Photoreflectance and surface photovoltage spectroscopy characterisation of an InGaP/InGaAsN/GaAs NpN DHBT structure

An InGaP/InGaAsnN/GaAs NpN double-heterojunction bipolar transistor (DHBT) structure has been characterised using the techniques of photoreflectance (PR), including the dependence of the signals on the polarisation {[1 1 0] and [1 1̄ 0]} of the incident radiation, and surface photovoltage spectroscopy (SPS). The ordering parameter of the InGaP is deduced from the polarisation dependence of the PR signals from the emitter region. The observed Franz-Keldysh oscillations have been used to evaluate the electric fields in the collector and emitter regions. The field in the collector region agrees well with the theoretical value, while the field in the emitter region is found to be about 25 kV/cm smaller than the theoretical value not taking into account the possible ordering-induced screening effect. The difference is ascribed to the influence of the piezoelectric field related to ordering. In addition, the InGaAsN band gap is determined to be 1.196 eV by analysing the PR and SPS spectra in the base region. The narrower band gap of InGaAsN has led to a lower turn-on voltage, which shows great potential for the application of InGaAsN in low-power electronics.