《General Theory of Transport in Silicon- Germanium Alloys》.pdf

Transpo rt in Silicon- Germanium Heterostructures Daniel Chrastina 3 General Theory of Transport in Silicon- Germanium Alloys Ignoring doping for the moment, there are two ways in which an alloy layer may be incorporated within a heterostructure. In a pseudomorphic structure (such as that described in Figure 3.1) a silicon-germanium alloy is grown directly on pure silicon. (It is normal practice not to grow the alloy directly onto the silicon substrate but to deposit a few hundred nanometres of pure silicon first.) The alloy layer is then generally capped by more pure silicon (there are issues regarding the oxidization of a 1 silicon-germanium alloy ). The alloy layer will match its lattice constant to that of the pure silicon, provided it is not too thick, the germanium concentration is not too high, or the growth temperature is not too high. For this reason, whilst pseudomorphic structures are generally simple to grow, they are generally limited to low germanium concentrations and thin alloy layers. Alternatively, to allow for greater flexibility regarding thicker alloys with higher germanium concentrations or pure silicon under tensile strain (for electron channels, see Chapter 6) a virtual substrate may be grown. A buffer layer of alloy is grown and allowed to relax, and then the active channels are grown on this. The germanium concentration within the buffer may be graded to increase upwards, but is generally constant for the few hundred nanometres below the active channel. It is important to ensure that the buffer layer is fully relaxed (by growing it at a high enough temperature) and that the defects are not migrating up into the active channel.2 3.1 Principle Advantages In Using A Silicon-Germanium Heterostructure