18
Strain-Engineered MOSFETs
on the application. For long-channel MOSFET devices, a mobility enhancement up to 10 times for holes and 2 times for electrons has been obtained
using this approach [2].
2.2 Strained SiGe Film Growth
The lattice constant of silicon-germanium can be engineered to be close
enough to that of silicon so that device quality epitaxial layers can be grown
on a silicon (100) surface. The lattice constant of germanium (100) is a Ge =
5.658 Å, and that of silicon (100) is a Si = 5.431 Å. A silicon-germanium alloy,
Si 1–x Ge x , has a lattice constant, a SiGe , that varies linearly between the lattice
constant of silicon and that of germanium. By growing epitaxial SiGe with a
considerable amount of silicon, for example, 70% Si, the lattice constant of the
Si 0.7 Ge 0.3 will be 5.5 Å, which is only 1.2% larger than the silicon lattice constant. A compressively strained pseudomorphic epitaxial layer of Si 0.7 Ge 0.3
can be grown on Si (100). As the thickness of the Si 0.7 Ge 0.3 layer increases, the
amount of strain in the layer increases. Eventually the film reaches a thickness at which its strain energy is greater than the energy needed to form
dislocations or change the surface morphology; this thickness is called the
critical thickness. Below this thickness the film remains fully strained and
does not create defects to relieve that strain.
Many methods have been used for deposition of epitaxial Si and alloys
incorporating Ge, C, and Sn on Si substrates. These can be broadly categorised into physical vapour deposition and chemical vapour deposition.
Both binary silicon-germanium (Si 1–x Ge x ) and ternary silicon-germaniumcarbon (Si 1–x–y Ge x C y ) alloys have found applications in Si CMOS technology. The advances in crystal growth technologies, such as molecular beam
epitaxy (MBE), gas source molecular beam epitaxy (GSMBE), organometallic vapour phase epitaxy (OMVPE), and chemical vapour deposition
(CVD), have enabled ultra-thin epitaxial semiconductor layers to be routinely grown with both monolayer precision in thickness and composition
control to about 1 atomic percent. The main physical vapour deposition
method is MBE, which is widely used because of its excellent control over
thickness and composition of layers. Two or more of them can form alloys,
which have lattice constants between those of the pure form of the constituents. Most of the early work on binary Si 1–x Ge x alloy films was performed
using MBE, whereas growth using CVD systems started much later. An
epitaxially grown SiGe film on a Si substrate contains a compressive strain
because SiGe has a larger lattice constant than Si. Because Ge has an atomic
spacing ~4.17% larger than that of Si, the incorporation of Ge into Si could
Strain-Engineered MOSFETs
on the application. For long-channel MOSFET devices, a mobility enhancement up to 10 times for holes and 2 times for electrons has been obtained
using this approach [2].
2.2 Strained SiGe Film Growth
The lattice constant of silicon-germanium can be engineered to be close
enough to that of silicon so that device quality epitaxial layers can be grown
on a silicon (100) surface. The lattice constant of germanium (100) is a Ge =
5.658 Å, and that of silicon (100) is a Si = 5.431 Å. A silicon-germanium alloy,
Si 1–x Ge x , has a lattice constant, a SiGe , that varies linearly between the lattice
constant of silicon and that of germanium. By growing epitaxial SiGe with a
considerable amount of silicon, for example, 70% Si, the lattice constant of the
Si 0.7 Ge 0.3 will be 5.5 Å, which is only 1.2% larger than the silicon lattice constant. A compressively strained pseudomorphic epitaxial layer of Si 0.7 Ge 0.3
can be grown on Si (100). As the thickness of the Si 0.7 Ge 0.3 layer increases, the
amount of strain in the layer increases. Eventually the film reaches a thickness at which its strain energy is greater than the energy needed to form
dislocations or change the surface morphology; this thickness is called the
critical thickness. Below this thickness the film remains fully strained and
does not create defects to relieve that strain.
Many methods have been used for deposition of epitaxial Si and alloys
incorporating Ge, C, and Sn on Si substrates. These can be broadly categorised into physical vapour deposition and chemical vapour deposition.
Both binary silicon-germanium (Si 1–x Ge x ) and ternary silicon-germaniumcarbon (Si 1–x–y Ge x C y ) alloys have found applications in Si CMOS technology. The advances in crystal growth technologies, such as molecular beam
epitaxy (MBE), gas source molecular beam epitaxy (GSMBE), organometallic vapour phase epitaxy (OMVPE), and chemical vapour deposition
(CVD), have enabled ultra-thin epitaxial semiconductor layers to be routinely grown with both monolayer precision in thickness and composition
control to about 1 atomic percent. The main physical vapour deposition
method is MBE, which is widely used because of its excellent control over
thickness and composition of layers. Two or more of them can form alloys,
which have lattice constants between those of the pure form of the constituents. Most of the early work on binary Si 1–x Ge x alloy films was performed
using MBE, whereas growth using CVD systems started much later. An
epitaxially grown SiGe film on a Si substrate contains a compressive strain
because SiGe has a larger lattice constant than Si. Because Ge has an atomic
spacing ~4.17% larger than that of Si, the incorporation of Ge into Si could