Influence of pressure on the properties of GaN/AlN multi-quantum wells – Ab initio study

• AlN and GaN under strain behave in nonlinear manner.
• Tetragonal misfit strain affects properties of AlN//GaN and GaN//AlN layers.
• Electric fields in GaN/AlN MQWs are drastically increased by tetragonal strain.
• Piezo fields are responsible for low pressure coefficients in polar GaN/AlN MQWs.

Pressure dependence of physical properties of GaN/AlN multi-quantum wells (MQWs) was investigated using ab intio calculations. The influence of pressure was divided into two main contributions: pressure affecting the properties of GaN and AlN bulk semiconductors and an influence on systems of polar quantum wells deposited on various substrates. An influence of hydrostatic, uniaxial, and tetragonal strain on the crystallographic structure, polarization (piezoelectricity), and the bandgap of the bulk systems is assessed using ab initio calculations. It was shown that when a partial relaxation of the structure is assumed, the tetragonal strain may explain an experimentally observed reduction of pressure coefficients for polar GaN/AlN MQWs. The MQWs were also simulated directly using density functional theory (DFT) calculations. A comparison of these two approaches confirmed that nonlinear effects induced by the tetragonal strain related to lattice mismatch between the substrates and the polar MQWs systems are responsible for a drastic decrease of the pressure coefficients of photoluminescence (PL) energy experimentally observed in polar GaN/AlGaN MQWs.