Photoreflectance characterization of an InAlAs/InGaAs heterostructure bipolar transistor

We have measured the photoreflectance spectrum at 300 K from a lattice-matched InAlAs/InGaAs heterostructure bipolar transistor grown by molecular beam epitaxy. The energy features of photoreflectance spectra have been identified and the built-in dc electric fields and associated doping profiles have been evaluated in the n-InAlAs emitter from the observed Franz-Keldysh oscillations. The undoped InGaAs spacer between emitter and base was added on to change the built-in electric field. The results showed that the energy features above the InGaAs band gap are the transitions from the valence band to the quantized state of the conduction band. The quantum well of the conduction band is in the interface of the InAlAs and InGaAs heterojunction. The interface charge densities in the spacer channel are determined to be 3.54×1011 cm-2 and 4.22×1011 cm-2, corresponding to the samples with spacer thicknesses of 300 and 500 Å, respectively. A triangular potential profile model was used to calculate the microstructure in the potential well and electron energy transition. The theoretical and experimental results were compared and good agreements were also found.