MOCVD n-type doping of GaAs and GaAlAs using silicon and selenium and fabrication of double heterostructure bipolar transistor

Abstract

n-Type doping of GaAs and GaAlAs was achieved using SiH 4 and H 2Se in atmospheric pressure MOCVD. We compare the doping behaviour of silicon and selenium with respect to aluminium content of the layers and the memory effects of the doping gases. Very high doping levels with good surface morphology have been obtained for silicon doped Ga 0.7 Al 0.3 As ( n = 10 19 cm -3) and GaAs ( n = 8 × 10 18 cm -3) by using a high growth temperature. For silicon doped GaAs, we could control the electron concentrations from 6 × 10 15 to 8 × 10 18 cm -3. The layers were characterized by Hall effect and photoluminescence measurements. We studied the influence of substrate temperature and of growth rate on doping level and we show, by SIMS measurements, that H 2Se has a strong memory effect, depending on the reactor design (with or without graphite baffles) while SiH 4 presents almost no memory effect. Nevertheless, by carefully adjusting the growth parameters, very good DHBT (double heterostructure bipolar transistor) were grown, with and without a graded composition interface between base-collector and base-emitter using selenium as n-type dopant. A current gain of 5500 has been obtained for a structure with 0.2 μm base thickness, with a graded interface. This is the highest current gain reported for GaAlAs/GaAs bipolar transistor obtained by either MOCVD or MFE techniques, and almost as high as the best LPE results.