Photoluminescence in heavily doped AlGaAs/GaAs heterojunction bipolar transistors

Low-temperature (15 K) photoluminescence (PL) in molecular-beam epitaxy-grown AlGaAs/GaAs heterojunction bipolar transistors (HBTs) has been studied. Both cw and pulsed lasers were used as excitation sources. These HBT samples are heavily doped with Si and Be in the n+-GaAs and p+-GaAs layers, respectively. The origin of each PL signal is identified by removing each layer. Three PL peaks from the n-AlGaAs layer were observed. We found a direct correlation between height of PL peaks in the n-AlGaAs layer and device performance (current gain). We have also studied the correlation between PL in the n-AlGaAs layer and the current gain and Be redistribution from the p+-GaAs base layer into emitter layers while these layers were being grown. Poor current gain was observed as the Be diffusion depth into the emitter layers became large. The Be diffusion depth was measured by means of secondary ion mass spectroscopy. A sharp Be-related peak in the PL spectra from the n-AlGaAs layer clearly indicates Be diffusion into the emitter layers.