Electronic Materials Laboratory

I- Purpose/Objective:

The purpose of this laboratory is to enable you to learn about techniques for testing and state-of-the-art tools for visualization of electronic materials.  The experiments include Ohmic contact preparation and resistivity and Hall effect measurements, plus tutorial information on scanning tunneling microscopy.   We will also determine the structure of semiconductor samples using x-ray diffraction.

For Ohmic contact preparation, we will anneal indium contacts to semiconductors in an annealing station. We will use either a wire bonder to attach Au wire to these contacts or directly solder the wires to the contacts using a small indium soldering iron.  We will then package our sample in a component carrier, using the small indium soldering iron to attach the gold wires to the posts for electrical contact.  We will also measure the current-voltage characteristics of the contacts using a semiconductor parameter analyzer.

In addition to attempting to verify that our contacts are Ohmic (i.e., having linear current-voltage characteristics), we will measure the resistivity and Hall coefficient in order to determine the concentration of free carriers and the ability of those carriers to respond to an electric field.  We will perform these measurements at room temperature and liquid nitrogen temperature, 77 K.

This lab involves five major tasks, which we have scheduled in a sequence in order to minimize bottlenecks:

  • Preparation of Ohmic contacts using the annealing station and wire bonder
  • Current-voltage measurements using the semiconductor parameter analyzer
  • Resistivity and Hall effect measurements using the Hall apparatus
  • Scanning tunneling microscopy tutorial
  • X-ray diffraction measurements of the semiconductors in the Miniflex system

II - Experimental Procedure:

A.  Ohmic Contacts

The laboratory instructor will supply each group with an approximately 0.25” x 0.25” square piece of a semiconductor.  You will need to use a caliper to measure the thickness of your semiconductor square and record this on the Van der Pauw resistivity and Hall measurement worksheet.  After cleaning your sample in solvents, apply indium contacts to the corners, reshape them, and anneal them at 300°C for 1.5 hours in the annealing station.

While the contacts are annealing, you can perform some current-voltage (I-V) measurements on one or more known semiconductor samples (e.g., n-type GaAs, undoped GaAs, n-type InAs, and p-type Si), using the semiconductor parameter analyzer. For each sample, measure, record, and save the I-V characteristics for all four combinations of contacts. Be sure to record your data filenames in your lab notebook. Plan to discuss which samples have contacts with linear I-V characteristics? That is, which are "Ohmic" contacts?  What are the contact resistances of the "Ohmic" contacts?  Use various programs as needed, e.g.,Excel or Kaleidagraph, to plot and analyze the data in order to determine the contact resistances.  Perform the same measurements in both the dark (cap on) and with light (cap off).  Note how the I-V characteristics and contact resistances compare in the dark and with light and offer possible explanations. 

After the contact annealing is complete, and the sample has cooled, use the K&S wirebonder or the small indium soldering iron to connect Au wires to each indium contact.   After re-shaping the indium contact, solvent clean the sample and the pins of a component carrier.  Mount the sample in the component carrier and use the soldering iron to attach the Au wires to the pins of the component carrier.  You are now ready to check if your contacts are Ohmic.

Measure, record, and save the I-V characteristics of all combinations of your contacts.  Are your contacts Ohmic?  Calculate and compare the contact resistances of your sample with those you measured on the known samples.  What is the light sensitivity of your contact?  How do the I-V characteristics and contact resistances compare in the dark and with the light?

B.  Resistivity and Hall Measurements

Check that the HP nanovoltmeter and the Keithley constant current source have been powered up (turned on) for more than one hour.

Measure the magnetic field of the magnets on the Hall set-up, using the Lakeshore gaussmeter, along with the proper probe.  Be sure to record the value on the Van der Pauw resistivity and Hall measurement worksheet.

Measure the resistivity and the Hall Effect at both room temperature and at 77 K in the dark and with light.  How do your results compare and why?  Use this and other information to additionally characterize your semiconductor.

C.  Scanning Tunneling Microscopy (STM) of Cleaved Graphite Surfaces [tutorial only]

The STM is a very important tool for atomic-level characterization of surfaces of many classes of materials, including and perhaps especially semiconductors. Because renovations of the undergraduate laboratories are not yet complete, we will not be able to perform the planned STM calibration experiments using cleaved surfaces of graphite. Instead, a brief tutorial on the technique will be given to each lab group at various times throughout the conduct of this experiment. The procedure for this part of the experiment, had we been able to do it, is given below.

The laboratory instructor will supply each group with a sample of highly oriented pyrolytic graphite (HOPG).  Use clipped Pt-Ir wire tips in the teaching STM to collect high quality images of your cleaved surface.  It may take a few iterations between tips and sample cleaves in order to obtain a stable atomic-resolution image.  Once you have achieved very stable graphite images, acquire several images with approximate sizes 100 x 100 nm, 50 x 50 nm, 10 x 10 nm, and 2 x 2 nm.  In addition, collect scans with the same varying sizes with at least two additional bias voltages, where the sign of the bias voltage has been changed for one of these.  Comment on the similarities and differences you observe. Print out the images and also save them on disk so that you can prepare micrographs for your report.

D. X-ray Diffraction

Each group will collect an x-ray powder pattern of intensity versus 2θ, over the approximate 2Θ range of approximately 20-80o (Why this range?) using the Miniflex system, with a step size of Δ(2Θ) = 0.05° and an integration time of 1 second.  Analyze the diffraction pattern with the JADE software and the JCPDS Powder Diffraction file.  Identify all the peaks in the pattern and index the pattern by labeling each peak with the d-spacing and the Miller index of the plane it represents.  This information will help you to additionally characterize your semiconducting sample. As time permits, obtain similar diffraction data for some of the other semiconductors for which I-V data were obtained in Part A. Comment on any similarities or differences with respect to your specimen.

III - Theory/Background Information:

IV - Theory/Background References:

There are many possible reference books that can be consulted for general background information. Here are a few that are on my bookshelf:

  1. S.T. Thorton and A.Rex, Modern Physics for Scientists and Engineers, Brooks/Cole - Thomson, 2002, chapters 8, 11. [MSE 242 text].
  2. W.D. Callister, Materials Science and Engineering: An Introduction, Wiley, 6th edition,  2003, Chapter 18. [MSE 220, 250 text; note error in the mobility data for GaAs].
  3. C.A. Wert and R.M. Thomson, Physics of Solids, 2nd edition, McGraw-Hill, 1970, chapters 9-13.
  4. C. Kittel, Introduction to Solid State Physics, 4th edition, Wiley, 1971, chapters 7-11.
  5. S.O. Kasap, Principles of Electronic Materials and Devices, 2nd edition, McGraw-Hill, 2002, chapter 2.
  6. R.H. Bube, Electrons in Solids: An Introductory Survey, 2nd edition, Academic Press, 1988, chapter 9.

Some of the specific papers to explore include:

  1. L.J. van der Pauw, A Method of Measuring Specific resistivity and Hall Effect of Discs of Arbitrary Shape, Philips Res. Repts. vol. 13, 1958, pp.. 1-9.
  2. 2. ASTM Standard F 76-86, Standard Test Method for measuring Resistivity and Hall Coefficient and Determining Hall Mobility in Single-Crystal Semiconductors, Annual Book of ASTM Standards, 1996.

V- Activity Schedule:

VI -Format and Important Questions for Lab Report: