Neutron depth profiling
Encyclopedia
Neutron depth profiling is a near-surface analysis technique that is commonly used to obtain profiles of concentration as a function of depth for certain technologically important light elements in nearly any substrate. The technique was first proposed by Ziegler et al. to determine the concentration profiles of boron
impurities in silicon
substrates, and later improved by Biersack and coworkers to much of its existing capabilities.
beam passes through a material and interacts with isotopes that emit monoenergetic charged particles upon neutron absorption; either a proton
or an alpha
, and a recoil nucleus
. Since the charged particles are equally likely to be emitted in any direction, reaction kinematics
are straightforward. Because low-energy neutrons are used, there is no significant momentum transfer from the neutron beam to the substrate, and the analysis is practically non-destructive. As charged particles move towards the surface, they are rapidly slowed down, primarily by interacting with the electrons of the substrate. The amount of energy loss is directly related to the thickness penetrated by the particle. The depth of the reaction site can be found by stopping power
correlations.
; most commonly either a surface barrier detector (SBD) or a passivated implanted planar silicon (PIPS) detector. In this configuration, the semiconductor detector is placed opposite to the surface of the sample being analyzed, and an energy spectrum of charged particles emitted by the neutron-induced reaction is acquired.
Boron
Boron is the chemical element with atomic number 5 and the chemical symbol B. Boron is a metalloid. Because boron is not produced by stellar nucleosynthesis, it is a low-abundance element in both the solar system and the Earth's crust. However, boron is concentrated on Earth by the...
impurities in silicon
Silicon
Silicon is a chemical element with the symbol Si and atomic number 14. A tetravalent metalloid, it is less reactive than its chemical analog carbon, the nonmetal directly above it in the periodic table, but more reactive than germanium, the metalloid directly below it in the table...
substrates, and later improved by Biersack and coworkers to much of its existing capabilities.
Neutron depth
In NDP, a thermal or cold neutronNeutron
The neutron is a subatomic hadron particle which has the symbol or , no net electric charge and a mass slightly larger than that of a proton. With the exception of hydrogen, nuclei of atoms consist of protons and neutrons, which are therefore collectively referred to as nucleons. The number of...
beam passes through a material and interacts with isotopes that emit monoenergetic charged particles upon neutron absorption; either a proton
Proton
The proton is a subatomic particle with the symbol or and a positive electric charge of 1 elementary charge. One or more protons are present in the nucleus of each atom, along with neutrons. The number of protons in each atom is its atomic number....
or an alpha
Alpha particle
Alpha particles consist of two protons and two neutrons bound together into a particle identical to a helium nucleus, which is classically produced in the process of alpha decay, but may be produced also in other ways and given the same name...
, and a recoil nucleus
Atomic nucleus
The nucleus is the very dense region consisting of protons and neutrons at the center of an atom. It was discovered in 1911, as a result of Ernest Rutherford's interpretation of the famous 1909 Rutherford experiment performed by Hans Geiger and Ernest Marsden, under the direction of Rutherford. The...
. Since the charged particles are equally likely to be emitted in any direction, reaction kinematics
Kinematics
Kinematics is the branch of classical mechanics that describes the motion of bodies and systems without consideration of the forces that cause the motion....
are straightforward. Because low-energy neutrons are used, there is no significant momentum transfer from the neutron beam to the substrate, and the analysis is practically non-destructive. As charged particles move towards the surface, they are rapidly slowed down, primarily by interacting with the electrons of the substrate. The amount of energy loss is directly related to the thickness penetrated by the particle. The depth of the reaction site can be found by stopping power
Stopping power (particle radiation)
In passing through matter, fast charged particles ionize the atoms or molecules which they encounter. Thus, the fast particles gradually lose energy in many small steps. Stopping power is defined as the average energy loss of the particle per unit path length, measured for example in MeV/cm...
correlations.
Profiling
Conventionally, the residual energies of charged particles and recoil nuclei have been measured by a silicon charged-particle detectorSemiconductor detector
This article is about particle detectors. For information about semiconductor detectors in radio, see Diode#Semiconductor_diodes, rectifier, detector and cat's-whisker detector....
; most commonly either a surface barrier detector (SBD) or a passivated implanted planar silicon (PIPS) detector. In this configuration, the semiconductor detector is placed opposite to the surface of the sample being analyzed, and an energy spectrum of charged particles emitted by the neutron-induced reaction is acquired.