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====== Basics ======
[[http://www.nist.gov/pml/div683/hall.cfm|Overview]] of Hall effect and measurement techniques from NIST.  Must read!

{{:hall:hall_procedure.pdf|Hall Procedure}} - Instructions for doing Hall effect measurements on the Lakeshore system.

{{:hall:lowthall.pdf|Low Temperature Procedure}} - Instructions for operating the MMR cryostat.

{{:hall:hightemperaturehallsystemguide.pdf|High Temperature Guide}} - Instructions for installing, operating, and troubleshooting the high temperature oven.

{{:hall:Instruction_for_manual_magnetic_power_supply.pdf|Magnet power supply guide}} - Instructions for manual operation of magnet power supply.

{{:Troubleshooting.pdf|Troubleshooting}} - Problems and solutions in the past.

Add reminder to update user log

After #4/installation, note pressure to at least 10^-4 Torr (should get to 10^-6) depending on last use (15 - 30 mins to 10^-4 Torr and a day or so to 10^-6, depending on previous usage and cleanliness).  This is noted under #9/installation, but should be repeated in #4/installation

Between steps 3&4 under operation, insert note about cleaning probe with alcohol/acetone before inserting, and put in general warning about keeping finger grease off probe during general use. gloves.

Add section on shutdown: temp -> RT; pump off; open vent valve; disconnect hoses/cables; put pole pieces back

Add section on safety - should the system be watched at all times?  verify that shutdown procedure is in place, Ar perssure higher than xx psi. New cylinder (2500psi) lasts 3-4 days. Getting low is 1000 psi.  500 psi = get a new cylinder. What are typical times? 

Add comment about iv curves not being done at all T- possibly good to go to high T to check whether different excitation currents will be necessary.
====== Users ======
These people are allowed users of the Hall system. Everyone is to (a) sign in on the Hall log in Weniger 481 every time the system is used (**failure to do so will result in suspension of privileges**), (b) book the system on the google calendar so that people can manage schedules (if someone has signed up and you are using the system, you can be bumped).  Do not switch between room temp Hall and high temp Hall probes unless authorized.
|^Name ^PI/group & dept^email^other contact^trained by/date^High-T Hall?|
|Janet Tate|Tate, Physics |tate@physics.oregonstate.edu|541-7371700 (office)|Janet 1995!|yes|
|Kelda Diffendaffer|Tate, Physics |diffendk@oregonstate.edu|?|Aaron,Janet 1/2018|yes|
|Acacia Patterson|Tate, Physics |patterac@oregonstate.edu|?|Janet 10/2018|no|
|Jenn Amador|Keszler, Chemistry |amadorj@oregonstate.edu|?|Kelda 7/2018|no|
|Aaron Dethlefs|PMIC |dethleaa@oregonstate.edu|?|Janet 2017|yes|
|Linda Engelbrecht|PMIC |lindaengelbrecht@pmiclab.com|?|Janet 2015|yes|
|Robert Kokenyesi|Tate/Keszler/Engineering |robert.kokenyesi@oregonstate.edu|?|Janet 2010|yes|
|Bokun Zhou|Wang, EECS |zhoubok@oregonstate.edu|?|Aaron?, 2018|no|
|Yunfei Bo|Wang, EECS |boyu@oregonstate.edu|?|Requested permission/NOT YET CONFIRMED, 2019|no|
|Dylan Kearney|Amorphyx|dkearney@amorphyx.com|?|Acacia 6/2019|no|





======= Web pages, presentations, etc ======
 {{:hall:hall_jmr_20141028.pdf|Some Hall effect pitfalls}}- Janet's presentation on some Hall effect pitfalls - includes some instrument limits.

{{:hall:keithley021711_halleffectmeasurements.pdf|Keithley Hall Measurement Presentation}} - More information about how to get good data.

{{:hall:manual_app_a_hallmeasurementsystem.pdf|Hall Manual Appendix A}} on Hall measurement geometries and general overview.

[[http://www.LakeShore.com|LakeShore]] website.  Information on many products from LakeShore, including the Hall Measurement System (HMS).  Vaden West (vwest@lakeshore.com) is the rep there.  He sold us the original system. 

[[https://software.lakeshore.com|LakeShore software site]].  Check here for upgrades to the software system.  Login and password are both LakeSoft.

{{:hall:hallcatalog_h.pdf|HMS (hi res) catalog}} describes HMS system and accessories.

