Si/SiGe/Si double hetero structure diodes for efficient Si photonics

Thomas Fromherz

So far, three main obstacles have prevented the demonstration of electrically pumped gain in the Si/SiGe material system: the small radiative emission rate in Si and SiGe alloys as a consequence of the indirect nature of the fundamental bandgap in k-space, the type-II bandedge profiles across Si/SiGe hetero-interfaces and the lattice mismatch between Si and SiGe alloys. However, all these obstacles can be overcome: In Ref. [1] it is predicted, that gain can be achieved in indirect bandgap materials like Si at least at cryogenic temperatures. In addition, in this work we show that despite the type II bandalignment, the double heterostructure (DHS) concept, decisive for the implementation of efficient laser diodes [2] and honoured with the 2000 Physics Noble Price for Z. I. Alferov and H. Kroemer, can be applied to Si/SiGe/Si diodes in order to increase the e-h recombination current over diffusion current. Practically, such SiGe layers with sufficiently low point defect density and excellent optical quality can be realized pseudomorphically on Si substrate by MBE growth under ultra low temperature (ULT) conditions, with thicknesses far above the limits predicted by People and Bean, suitable for sufficiently thick gain regions [3].

As example, we demonstrate the first p-n DHS light emitting diode realized in the group-IV material system. The DHS diode is based on a 16 nm thick Si0.6Ge0.4 layer, grown by ULT-MBE, sandwiched between Si bottom- and capping layers. The p-n transition, centred in the SiGe layer, is achieved by doping first with B and then with Sb, both at a concentration of 5x1018cm-3. Intense electroluminescence emission from the SiGe layer is observed in the telecom wavelength region even for temperatures in excess of room temperature. Self-consistent Poisson-current simulations of the electrical current through this type-II p-n DHS diode clearly indicate electron and hole accumulation in the SiGe layer of the biased hetero diode, despite the type-II band edge profile of an unbiased, intrinsic Si/SiGe hetero interface. Thus, similar effects as in a type-I DHS diodes are responsible for the observed enhanced electroluminescence efficiency. Moreover, for bias voltages in excess of EgSiGe/q, the simulations indicate e-h occupation inversion and, thus, gain in the SiGe layer. Here EgSiGe denotes the bandgap energy of the SiGe layer and q the elementary charge. In combination with the epitaxial growth and the self-assembly of colour centers in the Si material system, the DHS concept could enable the realization of efficiently driven quantum emitter diodes for the telecom spectral range.