Phys 317, Instrumentation LabD. W. Koon, Fall 2008

ELECTRONIC CHARGE TRANSPORT

(Cryogenics, Charge Transport, Evaporators, Vacuum systems)

WARNING: Liquid nitrogen is hazardous. Please treat it with respect. Any horseplay, or any kind of play for that matter involving liquid nitrogen puts you, your labmates, and your lab grade in peril.

NOTE: Some of these exercises are followed by homework [HW] questions I’ll ask you to hand in next week.

Week I. RESISTIVITY VS TEMPERATURE for metals and semiconductors.

Use a four-wire resistance meter to measure the resistance of a piece of wire. Compare its resistance at room temperature to its resistance when dunked in a dewar of liquid nitrogen. Repeat for a carbon resistor (very important: make sure it is NOT a metal-film resistor).

HW: (1) Describe the difference between the temperature behavior. Find out why this is characteristic of metals and semiconductors.

Week I. CRYOGENIC MEASUREMENT of resistivity and Hall coefficient of a metal.

  1. The cryostat has a glass slide with a palladium film of 40nm thickness attached to the sample mount. Locate it. Also locate the temperature diode that will be used to measure temperature. When you think you’ve found it, press your finger to it and verify that the temperature controller records an increase in temperature (unless you’re a reptile), and that T drops again when you stop touching it.
  2. Locate the LabVIEW software on the desktop of the lab computer. Start it up. Your instructor will show you the key features. Make sure that the metal specimen is connected to the instrumentation, and begin the program. You will be measuring both the Hall coefficient and the resistivity.
  3. Make sure that all the electrical connections to the specimen are working. Make sure it is connected to the instruments.
  4. Screw the inner (cylindrical) thermal shield into place. Place the cryostat inside the outer shield and clamp it shut.
  5. Mount the cryostat inside the magnet. Turn the magnet on, adjust the current to 2.000A.
  6. Turn on LabVIEW program. Be sure that you are saving the data to a new file (with a descriptive title) on the T: drive. Keep an eye on the program throughout the experiment to make sure that you still have full electrical contact with the specimen and that the voltmeter does not overload. If you need to fiddle with the meters or the wiring, be sure to turn off the “Collect data?” button and wait for the present measurement cycle to stop.
  7. Pump a vacuum on the outer thermal shield of the cryostat. Pump for several minutes before shutting off the valve again.
  8. Fill a portable dewar with liquid nitrogen.
  9. Place a funnel in the top [vertical] port of the cryostat and start to slowly fill it with LN2. Monitor the temperature and LabVIEW graphs as you do so.
  10. When the temperature inside the cryostat has settled near liquid nitrogen temperature (77K), you’re ready to use a trick to lower it further. Make sure the cryostat has a good amount of LN2 inside, then pump on the LN2 exhaust port while covering the filling port with a rubber stopper. Keep pumping (temperature should drop) until the temperature starts to rise again.
  11. At some point, the liquid nitrogen revaporizing will probably cause the stopper to come off. When that happens, repeat the last step (pumping on nitrogen). Make sure you have a good column of LN2 inside the cryostat first.
  12. Allow the specimen to warm up. You already know what the final [room temperature] values of resistivity and Hall effect will be, and what the lowest-T values will be, so this part won’t be that exciting. Your instructor will show you how to speed up the heating a bit with the cryostat’s heater. Leave the instruments on and the software working when you leave the lab today. Your instructor will stop the program and turn off the instruments either tonight or tomorrow morning. They will then be available on the drive where you recorded them. Check that the program has been terminated by checking the last time it was modified. You may now analyze your data. Please make a copy of the file first.

Analysis: Do some literature research on resistivity and the Hall coefficient. Find out how one calculates mobility, , and charge carrier density, n, from these two quantities. Plot these two quantities vs temperature for your specimen, a typical metal.

HW

(2) How does the cryostat work? Is the specimen in contact with the liquid nitrogen?

(3) How does pumping on liquid nitrogen work? The lowest limit for practical cooling through this technique is the freezing point of nitrogen. Look up what this value is. Is it likely that you actually froze any nitrogen?

[Still] Week I. EVAPORATING FILMS.

Your instructor will walk you through deposition of bismuth, a “semimetal”, onto a glass plate and then attaching electrical contacts to the metal. Take good notes about the vacuum pumps and the process of evaporation: you will have HW questions related to these. While you are doing this, your instructor may have you measure the resistivity of a copper wire in the cryostat.

After the film has been deposited and “rescued” from the evaporator, (a) scratch “cloverleaf slits” onto the slide, (b) attach silver paint leads to the corners of the cloverleaf, and (c) heat the specimen in an oven at about 100ºC to dry out the contacts. We will study this specimen next week.

HW

(4) What is the general operating principle behind a diffusion pump? \

(5) Why does the evaporator use such a pump rather than simply the mechanical (rotary) pump?

(6) Why does the diffusion pump need the rotary pump?

(7) What steps would you take if you had the diffusion pump on when (a) Bewkes Hall suddenly lost power? (b) Bewkes Hall suddenly lost all running water?

Week III. RESISTIVITY AND HALL EFFECT in a semimetal.

Repeat the procedures for measuring resistivity and Hall effect, this time for bismuth. Bismuth is a “semimetal”. In what qualitative and quantitative ways might its resistivity and Hall coefficient differ from a metal’s? If we don’t have the opportunity to repeat these measurements on a semiconductor, what qualitative behavior would you expect for these two quantities vs temperature?

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