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Resistivity measurement, Summaries of Physics

Indian Institute of TechnologyPhysics

study of resistivity measurement

Typology: Summaries

2017/2018

Uploaded on 11/05/2018

Sanjeev993
Sanjeev993 🇮🇳

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Objective:
To measure and study the temperature dependent resistivity of VO2 compound using home-made
four probe setup.
Materials/Apparatus required:
• Sample of VO2
• Home-made four probe setup as shown in Fig.2
• Current source (AC and DC current source-Keithley (Tektronics Company))
• Temperature controller (Lakeshore, Model-335)
• Nano-voltmeter (Keithley, Model-2182A)
• Combination of rotary and diffusion pump (from Hind High Vacuum Co. (P) Ltd.)
• Desktop with labview software installed in it.
Theory:
VO2 sample-
It is inorganic oxide where Vanadium is present in +4 oxidation state and Oxygen in -2 oxidation
state i.e. it is Vanadium(IV) oxide.
Structural properties:
At temperatures below Tc=340 K, VO2 has a monoclinic (space group P21/c) crystal structure
and above Tc, the structure is tetragonal, like rutile TiO2. In the monoclinic phase, the V4+ ions
form pairs along the c axis, leading to alternate short and long V-V distances of 2.65 Å and 3.12
Å. In comparison, in the rutile phase the V4+ ions are separated by a fixed distance of 2.96 Å. As
a result, the number of V4+ ions in the crystallographic unit cell doubles from the rutile to the
monoclinic phase.The equilibrium morphology of rutile VO2 particles is acicular, laterally
confined by (110) surfaces, which are the most stable termination planes. The surface tends to be
oxidized with respect to the stoichiometric composition, with the oxygen adsorbed on the (110)
surface forming vanadyl species. The presence of V5+ ions at the surface of VO2 films has been
observed by x-ray photoelectron spectroscopy (XPS) measurements.
Electronic properties:
At the rutile to monoclinic transition temperature,VO2 also exhibits a metal to semiconductor
transition in its electronic structure: the rutile phase is metallic while the monoclinic phase is
semiconducting. The optical band gap of VO2 in the low-temperaturemonoclinic phase is about
0.7 eV.
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Objective: To measure and study the temperature dependent resistivity of VO 2 compound using home-made four probe setup. Materials/Apparatus required:

  • Sample of VO 2
  • Home-made four probe setup as shown in Fig. 2
  • Current source (AC and DC current source-Keithley (Tektronics Company))
  • Temperature controller (Lakeshore, Model-335)
  • Nano-voltmeter (Keithley, Model-2182A)
  • Combination of rotary and diffusion pump (from Hind High Vacuum Co. (P) Ltd.)
  • Desktop with labview software installed in it. Theory: VO 2 sample - It is inorganic oxide where Vanadium is present in +4 oxidation state and Oxygen in - 2 oxidation state i.e. it is Vanadium(IV) oxide. Structural properties: At temperatures below Tc=340 K, VO 2 has a monoclinic (space group P2 1 /c) crystal structure and above Tc, the structure is tetragonal, like rutile TiO 2. In the monoclinic phase, the V4+^ ions form pairs along the c axis, leading to alternate short and long V-V distances of 2.65 Å and 3. Å. In comparison, in the rutile phase the V4+^ ions are separated by a fixed distance of 2.96 Å. As a result, the number of V4+^ ions in the crystallographic unit cell doubles from the rutile to the monoclinic phase.The equilibrium morphology of rutile VO 2 particles is acicular, laterally confined by (110) surfaces, which are the most stable termination planes. The surface tends to be oxidized with respect to the stoichiometric composition, with the oxygen adsorbed on the (110) surface forming vanadyl species. The presence of V5+^ ions at the surface of VO 2 films has been observed by x-ray photoelectron spectroscopy (XPS) measurements. Electronic properties: At the rutile to monoclinic transition temperature,VO 2 also exhibits a metal to semiconductor transition in its electronic structure: the rutile phase is metallic while the monoclinic phase is semiconducting. The optical band gap of VO 2 in the low-temperaturemonoclinic phase is about 0.7 eV.

