Conductive Atomic Force Microscopy Experiments Research Articles

Solid Platinum Nanoprobes for Highly Reliable Conductive Atomic Force Microscopy.

Conductive atomic force microscopy (CAFM) is a powerful technique to investigate electrical and mechanical properties of materials and devices at the nanoscale. However, its main challenge is the reliability of the probe tips and their interaction with the samples. The most common probe tips used in CAFM studies are made of Si coated with a thin (∼20 nm) film of Pt or Pt-rich alloys (such as Pt/Ir), but this can degrade fast due to high current densities (>102A/cm2) and mechanical frictions. Si tips coated with doped diamond and solid doped diamond tips are more durable, but they are significantly more expensive and their high stiffness often damages the surface of most samples. One growing alternative is to use solid Pt tips, which have an intermediate price and are expected to be more durable than metal-coated silicon tips. However, a thorough characterization of the performance of solid Pt probes for CAFM research has never been reported. In this article, we characterize the performance of solid Pt probes for nanoelectronics research by performing various types of experiments and compare them to Pt/Ir-coated Si probes. Our results indicate that solid Pt probes exhibit a lateral resolution that is very similar to that of Pt/Ir-coated Si probes but with the big advantage of a much longer lifetime. Moreover, the probe-to-probe deviation of the electrical data collected is small. The use of solid Pt probes can help researchers to enhance the reliability of their CAFM experiments.

Surface‐Dependent Properties and Tunable Photodetection of CsPbBr3 Microcrystals Grown on Functional Substrates

AbstractAll‐inorganic metal halide perovskites are highly attractive materials for optoelectronics and are typically implemented as thin films, nanocrystals, and single crystals with macroscopic and millimeter‐size dimensions. Single crystals with regular shapes in the micrometer range are less explored and present an interesting alternative when a well‐defined and spatially localized response is desired. Here, CsPbBr3 microcrystals are fabricated by simple and fast drop‐casting of a precursor solution on several substrates that for the growth process are heated to temperatures in the range from 80 to 150 °C. The microcrystals have a cubic shape and feature a pyramidal cavity on their top surface that forms due to faster growth at the edges once the growing crystal is attached to the substrate. Distinct heterogeneity in the photoluminescence (PL) and conductivity of the different facets are measured by µ‐PL and conductive atomic force microscopy experiments. Toward device applications, the authors contact single microcrystals on conductive substrates mechanically with a micromanipulator tip. They observe diode‐like current–voltage curves and good photodetector functionality, obtaining a high responsivity of 150–300 A W−1 in the blue–green spectral region under forward bias, and a sharp detection band centered around 540 nm with peak responsivity close to 0.7 A W−1 under reverse bias.

Understanding Current Instabilities in Conductive Atomic Force Microscopy.

Conductive atomic force microscopy (CAFM) is one of the most powerful techniques in studying the electrical properties of various materials at the nanoscale. However, understanding current fluctuations within one study (due to degradation of the probe tips) and from one study to another (due to the use of probe tips with different characteristics), are still two major problems that may drive CAFM researchers to extract wrong conclusions. In this manuscript, these two issues are statistically analyzed by collecting experimental CAFM data and processing them using two different computational models. Our study indicates that: (i) before their complete degradation, CAFM tips show a stable state with degraded conductance, which is difficult to detect and it requires CAFM tip conductivity characterization before and after the CAFM experiments; and (ii) CAFM tips with low spring constants may unavoidably lead to the presence of a ~1.2 nm thick water film at the tip/sample junction, even if the maximum contact force allowed by the setup is applied. These two phenomena can easily drive CAFM users to overestimate the properties of the samples under test (e.g., oxide thickness). Our study can help researchers to better understand the current shifts that were observed during their CAFM experiments, as well as which probe tip to use and how it degrades. Ultimately, this work may contribute to enhancing the reliability of CAFM investigations.

Analysis of screw dislocation mediated dark current in Al0.50Ga0.50N solar-blind metal-semiconductor-metal photodetectors

We report on a billion-fold reduction in reverse-bias leakage current density (11.3 A/cm2 to 8.5 nA/cm2 at 20 V) across Schottky contacts made to Al0.50Ga0.50N epilayers grown on sapphire. Interdigitated back-to-back Ni/Au Schottky contacts were made to unintentionally doped Al0.50Ga0.50N epilayers grown by metal organic chemical vapor deposition to realize photodetectors with metal-semiconductor-metal geometry. Testing on a self-consistent series of samples grown under different conditions revealed that a two-order reduction in screw dislocation density is primarily responsible for the significant reduction in the lateral leakage (dark) current. This observation is validated by conductive atomic force microscopy experiments. Analytical modelling by temperature-dependent current-voltage measurements confirm a screw dislocation mediated carrier transport mechanism. The anomalously high reverse-bias leakage current (μA-mA) in the highly defective samples is found to be dominated by thermionic field emission (TFE) at low biases and Poole-Frenkel emission (PFE) from a deep donor level at high biases. With a significant improvement in the crystalline quality, a solar-blind photodetector with an EQE of 47%, photo/dark current ratio of 1.4 × 105 and transient characteristics of <20 ms is demonstrated. This study towards understanding the leakage in Al0.50Ga0.50N is expected to benefit the development of various deep-UV devices and GaN-AlGaN based power transistors.

