The Lattice Spacing Variability of Intrinsic Float-Zone Silicon

Area of Science:

• Materials Science
• Metrology
• Crystallography

Background:

• The National Institute of Standards and Technology (NIST) conducts precision lattice spacing comparison measurements.
• These measurements establish traceability for X-ray wavelength and powder diffraction standards to the international system of units (SI).

Purpose of the Study:

• To summarize and document key lattice spacing measurements over the past two decades.
• To assess the internal consistency of measurements on intrinsic float-zone silicon.
• To provide reliable reference values for diffraction applications.

Main Methods:

• Comparison measurements linking unknown lattice spacing to a standard crystal (WASO 04).
• X-ray/optical interferometry used to determine the standard crystal's lattice spacing traceable to the meter definition.
• Analysis of six lots of intrinsic float-zone silicon, including unpublished data.

Main Results:

• The standard crystal WASO 04 has a lattice spacing known with a relative uncertainty of 5 × 10^-9.
• Individual lattice spacing comparison results exhibit typical uncertainties of 1 × 10^-8.
• Measurements accounting for material variability yield relative uncertainties of a few tens of nanometers for test materials.

Conclusions:

• Nearly perfect modern intrinsic float-zone silicon exhibits sufficiently small lattice spacing variability.
• A recommended reference value can be used for most diffraction applications.
• These findings reinforce the reliability of silicon as a standard in metrology.