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Magnetotransport Measurements

Magnetotransport Measurements

Created by ESW, Nov. 2017"

Background

Sample Prep

  • A vdp structure with In solder contacts can be used for all of these measurements, similar to room temperature Hall station. (Hall Probe Station)

  • For the PPMS, the sample should be <= 8 mm x 8 mm

. Double check with Banerjee student on exact dimensions, as this is dependent on the sample holder. Some sample holders support a rectangular geometry, but this is not a good idea- it makes the Rxy measurement very noisy.

  • Forming In solder contacts to transferred Bi films, or membranes (I assume) is difficult, as the heat makes the transferred film curl away from the iron. You will need to deposit metal contacts (i.e. in the CHA), and apply solder contacts to those or bond to them directly.

  • Developing a vdP mask and sample holder for small samples is currently ongoing

  • Loading into Ecopia Hall station is exactly like our room temp Hall station

  • For loading into the PPMS, need to wirebond to PPMS chuck (provided by Banerjee student who will be measuring your samples)

    • Do not wirebond directly to In solder contacts. You will clog the tip of the wire bonder. Even if it manages to make some contacts, it will clog eventually. If you do clog the tip of the wire bonder, or something else goes wrong tell Johnny, ESW, etc. so it can be changed. No one likes people who break the wirebonder. Wasserman's group has some extra tips, although they are a different brand than the standard one used.

    • Form a bond to the pad on the PPMS, extend the length of wire using the lab jack, cut the wire, and insert it into the In solder contact using tweezers (Rodolfo wirebonding method)

Rxy(B) (Hall Resistance) in samples with multiple carriers

  • The room temperature Hall station (upstairs at MER) uses a simplified version of the Hall equation that applies to samples with one type of carrier - i.e. most semiconductors, which are either dominantly n-type or p-type.

  • If you have one of the following types of films, multiple carriers are likely present, and you will need to use a more complete version of the Hall equation to calculate the mobility and carrier density (because the hall voltage is non-linear with the magnetic field)

    • Semi-metals: 2 carriers (electrons and holes)

    • Topological insulator: 2 carriers (bulk and surface)

    • Quantum-confined Bi: 3 carriers (bulk electrons, bulk holes, surface state carrier)

  • To determine Rxy(B), you need to use the PPMS, which is currently the only setup with a variable B)

Rxx(B) (Magnetoresistance) in high SOC materials (Bi, Bi1-xSbx, some metallic thin films)

  • Must be performed in the PPMS, which is currently the only setup with a variable B

  • Materials with high SOC often have a sharp dip in the magnetoresistance (Rxx(B)) at low temperatures, around zero field - this is called weak antilocalization (WAL)

  • Information about the number of transport channels present can be indirectly determined by fitting the WAL feature to the HLN equation.

    • The value of the alpha parameter loosely indicates whether surface states are present

    • Think of streaky RHEED indicating, but not concluding, good crystalline quality

    • Thickness-dependent measurements are necessary for a thorough determination of whether surface states are present

    • Our access to the PPMS is determined by how Banerjee students feel about measuring our samples, so it is difficult to measure enough thicknesses for a complete transport study. Instead, I usually combine PPMS measurement of a few samples with Gsh(T,d) measurements from the Ecopia to draw a general conclusion that can support other materials characterization or device measurement data.

Rxx(T) (Sheet Resistance)

  • Can be measured with Ecopia Hall station on main campus from 77 K - 300 K (LN2 cooled)

  • Or measured with PPMS from 2 K - 300 K (He cooled)

  • Used to determine whether sample is semiconducting or metallic

    • Gxx increases w/ increasing T - suggests semiconducting

    • Gxx decreases w/ increasing T - suggests metallic

    • When measured for various thicknesses can determine whether quantum confinement is occurring

    • For Bi, can help to determine whether surface states are present, when combined with other measurements

Reading

Ecopia Hall Station (Main Campus, Prof. Delia Milliron)

Contacts

  • Corey Staller (cstaller@che.utexas.edu ) - super user, contact for training before using system

  • Trained users as of Fall 2017: ESW, SDM, SEM (Sarah), AFB

Procedure

Setup

  • Testing contact quality on MER room temp hall station is advisable before driving to main campus with samples

  • Contact Corey to let him know when you'll be using Hall station, and so he can make sure LN2 is available in the lab (we do not have access to LN2 filling station)

  • Goggles are required everywhere in the lab

  • Do not wear gloves and touch the Hall computer

  • When arriving, fill small thermos or dewar with LN2

  • Cryo gloves and face shield are on top of cabinets in main hallway - LN2 is in room with chemical hoods

