Hall Measurement Procedure
Hall Measurements
Taken from NTS notes (March 2015) NTS, 05/16/2015
Last Updated/major overhaul: Aug 2017 by SDM
Sample Prep
Cleave into ~ 1cm^2 pieces, square
Use good material from center or near center of grown sample
Scratch back of sample before finishing cleaving to clearly indicate bottom side, as these are typically taken from center of doping cal
Solder In into sample corners
Do small triangular contacts at each corner
Too small can make it hard for Hall probes to make good contact
Is small triangles better than tiny crap triangles?
Use the "indium-only" soldering iron and indium material. Getting contaminants in the solder will degrade the contact material and lead to balling up of the solder, making it difficult/impossible to solder contacts to your sample. Run the soldering iron at 550 or 660 C.
Hall Measurements
Prep
Turn on water return
Turn on water supply
Check that flow meter is bobbling
Turn on power
Keithley Picoammeter -- small white box at top
Keithley Multimeter -- small white box at top
Keithley 706 Scanner -- large brown box at top, modules on the back control direction of current flow and contact flipping during measurements
Keithley 236 Source Measure Unit -- small brown box at middle
Sorensen power supply for electromagnet -- large black box at bottom
Hall power supply components
Mount Sample on holder carefully
Pins must contact each corner contact
Mount holder on carrier arm facing left
Slide into center of magnet
Be careful moving this so as not to slam holder into anything
The sample surface must be normal to the magnetic field
Cover the sample stage with foil. This limits any photocurrent, which will add error to your sample
Software
Make sure you are logged in under correct computer user: LASE
So the Hall software is the LASE version
If the computer has been sitting idle, the LASE user shouldn’t be logged in
Open Hall software
Enter Directory location where you want files saved
Enter sample name
Current Test number and Output File Location update automatically and can’t be set or altered in software. Current Test is to tell you which test instrument is on while running.
Setup parameters in Box 2
Magnetic field 5 kGauss
Excitation current 1.0000 mA
Film thickness: enter thickness of doped layer in um
Number of tests: usually 1
When to do multiple: If you want to calculate error bars
Weird or bad data
Notes are optional but suggested, e.g. GaAs Be doping cal 1e17 NTS
Double check parameters in all boxes are correct
Folder and file naming by software
Will take your sample name and truncate the last character to make a folder if it doesn’t already exist.
If the folder does exist, it will save files into that folder
Output filename will auto-update with each test run as set in box 2, with an underscore test number, i.e. _2 and so on
Completing your tests and re-running them will presumably over-write txt file results
You can rename your sample to re-run tests on it
Change the last character of the sample name to an unused one (e.g. E or R)
Rename as needed. It is easy to change filenames (F2) and move files as needed
Only Box 2 Data is saved into output txt file
If you change the file name more, be aware the output folder might change
Run test, arrow in upper left
Check current and voltage values during test
Check output folders and files
Check results as you go, before swapping samples
- Example of a good measurement for a 0.5 um Te-doped GaAs B170817A
Swap samples, reverse Steps 6 and 5
Slide carrier arm out and dismount sample holder, carefully
Dismount sample from holder carefully
Repeat process as needed for each sample (Steps 5-15)
Close Hall software
Log off LASE user when done
Troubleshooting
Don't ask Seth (or anyone one else, really) for help unless you can guarantee the contacts are ohmic
Seriously, he won't even entertain you unless you can prove it
The Hall station does not (despite what Seth thinks) generate an I-V curve for each contact. This would be a nice feature that a future student should implement.
You can use the Ecopia Temperature-dependent Hall station on main campus to generate an I-V curve if necessary. Contact Corey Staller / ESW / Sarah Muschinske.
If you want an I-V curve from this Hall Station...
Manually run the Hall measurement program for different excitation currents
Plot the I-V characteristic for the different contacts,
For example, should see a polarity change in the voltage between the 1->2 and 2->1 current directions (keep in mind the current in the output file will not explicitly be + or -, so just pick either 1->2 or 2->1 as "+" and the other as "-" when you decide to plot. Keep the same order for each subsequent contact pairing in the output file)
Alternatively, you can use a parameter analyzer, like the Keysight B1500, to test if the contacts are ohmic. This is a nice option because the resolution of the B1500 is much finer than the Hall station. The Hall station is limited to 100 nA excitation current resolution, which may be too coarse if your sample is super-resistive (i.e. low doping)
The most likely issue: bad contact(s)
Output file will show non-ohmic behavior
Excitation current specified by user does not match the current actually delivered to the sample
Schottky/rectifying behavior can only be correctly identified by plotting the I-V curve
Re-apply In
Sometimes just gently running the soldering iron over the existing contacts helps anneal the indium, making better contact
Possible the sample was damaged by the soldering iron (e.g. run too hot or touched sample without any indium on the iron) and need to make new sample. Do this if re-applying indium and annealing does not work
Contacts on back: Put contacts on reverse side of sample, Make new upright sample. For example, if you grow on an SI-GaAs sample, it is very difficult to make ohmic contacts to the SI side. Also, the background doping level of the SI-GaAs is ~1e12 cm^-3
Sample is too resistive (i.e. not doped enough) to be measured. Becomes an issue for sample near 1E15 cm^-3 and lower
The Hall station does not reliably measure doping levels below the mid to high 1E15 range
The easiest way to overcome this is lowering the excitation current. Note, the lowest value allowed is 100 nA. Also, the lower the current, the smaller the voltage signal, which may not be detected accurately detected (or possibly not at all...).
The compliance of the measurement system is 10 V. From Hall Effect theory, we can plot the expected Hall voltage vs. current (I_excitation), carrier concentration (n), doped region of thickness (t), and magnetic field (B). See image below for an example
Miscaluated doping range: you dramatically over- or under-shot the doping value you expected. The dopant cells do not follow a linear trend over orders of magnitude. Consult doping data from previous growth campaigns to get a ballpark for where the different doping ranges are.
Check compliance voltage and current in outputfile
Compliance voltage of the Hall station is 10 V
If the current measured is much smaller than the desired excitation, then the system might be hitting voltage compliance of, so lower the excitation current
Normally, hitting compliance occurs if (1) the contacts are bad or (2) your samples have low doping thus are more resistive, around 1e16 cm^-3 or less