1：Experimental Design and Method Selection:

The study involved the fabrication of undoped GaAs/AlxGa1 – xAs multiple quantum well heterostructures by molecular-beam epitaxy (MBE). The spectral position of absorption lines corresponding to allowed transitions between quantum-confined electron and hole levels was established. The influence of impurity–defect states on the luminescence and absorption spectra was studied.

2：Sample Selection and Data Sources:

Semi-insulating GaAs(100) substrates were used for epitaxial growth. The heterostructures consisted of GaAs quantum wells separated by AlxGa1 – xAs barriers, with the barrier-layer thickness varied from 50 to 100 nm at an Al content of 15–35%.

3：5%. List of Experimental Equipment and Materials:

3. List of Experimental Equipment and Materials: A Riber Epineat 3-5 setup was used for MBE. Effusion cells and a cracking source were used for evaporating Group-III and Group-V elements, respectively. The substrate temperature was monitored with a thermocouple and a pyrometer. The reflection high-energy electron diffraction (RHEED) technique was used for monitoring the growth of epitaxial layers.

4：Experimental Procedures and Operational Workflow:

The growth process started with a GaAs buffer layer, followed by the heterostructure composed of GaAs QWs separated by AlxGa1 – xAs barriers. The top of the structure was coated with a ~10-nm-thick GaAs layer to avoid oxidation.

5：Data Analysis Methods:

Photoluminescence (PL) and photoluminescence excitation (PLE) spectra were recorded in the temperature range 5–77 K. The PL spectra were excited by a continuous-wave laser emitting at 472 nm, and the recombination emission was analyzed using a grating spectrograph equipped with a multichannel CCD detector.