A INTRODUCTION

Injection currents across heterojunctions are influenced by multiple factors such as material properties (bandgap, dielectric constant, effective carrier masses), interface properties (conduction and valence band discontinuities) and electrostatic forces that are related to the specific doping situation and the presence of an external bias.

In Si/Si _1-xGe _x heterojunctions material properties are largely determined by the value of the Ge fraction x (0 < x < 1). In contrast, interface properties (conduction and valence band discontinuities) do not solely depend on x but are also strongly or even dominantly influenced by factors such as crystal orientation (the present data are related to (100) oriented interfaces) and the presence of layer strain. The latter is an inherent property of the lattice mismatched Si/Si _1-xGe _x heterosystem [1]. To illustrate the specific strain influence on band discontinuities FIGURE 1 shows the energy band diagram for Si and Si _1-xGe _x isolated (i.e. without space charges) if (a) Si is unstrained (Si _1-xGe _x is under compressive strain) and (b) Si _1-xGe _x is unstrained (Si is under tensile strain).

Figure 1: Band alignments in pseudomorphic Si/Si _1-xGe _x heterojunctions for (a) Si _1-xGe _x strained and (b) Si strained. Depending on whether strain is compressive (Si _1-xGe _x) or tensile (Si) the transverse (four-fold degenerate) or vertical (two-fold degenerate) conduction band valleys are dominantly...