Conductive Atomic Force Microscope Measurements Research Articles

Study on the electrical performance degradation mechanism of β -Ga2O3 p-n diode under heavy ion radiation

The impact of heavy ion irradiation on the β-Ga2O3 p-n diode and its physical mechanism have been studied in this Letter. After the irradiation fluence of 1 × 108 cm−2, it is observed that the electrical performance of the device is significantly degraded. The forward current density (JF) is reduced by 49.4%, the reverse current density (JR) is increased by more than two orders of magnitude, and the breakdown voltage (VBR) is decreased by 30%. Based on the results of the deep-level transient spectroscopy measurement, it is concluded that acceptor-like traps generated with an energy level of EC-0.75 eV in the β-Ga2O3 drift layer dominate the JF degradation of the device, which are most likely related to Ga vacancies. These acceptor-like traps result in the reduction of change carrier concentration, which in turn leads to a decrease in JF. In addition, according to the conductive atomic force microscope measurements and theoretical calculation, it is clearly observed that the latent tracks induced by heavy ion irradiation can act as leakage paths, leading to a significant degradation of JR and VBR.

The Growth of High-Quality Hexagonal GaTe Nanosheets Induced by ZnO Nanocrystals

The monoclinic and hexagonal gallium tellurides (m-GaTe and h-GaTe) show different applications in optoelectronic devices. Compared to the m-GaTe, the h-GaTe is a metastable phase, which generally exists in ultrathin samples and is difficult to obtain by direct chemical reaction. Herein, a hexagonal ZnO-induced crystal growth strategy was used for the design and fabrication of h-GaTe. The high-quality h-GaTe nanosheets were successfully grown on the (001) surface of hexagonal ZnO by the chemical vapor deposition method under ambient pressure. The SEM, XPS, XRD, and HRTEM characterizations uncovered a flower-like nanosheet morphology and a hexagonal crystal structure for the obtained GaTe samples. Meanwhile, the conductive atomic force microscope measurement indicates that the obtained h-GaTe nanosheet is a p-type semiconductor. Based on the electron localization function simulation, the lattice-induced crystal growth of h-GaTe was demonstrated. The results give an insight into the synthesis of metastable phase crystal and open an avenue for fabricating new two-dimensional devices by p-type h-GaTe.

Analog Resistive Switching in Reduced Graphene Oxide and Chitosan‐Based Bio‐Resistive Random Access Memory Device for Neuromorphic Computing Applications

Bio‐resistive random access memory (Bio‐RRAM) devices are important because they are biocompatible and biodegradable. Herein, analog unipolar resistive switching and bipolar resistive switching in chitosan and reduced graphene oxide + chitosan (RGO+chitosan)‐based Bio‐RRAM devices are demonstrated, respectively. Endurance and retentivity of the Ag/RGO+chitosan/FTO‐based Bio‐RRAM device demonstrate good stability and nonvolatile in nature. Conductive atomic force microscope measurements infer the formation of the 1D conduction channels in the device, which further confirms the conduction channel mechanism in the RGO+chitosan‐based Bio‐RRAM device. Further, synaptic learning rules such as long‐term potentiation (LTP), long‐term depression, and spike time‐dependent plasticity on Ag/RGO+chitosan/FTO‐based device are also demonstrated. Estimated relaxation time infers more time for forgetting than learning. These results suggest that Ag/RGO+chitosan/FTO synaptic Bio‐RRAM device would indeed be a potential candidate for future neuromorphic computing applications.

Multideck light-induced reset in a transparent bilayer synaptic device

The research of photoelectric memristors has been gaining momentum in recent years. Although resistive memory synaptic devices are usually electrically controlled, an opto-electronic one would certainly be advantageous. A light-responsive structure can extend the functionality of such devices and allow for coupling of light and electrical signals in the implementation of neuromorphic systems. This paper presents a detailed analysis of a transparent, bilayer synaptic device, capable of hybrid photonic and electronic response with multideck, erase functionality. Such steplike operation may allow for more degrees of freedom in the implementation of artificial vision systems based on these incremental conductance changes. Multilevel operation is demonstrated under different illumination intensities and functional methodologies (i.e., irradiation schemes). Statistical data are also presented to give a better foundation for this suggested functionality. Finally, the underlying physical mechanisms are discussed, supported by ultrahigh-vacuum conductive atomic force microscope measurements over a dedicated lateral test structure.

