"The group IV silicon-germanium random alloys differ in several respects from other material combinations treated in this volume. One of the most characteristic features of this material combination concerns bulk Si_1-xGe_x: Si and Ge are miscible over the complete range of compositions. However, the large splitting of the solidus/liquidus phase boundary makes it almost impossible to pull bulk crystals of acceptable radial and axial homogeneity in a composition range that differs from the pure materials by more than a few atomic percent. Hence, interest in bulk alloys, which undoubtedly existed in the 1960s and early 1970s for a variety of reasons, soon waned both because of the rapid switching from Ge to Si as the dominant device material and because of the fundamental difficulties of providing high-quality Si_1-xGe_x substrates. For these reasons, many of the available data concerning the physical properties of bulk Si_1-xGe_x were recorded more than 30 years ago, some of them on material of doubtful crystal quality, especially in the composition range around x = 50%. Material quality and composition-dependent dopant segregation are also the main reason for the almost complete lack of data concerning the dopant-dependence of basic material parameters, such as carrier mobility or fundamental energy gap. Except for a few new attempts toward employing Si_1-xGe_x : bulk alloys with moderate compositions for thermoelectric and optoelectronic devices, only minor activities became known in that direction in the last 20 years or so.
On the other hand, the development of low-temperature growth techniques, such as molecular beam epitaxy or chemical vapor deposition, and the new concepts of energy band engineering (first emerged for III-V materials in the seventies and eighties) led to a fast increase of the number of groups dealing with thin Si_1-xGe_x films. These films are usually deposited on an Si substrate. Because of the inherent lattice mismatch of around 4% between pure Si and pure Ge, such films are tetragonally distorted, when grown to a thickness below the critical value for the onset of misfit dislocations. These films begin to relax to their intrinsic cubic lattice constant, once the critical thickness is exceeded. Hence depending on the thickness of a Si_1-xGe_x film at a given composition (and other growth parameters that rule kinetic limitations), such films can be either biaxially strained or strain-relaxed. Strain-relaxed films can be considered as sort of a virtual "bulk55 substrate. With the art of epitaxial growth rapidly advancing, it was soon recognized that strain is an as important material parameter as composition in the Si_1-xGe_x heterostructure system. Many parameters, such as band gaps, band offsets, effective masses, and so on, are strongly strain-dependent, making strain control a vital necessity for any kind of energy band engineering conceivable in these materials. The advantages gained by the introduction of thin Si_1-xGe_x films in their basic compatibility with standard silicon technologies have made this heterostructure system an extremely interesting candidate for production devices. The first commercial products in the high-frequency analog market segment were introduced in spring 1998".                             Schaffler F. (2001)

This partiton reflects the contrast between the technical relevance of strained Si_1-xGe_x thin films and the quite limited interest in bulk alloys. Where strain-dependent data are given, they are restricted to biaxial strain in the (001) plane, which corresponds to pseudomorphic growth on a (001)-oriented substrate. This is presently the only orientation of technical relevance, but references to other surface orientations are given, when available. Also, data that are important for the relaxation of Si_1-xGe_x on Si, such as critical thickness or misfit dislocation glide velocities, are incorporated. The more detailed part of this chapter is preceded by a table of 300 K bulk data, which also repeats the most basic properties of elemental Si and Ge for comparison. Several parameters, such as the lattice constant, vary almost linearly between the constituents, expressing their close chemical similarity. Other parameters, such as the band gap or the effective electron masses, do not, because the general conduction band structure changes from Si-like to Ge-like at x = 85%. The variation of the room temperature bulk parameters with composition, as listed in the table, gives a quick guide to where linear variation can be expected and where not. In any case, the more detailed plots in the second part should be consulted, especially when dealing with strained films of Si_1-xGe_x alloys.

