MOCVD grown Si-doped n+ InP layers for the subcollector region in HBTs

Subcollector layers in emitter-up HBTs are used to make ohmic contacts to the collector region grown immediately above it. Ideally the n⁺ subcollector layer should have low electrical resistivity, good morphology and an abrupt high/low doping transition between it and the adjacent undoped collector region. Carrier concentrations in excess of 1×10¹⁹/cm³ are desirable for low resistance contacts to the subcollector layers. Doping above 1×10 ¹⁹/cm³ is reported for Sn, S, Se and Te dopants, but memory effects (i.e., continuing of the dopant incorporation into subsequently grown undoped layers) limit their usefulness. Si-doping of InP shows no memory problem, however maximum carrier concentrations seem to be limited to <1×10¹⁹/cm³, similar to Si-doped GaAs, above which degraded morphology occurs. Prospects for higher Si dopant saturation levels are seen from a few citations of carrier concentrations up to ~2×10¹⁹/cm³ and some of these results suggest that lower growth temperature may be the key to achieving higher Si-doping