{{:hall:the_hall_effect_look_s_paper.pdf|Look's paper}} on Hall measurements.

{{:hall:standard_test_methods.pdf |NIST Standard test methods}} for data collection from Hall measurements.

D.K Schroder Elec. Char. Mat. Ch. 8 on {{:hall:mobility.pdf|mobility}} of carriers.

{{:hall:vdpauw_phillresrep13_1958_vdpgeom.pdf|Van der Pauw's paper}} on determining resistivity of flat samples: Phillips Research Report 1958 

[[http://www.mmr-tech.com/|MMR]] presentation on [[http://www.mmr-tech.com/PDFs/jThomsonPPT.pdf|Joule-Thomson refrigeration]] and on [[http://www.mmr-tech.com/PDFs/opticalStudiesPPT.pdf|optical cryostats]] (which we adapted for Hall measurements).

====== About the Equipment ======
The Lakeshore 7504 **Hall Measurement System** (**HMS**) is located in WGR 481. It measures the Hall coefficient and resistivity in the van der Pauw and bar geometries.  It is set up for room temperature measurements of thin-film and pressed pellet samples (size of order 5-10 mm), but we have some variable temperature capabilities - 80 K (LN2 cryostat), 80K to ambient (MMR cryostat; not currently functional) and ambient to ~600 K (high-T Hall oven).

The **electronics rack** is almost always on, except for the magnet power supply (safer to leave a high current supply off). Turn on the **magnet power supply** on when you come in. Turn it off when you leave. Make sure that the MPS is set to **remote** mode rather than local (Two buttons on the top right of the MPS). Sometimes it will switch when power to the rack is lost. If the rack has been turned off, turn it on. Allow an hour for warm up of the rack. The magnet is cooled by water (supply on the roof).  Turn on the water supply on when you arrive (open out first, open in second) and off when you leave (close in first, close out second). The room is cooled by a floor AC unit set to 72 degrees.  Leave it on. Windows remain closed.

The **electromagnet** has a set of 4" poles and a set of 1" poles.  The 4" poles produce a field of 9.8 kG at standard separation (just enough to accommodate the sample shield plus Hall probe); the 1" poles achieve about 15.4 kG.  You can push it slightly higher by removing the sample shield (risk temperature gradients and electrical disturbances. You can also pull the Hall probe out a little to trick the software into thinking the field is lower than it actually is, but this shouldn't be necessary to get good data.  If you do decide to do this, you can't use the calculated values from the program.

The **sample** goes in facing north (the electronics rack).  Even though there is a missing pin, so things look asymmetric, it is possible to put the sample in in both ways.  You'll get nonsense if it's mounted the wrong way! Use the sample shield (black box that covers sample and screws in). It protects the sample from drafts (temperature variations) and electrical disturbances. Handle the sample probe by the blue junction box rather than the tube.  It's safer.

Make sure the **Hall probe** is in and right side up (the text should be oriented correctly).  If you forget to put it in, or take it out for too long (e.g. while loading a sample), or put it in upside down, the magnet power supply will ramp up to the maximum current in a failed attempt to get the field down to zero, causing it to beep and shut off.  If this happens, just reset the power supply and continue. The Hall probe must be carefully placed as close to the sample as possible.  Sight from the top down and from the side to ensure that it is in exactly the right spot. The Hall probe has been abused in the past.  Be very careful with it.

The **ammeter** maxes out at 2mA.  If you want an excitation current over 2mA, then you have to short it (back of the rack, move the BNC cable from "2mA max" to "Shorted."  The software will prompt you when you go over 2mA, but it will not prompt you when the excitation current is below 2mA, so if you're not able to measure a current, check that the last user didn't leave the ammeter shorted.

Sometimes the HMS software has trouble communicating with the equipment.  If this happens, you can try just restarting the program, but this usually doesn't work.  The most successful approach is: do Ctrl-Alt-Delete and bring up the Task Manager, then kill all the HMS* processes.  Failing that, restart everything.

**Software**: Open "HMS". You do a manual resistance measurement, field measurement or temperature measurement (top bar, right). First step is usually an I-V measurement ... to be continued.