Four probe method - Four probe apparatus is one of the standard and most widely used apparatus for the measurement of resistivity of semiconductors. This method is employed when the sample is in the form of a thin wafer, such as a thin semiconductor material deposited on a substrate. The sample is millimeter in size and having a thickness w. It consists of four probe arranged linearly in a straight line at equal distance S from each other. A constant current is passed through the two probes and the potential drop V across the middle two probes is measured. An oven is provided with a heater to heat the sample so that behavior of the sample is studied with increase in temperature as shown in Fig. 1. Fig.1. Schematic of Four Probe set-up At a constant temperature, the resistance, R of a conductor is proportional to its length L and inversely proportional to its area of cross section A. 𝑅 = 𝜌 𝑙 𝐴

where ρ is the resistivity of the conductor and its unit is ohm-meter. A semiconductor has electrical conductivity intermediate in magnitude between that of a conductor and insulator. Semiconductor differs from metals in their characteristic property of decreasing electrical resistivity with increasing temperature. According to band theory, the energy levels of semiconductors can be grouped into two bands, valence band and the conduction band. In the presence of an external electric field it is electrons in the valence band that can move freely, thereby responsible for the electrical conductivity of

Fig.2 Schematic of Four Probe Experimental method used to measure the resistivity of VO 2 Fig.2 shows the Experimental arrangement that we have used to measure the resistivity of VO 2 .Here, four contacts are made on sample which is attached to sample holder(having heater(made up of Nichrome wire) and temperature sensor(PT100)).The middle two contacts are connected to Nano-voltmeter( Keithley ) to measure the voltage between those probes to avoid any contact resistance and the outside two contacts are connected to current source meter( Keithley ) to supply current to sample. The heater and temperature sensor are connected to temperature controller ( Lakeshore )to measure the temperature at constant intervals. This whole setup is controlled by Lab-View program. Description of instrument made for resistivity measurement in high temperature region(300-600K ) Fig.3 Schematic of Home-made four probe set-up for high-temperature resistivity measurement It is simple cylindrical shape instrument. The top portion has wire connections which are to be connected to Nano-voltmeter, current source meter and temperature controller. To achieve vacuum, the whole instrument is to be connected to vacuum pump assembly consisting of rotary and diffusion pump (described later).The sample holder assembly consists of heater(Nichrome wire)and temperature sensor(PT-100).Sample(made up of copper because it is a good conductor and has less linear coefficient of thermal expansion) is placed on mica sheet ,not directly on

sample holder so that the bottom surface acts as an insulator to avoid flow of heat from bottom surface.PT-100 is used as temperature sensor as its resistant varies linearly with temperature. Description of various other electrical instrument and components used-

