Photoconductivity and electroreflectance study of cubic GaN/GaAs(001) heterostructures by optical-biasing technique

Photoconductivity (PC) and electroreflectance (ER) spectrum measurements were carried out for the cubic GaN/GaAs and cubic AlGaN/GaN/GaAs heterostructures grown by LP-MOVPE. Both spectra contained significantly overlapped signals from the c-(Al)GaN epitaxial layer and the underlying GaAs layer, but the latter was successfully suppressed and distinguished from the former by the use of an additional steady-state illumination of Ar⁺ laser as an optical bias. The temperature dependence of the optically-biased PC spectrum showed that there remained structural disorder in the c-GaN layer indicated by the temperature-independent Urbach energy. From the temperature dependence of the ER spectrum, the E₀ critical point of c-GaN observed likely corresponds not to excitonic transition but to free-carrier transition as it shifted by ∼30 meV to higher energies than the reported value, probably due to the dissociation of exciton by the relatively large modulation field applied to the c-(Al)GaN layer.