Substrate effect on electrical conductance at a nanoasperity-graphene contact

The use of graphene for applications such as micro- and nano-scale electronic devices often involves incorporating the two-dimensional material onto various substrates. However, the effects of the substrate's mechanical properties on electrical contact conductance are not fully understood. Here, we explore these effects by measuring the conductance between a nanoscale probe and a single layer of graphene with three different levels of substrate support: no substrate, i.e. free-standing graphene, an elastic substrate, and a rigid substrate. These three systems are studied using conductive atomic force microscopy experiments complemented by molecular dynamics simulations using the electrochemical dynamics with implicit degrees of freedom method. In both experiments and simulations, at a given normal force, current increases as: rigid substrate < elastic substrate < no substrate. We demonstrate that the substrate support influences graphene/tip contact conductance through substrate's elasticity, which determines contact size, as well as through variability of interatomic distances in the contact, which contributes to the interface resistivity.

Leakage current through the poly-crystalline HfO2: Trap densities at grains and grain boundaries

We investigate the role of grains and grain boundaries (GBs) in the electron transport through poly-crystalline HfO2 by means of conductive atomic force microscopy (CAFM) measurements and trap-assisted tunneling simulations. CAFM experiments demonstrate that the leakage current through a thin dielectric film preferentially flows via the GBs. The current I-V characteristics measured on both types of sites, grains, and GBs are successfully simulated by utilizing the multiphonon trap-assisted tunneling model, which accounts for the inelastic charge transport via the electron traps. The extracted density of electrically active traps, whose energy parameters match those of the positively charged oxygen vacancies in hafnia, is ∼3 × 1019 cm−3 at the grains, whereas a much higher value of (0.9÷2.1) × 1021 cm−3 is required to reproduce the leakage current through the GBs.

Microscopic Characterizations of Nanostructured Silicon Thin Films for Solar Cells

ABSTRACTMicroscopic characterization of mixed phase silicon thin films by conductive atomic force microscopy (C-AFM) was used to study the structure composed of conical microcrystalline grains dispersed in amorphous matrix. C-AFM experiments were interpreted using simulations of electric field and current distributions. Density of absorbed optical power was calculated by numerically solving the Maxwell equations. The goal of this study is to combine both models in order to simulate local photoconductivity for understanding the charge photogeneration and collection in nanostructured solar cells.

Note: Electrical resolution during conductive atomic force microscopy measurements under different environmental conditions and contact forces

Conductive atomic force microscopy experiments on gate dielectrics in air, nitrogen, and UHV have been compared to evaluate the impact of the environment on topography and electrical measurements. In current images, an increase of the lateral resolution and a reduction of the conductivity were observed in N(2) and, especially, in UHV (where current depends also on the contact force). Both effects were related to the reduction/elimination of the water layer between the tip and the sample in N(2)/UHV. Therefore, since current measurements are very sensitive to environmental conditions, these factors must be taken into consideration when comparisons between several experiments are performed.

Band-Gap Determination of the Native Oxide Capping Quantum Dots by Use of Different Kinds of Conductive AFM Probes: Example of InAs/GaAs Quantum Dots

In most of quantum-dot (QD) systems, the electrical behavior of a single QD is directly linked to the native oxide grown on their surface. Obtaining quantitative electrical measurements requires identifying this oxide well, which is not a trivial task. Due to the use of two conductive atomic force microscopy (C-AFM) probes of different behaviors, C-AFM experiments and local electrical measurements allow one to determine the barrier heights at the interface between InAs QD and the native oxide and then to deduce the oxide band gap. In the case of InAs/GaAs QDs, based on our work and on literature results, it may be assumed that the capping oxide is an InAs oxide enriched in indium.

Degeneracy and instability of nanocontacts between conductive tips and hydrogenatednanocrystalline Si surfaces in conductive atomic force microscopy

Conductive atomic force microscopy (C-AFM) has been extensively used for makingmeasurements of electrical properties of nanostructures, devices and multiphase materials.Despite its wide use, the mechanical and electrical interactions at the nanoscale betweenthe tip and the sample surface are not yet well understood. These phenomena should becarefully studied and modeled in order to avoid significant measurement artifacts.In the present contribution a study of the interactions occurring between conductive tipsand the surface of nanocrystalline silicon thin films that lead to measurement artifacts ispresented.A significant deterioration of the tip coating was detected after a few maps, resulting inmeaningless maps. The features of the map obtained dramatically depend on the tipcoating characteristics and on the load conditions. Moreover, under a constant bias voltage,the electrical current passing through the tip–sample junction degenerates strongly withtime.These phenomena were interpreted by considering the effect of strong electric fields presentduring C-AFM experiments.

Conduction mechanisms and charge storage in Si-nanocrystals metal-oxide-semiconductor memory devices studied with conducting atomic force microscopy

In this work, we demonstrate that conductive atomic force microscopy (C-AFM) is a very powerful tool to investigate, at the nanoscale, metal-oxide-semiconductor structures with silicon nanocrystals (Si-nc) embedded in the gate oxide as memory devices. The high lateral resolution of this technique allows us to study extremely small areas (∼300nm2) and, therefore, the electrical properties of a reduced number of Si-nc. C-AFM experiments have demonstrated that Si-nc enhance the gate oxide electrical conduction due to trap-assisted tunneling. On the other hand, Si-nc can act as trapping centers. The amount of charge stored in Si-nc has been estimated through the change induced in the barrier height measured from the I-V characteristics. The results show that only ∼20% of the Si-nc are charged, demonstrating that the electrical behavior at the nanoscale is consistent with the macroscopic characterization.

New insights on the post-BD conduction of MOS devices at the nanoscale

We have combined standard electrical tests with conductive-atomic force microscopy experiments to investigate the conduction of MOS devices after the dielectric breakdown (BD) of the SiO/sub 2/ gate oxide. In particular, the influence of the conduction nanometer scale parameters on the overall device post-BD current-voltage characteristics has been analyzed. The results show a nonuniform conductivity of the oxide at the BD area and that the total current flowing through the device is mainly driven by a very small fraction of that region.