====== Getting good data ======
There are a few ways to ensure you get good data.
  * Use the Hall mask to pattern the sample; contacts to mask-samples are less critical.  For an unpatterned sample, use small contacts on the sample periphery.  If your contacts are large or extend well into the sample, you're guaranteed to get garbage out, and it's basically impossible to fix once you've got too much indium on the sample.
  * Check the resistance through each pair of contacts.  If one contact seems to be particularly high, remake it.
  * Try to get the voltage as close to 5V as possible by modifying the excitation current.
  * Use a longer dwell time.  The default is 2 s, but 5 s will give you better data.  Note that increasing the dwell time will also slightly increase the voltage, so make sure to check the voltages with the dwell time you want to use.
  * If there are any red lights in the resistance window, fix them before continuing.  The current light usually means the ammeter is shorted.  Others generally mean you need to back the excitation current off a bit.
  * Do an IV scan over the range -I_exc to I_exc.  Use the default contacts.  Do this measurement first to make sure the contacts are Ohmic and to make sure that the voltage isn't going too high.
  * If your mobility is negative when the field is applied in one direction and positive when the field is applied in the other, check your contacts.  You can't reasonably claim to average -2 and +4 to get a mobility of 1.
  * Collect data at several fields (usually 15kG to 20kG in 2.5 kG steps is sufficient) and make sure your data doesn't have any wild fluctuations in it (be sure to check the y axis scale before deciding that your data is junk!).
====== Contacts ======
Good contacts are essential and are the most difficult part of the process. It helps immensely to use the microscope and to hold the sample with the stage.
  * Indium can be melted/soldered onto some materials. Use a very fine-tipped solder iron and keep the temperature as low as possible to avoid oxidizing the indium. Keep one solder tip for indium exclusively.
  * Linda Engelbrecht from PMIC has had good success with graphite paint connections to polished p-type Si.  She uses //Graphite Conductive Adhesive 112// (from EMS - Electron Microscopy Services) @ $12.50/30gm. Adhesive 112 is an air drying graphite coating of unusually high conductivity. It provides excellent static bleed properties and acts as a protective energy absorbing layer. It also offers good shielding performance (30-50 db over 50-450 MHz) at a coating thickness of 2 mils. It is water based and useful in solvent prohibited applications.
To use: Air dry until all water has flashed off. Air dries to touch in 20 minutes, to handle in 25 minutes. It will continue to harden for 24 hours. It can be forced dried at temperatures up to 160°F (71°C).
Service temp 177°C (350°F)
Sheet resistance **<50** Ohms/Square @ 1 mil

Good van der Paul contacts are as small as possible and located on the periphery of the sample.

Other EMS options are 

//Carbon Conductive Adhesive 502// @ $16/30gm, a High Temperature Conductive Paint. 
Conductive adhesive 502 is a combination of specially processed carbon particles in a fluoroelastomer resin system designed to provide high resistance values. In its cured form, it exhibits both high and low temperature flexibility and moisture resistance.

ADVANTAGES:
Withstands ambient temperatures of over 500°F (260°C)
Remains flexible over temperature range of –40°F to over 500°F (260°C)
Cures at room temperature
Good adhesion to a variety of substrate
Excellent oxidation resistance
Ready to use - easy to apply
Max service temperature 525°F (275°C)
Sheet resistance **130** ± 100 ohms/sq.in./1 mil dry film
 

//Silver Conductive Adhesive 503// @ $39/15gm, a High Temperature Conductive Paint.
Silver paint 503 is a flexible, high temperature conductive material designed for a wide variety of uses, and adheres to most substrates.

ADVANTAGES:
Withstands ambient temperatures of over 500°F (260°C)
Remains flexible over temperature range of –40°F to 500°F
Highly conductive – good adhesion to substrates
Dries at room temperature
Ready to use – easy to apply
Service temperature 525°F (275°C)
Sheet resistance **0.05** ohms/sq. in/ 1mil dry film

====== Upgrades ======
JT discussion with Vaden West 3/9/2012: Upgrade to high temperature capability (800K=527C) costs $30k, and includes a new insert (including breakout box) that interchanges with present room temperature probe.  Also includes LakeShore 340 Temp Controller + TC card (340 with inbuilt cernox card would work for low T accessory, too).  VW said that B-field obtainable is 1.3T at room temp with 4" polecaps and a 1" gap, and with oven inserted can get to 0.87T with 4" polecaps and 2" gap, and Lead time 3-4 months.  Software updates - last one in January 2011 ver_3.9. Check website for hardware requirements.
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