Electromodulation spectroscopy of metalorganic-vapor-phase-epitaxy-grown GaAs/AlxGa1-xAs multiple quantum wells

This paper describes 17-K, electric-field-dependent electroreflectance and electrotransmission measurements on two GaAs/Al0.3Ga0.7As multiple-quantum-well (MQW) samples, grown by metalorganic vapor-phase epitaxy (MOVPE). Photocurrent and transmission spectra are also taken at different biases for comparison. It is shown how each of these four spectroscopies provide complementary information and how this allows clear deductions to be made about the microscopic structure of the MQW stack. In particular, each technique shows splittings of the E1H1 and E1L1 features, which it is argued derive from sudden changes in the well width at certain depths in the stack. Several measurements demonstrate that the region with wider wells lies deeper in the structure. Under flatband conditions, these monolayer splittings would be smaller than the optical linewidth of the individual quantum wells and would thus be unresolvable. However, the unintentional background doping of the MQWs results in the field being larger at the back of the stack than that at the front, and this enhances the splitting. Hence we have been able to observe monolayer splittings in wider wells (87 and 158) than previously reported. The size of the splittings for both samples, the relative height of the split features in the different spectroscopies, and their dependence on the applied bias are all in agreement with our model for the structure of the stack. The imperfections of the stack have important consequences for MOVPE-grown optoelectronic devices that incorporate MQWs.