13.3.1
Thin-film silicon alloys
Thin-film silicon materials are usually deposited with chemical vapour deposition (CVD)
processes that we will discuss in more detail in Section 13.3.3. In chemical vapour
deposition different precursor gases are brought into the reaction chamber. Due to
chemical reactions, a layer is formed on the substrate. Depending on the precursors used
and other deposition parameters such as the gas flow rate, pressure, and temperature,
various different alloys with different electrical and optical parameters can be deposited.
We will discuss the most important alloys in the following paragraphs.
We start with two alloys consisting of silicon and hydrogen: hydrogenated amorphous
silicon (a-Si:H) and hydrogenated nanocrystalline silicon (nc-Si:H), which is also known
as microcrystalline silicon. The term hydrogenated indicates that some of the valence
electrons in the silicon lattice are passivated by hydrogen, which is indicated by the ‘:H’ in
the abbreviation. The typical atomic hydrogen content of these alloys is from 5% up to
about 15%. The hydrogen passivates most defects in the material, resulting in a defect
density around 10
16
cm
−3 [66], which is suitable for PV applications. Often, the term
‘hydrogenated’ is left out for simplicity. Pure amorphous silicon (a-Si) would have an
extremely high defect density (> 10
19
cm
−3
) [67], which would result in fast recombination
of photoexcited excess carriers. Similarly, we can make alloys from germanium and
hydrogen: hydrogenated amorphous germanium (a-Ge:H) and hydrogenated
nanocrystalline germanium (nc-Ge:H).
Let us now take a look at alloys of silicon with four valence electrons with other
elements with four valence electrons, carbon and germanium. In thin-film silicon solar
cells, both hydrogenated amorphous and nanocrystalline silicon-germanium alloys (aSiGe:H and nc-SiGe:H) are being used. Silicon is also mixed using the four valence
electron material, carbon, leading to hydrogenated amorphous silicon carbide (a-SiC:H).
Another interesting alloy is obtained when oxygen with six valence electrons is
incorporated into the lattice: hydrogenated nanocrystalline silicon oxide is often used in
thin-film silicon solar cells.
All these alloys can be doped, usually boron is used as a p− dopant while phosphorus
is the most common n− dopant.
Many of the alloys mentioned above are present as amorphous materials. It is hence
important to discuss the structure of amorphous lattices. In this discussion we will limit
ourselves to amorphous silicon, since it is the best studied amorphous semiconductor and
the general properties of the other amorphous alloys are similar. In Chapter 12 we
thoroughly discussed crystalline silicon, which has an ordered lattice in which the
Thin-film silicon alloys
Thin-film silicon materials are usually deposited with chemical vapour deposition (CVD)
processes that we will discuss in more detail in Section 13.3.3. In chemical vapour
deposition different precursor gases are brought into the reaction chamber. Due to
chemical reactions, a layer is formed on the substrate. Depending on the precursors used
and other deposition parameters such as the gas flow rate, pressure, and temperature,
various different alloys with different electrical and optical parameters can be deposited.
We will discuss the most important alloys in the following paragraphs.
We start with two alloys consisting of silicon and hydrogen: hydrogenated amorphous
silicon (a-Si:H) and hydrogenated nanocrystalline silicon (nc-Si:H), which is also known
as microcrystalline silicon. The term hydrogenated indicates that some of the valence
electrons in the silicon lattice are passivated by hydrogen, which is indicated by the ‘:H’ in
the abbreviation. The typical atomic hydrogen content of these alloys is from 5% up to
about 15%. The hydrogen passivates most defects in the material, resulting in a defect
density around 10
16
cm
−3 [66], which is suitable for PV applications. Often, the term
‘hydrogenated’ is left out for simplicity. Pure amorphous silicon (a-Si) would have an
extremely high defect density (> 10
19
cm
−3
) [67], which would result in fast recombination
of photoexcited excess carriers. Similarly, we can make alloys from germanium and
hydrogen: hydrogenated amorphous germanium (a-Ge:H) and hydrogenated
nanocrystalline germanium (nc-Ge:H).
Let us now take a look at alloys of silicon with four valence electrons with other
elements with four valence electrons, carbon and germanium. In thin-film silicon solar
cells, both hydrogenated amorphous and nanocrystalline silicon-germanium alloys (aSiGe:H and nc-SiGe:H) are being used. Silicon is also mixed using the four valence
electron material, carbon, leading to hydrogenated amorphous silicon carbide (a-SiC:H).
Another interesting alloy is obtained when oxygen with six valence electrons is
incorporated into the lattice: hydrogenated nanocrystalline silicon oxide is often used in
thin-film silicon solar cells.
All these alloys can be doped, usually boron is used as a p− dopant while phosphorus
is the most common n− dopant.
Many of the alloys mentioned above are present as amorphous materials. It is hence
important to discuss the structure of amorphous lattices. In this discussion we will limit
ourselves to amorphous silicon, since it is the best studied amorphous semiconductor and
the general properties of the other amorphous alloys are similar. In Chapter 12 we
thoroughly discussed crystalline silicon, which has an ordered lattice in which the