Pulsed laser deposition of ZnO/Mg <i> _x </i> Zn _{1- <i>x</i>} O superlattices for quantum cascade laser applications

We developed a precise molecular layer deposition technique using pulsed laser deposition (PLD) to realize a quantum cascade laser (QCL) structure using wide-bandgap semiconductors ZnO and MgxZn1-xO. Compared to GaAs and InP, ZnO has a larger optical bandgap (3.3 eV) and higher longitudinal optical phonon energy (72 meV). This combination suppresses leakage current and promotes fast nonradiative relaxation, making ZnO a promising material for high-temperature QCL devices operating near room temperature. Nanometer-order thickness control is essential for realizing a QCL device structure. However, PLD techniques suffer from fluctuations in the deposition rate caused by viewport contamination. In this study, a quartz crystal microbalance is used to continuously measure the deposition mass during the PLD process. The combination of a cooling and shielding design with an infrared heating system is used to suppress the thermal drift and plasma damage. Consequently, precise molecular layer control is achieved in the periodic structure of the ZnO/MgxZn1-xO superlattice. The prepared ZnO QCL structure is characterized by scanning transmission electron microscopy and X-ray diffraction, which confirms that the designed superstructure is reproduced with high precision. This study establishes a PLD process to realize ZnO-based QCL and paves the way for developing new QCL platforms using wide-bandgap materials.