Basic Parameters

			Remarks	Referens
Crystal structure	Si (x=0)	Diamond	300 K	see also Si. Basic Parameters
	Ge (x=1)	Diamond	300 K	see also Ge. Basic Parameters
	Si1-xGex	Diamond (random alloy)
Group of symmetry	Si (x=0)	Oh7-Fd3m	300 K	see also Si. Basic Parameters
	Ge (x=1)	Oh7-Fd3m	300 K	see also Ge. Basic Parameters
	Si1-xGex	Oh7-Fd3m
Number of atoms in 1 cm3	Si (x=0)	5.0 · 1022	300 K	see also Si. Basic Parameters
	Ge (x=1)	4.42 · 1022	300 K	see also Ge. Basic Parameters
	Si1-xGex	(5.00-0.58x) · 1022
			Remarks	Referens
Bulk modulus	Si1-xGex	(97.9 - 22.8x) GPa	300 K	Schaffler F. et al.(2001)
	Si (x=0)	98 GPa	300 K	see Si. Thermal properties
	Ge (x=1)	75 GPa	300 K	see Ge. Thermal properties
Linear thermal expansion coefficien	Si1-xGex	(2.6 + 2.55x) x 10-6 K-1	x <0.85, 300 K	Schaffler F. et al.(2001)
	Si1-xGex	(-0.89 + 7.53x) x 10-6 K-1	x >0.85, 300 K
	Si (x=0)	2.6 x 10-6 K-1	300 K	see Si. Thermal properties
	Ge (x=1)	5.9 x 10-6 K-1	300 K	see Ge. Thermal properties
Debye temperature	Si1-xGex	(640 - 266x) K	300 K	Schaffler F. et al.(2001)
	Si (x=0)	640 K	300 K	see Si. Thermal properties
	Ge (x=1)	374 K	300 K	see Ge. Thermal properties
Melting point	Si1-xGex    (solidus)	Ts(1412 - 738x + 263x2) oC	solidus, 300 K	Stohr & Klemm (1954)
	Si1-xGex   (liquidus)	Tl (1412 - 80x - 395x2) oC	liquidus, 300 K	Stohr & Klemm (1954)
	Si (x=0)	1412 K	300 K	see Si. Thermal properties
	Ge (x=1)	937 K	300 K	see Ge. Thermal properties
Specific heat	Si1-xGex	(19.6 + 2.9x) J mol-1 K-1		Schaffler F. et al.(2001)
	Si (x=0)	19.6 J mol-1 K-1 0.7 J g-1 K-1	300 K
	Ge (x=1)	22.5 J mol-1 K-1 0.31 J g-1 K-1	300 K
Thermal conductivity	Si1-xGex	(0.046 + 0.084x) W cm-1 K-1	0.2 < x <0.85; 300 K. see also Thermal conductivity vs. composition	Schaffler F. et al.(2001)
	Si (x=0)	1.3 W cm-1 K-1	300 K	see Si. Thermal properties
	Ge (x=1)	0.58 W cm-1 K-1	300 K	see Ge. Thermal properties
Thermal diffusivity	Si (x=0)	0.8 cm2 s-1	300 K	see Si. Thermal properties
	Ge (x=1)	0.36 cm2 s-1	300 K	see Ge. Thermal properties
Thermal expansion coefficient	Si1-xGex	α = (2.6 + 2.55x) x 10-6 K-1	x < 0.85, 300 K	Zhdanova et al. (1967).
	Si1-xGex	α = (7.53 - 0.89x) x 10-6 K-1	x > 0.85, 300 K	Zhdanova et al. (1967).

Dielectric constant (static)	Si (x=0)	11.7	300 K	Schaffler F. et al.(2001)
	Ge (x=1)	16.2	300 K
	Si1-xGexSi1-xGex	11.7 + 4.5x	300 K
Infrared refractive index n(λ)	Si1-xGexSi1-xGex	n 3.42 + 0.37x + 0.22 x2	300K	Schaffler F. et al.(2001)
	Si (x=0)Si (x=0)	n = 3.42	300K	see Si. Refractive index
		n = 3.38(1 + 3.9·10-5·T)	77K < T < 400 K
	Ge (x=1)	n = 4.0	300K	see Ge. Refractive index
Radiative recombination coefficient	Si (x=0)	1.1 x 10-14 cm3 s-1	300 K	see Si. Optical properties
	Ge (x=1)	6.4 x 10-14 cm3 s-1	300 K	see Ge. Optical properties
Optical photon energy	Si1-xGex	(63 - 8.7x) meV	Si - Si, 300 K	Schaffler F. et al.(2001)
		(35.+2.0x) meV	Ge-Ge, 300 K
		50 meV	Si-Ge, 300 K	see also Si1-xGex. Optical phonon Raman signals
	Si (x=0)	63 meV	300 K	see Si. Optical properties
	Ge (x=1)	37 meV	300 K	see Ge. Optical properties

Effective electron mass       (longitudinal)ml	Si1-xGex	0.92mo	300K, x < 0.85	Schaffler F.(2001)
		0.159mo	300K, x > 0.85
Effective electron mass       (transverse)mt	Si1-xGex	0.19mo	300K, x < 0.85	Schaffler F.(2001)
		0.08mo	300K, x > 0.85
Effective mass of density of states  mcd=M2/3 mc       (for all valleys of conduction band)	Si1-xGex	1.06mo	300K, x < 0.85	Son et al. (1994); Son et al. (1995)
		1.55mo	300K, x > 0.85
Effective mass of the density of states mc=(ml+mt2)1/3       (in one valley of conduction band)	Si1-xGex	0.32mo	300K, x < 0.85	Son et al. (1994); Son et al. (1995)
		0.22mo	300K, x > 0.85
Effective hole masses (heavy)   mhh	Si (x=0)	0.537 mo	4.2 K	see also Si. Effective Masses
	Ge (x=1)	0.33 mo		see also Ge. Effective Masses
Effective hole masses (light)   mlh	Si (x=0)	0.153 mo		see also Si. Effective Masses
	Ge (x=1)	0.0430 mo		see also Ge. Effective Masses
Effective hole masses (spin-orbit-split ) mso	Si1-xGex	(0.23-0.135x) mo	300 K	Schaffler F.(2001)
	Si (x=0)	0.234 mo		see also Si. Effective Masses
	Ge (x=1)	0.095(7) mo	30 K	see also Ge. Effective Masses
Effective mass of conductivity  mcc= 3/(1/ml+2/mt)	Si1-xGex	0.26mo	300K, x < 0.85	Son et al. (1994); Son et al. (1995)
		0.12m	300K, x > 0.85
Lattice constant a(x)	Si1-xGex	( 5.431 + 0.20x + 0.027x2) A	300 K	Dismukes et al. (1964b)