showed that the interface acted as an electrical insulator with a ~2-μm-thick carrier depletion layer. The carrier depletion layer contained a higher number of oxygen impurities than the bulk crystals due to the incorporation of oxygen from the native oxide film during diffusion bonding. Density functional theory calculations of the density of states as a function of the bandgap also supported these findings. The existence of a carrier depletion layer was also confirmed in a p-type polycrystalline SiC ceramic. These results suggest that the electrical conductivity of SiC ceramics was mostly affected by carrier depletion near the grain boundary rather than the grain boundary itself.

Optimization of signal intensity in intermittent contact scanning nonlinear dielectric microscopy

Scanning nonlinear dielectric microscopy (SNDM) is a useful method for the nano-scale carrier distribution imaging of semiconductor materials and devices. This method is normally combined with contact mode atomic force microscopy (AFM) but the combination of SNDM with intermittent contact mode AFM is preferable to avoid the damages of tips and samples. However, signal-to-noise (S/N) ratio may significantly reduce due to a shorter contact time in the intermittent contact mode. Here we discuss the S/N ratio of the intermittent contact mode and show that the existence of the optimal condition on contact time and measurement bandwidth to maximize S/N ratio. We also experimentally demonstrate that signal intensity is actually improved by controlling contact time in the carrier distribution imaging on a Si test sample and atomically-thin layered semiconductors.

Carrier distribution imaging using ∂C/∂z-mode scanning nonlinear dielectric microscopy.

Scanning nonlinear dielectric microscopy (SNDM) can be used to visualize the carrier distribution in semiconductors with high sensitivity and spatial resolution. We recently proposed a complementary method named ∂C/∂z-SNDM that avoids the problem of contrast reversal. This paper describes a methodology for calculating the signal intensity in ∂C/∂z-SNDM using examples. For the simulation, the capacitance of a conductive-probe metal/oxide/semiconductor model was calculated and then the response signal for various probe-sample distances was analyzed. The simulation results confirm that the ∂C/∂z-SNDM signal intensity increases monotonically with dopant concentration, avoiding contrast reversal. Moreover, in addition to the fundamental (1ω) signal, higher-harmonic (2ω, 3ω) signals have sufficient intensities to be detected. The results suggest that the detection sensitivity for low dopant concentrations can be improved by conducting the measurement under an appropriate dc bias.

Photoelectron spectroscopic study of electronic states and surface structure of an in situ cleaved In2O3 (111) single crystal

Surface electronic states and band-bending behavior of a melt-grown (111)-plane In2O3 single crystal were investigated using X-ray photoelectron spectroscopy (XPES). The In2O3 single crystal was cleaved under vacuum conditions below 5 × 10−5 Pa, and loaded to a measurement chamber with a vacuum level of approximately 5 × 10−7 Pa. After cleaving, density reduction of in-gap states relating to oxygen vacancies was confirmed. Nevertheless, the cleaved surface had a surface electron accumulation layer with the surface Fermi level pinned at approximately 0.76 eV above the conduction band minimum. The bulk and surface carrier density were estimated as 3.9 × 1017 and 6.0 × 1019 cm−3 respectively by combining the XPES result and the Poisson–Schrödinger solutions. Scanning nonlinear dielectric microscopy indicated a higher carrier density at the (100) surface than that at the (111) surface, suggesting a crystallographic plane dependence which may originate in defects relating to the dipole and surface instability of the In2O3 surface.

High resolution observation of defects at SiO2/4H-SiC interfaces using time-resolved scanning nonlinear dielectric microscopy

High resolution observation of density of interface states (Dit) at SiO2/4H-SiC interfaces was performed by time-resolved scanning nonlinear dielectric microscopy (tr-SNDM). The sizes of the non-uniform contrasts observed in the map of Dit were in the order of several tens of nanometres, which are smaller than the value reported in the previous study (>100 nm). The simulation of the tr-SNDM measurement suggested that the spatial resolution of tr-SNDM is down to the tip radius of the cantilever used for the measurement and can be smaller than the lateral spread of the depletion layer width.

Visualization of traps at SiO2/SiC interfaces near the conduction band by local deep level transient spectroscopy at low temperatures

Local deep level transient spectroscopy (local-DLTS) at low temperatures is performed to investigate microscopic distributions of traps at SiO2/SiC interfaces near the conduction band. We find that the distributions of the density of interface states (Dit) with the energy depth of 0.20 eV from the conduction band edge show nonuniform contrasts with the scale of several hundreds of nanometers. We also apply local-DLTS to reveal the contribution of Dit to the nonuniform contrasts observed in scanning nonlinear dielectric microscopy (SNDM) images of SiO2/SiC samples. We analyze the cross-correlation between SNDM and Dit images obtained by simultaneous measurement with SNDM and local-DLTS, respectively. The patterns observed in the SNDM and Dit images simultaneously obtained at 150 K show a weak negative correlation, suggesting that nonuniform distributions of Dit contribute to the formation of inhomogeneous contrasts in the SNDM images.