13.2.2
important than the cost. Further, using concentrator photovoltaics (CPV), which is
discussed in Section 15.8, allows the solar cell area to be reduced drastically.
The current world record conversion efficiency for all solar cell technologies on lab
scale is 46.0% for a 4-junction III-V solar cell under 508-fold concentrated sunlight
conditions. This result was achieved by cooperation of Soitec and CEA-Leti, France, with
the Fraunhofer Institute for Solar Energy Systems ISE [48].
Processing of III-V semiconductor materials
As already mentioned above, high quality III-V semiconductor materials can be deposited
using epitaxy deposition methods. In this method, crystalline overlayers are deposited on a
crystalline substrate, such that they adopt the crystal lattice structure of the substrate. The
precursor atoms from which the layers are grown are provided by various elemental
sources. For example, if GaAs is deposited with epitaxy, Ga and As atoms are directed to a
growth surface under ultra high vacuum conditions. For growing III-V semiconductors,
germanium substrates are usually are. On this substrate the GaAs crystalline lattice is
grown layer-by-layer and adopts the structure of the crystalline substrate.
As the layer-by-layer growth process is very slow, it allows the deposition of compact
materials without any vacancy defects. Furthermore, processing at high vacuum
conditions prevents the incorporations of impurities. Hence, III-V semiconductors can be
deposited up to a very high degree of purity. Dopants can be added to make it n- or p-type.
Typically, III-V semiconductor layers are deposited by using metal organic chemical
vapour deposition (MOCVD). Typical precursor gases are trimethylgallium (Ga(CH 3 ) 3 ),
trimethylindium (In(CH 3 ) 3 ), trimehtylaluminium (Al 2 (CH 3 ) 6 ), arsine gas (AsH 3 ) and
phosphine gas (PH 3 ). Surface reactions of the metal-organic compounds and hydrides,
which contain the required metallic chemical elements, create the right conditions for the
epitaxial crystalline growth. Epitaxial growth is a very expensive process; similar
techniques are also used in the microchip production process.
A big challenge of depositing III-V semiconductor materials is that the lattice
constants of the various materials are different, as seen in Figure 13.10. We see that every
III-V semiconductor has a unique bandgap-lattice constant combination. Hence, interfaces
between different III-V materials show a lattice mismatch, as illustrated in Fig. 13.11.
Because of this mismatch, not every valence electron is able to make a bond with a
neighbour. This problem can be solved by lattice matching, as was done in the triplejunction cell discussed in Section 13.2.1. To understand what this means, we take another
look at the phase diagram shown in Fig. 13.11. The triple junction is processed on a p-type
important than the cost. Further, using concentrator photovoltaics (CPV), which is
discussed in Section 15.8, allows the solar cell area to be reduced drastically.
The current world record conversion efficiency for all solar cell technologies on lab
scale is 46.0% for a 4-junction III-V solar cell under 508-fold concentrated sunlight
conditions. This result was achieved by cooperation of Soitec and CEA-Leti, France, with
the Fraunhofer Institute for Solar Energy Systems ISE [48].
Processing of III-V semiconductor materials
As already mentioned above, high quality III-V semiconductor materials can be deposited
using epitaxy deposition methods. In this method, crystalline overlayers are deposited on a
crystalline substrate, such that they adopt the crystal lattice structure of the substrate. The
precursor atoms from which the layers are grown are provided by various elemental
sources. For example, if GaAs is deposited with epitaxy, Ga and As atoms are directed to a
growth surface under ultra high vacuum conditions. For growing III-V semiconductors,
germanium substrates are usually are. On this substrate the GaAs crystalline lattice is
grown layer-by-layer and adopts the structure of the crystalline substrate.
As the layer-by-layer growth process is very slow, it allows the deposition of compact
materials without any vacancy defects. Furthermore, processing at high vacuum
conditions prevents the incorporations of impurities. Hence, III-V semiconductors can be
deposited up to a very high degree of purity. Dopants can be added to make it n- or p-type.
Typically, III-V semiconductor layers are deposited by using metal organic chemical
vapour deposition (MOCVD). Typical precursor gases are trimethylgallium (Ga(CH 3 ) 3 ),
trimethylindium (In(CH 3 ) 3 ), trimehtylaluminium (Al 2 (CH 3 ) 6 ), arsine gas (AsH 3 ) and
phosphine gas (PH 3 ). Surface reactions of the metal-organic compounds and hydrides,
which contain the required metallic chemical elements, create the right conditions for the
epitaxial crystalline growth. Epitaxial growth is a very expensive process; similar
techniques are also used in the microchip production process.
A big challenge of depositing III-V semiconductor materials is that the lattice
constants of the various materials are different, as seen in Figure 13.10. We see that every
III-V semiconductor has a unique bandgap-lattice constant combination. Hence, interfaces
between different III-V materials show a lattice mismatch, as illustrated in Fig. 13.11.
Because of this mismatch, not every valence electron is able to make a bond with a
neighbour. This problem can be solved by lattice matching, as was done in the triplejunction cell discussed in Section 13.2.1. To understand what this means, we take another
look at the phase diagram shown in Fig. 13.11. The triple junction is processed on a p-type