CBE growth of carbon doped InGaAs/InP HBTs for 25 Gbit/s circuits

The chemical beam epitaxy (CBE) growth of carbon doped InGaAs/InP HBTs for the fabrication of digital circuits using carbon and silicon tetrabromide as source of dopant is reported. Despite their low vapour pressure (CBr 4) or high efficiency (SiBr 4), these sources could be easily controlled at room temperature without carrier gas using a simple pressure regulation system. Carbon doping of InGaAs with CBr 4 could be enhanced up to 1×10 20 at/cm 3 using low growth temperature and V/III ratio. Even at such high doping level, the InGaAs resistivity could not be decreased below 1×10 −3 Ω cm. The electron lifetime in highly carbon-doped InGaAs layers were found slightly improved at a low growth temperature. By comparison with carbon doped GaAs, the ternary layers have inferior majority and minority carrier properties, which induce strong limitations in the base sheet resistance and the current gain of InGaAs/InP HBTs. The introduction of an In graduality in the base layer allowed to improve the HBT current gain. Small unit devices exhibit maximum f t and f max values of 84 and 54 GHz, respectively. Uniformity on 2 in wafer is better than 3% for frequencies and 6% for the gain. Digital circuits were fabricated and tested, such as a 2 : 1 selector showing a very clear eye diagram at 25 Gbit/s.