Perovskite Oxides Tunable by Electromechanical and Electrothermal Couplings

Objectives: Oxides hold the great promise for emerging electronic applications (other than conventional CMOS components such as gate oxides) because they can significantly increase the operational range of temperature and bias voltage due to the much higher bandgap energy than semiconductors. However, using the oxides for such novel purposes requires their electronic properties to be properly tuned and tailored to specific device applications. Therefore, this work seeks to develop the way to address this challenge by focusing on the tunable characteristics of perovskite oxides driven by their unique electromechanical and electrothermal couplings. Among various types of oxides, crystalline perovskite titanate structures such as STO (SrTiO3) have been chosen as the representative material platform in this study, due to their multifunctional properties and well-established growth techniques. New Results: Previously, researchers have demonstrated the heat-induced tunable electrical characteristics of STO doped with various impurities [1-2]. Due to the complex chemistry where both the thermal and doping effects contribute to the observed change in electrical properties, accurate physical origin of the STO’s tunable behavior has not been clearly identified. In this work, we focused on an undoped, 1 mm-thick STO crystal to quantitatively measure its response when solely exposed to an external stimulus such as mechanical pressure.Fig. 1 depicts the C-AFM (conductive atomic force microscope) experimental setup to study the tunable behavior of STO driven by an electromechanical coupling. The commercially available conductive tip (Asyelec.01-R2) was used to apply a bias voltage (Vbias = 2V) while in contact with the top surface of the STO sample. This enables simultaneous measurement of electrical response during the experiment. The force was applied and varied by changing the set-point voltage (Vsetpoint, kept below 3V to prevent any possible damage to the measurement system) that is used to make the cantilever tip bent with the varying height. Conversion from the control parameter (Vsetpoint) to the mechanical pressure (N/m2) was conducted through precise calibration of the C-AFM measurement setup, including spring constant (0.92 N/m) and deflection sensitivity (0.0576 mm/V) (Table 1).Fig. 2 shows the measured electrical conductivity vs. applied pressure plot for the undoped STO sample used in the experiment. To ensure statistically meaningful data, each experiment was performed repetitively (at least 10 times) and both the mean (data points) and standard deviation (error bars) values were presented in the figure. A clear trend of increase in electrical conductivity with increasing pressure was observed, which is best attributed to creation of oxygen vacancies by mechanical stress in STO [3]. This will surely help researchers study the fundamental electromechanical coupling-driven behavior of perovskite oxide thin films. Fig. 3 further presents the C-AFM image when pressure was applied to the STO surface (the white particles are believed to be due to pile-up of electronic charges upon the application of localized electrical bias).We next studied the electrothermal coupling-driven conductance change in STO by performing a systematic study of thermal annealing (at temperatures ranging from 200°C to 800°C and in dry air). For this study, a sufficient amount of oxygen vacancies was purposely introduced when the STO thin film was prepared (see Fig. 4 for the device schematic). Because subsequent annealing annihilated oxygen vacancies, decrease in electrical conductivity was observed in STO samples annealed at high temperatures (Fig. 5). The reasoning of relating the observed trend in Fig. 5 to the change in the number of oxygen vacancies upon thermal annealing is well supported by measuring the hysteretic behavior of the STO thin film. Fig. 6 shows a measured butterfly curve that is typical for memristor devices where removal and restoration of oxygen vacancies play a key role as the switching mechanism. Significance: This work is of significant importance for the development of future electronic devices with significantly enhanced reliability features. By demonstrating that electrical characteristics of multifunctional oxide thin films can be largely tuned by external stimuli such as mechanical pressure and thermal annealing, it will spark various research initiatives in using the oxide for novel, more versatile applications.

Non-volatile resistive switching in CuBi-based conductive bridge random access memory device

Resistive switching devices, which are dominated by metal cation based conductive filament formation/rupture, are called programmable memory, or conductive bridge random access memory (CBRAM), and are widely expected to replace existing memory devices. In this letter, CuBi alloy was used as an active electrode to control the over-diffusion of Cu ions into the solid electrolyte of a CBRAM. In addition, resistive switching performance was improved by inserting lutetium and dysprosium metals, which acted as a buffer layer at the interface of the active electrode (CuBi) and the dielectric layer (Al2O3). When optimized, the Cu0.55Bi0.45/Lu(Dy)/Al2O3/Pt showed excellent resistive switching performance. This improvement can be explained by the high controllability of Cu in Cu-Bi electrode with the Lu (Dy) buffer layer. The formation of intermediate oxide buffer layers at the CuBi/Lu(Dy) interface was analysed using XPS. Moreover, conductive-atomic force microscope measurements helped to define the inverted cone shape of the metallic conductive filament.

The mechanics-modulated tunneling spectrum and low-pass effect of viscoelastic molecular monolayer

Understanding the force-induced conductance fluctuation in molecules is essential for building molecular devices with high stability. While stiffness of molecule is usually considered to be desirable for stable conductance, we demonstrate mechanical dragging in viscoelastic molecules integrates both noise resistance and mechanical controllability to molecular conductance. Via conductive atomic force microscope measurement and theoretical modeling, it’s found that viscoelastic Azurin monolayer has spectrum-like pattern of conductance corresponding to the duration and strength of applied mechanical pulse under low-frequency excitation. Conductance fluctuation is prevented under high-frequency excitation by dragging dissipation, which qualifies molecular junction with electric robustness against mechanical noise.

