Molecular-beam epitaxy of GaN/AlxGa1−xN multiple quantum wells on R-plane (101̄2) sapphire substrates

$\mathrm{GaN}/{\mathrm{Al}}_{\text{0.15}}{\mathrm{Ga}}_{\text{0.85}}\mathrm{N}$ multiple quantum wells (MQWs) have been grown by plasma-assisted molecular-beam epitaxy on R-plane (101̄2) sapphire substrates. The orientation relationship was found to be (112̄0) $(\mathrm{Al})\mathrm{GaN}\text{∥(10\hspace{0.17em}1̄2)}$ ${\mathrm{Al}}_2{\mathrm{O}}_3,$ resulting in nonpolar GaN/AlGaN heterostructures. Room-temperature photoluminescence studies were performed to compare the optical properties of the MQWs grown on (0001) and (101̄2) ${\mathrm{Al}}_2{\mathrm{O}}_3$ substrates. The peak transition energy, as a function of well width for the (112̄0) MQWs, followed the trend for rectangular potential profiles indicating the absence of built-in electrostatic fields. In comparison, the peak transition energies for the (0001) MQWs showed a significant redshift due to the quantum-confined Stark effect, consistent with a built-in field value of 750 kV/cm. In addition, the photoluminescence intensity was 20 to 30 times higher for the (112̄0) MQWs compared to the (0001) MQWs.