Hydrogen diffusion in silicon has been studied for over three decades due to hydrogen's role in altering the electrical properties of silicon devices. Hydrogen can exist in silicon in various forms - atomic, molecular, or bound to defects or impurities - and the probability of each form depends on the defect/impurity concentrations and hydrogen concentration. As a result, hydrogen diffusion varies based on how it is introduced to silicon, such as through ion implantation or plasma exposure. Early experiments in the 1950s found hydrogen is very mobile in silicon with a diffusion coefficient that depends on temperature, silicon conductivity, and charge state, which can be positive, negative, or neutral depending on doping.

1. Introduction
Motivation of studying Si:H systems
The physics and chemistry of hydrogen in silicon has been the subject of considerable
scientific and technological interest for over three decades. This interest has been driven by
the omnipresent appearance of hydrogen in silicon processing which always leads to
hydrogen incorporation into the substrate, either intentionally or unintentionally. Importantly,
this hydrogen may strongly alter the electrical characteristics of the resultant device by
diffusion into active region and passivation of the dopant. Attention has been focused
primarily, although not exclusively, on the diffusion of atomic hydrogen, its molecule
formation, and its complex formation with and passivation of dopant impurities.
Hydrogen diffusion – the central issue of
Si:H system
The diffusion of H in Si is complex because of the presence of several charge states and the
fact that hydrogen in often present in different forms, namely atomic, molecular (or larger
clusters), or bound to a defect or impurity. The probability for the formation of these different
forms is dependent on the defect and impurity concentration in the material and the hydrogen
concentration itself. Thus, the apparent diffusivity is dependent on the method of hydrogen
insertion. Hydrogen can be introduced into silicon through various processes like reactive ion-
etching, glow discharge, plasma hydrogenation, or H-ion implantation and consequently each
method with its distinct influence on defect or impurity generation in the silicon crystal leads
to a different behavior of hydrogen in the material. For example, hydrogen appears to diffuse
more rapidly under conditions of low hydrogen concentration than it does under conditions of
high concentration, such as plasma exposure. Furthermore, H-diffusion turned out to be a
function of the silicon conductivity and type.
H diffusion in Si measurement
The earliest work on the behavior of hydrogen in crystalline silicon is that of Van Wiering
and Warmholz (1956) who studied the diffusion of hydrogen through the walls of cylinders
made of single crystal silicon at high temperatures between 900 and 1200 °C.1Their early high
temperature permeation experiments found that hydrogen
is very mobile in c-Si and has a diffusion coefficient of
DH = 9 × - 3 e -
.4 10 xp ( 0 8e
.4 V
kT
c 2s
m -1)
where k is the Boltzmann constant.
Pseudopotential density functional calculations showed that atomic hydrogen in silicon can
appear in all three charge states H+, H0, and H-.2 The charge state is dependent on the
position of the Fermi level. The positive charge state is more stable in p-type silicon, the
negative charge state more stable in highly n-type doped silicon, and hence, also charge state
dependent diffusion coefficients have been incorporated into many kinetic studies.
[Continue eventually based on Höchbauer thesis]
1
A. Van Wieringen and N. Warmoltz, Physica 22, 849 (1956).
2
T. Ichimiya and A. Furuichi, The International Journal of Applied Radiation and
Isotopes 19, 573 (1968).