Heterojunction bipolar transistor characterisation using non-contact optical spectroscopy

In this work we report the application of optical spectroscopic techniques namely photoreflectance (PR), ellipsometry and photoluminescence (PL) for qualification of InGaP/GaAs multi-layer heterojunction bipolar transistor (HBT) material. These techniques reveal important information regarding the quality of the different InGaP and GaAs layers for the emitter, base, collector and surface cap regions. In particular PR studies of non-optimal HBT material reveals InGaP (emitter) layer sub-lattice ordering effects, as correlated with selective area electron diffraction patterns (<311> & 〈511〉). Moreover, comparison of the emitter/base interface field levels and InGaP ordering data reveals further evidence of a non-abrupt inGaP/GaAs heterojunction, proving to have adverse consequences for HBT current gain characteristics and consistent with measured reduced common emitter current gain. Supporting evidence for such non-optimal, strained emitter/base region is provided from x-ray (004) & (002) diffraction but mainly from cleaved edge (g=002) dark field TEM, revealing significant interfacial non-uniformity, also likely correlated to the emitter layer ordering present. PR spectral information is compared with PL lineshape data - including Arrhenius (thermal) plots, while extracted interfacial electric field data are also supported by device finite-element (ANSYS) modelling. In summary this paper demonstrates the application of non-destructive and rapid techniques for evaluation and control of compound semiconductor materials for HBT technology.