  • Vacuum pump assembly- The vacuum pump assembly is used to maintain vacuum inside the sample chamber so that sample is maintained at constant set temperature and heat is not exchanged with the surroundings. The assembly consists of Rotary pump and diffusion pump. Here, rotary pump is used for roughing i.e. to achieve pressure upto 10-^3 mbar and diffusion pump as backing pump i.e. to achieve pressure upto 10-^6 mbar. Rotary pump- Fig.4. Rotary pump with schematic of various components of it.
  • It is used as roughing pump to achieve pressure upto 10-^3 mbar.A rotary vane vacuum pump is an oil-sealed rotary displacement pump.
  • The pumping system consists of a housing (1), an eccentrically installed rotor (2), vanes that move radially under spring force (3) and the inlet and outlet (4). The outlet valve is oil-sealed. The inlet valve is designed as a vacuum safety valve that is always open during operation. The working chamber (5) is located inside the housing. Rotor and vanes divide the working chamber into two separate spaces having variable volumes.as shown in Fig.4.
  • As the rotor turns, gas flows into the enlarging suction chamber until it is sealed off by the second vane. The enclosed gas is compressed until the outlet valve opens against atmospheric pressure. In the case of gas ballast operation, a hole to the outside is opened, which empties into the sealed suction chamber on the front side. The Rotor moves with the help of a motor attached to it.
  • Generally cold traps and baffles are utilized between the chamber and the diffusion pump to minimize backstreaming, although this results in some loss of pumping ability. The oil of a diffusion pump cannot be exposed to the atmosphere when hot. If this occurs, the oil will burn and has to be replaced. Temperature controller ( Lakeshore Model-335) Fig.6. Temperature controller (Lakeshore Model-335) front-view As the name implies, a temperature controller is an instrument used to control temperatures. The temperature controller takes an input from a temperature sensor and has an output that is connected to a control element such as a heater. The basic function of a controller is to compare the actual temperature with its set point and produces an output that will maintain the set point. There is different type temperature controller on the basis of various applications. There are three type of controller namely
  1. On/off type
  2. Proportional type
  3. PID type PID type -
  • This type of temperature controller is used in our experiment. PID controls provide more accurate and stable control than on/off or proportional controller typese.it is best for those systems that have a relatively small mass and which react quickly to changes in temperature added in process.
  • This controller combines proportional control with two additional adjustments, which helps the unit automatically compensate for changes in the system. These adjustments, integral and derivative, are expressed in time based unit; they are also referred to by their reciprocals, RESET and RATE, respectively.
  • Rate and reset are methods used by controllers to compensate for offset and shifts in temperature. The offset can be compensated for manually or automatically. Using manual reset. The user will shift the proportional band so that the process will stabilized at the set point. Automatic reset will integrate the deviation signal with respect to time and the integral is summed with the deviation signal to shift the proportional band.
  • The output power is automatically maintained to bring the process temperature back to set point. The rate or derivative function provides the controller with the ability to shift the proportional band to compensate for rapidly changing temperature. The amount of shift is proportional to the rate of temperature change. Nano-voltmeter (KeithleyModel-2182A) Fig.7 Nano-voltmeter (Keithley Model-2182A) A voltmeter is an instrument used for measuring electrical potential difference between two points in an electrical circuit. When extremely small voltages and currents are measured, special demands are imposed on the measuring equipment, which must detect a weak signal in the background of significant noises. Nano-voltmeter is optimized for making stable, lownoise voltage measurements and for characterizing low resistance materials. Current source meter ( Keithley AC and DC current source-Keithley (Tektronics Company) ) Fig.8 Current source meter(AC and DC current source-Keithley (Tektronics Company))

be wired to the addition function so that the indicator displays the sum of the two controls.

  • Thus a virtual instrument can be run as either a program, with the front panel serving as a user interface, or, when dropped as a node onto the block diagram, the front panel defines the inputs and outputs for the node through the connector pane. Procedure-
  1. Attach the mica sheet to sample holder using G varnish before placing the sample on it.
  2. Using G varnish stick the sample on mica sheet.
  3. Using silver paste make four contacts on the sample, inner two for current and outer two for voltage.
  4. After the contact gets dried, measure the contact resistance.Ensure that contact resistance is very small.
  5. Close the instrument and connect it to vacuum pump assembly. Make the electrical contacts to Nano-voltmeter, current source meter and Temperature controller. Once the vacuum in chamber reaches 10-^6 mbar. Start the experiment.
  6. Set the initial and final temperature in labview program and set the step size also. Initial temperature 300K Step size 2K Final temperature 600K
  7. Once all the required conditions are achieved and connections are proper. Start the program.
  8. Program will automatically note down the resistance as a function of temperature.
  9. Plot the obtained data using origin and before that convert the resistance into resistivity using eqn.1 and below mentioned parameters values. Distance between voltage probes(mm)

Thickness(mm) 0. Width(mm) 0.

Graph-

  • Plot between Resistivity Vs Temperature for VO 2 300 350 400 450 500 550 600

^ (ohm-m) X

T(K) As it is clear from above graph, there is sharp transition from metallic to semiconducting phase near 330K which may be accompanied by a structural change from tetragonal lattice in the M phase to monoclinic lattice in I phase as reported in research papers.(Ref.4 and Ref.5) Results and discussions

  • There is sharp electronic phase transition from high temperature metallic (M) phase to low temperature semi-conducting (I) phase near 330 K. References
  • https://en.wikipedia.org/wiki/LabVIEW
  • https://en.wikipedia.org/wiki/Vanadium(IV)_oxide
  • https://en.wikipedia.org/wiki/Vacuum_pump
  • Constant threshold resistivity in the metal – insulator transition- J.Cao,W.Fan (PRB B 82,241101(R)(2010))