The study of material properties is an important part of modern material science. The so-called material properties refer to the quantitative measurement and description of the functional characteristics and utility of materials, that is, the reaction of materials to electrical, magnetic, optical, thermal and mechanical loads. Source meter SMU plays an important role in the research of modern materials science. The selection of SMU suitable for electrical performance test of certain materials, how to reduce the test error, and what should be paid attention to in the test need to be focused on. Tektronix enjoys a high reputation among engineers and scientists in many disciplines around the world. Its high-precision source meter (SMU), multimeter, precision power supply, micro signal testing and data acquisition products, together with Tektronix’s original product line, provide a variety of testing solutions for contemporary materials science research.
The current materials electrical testing class series covers the cutting-edge electrical transport and quantum / superconducting materials testing, one-dimensional / carbon nanotube materials testing, two-dimensional materials and graphene testing, and nano materials application testing. Today, I’d like to share with you the first part of the series of “contemporary materials electrical testing class”: nano testing (Part I).
Nanomaterials refer to the materials with at least one dimension in the nanometer scale (1-100nm) in three-dimensional space scale. They are a new generation of materials composed of nanoparticles with sizes between atoms, molecules and macroscopic systems. Nanomaterials can be classified according to various scales, and can be divided into zero dimensional materials – quantum dots, nanopowders and nanoparticles; One dimensional materials – nanowires or carbon nanotubes; Two dimensional materials – nano film, graphene; Three dimensional measurement – nano solid materials. According to the composition, it can be divided into: metal nano materials, semiconductor nano materials, organic polymer nano materials and composite nano materials. The following figure shows the classification of nanomaterials according to their physical properties and the application of nanomaterials. It can be seen that nanomaterials have been widely used in many fields.
Characteristics of nano materials and electronic devices
Because the one-dimensional or multi-dimensional size of nanomaterials is nanometer, they have many characteristics different from macromaterials. The basic characteristics of nanomaterials include: surface and interface effects, such as lower melting point and higher specific heat; Small size effects, such as conductor becoming unable to conduct electricity; However, insulators begin to conduct electricity and have superhard properties; Quantum size effect and macroscopic quantum tunneling effect. The physical and chemical properties of nano materials are: high strength, high toughness; High specific heat and coefficient of thermal expansion; Abnormal conductivity and diffusivity; High magnetic susceptibility.
Based on the above characteristics, nano materials are widely used to make nano electronic devices. Nano electronic devices refer to electronic devices with nano scale and specific functions designed and prepared by using nano scale processing and preparation technology. Nano electronic devices include nano CMOS devices, such as silicon on insulator MOSFET, silicon germanium heterojunction MOSFET, low temperature MOSFET, bipolar MOSFET, intrinsic silicon channel tunnel MOSFET, etc; Quantum effect devices; Quantum interference devices, quantum dot devices; Resonant tunneling devices such as transverse resonant tunneling devices, resonant tunneling transistors, resonant tunneling field effect transistors (rteets), bipolar quantum resonant tunneling transistors, resonant tunneling thermoelectronic transistors, etc; Longitudinal resonant tunneling devices, such as tunneling barrier modulation transistors, etc; Single electron devices, such as single electron box, single electron transistor with capacitance and resistance coupling, single electron neural network transistor, single electron junction array, single electron pump, single electron trap and single electron revolving gate, etc; Single atom devices and single molecule devices, such as single electron switch, single atom point contact device, single molecule switch, molecular wire, quantum effect molecular electronic device, electrochemical molecular electronic device, etc.
Electrical properties test of nano materials
The characterization of nanomaterials includes composition analysis, particle analysis, structure analysis and performance analysis. The main analysis methods are electron microscopy, especially scanning tunneling microscopy (SMT), which has advantages in the analysis of conductor and semiconductor nanomaterials.
The measurement of electrical properties of nanomaterials is to analyze the density of state. The so-called density of States refers to the number of electrons allowed in the unit energy range, that is, the distribution of electrons in a certain energy range. The density of States is a microcosmic quantity, which is suitable to explain the characteristics caused by the size change of nanoparticles.
X-ray spectroscopy is a conventional method to measure the density of states. However, the density of states can be obtained by directly measuring the electrical properties of nano materials. The density of states (DOS) can be obtained from the curve of differential conductance (di / DV) versus voltage measured by scanning tunneling electron microscopy (STM). In this method, a low-level AC signal is used to modulate the quiescent current for testing, and there is a high resistance contact between the electron microscope electrode and the tested sample.
Because X-ray spectroscopy and scanning tunneling electron microscopy are expensive equipment, if it is not to prepare and characterize nano materials, it is only to carry out applied research on nano materials. Source surface (SMU) + nano probe stage is a cost-effective alternative. Different from the scanning tunneling electron microscopy (STM), the contact between the nanoprobe stage and the tested sample is low resistance, which requires that the SMU must have the ability of low-level testing, and change the working mode of the SMU according to the impedance of the tested sample. This method is mainly used to test the resistance, resistivity and Hall effect of the sample, which is more suitable for the testing of nano electronic devices.
Resistivity measurement of two dimensional nano materials
For two-dimensional nano materials (such as graphene), resistivity measurement is an important test item. The four point collinear probe method and the Vander Pauw method are the main test methods.
Hall effect measurement of two dimensional nanomaterials
When the current passes through the semiconductor perpendicular to the external magnetic field, the carrier will deflect, and an additional electric field will be generated perpendicular to the direction of the current and magnetic field, thus creating a potential difference at both ends of the semiconductor. This phenomenon is called Hall effect, which is also called Hall potential difference. By measuring the potential difference, the carrier concentration and carrier mobility of the material can be obtained. The Hall effect of two-dimensional nanomaterials is still measured by Vanderberg method, but the electrode connection is different from the resistivity measured by Vanderberg method, and the magnetic field is usually used to test the Hall effect.
Challenges in electrical testing of nanomaterials and electronic devices
·Nanometer size, performance is different from macro size materials and devices
·The state changes quickly, which requires the response speed of the test instrument
·Need to cooperate with nano probe stage
·It is necessary to prevent self heating, otherwise it is easy to burn the tested sample, so the SMU with pulse mode should be selected
·Nano materials bear and test current is very small (up to FA level), bear and test voltage is very low (up to NV level), different kinds of materials, resistance range is very wide, from u Ω to t Ω, we need to choose SMU which is suitable for the electrical performance of nano materials and devices to be tested, we need a variety of means to reduce error and noise, such as current measurement or pressure current measurement, four wire connection, shielding and filtering, Reduce thermal noise, etc.
The electrical test scheme of nanomaterials will be detailed in the “nanowire / carbon nanotube test scheme” and “two-dimensional / graphene material test scheme”. Schematic diagram of SMU application scenario, test characteristics and selection principles for electrical testing of nano materials, and select the most suitable SMU in combination with the types and test points of nano materials or nano electronic devices to be tested. 4200 – SCS is suitable for testing almost all kinds of nanomaterials. Of course, some special source tables are more suitable for some special applications.