Room temp check

  • Load sample onto sample holder

  • The sample holder is conductive, so place a piece of glass slide between sample and holder (should be on side of hall station)

  • Hold in place with pegs on solder contacts (similar to room temp Hall station)

  • Rotate stage from loading position to measurement position

  • "Go to I-V Measurement" --> I-V measurement to check contact quality if necessary

  • Room temp Hall measurement - 5-10 iterations at room temperature - make sure values are stable with repeated measurements

  • Save data

T-Dependent Measurement

  • With sample in measurement position, pour LN2 into reservoir (black cylinder on top), and directly into chamber (through funnel on bottom left)

  • Wait for temperature to cool to 77 K

  • Allow system to warm back up to ~ 80 K

  • Cool to 77 K again

  • Repeat warm up to 80 K and cool down again - this helps stabilize the temperature

  • Set measurement to begin at 80 K - first couple values from 77-80 are not very reliable

  • Cover hole where funnel was placed with a glass slide to prevent moisture from entering system (this is very important for Corey's samples, so remember to do this)

  • T-dependent sheet resistance will be measured as sample heats up

  • When measurement completes, save data

Trouble Shooting

  • In solder contacts are reliable from 77 - 300 K. Other applied contacts, notably silver paste, do not work well below 150 K.

  • Measurement error - reset

    • Stop measurement

    • Save measurement data so far

    • Chang measurement conditions to start at slightly higher T value

    • Press reset button on Ecopia box on left

    • Start new measurement

    • If this occurs multiple times, can be a problem with one of the Ecopia contact pegs - sometimes they loosen over time. Ask Corey for help adjusting them.

  • If the current temperature is hotter than the set temperature at which the measurement should begin, the system doesn't know what to do, and will not measure your sample.

  • It is important that the substrate has a significantly higher resistance than the film you are measuring. This seems obvious but is frequently overlooked. If you are measuring a sample for a collaborator, make sure they know the expected resistance of each layer.

PPMS (MER, Banerjee MBE lab)

  • Outside (non-Banerjee) users are not allowed to use the PPMS independently. This policy is mainly a result of the damage their Lakeshore and Cascade probe stations have taken from allowing outside users in the last few years. Convincing them to allow a single, well-trained user, may be possible if necessary. Prof. Akinwande's post doc Deepyanti is currently pursuing this.

  • In the meantime, contact a Banerjee student to measure your samples. Tanuj and Hema have experience measuring TI thin films, including WL and WAL effects. Tanmoy and Sarmita mostly have experience in magnetic hysteresis measurements, which is not useful for us. Amritesh is trained to measure TI thin films, but has not used the system recently.

  • Procedure is off the record - refer to it to make sure your sample is being loaded/measured correctly, and to understand the measurement for your own use

Measurement FYIs

  • For each config you want to measure, you need to measure two orientations - i.e. for Rxx, you need RxxA and RxxB, and for Rxy you need RxyA and RxyB

  • Before fitting the raw measured data, you need to symmetricize it, and for Rxx, usually convert to sheet resistance

    • Symetricizing removes Rxx component from Rxy, and vice versa, which significantly affects data

    • RxyA = (RxyA(B+) - RxyA(B-))/2

    • RxxA = (RxxA(B+) + RxyA(B-))/2

    • For Rxx, Calculate sheet resistance from RxxA and RxxB using vdP equation - use vdP code below / NIST measurement algorithm to include geometry correction.

    • For Rxy, average RxyA and RxyB.

  • Make sure Rxy measurement is set up so that both orientations agree in sign - RxyA(B+) should have the same sign as RxyB(B+).

Setup

  • Get PPMS sample holder from Banerjee student

  • Wire bond your sample to PPMS sample holder

  • Can store your sample in Banerjee lab desiccator if necessary

Loading

Connections for Rxx and Rxy measurements

Measurements

Fitting

  • Using Gsh and Rsh instead of conductivity and resistivity is a good idea because it allows you to avoid making assumptions about which conductance values should be divided by the film thickness if only some of them are 3D (i.e. surface and bulk states both present)

  • Gsh is often plotted in units of e^2/h

  • Simultaneously fitting Rxy and Rxx improves accuracy of mobility and concentrations, since there are so many fitting parameters

    • You can either convert measured data to Gxy and Gxx, and fit the Gxx/Gxy eqs, or convert the Gxx/Gxy equations to Rxx/Rxy

    • Fitting works better if you enforce the sign of each carrier, if you're confident enough about which signs are present to do so.

Code (MATLAB)

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