CAFM Experimental Considerations and Measurement Methodology for In-Line Monitoring and Quantitative Analysis of III–V Materials Defects

To continue technology scaling, a new generation of high-performance devices are considered to be implemented using III–V semiconductors, which need to be grown over the conventional Si substrate. However, due to the lattice mismatch between the III–V and silicon materials, the former tend to develop significant density of structural defects [specifically, threading dislocations (TDs)], which can adversely affect device electrical characteristics. Conductive atomic force microscope (CAFM) technique is among the most promising tools for the identification and analysis of TDs in a nanoscale range although obtaining reliable quantitative data requires precise controls over the measurements conditions. In this study, CAFM technique has been applied for TDs detection and analysis in III–V films, and tool requirements and measurement methodology are discussed.

Nanostructured fuzz growth on tungsten under low-energy and high-flux He irradiation.

We report the formation of wave-like structures and nanostructured fuzzes in the polycrystalline tungsten (W) irradiated with high-flux and low-energy helium (He) ions. From conductive atomic force microscope measurements, we have simultaneously obtained the surface topography and current emission images of the irradiated W materials. Our measurements show that He-enriched and nanostructured strips are formed in W crystal grains when they are exposed to low-energy and high-flux He ions at a temperature of 1400 K. The experimental measurements are confirmed by theoretical calculations, where He atoms in W crystal grains are found to cluster in a close-packed arrangement between {101} planes and form He-enriched strips. The formations of wave-like structures and nanostructured fuzzes on the W surface can be attributed to the surface sputtering and swelling of He-enriched strips, respectively.

Analysis of Factors in the Nanoscale Physical and Electrical Characterization of High-K Materials by Conductive Atomic Force Microscope

The conductive atomic force microscope (CAFM) has been shown to be a powerful tool to study the physical and electrical properties of high-k materials on nanoscale. However, such accurate measurements could be altered by external factors. In this work, the main factors affecting CAFM measurements including environmental conditions, tip coating materials and the tip-sample bias are evaluated by analyzing the topographic and current maps performed on HfO2 stacks in different conditions. It is found that environmental conditions have notable effects on the CAFM measurements due to the water meniscus formed by adsorbed water layers on the tip and sample surface in air. And the lateral resolution of the technique can be improved remarkably by measuring in a high vacuum environment. The mechanical and electrical properties of tip coating materials are found to be related to the stability of tip conductivity and the real voltage drop in the stack, hence have a key influence on the current measurement. Moreover, the tip-sample interaction can be altered by the electrostatic force induced by the applied bias, leading to variations of the topographic image of the surface with the bias voltage. Our results indicate that the impact of these factors must be taken into account when performing CAFM measurements under different conditions or comparing the results obtained in different experiments.

Nanoscale observations of resistive switching high and low conductivity states on TiN/HfO2/Pt structures

Resistive Switching (RS) phenomenon in Metal–Insulator–Metal (MIM) structures with polycrystalline HfO2 layers as dielectric has been studied at the nanoscale using Conductive Atomic Force Microscope (CAFM). The CAFM measurements reveal that (i) the conductive filaments (CFs) created at very small areas are the origin of the RS phenomenon observed at device level and (ii) RS conductive filaments are primarily formed at the grain boundaries, which exhibit especially low breakdown voltage. CAFM images obtained on MIM structures at the Low and High Resistive states also show that, although the current in the Low Resistive State is mainly driven by a completely formed single CF, the cell area dependence of the conductivity in the High Resistive State could be explained by considering the presence of multiple partially formed CFs.

Localized oxidation influence from conductive atomic force microscope measurement on nano-scale I-V characterization of silicon thin film solar cells

Nano-scale current–voltage (I-V) characteristics of hydrogenated microcrystalline silicon (μc-Si:H) prepared by Hot-Wire CVD (HWCVD) technique have been studied by Conductive Atomic Force Microscope (Conductive-AFM) under atmospheric conditions. It is demonstrated that a local modification is caused by the current, detected as a dramatic decrease in the forward biased current of I-V characteristics with the number of repeated scans. On the other hand, smaller change of reverse biased current is observed after the repeated scans. On the base of these results, we discuss and demonstrate the validity of our proposed new junction characterization method at the nanometer scale; that is, simultaneous nano-scale Topographical and Current–Voltage Imaging (TCVI) for Silicon (Si) thin film solar cells.

Self-organized InGaAs quantum dots grown on GaAs (3 1 1)B substrate studied by conductive atomic force microscope technique

We have used conductive atomic force microscope (AFM) in a high vacuum in order to investigate the electronic properties of self-organized In x Ga 1– x As quantum dots (QDs) on GaAs (3 1 1)B substrates. The QDs were fabricated by atomic H-assisted molecular beam epitaxy, and Si AFM tips coated with Au, which warrants electrical conductivity were used to measure both the topographic and current images of QDs surface simultaneously. The conductive AFM measurements were performed in vacuum at room temperature and at lowered temperatures. With this technique, the current–voltage ( I– V) characteristics of QDs of varying sizes, and of any other arbitrary positions on the QDs surface can also be studied by using the same conductive AFM tip. It was found that the center of a QD is more conductive than its periphery, and the surface in between the QDs is highly resistive. The differences in the conductance are thought to be due to the local modification of surface bending associated with the surface states. Further, we have shown that the conductance becomes spatially uniform at all points over the packed and ordered QDs at low temperatures, which could be explained by lateral coupling of these strained QDs.